Synthesis And Performances Study On Cathode Materials For Lithium Ion Batteries

The development of the Li-ion battery has been limited by its cathode material, which is one of the key components of the Li-ion battery system. It is known that the performance of cathode materials is greatly influenced by the synthesis methods. However, solid state method, which is used in industry widely, wastes more power and time. At the same time, the materials synthesized using this method seriously agglomerate and its crystals and particles can not be well controlled, which leads to the degradation of materials' performance. Therefore, researchers in increasing numbers focus on looking for new methods to prepare the cathode materials with excellent performance.In this thesis, according to cathode material's semiconductor characteristics and various requirements of physical and chemical performances used in Li-ion battery, a series of LiCoO₂, LiMn₂O₄ and LiFePO₄ cathode materials were successfully synthesized by templatemicrowave or solid state method which has not been reported in previous articles. And these materials were systematically study through various morphological, structural and electrochemical tests. Owing to the drawbacks of these materials, dopping or surface coating method was adopted to modify the materials' performance. The parameters of pilot-scale production were optimized, and the material performance fits to the designed requirement. That is to say, this method not only can be used to synthesis all kinds of the cathode materials used in Li-ion batteries, but also has the quality to be industrialized. At last, scientific theories were proposed about the mechanisms of synthesis and modification. In a word, it is a new technique, which is dependable, efficient, and practical, to synthesize structure-functional metal oxide materials as well as cathode materials used in Li-ion batteries. At the same time, this method enriches the control theory of solid state synthesis, and is favor to the research of the profound mystery.Main points and contributions are as follows:1. Synthesis and modification of layered LiNi_xCo_1-xO₂ material Cathode materials LiNi_xCo_1-xO₂ were successfully synthesized by microwave-solid compound heat technique using PAM as template. The influencing law of synthesis conditions on materials' performance was studied systematically. The LiNi_0.5Co_0.5O₂ material prepared by this method has higher capacity, lower cost, and identical cycleability. However, the material has drawbacks, for instance, the cycleability degenerates seriously under large current. Therefore, the LiNi_0.5Co_0.5O₂ material was modified by dopping rare earth element Nd. The results show that the cycleability LiNi_0.5-xCo_0.5Nd_xO₂ was greatly improved when x=0.01. In addition, LiNi_0.5Co_0.5O₂ was coated with Ni, and ZnO using different method. It indicates that only ZnO can be uniformly coated on material surface, and efficiently improve cycleability and stability of the material. The modified mechanism was tentatively investigated.2. Synthesis and modificationof spinel LiMn₂O₄ material Cathode material LiMn₂O₄ was successfully synthesized by microwave-template method. The particles of this material are sphere-like and distribute uniformly. There are rich active points on the particle surface, and reasonable distributed defects in the particles, which is in favor of the intercalation/disintercalation of Li⁺. The electrochemical tests show that the LiMn₂O₄ material exhibited better cycleablility and higher discharge capacity compared to LiMn₂O₄ material synthesized by solid state reaction. After 50 cycles, capacity retention of this kind of material was 90ï¼…. LiMn₂O₄ material was dopped by rare earth elements in order to reduce the capacity lost rate. The effect of dopping on the morphology, structural, and performance varies with the dopping elements and amount. And the performance of material is satisfying. At the same time, LiMn₂O₄ material was coated with LiCoO₂ and Co₃O₄ to prevent the contact of cathode material and electrolyte, so the dissolution of Mn in electrolyte was decreased. LiCoO₂-coated LiMn₂O₄ presented higher discharge capacity and better capacity retention.3. Synthesis and modification of olivine type LiFePO₄/C material Pure phase LiFePO₄ and physically composite LiFePO₄ with carbon black were synthesized by the traditional solid state method, and different LiFePO₄/C composites were prepared using polyacrylamide(PAM), sugar or phenolic resin as template. These materials were characterized by electrochemical tests and physical characterizations systematically. The results indicate that the electrochemical properties of the LiFePO₄/C composites were distinctively improved. Besides the polymers' template function, the materials' conductivity was greatly increased due to the carbonaceous residue of polymers dispersed in the composite materials. LiFePO₄/C composite using PAM as template was the most excellent one among those composites, and the optimal amount of carbon is 4.55wt.ï¼…. The charge-discharge mechanism of LiFePO₄ material was studied through XRD,XPS and Mossbauer, and the high security of material was explicated. The doped LiFe_0.99M_0.01PO₄/C(M=Nd, Co, Cr, Mn) cathode materials were synthesized and studied systematically. All conductivities of those doped material was increased about 10⁵-10⁷ times, compared with pure LiFePO₄ material. Among them, the material LiFe_0.99Nd_0.01PO₄/C has the best electrochemical performance owing to its relatively high conductivity, biggest cell parameters and the Fe doped site which can assure more expedite Li⁺ ion diffusion channels.4. Mechanism research of synthesis method Study the synthesis process of materials in detail using TG,IR and XRD techniques. The mechanism of PAM template and microwave was presented during synthesis process. It is verified that PAM is trifunctional during the synthesis process. Firstly, raw materials can be thorough mixed by electrostatic function with PAM, which can lower the activation energy. Secondly, PAM combined with microwave technique is in favor of the formation of nuclei. The last but not the last, carbonaceous residue formed in the later process of calcination can control the growth of crystal and the morphology of particles, and can increase the electric conductivity.5. Study and results of pilot-scale production The key equipments and processing parameters in large scale production were defined by the pilot study of LiFePO₄ material through the PAM template solid state approach. The specific area of the material synthesized by this method is larger than the material which was synthesized without PAM. And the former material's grain size is normal distribution. It is the most important that all the performances of this material meet the requirements of merchantable Li-ion batteries. The feasibility of this new method to synthesize LiFePO₄ material was proved, and the technology parameters and indexes also were fixed. A general method with low cost and high efficiency has been provided to produce cathode materials in large scale, which has great significance theoretically and practically.