WO1998037589A1

WO1998037589A1 - Polymeric electrolyte and electrochemical cell using same

Info

Publication number: WO1998037589A1
Application number: PCT/US1998/003264
Authority: WO
Inventors: Ganesh Venugopal; Anaba A. Anani; Dimitrios Vitsas
Original assignee: Motorola Inc.
Priority date: 1997-02-21
Filing date: 1998-02-20
Publication date: 1998-08-27

Abstract

An electrochemical cell (10) includes first and second electrodes (12 and 14) with an electrolyte system (26) disposed therebetween. The electrolyte system includes at least a first and second layer (52 and 70), the second layer (70) being used to absorb an electrolyte active species. The layers of material are fabricated of the same or a similar polymeric material, however, the physical characteristics of the different layers may vary.

Description

POLYMERIC ELECTROLYTE AND ELECTROCHEMICAL CELL USING SAME

Technical Field This invention relates in general to the field of electrolytes for electrochemical cells, and more particularly to electrochemical cells using polymer gel electrolytes.

Background of the Invention There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as cellular communication, satellites, portable computers, and electric vehicles to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective batteries having improved performance characteristics.

Rechargeable or secondary cells are more desirable than primary (non- rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being recharged by the application of an electrical charge thereto. Numerous advanced electrode systems have been developed for storing electrical charge. Concurrently much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities and performance of electrochemical cells.

Heretofore, electrolytes have been either liquid electrolytes as are found in conventional wet cell batteries, or solid films as are available in newer, more advanced battery systems. Each of these systems have inherent limitations and related deficiencies which make them unsuitable for various applications. Liquid electrolytes, while demonstrating acceptable ionic conductivity, tend to leak out of the cells into which they are sealed. While better manufacturing techniques have lessened the occurrence of leakage, cells still do leak potentially dangerous liquid electrolytes from time to time. Moreover, any leakage in the cell lessens the amount of electrolyte available in the cell, thus reducing the effectiveness of the device.

Solid electrolytes are free from problems of leakage, however, they have traditionally offered inferior properties as compared to liquid electrolytes. This is due to the fact that ionic conductivities for solid electrolytes are often one to two orders of magnitude poorer than a liquid electrolyte. Good ionic conductivity is necessary to insure a battery system capable of delivering usable amounts of power for a given application. Most solid electrolytes have not heretofore been adequate for many high performance battery systems. One class of solid electrolytes, specifically gel electrolytes, have shown great promise for high performance battery systems. Gel electrolytes contain a significant fraction of solvents and/or plasticizers in addition to the salt and polymer of the electrolyte itself. One processing route that can be used to assemble a battery with a gel electrolyte is to leave the electrolyte salt and solvent out of the polymer gel system until after the cell is completely fabricated. Thereafter, the solvent and the electrolyte salt may be introduced into the polymer system in order to swell and activate the battery. While this approach (which is described in, for example, U.S. Patent No. 5,456,000 issued October 10, 1995) has the advantage of allowing the cell to be fabricated in a non-dry environment (the electrolyte salt in a lithium cell is typically highly hygroscopic) it offers problems with respect to performance and assembly. First, the gel electrolyte may lack sufficient mechanical integrity to prevent shorting between the electrodes while they are being bonded or laminated together with the gel electrolyte. The gel electrolyte layer thickness is reported to be 75 μm, presumably to overcome this shorting problem and to help facilitate handling of the electrolyte material. When compared to the 25 μm typical thickness for separators used in liquid lithium ion cells, this results in a significant reduction in the volumetric energy density for the cell. Second, in order to create porosity in the electrolyte and electrode layers that will be used to absorb liquid electrolyte, a plasticizer is used. Unfortunately, the subsequent removal of this plasticizer to create the pores requires the use of flammable organic solvents. These problems, among others, are significant commercial limitations to the successful implementation of gel electrolytes in electrochemical cells.

Successful solutions to these problems are taught in, for example, commonly-assigned, co-pending patent application serial nos. 08/714,032 filed September 23, 1996, and 08/720,062 filed September 27, 1996, both to Eschbach, et al, which each disclose processes in which a first polymeric material is coated with layers of a second polymeric material. However, the commercial feasibility of these processes and structures is unproved, and the associated manufacturing costs are not known. Accordingly, there exists a need for a new polymeric electrolyte system which combines the properties of good mechanical integrity, as well as the ability to absorb sufficient amounts of an electrolyte active species so as to produce an electrolyte system with the high ionic conductivity characteristic of liquid electrolytes. The electrolyte so formed should be relatively easy to produce, without need for complex manufacturing processes-

Brief Description of the Drawings

FIG. 1 is a cross sectional side view of an electrochemical cell in accordance with the invention;

FIG. 2 is a cross-sectional side view of an electrolyte layer for use with an electrochemical cell, in accordance with the invention;

FIG. 3 is a flow chart illustrating a method for processing a laminated structure in accordance with the invention; and FIG. 4 is a chart illustrating cycle number versus discharge capacity in mAh for three test cells employing an electrolyte system, in accordance with the 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 a cross sectional side view of an electrochemical cell having a multi-layered electrolyte system, in accordance with the instant invention. The cell 10 includes first and second electrodes 12 and 14 respectively. The first electrode may be, for example, an anode in a lithium rechargeable cell. Accordingly, the anode may be fabricated of any of a number of different known materials for lithium rechargeable cells, examples of which include metallic lithium, lithium, lithium alloys, such as lithium: aluminum, and lithium intercalation materials such as carbon, petroleum coke, activated carbon, graphite, and other forms of carbon known in the art. In one preferred embodiment, the anode 12 is fabricated of an amorphous carbonaceous material such as that disclosed in commonly assigned, co-pending U.S. Patent Application Serial No. 08/561,641 entitled "Improved Carbon Electrode Materials For Lithium Battery Cells And Method of Making Same" filed on November 22, 1995, in the names of Jinshan Zhang, et al, the disclosure of which is incorporated herein by reference.

More particularly, the anode 12 comprises a layer of active material 16 such as a carbon material as described hereinabove deposited on a substrate 18. Substrate 18 may be any of a number of materials known in the art, examples of which include copper, gold, nickel, copper alloys, copper plated materials, and combinations thereof. In the embodiment of FIG. 1, the substrate 18 is fabricated of copper. The second electrode 14 may be adapted to be the cathode of a lithium rechargeable cell. In such an instance, the cathode is fabricated of a lithium intercalation material such as is known in the art, examples of which include lithiated magnesium oxide, lithiated cobalt oxide, lithiated nickel oxide, and combinations thereof. In one preferred embodiment, the cathode 14 is fabricated of a lithiated nickel oxide material such as is disclosed in commonly assigned, co-pending U.S. Patent Application Serial No. 08/464,440 in the name of Zhenhua Mao filed June 5, 1995, the disclosure of which is incorporated herein by reference.

More particularly, the cathode 14 comprises a layer of the cathode active material 20 disposed on a cathode substrate 22. The cathode material 20 maybe such as that described hereinabove, while the substrate may be fabricated from any of a number of materials known in the art, examples of which include aluminum, nickel, and combinations thereof- In one preferred embodiment, substrate 22 is fabricated of aluminum. Disposed between electrodes 12 and 14 is a polymeric electrolyte material system 26. The electrolyte system 26 comprises an electrolyte active species and a polymeric support structure consisting of a layer of polymeric material which has different characteristics at different regions thereof. For example, the polymeric support structure can comprise at least first and second layers 28 and 30 of a similar polymeric material, the first and second layers having differing physical characteristics. The differing physical characteristics can include, for example, porosity, crystallinity, molecular weight, composition, and combinations thereof, to name a few.

Alternatively, the polymeric support structure can be thought of as a single layer having, for example, first and second surfaces, and a central region. The physical characteristics of the material vary depending upon where in the material one looks. For example, the porosity of the material may be greater at the surfaces than in the central region. Alternatively, one of the other physical characteristics described above may be varied or graded across the layer of material.

Preferred materials from which to fabricated the layer or layers of the polymeric material include poly (vinyli dene fluoride), polypropylene, polyethylene, polytetrafluorethylene, polyethyleneterephthalate, hexafluoro propylene, polystyrene, ethylene propylene diene monomer, nylon, polyurethane, polyethylene oxide, polyacrylonitrile, polymethylacrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate, copolymers of any of the foregoing, and combinations thereof. In one preferred embodiment, the polymeric support structure is fabricated of poly (vinylidene fluoride), in which either the porosity of the outer regions or layers is greater than that of the inner layer or region, or in which the crystallinity of the inner region or layer is greater than that of the outer layer or regions.

More particularly, first polymeric layer, i.e., the inner layer or region may be fabricated of at least about 60 weight % crystalline polyvinylidene fluoride. Accordingly, second (and third) layer(s), i.e., the outer layers or surfaces, may be fabricated of polyvinylidene fluoride which is less than or equal to 40 weight % crystalline. Where porosity is the critical measure, i.e., the varied physical characteristic, the first layer or inner region may be fabricated of less than about 55 volume % porous polyvinylidene fluoride and the second or outer layers or regions are fabricated of at least about 70 volume % porous polyvinylidene fluoride. While the polymeric support structure may be fabricated of a single layer, it may alternatively be formed as a plurality of layers of similar polymeric material. Accordingly, the structure can be thought of as comprising a single inner layer 28 which provides mechanical integrity to the support structure, while the outer layer or layers sandwich the inner layer. The outer layers may be provided to better absorb the electrolyte active species, and to also form a gel for adhering to the electrodes in an electrochemical cell, such as that described above. It is to be understood however, that the inner layer will also absorb the electrolyte active species, and gel, however gelling and absorption is to a lower degree than the outer layers and takes place at higher temperatures. This is due to the fact that the inner layer is typically fabricated of a polymeric material with lower porosity or higher crystallinity. Accordingly, the inner layer is either non-gelling, or gels only slightly, while the outer layer or layers gel to a much greater degree owing to higher porosity or crystallinity.

The electrolyte active species is a liquid or solid component (or both) which provides ionic conductivity between the anode and the cathode. In the embodiment in which the electrochemical cell 10 is a lithium intercalation cell, the electrolyte active species consists of an alkali metal salt in a solvent. Typical alkali metal salts include, but are not limited to, salts having the formula M⁺X" where M⁺ is an alkali metal cation such as Li^4", Na⁺, K⁺, and combinations thereof; and X" is an anion such as. Cl^", Br", I", CIO4-, BF4-, PF6^", AsF6^", SbF6^", CH3CO2-, CF3SO3- , N(CF3Sθ2)2^_,

C(CF Sθ2)2" and combinations thereof. The solvent into which the salt is dispersed is typically an organic solvent including, but not limited to, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropylcarbonate, dimethylsulfoxide, acetonitrile, dimethoxyethane, tetrahydrofuran, n-methyl-2-pyrrolidone (NMP), acetone and combinations thereof. For other electrode combinations, i.e., Ni-Cd or Ni-metal hydride, other electrolyte active species may be used, such as OH.

Referring now to FIG. 2, there is illustrated therein a cross-sectional side view of the electrolyte system 26 of FIG. 1. The electrolyte system 26 comprises a first polymeric layer or region 52. The polymeric material is substantially non-gelling: That is the polymer will not form a gel in the presence of one or more solvent materials used in electrochemical cells, or if it does, gelling will not occur until after temperatures greater than that necessary to gel the outer layers described above. Layer 52 further includes first and second major surfaces 54 and 56, respectively. Layer 52 may preferably be fabricated of poly(vinylidene fluoride), having either low porosity or high crystallinity, (or both) each as defined above. As layer 52 is a layer of comparatively non-gelling polymeric material, it is hence provided to enhance the mechanical strength or integrity of the polymer electrolyte system. The central region or layer 52 may also include a filler material so as to, for example, enhance mechanical integrity. Exemplary filler materials include, hollow glass microspheres, amorphous silica, alumina, calcium carbonate, and combinations thereof. Plastic fillers like polyethylene, polypropylene, polytetrafluoroethylene, polystyrene, polyethyleneterephthalate, ethylene propylene diene monomer, nylon, hollow plastic microspheres and combinations thereof could also be used. Disposed on at least one of the first and second major surfaces, 54, 56 is a layer of a second, gelling, polymeric electrolyte carrier material adapted to absorb the electrolyte active species. The gel-forming polymer is preferably fabricated of a high porosity of low crystallinity (or both) poly(vinylidene fluoride) (PNDF). As illustrated in FIG. 2, the layer of the second polymeric material 70 is disposed on surface 54 of layer 52. A second layer 72 of a second polymeric material may be disposed on the second major surface 56 of layer 52. The layers of gel forming polymer 70, 72 may be fabricated of the same materials, as described hereinabove. It is also to be noted that the layers of polymeric material described above are all discrete layers which are stacked one atop the other into a layered structure. The layers may then be laminated into a unitary structure and subsequently disposed between opposing electrodes. Alternatively, the layers may simply be stacked adjacent one another, and between the electrodes, after which all the components are laminated and wound into an electrochemical cell.

Another advantage of the structure illustrated in FIG. 2 relates to the fact that the outside layers or region of the structure will, due to the higher porosity or lower crystallinity, have better gelling properties than the interior layer or region. This will allow the outer layers or regions to adhere well to the electrodes of an electrochemical cell into which the electrolyte structure may be incorporated. The low, or even zero degree of gelation of the middle layer or region allows it to maintain a robust, microporous structure within the overall structure. Referring now to FIG. 3, there is illustrated therein a flow chart of the steps that may be taken to fabricated a laminated multi-layered electrolyte structure in accordance with the instant invention. The flowchart 100 illustrates the process for fabricating a tri-layer structure in which the outside layers are fabricated of substantially the same materials, and have substantially the same physical characteristics. Accordingly, the step of preparing high porosity and/or low crystallinity polymer, i.e., PNDF, for the outside layers is illustrated by boxes 102 and 104, and includes the steps of blending the polymer with an appropriate plasticizer, such as dibutyl phthalate, diethyl phthalate, propylene carbonate, ethylene carbonate, and combinations thereof. The blend is then mixed in an extruder in a manner well known in the art. Concurrently, the polymer center layer is being fabricated of a similar polymer, though with different physical characteristics, i.e., lower porosity and/or high crystallinity. Accordingly, step 106 illustrates the step of preparing a different grade of the starting polymer by blending it with a plasticizer, such as that described above, and then mixing and extruding the blend.

The polymer blends prepared and extruded at steps 102, 104, and 106 are then formed into films at steps 108, 110 and 112 respectively. Films may be formed by any of a number of known techniques, examples of which include pressing, casting or extrusion, all as are well known in the art. The three discrete films are then brought together or integrated into a single structure at step 114, as by co-extrusion or lamination. Plasticizer is then removed from the integrated film structure, at step 116. Plasticizer is removed by exposing the film to a solvent extraction medium. Specifically, the laminated structure may be dipped in isopropanol for a period of time sufficient to dissolve out the plasticizer. Thereafter, the residual isopropanol may be dried, i.e., removed, by placing the film in an oven at 60°C.

The invention may be better understood from a perusal of the examples of which are attached hereto.

EXAMPLES

Example I

A pre-laminated porous trilayer PVDF electrolyte polymeric support structure was made using the following method. Two batches of PNDF and plasticizer (dibutyl phthalate) were mixed in a Haake mixer at around 180°C The first batch (I) had approximately 40 weight % crystalline PNDF (from Elf ATOCHEM, known as Kynar 461) and about 75 volume % of plasticizer while the second batch (II) had approximately 60 weight % crystalline PNDF, and about 60 volume % plasticizer. Sheets of plasticized polymers from both batches were made by hot pressing in a Carver press at around 200°C for about 3 minutes and then cooling under pressure in a separate Carver press.

The plasticizer from the sheets from each batch was extracted by dipping in isopropanol. The sheets were then dried in an oven at 60°C to remove excess isopropanol, and the porosities of the sheets were measured to be 70 % for Batch I and about 40 % for Batch H In order to make the trilayer, one sheet of plasticized PNDF from Batch II was sandwiched between two sheets of plasticized PNDF from Batch I and laminated in a benchtop roll laminator. The plasticizer from the laminate was extracted by dipping the laminate in isopropanol. The laminate was then dried at 60°C to form a porous trilayer with good interlayer adhesion.

Example II

Three 50 milliampere hour (mAh) lithium ion polymer flat cells were made using a polymer electrolyte support structure as shown above in FIG. 2. The structure included three discrete layers of PNDF, wherein the center layer was a 55 volume % porous PNDF, while the outside layers were about 70 volume % porous PNDF. Commercially available lithium cobalt oxide purchased from Nippon Chemical Corp. was used as the cathode active material, while the anode active material used was commercially available graphite known as Lonza SFG44, purchased from Timcal, Inc. The liquid electrolyte active species used in the system was a 1 Molar solution of lithium hexafluorophosphate in a mixture of 40:60 ratio of ethylene carbonate and diethyl carbonate. The electrolyte active species was added after the polymeric support structure and anode and cathode electrodes had been assembled into a stacked configuration. The stacks were packaged in an aluminum foil laminate pouch. Cell I was gelled by heating the packaged stack to a temperature of 90 degrees centigrade (°C) for three minutes, then cooled to room temperature, between stainless steel plates. Cells II and DI

were heated to 90°C for two minutes, and then cooled as Cell I. The performance results for the three test cells is summarized in Table 1 below.

The first cycle efficiencies for the three cells ranged from 82 to 87%, indicating good active material utilization. This in turn indicates that there is good interfacial adhesion between the electrodes, and the polymeric electrolyte system. All three cells completed at least 25 charge /discharge cycles with greater than 80% of the first cycle discharge capacity. Cycle number versus discharge capacity in mAh for cells I, II, and HI are illustrated by lines 120, 122, and 124 respectively in FIG. 4.

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 for an electrochemical cell, comprising a polymeric support structure having at least first and second layers of a similar polymeric material, said first and second layers having differing physical characteristics.

2. An electrolyte system as in claim 1, wherein said layers of polymeric material have differing degrees of porosity.

3. An electrolyte system as in claim 1, wherein said layers of polymeric material have differing degrees of crystallinity.

4. An electrolyte system as in claim 1, wherein said layers of polymeric material are selected from the group consisting of poly(vinylidene fluoride), polypropylene, polyethylene, hexafluoropropylene, polytetrafluorethylene, polyethyleneterephthalate, polystyrene, ethylene propylene diene monomer, nylon, polyurethane, polyethylene oxide, polyacrylonitrile, polymethylacrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate, copolymers of any of the foregoing, and combinations thereof.

5. An electrolyte system as in claim 1, wherein said polymeric support structure further comprises a third layer of polymeric material, said first layer of polymeric material being sandwiched between said second and third layers of a polymeric material, said second and third layers of polymeric material having similar physical characteristics, but differing from the physical characteristics of said first layer.

6. An electrolyte system as in claim 5, wherein said second and third layers are fabricated of less than or equal to about 40 weight % crystalline polyvinylidene fluoride and said first layer is fabricated of at least about 60 weight % crystalline polyvinylidene fluoride.

7. An electrolyte system as in claim 5, wherein said second and third polymeric layers are fabricated of at least about 70 volume % porous polyvinylidene fluoride and said first layer is fabricated of less than or equal to 55 volume % porous polyvinylidene fluoride.

8. An electrolyte system for an electrochemical cell, comprising a multi-layered structure including a non-gelling, polymeric layer sandwiched between second and third layers of a gelling, polymeric layer, said first, second and third layers being fabricated of the same polymeric material, but said second and third layers having a different degree of porosity' than said first polymeric layer.

9. An electrolyte system as in claim 8, wherein said first, second and third polymeric layers are fabricated of a material selected from the group consisting of poly (vinylidene fluoride), polyurethane, hexafluoropropylene, polyethylene oxide, polyacrylonitrile, polymethylacrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate, polypropylene, polyethylene, polytetrafluorethylene, polyethyleneterephthalate, polystyrene, ethylene propylene diene monomer, nylon, copolymers of any of the foregoing, and combinations thereof.

10. An electrolyte system for an electrochemical cell, comprising a multi-layered structure including a first polymeric layer sandwiched between second and third layers of a gelling, polymeric layer, said first, second and third layers being fabricated of the same polymeric material, but said second and third layers having a different degree of crystallinity than said first polymeric layer.

11. An electrolyte system for an electrochemical cell comprising a layer of a polymeric material having a central region and first and second surfaces wherein the degree of porosity at each said surface is greater than in the central region.

12. An electrochemical cell, comprising: an anode; a cathode; and an electrolyte system comprising a layer of a polymeric material having a central region and first and second surfaces wherein the degree of porosity at each said surface is greater than in the central region, and an electrolyte active species dispersed in at least a portion of said layer of polymeric material.