Synthesis, Structure And Performance Of Li-Ni-M-O Compounds As Cathode Materials For Lithium Ion Batteries

Synthesis, Structure and Performance of Li-Ni-M-O Compounds as Cathode Materials for Lithium Ion BatteriesLiNiO2-based compounds are promising candidates of cathode materials for lithium ion batteries. A study on the synthesis, structure and performance of LiNiO2-based compounds as cathode materials for lithium ion batteries was carried out systemically and in detail in this dissertation.As the first step of this study, a sol-gel method using citric acid as a chelating agent was developed. The reaction conditions in sol-gel process, pre-calcination process and calcination process, especially sintering temperature and sintering time in calcinations process, were analyzed and optimized carefully. LiNi0.8sCo0.2O2 cathode material, which was synthesized with 10% excess lithium under 800 ml/min oxygen flow and 725@ for 24 hours, shows a high initial discharge capacity of 181mAh/g at 0.1C current between 3.0V and 4.2V, and good capacity retention of 83% after 50 cycles.After that, a series of Co-doped LiNiO2 compounds was synthesized using the optimized sol-gel method. The effects of Co-doping on the structure and performance of LiNiO2 cathode material were investigated systemically. The results show that cationic displacement is decreased in Co-doped materials. The phase transitions of LiNiO2 during cycling are suppressed by Co doping. The structural stability of delithiated cathode materials is also increased after Co doping. Therefore,Co-doped materials show improved cycling performance and thermal stability. LiNi0.8Co0.2O2 compound is thought the most promising candidate of cathode material for lithium ion batteries, although its thermal stability and cycling performance still need to further improve.As an improved way, Ti-doped LiNi0.8-yTiyCo0.2O2 cathode materials were synthesized. The effects of Ti doping on the structural, electrochemical and thermal properties of LiNi0.8Co0.2O2 cathode material were studied systemically and in detail. Improved cycling performance and enhanced thermal stability are observed for Ti-doped cathode materials. These positive effects are attributed to the changes of cationic distribution and chemical bonds in the structure of Ti-doped materials. A significant suppression of phase transitions and lattice changes during cycling is occurred for Ti-doped materials, and a decrease of interface reaction activity between the cathode and electrolyte is also demonstrated for Ti-doped cathodes. As a result, the capacity losses, which are originated from structural changes and interface reactions during cycling, decrease and thereby cycling life increases for Ti-doped materials. Meanwhile, the structural stability of delithiated cathode materials is also improved by Ti doping. It results in the suppression of thermal decomposition reaction of delithiated cathode material, which will produce heat and oxygen gas as the fuse of electrolyte decomposition and combustion reaction. Hence, thermal stability of delithiated cathode material is also enhanced by Ti doping.Storage performance of LiNiO2-based materials was also investigated in depth in this dissertation. A deterioration mechanism of LiNiO2 during storage was proposed at the first time. LiNiO2 shows a distinct deterioration after storage in air for a period of time. Li2CO3 is observed on the surface of stored materials, accompanied with the adsorption of H2O and CO2. The spontaneous reduction of Ni3+ to Ni2+ is considered to the actual origin of chemical instability of LiNiO2during storage. The corresponding oxidization of lattice oxygen O2' to active oxygen species (O-, O2-) is thought to the direct cause of formation of Li2CO3 and adsorption of H2O and CO2 on the surface of stored materials. After storage for a long time, a layer of Li2CO3 and absorbed species (H2O, CO2and O2-) will appear on the surface, and a thin NiO-like layer will be formed on the near surface of LiNiO2 material. The two layers result in the degradation of LiNiO2 cathode material. Therefore, preventing the reduction of Ni3+ to Ni2+ and...