Negative electrode layer, battery, and method for manufacturing the negative electrode layer

Abstract

The primary objective of this disclosure is to provide a negative electrode layer in which the amount of volume change is suppressed. [Solution] The present disclosure provides a negative electrode layer comprising a composite active material, a carbon-based active material, and a binder, wherein the composite active material comprises a Si-based active material and a carbon-containing layer covering at least a portion of the surface of the Si-based active material, the carbon-based active material has hydroxyl groups on its surface, and the binder is a hydrophobic binder having aromatic rings, thereby solving the above problem.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode layer, a battery, and a method for manufacturing a negative electrode layer. [Background technology]

[0002] In recent years, battery development has been booming. For example, in the automotive industry, development is progressing on batteries used in electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and hybrid electric vehicles (HEVs).

[0003] For example, Patent Document 1 discloses a lithium-ion secondary battery comprising a negative electrode containing a silicon-based negative electrode active material, graphite, and a negative electrode binder. Patent Document 2 discloses a negative electrode in which a negative electrode mixture layer containing a Li-free negative electrode active material is doped with Li ions. Patent Document 3 discloses a negative electrode active material for a non-aqueous electrolyte secondary battery characterized by a carbon coating on the surface of a composite comprising a fine Si phase and a three-phase system of silicon oxide and carbonaceous material. Patent Document 4 discloses a non-aqueous electrolyte secondary battery comprising a negative electrode using a carbon material capable of intercalating and releasing lithium as the active material. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-050203 [Patent Document 2] Japanese Patent Publication No. 2018-116833 [Patent Document 3] Japanese Patent Publication No. 2006-092969 [Patent Document 4] Japanese Patent Application Publication No. 10-228896 [Overview of the project] [Problems that the invention aims to solve]

[0005] Si has a large theoretical capacity and is effective in increasing the energy density of batteries. On the other hand, Si undergoes a large volume change during charging and discharging. Therefore, there is a risk that the volume change of the negative electrode layer containing a Si-containing negative electrode active material (Si-based active material) will also be large during charging and discharging. If the volume change of the negative electrode layer is large, for example, it may be necessary to increase the strength and size of the restraining member in the battery, which may reduce the energy density of the battery. In this regard, the above-mentioned Patent Document 1 attempts to suppress the volume change of the negative electrode layer by using a predetermined negative electrode binder, but there is still room for further improvement regarding the volume change of the negative electrode layer.

[0006] This disclosure has been made in view of the above circumstances, and its main purpose is to provide a negative electrode layer in which the amount of volume change is suppressed. [Means for solving the problem]

[0007] [1] A negative electrode layer containing a composite active material, a carbon-based active material, and a binder, The above composite active material comprises a Si-based active material and a carbon-containing layer covering at least a portion of the surface of the Si-based active material. The above carbon-based active material has hydroxyl groups on its surface, The above binder is a hydrophobic binder having aromatic rings, which is the negative electrode layer.

[0008] [2] The negative electrode layer according to [1], wherein the ratio of the weight of Si in the composite active material to the weight of the hydrophobic binder is 0.5 or more and 8.5 or less.

[0009] [3] The negative electrode layer described in [2], wherein the above ratio is 0.8 or greater.

[0010] [4] The negative electrode layer according to [2] or [3], wherein the above ratio is 5.0 or less.

[0011] [5] The negative electrode layer according to any one of [1] to [4], wherein the amount of the hydroxyl group is 0.10 mmol / g or more.

[0012] [6] The negative electrode layer according to any one of [1] to [4], wherein the amount of the hydroxyl group is 0.50 mmol / g or more.

[0013] [7] The negative electrode layer according to any one of [1] to [6], wherein the carbon-based active material contains a phenolic hydroxyl group as the hydroxyl group.

[0014] [8] The negative electrode layer according to any one of [1] to [7], which contains polyimide as the hydrophobic binder.

[0015] [9] The negative electrode layer according to any one of [1] to [8], which contains graphite as the carbon-based active material.

[0016]

[10] A battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, The battery, wherein the negative electrode layer is the negative electrode layer according to any one of [1] to [9].

[0017]

[11] A method for manufacturing a negative electrode layer according to any one of [1] to [9], comprising: a first step of mixing the composite active material with the hydrophobic binder or a precursor of the hydrophobic binder to obtain a first mixture; a second step of adding and mixing the carbon-based active material to the first mixture to obtain a second mixture; and a third step of forming the negative electrode layer using the second mixture. [Advantages of the Invention]

[0018] This disclosure provides the effect of obtaining a negative electrode layer with suppressed volume changes. [Brief explanation of the drawing]

[0019] [Figure 1] This diagram illustrates the mechanism by which the negative electrode layer in this disclosure can solve the problem. [Figure 2] This is a schematic cross-sectional view illustrating a battery in this disclosure. [Figure 3] This is a flowchart illustrating the method for manufacturing the negative electrode layer in this disclosure. [Figure 4] This is a flowchart illustrating the manufacturing method of the negative electrode layer in Comparative Example 2 and Example 2. [Modes for carrying out the invention]

[0020] The negative electrode layer, battery, and method for manufacturing the negative electrode layer as described below will be explained in detail. Note that the figures shown below are schematic representations, and the size and shape of each part have been exaggerated as appropriate for ease of understanding.

[0021] A. Negative electrode layer The negative electrode layer in this disclosure contains a composite active material, a carbon-based active material, and a binder. The composite active material comprises a Si-based active material and a carbon-containing layer covering at least a portion of the surface of the Si-based active material, the carbon-based active material has hydroxyl groups on its surface, and the binder is a hydrophobic binder having aromatic rings.

[0022] According to this disclosure, the negative electrode layer volume change is suppressed because it contains a composite active material having a carbon-containing layer, a carbon-based active material having hydroxyl groups on its surface, and a hydrophobic binder having an aromatic ring.

[0023] Here, the mechanism by which the negative electrode layer in this disclosure can solve the problem will be explained in detail using Figure 1. Figure 1(a) is a schematic cross-sectional view illustrating a negative electrode layer different from that of the present invention, and Figure 1(b) is a schematic cross-sectional view illustrating a negative electrode layer in the present invention. First, from the viewpoint of high energy density of the battery and from the viewpoint of providing good electronic conductivity, the combined use of Si-based active material and carbon-based active material has been considered. In this regard, when a carbon-based active material without hydroxyl groups on its surface, Si, and a binder are used as shown in Figure 1(a), the binder is dispersed in the negative electrode layer, but if a hydrophobic binder is used as the binder, there is a risk that the binder will be localized around the carbon-based active material rather than around the Si. This is because hydrophobic binders have a high affinity for carbon (element C).

[0024] In contrast, as shown in Figure 1(b), when a composite active material having a carbon-containing layer, a carbon-based active material having hydroxyl groups on its surface, and a hydrophobic binder are combined, the hydrophobic binder is considered to exist locally around the composite active material. This is because the composite active material has a carbon-containing layer, resulting in a high affinity between the hydrophobic binder and the composite active material, while the carbon-based active material has hydroxyl groups on its surface, resulting in a low affinity between the hydrophobic binder and the carbon-based active material. Thus, in the negative electrode layer of this disclosure, the binder can be localized around the composite active material, allowing the binder to effectively absorb the expansion and contraction of the composite active material having a Si-based active material. As a result, volume changes in the negative electrode layer are suppressed.

[0025] 1.Composite active material The composite active material in this disclosure comprises a Si-based active material and a carbon-containing layer covering at least a portion of the surface of the Si-based active material.

[0026] Examples of Si-based active materials include elemental Si, Si alloys, Si oxides, and Si composites. Examples of Si alloys include Si-Al alloys, Si-Sn alloys, Si-In alloys, Si-Ag alloys, Si-Pb alloys, Si-Sb alloys, Si-Bi alloys, Si-Mg alloys, Si-Ca alloys, Si-Ge alloys, and Si-Pb alloys. Si alloys may be two-component alloys or multi-component alloys with three or more components. Examples of Si oxides include SiO.

[0027] In Si composites, the Si component and other components (other elements) usually exist independently. Examples of other elements include carbon (C). Si-C composites (SiC composites) are what are known as Si-C active materials. In Si-C active materials, for example, silicon is dispersed within carbon particles.

[0028] The shape of Si-based active materials is usually particulate. Furthermore, Si-based active materials may be solid particles, hollow particles, or porous particles. The porosity of porous particles is not particularly limited, but for example, it is between 4% and 40%. The average pore size (average pore diameter) of porous particles is not particularly limited, but for example, it is between 10 nm and 100 nm. Also, the pore volume of porous particles is not particularly limited, but for example, it is between 0.10 cm³. 3 / g or more, 0.50cm 3 It is less than / g.

[0029] The carbon-containing layer is a layer that contains at least carbon (element C). The carbon-containing layer may contain only carbon, or it may contain other components. Examples of other components include binders. Binders are described in "4. Negative Electrode Layer".

[0030] The carbon-containing layer may cover a portion of the surface of the Si-based active material, or it may cover the entire surface. In the former case, the coverage rate of the carbon-containing layer may be, for example, 40% or more, or 50% or more. Alternatively, the coverage rate of the carbon-containing layer may be, for example, 90% or less, or 80% or less, or 70% or less.

[0031] The thickness of the carbon-containing layer is not particularly limited, but for example, it is between 1 nm and 100 nm. The method for forming the carbon-containing layer is not particularly limited, but examples include vapor deposition methods such as CVD (Chemical Vapor Deposition) and wet coating methods using a carbon-containing slurry.

[0032] The composite active material may be primary particles or secondary particles formed by the aggregation of primary particles. The average particle size (D) of the composite active material. 50 ) is, for example, 10 nm or more and 50 μm or less. Average particle diameter (D 50 ) refers to the cumulative 50% particle size in the volume-based particle size distribution measured by a laser diffraction particle size distribution analyzer.

[0033] The proportion of the composite active material in the negative electrode layer is not particularly limited, but may be, for example, 15% by weight or more, 20% by weight or more, 30% by weight or more, or 40% by weight or more. On the other hand, the proportion of the composite active material may be, for example, 80% by weight or less, 70% by weight or less, 60% by weight or less, or 50% by weight or less.

[0034] 2. Carbon-based active materials The carbon-based active material in this disclosure has hydroxyl groups on its surface. The presence of hydroxyl groups can be confirmed, for example, by neutralization titration methods such as the Bohem method and spectroscopic methods such as XPS (X-ray photoelectron spectroscopy). However, hydroxyl groups may inevitably be present on the surface of the carbon-based active material. In this regard, the hydroxyl groups in this specification do not include such unavoidable hydroxyl groups. For example, if the amount of hydroxyl groups determined by neutralization titration is 0.05 mmol / g or less, it can be determined that the carbon-based active material does not have hydroxyl groups.

[0035] The amount of hydroxyl groups is, for example, 0.10 mmol / g or more, may be 0.30 mmol / g or more, or 0.50 mmol / g or more. On the other hand, the amount of hydroxyl groups is, for example, 1.00 mmol / g or less, may be 0.80 mmol / g or less, or 0.65 mmol / g or less.

[0036] Here, the carbon-containing layer in the composite active material described above may or may not have hydroxyl groups on its surface, but the latter is preferred because it increases the affinity between the composite active material and the hydrophobic binder. On the other hand, in the former case, it is preferable that the amount of hydroxyl groups present on the surface of the carbon-containing layer is less than the amount of hydroxyl groups present on the surface of the carbon-based active material.

[0037] The hydroxyl group may be part of a carboxyl group, an alcoholic hydroxyl group, or a phenolic hydroxyl group. Among these, the phenolic hydroxyl group is preferred.

[0038] Examples of carbon-based active materials include graphite, hard carbon, and soft carbon.

[0039] The proportion of carbon-based active material in the negative electrode layer is not particularly limited, but for example, it may be 10% by weight or more, 20% by weight or more, or 30% by weight or more. On the other hand, the proportion of carbon-based active material in the negative electrode layer may be 70% by weight or less, 60% by weight or less, or 50% by weight or less.

[0040] 3. Hydrophobic binder The hydrophobic binders in this disclosure have aromatic rings. Aromatic rings are typically highly hydrophobic. Examples of binders having aromatic rings include polyimides and polyamideimides.

[0041] Polyimide is represented by the following formula (1). In formula (1), R and R' are aromatic compounds (aromatic rings), and n is the number of repeats.

[0042] [ka]

[0043] The weight-average molecular weight of polyimide is not particularly limited, but for example, it is between 5,000 and 500,000.

[0044] Here, in the negative electrode layer, the weight of Si in the composite active material may be less than, the same as, or more than the weight of the hydrophobic binder. The ratio of the weight of Si to the weight of the hydrophobic binder may be, for example, 0.5 or more, 0.8 or more, 0.83 or more, 1.0 or more, 2.0 or more, 2.5 or more, or 3.0 or more. If the above ratio is too small, there is a risk that sufficient capacity cannot be obtained when used in a battery. On the other hand, the above ratio may be, for example, 8.5 or less, 8.0 or less, 7.0 or less, 5.0 or less, or 3.5 or less. If the above ratio is too large, there is a risk that the binder may not be able to sufficiently suppress the volume change of the composite active material. Here, the weight of Si refers to the weight of Si contained in the Si-based active material. If the Si-based active material is pure Si, the weight of Si can be considered as the weight of the Si-based active material. On the other hand, if the Si-based active material is a Si alloy, Si oxide, or Si composite and contains elements other than Si, the weight of Si can be considered as the weight of the Si-based active material minus the elements other than Si.

[0045] The proportion of hydrophobic binder in the negative electrode layer is, for example, 0.01% by weight or more and 5% by weight or less.

[0046] 4. Negative electrode layer The negative electrode layer in this disclosure may contain a conductive material as needed. Examples of conductive materials include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and Ketjenblack (KB), and fibrous carbon materials such as carbon fibers, carbon nanotubes (CNTs), and carbon nanofibers (CNFs). The proportion of the conductive material in the negative electrode layer is, for example, 1% by weight or more and 5% by weight or less.

[0047] Furthermore, the negative electrode layer may contain binders other than the hydrophobic binder having an aromatic ring as described above. Examples of binders include rubber-based binders and fluoride-based binders. The proportion of the hydrophobic binder having an aromatic ring to the total amount of binders is, for example, 60% by weight or more and 99% by weight or less.

[0048] The thickness of the negative electrode layer is not particularly limited, but for example, it is between 0.1 μm and 1000 μm. The negative electrode layer in this disclosure is typically used in batteries. Batteries are described in "B. Batteries". The method for manufacturing the negative electrode layer in this disclosure is not particularly limited, but for example, it can be manufactured by the method described in "C. Method for Manufacturing the Negative Electrode Layer".

[0049] B.Battery Figure 2 is a schematic cross-sectional view illustrating a battery in this disclosure. The battery 10 shown in Figure 2 includes a positive electrode layer 1, a negative electrode layer 2, an electrolyte layer 3 disposed between the positive electrode layer 1 and the negative electrode layer 2, a positive electrode current collector 4 for collecting current from the positive electrode layer 1, and a negative electrode current collector 5 for collecting current from the negative electrode layer 2. In this disclosure, the negative electrode layer 2 is the negative electrode layer described in "A. Negative Electrode Layer".

[0050] According to this disclosure, because the negative electrode layer described above is present, the battery exhibits suppressed volume changes. As a result, the need for large restraining members is reduced, and a decrease in the battery's energy density can be suppressed.

[0051] 1. Negative electrode layer Regarding the negative electrode layer, the description is omitted because it is the same as the content described in "A. Negative electrode layer".

[0052] 2. Positive electrode layer The positive electrode layer is a layer containing at least a positive electrode active material. Further, the positive electrode layer may contain at least one of an electrolyte, a conductive material, and a binder as needed. Regarding the conductive material and the binder, they are the same as the content described in "A. Negative electrode layer". Incidentally, the positive electrode layer may contain, as a binder, the hydrophobic binder having an aromatic ring described above. The electrolyte will be described in "3. Electrolyte layer".

[0053] Examples of the positive electrode active material include oxide active materials. Examples of the oxide active material include rock salt layer-type active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, spinel-type active materials such as LiMn2O4, Li4Ti5O 12 , Li(Ni 0.5 Mn 1.5 )O4, and olivine-type active materials such as LiFePO4, LiMnPO4, LiMnFePO4, LiNiPO4, and LiCoPO4.

[0054] Examples of the shape of the positive electrode active material include particulate. The average particle diameter (D 50 ) of the positive electrode active material is not particularly limited, but for example, it is 10 nm or more, and may be 100 nm or more. On the other hand, the average particle diameter (D 50 ) of the positive electrode active material is, for example, 50 μm or less, and may be 20 μm or less. D 50 is as described above.

[0055] The proportion of the positive electrode active material in the positive electrode layer is, for example, 20% by weight or more and 80% by weight or less. The thickness of the positive electrode layer is, for example, 0.1 μm or more and 1000 μm or less.

[0056] 3. Electrolyte layer The electrolyte layer is a layer formed between the positive electrode layer and the negative electrode layer, and contains at least an electrolyte. The electrolyte layer may also contain a binder as needed. The binder is the same as described in "A. Negative Electrode Layer".

[0057] The electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte, but the former is preferred.

[0058] The electrolyte preferably contains a supporting salt and a solvent. Examples of supporting salts (lithium salts) for lithium-ion conductive electrolytes include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, and organic lithium salts such as LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, and LiC(CF3SO2)3. Examples of solvents used in the electrolyte include cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and linear esters (linear carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0059] Examples of solid electrolytes include organic solid electrolytes such as polymer electrolytes and gel electrolytes, as well as inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, and halide solid electrolytes.

[0060] The electrolyte layer may be a layer in which the above-mentioned electrolyte solution is impregnated into a porous membrane separator. Alternatively, the electrolyte layer may be a solid electrolyte layer containing the above-mentioned solid electrolyte. The material of the separator may be an organic material or an inorganic material. Specifically, examples include polyethylene (PE), polypropylene (PP), cellulose, polyvinylidene fluoride, polyamide, polyimide, etc. The separator may be a nonwoven fabric such as a resin nonwoven fabric or a glass fiber nonwoven fabric, or a ceramic porous membrane. Furthermore, the separator may have a single-layer structure or a laminated structure. Examples of laminated separators include a PE-PP two-layer separator, a PP-PE-PP or PE-PP-PE three-layer separator.

[0061] 4. Other configurations The battery in this disclosure preferably has a positive electrode current collector for collecting current from the positive electrode layer and a negative electrode current collector for collecting current from the negative electrode layer. Examples of materials for the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. On the other hand, examples of materials for the negative electrode current collector include SUS, copper, nickel, and carbon.

[0062] The battery in this disclosure may further include a restraining jig that applies restraining pressure along the thickness direction to the positive electrode layer, electrolyte layer, and negative electrode layer. In particular, when the electrolyte layer is a solid electrolyte layer, it is preferable to apply restraining pressure in order to form good ion conduction paths and electron conduction paths. The restraining pressure may be, for example, 0.1 MPa or more, 1 MPa or more, or 5 MPa or more. On the other hand, the restraining pressure may be, for example, 100 MPa or less, 50 MPa or less, or 20 MPa or less.

[0063] 5.Battery The type of battery in this disclosure is not particularly limited, but is typically a lithium-ion battery. Furthermore, the battery in this disclosure may be a liquid battery in which the electrolyte layer contains an electrolyte solution, or a solid battery in which the electrolyte layer contains a solid electrolyte. The solid battery may be a semi-solid battery or a fully solid battery. In this disclosure, a semi-solid battery is a battery in which the electrolyte layer has a solid electrolyte and a liquid component (e.g., an ionic liquid). In this disclosure, a fully solid battery is a battery in which the electrolyte layer has only an inorganic solid electrolyte as the electrolyte. Furthermore, the battery in this disclosure may be a primary battery or a secondary battery, but a secondary battery is preferred because it can be repeatedly charged and discharged and is useful, for example, as an in-vehicle battery.

[0064] Applications of batteries include, for example, powering vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. In particular, they are preferred for use as a power source for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). Batteries may also be used as a power source for other mobile devices (e.g., trains, ships, aircraft), or as a power source for electrical products such as information processing devices.

[0065] C. Method for manufacturing the negative electrode layer Figure 3 is a flowchart illustrating a method for manufacturing the negative electrode layer in this disclosure. As shown in Figure 3, in the method for manufacturing the negative electrode layer in this disclosure, first, the composite active material is mixed with a binder solution (a solution in which a hydrophobic binder or a hydrophobic binder precursor is dissolved in a solvent) to obtain a first mixture (first step). Next, the carbon-based active material is added to the first mixture and mixed to obtain a second mixture (second step). Then, the negative electrode layer is formed using the second mixture (third step).

[0066] According to this disclosure, by mixing the composite active material with a hydrophobic binder (or its precursor) before the carbon-based active material, the binder can be more localized around the composite active material. As a result, with the manufacturing method described in this disclosure, a negative electrode layer with a more suppressed volume change can be produced. It should be noted that, as shown in Example 2 described later, even if the binder solution and the carbon-based active material are mixed first and then mixed with the composite active material, it is still possible to obtain a negative electrode layer with a suppressed volume change as described in this disclosure.

[0067] 1.First step In the first step, the composite active material is mixed with the hydrophobic binder or the hydrophobic binder precursor to obtain a first mixture. The composite active material and the hydrophobic binder are the same as those described in "A. Negative Electrode Layer".

[0068] In the first step, a precursor of the hydrophobic binder described above may be used. For example, if the hydrophobic binder is polyimide, a polyamic acid may be used in the first step. In this case, it is preferable to perform an imidation treatment in the third step, which will be described later.

[0069] The hydrophobic binder and precursor may be in the form of a binder solution dissolved in a solvent. In other words, the first mixture may be a slurry. The solvent is not particularly limited, but examples include organic solvents such as N-methyl-2-pyrrolidone (NMP).

[0070] The mixing method is not particularly limited, and known methods can be employed. The ratio of the composite active material to the binder solution is adjusted as appropriate according to the desired negative electrode layer.

[0071] 2.Second process In the second step, the carbon-based active material (carbon-based active material having hydroxyl groups on its surface) is added to the first mixture to obtain the second mixture. The carbon-based active material is the same as described in "A. Negative Electrode Layer".

[0072] In the second step, a solvent may be added. The solvent is as described above. The mixing method is not particularly limited, and known methods can be used. The proportion of carbon-based active material is adjusted as appropriate according to the desired negative electrode layer.

[0073] 3. 3rd process The third step is to form the negative electrode layer described above using the second mixture. The method for forming the negative electrode layer is not particularly limited as long as the negative electrode layer described above is obtained. For example, one method is to apply the second mixture to a metal plate and dry it. The metal plate may be a transfer substrate or a negative electrode current collector. In the third step, it is sufficient that the negative electrode layer is ultimately obtained. In other words, the third step may include a process of applying the second mixture to a substrate and drying it to form a precursor layer, and an imidation process in which polyimide is obtained from the polyamic acid by heating the precursor layer.

[0074] 4. Negative electrode layer The negative electrode layer produced by the method described above is the same as described in "A. Negative Electrode Layer," so it will not be described here.

[0075] This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of this disclosure and achieves similar effects is included within the technical scope of this disclosure. [Examples]

[0076] [Comparative Example 1] (Fabrication of the negative electrode layer) As a carbon-based active material, graphite that had not undergone the surface oxidation treatment described later was prepared. In addition, a composite active material having a carbon-containing layer fabricated by wet coating on the surface of SiO was prepared. Furthermore, a binder solution was prepared by dissolving polyamic acid in a solvent (NMP).

[0077] Using the materials described above, a second mixture (negative electrode slurry) was prepared according to the flow shown in Figure 3 (Method A). The amounts of composite active material and binder solution added were as shown in Table 1. The proportion of graphite was adjusted so that the ratio of graphite to composite active material (solid content ratio) to the binder solution was 10:90. The obtained negative electrode slurry was applied to a negative electrode current collector (Cu foil, 10 μm thick) and dried to form a precursor layer. The precursor layer was heated in an Ar flow at 350°C for 12 hours to imidize the polyamic acid and form a polyimide. This produced a negative electrode layer containing composite active material, carbon-based active material, and hydrophobic binder (polyimide: PI).

[0078] Here, the ratio of the weight of Si in the composite active material to the weight of the hydrophobic binder (PI) (Si / PI ratio) was determined as follows. A predetermined amount of the above composite active material was weighed, and Si was extracted by hydrofluoric acid treatment. Subsequently, the weight of Si was determined by ICP emission spectrometry. The weight of Si in the fabricated negative electrode layer was determined from the ratio of the weighed amount to the amount actually added (equivalent amount of added composition). A predetermined amount of the above binder solution was weighed and dried at 200°C. The solid fraction (amount of polyamic acid) in the binder solution was determined, and the amount of polyimide was calculated from the polyamic acid. The weight of polyimide in the fabricated negative electrode layer was determined from the ratio of the weighed amount to the amount actually added (equivalent amount of added composition). The Si / PI ratio was calculated from the Si element content and polyimide content as described above. The results are shown in Table 1.

[0079] (Preparation of evaluation batteries) An evaluation battery (liquid-based battery; wound-wound battery) was fabricated using the negative electrode layer described above. The materials of each component are as follows: The positive electrode layer is made of an oxide active material (NCM:LiNi 1 / 3 Co 1 / 3 Mn 1 / 3A layer containing O2, a carbon-based conductive material (AB), and a fluorine-based binder (PVdF) was used. The weight ratio of oxide active material, carbon-based conductive material, and fluorine-based binder in the positive electrode layer was 92:5:3. In addition, a 15 μm thick aluminum foil was used as the positive electrode current collector. As the electrolyte layer, a 24 μm thick separator with a three-layer structure of polypropylene (PP)-polyethylene (PE)-polypropylene (PP) was used. One side of the separator (the side facing the positive electrode layer) was coated with a 4 μm thick layer of ceramic. The electrolyte used consisted of an electrolyte containing ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC) in a volume ratio of 3:3:3:1, and an electrolyte containing an electrolyte salt (LiPF6; 1 mol / L).

[0080] Using the components described above, a battery was fabricated as follows. First, the negative electrode (negative electrode layer and negative electrode current collector) and positive electrode (positive electrode layer and positive electrode current collector) were wound together with a separator in between to create an electrode group. Next, current collector plates with covers were welded to both ends of the electrode group, inserted into a case, and the cover plates were welded to the case. The electrolyte was poured in through the injection hole, and then the injection hole was sealed using a screw. After standing for a certain period of time, the electrolyte was impregnated into each layer. Then, after the initial charge, aging was performed at 60°C. This created an evaluation battery.

[0081] [Comparative Example 2] The second mixture (negative electrode slurry) was prepared by changing the flow shown in Figure 4 (Method B). Specifically, graphite that had not undergone surface oxidation treatment was mixed with the binder solution, and then the composite active material was added and mixed to prepare the second mixture. An evaluation battery was prepared in the same manner as in Comparative Example 1, except that this negative electrode slurry was used.

[0082] [Example 1] A surface oxidation treatment was performed in an oil bath at 40°C for 1 hour using graphite and concentrated sulfuric acid. This prepared graphite having phenolic hydroxyl groups on its surface. An evaluation battery was prepared in the same manner as in Comparative Example 1, except for the use of this graphite.

[0083] [Examples 2-7] As shown in Table 1, an evaluation battery was prepared in the same manner as in Example 1, except that at least one of the following was changed: the amount of composite active material added, the amount of binder solution added, the Si / PI ratio, and the method of preparing the negative electrode slurry.

[0084] [evaluation] (Measurement of hydroxyl group content) The amount of hydroxyl groups in the carbon-based active materials used in Comparative Examples 1-2 and Examples 1-7 was quantified using the Boehm method. The results are shown in Table 1. Although a hydroxyl group content of 0.05 mmol / g was also confirmed in Comparative Examples 1 and 2, this is considered to be an unavoidably present hydroxyl group, and it was determined that the graphite used in Comparative Examples 1-2 does not contain hydroxyl groups.

[0085] (Measurement of expansion rate) The fabricated evaluation battery was mounted on a restraint jig equipped with a contact-type displacement sensor and charged. Charging was performed at 0.1C until the voltage reached 4.25V. The expansion rate (expansion rate of the negative electrode layer) (%) was calculated by dividing the displacement obtained during this process by the thickness of the battery during fabrication. The results are shown in Table 1.

[0086] [Table 1]

[0087] As shown in Table 1, Examples 1-7 showed a smaller expansion rate compared to Comparative Examples 1 and 2, confirming that the volume change of the anode layer was suppressed. Furthermore, from Comparative Examples 1-2 and Examples 1-2, the expansion rate was even more suppressed when the anode slurry was prepared using the manufacturing method (Method A) of this disclosure. This is presumed to be because the binder solution was mixed with the composite active material before the carbon-based active material, resulting in better affinity between the binder and the composite active material, and thus the binder becoming more localized around the composite active material in the anode layer. [Explanation of Symbols]

[0088] 1 ... Positive electrode layer 2 ... Negative electrode layer 3...electrolyte layer 4...Positive electrode current collector 5...Negative electrode current collector 10...battery

Claims

1. A negative electrode layer containing a composite active material, a carbon-based active material, and a binder, The composite active material comprises a Si-based active material and a carbon-containing layer covering at least a portion of the surface of the Si-based active material. The carbon-based active material has hydroxyl groups on its surface, The binder is a hydrophobic binder having aromatic rings, which is the negative electrode layer.

2. The negative electrode layer according to claim 1, wherein the ratio of the weight of Si in the composite active material to the weight of the hydrophobic binder is 0.5 or more and 8.5 or less.

3. The negative electrode layer according to claim 2, wherein the ratio is 0.8 or more.

4. The negative electrode layer according to claim 2, wherein the ratio is 5.0 or less.

5. The negative electrode layer according to claim 1, wherein the amount of hydroxyl groups is 0.10 mmol / g or more.

6. The negative electrode layer according to claim 1, wherein the amount of hydroxyl groups is 0.50 mmol / g or more.

7. The negative electrode layer according to claim 1, wherein the carbon-based active material contains a phenolic hydroxyl group as the hydroxyl group.

8. The negative electrode layer according to claim 1, wherein the hydrophobic binder contains polyimide.

9. The negative electrode layer according to claim 1, wherein the carbon-based active material contains graphite.

10. A battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, A battery in which the negative electrode layer is the negative electrode layer described in any of claims 1 to 9.

11. A method for manufacturing a negative electrode layer, comprising manufacturing a negative electrode layer according to any one of claims 1 to 9, The first step is to mix the composite active material with the hydrophobic binder or the hydrophobic binder precursor to obtain a first mixture. The second step involves adding the carbon-based active material to the first mixture and mixing it to obtain a second mixture. A method for producing a negative electrode layer, comprising: a third step of forming the negative electrode layer using the second mixture described above.

Images