Dual-site substitution enables high-voltage P2-Na0.67Ni0.33Mn0.67O2 cathodes with superior wide-temperature performance and enhanced ambient stability for sodium-ion batteries

Abstract

P2-type Na_xNi_yMn_zO₂ cathodes, while promising for rapid Na⁺ transport, suffer from irreversible phase transitions, severe high-voltage degradation, and intrinsic hygroscopicity. To address these limitations, a synergistic doping strategy is applied to synthesize the P2-Na_0.61Ca_0.03Ni_0.23Mg_0.10Mn_0.67O₂ cathode with enhanced sodium storage performance and excellent ambient stability. The material achieves a compaction density of 3.16 g cm⁻³ under 260 MPa, rivaling ternary lithium cathodes. Through Ca incorporation in Na layers and Mg substitution in transition metal frameworks, the cathode expands Na⁺ pathways, suppresses P2–O2 phase transition, and promotes highly reversible Ni²⁺/Ni⁴⁺ redox activity. It maintains 82.32% capacity after 200 cycles at 1 C and delivers 64.62 mAh g⁻¹ at 15 C within 2.0–4.3 V, significantly surpassing the 6.58% retention and 27.91 mAh g⁻¹ of the undoped Na_0.67Ni_0.33Mn_0.67O₂. Upon increasing the voltage to 4.5 V, 73.25% capacity retention is maintained, far exceeding that of the Pristine cathode (5.04%). Notably, exceptional wide-temperature performance is achieved, with 90.73% capacity retention after 200 cycles at −15 °C and 65.69% after 150 cycles at 55 °C. Moreover, in full cells the material still sustains 259.32 Wh kg⁻¹ after 150 cycles, underscoring the practical potential for advanced storage applications.