Carbon aerogel-based electrode material and its manufacturing method

Abstract

Nanoporous carbon-based scaffolds or structures, specifically carbon aerogels, and their manufacture and use are provided. An embodiment includes a silicon-doped anode material for lithium-ion batteries, which comprises polyimide-derived carbon aerogel beads. The carbon aerogel contains silicon particles and accommodates the silicon particles, which expand during lithiation. The anode material provides optimal properties for use in lithium-ion batteries.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Patent Application No. 16 / 803,348, filed February 27, 2020, and U.S. Provisional Patent Application No. 62 / 811,230, filed February 27, 2019, each of which is incorporated by reference in its entirety and the definitions of terms in this application are controlling. [Technical Field]

[0002] The present invention relates generally to nanoporous carbon-based materials, and more particularly to carbon aerogels suitable for use in environments involving electrochemical reactions, such as for use as electrode materials in lithium-ion batteries. [Background technology]

[0003] Aerogels are solid materials containing a highly porous network of micro- and meso-sized pores. Depending on the precursor materials used and the processing performed, aerogels often have pores that account for more than 90% of their volume, with a density of approximately 0.05 g / cc. Aerogels are typically prepared by removing solvent from a gel (a solid network containing its solvent) in a manner that minimizes or prevents the gel from shrinking at the surface due to capillary forces. Solvent removal methods include, but are not limited to, supercritical drying (drying using a supercritical fluid, e.g., replacing transient solvents within the gel with a supercritical fluid with low surface tension), solvent exchange with a supercritical fluid followed by solvent exchange with a fluid that is converted to a supercritical state, subcritical or near-critical drying, and sublimation of frozen solvents in a freeze-drying process (see, for example, PCT Patent Application Publication No. WO2016127084A1). It should be noted that drying at ambient conditions may result in gel shrinkage due to solvent evaporation and the formation of a xerogel. Thus, the preparation of aerogels by sol-gel or other polymerization methods typically proceeds through the following sequence of steps: dissolution of solute in solvent, formation of a sol / solution / mixture, formation of a gel (which may include additional cross-linking), and solvent removal by supercritical drying or any other method that removes the solvent from the gel without collapsing the pores.

[0004] Aerogels can be formed from inorganic and / or organic materials. When aerogels are formed from organic materials, such as phenol, resorcinol-formaldehyde (RF), phloroglucinol furaldehyde (PF), polyacrylonitrile (PAN), polyimide (PI), polyurethane (PU), polybutadiene, polydicyclopentadiene, and their precursors or polymer derivatives, they can be carbonized (e.g., by pyrolysis) to form carbon aerogels. Depending on the precursor materials and methods used, the aerogels can have different or overlapping properties (e.g., pore volume, pore size distribution, morphology, etc.). However, in all cases, certain drawbacks exist based on the material and application, such as low pore volume, wide pore size distribution, and low mechanical strength. In recent years, efforts have focused on the development and characterization of carbon aerogels as electrode materials with improved performance for applications in energy storage devices, such as lithium-ion batteries (LIBs).

[0005] LIBs are widely used in a variety of applications, from portable electronics to automobiles. They are a type of rechargeable battery in which lithium ions move from the anode to the cathode during discharge and from the cathode to the anode during charging. Traditionally, the cathode is made of lithium metal (e.g., cobalt, nickel, manganese) oxide, and the anode is made of graphite, where lithium ions are intercalated into the graphite layers during charging (energy storage). Graphite is widely used because it has a higher lithium intercalation rate than other known carbons.

[0006] Given the growing demand for high-capacity anode and cathode materials, a major drawback of conventional LIBs is the limited capacity of graphite, or in other words, its ability to accommodate only a limited amount of lithium. Silicon has a higher affinity for lithium compared to graphite (carbon), allowing it to store significantly more lithium than graphite during charging, theoretically resulting in higher capacity on the anode side of LIBs. By comparison, graphite has a theoretical capacity of 372 mAh / g when combined with lithium, while silicon has a theoretical capacity of 4200 mAh / g. These figures drive the desire to place as much silicon as possible within the anode. However, a key issue with silicon is that when fully lithiated, its volume expands by 3–4 times (often resulting in breakage or cracking), significantly limiting the amount of silicon that can be placed within the electrode.

[0007] Therefore, what is needed is an improved nanoporous carbon material that contains a functional morphology and an optimal porous structure while solving at least one of the problems described above. However, in view of the art as a whole at the time this invention was made, it was not apparent to those skilled in the art how the shortcomings of the prior art could be overcome.

[0008] Although certain aspects of the prior art have been discussed to facilitate disclosure of the present invention, applicants do not in any way reject these technical aspects and believe that the claimed invention may encompass one or more of the aspects of the prior art discussed herein, particularly in combination with the innovative aspects described herein.

[0009] The present invention may address one or more of the problems and shortcomings of the technology discussed above. However, it is believed that the present invention may prove useful in addressing other problems and shortcomings in many technical fields. Accordingly, the claimed invention should not be construed as limited to addressing the particular problems or shortcomings discussed herein.

[0010] Where any document, act or item of knowledge is referenced or discussed in this specification, this reference or discussion is not an admission that said document, act or item of knowledge, or any combination thereof, was publicly available, known to the public, part of common general knowledge, or constitutes prior art under any applicable legal provision, or was known to be relevant to any attempt to solve any problem or problem herein. Summary of the Invention

[0011] A long-felt need for improved nanoporous carbon materials, hitherto unrealized, is now realized through a new, useful, and non-obvious invention.

[0012] A first general aspect relates to a carbon composition comprising a carbon material, such as a nanoporous carbon material, and a silicon-based material, wherein the carbon material comprises a porous structure, the carbon composition comprises greater than about 10 wt% silicon-based material, and the silicon utilization is at least about 20%.

[0013] In an exemplary embodiment, the carbon material comprises a fibrillar morphology and has one or more porous structures with a Young's modulus of at least about 0.2 GPa, a conductivity of at least about 10 S / cm, and a density between about 0.15 g / cc and about 1.5 g / cc.

[0014] In another exemplary embodiment, the carbon composition comprises a silicon-doped nanoporous carbon material having a silicon utilization of at least about 20%, wherein the carbon material is doped with silicon at greater than about 25% by weight of the carbon material. Optionally, the electrical conductivity of the carbon material can be at least about 10 S / cm. Optionally, the Young's modulus of the carbon material can be at least about 0.2 GPa.

[0015] In a further exemplary embodiment, the carbon composition comprises a silicon-doped nanoporous carbon material having a porous structure with the following properties: a fibrillar morphology, a Young's modulus of at least about 0.2 GPa, a density of between about 0.15 g / cc and about 1.5 g / cc, and a silicon utilization of at least about 20%. Optionally, the electrical conductivity of the carbon material may be at least about 10 S / cm.

[0016] In another exemplary embodiment, the carbon composition comprises a silicon-doped nanoporous carbon material having a porous structure comprising a fibrillar morphology, a conductivity of at least 10 S / cm, a density of between about 0.15 g / cc and about 1.5 g / cc, and a silicon utilization of at least about 20%. Optionally, the carbon material may have a Young's modulus of at least about 0.2 GPa.

[0017] In any embodiment, the nanoporous carbon material can be a carbon aerogel, e.g., an imide-derived carbon aerogel, such as a polyimide-derived carbon aerogel. In further embodiments, the carbon aerogel can be in a monolithic structure or in particulate form, e.g., a powder. When the carbon aerogel is in monolithic form, the carbon material can be substantially binder-free or completely binder-free. Alternatively or additionally, the thickness of the monolithic carbon aerogel can be between about 10 micrometers and about 500 micrometers.

[0018] In exemplary embodiments in which the carbon aerogel is in particulate form, the particulate carbon aerogel can have a diameter of from about 1 micrometer to about 50 micrometers.

[0019] In any embodiment, the porous structure of the nanoporous carbon material may be characterized by pores of the carbon material that partially, substantially, or completely surround the silicon-based material, e.g., to form an interconnected structure around the silicon characterized by multiple connection points between the silicon and the pore walls. For example, the silicon-based material may be at least partially present within the porous structure of the carbon material.

[0020] In any embodiment, the nanoporous carbon material can be doped with about 5% to about 80% silicon by weight of the carbon material, for example, the carbon material can include about 25% to about 65% silicon by weight of the carbon material.

[0021] In any embodiment, the pore volume of the nanoporous carbon material may be at least 0.3 cc / g.

[0022] In any embodiment, the prelithiation porosity of the nanoporous carbon material can be between about 10% and about 80%.

[0023] In any embodiment, the residual nitrogen in the carbon material, such as the nanoporous carbon material, can be at least about 4% by weight.

[0024] In any embodiment, the capacity of the silicon-doped nanoporous carbon material can be at least about 800 mAh / g. For example, the capacity of the silicon-doped nanoporous carbon material can be up to about 2000 mAh / g.

[0025] In any embodiment, the full width at half maximum of the porous structure of the nanoporous carbon material may be about 50 nm or less (ie, a narrow pore size distribution).

[0026] In any embodiment, the pore size at the maximum peak from the distribution of the porous structure of the nanoporous carbon material may be about 100 nm or less.

[0027] In any embodiment, the average strut width of the fibrillar morphology of the nanoporous carbon material can be about 2-10 nm, or even more specifically about 2-5 nm.

[0028] In an exemplary embodiment, a collectorless, binderless interconnect anode material for lithium-ion batteries is provided. The anode material comprises an open-cell monolithic polyimide-derived nanoporous carbon material (also referred to as a CPI composite) having a fibrillar network and an array of pores. Silicon particles reside within the pores surrounded by the fibrillar network and comprise between about 20% and 80% by weight of the anode material. The average strut width of the fibrillar network is between about 2 and 10 nm. The porosity of the carbon material is between about 20% and about 50%, including pores that contain silicon particles in a non-lithiated state and can accommodate silicon particles in a lithiated, volume-expanded state. The carbon aerogel has the following properties: a pore volume of about 0.1 cc / g or greater, a substantially uniform pore size distribution with a full width at half maximum of about 50 nm or less, and a pore size at the largest peak from the distribution of about 100 nm or less. The resulting anode material has the following properties: density between about 0.50 g / cc and about 1.5 g / cc, conductivity of about 10 S / cm or greater, Young's modulus of about 0.5 GPa or greater, and thickness between about 10 micrometers and about 200 micrometers.

[0029] In an exemplary embodiment, a collectorless, binderless anode material for lithium-based energy storage devices is provided. The anode material comprises an open-cell, monolithic nanoporous carbon material having a fibril network and a pore array, with silicon particles present within the pores surrounded by the fibril network and comprising greater than 0% and less than about 95% by weight of the anode material. The fibril network acts as a carbon coating on the silicon particles, protecting them from breakage during lithiation, and has an average strut width of about 2-10 nm. The porosity of the carbon material is about 80% or less, and the porosity includes pores that surround the silicon particles in their unlithiated state and can accommodate the silicon particles in their lithiated volume-expanded state. The carbon aerogel has the following properties: a pore volume of about 0.3 cc / g or more, a substantially uniform pore size distribution with a full width at half maximum of about 50 nm or less, and a pore size at the largest peak from the distribution of about 100 nm or less. The resulting anode material has the following properties: density between about 0.50 g / cc and about 1.5 g / cc, conductivity of about 10 S / cm or greater, Young's modulus of about 0.5 GPa or greater, and thickness of between about 10 micrometers and about 4 cm.

[0030] Another embodiment relates to a binder-free composite comprising an open-cell porous carbon scaffold having a pore array, wherein an electrochemically active species is disposed within the pore array of the carbon scaffold and in direct contact with the carbon scaffold, such that the porosity of the carbon scaffold is about 90% or less. The electrochemically active species is present in an amount between about 5% and about 65% by weight of the composite material. The carbon aerogel has the following properties: a pore volume of about 0.3 cc / g or more, and a substantially uniform pore size distribution with a full width at half maximum of about 50 nm or less.

[0031] Another embodiment relates to a composite material comprising an open-cell nanoporous carbon network and an electrochemically active species disposed within the pores of the nanoporous carbon network. The carbon network has a porosity of about 90% or less and a substantially uniform pore size distribution with a full width at half maximum of about 50 nm or less. The composite material is formed as a monolith or powder. Optionally, the material has a conductivity of about 10 S / cm or more. Optionally, the material has a Young's modulus of about 0.5 GPa or more and a thickness between 10 micrometers and about 500 micrometers.

[0032] In another embodiment, there is provided a silicon-containing monolithic imide-derived carbon aerogel composite formed from a nanoporous carbon material, the silicon-containing monolithic imide-derived carbon aerogel composite being binder-free and including silicon particles embedded within the monolithic polyimide-derived carbon aerogel composite.

[0033] Optionally, the composite may be pre-doped with a metal oxide selected from the group consisting of tin, sulfur, phosphorus, nickel, copper, cobalt, manganese, lithium, magnesium, iron, zinc, boron, titanium, aluminum oxide, titanium oxide, niobium oxide, molybdenum oxide, silica, and aluminosilicates.

[0034] Optionally, the silicon particles may be pre-doped with a p-type acceptor selected from the group consisting of boron, aluminum, gallium, and indium, or with an n-type donor selected from the group consisting of phosphorus, lithium, arsenic, antimony, and bismuth.

[0035] In some cases, the carbon aerogel comprises a fibrillar morphology with an average strut width of about 2-10 nm.

[0036] In a further embodiment, an electrode is provided comprising the nanoporous carbon material described herein. The electrode may be an anode, and may optionally not include a separate current collector. In another embodiment, an electrochemical cell is provided comprising the carbon composition, nanoporous carbon material, and / or electrode described herein. In a further embodiment, an energy storage device, such as a battery, more particularly a lithium-ion battery, is provided comprising the carbon composition, nanoporous carbon material, and / or electrochemical cell described herein.

[0037] A further general aspect relates to a method for forming or manufacturing a carbon composition. In an exemplary embodiment, the method includes providing a mixture of a polyimide precursor and a silicon-based material, chemically or thermally imidizing the mixture, e.g., by adding an imidization catalyst or by heating, drying the imidized mixture to obtain a porous polyimide silicon composite, and carbonizing the porous polyimide silicon composite, e.g., by pyrolysis, to obtain a carbon composition containing greater than about 25% silicon by weight and having a porosity between about 10% and about 90%. In some embodiments, the method further includes combining the mixture with a medium immiscible with the mixture, e.g., a dispersing medium, to form droplets of the imidized mixture. For example, an emulsion can be formed with the imidized mixture as the dispersed phase. In an exemplary embodiment, the method further includes drying the droplets to form particles. In any embodiment, the carbon composition can include a carbon aerogel and can be formed as a monolith or particles.

[0038] In an exemplary embodiment, the method involves forming or producing a continuous porous carbon silicon composite, such as a carbon aerogel. For example, imide precursors, such as a diamine and a dianhydride, each of which can contain aromatic and / or aliphatic groups, are mixed in a suitable solvent (e.g., a polar, aprotic solvent). An additive, such as silicon particles in this embodiment, is mixed with the imide precursor in the solvent medium before adding an imidization catalyst. The imidization catalyst is then added to initiate imidization. In an alternative embodiment, imidization can be achieved by thermal imidization. A gel is formed in which the additive particles, such as silicon particles, are uniformly dispersed. The resulting mixture is then dried to obtain a continuous porous polyimide silicon composite, which may be dried using subcritical and / or supercritical carbon dioxide. Optionally, the polyimide silicon composite may be densified. For example, the polyimide silicon composite can preferably be compressed uniaxially (e.g., to 95% strain) to increase its density, which can be adjusted to about 1.5 g / cc based on the amount of compression. With or without compaction, the polyimide silicon composite may be pyrolyzed to obtain an open porous silicon carbon composite, the resulting composite containing greater than 0% and less than about 95% silicon by weight and having a porosity between about 5% and 99%. In certain embodiments, pyrolysis is carried out at a maximum temperature between about 750°C and about 1600°C, with optional graphitization occurring between about 1600°C and about 3000°C.

[0039] In another embodiment, a method for forming or manufacturing a continuous porous silicon carbon composite, such as a carbon aerogel, is provided. Polyimide precursors, such as a diamine and a dianhydride, each of which may contain aromatic and / or aliphatic groups, are mixed in a suitable solvent (e.g., a polar, aprotic solvent). A silicate (e.g., silicon dioxide, aluminosilicate, and / or halloysite) and a reducing agent (e.g., magnesium, lithium, sodium, potassium, aluminum, calcium, or a combination thereof) are mixed with the polyimide precursor in the solvent before adding an imidization catalyst. The imidization catalyst is then added. In an alternative embodiment, imidization can be achieved by thermal imidization. The resulting mixture is then dried to obtain a continuous porous polyimide silicate and reducing agent composite, which may be dried using subcritical and / or supercritical carbon dioxide. Optionally, the polyimide silicate and reducing agent composite can be compressed uniaxially (e.g., to 95% strain) to increase its density, which can be adjusted to about 1.5 g / cc based on the amount of compression. With or without compaction, the composite of polyimide silicate and reducing agent is heated under pyrolytic and reducing environmental conditions to produce an open-pore silicon carbon composite, the resulting composite containing greater than 0% and less than about 95% silicon by weight, with a porosity between about 5% and 99%. In certain embodiments, pyrolysis is carried out at a maximum temperature between about 750°C and about 1600°C, with optional graphitization carried out between about 1600°C and about 3000°C. Additionally, the silicate and reducing agent may be reacted with hydrogen gas under inert conditions at temperatures above about 700°C to form silicon within the carbon composite.

[0040] In a further embodiment, a method for forming or producing a porous carbon silicon composite, such as a silicon-doped carbon aerogel, is provided. Polyimide precursors, such as a diamine and a dianhydride, each of which may contain aromatic and / or aliphatic groups, are mixed in a suitable solvent (e.g., a polar, aprotic solvent). An imidization catalyst is then added. In an alternative embodiment, imidization can be achieved by thermal imidization. The resulting mixture is then dried to obtain a continuous porous polyimide, where drying may be performed under subcritical conditions and / or using supercritical carbon dioxide.

[0041] Optionally, the polyimide can be preferably compressed uniaxially (e.g., to 95% strain) to increase its density, which can be adjusted up to about 1.5 g / cc based on the amount of compression. Whether or not compression has occurred, the polyimide is pyrolyzed to obtain a continuous porous carbon. Silicon is then deposited on or into the silicon to obtain a continuous porous silicon composite containing greater than 0 wt. % and less than about 95 wt. % silicon and having a porosity between about 5% and 99%. In certain embodiments, pyrolysis is performed at a maximum temperature between about 750°C and about 1600°C, and optional graphitization may be performed between about 1600°C and about 3000°C.

[0042] In some cases, silicon may be deposited by dip-coating the porous carbon with a silicon-forming silane precursor, followed by heating under inert conditions to decompose the silane and form a conformal silicon coating within the porous carbon. This dip treatment can be performed multiple times, increasing the thickness and silicon content to about 95% by weight. In other embodiments, silicon can be deposited by atomic layer deposition or chemical vapor deposition (CVD).

[0043] In any of the above-described methods for producing a carbon composition, such as an open porous silicon carbon composite, the carbon composition may optionally be a monolith or free-standing structure, may be prepared with or without a substrate, may be formed as beads, or may be pulverized into powder form. Additionally, the composite may be reinforced with or without nonwoven or woven materials (e.g., fibers, foams, etc.).

[0044] These and other important objects, advantages, and features of the present invention will become apparent as the present disclosure proceeds.

[0045] The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the disclosure which follows, the scope of the invention being indicated in the claims. [Brief explanation of the drawings]

[0046] For a full and clear understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.

[0047] [Figure 1] 1 is a flow diagram illustrating the formation of carbon aerogel for use in battery applications. [Figure 2] FIG. 1 shows the discharge capacity over several cycles comparing silicon doped monoliths with silicon particles incorporated by conventional slurry processing methods. [Figure 3] 1 is a flow diagram illustrating the formation of polyimide-derived carbon aerogels. [Figure 4] The density of carbonized polyimide (CPI) composites is shown as a function of compressed thickness (initial thickness of about 250 micrometers). [Figure 5] The density of the CPI composites is shown as a function of compressed thickness (initial thickness of about 580 micrometers). [Figure 6A] Scanning electron microscope (SEM) image of silicon-doped uncompressed PI aerogel (LS1). [Figure 6B]SEM image of silicon-doped compressed PI aerogel (LS2). [Figure 7] Discharge capacity per dopant (silicon, LS2; graphite; LG2) compressed composite (half-cell battery test, 0.1° C. rate) is shown. [Figure 8] 1 shows the surface area and micropore area function of silicon content in CPI monoliths. [Figure 9] Pore ​​size distribution of Si-doped CPI monoliths. [Figure 10] Figure 1 shows the discharge capacity of CPI composites as a function of Si content at cycle 5. [Figure 11A] 1 is an SEM image of a CPI composite with a Si loading of 27% Si in the composite. [Figure 11B] 1 is an SEM image of a CPI composite with a Si loading of 46% Si in the composite. [Figure 11C] 1 is an SEM image of a CPI composite with a Si loading of 64% Si in the composite. [Figure 12A] The cycle capacity based on 27% Si content is shown (S27). [Figure 12B] Electrode-based cycling capacity compared to FIG. 12A. [Figure 12C] The cycle capacity based on 46% Si content is shown (S46). [Figure 12D] Electrode-based cycling capacity compared to FIG. 12C. [Figure 12E] The cycle capacity based on 64% Si content is shown (S64). [Figure 12F] Electrode-based cycling capacity compared to FIG. 12E. [Figure 13A] 1 is an SEM image of a CPI composite with a thickness of approximately 337 micrometers. [Figure 13B] 1 is an SEM image of a CPI composite with a thickness of approximately 180 micrometers. [Figure 14A] The discharge capacity is shown based on the electrode weight (thickness: approximately 323 μm). [Figure 14B] The discharge capacity is shown based on the electrode weight (thickness: approximately 170 μm). [Figure 15A] SEM image of the CPI composite prepared without dispersant (C45) before pyrolysis. [Figure 15B] 15B is an SEM image of the composite of FIG. 15A after pyrolysis. [Figure 16A] 1 is an SEM image of a CPI composite prepared without dispersant (C45-control). [Figure 16B] 1 is an SEM image of a CPI composite (B45) prepared with BYK384. [Figure 16C] 1 is an SEM image of a CPI composite (P45) prepared with Pluronic® F87 dispersant. [Figure 17A] Figure 16A shows the discharge capacity of the C45 composite. [Figure 17B] The discharge capacity of the B45 composite is shown in FIG. 16B. [Figure 17C] The discharge capacity of the P45 composite is shown in FIG. 16C. [Figure 18] 1 shows the density as a function of thickness of P45 CPI composites. [Figure 19] 1 shows the electrical conductivity of CPI composites doped with various Si concentrations as a function of density. [Figure 20] FIG. 1 is a schematic diagram showing polyamic acid formation. [Figure 21] Isotherms for four CPI samples are shown. [Figure 22] The pore size distribution of the CPI sample in FIG. [Figure 23] This is an SEM image of the MT material (uncompressed). [Figure 24] 1 is an SEM image of MTC material (compressed). [Figure 25A] The cycle capacity of the compressed CPI samples is shown based on the Si content (left) and the electrode (right). [Figure 25B] The cycle capacity based on the Si content (left) and electrode (right) of the uncompressed CPI sample is shown. [Figure 26A] Cycling capacity based on Si content (left) and electrode (right) is shown for a pressed CPI sample containing 29 wt% silicon per total solids. [Figure 26B] Cycling capacity based on Si content (left) and electrode (right) is shown for an uncompressed CPI sample containing 29 wt% silicon per total solids. [Figure 27] The characteristics and micrographs of carbon / Si infiltrated into carbon fiber (10 g / m2) are shown. [Figure 28] Characterization and micrographs of carbon / Si infiltrated into carbon fiber (4 g / m2) are shown. [Figure 29] Characterization and micrographs of carbon / Si infiltrated into carbon fiber (2 g / m2) are shown. [Figure 30] Cycling capacity of carbon fiber reinforced C / Si based on Si content (left) and electrode (right). [Figure 31] Cycling capacity of cellulose fiber reinforced C / Si based on Si content (left) and electrode (right). [Figure 32] SEM image of a thick composite (approximately 0.6 mm) of Si and PI mixed for 16 hours. [Figure 33] This is an SEM image of a thin composite (approximately 0.12 mm) of Si and PI mixed for 16 hours. [Figure 34] SEM image of a monolith sample mixed with Si and PI for 16 hours. [Figure 35] SEM cross-sectional images of C / Si monoliths, where the image on the left shows Si and PI mixed for 16 hours, and the image on the right shows Si and PI mixed for 4–6 minutes. [Figure 36] SEM cross-sectional images of C / Si composites. The image on the left shows Si and PI mixed for 16 hours, and the image on the right shows Si and PI mixed for 4–6 minutes. [Figure 37] Cycle capacity of uncompacted C / Si (16 h mixed) based on Si content (left) and electrode (right). [Figure 38] Cycling capacity of compacted C / Si (16 h mixing) based on Si content (left) and electrode (right). [Figure 39]Figure 1 shows compressed circular Si / C electrodes fabricated using a die cutter on aerogel. [Figure 40] Cycling capacity of uncompressed circular C / Si aerogels based on Si content (left) and electrode (right). [Figure 41] Cycling capacity of compressed circular C / Si aerogels based on Si content (left) and electrode (right). [Figure 42] A C / Si aerogel sample obtained from the PF / Si aerogel is shown. [Figure 43] Shown is an SEM image of the C / Si aerogel (obtained from the PF / Si aerogel). [Figure 44] Figure 1 shows the cycling capacity of PF aerogel composites with Si based electrodes. [Figure 45] 1 shows the effect of silicon content on Young's modulus of samples tested by nanoindentation. [Figure 46] 1 shows the effect of density on Young's modulus as tested by nanoindentation. [Figure 47] 1 is an SEM image of a particulate C / Si aerogel sample according to embodiments disclosed herein. [Figure 48] 1 shows the cycling performance of an electrode including CPI silicon beads according to embodiments disclosed herein. DETAILED DESCRIPTION OF THE INVENTION

[0048] In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and which show, by way of illustration, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0049] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term "or" is generally used in its sense including "and / or" unless the context clearly dictates otherwise.

[0050] As used herein, "about" means approximately or nearly, and refers to ±15% of the numerical value in a stated numerical value or range. In one embodiment, the term "about" can include conventional rounding to the significant digits of a numerical value. Also, "about 'x' to 'y'" includes "about 'x' to about 'y'."

[0051] In this disclosure, the term "aerogel" or "aerogel material" refers to a gel that includes a framework of interconnected structures and a corresponding network of interconnected pores embedded within the framework, and that contains a gas, such as air, as a dispersed pore medium, and that exhibits the following physical and structural properties (as determined by nitrogen porosimetry testing) that are attributed to aerogels: (a) an average pore size ranging from about 2 nm to about 100 nm, (b) a porosity of at least 80% or greater, and (c) a porosity of at least about 20 nm. 2 / g or greater. It will be appreciated that the inclusion of additives such as reinforcing materials or electrochemically active species may decrease the porosity of the resulting aerogel composite. Densification may also decrease the porosity of the resulting aerogel composite. This will become more apparent later in this specification.

[0052] Thus, the aerogel materials of the present disclosure include any aerogel or other open-cell compound that meets the defining elements set forth in the previous paragraph, including compounds that may be otherwise classified as xerogels, cryogels, ambigels, microporous materials, etc.

[0053] In this disclosure, the term "framework" or "framework structure" refers to the network of interconnected oligomeric, polymeric, or colloidal particles that form the solid structure of a gel or aerogel. The diameter of the polymers or particles that make up the framework structure is generally about 100 angstroms. However, the framework structure of this disclosure can also include a network of interconnected oligomeric, polymeric, or colloidal particles of any diameter size that form the solid structure within a gel or aerogel.

[0054] In this disclosure, the term "aerogel composition" refers to any composite material that includes an aerogel material as a component of the composite. Examples of aerogel compositions include, but are not limited to, fiber-reinforced aerogel composites, aerogel composites containing additives such as opacifiers and electrochemically active species, aerogel-foam composites, aerogel-polymer composites, and composite materials that incorporate aerogel particles, particles, granules, beads, or powders into a solid or semi-solid material such as a binder, resin, cement, foam, polymer, or similar solid material.

[0055] In this disclosure, the term "reinforced aerogel composition" refers to an aerogel composition that includes a reinforcing phase within the aerogel material, which is not part of the aerogel skeleton or can be modified to be covalently bonded to the aerogel framework. The reinforcing phase can be any material that improves the flexibility, resilience, conformability, or structural stability of the aerogel material. Examples of well-known reinforcing materials include, but are not limited to, open-cell foam reinforcement, closed-cell foam reinforcement, open-cell membranes, honeycomb reinforcement, polymeric reinforcement, and fibrous reinforcement, such as discrete fibers, woven materials, nonwoven materials, batting, webs, mats, and felts. Furthermore, the reinforcing material may be combined with one or more other reinforcing materials and may be continuously oriented throughout or within a desired portion of the composition. In other embodiments, a reinforcing phase may not be used at all if the aerogel material and / or aerogel framework is structurally stable (i.e., self-supporting). This self-supporting nature of certain carbon aerogels will become more apparent later in this specification.

[0056] In this disclosure, the term "wet gel" refers to a gel in which the mobile pore phase within a network of interconnected pores is composed primarily of a liquid phase, such as a conventional solvent, a liquefied gas, such as liquid carbon dioxide, or a combination thereof. Aerogels generally require the initial generation of a wet gel, followed by processing and extraction to replace the mobile pore liquid phase in the gel with air or another gas. Examples of wet gels include, but are not limited to, alcogels, hydrogels, ketogels, carbonogels, and any other wet gels known to those skilled in the art.

[0057] In this disclosure, the term "additive" or "additive element" refers to a material that can be added to a composition before, during, or after its manufacture. Additives can be added, for example, to alter or improve a desirable property in an aerogel composition, or to counteract or mitigate an undesirable property in an aerogel composition. Additives are typically added to an aerogel composition before or during gelation. Additives can also be added to an aerogel composition by atomic layer deposition or chemical vapor deposition (CVD). A specific example of an additive is an electrochemically active species such as silicon, e.g., silicon particles.

[0058] In this disclosure, the term "silicon particles" refers to silicon or silicon-based materials having a particle size range suitable for use with the aerogel compositions disclosed herein. Silicon particles of the present disclosure can be nanoparticles, e.g., two- or three-dimensional particles ranging from about 1 nm to about 150 nm. Silicon particles of the present disclosure can be particulates, e.g., two- or three-dimensional micron-sized particles, e.g., substantially spherical particles with diameters ranging from about 150 nm to about 10 micrometers or greater. For example, silicon particles of the present disclosure can be two- or three-dimensional particles, e.g., substantially spherical particles with diameters of about 10 nm, 50 nm, 100 nm, 150 nm, 200 nm, 500 nm, 1 micrometer, 1.5 micrometers, 2 micrometers, 3 micrometers, 5 micrometers, 10 micrometers, 20 micrometers, 40 micrometers, 50 micrometers, 100 micrometers, or a range between any two of these values. In some embodiments, silicon particles can be monodisperse or substantially monodisperse. In other embodiments, silicon particles can have a particle size distribution. In the present disclosure, the size of silicon particles is provided based on the median of the particle size distribution, i.e., D50. The silicon particles of the present disclosure can be silicon wire, crystalline silicon, amorphous silicon, silicon alloy, silicon oxide (SiOx), coated silicon, for example, carbon-coated silicon, and any combination of silicon particle materials disclosed herein.

[0059] In this disclosure, the term "freestanding" refers to the ability of an aerogel material or composition to be flexible and / or resilient, based primarily on the physical properties of the aerogel. Freestanding aerogel materials or compositions of the present disclosure can be distinguished from other aerogel materials, such as coatings, that rely on an underlying substrate or reinforcing material to impart flexibility and / or resilience to the material.

[0060] In this disclosure, the term "density" refers to a measure of the mass per unit volume of an aerogel material or composition. The term "density" typically refers to true density for aerogel materials and bulk density for aerogel compositions. Density is generally measured in kg / m 3 The density of an aerogel material or composition can be measured by methods known in the art, including, but not limited to, Standard Test Methods for Dimensions and Density of Preformed Block and Board Insulation (ASTM C303, ASTM International, West Conshohocken, Pa.), Standard Test Methods for Thickness and Density of Blanket or Batt Insulation (ASTM C167, ASTM International, West Conshohocken, Pa.), and Measurement of Apparent Density of Preformed Pipe Insulation (ISO 18098, International Organization for Standardization, Switzerland). In this disclosure, unless otherwise specified, density measurements are taken according to the ASTM C167 standard. Preferably, the density of the aerogel material or composition of the present disclosure is about 1.50 g / cc or less, about 1.40 g / cc or less, about 1.30 g / cc or less, about 1.20 g / cc or less, about 1.10 g / cc or less, about 1.00 g / cc or less, about 0.90 g / cc or less, about 0.80 g / cc or less, about 0.70 g / cc or less, about 0.60 g / cc or less, about 0.50 g / cc or less, about 0.40 g / cc or less, about 0.30 g / cc or less, about 0.20 g / cc or less, about 0.10 g / cc or less, or a range between any two of these values, for example, between about 0.15 g / cc and 1.5 g / cc, or more specifically, 0.50 g / cc and 1.5 g / cc.

[0061] According to certain embodiments, the production of aerogel typically involves the following steps: i) forming a solution containing gel precursors, ii) forming a gel from the solution, and iii) extracting the solvent from the gel material to obtain a dry aerogel material. According to certain embodiments, the production of aerogel beads follows a conventional process for the production of aerogels and typically involves the following steps: i) forming a solution containing gel precursors, ii) dispersing the gel precursors in a medium immiscible with the gel precursors, iii) forming gel beads in the immiscible medium from the gel precursor solution, iv) removing the gel beads from the medium, and v) extracting the solvent from the gel beads to obtain a dry aerogel material. These methods are described in more detail below, particularly with respect to the formation of organic aerogels, such as polyimide aerogels. However, the specific examples and diagrams provided herein are not intended to limit the present disclosure to any particular type of aerogel and / or preparation method. The present disclosure can include any aerogel formed by any relevant preparation method known to those skilled in the art.

[0062] An exemplary solution for producing silica aerogel is formed by combining at least one gelling precursor with a solvent. Suitable solvents for solution formation include lower alcohols containing 1 to 6 carbon atoms, preferably 2 to 4 carbon atoms, as known to those skilled in the art. Examples of useful solvents include, but are not limited to, methanol, ethanol, isopropanol, ethyl acetate, ethyl acetoacetate, acetone, dichloromethane, and tetrahydrofuran. Multiple solvents can also be combined to achieve a desired level of dispersion or to optimize the properties of the gel material. Therefore, the selection of the optimal solvent for the polymerization and gel formation process depends on the specific precursors, fillers, and additives incorporated into the solution, the target processing conditions for gelation and liquid-phase extraction, and the desired properties of the final aerogel material.

[0063] An exemplary solution for producing polyimide aerogels is formed by combining at least one diamine and at least one dianhydride in a common polar aprotic solvent. Further details regarding polyimide gel / aerogel formation can be found in Rhine et al., U.S. Pat. Nos. 7,074,880 and 7,071,287; Suzuki et al., U.S. Pat. No. 6,399,669; Leventis et al., U.S. Pat. No. 9,745,198; Leventis et al., Polyimide Aerogels by Ring-Opening Metathesis Polymerization (ROMP), Chem. Mater. 2011, 23, 8, 2250-2261; Leventis et al., Isocyanate-Derived Organic Aerogels: Polyureas. , Polyimides, Polyamides, MRS Processing, 1306(2011), Mrsf10-1306-bb03-01.doi:10.1557 / opl.2011.90; Chidambareswarapattar et al., One-step room temperature synthesis of fibrous polyimide aerogels from anhydrides and isocyanates and their conversion to isomorphous carbon, J. Mater. Chem. 2010,20,9666-9678; Guo et al., Crosslinked polyimide aerogels from amine-functionalized polyoligomeric silsesquioxanes, ACS Appl. Mater. Interfaces 2011, 3, 546-552; Nguyen et al., Development of High-Temperature Flexible Polyimide Aerogels, Proceedings of the American Chemical Society, 2011; Meador et al., Mechanically Strong, Flexible Polyimide Aerogels Crosslinked with Aromatic Triamines, ACS Appl. Mater. Interfaces, 2012, 4(2), pp536-544; Meador et al., Polyimide Aerogels with Amide Crosslinks: A Low-Cost Alternative to Mechanically Strong Polymer Aerogels, ACS Appl. Mater. Interfaces 2015, 7, 1240-1249; Pei et al., Preparation and Characterization of Polyimide-Based Highly Crosslinked Polyimide Aerogels Containing Trimethoxysilane Side Groups, Langmuir 2014, 30, 13375-13383, each of which is incorporated herein by reference in its entirety. Triamines, tetraamines, pentamines, hexamines, etc. may also be used in place of or in addition to the diamines or combinations thereof to optimize the properties of the gel material.To optimize the properties of the gel material, trianhydrides, tetraanhydrides, pentaanhydrides, hexanhydrides can also be used in place of or in addition to the dianhydrides or combinations thereof. Dehydrating agents and catalysts can be incorporated into the solution to initiate and advance the imidization.

[0064] The solution can include additional co-gelling precursors along with filler materials and other additives. The filler materials and other additives can be added to the solution at any time before or during gel formation. Alternatively, the filler materials and other additives can be incorporated into the gel material after gelation by various techniques known to those skilled in the art. The solution containing the gelling precursors, solvent, catalyst, water, filler materials, and other additives is preferably a homogeneous solution capable of effective gel formation under appropriate conditions.

[0065] Once the solution is formed and optimized, the gel-forming components in the solution can be converted into a gel material. The process of converting the gel-forming components into a gel material includes an initial gel formation step in which the gel solidifies to the gel point of the gel material. The gel point of the gel material can be considered the point at which the gelling solution exhibits resistance to flow and / or forms a substantially continuous polymeric framework throughout its volume. Various gel formation techniques are known to those skilled in the art. Examples include, but are not limited to, maintaining the mixture in a quiescent state for a sufficient period of time, adjusting the concentration of a catalyst, adjusting the temperature of the solution, applying a type of energy (ultraviolet light, visible light, infrared light, microwave, ultrasound, particle radiation, electromagnetic) to the mixture, or a combination thereof.

[0066] The process of forming gel beads from a gel solution can include combining the solution with a medium that is immiscible with the solution, such as a dispersion medium. Examples of the dispersion medium include silicone oil and mineral oil. The gel solution can be added, for example, by pouring or otherwise combining with the immiscible dispersion medium. The combined dispersion medium and gel precursor can be agitated, such as by mixing, before or during the process of converting the gel-forming components into a gel material to promote the formation of droplets, such as beads. For example, the dispersion medium and gel precursor can be combined to form an emulsion in which the gel precursor solution is the dispersed phase. An exemplary method for producing gel beads is found in U.S. Patent Application Publication No. 2006 / 0084707 to Ou et al., which is incorporated herein by reference in its entirety.

[0067] Interfacial tension causes spherical droplets of the gel precursor to form in the dispersion medium. The droplets gel and strengthen during the dispersion medium, such as silicone oil. Agitation of the mixture is typically performed to prevent the droplets from coalescing. Heat or radiation can also be applied to the dispersion medium to induce or enhance the gelation of the droplets, strengthening the gel beads to make them strong enough to withstand collisions. The ability to generate gel beads in a given space depends on precise control of the droplet gelation process.

[0068] The process further includes removing the gel beads from the dispersing medium, such as silicone oil. The gel beads are filtered from the dispersing medium and then washed or rinsed with a fluid, such as an alcohol, such as ethanol, methanol, isopropanol, or a higher alcohol. The rinse must be capable of removing the oil (or other dispersing medium) without chemically reacting with the gel. After removing the excess silicone oil, the gel beads can be placed in a solvent for aging, as discussed in more detail below. For example, the gel beads can be aged in ethanol. As described herein, the gel beads are suitable for crevice solvent removal using supercritical fluid drying methods. They can also be dried at ambient conditions to produce xerogels. As described in more detail below, dried gel beads, such as aerogel or xerogel beads, are suitable for heat treatment and carbonization. In an exemplary embodiment, the gel beads are substantially spherical.

[0069] The process of transferring gel-forming components into a gel material can also include an aging step (also called curing) prior to liquid-phase extraction. Aging the gel material after reaching the gel point increases the number of crosslinks within the network, further strengthening the gel framework. The duration of gel aging can be adjusted to control various properties within the resulting aerogel material. This aging procedure can be useful to prevent potential volume loss and shrinkage during liquid-phase extraction. Aging can involve maintaining the gel in a quiescent state for an extended period of time (prior to extraction), maintaining the gel at an elevated temperature, adding a crosslinking-promoting compound, or any combination of these. The preferred temperature for aging is typically between about 10°C and about 200°C. Aging of the gel material generally continues until liquid-phase extraction of the wet gel material.

[0070] The time for transitioning the gel-forming material into a gel material includes both the duration of initial gel formation (from the onset of gelation to the gel point) and the duration of any subsequent hardening and aging of the gel material prior to liquid-phase extraction (from the gel point to the onset of liquid-phase extraction). The total time for transitioning the gel-forming material into a gel material is generally between about 1 minute and several days, preferably about 30 hours or less, about 24 hours or less, about 15 hours or less, about 10 hours or less, about 6 hours or less, about 4 hours or less, about 2 hours or less, about 1 hour or less, about 30 minutes or less, or about 15 minutes or less.

[0071] The resulting gel material may be washed in a suitable secondary solvent to replace the primary reaction solvent present in the wet gel. Such secondary solvents include linear monohydric alcohols having one or more aliphatic carbon atoms, dihydric alcohols having two or more carbon atoms, branched alcohols, cyclic alcohols, alicyclic alcohols, aromatic alcohols, polyhydric alcohols, ethers, ketones, cyclic ethers, or derivatives thereof.

[0072] Once the gel material has been formed and processed, the liquid phase of the gel can then be at least partially extracted from the wet gel using extraction methods, including processing and extraction techniques, to form an aerogel material. Liquid phase extraction, among other things, plays an important role in manipulating aerogel properties, such as porosity and density, and related properties, such as thermal conductivity. Typically, aerogels are obtained by extracting the liquid phase from the gel in a manner that causes low shrinkage of the porous network and framework of the wet gel.

[0073] Aerogels are typically formed by removing a liquid mobile phase from a gel material at temperatures and pressures near or above the critical point of the liquid mobile phase. When the critical point is reached (near-critical) or exceeded (supercritical) (i.e., the system pressure and temperature are at or above the critical pressure and critical temperature, respectively), a new supercritical phase, distinct from the liquid or gas phase, appears in the fluid. The solvent can then be removed without encountering liquid-vapor interfaces, capillary pressure, or any of the related mass transfer limitations commonly associated with liquid-vapor boundaries. Furthermore, the supercritical phase is typically more miscible with organic solvents and therefore has better extraction capabilities. Cosolvents and solvent exchanges are also commonly used to optimize the supercritical fluid drying process.

[0074] If evaporation or extraction occurs below the supercritical point, capillary forces caused by liquid evaporation can cause shrinkage and pore collapse within the gel material. Maintaining the mobile phase near or above the critical pressure and temperature during the solvent extraction process reduces the adverse effects of such capillary forces. In certain embodiments of the present disclosure, the use of near-critical conditions, just below the critical point of the solvent system, allows for the production of aerogel materials or compositions with sufficiently low shrinkage to produce commercially viable final products.

[0075] Several additional aerogel extraction methods are known in the art, including various different methods for using supercritical fluids in aerogel drying and ambient drying methods. For example, Kistler (J. Phys. Chem. (1932) 36:52-64) describes a simple supercritical extraction process in which the gel solvent is maintained above its critical pressure and temperature, thereby reducing evaporation capillary forces and maintaining the structural integrity of the gel network. U.S. Patent No. 4,610,863 describes an extraction method in which the gel solvent is exchanged with liquid carbon dioxide, followed by extraction under conditions in which the carbon dioxide is in a supercritical state. U.S. Patent No. 6,670,402 teaches the production of aerogel by rapid solvent exchange and extraction of the liquid phase from the gel by injecting supercritical (rather than liquid) carbon dioxide into an extractor preheated and prepressurized to substantially the supercritical state or above. U.S. Patent No. 5,962,539 describes a method for obtaining aerogels from polymeric materials in sol-gel form in organic solvents by exchanging the organic solvent with a fluid whose critical temperature is below the polymer decomposition temperature and subjecting the fluid / sol-gel to supercritical extraction. U.S. Patent No. 6,315,971 discloses a method for producing gel compositions, which involves drying a wet gel containing gel solids and a desiccant to remove the desiccant under conditions sufficiently dry to reduce gel shrinkage during drying. U.S. Patent No. 5,420,168 describes a method for producing resorcinol / formaldehyde aerogels using a simple air-drying technique. U.S. Patent No. 5,565,142 describes a drying technique that modifies the gel surface to make it stronger and more hydrophobic, allowing the gel framework and pores to resist collapse during atmospheric drying or subcritical extraction. Other examples of extracting a liquid phase from aerogel materials are found in U.S. Patents Nos. 5,275,796 and 5,395,805.

[0076] One preferred embodiment for extracting the liquid phase from a wet gel uses supercritical carbon dioxide conditions, for example, by first substantially exchanging the primary solvent present in the gel's pore network with liquid carbon dioxide, then heating the wet gel (typically in an autoclave) above the critical temperature of carbon dioxide (approximately 31.06°C), and increasing the system pressure to a pressure higher than the critical pressure of carbon dioxide (approximately 1070 psig). Slight fluctuations in the pressure surrounding the gel material can facilitate the removal of the supercritical carbon dioxide fluid from the gel. Carbon dioxide can be recirculated through the extraction system to facilitate the continuous removal of the primary solvent from the wet gel. Finally, the temperature and pressure are slowly returned to ambient conditions to produce a dry aerogel material. Carbon dioxide can also be pretreated to a supercritical state before being injected into the extraction chamber. In other embodiments, extraction can be performed by any suitable mechanism, such as by varying the pressure, timing, and solvent described above.

[0077] In certain embodiments of the present disclosure, the dried polyimide aerogel composition may be cured for 3 hours or more, between 10 seconds and 3 hours, between 10 seconds and 2 hours, between 10 seconds and 1 hour, between 10 seconds and 45 minutes, between 10 seconds and 30 minutes, between 10 seconds and 15 minutes, between 10 seconds and 5 minutes, between 10 seconds and 1 minute, between 1 minute and 3 hours, between 1 minute and 1 hour, between 1 minute and 45 minutes, between 1 minute and 30 minutes, between 1 minute and 15 minutes, between 1 minute and 5 minutes, between 10 minutes and 3 hours. The material may be subjected to one or more heat treatments for durations of between 10 minutes and 1 hour, between 10 minutes and 45 minutes, between 10 minutes and 30 minutes, between 10 minutes and 15 minutes, between 30 minutes and 3 hours, between 30 minutes and 1 hour, between 30 minutes and 45 minutes, between 45 minutes and 3 hours, between 45 minutes and 90 minutes, between 45 minutes and 60 minutes, between 1 hour and 3 hours, between 1 hour and 2 hours, between 1 hour and 90 minutes, or between any two of these values.

[0078] In certain embodiments, the present invention involves the formation and use of nanoporous carbon-based scaffolds or structures, such as carbon aerogels, as electrode materials in energy storage devices, e.g., primary anode materials in LIBs. The pores of the nanoporous scaffold are designed, organized, and structured to accommodate particles of silicon, other metalloids, or metals, and the expansion of said particles upon lithiation, e.g., in LIBs. Alternatively, the pores of the nanoporous scaffold may be filled with sulfides, hydrides, any suitable polymer, or other additives, where it is beneficial to contact the additive with the conductive material (i.e., the scaffold / aerogel) to provide a more effective electrode. A typical process for utilizing silicon-doped carbon aerogels in battery applications is shown in Figure 1.

[0079] To further expand on the exemplary applications in LIBs, when carbon aerogel materials are utilized as the primary anode material, as in certain embodiments of the present invention, the aerogel nanoporous structure has a narrow pore size distribution, provides high electrical conductivity, high mechanical strength, and a morphology and sufficient pore volume (at final density) to accommodate a high weight percent of silicon particles and their expansion. Certain embodiments of the present invention structurally have a fibrillar morphology with, among other things, the narrow pore size distribution, high pore volume, and strut size described above that result in improved connectivity.

[0080] In additional or alternative embodiments, the carbon aerogel itself functions as a current collector due to its electrical conductivity and mechanical strength; therefore, in preferred embodiments, a separate current collector is not required on the anode side (if the anode is formed of carbon aerogel). Note that in conventional LIBs, copper foil is bonded to the anode as its current collector. However, depending on the application of the carbon aerogel, removing one or both of these components creates additional space for more electrode material, further increasing the capacity of the cell / individual electrodes and the overall energy density of the packaged battery system. However, in certain embodiments, existing current collectors can be integrated with the anode materials of various other embodiments to enhance the current-collection capability or capacity of the copper or aluminum foil.

[0081] In certain embodiments, nanoporous carbon-based scaffolds or structures, specifically carbon aerogels, can be used as a conductive network or current collector on the anode side of an energy storage device. The fully interconnected carbon aerogel network is filled with electrochemically active species, which are in direct contact with or physically connected to the carbon network. The loading of the electrochemically active species is tuned with respect to pore volume and porosity for high and stable capacity and improved safety of the energy storage device. When utilized on the anode side, the electrochemically active species can include, for example, silicon, graphite, lithium, or other metalloids or metals. In yet another embodiment, the anode can include a nanoporous carbon-based scaffold or structure, specifically carbon aerogels.

[0082] In this disclosure, the term "collectorless" refers to the absence of a separate current collector directly connected to the electrode. As mentioned above, in conventional LIBs, copper foil is typically bonded to the anode as its current collector. According to embodiments of the present invention, electrodes formed from nanoporous carbon-based scaffolds or structures (e.g., carbon aerogel) can be freestanding or otherwise capable of being collectorless, as the scaffold or structure itself functions as the current collector due to its high electrical conductivity. Within an electrochemical cell, collectorless electrodes can be bonded to form circuits by embedding solid, mesh, or woven tabs during the melting process that produces continuous porous carbon, or by soldering, welding, or metal depositing leads to portions of the porous carbon surface. Other mechanisms for contacting the carbon to the rest of the system are also contemplated herein. In alternative embodiments, nanoporous carbon-based scaffolds or structures, specifically carbon aerogels, may be disposed on or otherwise in communication with a dedicated current collecting substrate (e.g., copper foil, aluminum foil, etc.). In this method, the carbon aerogel is attached to a solid current collector using a conductive adhesive, and different amounts of pressure are applied.

[0083] It is further contemplated herein that nanoporous carbon-based scaffolds or structures, particularly carbon aerogels, can be in the form of monolithic structures. When essentially monolithic, carbon aerogels do not require any binder; in other words, the anode can be binderless. As used herein, the term "monolithic" refers to an aerogel material in which the majority (by weight) of the aerogel contained in the aerogel material or composition is in the form of a single, continuous, interconnected aerogel nanostructure. Monolithic aerogel materials include aerogel materials that are initially formed to have a single, interconnected gel or aerogel nanostructure, but may subsequently be cracked, broken, or segmented into non-single aerogel nanostructures. Monolithic aerogels can be in the form of free-standing structures or reinforced (fiber or foam) materials. Using silicon lithiation as an example, silicon incorporated into monolithic aerogels can be more efficiently utilized, in terms of theoretical capacity, than the same amount of silicon incorporated into a slurry using a conventional process (see Figure 2).

[0084] Monolithic aerogel materials are distinguished from granular aerogel materials. The term "granular aerogel material" refers to an aerogel material in which the majority (by weight) of the aerogel contained in the aerogel material is in the form of fine particles, particles, granules, beads, or powder, which may be mixed together (i.e., via a binder, such as a polymer binder) or compressed together, but there is no interconnected aerogel nanostructure between the individual particles. Collectively, aerogel materials in this form are said to have a powder or granular morphology (as opposed to a monolithic morphology). Note that, although individual particles of a powder have a unitary structure, the individual particles are not considered monoliths herein. Integrating aerogel powder into an electrochemical cell generally involves preparing a paste or slurry from the powder, casting it onto a substrate, and drying, which may optionally include calendering.

[0085] Particulate aerogel materials, such as aerogel beads, offer certain advantages. For example, particulate materials according to embodiments disclosed herein can be used as a direct replacement for other materials, such as graphite, in LIB anodes and anode manufacturing processes. Particulate materials according to embodiments disclosed herein can also provide improved lithium ion diffusion rates due to shorter diffusion paths within the particulate material. Particulate materials according to embodiments disclosed herein can also achieve electrodes with optimized packing densities, for example, by adjusting particle size and packing arrangement. Particulate materials according to embodiments disclosed herein can also provide improved silicon access due to inter- and intra-particle porosity.

[0086] In this disclosure, the terms "binderless" or "binder-free" (or derivatives thereof) refer to a material that is substantially free of a binder or adhesive to hold the material together. For example, a monolithic nanoporous carbon material does not contain a binder because its framework is formed as a single, continuous, interconnected structure. Benefits of being binderless include, for example, avoiding the effect of a binder on electrical conductivity and pore volume. On the other hand, aerogel particles require a binder to hold them together to form larger functional materials, and such larger materials are not considered monolithic herein. Furthermore, the term "binder-free" does not exclude all use of binders. For example, a binder or adhesive may be placed on a major surface of the aerogel material to secure a monolithic aerogel according to the present invention to another monolithic aerogel or non-aerogel material. In this manner, a binder is used to create a laminated composite, but the binder does not function to maintain the stability of the monolithic aerogel framework itself.

[0087] Furthermore, the monolithic polymer aerogel material or composition of the present disclosure may be compressed to strains of up to 95% without significantly destroying or fracturing the aerogel framework, while densifying the aerogel and minimizing reduction in porosity. In certain embodiments, the compressed polymer aerogel material or composition is subsequently carbonized using various methods described herein to form a nanoporous carbon material. It is understood that the amount of compression affects the thickness of the resulting carbon material, which in turn affects capacity, as will become clearer later in this specification. The examples described below illustrate various thicknesses formed and contemplated by the present invention, and thickness can be adjusted based on compression. Thus, the thickness of the composite (typically compressed) can be about 10 to 1000 micrometers, or any narrower range therein, depending on the desired benefits of the final composite. The present invention also contemplates powder or particulate forms of carbon aerogel when a binder is required and particle size is optimized. Particle size ranges can be about 1 to 50 micrometers.

[0088] Nanoporous carbons, such as carbon aerogels, according to the present invention can be formed from any suitable organic precursor material. Examples of such materials include, but are not limited to, RF, PF, PI, polyamide, polyacrylate, polymethyl methacrylate, acrylate oligomers, polyoxyalkylenes, polyurethanes, polyphenols, polybutadiene, trialkoxysilyl-terminated polydimethylsiloxane, polystyrene, polyacrylonitrile, polyfurfural, melamine-formaldehyde, cresol-formaldehyde, phenol-furfural, polyethers, polyols, polyisocyanates, polyhydroxybenzyl, polyvinyl alcohol dialdehyde, polycyanurates, polyacrylamides, various epoxies, agar, agarose, chitosan, and combinations and derivatives thereof. Any precursors to these materials may be used to fabricate and use the resulting material. In an exemplary embodiment, the carbon aerogel is formed from a pyrolyzed / carbonized polyimide-based aerogel, i.e., the polymerization of polyimides. More specifically, polyimide-based aerogels can be produced using one or more of the methods described in U.S. Patents 7,071,287 and 7,074,880 to Rhine et al., for example, by imidizing poly(amide) acids and drying the resulting gels using supercritical fluids. Other suitable methods for producing polyimide aerogels (and carbon aerogels derived therefrom) include, for example, U.S. Patents 6,399,669 to Suzuki et al.; 9,745,198 to Leventis et al., Polyimide Aerogels by Ring-Opening Metathesis Polymerization (ROMP), Chem. Mater. 2011, 23, 8, 2250-2261; and Leventis et al., Isocyanate-Derived Organic Aerogels: Polyureas, Polyimides, and Polyamides, MRS Methods, 1306 (2011), Mrsf10-1306-bb03-01. doi:10.1557·opl.2011.90; Chidambareswarapattar et al., One-step room-temperature synthesis of fibrous polyimide aerogels from anhydrides and isocyanates and their conversion to isomorphous carbon, J. Mater. Chem., 2010, 20, 9666-9678; Guo et al., Crosslinked polyimide aerogels with amine-functionalized polyoligomeric silsesquioxanes, ACS Appl. Mater. Interfaces 2011, 3, 546-552; Nguyen et al., Development of high-temperature flexible polyimide aerogels, American Chemical Society, proceedings, 2011; Meador et al., Mechanically strong and flexible polyimide aerogels crosslinked with aromatic triamines, ACS Appl. Mater. Interfaces, 2012, 4(2), pp536-544; Meador et al., Polyimide aerogels with amide crosslinks: a low-cost alternative for mechanically strong polymer aerogels, ACS Appl. Mater. Interfaces 2015, 7, 1240-1249; Pei et al., "Preparation and Characterization of Highly Crosslinked Polyimide Aerogels Based on Polyimides Containing Trimethoxysilane Side Groups," Langmuir 2014, 30, 13375-13383. The resulting polyimide aerogels are then pyrolyzed to form polyimide-derived carbon aerogels.

[0089] Carbon aerogels according to exemplary embodiments of the present disclosure, such as polyimide-derived carbon aerogels, can have a residual nitrogen content of at least about 4% by weight. For example, the residual nitrogen content of carbon aerogels according to embodiments disclosed herein can be at least about 0.1%, at least about 0.5%, at least about 1%, at least about 2%, at least about 3%, at least about 4%, at least about 5%, at least about 6%, at least about 7%, at least about 8%, at least about 9%, at least about 10%, or a range between any two of these values.

[0090] In certain embodiments of the present disclosure, for carbonization of organic (e.g., polyimide) aerogels, the dried polymer aerogel composition can be subjected to a processing temperature of 200° C. or higher, 400° C. or higher, 600° C. or higher, 800° C. or higher, 1000° C. or higher, 1200° C. or higher, 1400° C. or higher, 1600° C. or higher, 1800° C. or higher, 2000° C. or higher, 2200° C. or higher, 2400° C. or higher, 2600° C. or higher, 2800° C. or higher, or a range between any two of these values. Without being bound by theory, it is believed herein that the electrical conductivity of the aerogel composition increases as the carbonization temperature increases.

[0091] In this disclosure, the term "electrical conductivity" refers to a measurement of a material's ability to conduct electric current or otherwise allow electrons to pass through or flow through it. Conductivity is specifically measured as its electrical conductance / susceptance / admittance per unit size of the material. It is commonly reported as S / m (Siemens / meter) or S / cm (Siemens / centimeter). The conductivity or resistivity of a material can be measured by methods known in the art, including, but not limited to, the four-point in series resistivity method (using the ASTM F84-99 dual configuration test method). In this disclosure, unless otherwise specified, conductivity measurements are obtained according to the ASTM F84-resistivity (R) measurement, which is obtained by measuring voltage (V) and dividing by current (I). In certain embodiments, the electrical conductivity of the aerogel material or composition of the present disclosure is about 10 S / cm or greater, 20 S / cm or greater, 30 S / cm or greater, 40 S / cm or greater, 50 S / cm or greater, 60 S / cm or greater, 70 S / cm or greater, 80 S / cm or greater, or a range between any two of these values.

[0092] In this disclosure, the term "electrochemically active species" refers to an additive capable of accepting and releasing ions within an energy storage device. Using a LIB as an example, the electrochemically active species in the anode accepts lithium ions during charging and releases lithium ions during discharging. The electrochemically active species can be stabilized within the anode through direct / physical connection with the nanoporous carbon. In certain embodiments, the nanoporous carbon network forms an interconnected structure around the electrochemically active species. The electrochemically active species is bonded to the nanoporous carbon at multiple points. One example of an electrochemically active species is silicon, which, as mentioned above, can expand and crack or break upon lithiation. However, because silicon has multiple connection points with the nanoporous carbon (aerogel), it remains active within the nanoporous structure, e.g., within the pores, or enclosed within the structure, even if fracture or cracking occurs.

[0093] In this disclosure, the terms "compressive strength," "flexural strength," and "tensile strength" refer to a material's resistance to fracture or cracking under compressive, flexural, and tensile forces, respectively. These strengths are specifically measured as the amount of load / force per unit area resisting the load / force. This is commonly reported in pounds per square inch (psi), megapascals (MPa), or gigapascals (GPa). Among other things, the compressive, flexural, and tensile strengths of a material collectively contribute to the material's structural integrity, which is beneficial for withstanding the volume expansion of silicon particles during, for example, lithiation in LIBs. Specifically, reference is made to Young's modulus, an indicator of mechanical strength, which may be measured by methods known in the art, including, but not limited to, Standard Test Practice for Instrumented Indentation Testing (ASTM E2546, ASTM International, West Conshocken, PA) or Standardized Nanoindentation (ISO 14577, International Organization for Standardization, Switzerland). In this disclosure, Young's modulus measurements are obtained in accordance with ASTM E2546 and ISO 14577 unless otherwise specified. In certain embodiments, the Young's modulus of an aerogel material or composition of the present disclosure is about 0.2 GPa or greater, 0.4 GPa or greater, 0.6 GPa or greater, 1 GPa or greater, 2 GPa or greater, 4 GPa or greater, 6 GPa or greater, 8 GPa or greater, or a range between any two of these values.

[0094] In this disclosure, the term "pore size distribution" refers to the statistical distribution or relative amount of each pore size within a sample volume of a porous material. A narrower pore size distribution refers to a relatively large proportion of pores within a narrow range of pore sizes, thus optimizing the amount of pores that can accommodate electrochemically active species and maximizing the use of pore volume. Conversely, a wider pore size distribution refers to a relatively small proportion of pores within a narrow range of pore sizes. Therefore, pore size distribution is generally measured as a function of pore volume and reported as the unit size of the full width at half maximum of the main peak in a pore size distribution chart. The pore size distribution of a porous material may be measured by methods known in the art, including, but not limited to, surface area and porosity analyzers by nitrogen adsorption and desorption, which can calculate the pore size distribution. In this disclosure, pore size distribution measurements are obtained according to this method unless otherwise specified. In certain embodiments, the aerogel material or composition of the present disclosure has a relatively narrow pore size distribution (full width at half maximum) of about 50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, 15 nm or less, 10 nm or less, 5 nm or less, or a range between any two of these values.

[0095] In this disclosure, the term "pore volume" refers to the total volume of pores within a sample of porous material. Pore volume is specifically measured as the volume of void space within the porous material, which may be measurable by itself and / or by electrochemically active species such as silicon particles. This is generally expressed in cubic centimeters per gram (cm 3The pore volume of a porous material may be measured by methods known in the art, including, but not limited to, surface area and porosity analyzers by nitrogen adsorption / desorption, which can calculate the pore volume. In the present disclosure, pore volume measurements are obtained according to this method unless otherwise specified. In certain embodiments, the relatively large pore volume of an aerogel material or composition (not incorporating an electrochemically active species such as silicon) of the present disclosure is about 1 cc / g or more, 1.5 cc / g or more, 2 cc / g or more, 2.5 cc / g or more, 3 cc / g or more, 3.5 cc / g or more, 4 cc / g or more, or a range between any two of these values. In other embodiments, the pore volume of the aerogel material or composition (incorporating an electrochemically active species such as silicon) of the present disclosure is about 0.3 cc / g or more, 0.6 cc / g or more, 0.9 cc / g or more, 1.2 cc / g or more, 1.5 cc / g or more, 1.8 cc / g or more, 2.1 cc / g or more, 2.4 cc / g or more, 2.7 cc / g or more, 3.0 cc / g or more, 3.3 cc / g or more, 3.6 cc / g or more, or a range between any two of these values.

[0096] In this disclosure, the term "porosity" refers to the volume fraction of the pores that does not contain additional material (e.g., electrochemically active species such as silicon) bound to the pore walls. For clarity and illustrative purposes, it should be noted that in certain embodiments of silicon-doped carbon aerogels as primary anode materials in LIBs, porosity refers to the void space after including silicon particles. Thus, the porosity may be, for example, about 10% to 70% when the anode is in a pre-lithiation state (to accommodate ion transport and silicon expansion) and about 1% to 50% when the anode is in a post-lithiation state (to accommodate ion transport). More generally, porosity may be measured by methods known in the art, including, but not limited to, the ratio of the pore volume to the bulk density of the aerogel material. In this disclosure, porosity measurements are obtained according to this method unless otherwise specified. In certain embodiments, the porosity of the aerogel material or composition of the present disclosure is about 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, or a range between any two of these values.

[0097] It should be noted that pore volume and porosity are different measures of the same property of a porous structure, namely, the "empty space" within the porous structure. For example, when silicon is used as the electrochemically active species contained within the pores of a nanoporous carbon material, pore volume and porosity refer to the "empty" space, i.e., the space not utilized by silicon or carbon. As will be seen later, densification of the pre-carbonized nanoporous material, for example by compression, can also affect pore volume and porosity, among other things.

[0098] In the present disclosure, the term "pore size at the maximum peak from the distribution" refers to the value at a distinct peak on a graph showing the pore size distribution. The pore size at the maximum peak from the distribution is specifically measured as the pore size with the highest percentage formed. This is generally recorded in any unit length of pore size, such as micrometers or nm. The pore size at the maximum peak from the distribution is measured by methods known in the art, including, but not limited to, surface area and porosity analyzers by nitrogen adsorption / desorption, which can calculate the pore size distribution and measure the pore size at the maximum peak. In the present disclosure, unless otherwise specified, the measurement of the pore size at the maximum peak from the distribution is obtained according to this method. In certain embodiments, the pore size at the largest peak from the distribution of an aerogel material or composition of the present disclosure is about 150 nm or less, 140 nm or less, 130 nm or less, 120 nm or less, 110 nm or less, 100 nm or less, 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less, 40 nm or less, 30 nm or less, 20 nm or less, 10 nm or less, 5 nm or less, 2 nm or less, or a range between any two of these values.

[0099] In this disclosure, the term "strut width" refers to the average diameter of the nanostruts, nanorods, nanofibers, or nanofilaments that form aerogels with a fibrillar morphology. This is typically reported as an arbitrary unit length, such as micrometers or nm. Strut width is measured by methods known in the art, including, but not limited to, scanning electron microscope image analysis. In this disclosure, strut width measurements are obtained according to this method unless otherwise specified. In certain embodiments, the strut width of the aerogel materials or compositions of this disclosure is about 10 nm or less, 9 nm or less, 8 nm or less, 7 nm or less, 6 nm or less, 5 nm or less, 4 nm or less, 3 nm or less, 2 nm or less, or a range between any two of these values. An exemplary range of strut widths seen in the examples below (particularly in the ESM images in the figures) is about 2-5 nm. Smaller strut widths, such as these, allow a greater amount of struts to be present within the network, thus making contact with electrochemically active species, which in turn allows more electrochemically active species to be present within the composite. This improves electrical conductivity and mechanical strength.

[0100] In this disclosure, the term "fibrillar morphology" refers to a structural morphology of nanoporous carbon (e.g., aerogel) that includes struts, rods, fibers, or filaments. For example, in one embodiment, the choice of solvent, such as dimethylacetamide (DMAC), can influence the formation of such morphologies. Furthermore, in certain embodiments, when carbon aerogels are derived from polyimides, crystalline polyimides result from polyimides that form linear polymers. As will become more apparent in the examples below, in certain embodiments, the inclusion of fibrillar morphology as an interconnected polymer structure was surprisingly observed, where long linear structures were expected based on the known behavior of polyimide precursors. In comparison, the product morphology of nanoporous carbon can instead be essentially particulate or powder, with the fibrillar morphology of the carbon aerogel persisting. As will become clearer later in this specification, the fibrillar morphology can offer certain advantages over particulate morphology, such as mechanical stability / strength and electrical conductivity, particularly when nanoporous carbons are used in certain applications, such as anode materials in LIBs. It is noted that this fibrillar morphology is found in both monolithic and powder forms of nanoporous carbon, i.e., monolithic carbon can have a fibrillar morphology, and aerogel powders / particles can have a fibrillar morphology. Furthermore, in certain embodiments, when the nanoporous carbon material contains an additive such as silicon, the fibrillar nanostructure inherent to the carbon material is preserved and acts as a bridge between the additive particles.

[0101] In this disclosure, the term "cycle life" refers to the number of complete charge / discharge cycles that an anode or battery (e.g., a LIB) can support before its capacity drops below about 80% of its original rated capacity. Cycle life can be affected by various factors that are not significantly affected over time, such as the mechanical strength of the underlying substrate (e.g., a carbon aerogel), the connectivity of silicon particles within the aerogel, and the maintenance of aerogel interconnectivity. It is noted that it is a surprising aspect of certain embodiments of the present invention that these factors actually remain relatively unchanged over time. Cycle life can be measured by methods known in the art, including, but not limited to, cycle testing, in which a battery cell is subjected to repeated charge / discharge cycles at a predetermined current rate and operating voltage. In this disclosure, cycle life measurements are obtained according to this method unless otherwise specified. In certain embodiments of the present disclosure, the cycle life of an energy storage device, such as a battery, or an electrode thereof is about 25 cycles or more, 50 cycles or more, 75 cycles or more, 100 cycles or more, 200 cycles or more, 300 cycles or more, 500 cycles or more, 1000 cycles or more, or a range between any two of these values.

[0102] In this disclosure, the term "capacity" refers to the specific amount of energy or charge a battery can store. Capacity is specifically measured as the discharge current a battery can deliver over time per unit mass. It is typically reported as ampere-hours or milliampere-hours per gram of total electrode mass, Ah / g or mAh / g. The capacity of a battery (and particularly the anode) can be measured by methods known in the art, including, but not limited to, applying a fixed constant current load to a fully charged cell until the cell voltage reaches an end-of-discharge voltage value, where the time to reach the end-of-discharge voltage multiplied by the constant current is the discharge capacity, and measuring the specific capacity and volumetric capacity by dividing the discharge capacity by the weight or volume of the electrode material. In this disclosure, capacity measurements are obtained according to this method unless otherwise specified. In certain embodiments, the capacity of the aerogel material or composition of the present disclosure is about 200 mAh / g or greater, 300 mAh / g or greater, 400 mAh / g or greater, 500 mAh / g or greater, 600 mAh / g or greater, 700 mAh / g or greater, 800 mAh / g or greater, 900 mAh / g or greater, 1000 mAh / g or greater, 1200 mAh / g or greater, 1400 mAh / g or greater, 1600 mAh / g or greater, 1800 mAh / g or greater, 2000 mAh / g or greater, 2400 mAh / g or greater, 2800 mAh / g or greater, 3200 mAh / g or greater, or a range between any two of these values. Unless otherwise specified, when the present nanoporous carbon material is used in a battery, capacity is reported at cycle 10 of the battery.

[0103] In this disclosure, the term "silicon utilization" refers to the difference between the theoretical capacity of lithiated silicon and the measured capacity of the electrode based on the weight of silicon. Silicon utilization is specifically measured as the efficiency of silicon utilization in the electrode. It is reported herein as a percentage using the following formula: JPEG0007770921000001.jpg35170

[0104] To calculate silicon utilization, the electrode capacity and silicon capacity are measured as described above. In certain embodiments, the silicon utilization of the aerogel materials or compositions of the present disclosure is about 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or a range between any two of these values, with a higher percentage indicating better or more efficient silicon utilization. Unless otherwise specified, when the nanoporous carbon materials are used in batteries, the silicon utilization is reported at cycle 10 of the battery.

[0105] In one embodiment, the invention is a LIB anode comprising a silicon-doped polyimide-derived carbon aerogel, wherein the silicon particles are at least partially contained within the pores of the carbon aerogel. Typical reactions and processes for developing polyimide-derived carbon aerogels (i.e., silicon-free) are shown in Figure 3. As will be seen, the carbon aerogel pore structure can be tailored to have different properties (e.g., pore volume, pore size distribution) based on the required requirements (e.g., electrode size or capacity in a LIB). In another embodiment, the invention is a LIB comprising such an anode or an electrode within its electrochemical cell. In yet a further embodiment, the invention is a device or system incorporating such an energy storage device. Examples include, but are not limited to, electric vehicles and electronic devices (e.g., mobile devices and drones).

[0106] In a specific embodiment, the present invention is a method for forming or producing a continuous porous silicon carbon composite, such as a carbon aerogel. Polyimide precursors, such as a diamine and a dianhydride, each of which may contain aromatic and / or aliphatic groups, are mixed in a suitable solvent (e.g., a polar, aprotic solvent). Silicon particles are mixed with the polyimide precursor in the solvent before adding an imidization gelation catalyst. The imidization gelation catalyst is then added to initiate gelation of the mixture. In an alternative embodiment, imidization can be achieved by thermal imidization, with any suitable temperature and time range being contemplated (e.g., heating at about 100°C to 200°C for about 20 minutes to about 8 hours, followed by heating at about 300°C to 400°C for about 20 minutes to about 1 hour). The gelled mixture is then dried to yield a continuous porous polyimide silicon composite, where drying can be performed using subcritical and / or supercritical carbon dioxide. Optionally, the polyimide silicon composite can be compressed, preferably uniaxially (e.g., to 95% strain), to increase its density, which can be adjusted up to about 1.5 g / cc based on the amount of compression. In an exemplary embodiment, the polyimide silicon composite can be compressed to a strain of greater than about 80% before pyrolysis of the composite. Whether or not compression is performed, the polyimide silicon composite can be pyrolyzed to obtain an open-pore silicon carbon composite, the resulting composite containing greater than 0% and less than about 95% silicon by weight, with a porosity between about 5% and 99%. In certain embodiments, pyrolysis can be performed at a maximum temperature between about 750°C and about 1600°C, with optional graphitization at about 1600°C to about 3000°C.

[0107] In certain embodiments, the carbon silicon composite can be a monolith or a free-standing structure, prepared on or without a substrate, pulverized into powder form, or prepared as a particulate material such as beads. Furthermore, the composite can be reinforced with or without a nonwoven or woven material (e.g., fiber, foam, etc.). Optionally, the composite can be pre-doped with a metal or metal oxide, including, but not limited to, tin, sulfur, phosphorus, nickel, cobalt, manganese, lithium, magnesium, iron, zinc, boron, titanium, aluminum oxide, titanium oxide, niobium oxide, molybdenum oxide, silica, and aluminosilicates. Furthermore, the silicon particles can be pre-doped with p-type acceptors (e.g., boron, aluminum, gallium, and indium) or n-type donors (e.g., phosphorus, lithium, arsenic, antimony, bismuth).

[0108] In an alternative embodiment, porous silicon carbon composites may be formed or produced utilizing the above method, except that rather than mixing silicon particles with a mixture of polyimide precursors in a solvent, a silicate (e.g., silicon dioxide, aluminosilicate, and / or halloysite) and a reducing agent (e.g., magnesium, lithium, sodium, potassium, aluminum, calcium, or a combination thereof) are mixed with a mixture of polyimide precursors in a solvent. Upon drying, a continuous porous polyimide silicate and reducing agent composite is formed, which can optionally be compressed to adjust the density. In this case (i.e., when a silicate and reducing agent are used), the silicate and reducing agent are reacted with hydrogen gas under inert conditions at temperatures above about 700°C to form silicon within the carbon composite.

[0109] In a further alternative embodiment, the above method may be utilized, except that rather than adding silicon or silicate plus a reducing agent to a polyimide precursor, a continuous porous carbon may first be formed (i.e., polyimide precursor, imidization using a catalyst or heat, drying, and pyrolysis), followed by deposition of silicon onto or within the porous carbon. In this case, silicon is deposited by dip-coating the porous carbon into a silicon-forming silane precursor, followed by heating under inert conditions to decompose the silane and form a conformal silicon coating within the porous carbon. This dip process may be performed multiple times, increasing the thickness and silicon content to approximately 95% by weight. In other embodiments, silicon may be deposited by atomic layer deposition or CVD.

[0110] Furthermore, it is contemplated herein that the pore size can be adjusted as needed. There are five main ways to adjust the pore size taught herein. First, the amount of solids, specifically the amount of polyimide precursor monomers (e.g., aromatic or aliphatic diamines and aromatic or aliphatic dianhydrides), can adjust the pore size. Smaller pore sizes result from a higher amount of solids per unit volume of fluid, since there is less available space for closer interconnections. Note that the strut width remains constant to a certain extent regardless of the amount of solids used. The amount of solids is more related to the density of the network.

[0111] Another method for tailoring pore size is to apply radiation (e.g., radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays) to the composite in either the polyimide or carbon state. Radiation has an oxidative effect, resulting in an increase in surface area, an increase in pore size, and a broadening of the pore size distribution. Third, pore size is affected by macroscopic compression of the polyimide composite. As demonstrated in the examples below, pore size decreases with compression.

[0112] Yet another method for adjusting pore size is ion bombardment of the composite in either the polyimide or carbon state. The effect of ion bombardment depends on the specified method. For example, there is additive ion bombardment (e.g., CVD), which adds something and reduces the pore size. There is also destructive ion bombardment, which increases the pore size. Finally, the pore size can be adjusted (increased or decreased) by heat treatment in different gas environments, such as in the presence of carbon dioxide or carbon monoxide, chemically active environments, or hydrogen reducing environments. Carbon dioxide environments are known to produce, for example, activated carbons, where activation removes mass, increases pore size, and increases surface area.

[0113] While each of the above methods of adjusting pore size are contemplated, this disclosure focuses more on the variation of the solids content (polyimide precursor) and compression of the polyimide composite prior to carbonization. [Example]

[0114] The following examples are offered for illustrative purposes only and are not intended to limit the scope of the various embodiments of the present invention in any way.

[0115] Example 1: CPI Compounds with Low Levels of Doping A. CPI with 9% dopant dispersed in polyimide PI gel was prepared from pyromellitic dianhydride (PMDA) and 1,4-phenylenediamine (PDA) in DMAC solvent at a target density of 0.06 g / cc and a molar ratio of 1:1. The precursors were mixed at room temperature for 3 hours, and then acetic anhydride (AA) was added to the PMDA at a molar ratio of 4.3 and mixed with the solution for 2 hours. Powder dopants, such as graphite CNGT0112 with a thickness of approximately 40 nm and a length of approximately 400–600 nm and silicon BASIC005 with spherical particles of approximately 30 nm in diameter, were obtained from ACS Materials. The solution was doped with 4.5% graphite or silicon (Si) per total solids. The graphite was mixed with the solution for 10 minutes by stirring with a magnetic bar, and the doped mixture and imidization were catalyzed with pyridine (Py). Based on visual evaluation, the graphite was well dispersed. After stirring the silicon with the polyimide solution for 10 minutes, the dispersion appeared visually inferior (i.e., evidence of settling and / or particle aggregation in the solution), and the mixture was sonicated for an additional 3 minutes. Once the dispersion quality improved, a pyridine catalyst was added to the mixture. The molar ratio of Py to PMDA was 4.0.

[0116] To prepare the PI composites, the solution was cast between glass plates with spacers at the edges to control thickness. Other suitable methods of casting the solution are also contemplated herein. The spacers were made from 200-micrometer-thick aluminum foil. Monoliths approximately 2 inches in diameter were also cast in Teflon containers. The gelation time at ambient temperature was approximately 11.5 minutes for the graphite-doped sample and approximately 15.5 minutes for the silicon-doped sample. The gels were cured overnight at room temperature and then exchanged three times with ethanol at 68°C before supercritical CO2 extraction. The PI aerogel composites were compressed to various thicknesses, starting from approximately 250 micrometers, and carbonized by pyrolysis at 1050°C for 2 hours under an inert atmosphere to form CPI composites. The % dopant in the CPI was calculated to be approximately 9% based on the amount of dopant in the formulation and the weight of the composite retained after pyrolysis.

[0117] The densities of the approximately 80-50 micrometer thick compressed CPI composites ranged from approximately 0.24 to 0.36 g / cc (Table 1 and Figure 4).

[0118] JPEG0007770921000002.jpg41102

[0119] Porosity was calculated based on the actual density and skeletal density of the CPI composites. Because the density of amorphous carbon ranges from approximately 2.0 to 2.3 g / cc, and the density of silicon or graphite is approximately 2.3 g / cc, the skeletal density used in these calculations was approximately 2.2 g / cc for all composites. High porosities of 84 to 89% were calculated for these low-density composites.

[0120] B. CPI with 9% dopant dispersed in solvent by mixing Similar experiments were performed using a solution of polyimide with a target density of 0.10 g / cc. In this case, the dopant was mixed with a portion of DMAC for 10 minutes and added to the mixture before catalysis. A 500-micrometer-thick Teflon spacer was used for casting. The molar ratio of Py to PMDA was 2.0. The gelation time was approximately 2.5 minutes for the graphite-doped sample and approximately 4.0 minutes for the silicon-doped sample. The gel was extracted using supercritical CO2. PI aerogel composites were compressed to various thicknesses, starting from approximately 580 micrometers, and then pyrolyzed at 1050°C for 2 hours to form CPI composites.

[0121] The properties of the CPI composites are shown in Table 2. As expected, the more compacted composites exhibited slightly lower porosity.

[0122] JPEG0007770921000003.jpg90102

[0123] The densities of the compacted CPI composites (approximately 115–80 micrometers thick) ranged from approximately 0.57–0.87 g / cc. The densities of the silicon-doped CPI composites were slightly lower compared to the graphite-doped samples (Figure 5).

[0124] The density and shrinkage of the doped PI aerogel monoliths after pyrolysis are shown in Table 3. The densities of the uncompressed monoliths (LS1 and LG1) were lower compared to the densities of the compressed composites (LS2 and LG2).

[0125] JPEG0007770921000004.jpg46159

[0126] SEM images of the silicon-doped pyrolyzed composites are shown in Figures 6A-6B. Note that Figure 6B shows the fibrillar morphology of the silicon-doped CPI composite. Pockets of silicon aggregates and silicon nanowires were embedded in the carbon matrix. Figure 7 shows the anode discharge capacity per dopant content for half-cell tests. The initial capacity of silicon was significantly higher compared to the graphite dopant. Although the initial capacity decreased with cycling, its reversible capacity was still dramatically higher than that of graphite alone, as in conventional anodes. Furthermore, the capacity loss may be due to the foil electrode used in these half-cell tests, as the electrode was unable to operate at the high capacity of the silicon-containing electrode.

[0127] Example 2: High PI solids and highly silicon-doped CPI composites A. 27% silicon-doped CPI dispersed in solvent by ultrasonic treatment PI gel was prepared with a target density of 0.10 g / cc. PMDA precursor and PDA precursor were mixed at room temperature for 3 hours. Separately, 30 nm particle size silicon powder was sonicated in DMAC solvent for 20 seconds and added to the mixture at 15.0% based on total solids and stirred for 15 minutes. AA was added to the mixture doped at a 4.3 molar ratio relative to PMDA and mixed for 2 hours. Pyridine was used to catalyze the mixture at a 2.0 molar ratio relative to PMDA. Composites were cast between glass plates using 500 micrometer thick Teflon spacers. 2-inch diameter monoliths were also cast in Teflon containers. The gelation time at ambient temperature was approximately 3.5 minutes. The gel was allowed to cure overnight at room temperature, followed by three ethanol exchanges at 68°C before supercritical CO2 extraction. The PI aerogel composite was compressed and carbonized by pyrolysis at 1050°C for 2 hours to form CPI composites.

[0128] B. 46% and 64% silicon-doped CPI dispersed in solvent by ultrasonication PI gel was prepared with a target density of 0.08 g / cc. The PMDA and PDA precursors were mixed at room temperature for 4 hours. AA was then added and mixed with the solution for 2 hours. Separately, 30 nm particle size silicon powder was sonicated in DMCA solvent for 1 minute and added to the mixture 5 minutes before adding the pyridine catalyst. Silicon was added at approximately 29.7% and 49.6% total solids. A 3.2 molar ratio of Py to PMDA was used to catalyze the doped mixture. Composites were cast using 500 micrometer thick spacers. The gel time at ambient temperature was approximately 6.5 minutes. After processing and extraction, the PI aerogel composite was compressed and carbonized by pyrolysis at 1050°C for 2 hours to form the CPI composite.

[0129] A comparison of the surface area and porosimetry of pyrolyzed uncompacted monoliths doped with different amounts of silicon is shown in Table 4 and Figure 8. The surface area, micropore area, and pore volume decreased with increasing silicon content in the CPI.

[0130] JPEG0007770921000005.jpg53159

[0131] The pore size distribution at the maximum peak depended on the silicon content and the silicon dispersion mode (Figure 9). Generally, by increasing the silicon content, the pore size at the maximum peak from the distribution shifted to larger sizes. As the Si content increased from about 9% to about 46% and then to about 64%, respectively, a shift of the main peak in the maximum size distribution was observed from about 23 nm to about 26 nm and then to about 34 nm. The basal pore size distribution became wider as the silicon content increased.

[0132] However, when comparing the Td 0.10 g / cc samples, the 27% Si monolith, prepared by adding silicon before AA and mixing with the solution for a longer time, exhibited a pore size shift of approximately 15 nm in the main peak, compared to approximately 23 nm for the 9% Si monolith, prepared by mixing Si with the solution for a shorter time. The 27% Si content doped sample exhibited a bimodal pore size distribution, with another small broad band centered at approximately 30 nm. This may be due to incomplete dispersion of silicon in the high target density, high viscosity mixture. This was also true for the higher silicon content sample (64%) in this series, which also exhibited a bimodal pore size distribution.

[0133] The compressed pyrolyzed composites had higher densities and smaller surface areas and pore volumes compared to their uncompressed monolith counterparts (Table 5). Densities were calculated as the average of six samples.

[0134] JPEG0007770921000006.jpg50145

[0135] Half-cell units (2032 coin cells) were fabricated using the CPI composite as an electrode, lithium foil as a counter electrode, and CELGARD 2500 as a microporous separator between the electrodes. The electrolyte was 1.0 M LiPF6 in a 3:7 weight ratio of EC:EMC. Unless otherwise noted, all cells were tested using an ARBIN BT2043 tester at a charge / discharge rate of 0.1 C. The discharge capacity of the compressed CPI composite at the fifth cycle is shown in Table 6.

[0136] JPEG0007770921000007.jpg48159

[0137] As shown in Figure 10, the optimum performance of these samples was obtained at Si contents of 30–50% per CPI.

[0138] SEM images of the high Si content doped composites are shown in Figures 11A-11C, which also show the fibrillar morphology of the composites. In samples with higher doping concentrations, the silicon (light areas) becomes more compact. In all samples, Si nanowires can be observed at high magnification. As can be seen, there is direct contact and interpenetration between the silicon and the carbon porous structure (dark areas).

[0139] The cycling capacity based on Si and electrode content is shown in Figures 12A-12F, and the raw data is shown in Table 7. Increasing the Si loading of the CPI composite resulted in faster capacity loss with cycling. However, the capacity loss may be due to the foil electrode used in these half-cell tests, as the electrode was not able to operate at the high capacity of the silicon-containing electrode.

[0140] JPEG0007770921000008.jpg86159

[0141] As can be seen in Table 7, silicon utilization can be calculated to be about 20% to about 90%, or even more optimally about 50% to about 90%, depending on the amount of silicon incorporated into the electrode. Narrower ranges are also contemplated herein based on the desired benefit (e.g., desired amount of silicon) in the final composite. Overall, this broader range is significantly higher than that found in the prior art. Furthermore, it can be seen that capacity at cycle 10 can beneficially be about 800 mAh / g or greater, but can vary based on silicon concentration.

[0142] Example 3: Silicon-doped CPI composite with low PI solids content A. 66% silicon-doped CPI dispersed in solvent by ultrasonic treatment PI gel was prepared with a target density of 0.05 g / cc. The PMDA and PDA precursors were mixed at room temperature for 3 hours. AA was then added and mixed with the solution for 2 hours. Separately, silicon powder with a particle size of approximately 30 nm was sonicated in DMAC solvent for 2 minutes and added to the mixture 30 minutes before the addition of the pyridine catalyst. Approximately 60.5% silicon was added per total solids. The Py / PMDA molar ratio was 7.5. The gelation time at ambient temperature was approximately 5.5 minutes. A 500-micrometer-thick spacer was used to cast the composite. After processing and extraction, the PI aerogel composite was compressed and carbonized by pyrolysis at 1050°C for 2 hours to form the CPI composite. The Si content per CPI was 66%.

[0143] The CPI composites compressed to different thicknesses were examined by SEM, and Figures 16A-16B show the fibrillar morphology of the CPI composites. No silicon nanowires were observed in these images. The properties of the low-compression (LC) and high-compression (MC) composites tested in half-cells are shown in Table 8.

[0144] JPEG0007770921000009.jpg36150

[0145] Figures 14A-14B show that thickness plays a role in anode performance. By reducing the thickness by approximately half, the electrode capacity essentially doubled, with the 170 micrometer-thick electrode exhibiting a discharge capacity of over 1500 mAh / g after the fourth cycle. Although these high-capacity CPI composites with a large amount of silicon (66%) loading were not stable over multiple cycles, the capacity fade may be due to the foil electrode used in these half-cell tests, as the electrode was unable to operate at the high capacity of the silicon-containing electrode.

[0146] B. 45% silicon-doped CPI mixed and dispersed in solvent with and without dispersant. PI gels were prepared with a target density of 0.05 g / cc. The PMDA and PDA precursors were mixed at room temperature for 17 hours. AA was then added and mixed with the solution for 3 hours. Separately, 30 nm particle size silicon powder was mixed in DMAC solvent with or without a dispersant for 20 hours. A control C45 sample was prepared in the absence of a dispersant. BYK384 was used as the dispersant at 20% by weight of Si in sample B45. Pluronic® F87 was used as the non-ionic surfactant at 20% by weight of Si in sample P45. The dispersant was added to the mixture 30 minutes before the addition of the pyridine catalyst. Silicon was added at 27.3% by weight of total solids. The Py / PMDA molar ratio was 7.0. The gel time at ambient temperature was approximately 11 minutes. Composites were cast using a 500 micrometer thick spacer. After processing and extraction, the PI aerogel composite was compressed and carbonized by pyrolysis at 1050 °C for 2 h to form CPI composite, with a Si content of approximately 45% per CPI.

[0147] SEM images of the compacted C45 polyimide composites before and after pyrolysis are shown in Figures 15A-15B, with Figure 15B also showing the fibrillar morphology of the CPI composite. Si nanowires were only visible in the CPI and not in the non-pyrolyzed composite. SEM images of the CPI composites with and without dispersant are shown in Figures 16A-16C. The properties of the CPI composites subjected to battery testing are listed in Table 9. The conductivity of these samples was approximately 26-27 S / cm.

[0148] JPEG0007770921000010.jpg39150

[0149] Half-cell test results showed that the P45 composite had a higher discharge capacity and lower irreversible capacity loss compared to the other two samples (Figures 17A-17C). The capacity loss may also be due to the foil electrode used in these half-cell tests, as the electrode was unable to operate at the high capacity of the silicon-containing electrode.

[0150] Composites of the P45 formulation were cast at three thicknesses: 780 microns, 580 microns, and 370 microns. Each of these PI batches was cut into approximately 0.5 x 0.5 square inch composite sections, which were compressed to three different levels using a hydraulic press and then pyrolyzed. The results are shown in Table 10 and Figure 18.

[0151] JPEG0007770921000011.jpg45102

[0152] The density of the Si-doped CPI composites varied between 0.3 and 1.0 g / cc at thicknesses of approximately 50 to 130 micrometers (Figure 18). The density increased with increasing initial thickness of the PI composites. The porosity ranged from approximately 50 to 90%.

[0153] Conductivity of Silicon-Doped CPI Composites. The conductivity of CPI composites doped with various concentrations of silicon and pyrolyzed at 1050°C was measured using a Keithley four-point probe apparatus. The conductivity of the samples varied from about 5 to 80 S / cm (Table 11). The conductivity of the undoped carbon composite was about 13.6 S / cm, which, as known in the art, is within the expected range for amorphous carbon. The reported conductivity of silicon is 1.6 x 10 -5 S / cm, whereas silicon nanowires can exhibit three orders of magnitude higher conductivity (0.03 S / cm) (Sabar D. Hutagalung, Mohammed M. Fadhali, Raed A. Areshi and Fui D. Tan, Optical and Electrical Properties of Silicon Nanowires Prepared by Electroless Etching, Nanoscale Research Letters, 2017, 12:425).

[0154] JPEG0007770921000012.jpg52159

[0155] Typically, electrical conductivity increased with increasing density. However, at silicon concentrations above 60%, conductivity was lower than expected (Figure 19). At this high silicon content, the carbon network connectivity was disrupted, resulting in decreased conductivity. The sample doped with 27 wt% Si per CPI exhibited the highest conductivity, suggesting an optimal silicon dispersion for high conductivity. A broader optimal range of Si is found to be between about 5% and about 80 wt% Si per CPI, or more specifically, between about 5% and 50 wt% Si per CPI. Tables 9 and 11 show that by varying the silicon content, the conductivity can be adjusted up to about 80 S / cm. Thus, the conductivity can exceed about 5 S / cm, 10 S / cm, 15 S / cm, 25 S / cm, 50 S / cm, and 75 S / cm, where a wider range and more precise conductivity is believed to be tunable based on the silicon content.

[0156] Example 4: Carbonized polyimide aerogels with high pore volume and narrow pore size distribution PI gel is prepared by forming polyamic acid in 100 mL of DMCA by reacting 6 g of PMDA with 3 g of PDA at room temperature for 2–24 hours. Subsequently, 8.86 g of AA is added to the polyamic acid solution as a chemical imidization reagent (see Figure 20). The acidified polyamide solution is vigorously mixed for at least 2 hours. The resulting mixture is diluted with DMAC to the desired target density of the PI aerogel. 1–4 g of Py per 100 mL of mixture is added to the final solution to promote gelation, which occurs in 4–25 minutes. Prior to gelation, the mixture is cast into the desired form (e.g., film, monolith, reinforcing fiber, etc.). The resulting gel is then aged in an oven at 65–70 °C and washed / rinsed several times with ethanol before supercritical drying. The PI aerogel is converted to carbon aerogel by pyrolysis at 1,050 °C for 2 hours under an inert environment (nitrogen gas flow). Without being bound by theory, the physical and structural properties of the carbonized PI aerogel depend on the precursor mixing time and the amount of Py.

[0157] The structural properties of four CPI aerogels tested by nitrogen adsorption / desorption are reported in Table 12. The four samples differ in mixing time and amount of Py. The target density was fixed at 0.05 g / cc. Interestingly, all samples exhibit relatively similar surface BET, but the pore size distribution and pore volume appear to be affected by the synthesis parameters.

[0158] JPEG0007770921000013.jpg51159

[0159] Example 5: Si-loaded carbonized polyimide aerogel Si particles (30 nm) were added to polyamic acid solutions at different concentrations. The synthesis of the solutions was the same as that described in Example 4. However, for this composite system (PI / Si), the silicon particles were first dispersed in DMAC for at least 2 hours before being mixed with the polyamic acid solution. To avoid Si settling, the gelation time of the Si / polyamic acid solution was kept relatively short (4-6 min). To achieve the target gelation time, a mixture of 4 g of pyridine per 100 mL of solution was used. Prior to gelation, the mixture was cast into the desired form (e.g., film, monolith, or reinforcing fiber). The resulting gel was then aged in an oven at 65-70 °C and washed / rinsed several times with ethanol before supercritical drying.

[0160] A. 22-25 wt% silicon in polyimide-carbon aerogel composite The target density of the composite was fixed at 0.06 g / cc, and the Si loading during the polymerization process was approximately 11.88 wt%. The silicon content was adjusted after pyrolysis of the polyimide aerogel at 1050 °C for 2 h, as all samples recorded a weight loss of more than 50%. Four different thicknesses of composites were fabricated. MT (medium thickness composite approx. 0.3~0.4mm) T (thick composite about 0.6~0.8mm) MTC (medium thickness compressed composite approximately 0.07-0.09mm) TC (thick compressed composite about 0.12~0.16mm)

[0161] The different PI / Si samples were pyrolyzed at 1050°C for 2 hours and then battery tested. The physical properties of each sample are shown in Table 13. JPEG0007770921000014.jpg117159

[0162] An SEM image of the pyrolyzed composite (MT material) doped with about 25% Si can be seen in Figure 23. The SEM image shows at high magnification the aggregation of Si and silicon nanowires embedded in the carbon matrix.

[0163] The cycling capacity of the MTC5 and MT5 samples based on Si and electrode content is shown in Figures 25A-25B. MTC5 (compressed CPI aerogel) was cycled up to 400 cycles, while MT5 (uncompressed CPI aerogel) was cycled up to 150 cycles. The two samples exhibited different behavior. MTC5 exhibited a relatively stable capacity discharge up to 200 cycles, followed by a capacity decline, which, as previously discussed, is likely due to the foil electrode in the half-cell test.

[0164] B. 39 wt% silicon in polyimide carbon aerogel composite In the next synthetic route, the Si content was increased to 39 wt% in anticipation of reduced capacity loss during first discharge. Polyimide gels were prepared with a target density of 0.05 g / cc. Polyamic acid solutions were prepared by mixing PMDA and PDA precursors in DMAC for 16 hours. Separately, silicon powder was dispersed in DMAC for 2 hours and then added to the polyamic acid solution before adding pyridine. After pyridine was added, composites were prepared between Teflon plates using 500 micron spacers. The aerogel composites were compressed and then pyrolyzed at 1050 °C for 2 hours. The silicon content was 39 wt% based on total solids.

[0165] The properties of the compressed (PISi1NC(C)) and uncompressed (PISi7NC) composites tested in half-cells are shown in Table 14.

[0166] JPEG0007770921000015.jpg48159

[0167] Figures 26A-26B show the discharge capacity as a function of cycling for anodes fabricated using the two materials reported in Table 14. A clear improvement in the discharge capacity of the anode is evident when the silicon content is increased from 25 wt% (previous materials, i.e., MT5 and MYC5) to 39 wt%.

[0168] Example 6: Fiber-reinforced C / Si aerogel Different areal densities (2, 4, 10 g / m 2 Three different carbon fiber reinforcements were tested as reinforcements for C / Si aerogels. The synthesis and processing of PI / Si aerogels are the same as those previously described, with one exception: During the gelation process, the mixture is cast into fibers. After supercritical drying, the carbon fiber-reinforced PI / Si composites are cut into 15 mm (inner diameter (ID)) circular specimens using a dye cutter and pyrolyzed at 1050 °C for 2 h. Figures 27-29 show the properties and micrographs of the carbonized PI / Si / carbon fiber specimens.

[0169] After pyrolysis, 2 and 4 g / m 2 The carbon fiber reinforced samples at 10 g / m2 exhibited high shrinkage and reinforcement. Figures 28-29 show the high porosity that characterizes these two materials as being particularly unsuitable for battery testing. 2 The C / Si reinforced with carbon fiber showed a better microstructure and no evidence of voids. However, the Si content was low (about 21%) and the fiber density was high (>30 wt%). The charge and discharge capacities of this composite were tested in the system. Based on the Si and electrode content, the cycle capacities of the samples shown in Figure 27 are reported in Figure 30.

[0170] In a separate experiment, cellulose fibers were also tested as a reinforcement for C / Si. The synthetic route was similar to that used for carbon fibers. Cellulose fibers constituted 68% (by weight) of the C / Si composite, but after pyrolysis, much of this fiber was decomposed, as the % Si increased from 6% to 24% after pyrolysis. The physical properties of the cellulose fiber reinforced C / Si are reported in Table 15.

[0171] JPEG0007770921000016.jpg31159

[0172] The cycling capacity of cellulose fiber reinforced C / Si based on Si and electrode content is reported in Figure 31. The performance of this material is similar to that seen for carbon fiber reinforced C / Si.

[0173] Example 7: Improving silicon dispersion within PI aerogel PI composites and monolith gels with a target density of 0.05 g / cc were prepared with 47% silicon loading. To avoid agglomeration, a route was taken to better disperse the silicon within the polyimide matrix. The silicon was dispersed and mixed with PMDA and PDA from the beginning of the solution synthesis. The polyamic acid solution + silicon was mixed for 16 hours. Acetic acid was added and mixed with the solution for 4 hours. Rapid gelation (approximately 4 minutes) was ensured by adding an appropriate amount of Py to the acidified polyimide / Si mixture. Composites and thick monoliths approximately 200-300 microns thick were cast. After supercritical drying, the PI / Si aerogel composites were compressed and then pyrolyzed at 1050°C for 2 hours.

[0174] Subsequently, several different samples (e.g., compressed, uncompressed, monolith, etc.) were analyzed by SEM to evaluate the dispersion of silicon in the carbon matrix. Figure 32 shows three cross-sectional SEM images of a thick uncompressed composite (0.60 mm). At low magnification (left image), uniform and well-distributed silicon is evident. Large clusters of silicon are also visible. At higher magnification (right image), dense silicon is seen well embedded in the carbon matrix. Figure 33 shows three cross-sectional SEM images of a thin composite (0.12 mm). The same observations are obtained: good Si dispersion and proper impregnation into the carbon matrix. The cross-sectional SEM image of the monolith (Figure 34) clearly confirms the same structure as that shown in the thick and thin composites.

[0175] Figures 35-36 show a side-by-side comparison of two differently processed C / Si composites. The images on the left (in both figures) are for monoliths and composites cast from Si / polyamic acid solutions mixed for 16 hours (long contact). The photographs on the right are for monoliths and composites made from Si / polyamic acid solutions mixed for 4-6 minutes (short contact). The silicon distribution is observed to be different for the two processes. Indeed, long contact resulted in better Si dispersion than short contact.

[0176] Half-cell testing of compressed and uncompressed C / Si composites was performed. In both the PISi3 (uncompressed) and PISi6C (compressed) samples, the Si was well dispersed in the carbon aerogel matrix. The physical properties of both samples are reported in Table 16.

[0177] JPEG0007770921000017.jpg27159

[0178] The charge-discharge cycles of the two samples are shown in Figures 37-38. The samples performed surprisingly well, with the compressed sample (PISi6C) showing stable cycling performance.

[0179] Example 8: Fabrication of C / Si circular electrodes In the previous example, the shape of the C / Si electrode was square (approximately 1 cm 2 ) and tested the battery performance of circular cells (ID=15mm). To obtain higher efficiency and more reliable test results, testing of circular C / Si electrodes is necessary. Therefore, circular electrodes were made using a die cutter.

[0180] A 31.4% Si-loaded PI aerogel composite was produced with a target density of 0.13 g / cc (using 16 hours of mixing). After extraction, the density of the final aerogel was measured to be approximately 0.213 g / cc. A die cutter was used to produce several circular aerogel composites with an inner diameter of 15 mm (see Figure 39). The thickness of the samples was approximately 0.43 mm. Some of the samples were compressed and then pyrolyzed at 1050°C for 2 hours. The properties of the pyrolyzed circular electrodes were as follows: Weight loss: about 41% Uncompressed sample: approx. 0.261 g / cc (inner diameter 1.1 cm, thickness 0.38 mm) Compressed sample: approx. 0.652 g / cc (inner diameter 1.35 cm, thickness 0.11 mm)

[0181] The performance of the compressed and uncompressed half-cells is shown in Figures 40-41. The capacity of the uncompressed sample appears to be higher than the compressed sample, but the capacity of the uncompressed sample was also more erratic.

[0182] Example 9 Si-filled carbonized PF aerogel Regarding the fabrication of C / Si aerogel electrodes, carbon aerogels prepared from PF are also contemplated herein. For PF systems, aerogels with high target densities (>0.7 g / cc) can be achieved through synthetic routes. The resulting aerogels do not need to be compressed. A high-density PF gel composite was prepared as follows: 22 g of phloroglucinol was dissolved in 100 mL of ethanol, and 44 mL of 2-furaldehyde was added to the phloroglucinol solution. The resulting solution was mixed for 30 minutes. 5 wt% silicon was added to the mixture, and the mixture was vigorously mixed for an additional 30 minutes. The PF mixture can be gelled with a base (diamine or triamine), with a gelation time of approximately 20–40 minutes depending on the base concentration. In this case, in the presence of silicon, the gelation time should be shorter (less than 1 minute) to avoid precipitation of the silicon during gelation. Chloric acid (HCl) was used to catalyze the gelation of PF with a gelation time as short as 20 seconds. Therefore, 0.012 g of concentrated HCl per 100 mL of mixture was added to the mixture, mixed for 20 seconds, and cast between Teflon plates to produce a gel. After aging and solvent exchange, the PF gel composite was dried with supercritical CO2. The properties of the resulting PF / Si aerogel are as follows: Two composite thicknesses: approximately 0.2 mm and approximately 0.1 mm Final density of aerogel: approx. 0.7 g / cc

[0183] After pyrolysis at 1050°C for 2 hours under inert gas, the density of the resulting C / Si aerogel was adjusted to 0.77 g / cc and the Si content to 10 wt%. 2 ) (see Figure 42) was prepared.

[0184] The surface area of ​​the C / Si aerogel tested by liquid nitrogen adsorption / desorption was 541 m 2 / g, with a pore volume of 0.32 cc / g. Interestingly, the micropore area accounts for 80% of the total surface area, confirming the compact, dense structure shown in the SEM images (see Figure 43). Furthermore, the Si-containing carbon aerogel composite sample, in which the carbon was derived from PF, exhibited high capacity loss after the first cycle and very low overall capacity (see Figure 44). These results highlight the unique property of Si, which performs better in a fibrillar morphology, as seen in polyimide-derived nanoporous carbon / carbon aerogels.

[0185] Example 10: Nanoindentation as a measure of mechanical strength Using the method described above, several CPI composite samples were prepared, with silicon content and density being the variables within the samples. The Young's modulus of each sample was measured using nanoindentation, which tests the hardness of materials. More specifically, 20 indentations were made across the sample surface. Approximately 8-10 indentations were selected to obtain average data and standard deviations for each mechanical property. The indentation locations were selected under a microscope, and the surfaces were relatively clean and smooth, with few surface features, thus providing more reliable data. Samples 1 and 2 were softer than the other samples, so a maximum load of 50 mN was selected. An indentation load of 300 mN was used for samples 3-7. The results and other properties are shown in Table 18. Figure 45 shows the elastic modulus as a function of density, and Figure 46 shows the elastic modulus as a function of density.

[0186] JPEG0007770921000018.jpg61159

[0187] Example 11: 45% Silicon Doped CPI Beads PI gel beads were prepared with a target density of 0.10 g / cc. PMDA precursor and PDA precursor were mixed in DMAC solvent at room temperature for 3 hours. AA was then added and mixed with the solution for 2 hours. Separately, 30 nm particle size silicon powder was sonicated in DMCA solvent for 1 minute and added to the mixture 5 minutes before adding the pyridine catalyst. Silicon was added at approximately 24% of the total solids. The doped mixture was catalyzed using a 3.2 molar ratio of Py to PMDA. Prior to gelation, the catalyst sol containing the silicon particles was poured into a container containing previously stirred silicone oil as a dispersing medium (silicone oil:catalyst sol volume ratio of 10:1). The gelled PI beads were isolated from the silicone oil by filtration, subsequently rinsed with ethanol, and then dried by supercritical CO2 extraction. The PI aerogel-silicon composite beads were then pyrolyzed at 1050°C for 2 h to form CPI-silicon composite beads with a tap density of 0.7 g / cc and D50 = 15 μm.

[0188] Example 12: Electrodes prepared from CPI beads Using the CPI silicon composite beads prepared according to Example 11, an anode electrode was prepared on a Cu foil as a current collector by mixing a slurry containing 80 wt. % CPI silicon composite beads, 10 wt. % polyacrylic acid (PAA) binder, and 10 wt. % hard carbon conductive additive (C65) in water at a total solids content of approximately 36 wt. Using a doctor blade, the slurry was cast onto the Cu foil. After drying and calendering, a load of 3.1 mg / cm was obtained. 2 An electrode with a density of 0.7 g / cc was obtained.

[0189] Example 13: Half-cell units made from CPI electrodes Half-cell units (2032 coin cells) were fabricated with the CPI composite electrode prepared according to Example 12, using lithium foil as the counter electrode and CELGARD 2500 as the microporous separator between the electrodes. The electrolyte was 1.0 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (3:7 weight ratio) and 5 wt% fluoroethylene carbonate (FEC). All cells were tested in an ARBIN BT2043 tester. The cell testing protocol included four formation cycles at a charge / discharge rate of C / 20. Subsequent cycles were performed at various rates between C / 20 and 5C. Figure 48 shows the half-cell cycling characteristics of a CPI composite electrode containing 30 wt% silicon particles, prepared as an electrode from a slurry containing 80 wt% CPI silicon composite beads, 10 wt% polyacrylic acid (PAA) binder, and 10 wt% hard carbon conductive additive (C65). The graph in Figure 48 shows the discharge capacity (mAh / g based on total electrode weight) and coulombic efficiency versus cycle number for 1.0 M LiPF in ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (weight ratio 3:7) and 5 wt% fluoroethylene carbonate (FEC).

[0190] Example 14: Alternative Method for PI Aerogel Production The previous examples discussed herein teach specific methods for forming PI aerogel. In certain embodiments, the present invention also contemplates alternative methods for forming PI aerogel. A non-exhaustive and non-limiting set of examples of such alternative methods will now be discussed.

[0191] For example, U.S. Patent No. 6,399,669 to Suzuki et al. teaches four related methods for making PI dry gels (aerogels). The first method involves synthesizing a PI precursor followed by imide formation from the PI precursor to produce a polyimide. The second method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The second method involves gelling the solution / swollen bulk to produce a PI precursor wet gel. The second method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The third method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The second method involves gelling the PI precursor wet gel. The third method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The third method involves gelling the PI precursor followed by imide formation from the PI precursor to produce a PI wet gel. The third method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The third method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The third method involves gelling the PI precursor followed by PI wet gel. The third method involves synthesizing a PI precursor followed by PI precursor solution or swollen bulk. The solution / swollen bulk is gelled to produce a PI precursor wet gel, which is then dried to produce a PI precursor dry gel, which is then converted into an imide to form a PI dry gel (aerogel).

[0192] As a further example, Leventis et al. [Polyimide Aerogels by Ring-Opening Metathesis Polymerization (ROMP), Chem. Mater. 2011, 23, 8, 2250-2261] discuss the formation of PI aerogels using the ROMP method. A low molecular weight imidized oligomer end-capped with polymerizable groups is provided and mixed with a polymerization (e.g., ROMP) catalyst. Polymerization is then initiated to produce a crosslinked polyimide. The polyimide is gelled and dried to form a PI aerogel. Also, Leventis et al. (U.S. Pat. No. 9,745,198; Chidambareswarapattar et al., One-Step Room-Temperature Synthesis of Fibrous Polyimide Aerogels from Anhydrides and Isocyanates and Their Conversion to Isomorphous Carbon, J. Mater. Chem., 2010, 20, 9666-9678) teach the formation of PI aerogels by mixing a dianhydride (e.g., PMDA) with an isocyanate (e.g., 4,4'-diisocyanatodiphenylmethane or methylenediparaphenyldiisocyanate) to form a sol-gel material, which is then dried to produce the PI aerogel. Also, Leventis et al. [Isocyanate-derived organic aerogels: polyureas, polyimides, polyamides, MRS Methods, 1306 (2011), Mrsf10-1306-bb03-01.doi:10.1557 / opl.2011.90] note that DESMODUR N 3300A, DESMODUR RE, and MONDUR CD (all available from BAYER) can be used as isocyanates.

[0193] In another approach, Guo et al. [Polyimide Aerogels Crosslinked with Amine-Functionalized Polyoligomeric Silsesquioxanes, ACS Appl. Mater. Interfaces 2011, 3, 546-552] investigated the formation of PI aerogels by reacting aminosilsesquioxanes with polyamic acid oligomers end-capped with anhydride groups. The product was imidized with pyridine (although thermal imidization is also possible), gelled, and subsequently dried to yield PI aerogels. Nguyen et al. [Development of High-Temperature Flexible Polyimide Aerogels, American Chemical Society, Proceedings, 2011] described the production of branched polyimides by mixing diamines and dianhydrides, imidizing the mixture, and then reacting with multi-amino compounds (e.g., 1,3,5-tris(4-aminophenoxybenzene)). This product was then reacted with 4,4'-methylene diisocyanate and dried to form PI-urea aerogels.

[0194] In another embodiment, Meador et al. [Mechanically Strong, Flexible Polyimide Aerogels Crosslinked with Aromatic Triamines, ACS Appl. Mater. Interfaces, 2012, 4(2), pp. 536-544] study the crosslinking of anhydride-end-capped polyamic acid oligomers with aromatic triamines in solution followed by imidization to form PI gels. The resulting wet mass is dried to form PI aerogels. Furthermore, Meador et al. [Polyimide Aerogels with Amide Crosslinks: A Low-Cost Alternative to Mechanically Strong Polymer Aerogels, ACS Appl. Mater. Interfaces, 2015, 7, pp. 1240-1249] study the formation of PI gels by crosslinking amine-capped oligomers with 1,3,5-benzenetricarbonyl trichloride. The resulting gel is dried to form PI aerogels.

[0195] In yet another embodiment, Pei et al. [Preparation and Characterization of Highly Crosslinked Polyimide Aerogels Containing Trimethoxysilane Side Groups] (Langmuir 2014, 30, 13375-13383) fabricated PI aerogels from polyimides containing trimethoxysilane side groups, which are the condensation products of polyimides containing acid chloride side groups and 3-aminopropyltrimethoxysilane. The resulting gels were dried to form PI aerogels.

[0196] In any of these methods, a suspension of graphene can be added (see Zhang et al., Graphene / Carbon Aerogels Derived from Graphene-Crosslinked Polyimide as Electrode Material for Supercapacitors, RSC Adv., 2015, 5, 1301).

[0197] Each of these methods can result in polyimide aerogels, and the present invention contemplates any suitable method for producing such polyimide aerogels. Regardless of the method used to produce PI aerogels, according to certain embodiments of the present invention, the resulting PI aerogels can be pyrolyzed to form PI-derived carbon aerogels. Additives such as silicon can also be introduced, according to certain embodiments discussed herein.

[0198] All referenced publications are incorporated herein by reference in their entirety. Furthermore, to the extent that the definition or use of a term in a reference incorporated herein by reference contradicts or is contrary to the definition of that term provided herein, the definition of that term provided herein shall apply and the definition of that term in the reference shall be disregarded.

[0199] The above advantages, and those that will become apparent from the above description, are efficiently achieved. Since certain changes can be made to the above construction without departing from the scope of the invention, all matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense.

[0200] It is also to be understood that the following claims are intended to cover all of the general and specific features of the invention described herein, and all statements of the scope of the invention that may be said to lie therebetween as a matter of language. Some of the embodiments of the invention related to the present invention are shown below. [Embodiment 1] a carbon material having a porous structure; a silicon-based material at least partially within the porous structure of the carbon material; and A carbon composition comprising: A carbon composition comprising greater than about 10% by weight of silicon-based material, having a porosity between about 10% and about 80%, a pore size at the largest peak from the distribution of less than or equal to about 100 nm, and a silicon utilization of at least about 20%. [Embodiment 2] 2. The carbon composition of embodiment 1, wherein the carbon material has a porous structure comprising a fibrillar morphology, a Young's modulus of at least about 0.2 GPa, and a density of between about 0.15 g / cc and about 1.5 g / cc. [Embodiment 3] 2. The carbon composition of embodiment 1, wherein the carbon material has a porous structure comprising a fibrillar morphology, a conductivity of at least about 10 S / cm, and a density of between about 0.15 g / cc and about 1.5 g / cc. [Embodiment 4] 4. The carbon composition of any one of embodiments 1 to 3, wherein the carbon material comprises carbon aerogel. [Embodiment 5] 5. The carbon composition of embodiment 4, wherein the carbon material comprises a polyimide-derived carbon aerogel. [Embodiment 6] 4. The carbon composition of any one of the preceding claims, wherein the carbon material comprises at least about 4% by weight of residual nitrogen. [Embodiment 7] 4. The carbon composition of any one of embodiments 1 to 3, wherein the carbon composition is in monolithic form. [Embodiment 8] 8. The carbon composition of embodiment 7, wherein the monolithic carbon aerogel is binder-free. [Embodiment 9] 9. The carbon composition of embodiment 8, wherein the monolithic carbon aerogel has a thickness between about 10 micrometers and about 500 micrometers. [Embodiment 10] 4. The carbon composition of any one of embodiments 1 to 3, wherein the carbon composition is in particulate form. [Embodiment 11] 11. The carbon composition of embodiment 10, wherein the particulate carbon composition has a diameter of from about 1 micrometer to about 50 micrometers. [Embodiment 12] 4. The carbon composition of any one of embodiments 1 to 3, wherein the carbon material comprises about 25% to 65% silicon by weight of the carbon material. [Embodiment 13] 4. The carbon composition of any one of embodiments 1 to 3, having a capacity of at least about 800 mAh / g. [Embodiment 14] An electrode comprising the carbon composition according to any one of embodiments 1 to 3. [Embodiment 15] An energy storage device comprising the carbon composition according to any one of embodiments 1 to 3. [Embodiment 16] 16. The energy storage device of embodiment 15, wherein the energy storage device is a lithium ion battery. [Embodiment 17] providing a mixture of a polyimide precursor and a silicon-based material; chemically or thermally imidizing the mixture; drying the imidized mixture to obtain a porous polyimide silicon composite; and carbonizing the porous polyimide silicon composite to obtain a carbon composition, the method comprising: 1. A method of forming a carbon composition, wherein the carbon composition comprises greater than about 10% by weight silicon, has a porosity between about 10% and about 80%, and has a pore size at the largest peak from the distribution of about 100 nm or less, wherein the silicon-based material is at least partially within the porous structure of the carbon material. [Embodiment 18] 18. The method of embodiment 17, wherein the carbon composition comprises a carbon aerogel. [Embodiment 19] 18. The method of embodiment 17, wherein the carbon composition is formed as a monolith. [Embodiment 20] 18. The method of embodiment 17, further comprising mixing the mixture with a medium that is immiscible with the mixture to form droplets of the imidization mixture. [Embodiment 21] 21. The method of embodiment 20, further comprising drying the droplets to form particles. [Embodiment 22] 22. The method of embodiment 21, wherein the particles have a diameter of from about 1 micrometer to about 50 micrometers. [Embodiment 23] 18. The method of embodiment 17, wherein the maximum pyrolysis temperature is between about 750°C and about 1600°C. [Embodiment 24] 24. The method of any one of embodiments 17-23, wherein the capacity of the carbon composition is at least about 800 mAh / g. [Embodiment 25] 24. The method of any one of embodiments 17-23, wherein the carbon composition has a silicon utilization of at least about 20%. [Embodiment 26] 24. The method of any one of embodiments 17-23, wherein the carbon composition comprises a carbon aerogel.

Claims

1. a carbon material comprising a fibrillar morphology comprising a plurality of interconnected carbon materials containing a porous structure, the interconnected carbon struts defining a plurality of pores, the carbon material comprising a polyimide-derived carbon aerogel; a silicon-based material; and a carbon composition comprising: the carbon composition comprises greater than 10% by weight of a silicon-based material; The carbon material is a carbon composition characterized in that it has a Young's modulus of at least 0.2 GPa and a density of 0.15 g / cc to 1.5 g / cc.

2. 10. The carbon composition of claim 1, wherein the carbon composition has a conductivity of at least 10 S / cm.

3. The carbon composition of claim 1 or 2, wherein the carbon composition is in monolithic form.

4. The carbon composition of claim 3 , wherein the monolithic carbon composition is binder-free.

5. The carbon composition of claim 4, wherein the monolithic carbon composition has a thickness of 10 micrometers to 500 micrometers.

6. The carbon composition of claim 1 or 2, wherein the carbon composition is in particulate form.

7. The carbon composition of claim 6, wherein the particulate carbon composition has a diameter of from 1 micrometer to 50 micrometers.

8. 3. The carbon composition of claim 1 or 2, wherein the silicon-based material is at least partially present within one of a plurality of pores of the carbon material.

9. The carbon composition according to claim 1 or 2, wherein the carbon material comprises 25% to 65% silicon by mass of the carbon material.

10. 3. The carbon composition of claim 1 or 2, having a capacity of at least 800 mAh / g.

11. An electrode comprising the carbon composition of claim 1 or 2.

12. An energy storage device comprising the carbon composition of claim 1 or 2.

13. 13. The energy storage device of claim 12, wherein the energy storage device is a lithium ion battery.

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