Effect of sodium addition on lattice structure and tuning performance in sodium rich NaxTm2-xO2 type cathode materials (Tm=Mn and Cr; X=1.05–1.3) - a study

•Exploring gray area between Na deficient and Na-rich cathode materials.•Effect of sodium addition on lattice structure and oxygen loss is studied.•Rietveld and electron density mapping were used as a tool for study.•A commonly used Mn and Cr layered structures are systematically studied. Na-rich transition metal oxides are promising cathode materials owing to their high theoretical capacity and ability to be used in a solid-state sodium battery. However, one of their drawbacks is that they are prone to capacity loss. The reduction in capacity during cycling is attributed to oxygen loss, resulting in rapid capacity fading. To address this issue, a NaxTm2-xO2 type cathode material (Tm = Mn and Cr; X = 1.05–1.3) was synthesized successfully and analyzed for desired cathode attributes. The material was synthesized using the solid state method and quenching to achieve a crystalline cathode structure. Different sodium stoichiometries were used and electrochemically tested. The resultant was a Na1.1Cr0.9O2 electrode that achieved a high capacity of 110 mAh/g at 1000 mA/g compared with other compositions. The superior performance of Na1.1Cr0.9O2 over other Na-rich stoichiometries (1.05–1.3) with Mn as the transition metal was analyzed using X-ray characterization techniques and other electrochemical techniques. This high-capacity cathode was used to construct a full cell battery by coupling with a hard-carbon anode to demonstrate its applicability in the real world. The constructed battery with a Na1.1Cr0.9O2 cathode exhibited a high capacity with a high energy density of 223 Wh/kg and power density of 1100 kW/kg.