Composite material for electrochemical storage

Abstract

To provide a composite material, and a method for forming the composite material.SOLUTION: A composite material described herein can be utilized as an electrode material for a battery. In certain embodiments, the composite material includes silicon particles between 0 and 90 wt%, and one or more types of carbon phases between 0 and 90 wt%. At least one of the one or more types of carbon phases can be a substantially continuous phase. A method of forming the composite material can include the steps of: providing a mixture including a precursor and silicon particles; and pyrolyzing the precursor to convert the precursor into one or more types of carbon phases to form the composite material.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application claims the benefit of U.S. Provisional Application No. 61 / 315,845, filed Mar. 19, 2010, and U.S. Provisional Application No. 61 / 295,993, filed Jan. 18, 2010, each of which is hereby incorporated by reference in its entirety.

[0002] This application relates to composite materials containing silicon and carbon. In particular, this application relates to composite materials for use in battery electrodes.

Background Art

[0003] Typically, a lithium ion battery includes a separator and / or an electrolyte between an anode and a cathode. In certain types of batteries, the separator, cathode, and anode materials are formed individually in sheet or membrane form. The sheets of the cathode, separator, and anode are then laminated, or the cathode and anode (e.g., electrodes) are wound together with a separator that separates them to form the battery. In the wound cathode, separator, and anode, each sheet must be sufficiently deformable or flexible so as to be wound without causing damage such as cracks, breaks, mechanical damage, etc. A typical electrode includes an electrochemically active material layer on a conductive metal (e.g., aluminum and copper). The membranes are wound or cut into fine pieces and then laminated into the laminate. The laminate is composed of alternating electrochemically active materials with a separator therebetween.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Means for Solving the Problems

[0005] In one embodiment, a composite material is provided. The composite material can include silicon particles greater than 0 wt% and less than about 90 wt%, and one or more carbon phases greater than 0 wt% and less than about 90 wt%. Further, at least one of the one or more carbon phases is a substantially continuous phase.

[0006] The silicon particles can have an average maximum dimension of less than about 1 μm. In some embodiments, the silicon particles comprise from about 20 wt% to about 80 wt% of the composite material.

[0007] At least one of the one or more carbon phases that is a substantially continuous phase can be electrochemically active and can be conductive. In some embodiments, at least one of the one or more carbon phases that is a substantially continuous phase includes hard carbon.

[0008] Also, the composite material can include other particles. For example, the one or more carbon phases can include graphite particles. The composite material can include conductive particles, metal particles, and the like.

[0009] The composite material can be substantially electrochemically active. Further, the composite material can be self-standing. In one embodiment, a battery electrode including the composite material described herein is provided.

[0010] In one embodiment, a method of using the composite material described herein is provided. The method can include using the composite material at a weight capacity of less than about 70% of the maximum weight capacity of the composite material.

[0011] In one embodiment, a method of forming the composite material is provided. The method can include providing a mixture including a precursor and silicon particles, and pyrolyzing the precursor to convert the precursor to one or more carbon phases to form the composite material.

[0012] After pyrolyzing the precursor, the mixture can form a self-standing composite structure. At least one of the one or more carbon phases can be a substantially continuous phase. At least one of the one or more carbon phases that is a substantially continuous phase can contain hard carbon. In some embodiments, the silicon particles comprise from about 20 wt% to about 80 wt% of the composite material. In further embodiments, the silicon particles are greater than 0 wt% and less than about 80% in the mixture, and the precursor is about 5 wt% and less than about 80% in the mixture.

[0013] The mixture can further contain a solvent. The precursor can include hydrocarbon compounds, polyimides, phenolic resins, etc. This method can further include casting the mixture onto a substrate, drying the mixture to form a film, removing the film from the substrate, and curing the film with a hot press. Also, this method can include forming a battery electrode from the composite material.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0015] Typical carbon anode electrodes include a current collector such as a copper sheet. Carbon is deposited on the collector together with an inert binder material. Carbon is often used because it has excellent electrochemical properties and is conductive. When the current collector layer (e.g., copper layer) is removed, carbon can no longer mechanically support itself. Thus, conventional electrodes require a support structure such as a collector that can function as an electrode. The electrode (e.g., anode or cathode) composite materials described in this application can produce self-standing electrodes. Since conductive carbonized polymers are used in the mechanical support and the current collector in the anode structure, the requirements for metal foil current collectors are removed or minimized. In contrast to particulate carbon suspended in a non-conductive binder in some conventional lithium-ion battery electrodes, carbonized polymers can form a substantially continuous conductive carbon phase throughout the electrode. The advantages of carbon composite mixtures utilizing carbonized polymers can include, for example, 1) high capacity, 2) enhanced overcharge / discharge protection, 3) low irreversible capacity due to the removal (or minimization) of metal foil current collectors, and 4) cost reduction expected from simplified manufacturing.

[0016] The anode electrodes currently used in rechargeable lithium-ion cells typically have a specific capacity of about 200 milliampere hours per gram (including the metal foil current collector, conductive additive, and binder material). Graphite, the active material used in most lithium-ion battery anodes, has a theoretical energy density of 372 milliampere hours per gram (mAh / g). In comparison, silicon has a high theoretical capacity of 4200 mAh / g. However, silicon swells by more than 300% when lithium is inserted. Due to this expansion, for the anode containing silicon to maintain electrical contact with silicon, the anode containing silicon must expand for silicon.

[0017] Also, this application describes a novel method for manufacturing monolithic, free-standing anodes using carbonized polymers. Since the polymer is converted into a conductive and electrochemically active matrix, the resulting electrode is sufficiently conductive such that a metal foil or mesh current collector can be eliminated or minimized. Also, during cycling, the converted polymer functions as a swelling buffer for the silicon particles, enabling a high cycle life. In one embodiment, the resulting electrode consists essentially of active material. In a further embodiment, the resulting electrode is essentially active material. The electrode can have a high energy density between about 500 mAh / g and about 1200 mAh / g, for example, by 1) use of silicon, 2) removal or substantial reduction of the metal current collector, and 3) consisting entirely (or almost entirely) of active material.

[0018] The composite materials described herein can be used as anodes for most conventional lithium-ion batteries; also, they can be used as cathodes in some electrochemical couplings with additional additives. The composite materials can also be used in either secondary batteries (e.g., rechargeable) or primary batteries (e.g., non-rechargeable). In one embodiment, the composite material has a free-standing structure. In a further embodiment, the composite material has a free-standing monolithic structure. For example, a collector need not be included in the electrode consisting of the composite material. In one embodiment, the composite material can be used to form a carbon structure as discussed in Patent Document 1 entitled "Carbon Electrode Structures for Batteries", the entirety of Patent Document 1 being incorporated herein by reference. Further, the composite materials described herein can be, for example, silicon composite materials, carbon composite materials, and / or silicon-carbon composite materials.

[0019] FIG. 1 illustrates an embodiment of a method 100 for forming a composite material. For example, the method for forming a composite material can include forming a mixture including a precursor, which can include block 101. The method can further include pyrolyzing the precursor to convert the precursor to a carbon phase. The precursor mixture can include carbon additives such as graphite active material, etched or ground carbon fibers, carbon nanofibers, carbon nanotubes, and / or other carbon. After the precursor is pyrolyzed, the resulting carbon material can be a self-standing monolithic structure. In certain embodiments, one or more materials are added to the mixture to form the composite material. For example, silicon particles can be added to the mixture. The carbonized precursor results in an electrochemically active structure that holds the composite material together. For example, the carbonized precursor can be a substantially continuous phase. The silicon particles can be dispersed throughout the composite material. Advantageously, the carbonized precursor is a structural material and also an electrochemically active and conductive material. In certain embodiments, the material particles added to the mixture are uniformly dispersed throughout the composite material to form a homogeneous composite.

[0020] The mixture can contain various different components. The mixture can contain one or more precursors. In certain embodiments, the precursor is a hydrocarbon compound. For example, the precursor can include polyamic acid, polyimide, etc. Other precursors include phenolic resins, epoxy resins, and other polymers. The mixture can further contain 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, etc. Examples of precursor and solvent solutions include PI-2611 (HD Microsystems), PI-5878G (HD Microsystems), and VTEC PI-1388 (RBI, Inc.). PI-2611 consists of >60% n-methyl-2-pyrrolidone and 10-30% s-biphenyldianhydride / p-phenylenediamine. PI-5878G consists of >60% n-methylpyrrolidone, 10-30% polyamic acid of pyromellitic dianhydride / oxydianiline, and 10-30% aromatic hydrocarbon (petroleum distillate) containing 5-10% 1,2,4-trimethylbenzene. In certain embodiments, the amount of precursor in the solvent is from about 10 wt% to about 30 wt%. Additional materials can also be included in the mixture. For example, as described above, carbon particles or silicon particles containing graphite active material, etched or ground carbon fibers, carbon nanofibers, carbon nanotubes, and conductive carbon can be added to the mixture. Further, the mixture can be mixed to homogenize the mixture.

[0021] In one embodiment, in block 102 of FIG. 1, the mixture is cast onto a substrate. In some embodiments, the casting step includes the step of using a gap extrusion or blade casting technique. The blade casting technique can include the step of providing a coating on the substrate by using a controlled flat surface (e.g., a blade) at a certain distance above the substrate. A liquid or slurry can be applied to the substrate, and the blade can be traversed by the liquid so that the liquid spreads across the substrate. Since the liquid passes through the gap, the thickness of the coating can be controlled by the gap between the blade and the substrate. Since the liquid passes through the gap, excess liquid can also be trimmed off. For example, the mixture can be cast onto a polymer sheet, a polymer roll, or a foil or roll made of glass or metal. Next, in block 103, the mixture can be dried to remove the solvent. For example, the polyamic acid and NMP solution can be dried at about 110° C. for about 2 hours to remove the NMP solution. Next, the dried mixture can be removed from the substrate. For example, the aluminum substrate can be etched using HCl. Alternatively, the dried mixture can be removed from the substrate by peeling or other mechanical steps of removing the dried mixture from the substrate. In one embodiment, the dried mixture is a film or a sheet. In some embodiments, in block 104, the dried mixture is cured. A hot press can be used to cure and maintain the dried mixture flat. For example, the mixture dried from the polyamic acid and MNP solution can be hot pressed at about 200° C. for about 8 to 16 hours. Alternatively, the entire process including casting and drying can be implemented as a roll-to-roll process using a standard film-processing apparatus. The dried mixture can be rinsed to remove any remaining solvent or etchant.For example, deionized (DI) water can be used to wash away the dried mixture. In certain embodiments, tape casting techniques can be used for casting. In other embodiments, there is no substrate for casting and the anode film need not be removed from any substrate. The dried mixture can be cut or mechanically divided into small pieces.

[0022] In block 105, to convert the precursor to carbon, the mixture is further subjected to pyrolysis. In certain embodiments, the mixture is pyrolyzed under a reducing atmosphere. For example, an inert atmosphere, vacuum and / or a flow of argon, nitrogen, or helium gas can be used. In some embodiments, the mixture is heated to about 900 °C to about 1350 °C. For example, a polyimide formed from polyamic acid can be carbonized at about 1175 °C for about 1 hour. In certain embodiments, the heating rate and / or cooling rate of the mixture is about 10 °C / min. A holder can be used to keep the mixture in a specific shape. The holder can be graphite, metal, etc. In certain embodiments, the mixture is held flat. After the mixture has been pyrolyzed, tabs can be attached to the pyrolyzed material to form electrical contacts. For example, nickel, copper or their alloys can be used for the tabs.

[0023] In certain embodiments, one or more of the methods described herein are continuous processes. For example, casting, drying, curing and pyrolysis can be carried out in a continuous process; for example, the mixture can be coated onto a glass or metal cylinder. While rotating on the cylinder, the mixture can be dried to form a film. The film is transferred as a roll or peeled off and fed into another machine for further processing. Extrusion and other film manufacturing techniques known in the industry can also be utilized before the pyrolysis step.

[0024] The pyrolysis of the precursor yields a carbon material (at least one carbon phase). In certain embodiments, the carbon material is hard carbon. In some embodiments, the precursor can be any material that can be pyrolyzed to form hard carbon. In addition to the carbonized precursor, if the mixture contains one or more additional materials or phases, a composite material can be formed. Specifically, the mixture can contain silicon-carbon (e.g., at least one first phase containing silicon and at least one second phase containing carbon), or silicon-carbon-carbon (e.g., at least one first phase containing silicon, at least one second phase containing carbon, and at least one third phase containing carbon) composite materials forming silicon particles. The silicon particles can increase the specific lithium insertion capacity of the composite material. When silicon absorbs lithium ions, a large volume increase on the order of 300+ volume percent is observed, which can cause structural integrity problems in the electrode. In addition to the issues related to volume expansion, silicon is not inherently conductive, but becomes conductive when alloyed with lithium (e.g., lithiated). When silicon is delithiated, the surface of the silicon loses conductivity. Furthermore, when silicon is delithiated, its volume decreases, and as a result, the silicon particles may lose contact with the matrix. Also, the dramatic change in volume results in mechanical breakage of the silicon particle structure, and in turn, it becomes pulverized. The pulverization and loss of electrical contact cause problems for using silicon as an active material in lithium-ion batteries. A decrease in the initial size of the silicon particles can prevent further pulverization of the silicon powder and minimize the loss of surface conductivity. Furthermore, by adding a material to the composite material that can elastically deform with the volume change of the silicon particles, it is possible to ensure that electrical contact with the surface of the silicon is not lost. For example, the composite material can contain carbon such as graphite that can contribute to the ability of the composite material to absorb expansion and can insert lithium ions (e.g., chemically active) to increase the storage capacity of the electrode. Thus, the composite material can contain one or more carbon phases.

[0025] Embodiments of the silicon particles of maximum dimension include less than about 40 μm, less than about 1 μm, between about 10 nm and 40 μm, between about 10 nm and 1 μm, less than about 500 nm, less than about 100 nm, and about 100 nm. All, substantially all, or at least some of the silicon particles may include the above maximum dimensions. For example, the average or median maximum dimension of the silicon particles includes less than about 40 μm, less than about 1 μm, between about 10 nm and 40 μm, between about 10 nm and 1 μm, less than about 500 nm, less than about 100 nm, and about 100 nm. The amount of silicon in the composite material can be greater than zero weight percent of the mixture and composite material. In certain embodiments, the amount of silicon in the mixture is between greater than 0 weight percent and less than about 90 weight percent of the mixture, or between about 30 weight percent and about 80 weight percent. Embodiments of the amount of silicon in the composite material include greater than 0 weight percent and less than about 35 weight percent, greater than 0 weight percent and less than about 25 weight percent, between about 10 weight percent and about 35 weight percent, and about 20 weight percent. In a further embodiment, the amount of silicon in the mixture is at least about 30 weight percent. Additional embodiments of the amount of silicon in the composite material include about 50 weight percent or more, between about 30 weight percent and about 80 weight percent, between about 50 weight percent and about 70 weight percent, and between about 60 weight percent and about 80 weight percent. Further, the silicon particles may or may not be pure silicon. For example, the silicon particles can be substantially silicon or a silicon alloy. In one embodiment, the silicon alloy includes silicon as a first constituent material along with one or more other elements.

[0026] The amount of carbon obtained from the precursor can be about 50 weight percent from the polyamic acid. In certain embodiments, the amount of carbon from the precursor in the composite material is from about 10 to 25 weight percent. The carbon from the precursor can be hard carbon. Hard carbon is carbon that does not convert to graphite even when heated above 2800 degrees Celsius. Precursors that melt or flow during pyrolysis convert to soft carbon and / or graphite at sufficient temperature and / or pressure. Since soft carbon precursors flow and soft carbon and graphite are mechanically weaker than hard carbon, hard carbon can be selected. Other possible hard carbon precursors include phenolic resins, epoxy resins, and other polymers with extremely high melting points or that are cross-linked. Embodiments of the amount of hard carbon in the composite material include from about 10 weight percent to about 25 weight percent, about 20 weight percent, about 50 weight percent or more. 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 includes amorphous and crystalline carbon. The hard carbon phase can be the matrix phase in the composite material. Also, the hard carbon can be embedded in pores of additives containing silicon. Hard carbon can react with some additives to form some materials at the interface. For example, a silicon carbide layer can exist between silicon particles and hard carbon.

[0027] In one embodiment, graphite particles are added to the mixture. Advantageously, graphite is an electrochemically active material in the battery and an elastically deformable material capable of reacting to the volume change of silicon particles. Since it has a low irreversible capacity, graphite is a preferred active anode material in certain lithium-ion batteries currently sold on the market. Further, graphite is softer than hard carbon and can better absorb the volume expansion of silicon additives. In one embodiment, the maximum dimension of the graphite particles is between about 0.5 micrometers and about 20 micrometers. All, substantially all, or at least some of the graphite particles may include the maximum dimension described herein. In a further embodiment, the average or median maximum dimension of the graphite particles is between about 0.5 micrometers and about 20 micrometers. In one embodiment, the mixture comprises greater than 0 wt% and less than about 80 wt% graphite particles. In a further embodiment, the composite material comprises from about 40 wt% to about 75 wt% graphite particles.

[0028] In certain embodiments, conductive particles that may also be electrochemically active are added to the mixture. Such particles provide both a more conductive composite material and a more mechanically deformable composite material that is capable of absorbing the large volume changes that occur during lithiation and delithiation. In certain embodiments, the maximum dimension of the conductive particles is between about 10 nanometers and about 7 millimeters. All, substantially all, or at least some of the conductive particles may include the maximum dimension described herein. In further embodiments, the average or median maximum dimension of the conductive particles is between about 10 nm and about 7 millimeters. In certain embodiments, the mixture includes greater than zero and up to about 80 weight percent conductive particles. In further embodiments, the composite material includes from about 45 weight percent to about 80 weight percent conductive particles. The conductive particles can be conductive carbon including carbon black, carbon fibers, carbon nanofibers, carbon nanotubes, and the like. Many carbons considered to be non-electrochemically active conductive additives become active once pyrolyzed in the polymer matrix. Alternatively, the conductive particles can be a metal or an alloy including copper, nickel, or stainless steel.

[0029] In certain embodiments, the electrode can include the composite material described herein. For example, the composite material can form a self-standing monolithic electrode. The pyrolyzed carbon phase (e.g., hard carbon phase) of the composite material can coalesce and structurally support the particles added to the mixture. In certain embodiments, the self-standing monolithic electrode does not include a separate collector layer and / or other support structure. In some embodiments, the composite material and / or the electrode do not include more than trace amounts of polymer remaining after pyrolysis of the precursor. In further embodiments, the composite material and / or the electrode do not include a non-conductive binder. Also, the composite material can include pores. For example, the porosity can be volume porosity and can be from about 5% to about 40%.

[0030] In addition, the composite material can be formed in a powder form. For example, the composite material can be ground into a powder form. The composite material powder can be used as an active material for an electrode. For example, the composite material powder can be deposited on a collector in the same manner as manufacturing a conventional electrode structure known in the art.

[0031] In certain embodiments, the electrodes in a battery or an electrochemical cell can include the composite materials described herein. For example, the composite materials can be used for anodes and / or cathodes. 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.

[0032] Furthermore, the total capacity of the composite material may not be utilized during the use of the battery to improve the battery life (e.g., the number of charge and discharge cycles before the battery fails, or the performance of the battery degrades below a useful level). For example, a composite material containing about 70 wt% silicon particles, about 20 wt% carbon from a precursor, and about 10 wt% graphite has a maximum weight capacity of about 2000 mAh / g, while the composite material can only have a maximum weight capacity of about 550 - about 850 mAh / g used. The maximum weight capacity of the composite material may not be utilized, but by using the composite material at a lower capacity, it is still possible to obtain a higher capacity than a certain lithium-ion battery. In certain embodiments, the composite material is used at, or only used at, a weight capacity lower than about 70% of the maximum weight capacity of the composite material. For example, the composite material is not used at a weight capacity exceeding about 70% of the maximum weight capacity of the composite material. In further embodiments, the composite material is used at, or only used at, a weight capacity lower than about 50% of the maximum weight capacity of the composite material, or lower than about 30% of the maximum weight capacity of the composite material.

[0033] (Examples) The following exemplary process for anode manufacturing generally includes the steps of mixing the components together, casting them onto a removable substrate, drying, curing, removing the substrate, and then pyrolyzing the resulting sample. Typically, N-methyl-2-pyrrolidone (NMP) is used as the solvent to change the viscosity of any mixture and make it castable using the doctor blade technique.

Example

[0034] In Example 1, a polyimide liquid precursor (PI 2611 from HD Microsystems), graphite particles (SLP30 from Timcal), conductive carbon particles (Super P from Timcal), and silicon particles (Alfa Aesar) were mixed together at a weight ratio of 200:55:5:20 for 5 minutes using a Spex 8000D machine. Next, the mixture was cast onto aluminum foil and dried in an oven at 90 °C to remove the solvent, which was NMP for example. This was then followed by a curing step at 200 °C in a hot press under very slight pressure for at least 12 hours. Next, the backing of the aluminum foil was removed by etching in a 12.5% HCl solution. The remaining film was then rinsed in DI water, dried, and then pyrolyzed under a flow of argon at 1175 °C for about 1 hour. This process yielded a composite of 15.8 wt% carbon from PI 2611, 57.9 wt% graphite particles, 5.3 wt% carbon obtained from Super P, and 21.1 wt% silicon.

[0035] Next, the resulting electrode was tested in a pouch cell structure against a lithium NMC oxide cathode. A typical cycle graph is shown in Figure 2.

Example

[0036] In Example 2, first, using a Turbula mixer, silicon particles (from EVNANO Advanced Chemical Materials Co., Ltd.) were mixed with NMP at a weight ratio of 1:9 for a duration of 1 hour. Next, a polyimide liquid precursor (PI 2611 from HD Microsystems), graphite particles (SLP30 from Timcal), and carbon nanofibers (CNF from Pyrograf) were added to the Si:NMP mixture, at a weight ratio of 200:55:5:200, and vortexed for approximately 2 minutes. Next, the mixture was cast onto an aluminum foil coated with a 21 μm thick copper mesh. Next, the sample was dried in an oven at 90 °C to remove the solvent, e.g., NMP. This was followed by a step of curing at 200 °C in a hot press under very slight pressure for at least 12 hours. Next, the backing of the aluminum foil was removed by etching in a 12.5% HCl solution. Next, the remaining film was rinsed in DI water, dried, and then pyrolyzed under argon at 1000 °C for about 1 hour. This process yielded a composite of 15.8 wt% carbon from PI 2611, 57.9 wt% graphite particles, 5.3 wt% CNF, and 21.1 wt% silicon.

[0037] Next, the obtained electrode was tested in a pouch cell configuration against a lithium NMC oxide cathode. A typical cycle graph is shown in Figure 3.

Example

[0038] In Example 3, a polyimide liquid precursor (PI 2611 from HD Microsystems), and 325-mesh silicon particles (from Alfa Aesar) were mixed together at a weight ratio of 40:1 for a duration of 1 hour using a Turbula mixer. Next, the mixture was cast onto aluminum foil and dried in an oven at 90 °C to remove the solvent, which was NMP for example. This was followed by a step of curing at 200 °C in a hot press under very slight pressure for at least 12 hours. Next, the backing of the aluminum foil was removed by etching in a 12.5% HCl solution. Next, the remaining film was rinsed in DI water, dried, and then pyrolyzed at 1175 °C for about 1 hour under a flow of argon. This process yielded a composite of 75 wt% carbon from PI 2611 and 25 wt% silicon.

[0039] Next, the obtained electrode was tested in a pouch cell structure against a lithium NMC oxide cathode. A typical cycle graph is shown in Figure 4.

Example

[0040] In Example 4, silicon microparticles (from Alfa Aesar), polyimide liquid precursor (PI 2611 from HD Microsystems), graphite particles (SLP30 from Timcal), ground carbon fibers (from Fibre Glast Developments), carbon nanofibers (CNF from Pyrograf), carbon nanotubes (from CNANO Technology Limited), conductive carbon particles (Super P from Timcal), and conductive graphite particles (KS6 from Timca) were mixed using a vortex for 5 minutes at a weight ratio of 20:200:30:8:4:2:1:15. Next, the mixture was cast onto an aluminum foil. Next, the sample was dried in an oven at 90 °C to remove the solvent, e.g., NMP. This was followed by a step of curing at 200 °C in a hot press under very slight pressure for at least 12 hours. Next, the substrate of the aluminum foil was removed by etching in a 12.5% HCl solution. Next, the remaining film was rinsed in DI water and dried, and then pyrolyzed under argon at 1175 °C for about 1 hour. This process yielded a composite similar to the initial mixture but containing a carbon portion derived from PI 2611 that is 7.5% of the initial weight of the polyimide precursor.

[0041] Next, the obtained electrode was tested in a pouch cell configuration against a lithium NMC oxide cathode. A typical cycle graph is shown in Figure 5.

Example

[0042] In Example 5, a polyimide liquid precursor (PI 2611 from HD Microsystems), and silicon microparticles (from Alfa Aesar) were mixed together using a Turbula mixer at a weight ratio of 4:1 for a duration of 1 hour. Next, the mixture was cast onto an aluminum foil covered with a carbon veil (from Fibre Glast Developments Corporation) and dried in an oven at 90 °C to remove the solvent, for example NMP. This was followed by a step of curing at 200 °C in a hot press under very slight pressure for at least 12 hours. Next, the substrate of the aluminum foil was removed by etching in a 12.5% HCl solution. Next, the remaining film was rinsed in DI water, dried, and then pyrolyzed at 1175 °C for about 1 hour under a flow of argon. This process yielded a composite of about 23 wt% carbon from PI 2611, 76 wt% silicon, and a very small weight of the veil.

[0043] Next, the obtained electrode was tested in a pouch cell structure against a lithium nickel manganese cobalt oxide (NMC) cathode. A typical cycle graph is shown in FIG. 6.

Example

[0044] In Example 6, a polyimide liquid precursor (PI 2611 from HD Microsystems), graphite particles (SLP30 from Timcal), and silicon microparticles (from Alfa Aesar) were mixed together using a Spex 8000D machine at a weight ratio of 200:10:70 for 5 minutes. Next, the mixture was cast onto an aluminum foil and dried in an oven at 90 °C to remove the solvent (e.g., NMP). The dried mixture was cured at 200 °C in a hot press under very slight pressure for at least 12 hours. Next, the substrate of the aluminum foil was removed by etching in a 12.5% HCl solution. Next, the remaining film was rinsed in DI water, dried, and then pyrolyzed at 1175 °C for about 1 hour under an argon flow. This process yielded a composite of 15.8 wt% carbon from PI 2611, 10.5 wt% graphite particles, and 73.7 wt% silicon.

[0045] Next, the obtained electrode was tested in a pouch cell structure against a lithium NMC oxide cathode. The anode was charged to 600 mAh / g in each cycle, and the discharge capacity per cycle was recorded. A typical cycle graph is shown in Figure 7.

Example

[0046] In Example 7, PVDF and silicon particles (from EVNANO Advanced Chemical Materials Co), conductive carbon particles (Super P from Timcal), conductive graphite particles (KS6 from Timcal), graphite particles (SLP30 from Timcal), and NMP were mixed at a weight ratio of 5:20:1:4:70:95. Next, the mixture was cast onto a copper substrate and then placed in an oven at 90 °C to remove the solvent, which was NMP for example. Next, the obtained electrode was tested in a pouch cell structure against a lithium NMC oxide cathode. A typical cycle graph is shown in Figure 8.

Example

[0047] The proportion of graphite particles (SLP30 from Timcal) decreased while the proportion of silicon microparticles (from Alfa Aesar) was maintained at 20 wt.%. Many experiments were conducted to find the effect of the change in the proportion of carbon derived from polyimide (e.g., 2611c).

[0048] As shown in FIGS. 9A and 9B, the results were that more graphite and less 2611c were beneficial to cell performance by increasing the specific capacity while reducing the irreversible capacity. Minimizing 2611c has an adverse effect on the strength of the resulting anode. In one embodiment, a value close to 20 wt.% can be preferably a compromise.

Example

[0049] Similar to FIG. 8, when 2611c is maintained at 20 wt.% and the proportion of Si increases by consuming graphite particles, the first cycle discharge capacity of the resulting electrode increases. FIG. 10 shows that the anode can operate well with a high silicon capacity.

Example

[0050] A 1-mil thick polyimide sheet was thermally decomposed and tested according to the procedure of Example 1. The reversible capacity and irreversible capacity were plotted as a function of the thermal decomposition temperature. FIG. 11 shows that in one embodiment, it is preferable to thermally decompose the polyimide sheet (Upilex by UBE) at about 1175 °C.

[0051] (Additional Example) FIG. 12 is a photograph of a 4.3 cm × 4.3 cm composite anode film without a metal foil support layer. The composite anode film has a thickness of about 30 micrometers and a composition of about 15.8 wt. carbon derived from PI 2611, about 10.5 wt.% graphite particles, and about 73.7 wt.% silicon.

[0052] Figures 13 to 18 are micrographs of a scanning electron microscope (SEM) of the composite anode film. The composition of the composite anode film is about 15.8 wt% carbon from PI 2611, about 10.5 wt% graphite particles, and about 73.7 wt% silicon. Figures 13 and 14 show before being exposed to the cycle (the out-of-focus part is the bottom of the anode, and the in-focus part is the cut end of the composite film). Figures 15, 16, and 17 are SEM micrographs of the composite anode film after 10 cycles, 10 cycles, and 300 cycles, respectively. The SEM micrographs show that there is no sufficient pulverized part of silicon and that the anode does not have an excessive layer of the solid electrolyte interface / phase boundary (SEI) formed thereon after cycling. Figure 18 is an SEM micrograph of the cross-section of the composite anode film.

[0053] Various embodiments have been described above. Although the present invention has been described with reference to these specific embodiments, this description is intended to be illustrative and not restrictive. Those skilled in the art can recognize various modifications and applications without departing from the true spirit and scope of the present invention as defined in the appended claims.

Description of Reference Numerals

[0054] 100 Method for forming a composite material 101, 102, 103, 104, 105 Blocks

Claims

1. A composite material, comprising: Silicon particles greater than 0 wt% and less than 90 wt%; One or more carbon phases greater than 0 wt% and less than 90 wt%; wherein at least one of the one or more carbon phases is a substantially continuous phase; the carbon phase that is the substantially continuous phase contains hard carbon; the silicon particles are dispersed throughout the composite material; and the composite material is self - standing.

2. The composite material according to Claim 1, wherein the silicon particles have an average maximum dimension of less than 1 μm.

3. The composite material according to Claim 1 or 2, wherein the silicon particles constitute 20 wt% to 80 wt% of the composite material.

4. The composite material according to any one of Claims 1 to 3, wherein the carbon phase that is the substantially continuous phase is electrochemically active and conductive.

5. The composite material according to any one of Claims 1 to 4, wherein the one or more carbon phases contain graphite particles.

6. The composite material according to any one of Claims 1 to 5, further comprising conductive particles.

7. The composite material according to any one of Claims 1 to 6, further comprising metal particles.

8. The composite material according to any one of Claims 1 to 7, wherein the composite material is substantially electrochemically active.

9. The composite material according to any one of Claims 1 to 8, wherein the composite material is self - standing by the continuous phase.

10. A battery electrode comprising the composite material according to any one of Claims 1 to 9.

11. A method of using the composite material according to any one of Claims 1 to 9, comprising using the composite material only at a weight capacity of less than 70% of the maximum weight capacity of the composite material.

12. A method for forming a composite material, comprising: providing a mixture comprising a precursor and silicon particles; pyrolyzing the precursor to convert the precursor into one or more carbon phases to form the composite material; wherein the composite material comprises silicon particles greater than 0 wt% and less than 90 wt%; one or more carbon phases greater than 0 wt% and less than 90 wt%; wherein at least one of the one or more carbon phases is a substantially continuous phase; the carbon phase that is the substantially continuous phase contains hard carbon; and the silicon particles are dispersed throughout the composite material. A method in which the composite material is self-supporting.

13. The method according to claim 12, wherein after the step of pyrolyzing the precursor, the mixture forms a self-supporting composite material.

14. The method according to claim 12 or 13, wherein the silicon particles constitute 20% to 80% by weight of the composite material.

15. The method according to any one of claims 12 to 14, wherein the mixture further comprises a solvent.

16. The method according to any one of claims 12 to 15, wherein the precursor comprises a polyimide.

17. The method according to any one of claims 12 to 16, wherein the precursor comprises a phenolic resin.

18. The method according to any one of claims 12 to 17, wherein the precursor comprises a hydrocarbon compound.

19. The method according to any one of claims 12 to 18, wherein the silicon particles constitute more than 0% by weight and less than 80% by weight in the mixture, and the precursor constitutes 5% to 80% by weight in the mixture.

20. Casting the mixture onto a substrate; Drying the mixture to form a film; Removing the film from the substrate; Curing the film by hot pressing; The method according to any one of claims 12 to 19, further comprising.

21. The method according to any one of claims 12 to 20, further comprising forming a battery electrode from the composite material.

22. The method according to any one of claims 12 to 21, wherein the composite material is self-supporting by the continuous phase.

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