Composite Electrode Material and Method for Manufacturing the Same

Abstract

The present invention discloses a composite electrode material and a method for manufacturing the same. The composite electrode material may include a carbon-based matrix component and a silicon-based particulate component embedded in the carbon-based matrix component. The silicon-based particulate component may include a plurality of core-shell structures, wherein each core-shell structure includes: a silicon core, an intermetallic layer covering the core, and a graphite shell surrounding the silicon core and the intermetallic layer. In the method for manufacturing the composite electrode material, a metal catalyst layer may be deposited on a plurality of silicon particles to form a plurality of precursor structures in the form of particles. The precursor structures may be dispersed in an organic polymer material to form a precursor electrode material, which may be heated in an inert environment to pyrolyze the organic polymer material and convert the precursor electrode material into the composite electrode material.

Description

Technical Field

[0001] The present invention relates to composite electrode materials and methods of making composite negative electrode materials for use in lithium ion battery packs. Background Art

[0002] The present invention relates to secondary lithium ion battery packs.

[0003] A battery pack is a device that converts chemical energy into electrical energy via an electrochemical reduction-oxidation (redox) reaction. In a secondary battery pack or rechargeable battery pack, these electrochemical reactions are reversible, which allows the battery pack to undergo multiple charge and discharge cycles.

[0004] A secondary lithium ion battery pack typically includes one or more electrochemical cells, the one or more electrochemical cells including a negative electrode, a positive electrode, and an electrolyte that provides a medium for conducting lithium ions between the negative electrode and the positive electrode across the electrochemical cell. During charging, lithium ions are released from the positive electrode, transported through the electrolyte, and inserted into the negative electrode material for storage. During discharging, lithium ions are released or extracted from the negative electrode material and transferred back to the positive electrode. The amount of electric charge that a lithium ion battery pack can accommodate thus depends on the lithium ion storage capacity of the negative electrode material. In addition, the usable cycle life of such a battery pack depends on the number of times the negative electrode material can effectively absorb and release lithium ions without undergoing significant mechanical degradation and / or capacity loss.

[0005] Carbon-based materials (such as graphite) are commonly used as electrochemically active negative electrode materials in lithium ion battery packs because of their ability to effectively store lithium ions between the layers of graphite sheets via a mechanism called intercalation. Such carbon-based materials undergo low volume expansion and contraction when accepting (intercalating) and releasing (deintercalating) lithium ions, which provides suitable mechanical stability for such materials to be used as negative electrode materials for a long time. However, graphite-carbon-based negative electrode materials exhibit a relatively low theoretical specific capacity of about 372 mAh / g. In addition, during the initial battery pack cycling, due to the formation of a solid electrolyte interface (SEI) along the surface of the carbon-based negative electrode material, lithium ions can be consumed and fixed, which can lead to irreversible capacity loss and can also hinder subsequent lithium ion intercalation processes.

[0006] Silicon has been identified as a desirable electrochemically active negative electrode material for secondary lithium ion battery packs due to its high theoretical specific capacity (e.g., 4200 mAh / g). Silicon-based negative electrode materials store lithium ions in their structural framework by forming LiSi X alloy compounds. However, during the process of lithium ion insertion and removal, the volume of the silicon-based negative electrode material significantly expands and contracts, which can result in mechanical stress and promote degradation of the negative electrode material over time. Summary of the Invention

[0007] The composite electrode material may include a carbon-based matrix component and silicon-based particulate components embedded in the carbon-based matrix component. The silicon-based particulate components may include a plurality of core-shell structures. Each core-shell structure may include a silicon core, an intermetallic layer covering the core, and a graphite shell surrounding the silicon core and the intermetallic layer.

[0008] The silicon-based particulate components may account for 10% to 90% by weight of the composite electrode material.

[0009] The plurality of core-shell structures may be uniformly distributed throughout the carbon-based matrix component.

[0010] The silicon core may include more than 99% silicon (Si) by weight.

[0011] The intermetallic layer may include a metal silicide. The metal silicide may include at least one metal selected from copper, nickel, iron, or cobalt.

[0012] In each core-shell structure, the intermetallic layer may be disposed between the silicon core and the graphite shell.

[0013] The graphite shell may include crystalline graphite.

[0014] The carbon-based matrix component may include amorphous hard carbon.

[0015] The carbon-based matrix component may not include discrete particles or regions of crystalline graphite.

[0016] In a method of manufacturing the composite electrode material, a plurality of silicon particles may be provided, each of which has a surface. A metal catalyst layer may be deposited on the surface of each silicon particle to form a plurality of precursor structures. The precursor structures may be dispersed in an organic polymer material to form a precursor electrode material. The precursor electrode material may be heated in an inert environment: (i) to convert the organic polymer material into a carbon-based material, (ii) to convert at least a portion of the metal catalyst layer on the surface of each silicon particle into a metal silicide; and (iii) to form a graphite shell around each silicon particle.

[0017] The plurality of silicon particles may exhibit an average particle size of 10 nanometers to 40 micrometers.

[0018] The metal catalyst layer may be deposited on the surface of each silicon particle using wet chemical deposition techniques, chemical vapor deposition techniques, or high-energy ball milling techniques.

[0019] The metal catalyst layer may include at least one metal selected from copper, nickel, iron, or cobalt.

[0020] The metal catalyst layer may have a thickness of 2 nanometers to 200 nanometers.

[0021] The organic polymer material may include polyimide or polyacrylonitrile.

[0022] The precursor electrode material may be heated in an inert environment at a temperature of 400°C to 900°C to pyrolyze the organic polymer material.

[0023] During the process of heating the precursor electrode material in an inert environment, the metal catalyst layer on the surface of each silicon particle may promote the formation of a graphite shell around each silicon particle.

[0024] During the process of heating the precursor electrode material in an inert environment, the metal catalyst layer on the surface of each silicon particle may physically separate each silicon particle from the organic polymer material and prevent the formation of silicon carbide (SiC).

[0025] In a method of manufacturing a composite negative electrode material for a lithium-ion battery pack, a plurality of silicon particles may be provided, wherein each silicon particle has a surface. A metal catalyst layer may be deposited on the surface of each silicon particle to form a plurality of precursor structures. The precursor structures may be dispersed in an organic polymer material to form a precursor electrode material. The precursor electrode material may be heated in an inert environment to pyrolyze the organic polymer material and convert the precursor electrode material into a composite electrode material, the composite electrode material including a carbon-based matrix component and a silicon-based particulate component embedded in the carbon-based matrix component. The silicon-based particulate component may include a plurality of core-shell structures. Each core-shell structure may include a silicon core, an intermetallic layer covering the core, and a graphite shell surrounding the silicon core and the intermetallic layer.

[0026] The intermetallic layer may include a metal silicide. The metal silicide may include at least one metal selected from copper, nickel, iron, or cobalt.

[0027] The present invention discloses the following embodiments:

[0028] 1. A composite electrode material, comprising:

[0029] A carbon-based matrix component; and

[0030] A silicon-based particulate component embedded in the carbon-based matrix component,

[0031] wherein the silicon-based particulate component includes a plurality of core-shell structures, wherein each core-shell structure includes:

[0032] A silicon core,

[0033] An intermetallic layer covering the core, and

[0034] A graphite shell surrounding the silicon core and the intermetallic layer.

[0035] 2. The composite electrode material according to Embodiment 1, wherein the silicon-based particulate component accounts for 10% to 90% by weight of the composite electrode material.

[0036] 3. The composite electrode material according to Embodiment 1, wherein the plurality of core-shell structures are uniformly distributed throughout the carbon-based matrix component.

[0037] 4. The composite electrode material according to Embodiment 1, wherein the silicon core comprises greater than 99% silicon (Si) by weight.

[0038] 5. The composite electrode material according to Embodiment 1, wherein the intermetallic layer comprises a metal silicide, and wherein the metal silicide comprises at least one metal selected from copper, nickel, iron, or cobalt.

[0039] 6. The composite electrode material according to Embodiment 1, wherein in each core-shell structure, the intermetallic layer is disposed between the silicon core and the graphite shell.

[0040] 7. The composite electrode material according to Embodiment 1, wherein the graphite shell comprises crystalline graphite.

[0041] 8. The composite electrode material according to Embodiment 1, wherein the carbon-based matrix component comprises amorphous hard carbon.

[0042] 9. The composite electrode material according to Embodiment 1, wherein the carbon-based matrix component does not include discrete particles or regions of crystalline graphite.

[0043] 10. A method of manufacturing a composite electrode material, the method comprising:

[0044] providing a plurality of silicon particles, wherein each silicon particle has a surface;

[0045] depositing a metal catalyst layer on the surface of each silicon particle to form a plurality of precursor structures;

[0046] dispersing the precursor structures in an organic polymer material to form a precursor electrode material; and

[0047] heating the precursor electrode material in an inert environment: (i) to convert the organic polymer material into a carbon-based material, (ii) to convert at least a portion of the metal catalyst layer on the surface of each silicon particle into a metal silicide; and (iii) to form a graphite shell around each silicon particle.

[0048] 11. The method according to Embodiment 10, wherein the plurality of silicon particles exhibit an average particle size of 10 nanometers to 40 micrometers.

[0049] 12. The method according to embodiment 10, wherein the metal catalyst layer is deposited on the surface of each silicon particle using a wet chemical deposition technique, a chemical vapor deposition technique, or a high energy ball milling technique.

[0050] 13. The method according to embodiment 10, wherein the metal catalyst layer comprises at least one metal selected from copper, nickel, iron, or cobalt.

[0051] 14. The method according to embodiment 10, wherein the metal catalyst layer has a thickness of 2 nanometers to 200 nanometers.

[0052] 15. The method according to embodiment 10, wherein the organic polymer material comprises polyimide or polyacrylonitrile.

[0053] 16. The method according to embodiment 10, wherein the precursor electrode material is heated in an inert environment at a temperature of 400 °C to 900 °C to pyrolyze the organic polymer material.

[0054] 17. The method according to embodiment 10, wherein during the heating of the precursor electrode material in an inert environment, the metal catalyst layer on the surface of each silicon particle promotes the formation of a graphite shell around each silicon particle.

[0055] 18. The method according to embodiment 10, wherein during the heating of the precursor electrode material in an inert environment, the metal catalyst layer on the surface of each silicon particle physically separates each silicon particle from the organic polymer material and prevents the formation of silicon carbide (SiC).

[0056] 19. A method of manufacturing a composite negative electrode material for a lithium ion battery pack, the method comprising:

[0057] providing a plurality of silicon particles, wherein each silicon particle has a surface;

[0058] depositing a metal catalyst layer on the surface of each silicon particle to form a plurality of precursor structures;

[0059] dispersing the precursor structures in an organic polymer material to form a precursor electrode material; and

[0060] heating the precursor electrode material in an inert environment to pyrolyze the organic polymer material and convert the precursor electrode material into a composite electrode material, the composite electrode material comprising a carbon-based matrix component and silicon-based particulate components embedded in the carbon-based matrix component,

[0061] Wherein the silicon-based particulate component comprises a plurality of core-shell structures, wherein each core-shell structure comprises a silicon core, an intermetallic layer covering the core, and a graphite shell surrounding the silicon core and the intermetallic layer.

[0062] 20. The method according to embodiment 19, wherein the intermetallic layer comprises a metal silicide, and wherein the metal silicide comprises at least one metal selected from copper, nickel, iron, or cobalt.

[0063] The above summary is not intended to represent every possible embodiment or every aspect of the present disclosure. Rather, the foregoing summary is intended to illustrate some of the novel aspects and features disclosed herein. When taken in conjunction with the drawings and the appended claims, the above features and advantages of the present disclosure, as well as other features and advantages, will become apparent from the following detailed description of representative embodiments and modes for carrying out the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Embodiments will be described below with reference to the drawings, where like reference numerals denote like elements, and wherein:

[0065] Figure 1 is a schematic side cross-sectional view of an electrochemical cell for a secondary lithium-ion battery pack, which includes a negative electrode, a positive electrode, and a non-aqueous electrolyte in ionic contact with the negative electrode and the positive electrode;

[0066] Figure 2 is Figure 1 an enlarged cross-sectional view of a portion of the negative electrode of, depicting a negative current collector and a portion of a composite negative electrode material layer covering the negative current collector, the composite negative electrode material layer comprising a carbon-based matrix component and a silicon-based particulate component embedded in the carbon-based matrix component;

[0067] Figure 3 is according to one embodiment of the present disclosure Figure 2 an enlarged cross-sectional view of a portion of the silicon-based particulate component of the composite negative electrode material layer of; and

[0068] Figure 4 is according to another embodiment of the present disclosure Figure 2 an enlarged cross-sectional view of a portion of the silicon-based particulate component of the composite negative electrode material layer of.

[0069] The present disclosure may permit modifications and alternative forms, where representative embodiments are shown by way of example in the drawings and described in detail below. The inventive aspects of the present disclosure are not limited to the particular forms disclosed. Rather, the present disclosure is intended to cover modifications, equivalents, combinations, and alternative forms falling within the scope of the present disclosure as defined by the appended claims. DETAILED DESCRIPTION

[0070] The composite electrode material of the present disclosure includes a carbon-based matrix component and silicon-based particulate components embedded or dispersed in the carbon-based matrix component. The silicon-based particulate components are composed of a large number of discrete particles or regions dispersed throughout the carbon-based matrix component, wherein each discrete region exhibits a core-shell structure, and the core-shell structure includes a silicon core, an intermetallic layer covering the silicon core, and a graphite shell surrounding the silicon core and the intermetallic layer. Compared with electrode materials that do not include silicon, the silicon-based particulate components endow the composite electrode material with a relatively high specific capacity, and compared with electrode materials that include silicon particles but do not include silicon particles with an intermetallic layer formed thereon, the composite electrode material has excellent mechanical robustness, improved capacity retention, and increased cycle life.

[0071] Before forming the composite electrode material of the present disclosure, a large number of silicon particles are provided, and a metal catalyst layer (i.e., a layer of Cu, Ni, Fe, or Co and / or its alloy) is deposited on the surface of each silicon particle to form a large number of precursor structures. Subsequently, the precursor structures are dispersed in an organic polymer material to form a precursor electrode material, the precursor electrode material is heat-treated to pyrolyze the organic polymer material, and the precursor electrode material is converted into a composite electrode material.

[0072] Without intending to be bound by theory, it is believed that during the heat treatment process, the metal catalyst layer promotes the formation of a graphite carbon shell around each silicon particle, rather than the formation of non-graphite carbon (which would be formed near and around each silicon particle if the metal catalyst layer were absent). Compared with non-graphite carbon, the graphite carbon shell formed around each silicon particle can exhibit an improved ability to absorb the volume expansion experienced by the silicon particles during the lithiation process. In addition, during the heat treatment process, at least a portion of each metal catalyst layer can react with the underlying silicon particle associated therewith to produce a metal silicide, which can contribute to improving the electronic and particle conductivity of the composite electrode material. In addition, forming a metal catalyst layer on the silicon particles can also help prevent an undesirable chemical reaction from occurring between the silicon particles and the surrounding carbon-based matrix component, which could otherwise lead to the undesirable formation of silicon carbide (SiC). Given the following disclosure, those of ordinary skill in the art will readily understand these and other benefits.

[0073] Figure 1FIG. 0 shows a schematic cross-sectional view of an electrochemical cell 10 of a secondary lithium metal battery pack (not shown). The electrochemical cell 10 includes a negative electrode 12, a positive electrode 14, and a non-aqueous electrolyte 16 that is in ionic contact with the negative electrode 12 and the positive electrode 14. The negative electrode 12 includes a negative current collector 18 and an electrochemically active negative electrode material layer 20 that covers the negative current collector 18. The positive electrode 14 includes a positive current collector 22 and an electrochemically active positive electrode material layer 24 that covers the positive current collector 22. The positive and negative electrodes 12, 14 are spaced apart from each other and are physically separable from each other by a porous separator (not shown) during assembly. During assembly, the negative electrode 12 can be electrically connected to the positive electrode 14 via an external circuit (not shown) such that during cycling of the electrochemical cell 10, electrons can flow between the negative electrode and the positive electrodes 12, 14 while lithium ions travel through the non-aqueous electrolyte 16 between the negative electrode material layer 20 and the opposing positive electrode material layer 24.

[0074] Now referring to Figure 2 , the negative electrode material layer 20 exhibits a composite structure 26 that includes a carbon-based matrix component 28 and a silicon-based particulate component 30 that is embedded in or dispersed within the carbon-based matrix component 28. The matrix component 28 and the particulate component 30 exist as separate and discrete phases within the composite structure 26 of the negative electrode material layer 20 and have different chemical compositions and different chemical and mechanical properties. Accordingly, the matrix component 28 and the particulate component 30 each contribute a separate set of desirable attributes or characteristics to the negative electrode material layer 20 that can be combined to improve the overall performance of the electrochemical cell 10. For example, compared to an electrochemical cell that does not include a negative electrode material layer having the composite structure 26 of the present disclosure, the combination of the matrix component 28 and the particulate component 30 imparts improved mechanical integrity and robustness to the negative electrode material layer 20 and also imparts increased energy density and cycle stability to the electrochemical cell 10.

[0075] The carbon-based matrix component 28 can account for from about 5-40% by weight of the overall negative electrode material layer 20, and the silicon-based particulate component 30 can account for from about 60-95% by weight of the overall negative electrode material layer 20. In some embodiments, the matrix component 28 can account for from about 10-30% by weight of the negative electrode material layer 20, and the particulate component 30 can account for from about 70-90% by weight of the negative electrode material layer 20. As used herein, the phrase “overall negative electrode material layer 20” and “overall composite structure 26” denote the same thing.

[0076] The carbon-based matrix component 28 can include a continuous monolithic three-dimensional network in which the particulate component 30 is embedded or dispersed. The term “monolithic” refers to a solid three-dimensional structure that is not inherently particulate. The carbon-based matrix component 28 can be conductive and electrochemically active. For example, the carbon-based matrix component 28 can have the ability to undergo reversible insertion or intercalation of lithium ions.

[0077] The carbon-based matrix component 28 is a carbon-based material and can be derived from the pyrolysis of an organic polymer material. As used herein, the terms "carbon-based material" and "carbon material" refer to materials composed primarily of carbon, meaning that carbon is the single largest component of the material based on the overall weight of the material. This can include materials containing greater than 50% carbon (C) by weight, as well as those containing less than 50% carbon (C) by weight, provided that carbon (C) is the single largest component. In some embodiments, the carbon-based matrix component 28 can contain greater than 75% carbon, preferably greater than 90% carbon, and more preferably greater than 99% carbon by weight. The carbon-based matrix component 28 can contain one or more non-metallic elements (such as oxygen, hydrogen, and / or nitrogen), which can be present as residual by-products of the pyrolysis of the organic polymer material in the carbon-based matrix component 28. Such non-metallic elements can be present in the carbon-based matrix component 28 in relatively small amounts, such as less than 25%, preferably less than 10%, and more preferably less than 1% by weight based on the overall carbon-based matrix component 28.

[0078] The carbon-based matrix component 28 can contain amorphous hard carbon. The term "hard carbon" refers to a non-graphitizable carbon material, meaning that at elevated temperatures (such as temperatures greater than 1500 °C), the carbon material will remain substantially amorphous and cannot be transformed into crystalline graphite via heat treatment. On the other hand, "soft" carbon can be transformed into polycrystalline graphite when heated at such temperatures. In some embodiments, prior to lithiation, the carbon-based matrix component 28 can consist essentially of amorphous hard carbon and thus be substantially free of other forms (i.e., allotropes) of carbon. Specific allotropes of carbon that are preferably excluded from the carbon-based matrix component 28 include graphite. In some embodiments, the carbon-based matrix component 28 can contain greater than 75% amorphous hard carbon by weight.

[0079] In some embodiments, the carbon-based matrix component 28 can be porous, for example, and can exhibit a porosity of 1 - 70%, preferably 5 - 50%, and more preferably 10 - 40%.

[0080] The silicon-based particulate component 30 is a major component of the negative electrode material layer 20 and includes a large number of discrete particles or three-dimensional regions 32 in the composite structure 26 of the negative electrode material layer 20, which exhibit chemical and / or mechanical properties different from those of the surrounding carbon-based matrix component 28. As Figure 3As shown, the discrete particles or regions 32 of the silicon-based particulate component 30 that make up the negative electrode material layer 20 may each exhibit a core-shell structure that includes a silicon core 34, an intermetallic layer 36 that covers the core 34, and a graphite shell 38 that surrounds the core 34. The silicon-based particulate component 30 of the negative electrode material layer 20 will be further described herein with respect to one of its discrete particles or regions 32; however, it is to be understood that such description is equally applicable to all discrete particles or regions 32 of the composite structure 26 of the silicon-based particulate component 30 that make up the negative electrode material layer 20.

[0081] The silicon core 34 is electrochemically active and is capable of storing lithium ions in its structural framework during charging of the electrochemical cell 10 and is equally capable of releasing lithium ions therefrom during discharging of the electrochemical cell 10. The silicon core 34 is in indirect physical, electrical, and ionic contact with the carbon-based matrix component 28 of the negative electrode material layer 20 via the intermetallic layer 36 and the graphite shell 38. The silicon core 34 may be amorphous and / or crystalline and may contain, by weight, greater than 90%, preferably greater than 95%, and more preferably greater than 99% silicon prior to lithiation.

[0082] The intermetallic layer 36 covers the surface 40 of the silicon core 34 and physically separates the silicon core 34 from the graphite shell 38 and the surrounding carbon-based matrix component 28. In some embodiments, the silicon core 34 may be completely encapsulated by the intermetallic layer 36. Without intending to be bound by theory, it is believed that the intermetallic layer 36 may contribute to maintaining the mechanical integrity of the silicon core 34 during repeated lithiation and delithiation cycles and may also help prevent the formation of silicon carbide (SiC) in the region 32, which may contribute to maintaining the electrical conductivity and ionic conductivity of the negative electrode material layer 20.

[0083] The intermetallic layer 36 is formulated to exhibit good electrical conductivity and ionic conductivity and may comprise a metal, a metal alloy, and / or a metal silicide. For example, the intermetallic layer 36 may comprise copper (Cu), nickel (Ni), iron (Fe), or cobalt (Co), and / or the intermetallic layer 36 may comprise an alloy of Cu, Ni, Fe, and / or Co. Additionally or alternatively, the intermetallic layer 36 may comprise a metal silicide having the formula MeSi X where Me = Cu, Ni, Fe, and / or Co. In some embodiments, the intermetallic layer 36 may contain at least one metal silicide in an amount greater than 40%, preferably greater than 60%, and more preferably greater than 80% by weight of the overall intermetallic layer 36. In some embodiments, the intermetallic layer 36 may consist essentially of at least one metal silicide. In other embodiments, as Figure 4As best shown, the intermetallic layer 136 can be formed on the surface 40 of the silicon core 34 and includes an inner metal silicide layer 142 and an outer metal layer 144. In such cases, the inner metal silicide layer 142 can comprise or consist essentially of at least one metal silicide, and the outer metal layer can comprise or consist essentially of Cu, Ni, Fe, and / or Co, and / or can comprise or consist essentially of an alloy of Cu, Ni, Fe, and / or Co.

[0084] The intermetallic layers 36, 136 can have a thickness of from 2 nanometers to 200 nanometers.

[0085] The graphite shell 38 surrounds the silicon core 34 and the intermetallic layers 36, 136 and physically separates the silicon core 34 and the intermetallic layers 36, 136 from the surrounding carbon-based matrix component 28. The graphite shell 38 can comprise a graphite material. A "graphite material" refers to a material having a graphite surface with a hexagonal arrangement of carbon atoms and can include any material having a graphite surface, regardless of the physical, chemical, or structural properties of such material. Examples of graphite materials include crystalline graphite and highly oriented pyrolytic graphite (HOPG).

[0086] The graphite shell 38 can have a thickness of from 1 nanometer to 5 micrometers and can have a porosity of from 0.1% to 50%.

[0087] The negative electrode material layer 20 can be formed by a method including one or more of the following steps: (a) providing a plurality of silicon particles, (b) depositing a metal catalyst layer on the surface of each silicon particle to form a precursor structure in the form of a large number of particles, (c) dispersing the precursor structure in an organic polymer material to form a precursor electrode material, and (d) heating the precursor electrode material in an inert environment to pyrolyze the organic polymer material and convert the precursor electrode material into a composite electrode material. The silicon particles provided in step (a) can exhibit an average particle size of from 10 nanometers to 40 micrometers and can comprise greater than 90%, preferably greater than 95%, and more preferably greater than 99% silicon by weight.

[0088] The metal catalyst layer deposited on the surface of each silicon particle in step (b) can comprise a metal or a metal alloy. For example, the metal catalyst layer can comprise Cu, Ni, Fe, and / or Co, and / or the catalyst layer can comprise an alloy of Cu, Ni, Fe, and / or Co. The metal catalyst layer can be deposited on the surface of each silicon particle using a wet chemical deposition method, a physical vapor deposition method, a high energy ball milling method, or any other method capable of forming a thin continuous layer of a metal or a metal alloy on the surface of each silicon particle. The metal catalyst layer deposited on the surface of each silicon particle can exhibit a thickness of from 2 nanometers to 200 nanometers.

[0089] In embodiments where a wet chemical deposition method is employed, electroless deposition or electroplating techniques can be used. Electroless deposition techniques can be used, for example, to form a thin continuous layer of a metal or metal alloy on the surface of each silicon particle by immersing the silicon particles in a solution containing a salt of the metal to be deposited on the silicon particles and then adding a reducing agent (such as formaldehyde) thereto. The solution for the electroless deposition method can also contain a complexing agent, a buffer, an exaltant, and / or a stabilizer. Electroplating techniques can be used, for example, to form a thin continuous layer of a metal or metal alloy on the surface of each silicon particle by immersing the silicon particles in an electrolyte solution containing a salt of the metal to be deposited on the silicon particles and then applying a direct current to the silicon particles to reduce the metal ions in the electrolyte solution to the zero-valent state on the surface of the silicon particles. Examples of physical vapor deposition methods that can be used to deposit a metal catalyst layer on the surface of each silicon particle include cathodic arc deposition, electron beam physical vapor deposition, evaporation deposition, pulsed laser deposition, sputtering deposition, and pulsed electron beam deposition. In embodiments where a high-energy ball milling method is employed, the silicon particles can be combined with nano-sized particles of Cu, Ni, Fe, and / or Co in a ball mill.

[0090] In step (c), the precursor structure formed in step (b) is dispersed in an organic polymer material to form a precursor electrode material. The precursor structure can be dispersed in the organic polymer material, for example, by mixing the precursor structure with the organic polymer material. The organic polymer material can include an organic polymer or a combination of organic polymers. Organic polymers that can be included in the organic polymer material of the precursor electrode material include polyimide and / or polyacrylonitrile. In some embodiments, the organic polymer material can also include a solvent. In such cases, the solvent can account for 10-90% by weight of the organic polymer material. The precursor structure can account for 0.01-20% by weight of the overall precursor electrode material, and the organic polymer material can account for 0.01-20% by weight of the overall precursor electrode material.

[0091] Before step (d), the precursor electrode material can be deposited on the surface of a substrate in the form of a continuous film. In some embodiments, the substrate can include a metal foil or a metal mesh. For example, the substrate can include a metal foil or a metal mesh having the same chemical composition as the negative current collector 18. In such cases, the negative electrode material layer 20 can be formed and bonded to the negative current collector 18 during the pyrolysis of the organic polymer material in step (d). In other embodiments, the substrate can include a template made of an inert material that does not react with the precursor electrode material and / or interfere with the pyrolysis of the organic polymer material during step (d). In such cases, after step (d) is completed, the composite electrode material can be removed from the template and bonded to the negative current collector 18 to form the negative electrode 12 of the electrochemical cell 10.

[0092] In step (d), the precursor electrode material can be heated in an inert, oxygen-free environment (e.g., in argon, nitrogen, and / or hydrogen) and / or in an environment below atmospheric pressure at a temperature and for a duration sufficient to pyrolyze the organic polymer material and convert the precursor electrode material into the composite structure 26 of the negative electrode material layer 20. For example, the precursor electrode material can be heated in an inert environment and / or in an environment below atmospheric pressure at a temperature greater than or equal to 400 °C and less than or equal to 900 °C to pyrolyze the organic polymer material. Without being bound by theory, it is believed that the presence of a metal catalyst layer on the surface of the silicon particles can lower the temperature at which the organic polymer material can be heated to effect its complete pyrolysis. For example, in some embodiments, the pyrolysis of the organic polymer material can be carried out at a temperature greater than 600 °C and less than 800 °C. More specifically, in some embodiments, the pyrolysis of the organic polymer material can be carried out at a temperature greater than 700 °C and less than 750 °C.

[0093] During the pyrolysis of the organic polymer material in step (d), the organic polymer material thermally decomposes, carbon-heteroatom bonds are broken, volatile organic compounds and hydrocarbon radicals are released, and new carbon-carbon bonds are formed, thereby converting the organic polymer material into a solid carbon-based material. Some of the organic polymer material adjacent to and / or in physical contact with the location of the metal catalyst layer on the surface of the silicon particles will be converted into a graphite carbon-based material (i.e., the graphite shell 38). On the other hand, the remaining portion of the organic polymer material that is neither in physical contact nor adjacent to the metal catalyst layer will be converted into a continuous phase of a non-graphite carbon-based material (i.e., the carbon-based matrix component 28). Without being bound by theory, it is believed that the presence of a metal catalyst layer on the surface of the silicon particles can facilitate the conversion of the portion of the organic polymer material in physical contact with or in close proximity to it into a graphite carbon-based material rather than a non-graphite carbon-based material. During the course of step (d), the portion of the organic polymer material that is not in sufficiently close proximity to the location of the metal catalyst layer on the surface of the silicon particles will be converted into a non-graphite carbon-based material rather than a graphite carbon-based material.

[0094] Without being bound by theory, it is believed that the presence of a metal catalyst layer on the surface of the silicon particles can help to avoid a chemical reaction between the organic polymer material and the silicon particles during the pyrolysis of the organic polymer material in step (d). More specifically, it is believed that the presence of a metal catalyst layer on the surface of the silicon particles can inhibit or prevent the undesired formation of silicon carbide (SiC) in the negative electrode material layer 20. It is believed that the formation of SiC can impede the flow of electrons and lithium ions in the negative electrode material layer 20. Thus, by inhibiting or preventing the formation of SiC in the negative electrode material layer 20, the metal catalyst layer on the surface of the silicon particles can allow for the formation of a negative electrode material layer 20 having improved electrical conductivity and ionic conductivity compared to the composite negative electrode material formed without the use of a metal catalyst layer.

[0095] Heating the precursor electrode material in step (d) can also cause at least a portion of the metals in the metal catalyst layer (i.e., Cu, Ni, Fe, and / or Co) to react with the silicon in the underlying silicon particles to form an intermetallic metal silicide, which can be represented by the chemical formula MeSi X where Me = Cu, Ni, Fe, and / or Co. The metal catalyst layer can be partially or completely converted to an intermetallic metal silicide during step (d). The proportion of the metal in the metal catalyst layer that reacts with the silicon in the underlying silicon particles to form a metal silicide can depend, for example, on the thickness of the metal catalyst layer and / or the duration of the pyrolysis process.

[0096] The non-aqueous electrolyte 16 can comprise any material capable of effectively conducting lithium ions between the negative electrode and the positive electrodes 12, 14. For example, the non-aqueous electrolyte 16 can comprise a liquid electrolyte. In such cases, the electrolyte 16 can comprise a solution comprising a lithium salt dissolved or ionized in a non-aqueous aprotic organic solvent or a mixture of non-aqueous aprotic organic solvents. Lithium salts that can be used to fabricate the electrolyte 16 include LiClO4, LiAlCl4, LiI, LiBr, LiSCN, LiBF4, LiB(C6H5)4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiPF6, and mixtures thereof. The non-aqueous aprotic organic solvent in which the lithium salt is dissolved can be a cyclic carbonate (i.e., ethylene carbonate, propylene carbonate), an acyclic carbonate (i.e., dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate), an aliphatic carboxylic acid ester (i.e., methyl formate, methyl acetate, methyl propionate), a γ-lactone (i.e., γ-butyrolactone, γ-valerolactone), a cyclic ether (i.e., 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane), a cyclic ether (i.e., tetrahydrofuran, 2-methyltetrahydrofuran), or a mixture thereof. As another example, the non-aqueous electrolyte 16 can comprise a gel or a plasticized polymer electrolyte. In such cases, the electrolyte 16 can comprise a polymer matrix material impregnated with a liquid electrolyte solution. Examples of the polymer matrix material include polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyethylene oxide (PEO), polyacrylate, and poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP).

[0097] When present, the porous separator disposed between the negative and positive electrodes 12, 14 can comprise any material that physically separates and electrically insulates the electrodes 12, 14 from each other while allowing lithium ions to flow freely therebetween. For example, the porous separator can include a nonwoven material or a microporous polymer material. In particular, the porous separator can comprise a single polyolefin or a combination of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and / or polyvinyl chloride (PVC). In one form, the porous separator can comprise a laminate of one or more polymer materials, such as a laminate of PE and PP.

[0098] The negative and positive current collectors 18, 22 associated with the negative electrode material layer 20 and the positive electrode material layer 24, respectively, can comprise any material capable of collecting and reversibly transferring free electrons to and from their respective electrode material layers 20, 24. For example, the negative and positive current collectors 18, 22 can each comprise a conductive metal or metal alloy, such as a transition metal or an alloy thereof. In some specific examples, the negative current collector 18 can comprise copper, nickel, an iron alloy (such as stainless steel), or titanium, and the positive current collector 22 can comprise aluminum, nickel, or an iron alloy (such as stainless steel). Other conductive metals can of course be used if desired. The negative and positive current collectors 18, 22 can each be in the form of a thin and flexible porous or nonporous metal substrate. For example, the negative and positive current collectors 18, 22 can be in the form of a thin and flexible nonporous metal foil, a porous metal mesh, or a perforated metal plate. The specific configuration of the negative and positive current collectors 18, 22 can depend on the intended application of the electrochemical cell 10.

[0099] The positive electrode material layer 24 can comprise one or more electrochemically active materials that can undergo a reversible redox reaction with lithium at an electrochemical potential higher than that of the negative electrode material layer 20, such that an electrochemical potential difference exists between the positive electrode material layer 24 and the negative electrode material layer 20. In one form, the positive electrode material layer 24 can comprise an insertion matrix material in the form of a metal oxide that can undergo reversible insertion or intercalation of lithium ions. In such cases, the insertion matrix material of the positive electrode material layer 24 can comprise a layered oxide represented by the formula LiMeO2, an olivine-type oxide represented by the formula LiMePO4, a spinel-type oxide represented by the formula LiMe2O4, one or both of the hydroxytavorite represented by the formula LiMeSO4F or LiMePO4F, or a combination thereof, where Me is a transition metal (such as Co, Ni, Mn, Fe, Al, V, or a combination thereof). For example, the insertion matrix material can comprise a layered lithium transition metal oxide, such as lithium cobalt oxide (LiCoO2) and lithium-nickel-manganese-cobalt oxide [Li(Ni XMn Y Co Z )O2], spinel lithium transition metal oxides, such as spinel lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4) or lithium iron fluorophosphate (Li2FePO4F), lithium nickel oxide (LiNiO2), lithium aluminum manganese oxide (Li X Al Y Mn 1-Y O2), lithium vanadium oxide (LiV2O5), or combinations thereof. In another form, the positive electrode material layer 24 may comprise a conversion material, the conversion material including components that can undergo a reversible electrochemical reaction with lithium, wherein the components undergo a phase change or a change in crystal structure as the oxidation state changes. In such cases, the conversion material of the positive electrode material layer 24 may include sulfur, selenium, tellurium, iodine, halides (such as fluorides or chlorides), sulfides, selenides, tellurides, iodides, phosphides, nitrides, oxides, oxysulfides, oxyfluorides, sulfur fluorides, sulfur oxyfluorides, or lithium and / or metal compounds thereof. Metals suitable for inclusion in the conversion material of the positive electrode material layer 24 include iron, manganese, nickel, copper, and cobalt. The electrochemically active material of the positive electrode material layer 24 may be commingled with a polymeric binder material to provide a positive electrode material layer 24 having structural integrity. Examples of polymeric binders include polyvinylidene fluoride (PVdF), ethylene propylene diene monomer (EPDM) rubber, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid, and mixtures thereof. The positive electrode material layer 24 may optionally include particles of a conductive material, which may comprise very fine particles such as high surface area carbon black.

[0100] Although some best modes and other embodiments have been described in detail, there are various alternative designs and embodiments for practicing the teachings defined in the appended claims. Those skilled in the art will recognize that the disclosed embodiments may be modified without departing from the scope of the present disclosure. Additionally, this concept expressly includes combinations and subcombinations of the described elements and features. The detailed description and the drawings support and describe the teachings, the scope of which is defined only by the claims.

Claims

1. A method of manufacturing a composite electrode material, the method comprising: providing a plurality of silicon particles, wherein each silicon particle has a surface; depositing a metal catalyst layer on the surface of each silicon particle to form a plurality of precursor structures; dispersing the precursor structures in an organic polymer material to form a precursor electrode material; and heating the precursor electrode material in an inert environment: (i) to convert the organic polymer material into a carbon-based material, (ii) to convert at least a portion of the metal catalyst layer on the surface of each silicon particle into a metal silicide; and (iii) to form a graphite shell around each silicon particle.

2. The method according to claim 1, wherein the plurality of silicon particles exhibit an average particle size of 10 nanometers to 40 micrometers.

3. The method according to claim 1, wherein the metal catalyst layer is deposited on the surface of each silicon particle using a wet chemical deposition technique, a chemical vapor deposition technique, or a high-energy ball milling technique.

4. The method according to claim 1, wherein the metal catalyst layer comprises at least one metal selected from copper, nickel, iron, or cobalt.

5. The method according to claim 1, wherein the metal catalyst layer has a thickness of 2 nanometers to 200 nanometers.

6. The method according to claim 1, wherein the organic polymer material comprises polyimide or polyacrylonitrile.

7. The method according to claim 1, wherein the precursor electrode material is heated in an inert environment at a temperature of 400°C to 900°C to pyrolyze the organic polymer material.

8. The method according to claim 1, wherein during the heating of the precursor electrode material in an inert environment, the metal catalyst layer on the surface of each silicon particle promotes the formation of a graphite shell around each silicon particle.

9. The method according to claim 1, wherein during the heating of the precursor electrode material in an inert environment, the metal catalyst layer on the surface of each silicon particle physically separates each silicon particle from the organic polymer material and prevents the formation of silicon carbide (SiC).

Images