Systematic Exploration of the Benefits of Ni Substitution in Na–Fe–Mn–O Cathodes

Abstract Na‐ion batteries (SIBs) are receiving a great deal of attention as potential sustainable replacements for Li‐ion batteries in electric vehicles and grid storage applications. To date, commercialized SIBs offer inferior energy density with passable extended cycling. By contrast, next‐generation SIBs will likely utilize layered oxide cathodes that offer improved energy density but to date show inferior stability both during cycling and in terms of stability of the cathodes in air during cell assembly. These properties are highly tunable with composition and herein the promising P2 phases are systematically explored in the Na–Fe–Mn–O phase diagram by making 256 different compositions. The optimal material is a P2 material saturated with Ni (a modest 16% of the transition metal layer) and shows a highly competitive energy density of 640 Wh kg −1 while minimizing the amount of sacrificial sodium needed in full cells and also improving the air stability of the material. This study shows the vital role that thorough systematic screening will play in the continued development of these vital materials for sustainable secondary battery production and provides guidance toward sustainable Na‐ion cathodes by minimizing the nickel content required for high performance.