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 (NMC)/graphite cells with 1 M LiPF 6 in EC:EMC (3:7 by wt) electrolyte containing 2 wt% vinylene carbonate (VC) and/or 0.3 wt% trimethoxyboroxine (TMOBX) 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. The impact of TMOBX on cell performance depends on electrolyte and electrode choices within the cell. When TMOBX is added to control electrolyte in LiCoO 2 /graphite cells, the cycle life is improved and the impedance reduced. When added to VC-containing electrolyte the impedance is also reduced with the impact on lifetime not being clear at this time. When TMOBX is added to NMC/graphite cells with control electrolyte the impedance is decreased but when added to VC-containing electrolyte the impedance is unchanged. The impact on cycle life when TMOBX is added to NMC/graphite cells is a decrease in capacity fade but a lower measured coulombic efficiency. Therefore TMOBX appears to be an interesting additive with potential to decrease impedance and/or extend cycle life based on the electrolyte formulation and electrodes used in a Li-ion cell.