WO1998026466A1

WO1998026466A1 - Sulfonated polybenzimidazole polymer electrolyte and electrochemical cell using same

Info

Publication number: WO1998026466A1
Application number: PCT/US1997/022175
Authority: WO
Inventors: Changming Li
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1996-12-09
Filing date: 1997-11-26
Publication date: 1998-06-18
Anticipated expiration: 1999-06-09

Abstract

An electrochemical cell is provided with first (10) and second (11) electrodes and a solid polymer electrolyte (15) disposed therebetween. The solid polymer electrolyte is preferably fabricated by providing a polybenzimidazole polymeric material which is subjected to a sulfur containing acidic species, at temperatures sufficient to sulfonate the polybenzimidazole. Electrochemical cells fabricated using these devices demonstrate performance characteristics far better than those available in the prior art.

Description

SULFONATED POLYBENZIMIDAZOLE POLYMER ELECTROLYTE AND ELECTROCHEMICAL CELL USING SAME

Technical Field This invention relates in general to electrochemical cells, and more particularly to electrochemical cells having a polymer electrolyte comprising a polymeric matrix or support structure and an electrolyte active species dispersed therein.

Background

Energy generation and storage has long been a subject of study and development. Of special importance is the storage of electrical energy in a compact form that can be readily charged and discharged such as rechargeable electrochemical batteries and/or electrochemical capacitors. High power, high current pulse rechargeable electrochemical charge storage devices are also becoming increasingly important in applications in which electrical pulses are demanded of the battery cells. Examples of such devices include digital communication devices, power tools, and portable computers to name but a few. In each of these devices, high electrochemical kinetic rate, long cycle life of the electrode material and good ionic conductivity of the electrolyte are all extremely important considerations.

Most electrochemical cells have heretofore relied upon aqueous or liquid electrolytes to provide ionic conductivity between the electrodes thereof. Unfortunately, aqueous liquid electrolytes have problems associated with sealing, packaging, and electrolyte leakage, all of which are well known in the industry. Solid polymer electrolytes were developed by numerous different companies in an effort to address the problems associated with liquid aqueous electrolytes. Each of these different types of solid polymer electrolyte systems have met with varying degrees of success, typically owing to the fact that ionic conductivity is generally not as good as that found in a liquid aqueous system. Solid polymer electrolytes alleviate the problems experienced with respect to packaging and electrolyte leakage. In addition, polymer electrolytes have the advantage of being able to be formed into thin films to improve the energy density, and to act as an electrode spacer in order to eliminate an inert separator used in the prior art. One polymer electrolyte system which has received considerable interest particularly in electrochemical capacitor applications, is polyvinyl alcohol (PVA), having dispersed therein a proton conducting electrolyte active species such as H2SO4 or H3PO4. This system is described in, for example, U.S. Patent Application Serial No. 08/547,821 to Lian, et al, filed October 25, 1995 the disclosure of which is incorporated herein by reference. Unfortunately, the PVA/H3PO4 electrolytes developed heretofore are not completely stable at elevated temperatures. The mechanical strength of thin films of PVA based polymer electrolytes also needs further improvement for eliminating shorts during the assembly process. Further, the frequency response of certain polymer electrolyte based electrochemical capacitor devices is relatively narrow in comparison to dielectric capacitors. This performance differential may be partially improved by developing polymer electrolytes which have higher ionic conductivity.

A second polymer electrolyte which has received some interest is polybenzimidazole ("PBI"), as is disclosed in, for example, commonly assigned, co-pending patent application serial no. 08/693,780, filed July 22, 1996 in the name of Li, et al, the disclosure of which is incorporated herein by reference. While PBI has demonstrated very acceptable characteristics with respect to electrochemical performance, it has not, heretofore, demonstrated the very high thermal stability, and chemical resistance found in other types of materials, notably perfluorinated membranes such as Nafion.

Accordingly, there exists a need to provide novel electrochemical devices incorporating polymer electrolyte materials free from the limitations inherent in the prior art. Such polymer electrolyte materials should possess high thermal stability, and good chemical resistance, as well as the high ionic conductivity required by most electrochemical cells. Further, such a material should be easily manufactured and economical for use in consumer electrochemical devices. Finally, fabrication of such an electrolyte layer should be relatively simple, inexpensive and readily repeatable.

Brief Description of the Drawings

FIG. 1 is a schematic representation of an electrochemical charge storage device in accordance with the instant invention; and FIG. 2 is a schematic representation of a second electrochemical charge storage device in accordance with the instant invention. Detailed Description of the Preferred Embodiment

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.

Referring now to FIG. 1, there is illustrated therein an energy storage device such as an electrochemical charge storage device fabricated from a pair of electrode assemblies 10 and 11, which may be the anode and the cathode of the device. The electrochemical charge storage device may be an electrochemical capacitor or an electrochemical battery cell. The electrochemical capacitor is preferably an electrochemical capacitor characterized by an oxidation/reduction charge storage mechanism. Each electrode assembly 10 and 11 includes an electrode 13 which electrodes may either be fabricated from the same or different materials. In the instance in which the electrodes are fabricated of the same material, they are referred to as "symmetric electrodes". Conversely, if they are made from different materials, they are referred to as "asymmetric electrodes". Regardless of whether or not the electrodes are asymmetric or symmetric, they may be each made from one or more materials selected from the group consisting of ruthenium, iridium, platinum, cobalt, tungsten, vanadium, iron, nickel, molybdenum, silver, zinc, lead, manganese, alloys thereof, nitrides thereof, carbides thereof, sulfides thereof, oxides thereof, and combinations thereof. Alternatively, said electrodes may be fabricated of conducting polymers. Each electrode assembly may further include a current collector 12 which is electrically conducting. The current collector 12 is preferably chemically inert in the polymer electrolyte 15 described hereinbelow. A housing or gasket 14 may be employed to house the electrode and the electrolyte, but is optional. The electrolyte 15 is sandwiched between the electrodes and is in the form of a film, such as a polymer, and may also serve as a separator between the two electrodes. This structure thus affords free and unobstructed movement to the ions in the electrolyte. The combination electrolyte /separator prevents contact between the opposing electrodes since such a condition would result in a short circuit and malfunction of the electrochemical cell.

Referring now to FIG. 2, there is illustrated therein a second electrochemical device, such as an electrochemical capacitor, which may be adapted to employ an electrolyte material such as that disclosed hereinbelow. The device of FIG. 2 is a bipolar electrochemical capacitor device which includes a seven capacitor subassemblies 44, 46, 48, 50, 52, 54, and 55, which may be arranged in stacked configuration. As each subassembly is essentially identical, only one, assembly 44 will be described in detail.

Capacitor subassembly 44 includes a first layer 30 which is a bipolar metal substrate or foil. The bipolar metal foil is fabricated to be both the substrate upon which active electrode materials are deposited, as well as a current collector for the charge generated by the materials. Accordingly, layer 30 may be fabricated of a number of different materials selected from the group consisting of carbon, aluminum, tin, indium, titanium, copper, nickel, brass, stainless steel, silver, titanium /tantalum alloys, alloys thereof, and combinations thereof. Layer 30 includes first and second major surfaces upon which are deposited layers of electrode active material 32 and 34. The electrode active materials may be fabricated of symmetric or asymmetric materials such as those described hereinabove with respect to FIG. 1.

Disposed upon at least one of said electrodes is a layer of an electrolyte material 36 in accordance with the instant invention. The electrolyte material 36 as illustrated in FIG. 2 is disposed upon electrode layer 34. As maybe appreciated from FIG. 2, a completed single cell bipolar device 44 comprises a bipolar metal foil, with electrodes disposed on either side of said foil and a layer of electrolyte material disposed on at least one of said electrodes. Incorporated into a multicell device, a plurality of such single subassemblies may be arranged in stacked configuration. Accordingly, seven such devices , 44, 46, 48, 50, 52, 54, and 55 may be arranged in stacked configuration in order to increase the voltage output therefrom. It is to be understood that the number of cells arranged in stacked configuration may be varied. Disposed adjacent the outer most cells 44 and 55 are end plates 56 and

58 adapted to collect current generated by the stacked subassemblies. It is to be understood that while the devices illustrated with respect to FIGs. 1 and 2 are electrochemical capacitors, the invention is not so limited. Indeed, the electrolyte material described hereinbelow, may be readily adapted for use in capacitors, electrochemical battery cells, fuel cells, electrochemical sensors, and any other type of electrochemical cell requiring an electrolyte material for providing ionic conductivity. The electrolyte materials described herein may be fabricated by providing a polybenzimidazole ("PBI") polymeric material which has been sulfonated. The PBI material is subjected to a sulfur containing acidic species at temperatures sufficient to induce sulfonation of the PBI material. The PBI material may be provided in any number of different forms, but is preferably provided in a form selecting from the group consisting of powders, films, fibers, particles, fabric, powders, and combinations thereof.

The sulfur containing acidic species is selected from the group consisting of H2SO4, H2SO3, H2O + SO2, H2O + SO3, and combinations thereof. In a preferred embodiment, the sulfur containing acidic species is H2SO4. The sulfur containing acidic species is typically provided in molar concentrations of between 0.01 and 7.6, and preferably 5.0. The ratio of sulfur containing acidic species to polymeric material is between about 2:1 and 40:1, and preferably between 10:1 and 30:1. In order to induce sulfonation, the PBI material and sulfur containing acidic species are heated to temperatures of between 30 and 95°C, and preferably heated to temperatures of between 50 and 80°C. During heating, sulfuric acid or sulfur oxides plus water vapor, react with the PBI to bond an anion, providing both cationic and anionic sites on the polymer. Thus, sulfonation happens, and the anion (SO3") is bonded on the polymer. Further, the sulfonation process produces some cross-linking of the PBI, resulting in a more stable polymer. The process by which the PBI electrolyte may be fabricated can be better understood from the following formula:

Poly{2,2'-m-(phenylene)-5,5'-bibenzimidazole}

Heat treatment

Sulfonation Process of PBI

EXAMPLE

In order to demonstrate that sulfonated PBI functions adequately as a solid electrolyte, an electrochemical capacitor was fabricated using a commercially available, non-woven sulfonated PBI material sandwiched between a pair of Ruθ2 electrodes deposited on Ti substrates, as are well known in the art. Cyclic voltammogram ("CV") and conductivity measurements were run on this device to measure its performance as an electrochemical capacitor. As originally tested, the solid electrolyte film was dry and demonstrated an unacceptably high equivalent series resistance ("ESR"). The device was the "wetted" by exposing it to steam for 1 minute. The CV results illustrated a device having low ESR and high conductivity. Conductivity was specifically measured for this film, and was 2.8x10"3 Ohms^"! per centimeter (" ^"V m"). With additional steaming of up to four minutes, conductivity increased to 6x10^~2 -1/cm. This compares very favorably to Nafion which, after steaming in sulfuric acid for 8 hours demonstrates conductivity of 2xl0"3 -1/cm.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

What is claimed is:

Claims

1. A method of making an electrolyte material for an electrochemical cell, said method comprising the steps of: subjecting a polybenzimidazole polymeric material to a sulfur containing acidic species; and heating said polybenzimidazole polymeric material and said acidic species to temperatures sufficient to induce sulfonation of said polybenzimidazole polymeric material.

2. A method as in claim 1, including the further step of providing the polybenzimidazole polymeric material in a form selecting from the group consisting of powders, films, fibers, particles, fabric, powders, and combinations thereof.

3. A method as in claim 1, wherein said sulfur containing acidic species is selected from the group consisting of H2SO3, H2O + SO2, H2O + SO3, and combinations thereof.

4. A method as in claim 1, wherein the ratio of sulfur containing acidic species to polymeric material is between 2:1 and 40:1.

5. A method as in claim 1, wherein the ratio of sulfur containing acidic species to polymeric material is between 10:1 and 30:1.

6. An electrochemical cell comprising: first and second electrodes fabricated from materials selected from the group consisting of Ru, Ir, Pt, Co, W, V, Fe, Ni, Mo, Ag, Zn, Pb, Mn, conductive polymers, alloys of the foregoing, oxides of the foregoing, and combinations thereof; and an electrolyte material comprising a sulfonated poly(benzimidazole).

7. An electrochemical cell as in claim 6, wherein said electrodes are symmetric.

8. An electrochemical cell as in claim 6, wherein said electrodes are asymmetric.

9. An electrochemical cell as in claim 6, wherein said electrodes are fabricated of Ru.