Solid state electrolytes (SEs) in Li batteries are believed to be the ultimate solution to the electrode dissolution problems and safety hazards of liquid electrolyte (LE) systems. Sulfur based bulk type SE attracts great interest due to their relatively high ionic conductivities. Besides the bulk lithium ion conductivity, physical contact, structural evolution and redox reaction kinetics at SE and electrode interface during battery processes are other key factors that dictate the efficiency of battery cycling performances. The current challenge of studying the interfacial processes in operando in all solid battery systems is the difficulty of assembling an air-tight spectro-electrochemical cell with good electrical contact and optical accessibility to the interface of interest. In this work, a spectro-electrochemical cell is designed for an in operando Raman measurement at SE/Au interface during Li deposition and stripping processes. Three representative sulfur based SEs (β-Li₃PS₄, 70Li₂S-30P₂S₅ glass ceramic (LPS-GS) and Li₁₀GeP₂S₁₂ (LGPS)) were investigated. Spectroscopic data shows that, in general, partially reversible structural interconversion occurs among PS₄^3-, P₂S₆^3-, P₂S₇^3- and other unidentified P-S anion species. Corresponding variations in cell impedance also supports these interfacial structural evolutions. Result reveals that in all solid Li battery systems, oxidation/reduction of Li⁺ occurs along with breaking and reformation of the Li⁺-Anion interactions, which are usually accompanied by some unexpected, partially irreversible structural evolution of the counter ions. The resulted byproducts are later accumulated at the electrode/electrolyte interfaces. This result not only help build the electrochemical fundamentals of the molecular details at solid-solid interface but also guide the choice of materials and interfaces in all solid Li battery systems.

Lingzi Sang

University of Illinois

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