Mechanistic Insight into the Electrochemical Performance of Zn/VO2 Batteries with an Aqueous ZnSO4 Electrolyte

Abstract Rechargeable aqueous zinc‐ion batteries (ZIBs) are promising for cheap stationary energy storage. Challenges for Zn‐ion insertion hosts are the large structural changes of the host structure upon Zn‐ion insertion and the divalent Zn‐ion transport, challenging cycle life and power density respectively. Here a new mechanism is demonstrated for the VO 2 cathode toward proton insertion accompanied by Zn‐ion storage through the reversible deposition of Zn 4 (OH) 6 SO 4 ·5H 2 O on the cathode surface, supported by operando X‐ray diffraction, X‐ray photoelectron spectroscopy, neutron activation analysis, and density functional theory simulations. This leads to an initial discharge capacity of 272 mAh g −1 at a current density of 3.0 A g −1 , of which 75.5% is maintained after 945 cycles. It is proposed that the competition between proton and Zn‐ion insertion in the VO 2 host is determined by the solvation energy of the salt anion and proton insertion energetics, where proton insertion has the advantage of minimal structural distortion leading to a long cycle life. As the discharge kinetics are capacitive for the first half of the process and diffusion limited for the second half, the VO 2 cathode takes the middle road possessing both fast kinetics and high practical capacities revealing a reaction mechanism that provides new perspective for the development of aqueous ZIBs.