| Due to their large specific capacity, Li-Ni-Co-Mn-O ternary cathode materials of lithium-ion batteries, especially LiMn1/3Ni1/3Co1/3O2and Li1.2Mn0.54Ni0.13Co0.13O2, have won widely attention and been considered as a new generation of battery materials with high capacity. However, the poor electronic conductivity and deteriorated high-rate performance which caused by the cation disorder of LiMn1/3Ni1/3Co1/3O2resulted in a low electrochemical performance and limited its practical application. The poor rate capabilities and bad cyclability of Li1.2Mn0.54Ni0.13Co0.13O2have restricted their further commercialization. This paper reviewed the research development of cathode materals and emphasized on structures, mechanism of lithium ion intercalation/extraction and applications of serveral kinds, especially LiMn1/3Ni1/3Co1/3O2and Li1.2Mn0.54Ni0.13Co0.13O2. In the experimental section, we investigated their electrochemical performance in aqueous electrolyte and modified their performance by graphene in different forms, aiming to to improve high-rate capability and electrical conductivity.LiMn1/3Ni1/3Co1/3O2materials were synthesized by polymer-pyrolysis mehod (PP-LMNC) and sol-gel method (SG-LMNC), respectively and then investigated their electrochemical performance in a l M Li2SO4aqueous solution. In the process of polymer pyrolysis, acrylic acid worked as complexing agent of metal ions, while ammonium persulfate worked as acrylate polymerization initiator. Ammonium persulfate polymerization can quickly form the polyacrylic acid salt in which metal ions were regularly arranged as soon as complexation of soluble metal ions by acrylic acid. Playing the role of crystallographic controlling agent, intermedium polyacrylic acid could change the crystal morphology. In terms of discharge capacity, PP-LMNC was much lower than that of SG-LMNC. However, the reversibility, the capacity retention and cycle stability of PP-LMNC had been improved.A facile solvothermal solution was introduced to hybridize pre-sintered LiMn1/3Ni1/3Co1/3O2(LMNC) nanocrystals with reduced graphene oxides (RGO) nanosheets. Compared with liquid-based growth of metallic oxide on graphene sheets, this approach not only ensures highly crystallity of LMNC particles from pre-sintering, but also realizes intimate coupling of RGO with LMNC via covalent metal-oxygen bonds. The results reveal markedly improved reversibility of the RGO/LMNC cathode toward lithium ion intercalation/extraction, which results in large discharge capacity, high coulombic efficiency, and excellent rate performance. The elevated electrochemical capability of the RGO/LMNC cathode material was accounted for better electrical conduction of the RGO nanosheets, highly stable crystal structure of the pre-sintered LMNC particles, and firm construction of the RGO/LMNC composite thanking to solvothermal hybridization.To prepare the electrode with mixed conducting3D networks, ultrasonicated suspension of graphene flakes and N-methyl-pyrrolidinone (G/NMP suspension) was used to prepare Li1.2Mn0.54Ni0.13Co0.13O2(MNC) electrode. As organic electrolyte, NMP was not only applied to dissolve graphene flakes, but also worked as dispersant for elecrode preparation. Due to the special2D structure and the excellent electrical conductivity, the graphene nanosheets provided high-speed electrical channels which can extend in a long distance for MNC and acetylene black. The rate and cycling tests demonstrate that such structure can significantly enhance the rate capability and long term cycling performance of MNC. The EIS measurements confirm that the kinetics was substantially improved by the addition of graphene. Furthermore, the results reveal that the rate performance was improved specifically in the two-phase region by the enhanced electrical conductivity benefiting from graphene networks. |
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