Preparation Of The Lithium-ion Battery Cathode Materials Modification And Characterization

Our ceaseless demands for energy sources have greatly promoted the development of the lithium-ion batteries. This makes the electrode materials become very attractive in the field of materials research. The most important materials for the lithium-ion batteries are the cathode materials. However, some problems of the cathode materials such as safety and cost have hindered the applications of the lithium-ion batteries. Therefore, researches aiming at improving the electrochemical properties of various cathode materials are desirable and very necessary. In addition, safety concerns of lithium ion batteries have been the key problems in their practical applications. One of the effective strategies to address the safety issue is to reduce the electrolyte flammability and in the ideal situation to use a nonflammable electrolyte to avoid explosions of cells.In Chapter 1. a general introduction is given on following aspects: the development and status of the lithium-ion batteries, their working principle, the common cathode materials, the factors that affect the electrochemical properties of cathode materials and some preparation methods.In Chapter 2. we mainly introduce the experimental processes and equipments used in the project of this thesis. A detailed description on the process to making a coin cell is presented. The electrochemical and structural analysis methods are also included.As an example of doped electrode. LiCo_1-xZr_x/2Mg_x/2O₂ (0≤x≤0.2) powders are prepared by a solution-combustion route in Chapter 3. The maximum doping level is between 0.06 and 0.1. The optimal electrochemical performance of this doped UCoO₂ system is obtained when x = 0.06. Both specific capacity and the stability of 3.6V plateau efficiency are improved by the combined Zr-Mg doping.In Chapter 4. a 4.7V class cathode material. LiNi_0.5Mn_1.5O₄ powders have been prepared with radiated polymer gel method. The electrochemical properties of these powders are closely related to the calcination temperature. The sample calcined at 950℃ shows the best electrochemical performance with an initial capacity of 139 mAhg^-1 and 96% capacity retention after 50 cycles. Adopting a slow cooling procedure during the powder calcinations can increase the capacity of LiNi_0.5Mn_1.5O₄ at the 4.7V plateau. In addition, we have found that the approach of a simultaneous DC resistance measurement during the galvanostatic cell cycling can be used as a sensitive probe to the structural change of the electrodes.As safety concerns have limited the full utilization of Li-ion batteries in EV and other high power fields, we also attempt to reduce the flammability of the electrolytes by incorporating a flame-retardant. i.e. trimethyl phosphite (TMP(i)). as the additive in Chapter 5. The results show that 5wt% TMP(i) as an additive to the electrolyte 1M LiPF₆+ EC/DEC (1:1. w/w) has improved the electrochemical performance including cycle performance and the stability of 3.6V plateau efficiency of LiCoO₂ electrodes. In addition, the use of TMP(i) additive can obviously influence the composition of surface layer formed on LiCoO₂ during cycling. In addition, the thermal stability of the electrolyte with 5wt% TMP(i) additive and the thermal stability of the interphase between the cathode and electrolyte have been obviously improved.We also study the 4V-cathode material Li[Li_0.2Ni_0.2Mn_0.6]O₂ (LNM) and Li[Li_0.16Ni_0.21Mn_0.63]O₂ powders prepared with radiated polymer gel method in Chapter 6. Li[Li_0.16Ni_0.21Mn_0.63]O₂ shows excellent electrochemical performance with a high reversible 1C-rate capacity of 158 mAhg^-1 in the voltage range of 2.0 to 4.7V. The presence of lower Ni valence in Li[Li_0.16Ni_0.21Mn_0.63]O₂ is believed to play an important role in facilitating the lithium diffusivity. enhancing the electronic conductivity, and suppressing the surface layer resistance. This lithium-scant LNM is very promising as a potential cathode material for lithium secondary batteries.Finally, in Chapter 7 the author gives an overview on the achievements and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.