Inhibition of Heterogeneous Li Filaments in Inorganic Solid Electrolytes via Work Function Alignments at the Grain Boundary

All-solid-state lithium metal batteries (ASSLMBs) with inorganic solid-state electrolytes (ISEs) are promising with high energy density and excellent safety compared to conventional Li-ion batteries using flammable organic liquid electrolytes. However, the ISEs have a severe electrochemical stability issue at ISE boundaries where heterogeneous Li filaments are formed, which causes internal short circuits during cycles by inducing Li penetration through ISEs. From the common fabrication process of ISEs, sintering the solid electrolyte powders, the microstructure boundaries of ISEs, such as grain boundaries (GBs), are inevitably formed. Therefore, applying an appropriate method at the GBs, the interface between the grain interiors (GIs), based on the intrinsic properties of these microstructures, is important. Fundamentally, the GBs in ISEs have high electronic conductivity, low ion conductivity, and high interfacial resistance, which induce heterogeneous Li filaments at the GBs. Especially among the properties, the high electron conductivity that results from the non-uniform distribution of electronic structures at GBs dominantly contributes to the formation of Li filaments. However, the studies on the electronic structures at the GBs have not proceeded well, even though that is essential to reveal the formation of Li filaments.<br/>In this study, we revealed the mechanisms for the origin of Li filaments and introduced the work function alignment interlayer at GBs based on the mechanisms to improve electrochemical stability and ensure superior safety. Among the ISEs, cubic Li₇La₃Zr₂O₁₂ (LLZO) with low electronic conductivity (10^-7 ~ 10^-11 S cm^-1), high bulk ion conductivity (&gt; 10^-4S cm^-1) and high reduction stability (0 V vs Li/Li⁺) was used for research. From the non-uniform work function between GIs (4.2 eV) and GBs (4.3 eV) of LLZO, electrons transferred from GIs to GBs and accumulated at GBs as the energy levels of the GIs and the GBs were aligned by the Fermi level. Under electric fields, localized current occurred at the electron accumulated points, which caused Li-filament formations at the GBs by encountering Li-ion. Using laser-induced breakdown spectroscopy (LIBS) after the Li plating, we also identified the highly localized Li intensity on the surface and cross-section images of LLZO, which means Li filaments were formed heterogeneously. Based on the mechanisms of Li-filament formation, we introduced the work function alignment interlayer, LiF layer, for LLZO to prevent heterogeneous Li filaments via inhibiting leakage current points at GBs and strongly enhanced the electrochemical stability. With work function alignment interlayer LiF, Li-symmetrical cell (Li/LLZO@LiF/Li) maintains a low overpotential (4 mV) over 4000 cycles and increases the critical current density 5 times compared to the Li-symmetrical cell without the LiF layer (Li/LLZO/Li). This research reveals that microstructure interface engineering based on uniformizing electronic structures at ISE boundaries is necessary for superior safety for ASSLMBs, which can be applied to other ISEs.