Highly reversible potassium intercalation into graphite in carbonate ester solution at room temperature is achieved byelectrochemical reductionat the potential approaching to K + /Kstandard potential which islower than that of Li + /Li. The intercalation results in formation of stage-1 KC 8 compound with delivering 244 mAh g-1 of reversible capacity. The initial irreversible capacity is suppressed by polycarboxylate bindercompared to poly(vinyledene fluoride) binder.The lower potential, good cyclabilty, andexcellent rate capabilityare first demonstrated forenergy storage applications. Because of the lowest potential andweakest solvation among Li + , Na + , K + , Mg 2+ , and Ca 2+ ion carriers, potassium shuttlecock mechanism between two insertion materials as "potassium-ion battery" is advantageous for higher-voltage/-power rechargeable batteries.The excellent rate performance is beneficial for the application to hybrid-type capacitor, "potassium-ion capacitor,"as an alternative to lithium-ion capacitors.
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