Silicon-dominated battery electrode

Abstract

A method of forming a composite film may include providing a mixture including a carbon precursor and silicon particles. The method may also include pyrolyzing the carbon precursor to convert the precursor to one or more types of carbon phases to form the composite film, such that the precursor has a char yield of greater than about 0% to about 60%, and the composite film includes about 90% to about 99% by weight of the silicon particles.

Description

[0001] This application is a divisional application of Chinese patent application No. 202080040973.1. Technical Field

[0002] The present application generally relates to silicon-dominated battery electrodes. Specifically, the present application relates to composite materials for battery electrodes, the composite materials comprising greater than 50 weight percent silicon particles, and in some cases 90 weight percent or more silicon particles. Background Art

[0003] Lithium-ion batteries typically include a separator and / or electrolyte between the anode and cathode. In one type of battery, the separator, cathode, and anode materials are formed into sheets or films, respectively. The sheets of cathode, separator, and anode are stacked or rolled in sequence so that the separator separates the cathode from the anode (e.g., electrode) to form a battery. Typical electrodes include layers of electrochemically active materials on conductive metals (e.g., aluminum and copper). The membrane can be rolled or cut into components, which are then stacked into a stack. The stack has alternating electrochemically active materials and separators between them. Summary of the invention

[0004] In certain embodiments, a method for forming a composite film is provided. The method may include providing a mixture comprising a carbon precursor and silicon particles. The method may also include pyrolyzing the carbon precursor to convert the precursor into one or more types of carbon phases to form the composite film, so that the precursor has a carbonization rate greater than about 0% to about 60%, and the composite film comprises about 90% to about 99% by weight of silicon particles. For example, the composite film may comprise about 95% to about 99% by weight of silicon particles.

[0005] In some cases, the carbon precursor may include polyacrylonitrile (PAN). In some cases, the carbon precursor may include cellulose, glucose, sucrose, lignin, dextran or a combination thereof. In some cases, the carbon precursor may include polyimide, phenol formaldehyde resin or a combination thereof. In some cases, the carbon precursor may include polyamic acid. For example, the carbon precursor may include dianhydride and / or diamine. In some such examples, the carbon precursor may include pyromellitic dianhydride oxidized aniline (PMDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA) or a combination thereof.

[0006] In some cases, mixture can also include the solvent comprising N-Methylpyrrolidone (NMP).In some cases, mixture can also include aprotic solvent.For example, aprotic solvent can include any one or mixture in dimethylformamide (DMF), dimethoxymethamphetamine (DMMA), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), sulfolane, ethylene carbonate or its combination.

[0007] In some cases, the mixture may further include an inorganic salt. For example, the inorganic salt may include lithium bromide, sodium thiocyanate, zinc chloride, or a combination thereof. In some cases, the mixture may further include sulfuric acid, nitric acid, or a combination thereof.

[0008] In some embodiments, the method may further include coating the mixture on a substrate to form a green film. The method may further include removing the green film from the substrate before pyrolyzing the carbon precursor. In some instances, the substrate may include polyethylene terephthalate (PET), cyclic olefin copolymer (COC), or a combination thereof. In some instances, pyrolysis may include pyrolyzing the green film on the substrate. The substrate may include a polymer having a carbonization rate of about 0% to about 5%. For example, the substrate may include acetal, polypropylene, polyethylene, polystyrene, or a combination thereof.

[0009] In some embodiments, the method may further include oxidizing the mixture prior to pyrolyzing. In some examples, pyrolyzing may include heating the mixture at a temperature of about 350° C. to about 1350° C. In some cases, pyrolysis may form a composite film that is a self-supporting structure.

[0010] In certain embodiments, a composite film is provided. The film may include about 90% to about 99% by weight of silicon particles. The film may also include one or more types of carbon phases greater than 0% by weight and less than or equal to about 10% by weight. At least one of the one or more types of carbon phases may include hard carbon as a matrix phase, which holds the composite film together so that the silicon particles are distributed throughout the composite film. In some instances, the composite film may include about 95% to about 99% by weight of silicon particles of the composite film.

[0011] In some embodiments, the silicon particles may have an average particle size of about 10 nm to about 40 μm. In some cases, the hard carbon may include glassy carbon. Some films may also include a silicon carbide layer between the silicon particles and the hard carbon. In some instances, the matrix phase may be a substantially continuous phase. In some cases, the silicon particles may be evenly distributed throughout the hard carbon. The composite film may be self-supporting.

[0012] In some embodiments, at least one of the one or more types of carbon phases may be electrochemically active and electrically conductive. The one or more types of carbon phases may also include graphite particles. The composite film may be substantially electrochemically active.

[0013] In certain embodiments, a battery electrode is provided. The electrode may be an anode. The composite film may be self-supporting. In some examples, the electrode may further include a current collector. The electrode may further include a polymer binder between the composite film and the current collector.

[0014] In some embodiments, a battery is provided. The battery may include an anode, a cathode and an electrolyte containing a composite film. The battery may be a lithium ion battery. In some instances, the cathode may include nickel cobalt manganese (NCM), lithium cobalt oxide (LCO), nickel cobalt aluminum oxide (NCaO), lithium manganese oxide (LMO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO) or lithium iron phosphate (LFP). In some cases, the electrolyte may be in a liquid state. In some cases, the electrolyte may be in a solid state. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1A An embodiment of a method of forming a composite material is illustrated, comprising forming a mixture comprising a precursor, casting the mixture, drying the mixture, curing the mixture, and pyrolyzing the precursor;

[0016] Figure 1B is a schematic illustration of the formation of silicon carbide on silicon particles;

[0017] Figure 2A and Figure 2B is a SEM micrograph of one embodiment of micron-sized silicon particles ground from larger silicon particles;

[0018] Figure 2C and Figure 2D is a SEM micrograph of one embodiment of a micrometer-sized silicon particle having nanometer-sized features on the surface;

[0019] Figure 3 Illustrative embodiments of methods of forming composite materials are illustrated;

[0020] Figure 4 is a graph of discharge capacity at an average rate of C / 2.6;

[0021] Figure 5 is a graph of discharge capacity at an average rate of C / 3;

[0022] Figure 6 is a graph of discharge capacity at an average rate of C / 3.3;

[0023] Figure 7 is a graph of discharge capacity at an average rate of C / 5;

[0024] Figure 8 is a graph of discharge capacity at an average rate of C / 9;

[0025] Fig. 9 is a graph of discharge capacity;

[0026] Fig.10 is a graph of discharge capacity at an average rate of C / 9;

[0027] Fig.11A and Fig. 11B is a graph of reversible capacity and irreversible capacity as a function of various weight percentages of PI-derived carbon and graphite particles from 2611c for a fixed percentage of 20 wt % Si;

[0028] Fig.12 is a graph of the first cycle discharge capacity as a function of the weight percentage of carbon;

[0029] Fig.13 is a graph of the reversible (discharge) capacity and the irreversible capacity as a function of the pyrolysis temperature;

[0030] Fig.14 It is a photograph of a 4.3 cm × 4.3 cm composite anodic film without a metal foil support layer;

[0031] Fig.15 is a scanning electron microscope (SEM) micrograph of the composite anode membrane before cycling (the defocused part is the bottom part of the anode, while the focused part is the cracked edge of the composite membrane);

[0032] Fig.16 is another SEM micrograph of the composite anodic film before cycling;

[0033] Fig.17 is the SEM micrograph of the composite anodic film after 10 cycles;

[0034] Fig.18 is another SEM micrograph of the composite anodic film after 10 cycles;

[0035] Fig.19 is the SEM micrograph of the composite anodic film after 300 cycles;

[0036] Fig. 20 Includes SEM micrographs of cross sections of composite anodic membranes;

[0037] Fig.21 is the X-ray powder diffraction (XRD) pattern of the sample silicon particles;

[0038] Fig. 22 is a SEM micrograph of one embodiment of a silicon particle;

[0039] Fig.23 is another SEM micrograph of one embodiment of a silicon particle;

[0040] Fig.24 is a SEM micrograph of one embodiment of a silicon particle;

[0041] Fig.25 is a SEM micrograph of one embodiment of a silicon particle;

[0042] Fig.26 It is the chemical analysis of the sample silicon particles;

[0043] Fig.27A and Fig.27B is an exemplary particle size histogram of two types of micron-sized silicon particles with nanometer-sized features;

[0044] Fig.28 is a graph comparing the discharge capacity of two types of exemplary silicon particles during battery cycling;

[0045] Fig.29 The stabilization and carbonization rate of polyacrylonitrile under different heat treatment conditions are shown;

[0046] Fig.30 A graph showing capacity versus cycle number for a battery having an exemplary silicon-dominated anode;

[0047] Fig.31 a graph showing capacity retention versus cycle number for a battery having an exemplary silicon-dominated anode; and

[0048] Fig.32 Shown is a graph of cell resistance versus cycle number for a cell having an exemplary silicon-dominated anode. DETAILED DESCRIPTION

[0049] Certain embodiments include silicon electrodes (e.g., anodes and / or cathodes) that include silicon or a composite material containing silicon for battery applications (e.g., lithium ion battery applications). Silicon is considered to be a host material for lithium ion batteries with potentially high energy per unit volume. Batteries with silicon anodes exhibit faster capacity loss when cycled than batteries with graphite anodes. Repeated expansion and contraction of silicon particles during charge and discharge can lead to mechanical failure of the anode during cycling.

[0050] Silicon particles (nano- and micro-sized) can be dispersed in a slurry containing a carbon precursor polymer as an adhesive and some solvents. These slurries are coated on a suitable substrate, dried, and peeled off from the substrate. Heat treatment of raw electrodes without substrates in an inert or reducing atmosphere can produce an electrode film with up to 90% by weight of silicon. Such methods can produce electrodes containing up to 90% by weight of silicon particles, which are held together by a carbon network providing a conductive path. In some cases, these electrodes can be attached to a current collector coated with a polymer adhesive with or without heat treatment.

[0051] According to certain embodiments described herein, for example, silicon-dominated electrodes with 90 wt% or more silicon particles can be produced using low-charring polymers such as polyacrylonitrile (PAN) as binders / carbon precursors. Low-charring polymers can produce small amounts of carbon, allowing for large amounts of silicon in the composite. These heat-treated silicon composites exhibit low cell resistance and high capacity retention when cycled for more than 150 cycles.

[0052] In addition, in some embodiments, the oxidation process parameters (e.g., temperature, time, and air / oxygen flow) can be adjusted to control the oxidation level. According to the oxidation and pyrolysis process, the carbonization rate of the polymer precursor can be controlled, thereby controlling the final Si weight % in the silicon-carbon composite electrode.

[0053] A typical carbon anode electrode includes a current collector such as a copper sheet. Carbon is deposited on a collector together with an inactive binder material. Carbon is commonly used because carbon has excellent electrochemical properties and is also conductive. If the current collector layer (e.g., copper layer) is removed, the carbon may not be able to mechanically support itself. Therefore, conventional electrodes require a supporting structure such as a collector to be able to act as an electrode. The electrode (e.g., anode or cathode) combination described in this application can produce a self-supporting electrode. Since conductive carbonized polymers are used for current collection in the anode structure and for mechanical support, the need for metal foil current collectors is eliminated or minimized. In typical applications in the mobile industry, metal current collectors are usually added to ensure sufficient rate performance. In contrast to the particulate carbon suspended in a non-conductive binder in a class of conventional lithium-ion battery electrodes, carbonized polymers can form a substantially continuous conductive carbon phase throughout the electrode. Advantages of using carbon composite blends of carbonized polymers may include, for example, 1) higher capacity, 2) enhanced overcharge / discharge protection, 3) lower irreversible capacity due to elimination (or minimization) of metal foil current collectors, and 4) potential cost savings due to simpler manufacturing.

[0054] The anode electrode currently used for rechargeable lithium-ion batteries generally has a specific capacity of about 200 milliampere hours per gram (including metal foil current collector, conductive additives, and adhesive materials). Graphite, the active material used for most lithium-ion battery anodes, has a theoretical energy density of 372 milliampere hours per gram (mAh / g). In contrast, silicon has a high theoretical capacity of 4200 mAh / g. In order to increase the volume energy density and weight energy density of lithium-ion batteries, silicon can be used as an active material for cathodes or anodes. Various types of silicon materials (e.g., silicon nanopowders, silicon nanowires, porous silicon, and ball-milled silicon) have also been reported as viable candidates for active materials for negative or positive electrodes. Small particle sizes (e.g., nanometer-range sizes) can generally increase cycle life performance. They can also exhibit very high irreversible capacity. However, small particle sizes may also result in very low volume energy densities (e.g., for the entire battery stack) due to difficulty in wrapping active materials. Larger particle sizes (e.g., micrometer or micron-range sizes) can generally result in higher density anode materials. However, the expansion of silicon active materials can lead to poor cycle life due to particle fracture. For example, upon lithium insertion, silicon can expand by more than 300%. Due to this expansion, anodes containing silicon should allow for expansion while maintaining electrical contact between silicon particles.

[0055] As described herein and in U.S. Patent Application No. 13 / 008,800 (U.S. Patent No. 9,178,208) and U.S. Patent Application No. 13 / 601,976 (U.S. Patent Application Publication No. 2014 / 0170498) entitled "Composite Materials for Electrochemical Storage" and "Silicon Particles for Battery Electrodes", respectively (the entire contents of which are hereby incorporated by reference), certain embodiments employ methods of producing a monolithic self-supporting anode using a carbonized polymer. Because the polymer is converted into a conductive and electrochemically active matrix, the resulting electrode is sufficiently conductive so that a metal foil or mesh current collector can be omitted or minimized. The converted polymer also serves as an expansion buffer for silicon particles during cycling, so that a high cycle life can be achieved. In certain embodiments, the resulting electrode is an electrode consisting essentially of an active material. In other embodiments, the resulting electrode is essentially an active material. The electrode can have a high energy density of about 500 mAh / g to about 3500 mAh / g, which can be attributed to, for example, 1) the use of silicon, 2) the elimination or substantial reduction of metal current collectors, and 3) being composed entirely or substantially entirely of active materials.

[0056] As described in U.S. Patent Application No. 14 / 821,586 (U.S. Patent Application Publication No. 2017 / 0040598), entitled "Surface Modification of Silicon Particles for Electrochemical Storage" (the entire contents of which are hereby incorporated by reference), in certain embodiments, the carbonized polymer can react with the native silicon oxide surface layer on the silicon particles. In some embodiments, the surface of the particles is modified to form a surface coating thereon, which can further serve as an expansion buffer for the silicon particles during cycling. The surface coating can include silicon carbide.

[0057] The composite materials described herein can be used as anodes in most conventional lithium-ion batteries; they can also be used as cathodes in some electrochemical couples together with additional additives. Composite materials can also be used in secondary batteries (e.g., rechargeable) or primary batteries (e.g., non-rechargeable). In certain embodiments, the composite material is a self-supporting structure. In other embodiments, the composite material is a self-supporting monolithic structure. For example, a collector can be included in an electrode composed of the composite material. In certain embodiments, the composite material can be used to form a carbon structure discussed in U.S. Patent Application No. 12 / 838,368 (U.S. Patent Application Publication No. 2011 / 0020701) entitled "Carbon ElectrodeStructures for Batteries" (the entire contents of which are hereby incorporated by reference). In addition, the composite materials described herein can be, for example, silicon composite materials, carbon composite materials, and / or silicon-carbon composite materials. As described in U.S. Patent Application No. 13 / 799,405 (U.S. Patent No. 9,553,303), entitled "Silicon Particles for Battery Electrodes" (the entire contents of which are hereby incorporated by reference), certain embodiments may further include composite materials comprising micron-sized silicon particles. For example, in some embodiments, the micron-sized silicon particles have nano-sized features on the surface. Silicon particles with such geometric structures can have both the benefits of micron-sized silicon particles (e.g., high energy density) and the benefits of nano-sized silicon particles (e.g., good cycling behavior). As used herein, the term "silicon particles" can generally include micron-sized silicon particles with or without nano-sized features.

[0058] Some composite materials can be provided on the current collector. In some embodiments, composite materials can be attached to the current collector using an attachment material. Attachment material and current collector can be any of those known in the art or yet to be developed. For example, some composite materials can be provided on the current collector, as in U.S. Patent No. 13 / 333,864 (U.S. Patent No. 9,397,338) entitled "Electrodes, Electrochemical Cells, and Methods of Forming Electrodes and Electrochemical Cells" or U.S. Patent No. 13 / 796,922 (U.S. Patent No. 9,583,757) entitled "Electrodes, Electrochemical Cells, and Methods of Forming Electrodes and Electrochemical Cells", described in each of them incorporated herein by reference. Some anodes can be formed on a current collector, for example, as described in U.S. Patent Application No. 15 / 471,860, entitled “Methods of Forming Carbon-Silicon Composite Material on a Current Collector” (U.S. Patent Application Publication No. 2018 / 0287129), which is incorporated herein by reference.

[0059] Figure 1A An embodiment of a method 100 for forming a composite material is illustrated. For example, the method for forming a composite material may include forming a mixture containing a precursor, box 101. The method may also include pyrolyzing the precursor to convert the precursor into a carbon phase. The precursor mixture may include a carbon additive, such as a graphite active material, chopped or ground carbon fiber, carbon nanofiber, carbon nanotube and / or other carbon. After the precursor is pyrolyzed, the resulting carbon material may be a self-supporting monolithic structure. In certain embodiments, one or more materials are added to the mixture to form a composite material. For example, silicon particles may be added to the mixture. The carbonized precursor produces an electrochemically active structure that holds the composite material together. For example, the carbonized precursor may be a substantially continuous phase. Silicon particles including micron-sized silicon particles with or without nano-sized features may be distributed throughout the composite material. Advantageously, the carbonized precursor may be a structural material as well as an electrochemically active and conductive material. In certain embodiments, the material particles added to the mixture are uniformly or substantially uniformly distributed throughout the composite material to form a uniform or substantially uniform composite.

[0060] The mixture may include a variety of different components. The mixture may include one or more precursors. In certain embodiments, the precursor is a hydrocarbon compound. For example, the precursor may include polyacrylonitrile (PAN), a homopolymer or copolymer mixture of a monomer having polyacrylonitrile as a primary monomer. As other examples, the precursor may include cellulose, glucose, sucrose, lignin, dextran, or a combination thereof. As other examples, the precursor may include one or more polyamide-imide, polyamic acid, polyimide, etc. In some cases, the precursor may include dianhydride and / or diamine. For example, the precursor may include pyromellitic dianhydride oxidized aniline (PMDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA), or a combination thereof. Such monomers (e.g., PMDA-ODA, BPDA-PDA, PMDA-PDA, etc.) may be converted into polyamic acid by polycondensation. Polyamic acid may be imidized to form polyimide during thermal curing that may or may not include oxygen. Other precursors from which polyamic acid can be derived (eg, by reaction between a dianhydride and a diamine / diisocyanate) can also be used. Other precursors can include phenolic resins (eg, phenol formaldehyde resins), epoxy resins, and / or other polymers.

[0061] The mixture can also include a solvent. For example, the solvent can be N-methyl-pyrrolidone (NMP). As another example, an aprotic solvent such as any one of dimethylformamide (DMF), dimethoxymethamphetamine (DMMA), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), sulfolane, ethylene carbonate or a mixture thereof can be used to dissolve the precursor, for example, to dissolve PAN. As another example, an inorganic salt such as an aqueous solution of lithium bromide, sodium thiocyanate and / or zinc chloride can be used to dissolve the precursor, for example, to dissolve PAN. In some cases, the aqueous solution can be concentrated, for example, to about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, or to a range formed by any of these values ​​(e.g., concentrated to about 10 wt.% to about 30 wt.%, about 10 wt.% to about 40 wt.%, about 20 wt.% to 30 wt.%, about 20 wt.% to about 40 wt.%, etc.). As another example, an acid, such as sulfuric acid and / or nitric acid, can be used to dissolve the precursor, for example, to dissolve PAN. In some cases, the acid can be concentrated, for example, concentrated to about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, or concentrated to a range formed by any of these values ​​(e.g., concentrated to about 10 wt.% to about 30 wt.%, about 10 wt.% to about 40 wt.%, about 20 wt.% to 30 wt.%, about 20 wt.% to about 40 wt.%, etc.). Other possible solvents include acetone, ether, γ-butyrolactone, isopropanol, dimethyl carbonate, ethyl carbonate, dimethoxyethane, ethanol, methanol, etc. Examples of precursors and solvent solutions include PI-2611 (HD Microsystems), PI-5878G (HD Microsystems), and VTEC PI-1388 (RBI, Inc.). PI-2611 is composed of> 60% of normal methyl-2-pyrrolidone and 10% to 30% of s-biphenyl dianhydride / p-phenylenediamine. PI-5878G is composed of> 60% of normal methyl pyrrolidone, 10% to 30% of the polyamic acid of benzene tetracarboxylic anhydride / diaminodiphenyl ether, and 10% to 30% of aromatic hydrocarbons (petroleum distillates) containing 5% to 10% of 1,2,4-trimethylbenzene. In certain embodiments, the amount of the precursor in the solvent is about 10wt.% to about 30wt.%. Additional materials may also be included in the mixture. For example, as previously discussed, silicon particles or carbon particles may be added to the mixture, and the carbon particles include graphite active materials, chopped or ground carbon fibers, carbon nanofibers, carbon nanotubes, graphite, and other conductive carbons. In addition, the mixture may be mixed to homogenize the mixture.

[0062] In certain embodiments, the mixture is cast on a substrate. Figure 1A In the box 102. In some embodiments, the casting includes using gap extrusion, belt casting or blade casting technology. The blade casting technology may include applying the coating to the substrate by using a flat surface (e.g., a blade) controlled at a certain distance above the substrate. The liquid or slurry may be applied to the substrate, and the blade may be passed over the liquid to spread the liquid on the substrate. Since the liquid passes through the gap between the blade and the substrate, the thickness of the coating may be controlled by the gap between the blade and the substrate. As the liquid passes through the gap, the excess liquid may also be scraped off. For example, the mixture may be cast on a substrate including a polymer sheet, a polymer roller and / or a foil or roller made of glass or metal. The mixture may then be dried to remove the solvent, box 103. For example, the polyamic acid and NMP solution may be dried at about 110° C. for about 2 hours to remove the NMP solution. The dried mixture coated on the substrate may form a green film. As described herein, in some embodiments, the green film may be retained on the substrate to undergo the next step (e.g., pyrolysis). However, in other embodiments, the green film may be removed from the substrate. For example, the aluminum substrate may be etched away with HCl. Alternatively, the dried mixture can be removed by peeling the dried mixture from the substrate or otherwise mechanically removing the dried mixture from the substrate. In some embodiments, the substrate comprises polyethylene terephthalate (PET), including, for example In some embodiments, the substrate may include a cyclic olefin copolymer (COC). The substrate is not particularly limited. For example, any substrate that can withstand coating conditions (e.g., the temperature and type of solvent used) can be used. In certain embodiments, the dried mixture is a film or sheet. In some embodiments, the dried mixture is optionally cured, block 104. In some embodiments, the dried mixture can be further dried. For example, the dried mixture can be placed in a hot press (e.g., between graphite plates in an oven). The hot press can be used to further dry and / or cure and keep the dried mixture flat. For example, the dried mixture from polyamic acid and NMP solution can be hot pressed for about 8 hours to 16 hours at about 200°C. Alternatively, the entire process including casting and drying can be completed with a roll-to-roll process using standard film processing equipment. The dried mixture can be rinsed to remove any solvent or etchant that may remain. For example, deionized (DI) water can be used to rinse the dried mixture. In certain embodiments, a tape casting technique can be used for casting. In some embodiments, the mixture can be coated on a substrate by a slot die coating process (e.g., metering a constant or substantially constant weight and / or volume through a set or substantially set gap). In some other embodiments, there is no substrate for casting, and the anodic film does not need to be removed from any substrate. The dried mixture can be cut or mechanically divided into smaller parts.

[0063] The mixture with or without a substrate can further undergo pyrolysis to convert the polymer precursor into carbon, block 105. In certain embodiments, the mixture is pyrolyzed in a reducing atmosphere. For example, an inert atmosphere, vacuum and / or flowing argon, nitrogen or helium can be used. In some embodiments, the mixture is heated to a temperature of about 300°C to about 1350°C. For example, the mixture can be heated to about 300°C to about 1300°C, about 350°C to about 1300°C, about 400°C to about 1300°C, about 450°C to about 1300°C, about 500°C to about 1300°C, about 350°C to about 1350°C, about 400°C to about 1350°C, about 450°C to about 1350°C, about 500°C to about 1350°C, about 700°C to about 1350°C, about 900°C to about 1350°C, etc. In some cases, the mixture containing PAN can be heated to about 350°C to about 1350°C. In some cases, the mixture containing polyamideimide can be heated to about 400°C (e.g., about 420°C) to about 1350°C. In some cases, the mixture containing polyimide (PI) can be heated to about 500°C to about 1350°C. Other examples are possible. For example, a polyimide formed by polyamic acid can be carbonized at about 1175°C for about one hour. In certain embodiments, the heating rate and / or cooling rate of the mixture is about 10°C / min. A retainer can be used to hold the mixture in a specific geometry. The retainer can be graphite, metal, etc. In certain embodiments, the mixture is held flat. After the mixture is pyrolyzed, a tab can be attached to the pyrolyzed material to form an electrical contact. For example, nickel, copper, or an alloy thereof can be used for the tab.

[0064] In certain embodiments, one or more of the methods described herein can be performed in a continuous process. In certain embodiments, casting, drying, possible curing and pyrolysis can be performed in a continuous process. For example, the mixture can be applied to a glass or metal drum. The mixture can be dried while rotating on the drum to produce a film. The film can be transferred or peeled off as a roll and fed into another machine for other processing. Before the pyrolysis step, extrusion and other film manufacturing techniques known in the industry can also be used.

[0065] The pyrolysis of the precursor produces a carbon material (e.g., at least one carbon phase). In certain embodiments, the carbon material is hard carbon. In some embodiments, the precursor is any material that can be pyrolyzed to form hard carbon. When the mixture includes one or more additional materials or phases other than the carbonized precursor, a composite material can be produced. In particular, the mixture can include silicon particles that produce a silicon-carbon composite material (e.g., at least one first phase comprising silicon and at least one second phase comprising carbon) or a silicon-carbon-carbon composite material (e.g., at least one first phase comprising silicon, at least one second phase comprising carbon and at least one third phase comprising carbon).

[0066] Silicon particles can increase the lithium insertion capacity (specific lithium insertion capacity) of composite materials. When silicon absorbs lithium ions, it experiences a significant volume increase with the order of 300+ volume percent, which can cause electrode structural integrity problems. In addition to volume expansion related issues, silicon is not inherently conductive, but becomes conductive when it forms an alloy with lithium (e.g., lithiation). When silicon delithiates, the surface of silicon loses conductivity. In addition, when silicon delithiates, the volume decreases, which causes the possibility that silicon particles lose contact with the matrix. The significant change in volume also causes the mechanical failure of the silicon particle structure, which in turn causes it to be crushed. Crushing and losing electrical contact make it challenging to use silicon as an active material in lithium ion batteries. The reduction of the initial size of silicon particles can prevent further crushing of silicon powder, and minimize the loss of surface conductivity. In addition, adding a material that can be elastically deformed as the volume of silicon particles changes to the composite can reduce the chance of loss of electrical contact with the silicon surface. For example, the composite material may include carbon, such as graphite, which contributes to the composite's ability to absorb expansion and also to intercalate lithium ions to increase the storage capacity of the electrode (eg, chemically active).Thus, the composite material may include one or more types of carbon phases.

[0067] The shape of the silicon particles is not particularly limited. For example, the silicon particles can be spherical, wedge-shaped, irregularly shaped, or a combination thereof. The silicon particles can be untreated or can be surface-modified to promote adhesion to the carbon precursor.

[0068] In some embodiments, the particle size (e.g., the diameter or maximum dimension of the silicon particles) can be less than about 50 μm, less than about 40 μm, less than about 30 μm, less than about 20 μm, less than about 10 μm, less than about 1 μm, about 10 nm to about 50 μm, about 10 nm to about 40 μm, about 10 nm to about 30 μm, about 10 nm to about 20 μm, about 0.1 μm to about 20 μm, about 0.5 μm to about 20 μm, about 1 μm to about 20 μm, about 1 μm to about 15 μm, about 1 μm to about 10 μm, about 10 nm to about 10 μm, about 10 nm to about 10 μm, about 10 nm to about 1 μm, less than about 500 nm, less than about 100 nm, about 100 nm, etc. All, substantially all, or at least some of the silicon particles can include the particle sizes (e.g., diameters or maximum dimensions) described above. For example, the silicon particles may have an average particle size (e.g., average diameter or average maximum dimension) or a median particle size (or median diameter or median maximum dimension) of less than about 50 μm, less than about 40 μm, less than about 30 μm, less than about 20 μm, less than about 10 μm, less than about 1 μm, about 10 nm to about 50 μm, about 10 nm to about 40 μm, about 10 nm to about 30 μm, about 10 nm to about 20 μm, about 0.1 μm to about 20 μm, about 0.5 μm to about 20 μm, about 1 μm to about 20 μm, about 1 μm to about 15 μm, about 1 μm to about 10 μm, about 10 nm to about 10 μm, about 10 nm to about 10 μm, about 10 nm to about 1 μm, less than about 500 nm, less than about 100 nm, about 100 nm, etc. In some embodiments, the silicon particles may have a distribution of particle sizes. For example, at least about 95%, at least about 90%, at least about 85%, at least about 80%, at least about 70%, or at least about 60% of the particles can have a particle size described herein.

[0069] The amount of silicon provided in the mixture or in the composite material can be greater than 0% by weight of the mixture and / or composite material. In certain embodiments, the amount of silicon can be from about 0% to about 99% by weight of the composite material, including greater than about 0% to about 99% by weight, greater than about 0% to about 95% by weight, greater than about 0% to about 90% by weight, greater than about 0% to about 35% by weight, greater than about 0% to about 25% by weight, about 10% to about 35% by weight, at least about 30% by weight, about 30% to about 99% by weight, about 30% to about 95% by weight, about 30% to about 90% by weight, about 30% to about 80% by weight, at least about 50 wt %, about 50 wt % to about 99 wt %, about 50 wt % to about 95 wt %, about 50 wt % to about 90 wt %, about 50 wt % to about 80 wt %, about 50 wt % to about 70 wt %, at least about 60 wt %, about 60 wt % to about 99 wt %, about 60 wt % to about 95 wt %, about 60 wt % to about 90 wt %, about 60 wt % to about 80 wt %, at least about 70 wt %, about 70 wt % to about 99 wt %, about 70 wt % to about 95 wt %, about 70 wt % to about 90 wt %, and the like. In various embodiments described herein, the amount of silicon can be 90 wt % or greater, for example, about 90 wt % or greater to about 95 wt %, about 90 wt % or greater to about 97 wt %, about 90 wt % or greater to about 99 wt %, about 92 wt % or greater to about 99 wt %, about 95 wt % or greater to about 99 wt %, about 97 wt % or greater to about 99 wt %, and the like.

[0070] In addition, the silicon particles may or may not be pure silicon. For example, the silicon particles may be substantially silicon, or may be a silicon alloy. In one embodiment, the silicon alloy comprises silicon as a major component, together with one or more other elements.

[0071] As described herein, micron-sized silicon particles can provide good volumetric energy density and weight energy density, together with good cycle life. In certain embodiments, in order to obtain the benefits of micron-sized silicon particles (e.g., high energy density) and the benefits of nano-sized silicon particles (e.g., good cycle behavior), the silicon particles can have an average particle size or median particle size in the micron range and a surface including nano-sized features. In some embodiments, the silicon particles can have an average particle size (e.g., average diameter or average maximum size) or median particle size (e.g., median diameter or median maximum diameter) of all values ​​from about 0.1 μm to about 30 μm or from about 0.1 μm to up to about 30 μm. For example, the silicon particles may have an average or median particle size of about 0.1 μm to about 20 μm, about 0.5 μm to about 25 μm, about 0.5 μm to about 20 μm, about 0.5 μm to about 15 μm, about 0.5 μm to about 10 μm, about 0.5 μm to about 5 μm, about 0.5 μm to about 2 μm, about 1 μm to about 20 μm, about 1 μm to about 15 μm, about 1 μm to about 10 μm, about 5 μm to about 20 μm, etc. Thus, the average or median particle size may be any value from about 0.1 μm to about 30 μm, for example, 0.1 μm, 0.5 μm, 1 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 15 μm, 20 μm, 25 μm, and 30 μm.

[0072] Nano-sized features may include an average feature size (e.g., average diameter or average maximum dimension) of about 1 nm to about 1 μm, about 1 nm to about 750 nm, about 1 nm to about 500 nm, about 1 nm to about 250 nm, about 1 nm to about 100 nm, about 10 nm to about 500 nm, about 10 nm to about 250 nm, about 10 nm to about 100 nm, about 10 nm to about 75 nm, or about 10 nm to about 50 nm. The features may include silicon.

[0073] The amount of carbon obtained from the precursor can be about 50% by weight of the polyamic acid. In certain embodiments, the amount of carbon obtained from the precursor in the composite material can be greater than 0% by weight to about 95% by weight, such as about 1% by weight to about 95% by weight, about 1% by weight to about 90% by weight, about 1% by weight to about 80% by weight, about 1% by weight to about 70% by weight, about 1% by weight to about 60% by weight, about 1% by weight to about 50% by weight, about 1% by weight to about 40% by weight, about 1% by weight to about 30% by weight, about 5% by weight to about 95% by weight, about 5% by weight to about 90% by weight, about 5% by weight to about 80% by weight, about 5% by weight to about 90% by weight, about 5% by weight to about 100% by weight, about 5% by weight to about 15 ... %, about 5 wt % to about 60 wt %, about 5 wt % to about 50 wt %, about 5 wt % to about 40 wt %, about 5 wt % to about 30 wt %, about 10 wt % to about 95 wt %, about 10 wt % to about 90 wt %, about 10 wt % to about 80 wt %, about 10 wt % to about 70 wt %, about 10 wt % to about 60 wt %, about 10 wt % to about 50 wt %, about 10 wt % to about 40 wt %, about 10 wt % to about 30 wt %, about 10 wt % to about 25 wt %, etc. For example, the amount of carbon obtained from the precursor can be about 1 wt %, about 5 wt %, about 10 wt %, about 15 wt %, about 20 wt %, about 25 wt %, etc. of the precursor. When the amount of silicon is 90 wt % or greater, the amount of carbon may be 10 wt % or less, for example, about 0 wt % or greater to about 3 wt %, about 0 wt % or greater to about 5 wt %, about 0 wt % or greater to about 10 wt %, about 1 wt % or greater to about 3 wt %, about 1 wt % or greater to about 5 wt %, about 1 wt % or greater to about 8 wt %, about 1 wt % or greater to about 10 wt %, about 5 wt % or greater to about 10 wt %, and the like.

[0074] The carbon from the precursor can be a hard carbon (e.g., glassy carbon). Hard carbon can be a carbon that is not converted into graphite even when heated at more than 2800 degrees Celsius. The precursor that melts or flows during pyrolysis is converted into soft carbon and / or graphite at sufficient temperature and / or pressure. Hard carbon can be selected because the soft carbon precursor can flow, and soft carbon is mechanically weaker than hard carbon with graphite. Other possible hard carbon precursors can include phenolic resins, epoxy resins, and other polymers with very high melting points or crosslinking. In some embodiments, the amount of hard carbon in the composite material can have a value of greater than 0 wt % to about 95 wt %, such as about 1 wt % to about 95 wt %, about 1 wt % to about 90 wt %, about 1 wt % to about 80 wt %, about 1 wt % to about 70 wt %, about 1 wt % to about 60 wt %, about 1 wt % to about 50 wt %, about 1 wt % to about 40 wt %, about 1 wt % to about 30 wt %, about 5 wt % to about 95 wt %, about 5 wt % to about 90 wt %, about 5 wt % to about 80 wt %, about 5 ... %, about 5 wt % to about 60 wt %, about 5 wt % to about 50 wt %, about 5 wt % to about 40 wt %, about 5 wt % to about 30 wt %, about 10 wt % to about 95 wt %, about 10 wt % to about 90 wt %, about 10 wt % to about 80 wt %, about 10 wt % to about 70 wt %, about 10 wt % to about 60 wt %, about 10 wt % to about 50 wt %, about 10 wt % to about 40 wt %, about 10 wt % to about 30 wt %, about 10 wt % to about 25 wt %, etc. In some embodiments, the amount of hard carbon in the composite material can be about 1 wt %, about 5 wt %, about 10 wt %, about 20 wt %, about 30 wt %, about 40 wt %, about 50 wt %, or greater than about 50 wt %. When the amount of silicon is 90 wt % or more, the amount of hard carbon may be 10 wt % or less, for example, about 0 wt % or more to about 3 wt %, about 0 wt % or more to about 5 wt %, about 0 wt % or more to about 10 wt %, about 1 wt % or more to about 3 wt %, about 1 wt % or more to about 5 wt %, about 1 wt % or more to about 8 wt %, about 1 wt % or more to about 10 wt %, about 5 wt % or more to about 10 wt %, and the like.

[0075] In certain embodiments, the hard carbon phase is substantially amorphous. In other embodiments, the hard carbon phase is substantially crystalline. In other embodiments, the hard carbon phase includes amorphous carbon and crystalline carbon. The hard carbon phase can be a matrix phase in a composite material. The hard carbon can also be embedded in the pores of a silicon-containing additive. The hard carbon can react with some additives to produce some materials at the interface. For example, a silicon carbide layer can exist between the silicon particles and the hard carbon.

[0076] In order to produce an electrode with about 90% by weight or more silicon, the amount of carbon can be less than or equal to about 10% by weight (e.g., the ratio of silicon to carbon precursor can be high). In order to produce such electrodes, a polymer with a low carbonization rate such as PAN can be used for the carbon precursor. Other natural polymers with low carbonization rates (e.g., cellulose, glucose, sucrose, lignin and / or dextran) or synthetic polymers (e.g., polyimide, phenol formaldehyde resin (e.g., SU-8)) and the like can be used. In some cases, the low carbonization rate polymer can be heat treated to certain temperatures under an inert atmosphere. In some embodiments, a partial oxidation process is used so that the carbonization rate can be low.

[0077] As described herein, some embodiments may be pyrolyzed on a substrate (e.g., such that the green film is not self-supporting when subjected to heat treatment). In some embodiments, substrates with low char formation (e.g., acetal, polypropylene, polyethylene, polystyrene, etc.) may have about 0 wt. % or greater to about 5% carbon (e.g., only 2% carbon) remaining after pyrolysis and may be used as sacrificial substrates. The recipe may be adjusted to provide a higher ratio of silicon to carbon precursor than if a substrate were not used, thereby allowing the flexibility to use precursors whose char formation may be higher than, for example, PAN.

[0078] In some embodiments, before carbonization / pyrolysis in an inert atmosphere such as nitrogen, argon, vacuum, etc., the raw film can be partially or completely oxidized under air / oxygen supply. The oxidation level can be controlled so that the film will not flow back at any stage during the thermal treatment, thereby maintaining the integrity of the coating shape. The oxidation level can be controlled by stacking the raw film (single layer or multilayer), the size of the raw film, the degree of convection in the oven, and the compression pressure from the weight on the top of the stack. The oxidation of the raw film and the subsequent thermal treatment can make the total carbonization rate from the raw film to the carbon-silicon composite material be about 0% to about 60%. For example, the carbonization rate can be about 0% or more to about 30%, about 0% or more to about 40%, about 0% or more to about 50%, about 1% or more to about 30%, about 1% or more to about 40%, about 1% or more to about 50%, about 1% or more to about 60%, etc.

[0079] According to various embodiments, the composite film may include about 90% to about 99% by weight of silicon particles and greater than 0% and less than or equal to about 10% by weight of one or more types of carbon phases. At least one of the carbon phases may include hard carbon as a matrix phase, which holds the composite film together so that the silicon particles are distributed throughout the composite film. In some cases, the amount of silicon may be about 90% by weight or greater to about 95% by weight, about 90% by weight or greater to about 97% by weight, about 90% by weight or greater to about 99% by weight, about 92% by weight or greater to about 99% by weight, about 95% by weight or greater to about 99% by weight, about 97% by weight or greater to about 99% by weight, etc.

[0080] In certain embodiments, heating the mixture to a desired pyrolysis temperature can further result in surface modification of the silicon particles present in the mixture. In some embodiments, pyrolysis of the mixture can result in the formation of a surface coating on at least 50% of the silicon particles present in the mixture. In some embodiments, pyrolysis of the mixture can result in the formation of a surface coating on at least 60%, 70%, 80%, 90% or 99% of the silicon particles present in the mixture. In some embodiments, the surface coating forms a substantially continuous layer on the silicon particles.

[0081] In some embodiments, the carbonized precursor or resin can contact the surface of silicon particles. In certain embodiments, the carbonized precursor in contact with the silicon particle surface can be one or more types of carbon phases produced by the pyrolysis of the precursor. The carbon phase of one or more types of carbon phases of the carbonized precursor in contact with the silicon particle surface can react with the silicon particles during pyrolysis, thereby forming silicon carbide on the silicon particle surface. Therefore, in some embodiments, the surface coating can include a mixture of carbon, silicon carbide and / or carbon and silicon carbide.

[0082] In some embodiments, as further described below, the silicon particles present in the mixture may include native silicon oxides (SiO, SiO2, SiO x ) surface layer. In certain embodiments, the carbonized precursor in contact with the surface of the silicon particles can react with the naturally occurring natural silicon oxide surface layer to form silicon carbide. In some embodiments, the carbonized precursor in contact with the surface of the silicon particles can react with substantially all of the natural silicon oxide layer to form silicon carbide. Therefore, in some embodiments, the surface coating on the silicon particles can include carbon and silicon carbide, wherein the surface coating is substantially free of silicon oxide. In some embodiments, the first portion of the surface coating can include silicon carbide, and the second portion can include a mixture of silicon carbide and carbon. In some other embodiments, the carbonized precursor in contact with the surface of the silicon particles can incompletely convert the natural silicon oxide layer into silicon carbide, and the resulting one or more surface coatings can include carbon, silicon carbide and one or more silicon oxides, such as SiO, SiO2 and SiO x In some embodiments, the carbonized precursor in contact with the surface of the silicon particles may react completely to obtain a surface coating comprising silicon carbide. In some embodiments, substantially all of the surface coating may comprise silicon carbide. In some embodiments, such a surface coating may be substantially free of silicon oxide and / or carbon.

[0083] In certain embodiments, the pyrolyzed mixture can include silicon particles with a carbon and / or silicon carbide surface coating that produces a silicon-carbon-silicon carbide composite material (e.g., at least one first phase comprising silicon, at least one second phase comprising carbon, and at least one third phase comprising silicon carbide) or a silicon-carbon-carbon-silicon carbide composite material (e.g., at least one first phase comprising silicon, at least one second phase comprising carbon, at least one third phase comprising carbon, and at least one fourth phase comprising silicon carbide).

[0084] In addition, the surface coatings on the silicon particles described herein can help limit the outward expansion of the silicon particles during lithiation. By limiting the outward particle expansion during lithiation, the surface coating can help prevent mechanical failure of the silicon particles and ensure good electrical contact. The surface coating can further enhance the charge transfer within the electrode. Controlled and optimized surface modification of the silicon particles in the anode can also significantly improve the capacity retention during the relevant battery cell cycling.

[0085] In addition, the surface coating substantially affects the reaction between the anode material and the electrolyte in the battery. The surface coating can help reduce undesirable reactions. During high-temperature pyrolysis, the surface coating formed and the removal of undesirable natural oxides (SiO2) via conversion into more stable and non-reactive SiC can provide higher reversible capacity, minimizing irreversible capacity loss. The irreversible capacity loss may be due to the formation and accumulation of a solid electrolyte interface (SEI) layer that consumes lithium. For silicon particles, this becomes a more prominent problem because nano-scale silicon particles and micron-scale silicon particles have a large surface area, and larger silicon particles tend to crush during lithiation and delithiation, which can introduce additional particle surface area. In addition, the irreversible capacity loss may be due to the reaction of lithium with undesirable natural silicon oxides (reaction formula 1), which is inevitable during the processing and storage of silicon anode materials.

[0086] SiO x +yLi+ye→Si+Li y O x (Reaction 1)

[0087] Therefore, surface modification of silicon particles by carbon and / or silicon carbide can help form a relatively stable solid electrolyte interface layer and can reduce or eliminate the undesirable reaction of lithium with native silicon oxide on the surface of Si particles (Reaction 1).

[0088] Figure 1BSchematic diagram of forming silicon carbide on silicon particles as described above. First, silicon particles comprising a natural silicon oxide surface layer are provided in a mixture comprising a precursor as described above. In some embodiments, the mixture is pyrolyzed in a reducing atmosphere. For example, a reducing atmosphere, a vacuum and / or a flowing gas, including H2, CO or a hydrocarbon gas, may be used. In some embodiments, the mixture is heated to about 500°C to about 1350°C. In some embodiments, the mixture is heated to about 800°C to about 1200°C. In some embodiments, the mixture is heated to about 1175°C.

[0089] The pyrolyzed precursor in contact with the surface of the silicon particles reacts with the native silicon oxide layer of the silicon particles to form silicon carbide. The carbonized precursor in contact with the surface of the silicon particles is described herein as continuous and conformal, but may not be continuous or conformal in some other embodiments. Furthermore, in some embodiments, the silicon carbide layer formed by the reaction between the native silicon oxide layer and the carbonized precursor in contact with the surface of the silicon particles may take the form of a coating or dispersion within the composite anode film. Figure 1B , it is shown that in some embodiments, the silicon carbide may not be continuous or conformal on the silicon particles, whereas in some other embodiments, the silicon carbide may be a continuous and / or conformal coating.

[0090] In certain embodiments, graphite particles are added to the mixture. Advantageously, graphite can be an electrochemically active material in a battery and an elastically deformable material that can respond to volume changes of silicon particles. For certain types of lithium-ion batteries currently on the market, graphite is a preferred active anode material because it has a low irreversible capacity. In addition, graphite is softer than hard carbon and can better absorb the volume expansion of silicon additives. In certain embodiments, the particle size (e.g., diameter or maximum size) of the graphite particles can be about 0.5 microns to about 20 microns. All, substantially all or at least some of the graphite particles can include the particle size (e.g., diameter or maximum size) described herein. In some embodiments, the average particle size or median particle size (e.g., diameter or maximum size) of the graphite particles can be about 0.5 microns to about 20 microns. In some embodiments, the graphite particles can have a distribution of particle size. For example, at least about 95%, at least about 90%, at least about 85%, at least about 80%, at least about 70%, or at least about 60% of the particles can have a particle size described herein. In certain embodiments, the composite material may include graphite particles in an amount greater than 0 wt % and less than about 80 wt %, including about 40 wt % to about 75 wt %, about 5 wt % to about 30 wt %, about 5 wt % to about 25 wt %, about 5 wt % to about 20 wt %, about 5 wt % to about 15 wt %, etc. When the amount of silicon is 90 wt % or greater, the amount of graphite may be 10 wt % or less, for example, about 0 wt % or greater to about 3 wt %, about 0 wt % or greater to about 5 wt %, about 0 wt % or greater to about 10 wt %, about 1 wt % or greater to about 3 wt %, about 1 wt % or greater to about 5 wt %, about 1 wt % or greater to about 8 wt %, about 1 wt % or greater to about 10 wt %, about 5 wt % or greater to about 10 wt %, etc.

[0091] In certain embodiments, conductive particles that may also be electrochemically active are added to the mixture. Such particles can achieve a more conductive composite and a more mechanically deformable composite that can absorb the substantial volume changes that occur during lithiation and delithiation. In certain embodiments, the particle size (e.g., diameter or maximum dimension) of the conductive particles can be from about 10 nanometers to about 7 microns. All, substantially all, or at least some of the conductive particles can include the particle size (e.g., diameter or maximum dimension) described herein. In some embodiments, the average particle size or median particle size (e.g., diameter or maximum dimension) of the conductive particles can be from about 10 nm to about 7 microns. In some embodiments, the conductive particles can have a distribution of particle sizes. For example, at least about 95%, at least about 90%, at least about 85%, at least about 80%, at least about 70%, or at least about 60% of the particles can have a particle size described herein.

[0092] In certain embodiments, the mixture can include conductive particles greater than 0 wt % to an amount of up to about 80 wt %. In other embodiments, the composite material can include about 45 wt % to about 80 wt %. The conductive particles can be conductive carbon, including carbon black, carbon fiber, carbon nanofiber, carbon nanotube, etc. Many carbons considered as non-electrochemically active conductive additives become active once pyrolyzed in a polymer matrix. Alternatively, the conductive particles can be metals or alloys, including copper, nickel or stainless steel. When the amount of silicon is 90 wt % or greater, the amount of the conductive particles can be 10 wt % or less, for example, about 0 wt % or greater to about 3 wt %, about 0 wt % or greater to about 5 wt %, about 0 wt % or greater to about 10 wt %, about 1 wt % or greater to about 3 wt %, about 1 wt % or greater to about 5 wt %, about 1 wt % or greater to about 8 wt %, about 1 wt % or greater to about 10 wt %, about 5 wt % or greater to about 10 wt %, etc.

[0093] In certain embodiments, the electrode may include a composite material described herein. For example, the composite material may form a self-supporting monolithic electrode. The pyrolyzed carbon phase (e.g., hard carbon phase) of the composite material may be kept together and structurally support the particles added to the mixture. In some cases, the hard carbon phase may be a matrix phase (e.g., essentially glassy) as a substantially continuous phase. Silicon particles may be evenly distributed throughout the hard carbon. In certain embodiments, the self-supporting monolithic electrode does not include a separate substrate, a collector layer, and / or other supporting structures. In some embodiments, the composite material and / or the electrode does not include polymers other than traces remaining after the precursor is pyrolyzed. In other embodiments, the composite material and / or the electrode does not include a non-conductive adhesive. The composite material may also include porosity, for example, from about 1% to about 70% or from about 5% to about 50% by volume porosity. For example, the porosity may be from about 5% to about 40% by volume porosity.

[0094] In some embodiments, the composite film (with or without a substrate) can be attached to a current collector. For example, an electrode attachment material (e.g., a polymer adhesive) can be used to laminate the composite material on the current collector. In some embodiments, the composite material can also be formed into a powder. For example, the composite material can be ground into a powder. Composite powder can be used as an active material for an electrode. For example, composite powder can be deposited on a collector in a manner similar to that of preparing conventional electrode structures as known in the industry.

[0095] In certain embodiments, the electrode in the battery or electrochemical cell may comprise a composite material, including a composite material with silicon particles described herein. For example, the composite material may be used for an anode and / or a cathode. In some cases, the battery may include an anode, a cathode and an electrolyte. The anode may comprise a composite material described herein. The cathode is not particularly limited and may comprise nickel cobalt manganese (NCM), lithium cobalt oxide (LCO), nickel cobalt aluminum oxide (NCaO), lithium manganese oxide (LMO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium iron phosphate (LFP), etc. The electrolyte may be in liquid or solid state. In certain embodiments, the battery is a lithium ion battery. In other embodiments, the battery is a secondary battery, or in other embodiments, the battery is a primary battery.

[0096] In addition, during the use of the battery, the full capacity of the composite material may not be used to improve the life of the battery (e.g., the number of charge and discharge cycles before the battery fails or the battery performance drops below the usable level). For example, a composite material having about 70% by weight of silicon particles, about 20% by weight of carbon from a precursor, and about 10% by weight of graphite may have a maximum weight capacity of about 3000mAh / g, while the composite material may only use a weight capacity of up to about 550mAh / g to about 1500mAh / g. Although the maximum weight capacity of the composite material may not be used, using the composite material at a lower capacity can still achieve a higher capacity than some lithium-ion batteries. In certain embodiments, the composite material is used at or only at a weight capacity of less than about 70% of the maximum weight capacity of the composite material. For example, the composite material is not used at a weight capacity of more than about 70% of the maximum weight capacity of the composite material. In other embodiments, the composite material is used at or only at a weight capacity of less than about 50% of the maximum weight capacity of the composite material or less than about 30% of the maximum weight capacity of the composite material.

[0097] Silicon particles

[0098] Described herein are silicon particles for battery electrodes (e.g., anodes and cathodes). Anode electrodes currently used for rechargeable lithium-ion batteries typically have a specific capacity of about 200 milliampere hours per gram (including metal foil current collectors, conductive additives, and adhesive materials). Graphite, the active material for most lithium-ion battery anodes, has a theoretical energy density of 372 milliampere hours per gram (mAh / g). In contrast, silicon has a high theoretical capacity of 4200 mAh / g. However, when lithium is embedded, silicon expands by more than 300%. Due to this expansion, anodes containing silicon should be able to expand while allowing silicon to maintain electrical contact with silicon.

[0099] Some embodiments provide silicon particles that can be used as electrochemically active materials in electrodes. In addition to silicon particles, the electrode can contain a binder and / or other electrochemically active materials. For example, the silicon particles described herein can be used as silicon particles in the composite material described herein. In another example, the electrode can have an electrochemically active material layer on the current collector, and the electrochemically active material layer contains silicon particles. The electrochemically active material can also contain one or more types of carbon.

[0100] Advantageously, the silicon particles described herein can improve the performance of electrochemically active materials, such as improving capacity and / or cycling performance. In addition, electrochemically active materials having such silicon particles may not be significantly degraded due to lithiation of the silicon particles.

[0101] In certain embodiments, as described herein, the silicon particles can have an average particle size, such as an average diameter or an average maximum dimension, of about 10 nm to about 40 μm. Other embodiments may include an average particle size of about 1 μm to about 15 μm, about 10 nm to about 1 μm, and about 100 nm to about 10 μm. Silicon particles of various sizes can be separated by various methods, such as by air selection, sieving or other screening methods. For example, a mesh size of 325 can be used to separate particles with a particle size less than about 44 μm from particles with a particle size greater than about 44 μm.

[0102] In addition, the silicon particles can have a distribution of particle sizes. For example, at least about 90% of the particles can have a particle size, such as a diameter or maximum dimension, of about 10 nm to about 40 μm, about 1 μm to about 15 μm, about 10 nm to about 1 μm, and / or greater than 200 nm.

[0103] In some embodiments, the silicon particles may have an average surface area per unit mass of about 1 m 2 / g to about 100m 2 / g, about 1m 2 / g to about 80m 2 / g, about 1m 2 / g to about 60m 2 / g, about 1m 2 / g to about 50m 2 / g, about 1m 2 / g to about 30m 2 / g, about 1m 2 / g to about 10m 2 / g, about 1m 2 / g to about 5m 2 / g, about 2m 2 / g to about 4m 2 / g or less than about 5m 2 / g.

[0104] In certain embodiments, the silicon particles are at least partially crystalline, substantially crystalline, and / or completely crystalline. Additionally, the silicon particles may be substantially pure silicon.

[0105] Compared with the silicon particles used in conventional electrodes, the silicon particles described herein for some embodiments may generally have a larger average particle size. In some embodiments, the average surface area of ​​the silicon particles described herein may be generally smaller. Without being bound by any particular theory, the smaller surface area of ​​the silicon particles described herein may contribute to enhancing the performance of the electrochemical cell. The rechargeable battery anode of a typical lithium ion type will contain nano-sized silicon particles. In order to further increase the capacity of the battery, smaller silicon particles (e.g., silicon particles in the nano-size range) are used to prepare electrode active materials. In some cases, the silicon particles are ground to reduce the size of the particles. Sometimes grinding can produce a rough or scratched particle surface, which also increases the surface area. However, the increased surface area of ​​the silicon particles can actually contribute to the degradation of the electrolyte, which leads to an increase in irreversible capacity loss. Figure 2A and Figure 2B is a SEM micrograph of an exemplary embodiment of silicon particles ground from larger silicon particles. As shown in the figures, certain embodiments may have a rough surface.

[0106] As described herein, certain embodiments include silicon particles having a surface roughness in the nanometer size range, such as micrometer-sized silicon particles having nanometer-sized features on the surface. Figure 2C and Figure 2D It is an SEM micrograph of an exemplary embodiment of such silicon particles. Various such silicon particles can have an average particle size (e.g., average diameter or average maximum dimension) in the micrometer range (e.g., as described herein, about 0.1 μm to about 30 μm) and a surface including nanometer-sized features (e.g., as described herein, about 1 nm to about 1 μm, about 1 nm to about 750 nm, about 1 nm to about 500 nm, about 1 nm to about 250 nm, about 1 nm to about 100 nm, about 10 nm to about 500 nm, about 10 nm to about 250 nm, about 10 nm to about 100 nm, about 10 nm to about 75 nm, or about 10 nm to about 50 nm). Features can include silicon.

[0107] and Figure 2A and Figure 2B Compared to the exemplary embodiment shown in FIG. , a combined micron / nano-sized geometry (e.g., Figure 2C and Figure 2D ) silicon particles can have a higher surface area than ground particles. Therefore, the silicon particles to be used can be determined by the desired application and specifications.

[0108] Although the silicon particles of certain embodiments have nano-sized features on the surface, the total surface area of ​​the particles may be more similar to that of micron-sized particles than to nano-sized particles. For example, micron-sized silicon particles (e.g., silicon ground from larger particles) typically have a surface area greater than about 0.5 m 2 / g and less than about 2m 2 The average surface area per unit volume (e.g., using Brunauer Emmet Teller (BET) particle surface area measurement) is 1000 m / g, while nanosized silicon particles generally have an average surface area of ​​greater than about 100 m / g. 2 / g and less than about 500m 2 / g average surface area per unit mass. Certain embodiments described herein may have an average surface area per unit mass of about 1 m 2 / g to about 30m 2 / g, about 1m 2 / g to about 25m 2 / g, about 1m 2 / g to about 20m 2 / g, about 1m 2 / g to about 10m 2 / g, about 2m 2 / g to about 30m 2 / g, about 2m 2 / g to about 25m 2 / g, about 2m 2 / g to about 20m 2 / g, about 2m 2 / g to about 10m 2 / g, about 3m 2 / g to about 30m 2 / g, about 3m 2 / g to about 25m 2 / g, about 3m 2 / g to about 20m 2 / g, about 3m 2 / g to about 10m 2 / g (for example, about 3m 2 / g to about 6m 2 / g), about 5m 2 / g to about 30m 2 / g, about 5m 2 / g to about 25m 2 / g, about 5m 2 / g to about 20m 2 / g, about 5m 2 / g to about 15m 2 / g or about 5m 2 / g to about 10m 2 / g.

[0109] Various examples of micron-sized silicon particles having nanometer-sized features can be used to form certain embodiments of the composite materials described herein. For example, Figure 3 An exemplary method 200 for forming a composite material of certain embodiments is illustrated. The method 200 includes providing a plurality of silicon particles (e.g., silicon particles having an average particle size of about 0.1 μm to about 30 μm and a surface including nano-sized features), block 210. The method 200 also includes forming a mixture including a precursor and the plurality of silicon particles, block 220. The method 200 also includes pyrolyzing the precursor (block 230) to convert the precursor into one or more types of carbon phases to form a composite material.

[0110] With regard to block 210 of method 200, silicon having the characteristics described herein can be synthesized as a product or byproduct of a fluidized bed reactor (FBR) method. For example, in an FBR method, useful materials can be grown on silicon seed materials. Typically, particles can be removed from the reactor by gravity. Some finely divided silicon materials can leave the reactor from the top of the reactor or can be deposited on the wall of the reactor. The material (e.g., byproduct material) leaving the top of the reactor or deposited on the wall of the reactor can have nanoscale features on micron-sized particles. In some such methods, a gas (e.g., a nitrogen carrier gas) can pass through the silicon material. For example, the silicon material can be a plurality of granular silicon. The gas can pass through the silicon material at a sufficiently high speed to suspend the solid silicon material and make it behave like a fluid. The method can be carried out under an inert atmosphere, such as under nitrogen or argon. In some embodiments, silane gas can also be used, for example, to allow the growth of metallic silicon on the surface of silicon particles. The method of growing from the gas phase can give silicon particles unique surface properties, such as nano-size features. Because silicon typically cracks in smooth shapes (e.g., like glass), certain embodiments of silicon particles formed using the FBR method can advantageously obtain small features, such as small features in the nanometer size range, which may not be easily obtained in some embodiments where silicon particles are formed by grinding from larger silicon particles.

[0111] In addition, because the FBR process can be under an inert atmosphere, very high purity particles (e.g., greater than 99.9999% purity) can be obtained. In some embodiments, a purity of about 99.9999% to about 99.999999% can be obtained. In some embodiments, the FBR process can be similar to the process used in the production of solar-grade polysilicon, while using 85% less energy than the conventional Siemens process, where polysilicon can be formed at 1150°C as trichlorosilane decomposes and additional silicon material is deposited on high purity silicon rods. Because nano-sized silicon particles have shown enhanced cycle life performance in electrochemical cells, micron-sized silicon particles have not been considered for use as electrochemically active materials in electrochemical cells.

[0112] For blocks 220 and 230 of method 200, a mixture comprising a precursor and a plurality of silicon particles is formed, block 220, and the precursor is pyrolyzed, block 230, to convert the precursor into one or more types of carbon phases to form a composite material similar to blocks 101 and 105, respectively, described herein in method 100. In some embodiments, pyrolysis occurs at a temperature below the melting point of silicon (e.g., about 1414°C) (e.g., about 900°C to 1350°C) without affecting the nano-sized features of the silicon particles.

[0113] According to certain embodiments described herein, certain micron-sized silicon particles having nano-surface features can achieve high energy density and can be used in composite materials and / or electrodes in electrochemical cells to improve performance during battery cycling.

[0114] Example

[0115] The following exemplary method for anode fabrication generally includes mixing the components together, casting those components on a release liner, drying, curing, removing the liner, and then pyrolyzing the resulting sample. Typically, N-methyl-2-pyrrolidone (NMP) is used as a solvent to change the viscosity of any mixture and make it castable using a doctor blade method.

[0116] Example 1

[0117] In Example 1, a polyimide liquid precursor (PI 2611 from HDMicrosystems corp.), graphite particles (SLP30 from Timcal corp.), conductive carbon particles (Super P from Timcal corp.), and silicon particles (from Alfa Aesar corp.) were mixed together in a weight ratio of 200:55:5:20 for 5 minutes using a Spex 8000D machine. The mixture was then cast on an aluminum foil and dried in an oven at 90°C to remove the solvent, e.g., NMP. The curing step was then performed in a hot press at 200°C for at least 12 hours under negligible pressure. The aluminum foil liner was then removed by etching in a 12.5% ​​HCl solution. The remaining film was then rinsed in DI water, dried, and then pyrolyzed at 1175°C for about 1 hour under an argon stream. The process produced a composition of 15.8 wt% PI 2611 derived carbon, 57.9 wt% graphite particles, 5.3 wt% carbon produced from Super P, and 21.1 wt% silicon.

[0118] The resulting electrodes were then tested in a pouch cell configuration against a lithium NMC oxide cathode. Figure 4 A typical cycle diagram is shown in .

[0119] Example 2

[0120] In Example 2, silicon particles (from EVNANO Advanced Chemical Materials Co., Ltd.) in a weight ratio of 1:9 were first mixed with NMP using a Turbula mixer for 1 hour. Then, polyimide liquid precursor (PI 2611 from HD Microsystems corp.), graphite particles (SLP30 from Timcal corp.) and carbon nanofibers (CNF from Pyrograf corp.) were added to the Si:NMP mixture in a weight ratio of 200:55:5:200, and vortexed for about 2 minutes. Then, the mixture was cast on an aluminum foil covered with a 21 μm thick copper mesh. Then, the sample was dried in an oven at 90 ° C to remove solvents, such as NMP. Then, a curing step was performed at 200 ° C in a hot press under negligible pressure for at least 12 hours. Then, the aluminum foil liner was removed by etching in a 12.5% ​​HCl solution. The remaining membrane was then rinsed in DI water, dried, and then pyrolyzed at 1000°C for about 1 hour under argon. The process produced a composition of 15.8 wt% PI 2611 derived carbon, 57.9 wt% graphite particles, 5.3 wt% CNFs, and 21.1 wt% silicon.

[0121] The resulting electrodes were then tested in a pouch cell configuration against a lithium NMC oxide cathode. Figure 5 A typical cycle diagram is shown in .

[0122] Example 3

[0123] In Example 3, a polyimide liquid precursor (PI 2611 from HD Microsystems corp.) and 325 mesh silicon particles (from Alfa Aesar corp.) were mixed together at a weight ratio of 40:1 using a Turbula mixer for a duration of 1 hour. The mixture was then cast on an aluminum foil and dried in an oven at 90°C to remove the solvent, e.g., NMP. A curing step was then performed in a hot press at 200°C under negligible pressure for at least 12 hours. The aluminum foil liner was then removed by etching in a 12.5% ​​HCl solution. The remaining film was then rinsed in DI water, dried, and then pyrolyzed at 1175°C under an argon stream for about 1 hour. The process produced a composition of 75 wt% PI 2611 derived carbon and 25 wt% silicon.

[0124] The resulting electrodes were then tested in a pouch cell configuration against a lithium NMC oxide cathode. Figure 6 A typical cycle diagram is shown in .

[0125] Example 4

[0126] In Example 4, silicon microparticles (from Alfa Aesar corp.), polyimide liquid precursor (PI 2611 from HD Microsystems corp.), graphite particles (SLP30 from Timcal corp.), milled carbon fibers (from Fibre Glast Developments corp.), carbon nanofibers (CNF from Pyrograf corp.), carbon nanotubes (from CNANO Technology Limited), conductive carbon particles (Super P from Timcal corp.), conductive graphite particles (KS6 from Timca corp.) were mixed in a weight ratio of 20:200:30:8:4:2:1:15 for 5 minutes using a vortexer. The mixture was then cast on an aluminum foil. The sample was then dried in an oven at 90°C to remove the solvent, e.g., NMP. A curing step was then performed in a hot press at 200°C under negligible pressure for at least 12 hours. The aluminum foil liner was then removed by etching in a 12.5% ​​HCl solution. The remaining film was then rinsed in DI water, dried, and then pyrolyzed at 1175°C for about 1 hour under argon. The process produced a composition similar to the initial mixture, but with 7.5% of the PI 2611 derived carbon moiety of the initial weight of the polyimide precursor.

[0127] The resulting electrodes were then tested in a pouch cell configuration against a lithium NMC oxide cathode. Figure 7 A typical cycle diagram is shown in .

[0128] Example 5

[0129] In Example 5, a polyimide liquid precursor (PI 2611 from HD Microsystems corp.) and silicon microparticles (from Alfa Aesar corp.) were mixed together in a weight ratio of 4:1 using a Turbula mixer for a duration of 1 hour. The mixture was then cast on an aluminum foil covered with a carbon shield (from Fibre Glast Developments Corporation) and dried in an oven at 90°C to remove the solvent, e.g., NMP. The curing step was then performed in a hot press at 200°C under negligible pressure for at least 12 hours. The aluminum foil backing was then removed by etching in a 12.5% ​​HCl solution. The remaining film was then rinsed in DI water, dried, and then pyrolyzed at 1175°C for about 1 hour under an argon stream. The process produced a composition of about 23% by weight of PI 2611 derived carbon, 76% by weight of silicon, and the weight of the shield was negligible.

[0130] The resulting electrodes were then tested in a pouch cell configuration against a lithium nickel manganese cobalt oxide (NMC) cathode. Figure 8 A typical cycle diagram is shown in .

[0131] Example 6

[0132] In Example 6, a polyimide liquid precursor (PI 2611 from HDMicrosystems corp.), graphite particles (SLP30 from Timcal corp.), and silicon microparticles (from Alfa Aesar corp.) were mixed together in a 200:10:70 weight ratio for 5 minutes using a Spex 8000D machine. The mixture was then cast on an aluminum foil and dried in an oven at 90°C to remove the solvent, e.g., NMP. The curing step was performed in a hot press at 200°C for at least 12 hours under negligible pressure. The aluminum foil liner was then removed by etching in a 12.5% ​​HCl solution. The remaining film was then rinsed in DI water, dried, and then pyrolyzed at 1175°C for about 1 hour under an argon flow. The process produced a composition of 15.8 wt% PI 2611 derived carbon, 10.5 wt% graphite particles, 73.7 wt% silicon.

[0133] The resulting electrodes were then tested in a pouch cell configuration against a lithium NMC oxide cathode. The anode was charged to 600 mAh / g for each cycle and the discharge capacity was recorded for each cycle. Fig. 9 A typical cycle diagram is shown in .

[0134] Example 7

[0135] In Example 7, PVDF and silicon particles (from EVNANO Advanced Chemical Materials Co), conductive carbon particles (Super P from Timcal Corp.), conductive graphite particles (KS6 from Timcal Corp.), graphite particles (SLP30 from Timcal Corp.), and NMP were mixed in a weight ratio of 5:20:1:4:70:95. The mixture was then cast on an aluminum substrate and placed in an oven at 90° C. to remove the solvent, e.g., NMP. The resulting electrode was then tested in a pouch cell configuration against a lithium NMC oxide cathode. Fig.10 A typical cycle diagram is shown in .

[0136] Example 8

[0137] Several experiments were performed to obtain the effect of varying the percentage of polyimide derived carbon (eg, 2611c) while decreasing the percentage of graphite particles (SLP30 from Timcal corp.) and keeping the percentage of silicon microparticles (from AlfaAesar corp.) at 20 wt.%.

[0138] like Fig.11A and Fig. 11B The results show that more graphite and less 2611c benefit cell performance by increasing specific capacity while reducing irreversible capacity. Minimizing 2611c adversely affects the strength of the resulting anode, so in one embodiment, a value close to 20 wt.% may be preferred as a compromise.

[0139] Example 9

[0140] Similar to Example 8, if 2611c is maintained at 20 wt.%, and the percentage of Si is increased at the expense of graphite particles, the first cycle discharge capacity of the resulting electrode increases. Fig.12 It shows that higher silicon content can produce anodes with better performance.

[0141] Example 10

[0142] A 1 mil thick polyimide sheet was pyrolyzed and tested according to the procedure in Example 1. The reversible capacity and irreversible capacity were plotted as a function of the pyrolysis temperature. Fig.13 It is shown that in one embodiment, it is preferred to pyrolyze a polyimide sheet (Upilex from UBE corp) at about 1175°C.

[0143] Additional Embodiments

[0144] Fig.14 is a photograph of a 4.3 cm x 4.3 cm composite anodic film without a metal foil support layer. The composite anodic film has a thickness of about 30 microns and has a composition of about 15.8 wt% PI 2611 derived carbon, about 10.5 wt% graphite particles, and about 73.7 wt% silicon.

[0145] Figures 15 to 20 is a scanning electron microscope (SEM) micrograph of the composite anode film. The composition of the composite anode film is about 15.8 wt % PI 2611 derived carbon, about 10.5 wt % graphite particles, and about 73.7 wt % silicon. Fig.15 and Fig.16 Shown before cycling (the defocused portion is the bottom portion of the anode and the focused portion is the cleaved edge of the composite membrane). Fig.17 , Fig.18 and Fig.19 SEM micrographs of the composite anode film after cycling for 10 cycles, 10 cycles and 300 cycles respectively. The SEM micrographs show the absence of any significant pulverization of the silicon and the anode does not have an excessive layer of solid electrolyte interface / interphase (SEI) built up on top of it after cycling. Fig. 20 is a SEM micrograph of the cross section of the composite anodic membrane.

[0146] The measured properties of exemplary silicon particles are described below.These examples are discussed for exemplary purposes, but should not be construed as limiting the scope of the disclosed embodiments.

[0147] Fig.21 The XRD pattern of the silicon particles in the sample is shown in Figure 1. The XRD pattern shows that the silicon particles in the sample are basically crystalline or polycrystalline.

[0148] Figure 22 to Figure 25 is a scanning electron microscope (SEM) micrograph of sample silicon particles. Although the SEM micrograph appears to show that the silicon particles may have an average particle size larger than the measured average particle size of about 300 nm, without being bound by theory, the particles are believed to have agglomerated together to appear as larger particles.

[0149] Fig.26 It is a chemical analysis of the sample silicon particles. The chemical analysis shows that the silicon particles are essentially pure silicon.

[0150] Fig.27A and Fig.27B 2 are exemplary particle size histograms of micron-sized silicon particles with nanometer-sized features. The particles are prepared by the FBR method. The exemplary silicon particles can have a particle size distribution. For example, at least 90% of the particles can have a particle size, such as a diameter or maximum dimension, of about 5 μm to about 20 μm (e.g., about 6 μm to about 19 μm). At least about 50% of the particles can have a particle size of about 1 μm to about 10 μm (e.g., about 2 μm and about 9 μm). In addition, at least about 10% of the particles can have a particle size of about 0.5 μm to about 2 μm (e.g., about 0.9 μm and 1.1 μm).

[0151] Fig.28is a graph comparing the discharge capacity of two types of exemplary silicon particles during battery cycling. The performance of four samples of silicon particles (micrometer-sized particles with nanometer-sized features) prepared by the FBR method is compared with five samples of silicon particles prepared by grinding larger silicon particles. Therefore, certain embodiments of silicon particles having a combined micrometer / nanometer geometry (e.g., prepared by the FBR method) can have enhanced performance relative to various other embodiments of silicon particles (e.g., micrometer-sized silicon particles prepared by grinding larger silicon particles). The type of silicon particles to be used can be adjusted for the intended or desired application and specification.

[0152] Example of a silicon-dominated electrode

[0153] Resin Preparation: High molecular weight (e.g., about 150,000 g / mol) PAN powder is dispersed in the dipolar aprotic solvent NMP at 75°C overnight to obtain a resin with a 12% solids content in this case. Higher molecular weights (e.g., greater than 150,000 g / mol, such as up to about 700,000 g / mol or up to about 750,000 g / mol) can also be used. Lower molecular weights, such as about 50,000 g / mol to about 150,000 g / mol, can also be used. Solvents such as DMF, DMSO, and DMAc can also be used. In addition, higher temperatures below the gelation temperature and / or below the flash point of these solvents can also be used.

[0154] Slurry and anode preparation: Silicon nanoparticles / microparticles are dispersed in PAN resin under high shear conditions (e.g., using a centrifugal planetary mixer at 2000 rpm for 10 minutes) to obtain a uniform slurry with >20 wt% Si. A ball milling step of the Si particles in a solvent can also be used to achieve depolymerization of the Si particles, and they can be dispersed in the resin to produce a slurry. The slurry is cast on a polyethylene terephthalate substrate and dried to remove most of the residual solvent. Sacrificial substrates, such as substrates with zero, near-zero, or low char formation rates (e.g., polypropylene) can also be used. A thinly coated anode (dry loading of 3.63 mg / cm 2 ) is peeled off from a polyethylene terephthalate substrate, cut into smaller components, and stacked into stacks of 10. The stacked green anodes are oxidized by heating at a temperature of 200° C. for 15 hours in an air convection oven. Stacks of anodes in a self-supporting substrate-free form or on a low carbonization rate substrate may result in limited air / oxygen mass transmitted to the green anode. The oxidized / stabilized composite anode is pyrolyzed in a furnace at a temperature exceeding 1175° C. under an argon inert atmosphere to obtain a silicon-carbon composite anode.

[0155] The charring rate and final Si weight % in the anode can be controlled by controlling the oxidation and pyrolysis processes. The oxidation / stabilization conditions, such as temperature, heating rate and atmosphere, and the subsequent heat treatment conditions under an inert / reducing atmosphere can be controlled to change the PAN charring rate in the final substrate-free anode. Some different conditions for unstacked PAN-silicon raw anodes are shown in Table 1. In Conditions 8 and 9, the unoxidized PAN anode reflows (e.g., does not maintain the membrane structure) and cannot be further processed. The charring rate can be further reduced by reducing the oxidation temperature to 100°C to 200°C (for example) and increasing the duration to 24 hours to 48 hours, sufficient oxidation to avoid reflow, keeping the final pyrolysis heat treatment conditions the same.

[0156] Table 1.

[0157]

[0158]

[0159] Fig.29 The stabilization / oxidation and carbonization rates of PAN under different heat treatment conditions are shown. The stabilization / oxidation yield is calculated as the weight after stabilization / oxidation divided by the original weight before stabilization / oxidation. The carbonization rate is calculated as the weight after pyrolysis divided by the original weight before pyrolysis. The actual carbonization rate of the stacked green anode is much lower than Fig.29 The actual charring rate in the anodes (e.g., 39% for 84% silicon anodes and 29% for 94% Si anodes) is reduced because the stacking reduces the bulk oxygen / air flow between the anodes, causing them to be partially oxidized. In some cases, the degree of oxidation of the green anodes in stacked form may also depend on the size of the green anodes, the size of the stack, the degree of convection in the oven, and the compressive pressure from the weights on top of the stack. In the anodes cycled here, the anode dimensions were 12 cm×9 cm×30 μm, and the pressure on the stack was 0.6 psi. The oven used was a gravity oven at 200°C (e.g., no forced air). These anodes were constructed into 5-layer cells with a nickel-based cathode and a standard carbonate-based electrolyte, and tested under cycling conditions. The test carrier and conditions are provided below.

[0160] Test vector

[0161] Cathode: NMC 622 23mg / cm 2 load

[0162] Electrolyte: Carbonate based electrolyte

[0163] 5-layer battery, estimated capacity of 710mAh

[0164] Test conditions:

[0165]

[0166] Silicon-carbon composite anodes produced by coating a silicon-graphite (or similar carbon source, such as graphene, carbon black, etc.) slurry with some polymer binder dispersed in a solvent on a current collector substrate, followed by drying and pressing, have the following disadvantages: poor reversible capacity retention and capacity retention, with more than 50% of the capacity lost in the first 30 cycles. Certain embodiments of the silicon-dominated anodes described herein show much better capacity retention when cycled under a wide voltage window.

[0167] Fig.30 A graph showing capacity versus cycle number for a cell having an exemplary silicon-dominated anode. Fig.31 A graph showing capacity retention versus cycle number for a cell with an exemplary silicon-dominated anode. The cell resistance does not increase much during cycling, indicating a mechanically stable anode. Poor mechanical stability / structural integrity of silicon-dominated anodes due to large anode volume changes can be a major issue, which is detrimental to the cycle life of lithium-ion cells including such anodes. Without being bound by theory, a cell with a 94% Si anode may start with a slightly higher capacity due to more active ingredients (e.g., Si). Fig.30 and Fig.31 In the embodiment, the battery with 84% Si anode appears to have higher capacity and retention with cycling. Without being bound by theory, this may be because the 84% Si anode includes high surface area (4%) graphite material as an additive, which can provide better electrical contact during cycling. In some embodiments, the battery with 94% Si material can show better capacity and retention due to such additives. Fig.32 A graph of cell resistance versus cycle number for cells with exemplary silicon-dominated anodes is shown. At 150 cycles, the cell resistance of the cell with 94% Si anode is slightly higher than the cell with 84% Si anode. Without being bound by theory, this may be due to the lack of conductive graphite additive after cycling in the cell with 84% Si anode. After 150 cycles, the cell shows a much lower increase in cell resistance.

[0168] The embodiments of the present application include but are not limited to the following:

[0169] Embodiment 1. A method of forming a composite film, the method comprising:

[0170] providing a mixture comprising a carbon precursor and silicon particles; and

[0171] The carbon precursor is pyrolyzed to convert the precursor into one or more types of carbon phases to form the composite film, so that the precursor has a char yield greater than about 0% to about 60%, and the composite film contains about 90 wt% to about 99 wt% of the silicon particles.

[0172] Embodiment 2. The method of Embodiment 1, wherein the composite film comprises from about 95 wt % to about 99 wt % of the silicon particles.

[0173] Embodiment 3. A method as described in Embodiment 1, wherein the carbon precursor includes polyacrylonitrile (PAN).

[0174] Embodiment 4. A method as described in Embodiment 1, wherein the carbon precursor comprises cellulose, glucose, sucrose, lignin, dextran or a combination thereof.

[0175] Embodiment 5. A method as described in Embodiment 1, wherein the carbon precursor comprises polyimide, phenol formaldehyde resin or a combination thereof.

[0176] Embodiment 6. A method as described in Embodiment 1, wherein the carbon precursor comprises polyamic acid.

[0177] Embodiment 7. A method as described in Embodiment 6, wherein the carbon precursor includes dianhydride and / or diamine.

[0178] Embodiment 8. A method as described in Embodiment 7, wherein the carbon precursor includes pyromellitic dianhydride oxidized aniline (PMDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA) or a combination thereof.

[0179] Embodiment 9. The method of Embodiment 1, wherein the mixture further comprises a solvent comprising N-methylpyrrolidone (NMP).

[0180] Embodiment 10. The method of embodiment 1, wherein the mixture further comprises an aprotic solvent.

[0181] Embodiment 11. A method as described in Embodiment 10, wherein the aprotic solvent comprises any one or a mixture of dimethylformamide (DMF), dimethoxymethamphetamine (DMMA), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), cyclopentane, ethylene carbonate, or a combination thereof.

[0182] Embodiment 12. The method of embodiment 1, wherein the mixture further comprises an inorganic salt.

[0183] Embodiment 13. The method of embodiment 12, wherein the inorganic salt comprises lithium bromide, sodium thiocyanate, zinc chloride, or a combination thereof.

[0184] Embodiment 14. The method of Embodiment 1, wherein the mixture further comprises sulfuric acid, nitric acid, or a combination thereof.

[0185] Embodiment 15. The method of embodiment 1 further comprises coating the mixture on a substrate to form a green film.

[0186] Embodiment 16. The method of Embodiment 15 further comprises removing the green film from the substrate before pyrolyzing the carbon precursor.

[0187] Embodiment 17. The method of Embodiment 16, wherein the substrate comprises polyethylene terephthalate (PET), cyclic olefin copolymer (COC), or a combination thereof.

[0188] Embodiment 18. A method as described in Embodiment 15, wherein pyrolysis includes pyrolyzing the green film on the substrate.

[0189] Embodiment 19. The method of Embodiment 18, wherein the substrate comprises a polymer having a char formation rate of about 0% to about 5%.

[0190] Embodiment 20. The method of Embodiment 19, wherein the substrate comprises acetal, polypropylene, polyethylene, polystyrene, or a combination thereof.

[0191] Embodiment 21. The method of Embodiment 1, further comprising oxidizing the mixture prior to pyrolysis.

[0192] Embodiment 22. The method of Embodiment 1, wherein pyrolysis comprises heating the mixture at a temperature of about 350°C to about 1350°C.

[0193] Embodiment 23. The method of Embodiment 1, wherein pyrolysis forms the composite film as a self-supporting structure.

[0194] Embodiment 24. A composite film comprising:

[0195] From about 90% to about 99% by weight silicon particles; and

[0196] Greater than 0 wt. % and less than or equal to about 10 wt. % of one or more types of carbon phases, wherein at least one of the one or more types of carbon phases includes hard carbon as a matrix phase that holds the composite film together such that the silicon particles are distributed throughout the composite film.

[0197] Embodiment 25. The composite film of Embodiment 24, wherein the composite film comprises the silicon particles in an amount of about 95% to about 99% by weight of the composite film.

[0198] Embodiment 26. A composite film as described in Embodiment 24, wherein the silicon particles have an average particle size of about 10 nm to about 40 μm.

[0199] Embodiment 27. A composite film as described in Embodiment 24, wherein the hard carbon comprises glassy carbon.

[0200] Embodiment 28. The composite film of Embodiment 24 further comprises a silicon carbide layer between the silicon particles and the hard carbon.

[0201] Embodiment 29. A composite film as described in Embodiment 24, wherein the matrix phase is a substantially continuous phase.

[0202] Embodiment 30. The composite film of Embodiment 24, wherein the silicon particles are uniformly distributed throughout the hard carbon.

[0203] Embodiment 31. The composite film of Embodiment 24, wherein the composite film is self-supporting.

[0204] Embodiment 32. A composite film as described in Embodiment 24, wherein at least one of the one or more types of carbon phases is electrochemically active and conductive.

[0205] Embodiment 33. A composite membrane as described in Embodiment 24, wherein the one or more types of carbon phases also include graphite particles.

[0206] Embodiment 34. The composite membrane of Embodiment 24, wherein the composite membrane is substantially electrochemically active.

[0207] Embodiment 35. A battery electrode comprising the composite membrane of Embodiment 24, wherein the electrode is an anode.

[0208] Embodiment 36. A battery electrode as described in Embodiment 35, wherein the composite film is self-supporting.

[0209] Embodiment 37. The battery electrode as described in Embodiment 35 also includes a current collector.

[0210] Embodiment 38. The battery electrode of Embodiment 37 further comprises a polymer binder between the composite film and the current collector.

[0211] Embodiment 39. A battery comprising:

[0212] An anode comprising the composite membrane of embodiment 24;

[0213] a cathode; and

[0214] Electrolytes.

[0215] Embodiment 40. A battery as described in Embodiment 39, wherein the battery is a lithium-ion battery.

[0216] Embodiment 41. A battery as described in Embodiment 39, wherein the cathode comprises nickel cobalt manganese (NCM), lithium cobalt oxide (LCO), nickel cobalt aluminum oxide (NCAO), lithium manganese oxide (LMO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO) or lithium iron phosphate (LFP).

[0217] Embodiment 42. A battery as described in Embodiment 39, wherein the electrolyte is in liquid form.

[0218] Embodiment 43. A battery as described in Embodiment 39, wherein the electrolyte is in a solid state.

[0219] Various embodiments have been described above. Although the present invention has been described with reference to these specific embodiments, the description is intended to illustrate rather than to limit. Various modifications and applications may be made by those skilled in the art without departing from the true spirit and scope of the present invention as defined in the appended claims.

Claims

1. A method for forming a composite film, the method comprising: providing a mixture comprising a carbon precursor and silicon particles; as well as The carbon precursor is pyrolyzed to convert the precursor into one or more types of carbon phases to form the composite film, so that the precursor has a char yield greater than about 0% to about 60%, and the composite film contains about 90 wt% to about 99 wt% of the silicon particles.

2. The method of claim 1, wherein the composite film comprises about 95% to about 99% by weight of the silicon particles.

3. The method of claim 1, wherein the carbon precursor comprises polyacrylonitrile (PAN).

4. The method of claim 1, wherein the carbon precursor comprises cellulose, glucose, sucrose, lignin, dextran, or a combination thereof. 5 . The method of claim 1 , wherein the carbon precursor comprises polyimide, phenol formaldehyde resin, or a combination thereof. The method of claim 1 , wherein the carbon precursor comprises polyamic acid.

7. The method of claim 6, wherein the carbon precursor comprises a dianhydride and / or a diamine.

8. The method of claim 7, wherein the carbon precursor comprises pyromellitic dianhydride oxidized aniline (PMDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA), or a combination thereof.

9. The method of claim 1, wherein the mixture further comprises a solvent comprising N-methylpyrrolidone (NMP).

10. A composite film comprising: From about 90% to about 99% by weight silicon particles; and Greater than 0 wt. % and less than or equal to about 10 wt. % of one or more types of carbon phases, wherein at least one of the one or more types of carbon phases includes hard carbon as a matrix phase that holds the composite film together such that the silicon particles are distributed throughout the composite film.

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