Modification And Electrochemical Properties Of Cobalt-free Cathode Materials For Lithium Ion Batteries

In recent years,with the rapid growth of electric vehicles,global cobalt has been greatly consumed.If the amount of cobalt is not reduced or recycled,there will be no cobalt available in the next few decades.In addition,the global distribution of cobalt mining is very uneven,the supply chain is unstable,and cobalt price is rising year by year.The development of low-cobalt or cobalt-free cathodes has gradually become a consensus.Cobalt-free and high-nickel binary cathode materials(Li Ni_yMn_1-yO₂,0.6≤y≤0.9)are similar to commercial ternary cathode materials in structure,and display great potential to become the next-generation cathode materials.Taking Li Ni_0.6Mn_0.4O₂（NM）as an example,compared with the commercial cathode material Li Ni_0.6Co_0.2Mn_0.2O₂（NCM）,the material has better stability.However,the NM present lower capacity and worse rate performance because it does not contain cobalt with good conductivity,which has become the main obstacle to its commercial application.Therefore,in this paper,the surface of the NM cathode material was reconstructed,and the possibility of replacing cobalt with other elements was explored.The main contents are as follows:（1）Co can reduce the mixing of Li/Ni and improve the rate performance o f the material,but it is not conducive to the stability of the structure.A little amount of Co was used to reconstruct the surface,which not only consume d the residual lithium of the original surface that hinders lithium ion transport,but also turn ed the surface into a good lithium ion conductor Li Ni_1-x-yCo_xMn_yO₂（where,0<x<1,0<y<1,x gradually decreases from the surface to the interior）.The modified surface highly improves the dynamic performance of the electrode,while the interior still ma intains the original stable structure of the NM.The stability of the modified material was basically unchanged,and the capacity and rate performance were also greatly improved.At a current density of 1 C in a voltage range of 2.7-4.5 V,the capacity was 161.2 m Ah g^-1(NCM is 134.7 m Ah g^-1)after 100 cycles.At a current density of 1 C in a voltage range of 2.7-4.3 V,the capacity was 139.9 m Ah g^-1（the NCM is broken）after 300 cycles.（2）Under the same conditions as Co modification,the surface of the N M binary cathode material was modified with Zn,Al,Mg,Zr,Cd,etc.,and it was found that Zn can enter the NM lattice like Co.After re-sintering,surface gradient doping could be formed.Al could react with residual lithium on the surface,and a little was embedded in the original lattice.However,like Mg,it was easier to form a coating layer on the outside.Zr and Cd generated independent phase on the surface after coating and re-sintering.Elements with atomic numbers similar to Ni,Co,and Mn were more likely to enter the lattice of NM.All in all,surface doping to form a good conductive layer is suitable for NM,a material with good stability.Co is compatible with NM and can improve lithium ion conductivity.However,other elements such as Zn,Al,Mg,Zr,Cd are difficult to replace.