The Limited Effect of VC in Graphite/NMC Cells

Degradation at the electrode surfaces is one of the major reasons behind capacity fade in well-constructed batteries. The effect of electrolyte additives, in particular vinylene carbonate (VC), is studied extensively for different lithium-ion chemistries and is shown to improve columbic efficiency of some electrodes. We investigate the effect of VC additive in a graphite/NMC333 (lithium-nickel-manganese-cobalt oxide) cell. The addition of VC improves the rate performance, especially, at moderately high rates. A new three-electrode cell design with Li reference electrode was particularly useful in studying the rate performance of each electrode. The rate of side reactions is found to decrease with the addition of VC. Despite these important performance improvements, no significant improvement in the capacity retention is observed. This suggests that the side reactions in graphite/NCM cells consist of two types, (1) repairing cracked solid electrolyte interphase (SEI) on the negative electrode (results in a net consumption of Li from the positive electrode), (2) reforming SEI components that dissolve from the negative electrode and are oxidized at the positive electrode. The VC appears to reduce the second type but have negligible effect on the first. This indicates that columbic efficiency measurements are not a reliable indicator of cell cycle life.