The Study On Surface Coating Of Layered Cathode Materials For Li-ion Battery

Lithium ion battery is widely used for its high capacity, high voltage, environmental friendship and excellent cycle life. With the development of society, there is an increasing demand on lithium ion battery with higher performances, which requests better lithium ion battery materials, especially for cathode materials. During recent decades, researchers have tried various methods to improve the performance of cathode materials. Of them, surface coating has been proved to be an effective means. According to the report from argonne laboratory, the surface coating can be divided into three different types:one is rough coating, which is involved in coating a kind of new material on the surface of the active material through simple methods such as mechanical mixing, sol-gel method; the second is precursors coating by a coprecipitation process, in which one materials is coated onto the other materials by coprecipitation to form core-shell structured precursors, followed by mixing with lithium salt and sintering to obtain so-called core-shell structured cathode materials; the third type is ultra thin film coating, during which a coating layer with a nanoscaled thickness is precisely controlled by the atomic layer deposition technology by on the surface of particles of active material. In this thesis, the former two kinds of coating ways were studied.Firstly, during rough coating lithium niobate was selected as coating material. Lithium niobate has high ionic conductivity, and has been proven to be a very effective coating material to improve the performances of LiCoO2cathode material in all solid state battery with the sulfide as electrolyte. In the part, the coating effect of lithium niobate on LiCo1/3Ni1/3Mn1/3O2and Li-rich layered Li[Li0.2Co0.13Ni0.13Mn0.54]O2materials in lithium ion battery with liquid electrolyte was carried out.. In the study of LiCo1/3Ni1/3Mn1/3O2, the comparative investigations on structure, morphology and electrochemical properties between pristine and coated materials were performed by XRD, SEM, and charge-discharge tests. XRD results suggest the formation of LiNbO3, the mixture of LiNbO3/Li3NbO4and Li3NbO4on the surface of active materials during post-annealing, depending on sintering temperatures. Electrochemical results indicate coated material presented much better cycle performance and rate performance than prestine material. In addition, a similar coating effect of lithium niobate was observed in Li[Li0.2Co0.13Ni0.13Mn0.54]O2.Secondly, the precursors coprecipitation coating method is studied, by which the so-called core-shell structured material in composition is developed finally. Core-shell structure is an effective coating method. Professor Yang-Kook Sun from South Korea had made a series of core-shell structure research. Although he had done all performances comparisons between core-shell structure materials and core materials or shell materials, he ignored the change of performance caused by the change of material composition. In order to make up the deficiency of that comparison, we compare core-shell materials with the non core-shell material havingthe same components. LiNi.8C0.1Mn0.1O2has higher specific capacity, and LiNi0.4Co0.2Mn0.4O2has excellent cycle performance and thermal stability, thus we coated Ni0.4Co0.2Mn0.4(OH)2on the surface of Ni0.8Co0.1Mn0.1(OH)2during coprecipitation, and then calcinated mixing with lithium carbonate to form LiNi0.5Co0.1Mn0.1O2coated with LiNio0.4Co0.2Mn0.4O2. We found that Ni0.4Co0.2Mn0.4(OH)2completely covered on the surface of Ni0.8Co0.1Mn0.1(OH)2through XRD, DTG, SEM and particle size distribution analysis. The results showed that core-shell material of Li(Ni0.8Co0.1Mn0.1)0.7(Ni0.4Co0.2Mn0.4)0.3O2and noncore-shell material of LiNi0.68Co0.13Mn0.19O2display better cyclability than that of the core material of LiNi0.8Co0.1Mn0.1O2. However, the core-shell material showed obvious better cycle performance than what noncore-shell material. We conclude that the change of component will cause the change of material performance, but the core shell structure also function to improve the cycle performance.