Publications

Solid-state nuclear magnetic resonance (solid-state NMR) is an essential tool for probing local structure and dynamics in complex materials, yet its uptake in the broader chemistry community has remained limited by technical and operational barriers. The PANACEA project established a pan-European infrastructure to transform solid-state NMR into a community-ready analytical technique by combining state-of-the-art instrumentation, coordinated user access, and targeted technological innovation. Over three years, PANACEA enabled over 90 user projects across chemistry and materials science, while driving advances in DNP methods, probe design, ultra-fast MAS, and interoperable software platforms such as EasyNMR and CHEMeDATA. This article presents the main outcomes of the initiative, illustrating how infrastructure-driven research and guided access can broaden the impact of solid-state NMR and integrate it into mainstream chemical workflows.

Dynamic nuclear polarization (DNP) is a powerful route for overcoming the inherent sensitivity limitation of solid-state nuclear magnetic resonance (ssNMR) spectroscopy by transferring high electron spin polarization to surrounding nuclear spins. Cross-effect (CE) DNP is the most efficient mechanism in solids. CE requires several conditions to be met, primarily the presence of two coupled electron spins with resonance frequencies separated by the nuclear Larmor frequency. This condition is typically achieved through the presence of large anisotropic spin interactions, which shift the transition frequencies of the two coupled electron spins with respect to each other. Here we present an alternative approach, where the CE condition is met via isotropic interactions. This is advantageous as it makes CE independent of the sample orientation, thus making the enhancements independent of the MAS frequency and enabling the use of fast relaxing polarizing agents. We demonstrate the feasibility of the approach in experiments and simulations for Mn(II) dopants as polarizing agents, making use of the isotropic hyperfine interactions with its 55Mn nuclear spin to achieve the required frequency difference.

X-ray Photoelectron Spectroscopy (XPS), a surface-sensitive technique with atomic-scale sensitivity, provides unique insights into the elemental composition, as well as the chemical and electronic states of elements within a material. Given its ability to analyze surfaces with high precision, XPS has become a prominent tool in battery research. Despite its numerous advantages and high reliability, XPS has certain limitations caused by experimental requirements. Factors such as X-ray radiation, high vacuum conditions, and necessity for charge neutralization can potentially alter the sample composition during analysis, leading to misinterpretations of artificial reaction products as genuine battery reactions processes. This concern is particularly critical in the analysis of nonaqueous battery systems containing liquid electrolytes, where volatile components are highly sensitive to vacuum conditions and X-ray exposure. In this study, we systematically investigate the effects of XPS conditions on Lithium bis(Trifluoromethane)Sulfonimide (LiTFSI), a widely used component in nonaqueous battery electrolytes. We demonstrate how prolonged vacuum exposure, application of a low-energy electron flood gun (eFG) and extended X-ray irradiation contribute to the continuous degradation of LiTFSI. Additionally, we propose partial solutions to mitigate these decomposition processes, thereby improving the reliability of XPS measurements in battery research.

Interfacial chemistry plays a central role in the development of next-generation high-energy Li-ion electrode materials. Yet, the rational design of new surface treatments that serve as beneficial solid electrolyte interphases is hindered by the challenges involved in probing their interfacial ion transfer properties. Here, we demonstrate how⁷Li Dark-State Exchange Saturation Transfer (DEST) NMR can be used to directly measure the Li-ion surface adsorption process at the solidliquid interface. The development of an optimized model system composed of monodisperse submicron particles allowed for comparison between different surface functionalities, enabling the characterization of state-of-the-art electrode coating layers in terms of their Li-ion affinity. Numerical simulations based on BlochMcConnell equations enable a quantitative analysis of the surface exchange rates and binding properties, cementing DEST as a valuable tool for elucidating the structurefunction relationship in electrode materials.

Lithium metal batteries are a promising energy storage technology, but their commercialization is hindered by nonuniform lithium deposition, which is detrimental to the battery lifetime and safety. In particular, needle-like dendrites pose the greatest risk as they often lead to short-circuits; as such, it is essential to identify and mitigate their formation for enabling use of lithium metal anodes. Here we demonstrate that Overhauser dynamic nuclear polarization (DNP)- enhanced NMR, where the high polarization of the lithium conduction electrons increases the sensitivity of lithium NMR, is a powerful tool for determining the lithium morphology. By systematically controlling the deposited lithium structures within a polymer electrolyte system, we show that DNP enhancement correlates with morphology, allowing us to distinguish between micro- and nano-sized dendrites. Complementary electron paramagnetic resonance and electron microscopy measurements confirm the morphological interpretation. This work introduces a spectroscopic strategy for sensitively probing lithium dendritic structures with high specificity, offering a pathway to understand and control their formation across a range of battery systems and electrochemical formation conditions.

Exploring the next generation of batteries based on sustainable materials is crucial for transitioning to a post-lithium-technology era. Potassium-based technology is one of the candidates that could fulfil the sustainability criteria. The electrolyte plays a crucial role in battery performance, being responsible for ionic transport, cycling performance, working temperature, and safety. Polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) have been extensively studied. However, one of the drawbacks of SPEs is their poor ionic conductivity at room temperature (RT). Quasi-solid and solvent-free polymer electrolytes are a promising solution to this issue, combining the benefits of a liquid phase and SPEs. This work focuses on the development of cross-linked PEO, fluorinated K salt, and ionic liquid (IL)-based solvent-free SPEs for potassium batteries. The designed cross-linked ternary solvent-free SPEs are thoroughly characterized both physicochemically and electrochemically, achieving ionic conductivities of up to 1.6 x 10-3 S cm-1 at 20 degrees C. The solvent-free SPEs were tested in K cells at 20 degrees C, using a Prussian white (PW) cathode as a proof-of-concept. The effects that different fluorinated anions, such as bis(fluoromethanesulfonyl) imide (FSI-) and bis(trifluoromethanesulfonyl) imide (TFSI-), have on the electrochemical performance were analysed by investigating the solid electrolyte interphase (SEI) formed on the K metal surface through electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and magic-angle spinning solid-state nuclear magnetic resonance spectroscopy (MAS-ssNMR).

Development of functional nanocrystals requires precise control over their composition and structure. Particularly, surface composition, defects, and doping play a central role in our ability to develop functional nanomaterials. As such, there is great interest in capturing these properties. Solid-state NMR spectroscopy is a powerful tool for probing structural and compositional features at the atomic scale, in particular, when it is coupled with the high sensitivity gained by dynamic nuclear polarization (DNP). DNP enhances NMR sensitivity by transferring high electron spin polarization to the surrounding nuclear spins. This dramatically improves the signal intensity, making it a valuable tool for detecting subtle structural features. Utilizing metal ion dopants as polarization agents for DNP has been shown to be an excellent approach to increasing ssNMR sensitivity in the bulk of inorganic solids. Here, we demonstrate the implementation of this approach to nanocrystals, focusing on Mn(II)-doped CdS, where homogeneous doping is known to be challenging while being critical for the DNP process. The intricate nature of the doping was elucidated by quantitative electron microscopy and electron paramagnetic resonance spectroscopy. We confirmed that Mn(II) doping is confined to the core of the nanocrystals and that statistically dopants are homogeneously distributed within each nanocrystal. DNP from Mn(II) dopants is then shown to increase ¹¹³Cd NMR sensitivity by an order of magnitude, enabling distinction between core and surface environments as well as the detection of defects in the bulk of the nanocrystals. We expect that the approach can be extended to other nanocrystals, providing an efficient route for characterizing their bulk and surface properties.

Liquid organic hydrogen carriers (LOHCs) are an effective solution for the long-term storage of hydrogen and its long-distance transportation, but one that requires efficient catalysis for hydrogen uptake and release. Homogeneous molecular catalysts employed in LOHC systems usually exhibit high reactivity and selectivity but are difficult to recover and reuse, a fact that severely limits their practical application. Herein, we report an easily synthesized polymer-grafted pincer-type complex of earth-abundant manganese, which can catalyze the aqueous dehydrogenation of methyl formate, an emerging LOHC material. Importantly, this immobilized Mn-pincer catalyst retains the high reactivity of the corresponding molecular catalyst, yet is also recyclable, exhibiting high stability and achieving a total hydrogen-based turnover number of more than 230000 after 5 consecutive recycling rounds.

The sensitivity of solid state nuclear magnetic resonance spectroscopy can be enhanced via dynamic nuclear polarization (DNP) using unpaired electrons as polarizing agents. In metal ions based (MI)-DNP, paramagnetic metal ions are introduced as dopants into inorganic materials serving as endogenous polarizing agents. Having polarizing agents as part of the structure enables signal enhancements within the bulk of the material. Nuclear spins can be hyperpolarized either directly through their coupling to the polarizing agent or via homonuclear spin diffusion. In this work, we addressed what are the factors determining the relative sizes of the spin pools polarized by each of these two mechanisms and how changing their contribution to the polarization process affects the experimental outcome. Experimentally, we adjusted the spin diffusion rate through modifying the isotope ratio ⁶Li/⁷Li in otherwise identical samples, Li₄Ti₅O₁₂ doped with paramagnetic Fe(III). DNP experiments on samples with typical content of polarizing agents for MI-DNP, corroborated by simulations, evidenced that while the efficiency of spin diffusion has large effects on the polarization buildup times, the enhancements remain largely unaffected.

Silica polymerization from its soluble monomers is fundamental to many chemical processes. Although industrial methods require harsh conditions and concentrated precursors, biological silica precipitation occurs under ambient conditions from dilute solutions. The hallmark of biosilica is the presence of amine-rich organic macromolecules, but their functional role remains elusive. Here, we show a pH-dependent stimulatory effect of such polyamines on silica polymerization. Notably, this process is decoupled from the saturation degree, allowing the synthesis of polymer-silica hybrid products with controlled network morphologies from undersaturated solutions. The data suggest a two-step phase separation process. First, an associative liquid-liquid phase separation forms a micrometer-size dense phase. Second, silica undergoes a liquid-to-solid transition in the supersaturated condensates to form a bicontinuous silica structure. This study can inspire \u201csoft chemistry\u201d routes to design organic-inorganic nanomaterials with regulatory principles optimized by evolution.

In this protocol we outline the practical aspects and methodology for performing metal ions-based dynamic nuclear polarization (MI-DNP), focusing on materials science applications. In MI-DNP polarization is transferred from unpaired electrons of paramagnetic metal ions to nearby nuclear spins, thereby increasing the sensitivity of NMR spectroscopy. The protocol encompasses detailed steps for (i) selecting suitable metal ion dopant based on chemical, structural and electron paramagnetic resonance (EPR) considerations, (ii) characterizing the concentration, homogeneity and EPR properties of the dopant and (iii) performing the MI-DNP experiment itself, including optimization of the field position and reliable assessment of the DNP enhancement factors. By adhering to this protocol, the interested reader can implement the MI-DNP approach in an efficient way, facilitating spectroscopic studies of functional materials.

Polymer-ceramic composite electrolytes enable safe implementation of Li metal batteries with potentially transformative energy density. Nevertheless, the formation of Li-dendrites and its complex interplay with the Li-metal solid electrolyte interphase (SEI) remain a substantial obstacle which is poorly understood. Here we tackle this issue by a combination of solid-state NMR spectroscopy and Overhauser dynamic nuclear polarization (DNP) which boosts NMR interfacial sensitivity through polarization transfer from the metal conduction electrons. We achieve detailed molecular-level insight into dendrites formation and propagation within the composites and determine the composition and properties of their SEI. We find that the dendrites quantity and growth path depend on the ceramic content and correlated with batterys lifetime. We show that the enhancement of Li resonances in the SEI occurs through Li/Li⁺ charge transfer in Overhauser DNP, allowing us to correlate DNP enhancements and Li transport and directly determine the SEI lithium permeability. These findings have promising implications for SEI design and dendrites management which are essential for the realization of Li metal batteries.

The practical realization of Nickel-rich layered oxide cathode materials such as LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) is hampered by several structural and interfacial instabilities over prolonged cycling. Several reports have proposed surface passivation via an artificial cathode electrolyte interphase (ACEI) as a promising method for mitigating the parasitic reactions affecting NMC811 while simultaneously improving its electrochemical performance over prolonged cycling. Herein, we report an in-house designed (tBuMe₂Si)₂Zn single source precursor for developing Si_xZn_yO_z ternary CEI thin films on NMC811 via molecular layer deposition (MLD) in combination with O₃ or H₂O as oxidizing agent. We demonstrate that the single precursor (tBuMe₂Si)₂Zn avoids the need for two different precursors (Si & Zn). In-depth spectroscopic studies reveal the mechanism of the formation of organosiloxane/zinc-oxide composite thin film, via intermediates of unprecedented organo-silicon-zinc compounds. Understanding the reaction mechanism paved the path for a successful deposition of ACEI on NMC811. Rate capability studies shows the ACEI protected cathodes exhibit higher discharge capacity at 4 C than pristine NMC811. Furthermore, studies on full cells with graphite anode were conducted to evaluate the practical viability of Si_xZn_yO_z ACEI thin films on NMC811. After prolonged cycling the ACEI-coated NMC811 full cells significantly improved the electrochemical performance than pristine NMC811 by ~12%.

Sodium ion batteries (SIB) are among the most promising devices for large scale energy storage. Their stable and long-term performance depends on the formation of the solid electrolyte interphase (SEI), a nanosized, heterogeneous and disordered layer, formed due to degradation of the electrolyte at the anode surface. The chemical and structural properties of the SEI control the charge transfer process at the electrode-electrolyte interface, thus, there is great interest in determining these properties for understanding, and ultimately controlling, SEI functionality. However, the study of the SEI is notoriously challenging due to its heterogeneous nature and minute quantity. In this work, we present a powerful approach for probing the SEI based on solid state NMR spectroscopy with increased sensitivity from dynamic nuclear polarization (DNP). Utilizing exogenous (organic radicals) and endogenous (paramagnetic metal ion dopants) DNP sources, we obtain not only a detailed compositional map of the SEI but also, for the first time for the native SEI, determine the spatial distribution of its constituent phases. Using this approach, we perform a thorough investigation of the SEI formed on Li₄Ti₅O₁₂ used as a SIB anode. We identify a compositional gradient, from organic phases at the electrolyte interface to inorganic phases toward the anode surface. We find that the use of fluoroethylene carbonate as an electrolyte additive leads to performance degradation which can be attributed to formation of a thicker SEI, rich in NaF and carbonates. We expect that this methodology can be extended to examine other titanate anodes and new electrolyte compositions, offering a unique tool for SEI investigations to enable the development of effective and long-lasting SIBs.

Three new crystalline potassium salts of hybrid carboxylate/Schiff-base oligomers are reported as electrochemically active anode materials for potassium ion batteries. One of them is constituted by the phenyl \u201cdimer\u201d KOOC-ϕ-C=N-ϕ-COOK (BSK-1) and the other two by the isomer \u201ctrimers\u201d KOOC-ϕ-C=N-ϕ-N=C-ϕ-COOK (BSK-2 A) and KOOC-ϕ-N=C-ϕ-C=N-ϕ-COOK (BSK-2B). A structural model refined from powder X-ray diffraction data is discussed for the short (dimeric) salt. A pseudo-monoclinic unit cell, with space group P1¯ and lattice parameters a = 15.88(1) Å, b = 5.772(2) Å, c = 3.887(1) Å and β = 98.8(1)⁰, contains a single molecule. Crystal packing of the aromatic molecules can be described by a slipped π-stacking along the c-axis. All three oligomers are electrochemically active at voltages low enough to be used as anode in potassium ion batteries, at average voltages of 1.2 V vs K⁺/K for the dimer and 1.05 V vs K⁺/K for the trimers. Combined with 3.9 M KFSI in DME electrolyte, they show reversible capacities of 115 mAh/g (dimer), 233 mAh/g (BSK-2 A) and 104 mAh/g (BSK-2B) at C/10, with capacity retentions at 1C of 74% and 65% for the dimer and the trimers respectively. Based on the dQ/dV plots, the trimers react through two solid solution mechanisms, forming a stable radical intermediate, whereas the dimer changes from a single solid solution to a solid-solution+biphasic reaction. Future in-situ x-ray diffraction studies based on the solved crystal packing should enable to confirm these mechanisms.

Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4,4-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold ¹³C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research.

Potassium-ion batteries (PIBs) have attracted significant attention as a complement to lithium-ion and sodium-ion batteries (SIBs). PIBs can theoretically provide higher specific energy and power density than SIBs due to lower standard electrode potential of K/K+ and faster K+ ion diffusion, maintaining the benefits of low-cost and sustainability. However, research on PIBs is in its infancy; therefore, further efforts are necessary to enhance their performance and position them as a competitive technology. In this perspective, the remaining challenges and possible strategies to advance the development of PIBs are presented.

Over the last two decades magic angle spinning dynamic nuclear polarization (MAS DNP) has revolutionized NMR for materials characterization, tackling its main limitation of intrinsically low sensitivity. Progress in theoretical understanding, instrumentation, and sample formulation expanded the range of materials applications and research questions that can benefit from MAS DNP. Currently the most common approach for hyperpolarization under MAS consists in impregnating the sample of interest with a solution containing nitroxide radicals, which upon microwave irradiation serve as exogenous polarizing agents. On the other hand, in metal ion based (MI)-DNP, inorganic materials are doped with paramagnetic metal centres, which then can be used as endogenous polarizing agents. In this work we give an overview of the electron paramagnetic resonance (EPR) concepts required to characterize the metal ions and discuss the expected changes in the NMR response due to the presence of paramagnetic species. We highlight which properties of the electron spins are beneficial for applications as polarizing agents in DNP and how to recognize them, both from the EPR and NMR data. A theoretical description of the main DNP mechanisms is given, employing a quantum mechanical formalism, and these concepts are used to explain the spin dynamics observed in the DNP experiment. In addition, we highlight the main differences between MI-DNP and the more common approaches in MAS DNP, which use organic radicals as exogenous polarizing source. Finally, we review some applications of metal ions as polarizing agents in general and then focus particularly on research questions in materials science that can benefit from MI-DNP.

Lithium-sulfur (Li-S) batteries (LSBs) have high energy densities and employ inexpensive materials. However, the poor sulfur conductivity and rapid capacity fading hamper their applications. We developed a free-standing composite cathode based on multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs), whose fabrication follows a solution-based, scalable method. The two CNT types create a synergic effect: SWCNTs result in high conductivity, high surface area, and mechanical strength/flexibility; MWCNTs larger pores ensure facile ionic diffusion and trapping of lithium polysulfides. The composite cathode exhibits a peak discharge capacity of 1221 mAh/g, maintaining 876 mAh/g after 100 cycles at a 0.1C rate.

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li2OCaO·2SiO2 and Li2CaSiO4, respectively. Using Gd­(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both 6Li and 29Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

Surface degradation is a major limitation in the utilization of high energy cathodes. Cathode coatings provide a promising route for achieving interfacial stability. In this case, there is interest in developing coatings which will provide efficient ion transport across the cathode-electrolyte-interface (CEI). Aluminum based coatings, particularly metal fluorides, have shown great promise for stabilizing the high voltage cathode, LiNi0.5Mn1.5O4. Despite the clear practical advantages of coatings, the chemical basis for their improved performance remains ambiguous. Here we present an in-depth investigation of a nominal AlF3 coating on LiNi0.5Mn1.5O4 based on solid state NMR spectroscopy to determine the composition and function of the Al/F coating layer. NMR results, supported by X-ray photoelectron spectroscopy and electron microscopy, reveal that the actual composition of the deposited layer is an amorphous AlOF phase. Isotope exchange is used to follow the spontaneous exchange of lithium ions across the CEI, revealing improved transport in coated cathodes. Finally, NMR experiments provide evidence for insertion of lithium ions in the Al/F coating following electrochemical cycling. The results suggest that lithium insertion into a nominally lithium-less coating plays a role in its improved performance, highlighting lithium content as an important factor in designing beneficial coatings for higher energy cathodes.

In this chapter, an introduction to dynamic nuclear polarization (DNP) solid state nuclear magnetic resonance (NMR) is given. The main focus lies in presenting the use of paramagnetic metal ion dopants in inorganic materials as endogenous sources of hyperpolarization. Relevant concepts of NMR in the presence of paramagnetic centers are introduced and the main DNP mechanisms are explained with a quantum mechanical formalism. The presented theoretical basis allows an in-depth discussion of the limits and possibilities of metal ion-based DNP (MIDNP). Conceptual and practical differences of this approach and exogenous DNP are discussed, in particular the advantages offered by MIDNP as a tool for structural elucidation of inorganic solids are highlighted. Finally, practical considerations to help implementing this experimental technique are given.

Carbonaceous materials are ubiquitous in energy storage and conversion systems due to their versatile chemical and physical properties. The surface chemistry of carbonaceous materials strongly affects their properties, and there is therefore great interest in determining the chemical composition of different carbon allotropes. Solid-state nuclear magnetic resonance spectroscopy is well suited for providing atomic-level structural information, especially when equipped with sensitivity from dynamic nuclear polarization (DNP). Exogenous DNP from nitroxide biradicals is the most efficient approach, typically providing 10²-10⁴fold enhancement in surface sensitivity. Herein, we consider the application of DNP in the study of the surface chemistry of carbonaceous materials through natural abundance ¹³C detection. We found that with TEKPol biradicals, the polarization transfer via ¹H-¹³C cross-polarization is limited to the solvent and does not propagate to the sample surface. We thus investigated in detail polarization transfer directly to ¹³C nuclei and found multiple interfering mechanisms when employing the exogenous DNP approach: (a) direct ¹³C polarization from the biradicals, (b) opposite enhancements due to heteronuclear cross-relaxation leading to the solvent Overhauser effect, and (c) solid effect from defects and delocalized electrons within the carbons. While the endogenous electron polarization interferes with the utilization of exogenous DNP, it provides significant surface sensitivity with signal enhancements of up to 50 and 20 for ¹³C and ¹H, respectively. Moreover, we show that endogenous DNP can be used at a wide range of temperatures, providing close to a 10-fold increase in ¹³C and ¹H signals at room temperature through differing DNP mechanisms. This approach opens the way for efficient detection of carbon surface chemistry under ambient conditions.

Lithium metal anodes offer a huge leap in the energy density of batteries, yet their implementation is limited by solid electrolyte interphase (SEI) formation and dendrite deposition. A key challenge in developing electrolytes leading to the SEI with beneficial properties is the lack of experimental approaches for directly probing the ionic permeability of the SEI. Here, we introduce lithium chemical exchange saturation transfer (Li-CEST) as an efficient nuclear magnetic resonance (NMR) approach for detecting the otherwise invisible process of Li exchange across the metal-SEI interface. In Li-CEST, the properties of the undetectable SEI are encoded in the NMR signal of the metal resonance through their exchange process. We benefit from the high surface area of lithium dendrites and are able, for the first time, to detect exchange across solid phases through CEST. Analytical Bloch-McConnell models allow us to compare the SEI permeability formed in different electrolytes, making the presented Li-CEST approach a powerful tool for designing electrolytes for metal-based batteries.

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T1/T2, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMg1−xMnxPO4 and Fe(III) doped Li4Ti5O12, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300 K.

The increasing need for portable and large-scale energy storage systems requires development of new, long lasting and highly efficient battery systems. Solid state NMR spectroscopy has emerged as an excellent method for characterizing battery materials. Yet, it is limited when it comes to probing thin interfacial layers which play a central role in the performance and lifetime of battery cells. Here we review how Dynamic Nuclear Polarization (DNP) can lift the sensitivity limitation and enable detection of the electrode-electrolyte interface, as well as the bulk of some electrode and electrolyte systems. We describe the current challenges from the point of view of materials development; considering how the unique electronic, magnetic and chemical properties differentiate battery materials from other applications of DNP in materials science. We review the current applications of exogenous and endogenous DNP from radicals, conduction electrons and paramagnetic metal ions. Finally, we provide our perspective on the opportunities and directions where battery materials can benefit from current DNP methodologies as well as project on future developments that will enable NMR investigation of battery materials with sensitivity and selectivity under ambient conditions.

Solid-state nuclear magnetic resonance (NMR) spectroscopy has increasingly been used for materials characterization as it enables selective detection of elements of interest, as well as their local structure and dynamic properties. Nevertheless, utilization of NMR is limited by its inherent low sensitivity. The development of dynamic nuclear polarization (DNP) approaches, which provide enormous sensitivity gain in NMR through the transfer of polarization from electron spins, has transformed the application of solid-state NMR in materials science. In this review, we outline the opportunities for materials characterization that DNP has opened up. We describe the main DNP mechanisms available, their implementation, and the kinds of insight they can provide across different materials classes, from surfaces and interfaces to defects in the bulk of solids. Finally, we discuss the current limitations of the approach and provide an outlook on future developments for DNP-enhanced NMR spectroscopy in materials science.

Colloidal inorganic nanofluorides have aroused great interest for various applications with their development greatly accelerated thanks to advanced synthetic approaches. Nevertheless, understanding their colloidal evolution and the factors that affect their dispersion could improve the ability to rationally design them. Here, using a multimodal in situ approach that combines DLS, NMR, and cryogenic-TEM, we elucidate the formation dynamics of nanofluorides in water through a transient aggregative phase. Specifically, we demonstrate that ligand-cation interactions mediate a transient aggregation of as-formed CaF2 nanocrystals (NCs) which governs the kinetics of the colloids' evolution. These observations shed light on key stages through which CaF2 NCs are dispersed in water, highlighting fundamental aspects of nanofluorides formation mechanisms. Our findings emphasize the roles of ligands in NCs' synthesis beyond their function as surfactants, including their ability to mediate colloidal evolution by complexing cationic precursors, and should be considered in the design of other types of NCs.

Comprehending the oxygen vacancy distribution in oxide ion conductors requires structural insights over various length scales: from the local coordination preferences to the possible formation of agglomerates comprising a large number of vacancies. In Y-doped ceria, ⁸⁹Y NMR enables differentiation of yttrium sites by quantification of the oxygen vacancies in their first coordination sphere. Because of the extremely low sensitivity of ⁸⁹Y, longer-range information was so far not available from NMR. Herein, we utilize metal ion-based dynamic nuclear polarization, where polarization from Gd(III) dopants provides large sensitivity enhancements homogeneously throughout the bulk of the sample. This enables following ⁸⁹Y⁸⁹Y homonuclear dipolar correlations and probing the local distribution of yttrium sites, which show no evidence of the formation of oxygen vacancy rich regions. The presented approach can provide valuable structural insights for designing oxide ion conductors.

Degradation processes at the cathodeelectrolyte interface are a major limitation in the development of high-energy lithium-ion rechargeable batteries. Deposition of protective thin coating layers on the surface of high-energy cathodes is a promising approach to control interfacial reactions. However, rational design of effective protection layers is limited by the scarcity of analytical tools that can probe thin, disordered, and heterogeneous phases. Here we propose a new structural approach based on solid-state nuclear magnetic resonance spectroscopy coupled with dynamic nuclear polarization (DNP) for characterizing thin coating layers. We demonstrate the approach on an efficient alkylated LixSiyOz coating layer. By utilizing different sources for DNP, exogenous from nitroxide biradicals and endogenous from paramagnetic metal ion dopants, we reveal the outer and inner surface layers of the deposited artificial interphase and construct a structural model for the coating. In addition, lithium isotope exchange experiments provide direct evidence for the function of the surface layer, shedding light on its role in the enhanced rate performance of coated cathodes. The presented methodology and results advance us in identifying the key properties of effective coatings and may enable rational design of protective and ion-conducting surface layers.

Understanding inorganic nanocrystal (NC) growth dynamic pathways under their native fabrication environment remains a central goal of science, as it is crucial for rationalizing novel nanoformulations with desired architectures and functionalities. We here present an in-situ method for quantifying, in real time, NCs size evolution at sub-nm resolution, their concentration, and reactants consumption rate for studying NC growth mechanisms. Analyzing sequential high-resolution liquid-state ¹⁹F-NMR spectra obtained in-situ and validating by ex-situ cryoTEM, we explore the growth evolution of fluoride-based NCs (CaF₂ and SrF₂) in water, without disturbing the synthesis conditions. We find that the same nanomaterial (CaF₂) can grow by either a particle-coalescence or classical-growth mechanism, as regulated by the capping ligand, resulting in different crystallographic properties and functional features of the fabricated NC. The ability to reveal, in real time, mechanistic pathways at which NCs grow open unique opportunities for tunning the properties of functional materials.

The commercialization of the high energy, lithium, and manganese-rich NCM (LMR-NCM) is impeded by its complex interfacial electrochemical processes, oxygen release, and surface degradation. Here, we introduced t-butyl-dimethylsilyllithium as a single-source precursor for depositing Li_xSi_yO_z with an integrated network of siloxane moieties as an artificial cathode/electrolyte interphase (ACEI) which stabilizes LMR-NCM by mitigating oxygen release, electrolyte degradation and preventing fractures. Using solid-state NMR coupled with dynamic nuclear polarization, detailed molecular-level characterization of the ACEI is presented. The proposed CEI enabled improved energy-density at high rates (644 Wh.kg^-1, compared to uncoated material with 457 Wh.kg^-1 at 4C) with suppressed parasitic reactions and O₂ evolution. The efficacy of the CEI is demonstrated in full graphite/LMR-NCM pouch cells with ~ 35% enhanced capacity and >80% capacity retention over 200 cycles. Altogether, these results present the importance of careful selection and design of surface chemistry for stabilizing the electrode/electrolyte interphase in challenging battery chemistries.

Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing applications and quality of NMR as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal ions based DNP, the polarization agents are distributed in the bulk sample and act as both source of relaxation and of polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted and the entire sample can be directly polarized. We exploit this finding to measure high quality NMR spectra of 17O in the electrode material Li4Ti5O12 doped with Fe(III) despite its low abundance and long relaxation time.

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li 4Ti 5O 12. In addition for being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.

In recent years dynamic nuclear polarization (DNP) has greatly expanded the range of materials systems that can be studied by solid state NMR spectroscopy. To date, the majority of systems studied by DNP were insulating materials including organic and inorganic solids. However, many technologically-relevant materials used in energy conversion and storage systems are electrically conductive to some extent or are employed as composites containing conductive additives. Such materials introduce challenges in their study by DNP-NMR which include microwave absorption and sample heating that were not thoroughly investigated so far.Here we examine several commercial carbon allotropes, commonly employed as electrodes or conductive additives, and consider their effect on the extent of solvent polarization achieved in DNP from nitroxide biradicals. We then address the effect of sample conductivity systematically by studying a series of carbons with increasing electrical conductivity prepared via glucose carbonization. THz spectroscopy measurements are used to determine the extent of vw absorption. Our results show that while the DNP performance significantly drops in samples containing the highly conductive carbons, sufficient signal enhancement can still be achieved with some compromise on conductivity. Furthermore, we show that the deleterious effect of conductive additives on DNP enhancements can be partially overcome through pulse-DNP experiments.

Surface modification of electrode materials using chemical treatments and atomic layer deposition is documented as an efficient method to stabilize the lattice structure as well as to reinforce the electrode/electrolyte interface. Nevertheless, expensive instrumentation and intrinsic deterioration of the material under high-temperature conditions and aggressive chemical treatments limit their practical application. Here, we report enhanced electrochemical stability and performances by simple atomic surface reduction (ASR) treatment of Li- and Mn-rich 0.35Li(2)MnO(3). 0.65LiNi(0.35)Mn(0.45)Co(0.20)O(2) (HE-NMC). We provide mechanistic indications showing that ASR altered the electronic structure of surface Mn and Ni, leading to higher stability and reduced parasitic reactions. We demonstrate significant improvement in the battery performance with the proposed surface reduction, which is reflected by the enhanced capacity (290 mA h g(-1)), rate capabilities (similar to 15% enhancement at rates of 1 and 2 C), 50-60 mV narrow voltage hysteresis, and faster (twice) Li+ diffusion. Utilizing online electrochemical mass spectrometry (OEMS), we show in-operando that the reduced surface layer results in suppressed side reactions. We further characterized the surface coating with high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and solid-state NMR before and after cycling. The results presented herein address all the critical challenges associated with the complex HE-NMC material and thus provide a promising research direction for choosing relevant methodology for surface treatment.

In the present work, a simple and agile methodology for atomic surface reduction of interfaces is introduced. Using a surface directed vapor phase reaction, at relatively low temperature, we show that a highly reactive and volatile molecule can be used to selectively reduce the interface, without changing the bulk of the treated material, and without the need of alternating sequence of multiple precursors, normally involved in ALD. The model system we use to demonstrate the efficacy, and potential of our approach is trimethyl aluminum, and high energy Li and Mn rich cathode (HE-NCM) as the functional material of interest. We demonstrate that with the proposed method, the particles of HE-NMC were conformally coated with similar to 3 nm amorphous layer of the reduced surface in less than 1 h (including the cooling time), as witnessed using HR-TEM. XPS and solid-state NMR, further confirmed that surface treatment was successfully achieved using the proposed method and is well explained by DFT calculations. Utilizing online electrochemical mass spectrometry (OEMS), we show in-operando that this amorphous layer helps to suppress parasitic reactions under extreme electrochemical conditions as indicated by the significant reduction in oxygen and CO2 evolution. The surface treatment further resulted in enhancement in specific capacity during the first cycle. This methodology provides a non-conventional path to achieve thin layer surface modification under facile conditions, and opens a new way to meet the requirements of surface modification strategies for improving the performance of electrode materials without utilizing expensive instrumentation and high temperature processes.

In recent years magic angle spinning-dynamic nuclear polarization (MAS-DNP) has developed as an excellent approach for boosting the sensitivity of solid-state NMR (ssNMR) spectroscopy, thereby enabling the characterization of challenging systems in biology and chemistry. Most commonly, MAS-DNP is based on the use of nitroxide biradicals as polarizing agents. In materials science, since the use of nitroxides often limits the signal enhancement to the materials' surface and subsurface layers, there is need for hyperpolarization approaches which will provide sensitivity in the bulk of micron sized particles. Recently, an alternative in the form of paramagnetic metal ions has emerged. Here we demonstrate the remarkable efficacy of Mn(II) dopants, used as endogenous polarization agents for MAS-DNP, in enabling the detection of O-17 at a natural abundance of only 0.038%. Distinct oxygen sites are identified in the bulk of micron-sized crystals, including battery anode materials Li4Ti5O12 (LTO) and Li2ZnTi3O8, as well as the phosphor materials NaCaPO4 and MgAl2O4, all doped with Mn(II) ions. Density functional theory calculations are used to assign the resonances to specific oxygen environments in these phases. Depending on the Mn(II) dopant concentration, we obtain significant signal enhancement factors, 142 and 24, for Li-6 and Li-7 nuclei in LTO, respectively. We furthermore follow the changes in the Li-6,Li-7 LTO resonances and determine their enhancement factors as a function of Mn(II) concentration. The results presented show that MAS-DNP from paramagnetic metal ion dopants provides an efficient approach for probing informative nuclei such as O-17, despite their low gyromagnetic ratio and negligible abundance, without isotope enrichment.

Solid state NMR spectroscopy is an extremely versatile and powerful method for determining the chemical composition and the structural and dynamic properties of solids. Here we provide an overview of the rich arsenal of experimental approaches offered by this spectroscopy for materials characterization with selected examples of recent applications.

Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of x-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute (1 at. %) Mn-doped lithium titanate (Li4/3Ti5/3O4, or LTO) the dopant Mn2+ ions reside on tetrahedral (8a) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the 1s -> 4p transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intenisty of this peak accurately. This dopant location can explain the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result, the observed remarkable 20% decrease in electrochemical capacity of the 1% Mn-doped LTO compared to pristine LTO.

Rechargeable battery cells are composed of two electrodes separated by an ion-conducting electrolyte. While the energy density of the cell is mostly determined by the redox potential of the electrodes and amount of charge they can store, the processes at the electrode-electrolyte interface govern the battery's lifetime and performance. Viable battery cells rely on unimpeded ion transport across this interface, which depends on its composition and structure. These properties are challenging to determine as interfacial phases are thin, disordered, heterogeneous, and can be very reactive. The recent developments and applications of solid state NMR spectroscopy in the study of interfacial phenomena in rechargeable batteries based on lithium and sodium chemistries are reviewed. The different NMR interactions are surveyed and how these are used to shed light on the chemical composition and architecture of interfacial phases as well as directly probe ion transport across them is described. By combining new methods in solid state NMR spectroscopy with other analytical tools, a holistic description of the electrode-electrolyte interface can be obtained. This will enable the design of improved interfaces for developing battery cells with high energy, high power, and longer lifetime.

Dynamic nuclear polarization (DNP), a technique in which the high electron spin polarization is transferred to surrounding nuclei via microwave irradiation, equips solid-state NMR spectroscopy with unprecedented sensitivity. The most commonly used polarization agents for DNP are nitroxide radicals. However, their applicability to inorganic materials is mostly limited to surface detection. Paramagnetic metal ions were recently introduced as alternatives for nitroxides. Doping inorganic solids with paramagnetic ions can be used to tune material properties and introduces endogenous DNP agents that can potentially provide sensitivity in the particles' bulk and surface. Here we demonstrate the approach by doping Li4Ti5O12 (LTO), an anode material for lithium ion batteries, with paramagnetic ions. By incorporating Gd(III) and Mn(II) in LTO we gain up to 14 fold increase in signal intensity in static Li-7 DNP-NMR experiments. These results suggest that doping with paramagnetic ions provides an efficient route for sensitivity enhancement in the bulk of micron size particles.

Out of the many challenges in the realization of lithium-O-2 batteries (LOB), the major is to deal with the instability of the electrolyte and the cathode interface under the stringent environment of both oxygen reduction and evolution reactions. Lithium nitrate was recently proposed as a promising salt for LOB because of its capability to stabilize the lithium anode by the formation of a solid electrolyte interphase, its low level of dissociation in aprotic solvents, and its catalytic effect toward oxygen evolution reaction (OER) in rechargeable LOB. Nevertheless, a deeper understanding of the influence of nitrate on the stability and electrochemical response of the cathode in LOB is yet to be realized. Additionally, it is well accepted that carbon instability toward oxidation therefore, it is essential to investigate the effect of electrolyte components on this side of the battery. In the present work, we show that nitrate leads to interfacial changes, which result in the formation of a surface protection domain on the carbon scaffold of LOB cathode, which helps in suppressing the oxidative damage of the carbon. This effect is conjugated with an additional electrocatalytic effect of the nitrate ion on the OER Using in operando online electrochemical mass spectroscopy, we herein deconvolute these two positive effects and show how they are dependent on nitrate concentration and the potential of cell operation. We show that a low amount of nitrate can exhibit the catalytic behavior; however, in order to harness its ability to suppress the oxidative damage and passivate the carbon surface, an excess of LiNO3 is required.

Liquid-free solid polymer electrolyte (SPE) Li-O-2 batteries are considered advantageous power sources for multiple applications, albeit their cycle performance is far from being acceptable. A most challenging SPE stability in Li-O-2 battery operating at 80 degrees C is described here, presenting possible directions for this battery type future development. Hereby, we investigated poly(ethylene oxide) (PEO) stability in Li-O-2 batteries after cycling and determined that the polymer instability is originated from an accumulation of formate-based species, which required high decomposition potential and showed low decomposition efficiency. This poses a key challenging issue of unfavorable round-trip efficiency, dictating a poor cycle performance.

Spinels (AB(2)O(4)) form a niche class of ceramics, which is rich in structural (dis)ordering due to the inherent mixing between the constituent tetrahedral and octahedral sites. The cations (A and B) can form antisite defects under the influence of external parameters like pressure, temperature and nuclear irradiation. The current study reports the formation and evolution of disorder-order structural transition in hydrothermally prepared zinc aluminate spinel ZnAl2O4. The effect of final calcination temperature (300-900 degrees C for 9 h) on the degree of cation ordering has been investigated with powder X-ray diffraction, Raman and Al-27 solid-state NMR spectroscopy. Rietveld refinement revealed a gradual disorder to order structural transition accompanied by lower inversion parameter (i(s)) and smaller lattice parameter (a) with higher calcination temperature. It was further affirmed by Raman analysis and solid-state NMR spectroscopy probing the ZnO4 and AlO4 tetrahedra in spinel. Independent of the degree of cation ordering, nanometric particle size with high surface area was observed in ZnAl2O4 spinel.

Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs4PbX6 reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX6 octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Also, depending on the final size of the Cs4PbX6 nanoparticles as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites.

High-energy conversion electrodes undergo successive Li insertion and conversion during lithiation. A primary scientific obstacle to harnessing the potentially high lithium storage capabilities of conversion electrode materials has been the formation of insulating new phases throughout the conversion reactions. These new phases are chemically stable, and electrochemically irreversible if formed in large amounts with coarsening. Herein, we synthesized FeOF conversion material as a model system and mechanistically demonstrate that a thin solid electrolyte [lithium phosphorus oxynitride (LiPON)] atomic layer deposition-deposited on the composite electrode extends the Li insertion process to higher concentrations, delaying the onset of a parasitic chemical conversion reaction and rendering the redox reaction of the protected conversion electrode electrochemically reversible. Reversibility is demonstrated to at least 100 cycles, with the LiPON protective coating increasing capacity retention from 29 to 89% at 100 cycles. Pursuing the chemical mechanism behind the boosted electrochemical reversibility, we conducted electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and electrochemical measurements that unrevealed the suppression of undesired phase formation and extended lithium insertion of the coated electrode. Support for the delayed consequences of the conversion reaction is also obtained by high-resolution transmission electron microscopy. Our findings strongly suggest that undesired new phase formation upon lithiation of electrode materials can be suppressed in the presence of a thin protection layer not only on the surface of the coated electrode but also in the bulk of the material through mechanical confinement that modulates the electrochemical reaction.

Forming a stable solid electrolyte interphase (SEI) is critical for rechargeable batteries' performance and lifetime. Understanding its formation requires analytical techniques that provide molecular-level insight. Here, dynamic nuclear polarization (DNP) is utilized for the first time to enhance the sensitivity of solid-state NMR (ssNMR) spectroscopy to the SEI. The approach is demonstrated on reduced graphene oxide (rGO) cycled in Li-ion cells in natural abundance and ¹³C-enriched electrolyte solvents. Our results indicate that DNP enhances the signal of outer SEI layers, enabling detection of natural abundance ¹³C spectra from this component of the SEI on reasonable time frames. Furthermore, ¹³C-enriched electrolyte measurements at 100 K provide ample sensitivity without DNP due to the vast amount of SEI filling the rGO pores, thereby allowing differentiation of the inner and outer SEI layer composition. Developing this approach further will benefit the study of many electrode materials, equipping ssNMR with the necessary sensitivity to probe the SEI efficiently.

Symmetry plays an important role in the retention or annihilation of a desired interaction Hamiltonian in NMR experiments. Here, we explore the role of symmetry in the radio-frequency interaction frame Hamiltonian of the refocused-continuous-wave (rCW) pulse scheme that leads to efficient H-1 heteronuclear decoupling in solid-state NMR. It is demonstrated that anti-periodic symmetry of single-spin operators (I-x, I-y, I-z) in the interaction frame can lead to complete annihilation of the H-1-H-1 homonuclear dipolar coupling effects that induce line broadening in solid-state NMR experiments. This symmetry also plays a critical role in cancelling or minimizing the effect of H-1 chemical-shift anisotropy in the effective Hamiltonian. An analytical description based on Floquet theory is presented here along with experimental evidences to understand the decoupling efficiency of supercycled (concatenated) rCW scheme.

We have synthesized the products of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) via lithium naphthalenide reduction. By analyzing the resulting solid precipitates and gas evolution, our results confirm that both FEC and VC decomposition products include HCO2Li, Li2C2O4, Li2CO3, and polymerized VC. For FEC, our experimental data supports a reduction mechanism where FEC reduces to form VC and LiF, followed by subsequent VC reduction. In the FEC reduction product, HCO2Li, Li2C2O4, and Li2CO3 were found in smaller quantities than in the VC reduction product, with no additional fluorine environments being detected by solid-state nuclear magnetic resonance or X-ray photoelectron spectroscopy analysis. With these additives being practically used in higher (FEC) and lower (VC) concentrations in the base electrolytes of lithium-ion batteries, our results suggest that the different relative ratios of the inorganic and organic reduction products formed by their decomposition may be relevant to the chemical composition and morphology of the solid electrolyte interphase formed in their presence.

The solid electrolyte interphase (SEI) of the high capacity anode material Si is monitored over multiple electrochemical cycles by Li-7, F-19, and C-13 solid-state nuclear magnetic resonance spectroscopies, with the organics dominating the SEI. Homonuclear correlation experiments are used to identify the organic fragments -OCH2CH2O-, -OCH2CH2-, -OCH2CH3, and -CH2CH3 contained in both oligomeric species and lithium semicarbonates ROCO2Li, RCO2Li. The SEI growth is correlated with increasing electrode tortuosity by using focused ion beam and scanning electron microscopy. A two-stage model for lithiation capacity loss is developed: initially, the lithiation capacity steadily decreases, Li+ is irreversibly consumed at a steady rate, and pronounced SEI growth is seen. Later, below 50% of the initial lithiation capacity, less Si is (de)lithiated resulting in less volume expansion and contraction; the rate of Li+ being irreversibly consumed declines, and the Si SEI thickness stabilizes. The decreasing lithiation capacity is primarily attributed to kinetics, the increased electrode tortuousity severely limiting Li+ ion diffusion through the bulk of the electrode. The resulting changes in the lithiation processes seen in the electrochemical capacity curves are ascribed to non-uniform lithiation, the reaction commencing near the separator/on the surface of the particles.

The delithiation mechanisms occurring within the olivine-type class of cathode materials for Li-ion batteries have received considerable attention because of the good capacity retention at high rates for LiFePO₄. A comprehensive mechanistic study of the (de)lithiation reactions that occur when the substituted olivine-type cathode materials LiFe_xCo_1-xPO₄ (x = 0, 0.05, 0.125, 0.25, 0.5, 0.75, 0.875, 0.95, 1) are electrochemically cycled is reported here using in situ X-ray diffraction (XRD) data and supporting ex situ ³¹P NMR spectra. On the first charge, two intermediate phases are observed and identified: Li_1-x(Fe³⁺)_x(Co²⁺)_1-xPO₄ for 0 < x < 1 (i.e., after oxidation of Fe²⁺ to Fe³⁺) and Li_2/3Fe_xCo_1-xPO₄ for 0 ≤ x ≤ 0.5 (i.e., the Co-majority materials). For the Fe-rich materials, we study how nonequilibrium, single-phase mechanisms that occur discretely in single particles, as observed for LiFePO₄ at high rates, are affected by Co substitution. In the Co-majority materials, a two-phase mechanism with a coherent interface is observed, as was seen in LiCoPO₄, and we discuss how it is manifested in the XRD patterns. We then compare the nonequilibrium, single-phase mechanism with the bulk single-phase and coherent interface two-phase mechanisms. Despite the apparent differences between these mechanisms, we discuss how they are related and interconverted as a function of Fe/Co substitution and the potential implications for the electrochemistry of this system.

Olivine MnPO4 is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO4, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO4. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO4, which causes amorphization of olivine MnPO4. The properties of crystalline MnPO4 obtained from carbon-coated LiMnPO4 and of the amorphous product resulting from delithiation of pure LiMnPO4 were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP2O7 and MnH2P2O7 from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO4 is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal.

We present a bimodal Floquet analysis of the recently introduced refocused continuous wave (rCW) solid-state NMR heteronuclear dipolar decoupling method and compare it with the similar looking X-inverse X (XiX) scheme. The description is formulated in the rf interaction frame and is valid for both finite and ideal pi pulse rCW irradiation that forms the refocusing element in the rCW scheme. The effective heteronuclear dipolar coupling Hamiltonian up to first order is described. The analysis delineates the difference between the two sequences to different orders of their Hamiltonians for both diagonal and off-diagonal parts. All the resonance conditions observed in experiments and simulations have been characterised and their influence on residual line broadening is highlighted. The theoretical comparison substantiates the numerical simulations and experimental results to a large extent. (C) 2016 Elsevier Inc. All rights reserved.

The solid electrolyte interphase (SEI) passivating layer that grows on all battery electrodes during cycling is critical to the long-term capacity retention of lithium-ion batteries. Yet, it is inherently difficult to study because of its nanoscale thickness, amorphous composite structure, and air sensitivity. Here, we employ an experimental strategy using H-1, Li-7, F-19, and C-13 solid-state nuclear magnetic resonance (ssNMR) to gain insight into the decomposition products in the SET formed on silicon electrodes, the uncontrolled growth of the SET representing a major failure mechanism that prevents the practical use of silicon in lithium-ion batteries. The voltage dependent formation of the SET is confirmed, with the SEI growth correlating with irreversible capacity. By studying both conductive carbon and mixed Si/C composite electrodes separately, a correlation with increased capacity loss of the composite system and the low-voltage silicon plateau is demonstrated. Using selective C-13 labeling, we detect decomposition products of the electrolyte solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) independently. EC decomposition products are present in higher concentrations and are dominated by oligomer species. Lithium semicarbonates, lithium fluoride, and lithium carbonate products are also seen. Ab initio calculations have been carried out to aid in the assignment of NMR shifts. ssNMR applied to both rinsed and unrinsed electrodes show that the organics are easily rinsed away, suggesting that they are located on the outer layer of the SEI.

The pyrolysis of 1,2-diphosphinobenzene at 800 degrees C gives a phosphorus-doped graphite (P-DG) with an unprecedented high phosphorus content, ca. 20 at%. In contrast with previously studied boron and nitrogen doped graphite materials, thorough characterisation and analysis of this material demonstrates that it is extensively disordered and contains substitutional P-atoms along with P=O units in the host graphitic lattice, as well as P-4 molecules trapped between the graphitic sheets. This represents a stabilised form of P-4, which has been shown to covalently bind to lithium as Li3P in this material.

Mobi-Li-ty: Lithium mobility, as a function of temperature during battery cycling, can be simply monitored by using in situ T₂ relaxation measurements. Since Li dynamics are strongly related to structural properties, the changes in in situ T₂ can be used to detect structural changes, such as Li ordering.

The hydroboration 1,3- and 1,4-cyclic dienes has been systematically investigated. The behavior of such dienes towards mono and dihydroboration was monitored directly by B-11 NMR to identify the actual boron species formed, as opposed to the most common analysis of the resultant oxidation products. Quantitative dihydroboration was achieved for the full range of cyclic dienes investigated including dienes, which were previously reported to be resistant to dihydroboration, leading to the formation of new boron-containing polymeric materials. The conditions favoring dihydroboration are reported as well as full characterisation of the materials. Furthermore, a hydroboration cascade mechanism is proposed for the formation of such boron-containing polymers, supported by both experimental and theoretical data.

The rechargeable aprotic lithium-air (Li-O-2) battery is a promising potential technology for next-generation energy storage, but its practical realization still faces many challenges. In contrast to the standard Li-O-2 cells, which cycle via the formation of Li-2 O-2, we used a reduced graphene oxide electrode, the additive LiI, and the solvent dimethoxyethane to reversibly form and remove crystalline LiOH with particle sizes larger than 15micrometers during discharge and charge. This leads to high specific capacities, excellent energy efficiency (93.2%) with a voltage gap of only 0.2 volt, and impressive rechargeability. The cells tolerate high concentrations of water, water being the dominant proton source for the LiOH; together with LiI, it has a decisive impact on the chemical nature of the discharge product and on battery performance.

Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO2 looping, where the material reacts reversibly with CO2 to form Li2CO3, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li2CO3 has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li2CO3 over the temperature range of 293 973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li2CO3. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion rotations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations.

The Carr-Purcell-Meiboom-Gill (CPMG) sequence is commonly used in high resolution NMR spectroscopy and in magnetic resonance imaging for the measurement of transverse relaxation in systems that are subject to diffusion in internal or external gradients and is superior to the Hahn echo measurement, which is more sensitive to diffusion effects. Similarly, it can potentially be used to study dynamic processes in electrode materials for lithium ion batteries. Here we compare the Li-7 signal decay curves obtained with the CPMG and Hahn echo sequences under static conditions (i.e., in the absence of magic angle spinning) in paramagnetic materials with varying transition metal ion concentrations. Our results indicate that under CPMG pulse trains the lifetime of the Li-7 signal is substantially extended and is correlated with the strength of the electron-nuclear interaction. Numerical simulations and analytical calculations using Floquet theory suggest that the combination of large interactions and a train of finite pulses, results in a spin locking effect which significantly slows the signal's decay. While these effects complicate the interpretation of CPMG-based investigations of diffusion and chemical exchange in paramagnetic materials, they may provide a useful approach to extend the signal's lifetime in these often fast relaxing systems, enabling the use of correlation experiments. Furthermore, these results highlight the importance of developing a deeper understanding of the effects of the large paramagnetic interactions during multiple pulse experiments in order to extend the experimental arsenal available for static and in situ NMR investigations of paramagnetic materials.

Vanadium sulfide VS4 in the patronite mineral structure is a linear chain compound comprising vanadium atoms coordinated by disulfide anions [S-2](2-). V-51 NMR shows that the material, despite having V formally in the d(1) configuration, is diamagnetic, suggesting potential dimerization through metal metal bonding associated with a Peierls distortion of the linear chains. This is supported by density functional calculations, and is also consistent with the observed alternation in V V distances of 2.8 and 3.2 angstrom along the chains. Partial lithiation results in reduction of the disulfide ions to sulfide S2-, via an internal redox process whereby an electron from V4+ is transferred to [S-2](2-) in oxidation of V4+ to V5+ and reduction of the [S-2](2-) to S2- to form Li3VS4 containing tetrahedral [VS4](3-) anions. On further lithiation this is followed by reduction of the V5+ in Li3VS4 to form Li3+xVS4 (x = 0.5-1), a mixed valent V4+/V5+ compound. Eventually reduction to Li2S plus elemental V occurs. Despite the complex redox processes involving both the cation and the anion occurring in this material, the system is found to be partially reversible between 0 and 3 V. The unusual redox processes in this system are elucidated using a suite of short-range characterization tools including V-51 nuclear magnetic resonance spectroscopy (NMR), S K-edge X-ray absorption near edge spectroscopy (XANES), and pair distribution function (PDF) analysis of X-ray data.

Viscoelastic, gel-like, polymeric dispersions (HVPDs) can be prepared by crosslinking polyols with borax or boric acid in water under alkaline conditions. Rheologically similar HVPDs have been prepared in organic liquids containing no water or hydroxylic groups through crosslinking partially or fully hydrolysed poly(vinyl acetate)s with trimethyl borate, boric acid, or borax. The organo-HVPDs are water-sensitive and rheoreversible on exposure to water. They were characterised rheologically and by solution and solid-state ¹¹B NMR spectroscopy. Spectroscopic analyses show the presence of mono- and di-diol crosslinks, as well as non-crosslinked boron species in HVPDs prepared with trimethyl borate or boric acid. The number of crosslinks in organo-HVPDs prepared with borax increased over the course of several days. Results from solution and solid-state ¹¹B NMR spectroscopy are comparable; no solid-like component was detectable. We demonstrate that hydro, organo, or organo-aqueous HVPDs can be obtained from partially hydrolysed poly(vinyl acetate)s by 'tuning' the structure of the boron-based crosslinker.

Previous theoretical studies of C3B have suggested that boron-doped graphite is a promising H-2- and Li-storage material, with large maximum capacities. These characteristics could lead to exciting applications as a lightweight H-2-storage material for automotive engines and as an anode in a new generation of batteries. However, for these applications to be realized a synthetic route to bulk C3B must be developed. Here we show the thermolysis of a single-source precursor (1,3-BBr2)(2)C6H4) to produce graphitic C3B, thus allowing the characteristics of this elusive material to be tested for the first time. C3B was found to be compositionally uniform but turbostratically disordered. Contrary to theoretical expectations, the H-2- and Li-storage capacities are lower than anticipated, results that can partially be explained by the disordered nature of the material. This work suggests that to model the properties of graphitic materials more realistically, the possibility of disorder must be considered.

The high theoretical gravimetric capacity of the Li-S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li-S phase diagram using computational techniques and complement this with an in situ ⁷Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li₂S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li⁺-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li₂S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li₂S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.

In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO4 (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li-2/3(Co2+)(2/3)(Co3+)(1/3)PO4 phase. Two resonances are observed for Li2/3CoPO4 with intensity ratios of 2:1 and 1:1 in the P-31 and 7Li NMR spectra, respectively. An ordering of Co2+/Co3+ oxidation states is proposed within a (a X 3b X c) supercell, and Li+/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co3+ ions and Li vacancies are found to order along the b-axis. Two other low energy Li+/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO4CoPO4 convex hull and they may be readily interconverted by Li+ hops along the b-direction.

Micrometer-sized LiNi_xMn_2-xO₄ (0.3 ≤ x ≤ 0.5) single crystals with (111) surface facets were synthesized and characterized by 6Li magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and electrochemical studies. All three techniques were sensitive to cation disorder and the corroborated results showed that structural ordering improves with x. The transition from the ordered to the disordered spinel was triggered by an increase in Mn³⁺ content, which was accomplished either by a change in chemical composition or postsynthesis thermal treatment. Disordering led to increased solid solution behavior, reduced two-phase transformation domains, and improved transport properties during Li extraction and insertion. Further increasing Mn³⁺ content in already disordered structure extends the solid solution domain and eliminates the presence of phase II; however, this has limited effect on rate capability. The study demonstrates the dominant role of structural ordering in morphology-controlled LiMn1.5Ni0.5O4, and it reveals that the kinetic significance of Mn³⁺ lies in its ability in triggering structural disordering. The rate performance of the spinels is not directly proportional to the Mn³⁺ content or the domain size of solid solution transformation in samples where two-phase transition is also present.

Olivine-type LiCoPO₄ (LCP) is a high energy density lithium ion battery cathode material due to the high voltage of the Co²⁺/Co ³⁺ redox reaction. However, it displays a significantly poorer electrochemical performance than its more widely investigated isostructural analogue LiFePO₄ (LFP). The co-substituted LiFe_xCo _1-xPO₄ olivines combine many of the positive attributes of each end member compound and are promising next-generation cathode materials. Here, the fully lithiated x = 0, 0.25, 0.5, 0.75 and 1 samples are extensively studied using ³¹P solid-state nuclear magnetic resonance (NMR). Practical approaches to broadband excitation and for the resolution of the isotropic resonances are described. First principles hybrid density functional calculations are performed on the Fermi contact shift (FCS) contributions of individual M-O-P pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFe_xCo_1-xPO₄ experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFe_xCo_1-xPO₄ have been calculated and used to assign the NMR spectra. Due to the additional unpaired electron in d⁶ Fe²⁺ as compared with d ⁷ Co²⁺ (both high spin), LFP is expected to have larger Fermi contact shifts than LCP. However, two of the Co-O-P pathways in LCP give rise to noticeably larger shifts and the unexpected appearance of peaks outside the range delimited by the pure LFP and LCP ³¹P shifts. This behaviour contrasts with that observed previously in LiFe_xMn _1-xPO₄, where all ³¹P shifts lay within the LiMnPO₄-LFP range. Although there are 24 distinct local P environments in LiFe_xCo_1-xPO₄, these group into seven resonances in the NMR spectra, due to significant overlap of the isotropic shifts. The local environments that give rise to the largest contributions to the spectral intensity are identified and used to simplify the assignment. This provides a tool for future studies of the electrochemically- cycled samples, which would otherwise be challenging to interpret. This journal is

Conversion materials for lithium ion batteries have recently attracted considerable attention due to their exceptional specific capacities. Some metal fluorides, such as CuF₂, are promising candidates for cathode materials owing to their high operating potential, which stems from the high electronegativity of fluorine. However, the high ionicity of the metal-fluorine bond leads to a large band gap that renders these materials poor electronic conductors. Nanosizing the active material and embedding it within a conductive matrix such as carbon can greatly improve its electrochemical performance. In contrast to other fluorides, such as FeF₂ and NiF₂, good capacity retention has not, however, been achieved for CuF₂. The reaction mechanisms that occur in the first and subsequent cycles and the reasons for the poor charge performance of CuF₂ are studied in this paper via a variety of characterization methods. In situ pair distribution function analysis clearly shows CuF₂ conversion in the first discharge. However, few structural changes are seen in the following charge and subsequent cycles. Cyclic voltammetry results, in combination with in situ X-ray absorption near edge structure and ex situ nuclear magnetic resonance spectroscopy, indicate that Cu dissolution is associated with the consumption of the LiF phase, which occurs during the first charge via the formation of a Cu¹⁺ intermediate. The dissolution process consequently prevents Cu and LiF from transforming back to CuF₂. Such side reactions result in negligible capacity in subsequent cycles and make this material challenging to use in a rechargeable battery.

To what extent the presence of transition metal ions can affect the optical properties of structurally well-defined, metal-doped polyoxotitanium (POT) cages is a key question in respect to how closely these species model technologically important metal-doped TiO₂. This also has direct implications to the potential applications of these organically-soluble inorganic cages as photocatalytic redox systems in chemical transformations. Measurement of the band gaps of the series of closely related polyoxotitanium cages [MnTi₁₄(OEt)₂₈O₁₄(OH)₂] (1), [FeTi₁₄(OEt)₂₈O₁₄(OH)₂] (2) and [GaTi₁₄(OEt)₂₈O₁₅(OH)] (3), containing interstitial Mn(ii), Fe(ii) and Ga(iii) dopant ions, shows that transition metal doping alone does not lower the band gaps below that of TiO₂ or the corresponding metal-doped TiO₂. Instead, the band gaps of these cages are within the range of values found previously for transition metal-doped TiO₂ nanoparticles. The low band gaps previously reported for 1 and for a recently reported related Mn-doped POT cage appear to be the result of low band gap impurities (most likely amorphous Mn-doped TiO₂).

Li-substituted layered P2-Na_0.80[Li_0.12Ni _0.22Mn_0.66]O₂ is investigated as an advanced cathode material for Na-ion batteries. Both neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the local structure, and they reveal that most of the Li ions are located in transition metal (TM) sites, preferably surrounded by Mn ions. To characterize structural changes occurring upon electrochemical cycling, in situ synchrotron X-ray diffraction is conducted. It is clearly demonstrated that no significant phase transformation is observed up to 4.4 V charge for this material, unlike Li-free P2-type Na cathodes. The presence of monovalent Li ions in the TM layers allows more Na ions to reside in the prismatic sites, stabilizing the overall charge balance of the compound. Consequently, more Na ions remain in the compound upon charge, the P2 structure is retained in the high voltage region, and the phase transformation is delayed. Ex situ NMR is conducted on samples at different states of charge/discharge to track Li-ion site occupation changes. Surprisingly, Li is found to be mobile, some Li ions migrate from the TM layer to the Na layer at high voltage, and yet this process is highly reversible. Novel design principles for Na cathode materials are proposed on the basis of an atomistic level understanding of the underlying electrochemical processes. These principles enable us to devise an optimized, high capacity, and structurally stable compound as a potential cathode material for high-energy Na-ion batteries.

A multi-nuclear solid-state NMR approach is employed to investigate the lithium-air battery, to monitor the evolution of the electrochemical products formed during cycling, and to gain insight into processes affecting capacity fading. While lithium peroxide is identified by ¹⁷O solid state NMR (ssNMR) as the predominant product in the first discharge in 1,2-dimethoxyethane (DME) based electrolytes, it reacts with the carbon cathode surface to form carbonate during the charging process. ¹³C ssNMR provides evidence for carbonate formation on the surface of the carbon cathode, the carbonate being removed at high charging voltages in the first cycle, but accumulating in later cycles. Small amounts of lithium hydroxide and formate are also detected in discharged cathodes and while the hydroxide formation is reversible, the formate persists and accumulates in the cathode upon further cycling. The results indicate that the rechargeability of the battery is limited by both the electrolyte and the carbon cathode stability. The utility of ssNMR spectroscopy in directly detecting product formation and decomposition within the battery is demonstrated, a necessary step in the assessment of new electrolytes, catalysts, and cathode materials for the development of a viable lithium-oxygen battery.

Electrochemical cells, in the form of batteries (or L supercapacitors) and fuel cells, are efficient devices for energy storage and conversion. These devices show considerable promise for use in portable and static devices to power electronics and various modes of transport and to produce and store electricity both locally and on the grid. For example, high power and energy density lithium-ion batteries are being developed for use in hybrid electric vehicles where they improve the efficiency of fuel use and help to reduce greenhouse gas emissions. To gain insight into the chemical reactions involving the multiple components (electrodes, electrolytes, interfaces) in the electrochemical cells and to determine how cells operate and how they fail, researchers ideally should employ techniques that allow real-time characterization of the behavior of the cells under operating conditions. This Account reviews the recent use of in situ solid-state NMR spectroscopy, a technique that probes local structure and dynamics, to study these devices. In situ NMR studies of lithium-ion batteries are performed on the entire battery, by using a coin cell design, a flat sealed plastic bag, or a cylindrical cell. The battery is placed inside the NMR coil, leads are connected to a potentiostat, and the NMR spectra are recorded as a function of state of charge. Li-7 is used for many of these experiments because of its high sensitivity, straightforward spectral interpretation, and relevance to these devices. For example, Li-7 spectroscopy was used to detect intermediates formed during electrochemical cycling such as LixC and LiySiz species in batteries with carbon and silicon anodes, respectively. It was also used to observe and quantify the formation and growth of metallic lithium microstructures, which can cause short circuits and battery failure. This approach can be utilized to identify conditions that promote dendrite formation and whether different electrolytes and additives can help prevent dendrite formation. The in situ method was also applied to monitor (by B-11 NMR) electrochemical double-layer formation in supercapacitors in real time. Though this method is useful, it comes with challenges. The separation of the contributions from the different cell components in the NMR spectra is not trivial because of overlapping resonances. In addition, orientation-dependent NMR interactions, including the spatial- and orientation-dependent bulk magnetic susceptibility (BMS) effects, can lead to resonance broadening. Efforts to understand and mitigate these BMS effects are discussed in this Account. The in situ NMR investigation of fuel cells initially focused on the surface electrochemistry at the electrodes and the electrochemical oxidation of methanol and CO to CO2 on the Pt cathode. On the basis of the C-13 and Pt-195 NMR spectra of the adsorbates and electrodes, CO adsorbed on Pt and other reaction intermediates and complete oxidation products were detected and their mode of binding to the electrodes investigated. Appropriate design and engineering of the NMR hardware has allowed researchers to integrate intact direct methanol fuel cells into NMR probes. Chemical transformations of the circulating methanol could be followed and reaction intermediates could be detected in real time by either H-2 or C-13 NMR spectroscopy. By use of the in situ NMR approach, factors that control fuel cell performance, such as methanol cross over and catalyst performance, were identified.

To date, in situ nuclear magnetic resonance (NMR) studies of working batteries have been performed in static mode, i.e., in the absence of magic angle spinning (MAS). Thus, it is extremely challenging to apply the method to paramagnetic systems such as the cathodes spinels Li1+xMn2−xO4 primarily due to three factors: (1) the resonance lines are broadened severely; (2) spectral analysis is made more complicated by bulk magnetic susceptibility (BMS) effects, which depend on the orientation and shape of the object under investigation; (3) the difficulty in untangling the BMS effects induced by the paramagnetic and metallic components on other (often diamagnetic) components in the system, which result in additional shifts and line broadening. Here we evaluate the orientation-dependence of the BMS effect of Li1.08Mn1.92O4, analyzing the experimental results by using a simple long-distance Li-electron dipolar coupling model. In addition, we discuss the shape and packing density dependence of the BMS effect and its influence on the observed frequencies of other components, such as the Li metal and the electrolyte in the battery. Finally, we show that by taking these effects into account we are able to minimize the BMS induced shift by orienting the cell at a rotation angle, αi=54.7° which facilitates the interpretation of the in situ NMR spectra of a working battery with the paramagnetic Li1.08Mn1.92O4 cathode.

The solvothermal reactions of Ti(OEt) ₄ with LnCl ₃ (Ln=La, Ce) produced new Ti ₂₈Ln cages, in which the Ln ³⁺ ions are coordinated within a metallocrown arrangement, which represents the highest nuclearity cages of this type (see figure).

A closer look: Solid-state 7Li and 17O?NMR spectroscopy is a valuable tool in the characterization of products formed in the lithium-oxygen battery, a necessary step in the development of a viable cell. Since lithium peroxide, the desired discharge product, has a unique 17O?NMR signature, it can be clearly identified.

We present a Floquet theory approach for the analysis of homonuclear recoupling assisted by radio frequency (RF) irradiation of surrounding heteronuclear spins. This description covers a broad range of systems from fully protonated to deuterated proteins, focusing in detail on recoupling via protons and deuterons separately as well as simultaneously by the double nucleus enhanced recoupling (DONER) scheme. The theoretical description, supported by numerical simulations and compared to experimental results from a partially deuterated model compound, indicates that in perdeuterated systems setting the RF amplitude equal to the magic angle spinning (MAS) frequency is not necessarily optimal for recoupling via ¹H and/or ²H nuclei and modified recoupling conditions are identified.

The application of Floquet theory in solid-state nuclear magnetic resonance is discussed. The Floquet approach has been used in the study of various effects related to quadrupolar nuclei varying from the basic magic-angle spinning (MAS) Hamiltonian, rotational resonance, and rotary-resonance conditions to the effect of quadrupole nuclei on spin 1/2 coupled to them. An extension of Floquet theory, multimode-multipole Floquet theory (MMFT), in which a multipole operator basis has been used is applied to the analysis of heteronuclear decoupling, the calculation of the R² condition width, and depolarization effects in double-quantum recoupling experiments. Cross polarization (CP) is one of the most important techniques for the detection of rare spins in solid-state nuclear magnetic resonance (NMR). MAS leads to improvement in the spectral resolution making the various sites distinguishable and improving the signal to noise ratio but it complicates the analysis of the spectra resulting from dynamic processes.

Schemes such as phase-modulated Lee-Goldburg (PMLG) for homonuclear dipolar decoupling have been shown to yield high-resolution ¹H spectra at high magic-angle spinning (MAS) frequencies of 50-70 kHz. This is at variance to the commonly held notion that these methods require MAS frequencies not comparable to the cycle frequencies of the pulse schemes. Here, a theoretical argument, based on bimodal Floquet theory, is presented to explain this aspect together with conditions where PMLG type of schemes may be successful at high MAS frequencies.

We present a rf scheme designed to excite triple quantum (TQ) coherences for proton solid state NMR. This recoupling scheme is based on the phase modulated Lee Goldburg sequence combined with echo pulses and applied nonsynchronous with the magic angle spinning period. Based on the effective bimodal Floquet Hamiltonian we optimize the conditions for TQ coherence excitation. Numerical simulations are used to further adjust the recoupling conditions as well as define the sequence limitations. Experimental TQ filtered one-dimensional spectra and two-dimensional correlations of TQ to single quantum coherences are presented for standard amino acids. These results are compared with the crystal structures showing that this scheme can aid in resonance assignments and in resolving local spin topologies.

We report here high-resolution ¹H solid-state nuclear magnetic resonance spectra acquired by a combination of magic-angle spinning (MAS) and radiofrequency pulse methods up to MAS frequencies of 65 kHz. The details of the pulse methods and experimental conditions are outlined together with spectra from model compounds.

A homonuclear dipolar decoupling scheme based on windowed phase-modulated Lee-Goldburg (wPMLG) pulse sequences that causes a" z -rotation" of the spins for high-resolution proton NMR spectroscopy of solids is described and analyzed. This supercycled scheme suppresses the effect of pulse imperfections on the spectra and significantly relaxes the off-resonance dependence of the line-narrowing efficiency and scale factor. This leads to a broad spectral window that is free of artifacts such as zero lines, image peaks, and localized rotor-radio-frequency resonances. High-resolution H1 spectra and two-dimensional homonuclear H1 - H1 correlation spectra of standard amino acids, obtained by a combination of this supercycled scheme with magic angle spinning frequencies up to 25 kHz, are demonstrated.

High-resolution ¹H spectroscopy in solid-state NMR, rendered difficult due to the strong ¹H-¹H homonuclear dipolar coupling, has been made possible under magic-angle spinning with homonuclear dipolar decoupling schemes, such as windowed phase-modulated Lee-Goldburg. Here, we outline the theory and implementation of a modification of this scheme with which an effective z-rotation for the magnetisation is obtained over a wide range of spectral window. Experimental results are presented for samples, such as glycine, histidine, and tryosine.

A theoretical treatment of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is presented here based on bimodal Floquet theory. The conditions necessary for good heteronuclear decoupling are derived. An analysis of a few of the decoupling schemes implemented until date is presented with regard to satisfying such decoupling conditions and efficiency of decoupling. Resonance conditions for efficient heteronuclear dipolar decoupling are derived with and without the homonuclear H1 - H1 dipolar couplings and their influence on heteronuclear dipolar decoupling is pointed out. The analysis points to the superior efficiency of the newly introduced swept two-pulse phase-modulation (SWf -TPPM) sequence. It is shown that the experimental robustness of SWf -TPPM as compared to the original TPPM sequence results from an adiabatic sweeping of the modulation frequencies. Based on this finding alternative strategies are compared here. The theoretical findings are corroborated by both numerical simulations and representative experiments.

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the ¹H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional ¹H- ¹H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.