Interphase formation and degradation of charge transfer kinetics between a lithium metal anode and highly crystalline Li7P3S11 solid electrolyte

• Interphase formation between lithium metal and the solid ion conductor Li7P3S11 is studied by a combined analytical approach.
• Using a self-developed in situ XPS method, we identify reaction products and discuss the impact on the interfacial properties
• A qualitative SEI model for the highly crystalline Li7P3S11 is developed

The properties of the interface between solid electrolytes and electrode materials are of vital importance for the performance of all solid-state batteries (ASSB). Unwanted reactions between alkali metal electrodes and the solid electrolyte can lead to the formation of compounds that either facilitate or block the ion transfer kinetics. In particular for lithium solid electrolytes in the Li2S–P2S5 system with very high lithium ion conductivity only little is known about interfacial reactions with lithium metal. Here we monitor the formation of an interphase between Li7P3S11 and lithium metal by a combined analytical approach, comprising in situ photoelectron spectroscopy and time-dependent electrochemical impedance spectroscopy. Utilizing a self-developed XPS peak fit model for Li7P3S11, we identify the components of this interphase, discuss its properties and develop a qualitative model, which shows that the reaction between electrolyte and lithium metal, and hence, the interphase growth, is limited to a few nm. The solid electrolyte being used is a highly crystalline form of the superionic conductor Li7P3S11 without any residual glassy phase, and the synthesis of this Li7P3S11 phase is also reported.