Observation of Interfacial Degradation of Solid Electrolytes Against Lithium Metal and Layered Transition Metal Oxides Via in Situ Electrochemical Raman Microscopy

Abstract

Solid state batteries, utilizing ceramic solid-state electrolytes, are attracting intensive research for their improved safety and possible higher energy density. Solid-state electrolytes can have comparable ionic conductivities to the liquid carbonate electrolytes, for example, Li10GeP2S12 with an ionic conductivity at 1.2 × 10-2 S cm-1 at room temperature. However, the performance of solid state batteries requires improvement to enable long term cycling. The interface instability between the solid electrolytes and electrodes is one of the limiting factors for the utilization of solid state batteries. Traditionally, mostly ex situ surface characterisation techniques such as Raman or x-ray photoelectron spectroscopy can be used to study the buried interfaces within all-solid-state batteries. Raman microscopy, in particular, is a powerful surface characterisation technique and has been used to detect structural and chemical information on electrode interfaces primarily in liquid electrolyte cells. The ability to do in situ observation of the solid electrolyte – electrode interface is paramount for understanding the performance and design interfacial strategies for solid state batteries.In this study, in situ Raman microscopy and ex situ Raman imaging were adopted to study the interfacial evolution during cycling covering β-Li3PS4, Li10GeP2S12, and Li6PS5Cl electrolytes against Li metal and LiCoO2 or LiNi0.6Mn0.2Co0.2O2. Degradation products at both Li metal side and cathode side were detected. For example, the degradation products of the interface Li6PS5Cl at both Li metal side and LiCoO2 side were resolved [1]. Li2S was formed by Li deposition onto Li6PS5Cl and there were polysulfides, and P2Sx species formation between Li6PS5Cl and LiCoO2 interface. In situ Raman results were correlated with ex situ Raman microscopy from cycled cells. The ex situ Raman imaging also directly visualised the distribution of degradation products. We will demonstrate that in situ Raman microscopy as a powerful tool in the study of electrode/electrolyte interfaces within the solid-state cell. The technique can be translated to alternative solid electrolyte – cathode combinations to understand the operando interfacial evolutions for designing improved all-solid-state batteries. Y Zhou, C Doerrer, J Kasemchainan, P.G. Bruce, M. Pasta, L.J. Hardwick, Batteries & Supercaps 3(2020)1-7.