Structural Evolution And Electrochemical Studies Of Lithiation/Delithiation In LiNi_1-x-yCo_xMn_yO2

Lithium ion battery has been widely used in various portable electronic devices,and gradually extended to new energy vehicles,energy storage,military applications and other fields due to the merits of high energy density,long cycle life and environmentally free.Improving the energy density and reducing cost are the most challenging in development of lithium ion battery.Since the energy density of batteries greatly depends on cathode materials,it is urgent to exploit high specific energy and low cost cathode materials.Compared with traditional commercial materials like LiCoO2,LiFePO4 and LiMn2O4,LiNi1-x-yCoxMnyO2 has the advantages of high specific capacity,low price and good rate performance,which has caused extensive concern of researchers.Increasing the charge cut-off voltage or Ni content is an effective way to increase the specific capacity of ternary materials.However,this will do harm to the structural stability,cycle performance and thermal stability.Here,we choose LiNi1/3Co1/3Mn1/3O2 and LiNio.6Coo.2Mno.2O2 materials to investigate the electrochemical performance under different cut-off voltage.Besides,we systematically studied long-range structure,local structure and thermal stability during lithium insertion/de-intercalation process by using chemical delithiation,in/ex-situ XRD,6Li/1H ss-NMR and TG-MS linked techniques.For LiNi1/3Co1/3Mn1/3O2 material,when the cutoff voltage increases from 4.2 V to 4.5 V,the specific capacity increases from 141 mAh g-1 to 168 mAh g-1,but the capacity retention after 100 cycles decreases from 89%to 73%.To figure out the reason for worse electrochemical performance at high cut-off voltage,we studied LixNi1/3Co1/3Mn1/3O2(0<x?1)with different degree of lithium deintercalation.XRD results indicate that when x?0.22,the material remains hexagonal layered structure(R3m space group).As lithium ion deintercalates,the structure changes from hexagonal layered structure(R3m space group,O3 phase)to trigonal structure(P3ml space group,O1 phase)with lower symmetry.Since the phase transformation from O3 phase to O1 phase is irreversible,it is not strange to observe worse cycle performance at high cut-off voltage.6Li NMR results show that lithium exits at both lithium layer and transition metal layer.And the lithium would deintercalate completely from the transition metal layer during the lithium deintercalation.As for the lithium at lithium layer,the lithium whose local environment possesses more Mn4+ will deintercalate first,then comes the Ni2+ and finally the Co3+.Meanwhile,the TG-MS results indicate that the thermal stability is getting worse with lithium deintercalation.When x<0.22,there may be some proton absorbed on the surface of materials which would enhance the activity of lattice oxygen leading to the rapid decrease of thermal stability.This is the reason of the safety of charged materials at high cut-off voltage being worse.As for LiNio.6Coo.2Mno.2O2 material,it is found that the material calcined at 800?shows the lowest cation disorder(?1.97%)with a high initial Coulombic effiency of 92.2%and the capacity retention is 81.4%after 100 cycles.When cut-off voltage increases from 4.3 V to 4.7 V,the specific capacity increases from 172 mAh g-1 to 201 mAh g-1,but the capacity retention after 100 cycles decreased from 83.8%to 27.2%.For the chemical delithiated samples,when the lithium content is no less than 0.21,the structure would change from H1 to H2 and then H3.The unit cell parameters are changed accordingly,however the structure is still indexed to hexagonal layered structure.With lithium deintercalation,the structure changes from H3 phase to spinel-like phase(Fd3m).And the process:H2?H3?spinel-like phase is irreversible.During the electrochemical delithiation process(3.0-4.7 V vs.Li/Li+),the structure changes from H1 to H2 and then H3 with no spinel-like phase existing.6Li NMR results show that the lithium deintercalation order is the same as LiNi1/3Co1/3Mn1/3O2 and the intercalation order is just the opposite.After the first cycle,the NMR peaks of lithium become asymmetric,and the intensity of the resonance peaks located at lower chemical shift gets weaker.This may be caused by the structure whose local environment possesses more Co3+ being collapsed during the process of H2?H3 phase change.TG-MS results indicate that the thermal stability is getting worse with the lithium deintercalation.When the lithium content is no less than 0.34,proton may be absorbed on the surface of materials which would enhance the activity of oxygen thus resulting in the sharp decrease of thermal stability.Because of this,the highly charged materials'safety is poor.