ABSTRACT 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.
Xu et al. (2026) studied this question.