Mechanistic Insight into the Electrochemical Performance of Zn/VO ₂ Batteries with an Aqueous ZnSO ₄ Electrolyte

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.