The Limited Effect of VC in Graphite / NMC Cells

Degradation at the electrode surfaces is one of the major reasons behind capacity fade in a well-constructed battery. The presence of electrolyte additives is known to alter the electrode particle surface which may in turn lead to changes in rates of intercalation as well as side-reactions. In particular, VC has been studied extensively for different lithium-ion chemistries and been shown to improve columbic efficiency of some electrodes. In the present study, the effect of vinylene carbonate (VC) as an electrolyte additive in a graphite/NMC 333 (lithium-nickel-manganese-cobalt oxide) cell is investigated. The study shows that the addition of VC improves the rate performance especially at moderately high rates. A new three-electrode cell design with Li as a reference electrode was used in this study. The three-electrode cell was particularly useful in studying the effect of VC on 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 Gr./NCM cells consist of two types, those that go to repair cracked SEI on the anode and results in a net consumption of Li from the cathode and those that reform SEI components that dissolve from the anode and are oxidized at the cathode.  The VC appears to reduce the second type but have negligible effect on the first.   This also indicates that columbic efficiency measurements are not a reliable indicator of cell life with regard to short term capacity fade.