Compatibility of Graphite with 1,3-(1,1,2,2-Tetrafluoroethoxy)propane and Fluoroethylene Carbonate as Cosolvents for Nonaqueous Electrolyte in Lithium-Ion Batteries

The compatibility of graphite with 1,3-(1,1,2,2-tetrafluoroethoxy)-propane (HFE) and fluoroethylene carbonate (FEC) as cosolvents is investigated with Li/graphite cells. Having lower surface tension, HFE can act as a "surfactant" to reduce the surface tension of the electrolyte, while FEC increases it. Charge discharge tests validate that the Li/graphite cells show a superior cycling performance and rate ability in 1 M (M = mol L-1) LiPF6-ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/HFE (mixing ratio: 1/1/1 in weight). Cyclic voltammetry, electrochemical impedance spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy results reveal the mechanism that HFE is reduced with EC together in an EC-based electrolyte, forming a compact surface film at the graphite surface with lower interface resistance compared with FEC. In addition, we proposed that HFE will produce fluoroalkyl lithium compounds (LiOCF2CF2CF2CF2OLi) through radical termination reaction in the reduction process.