Patent Application
20150318590
In aspects, the instant application relates generally to electrochemical cells. The present invention relates to a non-aqueous lithium/oxygen battery, especially a non-aqueous electrolyte solution that benefits the performance of such batteries. An exemplary embodiment of the instant invention is a non-aqueous lithium/oxygen cell comprising a lithium anode, a carbon air cathode, and a non-aqueous electrolyte soaked in a microporous separator membrane.
Metal-oxygen batteries, which are also referred to as metal-air batteries, are a class of electrochemical cells in which oxygen, which is typically obtained from the ambient environment, is reduced at a catalytic cathode surface as part of the electrochemical cell reaction.
Theoretically, the operation of a lithium/air battery can last as long as the supply of oxygen from environmental air and metal lithium remain. However, in embodiments, the operation of a practical lithium-air battery is often halted by the insoluble oxygen reduction products lithium peroxide (Li2O2) and lithium oxide (Li2O) that deposit and accumulate on the surface of the carbon air cathode and eventually clog the porous channels used for the diffusion of gaseous oxygen.
Reduction of the oxygen forms an oxide or peroxide ion which reacts with a cationic metal species. Metal-oxygen batteries have been developed based upon iron (Fe), zinc (Zn), aluminum (Al), magnesium (Mg), calcium (Ca), and lithium.
Typically, metal/air batteries contain a metal anode, an air cathode, and a liquid electrolyte that provide a media for ionic conduction and electronically isolates the anode and air cathode. The air cathodes may, for example, comprise a highly porous carbon sheet with or without catalyst-loading. Among many metals suitable for the anode of metal/air batteries, lithium is attractive because of its low molecular weight and its negative electrode potential that offer high energy and power densities.
However, metal lithium requires a dry environment and a non-aqueous organic electrolyte to avoid reaction with moisture. Lithium/air batteries are different from other conventional metal/air batteries in that their discharge products (Li2O2) and lithium oxide (Li2O) are insoluble in organic electrolytes. Lithium-oxygen batteries represent one type of metal-oxygen battery. In devices of this type, an electro-active cathode and a lithium-containing anode are disposed in an electrolyte which provides for ionic communication. During the discharge of the cell, oxygen is reduced at the electro-active cathode to produce O2−1 and/or O2−2 anions which combine with the lithium ions to produce Li2O2 and/or Li2O which deposits on the cathode. Therefore, the oxides (Li2O2 and Li2O) are deposited on the surfaces of the carbon cathode, rather than dissolved into the electrolyte, which prevents oxygen from diffusing into the reaction sites of the carbon. The battery fails to function when the surfaces of the carbon are fully covered by the discharge products.
Therefore, specific capacities of lithium/air batteries are determined by the amount of the oxides that form on the surfaces of the carbon in the air cathode.
In lithium/air batteries, oxygen is catalytically reduced to form peroxide anions on the surfaces of carbon (serving as a catalyst), and the resulting peroxide anions combine with Li+ ions from the electrolyte to form lithium peroxides (Li2O2) that deposit on the surfaces of the carbon. Depending on the discharge current rates and electrolyte formulations, the deposited lithium peroxides can be further discharged to lithium oxide (Li2O). Theoretically, lithium/air batteries have a specific capacity of 3862 mAh/g Li and specific energy densities of 11,425 Wh/kg and 11,248 Wh/kg, based on their discharge products lithium peroxide and lithium oxide, respectively.
High polarization of lithium/oxygen batteries is due to slow catalytic reduction of oxygen, which is affected by two factors: (1) the kinetics of the catalytic reduction of oxygen and (2) diffusion of dissolved oxygen in the liquid electrolyte. The former can be enhanced by using a catalyst and modifying the electrolyte formulation. The latter can be improved by reducing the viscosity of the liquid electrolytes and minimizing the diffusion distance of the dissolved oxygen, which can be achieved by adopting low viscous electrolyte solvents and avoiding electrolyte-flooding. Power and energy densities of practical lithium/oxygen batteries are not close to the theoretical expectations. Therefore, there remains a need to develop better electrolytes for improvement of the performance of lithium/oxygen batteries.
A non-aqueous electrolyte solution comprising a salt having the formula MBF3X where M is an alkali metal cation or a quaternary organic ammonium cation; and X is the halide chloride, bromide or iodide and a solvent mixture consisting of a first solvent and a second solvent. A non-aqueous electrolyte solution and lithium salt have several characteristics making it well suited for inclusion within a lithium-ion battery.
Also provided is non-aqueous electrolyte lithium/oxygen battery that has a high discharge voltage when compared to the same lithium/oxygen battery using conventional or prior art electrolytes.
In one aspect, the present invention provides a non-aqueous electrolyte lithium/oxygen battery that has a higher discharge voltage than the prior art. The battery comprises a lithium anode, an air (oxygen) cathode, and a non-aqueous organic electrolyte soaked in a microporous separator membrane.
A key aspect of the present invention is a non-aqueous electrolyte solution that comprises a lithium salt with a general molecular formula of LiBF3X wherein X is fluorine, chlorine, iodine or bromine, respectively, and a solvent mixture consisting of a first solvent and a second solvent. The first solvent has a higher boiling point than the second solvent and a relatively high viscosity of from greater than about 1.5 centipoises at twenty-five (25) degrees Celsius. The first solvent may, for example, comprise ethylene carbonate (EC), propylene carbonate (PC), gamma-butyrlactone (GBL), N-methyl pyrrolidinone (NMP), tetramethylene sulfone (sulfolane), triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and mixtures thereof. The second solvent has lower boiling point and low viscosity, which is selected from the group of cyclic ethers, linear ethers having a molecular formula of CmH2m+1OCnH2n+1 wherein m and n are independently from 1 to 4, linear carbonates having a molecular formula of CxH2x+1OC(O)OCyH2y+1 wherein x and y are independently from 1 to 4, and mixtures thereof. Concentrations of lithium salt in the electrolytes are from about 0.1 molal (m) to about 1.5 molal (m), and ratio of the first solvent in the solvent mixtures is from about 10 percent to about 90 percent by volume. Alternatively, said non-aqueous electrolyte may be plasticized into a polymer to form a gel polymer electrolyte.
In accordance with embodiments of the present invention, the air cathode is comprised of a highly porous carbon sheet that is made by bonding carbon powder with high specific surface areas with a polymer binder. Alternatively, additional catalyst material can be added into the air cathode either by coating it onto the surfaces of carbon or by mixing it with a conductive carbon powder. Polymer binders are these commonly used in the cathode of lithium batteries and lithium-ion batteries, which may, for example comprise polyvinylidene fluoride (PVdF), hexafluropropylene-vinylidene fluoride copolymers (PVdF-HFP), and polytetrafluoroethylene (PTFE). Separators are microporous polyolefin membranes or non-woven clothes made of synthetic polymer resins.
The inventive composition has the attribute of forming X2, wherein X comprises fluorine, chlorine, or bromine, upon exposure to oxygen. Within a lithium-ion battery oxygen is released during overcharge. This inventive attribute has the advantage of protecting a non-aqueous electrolyte battery from cathodic overcharge exotherms.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the embodiments of the invention.
In accordance with the present invention, the lithium/oxygen cell is composed of a lithium anode, an air (oxygen) cathode, and a non-aqueous organic electrolyte soaked in a microporous separator membrane. The air cathode is a highly porous carbon sheet that is made by bonding a conductive carbon powder with high specific surface areas together with a polymer binder, wherein carbon not only is a conducting agent, but also serves as a catalyst for the catalytic reduction of oxygen. Alternatively, additional catalyst can be added into the air cathode by coating it onto the surfaces of carbon or by mixing it with carbon powder. Suitable catalysts comprise, for example, metal macrocycles such as cobalt phthalocyanine, iron phthalocyanine, other phthalocyanines for example manganese, copper, transition metal oxides such as MnO2, FeOx, CoO, NiO, silver (Ag), carbon-supported platinum, carbon-supported gold, and carbon-supported palladium and highly dispersed precious metals such as platinum/ruthenium alloys. Polymer binders used in the cathode of lithium batteries and lithium-ion batteries may comprise polyvinylidene fluoride (PVdF), hexafluropropylene-vinylidene fluoride copolymers (PVdF-HFP), and polytetrafluoroethylene (PTFE).
To avoid electrical circuit-shorting, a separator is placed between the lithium anode and the air cathode, and non-aqueous liquid electrolytes are soaked into the pores of the separators to supply ionic conduction. Separators may, for example, be a microporous membrane of polypropylene and polyethylene, a non-woven cloth of synthetic polymer resin such as polytetrafluoroethylene, polypropylene, or a woven porous body of such materials.
With respect to the electrolyte, it comprises a non-aqueous organic solution that comprises a lithium salt and a solvent mixture. In accordance with the present invention, the lithium salt has a general chemical formula of LiBF3X wherein X is fluorine, chlorine or bromine, respectively. Concentrations of lithium salt in the electrolytes range from about 0.1 m to about 1.5 m. The electrolyte solvent comprises a mixture consisting of a first solvent and a second solvent. The first solvent has high dielectric constant of greater than about thirty (30) and a high boiling point (i.e., low volatility) of about 200 degrees Celsius, and which may, for example, comprise ethylene carbonate (EC), propylene carbonate (PC), gamma-butyrlactone (GBL), N-methyl pyrrolidinone (NMP), tetramethylene sulfone, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and mixtures thereof. In general, these solvents are very viscous, for example, greater than 1.5 centipoises at about twenty-five (25) degrees Celsius. Non-aqueous electrolytes used in the-state-of-the-art lithium-ion batteries contain a solvent system that, in general, includes a cyclic carbonate compound, such as ethylene carbonate (EC) and propylene carbonate (PC), as well as a linear carbonate, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC). The cyclic carbonates are chemically and physically stable and have high dielectric constant, which is necessary for their ability to dissolve salts. The linear carbonates are also chemically and physically stable and have low dielectric constant and low viscosity, which is necessary to increase the mobility of the lithium ions in the electrolytes. “PC-based electrolyte system” contains PC as one of the components and, when the only cyclic carbonate present is EC, the electrolyte system is considered “EC-based”.
Therefore, a second solvent with low viscosity for example of less than about 0.8 centipoises at about twenty-five (25) degrees Celsius is added to reduce the viscosity of the solvent mixture. The second solvent generally has a lower boiling point (high volatility) for example, a boiling point of less than about ninety (90) degrees Celsius and a relatively low dielectric constant for example, a dielectric constant of less than ten (10) which may further comprise cyclic ethers, linear ethers having a molecular formula of CmH2m+1OCnH2n+1 wherein m and n are independently from 1 to 4, linear carbonates having a molecular formula of CxH2x+1OC(O)OCyH2y+1 wherein x and y are independently from 1 to 4, and mixtures thereof. Examples of cyclic ethers may comprise, for example, 1,3-dioxolane, methyldioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 4-tetrahydropyran. Examples of linear ethers include 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, and 1-methoxy-2-ethyoxyethane. Examples of linear carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate. In accordance with the present invention, the ratio of the first solvent in the solvent mixtures is ranged from 10 percent to 90 percent by volume.
In yet further contemplated aspects of the invention subject matter, said non-aqueous electrolyte solution may be plasticized into a polymer to form gel polymer electrolyte. Suitable polymers are selected from the group of poly(ethylene oxide) (PEO), polyvinylidene fluoride (PVdF), hexafluropropylene-vinylidene fluoride copolymers (PVdF-HFP), poly(methyl methacrylate) (PMMA), polyvinyl acetate (PVAC), Polyvinyl chloride (PVC), and polyacrylonitrile (PAN).
The following examples provide details illustrating advantageous properties and performance of lithium/oxygen batteries using a non-aqueous organic electrolyte in accordance with the present invention. These examples are provided to exemplify and more clearly illustrate aspects and embodiments of the present invention and are in no way intended to be limiting.
Air cathodes were prepared by using an activated carbon as the active material and a polytetrafluoroethylene emulsion (PTFE), solid content from about 61.5 percent. An air cathode with a composition of 98 percent activated carbon and 2 percent polytetrafluoroethylene by weight was fabricated as follows: weighed carbon was wetted with an appropriate amount of 98 percent ethanol, and then a calculated amount of polytetrafluoroethylene emulsion was added and mixed to form a uniform paste, followed by drying in an 80 degrees Celsius (° C.) oven. When most of the solvent was evaporated, the warm paste was rolled into a self-standing thin sheet. The resulting carbon sheet was punched into small discs with a diameter of 7/16 inch (equal to 0.97 cm2) and dried at 110 degrees Celsius (° C.) under vacuum for 8 hours. In embodiments the air cathode had a thickness range of from about 0.56 to about 0.63 mm, an activated carbon loading of from about 18.0 to about 19.0 mg/cm2, and a porosity of from about 2.9 to about 3.2 mL/g (vs. carbon). A series of electrolyte solutions with different formulations was prepared in a dry-room having a dew point of about −90 degrees Celsius (° C.) for performance comparison of lithium/oxygen cells.
Lithium/oxygen testing cells with a cathode-limited design were assembled as follows: in the dry-room, a lithium foil (⅝ inch diameter), a membrane (¾ inch diameter), an air (carbon) cathode made above ( 7/16 inch diameter), and a nickel mesh (⅝ inch diameter) with a nickel tab as the cathode current lead were in sequence laminated into a button cell cap (24 mm diameter), then a silicon rubber disc (having a 22 mm diameter and a 3 mm thickness) with a 7/16 inch diameter hollow as the oxygen window was placed on the top of the nickel mesh to obtain a dry cell stack. To activate the cell stack with electrolyte, 200 uL of liquid electrolyte was added through the air-window, and vacuum was applied to ensure complete wetting. Extra liquid electrolyte was removed by lightly pressing a filter paper on the nickel mesh. The electrolyte-activated cell was clamped in a button cell holder to maintain constant pressure, sealed in a foil laminate pouch, and finally filled with pure oxygen.
Lithium/oxygen cells were discharged at 0.2 mA/cm2 on a cycler with a 1.5 V cutoff voltage.
This application claims priority to and the benefit of U.S. patent application Ser. No. 13/304,784 entitled “Non-aqueous Electrolyte solutions and Lithium/Oxygen Batteries Using the Same”, filed Nov. 28, 2011 in the name of Shengshui Zhang, the inventor herein, which is incorporated herein by reference in its entirety. Attention is directed to commonly owned and assigned U.S. Pat. No. 7,147,967, issued Dec. 12, 2006, entitled “CATHODE FOR METAL-OXYGEN BATTERY”, wherein there is disclosed a cathode material for a metal-oxygen battery such as a lithium-oxygen battery. The material comprises, on a weight basis, a first component which is an oxide or a sulfide of a metal. The first component is capable of intercalating lithium, and is present in an amount of greater than about 20 percent and to about 80 percent of the material. The material includes a second component which comprises carbon. The carbon is an electro active catalyst which is capable of reducing oxygen, and comprises from about 10 to about 80 percent of the material. The material further includes a binder, such as a fluoropolymer binder, which is present in an amount of from about 5 to about 40 weight percent. U.S. Pat. No. 7,147,967, issued Dec. 12, 2006, entitled “FLUOROHALOBORATE SALTS, SYNTHESIS AND USE THEREOF”, wherein there is disclosed a composition as a salt having the formula MBF3X where M is an alkali metal cation and X is the halide fluoride, chloride, bromide or iodide. A lithium salt has several characteristics making the composition well suited for inclusion within a lithium-ion battery. A process for forming an alkali metal trifluorohaloborate salt includes the preparation of a boron trifluoride etherate in an organic solvent. An alkali metal halide salt where the halide is fluoride, chloride, bromide or iodide is suspended in the solution and reacted with boron trifluoride etherate to form an alkali metal trifluorohaloborate. The alkali metal trifluorohaloborate so produced is collected as a solid from the solution. The entire disclosures of each of the above mentioned patents are incorporated herein by reference in their entirety. The appropriate components and processes of these patents may be selected for the electrolyte and processes of the present invention in embodiments thereof.
The invention described herein may be manufactured, used and licensed by or for the United States Government.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13304784 | Nov 2011 | US |
| Child | 14798486 | US |
