Bulk-interface engineering of PEO-based solid-state electrolytes toward 4.6 V lithium metal batteries at 30 °C

Polyethylene oxide (PEO)-based solid-state electrolytes (SSEs) promise intrinsic safe and high energy density lithium-ion batteries due to their excellent thermal stability, film-forming property and compatibility with lithium metal anodes. However, their narrow electrochemical window and incompatibility with nickel-rich cathodes have limited their development. Herein, a new class PEO-based composite SSE with Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/Lithium difluoro(oxalate)borate (LiDFOB) dual-salts and fluoroethylene carbonate (FEC) additive (DF-PEO) is proposed and developed. The introduction of LiDFOB and FEC produces a synergistic effect on decreasing crystalline degree of the PEO matrix and constructing addition lithium-ion pathway, resulting in a high ambient ionic conductivity of 1.6 × 10^-4 S cm^-1 with a transference number of 0.56. More importantly, this DF-PEO produces a stable LiF and Li-B-O rich cathode-electrolyte-interphase (CEI) with a thickness of ∼8 nm against LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode, while constructing a gradient-structured robust solid-electrolyte-interphase (SEI) film on the lithium anode side. Consequently, with a cut-off voltage of 4.4 V, the Li||NCM811 battery shows a capacity of 150 mAh g^-1 and maintains a retention rate of 80% after 250 cycles at 0.2C and 30 °C. Even elevated the cut-off voltage to 4.6 V, and still delivered an initial discharge specific capacity of 192.5 mAh g^-1 at 0.2C and maintained relatively stable capacity retention after 50 cycles. The novel strategy that combines bulk solvation structure optimization and interfacial chemistry engineering provides a new insight for the development of high-energy-density solid-state lithium batteries.