Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery is provided with a negative electrode current collector (40) and a negative electrode mixture layer (41) formed on the negative electrode current collector (40). The negative electrode mixture layer (41) contains a silicate compound (48) and a negative electrode active material capable of absorbing and releasing lithium ions. The negative electrode mixture layer (41) has a first region (45) disposed on the negative electrode current collector (40) side with respect to the thickness direction, and a second region (46) disposed on the opposite side from the negative electrode current collector (40) side. The content of the silicate compound (48) in the first region (45) is greater than the content of the silicate compound (48) in the second region (46).

Description

Technical Field

[0001] This disclosure relates to non-aqueous electrolyte secondary batteries. Background Technology

[0002] In a negative electrode of a non-aqueous electrolyte secondary battery having a negative electrode flux layer formed on a current collector, the electrolyte permeating into the negative electrode flux layer may undergo charge-discharge reactions within the flux layer by reacting with the negative electrode active material particles. Due to the expansion or contraction of the negative electrode active material particles during charge-discharge, the negative electrode flux layer also expands or contracts. Consequently, the electrolyte permeating into the negative electrode flux layer is discharged to the outside, resulting in an uneven distribution of electrolyte within the flux layer and increased resistance. In particular, during high-rate charge-discharge, the volume change of the negative electrode active material particles becomes larger, and the electrolyte expands due to Joule heating, thus the increase in resistance becomes significant.

[0003] Patent Document 1 describes a negative electrode comprising a negative electrode binder layer containing composite particles and a binder. The composite particles have negative electrode active material particles and a coating, the coating covering at least a portion of the surface of the negative electrode active material particles. In this negative electrode, the nanofibers contained in the binder and the silicate minerals in the coating form a composite that inhibits the expansion of the negative electrode active material particles. It is believed that the increase in battery resistance is small during such high-rate cycling.

[0004] Patent Document 2 describes a negative electrode with a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode mixture layer has a first layer formed on the negative electrode current collector and a second layer formed on the first layer. The first layer contains a first carbon-based active material, a Si-based active material, polyacrylic acid or a salt thereof, and fibrous carbon. The second layer contains a second carbon-based active material with a tap density greater than that of the first carbon-based active material. As a result, the movement of lithium ions in the second layer becomes smoother, thus improving the input characteristics of the battery.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2020-57500

[0008] Patent Document 2: International Publication No. 2020 / 0271763 Summary of the Invention

[0009] In either of the patent documents 1 and 2, there is room for improvement in terms of enhancing the charge and discharge characteristics during high-rate cycling.

[0010] As one embodiment of the present disclosure, the non-aqueous electrolyte secondary battery has a negative electrode current collector and a negative electrode compound layer formed on the negative electrode current collector. The negative electrode compound layer contains a negative electrode active material capable of absorbing and releasing lithium ions and a silicate compound. The negative electrode compound layer has a first region disposed on the side of the negative electrode current collector with respect to the thickness direction, and a second region disposed on the opposite side of the negative electrode current collector side. The silicate compound content in the first region is greater than the silicate compound content in the second region.

[0011] The non-aqueous electrolyte secondary battery disclosed herein enables smooth lithium-ion transport in the negative electrode and reduces the uneven concentration of lithium-ions in the thickness direction of the negative electrode binder layer. This improves the charge-discharge characteristics of the secondary battery during high-rate cycling. Attached Figure Description

[0012] Figure 1 This is a longitudinal cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an implementation method.

[0013] Figure 2 It is a display Figure 1 A schematic cross-sectional view of the negative electrode on one side in the thickness direction of a non-aqueous electrolyte secondary battery.

[0014] Figure 3 yes Figure 2 The diagram shows a silicate compound, montmorillonite, contained in the negative electrode. Detailed Implementation

[0015] As described above, there is still room for improvement in the charge-discharge characteristics of non-aqueous electrolyte secondary batteries during high-rate cycling. The inventors conducted in-depth research on this issue and discovered that the problem can be solved by including a silicate compound in the negative electrode binder layer, and by including a first region disposed on the negative electrode current collector side and a second region disposed on the opposite side of the negative electrode current collector side in the negative electrode binder layer, with the silicate compound content being higher in the first region than in the second region. That is, the silicate compound exhibits high lithium-ion adsorption and high diffusion properties. Furthermore, by increasing the silicate compound content in the first region on the negative electrode current collector side compared to the second region on the opposite side, lithium-ion transport in the negative electrode is promoted, and the uneven concentration of lithium-ions in the thickness direction of the negative electrode binder layer is reduced. As a result, the charge-discharge characteristics of the secondary battery during high-rate cycling can be improved.

[0016] The following is for reference Figures 1-3This document provides a detailed description of an embodiment of a lithium-ion secondary battery, which is a non-aqueous electrolyte secondary battery according to the present disclosure. The following example illustrates a cylindrical battery in which a wound electrode body is housed within a cylindrical battery casing. However, the electrode body is not limited to a wound type; it can also be a stacked type, consisting of multiple positive and multiple negative electrode separators alternately layered one by one. Furthermore, the battery casing is not limited to a cylindrical shape; for example, it can be square, coin-shaped, or made of laminates comprising metal and resin layers.

[0017] Figure 1 This is a longitudinal cross-sectional view of a lithium-ion secondary battery 10 (hereinafter referred to as secondary battery 10) as an example of an embodiment. Figure 1 In the secondary battery 10 shown, the electrode body 14 and the non-aqueous electrolyte (not shown) are housed in the outer container 16. Furthermore, for ease of explanation, the side with the sealing body 17 will be referred to as "upper" and the bottom side of the outer container 16 as "lower" in the following description.

[0018] The electrode body 14 has a wound structure formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped elongated bodies, which are alternately stacked in the radial direction of the electrode body 14 by winding them into a vortex. The separator 13 is formed to be one size larger than the positive electrode 11 and the negative electrode 12, and two sheets are arranged to sandwich the positive electrode 11.

[0019] The opening at the top of the outer can 16 is sealed by the sealing body 17, thereby sealing the interior of the secondary battery 10. An upper insulating plate 18 and a lower insulating plate 19 are respectively provided above and below the electrode body 14. A positive electrode tab 20 extends vertically through a through hole in the upper insulating plate 18, connecting the terminal plate 23, which serves as the bottom plate of the sealing body 17, to the positive electrode 11 contained in the electrode body 14. Thus, the positive electrode 11 is connected to the sealing body 17, and in the secondary battery 10, the top plate, i.e., the cover 27, of the sealing body 17, which is electrically connected to the terminal plate 23, becomes the positive terminal. The positive electrode tab 20 is, for example, an aluminum tab. On the other hand, a negative electrode tab 21 extends towards the bottom of the outer can 16 through a through hole in the lower insulating plate 19 and is welded to the inner bottom surface of the outer can 16. This connects the negative electrode 12 to the outer can 16, and in the secondary battery 10, the outer can 16 becomes the negative terminal. The negative electrode tab 21 is, for example, a nickel tab.

[0020] The positive electrode tab 20 is located at the center of the positive electrode 11 along its length, away from the starting and ending ends of the winding of the electrode body 14.

[0021] A negative electrode tab 21 is provided at one end of the negative electrode 12 along its length, located on the winding start side. The negative electrode 12 has a first negative electrode exposed portion (not shown) provided at the winding start side end and exposing the negative electrode current collector 40. The negative electrode tab 21 engages with the first negative electrode exposed portion.

[0022] A negative electrode 12 is disposed on the outermost peripheral surface of the electrode body 14, and a second negative electrode exposure portion 44 is provided on the surface of the negative electrode current collector 40. Furthermore, the negative electrode exposure portion 44 abuts against the inner peripheral surface of the outer packaging can 16. By having the negative electrode exposure portion 44 abut against the inner peripheral surface of the outer packaging can 16, which serves as the negative terminal, both ends of the negative electrode 12 in the longitudinal direction are electrically connected to the outer packaging can 16, ensuring good current collection. The negative electrode exposure portion 44 may also be provided on a portion of the outermost peripheral surface of the electrode body 14, but it is preferable to provide it on the entire area of ​​the outermost peripheral surface. For example, starting from the winding end of the negative electrode 12, a negative electrode exposure portion is provided on both sides of the negative electrode current collector 40 for a length of one circumference or more of the electrode body 14.

[0023] The placement of the negative electrode tab is not limited to Figure 1 In the example shown, a negative electrode tab can be provided only near the end of the winding of the negative electrode 12, or a negative electrode tab can be provided near both the end of the winding of the negative electrode 12 and the end of the winding.

[0024] The outer can 16 is a bottomed cylindrical metal container with an opening on one axial side. An airtight gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness of the interior of the secondary battery 10. The outer can 16, for example, has a groove 22 formed by stamping the side surface from the outside to support the sealing body 17. The groove 22 is preferably formed in a ring shape along the circumference of the outer can 16, and the sealing body 17 is supported thereon.

[0025] The sealing body 17 comprises a terminal plate 23, a lower valve body 24, an insulating component 25, an upper valve body 26, and a cover 27, stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating component 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 central portions, with the insulating component 25 sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heating, for example, the lower valve body 24 breaks, causing the upper valve body 26 to expand towards the cover 27 and detach from the lower valve body 24, thereby severing the electrical connection between them. When the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0026] The following describes in detail the positive electrode 11, negative electrode 12, separator 13 and non-aqueous electrolyte constituting the secondary battery 10, and in particular the negative electrode compound layer 41 constituting the negative electrode 12.

[0027] [positive electrode]

[0028] The positive electrode 11 has a positive current collector 30 and positive electrode additive layers 31 formed on both sides of the positive current collector 30. The positive current collector 30 can be a foil of a metal that is stable within the potential range of the positive electrode, such as aluminum or aluminum alloy, or a film with the metal disposed on the surface. The positive electrode additive layer 31 includes, for example, a positive electrode active material, a binder, and a conductive material. The positive electrode additive layer 31 may be formed only on one side of the positive current collector 30, but it is preferable to form it on both sides of the positive current collector 30. The positive electrode 11 can be manufactured, for example, by coating a positive electrode additive layer slurry containing a positive electrode active material, a binder, a conductive material, etc., onto the positive current collector 30, drying, and calendering the coating to form the positive electrode additive layer 31 on both sides of the positive current collector 30.

[0029] The positive electrode active material contained in the positive electrode compound layer 31 is mainly composed of a lithium-containing metal composite oxide. Examples of metal elements contained in the lithium-containing metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Ca, Sb, Pb, Bi, and Ge. A preferred example of the lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

[0030] Examples of conductive materials included in the positive electrode binder layer 31 include carbon materials such as carbon black, acetylene black, Ketjen black, graphite, and carbon nanotubes. Examples of binders included in the positive electrode binder layer include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. These resins can be used in combination with carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0031] [negative electrode]

[0032] The negative electrode 12 has a negative electrode current collector 40 and a negative electrode binder layer 41 formed on both sides of the negative electrode current collector 40. The negative electrode current collector 40 can be a metal foil, such as copper or a copper alloy, that is stable within the potential range of the negative electrode, or a film with the metal disposed on the surface. The negative electrode binder layer 41 may be formed on only one side of the negative electrode current collector 40, but it is preferable to form it on both sides of the negative electrode current collector 40. The negative electrode 12 can be manufactured by coating a negative electrode binder layer slurry containing a negative electrode active material, a silicate compound, and a binder onto the negative electrode current collector, and then drying and calendering the coating, thereby forming the negative electrode binder layer 41 on both sides of the negative electrode current collector 40, for example.

[0033] As the negative electrode active material contained in the negative electrode mixture layer 41, any material that can reversibly occlude and release lithium ions can be used, and carbon-based active materials such as graphite below can be used.

[0034] As the negative electrode active material, metals that can alloy with Li such as Si and Sn, metal compounds containing Si, Sn, etc., lithium titanium composite oxides, etc. can be used. For example, as shown below, a Si-containing compound represented by SiO x (0.5 ≤ x ≤ 1.5), or a Si-containing compound such as a lithium silicate phase represented by Li 2z SiO (2+z) (0 < z < 2) in which Si fine particles are dispersed can be used in combination with graphite.

[0035] Examples of the binder contained in the negative electrode mixture layer 41 include styrene-butadiene rubber (SBR), nitrile rubber (NBR), carboxymethyl cellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., and partial neutral salts can also be used), polyvinyl alcohol (PVA), etc. They can be used alone, or two or more of them can be used in combination.

[0036] Figure 2 is a schematic cross-sectional view showing one side in the thickness direction of the negative electrode 12. The negative electrode mixture layer 41 has a double-layer structure composed of a first region 45 disposed on the side of the negative electrode current collector 40 with respect to the thickness direction and a second region 46 disposed on the opposite side of the negative electrode current collector 40. In the first region 45 and the second region 46, only the first region 45, or both the first region 45 and the second region 46 contain the silicate compound 48. Moreover, the content of the silicate compound 48 in the first region 45 is more than the content of the silicate compound 48 in the second region 46. In addition, Figure 1 only the negative electrode mixture layer 41 on one side in the thickness direction of the negative electrode 12 is shown, but with reference to the negative electrode current collector 40, the negative electrode mixture layer on the other side in the thickness direction of the negative electrode 12 is also formed symmetrically with respect to the negative electrode mixture layer 41 on one side in the thickness direction of the negative electrode 12. In addition, the negative electrode mixture layer 41 can also be formed only on one side in the thickness direction of the negative electrode 12.

[0037] Region 1 (45) and Region 2 (46) are, for example, layers containing carbon-based active materials, Si-based active materials, polyacrylic acid or its salts, and fibrous carbon, with Region 1 (45) being the lower layer and Region 2 (46) being the upper layer. Using a high-density carbon-based active material (carbon-based active material A) ensures sufficient capacity, while using a high-porosity carbon-based active material (carbon-based active material B) improves liquid circulation. By varying the ratio of carbon-based active materials A and B in Region 1 (45) and Region 2 (46), a balance between input / output characteristics and energy density can be achieved. Furthermore, to ensure sufficient capacity of the carbon-based active material in Region 1 (45), a high-density carbon-based active material (carbon-based active material A) is preferred; and to improve liquid circulation of the carbon-based active material in Region 2 (46), a high-porosity carbon-based active material (carbon-based active material B) is preferred.

[0038] The mass of the first region 45 relative to the mass of the negative electrode mixture layer 41 is 50% or more and less than 90% by mass, preferably 60% or more and less than 80% by mass. Furthermore, the mass of the second region 46 relative to the mass of the negative electrode mixture layer 41 is 10% or more and less than 50% by mass, preferably 20% or more and less than 40% by mass. That is, the mass ratio of the first region 45 to the second region 46 (second region 46 / first region 45) is 0.1 or more and less than 1, preferably 0.3 or more and less than 0.7.

[0039] When the mass of the first region 45 is 90% or more of the mass of the negative electrode compound layer 41, and the mass of the second region 46 is 10% or less of the mass of the negative electrode compound layer 41, for example, the proportion of the second region 46, which helps to improve input characteristics, decreases, and the input characteristics of the battery decrease. Furthermore, when the mass of the first region 45 is less than 50% of the mass of the negative electrode compound layer 41, and the mass of the second region 46 is more than 50% of the mass of the negative electrode compound layer 41, the proportion of the first region 45 decreases, making it difficult to achieve high battery capacity.

[0040] Considering factors such as improving battery capacity, the preferred filling density of the negative electrode agent layer 41 is 1.65 g / cm³. 3 The above. The packing density of the negative electrode mixture layer 41 is, for example, 1.65 g / cm³. 3 Above and 1.75g / cm 3 The packing densities of region 1 (45) and region 2 (46) may be the same or different from each other. For example, the packing density of region 2 (46) may be lower than that of region 1 (45). As an example, the packing density of region 2 (46) is 1.40 g / cm³. 3 Above and 1.55g / cm 3The following is an example of the packing density in region 45, which is 1.70 g / cm³. 3 Above and 1.95g / cm 3 the following.

[0041] The thickness of the negative electrode flux layer 41 is, for example, 30 μm or more and 100 μm or less on one side of the negative electrode current collector 40. The thicknesses of the first region 45 and the second region 46 can be the same or different, provided that the above-mentioned mass ratio is satisfied. The thickness of the first region 45 can be greater than or less than the thickness of the second region 46. For example, the ratio of the thickness of the first region 45 to the thickness of the negative electrode flux layer 41 is 50% or more and 80% or less, and the ratio of the thickness of the second region 46 to the thickness of the negative electrode flux layer 41 is 20% or more and 50% or less. In addition, without prejudice to the purpose of this disclosure, the negative electrode flux layer 41 may also include layers other than the first region 45 and the second region 46.

[0042] As a carbon-based active material, graphite, amorphous carbon, etc., can be used, with graphite being preferred. Examples of graphite include natural graphite such as flake graphite, blocky artificial graphite, and artificial graphite such as graphitized mesophase carbon microspheres; natural and artificial graphite can also be used in combination. Furthermore, a conductive coating such as amorphous carbon can be formed on the surface of the graphite particles.

[0043] As described above, in the first region 45, in addition to the carbon-based active material, there is also a Si-based active material, polyacrylic acid (PAA) or its salt, and fibrous carbon. PAA or its salt binds the particles of the negative electrode active material (Si-based and carbon-based active materials) firmly together, thus suppressing the increase of negative electrode active material particles isolated from the conductive pathways in the first region 45, even if the volume of the Si-based active material changes significantly with charge and discharge. Therefore, by adding PAA or its salt to the first region 45, the decrease in cycle performance can be suppressed. Furthermore, the fibrous carbon, like PAA or its salt, forms a good conductive pathway in the first region 45.

[0044] The Si-based active material is at least one of Si and Si-containing compounds, preferably a Si-containing compound whose volume change during charge and discharge is smaller than that of Si. There is no particular limitation on the Si-containing compound as long as it contains Si, but SiO2 is preferred as described above. x Compounds represented by (0.5 ≤ x ≤ 1.5). A single Si-containing compound may be used alone, or two or more may be used in combination. Preferably, a conductive film made of a material with higher conductivity than the compound is formed on the surface of the Si-containing compound particles. The average particle size (Dv50) of the Si-containing compound is, for example, 1 μm or more and 15 μm or less.

[0045] SiO x has a structure in which, for example, Si is dispersed in an amorphous SiO2 matrix. In addition, SiO x may contain a lithium silicate salt (e.g., Li 2z SiO (2+z) (lithium silicate salt represented by 0 < z < 2)) within the particles, or may have a structure in which Si is dispersed in the lithium silicate salt phase.

[0046] The conductive film is preferably a carbon film. The carbon film is formed, for example, in an amount of 0.5% by mass or more and 10% by mass or less based on the mass of the SiO x particles. Examples of the method for forming the carbon film include a method of mixing coal tar or the like with Si-containing compound particles and performing heat treatment, a chemical vapor deposition method (CVD method) using a hydrocarbon gas or the like, etc. In addition, a carbon film can also be formed by fixing and bonding carbon black, Ketjen black, etc. to the surface of the Si-containing compound particles using a binder.

[0047] The mass ratio of the carbon-based active material to the Si-based active material contained in the first region 45 is, for example, 97:3 or more and 50:50 or less, preferably 95:5 or more and 80:20 or less. If the mass ratio is within this range, it is possible to achieve a high capacity of the battery, and the volume change of the Si-based active material can be alleviated by the carbon-based active material, and it is easy to suppress the deterioration of the cycle characteristics. In the first region 45, the proportion of the Si-based active material in the negative electrode active material is preferably 5% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 15% by mass or less.

[0048] PAA or its salt contained in the first region 45 and the second region 46 functions as a binder. The salt of PAA is, for example, a lithium salt, a sodium salt, a potassium salt, or an ammonium salt. In addition to PAA and its salt, the first region 45 and the second region 46 preferably further contain a second binder. Examples of the second binder include CMC or its salt, styrene-butadiene copolymer (SBR), polyvinyl alcohol (PVA), PEO, etc. Among them, CMC or its salt and SBR are preferred.

[0049] The content of the binder contained in the first region 45 is, for example, preferably 0.5% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 5% by mass or less, based on the mass of the first region 45.

[0050] The fibrous carbon contained in region 1 45 and region 2 46 acts as a conductive aid, forming good conductive pathways in regions 1 45 and 2 46. The fibrous carbon is, for example, a carbon material with an aspect ratio of 60 or greater, and has a size suitable for addition to regions 1 45 and 2 46. Region 2 46 is a layer with a higher content of fibrous carbon than region 1 45. Because the density of the active material in region 2 46 is low, the inclusion of more fibrous carbon ensures a conductive pathway.

[0051] Examples of fibrous carbon include carbon nanotubes (CNTs) and carbon nanofibers. CNTs are particularly preferred because the expansion or contraction of the negative electrode active material particles during charging and discharging can potentially cause cracks in the negative electrode binder layer, leading to a loss of conductivity and a reduction in the active material that facilitates charging and discharging. However, by using fibrous carbon containing CNTs, conductivity is ensured even in the event of cracking, thus suppressing the deterioration of charge-discharge cycle characteristics. CNTs can be not only monolayer CNTs, but also bilayer CNTs, multilayer CNTs, and mixtures thereof. Additionally, CNTs can also be vapor-grown carbon fibers known as VGCF (registered trademark). The fibrous carbon, for example, has a diameter of 2 nm or more and 20 μm or less, and a total length of 0.03 μm or more and 500 μm or less. The content of fibrous carbon relative to the mass of region 45 (first region) and region 46 is preferably 0.01% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less.

[0052] Silicate compound 48 exhibits lithiophilicity and high lithium diffusivity. Silicate compound 48 contains, for example, layered silicate minerals. Layered silicate minerals comprise a structure composed of multiple stacked silicate layers. Layered silicate minerals can be, for example, particulate. Layered silicate minerals can be, for example, plate-like particles, spherical particles, blocky particles, rod-like particles, etc. A layered silicate mineral is, for example, montmorillonite (MMT). Layered silicate minerals such as montmorillonite exhibit water swelling, cation adsorption capacity, and cation diffusivity.

[0053] Figure 3 This is a schematic diagram of montmorillonite, a silicate compound 48 contained in the negative electrode. Montmorillonite has a composition of (Na, Ca). 0.33 (Al, Mg) x (Si4O 10 The composition represented by )(OH)2·nH2O.

[0054] Specifically, montmorillonite constitutes the basic layer 50 of the layered structure. Within this basic layer 50, two sheet-like tetrahedral layers 51 (composed of two-dimensional bonds of Si and O tetrahedra) and two sheet-like octahedral layers 52 (composed of two-dimensional bonds of Al and OH octahedra) share a portion of O ions. Two tetrahedral layers 51 symmetrically sandwich one octahedral layer 52. Montmorillonite expands between the two basic layers 50 by allowing water molecules to enter. Additionally, Na+ is formed between the two basic layers 50. + Ca 2+ K + An iso-exchangeable cation layer is present. Therefore, silicate compound 48 containing montmorillonite exhibits high adsorption capacity for lithium ions as cations. Furthermore, it is known that in silicate compound 48, the energy consumption for lithium ion diffusion within the crystal structure is reduced. Therefore, silicate compound 48 can be considered to possess high lithium ion diffusivity.

[0055] The layered silicate compound is preferably at least one smectite. Preferred smectites, besides montmorillonite, include halloysite, hectorite, baddeleyite, nontronite, saponite, and sauconite. Among the layered silicate compounds, smectite has a high affinity for lithium ions, thus more effectively improving the lithium ion diffusion of the negative electrode mixture layer 41. The negative electrode mixture layer 41 contains at least one smectite selected from, for example, montmorillonite, halloysite, hectorite, baddeleyite, nontronite, saponite, and sauconite. Montmorillonite is preferred.

[0056] In this embodiment, as described above, the content of silicate compound 48 in the first region 45 is greater than that in the second region 46. Furthermore, silicate compound 48 exhibits high lithium-ion adsorption and high diffusivity. Therefore, it can promote lithium-ion transport in the negative electrode 12 and reduce the uneven concentration of lithium-ions in the thickness direction of the negative electrode binder layer 41. As a result, the charge-discharge characteristics during high-rate cycling in the secondary battery 10 can be improved.

[0057] Furthermore, the content of silicate compound 48 relative to the negative electrode mixture layer 41 in the first region 45 is preferably 0.1% by mass or more and 3% by mass or less. Additionally, the content of silicate compound 48 relative to the negative electrode mixture layer 41 in the second region 46 is preferably 0% by mass or more and 1% by mass or less. Alternatively, it may be configured such that the content of silicate compound 48 relative to the negative electrode mixture layer 41 in the first region 45 is 0.1% by mass or more and 3% by mass or less, and silicate compound 48 is not present in the second region 46.

[0058] The negative electrode 12 is manufactured, for example, by the following method: A first negative electrode binder layer slurry containing a first region 45, including a carbon-based active material, a Si-based active material, a binder, fibrous carbon, and a silicate compound 48, is prepared. A second negative electrode binder layer slurry containing a second region 46, including a carbon-based active material, a Si-based active material, a binder, and fibrous carbon, is prepared. The second negative electrode binder layer slurry may contain silicate compound 48. Then, the first negative electrode binder layer slurry is coated onto the negative electrode current collector 40, and the coating is dried to form the first region 45 on the negative electrode current collector 40. Next, the second negative electrode binder layer slurry is coated onto the first region 45, and the coating is dried to form the second region 46 on the first region 45. Then, the first region 45 and the second region 46 are compressed. This yields a negative electrode 12 having a negative electrode binder layer 41 containing the first region 45 and the second region 46 formed on the negative electrode current collector 40.

[0059] [Septum]

[0060] The diaphragm 13 is a porous sheet with ion permeability and insulation properties. Specific examples of porous sheets include microporous films, fabrics, and nonwoven fabrics. Preferred materials for the diaphragm 13 include polyethylene, polypropylene, olefin resins such as copolymers containing at least one of ethylene and propylene, and cellulose. The diaphragm 13 can be either a single-layer structure or a multilayer structure. A heat-resistant layer may also be formed on the surface of the diaphragm 13.

[0061] [Non-aqueous electrolytes]

[0062] Non-aqueous electrolytes possess ionic conductivity (e.g., lithium-ion conductivity). A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Non-aqueous electrolytes are not limited to liquid electrolytes (non-aqueous solutions) and can also be solid electrolytes using gel polymers, etc. The electrolyte salt can be, for example, lithium salts such as LiBF4 and LiPF6. The non-aqueous solvent can be, for example, esters, ethers, nitriles, amides, and mixtures of two or more of these solvents, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propionate (MP), etc. The non-aqueous solvent may also contain a halogen-substituted form, wherein at least a portion of the hydrogen atoms of the solvent is replaced by halogen atoms such as fluorine.

[0063] Examples of halogen substitutes include fluorinated cyclic carbonates such as fluorinated ethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP). From the perspective of suppressing the decrease in charge-discharge cycle characteristics of the non-aqueous electrolyte secondary battery or improving its input characteristics, the non-aqueous electrolyte preferably contains 5% by mass or more of FEC relative to the mass of the non-aqueous electrolyte, and more preferably contains 5% by mass or more and 15% by mass or less of FEC.

[0064] As solid electrolytes, polymeric electrolytes and inorganic solid electrolytes, for example, in solid or gel form, can be used. Polymer electrolytes include, for example, lithium salts and matrix polymers, or non-aqueous solvents, lithium salts and matrix polymers. As matrix polymers, polymeric materials that can absorb non-aqueous solvents and gel can be used. As polymeric materials, fluoropolymers, acrylic resins, polyether resins, etc., can be used. As inorganic solid electrolytes, known materials such as all-solid-state lithium-ion secondary batteries (e.g., oxide solid electrolytes, sulfide solid electrolytes, halide solid electrolytes, etc.) can be used.

[0065] Example

[0066] The present disclosure is further illustrated below by way of examples, but the present disclosure is not limited to the following examples.

[0067] <Example 1>

[0068] [positive electrode]

[0069] Lithium transition metal oxide, represented by LiNiCoAlO2, was used as the positive electrode active material. 99.0 parts by mass of the positive electrode active material, 0.4 parts by mass of carbon nanotubes (CNTs), and 0.6 parts by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode slurry. Next, the positive electrode slurry was coated, leaving the portion that would connect to the positive electrode tabs of the aluminum foil current collector uncoated, and the coating was allowed to dry. The coating was calendered using a roller and then cut to the specified electrode size to fabricate a positive electrode 11 with positive electrode slurry layers formed on both sides of the positive electrode current collector.

[0070] [negative electrode]

[0071] To form the first region 45 on the negative electrode current collector side of the negative electrode mixture layer, graphite, a Si-containing compound, a lithium salt of polyacrylic acid (PAA), a sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), carbon nanotubes (CNTs), and montmorillonite are mixed in a solid component mass ratio of 89.75:7.32:0.22:0.81:1.44:0.01:0.45, and an appropriate amount of water is added to prepare the first negative electrode mixture layer slurry for the first region 45.

[0072] To form the second region 46 on the negative electrode current collector side of the negative electrode mixture layer, graphite, a Si-containing compound, a lithium salt of polyacrylic acid (PAA), a sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), and carbon nanotubes (CNTs) are mixed in a solid component mass ratio of 89.81:7.33:0.58:1.08:1.17:0.03, and an appropriate amount of water is added to prepare the second negative electrode mixture layer slurry for the second region 46.

[0073] Next, a first negative electrode mixture slurry is applied to both sides of the negative electrode current collector 40, which is made of copper foil, leaving the portion to be connected to the negative electrode tab. The coating is then dried, forming a first region 45 on both sides of the negative electrode current collector 40. Next, a second negative electrode mixture slurry is applied to the first region 45 formed on both sides of the negative electrode current collector 40, and the coating is dried to form a second region 46. Then, the coating is rolled using a roller and cut to a specified electrode size to fabricate a negative electrode 12 with a negative electrode mixture layer 41 containing the first region 45 and the second region 46 formed on both sides of the negative electrode current collector. Therefore, the montmorillonite content in the first region 45 (0.45% by mass) is more than the montmorillonite content in the second region 46 (0%). Furthermore, the mass of the first region 45 and the second region 46 of the negative electrode mixture layer 41 was measured, and the mass ratio of the second region 46 to the first region 45 was 0.33.

[0074] [Non-aqueous electrolytes]

[0075] A non-aqueous electrolyte was prepared by adding 4% by mass of vinylene carbonate (VC) to a mixed solvent containing ethylene carbonate (EMC), fluorinated ethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 10:10:5:75, and dissolving LiPF6 at a ratio of 1.35 mol / L.

[0076] [Test Battery Cell]

[0077] Negative electrode tabs and positive electrode tabs are respectively installed on the negative electrode 12 and the positive electrode 11. A separator 13 is used to fabricate electrode bodies by alternately stacking and winding the negative electrodes 12 and positive electrodes 11 one by one. A single-layer polypropylene separator 13 is used as the separator. The fabricated electrode bodies are inserted into an outer casing made of aluminum laminates, and the opening of the outer casing is sealed to fabricate a test battery cell (laminated battery cell). The design capacity of the test battery cell is 440 mAh.

[0078] <Example 2>

[0079] In the preparation of the first negative electrode slurry layer in region 1 45, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), carbon nanotubes (CNTs), and montmorillonite are mixed in a solids mass ratio of 89.75:7.32:0.22:0.81:1.44:0.01:0.45. Furthermore, in the preparation of the second negative electrode slurry layer in region 2 46, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), styrene-butadiene copolymer (SBR), carbon nanotubes (CNTs), and montmorillonite are mixed in a solids mass ratio of 89.57:7.31:0.58:1.07:1.16:0.03:0.27. The test battery cell was fabricated under the same conditions as in Example 1. Therefore, in Example 2, the montmorillonite content in region 45, i.e., 0.45% by mass, is greater than the montmorillonite content in region 46, i.e., 0.27% by mass.

[0080] <Comparative Example 1>

[0081] In the preparation of the negative electrode slurry, only a slurry was prepared by mixing graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), and carbon nanotubes (CNTs) in a solid component mass ratio of 90.07:7.35:0.32:0.88:1.37:0.01. This slurry was coated on both sides of the negative electrode current collector. Otherwise, the test battery cell was fabricated in the same manner as in Example 1. Therefore, in Comparative Example 1, the negative electrode slurry layer was not divided into a first region and a second region, and the montmorillonite content in the negative electrode slurry layer was the same throughout the thickness direction, which was 0%.

[0082] <Comparative Example 2>

[0083] In the preparation of the first negative electrode slurry in the first region, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), and carbon nanotubes (CNTs) were mixed at a solid content mass ratio of 90.16:7.36:0.23:0.81:1.44:0.01. In the preparation of the second negative electrode slurry for the second region, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), styrene-butadiene copolymer (SBR), and carbon nanotubes (CNTs) were mixed at a solid content mass ratio of 89.81:7.33:0.58:1.08:1.17:0.03. The test battery cell was prepared under the same conditions as in Example 1 for Comparative Example 2. Therefore, in Comparative Example 2, the content of montmorillonite in Region 1 was the same as that in Region 2, both being 0%.

[0084] <Comparative Example 3>

[0085] In the preparation of the first negative electrode slurry in the first region, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), and carbon nanotubes (CNTs) were mixed at a solid content mass ratio of 90.16:7.36:0.23:0.81:1.44:0.01. In the preparation of the second negative electrode slurry for the second region, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), styrene-butadiene copolymer (SBR), carbon nanotubes (CNTs), and montmorillonite were mixed at a solid content mass ratio of 89.41:7.30:0.58:1.07:1.16:0.03:0.45. The test battery cell was prepared under the same conditions as in Example 1 for Comparative Example 3. Therefore, in Comparative Example 3, the montmorillonite content in Region 1, which is 0%, is less than the montmorillonite content in Region 2, which is 0.45% by mass.

[0086] <Comparative Example 4>

[0087] In the preparation of the negative electrode slurry, only a slurry was prepared by mixing graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), carbon nanotubes (CNTs), and montmorillonite in a solid component mass ratio of 89.66:7.32:0.31:0.87:1.37:0.01:0.45. This slurry was coated on both sides of the negative electrode current collector. Otherwise, the test battery cell was fabricated in the same manner as in Comparative Example 1. Therefore, in Comparative Example 4, the negative electrode slurry layer was not divided into a first region and a second region, and the montmorillonite content in the negative electrode slurry layer was the same throughout the thickness direction, which was 0.45% by mass.

[0088] <Comparative Example 5>

[0089] In the preparation of the first negative electrode slurry in the first region, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), carbon nanotubes (CNTs), and montmorillonite were mixed at a solid content mass ratio of 89.75:7.32:0.22:0.81:1.44:0.01:0.45. In the preparation of the second negative electrode slurry for the second region, graphite, a Si-containing compound, lithium salt of polyacrylic acid (PAA), sodium salt of carboxymethyl cellulose (CMC), carbon nanotubes (CNTs), styrene-butadiene copolymer (SBR), and montmorillonite were mixed at a solid content mass ratio of 89.41:7.30:0.58:1.07:1.16:0.03:0.45. The test battery cell was prepared under the same conditions as in Example 1 for Comparative Example 5. Therefore, in Comparative Example 5, the content of montmorillonite in Region 1 was the same as that in Region 2, both being 0.45 by mass.

[0090] In Table 1, the structure of the negative electrode compound layer, the second region and the first region, or the amount of montmorillonite (MMT) added in the monolayer of the test battery cells of Examples 1-2 and Comparative Examples 1-5 are expressed in mass % (%).

[0091]

[0092] [Evaluation of Capacity Maintenance Rate]

[0093] The test battery cells of Examples 1-2 and Comparative Examples 1-5 were charged at a constant current of 1 It at a temperature of 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current value was 0.02 It. Then, they were discharged at a constant current of 1 It until the battery voltage reached 2.5V. This charge-discharge cycle was performed 100 times, and the capacity retention rate during the charge-discharge cycle was calculated according to the following formula.

[0094] Capacity retention rate = (Charge capacity at 100th cycle / Charge capacity at 1st cycle) × 100

[0095] [Evaluation of Input Characteristics]

[0096] The test battery cells of Examples 1-2 and Comparative Examples 1-5 were charged at a constant current of 0.3 It to half of their initial capacity at 25°C, then charged at a constant voltage to a current of 0.05 It, and then stopped and left to stand for 2 hours. The voltage was then measured after discharging at a current of 0.5 It for 30 seconds. Based on the voltage decrease and the current flowing through the cells after 30 seconds of discharge, the battery cell resistance was calculated using the following formula.

[0097] Battery cell resistance = (Voltage drop after 30 seconds of discharge / Current value (0.5It))

[0098] Measurements were taken at the beginning of the capacity retention evaluation and after 100 cycles to calculate the cell resistance. The cell resistance increase rate was then calculated using the following formula.

[0099] Battery cell resistance increase rate = (Battery cell resistance in the 100th cycle / Battery cell resistance in the 1st cycle) × 100

[0100] Table 1 above shows the test results of capacity retention and cell resistance increase rate of the test battery cells of Examples 1-2 and Comparative Examples 1-5.

[0101] As shown in Table 1, in Comparative Examples 1-5, the montmorillonite content in the first region 45 of the negative electrode mixture layer was less than that in the second region 46, resulting in a capacity retention rate of less than 95.6% after 100 cycles. Furthermore, in Comparative Examples 1-5, the increase in cell resistance after 100 cycles was more than 104.8%. On the other hand, in Examples 1-2, the capacity retention rate after 100 cycles was increased to more than 96.8%. Additionally, in Examples 1-2, the increase in cell resistance after 100 cycles was reduced to less than 104.4%. This can be attributed to the fact that the montmorillonite content in the first region 45 was greater than that in the second region 46, thereby promoting lithium-ion transport in the negative electrode 12 and reducing the uneven concentration of lithium-ions in the thickness direction of the negative electrode mixture layer 41.

[0102] (Postscript)

[0103] This disclosure is further illustrated by the following embodiments.

[0104] Component 1:

[0105] A non-aqueous electrolyte secondary battery comprising a negative electrode current collector and a negative electrode flux layer formed on the negative electrode current collector.

[0106] The negative electrode mixture layer contains a negative electrode active material capable of absorbing and releasing lithium ions, and silicate compounds.

[0107] The negative electrode mixture layer has a first region disposed on the side of the negative electrode current collector relative to the thickness direction, and a second region disposed on the opposite side to the negative electrode current collector side.

[0108] The content of the silicate compound in the first region is greater than the content of the silicate compound in the second region.

[0109] Composition 2:

[0110] As in the non-aqueous electrolyte secondary battery as described in 1, the content of the silicate compound is 0.1% by mass or more and 3% by mass or less relative to the negative electrode compound layer in the first region.

[0111] Composition 3:

[0112] As in the non-aqueous electrolyte secondary battery described in 1 or 2, the content of the silicate compound is 0% by mass or more and 1% by mass or less relative to the negative electrode compound layer in the second region.

[0113] Composition 4:

[0114] As in any of the non-aqueous electrolyte secondary batteries described in any of 1 to 3, the content of the silicate compound is 0.1% by mass or more and 3% by mass or less relative to the negative electrode compound layer in the first region.

[0115] The second region does not contain the silicate compound.

[0116] Component 5:

[0117] As in any of the non-aqueous electrolyte secondary batteries described in 1 to 4, the silicate compound comprises montmorillonite.

[0118] Configuration 6: A non-aqueous electrolyte secondary battery as described in any one of configurations 1 to 5, wherein the negative electrode compound layer further contains a conductive additive.

[0119] The conductive additive contains carbon nanotubes.

[0120] Explanation of reference numerals in the attached figures

[0121] 10 Lithium-ion secondary battery (secondary battery), 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18 Upper insulating plate, 19 Lower insulating plate, 20 Positive electrode tab, 21 Negative electrode tab, 22 Tank inlet, 23 Terminal board, 24 Lower valve body, 25 Insulating component, 26 Upper valve body, 27 Cover, 28 Airtight pad, 30 Positive current collector, 31 Positive electrode compound layer, 40 Negative current collector, 41 Negative electrode compound layer, 44 Second negative electrode exposed part, 45 First region, 46 Second region, 48 Silicate compound, 50 Basic layer, 51 Tetrahedral layer, 52 Octahedral layer.

Claims

1. A non-aqueous electrolyte secondary battery, comprising a negative electrode current collector and a negative electrode flux layer formed on said negative electrode current collector, The negative electrode mixture layer contains a negative electrode active material capable of absorbing and releasing lithium ions, and silicate compounds. The negative electrode mixture layer has a first region disposed on the side of the negative electrode current collector relative to the thickness direction, and a second region disposed on the opposite side to the negative electrode current collector side. The content of the silicate compound in the first region is greater than the content of the silicate compound in the second region.

2. The non-aqueous electrolyte secondary battery as claimed in claim 1, wherein the content of the silicate compound is 0.1% by mass or more and 3% by mass or less relative to the negative electrode compound layer in the first region.

3. The non-aqueous electrolyte secondary battery as claimed in claim 1, wherein the content of the silicate compound is 0% by mass or more and 1% by mass or less relative to the negative electrode compound layer in the second region.

4. The non-aqueous electrolyte secondary battery as claimed in claim 1, wherein the content of the silicate compound relative to the negative electrode compound layer in the first region is 0.1% by mass or more and 3% by mass or less. The second region does not contain the silicate compound.

5. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the silicate compound comprises montmorillonite.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode binder layer further comprises a conductive additive. The conductive additive contains carbon nanotubes.

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