Negative electrode plate and lithium ion secondary battery

Abstract

To provide a negative electrode plate that can produce a secondary battery with excellent high-temperature storage durability and excellent input / output characteristics.SOLUTION: A negative electrode plate includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is formed from a negative electrode mixture layer containing a negative electrode active material and a binder. The negative electrode active material contains amorphous coated graphite in which the surfaces of graphite particles are coated with amorphous carbon. The oil absorption of the amorphous coated graphite is 40 cc / 100 g or more and 53 cc / 100 g or less. When the negative electrode mixture layer formed on the negative electrode current collector is compressed at a pressure of 190 MPa, the density of the negative electrode mixture layer is 1.55 g / cc or more and 1.76 g / cc or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a negative electrode plate and a lithium-ion secondary battery including the same.

Background Art

[0002] In a lithium-ion secondary battery, when the reaction area of the negative electrode plate is increased, the input / output characteristics (high-rate characteristics) are improved while the cycle durability is reduced. When the reaction area of the negative electrode plate is decreased, the cycle durability is improved while the input / output characteristics tend to decrease. Thus, since there is a trade-off relationship between the input / output characteristics and the cycle durability of the lithium-ion secondary battery, it is difficult to achieve both. In order to obtain a lithium-ion secondary battery having excellent input / output characteristics and cycle durability, a negative electrode active material in which graphite particles are coated with a carbon material having low crystallinity such as amorphous carbon may be used (Patent Documents 1 and 2, etc.).

[0003] Patent Document 1 discloses that by adjusting the material physical properties of the negative electrode active material, the battery characteristics of the lithium-ion secondary battery are improved. Patent Document 2 discloses that by adjusting the content of a specific supporting salt contained in the non-aqueous electrolyte together with the material physical properties of the negative electrode active material, the battery characteristics of the lithium-ion secondary battery are improved.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described above, by using graphite particles coated with a carbon material such as amorphous carbon as the negative electrode active material, the input / output characteristics and cycle durability of a lithium-ion secondary battery can be improved. However, a negative electrode plate is usually produced by applying a negative electrode mixture slurry obtained by mixing a negative electrode active material and a binder or the like onto a current collector, drying it, and compressing it. When such a processing step is performed, the material physical properties of the negative electrode active material may change. Therefore, in the product design of the negative electrode plate, it is necessary to consider not only the material physical properties of the negative electrode active material but also the physical properties of the negative electrode plate obtained using the negative electrode active material.

[0006] An object of the present disclosure is to provide a negative electrode plate that can provide a secondary battery excellent in high-temperature storage durability capable of maintaining a good discharge capacity even when stored at a high temperature and excellent in input / output characteristics, and a lithium-ion secondary battery.

Means for Solving the Problems

[0007] The present disclosure provides the following negative electrode plate and lithium-ion secondary battery. 〔1〕 A negative electrode plate having a negative electrode current collector and a negative electrode active material layer, The negative electrode active material layer is formed from a negative electrode mixture layer containing a negative electrode active material and a binder, The negative electrode active material includes amorphous-coated graphite in which the surface of graphite particles is coated with amorphous carbon, The oil absorption amount of the amorphous-coated graphite is 41 cc / 100 g or more and 53 cc / 100 g or less, The density of the negative electrode mixture layer when the negative electrode mixture layer formed on the negative electrode current collector is compressed at a pressure of 190 MPa is 1.55 g / cc or more and 1.76 g / cc or less, a negative electrode plate. 〔2〕 The negative electrode plate according to 〔1〕, wherein the binder includes at least one of a cellulose-based binder and styrene-butadiene rubber. 〔3〕 The negative electrode plate according to 〔2〕, wherein the binder includes a cellulose-based binder. 〔4〕 The specific surface area per unit mass of the negative electrode active material layer is 1.77 m 2 / g or more and 2.05 m 2The negative electrode plate according to any one of [1] to [3], which is below / g. 〔5〕 The negative electrode plate according to any one of [1] to [4], wherein the graphite particles are artificial graphite. 〔6〕 The negative electrode plate according to any one of [1] to [5], wherein the negative electrode plate is a negative electrode plate for a lithium ion secondary battery. 〔7〕 A lithium ion secondary battery including the negative electrode plate according to any one of [1] to [5].

Advantages of the Invention

[0008] According to the negative electrode plate of the present disclosure, it is possible to provide a lithium ion secondary battery having excellent high-temperature storage durability and further improved input / output characteristics.

Embodiments for Carrying Out the Invention

[0009] (Negative Electrode Plate) The negative electrode plate of the present embodiment has a negative electrode current collector and a negative electrode active material layer. The negative electrode plate usually has a negative electrode active material layer on one or both sides of the negative electrode current collector. The negative electrode current collector is, for example, a metal foil formed using a copper material such as copper and copper alloy. The negative electrode plate can be a negative electrode plate for a secondary battery, and particularly can be a negative electrode plate for a lithium ion secondary battery.

[0010] The negative electrode active material layer is formed from a negative electrode mixture layer containing a negative electrode active material and a binder. Therefore, the negative electrode active material layer also contains a negative electrode active material and a binder. The negative electrode mixture layer is a layer formed on the negative electrode current collector to form the negative electrode active material layer of the negative electrode plate. When the negative electrode mixture slurry is applied on the negative electrode current collector, dried, and compressed to form the negative electrode active material layer as described later, the negative electrode mixture layer is the layer after the applied negative electrode mixture slurry is dried and before compression. The negative electrode active material layer is usually formed by compressing the negative electrode mixture layer as described above. Both the negative electrode active material layer and the negative electrode mixture layer only need to contain at least a negative electrode active material and a binder, and may contain a conductive auxiliary agent such as fibrous carbon.

[0011] The negative electrode active material includes amorphous coated graphite in which the surface of graphite particles is coated with amorphous carbon. By using a negative electrode plate provided with a negative electrode active material layer containing amorphous coated graphite, the negative electrode active material can favorably occlude and release lithium ions, and it is possible to suppress the deposition of decomposition products of the electrolyte or the like on the surface of the negative electrode active material and the inhibition of the occlusion and release of lithium ions. As a result, it is possible to obtain a secondary battery that is excellent in input / output characteristics and also excellent in high-temperature storage durability in which the discharge capacity can be favorably maintained even when stored at a high temperature.

[0012] The graphite particles contained in the amorphous coated graphite may be artificial graphite, natural graphite, or both, but artificial graphite is preferred. Since artificial graphite has a higher hardness and is less likely to deform than natural graphite, it is possible to suppress the cracking of the amorphous carbon of the amorphous coated graphite when the negative electrode mixture layer is compressed. As a result, it becomes easier to adjust the compression density of the negative electrode mixture layer described later within the range described later.

[0013] The amorphous carbon contained in the amorphous coated graphite refers to a carbon material having an amorphous structure, and examples thereof include carbon black and activated carbon. The amorphous carbon may cover a part of the surface of the graphite particles or may cover the entire surface of the graphite particles. The thickness of the amorphous carbon covering the surface of the graphite particles may be, for example, 1 nm or more and 1 μm or less. The amorphous coated graphite can be obtained, for example, by mixing graphite particles and pitch and firing and carbonizing the pitch.

[0014] The oil absorption amount of the amorphous coated graphite is 40 cc / 100 g or more and 53 cc / 100 g or less, preferably 41 cc / 100 g or more and 51 cc / 100 g or less, more preferably 43 cc / 100 g or more and 50 cc / 100 g or less, and still more preferably 45 cc / 100 g or more and 50 cc / 100 g or less. When the oil absorption amount of the amorphous coated graphite is within the above range, it is possible to suppress the excessive adsorption of the binder on the surface of the amorphous coated graphite and the inhibition of the occlusion and release of lithium ions, so that the input / output characteristics of the secondary battery can be further improved.

[0015] The oil absorption amount of the amorphous-coated graphite can be calculated by the following procedure. First, the change in viscosity characteristics when linseed oil is added to the amorphous-coated graphite at an addition rate of 2 cc / min is detected by a torque detector, and the output is converted to torque. Subsequently, the addition amount of linseed oil when the torque corresponding to 70% of the maximum torque among the obtained torques is generated is converted to the addition amount per 100 g of the amorphous-coated graphite, and this is calculated as the oil absorption amount of the amorphous-coated graphite. As the torque detector, for example, "S-500" manufactured by Asahi Riken Co., Ltd. can be used.

[0016] The oil absorption amount of the amorphous-coated graphite can be adjusted, for example, by adjusting one or more selected from the group consisting of the particle size, tap density, specific surface area (BET), and shape of the amorphous-coated graphite.

[0017] Among the negative electrode active materials contained in the negative electrode active material layer, the content of the amorphous-coated graphite having the above-described oil absorption amount may be 80% by mass or more, 90% by mass or more, 95% by mass or more, or 100% by mass with respect to the total amount of the negative electrode active materials. When the negative electrode active material layer contains two or more types of negative electrode active materials, the total amount of the negative electrode active materials refers to the total amount of the two or more types of negative electrode active materials.

[0018] When the negative electrode mixture layer formed on the negative electrode current collector is compressed at a pressure of 190 MPa, the density of the negative electrode mixture layer (hereinafter, also referred to as "compression density of the negative electrode mixture layer") is 1.55 g / cc or more and 1.76 g / cc or less, preferably 1.56 g / cc or more and 1.75 g / cc or less, more preferably 1.57 g / cc or more and 1.73 g / cc or less, and still more preferably 1.58 g / cc or more and 1.70 g / cc or less. When the compression density of the negative electrode mixture layer is 1.55 g / cc or more, it is easy to increase the capacity of the secondary battery using the negative electrode plate. When the compression density of the negative electrode mixture layer is 1.76 g / cc or less, it is considered that the amorphous carbon of the amorphous-coated graphite is less likely to crack when the negative electrode mixture layer is compression-molded to form the negative electrode active material layer. Thereby, it is possible to suppress an increase in the specific surface area (BET) per unit mass of the negative electrode active material layer, and it is easy to obtain a secondary battery having excellent high-temperature storage durability. Thus, in the negative electrode plate of the present embodiment, since the negative electrode plate is designed in consideration of not only the material physical properties of the negative electrode active material but also the physical properties when the negative electrode mixture layer is compressed, a secondary battery having excellent input / output characteristics and excellent high-temperature storage durability is easily obtained.

[0019] The compression density of the negative electrode mixture layer is calculated based on the basis weight (solid content per unit area) of the negative electrode mixture layer and the thickness of the negative electrode mixture layer after compression, by forming the negative electrode mixture layer on the negative electrode current collector and compressing a laminate of the negative electrode current collector and the negative electrode mixture layer at a pressure of 190 MPa. Specifically, first, a laminate of the negative electrode current collector and the negative electrode mixture layer is punched into a circle with a diameter of 20 mm, and the mass is measured to calculate the basis weight [mg / 10 cm 2 of the negative electrode mixture layer. Subsequently, a load of 60 kN is applied to the punched laminate using a hydraulic press and held for 30 seconds, thereby compressing the laminate at a pressure of 190 MPa. The thickness [μm] of the negative electrode mixture layer of the compressed laminate is calculated, and the compression density [g / cc] of the negative electrode mixture layer (= basis weight [mg / 10 cm 2 of the negative electrode mixture layer / thickness [μm] of the negative electrode mixture layer) is calculated.

[0020] The compression density of the negative electrode mixture layer can be adjusted, for example, by the type of graphite particles constituting the amorphous-coated graphite, the type of amorphous carbon, the heat treatment temperature of the amorphous carbon, the content of the negative electrode active material in the negative electrode mixture layer, the type of binder, and the like. As described above, by using artificial graphite as the graphite particles of the amorphous-coated graphite, when the negative electrode mixture layer is compressed to form the negative electrode active material layer, it is possible to suppress the cracking of the amorphous carbon, so that it becomes easier to adjust the compression density of the negative electrode mixture layer to the above range.

[0021] The specific surface area (BET) per unit mass of the negative electrode active material layer in the negative electrode plate is preferably 1.77 m 2 / g or more and 2.05 m 2 / g or less, more preferably 1.79 m 2 / g or more and 2.00 m 2 / g or less, and even more preferably 1.80 m 2 / g or more and 1.99 m 2 / g or less. The specific surface area per unit mass of the negative electrode active material layer is obtained by cutting out the negative electrode plate into a size of 1 mm × 1 mm, placing it in a glass container, and determining the specific surface area [m 2 from the adsorption and desorption amount of nitrogen (N2) gas using a fully automatic specific surface area meter, and calculating the specific surface area [m 2 / g] per unit mass. As the fully automatic specific surface area meter, for example, "Macsorb HM model-1208" manufactured by MOUNTECH can be used.

[0022] Generally, when the specific surface area per unit mass of the negative electrode active material layer increases, the input resistance of the secondary battery decreases and the input / output characteristics are improved, but the high-temperature storage durability of the secondary battery tends to decrease. Conversely, when the specific surface area per unit mass of the negative electrode active material layer decreases, the high-temperature storage durability of the secondary battery is improved, but the input / output characteristics of the secondary battery tend to decrease. Therefore, if it is possible to reduce the input resistance of the secondary battery while suppressing an increase in the specific surface area per unit mass of the negative electrode active material layer, it is considered that a secondary battery with improved input / output characteristics can be obtained while having excellent high-temperature storage durability. Since the compression density of the negative electrode mixture layer and the oil absorption amount of the amorphous-coated graphite of the negative electrode plate of the present embodiment are adjusted to the specific ranges described above, compared with a secondary battery using a negative electrode active material layer having a similar specific surface area per unit mass although these physical properties are not adjusted, a secondary battery with further improved input / output characteristics can be obtained while maintaining the high-temperature storage durability.

[0023] The negative electrode active material only needs to contain at least amorphous-coated graphite having the above-described oil absorption amount, and may contain amorphous-coated graphite outside the above-described oil absorption amount range and / or other negative electrode active materials other than amorphous-coated graphite. Examples of other negative electrode active materials include carbon-based active materials containing carbon (C) atoms other than amorphous-coated graphite such as artificial graphite or natural graphite (graphite), hard carbon, and soft carbon; metal-based active materials containing metal elements such as simple metals or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). Examples of the Si-based active material containing a silicon element include silicon simple substance, SiOx, LixSiyOz, and the like.

[0024] Examples of the binder include cellulose-based binders such as carboxymethyl cellulose (CMC), methyl cellulose (MC), and hydroxypropyl cellulose; styrene butadiene rubber (SBR), polyacrylic acid (PAA), acrylonitrile butadiene rubber (NBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and the like. The binder preferably contains at least one of a cellulose-based binder and SBR, and may contain both. The cellulose-based binder is preferably CMC. CMC and PAA may be in the form of an acid or a salt.

[0025] The content of the binder contained in the negative electrode active material layer may be 0.5% by mass or more and 10% by mass or less, 1.0% by mass or more and 8.0% by mass or less, or 1.5% by mass or more and 5.0% by mass or less with respect to the total amount of the negative electrode active material layer. When the negative electrode active material layer contains two or more binders, the content of the binder refers to the total content of the two or more binders.

[0026] Examples of the conductive assistant include carbon materials such as fibrous carbon, carbon black (e.g., acetylene black, ketjen black), coke, and activated carbon. Examples of the fibrous carbon include carbon nanotubes (hereinafter also referred to as "CNT"). The CNT may be a single-walled carbon nanotube (SWCNT) or a multi-walled carbon nanotube such as a double-walled carbon nanotube (DWCNT).

[0027] (Method for manufacturing a negative electrode plate) For example, a negative electrode plate can be obtained by forming a negative electrode mixture layer by applying and drying a negative electrode mixture slurry on a negative electrode current collector, and forming a negative electrode active material layer by compressing the negative electrode mixture layer on the negative electrode current collector. Compression of the negative electrode mixture layer may be performed, for example, by pressing a laminate in which the negative electrode current collector and the negative electrode mixture layer are laminated with a roll press.

[0028] The negative electrode mixture slurry contains a negative electrode active material, a binder, and a dispersion medium, and can be obtained by mixing these. The negative electrode mixture slurry contains amorphous coated graphite within the above-described oil absorption amount range. Examples of the dispersion medium contained in the negative electrode mixture slurry include water such as ion-exchanged water. The negative electrode mixture slurry may contain a negative electrode active material other than amorphous coated graphite within the above-described oil absorption amount range, and may also contain a conductive assistant. Examples of the negative electrode active material and the binder that the negative electrode mixture slurry may contain include those described above.

[0029] The negative electrode mixture slurry can be prepared, for example, in the following steps. First, after dry-blending amorphous coated graphite, a part of a binder such as a cellulose-based binder, and, if necessary, a conductive assistant, a dispersion medium is added and kneaded at a high solid content to perform solid kneading to adhere the binder to the surface of the amorphous coated graphite. Subsequently, more dispersion medium is added and kneaded at a solid content lower than that in the above solid kneading to disperse the amorphous coated graphite with the binder attached to its surface in the dispersion medium. Then, if necessary, the remaining binder is added and kneaded to obtain the negative electrode mixture slurry.

[0030] (Lithium Ion Secondary Battery) The lithium ion secondary battery can include the negative electrode plate described above. The lithium ion secondary battery usually has an electrode body having the negative electrode plate, the positive electrode plate, and the separator described above, and an electrolytic solution. The lithium ion secondary battery can further include an exterior body that houses the electrode body and the electrolyte.

[0031] The electrode body has a structure in which the negative electrode active material layer of the negative electrode plate and the positive electrode active material layer of the positive electrode plate are laminated with the separator interposed therebetween. The electrode body may be a wound type or a laminated type. The electrode body may be a flat electrode body.

[0032] The positive electrode plate has a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode current collector is, for example, a metal foil composed of an aluminum material such as aluminum and an aluminum alloy. The positive electrode active material layer contains a positive electrode active material. As the positive electrode active material, known materials can be used, and examples include lithium transition metal oxides such as layered or spinel-type (e.g., LiNiCoMnO2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4, LiCrMnO4, LiFePO4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2).

[0033] The positive electrode active material layer can contain a binder and a conductive aid, etc. as additives other than the positive electrode active material. Examples of the binder include styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVdF), and polytetrafluoroethylene (PTFE). Examples of the conductive aid include carbon materials such as fibrous carbon, carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. The fibrous carbon is as described above.

[0034] The positive electrode active material layer can be formed by applying a positive electrode mixture slurry prepared by mixing a positive electrode mixture containing a positive electrode active material and an additive with a dispersion medium to the positive electrode current collector, drying it, and compressing it. Examples of the dispersion medium include N-methyl-2-pyrrolidone (NMP).

[0035] Examples of the separator include porous sheets (films, non-woven fabrics, etc.) made of resins such as polyethylene, polypropylene, polyester, cellulose, and polyamide. The porous sheet may have a single-layer structure or a multi-layer structure of two or more layers.

[0036] Examples of the electrolyte include non-aqueous electrolytes, for example, those obtained by containing a supporting salt in a non-aqueous solvent such as an organic solvent. The electrolyte may further contain one or more additives selected from the group consisting of vinylene carbonate (VC), cyclohexylbenzene (CHB), and modified products of CHB in order to form a good film on the surface of the negative electrode active material layer and / or the positive electrode active material layer, or to ensure stability during overcharging.

[0037] Examples of the non-aqueous solvent include one or more selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), γ-butyrolactone (γ-BL), sulfolane, acetonitrile, 1,2-dimethoxyethane (DME), 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran.

[0038] Examples of the supporting salt include one or more selected from the group consisting of lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium trifluoromethanesulfonate (LiCF3SO3), and lithium bis(trifluoromethylsulfonyl)imide [LiN(CF3SO2)2].

[0039] The exterior body may be formed, for example, using a laminate sheet having an aluminum layer or the like, into a size and shape that cover the outer surface of the electrode body.

Examples

[0040] Hereinafter, the present disclosure will be described more specifically by showing examples and comparative examples. 〔Comparative Examples 1 to 9, Examples 1 to 4〕 (Preparation of negative electrode mixture slurry) Using a small-scale kneader (Primix's "High Bis Mix 2P-1 type"), a negative electrode mixture slurry with water as the dispersion medium was prepared. The negative electrode mixture slurry was prepared according to the following procedure so that the mixing ratio of the negative electrode active material / carboxymethyl cellulose (CMC) / styrene butadiene rubber (SBR) was 98.3 / 0.7 / 1.0 (mass ratio) as shown in Table 1. First, the negative electrode active material shown in Table 1 and CMC as a binder were dry-blended at a rotation speed of 25 rpm for 10 minutes, then water was added and kneaded at a rotation speed of 60 rpm for 7.5 minutes (solid kneading) to obtain a kneaded product. Further water was added to this kneaded product and kneaded at a rotation speed of 60 rpm for 5 minutes to obtain a dispersion in which the negative electrode active material with CMC attached was dispersed in water. To this dispersion, SBR was added and kneaded at a rotation speed of 60 rpm for 5 minutes to obtain a negative electrode mixture slurry.

[0041] (Fabrication of laminate of negative electrode current collector and negative electrode mixture layer) As the negative electrode current collector, a copper (Cu) foil with a thickness of 10 μm was prepared. The negative electrode mixture slurry was applied to both sides of this copper foil and dried to form a negative electrode mixture layer, and a laminate in which the negative electrode current collector and the negative electrode mixture layer were laminated was obtained. The coating amount of the negative electrode mixture slurry was adjusted so that the value obtained by dividing the capacity of the negative electrode active material layer by the capacity of the positive electrode active material layer (capacity of negative electrode active material layer / capacity of positive electrode active material layer) was the same in all examples and comparative examples. The basis weight of the negative electrode mixture layer (solid content per unit area of the negative electrode mixture slurry coated on the copper foil) was adjusted within the range of 255 mg / 10 cm 2 to 265 mg / 10 cm 2 below. In all examples and comparative examples, the capacity of the positive electrode active material layer of the positive electrode plate was adjusted to be the same.

[0042] (Fabrication of negative electrode plate) The laminate obtained above was compressed using a small roll press to form a negative electrode active material layer with a density of 1.56 g / cc, thereby obtaining a negative electrode plate. In the compression by the roll press, the thickness at which the negative electrode active material layer with the above density was obtained was calculated based on the following formula, and the compression load of the roll press and the size of the gap between the rolls (gap) were adjusted so that the negative electrode active material layer with the calculated thickness was obtained. Density of negative electrode active material layer [g / cc] = Coating weight of negative electrode binder layer [mg / 10 cm 2 / Thickness of negative electrode active material layer [μm]

[0043] (Preparation of positive electrode binder slurry) Using a small kneader ("High Bis Mix 2P-1 type" manufactured by PRIMIX), a positive electrode binder slurry using N-methyl-2-pyrrolidone (NMP) as a dispersion medium was prepared. The positive electrode binder slurry was prepared so as to have a mixing ratio of positive electrode active material (lithium nickel cobalt manganese composite oxide (NCM)) / conductive assistant (carbon black) / binder (polyvinylidene fluoride (PVdF)) = 97.5 / 1.5 / 1.0 (mass ratio).

[0044] (Fabrication of positive electrode plate) As a positive electrode current collector, an aluminum (Al) foil with a thickness of 13 μm was prepared. The positive electrode binder slurry was applied to both sides of this aluminum foil and dried to form a positive electrode binder layer, and a laminate in which the positive electrode current collector and the positive electrode binder layer were laminated was obtained. This laminate was compressed using a small roll press to form a positive electrode active material layer, and a positive electrode plate was obtained.

[0045] (Fabrication of test cell (laminate cell)) Leads were attached to each of the negative electrode plate and the positive electrode plate fabricated above. An electrode body was obtained by laminating the electrode plates with leads attached so that the negative electrode active material layer and the positive electrode active material layer faced each other through a separator. After accommodating the electrode body in an exterior body composed of an aluminum laminate sheet and injecting an electrolyte, the opening of the exterior body was sealed to obtain a test cell as a laminate cell. The electrolyte was prepared by mixing 1.15 M LiPF6 as a supporting salt, a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) = 30 / 30 / 40 (volume ratio) as a non-aqueous solvent, and 1.0 mass% vinylene carbonate (VC) with respect to the total mass of the mixed solvent as an additive.

[0046] [Measurement of oil absorption amount of negative electrode active material] 20 g of the negative electrode active material being stirred by two blades (rotation speed on the high-speed side: 125 rpm, rotation speed on the low-speed side: 62.5 rpm) was added with linseed oil at an addition rate of 2 cc / min. The change in viscosity characteristics at this time was detected with a torque detector ("S-500" manufactured by Asahi Research Co., Ltd.), and its output was converted into torque with a microcomputer. The addition amount of linseed oil when torque corresponding to 70% of the maximum torque among the obtained torques was generated was converted into the addition amount per 100 g of the negative electrode active material, and this was calculated as the oil absorption amount [cc / 100 g] of the negative electrode active material. The results are shown in Table 1.

[0047] [Measurement of Compression Density of Negative Electrode Binder Layer] The density of the negative electrode binder layer (compression density of the negative electrode binder layer) when the negative electrode binder layer formed on the negative electrode current collector was compressed at a pressure of 190 MPa was measured using the laminate of the negative electrode current collector and the negative electrode binder layer prepared above. Specifically, after punching out this laminate into a circle with a diameter of 20 mm, the mass was measured and the basis weight of the negative electrode binder layer was calculated. The laminate was compressed at a pressure of 190 Mpa by applying a load of 60 kN using a hydraulic press and holding it for 30 seconds. Then, the thickness of the negative electrode binder layer of the compressed laminate was measured. From the basis weight [mg / 10 cm 2 and the thickness [μm] of the negative electrode binder layer, the compression density [g / cc] of the negative electrode binder layer was calculated based on the following formula. The results are shown in Table 1. Compression density of negative electrode binder layer [g / cc] =(Basis weight of negative electrode binder layer [mg / 10 cm 2 ) / (Thickness of negative electrode binder layer [μm])

[0048] [Measurement of Specific Surface Area (BET) per Unit Mass of Negative Electrode Active Material Layer] The negative electrode plate was cut into pieces of 1 mm × 1 mm in size and placed in a glass container for measurement. Using a fully automatic specific surface area measuring device ("Macsorb HM model-1208" manufactured by MOUNTECH), the specific surface area [m 2 was obtained from the adsorption and desorption amounts of nitrogen (N2) gas. This specific surface area [m 2 was divided by the mass [g] of the negative electrode active material layer (specific surface area [m 2 / Mass of the negative electrode active material layer [g]), specific surface area per unit mass of the negative electrode active material layer [m 2 / g] was calculated. The results are shown in Table 1.

[0049] [Evaluation of the input resistance of the test cell] After adjusting the SOC (state of charge) of the test cell to 50%, the test cell was charged at a constant current, and the voltage at the time when 5 seconds had elapsed since the start of charging was recorded. By performing the constant-current charging at a plurality of current values, V (voltage) was plotted against I (current), and the input resistance (V / I) [mΩ] at the time when 5 seconds had elapsed since the start of charging was calculated from the slope. The smaller the value of the input resistance, the better the input / output characteristics. The results are shown in Table 1.

[0050] [Evaluation of the high-temperature storage durability of the test cell] The test cell was charged and discharged so that the SOC became 0%, 100%, and 0% in order, and after measuring the initial discharge capacity, the SOC of the test cell was adjusted to 95%. Subsequently, the test cell was placed in a constant-temperature bath at a temperature of 60°C and left for 30 days for a high-temperature storage test. After taking out the test cell from the constant-temperature bath after the high-temperature storage test and setting the SOC to 0%, it was charged and discharged again so that the SOC became 0%, 100%, and 0% in order, and the discharge capacity after the high-temperature storage test was measured. The discharge capacity retention rate during high-temperature storage was calculated according to the following formula. The larger the value of the discharge capacity retention rate during high-temperature storage, the better the high-temperature storage durability. The results are shown in the column of the discharge capacity retention rate in Table 1. Discharge capacity retention rate during high-temperature storage [%] =(Discharge capacity after the high-temperature storage test / Initial discharge capacity) × 100

[0051]

Table 1

Claims

1. A negative electrode plate having a negative electrode current collector and a negative electrode active material layer, The negative electrode active material layer is formed from a negative electrode mixture layer containing a negative electrode active material and a binder, The specific surface area per unit mass of the negative electrode active material layer is 1.77 m2 / g or more and 1.99 m2 / g or less, The negative electrode active material includes amorphous-coated graphite in which the surface of graphite particles is coated with amorphous carbon, The graphite particles are artificial graphite, The oil absorption amount of the amorphous-coated graphite is 40 cc / 100 g or more and 53 cc / 100 g or less, The density of the negative electrode mixture layer when the negative electrode mixture layer formed on the negative electrode current collector is compressed at a pressure of 190 MPa is 1.55 g / cc or more and 1.76 g / cc or less. Negative electrode plate.

2. The negative electrode plate according to claim 1, wherein the binder contains at least one of a cellulose-based binder and styrene-butadiene rubber.

3. The negative electrode plate according to claim 2, wherein the binder contains a cellulose-based binder.

4. The negative electrode plate according to any one of claims 1 to 3, wherein the negative electrode plate is a negative electrode plate for a lithium ion secondary battery.

5. A lithium ion secondary battery including the negative electrode plate according to any one of claims 1 to 3.

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