Graphite for lithium ion secondary battery negative electrode, lithium ion secondary battery negative electrode and lithium ion secondary battery

Abstract

To provide graphite for lithium ion secondary negative electrode graphite capable of achieving a low resistance lithium-ion secondary battery that searches for suitable specifications of a shape parameter such as particle diameter in graphite, and a parameter concerning crystal characteristics intensively and can suppress dendrite precipitation, a lithium ion secondary battery negative electrode, and a lithium ion secondary battery.SOLUTION: With graphite for a lithium ion secondary battery negative electrode, an average particle diameter D50 is 5 to 8 μm. A crystallite diameter is 3 to 12 nm that is obtained by a Scherrer equation from a half width of a peak attributed to a (102) plane by a mirror index whose 2θ exists within a range of 50 to 52° in an X-ray diffraction pattern obtained by an X-ray diffraction measurement using a Cu-K α ray.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to graphite for a negative electrode of a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. [Background technology]

[0002] In recent years, lithium-ion secondary batteries have become widely used due to their high energy density and other reasons, and are installed as power sources in small portable devices such as mobile phones, digital cameras, and laptop computers. Furthermore, in light of issues such as energy resource depletion and global warming, development of lithium-ion secondary batteries is being promoted for large-scale industrial applications, such as hybrid and electric vehicles, and for storing electricity generated by natural energy sources such as solar and wind power. To expand the use of these power sources, lithium-ion secondary batteries are required to have even higher energy densities and longer lifespans.

[0003] The negative electrode of such a lithium-ion secondary battery comprises a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material provided on one or both surfaces of the negative electrode current collector. The negative electrode active material may be metallic lithium, a lithium alloy, a carbon material such as graphite, or lithium titanium oxide (Li4Ti5O 12 In particular, graphite, which has a large potential difference with the positive electrode and high capacity, is generally used as the negative electrode active material.

[0004] Graphite anodes, which use graphite as the anode active material, have the advantage of being easy to increase electrode density and have a large specific capacity, making it easy to fabricate high-energy-density lithium-ion secondary batteries. However, they also have the problem of proneness to lithium dendrites (also called dendrites or dendritic crystals) during charging. Dendrite precipitation is particularly likely during rapid charging or charging at low temperatures, which is a major concern for lithium-ion secondary batteries, which require high safety and reliability. Therefore, there is a need for the development of graphite for lithium-ion secondary battery anodes that is less prone to dendrite precipitation.

[0005] To suppress dendrite deposition, it is preferable to reduce the resistance of the negative electrode as much as possible. Generally, dendrite deposition is likely to occur when the reaction of lithium ions on the negative electrode is diffusion-limited. It is thought that by reducing the resistance of the negative electrode, the diffusion of lithium ions during charging is less likely to become rate-limited, making it less likely for lithium to deposit in a dendrite-like form on the negative electrode.

[0006] In this specification, "resistance" does not refer to the ohmic electrical resistance of graphite itself, but refers to the reaction resistance of a battery system having a graphite negative electrode, which is caused by electrochemical reactions during charge and discharge, such as the diffusion of lithium ions and film formation. The reaction resistance during charge and discharge of a lithium-ion battery can be evaluated by AC impedance measurement or the like.

[0007] Parameters related to crystalline characteristics, such as the crystallite size, degree of graphitization, and intensity ratio of X-ray diffraction peaks, of graphite are known to affect the charge-discharge characteristics of lithium-ion secondary batteries using graphite for the negative electrode. For example, Patent Document 1 discloses that the cycle characteristics of lithium-ion secondary batteries are improved by using graphite with a specific crystallite size and degree of graphitization as the negative electrode active material. Furthermore, Patent Document 2 discloses a lithium-ion secondary battery that uses graphite with a specific crystallite size and intensity ratio of X-ray diffraction peaks as the negative electrode active material in order to improve the cycle characteristics.

[0008] However, when used as graphite for lithium-ion secondary battery anodes, the relationship between the above-mentioned parameters related to the crystalline properties of graphite has not been clarified in terms of suppressing dendrite precipitation or reducing resistance, and the required properties have not been fully satisfied. Furthermore, while the resistance of lithium-ion secondary batteries using graphite for lithium-ion secondary battery anodes is naturally thought to be affected by μm-scale shape parameters such as graphite particle size, the conditions under which these shape parameters and nm-scale crystalline property parameters such as graphite crystallite size and crystal orientation must simultaneously be satisfied were unclear. Therefore, graphite used in lithium-ion secondary battery anodes, which require high safety and high reliability, has not been able to fully satisfy the required performance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-091054 [Patent Document 2] International Publication No. 2012 / 017677 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in view of the above circumstances, and has an object to provide graphite for a lithium ion secondary battery negative electrode, a lithium ion secondary battery negative electrode, and a lithium ion secondary battery that can realize a low-resistance lithium ion secondary battery that can suppress dendrite precipitation.

[0011] As a result of extensive research conducted to solve the above problems, the inventors discovered that a lithium-ion secondary battery with sufficiently low resistance can be provided by ensuring that both the particle size and parameters related to the crystal properties of graphite used as a negative electrode active material satisfy predetermined conditions, and thus completed the present invention.

[0012] That is, the graphite for a lithium ion secondary battery negative electrode according to the present invention has an average particle diameter D 50 In an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, the crystallite diameter calculated by the Scherrer equation from the half-width of the peak assigned to the (102) plane with Miller indices in the range of 2θ of 50 to 52° relative to the Bragg angle θ of the diffraction line is 3 to 12 nm. In this specification, a to b means not less than a and not more than b, unless otherwise specified.

[0013] In the X-ray diffraction pattern, the interplanar spacing d calculated from the peak attributable to the (002) plane with Miller indices in the 2θ range of 25 to 27° 002 The graphitization degree P1, which satisfies the relationship of the following formula (1), may be 0.70 to 0.82.

[0014]

number

[0015] In the X-ray diffraction pattern, the intensity ratio (100) / (101) of the peak attributable to the (100) plane in Miller indices in the 2θ range of 42 to 43° and the peak attributable to the (101) plane in Miller indices in the 2θ range of 44 to 45° may be 0.50 to 0.80.

[0016] In the X-ray diffraction pattern, the intensity ratio (110) / (004) of the peak attributable to the (110) plane in Miller indices in the 2θ range of 77 to 78° and the peak attributable to the (004) plane in Miller indices in the 2θ range of 54 to 55° may be 0.50 to 0.70.

[0017] The graphite for a lithium ion secondary battery negative electrode according to the present invention has a BET specific surface area of ​​2.0 to 3.0 m as measured by a nitrogen adsorption method. 2 / g may also be used.

[0018] In order to solve the above-mentioned problems, the negative electrode for a lithium ion secondary battery according to the present invention includes a negative electrode current collector and a negative electrode composite layer formed on the negative electrode current collector, the negative electrode composite layer containing the graphite for a lithium ion secondary battery negative electrode according to the present invention, wherein the content of the graphite for a lithium ion secondary battery negative electrode in the negative electrode composite layer is 90 to 98 mass %.

[0019] The density of the negative electrode mixture layer after initial activation in the lithium ion secondary battery may be 1.0 to 1.2 g / cc. Note that, although the density of the negative electrode mixture layer generally varies depending on the state of charge, in this specification, the density of the negative electrode mixture layer after initial activation refers to the density at any state of charge of 0 to 30%, where a fully discharged state is SOC 0% and a fully charged state is SOC 100%. Within the above range of state of charge, the density of the negative electrode mixture layer does not change significantly.

[0020] The negative electrode mixture layer may contain at least one of acetylene black, carbon nanotubes, carbon nanofibers, and graphene as a conductive material.

[0021] From the viewpoint of mitigating the expansion and contraction of the negative electrode active material during charge and discharge, the negative electrode mixture layer preferably contains acetylene black, which has excellent cushioning properties as a conductive material.

[0022] In order to solve the above problems, the lithium ion secondary battery according to the present invention includes the negative electrode for a lithium ion secondary battery according to the present invention, a positive electrode, a separator, and a non-aqueous electrolyte solution. [Effects of the Invention]

[0023] According to the present invention, it is possible to obtain graphite for a lithium ion secondary battery negative electrode, a lithium ion secondary battery negative electrode, and a lithium secondary battery, which can realize a low-resistance lithium ion secondary battery that has low resistance during charge and discharge and, as a result, can suppress dendrite precipitation. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, one embodiment of the present invention will be described, but this is merely an example, and the present invention is not limited to this description. In addition, various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention.

[0025] [Graphite for negative electrodes of lithium-ion secondary batteries] The graphite for the negative electrode of lithium-ion secondary batteries has an average particle diameter of D 50 In an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, the crystallite diameter calculated by the Scherrer equation from the half-width of the peak assigned to the (102) plane with Miller indices in the 2θ range of 50 to 52° is 3 to 12 nm, where θ is the Bragg angle of the diffraction line.

[0026] The following reasons (1) to (3) can be given for using the half-value width of the peak attributable to the (102) plane when determining the crystallite diameter of graphite for a lithium-ion secondary battery negative electrode.

[0027] (1) There are no other peaks within a 2θ of 50.6°±3° at which the peak exists, making it possible to measure the exact half-width. (2) The range of diffraction angles at which the peak exists is high enough to ignore the background effect of the diffraction profile, and low enough to observe a peak of sufficient intensity. (3) Both the c-axis component, which is the stacking direction of graphite, and the a-axis component, which is the diffusion direction of lithium ions, are contained in nearly equal proportions, making it possible to calculate a more isotropic crystallite size.

[0028] In particular, with regard to (1), the half-width of the peak attributed to the (101) plane, as used in Patent Document 2, may be affected by a peak attributed to the nearby (100) plane, and therefore the crystallite diameter determined using that peak may not be accurate. Also, with regard to (3), in the case of anisotropic crystallites, the crystallite diameter determined from the peak attributed to the (100) or (101) plane may not be sufficiently correlated with the resistance during charge and discharge.

[0029] <Average particle size> Average particle size of graphite D 50 When the average particle diameter D is 8 μm or less, the distance that lithium ions need to travel to be uniformly distributed within the graphite in the lithium ion secondary battery state is shortened, so that diffusion is less likely to be the limiting factor. 50 If the average particle diameter D of the graphite is larger than 8 μm, the distance that the lithium ions must travel within the graphite becomes longer, which makes it more likely to become diffusion-limited, resulting in higher reaction resistance. 50 If the diameter is less than 5 μm, the interface area between the graphite particles increases, hindering the diffusion of lithium ions and electrons in the negative electrode mixture layer and increasing the reaction resistance. 50 must be 5 μm or more.

[0030] <Crystallite diameter> When the graphite crystallite diameter determined by the above method is 12 nm or less, lithium ions are more likely to be deintercalated and deintercalated not only between the graphite crystal layers but also between the crystallites. The deintercalation and deintercalation of lithium ions between crystallites tends to proceed more quickly than the deintercalation and deintercalation between the crystallites. Conversely, when the crystallite diameter is larger than 12 nm, deintercalation and deintercalation of lithium ions between the crystallites becomes more difficult, leading to diffusion-limited reactions and higher reaction resistance. Furthermore, when the graphite crystallite diameter is smaller than 3 nm, excessive deintercalation and deintercalation of lithium ions between the crystallites causes a bias in the distribution of lithium ions within the graphite, which in turn inhibits the diffusion of lithium ions. Therefore, the crystallite diameter must be 3 nm or greater.

[0031] Therefore, only when the two conditions of average particle size and crystallite size are simultaneously met will the diffusion and desorption reactions of lithium ions not become rate-limiting, thereby reducing the resistance of lithium-ion secondary batteries. As a result, dendrite precipitation is thought to be less likely to occur. On the other hand, if either of these two conditions is not met, the diffusion or desorption reaction of lithium ions will become rate-limiting, and the effect of reducing the resistance of lithium-ion secondary batteries may not be expected.

[0032] <Graphitization degree> The graphite according to the present invention has a lattice spacing d calculated from a peak assigned to the (002) plane with Miller indices in the range of 2θ from 25 to 27° in an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation. 002 The graphitization degree P1, which satisfies the relationship of the following formula (1), is preferably 0.70 to 0.82.

[0033]

number

[0034] As is clear from the above equation (1), a low P1 means that d 002 indicates that is large. d 002 Graphite with a larger P1 allows lithium ions to diffuse more quickly between the crystalline layers, resulting in lower resistance when used in a lithium-ion secondary battery. In particular, using graphite with a wide interplanar spacing, such that P1 is 0.82 or less, can produce low-resistance lithium-ion secondary batteries. Furthermore, if the graphitization degree of graphite is excessively low, excessive deintercalation and deintercalation of lithium ions between the crystallites occurs, resulting in a biased distribution of lithium ions within the graphite and actually hindering the diffusion of lithium ions. Therefore, P1 is preferably 0.70 or more.

[0035] <Peak intensity ratio (100) / (101)> The graphite according to the present invention more preferably has an intensity ratio (100) / (101) of 0.5 to 0.8 between the peak attributable to the (100) plane in Miller indices in the 2θ range of 42 to 43° and the peak attributable to the (101) plane in Miller indices in the 2θ range of 44 to 45° in an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation.

[0036] A large intensity ratio (100) / (101) is thought to indicate the presence of significant disorder in the stacking structure of graphite crystals. It is speculated that a moderate amount of disorder in the stacking structure reduces the activation energy required for lithium ions to be inserted into and removed from graphite, leading to a reduction in the resistance of lithium-ion secondary batteries. This resistance-reducing effect is clearly observed when the intensity ratio is 0.5 or greater. Furthermore, in the graphite manufacturing process, excessive disorder in the stacking structure can result in excessive insertion and removal of lithium ions between crystallites, resulting in a biased distribution of lithium ions within the graphite and, in turn, hindering the diffusion of lithium ions. Therefore, it is preferable that the intensity ratio be 0.8 or less.

[0037] <Peak intensity ratio (110) / (004)> The graphite according to the present invention more preferably has an intensity ratio (110) / (004) of 0.5 to 0.7 between the peak attributable to the (110) plane in Miller indices in the 2θ range of 77 to 78° and the peak attributable to the (004) plane in Miller indices in the 2θ range of 54 to 55° in an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation.

[0038] The intensity ratio (110) / (004) indicates the crystalline orientation of graphite, with a larger value indicating a lower degree of orientation. By controlling the intensity ratio (110) / (004) to 0.5 or greater, the crystalline orientation of graphite is reduced, and the anisotropy of the reaction with lithium ions in the electrolyte in a lithium-ion secondary battery is minimized, thereby enabling a reduction in the resistance during charging and discharging. Furthermore, excessively lowering the crystalline orientation of graphite can result in excessive intercalation and deintercalation of lithium ions between crystallites, resulting in a biased distribution of lithium ions within the graphite and, in turn, hindering the diffusion of lithium ions. Therefore, it is preferable to set the intensity ratio to 0.7 or less.

[0039] <BET specific surface area> The graphite according to the present invention has a BET specific surface area of ​​2.0 to 3.0 m as measured by a nitrogen adsorption method. 2 / g is more preferred.

[0040] If the BET specific surface area is small, when the battery is used as a lithium-ion secondary battery, undesirable side reactions with the electrolyte are less likely to proceed, and the formation of an excessive SEI film is suppressed, thereby reducing the resistance during charging and discharging. Note that if the specific surface area is too small, the reaction area between the electrolyte and graphite decreases, which may increase the resistance. Therefore, a specific surface area of ​​2.0 m 2 / g or more is preferable.

[0041] The average particle size mentioned above can be controlled, for example, by appropriately adjusting the rotation speed of a mill used for crushing coke lumps to produce the graphite. The crystallite size can be controlled, for example, by the time of the calcination treatment in the production of the graphite. The degree of graphitization can be controlled, for example, by the temperature of the calcination treatment in the production of the graphite. The peak intensity ratios (100) / (101) and (110) / (004) can be controlled, for example, by appropriately adjusting the type and degree of polymerization of the carbon precursor used in the production of the graphite. The BET specific surface area can be controlled, for example, by appropriately adjusting the rotation speed of a mill used for crushing when producing the graphite, or the type and degree of polymerization of the carbon precursor used in the surface treatment.

[0042] [Anode for lithium-ion secondary batteries] Next, the negative electrode for a lithium ion secondary battery according to the present invention will be described. The negative electrode includes at least a negative electrode current collector and a negative electrode mixture layer.

[0043] <Negative electrode current collector> The material constituting the negative electrode current collector is not particularly limited, but it is preferable to use a metal. Specific examples include copper, aluminum, nickel, stainless steel, titanium, and other alloys. Among these, copper is preferable from the viewpoint of electronic conductivity and battery operating potential. The thickness of the negative electrode current collector is preferably 1 to 50 μm.

[0044] <Negative electrode composite material layer> The negative electrode mixture layer contains the graphite for a lithium ion secondary battery negative electrode according to the present invention (hereinafter, sometimes referred to as "negative electrode active material") and is formed on the negative electrode current collector. For example, the negative electrode mixture layer may be provided on one or both sides of the negative electrode current collector. The negative electrode mixture layer may also contain a binder and a conductive material, which will be described below.

[0045] The content of the graphite for lithium-ion secondary battery negative electrodes in the negative electrode mixture layer is 90 to 98% by mass. If the content is less than 90% by mass, the volume and weight of the portion in the negative electrode mixture layer that contributes to charge / discharge reactions will be small, resulting in a small capacity of the lithium-ion secondary battery. If the content is more than 98% by mass, the proportion of conductive material and binder relative to the graphite content will be insufficient, and the effect of reducing the resistance of the lithium-ion secondary battery may not be expected.

[0046] (binding material) Examples of the binder used in the negative electrode mixture layer include polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, polytetrafluoroethylene, poly(meth)acrylate, polyvinylidene fluoride, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, etc., or a mixture thereof. The content of the binder in the negative electrode mixture is, for example, 1 to 2 mass %.

[0047] <Conductive material> From the viewpoint of reducing the resistance of a lithium-ion secondary battery by improving the conductivity, it is preferable that a carbon material be contained as a conductive material in the negative electrode composite layer. Examples of conductive materials include conductive carbon powder such as acetylene black, carbon nanotubes, carbon nanofibers, and graphene, and at least one of these can be contained. Among these, it is preferable to use acetylene black as the conductive material in the negative electrode according to the present invention. The content of the conductive material in the negative electrode composite is, for example, approximately 0.1 to 1.0 mass%.

[0048] A thickener may be added to the negative electrode mixture layer as needed. Examples of thickeners include carboxymethyl cellulose and polyvinyl alcohol. When a thickener is added, the content of the thickener in the negative electrode mixture is, for example, 1 to 2 mass %. In addition to this, other additives such as a dispersing agent may also be added.

[0049] <Density of negative electrode mixture layer> The density of the negative electrode composite layer is preferably 1.0 to 1.2 g / cc. In this case, "density" refers to the density of the negative electrode composite layer after pressing the negative electrode to fabricate a battery and initial activation. The reason for this is that if the density of the negative electrode composite layer is too high, adequate voids will not form in the negative electrode composite layer, making it difficult for the electrolyte to penetrate, resulting in poor diffusion of lithium ions. Furthermore, if the density is below 1.0 g / cc, adhesion between the active materials in the negative electrode composite layer or between the active materials and the conductive material will be poor, which may result in increased resistance. In the case of graphite, which is typically used in the negative electrodes of lithium-ion secondary batteries, the density must be increased to at least 1.3 to 1.4 g / cc to prevent poor adhesion between the active materials and between the active materials and the conductive material. However, in this case, adequate voids will not form in the negative electrode composite layer, making it difficult for the electrolyte to penetrate. On the other hand, when using the graphite for lithium-ion secondary battery anodes according to the present invention, the moderately small particle size facilitates the creation of voids between active material particles, and the small crystallite size results in a low specific gravity, ensuring good adhesion between active materials and between the active material and the conductive material, even at a relatively low density of 1.0 to 1.2 g / cc. In other words, by using the graphite for lithium-ion secondary battery anodes according to the present invention, good adhesion between active materials and between the active material and the conductive material can be achieved while ensuring a moderate amount of voids within the anode composite layer. The density can be controlled by known methods, such as by changing the pressure or gap value of the press during press processing. Furthermore, since graphite-containing anode composite layers generally form an SEI (solid oxide layer) before and after initial activation, resulting in a 5 to 10% decrease in density, the density during press processing must be determined taking this into account.

[0050] [Lithium-ion secondary battery] Next, a lithium ion secondary battery according to the present invention will be described. The lithium ion secondary battery includes the negative electrode for a lithium ion secondary battery according to the present invention, a positive electrode, a separator, and a non-aqueous electrolyte solution.

[0051] <Positive electrode> The positive electrode is capable of absorbing and releasing lithium ions, and is composed of a positive electrode current collector and a positive electrode mixture layer formed on one or both sides of the positive electrode current collector.

[0052] (Positive electrode current collector) Although there are no particular limitations on the material that constitutes the positive electrode current collector, it is preferable to use a metal. Specific examples include aluminum, nickel, stainless steel, titanium, and other alloys. Among these, aluminum is preferred from the viewpoint of electronic conductivity and battery operating potential. The thickness of the positive electrode current collector is preferably 1 to 50 μm.

[0053] (Positive electrode mixture layer) The positive electrode mixture layer can be made of materials conventionally used in lithium-ion secondary batteries. The positive electrode mixture layer preferably contains a positive electrode active material selected from the group consisting of lithium metal oxides having a layered rock salt structure or a spinel structure, lithium metal phosphates having an olivine structure, or a mixture thereof. Furthermore, the conductive material, graphite, binder, and other additives used in the negative electrode mixture layer may also be used.

[0054] <Separator> Examples of the separator include a porous sheet separator made of polymer or fiber, a nonwoven fabric separator, etc. The pore size of the separator is preferably 0.01 to 10 μm, and the thickness is preferably 5 to 30 μm. The separator may also have a ceramic layer laminated on a porous substrate as a heat-resistant insulating layer.

[0055] <Non-aqueous electrolyte> The non-aqueous electrolyte solution is composed of a lithium salt and a non-aqueous solvent in which other additives are dissolved. Examples of the lithium salt contained in the non-aqueous electrolyte solution include, but are not limited to, one or a mixture of two or more selected from LiBF, LiPF, Li(FSO), Li(CFSO)N, etc. The concentration of the lithium salt is preferably 0.5 mol / L or more and 5 mol / L or less, and more preferably 0.8 mol / L or more and 1.5 mol / L or less.

[0056] The nonaqueous solvent contained in the nonaqueous electrolyte is not particularly limited, but examples thereof include one or a mixture of two or more selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, benzonitrile, etc. In particular, DMC, DEC, DPC, EMC, EC, and PC are preferred, and in terms of forming a good coating on the negative electrode active material, it is particularly preferred to include EC.

[0057] In addition, it is preferable to include additives other than the lithium salts described above for the purpose of forming a high-quality coating on the surface of the negative electrode active material through reductive decomposition during charge and discharge. Examples of additives include, but are not limited to, vinylene carbonate, fluoroethylene carbonate, 1,3,2-dioxathiolane 2,2-dioxide (MMDS), 1,5,2,4-dioxadithiane 2,2,4,4-tetraoxide, tris(trimethylsilyl) phosphite, 1-propene 1,3-sultone, and Li2PO2F2. These additives may be used alone or in combination. They may also be used in combination with other additives. Furthermore, other additives may also be used alone. [Example]

[0058] The present invention will be described in more detail below by way of examples, but the present invention is not limited to the following embodiments.

[0059] [Method for measuring the physical properties of graphite for negative electrodes of lithium-ion secondary batteries] The physical properties of the graphite used in the preparation of the negative electrode for the lithium ion secondary battery were determined by the following method.

[0060] <Crystallite diameter> In the X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, the half-width B of the peak assigned to the (102) plane in the Miller indices in the 2θ range of 50 to 52° was measured, and the (102) crystallite diameter D was calculated by substituting this into the following Scherrer equation (Equation (2)).

[0061]

number

[0062] Here, K is the Scherrer constant (1.84), λ is the X-ray wavelength (1.54 Å), and θ is the Bragg angle of the diffraction line.

[0063] <Graphitization degree> First, the Bragg angle θ of the peak assigned to the (002) plane in the Miller index 002 is measured and substituted into the Bragg equation (Equation (3)) to obtain the interplanar spacing d 002 was calculated.

[0064]

number

[0065] And the d obtained here 002 The value of was substituted into the following formula (1) to calculate the degree of graphitization P1.

[0066]

number

[0067] <Peak intensity ratio> In addition, the peak intensities of the peaks attributable to the (100), (101), (110), and (004) planes in the Miller indices were measured by powder X-ray diffraction in the same manner as above, and the peak intensity ratios (100) / (101) and (110) / (004) were calculated.

[0068] <Average particle size> Average particle size of graphite D 50 The median diameter was measured by the laser diffraction / scattering method described in JIS standard Z8825: 2013. A laser diffraction particle size distribution analyzer SALD-2300 (Shimadzu Corporation) was used for the measurement.

[0069] <BET specific surface area> The BET specific surface area was calculated by applying the BET equation to a value measured by a nitrogen adsorption method.

[0070] [Preparation of negative electrodes for lithium-ion secondary batteries] In Examples 1 to 13 and Comparative Examples 1 to 16, graphite having the physical properties shown in Table 1 was used as the negative electrode active material. 96.7 mass% of this graphite, 0.3 mass% of acetylene black as a conductive material, 1.5 mass% of styrene butadiene rubber as a binder, and 1.5 mass% of carboxymethyl cellulose as a thickener were mixed with ion-exchanged water as a solvent to form a slurry, thereby obtaining a negative electrode composite slurry.

[0071] The obtained negative electrode composite slurry was applied to one side of a 10 μm thick copper foil as a negative electrode current collector, dried, and then pressed with a roll press to obtain a negative electrode. The coating amount of the negative electrode composite layer per side was 58 g / m. 2 The gap value of the roll press was adjusted to perform pressing so that a negative electrode composite layer having the density shown in the "Density after pressing" column in Table 1 was obtained.

[0072] [Fabrication of lithium-ion secondary batteries] A lithium metal foil having a thickness of 300 μm was attached to a stainless steel foil current collector having a thickness of 100 μm to form a positive electrode.

[0073] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a lithium salt at a ratio of 1.3 mol / L in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3.

[0074] Using the above-mentioned positive electrode, negative electrode, nonaqueous electrolyte, and a polyolefin microporous film as a separator, 2032-type coin-type lithium ion secondary batteries (hereinafter sometimes referred to as "coin batteries") were fabricated as coin batteries of Examples 1 to 13 and coin batteries of Comparative Examples 1 to 16. The coin batteries were fabricated in an argon atmosphere with a dew point of -50°C or lower.

[0075] [Initial battery activation] Examples 1 to 13 and Comparative Examples 1 to 16 were transferred to a thermostatic chamber set at 25°C, and five cycles of initial activation were performed. The first cycle consisted of a constant-current / constant-voltage charge with a current of 0.1C, a voltage of 0V, and a cutoff current of 0.05C, followed by a constant-current discharge of 0.1C and a cutoff voltage of 1.5V. The second to fifth cycles consisted of a constant-current / constant-voltage charge with a current of 0.2C, a voltage of 0V, and a cutoff current of 0.05C, followed by a constant-current discharge of 0.2C and a cutoff voltage of 1.5V. A 15-minute rest period was set after each charge and discharge. After the fifth cycle, the batteries were charged at 0.1C for five hours, and the SOC (State of Charge) was adjusted to 50%.

[0076] [Calculating resistance value] For each of the coin-type batteries of Examples 1 to 13 and Comparative Examples 1 to 16 that had completed the initial activation process, AC impedance measurements were performed in a thermostatic chamber at 25°C. The input voltage amplitude was 10 mV, and the measurement frequency was 1 MHz to 10 mHz. Curve fitting was performed on the spectra obtained from the measurements to separate the spectra resulting from the film resistance on the negative electrode and the charge / discharge reaction, and the resistance value was calculated. The resistance value results for each coin-type battery are shown in Table 1. Resistance values ​​less than 20 Ω were judged as good (◯), less than 18 Ω as very good (◎), and 20 Ω or greater as poor (×).

[0077] [Method for measuring density after initial activation] After the initial activation process and AC impedance measurement, each coin battery was adjusted to an SOC of 20% and then disassembled, and the negative electrode was removed and washed with DMC and dried. The electrode thickness and weight of the washed negative electrode were measured to calculate the density of the negative electrode composite layer after initial activation.

[0078] [Table 1]

[0079] The evaluation results shown in Table 1 indicate that the battery resistance is reduced by using graphite as the negative electrode active material, which has both an average particle size and a (102) crystallite size within the specified ranges. Furthermore, the battery resistance is particularly reduced by using graphite as the negative electrode active material, which has, in addition to the average particle size and (102) crystallite size, a degree of graphitization and a diffraction peak intensity ratio (100) / (101) and (110) / (004) within the specified ranges, and by ensuring that the density of the negative electrode composite layer after initial activation is within the specified ranges. Therefore, it is estimated that dendrite precipitation is less likely to occur even during rapid charging or charging at low temperatures.

[0080] On the other hand, the crystallite diameters determined from the peaks attributable to the (100) and (101) planes in the Miller indices of graphite used as a negative electrode active material did not show a sufficient correlation with the resistance value. From these results, in order to obtain a negative electrode for a lithium secondary battery with sufficiently low resistance, it is preferable to use graphite as a negative electrode active material whose crystallite diameter, determined from the peak attributable to the (102) plane in the Miller indices rather than from any diffraction plane among the peaks obtained by powder X-ray diffraction, falls within a predetermined range. [Industrial Applicability]

[0081] A lithium-ion secondary battery equipped with a negative electrode using the graphite of the present invention can achieve extremely low resistance during charging and discharging. Therefore, even when rapid charging or repeated charging at low temperatures is performed, dendrite precipitation is unlikely to occur, and it is believed that the risk of capacity loss and fire or explosion can be reduced. Therefore, by using the graphite of the present invention, it is possible to provide a lithium secondary battery for applications requiring high safety and high reliability, making it industrially useful. The inventions described in the original claims of this application are set forth below. [1] The average particle size D50 is 5 to 8 μm, Graphite for use in negative electrodes of lithium ion secondary batteries, having a crystallite diameter of 3 to 12 nm, as determined by the Scherrer equation from the half-width of the peak assigned to the (102) plane, with Miller indices in the range of 2θ from 50 to 52° relative to the Bragg angle θ of the diffraction line in an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation. [2] In the X-ray diffraction pattern, the interplanar spacing d calculated from the peak attributable to the (002) plane with Miller indices in the 2θ range of 25 to 27° 002 Graphite for a negative electrode of a lithium ion secondary battery according to [1], having a degree of graphitization P1 of 0.70 to 0.82, which satisfies the relationship of the following formula (1): [Number 1] d 002 =3.35P 1 +3.44(1-P 1 ) (1) [3] The graphite for a lithium-ion secondary battery negative electrode according to [1] or [2], wherein in the X-ray diffraction pattern, an intensity ratio (100) / (101) of a peak attributable to a (100) plane in Miller indices in the 2θ range of 42 to 43° to a peak attributable to a (101) plane in Miller indices in the 2θ range of 44 to 45° is 0.50 to 0.80. [4] The graphite for a lithium ion secondary battery negative electrode according to any one of [1] to [3], wherein in the X-ray diffraction pattern, an intensity ratio (110) / (004) of a peak attributable to a (110) plane in Miller indices in the 2θ range of 77 to 78° to a peak attributable to a (004) plane in Miller indices in the 2θ range of 54 to 55° is 0.50 to 0.70. [5] The BET specific surface area measured by nitrogen adsorption is 2.0 to 3.0 m 2 The graphite for a negative electrode of a lithium ion secondary battery according to any one of [1] to [4], wherein the graphite has a molecular weight of 1000 or more and a molecular weight of 1000 or more. [6] a negative electrode current collector; a negative electrode mixture layer formed on the negative electrode current collector, the negative electrode mixture layer including the graphite for a lithium ion secondary battery negative electrode according to any one of [1] to [5]; Equipped with The negative electrode for a lithium ion secondary battery, wherein the content of the graphite for a lithium ion secondary battery negative electrode in the negative electrode mixture layer is 90 to 98 mass %. [7] The negative electrode for a lithium ion secondary battery according to [6], wherein the density of the negative electrode mixture layer after initial activation in the lithium ion secondary battery is 1.0 to 1.2 g / cc. [8] The negative electrode for a lithium ion secondary battery according to [6] or [7], wherein the negative electrode mixture layer contains at least one of acetylene black, carbon nanotubes, carbon nanofibers, and graphene as a conductive material. [9] a negative electrode for a lithium ion secondary battery according to any one of [6] to [9]; A positive electrode and A separator; a nonaqueous electrolyte; A lithium-ion secondary battery comprising:

Claims

1. The average particle size D50 is 5 to 8 μm, In an X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, the crystallite diameter calculated by the Scherrer equation from the half-width of the peak assigned to the (102) plane with Miller indices in which 2θ with respect to the Bragg angle θ of the diffraction line is in the range of 50 to 52° is 3 to 12 nm; The following three requirements: (1) In the X-ray diffraction pattern, when the interplanar spacing d 002 calculated from the peak attributable to the (002) plane with Miller indices in the 2θ range of 25 to 27° is expressed as the following formula: d 002 = 3.35P 1 + 3.44 (1 - P 1 ), the degree of graphitization P 1 is 0.70 to 0.

82. (2) In the X-ray diffraction pattern, the intensity ratio (100) / (101) of the peak attributable to the (100) plane in Miller indices in the 2θ range of 42 to 43 degrees to the peak attributable to the (101) plane in Miller indices in the 2θ range of 44 to 45 degrees is 0.50 to 0.80; and (3) In the X-ray diffraction pattern, the intensity ratio (110) / (004) of the peak attributable to the (110) plane in Miller indices in the 2θ range of 77 to 78 degrees and the peak attributable to the (004) plane in Miller indices in the 2θ range of 54 to 55 degrees is 0.50 to 0.

70. Graphite for a negative electrode of a lithium ion secondary battery that satisfies at least two of the above requirements.

2. BET specific surface area measured by nitrogen adsorption method is 2.0 to 3.0 m ２ 2. The graphite for a lithium ion secondary battery negative electrode according to claim 1, wherein the average molecular weight of the graphite is 1 / g.

3. a negative electrode current collector; a negative electrode mixture layer formed on the negative electrode current collector, the negative electrode mixture layer comprising the graphite for a lithium ion secondary battery negative electrode according to claim 1 or 2; Equipped with The negative electrode for a lithium ion secondary battery, wherein the content of the graphite for a lithium ion secondary battery negative electrode in the negative electrode mixture layer is 90 to 98 mass %.

4. A negative electrode for a lithium ion secondary battery as described in claim 3, wherein the negative electrode composite layer has a density of 1.0 to 1.2 g / cc after initial activation.

5. 5. The negative electrode for a lithium ion secondary battery according to claim 3, wherein the negative electrode mixture layer contains at least one of acetylene black, carbon nanotubes, carbon nanofibers, and graphene as a conductive material.

6. The negative electrode for a lithium ion secondary battery according to any one of claims 3 to 5, A positive electrode and A separator; a nonaqueous electrolyte; A lithium-ion secondary battery comprising:

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