Nonflammable quasi-solid electrolyte for energy-dense and long-cycling lithium metal batteries with high-voltage Ni-rich layered cathodes

Nonflammable quasi-solid electrolyte is prepared by copolymerizing flexible chains of polymer vinyl acetate and rigid skeletons of triethylene glycol diacrylate with assistance of flame-retardant phosphate solvents. The quasi-solid electrolyte simultaneously features a high conductivity (1.02 mS cm−1), a high Li+ transference number (0.65), a wide electrochemical window (5.2 V vs. Li+/Li) and rigid-flexible coupling characteristics. Moreover, the electrolyte can capture the phosphate solvents with polar molecular interactions to weaken Li+–solvent interactions while promoting Li+–anion interactions, which creates more anion-derived protective layer on both electrodes. Thus, the electrolyte not only suppresses side reactions, irreversible phase transition, stress-corrosion cracking, and the dissolution of transition-metal ions on the cathode side, but also enables highly reversible Li metal stripping/plating, leading to low pulverization on the anode side. Employing the quasi-solid electrolyte, our LiNi0.8Co0.1Mn0.1O2||Li battery increases the upper cut-off voltage from current 4.3 V to 4.5 V (versus Li+/Li), therefore, delivers an ultrahigh specific capacity of > 224.7 mAh g−1, average Coulombic efficiency of > 99.6% and capacity retention of ∼81.2% over 500 cycles.