Negative electrode and battery

Abstract

A negative electrode 10 comprises a negative electrode mixture layer 12 containing a conducting agent and a silicon-containing material serving as a negative electrode active material. The negative electrode mixture layer 12 includes: a first negative electrode mixture layer 13 including the surface of the negative electrode 10; and a second negative electrode mixture layer 14. The silicon-containing material contains a first silicon-containing material and a second silicon-containing material having a greater coefficient of volume expansion than the first silicon-containing material. The negative electrode active material contained in the first negative electrode mixture layer 13 is a first negative electrode active material; the negative electrode active material contained in the second negative electrode mixture layer 14 is a second negative electrode active material; the conducting agent contained in the first negative electrode mixture layer is a first conducting agent; and the conducting agent contained in the second negative electrode mixture layer is a second conducting agent. The mass proportion of the first silicon-containing material in the first negative electrode active material is higher than the mass proportion of the first silicon-containing material in the second negative electrode active material. The mass proportion of the second silicon-containing material in the second negative electrode active material is higher than the mass proportion of the second silicon-containing material in the first negative electrode active material. The mass ratio of the first conducting agent to the first negative electrode active material is higher than the mass ratio of the second conducting agent to the second negative electrode active material. The porosity of the first negative electrode mixture layer is higher than the porosity of the second negative electrode mixture layer.

Description

Negative electrode and battery

[0001] This disclosure relates to a negative electrode and a battery.

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, such as automotive and energy storage. The electrodes that make up such batteries have a significant impact on their performance. For this reason, various studies have been conducted on electrodes.

[0003] It is known that silicon (Si)-containing negative electrode active materials are effective in increasing the capacity of batteries. However, negative electrode active materials containing Si undergo large volume changes with charging and discharging, so repeated charging and discharging can break the conductive path with surrounding active materials, resulting in capacity degradation with each charge-discharge cycle. Therefore, Patent Document 1 proposes a technique to improve the charge-discharge cycle characteristics of lithium secondary batteries that use a material containing Si and O as the negative electrode active material. Specifically, Patent Document 1 proposes improving the charge-discharge cycle characteristics by adjusting the balance between the amount of negative electrode active material used and the amount of positive electrode active material used to control the utilization rate of the negative electrode active material, and by using a non-aqueous electrolyte that has components capable of forming a good coating on the new surface even if the Si and O-containing material is crushed and a new surface is created.

[0004] Japanese Patent Publication No. 2011-233245

[0005] However, there is room for improvement in negative electrodes containing silicon-based negative electrode active materials in achieving both improved battery charge-discharge cycle characteristics and reduced internal resistance.

[0006] Therefore, this disclosure provides a technology that can simultaneously improve the charge-discharge cycle characteristics of a battery and reduce its internal resistance for a negative electrode containing a negative electrode active material containing Si.

[0007] The negative electrode of the present disclosure comprises: a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector and containing a silicon-containing material as a negative electrode active material and a conductive agent, wherein the negative electrode mixture layer includes a first negative electrode mixture layer including the surface of the negative electrode and a second negative electrode mixture layer located between the first negative electrode mixture layer and the negative electrode current collector; the silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a larger particle volume expansion coefficient than the first silicon-containing material; and when the negative electrode active material contained in the first negative electrode mixture layer is designated as the first negative electrode active material, the negative electrode active material contained in the second negative electrode mixture layer is designated as the second negative electrode active material, the conductive agent contained in the first negative electrode mixture layer is designated as the first conductive agent, and the conductive agent contained in the second negative electrode mixture layer is designated as the second conductive agent, The mass ratio of the first silicon-containing material in the first negative electrode active material is greater than the mass ratio of the first silicon-containing material in the second negative electrode active material, the mass ratio of the second silicon-containing material in the second negative electrode active material is greater than the mass ratio of the second silicon-containing material in the first negative electrode active material, the mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material, and the porosity of the first negative electrode mixture layer is greater than the porosity of the second negative electrode mixture layer. Here, the particle volume expansion rate of the first silicon-containing material is the ratio of the particle volume of the first silicon-containing material in the charged state to the particle volume of the first silicon-containing material in the discharged state, and the particle volume expansion rate of the second silicon-containing material is the ratio of the particle volume of the second silicon-containing material in the charged state to the particle volume of the second silicon-containing material in the discharged state.

[0008] According to the technology disclosed herein, a battery equipped with a negative electrode containing a negative electrode active material containing Si can achieve both improved charge-discharge cycle characteristics and reduced internal resistance.

[0009] Figure 1 is a cross-sectional view showing the schematic configuration of the negative electrode according to Embodiment 1. Figure 2 is a longitudinal cross-sectional view schematically showing an example of a battery according to Embodiment 2.

[0010] The embodiments of this disclosure will be described in detail below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0011] (Embodiment 1) Figure 1 is a cross-sectional view showing the schematic configuration of a negative electrode according to Embodiment 1. The negative electrode 10 according to Embodiment 1 comprises a negative electrode current collector 11 and a negative electrode mixture layer 12 disposed on the negative electrode current collector 11. The negative electrode mixture layer 12 contains a silicon-containing material as a negative electrode active material and a conductive agent. The silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a larger particle volume expansion coefficient than the first silicon-containing material.

[0012] The negative electrode mixture layer 12 includes a first negative electrode mixture layer 13 that includes the surface of the negative electrode 10, and a second negative electrode mixture layer 14 located between the first negative electrode mixture layer 13 and the negative electrode current collector 11. The negative electrode active material contained in the first negative electrode mixture layer 13 is designated as the first negative electrode active material, the negative electrode active material contained in the second negative electrode mixture layer 14 is designated as the second negative electrode active material, the conductive agent contained in the first negative electrode mixture layer 13 is designated as the first conductive agent, and the conductive agent contained in the second negative electrode mixture layer 14 is designated as the second conductive agent. In this case, the mass ratio of the first silicon-containing material in the first negative electrode active material is greater than the mass ratio of the first silicon-containing material in the second negative electrode active material, and the mass ratio of the second silicon-containing material in the second negative electrode active material is greater than the mass ratio of the second silicon-containing material in the first negative electrode active material. Furthermore, the mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material. In addition, the porosity of the first negative electrode mixture layer 13 is greater than the porosity of the second negative electrode mixture layer 14.

[0013] The mass ratio (%) of the first conductive agent to the first negative electrode active material is calculated as (mass of the first conductive agent / mass of the first negative electrode active material) × 100, and the mass ratio (%) of the second conductive agent to the second negative electrode active material is calculated as (mass of the second conductive agent / mass of the second negative electrode active material) × 100.

[0014] Here, the particle volume expansion coefficient of the first silicon-containing material is the ratio of the particle volume of the first silicon-containing material in the charged state to the particle volume of the first silicon-containing material in the discharged state. Similarly, the particle volume expansion coefficient of the second silicon-containing material is the ratio of the particle volume of the second silicon-containing material in the charged state to the particle volume of the second silicon-containing material in the discharged state.

[0015] In this specification, the particle volume expansion coefficients of the first silicon-containing material and the second silicon-containing material are measured by the following method: (1) The battery to be evaluated is disassembled and the negative electrode is cut out. A monoelectrode cell is prepared in which the particle cross-section of the silicon-containing material is exposed by using metallic Li as the counter electrode and an ionic liquid as the electrolyte. (2) The monoelectrode cell is charged at 0.002C in a temperature environment of 25°C until the cell voltage reaches 5mV, then discharged at 0.05C until the cell voltage reaches 1.0V, and the particle cross-section of the silicon-containing material is observed in situ with a scanning electron microscope (SEM). (3) For the particle volume expansion coefficient of the silicon-containing material, the particle volume (Va) of the silicon-containing material in the charged state (fully charged state) and the particle volume (Vb) of the silicon-containing material in the discharged state (fully discharged state) are determined from the particle cross-sectional area of ​​the silicon-containing material, and the particle volume expansion coefficient (Va / Vb) is calculated. The particle volume is obtained by raising the particle cross-sectional area of ​​the silicon-containing material obtained from the SEM image of the particle cross-section to the power of 3 / 2. (4) For the silicon-containing material contained in the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, the above measurement (2) and the above calculation of the particle volume expansion rate (3) are performed on 50 particles each to obtain a particle volume expansion rate distribution based on particle area. The particle area in the particle volume expansion rate distribution obtained here is the area of ​​the particle in a complete discharge state. (5) The area-based particle volume expansion rate distribution obtained in (4) above is separated into two peaks using a Gaussian function for each layer, and the group of particles corresponding to the first peak on the side with the smaller particle expansion rate is designated as the first silicon-containing material, and the group of particles corresponding to the second peak on the side with the larger particle expansion rate is designated as the second silicon-containing material. Here, in each layer, the particle volume expansion coefficient of the first silicon-containing material is the particle volume expansion coefficient at the peak of the first peak, and the particle volume expansion coefficient of the second silicon-containing material is the particle volume expansion coefficient at the peak of the second peak.

[0016] The fact that the mass proportion of the first silicon-containing material in the first anode active material is greater than the mass proportion of the first silicon-containing material in the second anode active material, and that the mass proportion of the second silicon-containing material in the second anode active material is greater than the mass proportion of the second silicon-containing material in the first anode active material, can be confirmed by the following method.

[0017] This can be confirmed from the area-based particle volume expansion rate distribution obtained by the method described above, specifically by comparing the ratio of the total area originating from the first peak in the first anode mixture layer 13 to the total area of ​​the first anode active material and the ratio of the total area originating from the first peak in the second anode mixture layer 14 to the total area of ​​the second anode active material, as well as the ratio of the total area originating from the second peak in the first anode mixture layer 13 to the total first anode active material and the ratio of the total area originating from the second peak in the second anode mixture layer 14 to the total second anode active material.

[0018] The fact that the mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material can be determined, for example, by distinguishing and quantifying the negative electrode active material and conductive agent in the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, and deriving the results from these.

[0019] Examples of conductive materials include carbon black such as acetylene black and Ketjenblack, fibrous carbon such as carbon nanotubes (CNTs), carbon nanofibers, and graphene. To further improve charge-discharge cycle characteristics, the conductive material may contain fibrous carbon or CNTs.

[0020] Furthermore, if the first negative electrode mixture layer 13 contains multiple types of conductive agents, the mass ratio of the first conductive agent to the first negative electrode active material is the ratio of the total mass of the conductive agents contained in the first negative electrode mixture layer 13 to the mass of the first negative electrode active material. Similarly, if the second negative electrode mixture layer 14 contains multiple types of conductive agents, the mass ratio of the second conductive agent to the second negative electrode active material is the ratio of the total mass of the conductive agents contained in the second negative electrode mixture layer 14 to the mass of the second negative electrode active material.

[0021] The negative electrode active material in the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 can be determined by elemental mapping of a cross-section along the thickness direction of the negative electrode mixture layer 12 if it is a silicon-containing material. Quantitative analysis of the negative electrode active material by elemental mapping can be performed using an electron beam microanalyzer (EPMA). If the negative electrode active material includes carbon materials such as graphite, it is necessary to quantify it separately from carbon materials such as carbon black included as a conductive agent. In this case, the two can be differentiated and quantified using the method described later.

[0022] When the negative electrode mixture layer 12 contains a carbon material such as graphite as the negative electrode active material, the distinction between it and a carbon material such as carbon black included as a conductive agent is made by determining the G / D ratio using the Raman spectrum obtained by micro-Raman spectroscopy of the cross-section of the negative electrode mixture layer. Materials with a G / D ratio of ≤1 are identified as carbon materials such as carbon black included as a conductive agent, and materials with a G / D ratio >1 are identified as carbon materials such as graphite included as the negative electrode active material. The volume ratio is calculated by raising the area ratio of the identified negative electrode active material and conductive agent to the power of 3 / 2, and the mass ratio is calculated from their respective true densities (graphite: 2.24 g / cc, carbon black: 1.8 g / cc). In this way, the carbon material such as graphite included as the negative electrode active material and the carbon material such as carbon black included as a conductive agent can be distinguished, and their respective mass ratios can be determined.

[0023] When CNTs are included as a conductive agent, the amount of CNTs relative to the total negative electrode active material in the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 may be determined by the following method. The amount of CNTs relative to the total negative electrode active material can be determined from a sample obtained by removing only the negative electrode mixture layers (the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, respectively) from a discharged secondary battery. Specifically, first, the discharged secondary battery is disassembled and the negative electrode is removed. Next, the negative electrode is washed with an organic solvent, then vacuum dried, and a sample is obtained by peeling off only the negative electrode mixture layer. The pulverized sample can be dispersed in a dispersion medium such as water and / or alcohol, and the CNTs can be separated by centrifugation. Furthermore, the ratio of binder components and conductive agent components other than the negative electrode active material can be calculated by performing thermal analysis such as TG-DTA on the sample. By performing micro-Raman spectroscopy on the cross-section of the negative electrode mixture layer 12, carbon species such as CNTs and acetylene black can be identified, and their proportions can be calculated from thermal analysis such as TG-DTA of the exfoliated sample.

[0024] Furthermore, by performing thermal analysis such as TG-DTA on the above sample obtained by peeling off only the negative electrode mixture layer, the ratio of binder components and conductive agent components other than the negative electrode active material can be calculated.

[0025] As described above, by determining the mass ratio of each component in the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 using an appropriate method for each component, the mass ratio of the first conductive agent to the first negative electrode active material and the mass ratio of the second conductive agent to the second negative electrode active material can be determined.

[0026] The porosity of the first negative electrode mixture layer 13 and the porosity of the second negative electrode mixture layer 14 are two-dimensional values ​​obtained from the ratio of the area of ​​voids to the cross-sectional area of ​​the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 in the cross-section of the negative electrode mixture layer 12.

[0027] In this specification, the porosity of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 is measured by the following method: (1) The battery to be evaluated is disassembled and the negative electrode is cut out to expose the cross-section of the negative electrode mixture layer 12. One method for exposing the cross-section is to cut out a part of the negative electrode 10 and process it with an ion milling device (for example, Hitachi High-Tech Corporation's IM4000PLUS) to expose the cross-section of the negative electrode mixture layer 12. (2) A backscattered electron image of the exposed cross-section of the negative electrode mixture layer 12 is taken for each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 using a SEM. The magnification when taking the backscattered electron image is, for example, 800x. The following processes (3) to (4) are performed for the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, and the porosity of each is calculated. (3) The cross-sectional image obtained above is imported into a computer and binarized using image analysis software (for example, ImageJ from the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are converted to black and the voids present in the particle cross-sections are converted to white. (4) In the binarized image obtained above, the area of ​​the voids is calculated by considering the parts of the voids converted to white that are excluded from the voids inside the particles (pores not connected to the particle surface) and pores with a width of 3 μm or less that are connected to the particle surface as voids. The void ratio is calculated based on the following formula: Void ratio (%) = Area of ​​voids / Area of ​​the cross-section of the negative electrode mixture layer × 100 (5) For each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, the void ratio is calculated three times according to (3) and (4) above, and the average value is taken as the void ratio of the first negative electrode mixture layer 13 and the void ratio of the second negative electrode mixture layer 14.

[0028] The porosity of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 can be controlled, for example, by the type and proportion of carbon material included, and the rolling conditions when manufacturing the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14.

[0029] As described above, in the negative electrode 10 according to Embodiment 1, the negative electrode active material includes two silicon-containing materials with different particle volume expansion coefficients. The mass ratio of the first silicon-containing material having a smaller particle volume expansion coefficient is greater in the first negative electrode mixture layer 13 located on the surface side of the negative electrode 10 than in the second negative electrode mixture layer 14 located on the negative electrode current collector 11 side. The mass ratio of the second silicon-containing material having a larger particle volume expansion coefficient is greater in the second negative electrode mixture layer 14 located on the negative electrode current collector 11 side than in the first negative electrode mixture layer 13 located on the surface side of the negative electrode 10. In addition to these features of the silicon-containing material, the negative electrode 10 according to Embodiment 1 also has the feature that the porosity of the first negative electrode mixture layer 13 is greater than the porosity of the second negative electrode mixture layer 14. In addition to the characteristics of the silicon-containing material and the porosity described above, the negative electrode 10 according to Embodiment 1 also has the further characteristic that the mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material.

[0030] With the above configuration, in the first negative electrode mixture layer 13 on the surface side where the charging depth is deep, the first silicon-containing material having a smaller particle volume expansion coefficient effectively suppresses capacity degradation caused by the isolation of the negative electrode active material due to volume changes associated with charging and discharging, thereby suppressing deterioration of charge-discharge cycle characteristics. Furthermore, if the mass ratio of the first silicon-containing material having a smaller particle volume expansion coefficient is large in the first negative electrode mixture layer 13 on the surface side, it becomes difficult to form spaces on the surface side, and it is conceivable that the penetration of the electrolyte into the negative electrode mixture layer 12 will not be sufficient. However, in the negative electrode 10 according to Embodiment 1, the porosity of the first negative electrode mixture layer 13 is configured to be larger, so the penetration of the electrolyte into the negative electrode mixture layer 12 is improved, and the internal resistance of the battery equipped with the negative electrode 10 can be reduced. The isolation of the negative electrode active material in the first negative electrode mixture layer 13 due to the increased porosity of the first negative electrode mixture layer 13 can be sufficiently suppressed by increasing the proportion of conductive agent in the first negative electrode mixture layer 13, thereby improving the charge-discharge cycle characteristics. Thus, the negative electrode 10 according to Embodiment 1 can achieve both improved charge-discharge cycle characteristics and reduced internal resistance of the battery.

[0031] Furthermore, the negative electrode mixture layer 12 in the negative electrode 10 according to Embodiment 1 may consist of two layers, a first negative electrode mixture layer 13 and a second negative electrode mixture layer 14, as shown in Figure 1. Also, the negative electrode mixture layer 12 in the negative electrode 10 according to Embodiment 1 is not limited to the two-layer structure shown in Figure 1, but may consist of three or more layers. The negative electrode mixture layer 12 may further include layers located between the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, or between the second negative electrode mixture layer 14 and the negative electrode current collector 11.

[0032] The first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 can be distinguished by differences in their constituent components and composition ratios. For example, the first negative electrode mixture layer 13 can be identified by identifying a region having similar constituent components and similar composition ratios in the depth direction of the negative electrode mixture layer 12 from the surface of the negative electrode 10. The second negative electrode mixture layer 14 can be identified by identifying a region that exists between the identified first negative electrode mixture layer 13 and the negative electrode current collector 11 and has the above-described structural relationship with the first negative electrode mixture layer 13.

[0033] Furthermore, if it is difficult to distinguish between the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 in the negative electrode mixture layer 12 due to differences in constituent components and composition ratios, for example, the two divided layers resulting from the negative electrode mixture layer 12 being divided equally into two in the thickness direction may be identified as the first negative electrode mixture layer 13, with the divided layer located on the surface side of the negative electrode mixture layer 12 being identified as the second negative electrode mixture layer 14.

[0034] The following describes in detail the configurations of the negative electrode 10 in Embodiment 1.

[0035] [Negative current collector] As the negative current collector 11, a sheet or film made of a metal material such as stainless steel, nickel, copper, or their alloys can be used. The sheet or film may be porous or non-porous. As the sheet or film, a metal foil, a metal mesh, etc. are used. A carbon material such as carbon may be coated on the surface of the negative current collector 11 as a conductive auxiliary material.

[0036] The thickness of the negative current collector 11 is not particularly limited, but from the viewpoint of the balance between the strength and weight reduction of the negative electrode 10, for example, it is 1 μm or more and 50 μm or less, and may be 5 μm or more and 20 μm or less.

[0037] [Negative electrode binder layer] As described above, the negative electrode binder layer 12 contains a silicon-containing material as a negative electrode active material, and the silicon-containing material includes a first silicon-containing material and a second silicon-containing material having different particle volume expansion rates.

[0038] In this specification, the silicon-containing material means a material containing Si. Examples of the silicon-containing material include Si, Si alloys, Si compounds, and composite materials containing Si.

[0039] The average particle size of the silicon-containing material is, for example, 1 μm or more and 20 μm or less, and may be 1 μm or more and 15 μm or less. The average particle size of the silicon-containing material means the particle size (hereinafter referred to as "D50 based on volume") at which the volume integration value becomes 50% in the particle size distribution measured by the laser diffraction scattering method. For the measuring device, for example, "MT3000II" manufactured by Microtrac Bell Co., Ltd. is used, and the measurement is carried out using, for example, water as a dispersion medium.

[0040] A silicon-containing material is preferably a composite material containing Si. The composite material containing Si is, for example, composite particles including an ion conductive phase and a Si phase dispersed in the ion conductive phase. The ion conductive phase includes, for example, at least one selected from the group consisting of an aluminate phase, a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The ion conductive phase may be composed of one phase or may be composed of a plurality of phases. The Si phase is, for example, formed in the form of fine particles of Si and dispersed in the ion conductive phase.

[0041] The silicate phase may satisfy the following condition (A) and / or (B). (A) The silicate phase includes at least one selected from the group consisting of an alkali metal element and a Group 2 element (Group 2 element in the long-period type periodic table). (B) The silicate phase includes an element L. The element L is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanoid, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. The lanthanoid is a general term for 15 elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71.

[0042] Regarding the above condition (A), examples of the alkali metal element include lithium (Li), potassium (K), and sodium (Na). Examples of the Group 2 element include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). By including an alkali metal element and / or a Group 2 element, the irreversible capacity of the silicate phase may be reduced. A silicate phase containing lithium (hereinafter sometimes referred to as "lithium silicate phase") is preferable, for example, in that it has a small irreversible capacity and a high initial charge-discharge efficiency.

[0043] The lithium silicate phase may be an oxide phase containing Li, Si, and O, and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4.

[0044] The lithium silicate phase is given by formula: Li 2z SiO (2+z) The material may contain a lithium silicate phase represented by (0 < z < 2), or may be composed of such lithium silicate phase. Preferably, z satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2 (i.e., Li2Si2O5).

[0045] The carbon phase may be composed of, for example, amorphous carbon. Examples of amorphous carbon constituting the carbon phase include hard carbon, soft carbon, and other amorphous carbons. Amorphous carbon is a carbon material in which the average interplanar spacing d002 of the (002) planes, as measured by Kα diffraction, exceeds 0.34 nm.

[0046] The silicide phase is a compound phase consisting of Si and an element that is more electrically positive than Si, and examples include NiSi, Mg2Si, and TiSi2.

[0047] The silicon oxide phase is composed of compounds of Si and O. The main component of the silicon oxide phase may be silicon dioxide. The main component of the silicon oxide phase is, for example, a component that accounts for 95% or more and 100% or less by mass of the silicon oxide phase.

[0048] The aluminate phase is, for example, a phase containing alkali aluminate containing at least one alkali metal element, Al, and O. The alkali metal element is, for example, Li. The composition of lithium aluminate is given by the formula: Li u AlO (3+u) / 2 It can be expressed as follows. From the viewpoint of ease of fabrication, stability, and ionic conductivity, etc., u in the formula may be, for example, greater than 0 and less than or equal to 5, or greater than 0 and less than or equal to 1. When u = 1 / 5, it can be expressed as LiAl5O8, and when u = 1 / 2, it can be expressed as Li2Al4O7. When u = 1, it can be expressed as LiAlO2, and when u = 5, it can be expressed as Li5AlO4.

[0049] The average size of the Si phase is, for example, between 1 nm and 200 nm, but may also be between 1 nm and 100 nm, or between 1 nm and 10 nm. The average size of the Si phase is calculated as the average of the diameters of the circumscribed circles of 100 Si phases extracted by image analysis after capturing SEM images of the particle cross-sections of the silicon-containing material.

[0050] The above composite material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is composed of a material with higher conductivity than the ion-conducting phase, for example, and forms good conductive paths in the negative electrode mixture layer 12. The conductive layer is, for example, a carbon film composed of a conductive carbon material. The conductive carbon material can be carbon black such as acetylene black or Ketjen black, graphite, or amorphous carbon with low crystallinity. The thickness of the conductive layer is, for example, 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less, taking into consideration the securing of conductivity and the diffusion of Li ions into the particle interior. The thickness of the conductive layer can be measured by cross-sectional observation of the composite material using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0051] The composite material may be at least one selected from the group consisting of a first silicon-containing material and a second silicon-containing material. That is, the composite material may be at least one selected from the group consisting of a first silicon-containing material and a second silicon-containing material, comprising an ionic conductive phase and a Si phase dispersed in the ionic conductive phase. Both the first silicon-containing material and the second silicon-containing material may be the composite material.

[0052] It is preferable that the negative electrode mixture layer 12 further contains a carbon material as the negative electrode active material. By including a carbon material as the negative electrode active material, conductive paths in the negative electrode active material can be secured even when the silicon-containing material expands and contracts with charging and discharging, thereby further suppressing capacity degradation associated with the charge-discharge cycle. In addition, the inclusion of a carbon material makes it easier to control the porosity in the negative electrode mixture layer 12, so it is possible to control the porosity while considering the permeability of the electrolyte.

[0053] The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. Preferably, the carbon material is graphite. The graphite may be natural graphite or artificial graphite. As graphite, for example, artificial graphite such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, or earthy graphite, or mixtures thereof can be used. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less.

[0054] As described above, the silicon-containing material included as the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material, each having different particle volume expansion rates. The particle volume expansion rate of the silicon-containing material can be controlled by changing, for example, the proportion of Si contained, the density of the silicon-containing material, the particle size of the silicon-containing material, and the particle shape of the silicon-containing material. For example, if the silicon-containing material is a composite material containing Si, the particle volume expansion rate can be controlled to a desired range by adjusting the size of the Si phase contained in the composite material, the proportion of the Si phase, the pore diameter and pore volume of the ion-conducting phase, etc.

[0055] If V1 is the particle volume expansion coefficient of the first silicon-containing material and V2 is the particle volume expansion coefficient of the second silicon-containing material, then V1 and V2 may satisfy the following relationships (1) and (2), respectively: 1.3 ≤ V1 < 2.2 ... (1) 1.7 ≤ V2 < 3 ... (2)

[0056] By ensuring that the particle volume expansion coefficient V1 of the first silicon-containing material and the particle volume expansion coefficient V2 of the second silicon-containing material satisfy the above-mentioned relational equations (1) and (2), respectively, the capacity degradation caused by the isolation of the negative electrode active material due to volume changes associated with charging and discharging is more effectively suppressed in the first negative electrode mixture layer 13 on the surface side, while better diffusion of the electrolyte is possible in the second negative electrode mixture layer 14 on the negative electrode current collector 11 side, thereby further improving the charge-discharge cycle characteristics. As mentioned above, since the second silicon-containing material has a larger particle volume expansion coefficient than the first silicon-containing material, the above-mentioned relational equations (1) and (2) are satisfied assuming V1 < V2. It is also possible that V1 satisfies 1.3 ≤ V1 < 2.0 and V2 satisfies 2.0 ≤ V2 < 3.

[0057] The ratio (V1 / V2) of the particle volume expansion coefficient V1 of the first silicon-containing material to the particle volume expansion coefficient V2 of the second silicon-containing material satisfies, for example, 0.5 < V1 / V2 < 1, and may also satisfy, for example, 0.5 < V1 / V2 < 0.8.

[0058] If the expansion rate of the first negative electrode mixture layer 13 during charging is A1 and the expansion rate of the second negative electrode mixture layer 14 during charging is A2, then A1 and A2 may satisfy the following relationship (3): A1 < A2 ... (3)

[0059] Here, the expansion rate of the first negative electrode mixture layer 13 during charging is the rate of increase in the thickness of the first negative electrode mixture layer 13 in the charged state relative to the thickness of the first negative electrode mixture layer 13 in the discharged state. Similarly, the expansion rate of the second negative electrode mixture layer 14 during charging is the rate of increase in the thickness of the second negative electrode mixture layer 14 in the charged state relative to the thickness of the second negative electrode mixture layer 14 in the discharged state.

[0060] In this specification, the expansion rate of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 during charging is measured by the following method: (1) The battery to be evaluated is disassembled and the negative electrode is cut out, and a unipolar cell is made using metallic Li as the counter electrode and an ionic liquid as the electrolyte. (2) The unipolar cell is charged at 0.1C in a temperature environment of 25°C until the cell voltage reaches 5mV, and then discharged at 0.1C until the cell voltage reaches 1.0V, and the thickness of the first negative electrode mixture layer 13 (T1a) and the thickness of the second negative electrode mixture layer 14 (T2a) in the charged state (fully charged state) and the thickness of the first negative electrode mixture layer (T1b) and the thickness of the second negative electrode mixture layer 14 (T2b) in the discharged state (fully discharged state) are determined. (3) Calculate the rate of increase in thickness of the first negative electrode mixture layer 13 in the charged state relative to the thickness of the first negative electrode mixture layer 13 in the discharged state (T1a × 100 / T1b - 100), and use this as the expansion rate (%) of the first negative electrode mixture layer 13 during charging. Calculate the rate of increase in thickness of the second negative electrode mixture layer 14 in the charged state relative to the thickness of the second negative electrode mixture layer 14 in the discharged state (T2a × 100 / T2b - 100), and use this as the expansion rate (%) of the second negative electrode mixture layer 14 during charging.

[0061] Here, the thickness of the first anode mixture layer 13 and the thickness of the second anode mixture layer 14 are determined from the cross-sectional SEM images of each layer. Specifically, the thickness of the first anode mixture layer 13 and the second anode mixture layer 14 is measured at five arbitrary locations, and the average value calculated from the five obtained measurements is taken as the thickness. The identification of the first anode mixture layer 13 and the second anode mixture layer 14 is as described above.

[0062] When the above relation (3) is satisfied, that is, when the second negative electrode mixture layer 14 has a larger expansion rate during charging than the first negative electrode mixture layer 13, the capacity degradation caused by the isolation of the negative electrode active material due to volume changes associated with charging and discharging is more effectively suppressed in the surface-side first negative electrode mixture layer 13, while better diffusion of the electrolyte is possible in the second negative electrode mixture layer 14 on the negative electrode current collector 11 side, thereby further improving the charge-discharge cycle characteristics.

[0063] To further improve the charge-discharge cycle characteristics, the charging expansion rate A1 of the first negative electrode mixture layer 13 and the charging expansion rate A2 of the second negative electrode mixture layer 14 may also satisfy the following relationship (4): 0.5 < A1 / A2 < 1 ... (4)

[0064] A1 / A2 may be, for example, 0.5 < A1 / A2 < 0.9, or 0.5 < A1 / A2 < 0.7.

[0065] The expansion rate of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 during charging can be controlled to a desired range by, for example, adjusting the content ratio of the first silicon-containing material and the second silicon-containing material, and the porosity.

[0066] If the porosity of the first negative electrode mixture layer 13 is ε1 and the porosity of the second negative electrode mixture layer 14 is ε2, then ε1 and ε2 may satisfy the following relationship (5): 1.0 < ε1 / ε2 < 2.0 ... (5)

[0067] When the above relation (5) is satisfied, the permeability of the electrolyte into the negative electrode mixture layer 12 is further improved, and the internal resistance of the battery can be further reduced.

[0068] The true density of the first silicon-containing material may be lower than the true density of the second silicon-containing material. This configuration can further improve the charge-discharge cycle characteristics.

[0069] In each of the first anode mixture layer 13 and the second anode mixture layer 14, the content of silicon-containing material is, for example, 40% by mass or less of the total mass of the anode active material, may be 35% by mass or less, or 30% by mass or less, from the viewpoint of improving cycle characteristics. In each of the first anode mixture layer 13 and the second anode mixture layer 14, the content of silicon-containing material is, for example, 5% by mass or more of the total mass of the anode active material, from the viewpoint of increasing capacity and improving cycle characteristics. An example of a preferred range for the content of silicon-containing material is, for example, 5% by mass or more and 35% by mass or less of the total mass of the anode active material, may be 10% by mass or more and 30% by mass or less, or 5% by mass or more and 15% by mass or less.

[0070] As described above, the negative electrode mixture layer 12 further contains a conductive agent. The mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material.

[0071] If X1 is the mass ratio of the first conductive agent to the first negative electrode active material, and X2 is the mass ratio of the second conductive agent to the second negative electrode active material, then X1 and X2 may satisfy the following relationship (6): 1 < X1 / X2 < 9 ... (6)

[0072] When the above relation (6) is satisfied, capacity degradation due to isolation of the negative electrode active material in the first negative electrode mixture layer 13 can be suppressed more effectively, thereby improving the charge-discharge cycle characteristics.

[0073] The negative electrode mixture layer 12 may further contain a binder. Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and styrene-butadiene rubber (SBR). In addition, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc.

[0074] (Embodiment 2) The battery in Embodiment 2 comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the same as the negative electrode in Embodiment 1. With this configuration, the battery in Embodiment 2 can achieve both improved charge-discharge cycle characteristics and reduced internal resistance.

[0075] Figure 2 is a schematic longitudinal cross-sectional view showing an example of a battery according to Embodiment 2. The battery 100 is a cylindrical battery comprising a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed within the battery case and is in contact with the electrolyte.

[0076] The battery case consists of a case body 25, which is a bottomed cylindrical metal container, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is placed between the case body 25 and the sealing body 26. The gasket 37 ensures that the battery case is airtight. Inside the case body 25, insulating plates 27 and 28 are placed at both ends of the electrode group 24 in the winding axis direction.

[0077] The case body 25 has, for example, a stepped portion 31. The stepped portion 31 can be formed by partially pressing the side wall of the case body 25 from the outside. The stepped portion 31 may be formed in an annular shape on the side wall of the case body 25 along the circumferential direction of a virtual circle defined by the case body 25. In this case, the sealing body 26 is supported, for example, by the opening side surface of the stepped portion 31.

[0078] The sealing body 26 comprises a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. In the sealing body 26, these members are stacked in this order. The sealing body 26 is installed in the opening of the case body 25 such that the cap 36 is located on the outside of the case body 25 and the filter 32 is located on the inside of the case body 25.

[0079] Each of the above-mentioned components constituting the sealing body 26 is, for example, disc-shaped or ring-shaped. Except for the insulating member 34, each of the above-mentioned components is electrically connected to one another.

[0080] The electrode group 24 includes a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, the separator 22, and the negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is positioned between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are wound in a spiral shape with the separator 22 interposed between them.

[0081] When observing a cross-section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrode 21 and the negative electrode 23 are alternately stacked in the radial direction of a virtual circle defined by the case body 25, with a separator 22 interposed between them.

[0082] The positive electrode 21 is electrically connected to the cap 36, which also serves as the positive electrode terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, near the center of the positive electrode 21 in the longitudinal direction. The positive electrode lead 29 extends from the positive electrode 21 to the filter 32 through a through hole formed in the insulating plate 27. The other end of the positive electrode lead 29 is welded, for example, to the electrode group 24 side of the filter 32.

[0083] The negative electrode 23 is electrically connected to the case body 25, which also serves as the negative electrode terminal, via a negative electrode lead 30. One end of the negative electrode lead 30 is connected, for example, to the end of the negative electrode 23 in the longitudinal direction of the negative electrode 23. The other end of the negative electrode lead 30 is welded, for example, to the inner bottom surface of the case body 25.

[0084] The components of battery 100 will be described in detail below.

[0085] The positive electrode 21 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0086] As the positive electrode current collector, a sheet or film made of a metallic material such as aluminum, stainless steel, titanium, or their alloys can be used. Aluminum and its alloys are suitable as materials for positive electrode current collectors because they are inexpensive and easy to make into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the positive electrode current collector as a conductive auxiliary material.

[0087] The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material may be a material that has the ability to intercept and release metal ions (e.g., lithium ions). As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc., can be used. In particular, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0088] The positive electrode mixture layer may further contain a binder. As the binder, the material described in Embodiment 1 as a binder usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0089] The positive electrode mixture layer may further contain a conductive agent. As the conductive agent, the material described in Embodiment 1 as a conductive agent usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0090] The negative electrode 23 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The negative electrode 23 is the negative electrode 10 according to Embodiment 1.

[0091] The electrolyte solution used as the electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte solution may be, for example, 0.5 mol / liter or more and 2 mol / liter or less. By controlling the lithium salt concentration within the above range, an electrolyte solution with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0092] As non-aqueous solvents, cyclic carbonate esters, linear carbonate esters, cyclic ethers, linear ethers, nitriles, amides, etc., may be used. One of these solvents may be used, or two or more may be used in combination.

[0093] Examples of lithium salts that can be used include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One of these electrolyte salts may be used, or two or more may be used in combination.

[0094] Typically, it is desirable to interpose a separator between the positive and negative electrodes. The separator 22 has high ion permeability and appropriate mechanical strength and insulating properties. As the separator 22, a microporous thin film, woven fabric, and nonwoven fabric can be used. As the material of the separator 22, for example, a polymer can be used. The polymer may be polyolefin such as polypropylene and polyethylene.

[0095] In the battery according to Embodiment 2, the electrolyte may be impregnated into a polymer provided as a separator, for example. That is, the battery according to Embodiment 2 may have a structure in which both the electrolyte and the polymer are used in combination.

[0096] The battery according to Embodiment 2 may further contain a solid electrolyte as the electrolyte. That is, the battery of this disclosure may have a hybrid structure in which an electrolyte and a solid electrolyte are used in combination. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, or organic polymer solid electrolytes. In this disclosure, "halide solid electrolyte" means a solid electrolyte in which a halogen element is the main component of the anions. "Sulfide solid electrolyte" means a solid electrolyte in which sulfur is the main component of the anions. "Oxide solid electrolyte" means a solid electrolyte in which oxygen is the main component of the anions. The main component of the anions means the anion with the largest amount of substance among all the anions that make up the solid electrolyte.

[0097] As an example of the structure of the battery according to Embodiment 2, Figure 2 describes a configuration example, namely a cylindrical non-aqueous electrolyte secondary battery in which a wound electrode group, in which a positive electrode and a negative electrode are wound around a separator, and an electrolyte are housed in an outer casing. However, the battery according to this disclosure is not limited to this configuration example. The battery according to Embodiment 2 may take any form, such as prismatic, coin-type, button-type, laminate-type, etc. Furthermore, as the electrode group in the battery according to Embodiment 2, other forms of electrode groups may be used instead of the wound electrode group, such as an electrode group in which a positive electrode and a negative electrode are stacked with a separator in between.

[0098] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0099] (Technical 1) A negative electrode comprising: a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector and containing a silicon-containing material as a negative electrode active material and a conductive agent, wherein the negative electrode mixture layer includes a first negative electrode mixture layer including the surface of the negative electrode and a second negative electrode mixture layer located between the first negative electrode mixture layer and the negative electrode current collector; the silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a larger particle volume expansion coefficient than the first silicon-containing material; the negative electrode active material contained in the first negative electrode mixture layer is designated as the first negative electrode active material, the negative electrode active material contained in the second negative electrode mixture layer is designated as the second negative electrode active material, the conductive agent contained in the first negative electrode mixture layer is designated as the first conductive agent, and the conductive agent contained in the second negative electrode mixture layer is designated as the second conductive agent, The mass ratio of the first silicon-containing material in the first negative electrode active material is greater than the mass ratio of the first silicon-containing material in the second negative electrode active material, the mass ratio of the second silicon-containing material in the second negative electrode active material is greater than the mass ratio of the second silicon-containing material in the first negative electrode active material, the mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material, and the porosity of the first negative electrode mixture layer is greater than the porosity of the second negative electrode mixture layer. Here, the particle volume expansion rate of the first silicon-containing material is the ratio of the particle volume of the first silicon-containing material in the charged state to the particle volume of the first silicon-containing material in the discharged state, and the particle volume expansion rate of the second silicon-containing material is the ratio of the particle volume of the second silicon-containing material in the charged state to the particle volume of the second silicon-containing material in the discharged state.

[0100] This configuration allows the negative electrode related to Technology 1 to achieve both improved battery charge / discharge cycle characteristics and reduced internal resistance.

[0101] (Technology 2) The anode according to Technology 1, wherein the anode mixture layer further comprises a carbon material as the anode active material.

[0102] This configuration allows the negative electrode related to Technology 2 to achieve both improved battery charge / discharge cycle characteristics and reduced internal resistance.

[0103] (Technology 3) When the particle volume expansion coefficient of the first silicon-containing material is V1 and the particle volume expansion coefficient of the second silicon-containing material is V2, V1 and V2 satisfy the following relationships (1) and (2) for the negative electrode described in Technology 1 or 2: 1.3 ≤ V1 < 2.2 ... (1) 1.7 ≤ V2 < 3 ... (2)

[0104] This configuration allows the negative electrode related to technology 3 to further improve the battery's charge-discharge cycle characteristics.

[0105] (Technical 4) When the expansion rate of the first negative electrode mixture layer during charging is A1 and the expansion rate of the second negative electrode mixture layer during charging is A2, A1 and A2 satisfy the following relational expression (3), which is the negative electrode described in any one of Technical 1 to 3: A1 < A2 ... (3) Here, the expansion rate of the first negative electrode mixture layer during charging is the rate of increase in the thickness of the first negative electrode mixture layer in the charging state relative to the thickness of the first negative electrode mixture layer in the discharge state, and the expansion rate of the second negative electrode mixture layer during charging is the rate of increase in the thickness of the second negative electrode mixture layer in the charging state relative to the thickness of the second negative electrode mixture layer in the discharge state.

[0106] With this configuration, the negative electrode according to technology 4 can further improve the charge-discharge cycle characteristics of the battery.

[0107] (Technical 5) The negative electrodes described in Technical 4, where A1 and A2 satisfy the following relation (4): 0.5 < A1 / A2 < 1 ... (4)

[0108] This configuration allows the negative electrode related to technology 5 to further improve the charge-discharge cycle characteristics of the battery.

[0109] (Technical 6) When the porosity of the first negative electrode mixture layer is ε1 and the porosity of the second negative electrode mixture layer is ε2, ε1 and ε2 satisfy the following relation (5), which is the negative electrode described in any one of Technical 1 to 5: 1.0 < ε1 / ε2 < 2.0 ... (5)

[0110] This configuration allows the negative electrode according to technology 6 to further reduce the internal resistance of the battery.

[0111] (Technical 7) The conductive agent is a negative electrode according to any one of Technical 1 to 6, comprising fibrous carbon.

[0112] This configuration allows the negative electrode related to technology 7 to further improve the charge-discharge cycle characteristics of the battery.

[0113] (Technical 8) The conductive agent is a negative electrode according to any one of Technical 1 to 7, comprising carbon nanotubes.

[0114] This configuration allows the negative electrode related to technology 8 to further improve the charge-discharge cycle characteristics of the battery.

[0115] (Technical 9) When the mass ratio of the first conductive agent to the first negative electrode active material is X1, and the mass ratio of the second conductive agent to the second negative electrode active material is X2, X1 and X2 satisfy the following relation (6), which is the negative electrode described in any one of Technical 1 to 8: 1 < X1 / X2 < 9 ... (6)

[0116] This configuration allows the negative electrode related to technology 9 to further improve the charge-discharge cycle characteristics of the battery.

[0117] (Technical 10) The negative electrode according to any one of Technical 1 to 9, wherein the true density of the first silicon-containing material is less than the true density of the second silicon-containing material.

[0118] With this configuration, the negative electrode related to technology 10 can further improve the charge-discharge cycle characteristics of the battery.

[0119] (Technical 11) The negative electrode according to any one of Technical 1 to 10, wherein at least one selected from the group consisting of the first silicon-containing material and the second silicon-containing material is a composite material comprising an ionic conductive phase and a Si phase dispersed in the ionic conductive phase, and the ionic conductive phase comprises at least one selected from the group consisting of an aluminate phase, a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

[0120] With this configuration, the negative electrode according to technology 11 can further improve the charge-discharge cycle characteristics of the battery.

[0121] (Technology 12) Regarding the two divided layers resulting from bisecting the negative electrode mixture layer in the thickness direction, the divided layer located on the surface side of the negative electrode mixture layer is the first negative electrode mixture layer, and the divided layer located on the side of the negative electrode current collector is the second negative electrode mixture layer. The negative electrode according to any one of Technologies 1 to 11.

[0122] With this configuration, the negative electrode according to Technology 12 can achieve both an improvement in the charge-discharge cycle characteristics of the battery and a reduction in internal resistance.

[0123] (Technology 13) A battery comprising the negative electrode according to any one of Technologies 1 to 12, a positive electrode, and an electrolyte.

[0124] With this configuration, the battery according to Technology 13 can achieve both an improvement in the charge-discharge cycle characteristics and a reduction in internal resistance.

[0125] Hereinafter, the present disclosure will be described in more detail using examples. The following examples are merely illustrative of one aspect and are not limited thereto.

[0126] [Example 1] (Production of Positive Electrode Active Material) The composite hydroxide represented by [Ni 0.88 Co 0.09 Al 0.03 (OH)2 obtained by the coprecipitation method was calcined at 500 °C for 8 hours to obtain an oxide (Ni 0.88 Co 0.09 Al 0.03 O2). Next, LiOH and the composite oxide were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.03:1 to obtain a mixture. This mixture was calcined in an oxygen stream with an oxygen concentration of 95% (flow rates of 2 mL / min per 10 cm 3 and 5 L / min per 1 kg of the mixture) at a heating rate of 2.0 °C / min from room temperature to 650 °C, and then at a heating rate of 0.5 °C / min from 650 °C to 780 °C to obtain a lithium-containing composite oxide represented by LiNi 0.88 Co 0.09 Al 0.03 O2.

[0127] (Preparation of the positive electrode) The above positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil, the coating was dried and compressed, and then the positive electrode current collector was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode mixture layers were arranged on both sides of the positive electrode current collector. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode current collector was exposed.

[0128] (Preparation of the first silicon-containing material) Tetraexisilane (TEOS) and cetyltrimethylammonium bromide (CTAB) were mixed in a mixed solution of ethanol / water / ammonia to produce CTAB-modified SiO2 nanoparticles. Resorcinol, formaldehyde, and a surfactant (Pluronic F-127) were added to these nanoparticles and polymerized to obtain polymer particles containing the aforementioned SiO2 nanoparticles. At this time, the molar ratio of surfactant to resorcinol (surfactant / resorcinol) was set to 0.005, and the mass ratio of resorcinol to TEOS (resorcinol / TEOS) was set to approximately 0.5 / 1. After drying these polymer particles, they were carbonized at 800°C in a nitrogen atmosphere, and then mixed with magnesium powder and heated at 650°C in an argon atmosphere to carry out a magnesium thermal reduction reaction. MgO was dissolved from the reaction-derived particles using a mixed solution of HCl / H2O / ethanol, and after washing with ethanol and drying, a mesoporous silicon-containing material with an average particle size of 8 μm, containing Si and C, was prepared. This silicon-containing material was designated as the first silicon-containing material.

[0129] (Preparation of Secondary Silicon-Containing Material) Coal pitch (manufactured by JFE Chemical Corporation, MCP250) and raw silicon (3N, average particle size 10 μm) were mixed as carbon raw materials in a mass ratio of coal pitch:raw material silicon = 2:1. The mixture was then pulverized into fine particles using a planetary ball mill (manufactured by Fritsch, P-5). Next, the finely pulverized powder mixture was calcined in an inert atmosphere to carbonize the carbon source and obtain a sintered product in which the Si phase was dispersed within the amorphous carbon phase. Subsequently, the sintered product was pulverized using a jet mill to obtain composite particles as a secondary silicon-containing material with an average particle size of 8 μm.

[0130] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.06 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0131] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.04 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0132] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0133] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for both the first and second anode composite layers. In Table 1, the first silicon-containing material is abbreviated as "First Si" and the second silicon-containing material as "Second Si". Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, determined from the charging ratio, for both the first and second anode composite layers.

[0134] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.2 mol / liter in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 3:3:4 (at 25°C).

[0135] (Preparation of test cell (secondary battery)) An aluminum lead was attached to the exposed part of the positive electrode and a nickel lead was attached to the exposed part of the negative electrode. The positive and negative electrodes were wound in a spiral shape via a polyolefin separator to create a wound electrode body. Insulating plates were placed above and below the electrode body, and the electrode body was housed in an outer casing. The negative electrode lead was welded to the bottom of the bottomed cylindrical outer casing, and the positive electrode lead was welded to the sealing body. Electrolyte was injected into the outer casing, and the opening of the outer casing was sealed with the sealing body via a gasket to create a secondary battery as a test cell.

[0136] [Example 2] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0137] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0138] (Preparation of the negative electrode) Graphite and a silicon-1 material were mixed in a mass ratio of graphite:silicon-1 material = 91:9 to form the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.06 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry for the first negative electrode mixture layer.

[0139] Graphite and a silicon-2 content material were mixed in a mass ratio of graphite:silicon-2 content material = 91:9, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.04 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0140] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0141] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer.

[0142] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0143] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 2 was used.

[0144] [Example 3] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0145] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0146] (Preparation of the negative electrode) Graphite and a silicon-1 material were mixed in a mass ratio of graphite:silicon-1 material = 86:14, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.06 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0147] Graphite and a silicon-2 content material were mixed in a mass ratio of graphite:silicon-2 content material = 86:14, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.04 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0148] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0149] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer.

[0150] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0151] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 3 was used.

[0152] [Example 4] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0153] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0154] (Preparation of negative electrodes) The negative electrode slurry for the first negative electrode slurry layer and the negative electrode slurry for the second negative electrode slurry layer were prepared in the same manner as in Example 1.

[0155] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.56. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0156] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Note that in Example 4, the linear pressure applied when compressing the coating film differed from that in Example 1.

[0157] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0158] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 4 was used.

[0159] [Example 5] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0160] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0161] (Preparation of negative electrodes) The negative electrode slurry for the first negative electrode slurry layer and the negative electrode slurry for the second negative electrode slurry layer were prepared in the same manner as in Example 1.

[0162] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.56. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0163] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Note that in Example 5, the linear pressure applied when compressing the coating film differed from that in Examples 1 and 4.

[0164] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0165] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 5 was used.

[0166] [Example 6] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0167] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0168] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.08 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0169] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.02 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0170] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0171] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer.

[0172] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0173] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 6 was used.

[0174] [Example 7] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0175] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0176] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.09 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0177] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.01 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0178] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0179] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer.

[0180] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0181] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 7 was used.

[0182] [Comparative Example 1] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0183] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0184] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:4.5:4.5 to form the negative electrode active material. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0185] Next, the prepared negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form a negative electrode mixture layer. The thickness of the negative electrode mixture layer was 100 μm on each side. In addition, an exposed portion of the negative electrode was provided where the surface of the negative electrode current collector was exposed.

[0186] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material for both the first and second negative electrode mixture layers, determined from the charging ratio.

[0187] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0188] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 1 was used.

[0189] [Comparative Example 2] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0190] (Preparation of the first silicon-containing material) The first silicon-containing material was prepared in the same manner as in Example 1. In Comparative Example 2, only the first silicon-containing material was used, and the second silicon-containing material was not used.

[0191] (Preparation of the negative electrode) Graphite and a silicon-1 content material were mixed in a mass ratio of graphite:silicon-1 content material = 91:9 to form the negative electrode active material. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0192] Next, the prepared negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form a negative electrode mixture layer. The thickness of the negative electrode mixture layer was 100 μm on each side. In addition, an exposed portion of the negative electrode was provided where the surface of the negative electrode current collector was exposed.

[0193] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material for both the first and second negative electrode mixture layers, determined from the charging ratio.

[0194] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0195] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 2 was used.

[0196] [Comparative Example 3] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0197] (Preparation of the second silicon-containing material) The second silicon-containing material was prepared in the same manner as in Example 1. In Comparative Example 3, only the first silicon-containing material was used, and the first silicon-containing material was not used.

[0198] (Preparation of the negative electrode) Graphite and a silicon-2 containing material were mixed in a mass ratio of graphite:silicon-1 containing material = 91:9 to form the negative electrode active material. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0199] Next, the prepared negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form a negative electrode mixture layer. The thickness of the negative electrode mixture layer was 100 μm on each side. In addition, an exposed portion of the negative electrode was provided where the surface of the negative electrode current collector was exposed.

[0200] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material for both the first and second negative electrode mixture layers, determined from the charging ratio.

[0201] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0202] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 3 was used.

[0203] [Comparative Example 4] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0204] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0205] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 86:7:7 to prepare the negative electrode active material. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0206] Next, the prepared negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form a negative electrode mixture layer. The thickness of the negative electrode mixture layer was 100 μm on each side. In addition, an exposed portion of the negative electrode was provided where the surface of the negative electrode current collector was exposed.

[0207] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material for both the first and second negative electrode mixture layers, determined from the charging ratio.

[0208] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0209] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 4 was used.

[0210] [Comparative Example 5] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0211] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0212] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:4.5:4.5, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.06 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0213] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:4.5:4.5, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.04 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0214] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.5. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0215] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material for both the first and second negative electrode mixture layers, determined from the charging ratio.

[0216] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0217] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 5 was used.

[0218] [Comparative Example 6] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0219] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0220] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:4.5:4.5, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0221] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 100:4.5:4.5, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0222] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.5. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0223] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material for both the first and second negative electrode mixture layers, determined from the charging ratio.

[0224] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0225] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 6 was used.

[0226] [Comparative Example 7] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0227] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0228] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:4.5:4.5, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.06 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0229] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:4.5:4.5, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.04 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0230] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.5. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0231] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the negative electrode active material, determined from the charging ratio, for the negative electrode mixture layer. Table 3 shows the mass ratio (%) of the conductive agent to the negative electrode active material, determined from the charging ratio, for both the first and second negative electrode mixture layers. Note that in Comparative Example 7, the linear pressure applied when compressing the coating film differed from that in Comparative Example 5.

[0232] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0233] (Preparation of test cell (secondary battery)) A secondary battery was prepared as a test cell in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 7 was used.

[0234] [Reference Example 1] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0235] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0236] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0237] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0238] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0239] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer.

[0240] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0241] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Reference Example 1 was used.

[0242] [Reference Example 2] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0243] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0244] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0245] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0246] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0247] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first and second anode mixture layers. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first and second anode mixture layers. Note that in Reference Example 2, the linear pressure applied when compressing the coating film differed from that in Reference Example 1.

[0248] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0249] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Reference Example 2 was used.

[0250] [Reference Example 3] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0251] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0252] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0253] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.05 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0254] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0255] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first and second anode mixture layers. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first and second anode mixture layers. Note that in Reference Example 3, the linear pressure applied when compressing the coating film differed from that in Reference Examples 1 and 2.

[0256] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0257] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Reference Example 3 was used.

[0258] [Reference Example 4] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0259] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0260] (Preparation of negative electrodes) The negative electrode slurry for the first negative electrode slurry layer and the negative electrode slurry for the second negative electrode slurry layer were prepared in the same manner as in Example 1.

[0261] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0262] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Note that in Reference Example 4, the linear pressure applied when compressing the coating film differed from that in Example 1.

[0263] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0264] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Reference Example 4 was used.

[0265] [Reference Example 5] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0266] (Preparation of the first silicon-containing material and the second silicon-containing material) The first silicon-containing material and the second silicon-containing material were prepared in the same manner as in Example 1.

[0267] (Preparation of the negative electrode) Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:6:3, and this was used as the negative electrode active material for the first negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.04 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0268] Graphite, a first silicon-containing material, and a second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6, and this was used as the negative electrode active material for the second negative electrode mixture layer. Then, 100 parts by mass of the negative electrode active material was mixed with 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and 0.06 parts by mass of carbon nanotubes (conductive agent), and an appropriate amount of water was added to prepare the negative electrode mixture slurry.

[0269] Next, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied onto the second negative electrode mixture layer, the coating was dried, and compressed at a different linear pressure than that used for forming the second negative electrode mixture layer to form the first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 100 μm on each side. Furthermore, the value of T1 / (T1+T2), calculated from the thickness of the first negative electrode mixture layer (T1) and the thickness of the second negative electrode mixture layer (T2), was 0.52. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0270] Table 1 shows the mass percentages of the first silicon-containing material and the second silicon-containing material in the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer. Table 3 also shows the mass ratio (%) of the conductive agent to the anode active material, respectively, determined from the charging ratio, for the first anode mixture layer and the second anode mixture layer.

[0271] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0272] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Reference Example 5 was used.

[0273] [Measurement of Particle Volume Expansion Rate of First and Second Silicon-Containing Materials] For the negative electrodes of each example, comparative example, and reference example, the particle volume expansion rates of the first and second silicon-containing materials were determined using the method described in Embodiment 1. However, instead of disassembling the battery and cutting out the negative electrode, a single-electrode cell was made using the fabricated negative electrode, and the particle volume expansion rates of the first and second silicon-containing materials were determined using that single-electrode cell. The results are shown in Table 1. However, for silicon-containing materials fabricated using the same method, the particle volume expansion rate of one representative silicon-containing material was measured, and that measurement value was used.

[0274] [Measurement of the expansion rate during charging of the first and second negative electrode mixture layers] For the negative electrodes of each example and comparative example, the expansion rate during charging of the first and second negative electrode mixture layers was determined using the method described in Embodiment 1. However, instead of disassembling the battery and cutting out the negative electrode, a single-electrode cell was made using the fabricated negative electrode, and the expansion rate during charging of the first and second negative electrode mixture layers was determined using that single-electrode cell. The results are shown in Table 2.

[0275] [Measurement of porosity of the first and second negative electrode mixture layers] For each example, comparative example, and reference example of the negative electrode, the porosity of the first and second negative electrode mixture layers was determined using the method described in Embodiment 1. However, instead of disassembling the battery and cutting out the negative electrode, the porosity of the first and second negative electrode mixture layers was determined using the fabricated negative electrode. The results are shown in Table 2.

[0276] [Measurement of Resistance (DC-IR))] The DC-IR of the cells was measured using the following method with the test cells of each example and comparative example.

[0277] Initial charging and discharging were performed on the test cells of each example, comparative example, and reference example under the following conditions: (Charging and discharging conditions) Initial charging and discharging conditions Constant current charging was performed with a current of 0.5 It (625 mA) until the battery voltage reached 4.2 V. Furthermore, constant voltage charging was performed at a voltage of 4.2 V until the current value reached 0.02 It (25 mA). Then, constant current discharging was performed with a current of 0.5 It (625 mA) until the battery voltage reached 2.5 V.

[0278] Next, charging and discharging were performed under the following conditions, and the initial DC-IR value was determined using the following formula (a). The results are shown in Table 1.

[0279] (Charging and discharging conditions) Under a temperature of 25°C, constant current charging was performed with a current of 0.3 It (375 mA) until the battery voltage reached 3.79 V. Furthermore, constant voltage charging was performed at a constant voltage of 3.79 V until the current value reached 0.02 It (25 mA). After a 2-hour rest period, the battery was discharged for 10 seconds with a current of 0.5 It (625 mA).

[0280] (Formula for calculating DC-IR) DC-IR (mΩ) = (Voltage immediately before discharge starts - Voltage 10 seconds after discharge starts) / (Discharge current density × Electrode area) ... (a)

[0281] The results are shown in Table 3. Table 3 also shows the relative values ​​when the DC-IR value of Comparative Example 1 is set as the baseline (100%).

[0282] [Evaluation of Charge / Discharge Cycle Characteristics] The test cells of each example, comparative example, and reference example were charged at a constant current of 0.3C at a temperature of 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current value was 0.02C. After that, the batteries were discharged at a constant current of 0.5C until the battery voltage reached 2.5V, and the discharge capacity at this time was defined as the initial discharge capacity. This charge / discharge cycle was considered one cycle, and 100 cycles were performed. The initial discharge capacity and the discharge capacity at the 100th cycle were determined, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = Discharge capacity at the 100th cycle / Initial discharge capacity × 100

[0283] The obtained capacity retention rate was defined as the cycle retention rate. The results are shown in Table 3. Table 1 shows the relative values ​​when the cycle retention rate of Comparative Example 1 is set as the baseline (100%).

[0284]

[0285]

[0286]

[0287] (Discussion) As shown in Table 3, all of the test cells in the examples showed improved charge-discharge cycle characteristics and reduced internal resistance compared to the test cell of Comparative Example 1. Furthermore, the test cells of Comparative Examples 2 and 3, in which the negative electrode mixture layer was formed using only one of either the first silicon-containing material or the second silicon-containing material, were able to improve either the charge-discharge cycle characteristics or the internal resistance compared to the test cell of Comparative Example 1, but could not improve both simultaneously. In addition, the test cells of Comparative Example 4, in which the proportion of silicon-containing material in the negative electrode active material was increased, Comparative Example 5, in which only the relationship of the mass ratio of the conductive agent to the negative electrode active material in the negative electrode mixture layer satisfies the relationship specified for the negative electrode in this disclosure, and Comparative Example 6, in which only the relationship of the porosity in the negative electrode mixture layer satisfies the relationship specified for the negative electrode in this disclosure, also could not simultaneously improve both the charge-discharge cycle characteristics and the internal resistance.

[0288] The negative electrodes of Reference Examples 1 to 5 have the relationship with respect to silicon-containing materials specified in the negative electrodes of this disclosure, namely, "the mass ratio of the first silicon-containing material in the first negative electrode active material is greater than the mass ratio of the first silicon-containing material in the second negative electrode active material, and the mass ratio of the second silicon-containing material in the second negative electrode active material is greater than the mass ratio of the second silicon-containing material in the first negative electrode active material." However, the negative electrodes of Reference Examples 1 and 3 do not satisfy the relationship with respect to porosity and conductive agent specified in the negative electrodes of this disclosure. The negative electrode of Reference Example 2 satisfies the relationship with respect to porosity specified in the negative electrodes of this disclosure, but does not satisfy the relationship with respect to conductive agent. The negative electrodes of Reference Examples 4 and 5 satisfy the relationship with respect to conductive agent specified in the negative electrodes of this disclosure, but do not satisfy the relationship with respect to porosity. The negative electrodes of Examples 1 to 7, while satisfying the relationships for silicon-containing materials specified in Reference Examples 1 to 5, i.e., the negative electrodes of the present disclosure, were able to achieve a higher level of improvement in charge-discharge cycle characteristics and reduction in internal resistance compared to negative electrodes that do not simultaneously satisfy both the relationships between porosity and conductive agent specified in the present disclosure.

[0289] The technology disclosed herein is useful for batteries such as lithium-ion secondary batteries that require high capacity and excellent charge-discharge cycle characteristics.

Claims

Negative electrode current collector and A negative electrode composite layer is disposed on the negative electrode current collector and comprises a silicon-containing material as a negative electrode active material and a conductive agent, A negative electrode equipped with, The negative electrode mixture layer includes a first negative electrode mixture layer including the surface of the negative electrode, and a second negative electrode mixture layer located between the first negative electrode mixture layer and the negative electrode current collector. The silicon-containing material comprises a first silicon-containing material and a second silicon-containing material having a larger particle volume expansion coefficient than the first silicon-containing material. When the negative electrode active material contained in the first negative electrode mixture layer is designated as the first negative electrode active material, the negative electrode active material contained in the second negative electrode mixture layer is designated as the second negative electrode active material, the conductive agent contained in the first negative electrode mixture layer is designated as the first conductive agent, and the conductive agent contained in the second negative electrode mixture layer is designated as the second conductive agent, The mass ratio of the first silicon-containing material in the first negative electrode active material is greater than the mass ratio of the first silicon-containing material in the second negative electrode active material. The mass ratio of the second silicon-containing material in the second negative electrode active material is greater than the mass ratio of the second silicon-containing material in the first negative electrode active material. The mass ratio of the first conductive agent to the first negative electrode active material is greater than the mass ratio of the second conductive agent to the second negative electrode active material. The porosity of the first negative electrode mixture layer is greater than the porosity of the second negative electrode mixture layer. Negative electrode. Here, the particle volume expansion coefficient of the first silicon-containing material is the ratio of the particle volume of the first silicon-containing material in the charged state to the particle volume of the first silicon-containing material in the discharged state, and the particle volume expansion coefficient of the second silicon-containing material is the ratio of the particle volume of the second silicon-containing material in the charged state to the particle volume of the second silicon-containing material in the discharged state. 　 The aforementioned negative electrode mixture layer further contains a carbon material as a negative electrode active material. The negative electrode according to claim 1. 　 When the particle volume expansion coefficient of the first silicon-containing material is V1 and the particle volume expansion coefficient of the second silicon-containing material is V2, V1 and V2 satisfy the following relational equations (1) and (2): The negative electrode according to claim 1. 1.3 ≤ V1 < 2.2 ... (1) 1.7 ≤ V2 < 3 ... (2) 　 When the expansion rate of the first negative electrode mixture layer during charging is A1 and the expansion rate of the second negative electrode mixture layer during charging is A2, then A1 and A2 satisfy the following relational expression (3): The negative electrode according to claim 1. A1<A2...(3) Here, the charging expansion rate of the first negative electrode mixture layer is the rate of increase in thickness of the first negative electrode mixture layer in the charging state relative to the thickness of the first negative electrode mixture layer in the discharge state, and the charging expansion rate of the second negative electrode mixture layer is the rate of increase in thickness of the second negative electrode mixture layer in the charging state relative to the thickness of the second negative electrode mixture layer in the discharge state. 　 A1 and A2 satisfy the following relation (4): The negative electrode according to claim 4. 0.5<A1 / A2<1...(4) 　 When the porosity of the first negative electrode mixture layer is ε1 and the porosity of the second negative electrode mixture layer is ε2, then ε1 and ε2 satisfy the following relational expression (5): The negative electrode according to claim 1. 1.0<ε1 / ε2<2.0 (5) 　 The conductive agent comprises fibrous carbon, The negative electrode according to claim 1. 　 The conductive agent includes carbon nanotubes, The negative electrode according to claim 1. 　 Let X1 be the mass ratio of the first conductive agent to the first negative electrode active material, and X2 be the mass ratio of the second conductive agent to the second negative electrode active material. Then X1 and X2 satisfy the following relation (6): The negative electrode according to claim 1. 1<X1 / X2<9...(6) 　 The true density of the first silicon-containing material is less than the true density of the second silicon-containing material. The negative electrode according to claim 1. 　 At least one selected from the group consisting of the first silicon-containing material and the second silicon-containing material is a composite material comprising an ionic conductive phase and a Si phase dispersed in the ionic conductive phase. The ion-conducting phase includes at least one selected from the group consisting of an aluminate phase, a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The negative electrode according to claim 1. 　 Regarding the two divided layers resulting from the division of the negative electrode mixture layer into two equal parts in the thickness direction, the divided layer located on the surface side of the negative electrode mixture layer is the first negative electrode mixture layer, and the divided layer located on the negative electrode current collector side is the second negative electrode mixture layer. The negative electrode according to claim 1. 　 A negative electrode according to any one of claims 1 to 12, Positive electrode and, Electrolytes, A battery equipped with this feature.

Images