Synthesis And Electrochemical Performances Of Polyanionic Compounds For Na-ion Batteries Cathodes

Polyanion compounds possess many advantages as Na-ion batteries cathodes.For example,various crystal structures with open channels for Na-ions are available,and the polyanion compounds are believed to have high thermal stabilities due to strong covalent bonding of oxygen atom in the polyanion polyhedral.Furthermore,the reaction potentials of the compounds in the process of charging and discharging are influenced by the environment around the polyanion themselves,so we can adjust the structure of polyanion to change the reaction routes and improve the electrochemical performance.Na3V2?PO4?3 is a promising cathode material for Na-ion batteries due to the unique NASICON framework and excellent ionic conduction.On the other hand,the conductivity of Na₃V₂?PO₄?₃ is poor due to the presence of PO₄^3-,therefore many works were done to improve the electrochemical performance of materials through coating amorphous carbon layer on Na₃V₂?PO₄?₃ particles or synthesizing composite with conductive graphene and so on.This work aims to seek a simple and safe route to synthesis Na₃V₂?PO₄?₃/C composite and NaVPO₄F/C composite by introducing the F^-,and to investigate their electrochemical performances.The main contents are as follows:1.Synthesizing Na₃V₂?PO₄?₃ gel precursor through a sol-gel method.In this process,oxalic acid is used as reductant to reduce V2O5.Sintering the gel precursor at high temperature,the excess oxalic acid will decompose and form an amorphous carbon coating.To explore the influence of different calcination temperatures on morphology,structure and performance of Na3V2?PO4?3.Although the results show that the morphology and structure of the materials sintered at 700 ^oC?750 ^oC and 850 ^oC are the same,the composite sintered at 700 ^oC achieved the best rate and cycle performance,the first discharge specific capacity is 92 mAh/g at 0.1 C.2.Synthesizing NaVPO₄F gel precursor through a sol-gel method,NH₄F was added after the V⁵⁺was reduced to V³⁺.To explore the influence of different calcination temperature and the thickness of carbon layer on the electrochemical performance of composites.The testing results show that the composite sintered at 850 ^oC with 3 nm carbon layer achieve the best rate and cycle performance.The first discharge specific capacity is 90mAh/g,the first coulomb efficiency is 82%.Especially,when the rate is higher at 10 C,the discharge specific capacity is still as high as 49.2 mAh g^-1.At a charge/discharge rate of 1 C,the NaVPO₄F-C composite sintered at 850 ^oC exhibits a 67 mAh/g capacity after 300 cycles,the capacity retention is 74%.3.Examining the performance of hard carbon at different current densities,and using hard carbon?HC?as anode,NaVPO₄F-C as cathode to make a full cell.Based on the discharge specific capacities of NaVPO₄F-C composite and hard carbon at 30 mA/g current density,adjust the mass of the two materials,and set theoretical capacity to practical capacity ratio to be 1:1.2.Examine electrochemical performances of the assembled NaVPO₄F-C//HC full cells.