Study On Synthesis And Modification Of Na₃V₂（PO₄）₂F₃ Cathode Material For Sodium-ion Batteries

As a cathode material for sodium-ion batteries,NASICON-type Na₃V₂（PO₄）₂F₃possesses high discharge voltage plateau,excellent structural stability and higher diffusion coefficient of Na⁺,and has been considered as one of the most promising cathode materials for sodium-ion batteries.However,the low electronic conductivity of Na₃V₂（PO₄）₂F₃ greatly impedes the electrochemical performance,especially rate capability,of Na₃V₂（PO₄）₂F₃.To solve this problem,we adopted the methods of carbon coating,doping in V sites,doping in Na sites,and doping both V and Na sites to improve the electrochemical performance of Na₃V₂（PO₄）₂F₃ in this dissertation.The prepared cathode materials were investigated by X-Ray diffraction（XRD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,galvanostatic charge and discharge,cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）tests.The effects of experimental conditions on the components,morphology and electrochemical performance of the synthesized materials were studied,and the experimental conditions were optimized to prepare the modified Na₃V₂（PO₄）₂F₃ with better electrochemical performance.Na₃V₂（PO₄）₂F₃/Ccompositeswere prepared by solid-state reaction,and the effects of carbon sources,calcination temperature and calcination time for the preparation were investigated,and hence the optimal experimental conditions were obtained.The optimal composite exhibits initial discharge capacity of 105mAh·g^-1 at 0.5C(64 mA?g^-1)in the voltage range of 3.0-4.5 V（vs Na⁺/Na）,and the capacity remains 105mAh·g^-1 after 30 cycles.Na₃V_2-xAl_x（PO₄）₂F₃/C（x=0,0.03,0.05,0.07）were synthesized by solid-state reaction based on the preparation of Na₃V₂（PO₄）₂F₃/Ccomposites.The results show that doping a small amount of Al³⁺does not change the tetragonal structure of Na₃V₂（PO₄）₂F₃and significantly improves the electrochemical performance of Na₃V₂（PO₄）₂F₃.The optimal Al-dopedNa₃V₂（PO₄）₂F₃/C composite,Na₃V_1.95Al_0.05（PO₄）₂F₃/C,displays initial discharge capacity of 113 mAh·g^-1 at 5C between 3.0 and 4.5 V,and the discharge keeps 103 mAh·g^-1after 100 cycles,while the pristine Na₃V₂（PO₄）₂F₃/C presents initial discharge capacity of only 103 mAh·g^-1 and the corresponding capacity retention of 78%under the same cycled conditions.On the basis of the preparation of Na₃V_1.95Al_0.05（PO₄）₂F₃/C,Na_3-xK_xV_1.95Al_0.05（PO₄）₂F₃/C（0,0.05,0.07,0.09）composites were prepared by solid state reaction.The results indicate that co-doping of hetero-ion in both Na and V sites does not alter the tetragonal structure of Na₃V₂（PO₄）₂F₃ and all the co-doped Na₃V₂（PO₄）₂F₃/C composites exhibits the improved cycle performance.And the best co-doped composite,Na_2.95K_0.05V_1.95Al_0.05（PO₄）₂F₃/C,presents the initial discharge capacity of 115mAh·g^-1 at 10Cin the voltage range of 3.0-4.5 V,and the capacity slowly decreases to 100mAh·g^-1 after 100 cycles.