Research On Modification And Electrochemical Performance Of Layered Li-rich(Li_1.2Mn_0.54Ni_0.13Co_0.13O₂) Cathode Material For Lithium Ion Batteries

The lithium-ion batteries has various advantages,high voltage,high specific capacity,high energy density,no memory effect,longer cycle and calendar life,environment friendly and so on.Therefore,the lithium-ion battery(LIB)is widely used in electric vehicle and hybrid electric vehicle.Cathode materials are the key materials for LIBs;they play the crucial role for the performance of LIB.Lithium rich layered oxide has attracted many attentions,because of its high specific capacity(>250 m Ah g-1),which could meet the requirements of energy density for lithium-ion battery as a power battery.However,there are still several challenges to overcome before their practical applications,including the high irreversible capacity loss in first charge-discharge process,rate performance and poor cyclic stability.Many strategies have been investigated to overcome this problem,such as metal ions doping,surface modification and particle size controlling.In this paper,based on the detailed summary of development of the cathode materials of lithium ion batteries,Li1.2Mn0.54Ni0.13Co0.13O2 was prepared by sol-gel method and coprecipitation method,and investigated the effect of the nano-particles,modification of fluorine-doped carbon,surface modification of Gadolinium doped cerium and surface modification of lithium iron phosphate on the microstructure,structure and electrochemical properties of the material.The Li1.2Mn0.54Ni0.13Co0.13O2 synthesized with different organic acids of citric acid,tartaric acid,citric acid & tartaric acid,and taking XRD,FTIR,Raman spectroscopy,SEM,TEM,charging and discharging test,CV and EIS are used in this investigation.The results confirm that the composite organic acid have stronger steric hindrance effect which effectively control the growth of particles.Li1.2Mn0.54Ni0.13Co0.13O2 shows excellent electrochemical performance with a discharge capacity of 263.1 m Ah g-1 at 0.1 C and 187.9 m Ah g-1 at 5 C.But Li1.2Mn0.54Ni0.13Co0.13O2 exhibits the poor cycle performance with the capacity retention of 71.2%.The citric acid is mixed with all metal salts firstly,the citric acid interacts with all the raw material and form a big gap between each other,and then tartaric acid can fill into the gap between citric acid after interacting with the metal salts,which will enhance the space steric effect and facilitate to effectively control growth of particles.As the particle size can only improve the material magnification performance,the Li1.2Mn0.54Ni0.13Co0.13O2 material synthesized by sol-gel method was modified by fluorine-doped carbon,and taking XRD,Raman spectroscopy,SEM,TEM,charging and discharging test,CV and EIS are used in this investigation.The results show that the modification of fluorine-doped carbon can improve the rate performance and cycle performance.The Fluorine-Doped carbon-modified sample exhibited better rate performance with discharge capacity of 284.5 m Ah g-1 and 79.3 m Ah g-1 at 0.1 C and 10 C.Moreover,as prepared sample show excellent cycling stability with the capacity retention is 80.2% at 5 C after 500 cycles.Fluorine-Doped carbon exist the particle surface and between the particles and the fluorine doping result in the carbon more electron-rich,and the electron clouds bias to bulk LMNCO,and form a strong electronic coupling between each other,which can improve the electrical conductivity of the particles surface and between the particles,and increase the bonding force between F-doped carbon layer and bulk LMNCO,and thus effectively suppressed the reaction between the electrode and electrolyte.Nano-particles only improve the magnification performance,and the fluorine-doped carbons only improve the rate performance and cycle performance.Li1.2Mn0.54Ni0.13Co0.13O2 material synthesized by the coprecipitation method was surface-modified by gadolinium-doped ceria,and taking XRD,Raman spectroscopy,SEM,TEM,charging and discharging test,CV and EIS are used in this investigation.The results show that the surface modification of gadolinium-doped cerium dioxide can improve the initial coulombic efficiency,rate performance and cycle stability.The initial columbic efficiency of 3% gadolinium-doped ceria coated sample can be as high as 83.3%.The 3% gadolinium-doped ceria coated sample exhibited better rate performance with discharge capacity of 267.5 m Ah g-1 and 158 m A h g-1 at 0.1 C and 5 C.Moreover,as prepared sample show excellent cycling stability with the capacity retention is 92.9% and 89.3 % at 0.5 C and 1 C after 100 and 200 cycles.Gadolinium doped cerium oxide is uniformly coated on the surface of the particles.Gadolinium doped cerium oxide has a large amount of oxygen vacancy,uniformly coated on the surface of the particles,and it could suppress the disappearance of oxygen on the surface of the material in the sintering process which provides active sites for electronic and lithium ion transport.The coating layer also has an oxygen storage capacity that can inhibit reduction of the oxygen vacancy and the Mn4+ during the initial charging process.In addition,the coated layer can prevent occurrence of side effects between the electrode material and the electrolytic solution,and inhibit the formation of the inactive surface layers of Li F and Li2CO3.The coated layer also could buffer material volume changes and prevent the collapse of the material particle structure during the cycling.In order to suppress the voltage attenuation of Li1.2Mn0.54Ni0.13Co0.13O2 during the cycle,the sample synthesized by sol-gel method was modified by lithium iron phosphate,and taking XRD,Raman spectroscopy,SEM,TEM,charging and discharging test,CV and EIS are used in this investigation.The result shows that the surface modification of lithium iron phosphate can improve the the initial coulombic efficiency,rate performance,cycle stability and the suppression of the voltage attenuation during the cycling.3% lithium iron phosphate coated sample was greatly improved,from 74.5% to 81%.The discharge capacity of 3% lithium iron phosphate coated sample reach 282.8 m Ah g-1 and 125.3 m Ah g-1 at 0.1 C and 0.5 C,respectively.The discharge capacity of 3% lithium iron phosphate coated sample reach 249.8 m Ah g-1 and 139.9 m Ah g-1 with capacity retention of 92.5% and 83.6% respectively after 100 cycles at 0.5 C and 2 C,and suppress the voltage decay.The lithium iron phosphate with good lithium ion transport and electronic conductivity uniformly coated on the surface of the particles,and a certain amount of Fe2+ /Fe3+ and PO43-diffused into the secondary surface of the bulk material during the high temperature to form the transition layer.After high temperature calcination,Fe ions would occupy the Li+ sites in the transition layer of the layered structure on citric acid treated LLMO surfaces and PO43-polyanions would occupy the cubic close-packed oxygen sites,thus can increase the electrical conductivity of the material electron and lithium ion and suppress the transformation of layer structure to the spinel structure.PO43-polyanions occupy the cubic close-packed oxygen sites to suppress the disappearance of oxygen vacancy during the charge process.In addition,lithium iron phosphate could inhibit the side reaction between sample and the electrolyte.