Leskes Lab

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.

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.

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.