Non-aqueous secondary battery

Abstract

To provide a non-aqueous secondary battery that has excellent binding strength and electronic conductivity in a negative electrode and suppresses deterioration in discharge rate characteristics.SOLUTION: A non-aqueous secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode is formed by laminating a negative electrode current collector, a binder layer, and a negative electrode mixture layer in this order. The binder layer contains a conductive assistant. The ratio (A / B) of the average length (A) of the conductive assistant to the average thickness (B) of the binder layer is 1.8 or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to non-aqueous secondary batteries. [Background technology]

[0002] Lithium secondary batteries using a non-aqueous electrolyte (hereinafter also referred to as "nonaqueous secondary batteries") are used in information and communication technologies (for example, personal computers, smartphones, etc.), in-vehicle applications, power storage, and the like.

[0003] Patent Document 1 discloses an electrode plate for a battery. The electrode plate is an electrode plate in which an adhesive layer (hereinafter also referred to as a "binder layer") and an active material layer (hereinafter also referred to as a "composite layer") are laminated in this order on at least one surface of a current collector. The binder layer is made of a polyamideimide resin containing carbon. Graphite, carbon black, or acetylene black is specifically disclosed as the carbon. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-273181 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the electrode body disclosed in Patent Document 1, if the carbon content in the binder layer is increased to increase the energy density (specifically, to improve the electronic conductivity of the negative electrode), the strength with which the binder layer binds the negative electrode mixture layer to the negative electrode current collector (hereinafter also referred to as "binding strength of the negative electrode") may decrease. Furthermore, there is a demand for a nonaqueous secondary battery in which the decrease in the discharge rate characteristics of the nonaqueous secondary battery is suppressed.

[0006] The present disclosure has been made in consideration of the above circumstances. An object of one embodiment of the present disclosure is to provide a nonaqueous secondary battery having excellent binding strength and electronic conductivity of a negative electrode and suppressing a decrease in discharge rate characteristics. [Means for solving the problem]

[0007] Means for solving the above problems include the following embodiments. <1> A battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte; the negative electrode is formed by laminating a negative electrode current collector, a binder layer, and a negative electrode mixture layer in this order, The binder layer contains a conductive assistant, The nonaqueous secondary battery has a ratio (A / B) of the average length (A) of the conductive assistant to the average thickness (B) of the binder layer of 1.8 or more. <2> The ratio (A / B) is 2.5 or more. <1> The non-aqueous secondary battery according to claim 1. <3> The content of the conductive assistant is 0.8 mass% or more with respect to the total amount of the binder layer. <1> or <2> The non-aqueous secondary battery according to claim 1. <4> The binder layer comprises a styrene-butadiene copolymer. <1> ~ <3> 13. The nonaqueous secondary battery according to claim 12, <5> The negative electrode mixture layer has a thickness of 100 μm or more. <1> ~ <4> 13. The nonaqueous secondary battery according to claim 12, Effect of the Invention

[0008] According to the present disclosure, there is provided a nonaqueous secondary battery having excellent binding strength and electronic conductivity of a negative electrode and suppressing deterioration of discharge rate characteristics. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] In the present disclosure, a numerical range indicated using "~" means a range including the numerical values ​​described before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper limit or lower limit described in a certain numerical range may be replaced with the upper limit or lower limit of another numerical range described in stages. In the numerical ranges described in the present disclosure, the upper limit or lower limit described in a certain numerical range may be replaced with a value shown in the examples. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, the amount of each component means the total amount of the multiple substances, unless otherwise specified, when there are multiple substances corresponding to each component. In the present disclosure, the term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0010] (1) Non-aqueous secondary battery The nonaqueous secondary battery of the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The negative electrode is formed by laminating a negative electrode current collector, a binder layer, and a negative electrode mixture layer in this order. The binder layer contains a conductive assistant. The ratio (A / B) of the average length (A) of the conductive assistant to the average thickness (B) of the binder layer is 1.8 or more.

[0011] Since the nonaqueous secondary battery of the present disclosure has the above-mentioned configuration, the negative electrode has excellent binding strength and electronic conductivity, and deterioration of the discharge rate characteristics is suppressed. This effect is believed to be due to, but not limited to, the following reasons. In the present disclosure, the ratio (A / B) is 1.8 or more. Therefore, even if the amount of the conductive assistant in the binder layer is smaller than in a configuration in which the ratio (A / B) is less than 1.8 (for example, a configuration in which the conductive assistant is composed of at least one of graphite, carbon black, and acetylene black), a conductive path is easily formed in the binder layer. As a result, it is presumed that the nonaqueous secondary battery of the present disclosure has excellent binding strength and electronic conductivity of the negative electrode, and the decrease in discharge rate characteristics is suppressed.

[0012] The cell structure of the nonaqueous secondary battery is not particularly limited, and examples thereof include a wound type, a stacked type, etc. The wound type is formed by winding a linear electrode body in which a negative electrode, a separator, and a positive electrode are stacked in this order. The stacked type is formed by stacking a leaf-shaped electrode body in which a negative electrode, a separator, and a positive electrode are stacked in this order.

[0013] (1.1) Negative electrode The negative electrode is formed by laminating a negative electrode current collector, a binder layer, and a negative electrode mixture layer in this order.

[0014] (1.1.1) Negative electrode current collector The negative electrode current collector may be a known current collector (for example, copper foil, etc.).

[0015] (1.1.2) Binder layer The ratio (A / B) of the binder layer is preferably 2.5 or more. This suppresses the deterioration of the discharge rate characteristics of the nonaqueous secondary battery. The ratio (A / B) may be 4.0 or more, or 5.0 or more. The ratio (A / B) may be 7.0 or less, or 4.0 or less.

[0016] The thickness (B) of the binder layer is not particularly limited as long as the ratio (A / B) is 1.8 or more. The thickness (B) of the binder layer may be 3 μm or more, 5 μm or more, or 8 μm or more. The thickness (B) of the binder layer may be 13 μm or less, or 8 μm or less. The thickness (B) of the binder layer is measured by observation with an electron microscope (SEM).

[0017] (1.1.2.1) Conductive additives The average length (A) of the conductive assistant is not particularly limited, and may be 8 μm or more, 13 μm or more, or 20 μm or more. The average length (A) of the conductive assistant may be 35 μm or less. The average length (A) of the conductive assistant is measured by observation with a scanning electron microscope (SEM).

[0018] The conductive assistant is not particularly limited as long as the ratio (A / B) is 1.8 or more, and preferably contains carbon nanotubes, and is preferably carbon nanotubes.

[0019] The content of the conductive assistant is not particularly limited. The content of the conductive assistant may be 0.3 mass% or more, 0.8 mass% or more, or 1.0 mass% or more, based on the total amount of the binder layer. The content of the conductive assistant may be 2.2 mass% or less, or 1.2 mass% or less.

[0020] When the ratio (A / B) is 2.5 or more, the content of the conductive assistant is preferably 0.8 mass % or more relative to the total amount of the binder layer, which leads to a lower penetration resistance of the negative electrode mixture layer.

[0021] (1.1.2.2) Binder The binder layer binds the negative electrode mixture layer and the negative electrode current collector. The binder layer usually further contains a binder in addition to the conductive assistant. Examples of the binder include halogenated vinyl resins, rubbers, polyolefin resins, and the like. Examples of the halogenated vinyl resins include polyvinylidene fluoride (PVdF), copolymers of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP), and the like. Examples of the rubbers include styrene-butadiene rubber (SBR), butadiene rubber (BR), acrylate butadiene rubber (ABR), acrylonitrile-butadiene rubber (NBR), butyl rubber (isobutylene-isoprene rubber), and the like. Examples of the polyolefin resins include polyethylene, polypropylene, and the like. The binder may be used alone or in combination of two or more types.

[0022] When the ratio (A / B) is 2.5 or more and the content of the conductive assistant is 0.8 mass % or more, it is preferable that the binder layer contains styrene-butadiene rubber, which improves the binding strength of the negative electrode mixture layer and further suppresses the deterioration of the discharge rate characteristics of the nonaqueous secondary battery.

[0023] The content of the binder is not particularly limited. The content of the binder may be 99.7 mass% or less, 99.2 mass% or less, or 99.0 mass% or less, based on the total amount of the binder layer. The content of the conductive assistant may be 97.8 mass% or more, or 98.8 mass% or more.

[0024] (1.1.2.3) Other ingredients The binder layer may or may not further contain other components, such as an electrolyte supporting salt (lithium salt) for increasing ion conductivity, a polymer electrolyte, and an additive (e.g., trifluoropropylene carbonate).

[0025] (1.1.3) Negative electrode composite layer The negative electrode mixture layer contains a negative electrode layer active material capable of absorbing and releasing charge carriers (e.g., carbon (e.g., natural graphite, artificial graphite), a compound capable of alloying with lithium (e.g., silicon, tin, etc.), etc.). The negative electrode mixture layer may further contain, as necessary, a conductive assistant (e.g., acetylene black, etc.) for enhancing electronic conductivity, a binder, an electrolyte supporting salt (lithium salt) for enhancing ionic conductivity, a polymer electrolyte, and an additive (e.g., trifluoropropylene carbonate, etc.). Examples of the binder include the same ones as those exemplified as the binder of the binder layer.

[0026] The thickness of the negative electrode mixture layer is not particularly limited, but is preferably 100 μm or more. This improves the energy density of the non-aqueous secondary battery. From the viewpoint of increasing the energy density, the thickness of the negative electrode mixture layer is more preferably 150 μm or more, and further preferably 180 μm or more. The thickness of the negative electrode mixture layer may be 250 μm or less.

[0027] (1.2) Positive electrode The positive electrode may have a positive electrode current collector (for example, aluminum foil or the like) and a positive electrode mixture layer. The positive electrode mixture layer is laminated on at least one main surface of the positive electrode current collector.

[0028] The positive electrode mixture layer includes a positive electrode active material. The positive electrode active material releases lithium ions to a non-aqueous electrolyte solution or absorbs lithium ions from the electrolyte solution. The positive electrode active material may be a known positive electrode active material (e.g., LiNiO 2 , LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 The positive electrode mixture layer may further contain a known conductive material (e.g., carbon black, etc.), trilithium phosphate, and a binder. Examples of the binder include the same binders as those exemplified for the binder layer.

[0029] (1.3) Separator The separator maintains the distance between the positive electrode and the negative electrode to prevent contact short circuit and allows lithium ions to pass through. Examples of the separator include a porous resin sheet or a nonwoven fabric. Examples of the material of the porous resin sheet include polyolefin (polypropylene, polyethylene, etc.). Examples of the material of the nonwoven fabric include polypropylene, polyethylene terephthalate, methyl cellulose, etc. The separator may have a known configuration.

[0030] (1.4) Non-aqueous electrolyte The non-aqueous electrolyte may include a non-aqueous solvent and a lithium salt. The lithium salt may be, for example, LiClO 4 , LiAsF 6 , LiPF 6 , LiBF 4 , LiCF 3 SO 3 , LiN(FSO 2 ) 2 , LiN(CF 3 SO 2 ) 2Examples of the non-aqueous solvent include cyclic carbonates (e.g., ethylene carbonate, etc.), chain carbonates (e.g., dimethyl carbonate, ethyl methyl carbonate, etc.), cyclic esters (e.g., γ-butyrolactone, γ-valerolactone, etc.), chain esters (e.g., methyl formate, methyl acetate, etc.), ethers (e.g., dimethoxyethane, ethoxymethoxyethane, etc.), etc. The non-aqueous electrolyte may contain additives (e.g., vinylene carbonate, lithium bis(oxalato)borate, etc.), etc.

[0031] (1.5) Case A non-aqueous secondary battery usually has a case. The case contains a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The case is not particularly limited, and examples thereof include a laminate film (e.g., an aluminum sheet, etc.) and a battery can (e.g., a cylindrical, rectangular, coin-shaped, etc.). EXAMPLES

[0032] The present disclosure will be described in more detail below with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0033] [1] Examples 1 to 11 and Comparative Examples 1 to 11 [1.1] Positive electrode LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 The slurry was prepared by mixing the conductive additive and a polyvinylidene fluoride (PVdF)-containing solution as a binder. The mass ratio of the positive electrode active material, the conductive additive, and the binder (positive electrode active material:conductive additive:binder) was 87:10:3. The "PVdF-containing solution" refers to a mixed solution of PVdF and a solvent. The slurry was applied to an aluminum foil as a positive electrode current collector to obtain a positive electrode sheet. The positive electrode sheet was punched using a φ12 (diameter: 12 mm) punch to obtain a positive electrode. The positive electrode is formed by laminating a positive electrode current collector and a positive electrode composite layer in this order.

[0034] [1.2] Negative electrode [1.2.1] Binder layer As solutions containing a binder (hereinafter also referred to as "binder-containing solutions"), a styrene-butadiene rubber (SBR)-containing solution and a polyvinylidene fluoride (PVdF)-containing solution were prepared. "SBR-containing solution" refers to a mixed solution of SBR and a solvent. "PVdF-containing solution" refers to a mixed solution of PVdF and a solvent.

[0035] As the conductive assistant, carbon nanotubes (CNTs) with an average length of 10 μm, CNTs with an average length of 15 μm, and CNTs with an average length of 30 μm were prepared.

[0036] A binder-containing solution containing a binder shown in Table 1 and a conductive assistant shown in Table 1 were mixed in the ratio shown in Table 1 to obtain a dispersion. The dispersion was applied onto a copper foil as a negative electrode current collector and dried at 80°C. In this way, a negative electrode current collector with a binder layer was obtained. The negative electrode current collector with a binder layer includes a negative electrode current collector and a binder layer formed on one main surface of the negative electrode current collector.

[0037] [1.2.2] Negative electrode composite layer An amorphous-coated artificial graphite material with an average particle size of 15 μm was prepared as the negative electrode active material. The negative electrode active material was mixed with styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion solvent to obtain a slurry. The slurry was applied onto the binder layer of a binder-layered negative electrode current collector to obtain a negative electrode sheet. The negative electrode sheet was punched using a φ12 (diameter: 12 mm) punch to obtain a negative electrode. The negative electrode was formed by laminating a negative electrode current collector, a binder layer, and a negative electrode composite layer in this order. The capacity of the negative electrode was about 1.1 times the capacity of the positive electrode.

[0038] [1.2.3] Non-aqueous electrolyte In the mixed solvent, LiPF as electrolyte 6A non-aqueous electrolyte solution was obtained by adding the above. The mixed solvent consisted of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The volume ratio of EC to EMC (EC:EMC) was 30:70. LiPF in the non-aqueous electrolyte solution 6 The concentration was 1.0 M (mol / L).

[0039] [1.2.4] Separators A three-layered porous membrane was prepared as a separator. The porous membrane was made by laminating a polypropylene layer, a polyethylene layer, and a polypropylene layer in this order. The air permeability of the porous membrane obtained by the Gurley test method was 300 seconds.

[0040] [1.2.5] Batteries A cell (electrode body) was formed by using a positive electrode and a negative electrode and opposing them with a separator interposed therebetween, and the cell was sealed together with a non-aqueous electrolyte solution by lamination to prepare a battery for evaluation.

[0041] [2] Evaluation [2.1]1.0C rated discharge rate [2.1.1] Activation The initial charge was performed in a 25° C. thermostatic chamber at a constant current of 0.1 C up to 4.30 V. Then, the battery was discharged at a constant current of 0.3 C down to 3.00 V. This was repeated three times.

[0042] [2.1.2] Initial Characterization Using a constant current constant voltage method, the battery was charged to 4.30 V at a current value of 0.1 C, and then charged at a constant voltage until the current value during constant voltage charging became 1 / 50 C, at which point the battery was fully charged. It was then discharged to 3.00 V at a current value of 0.2 C using a constant current method. The capacity at this time was taken as the initial capacity.

[0043] [2.1.3] Long-term input / output characteristic evaluation The battery was charged to 4.30 V at a current value of 0.1 C using a constant current method, and then charged at a constant voltage until the current value during constant voltage charging became 1 / 50 C, at which point the battery was fully charged. The battery was then discharged to 3.00 V at a current value of 1.0 C using a constant current method, and the capacity at this time was measured.

[0044] The ratio of the capacity measured in the long-term input / output characteristics to the initial capacity measured in the initial characteristic evaluation was defined as the "1.0C rated discharge rate." The measurement results are shown in Table 1. The allowable range of the 1.0C rated discharge rate is "57% or more."

[0045] [2.2] Peel strength A peel test was conducted to measure the peel strength of the negative electrode composite layer. Specifically, a specified tape was attached to the negative electrode, and the stress was measured when it was peeled off at a constant speed. The stress when the negative electrode composite layer peeled off was taken as the "peel stress." The measurement results are shown in Table 1. The acceptable range of peel strength is "0.18 N / cm or more."

[0046] [2.3] Composite formability The evaluation method for the composite viability was performed in accordance with JIS K6854-1 1999. More specifically, the composite viability (i.e., the binding property required to prepare the cells of the examples) was evaluated according to the following evaluation criteria using the measurement results of the peel strength in the peel test. The evaluation results are shown in Table 1. Note that in order to prepare the cells of the examples, a peel strength of 0.10 N / m 2 The peel strength is equal to or greater than this.

[0047] A: Peel strength is 0.10N / m 2 That was all. B: Peel strength is 0.10N / m 2 It was less than.

[0048] [2.4] Penetration resistance In order to evaluate the electronic conductivity between the negative electrode mixture layer and the negative electrode current collector (i.e., the electronic conductivity of the negative electrode), the penetration resistance was measured. In detail, one terminal of the measuring device was electrically connected to the negative electrode mixture layer of the negative electrode, and the other terminal of the measuring device was electrically connected to the negative electrode current collector of the negative electrode, and electricity was passed between the negative electrode mixture layer of the negative electrode and the negative electrode current collector of the negative electrode. The electronic resistance value between the terminals was taken as the "penetration resistance". The measurement results are shown in Table 1. The allowable range for the penetration resistance is "2.10 Ω / cm or less." [Table 1]

[0049] In Table 1, "binder amount" refers to the ratio (mass%) of binder to the total amount of the negative electrode composite layer. "SBR" refers to styrene-butadiene rubber. "PVdF" refers to polyvinylidene fluoride. "Amount of binder added" refers to the ratio (mass%) of CNT to the total amount of the binder layer. "*" at 1.0C rated discharge rate indicates that it cannot be measured.

[0050] In Comparative Example 1 to Comparative Example 11, the ratio (A / B) was not 1.8 or more. Therefore, the measured values ​​of the 1.0C rated discharge rate in Comparative Example 5 to Comparative Example 11 were not "57% or more." The peel strength in Comparative Example 1 to Comparative Example 5 was not "0.18 N / cm or more." The penetration resistance in Comparative Example 8 to Comparative Example 10 was not "2.10 Ω / cm or less." These results show that the nonaqueous secondary batteries of Comparative Examples 1 to 11 are not "nonaqueous secondary batteries excellent in binding strength and electronic conductivity of the negative electrode and suppressed deterioration in discharge rate characteristics."

[0051] In Examples 1 to 11, the ratio (A / B) was 1.8 or more. Therefore, the measured values ​​of the 1.0C rated discharge rate of Examples 1 to 11 were "57% or more." The peel strength of Examples 1 to 11 was "0.18 N / cm or more." The penetration resistance of Examples 1 to 11 was "2.10 Ω / cm or less." As a result, it was found that the nonaqueous secondary batteries of Examples 1 to 11 were "nonaqueous secondary batteries excellent in binding strength and electronic conductivity of the negative electrode and suppressed deterioration in discharge rate characteristics."

Claims

1. A battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte; the negative electrode is formed by laminating a negative electrode current collector, a binder layer, and a negative electrode mixture layer in this order, The binder layer contains a conductive assistant, The nonaqueous secondary battery has a ratio (A / B) of an average length (A) of the conductive assistant to an average thickness (B) of the binder layer of 1.8 or more.

2. 2. The nonaqueous secondary battery according to claim 1, wherein the ratio (A / B) is 2.5 or more.

3. The nonaqueous secondary battery according to claim 2 , wherein the content of the conductive assistant is 0.8 mass % or more with respect to the total amount of the binder layer.

4. 4. The nonaqueous secondary battery according to claim 3, wherein the binder layer comprises a styrene-butadiene copolymer.

5. The nonaqueous secondary battery according to any one of claims 1 to 4, wherein the negative electrode mixture layer has a thickness of 100 µm or more.

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