WO2023054098A1 - 二次電池用負極および二次電池 - Google Patents
二次電池用負極および二次電池 Download PDFInfo
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- WO2023054098A1 WO2023054098A1 PCT/JP2022/035117 JP2022035117W WO2023054098A1 WO 2023054098 A1 WO2023054098 A1 WO 2023054098A1 JP 2022035117 W JP2022035117 W JP 2022035117W WO 2023054098 A1 WO2023054098 A1 WO 2023054098A1
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- composite material
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- negative electrode
- material layer
- secondary battery
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Classifications
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Definitions
- the present disclosure relates to a negative electrode for secondary batteries and a secondary battery using the negative electrode.
- Patent Document 1 discloses a lithium ion secondary battery including a positive electrode including a positive electrode mixture layer having a pore tortuosity of 7 or more and 60 or less as measured by a mercury porosimeter. Patent Document 1 describes the effect that good rate characteristics are exhibited by using the positive electrode.
- An object of the present disclosure is to improve cycle characteristics during high-rate charge/discharge in a negative electrode for a secondary battery that uses a silicon material as a negative electrode active material.
- a negative electrode for a secondary battery includes a core and a mixture layer formed on the core, the mixture layer being disposed on the first mixture layer and the first mixture layer. and a second composite material layer, and containing graphite and a silicon material as active materials, the content of the silicon material being 0.5% by mass or more relative to the active material, and the first composite material layer A ratio ( ⁇ 2/ ⁇ 1) of the tortuosity ( ⁇ 2) of the second composite material layer to the tortuosity ( ⁇ 1) is less than 1.1.
- a secondary battery according to the present disclosure includes the electrode and an electrolyte.
- the negative electrode for a secondary battery according to the present disclosure it is possible to improve cycle characteristics during high-rate charge/discharge in a negative electrode for a secondary battery using a silicon material as a negative electrode active material.
- the secondary battery according to the present disclosure has, for example, high energy density and excellent rapid charging performance and cycle characteristics.
- FIG. 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery that is an example of an embodiment
- FIG. 1 is a cross-sectional view of an electrode body that is an example of an embodiment
- FIG. 1 is a cross-sectional view of an electrode body that is an example of an embodiment
- the present inventors made the negative electrode mixture layer a multilayer structure, and set the ratio of the tortuosity of the second mixture layer on the surface side of the mixture layer to the tortuosity of the first mixture layer on the core side to be 1. It has been found that by setting the ratio to less than 1, the cycle characteristics during high-rate charge/discharge can be significantly improved.
- the ratio ( ⁇ 2/ ⁇ 1) of the tortuous ratio ( ⁇ 2) of the second composite layer to the tortuous ratio ( ⁇ 1) of the first composite layer is 0.3 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 1
- the negative electrode is produced so as to satisfy By using this negative electrode, it is possible to achieve good cycle characteristics during high-rate charge/discharge in a secondary battery having a high energy density and using a silicon material as the negative electrode active material.
- the tortuosity of the composite material layer is an index that indicates the degree of curvature of the voids (pores) formed in the composite material layer through which the electrolyte passes. means.
- the tortuosity is a value obtained by dividing the distance (path length) from the start point to the end point of the void of the composite material layer by the straight line distance from the start point to the end point of the void of the composite material layer. If the path length is the same as the straight line distance from the start point to the end point of the voids in the composite layer, the tortuosity is 1.
- the inventors have found that the tortuosity of the second composite material layer has a dominant effect on the high-rate charge/discharge performance compared to the first composite material layer. Based on this knowledge, the tortuosity of the first composite layer is increased to increase the packing density of the active material particles, and the tortuosity of the second composite layer is decreased to improve high-rate charge-discharge performance.
- a cylindrical battery in which the wound electrode body 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer casing of the battery is not limited to a cylindrical outer can.
- the secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic outer can or a coin-shaped battery with a coin-shaped outer can, and is composed of a laminate sheet including a metal layer and a resin layer. It may also be a laminate battery having an outer package that has been laminated.
- the electrode assembly is not limited to the wound type, and may be a laminated electrode assembly in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated with separators interposed therebetween.
- FIG. 1 is a cross-sectional view of a secondary battery 10 that is an example of an embodiment.
- the secondary battery 10 includes a wound electrode body 14, an electrolyte, and an outer can 16 that accommodates the electrode body 14 and the 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 spirally wound with the separator 13 interposed therebetween.
- the outer can 16 is a bottomed cylindrical metal container that is open at one end in the axial direction.
- the side of the sealing member 17 of the battery will be referred to as the upper side
- the bottom side of the outer can 16 will be referred to as the lower side.
- the non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.
- non-aqueous solvents include esters, ethers, nitriles, amides, and mixed solvents of two or more thereof.
- the non-aqueous solvent may contain a halogen-substituted product obtained by substituting at least part of the hydrogen atoms of these solvents with halogen atoms such as fluorine.
- non-aqueous solvents examples include ethylene carbonate (EC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC), mixed solvents thereof, and the like.
- a lithium salt such as LiPF 6 is used as the electrolyte salt.
- the positive electrode 11, the negative electrode 12, and the separator 13, which constitute the electrode assembly 14, are all strip-shaped elongated bodies, and are alternately laminated in the radial direction of the electrode assembly 14 by being spirally wound.
- the negative electrode 12 is formed with a size one size larger than that of the positive electrode 11 in order to prevent deposition of lithium. That is, the negative electrode 12 is formed longer than the positive electrode 11 in the longitudinal direction and the width direction (transverse direction).
- the separator 13 is at least one size larger than the positive electrode 11, and two separators 13 are arranged so as to sandwich the positive electrode 11, for example. A porous sheet having ion permeability and insulation is used for the separator 13 .
- 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.
- Insulating plates 18 and 19 are arranged above and below the electrode body 14, respectively.
- the positive electrode lead 20 extends through the through hole of the insulating plate 18 toward the sealing member 17
- the negative electrode lead 21 extends through the outside of the insulating plate 19 toward the bottom of the outer can 16 .
- the positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, 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 to the inner surface of the bottom of the outer can 16 by welding or the like, and the outer can 16 serves as a negative electrode terminal.
- a gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure hermeticity inside the battery.
- the outer can 16 is formed with a grooved portion 22 that supports the sealing member 17 and has a portion of the side surface projecting inward.
- the grooved portion 22 is preferably annularly formed along the circumferential direction of the outer can 16 and supports the sealing member 17 on its upper surface.
- the sealing member 17 is fixed to the upper portion of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing member 17 .
- 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 layered in order from the electrode body 14 side.
- Each member constituting the sealing member 17 has, for example, a disk shape or a ring shape, and each member other than the insulating member 25 is electrically connected to each other.
- the lower valve body 24 and the upper valve body 26 are connected at their central portions, and an insulating member 25 is interposed between their peripheral edge portions.
- FIG. 2 is a diagram schematically showing a part of the cross section of the electrode assembly 14. As shown in FIG.
- the positive electrode 11 includes a core 30 and a composite material layer 31 formed on the core 30 .
- the composite material layer 31 of the positive electrode 11 has a single layer structure.
- the negative electrode 12 includes a core 40 and a composite material layer 41 formed on the core 40 .
- the mixture layer 41 of the negative electrode 12 has a two-layer structure including a first mixture layer 41A and a second mixture layer 41B disposed on the first mixture layer 41A.
- the first mixture layer 41A is arranged on the surface of the core body 40 and the second mixture layer 41B is arranged on the outermost surface of the mixture layer 41 .
- the composite layer 31 of the positive electrode 11 it is also possible for the composite layer 31 of the positive electrode 11 to have a two-layer structure similar to that of the negative electrode 12 .
- the two layers constituting each composite material layer of the negative electrode 12 have different tortuosity. It is preferable that the tortuosity is smaller than the tortuosity of the first composite material layer 41A, which is the lower layer of 41 located on the core body 40 side.
- the negative electrode 12 having such a two-layer structure, it is possible to greatly improve the cycle characteristics during high-rate charge/discharge while maintaining a high packing density of the negative electrode active material particles.
- layers other than the mixture layer may be formed on the core as long as the object of the present disclosure is not impaired.
- An example of another layer is a protective layer that contains inorganic particles, a conductive agent, and a binder and is interposed between the core and the composite material layer.
- the tortuosity of the composite material layer is calculated by the following formula.
- ⁇ is the tortuosity
- f is the path length (abbreviated as the path length) passed by the Medial Axis passing through the surfaces facing each other in the thickness direction
- s is the length of the straight line connecting the start point and the end point of the f path (straight line at the end point distance).
- ⁇ f/s
- the tortuosity of the first composite material layer and the second composite material layer can be calculated as follows.
- ⁇ 1 f 1 /s 1
- ⁇ 2 f 2 /s 2
- ⁇ 1 tortuosity of the first composite material layer
- f 1 path length of the first composite material layer
- s 1 straight line distance from the starting point of the first composite material layer
- ⁇ 2 tortuosity of the second composite material layer
- f 2 path length of the second composite material layer
- s 2 straight-line distance from the starting point of the second composite material layer
- the above-mentioned path length and linear distance are determined by cross-sectional observation and image analysis of the composite material layer using a 3D scanning electron microscope (3D SEM, for example, Ethos NX-5000 manufactured by Hitachi High-Tech Co., Ltd.).
- 3D SEM 3D scanning electron microscope
- Ethos NX-5000 manufactured by Hitachi High-Tech Co., Ltd.
- a specific method for calculating the tortuous road ratio is as follows. (1) Construction of a three-dimensional structure by 3D SEM A composite material is placed on the sample stage of the 3D SEM, and continuous cross-sectional slicing and cross-sectional observation are alternately performed. Observation is performed at an acceleration voltage of 5 kV.
- the obtained two-dimensional continuous images are binarized by three-dimensional image analysis software (eg, EX FACT VR manufactured by Nippon Visual Science Co., Ltd.), and these images are joined to construct a three-dimensional structure.
- the three-dimensional structure is preferably 100 ⁇ m ⁇ 100 ⁇ m ⁇ 100 ⁇ m or more.
- the material layer and the second composite material layer were separated, and the voids extracted by binarization were thinned to determine the axis (medial axis) passing through the center of the voids.
- the medial axis that exists in the cube is extracted in the direction perpendicular to the core surface, and the shortest path for those that have branches in one path is the path length of the composite material. (f 1 and f 2 ).
- the positive electrode 11 includes the core 30 and the composite material layer 31 formed on the core 30 as described above.
- a foil of a metal such as aluminum or an aluminum alloy that is stable in the potential range of the positive electrode 11, a film having the metal on the surface layer, or the like can be used.
- the composite layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both surfaces of the core 30 excluding the core exposed portion to which the positive electrode lead is connected.
- the positive electrode 11 is formed by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. to the surface of the core 30, drying the coating film, and then compressing the mixture layer 31 to the core. It can be produced by forming on both sides of 30 .
- the thickness of the composite material layer 31 on one side of the core 30 is, for example, 50 to 150 ⁇ m.
- the packing density of the composite material layer 31 is, for example, 3.3 g/cc or more, preferably 3.3 to 3.7 g/cc, or 3.5 to 3.7 g/cc. If the packing density of the composite material layer 31 is within this range, the secondary battery 10 with high energy density can be realized.
- the composite material layer 31 contains a lithium-transition metal composite oxide as a positive electrode active material.
- Elements other than Li contained in the lithium-transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In , Sn, Ta, W, Si, P and the like.
- the lithium-transition metal composite oxide preferably contains at least Ni from the viewpoint of increasing the capacity.
- An example of a suitable positive electrode active material has the general formula Li y Ni (1-x) M x O 2 (where 0 ⁇ x ⁇ 0.2, 0 ⁇ y ⁇ 1.2, M is Co, Al, Mn , Fe, Ti, Sr, Ca, and at least one element selected from the group consisting of B). Among them, it is preferable to contain at least one selected from the group consisting of Co, Al, and Mn as M.
- the content of Ni may be 85 mol % or more (where 0 ⁇ x ⁇ 0.15) with respect to the total molar amount of elements other than Li and O constituting the lithium-transition metal composite oxide.
- the positive electrode active material has, for example, the general formula Li y Ni (1-x) M x O 2 (where 0 ⁇ x ⁇ 0.12, 0 ⁇ y ⁇ 1.2, M is an element containing at least Co and Al ) including composite oxides represented by
- the volume-based median diameter (D50) of the positive electrode active material is, for example, 3 to 30 ⁇ m, preferably 5 to 25 ⁇ m.
- the positive electrode active material may be a particle composed of one or a small number of primary particles, or may be a secondary particle formed by aggregation of many primary particles.
- D50 means a particle size at which the cumulative frequency is 50% from the smaller particle size in the volume-based particle size distribution, and is also called median diameter.
- the particle size distribution of the composite oxide (Z) can be measured using a laser diffraction particle size distribution analyzer (eg MT3000II manufactured by Microtrack Bell Co., Ltd.) using water as a dispersion medium.
- Carbon materials such as carbon black, acetylene black, ketjen black, and graphite can be exemplified as the conductive agent contained in the composite material layer 31 .
- the content of the conductive agent is preferably 0.1 to 10% by mass, more preferably 0.2 to 5% by mass, based on the mass of the composite material layer 31 .
- binder contained in the composite material layer 31 examples include fluorine resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or salts thereof, polyethylene oxide (PEO), and the like.
- the content of the binder is, for example, 0.1 to 5 mass % with respect to the mass of the composite material layer 31 .
- the composite material layer 31 of the positive electrode 11 has a single layer structure, but it may have a two-layer structure similar to the composite material layer 41 of the negative electrode 12.
- the composite material layer 31 has a two-layer structure including the first composite material layer (lower layer) and the second composite material layer (upper layer)
- the ratio ( ⁇ 2/ ⁇ 1) of the tortuosity ( ⁇ 2) is preferably less than 1.1, more preferably 0.3 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 1.
- the negative electrode 12 includes the core 40 and the composite layer 41 formed on the core 40 .
- a foil of a metal such as copper or a copper alloy that is stable in the potential range of the negative electrode 12, a film having the metal on the surface layer, or the like can be used.
- the composite layer 41 contains a negative electrode active material, a binder, and optionally a conductive agent, and is preferably provided on both sides of the core 40 excluding the core exposed portion to which the negative electrode lead is connected.
- the thickness of the composite material layer 41 on one side of the core 40 is, for example, 50 to 150 ⁇ m.
- a negative electrode mixture slurry containing a negative electrode active material, a conductive agent, a binder, and the like is applied to the surface of the core body 40, the coating film is dried, and then compressed to form the mixture layer 41 on the core body. It can be produced by forming on both sides of 40 .
- the composite material layer 41 contains graphite and a silicon material as negative electrode active materials.
- the content of the silicon material is 0.5% by mass or more, preferably 2% by mass or more, more preferably 3% by mass or more, relative to the mass of the negative electrode active material.
- the upper limit of the silicon material content is, for example, 25% by mass, preferably 20% by mass.
- An example of a suitable silicon material content is 0.5 to 25% by mass, more preferably 2 to 20% by mass, or 3 to 15% by mass relative to the mass of the negative electrode active material.
- natural graphite such as flake graphite, massive artificial graphite, and artificial graphite such as graphitized mesophase carbon microbeads are used.
- D50 of graphite is preferably 1 to 30 ⁇ m, more preferably 5 to 20 ⁇ m.
- natural graphite and artificial graphite may be used in combination, or two or more types of graphite with different D50 may be mixed and used.
- silicon material examples include a silicon oxide phase and a silicon material containing Si dispersed in the silicon oxide phase (hereinafter referred to as "SiO").
- the D50 of silicon materials is generally smaller than that of graphite, eg, 1-15 ⁇ m.
- SiO is, for example, a compound represented by the general formula SiO x (0 ⁇ x ⁇ 2), which has a sea-island structure in which fine Si particles are substantially uniformly dispersed in an amorphous silicon oxide matrix.
- the silicon oxide phase is composed of aggregates of particles finer than Si particles.
- the content of Si particles is preferably 35 to 75% by mass with respect to the total mass of SiO from the viewpoint of compatibility between battery capacity and cycle characteristics.
- the average particle size of the Si particles dispersed in the silicon oxide phase is, for example, 500 nm or less, preferably 200 nm or less, or 50 nm or less before charging and discharging. After charging and discharging, it is, for example, 400 nm or less, or 100 nm or less.
- the average particle size of the Si particles is obtained by observing the cross section of the SiO particles using a SEM or transmission electron microscope (TEM) and averaging the longest diameters of 100 Si particles.
- a conductive layer composed of a highly conductive material may be formed on the surface of the SiO particles.
- a suitable conductive layer is a carbon coating composed of a carbon material.
- the thickness of the conductive layer is preferably from 1 to 200 nm, more preferably from 5 to 100 nm, in consideration of ensuring conductivity and diffusibility of Li ions into the particles.
- the binder contained in the composite material layer 41 may be fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but styrene-butadiene rubber (SBR) is particularly used. is preferred.
- the composite material layer 41 preferably contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), or the like. Among them, it is preferable to use SBR together with CMC or its salt or PAA or its salt.
- the content of the binder is, for example, 0.1 to 5 mass % with respect to the mass of the composite material layer 41 .
- the composite material layer 41 preferably contains a conductive agent.
- the conductive agent may be particulate carbon such as carbon black, acetylene black, ketjen black, graphite, etc., as in the case of the positive electrode 11, but the conductive agent preferably contains at least fibrous carbon. .
- fibrous carbon By adding fibrous carbon to the composite material layer 41, it is possible to suppress the generation of active material particles that are isolated from the conductive paths, and the effect of improving cycle characteristics becomes more pronounced.
- the content of the conductive agent is preferably 0.1 to 10 mass %, more preferably 0.2 to 5 mass %, relative to the mass of the composite material layer 41 . Note that particulate carbon and fibrous carbon may be used in combination as the conductive agent.
- CNT carbon nanotube
- CNH carbon nanohorn
- VGCF vapor grown carbon fiber
- electrospun carbon fiber polyacrylonitrile (PAN)-based carbon fiber
- PAN polyacrylonitrile
- the layer structure of the CNTs is not particularly limited, and may be either single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), but SWCNTs are preferred from the viewpoint of improving the conductivity of the composite layer 41 and the like.
- the average fiber length of CNT is preferably 0.1-40 ⁇ m, more preferably 0.3-20 ⁇ m, and particularly preferably 0.5-5 ⁇ m.
- the diameter of CNT is, for example, 1-100 nm.
- the composite layer 41 has a two-layer structure including the first composite layer 41A and the second composite layer 41B, and contains graphite and a silicon material as negative electrode active materials.
- the ratio ( ⁇ 2/ ⁇ 1) of the tortuosity ( ⁇ 2) of the second composite material layer 41B to the tortuosity ( ⁇ 1) of the first composite material layer 41A is less than 1.1.
- the tortuosity ratio ( ⁇ 2/ ⁇ 1) is 1.1 or more, for example, the permeability of the electrolytic solution into the composite material layer 41 deteriorates, and the cycle characteristics during high-rate charging/discharging tend to deteriorate.
- the curving ratio ratio ( ⁇ 2/ ⁇ 1) is preferably 0.3 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 1, more preferably 0.4 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 0.9, and 0.5 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 0.8 or 0.6 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 0.8 is particularly preferred.
- the tortuosity ratio satisfies this relationship, the effect of improving cycle characteristics during high-rate charge/discharge becomes more pronounced.
- the tortuosity ( ⁇ 1) of the first composite material layer 41A is, for example, 1.5 to 5.0, preferably 2.0 to 4.0.
- the tortuosity ( ⁇ 2) of the second composite material layer 41B is, for example, 1.2 to 4.5, preferably 1.5 to 3.0.
- the thickness ratio between the first mixture layer 41A and the second mixture layer 41B may be 1:1, and the thickness of the second mixture layer 41B may be greater than the thickness of the first mixture layer 41A. However, it is preferable that the thickness of the first mixture layer 41A ⁇ the thickness of the second mixture layer 41B.
- the thickness of the first mixture layer 41A is preferably 50 to 90%, more preferably 50 to 85%, particularly preferably 50 to 80% or 60 to 80% of the thickness of the mixture layer 41.
- the thickness of the second mixture layer 41B is preferably 10 to 50%, more preferably 15 to 50%, particularly preferably 20 to 50% or 20 to 40% of the thickness of the mixture layer 41. If the thickness of each composite material layer is within this range, the effect of improving cycle characteristics becomes more pronounced.
- the porosity ratio of the first mixture layer 41A and the second mixture layer 41B may be 1:1, and the porosity of the first mixture layer 41A is greater than the porosity of the second mixture layer 41B. However, it is preferable that the porosity of the first mixture layer 41A ⁇ the porosity of the second mixture layer 41B.
- the porosity of the first composite material layer 41A is preferably 10-40%, more preferably 20-30%.
- the porosity of the second composite material layer 41B is preferably 20-50%, more preferably 30-40%. If the porosity of each composite material layer is within this range, the effect of improving cycle characteristics becomes more pronounced.
- the packing density of the composite material layer 41 is, for example, 1.2 g/cc or more, preferably 1.3 to 1.7 g/cc. If the packing density of the composite material layer 41 is within this range, the secondary battery 10 with high energy density can be realized.
- the packing density of each composite material layer is not particularly limited, and the packing density of the composite material layer 41 as a whole may be within the range. It becomes larger than the packing density of 41B.
- the content rate of the conductive agent in the first mixture layer 41A and the second mixture layer 41B may be the same, and the content rate ( ⁇ 1) of the conductive agent in the first mixture layer 41A is the same as that in the second mixture layer 41B. may be greater than the content of the conductive agent ( ⁇ 2) in , but preferably ⁇ 1 ⁇ 2.
- the second composite layer 41B has a smaller tortuous path ratio ( ⁇ 2) and tends to have a lower packing density. is preferred.
- the first composite material layer 41A by suppressing the amount of the conductive agent added and relatively increasing the amount of the active material, it is possible to increase the capacity.
- the conductive agent, especially the fibrous carbon content ( ⁇ 2) in the second composite material layer 41B is greater than the fibrous carbon content ( ⁇ 1) in the first composite material layer 41A.
- the ratio ( ⁇ 2/ ⁇ 1) of the content ( ⁇ 2) to the content ( ⁇ 1) preferably satisfies 1 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 8, and preferably satisfies 2 ⁇ ( ⁇ 2/ ⁇ 1) ⁇ 6. more preferred. In this case, the effect of improving cycle characteristics during high-rate charge/discharge becomes more pronounced.
- the constituent materials of the first composite material layer 41A and the second composite material layer 41B may be the same or different as long as they satisfy the ratio ( ⁇ 2/ ⁇ 1) of the tortuous road ratio.
- the tortuosity ratio ( ⁇ 2/ ⁇ 1) can be controlled within a desired range.
- Each composite material layer may use graphite having substantially the same composition and different D50 in addition to or in addition to changing the compressive force in the rolling process of the composite material layer.
- the tortuosity ratio ⁇ 2/ ⁇ 1 is adjusted to the desired range. can also be controlled.
- a lithium transition metal composite oxide represented by LiNi 0.88 Co 0.09 Al **0.03 O 2 was used as the positive electrode active material.
- a positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 100:1:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium.
- NMP N-methyl-2-pyrrolidone
- the positive electrode mixture slurry was applied to both surfaces of a positive electrode core made of aluminum foil, and the coating film was dried and then compressed using a roller.
- the positive electrode core was cut into strips having a predetermined width to obtain a positive electrode having positive electrode mixture layers formed on both sides of the positive electrode core.
- the composite material layer of the produced positive electrode had a single-layer structure, a tortuosity of 1.7, a porosity of 23%, and a total thickness of the electrode including the core and the composite material layers on both sides of 150 ⁇ m. .
- the tortuosity, porosity, and thickness of the composite layer are measured by the methods described above.
- a mixture of graphite and a silicon material represented by SiO at a mass ratio of 97:3 was used as the negative electrode active material.
- a mixture of natural graphite and artificial graphite was used as the graphite.
- a negative electrode active material, a styrene-butadiene rubber (SBR) dispersion, a sodium salt of carboxymethyl cellulose, polyacrylic acid, and a single-walled carbon nanotube (CNT) were mixed at a ratio of 100:1:1:1:0.1. They were mixed at a solid content mass ratio, and water was used as a dispersion medium to prepare a negative electrode mixture slurry.
- Two types of first and second negative electrode mixture slurries were prepared in which the mixing ratios of natural graphite and artificial graphite were different from each other.
- the first negative electrode mixture slurry is applied to both sides of the negative electrode core made of copper foil, the first coating is dried, the first coating is rolled at a predetermined pressure, and then the second negative electrode mixture is applied.
- the slurry was applied over the first coating to form a second coating. After drying the second coating, rollers were used to compress the first and second coatings.
- the negative electrode core was cut into strips having a predetermined width to obtain a negative electrode having negative electrode mixture layers formed on both sides of the negative electrode core.
- the composite material layer of the produced negative electrode has a multilayer (two-layer) structure, the tortuosity is 2.5, the thickness of the entire electrode including the core and the composite material layers on both sides is 150 ⁇ m, and the packing density is 1. .5 g/cc.
- the first composite material layer (A layer) on the negative electrode core side has a tortuous rate ( ⁇ 1) of 3.5, the porosity is 24%, and the second composite layer (B layer) on the surface side of the composite material layer has a tortuous path.
- the ratio ( ⁇ 2) was 2.5 and the porosity was 33%.
- the tortuosity ratio ( ⁇ 2/ ⁇ 1) of each composite material layer was 0.7, and the thickness ratio was 1:1. Table 1 shows physical properties of the negative electrode.
- test cell The positive electrode to which an aluminum lead was welded and the negative electrode to which a nickel lead was welded were spirally wound with a separator interposed therebetween to prepare a wound electrode assembly.
- This electrode body is housed in a bottomed cylindrical outer can having a diameter of 18 mm and a height of 65 mm, and after the non-aqueous electrolyte is injected, the opening of the outer can is sealed with a sealing member through a gasket. , a test cell X1 (non-aqueous electrolyte secondary battery) was obtained.
- Examples 2 to 9, 17, 18> The compressive force in the rolling process of each composite layer is applied so that the tortuosity ratio ( ⁇ 2/ ⁇ 1) of each composite layer and the content of the silicon material with respect to the mass of the negative electrode active material are the values shown in Table 1.
- a negative electrode and test cells X2 to X9, X17, and X18 were produced in the same manner as in Example 1, except that two types of negative electrode mixture slurries were prepared by changing the addition amount of the silicon material.
- Examples 10, 11, 19, 20> The negative electrode and test cells X10 and X11 were prepared in the same manner as in Example 1, except that the coating amounts of the two types of negative electrode mixture slurries were changed so that the thickness ratio of each mixture layer was the value shown in Table 1. , X19 and X20.
- Example 12 to 16 Two types of negative electrode mixture slurries were prepared by changing the amount of CNTs added so that the CNT content ratio ( ⁇ 2/ ⁇ 1) in each mixture layer was the value shown in Table 1.
- a negative electrode and test cells X12 to X16 were prepared in the same manner as in Example 1.
- Example 2 A test cell Y2 was fabricated in the same manner as in Example 1, except that in the fabrication of the negative electrode, the negative electrode mixture layer having a single-layer structure was formed using the second negative electrode mixture slurry.
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Abstract
Description
τ=f/s
第1合材層および第2合材層の曲路率は、以下で算出可能である。
τ1=f1/s1
τ2=f2/s2
(τ1:第1合材層の曲路率、f1:第1合材層の経路長、s1:第1合材層の終始点の直線距離)
(τ2:第2合材層の曲路率、f2:第2合材層の経路長、s2:第2合材層の終始点の直線距離)
具体的な曲路率の算出方法は、下記の通りである。
(1)3DSEMによる3次元的構造の構築合材を3DSEMの試料台に載せて、連続断面スライスと断面観察を交互に行う。加速電圧は5kVで観察を行う。得られた2次元連続画像を3次元的画像解析ソフト(例えば日本ビジュアルサイエンス株式会社製 EX FACT VR)により、画像の2値化を行い、それらをつなぎ合わせ3次元的構造を構築する。3次元構造は100μm×100μm×100μm以上であることが好ましい。
(2)第1合材層と第2合材層の境界決定
(1)で得られた3次元構造画像より、厚み方向で空隙が異なる領域の境界を第1合材層と第2合材層の境界線に決定する。
(3)第1合材層と第2合材層におけるf1、f2、s1、s2の決定
(1)で取得3次元構造の画像を(2)より求めた比率で第1合材層と第2合材層とに分け、さらに2値化により抽出した空隙を細線化し、空隙の中心を通る軸(Medeial Axis)を決定した。立方体の中に存在するMedial Axisを芯体表面に対し、垂直な方向に貫通しているものを抽出し、ひとつの経路の中で分岐を持つものに対してその最短経路を合材の経路長(f1およびf2)として決定した。
(4)第1合材層および第2合材層の曲路率の算出
(3)で得られた第1合材層および第2合材層の平均経路長(f1およびf2)、経路を結ぶ平均直線距離(s1およびs2)を用いて、上記式より第1合材層および第2合材層の曲路率を算出した後、それらの比であるτ2/τ1を算出する。
正極11は、上記の通り、芯体30と、芯体30上に形成された合材層31とを備える。芯体30には、アルミニウム、アルミニウム合金など、正極11の電位範囲で安定な金属の箔、当該金属を表層に配置したフィルム等を用いることができる。合材層31は、正極活物質、導電剤、および結着剤を含み、正極リードが接続される部分である芯体露出部を除く芯体30の両面に設けられることが好ましい。正極11は、芯体30の表面に正極活物質、導電剤、および結着剤等を含む正極合材スラリーを塗布し、塗膜を乾燥させた後、圧縮して合材層31を芯体30の両面に形成することにより作製できる。
負極12は、上記の通り、芯体40と、芯体40上に形成された合材層41とを備える。芯体40には、銅、銅合金など、負極12の電位範囲で安定な金属の箔、当該金属を表層に配置したフィルム等を用いることができる。合材層41は、負極活物質、結着剤、および必要により導電剤を含み、負極リードが接続される部分である芯体露出部を除く芯体40の両面に設けられることが好ましい。合材層41の厚みは、芯体40の片側で、例えば50~150μmである。負極12は、芯体40の表面に負極活物質、導電剤、および結着剤等を含む負極合材スラリーを塗布し、塗膜を乾燥させた後、圧縮して合材層41を芯体40の両面に形成することにより作製できる。
[正極の作製]
正極活物質として、LiNi0.88Co0.09Al**0.03O2で表されるリチウム遷移金属複合酸化物を用いた。正極活物質と、アセチレンブラックと、ポリフッ化ビニリデンとを、100:1:1の固形分質量比で混合し、分散媒としてN-メチル-2-ピロリドン(NMP)を用いて正極合材スラリーを調製した。当該正極合材スラリーをアルミニウム箔からなる正極芯体の両面に塗布し、塗膜を乾燥した後、ローラを用いて圧縮した。正極芯体を所定の幅で短冊状に裁断して、正極芯体の両面に正極合材層が形成された正極を得た。
負極活物質として、黒鉛と、SiOで表されるシリコン材料とを、97:3の質量比で混合したものを用いた。黒鉛には、天然黒鉛と人造黒鉛の混合物を用いた。負極活物質と、スチレンブタジエンゴム(SBR)のディスパージョンと、カルボキシメチルセルロースのナトリウム塩と、ポリアクリル酸と、単層カーボンナノチューブ(CNT)とを、100:1:1:1:0.1の固形分質量比で混合し、分散媒として水を用いて負極合材スラリーを調製した。なお、天然黒鉛と人造黒鉛の配合比が互いに異なる2種類の第1および第2の負極合材スラリーを調製した。
エチレンカーボネートと、ジエチルカーボネートとを、1:1の体積比で混合した後、2質量%の濃度となるようにフルオロエチレンカーボネートを添加した。当該混合溶媒に、LiPF6を1mol/Lの濃度となるように添加して非水電解液を得た。
セパレータを介して、アルミニウム製のリードを溶接した上記正極、及びニッケル製のリードを溶接した上記負極を渦巻き状に巻回して、巻回型の電極体を作製した。この電極体を、直径18mm、高さ65mmの有底円筒形状の外装缶に収容し、上記非水電解液を注入した後、ガスケットを介して封口体により外装缶の開口部を封止して、試験セルX1(非水電解質二次電池)を得た。
各合材層の曲路率の比率(τ2/τ1)、および負極活物質の質量に対するシリコン材料の含有量が表1に示す値となるように、各合材層の圧延工程における圧縮力を変更し、シリコン材料の添加量を変更して2種類の負極合材スラリーを調製したこと以外は、実施例1と同様にして、負極および試験セルX2~X9,X17,X18を作製した。
各合材層の厚み比が表1に示す値となるように、2種類の負極合材スラリーの塗布量を変更したこと以外は、実施例1と同様にして、負極および試験セルX10,X11,X19,X20を作製した。
各合材層におけるCNTの含有率の比率(θ2/θ1)が表1に示す値となるように、CNTの添加量を変更して2種類の負極合材スラリーを調製したこと以外は、実施例1と同様にして、負極および試験セルX12~X16を作製した。
負極の作製において、負極合材層の曲路率が2.5となるように、合材層の圧延工程における圧縮力を変更して単層構造の負極合材層を形成したこと以外は、実施例1と同様にして、試験セルY1を作製した。
負極の作製において、第2の負極合材スラリーを用いて単層構造の負極合材層を形成したこと以外は、実施例1と同様にして、試験セルY2を作製した。
作製した各試験セルを、1Cの電流で電池電圧が4.2Vになるまで定電流充電を行った。その後、1Cの電流で電池電圧が2.5Vになるまで定電流放電を行った。この充放電サイクルを150サイクル行い、下記式より容量維持率を求めた。評価結果を表1に示す。
容量維持率(%)=(150サイクル目放電容量/1サイクル目放電容量)×100
Claims (8)
- 芯体と、前記芯体上に形成された合材層とを備える二次電池用負極であって、
前記合材層は、第1合材層と、前記第1合材層上に配置された第2合材層とを含み、かつ活物質として、黒鉛およびシリコン材料を含有し、当該シリコン材料の含有量は前記活物質に対して0.5質量%以上であり、
前記第1合材層の曲路率(τ1)に対する前記第2合材層の曲路率(τ2)の比率(τ2/τ1)が、1.1未満である、二次電池用電極。 - 前記第1合材層の前記曲路率(τ1)に対する前記第2合材層の前記曲路率(τ2)の比率(τ2/τ1)は、0.3≦(τ2/τ1)<1を満たす、請求項1に記載の二次電池用負極。
- 前記合材層は、導電剤を含有する、請求項1又は2に記載の二次電池用負極。
- 前記導電剤には、少なくとも繊維状炭素が含まれる、請求項3に記載の二次電池用負極。
- 前記第2合材層における前記導電剤の含有率(θ2)は、前記第1合材層における前記導電剤の含有率(θ1)より大きい、請求項3又は4に記載の二次電池用負極。
- 前記含有率(θ1)に対する前記含有率(θ2)の比率(θ2/θ1)は、1<(θ2/θ1)≦8を満たす、請求項5に記載の二次電池用負極。
- 前記第1合材層の厚みは、前記合材層の厚みに対して50~80%であり、
前記第2合材層の厚みは、前記合材層の厚みに対して20~50%である、請求項1~6のいずれか一項に記載の二次電池用負極。 - 請求項1~7のいずれか一項に記載の負極と、正極と、電解質とを備える、二次電池。
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