De/protonation associated sustainable conversion reaction applicable to high-capacity zinc storage in mildly acidic aqueous system

•K2V3O8 undergoes conversion reaction between amorphous V5+2O5 and V3+OOH upon cycling.•The high cutoff voltage up to 1.9V effectively facilitates the protonation, accompanied by V5+/3+ redox.•The conversion reaction results in a large capacity of 362 mAh g−1, ∼97 % theoretical capacity.•The reversible conversion reaction contributes to outstanding long-term cycling stability.