Non-aqueous electrolyte secondary batteries

Abstract

This separator for nonaqueous electrolyte secondary batteries contains a polymer compound and a solid electrolyte, and has a pore volume of 0.06 cm3 / g or less.

Description

[Technical Field]

[0001] This disclosure , non water electrolyte secondary electricity In the pond Regarding. [Background technology]

[0002] Separators for non-aqueous electrolyte secondary batteries may contain inorganic fillers for the purpose of improving heat resistance and ion permeability. Patent Document 1 discloses a separator that does not shrink even at high temperatures and has excellent shape stability by containing an inorganic filler and a water-soluble polymer having a predetermined structure. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2018 / 079474 Summary of the Invention

[0004] The properties of separators for non-aqueous electrolyte secondary batteries vary depending on the pore size, and there is often a trade-off between these properties. Separators with large pore sizes of several hundred nanometers have the advantage of allowing the electrolyte to penetrate quickly, but they are prone to internal short circuits because dendrites tend to grow through the pores. On the other hand, separators with small pore sizes can prevent internal short circuits, but allow the electrolyte to penetrate slowly.

[0005] A separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a polymer compound and a solid electrolyte, and has a pore volume of 0.06 cm 3 / g or less.

[0006] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes the separator for a non-aqueous electrolyte secondary battery, a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0007] A method for producing a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a slurry preparation step of preparing a slurry by mixing a solution in which a polymer compound is dissolved in an ionic liquid with a solid electrolyte; a gelation step of applying the slurry to a substrate surface to prepare a coating film, and then gelling the coating film by replacing the coating film with an organic poor solvent in which the polymer compound has a lower solubility than the ionic liquid to prepare a gelled film; and a drying step of drying the gelled film to obtain a composite film.

[0008] According to the separator for a nonaqueous electrolyte secondary battery according to one aspect of the present disclosure, it is possible to improve the permeation rate of the electrolyte solution while suppressing internal short circuits. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a longitudinal cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an example of an embodiment of a separator for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the separator for a nonaqueous electrolyte secondary battery will be described in detail. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical outer can with a bottom will be exemplified. However, the outer can is not limited to a cylindrical outer can and may be, for example, a rectangular outer can or an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0011] Fig. 1 is a longitudinal cross-sectional view of a cylindrical secondary battery 10 as an example of an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and a non-aqueous electrolyte (not shown) are housed in an exterior body 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. For ease of explanation, the following description will refer to the sealing body 16 side as the "top" and the bottom side of the exterior body 15 as the "bottom."

[0012] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0013] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0014] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0015] Hereinafter, the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the electrode body 14 will be described in detail, particularly the separator 13.

[0016] [Positive Electrode] The positive electrode 11 has a positive electrode core body and a positive electrode mixture layer provided on the surface of the positive electrode core body. For the positive electrode core body, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum, a film with the metal disposed on the surface layer, etc. can be used. The thickness of the positive electrode core body is, for example, 10 μm to 30 μm. The positive electrode mixture layer contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both sides of the positive electrode core body except for the portion where the positive electrode lead 19 is connected. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, and a conductive material, etc. on the surface of the positive electrode core body, drying the coating film, and then compressing it to form the positive electrode mixture layer on both sides of the positive electrode core body.

[0017] Examples of the positive electrode active material contained in the positive electrode mixture layer include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium transition metal oxide is, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y [[ID=YID=26]]M 1-y O z 、Li x Ni 1-y M y O z 、Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple kinds. In terms of achieving a higher capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is Li x NiO2, Lix Co y Ni 1-y O2, Li x Ni 1-y M y O z It is preferable to contain a lithium nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≦ 1.2, 0 < y ≦ 0.9, 2.0 ≦ z ≦ 2.3).

[0018] Examples of the conductive material contained in the positive electrode composite material layer include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes, carbon nanofibers, and graphite. These may be used alone or in combination of two or more.

[0019] Examples of the binder contained in the positive electrode composite material layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, and polyolefin resin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc. These may be used alone or in combination of two or more.

[0020] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode composite material layer provided on the surface of the negative electrode core. For the negative electrode core, a metal foil stable within the potential range of the negative electrode 12 such as copper, a film having the metal disposed on the surface layer, etc. can be used. The thickness of the negative electrode core is, for example, 5 μm to 15 μm. The negative electrode composite material layer contains a negative electrode active material and a binder, and is preferably provided on both surfaces of the negative electrode core excluding, for example, the portion to which the negative electrode lead 20 is connected. The negative electrode 12 can be produced, for example, by applying a negative electrode composite material slurry containing a negative electrode active material and a binder, etc. on the surface of the negative electrode core, drying the coating film, and then compressing to form the negative electrode composite material layer on both surfaces of the negative electrode core. Also, a conductive material may be added to the negative electrode composite material slurry. The conductive material can homogenize the conductive path.

[0021] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer may be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, or polyolefin, but is preferably styrene-butadiene rubber (SBR). These resins may also be used in combination with CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, or polyvinyl alcohol (PVA). Examples of conductive materials contained in the negative electrode mixture layer include carbon black, acetylene black, ketjen black, carbon nanotubes, and carbon nanofibers.

[0022] [Separator] The separator 13 contains a polymer compound and a solid electrolyte. In other words, the separator 13 contains a polymer compound as a matrix and a solid electrolyte as an inorganic filler. The pore volume of the separator 13 is 0.06 cm 3 / g or less, and 0.05 cm 3 / g or less is preferable. This ensures ionic conductivity between the positive and negative electrodes while suppressing internal short circuits. Here, the pore volume can be measured using nitrogen gas using a commercially available measuring device such as the BELSORP-miniX manufactured by Microtrack-Bell.

[0023] The thickness of the separator 13 is preferably 0.2 μm to 10 μm, more preferably 0.4 μm to 1 μm. If the thickness of the separator 13 is less than 0.2 μm, the strength is insufficient. If the thickness of the separator 13 is more than 10 μm, the volume of the separator that does not contribute to charge and discharge in the internal space of the battery becomes large, and the density of the battery decreases.

[0024] The mass ratio of the polymer compound to the solid electrolyte in the separator 13 is preferably 100:1 to 100:100000, more preferably 100:1 to 100:10000, and particularly preferably 100:1 to 100:400. Note that the separator 13 may contain additives other than the polymer compound and the solid electrolyte, as long as the object of the present disclosure is not impaired.

[0025] Examples of polymer compounds contained in separator 13 include olefin resins such as polyethylene and polypropylene, cellulose, and cellulose derivatives. Cellulose is a preferred polymer compound. Cellulose is inexpensive and dissolves in ionic liquids, making it easy to form a thin separator 13 using the manufacturing method described below.

[0026] The solid electrolyte contained in the separator 13 is Li7La3Zr2O having a garnet structure. 12 (LLZ), Li with NASICON-type structure 1+x Al x Ti 2-x P3O 12 (LATP), a perovskite-type La 2 / 3-x Li x TiO3 (LLT), polyethylene oxide containing lithium ions (Li + LLZ, LATP, and LLT include those in which some of the elements contained in the above general formula are replaced with other additive elements.

[0027] As the solid electrolyte, LATP is preferable from the viewpoint of ionic conductivity. 1+x+y Al x Ti 2-x Si y P 3-y O 12 Ohara's LICGC, represented by TM can be used.

[0028] The solid electrolyte may be a powder. Its average particle size is, for example, 0.1 μm to 10 μm, preferably 0.4 μm to 1 μm. Here, the average particle size refers to the particle size at which the cumulative frequency of the smallest particle size in a volume-based particle size distribution is 50%, also known as the median size. The particle size distribution of the solid electrolyte can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) with water as the dispersion medium.

[0029] A plurality of solid electrolytes may be in contact with each other and connected from one side of separator 13 to the other side, allowing lithium ions to move inside separator 13, or the thickness of separator 13 may be set to be equal to or less than the average particle size of the solid electrolyte so that a single particle of the solid electrolyte penetrates the separator. From the viewpoint of ease of lithium ion movement, it is preferable to set the thickness of the separator to be equal to or less than the average particle size of the solid electrolyte.

[0030] A heat-resistant layer containing a heat-resistant material may be formed on the surface of separator 13. Examples of the heat-resistant material include polyamide resins such as aliphatic polyamides and aromatic polyamides (aramids), and polyimide resins such as polyamideimides and polyimides.

[0031] An example of a method for manufacturing the separator 13 will now be described.

[0032] First, in the slurry preparation step, a solution in which a polymer compound is dissolved in an ionic liquid is mixed with a solid electrolyte to prepare a slurry. The ionic liquid is a salt containing an anion and a cation, and is liquid at room temperature. A commercially available ionic liquid for dissolving cellulose can be used as the ionic liquid for dissolving cellulose. As the ionic liquid, an alkylimidazolium salt or the like can be used, and examples of the salt include chloride, acetate, and phosphate.

[0033] The mass ratio of the ionic liquid to the polymer compound in the slurry is, for example, 100:0.2 to 100:15. Within this range, the polymer compound dissolves in the ionic liquid, allowing the separator 13 to be produced by film formation. The solid electrolyte can be added in an amount of, for example, 1 part by mass to 100,000 parts by mass per 100 parts by mass of the polymer compound.

[0034] Next, in the gelation step, the slurry is applied to the surface of a substrate to form a coating film, and the coating film is gelled by replacing the solvent with an organic poor solvent in which the polymer compound has a lower solubility than the ionic liquid, to form a gelled film. The substrate to which the slurry is applied may be, for example, a flat substrate made of resin, glass, or metal. An example of the organic poor solvent that replaces the ionic liquid is ethyl acetate. The substrate with the coating film formed on its surface may be immersed in acetone and then in ethyl acetate, thereby replacing the ionic liquid with ethyl acetate via the acetone.

[0035] In the drying step, the gelled film is dried to obtain a composite film. Drying causes the gelled film to shrink, but the above-described fabrication method suppresses shrinkage, particularly of the polymer compound in the gelled film, thereby preventing voids from forming between the polymer compound and the solid electrolyte. The resulting composite film may be used as separator 13 as is, or may be subjected to further post-processing.

[0036] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (electrolytic solution) and may be a solid electrolyte using a gel polymer or the like. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0037] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

[0038] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, and the like. and chain ethers such as ethyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl.

[0039] As the halogen-substituted compound, it is preferable to use a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, a fluorinated chain carboxylate such as methyl fluoropropionate (FMP), or the like.

[0040] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 1 or more}, and the like. The lithium salt may be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the solvent.

[0041] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0042] <Example> [Preparation of Separator] Cellulose having a mass average molecular weight of about 300,000 was dissolved in 1-ethyl-3-methylimidazolium diethyl phosphate, which is an ionic liquid for dissolving cellulose. 2 parts by mass of LICGC powder (manufactured by Ohara Co., particle size 1 μm) was added to 1 part by mass of cellulose and uniformly mixed to form a slurry. A film was formed on a glass plate by the casting method, the ionic liquid was removed with acetone, and after further replacing acetone with ethyl acetate, it was naturally dried to prepare a separator. Regarding the prepared separator, an adsorption isotherm by nitrogen adsorption was measured using BELSORP-miniX manufactured by MicrotracBEL Co., and the pore volume was determined by the BJH method. As a result, 0.047 cm 3The separator had a thickness of 60 μm.

[0043] [Preparation of positive electrode] LiNi as the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2 was used. The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 100:3:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added. The mixture was then kneaded to prepare a positive electrode composite slurry. The positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. An exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core.

[0044] [Preparation of negative electrode] The negative electrode was obtained by pressing and fixing Li metal as the negative electrode active material onto a Ni mesh and cutting it to the specified electrode size. Note that an exposed portion was provided on a part of the negative electrode where the surface of the negative electrode core was exposed.

[0045] [Preparation of electrolyte] An electrolyte solution (non-aqueous electrolyte) was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.0 mol / L in a mixed solvent of fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP) in a volume ratio of 2:8.

[0046] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were arranged facing each other with the separator interposed therebetween to prepare an electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell.

[0047] [Evaluation of electrolyte penetration rate] The separator prepared above was sandwiched between Hyper Sheet gaskets and fixed in the center of an H-shaped cell. LiTFSI / DME (molar ratio LiTFSI:DME = 1:10) was placed in the left chamber of the H-shaped cell, and LiTFSI / FEC + FMP (molar ratio LiTFSI:FMP = 1:10) was placed in the right chamber. After 5 hours, the liquid in the right chamber was collected and the DME concentration was quantitatively analyzed using GC-MS. A higher DME concentration in the liquid in the right chamber indicates a faster electrolyte penetration rate.

[0048] [Short circuit suppression evaluation] The test cells prepared above were charged at a constant current (CC) of 0.01 C in a temperature environment of 25°C until the battery voltage reached 4.7 V, and the presence or absence of a voltage drop during charging was confirmed. Cells that did not experience a voltage drop during charging were evaluated as "Good", indicating that no internal short circuit had occurred, and cells that experienced a voltage drop were evaluated as "Poor", indicating that an internal short circuit had occurred.

[0049] <Comparative Example> The slurry described in Example 1 was used to form a film on a glass plate by casting, and the ionic liquid was removed using ultrapure water. The separator was then naturally dried, and evaluation was carried out in the same manner as in Example 1, except that the film was formed.

[0050] The evaluation results of the examples and comparative examples are shown in Table 1. Table 1 shows the thickness and pore volume of the separator together with the evaluation results.

[0051] [Table 1]

[0052] As shown in Table 1, the separator of the example was found to be more effective in suppressing short circuits than the separator of the comparative example. Furthermore, the separator of the example had a faster electrolyte penetration rate than the separator of the comparative example. This is because the use of ethyl acetate as the solvent increased the number of nano-sized pores in the separator. On the other hand, the decrease in pore volume is presumably due to the greater influence of the reduction in voids between the solid electrolyte and the matrix than the increase in nano-sized pores. [Explanation of symbols]

[0053] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Exterior body 16 Sealing body 17,18 Insulating plate 19 Positive lead 20 Negative lead 21 Grooved part 22 filters 23 Lower valve body 24 Insulating material 25 Upper valve body 26 Cap 26a opening 27 Gasket

Claims

1. cellulose and a solid electrolyte, Pore ​​volume is 0.06 cm ３ / g or less, and A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness is 0.2 μm or more and 10 μm or less.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the solid electrolyte is LATP.

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