Optimisation of a P3 phase with superior high voltage reversibility

Activation of oxygen anion redox represents an effective method of increasing the specific capacity as well as raising the operating voltage of layered sodium transition metal oxides. However, these reactions are often accompanied by irreversible structural transformations and detrimental side-reactions between the electrolyte and electrode interface which accelerate degradation, thereby impeding their practical application. To optimise the oxygen anion reversibility for practical use and compare the effects of dopants, we investigated Zn- and Ti-substitution both separately and combined in P3-structure Na_0.7Mn_0.75Ni_0.25O₂, assisted by DFT calculations. The Zn-substituted materials, Na_0.7Mn_0.65Ni_0.25Zn_0.1O₂ and Na_0.7Mn_0.58Ni_0.25Zn_0.07Ti_0.1O₂ present superior cycling stability over the high voltage range 3.8–4.3 V and enhanced rate capability, delivering a reversible capacity of ∼80 mA h g⁻¹ at 500 mA g⁻¹ over the voltage window 2.2–4.3 V compared with 58.6 mA h g⁻¹ for the parent-phase. The improved electrochemical performance of the Zn-substituted materials is attributed to suppression of the P3 to O′3 phase transformation revealed by X-ray diffraction and the lower electronegativity and filled d-band of Zn. The presence of TiO₆ octahedra in the Ti-substituted materials relieves structural distortions/TM ordering, also improving the cycling stability. With Zn/Ti co-substitution these advantages may be combined, as demonstrated by the superior electrochemical performance observed for Na_0.7Mn_0.58Ni_0.25Zn_0.07Ti_0.1O₂.