JP2021144799A - Power storage device - Google Patents

Abstract

To provide a power storage device that achieves both high energy density and high input/output.SOLUTION: A power storage device includes columnar negative electrodes which are made of a carbon material and in which when the cross-sectional shape thereof is set to have a minor axis a and a major axis b, the average value of b/a is in the range of 5 or more and 60 or less, and the discharge capacity per unit length is 0.05 mAh/cm or more and 1.0 mAh/cm or less, a separation film provided so as to surround the periphery of each columnar negative electrode, and positive electrodes each of which contains a positive electrode active material and is provided so as to fill the space between adjacent separation films.SELECTED DRAWING: Figure 1

Description

This specification discloses a power storage device.

Conventionally, this type of power storage device includes a lithium ion supply core portion containing an electrolyte and a current collector having a three-dimensional network structure formed by surrounding the outer surface of the core portion and coated with an internal electrode active material on the outer surface. A cable-type secondary battery including an internal electrode and an external electrode formed by surrounding the outer surface of the internal electrode and including an external electrode active material layer has been proposed (see, for example, Patent Document 1). In this secondary battery, the electrolyte in the core portion easily penetrates into the active material of the electrode, and the capacity characteristics and cycle characteristics of the battery are excellent. Further, as a power storage device, a device having a structure in which a plurality of negative electrodes are bundled in a state of being adjacent to a positive electrode via a separation membrane has been proposed (see, for example, Patent Document 2). In this power storage device, it is possible to provide a new device having further improved output characteristics.

Japanese Patent Application Laid-Open No. 2014-532277 JP-A-2019-75198

However, the secondary battery of Patent Document 1 is said to be able to provide a secondary battery having a novel linear structure that is easily deformed and the electrolyte easily flows into the active material of the electrode, but the output characteristics are further improved. It was not fully considered about what to do. Further, the secondary battery of Patent Document 2 can further improve the output characteristics, but it is not yet sufficient, and further improvement has been desired.

The present disclosure has been made in view of such problems, and an object of the present disclosure is to provide a power storage device having both high energy density and high input / output.

As a result of diligent research to achieve the above-mentioned object, the present inventors have improved the input / output characteristics while maintaining high energy density by adopting a columnar electrode structure arranged by using a columnar negative electrode having a large aspect ratio. We have found that it is possible to complete the invention disclosed in the present specification.

That is, the power storage device of the present disclosure is
When the carbon fibers have a bound structure and the cross-sectional shapes are the minor axis a and the major axis b, the average value of b / a is in the range of 5 or more and 60 or less, and the discharge capacity per unit length is 0. A columnar negative electrode having a diameter of 05 mAh / cm or more and 1.0 mAh / cm or less,
A separation membrane provided so as to surround each columnar negative electrode,
A positive electrode containing a positive electrode active material and provided so as to fill the space between adjacent separation membranes,
It is equipped with.

Atsushi, the power storage device of this disclosure is
It has a structure in which carbon fibers are bound, and when the cross-sectional shape is a minor axis a and a major axis b, it has a b / a of 5 or more and a cross-sectional area of 0.005 mm ² or more and 0.4 mm ² or less. With a columnar negative electrode
A separation membrane provided so as to surround each columnar negative electrode,
A positive electrode containing a positive electrode active material and provided so as to fill the space between adjacent separation membranes,
It is equipped with.

The present disclosure can provide a power storage device having both high energy density and high input / output. The reason why such an effect can be obtained is presumed as follows, for example. The columnar negative electrode containing a carbon material has a flat shape in which the average value of b / a when the minor axis a and the major axis b are 5 or more, so that the ion diffusion distance can be shortened and the input / output characteristics can be shortened. It is presumed that it can be improved. However, when the discharge capacity or cross-sectional area per unit length of the columnar negative electrode is small, the ratio of the separation membrane in the positive electrode and the negative electrode to the volume in the cell increases, and the energy density decreases. On the other hand, when the discharge capacity or cross-sectional area per unit length of the columnar negative electrode is large, it takes time to diffuse the carrier ions to the inside, so that the input / output characteristics are deteriorated. Therefore, the columnar negative electrode has a columnar shape such that the discharge capacity per unit length is in the range of 0.05 mAh / cm or more and 1.0 mAh / cm or less, and the cross-sectional area is in the range of 0.005 mm ² or more and 0.4 mm ² or less. By controlling the shape of the negative electrode, it is possible to further suppress the above-mentioned decrease in energy density and input / output characteristics, and to provide a power storage device having both high energy density and high input / output.

The schematic diagram which shows an example of the power storage device 10. Explanatory drawing which shows an example of columnar negative electrode 12. The explanatory view of the electrode structure in which the columnar negative electrode 12 is arranged, and the conductive material of the positive electrode 16. SEM photographs of cross sections of Experimental Examples 1 and 9. Discharge curves at 0.2C and 3C of Experimental Examples 1 and 9.

The power storage device of the present disclosure described in the embodiment includes a plurality of columnar negative electrodes, a separation membrane, and a positive electrode. This power storage device may include a positive electrode current collector electrically connected to the positive electrode and a negative electrode current collector electrically connected to the negative electrode. This power storage device may be, for example, an electric double layer capacitor, a hybrid capacitor, a pseudo electric double layer capacitor, an alkali metal secondary battery, an alkali metal ion battery, or the like. Examples of carrier ions of the power storage device include alkali metal ions such as lithium ion, sodium ion and potassium ion, and group 2 ions such as magnesium ion, strontium ion and calcium ion. Further, the positive electrode may exist around the columnar negative electrode, or may be filled in the space between the columnar negative electrodes. Further, this power storage device may have a structure in which a plurality of columnar negative electrodes are bundled in a state of being adjacent to the positive electrode via a separation membrane. Further, the power storage device may contain an electrolytic solution in one or more of the columnar negative electrode, the positive electrode and the separation membrane. A current collecting member such as a current collecting wire may be embedded in the positive electrode and the columnar negative electrode, or the current collecting member may not be provided. Here, for convenience of explanation, a lithium ion secondary battery having a lithium ion as a carrier will be described below as a main example thereof.

Here, the power storage device disclosed in the present embodiment will be described with reference to the drawings. FIG. 1 is a schematic view showing an example of the power storage device 10. FIG. 2 is an explanatory diagram showing an example of the columnar negative electrode 12. The power storage device 10 includes a columnar negative electrode 12, a negative electrode current collector 13, a separation membrane 15, a positive electrode 16, and a positive electrode current collector 17. The single cell 11 is composed of a columnar negative electrode 12, a separation membrane 15, and a positive electrode 16. The power storage device 10 includes a columnar negative electrode 12 containing a negative electrode active material made of a carbon material, and a positive electrode 16 containing a positive electrode active material formed around the columnar negative electrode 12 via a separation film 15. The power storage device 10 may have a structure in which a plurality of single cells 11 including a columnar negative electrode 12 on which a separation film 15 and a positive electrode 16 are formed are bundled. Further, the power storage device 10 may have a structure in which 350 or more single cells 11 are united. Alternatively, the power storage device 10 may have a structure in which the columnar negative electrode 12, the separation film 15 formed on the surface of the columnar negative electrode 12, and the positive electrode 16 are filled between the columnar negative electrode 12 via the separation film 15. good.

The columnar negative electrode 12 is a member containing a negative electrode active material made of a carbon material. Here, the term "columnar" includes not only those having a non-bending thickness but also those having a flexible fibrous thickness. The columnar negative electrode 12 may have a flat shape in which the average value of the aspect ratios b / a of the minor axis a and the major axis b is 5 or more, and the cross section thereof may be elliptical. , The side surface may be a curved surface. The "average value" is not an absolute value of each columnar negative electrode 12, but an average value of measurements of a plurality of columnar negative electrodes 12. The columnar negative electrode 12 may have a shape in which rectangular corners are formed in an arc shape when viewed in cross section, or may have a shape in which the length in the minor axis a direction becomes shorter toward the end side. In the power storage device 10, a plurality of columnar negative electrodes 12 are arranged in a predetermined direction. The columnar negative electrode 12 may be arranged so that the direction of the long side of the power storage device 10 and the direction of the major axis b are arranged in the same direction. It is preferable that a plurality of columnar negative electrodes 12 are arranged at equal intervals in the major axis direction and offset in the minor axis direction. In the power storage device 10, the columnar negative electrodes 12 may be arranged so that the center of the columnar negative electrode 12 in the arranged row and the end of the columnar negative electrode 12 in the adjacent row alternate with each other (FIG. 1, FIG. 3). In such an electrode structure, the filling efficiency of the columnar negative electrode 12 is high, which is preferable from the viewpoint of improving the density of the active material.

The outer periphery of the columnar negative electrode 12 other than the end portion connected to the negative electrode current collector 13 is covered with the separation film 15. For example, the columnar negative electrode 12 may have a capacity of 1 / n of the negative electrode capacity of the entire power storage device 10, and n of them may be connected in parallel to the negative electrode current collector 13. The columnar negative electrode 12 preferably has a minor axis a of 20 μm or more, more preferably 30 μm or more, and further preferably 50 μm or more. The minor axis a is preferably 250 μm or less, more preferably 200 μm or less, and even more preferably 190 μm or less. Further, the columnar negative electrode 12 preferably has a major axis b of 200 μm or more, more preferably 300 μm or more, and further preferably 400 μm or more. The major axis b is preferably 1900 μm or less, more preferably 1800 μm or less, and even more preferably 1600 μm or less. Further, b / a preferably has an average value of 6 or more, and may be 7 or more. The average value of b / a is preferably 30 or less, more preferably 20 or less, and even more preferably 10 or less. In particular, b / a is preferably 6 or more and 10 or less on average. In the above range, it is preferable that the energy density of the power storage device 10 and the input / output characteristics can be improved in a suitable range. Further, when the diameter is longer, the strength of the electrode structure can be ensured and stable charging / discharging can be performed. Further, when the diameter is smaller, the moving distance of the carrier ion does not become too long, and high output performance can be obtained.

The columnar negative electrode 12 may have a discharge capacity of 0.05 mAh / cm or more and 1.0 mAh / cm or less per unit length. This discharge capacity refers to the capacity when discharged with a potential range of 0.005 V to 1.5 V at the Li reference potential and a constant current of 0.2 C, with Li metal as the counter electrode. The columnar negative electrode 12 preferably has as large a discharge capacity as possible, preferably 0.08 mAh / cm or more, more preferably 0.15 mAh / cm or more, and even more preferably 0.20 mAh / cm or more. The discharge capacity is preferably 0.9 mAh / cm or less, more preferably 0.85 mAh / cm or more, and may be 0.82 mAh / cm or less.

The columnar negative electrode 12 may have a cross-sectional area in the range of ^{0.005 mm 2} or more and 0.4 mm ^{2 or less.} The columnar negative electrode 12 is preferably the cross-sectional area as large as possible, preferably 0.015 mm ² or more, more preferably 0.018 mm ² or more, 0.020 mm ² or more is more preferable. The cross-sectional area of the columnar negative electrode 12 is preferably ^{0.25 mm 2 or less, more preferably 0.20 mm 2} or less, and even more preferably 0.50 mm ² or less. The length of the columnar negative electrode 12 in the longitudinal direction can be appropriately determined depending on the application of the power storage device 10, and may be, for example, in the range of 20 mm or more and 200 mm or less. When the length of the columnar negative electrode 12 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.

The columnar negative electrode 12 preferably contains a carbon material as a negative electrode active material, and may have one or more of a structure in which carbon fibers 14 are bound and a structure in which the carbon material is solidified. FIGS. 1 and 2 show the carbon fibers 14 bound together. The carbon material has high conductivity and becomes a negative electrode active material, and is preferable as the columnar negative electrode 12. 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, the carbon fiber 14 having a graphite structure may be used. The carbon fiber 14 is preferably one in which crystals are oriented in the longitudinal direction, which is the fiber direction, for example. 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). The diameter d of the carbon fiber 14 may be, for example, 5 μm or more, 7.5 μm or more, or 10 μm or more. Further, the diameter d of the carbon fiber 14 may be in the range of 50 μm or less, 25 μm or less, or 20 μm or less. The columnar negative electrode 12 may be obtained by twisting a plurality of carbon fibers 14, or may be obtained by binding a plurality of carbon fibers 14 with a binder. The binder is preferably one having conductivity of carrier ions, for example, polyvinylidene fluoride (PVdF), a copolymer of PVdF and hexafluoropropylene (PVdF-HFP), polymethyl methacrylate (PMMA), and the like. And a copolymer of PMMA and an acrylic polymer. The number of carbon fibers 14 depends on the thickness and the like, but for example, 200 or more is preferable, 350 or more is more preferable, and 500 or more may be used. The number of carbon fibers 14 is preferably 5000 or less, more preferably 3500 or less, and may be 3000 or less. Further, the columnar negative electrode 12 may be a carbonized integral product obtained by molding a raw material of a carbon material into a columnar shape, or may be a carbonized carbon material solidified with a binder or the like.

The negative electrode current collector 13 is a conductive member and is electrically connected to the columnar negative electrode 12. The negative electrode current collector 13 is arranged on the upper surface side of the power storage device 10. The negative electrode current collector 13 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 acid resistance. For the purpose of improving the conversion (reduction) property, 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 negative electrode current collector 13 is not particularly limited as long as the columnar negative electrode 12 can be connected, and for example, a plate shape, a foil shape, a film shape, a sheet shape, a net shape, a punched or expanded shape, or a lath body. , Porous body, foam, fiber group forming body and the like.

The separation membrane 15 has ionic conductivity of carrier ions (for example, lithium ions) and insulates the columnar negative electrode 12 and the positive electrode 16, and is provided around the columnar negative electrode 12. The separation film 15 is formed on the entire outer peripheral surface of the columnar negative electrode 12 facing the positive electrode 16 to prevent a short circuit between the columnar negative electrode 12 and the positive electrode 16. The separation membrane 15 may be formed by, for example, preparing a self-supporting film from a raw material solution containing a resin and coating the surface of the columnar negative electrode 12 with the self-supporting film, or immersing the columnar negative electrode 12 in the raw material solution. It may be formed by coating the surface of the metal. Examples of the resin of the separation film 15 include polyvinylidene fluoride (PVdF), a copolymer of PVdF and hexafluoropropylene (PVdF-HFP), polymethyl 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 of the separation membrane 15 is, for example, preferably 2 μm or more, more preferably 5 μm or more, and may be 8 μm or more. When this thickness is 2 μm or more, it is preferable in order to secure the insulating property. In particular, when the thickness of the separation membrane 15 is 2 μm or more, it is easy to prepare. The thickness of the separation membrane 15 is preferably 15 μm or less, and more preferably 10 μm or less. When this thickness 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. When the thickness of the separation membrane 15 is in the range of 2 to 15 μm, ionic conductivity and insulating property are suitable.

The separation membrane 15 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 this 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), ethyl methyl 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, ethoxymethoxyethane, 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 methyldioxolane. In this electrolytic solution, a supporting salt containing ions, which is a carrier of the electricity storage device 10, 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 is 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 positive electrode 16 contains a positive electrode active material and is provided so as to fill the space between adjacent separation films 15. The positive electrode 16 may include a positive electrode active material, a conductive material, and a binder, if necessary. The positive electrode 16 may be formed by being coated on the outer periphery of the columnar negative electrode 12 at the time of manufacturing the power storage device 10 (see FIG. 1). With this shape, it is preferable that the positive electrode active material binds the columnar negative electrode 12 formed on the outer periphery so that the positive electrode 16 is easily filled between the columnar negative electrodes 12. The positive electrode 16 may exist between the plurality of columnar negative electrodes 12. The positive electrode 16 may include a conductive material and may have conductivity by itself, and the current collecting member and the like may be omitted. A positive electrode current collector 17 is connected to any region of the positive electrode 16. The positive electrode 16 may be formed, for example, by forming a separation film 15 on the outer periphery of the columnar negative electrode 12 and then applying the raw material of the positive electrode 16 to the outer periphery thereof.

The positive electrode 16 may be made of, for example, a positive electrode mixture in which a positive electrode active material, a conductive material, and a binder, if necessary, are mixed. 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, and a phosphoric acid compound containing lithium and a transition metal element. Specifically, a lithium manganese composite oxide having a basic composition formula of Li _(1-x) MnO ₂ (0 ≦ x ≦ 1, etc., the same applies hereinafter) or Li _(1-x) Mn ₂ O _{4, etc., basic composition} Lithium cobalt composite oxide whose formula is Li _(1-x) CoO _2, etc., Lithium nickel composite oxide whose basic composition formula is Li _(1-x) NiO _2, etc., basic composition formula is Li _{(1-x) Co a Ni b Mn c O 2} (a> 0, b> 0, c> 0, a + b + c = 1), Li (1-x) Co a Ni b Mn c O 4 (0 <a <1,0 <b <1, 1 ≦ c <2, a + b + c = 2), etc. Lithium cobalt nickel manganese composite oxide, basic composition formula is LiV ₂ O _3, etc. Lithium vanadium composite oxide, basic composition formula is V ₂ O _5, etc. A transition metal oxide or 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 cobalt nickel-manganese composite oxides such as LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and LiNi _0.4 Co _0.3 Mn _0.3 O ₂ are preferable. The "basic composition formula" is intended to include other elements such as components such as Al and Mg.

The conductive material contained in the positive electrode 16 is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance. For example, graphite such as natural graphite (scaly graphite, scaly graphite) or artificial graphite, acetylene black, etc. One or a mixture of two or more of carbon black, Ketjen black, carbon whisker, graphite coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. As shown in FIGS. 1 and 3, the positive electrode 16 preferably contains carbon fibers as the conductive material 18, and the carbon fibers are preferably oriented in the major axis direction of the columnar negative electrode 12. In the positive electrode 16, the electron conductivity in the positive electrode mixture can be improved. Here, "orientation" means that the carbon fibers are along the direction of the major axis b, and there may be carbon fibers that are inclined in various directions. The number of positive electrode conductors 18 may be, for example, 40% or more, more preferably 50% or more along the direction of the major axis b. Here, it is assumed that the carbon fibers are oriented even if the carbon fibers are inclined in a direction other than the major axis direction as long as they are along the major axis direction. 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 kinds. Further, an aqueous dispersion of cellulose-based binder or styrene-butadiene rubber (SBR), which is an aqueous binder, can also be used.

In the positive electrode 16, the content of the positive electrode active material is preferably higher, preferably 70% by mass or more, and more preferably 80% by mass or more, based on the total mass of the positive electrode 16. The content of the conductive material is preferably in the range of 0% by mass or more and 20% by mass or less, and more preferably in the range of 0% by mass or more and 10% by mass or less with respect to the total mass of the positive electrode 16. 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 positive electrode 16. Is more preferable.

The positive electrode current collector 17 is a conductive member and is electrically connected to the positive electrode 16. The positive electrode current collector 17 is arranged on the side surface side of the power storage device 10. From the viewpoint of conductivity, the positive electrode current collector 17 is preferably arranged on a surface orthogonal to the major axis direction of the columnar negative electrode 12 (see FIG. 1). As the positive electrode current collector 17, any material or shape mentioned in the negative electrode current collector 13 can be adopted.

In the power storage device 10, the volumetric energy density is more preferably higher, for example, 400 Wh / L or more, more preferably 500 Wh / L or more, and further preferably 600 Wh / L or more. preferable. Further, in the power storage device 10, when the rate characteristic as an input / output characteristic is a discharge capacity of 3C (3C / 0.2C) with respect to a discharge capacity of 0.2C, this rate characteristic is preferably higher, and is 0. .50 or more is preferable, 0.60 or more is more preferable, 0.70 or more is further preferable, and 0.80 or more is most preferable. This discharge capacity refers to the capacity when discharged with a potential range of 0.005 V to 1.5 V at the Li reference potential and constant currents of 0.2 C and 3 C with Li metal as the counter electrode. In the power storage device 10, 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 positive electrode 16 is appropriately set according to the diameter D of the columnar negative electrode 12 and the capacity ratio of the positive electrode and the negative electrode, but may be in the range of 5 μm or more and 50 μm or less, for example. The formed thickness of the positive electrode 16 is, for example, the maximum thickness of the portions formed on the columnar negative electrode 12.

In the power storage device 10 of the embodiment described in detail above, both high energy density and high input / output can be achieved at the same time. The reason why such an effect can be obtained is presumed as follows, for example. The columnar negative electrode 12 containing a carbon material has a flat shape in which the average value of b / a when the minor axis a and the major axis b are 5 or more, so that the ion diffusion distance can be shortened and input / output can be performed. It is presumed that the characteristics can be improved. However, when the discharge capacity or cross-sectional area per unit length of the columnar negative electrode 12 is small, the ratio of the separation membrane 15 on the positive electrode and the negative electrode to the volume in the cell increases, and the energy density decreases. On the other hand, when the discharge capacity or the cross-sectional area per unit length of the columnar negative electrode 12 is large, it takes time to diffuse the carrier ions to the inside, so that the input / output characteristics are deteriorated. Therefore, the columnar negative electrode has a columnar shape such that the discharge capacity per unit length is in the range of 0.05 mAh / cm or more and 1.0 mAh / cm or less, and the cross-sectional area is in the range of 0.005 mm ² or more and 0.4 mm ² or less. By controlling the shape of the negative electrode 12, it is possible to further suppress the above-mentioned decrease in energy density and input / output characteristics, and to provide a power storage device having both high energy density and high input / output. In particular, by forming the columnar negative electrode 12 into a flat shape having a discharge capacity and a cross-sectional area in the above range, the advantages of the electrode structure in which the columnar columnar negative electrodes are bound and the advantages of the sheet-shaped electrode structure can be compatible with each other. It is presumed that it can be done.

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

For example, in the above-described embodiment, the carrier of the power storage device is lithium ion, but the carrier is not particularly limited to this, and alkali metal ions such as sodium ion and potassium ion, and group 2 element ions such as calcium ion and magnesium ion can also be used. good. Further, the positive electrode active material may contain carrier ions. Further, although the electrolytic solution is a non-aqueous electrolytic solution, it may be an aqueous electrolytic solution.

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 for example, activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, etc. Examples include polyacenes. Of these, activated carbons showing 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.

Hereinafter, an example in which the above-mentioned power storage device is specifically manufactured will be described as an experimental example. The result of considering the structure and performance of the power storage device will be described. Experimental Examples 1 to 8 correspond to the examples of the present disclosure, and Experimental Examples 9 to 11 correspond to Comparative Examples.

(Experimental Example 1)
A columnar electrode was prepared. First, 1000 carbon fiber yarns (YS-90A-10S manufactured by Nippon Graphite Fiber, diameter d = 7 μm) were prepared, cut to an appropriate length, and then dissolved in N-methylpyrrolidone (NMP). It was passed through a nozzle containing a (PVdF) solution. Then, it was stretched with a roll and dried to remove the solvent to obtain a carbon fiber bundle having a flattened cross-sectional structure (Experimental Example 1). In Experimental Examples 1 to 11, the aspect ratio b / a of the flat shape was controlled by adjusting the conditions for stretching the bound body after passing through the nozzle with a roll.

(SEM observation of columnar negative electrode of carbon fiber bundle)
The cross section of the columnar negative electrode of the carbon fiber bundle was observed with a scanning electron microscope (SEM). SEM observation was performed using a scanning electron microscope (S-3600N manufactured by Hitachi High-Technologies Corporation) at 10 kV and a magnification of 150. As an observation result, the cross-sectional area (mm ² ) and the minor axis a (μm) and the major axis b (μm) were determined. The measurement was performed at 5 points and calculated as an average value. In Experimental Example 1, the minor axis a was 110 μm, the major axis b was 660 μm, and the cross-sectional area was 0.057 mm ² .

(Experimental Examples 2-8)
Experimental Example 2 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 350, the minor axis a was 70 μm, and the major axis b was 350 μm. Experimental Example 3 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 3500, the minor axis a was 230 μm, and the major axis b was 1150 μm. Experimental Example 4 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 350, the minor axis a was 60 μm, and the major axis b was 420 μm. Experimental Example 5 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 3500, the minor axis a was 190 μm, and the major axis b was 1330 μm. Experimental Example 6 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 350, the minor axis a was 50 μm, and the major axis b was 500 μm. Experimental Example 7 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 3,500, the minor axis a was 160 μm, and the major axis b was 1600 μm. Experimental Example 8 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 3,500, the minor axis a was 30 μm, and the major axis b was 1800 μm.

(Experimental Examples 9 to 11)
Experimental Example 9 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 1000, the minor axis a was 270 μm, and the major axis b was 270 μm. Experimental Example 10 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 75, the minor axis a was 30 μm, and the major axis b was 180 μm. Experimental Example 11 was prepared in the same manner as in Experimental Example 1 except that the number of carbon fibers was 7,500, the minor axis a was 300 μm, and the major axis b was 1800 μm.

(Half-cell charge / discharge evaluation of columnar negative electrode of carbon fiber bundle)
After connecting both ends of the columnar negative electrode of the carbon fiber binding body to the Ni tab via Ag paste, a polyethylene separator and a metal lithium foil as a counter electrode were arranged around the carbon fiber binding body. Conditioning charge / discharge is performed in the potential range of 0.005V to 1.5V at the Li reference potential to evaluate the discharge capacity, and then the constant current discharge capacity is measured at 0.2C and 3C, with respect to the constant current discharge capacity of 0.2C. The ratio of the constant current discharge capacity of 3C (1 V at the upper limit Li reference potential) was measured. For the columnar negative electrode, a half cell was prepared with a total length of 6 cm and a length of a portion facing the metallic lithium foil of 4 cm.

(Estimation of cell energy density)
Based on the size of the columnar negative electrode of the carbon fiber bundle, the thickness of the separation film is 20 (μm), the density of the positive electrode mixture is 2.8 (g / cm ³ ), and the positive and negative electrode capacity ratio is 1.05. The energy density of the cell was calculated. The value of each sample was standardized with Experimental Example 9 as 100.

(Results and discussion)
FIG. 4 is an SEM photograph of a cross section of Experimental Example 9 (FIG. 4A) and Experimental Example 1 (FIG. 4B). FIG. 5 is a discharge curve at 0.2C and 3C of Experimental Example 9 (FIG. 5A) and a discharge curve at 0.2C and 3C of Experimental Example 1 (FIG. 5B). In addition, the minor axis a (μm), the major axis b (μm), the aspect ratio b / a, the number of fibers (lines), the cross-sectional area (mm ² ), and the discharge capacity per unit length of the columnar negative electrode of Experimental Examples 1 to 11 (MAh / cm), rate characteristics (3C / 0.2C), and relative values of energy density are shown together. Each value of the energy density was standardized with Experimental Example 9 as 100.

As shown in Table 1, when the flat carbon fiber bundles of Experimental Examples 1 to 8 were used as columnar negative electrodes, the rate characteristics were higher than those of Experimental Example 9 having a columnar shape. Moreover, in Experimental Examples 1 to 8, the energy density equivalent to that of Experimental Example 9 was shown. In particular, in a flat columnar negative electrode, the discharge capacity per unit length is 0.05 mAh / cm or more and 1.0 mAh / cm or less, and the cross-sectional area is 0.005 mm ² or more and 0.4 mm ² or less. It was found that in a columnar negative electrode having an aspect ratio b / a of 5 or more and 60 or less (Experimental Examples 1 to 8), both high energy density and high input / output can be achieved at the same time. This effect is achieved, for example, by having a flat shape, the ion diffusion path is shortened, so that the input / output characteristics are improved, and the volume ratio is reduced by the structure in which the columnar negative electrodes having the separation membrane are arranged. It was presumed to show high energy density. On the other hand, in Experimental Example 10 in which the discharge capacity per unit length is less than 0.05 mAh / cm and the cross-sectional area is ^{less than 0.005 mm 2} , the size is too small even if the aspect ratio b / a is flat. The effect of being flat could not be exhibited. Further, in Experimental Example 11 in which the discharge capacity per unit length exceeds 1.0 mAh / cm and the cross-sectional area exceeds 0.4 mm ² , even if the aspect ratio b / a is flat, the size is too large. , It was found that the above effect could not be obtained.

As shown in Table 1, in the range of b / a of 6 or more and 10 or less, both high energy density and high input / output were better compatible. Further, it was suggested that the minor axis a is preferably in the range of 20 μm to 250 μm and the major axis b is preferably in the range of 200 μm to 1900 μm, and that the number of carbon fibers in the carbon fiber having a diameter d of 7 μm is preferably in the range of 200 to 5000. .. Sectional area of the columnar negative electrode is more preferably in the range of 0.015 mm ² or more 0.25 mm ² or less, the discharge capacity of the columnar negative electrode, suggested range of 0.08 mAh / cm or more 0.9mAh / cm and more preferably Was done.

(Evaluation of resistivity of power storage device using columnar negative electrode of carbon fiber bundle: Experimental Example 12)
A power storage device was manufactured using the columnar negative electrode of Experimental Example 1, and the resistivity was evaluated. The columnar negative electrode of Experimental Example 1 was coated with a solution of vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) dissolved in N-methylpyrrolidone (NMP) by a dip method and dried to obtain 5 μm. A polymer film as a separation film was uniformly applied to the surface of the negative electrode in terms of film thickness. Next, the positive electrode active material (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ), high crystalline carbon fiber as a conductive material (Nippon Graphite Fiber XN-90-60S: fiber diameter 10 μm, average fiber length 200 μm), and conductivity. Gas-phase grown carbon fiber (VGCF manufactured by Showa Denko Corporation) as a material and vinylidene fluoride (PVdF7305 manufactured by Kureha Corporation) as a binder are blended in a mass ratio of 90: 4: 2: 4 to N-. Methylpyrrolidone was added to prepare a positive mixture paste. The positive electrode slurry is applied to the surface of the polymer-coated negative electrode so that the carbon fibers of the conductive material are oriented in the major axis direction of the polymer-coated negative electrode, and the positive electrode mixture layer is formed so that the thickness of the positive electrode mixture is 35 μm. Formed. The electrode structure is formed by laminating the single cells of the negative electrode / polymer film / positive electrode mixture layer thus produced in an offset shape in 12 columns × 7 rows (see FIG. 3) and pressing them with a hydrostatic press. Obtained. This electrode structure is housed in a case, and an Al foil (thickness 100 μm) as a positive electrode current collector is attached to the end face (see FIG. 1) in the direction orthogonal to the carbon fibers of the oriented positive electrode conductive material and exposed from the positive electrode. A copper foil (thickness 100 μm) as a negative electrode current collector was attached to the tip end side of the columnar negative electrode, and pressed to be thickened and fixed. Experimental Example 12 was a power storage device obtained by injecting a non-aqueous electrolytic solution into this case and sealing it. _{In the non-aqueous electrolytic 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.

(Experimental Examples 13 and 14)
The power storage device of Experimental Example 13 was prepared in the same manner as in Experimental Example 12 except that the Al foil as the positive electrode current collector was attached to the surface (front side in FIG. 1) along the carbon fiber as the positive electrode conductive material. And said. Further, Experimental Example 14 was prepared in the same manner as in Experimental Example 13 except that acetylene black (HS-100 manufactured by Denka Co., Ltd.) was used as the conductive material instead of the carbon fiber of the positive electrode conductive material.

(Measurement results and consideration of power storage device: Experimental Examples 12 to 14)
Five points on the positive electrode surface of Experimental Example 12 were observed by SEM to determine the orientation of the carbon fibers. The number of carbon fibers whose inclination angle was within ± 30 ° in the major axis direction (horizontal direction) of the columnar negative electrode with respect to the total number of carbon fibers in one field of view was obtained, and the average value of these was defined as the degree of orientation. The degree of orientation of Experimental Example 12 was 65%. Moreover, when the resistivity was measured using the four-terminal method, the resistivity of Experimental Example 12 was half that of Experimental Example 13. In addition, Experimental Example 14 showed a resistivity four times that of Experimental Example 13. From this result, it was found that the positive electrode conductive material is preferably carbon fiber, and it is preferable to orient the columnar negative electrode in the major axis direction. Further, it was found that it is more preferable to dispose the positive electrode current collector on the oriented end face of the carbon fiber as the oriented positive electrode conductive material.

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

10 power storage device, 11 single cell, 12 columnar negative electrode, 13 negative electrode current collector, 14 carbon fiber, 15 separation membrane, 16 positive electrode, 17 positive electrode current collector, 18 conductive material, a minor axis, b major axis, d diameter.

Claims (7)

When it is made of carbon material and its cross-sectional shape is minor axis a and major axis b, the average value of b / a is in the range of 5 or more and 60 or less, and the discharge capacity per unit length is 0.05 mAh / cm or more 1 With a columnar negative electrode of 0.0 mAh / cm or less,
A separation membrane provided so as to surround each columnar negative electrode,
A positive electrode containing a positive electrode active material and provided so as to fill the space between adjacent separation membranes,
Power storage device equipped with. The power storage device according to claim 1, wherein the columnar negative electrode has a cross-sectional area in ^{the range of 0.005 mm 2} or more and 0.4 mm ^{2 or less.} A columnar negative electrode ^{composed of a carbon material and having a cross-sectional area of 0.005 mm 2} or more and 0.4 mm ² or less, having b / a of 5 or more and a cross-sectional area of 0.005 mm 2 or more and 0.4 mm 2 or less when the cross-sectional shape is a minor axis a and a major axis b.
A separation membrane provided so as to surround each columnar negative electrode,
A positive electrode containing a positive electrode active material and provided so as to fill the space between adjacent separation membranes,
Power storage device equipped with. The power storage device according to any one of claims 1 to 3, wherein the columnar negative electrode has an average b / a of 6 or more and 10 or less. The power storage device according to any one of claims 1 to 4, wherein the columnar negative electrode has one or more of a structure in which carbon fibers are bound and a structure in which a carbon material is solidified. The power storage device according to any one of claims 1 to 4, wherein a plurality of columnar negative electrodes are arranged at equal intervals in the major axis direction and offset in the minor axis direction. The power storage device according to any one of claims 1 to 5, wherein the positive electrode contains carbon fibers as a conductive material, and the carbon fibers are oriented in the major axis direction of the columnar negative electrode.

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