Modification Of Cathode Materials For Vanadium-based Secondary Lithium-ion Batteries

At present,lithium-ion batteries are widely used in the fields of electronic products,power energy,and other fields due to their advantages.Among them,the electric new energy vehicle industry is developing rapidly.However,the capacity of commercial lithium-ion batteries is currently difficult to meet people’s requirements for the long-term durability of new energy vehicles.The capacity of lithium-ion batteries is mainly provided by cathode materials.Therefore,the study of cathode materials for lithium-ion batteries with high capacity and good cycle performance is of great significance for the development of the new energy vehicle industry.Among the cathode materials for lithium-ion batteries,vanadium based materials have received extensive attention from researchers due to their large resource reserves,multiple variable valence states and low cost.However,most vanadium based cathode materials are lithium insertion/lithium deintercalation materials.During the charging and discharging process,the lithium-ion transmission resistance is high,and the capacity decline is fast.Due to the lack of active lithium in the material,it is necessary to discharge first and then charge.The anode materials must be metal lithium or lithium alloy,and can not use graphite anode electrode.However,commercial cathode materials are lithium intercalation/lithium deintercalation materials,that charge first and then discharge,which fundamentally limits the development of vanadium based cathode materials.The results show that the carbothermal reduction method can effectively increase the material activity,improve the electronic conductivity and surface active site of the material,increase the lithium ion diffusion channel,and can be used to synthesize ithium intercalation/lithium deintercalation vanadium cathode materials.However,this method requires a high reaction temperature and takes a long time.At the same time,the material has poor uniformity in morphology and severe capacity degradation.In order to solve these problems,this paper aims for lithium intercalation/lithium delithiation vanadium cathode materials with high capacity.Several lithium intercalation/lithium delithiation vanadium composite metastable cathode materials were synthesized by the sol gel method,hydrogen reduction plus lithium salt calcination method,and the phase analysis,morphology observation and electrochemical performance test of the composite materials were carried out.The structure-activity relationship of the composite materials was explained,and the performance optimization was achieved.The composite materials prepared contain active lithium.This work provides valuable research results for the development of vanadium based cathode materials.The specific research contents are as follows:(1)Different kinds of vanadium composite cathode materials Li-V-M-O(M=Zn,Mn,Fe,Ni,Cr)were synthesized by sol gel method,atmosphere reduction plus lithium salt calcination method.The results show that the composite materials all contain active lithium.When the ratio of V/Zn is 1:0.5,the electrochemical performance of the vanadium zinc lithium composite material Li-V-Zn-O synthesized after reduction at 480 ℃ is the best.The initial discharging capacity is 134.1 m Ah/g,and the initial coulombic efficiency is93.32%.The coulombic efficiency during the cycle is above 95%,which preliminarily proves that the vanadium based composite material has commercial application potential.(2)On the basis of single element composite,vanadium based composite cathode materials Li-V-Fe-M-O(M=Ni,Zn)were synthesized by sol gel method and atmosphere reduction plus lithium salt calcination method.The effects of different composite proportions and synthesis conditions on the properties of the composite materials were explored.The experimental results show that When the ratio of Fe/Zn is 1:1,the composite material Li-V-Fe-Zn-O synthesized after reduction at 450 ℃ has the best cycling performance.After 20 cycles of charging and discharging,the discharge capacity has no decay.When the ratio of Fe/Ni is 1:1,the capacity of the vanadium iron nickel lithium composite material Li-V-Fe-Ni-O synthesized after reduction at 430 ℃ was significantly increased,with an initial discharging capacity of 246.2 m Ah/g and a coulombic efficiency of over 90% during the charging and discharging process.Compared to the discharging capacity of the vanadium zinc lithium composite material Li-V-Zn-O of 134.1 m Ah/g,the improvement was significant.(3)On the basis of the above research,adding P,Zn,Cr and Mo into the precursor of vanadium iron nickel,and controling the proportion of composite elements.Lithium intercalation/lithium delithiation vanadium based cathode materials Li-V-Fe-Ni-M-O(M=Zn,P,Mo,Cr)with different element combinations were prepared.The experimental results show that the Li-V-Fe-Ni-Mo-O composite material has the best cycling performance,with a discharge capacity retention rate of 99.82% after 30 cycles;when the ratio of V/Cr is 4:1,the initial specific discharging capacity of the vanadium iron nickel chromium lithium composite material Li-V-Fe-Ni-Cr-O synthesized after reduction at 480 ℃is 301.4 m Ah/g,and the first 10 discharge capacities are all above 200 m Ah/g.Compared with the vanadium iron nickel lithium composite material Li-V-Fe-Ni-O,the discharging capacity and coulombic efficiency have been significantly improved.