Research Of Vanadium-based Polyanionic Compounds As Cathode Materials For Sodium-ion Batteries

With the increasingly serious energy crisis and environmental pollution,clean energy has been developing vigorously,electric vehicles have been gradually replacing fuel vehicles.Thus,there is a large demand for energy storage devices.However,the reserves of lithium resources on the earth are not abundant,and the distribution is very uneven.The resulting imbalance between supply and demand has driven up the price of lithium resources,which has seriously hindered the application of lithium-ion batteries（LIBs）.Sodium-ion batteries（SIBs）have a similar energy storage mechanism to LIBs,sodium resources are abundant and evenly distributed.Therefore,SIBs are considered as one of the most likely alternatives to LIBs.However,due to the larger atomic mass of sodium atom,larger radius of Na⁺,and higher standard electrode potential of Na/Na⁺,the energy density and cycling stability of SIBs are far behind those of LIBs.Therefore,it is very important to develop electrode materials for SIBs with excellent electrochemical performance.Among all the cathode materials for SIBs,polyanionic compounds have excellent structural stability,good ionic conductivity and adjustable voltage.Vanadium-based polyanionic compounds have attracted extensive attention because of the high redox potential and theoretical capacity.However,there are still some problems,such as complex production methods,high vanadium cost and low electronic conductivity of materials.Meanwhile,for the sake of safety,it is particularly important to replace the traditional liquid electrolytes.Based on the above analysis,the main research contents of this paper are as follows:（1）Layered Na_0.5VOPO₄·2H₂O is successfully synthesized by a simple coprecipitation at room temperature and self-assembled to form nanoflower structure.The nanoflower structure increases the specific surface area of the material and makes the contact area with the electrolyte larger,which is conducive to the transmission of Na⁺.Layered Na_0.5VOPO₄·2H₂O has a large interlayer distance,which provides the channels for the rapid diffusion of Na⁺.The Na⁺diffusion coefficient during the charge-discharge process is around 10^-11cm² s^-1.The electrode exhibits the good electrochemical performancea with an average discharge voltage of around 3.3 V.It delivers a high reversible capacity(127 m A h g^-1 at 0.2 C)and excellent long-term cycling stability（71%capacity retention after 1000 cycles at 1C）.In addition,in-situ XRD is used to investigate the reaction mechanism,which reveals that the electrode exhibits a highly reversible evolution during the charge-discharge process.（2）Na₄VMn（PO₄）₃ cathode material is prepared by a sol-gel method.Although the addition of Mn reduces the amount of V and improves the voltage platform（～3.5 V）,but the Jahn-Teller effect reduces the structural stability.Therefore,Na₄VMn_0.9Ni_0.1（PO₄）₃and Na_3.8VMn_0.9Zr_0.1（PO₄）₃ cathode materials are prepared by Ni and Zr doping,respectively.The doping decreases the Mn content,effectively inhibits the Jahn-Teller effect,and improves the cycling stability of electrode materials.After cycling test at 5 C for 800 cycles,the capacity retention of Na₄VMn_0.9Ni_0.1（PO₄）₃,Na_3.8VMn_0.9Zr_0.1（PO₄）₃and Na₄VMn（PO₄）₃ are 81.10%,79.58%and 71.11%,respectively.In the Na_3.8VMn_0.9Zr_0.1（PO₄）₃crystal,Zr doping changes the lattice parameters and produces Na vacancies,which improves the Na⁺diffusion performance and rate performance.The Na_3.8VMn_0.9Zr_0.1（PO₄）₃electrode exhibits high Na⁺diffusion coefficient(1.19×10^-11～6.65×10^-11cm² s^-1)and excellent rate performance(61.7 m A h g^-1at 20 C).It delivers a high reversible capacity of 62.6 m A h g^-1after 2300 cycles at 10 C,and the average capacity decay of 0.028%per cycle.（3）PFSA-Na membrane with high ionic conductivity and thermal structural stability is prepared.Na₃V₂O₂（PO₄）₂F@r GO with an average discharge voltage of～3.8 V is synthesized and applied as the cathode,r GO coating improves the electrochemical performance.Quasi-solid-state sodium-ion half-cells（Na/PFSA-Na/NVOPF@r GO）are assembled with Na₃V₂O₂（PO₄）₂F@r GO cathode,PFSA-Na electrolyte and sodium anode,which exhibit excellent electrochemical performance.At room temperature,the half-cell exhibits a reversible capacity of 129.7 m A h g^-1 at 0.5 C and a high capacity retention of88.1%after 1000 cycles at 4 C.At-20oC,it delivers a high reversible specific capacity of 74.0 m A h g^-1at 1 C.In addition,quasi-solid-state sodium-ion full-cells（HC/PFSA-Na/NVOPF@r GO）are assembled with the pre-sodiated HC anode,which also exhibit good electrochemical performance.In summary,we prepare different kinds of vanadium-based polyanionic compounds as cathode materials for SIBs.We investigate their electrochemical performance,reaction kinetics and energy storage mechanism.Overall,the work provide research ideas for the development of polyanion-type cathode materials of SIBs.