Structured solid electrolyte interphase enable reversible Li electrodeposition in flame-retardant phosphate-based electrolyte

Tailoring non-flammable phosphate-based electrolytes with high safety and good Li reversibility are in urgent demand for practical lithium metal batteries. However, phosphate-ester exhibits a poor compatibility with active Li surface and the key to solve this challenge is to build a sustainable solid-electrolyte interphase. Herein, dual-layer crystalline/polymeric solid electrolyte interphase (SEI) is designed in phosphate ester/ether mixed electrolyte utilizing lithium difluoro(oxalato)borate (LiDFOB) as main salt. The coordinated ether in Li solvation sheath will be reduced in Li metal surface, forming alkoxides residue in the early cycling stage, which will further react with DFOB⁻ to form an outer Li₂O, inner amorphous polyether SEI layer. Consequently, the flame-retardant electrolyte exhibits a low self-extinguishing time of 6.10 s g⁻¹ and a satisfied Li reversibility of 98.31% in NCM/Cu configuration. The coin-type limited-excess Li/LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) full cell with negative to positive electrode capacity ratio (N/P) of 1.92 can cycle for more than 140 cycles. Meanwhile, a pouch cell with ∼0.2 Ah capacity, low N/P ratio of ∼2.22 shows a good capacity retention of 87.9% after 60 cycles without Li exhaustion.