Electrochemical Characterizations of Conducting Polymers from Thiophene Derivatives and LiFePO4 Cathode Materials in Lithium ion Batteries

Abstract

A few conjugated conducting polymers such as polythiophenes and poly-3,4-ethylenedioxythiophenes have been synthesized and characterized electrochemically with a perspective of elucidating their growth mechanisms and understanding their electrical properties with the monomers and solvents varied as parameters affecting the polymerization process. Also, a new cathode material, LiFePO4, for Li-ion batteries have been evaluated using various electrochemical techniques such as cyclic voltammetry and Fourier transform electrochemical impedance spectroscopy (FTEIS). In the first part, polythiophenes obtained by electrochemical polymerization of thiophene, 2,2’-bithiophene, 2,2’:5’,2”-terthiophene in 1.0 M LiClO4 propylene carbonate solutions have been studied using various experimental techniques including UV-vis absorption spectroscopy, the electrochemical quartz crystal microbalance method, and current sensing atomic force microscopy (CS-AFM). The polythiophene films were obtained by repeated potentiodynamic voltage scans on gold electrodes. Clear differences were observed for electrochemically prepared polymer films from the monomers in their electrical and optical properties due to various factors such as degradation of the polymer at high anodic potentials, conjugation lengths of polymers, incorporated oligomers, and solvated ion pairs in the polymer matrix. The poly(bithiophene) displayed the best conductivity, while the polythiophene showed almost no conductivity.In the second part, systematic studies on the electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) were conducted at gold electrodes in two room temperature ionic liquids (RTILs) sharing the 1-butyl-3-methylimidazolium cation (BMIM+) for one of two anions, i.e., tetrafluoroborate (BF4-) or bis(trifluoro-methylsulfonyl)imide (TFSI-), and in propylene carbonate (PC) containing tetra-n-butylammonium tetrafluoroborate (TBABF4) or tetra-n-butylammonium bis(trifluoromethylsulfonyl)imide (TBATFSI). EDOT oxidation in RTILs was shown to undergo a change in charge-transfer mechanism during the potential sweep, which must have led to differences in polymerization reactions in the two media. Differences were noted between poly-EDOT (PEDOT) films prepared in RTILs and those in PC

PEDOT films were characterized using techniques including UV-visible absorp-tion spectroscopy, scanning electron microscopy, and Fourier transform electrochemical impedance spectroscopy. The films prepared in PC and with TFSI- displayed the better electrochemical properties. Differences in electron and mass transfer kinetics in different media led to differences observed for film growth kinetics, chain lengths, and redox characteristics for the PEDOT films. Also revealed was that different oxidation mechanisms are at work in RTILs in different potential regimes.Finally, the real-time impedance behavior of LiFePO4 nanoparticles as cathode materials has been investigated using FTEIS (Fourier transform electrochemical impedance spectroscopy) techniques during potentiodynamic charging and discharging processes. To compare effects of nanostructures of the LiFePO4 cathode material, the hollow sphere secondary structured LiFePO4 prepared by sequential precipitation methods and commercially available non-hollow structured LiFePO4 were tested. The lithium ion cells constructed using the hollow sphere secondary structured LiFePO4 cathode material exhibited improved performances during charging and discharging processes as judged from impedance parameters such as lower polarization resistances higher interfacial capacitances, higher Warbug admittances and larger diffusion coefficients of Li+ compared to cells using its non-hollow counterpart. These results appear to have resulted from the enhancement of intrinsic capabilities for electron and charge transport characteristics of LiFePO4 by modifying its secondary structures. Also complete information on electrode kinetics of a cathode material was obtained from a large body of impedance data for the first time.