Study On The Preparation And Surface Modification Of Multiple Cathode Materials For Lithium Ion Batteries

Layered LiNi_0.5Mn_0.5O₂and LiNi_1/3Co_1/3Mn_1/3O₂combines the advantages of LiCoO₂,LiMnO₂andLiNiO₂, such as low cost, high specific capacity, stable structure and high safety performance, so it isconsidered to be the most promising new material and has become a research hotspot in recent years. Thematerial has some shortcomings, for example, metal cations confusion arrangement, manganese ionsdissolved, SEI film formation and so on. It is because of these advantages of the two materials that it iswidely noticed. Its disadvantages point out the clear direction for further research.The pristine samples were prepared by coprecipitated hydroxides. The required amounts of nickelnitrate, cobalt nitrate and manganese nitrate were dissolved in deionized water. After that, solution of1Msodium hydroxide was added and NH4OH was used to adjust the PH keeping for10.5. The mixed solutionwas sufficiently stirred in argon gas. The filtered precipitation was repeated washed with distilled water anddried at120℃for12h. Then5atom%excess lithium of lithium hydroxide was thoroughly ground withthe filtered precipitation. Finally, the precursors were annealed in air at900℃for24h. The pristinesamples were obtained.In order to obtain more depth study on the electrochemical performance of the LiNi_1/3Co_1/3Mn_1/3O₂material, we chose the the YPO₄as the surface coating of LiNi_1/3Co_1/3Mn_1/3O₂. LayeredLiNi_1/3Co_1/3Mn_1/3O₂is surface coated with different contents of YPO₄（0.5wt.%,1wt.%and3wt.%）through simple chemical deposition method. The pristine and YPO₄-coated LNCMO are characterized byX-ray diffraction （XRD）, Raman spectroscopy and field emission scanning electron microscopy （FESEM）.It is found that the YPO₄surface coating layers do not change the bulk structure of LNCMO. Theinvestigation in their cycling performances demonstrates that1wt.%YPO₄-coated LNCMO exhibits thebest cycling stability, with capacity retention ratio of88.6%after50cycles, much better than that of thepristine material （only80.1%）. After50cycles, the improvement in capacities are0.46%and11.29%for0.5and1wt.%YPO₄coated LNCMO compared with that of pristine material. While, the capacity of3wt.%YPO₄coated material is only72.82%of the pristine electrode at the same condition, less than thatbefore YPO₄surface modification. Furthermore, the1wt.%YPO₄surface modification improves the rate performance and thermal stability of layered LNCMO. Cyclic voltammetry （CV） and electrochemicalimpedance spectra （EIS） of pristine and1wt.%YPO₄-coated LNCMO are also measured and compared.The enhanced electrochemical performances of the surface-modified samples are attributed to theimprovement of structural stability and the facilitation of lithium ion diffusion derived from the reactionbetween YPO₄and solid electrolyte interfacial （SEI） layer.The values of the integrated area are19.17and15.34mW/mg, for the pristine and1wt.%YPO₄-coated LNCMO, respectively. Additionally, the exothermic reaction temperature of the1wt.%YPO₄-modified LNCMO is delayed9℃compared with pristine material. Taken these analyses intoaccount, it’s clear that YPO₄surface modification improves the thermal stability of the material.