Charge-to-Mass Ratio Regulation of Solvated Li+ Clusters for Ultrafast-Charging Lithium Battery Electrolytes

Extreme fast charging of high-energy lithium batteries is limited by coupled Li+ transport, interfacial desolvation, and electrode degradation. Here, we introduce q/Mcluster as a composition-derived prescreening descriptor, where q is the assigned charge number of the Li-centered carrier (+1 here) and Mcluster is the abundance-weighted composition mass of the cationic clusters in the four most populated solvent-separated ion-pair (SSIP) configurations. A higher q/Mcluster identifies a lower average cluster composition mass and prioritizes electrolyte candidates for transport and electrochemical testing. Guided by this descriptor, we develop a methyl acetate/fluoroethylene carbonate (MA/FEC) electrolyte that favors MA-rich solvated Li+ clusters with lower composition mass and dynamic ligand exchange, while FEC supports interphase stabilization. Simulations and experiments show weaker Li+–solvent association, a lower calculated cluster ligand-removal energy, and faster Li+ diffusion in the optimized formulation. It enables stable operation for 1000 cycles at 15C and 500 cycles in a 1 Ah pouch cell under 3C charging. Composition and cross-family controls support complementary roles for MA-regulated solvation and FEC-derived interphase stabilization, while revealing limits to a single-parameter ranking. Thus, q/Mcluster helps prioritize formulations for independent transport, interfacial, and electrochemical validation.