Salt-in-Salt Mediated Weak-Solvent Electrolyte Enabling Fast-Charging and Wide-Temperature Lithium-Ion Batteries

Realizing the practical application of spinel LiNi_0.5Mn_1.5O₄ (LNMO) cathodes requires electrolytes with fast-charging capability and wide-temperature adaptability, which conventional electrolytes lack due to insufficient high-voltage stability, sluggish ion transport, and unstable interphases. Herein, we propose a salt-in-salt mediated "strong-weak synergy" strategy for fluorinated weakly solvating electrolytes (WSEs), distinct from conventional ether-based or single-component WSEs. Harnessing the moderate Lewis acidity of Mg²⁺ from Mg(TFSI)₂, we promote LiDFOB dissociation to enrich anion-rich contact ion pair/aggregate (CIP/AGG) solvation structures, while concurrently inducing a "drag" effect on Li⁺-coordinated solvents/anions to synergistically accelerate Li⁺ desolvation. Notably, Mg²⁺ from inorganic MgF₂ dynamically captures interfacial anions, directing the formation of a thin, robust inorganic CEI. This dual-regulation mechanism simultaneously optimizes bulk electrolyte ion conduction and interfacial stability, overcoming the intrinsic limitations of poor oxidation resistance and sluggish kinetics in traditional WSEs. Consequently, LNMO||Li cells exhibit exceptional fast-charging capability and cycling stability across a wide temperature range (-30 to 70°C), with pouch cells retaining 88.9% capacity after 400 stable cycles. The developed electrolyte also exhibits non-flammability and broad compatibility for nickel-rich LiNi_0.8Co_0.1Mn_0.1O₂, LiNi_0.92Co_0.06Mn_0.02O₂ and olivine-type LiFePO₄ cathodes. This work offers fundamental insights into solvation chemistry and interfacial engineering toward safe, high-performance lithium-ion batteries.