JP7484871B2 - Energy Storage Devices - Google Patents

Description

This specification discloses an electricity storage device.

Conventionally, a proposed electric storage device has an active material containing Si and O as constituent elements, with the atomic ratio of Si to Si and O (Si/(Si+O)) being 30 atomic % to 75 atomic % on the surface of the active material (see, for example, Patent Document 1). Also proposed as an electric storage device is a spherical composite particle having a plurality of silicon-containing particles and a plurality of scaly graphite particles as the active material (see, for example, Patent Document 2). Also proposed as an electric storage device is a device in which a first power generating element and a negative electrode of an adjacent second power generating element are bonded together with a conductive adhesive that does not have ion conductivity (see, for example, Patent Document 3). Also proposed as an electric storage device is an electrode having a large number of voids in the electrode, and an electrolyte membrane in which the voids in the nonwoven fabric are filled with an electrolyte matrix in a nonwoven fabric made of nanofibers placed on the electrode (see, for example, Patent Document 4).

JP 2016-35939 A JP 2015-125794 A JP 2011-204510 A JP 2018-32484 A

However, in the power storage device of Patent Document 1, expansion and contraction mitigation is attempted by forming a core-shell of high-capacity SiOx and forming voids, and by having Si particles with large volume changes exist in the voids of spherical particles made of flake graphite, but this is still insufficient, and there are still problems in making it thinner. In addition, in Patent Document 3, as a method for removing the metal current collector foil with a large specific gravity, the positive electrode and the negative electrode are opposed to each other via an adhesive layer that does not exhibit ionic conductivity but only exhibits electronic conductivity, but on the other hand, there is a concern that the mass of the cell will increase due to the high specific gravity of the inorganic solid electrolyte itself and its thickness of 30 μm. In addition, in Patent Document 4, it is possible to realize a lightweight lithium-ion secondary battery by filling a nonwoven fabric substrate with a polymer solid electrolyte, but since the structure of the electrode body is the same as the conventional laminated structure, there is a limit to how thin the layer can be made.

This disclosure has been made in consideration of these issues, and its primary objective is to provide an electricity storage device that is lightweight, thin, and has greater freedom in shape.

After extensive research to achieve the above-mentioned objective, the inventors discovered that by adopting a structure in which a negative electrode has a flat cross section, is oriented in the plane direction, and a positive electrode is placed on the end face of the negative electrode, it is possible to create an electricity storage device that is lightweight, thin, and has a high degree of freedom in shape, with good charge and discharge efficiency, and thus completed the invention disclosed in this specification.

That is, the electricity storage device of the present disclosure is
a positive electrode having an end face including a short side shorter than the long side in a direction intersecting with a first surface including the long side, the positive electrode including a positive electrode active material;
a separator adjacent to at least an end surface of the positive electrode;
a negative electrode having an end face including a short side shorter than the long side in a direction intersecting a first surface including a long side, the negative electrode including a carbon material oriented in a predetermined orientation direction along the first surface as a negative electrode active material, the end face facing an end face of the positive electrode via the separator;
A power storage device comprising:

The present disclosure can provide a lightweight, thin, and highly flexible power storage device. The reason for such an effect is presumed to be as follows. In a negative electrode containing a carbon material, the carbon material can be oriented along the surface direction of the first surface having a large area by making the negative electrode have a flat shape in which the b/a value exceeds 1 when the short side is a and the long side is b. This structural feature alleviates stress concentration on the surface layer when the negative electrode is curved, and causes internal stress dispersion, making it possible to curve the cell itself, and improving the freedom of the cell structure. In addition, the presence of a positive electrode on the end face in the orientation direction of the negative electrode makes it possible to smoothly insert/remove carrier ions into the carbon material and diffuse within the negative electrode, thereby achieving high input/output. In addition, by using a carbon material as a negative electrode active material and current collector, charging and discharging can be performed without using a metal current collector, and it is also possible to increase the mass energy density of the power storage device.

FIG. 1 is a schematic diagram showing an example of an electricity storage device 10. FIG. 2 is a schematic diagram showing an example of an electricity storage device 10A. FIG. 4 is a schematic diagram showing an example of an electricity storage device 10B. Observation photograph of a free-standing film-like positive electrode. Cross-sectional SEM image of a cylindrical negative electrode. Curvature test results for cylindrical negative electrodes. X-ray diffraction measurement results for Experimental Examples 1 to 3. FIG. 11 is a graph showing the relationship between the aspect ratio and the diffraction peak intensity ratio in Experimental Examples 1 to 3. 1 is an explanatory diagram of an opposed arrangement structure and a parallel arrangement structure, and measurement results of battery characteristics. 1 is an explanatory diagram of cells of an embodiment and a comparative example, and a result of examining mass energy density.

The power storage device of the present disclosure described in the embodiment includes a positive electrode, a negative electrode, and a separator. The power storage device may include a positive electrode collector electrically connected to the positive electrode and a negative electrode collector electrically connected to the negative electrode. The 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 in the power storage device include alkali metal ions such as lithium ions, sodium ions, and potassium ions, and Group 2 ions such as magnesium ions, strontium ions, and calcium ions. The power storage device may also include an electrolyte in one or more of the positive electrode, the negative electrode, and the separator. The positive electrode and the negative electrode may have a current collecting member such as a current collecting wire embedded therein, or may not have this current collecting member. Here, for convenience of explanation, a lithium ion secondary battery using lithium ions as a carrier will be described below as a main example.

Here, the electric storage device disclosed in this embodiment will be described with reference to the drawings. FIG. 1 is a schematic diagram showing an example of an electric storage device 10. FIG. 2 is a schematic diagram showing an example of an electric storage device 10A. FIG. 3 is a schematic diagram showing an example of an electric storage device 10B. The electric storage device 10 includes a positive electrode 11, a separator 12, and a negative electrode 13. The positive electrode 11 has an end face 23 including a short side 24 that is shorter than the long side 25 in a direction intersecting with a first surface 21 including a long side 25, and includes a positive electrode active material 26. The separator 12 is adjacent to at least the end face 23 of the positive electrode 11. The negative electrode 13 has an end face 33 including a short side 34 that is shorter than the long side 35 in a direction intersecting with a first surface 31 including a long side 35, and includes a carbon material oriented in a predetermined orientation direction along the first surface 31 as a negative electrode active material 36, and this end face 33 faces the end face 33 of the positive electrode 11 via the separator 12.

The positive electrode 11 is a flat-shaped member that contains a positive electrode active material 26 and is adjacent to the negative electrode 13 via the separator 12. The positive electrode 11 may have a short side 24 in the thickness direction and a long side 25 in the direction aligned with the negative electrode 13. The long side 25 may be shorter than the length in the axial direction perpendicular to the alignment direction (FIG. 2) or longer (FIG. 3) as long as it is longer than the short side 24. The positive electrode 11 may be a free-standing film having flexibility. Here, "free-standing" means a structure that maintains its shape without collapsing even without any substrate or support, and has a strength sufficient to allow handling. The short side 24 as the thickness of the positive electrode 11 corresponds approximately to the thickness of the electric storage device 10, and may be, for example, 400 μm or less, more preferably 200 μm or less, even more preferably 100 μm or less, or 80 μm or less. The long side 25 of the positive electrode 11 may be, for example, 250 μm or more, 500 μm or more, or 1 mm or more.

The positive electrode 11 may be made of a positive electrode mixture obtained by mixing, for example, a positive electrode active material, a conductive material, and, if necessary, a binder. The positive electrode active material may be, for example, a material capable of absorbing and releasing lithium, which is a carrier. The positive electrode active material may be, for example, a compound having lithium and a transition metal, such as an oxide containing lithium and a transition metal element, or a phosphate compound containing lithium and a transition metal element. Specifically, lithium manganese composite oxides having a basic composition formula of Li _{(1-x) MnO2} (0≦x≦1, etc., the same applies below) or Li _{(1-x) Mn2O4 , etc.} , lithium cobalt composite oxides having a basic composition formula of Li _{(1-x) CoO2} , etc., lithium nickel composite oxides having a basic composition formula of Li _{(1-x) NiO2} , etc., lithium _cobalt nickel manganese composite oxides having a basic composition formula of Li _{(1-x) CoaNibMncO2} (a>0, _b >0, c>0, a+b+c=1), Li _{(1-x) CoaNibMncO4} ( ₀ <a<1, 0<b _{< 1 ,} 1≦c<2, a+b+c= ₂ ), etc., lithium vanadium composite oxides having a basic composition formula of _LiV2O3 , _etc. , ₅ , etc. can be used. In addition, a lithium iron phosphate compound having a basic composition formula of LiFePO ₄ , etc. can be used as the positive electrode active material. Among 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 preferred. The term "basic composition formula" means that other elements, such as Al and Mg, may be included.

The conductive material contained in the positive electrode 11 is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance, and for example, a mixture of one or more of graphite such as natural graphite (scale graphite, flake graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used. As shown in FIG. 1, the positive electrode 11 contains expanded graphite as the conductive material 27, and it is preferable that the expanded graphite is oriented in a direction along the first surface 21 and the second surface 22. In this positive electrode 11, the electronic conductivity in the positive electrode mixture can be improved. Here, "orientation" means that the flat conductive material 27 is aligned along the major axis direction, and there may be conductive materials that are inclined in various directions. The binder serves to hold the positive electrode active material 26 and the conductive material 27 together to maintain a predetermined shape. For example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluorine rubber, thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR) can be used alone or as a mixture of two or more kinds. In addition, aqueous binders such as cellulose-based and aqueous dispersions of styrene butadiene rubber (SBR) can also be used.

In the positive electrode 11, the content of the positive electrode active material is preferably higher, and is preferably 70% by mass or more, and more preferably 80% by mass or more, relative to the total mass of the positive electrode 11. 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, relative to the total mass of the positive electrode 11. In such a range, the decrease in battery capacity can be suppressed and sufficient conductivity can be imparted. In addition, the content of the binder is preferably in the range of 0.1% by mass or more and 5% by mass or less, and more preferably in the range of 0.2% by mass or more and 3% by mass or less, relative to the total mass of the positive electrode 11.

The separator 12 is a member interposed between the positive electrode 11 and the negative electrode 13 to provide insulation. The thickness of the separator 12 is preferably thicker from the viewpoint of preventing short circuits, and is preferably thinner from the viewpoint of energy density, etc. The thickness of the separator 12 is preferably 2 μm or more, more preferably 3 μm or more, and may be 5 μm or more. The thickness of the separator 12 is preferably 30 μm or less, more preferably 20 μm or less, and may be 10 μm or less. The separator 12 can be made of any material that can withstand the range of use of the secondary battery, without any particular limitation. For example, polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, Examples of the material include fluorine-based resins such as vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, and vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, polyethers such as polyethylene oxide and polypropylene oxide, celluloses such as carboxymethyl cellulose and hydroxypropyl cellulose, polymer compounds mainly composed of poly(meth)acrylic acid and other esters, derivatives thereof, and films made of copolymers or mixtures thereof. These may be used alone or in combination. These films may also contain additives that increase ion conductivity or various additives that increase strength and corrosion resistance. Of these microporous films, polyethylene, polypropylene, polyvinylidene fluoride, polysulfone, and the like are preferably used. It is preferable that the separator is made microporous so that the nonaqueous electrolyte can penetrate and ions can easily pass through.

As shown in Figures 1 to 3, the electric storage device 10 preferably has a structure in which the positive electrode 11, the separator 12, and the negative electrode 13 are repeated in the arrangement direction. That is, the separator 12 may be continuously adjacent to the first surface 21 of the positive electrode 11, the end surfaces 23, 33 of the positive electrode 11 and the negative electrode 13, and the second surface 32 opposite to the first surface 31 of the negative electrode 13. Similarly, the separator 12 may be continuously adjacent to the second surface 22 of the positive electrode 11, the end surfaces 23, 33 of the positive electrode 11 and the negative electrode 13, and the first surface 31 opposite to the second surface 32 of the negative electrode 13. In this way, a structure in which the front surface of the positive electrode 11 and the back surface of the negative electrode 13 are continuously adjacent to each other can be made flexible and curved, which is preferable. In addition, the separator 12 is not particularly limited to this structure, as long as it is interposed between the end surface 23 of the positive electrode 11 and the end surface 33 of the negative electrode 13, and may be adjacent to either the first surface 21 or the second surface 22 of the positive electrode 11 or the first surface 31 or the second surface 32 of the negative electrode 13, or may not be adjacent to any one or more of them.

The negative electrode 13 has an end surface 33 including a short side 34 shorter than the long side 35 in a direction intersecting with the first surface 31 including the long side 35, and contains a carbon material oriented in a predetermined orientation direction along the first surface 31 as the negative electrode active material 36. The negative electrode 13 also faces the end surface 23 of the positive electrode 11 via the separator 12 at the end surface 33. The negative electrode 13 is a flat-shaped member, and the cross section perpendicular to the axial direction may be a columnar member having a flat shape. Here, "columnar" includes not only those with a thickness that does not bend, but also those with a fibrous thickness that can be bent. The cross section may be, for example, rectangular, or may be a shape in which the corners of a rectangle are formed in an arc shape, or may be elliptical. The negative electrode 13 may have a short side 34 in the thickness direction, and the alignment direction with the positive electrode 11 may be the long side 35. In addition, the long side 35 may be shorter (FIG. 2) or longer (FIG. 3) than the length in the axial direction perpendicular to the alignment direction, as long as it is longer than the short side 34. The negative electrode 13 may be flexible. The negative electrode 13 has a flat cross section in a direction perpendicular to the axial direction, and the short side 34 of the thickness of the negative electrode 13 corresponds to the thickness of the electricity storage device 10, and is preferably 400 μm or less, more preferably 200 μm or less, and even more preferably 100 μm or less, and may be 80 μm or less. The long side 35 of the negative electrode 13 may be, for example, 250 μm or more, 300 μm or more, or 600 μm or more.

The negative electrode 13 includes a carbon material oriented in a predetermined orientation direction along the first surface 31. Here, "orientation" means that the flat carbon material is aligned along the direction of the major axis, and may be inclined in various directions. The negative electrode active material 36 may have flat surfaces arranged parallel to the surface direction of the first surface 31 and/or the second surface 32. The presence or absence of orientation can be determined by observing the cross section with a microscope. The degree of orientation of the negative electrode 13 can be measured by X-ray diffraction in a state where the horizontal direction is the major axis and the vertical direction is the minor axis, and the 002/110 peak intensity ratio of the 002 diffraction peak (26.5 °) and the 110 diffraction peak (77.5 °) of graphite can be used as an index of the degree of orientation. The larger the 002/110 peak intensity ratio, the higher the orientation, and it is preferable that it is 300 or more, more preferably 500 or more, even more preferably 1000 or more, and most preferably 1500 or more. In addition, the 002/110 peak intensity ratio may be set to 5000 or less due to the limit of the degree of orientation.

The negative electrode 13 may have a flat shape with an average aspect ratio b/a of 3 or more when the short side is a and the long side is b. The "average" does not refer to the absolute value of each negative electrode 13, but rather to an evaluation based on an average value of measurements of multiple negative electrodes 13. The average value may be, for example, the average value of measurements taken by selecting any 10 negative electrodes 13. The average value of this aspect ratio b/a is more preferably 4 or more, and may be 5 or more. The average value of this aspect ratio b/a may be 20 or less, or may be 10 or less.

The negative electrode 13 includes a carbon material as the negative electrode active material 36. Examples of the carbon material include one or more of graphites, cokes, glassy carbons, non-graphitizable carbons, and pyrolytic carbons. Among these, graphites such as artificial graphite and natural graphite are preferred, and flake graphite is particularly preferred due to its high orientation. The negative electrode 13 is preferably a mixture of a carbon material and a binder, which is then molded and compacted. Alternatively, the negative electrode 13 is preferably a mixture of a carbon material, a binder, and a plasticizer, which is then compacted. In particular, the negative electrode 13 can be extruded into a cylindrical or regular polygonal columnar shape, and then compacted using a hydraulic press or the like to give it a flat shape. The pressing pressure may be, for example, 5 MPa or more, 10 MPa or more, or 50 MPa or less per unit area. The negative electrode 13 may be heated in air at a temperature range of 150° C. to 250° C. to remove the plasticizer, or may be heated in an inert atmosphere at a temperature range of 800° C. to 1500° C. to carbonize the resin component. The heating time may be set appropriately depending on the components contained, and may be in the range of 0.5 hours to 24 hours. The binder is preferably one having carrier ion conductivity, such as polyvinylidene fluoride (PVdF), a copolymer of PVdF and hexafluoropropylene (PVdF-HFP), polymethyl methacrylate (PMMA), and a copolymer of PMMA and an acrylic polymer. The amount of the binder is preferably 10% by mass or less, and may be 5% by mass or less. Examples of the plasticizer include phthalates such as dioctyl phthalate (DOP) and dibutyl phthalate (DPB), epoxidized vegetable oils such as epoxidized soybean oil and epoxidized linseed oil, and polyester-based plasticizers such as polyesters of dibasic acids (adipic acid, sebacic acid, phthalic acid, etc.) and glycols (1,2-propanediol, butanediol, etc.). Of these, dioctyl phthalate is preferred as the plasticizer. The amount of the plasticizer may be in the range of 5% by mass to 25% by mass with respect to the negative electrode mixture, for example.

The electricity storage device 10 may include an electrolyte that conducts carrier ions to the positive electrode 11, the separator 12, and the negative electrode 13. Examples of the electrolyte include non-aqueous solvents. Examples of the solvent for the electrolyte include solvents for non-aqueous electrolytes. Examples of the solvent include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and 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, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. The electrolyte may have dissolved therein a supporting salt containing ions that are carriers for the electricity storage device 10. Examples of supporting salts include _LiPF6 , _{LiBF4 , LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3 , LiSbF6 , LiSiF6 , LiAlF4 , LiSCN , LiClO4 , LiCl} , LiF, LiBr, LiI, and _LiAlCl4 . Among these, it is preferable from the _viewpoint of electrical properties to use a combination of _{one or} more salts selected from the group consisting of inorganic salts such as _LiPF6 , _LiBF4 , _LiClO4 , and organic salts such as _LiCF3SO3 , LiN( _CF3SO2 ) ₂ , LiC( _CF3SO2 ) ₃ . The concentration of this supporting salt in the electrolyte 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.

In this energy storage device 10, the mass energy density is preferably higher, for example, preferably 80 mAh/g or more, more preferably 90 mAh/g or more, and even more preferably 100 mAh/g or more. In this energy 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 to 1.5, more preferably 1.2 or less.

In the embodiment described above, it is possible to provide a lightweight, thin, and highly flexible electric storage device 10. The reason why such an effect is obtained is presumed to be, for example, as follows. In the negative electrode 13 containing a carbon material, by making it into a flat shape in which the b/a value exceeds 1 when the short side is a and the long side is b, the carbon material can be oriented along the surface direction of the first surface 31 and/or the second surface 32, which have a large area. This structural feature alleviates stress concentration on the surface layer and disperses internal stress when the negative electrode 13 is curved, making it possible to curve the cell itself, and improving the freedom of the cell structure. In addition, the presence of the positive electrode 11 on the end surface 33 in the orientation direction of the negative electrode 13 makes it possible to smoothly insert/remove carrier ions into the carbon material and diffuse within the negative electrode 13, thereby achieving high input/output. In addition, by using a carbon material as a negative electrode active material and current collector, charging and discharging are possible without using a metal current collector, and it is also possible to increase the mass energy density of the electric storage device 10.

In addition, in the electricity storage device 10, the positive electrode 11 is a free-standing film and contains expanded graphite particles as a conductive material, which improves the electronic conductivity within the plane of the positive electrode 11 and increases the strength of the positive electrode 11 as a free-standing film. This makes it possible to further increase the mass energy density of the electricity storage device 10 without using a current collector for the positive electrode 11. Furthermore, when the electricity storage device 10 is a small-capacity cell with a thickness of 200 μm or less, in a conventional electrode facing structure (see Figure 8B described below), the contribution of the current collecting foil is large, so the mass energy density of the electricity storage device 10 is significantly reduced, but the battery structure of the electricity storage device 10 makes it possible to significantly suppress the reduction in mass energy density.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and may be implemented in various forms as long as they fall within 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 is not limited thereto, and may be alkali metal ions such as sodium ions and potassium ions, or Group 2 element ions such as calcium ions and magnesium ions. The positive electrode active material may contain carrier ions. The electrolyte is a non-aqueous electrolyte, but may be an aqueous electrolyte.

In the above-described embodiment, the positive electrode active material is a transition metal composite oxide, but is not particularly limited thereto, and may be, for example, a carbon material used in a capacitor. The carbon material is not particularly limited, but may be, for example, activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, polyacenes, and the like. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as the carbon material preferably has a specific surface area of 1000 m ² /g or more, 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 preparation. In addition, the positive electrode is considered to store electricity by adsorbing and desorbing at least one of anions and cations contained in the ion conductive medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ion conductive medium.

The following describes the results of concretely fabricating the above-mentioned energy storage device and examining its structure and performance as experimental examples. Note that experimental example 3 corresponds to the working example, and experimental examples 1 and 2 correspond to reference examples.

(Fabrication of columnar electrodes and evaluation of their bendability)
Graphite flakes as an electrode active material and polyvinyl chloride as a binder were mixed at a mass ratio of 90:10. Dioctyl phthalate was mixed as a plasticizer to this mixed powder at a mass ratio of 20%, kneaded, and then molded into a columnar shape by melt extrusion. A flattened columnar graphite electrode precursor was obtained by performing a consolidation treatment before cooling and solidifying. The size before the consolidation treatment was 175 μm in diameter. The consolidation treatment was performed under two conditions, low pressure (1 MPa) and high pressure (5 MPa), using a hydraulic press after five columnar graphite electrode precursors cut to a length of 70 mm were aligned in parallel, and three types of columnar graphite electrode precursors, untreated (A), 2 MPa (B), and 10 MPa (C), were obtained. In order to remove the plasticizer, heat treatment was performed in air at 180 ° C. for 5 hours. Then, in order to carbonize the resin component, firing was performed in nitrogen at 1000 ° C. for 1 hour. The obtained columnar graphite electrodes of levels A to C were designated as Experimental Examples 1 to 3, respectively, and their cross sections were observed by SEM using a scanning electron microscope (S-3600N manufactured by Hitachi High-Technologies Corporation) to evaluate the short side a (μm) and long side b (μm). An electrode with a length of 60 mm was fixed to a polypropylene plate and curved so that both ends had a width of 40 mm, after which the shape of the electrode was confirmed and the curvature of the columnar graphite electrode was evaluated.

(Evaluation of the degree of orientation of columnar electrodes)
The columnar graphite electrodes were subjected to X-ray diffraction measurement to examine the degree of orientation. Using an X-ray diffraction measuring device (Ultima IV manufactured by Rigaku), measurements were performed under the measurement conditions of 2θ = 10° to 90°, a scanning speed of 5°/min, and a sampling width of 0.02°. The samples were measured in a state where seven columnar graphite electrodes of the above levels A to C were arranged on a Si non-reflective plate, and the flat products were arranged in parallel so that the horizontal direction was the long axis and the vertical direction was the short axis. The 002/110 peak intensity ratio, which is the ratio of the 002 diffraction peak (26.5°) of graphite to the 110 diffraction peak (77.5°), was used as an index of the degree of orientation.

(Battery characteristic evaluation)
<Preparation of self-supporting positive electrode film>
A positive electrode active material (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ), a conductive material (carbon black and expanded graphite powder (average particle size 7 μm), and a binder (PVdF7305 manufactured by Kureha) were mixed in a mass ratio of 90:4:2:4, and N-methylpyrrolidone was added to the mixture to prepare a positive electrode composite paste. This positive electrode composite was applied to an Al foil (thickness 12 μm), and the obtained positive electrode was peeled off from the Al foil to obtain a positive electrode freestanding film. FIG. 4 shows an observation photograph of a freestanding film-like positive electrode, where FIG. 4A shows the result after coating and drying, FIG. 4B shows a photograph confirming the flexibility of the freestanding film after preparation, and FIG. 4C shows a photograph of the Al foil peeling treatment. In addition, when the obtained positive electrode was observed from the outside with a SEM, the SEM image showed that the expanded graphite was distributed along the planar direction. From this, it was expected that the expanded graphite was oriented in the planar direction in the freestanding film.

<Cell production and evaluation>
Using a columnar graphite electrode (level C) and a positive electrode freestanding film, a cell was prepared in a geometry (opposite arrangement structure or parallel arrangement structure) as shown in FIG. 7A. In the opposed arrangement structure, the second surface of the columnar graphite negative electrode and the first surface of the positive electrode freestanding film were arranged to face each other through a separator. In the parallel arrangement structure, the end surface of the columnar graphite negative electrode and the end surface of the positive electrode freestanding film were arranged to face each other through a separator. The separator used was made of polyethylene with a thickness of 10 μm and a porosity of 50%. This electrode structure was housed in a case, and an Al foil (thickness 100 μm) was attached as a positive electrode current collector to the end surface of the positive electrode, and a copper foil (thickness 100 μm) was attached as a negative electrode current collector to the end of the columnar graphite negative electrode, and the copper foil was pressed and fixed. A nonaqueous electrolyte was poured into this case and sealed to obtain an electricity storage device, which was used as a test cell. The non-aqueous electrolyte was a 1M solution of _LiPF6 in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30/40/30. The design capacity of this test cell was 1.1 mAh. The obtained test cell was charged at a constant current and constant voltage to 4.1 V at a current value of 0.05 mA, and then discharged at a constant current and constant voltage to 2 V at a current value of 0.05 mA. Thereafter, the voltage was adjusted to 3.7 V, and impedance measurement was performed at 25 ° C. The impedance measurement was performed using a measuring device (Celltest manufactured by Solartron).

<Estimated change in electrode thickness and mass energy density>
Using the physical properties of common electrode materials shown in Table 2 and the cell configuration shown in FIG. 8A, the mass energy density of the cell was estimated when the thickness of the electrode body of the cell of the present invention and the cell of the conventional structure was changed in the range of 50 to 600 μm.

(Results and discussion)
(Orientation and curvature of columnar graphite electrodes)
FIG. 5 is a cross-sectional SEM image of the flattened columnar negative electrode. FIG. 6 is a result of the bending test of the columnar negative electrode. Table 1 also summarizes the measured values of the short side and long side, and the results of the bending test. Table 1 shows the average values measured by arbitrarily selecting 10 negative electrodes. As shown in Table 1 and FIG. 5, it was found that in the columnar graphite electrode flattened by compaction treatment, the flake graphite particles are oriented along the long side direction. The highest compaction level C showed an aspect ratio of 3 or more. Also, as shown in Table 1 and FIG. 6, it was found that when the aspect ratio b/a is 3 or more, bending does not cause breakage, and flexibility can be imparted.

Figure 7 shows the results of XRD measurements of the columnar graphite electrodes of Experimental Examples 1 to 3, with Figure 7A showing an overall view, Figure 7B showing the 002 diffraction peak, and Figure 7C showing the 110 diffraction peak. Figure 8 shows the relationship between the aspect ratio and the 002/110 diffraction peak intensity ratio. Table 1 shows the diffraction peak intensities and their ratios. As shown in Table 1 and Figures 7 and 8, as the aspect ratio b/a increases, the 002 diffraction peak intensity tends to increase and the 110 diffraction peak tends to decrease, resulting in a larger 002/110 diffraction peak intensity ratio, i.e., a higher degree of orientation. It was inferred that the 002/110 diffraction peak intensity ratio is preferably 300 or more, more preferably 500 or more, even more preferably 1000 or more, and most preferably 1500 or more.

(Test cell battery characteristic evaluation results)
FIG. 9 is an explanatory diagram of the facing arrangement structure and the parallel arrangement structure, and the battery characteristic measurement results, FIG. 9A is an explanatory diagram of the battery structure, FIG. 9B is a charge/discharge curve, and FIG. 9C is an impedance measurement result. As a result of evaluating the evaluation cell in which the electrodes were arranged differently as shown in FIG. 9A, the facing arrangement structure had a low charge/discharge capacity despite the large positive and negative electrode facing area, whereas the parallel arrangement structure showed a charge/discharge capacity close to the design value. It was presumed that this was due to the structure of the negative electrode in which lithium ions cannot be inserted and removed from the surface side of the oriented scale-like graphite in the negative electrode, while they are easily inserted and removed from the end side. In particular, in the parallel arrangement structure, despite the small contact area between the positive electrode and the negative electrode, a capacity close to the theoretical capacity was shown, and therefore it was presumed that the rate characteristics were also good. In addition, it was revealed from the impedance measurement results that in the parallel arrangement structure, the electronic resistance on the high frequency side and the charge transfer resistance on the low frequency side were significantly reduced compared to the facing arrangement structure. It was presumed that this is also because the parallel arrangement structure allows ions to be smoothly transferred between the positive electrode and the negative electrode.

(Mass Energy Density)
FIG. 10 shows an example cell (FIG. 10A), an explanatory diagram of a comparative example cell (FIG. 10B), and the results of a study on the relationship between mass energy density and battery thickness (FIG. 10C). As a result of calculating the change in mass energy density depending on the thickness of the battery structure for the example cell using a columnar graphite electrode and a positive electrode freestanding film and the comparative example cell with a conventional facing structure, it was found that the mass energy density is significantly high when the thickness of the battery structure is 400 μm or less, further 200 μm or less, and particularly when the thickness is 100 μm or less. In the example cell, in the case of a small capacity cell with a thickness of 200 μm or less, in the conventional electrode facing structure, the contribution of the current collecting foil is large, so that the mass energy density of the battery is significantly reduced, but it was found that the structure of the example cell can significantly suppress the decrease in mass energy density. In the cell of this example, the entire cell has flexibility and the current collector can be omitted, so that a lightweight and thin battery structure can be obtained, and it was inferred that it is promising for applications such as wearable batteries that do not require high capacity at a relatively low rate.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and may be implemented in a variety of ways as long as they fall within the technical scope of the present disclosure.

10, 10A, 10B: Electric storage device, 11: Positive electrode, 12: Separator, 13: Negative electrode, 21: First surface, 22: Second surface, 23: End surface, 24: Short side, 25: Long side, 26: Positive electrode active material, 27: Conductive material, 31: First surface, 32: Second surface, 33: End surface, 34: Short side, 35: Long side, 36: Negative electrode active material, a: Short side, b: Long side, x: Thickness.

Claims (6)

a positive electrode having an end face including a short side shorter than the long side in a direction intersecting with a first surface including the long side, the positive electrode including a positive electrode active material;
a separator adjacent to at least an end surface of the positive electrode;
a negative electrode having an end face including a short side shorter than the long side in a direction intersecting a first surface including a long side, the negative electrode including a carbon material oriented in a predetermined orientation direction along the first surface as a negative electrode active material , the end face facing an end face of the positive electrode via the separator ,
The negative electrode has an average aspect ratio b/a of the short side a to the long side b along the arrangement direction of the positive electrode of 3 or more,
The positive electrode is a free-standing film and contains expanded graphite particles in addition to the positive electrode active material . The power storage device according to claim 1 , wherein the negative electrode contains flake graphite as the negative electrode active material. The power storage device according to claim 1 , wherein the positive electrode and the negative electrode have a thickness of 200 μm or less. The power storage device according to any one of claims 1 to 3, wherein the separator is continuously adjacent to a first surface of the positive electrode, end faces of the positive electrode and the negative electrode, and a second surface of the negative electrode opposite to the first surface. The power storage device according to any one of claims 1 to 4, wherein the power storage device has a flexible structure that is bendable. The power storage device according to any one of claims 1 to 5, wherein the power storage device does not include a support member that contacts and supports the positive electrode, the negative electrode, and the separator.