Electricity storage device and electrode for electricity storage device

Abstract

The purpose of the present technology is to provide an electrode for power storage device and a power storage device that make it possible to involve more lithium ions in a charge-discharge reaction. A lithium-ion secondary battery (LiB1) has: a positive electrode current collector (P1); a positive electrode active material layer (P2) on the positive electrode current collector (P1); a negative electrode current collector (N1); and a negative electrode active material layer (N2) on the negative electrode current collector (N1). The negative electrode active material layer (N2) has a carbon nanowall (CNW1). The carbon nanowall (CNW1) is capable of involving, in the charge-discharge reaction, two or more lithium ions per carbon atom in a single charge or discharge.

Description

[Technical Field]

[0001] The technical field of the present specification relates to an electricity storage device and an electrode for the electricity storage device that uses a carbon material. [Background technology]

[0002] Examples of chargeable and dischargeable electricity storage devices include secondary batteries, electric double layer capacitors, etc. Furthermore, examples of electricity storage devices that utilize lithium ions include lithium ion secondary batteries, lithium ion primary batteries, and lithium ion capacitors.

[0003] For example, Patent Document 1 discloses a lithium-ion secondary battery having a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. It discloses a technology using lithium cobalt oxide or lithium nickel oxide as the positive electrode active material and carbon as the negative electrode active material (claims and examples of Patent Document 1). Graphite is often used as the carbon material. Graphite can absorb or release one lithium ion per six carbon atoms in a six-membered ring. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 2668678 Summary of the Invention [Problem to be solved by the invention]

[0005] Currently, the maximum weight energy density of lithium-ion secondary batteries is approximately 250 Wh / kg. If the weight energy density of secondary batteries is improved, for example, the output and driving range of electric vehicles will be improved. Furthermore, electronic devices can be operated for a longer period of time. To this end, it is preferable that the positive electrode active material or the negative electrode active material can absorb or release a larger number of lithium ions. Alternatively, it is preferable that a larger number of lithium ions or lithium atoms can be involved in a chemical reaction by a method other than absorption or release.

[0006] The problem to be solved by the technology of the present specification is to provide an electrode for an electricity storage device and an electricity storage device that can allow more lithium ions to participate in charge / discharge reactions. [Means for solving the problem]

[0007] The electrode for an electricity storage device according to the first aspect comprises: Made of metal foil or metal plate The battery has a current collector and an active material layer on the current collector. It has a thin film structure mainly composed of six-membered ring carbon It has carbon nanowalls. The average angle between the surface of the current collector and the carbon nanowalls is 80 degrees or more and 90 degrees or less. Carbon nanowalls can involve two or more lithium ions per carbon atom in a charge-discharge reaction during a single charge or discharge.

[0008] This electrode for an electricity storage device contains carbon nanowalls, which are capable of accommodating two or more lithium ions per carbon atom in a charge / discharge reaction during a single charge or discharge.

[0009] In contrast, graphite, which is used in conventional electrodes, can absorb or release one lithium ion per six carbon atoms in a six-membered ring, i.e., graphite can absorb or release 1 / 6 of a lithium ion per carbon atom.

[0010] Electrodes for power storage devices using carbon nanowalls can involve 12 times more lithium ions in charge / discharge reactions than electrodes using graphite. Therefore, power storage devices with electrodes using carbon nanowalls have excellent volumetric energy density and weight energy density. Therefore, with a single charge, these power storage devices can operate electronic devices, home appliances, vehicles, etc. for long periods of time. [Effects of the Invention]

[0011] The present specification provides an electrode for an electricity storage device and an electricity storage device that can allow more lithium ions to participate in charge / discharge reactions. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic configuration diagram of a lithium-ion secondary battery LiB1 of a first embodiment. [Figure 2] 2 is a diagram conceptually showing the structure of a carbon nanowall CNW1 of the lithium ion secondary battery LiB1 of the first embodiment. FIG. [Figure 3] 2 is a diagram schematically illustrating a cross section of a carbon nanowall CNW1 of the lithium ion secondary battery LiB1 according to the first embodiment. FIG. [Figure 4] FIG. 2 is a diagram schematically showing the inclination of carbon nanowalls in the lithium ion secondary battery LiB1 of the first embodiment. [Figure 5] FIG. 2 is a view of carbon nanowalls of the lithium ion secondary battery LiB1 of the first embodiment, viewed from a direction perpendicular to the surface of the negative electrode current collector N1. [Figure 6] FIG. 10 is a diagram illustrating a case where the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowalls is small. [Figure 7] 1 is a diagram hypothetically showing how lithium ions are absorbed by the carbon nanowalls CNW1 of the lithium ion secondary battery LiB1 of the first embodiment and how lithium or a lithium compound is deposited. FIG. [Figure 8]1 is a schematic diagram showing the configuration of a manufacturing apparatus for growing carbon nanowalls CNW1 in the lithium ion secondary battery LiB1 of the first embodiment. FIG. [Figure 9] FIG. 10 is a schematic configuration diagram of a lithium ion capacitor LiC1 according to a second embodiment. [Figure 10] FIG. 10 is a schematic configuration diagram of a lithium ion capacitor LiC2 according to a modified example of the second embodiment. [Figure 11] 1 is a micrograph of carbon nanowalls viewed from a direction perpendicular to the surface of a metal plate. [Figure 12] 1 is a micrograph of a carbon nanowall showing a cross section perpendicular to the surface of a metal plate. [Figure 13] 1 is a graph showing the relationship between the capacity and voltage of a lithium ion secondary battery using carbon nanowalls with a height of 1 μm in the negative electrode. [Figure 14] 1 is a graph showing the relationship between the capacity and voltage of a lithium ion secondary battery using carbon nanowalls with a height of 4 μm in the negative electrode. [Figure 15] 1 is a graph showing the relationship between the capacity and voltage of a lithium ion secondary battery using carbon nanowalls with a height of 10 μm in the negative electrode. [Figure 16] 1 is a graph showing the relationship between the capacity and voltage of a lithium ion secondary battery using graphite in the negative electrode. [Figure 17] FIG. 1 is a graph comparing the charging voltages of lithium ion secondary batteries using carbon nanowalls and graphite in the negative electrode. [Figure 18] 1 is a graph showing the discharge characteristics of a lithium ion secondary battery using a negative electrode having carbon nanowalls with a height of 0 nm. [Figure 19] 1 is a graph showing the discharge characteristics of a lithium ion secondary battery using a negative electrode having carbon nanowalls with a height of 20 nm. [Figure 20] 1 is a graph showing the discharge characteristics of a lithium ion secondary battery using a negative electrode having carbon nanowalls with a height of 50 nm. [Figure 21]1 is a graph showing the discharge characteristics of a lithium ion secondary battery using a negative electrode having carbon nanowalls with a height of 100 nm. [Figure 22] 1 is a graph showing the discharge characteristics of a lithium ion secondary battery using a negative electrode having carbon nanowalls with a height of 200 nm. [Figure 23] 1 is a graph showing the discharge characteristics of a lithium ion secondary battery using a negative electrode having carbon nanowalls with a height of 500 nm. [Figure 24] 1 is a scanning electron microscope photograph showing the surface of a carbon nanowall with a height of 500 nm. [Figure 25] 1 is a scanning electron microscope photograph showing the cross section of a carbon nanowall with a height of 500 nm. [Figure 26] 1 is a scanning electron microscope photograph showing the surface of a 50 nm-high carbon nanowall. [Figure 27] 1 is a scanning electron microscope photograph showing the cross section of a carbon nanowall with a height of 50 nm. [Figure 28] This is a scanning electron microscope photograph (part 1) showing carbon nanowalls after repeated charging and discharging. [Figure 29] This is a scanning electron microscope photograph (part 2) showing carbon nanowalls after repeated charging and discharging. [Figure 30] This is a scanning electron microscope photograph (part 3) showing carbon nanowalls after repeated charging and discharging. [Figure 31] 1 is a scanning electron microscope photograph showing a cross section of a negative electrode of a lithium ion secondary battery after charging. [Figure 32] 1 is a graph showing the results of X-ray diffraction of the negative electrode of the lithium ion secondary battery after charging. DETAILED DESCRIPTION OF THE INVENTION

[0013] Specific embodiments will be described below with reference to the drawings, taking an electrode for a power storage device and a power storage device as examples. In this specification, a power storage device is a device that can be charged and discharged. Power storage devices include lithium ion primary batteries, lithium ion secondary batteries, lithium ion capacitors, and other devices that charge and discharge using lithium ions.

[0014] (First embodiment) 1. Lithium-ion secondary battery 1 is a schematic configuration diagram of a lithium ion secondary battery LiB1 of Embodiment 1. The lithium ion secondary battery LiB1 has a positive electrode PE, a negative electrode NE, a separator Sp1, an electrolytic solution ES1, and a container V1.

[0015] The positive electrode PE is the positive electrode of the lithium-ion secondary battery LiB1. The positive electrode PE has a positive electrode current collector P1 and a positive electrode active material layer P2. The positive electrode active material layer P2 is formed on the first surface P1a and the second surface P1b of the positive electrode current collector P1.

[0016] The positive electrode current collector P1 is, for example, a metal foil. The positive electrode current collector P1 may have other shapes. The material of the positive electrode current collector P1 is, for example, Al or Ti. The material of the positive electrode current collector P1 may also be a conductor such as another metal.

[0017] The positive electrode active material layer P2 contains a positive electrode active material, a conductive additive, and a binder. The positive electrode active material layer P2 may also contain a thickener. Examples of positive electrode active materials include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, and ternary materials. Examples of conductive additives include carbon black. Examples of binders include SBR. Examples of thickeners include carboxymethyl cellulose. Thus, the positive electrode active material layer P2 contains lithium atoms.

[0018] The negative electrode NE is a negative electrode of the lithium-ion secondary battery LiB1. The negative electrode NE includes a negative electrode current collector N1 and a negative electrode active material layer N2. The negative electrode active material layer N2 is formed on the first surface N1a and the second surface N1b of the negative electrode current collector N1.

[0019] The negative electrode current collector N1 is, for example, a metal foil. The negative electrode current collector N1 may have another shape. The material of the negative electrode current collector N1 is, for example, Cu. The material of the negative electrode current collector N1 may also be a conductor such as another metal.

[0020] The negative electrode active material layer N2 contains a negative electrode active material. The negative electrode active material layer N2 contains carbon nanowalls CNW1 as the negative electrode active material. The carbon nanowalls CNW1 will be described later.

[0021] The separator Sp1 serves to electrically insulate the positive electrode PE from the negative electrode NE, and is permeable to lithium ions in the electrolyte solution ES1.

[0022] The electrolyte ES1 has the property of transferring lithium ions between the positive electrode PE and the negative electrode NE. The electrolyte ES1 fills the container V1. The electrolyte ES1 is a liquid obtained by dissolving a lithium salt such as lithium hexafluorophosphate (LiPF6) in dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC).

[0023] The container V1 accommodates the positive electrode PE, the negative electrode NE, the separator Sp1, and the electrolyte solution ES1 inside. The container V1 is made of a material that is less reactive with the electrolyte solution ES1.

[0024] 2. Carbon nanowalls In this specification, a carbon nanowall is a conductive nanostructure composed mainly of carbon atoms arranged in a wall shape on a substrate such as the negative electrode current collector N1.

[0025] FIG. 2 is a diagram conceptually illustrating the structure of the carbon nanowall CNW1 of the lithium-ion secondary battery LiB1 of the first embodiment. FIG. 2 conceptually illustrates one graphene sheet GS1. The carbon nanowall CNW1 is electrically conductive. The carbon nanowall CNW1 may be composed of multiple graphene sheets GS1. The graphene sheet GS1 may not have a complete graphene structure, but may be a thin film mainly composed of carbon with a six-membered ring structure. The graphene sheet GS1 may have a mosaic structure mainly composed of carbon with a six-membered ring structure. A mosaic structure is a structure in which multiple regions having a six-membered ring structure are discretely arranged. In other words, the carbon nanowall CNW1 does not have to be entirely single-crystal six-membered rings.

[0026] The negative electrode NE includes a negative electrode current collector N1 and a negative electrode active material layer N2, which includes an amorphous carbon layer AC1 and carbon nanowalls CNW1.

[0027] The carbon nanowall CNW1 is a graphite-like material in which about 10 layers of graphene sheets GS1 are stacked in the thickness direction of the carbon nanowall CNW1. The number of layers may be other than the above. Because the carbon nanowall CNW1 is a graphite-like material, the carbon nanowall CNW1 has a higher electrical conductivity than carbon materials such as activated carbon.

[0028] The amorphous carbon layer AC1 is located between the negative electrode current collector N1, which is a conductor such as a metal, and the carbon nanowall CNW1. The amorphous carbon layer AC1 is a layer that can be the starting point for the growth of the graphene sheet GS1 that constitutes the carbon nanowall CNW1. The film thickness of the amorphous carbon layer AC1 is, for example, 10 nm or more and 300 nm or less, preferably 10 nm or more and 100 nm or less, and more preferably 12 nm or more and 30 nm or less. Note that depending on the carbon nanowall growth method, the amorphous carbon layer AC1 may not be necessary. The amorphous carbon layer AC1 is conductive.

[0029] The carbon nanowall CNW1 has a root portion R1 on the side of the negative electrode current collector N1 and a tip portion E1 on the opposite side of the negative electrode current collector N1. The root portion R1 is often a fixed portion that is fixed to the negative electrode current collector N1 via an amorphous carbon layer AC1. The root portion R1 is also a connecting portion that is electrically connected to the negative electrode current collector N1 or the amorphous carbon layer AC1.

[0030] In the carbon nanowall CNW1, the graphene sheet GS1 is formed in a direction intersecting the surfaces (first surface N1a and second surface N1b) of the negative electrode current collector N1. In FIG. 2, the graphene sheet GS1 and the negative electrode current collector N1 are almost perpendicular to each other. Therefore, the tip of the graphene sheet GS1 has a tip portion E1. The tip portion E1 is the portion located at the tip of the graphene sheet GS1.

[0031] As mentioned above, the carbon nanowall CNW1 is a graphite layer consisting of many stacked graphene sheets GS1. In reality, the graphene sheets GS1 do not extend completely parallel to each other. Because the graphene sheets GS1 grow in different directions at each initial growth nucleus, the graphene sheets GS1 actually merge and overlap at random (see Figure 11). As shown in Figure 2, the distance between adjacent wall-like graphite particles is referred to as the wall spacing.

[0032] The average wall spacing D1, which is the average value of the wall spacing, is related to the density of the carbon nanowalls CNW1. In other words, the wider the average wall spacing D1, the lower the density of the carbon nanowalls CNW1. Conversely, the narrower the average wall spacing D1, the higher the density of the carbon nanowalls CNW1.

[0033] 2-1. Wall size 3 is a diagram schematically illustrating a cross section of the carbon nanowalls CNW1 of the lithium-ion secondary battery LiB1 of the first embodiment. The average height H1 of the carbon nanowalls CNW1 is preferably 100 nm or more. Alternatively, the average height H1 of the carbon nanowalls CNW1 may be 200 nm or more. When the average height H1 of the carbon nanowalls CNW1 is 100 nm or more, lithium tends to precipitate starting from the carbon nanowalls CNW1.

[0034] There is no problem if the average height H1 of the carbon nanowalls CNW1 is high. However, forming tall carbon nanowalls CNW1 takes time. From the viewpoint of productivity, the average height H1 of the carbon nanowalls CNW1 is preferably 200 μm or less, more preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 10 μm or less.

[0035] Therefore, the average height H1 of the carbon nanowalls CNW1 is, for example, 100 nm or more and 200 μm or less. Preferably, it is 100 nm or more and 50 μm or less. More preferably, it is 100 nm or more and 10 μm or less. Alternatively, the average height H1 of the carbon nanowalls CNW1 may be 200 nm or more and 200 μm or less. Alternatively, it may be 200 nm or more and 100 μm or less. Alternatively, it may be 200 nm or more and 10 μm or less.

[0036] The average thickness W1 of the carbon nanowalls CNW1 is, for example, 0.5 nm or more and 100 nm or less, preferably 1 nm or more and 50 nm or less, and more preferably 1.5 nm or more and 30 nm or less.

[0037] The layer spacing of graphite is approximately 0.35 nm. Therefore, the thickness of the carbon nanowall CNW1, which is composed of 10 layers of graphene sheets GS1, is approximately 3.5 nm. Although it depends on the manufacturing conditions, the average thickness of the carbon nanowall CNW1 is thought to be approximately 3.5 nm. Typically, the carbon nanowall CNW1 is thought to be composed of 5 to 20 layers of graphene sheets GS1. The thickness of the carbon nanowall CNW1 is, for example, 1.5 nm or more and 7 nm or less.

[0038] 2-2.Wall spacing The average wall spacing D1 between adjacent carbon nanowalls CNW1 is, for example, 10 nm or more and 500 nm or less. Preferably, it is 15 nm or more and 100 nm or less. More preferably, it is 20 nm or more and 50 nm or less. These numerical ranges are examples, and other numerical values ​​may be used. Note that the long walls of the carbon nanowalls do not necessarily grow in parallel, and the walls may merge with each other (see FIG. 11). Therefore, the spacing between the carbon nanowalls CNW1 near this merging point is narrower than the spacing between the carbon nanowalls CNW1 at other points.

[0039] 2-3.Wall angle Fig. 4 is a diagram schematically showing the inclination of the carbon nanowalls in the lithium ion secondary battery LiB1 of the first embodiment, showing the carbon nanowalls CNW1 projected onto the first surface N1a of the negative electrode current collector N1.

[0040] The projected region PR1 of the carbon nanowall CNW1(a) projected onto the first surface N1a of the negative electrode current collector N1 does not include carbon nanowalls CNW1(b) other than the carbon nanowall CNW1(a). However, this does not apply to the junction of the carbon nanowalls CNW1. All that is required is that the electrolyte and lithium ions can enter the micro-region defined by the junction of the carbon nanowalls CNW1. If the electrolyte and lithium ions can enter the micro-region defined by the carbon nanowalls CNW1, the reactions necessary for the battery to operate occur in that micro-region.

[0041] As shown in Fig. 4, an intermediate region PR2 exists between the projected regions PR1. The intermediate region PR2 is a visible region that can be observed by an observer using a scanning electron microscope (SEM) or the like when viewing the negative electrode current collector N1 from the direction of arrow J1 in Fig. 4. Here, the direction of arrow J1 in Fig. 4 is perpendicular to the first surface N1a of the negative electrode current collector N1.

[0042] The average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowall CNW1 is 80° or more and 90° or less. Here, the average angle θ is the average value of angles less than 90°. If one angle θ1 between the carbon nanowall and the negative electrode current collector N1 is an acute angle, the other angle θ2 is an obtuse angle. Among such angles, the average angle θ is the average of the smaller angles θ1.

[0043] Because each carbon nanowall grows and is arranged almost vertically, the carbon nanowalls do not come into contact with each other at their tips E1, allowing the electrolyte and lithium ions to penetrate between the carbon nanowalls, allowing the entire carbon nanowall to function effectively as an electrode.

[0044] However, this does not apply to the vicinity of the portion where the carbon nanowalls meet. Also, when carbon nanowalls are formed on a large-area substrate, there may be some portions where the carbon nanowalls come into contact with each other at their tips E1, but this does not impair the overall function.

[0045] The average angle θ is determined by the growth conditions of the carbon nanowalls, and it is necessary to set the average height H1 and average wall spacing D1 of the carbon nanowalls so that the carbon nanowalls do not come into contact with each other at their upper ends based on the average angle θ. Depending on the values ​​of the average height H1 and average wall spacing D1, the electrolyte and lithium ions cannot enter the gaps between the carbon nanowalls, and some of the carbon nanowalls cannot function as electrodes.

[0046] For example, if the average height H1 of the carbon nanowalls is 5 μm and the average wall spacing D1 is 100 nm, an angle of 88.9° or more is required to avoid contact with adjacent vertical walls. Alternatively, if the average height H1 of the carbon nanowalls is 0.6 μm and the average wall spacing D1 is 100 nm, an angle of 80.4° or more is required to avoid contact with adjacent vertical walls.

[0047] The numerical values ​​indicating the structure of these carbon nanowalls CNW1 are shown in Table 1. However, these numerical ranges are merely examples and are not limited to these numerical ranges.

[0048] [Table 1] Wall height: 100 nm or more, 200 μm or less Wall thickness: 0.5 nm or more, 100 nm or less Wall spacing: 10nm or more, 500nm or less Wall angle: 80° or more, 90° or less

[0049] 2-4. Surface area of ​​carbon nanowalls Here, we will explain the surface area of ​​the carbon nanowall CNW1. For a simple understanding, we will assume that the shape of the carbon nanowall is a grid. In reality, the shape of the carbon nanowall deviates from the grid because it is rare for a wall to extend in a straight line.

[0050] 5 is a view of the carbon nanowalls of the lithium ion secondary battery LiB1 according to the first embodiment, viewed from a direction perpendicular to the surface of the negative electrode current collector N1. In FIG. 5, the carbon nanowalls are assumed to have a grid shape, as described above.

[0051] If the pitch interval of the carbon nanowalls is I1, the area SS1 of a square with a side interval of I1 is I1 2 The area SS1 is the surface area of ​​the carbon material when it is solidly coated on the surface of the negative electrode current collector N1, rather than the carbon nanowall. In Figure 5, the area SS2 of the side of the carbon nanowall that occupies the square of which one side is the repeating unit and has a spacing I1 is given by the following equation: SS2 = 8 × (D1 / 2) × H1 = 4 × D1 × H1 H1: Average height of carbon nanowalls D1: Average wall spacing

[0052] When carbon nanowalls are present, the surface area SS3 of the carbon nanowalls in a square region with a side spacing I1 is given by the following equation. SS3 = SS1 + SS2

[0053] Therefore, the ratio SS3 / SS1 indicates the rate of increase in surface area due to the presence or absence of carbon nanowalls. SS3 / SS1 = (4×D1×H1+I1 2 ) / I1 2 ≒ 4×D1×H1 / I1 2 ≒ 4×H1 / D1 Here, H1>>I1 and D1≒I1 are used.

[0054] Thus, the higher the carbon nanowalls are and the narrower the spacing between the carbon nanowalls is, the larger the surface area of ​​the carbon material of the lithium ion secondary battery LiB1 is.

[0055] Table 2 shows the relationship between the size and angle of the carbon nanowalls and the rate of increase in surface area. It is believed that the larger the surface area of ​​the carbon nanowalls, the more lithium ions react on the surface of the active material layer. Therefore, it is believed that the larger the surface area of ​​the carbon nanowalls, the faster the charge / discharge rate of the lithium-ion secondary battery LiB1.

[0056] As shown in Table 2, when the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowalls is 80° or more and 90° or less, the surface area of ​​the carbon material of the lithium-ion secondary battery LiB1 can be increased by 20 times or more. The average angle θ is preferably 83° or more. The average angle θ is more preferably 85° or more. The average angle θ is even more preferably 88° or more. Furthermore, when the average angle θ is 89° or more, the increase in surface area is approximately 400 times or more.

[0057] [Table 2] Wall height Wall spacing Wall angle Surface area increase rate (μm) (nm) (°) (times) 1.0 200 78.5 20 1.0 50 87.1 80 2.0 400 78.5 20 2.0 50 88.6 160 5.0 500 84.3 40 5.0 50 89.4 400 10.0 50 89.7 800 10.0 20 89.9 2000 20.0 50 89.9 1600 20.0 20 89.9 4000 30.0 50 89.9 2400 30.0 20 89.9 6000 50.0 50 89.9 4000 50.0 20 89.9 10000

[0058] 2-5. When the average angle is small Here, a case where the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowalls is small will be described.

[0059] 6 is a diagram illustrating a case where the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowalls is small. As shown in FIG. 6, a projected region PR3 obtained by projecting the carbon nanowall CNW1(c) onto the first surface N1a of the negative electrode current collector N1 includes the carbon nanowall CNW1(d) other than the carbon nanowall CNW1(c).

[0060] When the tip E1 of the carbon nanowall CNW1(c) is projected onto the first surface N1a of the negative electrode current collector N1, the tip E1 of the carbon nanowall CNW1(c) crosses the side surface of the adjacent carbon nanowall CNW1(d).

[0061] Thus, when the average angle θ is small, the electrolyte does not easily reach the base R1 of the carbon nanowall CNW1. If the electrolyte does not cover the entire carbon nanowall, the battery capacity decreases accordingly.

[0062] 3. Charge and discharge reactions involving lithium ions 3-1.Charge and discharge reaction The negative electrode NE has carbon nanowalls CNW1, which can cause two or more lithium ions per carbon atom to participate in a charge / discharge reaction during one charge or discharge.

[0063] Here, the charge / discharge reaction is, for example, a chemical reaction expressed by the following chemical reaction formula. Li + + e - ⇔ Li …(1) 6C + xLi + + xe- ⇔ C6Li x …(2) Li 1-x CoO2+ xLi + + xe - ⇔ LiCoO2…(3) Equation (1) or (2) represents a reaction that can occur, for example, inside the negative electrode active material layer N2. Equation (3) represents a reaction that can occur, for example, inside the positive electrode active material layer P2. Both reactions involve lithium ions and electrons. A charge / discharge reaction is a chemical reaction in the positive electrode PE or negative electrode NE in which lithium ions are involved and electrons are exchanged. This charge / discharge reaction can result in the absorption or release of lithium ions, as well as the precipitation, deposition, adsorption, or dissolution of lithium or lithium compounds. Note that if lithium or lithium compounds are precipitated, the charge / discharge reaction can occur outside the positive electrode active material layer P2 or negative electrode active material layer N2. Note that the type of charge / discharge reaction varies depending on the materials of the positive electrode active material layer P2 and negative electrode active material layer N2.

[0064] 3-2. Lithium deposition on carbon nanowalls The carbon nanowall CNW1 has a surface on which lithium can be deposited, and therefore, lithium may be deposited on the surface of the carbon nanowall CNW1 during charging or discharging.

[0065] Carbon nanowall CNW1 can cause two or more lithium ions per carbon atom to participate in the charge-discharge reaction during one charge or discharge. Carbon nanowall CNW1 can cause ten or more lithium ions per carbon atom to participate in the charge-discharge reaction during one charge or discharge. Carbon nanowall CNW1 can cause twenty or more lithium ions per carbon atom to participate in the charge-discharge reaction during one charge or discharge.

[0066] Carbon nanowalls CNW1 can deposit lithium. Therefore, in principle, there is no upper limit to the number of lithium ions that can be involved in charge-discharge reactions per carbon atom. However, the larger the number of lithium ions involved in charge-discharge reactions per carbon atom during a single charge or discharge, the larger the volume of lithium that is deposited. Therefore, the number of lithium ions involved in charge-discharge reactions per carbon atom may be, for example, 100,000 or less. Preferably, it is 10,000 or less. More preferably, it is 1,000 or less. Even more preferably, it is 150 or less.

[0067] The height of the deposited lithium may be, for example, 200 μm or less, preferably 100 μm or less, and more preferably 50 μm or less.

[0068] For example, when 30 lithium atoms per carbon atom are precipitated on the surface of the carbon nanowall CNW1, this state can be virtually represented by the following chemical formula (composition formula). Li 30 C

[0069] When lithium is precipitated on the surface of the carbon nanowall CNW1 in this way, it is virtually represented by the following chemical formula (composition formula). Li X C Here, X is a real number greater than or equal to 0, and varies with charge and discharge. Because lithium is deposited, the theoretical maximum value of X is infinity.

[0070] 3-3. Comparison with conventional methods Conventionally, one lithium ion is placed in each six-membered ring of carbon atoms. This state is expressed by the following formula: LiC6 Therefore, the carbon nanowalls CNW1 of the first embodiment can involve a much larger number of lithium ions in the charge / discharge reaction than conventional ones, which means that the performance of the lithium ion secondary battery LiB1 of the first embodiment is high.

[0071] 3-4.Occluded and precipitated states FIG. 7 is a hypothetical diagram showing how lithium ions are absorbed by the carbon nanowalls CNW1 of the lithium ion secondary battery LiB1 of the first embodiment and how lithium or a lithium compound is deposited.

[0072] A hypothetical example is shown in Figure 7. As shown in region LA1 in Figure 7, lithium ions can be thought of as penetrating the interior of the carbon nanowall CNW1. Theoretically, the maximum intercalation value of lithium ions is LiC6. Therefore, the number of lithium atoms per carbon atom cannot exceed 1 / 6.

[0073] As shown in the region LA2 in FIG. 7, it is thought that lithium atoms or lithium ions are adsorbed or deposited on the surface of the carbon nanowall CNW1, and metallic lithium or a lithium compound is precipitated.

[0074] As described above, the carbon nanowall CNW1 can absorb or deposit lithium. If the absorption of lithium is thermodynamically more favorable than the deposition of lithium, it is believed that the absorption of lithium occurs first, the absorption of lithium becomes saturated, and then the deposition of lithium occurs. If the deposition of lithium is thermodynamically more favorable than the absorption of lithium, it is believed that the absorption of lithium does not occur and the lithium is deposited.

[0075] As charging of the lithium-ion secondary battery LiB1 continues, lithium precipitates on the surface of the carbon nanowalls CNW1. As charging of the lithium-ion secondary battery LiB1 continues, lithium fills the gaps in the carbon nanowalls CNW1. It is thought that as charging of the lithium-ion secondary battery LiB1 continues, lithium precipitates beyond the height of the carbon nanowalls CNW1.

[0076] 3-5.Other conditions In addition to the absorption or desorption of lithium ions and the precipitation, deposition, and adsorption of lithium, other phenomena such as dissolution of lithium and precipitation, deposition, adsorption, and dissolution of lithium compounds may occur.

[0077] 4. Manufacturing equipment A manufacturing apparatus for forming carbon nanowalls CNW1 on the surface of the negative electrode current collector N1 will be described.

[0078] 8 is a schematic diagram showing the configuration of a manufacturing apparatus 1 for growing carbon nanowalls CNW1 in a lithium-ion secondary battery LiB1 according to the first embodiment. The manufacturing apparatus 1 has a plasma generation chamber 46 and a reaction chamber 10. The plasma generation chamber 46 is used to generate plasma therein and also to generate radicals to be supplied to the reaction chamber 10. The reaction chamber 10 is used to form carbon nanowalls CNW1 by utilizing the radicals generated in the plasma generation chamber 46.

[0079] The manufacturing apparatus 1 also has a waveguide 47, a quartz window 48, and a slot antenna 49. The waveguide 47 is for introducing the microwaves 39. The slot antenna 49 is for introducing the microwaves 39 from the quartz window 48 into the plasma generation chamber 46.

[0080] The plasma generation chamber 46 is for generating surface wave plasma (SWP) by microwaves 39. The plasma generation chamber 46 is provided with a radical source inlet 42. The radical source inlet 42 is for supplying a gas that serves as a radical source into the inside of the plasma 61 generated in the plasma generation chamber 46.

[0081] A partition wall 44 is provided between the plasma generation chamber 46 and the reaction chamber 10. The partition wall 44 separates the plasma generation chamber 46 from the reaction chamber 10. The partition wall 44 also serves as a first electrode 22 for applying a voltage. A through-hole 14 is formed in the partition wall 44. This is for supplying radicals generated in the plasma generation chamber 46 to the reaction chamber 10.

[0082] The reaction chamber 10 is used to generate capacitively coupled plasma (CCP). The reaction chamber 10 is also used to form carbon nanowalls CNW1 on the negative electrode current collector N1. The reaction chamber 10 has a second electrode 24, a heater 25, a raw material inlet 12, and an exhaust port 16. The second electrode 24 is used to apply a voltage between the first electrode 22 and the second electrode 24. The heater 25 is used to heat the negative electrode current collector N1 and control the temperature of the negative electrode current collector N1. The raw material inlet 12 is used to supply a carbon-based gas 32, which is a raw material for the carbon nanowalls. The exhaust port 16 is connected to a vacuum pump or the like. The vacuum pump is used to adjust the pressure inside the reaction chamber 10.

[0083] As described above, the partition wall 44 also serves as the first electrode 22 for applying a voltage between it and the second electrode 24. A power source and a circuit are connected to the first electrode 22 in order to control the potential of the first electrode 22 over time. The second electrode 24 is used to apply a voltage between it and the first electrode 22. The second electrode 24 also serves as a mounting base for placing the negative electrode current collector N1 thereon. The second electrode 24 is grounded. The distance between the first electrode 22 and the second electrode 24 is approximately 5 cm. Of course, this value is not limiting.

[0084] 5. Negative electrode manufacturing method 5-1. Amorphous carbon layer formation process First, the negative electrode current collector N1 before the carbon nanowalls CNW1 are formed is placed inside the manufacturing apparatus 1. At this time, the first surface N1a of the negative electrode current collector N1 faces up, and the second surface N1b is in contact with the second electrode 24. Next, microwaves 39 are introduced into the waveguide 47. The microwaves 39 are introduced into the plasma generation chamber 46 through the quartz window 48 by the slot antenna 49. This generates high-density plasma 60.

[0085] This high-density plasma 60 then diffuses inside the plasma generation chamber 46 to become plasma 61. This plasma 61 contains ions of the radical source supplied from the radical source inlet 42. Hydrogen is used as the radical source. Alternatively, oxygen, nitrogen, or other gases may be used. Most of the ions in the plasma 61 collide with the partition wall 44 and are neutralized to become radicals. The radicals 38 pass through the through-holes 14 in the partition wall 44 and enter the reaction chamber 10.

[0086] In addition to the radicals 38, a carbon-based gas 32 is supplied into the reaction chamber 10 from the raw material inlet 12. The carbon-based gas 32 is, for example, CH4 or C2F6. Of course, other gases may also be used. Then, a voltage is applied between the first electrode 22 and the second electrode 24. This generates a plasma 34 inside the reaction chamber 10.

[0087] The atmosphere of the plasma 34 contains a mixture of particles and the like derived from the carbonaceous gas 32, which is the raw material, and radicals 38. In this plasma 34 atmosphere, an amorphous carbon layer AC1 grows on the surface of the negative electrode current collector N1.

[0088] In this manner, the carbonaceous gas that has been converted into plasma inside the manufacturing apparatus 1 is supplied to the negative electrode current collector N1 to form the amorphous carbon layer AC1 on the negative electrode current collector N1.

[0089] The pressure inside the reaction chamber 10 is within a range of 5 mTorr to 2000 mTorr (0.65 Pa to 267 Pa). The temperature of the negative electrode current collector N1 is within a range of 100° C. to 800° C. Of course, these are merely examples and the present invention is not limited to these numerical ranges.

[0090] 5-2. Carbon nanowall growth process Next, carbon nanowalls CNW1 are grown on the amorphous carbon layer AC1 inside the manufacturing apparatus 1. As in the case of growing the amorphous carbon layer AC1, plasma 61 is generated. Hydrogen gas is used as a radical source of the radicals 38, and CH or C F , for example, is used as the carbon-based gas 32.

[0091] In this manner, carbonaceous gas that has been converted into plasma inside the manufacturing apparatus 1 is supplied to the negative electrode current collector N1 to grow carbon nanowalls on the amorphous carbon layer AC1.

[0092] The pressure inside the reaction chamber 10 is within a range of 5 mTorr to 2000 mTorr (0.65 Pa to 267 Pa). The temperature of the negative electrode current collector N1 is within a range of 100° C. to 800° C. Of course, these are merely examples and the present invention is not limited to these numerical ranges.

[0093] 5-3. Cleaning process After the growth of the carbon nanowalls CNW1 has progressed to a certain extent, the negative electrode current collector N1 is removed from the production apparatus 1. At this time, the height H1 of the carbon nanowalls CNW1 is, for example, 1000 nm.

[0094] Next, the inside of the manufacturing apparatus 1 is cleaned. The carbon material is scraped off from the inner wall surface. Alternatively, the carbon material on the inner wall surface is removed using hydrogen plasma or the like. In this way, in this step, the growth process is stopped and the inside of the manufacturing apparatus 1 is cleaned.

[0095] 5-4. Repetition of processes, etc. The carbon nanowall growing process and the cleaning process are then repeated. This allows carbon nanowalls CNW1 of sufficient height H1 to be obtained. The negative electrode current collector N1 is then turned over, and the carbon nanowalls CNW1 are formed on the second surface N1b of the negative electrode current collector N1. Note that even if the negative electrode current collector N1 is placed with the carbon nanowalls CNW1 facing downward, no problems occur with the carbon nanowalls CNW1.

[0096] 6. Manufacturing method of lithium-ion secondary battery 6-1. Negative electrode manufacturing process The negative electrode NE is manufactured as described above. The negative electrode active material layer N2 is formed on the negative electrode current collector N1.

[0097] 6-2. Positive electrode manufacturing process Next, the positive electrode PE is manufactured. A coating liquid is applied to the positive electrode current collector P1 and dried. The coating liquid contains a positive electrode active material, a conductive additive, and a binder. The positive electrode PE may also be subjected to a pressing process.

[0098] 6-3. Electrolyte injection process Next, the positive electrodes PE and the negative electrodes NE are alternately arranged inside the container V1 with separators Sp1 in between. Then, the electrolyte solution ES1 is poured into the container V1. After this, the opening of the container V1 is simply sealed.

[0099] 6-4. Other processes Other steps such as an aging step may also be carried out.

[0100] 7. Effects of the First Embodiment The negative electrode NE of the lithium-ion secondary battery LiB1 of the first embodiment has carbon nanowalls CNW1. The surface area of ​​the carbon nanowalls CNW1 is sufficiently large. The carbon nanowalls CNW1 can involve a large number of lithium ions per carbon atom in charge and discharge reactions. Therefore, the capacity of the lithium-ion secondary battery LiB1 is very large.

[0101] As mentioned above, carbon nanowalls CNW1 can involve a large number of lithium ions in the charge / discharge reaction. Therefore, the amount of carbon nanowalls CNW1 is small. This means that the size and weight of the negative electrode are smaller and lighter than conventional negative electrodes. Therefore, the volumetric energy density and weight energy density of the lithium-ion secondary battery LiB1 are improved compared to conventional batteries.

[0102] 8. Variations 8-1. Surface on which the positive electrode active material layer or the negative electrode active material layer is formed The positive electrode active material layer P2 may be formed on only one surface of the positive electrode current collector P1, and the negative electrode active material layer N2 may be formed on only one surface of the negative electrode current collector N1.

[0103] 8-2. Carbon nanowalls in the positive electrode Depending on the type of electricity storage device, carbon nanowalls may be formed on the positive electrode current collector P1. Even in this case, the electricity storage device electrode includes a current collector and an active material layer on the current collector. The active material layer includes carbon nanowalls.

[0104] 8-3.Amorphous carbon layer The negative electrode NE does not necessarily have to have the amorphous carbon layer AC1. In that case, the carbon nanowalls CNW1 are formed directly on the negative electrode current collector N1. The amorphous carbon layer AC1 may or may not function as a negative electrode active material.

[0105] 8-4. Amorphous carbon on carbon nanowall CNW1 Immediately after the growth of the carbon nanowalls CNW1, the surfaces of the carbon nanowalls CNW1 may be covered with amorphous carbon, which can be removed by H2O2.

[0106] 8-5.Laminates The electrode may be a stack of positive electrodes PE and negative electrodes NE, which alternate in the stack, with separators Sp1 disposed between the positive electrodes PE and negative electrodes NE.

[0107] 8-6. Cleaning process The cleaning step may be omitted depending on the height of the carbon nanowalls CNW1 and the manufacturing apparatus 1.

[0108] 8-7. Combination The above modifications may be freely combined.

[0109] (Second embodiment) A second embodiment will be described.

[0110] 1. Lithium-ion capacitor 9 is a schematic configuration diagram of a lithium ion capacitor LiC1 of Embodiment 2. The lithium ion capacitor LiC1 has a positive electrode PE2, a negative electrode NE, a separator Sp1, an electrolytic solution ES1, and a container V1.

[0111] The positive electrode PE2 includes a positive electrode current collector P1 and a positive electrode active material layer P3. The positive electrode active material layer P3 is, for example, activated carbon.

[0112] 2. Variations 10 is a schematic configuration diagram of a lithium ion capacitor LiC2 according to a modified example of Embodiment 2. The lithium ion capacitor LiC2 includes a positive electrode PE3, a negative electrode NE, a separator Sp1, an electrolytic solution ES1, and a container V1.

[0113] The positive electrode PE3 includes a positive electrode current collector P1 and a positive electrode active material layer P4. The positive electrode active material layer P4 includes carbon nanowalls CNW2. The carbon nanowalls CNW2 of the positive electrode PE3 are similar to the carbon nanowalls CNW1 of the negative electrode NE. Of course, the wall conditions may be changed.

[0114] (Third embodiment) A third embodiment will be described.

[0115] The basic structure of the lithium ion secondary battery of the third embodiment is the same as the basic structure of the lithium ion secondary battery LiB1 of the first embodiment.

[0116] 1. Amount of active material layer As described in the first embodiment, the carbon nanowalls CNW1 can involve a large number of lithium ions in the charge-discharge reaction, and therefore the negative electrode active material layer N2 is much lighter and has a smaller volume than the positive electrode active material layer P2.

[0117] The number of lithium atoms that can be contained per unit area in the positive electrode active material layer P2 is at least twice the number of carbon atoms that can be contained per unit area in the negative electrode active material layer N2. The number of lithium atoms that can be contained per unit area in the positive electrode active material layer P2 is preferably at least 100 times the number of carbon atoms that can be contained per unit area in the negative electrode active material layer N2. The number of lithium atoms that can be contained per unit area in the positive electrode active material layer P2 is preferably not more than 100,000 times the number of carbon atoms that can be contained per unit area in the negative electrode active material layer N2. The upper limit here is limited by the volume of deposited lithium.

[0118] 2. Effects of the Third Embodiment The lithium ion secondary battery of the third embodiment has a small number of carbon atoms, which results in high volumetric energy density and weight energy density, making it possible for the battery to contribute to a low-carbon society.

[0119] 3. Variations 3-1. Lithium-ion capacitor The technology of the third embodiment can be similarly applied to a lithium ion capacitor. [Example]

[0120] (experiment) 1. Carbon nanowalls on the current collector 1-1. Manufacturing method Using the production apparatus 1, carbon nanowalls were grown on a metal plate made of Ti.

[0121] 1-2. Carbon nanowalls Figure 11 is a micrograph of carbon nanowalls viewed from a direction perpendicular to the surface of a metal plate. As shown in Figure 11, the carbon nanowalls grow randomly, and the wall-like structures merge with each other as they grow. However, the spacing between them is fairly uniform.

[0122] Figure 12 is a micrograph of carbon nanowalls showing a cross section perpendicular to the surface of a metal plate. As shown in Figure 12, the carbon nanowalls are formed almost perpendicular to the substrate.

[0123] 2. Lithium-ion secondary battery 2-1. Manufacturing of lithium-ion secondary batteries As an example, a lithium-ion secondary battery LiB1 of the first embodiment was manufactured. The positive electrode current collector P1 was aluminum, and the positive electrode active material was lithium cobalt oxide. The negative electrode current collector N1 was copper, and the negative electrode active material was carbon nanowalls. The electrolyte was 1M LiPF6. The positive electrode active material layer had a diameter of 1.6 cm. The negative electrode active material layer had a diameter of 1.3 cm. The carbon nanowalls had three heights: 1 μm, 4 μm, and 10 μm.

[0124] The weight of lithium cobalt oxide is summarized in Table 3. The weight of the carbon nanowalls was calculated by subtracting the weight of the substrate before growing the carbon nanowalls from the weight of the substrate after growing the carbon nanowalls.

[0125] [Table 3] Lithium cobalt oxide 0.046g Carbon nanowall 0.00004g (height 1μm) Carbon nanowall 0.00016g (height 4μm) Carbon nanowall 0.00040g (height 10μm)

[0126] The positive electrode active material layer contained lithium cobalt oxide, a conductive additive, and a binder. The conductive additive was acetylene black. The binder was PVDF. The weight ratio of the lithium cobalt oxide, acetylene black, and PVDF was 100:5:3.

[0127] As a comparative example, a lithium ion secondary battery having a negative electrode in which graphite was used instead of carbon nanowalls was manufactured. The other conditions were the same as those in the examples. The weight of the graphite is summarized in Table 4.

[0128] [Table 4] Lithium cobalt oxide 0.046g Graphite 0.010g

[0129] 2-2. Capacity of Lithium-ion Secondary Battery FIG. 13 is a graph showing the relationship between the capacity and voltage of a lithium-ion secondary battery using 1 μm-high carbon nanowalls as the negative electrode. The horizontal axis of FIG. 13 represents charge / discharge capacity. The vertical axis of FIG. 13 represents voltage. The charge or discharge current was 0.5 mA. As shown in FIG. 13, the discharge capacity of the lithium-ion secondary battery was 9.0 mAh.

[0130] FIG. 14 is a graph showing the relationship between the capacity and voltage of a lithium-ion secondary battery using 4 μm-high carbon nanowalls as the negative electrode. The horizontal axis of FIG. 14 represents charge / discharge capacity. The vertical axis of FIG. 14 represents voltage. The charge or discharge current was 0.5 mA. As shown in FIG. 14, the discharge capacity of the lithium-ion secondary battery was 9.0 mAh.

[0131] FIG. 15 is a graph showing the relationship between the capacity and voltage of a lithium-ion secondary battery using 10 μm-high carbon nanowalls as the negative electrode. The horizontal axis of FIG. 15 represents charge / discharge capacity. The vertical axis of FIG. 15 represents voltage. The charge or discharge current was 0.5 mA. As shown in FIG. 15, the discharge capacity of the lithium-ion secondary battery was 9.0 mAh.

[0132] As shown in Figures 13 to 15, the discharge capacity of the lithium-ion secondary battery was 9.0 mAh, despite the difference in the height of the carbon nanowalls. This suggests that although there is some margin for the charge / discharge capacity of the negative electrode active material, it is limited by the charge / discharge capacity of the positive electrode active material. 9.0mAh / 0.046g = 196mAh / g As shown in Fig. 1, the measured discharge capacity reaches approximately 72% of the theoretical capacity of 274 mAh / g for lithium cobalt oxide, supporting the inference that the charge / discharge capacity is limited by the active material on the positive electrode side.

[0133] FIG. 16 is a graph showing the relationship between the capacity and voltage of a lithium-ion secondary battery using graphite as the negative electrode. The horizontal axis of FIG. 16 represents charge / discharge capacity. The vertical axis of FIG. 16 represents voltage. The charge or discharge current was 0.5 mA. As shown in FIG. 16, the discharge capacity of the lithium-ion secondary battery was 3 mAh.

[0134] Figure 17 is a graph comparing the charging voltage of lithium ion secondary batteries using carbon nanowalls and graphite as the negative electrode. The horizontal axis of Figure 17 represents charging capacity, and the vertical axis of Figure 17 represents voltage.

[0135] As shown in Figure 17, the charging voltage increases gradually when graphite is used as the negative electrode. Lithium ions intercalate between the graphene sheet layers of graphite. This intercalation stage is called stage 4 (LiC 24 ) to stage 1 (LiC6). The charging voltage also changes gradually in response to the time change in this stage.

[0136] In contrast, when carbon nanowalls are used as the anode, the charging voltage rises sharply. This suggests two possibilities. In the first possibility, the intercalation stage change is completed in a short time, and lithium is precipitated. In the second possibility, lithium is precipitated without intercalation occurring.

[0137] 2-3. Lithium-ion secondary battery As shown in FIG. 17, the charging voltage when carbon nanowalls are used for the negative electrode is about 0.1 V higher than the charging voltage when graphite is used for the negative electrode.

[0138] Li + + e - ⇔ Li …(1) -3.04V 6C + xLi + + xe - ⇔ C6Li x …(2) -2.90V Li 1-x CoO2+ xLi + + xe - ⇔ LiCoO2…(3) +0.90V

[0139] Equation (1) shows the case where lithium is deposited or ionized. Equation (2) shows the case where lithium ions are intercalated or deintercalated between layers of the graphene structure. Equation (3) shows the case where lithium cobalt oxide releases or absorbs lithium ions.

[0140] When the reaction of equation (1) occurs, the charging voltage is as follows: 0.90V - (-3.04V) = 3.94V

[0141] When the reaction of equation (2) occurs, the charging voltage is as follows: 0.90V - (-2.90V) = 3.80V

[0142] The difference in charging voltage between the case where carbon nanowalls are used for the negative electrode and the case where graphite is used for the negative electrode is thought to be due to the difference between formula (1) and formula (2). In other words, when carbon nanowalls are used for the negative electrode, the reaction in formula (1) is thought to occur mainly during charging and discharging, while when graphite is used for the negative electrode, the reaction in formula (2) is thought to occur mainly during charging and discharging.

[0143] 2-4.Capacity of carbon nanowalls The theoretical capacity of graphite is 372 mAh / g, and its chemical formula is LiC6.

[0144] The capacity of a carbon nanowall with a height of 1 μm is 2,250,000 mAh / g. 9.0mAh / 0.000040g = 2250000mAh / g

[0145] The capacity of a 1 μm-high carbon nanowall is approximately 600 times the theoretical capacity of graphite. 2250000mAh / g / 372mAh / g = 600

[0146] Therefore, the state of Li precipitated in the carbon nanowalls can be expressed by the following chemical formula (composition formula): Li 600 C6(Li 100 C)

[0147] 3. Carbon nanowall height dependence 3-1. Lithium-ion secondary battery A lithium-ion secondary battery was fabricated using a Li metal cathode and Cu coated with carbon nanowalls as the anode. The electrolyte was the same as that used in lithium-ion secondary batteries.

[0148] 3-2. Charge / discharge characteristics Figure 18 is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with carbon nanowalls of 0 nm height. The horizontal axis of Figure 18 represents capacity. The vertical axis of Figure 18 represents voltage. In this case, no carbon nanowalls are present in the negative electrode, and only copper foil is present. As shown in Figure 18, the voltage dropped immediately after the start of discharge.

[0149] Figure 19 is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with 20 nm-high carbon nanowalls. The horizontal axis of Figure 19 represents capacity. The vertical axis of Figure 19 represents voltage. As shown in Figure 19, the capacity was 1.6 mAh.

[0150] Figure 20 is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with carbon nanowalls 50 nm in height. The horizontal axis of Figure 20 represents capacity. The vertical axis of Figure 20 represents voltage. As shown in Figure 20, the capacity was 7.1 mAh.

[0151] Figure 21 is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with 100 nm-high carbon nanowalls. The horizontal axis of Figure 21 represents capacity. The vertical axis of Figure 21 represents voltage. As shown in Figure 21, the capacity was 13.2 mAh.

[0152] Figure 22 is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with carbon nanowalls 200 nm in height. The horizontal axis of Figure 22 represents capacity. The vertical axis of Figure 22 represents voltage. As shown in Figure 22, the capacity was 13.3 mAh.

[0153] Figure 23 is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with carbon nanowalls 500 nm in height. The horizontal axis of Figure 23 represents capacity. The vertical axis of Figure 23 represents voltage. As shown in Figure 23, the capacity was 13.2 mAh.

[0154] Table 5 summarizes the experimental data. As shown in Table 5, when the height of the carbon nanowalls is 100 nm or more, the capacity of the lithium-ion secondary battery is saturated. If the height of the carbon nanowalls is 100 nm or more, it is thought that all of the lithium in the positive electrode will be consumed. For this reason, it is desirable for the height of the carbon nanowalls to be 100 nm or more.

[0155] [Table 5] CNW height Capacity (nm) (mAh) 0 0.5 20 1.6 50 7.1 100 13.2 200 13.3 500 13.2 1000 12.5

[0156] 3-3.SEM images Figure 24 is a scanning electron micrograph showing the surface of a 500 nm-high carbon nanowall. Figure 25 is a scanning electron micrograph showing the cross section of a 500 nm-high carbon nanowall. The carbon nanowall has grown large.

[0157] Figure 26 is a scanning electron micrograph showing the surface of a 50 nm-high carbon nanowall. Figure 27 is a scanning electron micrograph showing the cross section of a 50 nm-high carbon nanowall. The carbon nanowall has not grown very much.

[0158] 4. Lithium Deposition (Part 1) 4-1. Lithium-ion secondary battery A lithium-ion secondary battery was fabricated using Li metal as the positive electrode and Cu with carbon nanowalls as the negative electrode. The electrolyte used was that used in lithium-ion secondary batteries. The carbon nanowalls were 200 nm high.

[0159] 4-2.SEM images Figure 28 is a scanning electron microscope photograph (part 1) showing the surface of carbon nanowalls after repeated charge and discharge. Figure 28 clearly shows the walls of the carbon nanowalls.

[0160] Figure 29 is a scanning electron microscope photograph (part 2) showing the surface of carbon nanowalls after repeated charge and discharge. Figure 29 shows that metallic lithium has precipitated in the gaps between the carbon nanowalls, filling most of the gaps.

[0161] Figure 30 is a scanning electron microscope photograph (part 3) showing the surface of the carbon nanowalls after repeated charge and discharge. Figure 30 shows how the deposited metallic lithium has completely filled the upper layer of the carbon nanowalls.

[0162] As shown in Figures 29 and 30, metallic lithium precipitates and fills the gaps between the carbon nanowalls. This allows more lithium ions to participate in the charge-discharge reactions than before. It is possible for two or more lithium ions per carbon atom to participate in the charge-discharge reactions.

[0163] The fabricated lithium-ion secondary battery was subjected to 30 charge-discharge cycles. Even after that, no dendrites were observed. This is presumably because metallic lithium precipitates from the surface of the carbon nanowalls, and the good crystallinity of this metallic lithium makes it difficult for dendrites to form.

[0164] 5. Lithium Deposition (Part 2) 5-1. Manufacturing of lithium-ion secondary batteries A coin-type lithium-ion secondary battery was fabricated. The positive electrode current collector P1 was aluminum, and the positive electrode active material was lithium cobalt oxide. The negative electrode current collector N1 was copper, and the negative electrode active material was carbon nanowalls. The electrolyte was 1 M LiPF6. The positive electrode active material layer had a diameter of 1.6 cm. The negative electrode active material layer had a diameter of 1.3 cm. The carbon nanowall height was 1 μm.

[0165] 5-2.Charging The coin-type lithium-ion secondary battery was charged at 0.5 mA for 18 hours.

[0166] 5-3. Lithium Deposition The charge Q is given by the following equation: Q = 0.5mA · 18h = 32.4C

[0167] The number of lithium ions that can receive charge, N, is given by: N = 32.4 / (1.6×10 -19 ) = 2.0×10 20 (pieces)

[0168] Assume that lithium ions receive electrons at the negative electrode and become lithium crystals. Lithium has a body-centered cubic lattice structure. Two lithium atoms fit into one lattice. The lattice constant of lithium crystals is 0.35 nm.

[0169] The number density n of lithium ions is given by the following equation: n = 2.0 × 10 20 / {2·(0.35×10 -9 ) 3} = 4.29 × 10 -9 m -3

[0170] The height HL of the deposited lithium is given by the following equation: HL = 4.29 x 10 -9 / (0.0065×0.0065×3.14) = 32 × 10 -6 m = 32 μm

[0171] 5-4.Cross-section of lithium crystal Figure 31 is a scanning electron microscope photograph showing the cross section of the negative electrode of a lithium-ion secondary battery after charging. As shown in Figure 31, the height of the lithium crystals after charging was 32 μm. Therefore, the height of the lithium crystals shown in Figure 31 agrees with the above calculation result.

[0172] 5-5.X-ray diffraction Figure 32 is a graph showing the results of X-ray diffraction of the negative electrode of a lithium-ion secondary battery after charging. Figure 32 shows X-ray peaks detected by the θ-2θ method. The horizontal axis of Figure 32 is d / Å. The vertical and horizontal axes of Figure 32 represent X-ray intensity.

[0173] As shown in Figure 32, peaks of Li(110), Li(200), Li(211), Li(220), and Li(310) were observed. The peak value of Li(110) was the largest.

[0174] Lithium From the above, it is clear that lithium is deposited on the carbon nanowalls.

[0175] (Addendum) A first aspect of the present invention provides an electrode for an electricity storage device, comprising a current collector and an active material layer on the current collector. The active material layer comprises carbon nanowalls. The carbon nanowalls are capable of accommodating two or more lithium ions per carbon atom in a charge / discharge reaction during a single charge or discharge.

[0176] The electrode for a power storage device according to the second embodiment includes a current collector and an active material layer on the current collector. The active material layer includes carbon nanowalls. The carbon nanowalls have a surface on which lithium can be deposited.

[0177] In the electrode for an electricity storage device according to the third embodiment, the carbon nanowalls can cause two or more lithium ions per carbon atom to participate in a charge / discharge reaction during one charge or discharge.

[0178] In the fourth embodiment of the electrode for a power storage device, the active material layer has an amorphous carbon layer between the current collector and the carbon nanowalls.

[0179] In the electricity storage device electrode according to the fifth aspect, the amorphous carbon layer has a film thickness of 10 nm or more and 300 nm or less.

[0180] In the sixth embodiment of the electrode for a power storage device, the projected area of ​​the carbon nanowalls projected onto the surface of the current collector does not include any carbon nanowalls other than the carbon nanowalls in question.

[0181] In the seventh embodiment of the electrode for a power storage device, the average angle between the current collector and the carbon nanowalls is 80° or more and 90° or less.

[0182] In the eighth aspect of the electrode for a power storage device, the carbon nanowalls have a height from the current collector of 100 nm or more and 10 μm or less.

[0183] A ninth aspect of the present invention provides an electricity storage device comprising a positive electrode current collector, a positive electrode active material layer on the positive electrode current collector, a negative electrode current collector, and a negative electrode active material layer on the negative electrode current collector. The negative electrode active material layer comprises carbon nanowalls. The carbon nanowalls are capable of accommodating two or more lithium ions per carbon atom in a charge / discharge reaction during a single charge or discharge.

[0184] A tenth aspect of the present invention provides an electricity storage device including a positive electrode current collector, a positive electrode active material layer on the positive electrode current collector, a negative electrode current collector, and a negative electrode active material layer on the negative electrode current collector. The negative electrode active material layer includes carbon nanowalls. The carbon nanowalls have surfaces on which lithium can be deposited.

[0185] In an eleventh aspect, the power storage device includes a positive electrode current collector, a positive electrode active material layer on the positive electrode current collector, a negative electrode current collector, and a negative electrode active material layer on the negative electrode current collector. The positive electrode active material layer includes lithium atoms. The negative electrode active material layer includes carbon nanowalls. The number of lithium atoms that can be contained per unit area of ​​the positive electrode active material layer is at least twice the number of carbon atoms that can be contained per unit area of ​​the negative electrode active material layer.

[0186] In the electricity storage device of the twelfth embodiment, the carbon nanowalls have a height from the negative electrode current collector of 100 nm or more and 10 μm or less. [Explanation of symbols]

[0187] LiB1...Lithium-ion secondary battery PE: Positive electrode P1…Positive electrode current collector P2...Positive electrode active material layer NE...negative electrode N1…Negative electrode current collector N2…Negative electrode active material layer CNW1...Carbon nanowall Sp1...Separator ES1…Electrolyte V1…Container E1…Tip R1: Base GS1...Graphene sheet

Claims

1. A current collector made of a metal foil or metal plate; an active material layer on the current collector; and The active material layer is The carbon nanowall has a thin film structure mainly composed of carbon with a six-membered ring structure, the average angle between the surface of the current collector and the carbon nanowalls is 80 degrees or more and 90 degrees or less; The carbon nanowalls are In one charge or discharge, It is possible to involve two or more lithium ions per carbon atom in the charge / discharge reaction. An electrode for an electricity storage device comprising:

2. A current collector made of a metal foil or a metal plate; an active material layer on the current collector; and The active material layer is The carbon nanowall has a thin film structure mainly composed of carbon with a six-membered ring structure, the average angle between the surface of the current collector and the carbon nanowalls is 80 degrees or more and 90 degrees or less; The carbon nanowalls are Having a surface on which metallic lithium crystals can be deposited An electrode for an electricity storage device comprising:

3. The electrode for an electricity storage device according to claim 2, The carbon nanowalls are In one charge or discharge, It is possible to involve two or more lithium ions per carbon atom in the charge / discharge reaction. An electrode for an electricity storage device comprising:

4. The electrode for an electricity storage device according to any one of claims 1 to 3, The active material layer is An amorphous carbon layer is provided between the current collector and the carbon nanowalls. An electrode for an electricity storage device comprising:

5. The electrode for an electricity storage device according to claim 4, The film thickness of the amorphous carbon layer is Between 10 nm and 300 nm An electrode for an electricity storage device comprising:

6. The electrode for an electricity storage device according to any one of claims 1 to 5, The projected area of ​​the carbon nanowalls projected onto the surface of the current collector is: Contains no carbon nanowalls other than the carbon nanowall in question An electrode for an electricity storage device comprising:

7. The electrode for an electricity storage device according to any one of claims 1 to 6, The height of the carbon nanowalls from the current collector is 100 nm or more and 10 μm or less An electrode for an electricity storage device comprising:

8. a positive electrode current collector; a positive electrode active material layer on the positive electrode current collector; a negative electrode current collector made of a metal foil or a metal plate; a negative electrode active material layer on the negative electrode current collector; and The negative electrode active material layer is The carbon nanowall has a thin film structure mainly composed of carbon with a six-membered ring structure, the average angle between the surface of the negative electrode current collector and the carbon nanowalls is 80 degrees or more and 90 degrees or less; The carbon nanowalls are In one charge or discharge, It is possible to involve two or more lithium ions per carbon atom in the charge / discharge reaction.

10. An electricity storage device comprising:

9. a positive electrode current collector; a positive electrode active material layer on the positive electrode current collector; a negative electrode current collector made of a metal foil or a metal plate; a negative electrode active material layer on the negative electrode current collector; and The negative electrode active material layer is The carbon nanowall has a thin film structure mainly composed of carbon with a six-membered ring structure, the average angle between the surface of the negative electrode current collector and the carbon nanowalls is 80 degrees or more and 90 degrees or less; The carbon nanowalls are Having a surface on which metallic lithium crystals can be deposited 10. An electricity storage device comprising:

10. a positive electrode current collector; a positive electrode active material layer on the positive electrode current collector; a negative electrode current collector made of a metal foil or a metal plate; a negative electrode active material layer on the negative electrode current collector; and The positive electrode active material layer is having lithium atoms, The negative electrode active material layer is The carbon nanowall has a thin film structure mainly composed of carbon with a six-membered ring structure, the average angle between the surface of the negative electrode current collector and the carbon nanowalls is 80 degrees or more and 90 degrees or less; The number of lithium atoms that can be contained in the positive electrode active material layer per unit area is The number of carbon atoms contained in the negative electrode active material layer per unit area is at least twice as large as the number of carbon atoms that can be contained in the negative electrode active material layer per unit area.

10. An electricity storage device comprising:

11. The electricity storage device according to any one of claims 8 to 10, The height of the carbon nanowalls from the negative electrode current collector is 100 nm or more and 10 μm or less 10. An electricity storage device comprising:

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