Novel high-voltage cathode for aqueous zinc ion batteries: Porous K0.5VOPO4·1.5H2O with reversible solid-solution intercalation and conversion storage mechanism

Cathode materials that possess high output voltage, as well as those that can be mass-produced using facile techniques, are crucial for the advancement of aqueous zinc-ion battery (ZIBs) applications. Herein, we present for the first time a new porous K0.5VOPO4·1.5H2O polyanionic cathode (P-KVP) with high output voltage (above 1.2 V) that can be manufactured at room temperature using straightforward coprecipitation and etching techniques. The P-KVP cathode experiences anisotropic crystal plane expansion via a sequential solid-solution intercalation and phase conversion pathway throughout the Zn2+ storage process, as confirmed by in-situ synchrotron X-ray diffraction and ex-situ X-ray photoelectron spectroscopy. Similar to other layered vanadium-based polyanionic materials, the P-KVP cathode experiences a progressive decline in voltage during the cycle, which is demonstrated to be caused by the irreversible conversion into amorphous VOx. By introducing a new electrolyte containing Zn(OTF)2 to a mixed triethyl phosphate and water solution, it is possible to impede this irreversible conversion and obtain a high output voltage and longer cycle life by forming a P-rich cathode electrolyte interface layer. As a proof-of-concept, the flexible fiber-shaped ZIBs based on modified electrolyte woven into a fabric watch band can power an electronic watch, highlighting the application potential of P-KVP cathode.