NASICON Structured Cathode And Alloy-type Anode Materials For Sodium Ion Battery

Compared with lithium-ion batteries,sodium resources have abundant reserves,uniform distribution,and low prices,making sodium-ion batteries an ideal choice in the field of large-scale energy storage,which has recently become a research hotspot.The electrode material is an important part of the sodium ion battery.Its physical and chemical properties and electrode performance directly affect the energy density and service life of the battery.Due to the large radius of sodium ions（0.102 nm）and the relative molar mass（23 g/mol）,this will limit the kinetics of Na⁺migration in the electrode material and limit its ability to charge and discharge quickly.Therefore,the development of advanced sodium ion anode and cathode materials is the key to the rapid development of sodium ion batteries.In the positive electrode material,the NASICON structure Na₃(VO_1-xPO₄)₂F_1+2x（0≤x≤1）has the advantages of strong structural stability,three-dimensional tunnel structure（conducive to ion diffusion）,and high working voltage.However,this polyanionic material Compared with its own conductivity,it will reduce the positive rate performance.Among the anode materials,metal antimony（Sb）is considered to be one of the most promising sodium ion battery anode materials due to its abundant resources,high theoretical capacity(660 mAh g^-1),and low operating potential.However,the conversion of Na⁺from Sb negative electrode into Na₃Sb will produce a large volume expansion（290%）,which will cause the powder of electrode material,and then greatly reduce the cycle stability,especially for micro-scale materials.Based on this,in view of the scientific problems faced by a variety of typical positive and negative materials above,we can significantly improve Na₃(VO_1-xPO₄)₂F_1+2x（0≤x≤1）rate performance and cycle stability through cathode material micro-nano structure control and carbon material compounding strategies.Through the electrolyte optimization method,stable cycling of micron-scale Sb anodes can be achieved,and the action mechanism of electrolyte additives and the energy storage mechanism of electrode materials are revealed.The research contents are as follows:1.Study on Micro-Nano Structure Regulation and Electrochemical Performance of Na₃(VO_1-xPO₄)₂F_1+2x（0≤x≤1）Cathode Materials.A series of Na₃(VO_1-xPO₄)₂F_1+2x（0≤x≤1）cathode materials can be synthesized by hydrothermal method.The value of x can be adjusted by adding the amount and valence of vanadium source.The effects of factors such as reaction solvent,temperature and time on the micro-morphology and structure were systematically investigated.Na₃V₂（PO₄）₂F₃ and Na₃V₂O₂（PO₄）₂F,which have higher working voltage and specific capacity,are selected for microstructure regulation.Studies have shown that ethanol is the reaction solvent,and Na₃V₂（PO₄）₂F₃ can be obtained at 180℃and 20h,which has a unique lamellar morphology structure;Na₃V₂O₂（PO₄）₂F can be obtained at 120℃and 10h under water as the reaction solvent.It has a spherical shape and a hollow structure.2.Na₃(VO_1-xPO₄)₂F_1+2x（x=0,x=1）/CNT electrochemical sodium storage behavior analysis.We use the carbon material composite strategy to further improve the rate performance of NASICON cathodes.Using carbon nanotubes as a conductive carbon source,composite materials were prepared by a hydrothermal method.Through SEM characterization,it can be found that carbon nanotubes are uniformly supported on the surfaces of Na₃V₂（PO₄）₂F₃ and Na₃V₂O₂（PO₄）₂F.Two representative Na₃V₂（PO₄）₂F₃-CNT（NVPF-CNT）and Na₃V₂O₂（PO₄）₂F-CNT（NVPOF-CNT）composite nanomaterials show excellent electrochemical performance.For example,NVPF-CNT has a capacity of 107 mAh g^-1 after1000 cycles at a current density of 0.5 C and a retention rate as high as 90.6%.The corresponding discharge capacity at a current density of 20 C is 76.3mAh g^-1;after 1000cycles at a current density of 0.5 C,NVPOF-CNT has a capacity of 98 mAh g^-1 and a retention rate of 83%.GITT tests show that sodium ion diffusion coefficient is 10^-9.In addition,we used ex-situ XRD to study the sodium storage mechanism of the NVPF and NVPOF cathodes.The results show that during the charging process,the release of Na⁺causes the unit cell to shrink,the main peak moves to a high angle,and the discharge process is the opposite.3.Research on micron-scale Sb anode materials.Through the electrolyte optimization strategy,the electrolyte stability of the micron-sized Sb anode was significantly improved by using electrolyte additives.The simple mechanical ball milling method was used to mix the commercial Sb powder and Super P conductive carbon uniformly to quickly prepare micron-sized Sb anode materials（about 1-3μm）.The samples were systematically characterized by morphology and structure.The effects of different levels of fluoroethylene carbonate electrolyte additives on the electrochemical performance of micron-sized Sb anodes were studied.The results show that when the FEC additive content is 10 vol.%,the micron-sized Sb anode shows excellent cycle stability and rate performance.For example,at a current density of 200 mA g^-1,after 150 cycles,the capacity retention rate is 95.4%.At a rate of 2000 mA g-1,the capacity is 565 mAh g-1.Through XPS,theoretical calculation,impedance analysis,SEM and other means,the mechanism of FEC was revealed systematically.Studies show that during the charge and discharge process,a NaF-rich SEI film is formed on the surface of the Sb anode,which is beneficial to alleviate the volume expansion of the electrode material and maintain its structural stability.In addition,compared with the electrolyte without FEC,the micron-level Sb anode material will gradually evolve into a porous intact body in the 10%FEC electrolyte,and there is no obvious phenomenon of active material shedding,which is beneficial to the cycling performance of the electrode.Finally,we used Na₃V₂O₂（PO₄）₂F as the positive electrode,micron-sized Sb as the negative electrode,and 1M NaClO4/PC+10%FEC as the electrolyte to construct a sodium ion full-battery system,which exhibited good cycle performance.The capacity retention rate after cycling 100 cycles at 0.2 C current density was 96%,which further proves the practicability of this electrolyte optimization strategy.