Key Factor Determining the Cyclic Stability of the Graphite Anode in Potassium-Ion Batteries

Graphite is the most commonly used anode material for not only commercialized lithium-ion batteries (LIBs) but also the emerging potassium-ion batteries (PIBs). However, the graphite anode in PIBs using traditional dilute ester-based electrolyte systems shows obvious capacity fading, which is in contrast with the extraordinary cyclic stability in LIBs. More interestingly, the graphite in concentrated electrolytes for PIBs exhibits outstanding cyclic stability. Unfortunately, this significant difference in cycling performance has not raised concern up to now. In this work, by comparing the cyclic stability and graphitization degree of the graphite anode upon cycling, we reveal that the underlying mechanism of the capacity fading of the graphite anode in PIBs is not the larger volume expansion of graphite caused by the intercalation of potassium ions but the continual accumulation of the solid electrolyte interphase (SEI) on the surface of graphite. By X-ray photoelectron and nuclear magnetic resonance spectroscopies combined with chemical synthesis, it is concluded that the accumulation of the SEI may mainly come from the continual deposition of a kind of oligomer component, which blocks intercalation and deintercalation of potassium ions in graphite anodes. The designed SEI-cleaning experiment further verifies the above conclusion. This finding clarifies the crucial factor determining the cyclic stability of graphite and provides scientific guidance for application of the graphite anode for PIBs.