Electrode/Electrolyte Interfacial Behaviors of LiCoO₂/Mixed Graphite Li-Ion Cells during Operation and Storage

Interfacial properties of Li-ion cells containing LiCoO2 and a mixture of natural and artificial graphite in a LiPF6-based nonaqueous electrolyte were studied during storage and operation under various temperatures. The graphite/electrolyte interface showed a higher activation energy of Li-ion transfer than the LiCoO2/electrolyte interface for a cell with 100% state of charge. Li-plating occurred on graphite in charging at low temperature, where the charge transfer resistance of graphite electrode became dominating in the fully charged cell. The instability of solid electrolyte interphase (SEI) film on graphite during storage and cycles led to a decrease in impedance and capacity loss. Mechanisms of reversible capacity loss of an aged cell during storage were discussed.