Negative electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

A negative electrode for a nonaqueous electrolyte secondary battery, according to one embodiment of the present invention, is provided with a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core. The negative electrode mixture layer has a first layer that includes: at least one of at least one kind of metal element selected from Si, Sn, Sb, Mg, and Ge, and a compound containing said metal element; at least one of multi-walled carbon nanotubes and single-walled carbon nanotubes; and a binding agent. The negative electrode mixture layer also has a second layer that includes graphite A with a particle fracture strength of 10-35 MPa. The porosity of the first layer is preferably 50-65%.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery using said negative electrode. [Background technology]

[0002] Metal elements that alloy with lithium, such as Si and Sn, and compounds containing these metal elements, are known to be able to absorb more lithium ions per unit volume compared to carbon materials such as graphite. Therefore, using these as negative electrode active materials can increase the capacity of batteries. Generally, from the viewpoint of battery charge-discharge cycle characteristics, it is preferable to use compounds containing metals such as Si rather than using metals such as Si directly. Furthermore, negative electrodes that use a combination of carbon materials and Si-containing compounds as negative electrode active materials are also known.

[0003] For example, Patent Document 1 discloses a negative electrode for a non-aqueous electrolyte secondary battery having a composite layer in which at least one layer each of a first active material containing a first active material capable of reversibly intercalating and releasing lithium and a second active material containing a second active material capable of reversibly alloying with lithium are alternately laminated, the first layer filling the pores formed in the second layer and a mixed layer formed at the interface between the two layers. Patent Document 1 states that the cycle characteristics of the battery are improved by using this negative electrode. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-179575 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, compounds containing metals such as Si undergo significant volume changes during charging and discharging. Therefore, batteries using such compounds as negative electrode active materials suffer from a decrease in charge-discharge cycle characteristics due to these volume changes. The technology described in Patent Document 1 fails to sufficiently suppress the volume change (expansion rate) of the negative electrode, and there is still room for improvement in terms of cycle characteristics.

[0006] The purpose of this disclosure is to provide a negative electrode for a non-aqueous electrolyte secondary battery using a high-capacity negative electrode active material, which improves the charge-discharge cycle characteristics of the battery. [Means for solving the problem]

[0007] A negative electrode for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core, wherein the negative electrode mixture layer comprises a first layer containing at least one metal element selected from Si, Sn, Sb, Mg, and Ge and at least one compound containing the metal element, 3 to 10% by mass of multilayer carbon nanotubes, and 6 to 18% by mass of a binder, and a second layer containing graphite A having a particle fracture strength of 10 to 35 MPa.

[0008] A negative electrode for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core, wherein the negative electrode mixture layer comprises a first layer containing at least one metal element selected from Si, Sn, Sb, Mg, and Ge and at least one compound containing the metal element, 0.2 to 0.7 mass% of single-walled carbon nanotubes, and 6 to 18 mass% of a binder, and a second layer containing graphite A having a particle fracture strength of 10 to 35 MPa.

[0009] A non-aqueous electrolyte secondary battery according to one aspect of this disclosure comprises the negative electrode, the positive electrode, and the non-aqueous electrolyte. [Effects of the Invention]

[0010] According to a negative electrode for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, the charge-discharge cycle characteristics of the battery can be improved. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. [Figure 2] This is a cross-sectional view of a negative electrode, which is an example of an embodiment. [Modes for carrying out the invention]

[0012] As a result of diligent research to solve the above-mentioned problems, the present inventors have found that by using a negative electrode having a multilayer negative electrode mixture layer comprising a first layer containing at least one of a metal element such as Si that is alloyed with lithium and a compound containing said metal element, a predetermined amount of carbon nanotubes and a predetermined amount of binder, and a second layer containing graphite A with a particle fracture strength of 10 to 35 MPa, the charge-discharge cycle characteristics of the battery are specifically improved. Preferably, the porosity of the first layer is controlled to 50 to 65%. One embodiment of the negative electrode of this disclosure has the characteristic of having a high capacity while having a small expansion rate during charging and discharging.

[0013] The first layer contains suitable voids that absorb the expansion of metal-containing compounds such as Si, thus enabling the suppression of expansion while ensuring high capacity. By using a predetermined amount of carbon nanotubes and a binder, voids suitable for suppressing expansion can be formed in the first layer. Furthermore, by using graphite A in the second layer, the graphite A is appropriately compressed during the manufacturing of the negative electrode, improving the packing density of the active material in the second layer and ensuring high capacity. In particular, graphite A and a BET specific surface area of ​​0.5 to 2.5 m² 2 When used in combination with graphite B at a concentration of / g, high capacity and good cycle characteristics can be obtained, and the high-temperature storage characteristics of the battery are also improved. Furthermore, if a negative electrode is used in which the second layer and then the first layer are arranged in that order from the negative electrode core side of the negative electrode mixture layer, the charging characteristics of the battery are effectively improved.

[0014] Hereinafter, an example of an embodiment of a negative electrode according to the present disclosure and a non-aqueous electrolyte secondary battery using the negative electrode will be described in detail with reference to the drawings. Note that selectively combining a plurality of embodiments and modification examples described below is included in the present disclosure.

[0015] Hereinafter, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer can 16 will be exemplified. However, the outer package of the battery is not limited to a cylindrical outer can. For example, it may be a rectangular outer can (rectangular battery), a coin-shaped outer can (coin-shaped battery), or an outer package (laminated battery) composed of a laminated sheet including a metal layer and a resin layer. Further, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated via a separator.

[0016] FIG. 1 is a diagram schematically showing a cross section of a non-aqueous electrolyte secondary battery 10 which is an example of an embodiment. As shown in FIG. 1, the non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an outer can 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer can 16 is a bottomed cylindrical metal container with one side in the axial direction open, and the opening of the outer can 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the side of the battery where the sealing body 17 is located is taken as the upper side, and the bottom side of the outer can 16 is taken as the lower side.

[0017] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more of these are used. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine. As an example of the non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixed solvents thereof can be mentioned. For the electrolyte salt, for example, a lithium salt such as LiPF6 is used. Note that the non-aqueous electrolyte is not limited to a liquid electrolyte and may be a solid electrolyte.

[0018] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode body 14 are all strip-shaped elongated bodies, and are alternately laminated in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed with dimensions slightly larger than those of the positive electrode 11 in order to prevent the precipitation of lithium. That is, the negative electrode 12 is formed longer than the positive electrode 11 in both the longitudinal direction and the width direction (short side direction). The separator 13 is formed with dimensions at least slightly larger than those of the positive electrode 11, and two sheets are arranged so as to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0019] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. In the example shown in FIG. 1, the positive electrode lead 20 extends toward the sealing body 17 through the through hole of the insulating plate 18, and the negative electrode lead 21 extends toward the bottom side of the outer can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected by welding or the like to the lower surface of the internal terminal plate 23 of the sealing body 17, and the cap 27, which is the top plate of the sealing body 17 electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the outer can 16, and the outer can 16 serves as the negative electrode terminal.

[0020] As described above, the outer can 16 is a bottomed cylindrical metal container with one side in the axial direction open. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the hermeticity inside the battery and the insulation between the outer can 16 and the sealing body 17. The outer can 16 is formed with a groove portion 22 that supports the sealing body 17, in which a part of the side surface portion projects inward. The groove portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the upper part of the outer can 16 by the groove portion 22 and the open end portion of the outer can 16 that is caulked to the sealing body 17.

[0021] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When a malfunction occurs in the battery and the internal pressure rises, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0022] The following describes in detail the positive electrode 11, negative electrode 12, and separator 13 that constitute the non-aqueous electrolyte secondary battery 10, with particular attention paid to the negative electrode 12.

[0023] [Positive electrode] The positive electrode 11 comprises a positive electrode core and a positive electrode mixture layer provided on the surface of the positive electrode core. The positive electrode core can be made of a metal foil that is stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core, excluding the exposed core portion where the positive electrode lead is connected. The thickness of the positive electrode mixture layer is, for example, 50 μm to 150 μm on one side of the positive electrode core. The positive electrode 11 can be manufactured by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the surface of the positive electrode core, drying the coating, and then compressing it to form the positive electrode mixture layer on both sides of the positive electrode core.

[0024] The positive electrode active material is mainly composed of a lithium transition metal composite oxide. Elements other than Li that may be included 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, W, Si, and P. A suitable example of a lithium transition metal composite oxide is one containing at least one of Ni, Co, and Mn. Specific examples include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0025] Examples of conductive agents included in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binders included in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may be used in combination with cellulose derivatives such as carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0026] [Negative electrode] Figure 2 is a cross-sectional view of the negative electrode 12. As shown in Figure 2, the negative electrode 12 has a negative electrode core 30 and a negative electrode mixture layer 31 provided on the surface of the negative electrode core 30. The negative electrode core 30 can be made of a metal foil that is stable in the negative electrode potential range, such as copper or a copper alloy, or a film with the metal arranged on its surface. The negative electrode mixture layer 31 is preferably provided on both sides of the negative electrode core 30, excluding the core exposed portion (not shown) where the negative electrode leads are connected. The thickness of the negative electrode mixture layer 31 is, for example, 50 μm to 150 μm on one side of the negative electrode core 30.

[0027] The negative electrode mixture layer 31 has a first layer 32 containing a Si-containing compound 34 and a second layer 33 containing graphite 35. During battery charging, the Si in the Si-containing compound 34 alloys with lithium, and the graphite 35 absorbs lithium. Both the Si-containing compound 34 and the graphite 35 function as negative electrode active materials. The first layer 32 may contain at least one metal element selected from Si, Sn, Sb, Mg, and Ge, and at least one compound containing the said metal element as negative electrode active material. In this embodiment, the Si-containing compound 34 is exemplified as the negative electrode active material in the first layer 32, but the first layer 32 may contain other metal elements or their compounds together with the Si-containing compound 34, or in place of the Si-containing compound 34. As will be described in detail later, the graphite 35 is graphite A with a particle fracture strength of at least 10 to 35 MPa.

[0028] The first layer 32 contains a Si-containing compound 34, 3 to 10 mass% of multi-walled carbon nanotubes (MWCNTs), and 6 to 18 mass% of a binder. Alternatively, it contains 0.2 to 0.7 mass% of single-walled carbon nanotubes (SWCNTs) and 6 to 18 mass% of a binder. The first layer 32 only needs to contain at least one of MWCNTs and SWCNTs. By adding a predetermined amount of CNTs and a binder together with the Si-containing compound 34, the porosity of the first layer 32 can be controlled to an appropriate range without the Si-containing compound 34 becoming isolated from the conductive path, thereby suppressing the expansion rate of the negative electrode 12. The second layer 33 is a layer containing, for example, graphite 35 (graphite A) and a binder, and is a layer densely packed with graphite 35.

[0029] The negative electrode mixture layer 31 may have a layer structure in which a first layer 32 and a second layer 33 are arranged in order from the negative electrode core 30 side, but preferably, as shown in Figure 2, it has a layer structure in which a second layer 33 and a first layer 32 are arranged in order from the negative electrode core 30 side. That is, it is preferable that the second layer 33 is formed on the negative electrode core 30 and the first layer 32 is formed on the second layer 33. When the latter layer structure is applied, the charging characteristics of the battery are improved compared to the former layer structure. Since the first layer 32 has a higher porosity than the second layer 33, it is thought that by placing the first layer 32 on the surface side of the negative electrode mixture layer 31, the penetration of the electrolyte into the negative electrode mixture layer 31 is improved, and the charging characteristics are improved.

[0030] In the example shown in Figure 2, there is no region where Si-containing compound 34 and graphite 35 are mixed at or near the interface between the first layer 32 and the second layer 33. The first layer 32 may contain graphite 35 and the second layer 33 may contain Si-containing compound 34, but in order to separate the functions of each layer and more effectively improve the cycle characteristics, it is preferable that the first layer 32 substantially does not contain graphite 35 and the second layer 33 substantially does not contain Si-containing compound 34. In addition, the negative electrode mixture layer 31 may be provided with layers other than the first layer 32 and the second layer 33, to the extent that it does not impair the purpose of this disclosure.

[0031] The negative electrode mixture layer 31 can be formed using a first negative electrode mixture slurry containing a Si-containing compound 34, CNTs, and a binder, and a second negative electrode mixture slurry containing graphite 35 and a binder. For example, a negative electrode 12 with a two-layer negative electrode mixture layer 31 can be manufactured by applying the second negative electrode mixture slurry to the surface of the negative electrode core 30, drying the first coating layer, applying the first negative electrode mixture slurry on this coating layer, drying the second coating layer, and then compressing the two layers together with a rolling mill. It is also possible to compress only the first coating layer, which will become the second layer 33, and then form the second coating layer, which will become the first layer 32.

[0032] The ratio of the thicknesses of the first layer 32 to the second layer 33 is not particularly limited, but an example of the thickness of the first layer 32 is, for example, 20-80%, 25-60%, or 30-50% of the total thickness of the first layer 32 and the second layer 33. The thicknesses of the first layer 32 and the second layer 33 may be substantially the same. The first layer 32 and the second layer 33 contain, for example, the same type of binder. However, it is preferable that the binder content in each layer is higher in the first layer 32. The binder content relative to the mass of the first layer 32 is, for example, 3-18 times, or 6-15 times, the binder content relative to the mass of the second layer 33.

[0033] As described above, the porosity of the first layer 32 is higher than that of the second layer 33. The voids in the negative electrode mixture layer 31 are spaces where no solids such as the negative electrode active material, conductive agent, and binder exist, and are mainly formed between the particles of the negative electrode active material. The porosity of each layer represents the ratio of the volume of voids to the volume of each layer and is calculated by the following formula. Porosity of each layer = [Apparent volume of each layer - Sum of volumes of materials constituting each layer] / Apparent volume of each layer The apparent volume is calculated from the mass of each layer and the true density of the constituent materials.

[0034] The porosity of the first layer 32 is, for example, 40-70%, preferably 50-65%, or 50-60%. If the porosity of the first layer 32 is within this range, the expansion rate can be sufficiently suppressed while ensuring the capacity of the negative electrode 12, thereby achieving a higher level of compatibility between high capacity and good cycle characteristics. By using a predetermined amount of CNTs and a binder, and adjusting the compressive force of the coating film as necessary, the porosity of the first layer 32 can be controlled to an appropriate range. It is also possible to increase the porosity by weakening the compressive force of the coating film without adding a predetermined amount of CNTs and a binder, but in that case, the improvement effect on cycle characteristics cannot be obtained. The second layer 33 has fewer voids than the first layer 32, and its porosity is, for example, 35% or less, preferably 15-30%.

[0035] The Si-containing compound 34 included in the first layer 32 is, for example, smaller in particle size than the graphite 35 included in the second layer 33. An example of the volume-based median diameter (D50) of the Si-containing compound 34 is 1 μm to 15 μm, or 3 μm to 10 μm. D50 means the particle diameter at which the frequency cumulative in the volume-based particle size distribution reaches 50% from the smaller particle diameter side, and is also called the median diameter. The particle size distribution can be measured using a laser diffraction type particle size distribution measuring device (for example, MT3000II manufactured by Microtrac Bell Co., Ltd.) with water as the dispersion medium. An example of D50 of the graphite 35 is 5 μm to 30 μm, or 10 μm to 25 μm.

[0036] The Si-containing compound 34 includes a silicon oxide phase and a compound (SiO) containing Si dispersed in the silicon oxide phase, a lithium silicate phase and a compound (LixSiOy composition) containing Si dispersed in the lithium silicate phase, a carbon phase and a compound (Si-C) containing Si dispersed in the carbon phase, and the like. One type of Si-containing compound 34 may be added to the first layer 32, or two or more types of Si-containing compounds 34 may be added. Also, a conductive layer made of a material with high conductivity may be formed on the particle surface of the Si-containing compound 34. An example of a suitable conductive layer is a carbon film made of a carbon material. The content of the Si-containing compound 34 varies slightly depending on the type of CNT and the like, but is, for example, 73 to 93% by mass with respect to the mass of the first layer 32.

[0037] SiO has a particle structure in which fine Si particles are dispersed in a silicon oxide phase. A suitable SiO has an island structure in which fine Si particles are substantially uniformly dispersed in an amorphous silicon oxide matrix, and is represented by the general formula SiO x (0 < x ≦ 2). The silicon oxide phase is composed of an aggregate of particles finer than the Si particles. From the viewpoint of achieving both battery capacity and cycle characteristics, the content of the Si particles is preferably 35 to 75% by mass with respect to the total mass of SiO.

[0038] A suitable LixSiOy composition having a particle structure in which fine Si particles are dispersed in a lithium silicate phase has a sea-island structure in which fine Si particles are substantially uniformly dispersed in a matrix of lithium silicate. The lithium silicate phase is composed of an aggregate of particles finer than the Si particles. The content of the Si particles is preferably 35 to 75% by mass based on the total mass of the LixSiOy composition, similar to the case of SiO. Also, the average particle size of the Si particles is, for example, 500 nm or less before charge and discharge, preferably 200 nm or less, or 50 nm or less.

[0039] The lithium silicate phase is generally Li 2z SiO (2+z) (0 < z < 2). That is, Li4SiO4 (Z = 2) is not included in the lithium silicate phase. Li4SiO4 is an unstable compound and reacts with water to exhibit alkalinity, which may cause deterioration of Si and lead to a decrease in charge-discharge capacity. From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., it is preferable that the lithium silicate phase is mainly composed of Li2SiO3 (Z = 1) or Li2Si2O5 (Z = 1 / 2). When Li2SiO3 or Li2Si2O5 is the main component, the content of the main component is preferably more than 50% by mass based on the total mass of the lithium silicate phase, and more preferably 80% by mass or more.

[0040] Si-C has a particle structure in which fine Si particles are dispersed in a carbon phase. A suitable Si-C has a sea-island structure in which fine Si particles are substantially uniformly dispersed in a matrix of the carbon phase. From the viewpoint of increasing the capacity, etc., the content of the Si particles is preferably 30 to 80% by mass based on the total mass of Si-C, and more preferably 35 to 75% by mass. The average particle size of suitable Si particles is generally 500 nm or less before charge and discharge, preferably 200 nm or less, and more preferably 100 nm or less. After charge and discharge, it is preferably 400 nm or less, and more preferably 100 nm or less.

[0041] The carbon coating described above is composed of, for example, carbon black, acetylene black, Ketjen black, graphite, and mixtures of two or more of these. Examples of methods for carbon coating the particle surface of the Si-containing compound 34 include CVD using acetylene, methane, etc., and a method of mixing coal pitch, petroleum pitch, phenolic resin, etc. with the Si-containing compound 34 particles and performing heat treatment. Alternatively, the carbon coating may be formed by fixing carbon powder such as carbon black to the particle surface using a binder. The thickness of the carbon coating is preferably 1 to 200 nm or 5 to 100 nm, taking into consideration the need to ensure conductivity and the diffusion of lithium ions into the particle interior.

[0042] As described above, the first layer 32 contains carbon nanotubes (CNTs). The CNTs function as conductive agents, forming good conductive paths within the first layer 32 and contributing to the low resistance of the first layer 32. The CNTs also play an important role in controlling the porosity of the first layer 32. The first layer 32 may also contain, to the extent that it does not impair the purpose of this disclosure, particulate conductive agents such as carbon black, acetylene black, Ketjen black, and graphite, fibrous conductive agents such as vapor-grown carbon fibers (VGCF), electrospun carbon fibers, polyacrylonitrile (PAN) carbon fibers, pitch carbon fibers, and graphene, along with the CNTs.

[0043] The carbon nanotubes (CNTs) may be either multiwalled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs), and both SWCNTs and MWCNTs may be added to the first layer 32. However, the amount added needs to be changed depending on the type of CNT. As MWCNTs, for example, tubular structure CNTs in which graphene sheets made of six-membered carbon rings are wound parallel to the fiber axis, poollet structure CNTs in which graphene sheets made of six-membered carbon rings are arranged perpendicular to the fiber axis, and herringbone structure CNTs in which graphene sheets made of six-membered carbon rings are wound at an oblique angle to the fiber axis can be used.

[0044] In the case of MWCNTs, the average diameter is, for example, 20 nm or less, preferably 15 nm or less, and more preferably 10 nm or less. In the case of SWCNTs, the average diameter is, for example, 1 to 3 nm. If the average diameter of the CNTs (MWCNTs, SWCNTs) is within this range, the improvement effect on cycle characteristics is enhanced compared to using CNTs whose average diameter falls outside this range.

[0045] The average fiber length of the CNTs is, for example, 0.5 μm or more, preferably 0.7 μm or more, more preferably 0.8 μm or more, or 1 μm or more. The upper limit of the average fiber length of the CNTs is not particularly limited, but one example is 10 μm or 5 μm. An example of a suitable range for the average fiber length of the CNTs is 1 to 10 μm or 1 to 5 μm. If the average fiber length of the CNTs is within this range, the improvement effect on cycle characteristics is enhanced compared to using CNTs whose average fiber length falls outside this range. The average diameter and average fiber length of the CNTs are determined by selecting 100 CNTs each from the cross-sectional image of the first layer 32, measuring their diameter and fiber length, and averaging these measured values.

[0046] The MWCNT content is 3 to 10% by mass, more preferably 3 to 7% by mass, relative to the mass of the first layer 32. If the MWCNT content is within this range, the porosity of the first layer 32 can be controlled to the appropriate range described above, improving the battery's cycle characteristics. For example, if the MWCNT content is less than 3% by mass, the improvement effect on the battery's cycle characteristics decreases significantly, and if it exceeds 10% by mass, the capacity density of the negative electrode 12 decreases significantly. When SWCNTs are used, their content is 0.2 to 0.7% by mass, more preferably 0.3 to 0.7% by mass, relative to the mass of the first layer 32. Similar to the case of MWCNTs, if the SWCNT content is within this range, the porosity of the first layer 32 can be controlled to the appropriate range described above, improving the battery's cycle characteristics.

[0047] The binder included in the first layer 32 may be fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferred. The first layer 32 may further contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. It is preferable to use SBR and CMC or its salt, or PAA or its salt in combination as the binder in the first layer 32.

[0048] The binder content is 6 to 18% by mass, more preferably 10 to 15% by mass, relative to the mass of the first layer 32. When two or more types of binders are used, the total mass is adjusted to fall within this range. If the binder content is within this range, the porosity of the first layer 32 can be controlled to the appropriate range described above, improving the battery's cycle characteristics. For example, if the binder content falls below 6% by mass, the improvement effect on the battery's cycle characteristics decreases significantly, and if it exceeds 18% by mass, the capacity density of the negative electrode 12 decreases significantly.

[0049] As described above, the second layer 33 contains graphite 35 and a binder. Conductive agents such as CNTs are not required in the second layer 33. Graphite 35 is, for example, natural graphite such as spheroidal graphite or flaky graphite, or artificial graphite such as lump artificial graphite or graphitized mesophase carbon microbeads. Graphite 35 contains graphite A with a particle fracture strength of at least 10-35 MPa. Since graphite A is appropriately crushed during the manufacturing of the negative electrode 12, the packing density of the active material in the second layer 33 is improved, ensuring a high capacity. The second layer 33 may contain two or more types of graphite, for example, graphite A and graphite with a BET specific surface area of ​​0.5-2.5 m². 2 It may also contain graphite B at a concentration of / g. In this case, the improvement in cycle characteristics is more pronounced, and high-temperature storage characteristics are also improved.

[0050] Graphite A is, for example, spheroidized natural graphite with a D50 of 10 μm to 25 μm. The BET specific surface area of ​​graphite A is larger than that of graphite B, for example, 3 to 7 m². 2 / g, or 4-6m 2It is / g. The BET specific surface area of graphite is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0051] The particle destruction strength of graphite A is 10 to 35 MPa, more preferably 15 to 30 MPa. If the particle destruction strength is within this range, both high capacity and good cycle characteristics can be achieved. The compression destruction strength of graphite A is measured under the following conditions using a micro-compression tester (manufactured by Shimadzu Corporation, "MCT-W20"). In the micro-compression tester, for one graphite particle, the deformation amount and load of the particle when a load is applied at the following load rate are measured. Then, the load (N) when the particle is deformed and reaches its destruction point (the point where the displacement begins to increase rapidly) and the particle diameter of the particle before deformation (the particle diameter measured by a CCD camera) are substituted into the following formula to calculate the compression destruction strength. Compression destruction strength (MPa) = 2.8 × load (N) / {π × (particle diameter (mm))2}

[0052] <Measurement conditions for compression strength> Test temperature: normal temperature (25 °C) Upper pressing indenter: flat indenter with a diameter of 50 μm (material: diamond) Lower pressing plate: SKS flat plate Measurement mode: compression test Test load: minimum 10 mN, maximum 50 mN Load rate: minimum 0.178 mN / second, most large 0.221 mN / second Displacement full scale: 10 μm

[0053] Graphite B is, for example, artificial graphite with a D50 of 10 μm to 25 μm. The D50 of graphite B may be larger or smaller than the D50 of graphite A. The BET specific surface area of graphite B is 0.5 to 2.5 m 2 / g, more preferably 0.7 to 1.5 m 2 / g. If the BET specific surface area is within this range, good high-temperature storage characteristics can be obtained. Also, the particle destruction strength of graphite B is greater than that of graphite A, and as an example, it is 100 to 200 MPa.

[0054] When graphite A and B are used in combination, the ratio of graphite A to the total mass of graphite A and B is preferably 25 to 75% by mass, and more preferably 30 to 60% by mass. The mass ratio of graphite A and B may be substantially 1:1. The graphite 35 content is, for example, 97 to 99.5% by mass, or 95 to 99% by mass, relative to the mass of the second layer 33. The binder for the second layer 33 can be the same as the binder for the first layer 32. The binder content is, for example, 0.5 to 3% by mass, or 0.7 to 1.5% by mass, relative to the mass of the second layer 33.

[0055] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polypropylene, polyolefins such as copolymers of ethylene and α-olefins, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. The surface of the separator 13 may have a heat-resistant layer containing inorganic particles, or a heat-resistant layer composed of a highly heat-resistant resin such as aramid resin, polyimide, or polyamide-imide. [Examples]

[0056] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0057] <Example 1> [Fabrication of the positive electrode] As a positive electrode active material, LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3A lithium transition metal composite oxide represented by O2 was used. A cathode slurry was prepared by mixing 98 parts by mass of cathode active material, 1 part by mass of acetylene black, and 1 part by mass of polyvinylidene fluoride, and using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the cathode slurry was applied to both sides of a cathode core made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to produce a cathode in which cathode slurry layers were formed on both sides of the cathode core. An exposed portion was provided in the longitudinal center of the cathode in which the core surface was exposed, and a cathode lead was welded to this exposed portion.

[0058] [Preparation of the first negative electrode mixture slurry] As the negative electrode active material, a Si-containing compound (LixSiOy composition) with a D50 of 10 μm, in which Si particles were dispersed in a lithium silicate phase mainly composed of Li2Si2O5, was used. The Si-containing compound was mixed with multi-walled carbon nanotubes (MWCNTs) with an average diameter of 10 nm and an average fiber length of 1 μm, a dispersion of carboxymethylcellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) in a solid content mass ratio of 83:5:6:6, and water was used as the dispersion medium to prepare the first negative electrode mixture slurry.

[0059] [Preparation of the second negative electrode mixture slurry] The negative electrode active material has a particle fracture strength of 25 MPa, a D50 of 20 μm, and a BET specific surface area of ​​5 m². 2 Spheroidized natural graphite was used at a concentration of / g. Spheroidized natural graphite, CMC-Na, and SBR dispersion were mixed in a solid content mass ratio of 98:1:1, and water was used as the dispersion medium to prepare a second negative electrode mixture slurry.

[0060] [Fabrication of the negative electrode] A first negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil. After drying and compressing the first coating layer, a second negative electrode mixture slurry was applied on top of the coating layer, and the second coating layer was dried and compressed. At this time, each slurry was applied so that the thickness ratio of the first layer containing the Si compound and the second layer containing graphite A was 1:2. Subsequently, the material was cut to a predetermined electrode size, and a negative electrode was fabricated having a two-layer negative electrode mixture layer on both sides of the negative electrode core, with the first layer (lower layer) and the second layer (upper layer) formed sequentially from the core side. An exposed portion where the core surface is exposed was provided at the longitudinal end of the negative electrode, and a negative electrode lead was welded to this exposed portion. The void ratio of the first layer was 55%.

[0061] [Preparation of non-aqueous electrolyte solution] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 1:3.

[0062] [Preparation of test cells] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to a predetermined position on the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator to create a wound electrode body. This electrode body was placed inside a bottomed cylindrical outer container, the non-aqueous electrolyte was injected, and a sealing body was attached to the opening of the outer container via a gasket to create a cylindrical test cell.

[0063] <Example 2> In the preparation of the first negative electrode mixture slurry, SWCNTs were added instead of MWCNTs, and the Si-containing compound, SWCNTs, CMC-Na, and SBR dispersion were mixed in a solid content mass ratio of 87.5:0.5:6:6. Otherwise, the negative electrode and test cell were prepared in the same manner as in Example 1.

[0064] <Example 3> A negative electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the application order of the first and second negative electrode mixture slurries was changed to form a two-layer negative electrode mixture layer in which the second layer (lower layer) and the first layer (upper layer) were formed sequentially from the negative electrode core side.

[0065] <Example 4> In the preparation of the second negative electrode slurry, a negative electrode and a non-aqueous electrolyte secondary battery were prepared in the same manner as in Example 3, except that a mixture of graphite A and B in a mass ratio of 40:60 was added as the negative electrode active material. Graphite B had a particle fracture strength of 150 MPa, a D50 of 15 μm, and a BET specific surface area of ​​1.0 m². 2 This is synthetic graphite in gram quantities.

[0066] <Example 5> In the preparation of the first negative electrode mixture slurry, SWCNTs were added, and the Si-containing compound, SWCNTs, CMC-Na, and SBR dispersion were mixed in a solid content mass ratio of 84.6:0.6:8:7 to adjust the porosity of the first layer to 65%, except that the negative electrode and non-aqueous electrolyte secondary battery were prepared in the same manner as in Example 3.

[0067] <Comparative Example 1> In the preparation of the first negative electrode mixture slurry, MWCNTs were not added, and a Si-containing compound, CMC-Na, and SBR dispersion were mixed in a solid content mass ratio of 97:1.5:1.5. The negative electrode and test cell were prepared in the same manner as in Example 1, except that the compressive force of the coating film in the negative electrode mixture layer formation process was adjusted to reduce the compressive force compared to Example 1, thereby adjusting the porosity of the first layer to 55%.

[0068] <Comparative Example 2> A negative electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Comparative Example 1, except that the compressive force of the coating film in the negative electrode mixture layer formation process was the same as in Example 1.

[0069] <Comparative Example 3> In the preparation of the first negative electrode mixture slurry, the negative electrode and test cell were prepared in the same manner as in Example 2, except that a Si-containing compound, SWCNT, CMC-Na, and SBR dispersion were mixed in a solid content mass ratio of 96.5:0.5:1.5:1.5.

[0070] <Comparative Example 4> In the preparation of the first negative electrode mixture slurry, the negative electrode and test cell were prepared in the same manner as in Example 2, except that the Si-containing compound, SWCNT, CMC-Na, and SBR dispersion were mixed in a solid content mass ratio of 96.9:0.1:1.5:1.5.

[0071] For each of the above test cells in the examples and comparative examples, the expansion rate of the negative electrode, cycle characteristics, and 2C charging characteristics were evaluated using the method described below. The evaluation results, along with the composition of the negative electrode mixture layer, are shown in Table 1. The evaluation results for cycle characteristics and 2C charging characteristics are relative values ​​with the evaluation results of Comparative Example 1 set as the baseline (100).

[0072] [Evaluation of the expansion rate of the negative electrode] The thickness of the obtained negative electrode plate was measured with a microgauge and defined as the initial negative electrode thickness W1. Next, the fabricated cell was charged at a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.2V. After charging at a constant voltage until the current value was 0.02It at 4.2V, the negative electrode plate was removed from the cell, and the negative electrode thickness W2 in the charged state was measured in the same manner. The expansion rate of the negative electrode was determined based on the following formula. The expansion rate of the negative electrode = [(thickness of the negative electrode in the charged state W2 - thickness of the core) / (initial thickness of the negative electrode W1 - thickness of the core)] × 100

[0073] [Evaluation of cycle characteristics (capacity retention rate)] The test cell was charged at a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage until the current value was 0.02It at 4.2V. Subsequently, it was discharged at a constant current of 0.2It until the battery voltage reached 3.0V. This charge-discharge cycle was performed 500 times, and the capacity retention rate during the charge-discharge cycle was determined based on the following formula. Capacity retention rate = [Discharge capacity at 500 cycles / Discharge capacity at 1 cycle] × 100

[0074] [Evaluation of 2C charging characteristics] The test cell was charged at a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage until the current value was 0.02It at 4.2V. Subsequently, it was discharged at a constant current of 0.2It until the battery voltage reached 2.5V, and this capacity was defined as the rated capacity. Next, at a temperature of 25°C, it was charged at a constant current of 2It until the battery voltage reached 4.2V, and this charging capacity was defined as the 2C charging capacity. The depth of charge for 2C charging was determined based on the following formula and used as an evaluation index for the 2C charging characteristics. 2C charging depth = [2C charging capacity / rated capacity] × 100

[0075] [Table 1]

[0076] As shown in Table 1, all of the examples showed a smaller negative electrode expansion rate and a higher capacity retention rate of the test cell compared to the comparative example. Furthermore, when the first layer was the upper layer on the surface side of the composite layer and the second layer was the lower layer on the core side, the 2C charging characteristics were greatly improved (Examples 3-5). In the second layer, the BET specific surface area was 1.0 m². 2 When graphite B was used in combination at a concentration of / g, the improvement in volume retention was more pronounced (Example 4). When CNTs were not present in the first layer, the volume retention rate decreased significantly compared to the example (Comparative Examples 1 and 2), and no improvement in volume retention was obtained even when the porosity of the first layer was adjusted to 55%, the same as in the example (Comparative Example 1). Furthermore, no improvement in volume retention was obtained when the amount of CNTs or binder added to the first layer was inappropriate (Comparative Examples 3 and 4). [Explanation of symbols]

[0077] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Negative electrode core body, 31 Negative electrode mixture layer, 32 First layer, 33 Second layer, 34 Si-containing compound, 35 Graphite

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery comprising a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core, The aforementioned negative electrode mixture layer is A first layer comprising at least one metal element selected from Si, Sn, Sb, Mg, and Ge, and at least one compound containing the metal element, 3 to 10% by mass of multilayer carbon nanotubes, and 6 to 18% by mass of a binder, A second layer containing graphite A with a particle fracture strength of 10 to 35 MPa, It has, The porosity of the first layer is 50-65%, The second layer comprises graphite A and graphite B having a BET specific surface area of ​​0.5 to 2.5 m² / g, and is a negative electrode for a non-aqueous electrolyte secondary battery.

2. A negative electrode for a non-aqueous electrolyte secondary battery comprising a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core, The aforementioned negative electrode mixture layer is A first layer comprising at least one metal element selected from Si, Sn, Sb, Mg, and Ge, and at least one compound containing the metal element, 0.2 to 0.7 mass% of single-walled carbon nanotubes, and 6 to 18 mass% of a binder, A second layer containing graphite A with a particle fracture strength of 10 to 35 MPa, It has, The porosity of the first layer is 50-65%, The second layer comprises graphite A and graphite B having a BET specific surface area of ​​0.5 to 2.5 m² / g, and is a negative electrode for a non-aqueous electrolyte secondary battery.

3. A negative electrode for a non-aqueous electrolyte secondary battery comprising a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core, The aforementioned negative electrode mixture layer is A first layer comprising at least one metal element selected from Si, Sn, Sb, Mg, and Ge, and at least one compound containing the metal element, 3 to 10% by mass of multilayer carbon nanotubes, and 6 to 18% by mass of a binder, A second layer containing graphite A with a particle fracture strength of 10 to 35 MPa, It has, The second layer consists of graphite A and a material with a BET specific surface area of ​​0.5 to 2.5 m². ２ A negative electrode for a non-aqueous electrolyte secondary battery containing graphite B at a concentration of / g.

4. A negative electrode for a non-aqueous electrolyte secondary battery comprising a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core, The aforementioned negative electrode mixture layer is A first layer comprising at least one metal element selected from Si, Sn, Sb, Mg, and Ge, and at least one compound containing the metal element, 0.2 to 0.7 mass% of single-walled carbon nanotubes, and 6 to 18 mass% of a binder, A second layer containing graphite A with a particle fracture strength of 10 to 35 MPa, It has, The second layer consists of graphite A and a material with a BET specific surface area of ​​0.5 to 2.5 m². ２ A negative electrode for a non-aqueous electrolyte secondary battery containing graphite B at a concentration of / g.

5. A negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the second layer and the first layer are arranged in that order from the negative electrode core side.

6. A negative electrode according to any one of claims 1 to 5, Positive electrode and, Non-aqueous electrolytes, A non-aqueous electrolyte secondary battery equipped with [a specific feature].

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