Lithium-ion batteries with liquid electrolytes are commonly employed for powering portable electronic devices. To expand the range of applications where Li ions batteries can be used (e.g., electric transportation), solid electrolytes are considered as a safer alternative to the liquid electrolytes and they may enable use of lithium metal anodes. In this study we focused on composite solid electrolytes which are based on solid polymer (Polyethylene Oxide) and ceramic particles (Li1.5Al0.5Ge1.5P3O12, LAGP). Previous studies revealed that the highest ionic conduction path in the composites is through the interface polymer - ceramic interface. However, the chemical nature of the interface and the reason for its higher conductivity remains unclear. We aim to gain molecular - atomic level insight into the nature of the polymer - ceramic interface from solid state NMR spectroscopy. Here, I will present the development of a solid - state NMR approach that can potentially be used to selectively probe the interface. To gain sensitivity and selectivity Dynamic Nuclear Polarization (DNP), a process in which high polarization from unpaired electrons is transferred to surrounding nuclear spins will be employed. Several metal ion dopants were tested for their DNP performance in LAGP powder, and Mn2+ ions were further examined in their efficacy in the composite electrolyte. The approach was tested for selectively enhancing the NMR signal of the PEO - LAGP interface. Electrochemical characterization and in - depth solid state NMR studies provided insight into the performance of the composite and degradation processes in the composite.