The Development of Vanadyl Phosphate Cathode Materials for Energy Storage Systems: A Review.

Various cathode materials have been proposed for high-performance rechargeable batteries. Vanadyl phosphate is an important member of the polyanion cathode family. VOPO4 has seven known crystal polymorphs with tunneled or layered frameworks, which allow facile cation (de)intercalations. Two-electron transfer per formula unit can be realized by using VV /VIV and VIV /VIII redox couples. The electrochemical performance is closely related to the structures of VOPO4 and the types of inserted cations. This Review outlines the crystal structures of VOPO4 polymorphs and their lithiated phases. The research progress of vanadyl phosphate cathode materials for different energy storage systems, including lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, multivalent batteries, and supercapacitors, as well as the related mechanism investigations are summarized. It is hoped that this Review will help with future directions of using vanadyl phosphate materials for energy storage.

Stabilization of VOPO4·2H2O voltage and capacity retention in aqueous zinc batteries with a hydrogen bond regulator.

The transformation of polyanion cathodes into oxides in aqueous Zn batteries results in voltage decay. Herein, we uncover a polyanion dissolution and oxide re-electrodeposition process for this transformation. Accordingly, the dissolution is inhibited by reducing the water activity in the electrolyte with the hydrogen bond regulator of glucose (Glu). In the 4 m Zn(OTf)2/5.5 m Glu electrolyte, the VOPO4·2H2O cathode maintains the redox reactions at a high voltage of 1.6 V/1.5 V with stable capacity retention during cycling at different rates. It also shows promising electrochemical activity and stability at temperatures down to -20 °C.

An amphoteric betaine electrolyte additive enabling a stable Zn metal anode for aqueous batteries.

The corrosion and dendritic growth of the Zn anode limit its electrochemical performance in aqueous Zn batteries. Here, we present an amphoteric betaine additive for 5 m ZnCl2 aqueous electrolyte. The carboxyl group on betaine forms hydrogen bonds with water and reduces the water activity. The molecule also experiences preferential adsorption on the Zn surface and separates the interactions between Zn and water. Side reactions at the Zn electrode are thus inhibited. The regulated interface also ensures uniform Zn deposition. As a result, the electrolyte with betaine additive allows reversible Zn plating/stripping for over 1400 h at 0.5 mA cm-2. A capacity retention of 94% is obtained after 3000 cycles for a VO2 cathode.

The back-deposition of dissolved Mn2+ to MnO2 cathodes for stable cycling in aqueous zinc batteries.

The Mn2+ dissolution of MnO2 cathode materials causes rapid capacity decay in aqueous zinc batteries. We herein show that the dissolved Mn2+ can be deposited back to the cathode with the aid of a suitable conductive agent. The active material is thus retained for energy storage, and this MnO2/Mn2+ redox process also provides capacity. In the Mn2+ free ZnSO4 electrolyte, MnO2 delivers 325 mA h g-1 capacity at 0.1 A g-1, and 90.4% capacity retention is achieved after 3000 cycles at 5 A g-1. Our work demonstrates an effective strategy to realize stable cycling of MnO2 cathodes in aqueous zinc batteries without Mn2+ additives.