Synthesis And Properties Research Of Polyanion Type Cathode Materials For Sodium-ion Batteries

Sodium-ion batteries,representative members of the post-lithium-battery club,are very attractive and promising for large-scale energy stroage application.The increasing attention in sodium-ion batteries（SIBs）are being driven by the demand of the Na-based electrode materials which are resource-abundant,cost-effective,and long lasting.Polyanion type compounds are among the most promising electrode materials for SIBs due to their stability,safety,and suitable operating voltages.However,the typical low conductivity and relatively low capacity performance of such materials still restrict their development.This theis adopted three methods,and compared the solid phase method,the precursor-based two-step method and the hydrothermal method,to synthesize the olivine-type sodium phosphate（NaMnPO₄）with a sodium ion migration channel.The ratio of its raw materials,hydrothermal temperature and carbon coated temperature were optimized.HT-NaMnPO4/C,prepared at the condition of Na:P:Mn=3:1:1,the hydrothermal and calcined temperature were 180℃and 500℃respectively,.has a specific capacity of 59.6 mAh·g^-1 at0.1C,with the Na ion diffusion coefficient of 1.31×10^-14 cm·s^-1.The Na_3-2xMg_x V₂（PO₄）₃/C（x=0,0.05,0.1,0.2）cathode material was synthesized by sol-gel method.The particle size is between 200⁶00 nm.The results show that when x=0.05,the electrochemical performance of Na_2.9Mg_0.05V₂（PO₄）₃/C sample is the best.The discharge specific capacities of Na_2.9Mg_0.05V₂（PO₄）₃/C at 10C,20C and 30C are 98.9%,91.5%and 76.2%at the valve of 1C,respectively.The capacity retention of Na_2.9Mg_0.05V₂（PO₄）₃/C at 10C after1200 cycles is 80.3%,which portends a good application prospect.The Na₃V_2-xCu_x（PO₄）₃/C（x=0,0.01,0.03,0.05 and 0.07）cathodes have been prepared by ball-milling and spray-drying assisted high-temperature solid-state reaction.The effects of Cu doping on Na₃V₂（PO₄）₃/C were characterized by XRD,SEM,XPS,EPR and electrochemical measurements.The results show that the Na₃V_1.95Cu_0.05（PO₄）₃/C delivers the highest discharge capacity of 90.6 mAh·g^-1 at 50 C and superior cyclic retention of 88.2%after 2000 cycles at 20 C,which could be mainly attributed to the high electronic conductivity(7.2×10^-2S·cm^-1)and Na ion diffusion coefficient(1.695×10^-12cm²·s^-1).The introduction of Cu²⁺into the Na₃V₂（PO₄）₃/C lattice produces oxygen vacancies with accompanying conduction electrons,which leads to the enhancement of both the electronic conductivity and D_Na+.Besides,less electroactive Cu²⁺acts as a pillar to reduce the negative effects on structure caused by the volume shrinking/swelling during the Na⁺extraction/insertion.