Carbon-coated silcon-graphite composite anode material for rechargeable lithium-ion batteries and its preparation method

Abstract

The present disclosure relates to a carbon-coated silicon-graphite composite anode material. The present disclosure also relates to a method for preparing the carbon-coated silicon-graphite composite anode material. The present disclosure also provides a lithium-ion coin cell. The carbon coating of the Si-Graphite composite material binds the Si nanoparticles on the graphite matrix during the lithiation / delithiation reaction, enhancing the electrochemical cycling stability of the Si-Graphite anode material, which meets the essential criteria for a Li-ion battery anode.

Description

[Technical Field]

[0001] The present disclosure relates to a carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries. The present disclosure also relates to a method for preparing the carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries. The present disclosure also discloses a lithium-ion coin cell. [Background technology]

[0002] The background discussion includes information that may be useful in understanding the present invention. No admission is made that any of the information provided herein is prior art or relevant to the present invention, or that any publication specifically or implicitly referenced is prior art.

[0003] In recent years, the need for rechargeable batteries in electronic devices, from mobile phones to smartwatches to eco-friendly e-vehicles, has led to a surge in demand for lithium-ion batteries due to their best electrochemical performance.

[0004] Graphite is a successful anode material for lithium-ion batteries. However, its theoretical capacity is 372 mAh / g, and commercially available materials reach a practical maximum level of closer to 350-365 mAh / g. This indicates the need for the development of alternative negative active materials with higher capacities.

[0005] Among all anode materials, group IV Si is the first choice for negative anode material, and the Li of Si alloys with Li during lithiation / delithiation. 4.4The lithiated Si phase is an excellent choice for anode materials, exhibiting a high theoretical capacity of 4300 mAh / g. However, Si exhibits poor life cycles due to its large expansion during lithiation / delithiation and a poorly stable SEI layer, resulting in reduced chronon efficiency. The approximately 300% volume expansion during the alloying of Li with Si and the crushing of the active material due to the large mechanical stress maximizes contact loss between the active materials, increases charge transfer resistance, and exhibits high capacity fading, resulting in poor electrochemical performance and a poor life cycle.

[0006] These factors limit the use of Si active anode materials in lithium-ion batteries. The above issues can be addressed by various approaches, such as reducing particle size to the nanometer level, such as Si nanoparticles, Si nanowires, and Si dispersed in a material matrix, as well as thin films of Si. Generally, carbon-based conductive material matrices are used to improve conductivity by accommodating volume expansion during electrochemical reactions, contributing to better cycling stability with high cell capacity.

[0007] Various methods for producing Si-graphite composites and their drawbacks include directly mixing nano-Si particles into graphite as a negative active material composite. However, the Si nanoparticles are not properly mixed into the composite, resulting in agglomeration of the Si particles, resulting in poor electrochemical performance. In another modified manufacturing process, a metal / metal oxide film of the negative electrode material is coated onto a substrate by magnetron sputtering deposition. The Si / carbon composite method involves mixing silicon particles with pyrolytic carbon from an organic compound, which undergoes carbonization, cracking, and fracture to improve electrochemical performance. The carbon coating method, which coats silicon alloys with a conductive carbon layer, provides sufficient void space for volume expansion during the lithiation / delithiation reaction. However, the carbon coating method requires high temperatures, up to 1200°C, for carbonization. As the carbon coating temperature increases, the crystallinity of the active material increases, rapidly causing volume expansion problems and poor electrochemical performance. Different methods have been proposed in the prior art for producing Si-graphite composites as negative materials for batteries.

[0008] There are several methods for dispersing Si nanoparticles on a conductive material matrix. The solid-phase mixing method involves dispersing Si nanoparticles on a graphite layer with Si nanoparticles on the surface, produced by mechanochemical milling using pitch as a binder. Dispersing nanosilicon into primary particles would be difficult using dry dispersion; it may require very high energy dispersion, which could damage the graphite particles. Si nanoparticles placed on the surface of graphite layers produced by solid-phase mixing have a negative effect on electrochemical performance if the Si and graphite particles do not come into intimate contact with the graphite, and a large amount of Si particles are exposed to the electrolyte. The electrochemical reaction occurring on the surface only increases the thickness of the SEI layer, which acts as a blocking layer, and volume expansion occurs with subsequent cycles. Further electrochemical reactions and capacity fade reduce cycling stability.

[0009] CN103730644B discloses a method for preparing a silicon-silicon oxide-carbon composite anode material for lithium ion batteries by mixing silicon oxide, silicon, and graphite ball milled pitch, followed by high-temperature heat treatment (500-1100°C) to obtain a silicon oxide-carbon composite anode element for lithium ion batteries. However, the life cycle efficiency is low.

[0010] Chemical vapor deposition of silicon nanoparticles on the surface of a graphite layer is another method for producing silicon-graphite composites. US20200148545 discloses the synthesis of carbon-coated silicon oxide graphite composites by chemical vapor deposition. This method requires higher capital costs for scale-up production.

[0011] CN105895873A discloses a Si-carbon composite anode material prepared by inductively coupled plasma (ICP) technology. Nano-Si and organic carbon sources are dispersed and mixed with graphite for carbonization. An induction plasma system, processed in a high-temperature environment at 5000-12000°C, gasifies the Si powder, producing gaseous Si. This process is expensive and difficult to scale for large-scale production.

[0012] A liquid-phase dispersion method is used to fabricate Si-graphite composites by embedding Si nanoparticles in the graphite layers, voids, and cracks. This liquid-phase method has advantages over other methods because it ultimately results in a more dispersed raw material. The third phase material, i.e., an amorphous carbon coating tightly bonded to the nano-Si-graphite composite, enhances the conductivity of the active material, improving electrochemical reactions. In the liquid-phase method, Si nanoparticles embedded in the graphite voids and pores and well-dispersed on the graphite layers increase the active sites for electrochemical reactions. Furthermore, the carbon coating on the Si-graphite composite prevents direct contact between the nano-Si and the electrolyte.

[0013] CN106328898A discloses a composite anode material for lithium-ion batteries via a template method. NaCl, artificial graphite, SiO, and an organic compound used as a carbon source are mixed into a homogeneous solution and then carbonized at 500-1200°C in an inert atmosphere to form a composite anode material. In this template method, temperature plays an important role in controlling the morphology; increasing the temperature rapidly increases the crystallinity of the particles, and the concentration of the organic compound limits the thickness of the carbon coating.

[0014] To overcome these obstacles, a novel method of the present invention is implemented to fabricate carbon conductive layer coated Si-graphite composite materials as negative active materials for energy applications. Summary of the Invention [Problem to be solved by the invention]

[0015] An object of the present invention is to provide a method for preparing a carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries.

[0016] Another object of the present disclosure is to provide a carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries.

[0017] Yet another object of the present disclosure is to provide a lithium-ion coin cell. [Brief explanation of the drawings]

[0018] The accompanying drawings, which are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the disclosure.

[0019] [Figure 1] A process flow diagram is shown.

[0020] [Figure 2]1 shows the surface morphology of carbon-coated Si-graphite composites.

[0021] [Figure 3] 1 shows elemental mapping of Example 1 (black: carbon; green: Si).

[0022] [Figure 4] 1 shows elemental mapping of Comparative Example 1 (black: carbon; green: Si).

[0023] [Figure 5] 1 shows elemental mapping of Comparative Example 2 (black: carbon; green: Si).

[0024] [Figure 6] Electrochemical studies of Si-Graphite composites are presented.

[0025] Means to solve the problem This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify all key features or essential features of the claimed subject matter, nor is it intended to be used solely as an aid in determining the scope of the claimed subject matter.

[0026] An aspect of the present disclosure is a method for preparing a carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries, comprising the steps of: i) dispersing 2-15% w / v silicon nanoparticles in a polar solvent, followed by ultrasonication to obtain a dispersed solution; ii) adding 30-50% w / v coal tar oil into the dispersed solution under stirring to obtain a first mixture; iii) adding 5-30% w / v carbon source to the first mixture under stirring to obtain a second mixture; and iv) homogenizing the second mixture to obtain a second mixture. v) dispersing 85-98% w / v graphite in a solvent and subsequently adding it to the homogenized mixture of step iv) to obtain a second homogenized mixture; vi) separating the second homogenized mixture by fractional distillation to evaporate the solvent and the coal tar oil, as well as a silicon-graphite-pitch composite residue; and vii) carbonizing the silicon-graphite-pitch composite residue to obtain a carbon-coated silicon-graphite composite anode material.

[0027] Another aspect of the present disclosure is to provide a carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries, comprising 2-15 wt. % silicon nanoparticles, 5-30 wt. % carbon, and 85-98 wt. % graphite.

[0028] Another aspect of the present disclosure is to provide a lithium-ion coin cell including a copper foil current collector and a slurry including 80-90% carbon-coated silicon-graphite composite anode material, 4-8% by weight of a conductive additive, 3-5% by weight of a dispersant, and 5-7% by weight of a binder, wherein the slurry is uniformly coated on the copper foil current collector.

[0029] Various objects, features, aspects and advantages of the present subject matter will become more apparent from the following detailed description of preferred embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0030] DETAILED DESCRIPTION OF THE INVENTION The detailed description of the invention is provided below. The embodiments are in sufficient detail to clearly communicate the present disclosure. However, the amount of detail provided is not intended to limit the possible variations of the embodiments; rather, the intent is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure, as defined by the appended claims.

[0031] Unless the context clearly requires otherwise, throughout this specification and the claims that follow, the word "comprise" and variations thereof, such as "comprises" and "comprising," are to be construed in an open, inclusive sense, i.e., "including, but not limited to."

[0032] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. Furthermore, the term "or" is generally used in its sense including "and / or" unless the content clearly dictates otherwise.

[0033] In some embodiments, numbers expressing characteristics, such as amounts of ingredients, concentrations, reaction conditions, and the like, used to describe and claim particular embodiments of the invention should be understood to be modified in some instances by the term "about." Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that may vary depending upon the desired properties sought to be obtained by a particular embodiment. In addition, the numerical parameters set forth herein should be construed in light of the number of reported significant digits, numerical precision, and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as possible.

[0034] Recitation of ranges of values ​​herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, and unless otherwise indicated herein, each separate value is incorporated herein as if it were individually set forth herein.

[0035] All methods described herein may be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. Any and all examples or exemplary language (e.g., "such as") provided herein are intended to better illustrate embodiments of the present disclosure and do not pose a limitation on the scope of the present disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the present disclosure.

[0036] The headings used herein are for convenience only and do not interpret the scope or meaning of the claimed embodiments.

[0037] The following description provides many exemplary embodiments of the inventive subject matter. Although each embodiment represents a single combination of the inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment includes elements A, B, and C and a second embodiment includes elements B and D, the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0038] All publications and patents referenced in this disclosure are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. If a definition or use of a term in an incorporated reference conflicts with the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0039] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member may be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group may be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is deemed to include the group as modified to meet the following written description and embodiments described herein by illustrating examples of specific embodiments of the principles and aspects of the present disclosure. These examples are offered for purposes of illustration, not limitation, of those principles and the present disclosure.

[0040] It should also be understood that the present invention can be implemented in numerous ways, including as a system, a method, or a device. These implementations, or any other form the present invention may take, may be referred to herein as a process. In general, the order of steps in a disclosed process may be varied within the scope of the present invention.

[0041] Various terms used in this specification are set forth below. Unless a term used in the claims is defined below, it should be given the broadest definition that one skilled in the art has given that term, as reflected in printed publications and issued patents at the time of filing.

[0042] A method for preparing a carbon-coated silicon graphite composite anode material for a rechargeable lithium-ion battery according to an embodiment of the present disclosure, comprising the steps of: i) dispersing 2-15% w / v silicon nanoparticles in a polar solvent, followed by ultrasonic treatment to obtain a dispersed solution; ii) adding 30-50% w / v coal tar oil into the dispersed solution under stirring to obtain a first mixture; iii) adding 5-30% w / v carbon source to the first mixture under stirring to obtain a second mixture; and iv) homogenizing the second mixture to obtain a second mixture. FIG. 1 shows a manufacturing process flow of the method, which includes the steps of obtaining a first homogenized mixture; v) dispersing 85-98% w / v graphite in a solvent and subsequently adding it to the homogenized mixture of step iv) to obtain a second homogenized mixture; vi) separating the second homogenized mixture by fractional distillation to evaporate the solvent and the coal tar oil, as well as a silicon-graphite-pitch composite residue; and vii) carbonizing the silicon-graphite-pitch composite residue to obtain a carbon-coated silicon-graphite composite anode material.

[0043] In a preferred embodiment, a method for preparing a carbon-coated silicon graphite composite anode material for rechargeable lithium-ion batteries includes the steps of: i) dispersing 5-10% w / v silicon nanoparticles in a polar solvent, followed by ultrasonic treatment to obtain a dispersed solution; ii) adding 30-50% w / v coal tar oil into the dispersed solution and stirring to obtain a first mixture; iii) adding 10-20% w / v carbon source to the first mixture and stirring under conditions to obtain a second mixture; iv) adding 10-20% w / v carbon source to the first mixture and stirring under conditions to obtain a second mixture; The method includes homogenizing the mixture of step 2 to obtain a first homogenized mixture; v) dispersing 90-95% w / v graphite in a solvent and then adding it to the homogenized mixture of step iv) to obtain a second homogenized mixture; vi) separating the second homogenized mixture by fractional distillation to evaporate the solvent and coal tar oil and obtain a silicon-graphite-pitch composite residue; and vii) carbonizing the silicon-graphite-pitch composite residue to obtain a carbon-coated silicon-graphite composite negative electrode material.

[0044] In one embodiment, the polar solvent is selected from any aqueous protic or aprotic solvent having a dielectric constant between 5 and 40 and high miscibility, compatibility with coal tar, and combinations thereof.

[0045] In one embodiment, the ultrasonic treatment in step i) is carried out for a period ranging from 30 to 180 minutes. Preferably, the ultrasonic treatment period is 60 minutes.

[0046] In one embodiment, the stirring in step ii) is carried out continuously for a period ranging from 30 to 180 minutes. Preferably, the stirring is carried out for 60 minutes.

[0047] In one embodiment, the carbon source in step iii) is selected from the group consisting of crushed coal tar, petroleum tar pitch, resin, and combinations thereof. Preferably, the carbon source is pitch.

[0048] In one embodiment, the conditions in step iii) comprise a temperature in the range of 50-100° C. for a period in the range of 30-180 minutes. Preferably, the conditions comprise a temperature of 80° C. for 60 minutes.

[0049] In one embodiment, the homogenization in step iv) is carried out for a period ranging from 30 to 180 minutes at a temperature ranging from 80 to 100° C. Preferably, the homogenization is carried out at a temperature of 90° C. for 60 minutes.

[0050] In one embodiment, the solvent in step v) is selected from the polar group consisting of acetone, ethanol, isopropyl alcohol, and n-butanol, and combinations thereof. Solvents having a dielectric constant of 10-30, such as acetone, ethanol, isopropyl alcohol, and n-butanol, and combinations thereof, can be used. Preferably, the solvent has a dielectric constant of 20-30, and the solvent is completely miscible with the coal tar-based oil.

[0051] In one embodiment, the graphite is dispersed in the solvent in step v) for a period ranging from 5 to 15 minutes. Preferably, the graphite is dispersed in the solvent for 10 minutes.

[0052] In one embodiment, the homogeneous mixture is fractionally distilled from room temperature (about 20-35°C) to 300°C. In one embodiment, the solvent in step vi) is evaporated at a temperature in the range of 70-90°C, and the coal tar oil is removed at a temperature in the range of 250-270°C. Preferably, the solvent is evaporated at 80°C, and the coal tar oil is removed at 260°C.

[0053] In one embodiment, the silicon-graphite-pitch composite residue is carbonized in step vii) at a temperature in the range of 600-1200° C., preferably 600-1000° C., under inert atmospheric conditions, which include Ar, N2 and / or CO2.

[0054] In one embodiment, the carbon coated silicon-graphite composite of step vii) has a size in the range of 15-20 μm.

[0055] Another aspect of the present disclosure is to provide a carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries, comprising 2-15 wt. % silicon nanoparticles, 5-30 wt. % carbon, and 85-98 wt. % graphite.

[0056] In a preferred embodiment, the carbon-coated silicon graphite composite anode material for rechargeable lithium-ion batteries comprises 2-8 wt. % silicon nanoparticles, 10-20 wt. % carbon, and 88-92 wt. % graphite.

[0057] Another aspect of the present disclosure is to provide a lithium-ion coin cell including a copper foil current collector and a slurry including 80-90% carbon-coated silicon-graphite composite anode material, 4-8% by weight of a conductive additive, 3-5% by weight of a dispersant, and 5-7% by weight of a binder, wherein the slurry is uniformly coated on the copper foil current collector.

[0058] In a preferred embodiment, a lithium-ion coin cell comprises a copper foil current collector and a slurry comprising 84 wt % carbon-coated silicon-graphite composite anode material, 6 wt % conductive additive, 4 wt % dispersant, and 6 wt % binder, the slurry being uniformly coated on the copper foil current collector.

[0059] In one embodiment, the conductive additive is carbon black, carbon nanotubes (CNTs), s ) and reduced graphene oxide (RGO s ) and combinations thereof. The conductive additive is selected from materials having high electronic conductivity. Preferably, the conductive additive is carbon black, carbon nanotubes (CNTs), or the like. s ), and reduced graphene oxide (RGO s ), and combinations thereof.

[0060] In one embodiment, the dispersant is selected from the group consisting of carboxymethylcellulose, N-methylpyrrolidine, and combinations thereof. The dispersant is selected from any aqueous or non-aqueous-based surfactant material and combinations thereof that tend to form structures or aggregates called micelles in the bulk aqueous or non-aqueous phase. Preferred are dispersants such as carboxymethylcellulose, N-methylpyrrolidine, and combinations thereof.

[0061] In one embodiment, the binder is selected from the group consisting of styrene butadiene (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and combinations thereof. The binder is selected from any aqueous or non-aqueous based polymer. The polymer has high elasticity, strong adhesive properties, self-healing properties, ionic or electronic conductivity, and combinations thereof. Preferably, the binder used is styrene butadiene (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and combinations thereof.

[0062] This disclosure includes a method for preparing a carbon-coated Si-graphite composite anode for rechargeable lithium-ion batteries. This process involves dispersing nanosilicon into a homogenous slurry, then adding a carbon binder to fix it to the graphite surface, and coating the silicon / graphite with carbon in the same step. The carbon coating on the silicon and graphite is achieved through the following steps: dispersion and homogenization of nanosilicon in a nonpolar solvent / tar-based oil, followed by the addition of graphite, followed by a second homogenization to disperse the nanosilicon on the graphite slurry. The mixture is then distilled to remove the solvent, and the composite is then carbonized to produce a graphite-Si composite electrode material. After carbonization, the final product is ground into a fine powder and investigated as an anode material for lithium-ion batteries. This slurry production process involves multiple mixing steps and sequences to achieve good homogenization, resulting in a carbon-coated Si-graphite composite with well-dispersed / embedded Si nanoparticles in the graphite matrix without agglomeration. The carbon coating of the Si-Graphite composite binds the Si nanoparticles on the graphite matrix during the lithiation / delithiation reaction, enhancing the electrochemical cycling stability of the Si-Graphite anode material, which meets the essential criteria for Li-ion battery anodes.

[0063] While the foregoing describes various embodiments of the present disclosure, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof. The present invention is not limited to the described embodiments, versions or examples which, when combined with information and knowledge available to those skilled in the art, are included to enable those skilled in the art to make and use the invention. [Example]

[0064] The present invention is further described in the form of the following examples. However, it should be understood that the following examples are merely illustrative and should not be construed as limitations on the scope of the present invention. Example 1

[0065] 5-10% w / v Si nanoparticles were dispersed in a polar solvent. The solution was sonicated / homogenized for 60 minutes to disperse the primary Si nanoparticles, resulting in a dispersion solution. 30-50% (w / v%) coal tar oil was added to the dispersion solution and stirred continuously for 60 minutes to obtain a first mixture. 10-20% (wt%) well-ground coal tar as a carbon source was added to the first mixture containing coal tar oil and nanosilicon, and stirred at 70°C for 60 minutes to obtain a second mixture. The second mixture was homogenized at 90°C for 60 minutes to obtain homogenized solution A. 90-95% (wt.% or gm) graphite was dispersed in acetone for 10 minutes and added to A to obtain homogenized mixture B. Homogeneous mixture B was fractionally distilled from room temperature to 300°C. Acetone was evaporated at 80°C, and the coal tar oil was removed at 260°C. All coal oil was removed, and the residue, a silicon-graphite-pitch composite, was recovered. The silicon-graphite-pitch composite was carbonized under inert conditions at 600-1000°C to obtain a carbon-coated silicon-graphite composite. The composite was crushed and sieved to prepare electrodes (lithium-ion coin cells) for testing. The electrodes (lithium-ion coin cells) were prepared from a copper foil current collector and a slurry containing 80-85 wt% carbon-coated silicon-graphite composite anode material, 4-6 wt% conductive additive, 3-4 wt% dispersant, and 5-6 wt% binder. The slurry was uniformly coated onto the copper foil current collector to obtain the electrode. 15 mm diameter electrodes were cut and assembled as 2032 coins using lithium metal as the reference electrode. Add a mixture of 1% 1,3-propane sultone and 10% fluoroethylene carbonate as an electrolyte additive with commercial electrolyte (EC / DMC / DEC (4:3:3) - VC < 2%). Comparative Example 1

[0066] A composite of 5-10% (by weight) nanosilicon and 90-95% graphite was prepared by solid-state mixing (direct method) to obtain a Si-graphite composite. An electrode was prepared by mixing a copper foil current collector with a slurry containing 80-85% by weight of carbon-coated silicon-graphite composite anode material, 4-6% by weight of conductive additive, 2-4% by weight of dispersant, and 3-6% by weight of binder. The slurry was uniformly coated on the copper foil current collector to obtain an electrode. 15 mm diameter electrodes were cut and assembled as 2032 coins using lithium metal as the reference electrode. A commercially available electrolyte (EC / DMC / DEC (4:3:3)-LiPF6 with <2% VC) was used, and a mixture of 1% 1,3-propane sultone and 10% fluoroethylene carbonate was added as an electrolyte additive. Comparative Example 2

[0067] 10–20% (wt%) of well-ground coal tar as a carbon source was added to coal tar oil and stirred at 70°C for 60 minutes to obtain a first homogenized mixture. 90–95% graphite was dispersed in a polar solvent for 10 minutes and added to the first homogenized solution to obtain a second homogenized mixture. The second mixture was homogenized at room temperature for 20 minutes and then added to the first homogenized solution. The resulting homogenized mixture was fractionally distilled from room temperature to 300°C. The polar solvent was evaporated at 80°C, and the coal tar oil was removed at 260°C. After removing all the coal oil, the residue, graphite pitch residue, was recovered and carbonized under inert conditions at 600–1000°C. The graphite-pitch composite and 5–10% (wt% or gm) Si nanoparticles were dispersed in a polar solvent at 10–50% (wt% / v). The graphite-pitch composite and Si mixture were sonicated / homogenized for 20 minutes to disperse the primary Si nanoparticles. The dispersion solution was then dried at 110 °C to remove the solvent. The composite was crushed and sieved into a fine powder to fabricate electrodes for testing. The electrodes were prepared from a copper foil current collector and a slurry containing 80-85 wt% silicon-graphite composite anode material, 4-6 wt% conductive additive, 2-4 wt% dispersant, and 3-6 wt% binder. The slurry was uniformly coated onto the copper foil current collector to obtain the electrode. 15 mm diameter electrodes were cut and assembled as 2032 coins, with lithium metal as the reference electrode. A commercially available electrolyte (EC / DMC / DEC (4:3:3)-LiPF6 with <2% VC) was used, with a mixture of 1% 1,3-propane sultone and 10% fluoroethylene carbonate added as an electrolyte additive. Advantages of the Invention

[0068] This liquid-phase composite method for preparing high-performance carbon-coated composite anode materials is easy to scale up and requires low capital investment. The electrochemical performance of the liquid-phase silicon-graphite composite preparation method is significantly improved over that of the solid-phase physical mixing method. The carbon-coated Si-graphite composite can be used as an anode material for energy storage devices, with improved electrochemical stability for long cycle life.

[0069] Second, Si-graphite composite anode materials were prepared by this method using low-viscosity polar solvents, such as acetone and coal tar oil, to produce a homogeneous mixture of well-dispersed / embedded Si nanoparticles in the graphite matrix. Nano-Si particles were well-dispersed in the graphite matrix and occupied the voids and spaces via the liquid-phase dispersion method (Example 1). The uniform dispersion of Example 1 is shown in the SEM image shown in Figure 2. Element mapping analysis of Example 1 and Comparative Examples 1 and 2, respectively, is shown in Figures 3 to 5. Si mapping analysis of materials made from solid-state physical mixing (Comparative Example 1) and the liquid-phase method in which Si was dispersed after carbonization (Comparative Example 2) is shown in Figure 4. This indicates that more Si is dispersed and agglomerated on the surface of the graphite. Figure 4 (Comparative Example 2) shows better dispersion of Si particles in the graphite via the solvent dispersion. However, Figure 3 shows less Si on the surface of the graphite and more dispersed particles embedded in the graphite matrix occupying the spaces and voids between the graphite layers. These non-agglomerated, well-dispersed Si nanoparticles improve the electrochemical performance of the Si-graphite composite in the subsequent capacity cycling reaction of lithiation / delithiation, as shown in Figure 6. Figure 6 shows carbon-coated 5% Si-graphite composite active materials prepared by different synthetic routes (Example 1, Comparative Examples 1 and 2) as anodes for lithium-ion coin cells. The composites synthesized by different routes as negative materials exceed 500 mAh / g in the initial cycles. Comparative Example 1 gradually decreases in capacity with subsequent cycles, showing 80% capacity retention at 120 cycles. Comparative Example 2 provides better electrochemical stability than Comparative Example 1, showing 80% capacity retention after 150 cycles. Here, in Example 1, the composite provides much better electrochemical stability than the comparative examples. Example 1 shows a high capacity retention of approximately 90% even after 300 cycles. This proves that the Si dispersion method in the graphite matrix is ​​a key part of achieving better electrochemical stability by the liquid-phase dispersion method. Furthermore, the dispersion of Si particles at an appropriate stage of synthesis is beneficial to avoid direct surface contact of Si nanoparticles with the electrolyte, which would result in capacity fade.

[0070] Third, the pores and voids within the graphite matrix provide enough space to accommodate the volume expansion caused by the lithiated phase of Si, resulting in a loss of contact between the negative active material and the Si dispersed on the graphite, while the voids and pores in the graphite maintain electrical conductivity.

[0071] Fourth, the carbon coating of the silicon-graphite composite provides high electronic conductivity for the active material. This electronic conductivity of the top layer of the Si-Graphite composite forms a good electronic network, allowing the Li + Reduces the intergranular distance for ion migration.

[0072] Fifth, the carbon coating on the Si-graphite composite provides an electrolyte barrier, and the internal void space of the carbon-coated Si-graphite composite provides space for the large-volume expansion of Si nanoparticles during lithiation / delithiation, enhancing the cycling stability of the Si-graphite composite. Therefore, the carbon coating acts as a conductive network that neutralizes the internal resistance increased by the volume expansion and provides a conductive framework between the Si-graphite composite anode.

[0073] The foregoing examples are merely illustrative and should not be construed as limitations on the scope of the present invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the scope of the present invention.

Claims

1. 1. A method for preparing a carbon-coated silicon-graphite composite anode material for a rechargeable lithium-ion battery, comprising: i) dispersing 2-15% w / v silicon nanoparticles in a polar solvent, followed by ultrasonic treatment to obtain a dispersed solution; ii) adding 30-50% w / v coal tar oil into the dispersion solution while stirring to obtain a first mixture; iii) adding 5-30% w / v of a carbon source to the first mixture under stirring to obtain a second mixture; iv) homogenizing the second mixture to obtain a first homogenized mixture; v) dispersing 85-98% w / v graphite in a solvent and subsequently adding it to the homogenized mixture of step iv) to obtain a second homogenized mixture; vi) separating the second homogenized mixture by fractional distillation to evaporate the solvent and the coal tar oil, as well as a silicon-graphite-pitch composite residue; and vii) carbonizing the silicon-graphite-pitch composite residue to obtain a carbon-coated silicon-graphite composite anode material.

2. 10. The method of claim 1, wherein the polar solvent is selected from any aqueous protic or aprotic solvent having a dielectric constant between 5 and 40 and high miscibility, compatibility with coal tar, and combinations thereof.

3. 10. The method of claim 1, wherein the ultrasonic treatment in step i) is carried out for a period ranging from 30 to 180 minutes.

4. 10. The method of claim 1, wherein the stirring in step ii) is carried out continuously for a period ranging from 30 to 180 minutes.

5. 10. The method of claim 1, wherein the carbon source in step iii) is selected from the group consisting of ground coal tar, petroleum tar pitch, resin, and combinations thereof.

6. 10. The method of claim 1, wherein the conditions of step iii) comprise a temperature in the range of 50 to 100°C for a period in the range of 30 to 180 minutes.

7. 2. The method of claim 1, wherein the homogenization in step iv) is carried out for a period ranging from 30 to 180 minutes at a temperature ranging from 80 to 100°C.

8. 10. The method of claim 1, wherein the solvent in step v) is selected from the polar group consisting of acetone, ethanol, isopropyl alcohol, and n-butanol, and combinations thereof.

9. 10. The method of claim 1, wherein the graphite is dispersed in the solvent in step v) for a period ranging from 5 to 15 minutes.

10. 2. The process of claim 1, wherein the solvent in step vi) is evaporated at a temperature in the range of 70 to 90°C and the coal tar oil is removed at a temperature in the range of 250 to 270°C.

11. 2. The method of claim 1, wherein the silicon-graphite-pitch composite residue is carbonized in step vii) at a temperature in the range of 600 to 1200° C. under inert atmospheric conditions.

12. 10. The method of claim 1, wherein the carbon-coated silicon-graphite composite of step vii) has a size in the range of 15 to 20 μm.

13. 2-15 wt% silicon nanoparticles; 5 to 30% by weight carbon; and A carbon-coated silicon-graphite composite anode material for rechargeable lithium-ion batteries comprising 85-98 wt. % graphite.

14. a copper foil current collector; a slurry comprising 80-90 wt % of the carbon-coated silicon-graphite composite anode material of claim 13, 4-8 wt % of a conductive additive, 3-5 wt % of a dispersant, and 5-7 wt % of a binder, the slurry being uniformly coated on a copper foil current collector.

15. 15. The lithium-ion coin cell of claim 14, wherein the conductive additive is selected from the group consisting of carbon black, carbon nanotubes (CNT), and reduced graphene oxide (RGO), and combinations thereof.

16. 15. The lithium-ion coin cell of claim 14, wherein the dispersant is selected from the group consisting of carboxymethyl cellulose, N-methylpyrrolidine, and combinations thereof.

17. 15. The lithium ion coin cell of claim 14, wherein the binder is selected from the group consisting of styrene butadiene (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and combinations thereof.

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