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

A promising lithium salt of Li difluorophosphate (LiPO2F2) is introduced and added to the basic electrolyte (1 M LiPF6 + 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 LiPO2F2 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 % LiPO2F2-containing electrolyte (2 LiPO2F2) 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 % LiPO2F2-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 LiPO2F2 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 LiPO2F2; 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 LiPO2F2 may meet the application requirements of ultra-low-temperature conditions.