p-n Heterojunction engineering in Na₃V₂(PO₄)₂F₃-based cathode: space-charge boosted carrier kinetics for high-rate and wide-temperature sodium-ion batteries

Sodium-ion batteries (SIBs) stand as promising candidates for grid-scale energy storage, with Na₃V₂(PO₄)₂F₃ (NVPF) emerging as a competitive cathode material owing to its robust Na superionic conductor (NASICON)-type structure and high operating voltage. However, its practical application is severely hindered by intrinsically low electronic conductivity and sluggish Na⁺ diffusion kinetics. Herein, we report an in-situ phase transition strategy to construct a p-n heterostructure within NVPF-based cathodes, integrating n-type NVPF and p-type Na₃V₂(PO₄)₃ (NVP) via thermally induced fluorine loss. This unique heterostructure harnesses the atomic-scale interfacial space-charge effect to establish a built-in electric field, which serves as a core driving force for boosted carrier kinetics. The inherent Fermi-level disparity between NVPF and NVP triggers directional carrier diffusion, forming a space-charge region that synergistically enhances bulk electronic conductivity and lowers the Na⁺ migration barriers. The resulting pn-NVPF-based cathode delivers exceptional electrochemical performance with a reversible capacity of 61.4 mAh g^-1 at an ultrahigh rate of 50 C (1 C = 128 mA g^-1), 80% state of charge achieved in merely 3.5 min at 10 C, and 83.3% capacity retention after 10,000 cycles at 10 C. Notably, it maintains stable operation over an extreme temperature range of - 40 to 60 °C. When assembled into a hard carbon//pn-NVPF full cell, it retains 94.2% capacity after 100 cycles under fast-charge (3 C)/slow-discharge (0.2 C) conditions and delivers a high energy density of 397.6 Wh kg^-1 at 0.2 C. This study establishes the space-charge effect as a versatile design principle for overcoming the dual kinetic bottlenecks in electrode materials, offering new insights into the development of high-performance SIBs.