Functional Electrolyte of Fluorinated Ether and Ester for Stabilizing Both 4.5 V LiCoO₂ Cathode and Lithium Metal Anode

In our work, fluoroethylene carbonate (FEC) with higher ionic conductivity and oxidative stability has been used to replace ethylene carbonate along with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) added to obtain a trisolvent electrolyte system of 1.2 M LiPF6 in FEC/dimethyl carbonate/HFE (1:1:1 by volume). In addition, on the basis of such an electrolyte, we have also added 0.15 M lithium difluoro(oxalato)borate as a film-forming additive to further stabilize both the cathode and anode, so that a new bifunctional electrolyte of 1.35 M has been obtained. The new electrolyte enables Li||LiCoO2 batteries to maintain nearly 84% capacity after 300 cycles when charged to 4.5 V under the current density of 200 mA g–1. Meanwhile, it can also make Li||Cu half-cells maintain about 98% coulombic efficiency after being charged/discharged 200 times under 1 mA cm–2 current density and 1 mAh cm–2 lithium deposition amount per cycle, showing an obvious advantage in the stability of lithium metal.