Low-valence Sn²⁺-doped Na₃V₂(PO₄)₃ cathodes: A highly stable cathode for long-cycle-life sodium-ion batteries

Owing to its high operating voltage and stable three-dimensional Na⁺ diffusion channels, Na₃V₂(PO₄)₃, a typical NASICON-type materials, has attracted a surge of interest for successfully configuring long-term ultra-stable sodium-ion batteries. However, the practical implementation of Na₃V₂(PO₄)₃ is severely impeded by its intrinsically low electronic conductivity and severe structural degradation, leading to inferior rate and cycle performances. Therefore, here a sol-gel-derived Sn²⁺-doping strategy is employed to resolve this dual kinetic-thermodynamic bottleneck of Na₃V₂(PO₄)₃ cathodes for sodium-ion batteries. Based on different characterizations, for example X-ray photoelectron spectrometer (XPS), High-Resolution Transmission Electron Microscopy (HR-TEM), X-ray Absorption Fine Structure (XAFS) and so on, it is evident that Sn²⁺-doping not only accelerates the Na2 occupation, but also largely mitigates the substrate lattice strain, thereby granting the Na₃V₂(PO₄)₃ cathodes excellent cycling structural stability. Specifically, the Sn²⁺-doped Na₃V₂(PO₄)₃ cathodes delivers an ultra-high initial discharging capacity of 102.73 mAh·g^-1 at 20C and maintains ∼81% capacity retention after 5000 cycles. After 100 cycles at 1C, the initial and retained discharge specific capacities of the full sodium-ion batteries with optimized Sn²⁺-doped Na₃V₂(PO₄)₃ and coal-based hard carbon as cathodes and anodes, are 99.53 and 93.46 mAh·g^-1, corresponding to a capacity retention of 93.9% and high coulombic efficiency of approximately 100%. Based on systematical theoretical calculations, experimental and characterization results, the working mechanism of Sn²⁺-doped Na₃V₂(PO₄)₃ cathodes is initially concluded, which is significant and important for promoting its practical application valid.