Modification Of LiNi_0.9Co_0.04Mn_0.03Al_0.03O₂ Cathode Materials For Lithium Ion Battery

Nickel-rich layered oxide cathodes,which have a reversible capacity of over 200m Ah·g^-1and a high operating voltage are considered as an adequate cathode material to meet market demand.However,traditional Ni-rich layered oxide cathodes will inevitably suffer from rapid structural degradation and capacity decline during the cycles,which becomes a stumbling block in its commercial application.This research adopts cation doping to enhance the properties of the LiNi_0.9Co_0.04Mn_0.03Al_0.03O₂cathode material,the results are as following.P doping narrows the band gap of NCMA,weakens the energy required for electronic transition between valence and conduction band,improves the conductivity of the material,which curtails the increment of internal resistance during cycling and inhibits thermal expansion,and finally enhances the cycle performance of the material.In addition,the addition of phosphorus expands the thickness of lithium layer and improves the rate performance of the material.Besides,different doping amounts of phosphorus have unequal effects on enhancing the electrochemical performance of NCMA.The doping rate of 2%P had the greatest effect on the first specific capacity(217.9m Ah·g^-1vs.213.9m Ah·g^-1).It also presented supreme capacity retention(93.7%/100^th)and rate performance(5C/162.2m Ah·g^-1).Subsequently,it was found that Ta-NCMA cathode material showed an excellent reversible capacity and capacity retention at a high cut-off voltage of 4.5V.The introduction of strong Ta-O bond inhibits the harmful effects of irreversible H2-H3 phase transition.The volume variation of the cell decreases during charging,which stabilizes the structure.Finally,Ta-NCMA material presents a high capacity retention rate of 91.8%at 4.5V cutoff voltage.In addition,the addition of Ta expands the lithium layer space and improves the lithium ion diffusion ability.This gives it a high rate performance of 167.2m Ah·g^-1under 5C.Finally,the DFT calculation were adopted to predict the change of electronic structure after tungsten doping,and the optimization effect of tungsten on the electronic structure of the original cathode material was proved through conventional electrochemical testing methods.The band gap of the material was reduced（1.3857e V vs.1.2479e V）,and the electron density near the Fermi level was improved to some extent.This enhances the electronic conductivity of the material.This lifting restrains the polarization of the material’s electrodes,thus slowing the decline of the material’s capacity during long cycles.Its capacity retention increased from the initial 32.3%to 95%.At the same time,tungsten doping also enhanced the ionic conductivity of the material,making the specific discharge capacity of the material increased from the initial 94m Ah·g^-1to 155m Ah·g^-1at the high rate of 5C.