Patent Application
20240396015
The present disclosure relates to a lithium-ion secondary battery.
An electrode of a lithium-ion secondary battery commonly has a metallic current collector and a mixture layer formed on a surface of the current collector. The mixture layer includes an active material, which is a main component, and a conductive agent interposed between the active material particles to form a conductive path. Patent Literature 1 discloses a technique of incorporating carbon nanotubes as a conductive agent in a negative electrode mixture layer in order to keep a conductive path between negative electrode active materials even after repeated charge and discharge.
In recent years, it has been investigated to use a silicon-based material, which has a larger capacity per volume than a carbon-based material such as graphite, as a negative electrode active material in order to increase the capacity of a negative electrode. The present inventors have found as a result of intensive study that, since the volume of the silicon-based material changes largely during charge and discharge, the effect by the carbon nanotubes is less likely to be achieved with a high proportion of the silicon-based material in the negative electrode active material. The technique in Patent Literature 1 does not consider a case where the negative electrode active material with large change in volume during charge and discharge is used, leaving room for improvement.
It is an advantage of the present disclosure to provide a lithium-ion secondary battery having a high capacity and a long lifetime.
A lithium-ion secondary battery of an aspect of the present disclosure comprises: a positive electrode; a negative electrode; a separator that separates the positive electrode and the negative electrode from each other; and an electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, the negative electrode mixture layer includes: a negative electrode active material having a discharge capacity of greater than or equal to 400 mAh/g and less than or equal to 750 mAh/g; and a conductive agent, the negative electrode active material includes a carbon-based material and a silicon-based material, the negative electrode current collector has a thickness of greater than or equal to 4 μm and less than or equal to 12 μm, and the negative electrode current collector has a 1%-proof strength of greater than or equal to 300 MPa and less than or equal to 700 MPa, and the conductive agent includes carbon nanotube.
The lithium-ion secondary battery of an aspect of the present disclosure has a high capacity, and may inhibit the decrease in the battery capacity due to repeated charge and discharge.
The present inventors have intensively investigated increase in a capacity of the lithium-ion secondary battery, and consequently found that the battery capacity is decreased considerably due to repeated charge and discharge with a high content rate of a silicon-based material in the negative electrode mixture layer even when the negative electrode mixture layer contains carbon nanotube. The present inventors have made further investigation and found that breakage and elongation of the negative electrode due to repeated charge and discharge may be inhibited by using carbon nanotube and a predetermined negative electrode current collector even when a proportion of the silicon-based material in the negative electrode active material is increased to set a discharge capacity of the negative electrode to greater than or equal to 400 mAh/g and less than or equal to 750 mAh/g. More specifically, expansion of the negative electrode may be inhibited by using a negative electrode current collector having a thickness of greater than or equal to 4 μm and less than or equal to 12 μm and a 1%-proof strength of greater than or equal to 300 MPa and less than or equal to 700 MPa, and deterioration of cycle characteristics may be inhibited by keeping a conductive path with the carbon nanotube. Here, the cycle characteristics refer to change in a battery capacity due to repeated charge and discharge.
Hereinafter, an example of embodiments of the negative electrode according to the present disclosure and the lithium-ion secondary battery using the negative electrode will be described in detail with reference to Drawings. Note that, the present disclosure includes selectively combining a plurality of embodiments and modified examples described below.
Hereinafter, a cylindrical battery in which a wound electrode assembly 14 is housed in a bottomed cylindrical exterior housing can 16 will be exemplified, but the exterior of the battery is not limited to a cylindrical exterior housing can, and may be, for example, a rectangular exterior housing can (rectangular battery), a coin-shaped exterior housing can (coin-shaped battery), or an exterior composed of laminated sheets including a metal layer and a resin layer (laminated battery). The electrode assembly may be a stacked electrode assembly in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator interposed therebetween.
The electrolyte, for example, includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, esters, ethers, nitriles, amides, a mixed solvent of two or more thereof, and the like are used, for example. The non-aqueous solvent may contain a halogen-substituted derivative in which hydrogen of these solvents is at least partially replaced with a halogen atom such as fluorine. An example of the non-aqueous solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), a mixed solvent thereof, or the like. For the electrolyte salt, a lithium salt such as LiPF6 is used, for example. The electrolyte is not limited to a liquid electrolyte but may be a solid electrolyte.
The positive electrode 11, the negative electrode 12, and the separator 13, which constitute the electrode assembly 14, are all a band-shaped elongated body, and spirally wound to be alternately stacked in a radial direction of the electrode assembly 14. To prevent precipitation of lithium, the negative electrode 12 is formed to be one size larger than the positive electrode 11. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in a longitudinal direction and a width direction (short direction). Two separators 13 are formed to be one size larger than at least the positive electrode 11, and disposed to sandwich the positive electrode 11. The electrode assembly 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 disposed on the upper and lower sides of the electrode assembly 14, respectively. In the example illustrated in
The exterior housing can 16 is a bottomed cylindrical metallic container having an opening on one side in an axial direction, as noted above. A gasket 28 is provided between the exterior housing can 16 and the sealing assembly 17, thereby sealability inside the battery and insulation between the exterior housing can 16 and the sealing assembly 17 are ensured. On the exterior housing can 16, a grooved portion 22 in which a part of a side wall thereof projects inward to support the sealing assembly 17 is formed. The grooved portion 22 is preferably formed in a circular shape along a circumferential direction of the exterior housing can 16, and supports the sealing assembly 17 with the upper face thereof. The sealing assembly 17 is fixed on the upper part of the exterior housing can 16 with the grooved portion 22 and with an end part of the opening of the exterior housing can 16 caulked to the sealing assembly 17.
The sealing assembly 17 has a stacked structure of the internal terminal plate 23, a lower vent member 24, an insulating member 25, an upper vent member 26, and the cap 27 in this order from the electrode assembly 14 side. Each member constituting the sealing assembly 17 has, for example, a disk shape or a ring shape, and each member except for the insulating member 25 is electrically connected to each other. The lower vent member 24 and the upper vent member 26 are connected at each of central parts thereof, and the insulating member 25 is interposed between each of the circumferential parts. If the internal pressure increases due to battery abnormality, the lower vent member 24 is deformed so as to push the upper vent member 26 up toward the cap 27 side and breaks, and thereby a current pathway between the lower vent member 24 and the upper vent member 26 is cut off. If the internal pressure further increases, the upper vent member 26 breaks, and gas is discharged through an opening of the cap 27.
Hereinafter, the positive electrode 11, the negative electrode 12, and the separator 13, which constitute the lithium-ion secondary battery 10, specifically the negative electrode 12, will be described in detail.
The positive electrode 11 has a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector. For the positive electrode current collector, a foil of a metal stable within a potential range of the positive electrode 11, such as aluminum and an aluminum alloy, a film in which such a metal is disposed on a surface layer thereof, and the like may be used. The positive electrode mixture layer includes a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both surfaces of the positive electrode current collector except for a current collector exposed portion, which is a portion where a positive electrode lead is to be connected. A thickness of the positive electrode mixture layer is, for example, greater than or equal to 50 μm and less than or equal to 150 μm on one side of the positive electrode current collector. The positive electrode 11 may be produced by applying a positive electrode mixture slurry including the positive electrode active material, the conductive agent, the binder, and the like on the surface of the positive electrode current collector, and drying and subsequently compressing the coating to form the positive electrode mixture layers on both the surfaces of the positive electrode current collector.
The positive electrode active material is constituted with a lithium-transition metal composite oxide as a main component. Examples of an element contained in the lithium-transition metal composite oxide and excluding Li include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Si, and P. An example of the preferable lithium-transition metal composite oxide is a composite oxide containing at least one of Ni, Co, and Mn. Specific examples thereof include a lithium-transition metal composite oxide containing Ni, Co, and Mn, and a lithium-transition metal composite oxide containing Ni, Co, and Al.
Examples of the conductive agent included in the positive electrode mixture layer include carbon-based particles such as carbon black, acetylene black, Ketjenblack, carbon nanotube, graphene, and graphite. Examples of the binder included in the positive electrode mixture layer include a fluororesin such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), a polyimide resin, an acrylic resin, and a polyolefin resin. With these resins, a cellulose derivative such as carboxymethylcellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like may be used in combination.
The negative electrode 12 has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector. The negative electrode mixture layer is preferably provided on both surfaces of the negative electrode current collector. A thickness of the negative electrode current collector is greater than or equal to 4 μm and less than or equal to 12 μm. A thickness of the negative electrode mixture layer is, for example, greater than or equal to 50 μm and less than or equal to 150 μm on one side of the negative electrode current collector.
For the negative electrode current collector, a foil of a metal stable within a potential range of the negative electrode, such as copper and a copper alloy, a film in which such a metal is disposed on a surface layer thereof, and the like may be used. The negative electrode current collector is preferably copper. A crystal grain size of copper as the negative electrode current collector is preferably greater than or equal to 0.2 μm and less than or equal to 2 μm. The crystal grain size may be measured by using SEM-EBSD (electron backscatter diffraction) and the like.
The negative electrode current collector has a 1%-proof strength of greater than or equal to 300 MPa and less than or equal to 700 MPa. The 1%-proof strength is a proof strength (1%) measured by a metal material tensile test method of JIS Z 2241.
The negative electrode current collector has a breakage elongation of, for example, greater than or equal to 2% and less than or equal to 9%. Here, the breakage elongation is a value of an amount of elongation of a specimen broken in a tensile test divided by a length of the specimen before the tensile test, and the breakage elongation is represented as percentage. In
The negative electrode mixture layer includes: a negative electrode active material having a discharge capacity of greater than or equal to 400 mAh/g and less than or equal to 750 mAh/g; and a conductive agent. The negative electrode active material includes a carbon-based material and a silicon-based material. The negative electrode active material may include a material other than the carbon-based material and the silicon-based material, which may occlude and release Li.
A proportion of the silicon-based material in the negative electrode active material may be greater than or equal to 5 mass % and less than or equal to 30 mass % relative to a total amount of the carbon-based material and the silicon-based material. The negative electrode active material mixed at such a proportion has the above discharge capacity.
The discharge capacity CA (mAh/g) of the negative electrode active material, the thickness CT (μm) of the negative electrode current collector, and the 1%-proof strength CM (N/mm2) of the negative electrode current collector preferably satisfy a relationship of CA/(CM×CT)<0.3.
The carbon-based material may be at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Particularly, natural graphite or artificial graphite, which has excellent charge-discharge stability and a small irreversible capacity, is preferable. Examples of the natural graphite include flake graphite, massive graphite, and amorphous graphite, and examples of the artificial graphite include massive artificial graphite and graphitized mesophase-carbon microbead.
Examples of the silicon-based material include Si metal that forms an alloy with Li, a metal compound including Si, and a composite oxide including Si. The silicon-based material includes, for example, an ion-conductive phase, silicon particles dispersed in the ion-conductive phase, and a coating layer that covers a surface of the ion-conductive phase.
The ion-conductive phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, and a silicide phase.
SiO, which is an example of the silicon-based material, has a particle structure in which fine silicon particles are dispersed in a silicate phase. A preferable SiO has a sea-island structure in which fine silicon particles are substantially uniformly dispersed in an amorphous silicon oxide matrix, and represented by the general formula SiOx (0<x≤2). The silicate phase is composed of aggregation of particles finer than the silicon particles. A content rate of the silicon particles is preferably greater than or equal to 35 mass % and less than or equal to 75 mass % relative to a total mass of SiO from the viewpoints of achievement of both a battery capacity and cycle characteristics, and the like.
The ion-conductive phase may include at least one element selected from the group consisting of alkali metal elements and Group II elements. For example, a compound represented by the general formula Li2zSiO(2+z) (0<z<2) is the silicon-based material, and has a sea-island structure in which fine Si particles are substantially uniformly dispersed in a lithium silicate matrix. The lithium silicate phase is composed of aggregation of particles finer than the silicon particles. A content rate of the silicon particles is preferably greater than or equal to 35 mass % and less than or equal to 75 mass % relative to a total mass of the Li2zSiO(2+z) compound, as in the case of SiO.
Si—C, which is another example of the silicon-based material, has a particle structure in which fine silicon particles are dispersed in an amorphous carbon phase. A preferable Si—C has a sea-island structure in which fine silicon particles are substantially uniformly dispersed in an amorphous carbon phase matrix. A content of the Si particles is preferably greater than or equal to 35 mass % and less than or equal to 75 mass % relative to a total mass of Si—C from the viewpoints of increasing the capacity, and the like.
The ion-conductive phase may include an element M, and the element M may be at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.
Examples of a constituent material of the coating layer include a carbon material, a metal, and a metal compound, and the constituent material is particularly preferably a carbon material such as amorphous carbon. The coating layer may be formed by: a CVD method using acetylene, methane, or the like; a method in which coal pitch, petroleum pitch, a phenol resin, or the like is mixed with silicon compound particles, and the mixture is subjected to a thermal treatment; and the like, for example. The coating layer may also be formed by adhering a conductive filler such as carbon black onto a surface of the ion-conductive phase using a binder.
The conductive agent included in the negative electrode mixture layer includes carbon nanotube (CNT). Although the conductive agent may include carbon black, acetylene black, Ketjenblack, graphite, and the like in addition to CNT, the conductive agent preferably includes only CNT. The CNT forms a good conductive path in the negative electrode mixture layer to contribute to reduction in resistance of the negative electrode mixture layer.
The CNT may be any of multiwall CNT (MWCNT) and single-wall CNT (SWCNT), and is preferably SWCNT. The negative electrode mixture layer may include both SWCNT and MWCNT.
An average fiber diameter of the CNT is, for example, greater than or equal to 0.01 nm and less than or equal to 5 nm. When the average fiber diameter of the CNT is within the above range, the effect of improving the cycle characteristics increases compared with a case where CNT having an average fiber diameter being out of the above range is used. The average fiber diameter of CNT may be determined by measuring diameters of 100 CNTs selected from a cross-sectional image of the negative electrode mixture layer to average these measured values.
A content rate of the CNT in the negative electrode mixture layer is, for example, greater than or equal to 0.005 mass % and less than or equal to 0.6 mass % relative to a total amount of the negative electrode active material. When the content rate of the CNT is within the above range, a content rate of the negative electrode active material may be increased while keeping the conductive path in the negative electrode mixture layer, and thereby the battery capacity may be further increased while improving the cycle characteristics.
The negative electrode mixture layer may further include a binder. For the binder included in the negative electrode mixture layer, a fluororesin, PAN, a polyimide, an acrylic resin, a polyolefin, and the like may be used as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferably used. The negative electrode mixture layer may further include CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), and the like. For the binder included in the negative electrode mixture layer, SBR; and CMC or a salt thereof, or PAA or a salt thereof are preferably used in combination.
The separator 13 separates the positive electrode 11 and the negative electrode 12 each other. For the separator 13, a porous sheet having an ion permeation property and an insulation property is used. Specific examples of the porous sheet include a fine porous thin film, a woven fabric, and a nonwoven fabric. As a material for the separator 13, a polyolefin such as polyethylene, polypropylene, and a copolymer of ethylene and an α-olefin, cellulose, and the like are preferable. The separator 13 may have any of a single layer structure and a stacked structure. On a surface of the separator 13, a heat-resistant layer including inorganic particles, a heat-resistant layer constituted with a highly heat-resistant resin such as an aramid resin, a polyimide, and a polyamide imide, and the like may be formed.
Hereinafter, the present disclosure will be further described with Examples, but the present disclosure is not limited to these Examples.
As a positive electrode active material, a lithium-transition metal composite oxide represented by LiCo1/3Mn1/3Ni1/3O2 was used. Mixing 98 parts by mass of the positive electrode active material, 1 part by mass of acetylene black, and 1 part by mass of polyvinylidene fluoride was performed, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Then, this positive electrode mixture slurry was applied on both surfaces of a positive electrode current collector made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to produce a positive electrode in which positive electrode mixture layers were formed on both the surfaces of the positive electrode current collector. An exposed portion where the current collector surface was exposed was provided at a central portion in a longitudinal direction of the positive electrode, and a positive electrode lead was welded to this exposed portion.
Artificial graphite and SiOx (0<x≤2) were mixed at a mass ratio of 92:8, and this mixture was used as a negative electrode active material. A discharge capacity of this negative electrode active material was 430 mAh/g. As a conductive agent, carbon nanotube (CNT) having an average fiber diameter of 0.1 nm was used. The negative electrode active material, the CNT, carboxymethylcellulose (CMC), and a dispersion of styrene-butadiene rubber (SBR) were mixed at a solid content mass ratio of 100:0.05:1:1, and water was used as a dispersion medium to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied on both surfaces of copper foil with 6 μm in thickness as a negative electrode current collector, and the coating was dried and compressed. Thereafter, the cutting was conducted to a predetermined electrode size to produce a negative electrode in which negative electrode mixture layers were formed on both the surfaces of the negative electrode current collector. On an end portion in a longitudinal direction of the negative electrode, an exposed portion where the current collector surface was exposed was provided, and a negative electrode lead was welded to this exposed portion.
Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 3:3:4. Into this mixed solvent, lithium hexafluorophosphate (LiPF6) was dissolved so that the concentration was 1 mol/L to prepare an electrolyte.
A lead made of aluminum was attached to the exposed portion of the positive electrode, a lead made of nickel was attached to a predetermined position of the negative electrode, and the positive electrode and the negative electrode were spirally wound via a separator made of polyethylene to produce a wound electrode assembly. This electrode assembly was housed in a bottomed cylindrical exterior housing can, the electrolyte was injected thereinto, and then a sealing assembly was attached to an opening of the exterior housing can via a gasket to produce a cylindrical test cell.
Under an environment temperature of 25° C., the above test cell was charged at a constant current of 0.2 It until 4.2 V, and then charged at a constant voltage of 4.2 V until 0.02 It. Thereafter, the test cell was discharged at a constant current of 0.2 It until 2.5 V. These charge and discharge were specified as one cycle, and 200 cycles were performed to determine a capacity deterioration rate with the following formula.
Capacity deterioration rate=(Discharge capacity at 1st cycle−Discharge capacity at 200th cycle)/Discharge capacity at 1st cycle×100
Test cells were produced and evaluated in the same manner as in Example 1 except that, in the production of the negative electrode, the proportion of the mass of SiO relative to the total mass of the artificial graphite and SiO was changed, copper foil having different characteristics (thickness, 1%-proof strength, and breakage elongation) was used, and the content rate of the CNT was changed, as shown in Tables 1 to 3. Table 1 also shows discharge capacities of the negative electrode active materials in Examples and Comparative Examples.
Tables 1 to 3 show the evaluation results of the test cells of Examples and Comparative Examples. Table 1 shows the results when the proportion of the silicon-based material is 8 mass %, and a capacity deterioration rate in Example 1 is a value relative to a capacity deterioration rate in Comparative Example 1 being a standard (100). Table 2 shows the results when the proportion of the silicon-based material is 15 mass %, and capacity deterioration rates in Examples 2 to 4 and Comparative Example 2 are values relative to a capacity deterioration rate in Comparative Example 3 being a standard (100). Table 3 shows the results when the proportion of the silicon-based material is 20 mass %, and a capacity deterioration rate in Example 5 is a value relative to a capacity deterioration rate in Comparative Example 4 being a standard (100).
As shown in Tables 1 to 3, the test cells of Examples had a lower capacity deterioration rate than the test cells of Comparative Examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-161551 | Sep 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/031487 | 8/22/2022 | WO |
