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lithium difluorophosphate is a high-performance electrolyte additive that significantly improves the cyclability of carbonate-based lithium-ion batteries. It is also effective in alleviating the increase of cathode resistance that occurs during cycling of batteries. This is attributed to the lithium ion-induced dispersal of silicon dioxide from the negative electrode surface. In addition, it suppresses the formation of lithium nitride in the anode, which is known to cause poor cycling performance.
However, the conventional preparation of lithium difluorophosphate involves expensive siloxanes and the production process generates inevitable byproducts such as chloride and insoluble matter. These impurities need to be removed by an additional purification process. This invention provides a simple process for producing lithium difluorophosphate with high yield and purity. The process uses a mixture of organosilyl halide reagents instead of the conventional use of expensive siloxane. The equivalence ratio of organosilyl halides is important for the change of HF and Cl contents in lithium difluorophosphate.
Lithium hexafluorophosphate (LiPF 6), sodium difluorophosphate (NaFO), and tetramethylammonium difluorophosphate (MAFO) were prepared by reacting LiPF 6 with water, phosphorus pentoxide, and organosilyl halides. NaFO and MAFO exhibit excellent solubility and are free of insoluble impurities due to the use of a solid-state reaction. Moreover, the cation-exchange reaction of MAFO with tetramethylammonium chloride results in the formation of NaFO with high purity. Electrochemical evaluation of MAFO in lithium Ni/LiNi1/3Co1/3Mn1/3O2/graphite pouch cells shows that the addition of 1 wt% LiDFP notably enhances the cyclability and rate capability at 4.5 V.