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

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‐K 0.5 Co 0.33 Mn 0.67 O 2 and P3‐K 0.5 Co 0.33 Mn 0.67 O 2 *0.5H 2 O with redox‐active Co (and Mn), and P2‐Na 0.7 Co 0.8 Ti 0.2 O 2 and P2‐K 0.1 Na 0.7 Co 0.8 Ti 0.2 O 2 with Co activity is presented. It is found that the presence of K significantly enhances the moisture sensitivity: P3‐K 0.5 Co 0.33 Mn 0.67 O 2 *0.5H 2 O is quickly formed in the air from K 0.5 Co 0.33 Mn 0.67 O 2 , while Na 0.7 Co 0.8 Ti 0.2 O 2 remains stable under the same conditions. In electrochemical Na‐cells, a partial K–Na cation exchange in P3‐K 0.5 Co 0.33 Mn 0.67 O 2 and P3‐K 0.5 Co 0.33 Mn 0.67 O 2 *0.5H 2 O occurs without applying current. The electrochemical performance of P3‐K 0.5 Co 0.33 Mn 0.67 O 2 is slightly increased in comparison to K‐free P2‐Na 0.67 Co 0.33 Mn O.67 O 2 from the literature, staying, however, behind the performance of Na 0.7 Co 0.8 Ti 0.2 O 2. K 0.1 Na 0.7 Co 0.8 Ti 0.2 O 2 shows weaker rate capability than isostructural Na 0.7 Co 0.8 Ti 0.2 O 2 , 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.