Suppression Of Li/Ni Mixing In Li-Rich Manganese-Based Cathode With Surface Engineering And Trace Cobalt Doping

Lithium-ion batteries have been widely used in many aspects of life,but with the development,the new energy field especially electric vehicles has put forward higher requirements for power batteries with the higher energy density and cycle performance.Thus,the lithium-ion battery has become one of the current research hotspots.Currently,one of the main factors limiting the electrochemical performance of Li-ion batteries is the cathode materials.Among them,Li-rich manganese-based materials have attracted much attention due to high theoretical specific capacity(250 m Ah g^-1)and good thermal stability.Based on the redox of oxygen and transition metal ions,Li-rich manganese-based materials can deintercalate more Li⁺to provide ultra-high capacity.However,due to the shortcomings of fast capacity loss,voltage hysteresis and poor rate performance,in order to meet the requirements of commercial applications,Li-rich manganese-based cathode materials still need to be further explored and improved.In this thesis,Li_1.2Ni_0.2Mn_0.6O₂,a lithium-rich manganese-based cathode material,is used as a template,and jointly modified with surface engineering and trace cobalt doping.The main research contents are as follows:（1）A trace amount of cobalt-doped sample Li_1.2+xNi_0.185Co_0.01Mn_0.6O₂（x=0～0.005）is prepared by the sol-gel method.Doping a trace amount of Co can inhibit the Li/Ni mixing in the positive electrode particle phase.At the same time,trace cobalt doping helps to maintain the stability of the layered structure due to the formation of uniform Co-O bonds in the bulk phase.In addition,due to the trace amount of cobalt doping,the rock-salt layer will not be excessively thinned,leading to the improved cycle performance of the cathode material.Therefore,the strategy of trace cobalt doping can not only save the cost,but also greatly reduce the damage of cobalt to the cathode material structure under high voltages.（2）The ammonium molybdate and trace cobalt-doped samples are uniformly mixed by solid-phase milling,and then annealed at high temperatures.Through surface modification,the disordered rock-salt shell on the surface of the sample can be transformed into a cationically ordered spinel phase and layered phase,and a Li₂Mo O₄nanolayer is simultaneously formed on the surface of the sample.Although it is difficult to uniformly coat the surface of the sample by solid-phase grinding,ammonium molybdate and the surface of the material can be annealed at high temperatures to form an eutectic solution,which helps to form a uniform coating layer under the action of surface tension.Meanwhile,the solid-phase method avoids the defects of the wet coating,where the solvent causes lithium leakage and changes the surface stoichiometry of the cathode material.（3）The results of XRD refinement show that the combined modification of surface engineering and trace cobalt doping can effectively reduce the Li/Ni mixing level of Li-rich manganese-based cathode materials.The proportion of Ni²⁺in the Li layer in Li_1.2Ni_0.2Mn_0.6O₂ is 4.57%,while in the modified sample,the proportion of Ni²⁺in the Li layer is reduced to 1.22%.The XPS characterization results after ion etching showe that the valence states of Ni and Mn are both reduced.In addition,the molybdenum ions also emerge,demonstrating that high temperature annealing can induce the conversion of surface coating to surface doping.The XRD refinement results also show that the occupied transition metal layer contains a small amount of Mo⁶⁺and leads to a significant increase in S_TMO2.This indicates that a small amount of Mo⁶⁺is doped into the TM layer during the high temperature annealing.