Negative electrode material for lithium ion secondary battery, method for manufacturing the same, negative electrode using the same, and lithium ion secondary battery

Abstract

To provide a negative electrode material for a lithium ion secondary battery, which is made of graphite carbon particles and has a high capacity and exhibits small expansion and contraction during charging and discharging, and a negative electrode using the same.SOLUTION: A negative electrode material for a lithium ion secondary battery includes a mixture of at least two types of graphite particles A having an average breaking strength of 5 MPa or more and less than 50 MPa, and graphite particles B having an average breaking strength of 50 MPa to 150 MPa, and the graphite particles A are spheroidized natural graphite particles and the graphite particles B are artificial graphite particles, the particle size ratio (dB / dA) of the particle size dB of the graphite particles B to the particle size dA of the graphite particles A is 0.24 or less, and the mass ratio (MB / (MA+MB)) of the mass of the graphite particles B to the total mass (MA+MB) of the mass MA of the graphite particles A and the mass MB of the graphite particles B is 0.30 or less, and there is also provided a negative electrode that uses the negative electrode material.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a negative electrode material for a lithium ion secondary battery, a method for manufacturing the same, a negative electrode using the same, and a lithium ion secondary battery.

Background Art

[0002] Lithium ion secondary batteries are widely used in mobile terminals such as mobile phones, smartphones, and tablets because of their high energy density and long life. Recently, their use as in-vehicle batteries for electric vehicles and hybrid vehicles has also been expanding.

[0003] In mobile applications, a change in shape due to battery swelling is a problem, and a material that is long-lived, safe, and has little swelling is required. Similarly, in in-vehicle applications, since the number of stacked batteries is large and the swelling of each battery is large, the overall swelling becomes large, so a low-swelling material is required.

[0004] Among negative electrode materials, artificial graphite, depending on the type, tends to have less expansion and contraction during charge and discharge compared to natural graphite-based materials. On the other hand, in terms of price, natural graphite is more economical than artificial graphite at present. Therefore, in natural graphite-based materials, technologies have been developed to coat the surface with low-crystalline carbonaceous material or spheroidize it to reduce swelling, improve cycle characteristics, and discharge load characteristics.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] In Patent Document 1, a low-crystalline carbonaceous coating is applied to pressure-treated natural graphite, and a composite carbon material is further mixed to reduce the expansion rate during charging. However, there is a problem that the manufacturing method is complex. In Patent Document 2, pressure-treated spherical natural graphite is coated, but the improvement range is narrow and the effect is insufficient. In Patent Document 3, the manufacturing process is simpler than that of Patent Document 1, but the improvement range of the cycle capacity retention rate is narrow, and there are unclear parts regarding the improvement of the initial charge-discharge efficiency, expansion rate, and others.

[0007] The present invention has been made in view of the need for suppressing the expansion of batteries, which will become increasingly important in the future, and aims to obtain a negative electrode material for a lithium-ion secondary battery that maintains sufficient battery performance even at high density and realizes low expansion.

Means for Solving the Problems

[0008] The inventors of the present invention intensively studied to achieve the above object. Conventionally, it has been understood that when a highly crystalline graphite material is used as a negative electrode, the expansion during charging is large. However, when a low-expansion artificial graphite or amorphous carbon particles are simply mixed with the graphite material, a phenomenon in which the expansion is larger than the additive rule is often experienced. Further detailed analysis revealed that when a highly crystalline graphite material is pressed to form an electrode, the particles are easily deformed during the process. Then, when charge and discharge are performed, the larger the deformation due to pressing, the larger the expansion of the electrode. That is, the idea of the present invention was reached that it is important to reduce the deformation of the negative electrode material due to pressing during electrode manufacturing (specifically, to make the deformation rate of the graphite particles A of the present invention 15.0% or less) in order to reduce the expansion of the electrode.

[0009] That is, it was found that an economical negative electrode material that suppresses the expansion and contraction of particles can be obtained by mixing a mixture of soft graphite particles and hard graphite particles with each particle in an optimal configuration, and the present invention was completed.

[0010] That is, the present invention provides the following [1] to [7]. [1] A mixture of at least two types of graphite particles A having an average breaking strength of 5 MPa or more and less than 50 MPa and graphite particles B having an average breaking strength of 50 MPa to 150 MPa, wherein the graphite particles A are spheroidized natural graphite particles, the graphite particles B are artificial graphite particles, and a coating of a carbonaceous material is present on the surface of the graphite particles A, and the average particle diameter d A The average particle diameter d of the graphite particles B B The particle size ratio (d B / d A ) is 0.24 or less, and the mass M of the graphite particles A is A and the mass M of the graphite particles B B The total mass (M A +M B ) the mass M of the graphite particles B B Mass ratio (M B / (M A +M B )) is 0.30 or less. [2] In the negative electrode material for lithium ion secondary batteries according to [1] above, the coating of the carbonaceous material is a sintered body of a carbonaceous precursor. [3] In the negative electrode material for lithium ion secondary batteries according to [1] or [2], the artificial graphite particles are spherulitic graphite particles and / or needle coke graphitized particles. [4] The negative electrode material for a lithium ion secondary battery according to any one of [1] to [3] above, wherein the graphite particles A have a deformation rate of 15.0% or less. [5] In the method for producing a negative electrode material for a lithium ion secondary battery according to any one of [1] to [4], the negative electrode material comprises at least two kinds of graphite particles A having an average fracture strength of 5 MPa or more and less than 50 MPa and graphite particles B having an average fracture strength of 50 MPa to 150 MPa, the graphite particles A are spheroidized natural graphite particles, the graphite particles B are artificial graphite particles, a coating of a carbonaceous material is present on the surface of the graphite particles A, and the average particle diameter d A The average particle diameter d of the graphite particles B B The particle size ratio (d B / dA ) is 0.24 or less, and the mass M of the graphite particles A A and the mass M of the graphite particles B B The total mass (M A +M B ) of the mass M of the graphite particles B with respect to B The mass ratio (M B / (M A +M B )) is 0.30 or less and they are mixed. A method for producing a negative electrode material for a lithium ion secondary battery, characterized by this. 〔6〕A negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to any one of 〔1〕to 〔4〕 above. 〔7〕A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to 〔6〕 above as an electrode.

Advantages of the Invention

[0011] According to the present invention, while suppressing expansion and contraction during charge and discharge, a negative electrode material for a lithium ion secondary battery, a negative electrode, and a lithium ion secondary battery excellent in rate characteristics (1C charge rate, 2C discharge rate) and cycle characteristics, and having a small charge expansion rate and discharge expansion rate can be obtained.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0013] Hereinafter, the present invention will be described in more detail.

[0014] [Negative Electrode Material] First, an embodiment of the negative electrode material for a lithium ion secondary battery of the present invention will be described. The negative electrode material of the present invention contains a mixture of at least two types of graphite particles A having an average fracture strength of 5 MPa or more and less than 50 MPa and graphite particles B having an average fracture strength of 50 MPa to 150 MPa, and the average particle diameter d of the graphite particles A is A The average particle diameter d of the graphite particles B B The particle size ratio (d B / d A ) is less than 0.25, and the mass M of the graphite particles A is A and the mass M of the graphite particles B B The total mass (M A +M B ) the mass M of the graphite particles B B Mass ratio (M B / (M A +M B )) is 0.30 or less.

[0015] [Graphite particles A] The graphite particles A of the present invention preferably have a coating of a carbonaceous material on the surface of spherical natural graphite particles serving as a base material, and this coating of the carbonaceous material is preferably a fired carbonaceous body.

[0016] [Spherical natural graphite particles] The spherical natural graphite particles refer to natural graphite, the primary particles of which are scaly, which is sphericalized by powder processing during a pulverization process or the like, or granulated (including pulverized and re-aggregated). Among these, those with high crystallinity and an aspect ratio (major axis / minor axis) close to 1.00 (for example, 1.67 or less, more preferably 1.25 or less) are preferred. In particular, spherical natural graphite particles that have been subjected to pressure treatment, whether isotropic or anisotropic, and have few internal voids are preferred.

[0017] Originally, flat scaly natural graphite particles have the property of orienting in one direction when pressure is applied. Even after spheroidization processing, if they are pressurized, deformed, and exist in an oriented state on the electrode, the graphene layers will align parallel to the current collector. Since lithium is occluded between the graphene layers, the interlayer distance increases during charging and contracts during discharging, making it easier for the negative electrode material to expand and contract in one direction. As a result, force is applied to the particles in the electrode, and the particles expand and contract in the direction perpendicular to the electrode, deteriorating the cycle life of the battery. That is, this phenomenon is presumed to be because highly crystalline graphite is easily deformed when pressed to form an electrode, and then, when charged and discharged, the greater the deformation, the greater the expansion of the electrode.

[0018] [Coating of carbonaceous material] The purpose of providing a coating of carbonaceous material on the surface of natural graphite particles is to suppress the decomposition of the electrolyte on the surface of the negative electrode material and improve the safety of the lithium-ion secondary battery. To suppress the decomposition of the electrolyte on the surface of the negative electrode material, it is usually preferable to use a material with a small specific surface area compared to the negative electrode material. However, natural graphite tends to have a larger specific surface area compared to artificial graphite. Therefore, as described later, after coating the surface with a carbonaceous material such as pitch and then performing heat treatment, the specific surface area is reduced and then used as the negative electrode material. Also, depending on the type and amount of the coating material, it is considered to be an effect of improving the characteristics by this coating that the hardness of the spheroidized natural graphite can be adjusted so that it does not deform easily.

[0019] [Carbonaceous material] The above-mentioned coating of carbonaceous material is, for example, a material obtained by heat-treating (firing) a carbonaceous precursor such as pitch such as coal-based pitch or petroleum-based pitch, resin such as phenolic resin or furan resin, and a mixture of these pitch and resin. The carbonaceous precursor is not particularly limited, and conventionally known ones can be used. From the viewpoint of economy, etc., pitch such as coal-based pitch and petroleum-based pitch is preferable. Specific examples of coal-based pitch include coal tar pitch and coal liquefaction pitch. When using the pitch, the quinoline-insoluble content (QI) is not particularly limited, but from the viewpoint of increasing the battery capacity, it is preferably 2% by mass or less.

[0020] [Coating amount of carbonaceous material coating] The coating amount of the carbonaceous material coated on the surface of the graphite particles A of the present invention is appropriately selected from the average particle size and the hardness of the particles. Preferably, it is 1% by mass to 10% by mass. If it is less than 1% by mass, the surface reactivity of the base material cannot be sufficiently suppressed, and deformation is promoted. If it exceeds 10% by mass, the charge-discharge efficiency and the discharge capacity are reduced. More preferably, it is 5% by mass to 8% by mass. This coating amount can be obtained by calculation from the residual carbon ratio of the carbonaceous precursor (for example, the addition amount of the carbonaceous precursor × the residual carbon ratio).

[0021] [Heat treatment of carbonaceous precursor] After the carbonaceous precursor is attached to the spherical natural graphite particles serving as the base material, heat treatment is performed to generate a carbonaceous fired body on the surface of the base material. The heat treatment temperature at this time is preferably 700°C to 1300°C. If it is less than 700°C, carbonization does not proceed, and in the battery, reaction decomposition with the electrolyte occurs, resulting in a decrease in characteristics. If it exceeds 1300°C, it is uneconomical, and the carbonaceous material hardens, and there is a risk that the base material will be exposed during pressing. More preferably, it is 900°C to 1250°C, and from the viewpoint of the stability of the coating, even more preferably, it is 1000°C to 1200°C.

[0022] The atmosphere of the heat treatment is preferably a non-oxidizing atmosphere. This is because in an oxidizing atmosphere, the coating material burns and disappears, and even a small amount of oxygen generates functional groups on the surface and promotes surface reactivity. Therefore, under a nitrogen stream, an argon stream, a helium stream, under vacuum, or in the presence of substances such as coke breeze around that oxidize themselves to suppress the oxidation of the fired body and reduce the oxygen concentration to substantially a non-oxidizing atmosphere, etc., the atmosphere is preferably selected.

[0023] [Average breaking strength] In order to fully exhibit the effects of the present invention, the hardness of the graphite particles A is important, and the average breaking strength is 5 MPa or more and less than 50 MPa. If it is less than 5 MPa, it is too soft and deformation progresses, resulting in deterioration of battery characteristics. If it is 50 MPa or more, it is too hard and cracks due to pressing, exposing a new surface, leading to deterioration of battery characteristics. More preferably, it is 25 MPa to 48 MPa. Even more preferably, it is 25 MPa to 35 MPa.

[0024] [Measurement method of average breaking strength] The breaking strength of a particle refers to the force required to compress and break a single particle. The measurement method of the average breaking strength of the graphite particles A of the present invention and the graphite particles B described later is as follows.

[0025] Using a micro particle compression test MCT-W500 manufactured by Shimadzu Corporation, a single particle was compressed with a 50 μmΦ diamond flat indenter at a speed of 10 mN / sec, and the breaking strength (MPa) was determined from the force at the time of breakage. Compression tests were performed on 10 particles, and the average value of the breaking strength of each was obtained.

[0026] [Average particle size] The average particle size d of the graphite particles A of the present invention A is preferably 7.0 μm to 40.0 μm. If it is less than 7.0 μm, it becomes difficult to fabricate the electrode, and due to an increase in the specific surface area, the initial charge-discharge efficiency in the charge-discharge test may be significantly reduced. If it exceeds 40.0 μm, large particles are scattered on the electrode, which may cause the battery reaction to become non-uniform and may also inhibit battery characteristics such as load characteristics. More preferably, it is 10.0 μm to 30.0 μm, and even more preferably, it is 15.0 μm to 25.0 μm.

[0027] [Measurement method of average particle size] The average particle size (μm) of the graphite carbon particles A of the present invention and the graphite particles B described later was measured in accordance with JIS Z 8825:2013 using a laser diffraction particle size distribution analyzer with a few drops of a 3% aqueous solution of Triton X-100, a surfactant, added to ion-exchanged water as the dispersion medium.

[0028] [Graphite particles B] The graphite particles B of the present invention are preferably artificial graphite particles, and more preferably spherulitic graphite particles and / or needle coke graphitized particles.

[0029] [Artificial graphite particles] The artificial graphite particles are not particularly limited, and conventionally known ones can be used. For example, those obtained by graphitizing needle coke, spherulitic graphite particles which are graphitized mesocarbon microspheres, bulk mesophase, graphitized mesophase pitch carbon fiber, pitch such as coal-based pitch and petroleum-based pitch after firing and then graphitized at 2500 °C or higher, etc. can be mentioned. Among them, spherulitic graphite particles or their pulverized products are particularly preferred because they are difficult to orient and can reduce the deformation rate of the graphite particles A.

[0030] [Average fracture strength] The hardness of the graphite particles B of the present invention can vary depending on the average particle size and hardness of the graphite particles A to be mixed, as well as the mixing ratio with the graphite particles A, but it is harder than the graphite particles A, and its average fracture strength is 50 MPa to 150 MPa. If it is less than 50 MPa, although it also depends on the hardness of the graphite particles A, the graphite particles B may be deformed. If it exceeds 150 MPa, the graphite particles A may be deformed more than necessary. Preferably it is 50 MPa to 130 MPa. More preferably, it is 100 MPa to 130 MPa. Particularly preferably, it is 110 MPa to 125 MPa.

[0031] [Average particle size] The average particle size d of the graphite particles B of the present invention B depends on the properties of the graphite particles A to be mixed, but is preferably 1.0 μm to 10.0 μm. If it is less than 1.0 μm, it absorbs the binder and the adhesion of the electrode decreases, promoting expansion. If the average particle size is too large, it is likely to expand. More preferably, it is 3.0 μm to 5.0 μm. Even more preferably, it is 3.5 μm to 4.5 μm.

[0032] [Ratio of particle size between graphite particles A and B] The purpose of determining the particle size ratio of the graphite particles A and the graphite particles B in the present invention is to prevent the graphite particles A from being deformed. A Average particle size d of graphite particles B B The particle size ratio (d B / d A ) is less than 0.25. If it is 0.25 or more, the proportion of graphite particles B becomes large, and the effect of preventing the deformation of graphite particles A becomes insufficient, and the deformation is promoted. More preferably, it is 0.24 or less. Also, the particle size ratio (d B / d A ) is preferably 0.025, more preferably 0.10, and even more preferably 0.15.

[0033] [Mass ratio of graphite particles A and B] The mass M of the graphite particles A of the present invention A and the mass M of the graphite particle B B The total mass (M A +M B ) the mass M of graphite particle B B Mass ratio (M B / (M A +M B )) is 0.30 or less. Preferably, it is 0.05 to 0.30. If it is less than 0.05 or exceeds 0.30, the deformation rate of the graphite particles A becomes high. More preferably, it is 0.08 to 0.20, further preferably, it is 0.10 to 0.20, and particularly preferably, it is 0.10 to 0.16.

[0034] According to the present invention, by mixing soft graphite particles A with hard graphite particles B in the above-mentioned range, it is presumed that a small amount of graphite particles B present around the graphite particles A during pressing to manufacture the electrode disperses the pressing force applied to the graphite particles A, thereby inhibiting plastic deformation of the graphite particles A and preventing expansion during charge and discharge.

[0035] [Specific surface area of graphite particles] Furthermore, the specific surface areas of the graphite particles A and the graphite particles B of the present invention are preferably 10.00 m 2 / g or less for the reason of suppressing the reactivity with the electrolytic solution. More preferably, they are 8.00 m 2 / g or less, still more preferably 5.00 m 2 / g or less, particularly preferably 4.00 m 2 / g or less. Also, the preferable lower limit value of the specific surface area is 1.00 m 2 / g. Note that the specific surface area in the present invention is determined by the BET method based on the adsorption of nitrogen gas.

[0036] [Manufacturing method of negative electrode material] The manufacturing method of the negative electrode material of the present invention includes at least two or more types of graphite particles A having an average breaking strength of 5 MPa or more and less than 50 MPa and graphite particles B having an average breaking strength of 50 MPa to 150 MPa. The average particle size d A of the graphite particles B with respect to the average particle size d B of the graphite particles A has a particle size ratio (d B / d A ) of less than 0.25, and the mass ratio of the mass M A of the graphite particles A to the mass M B of the graphite particles B with respect to the total mass (M A +M B ) of the graphite particles B, i.e., M B / (M B +M A +M B )) is 0.30 or less, and they are mixed.

[0037] By mixing the graphite particles A and the graphite particles B adjusted to the above ranges, a negative electrode material is manufactured. The mixing method is not particularly limited and can be carried out using a conventionally known general mixer. Examples of the mixer include a conical mixer such as a Nauta mixer (registered trademark), a kneader mixer, a planetary mixer, a Filmix (Primix Corporation), or a combination thereof.

[0038] In addition, in the negative electrode material of the present invention, in addition to the graphite particles A and the graphite particles B, an appropriate amount of graphite particles having another property or a similar carbon material may be added as long as the effects of the present invention are not inhibited.

[0039] [Negative electrode] Next, the negative electrode for a lithium ion secondary battery of the present invention is obtained by the manufacturing method described below using the above-mentioned negative electrode material. Further, the density of the negative electrode is 1.00 g / cm 3 ~2.00 g / cm 3 which is preferable.

[0040] The density of the negative electrode (g / cm 3 ) is an index indicating the mass of the negative electrode per specific volume, and can be obtained by measuring the mass (using an electronic balance) and the thickness (using a micrometer) after punching out the negative electrode to a certain area. (For the current collector on which the negative electrode material is coated), measure the mass of 10 pieces punched out to the same area, average to obtain the mass of the current collector, and further obtain the thickness of the current collector from the density of the metal of the current collector. That is, it becomes the following formula (1).

[0041] Density of negative electrode = (Mass of negative electrode - Mass of current collector) / (Thickness of negative electrode - Thickness of current collector) × (Punched area) ··· (1) The preferable range of the density of the negative electrode is 1.00 g / cm 3 ~2.00 g / cm 3 . More preferably, it is 1.10 g / cm 3 ~1.90 g / cm 3 , and further, 1.20 g / cm 3 ~1.80 g / cm 3 . Since the effect becomes clearer when the density is higher, 1.40 g / cm 3 ~1.80 g / cm 3 is particularly preferable.

[0042] [Manufacturing method of negative electrode] The method for manufacturing the negative electrode of the present invention preferably involves pressing the negative electrode material of the present invention. Other steps regarding the manufacturing method of the negative electrode are not particularly limited and can be carried out according to the usual manufacturing methods.

[0043] When manufacturing the negative electrode, first, a negative electrode mixture containing at least two or more kinds of mixtures of the graphite particles A and graphite particles B of the present invention is used as a negative electrode mixture to which a binder, a conductive material, a solvent, etc. are added. As this binder, it is preferable to use one having chemical stability and electrochemical stability with respect to the electrolyte. For example, sodium carboxymethyl cellulose, ammonium carboxymethyl cellulose, polyvinyl alcohol, polyvinylidene fluoride, polyacrylate, polyolefin, etc. can be mentioned. The addition amount of the binder is preferably used in an amount of about 1% by mass to 20% by mass in the total amount of the negative electrode mixture. Examples of the solvent include distilled water or N-methylpyrrolidone, and examples of the conductive material include carbon black and carbon fiber. The negative electrode mixture is prepared by adding graphite particles A, graphite particles B, a binder, a conductive material, and a solvent, and stirring and mixing to obtain a paste-like negative electrode mixture (paint).

[0044] Next, this negative electrode mixture is applied to one side or both sides of the current collector, and the solvent is volatilized and dried with a blower dryer or the like to form a negative electrode mixture layer on the current collector. The shape of this current collector is not particularly limited, and examples include foil-like, mesh-like, expanded metal-like, etc. Examples of the material of the current collector include copper, stainless steel, nickel, etc.

[0045] After the paste-like negative electrode mixture is dried, pressing is performed using a pressing machine such as a roller press, and it is formed to fit the battery structure to form a negative electrode.

[0046] [Deformation rate of graphite particles A] The above-mentioned deformation rate (%) is an index representing the degree of deformation of the graphite particles A before and after pressing when the negative electrode mixture is pressed and compressed to a desired density during negative electrode manufacturing, and means the ratio of the change in the aspect ratio of the particles before and after pressing. This deformation rate is preferably 15.0% or less. When it exceeds 15.0%, the deformation becomes large and the effect of suppressing expansion cannot be obtained. More preferably, it is 12.0% or less. With the configuration of the negative electrode material of the present invention, the normal negative electrode density (1.00 g / cm 3 ~2.00 g / cm3 ) The deformation rate is 15.0% or less.

[0047] [Method for measuring the deformation rate] The method for measuring the deformation rate is as follows. Before and after pressing, the electrodes are embedded in resin, polished to expose the electrode cross-section, and cross-sectional photographs are taken using a polarized light microscope or a laser microscope. Based on the image taken at a magnification of 50 times the field of view, around 10 particle images close to the average particle size of the used graphite particles A are selected within a range of 100 μm × 50 μm. Particles that are fused and stuck together are excluded from the selection. After measuring the major axis and minor axis of each particle, the aspect ratio (major axis / minor axis) of each is calculated to obtain the average aspect ratio. The deformation rate of the present invention represents the ratio of the average aspect ratios obtained from the particle images before and after pressing, and is expressed by the following formula (2). Deformation rate (%) = 100 - {Average aspect ratio (before pressing) / Average aspect ratio (after pressing)} × 100 ···· (2) Here, the major axis means the longest diameter of the particle to be measured, and the minor axis means the short diameter perpendicular to the major axis of the particle to be measured.

[0048] [Lithium-ion secondary battery] Next, a lithium-ion secondary battery using the negative electrode of the present invention as an electrode will be described. A lithium-ion secondary battery usually has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main battery components. For the positive electrode and the negative electrode, substances capable of occluding lithium ions as a layered compound, or as a lithium-containing compound or in a cluster form are used respectively. And the entry and exit of lithium ions during the charge and discharge process occur between the layers. It is a battery mechanism in which lithium ions are doped into the negative electrode during charging and de-doped from the negative electrode during discharging.

[0049] The lithium-ion secondary battery of the present invention contains at least two or more kinds of mixtures of the graphite particles A and graphite particles B of the present invention as a negative electrode material, and includes a negative electrode using the negative electrode material and other battery components (such as a positive electrode and a non-aqueous electrolyte). Regarding other battery components, there is no particular limitation, and they conform to the components of a general lithium-ion secondary battery.

[0050] [Positive electrode] As the material of the positive electrode (positive electrode active material), it is preferable to select one that can dope or de-dope a sufficient amount of lithium ions. Such positive electrode active materials include, for example, transition metal oxides, transition metal chalcogenides, vanadium oxides and their lithium-containing compounds, and the general formula M X Mo6S 8-Y (where X is a numerical value in the range of 0 ≦ X ≦ 4, Y is a numerical value in the range of 0 ≦ Y ≦ 1, and M represents a metal such as a transition metal), Chevrel phase compounds represented by, activated carbon, activated carbon fibers, etc. These may be used alone or in combination of two or more. For example, carbonates such as lithium carbonate can also be added to the positive electrode active material.

[0051] The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more kinds of transition metals. Specifically, this lithium-containing transition metal oxide is LiM(1) 1-P M(2) P O2 (P in the chemical formula is a numerical value in the range of 0 ≦ P ≦ 1, and M(1), M(2) are composed of at least one kind of transition metal element), or LiM(1) 2-Q M(2) Q O4 (Q in the chemical formula is a numerical value in the range of 0 ≦ Q ≦ 1, and M(1), M(2) are composed of at least one kind of transition metal element). Here, the transition metal elements represented by M(1) and M(2) include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc. In particular, Co, Fe, Mn, Ti, Cr, V, Al are preferable.

[0052] The above-mentioned lithium-containing transition metal oxide can be obtained, for example, by using oxides or salts of Li or transition metals as starting materials, mixing these starting materials according to the composition, and firing them in an oxygen atmosphere within a temperature range of 600°C to 1000°C. Note that the starting materials are not limited to oxides or salts, and can also be synthesized from hydroxides, etc.

[0053] As a method for forming a positive electrode using such a positive electrode material, for example, a paste-like positive electrode mixture coating composed of a positive electrode active material, a binder, a conductive agent, etc. is applied to one side or both sides of a current collector to form a positive electrode mixture layer. As the binder, those exemplified for the negative electrode can be used. As the conductive agent, for example, fine carbon materials, fibrous carbon materials, graphite, carbon black can be used. The shape of the current collector is not particularly limited, and those having the same shape as the negative electrode are used. Usually, materials such as aluminum, nickel, stainless steel, etc. can be used.

[0054] [Non-aqueous electrolyte] As the non-aqueous electrolyte, which is one of the components of a lithium-ion secondary battery, a normal non-aqueous electrolyte containing a lithium salt such as LiPF6, LiBF4, etc. as an electrolyte salt is used. The non-aqueous electrolyte may be a liquid non-aqueous electrolyte solution, or may be a polymer electrolyte such as a solid electrolyte or a gel electrolyte.

[0055] When using a liquid non-aqueous electrolyte solution, aprotic organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, etc. can be used as the non-aqueous solvent.

[0056] When using a polymer electrolyte, it contains a matrix polymer gelled with a plasticizer (non-aqueous electrolyte solution). As this matrix polymer, ether-based polymers such as polyethylene oxide and its cross-linked products, polymethacrylate-based, polyacrylate-based, fluorine-based polymers such as polyvinylidene fluoride and vinylidene fluoride - hexafluoropropylene copolymers, etc. can be used alone or in combination. Among them, from the viewpoint of redox stability, etc., fluorine-based polymers are preferred.

[0057] As the electrolyte salt and non-aqueous solvent constituting the plasticizer (non-aqueous electrolyte) contained in the polymer electrolyte, those that can be used in liquid electrolytes can be used.

[0058] [Structure of Lithium-Ion Secondary Battery] In the lithium-ion secondary battery of the present invention, a separator such as a microporous membrane of polypropylene or polyethylene, or a layer structure thereof, or a non-woven fabric is usually used. It is also possible to use a gel electrolyte. In this case, for example, a negative electrode made of the negative electrode material of the present invention, a gel electrolyte, and a positive electrode are laminated in this order and housed in a battery exterior material to form the battery. The structure of the lithium-ion secondary battery of the present invention is arbitrary, and its shape and form are not particularly limited, and can be arbitrarily selected from, for example, a cylindrical shape, a rectangular shape, and a coin shape.

Examples

[0059] Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited thereto.

[0060] <Example 1> (Graphitic Particles A) To 100 parts by mass of spherical natural graphite particles with an average particle size of 15.0 μm, 7 parts by mass of pulverized powder of coal tar pitch (residual carbon rate 50%) was added and mixed for 30 minutes using a Nauta mixer (registered trademark) as a mixer. The obtained mixture was heat-treated in a graphite crucible at 1100 °C for 3 hours under a nitrogen flow of 2 L / min using a tubular furnace to obtain graphitic particles A having a carbonaceous fired body film on the surface. The average particle size was 16.7 μm, and the specific surface area was 3.62 m 2 / g.

[0061] (Graphitic Particles B) Mesocarbon microbeads manufactured by JFE Chemical Co., Ltd. were pulverized to an average particle size of 3.0 μm, and then heat-treated at 2800 °C for 3 hours in an argon stream. The obtained graphitic particles were sieved to remove coarse particles to obtain graphitic particles B. The average particle size was 4.0 μm, and the specific surface area was 3.42 m 2 / g.

[0062] (Manufacture of the negative electrode material) To 8.5 parts by mass of the obtained graphite particles A, 1.5 parts by mass of graphite particles B was added to obtain the negative electrode material of Example 1.

[0063] <Example 2> (Graphite particles A) To 100 parts by mass of spherical natural graphite particles with an average particle diameter of 20.0 μm, 15 parts by mass of a pulverized product powder of coal tar pitch (residual carbon rate 50%) was added and mixed for 30 minutes using a Nauta mixer (registered trademark) as a mixer. The obtained mixture was heat-treated in a graphite crucible at 1300 °C for 3 hours under a nitrogen flow of 2 L / min using a tubular furnace to obtain graphite particles A having a carbonaceous fired body film on the surface. The average particle diameter was 19.1 μm and the specific surface area was 1.43 m 2 / g.

[0064] (Graphite particles B) After needle coke was pulverized to an average particle diameter of 3.0 μm, it was heat-treated at 2800 °C for 3 hours in an argon stream, and the obtained powder was sieved to remove coarse particles to obtain graphite particles B. The average particle diameter was 3.6 μm and the specific surface area was 3.53 m 2 / g.

[0065] (Manufacture of the negative electrode material) To 9.0 parts by mass of the obtained graphite particles A, 1.0 part by mass of graphite particles B was added to obtain the negative electrode material of Example 2.

[0066] <Comparative Example 1> In Comparative Example 1, the graphite particles A of Example 1 were used as they were without mixing with other graphite particles B.

[0067] <Comparative Example 2> (Graphite particles A) To 100 parts by mass of spherical natural graphite particles with an average particle size of 18.0 μm, 4 parts by mass of a tar middle oil dissolved matter with a coal tar pitch (residual carbon rate 50%) was added in terms of solid content, and the mixture was mixed for 30 minutes using a kneader mixer. The obtained mixture was heat-treated in a graphite crucible at 1100 °C for 3 hours under a nitrogen flow of 2 L / min using a tubular furnace to obtain graphite particles A having a carbonaceous fired body coating on the surface. The average particle size was 19.2 μm, and the specific surface area was 1.09 m 2 / g.

[0068] (Manufacture of negative electrode material) The obtained graphite particles A were used as they were without mixing with other graphite particles B.

[0069] (Comparative Example 3) (Graphite particles B) Mesocarbon microbeads manufactured by JFE Chemical Corporation were pulverized to an average particle size of 2.5 μm, and then heat-treated in an argon stream at 2800 °C for 3 hours. The obtained graphite particles were sieved to remove coarse particles to obtain graphite particles B. The average particle size was 2.8 μm, and the specific surface area was 4.14 m 2 / g.

[0070] (Manufacture of negative electrode material) In Comparative Example 3, the obtained graphite particles B were used as they were without mixing with other graphite particles A.

[0071] (Comparative Example 4) (Graphite particles B) Mesocarbon microbeads manufactured by JFE Chemical Corporation were pulverized to an average particle size of 3.0 μm, and then heat-treated in an argon stream at 2800 °C for 3 hours. The obtained graphite particles were sieved to remove coarse particles to obtain graphite particles B. The average particle size was 3.1 μm, and the specific surface area was 3.25 m 2 / g.

[0072] (Manufacture of negative electrode material) In Comparative Example 4, the obtained graphite particles B were used as they were without mixing with other graphite particles A.

[0073] <Comparative Example 5> (Graphitic Particle A) To 100 parts by mass of spherical natural graphite particles with an average particle size of 20.0 μm, 33 parts by mass of coal tar dissolved in medium oil of tar (residual carbon rate 60%) was added, and the mixture was mixed for 30 minutes using a Nauta mixer (registered trademark) as a mixer. The obtained mixture was heat-treated in a graphite crucible at 1200 °C for 3 hours under a nitrogen flow of 2 L / min using a tubular furnace to obtain graphitic particles A having a carbonaceous fired body film on the surface. The average particle size was 20.7 μm, and the specific surface area was 1.08 m 2 / g.

[0074] (Graphitic Particle B) Needle coke was pulverized to an average particle size of 4.0 μm, then heat-treated at 2800 °C for 3 hours in an argon gas stream, and the obtained powder was sieved to remove coarse particles to obtain graphitic particles B. The average particle size was 4.2 μm, and the specific surface area was 4.74 m 2 / g.

[0075] (Manufacture of Negative Electrode Material) To 6.5 parts by mass of the obtained graphitic particles A, 3.5 parts by mass of graphitic particles B were added to obtain the negative electrode material of Comparative Example 5.

[0076] <Comparative Example 6> (Graphitic Particle A) The graphitic particles A used in Example 2 were used.

[0077] (Graphitic Particle B) Natural graphite BF-5A manufactured by Chuo Graphite Co., Ltd. was used. The average particle size was 5.0 μm, and the specific surface area was 10.00 m 2 / g.

[0078] (Manufacture of Negative Electrode Material) To 8.5 parts by mass of the obtained graphitic particles A, 1.5 parts by mass of graphitic particles B were added to obtain the negative electrode material of Comparative Example 6.

[0079] <Evaluation> (Evaluation of Graphitic Particles) Regarding the graphite particles used in the above Examples and Comparative Examples, the average particle size (unit: μm), specific surface area (unit: m 2 / g), average fracture strength (unit: MPa), and deformation rate of graphite particles A (unit: %) were measured by the methods described above. The results are shown in Table 1.

[0080] Next, the graphite particles obtained in each Example and Comparative Example were used as the negative electrode material, and a coin-type secondary battery for evaluation shown in FIG. 1 was fabricated and various evaluations were performed. The results are shown in Table 2.

[0081] (Preparation of negative electrode mixture paste) First, using the obtained graphite particles as the negative electrode material, a paste-like negative electrode mixture (hereinafter also referred to as the negative electrode mixture paste) was prepared. Specifically, a planetary mixer was used as the mixer, and graphite particles (98 parts by mass) and 50 parts by mass of an aqueous solution of sodium carboxymethylcellulose (solid content: 2 parts by mass) were charged, and stirred at 50 rpm for 30 minutes. Further, distilled water was added to adjust the solid content ratio to 55%, and stirring was continued for 15 minutes. Then, a styrene-butadiene rubber emulsion (1 part by mass in terms of solid content) was added to prepare the negative electrode mixture paste.

[0082] (Fabrication of negative electrode) The prepared negative electrode mixture paste was applied onto a copper foil serving as a current collector so as to have a uniform thickness, and then placed in a blow dryer to volatilize the solvent at 100 °C to form a negative electrode mixture layer. Next, the negative electrode mixture layer was pressed by a roller press to have the negative electrode density shown in Table 1, and further punched into a circular shape with a diameter of 15.5 mm to fabricate a negative electrode (working electrode) having a negative electrode mixture layer adhered to the current collector made of copper foil. Note that before the evaluation, drying was performed in a vacuum at 100 °C for 8 hours or more.

[0083] (Preparation of electrolyte) As the electrolyte, LiPF6 was dissolved in a mixed solvent obtained by mixing ethylene carbonate (33% by volume) and methyl ethyl carbonate (67% by volume) at a concentration of 1 mol / dm 3 to prepare a non-aqueous electrolyte.

[0084] The separator 5 and the working electrode 2 of the coin-shaped secondary battery for evaluation (also simply referred to as the "evaluation battery") shown in FIG. 1 were impregnated with a liquid in advance by immersing them in a non-aqueous electrolyte solution.

[0085] (Fabrication of the evaluation battery) Next, using the fabricated working electrode (negative electrode), an evaluation battery shown in FIG. 1 was fabricated.

[0086] First, a lithium metal foil was pressed against a nickel net and punched out into a circular shape with a diameter of 15.5 mm, thereby fabricating a disk-shaped counter electrode 4 made of a lithium foil adhered to a current collector 7a made of a nickel net.

[0087] Next, the separator 5 was sandwiched and laminated between the working electrode (negative electrode) 2 adhered to the current collector 7b and the counter electrode (positive electrode) 4 adhered to the current collector 7a. Then, the working electrode 2 was housed in the outer package cup 1, and the counter electrode 4 was housed in the outer package can 3. The outer package cup 1 and the outer package can 3 were combined, and the peripheral portions of the outer package cup 1 and the outer package can 3 were caulked through an insulating gasket 6 to be sealed, thereby fabricating the evaluation battery.

[0088] In the fabricated evaluation battery, the peripheral portions of the outer package cup 1 and the outer package can 3 are caulked through the insulating gasket 6 to form a sealed structure. Inside the sealed structure, as shown in FIG. 1, in order from the inner surface of the outer package can 3 toward the inner surface of the outer package cup 1, a current collector 7a, a counter electrode (positive electrode) 4, a separator 5, a working electrode (negative electrode) 2, and a current collector 7b are laminated.

[0089] (Charge and discharge test) The fabricated evaluation battery was subjected to the following charge and discharge test at 25°C. In the charge and discharge test using lithium as the counter electrode, the process of doping lithium ions into the graphite particles was defined as "charging", and the process of de-doping from the graphite particles was defined as "discharging".

[0090] First, constant current charging was performed at a current value of 0.9 mA until the circuit voltage reached 0 mV. When the circuit voltage reached 0 mV, the charging was switched to constant voltage charging, and the charging was continued until the current value reached 20 μA. The charging capacity (also referred to as the "initial charging capacity") (unit: mAh / g) was determined from the amount of electricity conducted during that period. Thereafter, a rest was taken for 120 minutes. Next, constant current discharging was performed at a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity (also referred to as the "initial discharge capacity") (unit: mAh / g) was determined from the amount of electricity conducted during this period. This was taken as the first cycle. Table 2 shows the initial discharge capacity, initial charge-discharge efficiency, and expansion rate during charging when using an electrolytic solution.

[0091] (Initial charge-discharge efficiency) From the results of the charge-discharge test, the initial charge-discharge efficiency (unit: %) was determined by the following formula (3). Initial charge-discharge efficiency = (Initial discharge capacity / Initial charging capacity) × 100 ···· (3) Also, the 1C charge rate (%) was calculated from the following formula (4). 1C charge rate = (Charging capacity of the CC part at the 1C current value / Discharge capacity of the first cycle) × 100 ···· (4) Also, the 2C discharge rate (%) was calculated from the following formula (5). 2C discharge rate = (Discharge capacity at the 2C current value / Discharge capacity of the first cycle) × 100 ··· (5) Note that 1C is the current value at which the battery's capacity can be charged and discharged in 1 hour, and 2C is a current value twice that of 1C. Also, CC refers to constant current.

[0092] (Expansion rate) The expansion rate was determined by performing charge and discharge under the conditions shown below using a HS displacement cell manufactured by Takizawa Co., Ltd. shown in Fig. 2 with a lithium cobalt oxide electrode at the counter electrode, and measuring the total expansion thickness of the positive and negative electrodes. The HS displacement cell in Fig. 2 has a negative electrode 10 and a positive electrode 12 with a separator 11 sandwiched therebetween as electrodes in a container containing the electrolytic solution 13, and has an electrode presser 14 and a displacement transfer rod 15 above the electrodes, and the thickness by which the electrodes expand during the charge-discharge test is measured as the displacement amount by a displacement meter (not shown) provided above.

[0093] Specifically, the expansion rate was determined from the total expansion thickness of the positive and negative electrodes after charging and discharging was repeated 12 times as follows. · First charge-discharge condition: Charge to 4.2 V at 0.1C (a current value that is one-tenth of 1C), and when 4.2 V is reached, control the current value to maintain 4.2 V, and continue charging until the current value drops to 0.01C mA (constant current constant voltage charging: CCCV). Next, after a 10-minute pause, discharge at 0.1C and discharge at a constant current until the voltage drops to 3.0 V. · Second time: Charge and discharge at 0.2C. It was performed under the same conditions as the first time except that the current value was changed. · Third to twelfth times: Charge and discharge at 0.5C. It was performed under the same conditions as the first time except that the current value was changed.

[0094] The expansion rate was calculated using the following formulas (6) and (7). Charge expansion rate at 12 cycles (%) = { (electrode thickness during charging at 12 cycles (total thickness of positive and negative electrodes) - electrode thickness before the start of the test (total thickness of positive and negative electrodes))} / (electrode thickness of the used negative electrode before the start of the test) × 100 - 100 ···· (6) Discharge expansion rate at 12 cycles (%) = { (electrode thickness during discharging at 12 cycles (total thickness of positive and negative electrodes) - electrode thickness before the start of the test (total thickness of positive and negative electrodes))} / (electrode thickness of the used negative electrode before the start of the test) × 100 - 100 ···· (7)

[0095] [Table 1]

[0096] [Table 2]

[0097] Examples 1 and 2 have a high capacity, a high 2C discharge rate, and a low discharge expansion rate. On the other hand, Comparative Examples 1 and 2 have a low 2C discharge rate, and both the charge expansion rate and the discharge expansion rate are high. Comparative Examples 3 and 4 with only graphite particles B have low charge expansion rates and discharge expansion rates, but due to the characteristics of the material, the electrode density cannot be increased, and the discharge capacity of the material itself is also low, so a high-capacity battery that meets the object of the present invention cannot be produced. In Comparative Example 5, even though the deformation rate, charge expansion rate, and discharge expansion rate are low, the electrode density is low and the discharge capacity is small, so a high-capacity battery cannot be produced. Comparative Example 6 has a low 2C discharge rate, and both the charge expansion rate and the discharge expansion rate are high.

[0098] As described above, since the negative electrode material of the present invention is a material with a low deformation rate during pressing, the expansion during charge and discharge is suppressed to a low level, the cycle characteristics are improved, and it can be used over a long period of time.

Description of symbols

[0099] 1 Outer package cup 2 Working electrode (negative electrode) 3 Outer package can 4 Counter electrode (positive electrode) 5 Separator 6 Insulating gasket 7a, 7b Current collector 10 Negative electrode 11 Separator 12 Positive electrode 13 Electrolyte 14 Electrode presser 15 Displacement transmission rod

Claims

1. The graphite particles A include a mixture of at least two types of graphite particles A having an average breaking strength of 5 MPa or more and less than 50 MPa, and graphite particles B having an average breaking strength of 50 MPa to 150 MPa, the graphite particles A being spheroidized natural graphite particles having an average particle size d A of 7.0 μm to 40.0 μm, the graphite particles B being artificial graphite particles having an average particle size d B of 1.0 μm to 10.0 μm, a coating of a carbonaceous material being present on the surface of the graphite particles A, and the graphite particles A having an average particle size d Ａ The average particle diameter d of the graphite particles B Ｂ Particle size ratio (d Ｂ / d Ａ ) is 0.24 or less, and the mass M of the graphite particles A is Ａ and the mass M of the graphite particles B Ｂ The total mass (M Ａ +M Ｂ ) the mass M of the graphite particles B Ｂ Mass ratio (M Ｂ / (M Ａ +M Ｂ ) is 0.30 or less.

2. The coating of the carbonaceous material is a fired body of a carbonaceous precursor, and the negative electrode material for a lithium ion secondary battery according to Claim 1, characterized in that.

3. The artificial graphite particles are spherical graphite particles and / or needle coke graphitized particles, and the negative electrode material for a lithium ion secondary battery according to Claim 1 or 2, characterized in that.

4. The negative electrode material for a lithium ion secondary battery according to any one of Claims 1 to 3, characterized in that the deformation rate of the graphite particles A is 15.0% or less.

5. Graphitic particles A with an average breaking strength of 5 MPa or more and less than 50 MPa and graphitic particles B with an average breaking strength of 50 MPa to 150 MPa, including at least two or more kinds, wherein the graphitic particles A are spherical natural graphite particles with an average particle diameter dA of 7.0 μm to 40.0 μm, and the graphitic particles B are artificial graphite particles with an average particle diameter dB of 1.0 μm to 10.0 μm, and a carbonaceous material coating exists on the surface of the graphitic particles A, and the average particle diameter d Ａ of the graphitic particles B with respect to the average particle diameter d Ｂ of the graphitic particles A, the particle size ratio (d Ｂ / d Ａ ) is 0.24 or less, and the mass M Ａ of the graphitic particles A and the mass M Ｂ of the graphitic particles B, the mass ratio (M Ａ / (M Ｂ + M Ｂ ) of the mass M Ｂ of the graphitic particles B to the total mass (M Ａ + M Ｂ ) is 0.30 or less, and the method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, characterized by mixing.

6. A negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to any one of Claims 1 to 4.

7. A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to Claim 6 as an electrode.

Images