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

Abstract

Layered manganese-based oxide (LMBO) is promising cathode materials for sodium-ion batteries. However, their structural instability caused by the Jahn-Teller effect during cycling still remains 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 was synthesised via a co-precipitation method with the addition of oxalic acid. The Ti(III)/Ti(IV) doped samples exhibited larger crystallographic parameters compared to those doped solely with Ti(IV), which can be attributed to the larger ionic radius of Ti(III). Furthermore, an additional redox peak at approximately 1.8 V was observed in the dQ/dV curves of the cathodes with Ti(III), suggesting contributions to additional capacity. Ex-situ XPS analysis revealed that while multiple ionic species participated in the additional redox peak, the Ti(III)/Ti(IV) couple played 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 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 hard carbon anode at 0.2 C. 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.