Preparation Of Modified Sodium Vanadium Phosphate And Its Application In Sodium Ion Batteries

Sodium vanadium phosphate（Na₃V₂（PO₄）₃,NVP）as a polyanionic compound possesses the unique 3D NASICON framework to facilitate fast Na⁺migration,rendering it as a potential cathode with great performances.However,the inferior intrinsic conductive of NVP impedes its commercial application.Therefore,we chose two doping methods to modify NVP separately.One is doping with W⁴⁺of larger radius,and the other is doping NVP with high-entropy components（five equimolar amounts of elements）.On the one hand,ion doping can expand the lattice spacing and facilitate the diffusion of Na⁺in the crystal structure.On the other hand,doping can stabilize the crystal structure and alleviate the lattice distortion caused by Na⁺insertion/extraction.（1）The W-doped NVP was fabricated successfully by utilizing WS₂ as a tungsten source with a large ionic radius of W⁴⁺.The doping of W⁴⁺can expand the lattice spacing and increase the ionic conductivity of NVP,so that the Na⁺can migrate rapidly in the crystal and thus optimize the Na⁺diffusion performance.The results show that the W-doped NVP has excellent charge storage capacity,cycling stability and wide temperature tolerance.The initial specific capacity at 1 C was close to the theoretical value,with test data reaching 116.0 m A h g^-1;the material still maintained 94.3%specific capacity after 18000 cycles in the ultra-long cyclic test at 50 C;and it operated stably at a current density of 1 C for a longer period of time at-20℃ and 45℃.（2）Five high-entropy elements（Fe,Al,Cr,Co,and Nb）were used to prepare the high-entropy doping NVP.The high-entropy element replaces the V element in the material as a multi-component transition metal,thus allowing the material to adapt to the capacity drop caused by rapid changes in current and successfully increase the specific capacity of the battery.The high entropy-doped NVP has a larger cell volume,which confers stronger Na⁺diffusion properties and structural stability.The results demonstrate that NVP-HE0.1@C as a cathode material significantly improves the electrochemical performance of sodium ion batteries,including optimal capacity(107.5m A h g^-1 initial specific capacity at 1 C)as well as outstanding cyclic stability（97.4%and 87.5%capacity retention after 500 and 8000 cycles at 1 C and 50 C,respectively）.In addition,the diffusion rate of Na⁺was greatly enhanced during the cycling process.