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

High content of inorganic solid electrolytes (ISEs) is a key for semi-solid-state electrolytes (SSSEs), such as composite polymer electrolyte with Garnet-type Li₇La₃Zr₂O₁₂ (LLZO). However, the inevitably formed LiOH and Li₂CO₃ 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 Li₂CO₃ 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). ⁷Li MAS solid-state NMR and T₁ 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 Li₂CO₃ on the surface of LLZO, it will open a new avenue for the commercialization of semi-solid-state batteries.