Abstract
Introducing a coating layer (CL) at an active material (AM)/solid electrolyte (SE) interface is a pivotal approach to ensure interfacial stability in solid-state batteries (SSBs), thereby improving their durability and performance. To thermodynamically protect the interface, CLs must not only be chemically compatible with the SE and AM but also maintain Li chemical potential (µLi) at the SE/CL interface within the electrochemical window of the SE. However, a general CL design principle to achieve this remains unestablished. Here we theoretically elucidate the µLi distribution across the SE and CL in SSBs and examine the requirements for CLs to thermodynamically protect SEs. We show that the protective capability of CLs is not solely determined by their intrinsic characteristics and chemical compatibility with SEs and AMs, but is also governed by the µLi distribution within the SE and CL. We propose a quantitative approach based on the µLi distribution within the SE and CL to determine the required characteristics and geometries of CLs that ensure interfacial thermodynamic stability while minimizing ohmic resistance in SSBs, providing insights for CL design.