Novel Ti(iii)/Ti(iv) mixed-valence doping for enhancing the structural stability and specific capacity of a P2-layered Na0.67Ni0.2Mn0.8O2 cathode for sodium-ion batteries

Abstract

Layered manganese-based oxide (LMBO) is a promising cathode material for sodium-ion batteries. However, the structural instability caused by the Jahn–Teller effect during cycling remains a challenge. Although Ti-ion doping has been employed to stabilise the structure, the introduction of electrochemically inactive Ti ions leads to a reduction in capacity. To address this issue, Ti(III)/Ti(IV)-doped Na_0.67Ni_0.2Ti_0.125Mn_0.675O₂ materials were synthesised via a co-precipitation method with the addition of oxalic acid. Ti(III)/Ti(IV)-doped samples exhibited larger crystallographic parameters compared with those doped solely with Ti(IV) ions, which could be attributed to the larger ionic radius of Ti(III). Furthermore, an additional redox peak at ∼1.8 V was observed in the dQ/dV curves of the cathodes with Ti(III), suggesting contributions to additional capacity. Ex-situ XPS revealed that, while multiple ionic species participated in the additional redox peak, the Ti(III)/Ti(IV) couple had a critical role in this redox activity. Na_0.67Ni_0.2Ti_0.125Mn_0.675O₂ demonstrated an optimal balance between enhanced capacity attributed to the presence of Ti(III) and the structural stability provided by Ti(IV). With significantly enhanced cycling stability, this material delivered a high initial capacity of 154 mAh g⁻¹ in a half-cell and 98 mAh g⁻¹ in a full cell with a hard carbon anode at 0.2C. After 50 cycles, the capacity retention was 82% and 79%, respectively. This study presents an effective strategy for designing excellent cathode materials for sodium-ion batteries.