Secondary battery, negative electrode, and method for manufacturing a negative electrode

Abstract

To provide a negative electrode and secondary battery that can improve safety. [Solution] A secondary battery comprising a positive electrode and a negative electrode. The negative electrode comprises a negative electrode current collector, a first negative electrode layer provided on the negative electrode current collector and containing graphene, a lithium metal layer provided on the first negative electrode layer and containing lithium metal but not graphene, and a second negative electrode layer provided on the lithium metal layer and containing graphene. The potential of the negative electrode is 0V (vs Li / Li + In this state, the first negative electrode layer has lithium alloy particles containing lithium and a metal that forms an alloy with lithium, and the mass fraction of lithium alloy particles in the first negative electrode layer is greater than the mass fraction of lithium alloy particles in the second negative electrode layer.

Description

[Technical Field]

[0001] This disclosure relates to a secondary battery, a negative electrode, and a method for manufacturing a negative electrode. [Background technology]

[0002] Patent Document 1 discloses a lithium metal secondary battery in which a polymer is placed on a lithium metal negative electrode, and lithium metal is uniformly deposited to suppress the growth of lithium metal dendrites (dendritic projections). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2022 / 105614 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, even with the negative electrode of the lithium metal secondary battery described in Patent Document 1, if the negative electrode active material deteriorates, dendrites may still form on the surface of the lithium metal deposited on the surface during charging. Since dendrites can cause internal short circuits when they come into contact with the positive electrode, safety may be compromised.

[0005] This invention has been made in view of the above problems, and aims to provide a negative electrode and a secondary battery that can improve safety. [Means for solving the problem]

[0006] The secondary battery according to this embodiment comprises a positive electrode and a negative electrode, the negative electrode comprising a negative electrode current collector, a first negative electrode layer provided on the negative electrode current collector and containing graphene, a lithium metal layer provided on the first negative electrode layer and containing lithium metal but not graphene, and a second negative electrode layer provided on the lithium metal layer and containing graphene, wherein the potential of the negative electrode is 0V (vs Li / Li +In the state of , the first negative electrode layer has lithium alloy particles containing lithium and a metal that forms an alloy with the lithium, and the mass fraction of the lithium alloy particles in the first negative electrode layer is larger than the mass fraction of the lithium alloy particles in the second negative electrode layer.

Advantages of the Invention

[0007] According to the present invention, a negative electrode and a secondary battery capable of improving safety can be provided.

Brief Description of the Drawings

[0008] [Figure 1] FIG. 1 is a cross-sectional view showing an example of a secondary battery according to an embodiment. [Figure 2] FIG. 2 is an enlarged cross-sectional view showing a part of the cross-section of the electrode body according to FIG. 1. [Figure 3] FIG. 3 is a schematic cross-sectional view showing an example of a negative electrode according to an embodiment. [Figure 4] FIG. 4 is a schematic cross-sectional view showing an example of a negative electrode according to an embodiment after charging. [Figure 5] FIG. 5 is a schematic cross-sectional view showing a negative electrode according to a comparative example after charging. [Figure 6] FIG. 6 is a cutaway view showing a different example of a secondary battery according to an embodiment. [Figure 7] FIG. 7 is a schematic view of the cross-section taken along line VII-VII of FIG. 6. [Figure 8] FIG. 8 is a diagram showing charge curves for half-cells of a comparative example and an example. [[ID=3۸]] [Figure 9] FIG. 9 is a diagram showing Coulombic efficiencies for half-cells of a comparative example and an example. [Figure 10] FIG. 10 is a diagram showing a SEM observation image of the surface of the negative electrode according to Example 1 on the separator side. [Figure 11] FIG. 11 is a diagram showing a SEM observation image of the cross-section of the negative electrode according to Example 1. [Figure 12] FIG. 12 is a diagram showing a SEM observation image of the cross-section of the negative electrode according to Example 1. [Figure 13]Figure 13 shows an SEM image of the separator side surface of the negative electrode according to Comparative Example 1. [Figure 14] Figure 14 shows an SEM image of the cross-section of the negative electrode related to Comparative Example 1. [Modes for carrying out the invention]

[0009] This disclosure will be described in more detail based on embodiments. However, this will not limit the disclosure.

[0010] Figure 1 is a cross-sectional view showing an example of a secondary battery according to the first embodiment. The secondary battery 1 shown in Figure 1 is a laminate-type lithium metal secondary battery. In this disclosure, a lithium metal secondary battery refers to a secondary battery that performs charging and discharging using lithium metal deposited at the negative electrode. As shown in Figure 1, the secondary battery 1 comprises a battery element 20, an outer casing member 30, and an adhesive material 32.

[0011] The battery element 20 is provided inside the outer casing member 30. As shown in Figure 1, the battery element 20 comprises an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn out from the positive electrode 210 (described later) to the outside of the outer casing member 30. In other words, the positive electrode lead 21 is the terminal that becomes the positive electrode of the secondary battery 1. In Figure 1, the positive electrode lead 21 is provided on the end face of the electrode body 200. The negative electrode lead 22 is a terminal drawn out from the inside of the negative electrode 220 (described later) to the outside of the outer casing member 30. In other words, the negative electrode lead 22 is the terminal that becomes the negative electrode of the secondary battery 1. In Figure 1, the negative electrode lead 22 is provided on the end face of the electrode body 200. Details of the electrode body 200 will be described later.

[0012] 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 comprise an insulating layer, a metal layer, and an outermost layer. In the example shown in Figure 1, the exterior sheet 30a is provided with a recess 31. By housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b, the battery element 20 is housed within the exterior member 30.

[0013] The outer sheets 30a and 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in that order, starting from the inside, i.e., the side where the battery element 20 is installed, and then bonding them together by lamination or the like. The insulating layer of the outer sheets 30a and 30b is made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This allows the outer sheets 30a and 30b to reduce the moisture permeability of the secondary battery 1 and improve airtightness. The metal layer of the outer sheets 30a and 30b is made of a metal sheet or foil such as aluminum, stainless steel, nickel, or iron. The outermost layer may be made of any material, but it is preferable to make it of a material with high strength against tearing and punctures, such as a resin similar to the insulating layer or nylon.

[0014] The adhesive material 32 is a component for making the outer casing member 30 airtight. The adhesive material 32 is provided between the outer casing member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive material 32 preferably has good adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, if the positive electrode lead 21 and the negative electrode lead 22 are made of metal, the adhesive material 32 can be made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. As a result, the adhesive material 32 can seal the gap between the outer casing member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the inside of the outer casing member 30 airtight.

[0015] Figure 2 is an enlarged cross-sectional view showing a part of the cross-section of the electrode body according to Figure 1. More specifically, Figure 2 is a cross-sectional view showing a part of one layer of positive electrode 210 and one layer of negative electrode 220 of the electrode body 200. As shown in Figure 2, the electrode body 200 comprises a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode body 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 in between. The positive electrode 210 and the negative electrode 220 included in the electrode body 200 are layered members for the charge and discharge reaction of the secondary battery according to the first embodiment.

[0016] The positive electrode 210 comprises 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.

[0017] The positive electrode current collector 211 is a conductive layer, and can be made of, for example, aluminum foil or stainless steel foil. In the example shown in Figure 1, the shape of the positive electrode current collector 211, when viewed in plan in the thickness direction, is a rectangular sheet with a projection on the positive electrode lead 21 side. The projection of the positive electrode current collector 211 is connected to the positive electrode lead 21.

[0018] The positive electrode mixture layer 212 is a layer containing a positive electrode active material. The positive electrode mixture layer 212 contains 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 contain, for example, a dispersant.

[0019] 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 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, a crystal structure such as an olivine type. Specific examples of the lithium-containing composite oxide are LiNiO2, LiCoO2, LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O2, LiMn2O4, etc. Specific examples of the lithium-containing phosphate compound are LiFePO4, LiMnPO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.3 Mn 0.7 PO4, etc.

[0020] [[ID=5x1]] The positive electrode binder contained in the positive electrode binder layer 212 may contain any material, for example, any one or more of synthetic rubber and polymer compounds. The synthetic rubber is, for example, styrene-butadiene rubber, fluorine rubber, ethylene propylene diene, etc. The polymer compound is, for example, polyvinylidene fluoride (PVdF), polyimide, etc.

[0021] The conductive additive contained in the positive electrode mixture layer 212 may contain any material, for example, a carbon material. Examples of carbon materials 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, it may also be a metallic material, a conductive polymer, etc.

[0022] The separator 230 is a film that allows lithium ions to pass through while insulating the positive electrode 210 and the negative electrode 220. The separator 230 is placed 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 shown in Figure 1, the shape of the separator 230 is a rectangular sheet when viewed in plan in the thickness direction.

[0023] The material of the separator 230 is preferably electrically stable, chemically stable with respect to the positive electrode active material, negative electrode active material, and electrolyte, and also insulating. The separator 230 can be, for example, a polymer nonwoven fabric, a porous film, or a layer made of glass or ceramic fibers. It is more preferable that the material of the separator 230 includes a porous polyolefin film. This improves the safety of the secondary battery through short-circuit prevention and shutdown effects.

[0024] The electrolyte is impregnated into the separator 230. In the example shown in Figure 1, the electrolyte is filled into the space within the outer casing member 30. The electrolyte is a non-aqueous electrolyte containing an electrolyte salt and a solvent that dissolves this electrolyte salt.

[0025] Electrolyte salts include lithium salts such as lithium perchlorate (LiClO4), lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO2C2F5)2), and lithium hexafluoroarsenate (LiAsF6).

[0026] The solvents include, for example, lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate ester-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethylmethyl 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; sulforane-based solvents; phosphoric acids; phosphate ester solvents; and non-aqueous solvents including pyrrolidones.

[0027] The electrolyte may further contain additives such as fluorinated carboxylic acid esters, sulfonic acid esters, sulfonic acid anhydrides, and carboxylic acid anhydrides.

[0028] The negative electrode 220 according to the first embodiment will be described in more detail below.

[0029] Figure 3 is a schematic cross-sectional view showing an example of a negative electrode according to the first embodiment before charging. As shown in Figure 3, the negative electrode 220 according to the first embodiment before charging comprises a negative electrode current collector 221, a first negative electrode layer 222, and a second negative electrode layer 223. In the example in Figure 3, the negative electrode 220 is stacked in the order of negative electrode current collector 221, first negative electrode layer 222, and second negative electrode layer 223.

[0030] Figure 4 is a schematic cross-sectional view showing an example of a negative electrode according to the first embodiment after charging. As shown in Figure 4, the negative electrode 220 according to the first embodiment after charging comprises a negative electrode current collector 221, a first negative electrode layer 222, a second negative electrode layer 223, and a lithium metal layer 224. In the example of Figure 4, the negative electrode 220 is stacked in the order of negative electrode current collector 221, first negative electrode layer 222, lithium metal layer 224, and second negative electrode layer 223. That is, the lithium metal layer 224 is a layer generated by charging the negative electrode 220 shown in Figure 3. In this disclosure, the lithium metal layer 224 is a layer containing a single lithium metal and does not contain graphene.

[0031] Figure 5 is a schematic cross-sectional view showing the negative electrode of a comparative example after charging. The negative electrode in Figure 5 is the negative electrode 220X obtained by removing the second negative electrode layer 223 from the negative electrode 220 according to the first embodiment, and is the negative electrode of Comparative Example 1, which will be described later. In the example of Figure 5, the negative electrode 220 is stacked in the order of negative electrode current collector 221, first negative electrode layer 222, and lithium metal layer 224.

[0032] 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 secondary batteries, dendritic crystals (dendrites) may form on the surface of the lithium metal layer formed on the surface of the negative electrode active material during charging. In the following explanation, these dendritic crystals will be referred to as dendrites D. As shown in Figure 5, if dendrites D form on the surface of the deposited lithium metal layer 224X, the dendrites D will have a sharp shape and may penetrate the separator 230 and reach the positive electrode 210, causing an internal short circuit and potentially leading to overheating or ignition of the secondary battery.

[0033] The negative electrode current collector 221, the first negative electrode layer 222, and the second negative electrode layer 223 included in the negative electrode 220 according to the first embodiment will be described in detail below.

[0034] The negative electrode current collector 221 is mainly composed of copper. In this disclosure, "the negative electrode current collector 221 is mainly composed of copper" means that the copper content in the negative electrode current collector 221 is 90 mol% or more. For example, copper foil can be used for the negative electrode current collector 221. In the example shown in Figure 1, the shape of the negative electrode current collector 221 is a rectangular sheet with a projection on the negative electrode lead 22 side when viewed in plan in the thickness direction. The projection of the negative electrode current collector 221 is connected to the negative electrode lead 22.

[0035] The first negative electrode layer 222 and the second negative electrode layer 223 are layers containing graphene. In the first embodiment, the first negative electrode layer 222 and the second negative electrode layer 223 are layers obtained by laminating multiple layers of reduced graphene oxide. In this disclosure, reduced graphene oxide is graphene oxide with an oxygen content of 40% by mass or less. Since reduced graphene oxide has excellent dispersibility and conductivity in polar solvents, it is possible to improve conductivity while simplifying the manufacturing process of the first negative electrode layer 222 and the second negative electrode layer 223. Whether or not the first negative electrode layer 222 and the second negative electrode layer 223 contain reduced graphene oxide can be measured by Raman spectroscopy or X-ray photoelectron spectroscopy (XPS) on the first negative electrode layer 222 and the second negative electrode layer 223.

[0036] The negative electrode potential is 0V (vs Li / Li + In the state described above, the first negative electrode layer 222 contains lithium alloy particles 222b. Lithium alloy particles refer to particles made of lithium and a lithium alloy containing metal M. Metal M is a metal that forms an alloy with lithium. Preferably, metal M is at least one of zinc (Zn), silicon (Si), magnesium (Mg), aluminum (Al), gold (Au), platinum (Pt), and silver (Ag). The potential of the negative electrode is 0V (vs Li / Li + In this state, the first negative electrode layer 222 contains lithium alloy particles 222b, and the lithium alloy particles 222b act as the starting point for the formation of the lithium metal layer 224. As a result, the first negative electrode layer 222 and the lithium metal layer 224 are strongly bonded together by the anchoring effect of the lithium alloy particles 222b. This promotes the growth of the lithium metal layer 224 on the second negative electrode layer 223 side. Therefore, the deposition of lithium metal on the surface of the negative electrode 220 can be suppressed, and the generation of dendrites D can be suppressed.

[0037] Here, the potential of the negative electrode is 0V (vs Li / Li +In the state shown in Figure 3, in order for the first negative electrode layer 222 to contain lithium alloy particles 222b, the first negative electrode layer 222 should contain oxide particles 222a of metal M. Here, the oxide particles 222a of metal M are an example of metal compound particles of this disclosure, which contain a compound of metal M. This brings the potential of the negative electrode to 0V (vs Li / Li + In this state, the oxide particles 222a of metal M react with lithium ions in the electrolyte and are reduced by charging. This reaction proceeds inside the oxide particles 222a of metal M, forming lithium alloy particles 222b. That is, lithium alloy particles 222b are formed from the oxide particles 222a of metal M by charging. The formed lithium alloy particles 222b serve as nuclei for lithium metal, and lithium metal further adheres to them. As a result, the lithium alloy particles 222b promote the growth of the lithium metal layer 224 between the first negative electrode layer 222 and the second negative electrode layer 223.

[0038] The presence or absence and composition of lithium alloy particles 222b in the first negative electrode layer 222 and the second negative electrode layer 223 are such that the negative electrode potential is 0V (vs Li / Li + In this state, the first negative electrode layer 222 and the second negative electrode layer 223 are each taken out as samples, and the measurement can be performed by obtaining energy dispersive X-ray spectroscopy (EDS) mapping of the cross-section in the thickness direction. More specifically, EDS mapping of the cross-section in the thickness direction of the first negative electrode layer 222 and the second negative electrode layer 223 is obtained for Li and metal M, and if the distribution of Li and the distribution of metal M overlap at least partially, it can be determined that lithium alloy particles 222b containing Li and metal M have been formed. As another measurement method, the photoelectron spectra of the above sample for Li and metal M are obtained by X-ray photoelectron spectroscopy (XPS), and if peaks originating from Li and metal M appear in the photoelectron spectra, it can be determined that lithium alloy particles 222b containing Li and metal M have been formed.

[0039] The mass fraction of lithium alloy particles in the first negative electrode layer 222 is greater than the mass fraction of lithium alloy particles in the second negative electrode layer 223. As a result, the lithium metal layer 224 adheres more firmly to the first negative electrode layer 222 than to the second negative electrode layer 223 due to the anchoring effect of the lithium alloy particles 222b, thereby promoting the growth of the lithium metal layer 224 on the second negative electrode layer 223 side. Therefore, the deposition of lithium metal on the surface of the negative electrode 220 can be suppressed, and the generation of dendrites D can be suppressed.

[0040] Here, the potential of the negative electrode is 0V (vs Li / Li + In the state shown in Figure 3, to make the mass fraction of lithium alloy particles in the first negative electrode layer 222 greater than the mass fraction of lithium alloy particles in the second negative electrode layer 223, the mass fraction of metal M oxide particles 222a in the first negative electrode layer 222 should be made greater than the mass fraction of metal M oxide particles 222a in the second negative electrode layer 223. This makes it possible to make the absolute value of the lithium nucleation overpotential in the first negative electrode layer 222 less than the absolute value of the lithium nucleation overpotential in the second negative electrode layer 223. In this disclosure, the lithium nucleation overpotential is defined as the potential of the lithium metal deposited in the negative electrode 220 when charging is performed with lithium metal as the positive electrode and the object to be measured as the negative electrode, where the potential of the lithium metal is 0V (vs Li / Li + This refers to the voltage drop within the following range. Here, when the absolute value of the lithium nucleation overpotential is small, the energy barrier for the generation of lithium metal nuclei is low, so lithium metal is more easily deposited. As a result, when the secondary battery 1 is charged, lithium metal is more likely to be deposited on the surface of the first negative electrode layer 222 opposite to the negative electrode current collector 221 than on the surface of the second negative electrode layer 223 opposite to the negative electrode current collector 221. Therefore, in this embodiment, as shown in Figure 4, the lithium metal layer 224 can be formed between the first negative electrode layer 222 and the second negative electrode layer 223. Therefore, even if dendrites D are formed on the surface of the lithium metal layer 224 on the separator 230 side, the second negative electrode layer 223 can protect the separator 230 and the positive electrode 210 from the dendrites D, thereby improving the safety of the secondary battery 1.

[0041] The mass fraction of lithium alloy particles 222b in the first negative electrode layer 222 is preferably 10% by mass or more. This sufficiently improves the adhesion between the first negative electrode layer 222 and the lithium metal layer 224. The mass fraction of lithium alloy particles 222b in the first negative electrode layer 222 is preferably 50% by mass or less. This suppresses a decrease in ion conductivity in the first negative electrode layer 222. The mass fraction of lithium alloy particles in the second negative electrode layer 223 is preferably 0% by mass. That is, it is preferable that the second negative electrode layer 223 does not contain lithium alloy particles. This suppresses strong adhesion between the second negative electrode layer 223 and the lithium metal layer 224, and suppresses the generation of dendrites D due to the deposition of lithium metal on the surface of the negative electrode 220.

[0042] In the first negative electrode layer 222 and the second negative electrode layer 223, the mass fraction of lithium alloy particles 222b is such that the negative electrode potential is 0V (vs Li / Li + In this state, the first negative electrode layer 222 and the second negative electrode layer 223 are each taken out as samples, and the mass fraction can be calculated using inductively coupled plasma (ICP) emission spectroscopy. More specifically, after weighing the sample, it is dissolved in an acid solution, and the masses of metal M and Li are analyzed by ICP emission spectroscopy to calculate the mass fraction of lithium alloy particles 222b as the ratio of the mass of lithium alloy particles to the sum of the masses of graphene and lithium alloy particles. Another calculation method is to use a thermogravimetric differential thermal analyzer (TG-DTA). Since graphene volatilizes into carbon dioxide upon combustion, the mass of graphene can be measured from the weight change of the sample at the graphene combustion temperature, and the mass of metal M can be measured from the weight of the sample after combustion, so the mass fraction of lithium alloy particles 222b can be calculated in the same manner as above.

[0043] Note that the negative electrode potential is 0V (vs Li / Li +In this state, the first negative electrode layer 222 may contain substances other than carbon, lithium, and metal M, for example, it may contain oxide particles 222a of metal M.

[0044] Preferably, the adhesive strength between the first negative electrode layer 222 and the lithium metal layer 224 is greater than the adhesive strength between the second negative electrode layer 223 and the lithium metal layer 224. This promotes the growth of the lithium metal layer 224 between the lithium metal layer 224 and the second negative electrode layer 223, thereby suppressing the deposition of lithium metal on the side of the second negative electrode layer 223 opposite to the first negative electrode layer 222, and further suppressing the generation of dendrites D.

[0045] The adhesive strength between the first negative electrode layer 222 and the lithium metal layer 224 can be compared with the adhesive strength between the second negative electrode layer 223 and the lithium metal layer 224 by the tensile adhesive strength test method described in JIS K6849. More specifically, the negative electrode 220 with the lithium metal layer 224 generated is removed from the secondary battery 1, and the tensile adhesive strength test described above is performed by pulling on both sides of the stacking direction of the negative electrode 220. In this test, if the second negative electrode layer 223 peels off from the lithium metal layer 224 before the first negative electrode layer 222, it can be said that the adhesive strength between the first negative electrode layer 222 and the lithium metal layer 224 is greater than the adhesive strength between the second negative electrode layer 223 and the lithium metal layer 224. On the other hand, if the first negative electrode layer 222 peels off from the lithium metal layer 224 before the second negative electrode layer 223, then the adhesive strength between the first negative electrode layer 222 and the lithium metal layer 224 is less than the adhesive strength between the second negative electrode layer 223 and the lithium metal layer 224.

[0046] The secondary battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to the one shown in Figure 1.

[0047] For example, between the first negative electrode layer 222 and the second negative electrode layer 223, there may be elements of the metal M contained in the first negative electrode layer 222, or an alloy of metal M with Li.

[0048] Figure 6 is a cutaway view showing a different example of the secondary battery according to the first embodiment. Figure 7 is a schematic diagram of the cross-section of line VII-VII in Figure 6. The secondary battery according to the first embodiment may also be the secondary battery shown in Figures 6 and 7. The secondary battery 1A shown in Figures 6 and 7 differs from the example shown in Figure 1 in that the electrode body 200A is wound around the positive electrode lead 21A and the negative electrode lead 22A. In the following description, components similar to those in Figures 1 and 2 are denoted by reference numerals and their explanation is omitted.

[0049] The battery element 20A is provided inside the outer casing member 30. As shown in Figure 7, the battery element 20A comprises 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 out from inside the battery element 20A to the outside of the outer casing 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 out from inside the battery element 20A to the outside of the outer casing 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 wrap around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0050] In the example shown in Figure 7, the electrode body 200A is a laminate for the charge-discharge reaction of the secondary battery according to the first embodiment. The electrode body 200A includes a positive electrode 210A comprising a positive electrode current collector 211A and a positive electrode mixture layer 212A, a negative electrode 220A comprising a negative electrode current collector 221A, a first negative electrode layer 222A, a second negative electrode layer 223A and a lithium metal layer 224A, and a separator 230A. The electrode body 200A has a structure wound around the positive electrode lead 21A and the negative electrode lead 22A, and is stacked in the following order from the outside, i.e., the side of the protective material 23: negative electrode current collector 221A, first negative electrode layer 222A, second negative electrode layer 223A, separator 230A, positive electrode mixture layer 212A, positive electrode current collector 211A, positive electrode mixture layer 212A, separator 230A, second negative electrode layer 223A, lithium metal layer 224A, and first negative electrode layer 222A. Near the positive electrode lead 21A and the negative electrode lead 22A, the electrode body 200A does not have any layers other than the negative electrode current collector 221A, separator 230A, and positive electrode current collector 211A. 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.

[0051] As described above, the secondary battery 1 according to the embodiment is a secondary battery comprising a positive electrode 210 and a negative electrode 220. The negative electrode 220 comprises a negative electrode current collector 221, a first negative electrode layer 222 provided on the negative electrode current collector 221 and containing graphene, a lithium metal layer 224 provided on the first negative electrode layer 222 and containing lithium metal but not graphene, and a second negative electrode layer 223 provided on the lithium metal layer 224 and containing graphene. The potential of the negative electrode is 0V (vs Li / Li + In this state, the first negative electrode layer 222 has lithium alloy particles 222b containing lithium and a metal M that forms an alloy with lithium, and the mass fraction of lithium alloy particles 222b in the first negative electrode layer 222 is greater than the mass fraction of lithium alloy particles 222b in the second negative electrode layer 223.

[0052] In the secondary battery 1 according to this embodiment, the lithium metal layer 224 adheres more firmly to the first negative electrode layer 222 than to the second negative electrode layer 223 due to the anchoring effect of lithium alloy particles 222b, thereby promoting the growth of the lithium metal layer 224 on the second negative electrode layer 223 side. Consequently, the deposition of lithium metal on the surface of the negative electrode 220 can be suppressed, and the generation of dendrites D can be suppressed, thereby improving safety.

[0053] In a desirable embodiment, the mass fraction of lithium alloy particles 222b in the first negative electrode layer 222 is 10% by mass or more and 50% by mass or less. This suppresses a decrease in ionic conductivity in the first negative electrode layer 222 while sufficiently improving the adhesion between the first negative electrode layer 222 and the lithium metal layer 224.

[0054] In a preferred embodiment, at least one of the first negative electrode layer 222 and the second negative electrode layer 223 contains reduced graphene oxide. This allows for improved conductivity while simplifying the manufacturing process of at least one of the first negative electrode layer 222 and the second negative electrode layer 223.

[0055] In a desirable embodiment, the adhesive strength between the first negative electrode layer 222 and the lithium metal layer 224 is greater than the adhesive strength between the second negative electrode layer 223 and the lithium metal layer 224. This promotes the growth of the lithium metal layer 224 between the lithium metal layer 224 and the second negative electrode layer 223, thereby suppressing the deposition of lithium metal on the side of the second negative electrode layer 223 opposite to the first negative electrode layer 222, and further suppressing the generation of dendrites D, thus improving safety.

[0056] In a preferred embodiment, the metal M that forms an alloy with lithium is at least one of Zn, Si, Mg, Al, Au, Pt, and Ag. This allows the lithium metal layer 224 to adhere more firmly to the first negative electrode layer 222 than to the second negative electrode layer 223 due to the anchoring effect of the lithium alloy particles 222b, thereby promoting the growth of the lithium metal layer 224 on the second negative electrode layer 223 side. Consequently, the deposition of lithium metal on the surface of the negative electrode 220 can be further suppressed, and the generation of dendrites D can be further suppressed, thereby improving safety.

[0057] Examples Examples based on embodiments will be described. However, this disclosure is not limited by these embodiments. Table 1 is a table showing the configuration of the negative electrode and experimental results for Example 1 and Comparative Examples 1 and 2. Here, in the column "Presence or Absence of First Negative Electrode Layer" in Table 1, "Y" indicates the presence of a first negative electrode layer containing lithium alloy particles, and "N" indicates the absence of a first negative electrode layer containing lithium alloy particles. Also, in the column "Presence or Absence of Second Negative Electrode Layer" in Table 1, "Y" indicates the presence of a second negative electrode layer without lithium alloy particles, and "N" indicates the absence of a second negative electrode layer without lithium alloy particles. Also, in the column "Dendrite Generation on Negative Electrode Surface" in Table 1, "Y" indicates the generation of dendrites on the negative electrode surface, and "N" indicates the absence of dendrites on the negative electrode surface.

[0058] [Table 1]

[0059] The negative electrode current collector in Example 1 consists of a 2cm thick copper foil (Pred Materials). 2 It was made by punching out a piece to that size.

[0060] The first negative electrode layer in Example 1 was prepared by the following method. First, zinc oxide powder (Sigma-Aldrich Co. LLC) was added to a slurry (GO-3, Hangzhou Gaoxi Technology Co., Ltd.) containing 1.1% by mass of graphene oxide, and the mixture was stirred for 20 minutes in a rotating / revolving mixer (Thinky) to prepare the first negative electrode mixture. Here, the amount of zinc oxide powder added was adjusted to account for 33% by mass of the dried first negative electrode mixture. Then, the first negative electrode mixture was applied to a glass plate using a doctor blade with a gap set to 15 milli-inch (0.38 mm) and dried overnight at room temperature. The dried first negative electrode mixture was then peeled off the glass plate and reduced by contact with a hot plate heated to 390°C in a glove box under an argon atmosphere. Then, 1 cm 2 The first negative electrode layer according to Example 1 was fabricated by punching out a piece to the specified size.

[0061] The second negative electrode layer according to Example 1 was prepared by the following method. First, a slurry containing 1.1% by mass of graphene oxide (GO-3, manufactured by Hangzhou Gaoxi Technology Co., Ltd.) was used as the second negative electrode mixture. The mixture was applied to a glass plate using a doctor blade with a gap set to 15 milli-inch (0.38 mm) and dried overnight at room temperature. The dried second negative electrode mixture was then peeled off the glass plate and reduced by contacting it with a hot plate heated to 390°C in a glove box under an argon atmosphere. Then, 1 cm 2 The second negative electrode layer according to Example 1 was fabricated by punching out a piece to the specified size.

[0062] The electrolyte for Example 1 was prepared in a glove box under an argon atmosphere with an oxygen concentration of 0.2 ppm or less and a water concentration of 0.01 ppm or less. The electrolyte was prepared by adding bis(trifluoromethanesulfonyl)imide lithium (LiTFSI, Solvay) and lithium nitrate (LiNO3, Aldrich) as solutes to a mixture of dimethyl ether (Aldrich) and 1,3-dioxolane (Aldrich) in a volume ratio of 1:1, and stirring overnight. The electrolyte was prepared so that the concentration of LiTFSI was 1 mol / L and the concentration of LiNO3 was 1% by mass.

[0063] Subsequently, a half-cell was fabricated using the fabricated negative electrode. The half-cell was a 2023 type coin cell. The half-cell was fabricated in a glove box under an argon atmosphere with an oxygen concentration of 0.2 ppm or less and a moisture concentration of 0.01 ppm or less. Here, the positive electrode of the half-cell was prepared by scraping the surface of a 750 μm thick, 99.9% pure lithium foil (Alfa Aesar) to remove the oxide film, and then removing 1 cm 2 The lithium metal foil was punched out. For the half-cell separator, a 25μm thick, three-layer polypropylene-polyethylene-polypropylene separator (Celgard) was used. The spacer was made of 0.5mm thick stainless steel foil. First, the positive electrode was attached to one spacer and placed in the anode can with a 0.5mm thick spring and two spacers. Next, the negative electrode current collector, the first negative electrode layer, and the second negative electrode layer were stacked to form the negative electrode and placed in the cathode can. Then, the anode can and cathode can were stacked with the prepared electrolyte-impregnated separator in between, and then crimped to create a coin cell.

[0064] Charging test A charging test was performed on the fabricated half-cells. The charging test was conducted using a battery cycler (Arbin) in a glove box under an argon atmosphere with an oxygen concentration of 0.2 ppm or less and a moisture concentration of 0.01 ppm or less. The charging rate was 0.1 mA / cm². 2 CC charging is performed, and the voltage in the charging curve is 0V (vs Li / Li + ) 1mAh / cm² within the following range2 The process continued until it reached [a certain value]. In the charging test, the voltage in the charging curve was 0V (vs Li / Li + ) After the voltage dropped within the range below, charging was continued until the voltage rose again. At this time, in the charging curve obtained in the measurement, the voltage was 0V (vs Li / Li + In the following region, the voltage of the minimum value of the charging curve was measured as the lithium nucleation overpotential.

[0065] EDS measurement After the charging test, the half-cell was disassembled, and EDS mapping images of the cross-section in the thickness direction of the first negative electrode layer were acquired using EDS (EMAX Evolution X-Max20, Horiba, Ltd.) to check for the presence or absence of lithium alloy particles. As a result, it was confirmed that lithium alloy particles were present in the first negative electrode layer of both Example 1 and Comparative Example 1.

[0066] Charge / Discharge Test Charge and discharge tests were conducted on the fabricated half-cells, and the Coulomb efficiency was measured. The charge and discharge test yielded a value of 0.5 mA / cm². 2 1mAh / cm² 2 Charge until it reaches 0.5mA / cm². 2 The discharge cycle was repeated until the voltage reached a cutoff potential of 1V, and the Coulombic efficiency (CE) was calculated by dividing the discharge capacity by the charge capacity for each cycle number.

[0067] SEM observation After the charging test, the half-cell was disassembled, and the surface of the negative electrode on the separator side and the cross-section of the negative electrode were observed using a scanning electron microscope (SEM). The SEM observation was performed under the following conditions. SEM: S-4800 (Hitachi High-Technologies) Acceleration voltage: 3.0kV

[0068] Comparative Example 1 For Comparative Example 1, a half-cell was fabricated in the same manner as in Example 1, except that a second negative electrode layer was not laminated. A charging test, a charge-discharge test, and SEM observation were then performed.

[0069] Comparative Example 2 For Comparative Example 2, a half-cell was prepared in the same manner as in Example 1, except that the first negative electrode layer was not laminated. A charging test, a charge-discharge test, and SEM observation were then performed.

[0070] Figure 8 shows the charging curves for the half-cells of the examples and comparative examples. From the graph in Figure 8, the lithium nucleation overpotential was as shown in Table 1. As shown in Figure 8 and Table 1, in Comparative Example 2, in which only the second negative electrode layer without lithium alloy particles was laminated onto the negative electrode current collector, the absolute value of the lithium nucleation overpotential was smaller compared to Comparative Example 1, in which only the first negative electrode layer containing lithium alloy particles was laminated onto the negative electrode current collector. Therefore, it is considered that the surface of the first negative electrode layer containing lithium alloy particles is more prone to lithium metal layer formation than the surface of the second negative electrode layer without lithium alloy particles.

[0071] Furthermore, as shown in Figure 8 and Table 1, in Example 1, where a second negative electrode layer without lithium alloy particles is laminated onto a first negative electrode layer containing lithium alloy particles, the absolute value was closer to that of Comparative Example 1, where only the first negative electrode layer was laminated onto the negative electrode current collector, than to that of Comparative Example 2, where only the second negative electrode layer was laminated onto the negative electrode current collector. From this, it can be seen that when the negative electrode comprises a first negative electrode layer containing lithium alloy particles and a second negative electrode layer without lithium alloy particles (Example 1), its nucleation overpotential is the same as when it contains only the first negative electrode layer containing lithium alloy particles (Comparative Example 1). From this, it can be seen that even when a negative electrode is formed by laminating a second negative electrode layer without lithium alloy particles onto a first negative electrode layer containing lithium alloy particles, the lithium metal layer is more likely to form on the surface of the first negative electrode layer containing lithium alloy particles than on the surface of the second negative electrode layer without lithium alloy particles during charging.

[0072] Figure 9 shows the Coulomb efficiency for half-cells in the comparative example and the example. As shown in Figure 9, the negative electrode in Example 1, in which a second negative electrode layer without lithium alloy particles is laminated on a first negative electrode layer containing lithium alloy particles, showed improved Coulomb efficiency over 40 cycles compared to Comparative Examples 1 and 2, in which only one of the first or second negative electrode layers is laminated. This indicates that the negative electrode with the second negative electrode layer laminated on the first negative electrode layer has improved charge-discharge characteristics. This is thought to be because the growth of the lithium metal layer between the first and second negative electrode layers suppresses the formation of dendrites D, thereby suppressing side reactions with the electrolyte and the generation of isolated Li metal, and thus reducing irreversible capacity.

[0073] Figure 10 shows an SEM image of the separator-side surface of the negative electrode according to Example 1. Figures 11 and 12 show SEM images of the cross-section of the negative electrode according to Example 1. Here, Figure 11 is an SEM image of the negative electrode of Example 1 before the charging test, and Figure 12 is an SEM image of the negative electrode of Example 1 after the charging test. Figure 13 shows an SEM image of the separator-side surface of the negative electrode according to Comparative Example 1. Figure 14 shows an SEM image of the cross-section of the negative electrode according to Comparative Example 1. As shown in Figures 13 and 14, a lithium metal layer 224X with dendrites D on its surface was formed on the separator-side surface of the negative electrode according to Comparative Example 1. On the other hand, as shown in Figure 10, no lithium metal layer was formed on the separator-side surface of the negative electrode according to Example 1 (the surface of the second negative electrode layer 223). Furthermore, as shown in Figures 11 and 12, in the negative electrode according to Example 1, a lithium metal layer 224 was formed between the first negative electrode layer 222 and the second negative electrode layer 223 by charging. This indicates that, because the second negative electrode layer 223, which does not contain lithium alloy particles, is laminated on the first negative electrode layer 222, which contains lithium alloy particles, a lithium metal layer 224 is generated between the first negative electrode layer 222 and the second negative electrode layer 223 by charging, and the lithium metal layer 224 is covered by the second negative electrode layer 223. From the results in Figure 8, it is considered that the lithium metal layer is preferentially formed on the surface of the first negative electrode layer 222, which contains lithium alloy particles, rather than on the surface of the second negative electrode layer 223, which does not contain lithium alloy particles.

[0074] Various changes and modifications to the embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of this subject matter and without impairing the intended advantages. Accordingly, such changes and modifications are intended to be encompassed by the appended claims.

Claims

1. A secondary battery comprising a positive electrode and a negative electrode, The aforementioned negative electrode is Negative electrode current collector and The negative electrode current collector is provided with a first negative electrode layer containing graphene, The first negative electrode layer is provided with a lithium metal layer containing lithium metal but not graphene, A second negative electrode layer containing graphene is provided in the lithium metal layer. Equipped with, The potential of the negative electrode is 0V (vs Li / Li ＋ In the state of, The first negative electrode layer has lithium alloy particles containing lithium and a metal that forms an alloy with the lithium, A secondary battery in which the mass fraction of the lithium alloy particles in the first negative electrode layer is greater than the mass fraction of the lithium alloy particles in the second negative electrode layer.

2. The secondary battery according to claim 1, wherein the mass fraction of the lithium alloy particles in the first negative electrode layer is 10% by mass or more and 50% by mass or less.

3. The secondary battery according to claim 1, wherein at least one of the first negative electrode layer and the second negative electrode layer comprises reduced graphene oxide.

4. The secondary battery according to claim 1, wherein the adhesive force between the first negative electrode layer and the lithium metal layer is greater than the adhesive force between the second negative electrode layer and the lithium metal layer.

5. The secondary battery according to claim 1, wherein the metal that forms an alloy with the lithium is at least one of Zn, Si, Mg, Al, Au, Pt, and Ag.

6. The secondary battery according to claim 1, wherein the absolute value of the lithium nucleation overpotential in the first negative electrode layer is smaller than the absolute value of the lithium nucleation overpotential in the second negative electrode layer.

7. It is the negative electrode, Negative electrode current collector and The negative electrode current collector is provided with a first negative electrode layer containing graphene, The first negative electrode layer is provided with a lithium metal layer containing lithium metal but not graphene, A second negative electrode layer containing graphene is provided in the lithium metal layer. Equipped with, The potential of the negative electrode is 0V (vs Li / Li ＋ In the state of, The first negative electrode layer has lithium alloy particles containing lithium and a metal that forms an alloy with the lithium, A negative electrode in which the mass fraction of the lithium alloy particles in the first negative electrode layer is greater than the mass fraction of the lithium alloy particles in the second negative electrode layer.

8. The negative electrode according to claim 7, wherein the mass fraction of the lithium alloy particles in the first negative electrode layer is 10% by mass or more and 50% by mass or less.

9. The negative electrode according to claim 7, wherein at least one of the first negative electrode layer and the second negative electrode layer comprises reduced graphene oxide.

10. The negative electrode according to claim 7, wherein the adhesive force between the first negative electrode layer and the lithium metal layer is greater than the adhesive force between the second negative electrode layer and the lithium metal layer.

11. The negative electrode according to claim 7, wherein the metal that forms an alloy with the lithium is at least one of Zn, Si, Mg, Al, Au, Pt, and Ag.

12. The negative electrode according to claim 7, wherein the absolute value of the lithium nucleation overpotential in the first negative electrode layer is smaller than the absolute value of the lithium nucleation overpotential in the second negative electrode layer.

13. A method for manufacturing a negative electrode, A first negative electrode layer containing graphene is provided on the negative electrode current collector, A second negative electrode layer containing graphene is provided on the first negative electrode layer, By charging, a lithium metal layer is formed between the first negative electrode layer and the second negative electrode layer. Includes, The first negative electrode layer has metal compound particles containing a metal that forms an alloy with lithium, A method for manufacturing a negative electrode, wherein the mass fraction of the metal compound particles in the first negative electrode layer is greater than the mass fraction of the metal compound particles in the second negative electrode layer.

14. The method for manufacturing a negative electrode according to claim 13, further comprising forming lithium alloy particles containing lithium and a metal that forms an alloy with the lithium from the metal compound particles by the aforementioned charging.

15. The method for manufacturing a negative electrode according to claim 14, wherein the mass fraction of the lithium alloy particles in the first negative electrode layer is 10% by mass or more and 50% by mass or less.

16. The method for manufacturing a negative electrode according to claim 13, wherein the metal compound particles are metal oxide particles.

17. The method for producing a negative electrode according to claim 13, wherein at least one of the first negative electrode layer and the second negative electrode layer comprises reduced graphene oxide.

18. The method for manufacturing a negative electrode according to claim 13, wherein the adhesive force between the first negative electrode layer and the lithium metal layer is greater than the adhesive force between the second negative electrode layer and the lithium metal layer.

19. The method for manufacturing a negative electrode according to claim 13, wherein the metal that forms an alloy with the lithium is at least one of Zn, Si, Mg, Al, Au, Pt, and Ag.

20. The method for manufacturing a negative electrode according to claim 13, wherein the absolute value of the lithium nucleation overpotential in the first negative electrode layer is smaller than the absolute value of the lithium nucleation overpotential in the second negative electrode layer.

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