Enhanced structural stability of P2-Na0.75Ni0.25Mn0.75-xTixO2 cathodes with high-voltage non-phase transitions property by Mn-sites manipulation strategy

Ni/Mn-based cathode is regarded as a competitive candidate for sodium-ion batteries (SIBs) owing to its high theoretical capacity and low cost. Nevertheless, the issue of its poor cycling stability continues to be a significant obstacle. In this work, P2-Na _0.75Ni _0.25Mn _0.75-xTi _xO ₂ (NaNMT-x) cathode materials with high-voltage non-phase transitions property were successfully synthesized. Interestingly, Ti-doping strategy could induce a beneficial reinforcement in the occupancy of stabler Na _e sites. Especially, the Na _e/Na _f ratio of NaNMT-0.15 reached the maximum, which was beneficial for stability. Besides, NaNMT-0.15 with the lowest thermodynamic formation energy and narrower band gap could clearly elaborate excellent structural stability and conductivity via the calculation. The results demonstrated the underlying mechanisms. From the capacity-diffusing factor numerical model and GITT, NaNMT-0.15 have best Na ⁺ diffusion ability. Moreover, ex-situ XRD further demonstrated that NaNMT-0.15 cathode did not undergo obvious P2-O2 phase transition phenomenon in high voltage section. The “no phase transitions” effect enabled NaNMT-0.15 cathode to present distinguished initial capacity (126.0 mAh g ⁻¹ at 0.1 C) and stability (85.0 % over 100 cycles). These findings might offer a fresh perspective for designing novel Ni/Mn-based cathode materials with no P2-O2 phase transitions property.