Investigation of O3-type Na0.9Ni0.45MnxTi0.55-xO2 (0 ≤ x ≤ 0.55) as positive electrode materials for sodium-ion batteries

- The Na0.9Ni0.45MnxTi0.55-xO2 (0 ≤ x ≤ 0.55) series was synthesized as phase pure with the O3 structure.
- The structure, electrochemistry and Na+ diffusion in Na0.9Ni0.45MnxTi0.55-xO2 (0 ≤ x ≤ 0.55) were investigated.
- Ti substitution in Na0.9Ni0.45MnxTi0.55-xO2 suppresses phase transitions during electrochemical cycling.
- Ti substitution in Na0.9Ni0.45MnxTi0.55-xO2 results in improved cycling performance, but also reduces capacity.

The Na0.9Ni0.45MnxTi0.55-xO2 (0 ≤ x ≤ 0.55) series of layered oxides was synthesized using a solid-state method and the materials in this series were studied as positive electrode materials in sodium cells. All compositions resided in a phase pure O3 solid solution. Compared to the Ti-free composition, Na0.9Ni0.45Mn0.4Ti0.15O2 has improved capacity retention without sacrificing reversible capacity and average voltage. Na0.9Ni0.45Mn0.4Ti0.15O2 delivers ∼197 mAh/g of reversible capacity in a voltage range of 1.5-4.5 V with an average voltage of ∼3.27 V. Energy density available based on metallic sodium is estimated to be approximately 643 Wh/kg or 2805 Wh/L. Increasing the Ti content beyond this composition results in increased average voltage, increased polarization and a reduction in cycling performance.