Activating Li ₂ CO ₃ Passivation Layers via In Situ Chemical Reconstruction for Fast Li ⁺ Transport at Garnet Electrolyte Interfaces Toward High‐Performance Solid‐State Batteries

Garnet‐type solid electrolytes are promising active fillers for composite solid electrolytes, offering enhanced energy density and safety. However, the uncontrolled formation of passivating and ionically insulating Li 2 CO 3 layers on garnet surfaces severely compromises interfacial contact and blocks efficient Li + transport. Here, we present a mild, solution‐phase interfacial transformation strategy using magnesium trifluoromethanesulfonate (Mg(OTf) 2 ) to reconstruct the interface. This approach chemically reconstructs the resistive Li 2 CO 3 into an ion‐conductive, amorphous Li/Mg carbonate mixed interphase enriched with LiOTf. Mechanistic studies and molecular dynamics simulations show that Li + diffusion within this Mg‐incorporated mixed carbonate network is intrinsically more favorable than in native Li 2 CO 3 , highlighting mixed carbonate chemistry as a key factor regulating interfacial ion transport and enabling continuous, fast Li + conduction pathways. Furthermore, the strategy promotes the formation of a LiF/MgF 2 ‐rich solid‐electrolyte interphase on the Li metal anode, facilitating uniform Li deposition and stripping. Consequently, Li symmetric cells achieve stable cycling over 5000 h (0.1 mA cm −2 , 0.1 mAh cm −2 ), and LiFePO 4 full cells exhibit stable long‐term cycling with a capacity retention of 87.2% after 1000 cycles at 1 C. This work provides fundamental insights into interfacial ion transport and establishes an effective strategy for designing high‐performance, durable solid‐state batteries.