Synthesis And Modification Of LiNi_1/3Co_1/3Mn_1/3O₂ As A Cathode Material For Lithium-ion Batteries

Layered LiNi]/3Coi/3Mn1/3/3O2 has been considered to be one of the most promising candidates for cathode materials in high energy lithium ion batteries owing to its characteristic advantages, such as higher reversible capacity, excellent structural and thermal stability. However, its irreversible capacity loss, cycling performance under a high charge cutoff voltage and rate performance remain to be improved. In this paper, LiNi1/3Coi/3Mni/3O2 powers were prepared by different methods to optimize the synthesis conditions, and the as-prepared LiNii/3Coi/3M1/3/3O2 powders were coated by Al2O3 and graphene to improve its electrochemical performances and explore the functional mechanism.The LiNii/3Co1/3Mni/302 powders were prepared from Na2CO3, NaHCO3 and Na2CO3+NaHCO3 precipitant agents via a carbonate co-precipitation process, respectively. The material prepared from Na2CO3 precipitant agent had a higher discharge capacity (171.8 mAh·g-1 @ 0.5 C). The material prepared from Na2CO3+NaHCO3 precipitant agent possessed the best hexagonal a-NaFeO2 layered crystal structure with lowest cation mixing and delivered excellent cycling performance (86.3% @ 100th cycles,0.5 C) and rate performance. The results from CV and EIS indicated that the LiNi1/3Coi/3Mni/3O2 prepared from Na2CO3+NaHCO3 precipitant agent showed more intensity redox peaks with lowest AV value, and its Rf and Ret resistance value increased slowly and impedance values were smaller, resulting in better structural stability and reversibility during cycling.The LiNi1/3Co1/3Mn1/3 powders were prepared by ethylene glycol assisted sol-gel method to optimize the synthesis conditions by changing the pH value of the solution. LiNi1/3Co1/3Mni/3O2 prepared under the pH value around 5 possessed well hexagonal α-NaFeO2 layered crystal structure and its paticle size was small, thus it exhibited a higher discharge capacity (205.2 mAh·g-1@ 0.1 C). However, its cycling performance was poor.Layered LiNi1/3Co1/3Mn1/3O2 had been successfully coated with uniform Al2O3 film through homogeneous precipitation method with urea as precipitant. For the 1% Al2O3-coated LiNii/3Coi/3Mni/3O2 sample, XRD results showed that it possessed the best hexagonal a-NaFeO2 layered crystal structure with lowest cation mixing among all samples and SEM and TEM images showed a uniform and amorphous Al2O3 layer (13-20 nm) is formed on the LiNi1/3Co1/3Mn1/3O2 surface. Compared with bare sample, it exhibited superior electrochemical performance including the high discharge capacity (202.6 mAh g-1@0.1 C), highest initial coulombic efficiency (92.1%), excellent cycling performance (92% @ 25℃,93% @ 55℃) and rate performance, the results from CV, EIS and XPS showed that the improving on the electrochemical performance dued to the Al2O3 coating layer could be attributed to the suppressed electrolyte (LiPF6 or solvents) decomposition reaction on the cathode/electrolyte interface and thus reduced interfacial impedance value and impedance growth speed with cycling.The LiNi1/3Co1/3Mn1/3O2/RGO composites were prepared by solid state method and hydrothermal method. SEM and TEM images showed that the LiNi1/3Co1/3Mm/3O2 particles adhered to the surface of RGO layers or enwrapped into the RGO sheets. Compared with solid state method, LiNi1/3Co1/3Mn1/3O2/RGO composite prepared by hydrothermal method realized the intimate coupling of RGO with LiNi1/3Co1/3Mn1/3O2 via covalent metal-oxygen bonds, resulting in excellent electrochemical performance. The results from CV and EIS showed that the impedance of the battery had been reduced after RGO coating, thus the capacity and reversibility of the material had been improved. However, the Rf and Rct values were not significantly decreased, so its electrochemical performances were not obviously improved.