Cosolvent occupied solvation tuned anti-oxidation therapy toward highly safe 4.7 V-class NCM811 batteries†

Fluorinated electrolytes are promising for stabilizing the interfacial chemistry in high-voltage LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) batteries. However, the design of previous fluorinated electrolytes overlooked the essential role of the cathode–electrolyte interface (CEI) on de-solvation, relying heavily on weak solvation. Theoretically, the cosolvent occupied solvation structure characteristic of the highly antioxidative cosolvent and the easily oxidized salt additive in the first solvation shell is highly desirable to both widen the electrochemical window and promote the anion-enriched CEI to facilitate de-solvation. The key challenges lie in identifying ideal cosolvents that are highly polar, antioxidative, and have a stronger interaction with anions, to replace the solvation site of the main solvents without oxidation of itself and promote the oxidation of additive anions. Herein sulfone (SL) and DFOB⁻ are screened out following developed rules, and the interaction relationships are: (i) Li⁺–cosolvent > Li⁺–main solvent; (ii) DFOB⁻–cosolvent > DFOB⁻–main solvent; (iii) DFOB⁻–cosolvent > DFOB⁻–Li⁺. And an optimized fluorinated electrolyte composed of 10% SL and 0.02 M LiDFOB is therefore successfully developed. This occupied solvation design promotes both interfacial/anodic stability and de-solvation under an aggressive 4.7 V. Consequently, ∼400 W h kg⁻¹ NCM811/Li cells at 4.7 V demonstrate an 82% capacity retention after 200 cycles. Commercial NCM811/Gr pouch cells at 4.5 V achieve 92% capacity retention over 500 cycles, concurrently with unexpectedly high safety performance in terms of thermal, mechanical, and electrical abuse. This work underscores the critical impact of solvation site-occupied cosolvent on the CEI modification and kinetics optimization, opening a new avenue for high voltage electrolyte design.