Studies On Synthesis And Properties Of Polymer Positive Materials For Lithium Second Batteries

The development of lithium ion batteries and their cathode materials were reviewed. PTMA(poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate)), DMcT(2,5-dimercapto-1,3,4-thiadiazole), PAn(polyaniline) and their compounds with good electrochemical properties were synthesized. Their structure, physical and electrochemical properties were investigated by using various electrochemical methods in combination with modern analytic techniques.MTMP was prepared by a new way route using copper as inhibition and sodium as catalyst, which was favorable for the increase of yield, the purification of MTMP, and the polymerization reaction of MTMP. In order to obtain the optimal conditions of polymerization, the radical polymerization conditions of MTMP such as monomer concentration, initiator concentration, solvent, reaction time, reaction temperature, reaction atmosphere were investigated. The catalytic oxidation reaction mechanism of PMTMP was proposed by the experiments.The electrochemical properties of PTMA were investigated. The Cyclic voltammograms indicated the difference of the anodic and cathodic peak potential of PTMA was 63 mV. The result of EIS of PTMA showed that the charge-transfer resistance of PTMA was 19.5 Ω. In the charging/discharging curves, the maximum discharge specific capacity of PTMA is 78.4 mAh·g^-1 (0.2 C). The charge and discharge voltage plateaus of PTMA were 3.65 V and 3.56 V, respectively. The capacity decay of PTMA is about 4.8 % after 100 cycles. Furthermore, 85.5 % of the maximum specific capacity of PTMA could be charged just in six minutes at the discharge rate of 10 C. Button cells with PTMA as cathode materials also exhibited excellent electrochemical properties using carbon as anodes. The results showed that PTMA possessed an excellent electrochemical reversibility, stability and high rate performance. Therefore, PTMA was a promising polyradical cathode materials for lithium second batteries. The exchange current density of PTMA powder microelectrodes was examined by means of linear sweep voltammetry. The results showed that the exchange current density of PTMA at the state of full charge, half-charge and full discharge was 9.20 mA·cm^-2, 7.15 mA·cm^-2 and 1.02 mA·cm^-2, respectively.PTMA were doped with S, DMcT, PAn, LiMn₂O₄, V₂O₅ to improve its electrochemical properties. The results indicated that the doping of S, DMcT, PAn, and DMcT-PAn could raise greatly specific capacity at the expensive of the cycling performance. The initialdischarging specific capacity of PTMA/LiMn2O4 compound materials was 110.5 mAh-g"1 (0.2 C) when the mass ratio of PTMAand LiMn2O4 was 0.5:1. After 50 cycles, its specific capacity decreased from 106.3 mAh-g"1 to 96.1 mAh-g"1 at the discharge rate of 1C, and its capacity decay was about 9.6 %. The initial discharging specific capacity of PTMA/V2O5 compound materials was 188.9 mAh-g"1 (0.2 C) when the mass ratio of PTMA and LiMn2O4 was 0.75:1. After 50 cycles, its capacity decay was about 10.8 %. The initial discharging specific capacity of PTMA/V^Os-PAn (the mass ratio of PTMA, V2O5 and PAn was 0.75:1:0.25) compound materials was 220.3 mAh-g"1 (0.2 C) . After 50 cycles, its capacity decay was only 5.2 %. Therefore, PTMA/LiMn2O4, PTMA/V2O5 and PTMAA^Os-PAn compound materials were promising organic-inorganic composite positive materials with excellent electrochemical properties for lithium second batteries.DMcT was prepared by optimal approach, which increased the yield of DMcT and decreased the cost of acid and alkali. The difference of the anodic and cathodic peak potential in the Cyclic voltammograms indicated that the redox reaction of DMcT was rather slow at room temperature. The influence of neutral, acidic and basic condition on the redox behavior of DMcT was studied. The addition of triethylamine facilitated the oxidation of DMcT and made the peak current increased, but did not influence the reduction peak potential of DMcT. On the contrary, the addition of CF3COOH made the oxidation of DMcT harder than the neutral condition, but facilitated the reduction of DMcT.PAn with excellent electrochemical properties was synthesized by chemical and oxidized polymerization. The prepared PAn was characterized by infrared spectroscopy, ultraviolet-visible spectroscopy and elemental analysis. Cyclic voltammograms and charging/discharging curves showed that PAn possessed an excellent reversibility, cycling stability and high rate performance.DMcT/PAn cathode was prepared from NMP(N-methyl-2-pyrrolidone) solution of DMcT and PAn. The difference of the anodic and cathodic peak potential in the CV curves and the charge-transfer resistance of DMcT/PAn in EIS curves were less than that of DMcT, which indicated PAn had a good electrochemical catalysis on the redox reaction of DMcT. The redox reaction of DMcT was accelerated greatly by PAn. After 34 cycles, its specific capacity decreased from 186 mAh-g"1 to 82 mAh-g"1 at the discharge rate of 0.2 C, which indicated the worse cycle stability of DMcT/PAn. The electron transfer reaction between PAn and DMcT was confirmed by ultraviolet-visible spectroscopy. The redox reaction complexmechanism of DMcT/PAn was studied, and the electrochemical catalysis of PAn on the redox reaction of DMcT were explained.The electrochemical properties of DMcT/PAn composite materials doped with S or Q1C2O4 was investigated. The results indicated that the doping of S could raise specific capacity of DMcT/PAn. After the addition of CUC2O4 to DMcT/PAn cathode, the difference of the anodic and cathodic peak potential of DMcT/PAn cathode in the CV curves and the charge-transfer resistance of DMcT/PAn cathode in EIS curves decreased, the redox peak current in the CV curves increased, and the initial discharging specific capacity of DMcT/PAn cathode increased from 186 mAh-g"1 to 298 mAh-g"1 at the discharge rate of 0.2C. After 50 cycles, the capacity decay of DMcT/PAn cathode decreased from 68.8 % to 37.3 %. The results indicated that the doping of Q1C2O4 could improve the homogeousness of DMcT/PAn, increase the conductivity of DMcT/PAn, accelerate the catalysis of PAn on the redox reaction of DMcT, and raise the useful ratio of DMcT/PAn cathode. The doping of G1C2O4 could stabilize the redox processes of DMcT and decrease the solubility of DMcT and its coordination complex in an electrolyte. The doping of G1C2O4 could remove more of the solvent during drying of the cathode films so as to increase the overall specific capacity of DMcT/PAn.