ALKALI ION CONDUCTING SEPARATOR

RELATED APPLICATIONS

[0001] This application claims priority to, and the benefit of, provisional U.S. Patent Application No. 61/438,328 filed on February 1, 2011, which was a continuation in part, and claimed priority to, and the benefit of, U.S. Application No. 11/944,719 filed on November 26, 2007 entitled Nickel-Metal Hydride Battery Using Alkali Ion Conducting Separator. This application is also a continuation in part of, and claims priority to, both U.S. Application Nos. 13/189,176 and 13/189,177, each filed on July 22, 2011, which applications were divisional applications of U.S. Application No. 11/944,719. U.S. Application No. 11/944,719 issued on September 6, 2011 as U.S. Patent No. 8,012,621. These applications and patent are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a nickel-metal hydride/Hydrogen hybrid battery that includes a non-porous, alkali ion conducting separator.

[0003] Nickel-metal hydride storage batteries are widely used for the power sources of cordless electronic equipment, power tools, electric vehicles and the like. Conventional nickel-metal hydride batteries are composed of a positive electrode containing nickel hydroxide, a negative electrode containing a hydrogen-absorbing metal alloy, a microporous separator interposed between the positive and negative electrodes, and an electrolyte.

[0004] Nickel hydrogen battery (Ni-H ₂ ) is a choice battery in many aerospace applications, especially geo- synchronous (GEO) and low earth-orbit (LEO) satellites. Recently, nickel-hydrogen batteries have also been used in terrestrial applications. The difference with a nickel-metal hydride battery is the use of hydrogen in a pressurized cell of up to 1200 psi (82.7 bar). The Ni-H ₂ battery comprises a positive electrode containing nickel hydroxide, a negative hydrogen electrode utilizes a teflon-bonded platinum black catalyst, and a zirconia cloth separator. This battery has a long cycle life, high specific energy, high power density, and also exhibits tolerance for overcharge. Its disadvantages include an expensive initial cost, as well as low volumetric energy density. [0005] Self-discharge is a phenomenon in many rechargeable batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to have less charge than expected when actually put to use. How fast self-discharge in a battery occurs is dependent on the type of battery and temperature. Nickel-based batteries typically are significantly affected by self-discharge (nickel cadmium, 15-20% per month; nickel metal hydride, 30% per month; nickel hydrogen proportional to hydrogen pressure). Self-discharge is a chemical reaction and tends to occur more quickly at higher temperatures. Storing batteries at lower temperatures may reduce the rate of self-discharge and preserve the initial energy stored in the battery.

[0006] Without being bound by theory, it is believed the self-discharge problem associate with nickel metal hydride batteries is a result of hydrogen passing through the porous separator.

[0007] It would be an improvement in the art to provide a nickel metal hydride battery with reduced or limited self-discharge.

SUMMARY OF THE INVENTION

[0008] In accordance with the present invention, there is provided herein a nickel-metal hydride (hydrogen) hybrid battery that contains a positive electrode containing nickel hydroxide and a negative electrode containing a reversible hydrogen electrode. In one embodiment, the negative electrode is a combination electrode containing a hydrogen storage alloy electrode. The battery also contains a separator having a substantially non-porous alkali ion conducting material disposed between the positive electrode and the negative electrode.

[0009] In accordance with the present invention, there is also provided method of charging a nickel-metal hydride (hydrogen) hybrid battery having a positive electrode containing nickel hydroxide, a combination negative electrode containing a hydrogen storage alloy electrode and a reversible hydrogen electrode, an alkaline electrolyte, and a separator comprising a substantially non-porous alkali ion conducting material. In one embodiment, the method includes the steps of applying an electric charging potential to the positive and negative electrodes to cause the following reaction to occur at the positive electrode:

Ni(OH) ₂ + MeOH→ NiOOH + H ₂ 0 + Me ⁺ + e ^" and to cause the following reactions to occur at the negative electrode:

M + H ₂ 0 + e ^" + Me ⁺ → MH _ab + MeOH

H ₂ 0 + e ^" + Me ⁺ → l/2H ₂ + MeOH M in the forgoing reactions may be a hydrogen absorbing alloy, H _a may be absorbed hydrogen, and Me may be an alkali metal.

[0010] The method may also include the step of conducting Me ⁺ ions across the alkali ion conducting separator from the positive electrode to the negative electrode. In one embodiment, the positive electrode is the electrode bearing a positive charge on discharge and the negative electrode bears a negative charge on discharge.

[0011] In accordance with the present invention, there is also provided a method of discharging a nickel-metal (hydrogen) hybrid battery having a positive electrode containing nickel hydroxide, a combination negative electrode containing a hydrogen storage alloy electrode and a reversible hydrogen electrode, an alkaline electrolyte, and a separator comprising a substantially non-porous alkali ion conducting material. The method includes the step of generating an electric potential between the positive and negative electrodes due in part to the following reaction occurring at the positive electrode:

NiOOH + H ₂ 0 + Me ⁺ + e ^" → Ni(OH) ₂ + MeOH and due in part to the following reaction occurring at the negative electrode:

MH _ab + M _e OH→ M + H ₂ 0 + e ^" + Me ⁺

l/2H ₂ + MeOH→ H ₂ 0 + e ^" + Me ⁺

In one embodiment, M is a hydrogen absorbing alloy, ¾ is absorbed hydrogen, and Me is an alkali metal. The method includes the step of conducting Me ⁺ ions across the alkali ion conducting separator from the negative electrode to the positive electrode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0012] In order that the manner in which the above -recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0013] Figure 1 is a schematic representation of one embodiment of a Ni-MH/H ₂ hybrid battery within the scope of the invention.

[0014] Figure 2 is a schematic representation of another embodiment of a Ni-MH/H ₂ hybrid battery within the scope of the invention. DETAILED DESCRIPTION

[0015] Reference throughout this specification to "one embodiment," "an embodiment," or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0016] Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of cells, membranes, processes, methods, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details or method steps, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0017] The embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the nickel-metal hydride battery using an alkali metal conducting separator within the scope of the present invention as represented in Figures 1 and 2, is not intended to limit the scope of the invention, as claimed, but is merely representative of the embodiments of the invention.

[0018] Referring to Figure 1 , there is provided a schematic representation of a N1-MH/H ₂ hybrid battery within the scope of the invention. In this embodiment, battery includes a positive nickel hydroxide electrode, a negative electrode, and an alkali ion conducting separator. The battery also includes an alkaline electrolyte, such as an alkali metal hydroxide. The electrolyte may contain other minor constituents to enhance cell performance. The alkaline electrolytes may include by way of non-limiting example, sodium hydroxide, lithium hydroxide, sodium carbonate, lithium carbonate, and the like. In one embodiment, alkaline electrolytes may be positioned on either side of the separator and may be different from one another. The substantially non-porous nature of the separator allows for different electrolytes having different enhancers or constituents to be used on one side of the separator without effecting electrode performance on the other side of separator. Thus for example, different alkaline electrolytes having different alkalinities may be either side of the separator.

[0019] The positive electrode may comprise nickel hydroxide (Ni(OH) ₂ ) or other materials used in conventional nickel-metal hydride batteries. In some embodiments, the positive electrode may be a pasted or sintered-type material.

[0020] The negative electrode in the present invention may contain a reversible hydrogen electrode. In another embodiment, the negative electrode is a combination of MH and H ₂ electrodes, wherein different charge/discharge reactions happen at the respective electrodes. Figure 1 shows a combination cathode. The MH cathode is same as or similar to traditional Ni-MH battery cathode and the hydrogen cathode is a gas diffusion type of cathode.

[0021] The MH negative electrode may comprise a hydrogen-absorbing alloy. Such alloys are known in the art. Examples of early hydrogen-absorbing alloys include NiFe, Mg ₂ Ni, and LaNis. These hydrogen-absorbing alloys combine metal (A) whose hydrides generate heat exothermically with metal (B) whose hydrides generate heat endothermically to produce the suitable binding energy so that hydrogen can be absorbed and released at or around normal temperature and pressure levels. Depending on how metals A and B are combined, the alloys are classified into the following types: AB (TiFe, etc.), AB ₂ (ZnMn ₂ , etc.), AB ₅ (LaNi ₅ , etc.) and A ₂ B (Mg ₂ Ni, etc.).

[0022] Of the foregoing general types of hydrogen absorbing metal alloys, two general classes of metallic alloys are identified as possessing characteristics desirable for battery cell use because of their good charge and discharge efficiency and durability. These are rare earth/nickel alloys generally based around LaNis (the so-called AB ₅ class of alloys) and alloys consisting primarily of titanium and zirconium (designated as AB ₂ alloys). In both cases, some fraction of the base metals is often replaced with other metallic elements. The AB ₅ formulation appears to offer the best set of features for commercial nickel-metal hydride cell applications. The metal hydride electrode has a theoretical capacity approximately 40 percent higher than the cadmium electrode in a nickel-cadmium couple. As a result, nickel- metal hydride cells provide energy densities that are 20-40 percent higher than the equivalent nickel-cadmium cell.

[0023] The reversible hydrogen electrode comprises a catalyst comprising platinum dispersed on a carbon in the form of a gas diffusion electrode. The catalyst may be a platinum-type electro catalyst. Gas diffusion electrodes are used in chlor-alkali electrolysis, metal-air batteries, and fuel cells. In one embodiment, the reversible hydrogen may be a gas diffusion electrode that interfaces with an alkaline electrolyte and a gaseous phase for electrochemical oxidation of hydrogen, the gas diffusion electrode comprising at least one reaction layer having dispersed therein a platinum-type catalyst, wherein the reaction layer is in fluid communication with the alkaline electrolyte and wherein the gas diffusion layer is in fluid communication with a gas comprising hydrogen.

[0024] A gas diffusion electrode has a multilayer structure composed of a gas diffusion layer, a reaction layer, and a current collector for electrical connection. Gas phase hydrogen is exposed to the gas diffusion layer. The reaction layer resides between the gas diffusion layer and the electrolyte. After passing through the gas diffusion layer, hydrogen is consumed through a reduction reaction (on discharge) on an hydrogen reduction catalyst in the reaction layer.

[0025] The gas diffusion layer is required to allow the hydrogen to pass there through rapidly and to diffuse uniformly into the entire reaction layer. The gas diffusion layer is also required to prevent the electrolyte from permeating to the gas phase. The gas diffusion layer is comprised of a material formed of carbon particles bonded to each other with a material, such as polytetrafluoroethylene, having high water repellent properties. The gas diffusion layer must also conduct electrons from the current collector to the reaction layer.

[0026] The reaction layer contains uniformly dispersed hydrogen reduction catalyst particles in electronic continuity with the gas diffusion layer and current collector. In the reaction layer, a large interface area is formed among the oxygen, electrolyte, electrons, and the oxygen reduction catalyst.

[0027] Mainly noble metals such as platinum and silver, dispersed in or supported on carbon black in the reaction layer, have been used and investigated as hydrogen reduction catalysts for concentrated alkaline solution.

[0028] The current collector may be, for example, a wire mesh or a foam material, which is composed of nickel, silver, or the like.

[0029] Figure 1 shows that the negative electrode is made by placing the reversible hydrogen electrode adjacent to the hydrogen storage alloy electrode such that the reaction layer of the reversible hydrogen electrode is facing the hydrogen storage alloy electrode and the gas diffusion layer is facing the hydrogen gas.

[0030] Also understood is that the advantages of the present invention described herein also apply to a Ni-H2 battery without metal absorbing anode but with a substantially reversible hydrogen electrode with an alkaline conductive separator between anode and cathode, with sufficient alkali metal electrolyte to carry the current across the membrane with each charge and discharge. This embodiment is partially depicted in Figure 1 except in this embodiment there is no hydrogen absorbing metal and the negative electrode.

[0031] Figure 2 shows a second embodiment where the hydrogen absorbing alloy electrode material is combined with the reversible hydrogen electrode material to form the combination negative electrode. One way to form the combination negative electrode is to mix hydrogen absorbing alloy electrode material (mischmetal) with the platinum black catalyst (for reversible hydrogen reaction) and the alkaline electrolyte to form the combination negative electrode or anode as a unitary component. Alternatively the platinum black and/or the alkaline electrolyte may be dispersed on the hydrogen absorbing alloy electrode material (mischmetal) itself. In yet another embodiment, the combination negative electrode comprises hydrogen storage alloy material, an alkaline electrolyte, and the reaction layer material of the reversible hydrogen electrode. In yet another embodiment, the combination negative electrode includes the hydrogen storage alloy material, the alkaline electrolyte and the materials from both gas diffusion and reaction layers of the reversible hydrogen electrode mixed and formed into a single component. Another way to form the combination negative electrode is to mix hydrogen absorbing alloy electrode material (mischmetal) with the reaction layer material of the gas diffusion electrode (for reversible hydrogen reaction) and the alkaline electrolyte to form the combined anode. The mixtures or combinations of these embodiments may be homogeneous or nonhomogeneous. Accordingly, the hydrogen storage alloy electrode reversibly absorbs hydrogen. Thus, the terms hydrogen storage alloy electrode and hydrogen absorbing alloy electrode may be used interchangeably herein throughout as context permits.

[0032] The charge and discharge reactions for nickel-metal hydride (hydrogen) hybrid battery within the scope of the invention, using sodium as a representative alkali metal, are shown below:

Positive electrode: Ni(OH) ₂ + NaOH → NiOOH + H ₂ 0 + Na ⁺ + e ^"

(charge)

NiOOH + H ₂ 0 + Na ⁺ + e ^" → Ni(OH) ₂ + NaOH (discharge)

Negative electrode: M + H ₂ 0 + e ^" + Na ⁺ → MH _ab + NaOH

(charge)

Negative electrode: H ₂ 0 + e ^" + Na ⁺ → 1/2H ₂ + NaOH

(charge) MH _ab + NaOH → M + H ₂ 0 + e ^" + Na ⁺

(discharge)

1/2H ₂ + NaOH → H ₂ 0 + e ^" + Na ⁺ (discharge)

Overall reaction for the Ni-MH portion of the battery is:

Ni(OH) ₂ + M → NiOOH + MH _ab (charge)

NiOOH + MH _ab → Ni(OH) ₂ + M (discharge)

Where M is a hydrogen absorbing alloy and H _a is absorbed hydrogen.

Overall reaction for the Ni-H ₂ portion of the battery is:

Ni(OH) ₂ → NiOOH + 1/2H ₂

(charge)

NiOOH + 1/2H ₂ → Ni(OH) ₂ (discharge)

[0033] Accordingly, the reversible hydrogen anode oxidizes hydrogen to water during charge and reduces water back to hydrogen during discharge.

[0034] The capacity distribution between the two negative electrodes may be adjusted so that the either of the charge/discharge reactions for Ni-MH or Ni-H ₂ are predominant. This means that one of the negative electrode possesses a greater capacity than the other. It may be that one of the negative electrode will reach full capacity first as the cell is charged/discharged before the other one. Thus in one embodiment, the hydrogen storage alloy electrode and the reversible hydrogen electrode have different charge storage capacities.

[0035] In accordance with the present invention, there is provided herein a nickel-metal hydride (hydrogen) hybrid battery that contains an alkali ion conducting separator configured to selectively transport alkali ions. The nickel-metal hydride (hydrogen) combination battery is structurally similar to conventional nickel-metal hydride and nickel-hydrogen batteries and contains a positive electrode and a negative electrode. One difference between the traditional Ni-MH and Ni-H ₂ compared to the present battery is that an alkali ion conducting separator is disposed between the positive and negative electrodes. The separator may be substantially non-porous.

[0036] The separator in one embodiment is an alkali ion conducting solid electrolyte configured to selectively transport alkali ions. It may be a specific alkali ion conductor. For example, the separator may be a solid MeSICON (Metal Super Ion CONducting) material, where Me is Na, K, Li or a combination thereof. The alkali ion conducting separator may comprise a material having the formula Mei _+x Zr ₂ Si _x P ₃ _ _x Oi2 where 0<x<3, and where Me is Na, K, Li or a combination thereof. Other alkali ion conducting solid electrolytes may comprise a material having the formula MesRL^O^ where Me is Na, K, Li or combinations thereof, and where RE is Y, Nd, Dy, Sm, or any mixture thereof. The alkali ion conducting separator may comprise a non- stoichiometric alkali-deficient material having the formula (Me5RESi40i2)i-5(RE2C>3.2SiC>2)5, where Me is Na, K, Li, or a combination thereof, where RE is Nd, Dy, Sm, or any mixture thereof, and where δ is the measure of deviation from stoichiometry. In one embodiment, the separator comprises a material having the formula Nai _+x ¾Si _x P ₃ - _x Oi2 where 0<x<3. The alkali ion conducting separator may also be beta- alumina.

[0037] The alkali ion conducting separator may be configured in the form of a monolithic flat plate, a monolithic tube, a monolithic honeycomb, or supported structures of the foregoing. The alkali ion conducting separator may be a flexible sheet of the polymer configured in various forms applicable to the intended application. The alkali ion conducting separator may be a flexible sheet composed of a mixture of polymer and ceramic and configured in a variety of forms. The alkali ion conducting separator may be configured as a layered alkali ion conducting ceramic-polymer composite membrane comprising alkali ion selective polymers layered on alkali ion conducting ceramic solid electrolyte materials.

[0038] In the present battery using the alkali metal ion conducting separator, the current carrying species in the electrolyte are exclusively alkali metal ions. Also the concentrations of the electrolyte change at both electrodes during battery operation because the non porous separator prevents mixing of electrolyte from both the compartments. This advantageously prevents transport of unwanted species from one electrode to the other and substantially eliminates capacity loss and self discharge. In one embodiment of the invention, the separator is a substantially non-porous ceramic separator material. The substantially non- porous ceramic separator material may include pockets of porosity, but it should not have "through-porosity." "Substantially non-porous" in some embodiments, means less than or equal to 5% porosity. The substantially non-porous separator is preferably hermetic or gas- impermeable. However, the substantially non-porous separator used within the scope of the present invention may possess a trace amount of through porosity and/or gas permeability. The term substantially non-porous is intended to differentiate the prior art separators that are substantially porous. [0039] The separator conducts alkali ions, but is substantially impermeable to hydrogen. The term "substantially impermeable to hydrogen" means that the separator is greater than or equal to 95% impermeable to hydrogen. Without being bound by theory, it is presently believed that self-discharge of the nickel-metal hydride and nickel-hydrogen batteries may be substantially reduced or eliminated by preventing hydrogen from passing from the negative electrode to the positive electrode. The solid electrolyte separator being non porous prevents any hydrogen transport to the positive electrode while the polymer separator will allow some diffusion of hydrogen although lower than a microporous separator commonly used in the prior art. The Ni-MH/H ₂ hybrid battery within the scope of the present invention can be stored and used at higher temperature than the prior art because of the minimal self discharge.

[0040] The combination of highly reversible hydrogen negative electrode and hydrogen storage alloy electrode should result in longer charge/discharge cycle life than conventional Ni-MH battery. The presence of substantially hydrogen impermeable solid electrolyte separator prevents hydrogen consumption at the positive electrode and therefore the battery should have a longer shelf-life than conventional Ni-MH or Ni-H ₂ batteries

[0041] In some embodiments, the nickel-metal hydride battery may be operated at temperatures from about -40 °C to about 120°C.

[0042] A method of charging a nickel-metal hydride (hydrogen) hybrid battery having a positive electrode containing nickel hydroxide, a combination negative electrode containing a hydrogen storage alloy electrode and a reversible hydrogen electrode, an alkaline electrolyte, and a separator comprising a substantially non-porous alkali ion conducting material includes the steps of applying an electric charging potential to the positive and negative electrodes to cause the following reaction to occur at the positive electrode:

Ni(OH) ₂ + MeOH → NiOOH + H ₂ 0 + Me ⁺ + e ^" and to cause the following reactions to occur at the negative electrode:

M + H ₂ 0 + e ^" + Me ⁺ → MH _ab + MeOH

H ₂ 0 + e ^" + Me ⁺ → l/2H ₂ + MeOH

M in the forgoing reactions may be a hydrogen absorbing alloy, H _ab may be absorbed hydrogen, and Me may be an alkali metal.

[0043] The method may also include the step of conducting Me ⁺ ions across the alkali ion conducting separator from the positive electrode to the negative electrode. In one embodiment, the positive electrode is the electrode bearing a positive charge on discharge and the negative electrode bears a negative charge on discharge. [0044] A method of discharging a nickel-metal (hydrogen) hybrid battery having a positive electrode containing nickel hydroxide, a combination negative electrode containing a hydrogen storage alloy electrode and a reversible hydrogen electrode, an alkaline electrolyte, and a separator comprising a substantially non-porous alkali ion conducting material includes the step of generating an electric potential between the positive and negative electrodes due in part to the following reaction occurring at the positive electrode:

NiOOH + H ₂ 0 + Me ⁺ + e ^" → Ni(OH) ₂ + MeOH and due in part to the following reaction occurring at the negative electrode:

MH _ab + M _e OH → M + H ₂ 0 + e ^" + Me ⁺

l/2H ₂ + MeOH → H ₂ 0 + e ^~ + Me ⁺

In one embodiment, M is a hydrogen absorbing alloy, ¾ is absorbed hydrogen, and Me is an alkali metal. The method includes the step of conducting Me ⁺ ions across the alkali ion conducting separator from the negative electrode to the positive electrode.

[0045] Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment, but may refer to every embodiment.

[0046] Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

[0047] These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.