OXIDA TION-RESISTANT SEPARA TOR FOR ALKALINE BA TTERIES

CLAIM OF PRIORITY

[0001] The present application claims priority to U.S. provisional patent application serial no. 60/826,863, filed on September 25, 2006; U.S. provisional patent application serial no. 60/826,842, filed on September 25, 2006; and U.S. provisional patent application serial no. 60/826,890, filed on September 25, 2006, each of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention is concerned with electrical alkaline batteries, and in particular with separators for alkaline batteries and methods of making the same.

BACKGROUND

[0003] An electrical storage battery comprises one electrochemical cell or a plurality of electrochemical cells of the same type, the latter typically being connected in series to provide a higher voltage or in parallel to provide a higher charge capacity than provided by a single cell. An electrochemical cell comprises an electrolyte interposed between and in contact with an anode and a cathode. For a storage battery, the anode comprises an active material that is readily oxidized, and the cathode comprises an active material that is readily reduced. During battery discharge, the anode active material is oxidized and the cathode active material is reduced, so that electrons flow from the anode through an external load to the cathode, and ions flow through the electrolyte between the electrodes. [0004] Many electrochemical cells used for electrical storage applications also include a separator between the anode and the cathode is required to prevent reactants and reaction products present at one electrode from reacting and/or interfering with reactions at the other electrode. To be effective, a battery separator must be electronically insulating, and remain so during the life of the battery, to avoid battery self-discharge via internal shorting between the electrodes. In addition, a battery separator must be both an effective electrolyte transport barrier and a sufficiently good ionic conductor to avoid excessive separator resistance that substantially lowers the discharge voltage.

[0005] Electrical storage batteries are classified as either "primary" or "secondary" batteries. Primary batteries involve at least one irreversible electrode reaction and cannot be recharged with useful charge efficiency by applying a reverse voltage. Secondary batteries involve relatively reversible electrode reactions and can be recharged with acceptable loss of charge capacity over numerous charge-discharge cycles. Separator requirements for secondary batteries tend to be more demanding since the separator must survive repeated charge-discharge cycles.

[0006] For secondary batteries comprising a highly oxidative cathode, a highly reducing anode, and an alkaline electrolyte, separator requirements are particularly stringent. The separator must be chemically stable in strongly alkaline solution, resist oxidation in contact with the highly oxidizing cathode, and resist reduction in contact with the highly reducing anode. Since ions, especially metal oxide ions, from the cathode can be somewhat soluble in alkaline solutions and tend to be chemically reduced to metal on separator surfaces, the separator must also inhibit transport and/or chemical reduction of metal ions. Otherwise, a buildup of metal deposits within separator pores may increase the separator resistance in the short term and ultimately lead to shorting failure due to formation of a continuous metal path through the separator. In addition, because of the strong tendency of anodes to form dendrites during charging, the separator must suppress dendritic growth and/or resist dendrite penetration to avoid failure due to formation of a dendritic short between the electrodes. A related issue with anodes is shape change, in which the central part of the electrode tends to thicken during charge-discharge cycling. The causes of shape change are complicated and not well-understood but apparently involve differentials in the current distribution and solution mass transport along the electrode surface. The separator preferably mitigates zinc electrode shape change by exhibiting uniform and stable ionic conductivity and ionic transport properties.

[0007] In order to satisfy the numerous and often conflicting separator requirements for zinc-silver oxide batteries, a separator stack comprised of a plurality of separators that perform specific functions is needed. Some of the required functions are resistance to electrochemical oxidation and silver ion transport from the cathode, and resistance to electrochemical reduction and dendrite penetration from the anode.

[0008] Traditional separators decompose chemically in alkaline electrolytes, which limits the useful life of the battery. Traditional separators are also subject to chemical oxidation by soluble silver ions and electrochemical oxidation in contact with silver electrodes. Furthermore, some traditional separators exhibit low mechanical strength and poor resistance to penetration by dendrites.

[0009] To solve some of the problems caused by traditional separators, new separator materials have been developed.

SUMMARY OF THE INVENTION

[0010] The invention provides a separator e.g., an oxidation-resistant separator, for use in an alkaline battery. The separator comprises a polyether material, and the polyether material comprises an optionally substituted linear or branched polyether polymer. Separators of the

present invention may optionally include additives such a filler (e.g., metallic oxide filler), a surfactant (e.g., an anionic surfactant, a cationic surfactant, a non-ionic surfactant, an ampholytic surfactant, an amphoteric surfactant, a zwitterionic surfactant, or any combination thereof), a conductivity enhancer (e.g., a titanate of an alkali metal), a plasticizer, or the like. [0011] Another aspect of the present invention provides an alkaline battery comprising an electrolyte, an anode, a cathode, and a separator, such as those described above, comprising a polyether material, wherein the polyether material comprises an optionally substituted linear or branched polyether polymer.

[0012] Another aspect of the present invention provides an alkaline battery comprising an alkaline electrolyte; an anode comprising zinc; a cathode; a first separator (e.g., an oxidation- resistant separator) comprising polyether material wherein the polyether material comprises optionally substituted linear or branched polyether polymer; and a second separator comprising a cross-linked polyvinyl alcohol material and a cross-linking agent, wherein the polyvinyl alcohol material comprises an optionally substituted linear or branched polyvinyl alcohol polymer. The second separator is disposed between the first separator and the anode. [0013] Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] All of the drawings presented in this document are cross-sectional views. [0015] Figure IA depicts a cross-section of an exemplary separator comprising an optional porous substrate and a polyether material disposed thereon, according to one aspect of the invention.

[0016] Figure IB depicts a cross-section of an exemplary substrate comprising at least one pore traversing the substrate that is useful in separators according to the present invention. [0017] Figure 2 depicts an exemplary battery of the present invention, wherein the battery comprises two separators interposed between an anode and a cathode. [0018] Figure 3 depicts an exemplary battery comprising four separator layers according to the invention.

[0019] Figure 4 shows plots of charge capacity vs. cycle number for two zinc-silver oxide cells (A and B) constructed with a commercial three-layer laminate separator (175 μm total thickness).

[0020] Figure 5 shows plots of charge capacity vs. cycle number for two zinc-silver oxide cells (C and D) constructed with a separator stack (90 μm total thickness) according to the

invention.

[0021] Figure 6 depicts a cross-sectional view of a separator stack used to test the oxidation-resistant separator of the invention for resistance to silver ion transport as described in Example 2, below.

[0022] Figure 7 depicts a cross-sectional view of an alkaline battery, described in Example

3, comprising an oxidation-resistant separator, two dendrite-resistant separators, two porous separators, an anode, and a cathode.

[0023] These figures are not to scale and some features have been enlarged for better depiction of the features and operation of the invention. Furthermore, the figures are by way of example, and are not intended to limit the scope of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention provides a secondary battery comprising a cathode, an anode, an alkaline electrolyte, and a separator. The separator is formed from a polyether material and the polyether material comprises an optionally substituted linear or branched polyether polymer.

[0025] I. DEFINITIONS

[0026] The term "battery" encompasses electrical storage devices comprising one electrochemical cell or a plurality of electrochemical cells. A "secondary battery" is rechargeable, whereas a "primary battery" is not rechargeable. For secondary batteries of the present invention, a battery anode is designated as the positive electrode during discharge, and as the negative electrode during charge.

[0027] The term "alkaline battery" refers to a primary battery or a secondary battery, wherein the primary or secondary battery comprises an alkaline electrolyte. [0028] For convenience, both polymer names "polyether", "polyethylene oxide", "polypropylene oxide" and "polyvinyl alcohol" and their corresponding initials "PE", "PEO", "PPO" and "PVA", respectively, are used interchangeably as adjectives to distinguish polymers, solutions for preparing polymers, and polymer coatings. Use of these names and initials in no way implies the absence of other constituents. These adjectives also encompass substituted and co-polymerized polymers. A substituted polymer denotes one for which a substituent group, a methyl group, for example, replaces a hydrogen on the polymer backbone.

[0029] The symbol "M" denotes molar concentration.

[0030] The term "optionally substituted" means substituted or unsubstituted and refers to the optional substitution of one or more hydrogen atoms with one or more substituents on a

chemical species. For example, a hydrogen atom is substituted with an alkyl group (e.g., methyl, ethyl, or propyl) on a polymer such as a polyether polymer or a polyvinyl alcohol polymer.

[0031] The term "linear" describes a polymer in which the molecules form long chains without branches or cross-linked structures. The molecular chains of a linear polymer may be intertwined, but the forces tending to hold the molecules together are physical rather than chemical and thus can be weakened by energy applied in the form of heat. Exemplary linear polymers include thermoplastics and/or polyether polymers.

[0032] The term "branched" describes a polymer comprising a main chain with one or more substituent side chains or branches.

[0033] Batteries and battery electrodes are denoted with respect to the active materials in the fully-charged state. For example, a zinc-silver oxide battery comprises an anode comprising zinc and a cathode comprising silver oxide. Nonetheless, more than one species is present at a battery electrode under most conditions. For example, a zinc electrode generally comprises zinc metal and zinc oxide (except when fully charged), and a silver oxide electrode usually comprises silver oxide (AgO and/or Ag ₂ O) and silver metal (except when fully discharged).

[0034] The term "oxide" applied to alkaline batteries and alkaline battery electrodes encompasses corresponding "hydroxide" species, which are typically present, at least under some conditions.

[0035] As used herein "substantially stable" refers to a compound or component that remains substantially chemically unchanged in the presence of an alkaline electrolyte (e.g., potassium hydroxide) and/or in the presence of an oxidizing agent (e.g., silver ions present in the cathode or dissolved in the electrolyte).

[0036] The terms "curing", "cure", or "cured" refer to a process wherein polymeric units (e.g., monomers or polymers in a copolymer) become cross-linked and solidify or harden the polymer. It is noted that a cured polymer is a polymer that experiences at least some cross- linking between polymer units.

[0037] The term "zirconium oxide" encompasses any oxide of zirconium, including zirconium dioxide and yttria-stabilized zirconium oxide.

[0038] As used herein, the terms "first" and/or "second" do not refer an order or denote relative positions in space or time, but these terms are used to distinguish between two different elements or components. For example, a first separator does not necessarily proceed a second separator in time or space; however, the first separator is not the second

separator and vice versa. Although it is possible for a first separator to proceed a second separator in space or time, it is equally possible that a second separator proceeds a first separator in space or time.

[0039] As used herein "oxidation-resistant" refers to a separator that resists oxidation in an electrochemical cell of an alkaline battery and/or is substantially stable in the presence of an alkaline electrolyte and/or an oxidizing agent (e.g., silver ions).

[0040] As used herein "dendrite-resistant" refers to a separator that reduces the formation of dendrites in an electrochemical cell of an alkaline battery under normal operating conditions, i.e., when the batteries are stored and used in temperatures from about -20° C to about 70° C, and are not overcharged or charged above their rated capacity and/or is substantially stable in the presence of an alkaline electrolyte, and/or is substantially stable in the presence of a reducing agent (e.g., an anode comprising zinc). In some examples, a dendrite-resistant separator inhibits transport and/or chemical reduction of metal ions.

[0041] As used herein, a "titanate salt" refers to a chemical salt that includes in its chemical formula TiO ₃ . Examples of titanate salts include potassium titanate, sodium titanate, lithium titanate, rubidium titanate, or cesium titanate, without limitation.

[0042] As used herein, "adjacent" refers to the positions of at least two distinct elements

(e.g., at least one separator and at least one electrode (e.g., an anode and/or a cathode)).

When an element such as a separator is adjacent to another element such as an electrode or even a second separator, one element is positioned to contact or nearly contact another element. For example, when a separator is adjacent to an electrode, the separator electrically contacts the electrode when the separator and electrode are in an electrolyte environment such as the environment inside an electrochemical cell. The separator may be in physical contact or the separator may nearly contact the electrode such that any space between the separator and the electrode is void of any other separators or electrodes. It is noted that electrolyte may be present in any space between a separator that is adjacent to an electrode or another separator.

[0043] II. SEPARATORS AND BATTERIES

[0044] The separator (e.g., the oxidation-resistant separator) of the invention for use in an alkaline battery comprises a polyether material, and the polyether material comprises an optionally substituted linear or branched polyether polymer. In several embodiments, the polyether material is substantially stable in the presence of an alkaline battery electrolyte and in the presence of an oxidation agent (e.g., silver ions).

[0045] In several embodiments, the polyether material comprises a polyether (PE) polymer,

a polyethylene oxide (PEO) polymer, a polypropylene oxide (PPO) polymer, polytetrafluoroethylene (PTFE), a copolymer thereof, or any combination thereof, wherein each of the polyether (PE) polymer, the polyethylene oxide (PEO) polymer, the polypropylene oxide (PPO) polymer, polytetrafluoroethylene (PTFE), or the copolymer thereof may be optionally substituted or linear or branched. For example, the polyether material comprises a copolymer or a mixture of the polyether polymer with one or more polymer materials other than a polyether, for example, polyethylene, polypropylene, polyphenylene oxide, polysulfone, acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene, or any combination thereof.

[0046] In other embodiments, polyether polymer comprises polyethylene oxide, and the polyethylene oxide has an average molecular weight from about 500,000 amu to about 5,000,000 amu.

[0047] Separators of the present invention can optionally comprise additives such as a filler, a surfactant, a conductivity enhancer, or the like.

[0048] In one embodiment, the separator further comprises a filler that works to inhibit silver ion transport by forming a surface complex with silver ions. In one embodiment, the filler comprises a powder of a metallic oxide. For example, the filler comprises a powder of zirconium oxide, titanium oxide, aluminum oxide, or any combination thereof. In another example, the filler comprises zirconium oxide powder. Without wishing to be bound by theory, it is theorized that the zirconium oxide powder inhibits silver ion transport by forming a surface complex with silver ions. The zirconium oxide powder (or other metallic oxide) is dispersed throughout the PE film so as to provide a substantially uniform barrier to transport of silver ions. The average particle size of the zirconium oxide powder (or other metallic oxide powder) should be in the range from about 1 nm to about 5000 nm (e.g., from about 5 nm to about 200 nm, or from about 5 nm to about 100 nm). Zirconium oxide filler also tends to increase the ionic conductivity of the separator.

[0049] In other embodiments, the separator further comprises a conductivity enhancer to further improve the ionic conductivity of the separator. The conductivity enhancer is dispersed in the polyether material. For example, the conductivity enhancer comprises a titanate salt of an alkali metal that is dispersed in the polyether material. In other examples, the alkali metal comprises potassium, sodium, lithium, rubidium, cesium, or any combination thereof. In other examples, the conductivity enhancer comprises potassium titanate. [0050] In several embodiments, the separator comprises about 1 wt % to about 10 wt % of potassium titanate.

[0051] In some embodiments, the conductivity enhancer comprises an organic material. For example, organic conductivity enhancing materials include organic sulfonates and carboxylates. Such organic compounds of sulfonic and carboxylic acids, which may be used singly or in combination, comprise a wide range of polymer materials that may include salts formed with a wide variety of electropositive cations, K ⁺ , Na ⁺ , Li ⁺ , Pb ⁺² , Ag ⁺ , NH ₄ ⁺ , Ba ⁺² , Sr ⁺² , Mg ⁺ , Ca ⁺ or anilinium, for example. These compounds also include commercial perfluorinated sulfonic acid polymer materials, Nafion ^® and Flemion ^® sold by E. I. du Pont de Nemours and Company, for example. The conductivity enhancer may include a sulfonate or carboxylate copolymer, with polyvinyl alcohol, for example, or a polymer having a 2- acrylamido-2-methyl propanyl as a functional group. Any combination of one or more conductivity enhancing materials may also be used, including any combination of titanate salts of alkali earth metals, as described above, and organic material, as described above. [0052] Separators of the present invention can comprise from about 5 wt % to about 95 wt % of zirconium oxide and/or conductivity enhancer. In some embodiments, the separator comprises from about 20 wt % to about 60 wt % of zirconium oxide and/or conductivity enhancer. For example, the separator comprises from about 30 wt % to about 50 wt % of zirconium oxide.

[0053] In some embodiments, the separator further comprises a surfactant to improve dispersion of additives such as a filler, conductivity enhancer, or other additive by preventing agglomeration of small particles. The surfactant is dispersed in the polyether material. Any suitable surfactant may be used, including one or more anionic surfactants, cationic surfactants, non-ionic surfactants, ampholytic surfactants, amphoteric surfactants, zwitterionic surfactants, or any combination thereof. In one embodiment, the separator comprises an anionic surfactant. Anionic surfactants include salts of sulfate, sulfonate, carboxylate and sarcosinate. One useful surfactant comprises p-(l,l,3,3-tetramethylbutyl)- phenyl ether, which is commercially available under the trade name Triton X- 100 from Rohm and Haas.

[0054] In one example, the surfactant comprises alkyl benzene sulfonate, sodium lauryl sulfate, dioctyl sodium sulfosuccinate, or isooctyl phenyl polyethoxy ethanol. In another example, the surfactant comprises p-(l,l,3,3-tetramethylbutyl)-phenyl ether, which is commercially available under the trade name Triton X-100, from Rohm and Haas. In several examples, the separator comprises from about 0.01 wt % to about 1 wt % of surfactant. [0055] Separators of the present invention can be free-standing, or they can comprise a substrate. Referring to Figure IA, one exemplary separator 10 comprises a polyether

material 20, as described above, disposed on a porous substrate 30 to form a coating. In other examples, the separator comprises polyether material disposed on a non-porous substrate to form a coating. For example, the separator comprises a substrate comprising a polyolefin material (e.g., a porous polyolefin material). In several embodiments, at least a portion of the coating is disposed within pores in the first porous substrate. Figure 1 B depicts a porous substrate 40 having pores 50 that traverse the porous substrate. The porous substrate material is selected to be chemically stable in concentrated alkaline solutions and to resist electrochemical oxidation.

[0056] Without wishing to be bound by theory, it is theorized that by filling pores, depicted in Figure 1 B, in the porous substrate, the polyether material inhibits transport and reduction of silver ions within the pores. When both of the porous substrate film and the polyether material are stable in the battery electrolyte and resist electrochemical oxidation, the separator formed thereby is stable and oxidation-resistant.

[0057] Several embodiments of the present invention provide an oxidation-resistant separator comprising a polyether material, a porous substrate, a zirconium oxide powder filler, and a potassium titanate conductivity enhancer, wherein the polyether material is disposed on the substrate to form a coating. The polyether material, the porous substrate, the zirconium oxide powder and the potassium titanate conductivity enhancer are as described above. In other examples, the oxidation-resistant separator further comprises a surfactant. Useful surfactants include any described above.

[0058] The separator of the present invention is suitable for use in an alkaline storage battery comprising a zinc anode and a silver oxide cathode, but may be used with other anodes and other cathodes. For example, the separator may be used with anodes comprising zinc, cadmium or mercury, or mixtures thereof, for example, and/or with cathodes comprising silver oxide, nickel oxide, cobalt oxide or manganese oxide, or mixtures thereof. [0059] Alkaline battery separators of the present invention may be configured in a variety of ways. For example, a separator for a rectangular battery electrode may be in the form of a sheet or film comparable in size or slightly larger than the electrode, and may simply be placed on the electrode or may be sealed around the edges. The edges of the separator layer may be sealed to the electrode, an electrode current collector, a battery case, or another separator sheet or film on the backside of the electrode via an adhesive sealant, a gasket, or fusion (heat sealing) of the separator or another material. The separator layer may also be in the form of a sheet or film wrapped and folded around the electrode to form a single layer (front and back), an overlapping layer, or multiple layers. For a cylindrical battery, the

separator may be spirally wound with the electrodes in a jellyroll configuration. In many examples, a separator is included in an electrode stack comprising a plurality of separator layers. The separator of the present invention may be incorporated in a battery in any suitable configuration.

[0060] Another aspect of the present invention provides an alkaline battery that comprises a cathode, an anode, an alkaline electrolyte, and a separator. The separator comprises a polyether material, and the polyether material comprises an optionally substituted linear or branched polyether polymer.

[0061] Separators useful in the present alkaline battery include any separator described above that is stable in the presence of an alkaline electrolyte and in the presence of an oxidizing agent. For example, in several embodiments, the polyether polymer comprises polyethylene oxide (PEO), polypropylene oxide (PPO), a copolymer thereof, or a mixture thereof. In another example, polyether material comprises a copolymer and the copolymer comprises polyether polymer copolymerized with a second polymer, for example, polyethylene, polypropylene, polyphenylene oxide, polysulfone, acrylonitrile butadiene styrene (ABS), or polytetrafluoroethylene. In another example, the polyether material comprises a polyether polymer, which comprises polyethylene oxide. For instance, the polyethylene oxide has an average molecular weight of greater than about 5,000 amu (e.g., from about 500,000 amu to about 5,000,000 amu).

[0062] In other embodiments, the alkaline battery comprises a separator that comprises one or more additives such as a filler, a conductivity enhancer, a surfactant, or any combination thereof. For example, the alkaline battery comprises a separator that comprises a filler. In another example, the alkaline battery comprises a separator that comprises a polyether material, a filler comprising zirconium oxide, a surfactant, and a conductivity enhancer comprising potassium titanate.

[0063] In other embodiments, the separator comprises a substrate. For example, the alkaline battery comprises a separator comprising polyether material and a substrate, wherein the polyether (PE) material is disposed on the substrate to form a coating. In other examples, the substrate is a porous material or a non-porous material (e.g., a porous polyolefin material).

[0064] Several embodiments of the present invention are configured so that the separator

(e.g., the oxidation-resistant separator) is adjacent to the cathode.

[0065] Alkaline batteries of the present invention can comprise an anode formed from any suitable material and a cathode formed from any suitable material. In one embodiment, the

anode comprises zinc, cadmium, mercury, or any combination thereof. In another embodiment, the cathode comprises silver oxide, nickel oxide, cobalt oxide, manganese oxide, or any combination thereof. One exemplary battery of the present invention comprises a cathode that comprises silver oxide and an anode that comprises zinc. [0066] In several embodiments, the alkaline electrolyte of the present battery comprises an aqueous solution of a hydroxide of an alkali metal. In several embodiments, the alkali metal comprises potassium, sodium, lithium, rubidium, cesium, or any combination thereof. For instance, the alkaline battery comprises an alkaline electrolyte comprising potassium hydroxide. In other examples, the electrolyte comprises potassium hydroxide having a concentration of from about 10 M to about 16 M (e.g., from about 12 M to about 16 M, from about 13 M to about 16_M, or about 15 M). In one exemplary battery of the present invention, the battery comprises an anode comprising zinc, a cathode comprising silver oxide, and the electrolyte comprises about 15 M potassium hydroxide. However, in some embodiments, the electrolyte comprises a gelling agent, such as polyethylene oxide, polyvinyl alcohol, carboxyalkyl cellulose, polyacylonitrile, polyacrylic acid, polymethacrylic acid, polyoxazoline, polyvinylpyrrolidine, polyacrylates, or polymethacrylate, or any combination thereof.

[0067] In several embodiments, the alkaline battery further comprises a second separator (e.g., a dendrite-resistant separator), wherein the second separator comprises a polyvinyl alcohol material and a cross-linking agent. In several embodiments, the alkaline battery comprises a second separator, and the second separator is a dendrite-resistant separator. [0068] It is noted that batteries of the present invention that comprise this optional second separator have improved charge-discharge performance and an extended cycle life. [0069] In one exemplary embodiment, the alkaline battery comprises an alkaline electrolyte, an anode, a cathode, an oxidation-resistant separator comprising a polyether material, a dendrite-resistant separator comprising a cross-linked polyvinyl alcohol material and a cross-linking agent. In several examples, the oxidation-resistant separator comprises a polyether material comprising an optionally substituted linear or branched polyether polymer. In other examples, the dendrite-resistant separator comprises an optionally substituted linear or branched cross-linked polyvinyl alcohol material and a cross-linking agent, wherein the dendrite-resistant separator is formed by providing a mixture comprising polyvinyl alcohol precursor polymer, cross-linking agents, and/or optional additives; and curing the mixture to form the dendrite resistant separator. [0070[ In several embodiments, the alkaline battery comprises an oxidation-resistant

separator and a dendrite-resistant separator, and the dendrite-resistant separator comprises a cross-linked polyvinyl alcohol material formed from a mixture comprising a cross-linking agent and a PVA precursor polymer, wherein the PVA precursor polymer is linear or branched and is optionally substituted. In some embodiments, the PVA precursor polymer is at least partially hydrolyzed (e.g., at least about 70 % hydrolyzed, at least about 75 % hydrolyzed, or at least about 80 % hydrolyzed). In some embodiments, the PVA precursor polymer has a mean molecular weight of greater than about 5000 amu (e.g., from about 150,000 amu to about 190,000 amu). In other examples, the PVA precursor polymer is at least partially hydrolyzed (e.g., at least about 70 % hydrolyzed, at least about 75 % hydrolyzed, or at least about 80 % hydrolyzed). In some examples, the PVA precursor polymer has a mean molecular weight greater than about 5000 amu and is at least 80 % hydrolyzed.

[0071] In other embodiments, the cross-linked polyvinyl alcohol material comprises a copolymer, and the copolymer comprises a copolymerized polymer and at least 60 mole percent polyvinyl alcohol. The copolymer is formed by including the monomer of the copolymerized polymer in the mixture. Suitable monomers for forming a PVA copolymer include vinyl acetate, ethylene, vinyl butyral, and mixtures thereof. [0072] Cross-linking is necessary to render the polyvinyl alcohol polymer substantially insoluble in water. Suitable cross-linking agents may be added to the second mixture to effect cross-linking of the polyvinyl alcohol precursor polymer include monoaldehydes (e.g., formaldehyde, glyoxilic acid, or a combination thereof); aliphatic, furyl, or aryl dialdehydes (e.g., glutaraldehyde, 2,6 furyldialdehyde, or terephthaldehyde); dicarboxylic acids (e.g., oxalic acid or succinic acid); polyisocyanates; methylolmelamine; copolymers of styrene and maleic anhydride; germaic acid or its salts; boron compounds (e.g., boron oxide, boric acid or its salts, or metaboric acid or its salts); or salts of copper, zinc, aluminum or titanium. In one embodiment, the cross-linking agent comprises boric acid. For example, the alkaline battery comprises an oxidation-resistant separator and a dendrite-resistant separator, and the dendrite-resistant separator comprises a cross-linked polyvinyl alcohol material formed from a mixture comprising a cross-linking agent comprising boric acid and a PVA precursor polymer as described above.

[0073] In several embodiments, dendrite-resistant separators of the present alkaline battery comprise one or more additives such as a filler, a conductivity enhancer, a substrate, a surfactant, a plasticizer, or any combination thereof. In one example, the dendrite-resistant separator comprises zirconium oxide powder. In several embodiments, the dendrite-resistant

separator of the present alkaline battery comprises from about 1 wt % to about 99 wt % (e.g., from about 2 wt % to about 98 wt %, from about 20 wt % to about 60 wt %, or from about 30 wt % to about 50 wt %) of zirconium oxide.

[0074] Exemplary fillers, conductivity enhancers, substrates, and surfactants include any of those described above. For example, dendrite-resistant separator comprises a plasticizer. The plasticizer can facilitate removal of a separator layer from a casting tray or mold. For example, the dendrite-resistant separator comprises a plasticizer comprising glycerin, low- molecular-weight polyethylene glycols, aminoalcohols, polypropylene glycols, 1,3 pentanediol branched analogs, 1,3 pentanediol, any combination thereof, and/or water i.e., H ₂ O. In one example, the conductivity enhancer comprises greater than about 1 wt % of glycerin, low-molecular-weight polyethylene glycols, aminoalcohols, polypropylene glycols, 1,3 pentanediol branched analogs, 1,3 pentanediol, or any combination thereof and less than 99 wt % of water. For example, the conductivity enhancer comprises from about 1 wt % to about 10 wt % of glycerin, low-molecular- weight polyethylene glycols, aminoalcohols, polypropylene glycols, 1 ,3 pentanediol branched analogs, 1 ,3 pentanediol, or any combination thereof and from about 90 wt % to about 99 wt % water. [0075] In another exemplary alkaline battery of the present invention, the dendrite-resistant separator is disposed adjacent to the anode. In several configurations, the dendrite-resistant separator is disposed between the oxidation-resistant separator and the anode. [0076] Referring to Figure 2, the configuration of one exemplary alkaline battery 200 of the present invention comprises an oxidation-resistant separator 210 adjacent to a cathode 220, and a dendrite-resistant separator 230 adjacent to an anode 240. A plurality of oxidation- resistant separators and/or dendrite-resistant separators may be included in an alkaline battery of the present invention.

[0077] In one embodiment, the oxidation-resistant separator comprises a filler, and the dendrite-resistant separator comprises a filler. In both the oxidation-resistant separator and the dendrite-resistant separator, the filler comprises a powder of metallic oxide. In each separator, i.e., the oxidation-resistant separator and the dendrite-resistant separator, the filler is independently selected from zirconium oxide, titanium oxide and aluminum oxide. In other examples, both of the oxidation-resistant separator and the dendrite-resistant separator comprise zirconium oxide powder, in any form or amount described above. [0078] In another example, the dendrite-resistant separator has an improved ionic resistance. For example, the dendrite-resistant separator comprises an ionic resistance of less than about 20 mω/cm ² , (e.g., less than about 10 mω/cm ² , less than about 5 mω/cm ² , or less

than about 4mω/cm ² ). In another embodiment, the dendrite-resistant separator comprises an inorganic or organic conductivity enhancer that may be included in the PVA solution to improve the ionic conductivity of the corresponding separator layer. For example, the dendrite-resistant separator comprises an inorganic conductivity enhancer that comprises a titanate salt of an alkali metal selected from the group consisting of potassium, sodium, lithium, rubidium, cesium, and mixtures thereof. In another example, the dendrite resistant separator comprises a conductivity enhancer that comprises potassium titanate, which has been shown to increase the ionic conductivity of the oxidation-resistant separator layer. The organic conductivity enhancers discussed above for use in the oxidation-resistant separator may also be used for the dendrite-resistant separator.

[0079] In one embodiment, the dendrite-resistant separator of the present invention comprises a conductivity enhancer, and the conductivity enhancer comprises a copolymer of polyvinyl alcohol and a hydroxyl-conducting polymer. Suitable hydroxyl-conducting polymers, which have functional groups that facilitate migration of hydroxyl ions, include polyacrylates, polylactones, polysulfonates, polycarboxylates, polysulfates, polysarconates, polyamides, polyamidosulfonates, or any combination thereof. A solution containing a copolymer of a polyvinyl alcohol and a polylactone is sold commercially under the trade name Vytek® polymer by Celanese, Inc.

[0080] The dendrite-resistant separator may comprise a free-standing cross-linked PVA material, or the dendrite-resistant separator can comprise an optional porous substrate wherein the PVA material is disposed on the substrate to form a coating. [0081] The dendrite-resi stance of the separator comprising PVA material, a cross-linking agent, and zirconium oxide powder, and any optional additives, as described above, can be demonstrated comparatively against a separator that comprises a cross-linked PVA material and is substantially free of zirconium oxide powder (e.g., the separator comprises less than 1 wt % of zirconium oxide powder (e.g., less than 0.5 wt % of zirconium oxide powder)) using standard testing methods such as those described in "Study to Investigate and Improve the Zinc Electrode for Spacecraft Electrochemical Cells", James McBreen, August 1967 Accession Number N67-38923, Page 6, hereby incorporated in its entirety by reference. [0082] In many instances, the physical characteristics of the dendrite-resistant separator are analogous to those of the oxidation-resistant separator depicted in Figure IA. [0083] Referring to Figure 3, an exemplary alkaline battery 300 comprises a third separator 310 comprising pores interposed between cathode 320 and the oxidation-resistant separator 330, a fourth separator 340 comprising pores interposed between anode 350 and the dendrite-

resistant separator 350, or at least one each of the third separator and the fourth separator. In this example, the primary function of the third and fourth porous separators is to hold electrolyte within their pores but provide only a physical barrier to ionic transport. Thus, the third and fourth porous separators may ensure the supply of electrolyte at the electrode surface. It is noted that any number of porous separator layers may be included at any location in the separator stack of the invention.

[0084] The oxidation-resistant separator, dendrite-resistant separator, and the third and fourth porous separators may comprise substrates of any suitable organic polymer or inorganic material that is electronically insulating, provides sufficient structural integrity, and is chemically and electrochemically stable in concentrated alkaline solutions. Exemplary substrates comprise polyolefins (polyethylene or polypropylene, for example), polyethers (polyethylene oxide and polypropylene oxide, for example), polyfluorocarbons (polytetrafluoroethylene, for example), polyamides (nylon, for example), polysulfones (Udel ^® sold by Solvay, for example), polyethersulfones (Radel ^® sold by Solvay, for example), polyacrylates, polymethacrylates, polystyrenes, and mixtures, co-polymers and substituted polymers thereof. Porous films of commercial blended polymers, ABS (acrylonitrile butadiene styrene) or EPDM (ethylene-propylene-diene terpolymer), for example, may also be used. Suitable inorganic materials include metallic oxides, including aluminum oxide, titanium oxide, zirconium oxide, yttria-stabilized zirconium oxide, and mixtures thereof, and metallic nitrides, including titanium nitride, aluminum nitride, zirconium nitride, and mixtures thereof.

[0085] The optional substrates of the oxidation-resistant separator, the dendrite resistant separator, the third porous separator, and the fourth porous separators may be woven or unwoven and may comprise the same material or different materials in an alkaline battery of the present invention. Furthermore, multiple layers of the third or fourth porous separators may comprise the same material or different materials. In one example, the oxidation-resistant separator comprises a substrate comprising a non-woven polymer comprising a polyolefin, polyethylene, or polypropylene. For instance, the non-woven polyethylene film for use as the substrate for the oxidation-resistant separator is sold under the trade name Solupor ^® by DSM company. In another example, the dendrite-resistant separator comprises a substrate that comprises a non-woven polymer comprising a polyolefin, polyethylene or polypropylene. An exemplary non- woven polyolefin film for use as the substrate for the dendrite-resistant separator is sold under the trade name Hipore™ by Asahi company. Another exemplary substrate of the dendrite-resistant separator comprises a non-woven polypropylene film sold

under the trade name SciMAT by Freudenberg Company.

[0086] The PE and PVA solutions may be applied to their respective optional substrates by any suitable method, including a method selected from the group consisting of pouring, spreading, casting, pressing, backfilling, dipping, spraying, rolling, laminating, and combinations thereof. For example, the PE mixture or PVA mixture is mixed until it comprises a substantially uniform mixture, poured onto the porous substrate film, and spread with a coating rod. After the spreading operation, the oxidation-resistant or dendrite-resistant coating is dried in an oven at elevated temperature. [0087] III. METHODS

[0088] Another aspect of the present invention provides a method of producing a separator (e.g., an oxidation-resistant separator) comprising providing a mixture (e.g., solution) that comprises a polyether material, and curing the mixture to form a separator. The polyether material comprises an optionally substituted linear or branched polyether polymer. Separators formed using the present method can comprise one or more additives such as a filler, a conductivity enhancer, a surfactant, of a combination thereof, each of which can be added to the mixture before the mixture is cured or while the mixture is curing. The additives suited for the present method are, without limitation, described above. [0089] The concentrations in weight percent of the components in one exemplary mixture (e.g., solution) are within the ranges: about 87% to about 95% water; about 2% to about 6% polyethylene oxide; about 2% to about 6% yttria-stabilized zirconium oxide; about 0.2% to about 1.5% potassium titanate; and about 0.08% to about 0.2% Triton X-100. [0090] To form a separator of the present invention that comprises a substrate, the mixture can be applied to the substrate using any suitable means to form a coating. For example, the mixture can be applied to the substrate by pouring the PEO solution onto the substrate and spreading the mixture with a coating rod. In other examples, the mixture is applied to the substrate by dipping, painting, spraying, coextruding, or any combination thereof. [0091] In other examples, the separator may be cured using any suitable means. For example, the mixture may be cured by heating, air-drying, exposing to electromagnetic radiation the mixture. Every combination of these curing methods is also suitable to form a separator of the present invention.

[0092] In other examples, a separator (e.g., an oxidation-resistant separator and/or a dendrite-resistant separator) is a free-standing separator wherein the mixture is cast and cured. In another example, the separator comprises a substrate and the mixture is applied to the substrate to form a coating, or the mixture is cast, cured into a free-standing polymer,

which is bonded to a substrate. A free-standing polymer may be bonded to a porous substrate via use of one or more mechanical fasteners (e.g., clips, or the like), adhesives, heat bonding, pressure bonding, or any combination thereof.

[0093] IV. EXAMPLES

[0094] Example 1:

[0095] To fabricate one oxidation-resistant separator according to the invention, a PEO forming solution was prepared by adding 0.9 g of potassium titanate and one drop of Triton

X-IOO surfactant to 72.0 g of a 5% solution of polyethylene oxide, stirring the solution for 5 minutes, and then adding 24 g of zirconium dioxide. This mixture was thoroughly stirred and then evenly spread onto a porous polyolefinic substrate. The supported separator was dried in a convection oven at 75 ⁰ C.

[0096] Example 2:

[0097] To fabricate a free-standing separator according to the invention, a PEO forming solution was prepared by adding 15 g of a 13% PVA-co-AMPS polymer solution and 250 mg of poly(sodium 4-styrenesulfonate) conductivity enhancer to 40 g of a 3% solution of polyethylene oxide. After this solution was stirred, 3 g of yttria-stabilized zirconium (IV) oxide was added. The resulting mixture was thoroughly stirred, poured onto a glass tray, and dried.

[0098] The separators of Examples 1 and 2 were tested for resistance to silver ion transport.

As depicted in Figure 6, the test method involved a three-layer stack of separators 400.

Bottom layer 101 was an indicator layer comprising two identical layers of a cellulosic mat film that turns brown and then black due to precipitation of fine silver metal particles upon exposure to silver ions in alkaline solution. Middle layer 102 was a separator being tested, and top layer 103 was a porous separator. All of the layers were pre-soaked in alkaline battery electrolyte and were then stacked as depicted in Figure 6. A layer 104 of an AgO slurry in KOH solution was then placed on top layer 103, and the stack was heated at 50 ⁰ C

(to accelerate silver ion diffusion) for 24 hours.

[0099] When middle layer 102, i.e., the separator under test, was a microporous polyolefin battery separator, indicator layer 101 rapidly turned dark, indicating rapid diffusion of silver ions across test layer 102. When middle layer 102 was either of the separators of Examples 1 or 2, however, no discoloration was observed during the 24-hour test, indicating good resistance of the separators of Examples 1 and 2 to silver ion migration.

[00100] Example 3:

[00101] In example 3, depicted in Figure 7, an oxidation-resistant separator of the

invention is incorporated in an alkaline battery 500 comprising an alkaline electrolyte (not shown), a zinc anode 510, and a silver oxide cathode 520. The battery 500 includes an oxidation-resistant separator 530, two dendrite-resistant separators 540 and 550, and at least one porous separator 560 and 570 adjacent to each electrode. The porous separator for the cathode comprises a porous polyolefϊn film (SciMAT from Freudenberg Company). A porous separator for the anode comprises a polyolefin film (Solupor ^® Typecode E9H01 A). [00102] Example 4:

[00103] In example 4, an oxidation-resistant separator comprises a polyether material comprising a polyethylene oxide polymer disposed on a porous substrate. This exemplary embodiment was formed by providing a mixture of 91.1 wt % water, 3.7 wt % polyethylene oxide, wherein the polyethylene oxide had an average molecular weight of about 1,000,000 amu, 4.1 wt % zirconium oxide (ZrO ₂ of 100 nm average particle size), 0.9 wt % potassium titanate, and 0.1 wt% surfactant (Triton X-100). The polyethylene oxide was dissolved in the water using a high-speed mixer (FlackTek DAC 150 FVZ SpeedMixer); the potassium titanate and the surfactant were added to the mixture; which was mixed for about 5 minutes at 3300 rpm using the high-speed mixer; the zirconium oxide was added to the solution, and the mixture was mixed for another 5 minutes at 3300 rpm in the mixer. The mixture was poured onto a porous polypropylene film (Solupor Typecode E9H01 A) on a flat surface and spread evenly over the film surface using coating wire (wire size #50). The coated film was dried for 15 minutes in an oven (75°C). [00104] Example 5:

[00105] In example 5, a dendrite-resistant separator comprising a free-standing PVA polymer is formed. This exemplary embodiment was formed by providing a mixture of 95 wt % water, 3.1 wt % polyvinyl alcohol having an average molecular weight of 150,000amu, 1.9 wt % zirconium oxide (ZrO ₂ of 0.6 μm average particle size), and 0.06 wt % boric acid. The zirconium oxide was dispersed in the water using a high-speed mixer; the polyvinyl alcohol was added to the water and the mixture was mixed for 3 minutes at 3500 rpm in the high speed mixer. Boric acid was added (dissolved in 20 g of water) to the mixture; and mixture was mixed for another 4 minutes at 3500 rpm in the high speed mixer. A portion of the PVA polymer material was poured onto a glass tray and dried overnight in a controlled-humidity drying room at ambient temperature. [00106] Example 6:

[00107] The utility of the present invention was demonstrated by comparing the performance of a battery according to the invention with that of a commercial battery, acting

as a negative control, in zinc-silver oxide cells during charge-discharge cycling (0.4 - 1.1 V discharge and 0.3 - 2.03 V charge). The battery according to the invention comprised one oxidation-resistant separator and two dendrite-resistant separators, all of which had a combined thickness of 90 μm. The commercial battery comprised a Shilong PPAT-SL8(2) three-layer laminate having a total thickness of 175 μm.

[00108] Figure 4 shows plots of charge capacity vs. cycle number for two zinc-silver oxide batteries (A and B) constructed with the commercial three-layer laminate separator. A significant loss in charge capacity is evident for both cells in less than 10 cycles. [00109] Figure 5 shows plots of charge capacity vs. cycle number for two exemplary zinc- silver oxide batteries of the present invention (C and D) constructed with the separators according to the invention. No significant loss in capacity is evident for either of these cells over 30 cycles. These cells continued to cycle with minimal loss in capacity up to 80 cycles.

OTHER EMBODIMENTS

[00110] The preferred embodiments of the present invention have been illustrated and described above. Modifications and additional embodiments, however, will undoubtedly be apparent to those skilled in the art. Furthermore, equivalent elements may be substituted for those illustrated and described herein, parts or connections might be reversed or otherwise interchanged, and certain features of the invention may be utilized independently of other features. Consequently, the exemplary embodiments should be considered illustrative, rather than inclusive, while the appended claims are more indicative of the full scope of the invention.