Insights into the oxygen vacancies in transition metal oxides for aqueous Zinc-Ion batteries

Transition metal oxides are considered promising cathode materials for aqueous zinc-ion batteries (AZIBs) due to the advantages of high theoretical capacity, abundant reserves, low cost and rich valence states. However, the low conductivity, easy degradation of the structure and slow reaction kinetics of transition metal oxides hindered their further applications in AZIBs. The introduction of oxygen vacancies in transition metal oxides is a very effective strategy to regulate the electronic structure of the materials, which can facilitate ion/electron transfer to achieve high-performance AZIBs. In this review, firstly, a brief outline of various energy storage mechanisms for AZIBs is introduced. The research progress of oxygen vacancies in different transition metal oxides is summarized afterward, focusing on manganese- and vanadium-based oxides. In particular, the approaches to introducing oxygen vacancies to various types of oxides are exhibited, and their functional mechanisms are discussed. The AZIB performance would be improved by oxygen vacancies mainly through 1) enhancing electronic conductivity; 2) optimizing ion transport channels; 3) lowering ion intercalation barriers; 4) increasing active sites. After that, some commonly used characterization methods of oxygen vacancies are also presented. Summarized discussions are also provided.