Stacked lithium ion secondary battery

Abstract

To provide a laminated lithium ion secondary battery which is strong against impact from external force and has high reliability and high safety even if it has high capacity and high voltage.SOLUTION: The separator of the laminated lithium ion secondary battery includes a substrate and a heat-resistant coating layer formed on the substrate, wherein the separator has an impact energy absorbing capacity C of not less than 36.0 and not more than 55.0 in MD and TD, and a ratio (CMD / CTD) of a value CMD of the impact energy absorbing capacity C in MD to a value CTD in TD is not less than 0.80 and not more than 1.25. The impact energy absorbing capacity is an area value surrounded by a stress-strain curve and a horizontal axis from an origin to a breaking point in the stress-strain curve obtained by performing a tensile test under a condition of a tensile speed of 100mm / min using a test piece of test piece type 2 by a method in accordance with JISK7127 Plastics-Determination of tensile properties-Part 3: Test conditions for films and sheets.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a stacked lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries have traditionally been used as power sources for a wide range of electronic devices, including portable devices, electric vehicles, homes, drones, robots, and storage batteries for commercial facilities. Lithium-ion secondary batteries for drones, in particular, are required to have both high power density and high energy density. However, achieving both of these properties with current lithium-ion secondary batteries can lead to deterioration in cycle characteristics and battery swelling, so improved reliability is also required.

[0003] A stacked lithium-ion secondary battery includes an electrode group in which multiple positive and negative electrode plates are alternately stacked with separators between them, and an exterior housing that houses the electrode group. The electrode group is sealed within the exterior housing, and positive and negative terminals extend outside the exterior housing. Such stacked lithium ion secondary batteries are required to be resistant to external shocks, highly reliable, and safe, even if they have a high capacity and a high voltage.

[0004] Patent Document 1 describes that for large, high-capacity lithium-ion batteries, high reliability is as important as battery properties, and that separators used in these batteries are also required to have high impact resistance from the standpoint of safety. It also describes that to achieve this, separators are used whose tensile strength and tensile elongation in MD (the length direction of the product when the separator is manufactured, the machine direction) and TD (the width direction of the product when the separator is manufactured, the direction perpendicular to MD) satisfy a specific relationship.

[0005] Patent Document 2 describes the use of a polyolefin microporous membrane with a specified relationship between the average pore number and toughness calculated as "MD tensile strength (MPa) × tensile elongation (%) + TD tensile strength (MPa) × tensile elongation (%)" in order to improve safety and high-rate charge / discharge characteristics, particularly battery characteristics at low temperatures, using a thin-film separator. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2018 / 180714 Brochure [Patent Document 2] International Publication No. 2020 / 203908 Brochure Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a stacked lithium ion secondary battery that is resistant to external shocks, highly reliable, and safe, even when it has a high capacity and a high voltage. [Means for solving the problem]

[0008] In order to solve the above problems, a first aspect of the present invention provides a stacked lithium ion secondary battery having the following configurations (1) to (5). (1) The battery includes an electrode group in which a plurality of positive and negative electrode plates are alternately stacked with separators interposed therebetween, an exterior housing that houses the electrode group in a sealed state, and a positive electrode terminal and a negative electrode terminal that are continuous with the positive electrode plate and the negative electrode plate, respectively, and extend outside the exterior housing. (2) The separator includes a substrate and a heat-resistant coating layer formed on the substrate. (3) The separator has an impact energy absorption capacity C of 36.0 or more and 55.0 or less in MD and TD. (4) The value C of the impact energy absorption capacity in MD MD The value C of TD TDRatio to (C MD / C TD ) is between 0.80 and 1.25. (5) The impact energy absorption capacity is the area enclosed by the stress-strain curve from the origin to the breaking point and the horizontal axis in a stress-strain curve obtained by conducting a tensile test in accordance with JIS K 7127 Plastics - Test methods for tensile properties - Part 3: Test conditions for films and sheets, using a Type 2 test specimen, at a tensile speed of 100 mm / min. [Effects of the Invention]

[0009] According to the present invention, it is expected to provide a stacked lithium ion secondary battery that is resistant to external shocks, highly reliable, and safe, even when it has a high capacity and a high voltage. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a perspective view showing a stacked lithium ion secondary battery according to an embodiment of the present invention; [Figure 2] 2 is a cross-sectional view taken along the line AA in FIG. 1. [Figure 3] 1 is a perspective view showing an electrode group of a stacked lithium ion secondary battery according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. In the following embodiments, technically preferable limitations are imposed for carrying out the present invention, but these limitations are not essential requirements for the present invention.

[0012] [Overall structure] First, the overall configuration of the stacked lithium ion secondary battery of this embodiment will be described. 1 and 2, the stacked lithium-ion secondary battery 1 of this embodiment includes an exterior body 2, an electrode group 3 housed inside the exterior body 2, and a positive electrode terminal 4 and a negative electrode terminal 5 extending outside the exterior body 2. The exterior body 2 is composed of two laminate films 21 and 22 that have the same rectangular planar shape. One laminate film 21 is a rectangular flat plate, and the other laminate film 22 has a shape in which a central portion forms an accommodation recess 221 and its peripheral portion forms a flange 222.

[0013] 2, the laminate films 21 and 22 are formed by laminating, in this order from the inside to the outside, heat-sealable resin layers 21a and 22a, metal layers 21b and 22b, and protective layers 21c and 22c. The laminate films 21 and 22 are arranged so that the heat-sealable resin layers 21a and 22a face each other, and the electrode group 3 is housed in the space formed by the laminate film 21 and the housing recess 221 of the laminate film 22 (between the heat-sealable resin layer 21a and the heat-sealable resin layer 22a). The exterior body 2 has a sealing portion 20 formed by heat-sealing the peripheral edges of the heat-sealable resin layers 21a and 22a of the two laminate films 21 and 22 together (the flange portion 222 of the laminate film 21).

[0014] 2, the electrode group 3 is formed by alternately stacking a plurality of positive electrode plates 31 and negative electrode plates 32 with separators 33 interposed therebetween. The positive electrode plate 31 is formed by forming a positive electrode active material layer 312 on one or both surfaces of a positive electrode current collector plate 311. The negative electrode plate 32 is formed by forming a negative electrode active material layer 322 on one or both surfaces of a negative electrode current collector plate 321.

[0015] 2 and 3 , a positive electrode lead 313 and a negative electrode lead 323 are continuous from the positive electrode current collector plate 311 and the negative electrode current collector plate 321 that constitute the positive electrode plate 31 and the negative electrode plate 32. One end of the positive electrode terminal 4 and one end of the negative electrode terminal 5 are joined and electrically connected to the bundled portion where all the positive electrode leads 313 and the negative electrode leads 323 are bundled by ultrasonic welding, resistance welding, or the like. The other ends of the positive electrode terminal 4 and the negative electrode terminal 5 extend outside the exterior body 2. In other words, the positive electrode terminal 4 and the negative electrode terminal 5 are continuous from the positive electrode plate 31 and the negative electrode plate 32, respectively, and extend outside the exterior body 2. The positive electrode terminal 4 and the negative electrode terminal 5 are fixed to the sealing portion 20 of the exterior body 2 in a state where they are sandwiched between the sealant portion 6 .

[0016] That is, the electrode group 3 is housed in the exterior body 2 in a sealed state. 3, tape 7 is attached to two locations on each of two of the four side surfaces of the electrode group 3 on which the positive electrode lead 313 and the negative electrode lead 323 are not present, in order to prevent misalignment of the positive electrode plate 31, the negative electrode plate 32, and the separator 33 in the electrode group 3. The tape 7 is attached from the front surface (the uppermost surface when the stacking direction is the vertical direction) of the electrode group 3 through the side surfaces to the back surface (the lowermost surface when the stacking direction is the vertical direction).

[0017] [Separator configuration] The separator 33 is composed of a substrate and a heat-resistant coating layer formed on the substrate. The separator has an impact energy absorption capacity C of 36.0 or more and 55.0 or less in MD and TD. The value C of the impact energy absorption capacity C in MD MD The value C of TD TD Ratio to (C MD / C TD ) is between 0.80 and 1.25.

[0018] Impact energy absorption capacity C is the area enclosed by the stress-strain curve from the origin to the breaking point and the horizontal axis in a stress-strain curve obtained by conducting a tensile test using a Type 2 test specimen at a tensile speed of 100 mm / min in accordance with JIS K 7127 Plastics - Test methods for tensile properties - Part 3: Test conditions for films and sheets. The thickness of the heat-resistant coating layer per side is preferably 1 μm or more and 4 μm or less. If it is less than 1 μm, the heat-resistant function of the heat-resistant coating layer may not be exhibited, which may reduce safety, and if it is more than 4 μm, the migration of lithium ions during the battery reaction may be inhibited, which may reduce output characteristics.

[0019] The weight of the heat-resistant coating layer on one side is 0.25 mg / cm 2 More than 0.95mg / cm 2 The weight per unit area is preferably 0.25 mg / cm or less. 2 If the concentration is less than 0.95 mg / cm, the heat-resistant coating layer may not be able to exhibit its heat-resistant function, which may result in a decrease in safety. 2 If the amount is greater than this, the battery weight will increase, and there is a risk that lithium ion migration will be inhibited during the battery reaction, resulting in a decrease in output characteristics.

[0020] The thickness of the heat-resistant coating layer per side and the basis weight of the heat-resistant coating layer per side are both values ​​per separator.

[0021] The heat-resistant coating layer may be a layer formed from a heat-resistant resin, or a layer formed from a heat-resistant resin and an inorganic filler. Examples of heat-resistant resins that constitute the heat-resistant coating layer include water-soluble polymers such as polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, and polymethacrylic acid, and nitrogen-containing aromatic polymers such as aromatic polyaramid, aromatic polyimide, and aromatic polyamideimide.

[0022] Furthermore, the inorganic filler constituting the heat-resistant coating layer is preferably inorganic fine particles having electrical insulation properties, and specific examples include inorganic oxide fine particles such as iron oxide, silica (SiO2), alumina (Al2O3), TiO2, and BaTiO3; inorganic nitride fine particles such as aluminum nitride and silicon nitride; sparingly soluble ionic crystal fine particles such as calcium fluoride, barium fluoride, and barium sulfate; covalently bonded crystal fine particles such as silicon and diamond; and clay fine particles such as montmorillonite.

[0023] Here, the inorganic oxide fine particles may be fine particles of substances derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and mica, or artificial products thereof.

[0024] The inorganic compounds constituting the inorganic fine particles used as the inorganic filler may be element-substituted or solid-solutionized as necessary, and the inorganic fine particles may be surface-treated. The inorganic fine particles may also be particles made electrically insulating by coating the surface of a conductive material, such as a metal, a conductive oxide such as SnO2 or tin-indium oxide (ITO), or a carbonaceous material such as carbon black or graphite, with an electrically insulating material (for example, the inorganic oxides mentioned above).

[0025] The inorganic filler constituting the heat-resistant coating layer may be of one type or a mixture of two or more types, and ceramic powders of the same or different types with different particle sizes may be arbitrarily mixed and used. Furthermore, the tensile strength of the separator does not change depending on the material of the heat-resistant coating layer.

[0026] [Action, effect] As described above, by using the separator 33 having the above configuration, the stacked lithium ion secondary battery 1 of the embodiment is resistant to external impacts and is highly reliable and safe, even when it has a high capacity and a high voltage. [Example]

[0027] The present invention will be further described below with reference to specific examples. As the laminated lithium ion secondary battery 1 of the embodiment, prototype batteries No. 1 to No. 6, all of which had the same configuration except for the separator 33, were fabricated.

[0028] [Preparation of positive electrode plate] A positive electrode slurry was prepared by dispersing 93.7 mass% of lithium cobalt oxide (LiCoO2) as the positive electrode active material, 2 mass% of polyvinylidene fluoride (PVDF) as the binder, 4 mass% of carbon black as the conductive agent, and 0.3 mass% of polyvinylpyrrolidone (PVP) as the dispersant in N-methyl-2-pyrrolidone (NMP) as the solvent.

[0029] Next, the positive electrode slurry was applied to both sides of an aluminum foil (thickness: 12 μm) serving as a positive electrode current collector plate 311 in an amount of 31.6 g / m 2 The mixture was coated with a coater so that the thickness became 1.58 g / cc, and then dried at 80 to 130° C. Thereafter, the mixture was pressed until the electrode density became 3.58 g / cc. In this way, a positive electrode plate 31 was produced in which positive electrode active material layers 312 were formed on both sides of a positive electrode current collector plate 311.

[0030] In addition, a positive electrode plate 31 in which a positive electrode active material layer 312 was formed on one side of the positive electrode current collector 311 was produced in the same manner, except that the positive electrode slurry was applied to one side of the aluminum foil (thickness 12 μm) that served as the positive electrode current collector 311.

[0031] The positive electrode current collector plate 311 used had a rectangular planar shape and was provided with a positive electrode lead 313 protruding by a predetermined width from the center of one side of the rectangular planar shape.

[0032] [Preparation of negative electrode plate] Anode active materials (94.92 mass % graphite and 2.68 mass % silicon oxide) and binders (1 mass % styrene butadiene rubber (SBR) and 1.4 mass % carboxymethyl cellulose (CMC)) were dispersed in ion-exchanged water as a solvent to prepare anode slurry.

[0033] Next, the negative electrode slurry was applied to both sides of a copper foil (thickness 6 μm) serving as the negative electrode current collector plate 321 in an amount of 16.0 g / m 2 The mixture was coated with a coater so that the thickness became 1.52 g / cc, and then dried at 80 to 110° C. Thereafter, the mixture was pressed until the electrode density became 1.52 g / cc. In this way, a negative electrode plate 32 was produced in which a negative electrode active material layer 322 was formed on both sides of a negative electrode current collector plate 321.

[0034] The negative electrode current collector plate 321 used had a rectangular planar shape and was provided with a negative electrode lead 323 protruding by a predetermined width from the center of one side of the rectangular planar shape.

[0035] [Separator] Separator 33 was a microporous film made of polyethylene (PE) resin, and other components shown in Table 1 were used. Impact energy absorption capacity is the area enclosed by the horizontal axis and the stress-strain curve from the origin to the breaking point on the stress-strain curve obtained by conducting a tensile test using a Type 2 test specimen at a tensile speed of 100 mm / min in accordance with JIS K 7127 Plastics - Test methods for tensile properties - Part 3: Test conditions for films and sheets.

[0036] The tensile tester used was the "FGS-100VC" manufactured by Nidec-Shimpo Corporation, and the force gauge was the "FGP-20" manufactured by the same company. The tensile direction was 180°. The area value is calculated using the quadrature method of pieces from the stress and strain data obtained by performing a tensile test. n-1 +Stress n ) × (strain n -Strain n-1 ) / 2} from n=1 to the breaking point n.

[0037] [Preparation of electrode groups] The positive electrode plate 31, the negative electrode plate 32, and the separator 33 were cut into a rectangle of a predetermined area. Next, as shown in Fig. 2, n positive electrode plates and (n+1) negative electrode plates were alternately stacked with separators 33 interposed therebetween to obtain an electrode group 3. Here, n=19. The thickness of the electrode group 3 was 6 mm.

[0038] 2 and 3, the positive electrode leads 313 and the negative electrode leads 323 were bundled at their tip ends and joined together by ultrasonic welding. An aluminum tab was joined to the joint of the positive electrode lead 313 by ultrasonic welding as the positive electrode terminal 4. A copper tab was joined to the joint of the negative electrode lead 323 by ultrasonic welding as the negative electrode terminal 5.

[0039] Next, as shown in Figure 3, tape 7 was attached to two locations on each of the two side surfaces, from the surface of the electrode group 3 (the top surface when the stacking direction is vertical) through the side surfaces to the back surface (the bottom surface when the stacking direction is vertical).

[0040] [Exterior body] The laminate films 21 and 22 of the exterior package 2 were prepared with the following layers: The heat-sealable resin layers 21a and 22a were polyolefin resin films with a thickness of 80 μm. The metal layers 21b and 22b were aluminum foils with a thickness of 40 μm. The protective layers 21c and 22c were polyamide films with a thickness of 25 μm.

[0041] [Nonaqueous electrolyte] The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) as an electrolyte at a concentration of 1.3 mol / L in a solvent made by mixing ethylene carbonate (EC), diethyl carbonate (DEC), ethyl propionate (EP), and propyl propionate (PP) in a volume ratio of 30:10:45:15 (=EC:DEC:EP:PP).

[0042] In addition, as additives, 2.25 mass% of vinylene carbonate (VC), 1.0 mass% of allylsuccinic anhydride (ASAh), 1.0 mass% of 1,3-dioxane (DO), 1.25 mass% of adiponitrile (ADN), 1.25 mass% of suberonitrile (SBN), and 0.15 mass% of lithium tetrafluoroborate (LiBF) were added relative to the total mass of the electrolyte (EC+DEC+EP+PP+LiPF).

[0043] [Prototype battery assembly] The electrode group 3 to which the positive electrode terminal 4 and the negative electrode terminal 5 are welded is placed in the accommodation recess 221 of the other laminate film 22, and one laminate film 21 is placed over it, and the peripheral edge of one laminate film 21 is overlapped with the flange 222 of the other laminate film 22. As a result, the tip of the sealant portion 6 protrudes from the laminate films 21 and 22, and the positive electrode terminal 4 and the negative electrode terminal 5 extend from the tip of the sealant portion 6.

[0044] Next, the overlapping flange 222 of the laminate films 21, 22 was heat-sealed to seal the three sides, including the two sides from which the positive electrode terminal 4 and the negative electrode terminal 5 extended. Next, 2.0 g / Ah of nonaqueous electrolyte was injected through the portion corresponding to the unsealed side. The portion of the flange 222 corresponding to this side was then heat-sealed under a reduced pressure of 1 hPa to 100 hPa to obtain a prototype battery after the injection of the nonaqueous electrolyte. The theoretical capacity of this prototype battery was 6.4 Ah. After the non-aqueous electrolyte was poured into the prototype battery, the battery was left standing for 12 hours to allow the non-aqueous electrolyte to permeate the constituent members of the electrode group.

[0045] Next, the positive and negative terminals of the prototype battery were connected to a power source, and the prototype battery was initially charged to 10% of its rated capacity at a current of 0.25 ItA. After that, the prototype battery was placed in a decompression chamber with part of the sealing part of the exterior body open, to vent the gas generated inside the prototype battery.

[0046] The exterior was then sealed again, and the battery was charged a second time at a current of 0.5 ItA to 10-100% of its rated capacity. After leaving the battery in a high-temperature environment for 3 hours, it was charged at a constant current of 0.5 ItA to 4.40 V, and then charged at a constant voltage of 0.02 ItA.

[0047] Thereafter, the rated capacity was measured at a current value of 0.2 ItA, and the battery was charged to a state of charge (SOC) of 10% of the rated capacity to obtain a prototype battery. The charging voltage of the prototype batteries No. 1 to No. 6 is 4.4 V, and the energy density is 241 Wh / kg (over 240 Wh / kg).

[0048] [Crash test] The resulting prototype batteries No. 1 to No. 6 were subjected to a crash test in accordance with UL1642.

[0049] Although the upper limit charging voltage of prototype batteries No. 1 to No. 6 was 4.4 V, the upper limit voltage was increased to account for errors in voltage accuracy when packed, and then the crash test was conducted. In other words, before the crash test, the batteries were charged at a constant current of 1.0 ItA up to 4.45 V, and then at a constant voltage down to 0.1 ItA.

[0050] Specifically, the impact test involved placing a test battery on a flat surface, placing a 15.8mm±0.1mm diameter rod at the midpoint of the long side of the rectangular flat surface of the test battery, with its length perpendicular to the long side, and dropping a 9.1kg weight onto the rod from a height of 610mm±25mm to check for ignition. If ignition occurred, the test was marked "X" as a failure, and if only smoke was emitted but no ignition occurred, the test was marked "O" as a pass. The results are shown in Table 1 along with the configuration.

[0051] [Table 1]

[0052] As can be seen from the results in Table 1, the separator consists of a substrate and a heat-resistant coating layer formed on the substrate. MD and C TD is 36.0 or more and 55.0 or less, and the separator ratio (C MD / C TD The prototype batteries No. 1, No. 4, and No. 5, which used separators that satisfied both the requirement of 0.80 to 1.25, passed the impact test. In contrast, the prototype batteries No. 1, No. 4, and No. 5, which were composed of a substrate and a heat-resistant coating layer formed on the substrate, MD and C TD is 36.0 or more and 55.0 or less, and the separator ratio (C MD / C TD The prototype batteries No. 2, No. 3, and No. 6, which used separators that did not satisfy either of the conditions of 0.80 or more and 1.25 or less, failed the crash test. [Explanation of symbols]

[0053] 1. Stacked lithium-ion secondary battery 2. Exterior body 3 electrode groups 31 Positive electrode plate 32 negative electrode plate 311 Positive current collector plate 321 Negative current collector plate 312 Cathode active material layer 322 Negative electrode active material layer 33 Separator 313 Positive lead 323 Negative lead 4 Positive terminal 5 Negative terminal 6 Sealant section 7. Tape

Claims

1. an electrode group in which a plurality of positive electrode plates and negative electrode plates are alternately stacked with separators interposed therebetween; an exterior body that houses the electrode group in a sealed state; a positive electrode terminal and a negative electrode terminal that are continuous with the positive electrode plate and the negative electrode plate, respectively, and extend outside the exterior body; Including, the separator includes a substrate and a heat-resistant coating layer formed on the substrate, The separator has an impact energy absorption capacity C of 36.0 or more and 55.0 or less in MD and TD, The value C of the impact energy absorption capacity C in the MD MD The value C in TD TD Ratio to (C MD / C TD ) is 0.80 or more and 1.25 or less, The impact energy absorption capacity is the area value enclosed by the stress-strain curve from the origin to the breaking point and the horizontal axis in a stress-strain curve obtained by conducting a tensile test in accordance with JIS K 7127 Plastics - Test methods for tensile properties - Part 3: Test conditions for films and sheets, using a type 2 test specimen, at a tensile speed of 100 mm / min.

2. The thickness of the heat-resistant coating layer per side is 1 μm or more and 4 μm or less, and the weight per side of the heat-resistant coating layer is 0.25 mg / cm 2 0.95mg / cm or more 2 2. The stacked lithium ion secondary battery according to claim 1, wherein:

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