Patent Application
20070218364
Over the last few decades revolutionary advances have been made in electrochemical storage and conversion devices expanding the capabilities of these systems in a variety of fields including portable electronic devices, air and space craft technologies, and biomedical instrumentation. Current state of the art electrochemical storage and conversion devices have designs and performance attributes that are specifically engineered to provide compatibility with a diverse range of application requirements and operating environments. For example, advanced electrochemical storage systems have been developed spanning the range from high energy density batteries exhibiting very low self discharge rates and high discharge reliability for implanted medical devices to inexpensive, light weight rechargeable batteries providing long runtimes for a wide range of portable electronic devices to high capacity batteries for military and aerospace applications capable of providing extremely high discharge rates over short time periods.
Despite the development and widespread adoption of this diverse suite of advanced electrochemical storage and conversion systems, significant pressure continues to stimulate research to expand the functionality of these systems, thereby enabling an even wider range of device applications. Large growth in the demand for high power portable electronic products, for example, has created enormous interest in developing safe, light weight primary and secondary batteries providing higher energy densities. In addition, the demand for miniaturization in the field of consumer electronics and instrumentation continues to stimulate research into novel design and material strategies for reducing the sizes, masses and form factors of high performance batteries. Further, continued development in the fields of electric vehicles and aerospace engineering has also created a need for mechanically robust, high reliability batteries capable of good device performance in a useful range of operating environments (e.g., terrestrial, lower, mid and upper atmosphere, and space).
Many recent advances in electrochemical storage and conversion technology are directly attributable to discovery and integration of new materials for battery components. Lithium battery technology, for example, continues to rapidly develop, at least in part, due to the discovery of novel electrode and electrolyte materials for these systems. From the pioneering discovery and optimization of intercalation cathode materials, such as fluorinated carbon materials and nanostructured transition metal oxides, to the development of high performance non-aqueous electrolytes, the implementation of novel materials strategies for lithium battery systems have revolutionized their design and performance capabilities. As a result of these advances, lithium battery technology is positioned to be the preferred technology for the next generation of high-power, portable electronic systems. Accordingly, the identification and performance evaluation of novel electrode and/or electrolyte materials is currently a research priority in the development of new and improved lithium based electrochemical energy storage and conversion systems.
Commercial primary lithium battery systems typically utilize a lithium metal negative electrode for generating lithium ions which during discharge are transported through a liquid phase or solid phase electrolyte and undergo intercalation reaction at a positive electrode comprising an intercalation host material. The element lithium has a unique combination of properties that make it highly attractive for use in an electrochemical cell. First, it is the lightest metal in the periodic table having an atomic mass of 6.94 AMU. Second, lithium has a very high electrochemical reduction potential (i.e., 3.045 V). This unique combination of properties enables lithium based electrochemical cells to have very high specific capacities. Essential to the development and commercial implementation of lithium batteries, however, has been the development of high performance intercalation cathode materials, and nonaqueous electrolytes capable of providing useful ionic conductivities. Useful intercalation cathode materials for lithium batteries include layered transition metal oxides (e.g., MnO2, NiO2, CoO2), spinel transition metal oxides (e.g., Mn2O2, Ti2O2), transition metal sulfides (e.g. FeS2) and fluorinated carbon materials (e.g., CF1). Useful nonaqueous electrolytes for lithium batteries include solutions of lithium salts in polar organic or inorganic liquids, ionically conducting polymers and fused lithium salts.
Advances in materials strategies and electrochemical cell designs for lithium battery technology have realized electrochemical cells capable of providing useful device performance including: (i) high cell voltages (e.g. up to about 3 V), (ii) substantially constant (e.g., flat) discharge profiles, (iii) long shelf-life (e.g., up to 10 years), and (iv) compatibility with a range of operating temperatures (e.g., −20 to 60 degrees Celsius). As a result of these beneficial characteristics, primary lithium ion batteries are widely used as power sources in a range of portable electronic devices and in other important device applications including, biomedical engineering, sensing, military communications, and lighting. Dual intercalation lithium ion secondary batteries have also been developed. In these systems, lithium metal is replaced with an intercalation host material for the negative electrode, such as carbons (e.g., graphite, cokes etc.), metal oxides, metal nitrides and metal phosphides. State of the art lithium ion secondary batteries provide excellent charge-discharge characteristics, and thus, have also been widely adopted as power sources in portable electronic devices, such as cellular telephones and portable computers.
Given rapid advances and implementation of lithium based battery technology, these systems are currently identified as an important class of power sources for a range of aerospace applications. Target applications include electrochemical cells for satellites, spacecraft, aircraft, and defense technologies. Successful lithium battery implementation in these applications requires a combination of very high performance attributes, including high specific capacities at substantial discharge rates, good reliability and safety, and long operating lifetime. While many of these performance attributes have been demonstrated at room temperature, it is currently unclear if state of the art lithium based electrochemical systems are capable of meeting stringent performance requirements under the extreme operating conditions inherent to many of these applications, including low temperature (e.g., −30 degrees Celsius) environments.
The existing off-the-shelf suite of lithium batteries capable of providing energy at reasonable rates at these temperatures, including Li—SO2Cl2, Li—MnO2, and Li—SO2, have lingering safety issues and/or low temperature energy density values that hinder their implementation in applications requiring low temperature performance. Accordingly, there is currently a need for safe, high energy density primary batteries that can function at low temperatures (e.g., −30° C. and below) for a host of important aerospace applications.
Commercial Li/CFx batteries using active cathode materials consisting of fluorinated carbonaceous materials, such as coke or graphite comprise a potentially attractive system for low temperature energy source applications. Many of these systems utilize nonaqueous electrolyte solutions having low freezing points that are compatible with low temperature operation. Further, Li/CFx battery systems are know to be capable of delivery of up to 700 Wh/kg, 1000 Wh/l, at room temperature, and at a rate of C/100 (i.e., a battery current of a 1/100th that of the capacity of the battery per hour). (See, e.g., Bruce, G. Development of a CFx D Cell for Man Portable Applications. in Joint Service Power Expo. 2005; and Gabano, J. P., ed. Lithium Batteries, by M. Fukuda & T. lijima. 1983, Academic Press: New York). Cathodes in these systems typically have carbon—fluoride stoichiometries typically ranging from CF1.05 to CF1.1. This cathode material, however, is known to be discharge rate limited, and currents lower than C/50 (battery current 1/50th that of the capacity of the battery divided by 1 hour) are often necessary to avoid cell polarization and large capacity loss. Low electronic conductivity of CFx is a potential cause of the observed discharge rate limitations, as there is a strong correlation between cathode thickness and performance; thicker cathodes tend to be more rate-limited. (See, e.g., Gunther, R. G. in Fifth Power Sources Conference. Pages 713-728, 1975).
To address discharge rate limitations, a number of high performance Li—CFx batteries have been designed using thin cathode structures in a spirally-wound configuration. This development has provided some promising results both at room temperature and low temperature operating conditions. For example, a 15.5 Ah fully packaged D cell has been reported that delivers over 460 Wh/kg at better than a C/8 rate at room temperature, and that delivers about 12 Ah under a 2 A discharge, with an active cathode materials utilization of over 570 mAh/g at −20° F. (−29° C.). (See, e.g., Bruce, G. Development of a CFx D Cell for Man Portable Applications. in Joint Service Power Expo. 2005.). A similar CFx spirally-wound electrode design exhibited a bulk cathode impedance of 0.0002 ohms/mm2 and was able to deliver over 500 mAh/g active cathode material at a C/20 rate at −30° C. (See, e.g., T. Tan, P. Lam, and H. Yumoto. Li/(CF)n Battery for Low Temperature and High Power Application. in 208th Electrochemical Society Meeting. 2005. Los Angeles, Calif.) Substantial self heating under some discharge conditions has also been reported for the Li—CFx positive electrode system, which could also enhance low temperature operation of these electrochemical cells. (See, e.g., Rohde, D., M. J. Root, and D. Fostor. Li/CFx Cell and Material Development for High Rate Applications. in 37th Power Sources Conference. 1996
As will be clear from the foregoing, there exists a need in the art for electrochemical storage and conversion devices capable of satisfying the demanding device performance requirements inherent to many important aerospace applications. Specifically, electrochemical cells are needed that are capable of providing useful specific capacities under large discharge rate conditions and at low temperatures (e.g., less than about −30 degrees Celsius). A need exists for new electrode and electrolyte materials for primary and secondary lithium batteries that enhance system performance at low temperature.
The present invention provides electrochemical cells providing good electronic performance, particularly at low temperatures. Electrochemical cells of the present invention include lithium batteries with useful specific capacities under significant discharge rates for a wide range of temperatures, including temperatures as low as about −60 degrees Celsius. The present invention also provides methods for making electrochemical cells including a room temperature predischarge step that enhances the performance of batteries having subfluorinated carbonaceous positive electrode active materials at low temperatures.
Electrochemical cells of the present invention combine specially selected electrode and electrolyte materials, compositions and form factors to provide higher specific capacities than conventional state of the art electrochemical cells for substantial discharge rates (e.g., greater than or equal to about C/20) over a wide range of temperatures, including temperatures less than or equal to about −20 degrees Celsius. Electrochemical cells of the present invention include lithium batteries having a positive electrode comprising a multiphase, subfluorinated carbonaceous active material that are capable of providing high specific capacities (e.g., specific capacity greater than or equal to about 625 mAh g−1) for high current density discharge conditions over a wide range of temperatures. The present invention also provides electrolyte compositions for lithium batteries providing high ionic conductivities, good chemical stability and useful positive electrode wetting an conditioning characteristics during low temperature operation.
In an aspect, the present invention provides novel materials strategies for enhancing the performance of lithium batteries. For example, the present invention provides complementary subfluorinated carbonaceous positive electrode active materials and electrolyte compositions that provide synergistic performance enhancements important for improving the electronic performance of lithium batteries at low temperatures and/or for accessing advantageous electrode form factors, including thicker positive electrode configurations. In an embodiment, for example, the present invention provides subfluorinated carbonaceous positive electrode active materials comprising nanoscale intermixed fluorinated and unfluorinated domains providing enhanced cathode performance at low temperatures compared to conventional CF1 positive electrode active materials. The present invention also provides electrolyte compositions for lithium ion batteriess, including anion receptor additives, lithium salt concentrations, and nonaqueous solvent compositions, providing physical and chemical properties that extend the range of useful operating temperatures of electrochemical cells having subfluorinated carbonaceous positive electrode active materials. The combination of subfluorinated positive electrode active materials and electrolyte compositions of the present invention enhance the kinetics and charge transfer properties of the positive electrode and the ion conductivity of the electrolyte, thereby enabling lithium batteries exhibiting superior low temperature performance compared to state of the art lithium battery systems and enabling electrochemical cells having advantageous positive electrode form factors and configurations.
In an aspect, the present invention provides an electrochemical cell capable of low temperature operation having a positive electrode comprising a multiphase subfluorinated carbonaceous active material. In an embodiment, an electrochemical cell of this aspect comprises a positive electrode comprising a subfluorinated carbonaceous material having an average stoichiometry CFx, wherein x is the average atomic ratio of fluorine atoms to carbon atoms and is selected from the range of about 0.3 to about 1.0. The subfluorinated carbonaceous material comprising the positive electrode of this aspect of the present invention is a multiphase material having an unfluorinated carbon component and at least one fluorinated carbon component. An electrochemical cell of this embodiment further comprises a negative electrode; and a nonaqueous electrolyte that is provided between the positive and negative electrodes. Useful negative electrodes in electrochemical cells of this aspect of the present invention comprise a source of metal cations, such as lithium ions. Nonaqueous electrolytes are preferably provided such that ions in the electrolyte can efficiently interact with positive and negative electrodes of the electrochemical cell. Optionally, the electrolyte of this aspect comprises a high performance low temperature electrolyte having a composition selected to provide ionic conductivities and reaction kinetics at the subfluorinated carbonaceous positive electrode for enhanced low temperature performance.
As used herein, the expression “subfluorinated carbonaceous material” refers to a multiphase carbonaceous material having an unfluorinated carbonaceous component. As used herein an “unfluorinated carbonaceous component” includes unfluorinated carbon compositions and/or phases, such as graphite, coke, multiwalled carbon nanotubes, carbon nanofibers, carbon nanowhiskers, multi-layered carbon nanoparticles, carbon nanowhiskers, and carbon nanorods, and also includes slightly fluorinated carbon compositions and/or phases. Slightly fluorinated, in this context, refers to carbon that is weakly bound to fluorine, as opposed to compositions wherein carbon is covalently bonded to fluorine, as in CF1 and C2F phases. Multiphase subfluorinated carbonaceous materials may comprises a mixture of carbonaceous phases including, one or more unfluorinated carbonaceous phases, and one or more fluorinated phase (e.g., poly(carbon monoflouride (CF1); poly(dicarbon monofluoride) etc.). Subfluorinated carbonaceous materials include carbonaceous materials exposed to a fluorine source under conditions resulting in incomplete or partial fluorination of a carbonaceous starting material. Subfluorinated carbonaceous materials useful in the present invention and related methods of making subfluorinated carbonaceous materials are described in U.S. patent application Ser. Nos. 11/253,360, 11/422,564 and 11/560,570 filed Oct. 18, 2005, Jun. 6, 2006, and Nov. 16 , 2006, respectively, which are hereby incorporated by reference in their entirety to the extent not inconsistent with the present description.
Selection of the fluorine atom to carbon atom stoichiometry of the subfluorinated carbonaceous positive electrode active material in this aspect of the invention determines, at least in part, the specific capacity and discharge rate characteristics of the electrochemical cell, wherein a larger extent of fluorination (e.g. a larger value of x) provides for larger specific capacities. In some embodiments, for example, selection of a large average atomic ratio (x) of fluorine atoms to carbon atoms, for example x equal to about 0.5 to about 0.9, is useful for providing electrochemical cells capable of exhibiting specific capacities greater than about 500 mAh g−1 for discharge at about 2 V, and in some embodiments greater than about 625 mAh g−1 for discharge at about 2.1 V. The fluorine atom to carbon atom stoichiometry also, at least in part, determines the extent of the unfluorinated component of the subfluorinated carbonaceous positive electrode active material. In some embodiments, the unfluorinated carbon component is between about 5% to about 70% by mass of the subfluorinated carbonaceous material comprising the positive electrode active material, and preferably for some embodiments the unfluorinated carbon component is between about 10% to about 20% by mass of the subfluorinated carbonaceous material comprising the positive electrode active material. Use of subfluorinated carbonaceous positive electrode active materials having an unfluorinated carbon component of at least about 5% by mass is beneficial for providing electrodes having useful conductivities in some embodiments.
A range of carbonaceous materials are useful for subfluorinated active materials in positive electrodes of the present invention including graphite, coke, and carbonaceous nanomaterials, such as multiwalled carbon nanotubes, carbon nanofibers, multi-layered carbon nanoparticles, carbon nanowhiskers and carbon nanorods. In an embodiment, the subfluorinated carbonaceous material comprises a plurality of nanostructured particles; wherein each of the nanostructured particles comprise a plurality of fluorinated domains and a plurality of unfluorinated domains. In the context of this description a “domain” is a structural component of a material having a characteristic composition (e.g., unfluorinated or fluorinated), phase (e.g., amorphous, crystalline, C2F, CF1, graphite, coke, carbon fiber, carbon nanomaterials such as multiwalled carbon nanotube, carbon whisker, carbon fiber etc.), and/or morphology. Useful subfluorinated carbonaceous materials for positive electrode active materials comprise a plurality of different domains. Individual fluorinated and unfluorinated domains preferably for some applications have at least one physical dimension (e.g., lengths, depths, cross sectional dimensions etc.) less than about 50 nanometers, and more preferably for some applications at least one physical dimension less than about 10 nanometers. Positive electrode active materials particularly useful for electrochemical cells providing high performance at low temperatures include nanostructured particles having fluorinated domains and unfluorinated domains that are distributed throughout each nanostructured particle of the active material, and in some embodiments substantially uniformly distributed throughout each nanostructured particle of the active material. In some embodiments, fluorinated domains of particles of the positive electrode active material comprise a subfluorinated carbonaceous material having an average stoichiometry CFy, wherein y is the average atomic ratio of fluorine atoms to carbon atoms and is selected from the range of about 0.8 to about 0.9, and the unfluorinated domains of the particles of the positive electrode active material comprise a unfluorinated carbonaceous phase, such as graphite, coke, multiwalled carbon nanotubes, multi-layered carbon nanofibers, multi-layered carbon nanoparticles, carbon nanowhiskers and carbon nanorods.
Use of nanostructured subfluorinated carbonaceous particles as positive electrode active materials provides a number of benefits in electrochemical cells of the present invention. First, nanoscale intermixing of fluorinated and unfluorinated domains in these particles results in high interfacial surface areas between the fluorinated and unfluorinated domains. This attribute provides good electronic and interface properties between these domains facilitating electron transfer, particularly at low temperatures. In addition, the presence of an appreciable unfluorinated component in these particles enhances net electrode conductivity by providing nanoscale electrically conducting pathways in the electrode active material. Further, the distribution of nanoscale fluorinated domains in these materials provides an appreciable density of fluorinated domains having high fluorine ion loading exposed to the electrolyte, thereby resulting in a high interfacial surface area of fluorinated structural domains exposed to the electrolyte further enhancing the kinetics at the positive electrode.
Positive electrodes of the present electrochemical cell may further comprises a conductive diluent, such as acetylene black, carbon black, powdered graphite, coke, carbon fiber, and metallic powder, and/or may further comprises a binder, such polymer binder. Useful binders for positive electrodes in some embodiments comprise a fluoropolymer such as polyvinylidene fluoride (PVDF). Positive electrodes of the present invention may be provided in a range of useful configurations and form factors as known in the art of electrochemistry and battery science, including thin electrode designs, such as thin film electrode configurations. In an embodiment, the positive electrode comprises a thin composite film comprising a subfluorinated carbonaceous active material provided on metallic current collector, wherein the thickness of the thin composite film is selected from the range of about 20 microns to 120 microns. Electrode active materials and electrolytes of the present invention enable lithium batteries capable of providing useful discharge rates with electrode form factors having thicknesses greater than or equal to about 90 microns. In an embodiment, a positive electrode comprising a subfluorinated carbonaceous active material is provided in a spirally-wound electrode configuration.
Negative electrodes of the present invention comprise a source of ions of a metal selected from Groups 1, 2, and 3 of the Periodic Table of Elements. In embodiments of the present invention comprising a low temperature lithium battery the negative electrode comprises a source of lithium ions, such as lithium metal, a lithium alloy, a carbon-lithium material and lithium metal oxide intercalation compounds. Useful carbon-lithium materials include carbonaceous materials, such as graphite or coke, intercalated with lithium ions.
In another aspect, electrochemical cells of the present invention have a nonaqueous electrolyte comprising a high performance low temperature electrolyte. In the context of this description, the expression “high performance low temperature electrolyte” refers to electrolyte compositions having a low freezing point and one or more of the following properties at temperatures less than about −20 degrees Celsius: (i) ionic conductivity of greater than or equal to 0.0001 S cm−1, greater than or equal to 0.001 S cm−1, or greater than or equal to 0.005 S cm−1; (ii) good positive electrode conditioning and wetting properties for subfluorinated carbonaceous electrode active materials; and (iii) high chemical and electrochemical stability, particularly with respect to decomposition on positive and negative electrode surfaces. The present invention provides a range of high performance low temperature electrolytes having physical and chemical properties that enhance the low temperature performance of electrochemical cells having subfluorinated carbonaceous positive electrode active materials.
In an embodiment, a high performance low temperature nonaqueous electrolyte comprises a solution of a lithium salt and a solvent. Useful lithium salts for this aspect of the present invention include, but are not limited to, LiBF4, LiF, LiClO4, LiAsF6, LiSbF6 and LiPF6. The concentration of lithium salt in the nonaqueous electrolyte solution is an important parameter which is selected in some embodiments to enhance low temperature performance. In an embodiment, for example, the lithium salt, such as LiBF4, has a concentration in the nonaqueous electrolyte solution that is preferably less than 1.0 M for some applications, and more preferably less than 0.5 M for some applications. Useful lithium salt concentrations for some electrochemical cells of the present invention are selected from the range of about 0.75 M to about 0.25 M, for example when LiBF4 is the selected lithium salt.
Solvent composition is also selected in embodiments of the present invention to enhance low temperature electrochemical cell performance. Solvents useful in nonaqueous electrolytes of the present invention include, but are not limited to, propylene carbonate, 1,2-dimethoxy ethane, trifluoroethyl ether, diethyl ether, diethoxyethane, 1-3 dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, α-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, gamma-butyrolactone, diethoxyethane, acetonitrile, and methylacetate. Nonaqueous electrolytes of the present invention also include fluorine analogs of the solvents provided above. In the context of this description fluorine analogs include: (i) fully fluorinated analogs wherein each hydrogen atom of the solvent molecule is replaced by a fluorine atom, and (ii) partially fluorinated analogs wherein at least one hydrogen atom of the solvent molecule is replaced by a fluorine atom. Use of solvents containing fluorine analogs is preferred in some embodiments as these materials comprise a fluorous phase that can associatively interact with the positive electrode given the inherent fluorophilicity of subfluorinated and fully fluorinated carbonaceous electrode active materials. For example, fluorophilic subflourinated CFx active material undergo enhance wetting and conditioning at the positive electrode, thereby enabling high discharge rate performance, particularly at low temperatures.
Use of a solvent comprising a mixture of mixture of propylene carbonate (or alternatively ethylene carbonate) and 1,2-dimethoxy ethane provides a nonaqueous electrolyte solution exhibiting useful physical and chemical properties in the present invention. The relative proportions of propylene carbonate and 1,2-dimethoxy ethane in these solvent blends is selected to provide sufficient propylene carbonate to achieve a high dielectric constant so as to prevent significant self discharge, thereby enhancing battery stability. In some embodiments sufficient 1,2-dimethoxy ethane is provided in the electrolyte to achieve low viscosity and good coordinating properties with the lithium salt so as to provide useful ionic conductivities. In addition, propylene carbonate should be provided in sufficient proportion so as to impart effective passivation of the negative electrode. In an embodiment, for example, the solvent has a volume to volume ratio of propylene carbonate to 1,2-dimethoxy ethane is selected from the range of about 0.25 to about 1. In an embodiment, for example, the solvent has a volume to volume ratio of propylene carbonate to 1,2-dimethoxy ethane that is less than about 0.50.
Another strategy for enhancing the low temperature performance of nonaqueous electrolytes in electrochemical cells of the present invention includes use of solvents containing an ether having at least one fluroalkyl group, such as trifluoroethyl ether (TEE). Ethers having fluoroalkyl groups impart beneficial physical properties to electrolyte solutions of the present invention due to the low melting points, low viscosity and good solvating properties of these compounds. Further, addition of hydrofluorocarbon ethers to the electrolyte solution enhances the wetting of active electrode materials comprising subfluorinated carbonaceous materials. In an embodiment, the invention provides a nonaqueous electrolyte solution comprising an ether having at least one fluroalkyl group, such as trifluoroethyl ether, having a percent by volume of the solvent selected from the range of about 10% to about 40%. As described above, use of electrolyte materials comprising a fluorous phase (i.e., ethers having one or more fluoroalkyl groups) enhances wetting and/or conditioning of fluorophilic electrode active materials, such as subfluorinated carbonaceous materials.
In another aspect, the present invention provides high performance nonaqueous electrolyte solutions comprising an anion receptor. High performance nonaqueous electrolytes of the present invention are particularly beneficial for enhancing low temperature performance of lithium batteries having a fluorinated carbonaceous positive electrode active material, such as a subfluorinated carbonaceous material or fully fluorinate carbonaceous material. Incorporation of anion receptors that do not use hydrogen bonding to tie up the anion, such as a Lewis acid anion receptor, increases the solubility of the lithium salt so as to enhance the net ionic conductivity of the electrolyte solutions of the present invention, particularly at low temperatures. Incorporation of anion receptor also improves performance of the present high performance low temperature electrolytes by conditioning the positive electrode active material. In an embodiment, for example, anion receptors of the present invention are capable of dissolving discharge reactants that can buildup on the positive electrode surface upon discharge of the electrochemical cell. Dissolution of such discharge reactants is beneficial as buildup can reduce kinetic performance at the electrode, particularly at the positive electrode. Dissolution of discharge reactants by anion receptors in nonaqueous electroytes of electrochemical cells of the present invention enables the practical thickness of the active material positive electrode to be increased while still preserving useful discharge rate performance. (e.g., high discharge rate capability, no significant voltage delay or severe polarization effects).
In an embodiment, a high performance nonaqueous electrolyte of the present invention further comprises an anion receptor capable of coordinating fluoride ions. Use of a fluoride ion anion receptor in these aspects of the present invention is useful for reducing or eliminating unwanted LiF buildup on the positive electrode interface. During discharge of a lithium battery, LiF commonly precipitates on the surface of carbonaceous positive electrode active materials. LiF is virtually insoluble in many nonaqueous electrolytes and is also electrochemically inactive. Therefore, build up on an electrode surface impedes ionic and/or electronic conductivity at the positive electrode. Further, the equilibrium constant (Ksp) for the LiF precipitation reaction is small;
Li+soln+F−solnLiFppt;
thereby strongly favoring the formation of the solid precipitate LiFppt at the surface of the positive electrode. Moreover, LiFppt grain growth via Ostwald ripening is exceeding slow resulting in tiny grains that do not grow effectively, but rather agglomerate, thereby resulting in a dense fine coating at the cathode capable of degrading electron and ion conductivity.
Addition of a fluoride ion anion receptor in the electrolytes of the present invention is useful for initiating the dissolution of LiFppt via a fluoride ion complex reactions. By removing the buildup of LiFppt during discharge, the high performance low temperature electrolytes of the present invention enhance electronic conductivity by improving electrolyte access to the carbonaceous positive electrode active material. Furthermore, after LiF saturates in the nonaqueous electrolyte solution, grain growth of the LiFppt is much more favored, resulting in less conformal coating of the positive electrode. A beneficial result provided by the addition of fluoride ion anion receptors of the present invention is to modify the composition and/or morphology of the solid electrolyte interface (SEI) film so as to decrease interfacial resistance at the subfluorinated carbonaceous positive electrode.
Useful anion receptors in the present high performance nonaqueous electrolyte solutions include fluorinated boron-based anion receptors, such as boranes, boronates and borates having electron withdrawing ligands. Useful anion receptors in the electrochemical cells of the present invention include, but are not limited to, fluorinated borate-based compounds, phenyl boron-based compounds and aza-ether boron-based compounds. Anion receptor-containing nonaqueous electrolyte solutions of some aspects of the present invention have a concentration of anion receptor less than about 2 M, preferably for some embodiments a concentration of anion receptor less than about 1 M, and more preferably for some embodiments a concentration of anion receptor less than about 0.5 M.
In some embodiments, anion receptors of the present invention are capable of facilitating dissolution of low solubility lithium salts, such as LiF, in a non aqueous solvent. Anion receptors of the present invention are capable of increasing the specific capacity of subfluorinated carbonaceous positive electrodes. Anion receptors of the present invention are capable of increasing the conductivity of nonaqueous electrolyte solutions of the present invention, for example, to a value greater than or equal to about 0.001 S cm−1 for some embodiments or, a value greater than or equal to about 0.01 S cm−1 for some embodiments.
A high performance low temperature nonaqueous electrolyte solution of the present invention further comprises an anion receptor having the chemical structure AR1:
wherein R1, R2 and R3 are independently selected from the group consisting of alkyl, aromatic, ether, thioether, heterocyclic, aryl or heteroaryl groups which are optionally substituted with one or more halogens, including F, alkyl, alkoxide, thiol, thioalkoxide, aromatic, ether or thioether.
In a specific embodiment, for example, the present invention provides a high performance low temperature nonaqueous electrolyte solution comprising a borate-based anion receptor compound having the chemical structure AR2:
wherein R4, R5 and R6 are selected from the group consisting of alkyl, aromatic, heterocyclic, aryl or heteroaryl groups which are optionally substituted with one or more halogens, including F, alkyl, alkoxide, thiol, thioalkoxide, aromatic, ether or thioether. In an embodiment R4, R5 and R6 are identical. In an embodiment, each of R4, R5 and R6 are F-bearing moieties.
In a specific embodiment, for example, the present invention provides a high performance low temperature nonaqueous electrolyte solution comprising a phenyl boron-based anion receptor compound having the chemical structure AR3:
wherein R7 and R8 are selected from the group consisting of alkyl, aromatic, heterocyclic, aryl or heteroaryl groups which are optionally substituted with one or more halogens, including F, alkyl, alkoxide, thiol, thioalkoxide, aromatic, ether or thioether. In an embodiment R7 and R8 are identical. In an embodiment, each of R7 and R8 are F-bearing moieties. In an embodiment, R7 and R8 together from an aromatic, including a phenyl that is optionally substituted, including substituents that are F and substituents that are themselves F-bearing moieties, as shown by chemical formula AR4:
wherein XA and XB represent one or more hydrogens or non-hydrogen ring substituents independently selected from the group consisting of halogens, including F, alkyl, alkoxide, thiol, thioalkoxide, ether, thioether. In an embodiment, at least one of the substituents is a F-bearing moiety.
A high performance low temperature nonaqueous electrolyte solution of the present invention further comprises a Tris (hexafluoroisopropyl) borate (THFIB; MW=511.9 AMU) anion receptor having the chemical structure AR5:
or a Tris (2,2,2-trifluoroethyl) borate (TTFEB; MW=307.9 AMU) anion receptor having the chemical structure AR6:
or a Tris (pentafluorophenyl) borate (TPFPB; MW=511.98 AMU) anion receptor having the chemical structure AR7:
or a Bis (1,1,3,3,3-hexafluoroisopropyl) pentafluorophenyl boronate (BHFIPFPB; MW−480.8 AMU) anion receptor having the structure AR8:
Anion receptors useful in electrolytes of present invention include those having the formula selected from the group consisting of: (CH3O)3B, (CF3CH2O)3B, (C3F7CH2O)3B, [(CF3)2CHO]3B, [(CF3)2C(C6H5)O]3B, ((CF3)CO)3B, (C6H5O)3B, (FC6H4O)3B, (F2C6H3O)3B, (F4C6HO)3B, (C6F5O)3B, (CF3C6H4O)3B, [(CF3)2C6H3O]3B and (C6F5)3B.
The present invention provides lithium batteries capable of enhanced operation, particularly at low temperatures. A primary lithium battery of the present invention is capable of providing discharge rates equal to or greater than C/5 at temperatures lower than or equal to −40 degrees Celsius; wherein C is the capacity of the electrochemical cell. A primary lithium battery of the present invention is capable of providing a specific capacity equal to or greater than 500 mAh g−1 at a discharge rate of C/5 and with a discharge voltage greater than or equal to about 2.5 V at a temperature equal to −40 degrees Celsius. A primary lithium battery of the present invention is capable of providing a specific capacity of 625 mAh/g for discharge at C/40 at −40 C at 2.3V. A primary lithium battery of the present invention is capable of providing an energy density greater than or equal to about 1700 Wh kg−1, at a discharge rate equal to or greater than about C/40 and at −40 degrees Celsius.
In another aspect, the present invention provides a method of generating an electrical current at a temperature less than or equal to −40 degrees Celsius comprising the steps of: (i) providing an electrochemical cell comprising: a positive electrode comprising a subfluorinated carbonaceous material having an average stoichiometry CFx, wherein x is the average atomic ratio of fluorine atoms to carbon atoms and is selected from the range of about 0.3 to about 1.0; the subfluorinated carbonaceous material being a multiphase material having an unfluorinated carbon component; a negative electrode; and a nonaqueous electrolyte provided between the positive and negative electrodes; and (ii) discharging the electrochemical cell at a temperature less than or equal to −40 degrees Celsius. Optionally, the method of this aspect of the invention further comprising the step of discharging a portion of the capacity of the electrochemical cell at a temperature equal to about 20 to about 30 degrees Celsius prior to the step of discharging the electrochemical cell at a temperature less than or equal to −40 degrees Celsius. In an embodiment, for example, between about 0.5% to about 10% of the capacity of the electrochemical cell is discharged at a temperature equal to about 20 to about 30 degrees Celsius prior to the step of discharging the electrochemical cell at a temperature less than or equal to −40 degrees Celsius.
In another aspect, the present invention provides a method of making an electrochemical cell capable of low temperature operation comprising the steps of: (i) providing an electrochemical cell comprising: a positive electrode comprising a subfluorinated carbonaceous material having an average stoichiometry CFx, wherein x is the average atomic ratio of fluorine atoms to carbon atoms and is selected from the range of about 0.3 to about 1.0; the subfluorinated carbonaceous material being a multiphase material having an unfluorinated carbon component; a negative electrode; and a nonaqueous electrolyte provided between the positive and negative electrodes; and discharging a portion of the capacity of the electrochemical cell at a temperature equal to about 20 to about 30 degrees Celsius. In an embodiment of this aspect of the invention, between about 0.5 % to about 10% of the capacity of the electrochemical cell is discharged at a temperature equal to about 20 to about 30 degrees Celsius prior to the step of discharging the electrochemical cell at a temperature less than or equal to−40 degrees Celsius.
Use of a room temperature predischarge step are useful in these methods of the present invention for achieving optimal low temperature performance, including high specific capacities and stable discharge characteristics (e.g., reduced voltage delay, enhanced discharge stability). In some embodiments, room temperature predischarge conditioning results in formation of a fluorine-free carbonaceous layer at the positive electrode—electrolyte interface that enhances wetability and conductivity of the electrode. Room temperature predischarge conditioning provides a useful means, therefore, of preparing the positive electrode interfacial regions to accommodate high performance low temperature discharge.
Without wishing to be bound by any particular theory, there can be discussion herein of beliefs or understandings of underlying principles relating to the invention. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.
Referring to the drawings, like numerals indicate like elements and the same number appearing in more than one drawing refers to the same element. In addition, hereinafter, the following definitions apply:
The term “electrochemical cell” refers to devices and/or device components that convert chemical energy into electrical energy or electrical energy into chemical energy. Electrochemical cells typical have two or more electrodes (e.g., positive and negative electrodes) wherein electrode reactions occurring at the electrode surfaces result in charge transfer processes. Electrochemical cells include, but are not limited to, primary batteries, secondary batteries, lithium batteries and lithium ion batteries. General cell and/or battery construction is known in the art, see e.g., U.S. Pat. Nos. 6,489,055, 4,052,539, 6,306,540, Seel and Dahn J. Electrochem. Soc. 147(3) 892-898 (2000).
The term “capacity” is a characteristic of an electrochemical cell that refers to the total amount of electrical charge an electrochemical cell, such as a battery, is able to hold. Capacity is typically expressed in units of ampere-hours. The term “specific capacity” refers to the capacity output of an electrochemical cell, such as a battery, per unit weight. Specific capacity is typically expressed in units of ampere-hours kg−1.
The term “discharge rate” refers to the current at which an electrochemical cell is discharged. Discharge current can be expressed in units of ampere-hours. Alternatively, discharge current can be normalized to the rated capacity of the electrochemical cell, and expressed as C/(X t), wherein C is the capacity of the electrochemical cell, X is a variable and t is a specified unit of time, as used herein, equal to 1 hour.
“Current density” refers to the current flowing per unit electrode area.
The term “nanostructured” refers materials and/or structures have a plurality of discrete structural domains with at least one physical dimension (e.g., height, width, length, cross sectional dimension) that is less than about 1 micron. In this context, structural domains refer to features, components or portions of a material or structure having a characteristic composition, morphology and/or phase. Nanostructured materials useful as positive electrode active materials include nanostructured composite particles having a plurality of fluorinated carbon domains and unfluorinated carbon domains. In some embodiments, nanostructured materials of the present invention comprise a plurality of structural domains having different compositions, morphologies and/or phase intermixed on a very fine scale (e.g., at least smaller than 10's of nanometers).
“Active material” refers to the material in an electrode that takes part in electrochemical reactions which store and/or delivery energy in an electrochemical cell. The present invention provides electrochemical cells having a positive electrode with a subfluorinated carbonaceous active material.
As used herein, a carbon nanomaterial has at least one dimension that is between one nanometer and one micron. In an embodiment, at least one dimension of the nanomaterial is between 2 nm and 1000 nm. For carbon nanotubes, nanofibers, nanowhiskers or nanorods the diameter of the tube, fiber, nanowhiskers or nanorod falls within this size range. For carbon nanoparticles, the diameter of the nanoparticle falls within this size range. Carbon nanomaterials suitable for use with the invention include materials which have total impurity levels less than 10% and carbon materials doped with elements such as boron, nitrogen, silicon, tin and phosphorous.
As used herein, the term “nanotube” refers to a tube-shaped discrete fibril typically characterized by a diameter of typically about 1 nm to about 20 nm. In addition, the nanotube typically exhibits a length greater than about 10 times the diameter, preferably greater than about 100 times the diameter. The term “multi-wall” as used to describe nanotubes refers to nanotubes having a layered structure, so that the nanotube comprises an outer region of multiple continuous layers of ordered atoms and a distinct inner core region or lumen. The layers are disposed substantially concentrically about the longitudinal axis of the fibril. For carbon nanotubes, the layers are graphene layers. Carbon nanotubes have been synthesized in different forms as Single-, Double- and Multi-Walled Carbon Nanotubes noted SWCNT, DWCNT and MWCNT respectively. The diameter size ranges between about 2 nm in SWCNTs and DWCNTs to about 20 nm in MWCNTs. In an embodiment, the MWNT used in the invention have a diameter greater than 5 nm, greater than 10 nm, between 10 and 20 nm, or about 20 nm.
Electrodes are manufactured as disclosed herein and as known in the art, including as disclosed in, for example, U.S. Pat. Nos. 4,052,539, 6,306,540, 6,852,446. Briefly, the electrode is typically fabricated by depositing a slurry of the electrode material, an electrically conductive inert material, the binder, and a liquid carrier on the electrode current collector, and then evaporating the carrier to leave a coherent mass in electrical contact with the current collector.
“Room temperature” refers to a temperature selected over the range of about 293 to 303 degrees Celsius.
Aspects of the present invention are further set forth and described in the following Examples.
Overview
Commercial lithium/polycarbon monofluoride batteries [Li—(CF)n] are typically current-limited and are therefore not implemented in high-rate or low-temperature applications. Recent results suggest, however, that CF-based cathodes that use sub-fluorinated CFx (SFCFx) active materials in a thin electrode form factor are able to support very high currents (up to 5 C) while still providing a significant fraction of their specific capacity. In this Example, the low temperature efficacy of these materials is examined in a −40° C. environment. CF0.54 and CF0.65 powders were characterized using x-ray diffraction, scanning electron microscopy, and x-ray energy dispersive spectroscopy. These materials were then implemented in a spray-deposited electrode using a 1-mil (˜25 μm) aluminum foil current collector and PVDF as a binder. Electrochemical tests showed that these materials are able to deliver specific capacity values up to 5 times greater than commercial CF1.08 powder inserted into identically fabricated test cells tested at −40° C. Testing also indicated that a room-temperature pre-discharge step was necessary to condition the electrode materials before exposure to the low-temperature test environment.
Introduction
There is a need for safe, high energy density primary batteries that can function at low temperatures, −40° C. and below, for a host of aerospace (and other) applications. The existing off-the-shelf suite of batteries capable of providing energy at reasonable rates at these temperatures, including Li—SO2Cl2, Li—MnO2, and Li—SO2 have lingering safety issues or low temperature energy density values that make the further investigation of new low temperature battery chemistries important. The high energy density and inherently safe nature of the Li—CFx electrochemical couple make it very appealing, though this chemistry is thought to be severely rate limited, particularly at low temperatures. It is a goal of the present invention to provide high rate (C/10 to C/20) Li—CFx based batteries that can function well in environments no warmer than −40° C., while still providing high energy density. In doing so, we examined the performance of sub-fluorinated CFx materials as implemented in a thin electrode form factor.
Commercial Li/CFx batteries can deliver up to 700 Wh/kg, 1000 Wh/l, (at room temperature, C/100 rate) and typically use active cathode materials consisting of fluorinated carbonaceous materials such as coke or graphite. These cathodes have compositions typically ranging from CF1.05 to CF1.1 that can deliver specific capacities in excess of 800 mAh/g at 2.5 Volts under low-rate conditions at room temperature. This cathode material, however, is known to be discharge rate limited, and currents lower than C/50 (battery current 1/50th that of the capacity of the battery divided by 1 hour) are often necessary to avoid cell polarization and large capacity loss. Oftentimes, the low electronic conductivity of CF is cited as the cause of the observed discharge rate limitations, and it has been long been known that there is a strong correlation between cathode thickness and performance; thicker cathodes are prone to be more rate-limited. To this end, a number of Li—CFx battery manufacturers, have been developing high-rate CFx cells that use thin cathode structures in a spirally-wound configuration. Some very promising results have been recently described, including a 15.5 Ah fully packaged D cell that delivers over 460 Wh/kg at better than a C/8 rate. At −20° F. (−29° C.), these same cells delivered about 12 Ah under a 2 A discharge, with an active cathode materials utilization of over 570 mAh/g. A similar electrode design approach developed had a bulk cathode impedance of 0.0002 ohms/mm2 and was able to deliver over 500 mAh/g active cathode material at a C/20 rate at −30° C. Others have reported that the Li—CFx chemistry exhibits substantial self heating under some discharge conditions. While this is a liability at ambient or elevated temperature use, properly designed cells and batteries could take advantage of this effect for very low temperature operation.
To examine the possibility of further augmenting these results, we have taken advantage of the relatively high electronic conductivity sub-fluorinated CFx (SFCFx) cathode materials. Recently, substantial success has been reported wherein CFx with 0.3<x<0.8 has been used in test cells that are capable of discharge rates as high as 5 C at room temperature with excellent capacity retention. It was unknown, however, whether these cathodes would possess the good characteristics at very low temperatures or high rates. This is because it is possible that significantly slower cathode reaction kinetics and lower ion conductivity values at these temperatures could mitigate the benefits observed at room temperature for the enhanced electronic conductivity SFCFx materials. Furthermore, the factors that cause the well-known voltage delay effect in Li—(CF)n batteries are likely to be more prominent under high-rate, low temperature conditions and so must be examined and understood if a functional battery is to be eventually produced. The present Example describes the performance of Li—CFx test cells based on SFCFx active cathode materials and electrolytes designed to function well at low-temperatures. A series of test cells based on commercially-supplied CF1.08 powders were tested in parallel in identical test cells.
Experimental
Materials Fabrication and Analyses:
A monel boat, containing Madagascar natural graphite (30 g) with average particle size of about 10 μm, was placed in a nickel reactor and heated under pure fluorine gas flow at 375 and 390° C. during 17 hours to produce CF0.54 and CF0.65, respectively. The products and were analyzed using x-ray diffraction (Cu Kαradiation), scanning electron microscopy (SEM) and x-ray energy dispersive spectroscopy (XEDS).
Electrochemical Analyses:
Spray-deposited cathodes were fabricated using SFCFx powders that were prepared using a mix consisting of 80 wt %, 10 wt % carbon black in a NMP solution solvating 10 wt % PVDF (pre-stirred solution). As a control, a commercial CF1.08-based cathode mixture was also produced which had a similar overall fluorine mass fraction as in the CF0.54 mix (and so therefore had a higher carbon-diluent-to “active material” ratio). Specifically, these baseline cathodes contained 30 wt % C black, 60 wt % CF1.08 and 10 wt % PVDF. In all cases the fully intermixed slurry (as mechanically stirred for at least 12 hours) was spray-deposited in multiple layers onto a roughened 1-mil thick Al foil current collector. The resulting physically robust cathode structures, 1 to 3 mils in thickness, were vacuum-furnace dried for 24 to 48 hours at ˜105° C. 2032 size coin cell test cells were then fabricated using 16 mm diameter cathode disks, a single layer of Tonen separator, and a 16 mm diameter Li foil anode.
The electrolytes contained 1 M LiBF4 salt solvated in a mix of either propylene carbonate (PC)+1,2,-dimethoxy ethane (DME) in a 20/80 or 50/50 v/v % ratio. Since these electrodes were so thin, the variation in wettability between these two solvent compositions was not found to have a significant effect on cell performance. A series of electrolytes made using various solvent additives has also been performed, and will be reported on elsewhere in detail.
Electrochemical testing consisted of galvanostatic discharges at room temperature and −40° C. under currents consistent with rates ranging from 2 C to C/40. These rate values were calculated using the nominal room-temperature specific capacities that have been previously published for these SFCFx materials. A room-temperature pre-discharge step of 3% of the total estimated cathode capacity at a C/33 rate was employed in most cases. Electrochemical impedance spectroscopy (EIS) in the potentiostatic mode (5 mV signal amplitude, 10−2 to 105 Hz frequency range) was carried out to examine the effect of this pre-discharge on the cathode an anode.
Results
Materials Characterization
Scanning electron micrographs of this material are shown in
Electrochemical Characterization
Room Temperature characterization: A number of room-temperature discharges were conducted to characterize the CFx materials. In these cases, the electrolyte consisted of either 1.0 M LiBF4 in PC+DME (20/80 or 50/50 v/v %).
The baseline cathodes fabricated using commercial CF1.08 and 30 wt % carbon black material were also discharged at room temperature at high rates, as shown in
Room temperature discharge data from a CF0.65 cathode are shown in
Low Temperature Testing: Test cells made from the same materials as for the room temperature testing were discharged at −40° C.
Table 1 contains a partial list of test cells, composition, test conditions, and resulting capacities.
Discussion
Microstructure: The characterization data indicate that the SFCFx materials were comprised of pristine un-fluorinated graphite mixed intimately with fluorinated C, and are consistent with data reported previously. The SEM/XEDS analyses indicate that the C and F are intermixed at a very fine scale (at least smaller than 10's of nm), while the XRD data show that the graphitic material that has undergone fluorination is likely fluorinated to the same degree regardless of the total fluorination of the sample (i.e. x in CFx). There is also no evidence of graphitic gallery staging at the various levels of fluorination. The combination of these indicators suggests that the microstructure of this material is a collection of nano-dimensional graphite and CFy domains, where y is approximately 0.8 to 0.9. The small scale of these compositionally variant regions dictates that the material have a very high graphite-to-CFy surface area and subsequently will posses excellent electronic/interface transport properties at all temperatures.
Although thermodynamically predictable, the fluorination of graphite is kinetically unfavorable as it requires the following: i) separation of the graphene layers to allow for fluorine diffusion, ii) change in the carbon hybridization from stable sp2 to less stable sp3, and iii) dissociation of the F2 molecule. At constant temperature and fluorine pressure, the chemical activity of fluorine decreases with the depth of fluorine penetration within the layers. Consequently, the rate of the fluorination reaction (C+y/2F2=CFy) should decrease as the diffusion layer becomes thicker, which eventually leads to an equilibrium state in which possibly a fraction of carbon remains unreacted. At 375 and 390° C. used to prepare CF0.54 and CF0.64 samples respectively, under 1 atmosphere fluorine pressure, the practical end point is reached after 17-hours reaction. Both samples show unreacted graphite.
Electrochemical Behavior: To establish that this uniquely structured material offers some benefits compared to state-of-the-art (CF)n cathode materials, it was necessary to create a control cathode based on commercially available CF1.08 that had the same fraction of fluorine in the as-tested cathode structure. Relatively large quantities of C black were used to lower the total F content in these cathodes to the same level as found in the CF0.54-based cathodes (which used 10 wt % carbon black). At room temperatures, there was little difference between the SFCFx material and the industry baseline, as seen in
At −40° C., however, the commercial CF1.08 baseline and SFCFx cathode materials behaved differently. In particular, the CF1.08 yielded significantly lower capacities at lower discharge potentials than the SFCFx materials, even though the latter were in a slightly thicker electrode form factor, as summarized in table 1. The CF1.08-based cathodes typically delivered one-third the capacity of the CF0.65 material under identical conditions. The result is more pronounced when considering the fact that the baseline material cathodes also polarized to a greater degree, thereby offering a energy density.
This result indicates that the nano-scale intermixing of graphitic and CFy materials is important for low-temperature performance, and cannot be emulated by physically mixing high surface area carbon materials with CF1.08 as which was shown to be sufficient for higher temperature testing environments. The two main differentiating features of the SFCFx material compared to the CF1.08 were (1) higher surface-area-to-mass ratio for the active CFy domains, and (2) the existence of nano-scale electronically conductive pathways.
The capacity/energy density of a D-sized cell produced using the CF0.647 cathode material can now be estimated (for a −40° C., C/40 discharge condition). It is assumed that the CF0.647 active material may be inserted into the same cell structure the 15.5 Ah D-size LiCFx battery described in the introduction. Substituting 600 mAh/g CF0.647 into this cell format results in a cell with an energy density of about 400 Wh/kg when discharged at a C/40 rate at −40° C.
It was found that the low-temperature performance of all CFx materials was greatly enhanced if a small, room temperature pre-discharge was used. This protocol is used for industrially-fabricated Li—CFx batteries to condition the electrode/electrolyte interfaces. The observed effect could have been a result of the existing native Li anode passivation layer of LiOH species that is removed with a short discharge. To examine this possibility, several Li half-cells (using Cu foil as a counter electrode) were constructed with the same electrolyte/salt/separator as used in the full Li—CFx cells. −40° C. galvanostatic electrochemical tests were conducted on cells that both had and had not been subjected to the room temperature galvanostatic conditioning step identical to that used in the full cells.
Other tests using a flooded three-electrode glass cells indicated the same result.
Another relationship is indicated in
This Example shows that the nano-scale intermixing of graphitic domains and CFy found in sub-fluorinated CFx (SFCFx) materials greatly benefited the specific capacity of these materials when discharged at low temperatures using aggressive current densities. The performance of SFCFX materials was compared to industry-standard, fully-fluorinated CF1.08 powders inserted in otherwise identical test cells. The SFCFx and CF1.08 active material yielded nearly the same specific capacity values at room temperature at rates as high as 2 C. However the SFCFx gave a 3 times (or greater) capacity value at −40° C. using rates up to C/10. The results also indicate that a room-temperature pre-discharge is important to low-temperature performance, an effect attributed to the kinetically-motivated cathode depassivation/conditioning events that occur upon initial discharge. It is likely that the pre-discharge results in the evolution of a fluorine-free C layer at the electrode/electrolyte interface that would subsequently enhance electrolyte wetability and conductivity. These results show that the high specific capacity SFCFx cathode chemistry is a viable intermediate rate low-temperature battery cathode material if prepared in a sub-fluorinated state and implemented in a sufficiently thin-electrode cell design.
Overview
This example describes the continued examination of the low-temperature performance of the Li—CFx electrochemical couple in a −40° C. or colder environment. Previously, the efficacy of sub-fluorinated CFx (SFCFx) cathode active materials was demonstrated; preliminary results indicated that the material was functional at rates up to C/10 at −40° C., however there were often substantial voltage fluctuations during discharge, accompanied with inconsistent capacity yields and sometimes dramatic polarization events. In the research described herein, an investigation of various electrolyte and cathode compositions was conducted in an effort to optimize performance. In particular, several different electrolyte solvent formulations were examined using a salt content of either 1 M or 0.5 M LiBF4. A further modification consisted of addition of an anion-receptor to the electrolyte that was intended to be a LiF-solvating agent. The most promising electrolyte was tested with several different SFCFx compositions created using either graphitic or multi-wall nanotube precursor materials. Electrochemical evaluation showed that the best SFCFx-based test cells were able to deliver specific capacity values up to 5 times greater than control cells (containing conventional CF1.08 powder) at −40° C. under discharge currents as high as C/5 with composite electrodes thicker than 50 μm.
Introduction
Recently, it has been reported that sub-fluorinated CFx (SFCFx) materials (where 0.3<x<0.66) are capable of supporting discharge rates as high as 5 C at room temperature with excellent utilization. It was unknown, however, whether these cathodes would possess good characteristics at very low temperatures at aggressive rates (i.e. greater than C/100). The significantly slower cathode reaction kinetics and/or lower electrolyte ionic conductivity values at these temperatures could mitigate the benefits observed at room temperature. Furthermore, the factors that cause the voltage delay effect in Li—(CF)n batteries are likely to be more prominent under high-rate, low temperature conditions and therefore must be examined if a functional battery is to be eventually produced. Preliminary results show that the sub-fluorinated CFx materials did exhibit superior low-temperature performance at −40° C. compared to baseline CF1, though at current levels not exceeding a C/20 rate. Two electrolyte blends were compared: 1 M LiBF4 solvated in a DME/PC mix to a ratio of either 80/20 v/v % or 50/50 v/v %. The 80/20 v/v % DME/PC-based electrolyte had superior performance at low temperatures, though there were still significant variations in cell performance at higher rates at −40° C. It was unclear whether this erratic behavior was due to the inherent properties of the electrolyte, interfacial properties (i.e., cathode surface wettability), or the re-distribution of reaction byproducts (LiF) upon discharge.
The focus of the present Example is to further improve low-temperature functionality by matching the highest capacity sub-fluorinated cathode active material with the proper electrolyte blend and electrode form factor. In particular, three different electrolytes were tested at low temperatures with a standard Li—CF0.65 cell. The best electrolyte was then used to test several variations of sub-fluorinated CFx cathode material. These results were compared to those obtained from cathodes produced using commercially available CF1.08. In some cells, a solvating anion receptor additive was also inserted into the electrolyte with the intention of reducing cathode surface Li—F passivation, thereby increasing the electrode functionality. The most promising test cells were able to support substantial discharge currents at temperatures colder than −40° C., with composite cathode thicknesses exceeding 110 μm.
Experimental Procedure
A standard test vehicle was adopted that consisted of a spray-deposited cathode layer containing 10% PVDF binder, 10% carbon black, and 80% active material on a 1-mil thick Al current collector foil (and spray-deposited onto the heated Al current collector). These composite cathodes ranged in thickness from 10 μm to 120 μm, with the most commonly tested thickness being about 40 μm +/−5 μm. The relationship between electrode mass and thickness was determined using a precision caliper on the thicker samples (>50 μm). The standard test cell consisted of 2032 coin cells with a Li metal anode, and a single layer of polypropylene (Tonen™) separator. The control electrode, based on CF1.08 as received from a commercial vendor, contained 30 wt % carbon black and 10 wt % PVDF. This resulted in a composite electrode capable of similar room temperature performance (per unit mass of the total composite electrode) as a CF0.55-containing electrodes.
A representative SFCFx electrode composition and thickness, CF0.65 and 40 μm +/−5 μm thick, was first used to identify promising electrolyte blends for low temperature performance. The selected electrolyte blends are indicated in Table 2.
The lithium tetrafluoroborate used was battery grade and obtained from Mitsubishi Petrochemicals Co. and vacuum dried prior to use. The propylene carbonate (PC) and 1,2-dimethoxyethane (DME) were also high purity battery grade and obtained from Mitsubishi Petrochemicals Co. and used as received. The trifluoroethyl ether (TEE) was synthesized by a known method consisting of the acid catalyzed dehydration reaction of 2,2,2-trifluoroethanol.
The rationale for incorporating ethers bearing fluoroalkyl groups in electrolyte formulations is two-fold: (a) they are envisioned to impart beneficial physical properties to the electrolyte, due to their low viscosity, low melting points, and good solvating properties, and (b) the hydrofluorocarbon ethers are anticipated to better wet the CFx electrode improving the mass transport properties. The optimization of the PC+DME blends is based upon the balance of having adequate proportion of the high dielectric constant component (PC) and sufficient proportion of the low viscosity, good coordinating component (DME). During the course of optimization of these electrolytes for low temperature operation, higher conductivity is observed with electrolyte formulations containing low PC content and low electrolyte salt concentration. In addition to these factors, the primary solvent either EC or PC, needs to be in adequate proportion to impart passivation to the anode (and hence shelf life to the cell), but not high enough to contribute to higher viscosity at low temperatures.
Upon determining the highest performing electrolyte, at low temperatures, of those examined, test cells were made and tested using four different cathode active materials; three were SFCFx-based, and the last was the CF1.08-based standard. The CF0.65 electrodes used partially fluorinated Madagascar graphite precursor, while the other two SFCFx variations contained CF0.59 or CF0.82 material made using multi-wall carbon nano-tube material precursor materials supplied by MER Corporation. Characterization data indicated that all of the SFCFx materials were comprised of pristine un-fluorinated graphite domains mixed intimately with nano-dispersed near-fully fluorinated graphitic regions, and are consistent with data reported previously. The small scale of these regions dictates that the materials had a very high graphite-to-CF surface area and subsequently might possess excellent electronic/interfacial transport properties.
In several of the 0.5 M LiBF4 in DME/PC (80:20) containing cells, an anion receptor Lewis acid additive, tris(2,2,2-trifluoroethyl) borate (TTFEB), was also dissolved into the electrolyte solvent in an effort to decrease or eliminate the LiF (discharge reactant) buildup via a F−complexing reaction. Such an approach is also useful for enhancing the solubility of lithium salts. The nominal TTFEB concentration in the electrolyte solution was 1.5/98 v/v % TTFEB/(DME/PC (80/20).
Previous work (See, Example 1) indicated that a room-temperature pre-discharge step (consisting of a 1 hour, discharge at a C/33 current level) was necessary to prepare the electrode interfacial regions to accommodate low-temperature discharge. This preparation was used here for most test cells, however, the necessity of this pre-discharge was re-examined for the most promising electrode/electrolyte combinations. In all cases, the C rate was calculated based on the expected room-temperature capacity of the cathode if discharged at a C/40 rate.
Results
The execution of the pre-discharge step in the 0.5 M salt content cells was found to have no effect on low-temperature performance, much in contrast to the results obtained using electrolytes containing higher salt concentrations. EIS data collected from test coin cells with stainless-steel blocking electrodes indicate that the 1 M salt content electrolyte had approximately twice the high frequency, real-axis intercept at −40° C. compared to the 0.5 M salt containing blend.
The nominal maximum usable composite cathode thickness has previously been found to have a strong negative correlation with the maximum usable discharge rate or realized discharge capacity in the Li—CFx chemistry and was therefore examined here under low temperature conditions.
The use of the TTFEB anion-receptor additive resulted in even thicker composite electrodes that could be made functional at these temperatures.
Discussion
A decrease in the electrolyte salt content from 1 M to 0.5 M in an 80/20 v/v % DME/PC solution has further enhanced the low temperature performance of the Li/SFCFx electrochemical couple compared to previously published results. This improvement can be attributed to at least one of several mechanisms, including enhanced low temperature electrolyte conductivity, or a relative decrease in surface precipitation on the active cathode material surfaces. There is not as yet conclusive proof as to which of these mechanisms (if either) are at work, and this effect bears further examination. The impedance spectroscopy data indicate, however, that there is not a substantial difference between the electrode impedance as a function of salt content, which indicates that the enhancement mechanism is not directly related to the IR losses associated with an SEI layer. The cathode impedance is substantially reduced after discharge, in solutions with either 0.5 M or 1.0 M salt concentration, with or without the anion receptor additive. Furthermore, previous results on three-electrode cells indicate that much of the change in series resistance occurs on the cathode side of the cell under these conditions. Even though the series resistance, reflected in the x-axis intercept at high frequencies, does not change, the low frequency impedance is significantly reduced. This may be due to inadequate wetting of the interface before discharge, or could be due to the insulating nature of a native film that forms on the cathode before discharge.
The fact that a room-temperature pre-discharge is not required using the lower salt content electrolyte blend is consistent with the idea that some portion of the electrolyte salt reacts with the cathode material upon contact to form interfacial film that impedes the cathodic reaction in some way, particularly at discharge commencement. Lowering the salt content may result in a less robust cathode surface film that may subsequently reduce or negate the need for a pre-discharge step.
The fact that the addition of an anion receptor, tris (2,2,2-trifluoroethyl borate, known to dissolve LiF results in a further performance enhancement is also consistent with this notion. In intermediate cases (i.e., C/10 or higher rates, or around 50 μm in thickness), a substantial voltage delay was observed for cells without any anion receptor, such as that for the 57 μm thick CF0.65 cathode discharged at a C/5 rate at −40° C. (
The type of graphitic precursor used did not have a significant impact on test cell performance at low temperatures. The data in
The results illustrate that the nano-scale intermixing of pristine graphitic domains and CF0.8 domains observed in SFCFx materials greatly enhance the specific capacity of these materials at low temperatures at aggressive rates. Electrolyte screening identified that blends consisting of 20/80 v/v % PC/DME with 1 M LiBF4 offered superior low temperature performance compared to the baseline formulations, based on 1 M LiBF4 in either 50/50 v/v % or 20/80 v/v % PC/DME. Three variations of SFCFx cathode materials were inserted into similar ˜40 μm thick composite electrode structures and tested at −40° C. using several different electrolyte blends. The CF0.65 and CF0.82 based cells could deliver well over 600 mAh/g above 2 V under a C/5 discharge rate at −40° C. with the proper electrolyte blend. To further increase rate capability and/or functional cathode thickness, an anion receptor electrolyte additive, TTFEB, was evaluated and proved to be effective. With this additive, composite cathode structures 115 μm thick yielded over 500 mAh/g at a C/5 discharge rate at −40° C. These results indicate that both the CF/electrolyte interfacial area and the nature of any SEI that may form on these active areas are important to low temperature performance. In particular, the results indicate that this film was reaction-limiting early in the discharge process and could be mitigated by both lowering the electrolyte salt content and adding a LiF solvating agent. These modifications resulted in a greatly enhanced low temperature performance for this electrochemical couple, and have enabled cells that are functional to temperatures to −60° C. and possibly colder.
CFx Cathode Materials Development
Five variations of sub-fluorinated CFx cathode materials were examined and compared to commercially available CF1 in the present Example. The main focus of these studies was to improve low temperature functionality of this class of materials by matching the sub-fluorinated cathode with the proper electrolyte blend. Test cells based on commercially-supplied CF1.08 powders were tested in parallel in identical test fixtures.
In the Example, the generation I and II materials consisted of partially fluorinated graphite, where x=0.53, and 0.65 respectively. The generation III material was partially fluorinated carbon nano-tube material (precursor supplied by MER corporation), where x was equal to 0.59, 0.76 and 0.82. In qualifying these different cathodes, a standard test vehicle was adopted that consisted of a spray—deposited cathode layer containing 10% PVDF binder, 10% carbon black conductive diluent, and 80% active material on a 1-mil thick Al current collector. These composite cathodes ranged in thickness from 10 μm to 120 μm, with the most commonly tested thickness being about 35 μm. The standard test cell consisted of 2032 coin cells with a Li metal anode, and a Tonen™ separator.
Many electrolyte variations were examined through the course of this Example.
Materials Characterization
The characterization data indicate that the SFCFx materials were comprised of pristine un-fluorinated graphite mixed intimately with fluorinated C. The SEM/XEDS analyses indicate that the C and F are intermixed at a very fine scale (at least smaller than 10's of nm), while the XRD data show that the graphitic material that has undergone fluorination is likely fluorinated to the same degree regardless of the total fluorination of the sample (i.e. x in CFx). There is also no evidence of graphitic gallery staging at the various levels of fluorination. The combination of these indicators suggests that the microstructure of this material is a collection of nano-dimensional graphite and CFy domains, where y is approximately 0.8 to 0.9. The small scale of these compositionally variant regions dictates that the material have a very high graphite-to-CFy surface area and subsequently will posses excellent electronic/interface transport properties at all temperatures.
Although thermodynamically predictable, the fluorination of graphite is kinetically unfavorable as it requires the following: i) separation of the graphene layers to allow for fluorine diffusion, ii) change in the carbon hybridization from stable sp2 to less stable sp3, and iii) dissociation of the F2 molecule. At constant temperature and fluorine pressure, the chemical activity of fluorine decreases with the depth of fluorine penetration within the layers. Consequently, the rate of the fluorination reaction (C+y/2F2=CFy) should decrease as the diffusion layer becomes thicker, which eventually leads to an equilibrium state in which possibly a fraction of carbon remains unreacted. At 375 and 390° C. used to prepare CF0.54 and CF0.64 samples respectively, under 1 atmosphere fluorine pressure, the practical end point is reached after 17-hours reaction. Both samples show unreacted graphite.
Electrochemical Performance
Table 3 has a summary of some early test results taken from cells using the 1 M LiBF4 salt content electrolytes.
It is clear from these data that very inconsistent discharge capacities were obtained from cell to cell at higher discharge rates, though the best of the cells had very promising performance.
Promising new blend from the primary battery electrolyte sub-task were subsequently tested with these new cathodes and resulted in an important finding: a lower electrolytic salt content greatly enhanced the discharge stability and rate capability of the sub-fluorinated CFx materials at low temperatures.
All of the new cathode materials were studied in this electrolyte blend under similar conditions and can therefore be directly compared.
A key factor that will dictate the practicality of implementing these new cathodes in industrially fabricated batteries for low temperature use is the nominal composite cathode thickness. This parameter has been found to be strongly correlated to CFx battery rate capability at room temperature and was therefore studied here.
To establish that this uniquely structured material offers benefits compared to state-of-the-art (CF)n cathode materials, it was necessary to create a control cathode based on commercially available CF1.08 that had the same fraction of fluorine in the as-tested cathode structure. Relatively large quantities of C black were used to lower the total F content in these cathodes to the same level as found in the CF0.54-based cathodes (which used 10 wt % carbon black). At room temperatures, there was little difference between the SFCFx material and the industry baseline. Though the CF0.54 material had a higher potential initially, this material had a discharge plateau that was more sloping, and actually delivered less capacity at room temperature than the CF1.08 cathodes. This result is not unexpected, as the very thin cathode form factor used in these cells likely minimized the beneficial effects of the enhanced conductivity found in the SFCFx materials. It is possible that substantially thicker cathode structures would allow for the difference in performance between the CF1.08 and the CF0.54 to be resolved.
At −40° C., however, the commercial CF1.08 baseline and SFCFx cathode materials behaved differently. In particular, the CF1.08 yielded significantly lower capacities at lower discharge potentials than the SFCFx materials, even though the latter were in a slightly thicker electrode form factor, as summarized in Table 3. The CF1.08-based cathodes typically delivered one-third the capacity of the CF0.65 material under identical conditions. The result is more pronounced when considering the fact that the baseline material cathodes also polarized to a greater degree, thereby offering a energy density.
This result indicates that the nano-scale intermixing of graphitic and CFy materials is important for low-temperature performance, and cannot be emulated by physically mixing high surface area carbon materials with CF1.08 as which was shown to be sufficient for higher temperature testing environments. The two main differentiating features of the SFCFx material compared to the CF1.08 were (1) higher surface-area-to-mass ratio for the active CFy domains, and (2) the existence of nano-scale electronically conductive pathways.
Anion Receptor Electrolyte Additive For Enhanced Rate Capability
To further extend the low temperature performance, the possibility of using an anion receptor additive that might serve to solvate and remove excess inert LiF (discharge reaction product) from the electrode surface to allow for subsequent reaction has been examined. Specifically, we have found that tris-(2,2,2trifluoroethyl) borate, with relatively low molecular weight and viscosity, results in enhanced cell performance. This effect was observed when using sub-fluorinated CFx material. The difference in performance with and without the electrolyte additive was significant. For example, at −40° C., cells without the anion receptor additive were discharged at a rate of C/2.5, and were essentially nonfunctional. However, the same experiment repeated with cells incorporating 0.64 M tris (2,2,2 trifluoroethy) borate demonstrated that these cells were able to achieve nearly 50% of the room temperature capacity.
It is important to note that, in the limited testing performed using this additive, the improved performance was observed only when using the sub-fluorinated cathode materials: a total capacity of only 70 mAh/g was extracted from CF1.08-based cathodes under identical conditions (C/2.5 rate, −40° C.) using the same electrolyte+anion receptor, as shown in
Another key advantage to using this anion receptor is extending the practical thickness of the composite cathodes. Discharges on test cells containing 115 μm thick cathode structures showed that the anion-receptor additive enabled high (C/5) discharge rate without any voltage delay or severe polarization effects. This cathode thickness exceeds that typically found in industrial Li-ion batteries and so shows that it is, in principle, possible to make a practical, high energy density CFx battery that is functional to very low temperatures.
Summary of CFx Primary Battery Cathode Results
This Example shows that the nano-scale intermixing of graphitic domains and CFy found in sub-fluorinated CFx (SFCFx) materials greatly benefited the specific capacity of these materials when discharged at low temperatures using aggressive current densities. The performance of SFCFx materials was compared to industry-standard, fully-fluorinated CF1.08 powders inserted in otherwise identical test cells. The SFCFx and CF1.08 active material yielded nearly the same specific capacity values at room temperature at rates as high as 2 C. However the SFCFx gave a 3 times (or greater) capacity value at −40° C. using rates up to C/10. The results also indicate that a room-temperature pre-discharge is important to low-temperature performance, an effect attributed to the kinetically-motivated cathode depassivation/conditioning events that occur upon initial discharge. It is suggested that the pre-discharge results in the evolution of a fluorine-free C layer at the electrode/electrolyte interface that would subsequently enhance electrolyte wetability and conductivity. These results show that the high specific capacity SFCFX cathode chemistry is a viable intermediate rate low-temperature battery cathode material if prepared in a sub-fluorinated state and implemented in a sufficiently thin-electrode cell design.
Five variations of sub-fluorinated CFx cathode materials were tested at temperatures as cold as −60° C. using several different electrolyte variations. Multiple cells were examined for each materials combination, and different discharge currents were examined. With the proper electrolyte blend, 0.5 M LiBF4 in 20/80 PC/DME, the best of these materials could deliver up to 600 mAh/g at a C/5 rate. To further increase rate capability and/or functional cathode thickness, an anion receptor electrolyte additive was evaluated and proved be effective. Using this electrolyte blend and a 115 μm thick CF0.65-based cathode, a cell was fabricated that could deliver about 600 mAh/g at a C/5 rate.
The capacity/energy density of a D-sized cell produced using the CF0.647 cathode material can now be estimated (for a −40° C., C/5 discharge condition). It is assumed that the CF0.647 active material may be inserted into the same cell structure the 15.5 Ah D-size LiCFx battery under development at Eagle Picher. Substituting 600 mAh/g CF0.647 into this cell format results in a cell with an energy density of about 400 Wh/kg when discharged at a C/5 rate at −40° C. Approximately 120 μm thick composite cathodes containing CF0.7 combined with a Li metal anode in a 0.5 M LiBF4 8/2 DME/PC electrolyte with the tris-(2,2,2 trifluoroethyl) borate anion receptor additive offers excellent low temperature performance.
The present invention includes nonaqueous electrolyte compositions having anion receptor additives useful in lithium batteries. Anion receptors in some aspects of the present invention are capable of conditioning the surfaces of subfluorinated and fully fluorinated carbonaceous positive electrode active materials by dissolving discharge products, such as LiF, that can degrade the electrical and ion conductivity at the electrode. To demonstrate this attribute, the capability of anion receptors of the present invention to coordinate fluoride ions in propylene carbonate solvent was evaluated.
As shown in
The plots in
Furthermore, fluoride ion anion receptors of the present invention, such as tris (hexafluoroisopropyl) borate, decrease electrolyte viscosity and improve low temperature conductivity. The present anion receptors complex BF4−1, thereby increase the Li+ transport number in electrolyte systems having a LiBF4 lithium salt.
All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).
The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.
When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.
Many of the molecules disclosed herein contain one or more ionizable groups [groups from which a proton can be removed (e.g., —COOH) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein. With regard to salts of the compounds herein, one of ordinary skill in the art can select from among a wide variety of available counterions those that are appropriate for preparation of salts of this invention for a given application. In specific applications, the selection of a given anion or cation for preparation of a salt may result in increased or decreased solubility of that salt.
Every formulation or combination of components described or exemplified herein can be used to practice the invention, unless otherwise stated.
Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.
All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art as of their publication or filing date and it is intended that this information can be employed herein, if needed, to exclude specific embodiments that are in the prior art. For example, when composition of matter are claimed, it should be understood that compounds known and available in the art prior to Applicant's invention, including compounds for which an enabling disclosure is provided in the references cited herein, are not intended to be included in the composition of matter claims herein.
As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” may be replaced with either of the other two terms. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.
One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.
This application claims priority under 35 U.S.C. 119(e) to U.S. provisional Patent Application 60/774,262 filed Feb. 16, 2006, U.S. provisional Patent Application 60/784,957 filed Mar. 21, 2006, U.S. provisional Patent Application 60/784,960 filed Mar. 20, 2006, and to a U.S. provisional Patent Application filed on Feb. 9, 2007 for “Coke Based Subfluorinated Carbon Fluorides (CFx) Cathodes in Lithium Batteries” (serial number not yet assigned); this application is also a continuation-in-part of 11/253,360 and 11/422,564 filed Oct. 18, 2005 and Jun. 6, 2006, respectively, which claim the benefit of priority of U.S. Provisional Application 60/724,084 filed Oct. 5, 2005; this application is also a continuation-in-part of 11/560,570 filed Nov. 16 , 2006 which claims the benefit of priority of U.S. Provisional Application Nos. 60/737,186; 60/775,110 and 60/775,559 filed Nov. 16, 2005, Feb. 21, 2006 and Feb. 22, 2006, respectively, all of which are incorporated herein by reference in their entireties to the extent not inconsistent with the disclosure herein.
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.
| Number | Date | Country | |
|---|---|---|---|
| 60744262 | Apr 2006 | US | |
| 60784957 | Mar 2006 | US | |
| 60784960 | Mar 2006 | US | |
| 60724084 | Oct 2005 | US | |
| 60724084 | Oct 2005 | US | |
| 60737186 | Nov 2005 | US | |
| 60775110 | Feb 2006 | US | |
| 60775559 | Feb 2006 | US | |
| 60900409 | Feb 2007 | US | |
| 60798532 | May 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11253360 | Oct 2005 | US |
| Child | 11675308 | Feb 2007 | US |
| Parent | 11422564 | Jun 2006 | US |
| Child | 11675308 | Feb 2007 | US |
| Parent | 11560570 | Nov 2006 | US |
| Child | 11675308 | Feb 2007 | US |
