Influences of trace water on electrochemical performances for lithium hexafluoro phosphate- and lithium Bis(oxalato)borate-based electrolytes

Water has a fatal influence on the performance of lithium ion batteries. In this work, Lithium hexafluoro phosphate(LiPF6)-ethylene carbonate (EC)/diethyl carbonate (DEC) and lithium bis(oxalato)borate (LiBOB)-tetramethylene sulfone (SL)/DEC are taken as examples to investigate the influences of water concentration for electrochemical window, acidity, impedance and cycling performance for lithium ion batteries by adjusting water concentrations in the electrolytes. Results show that LiBOB-based electrolyte has better water tolerance compared with LiPF6-based system, due to the formation of LiBOB·xH2O compounds by consuming the additive trace water. Besides, inductively coupled plasma test result shows that Mn ion dissolution in LiPF6 systems is extremely severe, which is mainly caused by the corrosive reaction between LiMn2O4 and by-product HF acid. And we believe that Mn ion dissolution and the following deposition should be responsible for failure work of LiPF6-based cells. But for LiBOB-based cells, stable SEI layers and good electrochemical performances have been obtained, benefitting from the synergistic effect between LiBOB salt and SL solvent. And the presence of high-resistance B(C2O4) (OH) and LiB(C2O4) (OH)2 products on surface of graphite electrode is the main reason for a small number of capacity fading.