JP7047826B2 - Active material membrane and power storage device - Google Patents

Description

This specification discloses an active material membrane and a power storage device.

Conventionally, as this type of power storage device, for example, a slurry containing a positive electrode active material having magnetic susceptibility anisotropy (LiFePO ₄ or the like) is applied to a current collector and then left in a magnetic field to orient the active material. A lithium ion battery as a negative electrode has been proposed (see, for example, Patent Document 1). In this battery, the lithium ion conducting surfaces of the positive electrode active material crystal structure can be efficiently arranged adjacent to each other, and the charge / discharge capacity can be increased. Further, when a slurry composed of a positive electrode active material, scaly graphite powder and / or a gas phase growing carbon fiber and a binder is applied onto a positive electrode current collector, a magnetic field is applied and then dried to obtain scaly graphite and / or scaly graphite. / Or a vapor-grown carbon fiber oriented perpendicular to the current collector has been proposed (see, for example, Patent Document 2). It is said that this electrode can exhibit high current collector performance with a small amount of addition, and can suppress an increase in resistance between the current collector and the reaction layer even if the cycle is repeated. Further, a lithium secondary battery using a positive electrode containing 0.25 to 1.5% by mass of carbon fibers having a fiber length of 10 to 1000 nm and a fiber diameter of 1 to 100 nm in a positive electrode mixture layer has been proposed (for example). , Patent Document 3). It is said that this battery can provide a battery having a high capacity and excellent load characteristics and cycle characteristics. Further, a battery having a plurality of composite whiskers (carbon nanotubes and carbon fibers) in the positive electrode active material layer and the positive electrode active material being carried on the surface of the whiskers has been proposed (see, for example, Patent Document 4). ). It is said that this battery can provide a positive electrode body and a battery that are not easily damaged by repeated charging and discharging.

Japanese Unexamined Patent Publication No. 2017-4941 Japanese Unexamined Patent Publication No. 2006-127823 Japanese Unexamined Patent Publication No. 2011-243558 Japanese Unexamined Patent Publication No. 2011-14439

However, Patent Document 1 aims to improve the ion conductivity in the electrode by aligning the directions of lithium ion diffusion in the active material, but considering the ion diffusion coefficient of the active material, ion diffusion in the film thickness direction of the electrode. It is not realistic to cover the above only in the active material, and LiFePO ₄ that orients in a magnetic field has low electron conductivity, a carbon coating is essential on the surface of the active material, and ion diffusion may be hindered. Further, in Patent Document 2, the electron conduction in the film thickness direction is improved by orienting the scaly graphite or the vapor-grown carbon fiber, which are conductive materials in the positive electrode, in the magnetic field direction. In order to lower the slurry viscosity at the time of working, it is necessary to add an excessive amount of a solvent and then slowly dry it, which may cause migration of the binder component and cause poor bonding strength of the electrode. Further, Patent Document 3 defines the size of the carbon nanotubes contained in the positive electrode mixture layer, but the direction in which the carbon nanotubes are contained is not taken into consideration, and the electron conductivity in the film thickness direction is sufficiently improved. I couldn't say that. Further, in Patent Document 4, since a whiskers (carbon nanotubes and carbon fibers) are used as the positive electrode active material, the filling volume density of the active material in the electrode is low, and it is difficult to increase the energy density of the battery.

The present disclosure has been made in view of such problems, and an object of the present disclosure is to provide an active material film capable of further enhancing input / output characteristics. Another object of the present invention is to provide a power storage device capable of further increasing the energy density and achieving both high input / output characteristics.

As a result of diligent research to achieve the above-mentioned object, the present inventors configured the energy storage device by orienting the carbon fiber material in the plane direction of the active material film and with respect to the current collecting portion and facing the columnar electrodes. Then, it was found that the energy density could be further increased and the high input / output characteristics could be compatible with each other, and the invention disclosed in the present specification was completed.

That is, the active material membrane disclosed in the present specification is
An active material membrane that is used as an electrode of a power storage device and is a self-supporting membrane that can be handled independently.
It contains an active material and a carbon fiber material having an average fiber diameter of 1 μm or more and an average fiber length of 10 μm or more, and the carbon fiber material is contained along a direction parallel to the film plane of the active material film. Is.

The power storage device disclosed in this specification is
A plurality of columnar first electrodes arranged in a predetermined direction at predetermined intervals,
The second electrode containing the above-mentioned active material film and
A separation membrane that has ionic conductivity and insulating properties and covers the first electrode and is interposed between the first electrode and the second electrode.
It is equipped with.

The present disclosure can further increase the energy density and achieve both high input / output characteristics. The reason why such an effect is obtained is presumed as follows. For example, by using an active material film in which carbon fiber material is contained along a direction parallel to the film plane, it is possible to remove the current collecting foil while maintaining the required electron conductivity in the electrode surface. Will be. As a result, it is possible to realize a power storage device having a high energy density. This can ensure an electronic connection by orienting the carbon fiber material perpendicular to the current collector in the plane of the electrode. This effect can be exhibited even in a battery having a general laminated structure, but in a power storage device using a columnar electrode, ensuring electron conductivity around the columnar electrode is an issue, so this active material. It is presumed that the use of a film can more effectively achieve both high energy density and high input / output. Here, the carbon fiber material refers to a short carbon fiber having a long length (for example, 10 cm or more). The carbon fiber material has an average fiber diameter of 1 μm or more and an average fiber length of 10 μm or more, but may have an average fiber diameter of 5 μm or more and an average fiber length of 50 μm or more.

The schematic diagram which shows an example of an active material membrane 10. The schematic diagram which shows an example of a power storage device 20. It is explanatory drawing which shows an example of the manufacturing process of the power storage device 20 using the active material film 10. The schematic diagram which shows an example of a power storage device 20B. The schematic diagram which shows an example of a power storage device 120. SEM photographs of Experimental Examples 1 and 4.

(Active substance membrane)
The active material film described in the present embodiment is a self-standing film that is used as an electrode of a power storage device and can be handled independently. The electricity storage device may be, for example, an electric double layer capacitor, a hybrid capacitor, a pseudo electric double layer capacitor, a lithium secondary battery, a lithium ion battery, or the like. Here, a lithium secondary battery using a lithium ion as a carrier will be described below as a main example thereof. FIG. 1 is a schematic diagram showing an example of the active material membrane 10. The active material film 10 contains a carbon fiber material 11 and an active material 12. In the active material film 10, the carbon fiber material 11 is included along a direction parallel to the film plane 13 of the active material film 10. In the active material film 10, the carbon fiber material 11 is in a direction parallel to the film plane 13 and in a direction perpendicular to the current collecting unit (second current collecting unit 26) to which the active material film 10 is connected. It may be included along the line. By the orientation in this way, the input / output characteristics of the power storage device can be further improved by further improving the conductivity. Here, "along a predetermined direction" includes a completely vertical direction and a parallel direction, and allows the existence of an angle that is not completely along the direction. For example, "along the direction parallel to the film plane 13 of the active material film 10" means that 90% or more of the carbon fiber materials have an angle θ within ± 30 ° in the direction parallel to the film plane 13. It shall mean things. Further, it is assumed that 90% or more of the fibers are within ± 30 ° in the direction perpendicular to the current collector (second current collector 26) to which the active material film 10 is connected. Further, as the carbon fiber material, a long carbon fiber (for example, 10 cm or more) may be shortened by controlling the fiber length through a shortening step.

The active material film 10 contains a carbon fiber material 11 as a conductive material, and may further contain one or more of granular carbon and vapor phase grown carbon fibers. Further, it is more preferable that the conductive material contains three types of granular carbon, vapor phase-grown carbon fiber and carbon fiber material. This is because the conductivity can be further improved. The vapor-phase-grown carbon fiber is a material having a function and a shape different from that of the carbon fiber material, and may be grown by heating a hydrocarbon gas, for example. The vapor-phase-grown carbon fiber may be a carbon nanotube. The vapor phase-grown carbon fiber may have an average diameter of less than 1.0 μm, 0.5 μm or less, or 0.25 μm or less. Further, the vapor phase-grown carbon fiber may have an average length of less than 10 μm, 6 μm or less, or 1 μm or less. Since the vapor-phase-grown carbon fiber has an extremely smaller average diameter and average length than the carbon fiber material, it is presumed that the vapor-grown carbon fiber imparts conductivity characteristics close to those of granular carbon. The carbon fiber material 11 may have, for example, a true density of 2.0 g / cm ³ or more, preferably 2.1 g / cm ³ or more, and more preferably 2.12 g / cm ³ or more. preferable. Further, the carbon fiber material may have a true density of 3.0 g / cm ³ or less. The carbon fiber material 11 may have an average fiber length X (μm) longer than the thickness T (μm) of the active material film 10. Such a carbon fiber material 11 is preferable because it is easy to orient in a predetermined direction. The average fiber length X may be, for example, 1.5 times or more the length of the thickness T, 2 times or more, or 10 times or more the length. For example, the thickness T of the active material film 10 may be in the range of 10 μm or more and 200 μm or less, or may be in the range of 50 μm or more and 100 μm or less. The average fiber diameter of the carbon fiber material 11 is, for example, 1 μm or more, may be 5 μm or more, 7.5 μm or more, or 10 μm or more. Further, the average fiber diameter of the carbon fiber material 11 may be in the range of 50 μm or less, 25 μm or less, or 20 μm or less. The average fiber length X is 10 μm or more, but may be 50 μm or more, 100 μm or more, or 150 μm or more. Further, the average fiber length X may be in the range of 500 μm or less, 300 μm or less, or 250 μm or less. The average fiber diameter and the average fiber length X of the carbon fiber material 11 may be appropriately set based on the use required for the power storage device. The average fiber diameter and average fiber length of the carbon fiber material 11 are totaled by taking an image of the carbon fiber material 11 using a scanning electron microscope and measuring the thickness and length of the carbon fibers contained in the image. , It shall be obtained by dividing by the number of fibers.

The carbon fiber material 11 may be contained in a range of 1% by mass or more with respect to the entire active material film 10. At 1% by mass or more, the input / output characteristics of the power storage device can be further enhanced by further improving the conductivity. The content of the carbon fiber material 11 is preferably 3% by mass or more, more preferably 3.5% by mass or more, and may be 4% by mass or more. Further, the content of the carbon fiber material 11 may be contained in the range of 8% by mass or less. When the content is 8% by mass or less, the IV resistance of the power storage device can be further reduced, which is preferable. The content of the carbon fiber material 11 is more preferably 7.5% by mass or less, and may be 7% by mass or less. Further, the carbon fiber material 11 may have a content of 10% by mass or more with respect to all the conductive materials contained in the active material film 10. When this content is 10% by mass or more, the oriented carbon fiber material 11 can further improve the conductivity. This content is preferably 20% by mass or more, and may be 30% by mass or more. Further, the content of the carbon fiber material 11 may be 50% by mass or less with respect to all the conductive materials. When this content is 50% by mass or less, the IV resistance can be further reduced and the input / output characteristics of the power storage device can be further improved. This content is preferably 40% by mass or less, and may be 35% by mass or less.

The active material 12 may be, for example, a positive electrode active material or a negative electrode active material, but is preferably a positive electrode active material. The positive electrode active material is preferably a lithium transition metal oxide. Specifically, a lithium manganese composite oxide having a basic composition formula of Li _(1-x) MnO ₂ (0 ≦ x ≦ 1, the same applies hereinafter), Li _(1-x) Mn ₂ O ₄ , or the like, a basic composition formula. Lithium cobalt composite oxide with Li _(1-x) CoO ₂ etc., lithium nickel composite oxide with basic composition formula Li _(1-x) NiO ₂ etc., basic composition formula Li _(1-x) Ni _a Co _b Mn _c O ₄ (where 0 <a <1, 0 <b <1, 0 <c <2, a + b + c = 2) and the basic composition formula are Li _(1-x) Ni _a Co _b Mn. Lithium nickel-cobalt-manganese composite oxide with _c O ₂ (provided that 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), etc., and the basic composition formula is LiV ₂ O ₃ etc. Lithium vanadium composite oxide and the like can be used. Further, a lithium iron phosphate compound having a basic composition formula of LiFePO ₄ or the like can be used as the positive electrode active material. Of these, lithium transition metal composite oxides, such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , and the like are preferred. The "basic composition formula" means that the composition of each element may be different or may contain other elements (for example, Al, Mg, etc.).

(Power storage device)
The power storage device may include a first electrode, a second electrode, and a separation membrane. Further, the first electrode is preferably a negative electrode and the second electrode is preferably a positive electrode, but the first electrode may be a positive electrode and the second electrode may be a negative electrode. The first electrode may be flat or columnar, but columnar is preferable because it is advantageous in improving the energy density and the rate of occlusion and release of carrier ions. The first electrode may be a plurality of columnar members arranged in a predetermined direction at predetermined intervals. The first electrode may contain the first active material. Here, the term "columnar" includes not only those having a non-bending thickness but also those having a flexible fibrous thickness. The first electrode may be columnar, and its cross section may be circular or polygonal. The second electrode may include the above-mentioned active material film. The second electrode may be present around the first electrode or may be filled in the space between the first electrodes. The separation membrane has ionic conductivity and insulating property, and may be interposed between the first electrode and the second electrode. This separation membrane may cover the first electrode. In this power storage device, one or more of the first electrode, the second electrode, and the separation membrane may contain an electrolytic solution. Further, the positive electrode and the negative electrode may be embedded with a current collector member such as a current collector, or may not be provided with the current collector member. Here, for convenience of explanation, a lithium secondary battery having a columnar negative electrode as a columnar negative electrode, a second electrode as a positive electrode, and a lithium ion carrier will be described below as a main example thereof.

The power storage device disclosed in this embodiment will be described with reference to the drawings. FIG. 2 is a schematic diagram showing an example of the power storage device 20. As shown in FIG. 1, the power storage device 20 includes a first electrode 21 which is a negative electrode, a first current collector 25 connected to the first electrode 21, a separation film 24, and a second electrode 22 which is a positive electrode. , A second current collector 26 connected to the second electrode 22 is provided. The power storage device 20 includes a first electrode 21 made of a columnar negative electrode active material, and a second electrode 22 made of an active material film 10 formed around the first electrode 21 via a separation film 24. ing. In this power storage device 20, the first electrodes 21 on which the separation film 24 is formed are arranged at predetermined intervals, the active material films 10 are arranged above and below the first electrodes 21 to be compressed, and the active material 12 is filled around the first electrodes 21. Has a structure that has been modified.

The first electrode 21 is a columnar substance containing an active material. In the power storage device 20, a plurality of columnar negative electrodes are arranged in a predetermined direction. The outer periphery of the first electrode 21 other than the end face faces the second electrode 22 via the separation membrane 24. For example, the first electrode 21 may have a capacity of 1 / n of the negative electrode capacity of the entire cell, and n of them may be connected in parallel to the first current collector 25. The diameter D of the cross section perpendicular to the longitudinal direction of the first electrode 21 is preferably 10 μm or more, more preferably 15 μm or more, and may be 30 μm or more. Further, the diameter D of the first electrode 21 is preferably 800 μm or less, more preferably 500 μm or less, and may be 400 μm or less. When the diameter D is 10 μm or more, the strength of the electrode structure can be ensured and stable charging / discharging can be performed. Further, when the diameter D is 800 μm or less, the moving distance of the carrier ions does not become too long, and high output performance can be obtained. Further, when the diameter D is in the range of 10 to 500 μm, the energy density per unit volume can be further increased. Alternatively, in this range, the moving distance of the carrier ions can be made shorter, and charging / discharging can be performed with a larger current. The length of the columnar body in the longitudinal direction can be appropriately determined depending on the use of the secondary battery and the like, and may be, for example, in the range of 20 mm or more and 200 mm or less. When the length of the columnar body is 20 mm or more, the battery capacity can be further increased, and when it is 200 mm or less, the electric resistance of the negative electrode can be further reduced, which is preferable. This negative electrode may contain a carbon material as a negative electrode active material. Examples of the carbon material include one or more of graphites, cokes, glassy carbons, non-graphitizable carbons, and pyrolytic carbons. Of these, graphites such as artificial graphite and natural graphite are preferable. Further, it may be a carbon fiber having a graphite structure. As such carbon fibers, for example, those in which crystals are oriented in the longitudinal direction, which is the fiber direction, are preferable. Further, it is preferable that the crystals are radially oriented from the center to the outer peripheral surface side when viewed in cross section in a direction orthogonal to the longitudinal direction (fiber direction). Alternatively, the columnar first electrode 21 may be formed into a columnar body of a composite oxide capable of occluding and releasing carrier ions. Examples of the composite oxide include lithium titanium composite oxide and lithium vanadium composite oxide. The negative electrode may have a conductive component formed on at least a part of the surface thereof. With this conductive component, the conductivity can be further enhanced. This conductive component is not particularly limited as long as it is a highly conductive material, but may be, for example, a metal.

The first current collector 25 is a member having conductivity, and the end face of the first electrode 21 is electrically connected. It may be assumed that 50 or more first electrodes 21 are connected in parallel to the first current collector 25. The first current collecting unit 25 includes, for example, carbon paper, aluminum, copper, titanium, stainless steel, nickel, iron, platinum, calcined carbon, conductive polymer, conductive glass, etc., as well as adhesiveness, conductivity, and For the purpose of improving oxidation resistance (reduction), those whose surfaces such as aluminum and copper are treated with carbon, nickel, titanium, silver, platinum, gold and the like can also be used. The shape of the first current collector 25 is not particularly limited as long as a plurality of first electrodes 21 can be connected, and is, for example, plate-shaped, foil-shaped, film-shaped, sheet-shaped, net-shaped, punched or expanded. Examples include a lath body, a porous body, a foam body, and a fiber group forming body.

The separation membrane 24 has ionic conductivity of ions (for example, lithium ions) that are carriers and insulates the first electrode 21 and the second electrode 22. The separation film 24 is formed on the entire outer peripheral surface of the first electrode 21 facing the second electrode 22 to prevent a short circuit between the first electrode 21 and the second electrode 22. The separation membrane 24 may contain a resin having ionic conductivity and insulating property. The separation membrane 24 may be formed, for example, by preparing a self-supporting film from a raw material solution containing a resin and covering the surface of the first electrode 21 with the self-supporting film, or by adding the first electrode 21 to the raw material solution. It may be formed by immersing it and coating the surface thereof. Examples of the resin of the separation film 24 include polyvinylidene fluoride (PVdF), a copolymer of PVdF and hexafluoropropylene (PVdF-HFP), methyl methyl methacrylate (PMMA), and PMMA and an acrylic polymer. Examples include copolymers. For example, in a copolymer of PVdF and HFP, a part of the electrolytic solution swells and gels this membrane to become an ionic conduction membrane. The thickness L of the separation membrane 24 is, for example, preferably 2 μm or more, more preferably 5 μm or more, and may be 8 μm or more. When the thickness L is 2 μm or more, it is preferable in order to secure the insulating property. In particular, when the thickness of the separation membrane 24 is 2 μm or more, it is easy to manufacture. The thickness L of the separation membrane 24 is preferably 15 μm or less, and more preferably 10 μm or less. When the thickness L is 15 μm or less, it is preferable in that the decrease in ionic conductivity can be suppressed and the volume occupied in the cell can be further reduced. In the range of thickness L of 2 to 15 μm, ionic conductivity and insulating property are suitable.

The separation membrane 24 may contain an electrolytic solution that conducts ions as carriers. Examples of this electrolytic solution include non-aqueous solvents. Examples of the solvent of the electrolytic solution include a solvent of a non-aqueous electrolytic solution. Examples of the solvent include carbonates, esters, ethers, nitriles, furans, sulfolanes, dioxolanes and the like, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate, dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate, and ethyl. Chain carbonates such as -n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyl lactone and γ-valerolactone, Chain esters such as methyl formate, methyl acetate, ethyl acetate, methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxy ethane, diethoxyethane, nitriles such as acetonitrile and benzonitrile, furan such as tetrahydrofuran and methyl tetrahydrofuran, etc. Classes, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolan. In this electrolytic solution, a supporting salt containing ions, which is a carrier of the storage device 20, may be dissolved. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, and the like. Examples thereof include LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Of these, 1 selected from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiC (CF ₃ SO ₂ ) ₃ . It is preferable to use a seed or a combination of two or more kinds of salts from the viewpoint of electrical characteristics. The concentration of this supporting salt in the electrolytic solution is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less.

The second electrode 22 has a positive electrode active material and is formed on the outer periphery of the first electrode 21 via a separation membrane 24. The second electrode 22 may be obtained by stacking a plurality of active material films 10 at the time of manufacturing the power storage device 20 and arranging the first electrode 21 via the separation film 24 between them and compressing the second electrode 22 (the second electrode 22). See FIG. 3 below). The second electrode 22 may exist between the plurality of first electrodes 21. The end surface of the second electrode 22 may be directly connected to the second current collector 26, or the second current collector 26 may be connected to the entire side surface. The second electrode 22 is formed of the active material film 10 and contains the carbon fiber material 11 and the positive electrode active material. The second electrode 22 may be composed of, for example, an active material film 10 formed by mixing and molding a positive electrode active material, a carbon fiber material 11, and a binder, if necessary. Examples of the positive electrode active material include a material capable of occluding and releasing lithium as a carrier. Examples of the positive electrode active material include compounds having lithium and a transition metal, for example, an oxide containing lithium and a transition metal element, a phosphoric acid compound containing lithium and a transition metal element, and the like.

The conductive material contained in the second electrode 22 is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance, and is, for example, one or more of granular carbon, vapor phase grown carbon fiber, and carbon fiber material 11. May be included. This conductive material includes, for example, graphite such as natural graphite (scaly graphite, scaly graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, and metal (copper, nickel, aluminum, silver). , Gold, etc.) or a mixture of two or more types can be used. The binder serves to hold the active material particles and the conductive material particles together to maintain a predetermined shape, and contains, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, and the like. Fluororesin, thermoplastic resin such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Further, an aqueous dispersion of cellulose-based binder or styrene-butadiene rubber (SBR), which is an aqueous binder, can also be used.

In the second electrode 22, the content of the positive electrode active material is preferably larger, preferably 70% by mass or more, and more preferably 80% by mass or more with respect to the total mass of the second electrode 22. .. The content of the conductive material is preferably in the range of 0% by mass or more and 20% by mass or less with respect to the total mass of the second electrode 22, and more preferably in the range of 5% by mass or more and 10% by mass or less. .. In such a range, it is possible to suppress a decrease in battery capacity and sufficiently impart conductivity. The content of the binder is preferably in the range of 0.1% by mass or more and 5% by mass or less, and in the range of 0.2% by mass or more and 3% by mass or less with respect to the total mass of the second electrode 22. Is more preferable.

The second current collector 26 is a member having conductivity and is electrically connected to the second electrode 22. The second current collector 26 may be the same member as the first current collector 25.

In this power storage device 20, the ionic conductivity of the separation membrane 24 is preferably higher, but preferably 1.0 mS / cm or more, more preferably 3.0 mS / cm or more, and 3.5 mS. It is more preferably / cm or more. The higher the ionic conductivity, the higher the discharge capacity can be. In this power storage device 20, the discharge capacity is preferably larger, but preferably 0.175 mAh or more, more preferably 0.180 mAh or more, and even more preferably 0.182 mAh or more. The volume energy density of the power storage device 20 at this time is more preferably higher, for example, 650 Wh / L or more, more preferably 830 Wh / L or more, and 900 Wh / L or more. More preferred.

In this power storage device 20, the positive / negative electrode capacity ratio (negative electrode capacity / positive electrode capacity), which is the ratio of the capacity of the negative electrode active material to the capacity of the positive electrode active material, is preferably in the range of 1.0 or more and 1.5 or less. More preferably, it is in the range of 1.2 or less. The formation thickness of the second electrode 22 is appropriately set according to the diameter of the first electrode 21 and the positive / negative electrode capacity ratio, but may be, for example, in the range of 5 μm or more and 50 μm or less.

(Manufacturing method of power storage device 20)
Next, a method of manufacturing the power storage device 20 will be described. FIG. 3 is an explanatory diagram showing an example of a manufacturing process of the power storage device 20 using the active material film 10. 3A is a process of forming an active material film 10, FIG. 3B is a process of peeling off the active material film 10, FIG. 3C is a process of producing an electrode structure 18, and FIG. 3D is an explanatory view of the electrode structure 18, FIG. 3E. Is an explanatory diagram of the connection process of the second current collecting unit 26, and FIG. 3F is an explanatory diagram of the connection process of the first current collecting unit 25. Since this manufacturing method is a method for manufacturing the above-mentioned active material film 10 and the power storage device 20, the members and compounding ratios described in the above-mentioned active material film 10 and the power storage device 20 are appropriately used.

This manufacturing method may include a forming treatment, a peeling treatment, a manufacturing treatment, a connection treatment, and the like. In the forming treatment, an electrode mixture containing the carbon fiber material 11, the active material 12, and the binder, if necessary, is prepared and formed on the support 15 (FIG. 3A). The electrode mixture may be prepared, for example, by mixing a conductive material containing the active material 12 and the carbon fiber material 11 and a binder, and adding a solvent as necessary. The electrode mixture may be in the form of a paste or in the form of clay. The support 15 may have flexibility as long as the surface is smooth, or may be inflexible, and examples thereof include a metal foil, a resin sheet, a resin plate, and glass. Be done. The formation of the electrode mixture on the support 15 is performed by, for example, a comma coater or a doctor blade. It can be done by spin coating or the like. Since the carbon fiber material 11 is formed along the direction parallel to the film plane 13, a comma coater, a doctor blade, or the like is preferable. After forming the electrode mixture on the support 15, it may be dried if necessary. Next, the active material film 10 is peeled off from the support 15 (FIG. 3B). Since the active material film 10 contains the carbon fiber material 11, it has sufficient strength for handling and has flexibility, so that it can be handled in an independent state.

Next, in the fabrication process, a plurality of first electrodes 21 having formed the separation film 24 are arranged at predetermined intervals between the plurality of active material films 10 and pressed from above and below the film plane 13 (FIG. 3C). , Obtain an electric strength structure 18 (FIG. 3D). At this time, the active material film 10 arranges the active material film 10 so that the carbon fiber material 11 is perpendicular to the connection surface of the second current collector 26 to which the active material film 10 is connected. It is assumed that the separation film 24 is formed on the first electrode 21 in addition to the tip end. A part of the tip end side of the first electrode 21 and the separation membrane 24 may be exposed from the second electrode 22. Subsequently, the second current collector 26 is connected to the active material film 10 of the electrode structure 18 (FIG. 3E), and the first electrode 21 is also connected to the first current collector 25 (FIG. 3F). Obtain a power storage device 20. In this way, the power storage device 20 can be manufactured.

In FIG. 3, the carbon fiber material 11 is oriented in a direction orthogonal to the axis of the first electrode 21, the first current collector 25 is arranged in front, and the second current collector 26 is the electrode structure 18. Although it is arranged on the side surface, it is not particularly limited to this. FIG. 4 is a schematic diagram showing an example of another power storage device 20B. In this power storage device 20B, the carbon fiber material 11 is oriented in a direction parallel to the axis of the first electrode 21, the first current collector 25 is arranged in front, and the second current collector 26 is the electrode structure 18. It was supposed to be arranged on the back surface.

In the active material membrane 10 described in detail above, the input / output characteristics can be further enhanced. Further, in the power storage device 20, the energy density can be further increased and high input / output characteristics can be achieved at the same time. The reason why such an effect is obtained is presumed as follows. For example, by using the active material film 10, it is possible to remove the current collector foil while maintaining the required electron conductivity in the electrode surface. As a result, it is possible to realize a power storage device 20 having a high energy density. This can ensure an electronic connection by orienting the carbon fiber material 11 perpendicularly to the second current collector 26 in the plane of the electrode. This effect can be exhibited even in a battery having a general laminated structure, but in the power storage device 20 using the first electrode 21 which is a columnar electrode, it is a problem to secure the electron conductivity around the columnar electrode. Therefore, it is presumed that by using this active material film 10, it is possible to more effectively achieve both high energy density and high input / output. Further, in this power storage device 20, in order to secure an electronic connection by orienting the carbon fiber material 11 perpendicularly to the second current collector 26 in the plane of the second electrode 22, the electrode structure 18 is used. Conductivity can be ensured even if there is no current collector foil inside. For example, if there is a metal current collecting foil inside the power storage device 20, an oxidation reaction (for example, a thermite reaction) of the current collection foil may occur. However, in the above-mentioned power storage device 20, the electrode structure 18 is used. By removing the metal collecting foil inside, it is possible to prevent the oxidation reaction and make it safer.

It should be noted that the present disclosure is not limited to the above-described embodiment, and it goes without saying that the present disclosure can be carried out in various embodiments as long as it belongs to the technical scope of the present disclosure.

For example, in the above-described embodiment, in the power storage device 20, the negative electrode and the positive electrode do not include a current collector member, but the present invention is not particularly limited to this, and each electrode is embedded with a current collector member such as a current collector. You may be doing it.

In the above-described embodiment, the power storage device 20 having the electrode structure 18 in which the active material film 10 and the first electrode 21 are laminated in a plurality of layers has been described, but the first electrode 21 is one layer and is active above and below it. It may be a power storage device having an electrode structure in which the material film 10 is arranged.

Further, in the above-described embodiment, the carrier of the secondary battery is lithium ion, but the carrier is not particularly limited to this, and is used as an alkali metal ion such as sodium ion or potassium ion, or as a group 2 element ion such as calcium ion or magnesium ion. May be good. Further, the positive electrode active material may contain carrier ions. Further, although the electrolytic solution is a non-aqueous electrolytic solution, an aqueous solution-based electrolytic solution may be used.

In the above-described embodiment, the example in which the first electrode 21 of the columnar body has a cylindrical shape has been described, but the present invention is not particularly limited to this, and the first electrode 21 may have a shape such as a quadrangular column or a hexagonal column.

In the above-described embodiment, the positive electrode active material is a transition metal composite oxide, but the present invention is not particularly limited, and for example, it may be a carbon material used for a capacitor. The carbon material is not particularly limited, but is, for example, activated carbons, coke, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and the like. Examples include polyacenes. Of these, activated carbons having a high specific surface area are preferable. Activated carbon as a carbon material preferably has a specific surface area of 1000 m ² / g or more, and more preferably 1500 m ² / g or more. When the specific surface area is 1000 m ² / g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m ² / g or less, and more preferably 2000 m ² / g or less, from the viewpoint of ease of production. It is considered that at the positive electrode, at least one of the anion and the cation contained in the ion conduction medium is adsorbed and desorbed to store electricity, but further, at least one of the anion and the cation contained in the ion conduction medium is inserted and desorbed. It may be used to store electricity.

In the above-described embodiment, the active material 12 is a positive electrode active material, but the active material 12 is not particularly limited to this, and the active material 12 may be a negative electrode active material. Examples of the negative electrode active material include inorganic compounds such as lithium, lithium alloys, and tin compounds, carbon materials capable of storing and releasing lithium ions, composite oxides containing a plurality of elements, and conductive polymers. Examples of the composite oxide include lithium titanium composite oxide and lithium vanadium composite oxide.

Hereinafter, an example in which the above-mentioned secondary battery is specifically manufactured will be described as an experimental example. Experimental Examples 2 to 6 correspond to the examples of the present disclosure, and Experimental Examples 1, 7, and 8 correspond to Comparative Examples.

(Experimental Examples 1 to 6)
(Preparation of positive electrode active material film)
Positive positive active material (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ), acetylene black (HS-100 manufactured by Denka), vapor-grown carbon fiber (VGCF manufactured by Showa Denko), and carbon fiber material as conductive materials in a mass ratio of 85: 5: After mixing to a ratio of 2: 3, a binder (PVdF7305 manufactured by Kureha) corresponding to the remaining 5% by mass was added, and the viscosity was adjusted with N-methylpyrrolidone to obtain a positive electrode slurry. The carbon fiber material was prepared by crushing highly crystalline carbon fiber (XN-90-60S manufactured by Nippon Graphite Fiber: fiber diameter 10 μm) with a mixer so that the average fiber length was 200 μm. A high crystalline carbon fiber crushed to a particle size of 10 μm was also prepared. The average fiber length of the carbon fiber material was obtained by taking an image of the carbon fiber material using a scanning electron microscope, measuring the length of the carbon fiber contained in the image, totaling it, and dividing it by the number of fibers. .. The prepared positive electrode slurry was applied to an aluminum foil as a support with a comma coater and dried to prepare a coated positive electrode active material film, and then the foil was peeled off to obtain a positive electrode self-standing film having a thickness of 180 μm. The density of the positive electrode active material was adjusted to 2.5 g / cm ³ by a roll press, and the obtained self-supporting membrane was used as Experimental Example 4. In addition, a product in which the mass ratio of the carbon fiber material was changed was produced. After fixing 10% by mass of the conductive material in the positive electrode mixture, the mass ratio of the carbon fiber material was controlled to 0 to 8% by mass. The other two types of conductive materials (acetylene black and VGCF) had a fixed mass ratio of 5: 2, and the addition amount was adjusted according to the mass ratio of the carbon fiber material. The carbon fiber materials having a mass ratio of 0% by mass, 0.5% by mass, 1% by mass, 3% by mass, 5% by mass, and 8% by mass were designated as Experimental Examples 1 to 6, respectively. The obtained positive electrode self-standing film was observed by SEM to confirm the dispersed state of the conductive material. In addition, the in-plane electronic resistance of the positive electrode film was evaluated using the 4-terminal method.

(Experimental Examples 7 and 8)
After fixing 10% by mass of the conductive material in the positive electrode mixture and fixing the mass ratio of 5: 2 of the other two types of conductive materials (acetylene black and VGCF), the highly crystalline carbon fiber was excessively crushed. As Experimental Example 7, a carbon fiber material adjusted to an average particle size of 10 μm was used in an amount of 3% by mass. Further, as shown in FIG. 5, Experimental Example 8 was set so that the orientation direction of the carbon fiber material was parallel to the current collector 26 to which the layer containing the active material film was connected. FIG. 5 is a schematic view showing an example of a power storage device 120 in which the carbon fiber material 11 is oriented in a direction parallel to the second current collector 26.

(Formation of separator on carbon rod negative electrode)
A solution prepared by dissolving vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) in N-methylpyrrolidone is coated on a carbon rod having a diameter of 160 μm and a length of 6.0 cm by a dip method and dried. Then, a polymer film was uniformly applied to the surface of the carbon rod with a film thickness of about 7 μm.

(Battery production)
The polymer-coated carbon rods were arranged perpendicular to the coating direction of the pressed positive electrode self-standing film, and both ends of the carbon rods were connected to the Ni tab via Ag paste. Further, the end portion of the positive electrode self-standing film was connected to the Al tab via carbon paper (carbon fiber non-woven fabric). After setting on the Al laminate, a non-aqueous electrolytic solution was injected and sealed to prepare a cell. In the non-aqueous electrolyte solution, LiPF ₆ was dissolved at a concentration of 1 M in a mixed solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30/40/30. Was used.

(Battery charge / discharge evaluation)
After conditioning the battery, charging and discharging were performed, and then IV measurement was performed to obtain a resistance value from the slope of current-voltage. Conditioning charge / discharge was carried out with a final voltage of 2.5 to 4.1 V, a current value of 0.1 C, and a test temperature of 25 ° C. In the IV measurement, after adjusting to a state of charge (SOC = 50%) of 50% of the battery capacity, the battery is discharged at a measurement temperature of 25 ° C. with a current corresponding to 0.2C, 1C, 3C, and 5C, and the voltage after 10 seconds. By measuring the value, the applied current and voltage were linearly approximated, and the IV resistance was obtained from the gradient.

(Results and discussion)
6A and 6B are SEM photographs of Experimental Examples 1 and 4, FIG. 6A is an image of 5000 times that of Experimental Example 1, FIG. 6B is an image of 5000 times that of Experimental Example 4, and FIG. 6C is an image of 100 times that of Experimental Example 1. , FIG. 6D is an image 100 times larger than that of Experimental Example 4. Table 1 shows the compounding ratio (mass%) of the highly crystalline carbon fibers (carbon fiber material) to the positive electrode mixture, the compounding ratio (mass%) to the entire conductive material, and the fiber length X (μm) of Experimental Examples 1 to 8. ), Presence or absence of orientation, orientation of carbon fiber material, in-plane electron resistance, and IV resistance are shown together. Regarding the presence or absence of orientation, in the image observed by SEM, 90% or more of the fibers within ± 30 ° in the direction perpendicular to the current collector to which the active material film is connected were defined as “aligned”. Further, in Experimental Example 8, those having 90% or more of fibers within ± 30 ° in the parallel direction with respect to the current collector to which the active material membrane is connected were defined as “aligned”. The orientation of the carbon fiber material indicates the direction of the carbon fiber material with respect to the current collector to which the active material membrane is connected. The in-plane electronic resistance and IV resistance were standardized with Experimental Example 1 containing no carbon fiber material as 100.

As shown in FIG. 6, in Experimental Example 1 containing no carbon fiber material, a structure in which the particulate conductive material aggregates was observed like the black portion in the figure. On the other hand, in Experimental Example 4 produced by coating the positive electrode mixture slurry in a predetermined direction, the aggregation of the conductive material is suppressed and dispersed, and the carbon fiber material exists along the direction orthogonal to the current collecting portion. It turned out that there was.

As shown in Table 1, from the results of Experimental Examples 1 to 6, it is possible to reduce the in-plane electronic resistance of the positive electrode self-standing film containing the carbon fiber material, and the lithium ion secondary battery using this is possible. It was found that the input / output characteristics of can be significantly improved. In particular, in Experimental Examples 3 to 5, it was found that the IV resistance can be further reduced, which is more preferable. It was speculated that this effect was exhibited when the carbon fiber material was oriented in the free-standing film in a direction parallel to the membrane plane and in a direction perpendicular to the current collector. As in Experimental Example 7, if the carbon fiber material is shorter than the electrode film thickness, it cannot be oriented and the above effect cannot be obtained. Further, as in Experimental Example 8 (FIG. 5), the above effect was not obtained even when the fibers were oriented in the direction parallel to the current collector. It was also found that when the amount of the carbon fiber material was 0.5% by mass or more, more preferably 1% by mass or more, the in-plane electronic resistance was reduced with respect to the positive electrode mixture. It was also found that when the carbon fiber material is more than 50% by mass with respect to the total amount of the conductive material, the in-plane resistance is reduced, but the IV resistance as a battery is increased. It was presumed that it is more preferable to contain particulate acetylene black or VGCF in order to secure the electronic bonding between the active material and the conductive material.

It should be noted that the present disclosure is not limited to the above-mentioned examples, and it goes without saying that the present disclosure can be carried out in various embodiments as long as it belongs to the technical scope of the present disclosure.

10 Active material film, 11 Carbon fiber material, 12 Active material, 13 Membrane plane, 15 Support, 20, 20B, 120 Power storage device, 21 1st electrode, 22 2nd electrode, 24 Separation film, 25 1st current collector , 26 Second current collector.

Claims (7)

An active material membrane that is used as an electrode of a power storage device and is a self-supporting membrane that can be handled independently.
It contains an active material and a carbon fiber material having an average fiber diameter of 1 μm or more and an average fiber length of 10 μm or more, and the carbon fiber material is contained along a direction parallel to the film plane of the active material film. Active material film. The active material according to claim 1, wherein the carbon fiber material is contained along a direction parallel to the film plane and a direction perpendicular to the current collecting portion to which the active material film is connected. film. The active material film according to claim 1 or 2, wherein the carbon fiber material has an average length X longer than the thickness T of the active material film. The active material film according to any one of claims 1 to 3, which contains granular carbon, vapor-grown carbon fibers, and the carbon fiber material as the conductive material. One of claims 1 to 4, wherein the carbon fiber material is contained in the active material film in a range of 0.5% by mass or more and 5.0% by mass or less with respect to the entire active material film. The active material membrane described in. The activity according to any one of claims 1 to 5, wherein the content of the carbon fiber material in all the conductive materials contained in the active material film is 5% by mass or more and 50% by mass or less. Material film. A plurality of columnar first electrodes arranged in a predetermined direction at predetermined intervals,
The second electrode including the active material film according to any one of claims 1 to 6 and the second electrode.
A separation membrane that has ionic conductivity and insulating properties and covers the first electrode and is interposed between the first electrode and the second electrode.
Power storage device equipped with.

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