Study On Charge-discharge Performance And Structural Stability Of Ternary Cathode Materials For Lithium-ion Batteries

With the new energy vehicles into the market,the energy density,lifespan and safety of the batteries have attracted more attention in recent years.The development of new NCM ternary materials with high capacity,long life and good safety has become a current research hotspot.Aiming to improve the cycle life and thermal safety,this paper investigated the preparation,structure and properties of Li[Co_2xNi_0.5-xMn_0.5-x]O₂ and Li[Ni_0.6Co_0.2Mn_0.2]_1-xM_xO₂（M=Ga,Cr）cathode materials,respectively,based on the improvement of structural stability during the charge and discharge process.The following conclusions are obtained:（1）Li[Co_2xNi_0.5-xMn_0.5-x]O₂（x=1/6,0.2,0.25,0.3,0.35,0.4）materials with well layered structure were successfully prepared by rheological phase method,where the sample with x=0.25 has the best layered structure.The DSC results showed that the thermal stability of the sample with x=0.25 was also the best.The exothermic peak temperature of the material in charged state（4.5 V）increased by 27.16℃,and the heat release reduced by 377.8 J g^-1 while compared with that of the Li[Co_1/3Ni_1/3Mn_1/3]O₂ material.Electrochemical performance analysis revealed that as the cobalt content increases,the sample with Co content≤0.5（x≤0.25）material exhibited a decreased interface impedance,and enhanced capacity and cycling performance;however,the sample with Co content≥0.6（x≥0.3）displayed an increased interface impedance,and declined cycling stability.The sample with x=0.25 showed the best electrochemical performance.Its discharge ratio capacity of 3.0 to 4.5 V can reach 188.1 m Ah g^-1（0.2C）and 122.8 m Ah g^-1（5C）,0.5C after 50 and100 cycles capacity retention rate of up to 99.6%and 98.61%,respectively,showing excellent cycling performance under high cut-off voltage.In-site XRD analysis displayed that during the charging process in the range of3.0～4.5 V,the total unit cell volume change reduced from 1.21%for Li[Co_1/3Ni_1/3Mn_1/3]O₂ to 0.57%for Li[Co_0.5Ni_0.25Mn_0.25]O₂.The cell volume change decreased by 52.9%,indicating a remarkable improved structural stability.This should be the main reason why the material with x=0.25 has excellent cycling performance and thermal stability under high cut-off voltage.（2）The Li[Ni_0.6Co_0.2Mn_0.2]_1-xGa_xO₂ and Li[Ni_0.6Co_0.2Mn_0.2]_1-xCr_xO₂（x=0,0.01,0.02,0.03,0.05）materials with pureα-NaFeO₂ structure were prepared by high temperature solid phase method.Among them,Ga³⁺doping showed a significant effect in performance improvement.When the Ga³⁺content x=0.02,it was not only beneficial to promote the growth of crystal grains,but also reduce the charge transfer resistance,improve the stability of the interface structure and the body structure,thereby significantly improving the electrochemical performance and thermal safety.Compared with that of NCM622 raw material,the discharge capacity of Li[Ni_0.6Co_0.2Mn_0.2]_0.98Ga_0.02O₂ increased from 169.3 m Ah g^-1 to177 m Ah g^-1 at 0.5C rate,and the capacity retention increased from 82.8%to 89.8%after 50 cycles,in addition,its exothermic temperature increased from 245.13℃to above 271.24℃,and the total exotherm decreased from561.7 J g^-1 to 225.6 J g^-1 for the charged materials（4.3 V）.In-site XRD analysis showed that the total unit cell volume change of Li[Ni_0.6Co_0.2Mn_0.2]_0.98Ga_0.02O₂ during the charging process in the range of3.0～4.3 V was 1.94%,which was less than 2.6%of the NCM622 material,in addition,the unit cell volume change in the rapid shrinkage area along c direction（the y of Li_yMO₂ decreases from 0.45 to 0.3）was 0.72%,which is much smaller than 1.33%of the NCM622 material.This indicates that appropriate amount of Ga³⁺doping makes the structure of the cathode material more stable and the cycling performance is better.On the contrary,the modification effect of Cr³⁺doping is unsatisfactory.A small amount of Cr³⁺（x=0.02）doping could improve the layered structure and increase the initial discharge capacity,bur it significantly increased the charge transfer resistance and deteriorated the cycling performance and thermal stability of the material.Compared with that of undoped sample,the exothermic temperature of the sample with x=0.02 in charged state（4.3 V）decreased from 345.13℃to 232.82℃,and the total exotherm increased from 561.7J g^-1 to 707.3 J g^-1.XPS analysis confirmed that there were more unstable Cr⁶⁺generated during the charge and discharge process,which probably accelerated the structural instability,increased the charge transfer resistance,hindered the re-intercalation of lithium ions,and thus deteriorated the cycling performance and thermal stability.