Synthesis And Application Of Cathode Material Of Sodium Ion Battery

Due to the abundance and low cost of sodium,it is very important to probe sodium ion batteries to meet the development of large scale energy storage.Na₃V₂（PO₄）_3,which owns open NASICON framework,high specific energy density（⁴00 Wh/kg）and high thermal stability,is considered to be a prospective cathode of sodium ion batteries.However,similar to other lithium metal phosphates,the electronic conductivity of Na₃V₂（PO₄）₃ is very poor,which greatly affects its electrochemical properties.In order to improve its disadvantage of low electronic conductivity,several modification methods are adopted to enhance its electrochemical performance.The detailed research contents are as follows:（1）Budding willow branches shaped Na₃V₂（PO₄）₃/C nanofibers were successfully synthesized by a simple electrospinning technique with Poly（vinyl pyrrilidone）（PVP）.The Na₃V₂（PO₄）₃/C nanoparticles that anchored on the nanofibers surface seemed like the willow buds;the inner core of the nanofibers,which composed Na₃V₂（PO₄）₃,looked like willow twig and the uniform carbon layer was same with willow bark.Such special morphology played a vital role in improving cycle stability and rate capability of the electrode due to the conductive network built up by nanofibers.The Na₃V₂（PO₄）₃/C nanofibers cathode exhibited an initial specific capacity of 106.8 mAh g^-11 at a current density of 0.2 C,still stabling at 107.2 mAh g^-11 after 125 cycles with excellent cycle stability.Moreover,a capacity retention of 95.7%was obtained when Na₃V₂（PO₄）₃/C nanofibers cycled stepwise from 0.2 to 2 C.Good electrochemical performance should be ascribed to both the special morphology and preferential growth of the（113）plane.The simple synthesis technique and good electrochemical performance suggests that this material with the special shape of budding willow branches is a promising cathode for sodium ion batteries.（2）Na-ion batteries have been regarded as promising alternatives for Li-ion batteries due to the extensive sodium reserves in the world.Na₃V₂（PO₄）₃ has been proved to be a good candidate of the cathode materials in Na-ion batteries,but the intrinsic low electrical conductivity and sluggish kinetics handicapped its application.Here,3D hierarchical Na₃V₂（PO₄）₃ particles were synthesized by a facile hydrothermal method,constructed by carbon-coated 2D Na Na₃V₂（PO₄）₃ nanowalls.Superior cell performance of high rate capability and cycle stability were observed in the well-defined structure.As the cathode in Na-ion batteries,it delivered a high capacity almost reaching the theoretical one and exhibited high capacity retention.The enhanced rate capability and cycle performance can be attributed to the improved electrical conductivity from the interconnected carbon layer and the shortened ion diffusion length and high specific surface area from the nanowalls.（3）Structural and morphological control is an effective approach for improvement of electrochemical performance in rechargeable batteries.In this paper,three different morphological Na₃V₂（PO₄）₃（irregular shaped,the porous sponge-like and plate like）were successfully prepared through controlling the amount of oxalic acid by a simple two-step reduction method.It is found that the amount of oxalic acid had vital impacts on the morphology of Na₃V₂（PO₄）₃;moreover,the morphological evolution and formation mechanism were proposed based on the reactions of different amount of oxalic acid occurring in the two-step reduction process.The excellent electrochemical performances of the porous sponge-like Na₃V₂（PO₄）₃ were attributed to the unique morphology.The initial capacity of the porous sponge-like Na₃V₂（PO₄）₃ was 101.77 mAh g^-11 at 30 C;after 700cycles,it remains as high as 89.28 mAh g^-11 with only 12%capacity loss.When the current density increases to 50 C and 70 C,the capacity retentions of 81%after 600 cycles,and92.5%after 500 cycles are achieved,respectively.（4）In order to get element substituted Na₃V₂（PO₄）₃/C in appointed V site,the simple sol-gel method is used to design and prepare a series of Na-rich Na_3+xV_2-xNi_x（PO₄）₃/C（x=0,0.01,0.03,0.05 and 0.07）compounds.To get a charge balance,the ratio of Na,V and Ni would be changed differently if Ni goes into different site.Hence,ICP is applied to probe the real stoichiometry of the as-prepared Na_3+xV_2-xNi_x（PO₄）₃/C（x=0,0.01,0.03,0.05 and 0.07）.According to Na/V ratio from ICP result,it indicates that Ni²⁺goes to V site,and more Na⁺will be introduced into the crystal to keep the charge balance.In addition,the crystal structure changes are explored by XRD and Rietveld refinement,it can be indicated from the results that Ni²⁺doping does not destroy the lattice structure of Na₃V₂（PO₄）₃.When applied as Na-storage material,the electrochemical property of all Ni²⁺doped Na3+xV2-xNix（PO4）3/C composites have been significantly improved,especially for the Na_3.03V_1.97Ni_0.03（PO₄）₃/C sample.For example,107.1 mAh g^-11 can be obtained at the first cycle,after 100 cycles,the capacity retention is as high as 95.5%.Moreover,when charging/discharging at higher rate of 5 C,the capacity still remains 88.9 mAh g^-1,displaying good rate performance.The good electrochemical performance is ascribed to the optimized morphology,stabled crystal structure and improved ionic conductivity.（5）Based on the research of Ni²⁺doped Na₃V₂（PO₄）₃,Mg²⁺is also chosen to dope into Na₃V₂（PO₄）₃.A series of Na_3+xV_2-xMg_x（PO₄）₃/C（x=0,0.01,0.03,0.05,0.07 and 0.1）samples were prepared by simple sol-gel method.First principles calculation was applied to calculate the form energy for doping at different site,the result indicates that Mg²⁺prefers to go to V site.ICP is applied to probe the real stoichiometry of the as-prepared Na_3+xV_2-x-x Mg_x（PO₄）₃/C（x=0,0.01,0.03,0.05,0.07 and 0.1）.According to Na/V ratio from ICP result,it indicates that Mg²⁺goes to V site,and more Na⁺will be introduced into the crystal to keep the charge balance.The experimental result is similar with first principle calculation result.In addition,the comparison of crystal parameters based on XRD refinement and model from first principle demonstrated that Mg is not in the bulk of Na₃V₂（PO₄）₃.According to the sodium ion diffusion coefficient results,it can indicate again that most of Mg²⁺exist in the surface of Na₃V₂（PO₄）₃.When used as cathode of sodium ion batteries,all the doped samples show much better electrochemical performance than the undoped one,especially for Na_3.05V_1.95Mg_0.05（PO₄）₃/C.The initial capacity is 96.7mAh g^-11 when cycled at 10 C,after 180 cycles,the capacity is still as high as 86 mAh g^-1.However,the undoped sample only display 88.8 mAh g^-11 at the first cycle at 10 C,only 63mAh g^-1left after 180 cycles.