Method for forming a carbon-silicon composite on a current collector

Abstract

A method for forming an electrode is described. In some embodiments, the method can include providing a current collector. The method can include providing a first carbon precursor on the current collector and providing a mixture on the first carbon precursor. The mixture can include a second carbon precursor and silicon particles. The method can further include pyrolyzing the second carbon precursor to convert the second carbon precursor to one or more carbon phases to form a composite. The one or more carbon phases can be a substantially continuous phase with silicon particles distributed throughout the composite. The method can also include pyrolyzing the first carbon precursor to adhere the composite onto the current collector.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Application No. 15 / 471,860, filed March 28, 2017, and U.S. Application No. 16 / 430,306, filed June 3, 2019, which are incorporated herein by reference in their entireties.

[0002] FIELD OF THE DISCLOSURE The present disclosure relates to electrodes, electrochemical cells, and methods of forming electrodes and electrochemical cells. In particular, the present disclosure relates to methods of forming carbon-silicon composites on current collectors. [Background technology]

[0003] Lithium-ion batteries generally contain a separator and / or electrolyte between the anode and cathode. In some batteries, the separator, cathode, and anode materials are each formed into a sheet or film. The cathode, separator, and anode sheets are then stacked or wound together to form a battery, with the separator separating the cathode and anode (e.g., electrode). When the cathode, separator, and anode are wound, each sheet must be sufficiently deformable or flexible to avoid defects such as cracks, cuts, and mechanical defects. A typical electrode contains a layer of electrochemically active material on a conductive metal (e.g., aluminum and copper). For example, carbon can be deposited on a current collector with an inert binder material. Carbon is often used because it has excellent electrochemical properties and is electrically conductive. Electrodes can be wound or cut into individual pieces and then layered together to form a stack. A stack is a stack of electrochemically active materials, with separators between them. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Application Serial No. 15 / 471,860 [Patent Document 2] U.S. Patent Application No. 16 / 430,306 [Patent Document 3] U.S. Patent No. 9,178,208 [Patent Document 4] U.S. Patent Application Publication No. 2014 / 0170498 [Patent Document 5] U.S. Patent No. 9,553,303 [Patent Document 6] U.S. Patent No. 9,397,338 [Patent Document 7] U.S. Patent No. 9,583,757 Summary of the Invention [Means for solving the problem]

[0005] In certain embodiments, a method for forming an electrode is provided. The method can include providing a current collector. The method can also include providing a mixture on the current collector. The mixture can include a precursor and silicon particles. The method can further include pyrolyzing the mixture on the current collector to convert the precursor to one or more carbon phases to form a composite and adhere the composite to the current collector. The one or more carbon phases can be a substantially continuous phase with silicon particles distributed throughout the composite.

[0006] In various embodiments, providing the current collector can include providing a current collector comprising stainless steel. For example, providing the current collector can include providing a stainless steel foil. As another example, providing the current collector can include providing a clad foil comprising stainless steel on at least one side of the clad foil. Providing the mixture on the current collector can include providing the mixture on at least one side of a clad foil comprising stainless steel.

[0007] In various embodiments, providing the current collector can include providing a current collector comprising tungsten. For example, providing the current collector can include providing a tungsten foil. As another example, providing the current collector can include providing a clad foil comprising tungsten on at least one side of the clad foil. Providing the mixture on the current collector can include providing the mixture on at least one side of a clad foil comprising tungsten.

[0008] In various embodiments, providing the current collector can include providing a current collector having a polymer coated on at least one side of the current collector. In some such embodiments, providing the mixture can include providing the mixture on at least one side of the current collector including the polymer. In some embodiments, the polymer and the precursor can be the same material. The precursor can include polyamideimide, polyamic acid, polyimide, phenolic resin, or epoxy resin.

[0009] In some embodiments, providing the current collector may include providing a current collector having a carbon film coated on at least one surface of the current collector. In some such embodiments, providing the current collector having a carbon film coated on at least one surface of the current collector may include providing a carbon precursor on at least one surface of the current collector and pyrolyzing the carbon precursor to form a carbon film. The carbon precursor may include polyamideimide, polyamic acid, polyimide, phenolic resin, or epoxy resin. In some embodiments, providing the mixture may include providing the mixture on at least one surface of a current collector including a carbon film.

[0010] In some embodiments, providing the mixture can include preparing a slurry including the precursor and silicon particles. Providing the mixture can include slot-die coating the mixture onto a current collector. In some embodiments, the method can further include drying the mixture before pyrolyzing the mixture.

[0011] Providing the mixture can include including silicon particles such that the composite contains at least about 70% to about 90% silicon particles by weight. In some examples, providing the mixture can further include conductive particles. In some examples, providing the mixture can further include graphite.

[0012] In various embodiments, the electrode can be an anode. In some embodiments, the method can form a battery electrode.

[0013] In certain embodiments, a method for forming an electrode is provided. The method can include providing a current collector, providing a first carbon precursor on the current collector, and providing a mixture on the first carbon precursor. The mixture can include a second carbon precursor and silicon particles. The method can also include pyrolyzing the second carbon precursor to convert the second carbon precursor to one or more carbon phases and forming a composite comprising one or more carbon phases as a substantially continuous phase with silicon particles distributed throughout the composite. The method can also include pyrolyzing the first carbon precursor to adhere the composite to the current collector.

[0014] In various embodiments, pyrolysis of the first carbon precursor causes the pyrolyzed carbon to diffuse into the current collector. In some embodiments, the pyrolysis of the first and second carbon precursors can occur during the same heat treatment. In some examples, the pyrolysis of the first and second carbon precursors can occur at a temperature ranging from about 350°C to about 1350°C. For example, the pyrolysis of the first and second carbon precursors can occur at a temperature ranging from about 350°C to about 1275°C. As another example, the pyrolysis of the first and second carbon precursors can occur at a temperature ranging from about 700°C to about 1350°C. As another example, the pyrolysis of the first and second carbon precursors can occur at a temperature ranging from about 700°C to about 1275°C. As another example, the pyrolysis of the first and second carbon precursors can occur at a temperature ranging from about 900°C to about 1350°C. As another example, the pyrolysis of the first and second carbon precursors can occur at a temperature ranging from about 900°C to about 1275°C. In some embodiments, the first carbon precursor can have a carbide yield of 10% to 70%.

[0015] In various embodiments, the first carbon precursor can include polyamic acid, phenol formaldehyde resin, polypyrrole, polyacrylonitrile, polyamideimide, polyimide, polyimide precursor, or a combination thereof. For example, the polyimide precursor can include pyromellitic dianhydride oxydianiline (PMDA-ODA), biphenyltetracarboxylic dianhydride oxydianiline (BPDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA), or a combination thereof.

[0016] In some embodiments, providing the first carbon precursor can include coating the first carbon precursor on a current collector. In some examples, the method can further include drying the first carbon precursor before providing the mixture on the first carbon precursor. In some examples, the first carbon precursor on the current collector can have a thickness ranging from about 1 μm to about 1 mm. The first carbon precursor and the second carbon precursor can be chemically similar. Alternatively, the first carbon precursor can be chemically different from the second carbon precursor.

[0017] In some examples, providing the mixture can include preparing a slurry including the second carbon precursor and silicon particles. In some examples, the method can further include drying the mixture before pyrolyzing the second carbon precursor.

[0018] In various embodiments, the current collector can include a transition element and / or an alloy including a transition element. For example, the transition element or alloy can include chromium, molybdenum, iron, vanadium, tungsten, tantalum, niobium, or a combination thereof. In some examples, the alloy can include nickel and chromium. As an example, the alloy can include nichrome. In some examples, the alloy can include stainless steel. In some embodiments, the current collector can include a layer including a transition element and / or an alloy including a transition element on at least one side of the current collector, and a first carbon precursor can be provided on at least one side of the current collector. In some examples, the current collector can include nickel and / or copper.

[0019] In some embodiments, providing the mixture can include providing silicon particles such that the composite comprises about 60% to about 90% by weight of silicon particles. In some examples, the electrode can be an anode.

[0020] In certain embodiments, a method for forming an electrochemical device is provided. The method can include providing a first electrode. Providing the first electrode can include providing an electrode formed by the methods described herein. The method can also include providing a second electrode and providing an electrolyte. In some examples, the first electrode can be an anode and the second electrode can be a cathode. The electrochemical device can be a battery. [Brief explanation of the drawings]

[0021] [Figure 1] Images are shown before (Figure 1A) and after (Figure 1B) pyrolysis on copper foil. [Figure 2] Images of pyrolysis on nickel foil at 650°C (Figure 2A) and 750°C (Figure 2B) are shown. [Figure 3A] Shows an image of a silicon-carbon composite pyrolyzed at 700°C on copper foil. [Figure 3B] 1 shows a scanning electron microscope (SEM) image of a cross section of a silicon-carbon composite pyrolyzed on copper foil. [Figure 4] 1 illustrates an exemplary method of forming an electrode according to certain embodiments described herein. [Figure 5] An example of a slurry of carbon precursor and silicon particles coated onto a stainless steel foil and dried is shown. [Figure 6] An example of a pyrolytic composite on stainless steel is shown. [Figure 7] 1 illustrates an exemplary method for forming an electrode according to certain embodiments described herein. [Figure 8] Shows an image of a silicon-carbon composite pyrolyzed at 700°C on nichrome foil. [Figure 9] 1 is a graph showing the relationship between discharge capacity and cycle number for various samples. [Figure 10] 1 is a graph showing IEC (International Electrotechnical Commission) capacity versus cycle number for various samples. [Figure 11]1 shows SEM images at various magnifications of a silicon-carbon composite pyrolyzed on nichrome foil. [Figure 12] 1 shows SEM images at various magnifications of a silicon-carbon composite pyrolyzed on a stainless steel foil. [Figure 13] 1 is a graph showing coin cell capacity (mAh) versus cycle number for various samples. [Figure 14] 1 is a graph showing coin cell discharge capacity (mAh) versus cycle number for various samples. DETAILED DESCRIPTION OF THE INVENTION

[0022] This application describes certain embodiments of electrodes (e.g., anodes and cathodes) and electrochemical cells that may include a carbonized polymer and a silicon material. For example, a mixture containing a carbon precursor with a silicon material can be formed into a composite. Because this mixture can contain both carbon and silicon, it can be referred to as a carbon-silicon composite, a silicon-carbon composite, a carbon composite, or a silicon composite. This application also describes certain methods for forming the composite on a current collector. Mixtures containing a carbon precursor and a silicon material have not currently been pyrolyzed directly on a current collector (e.g., a copper or nickel current collector). During the carbonization process (e.g., by heat), the silicon and / or carbon can react directly with the metal current collector (e.g., forming a copper or nickel silicide or carbide). The metal silicide or carbide can prevent the composite from adhering to the current collector and / or destroy the current collector by converting it to a different material. For example, Figures 1A and 1B show images of silicon-carbon precursor slurries coated onto 15 μm copper foil, dried, and pyrolyzed at 650°C and 750°C under an argon atmosphere. Figure 1A shows the current collector before pyrolysis, and Figure 1B shows the current collector after pyrolysis. The copper decomposed at 750°C. Without being bound by theory, copper silicide formed due to reaction between the foil and silicon, resulting in pitting in the foil. Similarly, Figures 2A and 2B show images of slurries coated, dried, and pyrolyzed onto nickel foil at 650°C and 750°C, respectively. The nickel foil decomposed due to reaction with silicon at the composite / foil interface. Figure 3A shows a silicon-carbon composite pyrolyzed on copper foil at 700°C. Tape tests indicated poor adhesion at the composite / foil interface. Additionally, bend tests resulted in completely exposed, uncoated foil. 3B is a scanning electron microscope (SEM) image of a cross section of a silicon-carbon composite pyrolyzed on copper foil. The SEM image shows poor adhesion at the composite-to-foil interface. In various embodiments described herein, a carbon precursor containing silicon material can be advantageously pyrolyzed on a current collector while ensuring sufficient bonding to the current collector and / or with relatively little or no detrimental conversion of the current collector.

[0023] A typical carbon anode includes a current collector, such as a copper plate. Carbon is deposited on the current collector along with an inert binder material. Carbon is often used because it has excellent electrochemical properties and is electrically conductive. Anodes used in rechargeable lithium-ion batteries generally have a specific capacity of approximately 200 milliampere-hours per gram (including the metal foil current collector, conductive adhesive, and binder material). Graphite, the active material used in many lithium-ion battery anodes, has a theoretical energy density of 372 milliampere-hours per gram (mAh / g). Silicon, on the other hand, has a high theoretical capacity of 4200 mAh / g. However, silicon expands by more than 300% upon lithiation. This expansion causes anodes containing silicon to expand and contract, losing electrical contact with the rest of the anode. Therefore, silicon anodes must be designed to allow expansion while maintaining good electrical contact with the rest of the anode.

[0024] U.S. Patent No. 9,178,208, U.S. Patent Application Publication No. 2014 / 0170498, and U.S. Patent No. 9,553,303, each of which is incorporated herein by reference, describe specific embodiments of carbon-silicon composites using a carbonized polymer and a silicon material. The carbonized polymer can act as an expansion buffer for the silicon particles during cycling, enabling high cycle life to be achieved. In certain embodiments, the resulting electrode can be an electrode consisting essentially of the active material. For example, the carbonized polymer can form a substantially continuous conductive carbon phase throughout the electrode, as opposed to the particulate carbon suspended in a non-conductive binder in some conventional lithium-ion battery electrodes. Because the polymer can be converted into a conductive, electrochemically active matrix, in some embodiments, the resulting electrode can be sufficiently conductive that metal foil or mesh current collectors can be omitted, minimized, or reduced. Thus, U.S. Patent No. 9,178,208, U.S. Patent Application Publication No. 2014 / 0170498, and U.S. Patent No. 9,553,303 disclose specific embodiments of monolithic, free-standing electrodes that can have high energy densities of about 500 mAh / g to about 3500 mAh / g due to, for example, 1) the use of silicon, 2) the elimination or substantial reduction of metal current collectors, and 3) the electrodes being composed entirely or substantially entirely of active material.

[0025] Current collectors may be preferred in some applications, such as applications where current exceeds a certain threshold or additional mechanical support is desired. As mentioned above, mixtures containing carbon precursors and silicon materials are not currently pyrolyzed directly on current collectors because it is believed that the carbon and / or silicon may react with the metal current collector during the pyrolysis process. To overcome this challenge, the mixture can first be applied to a substrate, pyrolyzed, removed from the substrate, and then bonded to a current collector. U.S. Pat. Nos. 9,397,338 and 9,583,757, each of which is incorporated herein by reference, describe specific embodiments of composites bonded to a current collector using an electrode bonding substrate.

[0026] This application also describes certain embodiments of electrodes including current collectors, electrochemical cells including such electrodes, and methods of forming such electrodes and electrochemical cells. For example, in various embodiments, the electrodes include composites bonded to a current collector. The electrodes described herein can be used as anodes in lithium-ion batteries, and they can also be used as cathodes in electrochemical combination with additional additives. The electrodes can also be used in secondary (e.g., rechargeable) or primary (e.g., non-rechargeable) batteries. Various embodiments also include materials pyrolyzed on the current collector that are sufficiently adhesive to the current collector and exhibit relatively little or no reaction with the metal current collector.

[0027] FIG. 4 illustrates an exemplary method for forming an electrode according to certain embodiments described herein. The method 100 for forming an electrode can include providing a current collector, as shown in block 110. The method 100 can also include providing a mixture on the current collector, as shown in block 120. The mixture can include a precursor (e.g., a carbon precursor) and silicon particles. As shown in block 130, the method 100 can further include pyrolyzing the mixture on the current collector. Pyrolyzing the mixture can convert the precursor to one or more carbon phases with the silicon particles distributed throughout the composite as a substantially continuous phase, and adhere the composite to the current collector. Thus, various embodiments described herein can pyrolyze a mixture of carbon precursor and silicon particles on the current collector to form a carbon-silicon composite that adheres to the current collector. Such embodiments can advantageously result in higher yields due to less handling of the fragile electrode. Such embodiments can advantageously result in faster processing and lower costs.

[0028] Without being bound by any particular theory, the silicon and / or carbon in the mixture may react with metals, such as copper or nickel, in the current collector, possibly forming metal silicides or carbides that prevent adhesion to the current collector and / or destroy the current collector by converting it to a different material. In various embodiments described herein, the use of a current collector that reduces the probability of reaction with silicon and / or carbon reduces (and / or in some instances avoids) the formation of metal silicides and / or carbides, allowing the composite to adhere to the current collector while maintaining the conductive metallic properties of the current collector. Some embodiments may include applying and pyrolyzing a mixture of a carbon precursor and a silicon material on a current collector that includes a material that does not react with silicon and / or carbon. For example, instead of using a copper or nickel current collector, some embodiments may include applying and pyrolyzing a mixture of a carbon precursor and a silicon material on a current collector that includes stainless steel, tungsten, or a combination thereof. Stainless steel and tungsten are believed not to react with silicon and carbon in the same way as copper or nickel. As another example, some embodiments may include providing and pyrolyzing a mixture of a carbon precursor and a silicon material on a current collector coated with a layer of polymer or carbon. Without being bound by any particular theory, the presence of the coating layer on the current collector may separate the current collector from the silicon and / or carbon in the mixture, reducing and / or preventing the formation of metal silicides and / or carbides in some instances. The process of FIG. 4 will now be described.

[0029] Referring to block 110, a current collector is provided. The provided current collector may include a current collector comprising stainless steel. In some embodiments, the current collector may comprise primarily stainless steel. For example, the current collector may comprise stainless steel metal, e.g., stainless steel foil. In some other embodiments, the current collector may comprise stainless steel as one of a number of materials. For example, the current collector may comprise a clad material comprising stainless steel, e.g., a clad foil comprising stainless steel on at least one side (e.g., one or both sides) of the clad foil. In some embodiments, the current collector may comprise primarily tungsten. For example, the current collector may comprise tungsten metal, e.g., tungsten foil. In some other embodiments, the current collector may comprise tungsten as one of a number of materials. For example, the current collector may comprise a clad material comprising tungsten, e.g., a clad foil comprising tungsten on at least one side (e.g., one or both sides) of the clad foil. As another example, the current collector may comprise a clad material comprising tungsten on one side and stainless steel on the other side.

[0030] In some embodiments, the current collector may include a polymer coating. For example, the current collector may include a polymer coating on a copper or nickel current collector. As another example, the current collector may include a polymer coating on a stainless steel and / or tungsten current collector. At least one side of the current collector may be coated with a polymer. The polymer coating may include a carbon precursor, e.g., any of the precursors described herein, such as polyamideimide. In some embodiments, the polymer coating may be the same material as the precursor in the mixture. In some other embodiments, the polymer coating may not be the same material as the precursor in the mixture. In various embodiments, the polymer coating may include any one or more of the polymers disclosed herein, including polyamideimide, polyamic acid, polyimide, polyimide precursor, phenolic resin (e.g., phenol-formaldehyde resin), polypyrrole, polyacrylonitrile, and epoxy resin. The polyimide precursor can include pyromellitic dianhydride oxydianiline (PMDA-ODA), biphenyltetracarboxylic dianhydride oxydianiline (BPDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA), or combinations thereof. The thickness of the polymer coating in various embodiments can be in the range of about 200 nanometers to about 5 microns (e.g., about 200 nm, about 250 nm, about 300 nm, about 400 nm, about 500 nm, about 600 nm, about 700 nm, about 750 nm, about 800 nm, about 900 nm, about 1 micron, about 2 microns, about 3 microns, about 4 microns, about 5 microns, about 10 microns, about 50 microns, about 100 microns, about 250 microns, about 500 microns, about 750 microns, about 1 mm, or any value within this range, etc.), or a range formed by any of the values ​​within this range. In some examples, a thickness of about 1 μm to about 1 mm can reduce (and / or prevent) interaction of the silicon with the current collector during coating and pyrolysis.

[0031] In some embodiments, the current collector having the polymer coating may be heat-treated prior to further processing (e.g., prior to disposing the mixture on the current collector). The heat treatment can result in the formation of a carbon-coated current collector through a pyrolysis process. The pyrolysis process can be similar to the process of pyrolyzing the mixture described herein. Thus, in some embodiments, the current collector that is provided can include a current collector coated with a carbon material (e.g., a carbon film).

[0032] Referring to block 120, the mixture is applied to a current collector. In some embodiments, the mixture can be applied to a current collector, such as a current collector comprising stainless steel, tungsten, or a combination thereof. For example, the mixture can be coated (e.g., directly coated, in various embodiments) on a stainless steel or tungsten foil. As another example, the mixture can be coated (e.g., directly coated, in various embodiments) on at least one side of a clad foil comprising stainless steel, tungsten, or a combination thereof. The other side of the clad foil can comprise a different material, such as copper or nickel. In some embodiments, the clad foil can comprise stainless steel on both sides of the clad foil, tungsten on both sides of the clad foil, or stainless steel on one side and tungsten on the other side of the clad foil. In some such examples, the mixture can also be coated on both sides of the clad foil.

[0033] As another example, the mixture can be provided on a current collector coated with a polymer, carbon, or a combination thereof on at least one side of the current collector. The mixture can be provided on the side of the current collector coated with the polymer, carbon, or a combination thereof. The current collector can also be coated with a polymer, carbon, or a combination thereof on both sides (e.g., a polymer coating on both sides of the current collector, a carbon coating on both sides of the current collector, or a carbon coating on one side and a polymer coating on the other side, etc.), and the mixture can be provided on both sides of the current collector. In some examples, a current collector having a polymer coating, a carbon coating, or a combination thereof may include a current collector including stainless steel, tungsten, or a combination thereof. However, in some examples, a current collector having a polymer coating, a carbon coating, or a combination thereof does not necessarily include stainless steel, tungsten, or a combination thereof. For example, a current collector having a polymer or carbon coating can include copper or nickel in some embodiments.

[0034] The mixture disposed on the current collector may include any of the mixtures described in U.S. Pat. No. 9,178,208, U.S. Patent Application Publication No. 2014 / 0170498, and / or U.S. Pat. No. 9,553,303. The mixture may include a wide 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 polyamideimide, polyamic acid, polyimide, polyimide precursor, etc. Other precursors include phenolic resins (phenol formaldehyde resins), polypyrrole, polyacrylonitrile, epoxy resins, and other polymers. The polyimide precursor may include pyromellitic dianhydride oxydianiline (PMDA-ODA), biphenyltetracarboxylic dianhydride oxydianiline (BPDA-ODA), biphenyltetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA), or combinations thereof. The mixture can further include a solvent. For example, the solvent can be N-methyl-pyrrolidone (NMP). Other possible solvents include acetone, diethyl ether, gamma butyrolactone, isopropanol, dimethyl carbonate, ethyl carbonate, dimethoxyethane, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and dimethylacetamide (DMAc). As an example, a high molecular weight (e.g., greater than 200,000 g / mol) polyamideimide powder can be dispersed overnight in a dipolar aprotic solvent such as N-methyl-2-pyrrolidone (NMP) at 75°C. Higher temperatures below the gelation temperature and / or below the flash point of the solvent can also be used. Examples of precursor and solvent solutions include PI-2611 (HD Microsystems), PI-5878G (HD Microsystems), and VTEC PI-1388 (RBI). PI-2611 consists of over 60% n-methyl-2-pyrrolidone and 10-30% s-biphenyl dianhydride / p-phenylenediamine.PI-5878G is composed of greater than 60% n-methylpyrrolidone, 10-30% polyamic acid of pyromellitic dianhydride / oxydianiline, and 10-30% aromatic hydrocarbons (petroleum distillates) containing 5-10% 1,2,4-trimethylbenzene. In certain embodiments, the amount of precursor (e.g., solid polymer) in the solvent is about 10% to about 30% by weight.

[0035] The mixture can include silicon particles as described herein. The mixture can include about 5% to about 80% by weight of precursor and greater than 0% to about 99% by weight of silicon particles. Additional materials can also be included in the mixture. For example, graphite active material, chopped or crushed carbon fiber, carbon nanofibers, carbon nanotubes, and other carbon particles, including conductive carbon, can be added to the mixture. Conductive particles can also be added to the mixture. The mixture can also be mixed to homogenize the mixture. In some instances, the silicon particles can be dispersed in the precursor under high shear conditions. For example, a planetary mixer can be used. As another example, a ball mill can be used to achieve deagglomeration of the silicon particles in a solvent that can then be dispersed in a resin to produce a slurry mixture.

[0036] In certain embodiments, the mixture can be cast onto a current collector. In some embodiments, casting can involve using gap extrusion, tape casting, or blade casting techniques. Blade casting techniques can involve applying a coating to a current collector by using a flat surface (e.g., a blade) controlled to a fixed distance above the current collector. A slurry (or liquid) can be applied to the current collector, and a blade can be passed over the slurry to spread it over the current collector. As the slurry passes through the gap, the thickness of the coating can be controlled by the gap between the blade and the current collector. As the slurry passes through the gap, excess slurry can be scraped off. For example, the mixture can be cast onto a current collector. In some embodiments, the mixture can then be dried to remove the solvent. In some examples, the mixture can be dried in a conventional oven. For example, the polyamic acid and NMP solution can be dried at about 110°C for about 2 hours to remove the NMP solution. In some embodiments, the dried mixture can be further dried or cured. In some embodiments, the mixture can be heated and pressed (e.g., between graphite plates in an oven). A heated press can be used to dry and flatten the dried mixture. For example, the dried mixture of polyamic acid and NMP solution can be heated and pressed at about 200°C for about 8 to 16 hours. Alternatively, the entire process, including casting and drying, can be performed as a roll-to-roll process using standard film processing equipment. The dried mixture can be washed to remove any remaining solvent or etchant. For example, deionized (DI) water can be used to wash the dried mixture. The dried mixture can be cut or mechanically divided into smaller pieces. In some embodiments, the mixture can be coated onto a current collector by a slot die coating method (e.g., metering a constant or substantially constant weight and / or volume through a defined or substantially defined gap). Figure 5 shows an example of a slurry of carbon precursor and silicon particles coated and dried on a stainless steel foil.

[0037] Referring to block 130 of FIG. 4, the mixture further undergoes pyrolysis. In various embodiments, pyrolysis can convert the precursor to carbon, and the pyrolyzed material can be deposited on a current collector. For example, after drying the mixture, the material on the current collector can be drilled and pyrolyzed in a furnace. Different pyrolysis ramp rates and final hold temperatures can be utilized to achieve the desired electrode. FIG. 6 shows an example of a pyrolyzed composite in stainless steel. These samples were coated on one side, dried, and punched into circles prior to pyrolysis.

[0038] In certain embodiments, the mixture is pyrolyzed in a reducing atmosphere. For example, an inert atmosphere, a vacuum, and / or a flow of argon, nitrogen, or helium gas can be utilized. In some embodiments, the mixture is pyrolyzed at temperatures ranging from about 350°C to about 1275°C, about 400°C to about 1275°C, about 450°C to about 1275°C, about 500°C to about 1275°C, about 550°C to about 1275°C, about 600°C to about 1275°C, about 650°C to about 1275°C, about 700°C to about 1275°C, about 750°C to about 1275°C, about 800°C to about 1275°C, about 850°C to about 1275°C, about 900°C to about 1275°C, about 950°C to about 1275°C, about 1000°C to about 1275°C, or about 1275°C. The mixture may be heated to a temperature range of about 75°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 550°C to about 1350°C, about 600°C to about 1350°C, about 650°C to about 1350°C, about 700°C to about 1350°C, about 750°C to about 1350°C, about 800°C to about 1350°C, about 850°C to about 1350°C, about 900°C to about 1350°C, about 950°C to about 1350°C, or about 1000°C to about 1350°C. For example, a polyimide formed from a polyamic acid may be carbonized at about 1175°C for about 1 hour. In certain embodiments, the heating and / or cooling rate of the mixture is about 10°C / min. A holder may be used to hold the mixture in a particular geometry, and may be graphite or metal, for example. In certain embodiments, the mixture is held flat. After pyrolysis of the mixture, tabs can be bonded to the pyrolyzed material to form electrical contact. For example, nickel, copper, or alloys thereof can be used for the tabs.

[0039] Silicide formation can be reduced by adjusting the silicon content, thickness of the current collector, and / or by having a barrier layer between the composite and the current collector. As described herein, providing a current collector can include providing a current collector coated with a polymer or carbon layer. While providing such a layer can reduce reaction between the composite and the current collector, in some instances, the layer can also weaken adhesion at the interface. Various embodiments described herein can improve adhesion between the composite and the current collector while reducing (and / or preventing) reaction.

[0040] FIG. 7 illustrates an exemplary method of forming an electrode according to certain embodiments described herein. A method 200 for forming an electrode can include providing a current collector, as shown in block 210. Method 200 can also include providing a first carbon precursor on the current collector, as shown in block 215. Method 200 can also include providing a mixture on the first carbon precursor, as shown in block 220. The mixture can include a second carbon precursor and silicon particles. Method 200 can further include pyrolyzing the second carbon precursor to convert the second carbon precursor to one or more carbon phases and form a composite including one or more carbon phases and silicon particles, as shown in block 230. The composite can include the one or more carbon phases as a substantially continuous phase with the silicon particles distributed throughout the composite. Method 200 can also include pyrolyzing the first carbon precursor to adhere the composite onto the current collector, as shown in block 235.

[0041] Referring to block 210, a current collector is provided. The current collector can include any of those described herein. To improve adhesion between the composite and the current collector, the current collector can include a transition element and / or an alloy containing a transition element. Exemplary current collectors can include chromium (Cr), molybdenum (Mo), iron (Fe), vanadium (V), tungsten (W), tantalum (Ta), and niobium (Nb) metals, or alloys containing these materials. For example, the current collector can include stainless steel containing Fe and Cr. As another example, the current collector can include nichrome containing Ni, Cr, and optionally Fe. The current collector can primarily include the material, or can include a clad material containing the material. The current collector can also include a layer of these materials on at least one side of another material. For example, the material can be deposited on a common current collector (e.g., Ni or Cu). Without being bound by theory, the carbon (as pyrolyzed) may partially diffuse (e.g., by thermal diffusion) into the transition elements in the current collector, thereby improving adhesion between the pyrolyzed composite and the current collector. Other materials may also be used, such as materials that allow thermal diffusion of carbon at temperatures above the carbonization temperature (e.g., greater than 350°C for some precursors).

[0042] Referring to block 215, a first carbon precursor is applied to the current collector. As described herein, a first layer of carbon precursor material can be coated on the current collector. The first carbon precursor can be any of those described herein, including, but not limited to, polyamic acid, phenol-formaldehyde resin, polyimide (e.g., PMDA-ODA or BPDA-ODA), polyacrylonitrile, polypyrrole, and the like. Some such precursors can be carbonized to a char yield of about 10% to about 70%. In some examples, a char yield of less than 10% can result in voids that can affect adhesion. In some examples, method 201 can include, in block 220, drying the first carbon precursor before applying the mixture to the first carbon precursor. The first carbon precursor can be dried in a conventional oven at a temperature high enough to dry at least a portion or most of the solvent and low enough to reduce or avoid oxidation of the foil. The first carbon precursor on the current collector can have a thickness ranging from about 1 μm to about 1 mm. Some such thicknesses can reduce (and / or prevent) interaction between the silicon and the current collector during coating and pyrolysis. The thickness of the first carbon precursor and its layer may be selected, at least in part, based on the silicon particle size and / or the roughness of the composite coated with the first carbon precursor layer.

[0043] Referring to block 220, a mixture including a second carbon precursor and silicon particles can be provided on the first carbon precursor. The second carbon precursor can be any of those described herein. For example, the mixture can include a slurry including the second carbon precursor and silicon particles. The second carbon precursor can be chemically the same as or different from the first carbon precursor. The mixture can be dried similarly to the first carbon precursor. In some examples, the mixture is dried before pyrolyzing the second carbon precursor in block 230.

[0044] With respect to blocks 230 and 235, the second carbon precursor can be pyrolyzed to convert the second carbon precursor into one or more carbon phases and form a composite comprising carbon and silicon, and the first carbon precursor can be pyrolyzed to adhere the composite to the current collector. In some embodiments, pyrolyzing the first carbon precursor can cause the pyrolyzed carbon to diffuse into the current collector. The first and second carbon precursors can be advantageously pyrolyzed during the same heat treatment. For example, both coating layers of the precursor can be subjected to the same pyrolysis treatment. In some examples, the first carbon precursor may be pyrolyzed before the second carbon precursor. In some examples, the second carbon precursor may be pyrolyzed before the first carbon precursor. The precursors can be pyrolyzed as described herein. For example, the first and / or second carbon precursors can be pyrolyzed in a furnace under an inert or reducing atmosphere. The precursors can be pyrolyzed at temperatures described herein, for example, from about 350°C to about 1275°C, from about 350°C to about 1350°C, from about 700°C to about 1275°C, from about 700°C to about 1350°C, from about 900°C to about 1275°C, or from about 900°C to about 1350°C.

[0045] As described herein, without being bound by theory, a current collector including a transition element (e.g., Cr, Mo, Fe, V, W, Ta, Nb, etc.) or an alloy including a transition element can allow carbon (formed by pyrolysis of the first layer) to partially diffuse through the current collector. Such diffusion can help ensure sufficient adhesion (and / or good adhesion and / or excellent adhesion) between the silicon-carbon composite and the current collector. The barrier carbon layer can also reduce (and / or prevent) silicide formation with the current collector by reducing (and / or preventing) contact between the silicon and the current collector.

[0046] FIG. 8 shows a silicon-carbon composite pyrolyzed at 700°C on nichrome foil adjacent to the tape after tape testing. The tape test showed better adhesion compared to the silicon-carbon composite pyrolyzed on copper foil, shown in FIG. 3A. Without being bound by theory, the adhesion can be attributed to the diffusion of carbon (during pyrolysis) from the first layer to the Cr in the nichrome. For stainless steel, the adhesion can be attributed to the diffusion of carbon (during pyrolysis) from the first layer to the Fe and Cr in the stainless steel. Such diffusion can also apply to foils with Mo, W, Ta, Nb, V, etc.

[0047] In certain embodiments, one or more of the methods described herein are continuous processes. For example, casting, drying, possibly curing, and pyrolysis can be performed in a continuous process. For example, the mixture can be coated onto a current collector, dried, and pyrolyzed. The mixture can be dried while rotating in a cylinder to form a film. The dried mixture on the current collector can be transferred as a roll and sent to another machine for further processing. Extrusion and other membrane manufacturing techniques known in the industry can also be used prior to the pyrolysis step.

[0048] 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 in addition to the carbonized precursor, a composite can be formed. In particular, as described herein, the mixture can include silicon particles to form a silicon-carbon (e.g., at least one first phase comprising silicon and at least one second phase comprising carbon) or silicon-carbon-carbon (e.g., at least one first phase comprising silicon, at least one second phase comprising carbon, and at least one third phase comprising carbon) composite.

[0049] Silicon particles can increase the specific lithium insertion capacity of the composite. When silicon absorbs lithium ions, it experiences a significant volume increase of 300 volume percent, potentially posing problems for the structural integrity of the electrode. In addition to the volume increase issue, silicon is not inherently conductive, but it becomes conductive when alloyed with lithium (e.g., lithiated). When silicon is delithiated, its surface loses conductivity. Delithiation can also cause a volume decrease, potentially causing the silicon particles to lose contact with the matrix. The sudden change in volume can also lead to mechanical defects in the silicon particle structure, which then pulverize. Pulverization and loss of electrical contact have made it difficult to use silicon as an active material in lithium-ion batteries. Reducing the initial particle size of the silicon particles can prevent further pulverization of the silicon powder and minimize the loss of electrical conductivity. Additionally, adding a material to the composite that can elastically deform with the volume change of the silicon particles can reduce the likelihood of electrical contact loss to the silicon surface. For example, the composite may include carbon such as graphite, which contributes to the composite's ability to absorb expansion and may also intercalate (e.g., be chemically active with) lithium ions to increase the storage capacity of the electrode. Thus, the composite may include one or more carbon phases.

[0050] As described herein, silicon may be used as an active material for the cathode or anode to increase the volumetric and gravimetric energy density of lithium-ion batteries. Several types of silicon materials, such as silicon nanopowder, silicon nanofiber, porous silicon, and ball-milled silicon, are viable candidates as active materials for the cathode or anode.

[0051] In some embodiments, all, substantially all, or at least a portion of the silicon particles may have a particle size (e.g., diameter or largest dimension of the particle) 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, from about 10 nm to about 50 μm, from about 10 nm to about 40 μm, from about 10 nm to about 30 μm, from about 10 nm to about 20 μm, from about 0.1 μm to about 20 μm, from about 0.5 μm to about 20 μm, from about 1 μm to about 20 μm, from about 1 μm to about 15 μm, from about 1 μm to about 10 μm, from about 10 nm to about 10 μm, from about 10 nm to about 1 μm, less than about 500 nm, less than about 100 nm, and about 100 nm. For example, in some embodiments, the average particle size (or average diameter or average maximum dimension) or median particle size (or median diameter or median 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 1 μm, less than about 500 nm, less than about 100 nm, and about 100 nm. In some embodiments, the silicon particles may have a particle size distribution. 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 may have a particle size described herein.

[0052] The amount of silicon included in the mixture or composite can be greater than zero weight percent of the mixture and / or composite. In certain embodiments, the amount of silicon is in the range of about 0% to about 99% by weight of the composite, 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, or about 30% to about 80% by weight. , at least about 50% by weight, about 50% to about 99% by weight, about 50% to about 95% by weight, about 50% to about 90% by weight, about 50% to about 80% by weight, about 50% to about 70% by weight, at least about 60% by weight, about 60% to about 99% by weight, about 60% to about 95% by weight, about 60% to about 90% by weight, about 60% to about 80% by weight, at least about 70% by weight, about 70% to about 99% by weight, about 70% to about 95% by weight, or about 70% to about 90% by weight. In various embodiments described herein, the amount of silicon can be 90% or more by weight, e.g., from about 90% or more to about 95% by weight, from about 90% or more to about 97% by weight, from about 90% or more to about 99% by weight, from about 92% or more to about 99% by weight, from about 95% or more to about 99% by weight, from about 97% or more to about 99% by weight, etc.

[0053] According to certain embodiments described herein, certain micron-sized silicon particles with nanometer surface features can achieve high energy densities and can be used in composites and / or electrodes used in electrochemical cells to improve performance during cell cycling. Small silicon particle sizes (e.g., nanometer-range particle sizes) generally enable electrodes to have long cycle life. They can also exhibit very high irreversible capacity. However, small particle sizes can result in very low volumetric energy densities (e.g., across a cell stack) due to difficulties in packing the active material. Larger particle sizes (e.g., micrometer- or micron-range particle sizes) generally enable higher anode material density. However, swelling of the silicon active material can reduce cycle life due to particle cracking.

[0054] In some embodiments, micron-sized silicon particles can provide good volumetric and gravimetric energy density combined with good cycle life. In certain embodiments, to obtain both the advantages of micron-sized silicon particles (e.g., high energy density) and nanometer-sized silicon particles (e.g., good cycle behavior), the silicon particles can have an average particle size in the micron range and a surface including nanometer-sized shapes. In some embodiments, the silicon particles have an average particle size (e.g., average diameter or average maximum dimension) or intermediate particle size (e.g., intermediate diameter or intermediate maximum dimension) of about 0.1 μm to about 30 μm, or any value between about 0.1 μm and 30 μm. For example, in some embodiments, the silicon particles can have an average 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, or about 5 μm to about 20 μm, etc. Thus, the average or median particle size can be any value between about 0.1 μm and about 30 μm, such as 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. Nanometer-sized features can include an average feature size (e.g., average diameter or average largest 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 can include silicon material.

[0055] The silicon particles may also 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 may have a particle size described herein.

[0056] In certain embodiments, the silicon particles are at least partially crystalline, substantially crystalline, and / or fully crystalline. The silicon particles may or may not be substantially pure silicon. 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 along with one or more other elements. For example, these elements may include aluminum (Al), iron (Fe), copper (Cu), oxygen (O), or carbon (C).

[0057] In certain embodiments described herein, the average surface area per unit mass (e.g., by Brunauer-Emmett-Teller (BET) particle surface area measurement) is about 1 m 2 / g to about 30m 2 / g, approx. 1m 2 / g to about 25m 2 / g, approx. 1m 2 / g to about 20m 2 / g, approx. 1m 2 / g to about 10m 2 / g, approx. 2m 2 / g to about 30m 2 / g, approx. 2m 2 / g to about 25m 2 / g, approx. 2m 2 / g to about 20m 2 / g, approx. 2m 2 / g to about 10m 2 / g, approx. 3m 2 / g to about 30m 2 / g, approx. 3m 2 / g to about 25m 2 / g, approx. 3m 2 / g to about 20m 2 / g, approx. 3m 2 / g to about 10m 2 / g (for example, about 3m 2 / g to about 6m 2 / g), approx. 5m 2 / g to about 30m 2 / g, approx. 5m 2 / g to about 25m 2 / g, approx. 5m 2 / g to about 20m 2 / g, approx. 5m 2 / g to about 15m 2 / g, or approximately 5m 2 / g to about 10m 2 / g.

[0058] The silicon particles described herein generally have a larger average particle size than silicon particles used in conventional electrodes. In some embodiments, the silicon particles described herein generally have a smaller average surface area. Without being bound by any particular theory, the smaller surface area of ​​the silicon particles described herein may contribute to improved performance of electrochemical cells.

[0059] Advantageously, the silicon particles described herein can improve the performance of electrochemically active materials, for example, improving capacity and / or cycling performance, and electrochemically active materials comprising such silicon particles do not appear to significantly decompose upon lithiation of the silicon particles.

[0060] In some embodiments, the range of carbon yields and / or the quality of carbon can be based, at least in part, on the pyrolysis conditions (e.g., final hold temperature and time, ramp rate and atmosphere, etc.) and / or precursor material. In some examples, the amount of carbon obtained from the precursor can be greater than about 0 wt.% to about 80 wt.%, about 5 wt.% to about 80 wt.%, about 5 wt.% to about 70 wt.%, 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 50 wt.%, about 10 wt.% to about 40 wt.%, about 10 wt.% to about 30 wt.%, or about 10 wt.% to about 25 wt.%, etc. For example, the amount of carbon obtained from the precursor can be about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, etc. of the precursor. When the amount of silicon is 90% by weight or more, the amount of carbon can be 10% by weight or less, for example, from about 0% by weight to about 3% by weight, from about 0% by weight to about 5% by weight, from about 0% by weight to about 10% by weight, from about 1% by weight to about 3% by weight, from about 1% by weight to about 5% by weight, from about 1% by weight to about 8% by weight, from about 1% by weight to about 10% by weight, or from about 5% by weight to about 10% by weight, etc.

[0061] The carbon from the precursor can be hard carbon. Hard carbon is carbon that does not convert to graphite when heated above 2800 degrees Celsius. Precursors that melt or flow upon pyrolysis are converted to soft carbon and / or graphite with sufficient temperature and / or pressure. Hard carbon may be selected because soft carbon precursors may flow, and soft carbon and graphite are mechanically weaker than hard carbon. Other possible hard carbon precursors include phenolic resins, epoxy resins, and other polymers that have very high melting points and are crosslinked. The amount of hard carbon in the composite can be within any of the ranges described herein for the amount of carbon obtained from the precursor. For example, in some embodiments, the amount of hard carbon in the composite can be about 10% to about 25% by weight, about 10% to about 30% by weight, about 10% to about 40% by weight, about 10% to about 50% by weight, about 10%, about 20%, about 30%, about 40%, about 50%, or greater than 50% by weight. When the amount of silicon is 90% or greater by weight, the amount of hard carbon can be 10% or less by weight, such as about 0% to about 3% by weight, about 0% to about 5% by weight, about 0% to about 10% by weight, about 1% to about 3% by weight, about 1% to about 5% by weight, about 1% to about 8% by weight, about 1% to about 10% by weight, or about 5% to about 10% by weight, etc.

[0062] In certain embodiments, the hard carbon phase is substantially amorphous. In other embodiments, the hard carbon phase is substantially crystalline. In further embodiments, the hard carbon phase comprises amorphous carbon and crystalline carbon. The hard carbon phase can be the matrix phase of the composite. The hard carbon can also be embedded in the pores of the silicon-containing additive. The hard carbon can react with a portion of the additive to form some material at the interface. For example, a silicon carbide layer can form between the silicon particles and the hard carbon.

[0063] In some embodiments, graphite is one of the carbon phases from the precursor. In certain embodiments, graphite particles are added to the mixture. Graphite can advantageously be both an electrochemically active material in the battery and an elastically deformable material that can accommodate the volumetric changes of the silicon particles. Graphite is a preferred active anode material for certain currently commercially available lithium-ion batteries due to its low irreversible capacity. Graphite is also softer than hard carbons and can better accommodate the volumetric expansion of the silicon additive. In certain embodiments, all, substantially all, or at least a portion of the graphite particles may have a particle size (e.g., diameter or maximum dimension) of about 0.5 microns to about 20 microns. In some embodiments, the graphite particles have an average particle size (e.g., average diameter or average maximum dimension) or a median particle size (e.g., median diameter or median maximum dimension) of about 0.5 microns to about 20 microns. In some embodiments, the graphite particles may have a particle size distribution. 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 may have a particle size described herein. In certain embodiments, the composite may comprise greater than 0% to less than about 80% by weight of graphite particles, including 40% to about 75% by weight, about 5% to about 30% by weight, 5% to about 25% by weight, 5% to about 20% by weight, or 5% to about 15% by weight. When the amount of silicon is 90% by weight or more, the amount of graphite can be 10% by weight or less, for example, from about 0% by weight to about 3% by weight, from about 0% by weight to about 5% by weight, from about 0% by weight to about 10% by weight, from about 1% by weight to about 3% by weight, from about 1% by weight to about 5% by weight, from about 1% by weight to about 8% by weight, from about 1% by weight to about 10% by weight, or from about 5% by weight to about 10% by weight, etc.

[0064] In certain embodiments, conductive particles, which may be electrochemically active, are added to the mixture. Such particles can enable both a more conductive composite and a more mechanically deformable composite that can accommodate the large volume changes caused during lithiation and delithiation. In certain embodiments, all, substantially all, or at least a portion of the conductive particles can have a particle size (e.g., diameter or largest dimension) of about 10 nanometers to about 7 millimeters. In some embodiments, the conductive particles have an average particle size (e.g., average diameter or average largest dimension) or a median particle size (e.g., median diameter or median largest dimension) of about 10 nm to about 7 millimeters. 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.

[0065] In certain embodiments, the mixture contains conductive particles in an amount from greater than zero to 80% by weight. In some embodiments, the composite contains about 45% to about 80% by weight of conductive particles. The conductive particles can be conductive carbon, including carbon black, carbon fiber, carbon nanofibers, carbon nanotubes, etc. Many carbons, which are considered electrochemically inactive conductive additives, become active when pyrolyzed in the polymer matrix. Alternatively, the conductive particles can be metals or alloys, including copper, nickel, or stainless steel. When the amount of silicon is 90% or more by weight, the amount of conductive particles can be 10% or less by weight, such as from about 0% to about 3% by weight, from about 0% to about 5% by weight, from about 0% to about 10% by weight, from about 1% to about 3% by weight, from about 1% to about 5% by weight, from about 1% to about 8% by weight, from about 1% to about 10% by weight, or from about 5% to about 10% by weight.

[0066] After the precursor is pyrolyzed, the resulting material is a composite adhered to the current collector. The current collector can provide additional mechanical support, allowing the composite to become a self-supporting monolithic structure, such as a self-supporting composite film. For example, the carbonized precursor can provide an electrochemically active structure that holds the composite together. In some embodiments, the carbonized precursor can be a substantially continuous phase. Thus, the carbonized precursor can be both a structural material and an electrochemically active and conductive material. In certain embodiments, the silicon particles and / or other material particles added to the mixture are distributed throughout the composite. In some embodiments, the silicon particles and / or other material particles can be uniformly distributed throughout the composite to form a uniform composite.

[0067] In some embodiments, the composite and / or electrode contains only trace amounts of polymer remaining after pyrolysis of the precursor. In further embodiments, the composite and / or electrode does not contain a non-conductive binder. The composite may contain voids. In some embodiments, the composite (or film) may contain about 1% to about 70% by volume, or about 5% to about 50% by volume voids. For example, the voids may be about 5% to about 40% by volume voids.

[0068] In certain embodiments, an electrode in an electrochemical device, such as a battery or electrochemical cell, can comprise a composite, including a composite with silicon particles described herein. For example, the composite can be used in an anode and / or cathode. The electrochemical device can include an electrolyte and can be a battery. In certain embodiments, the battery is a lithium ion battery. In further embodiments, the battery is a secondary battery, or in other embodiments, the battery is a primary battery.

[0069] Furthermore, the full capacity of the electrode composite described herein cannot be utilized during battery use to improve the battery's lifespan (e.g., the number of charge-discharge cycles until the battery dies or its performance drops below a practical level). For example, a composite having about 70% by weight of silicon particles, about 20% by weight of carbon from a precursor, and about 10% by weight of graphite can have a maximum weight capacity of about 3000 mAh / g, but the composite can only be used to a weight capacity of about 550 mAh / g to about 1500 mAh / g. Although the maximum weight capacity of the composite cannot be utilized, higher capacities than certain lithium-ion batteries can be achieved by using the composite at lower volumes. In certain embodiments, the composite is used at or only at a weight capacity less than about 70% of the composite's maximum weight capacity. The composite is not used at a weight capacity greater than about 70% of the composite's maximum weight capacity. In further embodiments, the composite is used at or only at a weight capacity less than about 50% of the composite's maximum weight capacity or less than about 30% of the composite's maximum weight capacity. [Example]

[0070] The following examples are presented to demonstrate the benefits of several embodiments of the electrodes, electrochemical cells, and methods of forming them. These examples are provided for illustrative purposes and should not be construed as limiting the scope of the disclosed embodiments.

[0071] Various embodiments described herein can simplify the manufacturing process by pyrolyzing the active material on the current collector (in various embodiments, directly on the current collector) instead of forming the electrochemically active material on a substrate, removing the active material from the substrate, and bonding the active material to a current collector. Example coin cells were fabricated using a standard cathode, a standard electrolyte, and an anode formed using various embodiments described herein. The example coin cells were compared to coin cells fabricated using a standard cathode, a standard electrolyte, and an anode formed by laminating pyrolyzed material (e.g., already pyrolyzed material) onto a copper or stainless steel current collector. Test conditions for various samples are included in Table I.

[0072] [Table 1]

[0073] FIG. 9 is a graph showing the relationship between discharge capacity and cycle number for various samples. Every 50 cycles was plotted to produce a graph of IEC (International Electrotechnical Commission) capacity versus cycle number, Figure 10. As shown, the samples including anodes formed by coating and pyrolyzing the active material onto a stainless steel current collector had the highest capacity compared to samples in which the anode was first formed and then laminated onto a copper or stainless steel current collector. Additional Examples

[0074] Preparation of the first layer High molecular weight polyamideimide powder (e.g., greater than 200,000 g / mol) was dispersed in a dipolar aprotic solvent, N-methyl-2-pyrrolidone (NMP), at 75°C overnight to yield a resin with a 10% solids content. This resin was then coated onto 50 μm stainless steel (316H) and 50 μm nichrome foil (20% Cr) using a manual coating device. The coating was dried in a convection oven at 100°C for 30 minutes and then under vacuum at 100°C overnight.

[0075] Preparation of Slurry and Untreated Anode Silicon nanoparticles / microparticles were dispersed in polyamic acid resin under high shear conditions (using a planetary agitator at 2000 rpm for 10 minutes) to obtain a uniform slurry containing more than 20 wt% Si. N-methyl-2-pyrrolidone (NMP) solvent was used to dilute and adjust the slurry viscosity to approximately 2000 cP. The slurry was then cast onto a first layer coated on foil and dried to remove most of the residual solvent. These dried anodes were then drilled to obtain 16 mm diameter samples.

[0076] pyrolysis The perforated green anode was then pyrolyzed under Ar at a flow rate of 5 scfh at a slow ramp rate of 5°C / min until it reached 900°C, at which temperature it was held for 2 hours. Cooling was performed at a similar ramp-down rate. The anode load without the foil was approximately 3.8 mg / cm. 2 was controlled.

[0077] Figure 11 shows SEM images at different magnifications of a cross section of a pyrolyzed anode on nichrome foil, demonstrating good anode / foil interfacial adhesion. Figure 12 shows SEM images at various magnifications of a cross section of a pyrolyzed anode on stainless steel, demonstrating good composite / foil interfacial adhesion.

[0078] The resulting electrodes were electrically tested in coin cells and compared to anodes obtained by pyrolyzing and laminating free-standing films on copper or stainless steel foil. Details of the coin cell construction are included in Table II, and the test conditions for the various samples are included in Table III. Table II Coin cell consists of: Cathode: 95% LCO, 2.5% PVDF, 2.5% carbon, 28 mg / cm 2 Electrolyte: 1M LiPF6 + 2% adiponitrile in a 3:7 FEC:EMC by weight ratio Anode: 3.8 mg / cm 2 , 80%Si active material, 20% pyrolytic carbon

[0079] [Table 2]

[0080] Figure 13 shows a graph of coin cell capacity (mAh) versus cycle number. The cells were charged to 4.3 V at 0.5 C for 5 hours. Figure 14 shows the discharge capacity. The cells were discharged to 2.75 V at 0.2 C every 50 cycles. Cells with anodes prepared by pyrolysis on a current collector as described herein showed improved performance compared to free-standing films laminated on copper or stainless steel foil. The cells showed good cycling behavior when cycled down from high to low cell voltages, allowing for higher Si delithiation.

[0081] Coating and pyrolysis of silicon-carbon composites (including silicon-based composites) is feasible and manufacturable. Certain embodiments described herein can allow the electrode material to be pyrolyzed on the current collector, the active material to be Si-based, and the resin to be pyrolyzed at relatively high temperatures (e.g., temperatures above 700°C, 800°C, 900°C, or 1000°C). Various embodiments can coat and pyrolyze silicon-carbon composites (e.g., including silicon-based composites) at temperatures that allow the carbon precursor to carbonize and diffuse into the current collector, providing adhesion. The active material can maintain electrode material / foil interface adhesion, which in some cases can be critical to improving cycle life at severe full-cell discharge voltages where Si is highly delithiated.

[0082] Various embodiments have been described above. While the invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative and not limiting. Those skilled in the art will recognize that various modifications and adaptations may be made thereto without departing from the spirit and scope of the invention, as defined in the appended claims. [Explanation of symbols]

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Claims

1. 1. A method of forming an electrode, comprising: providing a current collector; providing a first carbon precursor on a current collector; providing a mixture comprising a second carbon precursor and silicon particles on the first carbon precursor; pyrolyzing the second carbon precursor to convert the second carbon precursor to one or more carbon phases to form a composite comprising the one or more carbon phases as a substantially continuous phase with the silicon particles distributed throughout the composite; and Pyrolyzing the second carbon precursor and then pyrolyzing the first carbon precursor to adhere the composite onto the current collector, wherein pyrolyzing the first carbon precursor causes the pyrolyzed carbon to diffuse into the current collector. Including, the first carbon precursor is a polyamideimide powder having a molecular weight greater than 200,000 g / mol, and pyrolysis of the first carbon precursor occurs at a temperature in the range of 900°C to 1350°C; The method wherein the second carbon precursor is a polyamic acid resin.

2. 10. The method of claim 1, wherein the pyrolysis of the first and second carbon precursors occurs during the same heat treatment.

3. The method of claim 1 , wherein the first carbon precursor has a carbide yield of 10% to 70%.

4. The method of claim 1 , wherein providing the first carbon precursor comprises coating the first carbon precursor onto a current collector.

5. The method of claim 1 , further comprising the step of drying the first carbon precursor before applying the mixture onto the first carbon precursor.

6. 10. The method of claim 1, wherein the first carbon precursor on the current collector has a thickness in the range of 1 μm to 1 mm.

7. The method of claim 1 , wherein the step of providing a mixture comprises providing a slurry comprising the second carbon precursor and silicon particles.

8. 10. The method of claim 1, further comprising drying the mixture before pyrolyzing the second carbon precursor.

9. The method of claim 1 , wherein the current collector comprises a transition element and / or an alloy containing a transition element.

10. The method of claim 9, wherein the transition element or alloy comprises chromium, molybdenum, iron, vanadium, tungsten, tantalum, niobium, or a combination thereof.

11. The method of claim 9 , wherein the alloy comprises nickel and chromium.

12. The method of claim 10 , wherein the alloy comprises stainless steel.

13. 10. The method of claim 9, wherein a current collector includes a layer including the transition element and / or the alloy on at least one surface of the current collector, and a first carbon precursor is provided on the layer included on the at least one surface.

14. The method of claim 1 , wherein the current collector comprises nickel and / or copper.

15. 10. The method of claim 1, wherein the step of providing a mixture includes including silicon particles such that the composite in the resulting electrode includes 60% to 99% silicon particles by weight.

16. The method of claim 1 , wherein the electrode is an anode.

17. 1. A method of forming an electrochemical device, comprising: providing a first electrode, the first electrode comprising providing an electrode formed by the method of claim 1; providing a second electrode; and Providing an electrolyte A method comprising:

18. 18. The method of claim 17, wherein the first electrode is an anode and the second electrode is a cathode.

19. 18. The method of claim 17, wherein the electrochemical device is a battery.

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