Electrochemical Exchange Reaction Mechanism and the Role of Additive Water to Stabilize the Structure of VOPO4 ⋅2 H2 O as a Cathode Material for Potassium-Ion Batteries.

VOPO4 ⋅2 H2 O is demonstrated as a cathode material for potassium-ion batteries in 0.6 m KPF6 in ethylene carbonate/diethyl carbonate, and its distinct exchange reaction mechanism between potassium and crystal water is reported. In an anhydrous electrolyte, the cathode shows an initial capacity of approximately 90 mAh g-1 , with poor capacity retention (32 % after 50 cycles). In contrast, the capacity retention dramatically improved (86 % after 100 cycles) in a wet electrolyte containing 0.1 m of additive water. VOPO4 ⋅2 H2 O contains two types of water (structural and crystal). Upon discharge, potassium ions are intercalated whereas the crystal water is simultaneously de-intercalated from the structure. Upon charging, a completely reverse reaction takes place in the wet electrolyte, resulting in high stability of the host structure and excellent cyclability. However, in the anhydrous electrolyte, some portion of the extracted crystal water molecules cannot be reinserted into the host structure because they are distributed over the anhydrous electrolyte. Keeping some concentration of water in the electrolyte turns out to be was the key to achieving such high reversibility. The potassium ions (90 %) and proton or hydronium ions (10 %) seem to be co-intercalated in the wet electrolyte. This work provides a general insight into the intercalation mechanism of crystal-water-containing host materials.