Simultaneous Modification of Al³⁺/F^- Cosubstitution to Construct a Solid Framework for Na₃V₂(PO₄)₃ with High Thermal...

Challenges related to poor electronic conductivity and cycling stability have impeded the development and utilization of Na₃V₂(PO₄)₃ (NVP). Therefore, this study focuses on enhancing the performance of NVP by employing a sol-gel method to design various gradients of F/Al-doped and carbon nanotube (CNT)-enwrapped NVP materials. The introduction of F doping replacing PO₄ tetrahedra reduces the occupied space, while F monomers can establish stronger bonds with VO₆ octahedral pillars closer to O atoms. Additionally, Al doping introduces a new AlO₆ octahedral structure at the V site, strengthening the 3D framework. The synergistic substitution of F and Al contributes to improving the stability of the framework, which enhances the Na⁺ migration channels and overall electrochemical performance. Furthermore, the coating of CNTs plays a crucial role in creating a favorable interface transition layer that facilitates efficient electron transport and enhances electronic conductivity. Comprehensively, the modified FAl-2 exhibits a high capacity of 115.8 mA h g^-1 at 0.1C. It reveals 89.3 mA h g^-1 at 60C and maintains 83.8 mA h g^-1 after 2000 cycles, indicating a capacity retention rate of 93.84%. Electrochemical ex situ X-ray diffraction (XRD) demonstrates that FAl-2 behaves at relatively low values (0.328%-1.075%) of volume shrinkage during the whole charge/discharge process, indicating its near-zero strain property. The postcycled XRD and X-ray photoelectron spectroscopy further verify the significantly enhanced crystal structural stability of FAl-2. Moreover, FAl-2 possesses a higher thermal runaway temperature, indicating a superior thermal stability. The self-releasing heat trend observed in FAl-2 can offer valuable insights into the design of battery management systems.