Abstract
The electrolyte additive lithium difluorophosphate improves the lifetime of lithium-ion cells. This work presents the synthesis and evaluation of alternative difluorophosphate salt electrolyte additives. Ammonium difluorophosphate is readily prepared via a solid-state, benchtop reaction of ammonium fluoride and phosphorus pentoxide that requires only gentle heating to initiate. The best yield of sodium difluorophosphate (NaFO) in the present study was obtained by reacting difluorophosphoric acid and sodium carbonate in 1,2-diemethoxyethane over 3 Å molecular sieves. Tetramethylammonium difluorophosphate was prepared from NaFO via cation-exchange with tetramethylammonium chloride. NaFO is here reported to be a very good electrolyte additive, with similar performance in NMC532/graphite pouch cells as the lithium salt. The beneficial nature of both additives is attributable to the difluorophosphate anion. In contrast, ammonium and tetramethylammonium difluorophosphates are found to be poor electrolyte additives. For the former, this is suggested to be due to the formation of lithium nitride and hydrogen gas.
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