Enabling good interfacial stability by dual-salt composite electrolyte for long cycle lithium metal batteries

The practical application of solid-state batteries is limited by the relatively low ionic conductivity of solid polymer electrolytes (SPEs) and the high charge transfer resistance resulting from poor interfacial wettability between electrodes and solid electrolytes of inorganic solid electrolytes (ISEs). In this manuscript, a dual-salt dual-network composite electrolyte is prepared by adjusting the content of lithium bis(oxalato)borate (LiBOB) and Li6.75La3Zr1.75Ta0.25O12 (LLZTO), and a composite electrolyte with excellent long cycle and interfacial stability is designed. The as-prepared composite electrolyte that includes 77 wt% gel electrolyte, 20 wt% LLZTO and 3 wt% LiBOB (assigned as PDFL-20LLZTO-3LiBOB) has a high ionic conductivity of 1.69 × 10−3 S cm−1 and a high lithium-ion migration number of 0.833 at 25 °C. The Li metal battery (LMB) with PDFL-20LLZTO-3LiBOB exhibits excellent cycling stability with 94% capacity retention at 25 °C for 100 cycles. The corresponding symmetrical Li|PDFL-20LLZTO-3LiBOB|Li can cycle for over 1200 h with excellent long-term stability. This impressive electrochemical performance is attributed to the in-situ formation of a dense and uniform solid electrolyte interface (SEI) rich in LiF and B–F at the interface under the dual action of LiBOB and fluoroethylene carbonate (FEC). In addition, the high ionic conductivity and high ionic mobility number are favorable for uniform deposition of Li and stable interfacial compatibility.