Flame-retardant and inactive hydrogen deep eutectic electrolytes for fast-charging and high-voltage lithium metal batteries

ABSTRACT

The development of safe and high-performance electrolytes is essential for realizing high-energy-density lithium-metal batteries (LMBs). Herein, we report a sulfone-based deep eutectic electrolyte incorporating dimethyl sulfone with inactive hydrogen (S-DEE) that overcomes key limitations of conventional deep eutectic electrolytes. The S-DEE exhibits exceptional non-flammability, a high Li⁺ transference number (0.78), and an electrochemical stability window >5.5 V. Tailored solvation structures promote inorganic-rich, uniform solid-electrolyte interphase formation, enabling dendrite-free Li plating/stripping. The LiFePO₄||Li cells with S-DEE achieve 93.0% capacity retention after 3000 cycles (1C) and 96.1% after 1000 cycles (5C). Notably, high-voltage cathodes (LiNi_0.8Co_0.1Mn_0.1O₂ and LiCoO₂) exhibited excellent performance in the S-DEE under 3.0‒4.6 V. Practical viability is demonstrated in LiFePO₄||graphite (94.3%, 1000 cycles) and LiNi_0.8Co_0.1Mn_0.1O₂||graphite (94.1%, 400 cycles) pouch cells. The S-DEE also exhibits outstanding thermal stability, as confirmed by nail penetration tests in 2 Ah LiFePO₄||Li pouch cells. The redox-inert S=O groups and absence of reactive hydrogens in dimethyl sulfone eliminate decomposition pathways, resolving interfacial instability inherent to eutectic electrolytes. This molecular design strategy establishes a scalable platform for high-energy LMBs.