Impact of Potassium in Layered Cobalt Oxide Cathodes on Electrochemical Performance in Sodium‐Ion Batteries

Abstract

Rechargeable sodium‐ion batteries are promising sustainable energy storage systems, owing to their low cost and high energy density. However, the existing cathode chemistries, particularly layered sodium transition metal oxides, exhibit restricted electrochemical performance, which hinders their extensive applications. To overcome challenges, originating from kinetically limited structural transformations, various cation substitutions are reported in the literature. Especially the partial Na–K replacement is discussed recently, which can increase bulk Na‐diffusivity and suppress undesirable phase transitions. Here, a critical study of potassium doping in different layered materials P3‐K0.5Co0.33Mn0.67O2 and P3‐K0.5Co0.33Mn0.67O2*0.5H2O with redox‐active Co (and Mn), and P2‐Na0.7Co0.8Ti0.2O2 and P2‐K0.1Na0.7Co0.8Ti0.2O2 with Co activity is presented. It is found that the presence of K significantly enhances the moisture sensitivity: P3‐K0.5Co0.33Mn0.67O2*0.5H2O is quickly formed in the air from K0.5Co0.33Mn0.67O2, while Na0.7Co0.8Ti0.2O2 remains stable under the same conditions. In electrochemical Na‐cells, a partial K–Na cation exchange in P3‐K0.5Co0.33Mn0.67O2 and P3‐K0.5Co0.33Mn0.67O2*0.5H2O occurs without applying current. The electrochemical performance of P3‐K0.5Co0.33Mn0.67O2 is slightly increased in comparison to K‐free P2‐Na0.67Co0.33MnO.67O2 from the literature, staying, however, behind the performance of Na0.7Co0.8Ti0.2O2. K0.1Na0.7Co0.8Ti0.2O2 shows weaker rate capability than isostructural Na0.7Co0.8Ti0.2O2, reflecting a negative impact of K on Na‐diffusivity. Therefore, effect of K‐incorporation in layered structures on the battery performance strongly depends on the cathode composition.