Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, lithium ion secondary battery, and manufacturing methods thereof

Abstract

To provide a negative electrode material for a lithium ion secondary battery, having good cycle characteristics while using silicon with a high charge capacity, and a negative electrode for a lithium ion secondary battery using the negative electrode material.SOLUTION: Problems can be solved by using a negative electrode material that includes a composite of crushed graphite particles with an average particle size of 0.1-30 μm and porous silicon particles. In addition, by combining the negative electrode material with a current collector that has a graphene layer formed thereon, cycle characteristics can be improved furthermore.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and methods for manufacturing these.

Background Art

[0002] From the perspective of meeting a wide range of energy demands such as consumer batteries for mobile electronic devices, etc., batteries for transportation such as electric vehicles, electric buses, and unmanned aerial vehicles (drones), and large batteries such as large-scale energy storage systems, lithium ion secondary batteries have been attracting attention for a long time. In the development of lithium ion secondary batteries, in order to achieve further increased capacity and higher energy density, active studies have been conducted on the electrode materials and electrolytes that make up the batteries. One such study is the search for negative electrode materials using silicon particles. A negative electrode material composed of a silicon-based material is said to have a theoretical capacity density of 4200 mAh / g, which is more than 11 times the theoretical capacity density (372 mAh / g) of the currently most widely used graphite negative electrode. For this reason, silicon has been regarded as a promising negative electrode material capable of realizing increased capacity of lithium ion secondary batteries. -1 However, there are also technical problems with negative electrode materials composed of silicon-based materials. Silicon contained in the negative electrode of a lithium ion secondary battery takes in lithium ions during charging to form a lithium silicon alloy represented by Li4Si, and releases lithium ions during discharging to return to silicon. During this process, silicon is said to undergo a volume change of more than 300%, and when this large volume change occurs, cracks and fractures occur in the negative electrode containing silicon, significantly deteriorating the cycle characteristics of the lithium ion secondary battery. -1 )

[0003]

[0004] In order to solve such problems, it has been proposed to use porous silicon particles as the negative electrode material (see, for example, Patent Document 1). Since the silicon particles contained in the negative electrode are porous, the volume increase accompanying the uptake of lithium ions into the silicon particles is absorbed in such a way that the pores provided in the silicon particles are filled, while the volume decrease accompanying the release of lithium ions is compensated in the form of regeneration of the filled pores. As a result, the volume change of the entire negative electrode is suppressed, cracks and fractures in the negative electrode are suppressed, and thus it is possible to suppress a significant impairment of the cycle characteristics of the lithium ion secondary battery.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, a protective film layer (SEI; solid electrolyte interface), which is a reduction decomposition product of the electrolyte solution, is usually formed on the surface of the negative electrode of a lithium ion secondary battery. This is formed during the initial charging of the lithium ion secondary battery, and the presence of the electrically insulating SEI on the negative electrode surface suppresses further decomposition of the electrolyte solution. On the other hand, as described above, when the negative electrode cracks due to volume change, new silicon particles not covered by the SEI are exposed to the electrolyte solution, and thus an SEI layer is formed on the surface thereof when charged. By repeating this crack formation and SEI layer formation, the electrically insulating SEI layer enters between the silicon particles, and the electrical conductivity between these particles decreases. Then, the degree of uptake and release of lithium ions in the negative electrode decreases, leading to performance degradation such as a decrease in the capacity and current of the secondary battery.

[0007] The present invention has been made in view of the above circumstances, and an object thereof is to provide a negative electrode material for a lithium ion secondary battery having good cycle characteristics while using silicon with a large charge capacity, and a negative electrode for a lithium ion secondary battery using the same.

Means for Solving the Problems

[0008] As a result of intensive studies to solve the above problems, the present inventors have found that these problems can be solved by using a negative electrode material comprising a composite of crushed graphite particles or graphene nanoplatelets having an average particle size of 0.1 μm to 30 μm and porous silicon particles, and have completed the present invention. Specifically, the present invention provides the following.

[0009] (1) The present invention relates to a negative electrode for a lithium ion secondary battery comprising a current collector and a negative electrode material for a lithium ion secondary battery provided on the surface thereof, wherein the current collector has a single-layer graphene layer on its surface, and on the surface of the single-layer graphene layer in the current collector, the average particle size is 0.1 μm to 30 μm Crushed graphite particles and a composite with porous silicon particles and the crushed graphite particles contain graphene flakes A negative electrode for a lithium ion secondary battery comprising a negative electrode material for a lithium ion secondary battery characterized by comprising.

[0010] (2) Further, the present invention is characterized in that the mass ratio of the porous silicon particles (Si) to the above Crushed graphite particles (G) is Si:G = 5:1 to 5:5, which is the negative electrode for a lithium ion secondary battery according to item (1).

[0011] (3) Further, the present invention is characterized in that the porous silicon particles are surface-coated with the above Crushed graphite particles The negative electrode for a lithium ion secondary battery according to item (1) or (2).

[0012] (4) The present invention The lithium-ion secondary battery also includes, as a negative electrode, the negative electrode for a lithium-ion secondary battery according to any one of items (1) to (3).

[0014] (5) The present invention includes a dispersion step of subjecting a mixture of graphite and a solvent to treatment by an external force to obtain a dispersion through a step of crushing the graphite in the solvent, the dispersion obtained in the dispersion step, porous silicon particles, a binder, and water, at least, to prepare a slurry of a negative electrode active material mixture, and a coating step of coating the slurry on the surface of a current collector having a single-layer graphene layer formed on the surface and drying the slurry. The present invention is also a method for manufacturing a negative electrode for a lithium-ion secondary battery, characterized by comprising these steps.

[0015] (6) The present invention is Graphene nanoplatelet By subjecting a mixture of [graphite] and a solvent to treatment by an external force, a dispersion step of obtaining a dispersion in which the [graphite] is dispersed in the solvent, a mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion obtained in the dispersion step, porous silicon particles, a binder, and water, and a coating step of coating the slurry on the surface of a current collector having a single-layer graphene layer formed on its surface and drying it. It is also a method for manufacturing a negative electrode for a lithium-ion secondary battery, characterized by comprising: Disperse the graphene nanoplatelet

[0016] (7) Further, the present invention is a method for manufacturing a negative electrode for a lithium-ion secondary battery according to the above, wherein the treatment by the external force is ultrasonic treatment. (5) or (6)

[0017] (8) Further, the present invention is a method for manufacturing a negative electrode for a lithium-ion secondary battery according to the above, characterized in that the graphite is expanded graphite flakes. (5)

[0018] (9) The present invention subjects a mixture of graphene nanoplatelets, porous silicon particles, and a solvent to treatment by an external force to obtain a dispersion step of obtaining a dispersion in which the graphene nanoplatelets and the porous silicon particles are dispersed in the solvent, a mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion obtained in the dispersion step, a binder, and water, and having a single-layer graphene layer formed on the surface A coating step of coating and drying the slurry on the surface of a current collector. It is also a method for manufacturing a negative electrode for a lithium-ion secondary battery, characterized by comprising:

Advantages of the Invention

[0020] It should be noted that the text seems to be incomplete in some parts where specific substances or operations are not fully described. The above translation is based on the existing text as accurately as possible.According to the present invention, there is provided a negative electrode material for a lithium ion secondary battery having good cycle characteristics while using silicon with a large charge capacity, and a negative electrode for a lithium ion secondary battery using the same.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0022] Hereinafter, one embodiment of the negative electrode material for a lithium ion secondary battery of the present invention, one embodiment of the negative electrode for a lithium ion secondary battery, one embodiment of the lithium ion secondary battery, the first embodiment and the second embodiment of the manufacturing method of the negative electrode material for a lithium ion secondary battery, and one embodiment of the manufacturing method of the negative electrode for a lithium ion secondary battery will be described. Note that the present invention is not limited to these embodiments or embodiments, and can be appropriately modified and implemented within the scope of the present invention.

[0023] <Negative Electrode Material for Lithium Ion Secondary Battery> First, an embodiment of the negative electrode material for a lithium-ion secondary battery of the present invention will be described. The negative electrode material for a lithium-ion secondary battery of the present invention (hereinafter, also simply referred to as the negative electrode material) is characterized by including a composite of crushed graphite particles or graphene nanoplatelets having an average particle diameter of 0.1 μm to 30 μm and porous silicon particles. The crushed graphite particles and graphene nanoplatelets have conductivity, and when the porous silicon particles, which are the negative electrode active material, are combined with such a conductive material, high electron conduction is realized between the negative electrode active material and the current collector, and the volume expansion of silicon accompanying the storage of lithium ions is alleviated. As a result, the negative electrode material of the present invention exhibits excellent specific charge and discharge capacity and good cycle characteristics. In particular, as a part of the graphite is crushed, it may be converted into graphene flakes, which are two-dimensional materials. When the porous silicon particles, which are the negative electrode active material, are surrounded by the crushed graphite particles containing such graphene flakes or graphene nanoplatelets, which are a type of graphene flakes, the above effects become more prominent.

[0024] The porous silicon particles contained in the negative electrode material of the present invention have a diameter of approximately 50 to 2000 nm and belong to the region from so-called nanoparticles to microparticles. As this diameter, 50 to 1000 nm is preferably mentioned.

[0025] Further, the porous silicon particles have a plurality of surface pores. As the diameter of the surface pores, 10 to 100 nm is preferably mentioned, 10 to 80 nm is more preferably mentioned, and 10 to 70 nm is even more preferably mentioned. Also, as the porosity of the silicon particles, about 30 to 60% is preferably mentioned, and about 30 to 50% is more preferably mentioned. The porosity is a parameter that can be obtained from, for example, BET nitrogen adsorption and density analysis, and indicates the ratio of the hollow part of the porous material to the total volume. By providing such surface pores in the porous silicon particles, which are the negative electrode material, the generation of cracks due to volume changes during charge and discharge can be suppressed in the negative electrode containing the same, and the cycle characteristics of the secondary battery can be improved.

[0026] The porous silicon particles may be undoped silicon or doped silicon doped with impurities. When using doped silicon, preferred impurities for doping include Group 13 or Group 15 elements. By including such impurities in the porous silicon particles, the conductivity of the silicon particles is improved and the cycle characteristics are enhanced. Examples of Group 13 elements include boron. Examples of Group 15 elements include phosphorus and arsenic. Among these, phosphorus is preferably mentioned. The dopant concentration in the porous silicon particles is preferably 0.01 atom% to 3 atom%, more preferably 0.05 atom% to 1 atom%, and even more preferably 0.05 atom% to 0.3 atom%.

[0027] The crushed graphite particles are prepared by crushing graphite, and their average particle size is 0.1 μm to 30 μm. The average particle size is preferably 0.1 μm to 10 μm, more preferably 0.1 μm to 5 μm, and even more preferably 0.5 to 3 μm. The means for crushing graphite is not particularly limited, but preferably, ultrasonic vibration can be applied to the graphite for crushing. Note that, as described above, it is preferable that the crushed graphite contains graphene formed by exfoliation of graphite. The crushed graphite containing graphene flakes can be obtained, for example, by crushing commercially available expanded graphite flakes by ultrasonic vibration.

[0028] Graphene nanoplatelets are graphene flakes, and various grades are commercially available. Instead of crushed graphite particles, such graphene nanoplatelets may form a composite. Although the thickness of graphene nanoplatelets is only a few nanometers, their size in the width direction, i.e., the average particle size, is on the order of micrometers. In fact, those with an average particle size ranging from 0. several micrometers to dozens of micrometers are commercially available. Therefore, even when using graphene nanoplatelets, as described above, those with an average particle size of 0.1 μm to 30 μm are used. Note that as the average particle size, 0.1 μm to 10 μm is preferably mentioned, 0.1 μm to 5 μm is more preferably mentioned, and 0.5 to 3 μm is even more preferably mentioned.

[0029] The mass ratio of porous silicon particles (Si) to crushed graphite particles or graphene nanoplatelets (G) in the composite is preferably Si:G = 5:1 to 5:5, and more preferably Si:G = 5:2 to 5:5.

[0030] <Negative electrode for lithium-ion secondary battery> The negative electrode for a lithium-ion secondary battery to which the negative electrode material of the present invention is applied (hereinafter, also simply referred to as the negative electrode) is also one of the present inventions. The negative electrode of the present invention is provided with the negative electrode material of the present invention on the graphene layer of a current collector having a graphene layer on its surface. By providing a graphene layer on the surface of the current collector, the electron conduction between the negative electrode material containing crushed graphite particles or graphene nanoplatelets and the current collector becomes smooth, and excellent specific charge-discharge capacity and good cycle characteristics are realized.

[0031] As the current collector, those conventionally used for lithium-ion secondary batteries can be mentioned without particular limitation. Examples of such current collectors include copper foil and aluminum foil.

[0032] The graphene layer provided on the surface of the current collector may be a single layer or multiple layers. To form a graphene layer on the surface of the current collector, it is preferably to use chemical vapor deposition (CVD method) with ethanol, methane, etc. as carbon sources, but it is not particularly limited.

[0033] The composite of porous silicon particles, which are the negative electrode material, and crushed graphite particles or graphene nanoplatelets is coated on the current collector in a state of being mixed with a binder and a conductive auxiliary agent, etc. to form a negative electrode active material layer.

[0034] As the binder, known ones can be listed without particular limitation. Such binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic acid, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers, etc. The content of the binder in the negative electrode active material layer is preferably about 5% by mass to 30% by mass, and more preferably about 5% by mass to 20% by mass. By setting the content of the binder within the above range, the above-mentioned negative electrode material in the form of fine particles can be stably held.

[0035] Examples of the conductive auxiliary agent include carbonaceous materials, metals, conductive ceramics, etc. Examples of the carbonaceous materials include graphitized carbon, non-graphitized carbon, graphene-based carbon, etc. Examples of the non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, carbon black, etc. Examples of the carbon black include furnace black, acetylene black, ketjen black, etc. Examples of the graphene-based carbon include graphene, carbon nanotubes (CNT), fullerenes, etc. Examples of the shape of the conductive auxiliary agent include powder form, fibrous form, etc. As the conductive auxiliary agent, one of these materials may be used alone, or two or more of them may be mixed and used. Further, these materials may be used in a composite form. For example, a material obtained by compositing carbon black and CNT may be used. Among these, carbon black is preferable from the viewpoints of electron conductivity and coatability, and acetylene black is particularly preferable. When a conductive auxiliary agent is used in the negative electrode active material layer, the proportion of the conductive auxiliary agent in the negative electrode active material layer is preferably 15% by mass or less, more preferably 10% by mass or less.

[0036] <Lithium Ion Secondary Battery> A lithium ion secondary battery including the negative electrode for a lithium ion secondary battery of the present invention as a negative electrode is also one of the present inventions. As described above, according to the negative electrode for a lithium ion secondary battery of the present invention, excellent specific charge-discharge capacity and cycle characteristics can be obtained. Therefore, the lithium ion secondary battery of the present invention also exhibits excellent specific charge-discharge capacity and cycle characteristics.

[0037] <First Embodiment of the Method for Manufacturing a Negative Electrode Material for a Lithium Ion Secondary Battery> Next, a first embodiment of the method for manufacturing a negative electrode material for a lithium-ion secondary battery according to the present invention will be described. The method for manufacturing a negative electrode material for a lithium-ion secondary battery according to this embodiment includes subjecting a mixture of graphite or graphene nanoplatelet and a solvent to treatment by an external force to obtain a dispersion liquid through a step of crushing and / or dispersing the graphite or graphene nanoplatelet in the solvent, and a mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion liquid obtained in this dispersion step, porous silicon particles, a binder, and water. That is, the negative electrode material for a lithium-ion secondary battery obtained by this manufacturing method becomes a slurry of a negative electrode active material mixture containing the negative electrode material for a lithium-ion secondary battery according to the present invention described above. Hereinafter, each step will be described.

[0038] [Dispersion step] First, the dispersion step will be described. The dispersion step is a step of obtaining a dispersion liquid through a step of subjecting a mixture of graphite or graphene nanoplatelet and a solvent to treatment by an external force to crush and / or disperse the graphite or graphene nanoplatelet in the solvent. In the following description, "graphite or graphene nanoplatelet" will be appropriately abbreviated as "graphite etc.", and "crushed graphite particles or graphene nanoplatelet" will be appropriately abbreviated as "crushed graphite particles etc.".

[0039] As for graphite, various types can be used without particular limitation. As such graphite, expanded graphite flakes can be preferably used. Graphite has a crystal structure in which turtle shell-like layer structures are bonded by weak van der Waals forces, and other elements or compounds can be inserted (intercalated) between these layer structures. Expanded graphite flakes are carbon materials in which substances such as sulfates are inserted between the layers of flaky graphite, and have the property of easily generating graphite flakes by peeling between layers when subjected to an external force. As described above, in the negative electrode material of the present invention, porous silicon particles are surrounded by crushed graphite particles, so that high electron conduction is realized between the negative electrode active material and the current collector, and the volume expansion of silicon accompanying the storage of lithium ions is alleviated. However, since these crushed graphite particles contain graphene flakes, these effects are further enhanced. Therefore, as the graphite used as a raw material, expanded graphite flakes are preferably used. Since various types of such expanded graphite flakes are commercially available, these commercial products may be obtained and used.

[0040] Graphene nanoplatelets are graphene flakes, and various grades are commercially available. Unlike crushed graphite, graphene nanoplatelets themselves are graphene flakes, so they can effectively surround porous silicon. Therefore, when using graphene nanoplatelets, it is expected that the above effects can be obtained at a higher level than when using crushed graphite.

[0041] When graphite is mixed with a solvent and an external force is applied, it is dispersed in the solvent and converted into crushed graphite particles. Examples of the solvent include various ones such as hydrocarbons, alcohols, water, ethers, N-methyl-2-pyrrolidone, etc. Among these, from the viewpoint of promoting the dispersion of graphite and obtaining crushed graphite particles with a finer particle size, a solvent having a certain viscosity and polarity is preferred, and N-methyl-2-pyrrolidone is preferably mentioned as such a solvent. Note that graphene nanoplatelets may be used instead of graphite. Although graphene nanoplatelets themselves are already graphene flakes with a fine particle size, they are usually in an aggregated state. Therefore, when using them, it is necessary to apply an external force in a state of being mixed with a solvent to disperse them in the solvent, similar to the above-mentioned graphite. The preferred solvent in this case is the same as that for the above-mentioned graphite.

[0042] The amount of the solvent is not particularly limited as long as the graphite or the like to be treated can be sufficiently dispersed. As an example of such an amount of the solvent, about 100 mL of the solvent can be used for 2 g of graphite or the like.

[0043] The mixture of the graphite or the like to be treated and the solvent is subjected to treatment by an external force. As described above, a dispersion liquid in which crushed graphite particles or the like are dispersed in the solvent can be obtained by this treatment by an external force. Examples of such an external force include ultrasonic vibration, kneading, stirring by a stirring device, etc. Among these, treatment by ultrasonic vibration is preferably mentioned, and as a means of such treatment by ultrasonic vibration, treatment by a commercially available ultrasonic homogenizer is preferably mentioned.

[0044] When performing treatment with an ultrasonic homogenizer, a mixture of graphite or the like and a solvent may be contained in an appropriate container, and the crushing horn of a commercially available ultrasonic homogenizer device may be brought into contact with the mixture for crushing. Examples of the crushing conditions using an ultrasonic homogenizer include a maximum amplitude of 60%, an oscillation time (ON / OFF) of 10 seconds / 5 seconds, and a treatment time (cumulative time of oscillation ON) of about 2 hours, but these conditions may be set as appropriate. When performing crushing by ultrasonic vibration, it is preferable to perform the treatment while cooling the container so that the liquid temperature of the mixture does not rise too much.

[0045] By performing treatment with an external force, a dispersion liquid in which crushed graphite particles or the like are dispersed in a solvent is subjected to a mixing step. Note that the crushed graphite particles or the like may be recovered from the dispersion liquid by filtration, heated and dried, then mixed with a solvent, and treatment with an external force may be performed again. By repeating such treatment, crushed graphite particles or the like with a finer particle size can be obtained, and by using such crushed graphite particles or the like, a negative electrode material exhibiting excellent characteristics can be obtained.

[0046] The average particle size of the crushed graphite in the dispersion liquid is preferably 0.1 μm to 30 μm. More preferably, the average particle size is 0.1 μm to 10 μm, still more preferably 0.1 μm to 5 μm, and particularly preferably 0.5 to 3 μm. Further, the average particle size of the graphene nanoplatelets in the dispersion liquid is preferably 0.1 μm to 30 μm. More preferably, the average particle size is 0.1 μm to 10 μm, still more preferably 0.1 μm to 5 μm, and particularly preferably 0.5 to 3 μm.

[0047] [Mixing Step] The mixing step is a step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion liquid obtained in the above dispersion step, porous silicon particles, a binder, and water.

[0048] The porous silicon particles are fine particles of silicon having a plurality of surface pores. As already described, these porous silicon particles have a diameter of approximately 50 to 2000 nm and belong to the region from so-called nanoparticles to microparticles. As this diameter, 50 to 1000 nm is preferably mentioned. Further, as the diameter of the surface pores, 10 to 100 nm is preferably mentioned, 10 to 80 nm is more preferably mentioned, and 10 to 70 nm is even more preferably mentioned. Further, as the porosity of this silicon particle, about 30 to 60% is preferably mentioned, and about 30 to 50% is more preferably mentioned.

[0049] The means for obtaining the porous silicon particles is not particularly limited. As an example of such means, surface porosification treatment of silicon particles by the metal-assisted chemical etching method can be mentioned. In the metal-assisted chemical etching method, transition metal ions and hydrofluoric acid are coexisted in an etching solution to etch silicon particles, and surface pores are formed on the surface of the silicon particles through the following process. Next, an example of the porosification treatment of silicon particles by the metal-assisted chemical etching method will be described.

[0050] In the process of making silicon particles porous by the metal-assisted chemical etching method, first, the silicon particles are immersed in a solution containing transition metal ions and hydrofluoric acid. Then, the hydrofluoric acid dissolves and removes the oxide film (SiO2) covering the surface of the silicon particles. As a result, silicon (Si) is exposed on the surface of the silicon particles, and this silicon comes into contact with the transition metal ions in the solution. Then, the electrons contained in the silicon are transferred to the transition metal ions, and the transition metal ions become particle nuclei of the transition metal and adhere to the surface of the silicon particles on the spot. On the other hand, at the contact part between the particle nuclei of the transition metal and the silicon particles, the silicon loses electrons and is locally oxidized. The locally oxidized silicon is removed by the hydrofluoric acid contained in the solution, and then, at that location, it is further oxidized by the transition metal ions, and the particle nuclei of the transition metal grow. By repeating this reaction, the transition metal particles are inserted into the inside of the silicon particles as if they were digging into the silicon surface on the silicon surface, and pores are formed at that location. When such reactions occur at various locations on the surface of the silicon particles, a plurality of surface pores are formed on the surface of the silicon particles.

[0051] As the silicon particles used as raw materials, commercially available silicon particles may be used, or silicon sludge, which is silicon cutting powder generated when a silicon wafer is cut out, may be used. Further, the silicon particles may be doped silicon in which impurities are doped, or may be undoped. As described above, when doped silicon is used, the impurities for doping preferably include Group 13 or Group 15 elements. Examples of the Group 13 element include boron and the like. Examples of the Group 15 element include phosphorus, arsenic and the like. Among these, phosphorus is preferably mentioned. The dopant concentration in the silicon particles is preferably 0.01 atom% to 3 atom%, more preferably 0.05 atom% to 1 atom%, and even more preferably 0.05 atom% to 0.3 atom%. When porous silicon particles are doped silicon, porous silicon particles may be obtained using the doped silicon as a raw material as described above, or porous silicon particles may be obtained using undoped silicon as a raw material and then doped with impurities.

[0052] The porous treatment of silicon particles by the metal-assisted chemical etching method includes immersing silicon particles in a solution containing hydrofluoric acid and transition metal ions to deposit transition metal particles on the surface of the silicon particles, and along with this deposition, etching the surface of the silicon particles where the contact portion with the transition metal particles is oxidized with hydrofluoric acid in a first etching sub-step, adding an oxidizing agent to the mixture after the first etching step and reacting them while mixing in a second etching sub-step, and removing the transition metal particles from the silicon particles by acid-treating the silicon particles after the second etching step in an acid-treatment sub-step. Hereinafter, these three sub-steps will be described.

[0053] · First etching sub-step The first etching sub-step is a step of immersing silicon particles in a solution containing hydrofluoric acid and transition metal ions to deposit transition metal particles on the surface of the silicon particles, and along with this deposition, etching the surface of the silicon particles where the contact portion with the transition metal particles is oxidized with hydrofluoric acid.

[0054] When performing this small step, an appropriate amount of water may be added to silicon particles to form a mixed solution, and hydrofluoric acid and transition metal ions may be added to this mixed solution. As the amount of the mixed solution in this case, about 150 mL per 1 g of silicon particles can be mentioned. Note that the addition order of hydrofluoric acid and transition metal ions may be either first, and is not particularly limited.

[0055] Examples of the above-mentioned transition metals include copper, silver, gold, iron, etc., and among these, silver is preferably mentioned. These transition metals are added into the mixed solution in the state of transition metal salts and become transition metal ions. Examples of such transition metal salts include hydrochlorides, nitrates, sulfates, etc., and among these, nitrates are preferably mentioned.

[0056] As the concentration of transition metal ions in the mixed solution, 0.7 mmol / L to 30 mmol / L is preferably mentioned, 5 mmol / L to 25 mmol / L is more preferably mentioned, and about 20 mmol / L is most preferably mentioned.

[0057] As the concentration of hydrofluoric acid in the mixed solution, about 2 mol / L to 4 mol / L is preferably mentioned, and about 3 mol / L is more preferably mentioned.

[0058] In the mixed solution to which transition metal ions and hydrofluoric acid are added, an etching reaction proceeds by appropriately stirring at room temperature (about 15°C to 30°C). As the stirring time, about 1 minute can be exemplified, but it is not particularly limited. The obtained mixed solution is subjected to a second small etching step.

[0059] · Second small etching step The second etching sub-step is a step of adding an oxidizing agent to the mixed solution that has undergone the first etching sub-step and reacting them while mixing. In the first etching sub-step, as described above, silicon particles are locally oxidized by the exchange of electrons between transition metal ions and silicon particles, and then the oxidized silicon is dissolved by hydrofluoric acid, resulting in local etching. However, as transition metal particles are generated on the surface of silicon particles, the supply of electrons gradually saturates, making it difficult for local etching of silicon particles to occur and preventing the increase of the pore depth required for pore formation. Therefore, in the second etching sub-step, an oxidizing agent is additionally added to the mixed solution. As a result, electrons in the transition metal particles are taken away by hydrogen peroxide, promoting local etching in the depth direction inside the silicon particles.

[0060] The oxidizing agent is not particularly limited, but hydrogen peroxide is preferably mentioned. When hydrogen peroxide is used as the oxidizing agent, the concentration of hydrogen peroxide in the mixed solution is preferably about 0.03 mol / L to 0.09 mol / L, and more preferably about 0.06 mol / L.

[0061] The mixed solution added with hydrogen peroxide water is stirred in a state heated to about 50 °C. During this time, surface pores will be formed on the surface of the silicon particles by etching. The reaction time at this time is preferably about 10 to 90 minutes, and more preferably about 30 to 60 minutes.

[0062] After the above reaction, an appropriate amount of water is added to the mixed solution to reduce the concentrations of hydrofluoric acid and hydrogen peroxide and stop the etching reaction. Then, the silicon particles contained in the mixed solution are subjected to an acid treatment sub-step.

[0063] · Acid treatment sub-step The acid treatment sub-step is a step of removing transition metal particles from the silicon particles by acid-treating the silicon particles that have undergone the second etching sub-step.

[0064] In this small process, the silicon particles that have undergone the above second etching sub-process are subjected to acid treatment to remove the transition metal particles. Specifically, the silicon particles contained in the mixed solution are put into and immersed in an acidic aqueous solution as it is or in a state filtered out from the mixed solution. As a result, the transition metal particles transfer from the silicon particles to the acidic aqueous solution. The acidic aqueous solution preferably used at this time is concentrated nitric acid, and as the treatment method at that time, the filtered silicon particles may be immersed in the acidic aqueous solution for about 20 minutes.

[0065] On the surface of the silicon particles that have undergone acid treatment in this small process, a plurality of surface pores are formed. The porous silicon particles thus obtained can be used in the mixing process in the manufacturing method of the present invention.

[0066] As the binder, known ones can be listed without particular limitation. Such binders include fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), thermoplastic resins such as polyethylene, polypropylene, polyacrylic acid, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; polysaccharide polymers, etc. The content of the binder in the slurry is preferably about 5% by mass to 30% by mass as the dry mass, and more preferably about 5% by mass to 20% by mass.

[0067] In addition, it is preferable to add a conductive auxiliary agent to the slurry. Known conductive auxiliary agents can be listed without particular limitation. Examples of such conductive auxiliary agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphitized carbon, non-graphitized carbon, and graphene-based carbon. Examples of non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. Examples of the shape of the conductive auxiliary agent include powder form and fibrous form. As the conductive auxiliary agent, one of these materials may be used alone, or two or more of them may be mixed and used. Further, these materials may be used in a composite form. For example, a material obtained by compositeizing carbon black and CNTs may be used. Among these, carbon black is preferable from the viewpoints of electron conductivity and coatability, and among them, acetylene black is particularly preferable. When using a conductive auxiliary agent, the content of the conductive auxiliary agent in the slurry is preferably 15% by mass or less, more preferably 10% by mass or less, in terms of dry mass.

[0068] A slurry of the negative electrode active material mixture is prepared by mixing a dispersion liquid containing crushed graphite particles and the like obtained in the dispersion step, porous silicon particles, a binder, water, and, if necessary, a conductive auxiliary agent. Prior to this mixing, it is preferable to mix the dispersion liquid obtained in the dispersion step and the porous silicon particles, perform ultrasonic treatment, collect the particles by filtration, and dry them to form a composite. As described above, the mass ratio of the porous silicon particles (Si) to the crushed graphite particles and the like (G) in this composite is preferably Si:G = 5:1 to 5:5, more preferably Si:G = 5:2 to 5:5.

[0069] The slurry thus prepared is a negative electrode active material mixture for a lithium-ion secondary battery, and a negative electrode for a lithium-ion secondary battery can be prepared by applying this to a base material serving as a current collector. Thus, the slurry obtained by the production method of the present invention can be preferably used as a negative electrode material for a lithium-ion secondary battery.

[0070] Next, a second embodiment of the method for manufacturing a negative electrode material for a lithium-ion secondary battery according to the present invention will be described. The method for manufacturing a negative electrode material for a lithium-ion secondary battery of this embodiment includes a dispersion step of subjecting a mixture of graphene nanoplatelets, porous silicon particles, and a solvent to treatment by an external force to obtain a dispersion liquid in which the graphene nanoplatelets and the porous silicon particles are dispersed in the solvent, and a mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion liquid obtained in this dispersion step, a binder, and water.

[0071] In the production method of the first embodiment, a mixture of graphite or graphene nanoplatelets and a solvent was subjected to treatment by an external force to prepare a dispersion liquid. However, in the production method of this embodiment, a mixture of graphene nanoplatelets, porous silicon particles, and a solvent is subjected to treatment by an external force to prepare a dispersion liquid. That is, this embodiment is different from the above-described first embodiment in that porous silicon particles are present in the dispersion step, and a dispersion liquid containing these and graphene nanoplatelets is prepared. Therefore, in this embodiment, a slurry is prepared without adding porous silicon particles in the subsequent mixing step. Since other points are the same as those of the first embodiment, the description thereof will be omitted here.

[0072] The average particle size of the graphene nanoplatelets in the dispersion liquid is preferably 0.1 μm to 30 μm. The average particle size is more preferably 0.1 μm to 10 μm, still more preferably 0.1 μm to 5 μm, and particularly preferably 0.5 to 3 μm.

[0073] <Method for manufacturing a negative electrode for a lithium-ion secondary battery> The method for manufacturing a negative electrode for a lithium-ion secondary battery using the slurry which is a negative electrode active material mixture obtained by the method for manufacturing a negative electrode material for a lithium-ion secondary battery of the present invention is also one of the present inventions. The method for manufacturing a negative electrode for a lithium-ion secondary battery of the present invention includes a graphene layer forming step of forming a single-layer graphene layer on the surface of a current collector, and a coating step of coating and drying the slurry obtained by the above method on the graphene layer of the current collector that has undergone the graphene layer forming step. It is characterized by comprising the above. As described in the explanation of the negative electrode for a lithium-ion secondary battery of the present invention above, by providing a graphene layer on the surface of the current collector, the electron conduction between the negative electrode material containing crushed graphite particles and the like and the current collector becomes smooth, and excellent specific charge and discharge capacity and cycle characteristics are realized. The method for manufacturing a negative electrode for a lithium-ion secondary battery of the present invention prepares such a negative electrode. Hereinafter, each step will be described.

[0074] [Graphene layer forming step] The graphene layer forming step is a step of forming a single-layer graphene layer on the surface of a current collector. Instead of performing this step, a metal foil having a graphene layer formed on its surface may be obtained and used. In this case, this step may be considered to have been completed, and the subsequent coating step may be proceeded with.

[0075] Examples of the current collector include those that have been conventionally used for lithium-ion secondary batteries without particular limitation. Examples of such current collectors include copper foil and aluminum foil.

[0076] The graphene layer provided on the surface of the current collector may be a single layer or a plurality of layers. To form a graphene layer on the surface of the current collector, it is preferably mentioned to use chemical vapor deposition (CVD method) using ethanol, methane, etc. as a carbon source, but it is not particularly limited.

[0077] The current collector having a graphene layer formed on its surface is subjected to the coating step.

[0078] [Coating step] The coating process is a process of coating and drying the slurry obtained by the method for manufacturing the negative electrode material for the lithium ion secondary battery on the graphene layer of the current collector that has undergone the graphene layer formation process.

[0079] As for the means for coating the slurry, known ones can be used without particular limitation. By going through this process, a negative electrode active material layer is formed on the surface of the graphene layer, and the negative electrode for the lithium ion secondary battery of the present invention is obtained.

Examples

[0080] Hereinafter, the present invention will be described more specifically by showing examples, but the present invention is not limited to the following examples at all.

[0081] [Preparation of Porous Silicon Particles] Using silicon sludge from industrial waste (obtained from NER Co., Ltd., Japan) having an average diameter of about 530 nm as a raw material, porous silicon particles were prepared. First, the silicon sludge was heat-treated at 600°C for 30 minutes in an argon atmosphere to graphitize the insulating carbon-based organic substances contained in the silicon sludge, thereby obtaining heat-treated silicon particles.

[0082] 1.2 g of the heat-treated silicon particles were washed by performing ultrasonic treatment in acetone and then in ethanol for 10 minutes each. Further, the silicon particles were washed with a piranha solution, which is a 3:1 (v / v) mixture of concentrated sulfuric acid and hydrogen peroxide heated to 110°C, for 60 minutes, and then washed with deionized water.

[0083] Next, the silicon particles were made porous by a metal-assisted chemical etching method according to the following procedure. First, 1 g of the silicon particles that had been washed as described above was added to deionized water, and these were treated with an ultrasonic homogenizer for 5 minutes to uniformly disperse the silicon particles in the deionized water. Then, an etching solution composed of 3.0 M hydrofluoric acid and 0.02 M silver nitrate was added to this dispersion, and the mixture was stirred at 50 rpm for 1 minute using a stirrer to deposit silver nanoparticles on the surface of the silicon particles. The amount of the reaction solution at this time was 150 mL. To this dispersion containing hydrofluoric acid, 0.06 M hydrogen peroxide was added, and the mixture was reacted at a stirring speed of 50 rpm for 60 minutes while heating to 50 °C to make the silicon particles porous. Then, the silicon particles were washed with deionized water and then washed with a concentrated nitric acid solution for 40 minutes to completely remove the residue of silver nanoparticles present inside the pores, and finally washed with deionized water to obtain porous silicon particles. The average pore diameter of the surface pores of the porous silicon particles measured with a specific surface area and pore size distribution measuring device (manufactured by MicrotracBEL Corp., product name BELSORP) was 13.985 nm. The specific surface area of the porous silicon particles was 89.543 m 2 / g, and the total pore volume was 0.3131 cm 3 / g, and the pore volume was 0.2972 cm 3 / g. These operations were repeated to obtain the amount of porous silicon particles required for the preparation described later.

[0084] [Preparation of crushed graphite particle dispersion] 2.0 g of expanded graphite flakes (manufactured by Ito Graphite Industry Co., Ltd., product name EC10) was added to N-methyl-2-pyrrolidone (100 mL) and treated with an ultrasonic homogenizer. The conditions for the ultrasonic homogenizer treatment were Amplitude: 60%, Pulse ON: 10.0 s, Pulse OFF: 5.0 s, and the total time of Pulse ON was 2 hours. Next, the crushed graphite particles were once recovered by vacuum filtration, and these particles were thoroughly dried at 150 °C and then added to fresh N-methyl-2-pyrrolidone (100 mL) and treated with an ultrasonic homogenizer. The conditions for the ultrasonic homogenizer treatment at this time were the same as above. Next, the crushed graphite particles were once recovered by vacuum filtration, and these particles were thoroughly dried at 150 °C.

[0085] 0.8 g of the obtained dried particles was added to fresh N-methyl-2-pyrrolidone (40 mL) and subjected to the third treatment with an ultrasonic homogenizer. The conditions for the ultrasonic homogenizer treatment at this time were Amplitude: 60%, Pulse ON: 10.0 s, Pulse OFF: 5.0 s, and the total time of Pulse ON was 6 hours. The dispersion liquid that had undergone this treatment was used as the crushed graphite particle dispersion liquid. When the obtained crushed graphite particles were observed with a scanning electron microscope, graphene flakes were contained, and the calculated average particle size was 0.93 μm.

[0086] [Preparation 1 of Porous Silicon Particle / Crushed Graphite Particle Composite (Si / G Composite)] 1.2 g of the porous silicon particles obtained by the above procedure was added to 12 mL of the crushed graphite particle dispersion liquid obtained by the above procedure (corresponding to 0.24 g of crushed graphite particles) and treated with an ultrasonic homogenizer. At this time, the mass ratio of Si:G (porous silicon particles: crushed graphite particles) was 5:1. Also, the conditions for the ultrasonic homogenizer treatment were Amplitude: 30%, Pulse ON: 10.0 s, Pulse OFF: 5.0 s, and the total time of Pulse ON was 30 minutes. After the dispersion liquid after the treatment was stirred with a high-shear mixer (22,000 rpm) for 30 minutes, the Si / G composite was recovered by vacuum filtration and thoroughly dried at 150 °C to obtain the Si / G composite (Si:G = 5:1).

[0087] [Preparation 2 of Porous Silicon Particle / Crushed Graphite Particle Composite (Si / G Composite)] An Si / G composite (Si:G = 5:2) was obtained in the same procedure as above, except that the volume of the crushed graphite particle dispersion was 25 mL (corresponding to 0.5 g of crushed graphite particles) and the amount of porous silicon particles was 1.25 g.

[0088] [Preparation 3 of Porous Silicon Particle / Crushed Graphite Particle Composite (Si / G Composite)] An Si / G composite (Si:G = 5:5) was obtained in the same procedure as above, except that the volume of the crushed graphite particle dispersion was 32 mL (corresponding to 0.64 g of crushed graphite particles) and the amount of porous silicon particles was 0.64 g.

[0089] [Preparation 1 of Anode Active Material Mixture Slurry] For each of the three Si / G composites (Si:G = 5:1, 5:2, and 5:5), the respective raw materials were mixed according to the formulation in Table 1 to prepare an anode active material mixture slurry. In Table 1, "carbon black" is conductive carbon black (product name SuperP) manufactured by MTI. The preparation procedure of the anode active material mixture slurry was as follows. First, polyacrylic acid was added to pure water and stirred for 10 minutes. Conductive carbon black was added to the resulting solution and stirred for an additional 10 minutes to obtain a mixed solution. The Si / G composite obtained by the above procedure was added to the resulting mixed solution and stirred for 30 minutes to obtain three types of anode active material mixture slurries A (Si:G = 5:1, 5:2, and 5:5). In the stirring for the preparation of these anode active material mixture slurries, a high-shear mixer (22,000 rpm) was used.

[0090] [Table 1]

[0091] [Preparation 2 of Anode Active Material Mixture Slurry] 0.7 g of porous silicon particles and 0.7 g of graphene nanoplatelets (manufactured by Nanografi, thickness 3 nm, average particle size 1.5 μm, specific surface area 800 m 2 / g) were added to N-methyl-2-pyrrolidone (10 mL), and the mixture was stirred at 22,000 rpm for 2 hours using a high-shear mixer to obtain a dispersion. Next, 347 mg of polyacrylic acid was dissolved in 5.95 mL of pure water, and 173 mg of conductive carbon black (manufactured by MTI, product name SuperP) was added thereto, followed by stirring for 10 minutes to prepare a mixed solution. The above dispersion was added to this mixed solution and stirred for 30 minutes to obtain slurry B (SiGNP).

[0092] [Preparation of negative electrode] For each of the above three types of negative electrode active material mixture slurries A (Si:G = 5:1, 5:2, and 5:5) and slurry B (SiGNP), negative electrodes were prepared using a copper foil on which a single-layer graphene layer was formed by CVD method and a copper foil on which no single-layer graphene layer was formed, respectively. For the copper foil on which a single-layer graphene layer was formed, the negative electrode active material mixture slurry was coated on the surface of the single-layer graphene layer so that the dry film thickness was 50 μm, dried in a constant temperature bath at 80 °C for 6 hours, and then dried at 120 °C for 12 hours using a vacuum dryer to obtain a negative electrode. For the copper foil on which no single-layer graphene layer was formed, the negative electrode active material mixture slurry was coated on the surface of the copper foil so that the dry film thickness was 50 μm, dried in a constant temperature bath at 80 °C for 6 hours, and then dried at 120 °C for 12 hours using a vacuum dryer to obtain a negative electrode. The negative electrode prepared using the negative electrode active material mixture slurry A is referred to as "negative electrode A", and the negative electrode prepared using the negative electrode active material slurry B is referred to as "negative electrode B".

[0093] [Evaluation of charge-discharge cycle characteristics] For each of the negative electrodes obtained by the above procedure, a CR2032 half-cell using a lithium foil as the counter electrode was assembled and connected to a battery charge / discharge device (manufactured by Hokuto Denko Corporation, product name HJ1001SD8) by the four-terminal method, and the charge / discharge cycle characteristics were evaluated. In the evaluation, charging was performed in the constant current constant voltage (CCCV) mode. Charging was carried out at a constant current until the voltage reached 0.01 V, and after the voltage reached 0.01 V, charging was continued at a constant voltage of 0.01 V until the current value fell below the lower limit current. Discharging was performed in the constant current (CC) mode, and discharging was carried out at a constant current until the voltage reached 2 V. Since this time is the evaluation of the negative electrode, the positive side of the battery charge / discharge device was connected to the negative electrode of the battery, and the negative side was connected to the positive electrode of the battery. Therefore, unlike the normal battery evaluation, the voltage decreases as charging progresses and the voltage increases as discharging progresses. Also, the current value was set to 0.05 C (lower limit current 0.001 C) for the first cycle and 0.2 C (lower limit current 0.004 C) for the second cycle and subsequent cycles with respect to the theoretical capacity of 1 C. The charge retention rate was calculated based on the charge capacity of the second cycle.

[0094] Plots of the charge retention rate against the number of cycles for each negative electrode A (Si:G = 5:1, 5:2, and 5:5) when using a negative electrode A on which CVD single-layer graphene is not formed on the current collector are shown in FIG. 1. As shown in FIG. 1, when using any of the negative electrodes A with Si:G = 5:1, 5:2, and 5:5, the charge capacity decreased rapidly when the number of cycles was about 10 to 40, and the charge / discharge cycle characteristics were not good. However, regarding the ratio of Si:G contained in the negative electrode, as the mixing ratio of G (crushed graphite) increased, the charge / discharge cycle characteristics tended to improve.

[0095] Figure 2 shows the plots of the charge retention rate against the number of cycles for each negative electrode A (Si:G = 5:1, 5:2, and 5:5) when using a negative electrode A with CVD single-layer graphene formed on the current collector. As shown in Figure 2, a significant improvement in the charge-discharge cycle characteristics was observed compared with the case of using a negative electrode A without CVD single-layer graphene formed on the current collector (Figure 1). Also, regarding the ratio of Si:G contained in the negative electrode A, as the mixing ratio of G (crushed graphite) increases, the charge-discharge cycle characteristics tend to improve, and the charge-discharge cycles were particularly good when Si:G = 5:2 to 5:5.

[0096] Figure 3 shows the plots of the charge retention rate against the number of cycles for a negative electrode B (SiGNP) with CVD single-layer graphene formed on the current collector. For comparison, Figure 3 also shows the plot of the negative electrode A (Si:G = 5:5) with CVD single-layer graphene formed. As shown in Figure 3, the negative electrode B using graphene nanoplatelets exhibited better cycle characteristics than the negative electrode A using crushed graphite, and the charge capacity retention rate at the 101st cycle was much higher than that of the negative electrode A.

[0097] As described above, it can be understood that a lithium-ion secondary battery with excellent cycle characteristics can be obtained by using a composite of porous silicon particles and crushed graphite particles or graphene nanoplatelets as the negative electrode material and forming a graphene layer on the negative electrode current collector.

Claims

1. A negative electrode for a lithium-ion secondary battery, comprising a current collector and a negative electrode material for a lithium-ion secondary battery provided on the surface thereof, wherein the current collector has a single-layer graphene layer on its surface, and on the surface of the single-layer graphene layer in the current collector, a composite of crushed graphite particles having an average particle size of 0.1 μm to 30 μm and porous silicon particles is included, and the crushed graphite particles contain graphene flakes. A negative electrode for a lithium-ion secondary battery comprising a negative electrode material for a lithium-ion secondary battery.

2. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the mass ratio of the porous silicon particles (Si) to the crushed graphite particles (G) is Si:G = 5:1 to 5:

5.

3. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the porous silicon particles are surface-coated with the crushed graphite particles.

4. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 3 as a negative electrode.

5. A dispersion step of subjecting a mixture of graphite and a solvent to external force treatment to obtain a dispersion liquid through a step of crushing the graphite in the solvent, A mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion liquid obtained in the dispersion step, porous silicon particles, a binder, and water, and a coating step of coating and drying the slurry on the surface of a current collector having a single-layer graphene layer formed on its surface. A method for manufacturing a negative electrode for a lithium-ion secondary battery, characterized by comprising:

6. A dispersion step of subjecting a mixture of graphene nanoplatelets and a solvent to external force treatment to obtain a dispersion liquid through a step of dispersing the graphene nanoplatelets in the solvent, A mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion obtained in the dispersion step, porous silicon particles, a binder, and water; and a coating step of coating and drying the slurry on the surface of a current collector having a single-layer graphene layer formed on the surface. A method for manufacturing a negative electrode for a lithium-ion secondary battery, characterized by comprising the steps.

7. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 5 or 6, wherein the treatment by the external force is ultrasonic treatment.

8. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 5, characterized in that the graphite is expanded graphite flakes.

9. A dispersion step of subjecting a mixture of graphene nanoplatelets, porous silicon particles, and a solvent to treatment by an external force to obtain a dispersion in which the graphene nanoplatelets and the porous silicon particles are dispersed in the solvent; A mixing step of preparing a slurry of a negative electrode active material mixture by combining at least the dispersion obtained in the dispersion step, a binder, and water; and a coating step of coating and drying the slurry on the surface of a current collector having a single-layer graphene layer formed on the surface. A method for manufacturing a negative electrode for a lithium-ion secondary battery, characterized by comprising the steps.

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