Carbon-silicon composite material, negative electrode active material, and battery

Abstract

A carbon-silicon composite material according to the present disclosure contains porous carbon particles and silicon particles dispersed inside the carbon particles, wherein the carbon-silicon composite material further contains a halogen element and at least one element selected from the group consisting of Na, K, Ca, Sr, and Ba. The content of the at least one element in the carbon-silicon composite material is 0.005 mass % or more, and the content of the halogen element in the carbon-silicon composite material is 0.1 mass % or more.

Description

Carbon-silicon composite material, negative electrode active material, and battery

[0001] The present disclosure relates to a carbon-silicon composite material, a negative electrode active material, and a battery.

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications that require high capacity, such as in-vehicle applications and power storage applications. The electrodes that make up such batteries have a great influence on the performance of the battery. Therefore, various studies have been conventionally conducted on electrodes.

[0003] It is known that a negative electrode active material containing silicon is effective for increasing the capacity of a battery. However, since the negative electrode active material containing silicon has a large volume change accompanying charge and discharge, the conductive path with the surrounding active material is cut off by repeated charge and discharge, resulting in capacity degradation accompanying the charge and discharge cycle. Therefore, Patent Document 1 proposes a negative electrode active material capable of improving cycle stability as a negative electrode active material containing silicon. Specifically, Patent Document 1 proposes silicon-carbon composite particles in which the concentration of an alkali metal or alkaline earth metal and the pH are within a predetermined range.

[0004] Japanese Patent Application Laid-Open No. 2024-503167

[0005] There is room for improvement in the charge and discharge efficiency of the battery with respect to the conventional negative electrode active material as described above.

[0006] Therefore, the present disclosure provides a technique suitable for improving the charge and discharge efficiency of a battery using a negative electrode active material containing silicon.

[0007] The present disclosure provides a carbon-silicon composite material including porous carbon particles and silicon particles dispersed inside the carbon particles, in which the carbon-silicon composite material contains at least one element selected from the group consisting of Na, K, Ca, Sr, and Ba and a halogen element, the content of the at least one element in the carbon-silicon composite material is 0.005% by mass or more, and the content of the halogen element in the carbon-silicon composite material is 0.1% by mass or more.

[0008] The technology of the present disclosure is suitable for improving the charge and discharge efficiency of a battery.

[0009] Figure 1 is a cross-sectional view showing the schematic configuration of a carbon-silicon composite material according to Embodiment 1. Figure 2 is a cross-sectional view showing the schematic configuration of a modified example of the carbon-silicon composite material according to Embodiment 1. Figure 3 is a flowchart showing an example of a method for manufacturing the carbon-silicon composite material according to Embodiment 1. Figure 4 is a cross-sectional view showing the schematic configuration of a battery according to Embodiment 3.

[0010] The embodiments of this disclosure will be described in detail below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0011] [Embodiment 1] Figure 1 is a cross-sectional view showing the schematic configuration of a carbon-silicon composite material 10 according to Embodiment 1. The carbon-silicon composite material 10 includes porous carbon particles 1 and silicon particles 2 dispersed inside the carbon particles 1. The silicon particles 2 are typically located inside the pores of the carbon particles 1. The carbon-silicon composite material 10 may be in the form of particles, or in other words, carbon-silicon composite particles. The carbon-silicon composite material 10 is suitable for use as a component of a battery, for example, as a negative electrode active material. In the carbon-silicon composite material 10, since the silicon particles 2 are dispersed inside the carbon particles 1, the expansion of the silicon particles 2 during charging is mitigated by the carbon particles 1. That is, the carbon-silicon composite material 10, with its configuration including carbon particles 1 and silicon particles 2 dispersed inside the carbon particles 1, is suitable for suppressing expansion during charging. Depending on the state of the pores, such as the size of the pores of the carbon particles 1, silicon particles 2 may be observed to be deposited in layers inside the pores.

[0012] Furthermore, the carbon-silicon composite material 10 further contains at least one element selected from the group consisting of Na (sodium), K (potassium), Ca (calcium), Sr (strontium), and Ba (barium) (hereinafter referred to as "element A"), and a halogen element. The content of element A in the carbon-silicon composite material 10 is 0.005% by mass or more. The content of halogen elements in the carbon-silicon composite material 10 is 0.1% by mass or more. The carbon-silicon composite material 10 is typically formed by precipitating silicon particles 2 in the pores of carbon particles 1. According to the inventors' studies, it is presumed that the presence of both element A and halogen elements in predetermined amounts or more during the precipitation of silicon particles 2 contributes to the precipitation of silicon particles 2 in a state suitable for improving the charge-discharge efficiency of the battery. Element A belongs to the alkali metal element or alkaline earth metal element and may have catalytic activity for the reaction that precipitates silicon particles 2, for example, the thermal decomposition reaction of the precursor gas. Due to their high molecular polarity, halogen elements may have the effect of promoting electron transfer in the above reaction. The precipitation of silicon particles 2 in the above state may be due to the catalytic activity of element A and the above effect of halogen elements. The content of element A at 0.005 mass% or more and the content of halogen elements at 0.1 mass% or more in the carbon-silicon composite material 10 are thought to reflect the presence of element A and halogen elements in amounts greater than the above-mentioned predetermined amounts at the time of the precipitation of silicon particles 2.

[0013] The halogen element content in the carbon-silicon composite material 10 may be 0.2% by mass or more, 0.3% by mass or more, 0.5% by mass or more, 0.8% by mass or more, 1% by mass or more, 1.3% by mass or more, 1.5% by mass or more, 1.8% by mass or more, 2% by mass or more, 2.2% by mass or more, 2.4% by mass or more, and even 2.5% by mass or more. The upper limit of the content is, for example, 20% by mass or less, and may be 15% by mass or less, 13% by mass or less, 10% by mass or less, 9% by mass or less, 7% by mass or less, 5% by mass or less, 4% by mass or less, and even 3% by mass or less. The content may be 1% by mass or more and 10% by mass or less.

[0014] The halogen element content in the silicon carbon composite material 10 can be determined by energy-dispersive X-ray spectroscopy (EDX). Specifically, the halogen element content can be determined by performing elemental mapping analysis by EDX on the backscattered electron image of the cross-section of the silicon carbon composite material 10. The observation magnification is, for example, 2,000x to 20,000x. To determine the content from the state in which it is contained in a battery, for example, elemental mapping analysis by EDX can be performed on the cross-section of the backscattered electron image of the negative electrode active material containing the silicon carbon composite material 10, or the negative electrode mixture layer containing the silicon carbon composite material 10.

[0015] The halogen element is at least one selected from the group consisting of fluorine, chlorine, bromine, and iodine. The halogen element may also contain chlorine. The silicon-carbon composite material 10 may contain one or more halogen elements. The silicon-carbon composite material 10 containing two or more halogen elements may contain chlorine as the halogen element with the highest content. When the silicon-carbon composite material 10 contains two or more halogen elements, the content of halogen elements in the silicon-carbon composite material 10 is the sum (total amount) of the content of each halogen element contained.

[0016] The upper limit of the content of element A in the carbon-silicon composite material 10 is, for example, 0.5 mass% or less, and may be 0.3 mass% or less, 0.2 mass% or less, 0.1 mass% or less, less than 0.1 mass%, 0.09 mass% or less, 0.07 mass% or less, 0.05 mass% or less, less than 0.05 mass%, 0.04 mass% or less, 0.02 mass% or less, and even 0.01 mass% or less.

[0017] The content of element A in the carbon-silicon composite material 10 can be determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Semi-quantitative analysis may also be used for determination.

[0018] Element A may include at least one selected from the group consisting of Na, K, and Ca, or it may include at least one selected from the group consisting of Na and K, and may also include K. The carbon-silicon composite material 10 may contain one or more elements A. The carbon-silicon composite material 10 containing two or more elements A may include K as the element A with the highest content. When the carbon-silicon composite material 10 contains two or more elements A, the content of element A in the carbon-silicon composite material 10 is the sum (total amount) of the content of each element A contained. In the carbon-silicon composite material 10 containing two or more elements A, the content of the element A with the highest content may be 0.005 mass% or more. Furthermore, the upper limit of the content of the element A with the highest content may be within the above-mentioned ranges as the upper limit of the content of element A in the carbon-silicon composite material 10.

[0019] The components of the carbon-silicon composite material 10 according to Embodiment 1 will be described in more detail below.

[0020] The carbon particles 1 are porous, and more specifically, have a porous framework. The framework of the carbon particles 1 is a carbon-containing matrix. The carbon particles 1 include, for example, a carbonaceous material. The carbonaceous material may be a material capable of adsorbing lithium ions. This embodiment is suitable for use as a negative electrode active material in a carbon-silicon composite material 10. The carbonaceous material may be amorphous. The carbon particles 1 may be composed of an amorphous carbonaceous material. Examples of carbonaceous materials include pitch-derived materials, amorphous carbon, carbon black, and organic polymer-derived materials. Examples of pitch include coal pitch and petroleum pitch, and coal pitch is, for example, coal tar pitch. The carbon particles 1 may contain multiple types of carbonaceous materials.

[0021] The proportion of silicon in the carbon-silicon composite material 10 may be 40% by mass or more and 80% by mass or less, 45% by mass or more and 75% by mass or less, or 50% by mass or more and 70% by mass or less. The proportion of silicon in the carbon-silicon composite material 10 can be determined by ICP-AES.

[0022] The silicon particle 2 may be formed in the phase of elemental silicon. The silicon particle 2 may also contain other elements besides silicon. The inclusion of other elements may contribute to improved electronic conductivity. When the silicon particle 2 contains other elements besides silicon, the state of the other elements in the silicon particle 2 is not particularly limited. For example, the other elements may be located inside the silicon particle 2 or on the surface of the silicon particle 2. The other elements may be contained in the silicon particle 2 in any form, such as atoms, elements, or compounds. Examples of other elements include Ge, Al, Ni, P, B, Sb, etc.

[0023] The silicon particle 2 may contain crystalline silicon. If the silicon particle 2 contains elements other than silicon, it may contain crystalline silicon in which atoms of the other elements are in solid solution. The silicon particle 2 may be composed of, for example, multiple crystallites. The crystallite size constituting the silicon particle 2 may be 10 nm or less, 5 nm or less, or 2 nm or less. The above embodiment is particularly suitable for reducing the volume change due to the expansion and contraction of the silicon particle 2 associated with charging and discharging. The crystallite size of the silicon particle 2 can be calculated from the full width at half maximum of the diffraction peak originating from the Si(111) plane in the X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα rays using Scherrer's formula.

[0024] The lower limit of the crystallite size of silicon particles 2 is not particularly limited, but one example is 1 nm. A suitable example of a crystallite size for silicon particles 2 may be 1 nm or more and 5 nm or less. A crystallite size of silicon particles 2 of 1 nm or more can, for example, keep the surface area of ​​silicon particles 2 small, which can contribute to suppressing the degradation of silicon particles 2 accompanied by the generation of irreversible capacitance. A crystallite size of 5 nm or less can contribute to homogenizing the expansion and contraction of silicon particles 2, thereby effectively relieving the stress generated in the carbon-silicon composite material 10.

[0025] The silicon particles 2 may be nanoparticles. The average particle diameter of the silicon particles 2 may be between 50 nm and 300 nm, between 100 nm and 200 nm, or between 100 nm and 150 nm. The average particle diameter of the silicon particles 2 can be determined by analyzing the observation image of the silicon-carbon composite material 10 using a scanning electron microscope (SEM). However, the analysis is performed on the observation image in which the cross-section of the silicon particles 2 is exposed. The longest diameter is evaluated for at least 10 silicon particles 2, and the average of these is taken as the average particle diameter.

[0026] The average particle size of the carbon-silicon composite material 10, which is the particle, may be 1 μm or more and 20 μm or less, or 1 μm or more and 10 μm or less. The average particle size of the carbon-silicon composite material 10 can be determined, for example, in the same way as the average particle size of the silicon particles 2.

[0027] The pH of the carbon-silicon composite material 10 may be less than 7.5, and may be 7.2 or less, 7 or less, 6.7 or less, 6.5 or less, 6.2 or less, or even 6 or less. The pH of the carbon-silicon composite material 10 can be determined in accordance with Method A as defined in ASTM D1512.

[0028] Figure 2 is a cross-sectional view showing a schematic configuration of a modified example of the carbon-silicon composite material according to Embodiment 1. The modified example shown in Figure 2 further comprises a coating layer 3 that covers the surface of the carbon particles 1. The coating layer 3 may contain carbon. The configuration of the carbon-silicon composite material 20 shown in Figure 2 may be the same as the configuration of the carbon-silicon composite material 10 shown in Figure 1, except for the presence of the coating layer 3. Repetitive explanations are omitted.

[0029] The carbon-silicon composite material 20 shown in Figure 2 has a coating layer 3 on its surface. The coating layer 3 covers at least a portion of the surface of the carbon-silicon composite material 20 (hereinafter also referred to as the "matrix material") which contains carbon particles 1 and silicon particles 2. The matrix material may be particles.

[0030] The coating layer 3 is composed of, for example, a carbonaceous material. Examples of carbonaceous materials include those described above for the carbon particles 1. The carbonaceous material contained in the coating layer 3 may be the same material as the carbonaceous material contained in the carbon particles 1. The coating layer 3 may also be composed of a carbonaceous material made from pitch-derived material. A carbon-silicon composite material 20 having a coating layer 3 containing a conductive carbon material on its surface is suitable for enhancing conductivity.

[0031] When the carbon-silicon composite material 20 is in the form of particles, it is desirable that the thickness of the coating layer 3 be thin enough not to substantially affect the average particle size of the carbon-silicon composite material 20. From the viewpoint of ensuring conductivity and ion diffusion that contributes to charging and discharging, the thickness of the coating layer 3 may be 0.1 nm or more and 10 nm or less, or 5 nm or less. The thickness of the coating layer 3 can be measured, for example, by cross-sectional observation of the carbon-silicon composite material 20 using a scanning electron microscope (SEM).

[0032] The carbon-silicon composite material according to Embodiment 1 does not necessarily have a coating layer 3, as shown in Figure 1. In other words, the carbon-silicon composite material may consist only of the matrix material.

[0033] The silicon-carbon composite material according to Embodiment 1 may further contain other components in addition to the components described above. The silicon-carbon composite material 10 may contain the other components in a proportion of more than 0% by mass and 10% by mass or less, more than 0% by mass and 5% by mass or less, more than 0% by mass and 1% by mass or less, or more than 0% by mass and 0.1% by mass or less.

[0034] (Method for manufacturing carbon-silicon composite material) An example of a method for manufacturing a carbon-silicon composite material according to Embodiment 1 will be described. Figure 3 is a flowchart showing an example of a method for manufacturing a carbon-silicon composite material according to Embodiment 1. As shown in Figure 3, an example of a method for manufacturing a carbon-silicon composite material according to Embodiment 1 includes: (I) preparing porous carbon particles containing element A (S1), and (II) using a precursor gas containing silicon and halogen elements as a raw material gas, and precipitating silicon particles in the pores of the carbon particles by chemical vapor deposition (CVD) (S2).

[0035] When manufacturing a carbon-silicon composite material 20 further comprising a coating layer 3 that covers the surface of carbon particles 1, the method for manufacturing the carbon-silicon composite material may include a step (III) of forming the coating layer 3 after step (II) above.

[0036] The following details each step.

[0037] (Step (I)) Porous carbon particles are prepared. However, carbon particles containing element A are selected. Examples of element A, including preferred embodiments, are as described above. The content of element A in the carbon particles is set so that the content of element A in the final obtained carbon-silicon composite material is 0.005 mass% or more, which is a desired value. Examples of the types of carbon particles to be prepared are the same as those described above in the description of the carbon-silicon composite material 10.

[0038] The carbon particles to be prepared may be made by carbonizing a carbon-containing material (carbon source). For example, carbonizable organic materials and polymer materials can be used as carbon sources. Examples of carbonizable organic materials include plant biomass containing lignocellulosic materials (such as coconut shells and wood), coal, and fossil carbon sources. Examples of polymer materials include phenolic resins, pitch, melamine, polystyrene, polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP).

[0039] An example of carbon particle production by carbonization is described below. First, a carbon source is thermally decomposed at a predetermined temperature to induce carbonization. Subsequently, a chemical activation process or gas activation may be performed to adjust the pore size of the carbon particles. For example, the pore volume and pore size (pore diameter) of the carbon material obtained by thermal decomposition can be adjusted by contacting it with water vapor, CO, CO2, KOH, etc., at a temperature of, for example, 600°C or higher and 1200°C or lower.

[0040] The pore volume of the carbon particles to be prepared is, for example, 0.5 cm. 3 / g or more and 3.1cm 3 It may be less than / g. The pore volume of carbon particles can be measured, for example, by the nitrogen adsorption amount measurement method (BET method).

[0041] The pores contained in the carbon particles to be prepared may have a diameter of, for example, 0.4 nm or more and 100 nm or less. The diameter of the pores can be measured, for example, by the method of nitrogen adsorption measurement (BJH method).

[0042] (Step (II)) The precursor gas used as a raw material gas for the deposition of silicon particles by CVD method contains silicon and a halogen element. Examples of the halogen element are as described above, including desirable embodiments. The precursor gas is, for example, a silane halide. The precursor gas may be chlorosilane. The silane halide is, for example, dichlorosilane, trichlorosilane. The precursor gas preferably contains at least one selected from the group consisting of dichlorosilane and trichlorosilane, and more preferably contains dichlorosilane. The halogen element contained in the precursor gas can become the halogen element contained in the finally obtained carbon-silicon composite material. The content of the halogen element in the finally obtained carbon-silicon composite material can vary, for example, depending on the amount of the halogen element contained in the precursor gas, the flow rate of the precursor gas in Step (II), etc.

[0043] In Step (II), silicon particles may be deposited in the pores of the carbon particles by CVD method at a temperature of 400°C or more and 700°C or less. The temperature may be 450°C or more and 700°C or less, 500°C or more and 650°C or less, 530°C or more and 650°C or less, or 530°C or more and less than 600°C.

[0044] In Step (II), for example, carbon particles are inserted into a reaction vessel, and an inert gas (e.g., Ar gas) is flowed to remove oxygen. The reaction vessel is heated to a reaction temperature of, for example, 400°C or more and 700°C or less. While flowing dichlorosilane gas diluted with Ar at a predetermined flow rate, the carbon particles are stirred with a blade in the vessel, and the time for depositing silicon of a target weight is adjusted. In this way, the reaction for depositing silicon particles is carried out.

[0045] (Step (III)) At least a part of the surface of the base material obtained in Step (II) may be coated with a conductive material to form a coating layer 3. The conductive material is preferably electrochemically stable, and a conductive carbon material is desirable. Examples of the method of coating at least a part of the surface of the base material with a conductive carbon material include the CVD method using hydrocarbon gases such as acetylene and methane as raw materials. Further, a method of mixing coal pitch, petroleum pitch, phenolic resin, etc. with the base material and heating and carbonizing them can also be exemplified. The mixture of the base material and the raw materials of the conductive carbon material such as coal pitch, petroleum pitch, phenolic resin, etc. is heated at 400 °C or higher and 950 °C or lower, for example, in an inert atmosphere (for example, an atmosphere of argon, nitrogen, etc.). Further, carbon black may be adhered to the surface of the base material. In the above manner, the carbon-silicon composite material 10 or the carbon-silicon composite material 20 is obtained.

[0046] Then, the obtained carbon-silicon composite material 10 or the carbon-silicon composite material 20 may be further heat-treated at 400 °C or higher and 950 °C or lower for the growth of the internal composition.

[0047] The manufacturing method of the carbon-silicon composite material according to Embodiment 1 is not limited to the above example. Further, the method of controlling the contents of the element A and the halogen element in the carbon-silicon composite material is not limited to the above example.

[0048] (Use of the carbon-silicon composite material) The carbon-silicon composite material according to Embodiment 1 may be used as a negative electrode active material. The negative electrode active material may be the negative electrode active material included in the battery. An example of the battery is shown in Embodiment 3. However, the use of the carbon-silicon composite material according to Embodiment 1 is not limited to the above example.

[0049] [Embodiment 2] The negative electrode active material according to Embodiment 2 includes the carbon-silicon composite material according to Embodiment 1. The negative electrode active material according to Embodiment 2 may also include materials other than the carbon-silicon composite material according to Embodiment 1. For materials other than the carbon-silicon composite material, materials known as negative electrode active materials may be used. An example of a material other than the carbon-silicon composite material is a carbon material such as graphite. The main component of the negative electrode active material according to Embodiment 2 may be the carbon-silicon composite material according to Embodiment 1, or a material other than the carbon-silicon composite material. In this specification, the main component means the component with the largest content. The content of the main component may be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, 97% by mass or more, and even 99% by mass or more.

[0050] [Embodiment 3] The battery according to Embodiment 3 comprises a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material according to Embodiment 2. With this configuration, the battery according to Embodiment 3 can suppress the expansion of the negative electrode due to charging and be in a state suitable for improving charge and discharge efficiency.

[0051] Figure 4 is a schematic longitudinal cross-sectional view showing an example of a battery according to Embodiment 3. The battery 100 is a cylindrical battery comprising a cylindrical battery case, a wound electrode group 54, and an electrolyte (not shown). The electrode group 54 is housed within the battery case and is in contact with the electrolyte.

[0052] The battery case consists of a case body 55, which is a bottomed cylindrical metal container, and a sealing body 56 that seals the opening of the case body 55. A gasket 67 is placed between the case body 55 and the sealing body 56. The gasket 67 ensures that the battery case is airtight. Inside the case body 55, insulating plates 57 and 58 are placed at both ends of the electrode group 54 in the winding axis direction, respectively.

[0053] The case body 55 has, for example, a stepped portion 61. The stepped portion 61 may be formed by partially pressing the side wall of the case body 55 from the outside. The stepped portion 61 may be formed in an annular shape on the side wall of the case body 55 along the circumferential direction of a virtual circle defined by the case body 55. In this case, the sealing body 56 is supported, for example, by the opening side surface of the stepped portion 61.

[0054] The sealing body 56 comprises a filter 62, a lower valve body 63, an insulating member 64, an upper valve body 65, and a cap 66. In the sealing body 56, these members are stacked in this order. The sealing body 56 is installed in the opening of the case body 55 such that the cap 66 is located on the outside of the case body 55 and the filter 62 is located on the inside of the case body 55.

[0055] Each of the above-mentioned components constituting the sealing body 56 is, for example, disc-shaped or ring-shaped. Except for the insulating member 64, each of the above-mentioned components is electrically connected to one another.

[0056] The electrode group 54 includes a positive electrode 51, a separator 52, and a negative electrode 53. The positive electrode 51, the separator 52, and the negative electrode 53 are all strip-shaped. The width direction of the strip-shaped positive electrode 51 and negative electrode 53 is, for example, parallel to the winding axis of the electrode group 54. The separator 52 is positioned between the positive electrode 51 and the negative electrode 53. The positive electrode 51 and the negative electrode 53 are wound in a spiral shape with the separator 52 interposed between them.

[0057] When observing a cross-section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 54, the positive electrode 51 and the negative electrode 53 are alternately stacked in the radial direction of a virtual circle defined by the case body 55, with a separator 52 interposed between them.

[0058] The positive electrode 51 is electrically connected to the cap 66, which also serves as the positive electrode terminal, via a positive electrode lead 59. One end of the positive electrode lead 59 is connected, for example, near the center of the positive electrode 51 in the longitudinal direction. The positive electrode lead 59 extends from the positive electrode 51 to the filter 62 through a through hole formed in the insulating plate 57. The other end of the positive electrode lead 59 is welded, for example, to the side of the filter 62 facing the electrode group 54.

[0059] The negative electrode 53 is electrically connected to the case body 55, which also serves as the negative electrode terminal, via a negative electrode lead 60. One end of the negative electrode lead 60 is connected, for example, to the end of the negative electrode 53 in the longitudinal direction. The other end of the negative electrode lead 60 is welded, for example, to the inner bottom surface of the case body 55.

[0060] The components of battery 100 will be described in detail below.

[0061] The positive electrode 51 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The positive electrode 51 includes, for example, a positive electrode active material. The positive electrode 51 comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0062] As the positive electrode current collector, a sheet or film made of a metallic material such as aluminum, stainless steel, titanium, or their alloys can be used. Aluminum and its alloys are suitable as materials for positive electrode current collectors because they are inexpensive and easy to make into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the positive electrode current collector as a conductive auxiliary material.

[0063] The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material may be a material that has the ability to intercept and release metal ions (e.g., lithium ions). As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc., can be used. In particular, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0064] The positive electrode mixture layer may optionally contain a conductive additive, an ion conductor, and a binder.

[0065] A binder is used to improve the bonding properties of the materials constituting the electrodes. Examples of binders include polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethylcellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one of these binders can be used.

[0066] Conductive additives and ionic conductors are used to reduce electrode resistance. Examples of conductive additives include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymer compounds include polyaniline, polypyrrole, and polythiophene. At least one of these conductive additives can be used. Examples of ionic conductors include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and Li7La3Zr2O 12 Examples include inorganic solid electrolytes. At least one of these ion conductors can be used.

[0067] The negative electrode 53 includes a carbon-silicon composite material according to Embodiment 1. In other words, the negative electrode 53 may also include a negative electrode active material according to Embodiment 2. The negative electrode 53 comprises, for example, a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector.

[0068] The negative electrode current collector is a foil made of a metallic material such as stainless steel, nickel, nickel alloy, copper, or copper alloy.

[0069] The negative electrode mixture layer contains the carbon-silicon composite material according to Embodiment 1. In other words, the negative electrode mixture layer may also contain the negative electrode active material according to Embodiment 2. The negative electrode mixture layer may optionally contain other materials such as conductive additives, ion conductors, and binders. The materials described above for the positive electrode mixture layer can also be used as conductive additives, ion conductors, and binders for the negative electrode mixture layer.

[0070] The electrolyte solution used as the electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte solution may be, for example, 0.5 mol / liter or more and 2 mol / liter or less. By controlling the lithium salt concentration within the above range, an electrolyte solution with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0071] As non-aqueous solvents, cyclic carbonate esters, linear carbonate esters, cyclic ethers, linear ethers, nitriles, amides, etc., may be used. One of these solvents may be used, or two or more may be used in combination.

[0072] Examples of lithium salts that can be used include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One of these electrolyte salts may be used, or two or more may be used in combination.

[0073] Typically, it is desirable to interpose a separator between the positive and negative electrodes. The separator 52 has high ion permeability and appropriate mechanical strength and insulating properties. As the separator 52, a microporous thin film, woven fabric, and nonwoven fabric can be used. As the material of the separator 52, for example, a polymer can be used. The polymer may be polyolefin such as polypropylene and polyethylene.

[0074] In the battery according to Embodiment 3, the electrolyte may be impregnated into a polymer provided as a separator, for example. That is, the battery according to Embodiment 3 may have a structure in which both the electrolyte and the polymer are used in combination.

[0075] The battery according to Embodiment 3 may further contain a solid electrolyte as the electrolyte. That is, the battery of this disclosure may have a hybrid structure in which an electrolyte and a solid electrolyte are used in combination. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, or organic polymer solid electrolytes. In this disclosure, "halide solid electrolyte" means a solid electrolyte in which a halogen element is the main component of the anions. "Sulfide solid electrolyte" means a solid electrolyte in which sulfur is the main component of the anions. "Oxide solid electrolyte" means a solid electrolyte in which oxygen is the main component of the anions. The main component of the anions means the anion with the largest amount of substance among all the anions that make up the solid electrolyte.

[0076] As an example of the structure of the battery according to Embodiment 3, Figure 4 describes a configuration example, namely a cylindrical non-aqueous electrolyte secondary battery in which a wound electrode group, in which a positive electrode and a negative electrode are wound around a separator, and an electrolyte are housed in an outer casing. However, the battery according to this disclosure is not limited to this configuration example. The battery according to Embodiment 3 may take any form, such as prismatic, coin-type, button-type, laminate-type, etc. Furthermore, instead of a wound electrode group, other forms of electrode groups may be used as the electrode group in the battery according to Embodiment 3, such as an electrode group in which a positive electrode and a negative electrode are stacked with a separator.

[0077] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0078] (Technology 1) A carbon-silicon composite material comprising porous carbon particles and silicon particles dispersed inside the carbon particles, wherein the carbon-silicon composite material further comprises at least one element selected from the group consisting of Na, K, Ca, Sr, and Ba, and a halogen element, the content of the at least one element in the carbon-silicon composite material is 0.005% by mass or more, and the content of the halogen element in the carbon-silicon composite material is 0.1% by mass or more.

[0079] (Technology 2) The carbon-silicon composite material according to Technology 1, wherein the content of the halogen element in the carbon-silicon composite material is 1% by mass or more and 10% by mass or less.

[0080] (Technology 3) The carbon-silicon composite material according to Technology 1 or 2, wherein the halogen element contains chlorine.

[0081] (Technical 4) The carbon-silicon composite material according to any one of Technical 1 to 3, wherein the content of at least one element in the carbon-silicon composite material is less than 0.1% by mass.

[0082] (Technical 5) The carbon-silicon composite material according to any one of Technical 1 to 4, wherein the content of at least one element in the carbon-silicon composite material is less than 0.05% by mass.

[0083] (Technical 6) A carbon-silicon composite material according to any one of Technical 1 to 5, wherein at least one of the elements includes K.

[0084] (Technical 7) A negative electrode active material comprising a carbon-silicon composite material as described in any one of Technical 1 to 6.

[0085] (Technical 8) A battery comprising a negative electrode containing the negative electrode active material described in Technical 7, a positive electrode, and an electrolyte.

[0086] The present disclosure will be described in more detail below with reference to examples. The following examples are merely illustrative and not limited to any one aspect.

[0087] <Example 1> [Preparation of Carbon-Silicon Composite Material] Porous carbon particles were prepared with a total content of 0.020 mass% of element A (at least one selected from the group consisting of Na, K, Ca, Sr, and Ba). The content of element A in the prepared carbon particles and the content of element A in the prepared carbon-silicon composite material were determined by semi-quantitative analysis using ICP-AES (the same applies to subsequent examples). Among the elements A contained in the prepared carbon particles, K was the most abundant.

[0088] Next, 0.05 g of the prepared carbon particles were placed in a quartz cylindrical tubular furnace with an inner diameter of 50 mm using a glass boat, and the atmosphere inside the furnace was replaced with nitrogen by flowing nitrogen through it. Then, after stopping the inflow of nitrogen, the furnace temperature was raised to 540°C while simultaneously flowing dichlorosilane gas at a flow rate of 3 sccm and hydrogen at a flow rate of 10 sccm into the furnace as silicon precursor gases, and the decomposition reaction of the silicon precursor gases was carried out for 80 minutes. As a result, silicon particles were deposited inside the carbon particles, and the carbon-silicon composite material of Example 1 was obtained.

[0089] The proportion of silicon in the total mass of the obtained carbon-silicon composite material was evaluated by thermogravimetric (TG) measurement. The silicon content was 72.7 mass%. Based on the content of element A in the carbon particles and the above silicon content, the content of element A in the carbon-silicon composite material was calculated to be 0.0057 mass%. Similarly, the K content in the carbon-silicon composite material was calculated to be 0.0056 mass%, the Ca content to be 0.00005 mass%, and the Na content to be 0.00003 mass%. The Sr and Ba content were both 0 mass%. Furthermore, the types and content of halogen elements in the carbon-silicon composite material were evaluated by EDS. Chlorine was detected as a halogen element, and its content was 2.7 mass%.

[0090] [Battery Fabrication and Evaluation] (Fabrication of Negative Electrode Plate for Battery) The carbon-silicon composite material and graphite fabricated above were mixed in a mass ratio of 1:9 to prepare the negative electrode active material. Next, the negative electrode active material, sodium carboxymethylcellulose (CMCNa), styrene-butadiene rubber (SBR), and lithium polyacrylate salt were mixed in a mass ratio of negative electrode active material:CMCNa:SBR:lithium polyacrylate salt = 96.5:1:1.5:1 to prepare the negative electrode mixture. Next, water was added to the negative electrode mixture and stirred with a mixer to prepare the negative electrode slurry. Next, the prepared negative electrode slurry was applied to the surface of a copper foil to a thickness of 200 μm and dried at 60°C. A doctor blade was used for application. After drying, the material was rolled to a density of 1.6 g / mL to obtain the negative electrode plate.

[0091] (Battery Fabrication) The negative electrode plate fabricated above was punched out into a rectangle measuring 25 cm x 50 cm. The composite layer was peeled off the edges after punching to expose the copper foil. After connecting tab leads to the exposed copper foil, a predetermined area around the outer circumference of the tab leads was covered with an insulating film. Lithium foil was selected as the counter electrode and cut to a size slightly larger than the negative electrode plate. Next, an electrode winding was fabricated by winding the negative electrode plate and the counter electrode with a separator in between. Next, the electrode winding was inserted into an Al laminate film together with a reference electrode using a small piece of Li, and an electrolyte was poured in to obtain an evaluation cell. For the electrolyte, an ethylene carbonate (EC) / ethyl methyl carbonate (EMC) solution of sodium hexafluorophosphate (LiPF6) was used. The concentration of LiPF6 was 1 mol / L. The mixing ratio of EC and EMC was 3:7 by mass.

[0092] (Battery Evaluation) The evaluation cells prepared as described above were subjected to charging and discharging, and the initial charge-discharge efficiency was calculated. Charging and discharging were performed in an atmosphere of 25°C. The ratio of discharge capacity to charge capacity was defined as the initial charge-discharge efficiency. The initial charge-discharge efficiency of Example 1 was 80.9%.

[0093] <Comparative Example 1> [Material Preparation] Porous carbon particles were prepared with a total element A content of 0.0056 mass%. Of the elements A contained in the prepared carbon particles, K was present in the largest amount. Next, 0.05 g of the prepared carbon particles were placed in a quartz cylindrical tubular furnace with an inner diameter of 50 mm using a glass boat, and the atmosphere inside the furnace was replaced with nitrogen by flowing nitrogen through it. Next, after stopping the inflow of nitrogen, the furnace temperature was raised to 540°C while flowing 4 L of monosilane gas as a silicon precursor gas into the furnace to allow the decomposition reaction of the silicon precursor gas to proceed. As a result, silicon particles were deposited inside the carbon particles to obtain the carbon-silicon composite material of Comparative Example 1.

[0094] The obtained silicon-carbon composite material was confirmed, in the same manner as in Example 1, to have a silicon content of 52.6% by mass of the total mass and an element A content of 0.0027% by mass. The potassium content in the silicon-carbon composite material was 0.00096% by mass, the sodium content was 0.00095% by mass, the ca content was 0.00074% by mass, and the barium content was 0.00001% by mass. The syrup content was 0% by mass. Halogen elements were not detected by EDS analysis.

[0095] [Battery Fabrication and Evaluation] A battery was fabricated in the same manner as in Example 1, except that the carbon-silicon composite material of Comparative Example 1 was used, and its initial charge-discharge efficiency was evaluated. The initial charge-discharge efficiency of Comparative Example 1 was 79.5%.

[0096] <Comparative Example 2> [Material Preparation] Using 0.05 g of the same carbon particles prepared in Comparative Example 1, the decomposition reaction of silicon precursor gas was carried out in the same manner as in Example 1. As a result, silicon particles were deposited inside the carbon particles to obtain the carbon-silicon composite material of Comparative Example 2.

[0097] The obtained silicon-carbon composite material was confirmed, in the same manner as in Example 1, to have a silicon content of 53.8% by mass, an element A content of 0.0026% by mass, and a halogen element content of 5.6% by mass. Chlorine was detected as the halogen element. The K content in the silicon-carbon composite material was 0.00093% by mass, the Na content was 0.00092% by mass, the Ca content was 0.00073% by mass, and the Ba content was 0.00001% by mass. The Sr content was 0% by mass.

[0098] [Battery Fabrication and Evaluation] A battery was fabricated in the same manner as in Example 1, except that the carbon-silicon composite material of Comparative Example 2 was used, and its initial charge-discharge efficiency was evaluated. The initial charge-discharge efficiency of Comparative Example 2 was 73.8%.

[0099] <Comparative Example 3> [Material Preparation] Using 0.5 g of the same carbon particles prepared in Comparative Example 1, the decomposition reaction of silicon precursor gas was carried out in the same manner as in Comparative Example 1. However, the amount of monosilane gas flowed into the furnace was 2.7 L. This caused silicon particles to precipitate inside the carbon particles, obtaining the carbon-silicon composite material of Comparative Example 3.

[0100] The obtained silicon-carbon composite material was confirmed, in the same manner as in Example 1, to have a silicon content of 33.0% by mass of the total mass and an element A content of 0.0038% by mass. The potassium content in the silicon-carbon composite material was 0.0014% by mass, the sodium content was 0.0013% by mass, the ca content was 0.0011% by mass, and the barium content was 0.00002% by mass. The syrup content was 0% by mass. Halogen elements were not detected by EDS analysis.

[0101] [Battery Fabrication and Evaluation] A battery was fabricated in the same manner as in Example 1, except that the carbon-silicon composite material of Comparative Example 3 was used, and its initial charge-discharge efficiency was evaluated. The initial charge-discharge efficiency of Comparative Example 3 was 36.0%.

[0102] <Comparative Example 4> [Material Preparation] Porous carbon particles were prepared with a total element A content of 0.11% by mass. Of the elements A contained in the prepared carbon particles, K was present in the largest amount. Next, the decomposition reaction of silicon precursor gas was carried out on 0.5 g of the procured carbon particles in the same manner as in Comparative Example 1. However, the amount of monosilane gas flowed into the furnace was 2.7 L. As a result, silicon particles were precipitated inside the carbon particles to obtain the carbon-silicon composite material of Comparative Example 4.

[0103] The obtained silicon-carbon composite material was confirmed, in the same manner as in Example 1, to have a silicon content of 28.1% by mass of the total mass and an element A content of 0.081% by mass. The potassium content in the silicon-carbon composite material was 0.046% by mass, the sodium content was 0.024% by mass, the ca content was 0.010% by mass, the sr content was 0.00019% by mass, and the barium content was 0.00012% by mass. Halogen elements were not detected by EDS analysis.

[0104] [Battery Fabrication and Evaluation] A battery was fabricated in the same manner as in Example 1, except that the carbon-silicon composite material of Comparative Example 4 was used, and its initial charge-discharge efficiency was evaluated. The initial charge-discharge efficiency of Comparative Example 4 was 31.4%.

[0105] The results of the examples and comparative examples are summarized in Table 1.

[0106]

[0107] As shown in Table 1, it was confirmed that the carbon-silicon composite material of Example 1, which satisfies both the total content of element A (0.005% by mass or more) and the halogen element content (0.1% by mass or more), can improve the initial charge-discharge efficiency of the battery compared to the comparative example that does not satisfy either or both of these conditions.

[0108] From the above, it has been confirmed that the charge and discharge efficiency of batteries can be improved by using the carbon-silicon composite material disclosed herein.

[0109] The technology disclosed herein can be used in batteries such as lithium-ion secondary batteries.

Claims

1. A carbon-silicon composite material comprising porous carbon particles and silicon particles dispersed inside the carbon particles, wherein the carbon-silicon composite material further comprises at least one element selected from the group consisting of Na, K, Ca, Sr, and Ba, and a halogen element, the content of the at least one element in the carbon-silicon composite material is 0.005% by mass or more, and the content of the halogen element in the carbon-silicon composite material is 0.1% by mass or more.

2. The carbon-silicon composite material according to claim 1, wherein the content of the halogen element in the carbon-silicon composite material is 1% by mass or more and 10% by mass or less.

3. The silicon-carbon composite material according to claim 1, wherein the halogen element includes chlorine.

4. The carbon-silicon composite material according to claim 1, wherein the content of at least one element in the carbon-silicon composite material is less than 0.1% by mass.

5. The carbon-silicon composite material according to claim 1, wherein the content of at least one element in the carbon-silicon composite material is less than 0.05% by mass.

6. The carbon-silicon composite material according to claim 1, wherein at least one of the elements includes K.

7. A negative electrode active material comprising the carbon-silicon composite material according to any one of claims 1 to 6.

8. A battery comprising a negative electrode containing the negative electrode active material described in claim 7, a positive electrode, and an electrolyte.

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