Study On The Design And Sodium Storage Properties Of The High-Performance NASICON-typed Phosphate Cathode Materials

With the increasing contradiction between the over-exploitation of traditional fossil fuels such as oil and gas and the rapid development of modern science and technology,adhering to the concept of green and low-carbon sustainable development,the development and exploration of large-scale energy storage equipment for renewable and clean energy is one of the most important scientific research.Sodium ion batteries（SIBs）is considered to be one of the ideal choices for future large-scale energy storage systems due to its similar electrochemical energy storage mechanism with lithium ion battery,more abundant natural resources,relatively high safety and low cost.However,because the electrochemical capacity of the cathode material is significantly lower than that of the anode,which leads to higher usage of the cathode material in SIBs than the anode material,the cost of SIBs is mainly limited by cathode.Moreover,the working potential and structural stability of the cathode material also seriously affect the energy density and cycle life of SIBs.Therefore,it is of great significance to explore and develop cathode materials with high working potential,high specific capacity and high structure stability to promote the commercialization and industrialization of SIBs.Phosphate polyanionic compounds with NASICON（Natrium Super Ionic Conductor）structure have been widely concerned and studied for their unique electrochemical properties due to the"induction effect"of its polar group.Na₃V₂（PO₄）₃（NVP）is a typical NASICON-typed phosphate compound,which has high working voltage（about3.4V）and theoretical capacity（about 117 m Ah/g）due to its V³⁺/V⁴⁺redox pair.Moreover,because of the existence of 3D open phosphate group skeleton,it has excellent lattice stability and can withstand the reversible extraction/insertion of a large number of sodium ions.However,due to the poor intrinsic electronic conductivity of NVP and the high toxicity and high cost of vanadium-containing compounds used for the synthesis of Na₃V₂（PO₄）₃,the further application and development of NVP in industrialization and commercialization are seriously restricted.In order to solve the scientific problems faced by the NASICON-typed Na₃V₂（PO₄）₃ cathode materials described above,this paper aims to"reduce the cost of electrode while simultaneously having high electrochemical performance"as the core goal.We use transition metal ions with low cost to gradually carry out minor doping,partial substitution and total substitution of the V sites of Na₃V₂（PO₄）₃ cathode materials,and then composite it with carbon-based materials with high conductivity to design and prepare a new NASICON-typed phosphate cathode material with high performance and low cost.（1）In order to solve the shortcomings of NVP such as low conductivity and poor rate performance,the Na_3+xV_2-xCo_x（PO₄）₃/C cathode（x=0.01-0.05）was designed and prepared by the sol-gel method,using Co²⁺ions to low doped the V³⁺site of Na₃V₂（PO₄）₃.The low content doping of Co²⁺ions significantly improves the electronic conductivity and lattice structure stability of NVP.Moreover,the coral-like porous morphology and the carbon coating on the surface formed by the decomposition of acetate and citric acid at high temperature increase the contact area between the electrode and the electrolyte,which significantly improves the long cycle life and rate performance of the doped samples.The optimized Na_3.01V_2.99Co_0.01（PO₄）₃/C cathode showed the best electrochemical performance.Its has a high initial capacity of 106m Ah/g at 10 C,and the capacity retention rate reached 83.2%after 1000 cycles.（2）Due to the high toxicity and high cost of vanadium containing compounds,which seriously limits the further development of NVP cathode.Therefore,a new NASICON-typed Na_3.833V_1.167Mn_0.583Ni_0.25（PO₄）₃/C cathode was designed and prepared by using low-cost and environmentally friendly Ni²⁺and Mn²⁺ions to partially replace the V³⁺site of Na₃V₂（PO₄）₃ through the sol-gel method.On the one hand,it provides high specific capacity and high working platform for the cathode by using the multi-electron transfer reaction of V,Mn and Ni transition metal elements.On the other hand,the Ni²⁺ions with electrochemical inertness significantly enhance the crystal structure and electronic conductivity of the material,which further enhances the cyclic stability of the electrode at high rate.Na_3.833V_1.167Mn_0.583Ni_0.25（PO₄）₃/C cathode can reach an excellent capacity of 80.1 m Ah/g at the high rate of 20 C,with a capacity retention rate of 63.5%after 10000 cycles.In addition,the results of in-situ XRD show that the high reversibility and structural stability of the Na⁺extraction/insertion process of Na₄VMn_0.5Ni_0.5（PO₄）₃/C cathode during charging and discharging（3）In order to further reduce the production cost of NASICON-typed phosphate cathode materials for SIBs,the NASICON-typed Na₄Mn Cr（PO₄）₃（NMCP）cathode was successfully prepared by using cheaper Cr³⁺and Mn²⁺ions to completely replace the V³⁺sites of Na₃V₂（PO₄）₃.In order to solve the problems of low conductivity and poor rate performance of NMCP cathode,carbon nanotubes（CNTs）and polyvinylpyrrolidone（PVP）were used to doped NMCP by sol-gel method and freeze drying technology,and Na₄Mn Cr（PO₄）₃@C@PVP@CNTs cathode with high performance and low cost was successfully synthesized.The electronic conductivity and surface carbon coating of the material are improved by using the synergistic effect produced by the co-doping of CNTs and PVP,and the rate performance and cycle performance of the electrode are significantly enhanced.Therefore,Na₄Mn Cr（PO₄）₃@C@PVP@CNTs cathode has excellent electrochemical performance.The average working potential of the material is about 3.8V,and its capacity is 114 m Ah/g at 0.5 C and 96 m Ah/g at 5 C based on Mn²⁺/Mn⁴⁺multi-electron transfer in the voltage range of 1.4-4.3 V.In addition,by extending the voltage range to 1.4-4.6 V,the additional Cr³⁺/Cr⁴⁺redox pair can further increase the working potential and capacity of the cathode material,thus enhancing the electrochemical performance of SIBs.