Method of conditioning of organic polymeric electrodes

Abstract: Cells are disclosed using doped electroconductive polymer electrodes and electrolytes of alkali metal cations and soft anions in polar organic solvents. The structure of the nega­ tive electrodes comprises layers of electroconductive polymers and ion-exchange resin. Conditioning the negative electrodes is effected with AC current, the energy of the negative pulses exceeding that of the positive pulses.

Description: The present invention concerns a method of conditioning an anode of a rechargeable storage battery having cells containing a non-aqueous alkali-metal salt solution electrolyte and current collector substrates coated with one or more polypyrrole layers as anode and cathode electrodes, in which said anode is immersed in a non-aqueous lithium salt solution with a counterelectrode and a conditioning current is applied to said anode against said counterelectrode. Electrodes coated with a composite polymer of polypyrrole and an ion-exchange resin such as Nafion (R) which may find use in polymer batteries are already disclosed by the JOURNAL OF PHYSICS D/APPLIED PHYSICS, vol.21, No 7, 14th July 1988, pages 1043-1054; G. BIDAN et al.: "Conductive polymers with immobilised dopants: ionomer composites and auto-doped polymers - a review and recent advances" as well as by J. CHEM. SOC. CHEM. COMMON., 1987 (5), pages 327-328; T. SHIMIDZU et al.: "A novel type of polymer battery using a polypyrrole-polyanion composite anode" and also by J. ELECTROCHEM. SOC., vol. 135, No 5, May 1988, pages 1132-1137; TAKAYUKI HIRA et al.: "Electrochemical behaviors of polypyrrole, poly-3-methylthiophene, and polyaniline deposited on nafion-coated electrodes". When the present inventors attempted to condition an anode of such a rechargeable polymer battery, they noted that for instance the amount of charging and doping was insignificant and the anode thus conditioned could not be used in the manufacture of commercially workable batteries. Hence the present inventors attempted to correct the aforementioned deficiencies and, after painstaking efforts, they obtained excellent results by improving the conditioning technique. This is defined in the annexed claims. The reason why an alternating charging current provides so much enhanced conditioning efficiency (which may exceed 0.1 electron per pyrrole unit) has not been explained yet. The invention is now explained in detail with reference to the annexed drawing in which: Fig. 1 illustrates schematically a cell arrangement to be used for conditioning electrodes according to the invention. Fig. 2 is a graph representing the potential of a polypyrrole coated electrode upon attempted charging with negative DC current. Fig. 3 is a graph representing a charge and discharge cycle for a polypyrrole coated negative electrode conditioned according to the invention. The cell represented in fig. 1 comprises a container 1 filled with an electrolyte solution 2, i.e. a solution of a supporting salt like LiClO₄ in a polar organic solvent like propylene carbonate. Two main electrodes 3 and 4, respectively, are immersed in the electrolyte solution 2. The electrodes 3 and 4 are made of a current collecting material, e.g. metal, including Ni-sponge, or graphite, coated with layers 3a and 4a of polypyrrole. The electrode 3a further comprises a layer of ion exchange resin 3b, for instance a resin with sulfonate groups like Amberlite® or Nafion®. It should be noted that in the presently illustrated case, the electrode 3 is used as the negative electrode, and electrode 4 as the positive one. It should also be noted that if the collector material of electrode 4 is of carbon or graphite capable to form intercalates with the electrolyte anions, the layer of polypyrrole 4a can be omitted. Finally, in the instant illustrated embodiment, the electrode 4 with a polypyrrole layer could also be optionally provided with a layer of ion exchange resin, whereby the two electrodes would be formally symmetrical. The present cell further comprises a reference electrode 5 which can be any conventional reference potential element. Since the present embodiment operates in a lithium salt solution a lithium foil reference electrode is convenient as it will provide a Li/Li⁺ couple to be used as reference potential. The three electrodes in the system are connected to a potentiostat instrument 6, i.e. a conventional circuit which can continuously control and record (via a display or recorder not represented) the current (charge and discharge) in the main electrode circuit (this is measured on ammeter 7) as well as the voltage between the electrodes 3 and 4 and that relative to the reference couple 5. The polypyrrole coating of the electrodes can be obtained conventionally, for instance by mechanically coating (pressing or painting) with chemically prepared polypyrrole. Polypyrrole can be prepared for instance by oxidizing pyrrole monomer with a ferric salt in aqueous or anhydrous media as disclosed in co-pending WO 87/01504 or by electrooxidation. A convenient way to do this is to dip a metal, including Ni-sponge or carbon collector plate or sheet in an aqueous solution of pyrrole and an electrolyte salt, for instance tosylate or mesylate, and submit the collector to anodization against a counterelectrode, of, for instance, an inert metal like platinum. A layer of positively charged (p) polypyrrole will then precipitate on the collector, doping being achieved here by the tosylate ions. In the embodiment of fig. 1, both electrodes 3 and 4 can be obtained this way. Then, electrode 3 is further coated with a film of ion exchange resin, for instance by dipping into a solution or suspension thereof and thereafter drying. Of course, forming a new polypyrrole film over the resin and applying a new resin layer over the polypyrrole can be repeated to obtain a multilayer structure. Otherwise, the electroprecipitation of pPy can be brought about in the presence of a solution or dispersion of the ion exchange resin, whereby a multi-monomolecular layer structure will result. For conditioning the electrode 3 as an anode, the latter should be first undoped by removing the positive charges and doping anions and then reloaded with electrons. As said before, using DC as disclosed in JP 61-128478 for effecting conditioning was ineffective as depicted in fig. 2. In the experiment illustrated by fig. 2, a negative current (1 mA/cm²) was applied to electrode 3 for a period of about 30 min while recording the potential relative to the reference electrode (in the graph the origin corresponds to the reference potential of a saturated calomel element). So the initial potential was more than 5 Volt versus the Li/Li⁺ reference. Undoping did proceed smoothly (letter A) then a strong dip was observed and the potential progressively decreased beyond that of Li/Li⁺. After ∼ 30 min the cell was allowed to discharge, whereby the negative electrode returned instantaneously to neutral (letter D) showing that the coulomb efficiency was substantially nought. However when the conditioning was effected by a series of alternating negative and positive pulses (i.e. rapidly changing charging and discharging current), the electrode was efficiently converted to an electronegative electrode (coulomb efficiency exceeding 0.1 e/pyrrole unit) and excellent long-term cycling behaviour was noted. This is illustrated in fig. 3 which shows the typical charge/discharge cycle of such conditioned electrode in terms of voltage against the Li/Li⁺ couple with time under current densities of about 1 mA/cm². The considerable symmetry between the two legs of the curve can be noted, this symmetry being conserved for more than 100 cycles, which indicates excellent stability of the electrode materials. This is probably due to the efficiency of the ion exchange resin membrane in preventing the anions in the electrolyte from discharging at the anode. The parameters pertaining to the alternating conditioning current in this invention vary between wide limits. Preferably the frequency is comprised between about 0.1 and 100 Hz but this can be exceeded if necessary. The average current (rms) is preferably in the range of 5 - 500 mA/cm².g but again these limits can be exceeded depending on electrode construction, thickness of the active layers and structure of the coatings. The negative pulses should exceed the positive pulses by preferably at least 10% but can go to an upper limit of 3 or 4 times. It should be noted that the unbalance of the negative pulses versus the positive ones may not necessarily be in terms of intensity values but can also be in terms of time, i.e. for instance the intensities may be the same but the negative pulses may last longer than the positive pulses, e.g. 10% longer or twice as long, or the like. This distribution of the pulses can be effected conventionally by usual electronic means with which skilled ones are familiar and which need not be discussed in detail here. The other parameters like selection of cell solvents, supporting salts, electrode collector materials are all conventional and in conformity with the data from the prior art (see the references cited in the introduction). The examples below illustrate the invention. Example 1 An electrode made of a bundle of strands of about 10.000 graphite fibers (SEROFIM, Gennevillers, France), size 30 x 2 mm, thickness 2 mm was connected to a stainless steel current lead wire and dipped into a 1 molar aqueous solution of pyrrole containing 0,5 moles/L of sodium tosylate. The lead wire was protected against attack by the electrolyte. Naturally, other electroconductive materials, such as Pt, Ni, Cu and the like can also be used as lead wires. A positive current (counterelectrode, a platinum plate of 40 x 10 mm) of 10 mA/cm² was applied to the carbon electrode for 1 hr, whereby a quantity of about 20 mg of polypyrrole doped with tosylate ions was deposited thereon as a black layer (current yield about 2 g/Ahr). The electrode thus doped in positive form can be stored dry for any period of time. The above polypyrrole coated electrode was then coated with a layer of NAFION® by dipping into a 50% by weight solution or suspension of this material (commercially available from Aldrich Chemical Co., Milwaukee, Wi, USA) and allowed to dry in air. This provided an approximately 50 µm layer of ion-exchange resin over the polypyrrole. It should be noted that these operations can be repeated several times and that the order of deposition of the layers (polypyrrole and NAFION®) can be reversed. Other commercial ion-exchange resins are also convenient. A NAFION® coated polypyrrole electrode as prepared by the foregoing procedure and another larger polypyrrole electrode (not carrying a NAFION® overcoat) were used to make a cell of the kind illustrated in fig. 1. The electrolyte was a 1 molar solution of LiClO₄ in propylene carbonate. For conditioning, an alternating current (f = 50 HZ) with 1 mA negative pulses and 0.5 mA positive pulses was applied until the voltage was about equal to the reference (Li/Li⁺); this corresponds to about -2 V with respect to the H/H⁺ standard potential. Then the cell was allowed to discharge through the potentiostat 6 as indicated earlier to give the voltage/current curve depicted in fig. 3. Afterwards, the cycles of charge and discharge (DC only) could be repeated an indefinite number of times (>150 times) without significant loss of electrode performance. The initial charge of more than 0.6 C could be steadily increased to more than 5 C between 0.03 and 2 V, showing the remarkable advantages of the invention. Example 2 A piece of fine-mesh expanded Ni screen was cut so as to provide a circular current collector. 10 mg of chemically prepared polypyrrole (tosylate-doped) was pressed under 15 t pressure into this screen. A nickel wire was the current lead; it was welded to the screen beforehand. A Nafion coating was applied to this electrode by dipping into a Nafion® solution (see Example 1) and allowing to dry. After complete drying, the electrode was arranged as shown in Fig. 1 using an electrolyte and a counterelectrode like in Example 1 to make a complete cell. Again the counterelectrode was larger than the main electrode so any limitations in performance are attribuable to the structure with Nafion. The electrolyte was 1 M LiClO₄ in a mixture of 80% propylene carbonate and 20% ethylene carbonate. For conditioning, 100 pulses of alternating direction (0.5 mA negative, 0.3 mA positive) were applied. The potential of the electrode tested as battery anode were observed during the pulses. It shifted in the negative direction, first slowly and then more markedly, until the negative end potential reached during pulses was in the vicinity of the Li/Li⁺ potential. After conditioning, the cell was able to sustain charge-discharge cycles in which the "negative", i.e. the nickel-polypyrrole-Nafion structure, functioned with a discharging capacity increasing from 15 C/g to over 80 C/g of active material with increasing number of cycles between 0.01 and 1.8 V vs Li/li⁺. The testing was carried to about 40 cycles, the faradaic round trip efficiency was 90 - 96%

Claims: Method of conditioning an anode of a rechargeable storage battery having cells containing a non-aqueous alkali-metal salt solution electrolyte and current collector substrates coated with one or more polypyrrole layers and one or more ion-exchange resin layers placed over each other as anode and cathode electrodes, in which said anode is immersed in a non-aqueous lithium salt solution with a counterelectrode and a conditioning current is applied to said anode against said counterelectrode, characterized in that the conditioning current comprises balanced or unbalanced alternating positive and negative pulses. Method of claim 1 characterized in that the conditioning current comprises unbalanced alternating positive and negative pulses. Method of claim 2, wherein the negative pulses predominate over the positive pulses by 10% or more. Method of claim 3, wherein the negative pulses are of 1-500mA/cm².g of polypyrrole and the positive pulses are about half the negative pulses. Method of claim 2, wherein the negative pulses last longer than the positive pulses. Method of claim 2, wherein the anode current collector is made of carbon, the ion-exchange resin is a perfluorinated sulfonic ion-exchange resin and the electrolyte is LiClO₄ in a polar organic solvent.

Inventors: KULHANEK ANTONIN, MUELLER KLAUS, KULHANEK, ANTONIN, MUELLER, KLAUS, Müller, Klaus 
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: H01M4
IPC codes: C08G61, H01M10, H01M4