Study On Sodium Vanadium Phosphate/Carbon Composite Cathode Material

In this paper,Na₃V₂?PO₄?₃?NVP?was used as the basic material,and chemical synthesis method was used to synthesize composite materials through carbon coating,grain nano-crystallization,vanadium iron doping,sodium lithium doping and other modification measures.The electrochemical properties of composite materials were improved,and the synthesis mechanism,composition,structure and electrochemical properties of composite materials were explored through testing and characterization.Na₃V₂?PO₄?₃/carbon composites with Superlattice Quantum dot structure were synthesized by hydrothermal treatment using bamboo cellulose as carbon coating agent.As nucleating agent and hard carbon source,multifunctional bamboo cellulose can form uniformly distributed 5-10 nm Na₃V₂?PO₄?₃ quantum dot grains in hard carbon.Multifunctional bamboo cellulose forms hard carbon with layered structure,which significantly improves the specific surface area and ion transport of the composites.The results of electrochemical analysis show that the specific discharge capacity of the first round is 149 mAh g^-1 at 0.1 C,66.48 mAh g^-1 at 20 C and 37.6 mAh g^-1 after 3000cycles.Quantum dots improve the composition and structure of materials,improve the electrochemical properties.Na₃V₂?PO₄?₃/Carbon composites were prepared by sol-gel method using gluconic acid delta internal fat as carbon source and nucleating agent.The results show that the petal-like hard carbon structure is formed in the composite by the synergistic action of gluconic acid-delta-lactone and sodium vanadium phosphate.The 5-20 nm grains are uniformly dispersed in the hard carbon structure,which significantly improves the ratio performance and sodium ion diffusivity of the composite.The specific discharge capacity of the first round is 106 mAh g-1 at 0.5 C,72.4 mAh g^-1 at 50 C and 33.4mAh g^-1 at 10 000 cycles,with a capacity retention rate of 46.3%.The petal-like hard carbon structure controls the size of crystal particles and significantly improves the cyclic stability of materials.Na₃V_2-xFe_x?PO₄?₃/HCS composites were synthesized by sol-gel method.The results show that with the increase of iron content,the peak value of sodium ferric phosphate in the lattice becomes more and more obvious;under the influence of iron element,flaky carbon layer structure is formed,and the grains grow in the flaky carbon layer structure.The composite has a large specific surface area,which is conducive to the sodium ion intercalation and removal during charging and discharging process,so that the composite has excellent electrochemical properties.When the iron content is10%,the first discharge capacity reaches 105.6 mAh g-1 at 0.5 C,79.17 mAh g^-1 at 50C,77.7 mAh g^-1 at 100 cycles,and 98.14%capacity retention.The doping of iron leads to the formation of lamellar carbon layer structure,and the specific capacity of the material is obviously increased.Na_3-xLi_xV₂?PO₄?₃/C composite cathode materials were prepared by sol-gel method.The results show that when a small amount of lithium is added,the rhombic lithium vanadium phosphate?LVP?phase and rhombic sodium vanadium phosphate?NVP?phase are formed;with the increase of lithium content,the rhombic NVP phase decreases while the rhombic LVP phase increases gradually.Finally,the composite contains three crystalline phases:monoclinic LVP,rhombic LVP and rhombic NVP,whose grains are wrapped in flaky carbon.When the molar ratio of lithium to sodium is 9:1,the composite cathode material has the best electrochemical performance.The first discharge capacity of the battery is 67.02 mAh g^-1 at 0.5 C,50.45 mAh g^-1 at 50 C,46.9 mAh g^-1 at 100 cycles,and the coulomb efficiency is close to 100%.The doping of lithium element results in the formation of sheet carbon layer structure,which improves the electrochemical performance of the whole battery significantly.