Co2+ species dissolved from LiCoO2 in lithium-ion batteries have been well-established to be responsible for the cell performance fading, especially when the cells are charged to high voltage or at elevated temperatures. The accepted underlying mechanism is the deposition of Co2+ on the graphite anode that destroys the interphase. In this work, we report that the dissolved Co2+ exists in the form of both Co0 and Co2+ on the graphite anode surface, while Co0 formed at lithium insertion potential can be reoxidized to Co2+ during charging. Moreover, Co0 shows a higher catalytic activity than Co2+ toward the reductive decomposition of carbonate electrolyte. An interphase of ∼4 nm was thus engineered from a film-forming additive 3-sulflone, which completely eliminates the destructive effect of the deposited Co species. The understanding of the destructive role of the dissolved Co2+ on the interphasial stability of the graphite electrode and an effective strategy to suppress such a failure mechanism provides fresh insight into the failure mechanism of manganese-based cathode chemistries, which serves as a better guideline for electrolyte design for future batteries.
Keywords: 3-sulflone; cobalt(II) ions; film-forming electrolyte additive; graphite electrode; lithium-ion battery.