Patent Application
20200350558
Lithium ion batteries describe a class of rechargeable batteries in which lithium ions move between a negative electrode (i.e., anode) and a positive electrode (i.e., cathode). Liquid, solid, and polymer electrolytes can facilitate the movement of lithium ions between the anode and cathode. Lithium-ion batteries are growing in popularity for defense, automotive, and aerospace applications due to their high energy density and ability to undergo successive charge and discharge cycles.
Methods for fabricating electrodes are provided and can include coating a current collector with a slurry to form a coated current collector, and pyrolyzing the coated current collector to produce the electrode comprising a layer of silicon-based host material. The slurry can include a dry fraction and one or more solvents. The dry fraction can include silicon particles, one or more polymeric binders, and carbon fibers. Pyrolyzing the coated current collector can include heating at a first temperature, and subsequently heating at a second temperature wherein the first temperature is higher than the first temperature. The silicon particles can be single-phase silicon and Li2Si. The silicon particles can have an average particle diameter of less than about 10 μm. The dry fraction can include at least about 70 wt. % silicon particles. The polymeric binders can include polyacrylonitrile, and/or one or more fluorinated polymers. The polymeric binders can be only polyacrylonitrile. The polymeric binder can be up to about 10 wt. % of the dry fraction. The carbon fibers can have an average diameter of at least about 50 nm. The carbon fibers can have an average length of at least about 1 μm. The carbon fibers can be up to about 15 wt. % of the dry fraction. The solvents can include N-Methyl-2-pyrrolidone, dimethyl formamide, dimethyl sulfoxide, propylene carbonate, ethylene carbonate, acetone, and/or methyl ethyl ketone. The method can further include, after coating and prior to pyrolyzing, drying the coated current collector. Drying can occur at less than about 100° C. The first temperature can be about 250° C. to about 400° C., and the second temperature can be less than about 750° C. Pyrolyzing can occur in an environment substantially free of oxygenated gases. The thickness of the silicon-based host material layer can be about 20 μm to about 50 μm.
Methods for fabricating battery cells are provided and can include coating a current collector with a slurry to form a coated current collector, pyrolyzing the coated current collector to produce a negative electrode comprising a layer of silicon-based host material, and subsequently assembling the battery cell by disposing the negative electrode and a positive electrode in an electrolyte. The slurry can include a dry fraction and one or more solvents. The dry fraction can include silicon particles, wherein the silicon particles include Li2Si and single-phase silicon, one or more polymeric binders comprising polyacrylonitrile, polyvinylidiene fluoride, polytetrafluoroethylene, fluorinated ethylene propylene, and/or perfluoroalkoxy alkanes, and carbon fibers, wherein the carbon fibers have an average diameter of about 100 nm to about 200 nm and an average length of about 1 um to about 10 um. Pyrolyzing the coated current collector can include heating at a first temperature of up to about 400° C. and subsequently heating at a second temperature between about 450° C. and about 750° C. The silicon particles can have an average particle diameter of less than about 10 um. The dry fraction can be at least about 70 wt. % silicon particles. The one or more pyrolyzed polymeric binders can create a carbon layer around the silicon particles.
Other objects, advantages and novel features of the exemplary embodiments will become more apparent from the following detailed description of exemplary embodiments and the accompanying drawings.
Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.
Provided herein are methods for fabricating electrodes and battery cells comprising silicon-based host materials. The methods provide high capacity electrodes with high mechanical strength, minimal irreversible capacity loss during the first formation cycle, and improved capacity retention during prolonged cycling.
Battery cell 10 can be used in any number of applications. For example,
Battery cell 10 generally operates by reversibly passing lithium ions between Anode 11 and cathode 14. Lithium ions move from cathode 14 to Anode 11 while charging, and move from Anode 11 to cathode 14 while discharging. At the beginning of a discharge, Anode 11 contains a high concentration of intercalated/alloyed lithium ions while cathode 14 is relatively depleted, and establishing a closed external circuit between Anode 11 and cathode 14 under such circumstances causes intercalated/alloyed lithium ions to be extracted from Anode 11. The extracted lithium atoms are split into lithium ions and electrons as they leave an intercalation/alloying host at an electrode-electrolyte interface. The lithium ions are carried through the micropores of separator 18 from Anode 11 to cathode 14 by the ionically conductive electrolyte 17 while, at the same time, the electrons are transmitted through the external circuit from Anode 11 to cathode 14 to balance the overall electrochemical cell. This flow of electrons through the external circuit can be harnessed and fed to a load device until the level of intercalated/alloyed lithium in the negative electrode falls below a workable level or the need for power ceases.
Battery cell 10 may be recharged after a partial or full discharge of its available capacity. To charge or re-power the lithium ion battery cell, an external power source (not shown) is connected to the positive and the negative electrodes to drive the reverse of battery discharge electrochemical reactions. That is, during charging, the external power source extracts the lithium ions present in cathode 14 to produce lithium ions and electrons. The lithium ions are carried back through the separator by the electrolyte solution, and the electrons are driven back through the external circuit, both towards Anode 11. The lithium ions and electrons are ultimately reunited at the negative electrode, thus replenishing it with intercalated/alloyed lithium for future battery cell discharge.
Lithium ion battery cell 10, or a battery module or pack comprising a plurality of battery cells 10 connected in series and/or in parallel, can be utilized to reversibly supply power and energy to an associated load device. Lithium ion batteries may also be used in various consumer electronic devices (e.g., laptop computers, cameras, and cellular/smart phones), military electronics (e.g., radios, mine detectors, and thermal weapons), aircrafts, and satellites, among others. Lithium ion batteries, modules, and packs may be incorporated in a vehicle such as a hybrid electric vehicle (HEV), a battery electric vehicle (BEV), a plug-in HEV, or an extended-range electric vehicle (EREV) to generate enough power and energy to operate one or more systems of the vehicle. For instance, the battery cells, modules, and packs may be used in combination with a gasoline or diesel internal combustion engine to propel the vehicle (such as in hybrid electric vehicles), or may be used alone to propel the vehicle (such as in battery powered vehicles).
Returning to
The microporous polymer separator 18 can comprise, in one embodiment, a polyolefin. The polyolefin can be a homopolymer (derived from a single monomer constituent) or a heteropolymer (derived from more than one monomer constituent), either linear or branched. If a heteropolymer derived from two monomer constituents is employed, the polyolefin can assume any copolymer chain arrangement including those of a block copolymer or a random copolymer. The same holds true if the polyolefin is a heteropolymer derived from more than two monomer constituents. In one embodiment, the polyolefin can be polyethylene (PE), polypropylene (PP), or a blend of PE and PP. The microporous polymer separator 18 may also comprise other polymers in addition to the polyolefin such as, but not limited to, polyethylene terephthalate (PET), polyvinylidene fluoride (PVdF), and or a polyamide (Nylon). Separator 18 can optionally be ceramic-coated with materials including one or more of ceramic type aluminum oxide (e.g., Al2O3), and lithiated zeolite-type oxides, among others. Lithiated zeolite-type oxides can enhance the safety and cycle life performance of lithium ion batteries, such as battery cell 10. Skilled artisans will undoubtedly know and understand the many available polymers and commercial products from which the microporous polymer separator 18 may be fabricated, as well as the many manufacturing methods that may be employed to produce the microporous polymer separator 18.
Active material 16 can include any lithium-based active material that can sufficiently undergo lithium intercalation and deintercalation while functioning as the positive terminal of battery cell 10. Active material 16 can also include a polymeric binder material to structurally hold the lithium-based active material together. The active material 16 can comprise lithium transition metal oxides (e.g., layered lithium transitional metal oxides) or chalcogen materials. Cathode current collector 15 can include aluminum or any other appropriate electrically conductive material known to skilled artisans, and can be formed in a foil or grid shape. Cathode current collector 15 can be treated (e.g., coated) with highly electrically conductive materials, including one or more of conductive carbon black, graphite, carbon nanotubes, carbon nanofiber, graphene, and vapor growth carbon fiber (VGCF), among others. The same highly electrically conductive materials can additionally or alternatively be dispersed within the host material 13.
Lithium transition metal oxides suitable for use as active material 16 can comprise one or more of spinel lithium manganese oxide (LiMn2O4), lithium cobalt oxide (LiCoO2), a nickel-manganese oxide spinel (Li(Ni0.5Mn0.5)O2), a layered nickel-manganese-cobalt oxide (having a general formula of xLi2MnO3.(1−x)LiMO2, where M is composed of any ratio of Ni, Mn and/or Co). A specific example of the layered nickel-manganese oxide spinel is xLi2MnO3.(1−x)Li(Ni1/3Mn1/3Co1/3)O2. Other suitable lithium-based active materials include Li(Ni1/3Mn1/3Co1/3)O2), LiNiO2, Lix+yMn2−yO4 (LMO, 0<x<1 and 0<y<0.1), or a lithium iron polyanion oxide, such as lithium iron phosphate (LiFePO4) or lithium iron fluorophosphate (Li2FePO4F). Other lithium-based active materials may also be utilized, such as LiNixM1−xO2 (M is composed of any ratio of Al, Co, and/or Mg), LiNi1−xCo1−yMnx+yO2 or LiMn1.5−xNi0.5−yMx+yO4 (M is composed of any ratio of Al, Ti, Cr, and/or Mg), stabilized lithium manganese oxide spinel (LixMn2−yMyO4, where M is composed of any ratio of Al, Ti, Cr, and/or Mg), lithium nickel cobalt aluminum oxide (e.g., LiNi0.8Co0.15Al0.05O2 or NCA), aluminum stabilized lithium manganese oxide spinel (LixMn2−xAlyO4), lithium vanadium oxide (LiV2O5), Li2MSiO4 (M is composed of any ratio of Co, Fe, and/or Mn), and any other high efficiency nickel-manganese-cobalt material (HE-NMC, NMC or LiNiMnCoO2). By “any ratio” it is meant that any element may be present in any amount. So, for example, M could be Al, with or without Co and/or Mg, or any other combination of the listed elements. In another example, anion substitutions may be made in the lattice of any example of the lithium transition metal based active material to stabilize the crystal structure. For example, any 0 atom may be substituted with an F atom.
Chalcogen-based active material can include one or more sulfur and/or one or more selenium materials, for example. Sulfur materials suitable for use as active material 16 can comprise sulfur carbon composite materials, S8, Li2S8, Li2S6, Li2S4, Li2S2, Li2S, SnS2, and combinations thereof. Another example of sulfur-based active material includes a sulfur-carbon composite. Selenium materials suitable for use as active material 16 can comprise elemental selenium, Li2Se, selenium sulfide alloys, SeS2, SnSexSy (e.g., SnSe0.5S0.5) and combinations thereof. The chalcogen-based active material of the positive electrode 22′ may be intermingled with the polymeric binder and the conductive filler. Suitable binders include polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), an ethylene propylene diene monomer (EPDM) rubber, carboxymethyl cellulose (CMC)), styrene-butadiene rubber (SBR), styrene-butadiene rubber carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA), cross-linked polyacrylic acid-polyethylenimine, polyimide, or any other suitable binder material known to skilled artisans. Other suitable binders include polyvinyl alcohol (PVA), sodium alginate, or other water-soluble binders. The polymeric binder structurally holds the chalcogen-based active material and the conductive filler together. An example of the conductive filler is a high surface area carbon, such as acetylene black or activated carbon. The conductive filler ensures electron conduction between the positive-side current collector 26 and the chalcogen-based active material. In an example, the positive electrode active material and the polymer binder may be encapsulated with carbon. In an example, the weight ratio of S and/or Se to C in the positive electrode 22′ ranges from 1:9 to 9:1.
The anode current collector 12 can include copper, nickel, copper-nickel alloys, or any other appropriate electrically conductive material known to skilled artisans. Anode current collector 12 can be treated (e.g., coated) with highly electrically conductive materials, including one or more of chromium, conductive carbon black, graphite, carbon nanotubes, carbon nanofiber, graphene, and vapor growth carbon fiber (VGCF), among others. The current collector surfaces can be roughened, and/or the current collector can be perforated, for example. Silicon has the highest known theoretical charge capacity for lithium, making it one of the most promising anode host materials 13 for rechargeable lithium-ion batteries. Accordingly, provided herein are electrodes comprising silicon-based host materials (13), and methods for fabricating the same.
The slurry 103 comprises a dry fraction and one or more solvents. The dry fraction comprises silicon particles, one or more polymeric binders, and carbon fibers. The amount of solvent utilized in the slurry is tailored to achieve a particular slurry viscosity and/or any other physical characteristic suitable for the application of the slurry 103 to the current collector. The slurry can comprise about 5 wt. % dry fraction to about 50 wt. % dry fraction. For example, the slurry can comprise about 25 wt. % to about 30 wt. % dry fraction and about 70 wt. % to about 75 wt. % solvent(s), in one embodiment. The solvent(s) can comprise any polar solvents, including N-Methyl-2-pyrrolidone (NMP), dimethyl formamide, dimethyl sulfoxide, propylene carbonate, ethylene carbonate, acetone, and methyl ethyl ketone, among others. In some embodiments, the slurry comprises 100 wt. % dry fraction and no solvent(s).
The silicon particles can comprise elemental silicon particles, period table Group V (e.g., P, As, Sb, Bi) n-type doped silicon particles, and/or Li2Si particles. Silicon particles can additionally or alternatively comprise SiOx, wherein generally x≤2. In some embodiments, for some SiOx particles, x≈1. For example, x can be about 0.9 to about 1.1, or about 0.99 to about 1.01. Within a body of SiOx particles, SiO2 and/or Si domains may further exist. In some embodiments, the silicon particles can be considered “single phase” and not include any added conductive carbon (e.g., graphite). In other words, in such embodiments, the carbon present in the host material 13 can consist essentially of carbon contributed by the carbon fibers, the carbon nanotubes, carbon present in the current collector, and carbon contributed by the pyrolyzed polymeric binders.
Utilizing Li2Si particles can help prevent volumetric expansion of silicon particles, and irreversible capture of lithium from the cathode 14 during initial cycling of a battery cell 10. The silicon particles can have an average particle diameter of less than about 10 μm, about 50 nm to about 10 μm, or about 3 μm to about 10 μm in some embodiments. In some embodiments, particularly battery cells 10 configured for fast charging, the silicon particles can have an average particle diameter of about 1 μm to about 3 μm, or about 0.5 μm to about 1 μm. The dry fraction can comprise at least about 70 wt. % silicon particles. In some embodiments, the dry fraction can comprise about 70 wt. % to about 95% silicon particles. In some embodiments, the dry fraction can comprise about 75 wt. % to about 85% silicon particles. In embodiments utilizing Li2Si particles, the amount of lithium atoms can substantially equal the amount of silicon atoms in the dry fraction. For example, the silicon particles can comprise about 45% to about 50% (by count) of the total of silicon and lithium atoms.
The one or more polymeric binders can comprise polyacrylonitrile (PAN), and/or one or more fluorinated polymers (e.g., polyvinylidiene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and perfluoroalkoxy alkanes (PFA)). In one embodiment the polymeric binder can consist of PAN. The polymeric binder can comprise up to about 10 wt. % or about 5 wt. % to about 10 wt. % of the dry fraction. As will be described below, the amount of polymeric binder in the dry fraction can be tuned to achieve desirable caging of silicon particles by pyrolyzed polymeric binder.
The carbon fibers can have an average diameter of at least about 50 nm, about 50 nm to about 200 nm, or about 100 nm to about 200 nm. The carbon fibers can have an average length of at least about 1 μm, or about 1 μm to about 20 μm. The carbon fibers provide stiffness and mechanical integrity to the host material 13 while being electrically conductive. The dry fraction can optionally further include carbon nanotubes. Carbon nanotubes can have an average diameter of about 20 nm to about 50 nm and an average length of about 1 μm to about 2 μm, for example. Carbon nanotubes are very flexible and provide minimal strength relative to carbon fibers but increase electrical connections between individual silicon particles and between silicon particles and the current collector 12. In some embodiments, the weight ratio of carbon fibers to carbon nanotubes can be about 50:1 to about 4:1. The carbon fibers (and optionally the carbon nanotubes, collectively with the carbon fibers) can comprise up to about 15 wt. %, or about 2 wt. % to about 15 wt. % of the dry fraction. The upper limit of the amount of carbon fibers (and optionally the carbon nanotubes, collectively with the carbon fibers) can be defined by the amount of polymeric binder required to maintain the structural integrity of the resulting host material 13.
Subsequent to coating 101 and prior to pyrolyzing 110, the coated current collector 102 can be dried 105. Drying 105 substantially removes the solvent(s) from the slurry, generally by evaporation, and accordingly suitable solvents can be considered volatile organic compounds. Drying 105 can occur at temperatures below about 100° C. or below about 200° C., and can occur in an open air (e.g., non-inert) environment. In some embodiments, drying can occur at higher temperatures (e.g., up to about 500° C.), and utilize significantly shorter drying times relative to lower temperature (e.g., 100° C.) drying methods. Drying prevents the slurry solvent(s) from introducing oxides during pyrolyzing. Pyrolyzing 110 preferably occurs in an environment substantially free of oxygenated gases (e.g., O2, CO, CO2, etc.), or an “inert atmosphere”. An inert atmosphere can comprise an N2, Ar, and/or He atmosphere, or a vacuum, for example.
Pyrolyzing 110 can comprise heating at a first temperature, and subsequently heating at a second temperature wherein the first temperature is higher than the first temperature.
Excessive pyrolysis temperatures can create a brittle host material 13 which is not mechanically robust. Accordingly, the first temperature can be up to about 400° C., or about 250° C. to about 400° C., and the second temperature can be less than about 750° C. and higher than the first temperature, about 450° C. to about 750° C., about 500° C. to about 750° C., or about 700° C. to about 750° C. Pyrolyzing 110 at the first temperature can occur for about 1 hour, or about 0.25 hours to about 2 hours. Pyrolyzing at the second temperature can occur for about 1 hour, or about 0.25 hours to about 2 hours. The pyrolyzing duration can be tuned to the thickness T of the host material layer 13, wherein a thinner host material 13 layer generally requires a shorter pyrolyzing duration. In some embodiments, the thickness T of the silicon-based host material 13 layer is about 20 μm to about 50 μm, or up to about 50 μm.
Multiple electrodes were formed by applying a slurry to a copper current collector with a dry fraction comprising 73.4 wt. % Si particles, 2.8 wt. % carbon nanotubes, 7.3 wt. % carbon nanofibers, 13.8 wt. % PVDF, and 2.7 wt. % PAN. One such coated current collector was pyrolyzed for 1 hour in an inert gas atmosphere (Ar or N2) at a temperature greater than 750° C. After pyrolyzing, the current collector had degraded due to the substantially complete conversion of copper to copper silicide. Another such coated current collector was pyrolyzed for 1 hour in an inert gas atmosphere (Ar or N2) at a temperature greater than 800° C. After pyrolyzing, the current collector was similarly degraded due to the substantially complete conversion of copper to copper silicide, and the electrode exhibited no mechanical strength and crumbled under nominal mechanical impact.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.
