Low-cost molten salt coating enabling robust Li/garnet interface for dendrite-free all-solid-state lithium batteries

• A robust Li/garnet interface is constructed by a low-cost SnF 2 molten salt coating. • In-situ formed LiF/Li 22 Sn 5 interphase significantly suppressed the Li penetration. • The interphase endows LLZO-based Li|LFP cell with good rate and cycling capability. • The abundance of salt species gives salt-derived interphase more design freedom. Garnet-based solid-state electrolytes (SSEs) have received great attention due to their high ionic conductivity and superior chemical/electrochemical stabilities. However, the high interfacial resistance induced by the poor wettability of the Li/SSEs interface and the resulting Li dendrite penetration severely hinder the practical application of garnet-based all-solid-state Li metal batteries (ASSLMBs). Herein, we explore a low-cost and facile molten salt coating method to uniformly coat an ultrathin SnF 2 film on the surface of garnet-type Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) SSEs. After lithiation at elevated temperature, the SnF 2 film is in-situ transformed into hybrid interphase composed of LiF and Li 22 Sn 5 . Highly lithiophilic Li 22 Sn 5 alloy provides intimate contact between Li and LLZTO, while electron-blocking LiF prevents Li deposition within LLZTO. Therefore, assembled Li|SnF 2 -coated LLZTO|Li cells can cycle stably at 0.3 mA cm -2 over 1000 h and achieve a high critical current density of 1.0 mA cm -2 . Moreover, the Li|SnF 2 -coated LLZTO|LiFePO 4 full cells exhibit superior cycling (88.9 % capacity retention over 400 cycles at 0.5 C) and rate performance. Notably, the proposed salt coating strategy endows an extended degree of freedom for functional interfacial design and potential for large-scale application, given the abundance of salt species and low cost and complexity of heating treatment, respectively.