Potential-Controlled Pre-SEI Regulation for Improved Lithium Reversibility in Anode-Free Solid-State Lithium Metal Batteries

Anode-free solid-state lithium metal batteries (AF-SSLMBs) with high safety and improved energy density receive increasing attention but are restricted by the low Coulombic efficiencies (CEs) that result from undesirable solid electrolyte interface (SEI) formation and irreversible Li deposition/dissolution. Herein, a pre-SEI is designed by a potentiostatic controlling electrolyte decomposition method to reduce Li loss for SEI formation and smooth Li deposition/dissolution behavior. When holding the potential at 0.5 V, the electrolyte additive ethoxy-pentafluoro-cyclotriphosphazene (PFPN) and lithium salts simultaneously decompose to form a dense double-layered pre-SEI with high ionic conductivity, enabling fast Li⁺ transport across the interface and suppressing the following Li loss of building SEI. As a result, a high initial CE (ICE: 95.5%) and stable CE of 98.7% in Li|Cu cells are achieved, which is a 12.7% and 0.7% improvement compared with the counterpart without pre-SEI. Moreover, the cycling life of the assembled AF-SSLMB pouch cell (Cu||LFP) with pre-SEI is prolonged by 5 times, with a capacity retention rate of 44.9% after 100 cycles. This work provides a scalable strategy to reduce Li loss for both building SEI and following the Li plating/stripping process in AF-SSLMBs.