Sodium/Vanadium Site Control,structural Design And Electrochemical Properties Of Sodium Vanadium Phosphate Cathode

Sodium-ion batteries have been widely studied due to their abundant sodium resources,low cost and similar physical and chemical properties to lithium in recent years.The NASICON-type polyanion compound material Na₃V₂（PO₄）₃is considered to be one of the ideal cathode material for sodium ion batteries due to its high stability and three-dimensional open frame structure for fast sodium ion transport.However,its low intrinsic electronic conductivity and limited sodium ion diffusion kinetics limit its practical application.In response to the above problems,in this paper,the NVP-based cathode materials with excellent rate capability and cycle performance were developed through the control of sodium/vanadium sites and the design of three-dimensional carbon network structure to synergistically improve the NVP sodium ion diffusion kinetics and electronic conduction.The detailed research contents are as follows:（1）Na-site doping and three-dimensional carbon network design to construct high-performance NVP-based cathodes.A series of Na_3-2xCa_xV₂（PO₄）₃materials were prepared by adding calcium salts to the precursor solution.At the same time,a 3D framework carbon network structure was prepared by introducing a g-C₃N₄layered template.Studies have shown that the introduction of Ca²⁺did not change the crystal structure of Na₃V₂（PO₄）₃,but improves the electronic conductivity and sodium ion diffusion kinetics of Na₃V₂（PO₄）₃.Electrochemical characterization shows that when the Ca²⁺doping amount is 0.1,that is When the chemical formula is Na_2.8Ca_0.1V₂（PO₄）₃（CNVP@C/3DNC-0.1）,its rate performance and cycling stability are the best;and the introduction of 3D framework carbon network structure forms an efficient electron conduction network,which can ensure effective electron transfer between active particles,and also provide more active sites for sodium ion diffusion.Thus,the comprehensive electrochemical performance of NVP-based cathode materials is improved.According to the electrochemical test results,the specific discharge capacities of CNVP@C/3DNC-0.1 at 0.1 C,1 C and 10 C are 106.4 m Ah g^-1,95.4 m Ah g^-1and 84.2 m Ah g^-1,respectively;the specific discharge capacity after5000 cycles at 15 C is 77.3 m Ah g^-1,and the capacity retention rate is as high as93.1%.（2）Development of high-performance NVP-based cathodes through vanadium site modulation and three-dimensional porous carbon network structure design.By doping Mn element at the V site of Na₃V₂（PO₄）₃,not only the voltage platform of the electrode material can be improved(Mn^3+/2+=3.6 V),but also the material cost can be reduced.By adding PVP and graphene oxide（GO）suspensions in the precursor solution to construct a porous three-dimensional carbon framework supported NVP composite（HP-NVMP@C-GO）,this unique microstructure not only has better structural stability,it also promotes the rapid penetration and ion diffusion of the electrolyte and greatly improves the electron transport efficiency,thereby improving the rate performance and cycle performance.By analyzing the CV test and EIS test results of the electrode material,HP-NVMP@C-GO has higher sodium ion diffusion coefficient and smaller charge transfer resistance,showing better electrochemical performance.HP-NVMP@C-GO exhibits an activation-discharge specific capacity of113.3 m Ah g^-1at 0.1 C under the charge-discharge voltage window of 2.5-4.1 V and a discharge specific capacity of 72.9 m Ah g^-1at a high rate of 20 C and the capacity retention rate is 76.0%after 5000 cycles.The high and low temperature electrochemical performance tests of the electrode materials were carried out in extreme temperature environments of low temperature（-20°C）and high temperature（50°C）.The HP-NVMP@C-GO was charged and discharged at a constant current rate of 1 C at-20°C with an initial discharge specific capacity of 83.8 m Ah g^-1,and its capacity retention rate was 96.8%after 500 cycles.HP-NVMP@C-GO also still exhibits excellent cycling performance at 50°C(the discharge specific capacity of106.1 m Ah g^-1at 1 C and capacity retention is 63.6%after 1,500 cycles)and rate capability(the reversible discharge specific capacity of 51.8 m Ah g^-1at 15 C).