In Situ DRIFTS Analysis of SEI Evolution over Electrode Surface with Solid Polymer Electrolyte for Lithium Batteries

We present the design and implementation of in situ diffuse reflectance Fourier transform spectroscopy (DRIFTS) method for the analysis of solid electrolyte interphase (SEI) over electrodes when using solid polymer electrolyte (SPE). A PVDF-HFP/LiTFSI film SPE is used as an example. We validate the electrochemical performance of our design of DRIFTS cell by comparing cyclic voltammetry (CV) measurement with a conventional cell. The thickness of SPE needs to be thin enough for infrared penetration resolve SEI over electrode surfaces. The DRIFTS spectrum over Cu anode at open-circuit voltage (OCV) confirms the absence of native SEI formation at OCV and the observed characteristics of the used SPE including the NMP-Li+ interaction, the complexation of LiTFSI with polymer matrix, and the polymer crystal phases. This demonstrates the capability of our cell design and methodology. The difference spectra from in situ DRIFTS reveal a two-stage electrochemical decomposition of LiTFSI over Cu anode in consistence with CV reduction peaks. The observed LiTFSI decomposition species are consistent with that previously reported using liquid electrolytes over Cu surface, underscoring the applicability of DRIFTS for in situ analysis of interfacial phenomena using solid polymer electrolytes.