Uniformization of Electron Distribution at Grain Boundaries via Work Function Adjusting Interlayer to Prevent Li‐Filament Formation Within Solid Electrolytes

Abstract With the increasing demand for safe all‐solid‐state lithium metal batteries (ASSLMBs), preventing Li‐filament formation has become a critical issue in inorganic solid‐electrolytes (ISEs). Fundamentally, uniformizing electronic properties at the microstructure is key to preventing the reduction of Li‐ions, which strongly induces Li‐filament formation. However, little information on the electronic properties of ISEs makes interpreting the source of Li‐filament formation difficult. Herein, a mechanism for Li‐filament formation is revealed by confirming that work function differences at the interface between the grain interiors (LLZO) (≈ 4.2 eV) and grain boundaries (Li_xAlO_y) (≈ 4.32 eV), cause localized currents, reducing Li‐ions at the grain boundaries of ISEs. Introduction of a work function adjusting interlayer, a thin LiF layer that uniformizes the work function evenly across the grain interiors and grain boundaries (≈ 4.08 eV), dramatically suppresses Li‐filament formation by preventing localized currents. The prevention of Li‐filament formation is identified through surface and cross‐section images of laser‐induced breakdown spectroscopy (LIBS). Furthermore, the electrochemical stability of Li‐symmetrical cells with the LiF layer is enhanced by increasing the critical current density five times higher than the cell without the LiF layer. This suggested mechanism verifies that microstructure interface engineering with uniformizing electronic properties is essential for the safety of ASSLMBs.