SEPARATOR FOR AN ELECTROCHEMICAL CELL

Field

This relates to separators for use in an electrochemical cell, and cells.

Background

Dimensional stability often is a desirable feature of an electrochemical cell. Thus, good dimensional stability of the individual components often is necessary. Several factors can affect the dimensional stability of the components. Frequently, due to high solubility of one or more components of the electrolyte, the anode and/or cathode may swell significantly and, in turn, cause an overall cell to swell an unacceptable amount. In secondary batteries, i.e., rechargeable batteries, this swelling occurs during charge- discharge cycling. Even when swelling of the anode and/or cathode is negligible prior to charge-discharge cycling, swelling of the anode and/or cathode during charge-discharge cycling may be significant, leading to undesirably high swelling of the cell.

Another issue associated with dimensional stability involves a thin, porous separator that conducts ions while maintaining appropriate separation between the cathode and anode. Swelling of the separator by one or more components of an electrolyte may also cause undesirable swelling of the battery cell. Additionally, in some battery constructions, swelling of the anode and/or cathode can create large compressive forces on the separator causing the separator to fail.

One important class of batteries uses one or more rechargeable lithium ion electrochemical cells. Prior commercially available anode and cathode materials used in the fabrication of secondary lithium ion batteries have been characterized as exhibiting relatively good dimensional stability during charge-discharge cycling. Anode volume swell typically is less than 10% and cathode volume swell typically is less than 5%. However, the desired charge capacity for secondary lithium ion batteries continues to increase. To achieve these higher charge capacities requires new anode and/or cathode materials.

Powdered alloys of main group elements and conductive powders such as carbon black have been used to make thin film electrodes for rechargeable lithium-ion electrochemical cells in a process that involves mixing the powdered active ingredients with a polymeric binder such as polyvinylidene fluoride. The mixed ingredients are prepared as a dispersion in a solvent for the polymeric binder, and coated onto a thin metal foil substrate, or current collector. The resulting composite thin film electrode contains the powdered active ingredient in the binder, adhered to the thin metal foil substrate.

One challenge in designing rechargeable electrochemical cells for use in electronic devices is being able to store enough energy in the cell or a battery made up of multiple cells to operate the electronic device for a useful amount of time between recharging. The amount of energy (charge capacity) that can be stored in an electrochemical cell is proportional to the amount of negative electrode and positive electrode material in the electrodes. It is important, therefore, to pack as much negative electrode and positive electrode material into the volume occupied by the electrochemical cell as possible. This is usually achieved by rolling thin film sheets of the positive electrode and negative electrode into a cylinder between two separators which are porous membranes capable of preventing direct contact between the negative electrode and the positive electrode in the roll. In some cells this roll is flattened.

Rechargeable electrochemical cells, such as lithium-ion cells, are capable of being reversibly charged and discharged multiple times. In the case of lithium-ion batteries, the charging and the discharging of the lithium-ion electrochemical cells are accomplished by lithiating and delithiating the cell electrodes. When lithium-ion cells are constructed, they usually contain excess lithium-ions in the positive electrode and no lithium-ions in the negative electrode. During the initial cycling reaction of the cells (charging), lithium transfers from the positive electrode to the negative electrode until the negative electrode has reached its capacity of absorbing lithium-ions. This can cause a significant volume change in the negative electrode as it absorbs lithium ions. Upon the first discharge, the lithium-ions migrate from the lithiated negative electrode back to the positive electrode and the negative electrode typically decreases in volume. This expansion and contraction of the negative electrode and/or the positive electrode also occurs during repeated lithiation and delithiation (i.e., during use, or discharge, and during recharging). Such expansion and contraction can cause significant stress on a rolled electrochemical cell that

is confined in a battery case. In some cases this stress can cause premature failure of the electrochemical cell.

Summary Many of the newer electrode materials leading to higher charge capacities exhibit high volume swell during lithiation. The volume swell is often greater than 20% and creates a volume swell of the entire cell that is detrimental to performance features. There is a need for rechargeable electrochemical lithium-ion cells that have high charge capacity and can be repeatedly fully charged and discharged without having volume changes that build up large stresses in the cell, especially when the cells are confined to a battery case.

Articles useful as a battery are disclosure wherein the volume increase associated with a swelling electrode is at least partially offset by a compressible separator, leading to a final separator thickness and porosity in the desired operational range, while minimizing the volume change of the entire cell. In one aspect, provided is a separator for use in a rechargeable battery comprising a microporous membrane, wherein the separator maintains porosity after thickness compression during electrochemical cell cycling.

In another aspect, provided is a separator for use in a rechargeable battery comprising a microporous membrane having an initial porosity of at least about 10%, and retaining at least about 25% of the initial porosity after thickness compression of at least about 18%.

In another aspect, provided is a rechargeable battery comprising a negative electrode having a volume change greater than about eighteen percent after lithiation, and a separator having a volume change opposite that of the negative electrode, such that, the volume change of the negative electrode is at least partially offset by the volume change of the separator, resulting in a volume change of the battery that is less than twenty percent after at least one room temperature, charge-discharge cycle.

In another aspect, provided is a method of producing a battery comprising (a) sandwiching a separator as described herein between a positive electrode and a negative electrode, (b) packaging the sandwich into a battery case, (c) adding an electrolyte into the battery case, and (d) sealing the battery case. In this disclosure:

"a", "an", and "the" are used interchangeably with "at least one" to mean one or more of the elements being described;

"metal" refers to both metals and to metalloids, such as silicon and germanium, whether in an elemental or ionic state; "alloy" refers to a mixture of two or more metals;

"lithiate" and "lithiation" refer to a process for adding lithium to an electrode material;

"delithiate" and "delithiation" refer to a process for removing lithium from an electrode material; "active" refers to a material that can undergo lithiation and delithiation;

"charge" and "charging" and "recharging" refer to a process for providing electrochemical energy to a cell;

"discharge" and "discharging" refer to a process for removing electrochemical energy from a cell, e.g., when using the cell to perform desired work; "positive electrode" refers to an electrode (often called a cathode) where electrochemical reduction and lithiation occurs during a discharging process;

"negative electrode" refers to an electrode (often called an anode) where electrochemical oxidation and delithiation occurs during a discharging process; and

"porosity" refers to a measure of the void spaces in a material, and is measured as a fraction, between 0 and 1, or as a percent, between 0 and 100%.

Advantages of one or more embodiments of the present disclosure include one or more of longer cycle life electrochemical cells, greater capacity cells within standard sizes and shapes, and greater thermal stability.

Other features and advantages of various embodiments of the invention will be apparent from the following detailed description and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The detailed description that follows more particularly exemplifies certain preferred embodiments using the principles disclosed herein.

Detailed Description

All numbers are herein assumed to be modified by "about". The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). Rechargeable lithium ion electrochemical cells of this disclosure include a negative electrode, a positive electrode, and a separator. The cells of this invention can further comprise a rigid battery case. The negative electrode and the positive electrode can comprise active materials in electrical contact with metal foil substrates or current collectors. The active materials can be in the form of powders or vapor deposited thin films. Typically, the thin film negative electrodes and positive electrodes can be separated (so that they do not make direct electrical contact) by a microporous membrane. When high current capacity is needed from the electrochemical cell, the negative electrode and the positive electrode can be made as long sheets that are rolled together with a separator between them. It is typical when rolling the negative electrodes and positive electrodes to use two separators so that both sides of each electrode (negative electrode and positive electrode) are prevented from making direct electrical contact and shorting out.

A variety of powdered active materials can be employed to make the electrode compositions. Powdered materials can contain materials known to those skilled in the art, e.g., silicon, silver, lithium, tin, bismuth, lead, antimony, germanium, zinc, gold, platinum, palladium, arsenic, aluminum, gallium, indium, thallium, molybdenum, niobium, tungsten, tantalum, iron, copper, titanium, vanadium, chromium, nickel, cobalt, zirconium, yttrium, a lanthanide, an actinide or an alloy containing any of the foregoing metals or metalloids and other powdered active metals and metalloids that will be familiar to those skilled in the art. Graphitic carbon powder can also be used to make the disclosed electrode compositions. Exemplary powders can have a maximum length in one dimension that is no greater than 60 μm, no greater than 40 μm, or no greater than 20 μm, or even smaller. The powders can, for example, have a maximum particle diameter that is submicron, at least 1 μm, at least 2 μm, at least 5 μm, or at least 10 μm or even larger. For example, suitable powders often have a maximum dimension of 1 to 60 μm, 10 to 60 μm, 20 to 60 μm, 40 to 60 μm, 1 to 40 μm, 2 to 40 μm, 10 to 40 μm, 5 to 20 μm, or 10 to 20 μm.

The powdered materials can contain optional matrix formers with the powder particles. Each phase originally present in the particle (i.e., before a first lithiation) can be

in contact with the other phases in the particle. For example, in particles based on a silicon:copper:silver alloy, a silicon phase can be in contact with both a copper suicide phase and a silver or silver alloy phase. Each phase in a particle can for example have a grain size less than 500 A, < 400 A, < 300 A, < 200 A, < 150 A, or even smaller. Exemplary silicon-containing powdered materials useful in this invention include silicon alloys comprising from about 65 to about 85 mole percent silicon, from about 5 to about 12 mole percent iron, from about to about 12 mole percent titanium, and from about 5 to about 12 mole percent carbon. Additional examples of useful silicon alloys include compositions that include silicon, copper, and silver or silver alloy such as those discussed in U.S. Pat. Appl. Publ. No. 2006/0046144 Al (Obrovac et al.); multiphase, silicon- containing electrodes such as those discussed in U.S. Pat. Appl. Publ. No. 2005/0031957 Al (Christensen et al.); silicon alloys that contain tin, indium and a lanthanide, actinide element or yttrium such as those described in U.S. Pat. Appl. Publ. Nos. 2007/0020521, 2007/0020522, and 2007/0020528 (all to Obrovac et al.); amorphous alloys having a high silicon content such as those discussed in U.S. Pat. Appl. Publ. No. 2007/0128517

(Christensen et al.); and other powdered materials used for negative electrodes such as those discussed in U.S. Serial No. 11/419,564 (Krause et al.) filed May 22, 2006 and PCT Publ. No. WO 2007/044315 (Krause et al.) filed Oct. 2, 2006.

Other useful exemplary powdered materials for making positive electrodes of the invention include lithium alloys such as 1^4/3^5/304, Liλ^Og, L1V2O5,

LiCoo.2Nio.8°2 _> LiMθ2, LiFePθ4, LiMnPθ4, LiCoPθ4, LiMn2θ4, and LiCoθ2; lithium atoms intercalated within a lithium transition metal oxide such as lithium cobalt dioxide, lithium nickel dioxide, and lithium manganese dioxide; the lithium alloy compositions that include mixed metal oxides (e.g., two or three of cobalt, manganese, and nickel) such as those described in U.S. Patent Nos. 6,964,828 and 7,078, 128 (Lu et al),

6,203,944 (Turner), and 6,680,145 B2 (Obrovac et al.).

Exemplary powdered materials useful for making negative electrodes of his invention include U. S. Patent No. 6,699,336 B2 (Turner et al.); U.S. Pat. Appl. Publ. No.

2003/0211390 Al (Dahn et al.); U.S. Patent Nos. 6,255,017 Bl (Turner) and 6,436,578 B2 (Turner et al.); graphitic carbon in forms such as powders, flakes, fibers or spheres (e.g.,

mesocarbon microbeads or "MCMB"); combinations thereof and other powdered materials that will be familiar to those skilled in the art.

Useful negative electrodes can also be provided as a thin film of active material directly adhered to and in electrical contact with the current collector. The thin film can be applied to the current collector, for example, by means of evaporative or chemical vapor deposition, plasma deposition or sputtering. The thin film can be in the form of a pure element or an alloy. The thin film can be pure silicon. The thin film can be an alloy that includes only active elements or both active and inactive elements. Examples of useful negative thin film electrodes are described in U.S. Patent Nos. 6,203,944, 6,255,017, 6,436,578, and 6,699,336 (all to Turner or Turner et al.) and U.S. Pat. Appl. Publ. No. 2007/0020528 (Obrovac et al.).

Powdered alloy particles can include a conductive layer. For example, a particle that contains silicon, copper, and silver or a silver alloy can be coated with a layer of conducting material (e.g., with the alloy composition in the particle core and the conductive material in the particle shell). Suitable conductive materials include, for example, carbon, copper, silver, or nickel.

Exemplary powdered alloy materials can be prepared by any known means, for example, by physically mixing and then milling the various precursor components to the alloys. The mixing can be by simple blending or by using a melt spinning process. According to this process, ingots containing the alloy composition can be melted in a radio frequency field and then ejected through a nozzle onto a surface of a rotating wheel (e.g., a copper wheel). Because the surface temperature of the rotating wheel is substantially lower than the temperature of the melted alloy, contact with the surface of the rotating wheel quenches the melt. Quenching minimizes the formation of large crystallites that can be detrimental to electrode performance. When conductive coatings are employed, they can be formed using techniques such as electroplating, chemical vapor deposition, vacuum evaporation or sputtering. Suitable milling can be done by using various techniques such as vertical ball milling, horizontal ball milling or other milling techniques known to those skilled in the art. The electrode composition can contain additives such as will be familiar to those skilled in the art. The electrode composition can include an electrically conductive diluent to facilitate electron transfer from the powdered material to a current collector.

Electrically conductive diluents include, but are not limited to, carbon (e.g., carbon black for negative electrodes and carbon black, flake graphite and the like for positive electrodes), metal, metal nitrides, metal carbides, metal suicides, and metal borides. Representative electrically conductive carbon diluents include carbon blacks such as Super P and Super S carbon blacks (both from MMM Carbon, Belgium), Shawanigan

Black (Chevron Chemical Co., Houston, TX), acetylene black, furnace black, lamp black, graphite, carbon fibers and combinations thereof.

Useful electrode compositions can include an adhesion promoter that promotes adhesion of the powdered material or electrically conductive diluent to the binder. The combination of an adhesion promoter and binder can help the electrode composition better accommodate volume changes that can occur in the powdered material during repeated lithiation/delithiation cycles. The disclosed binders can offer sufficiently good adhesion to metals and alloys so that addition of an adhesion promoter may not be needed. If used, an adhesion promoter can be included with the lithium polysulfonate fluoropolymer binder (e.g., in the form of an added functional group), can be a coating on the powdered material, can be added to the electrically conductive diluent, or can be a combination of such measures. Examples of useful adhesion promoters include silanes, titanates, and phosphonates as described in U.S. Pat. Appl. Publ. No. 2004/0058240 Al (Christensen). Electrodes of this disclosure can include a binder. Exemplary polymer binders include polyolefins such as those prepared from ethylene, propylene, or butylene monomers; fluorinated polyolefins such as those prepared from vinylidene fluoride monomers; perfluorinated polyolefins such as those prepared from hexafluoropropylene monomer; perfluorinated poly(alkyl vinyl ethers); perfluorinated poly(alkoxy vinyl ethers); or combinations thereof. Specific examples of polymer binders include polymers or copolymers of vinylidene fluoride, tetrafluoroethylene, and propylene; and copolymers of vinylidene fluoride and hexafluoropropylene. Lithium polysalts are also examples of polymer binders that are useful in various embodiments of this disclosure.

In some electrodes, the binders can be crosslinked. Crosslinking can improve the mechanical properties of the binders and can improve the contact between the active material composition and any electrically conductive diluent that can be present. Other binders include polyimides such as the aromatic, aliphatic or cycloaliphatic polyimides described in U.S. Pat. Appl. Publ. No. 2006/0099506 (Krause et al).

To make a positive or a negative thin film electrode, the active powdered material, any selected additives such as binders, conductive diluents, fillers, adhesion promoters, thickening agents for coating viscosity modification such as carboxymethylcellulose and other additives known by those skilled in the art are mixed in a suitable coating solvent such as water, ethanol, methanol, isopropanol, n-propanol or N-methylpyrrolidinone (NMP) to form a coating dispersion or coating mixture. The dispersion is mixed thoroughly and then applied to a foil current collector by any appropriate dispersion coating technique, e.g., knife coating, notched bar coating, dip coating, spray coating, electrospray coating, or gravure coating. The current collectors are typically thin foils of conductive metals such as, for example, copper, aluminum, stainless steel, or nickel foil. After the slurry is coated onto the current collector foil it is allowed to dry followed usually by drying in a heated oven, typically set at about 80 ⁰ C to about 300 ⁰ C, for about an hour to remove solvent. The electrode can be compressed by pressing between two plates or rollers and known by those skilled in the art. The electrode can also be provided with a raised pattern as disclosed in U.S. Serial No. 11/696,979 (Obrovac et al.), filed April 5, 2007.

A variety of electrolytes can be employed in the disclosed lithium-ion cell. Representative electrolytes contain one or more lithium salts and a charge-carrying medium in the form of a solid, liquid or gel. Exemplary lithium salts are stable in the electrochemical window and temperature range (e.g. from about -30 ⁰ C to about 70 ⁰ C) within which the cell electrodes can operate, are soluble in the chosen charge-carrying media, and perform well in the chosen lithium-ion cell. Exemplary lithium salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , lithium bis(oxalato)borate, LiN(CF ₃ SC^, LiN(C2F ₅ SC>2)2, LiAsFg, LiC(CF ₃ SC^) ₃ , and combinations thereof. Exemplary charge-carrying media are stable without freezing or boiling in the electrochemical window and temperature range within which the cell electrodes can operate, are capable of solubilizing sufficient quantities of the lithium salt so that a suitable quantity of charge can be transported from the positive electrode to the negative electrode, and perform well in the chosen lithium-ion cell. Exemplary solid charge carrying media include polymeric media such as polyethylene oxide, polytetrafluoroethylene, polyvinylidene fluoride, fluorine-containing copolymers, polyacrylonitrile, combinations thereof and other solid media that will be

familiar to those skilled in the art. Exemplary liquid charge carrying media include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl- methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate, γ-butylrolactone, methyl difluoroacetate, ethyl difluoroacetate, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), tetrahydrofuran, dioxolane, combinations thereof and other media that will be familiar to those skilled in the art. Exemplary charge carrying media gels include those described in U.S. Patent Nos. 6,387,570 (Nakamura et al.) and 6,780,544 (Noh). The charge carrying media solubilizing power can be improved through addition of a suitable cosolvent. Exemplary cosolvents include aromatic materials compatible with Li-ion cells containing the chosen electrolyte. Representative cosolvents include toluene, sulfolane, dimethoxyethane, combinations thereof and other cosolvents that will be familiar to those skilled in the art. The electrolyte can include other additives that will familiar to those skilled in the art. For example, the electrolyte can contain a redox chemical shuttle such as those described in U.S. Patent Nos. 5,709,968 (Shimizu), 5,763,119 (Adachi), 5,536,599 (Alamgir et al.), 5,858,573

(Abraham et al.), 5,882,812 (Visco et al.), 6,004,698 (Richardson et al.), 6,045,952 (Kerr et al.), and 6,387,571 Bl (Lain et al.); and in U.S. Pat. Appl. Publ. Nos. 2005/0221168 Al, 2005/0221196 Al, 2006/0263696 Al, and 2006/0263697 Al (all to Dahn et al.).

Electrochemical cells of this invention can be made by taking at least one each of a positive electrode and a negative electrode as described above and placing them in an electrolyte. A microporous separator, such as those described below, is used to prevent the contact of the negative electrode directly with the positive electrode. When high current capacity is needed from the electrochemical cell, the negative electrode and the positive electrode can be made as long sheets that are rolled together with a separator between them. It is typical when rolling the negative electrodes and positive electrodes to use two separators so that both sides of each electrode (negative electrode and positive electrode) are prevented from making direct electrical contact and shorting out.

Electrochemical cells of this invention can include rechargeable lithium ion cells comprising an anode having a volume swell during lithiation and volume decrease during delithiation. This change in volume can be at least about 10%, at least about 15%, at least about 18%, at least about 20%, at least about 25%, or even greater. In the present disclosure, a separator that can accommodate the volume change by being compressible

such that the volume change is at least partially offset by the volume change of the separator. In some embodiments, this volume change is substantially offset by a commensurate volume change in the separator. This offset, or volume balance, is more important when the electrochemical cell is compacted into a rigid battery case such as a coin cell (such as, for example, a 2325 coin cell) or a cylindrical battery can or case (such as, for example an 18650 battery case). To reduce or minimize stress inside of a rigid battery case it can be important that the total volume change of the electrochemical cell that is less than about 20%, less than about 15%, less than about 10%, less than about 5%, or even less, after at least one room temperature, charge-discharge cycle. In some embodiments, the electrochemical cells of the present disclosure further comprise a rigid case and can be produced by deliberate selection of the anode and separator that comprise the cell. This selection of anode and separator, guided by the present disclosure, balances volume changes and reduces internal stresses in the electrochemical cell within a rigid case due to lithiation and delithiation. To relieve stress within the rigid case, space can be provided within the case to accommodate the volume swell of the anode. This space can be provided by appropriate choice of properties of the separator. Properties that can be important include the thickness and the porosity of the separator.

Separators useful in this disclosure include a porous film, such as a microporous membrane. Microporous membranes are polymers in which a fraction of the volume encompassed by their mass is comprised of small pores. Typically the separator's pores are connected and large enough to allow for the flow of gas, liquid, and/or ions through the separator. The separator can be formed from a single layer of a microporous material or from multiple layers of microporous materials of similar or differing chemical or physical properties. If a single layer is employed, the mechanical integrity of the separator must be maintained during and after the compressing phase.

In some embodiments, separators of this disclosure can comprise multiple layers of microporous membranes. In one such an embodiment, a first layer can provide mechanical integrity of the separator while a second layer having differing properties from the first layer can provide desirable compressibility characteristics. Thus, a multi-layer separator with a highly porous layer designed to provide the desired compressibility can be mated, or laminated, with a stronger, lower porosity separator designed to provide

mechanical integrity. The multi-layer separator can thus be specifically designed to offset volumetric changes upon use and recharging. In another embodiment, a separator can include more than two layers. For example, a three layer design is an embodiment wherein a highly porous layer designed for compressibility is located between two less- porous layers designed for mechanical integrity. In another embodiment, a thermal- shutdown membrane can be used as the separator or as a layer in a multi-layer separator. Other useful designs are within the scope of the present disclosure is and will be apparent to those of ordinary skill in the art guided by it.

Microporous membranes are well known to those skilled in the art and are available in a variety of chemistries and in a variety of thicknesses and porosities. These include multi-zone membranes such as those disclosed in U.S. Patent Nos. 6,706,184 (Sale et al), 6,090,441 (Vining et al), and 6,264,044 (Meyering et al.) and TIPS (thermally induced phase separation) membranes, such as those described in U.S. Pat. Appl. Publ. Nos. 2002/0135087 (Yapel et al.) and 2005/0058821 (Smith et al.), and membranes such as taught in U.S. Patent Nos. 5,993,954 and 6,461,724 (Radovanovic et al.). Additionally, a number of commercially available microporous membranes can be used in one or more layers of various embodiments of the present disclosure is, e.g., Duropore membranes, available from Millipore, Corp., Billerica, MA and Osmonics membranes, available from General Electric Co. Separators can be formed by any process that allows the desired porosity and thickness to be obtained. Typically, separators are formed by a dry stretch process or a wet process. In the dry stretch process, the pore structure is formed by stretching a separator precursor. In the wet process, the pore structure is formed by extracting one or more components from the separator precursor. Porous materials may be used in either a single layer construction or multilayer construction as separators in various embodiments of this disclosure.

Final separator porosity, i.e. after compression, may be 0.25 or larger, preferably 0.30 or larger. Final separator porosity may be 0.45 or smaller, preferably 0.40 or smaller. Initial separator porosity may be 0.50 or larger. Initial separator porosity may be 0.90 or smaller. When multi-layer separators are used, the porosity value should be based on a volumetric average of the appropriate layers.

Final separator thickness may be 15 micrometers (μm) or even larger. Final separator thickness may be 40 μm or even lower. When multi-layer separators are used, the thickness value is based on the thickness of the entire multi-layer construction. The pores of the separator can offer a significant volume space to compensate for some of the swelling of the electrode(s), such as the negative electrode. Thus, the separator can help to reduce the internal stresses in a cell during lithiation and delithiation.

In various embodiments of the present disclosure, the pores of the separator offer a significant volume space to accommodate the swelling of the electrode, such as the anode. In some embodiments, due to the compression of the separator and loss in pore volume, the initial porosity of the separator needs to be higher than the required operating value, which is generally from 0.25 to 0.45, in order for the compressed separator to have a final porosity in this operational range. Hence, in a specific battery design, the expected volume swell of the anode guides the initial required thickness and porosity of the compressible separator. In the design of an electrochemical cell of one embodiment of this invention, the expected volume swell of the anode upon lithiation guides the initial required thickness and porosity of the compressible separator used to make the cell. Assuming the change in dimension of the electrochemical cell components is predominantly due to volume expansion normal to the plane of the thin film electrode thickness the following equation can be written relating various electrochemical cell parameters:

AV _A ^ V _1S P ₁ (I-[P ₂ T ₂ Z P ₁ T ₁ ]) (1) where P ₁ is the initial porosity of the separator, P ₂ is the final porosity of the separator, T ₁ is the initial thickness of the separator, T ₂ is the final thickness of the separator, and V ₁ S is the initial volume of the separator. If the appropriate number of parameters are known or designated, Equation (1) is used to determine the values of the corresponding undetermined parameters. Assuming that the area of the anode is approximately equal to the area of the separator, and that these values do not change significantly prior to or after lithiation of the anode and at least one charge-discharge cycle, then Equation (1) can be simplified as shown in Equation (2): δT _A = (P ₁ T ₁ -P ₂ T ₂ ) (2)

Equation (2) can be written in another useful form that is shown as Equation (3):

Pi ≥ P ₂ + (AT _A Z T ₁ )( I- P ₂ ) (3) where δT _A is the change in thickness of the anode (the anode thickness after lithiation of the anode and at least one charge-discharge cycle minus the initial anode thickness). With knowledge of the desired final porosity of the separator, the final thickness of the separator and the change in thickness of the anode; a set of initial separator porosities and corresponding thicknesses can be determined that will enable the volume swell of the anode to be compensated by the compression of the separator, thus relieving internal stresses when the electrochemical cell is confined to a rigid cell case.

As an example, assume that the anode thickness swells 60 μm and that the final separator thickness is desired to be 20 μm with a porosity of 0.35 (35% of the volume of the separator is pores). Then, Ti is 80 μm and the initial separator porosity is calculated to be 0.84 (Equation (3)). Thus, to compensate for a 60 μm increase in anode thickness due to swell while still having a separator with a final thickness and porosity in the desired operational range (20 μm with 0.35 porosity), an 80 μm thick separator with an initial porosity of 0.84 is used. This approach provides that the separator pores have an initial free volume approximately equal to the volume swell of the anode. The volume of electrolyte typically added to the cell may need to be reduced to insure the desired free volume within the pores is available. Final separator porosity, i.e. after compression, may be 0.25 or larger, preferably 0.30 or larger. Final separator porosity may be 0.45 or smaller, preferably 0.40 or smaller. Initial separator porosity may be 0.50 or larger. Initial separator porosity may be 0.90 or smaller. When multi-layer separators are used, the porosity value should be based on a volumetric average of the appropriate layers. Final separator thickness may be 15 μm or even larger. Final separator thickness may be 40 μm or even lower. When multi-layer separators are used, the thickness value should be based on the thickness of the entire multi-layer construction.

Generally, the amount of anode swelling may be greater than 18%, in some instances greater than 20, 25, 30, 35, 40%, or even higher, and in other cases as high as 120%. In some instances, the initial anode thickness may be 10, 15, 20, 25 μm or even greater. In some instances the initial anode thickness may be 150 μm, 125 μm, or less.

In other embodiments, this invention can also be applied to electrochemical cells in which the cathode exhibits a significant degree of swell or the situation where both the cathode and anode exhibit a significant degree of swell.

Electrochemical cells can be constructed in any useful geometry. Electrodes often are used in cells as rectangular or circular sheets. It is typical to make a stack that includes a negative electrode sheet and a positive electrode sheet with a separator sandwiched between the negative electrode and the positive electrode to form a layered structure. In coin cells, such as 2325 coin cells, the components of the layered structured can be cut into rounded disks so that they form a substantially vertical stack that can then be inserted into the body of a coin cell. It is also possible to form substantially vertical stacks that include a negative electrode, a separator, and a positive electrode that are in various geometric shapes such as, squares, rectangles, triangles, polygons of any shape. The substantially vertical stack can also be rectangular with the length and width being different. One of these electrodes can be combined with a similarly shaped positive electrode and separator (sandwiched in between) to form a vertical stack that has a short dimension and a long dimension.

As described in U.S. Serial No. 11/696979 (Obrovac et al.), some electrochemical cell designs have the elongated vertical stack described above tightly rolled into what is known as a "jellyroll". The jellyroll can then be placed into a container or housing, such as a can or pouch, which can then be filled with electrolyte to form an electrochemical cell. The jellyroll can also be flattened and placed in a container to form a prismatic cell.

It is also contemplated that the vertical stack can include multiple layers. For example, it is possible to take a negative electrode, separator, and positive electrode vertical stack and place it upon another vertical stack by placing an additional separator between the outside electrodes so that they do not make electrical contact with one another. Thus, multiple vertical stacks can be made in this manner.

The disclosed cells can be used in a variety of devices, including portable computers, tablet displays, personal digital assistants, mobile telephones, motorized devices (e.g., personal or household appliances and vehicles), instruments, illumination devices (e.g., flashlights), and heating devices. One or more electrochemical cells of this invention can be combined to provide battery pack. Further details regarding the

construction and use of rechargeable lithium-ion cells and battery packs will be familiar to those skilled in the art.

It is apparent to those skilled in the art from the above description that various modifications can be made without departing from the scope and principles of this invention disclosure, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth hereinabove. All publications and patents are herein incorporated by reference to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference.