(527a) Advancing Nondestructive in Situ Infrared Nanospectroscopy of Buried Electrochemical Interfaces in Li-Ion and Li-Metal Solid State Batteries

The interface between solid/solid and solid/liquid phases plays a fundamental role in natural and engineered materials. In fact, most rechargeable batteries performance is largely determined by thermodynamic, kinetic, and mechanical properties of such electrochemical interfaces. Furthermore, secondary batteries operate far away from equilibrium and a thin passive film tend to form at the electrode/electrolyte interface. Situated at the surface of the electrode, and formed during battery assembly and/or operation, this so-called solid electrolyte interphase (SEI) layer is critical for the battery stable function and operation. The SEI’s inhomogeneous structure and chemistry influence localized current density distribution and the resulting slow electrode and device degradation during charge–discharge processes. Our overall understanding of heterogeneous ionic interfaces and interphases is still very limited due to their chemical and physical complexity and lack of effective in operando characterization techniques. In this work, we exploit the nanoscale spatial resolution, chemical selectivity, and surface sensitivity of near-field infrared nano-spectroscopy, XPS and ATR FTIR to characterize buried interfaces in model carbonaceous Li-ion anodes and Li -metal electrodes. We demonstrate that even an atomically flat surface of graphene in contact with an SPE is still prone to develop a heterogeneous surface film composition and structure, which then results in nonuniform Li-plating currents and distributions at the nano- and micro-scale. This study provides a unique insight into the mechanisms of early-stage interphase formation at electrochemically active buried interfaces, and an experimental diagnostic means to aid in the development of methods to control local nanoscale variations in electrolyte chemistry, structure, and ionic conductivity at the surface of the electrode. Our approach also paves a way for interface optimization by providing a method for unprecedented nanoscale measurements of crystallinity, structure, conductivity, and chemistry of intact and buried electrochemical interfaces in their native environments.