Harnessing Native Li ₂ CO ₃ Layers on Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO) for High‐Performance Tri‐Layer Composite Electrolytes in Semi‐Solid‐State Batteries

Abstract High content of inorganic solid electrolytes (ISEs) is a key for semi‐solid‐state electrolytes (SSSEs), such as composite polymer electrolyte with Garnet‐type Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO). However, the inevitably formed LiOH and Li ₂ CO ₃ layer on its surface triggers gelation of the slurry when mixed with PVDF‐HFP, which limits the practicality of LLZO for mass production and commercialization. Herein, the tri‐layer structure is reported where ethylene vinyl acetate (EVA) copolymer, which exhibits high elasticity, thermal stability, and ability to interact with LiOH and Li ₂ CO ₃ by hydrogen bonding, is initially used to create an LLZO scaffold, followed by coating PVDF‐HFP on both sides. As a result, the PVDF‐HFP/LLZO‐EVA/PVDF‐HFP tri‐layer structure suppresses gelation reaction, as well as showing high Li transference number (0.89) and effective ionic conductivity (0.38 mS cm ⁻¹ ). ⁷ Li MAS solid‐state NMR and T ₁ relaxation time measurement provide a detailed mechanism for Li‐ion transport in the tri‐layer structure. Furthermore, Li/NCM half cell and Graphite/LFP pouch full cell with the PVDF‐HFP/LLZO‐EVA/PVDF‐HFP tri‐layer structure as an electrolyte exhibits superior cycle properties over 200 cycles. Considering that our tri‐layer structure can be easily fabricated without the process for removing Li ₂ CO ₃ on the surface of LLZO, it will open a new avenue for the commercialization of semi‐solid‐state batteries.