Patent Application
20110104576
This invention relates to energy storage devices, notably electrochemical cells and batteries and, more particularly, lithium-oxygen electrochemical cells. The present invention provides electrochemical cells and batteries that include a carbon nanotube-based cathode.
Metal-oxygen (e.g., metal-air) batteries combine a metal anode, similar to that used in conventional primary batteries, and an oxygen (air) gas-diffusion cathode similar to that used in fuel cells. During operation, the metal anode typically based on Zn, Al, Mg, Ca, or Li is electrochemically oxidized for the expense of the oxygen from air, which is reduced on the gas-diffusion cathode. Some properties of metal-oxygen electrochemical cells are presented in Table 1. A classic example of a household zinc-air system is a so-called “hearing aid” cell. Besides that, these power sources occupy several other niches in the energy market.
In the case of lithium-oxygen cells, the possible discharge cell reactions and the associated cell voltages are:
2Li+O2→Li2O2;G0=−145Kcal(E0=3.1V) and
4Li+O2→2Li2O;G0=−268Kcal(E0=2.91V);
based on experimental discharge data from a Li/O2 test cell. It has been reported that the main discharge reaction is the reduction of oxygen to form Li2O2 based on Raman spectroscopic analysis of the products in a non-aqueous cell.
A lithium-oxygen (Li—O2) battery can be rechargeable when the carbon cathode contains catalysts derived from complexes or oxides of metals such as cobalt. The catalyst can be viewed as lowering the over-voltage for the oxidation of Li2O2 or Li2O to form metallic Li and oxygen. The first example of a Li—O2 rechargeable cell utilized non-aqueous conducting gel polymer electrolytes that were used to construct polymer-Li-air cells based on poly(acrylonitrile)(PAN) and poly(vinylidene difluoride) (PVdF). Non-aqueous lithium-air batteries represent a class of potentially ultrahigh energy density power sources useful for a variety of civilian applications. However, such systems generally exhibit a limited number of recharge cycles.
There are a number of factors that can limit the rechargeablity of Li—O2 cells. These include the structural and chemical design of the air-diffusion cathode, the kinetics of the redox reactions, the solubility and rates of dissolution of the reaction products during charging, and the need for high oxygen solubility in the non-aqueous electrolyte.
The cathode in all known types of metal-oxygen cells and batteries is an gas-diffusion electrode, in which oxygen (either pure or diluted in another gas, e.g., air) diffuses through the cathode, generally on a continuous basis. Such gas-diffusion electrodes typically comprise a porous, thin plate or sheet, which serves as a wall for the metal-oxygen cell and separates the electrolyte in the cell from the surrounding oxygen-containing gas (e.g., air). The dual purpose of the porous plate poses some conflicting requirements for cathode design. For example, the cathode must be highly porous and permeable to gaseous oxygen while simultaneously preventing leakage of the electrolyte through the porous cathode. The cathode must be electrically conductive and must possess enough mechanical strength to withstand the hydrostatic pressure of the electrolyte and any hydrodynamic shocks that may eventually occur. If the Li-oxygen cell is to be rechargeable, the porous cathode must contain an active catalyst for electrochemical oxidation of oxide and peroxide ions to form oxygen in contact with the electrolyte. Stable operation of the oxygen (air) cathode with time is also needed. Cathodes for metal-oxygen cells can contain various forms of carbon, for example, in the electrochemically active material, as conductive additive, and as part of the current collector.
Lithium-oxygen (e.g., Li-air) electrochemical cells and batteries have been targeted as the next generation energy storage system, possessing a very high theoretical specific energy that renders such batteries attractive for a number of power source applications including electric vehicles (EVs). Based on the high electropositive voltage of lithium and its low atomic weight, and the ready source of atmospheric oxygen, the theoretical specific energy for a lithium-air cell is 5200 Wh/kg, including oxygen. This is much higher than is achievable with a typical Li-ion battery, which has a specific energy value near 600 Wh/kg. In practice, oxygen is not stored in the battery, but is rather supplied by ambient air. The theoretical specific energy of a lithium-air cell, excluding oxygen, is 11140 Wh/kg. Oxygen from the ambient atmosphere enters the pores of the carbon cathode to serve as the cathode active material. In the discharge of a Li-air battery utilizing a porous carbon electrode, this oxygen is reduced, and the products are stored in the pores of the carbon electrode. As a result, the cell capacity is expressed as ampere-hour per kilogram of the carbon in the cathode. The specific characteristics, such as Ah/kg and AWL of metal-air batteries are significantly higher than that of the classical electrochemical systems with the same metal anode. Theoretical data show that Li and Ca possess very high energy densities of 13172 and 4560 Ah/kg, respectively, but these metals are not suitable to be used as anodes when aqueous electrolytes are utilized. Consequently, non-aqueous electrolytes must be used in such systems. When fully developed, lithium-oxygen batteries could exhibit practical specific energies of 1000-3000 Wh/kg.
In a primary Li—O2 cell, Li metal is electrochemically oxidized at expense of oxygen from air, which is reduced at the cathode, producing either Li2O2 (3.1 V vs. Li+/VLi0) or Li2O (2.91 V vs. Li+/Li0) or a combination of both. In the discharge of the Li-air battery, this oxygen is reduced and the products are stored in the cathode (typically high surface area carbon black). The Li—O2 battery may also be charged in the presence of a high-surface area carbon black and a manganese oxide catalyst such as α-MnO2 (hollandite structure) that is used to assist in the oxidation of the discharge product, Li2O2 and/or Li2O to oxygen. While high values of energy density have been realized for the Li—O2 cell as a primary power source, the utilization of the device as a rechargeable cell have been hindered by many obstacles.
Electrodes that utilize nanostructures for energy storage are increasingly prevalent. An example is synthesized nanostructured carbon-free LiFePO4 olivine, which reportedly has a high-rate for Li-ion battery applications. Nanostructured materials that feature high-surface areas, fast ion diffusion and high electronic conductivity due to their very small particle size have led to higher rates of electrochemical reactions in various applications. The evolution and improvement of Li-air batteries ultimately will involve tailored nanostructured materials that can support the rigors of the air-cathode during operation.
A lithium-oxygen electrochemical cell of the present invention comprises a lithium-containing anode, an oxygen-permeable cathode, a non-aqueous electrolyte comprising a lithium salt in a non-aqueous liquid between the anode and the cathode, and a source of gaseous oxygen in fluid communication with the cathode. The cathode comprises an oxygen-permeable support bearing carbon nanotubes having at least one open end.
A rechargeable lithium-oxygen electrochemical cell of the present invention comprises a lithium-containing anode, an oxygen-permeable cathode in fluid communication with an oxygen source (e.g., ambient air), and a non-aqueous electrolyte comprising a lithium salt in a non-aqueous liquid between the anode and the cathode. The cathode comprises an oxygen-permeable support bearing carbon nanotubes having at least one open end and a nanoparticulate catalyst in contact with the carbon nanotubes. The catalyst is adapted to facilitate the reversible interconversion between oxygen gas and an oxygen anion (e.g., oxide ion, peroxide ion, or both), during charge and discharge of the cell.
Batteries of the invention comprise two or more electrochemical cells connected in series, in parallel, or both.
The invention includes certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in various aspects of the invention, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the described invention.
The present invention provides lithium-oxygen (e.g., lithium-air) electrochemical cells and batteries formed from such cells. A lithium-oxygen electrochemical cell of the present invention comprises a lithium-containing anode, an oxygen-permeable cathode, a non-aqueous electrolyte comprising a lithium salt in a non-aqueous liquid between the anode and the cathode, and a source of gaseous oxygen in fluid communication with the cathode. The cathode comprises an oxygen-permeable support bearing carbon nanotubes having at least one open end.
A rechargeable lithium-oxygen electrochemical cell of the present invention comprises a cathode that also contains a nanoparticulate catalyst in contact with the CNTs, which is adapted to facilitate the reversible interconversion between oxygen gas and an oxygen anion e.g., oxide ion, peroxide ion, or a combination thereof, during charge and discharge of the cell. One preferred catalyst comprises cobalt metal. Another preferred catalyst comprises a cobalt oxide. Yet other preferred catalysts include nanoparticulate manganese dioxide (MnO2), ferric oxide (Fe2O3), WO3, CeO2, MoO2, MoO3, ferric/ferrous oxide (Fe3O4), Li4Ti5O12 (2Li2O.5TiO2), TiO2 or their mixtures with noble metals such as Pt, Au, Ru and the like. The catalyst can be located at the tip of the carbon nanotubes, within the nanotubes, or intermixed with the carbon nanotubes, nanofibers and grapheme sheets.
Suitable carbon nanotubes can be obtained by any method known to produce open-ended carbon nanotubes. One such method is described in U.S. Patent Application Publication No. 2010/0178232, which is incorporated herein by reference in its entirety, and which involves thermal decomposition of low density polyethylene in an autogenic pressure reactor in the presence of a metal salt such as a cobalt salt. This method provides CNTs containing nanoparticulate cobalt. Another method of producing CNTs, e.g., from recycled polymeric materials (e.g., LDPE and HDPE) and a catalyst such as Ni, Co, Fe, and oxides thereof, is described in U.S. 2010/0178232. The carbon nanotubes of the electrochemical cells of the invention typically have a average tube diameter in the range of about 50 to about 100 nm, preferably in the range of about 80 to about 90 nm. Typically, the carbon nanotubes have an average length in the range of about 200 nm to about 7 μm, preferably about 1000 nm to about 10 μm. The carbon nanotubes preferably have an average aspect ratio (length-to-diameter) in the range of about 20 to about 100.
The carbon nanotubes can be intermixed with an inert material, or with another active cathode material, if desired. In some preferred embodiments, the carbon nanotubes are intermixed another porous carbon material, such as carbon black.
The carbon nanotube-based cathode can include a binder to help maintain the cathode in a particular shape or configuration and to adhere the carbon nanotubes and catalyst, when present, to the support. Any binder that is stable to the electrochemical conditions of a lithium-oxygen battery can be utilized in the present invention. Many such materials are known in the art. One preferred binder is polyvinylidene difluoride (PVdF).
The support for the cathode can be any gas-permeable material suitable for use in metal-oxygen cells (e.g., materials that are not chemically reactive with oxygen and with typical electrolyte materials under the electrochemical conditions of cell charge and discharge), many of which are well known in the art. In some preferred embodiments, the support comprises a metal mesh (e.g., a nickel mesh). In other embodiments, the support comprises a non-woven fibrous material, such as a glass fiber mat or paper. The carbon nanotubes and optional catalyst can be applied to the support in any way that will provide a thin, even coating on the support (e.g., by spraying, dipping, painting, etc.). For example, the carbon nanotubes, catalyst, and a binder can be suspended in a volatile liquid (e.g., water, an organic solvent, and the like) and then painted onto the surface of the support. The liquid is then evaporated to leave a coating of carbon nanotubes and catalyst on the support surface.
The anode of the electrochemical cell can be any lithium-containing material suitable for use in lithium-oxygen cells and batteries. Some exemplary materials include metallic lithium, and lithium oxide containing materials that are known to be useful as anodes in lithium-oxygen or lithium ion electrochemical cells.
The electrochemical cells can include any non-aqueous electrolyte suitable for use in lithium-oxygen (e.g., lithium-air) cells and batteries, many of which are well known in the art. Typically, these non-aqueous electrolytes comprise an inert polar organic liquid, such as an organic carbonate (e.g., dimethyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, and the like), or a polymer-based gel. The liquid or gel contains a lithium salt (e.g., LiPF6, LiB(C2O4)2, LiBF2C2O4, LiBF4, LiPF2(C2O4)2, LiPF4C2O4, and the like) that is soluble in the liquid or gel. The following examples describe the principles of the invention as contemplated by the inventors, but they are not to be construed as limiting examples.
Carbon nanotubes containing a nanoparticulate cobalt catalyst (Co-CNTs) were prepared via thermal decomposition of low density polyethylene (LDPE) containing about 20 percent by weight (wt %) cobalt acetate (CoAc; Co(C2H4O2)2) catalyst in a sealed autoclave (made up of Haynes 230 alloy) under a nitrogen atmosphere. The autoclave was heated at about 700° C. to about 750° C. for about 2 to 3 hours followed by gradual cooling. The mixture of LDPE and catalyst in the autoclave generated about 50 pounds-per-square inch (psi) of pressure upon heating up to about 680° C., which increased to about 1000 psi at about 700° C. A chemical reaction took place under the autogenic pressure generated in the autoclave during the thermolysis of LDPE in the presence of CoAc, leading to the growth of Co-CNTs in about 40% yield.
Panel (a) of
Although the SEMs taken using secondary electrons demonstrate the one-dimensional fiber-like nanotubes, a transmission electron micrograph (TEM) further confirmed the hollow tubular structures of the CNTs, as shown in
The coated cathode described in Example 1 was evaluated in a lithium-oxygen electrochemical cell with a lithium metal foil as the anode, and an electrolyte consisting of 1.2M LiPF6 in a 3:7 (w/w) mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC), between the anode and the cathode. The cells were assembled in helium-filled glove box. After bringing the lithium-air cell out of the glove box, it was purged with oxygen, and then filled with O2 at about 20 psi pressures.
A charge-discharge cycling evaluation was also carried out on cells of the same construction described in Example 2. The cell was galvanostatically discharged and charged between 1.5 V and 4.7 V, respectively. The observed current density was about 75 mA/g at a C/4 rate, as shown in
Another cathode was prepared on a nickel mesh, with a coating containing about 15 wt % electrolytic MnO2 catalyst, about 35.5 wt % Co-CNTs comprising 4.5 wt % Co nanoparticles, about 30 wt % carbon black, and about 15 wt % PVdF binder. The CNTs preparation and dimensions are discussed in Example 1. The cathode was evaluated in a cell of the same design as described in Example 1, as well.
Another cathode was prepared comprising a nickel mesh coated with about 24 wt % of CNTs (purchased from Stream Chemicals), about 33 wt % KYNAR® 2801 vinyl fluoride resin binder, and about 42 wt % of Li4Ti5O12 catalyst (40 nm), with a propylene carbonate (PC) plasticizer. The CNTs had a diameter of about 50 nm and a length of several micrometers.
A cathode comprising a coating of 50 wt % CNTs (purchased from Stream Chemicals) and 50 wt % KYNAR® 2801 vinyl fluoride resin binder coated on glass fiber filter paper was evaluated in a cell of similar design to that described in Example 2, with a lithium foil anode and an electrolyte comprising 1M LiPF6 in a PC (pressurized with 20 psi of oxygen). The CNTs had a diameter of about 50 nm and a length of several micrometers.
A cathode was composed of about 33 wt % CNTs, about 33 wt % Fe3O4 nanoparticles, and about 33 wt % KYNAR® 2801 vinyl fluoride resin binder, coated on a glass fiber filter paper was prepared as described above. Panel (a) of
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
This application claims the benefit of U.S. Provisional Application Ser. No. 61/360,027, filed on Jun. 30, 2010, and of U.S. Provisional Application Serial No. 61/280,025, filed on Oct. 29, 2009, each of which is incorporated herein by reference in its entirety.
The United States Government has rights in this invention pursuant to Contract No. DE-ACO2-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.
| Number | Date | Country | |
|---|---|---|---|
| 61360027 | Jun 2010 | US | |
| 61280025 | Oct 2009 | US |
