Enhancing the cycling stability of layered cathodes for sodium-ion batteries via phase transition regulation

Abstract

Sodium-ion batteries have been deemed as a sustainable alternative to lithium-ion systems due to the abundance and affordability of sodium sources. Nevertheless, developing high-energy–density P2-type layered oxide cathodes with long-term cycling stability poses challenges, stemming from irreversible phase transitions, structural degradation, and lattice oxygen instability during electrochemical cycling. Here, we propose a one-step NbB₂ modification strategy that enhances both bulk and surface properties of Na_0.8Li_0.12Ni_0.22Mn_0.66O₂ cathodes. By exploiting different techniques, we disclose that bulk Nb and B doping combined with a Nb-Transition Metal-BO₃ surface layer reconstruction enable a reversible P2-OP4 phase transition and, meanwhile, improve anionic redox reversibility. In addition, Li⁺ migrates into alkali-metal layers and underpins the layered structure through the “pillar effect”, thereby facilitating the Na⁺ diffusion in Na_0.8Li_0.12Ni_0.22Mn_0.66O₂ cathodes and retaining their structural integrity at high voltage. As a result, the modified cathodes achieve 93.6% capacity retention after 500 cycles at 1C and deliver specific capacities above 114 mA h g⁻¹ at 10C within 2.0–4.3 V. Contrary to the previous studies reporting that OP4 phase are detrimental to the structural stability of layered cathodes, we experimentally validate that a well-regulated P2-OP4 phase transition is beneficial for structural and electrochemical stabilities.