WO1998042037A1

WO1998042037A1 - Electrochemical cell having a polymer blend electrolyte

Info

Publication number: WO1998042037A1
Application number: PCT/US1998/005123
Authority: WO
Inventors: Changming Li; Ke Keryn Lian; Han Wu; Marc Chason
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1997-03-17
Filing date: 1998-03-16
Publication date: 1998-09-24
Anticipated expiration: 1999-09-17

Abstract

An electrochemical cell is provided with first and second electrodes (10 and 11), and a solid polymer electrolyte (15) disposed therebetween. The electrodes may either be of the same or different materials and may be fabricated from ruthenium, iridium, cobalt, tungsten, vanadium, iron, molybdenum, hafnium, nickel, silver, zinc, and combinations thereof. The solid polymer electrolyte is in intimate contact with both the anode and the cathode, and is made from a homogeneous polymer blend of two or more polymers which are all ion conducting, and having doped or dispersed therein an electrolyte active species.

Description

ELECTROCHEMICAL CELL HAVING A POLYMER BLEND ELECTROLYTE

Cross Reference to Related Applications This application is related to commonly assigned, co-pending patent application serial no. 08/641,716 to Li , et al, filed May 2, 1996 , entitled "ELECTROCHEMICAL CELL HAVING MULTILAYERED POLYMER ELECTROLYTE"; and application serial no. 08/638,706 to Oliver, et al, filed April 29, 1996, entitled POLYMER GEL ELECTROLYTE, the disclosures of which are incorporated herein by reference.

Technical Field

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

Background of the Invention

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 liquid electrolytes (either aqueous or non-aqueous) 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 system. Solid polymer electrolytes alleviate the problems experienced with respect to packaging and electrolyte leakage. In addition, polymer electrolytes have the additional 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 prior art.

Polymer electrolyte systems which have received considerable interest, particularly in electrochemical capacitor applications, include polyvinyl alcohol (PVA) and polybenzimidazole (PBI), each having dispersed therein a proton conducting electrolyte active species such as H2SO4 or H3PO4. 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. PBI/H3PO4 systems address the shortcomings of PVA, but lack adhesive properties to aid in fabrication of stacked cells. A multi-layer of PVA/PBI is described in the above referenced '716 application, but that solution has likewise failed to meet all the requirements of high performance electrochemical systems. Accordingly, there exists a need to provide novel electrochemical capacitor devices free of limitations inherent in the prior art. Such a device should be characterized by a polymer electrolyte system in which the polymeric support structure or matrix thereof is stable at higher temperatures, possesses a relatively wide frequency response, and has relatively high ionic conductivity. Moreover, fabrication of such an electrolyte layer should be simple, inexpensive and readily repeatable.

Brief Description of the Drawings

FIG. 1 is a schematic representation of an electrochemical device in accordance with the instant invention; and

FIG. 2 is a CV for a device using an electrolyte system 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.

There is illustrated therein an electrochemical device, such as an electrochemical energy storage device fabricated from a pair of electrode assemblies 10 and 11. The electrode assemblies may be the anode and the cathode of the electrochemical device. The electrochemical charge storage device may be either an electrochemical capacitor or an electrochemical battery cell. The electrochemical capacitor is preferably one characterized by an oxidation/reduction charge storage mechanism, though other types of capacitors, such as double layer capacitors, may be employed. Each electrode assembly 10 and 11 includes an electrode 13 which 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 are each made from one or more materials selected from the group consisting of ruthenium, iridium, platinum, cobalt, tungsten, vanadium, iron, nickel, aluminum, antimony, bismuth, indium, tin, hafnium, molybdenum, silver, zinc, lead, manganese, alloys thereof, nitrides thereof, carbides thereof, sulfides thereof, oxides thereof, hydroxides 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 system 15 described hereinbelow. A housing or gasket 14 may be employed to house the electrode and the electrolyte, but is optional. The electrolyte system 15 is sandwiched between the electrodes and is in the form of a film, which 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, the polymer electrolyte system 15 is a polymer blend system which is disposed between and in contact with both electrode assemblies. The polymer blend electrolyte system includes at least first and second polymer components homogeneously mixed. The first polymer component may be provided, for example, to enhance conductivity, temperature tolerance, and to improve mechanical strength. The second polymer component may therefore be more formable (or less viscous) than the first polymer component, so that it can easily fill pores in the adjacent electrodes. The second polymer component may also provide an adhesive property so as to "glue" the electrolyte system together, and to the adjacent electrode assemblies. It is important to note that both polymers should be able to function as an electrolyte. This is in distinction to certain prior attempts to address the issue, in which one polymer of the blend has been inert to provide, for example, mechanical integrity, while the second polymer provided conductivity. The overall result was lower conductivity since part of the total system did not function as an electrolyte.

The polymer electrolyte comprises a polymer support structure or matrix which has an electrolyte active species doped, disposed or dispersed therein. As described above, the polymer support structure or matrix preferably is fabricated as a polymer blend including at least a first and second polymer component. It is to be understood that more than two polymers may be blended together to create the polymer electrolyte system of the instant invention. An electrolyte active species is doped or dispersed in said polymeric support structure. The polymer components of the electrolyte system may be selected from any of a number of polymers, and are preferably selected from the group of polybenzimidazoles (PBI), nafion, poly vinyl alcohol (PVA), poly(ethylene glycol), acrylated epoxy, acrylated urethane, polyethyleneimine (PEI), polyethylene oxide (PEO), poly(acrylic acid) (Paa), poly(acrylamide) (PAAM), and poly(2-hydroxyethyl methacrylate), poly(vinyl pyridine) (P2VP), poly(vinyl pyrrolidone) (PVP), poly(vinyl fluoride) (PVF), polyimide, polyamide, poly(vinyl methylethyl ether), phenol formaldehyde, and combinations thereof. The polymers may be mixed in any functioning ratio, but is typically mixed in a ratio of about 1:1. PBI type electrolytes, and preferably poly{2,2'-m-(phenylene)-5,5'- bibenzimidazole type PBFs are described in commonly assigned, copending application serial no. 08/629,174 in the names of Li, et al, the disclosure of which is incorporated by reference are preferred as the first polymer. Similarly, the second polymer material may be any one of a number of polymers, and is preferably selected from the group of polymers described above, with particular preference being for PVA or PAAM, and combinations thereof. In one preferred embodiment, the first polymer material is PBI, while the second polymer is PAAM.

Dispersed within or doping the polymer support structure is an electrolyte active species, for example a proton conducting electrolyte active species. The proton conducting electrolyte active species may be selected from the group of materials consisting of H₃P0₄ (phosphoric acid), H₂S0₄ (sulfuric acid), HC1 (hydrochloric acid), HN0₃, Boric acid, hetero polyacids, and combinations thereof. Alternatively, the electrolyte active species may be a metal hydroxide such as KOH, NaOH, LiOH, CeOH, and combinations thereof. Further, modifiers may be added to the polymer blends to increase electrolyte conductivity, and wetability of the electrodes. Preferred modifiers are porphines and/or porphyrins.

EXAMPLES Example 1: Preparation of PBI-PAAM-acrylic acid blend proton-conducting polymer electrolyte

10 g of Celazole Powders, 100 mesh, and PBI polymer 0.55 (Hoechst

Celanese) were mixed with 240 g of 85% H₃P0₄ in a glass beaker. The acid and

PBI powder completely mixed (about 2 - 5 minutes) until a paste was formed. The mixture was heat treated at 200°C in the covered beaker. After about 1 hour, when the mixture in the furnace became extremely viscous and a uniform one-phase "solution," it was removed from the furnace. The

"solution" in the beaker looked like a colored butter and behaved

Theologically similarly, but it was not adhesive.

10 g of poly(acrylamide-co-acrylic acid) (10 wt. % acrylamide) (Aldrich, Catalog No. 19,197-3) (PAAM), was weighed and immediately and slowly added to the beaker containing the hot PBI butter electrolyte out of the oven, while stirring with a glass stirring rod. Stirring continued with the PBI- PAAM butter electrolyte until a one-phase uniform "solution" formed.

This blended polymer electrolyte was adhesive and convenient for assembly of electrolyte-coated or printed electrodes into an integrated device through bonding between electrolyte films and electrodes.

Example 2: Preparation of porphine -modified PBI-PAAM-acrylic acid blend proton-conducting polymer electrolyte

Meso-Tetra(N-Methyl-4-Pyridyl)-Porphine was directly added into the blended polymer electrolyte described above and was stirred to form a uniform 0.05 wt.% porphine-modified electrolyte. The conductivity measured by making Ru0₂/Ti foil electrode-based single cell capacitor devices was improved by 20-50% depending on the sample. In principle, any porphine and/or porphyrins could be added in the blended polymer electrolytes to improve the electrolyte performance.

Example 3: Different blend proton-conducting polymer electrolyte systems

Ru0₂/Ti foil electrodes were used to make single cell capacitor devices with different multipolymer electrolytes for characterization. To make single cell devices, each single sided electrode surface was coated with the multi-polymer electrolyte and two of them were laminated together. The conductivities for different multi-polymer electrolyte systems were measured by a HP milliohmmeter. The measured results are shown in Table I.

TABLE I

The results demonstrate that multipolymer electrolytes have better conductivity than those made from individual polymers. This may be due to more free volume created by the polymer mixtures than by individual polymers. Further, the multi-polymer electrolytes are not homogeneous but rather are acid (or salt, alkali)-doped non-gelatinous polymer particles bonded by gelatinous polymer gel. This property makes these electrolytes castable and printable for manufacturing processes and can eliminate shorts during the assembly processes due to the polymer particle spacers. The electrolyte-coated or printed electrodes can significantly reduce interfacial effects for low ESR devices.

Example 4: Alkaline-based blended polymer electrolyte

A four component multipolymer electrolyte was made by directly mixing 35% PVP, 30% PEO, 30% PAAM, and 5% Paa with 31% KOH. All of these polymers can be used separately to make polymer gel electrolytes. However, these polymers have to be made as films and then doped by salt, alkali or acid for electrolyte applications. If they are separately mixed with an acid or alkali, PEO, PAAM, and Paa would immediately form solid gels, which are too gelatinous for manufacturing processes such as coating, casting, printing etc. for electrolyte applications. PVP remains a powder with doped acid or alkali and is not adhesive, and hence is very difficult to cast and print. However, when all four components are mixed together, a homogenous phase will form even in 31% KOH solution. A conventional Ag/Zn cell was made with this electrolyte. The cell had an open circuit voltage of 1.5 V. CV experiments showed that the cell had good performance, the current density could reach about 115 mA/cm² without significant shape distortion of CV curves, as is shown in FIG. 3. The measured conductivity of the cell was 4xl0^"2 S/cm. Further, no dendrite growth was found when the cell was opened after 1500 cycles (line 60), indicating this electrolyte could prevent dendrites in Ag/Zn cells, the most prevalent failure mode. The cell was further tested for an additional 3500 cycles. After a total of 5000 cycles (line 62), the cell showed satisfactory performance. This result far exceeds the previous results on PAAM-KOH and PVA-KOH systems, which only showed a life of only a few hundred cycles. Therefore, this type of electrolytes could have a great application in rechargeable Ag/Zn batteries.

Although electrochemical capacitors are described herein, it is to be understood that the polymer blend electrolyte of the instant invention is not so limited. Indeed, it may be used equally well in any type of electrochemical device, examples of which include, but are not limited to, electrochemical batteries, fuel cells, electrochromic devices, and electrolytic devices, to name a few. Indeed the device of Example 4 is an electolchemical battery which benefited greatly from the instant invention.

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. An electrolyte system comprising: a polymer blend support structure including at least first and second polymer components, said polymer blend comprising a homogeneous mixture of at least two polymers selected from the group consisting of polybenzimidazoles (PBI), nafion, poly vinyl alcohol (PVA), poly(ethylene glycol), acrylated epoxy, acrylated urethane, polyethyleneimine (PEI), polyethylene oxide (PEO), poly(acrylic acid) (Paa), poly(acrylamide) (PAAM), poly(2-hydroxyethyl methacrylate), poly(vinyl pyridine) (P2VP), poly(vinyl pyrrolidone) (PVP), poly(vinyl fluoride) (PVF), polyamide, polyimide, poly(vinyl methylethyl ether), phenol formaldehyde, and combinations thereof; and an electrolyte active species dispersed in said polymer blend support structure.

2. An electrolyte system as in claim 1, wherein said first polymer component is poly{2,2'-m-(phenylene)-5,5'-bibenzimidazole}, and said second polymer component is poly (aery lamide).

3. An electrolyte system as in claim 1, wherein said electrolyte active species is a proton conducting electrolyte active species.

4. An electrolyte system as in claim 1, wherein said first polymer component is PBI, and said second polymer component is PAAM, mixed in a ratio of 1:1.

5. An electrolyte system as in claim 1, wherein said first polymer component is PBI and said second polymer component is PVP, mixed in a ratio of 1:1.

6. An electrolyte system as in claim 1, further including a modifier.

7. An electrolyte system as in claim 6, wherein said modifier is a porphine or a porphyrin modifier.

8. An electrochemical device comprising: first and second electrodes; and an electrolyte system including an electrolyte active species dispersed in a polymer blend support structure which includes at least first and second polymer components, said polymer blend comprising a homogeneous mixture of at least two polymers selected from the group consisting of polybenzimidazoles (PBI), nafion, poly vinyl alcohol (PVA), poly(ethylene glycol), acrylated epoxy, acrylated urethane, polyethyleneimine (PEI), polyethylene oxide (PEO), poly(acrylic acid) (Paa), poly(acrylamide) (PAAM), poly(2-hydroxyethyl methacrylate), poly(vinyl pyridine) (P2VP), poly(vinyl pyrrolidone) (PVP), poly(vinyl fluoride) (PVF), poly(vinyl methylethyl ether), phenol formaldehyde, and combinations thereof.

9. An electrochemical device as in claim 8, wherein said first and second electrodes are fabricated of materials selected from the group of Ru,

Ir, Co, W, V, Fe, Mo, Ni, Ag, Zn, Pb, Hf, Mn alloys thereof, oxides thereof, carbides thereof, nitrides thereof, sulfides thereof, and combinations thereof.

10. An electrochemical device as in claim 8, wherein said device is a battery.

11. An electrochemical device as in claim 8, wherein said device is a capacitor.