Application And Research Of Polyanionic Compounds As Electrode Materials For Sodium Ion Batteries

In recent years,with the rapid development of mobile electronic products and the electric vehicle industry,lithium resources are rapidly being consumed and its costs keep rising.Therefore,it is imperative to develop and utilize other low-cost,high-performance energy storage systems.Sodium-ion batteries have become one of the strong competitors to replace lithium-ion batteries because of their abundant resources,low price and controllable working voltage.Polyanionic compounds is characterized by their high ionic conductivity due to their special framework structure,the controllable voltage platform brought about by the "induction effect" and their excellent structure and thermal stability during electrochemical process.It is considered as the ideal material for developing low cost,high performance sodium ion batteries.Although the polyanionic compounds have a perfect application prospect in theory,it is somewhat limited by its poor electronic conductivity and low theoretical specific capacity whenever it is used as a positive electrode or a negative electrode of a sodium ion battery.Therefore,how to effectively improve the conductivity and electrochemical performance of polyanionic compounds have become the top priority of current research.In addition,most of the research on sodium ion batteries is focused on half-batteries,but the exploration of full sodium-ion batteries is slightly insufficient.It will undoubtedly delay the process of large-scale promotion of full sodium-ion batteries,so it is equally significant to explore how to apply the developed electrode materials to a full sodium-ion battery system.Therefore,based on the above points,this paper aims to develop a new system of anode materials and improve the electrochemical performance of existing polyanionic cathode materials and apply them into sodium ion battery systems.The main research contents and results are as follows:1.A novel Mn_0.5Ti₂（PO₄）₃ polyanionic compound with NASICON structure was synthesized by sol-gel method,and then modified by in-situ reduction of graphene oxide to obtain the MTP@rGO composite possessing larger specific surface area and smaller MTP particles.This composite material（MTP@rGO）is the first applied to the sodium ion battery system.The electrochemical test results show that the MTP@rGO composite has a discharge specific capacity of 201.7 mAhg^-1 at a current density of 0.05 Ag^-1,and still remain 59.6 mAhg^-1 when the current density is increased to 2 A g^-1.When current density at 0.2 Ag^-1,the capacity retention rate can up to 84.6% after 300 cycles.Cyclic voltammetry（CV） and AC impedance analysis（EIS） showed that the modified composite has a faster sodium ion diffusion rate and a smaller electron transfer resistance.Therefore,reducing the particle size,increasing the specific surface area and improving the conductivity of the material are effective ways to improve the electrochemical performance of the polyanionic compound.2.A carbon-coated Na₃Fe_0.5V_1.5（PO₄）₃ nanoparticle composite（NFVP@C） was synthesized by a simple solid phase method.The CV data indicates that the composite has two pairs of redox peaks corresponding to Fe²⁺/Fe³⁺ and V³⁺/V⁴⁺redox couples,respectively.When used as a positive electrode material for sodium ion battery,it delivers a discharge specific capacity of 111 mAhg^-1 at a current density of 0.5 C.The electrode can still maintain a reversible capacity of 88 mAhg^-1 when the current density is increased to 20 C.The capacity,after 5000 cycles,still retains the reversible specific capacity of 72 mAhg^-1.In addition,the full cell NFVP@C//HC assembled with commercial hard carbon and the composite material also presents excellent electrochemical performance.Electrochemical test results reveals that NFVP@C//HC full cell also has two pairs of redox peaks,the peak potential of Fe²⁺/Fe³⁺ redox couple is about 2.4 V,and the peak potential of V³⁺/V⁴⁺ redox couple is about 3.3 V.It indicates that the hard carbon anode is still electrochemically active in a full cell system.At a current density of 0.5 C,the NFVP@C//HC full-cell discharge specific capacity is as high as 110 mAhg^-1,and the capacity retention rate is up to 93.8% after 200 cycles.When the current density is increased to 5 C,the discharge specific capacity still has 79 mAh g^-1.NFVP@C exhibits good electrochemical performance in both half and full-cell systems,suggesting that the method used in this paper can simply and effectively improve the electrochemical performance of polyanionic compounds,which provides more possibilities for the further development of sodium ion batteries.3.Sponge-carbon coated Na_3.32Fe_2.34（P₂O₇）₂ nanoparticles（NFPO@SC） with 3D porous structure were prepared by simple sol-gel method and multi-stage calcination.Compared with conventional NFPO@C,the sponge-like NFPO@SC electrode shows superior electrochemical performance compared to conventional NFPO@C.The material has a reversible specific capacity of 115.8 mAhg^-1 at a current density of 0.5C and 60.9 mAhg^-1 at a current density of 20 C.The capacity retention rate of 87.3% after 5500 cycles.The excellent electrochemical performance of NFPO@SC can be attributed to the unique spongy 3D structure that promotes electrolyte penetration,reduces sodium ion diffusion paths,increases ion diffusion rates and electron transfer rates.The full battery NFPO@SC//HC assembled by using commercial hard carbon and NFPO@SC respectively as negative and positive electrode also displays excellent electrochemical performance.The reversible specific capacity of 112.2 mAhg^-1 can be provided at a small current density,and the capacity retention rate is as high as 93.9% after 1000 cycles.Therefore,we believe that this paper provides a viable method for preparing high performance sodium ion full cells.In summary,the low-cost and good electrochemical performance of polyanionic materials prepared by simple and easy methods in this paper provides a new idea for the further development of high performance sodium ion batteries.