PTC film, secondary battery electrode provided with same, and secondary battery

Abstract

The present disclosure provides a PTC film with faster response. Disclosed is a PTC film containing a conductive material, a binder, polyethylene particles, and an insulating inorganic material, wherein a volume percentage of the conductive material relative to a total volume of solids included in the PTC film is 15 vol% or less, a volume ratio of the polyethylene particles to the volume of the conductive material is 4 or more, and a volume ratio of the insulating inorganic material to the volume of the conductive material is 0.5 or more.

Description

PTC film, secondary battery electrode provided with the same, and secondary battery

[0001] The present invention relates to a PTC film, a secondary battery electrode provided with the same, and a secondary battery.

[0002] For the purpose of improving the safety of secondary batteries, a technique of providing a film (PTC film) having a function of increasing electron resistance as the temperature rises (Positive Temperature Coefficient function) on an electrode is known. For example, Japanese Patent Application Laid-Open No. 2018-14286 discloses a technique of providing a PTC film containing a conductive material, an insulating inorganic substance, and a polymer between an electrode active material layer (negative electrode active material layer or positive electrode active material layer) and a current collector (negative electrode current collector or positive electrode current collector) in an all-solid-state battery, and setting the content of the insulating inorganic substance in the PTC film to 50% by volume or more. According to this document, it is said that the decrease in electron resistance due to the influence of the restraint pressure can be suppressed by the all-solid-state battery having the above configuration.

[0003] However, from the viewpoint of further improving the safety of secondary batteries, a PTC film having faster responsiveness (higher resistance increase rate) has been demanded.

[0004] Therefore, an object of the present invention is to provide a PTC film having faster responsiveness.

[0005] The inventor of the present invention conducted intensive studies in view of the above problems. In the process, it was found that the above problems can be solved by controlling the mixing ratios of the conductive material, PE particles, and insulating inorganic substance within a predetermined range in a PTC film containing a conductive material, a binder, polyethylene (PE) particles, and an insulating inorganic substance, and the present invention was completed.

[0006] That is, one aspect of the present invention relates to a PTC film containing a conductive material, a binder, polyethylene particles, and an insulating inorganic substance. In the PTC film, the percentage of the volume of the conductive material with respect to the volume of all solid components contained in the PTC film is 15% by volume or less, the ratio of the volume of the polyethylene particles to the volume of the conductive material is 4 or more, and the ratio of the volume of the insulating inorganic substance to the volume of the conductive material is 0.5 or more.

[0007] This is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (stacked secondary battery), which is one embodiment of the present invention. This is a perspective view of a stacked secondary battery according to one embodiment of the present invention. This is a side view taken from direction A shown in Figure 2. This is a graph showing the results of a heating resistance measurement test.

[0008] The embodiments of the present invention will be described below, but the technical scope of the present invention should be determined based on the claims and is not limited to the following forms. Note that "X to Y" indicating a range means "X or more and Y or less". Furthermore, unless otherwise specified, operations and measurements of physical properties, etc., are performed under conditions of room temperature (25°C) and relative humidity of 40-50% RH.

[0009] <PTC film> One embodiment of the present invention is a PTC film comprising a conductive material, a binder, polyethylene particles, and an insulating inorganic substance, wherein the percentage of the volume of the conductive material relative to the volume of the total solids contained in the PTC film is 15% by volume or less, the ratio of the volume of the polyethylene particles to the volume of the conductive material is 4 or more, and the ratio of the volume of the insulating inorganic substance to the volume of the conductive material is 0.5 or more. The PTC film according to this embodiment has faster response.

[0010] The inventors speculate that the mechanism by which these effects are achieved is as follows: In the PTC film according to this embodiment, the presence of polyethylene (PE) particles causes the PE particles to melt as the temperature rises. The melted PE covers the surface of the conductive material, blocking the conductive paths and increasing the electronic resistance. By setting the ratio of the conductive material to the solid content in the PTC film (percentage by volume) to 15% by volume or less, the ratio of PE particles to the conductive material (volume ratio) to 4 or more, and the ratio of insulating inorganic material to the conductive material (volume ratio) to 0.5 or more, more conductive paths are blocked, and the rate of resistance increase is improved. As a result, it is believed that a PTC film with faster response can be obtained.

[0011] From the viewpoint of further improving the responsiveness of the PTC film, it is preferable that the percentage of the volume of the conductive material relative to the volume of the total solids contained in the PTC film is 5% by volume or more; the ratio of the volume of polyethylene particles to the volume of the conductive material is 15 or less; and the ratio of the volume of insulating inorganic material to the volume of the conductive material is 2 or less.

[0012] The following describes each component that constitutes the PTC film in this embodiment.

[0013] [Conductive Material] The conductive material has the function of imparting conductivity to the PTC film. The conductivity of the conductive material is not particularly limited as long as it imparts sufficient conductivity to the PTC film, but it is preferably 1 S / m or more, and 1 × 10 ２ It is more preferable that the ratio be S / m or higher, and 1 × 10 ４ It is even more preferable that the ratio be S / m or higher, and 1 × 10 ５ It is particularly preferable that the conductivity is S / m or higher. There is no particular upper limit to the conductivity of the conductive material, but it is usually 1 × 10⁻⁶. ７ It is less than or equal to S / m.

[0014] The materials constituting the conductive material are not particularly limited, but carbon materials are preferred. Examples of conductive materials made of carbon include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.), graphite, hard carbon, carbon nanotubes, carbon nanohorns, carbon nanofibers, carbon nanofilaments, carbon fibrils, and vapor-grown carbon fibers. These conductive materials may be used individually or in combination of two or more. From the viewpoint of further accelerating the response of the PTC film, the conductive material is more preferably composed of at least one selected from the group consisting of carbon black, and preferably contains acetylene black.

[0015] When the conductive material contains carbon black (preferably acetylene black), the average primary particle diameter of the carbon black is not particularly limited, but is preferably 10 to 200 nm, more preferably 15 to 150 nm, and even more preferably 20 to 100 nm. When the average primary particle diameter of the carbon black is within the above range, the responsiveness of the PTC film can be made faster. In this specification, the average particle diameter (average primary particle diameter) of the particles refers to the arithmetic mean of the particle diameters of the particles observed in several to tens of fields of view when a cross-section of the layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the contour line of the observed particles).

[0016] The conductive material content in the PTC film is such that the percentage of the volume of the conductive material relative to the total volume of solids contained in the PTC film is 15 volume% or less. If this percentage exceeds 15 volume%, the effect of improving the responsiveness of the PTC film may not be obtained. From the viewpoint of making the responsiveness of the PTC film faster, the above percentage is preferably 14 volume% or less, more preferably 13 volume% or less, even more preferably 12 volume% or less, and particularly preferably 11 volume% or less. The lower limit of the above percentage is not particularly limited, but from the viewpoint of reducing the resistance during normal operation of the battery, it is preferably 5 volume% or more, more preferably 8 volume% or more, even more preferably 9 volume% or more, and particularly preferably 10 volume% or more. The preferred numerical ranges for the above percentages are 5 to 15 volume%, 8 to 14 volume%, 9 to 13 volume%, 10 to 12 volume%, and 10 to 11 volume%.

[0017] [Binder] The binder has the function of binding the various components in the PTC film together to maintain the strength of the film. The type of binder is not particularly limited, and any known in the art can be used as appropriate. Examples include styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), and carboxymethylcellulose (CMC). These binders may be used individually or in combination of two or more. From the viewpoint of maintaining film strength even after the PE particles have melted due to rising temperature and further increasing electronic resistance, the binder preferably contains at least one selected from the group consisting of styrene-butadiene rubber, polytetrafluoroethylene, and polyvinylidene fluoride, and more preferably contains polyvinylidene fluoride.

[0018] The binder content in the PTC film is not particularly limited, but the percentage of binder volume relative to the total solid content volume in the PTC film is preferably 1 to 5 volume%, and more preferably 2 to 4 volume%. When the percentage is within this range, the film strength can be sufficiently maintained, and contact between the conductive material and PE is good, resulting in more conductive paths being blocked when the temperature rises.

[0019] [Polyethylene particles] Polyethylene (PE) particles melt as the temperature of the secondary battery rises, and by covering the surface of the conductive material, they block the conductive path and have the function of increasing the electronic resistance.

[0020] The PE constituting the PE particles is not particularly limited and may be low-density polyethylene (LDPE), high-density polyethylene (HDPE), or linear low-density polyethylene (LLDPE). The particle size of the PE particles is also not particularly limited, but the median diameter (D50) in the volume-based particle size distribution is preferably 1 to 10 μm, and more preferably 2 to 7 μm. In this specification, the median diameter (D50) of the particles is measured by laser diffraction scattering.

[0021] The PE particle content in the PTC film is not particularly limited, but the percentage of the volume of PE particles relative to the total volume of solids contained in the PTC film is preferably 50 to 90 vol%, more preferably 60 to 85 vol%, and even more preferably 70 to 80 vol%. When the percentage is within this range, the responsiveness of the PTC film can be made faster.

[0022] In the PTC film according to this embodiment, the ratio of the volume of PE particles to the volume of conductive material must be 4 or more. If the ratio is less than 4, the effect of improving the responsiveness of the PTC film may not be obtained. From the viewpoint of making the responsiveness of the PTC film faster, the above ratio is preferably 5 or more, more preferably 6 or more, and even more preferably 7 or more. There is no particular upper limit to the above ratio, but from the viewpoint of reducing the resistance during normal operation of the battery, it is preferably 15 or less, more preferably 14 or less, even more preferably 13 or less, and particularly preferably 12 or less. The preferred numerical ranges for the above ratio are 4 to 15, 5 to 14, 6 to 13, and 7 to 12.

[0023] The ratio of the volume of polyethylene particles to the volume of binder in a PTC film is not particularly limited, but is preferably greater than 20, more preferably greater than 25, and even more preferably greater than 27. When the above ratio is within this range, contact between the conductive material and PE is good, and more conductive paths are blocked when the temperature rises. The upper limit of the above ratio is not particularly limited, but from the viewpoint of improving film strength, it is preferably less than 40, more preferably less than 35, and even more preferably 33 or less. The preferred numerical range for the above ratio is greater than 20 and less than 40, greater than 25 and less than 35, and greater than 27 and 33 or less.

[0024] [Insulating Inorganic Material] Insulating inorganic material has the function of preventing the PTC film from deforming due to the melting of PE particles as the temperature rises and the resulting flow of PE. In secondary batteries containing a solid electrolyte, a constraining pressure is applied in the stacking direction when the battery is operating. By incorporating insulating inorganic material into the PTC film, it is possible to suppress the deformation of the PTC film when the temperature rises, even when constraining pressure is applied, and to achieve the desired PTC function.

[0025] The type of insulating inorganic material is not particularly limited, but metal oxides and metal nitrides are preferred. Examples of metal oxides include alumina, zirconia, magnesia, silica, titania, barium titanate, and strontium titanate. Examples of metal nitrides include silicon nitride and boron nitride. In particular, the insulating inorganic material preferably contains at least one selected from the group consisting of metal oxides, more preferably contains at least one selected from alumina, zirconia, and silica, and even more preferably contains alumina.

[0026] The particle size of the insulating inorganic material is not particularly limited, but the median diameter (D50) in the volume-based particle size distribution is preferably 0.05 to 10 μm, more preferably 0.1 to 9 μm, and even more preferably 2 to 8 μm. When the particle size is within this range, the responsiveness of the PTC film can be accelerated.

[0027] The content of insulating inorganic matter in the PTC film is not particularly limited, but it is preferable that the percentage of the volume of insulating inorganic matter relative to the volume of total solid matter in the PTC film is less than 50 vol%, more preferably 30 vol% or less, even more preferably 15 vol% or less, and particularly preferably 13 vol% or less. When the above percentage is within this range, the resistance during normal operation of the battery can be reduced. The lower limit of the above percentage is not particularly limited, but from the viewpoint of making the response of the PTC film faster, it is preferable that it is 2 vol% or more, more preferably 3 vol% or more, even more preferably 5 vol% or more, and particularly preferably 7 vol% or more. The preferred numerical ranges for the above percentage are 2 vol% or more and less than 50 vol%, 3 to 30 vol%, 5 to 15 vol%, and 7 to 13 vol%.

[0028] In the PTC film according to this embodiment, the ratio of the volume of insulating inorganic material to the volume of conductive material is 0.5 or more. If this ratio is less than 0.5, the effect of improving the responsiveness of the PTC film may not be obtained. From the viewpoint of making the responsiveness of the PTC film faster, the above ratio is preferably 0.6 or more, more preferably 0.7 or more, and even more preferably 0.8 or more. There is no particular upper limit to the above ratio, but from the viewpoint of reducing the resistance during normal operation of the battery, it is preferably 2 or less, more preferably 1.7 or less, even more preferably 1.6 or less, and particularly preferably 1.5 or less. The preferred numerical ranges for the above ratio are 0.5 to 2, 0.6 to 1.7, 0.7 to 1.6, and 0.8 to 1.5.

[0029] The components constituting the PTC film according to this embodiment have been described above, but the PTC film may also contain components other than the conductive material, binder, PE particles, and insulating inorganic material mentioned above (hereinafter referred to as "other components"). However, from the viewpoint of accelerating the responsiveness of the PTC film, the percentage of the volume of other components relative to the volume of the total solids contained in the PTC film is preferably less than 5 volume%, more preferably 3 volume% or less, even more preferably 1 volume% or less, and particularly preferably 0 volume% (no other components).

[0030] The thickness of the PTC film is not particularly limited, but is preferably 1 to 20 μm, more preferably 1 to 15 μm, and even more preferably 1 to 13 μm. A thickness within this range allows for faster response of the PTC film.

[0031] The ratio of the median diameter (D50) in the volume-based particle size distribution of the insulating inorganic material to the thickness of the PTC film is not particularly limited, but is preferably 0.5 or more and 1 or less, and more preferably 0.5 or more and 0.8 or less. When the above ratio is within this range, it becomes easier to maintain the shape of the PTC film, and the responsiveness of the PTC film when constraining pressure is applied can be improved.

[0032] The porosity of the PTC film is not particularly limited, but is preferably 20% or less, more preferably 5-20%, more preferably 10-20%, and even more preferably 12-18%. When the porosity is within this range, the responsiveness of the PTC film can be made faster. In this specification, the porosity of the PTC film is calculated from the bulk density determined from the volume and mass of the PTC film and the true density of the PTC film determined from the true density and mixing ratio of the solid components constituting the PTC film using the formula: Porosity (%) = {1 - (bulk density / true density)} × 100.

[0033] The method for manufacturing the PTC film according to this embodiment is not particularly limited, and conventionally known methods can be referenced as appropriate. For example, a slurry is prepared by dispersing a conductive material, a binder, PE particles, and an insulating inorganic material in a solvent such as N-methyl-2-pyrrolidone (NMP). The slurry is coated onto a support (e.g., a current collector), and the solvent is removed. Then, if necessary, the dried coating is pressed to adjust the porosity to a predetermined range, thereby obtaining a PTC film.

[0034] <Electrodes for Secondary Batteries and Secondary Batteries> The PTC film can function as a safety device for secondary batteries by being interposed between the electrode active material layer and the current collector in the electrode for secondary batteries. More specifically, during normal operation of the battery, the PTC film has the function of mediating the movement of electrons between the current collector and the electrode active material layer. On the other hand, when the temperature rises due to abnormal heat generation of the battery, the PTC film becomes a high-resistivity material and has the function of suppressing the movement of electrons between the current collector and the electrode active material layer. As described above, the PTC film according to the present invention has faster response (higher resistance increase rate), and by incorporating it into the electrode, a secondary battery with superior safety can be provided. That is, according to another embodiment of the present invention, an electrode for secondary batteries is provided in which a current collector, the above-mentioned PTC film, and an electrode active material layer containing electrode active material are stacked in this order. Furthermore, according to yet another embodiment of the present invention, a secondary battery is provided that comprises a power generation element having a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein at least one of the positive electrode and the negative electrode is the electrode for the secondary battery described above.

[0035] Hereinafter, an embodiment of a secondary battery according to one aspect of the present invention will be described with reference to the attached drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted. Also, the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios.

[0036] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "stacked secondary battery"), which is one embodiment of the present invention. The stacked secondary battery 10a shown in Figure 1 has a structure in which a substantially rectangular power generation element 21, in which the charge and discharge reaction actually proceeds, is sealed inside a laminate film 29 which is the battery casing. Note that Figure 1 shows a cross-section of the stacked secondary battery during charging, and therefore a negative electrode active material layer 13 exists between the negative electrode current collector 11' and the solid electrolyte layer 17. In addition, a restraining pressure is applied to the stacked secondary battery 10a by a pressurizing member in the stacking direction of the power generation element 21 (not shown). Therefore, the volume of the power generation element 21 is kept constant.

[0037] As shown in Figure 1, the power generation element 21 of the stacked secondary battery 10a in this embodiment has a configuration in which a negative electrode has a negative electrode active material layer 13 arranged on both sides of a negative electrode current collector 11' via a PTC film (not shown), a solid electrolyte layer 17, and a positive electrode has a positive electrode active material layer 15 arranged on both sides of a positive electrode current collector 11'' via a PTC film (not shown). Specifically, one negative electrode active material layer 13 and an adjacent positive electrode active material layer 15 face each other via a solid electrolyte layer 17, and the negative electrode, solid electrolyte layer, and positive electrode are stacked in this order. As a result, adjacent negative electrodes, solid electrolyte layers, and positive electrodes constitute one single cell layer 19. Therefore, the stacked secondary battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel.

[0038] The negative electrode current collector 11' and the positive electrode current collector 11'' are each fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, respectively, which are electrically connected to the respective electrodes (negative and positive electrodes), and are structured to be led out of the laminate film 29 by being sandwiched between the edges of the laminate film 29. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be attached to the negative electrode current collector 11' and the positive electrode current collector 11'' of each electrode via negative electrode leads and positive electrode leads (not shown) as needed, by ultrasonic welding, resistance welding, or the like.

[0039] In the above description, a stacked (internal parallel connection type) all-solid-state lithium secondary battery was used as an example to illustrate one embodiment of a secondary battery according to one aspect of the present invention. However, the type of secondary battery to which the present invention can be applied is not particularly limited, and it can also be applied to, for example, a bipolar lithium secondary battery.

[0040] Figure 2 is a perspective view of a stacked secondary battery according to one embodiment of the present invention. Figure 3 is a side view taken from direction A shown in Figure 2. As shown in Figures 2 and 3, the stacked secondary battery 100 according to this embodiment includes a stacked secondary battery 10a as shown in Figure 1, two metal plates 200 that sandwich the stacked secondary battery 10a, and a bolt 300 and a nut 400 as fastening members. These fastening members (bolt 300 and nut 400) have the function of fixing the metal plates 200 in a state where they are sandwiching the stacked secondary battery 10a. As a result, the metal plates 200 and the fastening members (bolt 300 and nut 400) function as pressurizing members that pressurize (restrain) the stacked secondary battery 10a in its stacking direction. The pressurizing members are not particularly limited as long as they are members that can pressurize the stacked secondary battery 10a in its stacking direction. Typically, a combination of a plate made of a rigid material such as the metal plate 200 and the fastening members described above is used as the pressurizing members. Furthermore, regarding the fastening members, not only bolts 300 and nuts 400 may be used, but also tension plates that fix the ends of the metal plate 200 so as to restrain the stacked secondary battery 10a in its stacking direction.

[0041] The lower limit of the load applied to the stacked secondary battery 10a (restraining pressure in the stacking direction of the stacked secondary battery) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the restraining pressure in the stacking direction of the stacked secondary battery is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.

[0042] The main components of the stacked secondary battery 10a described above will be explained below.

[0043] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the materials that make up the current collector. Examples of materials that can be used to make up the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are no particular restrictions on the thickness of the current collector, but one example is 10 to 100 μm.

[0044] [Electrode Active Material Layer (Negative Electrode Active Material Layer, Positive Electrode Active Material Layer)] The negative electrode active material layer contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. In addition, lithium-containing active materials such as lithium metal and lithium-containing alloys may be used as the negative electrode active material. Examples of lithium-containing alloys include alloys of Li and at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. When lithium metal or lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery is preferably of the so-called lithium deposition type, in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. The layer made of lithium metal deposited on the negative electrode current collector during this charging process becomes the negative electrode active material layer. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharge process progresses. While a negative electrode active material layer does not necessarily need to be present during complete discharge, in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during complete discharge.

[0045] The shape of the negative electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, its average particle diameter is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm.

[0046] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably 40 to 100% by mass, and more preferably 50 to 90% by mass.

[0047] In addition to the negative electrode active material, the negative electrode active material layer may further contain a solid electrolyte, a binder, and / or a conductive aid. The types of the solid electrolyte, the binder, and / or the conductive aid are not particularly limited, and those known in the art can be appropriately adopted. Examples of the solid electrolyte include those described in the section of the solid electrolyte layer below. Examples of the binder include those described in the section of the PTC film above. Examples of the conductive aid include the conductive materials described in the section of the PTC film above.

[0048] The thickness of the negative electrode active material layer (in the case of a lithium deposition type, the thickness at full charge) varies depending on the configuration of the target secondary battery, but is preferably in the range of 0.1 to 1000 μm, for example.

[0049] [Positive electrode active material layer] The positive electrode active material layer essentially contains a positive electrode active material. The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but lithium-containing metal oxides are preferred. Specific examples of the lithium-containing metal oxides include layered rock salt type active materials such as LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ etc., spinel type active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ etc., olivine type active materials such as LiFePO ４ , LiMnPO ４ etc., Si-containing active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ etc. In addition, examples of oxide active materials other than the above include, for example, Li ４ Ti ５ O １２ , LiVO ２ . Among them, Li(Ni-Mn-Co)O ２Furthermore, those in which some of these transition metals are substituted with other elements (NMC composite oxides) are preferably used as positive electrode active materials. These positive electrode active materials may be used individually or in combination of two or more types.

[0050] Another preferred embodiment involves the use of a sulfur-based positive electrode active material. Examples of sulfur-based positive electrode active materials include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material that can release lithium ions during charging and absorb lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur is acceptable.

[0051] The shape of the positive electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, its average particle diameter is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm.

[0052] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably 50 to 99% by mass, more preferably 70 to 99% by mass or less, and even more preferably 80 to 99% by mass or less.

[0053] The positive electrode active material layer may further contain a solid electrolyte, a binder, and / or a conductive additive in addition to the positive electrode active material. The type of solid electrolyte, binder, and / or conductive additive is not particularly limited, and those known in the art can be used as appropriate. One example is the same as that described in the section on the negative electrode active material layer.

[0054] The thickness of the positive electrode active material layer varies depending on the configuration of the secondary battery, but is, for example, 0.1 to 1000 μm, preferably 30 to 300 μm, more preferably 50 to 200 μm, and even more preferably 70 to 150 μm.

[0055] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as the main component). The solid electrolyte is preferably a sulfide solid electrolyte containing S element, more preferably containing Li element, M element and S element, wherein the M element is a sulfide solid electrolyte containing at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl and I, and even more preferably a sulfide solid electrolyte containing S element, Li element and P element.

[0056] Sulfide solid electrolytes are Li ３ PS ４ It may have a skeleton, Li ４ P ２ S ７ It may have a skeleton, Li ４ P ２ S ６ It may have a skeleton. ３ PS ４ Examples of sulfide solid electrolytes with a skeleton include LiI-Li ３ PS ４ , LiI-LiBr-Li ３ PS ４ Li ３ PS ４ This can be cited. Also, Li ４ P ２ S ７ Examples of sulfide solid electrolytes having a framework include Li-P-S system solid electrolytes called LPS. Also, as sulfide solid electrolytes, for example, Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ You may also use LGPS, etc., which is represented as (where x satisfies 0 < x < 1). More specifically, for example, LPS(Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ Li ３．２ P ０．９６ S, Li ３．２５ Ge０．２５ P ０．７５ S ４ Li １０ GeP ２ S １２ , or Li ６ PS ５ Examples include X (where X is Cl, Br, or I). ２ S-P ２ S ５ The description of " is Li ２ S and P ２ S ５ This refers to a sulfide solid electrolyte made using a raw material composition containing the above, and the same applies to other descriptions. In particular, the sulfide solid electrolyte is preferably LPS (Li) because it has high ionic conductivity and a low bulk modulus, and can therefore follow the volume change of the electrode active material accompanying charging and discharging. ２ S-P ２ S ５ ), Li ６ PS ５ X (where X is Cl, Br, or I), Li ７ P ３ S １１ Li ３．２ P ０．９６ S and Li ３ PS ４ The solid electrolytes are selected from the group consisting of the following. These sulfide solid electrolytes may be used individually or in combination of two or more. Of course, other solid electrolytes may also be used.

[0057] The solid electrolyte content in the solid electrolyte layer is preferably 50% by mass or more and 100% by mass or less, and more preferably 90% by mass or more and 99% by mass.

[0058] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. An example of such a binder is the one described in the section on the PTC film above.

[0059] The thickness of the solid electrolyte layer varies depending on the intended configuration of the secondary battery, but is usually 0.1 to 1000 μm, and preferably 10 to 40 μm.

[0060] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Preferred materials for the current collector plates are, for example, metallic materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof. From the viewpoint of lightness, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material, or different materials may be used.

[0061] [Positive and Negative Leads] Although not shown in the diagram, the current collectors (11'', 11'') and the current collector plates (27'', 25) may be electrically connected via positive and negative leads. The materials used for the positive and negative leads may be the same as those used in known secondary batteries. It is preferable to cover the parts that are removed from the casing with heat-resistant insulating heat shrink tubing or the like to prevent leakage current from coming into contact with peripheral equipment or wiring and affecting the product (for example, automotive parts, especially electronic equipment).

[0062] [Battery casing material] As the battery casing material, a known metal can case can be used, or, as shown in Figure 1, a bag-shaped case made of a laminate film 29 containing aluminum that can cover the power generation element can be used. For example, a three-layer laminate film made by laminating PP, aluminum, and nylon in that order can be used, but there is no limit to these. Laminate film is desirable from the viewpoint of being able to increase power output and have excellent cooling performance, and can be suitably used for batteries in large equipment for EVs and HEVs. Furthermore, a laminate film containing aluminum is more preferable as the casing material because it allows for easy adjustment of the group pressure applied to the power generation element from the outside.

[0063] The secondary battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the secondary battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0064] Although embodiments of the present invention have been described above, the present invention is not limited to the configurations described in the embodiments described above, and can be modified as appropriate based on the claims.

[0065] For example, the secondary battery according to this embodiment does not have to be all-solid type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte). There are no particular restrictions on the amount of liquid electrolyte (electrolyte) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolyte) does not occur.

[0066] Furthermore, the following items are also included in the scope of the present invention: Item 1: A PTC film comprising a conductive material, a binder, polyethylene particles, and an insulating inorganic material, wherein the percentage of the volume of the conductive material relative to the volume of the total solids contained in the PTC film is 15 volume% or less (preferably 5 to 15 volume%, more preferably 8 to 14 volume%, even more preferably 9 to 13 volume%, particularly preferably 10 to 12 volume%, most preferably 10 to 11 volume%), the ratio of the volume of the polyethylene particles to the volume of the conductive material is 4 or more (preferably 4 to 15, more preferably 5 to 14, even more preferably 6 to 13, particularly preferably 7 to 12), and the ratio of the volume of the insulating inorganic material to the volume of the conductive material is 0.5 or more (preferably 0.5 to 2, more preferably 0.6 to 1.7, even more preferably 0.7 to 1.6, particularly preferably 0.8 to 1.5); Item 2: The PTC film according to Item 1, wherein the porosity is 20% or less (preferably 5 to 20%, more preferably 10 to 20%, and even more preferably 12 to 18%); Item 3: The PTC film according to Item 1 or 2, wherein the ratio of the median diameter (D50) in the volume-based particle size distribution of the insulating inorganic material to the thickness of the PTC film is 0.5 or more and 1 or less (preferably 0.5 or more and 0.8 or less); Item 4: The PTC film according to any one of Items 1 to 3, wherein the ratio of the volume of the polyethylene particles to the volume of the binder is greater than 20 (preferably greater than 25); Item 5: The PTC film according to any one of Items 1 to 4, wherein the conductive material comprises at least one selected from the group consisting of carbon black (preferably acetylene black); Item 6: The PTC film according to any one of Items 1 to 5, wherein the binder comprises polyvinylidene fluoride; Item 7: A PTC film according to any one of items 1 to 6, wherein the insulating inorganic material comprises at least one selected from the group consisting of metal oxides (preferably at least one selected from alumina, zirconia, and silica, more preferably alumina); Item 8: A PTC film according to any one of items 1 to 6, wherein the percentage of the volume of the insulating inorganic material relative to the volume of the total solids contained in the PTC film is less than 50 volume percent (preferably 2 volume percent or more and less than 50 volume percent, more preferably 3 to 30 volume percent, even more preferably 5 to 15 volume percent, and particularly preferably 7 to 13 volume percent);Item 9: An electrode for a secondary battery, comprising a current collector, a PTC film according to any one of items 1 to 8, and an electrode active material layer containing an electrode active material, stacked in this order; Item 10: A secondary battery comprising a power generation element having a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein at least one of the positive electrode and the negative electrode is the electrode for a secondary battery according to item 9;

[0067] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples.

[0068] <Examples of PTC film fabrication> [Comparative Example 1-1] 42.7 vol% acetylene black (AB; average primary particle size 35 nm) as a conductive material, 18.8 vol% polyvinylidene fluoride (PVDF) as a binder, 34.3 vol% polyethylene particles (PE; particle size (D50) 5 μm), and alumina (Al) as an insulating inorganic material. ２ O ３ A slurry was prepared by dispersing 4.2% by volume of a particle with a particle size (D50) of 6.81 μm in N-methyl-2-pyrrolidone (NMP) to a solid content concentration of 23% by mass. The slurry was coated onto aluminum foil using an applicator, and the coating on the aluminum foil was dried on a hot plate at 100°C. The dried coating was pressed using a roll press at room temperature (25°C) with a linear pressure of 0.1 kN / cm to obtain the PTC film of this comparative example (thickness 7 μm, porosity 34.8%) formed on the aluminum foil.

[0069] [Comparative Example 1-2] The PTC film of this comparative example (thickness 5 μm, porosity 8.7%) was obtained using the same method as in Comparative Example 1-1, except that the linear pressure when pressing the dried coating film was set to 0.2 kN / cm.

[0070] [Comparative Example 2-1] The mixing ratio of solids contained in the slurry was AB:PVDF:PE:Al ２ O ３ Except for the volume ratio being 23.1:5.1:65.0:6.8, the PTC film of this comparative example (thickness 10 μm, porosity 40.3%) was obtained by the same method as in Comparative Example 1-1 above.

[0071] [Comparative Example 2-2] A PTC film (thickness: 8 μm, porosity: 25.4%) of this comparative example was obtained by the same method as in Comparative Example 2-1, except that the linear pressure during pressing the dried coating film was 0.2 kN / cm.

[0072] [Example 1-1] A PTC film (thickness: 11 μm, porosity: 32.6%) of this example was obtained by the same method as in Comparative Example 1-1, except that the mixing ratio of the solid components contained in the slurry was AB:PVDF:PE:Al ２ O ３ = 13.7:3.5:72.9:9.9 (volume ratio).

[0073] [Example 1-2] A PTC film (thickness: 9 μm, porosity: 17.6%) of this example was obtained by the same method as in Example 1-1, except that the linear pressure during pressing the dried coating film was 0.2 kN / cm.

[0074] [Example 2-1] A PTC film (thickness: 1 μm, porosity: 30.7%) of this example was obtained by the same method as in Comparative Example 1-1, except that the mixing ratio of the solid components contained in the slurry was AB:PVDF:PE:Al ２ O ３ = 10.3:2.6:76.5:10.5 (volume ratio).

[0075] [Example 2-2] A PTC film (thickness: 10 μm, porosity: 16.9%) of this example was obtained by the same method as in Example 2-1, except that the linear pressure during pressing the dried coating film was 0.2 kN / cm.

[0076] [Example 3-1] A PTC film (thickness: 13 μm, porosity: 27.3%) of this example was obtained by the same method as in Comparative Example 1-1, except that the mixing ratio of the solid components contained in the slurry was AB:PVDF:PE:Al ２ O ３ = 7.2:2.4:79.2:11.3 (volume ratio).

[0077] [Example 3-2] A PTC film (thickness: 11 μm, porosity: 14.1%) of this example was obtained by the same method as in Example 3-1, except that the linear pressure during pressing the dried coating film was 0.2 kN / cm.

[0078] The specifications of the PTC film in each example and comparative example are shown in Table 1 below.

[0079]

[0080] <Heating Resistance Measurement Test> The laminate of PTC film and aluminum foil prepared above was cut, and copper foil was placed on the exposed surface of the PTC film as a counter electrode. Tabs were attached to both the aluminum foil and the copper foil, and a cell was created by sealing it with a laminate film. The cell was sandwiched between plate heaters, and a load was applied to the cell using a jig so that the surface pressure on the cell was 3 MPa. Then, while measuring the AC resistance from 10 kHz to 100 kHz, the heater temperature was increased at 5°C / min, and the change in resistance with increasing temperature was observed. The results are shown in Figure 4.

[0081] As shown in Figure 4, in a PTC film containing a conductive material, a binder, PE particles, and an insulating inorganic material, it can be seen that the resistance increase rate is improved when the mixing ratio of the conductive material, PE particles, and insulating inorganic material is within a predetermined range. Therefore, it can be said that the PTC film according to the present invention has a faster response.

[0082] Furthermore, comparing Examples 1-1, 2-1, and 3-1 with Examples 1-2, 2-2, and 3-2, it can be seen that lowering the porosity of the PTC film further accelerates the response.

[0083] 10a, 100 stacked secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 20 insulating layer, 21 power generation element, 25 negative electrode current collector plate (negative electrode tab), 27 positive electrode current collector plate (positive electrode tab), 29 laminate film, 200 metal plate, 300 volt, 400 nut.

Claims

1. A PTC film comprising a conductive material, a binder, polyethylene particles, and an insulating inorganic substance, wherein the percentage of the volume of the conductive material relative to the volume of the total solids contained in the PTC film is 15% by volume or less, the ratio of the volume of the polyethylene particles to the volume of the conductive material is 4 or more, and the ratio of the volume of the insulating inorganic substance to the volume of the conductive material is 0.5 or more.

2. The PTC film according to claim 1, wherein the percentage of the volume of the conductive material relative to the volume of the total solids contained in the PTC film is 5% by volume or more, the ratio of the volume of the polyethylene particles to the volume of the conductive material is 15 or less, and the ratio of the volume of the insulating inorganic material to the volume of the conductive material is 2 or less.

3. The PTC film according to claim 1 or 2, wherein the porosity is 20% or less.

4. The PTC film according to claim 1 or 2, wherein the ratio of the median diameter (D50) in the volume-based particle size distribution of the insulating inorganic material to the thickness of the PTC film is 0.5 or more and 1 or less.

5. The PTC film according to claim 1 or 2, wherein the ratio of the volume of polyethylene particles to the volume of the binder is greater than 20.

6. The PTC film according to claim 5, wherein the ratio of the volume of polyethylene particles to the volume of the binder is greater than 25.

7. The PTC film according to claim 1 or 2, wherein the conductive material comprises at least one selected from the group consisting of carbon black.

8. The PTC film according to claim 1 or 2, wherein the binder comprises polyvinylidene fluoride.

9. The PTC film according to claim 1 or 2, wherein the insulating inorganic material comprises at least one selected from the group consisting of metal oxides.

10. The PTC film according to claim 1 or 2, wherein the percentage of the volume of the insulating inorganic material relative to the volume of the total solids contained in the PTC film is less than 50% by volume.

11. An electrode for a secondary battery, comprising a current collector, a PTC film according to claim 1 or 2, and an electrode active material layer containing an electrode active material, stacked in this order.

12. A secondary battery comprising a power generation element having a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein at least one of the positive electrode and the negative electrode is an electrode for a secondary battery according to claim 11.

Images