Synthesis And Modification Of Co-free And Low-Ni LiNi_0.5Mn_0.5O₂ Cathode Material

Lithium-ion batteries（LIBs）,as an important energy storage device,have been widely used in new energy vehicles,mobile phones and laptops.The high nickel layered LiNi_xCo_yMn_zO₂（x≥0.6,z=1-x-y）cathode materials have attracted widely attention because of its high capacity and good cycle performance.It is generally believed that Ni is the active element in high nickel cathode materials,which provides charge compensation in the electrochemical reaction process.The higher the content of Ni in the material,the higher the specific capacity of the material.Therefore,studies on high-nickel cathode materials have developed from x=0.6 to 0.7,0.8 and even greater than0.9.However,Ni mainly exists in the form of Ni³⁺in high nickel cathode materials and the charge compensation in the process of Li⁺deintercalation is realized by the single-electron electrochemical reaction(Ni³⁺/Ni⁴⁺).However,Ni can realize multi-electron electrochemical reaction(Ni²⁺/Ni⁴⁺)to provide charge compensation in the process of Li⁺deintercalation in the medium and low nickel LiNi_0.5Mn_0.5O₂cathode material.It has similar theoretical capacity with high nickel cathode materials.In addition,LiNi_0.5Mn_0.5O₂ cathode material has low element cost,higher potential than the high nickel cathode materials at high delithium state and thereby high specific energy.It is considered as a promising high specific energy LIBs cathode material that can replace the current high nickel cathode materials.However,LiNi_0.5Mn_0.5O₂ cathode material has serious Li/Ni mixing and poor cycle performance at high operating voltage.Based on this,LiNi_0.5Mn_0.5O₂ cathode material was modified from two perspectives of ion doping and surface coating in this paper.The main research contents are as follows:（1）LiNi_0.5Mn_0.5O₂ and LiNi_0.5Mn_0.5-xSi_xO₂ cathode materials were prepared by sol-gel method combined with high temperature solid phase method.The effects of Si doping on the crystal structure,surface morphology and electrochemical properties of LiNi_0.5Mn_0.5O₂ were investigated.The results showed that Si element can be doped into the lattice of layered structure and the LiNi_0.5Mn_0.5-xSi_xO₂（x=0.01,0.02 and 0.03）materials all have well-orderedα-NaFeO₂ layered structure.The electrochemical test results showed that the initial discharge capacity of LiNi_0.5Mn_0.5O₂material was 170.6mAh·g^-1 at 0.1 C between 3.0-4.7 V,while the LiNi_0.5Mn_0.48Si_0.02O₂ material showed excellent electrochemical performance with the discharge capacity of 193.4 mAh·g^-1under the same conditions.This was mainly attributed to the fact that Si doping inhibits Li⁺/Ni⁺cations mixing and improves the structural stability of the material.（2）The LiNi_0.5Mn_0.5O₂ cathode material was prepared by coprecipitation combined with high temperature solid phase method and Ta₂O₅ coating was performed on LiNi_0.5Mn_0.5O₂ cathode material.The effects of Ta₂O₅ coating on the crystal structure,surface morphology and electrochemical properties of LiNi_0.5Mn_0.5O₂ were investigated.The results showed that the Ta₂O₅ coating has no significant effect on the layered structure of the cathode material.The electrochemical performance of the cathode material was improved after Ta₂O₅coating,which mainly attributed to the Ta₂O₅ coating layer protects the cathode material from the electrolyte erosion,reducing the dissolution of transition metal ions and the occurrence of side reactions.When the coating amount is 1 wt.%,the initial discharge capacity of the material reached 200.4mAh·g^-1 at 0.1 C between 3.0-4.7 V.The discharge capacity and capacity retention rate of the material was 191.2 mAh·g^-1and 98.9%after 100 cycles at 0.2 C,while the uncoated material was 157.3 mAh·g^-1 and 91.5%,respectively.In conclusion,the LiNi_0.5Mn_0.5O₂ cathode material was modified by Si doping and Ta₂O₅ coating in this paper,respectively.Through the comparative study of the structure and electrochemical properties of the material before and after modification,it was found that Si doping can inhibit the Li⁺/Ni²⁺cations mixing in the material and improve the structural stability of the material.Ta₂O₅ coating can protect the cathode material from the erosion of electrolyte and restrain the interface side reaction.Therefore,two modification methods are beneficial to realize multi-electron electrochemical reaction of LiNi_0.5Mn_0.5O₂ cathode in the process of lithium declination,improving the specific capacity and cycle performance of the material.These studies provide a reference for improving the properties of LiNi_0.5Mn_0.5O₂ cathode material.