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.
Prussian blue (PB) is a very promising cathode for K-ion batteries but its low electronic conductivity and deficiencies in the framework aggravate electrochemical performances. Compositing with conductive reduced graphene oxide (rGO) is an effective solution to address this problem. Nevertheless, little attention was paid to the loss of oxygen-containing functional groups on the rGO substrate during the compositing process, which weakens the interaction between PB and rGO and leads to poor electrochemical performance of PB/rGO. Herein, this interaction effect associated with surface functional groups is first openly debated. Two commonly used carbon substrates, graphene oxide (GO) and rGO, are investigated. A more stable interaction between PB and GO contributes to a higher capacity retention (91.8%) than that of PB/rGO (69.7%) after 300 cycles at a current density of 5 C. Meanwhile, polyvinylpyrrolidone (PVP) is employed to repair the weak interaction between PB and rGO substrates. PB is anchored to the rGO surface through the stable covalent linking of amide groups in PVP. A superior rate capability of 72 mA h g −1 at 10 C and an improved capacity retention of 96.5% over 800 cycles at 5 C are obtained by as-prepared PB/PVP-rGO. This study provides a deeper understanding of fabricating PB/carbon composites with a robust connection.
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