Preparation,Surface Modification And Electrochemical Performance Of Lithium-rich Cathode Material Li_1.2Mn_0.6Ni_0.2O₂

Under the circumstances that it is difficult for current lithium-ion battery cathode materials to achieve a breakthrough in energy density,lithium-rich cathode materials with the discharge specific capacity more than 250 m Ah g^-1 are regarded as a hot candidate for the next generation cathode materials of lithium-ion batteries.Meanwhile,due to the pursuit to lower cost and the harm caused by Co itself to material performance,lithium-rich materials are developing towards low-cobalt and cobalt-free.However,the inherent defects that limit the commercial application of lithium-rich cathode materials have nothing to do with Co.On the one hand,the irreversible release of lattice oxygen and transition metals migration caused cations mixed dislocation and the collapse of the layered structure,which led to the rapid decay of the capacity and voltage among cycles.On the other hand,residual lithium on the surface and low conductivity further hinder the development of the material.In this work,cobalt-free Li_1.2Mn_0.6Ni_0.2O₂ cathode materials were synthesized by co-precipitation method suitable for mass production,following the Co-free trend of lithium-rich materials.And through SEM,XRD,electrochemical testing and other methods to explore the influence laws of the precursor drying temperature,lithium content and roast temperature on the final cathode material,in order to obtain Li_1.2Mn_0.6Ni_0.2O₂ cathode material with the most excellent electrochemical performance.The results show that the ideal lithium-rich cathode materials can be prepared at the precursor drying temperature of 120℃,lithium content of 1.20 and roast temperature of 850℃.At the same time,this work attempts to use SO₃ anion substitution at high temperature to form a lithium-containing functional layer on the surface of the lithium-rich Li_1.2Mn_0.6Ni_0.2O₂ cathode particles to realize the reuse of the residual lithium on the surface.The surface functional layer mainly performs four functions.The first is to significantly reduce the surface Li OH and Li₂CO₃ content,effectively inhibit the formation of non-conductive SEI film,and slow down the increase in charge transfer resistance of the material during the cycle.The second is to change the electrochemical stable state of the material under 1 C current,guide its phase evolution process,and promote the material to form a more stable structure,improve its capacity retention rate of the materials after 300 cycles reaches 93.3%.The third is to improve the Li⁺diffusion kinetics and the high-rate performance of the materials significantly.The fourth is to suppress the process of the lattice oxygen in oxidation state pairing to generate O₂ during the first charging process,and promote the first coulombic efficiency of the material to 84.4%.