Research Progresses of Sodium Cobalt Oxide as Cathode in Sodium Ion Batteries

Abstract

Sodium ion batteries have regained widespread attention in the field of large-scale electrochemical energy storage by virtue of their widely distributed and low-cost sodium resources. Among many of the cathode materials (layered, tunnel- like, polyanionic type and Prussian-blue type, etc.), layered transition metal oxide materials have received extensive research attention due to the features of high specific capacity, relatively good electrochemical reversibility, and rich and adjustable chemical composition. Sodium cobalt oxide is one of the most typical layered transition metal oxides. A lot of research has been done about sodium cobalt oxide since the 1980s. Although compared with other energy storage systems (such as lithium cobalt oxide materials which has the similar composition), sodium cobalt oxide does not take more advantage in electrochemical performance (like rate performance, cycle performance, etc.), but it can be observed from the charge and discharge curve that sodium cobalt oxide has undergone complex electrochemical processes, which means that it has a bunch of information on the degradation mechanisms during the charge and discharge processes. The correlation studies of the failure mechanism during the charging and discharging processes of sodium cobalt oxide (including the structure changes and the charge compensation mechanisms) are of great significance for the deep understanding of the layered oxide systems in sodium ion batteries. Therefore, on the basis of introducing the common crystal structure types and the synthesis phase diagram of sodium cobalt oxides, this article focuses on reviewing the structure changes (including phase transition and Na+/ vacancy ordering) and charge compensation mechanisms of sodium cobalt oxides with different crystal structures during the charging and discharging. At the same time, the correlation between the mechanisms above and electrochemical performance is discussed. This review aims to provide support for the in-depth study and establishment of the structure-activity relationship in the electrochemical processes of layered transition metal oxide cathode materials.