The Impact of Intentionally Added Water to the Electrolyte of Li-Ion Cells

Wound prismatic LiCoO2/Li4Ti5O12 cells with 1 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 by wt) baseline electrolyte had water intentionally added to the electrolyte and were studied using the High Precision Charger at Dalhousie University, automated cycling/storage, AC impedance and extended cycling. Water was added at 200, 1000 and 2000 ppm to the control electrolyte. The control cells have been shown by the manufacturer to cycle for over four years with only about 1% capacity loss in on-going tests. Swelling during the formation cycle increased gradually when 200 and 1000 ppm water was added and substantially when 2000 ppm water was added. A corresponding increase in first cycle irreversible capacity with increasing water content was observed. Adding up to 2000 ppm water resulted in minimal change to measured coulombic efficiency, charge end point slippage, voltage drop during storage and average charge voltage increase. In fact, most measured parameters were slightly improved with the addition of up to 1000 ppm water compared to control. Measured cell impedance was reduced compared to control cells for all cells containing water. There was no detrimental effect on capacity retention due to the added water for cells tested for 400 charge-discharge cycles. At these relatively low loading levels of water in the electrolyte there were no obvious detrimental effects to cell performance and therefore it may be possible to relax water content specifications in Li-ion battery electrolyte leading to one avenue for cost reduction.