Integrated polyanion-layered oxide cathodes enabling 100 000 cycle life for sodium-ion batteries

Abstract

The practical application of Na₃V₂(PO₄)₃, a polyanionic cathode for sodium-ion batteries, is constrained by its poor electronic conductivity, limited specific capacity, and slow kinetics. In this study, an integrated polyanion-layered oxide cathode embedded within a porous carbon framework is designed. This cathode features an intergrown biphasic heterostructure, consisting of a Na-rich polyanionic compound, Na_3.5V_1.5Fe_0.5(PO₄)₃ (NVFP), and a layered oxide, V₂O₃ (NVFP–VO), which is optimized to enhance Na-ion storage performance. Fe doping reduces the bandgap of Na₃V₂(PO₄)₃ and activates its V⁴⁺/V⁵⁺ redox couple, enhancing both electronic conductivity and specific capacity. The porous carbon framework further improves the electronic conductivity of the integrated cathode and accommodates volume fluctuations during cycling. The heterostructure lowers ion transport barriers and accelerates reaction kinetics. Additionally, the low-strain V₂O₃ phase functions as a stabilizer, effectively buffering volume fluctuations and stress gradients in NVFP. The spontaneous activation of V₂O₃ further increases the capacity of the integrated cathode. Consequently, the cathode achieves a high reversible capacity of over 130 mA h g⁻¹ at 0.1C and exhibits unprecedented cyclability, maintaining over 100 000 cycles with 72.6% capacity retention at 100C in half-cells. This represents the longest cycle life reported among polyanion-based cathodes. In addition, our prepared Ah-level pouch cells exhibit a high energy density of 153.4 W h kg⁻¹ and a long cycle life exceeding 500 cycles. This study demonstrates that synergistic effects in multiphase integrated cathodes promote the development of advanced cathode materials for high-energy-density, fast-charging, and long-life sodium-ion batteries.