Coating layer design principles considering lithium chemical potential distribution within solid electrolytes of solid-state batteries

Introducing a coating layer at an active material /solid electrolyte interface is crucial for ensuring thermodynamic stability of the solid electrolyte at interfaces in solid-state batteries. To thermodynamically protect the solid electrolyte, coating layers must maintain lithium chemical potential (μ_Li) at coating layer/solid electrolyte interfaces within the electrochemical window of the solid electrolyte. However, a general coating layer design principle to achieve this remains unestablished. Here we theoretically elucidate the µ_Li distribution across the solid electrolyte and coating layer, examining requirements for thermodynamic protection. We show that the protective capability of coating layers is not solely determined by their intrinsic characteristics, but also by the µ_Li distribution within the solid electrolyte and coating layer. We propose a quantitative approach based on µ_Li distribution to determine the required characteristics and geometries of coating layers that ensure the thermodynamic stability of the solid electrolyte while minimizing ohmic resistance, providing insights for coating layer design.