Negative electrode composite material and lithium secondary battery

Abstract

The invention relates to a negative electrode composite material and a lithium secondary battery. This negative electrode composite material contains a plurality of carbon particles and a plurality of Si-based particles. The plurality of carbon particles include coated graphite particles. The coated graphite particles have scaly graphite particles and a carbon film doped with boron. The carbon film covers at least a portion of the surface of the scale-like graphite particles. The content of the plurality of Si-based particles relative to the plurality of carbon particles is 5-60 mass%. The content of the coated graphite particles relative to the plurality of carbon particles is 0.8-85.0 mass%. The Raman spectrum intensity ratio (D / G) of the coated graphite particles is 0.3-0.8.

Description

Technical Field

[0001] The present invention relates to a negative electrode composite material and a lithium secondary battery. Background Technology

[0002] Lithium secondary batteries are used in information and communication technology (such as personal computers, smartphones, etc.), in vehicles, and for power storage.

[0003] Japanese Patent Publication No. 2004-103391 discloses a non-aqueous electrolyte lithium secondary battery. The non-aqueous electrolyte lithium secondary battery comprises a negative electrode, a specific positive electrode and a non-aqueous electrolyte. The negative electrode contains a carbon material capable of doping and dedoping lithium. The carbon material is composed of a specific graphite that does not contain boron and a specific graphite that contains boron. SUMMARY OF THE INVENTION

[0004] However, the capacity retention rate of the non-aqueous electrolyte lithium secondary battery disclosed in Japanese Patent Application Laid-Open No. 2004-103391 has room for improvement.

[0005] The present invention is completed in view of the above situation.

[0006] The problem to be solved by one embodiment of the present invention is to provide a negative electrode composite material capable of improving the capacity retention rate of a lithium secondary battery and a lithium secondary battery with excellent capacity retention rate.

[0007] The method for solving the above-mentioned problem includes the following implementation methods.

[0008] <1>A negative electrode composite material comprising a plurality of carbon particles (A) and a plurality of Si-based particles (B),

[0009] The plurality of carbon particles (A) include coated graphite particles (A1),

[0010] The coated graphite particles (A1) include flaky graphite particles (a1) and a boron-doped carbon film (a2) covering at least a portion of the surface of the flaky graphite particles (a1),

[0011] The intensity ratio (D / G) of the Raman spectrum of the coated graphite particles (A1) is 0.3 to 0.8,

[0012] The content (B / A) of the plurality of Si-based particles (B) relative to the plurality of carbon particles (A) is 5% to 60% by mass,

[0013] The content (A1 / A) of the coated graphite particles (A1) relative to the plurality of carbon particles (A) is 0.8% by mass to 85.0% by mass.

[0014] <2> The negative electrode composite material according to <1> above, wherein the flaky graphite particles (a1) are doped with boron.​​

[0015] <3> The negative electrode composite material according to <1> or <2> above, wherein the doping amount of boron in the carbon film (a2) is 1.0 atomic% or more.

[0016] <4> The negative electrode composite material according to any one of <1> to <3> above, wherein

[0017] the plurality of carbon particles (A) further include a plurality of spherical graphite particles (A2),

[0018] the doping amount of boron in the carbon film (a2) is 0.2 atomic% to 2.4 atomic%.

[0019] <5> A lithium secondary battery, which includes a negative electrode containing the negative electrode composite material according to any one of <1> to <4> above.

[0020] According to the present invention, a negative electrode composite material capable of improving the capacity retention rate of a lithium secondary battery and a lithium secondary battery with excellent capacity retention rate can be provided. Detailed Embodiments

[0021] In the present invention, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical range described in stages in the present invention, the upper limit value or the lower limit value described in a certain numerical range can be replaced with the upper limit value or the lower limit value of another numerical range described in stages. In the numerical range described in the present invention, the upper limit value or the lower limit value described in a certain numerical range can be replaced with the value shown in the examples. In the present invention, the combination of two or more preferred modes is a more preferred mode. In the present invention, regarding the amount of each component, when there are two or more kinds of substances corresponding to each component, unless otherwise specified, it means the total amount of two or more kinds of substances. In the present invention, the term "process" not only refers to an independent process, but also includes this term as long as the desired purpose of the process can be achieved even when it cannot be clearly distinguished from other processes.

[0022] (1) Negative electrode composite material

[0023] The negative electrode composite material of the present invention contains a plurality of carbon particles (A) and a plurality of Si-based particles (B). The above-mentioned plurality of carbon particles (A) include coated graphite particles (A1). The above-mentioned coated graphite particles (A1) have flaky graphite particles (a1) and a boron-doped carbon film (a2). The carbon film (a2) covers at least a part of the surface of the above-mentioned flaky graphite particles (a1). The intensity ratio (D / G) of the Raman spectrum of the above-mentioned coated graphite particles (A1) (hereinafter also simply referred to as "intensity ratio (D / G)") is 0.3 to 0.8. The content (B / A) of the above-mentioned plurality of Si-based particles (B) relative to the above-mentioned plurality of carbon particles (A) (hereinafter also simply referred to as "content (B / A)") is 5% by mass to 60% by mass. The content (A1 / A) of the coated graphite particles (A1) relative to the above-mentioned plurality of carbon particles (A) (hereinafter also simply referred to as "content (A1 / A)") is 0.8% by mass to 85.0% by mass.

[0024] In the present invention, the "negative electrode composite material" refers to the solid component of the negative electrode composite material layer contained in the negative electrode of a lithium secondary battery. The lithium secondary battery may be a battery having a solid electrolyte or a battery containing a non-aqueous electrolyte. The "carbon particle" refers to a particle containing carbon. The "graphite particle" refers to a particle containing graphite. The "flaky" refers to a shape with an aspect ratio of 2.1 or more. The "aspect ratio" refers to the ratio of the major axis of the particle to the minor axis of the particle. The "Si-based particle" refers to a particle containing silicon (Si). The "carbon film" refers to a film containing carbon. The "intensity ratio (D / G) of the Raman spectrum" refers to the ratio of the peak intensity of the D band (around 1360 cm -1 -1) from the defect structure to the peak intensity of the G band (around 1580 cm -1 -1) from the graphite structure (SP2 bond) in the Raman spectrum on the surface of the coated graphite particles (A1). The lower the intensity ratio (D / G) of the Raman spectrum, the higher the crystallinity of the surface of the coated graphite particles (A1) (i.e., the carbon film (a2)).

[0025] Since the negative electrode composite material of the present invention has the above structure, the capacity retention rate of the lithium secondary battery can be improved.

[0026] It is speculated that this effect is due to the following reasons, but is not limited thereto.

[0027] When a boron-doped carbon film is coated on the flaky graphite particles, the electronic conductivity is improved. Therefore, it is easy to form a conductive path for the Si-based particles. As a result, it is speculated that the negative electrode composite material of the present invention can improve the capacity retention rate of the lithium secondary battery.

[0028] (1.1) Carbon particles (A)

[0029] The negative electrode composite material of the present invention contains a plurality of carbon particles (A). The plurality of carbon particles (A) function as a negative electrode active material. Examples of the material constituting the carbon particles (A) include graphite, hard carbon, soft carbon, etc. Among them, graphite is preferred. The graphite can be natural graphite or artificial graphite. The carbon particles (A) can be coated with an amorphous carbon material.

[0030] (1.1.1) Coated graphite particles (A1)

[0031] The plurality of carbon particles (A) include coated graphite particles (A1). The coated graphite particles (A1) have flaky graphite particles (a1) and a boron-doped carbon film (a2). The carbon film (a2) can cover the entire surface of the flaky graphite particles (a1) or a part of the surface of the flaky graphite particles (a1).

[0032] The intensity ratio (D / G) is 0.3 to 0.8. The intensity ratio (D / G) being 0.3 to 0.8 indicates that the carbon film (a2) is of low crystallinity. The intensity ratio (D / G) can be 0.4 or more, and can also be 0.5 or more. The intensity ratio (D / G) can be 0.7 or less, can be 0.5 or less, and can also be 0.4 or less.

[0033] The content (A1 / A) is 0.8 mass% to 85.0 mass%. The content (A1 / A) can be 5.0 mass% or more, can be 20.0 mass% or more, and can also be 40.0 mass% or more. The content (A1 / A) can be 50.0 mass% or less, can be 35.0 mass% or less, and can also be 25.0 mass% or less.

[0034] (1.1.1.1) Flaky graphite particles (a1)

[0035] The graphite constituting the flaky graphite particles (a1) can be natural graphite or artificial graphite.

[0036] Preferably, boron is doped in the flaky graphite particles (a1). Thus, compared with the structure in which boron is not doped in the flaky graphite particles (a1), the negative electrode composite material can improve the capacity retention rate of the lithium secondary battery.

[0037] When boron is doped in the flaky graphite particles (a1), the doping amount of boron in the flaky graphite particles (a1) is not particularly limited and can be 0.2 atomic% or more. The doping amount of boron in the flaky graphite particles (a1) can be 1.0 atomic% or more, and can also be 2.0 atomic% or more. The doping amount of boron in the flaky graphite particles (a1) can be 3.6 atomic% or less, and can also be 3.5 atomic% or less. The measurement method of the boron doping amount is the same as the method described in the examples.

[0038] The aspect ratio of the flaky graphite particles (a1) is 2.1 or more, may be 3.0 or more, and may also be 3.4 or more. The aspect ratio of the flaky graphite particles (a1) may be 10.0 or less, may be 4.0 or less, and may also be 3.8 or less.

[0039] The average particle size of the plurality of flaky graphite particles (a1) is not particularly limited, may be 2 μm to 40 μm, and may also be 4 μm to 26 μm. "Average particle size" means the particle size (median particle size) corresponding to 50% by volume of the cumulative frequency from the side of the fine particles with a small particle size in the particle size distribution based on the volume basis of the laser diffraction light scattering method.

[0040] (1.1.1.2) Carbon film (a2)

[0041] The carbon film (a2) may be an amorphous carbon film. The carbon film (a2) can be obtained, for example, by mixing pitch in the flaky graphite particles (a1) and firing.

[0042] The ratio (a2 / A1) of the carbon film (a2) to the total amount of the coated graphite particles (A1) may be 2% by mass or more, and may also be 5% by mass or more. The ratio (a2 / A1) may be 20% by mass or less, and may also be 10% by mass or less.

[0043] The doping amount of boron in the carbon film (a2) is not particularly limited, may be 0.2 atomic% or more, may be 1.0 atomic% or more, and may also be 1.4 atomic% or more. The doping amount of boron in the carbon film (a2) may be 2.5 atomic% or less, and may also be 2.4 atomic% or less. The method for measuring the doping amount of boron is the same as the method described in the examples.

[0044] When boron is doped in the flaky graphite particles (a1), the doping amount of boron in the carbon film (a2) is preferably 1.0 atomic% or more. Thus, compared with the structure in which the doping amount of boron in the carbon film (a2) is less than 1.0 atomic%, the negative electrode composite material can improve the capacity retention rate of the lithium secondary battery.

[0045] (1.1.2) Spherical graphite particles (A2)

[0046] The plurality of carbon particles (A) may further contain spherical graphite particles (A2). "Spherical" means a shape with an aspect ratio of less than 2.1. The graphite constituting the spherical graphite particles (A2) may be natural graphite or artificial graphite.

[0047] The spherical graphite particles (A2) may or may not be doped with boron. When boron is doped in the spherical graphite particles (A2), the doping amount of boron in the spherical graphite particles (A2) may be the same as the doping amounts exemplified as the doping amount of boron in the flaky graphite particles (a1).

[0048] The aspect ratio of the spherical graphite particles (A2) is less than 2.1, and may be 1.9 or less, or 1.7 or less. The aspect ratio of the spherical graphite particles (A2) may be 1.0 or more.

[0049] The average particle size of the plurality of spherical graphite particles (A2) is not particularly limited and may be 2 μm to 30 μm or 4 μm to 26 μm. The method for measuring the average particle size is the same as the method for measuring the average particle size of the scaly graphite particles (a1).

[0050] The content (A2 / A) of the plurality of spherical graphite particles (A2) relative to the plurality of carbon particles (A) may be 60.0% by mass or more, 75.0% by mass or more, or 85.0% by mass or more. The content (A2 / A) may be 98.5% by mass or less, 92.0% by mass or less, or 90.0% by mass or less.

[0051] (1.1.3) Optimal method

[0052] Preferably, the plurality of carbon particles (A) further include a plurality of spherical graphite particles (A2), and the boron doping amount of the carbon film (a2) is 0.2 atomic % to 2.4 atomic %. Thus, the negative electrode composite material can further improve the capacity retention rate of the lithium secondary battery.

[0053] (1.2)Si-based particles (B)

[0054] The negative electrode composite material of the present invention contains a plurality of Si-based particles (B). The plurality of Si-based particles (B) function as negative electrode active materials. Examples of materials constituting the Si-based particles (B) include Si single substance (silicon), Si alloys, silicon monoxide (SiO), silicon dioxide (SiO 2 ) etc. Si alloy preferably contains Si as the main component.

[0055] The aspect ratio of the Si-based particles (B) is not particularly limited and may be less than 2.1, 1.9 or less, or 1.7 or less. The aspect ratio of the Si-based particles (B) may be 1.0 or more.

[0056] The average particle size of the plurality of Si-based particles (B) is not particularly limited and may be 0.01 μm to 10.0 μm or 0.5 μm to 8.0 μm. The method for measuring the average particle size is the same as the method for measuring the average particle size of the flaky graphite particles (a1).

[0057] The content (B / A) is 5% to 60% by mass. The content (B / A) may be 10% by mass or more, or 15% by mass or more. The content (B / A) may be 40% by mass or less, 30% by mass or less, or 25% by mass or less.

[0058] (1.3) Binder

[0059] The negative electrode composite material may further contain a binder. Examples of the binder include polyvinylidene fluoride (PVDF), carboxymethyl cellulose, rubber-based binders (such as butadiene rubber, hydrogenated butadiene rubber, etc.), fluoride-based binders (such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene (PTFE), etc.), polyolefin-based thermoplastic resins (such as polyethylene, polypropylene, polystyrene, etc.). Regarding the content of the binder, it may be 0.1% by mass to 10% by mass relative to the total amount of the negative electrode composite material.

[0060] (1.4) Conductive additive

[0061] The negative electrode composite material may further contain a conductive additive. Examples of the conductive additive include carbon nanotubes (CNT), carbon black (such as acetylene black, furnace black, Ketjen black, etc.). Regarding the content of the conductive additive, it may be 0.1% by mass to 10% by mass relative to the total amount of the negative electrode composite material.

[0062] (2) Lithium secondary battery

[0063] The lithium secondary battery of the present invention includes a negative electrode. The negative electrode contains the negative electrode composite material of the present invention. Thus, the capacity retention rate of the lithium secondary battery of the present invention is excellent.

[0064] The lithium secondary battery of the present invention generally includes a positive electrode and an ion conduction medium in addition to the negative electrode. The ion conduction medium is interposed between the positive electrode and the negative electrode and conducts carrier ions. Examples of the ion conduction medium include non-aqueous electrolytes, non-aqueous gel electrolytes, solid ion conductive polymers, inorganic solid electrolytes, etc.

[0065] Hereinafter, a lithium secondary battery using a non-aqueous electrolyte (hereinafter also referred to as a "non-aqueous battery") will be described.

[0066] (2.1) Non-aqueous battery

[0067] The non-aqueous battery includes the negative electrode of the present invention, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0068] (2.1.1) Negative electrode

[0069] The negative electrode has a negative electrode composite material layer and may further have a negative electrode current collector (such as a copper foil, etc.). The negative electrode composite material layer is laminated on at least one main surface of the negative electrode current collector. The negative electrode composite material layer contains the negative electrode composite material of the present invention.

[0070] (2.1.2) Positive electrode

[0071] The positive electrode has a positive electrode composite material layer and may also have a positive electrode current collector (such as aluminum foil, etc.). The positive electrode composite material layer is laminated on at least one main surface of the positive electrode current collector.

[0072] The positive electrode composite material layer contains a positive electrode active material. The positive electrode active material releases lithium ions to the non-aqueous electrolyte or occludes lithium ions from the electrolyte. The positive electrode active material is a well-known positive electrode active material (such as LiNiO 2 、LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 etc.). The positive electrode composite material layer may also contain a well-known conductive material (such as carbon black, etc.), lithium phosphate, and a binder (such as polyvinylidene fluoride, etc.).

[0073] (2.1.3) Separator

[0074] The separator maintains the interval between the positive electrode and the negative electrode to prevent the occurrence of contact short circuit and allows lithium ions to pass through. Examples of the separator include a porous resin sheet or a non-woven fabric. Examples of the material of the porous resin sheet include polyolefins (such as polypropylene, polyethylene, etc.). Examples of the material of the non-woven fabric include polypropylene, polyethylene terephthalate, methyl cellulose, etc. The separator may have a well-known structure.

[0075] (2.1.4) Non-aqueous electrolyte

[0076] The non-aqueous electrolyte may contain a non-aqueous solvent and a lithium salt. Examples of the lithium salt include LiClO 4 、LiAsF 6 、LiPF 6 、LiBF 4 、LiCF 3 SO 3 、LiN(FSO 2 ) 2 、LiN(CF 3 SO 2 ) 2 etc. Examples of the non-aqueous solvent include cyclic carbonates (such as ethylene carbonate, etc.), chain carbonates (such as dimethyl carbonate, ethyl methyl carbonate, etc.), cyclic esters (such as γ-butyrolactone, γ-valerolactone, etc.), chain esters (such as methyl formate, methyl acetate, etc.), ethers (such as 1,2-dimethoxyethane, ethoxymethoxyethane, etc.). The non-aqueous electrolyte may contain additives (such as vinylene carbonate, lithium bis(oxalato)borate, etc.).

[0077] (2.1.5) Case

[0078] Non-aqueous batteries generally have a casing. The casing houses a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. As the casing, there is no particular limitation, and examples include a laminated film (such as an aluminum sheet, etc.), a battery can (such as cylindrical, square, coin-shaped, etc.).

[0079] Hereinafter, the present invention will be described in more detail by way of examples, but the invention disclosed herein is not limited to these examples.

[0080] [1] Examples and Comparative Examples

[0081] [1.1] Raw Materials

[0082] As raw materials for the negative electrode composite material, the following materials were prepared.

[0083] [1.1.1] Flaky Graphite Particles (a1)

[0084] [1.1.1.1] Flaky Graphite Particles (a1-1)

[0085] As the flaky graphite particles (a1) (negative electrode active material), a plurality of flaky graphite particles (a1-1) not doped with boron were prepared (particle material: graphite, particle aspect ratio: 3.3).

[0086] [1.1.1.2] Flaky Graphite Particles (a1-2)

[0087] As the flaky graphite particles (a1) (negative electrode active material), a plurality of flaky graphite particles (a1-2) doped with boron were prepared. Specifically, boron carbide (B 4 C) and a plurality of flaky graphite particles (particle material: graphite, particle aspect ratio: 3.5) were mixed to obtain a mixture. The mixing ratio of boron carbide (B 4 C) was 3.0% by mass relative to the plurality of flaky graphite particles. The mixture was fired at 2800 °C for 2 hours in an argon atmosphere. Thereby, a plurality of flaky graphite particles (a1-2) were obtained.

[0088] The boron doping amount of the plurality of flaky graphite particles (a1-2) was measured using XPS (X-ray photoelectron spectroscopy). Specifically, the boron doping amount was calculated from the peak at 188 eV (attributed to the B-C bond). The measurement result of the boron doping amount of the plurality of flaky graphite particles (a1-2) was 1.4 atomic %.

[0089] [1.1.1.3] Flaky Graphite Particles (a1-3)

[0090] As the flaky graphite particles (a1) (negative electrode active material), a plurality of flaky graphite particles (a1-3) doped with boron were prepared. Specifically, boron carbide (B 4The mixing ratio of (C) was changed to 0.5 mass% relative to a plurality of flaky graphite particles, and the rest was carried out in the same manner as the production of the flaky graphite particles (a1-2) to obtain a plurality of flaky graphite particles (a1-3). The measurement result of the boron doping amount of the flaky graphite particles (a1-3) was 0.2 atomic%.

[0091] [1.1.1.4] Flaky graphite particles (a1-4)

[0092] As the flaky graphite particles (a1) (negative electrode active material), a plurality of boron-doped flaky graphite particles (a1-4) were prepared. Specifically, boron carbide (B 4 C)'s mixing ratio was changed to 10.0 mass% relative to a plurality of flaky graphite particles, and the rest was carried out in the same manner as the production of the flaky graphite particles (a1-2) to obtain a plurality of flaky graphite particles (a1-4). The measurement result of the boron doping amount of the flaky graphite particles (a1-4) was 3.5 atomic%.

[0093] [1.1.2] Coated graphite particles (A1)

[0094] [1.1.2.1] Coated graphite particles (X1)

[0095] As the coated graphite particles (X) having a carbon film, coated graphite particles (X1) were prepared. Specifically, coal tar pitch as a carbon raw material and a plurality of flaky graphite particles (a1-1) were mixed to obtain a mixture. The content of coal tar pitch was 5 mass% relative to a plurality of flaky graphite particles (a1-1). The mixture was fired at 1000 °C in an argon atmosphere. Thus, a plurality of coated graphite particles (X1) were obtained.

[0096] [1.1.2.2] Coated graphite particles (A1-1)

[0097] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-1) were prepared. Specifically, coal tar pitch as a carbon raw material, a plurality of flaky graphite particles (a1-1), and boron carbide (B 4 C) were mixed to obtain a mixture. The content of coal tar pitch was 5 mass% relative to a plurality of flaky graphite particles (a1-1). The content of boron carbide (B 4 C) was 1 mass% relative to a plurality of flaky graphite particles (a1-1). The mixture was fired at 1000 °C in an argon atmosphere. Thus, a plurality of coated graphite particles (A1-1) were obtained.

[0098] The boron doping amount of the boron-doped low-crystalline carbon film (a2) of multiple coated graphite particles (A1-1) was measured using XPS (X-ray photoelectron spectroscopy). Specifically, the boron doping amount was calculated from the peak at 188 eV (attributed to the B-C bond). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of multiple coated graphite particles (A1-1) was 0.2 atomic %.

[0099] Using a Raman spectrometer (product number: DXR3) manufactured by Thermo Fisher Scientific, the intensity ratio (D / G) of the Raman spectrum of the coated graphite particles (A1-1) was measured. The laser wavelength was 532 nm. The intensity ratio (D / G) of the coated graphite particles (A1-1) was 0.3.

[0100] [1.1.2.3] Coated graphite particles (A1-2)

[0101] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-2) were prepared. Specifically, the flaky graphite particles (a1-1) were changed to flaky graphite particles (a1-2), and otherwise the same as the production of the coated graphite particles (A1-1) was carried out to obtain multiple coated graphite particles (A1-2). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of multiple coated graphite particles (A1-2) was 0.2 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-2) was 0.4.

[0102] [1.1.2.4] Coated graphite particles (A1-3)

[0103] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-3) were prepared. Specifically, the content of boron carbide (B 4 C) was changed to 3 mass % relative to multiple flaky graphite particles (a1-2), and otherwise the same as the production of the coated graphite particles (A1-1) was carried out to obtain multiple coated graphite particles (A1-3). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of multiple coated graphite particles (A1-3) was 1.4 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-3) was 0.3.

[0104] [1.1.2.5] Coated graphite particles (A1-4)

[0105] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-4) were prepared. Specifically, the boron carbide (B 4The content of (C) was changed to 10% by mass relative to the plurality of flaky graphite particles (a1-2), and the rest was carried out in the same manner as the production of the coated graphite particles (A1-1) to obtain a plurality of coated graphite particles (A1-4). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-4) was 2.4 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-4) was 0.4.

[0106] [1.1.2.6] Coated graphite particles (A1-5)

[0107] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-5) were prepared. Specifically, the content of boron carbide (B 4 C) was changed to 3% by mass relative to the plurality of flaky graphite particles (a1-2), and the rest was carried out in the same manner as the production of the coated graphite particles (A1-2) to obtain a plurality of coated graphite particles (A1-5). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-5) was 1.3 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-5) was 0.3.

[0108] [1.1.2.7] Coated graphite particles (A1-6)

[0109] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-6) were prepared. Specifically, the content of boron carbide (B 4 C) was changed to 10% by mass relative to the plurality of flaky graphite particles (a1-2), and the rest was carried out in the same manner as the production of the coated graphite particles (A1-2) to obtain a plurality of coated graphite particles (A1-6). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-6) was 2.4 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-6) was 0.5.

[0110] [1.1.2.8] Coated graphite particles (A1-7)

[0111] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-7) were prepared. Specifically, the firing temperature was changed to 1300 °C, and the rest was carried out in the same manner as the production of the coated graphite particles (A1-2) to obtain a plurality of coated graphite particles (A1-7). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-7) was 0.2 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-7) was 0.3.

[0112] [1.1.2.9] Coated graphite particles (A1-8)

[0113] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-8) were prepared. Specifically, the firing temperature was changed to 900 °C, and otherwise the same as the production of the coated graphite particles (A1-2) was carried out to obtain a plurality of coated graphite particles (A1-8). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-8) was 0.2 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-8) was 0.8.

[0114] [1.1.2.10] Coated graphite particles (X2)

[0115] As the coated graphite particles (X) having a carbon film, coated graphite particles (X2) were prepared. Specifically, the firing temperature was changed to 700 °C, and otherwise the same as the production of the coated graphite particles (A1-2) was carried out to obtain a plurality of coated graphite particles (X2). The measurement result of the boron doping amount of the carbon film of the plurality of coated graphite particles (X2) was 0.2 atomic %. The intensity ratio (D / G) of the coated graphite particles (X2) was 0.9.

[0116] [1.1.2.11] Coated graphite particles (A1-9)

[0117] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-9) were prepared. Specifically, the flaky graphite particles (a1-2) were changed to flaky graphite particles (a1-3), and otherwise the same as the production of the coated graphite particles (A1-2) was carried out to obtain a plurality of coated graphite particles (A1-9). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-9) was 0.2 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-9) was 0.4.

[0118] [1.1.2.12] Coated graphite particles (A1-10)

[0119] As the coated graphite particles (A1) having a boron-doped low-crystalline carbon film (a2), coated graphite particles (A1-10) were prepared. Specifically, the flaky graphite particles (a1-2) were changed to flaky graphite particles (a1-4), and otherwise the same as the production of the coated graphite particles (A1-2) was carried out to obtain a plurality of coated graphite particles (A1-10). The measurement result of the boron doping amount of the boron-doped low-crystalline carbon film (a2) of the plurality of coated graphite particles (A1-10) was 0.2 atomic %. The intensity ratio (D / G) of the coated graphite particles (A1-10) was 0.4.

[0120] [1.1.3] Spherical graphite particles (A2)

[0121] As the undoped boron spherical graphite particles (A2) (negative electrode active material), a plurality of spherical graphite particles (A2-1) (particle material: artificial graphite) are prepared.

[0122] [1.1.4] Si-based particles (B)

[0123] As the Si-based particles (B) (negative electrode active material), a plurality of silicon particles (B-1) (particle material: silicon) are prepared.

[0124] [1.2] Examples 1 to 17 and Comparative Examples 1 to 6

[0125] A plurality of coated graphite particles (A1) shown in Table 1 and a plurality of spherical graphite particles (A2-1) are mixed to obtain a plurality of carbon particles (A). The ratio (A1 / A) of the plurality of coated graphite particles (A1) to the total of the plurality of coated graphite particles (A1) and the plurality of spherical graphite particles (A2-1) (i.e., the plurality of carbon particles (A)) is the ratio shown in Table 1. A plurality of carbon particles (A) and a plurality of Si-based particles (B) are mixed to obtain a negative electrode composite material. The content (B / A) of the plurality of Si-based particles (B) relative to the plurality of carbon particles (A) is the ratio shown in Table 1.

[0126] [2] Evaluation of capacity retention rate

[0127] [2.1] Fabrication of lithium secondary battery

[0128] A lithium secondary battery is fabricated as follows.

[0129] [2.1.1] Positive electrode

[0130] As the positive electrode active material, LiNiCoMnO 2 . As the conductive additive, acetylene black (AB) is prepared. As the binder, polyvinylidene fluoride (PVdF) is prepared.

[0131] The positive electrode active material, conductive additive, binder, and solvent are mixed to prepare a positive electrode composite paste. The mass ratio of the positive electrode composite paste (positive electrode active material: conductive additive: binder) is 92:5:3. The positive electrode composite paste is coated on an aluminum foil (thickness: 15 μm), dried, and pressed. Thus, a positive electrode is obtained. The thickness of the positive electrode active material layer of the positive electrode is a specified thickness.

[0132] [2.1.2] Negative electrode

[0133] As binder A, carboxymethyl cellulose (CMC) is prepared. As binder B, styrene-butadiene rubber (SBR) is prepared.

[0134] Mix the obtained negative electrode composite material, conductive additive, binder A, binder B and solvent to prepare a negative electrode composite material paste. The mass ratio of the negative electrode composite material paste (negative electrode active material: conductive additive: binder A: binder B) is 97:1:1:1. It should be noted that insoluble CMC is calculated based on the weight of the active material. Coat the negative electrode composite material paste on a copper foil (thickness: 10 μm), and perform drying and pressing. Thus, a negative electrode is obtained. The thickness of the negative electrode active material layer of the negative electrode is a specified thickness.

[0135] [2.1.3] Separator

[0136] As the separator, prepare a porous sheet with a three-layer structure (thickness: 24 μm). The porous sheet is formed by sequentially laminating a polypropylene (PP) layer, a polyethylene (PE) layer, and a PP layer. A layer (4 μm) of ceramic (such as alumina, boehmite, etc.) is coated on one side of the separator.

[0137] [2.1.4] Non-aqueous electrolyte

[0138] As the non-aqueous electrolyte, prepare a mixed solution of a mixed solvent and LiPF as a supporting salt 6 The mixed solvent is composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The volume ratio of the mixed solvent (EC: DMC: EMC) is 3:3:4. The concentration of LiPF 6 is 1 mol / L.

[0139] [2.1.5] Assembly

[0140] Wind the positive electrode and the negative electrode with the separator in between to form an electrode group. The ceramic layer of the separator faces the positive electrode. Weld a current collector plate with a cover at both ends of the electrode group, insert the electrode group into the housing, and weld the cover plate to the housing. Inject a specified amount of non-aqueous electrolyte through the liquid injection hole of the housing, and tighten the sealing screw at the liquid injection hole. After injecting the non-aqueous electrolyte, leave it for an appropriate time to allow the non-aqueous electrolyte to impregnate the electrode group, charge the electrode group, and perform aging at 60 °C. Thus, a lithium secondary battery is obtained.

[0141] [2.2] Evaluation of life (cycle) characteristics

[0142] For a lithium-ion secondary battery, charge and discharge are carried out at a rate load of 2C in an atmosphere of 60°C. Within the SOC range of 0% to 100%, 300 cycles are implemented, and the battery capacity at the first discharge (hereinafter also referred to as "initial capacity") and the battery capacity after 300 cycles (hereinafter also referred to as "capacity after 300 cycles") are measured. The capacity retention rate is calculated by the following formula (A). The results are shown in Table 1. The higher the capacity retention rate, the better the battery characteristics can be evaluated. The allowable range of the capacity retention rate is 79% or more.

[0143] Formula (A): Capacity retention rate (%) = (Capacity after 300 cycles / Initial capacity) × 100

[0144] [3] Results

[0145]

[0146] In Table 1, "carbon film (a2)" represents a boron-doped low-crystalline carbon film (a2). The "-" for the carbon film (a2) in Comparative Example 1 indicates that the boron-doped low-crystalline carbon film (a2) was not formed.

[0147] The negative electrode composite materials of Comparative Examples 1 to 3 do not contain coated graphite particles (A1). Therefore, the capacity retention rate of Comparative Example 1 is not 79% or more.

[0148] In the negative electrode composite material of Comparative Example 4, the content (A1 / A) is not 0.8 mass% to 85.0 mass%. Therefore, the capacity retention rate of Comparative Example 1 is not 79% or more.

[0149] From these results, it can be seen that the negative electrode composite materials of Comparative Examples 1 to 4 are not "negative electrode composite materials capable of improving the capacity retention rate of lithium secondary batteries".

[0150] The negative electrode composite materials of Examples 1 to 17 contain a plurality of carbon particles (A) and a plurality of Si-based particles (B). The plurality of carbon particles (A) include coated graphite particles (A1). The coated graphite particles (A1) have flaky graphite particles (a1) and a boron-doped low-crystalline carbon film (a2). The content (B / A) is 5 mass% to 60 mass%. The content (A1 / A) is 0.8 mass% to 85.0 mass%. The intensity ratio (D / G) of the Raman spectrum of the coated graphite particles (A1) is 0.3 to 0.8. Therefore, the capacity retention rate of Examples 1 to 17 is 79% or more.

[0151] From these results, it can be seen that the negative electrode composite materials of Examples 1 to 17 are "negative electrode composite materials capable of improving the capacity retention rate of lithium secondary batteries".

Claims

1. A negative electrode composite material comprising a plurality of carbon particles and a plurality of Si-based particles, The plurality of carbon particles include coated graphite particles, The coated graphite particles include flaky graphite particles and a boron-doped carbon film covering at least a portion of the surface of the flaky graphite particles. The intensity ratio of the Raman spectrum of the coated graphite particles is 0.3 to 0.8, The content of the plurality of Si-based particles relative to the plurality of carbon particles is 5 mass % to 60 mass %, The content of the coated graphite particles relative to the plurality of carbon particles is 0.8 mass % to 85.0 mass %.

2. The negative electrode composite material according to claim 1, wherein The flaky graphite particles are doped with boron.

3. The negative electrode composite material according to claim 2, wherein: The boron doping amount of the carbon film is 1.0 atomic % or more.

4. The negative electrode composite material according to claim 1, wherein The plurality of carbon particles further include a plurality of spherical graphite particles, The boron doping amount of the carbon film is 0.2 atomic % to 2.4 atomic %. 5 . A lithium secondary battery comprising a negative electrode comprising the negative electrode composite material according to claim 1 .

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