Negative electrode and nonaqueous electrolyte secondary battery provided with the same

Abstract

The present application relates to a negative electrode and a nonaqueous electrolyte secondary battery provided with the same. Provided is a negative electrode that achieves an improvement in cycle life and high output of a secondary battery. The negative electrode disclosed herein is provided with a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer contains silicon oxide containing at least one alkaline earth metal. The negative electrode active material layer contains at least a first layer and a second layer. The first layer is disposed between the second layer and the negative electrode current collector. In the case of the second layer, when the negative electrode active material of the second layer is taken as 100% by mass, the silicon oxide containing the alkaline earth metal is 2% by mass or less. Here, the amount of the alkaline earth metal in the first layer calculated based on energy dispersive X-ray analysis using a scanning electron microscope image is higher than the amount of the alkaline earth metal in the second layer.

Description

Technical Field

[0001] This invention relates to a negative electrode. This invention also relates to a non-aqueous electrolyte secondary battery having the negative electrode. Background Technology

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion batteries have become suitable for use as portable power sources for personal computers, portable terminals, electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] The negative electrode of a non-aqueous electrolyte secondary battery typically consists of a layer of negative electrode active material supported on a negative electrode current collector. In recent years, with the aim of increasing the capacity of the negative electrode, research has focused on silicon (Si)-based negative electrode active materials, such as silicon and silicon compounds, which can absorb and release chemical species (e.g., lithium ions) that act as charge carriers. While Si-based negative electrode active materials are known to have high theoretical capacities, they also experience significant expansion and contraction (volume change) during charge-discharge cycles, resulting in a substantial decrease in capacity retention after charge-discharge cycles.

[0004] In Patent Document 1, in order to improve the charge and discharge capacity, initial charge and discharge efficiency and capacity retention of a secondary battery, a silicon composite oxide comprising MgSiO3 crystals, a negative electrode material coated with carbon material on its surface and a negative electrode using the oxide are disclosed.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-156922 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] However, the inventors conducted in-depth research and found that, in the case of a negative electrode using only silicon composite oxides containing MgSiO3 crystals and coated with carbon material, although the capacity retention rate (cycle life) of the secondary battery after cycling is improved, the initial resistance of the secondary battery is high, and there is still room for improvement from the viewpoint of high output.

[0010] The present invention was made in view of the above circumstances, and its main objective is to provide a negative electrode that improves the cycle life and increases the output of a secondary battery. Another objective is to provide a non-aqueous electrolyte secondary battery having this negative electrode.

[0011] The disclosed negative electrode comprises a negative current collector and a negative active material layer formed on the surface of the negative current collector, wherein the negative active material layer comprises silicon oxide containing at least one alkaline earth metal. The negative active material layer comprises at least a first layer and a second layer, wherein the first layer is disposed between the second layer and the negative current collector. Regarding the second layer, when the negative active material of the second layer is set to 100% by mass, the amount of silicon oxide containing the alkaline earth metal is 2% by mass or less. The amount of alkaline earth metal in the first layer, calculated based on energy-dispersive X-ray analysis using scanning electron microscopy images, is higher than the amount of alkaline earth metal in the second layer.

[0012] According to this configuration, by predominantly placing alkaline earth metals in the first layer and limiting the mass ratio of silicon oxide containing alkaline earth metals in the second layer, the entire negative electrode active material layer can effectively contribute to the charge-discharge reaction. Therefore, a negative electrode that improves the cycle life and increases the output of a secondary battery can be provided.

[0013] In a preferred embodiment of the negative electrode disclosed herein, when the negative electrode active material of the first layer is set to 100% by mass, the silicon oxide containing alkaline earth metal contained in the first layer is 2% by mass or more. According to this configuration, a negative electrode that more appropriately improves the cycle life of a secondary battery can be provided.

[0014] In a preferred embodiment of the negative electrode disclosed herein, the average thickness of the first layer is 20% or more and 70% or less relative to the average thickness of the negative electrode active material layer. According to this configuration, a negative electrode can be provided that achieves a higher level of balance between improved cycle life and high output of the secondary battery.

[0015] In a preferred embodiment of the disclosed negative electrode, the aforementioned alkaline earth metal-containing silicon oxide comprises magnesium-containing silicon oxide and / or calcium-containing silicon oxide. According to this configuration, a negative electrode can be provided that achieves a higher level of balance between improved cycle life and high output of the secondary battery.

[0016] In a preferred embodiment of the negative electrode disclosed herein, the second layer comprises silicon oxide containing an alkali metal. Based on this configuration, a negative electrode that more appropriately achieves high output in a secondary battery can be provided.

[0017] In a preferred embodiment of the negative electrode disclosed herein, the first layer comprises silicon oxide containing an alkali metal. Based on this configuration, a negative electrode that more appropriately achieves high output in a secondary battery can be provided.

[0018] In a preferred embodiment of the disclosed negative electrode, the aforementioned alkali metal-containing silicon oxide includes lithium-containing silicon oxide. According to this configuration, the diffusivity of Li ions is increased, more appropriately achieving high output in the secondary battery.

[0019] In a preferred embodiment of the negative electrode disclosed herein, the aforementioned negative electrode active material layer comprises a carbon material. Based on this configuration, a negative electrode can be provided that achieves a higher level of balance between improved cycle life and high output in secondary batteries.

[0020] In another aspect, the non-aqueous electrolyte secondary battery disclosed herein includes the negative electrode, positive electrode, and non-aqueous electrolyte described above. Based on this configuration, a non-aqueous electrolyte secondary battery with excellent cycle life and output characteristics can be provided. Attached Figure Description

[0021] Figure 1 A diagram illustrating the structure of the negative electrode in one embodiment.

[0022] Figure 2 A diagram illustrating the cross-sectional structure of a lithium-ion secondary battery according to one embodiment is provided.

[0023] Figure 3 The diagram illustrates the configuration of the wound electrode body of a lithium-ion secondary battery according to one embodiment.

[0024] Explanation of reference numerals in the attached figures

[0025] 20. Winded electrode body

[0026] 30 Battery casing

[0027] 32 Safety valve

[0028] 42 Positive extremes

[0029] 42a Positive Current Collector

[0030] 44 Negative extremes

[0031] 44a Negative Current Collector

[0032] 50 Positive electrode sheet (positive electrode)

[0033] 52 Positive current collector

[0034] 54 Positive electrode active material layer

[0035] 56. Non-forming portion of the positive electrode active material layer

[0036] 60 Negative electrode sheet (negative electrode)

[0037] 62 Negative current collector

[0038] 64 Negative Electrode Active Material Layer

[0039] 64A First Floor

[0040] 64B Second Layer

[0041] 66. Non-forming portion of the negative electrode active material layer

[0042] 70 Separator

[0043] 100 Lithium-ion Secondary Battery Detailed Implementation

[0044] The following is a reference to the appendix. Figure 1 The embodiments of the present invention will be described below. It should be noted that matters not mentioned in this specification but necessary for the implementation of the present invention can be understood by those skilled in the art based on prior art in this field. The present invention can be implemented based on the content disclosed in this specification and common technical knowledge in this field. Furthermore, in the following drawings, components and parts that perform the same function are labeled with the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships.

[0045] It should be noted that in this specification, "secondary battery" refers to an energy storage device capable of repeated charging and discharging, encompassing energy storage components such as batteries and double-layer capacitors. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that utilizes lithium ions as charge carriers, achieving charging and discharging through the movement of lithium ions between the positive and negative electrodes.

[0046] Figure 1 The diagram illustrates the negative electrode disclosed herein. As shown, the negative electrode 60 includes a negative current collector 62 and a negative active material layer 64 supported on the negative current collector 62. In the example shown, the negative active material layer 64 is disposed on one side of the negative current collector 62, but it may also be disposed on both sides. Preferably, the negative active material layer 64 is disposed on both sides of the negative current collector 62.

[0047] As the negative current collector 62, sheets or foils made of metals such as copper, nickel, titanium, and stainless steel can be used, with copper foil being preferred. When using copper foil as the negative current collector 62, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0048] As shown in the figure, the negative electrode active material layer 64 includes at least a first layer 64A and a second layer 64B. The first layer 64A is formed between the second layer 64B and the negative electrode current collector 62. That is, the first layer 64A is located on the side of the negative electrode current collector 62, and the second layer 64B is located on the surface side of the negative electrode active material layer 64. The first layer 64A is typically formed on the surface of the negative electrode current collector 62. The negative electrode active material layer 64 can be a multilayer structure with at least two layers, or it can be a multilayer structure with three or more layers.

[0049] The negative electrode active material layer 64 comprises silicon oxide containing at least one alkaline earth metal as the negative electrode active material. In the negative electrode 60 disclosed herein, the alkaline earth metal content of the first layer 64A is higher than that of the second layer 64B based on energy dispersive X-ray analysis using scanning electron microscopy images.

[0050] The "alkaline earth metal content (mass%)" in this specification can be determined using energy-dispersive X-ray analysis (SEM-EDS) of scanning electron microscope images. Specifically, first, a SEM image of a cross-section along the thickness direction of the negative electrode active material layer is taken. Then, EDS analysis is performed on the SEM image to calculate the proportion (mass%) of each constituent element contained in the negative electrode active material layer. The proportion of alkaline earth metal elements (Mg, Ca, etc.) calculated at this time (i.e., the proportion of alkaline earth metal elements relative to all constituent elements of the negative electrode active material layer) is set as the "alkaline earth metal content (mass%)" in this specification.

[0051] The alkaline earth metal content (mass%) of the first and second layers can be calculated, for example, as follows. In a cross-section along the thickness direction of the negative electrode active material layer, the thickness of 20% from the current collector toward the interior of the active material layer is defined as the first layer, and the thickness of 20% from the surface layer toward the interior of the active material layer is defined as the second layer. Then, following the same procedure as above, EDS analysis is performed on the first and second layers respectively to calculate the proportion (mass%) of each constituent element in each layer. The proportion of alkaline earth metal elements relative to all constituent elements of the first layer is defined as the "alkaline earth metal content (mass%) of the first layer" in this specification, and the proportion of alkaline earth metal elements relative to all constituent elements of the second layer is defined as the "alkaline earth metal content (mass%) of the second layer" in this specification.

[0052] The alkaline earth metal content of the first layer 64A is preferably 0.5% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less. Furthermore, the alkaline earth metal content of the second layer 64B can be less than 2% by mass or less than 1% by mass. It should be noted that, typically, the region containing an alkaline earth metal content of 0.5% by mass or more as calculated using SEM-EDS is defined as the first layer 64A. Furthermore, the alkaline earth metal content of the second layer 64B is not limited to the technology disclosed herein. That is, the alkaline earth metal content of the second layer 64B can also be 0% by mass.

[0053] The average thickness of the negative electrode active material layer 64 is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less. In a preferred embodiment, the average thickness of the first layer 64A is preferably 15% or more and 75% or less, more preferably 20% or more and 70% or less, relative to the average thickness of the negative electrode active material layer 64.

[0054] The negative electrode active material layer 64 contains at least a negative electrode active material capable of reversibly adsorbing and releasing chemical species (lithium ions in a lithium-ion secondary battery) that serve as charge carriers. In the disclosed technology, the negative electrode active material layer 64 comprises silicon oxide containing at least one alkaline earth metal as the negative electrode active material. The alkaline earth metal-containing silicon oxide is typically silicon oxide (SiO₂) doped with an alkaline earth metal (Mg, Ca, etc.) and containing silicon (Si) and oxygen (O) as essential components. y The state in ) is preferred. For example, it preferably has the general formula: M x SiO y (In the formula, x and y satisfy 0 < x ≦ 0.25 and 0 < y ≦ 2, respectively. M is at least one element selected from Mg, Ca, Be, Sr, Ba, and Ra.) Preferably, it is silicon dioxide containing Mg and / or silicon dioxide containing Ca.

[0055] There is no particular limitation on the average particle size (median particle size D50) of silica containing alkaline earth metals; for example, it can be greater than 0.5 μm and less than 15 μm. It should be noted that in this specification, "average particle size (median particle size D50)" refers to the particle size that corresponds to 50% of the cumulative frequency from the smaller particle side in a particle size distribution based on a general laser diffraction / light scattering method.

[0056] As a type of silicon dioxide containing Mg, it is typically a Mg-Si-O compound, which is silicon dioxide (SiO₂) doped with Mg, an alkaline earth metal. y ). In Mg doped SiO y In this case, as a crystal structure, Si phase and SiO can be formed. y Mg-containing silicon oxide typically contains the MgSiO3 phase. In the disclosed technology, Mg-containing silicon oxide preferably has a phase composed of the general formula: Mg α SiO y (In the formula, α and y satisfy 0<α≦0.25 and 0<y≦2 respectively) represents the composition.

[0057] Similarly, as a Ca-containing silicon dioxide, a typical example is a Ca-Si-O compound, which is silicon dioxide (SiO₂) doped with Ca, an alkaline earth metal. y In the disclosed technology, silicon dioxide containing Ca is preferably composed of the general formula: Ca β SiO y (In the formula, β and y satisfy 0<β≦0.25 and 0<y≦2 respectively) represents the composition.

[0058] The first layer 64A contains silicon oxide containing at least one alkaline earth metal as the negative electrode active material. When the negative electrode active material of the first layer 64A is set to 100% by mass, the mass percentage of silicon oxide containing an alkaline earth metal in the first layer 64A is preferably 1% by mass or more and 20% by mass or less, more preferably 1.5% by mass or more and 20% by mass or less, and particularly preferably 2% by mass or more and 20% by mass or less. When the negative electrode active material of the second layer 64B is set to 100% by mass, the mass percentage of silicon oxide containing an alkaline earth metal in the second layer 64B is preferably less than 3% by mass, more preferably 2% by mass or less, and particularly preferably 1% by mass or less. It should be noted that whether or not the second layer 64B contains silicon oxide containing an alkaline earth metal is not a limitation of the technology disclosed herein. That is, the silicon oxide containing an alkaline earth metal in the second layer 64B may also be 0% by mass.

[0059] It should be noted that the mass ratio of alkaline earth metal-containing silicon dioxide in each layer can be determined, for example, by setting the first and second layers as described above and performing ICP analysis.

[0060] By adjusting the mass ratio of alkaline earth metal-containing silicon oxide in the first layer 64A and the second layer 64B to the range mentioned above, it is possible to appropriately balance the improvement of cycle life and high output of the secondary battery. The rationale is not specifically limited, but is speculated as follows.

[0061] By incorporating alkaline earth metals into silicon oxide through doping, the diffusion of charge-carrying chemical species (lithium ions in lithium-ion secondary batteries) tends to slow down. When the second layer contains a large amount of alkaline earth metal-containing silicon oxide, a short-term decrease in the secondary battery's output occurs. In contrast, in the disclosed technology, by limiting the amount of alkaline earth metal-containing silicon oxide in the second layer disposed on the surface side of the negative electrode active material layer, high resistivity can be suppressed. This suppresses short-term decreases in the secondary battery's output. Furthermore, by having the alkaline earth metal-containing silicon oxide predominate in the first layer, which serves as the current collector, ions penetrating from the surface side diffuse towards the current collector side during long-term use, making the entire negative electrode active material layer conducive to the charge-discharge reaction. This improves cycle life. By appropriately adjusting the mass ratio of alkaline earth metal-containing silicon oxide within the negative electrode active material layer, high output and improved cycle life of the secondary battery can be achieved.

[0062] The negative electrode active material layer 64 may contain silicon oxide containing at least one alkali metal as the negative electrode active material. Typically, the alkali metal-containing silicon oxide is silicon oxide (SiO₂) doped with an alkali metal (Li, Na, etc.) and containing silicon (Si) and oxygen (O) as essential components. y The state in ) . For example, it is preferable to have the general formula: Q γ SiO y(In the formula, γ and y satisfy 0 < γ ≦ 2 and 0 < y ≦ 2, respectively. Q is at least one element selected from Li, Na, K, Rb, Cs, and Fr.) Preferably, it is silicon oxide containing Li.

[0063] The second layer 64B may contain silicon oxide containing an alkali metal. When the negative electrode active material of the second layer 64B is set to 100% by mass, the mass percentage of silicon oxide containing an alkali metal in the second layer 64B may be 20% by mass or less, or 18% by mass or less. Furthermore, the first layer 64A may contain silicon oxide containing an alkali metal. When the negative electrode active material of the first layer 64A is set to 100% by mass, the mass percentage of silicon oxide containing an alkali metal in the first layer 64A may be 18% by mass or less, or 16% by mass or less. It should be noted that in the technology disclosed herein, the mass percentage of silicon oxide containing an alkali metal in the negative electrode active material layer 64 is not limited to the technology disclosed herein. That is, the mass percentage of silicon oxide containing an alkali metal in the negative electrode active material layer 64 may also be 0% by mass.

[0064] It should be noted that the mass ratio of alkali metal-containing silicon dioxide in each layer can be determined, for example, by the ICP analysis described above.

[0065] Silicon oxide containing alkaline earth metals and silicon oxide containing alkali metals can be prepared, for example, by the following method. First, prepare SiO₂. y The raw material powders are made from alkaline earth metals (e.g., Mg, Ca, etc.) or alkali metals (e.g., Li, Na, etc.). Examples of alkaline earth metal raw material powders include Mg powder and Ca powder. Examples of alkali metal raw material powders include LiH powder. SiO₂ is milled using a ball mill or similar equipment. y The powder is mixed with alkaline earth metal or alkali metal raw material powder to obtain a mixed powder. Under an argon (Ar) atmosphere, the mixed powder is heated at approximately 1000°C for about 1 hour. This allows for the formation of SiO₂. y It is doped with alkaline earth metals or alkali metals.

[0066] In addition to silicon oxide containing alkaline earth metals, the negative electrode active material layer 64 also includes carbon materials such as graphite, hard carbon, and soft carbon as negative electrode active materials. The graphite can be natural graphite, artificial graphite, or graphite coated with amorphous carbon materials in the form of amorphous carbon-coated graphite.

[0067] There are no particular limitations on the morphology (average particle size, BET specific surface area, etc.) of carbon materials. Carbon materials are typically in particulate form. The average particle size D50 of particulate carbon materials can be 1 μm or more and 20 μm or less, for example, 5 μm or more and 15 μm or less. Furthermore, the BET specific surface area obtained using the BET method can preferably be, for example, 0.5 cm². 2 / g or more and 3cm 2 BET specific surface area below / g.

[0068] In addition to the materials described above, the negative electrode active material layer 64 may also contain other negative electrode active materials without impairing the technical effects disclosed herein. Examples of other negative electrode active materials include, for example, Si-based negative electrode active materials. Examples of Si-based negative electrode active materials include metallic Si and oxides with Si as a constituent element (e.g., SiO₂). y Alloys with Si as a constituent element, etc.

[0069] While not specifically limited, the content of the negative electrode active material in the negative electrode active material layer 64 (i.e., the ratio of the negative electrode active material to the total mass of the negative electrode active material layer) can be 80% by mass or more and 99% by mass or less, or 85% by mass or more and 98% by mass or less. When the negative electrode active material in the negative electrode active material layer 64 is set to 100% by mass, the mass ratio of the Si-based negative electrode active material (including silicon oxide containing alkaline earth metals and silicon oxide containing alkali metals) is preferably 2% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 18% by mass or less, and particularly preferably 4% by mass or more and 16% by mass or less. Furthermore, when the negative electrode active material in the negative electrode active material layer 64 is set to 100% by mass, the mass ratio of the carbon material is preferably 80% by mass or more and 98% by mass or less, more preferably 82% by mass or more and 97% by mass or less, and particularly preferably 84% by mass or more and 96% by mass or less.

[0070] While not specifically limited, the content of the negative electrode active material in the first layer 64A can be 80% by mass or more and 99% by mass or less, or 85% by mass or more and 98% by mass or less. When the negative electrode active material of the first layer 64A is set to 100% by mass, the mass ratio of the Si-based negative electrode active material (including silicon oxide containing alkaline earth metals and silicon oxide containing alkali metals) is preferably 2% by mass or more and 25% by mass or less, more preferably 5% by mass or more and 20% by mass or less, and particularly preferably 10% by mass or more and 20% by mass or less. Furthermore, when the negative electrode active material of the first layer 64A is set to 100% by mass, the mass ratio of the carbon material is preferably 75% by mass or more and 98% by mass or less, more preferably 80% by mass or more and 95% by mass or less, and even more preferably 80% by mass or more and 90% by mass or less.

[0071] While not specifically limited, the content of the negative electrode active material in the second layer 64B can be 80% by mass or more and 99% by mass or less, or 85% by mass or more and 98% by mass or less. When the negative electrode active material of the second layer 64B is set to 100% by mass, the mass ratio of the Si-based negative electrode active material (including silicon oxide containing alkaline earth metals and silicon oxide containing alkali metals) is preferably 2% by mass or more and 25% by mass or less, more preferably 10% by mass or more and 20% by mass or less. Furthermore, when the negative electrode active material of the second layer 64B is set to 100% by mass, the mass ratio of the carbon material is preferably 75% by mass or more and 98% by mass or less, more preferably 80% by mass or more and 90% by mass or less.

[0072] The negative electrode active material layer 64 may contain components other than the aforementioned negative electrode active material, such as adhesives and tackifiers. As adhesives, examples include styrene-butadiene rubber (SBR) and its modifiers, acrylonitrile-butadiene rubber and its modifiers, acrylic rubber and its modifiers, and fluororubber. SBR is preferred. The content of the adhesive in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.2% by mass or more and 3% by mass or less.

[0073] As a thickener, cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropyl methyl cellulose (HPMC) can be used; polyvinyl alcohol (PVA) is also acceptable. CMC is preferred. The content of the thickener in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.4% by mass or more and 2% by mass or less.

[0074] The negative electrode constructed as described above can improve the cycle life and increase the output of the secondary battery. The negative electrode constructed as described above can be used as the negative electrode of a secondary battery according to known methods. Therefore, the negative electrode disclosed herein is suitable for use in secondary batteries. This secondary battery is suitable as a non-aqueous electrolyte secondary battery.

[0075] Non-aqueous electrolyte secondary batteries

[0076] Therefore, on the other hand, the non-aqueous electrolyte secondary battery disclosed herein has the aforementioned negative electrode, positive electrode, and non-aqueous electrolyte.

[0077] The following describes in detail one embodiment of the non-aqueous electrolyte secondary battery disclosed herein, using a flat square lithium-ion secondary battery having a flat-shaped wound electrode body and a flat-shaped battery casing as an example. However, it is not intended to limit the non-aqueous electrolyte secondary battery disclosed herein to the contents described in this embodiment.

[0078] Figure 2 The lithium-ion secondary battery 100 shown is a sealed battery constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte (not shown) within a flat, square battery casing (i.e., outer packaging container) 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 32 configured to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is made of a lightweight and thermally conductive metal material, such as aluminum.

[0079] like Figure 2 and Figure 3 As shown, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped and wound along the length direction, separated by two elongated separators 70. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed on one or both (in this case, both) sides of the elongated positive electrode current collector 52 along the length direction. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed on one or both (in this case, both) sides of the elongated negative electrode current collector 62 along the length direction. The non-formed portions 56 of the positive electrode active material layer (i.e., the portions where the positive electrode active material layer 54 is not formed and thus the positive electrode current collector 52 is exposed) and the non-formed portions 66 of the negative electrode active material layer (i.e., the portions where the negative electrode active material layer 64 is not formed and thus the negative electrode current collector 62 is exposed) are formed to extend outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned length direction). The non-forming portion 56 of the positive electrode active material layer and the non-forming portion 66 of the negative electrode active material layer are respectively bonded with a positive electrode current collector 42a and a negative electrode current collector 44a.

[0080] The negative electrode sheet 60 uses the above-mentioned negative electrode.

[0081] The positive current collector 52 constituting the positive electrode sheet 50 can be made of sheet or foil material of metals such as aluminum, nickel, titanium, or stainless steel, with aluminum foil being suitable. When using aluminum foil as the positive current collector 52, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0082] There are no particular limitations on the positive electrode active material included in the positive electrode active material layer 54. One or more positive electrode active materials that have been conventionally used as positive electrode active materials in non-aqueous electrolyte secondary batteries, especially as positive electrode active materials in lithium-ion secondary batteries, can be used. For example, lithium composite oxides and lithium transition metal phosphate compounds (e.g., LiFePO4) are preferred as positive electrode active materials. Examples of lithium composite oxides include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, and lithium nickel manganese composite oxides (e.g., LiNi). 0.5 Mn 1.5 O4), lithium nickel manganese cobalt composite oxides (e.g., LiNi) 1 / 3 Co 1 / 3 Mn 1 / 3 O2), etc.

[0083] There is no particular limitation on the average particle size of the positive electrode active material; it can be approximately 0.5 μm or larger and 50 μm or smaller, for example, 1 μm or larger and 20 μm or smaller.

[0084] The positive electrode active material layer 54 may contain substances other than the positive electrode active material, such as conductive materials and adhesives. As conductive materials, carbon black such as acetylene black (AB) and other carbon materials (such as graphite) are preferred. As adhesives, fluorine-based adhesives such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), and rubber-based adhesives such as styrene-butadiene rubber (SBR) are preferred. Furthermore, the positive electrode active material layer 54 may also contain materials other than those mentioned above (such as various additives) as long as it does not impair the effects of the present invention.

[0085] From the viewpoint of energy density, the content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the ratio of the positive electrode active material to the total mass of the positive electrode active material layer) is preferably approximately 70% by mass or more. More preferably, it is 75% by mass or more and 99% by mass or less, and even more preferably 80% by mass or more and 97% by mass or less. Furthermore, the content of the conductive material in the positive electrode active material layer 54 is preferably, for example, 0.1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 15% by mass or less. The content of the binder in the positive electrode active material layer 54 is preferably, for example, 0.5% by mass or more and 15% by mass or less, more preferably 1% by mass or more and 10% by mass or less. Additionally, when various additives such as thickeners are included, the content of additives in the positive electrode active material layer 54 is preferably, for example, 7% by mass or less, more preferably 5% by mass or less.

[0086] Examples of separators 70 include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. These porous sheets can be single-layered or multi-layered (e.g., a three-layered structure with PP layers laminated on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0087] There is no particular limitation on the thickness of the separator 70, for example, it is 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less.

[0088] Non-aqueous electrolytes are typically liquids in which electrolyte salts (in other words, supporting salts) are dissolved or dispersed in a non-aqueous solvent. Alternatively, polymers can be added to the non-aqueous electrolyte to create a solid (typically a so-called gel) non-aqueous electrolyte.

[0089] As a non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in electrolytes of lithium-ion secondary batteries can be used without particular limitation. Among them, carbonates are preferred, and specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene monofluorocarbonate (MFEC), ethylene difluorocarbonate (DFEC), difluoromethyl difluoromethyl carbonate (F-DMC), and dimethyl trifluorocarbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations.

[0090] As the electrolyte salt, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) can be used, with LiPF6 being preferred. The concentration of the electrolyte salt is not particularly limited, but is preferably 0.7 mol / L or higher and 1.3 mol / L or lower. It should be noted that, provided it does not significantly impair the effects of the present invention, the above-mentioned non-aqueous electrolyte may also contain components other than those described above, such as film-forming agents like oxalic acid complexes, gas generators like biphenyl (BP) and cyclohexylbenzene (CHB), and various additives such as thickeners.

[0091] The lithium-ion secondary battery 100 configured as described above achieves improved cycle life and high output. The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for driving electric vehicles (BEVs), hybrid electric vehicles (HVs), and plug-in hybrid electric vehicles (PHEVs). In HEVs, where the secondary battery is used in conjunction with an engine (internal combustion engine), and high output is required for short periods, the negative electrode disclosed herein and a non-aqueous electrolyte secondary battery having this negative electrode are more suitable. Furthermore, the lithium-ion secondary battery 100 can typically be used in the form of a battery pack consisting of multiple batteries connected in series and / or in parallel.

[0092] Furthermore, as an example, a square lithium-ion secondary battery 100 having a flat, wound electrode body 20 has been described. However, the lithium-ion secondary battery disclosed herein can also be configured as a lithium-ion secondary battery having a stacked electrode body (i.e., an electrode body formed by alternating stacking of multiple positive electrodes and multiple negative electrodes). Additionally, the non-aqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium-ion secondary battery, a laminated shell type lithium-ion secondary battery, a coin-shaped lithium-ion secondary battery, etc.

[0093] In addition, according to known methods, the above-mentioned negative electrode can be used to construct all-solid-state batteries, sodium-ion secondary batteries, etc., which include a solid electrolyte layer, a gel electrolyte to replace the non-aqueous electrolyte and a separator.

[0094] The following describes experimental examples related to the present invention, but it is not intended to limit the present invention to the contents shown in these experimental examples.

[0095] <Example 1>

[0096] As the negative electrode active material, 10 parts by mass of silica containing magnesium (Mg) and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Mg silica and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier are mixed in ion-exchanged water to prepare a first negative electrode composite slurry.

[0097] In addition, as a negative electrode active material, 10 parts by mass of lithium (Li)-containing silicon dioxide and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Li-containing silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier are mixed in ion-exchanged water to prepare a second negative electrode composite slurry.

[0098] A first negative electrode composite slurry is coated on both sides of a copper foil negative electrode current collector and dried. The coating is then pressed and calendered using calendering rollers. Next, a second negative electrode composite slurry is coated on the dried coating of the first negative electrode composite slurry, and dried and calendered in the same manner. This yields the negative electrode sheet of Example 1, comprising a first layer formed of the first negative electrode composite slurry and a second layer formed of the second negative electrode composite slurry supported on the negative electrode current collector. It should be noted that the first negative electrode composite slurry is coated such that the average thickness of the first layer is 50% of the average thickness of the negative electrode active material layer.

[0099] In addition, LiNi will be used as the positive electrode active material. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed in N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 97:2:1 to prepare a positive electrode composite slurry. This positive electrode composite slurry is then coated onto aluminum foil. After drying, it is rolled to a specified thickness to produce a positive electrode sheet.

[0100] A porous polyolefin sheet with a three-layer structure of PP / PE / PE is prepared as a separator. The positive and negative electrode sheets are overlapped with the separator in between and wound together to obtain a wound body. This wound body is then pressed to create a flat wound electrode body.

[0101] The electrode terminals were installed on the electrode body, inserted into the aluminum laminate housing, and fused together. Then, a non-aqueous electrolyte was injected. The non-aqueous electrolyte used was a solution prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a 3:4:3 volume ratio. The laminate housing was then sealed, thus obtaining the lithium-ion secondary battery for evaluation in Example 1.

[0102] <Example 2 and Example 3>

[0103] As shown in Table 1, the mass ratio (mass%) of Mg-containing silicon oxide in the first layer was changed. Otherwise, the same procedure as in Example 1 was followed to produce the evaluation lithium-ion secondary batteries for Examples 2 and 3.

[0104] <Example 4>

[0105] As the negative electrode active material, 1 part by mass of Mg-containing silicon oxide, 9 parts by mass of Li-containing silicon oxide, and 90 parts by mass of graphite (C) were mixed to prepare a mixed negative electrode active material containing Mg-containing silicon oxide, Li-containing silicon oxide, and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier were mixed in deionized water to prepare a second negative electrode slurry. Except for the second negative electrode slurry, the same procedure as in Example 1 was followed to prepare the evaluation lithium-ion secondary battery of Example 4.

[0106] <Example 5 and Example 6>

[0107] As shown in Table 1, the mass ratio (mass%) of Mg-containing silicon oxide and Li-containing silicon oxide in the second layer was changed. Otherwise, the same procedure as in Example 4 was followed to produce the evaluation lithium-ion secondary batteries of Examples 5 and 6.

[0108] <Example 7~Example 9>

[0109] The first negative electrode composite slurry was coated in such a way that the average thickness of the first layer was relative to the average thickness of the negative electrode active material layer as shown in Table 1. Otherwise, the same procedure as in Example 1 was followed to produce the evaluation lithium-ion secondary batteries of Examples 7 to 9.

[0110] <Example 10>

[0111] As the negative electrode active material, 10 parts by mass of Mg-containing silicon dioxide and 90 parts by mass of graphite (C) were mixed to prepare a mixed negative electrode active material containing Mg-containing silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier were mixed in deionized water to prepare a second negative electrode slurry. Except for the second negative electrode slurry, the same procedure as in Example 1 was followed to prepare the evaluation lithium-ion secondary battery of Example 10.

[0112] <Example 11>

[0113] Ten parts by mass of Li-containing silicon dioxide and 90 parts by mass of graphite (C) were mixed to prepare a mixed negative electrode active material containing Li-containing silicon dioxide and graphite. One hundred parts by mass of this mixed negative electrode active material, one part by mass of styrene-butadiene rubber (SBR) as a binder, and one part by mass of carboxymethyl cellulose (CMC) as a tackifier were mixed in deionized water to prepare a first negative electrode slurry. Except for the first negative electrode slurry, the same procedure as in Example 1 was followed to prepare the evaluation lithium-ion secondary battery of Example 11.

[0114] <Evaluation of activation of lithium-ion secondary batteries>

[0115] The evaluation lithium-ion secondary batteries prepared in Examples 1 to 11 were placed in an environment at 25°C. For activation (initial charging), a constant current-constant voltage method was used. Each evaluation lithium-ion secondary battery was charged at a constant current value of 1 / 3C until 4.1V, and then charged at a constant voltage until the current value became 1 / 50C, reaching a fully charged state. Then, each evaluation lithium-ion secondary battery was discharged at a constant current value of 1 / 3C until 3.0V.

[0116] <Determination of volume retention>

[0117] The activated lithium-ion secondary batteries for evaluation were placed in an environment of 25°C. One cycle consisted of constant current charging at 0.5C to 4.1V and constant current discharging at 0.5C to 3.0V, repeated for 500 cycles. The discharge capacity of the first and 500th cycles was measured, and the ratio of the discharge capacity of the 500th cycle to the discharge capacity of the first cycle was calculated as the capacity retention rate (%). If the capacity retention rate was 90% or higher, it was rated "◎"; if it was 80% or higher but less than 90%, it was rated "○"; and if it was less than 80%, it was rated "×". The results are shown in Table 1. It is noted that a good capacity retention rate after 500 cycles indicates a high cycle life for the secondary battery.

[0118] <Battery Resistance Measurement>

[0119] The activated lithium-ion secondary batteries for evaluation were adjusted to a SOC of 50%. They were then left to stand at 25°C for 1 hour. Next, a constant current discharge of 5C was applied for 10 seconds. The voltage drop ΔV was recorded, and the battery resistance was calculated by dividing ΔV by the current value (5C). When the resistance of the lithium-ion secondary battery used for evaluation in Example 11 was set to 100%, a resistance less than 110% was rated "◎", a resistance greater than or equal to 110% but less than 120% was rated "○", and a resistance greater than or equal to 120% was rated "×". The results are shown in Table 1. Furthermore, a high output was achieved when the battery resistance was good.

[0120] [Table 1]

[0121] Table 1

[0122]

[0123] As shown in Table 1, it can be seen that when the negative electrode active material layer contains silicon oxide with at least one alkaline earth metal, the amount of alkaline earth metal in the first layer is higher than that in the second layer, and the amount of silicon oxide containing alkaline earth metal in the second layer is less than 2% by mass when the negative electrode active material in the second layer is 100% by mass, the capacity retention and battery resistance are evaluated well (“◎” or “○”). In particular, it can be seen that when the negative electrode active material in the second layer is set to 100% by mass and the amount of silicon oxide containing alkaline earth metal in the second layer is less than 1% by mass, the capacity retention and battery resistance are evaluated particularly well. In addition, it can be seen that when the average thickness of the first layer is more than 20% and less than 70% of the average thickness of the negative electrode active material layer, the capacity retention and battery resistance are also evaluated particularly well.

[0124] On the other hand, it can be seen that when the negative electrode active material of the second layer is set to 100% by mass, the battery resistance of Examples 6 and 10, which contain more than 3% by mass of alkaline earth metal silicon oxide in the second layer, is evaluated as "×". In addition, it can be seen that the capacity retention rate of Example 11, whose negative electrode active material layer only contains silicon oxide containing alkali metal, is evaluated as "×".

[0125] <Example 12>

[0126] As the negative electrode active material, 1 part by mass of silicon oxide containing Mg, 9 parts by mass of silicon oxide containing Li, and 90 parts by mass of graphite (C) were mixed to prepare a mixed negative electrode active material containing Mg, silicon oxide containing Li, and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier were mixed in deionized water to prepare a first negative electrode composite slurry. Except for the first negative electrode composite slurry, the same procedure as in Example 1 was followed to prepare the evaluation lithium-ion secondary battery of Example 12.

[0127] <Example 13 and Example 14>

[0128] As shown in Table 2, the mass ratio (mass%) of Mg-containing silicon oxide and Li-containing silicon oxide in the first and second layers were changed. Otherwise, the same procedure as in Example 12 was followed to produce the evaluation lithium-ion secondary batteries of Examples 13 and 14.

[0129] <Example 15>

[0130] As the negative electrode active material, 10 parts by mass of Li-containing silicon dioxide and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Li-containing silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier are mixed in ion-exchanged water to prepare a first negative electrode composite slurry.

[0131] In addition, as a negative electrode active material, 10 parts by mass of Mg-containing silicon dioxide and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Mg-containing silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier are mixed in ion-exchanged water to prepare a second negative electrode composite slurry.

[0132] Using the first negative electrode slurry and the second negative electrode slurry prepared above, except that the same procedure as in Example 1 is followed to produce the evaluation lithium-ion secondary battery of Example 15.

[0133] The lithium-ion secondary batteries used for evaluation in Examples 12 to 15, prepared as described above, were activated (initial charge). The capacity retention rate and battery resistance of each activated lithium-ion secondary battery were measured in the same manner as described above. If the capacity retention rate was 90% or higher, it was rated "◎"; if it was 80% or higher but less than 90%, it was rated "○"; and if it was less than 80%, it was rated "×". The results are shown in Table 2. When the resistance of the lithium-ion secondary battery used for evaluation in Example 11 was set to 100%, if the resistance was less than 110%, it was rated "◎"; if it was 110% or higher but less than 120%, it was rated "○"; and if it was 120% or higher, it was rated "×". The results are shown in Table 2.

[0134] [Table 2]

[0135] Table 2

[0136]

[0137] As shown in Table 2, it can be seen that even when the first layer contains silicon oxide containing alkali metals, the capacity retention rate and battery resistance are well evaluated when the negative electrode active material layer contains silicon oxide containing at least one alkaline earth metal, the amount of alkaline earth metal in the first layer is higher than that in the second layer, and the amount of silicon oxide containing alkaline earth metal in the second layer is less than 2% by mass when the negative electrode active material in the second layer is set to 100% by mass.

[0138] <Example 21>

[0139] As the negative electrode active material, 10 parts by mass of Ca-containing silicon dioxide and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Ca and silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose as a tackifier are mixed in ion-exchanged water to prepare a first negative electrode composite slurry.

[0140] In addition, as a negative electrode active material, 10 parts by mass of Li-containing silicon dioxide and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Li-containing silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier are mixed in ion-exchanged water to prepare a second negative electrode composite slurry.

[0141] A first negative electrode composite slurry is coated on both sides of a copper foil negative electrode current collector and dried. The coating is then pressed and calendered using calendering rollers. Next, a second negative electrode composite slurry is coated on the dried coating of the first negative electrode composite slurry, and dried and calendered in the same manner. This yields a negative electrode sheet of Example 21, comprising a first layer formed of the first negative electrode composite slurry and a second layer formed of the second negative electrode composite slurry, supported on a negative electrode current collector. It should be noted that the first negative electrode composite slurry is coated such that the average thickness of the first layer is 50% of the average thickness of the negative electrode active material layer.

[0142] In addition to the negative electrode sheet mentioned above, the same procedure as in Example 1 was followed to produce the evaluation lithium-ion secondary battery of Example 21.

[0143] <Example 22 and Example 23>

[0144] As shown in Table 3, the mass ratio of silicon oxide containing alkaline earth metals in the first layer was changed. Otherwise, the same procedure as in Example 21 was followed to produce the evaluation lithium-ion secondary batteries of Examples 22 and 23.

[0145] <Example 24>

[0146] As the negative electrode active material, 10 parts by mass of Li-containing silicon dioxide and 90 parts by mass of graphite (C) are mixed to prepare a mixed negative electrode active material containing Li-containing silicon dioxide and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a tackifier are mixed in ion-exchanged water to prepare a first negative electrode composite slurry.

[0147] As the negative electrode active material, 10 parts by mass of Ca-containing silicon dioxide and 90 parts by mass of graphite (C) were mixed to prepare a mixed negative electrode active material containing Ca and graphite. 100 parts by mass of this mixed negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose as a tackifier were mixed in deionized water to prepare a second negative electrode slurry. Otherwise, the same procedure as in Example 21 was followed to prepare the evaluation lithium-ion secondary battery of Example 24.

[0148] The lithium-ion secondary batteries used for evaluation in Examples 21 to 24, prepared as described above, were activated (initial charge). The capacity retention rate and battery resistance of the activated lithium-ion secondary batteries were measured in the same manner as described above. If the capacity retention rate was 90% or higher, it was rated "◎"; if it was 80% or higher but less than 90%, it was rated "○"; and if it was less than 80%, it was rated "×". The results are shown in Table 3. When the resistance of the lithium-ion secondary battery used for evaluation in Example 11 was set to 100%, if the resistance was less than 110%, it was rated "◎"; if it was 110% or higher but less than 120%, it was rated "○"; and if it was 120% or higher, it was rated "×". The results are shown in Table 3.

[0149] [Table 3]

[0150] Table 3

[0151]

[0152] As shown in Table 3, even when the type of silicon oxide containing alkaline earth metals is changed, the same trend as in Examples 1 to 3 of Table 1 can be observed. Therefore, regardless of the type of silicon oxide containing alkaline earth metals, a negative electrode that can improve the cycle life of the secondary battery and achieve high output can be provided.

[0153] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described within the scope of the claims includes technologies obtained by various modifications and alterations to the specific examples described above.

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode active material layer formed on a surface of the negative electrode current collector, the negative electrode active material layer contains silicon oxide containing at least one alkaline earth metal, the negative electrode active material layer contains at least a first layer and a second layer, the first layer is disposed between the second layer and the negative electrode current collector, in terms of the second layer, the silicon oxide containing the alkaline earth metal is 1% by mass or less when the negative electrode active material of the second layer is taken as 100% by mass, the silicon oxide containing the alkaline earth metal contained in the first layer is 2% by mass or more and 20% by mass or less when the negative electrode active material of the first layer is taken as 100% by mass, wherein the amount of the alkaline earth metal in the first layer calculated based on energy dispersive X-ray analysis using a scanning electron microscope image is higher than the amount of the alkaline earth metal in the second layer, the second layer contains silicon oxide containing an alkali metal, the silicon oxide containing the alkali metal is more than 0% by mass and 10% by mass or less when the negative electrode active material of the second layer is taken as 100% by mass.

2. The negative electrode for a secondary battery according to claim 1, wherein the average thickness of the first layer is 20% or more and 70% or less relative to the average thickness of the negative electrode active material layer.

3. The negative electrode for a secondary battery according to claim 1, wherein as the silicon oxide containing the alkaline earth metal, silicon oxide containing magnesium and / or silicon oxide containing calcium is contained.

4. The negative electrode for a secondary battery according to claim 1, wherein the first layer contains silicon oxide containing an alkali metal.

5. The negative electrode for a secondary battery according to claim 1 or 4, wherein as the silicon oxide containing the alkali metal, silicon oxide containing lithium is contained.

6. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode active material layer contains a carbon material.

7. A nonaqueous electrolyte secondary battery, comprising: the negative electrode according to any one of claims 1 to 6, a positive electrode, and a nonaqueous electrolyte.

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