Negative electrode and battery

Abstract

A negative electrode 10 according to the present disclosure comprises: a negative electrode current collector 11; and a negative electrode mixture layer 12 that is disposed on the negative electrode current collector 11 and contains a silicon-containing material as a negative electrode active material. The negative electrode mixture layer 12 includes: a first negative electrode mixture layer 13 including a surface of the negative electrode 10; and a second negative electrode mixture layer 14 positioned between the first negative electrode mixture layer 13 and the negative electrode current collector 11. 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. When the negative electrode active material included in the first negative electrode mixture layer 13 is used as a first negative electrode active material, and the negative electrode active material included in the second negative electrode mixture layer 14 is used as a second negative electrode active material, the mass ratio of the first silicon-containing material in the second negative electrode active material is larger than the mass ratio of the first silicon-containing material in the first negative electrode active material, and the mass ratio of the second silicon-containing material in the first negative electrode active material is larger than the mass ratio of the second silicon-containing material in the second negative electrode active material. The porosity of the first negative electrode mixture layer 13 is larger than the porosity of the second negative electrode mixture layer 14.

Description

Anodes and Batteries

[0001] The present disclosure relates to anodes and batteries.

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

[0003] It is known that using a negative electrode active material containing silicon (Si) and further improving the packing density of the negative electrode are effective in increasing the capacity of a battery. Patent Document 1 discloses a lithium secondary battery that uses a material containing Si and O as the negative electrode active material.

[0004] JP 2011-233245 A

[0005] However, increasing the packing density of a negative electrode using a negative electrode active material containing Si deteriorates the permeability of the electrolyte, resulting in an increase in the internal resistance of the battery.

[0006] Therefore, the present disclosure provides a technique capable of reducing the internal resistance of a battery that uses a negative electrode active material containing Si.

[0007] 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, wherein the negative electrode mixture layer includes a first negative electrode mixture layer including a 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, and the silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a particle volume expansion coefficient greater than that of the first silicon-containing material, and when the negative electrode active material contained in the first negative electrode mixture layer is defined as a first negative electrode active material and the negative electrode active material contained in the second negative electrode mixture layer is defined as a second negative electrode active material, a mass ratio of the first silicon-containing material in the second negative electrode active material is greater than a mass ratio of the first silicon-containing material in the first negative electrode active material, the mass proportion of the second silicon-containing material in the first negative electrode active material is greater than the mass proportion of the second silicon-containing material in 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, wherein 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 a charged state to the particle volume of the first silicon-containing material in a 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 a charged state to the particle volume of the second silicon-containing material in a discharged state.

[0008] According to the technology of the present disclosure, it is possible to reduce the internal resistance of a battery that uses a negative electrode active material containing Si.

[0009] Fig. 1 is a cross-sectional view showing a schematic configuration of a negative electrode according to embodiment 1. Fig. 2 is a longitudinal cross-sectional view showing a schematic example of a battery according to embodiment 2.

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0011] 1 is a cross-sectional view showing a schematic configuration of a negative electrode according to Embodiment 1. The negative electrode 10 according to Embodiment 1 includes 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. The silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a particle volume expansion coefficient greater than that of the first silicon-containing material.

[0012] 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 located between the first negative electrode mixture layer 13 and the negative electrode current collector 11. When the negative electrode active material included in the first negative electrode mixture layer 13 is the first negative electrode active material and the negative electrode active material included in the second negative electrode mixture layer 14 is the second negative electrode active material, the mass ratio of the first silicon-containing material in the second negative electrode active material is greater than the mass ratio of the first silicon-containing material in the first negative electrode active material, and the mass ratio of the second silicon-containing material in the first negative electrode active material is greater than the mass ratio of the second silicon-containing material in the second negative electrode active material. Furthermore, the porosity of the first negative electrode mixture layer 13 is greater than the porosity of the second negative electrode mixture layer 14.

[0013] 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 a charged state to the particle volume of the first silicon-containing material in a 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 a charged state to the particle volume of the second silicon-containing material in a discharged state.

[0014] 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) A battery to be evaluated is disassembled, the negative electrode is cut out, and a monopolar cell is prepared in which the particle cross-sections of the silicon-containing material are exposed using metallic Li as the counter electrode and an ionic liquid as the electrolyte. (2) The monopolar cell is charged at 0.002 C in a 25°C environment until the cell voltage reaches 5 mV, and then discharged at 0.05 C until the cell voltage reaches 1.0 V, and the particle cross-sections of the silicon-containing material are observed in situ using a scanning electron microscope (SEM). (3) Regarding the particle volume expansion coefficient of the silicon-containing material, the particle volume (Va) of the silicon-containing material in a charged state (fully charged state) and the particle volume (Vb) of the silicon-containing material in a 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 an SEM image of the particle cross-section to the 3 / 2 power. (4) For 50 particles of the silicon-containing material contained in the first negative electrode composite layer 13 and the second negative electrode composite layer 14, the measurement (2) above and the calculation (3) above of the particle volume expansion coefficient are performed to obtain a particle area-based particle volume expansion coefficient distribution. Note that the particle area in the particle volume expansion coefficient distribution obtained here refers to the particle area in a fully discharged state. (5) The area-based particle volume expansion coefficient distribution obtained in (4) above is separated into two peaks using a Gaussian function for each layer, and the particle group corresponding to the first peak with the smaller particle expansion coefficient is designated as the first silicon-containing material, and the particle group corresponding to the second peak with the larger particle expansion coefficient 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 apex of the first peak, and the particle volume expansion coefficient of the second silicon-containing material is the particle volume expansion coefficient at the apex of the second peak.

[0015] Furthermore, it can be confirmed by the following method that the mass proportion of the first silicon-containing material in the second negative electrode active material is greater than the mass proportion of the first silicon-containing material in the first negative electrode active material, and that the mass proportion of the second silicon-containing material in the first negative electrode active material is greater than the mass proportion of the second silicon-containing material in the second negative electrode active material.

[0016] This can be confirmed from the size of the ratio of the total area derived from the first peaks in the first anode composite layer 13 to the total area of ​​the first anode active material and the size of the ratio of the total area derived from the first peaks in the second anode composite layer 14 to the total area of ​​the second anode active material, which are obtained from the area-based particle volume expansion coefficient distribution obtained by the above-mentioned method, and the size of the ratio of the total area derived from the second peaks in the first anode composite layer 13 to the total area of ​​the first anode active material and the size of the ratio of the total area derived from the second peaks in the second anode composite layer 14 to the total area of ​​the second anode active material.

[0017] The porosity of the negative electrode mixture layer 12 is a two-dimensional value calculated from the ratio of the area of ​​voids to the cross-sectional area of ​​the negative electrode mixture layer 12 in the cross section of the negative electrode mixture layer 12 .

[0018] In this specification, the porosity of the first anode mixture layer 13 and the second anode mixture layer 14 is measured by the following method. (1) The battery to be evaluated is disassembled, the anode is cut out, and the cross section of the anode mixture layer 12 is exposed. For example, a method of exposing the cross section includes cutting out a portion of the anode 10 and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the anode mixture layer 12. (2) Using an SEM, backscattered electron images of the cross sections of the exposed anode mixture layer 12 are taken for each of the first anode mixture layer 13 and the second anode mixture layer 14. The magnification for taking the backscattered electron images is, for example, 800 times. The following steps (3) and (4) are performed on the first anode mixture layer 13 and the second anode 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 (e.g., ImageJ, manufactured by the National Institutes of Health), to obtain a binarized image in which particle cross sections in the cross-sectional image are colored black and voids present in the particle cross sections are colored white. (4) In the binarized image obtained above, of the voids converted to white, voids inside the particles (pores not connected to the particle surface) and pores connected to the particle surface with a width of 3 μm or less are considered to be voids, and the area of ​​the voids is calculated. The porosity is calculated based on the following formula: Porosity (%) = Area of ​​voids / Area of ​​cross section of 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 porosity calculations according to (3) and (4) above are performed three times, and the average values ​​are used as the porosity of the first negative electrode mixture layer 13 and the porosity of the second negative electrode mixture layer 14.

[0019] As described above, in the negative electrode 10 according to the first embodiment, the negative electrode active material includes two types of silicon-containing materials having different particle volume expansion coefficients, and the mass proportion of the second silicon-containing material having a larger 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, and the mass proportion of the first silicon-containing material having a smaller 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. The negative electrode 10 according to the first embodiment further includes a configuration in which the porosity of the first negative electrode mixture layer 13 is greater than the porosity of the second negative electrode mixture layer 14. A negative electrode active material containing Si undergoes a large volume change upon charge and discharge, and thus voids are formed between the negative electrode active material and the surrounding active material due to repeated charge and discharge. According to the above configuration, interparticle voids are provided in advance in the first negative electrode mixture layer 13 located on the surface layer side, and therefore spaces are efficiently formed by stress caused by expansion and contraction of the second silicon-containing material, which has a larger particle volume expansion coefficient, thereby improving the permeability of the electrolyte into the negative electrode mixture layer 12 and reducing the internal resistance of the battery.

[0020] 1 is configured of two layers, the first anode mixture layer 13 and the second anode mixture layer 14, but is not limited to this and may be configured of three or more layers. The anode mixture layer 12 may further include a layer located between the first anode mixture layer 13 and the second anode mixture layer 14, or between the second anode mixture layer 14 and the anode current collector 11.

[0021] The first anode mixture layer 13 and the second anode mixture layer 14 can be distinguished by differences in constituent components, composition ratios, etc. The first anode mixture layer 13 can be identified, for example, by identifying a region having similar constituent components and similar composition ratios from the surface of the anode 10 to the depth direction of the anode mixture layer 12. The second anode mixture layer 14 can be identified by identifying a region that exists between the identified first anode mixture layer 13 and the anode current collector 11 and has the above-mentioned structural relationship with the first anode mixture layer 13.

[0022] In addition, in the anode mixture layer 12, if it is difficult to identify the first anode mixture layer 13 and the second anode mixture layer 14 due to differences in the constituent components and composition ratios, for example, the anode mixture layer 12 is divided into two equal parts in the thickness direction, and the divided layer located on the surface side of the anode mixture layer 12 may be identified as the first anode mixture layer 13, and the divided layer located on the anode current collector 11 side may be identified as the second anode mixture layer 14.

[0023] Each component of the negative electrode 10 of the first embodiment will be specifically described below.

[0024] [Negative Electrode Current Collector] The negative electrode current collector 11 may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 11 as a conductive auxiliary material.

[0025] The thickness of the negative electrode current collector 11 is not particularly limited, but may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 20 μm or less, from the viewpoint of balancing the strength and weight reduction of the negative electrode 10.

[0026] [Negative Electrode Mixture Layer] As described above, the negative electrode mixture 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 expansion coefficients.

[0027] In this specification, the term "silicon-containing material" refers to a material containing Si. Examples of silicon-containing materials include Si, Si alloys, Si compounds, and composite materials containing Si.

[0028] The average particle size of the silicon-containing material is, for example, 1 μm or more and 20 μm or less, or may be 1 μm or more and 15 μm or less. The average particle size of the silicon-containing material means the particle size at which the volume integrated value is 50% in the particle size distribution measured by a laser diffraction scattering method (hereinafter referred to as "volume-based D50"). For example, an "MT3000II" manufactured by Microtrac Bell Co., Ltd. is used as a measuring device, and the measurement is performed using, for example, water as a dispersion medium.

[0029] The silicon-containing material is preferably a composite material containing Si. The composite material containing Si is, for example, a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The ion-conducting 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-conducting phase may be composed of one phase or multiple phases. The Si phase is, for example, formed of fine Si particles dispersed in the ion-conducting phase.

[0030] The silicate phase may satisfy the following conditions (A) and / or (B): (A) The silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long periodic table). (B) The silicate phase contains element L. The element L is at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanoids, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanoids is a collective term for 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0031] Regarding the above condition (A), examples of alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The inclusion of an alkali metal element and / or a Group 2 element may reduce the irreversible capacity of the silicate phase. A silicate phase containing lithium (hereinafter, sometimes referred to as a "lithium silicate phase") is preferable in that it has a small irreversible capacity and a high initial charge / discharge efficiency.

[0032] 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.

[0033] The lithium silicate phase has the formula: Li 2z SiO (2+z) The lithium silicate phase may contain or be composed of a lithium silicate phase represented by the formula (0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2 (i.e., Li2Si2O5).

[0034] 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 carbon. Amorphous carbon is a carbon material having an average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeding 0.34 nm.

[0035] The silicide phase is a phase of a compound consisting of Si and an element that is more electropositive than Si, and examples thereof include NiSi, Mg2Si, and TiSi2.

[0036] The silicon oxide phase is composed of a compound 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 mass % or more and 100 mass % or less of the silicon oxide phase.

[0037] 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 represented by the formula: Li u AlO (3+u) / 2 From the viewpoints of ease of preparation, stability, ionic conductivity, and the like, u in the formula may be, for example, greater than 0 and 5 or less, or greater than 0 and 1 or less. 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.

[0038] The average size of the Si phase is, for example, 1 nm to 200 nm, or may be 1 nm to 100 nm, or 1 nm to 10 nm. The average size of the Si phase is calculated by taking an SEM image of the particle cross section of the silicon-containing material and averaging the diameters of the circumscribed circles of 100 Si phases extracted by image analysis.

[0039] The composite material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is, for example, composed of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer 12. The conductive layer is, for example, a carbon coating composed of a conductive carbon material. Examples of conductive carbon materials that can be used include carbon black such as acetylene black and ketjen black, graphite, and amorphous carbon (amorphous carbon) with low crystallinity. The thickness of the conductive layer is, for example, 1 nm or more and 200 nm or less, or may be 5 nm or more and 100 nm or less, taking into consideration ensuring conductivity and the diffusibility of Li ions into the particles. The thickness of the conductive layer can be measured by observing the cross section of the composite material using a SEM or a transmission electron microscope (TEM).

[0040] At least one selected from the group consisting of the first silicon-containing material and the second silicon-containing material may be the composite material. That is, at least one selected from the group consisting of the first silicon-containing material and the second silicon-containing material may be a composite material containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. Both the first silicon-containing material and the second silicon-containing material may be the composite material.

[0041] The negative electrode mixture layer 12 preferably further contains a carbon material as a negative electrode active material. By including a carbon material as the negative electrode active material, a conductive path in the negative electrode active material can be ensured even when the silicon-containing material expands and contracts during charge and discharge, thereby further reducing the internal resistance of the battery. Furthermore, by including a carbon material, it becomes easier to control the porosity of the negative electrode mixture layer 12, which allows for control of the porosity in consideration of the permeability of the electrolyte.

[0042] The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. The carbon material is preferably graphite. The graphite may be natural graphite or artificial graphite. Examples of graphite that can be used include artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, and mixtures thereof. The volume-based D50 of the carbon material may be, for example, 1 μm or more and 30 μm or less, or 5 μm or more and 25 μm or less.

[0043] The carbon material contained as the negative electrode active material in the negative electrode mixture layer 12 may include, for example, first graphite particles and second graphite particles. Here, the internal porosity of the first graphite particles is smaller than that of the second graphite particles. In this case, the mass ratio of the first graphite particles to the total of the first graphite particles and the second graphite particles is desirably larger in the first negative electrode mixture layer 13 than in the second negative electrode mixture layer 14. According to this configuration, the first graphite particles with a smaller internal porosity, i.e., the harder first graphite particles, are contained more in the first negative electrode mixture layer 13 located on the surface layer side, which makes it easier to form particle interspaces on the surface layer side. This can further improve the permeability of the electrolyte and further reduce the internal resistance of the battery.

[0044] In this specification, the fact that the internal porosity of the graphite particles and the mass ratio of the first graphite particles to the total of the first and second graphite particles are greater in the first negative electrode mixture layer 13 than in the second negative electrode mixture layer 14 is confirmed by the following method. (1) The battery to be evaluated is disassembled, the negative electrode is cut out, and a cross section of the negative electrode mixture layer 12 is exposed. For example, a method of exposing the cross section includes cutting out a portion of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer. (2) A backscattered electron image of the exposed cross section of the negative electrode mixture layer 12 is taken using an SEM. The magnification when taking the backscattered electron image is 3,000 to 5,000 times. (3) The cross-sectional image obtained as described above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA) to obtain a binarized image in which particle cross sections in the cross-sectional image are colored black and voids present in the particle cross sections are colored white. (4) From the binarized image, graphite particles having a particle size of 5 μm to 50 μm are selected, and the area of ​​the graphite particle cross section and the area of ​​the internal voids present in the graphite particle cross section are calculated. Here, the area of ​​the graphite particle cross section refers to the area of ​​the region surrounded by the outer periphery of the graphite particle, i.e., the area of ​​the entire cross-sectional portion of the graphite particle. Furthermore, for voids present in the graphite particle cross section that are 3 μm or less in width, it may be difficult to distinguish between internal and external voids in image analysis, so voids with a width of 3 μm or less may be considered internal voids. Then, the internal particle porosity of the graphite particle (area of ​​internal voids in graphite particle cross section × 100 / area of ​​graphite particle cross section) is calculated from the calculated area of ​​the graphite particle cross section and the area of ​​the internal voids in the graphite particle cross section. (5) The internal particle porosity is calculated for all graphite particles with particle sizes of 5 μm to 50 μm present in the cross-sectional image of the negative electrode composite layer 12. The distribution of the internal porosity of the graphite particles is confirmed and converted to particle area. Fitting is performed using two peaks using a Gaussian function. The graphite particles corresponding to the first peak on the side with smaller internal porosity are referred to as first graphite particles, and the graphite particles corresponding to the second peak on the side with larger internal porosity are referred to as second graphite particles. Here, the internal porosity of the first graphite particle is the internal porosity at the apex of the first peak, and the internal porosity of the second graphite particle is the internal porosity at the apex of the second peak.The mass proportions of the first graphite particles and the second graphite particles are determined based on the magnitude of the two peak areas.

[0045] As described above, the silicon-containing material contained as the negative electrode active material includes a first silicon-containing material and a second silicon-containing material having different particle expansion coefficients. The particle volume expansion coefficient of the silicon-containing material can be controlled, for example, by changing the proportion of Si contained, the density of the silicon-containing material, the particle size of the silicon-containing material, the particle shape of the silicon-containing material, etc. For example, when the silicon-containing material is a Si-containing composite material, the particle volume expansion coefficient can be controlled within a desired range by adjusting the size of the Si phase contained in the composite material, the proportion of the Si phase, the pore size and pore volume of the ion-conducting phase, etc.

[0046] 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 may satisfy the following relational expressions (1) and (2), respectively: 1.3≦V1<2.2 (1) 1.7≦V2<3 (2)

[0047] When 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 relationship (1) and (2), respectively, the stress caused by the expansion and contraction of the first silicon-containing material and the second silicon-containing material during charge and discharge can more efficiently form spaces. This further improves the permeability of the electrolyte into the negative electrode mixture layer 12, thereby further reducing the internal resistance of the battery. As described above, since the second silicon-containing material has a particle volume expansion coefficient greater than that of the first silicon-containing material, the relationship (1) and (2) are satisfied, assuming V1 < V2. V1 may satisfy 1.3 ≦ V1 < 2.0, and V2 may satisfy 2.0 ≦ V2 < 3.

[0048] 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 be, for example, 0.5<V1 / V2<0.8.

[0049] When the expansion coefficient of the first negative electrode mixture layer 13 during charging is A1 and the expansion coefficient of the second negative electrode mixture layer 14 during charging is A2, A1 and A2 may satisfy the following relational expression (3): A1>A2 (3)

[0050] Here, the expansion rate of the first anode mixture layer 13 during charging refers to the rate of increase in thickness of the first anode mixture layer 14 in a charged state relative to the thickness of the first anode mixture layer 13 in a discharged state. Also, the expansion rate of the second anode mixture layer 14 during charging refers to the rate of increase in thickness of the second anode mixture layer 14 in a charged state relative to the thickness of the second anode mixture layer 14 in a discharged state.

[0051] In this specification, the expansion coefficients during charge of the first anode mixture layer 13 and the second anode mixture layer 14 are measured by the following method. (1) A battery to be evaluated is disassembled, and the anode is cut out. A single-electrode cell is fabricated using metallic Li as a counter electrode and an ionic liquid as an electrolyte. (2) The single-electrode cell is charged at 0.1 C in a temperature environment of 25°C until the cell voltage reaches 5 mV, and then discharged at 0.1 C until the cell voltage reaches 1.0 V. The thickness (T1a) of the first anode mixture layer 13 and the thickness (T2a) of the second anode mixture layer 14 in a charged state (fully charged state) and the thickness (T1b) of the first anode mixture layer 13 and the thickness (T2b) of the second anode mixture layer 14 in a discharged state (fully discharged state) are determined. (3) The thickness increase rate (T1a×100 / T1b−100) of the first anode mixture layer 13 in the charged state relative to the thickness of the first anode mixture layer 13 in the discharged state is calculated, and this is defined as the expansion rate (%) of the first anode mixture layer 13 during charge. The thickness increase rate (T2a×100 / T2b−100) of the second anode mixture layer 14 in the charged state relative to the thickness of the second anode mixture layer 14 in the discharged state is calculated, and this is defined as the expansion rate (%) of the second anode mixture layer 14 during charge.

[0052] Here, the thickness of the first negative electrode mixture layer 13 and the thickness of the second negative electrode mixture layer 14 are determined from cross-sectional SEM images of each layer. Specifically, the thickness of each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 is measured at any five locations, and the average value calculated from the five measured values ​​is used as the thickness.

[0053] When the above relational expression (3) is satisfied, i.e., when the second anode mixture layer 14 has a larger expansion rate during charge than the first anode mixture layer 13, spaces are formed more efficiently by stress caused by expansion and contraction of the first silicon-containing material and the second silicon-containing material during charge and discharge, which further improves the permeability of the electrolyte into the anode mixture layer 12, thereby further reducing the internal resistance of the battery.

[0054] In order to further reduce the internal resistance of the battery, the expansion rate A1 of the first negative electrode mixture layer 13 during charging and the expansion rate A2 of the second negative electrode mixture layer 14 during charging may further satisfy the following relational expression (4): 1<A1 / A2<1.8 (4)

[0055] A1 / A2 may be, for example, 1.2<A1 / A2<1.8, or 1.3<A1 / A2<1.8.

[0056] The expansion coefficients of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 during charging can be controlled within a desired range, for example, by adjusting the content ratio of the first silicon-containing material and the second silicon-containing material and the porosity.

[0057] When 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, ε1 and ε2 may satisfy the following relational expression (5): 1.0<ε1 / ε2<2.0 (5)

[0058] When the above relational expression (5) is satisfied, the permeability of the electrolyte solution into the negative electrode mixture layer 12 is further improved, and the internal resistance of the battery can be further reduced.

[0059] 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 of carbon material contained and its content ratio, as well as the rolling conditions when producing the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14.

[0060] The true density of the first silicon-containing material may be lower than the true density of the second silicon-containing material, which can further reduce the internal resistance of the battery.

[0061] In each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, the content of the silicon-containing material is, for example, 40 mass% or less, or may be 35 mass% or less, or 30 mass% or less, of the total mass of the negative electrode active material, from the viewpoint of improving cycle characteristics. In each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, the content of the silicon-containing material is, for example, 5 mass% or more, of the total mass of the negative electrode active material, from the viewpoint of increasing capacity and improving cycle characteristics. An example of a suitable range of the content of the silicon-containing material is, for example, 5 mass% or more and 35 mass% or less, or 10 mass% or more and 30 mass% or less, or 5 mass% or more and 15 mass% or less, of the total mass of the negative electrode active material.

[0062] The negative electrode mixture layer 12 may further contain a binder. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and styrene-butadiene rubber (SBR). These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0063] The negative electrode mixture layer 12 may further contain a conductive agent. Examples of the conductive agent include carbon materials such as carbon black, such as acetylene black or ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, and graphene.

[0064] (Embodiment 2) A battery according to embodiment 2 includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode according to embodiment 1. With this configuration, the battery according to embodiment 2 can reduce internal resistance.

[0065] 2 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to embodiment 2. The battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed in the battery case and is in contact with the electrolyte.

[0066] The battery case is composed of a case body 25, which is a cylindrical metal container with a bottom, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is disposed between the case body 25 and the sealing body 26. The gasket 37 ensures the airtightness of the battery case. Within the case body 25, insulating plates 27 and 28 are disposed on both ends of the electrode group 24 in the direction of the winding axis of the electrode group 24, respectively.

[0067] Case body 25 has, for example, a step portion 31. Step portion 31 can be formed by partially pressing the side wall of case body 25 from the outside. Step portion 31 may be formed in an annular shape on the side wall of case body 25 along the circumferential direction of an imaginary circle defined by case body 25. In this case, sealing body 26 is supported by, for example, the surface of step portion 31 on the opening side.

[0068] Sealing body 26 includes a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. These components are stacked in this order in sealing body 26. Sealing body 26 is attached to the opening of case body 25 so that cap 36 is located on the outside of case body 25 and filter 32 is located on the inside of case body 25.

[0069] Each of the above-mentioned members constituting the sealing body 26 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 34, are electrically connected to one another.

[0070] The electrode group 24 has 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 the negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is disposed between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are spirally wound with the separator 22 interposed between these electrodes.

[0071] When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrodes 21 and negative electrodes 23 are stacked alternately in the radial direction of an imaginary circle defined by the case body 25, with a separator 22 interposed between these electrodes.

[0072] The positive electrode 21 is electrically connected to a cap 36, which also serves as a positive electrode terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, to near the center of the positive electrode 21 in the longitudinal direction of the positive electrode 21. The positive electrode lead 29 passes through a through-hole formed in the insulating plate 27 and extends from the positive electrode 21 to the filter 32. The other end of the positive electrode lead 29 is welded, for example, to the surface of the filter 32 on the electrode group 24 side.

[0073] The negative electrode 23 is electrically connected to the case body 25, which also serves as a negative electrode terminal, via a negative electrode lead 30. One end of the negative electrode lead 30 is connected to, for example, an 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 to, for example, the inner bottom surface of the case body 25.

[0074] Each component of the battery 100 will be specifically described below.

[0075] The positive electrode 21 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0076] The positive electrode current collector can be, for example, a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable as materials for the positive electrode current collector because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, or the like may be used as the sheet or film. A carbon material such as carbon may be applied to the surface of the positive electrode current collector as a conductive auxiliary material.

[0077] The positive electrode mixture layer includes a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing metal ions (e.g., lithium ions). Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce battery manufacturing costs and increase average discharge voltage. 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.

[0078] The positive electrode mixture layer may further contain a binder. As the binder, the materials described in the first embodiment as binders usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0079] The positive electrode mixture layer may further contain a conductive agent. As the conductive agent, the materials described in the first embodiment as conductive agents usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0080] The negative electrode 23 includes a material having the property of absorbing and releasing metal ions (for example, lithium ions). The negative electrode 23 is the negative electrode 10 according to the first embodiment.

[0081] 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 / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0082] The non-aqueous solvent may be a cyclic carbonate, a chain carbonate, a cyclic ether, a chain ether, a nitrile, an amide, etc. One selected from these solvents may be used, or two or more may be used in combination.

[0083] Examples of lithium salts that can be used include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these electrolyte salts may be used, or two or more may be used in combination.

[0084] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 22 has high ion permeability and adequate mechanical strength and insulating properties. The separator 22 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 22 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0085] In the battery according to the second embodiment, the electrolyte may be impregnated into a polymer provided as a separator, for example, i.e., the battery according to the second embodiment may have a structure in which the electrolyte and the polymer are used in combination.

[0086] The battery according to the second embodiment may further include a solid electrolyte as the electrolyte. That is, the battery according to the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In the present disclosure, the term "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. The term "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. The term "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The term "main component of the anions" refers to the anion with the largest mass among all the anions constituting the solid electrolyte.

[0087] As an example of the structure of the battery according to the second embodiment, the configuration example shown in FIG. 2 is described, i.e., a cylindrical nonaqueous electrolyte secondary battery in which a wound electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are housed in an outer casing. However, the battery according to the present disclosure is not limited to this configuration example. The battery according to the second embodiment may have any shape, such as a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of the wound electrode group in the battery according to the second embodiment, an electrode group of another shape, such as an electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be used.

[0088] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0089] (Technology 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, wherein the negative electrode mixture layer includes a first negative electrode mixture layer including a 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, wherein the silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a particle volume expansion coefficient greater than that of the first silicon-containing material, and wherein when the negative electrode active material contained in the first negative electrode mixture layer is defined as a first negative electrode active material and the negative electrode active material contained in the second negative electrode mixture layer is defined as a second negative electrode active material, a mass ratio of the first silicon-containing material in the second negative electrode active material is greater than a mass ratio of the first silicon-containing material in the first negative electrode active material, a negative electrode, wherein the mass proportion of the second silicon-containing material in the first negative electrode active material is greater than the mass proportion of the second silicon-containing material in 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, wherein 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 a charged state to the particle volume of the first silicon-containing material in a 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 a charged state to the particle volume of the second silicon-containing material in a discharged state.

[0090] With this configuration, the negative electrode according to the first technique can reduce the internal resistance of the battery.

[0091] (Technology 2) The negative electrode according to Technology 1, wherein the negative electrode mixture layer further contains a carbon material as a negative electrode active material.

[0092] With this configuration, the negative electrode according to Technique 2 can further reduce the internal resistance of the battery.

[0093] (Technology 3) The negative electrode according to Technology 2, wherein the carbon material includes first graphite particles and second graphite particles, the first graphite particles have an internal porosity smaller than an internal porosity of the second graphite particles, and the mass ratio of the first graphite particles to the total of the first graphite particles and the second graphite particles is larger in the first negative electrode mixture layer than in the second negative electrode mixture layer.

[0094] With this configuration, the negative electrode according to Technique 3 can further reduce the internal resistance of the battery.

[0095] (Technology 4) The negative electrode according to any one of Technologies 1 to 3, wherein 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, and V1 and V2 satisfy the following relational expressions (1) and (2): 1.3≦V1<2.2 (1) 1.7≦V2<3 (2)

[0096] With this configuration, the negative electrode according to Technique 4 can further reduce the internal resistance of the battery.

[0097] (Technology 5) The negative electrode according to any one of Technologies 1 to 4, wherein, when the expansion coefficient during charge of the first negative electrode mixture layer is A1 and the expansion coefficient during charge of the second negative electrode mixture layer is A2, A1 and A2 satisfy the following relational expression (3): A1>A2 (3) Here, the expansion coefficient during charge of the first negative electrode mixture layer is a thickness increase rate of the first negative electrode mixture layer in a charged state relative to the thickness of the first negative electrode mixture layer in a discharged state, and the expansion coefficient during charge of the second negative electrode mixture layer is a thickness increase rate of the second negative electrode mixture layer in a charged state relative to the thickness of the second negative electrode mixture layer in a discharged state.

[0098] With this configuration, the negative electrode according to Technique 5 can further reduce the internal resistance of the battery.

[0099] (Technology 6) The negative electrode according to Technology 5, wherein A1 and A2 satisfy the following relational expression (4): 1<A1 / A2<1.8 (4)

[0100] With this configuration, the negative electrode according to Technique 6 can further reduce the internal resistance of the battery.

[0101] (Technology 7) The negative electrode according to any one of Technologies 1 to 6, wherein, when the porosity of the first negative electrode mixture layer is ε1 and the porosity of the second negative electrode mixture layer is ε2, the ε1 and the ε2 satisfy the following relational expression (5): 1.0<ε1 / ε2<2.0 (5)

[0102] With this configuration, the negative electrode according to Technique 7 can further reduce the internal resistance of the battery.

[0103] (Technology 8) The negative electrode according to any one of Technologies 1 to 7, wherein the true density of the first silicon-containing material is lower than the true density of the second silicon-containing material.

[0104] With this configuration, the negative electrode according to Technology 8 can further reduce the internal resistance of the battery.

[0105] (Technology 9) The negative electrode according to any one of Technologies 1 to 8, 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 including an ion-conducting phase and a Si phase dispersed in the ion-conducting phase, and 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.

[0106] With this configuration, the negative electrode according to Technique 9 can further reduce the internal resistance of the battery.

[0107] (Technology 10) The anode according to any one of Technologies 1 to 9, wherein the anode mixture layer is divided into two equal parts in a thickness direction to form two divided layers, the divided layer located on a surface side of the anode mixture layer is the first anode mixture layer, and the divided layer located on a side of the anode current collector is the second anode mixture layer.

[0108] With this configuration, the negative electrode according to Technique 10 can reduce the internal resistance of the battery.

[0109] (Technology 11) A battery comprising: the negative electrode according to any one of Technologies 1 to 10; a positive electrode; and an electrolyte.

[0110] With this configuration, the battery according to Technique 11 can further reduce the internal resistance.

[0111] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0112] [Example 1] (Preparation of Positive Electrode Active Material) [Ni 0.88 Co 0.09 Al 0.03 ] (OH)2 was calcined at 500°C for 8 hours to form an oxide (Ni 0.88 Co 0.09 Al 0.03 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 placed under an oxygen stream (10 cm) with an oxygen concentration of 95%. 3 The mixture was heated from room temperature to 650°C at a rate of 2.0°C / min, and then heated from 650°C to 780°C at a rate of 0.5°C / min to obtain LiNi. 0.88 Co 0.09 Al 0.03 A lithium-containing composite oxide represented by O2 was obtained.

[0113] (Preparation of Positive Electrode) The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of positive electrode active material: acetylene black: polyvinylidene fluoride = 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, and the coating was dried and compressed. The positive electrode current collector was then cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was disposed on both sides of the positive electrode current collector. An exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode current collector.

[0114] (Preparation of First Silicon-Containing Material) Tetraethylorthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) were mixed in an ethanol / water / ammonia mixed solution to prepare CTAB-modified SiO nanoparticles. Resorcinol, formaldehyde, and a surfactant (Pluronic F-127) were added to the mixture and polymerized to obtain polymer particles encapsulating the aforementioned SiO nanoparticles. 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, the polymer particles were carbonized at 800°C in a nitrogen atmosphere, mixed with magnesium powder, and heated at 650°C in an argon atmosphere to undergo a magnesium thermal reduction reaction. MgO was dissolved from the particles after the reaction in a mixed solution of HCl / HO / ethanol, washed with ethanol, and then dried to produce a mesoporous silicon-containing material containing Si and C and having an average particle size of 8 μm. This silicon-containing material was designated as the first silicon-containing material.

[0115] (Preparation of second silicon-containing material) Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon raw material and raw silicon (3N, average particle size 10 μm) were mixed in a mass ratio of coal pitch: raw silicon = 2:1, and the mixture was pulverized and micronized using a planetary ball mill (P-5, manufactured by Fritsch). Next, the micronized powder mixture was fired in an inert atmosphere to carbonize the carbon source and obtain a sintered product in which a Si phase was dispersed within an amorphous carbon phase. Thereafter, the sintered product was pulverized using a jet mill to obtain composite particles as a second silicon-containing material having an average particle size of 8 μm.

[0116] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 91:3:6 to form the negative electrode active material for the first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0117] In the negative electrode mixture slurry for the first negative electrode mixture layer, a mixture of artificial graphite as the first graphite particles having a smaller internal porosity and natural graphite as the second graphite particles having a larger internal porosity was used as the graphite. For the graphite used in the negative electrode mixture slurry for the first negative electrode mixture layer, the mass ratio of the first graphite particles to the total of the first graphite particles and the second graphite particles was 70 mass%, as shown in Table 2. In the following examples and comparative examples, the same artificial graphite as in Example 1 was used as the first graphite particles, and the same natural graphite as in Example 1 was used as the second graphite particles. Table 2 shows the mass ratio of the first graphite particles to the total of the first graphite particles and the second graphite particles for the graphite in the negative electrode mixture slurry for the first negative electrode mixture layer and the negative electrode mixture slurry for the second negative electrode mixture layer in each of the following examples and comparative examples.

[0118] Graphite, the first silicon-containing material, and the second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material=91:6:3 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0119] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0120] For each of the first negative electrode mixture layer and the second negative electrode mixture layer, the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charging ratio are shown in Table 1. In Table 1, the first silicon-containing material is referred to as "first Si" and the second silicon-containing material is referred to as "second Si."

[0121] (Preparation of non-aqueous electrolyte (electrolyte solution)) A non-aqueous electrolyte solution was prepared by dissolving LiPF at a concentration of 1.2 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio (25°C) of EC:EMC:DMC = 3:3:4.

[0122] (Preparation of Test Cell (Secondary Battery)) An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator between them to prepare a wound electrode body. Insulating plates were placed on the top and bottom of the electrode body, and the electrode body was housed in an outer can. The negative electrode lead was welded to the bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member. An electrolyte was poured into the outer can, and the opening of the outer can was sealed with a sealing member via a gasket to prepare a secondary battery as a test cell.

[0123] 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.

[0124] (Preparation of First Silicon-Containing Material and 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.

[0125] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0126] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0127] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0128] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0129] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0130] (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 Example 2 was used.

[0131] 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.

[0132] (Preparation of First Silicon-Containing Material and 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.

[0133] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0134] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0135] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.56. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode. Note that Example 3 differed from Example 2 in the linear pressure applied when compressing the coating film.

[0136] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0137] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0138] (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 Example 3 was used.

[0139] 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.

[0140] (Preparation of First Silicon-Containing Material and 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.

[0141] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0142] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0143] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.54. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode. Note that Example 4 differed from Examples 2 and 3 in the linear pressure applied when compressing the coating film.

[0144] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0145] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0146] (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 Example 4 was used.

[0147] 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.

[0148] (Preparation of First Silicon-Containing Material and 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.

[0149] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0150] Graphite and the second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0151] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.53. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode. Note that Example 5 differed from Examples 2 to 4 in the linear pressure applied when compressing the coating film.

[0152] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0153] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0154] (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 Example 5 was used.

[0155] 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.

[0156] (Preparation of First Silicon-Containing Material and 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.

[0157] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 86:3:11 to form the negative electrode active material for the first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0158] Graphite, the first silicon-containing material, and the second silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material:second silicon-containing material = 86:11:3 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0159] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0160] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0161] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0162] (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 Example 6 was used.

[0163] 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.

[0164] (Preparation of First Silicon-Containing Material and 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.

[0165] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 86:14 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0166] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 86:14 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0167] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0168] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0169] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0170] (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 Example 7 was used.

[0171] Example 8 (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.

[0172] (Preparation of First Silicon-Containing Material) Tetraethylorthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) were mixed in an ethanol / water / ammonia mixed solution to prepare CTAB-modified SiO nanoparticles. Resorcinol, formaldehyde, and a surfactant (Pluronic F-127) were added to the mixture and polymerized to obtain polymer particles encapsulating the aforementioned SiO nanoparticles. The molar ratio of surfactant to resorcinol (surfactant / resorcinol) was set to 0.003, and the mass ratio of resorcinol to TEOS (resorcinol / TEOS) was set to approximately 0.5 / 1. After drying, the polymer particles were carbonized at 800°C in a nitrogen atmosphere, mixed with magnesium powder, and heated at 650°C in an argon atmosphere to undergo a magnesium thermal reduction reaction. MgO was dissolved from the particles after the reaction in a mixed solution of HCl / HO / ethanol, washed with ethanol, and then dried to produce a mesoporous silicon-containing material containing Si and C and having an average particle size of 8 μm. This silicon-containing material was designated as the first silicon-containing material.

[0173] (Preparation of Second Silicon-Containing Material) A second silicon-containing material was prepared in the same manner as in Example 1.

[0174] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0175] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0176] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0177] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0178] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0179] (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 Example 8 was used.

[0180] Example 9 (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.

[0181] (Preparation of First Silicon-Containing Material) A first silicon-containing material was prepared in the same manner as in Example 8.

[0182] (Preparation of second silicon-containing material) Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon raw material and raw silicon (3N, average particle size 12 μm) were mixed in a mass ratio of coal pitch: raw silicon = 2:1, and the mixture was pulverized and micronized using a planetary ball mill (P-5, manufactured by Fritsch). Next, the micronized powder mixture was fired in an inert atmosphere to carbonize the carbon source and obtain a sintered product in which a Si phase was dispersed within an amorphous carbon phase. Thereafter, the sintered product was pulverized using a jet mill to obtain composite particles as a second silicon-containing material having an average particle size of 8 μm.

[0183] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0184] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0185] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0186] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0187] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0188] (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 Example 9 was used.

[0189] Example 10 (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 First Silicon-Containing Material) A first silicon-containing material was prepared in the same manner as in Example 8.

[0191] (Preparation of second silicon-containing material) Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon raw material and raw silicon (3N, average particle size 8 μm) were mixed in a mass ratio of coal pitch: raw silicon = 2:1, and the mixture was pulverized and micronized using a planetary ball mill (P-5, manufactured by Fritsch). Next, the micronized powder mixture was fired in an inert atmosphere to carbonize the carbon source and obtain a sintered product in which a Si phase was dispersed within an amorphous carbon phase. Thereafter, the sintered product was pulverized using a jet mill to obtain composite particles as a second silicon-containing material having an average particle size of 8 μm.

[0192] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0193] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0194] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0195] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0196] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0197] (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 Example 10 was used.

[0198] Example 11 (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.

[0199] (Preparation of First Silicon-Containing Material) Tetraethylorthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) were mixed in an ethanol / water / ammonia mixed solution to prepare CTAB-modified SiO nanoparticles. Resorcinol, formaldehyde, and a surfactant (Pluronic F-127) were added to the mixture and polymerized to obtain polymer particles encapsulating the aforementioned SiO nanoparticles. The molar ratio of surfactant to resorcinol (surfactant / resorcinol) was set to 0.006, and the mass ratio of resorcinol to TEOS (resorcinol / TEOS) was set to approximately 0.5 / 1. After drying, the polymer particles were carbonized at 800°C in a nitrogen atmosphere, mixed with magnesium powder, and heated at 650°C in an argon atmosphere to undergo a magnesium thermal reduction reaction. MgO was dissolved from the particles after the reaction in a mixed solution of HCl / HO / ethanol, washed with ethanol, and then dried to produce a mesoporous silicon-containing material containing Si and C and having an average particle size of 8 μm. This silicon-containing material was designated as the first silicon-containing material.

[0200] (Preparation of second silicon-containing material) Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon raw material and raw silicon (3N, average particle size 13 μm) were mixed in a mass ratio of coal pitch: raw silicon = 2:1, and the mixture was pulverized and micronized using a planetary ball mill (P-5, manufactured by Fritsch). Next, the micronized powder mixture was fired in an inert atmosphere to carbonize the carbon source and obtain a sintered product in which a Si phase was dispersed within an amorphous carbon phase. Thereafter, the sintered product was pulverized using a jet mill to obtain composite particles as a second silicon-containing material having an average particle size of 8 μm.

[0201] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0202] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0203] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0204] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0205] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0206] (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 Example 11 was used.

[0207] 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.

[0208] (Preparation of First Silicon-Containing Material and 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.

[0209] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the 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 prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0210] Next, the prepared negative electrode mixture layer 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 where the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0211] For the negative electrode mixture layer, the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charging ratios are shown in Table 1.

[0212] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0213] (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 1 was used.

[0214] 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.

[0215] (Preparation of First Silicon-Containing Material and 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.

[0216] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the 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 prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0217] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.48. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0218] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0219] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0220] (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 2 was used.

[0221] 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.

[0222] (Preparation of First Silicon-Containing Material and 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.

[0223] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the 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 prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0224] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.5. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0225] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0226] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0227] (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 3 was used.

[0228] 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.

[0229] (Preparation of First Silicon-Containing Material and 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.

[0230] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the 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 a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0231] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.50. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in part of the negative electrode.

[0232] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0233] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0234] (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 4 was used.

[0235] 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.

[0236] (Preparation of First Silicon-Containing Material and 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.

[0237] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the 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 prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0238] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0239] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0240] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0241] (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 5 was used.

[0242] 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.

[0243] (Preparation of First Silicon-Containing Material) The first silicon-containing material was prepared in the same manner as in Example 1. In Comparative Example 6, only the first silicon-containing material was used, and the second silicon-containing material was not used.

[0244] (Preparation of Negative Electrode) Graphite and a first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0245] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0246] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0247] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0248] (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 6 was used.

[0249] 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.

[0250] (Preparation of second silicon-containing material) The second silicon-containing material was prepared in the same manner as in Example 1. In Comparative Example 7, only the second silicon-containing material was used, and the first silicon-containing material was not used.

[0251] (Preparation of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0252] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.52. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0253] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0254] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0255] (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.

[0256] Comparative Example 8 (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.

[0257] (Preparation of First Silicon-Containing Material and 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.

[0258] (Fabrication of Negative Electrode) Graphite and a second silicon-containing material were mixed in a mass ratio of graphite:second silicon-containing material = 91:9 to form a negative electrode active material for a first negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer.

[0259] Graphite and the first silicon-containing material were mixed in a mass ratio of graphite:first silicon-containing material = 91:9 to form the negative electrode active material for the second negative electrode mixture layer. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer.

[0260] Next, the prepared negative electrode mixture layer 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 a second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer to the slurry for the second negative electrode mixture layer was 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.5. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

[0261] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0262] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0263] (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 8 was used.

[0264] Comparative Example 9 (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.

[0265] (Preparation of First Silicon-Containing Material and 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.

[0266] (Preparation of Negative Electrode) Graphite, the first silicon-containing material, and the 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 a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0267] Next, a portion of the prepared negative electrode mixture layer 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 second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, and the coating was dried and compressed with a linear pressure different from that used for forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was first negative electrode mixture layer:second negative electrode mixture layer = 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. The value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.58. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in part of the negative electrode.

[0268] Table 1 shows the mass proportions of the first silicon-containing material and the second silicon-containing material calculated from the charge ratios for the first negative electrode mixture layer and the second negative electrode mixture layer, respectively.

[0269] (Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)) A non-aqueous electrolyte solution was prepared in the same manner as in Example 1.

[0270] (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 9 was used.

[0271] [Measurement of particle volume expansion coefficient of the first silicon-containing material and the second silicon-containing material] For the negative electrodes of each example and comparative example, the particle volume expansion coefficient of the first silicon-containing material and the second silicon-containing material was measured using the method described in embodiment 1. However, instead of disassembling the battery and cutting out the negative electrode, a single-electrode cell was produced using the produced negative electrode, and the particle volume expansion coefficient of the first silicon-containing material and the second silicon-containing material was measured using this single-electrode cell. The results are shown in Table 1.

[0272] [Measurement of the expansion coefficient during charge of the first negative electrode mixture layer and the second negative electrode mixture layer] For the negative electrodes of each Example and Comparative Example, the expansion coefficient during charge of the first negative electrode mixture layer and the second negative electrode mixture layer 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 produced using the produced negative electrode, and the expansion coefficient during charge of the first negative electrode mixture layer and the second negative electrode mixture layer was determined using this single-electrode cell. The results are shown in Table 2.

[0273] [Measurement of porosity of first negative electrode mixture layer and second negative electrode mixture layer] For the negative electrodes of each example and comparative example, the porosity of the first negative electrode mixture layer and the second negative electrode mixture layer 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 negative electrode mixture layer and the second negative electrode mixture layer was determined using the fabricated negative electrode. The results are shown in Table 2.

[0274] [Measurement of Direct Current Internal Resistance (DC-IR)] Using the test cells of each Example and Comparative Example, the DC-IR of the cells was measured by the following method.

[0275] The test cells of each example and comparative example were initially charged and discharged under the following conditions. (Charge and Discharge Conditions) Initial charge and discharge conditions: Constant current charging was performed at a current of 0.5 It (625 mA) until the battery voltage reached 4.2 V. This was followed by constant voltage charging at a voltage of 4.2 V until the current reached 0.02 It (25 mA). Constant current discharging was then performed at a current of 0.5 It (625 mA) until the battery voltage reached 2.5 V.

[0276] Next, the batteries were charged and discharged under the following conditions, and the initial DC internal resistance (DC-IR) was measured using the following formula (a). The results are shown in Table 2.

[0277] (Charge / Discharge Conditions) At a temperature of 25° C., constant current charging was performed at a current of 0.3 It (375 mA) until the battery voltage reached 3.79 V. Further, 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 pause, the battery was discharged at a current of 0.5 It (625 mA) for 10 seconds.

[0278] (Calculation formula for DC-IR) DC-IR (mΩ) = (voltage immediately before the start of discharge - voltage 10 seconds after the start of discharge) / (discharge current density x electrode area) (a)

[0279] The results are shown in Table 2. Table 2 shows relative values ​​when the DC-IR value of Comparative Example 1 is taken as the reference (100%).

[0280]

[0281]

[0282] (Discussion) As shown in Table 2, the internal resistance of all of the test cells of the examples was lower than that of the test cells of comparative examples 1 to 9. That is, a negative electrode having a configuration in which two types of silicon-containing materials, a first silicon-containing material having a smaller particle volume expansion coefficient and a second silicon-containing material having a larger particle volume expansion coefficient, were included as negative electrode active materials, the mass ratio of the second silicon-containing material in the first negative electrode mixture layer located on the surface layer side was larger than the mass ratio of the second silicon-containing material in the second negative electrode mixture layer located on the current collector side, the mass ratio of the first silicon-containing material in the second negative electrode mixture layer was larger than the mass ratio of the first silicon-containing material in the first negative electrode mixture layer, and the porosity of the first negative electrode mixture layer was larger than the porosity of the second negative electrode mixture layer, was able to reduce the internal resistance of the battery.

[0283] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries that require high capacity.

Claims

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, wherein the negative electrode mixture layer includes a first negative electrode mixture layer including a 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, wherein the silicon-containing material includes a first silicon-containing material and a second silicon-containing material having a particle volume expansion coefficient greater than that of the first silicon-containing material, and wherein when the negative electrode active material contained in the first negative electrode mixture layer is defined as a first negative electrode active material and the negative electrode active material contained in the second negative electrode mixture layer is defined as a second negative electrode active material, a mass ratio of the first silicon-containing material in the second negative electrode active material is greater than a mass ratio of the first silicon-containing material in the first negative electrode active material, a negative electrode, wherein the mass proportion of the second silicon-containing material in the first negative electrode active material is greater than the mass proportion of the second silicon-containing material in 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, wherein 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 a charged state to the particle volume of the first silicon-containing material in a 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 a charged state to the particle volume of the second silicon-containing material in a discharged state.

2. The negative electrode according to claim 1, wherein the negative electrode mixture layer further contains a carbon material as a negative electrode active material.

3. The negative electrode according to claim 2, wherein the carbon material includes first graphite particles and second graphite particles, the internal porosity of the first graphite particles is smaller than the internal porosity of the second graphite particles, and the mass ratio of the first graphite particles to the total of the first graphite particles and the second graphite particles is greater in the first negative electrode mixture layer than in the second negative electrode mixture layer.

4. The negative electrode according to claim 1, wherein 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, and V1 and V2 satisfy the following relationship (1) and (2): 1.3≦V1<2.2 (1) 1.7≦V2<3 (2) 5. The negative electrode according to claim 1, wherein, when the expansion coefficient during charge of the first negative electrode mixture layer is A1 and the expansion coefficient during charge of the second negative electrode mixture layer is A2, A1 and A2 satisfy the following relational expression (3): A1>A2 (3) Here, the expansion coefficient during charge of the first negative electrode mixture layer is the rate of increase in thickness of the first negative electrode mixture layer in a charged state relative to the thickness of the first negative electrode mixture layer in a discharged state, and the expansion coefficient during charge of the second negative electrode mixture layer is the rate of increase in thickness of the second negative electrode mixture layer in a charged state relative to the thickness of the second negative electrode mixture layer in a discharged state.

6. The negative electrode according to claim 5, wherein A1 and A2 satisfy the following relational expression (4): 1<A1 / A2<1.8 (4) 7. The negative electrode according to claim 1, wherein, 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 relational expression (5): 1.0<ε1 / ε2<2.0 (5) 8. The negative electrode according to claim 1, wherein the true density of the first silicon-containing material is less than the true density of the second silicon-containing material.

9. The negative electrode according to claim 1, 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 including an ion-conducting phase and a Si phase dispersed in the ion-conducting phase, and 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.

10. The anode according to claim 1, wherein the anode mixture layer is divided into two equal parts in the thickness direction to produce two divided layers, the divided layer located on the surface side of the anode mixture layer is the first anode mixture layer, and the divided layer located on the anode current collector side is the second anode mixture layer.

11. A battery comprising: the negative electrode according to any one of claims 1 to 10; a positive electrode; and an electrolyte.

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