Evolution of the Interphase between Argyrodite-Based Solid Electrolytes and the Lithium Metal Anode─The Kinetics of Solid Electrolyte Interphase Growth

Argyrodite-based solid electrolytes (SEs) are promising candidates for application in solid-state batteries (SSBs) due to their high ionic conductivity and mechanical malleability. However, they are reduced by lithium and form interphases when they are in contact with a lithium metal anode, which are needed to construct cells with high energy density. If the interphase is electronically insulating, a so-called solid electrolyte interphase (SEI) is formed that can protect the solid electrolyte from further degradation and grows only slowly. Careful evaluation of individual lithium metal anode|SE interface reactions and their growth kinetics is necessary to advance the concept of lithium metal batteries. Here, the interphase growth in symmetric Li|Li6PS5Cl|Li cells is studied quantitatively by impedance spectroscopy using a three-electrode cell setup. Unidirectional plating experiments show that the three-electrode cell is well suited to study the SEI evolution. Passivated and freshly prepared lithium foils are investigated, and the impedance evolution is studied to explore the influence of the lithium metal anode surface on SEI growth. The study reveals that an inherent passivation layer, present on most commercial lithium foils, influences the rate of SEI formation and causes high internal cell resistance. The lithium reservoir-free anode ("anode-free concept") is recommended to overcome the issues caused by chemically poorly defined lithium foil surfaces.