Lithium ion secondary battery

Abstract

The lithium ion secondary battery has a positive electrode and a negative electrode, the negative electrode has a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, and the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer. The first negative electrode active material layer and the second negative electrode active material layer contain Si-based particles, and the weight fraction of the Si-based particles in the second negative electrode active material layer is greater than the weight fraction of the Si-based particles in the first negative electrode active material layer. The negative electrode active material layer has the following constitution (1) or the like. Configuration (1): the first negative electrode active material layer and the second negative electrode active material layer have carbon nanotubes, and the weight fraction of the carbon nanotubes in the second negative electrode active material layer is greater than the weight fraction of the carbon nanotubes in the first negative electrode active material layer. This makes it possible to improve the charge / discharge characteristics of a lithium ion secondary battery having a negative electrode containing Si-based particles containing SiOx (0 < x < 2) or the like as a silicon oxide-based active material.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a lithium-ion secondary battery. BACKGROUND

[0002] As a power source for an electric vehicle (EV), a hybrid vehicle (HV), or the like, or a stationary power source, a lithium-ion secondary battery or the like is used. As the lithium-ion secondary battery, a lithium-ion secondary battery having a negative electrode provided with a plurality of active material layers is known.

[0003] In Patent Literature 1, a negative electrode for a secondary battery is disclosed, which includes a current collector, a first negative electrode active material layer formed on the current collector and containing a first active material, and a second negative electrode active material layer formed on the first negative electrode active material layer and containing a second active material, wherein the second active material is a bimodal active material composed of active materials having mutually different specific surface areas, and the first active material and the second active material contain natural graphite, artificial graphite, or the like, and at least one of the first negative electrode active material layer and the second negative electrode active material layer further contains a silicon oxide-based active material (SiOx (0 < x < 2)), and the content of the silicon oxide-based active material in the upper layer (the second negative electrode active material layer) exceeds twice the content in the lower layer (the first negative electrode active material layer). x (0 < x < 2), and the content of the silicon oxide-based active material in the upper layer (the second negative electrode active material layer) exceeds twice the content in the lower layer (the first negative electrode active material layer).

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature: Japanese Patent Application Laid-Open No. 2022-74046 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] The silicon oxide-based active material used in the negative electrode for a secondary battery described in Patent Literature 1 sometimes swells and shrinks along with charge and discharge.

[0009] Therefore, there is a need to improve the charge and discharge characteristics of a negative electrode having a silicon oxide-based active material.

[0010] An object of the present application is to improve the charge and discharge characteristics of a lithium-ion secondary battery having a negative electrode containing a Si-containing particle including SiOx (0 < x < 2) as a silicon oxide-based active material. x (0 < x < 2) or the like.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The lithium ion secondary battery of the present application has a positive electrode and a negative electrode, the negative electrode has a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, and the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer. Also, the first negative electrode active material layer and the second negative electrode active material layer contain Si-based particles, the weight fraction of the Si-based particles of the second negative electrode active material layer is greater than the weight fraction of the Si-based particles of the first negative electrode active material layer. Also, the negative electrode active material layer has at least one or more of the following configurations (1) to (4).

[0013] Configuration (1): The first negative electrode active material layer and the second negative electrode active material layer contain carbon nanotubes, and the weight fraction of the carbon nanotubes of the second negative electrode active material layer is greater than the weight fraction of the carbon nanotubes of the first negative electrode active material layer.

[0014] Configuration (2): The first negative electrode active material layer and the second negative electrode active material layer contain a binder, and the weight fraction of the binder of the second negative electrode active material layer is greater than the weight fraction of the binder of the first negative electrode active material layer.

[0015] Configuration (3): The first negative electrode active material layer and the second negative electrode active material layer contain amorphous carbon, and the weight fraction of the amorphous carbon of the second negative electrode active material layer is greater than the weight fraction of the amorphous carbon of the first negative electrode active material layer.

[0016] Configuration (4): The first negative electrode active material layer and the second negative electrode active material layer contain graphite as a main component, and the hardness of the graphite of the first negative electrode active material layer is greater than the hardness of the graphite of the second negative electrode active material layer.

[0017] Effects of the Invention

[0018] According to the present application, the charge-discharge characteristics of a lithium ion secondary battery having a negative electrode containing Si-based particles including SiO x (0 < x < 2), and the like. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is an appearance perspective view of a lithium ion secondary battery of an embodiment.

[0020] Figure 2 is a perspective view of a charge-discharge body built in the lithium ion secondary battery 1 of Figure 1

[0021] Figure 3 is a perspective view of the charge-discharge body 100 partially expanded to show Figure 2

[0022] Figure 4 is a perspective view of​​Figure 3 schematic partial enlarged sectional view of the negative electrode 120 of

[0023] Figure 5 is a schematic partial enlarged sectional view of another mode of the negative electrode 120 of Figure 3 DETAILED DESCRIPTION

[0024] Hereinafter, embodiments of the present application will be described with reference to the drawings. In each drawing, the size and the ratio of the constituent members are sometimes exaggerated for the sake of easy understanding of the embodiments. In each drawing, the same constituent is given the same symbol. In each drawing, the lateral width direction X, the depth direction Y, and the height direction Z of the lithium-ion secondary battery and the constituent members of the lithium-ion secondary battery are indicated by arrows. However, in each drawing, the lateral width direction X, the depth direction Y, and the height direction Z indicate relative directional relationships. That is, for example, in a case where the lithium-ion secondary battery is rotated by 180 degrees so that the upper surface and the lower surface are arranged in reverse, or in a case where the lithium-ion secondary battery is rotated by 90 degrees so that the upper surface is arranged as a side surface, the lateral width direction X, the depth direction Y, and the height direction Z of the lithium-ion secondary battery change.

[0025] (Structure of lithium-ion secondary battery having positive electrode of embodiment)

[0026] Referring to Figures 1 to 4 the structure of the lithium-ion secondary battery having the positive electrode of the embodiment will be described.

[0027] Figure 1 is an external perspective view of the lithium-ion secondary battery of the embodiment.

[0028] In the drawing, the lithium-ion secondary battery 1 has a structure in which a charge-discharge body (not shown) is housed in a container constituted by a case 201 and a lid 202. The positive electrode terminal 301 and the negative electrode terminal 302 are mounted on the lid 202 in an insulating state. The lid 202 is engaged with the opening of the case 201, and seals the charge-discharge body together with the case 201.

[0029] In addition, in a case where a plurality of lithium-ion secondary batteries 1 are used to constitute a battery pack, the positive electrode terminal 301 of an adjacent lithium-ion secondary battery 1 and the negative electrode terminal 302 of another lithium-ion secondary battery 1 arranged adjacent thereto are engaged via a bus bar.

[0030] Figure 2 is a perspective view of the charge-discharge body built in the lithium-ion secondary battery 1 of Figure 1

[0031] Figure 2 ​​The charge-discharge body 100 shown has a configuration in which a separator is interposed between a positive electrode and a negative electrode and wound. The charge-discharge body 100 has a positive electrode tab 111b and a negative electrode tab 121b. The positive electrode tab 111b is connected to the positive terminal 301 of the lithium ion secondary battery 1. The negative electrode tab 121b is connected to the negative terminal 302 of the lithium ion secondary battery 1. Figure 1 Figure 1

[0032] The charge-discharge body 100 is injected with an electrolyte (electrolytic solution) from a liquid injection port provided on the lid 202 in a state in which the lithium ion secondary battery 1 is enclosed in the case 201. Thus, the charge-discharge body 100 becomes a state in which it is immersed in the electrolyte. After the electrolyte is injected, the liquid injection port is sealed with a liquid injection plug. The electrolyte includes an organic solvent, an electrolyte salt, and an additive. The organic solvent is used, for example, a carbonate or the like. The electrolyte salt is used, for example, a lithium salt. The additive includes a material for forming a negative electrode coating film. The material for forming a negative electrode coating film is, for example, vinylene carbonate. Figure 1

[0033] Figure 3 is a perspective view of the charge-discharge body 100 shown in Figure 2

[0034] As shown in Figure 3 , the positive electrode 110, the negative electrode 120, and the separator 130 that constitute the charge-discharge body are each formed into a long strip shape that extends in the X-axis direction. The positive electrode tab 111b is provided on the positive electrode 110. The negative electrode tab 121b is provided on the negative electrode 120. The positive electrode tab 111b and the negative electrode tab 121b protrude in the same direction (the positive direction of the Z-axis).

[0035] The separator 130 is interposed between the positive electrode 110 and the negative electrode 120. Furthermore, in order to prevent the positive electrode 110 and the negative electrode 120 from directly contacting each other when they are wound, the separator 130 is also arranged on the opposite side of the negative electrode 120. In other words, in this drawing, the negative electrode 120 is sandwiched between two separators 130. The separator 130 insulates the positive electrode 110 and the negative electrode 120 from each other. The separator 130 allows lithium ions to pass therethrough via an electrolyte.

[0036] The negative electrode current collecting portion of the negative electrode 120 is longer in width in the short side direction (the Z-axis direction) than the positive electrode current collecting portion of the positive electrode 110. The separator 130 is longer in width in the short side direction (the Z-axis direction) than the positive electrode 110 and the negative electrode 120.

[0037] In addition, the positive electrode current collecting layer of the positive electrode 110 is formed of, for example, an aluminum alloy. The separator 130 is constituted of a porous material. The separator 130 is formed of, for example, polyethylene (PE), polypropylene (PP), or the like.

[0038] Figure 4 is a perspective view of the charge-discharge body 100 shown in Figure 3 ​​​​schematic partial enlarged cross-sectional view of the negative electrode 120.

[0039] As Figure 4 illustrated, the negative electrode 120 includes a negative electrode current collecting layer 121 (negative electrode current collector), a first negative electrode active material layer 122, and a second negative electrode active material layer 123. On one face (upper face in the drawing) of the negative electrode current collecting layer 121, the first negative electrode active material layer 122 and the second negative electrode active material layer 123 are formed in this order. That is, the negative electrode 120 includes a plurality of active material layers.

[0040] In addition, the negative electrode current collecting layer 121 is formed of, for example, a copper alloy.

[0041] The first negative electrode active material layer 122 corresponds to the lower layer of the two active material layers when taking the negative electrode current collecting layer 121 as a reference.

[0042] The first negative electrode active material layer 122 includes graphite particles 122a, Si-based particles 122b, carbon nanotubes 122c (CNT), and a binder 122d.

[0043] The second negative electrode active material layer 123 corresponds to the upper layer of the two active material layers when taking the negative electrode current collecting layer 121 as a reference.

[0044] The second negative electrode active material layer 123 includes graphite particles 123a, Si-based particles 123b, carbon nanotubes 123c (CNT), and a binder 123d.

[0045] In the second negative electrode active material layer 123, the content of the Si-based particles 123b is more than that of the first negative electrode active material layer 122. In addition, the content of the binder 123d of the second negative electrode active material layer 123 is preferably equal to or relatively more than that of the binder 122d of the first negative electrode active material layer 122. The content of the carbon nanotubes 123c in the second negative electrode active material layer 123 is preferably equal to or relatively more than that of the carbon nanotubes 122c in the first negative electrode active material layer 122.

[0046] As Figure 4 illustrated, in the case where the content of the Si-based particles in the upper layer (second negative electrode active material layer) is large, the upper layer easily accepts lithium ions, and the input / output characteristics are improved. On the other hand, in the case where the content of the Si-based particles in the lower layer (first negative electrode active material layer) is large, the Si-based particles are difficult to react with the electrolyte, and the charge / discharge cycle characteristics are improved.

[0047] In addition, Figure 3The positive electrode 110 shown includes a positive electrode current collector and a positive electrode active material layer laminated on the positive electrode current collector. The positive electrode active material layer can be a single layer, or can be configured to include a first positive electrode active material layer laminated on the positive electrode current collector and a second positive electrode active material layer laminated on the first positive electrode active material layer. The single layer includes a positive electrode active material, a conductive aid, and a positive electrode binder. In addition, in the case of the configuration including the first positive electrode active material layer and the second positive electrode active material layer laminated on the first positive electrode active material layer, the first positive electrode active material layer contains a first positive electrode active material, a first conductive aid, and a first positive electrode binder. The second positive electrode active material layer contains a second positive electrode active material, a second conductive aid, and a second positive electrode binder.

[0048] The positive electrode active material, the first positive electrode active material, and the second positive electrode active material configuring the single layer are composed of a lithium-containing composite oxide. The lithium-containing composite oxide can contain, for example, metal elements such as lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn). The first conductive aid is also referred to as a "first conductive material", and the second conductive aid is also referred to as a "second conductive material".

[0049] Next, the measurement method of the composition, the particle diameter, and the specific surface area of the particles will be described.

[0050] <Measurement method of negative electrode active material and the like>

[0051] The average composition of the particles of the negative electrode active material can be confirmed by high-frequency inductively coupled plasma (ICP), atomic absorption spectrometry (AAS), or the like. The average particle diameter of the primary particles of the negative electrode active material is calculated by dividing the length of the cross line when a straight line is drawn in a prescribed direction of the cross-sectional observation image of the secondary particles by the number of primary particles contained in the cross line, using a scanning electron microscope (SEM), and using the average value of 10 secondary particles as the average particle diameter of the primary particles. In addition, the straight line in the prescribed direction refers to a straight line until the connection of the primary particles is interrupted, considering the case where the cross section of the secondary particles has a void or the like, and is referred to as a "cross line". The average particle diameter of the particles in the raw material slurry or the secondary particles of the negative electrode active material can be measured, for example, by a laser diffraction type particle size distribution measuring device or the like. The BET specific surface area can be calculated by a gas adsorption method using an automatic specific surface area measuring device.

[0052] The average particle diameter of the graphite particles 122a and the graphite particles 123a is preferably 5 μm or more and 25 μm or less. The average particle diameters of the graphite particles 122a and the graphite particles 123a can be the same or different. When the average particle diameters of the graphite particles 122a and the graphite particles 123a are different, the average particle diameter of the graphite particles 122a can be, for example, 15 μm or more and 25 μm or less, and the average particle diameter of the graphite particles 123a can be, for example, 5 μm or more and 10 μm or less. Alternatively, the average particle diameter of the graphite particles 123a can be, for example, 15 μm or more and 25 μm or less, and the average particle diameter of the graphite particles 122a can be, for example, 5 μm or more and 10 μm or less.

[0053] The average particle diameter of the positive electrode active material can also be measured using the method described above for measuring the average particle diameters of the primary particles and the secondary particles of the negative electrode active material.

[0054] In addition, the residual alkali component of the positive electrode active material can be calculated by neutralization titration. The compression of the positive electrode active material can be performed using a press machine or an AUTOGRAPH, or the like.

[0055] The graphite particles 122a and the graphite particles 123a can be easily graphitized carbon or hardly graphitized carbon. The easily graphitized carbon corresponds to soft carbon. The hardly graphitized carbon corresponds to hard carbon.

[0056] The first negative electrode active material layer 122 and the second negative electrode active material layer 123 can include amorphous carbon. The amorphous carbon is, for example, acetylene black or carbon black. The amorphous carbon includes low-crystallinity carbon.

[0057] The graphite particles 122a and the graphite particles 123a can be pitch-coated natural graphite, natural graphite without pitch coating, artificial graphite, or the like. The pitch-coated natural graphite is harder than the natural graphite without pitch coating. The natural graphite is composed of a plurality of layers, and the end portions of the plurality of layers in the stacking direction are not coated with resin. The resin contains, for example, carbon having electrical conductivity.

[0058] The Si-based particles 122b and the Si-based particles 123b are particles of Si or SiO x (0 < x < 2), particles containing Si or SiO x (0 < x < 2), or the like. As such particles, for example, silicon oxide in which lithium is pre-doped in silicon (Li-SiO x (0 < x < 2) is preferably used. In addition, the Si-based particles can not only be an oxide of silicon (Si), but also a silicon compound containing other anions, or the like. In the present specification, such an oxide and a silicon compound are collectively referred to as "Si-containing compound". Furthermore, the particles of Si and the particles of the Si-containing compound are collectively referred to as "Si-based particles".

[0059] The binder 122d and the binder 123d include, for example, any one or more of rubber-based, acrylic, polyamide-imide, and polyimide.

[0060] (Structure of the negative electrode of Examples 1 to 7 and Comparative Example 1 and effects of the structure)

[0061] In the negative electrode of Examples 1 to 7 and Comparative Example 1, the negative electrode active material layer has a two-layer structure of a first negative electrode active material layer (lower layer) and a second negative electrode active material layer (upper layer).

[0062] Further, the first negative electrode active material layer and the second negative electrode active material layer contain Si-based particles, and the weight fraction of the Si-based particles in the second negative electrode active material layer is greater than the weight fraction of the Si-based particles in the first negative electrode active material layer.

[0063] In addition, the negative electrode active material layer has at least one or more of the following configurations (1) to (4).

[0064] Configuration (1): The first negative electrode active material layer and the second negative electrode active material layer contain CNTs, and the weight fraction of the CNTs in the second negative electrode active material layer is greater than the weight fraction of the CNTs in the first negative electrode active material layer.

[0065] Configuration (2): The first negative electrode active material layer and the second negative electrode active material layer contain a binder, and the weight fraction of the binder in the second negative electrode active material layer is greater than the weight fraction of the binder in the first negative electrode active material layer.

[0066] Configuration (3): The first negative electrode active material layer and the second negative electrode active material layer contain amorphous carbon, and the weight fraction of the amorphous carbon in the second negative electrode active material layer is greater than the weight fraction of the amorphous carbon in the first negative electrode active material layer.

[0067] Configuration (4): The first negative electrode active material layer and the second negative electrode active material layer contain graphite as a main component, and the hardness of the graphite in the first negative electrode active material layer is greater than the hardness of the graphite in the second negative electrode active material layer.

[0068] The Si-based particles generate fine cracks in the active material layer due to expansion and contraction. Through the fine cracks, the electrolyte solution easily penetrates, and thus the charge and discharge characteristics can be improved.

[0069] According to configuration (1), it is difficult to hinder the expansion of the first negative electrode active material layer (lower layer). Thus, the electrolyte solution easily penetrates, and the charge and discharge characteristics can be improved.

[0070] According to configuration (2), it is difficult to hinder the expansion of the first negative electrode active material layer (lower layer). Thus, the electrolyte solution easily penetrates, and the charge and discharge characteristics can be improved.

[0071] According to the configuration (3), the electrolytic solution easily penetrates into the first negative electrode active material layer (lower layer), and the charge / discharge characteristics can be improved.

[0072] According to the configuration (4), fine cracks are easily generated in the first negative electrode active material layer (lower layer), the electrolytic solution easily penetrates, and the charge / discharge characteristics can be improved.

[0073] (Composition of the negative electrode of Examples 8 to 14 and Comparative Example 2 and effects of the composition)

[0074] In the negative electrode of Examples 8 to 14 and Comparative Example 2, the negative electrode active material layer also has a two-layer structure of the first negative electrode active material layer (lower layer) and the second negative electrode active material layer (upper layer).

[0075] Further, the first negative electrode active material layer and the second negative electrode active material layer contain Si-based particles, and the weight fraction of the Si-based particles of the first negative electrode active material layer is larger than the weight fraction of the Si-based particles of the second negative electrode active material layer.

[0076] In addition, the negative electrode active material layer has at least one or more of the following configurations (5) to (8).

[0077] Configuration (5): The first negative electrode active material layer and the second negative electrode active material layer contain CNTs, and the weight fraction of the CNTs of the second negative electrode active material layer is smaller than the weight fraction of the CNTs of the first negative electrode active material layer.

[0078] Configuration (6): The first negative electrode active material layer and the second negative electrode active material layer contain binders, and the weight fraction of the binder of the second negative electrode active material layer is smaller than the weight fraction of the binder of the first negative electrode active material layer.

[0079] Configuration (7): The first negative electrode active material layer and the second negative electrode active material layer contain amorphous carbon, and the weight fraction of the amorphous carbon of the second negative electrode active material layer is smaller than the weight fraction of the amorphous carbon of the first negative electrode active material layer.

[0080] Configuration (8): The first negative electrode active material and the second negative electrode active material contain graphite as a main component, and the hardness of the graphite of the second negative electrode active material layer is larger than the hardness of the graphite of the first negative electrode active material layer.

[0081] The Si-based particles generate fine cracks in the active material layer due to expansion and contraction.

[0082] Through the fine cracks, the electrolytic solution easily penetrates, and the charge / discharge characteristics can be improved.

[0083] According to the configuration (5), expansion of the second negative electrode active material layer (upper layer) is less likely to be hindered. Thus, the electrolytic solution easily penetrates, and the charge / discharge characteristics can be improved.

[0084] According to the configuration (6), it is difficult to hinder the expansion of the second negative electrode active material layer (upper layer). Thus, the electrolytic solution is easily impregnated, and the charge / discharge characteristics can be improved.

[0085] According to the configuration (7), the electrolytic solution is easily impregnated into the second negative electrode active material layer (upper layer), and the charge / discharge characteristics can be improved.

[0086] According to the configuration (8), fine cracks are easily generated in the second negative electrode active material layer (upper layer), the electrolytic solution is easily impregnated, and the charge / discharge characteristics can be improved.

[0087] Examples and comparative examples were produced using Figure 5 The configurations of the examples and comparative examples will be described.

[0088] The first negative electrode active material layer 124 is a lower layer, and contains graphite particles 124a, Si-based particles 124b, carbon nanotubes 124c (CNT), and a binder 124d.

[0089] The second negative electrode active material layer 125 is an upper layer, and contains graphite particles 125a, Si-based particles 125b, carbon nanotubes 125c (CNT), and a binder 125d.

[0090] The Si-based particles 124b and the Si-based particles 125b are SiO. The weight fraction of the Si-based particles 124b and the Si-based particles 125b is calculated based on the weight of SiO.

[0091] The graphite particles 124a and the graphite particles 125a are each given a prescribed compressive load.

[0092] The binder 124d and the binder 125d are a mixture of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and carboxymethyl cellulose (CMC).

[0093] Examples 1 to 7 and Comparative Example 1 in which the weight fraction of SiO contained in the second negative electrode active material layer is large, and the like are summarized in Table 1. Here, the weight fraction is expressed in percentage as the proportion of graphite particles and Si-based particles, CNT, a binder, and the like that constitute the negative electrode active material layer. In other words, the weight fraction is expressed in percentage as the proportion of graphite particles, Si-based particles, CNT, a binder, and the like with respect to the total weight of the positive electrode active material layer. The unit of the weight fraction is "wt%" (weight percent). In addition, the particle diameter is the average particle diameter of primary particles.

[0094] Table 1

[0095]

[0096] In this table, the weight fraction of SiO, CNT and binder as an example of Si-based particles, and the compression load required to make the density of graphite corresponding to the hardness of graphite particles 1.7 g / cc, the weight fraction of acetylene black added, and the percentage of (3C charge rate) / (0.2C charge rate) as a charge rate ratio are shown. In addition, since the binder is a mixture of three kinds, the weight fraction of the mixture is described in the upper row, and the weight fraction of each of SBR, PAA and CMC constituting the mixture is described below.

[0097] Specifically, the weight fraction of SiO is 25 to 35 wt% in the upper layer and 5 to 15 wt% in the lower layer. The weight fraction of CNT is 0.10 to 0.17 wt% in the upper layer and 0.03 to 0.10 wt% in the lower layer. The weight fraction of the binder is 5 to 8 wt% in the upper layer and 2 to 5 wt% in the lower layer. The compression load of graphite is 1.5 to 2.5 kN / cm 2 in the upper layer and 2.5 to 3.5 kN / cm 2 in the lower layer. The weight fraction of acetylene black is 2.0 to 3.5 wt% in the upper layer and 0.5 to 2.0 wt% in the lower layer.

[0098] Table 2 shows Examples 8 to 14 and Comparative Example 2 in which the weight fraction of SiO contained in the second negative electrode active material layer and the like are compared.

[0099] Table 2

[0100]

[0101] In this table, the weight fraction of SiO, CNT and binder as an example of Si-based particles, and the compression load required to make the density of graphite corresponding to the hardness of graphite particles 1.7 g / cc, the weight fraction of acetylene black added, and the percentage of (3C charge rate) / (0.2C charge rate) as a charge rate ratio are shown.

[0102] Specifically, the weight fraction of SiO is 25 to 35 wt% in the upper layer and 5 to 15 wt% in the lower layer. The weight fraction of CNT is 0.10 to 0.17 wt% in the upper layer and 0.03 to 0.10 wt% in the lower layer. The weight fraction of the binder is 5 to 8 wt% in the upper layer and 2 to 5 wt% in the lower layer. The compression load of graphite is 1.5 to 2.5 kN / cm 2 in the upper layer and 2.5 to 3.5 kN / cm 2 in the lower layer. The weight fraction of acetylene black is 2.0 to 3.5 wt% in the upper layer and 0.5 to 2.0 wt% in the lower layer.

[0103] (Other Embodiments)

[0104] The constitution of the negative electrode and the lithium ion secondary battery of the present application is not limited to the constitution of the lithium ion secondary battery described in the embodiments, and can be applied to batteries having various shapes and constitutions.

[0105] The embodiments are described in detail or simplified for the purpose of clearly describing the present application, and do not necessarily include all the described constitutions, or can include constitutions not illustrated. In addition, a part of the constitution of the embodiments can be deleted, replaced with the constitution of other embodiments, or combined with the constitution of other embodiments.

[0106] The lithium ion secondary battery of the present application can be applied to a driving motor for an electric automobile, a power source for a portable electronic device such as a smart phone, and a charging device for a stationary power generation device.

[0107] The negative electrode is not limited to the constitution in which the negative electrode tab is provided to protrude from the negative electrode current collecting portion. For example, the negative electrode can be a constitution in which the end portion of the wound negative electrode current collecting portion is made to be in conduction with the negative electrode terminal via the negative electrode current collecting plate. In addition, the negative electrode can be a constitution in which the end portion of the wound negative electrode current collecting portion is made to be in conduction with the negative electrode terminal.

[0108] The charge-discharge body is not limited to Figure 2 The charge-discharge body illustrated in the drawing is a wound type in which the long strip-shaped positive electrode and the long strip-shaped negative electrode are wound with the long strip-shaped separator therebetween. For example, the charge-discharge body can be a stacked type in which a plurality of positive electrodes, a separator, and a negative electrode each formed in a rectangular shape are stacked. In addition, the charge-discharge body can be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes each formed in a relatively short strip shape are opposed and alternately arranged with a separator therebetween with respect to one piece of the separator formed in a long strip shape. The charge-discharge body of such a constitution is made to be in conduction with the positive electrode and the negative electrode with the separator therebetween by folding and stacking the separator.

[0109] The lithium ion secondary battery of the present application is not limited to the constitution in which the charge-discharge body is sealed by the case and the lid. For example, the lithium ion secondary battery can be constituted by sealing the charge-discharge body with a laminated film.

[0110] The separator that insulates the positive electrode and the negative electrode can be constituted by an insulating member stacked on the electrode. The insulating member is constituted by being joined to the positive electrode or the negative electrode. The insulating member preferably has heat resistance. In this case, the insulating member is, for example, ceramic. Such a constitution corresponds to a so-called separatorless constitution. In addition to the separator, the constitution can be a constitution using the insulating member, that is, a constitution using both the separator and the insulating member.

[0111] Hereinafter, the effects of the lithium ion secondary battery of the present application will be described.

[0112] The silicon oxide-based active material used in the negative electrode of the lithium ion secondary battery sometimes swells and shrinks with charge and discharge, and sometimes generates fine cracks in the negative electrode active material layer. Due to the fine cracks, the electrolyte sometimes easily penetrates. In this case, when carbon nanotubes are mixed as a constituent element of the negative electrode active material layer, sometimes the swelling and shrinking of the negative electrode active material layer is inhibited. When the swelling and shrinking of the negative electrode active material layer is inhibited, the generation of fine cracks is inhibited, and the penetration of the electrolyte is inhibited. If the generation of fine cracks is thus inhibited, there is a case where the charge and discharge characteristics are lowered as a result.

[0113] According to the present application, in the negative electrode used in the lithium ion secondary battery, the generation of fine cracks of the negative electrode active material layer that hinders the swelling and shrinking of particles and the like containing Si or SiO x (0 < x < 2) can be inhibited. Thereby, the charge and discharge characteristics of the lithium ion secondary battery can be improved.

[0114] Explanation of symbols

[0115] 1: lithium ion secondary battery, 100: charge and discharge body, 110: positive electrode, 111: positive electrode current collecting layer, 111b: positive electrode tab, 112: first positive electrode active material layer, 112a, 112b, 113a: positive electrode active material, 112d, 113c: positive electrode binder, 113: second positive electrode active material layer, 120: negative electrode, 121: negative electrode current collecting layer, 121b: negative electrode tab, 122, 124: first negative electrode active material layer, 122a, 123a, 124a, 125a: graphite particle, 122b, 123b, 124b, 125b: Si-based particle, 122c, 123c, 124c, 125c: carbon nanotube, 122d, 123d, 124d, 125d: binder, 123: second negative electrode active material layer, 130: separator, 201: case, 202: lid, 301: positive electrode terminal, 302: negative electrode terminal.

Claims

1. A lithium-ion secondary battery, characterized in that, It has positive and negative electrodes. The negative electrode has a negative electrode current collector and a layer of negative electrode active material stacked on the negative electrode current collector. The negative electrode active material layer includes a first negative electrode active material layer stacked on the negative electrode current collector and a second negative electrode active material layer stacked on the first negative electrode active material layer. The first negative electrode active material layer and the second negative electrode active material layer contain Si-based particles. The weight fraction of Si-based particles in the second negative electrode active material layer is greater than that in the first negative electrode active material layer. The negative electrode active material layer has at least one of the following configurations (1) to (4). Composition (1): The first negative electrode active material layer and the second negative electrode active material layer contain carbon nanotubes, and the weight fraction of the carbon nanotubes in the second negative electrode active material layer is greater than the weight fraction of the carbon nanotubes in the first negative electrode active material layer; Composition (2): The first negative electrode active material layer and the second negative electrode active material layer contain a binder, wherein the weight fraction of the binder in the second negative electrode active material layer is greater than the weight fraction of the binder in the first negative electrode active material layer; Composition (3): The first negative electrode active material layer and the second negative electrode active material layer contain amorphous carbon, and the weight fraction of the amorphous carbon in the second negative electrode active material layer is greater than the weight fraction of the amorphous carbon in the first negative electrode active material layer; Composition (4): The first negative electrode active material layer and the second negative electrode active material layer contain graphite as the main component, and the hardness of the graphite in the first negative electrode active material layer is greater than that of the graphite in the second negative electrode active material layer.

2. The lithium-ion secondary battery as described in claim 1, characterized in that, The Si-based particles contain Si or SiO x wherein 0 < x < 2.

3. A lithium-ion secondary battery, characterized in that, It has a negative electrode current collector and a layer of negative electrode active material stacked on the negative electrode current collector. The negative electrode active material layer includes a first negative electrode active material layer stacked on the negative electrode current collector and a second negative electrode active material layer stacked on the first negative electrode active material layer. The first negative electrode active material layer and the second negative electrode active material layer contain Si-based particles. The weight fraction of Si-based particles in the second negative electrode active material layer is smaller than that in the first negative electrode active material layer. The negative electrode active material layer has at least one of the following configurations (5) to (8). Composition (5): The first negative electrode active material layer and the second negative electrode active material layer contain carbon nanotubes, and the weight fraction of the carbon nanotubes in the second negative electrode active material layer is smaller than the weight fraction of the carbon nanotubes in the first negative electrode active material layer. Composition (6): The first negative electrode active material layer and the second negative electrode active material layer contain a binder, wherein the weight fraction of the binder in the second negative electrode active material layer is smaller than the weight fraction of the binder in the first negative electrode active material layer; Configuration (7): the first negative electrode active material layer and the second negative electrode active material layer contain amorphous carbon, and the weight fraction of the amorphous carbon of the second negative electrode active material layer is smaller than the weight fraction of the amorphous carbon of the first negative electrode active material layer; Configuration (8): the first negative electrode active material layer and the second negative electrode active material layer contain graphite as a main component, and the hardness of the graphite of the second negative electrode active material layer is greater than the hardness of the graphite of the first negative electrode active material layer.

4. The lithium-ion secondary battery according to claim 3, wherein The Si-based particles contain Si or SiO x wherein 0 < x < 2.

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