Synthesis And Modification Of LiNi_0.3+yCo_{（0.7-y）/2}Mn_{（0.7-y）/2}O₂

With increasing energy consumption,it is particularly important to develop sustainable new energy sources.Rechargeable lithium-ion batteries have become a research hotspot as a green new energy storage device.In this dissertation,LiNi_0.3+yCo_{（0.7-y）/2}Mn_{（0.7-y）/2}O₂ ternary cathode material was synthesized using the coprecipitation method.It can be used as a cathode material for lithium-ion battery.By adjusting the proportion of Ni,Co and Mn and the synthesis process conditions to study the changes in the electrochemical performance of materials,it was concluded that the electrochemical performance of LiNi_0.5Co_0.25Mn_0.25O₂ was the best.Aiming at the problems of poor rate performance and low cycle stability caused by cation misalignment and phase change of LiNi_0.5Co_0.25Mn_0.25O₂,the doping modification of metal ions was studied.And the structure,morphology,composition and electrochemical performance of the modified material were characterized.The precursors were prepared by using nickel,cobalt and manganese sulfate as raw materials,NaOH as precipitating agent,and ammonia as complexing agent.The precursors were ball-milled,pre-calcined and calcined to obtain materials,and their electrochemical properties were studied.The results show that the electrochemical performance of LiNi_0.5Co_0.25Mn_0.25O₂ which is synthesized under the conditions of ammonia concentration of 0.4 mol/L,pre-calcination temperature of 450℃,calcination temperature of 850℃and calcination time of 12 h is better.And LiNi_0.5Co_0.25Mn_0.25O₂has anα-NaFeO₂ structure and no other impurity phases;the valences of transition metal ions are Ni²⁺/Ni³⁺,Co³⁺,and Mn⁴⁺.When the voltage range is 2.7-4.3 V and the discharge current density is 0.1C,the specific discharge capacity of the material is 178.9mAh/g;at1.0C,the specific discharge capacity is 110.7mAh/g,after 10 cycles,the specific discharge capacity is 104.8mAh/g,capacity retention rate is 94.7%.LiNi_0.5Co_0.25Mn_0.25O₂ was doped with Zr⁴⁺to obtain Li(Ni_0.5Co_0.25Mn_0.25)_0.99Zr_0.01O₂material.Its specific discharge capacities are 151.0mAh/g and 138.9mAh/g at 0.5C and1.0C,respectively.After large-rate discharge,the discharge specific capacity is 173.3mA/g when the current density returns to 0.1C.It is cycled 45 times at a current density of 0.1C,and the first discharge specific capacity is 179.6mAh/g,and the cycle retention rate is 87.5%.This is because Zr⁴⁺doping stabilizes the structure of the material and expands the interlayer distance,thereby improving the electrochemical performance of the material.Co-doped with Al³⁺and Zr⁴⁺to obtain Li(Ni_0.5Co_0.24Mn_0.25Al_0.01)_0.99Zr_0.01O₂material,its specific discharge capacities are 182.0mAh/g,148.3mAh/g,123.5mAh/g and 110.3mAh/g at 0.1C,0.2C,0.5C,and 1.0C,respectively.After 45 cycles at 0.1C,the capacity retention rate is 49.0%.LiNi_0.5Co_0.25Mn_0.25O₂ was doped with Mg²⁺and Nb⁵⁺,and the changes of the electrochemical properties of the obtained materials were studied.Appropriate amount of Mg²⁺can reduce the degree of mixed cations,so better layered structure material was obtained,and the cycle stability of the material is ultimately improved.Li(Ni_0.5Co_0.25Mn_0.25)_0.98Mg_0.02O₂ have a charge specific capacity of 193.7mAh/g and discharge specific capacity was 177.3mAh/g for the first time at 0.1C,the coulomb efficiency was 91.6%.Nb⁵⁺doping will not damage the crystal structure.The increase of unit cell parameters which are a,c and V are beneficial to the diffusion of lithium ions.Furthermore,Li(Ni_0.5Co_0.25Mn_0.25)_0.99Nb_0.01O₂ has more lattice oxygen and less oxygen on the surface lithium salt impurities than LiNi_0.5Co_0.25Mn_0.25O₂.Li(Ni_0.5Co_0.25Mn_0.25)_0.99Nb_0.01O₂ material has excellent rate performance,and the specific discharge capacities at 0.1C,0.2C,0.5C and 1.0C are 204.6mAh/g,186.0mAh/g,163.5mAh/g and 141.6mAh/g,respectively.When the current density returns to 0.1C again,the discharge specific capacity is as high as 174.1mAh/g.