Layered CrO2·nH2O as a cathode material for aqueous zinc-ion batteries: ab initio study†

Aqueous zinc-ion batteries are considered potential large-scale energy storage systems due to their low cost, environmentally friendly nature, and high safety. However, the development of high energy density cathode materials and uncertain reaction mechanisms remains a major challenge. In this work, the reaction mechanism, discharge voltage and diffusion properties of layered CrO₂ as a cathode material for aqueous zinc-ion batteries were studied using first-principles calculations, and the effect of pre-intercalated structural water on the electrochemical performance of CrO₂ electrodes is also discussed. The results show that CrO₂ exhibits high average discharge voltages (2.65 V for H insertion (pH = 7) and 1.97 V for Zn insertion) and medium theoretical capacities (319 mA h g⁻¹ (H and Zn)). The H intercalation voltage strongly depends on the pH value of the electrolyte. The H/Zn co-insertion mechanism occurs at low hydrogen concentrations (c(H) ≤ 0.125), where the initial insertion of H reduces the total amount of subsequent Zn insertion. For the substrate containing structured water (CrO₂·nH₂O, n ≥ 0.5), the average voltage of Zn insertion is significantly increased, while the average voltage of H slightly decreases. In addition, the pre-intercalated water strategy significantly improved the diffusion properties of H and Zn. This study shows that layered CrO₂·nH₂O is a promising cathode material for aqueous zinc-ion batteries, and also provides theoretical guidance for the development of high-performance cathode materials for aqueous zinc-ion batteries.