Silicon-containing amorphous carbon material and its manufacturing method, and lithium-ion secondary battery

Abstract

To provide a negative electrode carbon material for a lithium ion secondary battery or the like, the negative electrode carbon material having an excellent capacity retention rate, and a manufacturing method therefor.SOLUTION: There is provided a silicon-containing amorphous carbon material containing silicon oxide particles in easily graphitized amorphous carbon. The silicon-containing amorphous carbon material are spherical particles each of which comprises a core particle at a particle center of a particle and a shell layer outside the core particle. Voids are present between the core particle and the shell layer and also in the shell layer. The silicon-containing amorphous carbon material can be obtained by a manufacturing method including: a step of mixing a carbon raw material, silicon particles or silicon oxide particles, and a porosity agent to perform dry granulation; and a step of carbonizing granulated particles at a carbonization temperature of 800-1,200°C under an inert gas atmosphere.SELECTED DRAWING: None

Description

Technical Field

[0001] The technology disclosed in this specification relates to a silicon-containing amorphous carbon material used for the negative electrode of a lithium-ion secondary battery and a method for producing the same.

Background Art

[0002] Lithium-ion secondary batteries have been put into practical use as power sources for driving portable electronic devices such as mobile phones and notebook computers because they are lighter and have a higher capacity compared to conventional secondary batteries such as nickel-cadmium batteries, nickel-metal hydride batteries, and lead batteries. They are also used as power sources for electric vehicles and hybrid vehicles.

[0003] As materials for the negative electrode, silicon, tin, germanium, which alloy with lithium, and their oxides can be used. However, these materials expand in volume during charging when they occlude lithium ions, and contract in volume during discharging when they release lithium ions. Therefore, due to the volume change during repeated charge-discharge cycles, the negative electrode material may fall off and disintegrate from the electrode.

[0004] Patent Document 1 describes an active material for a lithium-ion secondary battery containing silicon oxide and a carbon material. Since this active material has voids inside, the volume change during charge and discharge is suppressed to a small value.

[0005] Also, Patent Document 2 describes a technique for preventing electrode breakage during charge and discharge by embedding lithium occluding material particles in a carbon material and reducing the size of the lithium occluding material particles.

[0006] Also, Patent Document 3 describes an amorphous carbon material containing SiOx (0 < x < 2) in easily graphitizable amorphous carbon, which is obtained by mixing and granulating green coke and silicon particles or silicon oxide particles and then carbonizing them. By using this amorphous carbon material, the volume change during charge and discharge is small, and the cycle characteristics can be improved.

Prior Art Documents

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-30428 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-71938 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-90748 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the active material for lithium-ion secondary batteries described in Patent Document 1 is obtained by carbonizing a sprayed resin aqueous solution together with colloidal silica, and therefore is nearly spherical and has a sharp particle size distribution. Therefore, when an electrode is produced, there are few inter-particle contacts, and it is necessary to incorporate a large amount of conductive material. Furthermore, the method described in Patent Document 1 requires many steps to produce the active material, which is considered impractical.

[0009] Furthermore, although the technology described in Patent Document 2 can suppress to some extent the volume change that occurs when lithium is absorbed and released in the lithium absorption material particles, it cannot sufficiently suppress the change, making it difficult to sufficiently prevent destruction of the negative electrode and improve the cycle characteristics.

[0010] Furthermore, although the technology described in Patent Document 3 improves cycle characteristics, the initial discharge capacity and initial efficiency are low.

[0011] In view of the above problems, an object of the present invention is to provide a material for a negative electrode of a lithium ion secondary battery or the like, which has an improved capacity retention rate. [Means for solving the problem]

[0012] That is, the first aspect of the present invention is a carbon fiber composite material comprising easily graphitizable amorphous carbon, A silicon-containing amorphous carbon material containing silicon oxide particles represented by SiOx (0 < x < 2) in the easily graphitizable amorphous carbon, The silicon-containing amorphous carbon material is spherical particles and consists of a core particle at the center of the particle and a shell layer outside the core particle, There is a void between the core particle and the shell layer, and there are also voids in the shell layer. The present invention relates to a silicon-containing amorphous carbon material characterized by this. Further, a second gist of the present invention resides in a method for producing a silicon-containing amorphous carbon material according to the first gist, which comprises a step of mixing a carbon raw material, silicon particles or silicon oxide particles, and a pore-forming agent and performing dry granulation, and a step of carbonizing the granulated particles in an inert gas atmosphere. Further, a third gist of the present invention resides in a lithium-ion secondary battery using the silicon-containing amorphous carbon material according to the first gist as a carbon material for a negative electrode.

Effects of the Invention

[0013] According to the silicon-containing amorphous carbon material of the present invention, a secondary battery excellent in capacity retention rate can be obtained without reducing the initial discharge capacity and initial efficiency.

Brief Description of the Drawings

[0014] [Figure 1] FIG. 1 is a diagram showing a microscopic photograph of a cross section of the amorphous carbon material according to Example 3. [Figure 2] FIG. 2 is a diagram showing a microscopic photograph of a cross section of the amorphous carbon material according to Comparative Example 1.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, the present invention will be described in detail. Note that what is described below is an example of an embodiment, and the constituent materials, the shapes of the constituent materials or members, the conditions of processing and heat treatment, etc. can be appropriately changed without departing from the gist of the present invention.

[0016] The "roundness" used in this specification is an index of the roundness of particles or the like, and is a value obtained by the following formula (1).

[0017] [Number 1] (Circularity) = {4 × π × (projected area)} / {(perimeter length) × 2} ··· (1)

[0018] -Explanation of silicon-containing amorphous carbon material- FIG. 1 is a diagram showing a micrograph of a cross-section of a silicon-containing amorphous carbon material according to an embodiment of the present invention.

[0019] The silicon-containing amorphous carbon material according to this embodiment includes amorphous carbon, and silicon oxide particles represented by SiOx (0 < x < 2) are contained in the amorphous carbon. The silicon oxide particles in the amorphous carbon exist, for example, in a dispersed state. The amorphous carbon is easily graphitizable carbon, so-called soft carbon. Each silicon-containing amorphous carbon material is composed of a plurality of carbon particles derived from raw materials.

[0020] According to this configuration, since the amorphous carbon contains silicon oxide particles, when used as a negative electrode material of a lithium ion secondary battery, it is possible to improve the initial discharge capacity.

[0021] The silicon-containing amorphous carbon material according to this embodiment is a spherical particle and is composed of a core particle at the center of the particle and a shell layer outside the core particle. The core particle is mainly composed of amorphous carbon, and the shell layer contains silicon oxide particles in the amorphous carbon. Further, there is a void between the core particle and the shell layer, and there are also voids in the shell layer. With such a configuration, it is possible to suppress the volume change when lithium is occluded and released.

[0022] In the silicon-containing amorphous carbon material according to this embodiment, the void existing between the core particle and the shell layer is crescent-shaped, and the length in the major axis direction is preferably 1 to 10 μm. The presence of a void between the core particle and the shell layer can relax the expansion of SiO during Li occlusion and maintain the particle shape. The length in the minor axis direction is preferably 0.1 to 1 μm.

[0023] In the silicon-containing amorphous carbon material according to this embodiment, the voids present in the shell layer preferably have a length in the major axis direction of 0.01 to 8 μm. The presence of voids in the shell layer alleviates the expansion of SiO during Li absorption, thereby maintaining the particle shape. The length in the minor axis direction is preferably 0.01 to 0.5 μm.

[0024] In the silicon-containing amorphous carbon material according to this embodiment, it is preferable that each particle have 20 or more voids each having a major axis length of 1 μm or more, and 100 or more voids each having a major axis length of 1 μm or less.

[0025] Furthermore, in the silicon-containing amorphous carbon material of this embodiment, it is more preferable that the molar ratio x of oxygen to silicon (SiOx) is 0.2 or more and less than 2.0, since this allows for a good balance between improving the initial discharge capacity and providing a certain level or more of cycle performance. It is even more preferable that the molar ratio x of silicon to oxygen (SiOx) is 0.3 to 1.7.

[0026] The silicon-containing amorphous carbon material has an average particle size D50 of, for example, about 5 to 40 μm. If the average particle size exceeds 40 μm, the strength of the carbon material may decrease, and it may be difficult to form an electrode with an appropriate film thickness when producing a negative electrode. Furthermore, if the carbon material has an average particle size of less than 5 μm, it is difficult to disperse silicon oxide particles in the amorphous carbon particles. The average particle size of the silicon-containing amorphous carbon material is more preferably 10 to 30 μm. The maximum particle size of the silicon-containing amorphous carbon material is about 45 μm or less.

[0027] The silicon content in the silicon-containing amorphous carbon material is preferably 1% by weight or more and 50% by weight or less. This is because granulation is easy if the silicon content is 50% by weight or less. In order to fully obtain the effect of improving capacity, the silicon content is preferably 5% by weight or more, and more preferably 2 to 40% by weight.

[0028] The carbon content in the silicon-containing amorphous carbon material is preferably 50 to 100% by weight. If it is less than 50% by weight, it is difficult to granulate, which is inconvenient. It is more preferably 52% by weight or more.

[0029] Furthermore, the circularity of the silicon-containing amorphous carbon material of this embodiment is preferably about 0.60 to 1.0, and more preferably 0.70 to 0.98. This configuration allows for increased packing density and electrode density. If the circularity is less than 0.6, the composite effect cannot be fully achieved, and particles tend to get caught on each other, resulting in low packing density and electrode density. The circularity does not exceed 1.0, and even a material with a circularity of 1.0 can achieve the effects of the present invention. However, in order to improve packing density and increase contact points between particles, a circularity of 0.98 or less is more preferable. However, even if the circularity of the silicon-containing amorphous carbon material is outside the above-mentioned range, it still has the effect of suppressing volume change during charge and discharge to a smaller extent than conventional carbon materials, and therefore can be used as a negative electrode material for lithium-ion secondary batteries.

[0030] The BET specific surface area of ​​the silicon-containing amorphous carbon material of this embodiment is 0.3 to 10 m 2 By having the BET specific surface area in the above range, the activity of the particle surface can be reduced, which is preferable in terms of reducing side reactions during charge and discharge when used in a secondary battery.

[0031] The amorphous carbon contained in the silicon-containing amorphous carbon material produced using raw coke preferably contains approximately 700 to 2500 ppm of transition metals.

[0032] In this way, it is believed that the inclusion of a transition metal in amorphous carbon has the effect of promoting the insertion or desorption of lithium, and the doping of silicon oxide with a transition metal can mitigate the expansion or contraction of silicon oxide particles.

[0033] According to the silicon-containing amorphous carbon material described above, since high-capacity silicon oxide particles are dispersed in the amorphous carbon, the initial charge capacity and initial discharge capacity can be made larger than when the material is composed of amorphous carbon alone.

[0034] Here, silicon oxide particles or silicon particles are used as the silicon source for the silicon-containing amorphous carbon material, as described below, and the silicon-containing amorphous carbon material can be obtained by mixing the materials in an appropriate compounding ratio in each case.

[0035] Furthermore, the carbon material inside the spherical particles fuses together during firing, eliminating grain boundaries, ensuring sufficient diffusion paths for lithium, allowing lithium to be inserted and extracted quickly.

[0036] Furthermore, the silicon-containing amorphous carbon material of this embodiment can absorb and release lithium ions isotropically in the amorphous carbon portion, allowing for faster charge and discharge than graphite. Furthermore, the inclusion of silicon oxide provides a high capacity. Therefore, the silicon-containing amorphous carbon material of this embodiment is particularly suitable for use in lithium-ion secondary batteries for electric vehicles.

[0037] Furthermore, since the absorption and desorption of lithium ions occurs in the same direction, the volume change in one direction is small, making the negative electrode less susceptible to damage than when a highly crystalline graphite material is used.

[0038] The silicon-containing amorphous carbon material of this embodiment can be used not only in lithium ion secondary batteries but also as a negative electrode material for lithium ion capacitors and the like.

[0039] -Method for producing silicon-containing amorphous carbon material (negative electrode material)-

[0040] [Gaining agent] In the present invention, a voiding agent is used to produce a carbon material, thereby forming appropriate voids in the carbon material. Examples of the voiding agent include polysaccharides that have an exothermic peak of 300°C or higher during thermal decomposition, and mineral oils that have a fixed carbon content of 10% or less when fired at 1000°C.

[0041] The polysaccharide is preferably crystalline cellulose. Crystalline cellulose remains almost unchanged up to around 300°C, but decomposes rapidly at 300-350°C. This rapid thermal decomposition rapidly generates volatile gases, temporarily trapping high-pressure gas inside the particles and enhancing the effect of void formation.

[0042] As the mineral oil, coal tar, ethylene bottom oil, paraffinic, or naphthenic mineral oil can be used.

[0043] The amount of the gap agent added is preferably 10 to 50 wt% based on the total amount of the carbon raw material and silicon oxide particles or silicon particles. If the amount of the gap agent added is less than 10 wt%, it becomes difficult to form the desired voids. If the amount of the gap agent added exceeds 50 wt%, it is not preferable because a sufficient spherical shape cannot be obtained in the granulation process and the particle strength cannot be obtained even after firing. A more preferable amount of the gap agent added is 15 to 45 wt%.

[0044] [Carbon raw materials] The silicon-containing amorphous carbon material of this embodiment can be produced using green coke such as needle coke or mosaic coke, etc. Green coke can be obtained by heating heavy oil to about 300°C to 700°C using coking equipment such as a delayed coker to cause thermal decomposition and polycondensation.

[0045] For example, a petroleum-based raw coke can be used that has an optically isotropic texture of 75% or more, more preferably 85% or more, evenly dispersed in a cross section observed under a polarizing microscope, and a total transition metal content of 700 ppm to 2500 ppm. Because this raw coke contains a large amount of transition metals and other impurities, it is believed that when used as a negative electrode material for a lithium-ion secondary battery, the efficiency of Li insertion / extraction will be improved.

[0046] The carbon raw material used in the present invention has a volatile component content (when fired at 1000°C) of 10 to 45% by weight. The volatile component content within this range is preferred in terms of improving the effect of reducing silicon oxide. The volatile component content is more preferably 15 to 38%.

[0047] The impurity content (ash content) of the carbon raw material used in the present invention is preferably 0.5 to 2% by weight. When the impurity content is within this range, it is considered that the efficiency of Li insertion / extraction is improved when the carbon raw material is used as a negative electrode material for a lithium ion secondary battery. The impurity content is more preferably 0.5 to 1.0% by weight.

[0048] Petroleum-based raw coke is pulverized using a mechanical pulverizer, such as a Super Rotor Mill (manufactured by Nisshin Engineering Co., Ltd.) or a Jet Mill (manufactured by Nippon Pneumatic Mfg. Co., Ltd.).

[0049] The average particle size (D50) after pulverization is 1 μm or more and 15 μm or less, and more preferably 3 μm or more and 10 μm or less. The average particle size is based on measurements using a laser diffraction particle size analyzer. If the D50 is less than 1 μm, the required pulverization energy becomes enormous, which is not practical. If the D50 is less than 3 μm, it may not be possible to impart sufficient mechanical energy to the particles during dry granulation. Furthermore, if the D50 exceeds 15 μm, the number of particles of a size suitable for use as a negative electrode material for lithium-ion secondary batteries after granulation is reduced, which is undesirable.

[0050] The pulverized product can be further classified using a precision air classifier such as Turbo Classifier (manufactured by Nisshin Engineering Co., Ltd.), Elbow Jet (manufactured by Nittetsu Mining Co., Ltd.), or Cruseal (manufactured by Seishin Enterprise Co., Ltd.).

[0051] [Silicon raw material] Silicon oxide particles or silicon particles are used. Here, the average particle size of the silicon raw material is not particularly limited, but by setting it to 1 μm or less, the expansion width of the silicon oxide particles during charge and discharge of the silicon-containing amorphous carbon material becomes small, and the carbon layer can suppress volume change.

[0052] <When using silicon oxide particles> Here, as an example, a case where silicon oxide particles having an average particle size of about 20 to 30 nm are used will be described. Note that, since the preferred embodiment differs depending on the type of silicon raw material, the case where silicon particles are used will be described later.

[0053] (Dry granulation process) First, the carbon raw material and silicon oxide particles are thoroughly mixed and dry granulated. The mixing ratio of the carbon raw material and the silicon raw material is not particularly limited, but the ratio of the silicon oxide particles is preferably 1 to 50 wt% when the sum of the weights of the carbon raw material and the silicon oxide particles is 100%. The ratio of the silicon oxide particles is more preferably 10 to 50 wt%, and even more preferably 30 to 50 wt%.

[0054] For this treatment, an apparatus capable of sphering treatment by simultaneously applying stresses such as shearing, compression, and collision can be used, but the treatment apparatus is not limited to an apparatus using such a structure and principle.

[0055] Examples of equipment used in this process include ball-type kneaders such as rotary ball mills, wheel-type kneaders such as edge runners, Hybridization System (manufactured by Nara Machinery Works), Mechanofusion (manufactured by Hosokawa Micron Corporation), Nobilta (manufactured by Hosokawa Micron Corporation), and COMPOSI (manufactured by Nippon Coke and Engineering Co., Ltd.). Equipment with a structure in which consolidation stress or compression stress is applied to the powder in the gap between the rotating blades and the housing is particularly preferred. By controlling the temperature applied to the powder during treatment to 60°C to 300°C, the volatile matter contained in the raw coke creates an appropriate degree of adhesion, causing particles to instantly adhere to each other, thereby promoting powder growth.

[0056] Controlling the circularity of the carbon raw material used as the raw material allows for control of the circularity of the powder obtained after shaping by compressive shear stress. When the circularity of the carbon raw material used as the raw material is approximately 0.5 to 0.8, the circularity of the powder obtained after shaping by compressive shear stress can be controlled to greater than 0.60 and 1.0 or less. The circularity of the obtained powder is preferably 0.65 to 0.98. Even if the circularity of the powder is 1.0, the effect of mitigating the effects of expansion and contraction of silicon oxide particles can be obtained. However, particles processed to a circularity of more than 0.98 are close to perfect spheres, resulting in fewer contact points between particles. In particular, the circularity of the particles is preferably in the range of 0.80 to 0.96.

[0057] Furthermore, in the present invention, it is not necessary to use an apparatus capable of spheronization. To produce a molded body, for example, a method can be used in which a pulverized carbon raw material and silicon oxide particles are mixed with good dispersibility, a molded body of a predetermined shape is produced using a hydraulic press, and the resulting product is then fired and pulverized in a pulverizer. Examples of pulverizers include a bead mill (MSC mill; Nippon Coke) and a jet mill (manufactured by Nippon Pneumatic Industrial Co., Ltd.).

[0058] When mixing silicon oxide particles with a carbon raw material, the entire amount of silicon oxide particles may be mixed with the carbon raw material. However, since a large amount of silicon oxide particles makes granulation difficult, it is also possible to mix the carbon raw material with a portion of the silicon oxide particles to start granulation, and then add the silicon oxide particles in multiple batches (for example, three or more batches). Alternatively, the silicon oxide particles and the carbon raw material may be added after the silicon oxide particles and the like are added at the start of granulation, or only the carbon raw material may be added at the end of granulation to coat the surfaces of the silicon oxide particles with the carbon raw material. In addition, in this process, a portion of the silicon oxide may be replaced with elemental silicon.

[0059] Furthermore, it is also possible to composite the raw coke with a different material by replacing a portion of the raw coke used for granulation with a carbon material such as acetylene black, or an inorganic or organic compound such as a transition metal compound. A portion of the raw coke added at the start of granulation or during granulation may be replaced with the different material, as long as it does not interfere with granulation, or the different material alone may be added during granulation. The amount of the different material added is not particularly limited as long as it does not interfere with granulation. The average particle size of the different material is not particularly limited as long as it does not interfere with granulation, but it is preferably ½ or less of the granulated particle size at the time of addition.

[0060] (carbonization process) Next, the granulated particles are carbonized by any method, but examples thereof include a method of heat treating the particles in an inert gas atmosphere such as nitrogen or argon at a maximum temperature of 800 to 1200°C for a holding time at the maximum temperature of more than 0 hours and not more than 10 hours.

[0061] If the carbonization temperature is 800°C or higher, the amount of low-molecular-weight hydrocarbons and functional groups remaining in the raw coke can be reduced, effectively suppressing the increase in irreversible capacity due to these impurities. If the carbonization temperature is 1200°C or lower, the generation of insulating silicon carbide in the material can be suppressed, which is preferable. A carbonization temperature of approximately 900 to 1100°C is particularly preferable. By setting the carbonization temperature to 900°C or higher, the increase in irreversible capacity due to the remaining low-molecular-weight hydrocarbons, etc. can be more effectively suppressed.

[0062] In the carbonization step, the holding time at the maximum reaching time may be longer than 10 hours, but this is not economical because the heat treatment will continue after the carbonization is completed.

[0063] It is believed that the carbonization process promotes the reduction of silicon oxide by the volatile components in the raw coke. Furthermore, when the gases from the volatile components generated during carbonization escape to the outside, gas release pathways are formed within the particles. When the raw coke is used as a negative electrode material for lithium-ion secondary batteries, these pathways act as lithium diffusion pathways and also act as buffers for the expansion and contraction of silicon oxide particles.

[0064] According to the above method, compared to the method described in Patent Document 1, the material used for the negative electrode of a lithium ion secondary battery can be produced more easily.

[0065] <When using silicon particles> As another example, a case where silicon particles are used instead of silicon oxide particles will be described.

[0066] Silicon particles are prone to forming an oxide film on their surface when handled in air, and in some cases, an oxide film is formed on the surface of the silicon particles in advance to prevent excessive oxidation of the silicon particles. However, these silicon particles can also be used in the present invention.

[0067] (Dry granulation process) First, raw coke particles and silicon particles are thoroughly mixed and dry granulated. The mixing ratio of the carbon raw material and the silicon raw material is not particularly limited, but it is preferable that the ratio of silicon particles is 2 to 90 wt% when the sum of the weights of the carbon raw material and the silicon particles is 100%. In particular, silicon particles with a low oxidation number expand and contract significantly, so the ratio of silicon particles is preferably 5 to 50 wt%, more preferably 5 to 35 wt%.

[0068] In this treatment, as in the above-mentioned method, an apparatus capable of simultaneously applying stresses such as shear, compression, and collision can be used.

[0069] Controlling the circularity of the raw coke used as a raw material allows for control of the circularity of the powder obtained after shaping by compressive shear stress. When the circularity of the raw coke used as a raw material is approximately 0.5 to 0.8, the circularity of the powder obtained after shaping by compressive shear stress can be controlled to greater than 0.60 and less than 1.0. The circularity of the obtained powder is preferably 0.65 to 0.98. Even if the circularity of the powder is 1.0, the effect of mitigating the effects of expansion and contraction of silicon particles can be obtained. However, particles processed to a circularity of more than 0.98 are close to perfect spheres, resulting in fewer contact points between particles. In particular, the circularity of the particles is preferably in the range of 0.80 to 0.96.

[0070] Here, the entire amount of silicon particles may be mixed with raw coke, but if the amount of silicon particles is too large, granulation becomes difficult. Therefore, after mixing raw coke with a portion of the silicon particles and starting granulation, the silicon particles may be added in multiple batches (for example, three or more batches). Alternatively, silicon particles and raw coke may be added after the silicon particles are introduced at the start of granulation, or only raw coke may be added at the end of granulation to coat the surfaces of the silicon particles with raw coke. In addition, in this process, a portion of the silicon may be replaced with silicon oxide.

[0071] Furthermore, it is also possible to composite the raw coke with a different material by replacing a portion of the raw coke used for granulation with a carbon material such as acetylene black, or an inorganic or organic compound such as a transition metal compound. A portion of the raw coke added at the start of granulation or during granulation may be replaced with the different material, as long as it does not interfere with granulation, or the different material alone may be added during granulation. The amount of the different material added is not particularly limited as long as it does not interfere with granulation. The average particle size of the different material is not particularly limited as long as it does not interfere with granulation, but it is preferably ½ or less of the granulated particle size at the time of addition.

[0072] (carbonization process) Next, the granulated particles are carbonized by any method, but examples thereof include a method of heat treating the particles in an inert gas atmosphere such as nitrogen or argon at a maximum temperature of 800°C to 1200°C for a holding time at the maximum temperature of more than 0 hours and not more than 10 hours.

[0073] A carbonization temperature of 800°C or higher can reduce the amount of low-molecular-weight hydrocarbons and functional groups remaining in the raw coke, effectively suppressing the increase in irreversible capacity due to these impurities.A carbonization temperature of 1200°C or lower is preferable because it can suppress the formation of insulating silicon carbide in the material.

[0074] It is particularly preferable if the carbonization temperature is about 900 to 1100° C. By setting the carbonization temperature to 900° C. or higher, it is possible to suppress an increase in irreversible capacity due to the residue of low-molecular-weight hydrocarbons and the like.

[0075] In the carbonization step, the holding time at the maximum reaching time may be longer than 10 hours, but this is not economical because the heat treatment will continue after the carbonization is completed.

[0076] It is believed that this carbonization treatment involves the action of the volatile components in the raw coke reducing the oxide film on the surface of the silicon particles. Carbon materials containing silicon particles with a low oxidation number exhibit high capacity and are therefore preferred as negative electrode materials, but the problem is that silicon particles with a lower oxidation number tend to expand and contract more. According to the present invention, voids formed when volatile gases generated during carbonization escape to the outside buffer the expansion and contraction of the silicon particles, thereby providing a high-capacity silicon-containing amorphous carbon material. Furthermore, when volatile gases generated during carbonization escape to the outside, gas release paths are formed within the particles, and these release paths become paths for lithium diffusion when the material is used as a negative electrode material for a lithium-ion secondary battery.

[0077] The above method also makes it possible to easily produce a material to be used for the negative electrode of a lithium ion secondary battery.

[0078] Furthermore, in the manufacturing method of this embodiment, the magnitude of the surface irregularities of the granulated particles can be adjusted. Specifically, in the granulation step, the surface irregularities can be increased by shortening the granulation time, lowering the pressure during granulation, increasing the blending ratio of silicon oxide, or by adding, during granulation, green coke particles with a larger particle size than the green coke particles added at the beginning of granulation. Conversely, the surface irregularities can also be reduced by adding, during granulation, green coke particles with a smaller particle size than the green coke particles added at the beginning of granulation.

[0079] - Structure of lithium-ion secondary battery - The lithium ion secondary battery according to this embodiment includes a negative electrode, a negative electrode current collector, a positive electrode, a positive electrode current collector, a separator interposed between the negative electrode and the positive electrode, and an exterior made of an aluminum laminate film or the like.

[0080] The negative electrode may be, for example, a metal foil coated on one or both sides with the amorphous carbon-containing material of the present embodiment. The average particle size and circularity of the coated silicon-containing amorphous carbon material are almost unchanged before and after the battery manufacturing process, being 5 to 40 μm and 0.60 to 1.0, respectively.

[0081] To fabricate the negative electrode, the granulated silicon-containing amorphous carbon material was mixed with conductive additives such as acetylene black (AB; Denka) and carbon nanofiber (VGCF-H; Showa Denko), as well as KF polymer (polyvinylidene fluoride (PVdF; Kureha Chemical)). The mixture was then mixed in N-methylpyrrolidone (NMP) as a solvent using a planetary centrifugal mixer (Thinky Corporation), and the resulting paste was applied to a copper foil current collector.

[0082] The shapes and materials of the members other than the negative electrode, such as the negative electrode current collector, positive electrode, positive electrode current collector, separator, and exterior, may be of general shape and material.

[0083] The lithium ion secondary battery according to this embodiment has a negative electrode coated with the silicon-containing amorphous carbon material, which allows for rapid charge and discharge, has a large capacity, and is less likely to collapse even after repeated charge and discharge. Furthermore, the battery has a high energy density, a small irreversible capacity, and improved cycle characteristics.

[0084] This is just one example of a lithium ion secondary battery, and the shape of each component, the number of electrodes, size, etc. may be changed as appropriate. [Example]

[0085] The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples in any way.

[0086] (a) Measurement of volatile components and ash content in carbon raw materials: The carbon raw material was calcined in an electric furnace at 1000°C for 5 hours in an inert gas atmosphere, and the weight change before and after calcination was confirmed to quantitatively analyze the volatile components in the carbon raw material. In addition, the material was calcined in an electric furnace at 1000°C for 5 hours in an air atmosphere, and the weight change before and after calcination was confirmed, and a quantitative analysis of the ash content was performed.

[0087] (b) Measurement of mean particle size (D50): Measurement was performed using a laser diffraction scattering particle size distribution analyzer LMS-2000e (Malvern Instruments).

[0088] (c) BET specific surface area measurement: The BET specific surface area was measured using a Multisorb (Malvern Instruments).

[0089] (d) Measurement of carbon content of amorphous carbon materials: The carbon content in the samples was quantitatively analyzed using a carbon and sulfur analyzer (EMIA-U520; Horiba Seisakusho Co., Ltd.) that uses the combustion-infrared absorption method.

[0090] (e) Measurement of oxygen content in amorphous carbon materials: The oxygen content in the samples was quantitatively analyzed using an oxygen / nitrogen analyzer (EMGA-920; Horiba Seisakusho Co., Ltd.) that uses an inert gas fusion-infrared absorption method.

[0091] (f) Measurement of silicon content of amorphous carbon materials: The sample was ashed at 1050°C, and the remaining amount was used as the silicon dioxide content to calculate the silicon content. The O / Si ratio was calculated based on the molar concentrations in the sample obtained from the oxygen and silicon contents.

[0092] (g) Measurement of circularity: The sheet was dispersed and fixed so that the particles did not stack and the flat surfaces of the flat particles were aligned parallel to the sheet, and the image was taken from directly above the sheet using a scanning electron microscope (S-4800, Hitachi High-Tech Corporation), and the image was analyzed using an A-zo-kun microscope (Asahi Kasei Engineering Co., Ltd.). In this example and comparative example, the projected area and projected perimeter were measured for 300 particles, and the circularity was calculated to determine the average circularity.

[0093] (h) Measurement of true specific gravity Place approximately 1.5 g of sample into a 10 ml pycnometer (Wa) and weigh it to an accuracy of four decimal places (Wb). Add approximately 4 ml of n-butanol to the pycnometer and process in an ultrasonic bath for 5 minutes. Add n-butanol up to the mark on the pycnometer, leave it in a thermostatic bath at 25°C for 5 minutes, then put the lid on and measure the weight (Wc). The true specific gravity was calculated using the following formula: True density = (Wb - Wa) / (Wb - Wa - Wc + Wd) × density of n-butanol (25°C) Wa: pycnometer weight Wb: Pycnometer + sample weight Wc: Weight of the pycnometer + sample + n-butanol up to the gauge line Wd: Weight of the pycnometer + n-butanol up to the gauge line

[0094] (i) Measurement of porosity 1) Measure the true specific gravity of each material: raw coke baked at 1000°C, nano Si powder, and nano SiO2 powder. 2) The theoretical value of true specific gravity is the sum of the true specific gravity of each material multiplied by the composition (%) of each material to be compounded. (ρa) In addition, measure the actual true specific gravity of the composite granulated sample (ρb). 3) Calculate the ratio (X) of the theoretical true specific gravity of each material to be compounded to the measured true specific gravity of the compounded sample. ρb / ρa X 4) Calculate the void ratio (Y) of the composite granulation sample when the theoretical volume of the composite granulation material is taken as 1. X-1 Y 5) The porosity (Z) of the composite granulated sample was calculated using the following formula. Y / X ×100(%)...Z(porosity)

[0095] (j) Cross-sectional observation of particles: Cross-sectional photographs of the particles were taken using a scanning electron microscope (S-4800, Hitachi High-Technologies Corporation) after processing the particles embedded in resin with a cross-section polisher (CP).

[0096] (k) Battery fabrication and evaluation test for half-cell evaluation: Single-electrode battery evaluation was performed using a CR2032 coin cell.

[0097] Preparation of paste for electrode sheet fabrication: To 1 part by weight of the sample, 0.039 parts by weight of acetylene black (AB), 0.13 parts by weight of carbon nanofiber (VGCF-H), and 0.13 parts by weight of 8% KF polymer (polyvinylidene fluoride (PVdF)) manufactured by Kureha Chemical Industry Co., Ltd. were added and mixed in N-methylpyrrolidone (NMP) as a solvent using a centrifugal mixer (Thinky Corporation). The mixture was then applied to a Cu metal foil and dried. The resulting sheet was rolled and punched to the desired size to prepare an electrode for evaluation. Metallic lithium was used as the counter electrode, and the electrolyte was a mixture of 1 mol / L LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio: 1:2). The following coin cell assembly was performed in a dry argon atmosphere with a dew point below -80°C.

[0098] Single-electrode charge / discharge test: Charging was performed at a constant current of 1.5 mA down to 0.05 V, and charging was completed when the current decayed to 0.04 mA. (CC-CV charging) Discharging was performed at a constant current of 1.5 mA (CC discharge) with cutoff at 2.0 V. This charge / discharge cycle was repeated 10 times.

[0099] -Preparation of silicon-containing amorphous carbon materials according to examples and comparative examples- In the following examples and comparative examples, green coke, which is a petroleum-based non-acicular coke, was used as the carbon material. The silicon raw material used was silicon dioxide obtained by precipitation (Npsil LP, Nipsil HD (Tosoh Silica Corporation)). The voiding agents used were crystalline cellulose agent (CEOLUS TG-F20 (Asahi Kasei Corporation)), ethylene bottom oil (EBO (Maruzen Petrochemical Co., Ltd.)), etc.

[0100] Example 1 The raw coke was crushed and classified to a D50 of 5.2 μm. The raw coke, silicon dioxide particles, and ethylene bottom oil (a voiding agent) were mixed and dry granulated. The primary particle size of the silicon dioxide particles was 20-30 nm. The amount of silicon dioxide particles added was 50 wt% when the sum of the weights of the silicon dioxide particles and raw coke was 100 wt%. Furthermore, the amount of ethylene bottom oil added as a voiding agent was 10 wt% when the sum of the weights of the silicon dioxide particles and raw coke was 100 wt%.

[0101] The raw coke, silicon dioxide particles, and a portion of the void agent ethylene bottom oil were charged into a COMPOSI CP15 (manufactured by Nippon Coke & Engineering Co., Ltd.), and spheronization treatment was initiated at low speed. The entire amount of silicon dioxide particles and ethylene bottom oil was then charged in several portions. After the entire amount was charged, the peripheral speed was set to 70 m / s, and treatment was continued for 420 minutes to obtain granulated particles.

[0102] Next, the granulated particles were carbonized at 1000°C for a holding time (carbonization time) of 5 hours at the maximum temperature.

[0103] The spherical composite material according to Example 1 thus obtained had a D50 of 17.5 μm and a BET of 0.4 m 2 The O / Si ratio (molar ratio) was 1.08, and the Si content in the obtained material was 28 wt%.

[0104] Examples 2 to 6, Comparative Examples 1 to 3 Silicon-containing amorphous carbon materials were obtained in the same manner as in Example 1, except that the type and amount of silicon oxide particles or silicon particles, the type and amount of void agent, the spheroidizing treatment conditions, and the carbonization treatment conditions were variously changed.

[0105] The properties of the carbon raw material are shown in Table 1, the properties of the silicon raw material in Table 2, and the properties of the gapping agent in Table 3. The production conditions are shown in Table 4, and the properties of the obtained silicon-containing amorphous carbon material are shown in Table 5.

[0106] [Table 1]

[0107] [Table 2]

[0108] [Table 3]

[0109] [Table 4]

[0110] [Table 5]

[0111] A cross-sectional photograph of the silicon-containing amorphous carbon material obtained in Example 3 is shown in Figure 1. As shown in Figure 1, there is a core particle made of soft carbon with a particle diameter of 5 to 6 μm, surrounded by a shell (second layer) portion in which SiOx particles of 20 to 100 nm and soft carbon of 1 μm or less are highly dispersed, and the particle is a spherical particle with an outer diameter of 10 to 20 μm. Voids exist inside spherical particles. Crescent-shaped voids exist concentrically around the core particle. In particular, the void at the boundary between the core particle and the second layer is large, measuring 1 to 10 μm in the long axis direction (0.1 to 1 μm in the short axis direction). Voids inside the second layer are 0.01 to 8 μm in the long axis direction (0.01 to 0.5 μm in the short axis direction). There are 20 or more voids with a long axis length of 1 μm or more per particle. There are 1,000 or more voids with a length of 1 μm or less. Figure 2 shows a cross-sectional photograph of the silicon-containing amorphous carbon material obtained in Comparative Example 1. As shown in Figure 2, many crescent-shaped voids exist between the core particle and the shell portion, but there are almost no voids in the shell portion.

[0112] -Battery evaluation measurement results- Table 6 shows the test results for the silicon-containing amorphous carbon materials according to Examples 1 to 6 and Comparative Examples 1 to 3.

[0113] [Table 6]

[0114] As shown in Table 6, when comparing the cells using the same silicon raw material, it was confirmed that the initial efficiency was almost the same for Examples 1 to 3 compared to Comparative Example 1, for Examples 4 and 5 compared to Comparative Example 2, and for Examples 5 and 6 compared to Comparative Example 3, and that the capacity retention rate was improved.

[0115] Furthermore, when the change in electrode thickness before and after measuring the capacity retention rate (after 30 cycles / before charge / discharge) was confirmed, it was 1.19 in Example 5, which had an Si content of 20 wt%, while it was 1.32 in Comparative Example 3, which had an Si content of 10 wt%, and it was confirmed that the change in electrode thickness due to charge / discharge was suppressed in the Examples. [Industrial Applicability]

[0116] The silicon-containing amorphous carbon material according to one example of this embodiment is useful as a negative electrode material for lithium ion secondary batteries or lithium ion capacitors used in, for example, electric vehicles, power storage systems for solar power generation, wind power generation, and the like.

Claims

1. Easily graphitizable amorphous carbon, a silicon-containing amorphous carbon material in which silicon oxide particles represented by SiOx (0<x<2) are contained in the graphitizable amorphous carbon, the silicon content in the silicon-containing amorphous carbon material is 1% by weight or more and 50% by weight or less; the silicon-containing amorphous carbon material is a spherical particle comprising a core particle at the center of the particle and a shell layer on the outside of the core particle; A silicon-containing amorphous carbon material, characterized in that a void exists between the core particle and the shell layer, and a void also exists in the shell layer.

2. 2. The silicon-containing amorphous carbon material according to claim 1, wherein the voids present between the core particles and the shell layer have a length in the major axis direction of 1 to 10 μm, and the voids present in the shell layer have a length in the major axis direction of 0.01 to 8 μm.

3. 3. The method for producing a silicon-containing amorphous carbon material according to claim 1, further comprising the steps of: mixing a carbon raw material, silicon particles or silicon oxide particles, and a gap agent, followed by dry granulation; and carbonizing the granulated particles under an inert gas atmosphere.

4. 4. The method for producing a silicon-containing amorphous carbon material according to claim 3, wherein the gapping agent is at least one selected from the group consisting of polysaccharides having an exothermic peak at 300°C or higher upon thermal decomposition and mineral oils having a fixed carbon content of 10% or less upon firing at 1000°C.

5. The method for producing a silicon-containing amorphous carbon material according to claim 3 or 4, A method for producing a silicon-containing amorphous carbon material, wherein the amount of a voiding agent added is 10 to 50 wt % based on the total amount of the carbon raw material and the silicon particles or silicon oxide particles.

6. A lithium ion secondary battery using the silicon-containing amorphous carbon material according to claim 1 or 2 as a carbon material for a negative electrode.

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