Lithium Difluorophosphate (LiPO2F2): An Electrolyte Additive to Help Boost Low-Temperature Behaviors for Lithium-Ion Batteries

A promising lithium salt of Li difluorophosphate (LiPO 2 F 2 ) is introduced and added to the basic electrolyte (1 M LiPF 6 + dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC)/propylene carbonate (PC)/fluoroethylene carbonate (FEC)) to enhance the electrochemical performance of lithium-ion batteries by changing its concentration at low temperatures. Experiments show that LiPO 2 F 2 can obviously improve the electrolyte’s ionic conductivity, especially. Compared with the baseline (6.29, 5.14, 4.62, 3.77, and 3.15 mS cm –1 ) under the same conditions, the ionic conductivities of the 2 wt % LiPO 2 F 2 -containing electrolyte (2 LiPO 2 F 2 ) are 7.18, 5.32, 5.23, 4.04, and 3.57 mS cm –1 . From this, it is clear that the integrated low-temperature electrochemical performances have been significantly improved. The specific discharge capacities of the 2 wt % LiPO 2 F 2 -added electrolyte at 0.2C are 161.4, 157.2, 148.9, and 137.5 mAh g –1 in the range from 25 to −40 °C, which are significantly much higher compared with the basic electrolyte (145.3, 149.4, 135.1, and 107.0 mAh g –1, respectively). Additionally, the addition of LiPO 2 F 2 can promote profoundly the composition and morphology characteristics of the cathode–electrolyte interface (CEI) film; namely, it is a uneven lithium alkyl carboxylate before the addition of LiPO 2 F 2; after that, it becomes an even LiF film, and meanwhile, the new film will facilitate Li + transport in the electrolyte, further boosting the eletrochemical performance of the cells under low temperatures. At −40 °C under the same current density, the specific discharge capacity is 81.97 mAh g –1 after 50 cycles, while at the same temperature, the specific discharge capacity of the basic electrolyte battery is only 33.37 mAh g –1; that is, the discharge capacity has a staggering 145.6% improvement at −40 °C. In conclusion, the lithium-ion batteries containing LiPO 2 F 2 may meet the application requirements of ultra-low-temperature conditions.