Impedance Reducing Additives and Their Effect on Cell Performance

Wound LiCoO 2 /graphite and Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 /graphite cells with 1 M LiPF 6 in EC:EMC (3:7 by wt) electrolyte containing 2 wt% vinylene carbonate (VC) and/or lithium bis (trifluoromethanesulphonyl) imide (LiTFSI or 3M Fluorad Lithium HQ-115) were studied using the High Precision Charger at Dalhousie University, automated cycling/storage, AC impedance and long-term cycling at elevated temperature. The additive, VC, improves lifetime performance by increasing coulombic efficiency and decreasing charge and discharge end point capacity slippage, while the impact of HQ-115 alone on lifetime is minimal. However, HQ-115 reduces cell impedance when added to either control or VC-containing electrolyte. Therefore it serves as a useful additive for improving the rate capability of cells and capacity retention during high rate cycling at elevated temperature. It is our opinion that Li-ion cells incorporating VC can benefit from a simultaneous addition of HQ-115 due to a synergistic effect observed between HQ-115 and VC additives. This report is the first of two companion reports studying impedance reducing additives and their impact on cell lifetime.