Study On Modification Of Sodium Vanadium Phosphate As Cathode Material For Sodium Ion Batteries With High Safety

In the background of current resource shortage and high cost of lithium-ion batteries,sodium-ion batteries are gradually being produced commercially,and SIBs are an ideal alternative to lithium-ion batteries.The cathode material directly affects the electrochemical properties of sodium-ion battery system,the energy density of the cathode material decides the upper limit of the energy density of the cell,and the ability of the cathode material to accommodate Na⁺and the transmission capacity of Na⁺determine the power density of the battery,thus the research on the cathode material is necessary.Among the current cathode material systems,Na₃V₂（PO₄）₃（NVP）with sodium-ion superconductor structure（NASICON）has excellent thermal stability and structural stability.Phosphate materials with low intrinsic ionic and electronic conductivity limits its development.In current,the material has received less attention in the market.In this paper,NVP is modified by combining bulk phase ion doping and compound with carbon nanotubes（CNTs）,which can minified the size of the particles and modulate the crystal structure,then improve the transport efficiency of Na⁺in the crystal structure of NVP,and optimize the electrochemical properties of the material.The stability of the material is explored on the basis of ensuring the performance has been improved.The contents and specific results of this paper are shown below:（1）The modified method is doping Ba²⁺with larger radius and compounding with CNTs,which can modulate the crystal structure of NVP.In this paper,the results shown that the Na_3.67V_1.73Ba_0.07（PO₄）₃@CNTs sample exhibits the best performance,which can provide high specific capacity and maintain long cyclic stability.During the first cycle of charge/discharge,the specific capacity of Na_3.67V_1.73Ba_0.07（PO₄）₃@CNTs is up to 117.6 mA·h·g^-1,which is much higher than the initial specific capacity of NVP(69.4 mA·h·g^-1).Moreover,the retention rate of specific capacity is 63%after 5000 cycles at 50 C.The analysis results of thermal stability indicate that the thermal stability of Na_3.67V_1.73Ba_0.07（PO₄）₃@CNTs is improved compared with NVP and does not decompose at high temperature.（2）Double ions of Mg and Ti with different valence state are co-doped into NVP,and synergistically compounded with CNTs.The efficiency of sodium ion transport in the structure is improved by generating p-n-type effects,thus the kinetic properties of the material are optimized.Meanwhile,Mg²⁺and Ti⁴⁺have smaller radius than V³⁺.Doping with ions with smaller radiu can broaden the Na⁺transport channels.In this paper,Na₃V_1.86Mg_0.07Ti_0.07（PO₄）₃@CNTs samples are prepared by sol-gel method,and the results shows that Na₃V_1.86Mg_0.07Ti_0.07（PO₄）₃@CNTs sample provide the specific capacities of 72.3mA·h·g^-1 and 71.9 mA·h·g^-1 at the current densities of 50 C and 100 C,respectively.The thermal stability of Na₃V_1.86Mg_0.07Ti_0.07（PO₄）₃@CNTs shows higher exothermic temperature than NVP,which has excellent safety stability and is less prone to decomposition at high temperatures.（3）P-n type Mn²⁺and Ti⁴⁺with different radius are simultaneously doped into NVP,and coated carbon layer and enwrapped CNTs around NVP.Two transition metal ions,one larger and one smaller than V³⁺,are introduced into the V site,and the n-type effect produced by small radius Ti⁴⁺generates a certain number of holes in the structure for carrier transport.Larger radius Mn²⁺supports the structure and expands the Na⁺transport channel.In this paper,1000cycles of Na₃V_1.7Mn_0.15Ti_0.15（PO₄）₃/C@CNTs are conducted at 50 C and 100 C,and the capacity decay rates for a single cycle are 0.018%and 0.021%,respectively.At the same time,at high temperatures,the material does not collapse,and the structure remains stable.