High‐Reversibility Li‐FeF ₃ Conversion Batteries Benefiting from Interface Welding of Garnet Electrolyte via Strong‐Acid‐Weak‐Base Salt Hydrolysis Cleaning

ABSTRACT Garnet‐type electrolyte Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 suffers from the poor contact with Li metal anode owing to the formation of Li 2 CO 3 passivation layer at interface, which causes the frustrating interface resistance and hinders the development of garnet‐based solid‐state batteries. Here, a strong‐acid‐weak‐base salt hydrolysis cleaning strategy is proposed for the elimination of Li 2 CO 3 and building of lithophilic ZnO modified layer. The hydrolysis of zinc salt solution is a kinetics dominant process, controllably releasing H + dependent on ambient temperature, to achieve the fast removing of Li 2 CO 3 and meanwhile avoid the excessive acid corrosion against garnet electrolyte. Benefiting from the thermodynamic reactivity of ZnO with Li, the Li 2 O and LiZn‐alloy nanodomains with heterojunction interface are formed after lithiation, and the built‐in electric field driven by the difference of work functions of Li 2 O and LiZn endows the interface with fast Li + /electron shunt migration. The interface exhibits a superior wettability toward garnet electrolyte and Li metal and an interfacial area specific resistance is as small as 5 Ω cm 2 at room temperature. The symmetric cells enable the stable plating/stripping behavior over 1000 h with a small unilateral polarization overpotential of 14 mV. The reversible solid‐state Li/FeF 3 batteries are successfully operated, displaying the high conversion capacity of ∼560 mAh g −1 and rate performance (∼400 mAh g −1 at 0.5 mA cm −2 ). The discharge conversion products display a microstructure consisting of amorphous LiF phase and fine nanodomains with poor crystallinity. Compared with the crystallized LiF in liquid system, the amorphous LiF in solid‐state system enables the faster bulk‐ion diffusion ability and easier LiF dissociation during charge. This passivation cleaning approach provides a universal solution to high energy‐density and reversible conversion‐type solid‐state Li metal batteries.