Solid polymer electrolytes, energy storage elements, and energy storage devices

Abstract

A solid polyelectrolyte according to one aspect of the present invention is provided with: a lithium salt; a polymer material that is mixed with the lithium salt and that has lithium-ion conductivity; and a porous base material to which the lithium salt and the polymer material adhere and that has a polysaccharide, wherein the content of the polysaccharide is 10-40 mass%.

Description

[Technical Field]

[0001] The present invention relates to a solid polymer electrolyte, an energy storage element, and an energy storage device. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles, due to their high energy density. Generally, these non-aqueous electrolyte secondary batteries consist of a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between these electrodes. They are configured to charge and discharge by transferring lithium ions between the two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double-layer capacitors are also widely used as energy storage elements.

[0003] In recent years, solid non-aqueous electrolytes have been proposed because they do not pose a risk of leakage. For example, one such non-aqueous electrolyte is a solid electrolyte containing a polymer, cellulose nanofibers, and a metal salt (see Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2018 / 008500 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, the solid electrolyte described in Patent Document 1 above does not have sufficiently high lithium ion conductivity, and therefore, the lithium ion conductivity of the energy storage element equipped with it cannot be said to be sufficiently high.

[0006] The object of the present invention is to provide a solid polymer electrolyte with high lithium-ion conductivity, an energy storage element and an energy storage device equipped therewith.

Means for Solving the Problem

[0007] The solid polymer electrolyte according to one aspect of the present invention includes a lithium salt, a polymer material in which the lithium salt is mixed and has lithium ion conductivity, and a porous substrate to which the lithium salt and the polymer material adhere and has a polysaccharide, and the content of the polysaccharide is 10% by mass or more and 40% by mass or less.

[0008] The power storage element according to another aspect of the present invention includes a positive electrode, a negative electrode, and the solid polymer electrolyte interposed between the positive electrode and the negative electrode.

[0009] The power storage device according to another aspect of the present invention includes two or more power storage elements and one or more power storage elements according to another aspect of the present invention.

Advantages of the Invention

[0010] The solid polymer electrolyte according to one aspect of the present invention has high lithium ion conductivity.

[0011] The power storage element and the power storage device according to another aspect of the present invention have high lithium ion conductivity.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a perspective view showing one embodiment of a power storage element. [Figure 2] FIG. 2 is a schematic view showing one embodiment of a power storage device formed by assembling a plurality of power storage elements.

Embodiments for Carrying Out the Invention

[0013] First, an overview of the solid polymer electrolyte, the power storage element, and the power storage device disclosed in this specification will be described.

[0014] The solid polymer electrolyte according to one aspect of the present invention includes a lithium salt, a polymer material in which the lithium salt is miscible and has lithium ion conductivity, and a porous substrate to which the lithium salt and the polymer material adhere and has polysaccharides, and the content of the polysaccharides is 20% by mass or more and 40% by mass or less.

[0015] By including the lithium salt, the polymer material, and the porous substrate, this solid polymer electrolyte has high lithium ion conductivity. The reason for such high lithium ion conductivity is not necessarily clear, but it is speculated as follows, for example.

[0016] That is, the polysaccharides contained in the porous substrate have a large number of hydroxyl groups. These hydroxyl groups are likely to coordinate with cations such as lithium ions due to the high electronegativity of the oxygen atoms of the hydroxyl groups, thus promoting the dissociation of the lithium salt. When these hydroxyl groups are dispersed in the porous substrate, the dissociation of the lithium salt is promoted by the hydroxyl groups exposed on the surface among the dispersed hydroxyl groups. In addition, since the polymer material has lithium ion conductivity, the lithium ions generated by the dissociation can be easily conducted through the solid polymer electrolyte while maintaining the dissociated state. Thus, the dissociation of the lithium salt mixed in the polymer material is promoted by the hydroxyl groups of the polysaccharides contained in the porous substrate, and the lithium ions generated by the dissociation can move within the solid polymer electrolyte while maintaining the dissociated state by the polymer material. Therefore, when comparing the lithium ion conductivity in the same length of path, the lithium ion conductivity is relatively high. Therefore, it is speculated that the lithium ion conductivity of the solid polymer electrolyte is high.

[0017] Furthermore, by using a porous substrate, polysaccharides can be incorporated into this porous substrate, making it possible to include a relatively large amount of polysaccharides in the solid polymer electrolyte, between 10% and 40% by mass. This relatively large amount of polysaccharides in the solid polymer electrolyte promotes the dissociation of lithium ions. Additionally, because the polysaccharides in the porous substrate are arranged relatively continuously due to their shape, relatively long lithium ion conduction paths can be formed. These factors also suggest that the lithium ion conductivity of the solid polymer electrolyte will be enhanced.

[0018] In addition, the solid polymer electrolyte having a porous substrate has higher strength compared to solid polymer electrolytes without a porous substrate (including those containing polysaccharides as fillers (particles that reinforce the strength of the solid polymer electrolyte)), and therefore exhibits excellent shape retention. Moreover, because the solid polymer electrolyte has excellent shape retention, it can suppress the occurrence of short circuits when the energy storage element equipped with the solid polymer electrolyte is compressed.

[0019] Here, "polysaccharide content" in the solid polymer electrolyte refers to the mass of polysaccharides contained in the porous substrate relative to the mass of the solid polymer electrolyte.

[0020] Here, the polysaccharides mentioned above may be β-glucans.

[0021] Thus, when the polysaccharide in question is β-glucan, its molecular structure is chemically more stable than that of other polysaccharides. Therefore, when used in lithium-ion secondary batteries and the like, it is expected that the oxidation-reduction decomposition of porous substrates will be suppressed.

[0022] Here, the polymer material may have at least one of an ether oxygen and a carbonate structure.

[0023] Thus, when the polymer material has at least one of an ether oxygen and a carbonate structure, the state in which lithium ions are dissociated is easily maintained by the oxygen contained in the ether oxygen and carbonate structure, and the lithium ion conductivity of the solid polymer electrolyte is increased.

[0024] Here, the content of the carbonate structure in the polymer material may be 60 mol% or more.

[0025] When the content of the carbonate structure in the above polymer material is 60 mol% or more, the state in which lithium ions are dissociated by the oxygen contained in the carbonate structure is more easily maintained, and thus the lithium ion conductivity of the solid polymer electrolyte is increased.

[0026] Here, the polysaccharide mentioned above may be cellulose.

[0027] Thus, when the polysaccharide is cellulose, it can be easily obtained compared to other β-glucans, making it possible to supply more inexpensive energy storage elements.

[0028] A power storage element according to another aspect of the present invention comprises a positive electrode, a negative electrode, and a solid polymer electrolyte interposed between the positive electrode and the negative electrode.

[0029] Because this energy storage element is equipped with the aforementioned solid polymer electrolyte, it exhibits high lithium-ion conductivity.

[0030] A power storage device according to another aspect of the present invention comprises two or more power storage elements, and one or more power storage elements according to the other aspect of the present invention.

[0031] This energy storage device has high lithium-ion conductivity because it is equipped with two or more of the aforementioned high lithium-ion conductive energy storage elements.

[0032] The configuration of the solid polymer electrolyte, the configuration of the energy storage element, the configuration of the energy storage device, and the method for manufacturing the energy storage element, as well as other embodiments, according to one embodiment of the present invention, will be described in detail. Note that the names of the components (each element) used in each embodiment may differ from the names of the components (each element) used in the background art.

[0033] <Solid polymer electrolyte> The solid polymer electrolyte is a solid polymer electrolyte for energy storage elements and comprises a lithium salt, a polymer material to which the lithium salt is mixed and which has lithium ion conductivity, and a porous substrate to which the lithium salt and the polymer material are attached and which contains polysaccharides, wherein the polysaccharide content is 10% by mass or more and 40% by mass or less.

[0034] One way in which the mixture of the lithium salt and the polymer material is attached to the porous substrate is that the mixture is impregnated into the pores of the porous substrate. By impregnating the pores of the porous substrate with the mixture, the pores of the porous substrate are filled with the mixture, making it easier to increase lithium ion conductivity. Alternatively, the mixture may be impregnated into the pores of the porous substrate and laminated on one or both surfaces of the porous substrate. The mixture of the lithium salt and the polymer material may be, for example, in the form of a composite obtained by dissolving or dispersing the lithium salt and the polymer material with a solvent by mixing them, and then removing the solvent.

[0035] The average thickness of the solid polymer electrolyte is preferably 1 μm or more from the viewpoint of the strength of the solid polymer electrolyte, and preferably 200 μm or less from the viewpoint of the internal resistance of the solid polymer electrolyte. Here, this "average thickness" is obtained by measuring the thickness at 10 arbitrary locations and calculating the average value of the measurement results.

[0036] (Lithium salt) The lithium salts described above function as electrolyte salts. Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and lithium bis(fluorosulfonyl)imide (LiN(SO2F)2, LiFSI); lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); and lithium salts having halogenated hydrocarbon groups such as LiSO3CF3, lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2, LiTFSI), LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, sulfonylimide lithium salts such as LiFSI and LiTFSI are preferred, and LiFSI and LiTFSI are more preferred. The lithium salts mentioned above may be used individually or in combination of two or more types.

[0037] The lithium salt content in the solid polymer electrolyte is preferably 10% to 90% by mass, more preferably 20% to 80% by mass, and even more preferably 40% to 70% by mass. When the lithium salt content is within the above range, the lithium ion conductivity of the solid polymer electrolyte can be further enhanced.

[0038] (Polymer materials) The polymer material described above is miscible with the lithium salt and is lithium ion conductive. The lithium ion conductive properties of the polymer material impart lithium ion conductivity to the solid polymer electrolyte. The polymer material is not particularly limited as long as it is lithium ion conductive. For example, the polymer material is preferably a compound having at least one of an ether oxygen and a carbonate structure. When the polymer material has at least one of an ether oxygen and a carbonate structure, the dissociated state of lithium ions is easily maintained by the oxygen contained in the ether oxygen and carbonate structure, so the lithium ion conductivity of the solid polymer electrolyte is increased. Examples of compounds having the above-mentioned carbonate structure include polycarbonates containing a carbonate structure as a constituent unit of a chain structure, such as polypropylene carbonate (PPC), polyethylene carbonate (PEC), and polytrimethylene carbonate (PTMC), and polycarbonates containing a carbonate structure as a constituent unit of a cyclic structure, such as polyvinylene carbonate (PVCA). Of these, polycarbonates containing a carbonate structure as a constituent unit of a chain structure are preferred. Examples of compounds having the above-mentioned ether oxygen include polyethylene oxide (PEO) and polypropylene oxide (PPO).

[0039] The content of the carbonate structure in the polymer material is preferably 60 mol% or more, and more preferably 70 mol% or more. When the content of the carbonate structure in the polymer material is above the lower limit, the state in which lithium ions are dissociated by the oxygen contained in the carbonate structure is more easily maintained, thereby increasing the lithium ion conductivity of the solid polymer electrolyte. On the other hand, the upper limit of the content of the carbonate structure in the polymer material may be 100 mol%, and the polymer material may be a compound having substantially only the carbonate structure.

[0040] The content of the polymer material in the solid polymer electrolyte is preferably 5% by mass or more and 80% by mass or less, more preferably 10% by mass or more and 70% by mass or less, and even more preferably 15% by mass or more and 60% by mass or less. When the content of the polymer material is within the above range, the lithium ion conductivity of the solid polymer electrolyte can be further enhanced.

[0041] The above polymer material is mixed with the above lithium salt. In addition to the above lithium salt, the above polymer material may also be mixed with inorganic fillers such as aluminum oxide or silicon dioxide. The content of the above inorganic fillers in the solid polymer electrolyte is preferably, for example, 1% by mass or more and 20% by mass or less. When the content of the above inorganic fillers is within the above range, there is an advantage that the lithium ion conductivity of the solid polymer electrolyte is increased.

[0042] (porous base material) The porous substrate described above contains polysaccharides. The inclusion of these polysaccharides in the porous substrate allows for the formation of relatively long lithium ion conduction paths due to the shape of the porous substrate. Furthermore, the presence of the porous substrate allows for higher strength of the solid polymer electrolyte compared to a solid polymer electrolyte without the porous substrate.

[0043] Examples of the porous substrate shape include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, nonwoven fabric is preferred from the viewpoint of ease of adhesion of the lithium salt and polymer material, and more specifically, ease of impregnation. A porous substrate having such a shape can be manufactured by known methods. For example, if the porous substrate is in the form of a nonwoven fabric, a nonwoven fabric can be manufactured using a known nonwoven fabric manufacturing machine with fibers containing the polysaccharide.

[0044] It is preferable that the porous substrate has polysaccharides as its main component. Here, "main component" refers to the component with the largest content (mass%), for example, a component with a content of 50% by mass or more. For example, the content of the polysaccharides in the porous substrate is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. When the content of the polysaccharides in the porous substrate is above the lower limit, the content of the hydroxyl groups becomes relatively large, and the frequency of proximity between the hydroxyl groups and the lithium salt increases, thus further promoting the dissociation of lithium ions in the lithium salt. On the other hand, the porous substrate may contain substantially only the polysaccharides, and the content of the polysaccharides in the porous substrate may be 100% by mass.

[0045] The content of the polysaccharide in the solid polymer electrolyte is 10% by mass or more and 40% by mass or less, preferably 15% by mass or more and 35% by mass or less. When the content of the polysaccharide in the solid polymer electrolyte is above the lower limit, the content of the hydroxyl group becomes relatively large, and the dissociation of lithium ions in the lithium salt is further promoted. In addition, the strength of the solid polymer electrolyte can be increased. On the other hand, when the content of the polysaccharide in the solid polymer electrolyte is below the upper limit, the content of other components such as lithium salt and polymer material can be relatively increased, so the decrease in lithium ion conductivity caused by the decrease in the content of these other components can be suppressed.

[0046] Examples of the above polysaccharides include β-glucan, α-glucan such as amylose and amylopectin, and inulin, with β-glucan being preferred. When the above polysaccharide is β-glucan, it has the advantage of high chemical stability, and when a lithium-ion secondary battery is formed using the solid polymer electrolyte, for example, oxidation-reduction decomposition of the porous substrate is suppressed, thereby suppressing a decrease in Coulomb efficiency. Examples of the above β-glucan include cellulose and callose, with cellulose being preferred. Cellulose may also be in the form of salts such as sodium salts and calcium salts.

[0047] The porous substrate described above may contain components other than the polysaccharides described above. Examples of such components include polyethylene, polypropylene and other polyolefins, polyimides, and aramids. The content of the components other than the polysaccharides in the porous substrate is preferably, for example, 10% by mass or more and 50% by mass or less. The content of the components other than the polysaccharides in the porous substrate within the solid polymer electrolyte is preferably, for example, 1% by mass or more and 40% by mass or less. When the content of the components other than the polysaccharides is within the above range, the solid polymer electrolyte can contain a large amount of the polymer material and the lithium salt while maintaining the strength of the porous substrate, which has the advantage of ensuring a more sufficient lithium ion conductivity of the solid polymer electrolyte.

[0048] The porosity of the above porous substrate is preferably 80 volume% or less from the viewpoint of strength, and preferably 20 volume% or more from the viewpoint of discharge performance. Here, "porosity" is a volume-based value and refers to the measurement value using a mercury porosimeter.

[0049] The average thickness of the porous substrate described above is preferably 10 μm or more from the viewpoint of strength, and preferably 200 μm or less from the viewpoint of internal resistance. Here, this "average thickness" is obtained by measuring the thickness at 10 arbitrary locations and calculating the average value of the measurement results.

[0050] When the porous substrate is impregnated with the mixture of the lithium salt and the polymer material, the degree of impregnation is not particularly limited and can be appropriately set so that lithium ions can move between the positive and negative electrodes through the solid polymer electrolyte. For example, all the voids in the porous substrate may be filled with the mixture, or some of the voids in the porous substrate may not be filled with the mixture.

[0051] (Method for producing solid polymer electrolytes) The solid polymer electrolyte comprises, for example, mixing the lithium salt and the polymer material, and attaching the mixture obtained by the mixing to the porous substrate.

[0052] The above-mentioned mixing can be performed, for example, by dissolving or dispersing the lithium salt and the polymer material in a solvent using a known mixer. Examples of the solvent include acetonitrile, dimethylformamide, dimethoxysulfoxide, chloroform, methylene chloride, and N-methylpyrrolidone.

[0053] Examples of the methods used to achieve the above-mentioned adhesion include impregnating the porous substrate with the mixture obtained by the above-mentioned mixing, and then removing the solvent; or applying the mixture to one or both surfaces of the porous substrate and then removing the solvent. Examples of removing the solvent include drying using a known drying oven.

[0054] <Configuration of energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode body having a positive electrode, a negative electrode, and a solid polymer electrolyte, and a container for housing the electrode body. The electrode body is usually a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked via a solid polymer electrolyte, or a wound type in which the positive electrode and negative electrode are wound in a stacked state via a solid polymer electrolyte. As an example of an energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0055] (positive electrode) The positive electrode comprises a positive electrode substrate and a positive electrode active material layer disposed directly on the positive electrode substrate or via an intermediate layer.

[0056] The positive electrode substrate is conductive. Whether or not it is conductive is determined by the volume resistivity measured in accordance with JIS-H-0505 (1975), which is 10 7The determination is made using Ω·cm as the threshold. The positive electrode substrate material can be a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoint of high potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30 as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0057] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per unit volume of the secondary battery. "Average thickness of the substrate" refers to the value obtained by dividing the punched mass when punching out a predetermined area of ​​the substrate by the true density of the substrate and the punched area, and the same applies to the negative electrode substrate.

[0058] The intermediate layer is a layer placed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The composition of the intermediate layer is not particularly limited and may include, for example, a binder and a conductive agent. The positive electrode active material layer may further contain a mixture of a lithium salt and a polymer material having lithium ion conductivity. The lithium salt and polymer material are not particularly limited but can be selected from the composition of the lithium salt and polymer material provided by the solid polymer electrolyte.

[0059] The positive electrode active material layer contains the positive electrode active material. The positive electrode active material layer may optionally contain conductive agents, binders, thickeners, fillers, and other optional components.

[0060] The positive electrode active material can be appropriately selected from known positive electrode active materials. As the positive electrode active material for a lithium ion secondary battery, a material that can occlude and release lithium ions is usually used. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal composite oxides having a spinel type crystal structure, polyanion compounds, chalcogen compounds, sulfur, and the like. Examples of the lithium transition metal composite oxide having an α-NaFeO2 type crystal structure include Li[Li x Ni (1-x) O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al (1-x-γ-β) O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), etc. Examples of the lithium transition metal composite oxide having a spinel type crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples include O4. Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. Some atoms or polyanions in these materials may be substituted with atoms or anions of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0061] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle size of the positive electrode active material above the lower limit makes it easier to manufacture or handle the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. When a composite material of the positive electrode active material and other materials is used, the average particle size of the composite material is used as the average particle size of the positive electrode active material. "Average particle size" refers to the value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001), based on the particle size distribution measured by laser diffraction / scattering method on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013), becomes 50%.

[0062] To obtain powder with a predetermined particle size, grinders and classifiers are used. Examples of grinding methods include using mortars, ball mills, sand mills, vibrating ball mills, planetary ball mills, jet mills, counter-jet mills, swirling airflow jet mills, or sieves. Wet grinding, which involves the coexistence of water or organic solvents such as hexane, can also be used during grinding. For classification, sieves and wind classifiers are used as needed, both dry and wet.

[0063] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% to 99% by mass, more preferably 70% to 98% by mass, and even more preferably 80% to 95% by mass. By setting the content of the positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the positive electrode active material layer.

[0064] The conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and Ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent can take the form of powder or fiber. One of these materials may be used alone as the conductive agent, or two or more may be used in mixture form. These materials may also be used in composite form. For example, a composite material of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coating properties, and acetylene black is particularly preferred.

[0065] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the secondary battery can be increased.

[0066] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0067] The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within the above range, the active material can be stably retained.

[0068] Examples of thickening agents include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. If the thickening agent has a functional group that reacts with lithium or the like, this functional group may be deactivated beforehand by methylation or the like.

[0069] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral resource-derived materials such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof.

[0070] The positive electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0071] (Negative electrode) The negative electrode comprises a negative electrode substrate and a negative electrode active material layer disposed directly on the negative electrode substrate or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from, for example, the configurations exemplified in the positive electrode.

[0072] The negative electrode substrate is electrically conductive. Suitable materials for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, as well as carbonaceous materials. Among these, copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials, with foil being preferred from a cost perspective. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0073] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per unit volume of the secondary battery.

[0074] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer optionally contains optional components such as a conductive agent, binder, thickener, and filler. The optional components such as the conductive agent, binder, thickener, and filler can be selected from the materials exemplified above for the positive electrode. The negative electrode active material layer may further contain a mixture of a lithium salt and a polymer material having lithium ion conductivity. The lithium salt and polymer material are not particularly limited, but can be selected from the composition of the lithium salt and polymer material provided by the solid polymer electrolyte.

[0075] The negative electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0076] The negative electrode active material can be appropriately selected from known negative electrode active materials. For lithium-ion secondary batteries, materials capable of intercalating and releasing lithium ions are typically used as negative electrode active materials. Examples of negative electrode active materials include: metallic Li; metals or metalloids such as Si and Sn; metal oxides or metalloid oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 LiTiO 2、 Examples of materials include titanium-containing oxides such as TiNb2O7; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or poorly graphitizable carbon). Among these materials, graphite and non-graphitizable carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0077] "Graphite" refers to the average lattice plane spacing (d) of the (002) plane, determined by X-ray diffraction before charging or discharging, or during the discharge state. 002 ) refers to carbon materials with a n-scale between 0.33 nm and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the standpoint of obtaining materials with stable physical properties.

[0078] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, which is determined by X-ray diffraction before charging / discharging or during the discharge state. 002 This refers to carbon materials with a nautical radius of 0.34 nm or more and 0.42 nm or less. Non-graphitized carbons include poorly graphitizable carbons and easily graphitizable carbons. Examples of non-graphitized carbons include resin-derived materials, petroleum pitch or materials derived from petroleum pitch, petroleum coke or materials derived from petroleum coke, plant-derived materials, and alcohol-derived materials.

[0079] Here, "discharge state" refers to a state in which sufficient lithium ions capable of being absorbed and released during charging and discharging are released from the carbon material, which is the negative electrode active material. For example, in a cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode, this is a state in which the open-circuit voltage is 0.7V or higher.

[0080] "Non-graphitizable carbon" refers to the above d 002 This refers to carbon materials with a wavelength between 0.36 nm and 0.42 nm.

[0081] "Easily graphitizable carbon" refers to the above d 002 This refers to carbon materials with a wavelength of 0.34 nm or more and less than 0.36 nm.

[0082] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, between 1 nm and 100 μm. If the negative electrode active material is a carbon material, titanium-containing oxide, or polyphosphate compound, its average particle size may be between 1 μm and 100 μm. If the negative electrode active material is Si, Sn, Si oxide, or Sn oxide, its average particle size may be between 1 nm and 1 μm. Setting the average particle size of the negative electrode active material above the lower limit makes it easier to manufacture or handle. Setting the average particle size of the negative electrode active material below the upper limit improves the electronic conductivity of the active material layer. To obtain powder with a predetermined particle size, a pulverizer or classifier is used. The pulverizing method and classification method can be selected from, for example, the methods exemplified above for the positive electrode. If the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of foil.

[0083] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the negative electrode active material layer.

[0084] (solid polymer electrolyte) The solid polymer electrolyte described above is used as the solid polymer electrolyte provided in the energy storage element. In the energy storage element, the solid polymer electrolyte is arranged as a layer (solid polymer electrolyte layer) placed between the positive electrode and the negative electrode. The solid polymer electrolyte has both the functions of an electrolyte and a separator.

[0085] The shape of the energy storage element in this embodiment is not particularly limited, and examples include cylindrical batteries, prismatic batteries, flat batteries, coin-type batteries, button-type batteries, and the like. Figure 1 shows an example of a rectangular battery, specifically an energy storage element 1. Note that this figure is a transparent view of the inside of the container. An electrode body 2, having a positive electrode and a negative electrode wound around a solid polymer electrolyte, is housed in a rectangular container 3. The positive electrode is electrically connected to the positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to the negative electrode terminal 5 via a negative electrode lead 51.

[0086] <Configuration of the energy storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) composed of multiple energy storage elements 1 in power supplies for vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power supplies for electronic devices such as personal computers and communication terminals, or power storage power supplies. In this case, it is sufficient that the technology of the present invention is applied to at least one of the energy storage elements included in the energy storage unit. An energy storage device according to one embodiment of the present invention comprises two or more energy storage elements and one or more energy storage elements according to the above embodiment of the present invention (hereinafter referred to as the "second embodiment"). The energy storage device according to the second embodiment only needs to have the technology according to one embodiment of the present invention applied to at least one energy storage element included in the energy storage device, and may comprise one energy storage element according to the above embodiment of the present invention and one or more energy storage elements not relating to the above embodiment of the present invention, or may comprise two or more energy storage elements according to the above embodiment of the present invention. Figure 2 shows an example of a second embodiment of an energy storage device 30, which is formed by further assembling energy storage units 20, each comprising two or more electrically connected energy storage elements 1. The energy storage device 30 may include busbars (not shown) that electrically connect two or more energy storage elements 1, busbars (not shown) that electrically connect two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a condition monitoring device (not shown) that monitors the state of one or more energy storage elements.

[0087] <Manufacturing method for energy storage elements> The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode body and housing the electrode body in a container. Preparing the electrode body includes preparing a positive electrode, a negative electrode and a solid polymer electrolyte, and forming an electrode body by stacking or winding the positive electrode and negative electrode via the solid polymer electrolyte.

[0088] <Other Embodiments> Furthermore, the energy storage element of the present invention is not limited to the embodiments described above, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or with well-known technology. In addition, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0089] In the above embodiment, the case in which the energy storage element is used as a rechargeable non-aqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery) has been described, but the type, shape, dimensions, capacity, etc. of the energy storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double-layer capacitors, or capacitors such as lithium-ion capacitors. [Examples]

[0090] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples.

[0091] <Example 1> A cellulose nonwoven fabric (specifically, a nonwoven fabric manufactured by Nippon Kodo Paper Industry Co., Ltd. (disc-shaped with a diameter of 26 mm and an average thickness of 35 μm)) was used as the porous substrate. A mixture was prepared by mixing PPC as the polymer material and LiTFSI as the lithium salt in a mass ratio of 60:40 in dimethylformamide as the solvent. The resulting mixture was impregnated into the cellulose nonwoven fabric, and the solvent was removed using a hot plate and a vacuum dryer to produce the solid polymer electrolyte of Example 1. In the solid polymer electrolyte of Example 1, the lithium salt content was 35.6% by mass, the polymer material content was 53.3% by mass, and the cellulose (i.e., porous substrate) content as a polysaccharide was 11.1% by mass. The average thickness of the solid polymer electrolyte of Example 1 was 50 μm.

[0092] The obtained solid polymer electrolyte was sandwiched between two 26 mm diameter stainless steel (SUS) plates to create a simulated battery with one SUS plate as the working electrode and the other as the counter electrode. The lithium ion conductivity of the solid polymer electrolyte was measured using the method described below. The results are shown in Table 1.

[0093] (Measurement of lithium-ion conductivity) For the obtained simulated battery, electrochemical impedance measurements were performed at 25°C to obtain Cole-Cole plots at an amplitude of 10mV and frequencies from 7MHz to 100mHz. The lithium-ion conductivity was calculated from the actual resistance of the first arc obtained.

[0094] <Examples 2 to 4> Solid polymer electrolytes for Examples 2 to 4 were prepared in the same manner as in Example 1, except that the polymer materials and lithium salts shown in Table 1 were used in the mass ratios shown in Table 1.

[0095] In Example 2, the lithium salt content in the solid polymer electrolyte was 71.1% by mass, the polymer material content was 17.8% by mass, and the cellulose (i.e., porous substrate) content as a polysaccharide was 11.1% by mass. The average thickness of the solid polymer electrolyte in Example 2 was 50 μm.

[0096] In Example 3, the lithium salt content in the solid polymer electrolyte was 35.6% by mass, the polymer material content was 53.3% by mass, and the cellulose (i.e., porous substrate) content as a polysaccharide was 11.1% by mass. The average thickness of the solid polymer electrolyte in Example 3 was 50 μm.

[0097] In Example 4, the lithium salt content in the solid polymer electrolyte was 71.1% by mass, the polymer material content was 17.8% by mass, and the cellulose (i.e., porous substrate) content as a polysaccharide was 11.1% by mass. The average thickness of the solid polymer electrolyte in Example 4 was 50 μm.

[0098] The lithium ion conductivity of the solid polymer electrolytes from Examples 2 to 4 was measured in the same manner as in Example 1. The results are shown in Table 1.

[0099] <Comparative Example 1> A mixture was prepared by mixing the polymer materials and lithium salts shown in Table 1 with dimethylformamide as a solvent in the mass ratios shown in Table 1. The resulting mixture was applied to one side of two SUS plates, similar to those used in Example 1, using an applicator with a 400 μm gap. The solvent was then removed using a hot plate and a vacuum dryer to prepare the solid polymer electrolyte of Comparative Example 1. Next, a simulated battery was prepared by covering the side of the solid polymer electrolyte opposite to the first SUS plate with the other SUS plate, and the lithium ion conductivity was measured in the same manner as in Example 1. The results are shown in Table 1.

[0100] <Comparative Examples 2 to 4> Solid polymer electrolytes for Comparative Examples 2 to 4 were prepared in the same manner as for Comparative Example 1, except that the polymer materials and lithium salts shown in Table 1 were used in the mass ratios shown in Table 1. Then, in the same manner as for Comparative Example 1, the other SUS plate was placed over the side of the obtained solid polymer electrolyte opposite to the first SUS plate. As a result, the solid polymer electrolyte flowed out from between the SUS plates under pressure sufficient to sandwich it between the two SUS plates, causing the SUS plates to come into contact with each other. Therefore, it was determined that a short circuit would occur in the solid polymer electrolytes of Comparative Examples 2 to 4 without even needing to measure the lithium ion conductivity. The results are shown in Table 1.

[0101] <Comparative Example 5> A glass fiber aggregate (disc-shaped with a diameter of 26 mm and an average thickness of 40 μm) was used as the porous substrate. A mixture was prepared by mixing the polymer material and lithium salt shown in Table 1 with dimethylformamide as the solvent in the mass ratio shown in Table 1. After impregnating the glass fibers with the resulting mixture, the solvent was removed using a hot plate and a vacuum dryer to prepare the solid polymer electrolyte of Comparative Example 5. The lithium salt content in the solid polymer electrolyte of Comparative Example 5 was 31.5% by mass, the polymer material content was 47.2% by mass, and the glass fiber content was 21.3% by mass. The average thickness of the solid polymer electrolyte of Comparative Example 5 was 50 μm. The lithium ion conductivity of the solid polymer electrolyte of Comparative Example 5 was measured in the same manner as in Example 1. The results are shown in Table 1.

[0102] <Comparative Example 6> A nonwoven polypropylene (PP) fabric (disc-shaped with a diameter of 26 mm and an average thickness of 17 μm) was used as the porous substrate. A mixture was prepared by mixing the polymer material and lithium salt shown in Table 1 with dimethylformamide as the solvent in the mass ratio shown in Table 1. The resulting mixture was impregnated into the PP nonwoven fabric, and the solvent was removed using a hot plate and a vacuum dryer to prepare the solid polymer electrolyte of Comparative Example 6. The lithium salt content in the solid polymer electrolyte of Comparative Example 6 was 36.3% by mass, the polymer material content was 54.5% by mass, and the PP nonwoven fabric content was 9.2% by mass. The average thickness of the solid polymer electrolyte of Comparative Example 6 was 50 μm. The lithium ion conductivity of the solid polymer electrolyte of Comparative Example 6 was measured in the same manner as in Example 1. The results are shown in Table 1.

[0103] [Table 1]

[0104] As shown in Table 1, the solid polymer electrolytes of Examples 1 to 4 exhibited significantly higher lithium ion conductivity compared to the solid polymer electrolytes of Comparative Examples 1, 5, and 6, which are capable of functioning as solid polymer electrolytes in energy storage devices. Furthermore, the solid polymer electrolytes of Comparative Examples 2 to 4 were shown to be unsuitable for use as solid polymer electrolytes in energy storage devices. As a result, it was demonstrated that the solid polymer electrolytes of Examples 1 to 4 can be used to fabricate energy storage devices, and moreover, that the lithium ion conductivity of these energy storage devices can be increased. [Industrial applicability]

[0105] This invention can be applied to energy storage elements and devices used as power sources for electronic devices such as personal computers and communication terminals, and for automobiles, etc. [Explanation of symbols]

[0106] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage units 30 Energy storage devices

Claims

1. Lithium salts and A polymer material having lithium ion conductivity and in which the above lithium salt is mixed, A porous substrate to which the above lithium salt and the above polymer material are attached and which also contains polysaccharides. Equipped with, The above lithium salt is lithium bis(fluorosulfonyl)imide, The above polymer material has a carbonate structure, A solid polymer electrolyte for energy storage devices, wherein the above-mentioned polysaccharide content is 10% by mass or more and 40% by mass or less.

2. The solid polymer electrolyte according to claim 1, wherein the above polysaccharide is β-glucan.

3. The solid polymer electrolyte according to claim 1 or claim 2, wherein the polymer material has ether oxygen.

4. The solid polymer electrolyte according to any one of claims 1 to 3, wherein the content of the carbonate structure in the polymer material is 60 mol% or more.

5. The solid polymer electrolyte according to any one of claims 1 to 4, wherein the above polysaccharide is cellulose.

6. Positive electrode and, The negative electrode and, A solid polymer electrolyte according to any one of claims 1 to 5 interposed between the positive electrode and the negative electrode A storage element equipped with the following features.

7. A power storage device comprising two or more energy storage elements, and one or more of the energy storage elements described in claim 6.

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