Deciphering Kinetic Principles of Dual-Anion Electrolytes for Extreme Fast-Charging Lithium-Ion Batteries

Tailoring Li⁺ solvation coordination has been recognized as a strategy to enhance the electrochemical performance of lithium-ion batteries (LIBs) under extreme fast-charging (XFC) conditions. Beyond weakening Li⁺ solvation, increased Li-anion pairing plays a crucial role in the formation of anion-derived, inorganic-rich electrode-electrolyte interfaces (EEIs). In this study, we propose an anion-screening guideline leveraging transport in bulk electrolyte, desolvation energy and interfacial kinetics. We investigated mechanisms governed by dual-anion electrolytes in both carbonate- and ester-based solvents, aiming to address key challenges such as interfacial instability, lithium plating, and structural degradation. Integrated computational and experimental studies reveal that optimized dual-anion systems create partially ion-paired solvation structures and robust anion-derived EEI on both electrodes, enabling principal merits of improved kinetics under XFC conditions. In LiNi_0.6Mn_0.2Co_0.2 || graphite pouch cells, the optimized dual-anion formulation, PF₆ ^-/TFSI^-, in dimethyl carbonate-based electrolyte retains over 85% of its original capacity and 94% retention after 500 cycles at 4C, while the ester-based variant in methyl propionate achieves 94%/83% retention after 500/1000 cycles at 4C. These improvements are attributed to reduced charge-transfer impedance with enriched inorganic fluorides and sulfates interface. Overall, this work provides a framework for anion regulations and offers a promising pathway to realizing fast-charging, high-energy-density LIBs.