Secondary battery

Abstract

The present invention facilitates improvement in charge / discharge cycle characteristics of a secondary battery including an electrode body in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. This secondary battery is characterized by comprising: an electrode body in which a positive electrode and a negative electrode (12) are wound with a separator interposed therebetween; and an electrolytic solution. The secondary battery is characterized in that: the negative electrode (12) is provided with a negative electrode current collector (40) and negative electrode active material layers (42) disposed on both surfaces of the negative electrode current collector (40); the negative electrode active material layers (42) include a first negative electrode active material layer (42a) that is disposed on, of the two surfaces of the negative electrode current collector (40), an inner circumferential-side surface (40a) of the negative electrode current collector (40) on the winding inner circumferential side, and a second negative electrode active material layer (42b) that is disposed on an outer circumferential-side surface (40b) of the negative electrode current collector (40) on the winding outer circumferential side; and the ratio (D1 / D2) of the volume density (D1) of the first negative electrode active material layer (42a) to the volume density (D2) of the second negative electrode active material layer (42b) is not less than 0.85 but less than 1.0.

Description

secondary battery

[0001] The present disclosure relates to secondary batteries.

[0002] BACKGROUND ART In recent years, secondary batteries have been widely used as high-power, high-energy density batteries. These secondary batteries have a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and charge and discharge are performed by transferring lithium ions or the like between the positive electrode and the negative electrode.

[0003] For example, Patent Document 1 discloses a secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, in which the thickness of the positive electrode active material layer on the inside of the winding is thinner than the thickness of the positive electrode active material layer on the outside of the winding, and the thickness of the negative electrode active material layer on the outside of the winding is thinner than the thickness of the negative electrode active material layer on the inside of the winding. Patent Document 1 also describes that by making the thickness of the positive electrode active material layer on the inside of the winding thinner than the thickness of the positive electrode active material layer on the outside of the winding, stress applied to the positive electrode active material layer on the inside of the winding can be alleviated and cracks and breaks can be suppressed, and that by making the thickness of the negative electrode active material layer on the outside of the winding thinner than the thickness of the negative electrode active material layer on the inside of the winding, unnecessary volume can be eliminated and capacity can be improved.

[0004] JP 2009-54598 A

[0005] When an electrode assembly is fabricated by winding a positive electrode and a negative electrode, the volume density of the negative electrode active material layer on the inner side of the winding increases, for example, near the center of the electrode assembly where the radius of curvature is small, and the voids that serve as paths for the electrolyte to move may decrease. If the voids that serve as paths for the electrolyte to move in the negative electrode active material layer on the inner side of the winding decrease, this leads to a deterioration in charge-discharge cycle characteristics.

[0006] Therefore, an object of the present disclosure is to improve the charge-discharge cycle characteristics of a secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween.

[0007] A secondary battery according to one aspect of the present disclosure includes an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and an electrolyte solution. The negative electrode includes a negative electrode current collector and negative electrode active material layers disposed on both surfaces of the negative electrode current collector. The negative electrode active material layers include a first negative electrode active material layer disposed on one of both surfaces of the negative electrode current collector, the inner surface of the negative electrode current collector being on the inner side of the winding, and a second negative electrode active material layer disposed on the outer surface of the negative electrode current collector being on the outer side of the winding, and the ratio (D1 / D2) of the volume density (D1) of the first negative electrode active material layer to the volume density (D2) of the second negative electrode active material layer is 0.85 or more and less than 1.0.

[0008] According to the present disclosure, it is possible to improve the charge-discharge cycle characteristics of a secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween.

[0009] Fig. 1 is a cross-sectional view of a secondary battery according to an embodiment; Fig. 2 is a schematic cross-sectional view showing the configuration of a negative electrode before winding; Fig. 3 is a partial cross-sectional view of a negative electrode when the negative electrode is wound, as seen from the direction of the winding axis; Fig. 4 is a schematic cross-sectional view showing another example of the configuration of a negative electrode before winding.

[0010] Hereinafter, an example of an embodiment will be described with reference to the drawings. Note that the secondary battery of the present disclosure is not limited to the embodiment described below. Also, the drawings referred to in the description of the embodiment are schematic.

[0011] Fig. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in Fig. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, an electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, and a battery case 15 serving as an exterior body. The battery case 15 is composed of a case body 16 that houses the electrode assembly 14, the electrolyte, and the like, and a sealing body 17 that closes the opening of the case body 16. The battery case 15 is not limited to a cylindrical, rectangular, or other metal case, and may also be a resin case formed by laminating a resin sheet (a so-called laminate type).

[0012] The electrolyte may be an aqueous electrolyte, but is preferably a nonaqueous electrolyte containing a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. Examples of nonaqueous solvents include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of nonaqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The nonaqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF ６ Lithium salts such as

[0013] The case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0014] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 may deform and rupture, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 may rupture, and gas may be discharged from the opening of the cap 27.

[0015] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom side of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0016] The positive electrode 11, the negative electrode 12, and the separator 13 will be described in detail below.

[0017] [Positive Electrode] The positive electrode 11 has, for example, a positive electrode current collector and positive electrode active material layers disposed on both surfaces of the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface layer. The positive electrode active material layer is composed of, for example, a positive electrode active material, a binder, a conductive material, etc. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, etc. to the surface of the positive electrode current collector, drying the coating, and then rolling it to form positive electrode active material layers on both surfaces of the positive electrode current collector.

[0018] Examples of conductive materials contained in the positive electrode active material layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes, etc. Examples of binders contained in the positive electrode active material layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, styrene-butadiene rubber (SBR), cellulose derivatives such as carboxymethyl cellulose (CMC) or a salt thereof, and polyethylene oxide (PEO).

[0019] As the positive electrode active material, for example, a lithium transition metal composite oxide or the like is used. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among them, it is preferable to contain at least one of Ni, Co, and Mn. An example of a suitable lithium transition metal composite oxide is represented by the general formula LiMO ２ (M is Ni and X, X is a metal element other than Ni, and the proportion of Ni is 50 mol % to 95 mol % based on the total number of moles of the metal elements excluding Li). Examples of X in the above formula include Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, etc.

[0020] [Negative Electrode] Fig. 2 is a schematic cross-sectional view showing the configuration of the negative electrode before winding. As shown in Fig. 2, the negative electrode 12 includes a negative electrode current collector 40 and a negative electrode active material layer 42 disposed on both surfaces of the negative electrode current collector 40. For the negative electrode current collector 40, a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, a film having such a metal disposed on the surface layer, or the like can be used.

[0021] 3 is a partial cross-sectional view of the negative electrode as viewed from the direction of the winding axis when the negative electrode is wound. As shown in FIG. 3 , the negative electrode active material layer 42 has a first negative electrode active material layer 42a disposed on the inner circumferential surface 40a of the negative electrode current collector 40, which is on the inner circumferential side of the winding, and a second negative electrode active material layer 42b disposed on the outer circumferential surface 40b of the negative electrode current collector 40, which is on the outer circumferential side of the winding, of both surfaces of the negative electrode current collector 40. Note that the inner circumferential surface 40a of the negative electrode current collector 40 refers to the surface of the negative electrode current collector 40 located radially inward of the wound negative electrode 12 when the negative electrode 12 is wound, and the outer circumferential surface 40b of the negative electrode current collector 40 refers to the surface of the negative electrode current collector 40 located radially outward of the wound negative electrode 12.

[0022] In this embodiment, the ratio (D1 / D2) of the volume density (D1) of the first negative electrode active material layer 42a to the volume density (D2) of the second negative electrode active material layer 42b is 0.85 or more and less than 1.0. By making D1 / D2 less than 1, for example, near the center of the electrode assembly 14 where the radius of curvature is small, an increase in the volume density of the first negative electrode active material layer 42a on the inside of the winding is reduced, and voids that serve as paths for the movement of the electrolyte are secured. Furthermore, by making D1 / D2 0.85 or more, voids that serve as paths for the movement of the electrolyte are secured on the side of the second negative electrode active material layer 42b on the outside of the winding. In other words, by making D1 / D2 0.85 or more and less than 1.0, voids that serve as paths for the movement of the electrolyte in the negative electrode active material layer 42 are secured, thereby enabling the charge / discharge cycle characteristics of a secondary battery including the wound electrode assembly 14 to be improved.

[0023] The upper limit of D1 / D2 is preferably 0.97 or less, and more preferably 0.93 or less, in order to increase the voids in the first anode active material layer 42a and further improve the charge-discharge cycle characteristics of the secondary battery. The lower limit of D1 / D2 is preferably 0.88 or more, in order to ensure the voids in the second anode active material layer 42b and further improve the charge-discharge cycle characteristics of the secondary battery. The volume density (D1) of the first anode active material layer 42a is, for example, 1.4 g / cm3, in order to ensure that voids serving as paths for the electrolyte are easily ensured. ３ Above, 1.6g / cm ３ It is preferable that:

[0024] A method for measuring the volume densities of the first negative electrode active material layer 42a and the second negative electrode active material layer 42b will be described. First, a cutter is used to punch out a 1 cm x 1 cm piece from the negative electrode 12 to obtain a negative electrode sample I. The thickness of the negative electrode sample I is measured using a thickness gauge. Next, the mass of the negative electrode sample I is measured. Next, the first negative electrode active material layer 42a is removed from the negative electrode sample I to obtain a negative electrode sample II in which only the second negative electrode active material layer 42b is disposed on the negative electrode current collector 40. The thickness and mass of the negative electrode sample II are measured. Next, the second negative electrode active material layer 42b is removed from the negative electrode sample II to obtain a negative electrode sample III made of the negative electrode current collector 40. The thickness and mass of the negative electrode sample III are measured.

[0025] Next, the thickness of the first anode active material layer 42a is calculated by subtracting the thickness of the anode sample II from the thickness of the anode sample I, and the volume of the first anode active material layer 42a is calculated. In addition, the mass of the first anode active material layer 42a is calculated by subtracting the mass of the anode sample II from the mass of the anode sample I. Then, the volume density (g / cm) of the first anode active material layer 42a is calculated by dividing the mass of the first anode active material layer 42a by the volume of the first anode active material layer 42a. ３ ) is calculated.

[0026] The thickness of the second anode active material layer 42b was calculated by subtracting the thickness of the anode sample III from the thickness of the anode sample II, and the volume of the second anode active material layer 42b was calculated. The mass of the second anode active material layer 42b was also calculated by subtracting the mass of the anode sample III from the mass of the anode sample II. The volume density (g / cm) of the second anode active material layer 42b was then calculated by dividing the mass of the second anode active material layer 42b by the volume of the second anode active material layer 42b. ３ ) is calculated.

[0027] In this embodiment, the thickness of the first anode active material layer 42a is preferably greater than the thickness of the second anode active material layer 42b. Generally, as the thickness of the first anode active material layer 42a decreases, the volume density of the first anode active material layer 42a on the inner side of the winding increases, making it difficult to ensure voids that serve as paths for the electrolyte. On the other hand, as in this embodiment, by setting D1 / D2 to 0.85 or greater and less than 1.0, the thickness of the first anode active material layer 42a on the inner side of the winding is increased, thereby mitigating the increase in the volume density of the first anode active material layer 42a and ensuring voids that serve as paths for the electrolyte. Furthermore, increasing the thickness of the first anode active material layer 42a leads to improved battery capacity. The thickness of the first anode active material layer 42a is determined by measuring the thickness of the first anode active material layer at 10 arbitrary positions and averaging the measured values. The same applies to the second anode active material layer 42b. The thickness of each of the first negative electrode active material layer 42a and the second negative electrode active material layer 42b depends on the size of the secondary battery, but is set, for example, in the range of 20 μm or more and 200 μm or less.

[0028] In this embodiment, the specific surface area of ​​the first anode active material layer 42a is preferably smaller than the specific surface area of ​​the second anode active material layer 42b, since this makes it easier to ensure voids that serve as paths for the electrolyte to move in the first anode active material layer 42a, thereby further improving the charge-discharge cycle characteristics. Furthermore, in this embodiment, the ratio (B1 / B2) of the specific surface area (B1) of the first anode active material layer 42a to the specific surface area (B2) of the second anode active material layer 42b is preferably 0.7 or more and 0.95 or less, and more preferably 0.75 or more and 0.85 or less. The specific surface areas of the first anode active material layer 42a and the second anode active material layer 42b are each, for example, 0.5 m ２ / g to 10m ２ / g. The specific surface areas of the first negative electrode active material layer 42a and the second negative electrode active material layer 42b are measured by the BET method using a conventionally known specific surface area measurement device (for example, Macsorb (registered trademark) HM model-1201 manufactured by Mountech Co., Ltd.). The specific surface area measurement by the BET method using the specific surface area measurement device is performed on the negative electrode active material layers after initial charging and discharging. For example, a measurement sample is prepared by punching out a negative electrode into a predetermined shape after initial charging and discharging. Then, the specific surface area of ​​the first negative electrode active material layer or the second negative electrode active material layer of the measurement sample is measured by the BET method using the specific surface area measurement device. Note that the measurement sample only needs to have the first negative electrode active material layer or the second negative electrode active material layer on its surface, and may also have other components such as a negative electrode current collector.

[0029] In this embodiment, the pore size distribution of the first negative electrode active material layer 42 a measured by mercury porosimetry has a maximum value (V Ｒ１ ) in the pore size distribution of the second negative electrode active material layer 42b measured by mercury porosimetry. The pore size distribution of the second negative electrode active material layer 42b has a maximum value (V Ｒ２ ) and the maximum value of the peak R2 (V Ｒ２ ) the maximum value of peak R1 (V Ｒ１ ) ratio (V Ｒ１ / V Ｒ２) is preferably greater than 1, more preferably 1.2 or more and 10.0 or less, and even more preferably 1.2 or more and 2.0 or less. Ｒ１ / V Ｒ２ By making V larger than 1, it is easier to ensure voids that serve as paths for the movement of the electrolyte in the first negative electrode active material layer 42a on the inside of the winding, and the charge-discharge cycle characteristics are further improved. Ｒ１ / V Ｒ２ By setting the upper limit to 10.0 or less, voids that serve as paths for the movement of the electrolyte in the second negative electrode active material layer 42b on the outer side of the winding are easily ensured, and the charge-discharge cycle characteristics are further improved.

[0030] In mercury intrusion porosimetry, mercury is forced into the pores of a solid sample by pressurizing the mercury, and the diameter and volume of the pores are calculated from the pressure applied to the mercury and the amount of mercury forced into the pores. When mercury to which pressure P is applied can penetrate into a pore of diameter D, the diameter D of the pore can be determined from the pressure P, the contact angle θ of the mercury, and the surface tension σ of the mercury according to the following formula. The pore volume is also calculated from the amount of mercury forced into the pore. In this disclosure, the pore diameter refers to the diameter of the pore. -4σ cos θ = PD

[0031] The pore size distribution measured by mercury intrusion porosimetry is expressed as the log differential pore volume (cm ３ / g) against the average pore diameter (μm) in each measurement section (pore distribution curve), where the horizontal axis represents the pore diameter (μm) and the vertical axis represents the log differential pore volume (cm ３ In the present disclosure, the maximum value of the peak in the pore size distribution means the value of the log differential pore volume at the apex of the peak.

[0032] The pore size distribution measurement by mercury porosimetry is performed on the negative electrode active material layer after initial charging and discharging. For example, a measurement sample is prepared by punching out a negative electrode into a predetermined shape after initial charging and discharging. Then, the pore size distribution is measured by mercury porosimetry on the first negative electrode active material layer or the second negative electrode active material layer of the measurement sample. Note that the measurement sample only needs to have the first negative electrode active material layer or the second negative electrode active material layer on its surface, and may also have other components such as a negative electrode current collector.

[0033] Measurement of pore size distribution by mercury intrusion porosimetry can be performed using an apparatus such as the Autopore IV9500 series manufactured by Micromeltix. During measurement, a measurement sample is sealed in a sample container under an inert atmosphere, and mercury is injected into the sample container to apply pressure to the mercury. The pressure applied to the mercury is adjusted appropriately depending on the pore size that the measurement sample may have, and is not particularly limited. For example, it is preferable to perform measurement by changing the pressure from 0.5 psi (3.4 kPa) to 60,000 psi (413,400 kPa) in order to be able to measure pore sizes over a wide range.

[0034] The negative electrode active material layer 42 includes, for example, a negative electrode active material, a binder, etc. The negative electrode active material preferably includes a first carbon material and a second carbon material having different BET specific surface areas. The BET specific surface area of ​​the first carbon material is smaller than the BET specific surface area of ​​the second carbon material. In this way, by using a first carbon material having a small BET specific surface area and a second carbon material having a large BET specific surface area, it becomes easier to adjust the volume density of the first negative electrode active material layer 42a and the second negative electrode active material layer 42b, for example.

[0035] Examples of the first carbon material and the second carbon material include natural graphite such as flake graphite, lump graphite, and amorphous graphite; artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB); and non-graphitizable carbon materials. Among these, graphite materials are preferred in terms of charge / discharge cycle characteristics. As with the positive electrode 11, the binder contained in the negative electrode active material layer 42 may be a fluororesin, PAN, polyimide resin, acrylic resin, polyolefin resin, or the like, but preferably styrene-butadiene rubber (SBR) or a modified product thereof. The negative electrode active material layer 42 may contain, in addition to SBR, CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, or the like.

[0036] The BET specific surface area of ​​the first carbon material is, for example, 0.5 m ２ / g or more, 3.0m ２ / g, and preferably less than 1.0 m ２ / g or more, 2.5m ２The BET specific surface area of ​​the second carbon material is preferably 3.0 m / g or less. ２ / g or more, 10.0m ２ / g or less, and ２ / g or more, 9.0m ２ / g or less is more preferable. When the BET specific surface area of ​​the first carbon material is smaller than the BET specific surface area of ​​the second carbon material and the BET specific surface areas of the first carbon material and the second carbon material satisfy the above range, for example, it becomes easy to adjust the volume density of the first negative electrode active material layer 42a and the second negative electrode active material layer 42b. The BET specific surface area of ​​the carbon material is measured by the BET method using a conventionally known specific surface area measuring device (for example, Macsorb (registered trademark) HM model-1201 manufactured by Mountech Co., Ltd.), similar to the measurement of the specific surface area of ​​the negative electrode active material layer described above.

[0037] The content of the first carbon material in the first anode active material layer 42a is preferably greater than the content of the first carbon material in the second anode active material layer 42b. Increasing the content of the first carbon material in the first anode active material layer 42a facilitates obtaining a first anode active material layer 42a with a low volume density, and facilitates ensuring a greater number of voids in the first anode active material layer 42a that serve as paths for electrolyte migration. As a result, the charge / discharge cycle characteristics of the secondary battery may be further improved. The content of the first carbon material in the first anode active material layer 42a is, for example, preferably 20% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 30% by mass or less, relative to the mass of the first anode active material layer 42a. The content of the first carbon material in the second anode active material layer 42b is, for example, preferably less than 20% by mass, and more preferably 10% by mass or more and less than 20% by mass, relative to the mass of the second anode active material layer 42b.

[0038] FIG. 4 is a schematic cross-sectional view showing another example of the configuration of a negative electrode. The negative electrode 12 shown in FIG. 4 is in a state before winding. In FIG. 4, of the two surfaces of the negative electrode current collector 40, the surface facing downward in FIG. 4 is referred to as the inner circumferential surface 40a of the negative electrode current collector 40, and the surface facing upward in FIG. 4 is referred to as the outer circumferential surface 40b of the negative electrode current collector 40. As shown in FIG. 4, the first negative electrode active material layer 42a has a lower layer 43 disposed on the inner circumferential surface 40a of the negative electrode current collector 40 and an upper layer 44 disposed on the lower layer 43. The content of the first carbon material in the upper layer 44 is preferably greater than the content of the first carbon material in the lower layer 43. Increasing the content of the first carbon material in the upper layer 44 makes it easier to obtain an upper layer 44 with a low volume density, and makes it easier to ensure more voids in the upper layer 44 that serve as paths for the electrolyte to move. That is, the electrolyte solution is more likely to penetrate from the surface of the first negative electrode active material layer 42a to the interior. As a result, the charge / discharge cycle characteristics of the secondary battery may be further improved. The content of the first carbon material in the upper layer 44 is, for example, preferably 50 mass% or more, and more preferably 50 mass% or more and less than 95 mass%, relative to the mass of the upper layer 44. The content of the first carbon material in the lower layer 43 is, for example, preferably 0 mass% or more and less than 50 mass%, and more preferably 3 mass% or more and less than 25 mass%, relative to the mass of the lower layer 43.

[0039] The thickness of the upper layer 44 is, for example, 20% to 50% of the thickness of the first negative electrode active material layer 42a, and the thickness of the lower layer 43 is, for example, 50% to 80% of the thickness of the first negative electrode active material layer 42a.

[0040] The porosity of the upper layer 44 is preferably higher than that of the lower layer 43. This allows the electrolyte to easily penetrate from the surface of the first negative electrode active material layer 42a to the interior thereof. As a result, the charge / discharge cycle characteristics of the secondary battery may be further improved. The porosities of the upper layer 44 and the lower layer 43 are each, for example, in the range of 10% or more and 30% or less.

[0041] The porosity of the upper layer 44 and the lower layer 43 can be determined by analyzing SEM images observed with a scanning electron microscope (SEM). For example, a portion of the negative electrode 12 is cut out and processed using an ion milling device (e.g., IM4000PLUS, manufactured by Hitachi High-Technologies Corporation) to expose a cross section of the first negative electrode active material layer 42a. Using a scanning electron microscope, backscattered electron images of the upper layer 44 and the lower layer 43 are captured from the cross section of the exposed first negative electrode active material layer 42a. The cross-sectional images of the upper layer 44 and the lower layer 43 obtained above are imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health). A binarized image is obtained in which particle cross sections in the cross-sectional images are converted into black and voids, such as voids between particles, are converted into white. The white area is determined for each of the binarized images of the upper layer 44 and the lower layer 43. The porosity of the upper layer 44 and the lower layer 43 can be determined by calculating the ratio of the area of ​​the voids in the upper layer 44 (white area) to the area of ​​the upper layer 44 (area of ​​the cross-sectional image of the upper layer), and the ratio of the area of ​​the voids in the lower layer 43 (white area) to the area of ​​the lower layer 43 (area of ​​the cross-sectional image of the lower layer).

[0042] The negative electrode active material preferably contains a Si-based material in order to increase the capacity of the secondary battery, etc. In addition to the carbon material and the Si-based material, the negative electrode active material may contain, for example, a substance capable of reversibly absorbing and releasing lithium ions.

[0043] Examples of the Si-based material include Si, Si alloys, and Si compounds. The Si-based material may also be, for example, composite particles containing an ion-conducting phase and a silicon phase (silicon particles in one respect) dispersed within the ion-conducting phase. The ion-conducting phase is a phase that conducts ions, and includes, for example, at least one of a silicate phase, a carbon phase, and a silicon oxide phase.

[0044] The carbon phase may be composed of, for example, amorphous carbon. Examples of amorphous carbon constituting the carbon phase include hard carbon, soft carbon, and other amorphous carbon. Amorphous carbon has an average interplanar spacing d of (002) planes measured by X-ray diffraction. ００２ The carbon material has a particle size of more than 0.34 nm.

[0045] The main component of the silicon oxide phase (for example, 95% by mass or more and 100% by mass or less) may be silicon dioxide. The composition of the composite particles containing the silicon oxide phase and the silicon phase dispersed therein is generally SiO ｘ It can be expressed as: SiO ｘ is a material in which silicon particles are amorphous SiO ２ The oxygen content ratio x to silicon is preferably, for example, 0.5≦x<2.0, and more preferably 0.8≦x≦1.5.

[0046] The silicate phase may satisfy the following conditions (1) and / or (2): (1) The silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long periodic table). (2) The silicate phase contains element L. The element L is at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanoids, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanoids is a collective term for 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0047] Regarding the above condition (1), examples of alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The inclusion of an alkali metal element and / or a Group 2 element may reduce the irreversible capacity of the silicate phase. A silicate phase containing lithium (hereinafter, sometimes referred to as a "lithium silicate phase") is preferable in terms of, for example, a small irreversible capacity and a high initial charge / discharge efficiency.

[0048] The lithium silicate phase may be an oxide phase containing Li, Si, and O, and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4.

[0049] The lithium silicate phase has the formula: Li ２ｚ SiO （２＋ｚ） The lithium silicate phase may contain or be composed of a lithium silicate phase represented by (0<z<2). Preferably, z satisfies the relationship 0<z<1, and z=1 / 2 (i.e., Li ２ Si ２ O ５ ) is more preferred.

[0050] The Si-based material may also include composite particles containing an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase, and a coating layer covering at least a portion of the surface of the composite particles.

[0051] The coating layer present on the surface of the composite particle may include, for example, a conductive layer. Forming a conductive layer on the surface of the composite particle may increase the conductivity of the Si-based material. A conductive material containing carbon is preferred as the conductive material constituting the conductive layer. Examples of conductive materials containing carbon include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, and amorphous carbon (amorphous carbon) with low crystallinity. Amorphous carbon is preferred because it has a strong buffering effect on the silicon phase, which changes in volume during charging and discharging. The amorphous carbon may be either easily graphitized carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Examples of carbon black include acetylene black and ketjen black. The thickness of the conductive layer may be, for example, in the range of 1 to 200 nm. The thickness of the conductive layer can be measured by observing the cross section of the Si-containing material using a SEM or TEM (transmission electron microscope).

[0052] The content of the Si-based material is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more, relative to the total amount of the negative electrode active material, from the viewpoint of increasing the capacity of the secondary battery, etc. Furthermore, from the viewpoint of increasing the capacity of the secondary battery and suppressing swelling of the negative electrode, the content of the Si-based material is preferably 3% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 25% by mass or less, and even more preferably 8% by mass or more and 20% by mass or less, relative to the total amount of the negative electrode active material.

[0053] The average particle size of the Si-based material is preferably 3 μm or more and 15 μm or less, more preferably 4 μm or more and 12 μm or less, and even more preferably 6 μm or more and 10 μm or less, for example, in terms of improving charge-discharge cycle characteristics, etc. The average particle size is the volume average particle size D50 at which the volume integrated value becomes 50% in the particle size distribution obtained by the laser diffraction scattering method.

[0054] An example of a method for fabricating the negative electrode 12 will be described. A first negative electrode active material layer slurry and a second negative electrode active material layer slurry are prepared by adding a negative electrode active material containing a first carbon material, a second carbon material, and an Si-based material, a binder, and the like to a dispersion medium. The first negative electrode active material layer slurry is applied to one surface of the negative electrode current collector 40 and dried to form a first negative electrode active material layer 42a. The second negative electrode active material layer slurry is applied to the other surface of the negative electrode current collector 40 and dried to form a second negative electrode active material layer 42b. The first negative electrode active material layer 42a and the second negative electrode active material layer 42b are rolled at a predetermined linear pressure using a rolling roller or the like. In this manner, the negative electrode 12 is fabricated. The rolling may be performed after forming the first anode active material layer 42a on one surface of the anode current collector 40, and then additional rolling may be performed after forming the second anode active material layer 42b on the other surface of the anode current collector 40, or may be performed after forming the second anode active material layer 42b on the other surface of the anode current collector 40, and then additional rolling may be performed after forming the first anode active material layer 42a on one surface of the anode current collector 40. Furthermore, as described above, rolling may be performed after forming the first anode active material layer 42a and the second anode active material layer 42b on both surfaces of the anode current collector 40.

[0055] For example, by adjusting the content of the first carbon material in the slurry for the first negative electrode active material layer and the content of the first carbon material in the slurry for the second negative electrode active material, it is possible to adjust the volume density of the first negative electrode active material layer 42a and the volume density of the second negative electrode active material layer 42b. Increasing the content of the first carbon material in the slurry makes it possible to lower the volume density of the active material layer. Furthermore, by adjusting the number of times of rolling and the linear pressure of the first negative electrode active material layer 42a and the second negative electrode active material layer 42b, it is also possible to adjust the volume density of the first negative electrode active material layer 42a and the volume density of the second negative electrode active material layer 42b. Increasing the number of times of rolling and increasing the linear pressure of the active material layer makes it possible to increase the volume density of the active material layer. Note that by adjusting the content of the first carbon material in the slurry and adjusting the number of times of rolling and the linear pressure of the active material layer, the specific surface area of ​​the first negative electrode active material layer 42a and the second negative electrode active material layer 42b and the maximum value V of the peak R1 can be adjusted. Ｒ１ , the maximum value of peak R2 V Ｒ２ is also adjustable.

[0056] When forming the first negative electrode active material layer 42a having the upper layer 44 and the lower layer 43, for example, a lower layer slurry and an upper layer slurry are prepared by adding a negative electrode active material, a binder, and the like to a dispersion medium, and the lower layer 43 is formed by applying the lower layer slurry to one surface of the negative electrode current collector 40 and drying it. The upper layer 44 is then formed by applying the upper layer slurry to the surface of the lower layer 43 and drying it. While the above describes a case in which the lower layer slurry is applied and then the upper layer slurry is applied, this is not limited thereto, and two-layer coating in which the lower layer slurry and the upper layer slurry are applied simultaneously may also be used. Alternatively, rolling may be performed after forming the lower layer 43, and then additional rolling may be performed after forming the upper layer 44, or rolling may be performed after forming the lower layer 43 and the upper layer 44.

[0057] For example, the porosity of the upper layer 44 and the porosity of the lower layer 43 can be adjusted by adjusting the content of the first carbon material in the lower layer slurry and the content of the first carbon material in the upper layer slurry. The porosity can be increased by increasing the content of the first carbon material in the slurry. The porosity of the upper layer 44 and the porosity of the lower layer 43 can also be adjusted by adjusting the number of times the upper layer 44 and the lower layer 43 are rolled or the linear pressure. The porosity of the layer can be reduced by increasing the number of times the layer is rolled or the linear pressure.

[0058] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator.

[0059] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0060] Example 1 [Fabrication of Negative Electrode] A low BET graphite material (specific surface area 1.3 m ２ / g) and high BET graphite material (specific surface area 4.2 m ２ / g) and SiO were mixed in a mass ratio of 23:67:10. This mixture was used as the negative electrode active material. These were then mixed so that the mass ratio of the negative electrode active material:carboxymethyl cellulose (CMC):polyacrylic acid (PAA):styrene-butadiene copolymer rubber (SBR) was 100:1:1:1, and mixed with any amount of water to prepare a slurry for a first negative electrode active material layer.

[0061] Low BET graphite material (specific surface area 1.5 m ２ / g) and high BET graphite material (specific surface area 4.2 m ２ / g) and SiO were mixed in a mass ratio of 17:73:10. This mixture was used as the negative electrode active material. These were then mixed so that the mass ratio of the negative electrode active material:carboxymethyl cellulose (CMC):polyacrylic acid (PAA):styrene-butadiene copolymer rubber (SBR) was 100:1:1:1, and mixed with any amount of water to prepare a slurry for a second negative electrode active material layer.

[0062] The slurry for the first negative electrode active material layer was applied to one surface of a negative electrode current collector made of copper foil, and the slurry for the second negative electrode active material layer was applied to the other surface of a negative electrode current collector. After these coatings were dried, the coatings were rolled with a rolling roller to produce a negative electrode having the first negative electrode active material layer formed on one surface of the negative electrode current collector and the second negative electrode active material layer formed on the other surface.

[0063] Examples 2 to 4 and Comparative Examples 1 to 5 Negative electrodes were fabricated in the same manner as in Example 1, except that the amounts of the low BET graphite material and the high BET graphite material added to the first negative electrode active material layer slurry and the second negative electrode active material layer slurry and the coating thickness were changed.

[0064] The thickness, coating amount (area density), volume density, content of low BET graphite material, and specific surface area of ​​the first and second negative electrode active material layers in Examples 1 to 4 and Comparative Examples 1 to 5 are shown in Table 1. In addition, in Examples 1 to 4 and Comparative Examples 1 to 5, the pore diameters and maximum values ​​(V Ｒ１ , V Ｒ３ ), and the pore diameters and maximum values ​​(V Ｒ２ , V Ｒ４ ) are shown in Table 2. The specific surface area and pore size distribution are values ​​measured on the negative electrode after initial charging and discharging of the secondary battery prepared by the following procedure. The initial charging and discharging conditions were the same as those of the charge-discharge cycle test described below.

[0065]

[0066]

[0067] [Preparation of Positive Electrode] As the positive electrode active material, LiNi ０．８８ Co０．０９ Al ０．０３ O ２ A lithium transition metal oxide represented by the formula (I) was used. The above positive electrode active material, carbon black, and polyvinylidene fluoride powder were mixed in a mass ratio of 100:1:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. This slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness 15 μm). After drying the coating, the coating was rolled using a rolling roller to produce a positive electrode in which a positive electrode active material layer was formed on both sides of the positive electrode current collector.

[0068] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate were mixed in a volume ratio of 1:3, and LiPF ６ was dissolved in the nonaqueous electrolyte at a concentration of 1.5 mol / L.

[0069] [Secondary Battery Fabrication] (1) A positive electrode lead was attached to the positive electrode, and a negative electrode lead was attached to the negative electrode of Example 1. The positive and negative electrodes were then wound with a separator made of a microporous polyethylene film between them to fabricate a wound electrode assembly. When winding the negative electrode, the negative electrode was positioned so that the first negative electrode active material layer was on the inner periphery of the winding and the second negative electrode active material layer was on the outer periphery of the winding. (2) Insulating plates were placed above and below the electrode assembly, and the negative electrode lead was welded to the case body and the positive electrode lead was welded to the sealing member to house the electrode assembly within the case body. (3) A nonaqueous electrolyte was injected into the case body under reduced pressure, and the opening of the case body was sealed with the sealing member via a gasket. This resulted in a secondary battery. (4) Secondary batteries were similarly fabricated using the negative electrodes of Examples 2 to 4 and Comparative Examples 1 to 5.

[0070] <Charge-Discharge Cycle Test> At an ambient temperature of 25°C, secondary batteries using negative electrodes of each Example and Comparative Example were charged at a constant current of 0.3 C to 4.2 V, and then charged at a constant voltage of 1 / 50 C from 4.2 V. Subsequently, the secondary batteries were discharged at a constant current of 0.3 C to 2.5 V. The discharge capacities at this time were measured and summarized in Table 3 as initial discharge capacities. The initial discharge capacities in Table 3 are shown relative to the initial discharge capacity of Example 1 (100%), and the initial discharge capacities of the other Examples and Comparative Examples are shown. The above charge-discharge cycle was counted as one cycle, and 500 cycles were performed. The capacity retention rate of the secondary batteries during the charge-discharge cycle was calculated using the following formula, and the results are summarized in Table 3. Capacity retention rate = (discharge capacity at 500th cycle / initial discharge capacity) × 100

[0071]

[0072] As shown in Table 3, the secondary batteries using the negative electrodes of Examples 1 to 4 all had higher capacity retention rates in charge-discharge cycle tests than the secondary batteries using the negative electrodes of Comparative Examples 1 to 5. That is, by setting the ratio (D1 / D2) of the volume density (D1) of the first negative electrode active material layer on the inner side of the winding to the volume density (D2) of the second negative electrode active material layer on the outer side of the winding to 0.85 or more and less than 1.0, as in the negative electrodes of Examples 1 to 4, it is possible to improve the charge-discharge cycle characteristics of secondary batteries equipped with a wound electrode assembly. Considering that the capacity retention rates of Examples 3 and 4 were particularly good among Examples 1 to 4, it is considered more preferable that the D1 / D2 ratio be 0.88 or more and 0.93 or less.

[0073] [Note] (1) A secondary battery having an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and an electrolyte, wherein the negative electrode comprises a negative electrode current collector and negative electrode active material layers disposed on both surfaces of the negative electrode current collector, the negative electrode active material layers having a first negative electrode active material layer disposed on one of both surfaces of the negative electrode current collector, the first negative electrode active material layer being disposed on the inner peripheral surface of the negative electrode current collector that is on the inner peripheral side of the winding, and a second negative electrode active material layer disposed on the outer peripheral surface of the negative electrode current collector that is on the outer peripheral side of the winding, and wherein a ratio (D1 / D2) of a volume density (D1) of the first negative electrode active material layer to a volume density (D2) of the second negative electrode active material layer is 0.85 or more and less than 1.0. (2) The secondary battery according to (1), wherein the negative electrode active material layer includes a first carbon material and a second carbon material, and the BET specific surface area of ​​the first carbon material is smaller than the BET specific surface area of ​​the second carbon material. ２ / g or more, 3.0m ２ / g, and the BET specific surface area of ​​the second carbon material is less than 3.0 m ２ / g or more, 10.0m ２ / g or less. (4) The secondary battery according to (2) above, wherein the content of the first carbon material in the first negative electrode active material layer is greater than the content of the first carbon material in the second negative electrode active material layer. (5) The secondary battery according to any one of (2) to (4) above, wherein the first negative electrode active material layer has a lower layer disposed on the inner circumferential surface of the negative electrode current collector and an upper layer disposed on the lower layer, and wherein the content of the first carbon material in the upper layer is greater than the content of the first carbon material in the lower layer. (6) The secondary battery according to (5) above, wherein the porosity of the upper layer is greater than the porosity of the lower layer. (7) The secondary battery according to any one of (1) to (6) above, wherein the thickness of the first negative electrode active material layer is greater than the thickness of the second negative electrode active material layer. (8) The secondary battery according to any one of (1) to (7) above, wherein the specific surface area (B1) of the first negative electrode active material layer is smaller than the specific surface area (B2) of the second negative electrode active material layer. (9) The secondary battery according to (8) above, wherein the ratio (B1 / B2) of the specific surface area (B1) of the first negative electrode active material layer to the specific surface area (B2) of the second negative electrode active material layer is 0.7 or more and 0.95 or less. (10) The pore size distribution of the first negative electrode active material layer measured by mercury intrusion porosimetry has a maximum value (V Ｒ１ ) in the pore size distribution of the second negative electrode active material layer measured by mercury porosimetry. The pore size distribution of the second negative electrode active material layer measured by mercury porosimetry has a maximum value (V Ｒ２ ) having a peak R2, and the maximum value of the peak R2 (V Ｒ２ ) the maximum value of the peak R1 (V Ｒ１ ) ratio (V Ｒ１ / V Ｒ２ (11) The secondary battery according to any one of (1) to (9), wherein the maximum value of the peak R2 (V Ｒ２ ) the maximum value of the peak R1 (V Ｒ１ ) ratio (V Ｒ１ / V Ｒ２ (12) The secondary battery according to (10), wherein the maximum value of the peak R2 (V Ｒ２ ) the maximum value of the peak R1 (V Ｒ１) ratio (V Ｒ１ / V Ｒ２ ) is 1.2 or more and 2.0 or less. (13) The secondary battery according to any one of (1) to (12) above, wherein the negative electrode active material layer contains a Si-based material. (14) The secondary battery according to (13) above, wherein the Si-based material has an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase, and the silicon phase has composite particles containing at least one of a silicon oxide phase, a silicate phase, and a carbon phase.

[0074] REFERENCE SIGNS LIST 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 case body, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 protruding portion, 23 filter, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 40 negative electrode current collector, 42 negative electrode active material layer, 42a first negative electrode active material layer, 42b second negative electrode active material layer, 43 lower layer, 44 upper layer.

Claims

1. A secondary battery having an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and an electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer disposed on both surfaces of the negative electrode current collector, the negative electrode active material layer having a first negative electrode active material layer disposed on one of both surfaces of the negative electrode current collector, the first negative electrode active material layer being disposed on the inner peripheral surface of the negative electrode current collector which is on the inner peripheral side of the winding, and a second negative electrode active material layer disposed on the outer peripheral surface of the negative electrode current collector which is on the outer peripheral side of the winding, and a ratio (D1 / D2) of a volume density (D1) of the first negative electrode active material layer to a volume density (D2) of the second negative electrode active material layer is 0.85 or more and less than 1.

0.

2. The secondary battery according to claim 1, wherein the negative electrode active material layer comprises a first carbon material and a second carbon material, and the BET specific surface area of ​​the first carbon material is smaller than the BET specific surface area of ​​the second carbon material.

3. The BET specific surface area of ​​the first carbon material is 0.5 m ２ / g or more, 3.0m ２ / g, and the BET specific surface area of ​​the second carbon material is less than 3.0 m ２ / g or more, 10.0m ２ The secondary battery according to claim 2 , wherein the capacitance is 0.1 μm or less.

4. The secondary battery according to claim 2 or 3, wherein the content of the first carbon material in the first negative electrode active material layer is greater than the content of the first carbon material in the second negative electrode active material layer.

5. The secondary battery described in claim 2 or 3, wherein the first negative electrode active material layer has a lower layer disposed on the inner circumferential surface of the negative electrode current collector and an upper layer disposed on the lower layer, and the content of the first carbon material in the upper layer is greater than the content of the first carbon material in the lower layer.

6. The secondary battery according to claim 5, wherein the porosity of the upper layer is higher than the porosity of the lower layer.

7. The secondary battery according to any one of claims 1 to 3, wherein the first negative electrode active material layer is thicker than the second negative electrode active material layer.

8. The secondary battery according to any one of claims 1 to 3, wherein the specific surface area (B1) of the first negative electrode active material layer is smaller than the specific surface area (B2) of the second negative electrode active material layer.

9. The secondary battery according to claim 8, wherein a ratio (B1 / B2) of the specific surface area (B1) of the first negative electrode active material layer to the specific surface area (B2) of the second negative electrode active material layer is 0.7 or more and 0.95 or less.

10. The pore size distribution of the first negative electrode active material layer measured by mercury intrusion porosimetry has a maximum value (V Ｒ１ ) in the pore size distribution of the second negative electrode active material layer measured by mercury intrusion porosimetry, the pore size distribution has a maximum value (V Ｒ２ ) having a peak R2, and the maximum value of the peak R2 (V Ｒ２ ) the maximum value of the peak R1 (V Ｒ１ ) ratio (V Ｒ１ / V Ｒ２ 4. The secondary battery according to claim 1, wherein:

11. The maximum value of the peak R2 (V Ｒ２ ) the maximum value of the peak R1 (V Ｒ１ ) ratio (V Ｒ１ / V Ｒ２ 11. The secondary battery according to claim 10, wherein the ratio of the surface area to the surface area is 1.2 or more and 10.0 or less.

12. The maximum value of the peak R2 (V Ｒ２ ) the maximum value of the peak R1 (V Ｒ１ ) ratio (V Ｒ１ / V Ｒ２ 11. The secondary battery according to claim 10, wherein the ratio of the capacitance to the capacitance of the secondary battery is 1.2 or more and 2.0 or less.

13. The secondary battery according to any one of claims 1 to 3, wherein the negative electrode active material layer contains a Si-based material.

14. The secondary battery described in claim 13, wherein the Si-based material is a composite particle having an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase, and the ion-conducting phase includes at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase.

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