Negative electrode for secondary battery, secondary battery, and method for manufacturing negative electrode for secondary battery

Abstract

To provide a negative electrode for a secondary battery in which liquid shortage hardly occurs.SOLUTION: The negative electrode for a secondary battery disclosed herein has a negative electrode current collector and a negative electrode active material layer fixed to the negative electrode current collector. The negative electrode active material layer includes graphite particles and Si-containing particles as a negative electrode active material, and void holding particles that are present more around the Si-containing particles than around the graphite particles and hold voids around the Si-containing particles.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a secondary battery, a secondary battery, and a method for producing a negative electrode for a secondary battery. [Background technology]

[0002] Conventionally, in order to realize a high-capacity secondary battery, a technique of using Si-containing particles as a negative electrode active material has been known (see Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-103052 [Patent Document 2] Patent No. 7121449 [Patent Document 3] Patent No. 5637257 [Patent Document 4] International Publication No. 2014 / 181447 Summary of the Invention [Problem to be solved by the invention]

[0004] However, Si-containing particles undergo a larger volume change (expansion / contraction) during charging and discharging than, for example, graphite particles, which are commonly used as negative electrode active materials. Therefore, according to the inventors' investigations, in negative electrodes containing Si-containing particles, the electrolyte tends to leak out when the Si-containing particles expand. Therefore, the above technology has a problem in that repeated charging and discharging can cause a shortage of electrolyte around the Si-containing particles, which is known as liquid starvation. This problem is particularly likely to occur in high-input / output secondary batteries installed in moving objects such as vehicles.

[0005] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide a negative electrode for a secondary battery which contains Si-containing particles and is resistant to liquid drying up. [Means for solving the problem]

[0006] The present invention provides a negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode active material layer fixed to the negative electrode current collector. The negative electrode active material layer contains graphite particles and Si-containing particles as negative electrode active materials, and void-retaining particles that are present in greater amounts around the Si-containing particles than around the graphite particles and retain voids around the Si-containing particles.

[0007] In the present invention, many void-retaining particles are present (distributed unevenly) around the Si-containing particles. As a result, even if the Si-containing particles expand and contract during charge and discharge, voids can be maintained around the Si-containing particles, and the voids can be kept impregnated with the electrolyte. Therefore, the liquid retention around the Si-containing particles is improved, and the occurrence of liquid depletion can be suppressed. As a result, a secondary battery can be realized in which the battery capacity is less likely to decrease even after repeated high-rate charge and discharge (excellent high-rate cycle characteristics). [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing one Si-containing particle and its surroundings. [Figure 2] FIG. 2 is a schematic vertical cross-sectional view of a secondary battery according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Preferred embodiments of the technology disclosed herein are described below. Matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein (for example, the general configuration and manufacturing process of a secondary battery that does not characterize the technology disclosed herein) can be understood as design matters for those skilled in the art based on prior art 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. Note that the expression "A to B" indicating a range in this specification means greater than or equal to A and less than or equal to B, and also encompasses the meanings of "preferably greater than A" and "preferably smaller than B."

[0010] [Negative electrode for secondary batteries] First, the negative electrode for a secondary battery disclosed herein will be described. In this specification, the term "secondary battery" refers to a general term for an electricity storage device capable of repeated charging and discharging, and is a concept that encompasses not only so-called secondary batteries such as lithium-ion secondary batteries and nickel-metal hydride secondary batteries, but also capacitors that utilize chemical reactions, such as lithium-ion capacitors and pseudo-capacitor capacitors. The negative electrode for a secondary battery disclosed herein is preferably a negative electrode for a nonaqueous electrolyte secondary battery, and more preferably a negative electrode for a lithium-ion secondary battery.

[0011] The negative electrode for a secondary battery disclosed herein has a negative electrode current collector and a negative electrode active material layer fixed to at least one surface of the negative electrode current collector. From the viewpoint of increasing capacity, the negative electrode active material layer is preferably provided on both surfaces of the negative electrode current collector. The negative electrode current collector is preferably made of a metal, more preferably a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The negative electrode current collector is preferably a metal foil, more preferably a copper foil or a copper alloy foil. Although not particularly limited, the thickness of the metal foil is preferably 5 to 35 μm, more preferably 6 to 20 μm.

[0012] The negative electrode active material layer contains at least (1) a negative electrode active material and (2) void-retaining particles. The negative electrode active material layer may further contain other optional components. In this embodiment, the negative electrode active material layer further contains (3) a conductive material and (4) a binder. The negative electrode active material layer preferably contains (3) a conductive material and (4) a binder. Each component will be described below in order.

[0013] (1) The negative electrode active material is made of a material capable of reversibly absorbing and releasing charge carriers. In this embodiment, the negative electrode active material essentially contains (1a) graphite particles and (1b) Si-containing particles. This allows the battery to have a high level of various characteristics (e.g., high capacity and high-rate cycle characteristics).

[0014] (1a) The graphite particles are materials with a smaller volume change (expansion and contraction amount) during charge and discharge compared to the (1b) Si-containing particles. The graphite particles are not particularly limited and may be natural graphite, artificial graphite, or amorphous carbon-coated graphite in which the core graphite particles are coated with an amorphous carbon material. Although not particularly limited, the average particle diameter of the graphite particles is preferably 1 to 100 μm, more preferably 5 to 50 μm, and even more preferably 10 to 30 μm. In this specification, the "average particle diameter" refers to the particle diameter (D50) at the integrated value of 50% in the volume-based particle size distribution measured by a particle size distribution measuring device based on the laser diffraction and scattering method.

[0015] (1b) The Si-containing particles are materials with a larger volume change (expansion and contraction amount) during charge and discharge compared to the (1a) graphite particles. The Si-containing particles are not particularly limited as long as they contain Si (silicon) and are known to be usable as a negative electrode active material. As an example, Si particles, SiC composite particles, SiO x silicon oxide particles represented by (where 0.05 < x < 1.95), SiC y silicon carbide particles represented by (where 0 < y < 1), SiN z silicon nitride particles represented by (where 0 < z < 4 / 3), etc. may be mentioned. Among them, SiC composite particles are preferable. The SiC composite particles typically have a structure in which Si particles are arranged in the pores of a porous carbon skeleton.

[0016] Although not particularly limited, the average particle diameter D1 of the Si-containing particles is preferably smaller than the average particle diameter of the above-mentioned graphite particles, more preferably 1 / 2 or less of the average particle diameter of the graphite particles, and even more preferably 1 / 3 or less. The average particle diameter D1 of the Si-containing particles is preferably 1 to 20 μm, more preferably 3 to 15 μm, and even more preferably 5 to 10 μm.

[0017] FIG. 1 is a schematic diagram showing one Si-containing particle and its surroundings. As shown in FIG. 1, it is preferable that at least a portion of the surface of the Si-containing particle is covered with (2) void-retaining particles, which will be described later. The coverage ratio b of the Si-containing particle by the void-retaining particles is preferably 50% or more, and more preferably 60 to 90%. It is preferable that more than half of the entire surface of the Si-containing particle is covered with the void-retaining particles, and it is particularly preferable that almost the entire surface (80% or more) is covered with the void-retaining particles. This allows more voids to be secured around the Si-containing particle, thereby enabling the effects of the technology disclosed herein to be exerted at a higher level.

[0018] In this specification, the term "coverage rate" refers to the arithmetic mean of the values ​​obtained by determining the outlines of a number of arbitrarily selected particles in an observation image using an electron microscope such as an SEM (scanning electron microscope), calculating the percentage of the total area of ​​the part of each particle to which the void-holding particles are attached to the total surface area, and then calculating the arithmetic mean of the obtained values.

[0019] Although not particularly limited, the mass ratio of (1a) graphite particles to (1b) Si-containing particles is preferably graphite particles:Si-containing particles=95:5 to 40:60, and more preferably 90:10 to 60:40. (1) The negative electrode active material preferably contains graphite particles as the first component (the component with the largest mass proportion; the same applies hereinafter), and more preferably contains graphite particles as the main component (a component accounting for 50 mass% or more; the same applies hereinafter).

[0020] When the total amount of the (1) negative electrode active material is taken as 100% by mass, the content of the (1a) graphite particles is preferably 50 to 95% by mass, and more preferably 60 to 90% by mass.When the total amount of the (1) negative electrode active material is taken as 100% by mass, the content of the (1b) Si-containing particles is preferably 5 to 50% by mass, and more preferably 10 to 30% by mass.

[0021] From the perspective of achieving a higher level of the effects of the technology disclosed herein, the total of (1a) graphite particles and (1b) Si-containing particles preferably occupies 80% by mass or more of the entire negative electrode active material, more preferably 90% by mass or more, still more preferably 95% by mass or more, and may be substantially composed of (1a) graphite particles and (1b) Si-containing particles (the total of the two components may occupy 98% by mass or more).

[0022] However, the negative electrode active material may further contain other particles that are typically present in a lower proportion than (1a) graphite particles and (1b) Si-containing particles and are known to be usable as the negative electrode active material. The content ratio of these other particles is preferably 10% by mass or less of the entire negative electrode active material, more preferably 5% by mass or less.

[0023] (2) The void-holding particles are typically particles distinguishable from the negative electrode active material. In other words, they are particles that do not have the function of reversibly occluding and releasing charge carriers (do not undergo volume change (expansion and contraction) during charge and discharge). In the technology disclosed herein, the void-holding particles are particles that are present (biased) more around (1b) Si-containing particles than around (1a) graphite particles and have the function of holding voids around the Si-containing particles. Thereby, even when the (1b) Si-containing particles expand and contract violently during charge and discharge, the electrolyte can be stably held in the voids around the Si-containing particles, improving the liquid retention property of the Si-containing particles. As a result, the drying out of the Si-containing particles can be suppressed.

[0024] In this specification, "present (biased) more around (1b) Si-containing particles than around (1a) graphite particles" means that, as shown in FIG. 1, the void-maintaining particles present around the (1b) Si-containing particles are more than the void-maintaining particles present around the (1a) graphite particles. In other words, when the coverage rate of the (1a) graphite particles by the void-holding particles is a and the coverage rate of the (1b) Si-containing particles by the void-holding particles is b, it means that a < b is satisfied.

[0025] Although not particularly limited, from the viewpoint of achieving the effects of the technology disclosed herein at a higher level, the coverage a is preferably less than 50%, more preferably 10 to 30%. Also, from the viewpoint of achieving the effects of the technology disclosed herein at a higher level, the difference (ba) between the coverage b and the coverage a is preferably 10% or more, more preferably 20% or more, even more preferably 30% or more, and particularly preferably 50% or more. The difference (ba) between the coverage b and the coverage a may be 80% or less, or 70% or less.

[0026] To achieve a higher level of effectiveness of the technology disclosed herein, the ratio of coverage a to coverage b (a / b) is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.6 or less, and particularly preferably 0.5 or less. To ensure a good conductive path for the (1b) Si-containing particles, the (a / b) ratio is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.25 or more. This reduces the likelihood of conductive path breakage even with repeated charge / discharge cycles, thereby preventing irreversible battery capacity loss. The (a / b) ratio can be suitably adjusted by mixing at least a portion of the Si-containing particles with the void-retaining particles before the graphite particles, and by adjusting the proportion of the Si-containing particles mixed first, as described in the manufacturing method (first mixing step) described below.

[0027] The material and properties of the void-retaining particles are not particularly limited as long as they can maintain voids around the Si-containing particles. In some embodiments, the void-retaining particles are preferably spherical with an average aspect ratio (length of long side / length of short side; the same applies below) of 1.5 or less. This makes it easier to maintain stable voids between particles (between the void-retaining particles and the Si-containing particles, and / or between the void-retaining particles themselves; the same applies below).

[0028] The type of void-holding particles is not particularly limited. The void-holding particles are preferably insoluble in the dispersion solvent used for the negative electrode composite in the manufacturing method (second mixing step) described below. The void-holding particles may be, for example, inorganic particles such as ceramic particles or metal particles, organic particles such as resin particles, or organic-inorganic composite particles. Among these, inorganic particles are preferred because they have higher hardness and strength than organic particles and are less likely to deform due to external forces. This makes it easier to stably maintain predetermined voids around the (1b) Si-containing particles, further improving cycle characteristics. Examples of ceramic particles include silica, alumina (anhydride), alumina hydrate (e.g., boehmite), titania, and zirconia. Among these, silica is preferred because it is relatively inexpensive and easily available. Furthermore, cellulose is preferred as the resin particles.

[0029] In some embodiments, the gap-holding particles preferably include insulating particles (e.g., ceramic particles or resin particles). This reduces side reactions with charge carriers (here, lithium ions) and the electrolyte. This in turn prevents the unintended formation of an SEI coating on the surface of the Si-containing particles, which can irreversibly reduce battery capacity. However, the gap-holding particles may also include conductive particles.

[0030] From the viewpoint of achieving the effects of the technology disclosed herein at a higher level, it is preferable that the gap-holding particles are mainly composed of insulating particles. When the total amount of the gap-holding particles is taken as 100 mass %, the content of insulating particles is more preferably 80 mass % or more, even more preferably 95 mass % or more, and it is particularly preferable that the gap-holding particles are essentially composed of insulating particles (98 mass % or more being insulating particles).

[0031] In some embodiments, the void-holding particles preferably include particles that have voids within them and can retain the electrolyte, such as porous particles (preferably continuous porous particles having a continuous three-dimensional network-like pore structure within the particle) or hollow particles (particles having a shell and a hollow portion formed inside the shell). Among these, the void-holding particles preferably include porous particles (e.g., ceramic particles). By including porous particles, voids that allow the electrolyte to penetrate can be secured within the particles, further improving the liquid retention around the Si-containing particles. However, the void-holding particles may also include particles that can retain voids only between particles, such as solid particles (particles that do not have the hollow portion) or non-porous particles.

[0032] Although not particularly limited, the porosity of porous particles is preferably 5% or more, more preferably 10% or more, and even more preferably, for example, 10 to 60%. In this specification, the term "porosity" refers to the value calculated as a percentage by defining the outline of an arbitrarily selected particle and calculating the ratio of the total area of ​​void portions to the area of ​​the entire particle (the sum of the area occupied by the particle and the total area of ​​void portions).

[0033] From the viewpoint of achieving the effects of the technology disclosed herein at a higher level, it is preferable that the void-holding particles are mainly porous particles. When the total amount of the void-holding particles is taken as 100 mass%, the content of porous particles is more preferably 80 mass% or more, even more preferably 95 mass% or more, and particularly preferably substantially composed of porous particles (98 mass% or more being porous particles).

[0034] Although not particularly limited, the ratio (D2 / D1) of the average particle diameter D2 of the void-retaining particles to the average particle diameter D1 of the (1b) Si-containing particles is preferably 0.01 to 1, more preferably 0.05 to 0.8, and even more preferably 0.1 to 0.5. This makes it possible to secure more voids around the (1b) Si-containing particles (especially between the Si-containing particles), thereby enabling the effects of the technology disclosed herein to be exerted at a higher level.

[0035] The average particle diameter D2 of the void-retaining particles is preferably determined in relation to the average particle diameter D1 of the Si-containing particles, and is not particularly limited, but is preferably smaller than the average particle diameter D1 of the (1b) Si-containing particles, and is preferably approximately 0.1 to 10 μm, more preferably 0.4 to 7 μm, and approximately 5 μm or less, for example, 0.5 to 5 μm is even more preferable, and 0.7 to 3.5 μm is particularly preferable.

[0036] Although not particularly limited, the content of the void-retaining particles is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 8 parts by mass, even more preferably 0.1 to 5 parts by mass, and generally 3 parts by mass or less, for example, 0.1 to 3 parts by mass, per 100 parts by mass of the negative electrode active material. By setting the content at a predetermined value or more, more voids can be secured around the Si-containing particles, and the effects of the technology disclosed herein can be exerted to a higher level. Furthermore, by setting the content at a predetermined value or less, the Si-containing particles are less likely to break their conductive paths even after repeated charge and discharge. This prevents irreversible reduction in battery capacity.

[0037] In some embodiments, the content of the gap-maintaining particles is preferably greater than the content of the conductive material (3) described below, and in some embodiments, the content of the binder is preferably less than the content of the binder (4) described below.

[0038] The (3) conductive material is a component that increases the conductivity within the negative electrode active material layer. In particular, when the (2) gap-retaining particles contain insulating particles, it is difficult to form a conductive path within the negative electrode active material layer, so it is preferable to include a conductive material. Although the conductive material is not shown in FIG. 1, it is preferable that the conductive material be uniformly distributed within the negative electrode active material layer. In other words, unlike the (2) gap-retaining particles described above, it is preferable that the conductive material be distributed in a balanced manner around the (1a) graphite particles and the (1b) Si-containing particles, and not unevenly distributed around the (1b) Si-containing particles.

[0039] The conductive material is not particularly limited, and one or more materials that are known to be usable for this type of application can be used without any particular limitation. Examples include amorphous carbon materials such as carbon nanotubes (CNTs), carbon fibers, carbon nanofibers, carbon black, activated carbon, hard carbon, and soft carbon. The conductive material preferably has higher electrical conductivity than the (2) void-retaining particles. In particular, it is preferable to include CNTs because of their excellent conductivity.

[0040] CNTs are fibrous carbons with a structure in which graphite forming a carbon hexagonal network is rolled into a cylindrical shape. CNTs may be single-walled carbon nanotubes (SWCNTs) with a structure in which one layer of graphite is rolled into a cylindrical shape, multi-walled carbon nanotubes (DWCNTs) with a structure in which two layers of graphite are rolled into a cylindrical shape, or multi-walled carbon nanotubes (MWCNTs) with a structure in which three or more layers of graphite are rolled into a cylindrical shape. CNTs may contain impurities (e.g., catalysts and amorphous carbon) derived from, for example, the manufacturing process.

[0041] The conductive material preferably has a highly anisotropic shape. In some embodiments, the conductive material preferably includes a carbon material (fibrous carbon) in a fibrous form with an average aspect ratio of 10 or more. Specifically, the conductive material preferably includes CNT, carbon fiber, carbon nanofiber, etc. Fibrous carbon has excellent conductivity and is less likely to aggregate than, for example, carbon black, which is commonly used as a conductive material, and therefore is easily distributed uniformly within the negative electrode active material layer. The average aspect ratio of the fibrous carbon is preferably 20 or more, more preferably 50 or more, and even more preferably 100 or more.

[0042] Although not particularly limited, the content of the conductive material is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and even more preferably 0.1 to 2 parts by mass, per 100 parts by mass of the negative electrode active material.

[0043] The (4) binder is a component that enhances the integrity of the negative electrode active material layer. The binder is preferably uniformly distributed within the negative electrode active material layer. In other words, unlike the above-described (2) void-retaining particles, the binder is preferably distributed in a balanced manner around the (1a) graphite particles and the (1b) Si-containing particles, and is not unevenly distributed around the (1b) Si-containing particles.

[0044] The binder is not particularly limited, and one or more materials known to be usable for this type of application can be used without any particular limitation. Examples include rubbers such as styrene butadiene rubber (SBR), celluloses such as carboxymethyl cellulose (CMC), and acrylic resins (resins obtained by polymerizing monomers having acryloyl groups) such as polyacrylic acid (PAA). It is more preferable that the negative electrode binder contains all of SBR, CMC, and PAA.

[0045] Although not particularly limited, the content of the binder is preferably 0.5 to 10 parts by mass, more preferably 1 to 8 parts by mass, and even more preferably 2 to 5 parts by mass, relative to 100 parts by mass of the negative electrode active material.

[0046] [Method of manufacturing a negative electrode for secondary batteries] The negative electrode for a secondary battery disclosed herein can be produced, for example, by a production method including a first mixing step (step 1), a second mixing step (step 2), an application step (step 3), and a pressing step (step 4) in this order. However, the pressing step (step 4) is not essential and can be omitted in other embodiments. The production method disclosed herein may further include other steps at any stage. For example, a heat treatment step may be included after the pressing step (step 4).

[0047] The first mixing step (step 1) is a step of mixing (1b) Si-containing particles with (2) void-retaining particles to obtain a first mixture in which the void-retaining particles are arranged around the Si-containing particles. The mixing method is not particularly limited, and a conventionally known dry mixing method or wet mixing method can be appropriately adopted. From the viewpoints of simplicity, cost, etc., a dry mixing method (dry mix) is preferred. The mixing can be performed using, for example, an agitator granulator, a mortar, a ball mill, a jet mill, a planetary mixer, a disperser, etc. In a preferred embodiment, the Si-containing particles and the void-retaining particles are mixed by a dry mixing method (dry mix) to obtain a powdery first mixture.

[0048] The second mixing step (step 2) is a step of mixing the first mixture obtained in the first mixing step (step 1) with (1a) graphite particles to obtain a negative electrode composite. The mixing method may be the same as or different from that in the first mixing step (step 1). In this step, components other than the graphite particles may be further mixed and contained in the negative electrode composite. Examples include (3) a conductive material, (4) a binder, a dispersion solvent, etc. The negative electrode composite is preferably prepared in a paste form (including a slurry form and an ink form) containing a dispersion solvent to improve the coatability in the application step (step 3) described below. In a preferred embodiment, this step includes a pre-paste preparation step (step 21), a pre-mixing step (step 22), and a paste preparation step (step 23).

[0049] The preliminary paste preparation step (step 21) is a step of mixing the powdery first mixture obtained in the first mixing step (step 1) with a predetermined dispersion solvent to prepare a paste. The dispersion solvent is not particularly limited and may be an aqueous solvent containing water or a non-aqueous solvent such as N-methyl-2-pyrrolidone (NMP). From the viewpoint of reducing the environmental load, the dispersion solvent is preferably water or a mixed solvent mainly composed of water.

[0050] The pre-mixing step (step 22) is a step of dry-mixing (1a) graphite particles with a powdered (4) binder (e.g., CMC or PAA) to obtain a powdered second mixture, which can improve the homogeneity and integrity of the negative electrode active material layer.

[0051] The paste preparation step (step S23) is a step of mixing the paste-like first mixture obtained in the preliminary paste preparation step (step S21) with the second mixture obtained in the preliminary mixing step (step S22), and further adding (3) a conductive material and a liquid (4) binder (e.g., SBR) to obtain a negative electrode composite. In a preferred embodiment, the negative electrode composite is prepared in a paste form by diluting and mixing with a dispersion solvent. The solid content concentration of the paste may be appropriately determined depending on, for example, the application method in the application step (step S3).

[0052] The application step (step 3) is a step of applying the negative electrode composite obtained in the second mixing step (step 2) to a negative electrode current collector. The method for applying the negative electrode composite is not particularly limited and may be the same as conventional methods. The negative electrode composite can be applied (typically coated) to the surface (one or both sides) of the current collector using a coating device such as a gravure coater, slit coater, die coater, comma coater, or dip coater. The amount of coating may be determined appropriately depending on, for example, the solid content concentration of the negative electrode composite so that the negative electrode active material layer has the desired properties (thickness, etc.). Furthermore, if the negative electrode composite contains a dispersion solvent, it is preferable to dry the negative electrode composite to remove the dispersion solvent. The drying of the negative electrode composite can be performed in the same manner as conventional methods. This fixes the negative electrode composite, which includes (1a) graphite particles, (1b) Si-containing particles, (2) void-retaining particles, (3) a conductive material, and (4) a binder, to the surface of the negative electrode current collector.

[0053] The pressing step (step 4) is a step of pressing the negative electrode composite on the negative electrode current collector. The pressing method and pressing conditions are not particularly limited and may be the same as conventional methods. In one example, the pressing can be performed using a pressing machine such as a roll press. The pressing conditions (e.g., pressure, holding time, etc.) may be appropriately set so that the negative electrode active material layer has the desired properties, such as thickness and density. Pressing may be performed at room temperature or while heating (at a high temperature). In this manner, the negative electrode disclosed herein can be produced.

[0054] [Secondary battery] FIG. 2 is a schematic longitudinal cross-sectional view of a secondary battery 100. As shown in FIG. 2, the secondary battery 100 includes a case 10, an electrode assembly 20, a positive electrode terminal 30, a negative electrode terminal 40, and a non-aqueous electrolyte (not shown). The secondary battery 100 here is a non-aqueous electrolyte secondary battery. The secondary battery 100 is preferably a lithium-ion secondary battery. The secondary battery 100 is characterized by including the negative electrode for a secondary battery disclosed herein, and may otherwise be similar to conventional batteries.

[0055] The case 10 is a container that houses the electrode assembly 20 and the non-aqueous electrolyte. Here, the case 10 includes a case body 12 having an opening 12h, and a sealing plate (lid) 14 that seals the opening 12h. The case 10 is integrated by joining the sealing plate 14 to the periphery of the opening 12h of the case body 12. The case 10 is hermetically sealed (sealed). The sealing plate 14 has two terminal lead-out holes 18, 19. The terminal lead-out holes 18, 19 penetrate the sealing plate 14.

[0056] The positive electrode terminal 30 is electrically connected to the positive electrode tab 23 of the electrode assembly 20 inside the case 10 via the positive electrode current collecting portion 50. The positive electrode terminal 30 extends from the inside to the outside of the sealing plate 14 through the terminal pull-out hole 18. The positive electrode terminal 30 is disposed at one end of the sealing plate 14 (the left end in FIG. 2 ). Here, the positive electrode terminal 30 is crimped to the peripheral portion of the sealing plate 14 that surrounds the terminal pull-out hole 18 by crimping.

[0057] The negative electrode terminal 40 is electrically connected to the negative electrode tab 25 of the electrode assembly 20 via the negative electrode current collecting portion 60 inside the case 10. The negative electrode terminal 40 extends from the inside to the outside of the sealing plate 14 through the terminal pull-out hole 19. The negative electrode terminal 40 is disposed at the other end of the sealing plate 14 (the right end in FIG. 2 ). Here, the negative electrode terminal 40 is crimped to the peripheral portion of the sealing plate 14 that surrounds the terminal pull-out hole 19 by crimping.

[0058] Here, the electrode assembly 20 is a wound electrode assembly formed by stacking a strip-shaped positive electrode and a strip-shaped negative electrode with a strip-shaped separator interposed therebetween and winding them around a winding axis. The electrode assembly 20 has a flat outer shape. Here, the electrode assembly 20 is disposed inside the case 10 with the winding axis oriented along the bottom surface 12a of the case 10. However, in other embodiments, the electrode assembly 20 may be disposed inside the case 10 with the winding axis oriented along the side surface 12b of the case 10. The electrode assembly 20 may also be a laminated electrode assembly formed by stacking multiple square-shaped (typically rectangular) positive electrodes and multiple square-shaped (typically rectangular) negative electrodes in an insulated state. The number of electrode assemblies 20 disposed inside one case 10 may be one, or two or more (multiple).

[0059] The configuration of the positive electrode is not particularly limited and may be the same as that of a conventional positive electrode. The positive electrode typically has a positive electrode current collector and a positive electrode active material layer fixed to at least one surface of the positive electrode current collector. A positive electrode tab 23 is attached to the left end of the positive electrode current collector and is electrically connected to a positive electrode terminal 30 via a positive electrode current collector 50. The positive electrode active material layer contains a positive electrode active material. An example of the positive electrode active material is a lithium transition metal composite oxide such as a lithium nickel cobalt manganese composite oxide.

[0060] As described above, the negative electrode includes a negative electrode current collector and a negative electrode active material layer fixed to at least one surface of the negative electrode current collector. A negative electrode tab 25 is attached to the right end of the negative electrode current collector and is electrically connected to a negative electrode terminal 40 via a negative electrode current collector 60.

[0061] The non-aqueous electrolyte may be the same as conventional ones and is not particularly limited. The non-aqueous electrolyte typically contains a non-aqueous solvent (organic solvent) and a supporting salt (electrolyte salt). Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate (EC) and monofluoroethylene carbonate (FEC), and chain carbonates such as dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). Examples of supporting salts include fluorine-containing lithium salts such as lithium hexafluorophosphate (LiPF6).

[0062] [Uses of secondary batteries] The secondary battery 100 can be used for a variety of purposes, but is preferably used in applications requiring high capacity and high input / output characteristics, such as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car, truck, etc. The type of vehicle is not particularly limited, and examples thereof include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV).

[0063] Some test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to these test examples.

[0064] <Test Example I> [Fabrication of Negative Electrode] Here, negative electrodes were fabricated in which the negative electrode active material layer contained the following components in common. Negative electrode active material: graphite particles, Si-containing particles (specifically, SiC composite particles) Conductive material: fibrous carbon (specifically, SWCNT) Binder: CMC, PAA, SBR

[0065] In Example 1, first, an agitation granulator was used to dry-mix Si-containing particles (SiC composite particles) having an average particle size D1 shown in Table 1 with void-retaining particles (silica, which are insulating and porous particles) having an average particle size D2 shown in Table 1 to obtain a first mixture in which the void-retaining particles were arranged around the SiC composite particles (first mixing step). Note that in Table 1, the fact that the SiC composite particles were first mixed with the void-retaining particles (in other words, the inclusion of the first mixing step) is indicated as "pre-mixing."

[0066] Next, in the second mixing step, a dispersion solvent (water) was added to the first mixture and a paste was prepared using a disperser (pre-paste preparation step). Next, graphite particles were dry-mixed with carboxymethyl cellulose particles (CMC) and polyacrylic acid particles (PAA) as binders using an agitation granulator to obtain a powdery second mixture (pre-mixing step). Next, the paste-like first mixture obtained in the pre-paste preparation step was mixed with the second mixture obtained in the pre-mixing step, and then fibrous carbon (SWCNT) as a conductive material and a dispersion solvent (water) were added and kneaded. Next, styrene butadiene rubber (SBR) as a binder was added and diluted and mixed with a dispersion solvent (water) to prepare a paste-like negative electrode composite (paste preparation step).

[0067] In the negative electrode composite, the blending ratio of graphite particles to SiC composite particles was 85:15. The blending ratio of other components was void-retaining particles (silica):SWCNT:CMC:PAA:SBR=1:0.1:1:1:1.5 (parts by mass) relative to the total amount (100 parts by mass) of the negative electrode active material.

[0068] During the above-mentioned kneading, the moisture content was adjusted as follows in order to coat the negative electrode active material with binder (CMC, PAA). When water is added to a powder mixture of graphite particles and SiC composite particles with a specified composition, the moisture content at which the torque required for mixing is maximum is defined as A0%. When the amount of moisture per 100g of mixed powder corresponding to a moisture content of A0% is A1 (ml), the ideal solid content of B0% that gives the maximum torque was calculated using the following formula. B0=100-A0=100 / (100+A1)×100 (%)

[0069] Next, the negative electrode composite paste prepared in the paste preparation step was applied to a Cu foil (thickness: 10 μm) as a negative electrode current collector and dried (application step). Next, the negative electrode composite on the negative electrode current collector was pressed to a predetermined thickness to densify the negative electrode composite (pressing step). Then, by processing to a predetermined dimension, a negative electrode having a negative electrode active material layer on the negative electrode current collector was produced.

[0070] In Example 2, only a portion of the SiC composite particles was "pre-mixed." Specifically, of a total of 1 part by mass of SiC composite particles, only 0.8 parts by mass of the SiC composite particles were dry-mixed with the void-retaining particles in the first mixing step, and the remaining 0.2 parts by mass of the SiC composite particles were added together with the graphite particles in the second mixing step. Except for this, a negative electrode was fabricated in the same manner as in Example 1.

[0071] In Comparative Example 1, a negative electrode was fabricated in the same manner as in Example 1, except that the negative electrode composite was prepared without adding void-retaining particles. Specifically, first, graphite particles, SiC composite particles, CMC, and PAA were dry-mixed all at once using a stirring granulator, and then SWCNTs and a dispersion solvent (water) were added and kneaded. Next, SBR was added and diluted with the dispersion solvent (water) to prepare a paste-like negative electrode composite.

[0072] In Comparative Example 2, a negative electrode was produced in the same manner as in Example 1, except that in the first mixing step, void-retaining particles were arranged around the graphite particles. Specifically, first, a first mixture was obtained by dry-mixing the graphite particles and void-retaining particles using an agitation granulator. Next, a solvent (water) was added to the first mixture, and a paste was prepared using a disperser. Next, a powdery second mixture was obtained by dry-mixing the SiC composite particles, CMC, and PAA using an agitation granulator. Next, the second mixture was mixed with the paste-like first mixture, and SWCNT and a dispersion solvent (water) were further added and kneaded. Next, SBR was added, and the mixture was diluted and mixed with the dispersion solvent (water) to prepare a paste-like negative electrode composite.

[0073] [Evaluation of Coverage Rate] For each negative electrode, the coverage rate a (%) of graphite particles by the void-retaining particles and the coverage rate b (%) of SiC composite particles by the void-retaining particles were calculated as follows. Specifically, first, multiple graphite particles and SiC composite particles were observed using a scanning electron microscope (SEM), and SEM images were obtained. Next, the particle outlines of multiple arbitrarily selected graphite particles and SiC composite particles were determined using analysis software. Then, for each particle, the ratio of the total area of ​​the portion where the void-retaining particles adhered to the total surface area was calculated as a percentage. The obtained values ​​were arithmetically averaged to calculate the coverage rates a and b, respectively. The results are shown in Table 1.

[0074] [Fabrication of Secondary Battery] First, a positive electrode was prepared. For the positive electrode, LiNiCoMnO2 (NCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and PVdF as a binder were mixed in a mass ratio of NCM:AB:PVdF = 100:1:1. Next, the fluidity was adjusted with a dispersion solvent (NMP) to prepare a paste-like positive electrode mixture. Next, the prepared positive electrode mixture was applied to an Al foil (thickness: 15 μm) as a positive electrode current collector, dried, and pressed to a predetermined thickness. Then, by processing to the predetermined dimensions, a positive electrode comprising a positive electrode active material layer on a positive electrode current collector was fabricated.

[0075] Next, the prepared positive and negative electrodes were placed opposite each other with a separator interposed therebetween to prepare an electrode assembly. The prepared electrode assembly was then housed in a case made of an aluminum laminate sheet, and a non-aqueous electrolyte was poured into it. The non-aqueous electrolyte was prepared by dissolving LiPF6 as a supporting salt (Li salt) at a concentration of 1.0 mol / L in a mixed solvent of EC, FEC, EMC, and DMC in a volume ratio of EC:FEC:EMC:DMC=15:5:40:40. The opening of the case was then sealed to prepare a test cell (laminated cell).

[0076] [Evaluation of High-Rate Cycle Characteristics] First, the test cell was charged at a constant current of 1.5 C in an environment of 25°C until the state of charge (SOC) reached 50%. Next, the test cell was stored in an environment of 25°C for 1 hour, and then the test cell was discharged at a constant current of 1.0 C for 10 seconds. Then, the initial resistance (Ω) was calculated by dividing the difference between the open circuit voltage (OCV) and the closed circuit voltage (CCV) 10 seconds after discharge by the discharge current value 10 seconds after discharge, as shown in the following formula. Initial resistance = (OCV-CCV) / Discharge current value 10 seconds after discharge

[0077] Next, in a 25°C environment, the test cell was subjected to CCCV charging (constant current charging at a constant current of 1.5C up to 4.2V, followed by constant voltage charging until the current value reached 0.1C), followed by CC discharging (constant current discharging at a constant current of 0.4C down to 2.5V), and 50 cycles of high-rate charge and discharge were repeated. Next, the post-cycle resistance of the test cell after high-rate charge and discharge was measured in the same manner as the initial resistance described above. The ratio of the resistance value after high-rate charge and discharge to the initial capacity was calculated as the resistance increase rate (%). The results are shown in Table 1.

[0078] [Table 1]

[0079] As shown in Table 1, Comparative Example 1, in which no void-retaining particles were added, had the highest resistance increase rate after high-rate cycling. The reason for this is thought to be that the Si-containing material in the negative electrode repeatedly expanded and contracted during charging and discharging, causing a shortage of electrolyte around the Si-containing particles and resulting in liquid depletion. Furthermore, Comparative Example 2, in which void-retaining particles were not unevenly distributed around the SiC composite particles despite containing void-retaining particles in the negative electrode, still had a high resistance increase rate.

[0080] In contrast to these comparative examples, Examples 1 and 2, in which the void-retaining particles were unevenly distributed around the SiC composite particles (the ratio (a / b) was less than 1), showed little increase in resistance even after repeated high-rate charge and discharge, and had relatively excellent high-rate cycle characteristics. This demonstrates that the effects of the technology disclosed herein are supported by experimental results.

[0081] <Test Example II: Examination of the ratio (D2 / D1) and the proportion of void-retaining particles added> In this test example, negative electrodes were prepared and evaluated in the same manner as in Example 1 of Test Example I, except that the average particle diameter D2 of the void-holding particles, the ratio (D2 / D1), and the proportion of void-holding particles added were varied as shown in Table 2. The results are shown in Table 2.

[0082] [Table 2]

[0083] As shown in Table 2, it was found that the effects of the technology disclosed herein can be obtained at least when the ratio (D2 / D1) is in the range of 0.05 to 1. Furthermore, Example 6, in which the ratio (D2 / D1) is 1, and Example 7, in which the ratio (D2 / D1) is 0.06, had a slightly higher resistance increase rate than, for example, Example 1, and therefore it was found that the ratio (D2 / D1) is preferably less than 1, and more preferably 0.1 to 0.5. It was also found that the average particle diameter D2 of the void-retaining particles is preferably approximately 0.1 to 10 μm (for example, 0.4 to 7 μm), more preferably 0.5 to 5 μm, and even more preferably 0.7 to 3.5 μm.

[0084] Furthermore, the results of Examples 4 and 5 show that the effects of the technology disclosed herein can be obtained at least when the addition ratio of the void-holding particles is in the range of 0.1 to 5 parts by mass. Furthermore, Example 5, in which the addition ratio of the void-holding particles is 5 parts by mass, had a slightly higher resistance increase rate than, for example, Example 1, and therefore it was found that the addition ratio of the void-holding particles is preferably 4 parts by mass or less, and more preferably 3 parts by mass or less.

[0085] <Test Example III: Examination of types of void-retaining particles> In this test example, a negative electrode was prepared and evaluated in the same manner as in Example 1 of Test Example I, except that non-porous alumina was used as the void-maintaining particles instead of porous silica. The results are shown in Table 3.

[0086] [Table 3]

[0087] As shown in Table 3, when non-porous particles were used as the void-maintaining particles, the resistance increase rate increased slightly, indicating that porous particles are more preferable as the void-maintaining particles.

[0088] Although the preferred embodiment of the present invention has been described above, the above embodiment is merely an example. 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 technical common sense in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiment. For example, it is possible to replace part of the above-described embodiment with other modifications, or to add other modifications to the above-described embodiment. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0089] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer fixed to the negative electrode current collector, wherein the negative electrode active material layer contains, as a negative electrode active material, graphite particles and Si-containing particles, and void-retaining particles that are present in greater amounts around the Si-containing particles than around the graphite particles and that retain voids around the Si-containing particles. Item 2: The negative electrode for a secondary battery according to Item 1, wherein the void-holding particles include insulating particles. Item 3: The negative electrode for a secondary battery according to Item 1 or 2, wherein the void-holding particles include porous particles. Item 4: The negative electrode for a secondary battery according to any one of Items 1 to 3, wherein the ratio (D2 / D1) of the average particle diameter D2 of the void-retaining particles to the average particle diameter D1 of the Si-containing particles is 0.1 or more and 0.5 or less. Item 5: The negative electrode for a secondary battery according to any one of Items 1 to 4, wherein the void-holding particles have an average particle diameter D2 of 0.5 μm or more and 5 μm or less. Item 6: The negative electrode for a secondary battery according to any one of Items 1 to 5, wherein the content of the void-forming particles is 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the negative electrode active material. Item 7: The negative electrode for a secondary battery according to any one of Items 1 to 6, wherein when the coverage of the graphite particles with the void-holding particles is a and the coverage of the Si-containing particles with the void-holding particles is b, the ratio (a / b) of the coverage a to the coverage b is 0.25 or more and 0.5 or less. Item 8: The negative electrode for a secondary battery according to any one of Items 1 to 7, wherein the negative electrode active material layer further contains carbon nanotubes as a conductive material. Item 9: A secondary battery comprising an electrode assembly and a non-aqueous electrolyte solution, wherein the electrode assembly includes the negative electrode for secondary batteries according to any one of items 1 to 8. Item 10: A method for producing a negative electrode for a secondary battery, comprising: a first mixing step of mixing Si-containing particles and void-maintaining particles to obtain a first mixture in which the void-maintaining particles are arranged around the Si-containing particles; a second mixing step of mixing the first mixture with graphite particles to obtain a negative electrode composite; and an application step of applying the negative electrode composite onto a negative electrode current collector. [Explanation of symbols]

[0090] 10 cases 20 Electrode body 100 Secondary battery

Claims

1. a negative electrode current collector; and a negative electrode active material layer fixed to the negative electrode current collector, The negative electrode active material layer is graphite particles and Si-containing particles as negative electrode active materials; void-retaining particles that are present in greater amounts around the Si-containing particles than around the graphite particles and that retain voids around the Si-containing particles; Including, Negative electrode for secondary batteries.

2. The gap-retaining particles include insulating particles. The negative electrode for a secondary battery according to claim 1 .

3. The void-retaining particles include porous particles. The negative electrode for a secondary battery according to claim 1 .

4. the ratio (D2 / D1) of the average particle diameter D2 of the void-retaining particles to the average particle diameter D1 of the Si-containing particles is 0.1 or more and 0.5 or less; The negative electrode for a secondary battery according to any one of claims 1 to 3.

5. The average particle diameter D2 of the void-retaining particles is 0.5 μm or more and 5 μm or less. The negative electrode for a secondary battery according to claim 4 .

6. The content of the void-retaining particles is 0.1 parts by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the negative electrode active material. The negative electrode for a secondary battery according to any one of claims 1 to 3.

7. a is the coverage rate of the graphite particles by the void-retaining particles, When the coverage of the Si-containing particles by the void-retaining particles is b, a ratio (a / b) of the coverage a to the coverage b is 0.25 or more and 0.5 or less; The negative electrode for a secondary battery according to any one of claims 1 to 3.

8. The negative electrode active material layer further contains carbon nanotubes as a conductive material. The negative electrode for a secondary battery according to any one of claims 1 to 3.

9. An electrode assembly and a non-aqueous electrolyte solution, The electrode body includes the negative electrode for a secondary battery according to any one of claims 1 to 3. Secondary battery.

10. a first mixing step of mixing Si-containing particles and void-retaining particles to obtain a first mixture in which the void-retaining particles are arranged around the Si-containing particles; a second mixing step of mixing the first mixture with graphite particles to obtain a negative electrode composite; an application step of applying the negative electrode composite material onto a negative electrode current collector; Including, A method for producing a negative electrode for a secondary battery.

Images