Silicon-based anode material for lithium ion batteries

Abstract

To provide a method for preparing an improved silicon-based anode material and such a silicon-based anode material.SOLUTION: A method of making a porous reduced silica material comprising forming a precursor fiber comprising silica, and magnesiothermically reducing the precursor fiber to form the porous reduced silica material, wherein the porous reduced silica material has a diameter of 0.1 to 20 microns, a surface area of 5m2 / g to 400m2 / g, and a porosity of 0.001 cm 3 / g to 1.5cm3 / g, and comprises greater than 20 weight percent silicon.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Application No. 62 / 558,107, filed September 13, 2017, entitled "Battery Electrode Materials," which is incorporated herein by reference in its entirety. [Background technology]

[0002] This disclosure relates generally to high-capacity silicon-based anode active materials for lithium-ion batteries. In particular, this disclosure describes silicon-based fibrous materials that exhibit improved cycle life over standard commercial-grade silicon and improved capacity over graphite anode materials.

[0003] Lithium-ion batteries have exploded in popularity over the past decade and are now the power source of choice for providing portable power for electronic devices, cordless appliances, and vehicles. As technology becomes increasingly reliant on lithium-ion battery power, the lithium-ion battery industry has worked to extend the performance of the cells to offer maximum versatility to end users.

[0004] Graphite is commonly used in lithium-ion batteries due to its ability to maintain stability and perform its function for hundreds of cycles with little to no capacity loss. Silicon is highly promising as an anode material due to its extremely high capacity (4000 mAh / g) compared to the current industry standard, graphite (372 mAh / g). However, silicon has the limitation of swelling by 350% upon lithiation. This swelling can cause severe disruption to the internal cell structure, damaging cell components and causing the anode to shatter itself and ultimately lose electrical connection, resulting in a rapid loss of capacity.

[0005] Thus, there is a continuing need for improved silicon-based anode materials and methods for preparing such silicon-based anode materials. Summary of the Invention [Means for solving the problem]

[0006] The following figures are included to illustrate certain aspects of the invention and should not be considered as exclusive embodiments. The disclosed subject matter is capable of possible modification, alteration, and equivalents in form and function, as will occur to those skilled in the art having the benefit of this disclosure. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 shows a schematic diagram of a lithium-ion battery including an anode and a cathode. [Figure 2] FIG. 2 is an X-ray diffraction (XRD) scan of the reduced mixture of Example 1. [Figure 3] FIG. 3 is an X-ray diffraction (XRD) scan of the reduced mixture of Example 1 after washing with acid. [Figure 4] FIG. 4 shows the effect of the Mg / SiO ratio on the percentage of SiO converted to Si, according to an embodiment of the present disclosure. [Figure 5] FIG. 5 shows the effect of Mg / SiO 2 ratio on the surface area of ​​silicon-based fibers, according to an embodiment of the present disclosure. [Figure 6] FIG. 6 shows the effect of Mg / SiO2 ratio on the median pore diameter of silicon-based fibers, according to an embodiment of the present disclosure. [Figure 7] FIG. 7 shows the effect of Mg / SiO2 ratio on the porosity of silicon-based fibers, according to an embodiment of the present disclosure. [Figure 8] FIG. 8 is a graph of the first cycle results for commercial silicon. [Figure 9] FIG. 9 is a graph of first cycle results for commercial graphite. [Figure 10] FIG. 10 is a graph of first cycle results for Sample 1 of Examples 3 and 4 according to an embodiment of the present disclosure. [Figure 11] FIG. 11 is a graph of first cycle results for Sample 5 of Examples 3 and 4 according to an embodiment of the present disclosure. [Figure 12] FIG. 12 is a graph of first cycle results for Sample 2 of Examples 3 and 4 according to an embodiment of the present disclosure. [Figure 13] FIG. 13 is a graph of first cycle results for Sample 10 of Examples 3 and 4, according to an embodiment of the present disclosure. [Figure 14] FIG. 14 is a graph of first cycle results for Sample 11 of Examples 3 and 4 according to an embodiment of the present disclosure. [Figure 15] FIG. 15 is a graph of capacity versus cycles for commercial silicon. [Figure 16] FIG. 16 is a graph of capacity versus cycles for commercial graphite. [Figure 17] FIG. 17 is a graph of capacity versus cycles for Sample 1 of Examples 3 and 4, according to an embodiment of the present disclosure. [Figure 18] FIG. 18 is a graph of capacity versus cycles for Sample 5 of Examples 3 and 4, according to an embodiment of the present disclosure. [Figure 19] FIG. 19 is a graph of capacity versus cycles for Sample 2 of Examples 3 and 4, according to an embodiment of the present disclosure. [Figure 20] 20 is a graph of capacity versus cycles for Sample 10 of Examples 3 and 4, according to an embodiment of the present disclosure; and [Figure 21] FIG. 21 is a graph of capacity versus cycles for Sample 11 of Examples 3 and 4 according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0008] The present disclosure provides nanoparticles having diameters of about 0.1 microns to about 20 microns, high surface area (e.g., about 5 to about 400 m 2The present invention describes porous reduced silica fibers having a surface area of ​​about 30 to about 300 m / g, and interconnected or discrete porosity. As used herein, "surface area" refers to the surface area determined by the Brunauer-Emmett-Teller (BET) technique. In some embodiments, the porous reduced silica fibers have a surface area of ​​about 30 to about 300 m / g. 2 / g. Where ranges of numbers are mentioned herein, all numbers within the range are specifically included herein.

[0009] In various embodiments, the porous reduced silica fibers are substantially free of silica, where "substantially free" means less than 20 weight percent, e.g., less than 10 weight percent, less than 5 weight percent, or less than 1 weight percent.

[0010] In some embodiments, the porous reduced silica fibers are from about 0.1 to about 0.7 cm 3 In other embodiments, the porous reduced silica fibers have a porosity of about 0.01 to about 1.5 cm / g. 3 / g. By providing internal porosity within the porous reduced silica fibers, an internal volume is created that can accommodate at least a portion of the silicon expansion without destroying the exterior of the fiber or the electrode structure. The porous reduced silica fibers are capable of accepting and releasing lithium ions from and into the electrolyte.

[0011] This disclosure also describes methods for producing porous reduced silica fibers for use as anodes in lithium-ion batteries. These methods rely on forming a SiO2-based fiber (i.e., a silica-based precursor) and subjecting it to various processes, primarily to refine silica chemistry or to create internal porosity in the silica-based precursor fiber. The silica-based precursor fiber is then subjected to a thermochemical reduction process to produce reduced silica fibers containing silicon with optional inclusions of SiO2 and / or SiO2. As described further below, silica-based precursor fibers can be produced by chemical methods such as glass melting or sol-gel processes. In various embodiments, porosity in the silica-based precursor fiber is intentionally introduced by chemical leaching, by heat treatment, by forming silica-based precursor fibers by a sol-gel process from the assembly of colloidal particulates, or by forming silica-based precursor fibers from solution and introducing a finely dispersed polymer into the fiberization solution, which upon burnout leaves porous cavities in the fiber (i.e., polymer template), or any combination thereof.

[0012] A chemical reduction process and subsequent purification methods are then performed to remove some or all of the chemically bound oxygen from the silica-based precursor fibers, leaving behind a material containing chemicals containing silicon (Si) and / or silicon monoxide (SiO). The material also contains residual silica or silicon dioxide (SiO). This material is referred to as reduced silica fiber.

[0013] In some embodiments, electrodes are formed from reduced silica fibers using standard battery manufacturing techniques. Advantageously, the electrodes have higher capacity than existing graphite electrodes and improved cycle life over comparable commercial silicon electrodes when used in lithium-ion half cells or full batteries. In other embodiments, fiber paper formed from reduced silica fibers is utilized as the anode, thus eliminating the need for a current collector.

[0014] In certain embodiments, melt-derived silica-based precursor fibers are produced with diameters of about 0.1 microns to about 20 microns. The fibers can then be chemically treated (i.e., leached) to remove non-silica components, leaving behind fibers of primarily silica composition. The fibers can then be subjected to a thermal reduction process to convert the silica to metallurgical silicon via oxygen removal.

[0015] In another embodiment, silica-based precursor fibers with controlled porosity and diameters of about 0.1 microns to about 20 microns are produced by a sol-gel process and then subjected to a reduction step to produce reduced silica fibers. The reduced silica fibers may have sufficient size and / or porosity to accommodate high expansion upon lithiation, thereby avoiding damage to the anode and enabling long cell life by taking advantage of the high capacity of silicon.

[0016] As used herein, "capacity" refers to a measure of the overall charge that can be delivered by a battery and determines the run time that the battery can provide under given load conditions. In effect, capacity determines the usage time of the battery. Capacity is measured as the product of the current delivered multiplied by the time of delivery. It is usually quoted as ampere-hours or milliampere-hours (mAh).

[0017] Specific capacity is defined by the overall capacity per unit weight or volume of the cell. Specific capacity can be quoted for the active material only (anode or cathode material), for an individual cell (including the active material, current collectors, electrolyte, and "can" or housing), or for the entire battery pack. Units are usually amp-hours per gram (A'h / g) or amp-hours per cubic centimeter (A'h / cm 3 )

[0018] As used herein, "cycle life" means the number of complete charge / discharge cycles a battery can support before its capacity drops below 80% of its original capacity. As used herein, "capacity retention" means the percentage of total capacity available from a battery under specified discharge conditions after being cycled or stored at a constant rate.

[0019] With use and over time, batteries experience physical changes in their internal components. This is driven by the consumption / conversion of electrolyte, the formation of reaction layers at the anode and cathode (called a solid electrolyte interface, or SEI), or charge / discharge cycling (usually resulting in the loss of electrical connection between the particles that make up the anode or cathode). The overall result is a decrease in overall cell capacity. A discharge curve taken over the operating life of a cell shows a gradual decrease in capacity. The definition of a cell's useful life is somewhat arbitrary, but it is typically expected to result in a 20% decrease in capacity, with 80% of the original capacity being usable.

[0020] In addition to higher capacity and improved cycle life, electrodes made from reduced silica fibers can facilitate improved charge / discharge rates. Reduced silica fibers have a high surface area and low volume (high SA / V ratio). This allows lithium to quickly enter and exit the structure (surface-limited reaction) and also reduces the distance lithium must diffuse once inside the structure.

[0021] As used herein, "charge rate" refers to the rate at which a current is applied to a cell to restore its capacity. Battery rate capability is a measure of a cell's ability to deliver a specific current to a load or accept a specific current during charging. Rate capability is represented by a discharge curve, which plots cell voltage (vertical axis) versus time (horizontal axis) under a specific charge / discharge current. As current increases, the battery's ability to accept or provide a stable voltage over an extended period of time decreases. In this sense, battery capacity tends to decrease as the rate increases. The ability to transfer lithium into and out of the cathode and anode materials is the limiting factor for achieving stable voltage and long run times at high rates. As a kinetic parameter, charge / discharge rate is also related to the cell's temperature performance. The industry demands batteries that can operate smoothly at extreme temperatures commonly encountered in use.

[0022] Rates are usually referred to as "C" values. A "C" charge rate will charge a particular cell to its full capacity in 1 hour. A "2C" charge rate will charge the same cell in 30 minutes. A "C / 2" charge rate will charge the same cell in 2 hours.

[0023] Lithium-ion battery structure A typical lithium-ion battery cell includes an outer metal housing. Enclosed within the outer metal housing are a cathode (i.e., positive electrode), an anode (i.e., negative electrode), and a separator. In a typical cylindrical lithium-ion battery, the cathode, anode, and separator are provided in the form of long spiral rolls of thin sheets. Alternatively, for pouch or coin cells, they may be provided in a stacked or Z-fold configuration. The cathode, anode, and separator sheets are submerged in a solvent that acts as the electrolyte. The separator physically and electrically separates the anode and cathode while allowing lithium ions to pass through.

[0024] In a typical lithium-ion battery, the cathode is made from a mixed lithium metal oxide material such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), or lithium manganese oxide (LiMn2O4). Other cathode materials include lithium manganese oxide (Li2MnO3), lithium titanate (Li4Ti5O 12 ) etc.

[0025] In a typical conventional lithium-ion battery, the anode is made of carbon, such as graphite. During the charging process, when the cell is absorbing power, lithium ions migrate from the cathode to the anode through the electrolyte and attach or bond to the carbon. During the discharging process, when the cell is discharging power, the lithium ions return through the electrolyte from the carbon anode to the lithium metal oxide cathode. The liquid electrolyte in a lithium-ion battery contains lithium salts, such as lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), or lithium perchlorate (LiClO4), in an organic solvent, such as ethylene carbonate, dimethyl carbonate, or diethyl carbonate. During both the charging and discharging processes, the lithium ions pass through a separator layer, which is typically a thin, perforated plastic sheet that separates the cathode and anode and allows the flow of lithium ions.

[0026] Reference is now made to Figure 3. Referring to Figure 1, a schematic diagram of a lithium-ion battery including an anode and a cathode is shown. The lithium-ion battery 10 includes a negative electrode or anode 12 and a positive electrode or cathode 14. According to the present disclosure, the anode 12 includes reduced silica fibers having a fine diameter, a high surface area, and interconnected or discrete porosity. The cathode 14 can include lithium-containing mixed metal oxide fibers having a fine diameter, a high surface area, and interconnected or discrete porosity. A copper current collector 16 is in electrical contact with and associated with the anode 12, and an aluminum current collector 18 is in electrical contact with and associated with the cathode 14.

[0027] Lithium ions 22 associate with the anode and cathode materials. A permeable polymer separator 24 separates the positive electrode material from the negative electrode material but allows the lithium ions 22 to pass through the electrolyte 26 between the electrodes 12, 14. During discharge of the lithium-ion battery 10, the lithium ions 22 travel from the anode 12 through the electrolyte 26 to the cathode 14, moving electrons 28 in the opposite direction around the circuit to power the load. When the battery 10 is charged, the lithium ions 22 are forced to migrate across the separator 24 to associate with the anode 12, enabling the next discharge cycle.

[0028] Generally, the method of forming the silicon-based anode of the present disclosure includes four steps: (1) forming precursor silica fibers; (2) subjecting the precursor silica fibers to a reduction process to remove oxygen and form reduced silica fibers; (3) washing and purifying the reduced silica fibers; and (4) using the washed and purified reduced silica fibers to form an electrode for use in a lithium-ion cell.

[0029] Preparation of silica-based precursors by fiber solution spinning. According to some embodiments, silica-based precursor fibers may be produced by utilizing a sol-gel or solution spinning process to produce silica fibers, which may then be further reduced to silicon. In this process, silica precursors such as colloidal silica, siloxanes such as pentaethoxypropanedisiloxane (PEDS), water glass (a soluble alkali silicate such as sodium silicate), or other chemicals such as tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), methyltrimethoxysilane (MTMS), or silicon alkoxides (MTES) are placed in a solution or dispersion (sol) to undergo a condensation / crosslinking reaction. Colloidal silicas include Levasil®, available from AkzoNobel, or other colloidal silica products available from CWK, Grace, Merck, Bayer, Nalco, or Dow.

[0030] Additionally, templated porosity can be created in the spun product by incorporating polymers / surfactants with appropriate solubility / cloud point properties into the spinning solution, as described in further detail below and in European Patent Application No. 0318203 and U.S. Patent No. 5,176,857, the disclosures of each of which are incorporated herein by reference in their entireties. Examples of surfactants include alkyl ethoxylates, alkylphenyl ethoxylates, polypropylene oxide-polyethylene oxide block copolymers, comb-type siloxane-polyethoxylate copolymers, and polyethoxylated amines. This introduces micelles into the as-formed fiber, leaving pores of controlled dimensions in the final fiber structure.

[0031] By controlling the addition of surfactants and the resulting micelle size in the solution or sol, the volume and size of the pores in the final product can be directly controlled. Pore size can be controlled down to the level of tens of angstroms or nanometers.

[0032] By controlling the solution temperature, fiberization can occur at or just above the polymer / surfactant cloud point. The "polymer cloud point" is defined as the temperature at which the polymer becomes insoluble in solution and begins to form as a finely divided second phase. The resulting polymer micelles serve to form templated polymer phase domains in the solution. As the material decays (i.e., stretches) into fibers, the polymer phase elongates, forming axially oriented porosity. The size and volume of the pores can be controlled using specific concentrations of polymer and variables such as the temperature in the solution, the fiberization and fiberization atmosphere, shear rate, and the relative humidity of the fiberization atmosphere. Once formed, the fibers are subjected to specific heat treatment protocols to decompose the organic materials (e.g., fiberization aids and template polymers) and affect cross-linking and bonding within the colloidal silica.

[0033] Certain polymers, such as polyethylene oxide, polyvinyl alcohol, polyethylene glycol, celluloses such as methyl or ethyl cellulose, polyvinylpyrrolidone or starch, can be added to the solution or sol to provide the necessary rheology for fiberization. Fiberization may be performed by any method known to those skilled in the art for sol-gel fiberization, such as solution spinning, extrusion, blowing or electrospinning.

[0034] In some embodiments, the sol-gel process for producing precursor silica fibers involves fiberizing a liquid having a viscosity between about 1 and about 1000 poise, such as between about 70 and about 300 poise, and in another example, about 100 and about 450 poise. One suitable solvent for use in the sol-gel process is water due to its low cost, availability, and low toxicity. Other polar solvents, such as methanol, ethanol, and acetone, can be used under certain circumstances, provided that the appropriate oxide precursor is selected. Nonpolar compounds, such as octane, benzene, and toluene, can also be used.

[0035] The liquid is converted into fibers by any convenient method, such as centrifugal spinning, electrospinning, drawing, blowing, tuck spinning, extruding the liquid through a spinneret, or a suitable combination thereof. Any means apparent to one skilled in the art can be used to spin the liquid.

[0036] After the liquid is converted into fiber, such as by spinning, the resulting fiber is dried and heated, optionally in an oxygen-containing atmosphere, for a time and at a temperature sufficient to convert the compounds to oxides in the fiber. The fiber is dried by any suitable means, for example, by heating the fiber at a temperature of about 30°C to about 150°C, optionally under reduced pressure. Any other suitable means for drying the fiber can be used, for example, by circulating dehumidified air or gas around the fiber.

[0037] The dried fibers are heated for a time and at a temperature sufficient to remove the organic components of the sol. Heating temperatures for silica / silicon fibers can be from about 400° C. to about 1000° C., for example, from about 650° C. to about 750° C. Heating times can be greater than about 15 minutes, and can be greater than about 1 hour.

[0038] In certain embodiments, the sol-gel process provides for the production of silica-based precursor fibers with a high proportion of axially aligned pores. In various embodiments, at least 25%, and typically much more than 25%, of the total porosity in the silica-based precursor fibers is provided by axially aligned pores.

[0039] In some embodiments, axially aligned pores are achieved by spinning a solution or sol containing a compound decomposable to silica upon heating and a nonionic surfactant having a cloud point in the spinning solution into a fiber at a temperature not more than 20° C. above the spinning temperature. The fiber is then heated to convert the compound to silica.

[0040] In one embodiment, the spinning temperature is about 25° C. and the surfactant has a cloud point in the spinning solution of less than 45° C. As the spinning temperature increases, the acceptable cloud point of the surfactant increases. For example, for a spinning temperature of about 50° C., the cloud point of the surfactant will be less than 70° C. In certain embodiments, the cloud point of the surfactant is at least 5° C. above the spinning temperature and no more than 15° C. above the spinning temperature.

[0041] As used herein, "spinning temperature" means the maximum temperature that the spinning solution or sol reaches during the extrusion and drawdown (stretching) stages of the fiber formation process. Thus, extrusion of the solution or sol into a heated environment results in an increase in the temperature of the solution or sol during drawdown (stretching) into green fiber. The spinning temperature in such a case is the maximum temperature that the solution or sol (green fiber) reaches before the drawdown (stretching) is complete. Generally, the spinning temperature is the wet-bulb temperature of the attenuated air.

[0042] As used herein, "cloud point of a surfactant" refers to the temperature at which a spinning solution containing the surfactant becomes cloudy when heated. Aqueous solutions of nonionic surfactants become cloudy when heated. These cloud points are determined by immersing a boiling tube containing approximately 10 mL of the solution in a cold, stirred water bath and then heating the bath at a rate of 1 °C / min. The solution in the tube is either not stirred or stirred gently with a spatula (to avoid air bubbles). The solution may become cloudy over a narrow temperature range of 1-2 °C, and the cloud temperature or cloud point may be recorded.

[0043] Several chemical types of surfactants (as described above) can be used. It will be understood that within each of these chemical classes of surfactants, some may have cloud points that are too high to be useful in the process, as well as some that are useful in the process. However, measuring the cloud point of a surfactant to determine its suitability for use in the process is a matter of simple routine experimentation. Blends or mixtures of surfactants can be used to provide an appropriate cloud point.

[0044] (The amount of surfactant used in the spinning solution can vary within wide limits, but is typically at least 1% by weight based on the spinning solution. This amount may be from 1% to 30% by weight of the solution, and in certain embodiments, from about 3% to about 10% by weight of the solution.

[0045] In some embodiments, the spinning solution or sol contains, in addition to a surfactant, a precursor of a stabilizing and / or sintering additive for the fiber. In certain embodiments, the surfactant itself can be the source of the phase stabilizer or sintering agent in the resulting fiber. For example, the use of a siloxane copolymer as a surfactant results in the formation of silica in the fiber when the fiber is heated to decompose the silicon oxide precursor into silica. Thus, a stabilizer precursor that can be incorporated into the spinning solution or sol is silica for silica fiber, such as a silica sol.

[0046] The diameter of the silica-based precursor fibers produced by this method can be from about 0.1 microns to about 20 microns, particularly less than about 10 microns. In one embodiment, the diameter of the silica fibers is from about 2 microns to about 15 microns. Such diameters are determined by the fiberization conditions, such as the size of the spinneret orifice, and the properties of the liquid being fiberized, particularly the viscosity of the liquid and the proportion of oxidizable compounds in the liquid. Higher liquid viscosity and higher solids content result in larger fiber diameters.

[0047] The density of silica fibers is highly dependent on the heat treatment the fibers undergo. After spinning and at least partial drying, the silica-based precursor fibers are typically heated to temperatures between about 200°C and about 600°C to decompose the organic oxide precursor. This heating step may be performed with or without steam. The fibers are further heated to burn off any organic residues and crosslink the resulting silica fibers, which are then further heated to sinter them. After steam treatment, the silica fibers are highly porous, and in the case of silica fibers, this high porosity is maintained during heating, for example, up to 600°C and 700°C. Therefore, by controlling the sintering temperature and the amount of phase stabilizer present, low-density fibers with high porosity can be obtained. Generally, higher treatment temperatures result in fiber shrinkage and reduced porosity.

[0048] The fibers may be produced by any technique of fiber formation known to those skilled in the art. For example, both short (staple) and nominally continuous fibers can be produced by blow spinning or centrifugal spinning techniques. Continuous fibers can be produced by conventional extrusion / winding techniques.

[0049] In silica-based precursor fibers produced by blow spinning or centrifugal spinning techniques, the spin blend may be formed into multiple fiber precursor streams that are at least partially dried in flight to produce gel fibers, which are collected on a suitable device such as a wire screen in the case of short fibers (staple) or a rapidly rotating take-up drum in the case of nominally continuous fibers.

[0050] The spinning formulation can be any known to those skilled in the art for producing sol-gel oxide fibers, and is typically a spinning solution or sol that is free or essentially free of suspended solid or gel particles greater than 10 microns in size. In certain embodiments, the solution or sol contains no particles greater than 5 microns in size.

[0051] The resulting silica-based precursor fibers are essentially SiO2 and may contain porosity resulting from their colloidal nature, from the polymer templating process, or both.

[0052] Fabrication of silica-based precursor fibers via leached glass According to another embodiment, silica-based precursor fibers can be produced from melt-derived silica-based glass fibers, compositions that can be chemically leached to remove non-silica components and leave a silica framework. Such compositions are known in the industry and may include Vycor® compositions (sodium borosilicate), sodium silicate, or other compositions that can leach non-silica components by chemical treatment, typically an acid treatment, and any feasible method can be used. One such composition includes sodium aluminosilicate (manufactured by Unifrax Specialty Fibers, Inc., Unifrax I LLC, Tonawanda, NY) and is used as a precursor to leached silica fibers.

[0053] Silica-based precursor fibers can be produced from silica-based glass melts by any applicable means, including but not limited to blowing, centrifugal spinning, rotary spinning, flame blowing, bushing extrusion, single filament extrusion, or electrospinning.

[0054] Silica-based precursor fibers can have porosity as a result of chemical leaching, which is considered an advantage because it increases the surface area, thus increasing the lithiation kinetics, and providing space for expansion during lithiation. Vycor® compositions, in particular, have the advantage of being phase-separated. The soluble components of the glass exist as separate phases, and their microstructure can be controlled by varying the time and temperature of the heat treatment. This microstructure consists of one phase containing high concentrations of sodium oxide (Na2O) and boron trioxide (BO3), which is highly soluble in acid, and approximately 96% SiO2, which is relatively insoluble in acid. By subjecting this material to an acid leaching process, the acid-soluble phase of Na2O and BO3 can be removed, leaving only the 96% SiO2 phase. The resulting material is not only rich in SiO2, but also contains substantial internal porosity. The porosity can be tailored by further heat treatment, causing the porosity to coalesce. This can result in an interconnected or non-interconnected structure.

[0055] In Vycor® compositions, the size and structure of the pores can be tailored to achieve desirable results. The pores can be tailored to be large or small, such as from about 1 Angstrom to about 200 Angstroms in diameter, e.g., from 40 Angstroms to about 80 Angstroms, and / or interconnected or discrete. This feature is applicable to phase-separated glass (melt or fiber) compositions containing materials in which one phase preferentially dissolves.

[0056] An additional example is the production of leached silica glass, such as materials commercially available as Belchem® fiber material or Valmiera fiber. In these materials, glass fibers are produced from a leachable glass composition. These fibers are then subjected to a leaching step with acid (usually HCl) to leach out NaO or other soluble components via an ion exchange or leaching process, leaving a fibrous silica structure.

[0057] This strategy can be extended to any range of similar soluble glass fibers consisting of a silica matrix to which leachable components such as Na2O, BO3, potassium oxide (KO), lithium oxide (LiO), calcium oxide (CaO), phosphorus pentoxide (PO5), or magnesia (MgO), among others, have been added. These fibers can be leached with acidic, neutral, or basic solutions to produce the leaching required to produce silica fibers. Depending on the glass and leaching conditions, these materials can also exhibit high surface areas and substantial internal porosity. Examples of glass fibers used as starting materials include low biopersistent refractory fibers such as Isofrax® fiber, Insulfrax® fiber, Superwool fiber, or Superwool HT fiber, or Stonewool-type products such as Stonewool HT fiber.

[0058] In some embodiments, the silica-based precursor fibers are prepared from glass fibers. Methods for preparing silica fibers from glass fibers are described in U.S. Patent Nos. 2,215,039, 2,221,709, 2,461,841, 2,491,761, 2,500,092, 2,624,658, 2,635,390, 2,686,954, 2,718,461, and 2,730,475, the disclosures of each of which are incorporated herein by reference in their entirety.

[0059] Glass fibers with high specific surface area, high tensile strength, and consistent glass chemistry and purity are available from Uniflax Specialty Fibers (Tonawanda, NY). These fibers are produced by a spinning and flame-attenuated manufacturing process. Average fiber diameters range from a very fine 0.1 μm to 5.0 μm. Typical glass fiber compositions are shown in the table below. [Table 1] *B2O3 contains 31.1% boron by weight. The maximum allowable boron content in A-glass is 0.028%.

[0060] Glass fibers can be converted to purer, high-silica fibers by extracting monovalent, divalent, and trivalent metal oxide components from them, leaving fibers essentially composed of silica, with a proportion of such metal oxides ranging from less than 10 parts to 90 parts silica and even 1 or 2 parts such oxides, leaving the remaining oxide portion of the fiber being silica. This is done by leaching the fibers, either in loose mass or processed form, with an acid other than hydrofluoric or phosphoric acid, for a time and temperature sufficient to extract substantially all oxides other than silica. The treated fibers are then washed until substantially acid-free and, if necessary, heated at elevated temperatures to dehydrate and shrink the fibers. This process can result in shrinkage of the fibers both in the diameter and length directions.

[0061] Suitable leaching acids include, for example, hydrochloric acid (HCl), sulfuric acid (HSO), nitric acid (HNO), acetic acid (CHCOOH), chloroacetic acid (ClCHCOOH), and chlorinated low molecular weight fatty acids, such as trichloroacetic acid. Acid strength can vary over a wide range, for example, from 0.1 N to 5 N or more. The leaching process can be carried out at elevated temperatures ranging from 100°F up to the boiling point of the acid, or above the boiling point by conducting the leaching in an autoclave under superatmospheric pressure.

[0062] It may be desirable to shrink the fibers by dehydrating them at high temperatures, for example, by heating to a temperature of about 400°F to 500°F, or higher, for a time sufficient to remove substantially all of the water of hydration and any adsorbed water, without substantially shrinking the fibers.

[0063] Borosilicate glass fibers can be readily leached with acid to remove metal oxides other than silica and can be dehydrated by heating to temperatures of about 1000° F., e.g., in the range of about 1400° F. to about 1600° F. By properly controlling the acid extraction process, the resulting fibers after calcination contain a high silica content, which can be as high as 90% and even essentially 99.9% silica, as measured by wet chemical methods.

[0064] Producing reduced silica fibers from silica-based precursor fibers After forming the silicon-based precursor fibers, at least a portion of the chemically bonded oxygen is removed to arrive at fibers containing elemental silicon. For use as a battery anode, the resulting product may contain silicon metal, silicon monoxide, and / or silica. Silicon, silicon monoxide, and their alloys and combinations are electrochemically active and can be used as anode materials in lithium-ion batteries. In some cases, mixed alloying may be particularly desirable because it allows for controlled expansion of the material upon lithiation (depending on the alloying state) and allows it to match the available pore space.

[0065] The thermochemical method of removing bound oxygen from the material is referred to herein as "reduction." This can be accomplished by a variety of established methods, including reduction in a carbon monoxide (CO) atmosphere, hydrogen atmosphere, or other effective means in contact with magnesium, aluminum, or carbon. The temperature of this reaction can be closely controlled to affect or maintain the porous structure of the fiber and ensure it remains within a desired or optimal range.

[0066] This type of reaction is commonly used in the mining and metals industries to convert materials in their native oxide state into a reduced state where they are no longer combined with oxygen and exist in a metallic state. The conversion of iron ore containing ferric oxide (Fe2O3) to metallic iron is an example of a reduction reaction. Another is the thermite reaction, in which iron oxide is mixed with aluminum powder and ignited, removing the oxygen from the iron oxide and combining with the aluminum, leaving aluminum oxide and metallic iron.

[0067] Reduction can be achieved by mixing the oxide to be reduced with the metallic form of a second material that has a higher chemical affinity for oxygen. This mixture is then heated to a high temperature, allowing the reaction to occur spontaneously. In the case of silica, materials such as carbon (carbothermic reduction), aluminum (aluminothermic reduction), or calcium can be used. However, these processes have the disadvantage of requiring excessively high temperatures that affect the physical state of the resulting silicon, causing it to melt or sinter, resulting in severe porosity or surface area reduction and grain growth. In addition to high-oxygen-affinity solids, gases such as hydrogen (H2) and carbon monoxide can also be used to affect the reduction. However, their ability to reduce silica is somewhat limited.

[0068] Silica reduction using magnesium (magnesiothermic reduction) is a very useful method because it can be achieved at relatively low temperatures (600-700°C) and allows for the preservation of microstructures that may be present in the precursor silica material. In particular, porous, high-surface-area structures, such as those obtained by polymer templating or other methods, can be preserved and / or enhanced by low-temperature magnesiothermic reduction.

[0069] In one embodiment, silica-based precursor fibers may be reduced by high-temperature treatment with magnesium powder to convert them into structures containing silicon (Si) or other reduced forms of SiO2. Reduction of silica with magnesium operates at much lower temperatures (650°C) and in shorter times (30 minutes) than other methods, such as carbothermal reduction (>1600°C) or electrochemical reduction. Reduced silica fibers can be extracted from the reduction mixture by immersing the material in a 1-2 M HCl solution. This dissolves the Mg-based components, leaving the Si species unaffected and preserving the fiber morphology.

[0070] Cleaning / purification of reduced silica fibers After reduction, the desired Si / SiO material exists in intimate combination with other reaction products from the reduction, such as magnesium oxide (MgO), magnesium silicide (Mg2Si), and forsterite (MgSiO4), to name a few. This material must be purified to remove unwanted materials, leaving relatively pure Si / SiO. This can be accomplished by washing the material with acid, specifically HCl. Reaction products such as MgO, Mg2Si, and MgSiO4 are soluble in acid, while Si, SiO, and SiO2 are not. Therefore, the washing process removes the unwanted components, leaving behind a silicon alloy, which can be recovered by filtering the solution.

[0071] Fabrication of electrodes using washed / purified reduced silica fibers for use in lithium-ion batteries SiO synthesized using the method described above x Reduced silica fibers containing (x=0-2) are SiO xSilicon can be used in any application where its application would be useful, including sensors, microelectronics, and especially lithium-ion cells. Silicon finds application in lithium battery cells as the anode material used to store lithium when the battery is in a charged state. When current is demanded from the cell, the anode discharges its lithium content into the electrolyte, and when the cell is discharged, the anode diffuses to the counter electrode for storage. Graphite is commonly used in lithium-ion cells because it maintains stability and can perform for hundreds of cycles without losing capacity. Silicon has the advantage of being able to hold more than 10 times the amount of lithium per unit weight compared to graphite. However, as mentioned above, silicon has the disadvantage of swelling substantially upon lithiation. Synthesizing silicon materials with internal porosity is expected to mitigate the adverse effects of swelling by allowing the expansion to occupy internal space.

[0072] In constructing anodes for lithium battery cells, electrochemically active anode materials are typically combined with other ingredients, formed into a viscous slurry, and applied to a copper foil current collector. Once applied to the current collector, the combination of electrochemically active materials and additives with the current collector is typically referred to as the anode.

[0073] Reduced silica fibers are used in conjunction with standard graphite and silicon / SiO2 anodes to prepare anodes. x It may be utilized in a similar manner to the powder: it can be used in fiber form, crushed to any fiber length range, or crushed and ground to a powder so that the fiber aspect ratio is non-existent.

[0074] Reduced silica fibers can be used as the sole active material or may be blended with other active materials, including, but not limited to, graphite, other silicon alloy materials, tin oxide (SnO), or materials for other uses as anode materials. These active materials may be further mixed with other electrode components. For example, they may be mixed with carbon black or carbon nanotubes to provide electrical connection through the electrode material. They may also be mixed with a binder to help hold the material together once it is placed on the current collector. The anode material and binder / viscosity modifier mixture is then applied to a copper foil current collector by a suitable method, such as roll coating, dip coating, or doctor blade.

[0075] In some instances, the reduced silica fibers may be formed into a paper consisting of the fibers and an optional binder, or the fibers may simply be self-entangled without a binder. Silicon-based fiber papers may contain other additives, such as carbon black or carbon nanotubes for electrical connections, as previously mentioned, carrier fibers for mechanical integrity, or other additives as deemed necessary for any reason.

[0076] The reduced silica fibers may also be coated with any material deemed advantageous. In particular, a thin carbon coating may be applied to the fibers to enhance their electrical conductivity and / or inhibit excessive SEI formation, thereby aiding cycle life. Coatings may also be applied for other reasons.

[0077] One advantageous method for applying a carbon coating to reduced silica fibers consists of a pyrolytic deposition process. In this process, reduced silica fibers are maintained at high temperatures (e.g., 950 °C in argon / hydrogen gas (Ar / H2) (forming gas)) in a controlled inert atmosphere. A gas containing a carbon-containing component (e.g., acetylene or other hydrocarbon gas) is introduced and decomposes, leaving a carbon deposit on the fiber surface. Various inert atmospheres, such as helium (He) or argon (Ar), or combinations thereof, can be used.

[0078] The electrodes thus prepared can be used for use in lithium ion batteries in the role of anode or cathode, depending on the nature of the counter electrode.

[0079] Cathode Fiber In various embodiments, a lithium ion battery cathode material is provided that includes mixed metal oxide fibers having diameters ranging from about 0.1 to about 20 microns, high surface area, and interconnected or discrete porosity, and is capable of accepting lithium ions from an electrolyte and releasing lithium ions into the electrolyte.

[0080] According to certain embodiments, the mixed metal oxide fibers are from about 5 to about 400 mm. 2 / g, or in some embodiments, from about 60 to about 140 m 2 / g of surface area.

[0081] According to certain embodiments, the mixed metal oxide fibers may have a porosity of from about 5% to about 60%.

[0082] According to certain embodiments, the mixed metal oxide fibers may include oxides of lithium and at least one of aluminum, cobalt, iron, manganese, nickel, titanium, and vanadium.

[0083] According to certain embodiments, the mixed metal oxide fibers may be made of lithium cobalt oxide (LiCoO), lithium nickel manganese cobalt oxide (LiNi0.33 Mn 0.33 Co 0.33 O2), lithium nickel cobalt aluminum oxide (LiNi 0.8 Co O.15 Al 0.05 O2), lithium permanganate (LiMnO4), lithium iron phosphate (LiFePO4), lithium manganese oxide (LiMn2O4), lithium manganese oxide (Li2MnO3), lithium titanate (Li4Ti5O 12 ) may include at least one of:

[0084] According to certain embodiments, the mixed metal oxide fibers include pores having diameters of 0.1 to about 100 nm or about 0.5 to about 25 nm.

[0085] Cathode fiber formation A wide variety of materials are used across the industry for lithium-ion battery cathodes. These materials are typically lithium-containing metal oxides, such as lithium titanium disulfide (LiTiS2), lithium cobalt oxide (LiCoO2), and lithium nickel manganese cobalt oxide (LiNi 0.33 Mn 0.33 Co0. 33 O2), lithium nickel cobalt aluminum oxide (LiNi 0.8 Co0. 15 Al 0.05 Examples of suitable cathode materials include, but are not limited to, lithium permanganate (LiMnO2), lithium iron phosphate (LiFePO4), or lithium ferric phosphate (LiFePO4). Many synthetic routes have been used to produce cathode materials, including the sol-gel method, which is described further below. In the sol-gel method, metal salts can be dissolved and mixed into a solution, then precipitated into a colloidal suspension (sol). This colloidal suspension can be gelled by reducing the water content. The material is then dried, allowing inorganic polymerization reactions to occur between the components. Further heat treatment removes salt anions and organic materials, densifying and crystallizing the remaining oxide material.

[0086] In various embodiments, high surface area cathode materials are prepared by the following method. First, a solution or sol is formed containing the desired metal salts or compounds in the proportions necessary to achieve the proper stoichiometry of the cathode material. In certain embodiments, the liquid contains about 10 to about 70 weight percent of an oxidizable precursor compound of the inorganic oxide contained in the lithium-ion battery electrode. The precursor compound can include a solvent-soluble salt of the electrode metal. Examples of suitable inorganic salts are metal nitrates, chlorides, and oxychlorides. Examples of suitable organic salts are metal salts of lower alkyl organic acids, such as acetates, chloroacetates, formates, oxyacetates, propionates, or butyrates, or metal salts of lower alkyl hydroxy acids, such as lactates. Additionally, some salts can be utilized in blends or mixes. Essentially any compound that can be dissolved in a solvent to form a fiberizable solution and that will yield metal oxide fibers when heated to a sufficient temperature in an oxygen-containing atmosphere can be used as the inorganic oxide precursor, including alkoxides of silicon and metals, aluminum (Al), cobalt (Co), iron (Fe), manganese (Mn), nickel (Ni), titanium (Ti), vanadium (V), and the like.

[0087] In one embodiment, spinning aids such as organic polymers are included in the solution or sol as spinning aids to provide the necessary rheology for fiberization. Other materials deemed desirable in the final product may also be included, including, but not limited to, phase change inhibitors or metal oxide powders for improved performance.

[0088] The solution or sol derived material may be fiberized using established fiberization methods, including, but not limited to, (1) providing the solution or sol to a rotating disk where it can be extruded into fibers by centrifugal force, (2) providing the solution or sol to an extrusion nozzle (or assembly thereof) and extruding the solution or sol as continuous or discontinuous fibers with or without attenuating airflow, or (3) electrospinning.

[0089] In the manufacturing process of fibrous cathode materials, as described in more detail, porosity can be introduced into the fibrous structure by including in the solution phase a nonionic surface-active polymeric agent containing a precursor metal salt. These are described below and in European Patent Application No. 0318203 and U.S. Patent No. 5,176,857, the disclosures of each of which are incorporated herein by reference in their entirety. Examples of surfactants include alkyl ethoxylates, alkylphenyl ethoxylates, polypropylene oxide-polyethylene oxide block copolymers, comb-type siloxane-polyethoxylate copolymers, and polyethoxylated amines.

[0090] By controlling the addition of surfactants and the resulting micelle size in the solution or sol, the volume and size of the pores in the final product can be directly controlled. Pore size can be controlled down to the level of tens of angstroms or nanometers.

[0091] The resulting fibers are subjected to a drying and calcination process to remove water, residual organic components, and volatile salt anions. In some embodiments, the fibers may undergo further heat treatment to develop a crystalline structure for optimal cathode performance.

[0092] Finally, the fibers are arranged into an electrode structure suitable for use as or inclusion in a cathode in a lithium ion battery.

[0093] The resulting fibrous materials can range in diameter from about 0.1 microns to about 20 microns. Driving the active material particle structure to a smaller size increases the surface area-to-volume ratio, allowing for faster lithium ion insertion or removal rates by shortening the diffusion distance and increasing the surface area for lithium ions to enter and exit the structure. The presence of porosity allows for a higher surface area-to-volume ratio than that of the fibers alone, further improving charge / discharge kinetics, i.e., lithium ion removal and insertion into the electrode structure.

[0094] Use as a cathode The porous, high-surface-area cathode fibers can be processed into electrodes for use in lithium-ion batteries. The fibers can be used as the sole active cathode material or mixed with other active cathode materials. The material may also be mixed with other components, such as conductive additives such as carbon black or carbon nanofibers or binders. These mixtures can then be applied to an aluminum current collector and utilized as the cathode portion of a lithium-ion battery.

[0095] According to some exemplary embodiments, the diameter of the nanoparticles ranges from about 0.1 to about 20 microns and the nanoparticles range from about 5 microns to about 10 microns. 2 / g~about 400m 2 A porous reduced silica fibrous material is provided having a surface area of ​​1 / g.

[0096] According to some exemplary embodiments, the porous reduced silica fibrous material has a diameter of about 2 to about 15 microns. According to some exemplary embodiments, the porous reduced silica fibrous material comprises silicon, silicon monoxide, and silica, wherein the silicon is present in an amount greater than about 20 weight percent.

[0097] According to some exemplary embodiments, the porous reduced silica fibrous material has a pore size of about 0.1 nm to about 150 nm. According to some exemplary embodiments, the porous reduced silica fibrous material has a pore size of about 1 nm to about 100 nm.

[0098] According to some exemplary embodiments, a process for producing a porous reduced silica fiber material includes providing a solution including a silica precursor, a polymer, and a surfactant, fiberizing the solution to produce fibers, heating the fibers to produce silica-based precursor fibers, subjecting the silica-based precursor fibers to magnesiothermic reduction to remove at least some oxygen from the silica-based precursor fibers and produce reduced silica fibers, and washing the reduced silica fibers with an acid to remove magnesium-containing reaction products and produce a porous reduced silica fiber material.

[0099] According to some exemplary embodiments, a process for manufacturing a porous reduced silica fiber material includes providing a melt comprising silica (SiO2) and at least one of sodium oxide (Na2O), boron trioxide (B2O3), lithium oxide (Li2O), calcium oxide (CaO), and magnesia (MgO), fiberizing the melt to produce fibers, treating the fibers to remove non-silica components to produce silica-based precursor fibers, subjecting the silica-based precursor fibers to magnesiothermic reduction to remove at least some oxygen from the silica-based precursor fibers to produce reduced silica fibers, and washing the reduced silica fibers with an acid to remove magnesium-containing reaction products to produce a porous reduced silica fiber material.

[0100] According to some exemplary embodiments, a porous reduced silica fiber material having a diameter of about 0.1 to about 20 microns and a surface area of about 5 m 2 / g to about 400 m 2 / g and substantially free of silicon dioxide is provided.

[0101] According to some exemplary embodiments, the porous reduced silica fiber material has a surface area of from about 30 m 2 / g to about 300 m 2 / g. According to some exemplary embodiments, the porous reduced silica fiber material has a porosity of from about 0.01 cm 3 / g to about 1.5 cm 3 / g. According to some exemplary embodiments, the porous reduced silica fiber material has a median pore diameter of from about 1 nm to about 90 nm.

[0102] According to some exemplary embodiments, the porous reduced silica fiber material has a plurality of axially aligned pores. According to some exemplary embodiments, the porous reduced silica fiber material has a pore diameter of from about 0.1 nm to about 80 nm. According to some exemplary embodiments, the porous reduced silica fiber material has a pore diameter of from about 4 nm to about 40 nm.

[0103] According to some exemplary embodiments, a battery electrode is provided that includes a binder and porous reduced silica fibers. According to some exemplary embodiments, the battery electrode has a coulombic efficiency of about 30% to about 100%. According to some exemplary embodiments, the battery electrode has a capacity ratio C3 / C1 of about 0.4 to about 1.0. According to some exemplary embodiments, the battery electrode has a capacity ratio C10 / C1 of about 0.1 to about 1.0.

[0104] According to some exemplary embodiments, a lithium-ion battery is provided that includes a battery electrode.

[0105] According to some exemplary embodiments, the diameter is about 0.1 to about 20 microns and the thickness is about 5 m / g to about 400 m 2 A porous mixed metal oxide fibrous material is provided having a surface area of ​​1 / g.

[0106] According to some exemplary embodiments, the porous mixed metal oxide fiber material has a thickness of about 30 mm. 2 / g~about 300m 2 / g of surface area. According to some exemplary embodiments, the porous mixed metal oxide fibrous material has a porosity of about 5% to 50% by volume.

[0107] According to some exemplary embodiments, the porous mixed metal oxide fiber material includes an oxide of lithium and at least one of aluminum, cobalt, iron, manganese, nickel, titanium, and vanadium. According to some exemplary embodiments, the porous mixed metal oxide fiber material includes an oxide of lithium and at least one of aluminum, cobalt, iron, manganese, nickel, titanium, and vanadium. 0.33 Mn 0.33 Co 0.33 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiMnO4, LiFePO4, LiMn2O4, Li2MnO3, and Li4Ti5O 12 Contains at least one of the following:

[0108] According to some exemplary embodiments, the porous mixed metal oxide fibrous material has a pore size of about 0.1 nm to about 150 nm. According to some exemplary embodiments, the porous mixed metal oxide fibrous material has a pore size of about 1 nm to about 100 nm.

[0109] According to some exemplary embodiments, the porous mixed metal oxide fiber material is produced by sol-gel fiberization.

[0110] According to some exemplary embodiments, a lithium-based battery electrode is provided that includes a binder and a porous mixed metal oxide fiber material. According to some exemplary embodiments, a lithium-ion battery is provided that includes the lithium-based battery electrode.

[0111] The following examples are illustrative of the materials and methods discussed above and are not intended to be limiting. [Example]

[0112] Example 1: Preparation of silica-based precursor fibers Formation of silica-based precursor fibers by solution spinning Levasil® 200s colloidal silica (a solution containing 30% colloidal silica) with a colloidal size of approximately 15 nm was used as the silica source for silica fibers. Siloxane (2 parts Evonik 5850 and 1 part Xiameter® OFX-0193 fluid) was added to the colloidal silica to achieve polymer template porosity. Polyethylene oxide (PEO) (Polyo® WSR N-750) was added as a spinning aid to impart rheology to the solution to enable fiber formation via an extrusion / blowing process. The resulting solution was thoroughly mixed in a paddle blender. This resulted in a solution with the following solids ratio: 450 g SiO2 (from colloidal silica), 22.5 g SiO2 (from siloxane), and 39 g PEO. The solution had a viscosity of 1-5 poise at 30°C. The solution was then converted into fibers via an extrusion / blowing process. This gives a median diameter of approximately 3 microns, a standard deviation of 1.3 microns, and a surface area of ​​153 m 2 / g, pore volume 0.285 cm 3 / g and a median diameter of 7.6 nm.

[0113] Reduction of silica-based precursor fibers 2.2 grams of the silica-based precursor fibers prepared as described above were thoroughly mixed with 1.74 grams of magnesium metal powder (available from Alfa Aesar, 99.6% metal basis, particle size -100 to +200 mesh) in an argon atmosphere glove box. This mixture was packed into a stainless steel tube (½ inch outer diameter, 0.035 inch wall thickness) and sealed using a stainless steel end cap. The assembled gas-tight unit was then placed in a Lindberg / Blue M® high-temperature furnace, ramped to 700°C at 5°C / min, and held at 700°C for 2 hours to allow the reduction reaction to occur.

[0114] At the end of the reduction process, the material was furnace-cooled and the resulting mixture was recovered. Subsequent X-ray diffraction (XRD) analysis revealed that the resulting material contained 17.4 wt% Si, 61.1 wt% MgO, 10.9 wt% Mg2Si, 2.0 wt% Al, 5.9 wt% Mg2SiO4, and 2.7 wt% MgAl2O4. Figure 2 shows the XRD scan of the resulting mixture.

[0115] In fact, any suitable vessel can be used, provided that it maintains the oxygen-free atmosphere required for the reduction. Furthermore, other gases besides argon, such as H2, helium (He), or a combination of Ar and H2 (i.e., forming gas), can be used, provided they do not react with the silica or magnesium used in the process. Also, a sealed vessel need not be used. The reaction can also proceed in an open or flow-through environment, provided the appropriate atmosphere is maintained.

[0116] Acid washing of reduced material The reduced material from above was then acid washed to remove non-silicon components. This was accomplished by adding the material to 2M HCl and stirring for three hours. At the end of the acid step, the material was filtered through Whatman® 44 filter paper, and the filtrate was washed twice with deionized water to remove any acid residue. After a final filtration, the material was dried at 80°C for three hours. The recovered material was then subjected to XRD analysis, which revealed the following: 41.7 wt% Si, 2.1 wt% MgAl2O4, 6.8 wt% Mg2SiO4, and 0.1 wt% MgO. An amorphous peak was found to be present at 49.3 wt%. Figure 3 shows an XRD scan of the reduced material after the acid wash. Those skilled in the art will recognize that any of a number of acids and concentrations can be used for the wash step, provided that Si metal and SiO are relatively insoluble in the solution, and MgO, MgSiO3, Mg2Si, and other materials are relatively insoluble in the solution, and the by-products are relatively soluble.

[0117] During the acid cleaning process, sparks were observed when the material came into contact with the acid, which was determined to be due to the formation of silane gas (SiH4) resulting from the decomposition of Mg2Si in the acid. Silane is highly flammable and is known to spontaneously combust in the presence of oxygen. This oxidation reaction could result in the formation of SiO2, potentially contaminating the product. In later experiments, this reaction was suppressed by adding a reducing mixture to the acid under a nitrogen blanket. In this way, oxygen was kept away from the silane, thereby inhibiting its oxidation and conversion to SiO2. In practice, any suitable inert gas could be used, including argon, helium, carbon dioxide, etc. [Example]

[0118] Example 2: Creation and testing of half-cells To evaluate the performance of the reduced silica fiber material as an anode, a set of half-cells was constructed to measure the electrochemical capacity and cycle life performance of cells utilizing the reduced silica fiber material.

[0119] The reduced silica fibers from Example 1 were mixed at 70% by weight with 20% Lith-X® 50 carbon black conductive additive and 10% styrene-butadiene rubber (SBR) binder available from Targray Technology International Inc. The mixture was thoroughly blended to achieve a uniform mixture, necessary to achieve the appropriate viscosity. This was then doctor bladed at a thickness of 200 microns onto a copper foil sheet, which served as a current collector. The coating was then dried at approximately 90°C under partial vacuum.

[0120] The anode material was then used in combination with a Celgard® 2325 separator and a lithium foil counter electrode to fabricate a coin-shaped half-cell. An electrolyte consisting of 1.2 M lithium hexafluorophosphate (LiPF) in 3:7 ethylene carbonate (EC):ethyl methyl carbonate (EMC) was added, and the cell was sealed and allowed to equilibrate internally for approximately 24 hours.

[0121] The coin-shaped half-cells were then subjected to cycling tests by undergoing charge and discharge to insert or extract lithium ions from the silicon material. In this particular case, the first two cycles were performed at a charge / discharge rate of C / 40 (the cell was fully charged / discharged in 40 hours), followed by three cycles at C / 20 (the cell was charged / discharged in 20 hours), and subsequent cycles at C / 10. This allowed us to measure not only the initial capacity (mAh / g of silicon material) but also the coulombic efficiency (the ratio of lithium insertion to lithium extraction) of the first cycle and the cycle life, which was determined by tracking the extraction capacity and cycles.

[0122] Similar cells utilizing a standard commercially available graphite anode (MAGE3 graphite available from Hitachi Chemical) and a commercially available battery-grade silicon anode available from Paraclet Energy were also tested to provide comparative samples. The results are summarized as follows: [Table 2]

[0123] In Table 2, "Coulombic efficiency" or "reversible capacity" is defined as the ratio of the extracted capacity to the inserted capacity for a given cycle. The column labeled "Capacity Ratio" indicates the amount of capacity fade that occurs with cycling by calculating the ratio of the extracted capacity for a particular cycle (in this case, cycle 3, cycle 10, and cycle 40) to the extracted capacity for the first cycle. Thus, a capacity ratio of 1 indicates no capacity fade with cycling, a ratio of 0.5 indicates 50% capacity fade at the specified cycle, and 0 indicates no capacity remaining at the specified cycle (i.e., complete cell failure).

[0124] The data show that graphite has an extraction capacity of 342.9 mAh / g, commercial silicon dioxide has an extraction capacity of 1648.0 mAh / g, and reduced silica fiber has an extraction capacity of 1369.0 mAh / g, indicating that the reduced silica fiber has a similar extraction capacity to silicon dioxide. Regarding cycling stability, graphite exhibits very stable cycling with a coulombic efficiency of 93.1% and a C3 / C1 capacity ratio of 1, while silicon dioxide has a lower coulombic efficiency of 43.3% and no residual capacity after three cycles. In comparison, the reduced silica fiber exhibits a coulombic efficiency of 68.2% and a much slower decay during cycling, which is superior to commercial silicon dioxide. [Example]

[0125] Example 3: Reduction test Because material processing and microstructure are known to play important roles in performance, further studies were conducted using the silica-based precursor fiber of Example 1. This material involved various reduction conditions: (1) varying the silica-to-magnesium ratio (amounts ranging from stoichiometric (1 Mg per O) to 50% stoichiometric (1 Mg per 2 O); (2) varying the heating rate above 400°C from 1°C / min to 5°C / min; (3) varying the maximum temperature from 650°C to 700°C; and (4) varying the hold time at temperature from 2 hours to 7 hours. After the reduction process, the material was examined by XRD to determine the crystalline phase composition. After acid washing as described in Example 1, the material underwent BET analysis to measure the surface area, median pore size, and overall porosity. The following table shows the sample matrix for the study. While this study represents a specific, limited range of reduction variables, it will be understood that material ratios, ramp rates, hold temperatures, and hold times beyond these may also be feasible. [Table 3]

[0126] The resulting phases and morphologies are shown in the table below. Figure 4 shows the effect of the Mg / SiO2 mass ratio on the percentage of SiO2 converted to silicon. At low and high ratios, the percentage of SiO2 converted is lower than when the ratio is 0.813 (nearly stoichiometric, i.e., one magnesium atom per oxygen atom, resulting in the oxidation of silica-based precursor fibers and magnesium). Figure 5 shows the effect of the Mg / SiO2 ratio on the surface area of ​​the reduced silicon material. As the ratio increases, the surface area decreases. Figure 6 shows the effect of the Mg / SiO2 ratio on the median pore size. As the ratio increases, the median pore size increases. Figure 7 shows the effect of the Mg / SiO2 ratio on the pore volume of the reduced silica material. At low and high ratios, the porosity is lower than when the ratio is closer to 0.813 (stoichiometry). [Table 4]

[0127] Because the microstructure of reduced silica fibers is likely related to the electrochemical and cycling performance of the material in the cell, it is desirable to be able to control the final microstructure of the reduced fibers. Therefore, it is important to preserve the microstructural features of the initial silica-based precursor fibers (created by leaching, polymer templating, or other processes) and to enhance these features by creating additional pore volume and surface area. Control of the reduction process is essential to create the optimal microstructure.

[0128] Samples 1 and 5 were reduced under similar conditions (700 °C for 2 and 3 hours, respectively) but at different heating rates. This resulted in similar phase structures (Si, MgO, Mg2Si, and MgSiO4), but very different morphological properties. Sample 1, heated at 5 °C / min, had a very small surface area (35 m 2 / g), with a relatively coarse median pore size (24 nm) and a porosity of 0.107 cm 3 In contrast, sample 5, which has a lower heating rate, has a surface area of ​​91 m 2 / g, median pore size of 21 nm, more than three times the porosity of 0.388 cm 3 / g. XRD shows that the silicon crystallite sizes for samples 1 and 5 are on the order of 132 nm and 31 nm, respectively.

[0129] This effect is likely due to the exothermic nature of the reduction reaction. A rapid heating rate (5°C / min) allows the reaction to occur over a short temperature period. If the reaction is completed quickly, heat from the reaction can accumulate within the reactor, causing the reactor temperature to become much higher than the ambient environment. This promotes sintering of the sample, grain growth, and other microstructural consolidation. A slower heating rate (1°C / min) allows the reaction to occur over a longer period, releasing the heat of reaction to the environment, thereby maintaining a lower reactor temperature and suppressing temperature-induced microstructural changes.

[0130] In another comparative study, samples 2, 9, 10, and 11 all underwent the same heat treatment cycle but with different Mg to SiO2 ratios. Sample 2 had a mass ratio of Mg to SiO2 (Mg / SiO2) of 0.813, which is stoichiometric for complete removal of oxygen from SiO2. Sample 9 had a ratio of 1.220, which contained a 50% excess of MgO relative to the amount required to completely reduce SiO2. Sample 10 had a ratio of 0.610, which contained SiO2. 2 This is 25% less Mg than needed to completely reduce the SiO2 (thus only 75% of the oxygen atoms need to be removed). Sample 11 required a ratio of 0.405 Mg to completely reduce the SiO2. As can be seen, the Mg-rich material had the lowest surface area and porosity values ​​and the coarsest pore size in this set. As shown in Figure 2, the surface area shows a gradual increase as the Mg content becomes thinner. [Example]

[0131] Example 4: Half-cell testing of selected samples from Example 3 While not wishing to be bound by theory, high pore volume is believed to be beneficial for anode performance. As silicon becomes increasingly lithiated, it expands to levels up to 3.5 times its unlithiated equivalent. This volume expansion must be accommodated within the anode and cell design. The data show that cells constructed with commercially available silicon, which exists as particles and agglomerates in the 0.1-1.0 micron diameter range, lose all capacity very quickly. This is believed to be because the repeated expansion and contraction of the silicon deforms the electrode coating, resulting in a loss of electrical conductivity within the electrode and an inability to charge or discharge current. By providing internal porosity within silicon-based fibers, an internal volume is created that can accommodate at least some of that expansion without destroying the exterior of the fiber or the electrode structure.

[0132] Selected samples were anodized and subjected to half-cell testing as described in Example 2, except that the first three cycles were performed at C / 20 and subsequent cycles were performed at C / 10 as before. [Table 5]

[0133] In this dataset, the intercalation capacity appears to be the highest of the commercially available silicon samples, with lower values ​​for samples 10 and 11 due to the lower amount of Mg used in their reduction. Samples 10 and 11 were not fully reduced and therefore likely contain a combination of Si (3500 mAh / g), SiO (1710 mAh / g), and SiO.

[0134] The cycle life, as indicated by the capacity ratio, is highest for graphite, which is known to have long-term stability. Commercial silicon lost all capacity after 10 cycles. However, silicon-based fiber materials maintained cycle life for 10 and 40 cycles, with samples 10 and 11 exhibiting the greatest capacity retention. Referring to the previous discussion of the potential role of internal porosity in accommodating expansion, samples 10 and 11 may expand less due to the presence of oxide materials and therefore expand at a level that can be easily accommodated by the existing porosity.

[0135] Various graphs of the first cycle results are provided in Figures 8-14. Figures 8-14 show the amount of lithium inserted into and extracted from the anode as the anode is cycled between 0.01 volts and 1.5 volts. The lower curve and lower scale show the amount of lithium inserted into the anode, or "insertion capacity." The upper curve shows the amount of lithium extracted from the anode, or "extraction capacity." Cells were tested in triplicate, so there are three sets of curves for each test.

[0136] Various graphs of cycle life are provided in Figures 15-21. Figures 15-21 show the extraction (or discharge) capacity of a half-cell and how it changes as the cell is progressively cycled. Each data point represents the extraction capacity of a particular test. The test was performed three times, so each graph displays three separate data sets.

[0137] Although only a few exemplary embodiments have been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present invention, and all such modifications are therefore intended to be included within the scope of the present invention as defined in the following claims.

Claims

1. 1. A method for producing a porous reduced silica material, comprising: forming a precursor fiber comprising silica; magnesiothermically reducing the precursor fibers to form the porous reduced silica material; Including, The porous reduced silica material has a diameter of 0.1 to 20 microns, a thickness of 5 mm ２ / g~400m ２ / g surface area and 0.01 cm ３ / g to 0.15 cm ３ / g porosity and greater than 20 weight percent silicon.

2. 10. The method of claim 1, wherein the precursor fiber is formed from a silica-based glass melt; and The method further comprises leaching the precursor fibers to remove non-silica components prior to magnesiothermic reduction of the precursor fibers. method.

3. 3. The method of claim 2, wherein the precursor fiber comprises a silica matrix containing the non-silica component, the non-silica component being sodium oxide (Na ２ O), boron trioxide (B ２ O ３ ), potassium oxide (K ２ O), lithium oxide (Li ２ O), calcium oxide (CaO), phosphorus pentoxide (P ２ O ５ ), magnesia (MgO), or a combination thereof.

4. 3. The method of claim 2, wherein the precursor fiber is leached using hydrochloric acid (HCl), sulfuric acid (H ２ SO ４ ), nitric acid (HNO ３ ), acetic acid (CH ３ COOH), chloroacetic acid (ClCH ２ COOH), trichloroacetic acid (CCl ３ COOH), or a combination thereof.

5. The method of claim 2 , wherein leaching the precursor fiber comprises leaching with a basic or neutral solution.

6. 10. The method of claim 1, wherein the precursor fiber is formed from a solution containing colloidal silica, the solution not containing particles larger than 5 microns.

7. The method of claim 6 , wherein the precursor fiber comprises axially aligned pores.

8. 8. The method of claim 7, wherein the axially aligned pores account for at least 25% of the total porosity of the precursor fiber.

9. 10. The method of claim 1, further comprising the step of heating the precursor fiber at a temperature of 400°F or greater prior to magnesiothermic reduction of the precursor fiber.

10. 10. The method of claim 9, wherein after heating, the precursor fiber comprises 90 weight percent or more silica.

11. 10. The method of claim 1, further comprising the step of grinding the porous reduced silica material into a powder that does not have a fiber aspect ratio.

12. 10. The method of claim 1, further comprising the step of acid washing said porous reduced silica material.

13. 13. The method of claim 12, wherein the porous reduced silica material comprises at least 0.1 weight percent of a magnesium compound.

14. 10. The method of claim 1, wherein the porous reduced silica material is 0.1 cm ３ / g to 0.7 cm ３ / g porosity.

15. The method of claim 1, wherein the porous reduced silica material has a median pore size of from 1 nm to 90 nm.

16. 10. The method of claim 1, further comprising a pyrolytic deposition process to form a carbon coating on the porous reduced silica material.

17. 1. An anode material comprising a porous reduced silica material, said porous reduced silica material having a diameter of 0.1 to 20 microns, a thickness of 5 mm or less, ２ / g~400m ２ / g surface area and 0.01 cm ３ / g to 0.15 cm ３ / g porosity and containing greater than 20 weight percent silicon.

18. 20. The anode material of claim 17, wherein the porous reduced silica material is a powder having no fibrous aspect ratio.

19. 20. The anode material of claim 17, wherein the porous reduced silica material is fibrous and the anode material is a fibrous paper.

20. 20. The anode material of claim 17, further comprising a carbon coating on the porous reduced silica material.

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