WO2000025376A1

WO2000025376A1 - Electrode for high surface area electrochemical charge storage device and method of producing same

Info

Publication number: WO2000025376A1
Application number: PCT/US1999/024742
Authority: WO
Inventors: Michael G. Semmens; Benny E. Jay; Joel Barlow; Fardad Forouzan
Original assignee: Electrosource, Inc.
Priority date: 1998-10-23
Filing date: 1999-10-22
Publication date: 2000-05-04
Also published as: AU1130400A

Abstract

A process for the manufacture of electrodes (8) for electrochemical storage devices that have charge storage material particles uniformly dispersed throughout the electrode to help facilitate the formation of a uniform network of interconnected pores. The process includes the steps of depositing a layer of powder on a current collector foil (2), the powder (4) including particles of charge storage material uniformly dispersed in particles of polymer material, and exposing the layer of powder to an energy source (6) to bond the polymer particles to one another and to the current collector foil (2). The layer of powder (4) is transformed into a charge storage material layer (9) that contains charge storage material particles that are uniformly dispersed among a matrix of bonded polymer particles, and additionally contains a substantially uniform network of interconnected pores extending therethrough.

Description

ELECTRODE FOR HIGH SURFACE AREA ELECTROCHEMICAL CHARGE STORAGE DEVICE AND METHOD OF PRODUCING SAME

Field of the Invention

The present invention relates to an electrode for a high surface area electrochemical charge storage device and a method of producing the same. In particular, the invention relates to the electrodes for a lithium polymer battery and a method of producing the same. "High surface area" refers to the electrochemically active surface area of charge storage material in the electrode.

Background of the Invention

A typical electrochemical storage device, such as a battery, includes one or more identical cells, each one of which consists of a cathode, an anode, a separator interposed between the cathode and anode, and an appropriate electrolyte. In a lithium polymer battery, the anode typically includes a copper current collector to which an anodic charge storage material layer is electronically bonded; the cathode typically includes an aluminum current collector to which a cathodic charge storage material layer is electronically bonded; and the separator and electrolyte functions are merged into a single material typically consisting of a mixture of polymer, lithium salt and organic solvent. The mixture exhibits the high dielectric strength required of a separator and the good conductivity (to lithium ion) required of the electrolyte. Such a composite separator/electrolyte material must maintain structural and electrochemical stability over the potential excursions associated with charging and discharging the cell. In an ideal battery electrode, the distribution of current through the charge storage material layer would be perfectly uniform in order to maximize the coulombic capacity of the electrode, and avoid local over-utilization of the charge storage material. In practice, however, the distribution of current through an electrode, is dependent largely upon the spatial distribution of charge storage materials (i.e., particles) throughout the structure itself and the distribution of separator and electrolyte materials. Additionally, spatial uniformity of current in the electrodes is strongly dependent upon the quality of the adhesion interface between the charge storage material layer and its underlying current collector. Conventional Processes

It is conventional in the field of manufacturing electrodes for lithium polymer batteries to prepare a paste mixture consisting of charge storage material particles, polymer, and pore formers dispersed in an appropriate solvent. These components are mixed thoroughly to achieve a uniform dispersion of binder, pore-former and charge storage material particles. The reaction of solvent and polymer creates a polymeric binder to fix the relative positions of charge storage material and pore former. The paste is then applied, typically in thin layers, to an underlying current collector either by extrusion through a slot die or by use of a knife coater. The applied layer of paste is heated to drive off the solvent. The dried residual material consists of charge storage material, pore former, and polymer, which serves as the structural host matrix. Thereafter, an additional solvent is applied to the dried residual layer to dissolve the pore-forming agent leaving a porous electrode structure having uniformly distributed charge storage material particles suspended in the host matrix and electronically connected to the current collector. In a final step, a solution consisting of an appropriate lithium salt and solvent is pressed into the dry, porous structure, and thermally cycled to encourage the attachment of lithium ions to certain target sites in the polymer structure, rendering it ionically conductive of lithium ions, which are the principal charge carriers in charging and discharging the electrode.

A significant problem with this particular conventional process arises from its use of a pore forming agent to achieve the porosity required for the process of loading the polymer with the appropriate lithium ion concentration. The pore forming agent first must be added to the paste and then subsequently removed from the dried residual material by application of an additional solvent. Since solvents used in this process are toxic, costly safety measures must be employed to recover the solvent as it escapes during the drying process. Excess solvent also must be disposed of in an environmentally safe manner. These additional procedures inflate the overall cost of manufacturing the electrode. In yet another conventional process employed for manufacturing lithium polymer electrodes, the need for pore formers has been eliminated. In this process, during paste preparation charge storage material particles, polymer, solvent(s) and lithium salt are combined and mixed thoroughly to achieve uniformly dispersed components. The solvent/salt ratio is selected to achieve a target lithium ion concentration in the polymer (binder) which renders the polymer ionically conductive, and also to provide paste rheological properties required for thin film extrusion of the paste mixture onto the current collector. At an appropriate point in the paste preparation process, typically prior to extrusion, the paste is cycled thermally to encourage lithium ion attachment to target sites in the polymer structure, which facilitates ionic conduction. An obvious advantage of this particular convention process is the introduction of solvent/salt solution to the polymer and charge storage material during paste mixing, which eliminates the need for separate pore forming, pore-material removal and pore filling procedures to make the polymer conductive. A significant disadvantage of this particular conventional process, however, arises from the rheological properties the paste mixture must have to facilitate production of thin film extrusions (the electrode) at high speed. That is, to extrude thin film lithium polymer electrodes at high speed, the paste mixture typically has a relatively low ratio of charge storage material to polymer, which results in less than ideal charge storage material concentration. At such low concentration of charge storage particles in the extruded electrode, the charge storage particles are not in intimate contact, with the result being that such electrodes typically exhibit high resistance to both charge and discharge polarization procedures. To compensate for the poor charge storage material particle contact, secondary (rechargeable) cells assembled from such electrodes require static pressures on the order of 100 psi to accept charge, and/or temperatures elevated well above room temperature (70-100°C).

Either of these requirements is costly to implement in practice and significantly reduces the intrinsically high specific energy and power potential of lithium polymer cells made with such processes.

It would be desirable to develop a process for the manufacture of electrodes for electrochemical energy storage devices, in particular, lithium polymer electrical energy storage devices, which can ensure uniform distribution of charge storage material particles in intimate contact with one another, and a uniformly interconnected ionically conductive polymer network throughout the electrode structure. It also would be desirable to develop a process for manufacturing an ionically conductive, interconnected polymer network distributed uniformly throughout the charge storage material that does not require the use of pore-forming agents and secondary solvent carriers. As an integral part of this new process, it would be desirable to develop a process for manufacturing in-situ an ionically conductive polymer separator layer at the electrode interfacing surfaces(s) simultaneously with the manufacture of the anode and cathode structures.

Summary of the Invention

It is an object of the present invention to provide a process for the manufacture of electrodes for electrochemical storage devices that have charge storage material particles uniformly dispersed throughout the electrode to help facilitate the formation of a uniform network of interconnected pores. The uniform distribution of charge storage material provides uniform current distribution throughout the electrode. The uniform distribution of pores provides uniform distribution of electrolyte if the electrode is used to make a battery, such as a lithium battery. It is another object of the present invention to provide a process that can achieve the electrode structure described above without the use of environmentally harmful solvents and other additives such as pore-forming agents.

The process in accordance with a preferred embodiment of the present invention includes the steps of depositing a layer of powder on a current collector foil, the powder comprising particles of charge storage material uniformly dispersed in particles of polymer material, and exposing the layer of powder to an energy source to bond the polymer particles to one another and to the current collector foil. The layer of powder is transformed into a charge storage material layer that contains charge storage material particles uniformly dispersed among a matrix of bonded polymer particles, and additionally contains a substantially uniform network of interconnected pores extending therethrough. The presently claimed process overcomes the above-discussed drawbacks associated with conventional processes for making polymer electrodes for electrochemical storage devices. The particle size distributions of both powder ingredients needed to make the electrode structure can be selected to ensure uniform distribution of the charge storage material throughout the entirety of the electrode structure. The presence of the polymer particles in powder form effectively holds the charge storage material particles in place when applied to the underlying current collector. There is no substantial liquid ingredient that would allow the particles to shift and otherwise settle under the influence of gravity. Accordingly, the uniform dispersion of particles as initially applied to the underlying current collector is maintained throughout the manufacturing process.

The process in accordance with the present invention also alleviates the need for a separate pore- forming agent since the manner in which the powder ingredients pack when applied onto the underlying current collector defines a uniform network of interconnected pores between and among the powder particles. The process for forming the electrode structure is thus simplified, as the removal of a separate pore-forming agent is no longer necessary.

The process in accordance with the present invention also alleviates the need for using environmentally harmful solvents, in that the powder ingredients used to make the electrode structure are applied to the underlying current collector in dry form and then integrated in situ. This solvent- free aspect of the present invention obviates the need for a separate solvent removal step, and also makes the manufacturing process much cheaper as it is no longer necessary to employ strict safety measures to recover and discard the toxic solvent. Still yet another advantage of the process in accordance with the present invention is that the adhesion interface between the charge storage material/polymer layer and the underlying current collector is vastly improved over that of electrodes made in accordance with conventional techniques. Specifically, the polymer particles bond not only to one another to secure the charge storage material particles in place, but also to the underlying current collector itself. The uniform dispersion of polymer particles and subsequent bonding to the current collector provide uniform adhesion of the entire layer to the underlying current collector, and thus, enhance the longevity of the overall electrode.

In accordance with a preferred embodiment of the present invention in the context of forming an electrode for use in a battery, such as a lithium battery, it is preferred to deposit a layer of polymer particles on the upper surface of the charge storage material/polymer layer while still in powder form, and then integrate the whole assembly to form simultaneously the active electrode layer bonded to the underlying current collector and the separator bonded to the upper surface of the active electrode layer. It is also preferred that the polymer particles used to form the separator layer are the same as those in the active electrode layer in order to ensure good adhesion of the separator layer to the underlying active electrode layer.

In accordance with yet another preferred embodiment of the present invention in the context of forming a battery electrode, a bimetallic strip is used as the current collector, and the anode is formed on one surface thereof as described above and the cathode is formed on the other surface thereof as described above.

Separators are also formed on the top surface of the anode and cathode to provide a bipolar cell stock material that can be diced into individual bipolar cells and stacked one on top of the other to form a high voltage polymer battery, such as a lithium polymer battery. These and other objects of the present invention will be better understood by reading the following detailed description in combination with the attached drawings of a preferred embodiment of the invention.

Brief Description of the Drawings For a more complete understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred mode of practicing the invention, read in connection with the accompanying drawings, in which:

FIG.l is a diagrammatic view depicting a preferred method of forming an electrode according to the invention; FIG. 2 is a cross-sectional view of an electrode produced by the method of FIG. 1;

FIG. 3 is a side view of a single-sided plate electrochemical cell;

FIG. 4 is a diagrammatic view depicting a preferred method of forming the electrodes used to make the cell shown in FIG.3;

FIG. 5 is a side view of a battery formed by stacking a plurality of the cells in FIG.3;

FIG. 6 is a side view of a double-sided plate electrochemical cell;

FIG. 7 is a diagrammatic view depicting a preferred method of forming the electrodes used to make the cell shown in Fig. 6;

FIG. 8 is a side view of a battery formed by stacking a plurality of the cells shown in FIG.6;

FIG. 9 is a side view of a bipolar cell; and

FIG. 10 is a side view of a battery formed by stacking a plurality of the cells shown in FIG.9.

Detailed Description of the Invention

FIG. 1 shows a preferred method for manufacturing a single-sided electrode in accordance with the present invention. A roll of metal foil 1 is used to provide a continuous supply of material for the current collector 2 of the electrode- The foil is conveyed under a first powder hopper 3 a to supply a mixture 4 of charge storage particles and polymer particles to the foil. The foil and powder are then passed under a first reverse roller 5a to adjust the thickness of the powder layer. The powder layer is then passed under an energy source 6 to cause the polymer particles to either melt partially or begin cross-linking so that the particles bond to one another to form an integrated structure in which the charge storage particles are uniformly dispersed and supported. The continuous sheet of electrode stock material can then be wound in a roll 7 for subsequent dicing.

Although not shown, the metal foil preferably is supported by a vacuum web transport conveyor to hold it in a stable horizontal plane while passing under hopper

3 a, roller 5 a and energy source 6. Typically, energy source 6 supplies radiant thermal energy from a laser or other lower intensity energy source. Electron beams can be used when it is desired to cross-link, instead of thermally fuse, the polymer particles to one another.

The electrode stock material produced by the method depicted in Fig. 1 can be diced into individual units 8, as shown in FIG. 2. Each electrode unit 8 includes a current collector 2 and an active electrode layer 9 integrally bonded thereto. The polymer particles of layer 9 have been fused or bonded to one another to form an interconnected matrix in which the charge storage particles are uniformly dispersed and supported. This is important to insure uniform current distribution throughout layer 9. The charge storage and polymer particles collectively define an interconnected pore network uniformly distributed throughout electrode layer 9. This is important to insure uniform distribution of electrolyte throughout layer 9. The process shown in FIG.l can be used to form anodes and cathodes, depending upon the type of metal foil and charge storage material used. Preferably the same polymer material is used in the anodes and cathodes.

Fig. 3 shows an example of an electrochemical cell employing two electrode units 8 stacked on one another through an interposed separator 12. The upper electrode unit 13 constitutes the anode, and is prepared by applying anodic charge storage material through hopper 3a of FIG.l . The lower electrode unit 14 constitutes the cathode, and is prepared by applying cathodic charge storage material through hopper 3a of Fig. 1.

Preferably, half of separator 12 is formed integrally on the active electrode layer of the anode 13 and the other half of separator 12 is formed integrally on the active electrode layer of cathode 14 by a more preferred method of the invention as shown in Fig. 4. Specifically, the foil/powder laminate produced in Fig. 1 is passed under a second powder hopper 3b to supply a layer 15 of polymer particles on top of the thickness-adjusted layer of mixed charge storage material and polymer particles. A second reverse roller 5b is then used to adjust the thickness of the polymer particle layer 15. Energy source 6 would bond (either by partial melting or cross-linking) the polymer particles in active electrode layer 9 to one another and to underlying current collector foil 2. Energy source 6 would also simultaneously bond the particles in the upper polymer layer 15 to one another and to the polymer particles in layer 9.

The resultant structure includes current collector foil 2 with active electrode layer 9 bonded thereto and one-half of separator layer 12 bonded to layer 9. The separator layer 12 becomes whole when the anode 13 and cathode 14 are stacked, as shown in Fig. 3. A plurality of the cells shown in Fig. 3 would then be loaded with electrolyte, and stacked and interconnected to produce a battery as shown in Fig. 5.

The process described above with reference to Fig. 1 could be modified to form a double-sided electrode cell, as shown in Fig. 6. In such a case, the process shown in Fig. 1 would be repeated on the reverse side of current collector 2 as shown in Fig. 7. When forming a double-sided anode, anodic charge storage material would be applied to both sides of current collector 2, and a full-thickness separator would be formed over one of the two active electrode layers. The process would be repeated to form a double-sided cathode. Individual anode and cathode double-sided units would then be stacked to form the cell shown in Fig. 6. A plurality of these double-sided electrode cells would then be loaded with electrolyte, and stacked and interconnected to produce a battery as shown in Fig. 8.

The process shown in Fig. 7 could also be used to form a bipolar cell as shown in Fig. 9. In this case, a bimetallic foil would be used as the current collector for the anode and cathode. The anode would first be formed on the anode side of the bimetallic foil, and then the cathode would be formed on the cathode side of the bimetallic foil. A full-thickness separator 12 would be formed on top of the cathode.

After loading with electrolyte, a plurality of these bipolar cells could be stacked to form a battery as shown in Fig. 10. The separator 12 would have to be removed from the uppermost cell to allow for proper interconnection.

The materials used to form the current collector and charge storage material layer are limited only by the specific type of electrochemical storage device in which the electrode is to be used. One of the main advantages of the present method is that it can be used to form electrodes from any type of particulate charge storage material that can remain stable when exposed to energy source 6. The following description is in the specific context of lithium ion and lithium polymer batteries.

Anode The current collector for the anode is typically formed from copper foil, an example of which is electrochemically surface roughened copper foil manufactured by Metal Foils, Inc.

The charge storage material layer typically contains coke as the charge storage material. A preferred material is a low temperature disordered carbon, such as that produced by pyrolyzing PAN (poly (acrylonitrile)) a method employed by

Bethlehem Advanced Materials of Easton, Pennsylvania. Graphite and mixtures/composites of coke and graphite also can be used as the charge storage material in the anode.

The polymer used to bond the charge storage material particle in the anode can be any one of a number of polymers that are stable in lithium cells. For example, poly (vinylidene fluoride) ("PVdF") and poly(acrylonitrile)("PAN") have been used widely in lithium batteries. Both materials are semicrystalline and have fairly high melting temperatures. PAN is more strongly self-bonded through hydrogen bonds, as evidenced by its higher melting temperature, T_m, higher glass transition temperature, T_g, and high solubility parameter, δ, and has higher strength

(in fiber form) relative to PVdF. Consequently, a PVdF based system has superior low temperature flexibility and practical toughness, relative to a PAN based system, and also has superior room temperature ductility.

Both of these materials have relatively high dielectric constants, and thus are substantially electronically non-conductive. This is important to insure that electronic conduction occurs only through the active particles of the charge storage material.

Other materials such as FPM, a PVdF copolymer, could also be used. FPM is a commercially available, PVdF-containing copolymer with typically 30% hexafluoropropylene. The primary difference between FPM and PVdF is that FPM is non-crystalline and has a much lower T_g. This feature permits much higher solids loading per unit binder. The FPM can be cross-linked to form an insoluble, non- fusible (i.e., it will not soften after cross-linked), three-dimensional network that can provide continuous service at temperatures to 200 °C with less than 15% compression set or creep.

The non- fusibility of the cross-linked FPM network is particularly useful for preparing double-sided coatings of electrode material on the metal foil current collector by the processes described above. Specifically, since the FPM network is cured on one side of the current collector, it will not soften when the opposed side of the current collector is heated (Fig. 7).

A typical formulation for cross-linking FPM is shown in Table 1. The material can be gelled, thereby rendering it non-fusible, by heating it to 177°C and holding it at that temperature for 5 minutes. Since chemical reaction rates roughly double for every 10°C increase in reaction temperature, faster gel rates are probably possible, especially in the thin films associated with the belt sintering process described above, where good heat transfer can prevent the local overheating that can occur when curing thick sections.

Table 1

The cross-linking agent typically used is a fluorinated bis-phenol. Catalyzed by the phosphonium compound, the acidic phenol OH groups react with the halogen on the FPM to form the cross-link and HF by-product which is reacted, in turn, with the MgO and Ca(OH)₂ inorganic bases to form fluoride salts and water. FPM compositions that are based on the polyhydroxy ether of bisphenol A, Phenoxy™ (Union Carbide), a thermoplastic polymer that is terminated on both ends with acidic phenol units, are believed to provide cured films that are firmly adhered to various metallic and inorganic substrates. Incorporation of Phenoxy™ as a cross- linking agent should help to improve both substrate adhesion and uncured FPM film properties.

Any polymer materials could be used so long as the mechanical and electronic/ionic performance characteristics are satisfied.

The charge storage material layer preferably includes 45-75 weight %, more preferably 55-65 weight %, and most preferably 60 weight %, charge storage material particles mixed with 25-55 weight %, more preferably 35-45 weight %, and most preferably 40 weight % polymer particles, respectively. The particle size and particle size distribution of each particle material can be selected easily to provide the appropriate porosity in the charge storage material layer. A porosity of about 40- 50 %, most preferably about 45%, is preferred in the context of a lithium battery.

Preferably, the charge storage material has an average particle diameter ranging from 5 nanometers to 1 micron, and the polymer particles have an average particle diameter ranging from 0.1-50 microns, more preferably 1-20 microns, and most preferably 1-5 microns. There is no particular limitation on the thickness of the current collector or charge storage material layer. However, the current collector should be as thin as possible in order to maximize the ratio of active material to current collector. Preferably, the thickness of the current collector ranges from 1 to 20 mils, more preferably 3 to 8 mils. The use of a reverse roller to adjust the thickness of the charge storage material layer permits the use of a very thin current collector, because the reverse roller exerts relatively low shear stress on the current collector. The use of a vacuum support under the current collector foil during fabrication also alleviates the need for tensioning the foil. This in turn also permits the thickness of the foil to be decreased substantially compared to the foil thickness required in the conventional processes described above. By reducing the thickness of the current collector foil, the thickness of the charge storage material layer can be increased to increase the power-to-energy ratio of the electrode without increasing the overall thickness of the electrode (e.g., 5 to 20 mils). The amount of charge storage material can be increased further by using a perforated current collector foil, which has the added benefit of increasing the adhesion strength of the charge storage material layer to the foil.

The thickness of the charge storage material layer is also not particularly limited, and is dictated largely by the particular power requirement of the cell. Generally speaking, the thickness of the charge storage material ranges from 5 to 20 mils, depending upon the thickness of the current collector.

Cathode

The current collector is typically formed from aluminum foil, an example of which is tie-coated aluminum foil manufactured by Metal Foils, Inc. The charge storage material layer typically contains Li, _+xMn₂O₄ (spinel),

LiC₀O₂ or variations of these compounds, as the active material mixed with the same type of polymer used in the anode. Other active materials include vanadium pentoxide, and other recently developed polylithiated materials.

The ratio of charge storage material to polymer and the particle size distributions are generally the same as with the anode. Additionally, the thicknesses of the current collector and charge storage material layer for the cathode are generally the same as those used in the anode.

While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.

Claims

We claim: L A process for making an electrode for an electrochemical charge storage device, comprising the steps of: (a) depositing a layer of powder on a current collector foil, said powder comprising particles of charge storage material uniformly dispersed in particles of polymer material; and (b) exposing the layer of powder to an energy source to bond the polymer particles to one another and to the current collector foil; whereby the layer of powder is transformed into a charge storage material layer that contains charge storage material particles uniformly dispersed among a matrix of bonded polymer particles, and additionally contains a substantially uniform network of interconnected pores extending therethrough.

2. The process of claim 1 , wherein the energy source heats the layer of powder to soften the polymer particles sufficiently to cause the polymer particles to wet one another and wet the current collector foil, and the process further comprises the step of: (c) cooling the layer of powder to solidify and bond the polymer particles to one another and to the current collector to thereby form the charge storage material layer.

3. The process of claim 2, further comprising the step of: (d) applying a layer of polymer particles on top of the layer of powder, and then performing steps (b) and (c) to bond the polymer particles to one another and to the polymer particles in the layer of powder, whereby the layer of polymer particles is transformed into a separator layer integrally bonded to the charge storage material layer.

4. The process of claim 3, wherein step (d) is performed after step (a) and before step (b).

5. The process of claim 1, wherein the electrode is bipolar and the current collector foil is a laminate of two sheets of dissimilar metal, and steps (a) - (b) are performed on one of the metal sheets on one side of the current collector foil to form the anode of the bipolar electrode and then performed on the other one of the metal sheets on the other side of the current collector foil to form the cathode of the bipolar electrode.