| Due to the limited global lithium resources,the growing demand for lithium ion secondary batteries cannot be met.Sodium has the advantages of rich content in the earth,wide distribution and low cost.And sodium ion battery(SEBs)is a promising alternative to lithium ion battery(LIBs).Currently,Cathode material performance is a key factor limiting the performance of sodium ion batteries.In this thesis,different kinds of water-soluble binders were investigated in P2-type cathode material to improve the electrochemical properties.And modification of Na3V2(PO4)3 cathode was carried out by synthetic method and Co ion doping.In this thesis,xanthan gum,sodium alginate and guar gum were investigated as binders to improve the electrochemical performance of Na2/3Mn2/3Ni1/3O2 cathode material in sodium ion batteries system.The aqueous binder has good chemical and electrochemical stability in the battery system.Electrode samples using aqueous binders have superior reversible specific capacity and capacity retention compared to the electrode samples with PVDF binder Among them,the electrodes use XG as binder has the best electrical properties.The sample with XG binder exhibits a capacity of 115.9 mAh g-1 after 80 cycles at a current density of 40 mA g-1,corresponding to a capacity retention rate of 77.6%,and a capacity of 75.4 mAh g-1 after 300 cycles at 100 mA g-1.Experimental characterization indicates that XG has the best electrochemical performance due to:(1)XG solution has the highest viscosity,(2)XG binder can make the electrode have a larger ion diffusion coefficient,(3)XG binder can inhibit the transition metal loss on the surface of the electrode material,(4)XG binder can help maintain a complete conductive network more effectively in the electrochemical cycle.Besides,Na3V2(PO4)3 cathode materials were synthesized by a solid phase method and a sol-gel hydrothermal method,which presents a regular sphere with a diameter of 1-2.5 ?m.The electrochemical performance characterization showed that the Na3V2(PO4)3 material synthesized by sol-gel hydrothermal method has high reversible specific capacity and excellent long-cycle stability with capacity retention of 94.80%after 1000 cycles at 117.6 mA g'1.Its capacity-voltage curve reflects its excellent long cycle stability and small polarization.Then,Co-doping Na3V2-xCox(PO4)3 material was prepared by a simple ball-milling solid phase method.XRD refinement results show that due to the similar ionic radius of V3+(0.64 A)and Co2+(0.65 A),the incorporation of Co does not have an obvious impact on the overall lattice parameters of the material.The surface of the particles gradually became rough with the increase of cobalt incorporation,and when the amount of cobalt incorporated was large,agglomeration of material is observed.The Na3V2-xCox(PO4)3 material exhibits the best electrochemical performance when the doping ratio of cobalt is 0.1 with capacity retention of 92.15%after 250 cycles at 117.6 mA g-1.The EIS characterization shows that the transfer resistance of Na3V2-xCOx(PO4)3 material is smaller than that of other samples when the doping ratio of cobalt is 0.1 and it also has a larger sodium ion diffusion coefficient,which provides faster sodium ion transport.This may be related to the incorporation of cobalt effectively increases the overall conductivity of the material. |
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