Composition Optimization Of Li-Rich Manganese-Based Layered Oxide Cathode Materials And The Modification Studies

Li-rich manganese-based layered oxides,which can be designed as Li₂Mn O₃·（1-x）Li MO₂（M=Ni,Co,Mn,etc.）,have been regarded as a solid solution that composed of Li₂Mn O₃and Li MO₂with different molar ratios and a complete layered structure.This kind of cathode materials have been intensively studied in recent decade because of its advantages such as high energy density,low cost and environmental friendliness.Owing to the defects such as low initial coulomb efficiency,poor rate performance and voltage degradation during cycling,they cannot be successfully applied commercially up to know.In order to overcome these defects,the composition is optimized by regulating the proportion of Li₂Mn O₃and Li Mn O₂in this paper.The optimized sample is modified via co-doping of F^-/K⁺ions,and then carbon coating of the co-doped sample to explore the effects of co-doping and carbon coating on the electrochemical properties of Li-rich cathode materials.The main contents include:Firstly,by controlling the proportion of lithium and manganese sources,three molar ratios of Li-rich manganese-based cathode materials were prepared by a solid-phase method.SEM and TEM images show that three samples are microspheres composed of nanoparticles.XRD patterns indicate that all samples have a layered structure,meanwhile the 5-LMO sample has the best crystallinity and high purity.The electrochemical test results show that the 5-LMO has the best cycling stability.It can deliver the maximum charge/discharge capacity of 266/187 m Ah g^-1and an initial coulombic efficiency of 70.3%at 0.1C rate.After 100 cycles,the discharge capacity retains 78 m Ah g^-1and the retention rate of is 41.7%.At the rates of 0.1 C,0.2 C,0.5C,1 C and 5 C,the specific discharge capacities output by 5-LMO sample are 158,117,63,25 and 10 m Ah g^-1,respectively.The discharge capacity recovers to 126.2m Ah g^-1when the current density returns to the initial 0.1 C rate.The results indicate that the electrochemical properties of 5-LMO are unsatisfied at this stage,which need modified furthermore.Secondly,the 5-LMO（0.5Li₂Mn O₃·05Li Mn O₂）was co-doped with F^-/K⁺,and the doping amounts were 0,1%,3%and 5%.SEM and TEM images display that the four cathode materials doped with various F/K contents are still microspheres composed of nanoparticles.Meanwhile,the sample particles doped with 3%F/K（LMO-F/K3）are smaller in size and more regular in shape.XRD results showed that F^-and K⁺are successfully doped into the material.The electrochemical test results indicate that the LMO-F/K3 has the best electrochemical performance.It can deliver the initial charge/discharge capacities of 266/205 m Ah g^-1at 0.1C rate accompanying with an initial coulombic efficiency of 77.1%.The discharge capacity remains 150m Ah g^-1after 100 cycles at 0.1C rate.The discharge specific capacities at the rates of0.1 C、0.2 C,0.5 C,1 C and 5 C are 186,171,124,73 and 40 m Ah g^-1,respectively.When the current density returns to 0.1C rate,the discharge specific capacity goes back to 197 m Ah g^-1.The results demonstrate that co-doping of F^-and K⁺can improve the structural stability of 5-LMO materials,suppress the degradation of voltage platform,and enhance cycling performance and rate capability.Finally,in order to further improve the electrochemical performance of the LMO-F/K3 sample,citric acid was used as carbon source to coat the LMO-F/K3particles.SEM and TEM images show that the morphology of the coated material（C@FK-LMO）is not changed.XRD patterns reveal that the structure maintains after carbon coating.Electrochemical tests display that the charge-discharge capacities and cycling stability of the carbon coated material are significantly improved.The capacity retention reaches 90.4%at 0.1C rate after 100 cycles.The C@FK-LMO sample can deliver the specific capacities of 229,202,161,120 and 66 m Ah g^-1at the rates of 0.1 C,0.2 C,0.5 C,1 C and 5 C,respectively.When the current density returns to 0.1C rate,the specific discharge capacities recovers to 220 m Ah g^-1.The results indicate that the carbon-coated sample has an significantly improved electrochemical performance.The results suggest that the components in the Li-Mn-rich cathode materials show their own advantages and interact with each other.The electrochemical performance is the best when the molar ratio of Li₂Mn O₃to Li Mn O₂is 1:1.The electrochemical properties and structural stability were improved greatly after the Li-Mn-rich cathode materials were co-doped with F^-/K⁺ions.Meanwhile,F^-ion doping inhibited cation mixing while K⁺ion doping expanded the lithium layer spacing,promoting the mobility of electrons and ions.Carbon coating can not only provide a physical barrier on the surface of cathode material,but also construct three-dimensional electron-conductive networks within the active particles,thus greatly improving the rate capability and cycling performance.