Patrick Soudan

Synthesis, chemical polymerization and electrochemicalproperties of low band gap conducting polymers for use in supercapacitors

A set of nine monomers derived from diaryl-cyanovinylene, -carboxyvinylene nand -cyanobutadiene were synthesized as were the corresponding polymers resulting nfrom the chemical polymerization of the monomers in the presence of an almost nquantitative amount of FeCl3 in chloroform. The aim of this work nwas to investigate the effect of the chemical structure of the polymers on ntheir charge capacitance and stability upon galvanostatic charge/discharge ncycling. The electrochemical performances of composite electrodes based on npolymer, acetylene black and PTFE have been investigated in acetonitrile containing n1xa0M Et4NBF4 using cyclic voltammetry and galvanostatic ncharge/discharge cycling experiments. The best performances in terms of ncharge capacitance for both the p- and n-doping processes were demonstrated nwith poly(7) n(2E,4E)-2,5-di-2-thienylpenta-2,4-dienenitrile nand poly(9) n(2E)-3-(2,2′-bithienyl-5-yl)-2-(2-thienyl)prop-2-enenitrile nsince values as high as 245xa0Cxa0g−1 were obtained nwith poly(7) in its n-doped nstate and 325xa0Cxa0g−1 with p-doped poly(9). The energy density (68xa0Whxa0kg−1) nand power density (24xa0kWxa0kg−1) delivered nby a poly(9) capacitor nare in good agreement with those expected from cyclic voltammetry and galvanostatic ncharge/discharge experiments performed with single electrodes. Unfortunately, na capacitance loss was observed upon cycling and was ascribed exclusively nto the n-doping process occurring at the negative electrode since the capacitance nof the positive electrode remained almost unchanged during these experiments.

Aging of the LiNi1 ∕ 2Mn1 ∕ 2O2 Positive Electrode Interface in Electrolyte

The evolution of lithium-containing species on the surface of grains of layered LiNi 1/2 Mn 1/2 O 2 material during the aging process in LiPF 6 (ethylene carbonate/dimethyl carbonate, 1 M) electrolyte has been followed using 7 Li magic angle spinning NMR spectroscopy. Materials displaying different surface areas have been investigated in order to study the influence of the surface/ volume ratio. The evolution of the NMR signal shows that the reaction of the active material with the electrolyte is extremely fast during the first moments of exposure and tends to slow down for longer exposure times. Coupled NMR, electrochemical impedance spectroscopy, and transmission electron microscopy experiments showed that the surface of the material grains is not covered by a homogeneous layer, indicating that the reaction with electrolyte cannot be considered as a real passivation reaction. The aging process performed on a sample initially stored in ambient atmosphere clearly demonstrates the dissolution of a pristine Li 2 CO 3 surface layer and the growth of an interphase made primarily of fluorinated compounds.

Chemical Synthesis and Electrochemical Properties of Poly(cyano-substituted-diheteroareneethylene) as Conducting Polymers for Electrochemical Supercapacitors

Polymers derived from diheteroaryl-(cyanovinylene), synthesized by chemical polymerization of the monomers in the presence of 6 equivalents of FeCl 3 in chloroform, were characterized by elemental analysis, energy-dispersive analysis by X-ray, and X-ray photoelectron spectroscopy. The electrochemical performances of composite electrodes prepared from chlorinated poly-(E)-α-[(2-thienyl)methylene]-2-thiopheneacetonitrile (poly-1), poly-(E)-α-[(3-methyl-2-thienyl)methylene]-2-thiopheneacetonitrile (poly-2), and poly-(E)-α-[(2-furanyl)methylene]-2-thiopheneacetonitrile (poly-3), with acetylene black (A.B., 45 wt %) and polytetrafluoroethylene (5 wt %) have been investigated in 1 M Et 4 NBF 4 /acetonitrile using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge cycling. The effect of the structure of the polymers on their electrochemical properties is in good agreement with anticipated effects for methyl and furan groups. The results indicated p- and n-doping levels up to 0.3 electrons per heterocycle for the three polymers. These values make those polymers interesting candidates for use in electrochemical supercapacitors in which specific capacity and energy of about 30 Ah/kg and 55 Wh/kg of polymer may be achieved. The best cyclahility was demonstrated with poly-1 and poly-2. in particular during cycling in their n-doping state with a doping level of 0.16 and 0.17 electrons per thiophene unit, respectively, maintained after 1000 cycles. Preliminary charge/ discharge galvanostatic cycling with a poly-1-based supercapacitor yielded specific energy and power of 42 Wh/kg and 11 kW/kg of polymer, respectively, during the first 60 cycles and 30 Wh/kg and 9 kW/kg after 1800 cycles, for a discharge time of about 10 s. Moreover, cycling experiments performed separately on negative and positive electrodes have shown that the capacity loss is associated essentially with the n-doping process at the negative electrode.