secondary batteries

Abstract

To provide a secondary battery comprising a negative electrode including a Si-based materia and hard carbon as an electrode active material, in which the cycle characteristic and the energy density are improved.SOLUTION: A secondary battery disclosed herein comprises an electrode body having a positive electrode and a negative electrode 60. The negative electrode 60 comprises a negative electrode current collector 62, and a negative electrode active material layer 64 disposed on the negative electrode current collector. The negative electrode active material layer 64 includes Si-containing particles 66 and hard carbon 68 as a negative electrode active material. The Si-containing particle 66 is a porous body containing Si nanoparticles 66a with a mesh structure. In the secondary battery, a ratio (D1 / D2) of an average particle diameter D1 of the Si-containing particles 66 to an average particle diameter D2 of the hard carbon 68 is 0.1 to 0.7. A weight ratio between the Si-containing particles 66 and the hard carbon 68 is 15:85 to 55:45.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc. The negative electrodes used in such secondary batteries generally have a configuration in which a negative electrode active material layer containing a negative electrode active material is disposed on a negative electrode current collector.

[0003] In recent years, the use of Si-based materials as negative electrode active materials has been investigated with the aim of increasing the capacity of secondary batteries (e.g., Patent Documents 1 to 3). Patent Document 1 discloses a negative electrode containing a Si-based material-containing negative electrode active material, carbon nanotubes with an outermost diameter of 5 nm or less, and carboxymethyl cellulose with a weight-average molecular weight of 150,000 to 450,000. Patent Document 2 discloses a silicon material whose precursor is amorphous silica produced from plant-derived raw materials. Patent Document 3 discloses a non-graphitizable carbonaceous material for use in a fully charged secondary battery, the non-graphitizable carbonaceous material having an oxygen element content of 0.25% by mass. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-38114 [Patent Document 2] International Publication No. 2022 / 070895 [Patent Document 3] Patent No. 6910296 Summary of the Invention [Problem to be solved by the invention]

[0005] Although Si-based materials have a larger specific capacity than carbon materials such as graphite particles, they also tend to expand and contract significantly during charging and discharging, making the conductive paths more susceptible to breakage. Therefore, while the use of Si-based materials can increase the capacity, the cycle characteristics of the secondary battery are likely to deteriorate. The inventors' research has revealed that when Si-based materials with many voids (pores) are used to mitigate expansion and contraction, the apparent density of the negative electrode active material layer (so-called negative electrode density) tends to decrease, resulting in a decrease in energy density per volume.

[0006] The present invention has been made in view of the above points, and aims to provide a secondary battery having a negative electrode containing a Si-based material and hard carbon as negative electrode active materials, and having improved cycle characteristics and energy density. [Means for solving the problem]

[0007] The secondary battery disclosed herein is a secondary battery including an electrode assembly having a positive electrode and a negative electrode. The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer contains Si-containing particles and hard carbon as negative electrode active materials. The Si-containing particles are porous bodies containing Si nanoparticles with a network structure. In the secondary battery, the ratio (D1 / D2) of the average particle diameter D1 of the Si-containing particles to the average particle diameter D2 of the hard carbon is 0.1 to 0.7, and the weight ratio of the Si-containing particles to the hard carbon is 15:85 to 55:45.

[0008] By using Si-containing particles, which are porous bodies containing Si nanoparticles with a network structure, and hard carbon, expansion and contraction can be suppressed, thereby improving the cycle characteristics of the secondary battery. Furthermore, by adjusting the ratio of the average particle size of the Si-containing particles to the average particle size of the hard carbon and the weight ratio of the Si-containing particles to the hard carbon, the negative electrode density can be improved, thereby improving the energy density per volume. Therefore, this configuration can achieve improvements in the cycle characteristics and energy density of the secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery according to one embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly according to one embodiment. [Figure 3] FIG. 3 is a diagram schematically illustrating a negative electrode according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. Matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification (e.g., the general configuration and manufacturing process of a secondary battery that do not characterize the technology disclosed herein), can be understood as design matters for a person skilled in the art based on conventional technology in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Each drawing is a schematic representation, and dimensional relationships (e.g., length, width, thickness) do not necessarily reflect actual dimensional relationships. In the drawings described below, components and parts that perform the same function are designated by the same reference numerals, and redundant descriptions may be omitted or simplified. In this specification, the notation "A to B" (A and B are arbitrary numbers) indicating a range means A or greater and B or less.

[0011] In this specification, the term "secondary battery" refers to a battery that can be repeatedly charged and discharged by the movement of charge carriers between a positive electrode and a negative electrode. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the movement of charge associated with the lithium ions between the positive and negative electrodes.

[0012] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery 100 according to one embodiment. As shown in FIG. 1, the secondary battery 100 includes an electrode assembly 20 having a positive electrode 50 and a negative electrode 60, an electrolyte (not shown), and a battery case 30 that accommodates the electrode assembly 20 and the electrolyte. The secondary battery 100 shown in FIG. 1 is a lithium-ion secondary battery. The negative electrode 60 disclosed herein is preferably used as a negative electrode for a lithium-ion secondary battery.

[0013] The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is configured to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting a non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0014] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly 20. Here, the electrode assembly 20 is a flat-shaped wound electrode assembly. As shown in FIG. 2, the electrode assembly 20 has a configuration in which a long sheet-shaped positive electrode 50 (hereinafter also referred to as "positive electrode sheet 50") and a long sheet-shaped negative electrode 60 (hereinafter also referred to as "negative electrode sheet 60") are stacked together with two long separators 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. 1 and 2, the positive electrode current collector exposed portion 52a (i.e., a portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54) and the negative electrode current collector exposed portion 62a (i.e., a portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the electrode body 20. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively.

[0015] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium-ion secondary batteries, and is not particularly limited. For example, a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.) can be used. Aluminum foil is preferred as the positive electrode current collector 52. The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0016] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a positive electrode active material of a known composition used in lithium ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds (e.g., lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4)). The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0017] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more. Among them, lithium nickel cobalt manganese composite oxide is preferably used as the positive electrode active material.

[0018] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as Y / Z, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0019] Although not particularly limited, it is preferable to use a positive electrode active material with a high Ni content and a low Co content from the viewpoint of increasing the capacity of the secondary battery 100 and reducing CO2 emissions during the production of the secondary battery 100. For example, it is preferable to use a composite oxide containing at least Li and Ni, in which the Ni content is 70 mol % to 100 mol % (more preferably 70 mol % to 90 mol %) and the Co content is 5 mol % or less (more preferably 3 mol % or less) relative to the total (total number of moles) of metal elements excluding Li.

[0020] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF).

[0021] Although not particularly limited, the proportion of the conductive material is preferably 0.1 to 10 parts by weight, and more preferably 1 to 5 parts by weight, based on 100 parts by weight of the positive electrode active material, and the proportion of the binder is preferably 0.1 to 10 parts by weight, and more preferably 1 to 5 parts by weight, based on 100 parts by weight of the positive electrode active material.

[0022] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

[0023] As the separator 70, various conventional microporous sheets can be used, such as microporous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such microporous resin sheets may have a single-layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). Separator 70 may also have a heat-resistant layer (HRL).

[0024] Conventional electrolytes can be used, for example, a nonaqueous electrolyte solution containing a supporting salt in an organic solvent (nonaqueous solvent). Examples of nonaqueous solvents that can be used include aprotic solvents such as carbonates, esters, and ethers. Among these, carbonates, such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), are particularly suitable. Alternatively, fluorine-based solvents such as fluorinated carbonates, such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC), can be preferably used. These nonaqueous solvents can be used alone or in appropriate combinations of two or more. Furthermore, although not particularly limited, from the viewpoint of reducing CO2 emissions during the production of the secondary battery 100, it is preferable to use CO2-derived DMC or EC. As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, LiClO4, etc. The concentration of the supporting salt is not particularly limited, but is preferably about 0.7 mol / L or more and 1.3 mol / L or less. The nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, as long as the effects of the present technology are not significantly impaired. For example, the nonaqueous electrolyte may contain various additives such as a gas generating agent, a film-forming agent, a dispersant, and a thickener.

[0025] The negative electrode 60 of the secondary battery disclosed herein will now be described. FIG. 3 is a schematic diagram illustrating the negative electrode 60 of the secondary battery 100 disclosed herein. As shown in FIG. 3, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 disposed on the negative electrode current collector 62. The negative electrode current collector 62 may be a conventionally known material and is not particularly limited. Examples include a sheet or foil made of a metal such as copper, nickel, titanium, or stainless steel. When copper foil is used as the negative electrode current collector 62, its average thickness is not particularly limited, but is, for example, 5 μm to 30 μm, preferably 5 μm to 20 μm, and more preferably 5 μm to 15 μm. Although not particularly limited, from the viewpoint of reducing the burden on the global environment, it is preferable to use copper foil recycled from scrap copper or the like as the negative electrode current collector 62.

[0026] The negative electrode active material layer 64 contains at least a negative electrode active material. The negative electrode active material layer 64 contains at least Si-containing particles 66 and hard carbon 68 as the negative electrode active material. The average particle diameter (D 50 Particle diameter)D1 and the average particle diameter of Hard Carbon 68 (D 50 By adjusting the ratio of the Si-containing particles 66 to the particle diameter D2 and the weight ratio of the Si-containing particles 66 to the hard carbon 68, the cycle characteristics and energy density of the secondary battery 100 can be improved.

[0027] The Si-containing particles 66 are porous bodies containing Si nanoparticles 66a with a network structure. The Si-containing particles 66 may contain components other than Si, as long as they contain Si. Examples of the Si-containing particles 66 include SiOx, Si-C composites, and porous Si particles with Si nanoparticles dispersed therein. The porous portion of the Si-containing particles 66 may be composed primarily of Si or C (carbon). For example, the Si-containing particles 66 are preferably composed of Si-C composites containing Si nanoparticles with a network structure and porous carbon particles. Alternatively, the Si-containing particles 66 are preferably composed of Si particles containing Si nanoparticles with a network structure and porous Si particles. In this specification, the phrase "A is mainly composed of B" means that, among the components constituting A, B is the largest component by weight.

[0028] The Si-containing particles 66 are porous bodies having a plurality of pores 66b. The Si-containing particles 66 may have, for example, micropores, mesopores, and macropores. Here, micropores are pores with a diameter of 2 nm or less, mesopores are pores with a diameter of more than 2 nm but less than 50 nm, and macropores are pores with a diameter of 50 nm or more. If the pore size is too large, there is a risk that the cycle characteristics of the secondary battery 100 will deteriorate due to corrosion by the electrolyte. From this perspective, the pores 66b of the Si-containing particles 66 are preferably, for example, 1 nm or more and 300 nm or less, and may be, for example, 1 nm or more and 250 nm or less. The Si-containing particles 66 may have, for example, a nanoporous structure having a nano-sized porous structure.

[0029] Although not particularly limited, it is preferable that the Si-containing particles 66 have a plurality of pores 66b with a relatively small diameter. This can mitigate expansion and contraction of the Si-containing particles 66 during charge and discharge, thereby improving the cycle characteristics of the secondary battery 100. Furthermore, even when pressed during the preparation of the negative electrode 60, the Si-containing particles 66 have a property of being resistant to crushing due to the presence of a plurality of relatively small pores 66b. In other words, the Si-containing particles 66 have a certain degree of durability due to the presence of a plurality of such pores 66b. The Si-containing particles 66 preferably have a nanoporous structure in which the number of 10 nm diameter pores is greater than the number of 100 nm diameter pores. Specifically, the Si-containing particles 66 have a log differential pore volume V of 100 nm diameter pores. 100 Log differential pore volume V of pores with a diameter of 10 nm 10 Ratio of (V 10 / V 100 ) should be adjusted to be 1 or more. 100 V against 10 Ratio of (V 10 / V 100 ) is preferably greater than 1, more preferably 1.2 or more, and may be 1.5 or more. 100 V against 10 Ratio of (V 10 / V 100 ) is preferably, for example, 20 or less, and may be 10 or less.

[0030] Log differential pore volume V of a pore with a diameter of 100 nm 100 and the log differential pore volume V of a pore with a diameter of 10 nm 10 can be calculated based on the BJH method using a specific surface area and pore size distribution measurement device. First, Si-containing particles are heated and dried under vacuum to prepare a measurement sample. Next, an adsorption isotherm of the measurement sample is obtained using liquid nitrogen as a refrigerant and nitrogen gas (N2 gas) as the adsorption gas. The obtained adsorption isotherm is analyzed by the BJH method to determine the log differential pore volume distribution. Then, from the log differential pore volume distribution, the log differential pore volume V of pores with a diameter of 100 nm is calculated.100 and the log differential pore volume V of a pore with a diameter of 10 nm 10 It is possible to find:

[0031] The Si-containing particles 66 have Si nanoparticles 66a with a mesh-like structure. The Si nanoparticles 66a are nano-sized (i.e., less than 1 μm) Si particles. The Si nanoparticles 66a may be present on the surface of the porous body and / or inside the pores 66b of the porous body. The Si nanoparticles 66a are preferably 100 nm or less, more preferably 50 nm or less. This reduces the amount of expansion and contraction per Si nanoparticle 66a during charge and discharge, making them less likely to break even with repeated expansion and contraction. Furthermore, although not particularly limited, the average particle diameter of the Si nanoparticles 66a may be, for example, 5 nm or more. Note that, in this specification, the "average particle diameter of Si nanoparticles" can be determined as follows. First, a sample for observation with a scanning transmission electron microscope (STEM) is prepared by FIB (focused ion beam) processing of the negative electrode active material layer. Then, the sample is subjected to elemental analysis by EDX element mapping, and a BF image (bright-field image) and a HAADF image (high-angle annular dark-field image) are acquired. The diameter of the Si nanoparticles can be determined from the contrast and shape obtained from the BF and HAADF images. The arithmetic mean of the diameters of at least 10 Si nanoparticles is defined as the "average particle size of Si nanoparticles" here.

[0032] The Si nanoparticles 66a have a mesh-like structure. In this mesh-like structure, a plurality of voids are formed randomly or regularly. The mesh-like structure of the Si nanoparticles 66a favorably improves the conductive path. Furthermore, the mesh-like structure of the Si nanoparticles 66a prevents the Si nanoparticles 66a from excessively expanding and contracting during charging and discharging.

[0033] Although not particularly limited, it is preferable that the Si-containing particles 66 have a plurality of pores 66b around the above-mentioned Si nanoparticles 66a. In particular, it is preferable that many small-diameter pores (e.g., pores with a diameter of 10 nm or less) exist around the Si nanoparticles 66a. This can mitigate expansion and contraction caused by charge and discharge while suitably suppressing erosion of the electrolyte.

[0034] Although not particularly limited, the oxygen content of the Si-containing particles is preferably, for example, 10 wt% or less when the entire Si-containing particles are taken as 100 wt%. This can reduce side reactions caused by excessive oxygen content, and can suitably improve the capacity and cycle characteristics of secondary batteries. The oxygen content can be measured by heating and melting the particles in an inert gas using an oxygen analyzer.

[0035] The above-mentioned Si-containing particles 66 can be obtained, for example, by firing a plant containing Si. That is, the Si-containing particles 66 are preferably derived from a plant. Specifically, plants such as rice husks (rice), barley, wheat, rye, etc., coconut husks, tea leaves, sugarcane, and corn are preferably used as raw materials. Among these, rice husks are preferably used as raw materials for the Si-containing particles 66. However, the Si-containing particles 66 may also be prepared by preparing a porous body mainly composed of Si or C and Si nanoparticles having a network structure, and then introducing the Si nanoparticles into the porous body.

[0036] When the Si-containing particles 66 are derived from plants, they tend to have more fine pores (voids) 66b than Si-containing particles that are not derived from plants. The amount of fine pores and the particle size of the Si nanoparticles 66a can be adjusted by appropriately changing the conditions for firing the plant. By using plant-derived Si-containing particles 66, Si-containing particles 66 with low expansion and high durability can be achieved. The plant-derived Si-containing particles 66 can further improve the cycle characteristics of the secondary battery 100. Furthermore, plants accumulate silicic acid absorbed from the soil around their cell walls. By firing this, a porous body containing Si nanoparticles 66a with a plant-derived network structure can be obtained.

[0037] Hard carbon 68, also known as non-graphitizable carbon, is solid carbon that does not change into graphite (graphite) through heat treatment, even at high temperatures of, for example, 3000°C or higher. Hard carbon 68 has smaller crystallites than graphite. Furthermore, hard carbon 68 has a stacked structure of several layers of graphene and a turbostratic structure. Hard carbon 68 has a structure with more pores than graphite. Therefore, hard carbon 68 can mitigate expansion and contraction associated with charge and discharge, thereby improving the cycle characteristics of the secondary battery 100. Furthermore, hard carbon 68 has many points where ions can be inserted and removed, which tends to improve the input / output characteristics of the secondary battery 100.

[0038] The hard carbon 68 of the secondary battery 100 disclosed herein preferably has a lattice spacing (d002) of 0.37 nm to 0.39 nm in the d(002) plane as determined by X-ray diffraction (XRD). This allows for rapid ion intercalation and deintercalation, improving the input / output characteristics of the secondary battery 100. Furthermore, since the hard carbon 68 is less rigid than graphite, it tends to have higher durability against expansion and contraction.

[0039] Although not particularly limited, the true density of hard carbon 68 based on the butanol method is 1.4 g / cm 3 ~1.7g / cm 3It is preferable that the density is 1.4 g / cm 3 ~1.6g / cm 3 By having a true density in this range, low expansion and high durability can be achieved.

[0040] The hard carbon 68 described above can be prepared by acid-treating a carbonaceous precursor and then heat-treating it. By appropriately changing the conditions of the acid treatment, it is possible to produce hard carbon 68 with even higher capacity. Here, the carbonaceous precursor may be a thermoplastic resin such as a phenolic resin, or may be derived from a plant. Preferably, the hard carbon 68 is prepared from a plant-derived carbonaceous precursor. Plant-derived carbonaceous precursors may be made from plants such as coconut shells, tea leaves, coffee beans, and sugarcane. Of these, coconut shells are the preferred raw material for the hard carbon 68.

[0041] When the hard carbon 68 is plant-derived, it has a higher capacity than general graphite materials. Furthermore, plant-derived hard carbon tends to have a lower true density than general graphite materials. Plant-derived hard carbon has low expansion during charging and discharging, is highly durable, and its conductive path is less likely to be broken. In other words, by using plant-derived hard carbon 68, a secondary battery 100 with high capacity and improved cycle characteristics can be realized.

[0042] In the secondary battery 100 disclosed herein, as described above, the Si-containing particles 66, which are porous bodies containing Si nanoparticles 66a with a network structure, and the hard carbon 68, which has more pores than graphite, are used, thereby improving the cycle characteristics of the secondary battery 100. On the other hand, since these materials have a relatively large number of pores (voids), the density of the negative electrode 60 tends to be low when pressed. Therefore, in the secondary battery 100 disclosed herein, the average particle diameter (D 50 High energy density can be achieved by adjusting the particle size ratio and weight ratio within a specified range.

[0043] In the secondary battery 100 disclosed herein, the ratio (D1 / D2) of the average particle diameter D1 of the Si-containing particles 66 to the average particle diameter D2 of the hard carbon 68 is 0.1 to 0.7. The ratio (D1 / D2) of the average particle diameter D1 to the average particle diameter D2 is preferably 0.15 to 0.65, and more preferably 0.17 to 0.60. By adjusting the ratio of the average particle diameters of the Si-containing particles 66 and the hard carbon 68 within the above range, each material is suitably arranged when the negative electrode 60 is pressed, thereby improving the energy density.

[0044] The average particle diameter of the Si-containing particles 66 (D 50 The particle diameter (D1) of the hard carbon 68 is not particularly limited as long as it satisfies the above-mentioned (D1 / D2). For example, the average particle diameter (D1) of the Si-containing particles 66 is preferably 2 μm or more and 10 μm or less, and more preferably 2 μm or more and 7 μm or less. 50 The particle diameter (D2) is not particularly limited as long as it satisfies the above-mentioned (D1 / D2). For example, the average particle diameter D2 of the hard carbon 68 is preferably 10 μm or more and 25 μm or less, and more preferably 12 μm or more and 20 μm or less. In this specification, the "average particle diameter of Si-containing particles" and the "average particle diameter of hard carbon" refer to the particle diameter (D2) corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on the laser diffraction / light scattering method. 50 This refers to the particle size.

[0045] In the secondary battery 100 disclosed herein, the weight ratio of the Si-containing particles 66 to the hard carbon 68 is 15:85 to 55:45. From the viewpoint of further improving the cycle characteristics, it is preferable to increase the content of the hard carbon 68. For example, it is more preferable that the weight ratio of the Si-containing particles 66 to the hard carbon 68 is 25:75 to 45:55. This realizes a secondary battery 100 that achieves both improved cycle characteristics and improved energy density.

[0046] A high apparent density of the negative electrode active material layer 64 (hereinafter also simply referred to as "negative electrode density") increases the battery capacity per volume. In other words, a high negative electrode density improves the volumetric energy density. From this perspective, the negative electrode density is set to 1.5 g / cm 3 It is preferable that the concentration is 1.55 g / cm or more. 3 More preferably, it is 1.58 g / cm or more. 3 The upper limit of the negative electrode density is not particularly limited, but is, for example, 2.5 g / cm 3 By adjusting the ratio of the average particle diameters of the Si-containing particles 66 and the hard carbon 68 and the weight ratio, such a negative electrode density can be achieved even for the negative electrode active material layer 64 containing the Si-containing particles 66 and the hard carbon 68 as described above. The apparent density of the negative electrode active material layer 64 is calculated by the apparent volume (cm) of the negative electrode active material layer 64 including the voids. 3 The weight (g) of the negative electrode active material layer 64 is the ratio of the weight (g) of the negative electrode active material layer 64 to the weight (g) of the negative electrode active material layer 64. For example, by measuring the weight per unit area of ​​the negative electrode active material layer 64 and the thickness of the negative electrode active material layer 64, the weight per unit area of ​​the negative electrode active material layer 64 can be easily calculated as ((weight per unit area of ​​the negative electrode active material layer 64) / (thickness of the negative electrode active material layer 64)).

[0047] The negative electrode active material layer 64 may contain, as the negative electrode active material, a material other than the above-described Si-containing particles 66 and hard carbon 68 (for example, SiOx or graphite particles not having the above-described structure) as long as the effect of the present technology is not significantly impaired.

[0048] Although not particularly limited, when the total weight of the negative electrode active material (the total weight of the Si-containing particles 66, the hard carbon 68, and other components that may be included as the negative electrode active material) is taken as 100 wt%, the content of the Si-containing particles 66 is preferably 10 wt% to 60 wt%, more preferably 10 wt% to 45 wt%, and even more preferably 25 wt% to 45 wt%. Furthermore, when the total weight of the negative electrode active material is taken as 100 wt%, the content of the hard carbon 68 is preferably 40 wt% to 90 wt%, more preferably 65 wt% to 90 wt%, and even more preferably 65 wt% to 75 wt%. By adjusting the contents of the Si-containing particles 66 and the hard carbon 68 to these values, improved cycle characteristics and improved energy density are achieved.

[0049] As described above, the secondary battery 100 disclosed herein preferably uses plant-derived Si-containing particles 66 and plant-derived hard carbon 68. By adjusting the average particle size ratio and weight ratio of the plant-derived Si-containing particles 66 and the plant-derived hard carbon 68, the cycle characteristics and energy density of the secondary battery 100 can be improved. Furthermore, compared to non-plant-derived Si-containing particles, plant-derived Si-containing particles can be produced at lower temperatures and with lower power consumption, thereby reducing CO2 emissions. Furthermore, compared to non-plant-derived hard carbon, plant-derived hard carbon can reduce the CO2 emission unit cost of the raw material. This allows for the realization of a secondary battery 100 with improved cycle characteristics and high energy density while reducing CO2 emissions from the material.

[0050] The negative electrode active material layer 64 may contain components other than the above-described negative electrode active material (Si-containing particles 66 and hard carbon 68), such as a conductive material, a binder, etc. A conventionally known conductive material can be used. Examples of the conductive material that can be used include carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), carbon black such as acetylene black (AB), and carbon fibers. Among these, carbon nanotubes are preferred, and single-walled carbon nanotubes are more preferred. By using carbon nanotubes as the conductive material, the conductive path can be more suitably maintained, and the cycle characteristics of the secondary battery 100 can be more suitably improved.

[0051] The proportion of the conductive material may be, for example, 0.01 parts by weight or more, such as 0.05 parts by weight or more, or 2 parts by weight or less, such as 1 part by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less, based on 100 parts by weight of the negative electrode active material.

[0052] As the binder, conventionally known binders can be used. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), etc. Among them, CMC, PAA, and SBR can be preferably used. Although not particularly limited, it is more preferable to use CMC, PAA, and SBR in combination.

[0053] The total amount of binder is, for example, 1 part by weight or more, preferably 3 parts by weight or more, and more preferably 3.5 parts by weight or more, per 100 parts by weight of the negative electrode active material, and is 10 parts by weight or less, preferably 8 parts by weight or less, and more preferably 5 parts by weight or less, per 100 parts by weight of the negative electrode active material.

[0054] The thickness of the negative electrode active material layer 64 per side is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

[0055] Although not particularly limited, the proportion of the negative electrode active material in the entire negative electrode active material layer 64 is, for example, 80 mass % or more, preferably 90 mass % or more, and more preferably 95 mass % or more. Furthermore, although not particularly limited, the proportion of the negative electrode active material in the entire negative electrode active material layer 64 may be, for example, 98 mass % or less.

[0056] The negative electrode active material layer 64 can be formed by dispersing Si-containing particles 66 and hard carbon 68 as the negative electrode active material, and materials used as needed (for example, a conductive material or a binder), in an appropriate solvent (for example, water) to prepare a paste-like (or slurry-like) composition, applying the composition to the surface of the negative electrode current collector 62, and drying it. Thereafter, the thickness and density of the negative electrode active material layer 64 can be adjusted by pressing as needed.

[0057] The configuration of the negative electrode 60 and the secondary battery 100 according to one embodiment have been described above. The negative electrode 60 is suitable for use in nonaqueous electrolyte secondary batteries. The negative electrode 60 has high durability against expansion and contraction due to repeated charge and discharge, and the particle size ratio and weight ratio of the Si-containing particles 66 and the hard carbon 68 are adjusted to improve the negative electrode density. Therefore, a secondary battery 100 with improved cycle characteristics and volumetric energy density is realized. The secondary battery 100 can be used for various purposes, and is suitable, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. The type of vehicle is not particularly limited, and examples include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and an electric vehicle (BEV). The secondary battery 100 can also be suitable for use in a battery pack.

[0058] Furthermore, in the above-described secondary battery 100, a wound electrode body is exemplified as the electrode body 20, but this is not limited thereto, and the electrode body 20 may be, for example, a laminated electrode body, which is an electrode body in which a plurality of approximately rectangular positive electrodes and a plurality of approximately rectangular negative electrodes are alternately stacked with separators interposed therebetween.

[0059] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples.

[0060] Example 1 First, as the negative electrode active material, Si-containing particles (D 50 Particle size: 5 μm) and hard carbon (D 50The Si-containing particles of Example 1 were plant-derived Si-C composite particles made from rice husks. The hard carbon of Example 1 was plant-derived hard carbon made from coconut shells. Single-walled carbon nanotubes (SWCNTs) were prepared as the conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders. These were kneaded with water as a solvent in a weight ratio of hard carbon:Si-containing particles:SWCNT:CMC:PAA:SBR=65:35:0.1:1:1:2 to prepare a slurry for forming a negative electrode active material layer.

[0061] Specifically, the mixing and kneading of the slurry for forming the negative electrode active material layer was carried out as follows. First, Si-containing particles, hard carbon, CMC, and PAA were dry-mixed. Next, the dry-mixed mixed powder, a paste of SWCNT (solid content 2%), and a dispersion medium were kneaded together. The solid content during the kneading was 60%. SBR and a solvent (water) were further added to the kneaded mixture and mixed. In this way, the slurry for forming the negative electrode active material layer was prepared. This slurry was applied in strips to both sides of copper foil (thickness 10 μm). The slurry on the copper foil was then dried and pressed under a linear pressure of 1.1 kN / cm, and then processed to the specified dimensions to produce a negative electrode sheet.

[0062] Next, lithium nickel cobalt manganese composite oxide (NCM) was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and PVDF as the binder. These were mixed with N-methylpyrrolidone (NMP) as the solvent in a weight ratio of NCM:AB:PVDF = 100:1:1 to prepare a slurry for forming the positive electrode active material layer. This slurry was applied in strips to both sides of an aluminum foil (thickness 15 μm). The slurry on the aluminum foil was then dried, pressed to a predetermined thickness, and processed to the predetermined dimensions to produce a positive electrode sheet.

[0063] The negative electrode sheet and the positive electrode sheet prepared above were stacked with a separator interposed therebetween to produce a laminated electrode assembly. Current-collecting leads were attached to the positive and negative electrode plates, respectively, and the laminated electrode assembly was inserted into an exterior housing made of an aluminum laminate sheet. A nonaqueous electrolyte was poured into the interior of the exterior housing, and the opening of the exterior housing was sealed to produce a test battery of Example 1. A porous polyolefin sheet with a three-layer structure of PP / PE / PP was used as the separator. The nonaqueous electrolyte was prepared by dissolving LiPF6 as a supporting electrolyte at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC=15:5:40:40.

[0064] <Examples 2 and 3, Comparative Examples 1 and 2> Evaluation batteries of Examples 2 and 3 and Comparative Examples 1 and 2 were fabricated in the same manner as in Example 1, except that the weight ratio of the Si-containing particles to the hard carbon was changed as shown in Table 1.

[0065] <Examples 4 to 7, Comparative Example 5> The average particle size of Si-containing particles (D 50 particle diameter)D1 and the average particle diameter of hard carbon (D 50 The particle diameter D2 and the particle diameter D1 were changed as shown in Table 1. Except for this, the evaluation batteries of Examples 4 to 7 and Comparative Example 5 were fabricated in the same manner as in Example 1.

[0066] <Comparative Example 3> An evaluation battery of Comparative Example 3 was fabricated in the same manner as in Example 1, except that only Si-containing particles were used as the negative electrode active material (that is, no hard carbon was included).

[0067] <Comparative Example 4> An evaluation battery of Comparative Example 4 was fabricated in the same manner as in Example 1, except that only hard carbon was used as the negative electrode active material (that is, no Si-containing particles were included).

[0068] <Evaluation of negative electrode density> The negative electrode density was calculated when pressed at a linear pressure of 1.1 kN / cm. The negative electrode density was calculated by measuring the coating weight and thickness of the negative electrode active layer and calculating the formula: negative electrode density (g / cm 3 The results are shown in Table 1.

[0069] <Evaluation of volumetric energy density> The discharge capacity and average voltage were obtained during the first cycle, which consisted of CCCV charging (at a rate of 0.05C up to 4.2V, then cut off at 0.05C) at 25°C, followed by CC discharging (at a rate of 0.05C to 2.5V). The electric energy (Wh) was then calculated using the formula: electric energy (Wh) = (discharge capacity) × (average voltage). The electrode thickness and area were also measured, and the electrode volume (L) was calculated using the formula: electrode volume (L) = (electrode thickness) × (electrode area). The volumetric energy density (Wh / L) was then calculated using the formula: volumetric energy density (Wh / L) = (electric energy) / (electrode volume). The results are shown in Table 1.

[0070] <Evaluation of cycle capacity retention rate> A cycle test was conducted in which 250 charge-discharge cycles were repeated at 25°C, with CCCV charging (at a rate of 0.4C up to 4.2V, then cut off at 0.1C) followed by CC discharging (at a rate of 0.4C and cut off at 2.5V). The discharge capacity at the first cycle (initial capacity) and the discharge capacity at the 250th cycle were measured, and the cycle capacity retention was calculated using the formula: cycle capacity retention (%) = ((discharge capacity at the 250th cycle) / (discharge capacity at the first cycle)) × 100. The higher the cycle capacity retention, the better the cycle characteristics of the secondary battery. The results are shown in Table 1.

[0071] [Table 1]

[0072] As shown in Table 1, in the evaluation batteries of Examples 1 to 7, the negative electrode density was 1.58 g / cm 3The results show that the volumetric energy density is 780 Wh / L or more and the capacity retention rate is 80% or more. These results show that a secondary battery that combines high energy density with improved cycle characteristics can be realized by including Si-containing particles and hard carbon as the negative electrode active material, with the ratio (D1 / D2) of the average particle diameter D1 of the Si-containing particles to the average particle diameter D2 of the hard carbon being 0.1 to 0.7, and the weight ratio of the Si-containing particles to the hard carbon being 15:85 to 55:45.

[0073] It is also apparent that the negative electrode density and volume energy density are further improved when the ratio (D1 / D2) of the average particle diameter D1 of the Si-containing particles to the average particle diameter D2 of the hard carbon is 0.15 to 0.65.

[0074] Although several embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0075] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A secondary battery including an electrode assembly having a positive electrode and a negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer including Si-containing particles and hard carbon as negative electrode active materials, the Si-containing particles being a porous body containing Si nanoparticles in a network structure, a ratio (D1 / D2) of an average particle diameter D1 of the Si-containing particles to an average particle diameter D2 of the hard carbon being 0.1 to 0.7, and a weight ratio of the Si-containing particles to the hard carbon being 15:85 to 55:45. Item 2: The secondary battery according to Item 1, wherein the Si-containing particles and the hard carbon are derived from plants. Item 3: The secondary battery according to item 1 or 2, wherein the content of the Si-containing particles is 10 wt % to 60 wt % when the negative electrode active material is taken as 100 wt %. Item 4: The secondary battery according to any one of Items 1 to 3, wherein the Si-containing particles have an average particle size of 2 μm to 10 μm. Item 5: The secondary battery according to any one of Items 1 to 4, wherein the hard carbon has an average particle size of 10 μm to 25 μm. [Explanation of symbols]

[0076] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode (positive electrode sheet) 52 Positive electrode current collector 52a Exposed part of positive electrode current collector 54 Cathode active material layer 60 Negative electrode (negative electrode sheet) 62 Negative electrode current collector 62a Exposed part of negative electrode current collector 64 Negative electrode active material layer 66 Si-containing particles 66a Si nanoparticles 66b Pore (void) 68 Hard Carbon 70 Separator 100 Secondary battery

Claims

1. A secondary battery including an electrode assembly having a positive electrode and a negative electrode, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer contains, as a negative electrode active material, Si-containing particles and hard carbon; The Si-containing particles contain a porous body and network-structured Si nanoparticles, the network-structured Si nanoparticles are arranged inside pores of the porous body, a ratio (D1 / D2) of an average particle diameter D1 of the Si-containing particles to an average particle diameter D2 of the hard carbon is 0.1 to 0.7; The secondary battery has a weight ratio of the Si-containing particles to the hard carbon of 15:85 to 55:

45.

2. 2. The secondary battery according to claim 1, wherein the content of said Si-containing particles is 10 wt % to 60 wt % when said negative electrode active material is taken as 100 wt %.

3. 3. The secondary battery according to claim 1, wherein the Si-containing particles have an average particle size of 2 μm to 10 μm.

4. 3. The secondary battery according to claim 1, wherein the hard carbon has an average particle size of 10 μm to 25 μm.

5. A secondary battery as described in claim 1 or 2, wherein the porous body of the Si-containing particles is composed primarily of Si or carbon.

6. The secondary battery of claim 1, wherein the weight ratio of the Si-containing particles to the hard carbon is 25:75 to 45:55.

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