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

Abstract

To provide a method for manufacturing a negative electrode for a secondary battery that is resistant to capacity degradation even after repeated charge and discharge.SOLUTION: A manufacturing method disclosed herein includes a preparation step of preparing a negative electrode active material and a film-forming agent, and a formation step of forming a negative electrode active material layer on a negative electrode current collector. In the preparation step, prepared are a negative electrode active material having an average aspect ratio (longer axis length / minor axis length) of 1.5 or less and a film-forming agent having an average particle diameter D2 that satisfies the following formula 1: D2≤0.155×D1 (where D1 is the average particle diameter of the negative electrode active material). In the formation step, the proportion of the film-forming agent is 0.1 parts by volume or more and 30 parts by volume or less, relative to 100 parts by volume of the negative electrode active material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A known technique involves adding a compound (film-forming agent) that is electrolyzed to form a film during charging to a battery assembly to form a solid electrolyte interphase (SEI) film on the negative electrode. Patent Document 1 (Patent Document 1) is a related art document. Patent Document 1 describes a method for fabricating a negative electrode, including the steps of: applying a first electrode slurry containing negative electrode active material particles but not containing film-forming agent particles onto a negative electrode current collector to form a first electrode layer; and applying a second electrode slurry containing negative electrode active material particles and film-forming agent particles onto the first electrode layer to form a second electrode layer. Figure 1 of Patent Document 1 illustrates the negative electrode active material particles and film-forming agent particles contained in the second electrode layer as being approximately the same size. Patent Document 1 also describes that the film-forming agent contained in the second electrode layer is electrolyzed during the activation step, resulting in voids in the areas of the second electrode layer where the film-forming agent was mixed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-89869 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the inventors' extensive research, the above-mentioned technology has revealed that the voids generated when the coating agent is decomposed are comparable in size to or larger than the size of the negative electrode active material particles. Negative electrode active material particles repeatedly expand and contract during charging and discharging, causing changes in volume. If large voids are present during this process, the contact points between the negative electrode active material particles are lost, making the conductive paths more likely to be interrupted. This results in a problem of reduced battery capacity. This problem is particularly pronounced when the negative electrode active material is spherical, as the particles are more likely to contact each other in a point-like manner.

[0005] The present invention has been made in view of the above circumstances, and its main object is to provide a method for producing a negative electrode for a secondary battery that is less likely to lose capacity even after repeated charge and discharge. [Means for solving the problem]

[0006] The present invention provides a method for manufacturing a negative electrode for a secondary battery, the method comprising: a preparation step of preparing a powdered negative electrode active material and a powdered film-forming agent; and a formation step of forming a negative electrode active material layer containing the negative electrode active material and the film-forming agent on a negative electrode current collector. In the preparation step, a negative electrode active material having an average aspect ratio (length of major axis / length of minor axis) of 1.5 or less and a film-forming agent having an average particle diameter D2 that satisfies the following formula 1: D2≦0.155×D1 (where D1 is the average particle diameter of the negative electrode active material). In the formation step, the proportion of the film-forming agent is 0.1 parts by volume or more and 30 parts by volume or less, relative to 100 parts by volume of the negative electrode active material.

[0007] The film-forming agent having a size satisfying the above formula (1) is suitably arranged in the voids of the hexagonal close-packed structure of the negative electrode active material particles during the formation process. Therefore, even if the film-forming agent is electrolyzed, the formation of large voids in the negative electrode active material layer can be suppressed. This makes it possible to realize a secondary battery in which the conductive paths in the negative electrode active material layer are less likely to be broken even with repeated charge and discharge, and thus the battery capacity is less likely to decrease. [Brief explanation of the drawings]

[0008] [Figure 1]1(A) and (B) are schematic diagrams showing the hexagonal close-packed structure of negative electrode active material particles, where (A) shows T vacancies and (B) shows R vacancies. [Figure 2] FIG. 2 is a schematic cross-sectional view of the negative electrode active material layer after the formation step. [Figure 3] FIG. 3 is a schematic cross-sectional view of the negative electrode active material layer after the activation step. [Figure 4] FIG. 4 is a schematic vertical cross-sectional view of a secondary battery according to one embodiment. [Figure 5] FIG. 5 is a graph showing the capacity retention rates of Examples 3-6 and Comparative Example 3. 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] [Method of manufacturing a negative electrode for secondary batteries] First, a method for manufacturing the negative electrode for a secondary battery disclosed herein will be described. In this specification, the term "secondary battery" refers to any power 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] The negative electrode for a secondary battery disclosed herein can be produced, for example, by a production method including, in this order, a preparation step (step 1), a formation step (step 2), and a compression step (step 3). 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.

[0012] (Step 1) The preparation step is a step of preparing (1) a powdered negative electrode active material and (2) a powdered film-forming agent. The powdered negative electrode active material contains a plurality of negative electrode active material particles. The powdered film-forming agent contains a plurality of film-forming agent particles.

[0013] (1) The negative electrode active material is a material capable of reversibly absorbing and releasing charge carriers. The type of negative electrode active material is not particularly limited, and one or more materials known to be useful for this type of application can be used without any particular limitation. Examples include carbon materials such as graphite, hard carbon, soft carbon, and carbon nanotubes, as well as Si-containing materials such as silicon oxide, silicon carbide, and silicon nitride. Of these, graphite is preferred. The graphite may be natural graphite, artificial graphite, or amorphous carbon-coated graphite, in which core graphite particles are coated with an amorphous carbon material. To achieve the effects of the technology disclosed herein at a higher level, the negative electrode active material preferably contains graphite as the main component (a component accounting for 50% by mass or more), more preferably 80% by mass or more, even more preferably 95% by mass or more, and particularly preferably consists essentially of graphite (98% by mass or more).

[0014] In this embodiment, the negative electrode active material has a spherical shape with an average aspect ratio (length of long side / length of short side) of 1.5 or less. The average aspect ratio of the negative electrode 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 exhibited at a higher level. The average aspect ratio of the negative electrode active material is 1.00 or more, and may be, for example, 1.1 or more from the viewpoint of ease of production, etc.

[0015] In this specification, the term "aspect ratio" refers to the ratio of the length of the long axis to the length of the short axis of a negative electrode active material particle. The aspect ratio of a negative electrode active material particle 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 short axis and long axis of the negative electrode active material particle in the image, and calculating the ratio (long axis length / short axis length). Measuring the lengths of the short axis and long axis of a negative electrode active material particle and calculating the 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 negative electrode 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 particle is.

[0016] Although not particularly limited, the average particle diameter D1 of the negative electrode active material is preferably 1 to 100 μm, more preferably 5 to 50 μm, even more preferably 10 to 30 μm, and particularly preferably 20 μm or less. By setting the average particle diameter D1 within the above range, it is possible to improve cycle characteristics and achieve the effects of the technology disclosed herein at a higher level. In this specification, the term "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 measuring device based on a laser diffraction / scattering method.

[0017] (2) The film-forming agent is a compound that is electrolyzed to form a film mainly during charging. The type of film-forming agent is not particularly limited, and one or more materials that have been known to be usable for this type of application can be used without any particular limitation. As an 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 with 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 negative electrode active material.

[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 structural moiety 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 achieving the effects of the technology disclosed herein at a higher level, oxalato complex compounds containing boron are preferred, and LiBOB is particularly preferred.

[0019] In this embodiment, the average particle diameter D2 of the film-forming agent satisfies the following formula (1): D2≦0.155×average particle diameter D1 of the negative electrode active material. This makes it easier for the film-forming agent to be arranged in the voids of the hexagonal close-packed structure of the negative electrode active material particles in the formation process described later. This will be explained in detail below with reference to the drawings. FIGS. 1(A) and 1(B) are schematic diagrams showing the hexagonal close-packed structure of the negative electrode 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., Fine 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.

[0020] As shown in FIG. 1(A), in the hexagonal close-packed structure, the radius r1 of the largest inscribed circle of a T void surrounded by three spherical negative electrode active material particles A1 to A3 can be expressed as 0.155r, where r is the radius of the negative electrode active material particle. 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 negative electrode active material particles A4 to A7 can be expressed as 0.414r, where r is the radius of the negative electrode active material particle. Therefore, as long as the film-forming agent 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 disposed within the T void and / or R void in the hexagonal close-packed structure of the negative electrode active material particle. Therefore, even if the film-forming agent is electrolyzed, the formation of large voids in the negative electrode active material layer can be suppressed. Conversely, if the average particle diameter D2 of the coating agent is larger than this, the coating agent will not fit into the pores and will tend to be interposed between the negative electrode active material particles, resulting in large voids after the coating agent is electrolyzed, which will tend to cause loss of contact between the negative electrode active material particles. Therefore, the average particle diameter D2 of the coating agent needs to satisfy the above (Equation 1).

[0021] Although not particularly limited, it is preferable that the average particle diameter D2 of the film-forming agent further satisfies the following formula (2): 0.00155 × average particle diameter D1 of the negative electrode active material ≦ D2. When formula (2) is satisfied, that is, when the average particle diameter D2 is equal to or greater than a predetermined value, the film-forming agent particles are less likely to dissolve into the solvent in the forming step (more specifically, the slurry preparation step (step 2-B)) described below. Therefore, the film-forming agent is less likely to be disposed outside the pores, and the effects of the technology disclosed herein can be exhibited at a higher level.

[0022] The average particle diameter D2 of the film-forming agent is not particularly limited because it is determined in relation to the average particle diameter D1 of the negative electrode active material, but is preferably approximately 0.1 to 20 μm, more preferably 0.5 to 15 μm, even more preferably 1 to 10 μm, and 5 μm or less, for example, 1 to 2.5 μm, particularly preferably. Furthermore, the film-forming agent is more preferably spherical with an average aspect ratio (long side length / short side length) of 1.5 or less. The average aspect ratio of the film-forming agent can be determined in the same manner as for the negative electrode active material.

[0023] The (Step 2) forming step is a step of forming a negative electrode active material layer containing the negative electrode active material prepared in the (Step 1) preparing step and a film-forming agent 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.

[0024] 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.

[0025] In the slurry preparation step (step 2-B), a negative electrode composite slurry containing at least the negative electrode active material and the film-forming agent prepared in the preparation step (step 1) is prepared. Specifically, the solid materials for the negative electrode active material layer (e.g., the negative electrode active material, the film-forming agent, the binder, the 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 pastes, inks, etc.

[0026] 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).

[0027] In this embodiment, the proportion of the film-forming agent is 0.1 to 30 parts by volume when the negative electrode active material is taken as 100 parts by volume. By setting the proportion of the film-forming agent to a predetermined value or more, a larger amount of SEI film can be formed on the surface of the negative electrode active material, and the effects of the technology disclosed herein can be exhibited at a higher level. From this perspective, the proportion of the film-forming agent is preferably 0.5 parts by volume or more, more preferably 1 part by volume or more, even more preferably 5 parts by volume or more, and particularly preferably 10 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. The proportion of the film-forming agent may be 25 parts by volume or less.

[0028] Although not particularly limited, the total proportion of the negative electrode active material (e.g., graphite) and the coating agent 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 negative electrode active material and the coating agent is preferably 99% by mass or less, more preferably 98.5% by mass or less, and even more preferably 98% by mass or less.

[0029] 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 (the proportion relative to the total solid content is preferably less than 0.1 mass%). This makes it easier for the coating agent to be arranged in the T pores and / or R pores in the hexagonal close-packed structure of the negative electrode active material particles.

[0030] 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 coating device and dried. This causes a negative electrode active material layer containing the negative electrode active material and the coating film-forming agent to adhere to the surface of the negative electrode current collector. The coating 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.

[0031] 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:

[0032] The negative electrode active material layer preferably contains 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 the lower layer and the upper layer each contain a film-forming agent. This allows for a relatively high level of suppression of a decrease in battery capacity and improved 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.

[0033] (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 and chipping of the negative electrode active material. 3 In this manner, the negative electrode disclosed herein can be produced.

[0034] [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. 2. The negative electrode active material layer includes a negative electrode active material and a film-forming agent. The negative electrode active material has an average aspect ratio (long side / short side) of 1.5 or less, and the film-forming agent has an average particle diameter D2 that satisfies the following formula 1: D2≦0.155×D1 (where D1 is the average particle diameter of the negative electrode active material). The proportion of the film-forming agent is 0.1 to 30 parts by volume when the negative electrode active material is taken as 100 parts by volume.

[0035] [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.

[0036] (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.

[0037] 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 includes a positive electrode active material capable of reversibly absorbing and releasing charge carriers. Compounds conventionally used for this type of application can be used as the positive electrode active material without particular limitation. Examples include lithium transition metal composite oxides such as lithium nickel cobalt manganese composite oxide. The positive electrode active material layer may include optional components other than the positive electrode active material, such as various additive components such as binders and conductive materials.

[0038] 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.

[0039] 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.

[0040] 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 an additive, if necessary. The nonaqueous electrolyte is typically liquid, but may also be gel-like.

[0041] 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 the film-forming agent in at least the negative electrode, and form an SEI film (coating) derived from the film-forming agent on the surface of the negative electrode active material, as shown in FIG. 3. 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 held at a predetermined voltage. This step may also include other steps at any stage.

[0042] 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 negative electrode active material. Here, 1 C refers to a current value that can fully charge the theoretical capacity (Ah) of the battery in one hour.

[0043] 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 then maintained in this charged state. Maintaining the negative electrode at a potential above the decomposition onset potential allows the film-forming agent in the negative electrode to be sufficiently decomposed, allowing an SEI film to be formed on the surface of the negative electrode active material. In one example, the battery assembly may be charged until its SOC reaches approximately 60% or more, e.g., 80 to 100%. In another example, the battery assembly may be charged until its voltage reaches approximately 4 V or more, e.g., 4.1 to 4.7 V.

[0044] 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.

[0045] The aging time cannot be generalized because it can vary depending on factors such as the aging temperature and the average particle diameter D2 of the coating agent, but from the standpoint of work efficiency, it may be, for example, 1 to 36 hours, and can be set to 5 to 24 hours as a guideline. As an example, when the aging temperature is about 40 to 75°C, the aging time should be set to 6 to 12 hours as a guideline. In this manner, a secondary battery can be manufactured.

[0046] Although not intended to be particularly restrictive, in the formation step (step 2), the film-forming agent having a size satisfying the above formula (1) is suitably arranged in the voids of the hexagonal close-packed structure of the spherical negative electrode active material particles, as shown in FIG. 2. As a result, as shown in FIG. 3, even when the film-forming 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, thereby achieving a high energy density. Furthermore, the voids generated when the film-forming agent is electrolyzed are smaller in size than the voids of the hexagonal close-packed structure of the negative electrode active material particles. This makes it easier to maintain contact between the negative electrode active material particles even after repeated charge / discharge cycles, and prevents disconnection of the conductive path within the negative electrode active material layer. Therefore, the conductive network can be stably maintained over a long period of time. Furthermore, by arranging the film-forming agent in each void, a balanced SEI film can be formed throughout the negative electrode active material layer, as shown in FIG. 3. Due to the combined effects described above, the technology disclosed herein can realize a secondary battery that is less likely to lose battery capacity even after repeated charge / discharge cycles and has excellent cycle characteristics.

[0047] [Secondary battery] Fig. 4 is a schematic longitudinal sectional view of a secondary battery 100. As shown in Fig. 4, 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.

[0048] 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.

[0049] 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. 4 ). 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.

[0050] 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. 4 ). Here, the negative electrode terminal 40 is crimped to the peripheral portion surrounding the terminal pull-out hole 19 of the sealing plate 14 by crimping.

[0051] 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).

[0052] 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.

[0053] 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 negative electrode active material and a coating (SEI film) disposed on the surface of the negative electrode active material.

[0054] [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.

[0055] 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.

[0056] [Preparation of negative electrode] First, 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 D1 shown in Table 1 was prepared as the negative electrode active material. Furthermore, a compound of the type and average particle diameter D2 shown in Table 1 was prepared as the film-forming agent (preparation step). Next, the film-forming agent was mixed with 100 parts by volume of the negative electrode active material in the ratio (parts by volume) 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.

[0057] [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.

[0058] 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.

[0059] 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.

[0060] [Evaluation of cycle characteristics] 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.

[0061] [Table 1]

[0062] First, Example 1-3 and Comparative Example 1-3 are compared. As shown in Table 1, Comparative Examples 1 and 2, in which the average particle diameter D1 of the negative electrode active material and the average particle diameter D2 of the film-forming agent did not satisfy (Equation 1), and Comparative Example 3, in which the proportion of the film-forming agent exceeded 30 parts by volume (the maximum value based on the void ratio when the negative electrode active material particles are closest packed), had relatively low capacity retention rates. The reason for this is thought to be that large voids were generated in the negative electrode active material layer when the film-forming agent was electrolyzed, resulting in a loss of contact between the negative electrode active material particles due to repeated charge and discharge, and thus a break in the conductive path.

[0063] In contrast to these comparative examples, Example 1-3, in which the average particle diameter D1 of the negative electrode active material and the average particle diameter D2 of the coating film forming agent satisfied Formula 1, showed little capacity loss even after repeated charge and discharge, and relatively excellent cycle characteristics. This demonstrates the effectiveness of the technology disclosed herein.

[0064] Next, Examples 3-6 are compared. Figure 5 is a graph showing the relationship between the proportion of the film-forming agent and the capacity retention rate for Examples 3-6 and Comparative Example 3. As shown in Table 1 and Figure 5, when the proportion of the film-forming agent was in the range of 10 to 30 parts by volume, the capacity retention rate was relatively high, at 90% or more in all cases, and the cycle characteristics were excellent. Furthermore, the effect of improving the capacity retention rate generally plateaued when the proportion of the film-forming agent reached 10 parts by volume.

[0065] Next, Examples 7-9 are compared. As shown in Table 1, it was found that the effects of the technology disclosed herein can be obtained at least when the average particle diameter D1 of the negative electrode active material is in the range of 10 to 30 μm. In addition, a graph was created in which the average particle diameter D1 of the negative electrode active material of Examples 7-9 is plotted on the X axis and the capacity retention rate is plotted on the Y axis, and an approximate curve was obtained, where y = -0.025x + 94.267(R 2 =0.9868) was obtained. Therefore, it was inferred that even if the average particle diameter D1 of the negative electrode active material is, for example, about 100 μm, the capacity retention rate can be maintained at 90% or more.

[0066] 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.

[0067] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A method for manufacturing a negative electrode for a secondary battery, comprising: a preparation step of preparing a powdered negative electrode active material and a powdered film-forming agent; and a formation step of forming a negative electrode active material layer containing the negative electrode active material and the film-forming agent on a negative electrode current collector, wherein the preparation step prepares a negative electrode active material having an average aspect ratio (length of major axis / length of minor axis) of 1.5 or less, and a film-forming agent having an average particle diameter D2 that satisfies the following (Formula 1): D2≦0.155×D1 (where D1 is the average particle diameter of the negative electrode active material); and the formation step adjusts the proportion of the film-forming agent to 0.1 parts by volume or more and 30 parts by volume or less relative to 100 parts by volume of the negative electrode active material. Item 2: The manufacturing method according to Item 1, wherein in the preparing step, the negative electrode active material is prepared having an average particle diameter D1 of 1 μm or more and 100 μm or less. Item 3: The manufacturing method according to Item 1 or 2, wherein in the forming step, the proportion of the coating agent is 10 parts by volume or more and 30 parts by volume or less when the negative electrode active material is 100 parts by volume. Item 4: The production method according to any one of Items 1 to 3, wherein in the preparation step, at least lithium bis(oxalato)borate is prepared as the film-forming agent. Item 5: In the forming step, the coating weight of the negative electrode active material layer is 10 mg / cm 2 5. The manufacturing method according to any one of Items 1 to 4, wherein Item 6: 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 6. The manufacturing method according to any one of Items 1 to 5, wherein: Item 7: 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 1 to 6; 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 and form a film derived from the film-forming agent on a surface of the negative electrode active material. Item 8: The manufacturing method according to Item 7, 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 9: The manufacturing method according to Item 8, wherein the battery assembly is maintained at a temperature of 40°C or higher and 75°C or lower in the aging step. Item 10: 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 including a negative electrode active material and a film-forming agent, the negative electrode active material having an average aspect ratio (long side / short side) of 1.5 or less, the film-forming agent having an average particle diameter D2 that satisfies the following (Formula 1): D2≦0.155×D1 (where D1 is the average particle diameter of the negative electrode active material), and the proportion of the film-forming agent is 0.1 parts by volume or more and 30 parts by volume or less when the negative electrode active material is taken as 100 parts by volume. Item 11: The negative electrode for a secondary battery according to Item 10, wherein the average particle diameter D1 is 1 μm or more and 100 μm or less. Item 12: The negative electrode for a secondary battery according to Item 10 or 11, wherein the film-forming agent contains lithium bis(oxalato)borate. [Explanation of symbols]

[0068] 10 cases 20 Electrode body 100 Secondary battery A1~A7 Negative electrode active material particles

Claims

1. a preparation step of preparing a powdered negative electrode active material and a powdered coating agent; a forming step of forming a negative electrode active material layer containing the negative electrode active material and the film-forming agent on a negative electrode current collector; Including, In the preparing step, a negative electrode active material having an average aspect ratio (length of major axis / length of minor axis) of 1.5 or less; a film-forming agent having an average particle diameter D2 that satisfies the following (formula 1): D2≦0.155×D1 (where D1 is the average particle diameter of the negative electrode active material); Prepare In the forming step, the ratio of the film-forming agent is set to 0.1 parts by volume or more and 30 parts by volume or less relative to 100 parts by volume of the negative electrode active material. A method for producing a negative electrode for a secondary battery.

2. In the preparing step, the negative electrode active material having the average particle diameter D1 of 1 μm or more and 100 μm or less is prepared. The method of claim 1.

3. In the forming step, the ratio of the coating agent is set to 10 parts by volume or more and 30 parts by volume or less when the negative electrode active material is set to 100 parts by volume. The method according to claim 1 or 2.

4. In the preparing step, at least lithium bis(oxalato)borate is prepared as the film-forming agent. The method according to claim 1 or 2.

5. In the forming step, the negative electrode active material layer has a basis weight of 10 mg / cm ２ That is all. The method according to claim 1 or 2.

6. 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 according to claim 1 or 2.

7. an electrode assembly manufacturing step of manufacturing an electrode assembly using the negative electrode manufactured by the manufacturing method according to claim 1 or 2; 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 and form a film derived from the film-forming agent on the surface of the negative electrode active material; A method for manufacturing a secondary battery, comprising:

8. 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 film-forming agent; The method of claim 7.

9. 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 8.

10. 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 a negative electrode active material and a film-forming agent, the negative electrode active material has an average aspect ratio (long side / short side) of 1.5 or less, The coating agent has an average particle diameter D2 that satisfies the following (Formula 1): D2≦0.155×D1 (where D1 is the average particle diameter of the negative electrode active material); the ratio of the film-forming agent is 0.1 parts by volume or more and 30 parts by volume or less when the negative electrode active material is taken as 100 parts by volume; Negative electrode for secondary batteries.

11. The average particle diameter D1 is 1 μm or more and 100 μm or less. The negative electrode for a secondary battery according to claim 10.

12. the film-forming agent comprises lithium bis(oxalato)borate; The negative electrode for a secondary battery according to claim 10 or 11.

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