Power storage device and electrode for power storage device

Abstract

The present technology aims to provide a power storage device and electrode for a power storage device that can cause more lithium ions to participate in charge / discharge reactions. 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) can cause two or more lithium ions per one carbon atom to participate in charge / discharge reactions in one charge or discharge.

Description

Technical Field

[0001] This specification relates to the technical field of energy storage devices using carbon materials and electrodes for energy storage devices. Background Technology

[0002] Examples of rechargeable and dischargeable energy storage devices include secondary batteries and double-layer capacitors. Additionally, examples of energy storage devices utilizing 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 embodiments of Patent Document 1). Graphite is commonly used as the carbon material. Graphite allows each of the six carbon atoms in a six-membered ring to absorb or release one lithium ion.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 2668678 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] Currently, the maximum gravimetric energy density of lithium-ion rechargeable batteries is around 250 Wh / kg. Increasing the gravimetric energy density of rechargeable batteries would improve, for example, the output and driving range of electric vehicles. Furthermore, it would enable electronic devices to operate for extended periods. Therefore, it is preferable that the positive or negative electrode active material can absorb or release more lithium ions. Alternatively, even if the method is not absorption or release, it is preferable that it allows more lithium ions or lithium atoms to participate in the chemical reaction.

[0009] The technical problem to be solved by this specification is to provide electrodes and energy storage devices that enable more lithium ions to participate in the charge and discharge reaction.

[0010] Methods for solving problems

[0011] The electrode of the energy storage device in the first method has a current collector and an active material layer on the current collector. The active material layer has carbon nanowalls. In a single charge or discharge cycle, the carbon nanowalls enable more than two lithium ions per carbon atom to participate in the charge / discharge reaction.

[0012] The electrodes of this energy storage device have carbon nanowalls. During a single charge or discharge cycle, the carbon nanowalls enable each carbon atom to have more than two lithium ions participate in the charge / discharge reaction.

[0013] In contrast, the graphite used in conventional electrodes allows each of the six carbon atoms in a six-membered ring to absorb or release one lithium ion. That is, graphite allows each carbon atom to absorb or release 1 / 6 of a lithium ion.

[0014] Electrodes using carbon nanowalls in energy storage devices can enable more than 12 times more lithium ions to participate in the charge-discharge reaction compared to electrodes using graphite. Therefore, energy storage devices with carbon nanowall electrodes exhibit superior volumetric energy density and gravimetric energy density. Consequently, these devices can enable electronic devices, home appliances, vehicles, and other similar products to operate for extended periods on a single charge.

[0015] Invention Effects

[0016] This specification provides electrodes and energy storage devices that enable more lithium ions to participate in the charging and discharging reaction. Attached Figure Description

[0017] Figure 1 This is a schematic structural diagram of the lithium-ion secondary battery LiB1 according to the first embodiment.

[0018] Figure 2 This is a diagram conceptually illustrating the structure of the carbon nanowall CNW1 of the lithium-ion secondary battery LiB1 according to the first embodiment.

[0019] Figure 3 This is a schematic diagram showing a cross-section of the carbon nanowall CNW1 of the lithium-ion secondary battery LiB1 according to the first embodiment.

[0020] Figure 4 This is a schematic diagram showing the tilt of the carbon nanowalls of the LiB1 lithium-ion secondary battery according to the first embodiment.

[0021] Figure 5 This is a diagram showing the carbon nanowall of the lithium-ion secondary battery LiB1 in the first embodiment, viewed from a direction perpendicular to the surface of the negative electrode current collector N1.

[0022] Figure 6 This diagram illustrates the case where the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowall is small.

[0023] Figure 7 This is a hypothetical diagram illustrating the adsorption of lithium ions and the precipitation of lithium or lithium compounds by the carbon nanowall CNW1 of the lithium-ion secondary battery LiB1 according to the first embodiment.

[0024] Figure 8 This is a schematic structural diagram showing the structure of a manufacturing apparatus for growing carbon nanowalls CNW1 in a lithium-ion secondary battery LiB1 according to the first embodiment.

[0025] Figure 9 This is a schematic structural diagram of the lithium-ion capacitor LiC1 according to the second embodiment.

[0026] Figure 10 This is a schematic structural diagram of a lithium-ion capacitor LiC2, a modified example of the second embodiment.

[0027] Figure 11 This is a microscope image of the carbon nanowall as viewed from a direction perpendicular to the surface of the metal plate.

[0028] Figure 12 This is a microscope image of a carbon nanowall in a cross-section perpendicular to the surface of a metal plate.

[0029] Figure 13 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery using carbon nanowalls with a height of 1 μm as the negative electrode.

[0030] Figure 14 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery using a 4μm high carbon nanowall as the negative electrode.

[0031] Figure 15 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery using carbon nanowalls with a height of 10 μm as the negative electrode.

[0032] Figure 16 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery that uses graphite as the negative electrode.

[0033] Figure 17 This is a graph comparing the charging voltage of lithium-ion secondary batteries when carbon nanowalls and graphite are used as the negative electrode.

[0034] Figure 18 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 0 nm in height.

[0035] Figure 19 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 20 nm in height.

[0036] Figure 20 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 50 nm in height.

[0037] Figure 21 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 100 nm in height.

[0038] Figure 22This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 200 nm in height.

[0039] Figure 23 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 500 nm in height.

[0040] Figure 24 This is a scanning electron microscope image showing the surface of a carbon nanowall with a height of 500 nm.

[0041] Figure 25 This is a scanning electron microscope image showing a cross-section of a carbon nanowall with a height of 500 nm.

[0042] Figure 26 This is a scanning electron microscope image showing the surface of a carbon nanowall with a height of 50 nm.

[0043] Figure 27 This is a scanning electron microscope image showing a cross-section of a carbon nanowall with a height of 50 nm.

[0044] Figure 28 The image shown is a scanning electron microscope image of a carbon nanowall after repeated charging and discharging (1).

[0045] Figure 29 The image shown is a scanning electron microscope image of a carbon nanowall after repeated charging and discharging (image 2).

[0046] Figure 30 The image shown is a scanning electron microscope image of a carbon nanowall after repeated charging and discharging (3).

[0047] Figure 31 This is a scanning electron microscope image showing the cross-section of the negative electrode of a charged lithium-ion secondary battery.

[0048] Figure 32 This is a graph showing the X-ray diffraction results of the negative electrode of a charged lithium-ion secondary battery. Detailed Implementation

[0049] The following description uses electrodes and energy storage devices as examples, with reference to the accompanying drawings, to illustrate specific embodiments. In this specification, an energy storage device refers to a device capable of charging and discharging. Energy storage devices include lithium-ion primary batteries, lithium-ion secondary batteries, lithium-ion capacitors, and other devices that utilize lithium ions for charging and discharging.

[0050] (First Implementation)

[0051] 1. Lithium-ion secondary battery

[0052] Figure 1This is a schematic structural diagram of the lithium-ion secondary battery LiB1 according to the first embodiment. The lithium-ion secondary battery LiB1 has a positive electrode PE, a negative electrode NE, a separator Sp1, an electrolyte ES1, and a container V1.

[0053] The positive electrode PE is the positive electrode of the lithium-ion secondary battery LiB1. The positive electrode PE has a positive current collector P1 and a positive active material layer P2. The positive active material layer P2 is formed on the surface of the first surface P1a and the second surface P1b of the positive current collector P1.

[0054] The positive current collector P1 is, for example, a metal foil. The shape of the positive current collector P1 can also be other shapes. The material of the positive current collector P1 is, for example, Al or Ti. The material of the positive current collector P1 can also be other metals or other conductive materials.

[0055] The positive electrode active material layer P2 contains a positive electrode active material, a conductive agent, and a binder. The positive electrode active material layer P2 may also contain a thickener, etc. Examples of positive electrode active materials include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, and ternary materials. Examples of conductive agents 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.

[0056] The negative electrode NE is the negative electrode of the lithium-ion secondary battery LiB1. The negative electrode NE has a negative current collector N1 and a negative active material layer N2. The negative active material layer N2 is formed on the surface of the first surface N1a and the second surface N1b of the negative current collector N1.

[0057] The negative current collector N1 is, for example, a metal foil. The shape of the negative current collector N1 can also be other shapes. The material of the negative current collector N1 is, for example, Cu. The material of the negative current collector N1 can also be other metals or other conductive materials.

[0058] The negative electrode active material layer N2 contains negative electrode active material. The negative electrode active material layer N2 includes carbon nanowalls CNW1 as the negative electrode active material. The carbon nanowalls CNW1 will be described later.

[0059] The separator Sp1 is used to electrically insulate the positive electrode PE from the negative electrode NE. The separator Sp1 allows lithium ions in the electrolyte ES1 to pass through.

[0060] Electrolyte ES1 has the characteristic of transferring lithium ions between the positive electrode PE and the negative electrode NE. Electrolyte ES1 fills container V1. Electrolyte ES1 is, for example, a liquid formed by dissolving lithium salts such as lithium hexafluorophosphate (LiPF6) in dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), etc.

[0061] The container V1 contains the positive electrode PE, the negative electrode NE, the separator Sp1, and the electrolyte ES1. The container V1 is made of a material that is difficult to react with the electrolyte ES1.

[0062] 2. Carbon nanowalls

[0063] In this specification, carbon nanowalls refer to conductive nanostructures composed primarily of carbon atoms that are configured as walls on substrates such as negative electrode current collector N1.

[0064] Figure 2 This is a conceptual diagram illustrating the structure of the carbon nanowall CNW1 in the LiB1 lithium-ion secondary battery of the first embodiment. Figure 2 The diagram conceptually illustrates a single graphene sheet GS1. The carbon nanowall CNW1 exhibits electrical conductivity. The carbon nanowall CNW1 can be composed of multiple graphene sheets GS1. Furthermore, the graphene sheet GS1 may not be a completely graphene structure, but rather a thin film primarily composed of carbon with a six-membered ring structure. Additionally, the graphene sheet GS1 can also be a mosaic structure primarily composed of carbon with a six-membered ring structure. A mosaic structure refers to a structure in which multiple regions with a six-membered ring structure are discretely arranged. That is, the carbon nanowall CNW1 may not be a single crystal with a complete six-membered ring surface.

[0065] The negative electrode NE has a negative current collector N1 and a negative active material layer N2. The negative active material layer N2 has an amorphous carbon layer AC1 and a carbon nanowall CNW1.

[0066] Carbon nanowall CNW1 is a graphite-like material formed by stacking approximately 10 layers of graphene sheets GS1 along the thickness direction of the carbon nanowall CNW1. The number of layers can also be other than those mentioned above. Because carbon nanowall CNW1 is a graphite-like material, it possesses higher electrical conductivity than carbon materials such as activated carbon.

[0067] An amorphous carbon layer AC1 is located between the negative current collector N1, which is a conductor such as a metal, and the carbon nanowall CNW1. The amorphous carbon layer AC1 is the starting layer 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, it is 10 nm or more and 100 nm or less. More preferably, it is 12 nm or more and 30 nm or less. Furthermore, depending on the carbon nanowall growth method, the amorphous carbon layer AC1 may sometimes be absent. The amorphous carbon layer AC1 is conductive.

[0068] In the carbon nanowall CNW1, a root R1 is located on the side of the negative current collector N1, and a front end E1 is located on the opposite side of the negative current collector N1. In most cases, the root R1 is a fixing part that is fixed to the negative current collector N1 by means of the amorphous carbon layer AC1. Alternatively, the root R1 is a connecting part that is electrically connected to either the negative current collector N1 or the amorphous carbon layer AC1.

[0069] In the carbon nanowall CNW1, graphene sheets GS1 are formed in a direction intersecting with the surfaces (first surface N1a, second surface N1b) of the negative electrode current collector N1. Figure 2 In the diagram, the graphene sheet GS1 is approximately perpendicular to the negative current collector N1. Therefore, the graphene sheet GS1 has a front end portion E1 at its front end. The front end portion E1 is the portion located at the front end of the graphene sheet GS1.

[0070] Furthermore, as mentioned above, the carbon nanowall CNW1 is graphite formed by stacking multiple 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, they actually randomly merge to form an overlapping shape (see reference). Figure 11 ).like Figure 2 As shown, the distance between adjacent wall-shaped graphite is called the wall spacing.

[0071] The average wall spacing D1, which represents the average value of the wall spacing, is related to the density of carbon nanowalls CNW1. That is, the wider the average wall spacing D1, the lower the density of carbon nanowalls CNW1. Conversely, the narrower the average wall spacing D1, the higher the density of carbon nanowalls CNW1.

[0072] 2-1. Wall dimensions

[0073] Figure 3 This diagram schematically illustrates a cross-section of the carbon nanowall CNW1 in the lithium-ion secondary battery LiB1 according to the first embodiment. The average height H1 of the carbon nanowall CNW1 can be 100 nm or more. Alternatively, the average height H1 of the carbon nanowall CNW1 can be 200 nm or more. When the average height H1 of the carbon nanowall CNW1 is 100 nm or more, lithium is easily deposited starting from the carbon nanowall CNW1.

[0074] Even if the average height H1 of the carbon nanowalls CNW1 is high, there is no problem. However, it takes time to form carbon nanowalls CNW1 with a high height. From a productivity point of view, the average height H1 of the carbon nanowalls CNW1 can be 200 μm or less. Preferably, it is 100 μm or less. More preferably, it is 50 μm or less. Even more preferably, it is 10 μm or less.

[0075] Therefore, the average height H1 of the carbon nanowall 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 nanowall CNW1 may also be 200 nm or more and 200 μm or less. Alternatively, it may also be 200 nm or more and 10 μm or less. Alternatively, it may also be 200 nm or more and 10 μm or less.

[0076] The average thickness W1 of the carbon nanowall CNW1 is, for example, 0.5 nm or more and 100 nm or less. Preferably, it is 1 nm or more and 50 nm or less. More preferably, it is 1.5 nm or more and 30 nm or less.

[0077] The interlayer spacing of graphite is approximately 0.35 nm. Therefore, the thickness of a carbon nanowall CNW1 composed of 10 layers of graphene sheets GS1 is approximately 3.5 nm. Although dependent on manufacturing conditions, the average thickness of the carbon nanowall CNW1 is considered to be approximately 3.5 nm. Typically, the carbon nanowall CNW1 is considered to consist of 5 to 20 layers of graphene sheets GS1. The thickness of the carbon nanowall CNW1 is, for example, greater than 1.5 nm and less than 7 nm.

[0078] 2-2. Wall partition

[0079] 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 illustrative, and values ​​other than those mentioned above are also possible. It should be noted that the long walls of the carbon nanowalls do not necessarily grow parallel to each other; sometimes the walls merge together (see [reference]). Figure 11 Therefore, the spacing between the carbon nanowalls CNW1 near the confluence site is narrower than the spacing between the carbon nanowalls CNW1 at other sites.

[0080] 2-3. Angle of the wall

[0081] Figure 4 This is a schematic diagram showing the tilt of the carbon nanowalls of the LiB1 lithium-ion secondary battery according to the first embodiment. Figure 4 The diagram shows the projection of the carbon nanowall CNW1 onto the first surface N1a of the negative electrode current collector N1.

[0082] The projection region PR1 formed by projecting the carbon nanowall CNW1(a) onto the first surface N1a of the negative electrode current collector N1 does not include carbon nanowall CNW1(b) other than CNW1(a). However, the confluence of carbon nanowall CNW1 is not limited to this. As long as the electrolyte and lithium ions can enter the tiny region defined by the confluenced carbon nanowall CNW1, the necessary reactions for battery operation will occur in that tiny region.

[0083] like Figure 4 As shown, there is an intermediate region PR2 between projection regions PR1 and PR2. The intermediate region PR2 is located between... Figure 4 When observing the negative current collector N1 in the direction of arrow J1, the observer can use a scanning electron microscope (SEM) or similar device to observe the visible area. Figure 4 The direction of arrow J1 is perpendicular to the first surface N1a of the negative current collector N1.

[0084] The average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowall CNW1 is greater than 80° and less than 90°. Here, the average angle θ is the average of angles less than 90°. If one angle θ1 of the angles formed by the carbon nanowall and the negative electrode current collector N1 is an acute angle, then the other angle θ2 is an obtuse angle. The average angle θ is obtained by averaging the smaller angle θ1 within such an angle range.

[0085] By growing the individual carbon nanowalls approximately vertically and arranging them such that they do not contact each other at their leading ends (E1), the electrolyte and lithium ions can enter between the carbon nanowalls. Therefore, the carbon nanowalls as a whole can effectively function as electrodes.

[0086] However, this is not necessarily the case near the areas where carbon nanowalls converge. Furthermore, when carbon nanowalls are formed on a large-area substrate, localized contact points may also form at the leading ends (E1) of the carbon nanowalls, but this does not reduce the overall functionality.

[0087] The average angle θ is determined by the growth conditions of the carbon nanowalls, but based on this average angle θ, it is necessary to set the average height H1 and average wall spacing D1 of the carbon nanowalls so that they do not contact each other at their upper ends. According to these values ​​of average height H1 and average wall spacing D1, the electrolyte and lithium ions cannot enter the gaps between the carbon nanowalls, and a portion of the carbon nanowalls cannot function as electrodes.

[0088] For example, when the average height H1 of the carbon nanowall is 5 μm and the average wall spacing D1 is 100 nm, an angle of 88.9° or greater is required to avoid contact with adjacent vertical walls. Alternatively, when the average height H1 of the carbon nanowall is 0.6 μm and the average wall spacing D1 is 100 nm, an angle of 80.4° or greater is required to avoid contact with adjacent vertical walls.

[0089] The numerical values ​​representing the structure of these carbon nanowalls CNW1 are shown in Table 1. However, these numerical ranges are illustrative and are not limited to these ranges.

[0090] [Table 1]

[0091] The wall height is above 100 nm and below 200 μm.

[0092] The wall thickness is between 0.5 nm and 100 nm.

[0093] The wall spacing is above 10nm and below 500nm.

[0094] The angle of the wall is between 80° and 90°.

[0095] 2-4. Surface area of ​​carbon nanowalls

[0096] Here, the surface area of ​​the carbon nanowall CNW1 is explained. For simplicity, the shape of the carbon nanowall is assumed to be lattice-like. The walls rarely extend in straight lines, so the actual shape of the carbon nanowall deviates from a lattice-like structure.

[0097] Figure 5 This is a diagram showing 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. Additionally, in Figure 5 In this context, as mentioned above, the shape of the carbon nanowalls is assumed to be lattice-like.

[0098] If the spacing between the carbon nanowalls is I1, then the area SS1 of a square with one side spaced I1 is I1. 2 The area SS1 is not the surface area of ​​the carbon nanowall, but rather the surface area of ​​the carbon material when the surface of the negative electrode current collector N1 is fully coated with carbon material. Figure 5 In the equation, the area SS2 occupied by the side of the carbon nanowall in a square with one side spaced I1 as a repeating unit is given by the following formula.

[0099] SS2 = 8 × (D1 / 2) × H1

[0100] =4×D1×H1

[0101] H1: Average height of the carbon nanowall

[0102] D1: Average wall spacing

[0103] The surface area SS3 of a carbon nanowall in the presence of a square region with one side spaced I1 is given by the following formula.

[0104] SS3 = SS1 + SS2

[0105] Therefore, SS3 / SS1 represents the rate of increase in surface area caused by the presence or absence of carbon nanowalls.

[0106] SS3 / SS1=(4×D1×H1+I1 2 ) / I1 2

[0107] ≈4×D1×H1 / I1 2

[0108] ≈4×H1 / D1

[0109] Here, we use H1 >> I1 and D1 ≈ I1.

[0110] Thus, the higher the carbon nanowalls are and the narrower the spacing between them, the larger the surface area of ​​the carbon material in the LiB1 lithium-ion secondary battery.

[0111] Table 2 shows the relationship between the size and angle of the carbon nanowalls and the rate of increase in surface area. It is assumed 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 and discharge rate of the LiB1 lithium-ion secondary battery.

[0112] As shown in Table 2, if the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowall is 80° or more and 90° or less, the surface area of ​​the carbon material in the lithium-ion secondary battery LiB1 can be increased by more than 20 times. 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 angle is 89° or more, the increase in surface area is approximately 400 times or more.

[0113] [Table 2]

[0114]

[0115] 2-5. Cases with small average angles

[0116] Here, we will explain the case where the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowall is small.

[0117] Figure 6 This diagram illustrates the case where the average angle θ between the first surface N1a of the negative electrode current collector N1 and the carbon nanowall is small. (See diagram for example.) Figure 6 As shown, the projection area PR3 formed 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).

[0118] When the front end E1 of the carbon nanowall CNW1(c) is projected onto the first surface N1a of the negative current collector N1, the front end E1 of the carbon nanowall CNW1(c) traverses the side of the adjacent carbon nanowall CNW1(d).

[0119] Thus, when the average angle θ is small, the electrolyte has difficulty penetrating to the vicinity of the root R1 of the carbon nanowall CNW1. Consequently, the battery capacity decreases when the electrolyte does not completely cover the carbon nanowall.

[0120] 3. Lithium-ion-mediated charge-discharge reaction

[0121] 3-1. Charge and discharge reaction

[0122] The negative electrode NE has carbon nanowalls CNW1. In a single charge or discharge cycle, the carbon nanowalls CNW1 enable more than two lithium ions to participate in the charge and discharge reaction for each carbon atom.

[0123] Here, charge-discharge reaction refers to a chemical reaction, for example, represented by the following chemical reaction formula.

[0124]

[0125]

[0126]

[0127] Formula (1) or (2) is, for example, a reaction that can occur inside the negative electrode active material layer N2. Formula (3) is, for example, a reaction that can occur inside the positive electrode active material layer P2. Both reactions involve lithium ions and electrons. The charge-discharge reaction refers to the chemical reaction in which lithium ions are introduced into the positive electrode PE or the negative electrode NE, and electron donation and acceptance occur. Through this charge-discharge reaction, phenomena such as the absorption or release of lithium ions, and the precipitation, accumulation, adsorption, and dissolution of lithium or lithium compounds occur. In addition, in the case of lithium or lithium compound precipitation, the charge-discharge reaction occurs outside the positive electrode active material layer P2 or the negative electrode active material layer N2. Furthermore, the type of charge-discharge reaction varies depending on the materials of the positive electrode active material layer P2 and the negative electrode active material layer N2.

[0128] 3-2. Lithium deposition in carbon nanowalls

[0129] The carbon nanowall CNW1 has a surface capable of lithium deposition. Therefore, lithium can sometimes be deposited on the surface of the carbon nanowall CNW1 during charging or discharging.

[0130] In a single charge-discharge cycle, the CNW1 carbon nanowall enables more than two lithium ions per carbon atom to participate in the charge-discharge reaction. In a single charge-discharge cycle, the CNW1 carbon nanowall enables more than ten lithium ions per carbon atom to participate in the charge-discharge reaction. In a single charge-discharge cycle, the CNW1 carbon nanowall enables more than twenty lithium ions per carbon atom to participate in the charge-discharge reaction.

[0131] The carbon nanowall CNW1 can precipitate lithium. Therefore, in principle, there is no upper limit to the number of lithium ions that can participate in the charge-discharge reaction per carbon atom. However, the greater the number of lithium ions participating in the charge-discharge reaction per carbon atom in a single charge or discharge cycle, the larger the volume of deposited lithium. Therefore, the number of lithium ions participating in the charge-discharge reaction per carbon atom can 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.

[0132] The height of the deposited lithium can be, for example, 200 μm or less. Preferably, it is 100 μm or less. More preferably, it is 50 μm or less.

[0133] For example, in the case where 30 lithium atoms are deposited on the surface of the carbon nanowall CNW1 for every 1 carbon atom, its state can be hypothetically represented by the following chemical formula (composition formula).

[0134] Li 30 C

[0135] Thus, when lithium is deposited on the surface of the carbon nanowall CNW1, it is hypothetically represented by the following chemical formula (composition formula).

[0136] Li X C

[0137] Here, X is a real number greater than 0, which varies with charging and discharging. Due to lithium deposition, the theoretical maximum value of X is infinite.

[0138] 3-3. Compared with existing technologies

[0139] Previously, one lithium ion entered the six-membered ring of each carbon atom. This state was represented by the following formula.

[0140] LiC6

[0141] Therefore, the carbon nanowall CNW1 of the first embodiment can enable a much larger number of lithium ions to participate in the charge-discharge reaction compared to conventional methods. In other words, the lithium-ion secondary battery LiB1 of the first embodiment has higher performance.

[0142] 3-4. Absorption and Precipitation States

[0143] Figure 7 This is a hypothetical diagram illustrating the adsorption of lithium ions and the precipitation of lithium or lithium compounds by the carbon nanowall CNW1 of the lithium-ion secondary battery LiB1 according to the first embodiment.

[0144] Figure 7 A hypothetical example is shown. For example... Figure 7 As shown in region LA1, lithium ions are assumed to enter the interior of the carbon nanowall CNW1. Theoretically, the maximum value for lithium ion intercalation is in the case of LiC6. Therefore, the number of lithium atoms per carbon atom cannot exceed 1 / 6.

[0145] like Figure 7 As shown in region LA2, it is believed that lithium atoms or lithium ions are adsorbed or accumulated on the surface of carbon nanowall CNW1, and lithium or lithium compounds in the metallic state are precipitated.

[0146] As described above, the carbon nanowall CNW1 can adsorb or deposit lithium. If lithium adsorption is thermodynamically more favorable than lithium deposition, lithium adsorption is assumed to occur first, followed by lithium deposition once adsorption is saturated. If lithium deposition is thermodynamically more favorable than lithium adsorption, lithium deposition is assumed, and lithium adsorption does not occur.

[0147] As the lithium-ion secondary battery LiB1 continues to charge, lithium is deposited on the surface of the carbon nanowalls CNW1. Lithium also fills the gaps between the carbon nanowalls CNW1. When the lithium-ion secondary battery LiB1 continues to charge, it is considered that the lithium deposition exceeds the height of the carbon nanowalls CNW1.

[0148] 3-5. Other states

[0149] In addition to the absorption or release of lithium ions and the precipitation, accumulation, and adsorption of lithium, phenomena such as lithium dissolution, precipitation, accumulation, adsorption, and dissolution of lithium compounds also occur.

[0150] 4. Manufacturing equipment

[0151] An apparatus for manufacturing carbon nanowalls CNW1 formed on the surface of the negative electrode current collector N1 is described.

[0152] Figure 8 This is a schematic structural diagram showing the structure 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 includes a plasma generation chamber 46 and a reaction chamber 10. The plasma generation chamber 46 is used to generate plasma within itself and also generates free radicals supplied to the reaction chamber 10. The reaction chamber 10 is used to form carbon nanowalls CNW1 using the free radicals generated in the plasma generation chamber 46.

[0153] In addition, the manufacturing apparatus 1 has a waveguide 47 and a quartz window 4.

[0154] 8 and slot antenna 49. Waveguide 47 is used to introduce microwaves 39. Slot antenna 49 is used to introduce microwaves 39 from quartz window 48 into plasma generation chamber 46.

[0155] The plasma generation chamber 46 is used to generate surface wave plasma (SWP) via microwaves 39. A radical source inlet 42 is provided in the plasma generation chamber 46. The radical source inlet 42 is used to supply gas, which serves as a radical source, into the interior of the plasma 61 generated in the plasma generation chamber 46.

[0156] 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 and the reaction chamber 10. The partition wall 44 also serves as the first electrode 22 to which the voltage is applied. Furthermore, a through hole 14 is formed in the partition wall 44. This is to supply the free radicals generated in the plasma generation chamber 46 to the reaction chamber 10.

[0157] The reaction chamber 10 is used to generate capacitively coupled plasma (CCP). Additionally, 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 feed inlet 12, and an exhaust port 16. The second electrode 24 is used to apply a voltage between itself and the first electrode 22. The heater 25 is used to heat the negative electrode current collector N1 to control its temperature. The feed inlet 12 is used to supply carbon-based gas 32, which forms the carbon nanowalls. The exhaust port 16 is connected to a vacuum pump, etc. The vacuum pump is used to adjust the pressure inside the reaction chamber 10.

[0158] As described above, partition 44 also functions as the first electrode 22 for applying voltage between itself and the second electrode 24. A power supply and circuit are connected to the first electrode 22. This is to control the potential of the first electrode 22 in time. The second electrode 24 is used to apply voltage between itself and the first electrode 22. Furthermore, the second electrode 24 also serves as a mounting platform for the negative current collector N1. The second electrode 24 is grounded. The distance between the first electrode 22 and the second electrode 24 is approximately 5 cm. However, this value is not strictly limited.

[0159] 5. Manufacturing method of negative electrode

[0160] 5-1. Amorphous carbon layer formation process

[0161] First, a negative electrode current collector N1, formed before the carbon nanowall CNW1 is formed, is placed inside the manufacturing apparatus 1. At this time, the first surface N1a of the negative electrode current collector N1 faces upward, 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 via the slot antenna 49. As a result, a high-density plasma 60 is generated.

[0162] The high-density plasma 60 then diffuses within the plasma generation chamber 46, becoming plasma 61. Plasma 61 contains ions from a free radical source supplied from the free radical source inlet 42. Hydrogen is used as the free radical source. Alternatively, oxygen, nitrogen, or other gases may also be used. Most of the ions in plasma 61 are neutralized by collisions with the partition wall 44, becoming free radicals. Free radicals 38 enter the reaction chamber 10 through the through-hole 14 in the partition wall 44.

[0163] Inside the reaction chamber 10, in addition to free radicals 38, a carbonaceous gas 32 is supplied from the feed inlet 12. The carbonaceous 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 plasma 34 inside the reaction chamber 10.

[0164] Particles and free radicals 38 generated from carbon-based gas 32 as a raw material are mixed in the atmosphere of plasma 34. Then, in the atmosphere of plasma 34, an amorphous carbon layer AC1 grows on the surface of the negative electrode current collector N1.

[0165] In this way, the carbon-based gas plasma inside the manufacturing apparatus 1 is supplied to the negative electrode current collector N1, thereby forming an amorphous carbon layer AC1 on the negative electrode current collector N1.

[0166] The internal pressure of reaction chamber 10 is in the range of 5 mTorr or more and 2000 mTorr or less (0.65 Pa or more and 267 Pa or less). Additionally, the temperature of the negative electrode current collector N1 is in the range of 100°C or more and 800°C or less. Of course, these are merely examples and are not limited to these numerical ranges.

[0167] 5-2. Carbon Nanowall Growth Process

[0168] Next, inside the manufacturing apparatus 1, carbon nanowalls CNW1 are grown on an amorphous carbon layer AC1. Similar to the growth of the amorphous carbon layer AC1, plasma 61 is generated. Hydrogen is used as the radical source for radical 38, and carbonaceous gases 32, such as CH4 or C2F6, are used.

[0169] In this way, the carbon-based gas plasma inside the manufacturing device 1 is supplied to the negative electrode current collector N1, so that the carbon nanowall grows on the amorphous carbon layer AC1.

[0170] The internal pressure of reaction chamber 10 is in the range of 5 mTorr or more and 2000 mTorr or less (0.65 Pa or more and 267 Pa or less). Additionally, the temperature of the negative electrode current collector N1 is in the range of 100°C or more and 800°C or less. Of course, these are merely examples and are not limited to these numerical ranges.

[0171] 5-3. Cleaning process

[0172] After the carbon nanowall CNW1 has grown to a certain extent, the negative current collector N1 is removed from the manufacturing apparatus 1. At this point, the height H1 of the carbon nanowall CNW1 is, for example, 1000 nm.

[0173] Next, the interior of manufacturing apparatus 1 is cleaned. Carbon material is scraped off from the inner wall surface. Alternatively, hydrogen plasma or the like is used to remove the carbon material from the inner wall surface. In this process, the growth process is stopped, and the interior of manufacturing apparatus 1 is cleaned.

[0174] 5-4. Repeated processes, etc.

[0175] Then, the above-described carbon nanowall growth and cleaning processes are repeated. This yields a carbon nanowall CNW1 with a sufficient height H1. Furthermore, the negative electrode current collector N1 is flipped over, and the carbon nanowall CNW1 is formed on its second surface N1b. Even if the negative electrode current collector N1 is positioned with the carbon nanowall CNW1 as the bottom side, the carbon nanowall CNW1 will not cause any problems.

[0176] 6. Manufacturing method of lithium-ion secondary batteries

[0177] 6-1. Negative electrode manufacturing process

[0178] The negative electrode NE is manufactured as described above. A layer of negative electrode active material N2 is formed on the negative electrode current collector N1.

[0179] 6-2. Positive electrode manufacturing process

[0180] Next, the positive electrode PE is manufactured. A coating solution is applied to the positive electrode current collector P1 and allowed to dry. The coating solution contains positive electrode active material, conductive agent, and binder. Alternatively, a pressing process can be performed on the positive electrode PE.

[0181] 6-3. Electrolyte Injection Process

[0182] Next, the positive electrode PE and the negative electrode NE are alternately arranged inside the container V1, separated by a diaphragm Sp1. Then, the electrolyte ES1 is injected into the container V1. After that, the opening of the container V1 is sealed.

[0183] 6-4. Other processes

[0184] Other processes, such as aging, can also be implemented.

[0185] 7. Effects of the first implementation method

[0186] The negative electrode NE of the lithium-ion secondary battery LiB1 in the first embodiment has a carbon nanowall CNW1. The surface area of ​​the carbon nanowall CNW1 is sufficiently large. The carbon nanowall CNW1 enables a large number of lithium ions to participate in the charge-discharge reaction for each carbon atom. Therefore, the capacity of the lithium-ion secondary battery LiB1 is very large.

[0187] As mentioned above, carbon nanowalls (CNW1) enable a large number of lithium ions to participate in the charge-discharge reaction. Therefore, the amount of CNW1 can be reduced. That is, the negative electrode is smaller and lighter than before. Consequently, the volumetric energy density and gravimetric energy density of the lithium-ion secondary battery LiB1 are improved compared to existing technologies.

[0188] 8. Variations

[0189] 8-1. Formation surface of the positive electrode active material layer or the negative electrode active material layer

[0190] The positive electrode active material layer P2 can also be formed only on one side of the positive electrode current collector P1. The negative electrode active material layer N2 can also be formed only on one side of the negative electrode current collector N1.

[0191] 8-2. Carbon nanowalls at the positive electrode

[0192] Depending on the type of energy storage device, carbon nanowalls may sometimes be formed on the positive current collector P1. Even in this case, the electrode for the energy storage device has a current collector and an active material layer on the current collector. The active material layer has carbon nanowalls.

[0193] 8-3. Amorphous carbon layer

[0194] The negative electrode NE may also lack the amorphous carbon layer AC1. In this case, a carbon nanowall CNW1 is directly formed on the negative electrode current collector N1. Furthermore, the amorphous carbon layer AC1 may or may not function as the negative electrode active material.

[0195] 8-4. Amorphous carbon on carbon nanowall CNW1

[0196] Furthermore, immediately after the growth of the carbon nanowall CNW1, its surface can also be covered by amorphous carbon. This amorphous carbon can then be removed using H2O2.

[0197] 8-5. Laminated bodies

[0198] The electrode can also be a laminate formed by stacking a positive electrode PE and a negative electrode NE. In the laminate, the positive electrode PE and the negative electrode NE alternate, and a separator Sp1 is disposed between the positive electrode PE and the negative electrode NE.

[0199] 8-6. Cleaning process

[0200] The cleaning process can be omitted depending on the height of the carbon nanowall CNW1. Furthermore, depending on the manufacturing apparatus 1, the cleaning process can sometimes be omitted.

[0201] 8-7. Combinations

[0202] The aforementioned variations can also be freely combined.

[0203] (Second Implementation)

[0204] The second embodiment will be described.

[0205] 1. Lithium-ion capacitors

[0206] Figure 9 This is a schematic structural diagram of the lithium-ion capacitor LiC1 according to the second embodiment. The lithium-ion capacitor LiC1 includes a positive electrode PE2, a negative electrode NE, a separator Sp1, an electrolyte ES1, and a container V1.

[0207] The positive electrode PE2 has a positive current collector P1 and a positive active material layer P3. The positive active material layer P3 is, for example, activated carbon.

[0208] 2. Variations

[0209] Figure 10 This is a schematic structural diagram of a modified example of the second embodiment of a lithium-ion capacitor LiC2. The lithium-ion capacitor LiC2 has a positive electrode PE3, a negative electrode NE, a separator Sp1, an electrolyte ES1, and a container V1.

[0210] The positive electrode PE3 has a positive current collector P1 and a positive active material layer P4. The positive active material layer P4 has a carbon nanowall CNW2. The carbon nanowall CNW2 of the positive electrode PE3 is the same as the carbon nanowall CNW1 of the negative electrode NE. Of course, the conditions of the wall can also be changed.

[0211] (Third Implementation)

[0212] The third embodiment will be described.

[0213] The basic structure of the lithium-ion secondary battery in the third embodiment is the same as that of the lithium-ion secondary battery LiB1 in the first embodiment.

[0214] 1. Amount of active substance layer

[0215] As described in the first embodiment, the carbon nanowall CNW1 enables a large number of lithium ions to participate in the charge-discharge reaction. Therefore, the negative electrode active material layer N2 is much lighter and has a smaller volume compared to the positive electrode active material layer P2.

[0216] The number of lithium atoms per unit area of ​​the positive electrode active material layer P2 is more than twice the number of carbon atoms per unit area of ​​the negative electrode active material layer N2. Preferably, the number of lithium atoms per unit area of ​​the positive electrode active material layer P2 is more than 100 times the number of carbon atoms per unit area of ​​the negative electrode active material layer N2. Preferably, the number of lithium atoms per unit area of ​​the positive electrode active material layer P2 is less than 100,000 times the number of carbon atoms per unit area of ​​the negative electrode active material layer N2. These upper limits are limited by the volume of deposited lithium.

[0217] 2. Effects of the third implementation method

[0218] The lithium-ion secondary battery of the third embodiment has a low number of carbon atoms. Therefore, this lithium-ion secondary battery has high volumetric energy density and high gravimetric energy density. Thus, this lithium-ion secondary battery can contribute to a low-carbon society.

[0219] 3. Variations

[0220] 3-1. Lithium-ion capacitors

[0221] The technology of the third embodiment can also be applied to lithium-ion capacitors in the same way.

[0222] Example

[0223] (experiment)

[0224] 1. Carbon nanowalls on current collectors

[0225] 1-1. Manufacturing method

[0226] Using manufacturing apparatus 1, carbon nanowalls are grown on a metal plate made of Ti.

[0227] 1-2. Carbon nanowalls

[0228] Figure 11 This is a microscope image of the carbon nanowall as viewed from a direction perpendicular to the surface of the metal plate. For example... Figure 11 As shown, the carbon nanowalls grow randomly. Furthermore, the wall-like structures merge with each other as they grow. However, the spacing is relatively uniform.

[0229] Figure 12 This is a microscopic image of a carbon nanowall in a cross-section perpendicular to the surface of a metal plate. (Example) Figure 12 As shown, the carbon nanowalls are formed approximately perpendicular to the substrate.

[0230] 2. Lithium-ion secondary batteries

[0231] 2-1. Manufacturing of Lithium-ion Secondary Batteries

[0232] As an example, a lithium-ion secondary battery LiB1 according to the first embodiment was manufactured. The positive electrode current collector P1 is aluminum, and the positive electrode active material is lithium cobalt oxide. The negative electrode current collector N1 is copper, and the negative electrode active material is carbon nanowalls. The electrolyte is 1M LiPF6. The positive electrode active material layer is a region with a diameter of 1.6 cm. The negative electrode active material layer is a region with a diameter of 1.3 cm. The heights of the carbon nanowalls are 1 μm, 4 μm, and 10 μm.

[0233] The weights of lithium cobalt oxide are summarized in Table 3. The weight of the carbon nanowalls is calculated by subtracting the weight of the substrate before carbon nanowall growth from the weight of the substrate after carbon nanowall growth.

[0234] [Table 3]

[0235] Lithium cobalt oxide 0.046g

[0236] 0.00004g carbon nanowall (1μm height)

[0237] 0.00016g carbon nanowall (4μm height)

[0238] 0.00040g carbon nanowall (10μm height)

[0239] The positive electrode active material layer contains lithium cobalt oxide, a conductive agent, and a binder. The conductive agent is acetylene black. The binder is PVDF. The weight ratio of lithium cobalt oxide, acetylene black, and PVDF is 100:5:3.

[0240] In addition, as a comparative example, a lithium-ion secondary battery with a negative electrode using graphite instead of carbon nanowalls was manufactured. All other conditions were the same as in the examples. The weights of the graphite are summarized in Table 4.

[0241] [Table 4]

[0242] Lithium cobalt oxide 0.046g

[0243] 0.010g of graphite

[0244] 2-2. Capacity of Lithium-ion Secondary Batteries

[0245] Figure 13This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery using carbon nanowalls with a height of 1 μm as the negative electrode. Figure 13 The horizontal axis represents the charge / discharge capacity. Figure 13 The vertical axis represents voltage. The charging or discharging current is 0.5mA. For example... Figure 13 As shown, the discharge capacity of the lithium-ion secondary battery is 9.0 mAh.

[0246] Figure 14 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery using a 4μm high carbon nanowall as the negative electrode. Figure 14 The horizontal axis represents the charge / discharge capacity. Figure 14 The vertical axis represents voltage. The charging or discharging current is 0.5mA. For example... Figure 14 As shown, the discharge capacity of the lithium-ion secondary battery is 9.0 mAh.

[0247] Figure 15 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery using carbon nanowalls with a height of 10 μm as the negative electrode. Figure 15 The horizontal axis represents the charge / discharge capacity. Figure 15 The vertical axis represents voltage. The charging or discharging current is 0.5mA. For example... Figure 15 As shown, the discharge capacity of the lithium-ion secondary battery is 9.0 mAh.

[0248] like Figures 13 to 15 As shown, despite variations in the height of the carbon nanowalls, the discharge capacity of the lithium-ion secondary battery is 9.0 mAh. This suggests that while the active material on the negative electrode side has sufficient charge / discharge capacity, it is limited by the charge / discharge capacity of the active material on the positive electrode side. Furthermore, as shown in the following equation...

[0249] 9.0mAh / 0.046g = 196mAh / g

[0250] The measured discharge capacity reached approximately 72% of the theoretical capacity of lithium cobalt oxide, which is 274 mAh / g. This confirms the inference that the charge / discharge capacity is limited by the active material on the positive electrode side.

[0251] Figure 16 This is a graph showing the relationship between capacity and voltage in a lithium-ion secondary battery that uses graphite as the negative electrode. Figure 16 The horizontal axis represents the charge / discharge capacity. Figure 16 The vertical axis represents voltage. The charging or discharging current is 0.5mA. For example... Figure 16 As shown, the discharge capacity of the lithium-ion secondary battery is 3mAh.

[0252] Figure 17 This is a graph comparing the charging voltage of lithium-ion secondary batteries when carbon nanowalls and graphite are used as the negative electrode. Figure 17 The horizontal axis represents the charging capacity. Figure 17 The vertical axis represents voltage.

[0253] like Figure 17 As shown, the charging voltage rises slowly when graphite is used as the negative electrode. Lithium ions are intercalated between the graphene sheets of graphite. This intercalation occurs in stages from stage 4 (LiC). 24 The charging voltage changes gradually from stage 1 (LiC6) to stage 2. Corresponding to the time change in this stage, the charging voltage also changes slowly.

[0254] In contrast, the charging voltage increases sharply when carbon nanowalls are used as the negative electrode. This suggests two possibilities. The first possibility is that the intercalation phase ends quickly, and lithium is deposited. The second possibility is that lithium is deposited without intercalation.

[0255] 2-3. Lithium-ion secondary batteries

[0256] like Figure 17 As shown, the charging voltage when carbon nanowalls are used as the negative electrode is about 0.1V higher than that when graphite is used as the negative electrode.

[0257]

[0258] -3.04V

[0259]

[0260] -2.90V

[0261]

[0262] +0.90V

[0263] Equation (1) represents the case of lithium deposition or lithium ionization. Equation (2) represents the case of lithium ions intercalating or deintercalating into the interlayer of the graphene structure. Equation (3) represents the case of lithium cobalt oxide releasing or absorbing lithium ions.

[0264] In the case of the reaction of formula (1), the charging voltage is as follows.

[0265] 0.90V - (-3.04V) = 3.94V

[0266] In the case of the reaction in equation (2), the charging voltage is as follows.

[0267] 0.90V - (-2.90V) = 3.80V

[0268] It is believed that the difference in charging voltage when carbon nanowalls are used as the negative electrode and when graphite is used as the negative electrode is due to the difference between equation (1) and equation (2). That is, it is believed that when carbon nanowalls are used as the negative electrode, the reaction of equation (1) mainly occurs during charging and discharging, while when graphite is used as the negative electrode, the reaction of equation (2) mainly occurs during charging and discharging.

[0269] 2-4. Capacity of carbon nanowalls

[0270] The theoretical capacity of graphite is 372 mAh / g. Its chemical formula at this point is represented by LiC6.

[0271] The capacity of the carbon nanowall with a height of 1 μm is 2250000 mAh / g.

[0272] 9.0mAh / 0.000040g=2250000mAh / g

[0273] The capacity of a carbon nanowall with a height of 1 μm is about 600 times that of the theoretical capacity of graphite.

[0274] 2250000mAh / g / 372mAh / g=600

[0275] Therefore, if the precipitation state of Li in the carbon nanowall is represented by a chemical formula (composition formula), it becomes the following formula.

[0276] Li 600 C6(Li 100 C)

[0277] 3. High dependence of carbon nanowalls

[0278] 3-1. Lithium-ion secondary battery

[0279] A lithium-ion secondary battery was fabricated, using Li metal as the positive electrode and a material formed by carbon nanowalls on Cu as the negative electrode. An electrolyte was used in the lithium-ion secondary battery.

[0280] 3-2. Charge and discharge characteristics

[0281] Figure 18 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 0 nm in height. Figure 18 The horizontal axis represents capacity. Figure 18 The vertical axis represents voltage. In this case, there are no carbon nanowalls at the negative electrode, only copper foil. For example... Figure 18 As shown, the voltage drops immediately after the discharge begins.

[0282] Figure 19 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 20 nm in height. Figure 19 The horizontal axis represents capacity. Figure 19 The vertical axis represents voltage. For example... Figure 19 As shown, the capacity is 1.6mAh.

[0283] Figure 20 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 50 nm in height. Figure 20 The horizontal axis represents capacity. Figure 20 The vertical axis represents voltage. For example... Figure 20 As shown, the capacity is 7.1mAh.

[0284] Figure 21 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 100 nm in height. Figure 21 The horizontal axis represents capacity. Figure 21 The vertical axis represents voltage. For example... Figure 21 As shown, the capacity is 13.2mAh.

[0285] Figure 22 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 200 nm in height. Figure 22 The horizontal axis represents capacity. Figure 22 The vertical axis represents voltage. For example... Figure 22 As shown, the capacity is 13.3mAh.

[0286] Figure 23 This is a graph showing the discharge characteristics of a lithium-ion secondary battery using a negative electrode with a carbon nanowall of 500 nm in height. Figure 23 The horizontal axis represents capacity. Figure 23 The vertical axis represents voltage. For example... Figure 23 As shown, the capacity is 13.2mAh.

[0287] Table 5 summarizes the experimental data. As shown in Table 5, the lithium-ion secondary battery capacity saturates when the height of the carbon nanowall is above 100 nm. It is assumed that when the height of the carbon nanowall is above 100 nm, all the lithium in the positive electrode is consumed. Therefore, the preferred height of the carbon nanowall is above 100 nm.

[0288] [Table 5]

[0289]

[0290] 3-3. SEM Images

[0291] Figure 24 This is a scanning electron microscope image showing the surface of a carbon nanowall with a height of 500 nm. Figure 25 This is a scanning electron microscope image showing a cross-section of a carbon nanowall with a height of 500 nm. The carbon nanowall has grown to a relatively large size.

[0292] Figure 26 This is a scanning electron microscope image showing the surface of a carbon nanowall with a height of 50 nm. Figure 27 This is a scanning electron microscope image showing a cross-section of a carbon nanowall with a height of 50 nm. The carbon nanowall does not grow very well.

[0293] 4. Lithium deposition (part 1)

[0294] 4-1. Lithium-ion secondary battery

[0295] A lithium-ion secondary battery was fabricated, using Li metal as the positive electrode and a material formed by carbon nanowalls on Cu as the negative electrode. The electrolyte was used in the lithium-ion secondary battery. The height of the carbon nanowalls was 200 nm.

[0296] 4-2. SEM Images

[0297] Figure 28 The image shows a scanning electron microscope image (1) of the surface of the carbon nanowall during repeated charging and discharging. Figure 28 The carbon nanotube wall was clearly photographed.

[0298] Figure 29 The image shown is a scanning electron microscope image of the surface of the carbon nanowalls during repeated charging and discharging (image 2). Figure 29 This indicates that metallic lithium is deposited in the gaps between carbon nanowalls, and that metallic lithium fills most of the gaps.

[0299] Figure 30 The image shown is a scanning electron microscope image of the surface of the carbon nanowalls during repeated charging and discharging (image 3). Figure 30 The image shows a case where the upper layer of the deposited lithium metal connected to the carbon nanowall is completely buried.

[0300] like Figure 29 and Figure 30 As shown, metallic lithium is deposited, filling the gaps in the carbon nanowalls. Therefore, more lithium ions participate in the charge-discharge reaction compared to previous methods. This allows for more than two lithium ions to participate in the charge-discharge reaction for each carbon atom.

[0301] The fabricated lithium-ion secondary battery was subjected to 30 charge-discharge cycles. Even after these cycles, no dendrites were observed. It is speculated that this is because metallic lithium precipitates from the surface of carbon nanowalls, and this metallic lithium has good crystallinity, thus making it difficult for dendrites to form.

[0302] 5. Lithium deposition (part 2)

[0303] 5-1. Manufacturing of Lithium-ion Secondary Batteries

[0304] A coin-shaped lithium-ion secondary battery was fabricated. The positive electrode current collector P1 is aluminum, and the positive electrode active material is lithium cobalt oxide. The negative electrode current collector N1 is copper, and the negative electrode active material is a carbon nanowall. The electrolyte is 1M LiPF6. The positive electrode active material layer is a region with a diameter of 1.6 cm. The negative electrode active material layer is a region with a diameter of 1.3 cm. The height of the carbon nanowall is 1 μm.

[0305] 5-2. Charging

[0306] The aforementioned coin-shaped lithium-ion secondary battery was charged at 0.5mA for 18 hours.

[0307] 5-3. Lithium deposition

[0308] The charge Q is given by the following formula.

[0309] Q = 0.5 mA · 18 h = 32.4 C

[0310] The number of lithium ions N that accept the charge is given by the following formula.

[0311] N = 32.4 / (1.6 × 10) -19 ) = 2.0 × 10 20 (indivual)

[0312] Assume that lithium ions accept electrons at the negative electrode to form a lithium crystal. Lithium has a body-centered cubic crystal structure. Two lithium atoms are present in each lattice. The lattice constant of a lithium crystal is 0.35 nm.

[0313] The number density n of lithium ions is given by the following formula.

[0314] n = 2.0 × 10 20 / {2·(0.35×10 -9 ) 3}

[0315] =4.29×10 -9 m -3

[0316] The height HL of the deposited lithium is given by the following formula.

[0317] HL = 4.29 × 10 -9 / (0.0065×0.0065×3.14)

[0318] =32×10 -6 m

[0319] =32μm

[0320] 5-4. Cross-section of a lithium crystal

[0321] Figure 31This is a scanning electron microscope image showing a cross-section of the negative electrode of a charged lithium-ion secondary battery. (Example) Figure 31 As shown, the height of the charged lithium crystal is 32 μm. Therefore, Figure 31 The height of the lithium crystal shown is consistent with the calculation results above.

[0322] 5-5. X-ray diffraction

[0323] Figure 32 This is a graph showing the X-ray diffraction results of the negative electrode of a charged lithium-ion secondary battery. Figure 32 The peak X-ray value detected by the θ-2θ method is shown. Figure 32 The horizontal axis is Figure 32 The horizontal and vertical axes represent the intensity of X-rays.

[0324] like Figure 32 As shown, peak values ​​were observed for Li(110), Li(200), Li(211), Li(220), and Li(310). Furthermore, Li(110) exhibited the largest peak value.

[0325] 5-6. Lithium

[0326] As can be seen from the above, lithium was deposited on the carbon nanowall.

[0327] (Postscript)

[0328] The electrode of the energy storage device in the first method has a current collector and an active material layer on the current collector. The active material layer has carbon nanowalls. In a single charge or discharge cycle, the carbon nanowalls enable more than two lithium ions per carbon atom to participate in the charge / discharge reaction.

[0329] In the electrode of the second type of energy storage device, there is a current collector and an active material layer on the current collector. The active material layer has carbon nanowalls. The carbon nanowalls have a surface capable of lithium deposition.

[0330] In third-party electrodes for energy storage devices, carbon nanowalls enable each carbon atom to have more than two lithium ions participate in the charge-discharge reaction during a single charge or discharge cycle.

[0331] In the fourth type of electrode for energy storage devices, the active material layer has an amorphous carbon layer between the current collector and the carbon nanowall.

[0332] In the fifth type of electrode for energy storage devices, the thickness of the amorphous carbon layer is 10 nm or more and 300 nm or less.

[0333] In the sixth type of electrode for energy storage devices, the projection area formed by projecting a carbon nanowall onto the surface of the current collector does not include any carbon nanowalls other than the carbon nanowall itself.

[0334] In the seventh type of electrode for energy storage devices, the average angle between the current collector and the carbon nanowall is 80° or more and 90° or less.

[0335] In the eighth type of electrode for energy storage devices, the height of the carbon nanowall from the current collector is more than 100 nm and less than 10 μm.

[0336] The energy storage device in the ninth method has a positive current collector, a positive active material layer on the positive current collector, a negative current collector, and a negative active material layer on the negative current collector. The negative active material layer has carbon nanowalls. In one charge or discharge cycle, the carbon nanowalls enable more than two lithium ions per carbon atom to participate in the charge and discharge reaction.

[0337] The energy storage device in the tenth embodiment has a positive current collector, a positive active material layer on the positive current collector, a negative current collector, and a negative active material layer on the negative current collector. The negative active material layer has carbon nanowalls. The carbon nanowalls have a surface capable of lithium deposition.

[0338] The energy storage device in the eleventh method comprises a positive current collector, a positive active material layer on the positive current collector, a negative current collector, and a negative active material layer on the negative current collector. The positive active material layer contains lithium atoms. The negative active material layer contains carbon nanowalls. The number of lithium atoms per unit area of ​​the positive active material layer is more than twice the number of carbon atoms per unit area of ​​the negative active material layer.

[0339] In the energy storage device of the twelfth type, the height of the carbon nanowall from the negative electrode current collector is more than 100 nm and less than 10 μm.

[0340] Symbol Explanation

[0341] LiB1…Lithium-ion secondary battery

[0342] PE…positive electrode

[0343] P1…Positive current collector

[0344] P2…Positive electrode active material layer

[0345] NE… Negative electrode

[0346] N1… Negative electrode current collector

[0347] N2…Negative electrode active material layer

[0348] CNW1…carbon nanowall

[0349] Sp1…Septum

[0350] ES1… Electrolyte

[0351] V1…container

[0352] E1…Front end

[0353] R1…root

[0354] GS1…Graphene Sheets

Claims

1. An electrode for an energy storage device, comprising: Current collector; and The active material layer on the current collector The active material layer has carbon nanowalls. The carbon nanowall has a surface capable of lithium deposition, and an amorphous carbon layer exists between the current collector and the carbon nanowall. The projection area formed by projecting the carbon nanowall onto the surface of the current collector does not include carbon nanowalls other than the carbon nanowall itself. The average angle between the current collector and the carbon nanowall is greater than 89° and less than 90°, and the increase rate of surface area due to the presence or absence of the carbon nanowall is set as 4×H1 / D1, where, H1 is the average height of the carbon nanowalls, and D1 is the average spacing of the carbon nanowalls. The setting of H1 and D1 makes the surface area increase by more than 400 times.

2. The electrode for a storage device according to claim 1, wherein, The carbon nanowall enables each carbon atom to have more than two lithium ions participate in the charge-discharge reaction during a single charge or discharge cycle.

3. The electrode for a storage device according to claim 1, wherein, The thickness of the amorphous carbon layer is greater than 10 nm and less than 300 nm.

4. The electrode for a storage device according to claim 1 or 2, wherein, The height of the carbon nanowall from the current collector is greater than 100 nm and less than 10 μm.

5. An energy storage device, comprising: Positive current collector; The positive electrode active material layer on the positive electrode current collector; Negative current collector; and The negative electrode active material layer on the negative electrode current collector The negative electrode active material layer has carbon nanowalls. The carbon nanowall has a surface capable of depositing lithium. An amorphous carbon layer exists between the negative electrode current collector and the carbon nanowall. The angle between the carbon nanowall and the surface of the negative electrode current collector is such that the projection area formed by projecting the carbon nanowall onto the surface of the negative electrode current collector does not include any carbon nanowalls other than the carbon nanowall itself. The average angle between the negative electrode current collector and the carbon nanowall is greater than 89° and less than 90°, and the increase rate of surface area due to the presence or absence of the carbon nanowall is set as 4×H1 / D1, where, H1 is the average height of the carbon nanowalls, and D1 is the average spacing of the carbon nanowalls. The setting of H1 and D1 makes the surface area increase by more than 400 times.

6. The energy storage device according to claim 5, wherein, The positive electrode active material layer contains lithium atoms. The number of lithium atoms per unit area of ​​the positive electrode active material layer is more than twice the number of carbon atoms per unit area of ​​the negative electrode active material layer.

7. The energy storage device according to claim 5 or 6, wherein, The height of the carbon nanowall from the negative electrode current collector is greater than 100 nm and less than 10 μm.

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