Additive for secondary battery negative electrode, negative electrode for secondary battery, and methods for manufacturing negative electrode for secondary battery and secondary battery

Abstract

To provide an additive that can balance negative electrode capacity and cycle characteristics.SOLUTION: An additive 1 for a secondary battery negative electrode disclosed herein has a core-shell structure having a core portion 2 containing a silicon material as a first active material, and a coating portion 4 containing a film-forming agent and coating at least a portion of the surface of the core portion 2. The average particle diameter of the additive 1 is 0.5 to 3.0 μm. Regarding the average particle diameter of the first active material, the ratio (D1 / D) of the average particle diameter D1 of the first active material to the average particle diameter D of the additive is 0.6 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] As secondary batteries become more widespread, there is a demand for ever higher performance. Patent Document 1, for example, discloses a method for achieving this by adding a compound (film-forming agent) that is electrolyzed to form a film mainly upon charging to the negative electrode active material layer. Patent Document 2 also discloses an additive for a negative electrode that includes a carbon-containing Si-based nanostructure (e.g., a nanowire with a core-shell structure that contains Si in the core and carbon in the coating). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-89869 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-527735 Summary of the Invention [Problem to be solved by the invention]

[0004] Secondary batteries are required to have a negative electrode that has a high capacity, a small capacity loss even after repeated charge and discharge, and an excellent balance between capacity and cycle characteristics.

[0005] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide an additive for a secondary battery negative electrode that can balance the negative electrode capacity and cycle characteristics. [Means for solving the problem]

[0006] The present invention provides an additive for a secondary battery negative electrode having a core-shell structure, the core including a silicon material as a first active material, and a coating including a film-forming agent that coats at least a portion of the surface of the core. The additive has an average particle size of 0.5 μm to 3.0 μm. The ratio (D1 / D) of the average particle size D of the additive to the average particle size D of the first active material is 0.6 or less.

[0007] The use of the additive makes it possible to realize a negative electrode with an excellent balance between capacity and cycle characteristics, and thus a secondary battery that can exhibit high energy density over a long period of time. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram of an additive for a secondary battery negative electrode according to one embodiment. [Figure 2] 2(A) and (B) are schematic diagrams showing the hexagonal close-packed structure of the second active material particles, where (A) shows a T vacancy and (B) shows an R vacancy. [Figure 3] FIG. 3 is a schematic cross-sectional view of the negative electrode active material layer after the formation step. [Figure 4] FIG. 4(A) is a schematic cross-sectional view of the negative electrode active material layer after the activation step, and FIG. 4(B) is an enlarged view of the second active material particle in FIG. 4(A). [Figure 5] FIG. 5 is a schematic vertical cross-sectional view of a secondary battery according to one embodiment. [Figure 6] FIG. 6 is a graph showing the negative electrode capacity and capacity retention rate of Examples 3-7. 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] [Additives for secondary battery negative electrodes] First, the additive for the negative electrode of 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 for a nonaqueous electrolyte secondary battery, and particularly preferably for a lithium-ion secondary battery.

[0011] FIG. 1 is a schematic diagram of an additive 1 for a negative electrode of a secondary battery. As shown in FIG. 1, the additive 1 disclosed herein has a core-shell structure having a core 2 containing a silicon material as a first active material and a coating 4 containing a film-forming agent and coating at least a portion of the surface of the core 2. In this example, the coating 4 covers the entire surface of the core 2. The additive 1 is preferably used in a negative electrode containing a second active material (typically, a carbon material such as graphite). It is more preferable that the additive 1 has a spherical shape with an average aspect ratio (length of long side / length of short side) of 1.5 or less. The method for calculating the average aspect ratio will be described later.

[0012] In this embodiment, the average particle diameter D of additive 1 is 0.5 to 3.0 μm. As will be described in detail in the section on the negative electrode manufacturing method, the particle diameter of the second active material (e.g., graphite) is generally 5 μm or more, for example, approximately 5 to 30 μm. In this case, when the average particle diameter D of additive 1 is a predetermined value or less, additive 1 is easily arranged in the pores of the hexagonal close-packed structure of the second active material particles (e.g., graphite particles). This makes it difficult for a large change in density to occur in the negative electrode active material layer even when the coating agent is electrolyzed. Furthermore, when the average particle diameter D is a predetermined value or more, the coating portion 4 (coating agent) is less likely to dissolve in the solvent in the formation step (more specifically, the slurry preparation step (step 2-B)) described later. Therefore, the effects of the technology disclosed herein can be stably exhibited. Note that, in this specification, the "average particle diameter" refers to the particle diameter (D50) at 50% of the cumulative value in the volume-based particle size distribution measured by a particle size distribution analyzer based on the laser diffraction / scattering method.

[0013] The core portion 2 contains a silicon material as a first active material. The silicon material may be one or more of materials known to be useful as negative electrode active materials for secondary batteries, without any particular limitations. The silicon material may be silicon or a silicon-containing compound such as silicon oxide, silicon carbide, or silicon nitride. To achieve the effects of the technology disclosed herein at a higher level, the silicon material preferably contains silicon as the main component (a component accounting for 50% by mass or more; the same applies hereinafter), more preferably 80% by mass or more of silicon, even more preferably 95% by mass or more of silicon, and particularly preferably consists essentially of silicon (98% by mass or more of silicon).

[0014] In this embodiment, the ratio (D1 / D) of the average particle diameter D1 of the first active material to the average particle diameter D of the additive 1 is 0.6 or less. As will be described in detail in the section on the negative electrode manufacturing method, by setting the ratio (D1 / D) to a predetermined value or less, voids that can accommodate the expansion of the first active material particles (silicon material) during charge and discharge can be suitably secured. Therefore, even if the first active material particles expand significantly during charge and discharge, they tend to fit within the voids of the hexagonal close-packed structure of the second active material particles (e.g., graphite particles). In other words, the first active material particles tend to expand so as to fill the voids of the second active material particles. Therefore, the negative electrode active material layer itself is less likely to undergo volumetric changes, improving cycle characteristics. From this perspective, the ratio (D1 / D) is more preferably 0.5 or less, and even more preferably 0.45 or less. Moreover, from the viewpoint of increasing the negative electrode capacity derived from the silicon material and achieving a high level of balance between the negative electrode capacity and the cycle characteristics, the ratio (D1 / D) is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.4 or more.

[0015] The average particle diameter D1 of the first active material (silicon material) is not particularly limited because it is determined in relation to the average particle diameter D of the additive 1, but from the viewpoint of ease of production, etc., it is preferably approximately 0.1 to 1.5 μm, more preferably 0.3 to 1.3 μm, and even more preferably 0.5 to 1.0 μm. Note that the average particle diameter D1 of the first active material is the same as the diameter of the core portion 2 here, as shown in FIG.

[0016] The core portion 2 may contain a material other than the first active material (silicon material), but from the viewpoint of achieving the effects of the technology disclosed herein at a higher level, it is preferable that the core portion 2 is mainly made of the first active material, more preferably that 80% by mass or more of the first active material is the first active material, even more preferably that 95% by mass or more of the first active material is the first active material, and it is particularly preferable that the core portion 2 is essentially made of the first active material (98% by mass or more of the first active material).

[0017] The coating portion 4 contains a film-forming agent. The film-forming agent is a compound that is electrolyzed to form a film mainly during charging. As the film-forming agent, one or more materials that have been known to be usable for this type of application can be used without any particular limitation. For example, a film-forming agent containing at least one oxalate ion (CO4 2- Examples of suitable compounds include oxalato complex compounds in which ZnO (Cu) is coordinately bonded to a central element (coordinating atom) such as boron (B) or phosphorus (P), and carbonate compounds such as vinylene carbonate (VC), vinylethylene carbonate (VEC), and fluoroethylene carbonate (FEC). Among these, oxalato complex compounds are preferred from the viewpoint of achieving a higher level of the effects of the technology disclosed herein. The film-forming agent is preferably a compound having a higher decomposition onset potential than the non-aqueous solvent (e.g., EC) used in the construction step of secondary battery production, which will be described later. This allows the film-forming agent to be decomposed before the non-aqueous solvent in the activation step, which will be described later, and allows a film derived from the film-forming agent to be suitably formed on the surface of the active material (e.g., a silicon material as the first active material and / or a carbon material as the second active material, which will be described later).

[0018] An example of an oxalato complex is a boron (B) central atom with at least one oxalate ion (CO). 2- Compounds with a four-coordinated structure in which at least one oxalate ion (CO) is coordinated to a phosphorus (P) central atom, such as lithium bis(oxalato)borate (Li[B(CO)]; LiBOB), lithium difluorooxalatoborate (Li[BF(CO)]; LiDFOB); 2- Examples of suitable compounds include compounds having a hexacoordinated structure in which 1,2,3,4,5,6,6-coordinated bis(oxalato)phosphate (Li[P(C2O4)3]) and lithium difluorobis(oxalato)phosphate (Li[PF2(C2O4)2]; LPFO). Among these, from the viewpoint of improving cycle characteristics, oxalato complex compounds containing boron are preferred, and LiBOB is particularly preferred.

[0019] The coating portion 4 may contain materials other than the film-forming agent, but from the viewpoint of achieving the effects of the technology disclosed herein at a higher level, it is preferable that the coating portion 4 is mainly composed of the film-forming agent, more preferably 80% by mass or more of the film-forming agent, even more preferably 95% by mass or more of the film-forming agent, and particularly preferably consists essentially of the film-forming agent (98% by mass or more of the film-forming agent).

[0020] The method for producing the additive 1 having a core-shell structure is not particularly limited, but from the viewpoint of ease of production, for example, a method in which a powdered silicon material constituting the core portion 2 and a powdered coating agent constituting the coating portion 4 are made into composite particles (granulated) by mechanochemical treatment is preferred. Examples of devices capable of performing mechanochemical treatment include the "Multipurpose Mixer" manufactured by Nippon Coke & Engineering Co., Ltd. and the "Nobilta" manufactured by Hosokawa Micron Corporation.

[0021] [Method of manufacturing a negative electrode for secondary batteries] Next, a method for producing the negative electrode for a secondary battery disclosed herein will be described. The negative electrode for a secondary battery disclosed herein can be produced, for example, by a production method including a preparation step (step 1), a formation step (step 2), and a compression step (step 3) in this order. However, the compression step (step 3) is not essential and can be omitted in other embodiments. Furthermore, the production method disclosed herein may further include other steps at any stage.

[0022] (Step 1) The preparation step is a step of preparing a second active material and the above-mentioned additive 1. The second active material contains a carbon material. The carbon material is typically in powder form. As the carbon material, one or more materials known to be usable as negative electrode active materials for secondary batteries can be used without any particular limitation. The carbon material is typically graphite. The graphite may be natural graphite, artificial graphite, or amorphous carbon-coated graphite in which a core of graphite is coated with an amorphous carbon material. The carbon material may contain carbon materials other than graphite particles, such as hard carbon, soft carbon, or carbon nanotubes. However, from the viewpoint of achieving the effects of the technology disclosed herein at a higher level, it is preferable that the carbon material be mainly composed of graphite, more preferably 80% by mass or more of graphite, even more preferably 95% by mass or more of graphite, and particularly preferably substantially composed of graphite (98% by mass or more of graphite).

[0023] In some embodiments, the second active material (carbon material) may have a spherical shape with an average aspect ratio (longer side length / short side length) of 1.5 or less. The average aspect ratio of the second active material is preferably 1.45 or less, more preferably 1.4 or less, even more preferably 1.3 or less, and particularly preferably 1.25 or less. In such cases, the technology disclosed herein can be utilized at a higher level. The average aspect ratio of the second active material is 1.00 or more, and may be, for example, 1.1 or more from the viewpoint of ease of production, etc.

[0024] In this specification, the term "aspect ratio" refers to the ratio of the length of the major axis to the length of the minor axis of the second active material particles. The aspect ratio of the second active material particles can be determined, for example, by taking an electron microscope image of the cross section of the negative electrode active material layer using a scanning electron microscope (SEM) or the like, determining the lengths of the minor axis and major axis of the second active material particles in the image, and calculating their ratio (length of major axis / length of minor axis). Measuring the lengths of the minor axis and major axis of the second active material particles and calculating their ratio can be easily done using commercially available image analysis software (e.g., "ImageJ"). The average aspect ratio can be determined by arbitrarily selecting 25 or more second active material particles from the electron microscope image of the cross section and arithmetically averaging their aspect ratios. The closer the aspect ratio is to 1, the more spherical the particles are.

[0025] Although not particularly limited, the average particle diameter D2 of the second active material (carbon material) is typically larger than the average particle diameter D1 of the first active material (silicon material), and is preferably 1 to 100 μm, more preferably 5 to 50 μm, even more preferably 5 to 30 μm, and particularly preferably 15±5 μm. By setting the average particle diameter D2 of the second active material within the above range, it is possible to improve cycle characteristics and to achieve the effects of the technology disclosed herein at a higher level.

[0026] In some embodiments, it is preferable that the second active material (carbon material) has an average particle diameter D2 that satisfies the following formula 1: [average particle diameter D of the first active material (silicon material) / 0.155]≦D2. This makes it easier for additive 1 to be placed in the voids of the hexagonal close-packed structure of the second active material particles (e.g., graphite particles) in the formation process described below. This will be explained in detail below with reference to the drawings. Figures 2(A) and 2(B) are schematic diagrams showing the hexagonal close-packed structure of the second active material particles. The size of the voids in the hexagonal close-packed structure can be determined from known literature, for example, Ashizawa Finetech Co., Ltd., Particle Technology Column, Regarding Bead Diameter and Material [online] Internet<URL:https: / / ashizawa.com / column / 16.html> and Shigeo Miwa, General Theory of Powder Technology, Nikkan Kogyo Shimbun, 1981, pp. 48-49.

[0027] As shown in FIG. 2(A), in the hexagonal close-packed structure, the radius r1 of the largest inscribed circle of a T void surrounded by three spherical second active material particles A1 to A3 can be expressed as 0.155r, where r is the radius of the second active material particles. Also, as shown in FIG. 1(B), in the hexagonal close-packed structure, the radius r2 of the largest inscribed circle of an R void surrounded by four spherical second active material particles A4 to A7 can be expressed as 0.414r, where r is the radius of the second active material particles. Therefore, as long as Additive 1 has a size equal to or smaller than the radius r1 of the smaller T void (i.e., 0.155r), it is likely to be located within the T void and / or R void in the hexagonal close-packed structure of the second active material particles. Therefore, even if Additive 1 is electrolyzed, the formation of large voids in the second active material layer can be effectively suppressed.

[0028] The (Step 2) forming step is a step of forming a negative electrode active material layer containing the second active material prepared in the (Step 1) preparing step and additive 1 on a negative electrode current collector. This step includes, for example, a current collector preparing step (Step 2-A) of preparing a negative electrode current collector, a slurry preparing step (Step 2-B) of preparing a negative electrode composite slurry, and an applying step (Step 2-C) of applying the negative electrode composite slurry to the negative electrode current collector. The order of the current collector preparing step (Step 2-A) and the slurry preparing step (Step 2-B) is not particularly limited. This step may further include other steps at any stage.

[0029] In the current collector preparation step (step 2-A), a negative electrode current collector is prepared. The negative electrode current collector is preferably made of metal, and more preferably made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The negative electrode current collector is preferably made of metal foil, and more preferably made of copper foil or a copper alloy foil. Although not particularly limited, the thickness of the metal foil is, for example, 5 to 35 μm, and preferably 6 to 20 μm.

[0030] In the slurry preparation step (step 2-B), a negative electrode composite slurry is prepared containing at least the second active material and additive 1 prepared in the preparation step (step 1) described above. Specifically, solid materials for the negative electrode active material layer (e.g., second active material, additive 1, binder, thickener, etc.) are mixed with a predetermined solvent. In this specification, the term "slurry" refers to a mixture in which some or all of the solids are dispersed in a solvent, and is a term that encompasses paste, ink, etc.

[0031] Examples of binders that can be used include styrene butadiene rubber (SBR) and its modified products, acrylonitrile butadiene rubber and its modified products, and polyvinylidene fluoride (PVdF). Examples of thickeners that can be used include cellulose-based polymers such as carboxymethyl cellulose (CMC). Examples of solvents that can be used include aqueous solvents containing water and non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP).

[0032] In this embodiment, the proportion of additive 1 is 0.1 to 30 parts by volume when the second active material is 100 parts by volume. By setting the proportion of additive 1 at a predetermined value or more, a larger amount of SEI film can be formed on the surface of the active material (the silicon material as the first active material and / or the carbon material as the second active material), thereby achieving the effects of the technology disclosed herein at a higher level. From the viewpoint of improving the negative electrode capacity, the proportion of additive 1 is preferably 0.5 parts by volume or more, more preferably 1 part by volume or more, and even more preferably 5 parts by volume or more. The upper limit of 30 parts by volume is based on the void ratio when the negative electrode active material particles are closest packed as described above. From the viewpoint of improving cycle characteristics, the proportion of additive 1 is preferably 25 parts by volume or less, more preferably 20 parts by volume or less, and even more preferably 10 parts by volume or less.

[0033] Although not particularly limited, the total proportion of the second active material (e.g., graphite) and additive 1 in the total solid content of the negative electrode mixture slurry is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, from the viewpoint of achieving a high energy density and achieving the effects of the technology disclosed herein at a higher level. The total proportion of the second active material and additive 1 is preferably 99% by mass or less, more preferably 98.5% by mass or less, and even more preferably 98% by mass or less.

[0034] The proportion of the binder (e.g., SBR) relative to the total solid content of the negative electrode mixture slurry is preferably 0.1 to 2 mass%, more preferably 0.5 to 1 mass%. The proportion of the thickener (e.g., CMC) relative to the total solid content of the negative electrode mixture slurry is preferably 0.1 to 2 mass%, more preferably 0.5 to 1 mass%. In some embodiments, the negative electrode mixture slurry preferably does not substantially contain a conductive agent such as carbon black (its proportion relative to the total solid content is less than 0.1 mass%). This makes it easier for the additive 1 to be arranged in the T pores and / or R pores in the hexagonal close-packed structure of the second active material described above.

[0035] In the application step (step 2-C), the prepared negative electrode composite slurry is applied to the surface of the negative electrode current collector using a conventionally known application device and dried. This causes a negative electrode active material layer containing the second active material (e.g., graphite) and additive 1 to be fixed to the surface of the negative electrode current collector. The application device is not particularly limited, and examples that can be used include a gravure coater, a comma coater, a slit coater, and a die coater. Drying can be performed according to a known method.

[0036] When forming negative electrode active material layers on both sides of the negative electrode current collector, the negative electrode composite slurry is applied to both sides of the negative electrode current collector. The coating weight of the negative electrode composite slurry (amount applied per unit area on one side) is set to 5 mg / cm from the viewpoint of achieving high energy density. 3 More than 10 mg / cm is preferable. 2 More preferably, 20 mg / cm or more 2 The weight of the negative electrode mixture slurry is more preferably 100 mg / cm.3 It may be the following:

[0037] The negative electrode active material layer preferably contains Additive 1, which includes a film-forming agent, uniformly in the thickness direction. For example, when the negative electrode active material layer is virtually divided into two equal parts in the thickness direction, with the side relatively closer to the negative electrode current collector defined as the lower layer and the side relatively farther from the negative electrode current collector defined as the upper layer, it is preferable that both the lower layer and the upper layer contain Additive 1. This makes it possible to suppress a decrease in battery capacity to a relatively high level and improve cycle characteristics, compared to an embodiment in which the film-forming agent is contained only on the outermost surface of the negative electrode active material layer, as in Patent Document 1, for example.

[0038] (Step 2) The compression step is a step of compressing the negative electrode active material layer formed on the negative electrode current collector. The compression treatment can be carried out by a known method, for example, using a roll press. In some embodiments, the packing density of the negative electrode active material layer after the compression step is 1.30 to 1.69 g / cm. 3 It is preferable to perform the compression process so that the packing density of the negative electrode active material layer after the compression process is equal to or greater than a predetermined value, thereby achieving a high energy density. Furthermore, the higher the packing density, the more likely the volume of the negative electrode (negative electrode active material layer) to change when repeatedly charged and discharged, and the more likely the conductive path within the negative electrode to break, so that it is particularly effective to apply the technology disclosed herein. From the viewpoint of achieving a high energy density, the packing density of the negative electrode active material layer after the compression process is set to 1.40 g / cm. 3 More than 1.45 g / cm is preferable. 3 The packing density of the negative electrode active material layer is preferably 1.60 g / cm from the viewpoint of preventing cracking or chipping of the second active material. 3 In this manner, the negative electrode disclosed herein can be produced.

[0039] [Negative electrode for secondary batteries] According to the above manufacturing method, it is possible to manufacture a negative electrode for a secondary battery, which 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, as shown in Fig. 3. The negative electrode active material layer contains a carbon material as a second active material and Additive 1. The proportion of Additive 1 is 0.1 to 30 parts by volume when the second active material is taken as 100 parts by volume.

[0040] [Secondary battery manufacturing method] Next, a method for manufacturing the secondary battery disclosed herein will be described. The secondary battery disclosed herein can be manufactured by a manufacturing method including, for example, an electrode body fabrication step (step 10), a construction step (step 20), and an activation step (step 30) in this order. The manufacturing method disclosed herein may further include other steps at any stage.

[0041] (Step 10) The electrode assembly fabrication step is a step of fabricating an electrode assembly using the negative electrode fabricated by the above-mentioned fabrication method. More specifically, a positive electrode and a separator are separately prepared, and for example, the negative electrode fabricated by the above-mentioned fabrication method is placed opposite the positive electrode with the separator interposed therebetween and wound together. This produces an electrode assembly for a secondary battery.

[0042] The positive electrode may be the same as a conventional one and is not particularly limited. The positive electrode typically includes a positive electrode current collector and a positive electrode active material layer adhered to at least one surface of the positive electrode current collector. The positive electrode current collector is preferably made of a metal, more preferably aluminum, an aluminum alloy, nickel, stainless steel, or the like. The positive electrode active material layer contains a positive electrode active material. Compounds conventionally used for this type of application can be used as the positive electrode active material without any particular limitations. Examples include lithium transition metal composite oxides such as lithium nickel cobalt manganese composite oxide. The positive electrode active material layer may contain optional components other than the positive electrode active material, such as various additive components such as binders and conductive materials.

[0043] The separator is a member that insulates the positive electrode from the negative electrode. The separator may be the same as a conventional separator and is not particularly limited. A suitable separator is, for example, a resin porous sheet (microporous membrane) made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator may have a functional layer (for example, an adhesive layer or a heat resistance layer (HRL)) on the surface of the resin porous sheet.

[0044] The construction step (step 20) is a step of constructing a battery assembly for a secondary battery by housing the electrode body 20 produced in the electrode body production step (step 10) and a separately prepared non-aqueous electrolyte in a case. In a preferred embodiment, first, a case body having an opening and a sealing plate (lid) having an inlet hole and sealing the opening of the case body are prepared. Next, the electrode body is placed inside the case body. Next, the sealing plate is welded to the periphery of the opening of the case body to integrate the case body and the sealing plate. Next, a non-aqueous electrolyte is prepared and injected into the case body through the inlet hole of the sealing plate.

[0045] The nonaqueous electrolyte may be the same as conventional ones and is not particularly limited. The nonaqueous electrolyte is typically a nonaqueous electrolytic solution containing a nonaqueous solvent and a supporting salt (electrolyte salt, for example, Li salt or Na salt). Examples of the nonaqueous solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Such nonaqueous solvents may be fluorinated. As the nonaqueous solvent, only one of the above-mentioned compounds may be used, or two or more of them may be mixed and used as a mixed solvent. When a mixed solvent is used, one containing a cyclic carbonate and a chain carbonate is preferred. The nonaqueous electrolyte may further contain additional components as necessary. The nonaqueous electrolyte is typically liquid, but may also be gel-like.

[0046] The activation step (step 30) is a step in which the battery assembly constructed in the construction step (step 20) is charged at least once to decompose at least the film-forming agent of Additive 1, and form an SEI film (coating) derived from the film-forming agent on the surface of the active materials (a silicon material as the first active material and a carbon material as the second active material), as shown in Figures 4(A) and 4(B). This step includes, for example, an initial charging step (step 30-A) and an aging step (step 30-B) in which the battery assembly is maintained at a predetermined voltage. This step may also include other steps at any stage.

[0047] In the initial charging step (step 30-A), the constructed battery assembly is charged at least once. Preferably, the constructed battery assembly is charged and discharged at least once. The charging and discharging of the battery assembly can be performed in a conventional manner. Typically, an external power source is connected between the positive and negative terminals of the battery assembly, and charging or discharging is performed, typically at room temperature (typically about 25°C), until a predetermined state of charge (SOC) is reached between the positive and negative terminals. The voltage reached during charging is preferably adjusted so that the negative electrode is at the decomposition starting potential of the film-forming agent. In one example, the battery assembly is charged at a constant current until the SOC reaches approximately 60% or more, for example, 80 to 100%. Although not particularly limited, a charge rate of 1 C or less, for example, about 0.1 to 0.5 C, is preferred from the viewpoint of slowly decomposing the film-forming agent and forming a uniform or high-quality SEI film on the surface of the active material. Note that 1 C refers to a current value that can fully charge the theoretical capacity (Ah) of the battery in one hour.

[0048] In the aging step (step 30-B), the battery assembly is charged until the potential of the negative electrode reaches or exceeds the decomposition onset potential of the film-forming agent, and is maintained in this charged state. By maintaining the potential of the negative electrode at or above the decomposition onset potential, the film-forming agent is sufficiently decomposed, allowing an SEI film to be formed on the surface of the active material. In one example, the battery assembly is charged until its SOC reaches approximately 60% or more, for example, 80 to 100%. In another example, the battery assembly is charged until its voltage reaches approximately 4 V or more, for example, 4.1 to 4.7 V.

[0049] In this step, it is preferable to maintain the battery assembly at a temperature of 40 to 75°C. That is, it is preferable to perform high-temperature aging. By setting the aging temperature to a predetermined value or higher, it becomes easier to form an SEI film in a short time. From this perspective, the aging temperature is preferably 50°C or higher. By setting the aging temperature to a predetermined value or lower, the SEI film can be formed gradually, and a uniform SEI film can be formed on the surface of the negative electrode active material. Furthermore, the occurrence of unintended side reactions inside the battery assembly can be suppressed. From this perspective, the aging temperature may be 65°C or lower.

[0050] The aging time cannot be generalized because it varies depending on, for example, the aging temperature and the average particle diameter D of additive 1, but from the viewpoint of work efficiency, it may be, for example, 1 to 36 hours, and can be set as a guideline for 5 to 24 hours. As an example, when the aging temperature is about 40 to 75°C, the aging time should be set as a guideline for 6 to 12 hours. In this manner, a secondary battery can be manufactured.

[0051] Although not intended to be particularly restrictive, in the formation step (step 2), additive 1 is suitably arranged in the pores of the hexagonal close-packed structure of the second active material particles, as shown in FIG. 3. As a result, even if the coating agent is electrolyzed in the activation step (step 30), a large change in density of the negative electrode active material layer is unlikely to occur, as shown in FIG. 4(A). Furthermore, because additive 1 contains a silicon material as the first active material particles, the technology disclosed herein allows the first and second active materials to achieve a high negative electrode capacity, thereby enabling a secondary battery with a high energy density.

[0052] In addition, the voids generated when the film-forming agent is electrolyzed are smaller than the voids in the hexagonal close-packed structure of the second active material particles. Furthermore, when the film-forming agent is decomposed, the portions where the film-forming agent of Additive 1 was located become voids. Even if the silicon material expands during charging and discharging, these voids can absorb the expansion, thereby suppressing volumetric changes in the negative electrode active material layer. This facilitates maintaining contact between the second active material particles even with repeated charging and discharging, reducing the likelihood of conductive path breaks within the negative electrode active material layer. Therefore, the conductive network can be stably maintained over a long period of time. Furthermore, by disposing the film-forming agent in each void of the second active material particles, a balanced SEI film can be formed throughout the negative electrode active material layer, as shown in Figure 4(A). The combined effects of these two aspects make it possible to realize a secondary battery with excellent cycle characteristics and with little loss in battery capacity even with repeated charging and discharging.

[0053] [Secondary battery] Fig. 5 is a schematic vertical cross-sectional view of a secondary battery 100. As shown in Fig. 5, 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.

[0054] 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 airtightly sealed (hermetically 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.

[0055] The positive electrode terminal 30 is electrically connected to the positive electrode tab 23 of the electrode assembly 20 via the positive electrode current collecting portion 50 inside the case 10. 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. 5 ). 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.

[0056] 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. 5 ). 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.

[0057] 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).

[0058] As described above, the positive electrode typically includes 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.

[0059] As described above, the negative electrode typically 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. The negative electrode active material layer includes a silicon material as a first active material, a carbon material as a second active material, and a coating (SEI film) disposed on the surfaces of the first active material and the second active material.

[0060] [Uses of secondary batteries and energy storage modules] The secondary battery 100 can be used for a variety of purposes, but is preferably used in applications that require high energy density over a long period of time, such as a power source (driving power source) for motors mounted on vehicles such as passenger cars, trucks, etc. The type of vehicle is not particularly limited, and examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), battery electric vehicles (BEVs), etc.

[0061] 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.

[0062] [Preparation of additives] First, in Examples 1-8 and Comparative Example 1, silicon powder (first active material) having an average particle diameter D1 shown in Table 1 was prepared as the core material, and the type of film-forming agent powder shown in Table 1 was prepared as the coating material. Next, the prepared silicon powder and film-forming agent powder were mixed and subjected to mechanochemical treatment. This produced an additive with a core-shell structure having a core made of silicon particles and a coating made of a film-forming agent. The average particle diameter D of the entire additive is shown in Table 1. In Comparative Example 2, simple silicon having an average particle diameter of 1.0 μm was prepared instead of the above additive.

[0063] [Preparation of negative electrode] Next, natural graphite having an average aspect ratio (length of major axis / length of minor axis) of 1.5 or less and an average particle diameter D2 of 15 μm was prepared as the second active material (preparation step). Next, in Examples 1-8 and Comparative Example 1, the additive prepared above (simple Si in Comparative Example 2) was mixed with 100 parts by volume of the second active material in the ratio (volume parts) shown in Table 1 to prepare a mixed powder. Next, this mixed powder was mixed with CMC as a thickener and SBR as a binder in a mass ratio of mixed powder:thickener:binder = 98:1:1, and an appropriate amount of ion-exchanged water as a solvent was added to prepare a negative electrode composite slurry. Next, the prepared negative electrode composite slurry was applied to a surface of 10 mg / cm2 per side. 2 The negative electrode active material layer was then rolled using a roll press to a packing density of 1.50 g / cm. 3 (compression step) In this manner, a negative electrode was obtained.

[0064] [Secondary battery production] First, a positive electrode and a separator were prepared. For the positive electrode, LiNiCoMnO2 as the positive electrode active material, acetylene black (AB) as the conductive material, and PVdF as the binder were mixed in a mass ratio of positive electrode active material:conductive material:binder = 92:5:3, and an appropriate amount of NMP as the solvent was added to prepare a positive electrode composite slurry. Next, the prepared positive electrode composite slurry was applied to a positive electrode current collector (aluminum foil) and dried to form a positive electrode active material layer. The formed positive electrode active material layer was then rolled using a roll press. In this way, a positive electrode was obtained. A heat-resistant separator was prepared for the separator, consisting of a 4 μm-thick ceramic-containing HRL layer on one side of a porous sheet with a three-layer structure of PP / PE / PP.

[0065] Next, the prepared negative electrode was opposed to the positive electrode via the separator to prepare an electrode assembly (electrode assembly preparation step). The separator was arranged so that the HRL layer faced the positive electrode. Next, the electrode assembly and a non-aqueous electrolyte were housed in a case to construct a battery assembly (construction step). The non-aqueous electrolyte used was a mixed solvent (nonaqueous solvent) containing EC, DMC, and EMC in a volume ratio of 3:3:4, with LiPF6 as a supporting electrolyte at a concentration of 1 mol / L.

[0066] [Evaluation of cycle characteristics] Next, the battery assembly constructed as described above was placed in a thermostatic chamber at 25°C and subjected to constant current charging (CC charging) at a charge rate of 0.1C until the voltage between the positive and negative electrodes reached 4.25V, followed by constant current discharging (CC discharging) at a discharge rate of 0.1C until the voltage between the positive and negative electrodes reached 2.5V (initial charging step, activation step). Next, constant current charging (CC charging) was performed at a charge rate of 0.1C until the voltage between the positive and negative electrodes reached 4.25V, followed by constant voltage charging (CV charging) until the current value reached 1 / 50C, resulting in a fully charged state. Next, while in the fully charged state, aging was performed at 60°C for 12 hours (aging step, activation step). After aging, constant current discharging (CC discharging) was performed at a discharge rate of 0.1C until the voltage between the positive and negative electrodes reached 2.5V. This produced a lithium-ion secondary battery.

[0067] Next, the activated battery was charged at 25°C to 4.25 V at 0.1 C, followed by constant current charging until the current reached 1 / 50 C, and then fully charged. It was then discharged at 0.1 C to 2.5 V at constant current, and the discharge capacity at this point was recorded as the initial capacity. Next, charge-discharge cycles were performed to measure the capacity retention. Specifically, the battery was charged at 0.3 C to 4.25 V, then charged at 0.3 C to 1 / 20 C, followed by constant current discharging at 0.3 C to 2.5 V. This cycle was repeated 199 times. The ratio of the battery capacity at the 199th cycle to the initial capacity was calculated as the capacity retention (%). The results are shown in Table 1.

[0068] [Evaluation of negative electrode capacity] First, a coin cell was fabricated by placing the above-prepared negative electrode opposite a metallic Li electrode. Next, the negative electrode was charged in a CC mode at a charge rate of 0.1 C until the voltage dropped to 3 mV (vs. Li / Li+) relative to the metallic Li counter electrode, and then charged in a CV mode until the current dropped to 1 / 50 C. The negative electrode was then discharged in a CC mode at a discharge rate of 0.1 C until the voltage dropped to 1.6 V (vs. Li / Li+) relative to the metallic Li counter electrode, and then discharged in a CV mode until the current dropped to 1 / 50 C. The capacity [mAh] during CCCV discharge was then divided by the combined weight [g] of the first active material (silicon material) and the second active material (carbon material) to determine the negative electrode capacity (single-electrode capacity). The results are shown in Table 1.

[0069] [Table 1]

[0070] First, Examples 1-3 are compared with Comparative Examples 1 and 2. As shown in Table 1, Comparative Example 2, in which simple silicon was used instead of an additive, had a high negative electrode capacity but a low capacity retention rate of 62.3%. Furthermore, Comparative Example 1, in which the average particle diameter D of the additive was relatively large at 15 μm and the ratio (D1 / D) of the average particle diameter D1 of the first active material to the average particle diameter D of the additive exceeded 0.6, had a significantly low capacity retention rate of 45.5%. The reason for this is thought to be that when the silicon material expanded during charge and discharge, the volume of the negative electrode active material layer changed significantly, causing the conductive paths within the negative electrode active material layer to be interrupted.

[0071] In contrast to these comparative examples, in Example 1-3 in which the average particle diameter D of the additive was 0.5 to 3.0 μm and the ratio (D1 / D) of the average particle diameter D of the first active material to the average particle diameter D of the additive was 0.6 or less, the capacity loss was small even after repeated charge and discharge, and the cycle characteristics were significantly improved. These results demonstrate the technical significance of the technology disclosed herein.

[0072] Next, Examples 3-7 are compared. Figure 6 is a graph showing the negative electrode capacity and capacity retention rate of Example 3-7. As shown in Table 1 and Figure 6, the negative electrode capacity increased as the amount of additive containing a silicon material increased, but the capacity retention rate tended to decrease. When the proportion of the additive and the proportion of the film-forming agent were in the range of 5 to 10 parts by volume, a particularly excellent balance between the negative electrode capacity and the cycle characteristics was achieved.

[0073] 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.

[0074] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: An additive for a secondary battery negative electrode having a core-shell structure, the core portion including a silicon material as a first active material, and a coating portion including a film-forming agent and coating at least a portion of the surface of the core portion, wherein the additive has an average particle diameter of 0.5 μm or more and 3.0 μm or less, and the ratio (D1 / D) of the average particle diameter D of the additive to the average particle diameter D of the first active material is 0.6 or less. Item 2: The additive according to Item 1, wherein the coating portion contains lithium bis(oxalato)borate as the film-forming agent. Item 3: A method for manufacturing a negative electrode for a secondary battery, comprising: a preparation step of preparing a carbon material as a second active material and the additive according to claim 1 or 2; and a formation step of forming a negative electrode active material layer containing the second active material and the additive on a negative electrode current collector, wherein in the formation step, a ratio of the additive is 0.1 parts by volume or more and 30 parts by volume or less when the second active material is taken as 100 parts by volume. Item 4: The manufacturing method according to Item 3, wherein in the preparing step, a carbon material having an average particle size of 5 μm or more and 30 μm or less is prepared. Item 5: The manufacturing method according to Item 3 or 4, wherein in the forming step, the proportion of the additive is 5 parts by volume or more and 10 parts by volume or less when the second active material is taken as 100 parts by volume. Item 6: In the forming step, the negative electrode active material layer has a basis weight of 10 mg / cm 2 6. The manufacturing method according to any one of Items 3 to 5, wherein Item 7: After the forming step, a compressing step of compressing the formed negative electrode active material layer is further included, and in the compressing step, the packing density of the negative electrode active material layer is reduced to 1.30 g / cm 3 More than 1.69g / cm 3 Item 7. The manufacturing method according to any one of Items 3 to 6, wherein: Item 8: A method for producing a secondary battery, comprising: an electrode assembly fabrication step of fabricating an electrode assembly using a negative electrode produced by the manufacturing method according to any one of items 3 to 7; a construction step of housing the electrode assembly and a non-aqueous electrolyte in a case to construct a battery assembly; and an activation step of charging the battery assembly at least once to decompose the film-forming agent in the additive and form a film derived from the film-forming agent on surfaces of the first active material and the second active material. Item 9: The manufacturing method according to Item 8, wherein the activation step includes an aging step of charging and maintaining the battery assembly until the potential of the negative electrode becomes equal to or higher than the decomposition starting potential of the coating agent. Item 10: The manufacturing method according to Item 9, wherein the battery assembly is maintained at a temperature of 40°C or higher and 75°C or lower in the aging step. Item 11: 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 a carbon material as a second active material and the additive according to item 1 or 2, and the proportion of the additive is 0.1 parts by volume or more and 30 parts by volume or less when the second active material is taken as 100 parts by volume. Item 12: The negative electrode for a secondary battery according to Item 11, wherein the second active material has an average particle size of 5 μm or more and 30 μm or less. [Explanation of symbols]

[0075] 1 Additives 2 Core 4 Covering 10 cases 20 Electrode body 100 Secondary battery A1~A7 Second active material particles

Claims

1. An additive for a secondary battery negative electrode having a core-shell structure, the core portion including a silicon material as a first active material, and a coating portion including a film-forming agent and coating at least a part of the surface of the core portion, The average particle diameter D of the additive is 0.5 μm or more and 3.0 μm or less, a ratio (D1 / D) of an average particle diameter D1 of the first active material to an average particle diameter D of the additive is 0.6 or less; Additive for secondary battery negative electrodes.

2. the coating portion contains lithium bis(oxalato)borate as the film-forming agent; The additive of claim 1.

3. a preparation step of preparing a carbon material as a second active material and the additive according to claim 1 or 2; a forming step of forming a negative electrode active material layer containing the second active material and the additive on a negative electrode current collector; Including, In the forming step, the ratio of the additive is set to 0.1 parts by volume or more and 30 parts by volume or less when the second active material is set to 100 parts by volume. A method for producing a negative electrode for a secondary battery.

4. In the preparing step, a carbon material having an average particle size of 5 μm or more and 30 μm or less is prepared. The method of claim 3.

5. In the forming step, the ratio of the additive is set to 5 parts by volume or more and 10 parts by volume or less when the second active material is set to 100 parts by volume. The method of claim 3.

6. In the forming step, the negative electrode active material layer has a basis weight of 10 mg / cm ２ That is all. The method of claim 3.

7. The method further includes a compression step of compressing the formed negative electrode active material layer after the formation step, In the compression step, the packing density of the negative electrode active material layer is set to 1.30 g / cm ３ 1.69g / cm or more ３ The following shall apply: The method of claim 3.

8. an electrode assembly manufacturing step of manufacturing an electrode assembly using the negative electrode manufactured by the manufacturing method according to claim 3; a construction step of housing the electrode assembly and the non-aqueous electrolyte in a case to construct a battery assembly; an activation step of charging the battery assembly at least once to decompose the film-forming agent in the additive and form a coating derived from the film-forming agent on the surfaces of the first active material and the second active material; A method for manufacturing a secondary battery, comprising:

9. the activation step includes an aging step of charging and maintaining the battery assembly until the potential of the negative electrode becomes equal to or higher than the decomposition starting potential of the coating-forming agent; The method of claim 8.

10. In the aging step, the battery assembly is maintained at a temperature of 40°C or higher and 75°C or lower. The method of claim 9.

11. 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 a carbon material as a second active material and the additive according to claim 1 or 2; When the second active material is taken as 100 parts by volume, the ratio of the additive is 0.1 parts by volume or more and 30 parts by volume or less. Negative electrode for secondary batteries.

12. The average particle size of the second active material is 5 μm or more and 30 μm or less. The negative electrode for a secondary battery according to claim 11 .

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