The Impact of Intentionally Added Water to the Electrolyte of Li-ion Cells: I. Cells with Graphite Negative Electrodes

Abstract

Wound LiCoO2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite cells with 1 M LiPF6 in EC:EMC (3:7 by wt) with selected electrolyte additives and intentionally added water were studied using the High Precision Charger at Dalhousie University, automated cycling/storage, AC impedance and long term cycling. Water was added at 100 and 1000 ppm levels to control electrolyte and to electrolyte containing 2 wt% vinylene carbonate (VC) and/or 2 wt% lithium bis (trifluoromethanesulphonyl) imide (LiTFSI or 3M Fluorad Lithium HQ-115). The addition of 100 ppm water to control electrolyte had minimal impact on first cycle irreversible capacity loss, cell swelling, measured coulombic efficiency and charge end point capacity slippage rate. The addition of 100 and 1000 ppm water to either VC or VC + HQ-115-containing electrolyte resulted in small changes in coulombic efficiency, charge end point capacity slippage, voltage drop during storage and charge transfer resistance, however in all cases the addition of 1000 ppm water was, surprisingly, beneficial compared to no added water. The only exception was that the addition of 1000 ppm water led to an increase in cell swelling. As predicted by the early high precision measurements, the cells containing water did not show major differences in long term cycling at both 40 and 55°C. This shows that at these relatively low loading levels of water in the electrolyte there are 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. Interestingly, the addition of HQ-115 reduced the impedance rise and capacity loss during long term cycling of cells containing both VC and water, suggesting that HQ-115 is a useful additive under conditions where moisture may not be controlled to tight tolerances.