Substitution of magnesium towards stabilizing low-nickel layered oxides for high voltage and cost-effective sodium-ion batteries†

The development and advancement of low-nickel layered oxides for cost-effective sodium-ion batteries are hindered by the lack of comprehensive studies on structural stability and the specific phase transition mechanisms during multiple irreversible phase transitions, especially under high-voltage conditions. Herein Mg substitution for Ni in O3–NaNi_0.25Fe_0.25Mn_0.5O₂ (NNFM) is proposed to mitigate the structural degradation under high voltage and long-term cycling. Through in situ XRD analysis, the complete structural evolution of NNFM and NMNFM under high-voltage conditions was revealed. Most importantly, it is revealed that Mg substitution suppresses the complex phase transitions of low-nickel cathodes under high voltage conditions and mitigates the phenomenon of phase transition hysteresis. NMNFM exhibits a high reversible capacity of 153 mA h g⁻¹ at 0.1C, decent capacity retention after 100 cycles and good rate capability. Last but not least, the fabricated hard carbon//O3-NMNFM full cell delivers an initial discharge capacity of 144 mA h g⁻¹ at 0.1C within a voltage range of 2.0–4.1 V and a capacity retention of 87.8% after 100 cycles.