Negative electrode and method of producing the same, and nonaqueous electrolyte secondary battery including negative electrode and method of producing the same

Abstract

To provide a negative electrode with which a nonaqueous electrolyte secondary battery having excellent cycling performance can be obtained.SOLUTION: A negative electrode has an active material layer including active material particles, carbon nanotubes (CNTs), and a first binder. The active material particles include metal-based active material particles and carbon-based active material particles. An average length of the CNTs determined by analysis of a scanning transmission electron microscope image of the active material layer is 1.5 μm or more. At least part of the first binder covers at least part of a surface of the active material particle while being not mixed with the CNTs. The CNTs include: first CNTs adhered to the first binder so as to cover the first binder; and second CNTs each connecting two of the active material particles to each other.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a negative electrode and a method for producing the same, and to a non-aqueous electrolyte secondary battery including the negative electrode and a method for producing the same. [Background technology]

[0002] A non-aqueous electrolyte secondary battery has a negative electrode including an active material layer. The active material layer may contain active material particles such as graphite or SiOx, and fibrous carbon such as carbon nanotubes (for example, Patent Document 1, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-99955 Summary of the Invention [Problem to be solved by the invention]

[0004] Metal-based active material particles such as SiOx have a lower electrical conductivity than graphite and expand and contract significantly during charging and discharging of a secondary battery, which deteriorates the conductive path formed between the surrounding active material particles and tends to cause deterioration of the cycle characteristics of the secondary battery. Carbon nanotubes can form a good conductive path by bridging between the active material particles, so they are used to suppress the deterioration of the conductive path. However, even when the active material layer contains carbon nanotubes, the cycle characteristics of the secondary battery may deteriorate.

[0005] An object of the present disclosure is to provide a negative electrode with which a secondary battery having excellent cycle characteristics can be obtained and a method for producing the same, as well as a nonaqueous electrolyte secondary battery including a negative electrode and a method for producing the same. [Means for solving the problem]

[0006] [1] A negative electrode for a non-aqueous electrolyte secondary battery, an active material layer including active material particles, carbon nanotubes, and a first binder; The active material particles include metal-based active material particles and carbon-based active material particles, the average length of the carbon nanotubes, as determined by analysis of a scanning transmission electron microscope image of the active material layer, is 1.5 μm or more; at least a portion of the first binder covers at least a portion of the surface of the active material particles without being mixed with the carbon nanotubes; The carbon nanotubes include first carbon nanotubes that are attached to the first binder so as to cover the first binder that covers the surfaces of the active material particles while not being mixed with the carbon nanotubes, and second carbon nanotubes that span between two of the active material particles. [2] The negative electrode according to [1], wherein the proportion of the carbon nanotubes not mixed with the first binder to the total amount of the carbon nanotubes in the active material layer is greater than 60%, as determined by analysis of the scanning transmission electron microscope image of the active material layer. [3] The negative electrode according to [1] or [2], wherein the metal-based active material particles include one or more particles selected from the group consisting of Si, SiOx, and a composite of Si and C. [4] The negative electrode according to any one of [1] to [3], wherein the metal-based active material particles include particles of a composite of Si and C. [5] The negative electrode according to any one of [1] to [4], wherein the carbon nanotubes are single-walled carbon nanotubes. [6] The negative electrode according to any one of [1] to [5], wherein the carbon nanotubes have a G / D ratio of 80 or less. [7] The negative electrode according to any one of [1] to [6], wherein the first binder is at least one of carboxymethyl cellulose and polyacrylic acid. [8] The active material layer includes a second binder different from the first binder, The negative electrode according to [7], wherein the second binder contains styrene-butadiene rubber. [9] A non-aqueous electrolyte secondary battery comprising the negative electrode according to any one of [1] to [8].

[10] A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, comprising: A first step of preparing a slurry; A second step of forming an active material layer of the negative electrode using the slurry, the slurry includes active material particles, a first binder, carbon nanotubes, and water; The active material particles include metal-based active material particles and carbon-based active material particles, The first step comprises: A step (1a) of obtaining a first kneaded body by kneading the active material particles, the first binder, and water so that the solid content is within a range of 60 to 65% by weight; (1b) adding water to the first kneaded body and kneading the first kneaded body so that the solid content is 50% by weight or less to obtain a second kneaded body; and (1c) kneading the second kneaded body and the carbon nanotubes to obtain a third kneaded body.

[11] The first binder is at least one of carboxymethyl cellulose and polyacrylic acid, The first step further includes a step of kneading the third kneaded body with a second binder, The method for producing a negative electrode according to

[10] , wherein the second binder contains styrene-butadiene rubber.

[12] The step (1a) dry mixing the active material particles and the first binder to obtain a mixture; and kneading the mixture and water so that the solid content is within the range of 60 to 65% by weight.

[13] The method for producing a negative electrode according to any one of

[10] to

[12] , wherein the metal-based active material particles contain one or more types of particles selected from the group consisting of Si, SiOx, and a composite of Si and C.

[14] The method for producing a negative electrode according to any one of

[10] to

[13] , wherein the first binder is at least one of carboxymethyl cellulose and polyacrylic acid.

[15] The method for producing a negative electrode according to any one of

[10] to

[14] , wherein the carbon nanotubes have a G / D ratio of 80 or less.

[16] A method for producing a non-aqueous electrolyte secondary battery, comprising a step of producing a negative electrode by the method for producing a negative electrode according to any one of

[10] to

[15] . Effect of the Invention

[0007] According to the negative electrode of the present disclosure, a nonaqueous electrolyte secondary battery having excellent cycle characteristics can be obtained. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is an explanatory diagram illustrating an example of a state in an active material layer of a negative electrode according to an embodiment. [Diagram 2] FIG. 2 is an explanatory diagram illustrating an example of the state of an active material layer of a negative electrode prepared by the procedure described in the Comparative Example. [Diagram 3] 2 is a flowchart showing a method for manufacturing a negative electrode according to an embodiment. [Figure 4] 2 is a scanning transmission electron microscope image of the surface of the active material layer of the negative electrode obtained in Example 1. [Diagram 5] 1 is a scanning transmission electron microscope image of the surface of the active material layer of the negative electrode obtained in Example 2. [Figure 6] 1 is a scanning transmission electron microscope image of the surface of the active material layer of the negative electrode obtained in Comparative Example 1. [Figure 7] 1 is a scanning transmission electron microscope image of the surface of the active material layer of the negative electrode obtained in Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] In this specification, a numerical range such as "x to y" includes an upper limit and a lower limit unless otherwise specified. That is, "x to y" represents a numerical range of "not less than x and not more than y". A numerical value arbitrarily selected from within the numerical range may be set as a new upper limit or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described in another part of this specification, in a table, or in a figure, etc.

[0010] (Negative electrode) Fig. 1 is an explanatory diagram showing an example of the state in the active material layer of a negative electrode according to an embodiment, and Fig. 2 is an explanatory diagram showing an example of the state in the active material layer of a negative electrode produced by the procedure described in the comparative example.

[0011] The negative electrode of this embodiment (hereinafter also referred to as "the present negative electrode") is a negative electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "secondary battery") and includes an active material layer. The active material layer includes active material particles, carbon nanotubes (hereinafter also referred to as "CNTs"), and a first binder. The active material particles include metal-based active material particles (hereinafter also referred to as "metal-based particles") and carbon-based active material particles (hereinafter also referred to as "carbon-based particles"). The average length of the CNTs obtained by analyzing a scanning transmission electron microscope image (hereinafter also referred to as "SEM image") of the active material layer is 1.5 μm or more. At least a portion of the first binder covers at least a portion of the surface of the active material particles without being mixed with the CNTs. The CNTs include a first carbon nanotube (hereinafter also referred to as "first CNT") that is attached to the first binder so as to cover the surface of the first binder that covers the surface of the active material particles while not being mixed with the CNTs, and a second carbon nanotube (hereinafter also referred to as "second CNT") that spans between the two active material particles.

[0012] The negative electrode may have an active material layer on a negative electrode current collector. The active material layer may be formed on only one side of the negative electrode current collector, or on both sides. The negative electrode current collector is, for example, a metal foil made of a copper material such as copper or a copper alloy.

[0013] The active material layer includes active material particles 1, at least a portion of the surface of which is covered with a first binder 2 (FIG. 1). The first binder 2 can be directly attached to the surface of this active material particle 1. It is sufficient that at least a portion of the entire surface of the active material particle 1 is covered with the first binder 2. For example, the first binder 2 may be scattered over the entire surface of the active material particle 1, or the first binder 2 may be present over the entire surface of the active material particle 1.

[0014] The first binder 2 covering the active material particles 1 includes at least a first binder (hereinafter also referred to as "first binder (s)") that is not mixed with CNTs 3, but may also include a first binder (hereinafter also referred to as "first binder (m)") that is mixed with CNTs 3. When the first binder 2 includes the first binder (m), the area occupied by the first binder (s) is preferably larger than the area occupied by the first binder (m) in an SEM image of the surface of the active material layer. In this specification, when all CNTs present on the surface and inside of the first binder are identified in an SEM image of the surface of the active material layer, and the total area of ​​the CNTs not covered by the first binder and the total area of ​​the CNTs covered by the first binder are calculated, the region of the first binder in which the ratio of the former to the sum of the former and the latter is 80% or more is defined as the first binder (s), and the region of the first binder in which the ratio is less than 20% is defined as the first binder (m).

[0015] In the active material layer, the first CNTs 31 are attached to the first binder (s) so as to cover the surface of the first binder (s) covering the active material particles 1 (FIG. 1). The first CNTs 31 may be attached to the first binder (s) over the entire length, or may be attached to the first binder (s) over a portion of the length. In this specification, the first CNTs 31 refer to CNTs other than the second CNTs 32, and 80% or more of the total length of which is present on the surface of the first binder (s). A high ratio of the first CNTs 31 to all CNTs in the active material layer suggests that the slurry for forming the active material layer was prepared under conditions that make it difficult for the first binder and the CNTs to mix together. Examples of such conditions include a relatively low solid content concentration when the first binder and the CNTs are mixed, as described in the manufacturing method described later.

[0016] In the active material layer, the second CNTs 32 span between two active material particles 1 (FIG. 1). Both ends of the second CNTs 32 can directly contact the surfaces of the two active material particles 1 or the surface of the first binder 2 covering the active material particles 1. Since the average length of the CNTs contained in the active material layer is 1.5 μm or more, the second CNTs 32 easily form good conductive paths between the active material particles 1. This allows a secondary battery using this negative electrode to have excellent cycle characteristics.

[0017] The active material layer containing the first CNTs and the second CNTs can be obtained by adjusting the order in which the CNTs are mixed, the solid content concentration when mixing the CNTs, and other conditions when preparing a slurry used to form the active material layer (described later).The active material layer containing the first CNTs and the second CNTs can be obtained, for example, using a slurry prepared through a process of kneading a kneaded body containing active material particles and a first binder and having a relatively small solid content rate with CNTs, as described in the examples described later.

[0018] In contrast, when an active material layer is obtained using a slurry prepared through a process of kneading active material particles, a first binder, and CNTs at a relatively high solid content, as described in a comparative example below, it is difficult to obtain an active material layer containing the first CNTs and the second CNTs. In this case, as shown in Fig. 2, an active material layer is easily obtained in which a mixture of CNTs 3 and the first binder 2 covers the surface of the active material particles 1 and bridges between the two active material particles 1. In the active material layer shown in Fig. 2, the proportion of the CNT surface exposed on the surface of the first binder 2 is small, and the proportion of the surface of the CNTs 3 covered by the first binder 2 is large.

[0019] The CNTs contained in the active material layer may include third carbon nanotubes (third CNTs) attached to the first binder (m) so as to cover the surface of the first binder (m) that covers the active material particles.

[0020] The ratio of CNTs not mixed with the first binder to the total amount of CNTs in the active material layer (hereinafter also referred to as "CNT ratio") is preferably more than 60%, more preferably 70% or more, further preferably 80% or more, and may be 85% or more. The CNT ratio is preferably more than 60% and 100% or less, and may be 70 to 99%, or may be 80 to 95%. When the CNT ratio is within the above range, a good conductive path can be formed between the active material particles, and therefore the cycle characteristics of the secondary battery can be improved. The CNT ratio can be determined by analyzing an SEM image of the active material layer as described in the examples below.

[0021] An example of a method for adjusting the CNT content to the above ratio is to adjust conditions such as the solids concentration when mixing the CNTs when preparing a slurry used to form the active material layer.

[0022] The active material layer includes active material particles for a negative electrode. The active material particles include metal-based particles and carbon-based particles, and may include active material particles other than metal-based particles and carbon-based particles. The total content of the carbon-based particles and metal-based particles in the active material particles may be 90 to 100% by weight, 90 to 99% by weight, or 92 to 95% by weight based on the total amount of the active material particles. The content ratio of the carbon-based particles to the metal-based particles in the active material particles, carbon-based particles:metal-based particles (weight ratio), may be 70:30 to 95:5, 80:20 to 92:8, or 85:15 to 95:5.

[0023] When active material layers are formed on both sides of the negative electrode current collector, the weight of the active material layers on each side (total of both sides) is, for example, 150 to 250 g / m 2 and 170 to 230 g / m 2 and 180 to 210 g / m 2 The weight per surface of the negative electrode current collector may be, for example, half of the weight per surface of both surfaces described above. The thickness of the active material layer per surface may be, for example, 90 to 200 μm, 100 to 180 μm, or 120 to 150 μm. The packing density of the active material layer can be calculated by the following formula, and may be, for example, 1.3 to 1.9 g / cc, 1.4 to 1.8 g / cc, or 1.5 to 1.7 g / cc. Packing density [g / cc] = active material layer weight [g / m 2 ] / active material layer thickness [μm]

[0024] The metal-based particles include particles of metal elements such as metal simple substances or metal oxides containing an element selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). The metal-based particles preferably include or are one or more particles selected from the group consisting of Si, SiOx (x=0.5-1.5), a composite of Si and C (hereinafter also referred to as "SiC composite"), and Sn, and more preferably are one or more particles selected from the group consisting of Si, SiOx, and a SiC composite. The metal-based particles are more preferably SiC composite particles. The SiC composite is, for example, a composite in which Si is dispersed in a carbon matrix.

[0025] The carbon-based particles preferably include or are one or more particles selected from the group consisting of carbon (C) such as graphite, hard carbon, soft carbon, and amorphous coated graphite, and are preferably graphite particles. The average particle diameter D50 of the graphite particles may be 5 to 25 μm, 8 to 23 μm, or 10 to 20 μm. The average particle diameter D50 in this specification is the particle diameter at which the cumulative frequency from the smaller particle diameter in the volume-based particle size distribution is 50%. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution measuring device. The specific surface area (BET) of the graphite particles is 1.0 m 2 / g or more 3.0m 2 / g or less, and 2 / g or more 2.5m 2 The specific surface area can be calculated by the BET multipoint method on an adsorption isotherm measured by a gas adsorption method using a fully automatic specific surface area meter or the like.

[0026] The CNT may be a single-walled carbon nanotube (hereinafter also referred to as "SWCNT") or a multi-walled carbon nanotube such as a double-walled carbon tube (DWCNT). From the viewpoint of having fewer defects in the CNT structure and being easy to obtain CNTs with a large G / D ratio, the CNT is preferably a SWCNT.

[0027] The average length of the CNTs in the active material layer is 1.5 μm or more, may be 1.6 μm or more, may be 1.7 μm or more, may be 1.8 μm or more, may be 1.9 μm or more, or may be 2.0 μm or more. The average length of the CNTs may be 1.5 to 5.0 μm, may be 1.8 to 3.0 μm, or may be 2.0 to 2.5 μm. The average length of the CNTs can be determined by analyzing SEM images as described in the examples below. It is believed that the presence of CNTs with the above average length in the active material layer can form a good conductive path between the active material particles, thereby improving the cycle characteristics of the secondary battery.

[0028] The G / D ratio of the CNT may be 80 or less, 70 or less, or 50 or less, preferably 30 or less, or 20 or less. The G / D ratio of the CNT may be 5 to 80, 10 to 50, or 10 to 30. The G / D ratio is a value determined by Raman spectroscopy. In the Raman spectrum of the CNT, -1 The Raman shift seen around 1350 is called the G band, which is derived from graphite. -1 The Raman shift observed around the G / D ratio is called the D band, which is derived from defects in amorphous carbon or graphite. From this, it can be said that the CNT with a larger G / D ratio has higher crystallinity and greater strength. When using a slurry containing CNTs, active material particles, and a first binder to form an active material layer as described later, if the above components are kneaded by applying a strong shearing force to increase the dispersibility of the CNTs, the CNTs may be torn off, resulting in a shortened length of the CNTs. However, in this embodiment, since the timing of adding the CNTs is adjusted when preparing the slurry as described later, the shortening of the CNTs can be suppressed even when CNTs with a small G / D ratio (low strength) are used. As a result, a good conductive path can be formed between the active material particles, which can improve the cycle characteristics of the secondary battery. The G / D ratio in this specification is a value obtained by dividing the G / D ratio by the G / D ratio by the G / D ratio of 1550 to 1650 cm in the Raman spectrum of the CNTs. -1 The maximum peak intensity in the range of 1300 to 1400 cm is defined as G.-1 The maximum peak intensity within this range was calculated as D. The G / D ratio of the CNTs can be determined by the procedure described in the Examples below.

[0029] The content of CNT in the active material layer may be 0.01 to 1 part by weight, 0.05 to 0.1 parts by weight, or 0.1 to 0.5 parts by weight, relative to 100 parts by weight of the active material particles.

[0030] The first binder may be one or more selected from the group consisting of cellulose-based binders such as carboxymethyl cellulose (hereinafter also referred to as "CMC"), methyl cellulose (hereinafter also referred to as "MC"), and hydroxypropyl cellulose, and polyacrylic acid (hereinafter also referred to as "PAA"). The first binder preferably contains at least one of CMC and PAA, and more preferably contains CMC and PAA. CMC, MC, and PAA may be in the form of an acid or a salt, but are preferably in the form of a salt. Examples of the salt include alkali metal salts such as potassium and sodium.

[0031] The content of the first binder in the active material layer may be 0.5 to 5 parts by weight, 1 to 4 parts by weight, or 2 to 3 parts by weight, relative to 100 parts by weight of the active material particles. When two or more types of first binders are used, the content of the first binders refers to the total amount thereof.

[0032] The active material layer may contain, in addition to the active material particles, CNTs, and the first binder, a second binder other than the first binder, and a conductive material other than CNTs. When the first binder contains at least one of CMC and PAA, or is at least one of CMC and PAA, the second binder may contain at least one of styrene butadiene rubber (SBR) and polyvinyl alcohol (PVA), and preferably contains SBR. Examples of conductive materials other than CNTs include carbon materials such as carbon black (e.g., acetylene black, ketjen black), coke, and activated carbon.

[0033] (Method of manufacturing negative electrode) Fig. 3 is a flow chart showing a method for manufacturing a negative electrode according to an embodiment. The method for manufacturing a negative electrode according to this embodiment can manufacture a negative electrode for a secondary battery, and can also manufacture the present negative electrode. The method for manufacturing a negative electrode includes a first step of preparing a slurry, and a second step of forming an active material layer of the negative electrode using the slurry (Fig. 3). The slurry contains active material particles, a first binder, CNTs, and water. The active material particles contain metal-based particles and carbon-based particles.

[0034] The metal particles and carbon particles may be the materials described above. The first binder may be the materials described above. The CNT may be the materials described above, preferably SWCNT. The G / D ratio of the CNT may be within the range described above.

[0035] The slurry may further contain a second binder and may contain a conductive material other than CNTs. Examples of the second binder and conductive material include the materials described above.

[0036] The solid content of the slurry prepared in the first step may be 35 to 55% by weight, 40 to 50% by weight, or 43 to 48% by weight.

[0037] The first step is A step (1a) of obtaining a first kneaded body by kneading active material particles, a first binder, and water so that the solid content is within a range of 60 to 65% by weight; (1b) adding water to the first kneaded body and kneading the mixture so that the solid content is 50% by weight or less to obtain a second kneaded body; (1c) obtaining a third kneaded body by kneading the second kneaded body and the CNTs; Includes (Figure 3).

[0038] In the method for producing a negative electrode, the active material particles and the first binder are kneaded at a relatively high solid content rate to obtain a first kneaded body (step (1a)), and the first kneaded body is diluted to obtain a second kneaded body (step (1b)). The first kneaded body does not contain CNTs. Since the second kneaded body has a relatively low solid content rate, the shear force for dispersing the CNTs can be reduced in step (1c). As a result, the load applied to the CNTs by the kneading in step (1c) can be reduced, and the length of the CNTs can be prevented from being shortened due to the CNTs being torn off or the like. This increases the ratio of CNTs bridging between the active material particles, and therefore good conductive paths can be formed between the active material particles, improving the cycle characteristics of the secondary battery. In addition, since the second kneaded body with a relatively low solid content rate is kneaded with the CNTs, an active material layer having a high ratio of the above-mentioned first CNTs to the total CNTs can be formed.

[0039] The solid content in step (1a) (solid content of the first kneaded body) may be 61 to 65% by weight, 62 to 65% by weight, or 63 to 64% by weight. By setting the solid content in step (1a) within the above range, the surfaces of the active material particles are easily covered with the first binder.

[0040] Step (1a) may include a step of dry-mixing the active material particles and the first binder to obtain a mixture, and a step of kneading the mixture and water to a solid content of 60 to 65% by weight. The solid content in the step of kneading the mixture and water may be 61 to 65% by weight, 62 to 65% by weight, or 63 to 64% by weight.

[0041] The solid content in step (1b) (solid content of the second kneaded body) may be 49% by weight or less, 48% by weight or less, 35 to 50% by weight, 40 to 49% by weight, or 43 to 48% by weight.

[0042] The first step may further include a step of kneading the third kneaded body with a second binder (hereinafter also referred to as "step (1d)"). In this case, it is preferable that the first binder is at least one of CMC and PAA, and the second binder is SBR.

[0043] The first step may be carried out using a stirrer having a stirring blade. Examples of the stirrer include a kneader and a homodisper, which may be used alone or in combination. For example, a kneader may be used in steps (1a) and (1b), and a homodisper may be used in steps (1c) and (1d).

[0044] The rotation speed of the stirring blade in each step included in the first step is not particularly limited. The rotation speed of the stirring blade in step (1a) may be 10 to 65 rpm, 20 to 60 rpm, or 30 to 55 rpm. The kneading time in step (1a) may be 10 to 200 min, 70 to 150 min, or 80 to 120 min.

[0045] The rotation speed of the stirring blade in step (1b) may be 5 to 50 rpm, 10 to 40 rpm, or 20 to 35 rpm. The kneading time in step (1b) may be, for example, 10 to 180 minutes, 20 to 150 minutes, or 25 to 120 minutes.

[0046] The rotation speed of the stirring blade in step (1c) may be 500 to 3000 rpm, 1000 to 2000 rpm, or 1200 to 1800 rpm. The kneading time in step (1c) may be, for example, 10 to 180 minutes, 20 to 150 minutes, or 25 to 100 minutes.

[0047] The rotation speed of the stirring blade in step (1d) may be 500 to 4000 rpm, 1000 to 3000 rpm, or 1500 to 2500 rpm. The kneading time in step (1d) may be, for example, 0.5 to 100 minutes, 1 to 60 minutes, or 3 to 30 minutes.

[0048] The second step is a step of forming an active material layer using the slurry prepared in the first step. The second step may include a step of applying the slurry onto a negative electrode current collector to form a coating layer, drying the coating layer, and may further include a step of compressing the coating layer after drying. The negative electrode current collector may be made of the above-mentioned materials.

[0049] (Nonaqueous electrolyte secondary battery) The nonaqueous electrolyte secondary battery of this embodiment (hereinafter also referred to as "the battery") includes the present negative electrode, and typically includes an electrode assembly including the present negative electrode, and a nonaqueous electrolyte. The battery may have a battery case that houses the electrode assembly and the nonaqueous electrolyte. The battery case may include an exterior body having an opening, and a sealing plate that seals the opening. The exterior body and the sealing plate are preferably made of metal, and may be formed using aluminum, an aluminum alloy, iron, an iron alloy, or the like, and may be formed using, for example, an aluminum laminate film. A resin sheet serving as an electrode holder may be disposed between the electrode body and the exterior body.

[0050] The electrode body may include the present negative electrode, a positive electrode, and a separator. In the electrode body, the active material layer of the present negative electrode and the positive electrode active material layer of the positive electrode face each other via the separator. The electrode body may be a laminated type in which the present negative electrode, the positive electrode, and the separator are laminated, or a wound type in which a laminate in which the present negative electrode, the positive electrode, and the separator are laminated is wound.

[0051] The positive electrode usually has a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector is, for example, a metal foil made of an aluminum material such as aluminum or an aluminum alloy. The positive electrode active material layer contains positive electrode active material particles, and may further contain a conductive material, a binder, and the like. The positive electrode active material particles include, for example, layered or spinel-based lithium transition metal oxides (e.g., LiNiCoMnO 2 , LiNiO 2 , LiCoO 2 , LiFeO 2 , LiMn 2 O 4 , LiNi 0.5 Mn 1.5 O 4 , LiCrMnO 4 , LiFePO 4 , LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 ) particles. The lithium transition metal oxide may be a lithium nickel cobalt manganese composite oxide (NCM). Examples of the conductive material include carbon materials such as fibrous carbon (CNT (SWCNT, DWCNT)), carbon black (e.g., acetylene black, ketjen black), coke, and activated carbon. Examples of the binder include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and SBR.

[0052] The separator may have a substrate and a functional layer on at least one side of the substrate. The substrate may be a film made of a resin such as polyethylene, polypropylene, polyester, cellulose, polyamide, or the like, or a porous sheet such as a nonwoven fabric. The substrate may have a single-layer structure or a multi-layer structure. Examples of the functional layer include an adhesive layer and a heat-resistant layer, and the separator may have one or both of these. The adhesive layer may be formed, for example, by an adhesive. The heat-resistant layer may include, for example, a filler and a binder.

[0053] The non-aqueous electrolyte is preferably a non-aqueous solvent such as an organic solvent containing a supporting salt. 6, LiBF 4 , LiClO 4 , LiFSO 3 , LiBOB (lithium bis(oxalato)borate), etc. The non-aqueous electrolyte may contain one or more of these supporting salts. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate (PC), butylene carbonate (BC), and diethyl carbonate (DEC). The non-aqueous electrolyte may contain one or more of these non-aqueous solvents.

[0054] (Method of manufacturing non-aqueous electrolyte secondary battery) The method for producing the present battery includes a step of producing a negative electrode by the method for producing a negative electrode of the present invention. The method for producing the present battery may further include a step of obtaining an electrode assembly using the present negative electrode, a positive electrode, and a separator, and a step of housing the electrode assembly and a nonaqueous electrolyte in a battery case. EXAMPLES

[0055] The present disclosure will be described in more detail below with reference to examples and comparative examples. Figures 4 to 7 are scanning transmission electron microscope (SEM) images of the surfaces of the active material layers of the negative electrodes obtained in Examples 1 and 2, and Comparative Examples 1 and 2, respectively. Example 1 (Preparation of negative electrode (1)) Graphite particles were used as the carbon particles, and composite particles of Si and C (SiC composite particles) were used as the metal particles. The average particle diameter D50 of the graphite particles was 17 μm, and the specific surface area (BET) of the graphite particles was 2.2 m 2 / g. The CNTs had the G / D ratio shown in Table 1. Slurry (1) was prepared using a kneader (PRIMIX Corporation, Hivismix 2P-1 model) and a homodisper (PRIMIX Corporation, Robomix) as the agitators according to the following procedure.

[0056] First, carbon-based particles, metal-based particles, CMC and PAA as the first binder were dry-mixed to obtain a mixture. Next, water was added to the mixture to make the solid content 65% by weight, and the kneader was rotated at 50 rpm for 90 minutes to obtain a first kneaded body (step (1a)). Water was further added to the first kneaded body to make the solid content 48%, and the kneader was rotated at 30 rpm for 30 minutes to obtain a second kneaded body (step (1b)). CNT (a water-soluble paste with a solid content of 1% by weight) was added to the second kneaded body, and the homodisper was rotated at 1500 rpm for 30 minutes to obtain a third kneaded body (step (1c)). SBR was added to the third kneaded body, and the homodisper was rotated at 1500 rpm for 30 minutes to obtain a slurry (1) (step (1d)). The compounding ratio of the slurry (1) was graphite particles:SiC composite particles:CNT:CMC:PAA:SBR=91:9:0.05:1:1:1 (weight ratio).

[0057] The slurry (1) obtained above was applied to both sides of a copper foil having a thickness of 8 μm as a negative electrode current collector, dried, and compressed to obtain a negative electrode (1). The active material layer of the negative electrode (1) had a basis weight (total basis weight of both sides) of 195 g / m 2 The active material layer had a thickness (per side) of 130 μm and a packing density of 1.52 g / cc. An SEM image of the surface of the active material layer of negative electrode (1) is shown in FIG. 4. The active material layer of negative electrode (1) contained CMC and PAA (first binder) that covered at least a portion of the surface of the active material particles and did not contain CNTs, and contained CNTs (first CNTs) covering the surfaces of the CMC and PAA, and CNTs (second CNTs) bridging between two active material particles.

[0058] (Preparation of positive electrode) The positive electrode active material particles were lithium nickel cobalt manganese composite oxide (NCM), the conductive material was acetylene black (AB), and the binder was polyvinylidene fluoride (PVdF) in a ratio of positive electrode active material particles:AB:PVdF=100:1:1 (weight ratio), which was mixed with N-methylpyrrolidone (NMP) as a solvent to obtain a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum foil with a thickness of 15 μm as a positive electrode current collector, dried, and compressed to obtain a positive electrode.

[0059] (Preparation of non-aqueous electrolyte secondary battery) Leads were attached to the negative and positive electrodes obtained above, and the negative and positive electrodes were laminated with a separator between them to obtain an electrode body. The electrode body was inserted into an exterior body made of an aluminum laminate sheet, a non-aqueous electrolyte was injected, and the opening of the exterior body was sealed to obtain a test cell (laminated cell) (1). The non-aqueous electrolyte was a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC=20:40:40, LiPF as a lithium salt, and 100% ethylene carbonate (EC) was added. 6 was dissolved at a concentration of 1 mol / L.

[0060] Example 2 (Creating negative electrode (2)) Negative electrode (2) was produced in the same manner as in Example 1, except that CNTs having the G / D ratio shown in Table 1 were used. An SEM image of the surface of the active material layer of negative electrode (2) is shown in Fig. 5. The active material layer of negative electrode (2) contained CMC and PAA (first binder) that covered at least a portion of the surface of the active material particles and did not contain CNTs, CNTs (first CNTs) covering the surfaces of the CMC and PAA, and CNTs (second CNTs) bridging between two active material particles.

[0061] (Preparation of non-aqueous electrolyte secondary battery) A test cell (2) was obtained in the same manner as in Example 1, except that the negative electrode (2) was used instead of the negative electrode (1).

[0062] Comparative Example 1 (Preparation of negative electrode (c1)) The carbon-based particles and metal-based particles used in Example 1 were dry-mixed with CMC and PAA as the first binder to obtain a mixture. Next, CNTs (water-soluble paste with a solid content of 1% by weight) having the G / D ratio shown in Table 1 and water were added to the mixture to make the solid content 64% by weight, and the kneading machine was rotated at 50 rpm for 90 minutes. Thereafter, water was further added and the kneading machine was rotated at 30 rpm for 30 minutes, and then SBR was added as the second binder and the kneading machine was rotated at 30 rpm for 5 minutes to obtain a slurry (c1) with a solid content of 46% by weight. The compounding ratio of the slurry (c1) was graphite particles:SiC composite particles:CNT:CMC:PAA:SBR=91:9:0.05:1:1:1 (weight ratio).

[0063] A negative electrode (c1) was obtained in the same manner as in Example 1, except that slurry (c1) was used instead of slurry (1). An SEM image of the surface of the active material layer of the negative electrode (c1) is shown in Figure 6. In the active material layer of the negative electrode (c1), a mixture of CNT and binder (CMC and PAA) covered the surface of the active material particles and spanned between two active material particles.

[0064] (Preparation of non-aqueous electrolyte secondary battery) A test cell (c1) was obtained in the same manner as in Example 1, except that the negative electrode (c1) was used instead of the negative electrode (1).

[0065] Comparative Example 2 (Preparation of negative electrode (c2)) A negative electrode (c2) was produced in the same manner as in Comparative Example 1, except that CNTs having the G / D ratio shown in Table 1 were used. An SEM image of the surface of the active material layer of the negative electrode (c2) is shown in Figure 7. In the active material layer of the negative electrode (c2), a mixture of CNTs and binders (CMC and PAA) covered the surfaces of the active material particles and spanned the space between two active material particles.

[0066] (Preparation of non-aqueous electrolyte secondary battery) A test cell (c2) was obtained in the same manner as in Example 1, except that the negative electrode (c2) was used instead of the negative electrode (1).

[0067] [Determination of G / D ratio of CNTs] The CNT dispersion liquid was dispensed onto a cover glass, and a Raman spectrum was obtained by irradiating a 300 μm diameter area with a laser beam having an excitation wavelength of 532 nm using a Raman microscope (manufactured by Horiba, Ltd.). -1 The maximum peak intensity in the range of 1300 to 1400 cm is defined as G. -1 The maximum peak intensity within this range was defined as D, and the G / D ratio was calculated. The results are shown in Table 1.

[0068] [Determination of average CNT length and CNT percentage] (SEM image acquisition) Using a scanning transmission electron microscope (SEM, Hitachi High-Technologies) at an accelerating voltage of 1 kV, the surface of the negative electrode active material layer was confirmed by SEM images with a field of view of 9 × 7 μm.

[0069] (Determination of the average length of CNTs) The CNTs in the SEM image were painted in lines using image software, the circumference of the lines was measured, and half of that value was taken as the length of the CNT. Since CNTs have an extremely long aspect ratio in terms of length compared to diameter, the influence of the line width on the circumference can be ignored. The lengths of all CNTs in the SEM image were determined, and the average value was taken as the average length of the CNTs. The results are shown in Table 1.

[0070] (Determination of CNT ratio) For all CNTs in the SEM images obtained above, the percentage [%] of CNTs not mixed with CMC and PAA (first binder) (CNT percentage [%]) was calculated according to the following formula. The results are shown in Table 1. CNT ratio [%] = (area of ​​CNTs not mixed with the first binder in the SEM image / area of ​​all CNTs in the SEM image) × 100

[0071] [Evaluation of cycle characteristics (1)] In an environment of 25°C, the test cell was charged to 4.2V by constant current / constant voltage charging (CCCV) at a current value of 0.2C (cut at 0.1C), and then discharged to 2.5V at a current value of 0.33C (CC discharge), with N [cycles] of charge / discharge being repeated. For the test cells obtained in Example 1 and Comparative Example 1, the number of charge / discharge cycles N was 100 cycles, and for the test cells obtained in Example 2 and Comparative Example 2, the number of charge / discharge cycles N was 150 cycles. The discharge capacity at the Nth cycle relative to the discharge capacity at the 1st cycle was calculated as the capacity retention rate [%] according to the following formula. The results are shown in Table 1. Capacity retention rate [%] = (discharge capacity at Nth cycle / discharge capacity at 1st cycle) x 100

[0072] [Table 1] [Explanation of symbols]

[0073] 1 active material particles, 2 first binder, 3 CNT (carbon nanotube), 31 first CNT (first carbon nanotube), 32 second CNT (second carbon nanotube).

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: an active material layer including active material particles, carbon nanotubes, and a first binder; The active material particles include metal-based active material particles and carbon-based active material particles, the average length of the carbon nanotubes, as determined by analysis of a scanning transmission electron microscope image of the active material layer, is 1.5 μm or more; At least a portion of the first binder covers at least a portion of the surface of the active material particles without being mixed with the carbon nanotubes; The carbon nanotubes include first carbon nanotubes that are attached to the first binder so as to cover the first binder that covers the surfaces of the active material particles while not being mixed with the carbon nanotubes, and second carbon nanotubes that span between two of the active material particles.

2. 2. The negative electrode of claim 1, wherein a percentage of the carbon nanotubes not intermixed with the first binder relative to a total amount of the carbon nanotubes in the active material layer, as determined by analysis of the scanning transmission electron microscope image of the active material layer, is greater than 60%.

3. 2. The negative electrode according to claim 1, wherein the metal-based active material particles include particles of one or more types selected from the group consisting of Si, SiOx, and a composite of Si and C.

4. The negative electrode according to claim 1 , wherein the metal-based active material particles include particles of a composite of Si and C.

5. The negative electrode according to claim 1 , wherein the carbon nanotubes are single-walled carbon nanotubes.

6. The negative electrode of claim 1 , wherein the carbon nanotubes have a G / D ratio of 80 or less.

7. 2. The negative electrode of claim 1, wherein the first binder is at least one of carboxymethyl cellulose and polyacrylic acid.

8. the active material layer includes a second binder different from the first binder, The negative electrode of claim 7 , wherein the second binder comprises styrene butadiene rubber.

9. A non-aqueous electrolyte secondary battery comprising the negative electrode according to any one of claims 1 to 8.

10. A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, comprising the steps of: A first step of preparing a slurry; A second step of forming an active material layer of the negative electrode using the slurry, the slurry includes active material particles, a first binder, carbon nanotubes, and water; The active material particles include metal-based active material particles and carbon-based active material particles, The first step comprises: (1a) obtaining a first kneaded body by kneading the active material particles, the first binder, and water to a solid content of 60 to 65% by weight; (1b) adding water to the first kneaded body and kneading the first kneaded body so that the solid content is within a range of 50% by weight or less to obtain a second kneaded body; and (1c) kneading the second kneaded body and the carbon nanotubes to obtain a third kneaded body.

11. the first binder is at least one of carboxymethyl cellulose and polyacrylic acid; The first step further includes a step of kneading the third kneaded body with a second binder, The method for producing a negative electrode according to claim 10 , wherein the second binder contains styrene-butadiene rubber.

12. The step (1a) dry mixing the active material particles and the first binder to obtain a mixture; and kneading the mixture and water so that the solid content is within a range of 60 to 65% by weight.

13. The method for producing a negative electrode according to claim 10 , wherein the metal-based active material particles contain one or more types of particles selected from the group consisting of Si, SiO x , and a composite of Si and C.

14. The method for producing a negative electrode according to claim 10 , wherein the first binder is at least one of carboxymethyl cellulose and polyacrylic acid.

15. The method for producing a negative electrode according to claim 10 , wherein the carbon nanotubes have a G / D ratio of 80 or less.

16. A method for producing a non-aqueous electrolyte secondary battery, comprising the step of producing a negative electrode by the method for producing a negative electrode according to any one of claims 10 to 15.

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