Negative electrode and lithium-ion secondary battery

Abstract

To improve safety, this negative electrode of a lithium ion secondary battery is equipped with: a negative electrode current collector; a negative electrode mixture layer provided on the negative electrode current collector and containing graphite as a negative electrode active material; and a dendrite inhibiting layer provided on the side opposite from the negative electrode current collector with respect to the negative electrode mixture layer and including carbon nanotubes.

Description

Anode and lithium-ion secondary battery

[0001] The present disclosure relates to a negative electrode and a lithium ion secondary battery.

[0002] Patent Document 1 describes a negative electrode of a lithium secondary battery in which two negative electrode active material layers are formed using negative electrode active materials with different reaction initiation voltages in order to prevent lithium from being deposited on the negative electrode and ensure safety.

[0003] JP 2010-020912 A

[0004] However, in the negative electrode of the lithium secondary battery described in Patent Document 1, if the negative electrode active material deteriorates, metallic lithium may be deposited on the surface during charging. The deposited metallic lithium may cause an internal short circuit when it comes into contact with the positive electrode, which may result in insufficient safety.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a negative electrode and a lithium ion secondary battery that can improve safety.

[0006] A negative electrode according to one aspect of the present disclosure is a negative electrode for a lithium-ion secondary battery, and includes: a negative electrode current collector; a negative electrode mixture layer provided on the negative electrode current collector and containing graphite; and a dendrite suppression layer provided on the opposite side of the negative electrode mixture layer from the negative electrode current collector and containing carbon nanotubes.

[0007] A lithium ion secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode according to one aspect of the present disclosure, and an electrolyte.

[0008] According to the present invention, it is possible to provide a negative electrode and a lithium ion secondary battery that can improve safety.

[0009] FIG. 1 is a cross-sectional view showing an example of a lithium-ion secondary battery according to the first embodiment. FIG. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly according to FIG. 1. FIG. 3 is a schematic cross-sectional view showing an example of a negative electrode according to the first embodiment. FIG. 4 is a schematic cross-sectional view showing an example of a negative electrode according to the first embodiment after charging. FIG. 5 is a schematic cross-sectional view showing a negative electrode according to a comparative example after charging. FIG. 6 is a schematic cross-sectional view showing a negative electrode according to a comparative example after charging. FIG. 7 is a diagram showing an image obtained by binarizing an SEM (Scanning Electron Microscope) image of the dendrite prevention layer according to the first embodiment, viewed in a plan view in the thickness direction, and then reversing the black and white image. FIG. 8 is a cutaway view showing another example of a lithium-ion secondary battery according to the first embodiment. FIG. 9 is a schematic cross-sectional view taken along line IX-IX in FIG. 8. FIG. 10 is a diagram showing charging curves for half cells according to a comparative example and an example. FIG. 11 is a diagram showing an SEM image of the separator-side surface of a negative electrode according to comparative example 1. FIG. 12 is a diagram showing an SEM image of the separator-side surface of a negative electrode according to comparative example 2. Fig. 13 is a diagram showing an SEM image of the separator-side surface of the negative electrode according to the example. Fig. 14 is a diagram showing an SEM image of a cross section of the negative electrode according to the example. Fig. 15 is a diagram showing an enlarged SEM image of the area between the negative electrode mixture layer and the dendrite prevention layer in the cross section of the negative electrode according to the example.

[0010] Hereinafter, embodiments of the present disclosure will be described, but the invention according to the present disclosure is not limited to these embodiments.

[0011] (Lithium-ion secondary battery) FIG. 1 is a cross-sectional view showing an example of a lithium-ion secondary battery according to the first embodiment. The lithium-ion secondary battery 1 shown in FIG. 1 is a laminated lithium-ion secondary battery. In the present disclosure, a lithium-ion secondary battery refers to a secondary battery that is charged and discharged by an intercalation reaction between lithium atoms and a negative electrode active material. In other words, the lithium-ion secondary battery according to the present disclosure does not include a lithium metal battery, i.e., a battery that uses metallic lithium as the positive electrode or negative electrode. As shown in FIG. 1, the lithium-ion secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive material 32.

[0012] The battery element 20 is provided inside an exterior member 30. As shown in FIG. 1 , the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. That is, the positive electrode lead 21 is a terminal that serves as a positive electrode of the lithium ion secondary battery 1A. In FIG. 1 , the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. That is, the negative electrode lead 22 is a terminal that serves as a negative electrode of the lithium ion secondary battery 1A. In FIG. 1 , the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

[0013] The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1 , the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b.

[0014] The exterior sheets 30a, 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in this order from the inside, i.e., the side where the battery element 20 is provided, and then bonding them together by lamination or other processing. The insulating layers of the exterior sheets 30a, 30b are made of resins such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, and polyolefin resins containing ethylene or propylene as monomers. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the lithium-ion secondary battery 1A and improve its airtightness. The metal layers of the exterior sheets 30a, 30b are metal plate or foil materials such as aluminum, stainless steel, nickel, and iron. The outermost layer may be made of any material, but is preferably made of the same resin as the insulating layer or a material with high resistance to tearing and punctures, such as nylon.

[0015] The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive 32 preferably has adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This allows the adhesive 32 to seal the gaps between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the exterior member 30 airtight.

[0016] Fig. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly in Fig. 1. More specifically, Fig. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode assembly 200. As shown in Fig. 2, the electrode assembly 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the lithium-ion secondary battery 1, the electrode assembly 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode assembly 200 are layered members for the charge / discharge reaction of the lithium-ion secondary battery according to the first embodiment.

[0017] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode mixture layer 212. In the positive electrode 210, the positive electrode current collector 211 is laminated between the positive electrode mixture layers 212. In other words, the positive electrode mixture layers 212 are formed on both sides of the positive electrode current collector 211.

[0018] The positive electrode current collector 211 is a conductive layer, and may be made of, for example, aluminum foil, stainless steel foil, etc. In the example of Fig. 1 , the positive electrode current collector 211 has a rectangular shape in plan view in the thickness direction, with protrusions on the positive electrode lead 21 side. The protrusions of the positive electrode current collector 211 are connected to the positive electrode lead 21.

[0019] The positive electrode mixture layer 212 is a layer containing a positive electrode active material. The positive electrode mixture layer 212 includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive additive. The positive electrode mixture layer 212 is not limited to the materials listed above, and may further include, for example, a dispersant.

[0020] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. The lithium-containing phosphate compound is a phosphate compound containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing phosphate compound has, for example, an olivine type crystal structure. A specific example of the lithium-containing composite oxide is LiNiO ２ , LiCoO ２ , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O ２ , Li １．１５ (Mn ０．６５ Ni ０．２２ Co ０．１３ ) O ２ , LiMn ２ O ４ Specific examples of lithium-containing phosphate compounds include LiFePO ４ , LiMnPO ４ , LiFe ０．５ Mn ０．５ P.O. ４ , LiFe ０．３ Mn ０．７ P.O. ４ And so on.

[0021] The positive electrode binder contained in the positive electrode mixture layer 212 may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0022] The conductive additive contained in the positive electrode mixture layer 212 may be any material, and may include, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the conductive additive contained in the positive electrode mixture layer 212 is not limited to these materials as long as it is a conductive material, and may also be a metal material, a conductive polymer, or the like.

[0023] Fig. 3 is a schematic cross-sectional view showing an example of a negative electrode according to the first embodiment. As shown in Fig. 3, the negative electrode 220 includes a negative electrode current collector 221, a negative electrode mixture layer 222, and a dendrite prevention layer 223. In the negative electrode 220, the negative electrode current collector 221 is stacked between the negative electrode mixture layers 222, and the dendrite prevention layer 223 is stacked on the opposite side of the negative electrode current collector 221 with respect to the negative electrode mixture layer 222.

[0024] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used. In the example of Fig. 1 , the shape of the negative electrode current collector 221 is a rectangular sheet having protrusions on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusions of the negative electrode current collector 221 are connected to the negative electrode lead 22.

[0025] The negative electrode mixture layer 222 is a layer containing a negative electrode active material. The negative electrode active material refers to a material that can absorb and desorb lithium through a charge / discharge reaction. The negative electrode mixture layer 222 contains graphite as the negative electrode active material. In the first embodiment, the negative electrode mixture layer 222 is made of graphite particles. Here, the graphite may be artificial graphite or natural graphite. Note that the negative electrode mixture layer 222 is not limited to being made only of the negative electrode active material, and may also contain, for example, a conductive additive and a binder.

[0026] The dendrite prevention layer 223 is a layer that prevents lithium dendrites from growing away from the negative electrode mixture layer 222 (upward in FIG. 3 ) even if they occur on the negative electrode mixture layer 222. The dendrite prevention layer 223 is a layer containing carbon nanotubes (CNTs). In the first embodiment, the dendrite prevention layer 223 is a layer formed by stacking multiple CNT films. The CNT film is a sheet-like material containing fibrous CNTs (CNT fibers). The diameter of the CNT fibers is preferably 50 nm or more and 500 nm or less. Furthermore, the CNT film is not limited to a structure in which multiple CNT fibers extend in the same direction, but may also have a structure in which multiple CNT fibers are entangled with each other. When the CNT fibers are entangled with each other, the average size and porosity of the gaps G, which will be described later, can be appropriate. Details of the dendrite prevention layer 223 will be described later.

[0027] The separator 230 is a membrane that allows lithium ions to pass through while insulating the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the main surface of the positive electrode 210 and the main surface of the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other. In the example of FIG. 1 , the shape of the separator 230 is a rectangular sheet when viewed in a plan view in the thickness direction.

[0028] The separator 230 is preferably made of a material that is electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and is insulating. The separator 230 can be made of, for example, a polymer nonwoven fabric, a porous film, or a layer of glass or ceramic fibers. The separator 230 is more preferably made of a porous polyolefin film. This improves battery safety by preventing short circuits and providing a shutdown effect.

[0029] The electrolyte solution is impregnated into the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0030] The electrolyte salt is, for example, lithium perchlorate (LiClO４ ), lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ２ CF ３ ) ２ ), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO ２ C ２ F ５ ) ２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0031] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0032] The electrolytic solution may further contain an additive such as a fluorinated carboxylic acid ester, a sulfonic acid ester, a sulfonic acid anhydride, or a carboxylic acid anhydride.

[0033] The dendrite suppression layer 223 according to the first embodiment will be described in detail below. FIG. 4 is a schematic cross-sectional view showing an example of the anode according to the first embodiment after charging. FIGS. 5 and 6 are schematic cross-sectional views showing anodes according to comparative examples after charging. Here, the anode according to FIG. 5 is the anode 220X obtained by removing the dendrite suppression layer 223 from the anode 220 according to the first embodiment, and corresponds to the anode according to comparative example 1 described below. The anode according to example 6 is the anode 220Y obtained by removing the anode mixture layer 222 from the anode 220 according to the first embodiment, and corresponds to the anode according to comparative example 2 described below.

[0034] Lithium-ion secondary batteries are manufactured so that the theoretical capacities of the positive electrode active material and the negative electrode active material are the same in order to improve energy density. In such lithium-ion secondary batteries, if the negative electrode active material deteriorates during charging and discharging, resulting in a negative electrode active material that does not contribute to the intercalation reaction, dendritic lithium crystals (lithium dendrites) may form on the surface of the negative electrode active material during charging. In the following explanation, these dendritic lithium crystals will be referred to as lithium dendrites LD. As shown in FIGS. 5 and 6 , if lithium dendrites LD form on the surface of the negative electrodes 220X and 220Y, the lithium dendrites LD have a sharp shape and may penetrate the separator 230 and reach the positive electrode 210, causing an internal short circuit, which may result in heat generation or fire in the battery.

[0035] In the first embodiment, a dendrite prevention layer 223 containing CNTs is laminated on a negative electrode mixture layer 222 containing graphite. This allows the absolute value of the nucleation overpotential of lithium in the dendrite prevention layer 223 to be greater than the absolute value of the nucleation overpotential of lithium in the negative electrode mixture layer 222. Here, the nucleation overpotential of lithium refers to the potential at which the potential of metallic lithium relative to lithium ions becomes 0 V (vs Li / Li) when charging is performed with metallic lithium as the positive electrode and the measurement target as the negative electrode. ＋) or less. Here, when the absolute value of the lithium nucleation overpotential is large, the energy barrier for the nucleation of metallic lithium is low, and thus lithium dendrites LD are likely to occur. As a result, during charging of the lithium-ion secondary battery 1, lithium dendrites LD are more likely to occur on the surface of the negative electrode mixture layer 222 opposite the negative electrode current collector 221 than on the surface of the dendrite suppression layer 223 opposite the negative electrode current collector 221. Therefore, even if lithium dendrites occur on the negative electrode mixture layer 222, as shown in FIG. 4, lithium dendrites LD occur between the negative electrode mixture layer 222 and the dendrite suppression layer 223. As a result, the dendrite suppression layer 223 can suppress the growth of lithium dendrites LD in a direction away from the negative electrode mixture layer 222 (upward in FIG. 4). Therefore, the dendrite suppression layer 223 can protect the separator 230 and the positive electrode 210 from lithium dendrites LD, thereby improving the safety of the lithium ion secondary battery 1 .

[0036] The dendrite suppression layer 223 is preferably made of a material that does not undergo a lithium intercalation reaction. When a half cell is fabricated using metallic lithium as the positive electrode and the material as the negative electrode, the potential difference of lithium ions relative to metallic lithium is 0 V (vs Li / Li ＋ ) or more, 1.0 mAh / cm ２ This means that the dendrite suppression layer 223 does not have a reversible capacity equal to or greater than the above. As a result, the reactivity of the dendrite suppression layer 223 with lithium is low, so the absolute value of the lithium nucleation overpotential in the dendrite suppression layer 223 can be made greater than the absolute value of the lithium nucleation overpotential in the negative electrode mixture layer 222, and the formation of lithium dendrites on the surface of the dendrite suppression layer 223 facing the separator 230 can be further suppressed. The lithium nucleation overpotential of the dendrite suppression layer 223 can be measured by the following method. A half cell is fabricated using the dendrite suppression layer 223 removed from the lithium ion secondary battery as the negative electrode. Then, the fabricated half cell is subjected to a charging test in the first charging mode and the second charging mode described below, whereby the lithium nucleation overpotential of the dendrite suppression layer 223 can be measured.

[0037] In the first embodiment, the dendrite suppression layer 223 is preferably conductive. In the present disclosure, the dendrite suppression layer 223 being conductive means that the sheet resistance of the dendrite suppression layer 223 is 500 Ω / cm ２ This means that, during discharge, electrons are removed from the lithium dendrites LD (oxidized) via the dendrite suppression layer 223, and the lithium dendrites LD can be dissolved to form lithium ions. As a result, the lithium ions dissolved during discharge contribute again to charge and discharge. This prevents the lithium dendrites LD from accumulating between the negative electrode mixture layer 222 and the dendrite suppression layer 223, thereby preventing short circuits caused by the lithium dendrites LD. The sheet resistance can be measured by the following method. The dendrite suppression layer 223 is peeled off from the negative electrode 220, and the resistivity of the peeled dendrite suppression layer is measured according to ASTM F 1529-97, thereby measuring the sheet resistance.

[0038] 7 is a diagram showing a black-and-white inverted image obtained by binarizing an SEM (Scanning Electron Microscope) image of the dendrite suppression layer according to the first embodiment, viewed in plan in the thickness direction. As shown in FIG. 7, in the first embodiment, the dendrite suppression layer 223 has a plurality of gaps G surrounded by CNT fibers. The average size of these gaps G is 0.01 μm when viewed in plan in the thickness direction of the dendrite suppression layer 223. ２ 0.25 μm or more ２or less. This allows lithium ions to pass through the dendrite suppression layer 223 efficiently, accelerating the dissolution of lithium dendrites LD during discharge and improving the safety of the lithium-ion secondary battery 1. The average size of the gaps G can be measured by the following method. First, the surface of the dendrite suppression layer 223 is observed using an SEM to obtain a secondary electron image (acceleration voltage 3.0 kV) measuring 3.5 μm in width and 2.5 μm in height. From the obtained image, as shown in FIG. 7 , the area occupied by the gaps G on the surface of the dendrite suppression layer 223 is extracted by binarizing the image using image processing software (ImageJ). More specifically, the binarization threshold is set to the peak color of the grayscale distribution of the SEM image, i.e., the color with the largest number of pixels among the colors appearing in the image, and the area of ​​the color that is blacker than the threshold is extracted as the gap G area (white area in FIG. 7 ). The area occupied by the extracted gap G is 0.01 μm ２ The above regions are extracted, and the arithmetic mean of the areas of the regions is defined as the size of the gap G.

[0039] The porosity of the dendrite suppression layer 223 is preferably 20% or more and 70% or less. This allows lithium ions to pass through the dendrite suppression layer 223 efficiently, promoting the dissolution of lithium dendrites LD during discharge and improving the safety of the lithium-ion secondary battery 1. The porosity can be measured as follows: The surface of the dendrite suppression layer 223 is observed using an SEM to obtain a secondary electron image (acceleration voltage: 3.0 kV) measuring 3.5 μm in width and 2.5 μm in height. From the obtained image, as shown in FIG. 7 , the area occupied by the gaps G on the surface of the dendrite suppression layer 223 is binarized and extracted by image processing using image analysis software (ImageJ). More specifically, the binarization threshold is set to the peak color of the grayscale distribution of the SEM image, i.e., the color with the largest number of pixels among the colors appearing in the image, and the area of ​​the color on the black side of the threshold is extracted as the gap G area (white area in FIG. 7 ). The sum of the areas of the extracted gaps G is then measured, and the ratio of this sum to the area of ​​the entire observation image is calculated to obtain the porosity of the dendrite suppression layer.

[0040] The thickness of the dendrite suppression layer 223 is preferably 1 μm or more. This makes it possible to prevent lithium dendrites LD formed between the negative electrode mixture layer 222 and the dendrite suppression layer 223 from penetrating through the dendrite suppression layer 223. The thickness of the dendrite suppression layer 223 is preferably 50 μm or less. This makes it possible to prevent a decrease in the conductivity of lithium ions from the positive electrode 210 to the negative electrode mixture layer 222. Here, the thickness of the dendrite suppression layer 223 is calculated by observing the range including the dendrite suppression layer 223 in the thickness direction with an SEM and calculating the arithmetic mean of the length of the dendrite suppression layer 223 in the thickness direction measured from the observation image obtained with the SEM.

[0041] The dendrite prevention layer 223 does not have to be made of CNTs alone and may contain other materials. For example, the dendrite prevention layer 223 may contain multiple CNT films and metal particles between the CNT film layers. The metal particles are preferably particles of a metal that does not form an alloy with metallic lithium, such as copper (Cu), nickel (Ni), iron (Fe), cobalt (Co), manganese (Mn), or chromium (Cr).

[0042] The battery according to the first embodiment has been described above, but the lithium-ion secondary battery according to the first embodiment is not limited to that shown in Fig. 1. Other examples will be described below with reference to the drawings, but the same components as those in Figs. 1 and 2 will be designated by reference numerals and will not be described again.

[0043] Fig. 8 is a cutaway view showing a different example of the lithium ion secondary battery according to the first embodiment. Fig. 9 is a schematic view of a cross section taken along line IX-IX in Fig. 8. The lithium ion secondary battery 1A shown in Figs. 8 and 9 differs from the example shown in Fig. 1 in that the electrode body 200 has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center.

[0044] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 9 , the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23 is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0045] 9, the electrode assembly 200A is a laminate for the charge / discharge reaction of the lithium-ion secondary battery according to Embodiment 1. The electrode assembly 200A includes a positive electrode 210A including a positive electrode current collector 211A and a positive electrode mixture layer 212A, a negative electrode 220A including a negative electrode current collector 221A, a negative electrode mixture layer 222A, and a dendrite prevention layer 223A, and a separator 230A. The electrode body 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center, and is stacked in the following order from the outside, i.e., from the protective material 23 side: a negative electrode current collector 221A, a negative electrode mixture layer 222A, a dendrite suppression layer 223A, a separator 230A, a positive electrode mixture layer 212A, a positive electrode current collector 211A, a positive electrode mixture layer 212A, a separator 230A, a dendrite suppression layer 223A, and a negative electrode mixture layer 222A. The electrode body 200A does not have any layers other than the negative electrode current collector 221A, the separator 230A, and the positive electrode current collector 211A near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode current collector 211A is connected to the positive electrode lead 21A, and the negative electrode current collector 221A is connected to the negative electrode lead 22A.

[0046] As described above, the negative electrode 220 according to the first embodiment is a negative electrode of a lithium-ion secondary battery, and includes a negative electrode current collector 221, a negative electrode mixture layer 222 provided on the negative electrode current collector 221 and containing graphite as a negative electrode active material, and a dendrite suppression layer 223 provided on the opposite side of the negative electrode current collector 221 with respect to the negative electrode mixture layer 222, containing carbon nanotubes, and so forth.

[0047] The negative electrode according to the first embodiment includes the negative electrode mixture layer 222 containing graphite and the dendrite prevention layer 223 containing carbon nanotubes (CNTs), and thus lithium dendrites LD are generated between the negative electrode mixture layer 222 and the dendrite prevention layer 223 during charging, and the lithium dendrites LD are covered by the dendrite prevention layer 223. As a result, even if lithium dendrites are generated on the negative electrode mixture layer 222, the dendrites LD grow in a direction away from the negative electrode mixture layer 222, and as a result, the lithium dendrites LD can be prevented from reaching the positive electrode 210, thereby improving safety.

[0048] In a preferred embodiment, the dendrite suppression layer 223 is electrically conductive. This allows electrons to be removed from the lithium dendrites LD (oxidized) via the dendrite suppression layer 223 during discharge, dissolving the lithium dendrites LD into lithium ions. As a result, the lithium ions dissolved during discharge contribute again to charge and discharge. This prevents lithium dendrites LD from accumulating between the negative electrode mixture layer 222 and the dendrite suppression layer 223, thereby preventing short circuits caused by lithium dendrites LD.

[0049] In a preferred embodiment, the dendrite suppression layer 223 is made of a material that does not undergo a lithium intercalation reaction. As a result, the reactivity of the dendrite suppression layer 223 with lithium is low, and therefore lithium dendrites LD are generated between the negative electrode mixture layer 222 and the dendrite suppression layer 223 during charging. The lithium dendrites LD are easily covered by the dendrite suppression layer 223, which further prevents the lithium dendrites LD from reaching the positive electrode 210 and further improves safety.

[0050] In a preferred embodiment, the dendrite suppression layer 223 has a plurality of gaps G. The average size of the plurality of gaps G is 0.01 μm. ２ 0.25 μm or more ２This allows lithium ions to easily pass through the negative electrode short-circuit prevention film and promotes dissolution of lithium dendrites LD during discharge, thereby preventing the lithium dendrites LD from accumulating in the negative electrode and further improving safety.

[0051] In a preferred embodiment, the porosity of the dendrite suppression layer 223 is 20% or more and 70% or less, which allows lithium ions to easily pass through the negative electrode short-circuit prevention film and promotes dissolution of lithium dendrites LD during discharge, thereby suppressing the accumulation of lithium dendrites LD in the negative electrode and further improving safety.

[0052] In a preferred embodiment, the thickness of the dendrite suppression layer 223 is 1 μm or more and 50 μm or less, which can further suppress lithium dendrites LD from reaching the positive electrode 210 while suppressing a decrease in the conductivity of lithium ions from the positive electrode 210 to the negative electrode mixture layer 222.

[0053] The lithium ion secondary battery according to the first embodiment includes the positive electrode 210, the negative electrode 220 according to the first embodiment, and an electrolyte, thereby improving safety.

[0054] EXAMPLES Examples will be described below, but the present invention is not limited to these examples.

[0055] Comparative Example 1 The negative electrode according to Comparative Example 1 was fabricated by laminating only a layer made of graphite on a negative electrode current collector. More specifically, it was fabricated by the following method. Graphite was dispersed in N-methyl-2-pyrrolidone as a negative electrode active material to prepare a paste-like negative electrode mixture slurry. Subsequently, the negative electrode mixture slurry was applied to both sides of a copper foil serving as a negative electrode current collector using a coating device, and the negative electrode mixture slurry was then dried with hot air to form a negative electrode mixture layer on the negative electrode current collector. The negative electrode mixture layer was then compression-molded using a hydraulic press to obtain the negative electrode according to Comparative Example 1.

[0056] Then, a half cell was fabricated using the fabricated negative electrode. The positive electrode of the half cell was a metallic lithium foil. The separator of the half cell was a porous polyolefin film. The electrolyte solution of the half cell was a liquid mixture of ethyl carbonate and dimethyl carbonate in a volume ratio of 3:7 as a solvent, and lithium hexafluorophosphate (LiPF ) as an electrolyte salt. ６ ) was dissolved in the solvent. The content of the electrolyte salt in the solvent was 1 mol / cm ３ The half-cell was assembled as follows: The separator was impregnated with the electrolyte and then laminated on the positive electrode. The prepared negative electrode was then laminated on the separator so that the negative electrode mixture layer side of the negative electrode faced the positive electrode.

[0057] <Charging Test> A charging test was carried out on the fabricated half-cell. In the test, a cutoff current was reached by CCCV charging under the following conditions as the first charging mode, and then a current of 0.1 mA / cm was charged under the following conditions as the second charging mode. ２ CC charging was continued. In the second charging mode, the voltage was 0 V (vs Li / Li) in the charging curve. ＋ After the voltage dropped below 0 V (vs. Li / Li), charging was continued until the voltage rose again. ＋ In the region below 1000 kJ / s, the minimum voltage of the charging curve was measured as the nucleation overpotential of lithium. First charging mode: Charging method: CCCV charging Charging rate: 1 C (3 mA / cm ２ ) Cutoff current: 0.01 mA / cm ２ Second charging mode: Charging method: CC charging Charging rate: 0.1mA / cm ２

[0058] <<SEM Observation>> After the charging test, the half-cell was disassembled, and the surface of the negative electrode was observed from the separator side using an SEM. The SEM observation was performed under the following conditions: SEM: S-4800 (Hitachi High-Technologies Corporation) Acceleration voltage: 3.0 kV

[0059] (Comparative Example 2) The negative electrode according to Comparative Example 2 was fabricated by laminating only a plurality of CNT films (Hamamatsu Carbonix Co., Ltd.) on a negative electrode current collector. At this time, the thickness of the layer made of the CNT film was 4 μm. In Comparative Example 2, a half cell was fabricated in the same manner as in Comparative Example 1, and a charging test and SEM observation were performed.

[0060] (Example) The negative electrode according to the example was fabricated by laminating a negative electrode mixture layer containing graphite and a dendrite prevention layer containing a CNT film on a negative electrode current collector. In the example, a plurality of CNT films (Hamamatsu Carbonix Co., Ltd.) were laminated as dendrite prevention layers on the negative electrode mixture layer side of the negative electrode according to Comparative Example 1, and the layers were then pressed together to fabricate the negative electrode according to the example. At this time, the thickness of the dendrite prevention layer was 4 μm. In the example, a half cell was fabricated in the same manner as in Comparative Example 1, and a charging test was performed.

[0061] In the examples, after the charging test, the half-cell was disassembled, and the surface of the negative electrode and a cross section along the thickness direction of the negative electrode were observed from the separator side using an SEM. The SEM observation was performed under the same conditions as in Comparative Example 1.

[0062] Fig. 10 shows the charging curves for the half-cells of the comparative example and the example. As shown in Fig. 10, in Comparative Example 1, in which only graphite was laminated on the negative electrode current collector, the nucleation overpotential of lithium was -0.015 V. On the other hand, in Comparative Example 2, in which only a CNT film was laminated on the negative electrode current collector, the nucleation overpotential of lithium was -0.04 V. This shows that the absolute value of the nucleation overpotential of lithium is smaller for graphite than for the CNT film.

[0063] 10 , in the example in which graphite and a CNT film were laminated on the negative electrode current collector, the nucleation overpotential of lithium was −0.015 V, similar to Comparative Example 1 in which only graphite was laminated on the negative electrode current collector. This shows that when the negative electrode includes a negative electrode mixture layer containing graphite and a dendrite suppression layer containing CNT, the nucleation overpotential is similar to that when only graphite is included. From this, it is thought that even when the negative electrode is formed by laminating a dendrite suppression layer containing CNT on a negative electrode mixture layer containing graphite, lithium dendrites are more likely to be generated on the surface of the negative electrode mixture layer containing graphite than on the surface of the dendrite suppression layer containing CNT during charging.

[0064] Furthermore, as shown in FIG. 10, in Comparative Example 2 in which only the CNT film was laminated on the negative electrode current collector, the charge capacity was 1.0 mAh / cm ２ This indicates that the CNT film is a material in which the lithium intercalation reaction does not occur.

[0065] Fig. 11 is a view showing an SEM image of the separator-side surface of the negative electrode according to Comparative Example 1. Fig. 12 is a view showing an SEM image of the separator-side surface of the negative electrode according to Comparative Example 2. As shown in Figs. 11 and 12, lithium dendrites LD were formed on the separator-side surfaces of the negative electrodes according to Comparative Examples 1 and 2.

[0066] Fig. 13 is a diagram showing an SEM image of the separator-side surface of the negative electrode according to the example. Fig. 14 is a diagram showing an SEM image of a cross section of the negative electrode according to the example. Fig. 15 is a diagram showing an enlarged SEM image of the cross section of the negative electrode according to the example, showing the area between the negative electrode mixture layer and the dendrite prevention layer. As shown in Fig. 13, lithium dendrites LD were not formed on the separator-side surface of the negative electrode according to the example (the surface of the dendrite prevention layer 223). Furthermore, as shown in Figs. 14 and 15, in the negative electrode according to the example, lithium dendrites LD were formed between the negative electrode mixture layer 222 and the dendrite prevention layer 223. This shows that by laminating the negative electrode in the order of negative electrode current collector 221, negative electrode mixture layer containing graphite, and dendrite prevention layer containing CNT, lithium dendrites LD are generated between the negative electrode mixture layer 222 and the dendrite prevention layer 223 during charging, and the lithium dendrites LD are covered by the dendrite prevention layer 223. From the results of Fig. 10, this is thought to be because lithium dendrites are generated preferentially on the surface of the negative electrode mixture layer 222 containing graphite rather than on the dendrite prevention layer 223 containing CNT.

[0067] The above-described embodiments are intended to facilitate understanding of the invention according to the present disclosure, and are not intended to limit the invention according to the present disclosure. The invention according to the present disclosure may be modified or improved without departing from the spirit thereof, and the present disclosure also includes equivalents thereof.

[0068] The invention according to the present disclosure may take the following forms. (1) A negative electrode for a lithium ion secondary battery, comprising: a negative electrode current collector; a negative electrode mixture layer provided on the negative electrode current collector and containing graphite as a negative electrode active material; and a dendrite prevention layer provided on the opposite side of the negative electrode mixture layer from the negative electrode current collector and containing carbon nanotubes. (2) The negative electrode according to (1), wherein the dendrite prevention layer is conductive. (3) The negative electrode according to (1) or (2), wherein the dendrite prevention layer is made of a material that does not undergo a lithium intercalation reaction. (4) The dendrite prevention layer has a plurality of gaps, and the average size of the plurality of gaps is 0.01 μm２ 0.25 μm or more ２ (5) The negative electrode according to any one of (1) to (3), wherein the porosity of the dendrite suppression layer is 20% or more and 70% or less. (6) The negative electrode according to any one of (1) to (5), wherein the thickness of the dendrite suppression layer is 1 μm or more and 50 μm or less. (7) A lithium ion secondary battery comprising a positive electrode, the negative electrode according to any one of (1) to (6), and an electrolyte.

[0069] REFERENCE SIGNS LIST 1, 1A Lithium ion secondary battery 20, 20A Battery element 21, 21A Positive electrode lead 22, 22A Negative electrode lead 23 Protective material 30 Exterior member 30a, 30b Exterior sheet 31 Recess 32 Adhesive material 200, 200A Electrode body 210, 210A Positive electrode 211, 211A Positive electrode current collector 212, 212A Positive electrode mixture layer 220, 220A Negative electrode 221, 221A Negative electrode current collector 222, 222A Negative electrode mixture layer 223, 223A Dendrite suppression layer 230, 230A Separator

Claims

1. A negative electrode of a lithium-ion secondary battery, comprising: a negative electrode current collector; a negative electrode mixture layer provided on the negative electrode current collector and containing graphite as a negative electrode active material; and a dendrite suppression layer provided on the side opposite to the negative electrode current collector with respect to the negative electrode mixture layer and containing carbon nanotubes.

2. The negative electrode according to claim 1, wherein the dendrite suppression layer has conductivity.

3. The negative electrode according to claim 1 or 2, wherein the dendrite suppression layer is made of a material in which an intercalation reaction of lithium does not occur.

4. The dendrite suppression layer has a plurality of gaps, and an average size of the plurality of gaps is 0.01 μm ２ or more and 0.25 μm ２ or less. The negative electrode according to any one of claims 1 to 3.

5. The negative electrode according to any one of claims 1 to 4, wherein the porosity of the dendrite suppression layer is 20% or more and 70% or less.

6. The negative electrode according to any one of claims 1 to 5, wherein the thickness of the dendrite suppression layer is 1 µm or more and 50 µm or less.

7. A lithium-ion secondary battery comprising a positive electrode, the negative electrode according to any one of claims 1 to 6, and an electrolyte.

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