Polyanion-type Compounds As Cathodes For Sodium Ion Batteries:From Materials Design To Device Manufacture

As alternative storage devices of lithium-ion batteries in the future,sodium-based secondary batteries have enormous commercial value.Polyanion-type compounds-based sodium ion batteries electrode materials can play an active role in the low-speed electric vehicle industry and large-scale energy storage field.However,the poor electronic conductivity at room temperature restricts the application.In this paper,reducing the particle size and combining with high-conductivity materials are two methods to improve the performance of the polyanion-type compounds material.The main research contents are as follows:?1?Flower-like nanostructured Na₃V₂?PO₄?₃/C was synthesized by sol-gel method using triblock copolymer F127 as morphology directing agent.On the one hand,the three-dimensional flower-like nanostructure ensure sufficient contact between electrolyte and material,on the other hand,the carbon-coating improve the electronic conductivity.The flower-like nanostructured Na₃V₂?PO₄?₃/C exhibits excellent electrochemical performance.The first cycle discharge specific capacity is 108 mA h g^-1 at0.1 C,and the initial coulombic efficiency is 99%,even at 30 C,the capacity could still matain 50 mA h g^-1.At the same time,the cycle performance of the electrode were tested in EC:PC and PC electrolyte,respectively.It was found that introduce EC cosolvent into the PC electrolyte system,the electrodes exhibit excellent cycle performance.After 1000 cycle at 5 C,the capacity retention is still 99%.?2?The Na₃V₂O₂?PO₄?₂F@PEDOT core-shell structure nanorod was successfully prepared by low temperature synthesis method.The conductive polymer poly?3,4-ethylenedioxythiophene?coated Na₃V₂O₂?PO₄?₂F cathode showed an impressive electrochemical performance in half cells.The discharge specific capacity is 120 mA h g^-1 at 0.1 C,even up to 20 C,the capacity could still matain 51mA h g^-1.After 8000 super long cycle tests at current density of 5 C,the capacity retention is still83.8%.And we found that two major effects of adding FEC additives into electrolyte in two-electrode system.The initial irreversible capacity was reduced,however,the polarization enlarged during the discharge process.In addition,we combine the Na₃V₂O₂?PO₄?₂F@PEDOT cathode and the peanut shell derived carbon anode to construct the high-performance"rocking-chair"sodium ion hybrid capacitor.Compared with the traditional sodium ion hybrid capacitor,the device showed excellent performance,which delivers a high energy density of 158 W h kg^-1 at 230 W kg^-1 and preserves at 25 W h kg^-1 even at 7000 W kg^-1 and a good cycling performance over 1000 cycles?capacity retention of 85.1%?at a current density of 1 A g^-1.?3?The Na₃V₂O₂?PO₄?₂F nanoparticle cathode material was prepared by one-step solvothermal method,which electrochemical performance was improved by decreasing the particle size.The discharge specific capacity of first cycle at 0.1 C is 112 mA h g^-1,and the initial coulombic efficiency is 84.8%,even up to 20 C,the capacity could still retain 29 mA h g^-1.After 600 cycle at the current density of 5 C,the capacity retention is still 99.7%.The sodium ion diffusion coefficients of the Na₃V₂O₂?PO₄?₂F nanoparticle electrode were tested by CV,GITT and EIS.The GITT and EIS technique test the diffusion coefficients at different SOCs,we found that the lowest diffusion coefficients are at the middle of charge-discharge plateau.