A weak-solvation strategy for modulating Li ion solvation sheath to enable fast-charging Li-ion batteries

Graphite is the common anode material used in lithium-ion batteries (LIBs). However, they show poor kinetics in traditional carbonate-based electrolytes, which limits the fast-charging performance of the corresponding LIBs. Here, a weak-solvation strategy is introduced to modulate the local environment around the Li ions and the electrode/electrolyte interfacial chemistry to facilitate Li⁺ transport. Specifically, the study finds the use of methyl acetate (MA) as a co-solvent with lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethyl methyl carbonate (EMC) can weaken the interactions of Li⁺ with the solvent molecules, generating a LiF-rich solid electrolyte interphase on the graphite anode, thereby improving its fast-charging performance. In addition, the desolvation energy of Li⁺ is further reduced by fluorinating EMC (FEMC), owing to the strong electron-withdrawing effect of fluorine. With a 1 M LiFSI FEMC/MA (1:1, v) + 10 wt% fluorinated ethylene carbonate electrolyte, graphite anode shows a lithiation capacity of about 230 mAh g^-1 at 1488 mA g^-1 (4 C_graphite rate), significantly higher than that obtained in a LiPF₆-based conventional electrolyte. The weakly-solvating electrolyte with LiFSI, FEMC and MA is also compatible with LiFePO₄ cathodes, where a LiFePO₄ | graphite full cell can be cycled even at a charging rate of 4.5 C_fullcell.