Studies On Organodisulfide Polymers As Cathode Active Materials For Lithium Secondary Batteries

Recently, organodisulfide polymers with S-S groups in the molecule have attracted enhancing research interests as cathodic materials with high specific capacity, low cost, light weight and are environmentally benign for high power secondary lithium batteries. The disulfide (S-S) bond provides reversible two-electron reactions during the electrocheical redox reaction, which is the key moiety contributing high specific capacity to the organodisulfide polymers used in the batteries. These materials have much higher theoretical capacities than that of conventional battery materials such as LiCoO₂, liNiO₂, LiMn₂O₄, etc.Among them PDMcT is one of the most promising candidates due to its high theoretical specific capacity (362 mAh/g) and high stability to temperature. Unfortunately, its redox kinetics is rather slow at room temperature and it is not electrically conducting, which may lead to significantly lower power density for use in batteries. In addition, the reduction products of the PDMcT are soluble in the organic electrolyte solution. These problems should be overcome in order to find practical applications under normal conditions. Recently, appropriate electrocatalysis such as transition metal thiolate salts, and conducting polymers have been utilized to enhance the redox kinetics of this organic disulfide. However, only the composite material of blended PAn/DMcT could have a high theoretical specific capacity of about 224 mAh/g. There is, therefore, much room for the improvement of these types of composite cathode materials.By coating a conductive material on the surface of LiFePO4 with conductivity as poor as PDMcT, a promising cathode material with a fascinating electrochemical performance has been obtained. So it is suggestive for us to improve the redox kinetics and electrical conductivity of PDMcT by using a thin conductive PPy film coated on the surface of PDMcT particles. And it was also expected that the coating film would inhibit the small molecules of DMcT, the reduction products of PDMcT, bleeding from the electrode into the electrolyte solution.On the other hand, a lot of new electrical conducting organodisulfide polymers have been prepared recently. However, to our previous studies, most of these disulfide materials generally show a redox process that is accompanied by the depolymerization and the structural changes of the main polymer chains. Thus the recombination efficiency of S-S bonds is very low, which causes poor reversibility of the cathodic material. The efficiency depends on the structure of the polymers strongly. An improved efficiency can be achieved when the main polymer chain remainsunchanged during redox reaction of S-S bond. In this case, the cleavage and recombination of S-S bonds is expected to be facile.In this thesis, the work is foucused on the improvement of the electrochemical property of PDMcT with conducting polymer thin film coating and the development of novel organodisulfide polymers used for cathodic materials in secondary lithium batteries. The main results are as follows:1. The investigation of thin film conducting PPy coated PDMcT with TFST technique and their electrochemical properties.The effect of synthesis methods on the electrochemistry of PDMcT has been studied, and the coating process is based on the PDMcT particles with favorite polymerization degree. The effect of pH on the ^-potentials values what determine the charge densities of the particulate surface was determined. Static SDBS adsorption isotherms on poly (2, 5-dimercapto-l, 3, 4-thiadiazole) (PDMcT) particles were obtained in neutral water solution with known concentrations of NaCl. NaCl was found to increase surfactant adsorption on the PDMcT particles.The PPy-coated PDMcT was prepared with the thin film via surfactant template (TFST) technique, and the formation of the uniform and well-connected film on the surface of PDMcT particles was confirmed by FT-IR and TEM. However, the effect of pH and added NaCl on the charateristics of polypyrrole (PPy) coated PDMcT was not obvious. The thickness of PPy coating film is increased with adding higher quantities of pyrrole, enabling the formation of uniform and well-connected conductive film of PPy. The conductivity of the PPy-coated PDMcT particles increased by a few orders of magnitude with increasing concentration of Py. Conductive PPy-coated PDMcT with enhanced redox process at room temperature was also obtained.A new conductive composite electrode material with initial specific capacity of 250mAb/g and conductivity of ca. 10'3 S cm'1 was prepared with the thin film via surfactant template (TFST) technique. Moreover, it showed a slower fading of discharge capacity than PDMcT when used as cathode material in secondary lithium batteries with liquid electrolyte solution. However, there is still a fast capacity fading of PPy-coated PDMcT with the cycle, which might be eliminated by the use of polymer electrolyte.2. Studies of synthesis and electrochemistry of PDTAn.A novel redox system, Poly (a,a'-Dithio-3-amino-o-xylene) (PDTAn) has been presented as a cathode material for high-energy secondary lithium batteries. PDTAn,containing one six-member ring with S-S bond on the side chain of aniline, have been electrochemically polymerized and chemically polymerized with (NH^SaOg from the monomer of DTAn (a,a'-Dithio-3-amino-o-xylene), respectively. It has been confirmed by IR and in situ Raman that the main chain of PDTAn is similar to that of polyaniline, and the S-S bond in the moiety of aniline is preserved after polymerization. This system has many advantages, such as high theoretical charge density (ca. 370mAh/g), fast redox process and enhanced reversibility. The fast redox process and enhanced reversibility are due to the intramolecular cleavage and recombination of the S-S bond, and the intramolecular electrocatalytic effect of polyaniline chain. The charge-discharge tests of Li/PDTAn cell show an initial capacity of 225 mAh/g and the charge efficiency of more than 80% after the first cycle activation.3. synthesis and electrochemistry of PABTHFrom facile preparation method and available low-cost raw materials, we have synthesized a novel kind of organodisulfide polymer cathode materials based on anthracene for lithium secondary batteries-polyanthra [l',9',8'-b,c,d,e] [4',10',5'-b',c',d',e']bis-[l,6,6a(6a-SIV)-trithia]pentalene (PABTH). PABTH has a rather high theoretical specific capacity of 442 mAh/g. This value has been calculated assuming the transfer of six electrons per monomeric unit as the concept of the redox of PABTH. The electorchemical reactions occurring in the polymer of anthrax[l',9',8'-b,c,d,e][4',10',5'-b',c',d',e']bis-[l,6,6a(6a-SIV)-trithia]pentalene (PABTH) have been investigated by cyclic voltammetric tests (CV) on cavity microelectrodes in 1M LiC104, EC/DMC solution. The mechanisms of current peaks drop in CV and capacity fading in galvanostatic cycling have been studied by XPS experiments and simultaneous electrochemical-electronic conductivity measurements. It is suggested that the existence of m-conjugated structure in the backbone of PABTH prompts its electrochemical performance, which is confirmed by the results of XPS, EIS and galvanostatic charge/discharge experiments. An initial discharge capacity of ca. 400mAhg"1 was obtained with the charging/discharging efficiencies of ca. 100% in 10 cycles.