Understanding the electrochemical properties and phase transformations of layered VOPO4⋅H2O as a potassium-ion battery cathode

Layered oxovanadium phosphate hydrate (VOPO·xHO) is considered to be a promising cathode material for potassium-ion batteries. However, its correlation between electrochemical properties and structural changes during electrochemical processes still need further exploration. Herein, monohydrate VOPO·HO is used as an alkali ion (Li, Na, K) host material that gives superior electrochemical performances in potassium-ion batteries. It has a high discharge capacity of 90 mA h g with a high average potential of 3.5 V and a lifespan of over 650 cycles with 61% retention at 1C rate. Three phase transitions during the potassiation/depotassiation process are observed: KVOPO (0 < x < 0.12, d = 7.06 Å) → α-KVOPO (0.12 < x < 0.48, d = 6.73 Å) → α-KVOPO (0.30 < x < 0.55, d = 6.55 Å). The small structural change (≤6% on the c-axis) and the full reversibility of the structural transformation ensure the stable cycling; the large interlayer distance allows a fast K-ion diffusion (10–10 cm s). This study provides a comprehensive understanding on electrochemical performances and structural evolutions of VOPO·HO. Meanwhile, the small volume variation, high operating potential, and excellent cycling stability of VOPO·HO can attract more attention to develop other layered materials beyond transition metal oxides.