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

In the last 20 years, lithium-ion batteries have been widely commercialized in industry for their relatively high and particularly reversible specific capacity, high power density and long cyclic life characteristics. They are successfully applied in various communication appliances and digital devices. However, with the requirements of higher battery safety, longer cycle life and lower pollutions, the present cell chemistry (LiCoO₂ as active cathode material, graphite as active anode material, and liquid electrolyte) can hardly meet these needs. Recently, the focus of research on lithium-ion battery is to improve the performance of present industrialized materials and search for new materials. It is well-known that the cathode material is the most important among all of the components in a battery, because it is the only source of lithium-ions and it to a large extent decides the cost, safety characteristic and the cycle life. Therefore, in this thesis, several modification approaches have been employed to improve the performance of Co-based cathode material (LiCoO₂). Furthermore, a systematic study on the solid-state reaction synthesis and up-scale production of the safer Fe-based material (LiFePO₄) is conducted. An exploration on a potential new polymer electrolyte is also attempted.In Chapter 1, a general introduction is given on following aspects: the development and status of traditional electrochemical power sources and lithium-ion batteries, the structure and working mechanism of lithium-ion battery, the common cathode materials and methods to synthesize and modify cathode materials reported in literature.In Chapter 2, the author mainly introduces the experimental processes and equipment used in the project of this thesis. A detailed description on the process to making a coin cell is presented. The structural, electrical and electrochemical analysis methods are also summarized.In Chapter 3, Si-doped LiCoO₂ powders are synthesized through a co-precipitation method. The optimal composition is found to be LiCo_0.99Si_0.01O₂. In the voltage region from 2.8V to 4.2V, its initial discharge specific capacity is 137mAh/g and after 50 cycles its capacity retention is even as high as 100%. The 3.6V-plateau efficiency of the optimal sample is also improved compared with the pristine LiCoO₂ and reaches 97% after 25 cycles.Chapter 4 presents four methods that are employed to modify the surface of a commercial LiCoO₂ powder. Through a solid-state reaction process, the cathode material LiCoO₂ can be surface modified by a Li₃PO₄ nano-powder. The electrochemical performance of LiCoO₂, especially its capacity retention in a wide voltage window from 2.8 to 4.5V, can be significantly improved. After heat-treated at the optimal temperature of 450℃, 1wt%-Li₃PO₄ modified LiCoO₂ can deliver a reversible capacity of about 164mAh/g at 1C rate during 50 cycles. A phosphorus-containing surface layer is believed to act as a separation layer to suppress the reactions between the Co⁴⁺ and electrolyte.In Chapter 5, with an Al₂O₃ powder as a model material for carbon-coating, the author investigates the variations of residual carbon from four different organic precursors and obtains that the best precursor among them is PVDF. Then the amount of PVDF as a carbon precursor in the synthesis of C-coated LiFePO₄ (LiFePO₄/C) is investigated. Under the optimal conditions that the amount of PVDF in the starting material is 30% and the sintering temperature is 710℃, the initial discharge capacity of synthesized LiFePO₄/C is above 140mAh/g at 0.2 C rate and the specific capacity even stabilizes at 120mAh/g after 30 cycles at 1-2 C rate.In Chapter 6, the author synthesizes a lithium-containing complex metal oxideβ,γ-Li₂CuZrO₄ with a double rock-salt structure. It may electrochemically react with lithium at potentials below 1.5V vs. Li/Li⁺. The final reaction products are Li₂O, Cu and ZrO₂. As the results of the electrical analysis, theγ-phase is found to be a pure electronic semiconductor with a rather high conductivity and it transforms intoβ-phase at a temperature over 800℃. The activation energy ofγ-Li₂CuZrO₄ is 14.4kJ/mol in the temperature region of 133-1273K.In Chapter 7, lithiated hyperbranched polymers LHn (n=20, 30 and 40) are synthesized from the reactions between the commercial hyperbranched polyester Boltorn^? Hn and lithium metal under an inert atmosphere. Structural analysis and conductivity measurements indicate that their conductivity is up to 5.9×10^-6 S/cm at room temperature and 1.8×10^-4 S/cm at 120℃. LH20 is stable in the voltage window from 0 to 5 V versus Li. This lithiated polyester material is very promising for future all-solid-state lithium-ion batteries.Finally, in Chapter 8, the author gives an overview on the originality work and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.