Study On Synthesis And Modification Of Layered Structure LiV₃O₈ Cathode Materials

Since the 21st century,with the consumption and pollution of non-renewable energy sources such as coal and petroleum,vehicles,ships and other transportation vehicles urgently need to develop new green and environmentally friendly energy storage devices.Lithium-ion batteries,as a new type of energy storage device,are widely used in portable equipment,but they are still insufficient as a power source for vehicles.This requires people to develop higher power and energy density lithium-ion batteries.The layered structure LiV3O8 material has the advantages of high specific capacity and low cost,and is one of the positive electrode materials for power lithium-ion batteries with promising applications.This thesis focuses on the preparation of layered LiV3O8 cathode materials and the investigation of their electrochemical performance.The effects of calcination temperature on the phase structure,micro-morphology and electrochemical performance of the samples were studied;Afterwards,it is modified by compounding with graphene oxide and Nb element doping,which further improving the electrochemical properti es of the material.The main results are as follows:Study on the performance of LiV3O8 cathode materials synthesized at different calcination temperatures.In Chapter 3,LiV3O8 electrode materials(denoted as LVO-350,LVO-400 and LVO-450)were prepared by sol-gel method at different temperatures,its structure is layered.LVO-350 sample size is between 200-400nm,which belongs to sub-micron particles,and the dispersion is more uniform.The morphology of the LVO-400 sample is a micrometer rod with a length of about 600 nm and a width of about 200 nm.The LVO-450 sample is a lumpy material with very uneven particle size.The LVO-350 electrode material has the best electrochemical performance.The initial discharge specific capacity is 330.9mAh/g at a current density of 60mA/g,and the specific capacity after 100 cycles is 225.5mAh/g,and the capacity retention rate is 68.1%.Under the large current density of 1200mA/g,the discharge specific capacity of the LVO-350 electrode is 119.4mAh/g.Study on synthesis and properties of LiV3O8/GO composites.In the fourth chapter,LiV3O8 material was prepared by the method in Chapter 3,and then it was compounded with graphene oxide to prepare LiV3O8/GO composite material.LiV3O8/GO composites show good cycling performance.The initial discharge specific capacity of LiV3O8/GO composite material is 335.2mAh/g.After 100 cycles,the discharge specific capacity of the composite material is still 280.2mAh/g,and the capacity retention rate is 83.5%.When the current density increases to 3A/g,the discharge specific capacity can still reach 118.4mAh/g,which is significantly higher than the 77.2mAh/g of LiV3O8 material.After compounding,the rate performance of the material is improved.The results of AC impedance spectroscopy show that the composition of GO reduces the charge transfer resistance in the process of lithium ion disentrapment,which is conducive to the diffusion of lithium ion.The Chapter 5 is about the preparation and performance of LiV3-yNbyO8 material.The electrochemical performance of Nb-doped materials has improved,and LiV2.94Nb0.06O9 has the best electrochemical performance.At a current density of 100mA/g,the initial discharge specific capacity is as high as 396.5mAh/g,and after 50 cycles,the specific capacity remains at 250.5mAh/g;when the current density is 5A/g,its discharge specific capacity can still reach 124.9mAh/g,which shows better multiplier performance.The results of AC impedance spectroscopy show that the charge transfer resistance of LiV2.94Nb0.06O8 material is significantly lower than that of undoped LiV3O8 material.After 50 cycles,its Rct resistance value only changes from 34.68Ω to 42.54Ω,Nb element can stabilize the structure of LiV3O8 material.In addition,the diffusion coefficients of the four samples were calculated,and the maximum Li+diffusion coefficient of the LiV2.94Nb0.06O8 electrode material was 7.29×10-15 cm2/s.It shows that the proper doping of Nb element can increase the diffusion coefficient of lithium ions.