Secondary batteries

Abstract

Disclosed is a secondary battery including: a positive electrode; and a negative electrode that includes a negative electrode active material. The negative electrode active material includes a carbon material and a silicon-containing material. The silicon-containing material includes a first silicon-containing material and a second silicon-containing material. The average particle size of the first silicon-containing material and the average particle size of the second silicon-containing material are both smaller than the average particle size of the carbon material. The average circularity Zc of the carbon material, the average circularity Zs1 of the first silicon-containing material, and the average circularity Zs2 of the second silicon-containing material satisfy Zs1<Zc and Zs1<Zs2.

Description

[Technical Field]

[0001] This disclosure relates to secondary batteries. [Background technology]

[0002] Silicon-containing materials, such as silicon (Si) and silicon oxides, are known to be able to absorb more lithium ions per unit volume compared to carbon materials such as graphite. Therefore, the use of silicon-containing materials as the negative electrode in lithium-ion secondary batteries has been proposed.

[0003] Patent Document 1 (Japanese Unexamined Patent Publication No. 2016-110969) discloses "a negative electrode active material for a lithium-ion secondary battery comprising Si or a Si alloy and a carbonaceous material or a carbonaceous material and graphite, characterized in that the average particle size (D50) of the negative electrode active material is 1 to 40 μm and the average circularity is 0.7 to 1.0 and the negative electrode active material is a substantially spherical composite particle." [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2016-110969 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Currently, there is a demand for further improvements in the charge-discharge cycle characteristics of secondary batteries. In this context, one of the objectives of this disclosure is to provide a secondary battery with a high capacity retention rate during charge-discharge cycles. [Means for solving the problem]

[0006] One aspect of the present disclosure relates to a secondary battery. The secondary battery includes a positive electrode and a negative electrode containing a negative electrode active material. The negative electrode active material includes a carbon material and a silicon-containing material. The silicon-containing material includes a first silicon-containing material and a second silicon-containing material. The average particle diameter of the first silicon-containing material and the average particle diameter of the second silicon-containing material are each smaller than the average particle diameter of the carbon material. The average roundness Zc of the carbon material, the average roundness Zs1 of the first silicon-containing material, and the average roundness Zs2 of the second silicon-containing material satisfy Zs1 < Zc and Zs1 < Zs2.

Advantages of the Invention

[0007] According to the present disclosure, a secondary battery with a high capacity retention rate in charge-discharge cycles can be obtained. The novel features of the present invention are described in the appended claims. However, the present invention relates to both the configuration and the content, and will be better understood by the following detailed description in combination with the drawings, together with other objects and features of the present invention.

Brief Description of the Drawings

[0008] [Figure 1] It is a cross-sectional view schematically showing a silicon-containing material according to an embodiment of the present invention. [Figure 2] It is a schematic perspective view of a part of a secondary battery according to an embodiment of the present invention with a cutout.

Modes for Carrying Out the Invention

[0009] Hereinafter, embodiments according to the present disclosure will be described with examples, but the embodiments according to the present disclosure are not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and other materials may be applied as long as the invention according to the present disclosure can be implemented. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit are exemplified for numerical values such as specific physical properties and conditions, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be made as long as the lower limit is not more than the upper limit.

[0010] (Secondary battery) The secondary battery according to this embodiment includes a positive electrode and a negative electrode containing a negative electrode active material. The negative electrode active material includes a carbon material and a silicon-containing material. The silicon-containing material includes a first silicon-containing material and a second silicon-containing material. Hereinafter, the carbon material and the silicon-containing material may be referred to as "carbon material (C)" and "silicon-containing material (S)", respectively. The secondary battery according to this embodiment satisfies the following conditions (1) and (2). (1) The average particle diameter of the first silicon-containing material and the average particle diameter of the second silicon-containing material are each smaller than the average particle diameter of the carbon material (C). (2) The average circularity Zc of the carbon material (C), the average circularity Zs1 of the first silicon-containing material, and the average circularity Zs2 of the second silicon-containing material satisfy Zs1 < Zc and Zs1 < Zs2.

[0011] The silicon-containing material (S) may contain carbon, but the carbon material in the silicon-containing material (S) is not included in the carbon material (C) and is included in the silicon-containing material (S). Also, in this specification, the description of the silicon-containing material (S) can be applied to both the first silicon-containing material and the second silicon-containing material.

[0012] The silicon-containing material (S) and the carbon material (C) are each present in the negative electrode in a particulate state. Generally, the circularity of particles is obtained by the following formula (M1). Circularity = 4πS / L2 (M1) [In equation (M1), S represents the area of ​​the particle's projection image, and L represents the perimeter of that projection image.]

[0013] In this specification, when measuring the circularity of the silicon-containing material (S) and carbon material (C) in the negative electrode, the circularity of the silicon-containing material (S) and carbon material (C) in the cross-section of the negative electrode is measured. In this case, the area of ​​the particle cross-section is used as S in equation (M1), and the perimeter of the cross-section is used as L in equation (M1). In one view, the circularity measured in this manner is the circularity of the cross-section of the silicon-containing material (S) and carbon material (C).

[0014] The values ​​S (area of ​​the particle's cross-section) and L (perimeter of the particle's cross-section) used in equation (M1) are obtained by the following method. First, the battery is disassembled and the negative electrode is removed, exposing the cross-section of the negative electrode. Next, the cross-section is photographed with a scanning electron microscope to obtain an image. Then, energy-dispersive X-ray spectroscopy (EDS) or the like is used to distinguish between carbon material and silicon-containing material in the image. By performing image analysis on the particle images (cross-sections of particles) in the image, the area of ​​the particle's cross-section and the perimeter of the particle's cross-section are obtained.

[0015] The average circularity is calculated by measuring the circularity of 100 arbitrarily selected particles and taking the arithmetic mean of the resulting circularity values. If the projected image of a particle is a perfect circle, the circularity is 1. Therefore, it can be assumed that the higher the circularity, the closer the particle is to a perfect sphere.

[0016] The average particle size of each particle, such as silicon-containing material (S) and carbon material (C), can also be determined by image analysis. In this case, first, 100 arbitrary particles in the image are selected and their maximum diameters are measured. Next, the arithmetic mean of the measured maximum diameters of the 100 particles is taken, and the resulting average value is taken as the average particle size.

[0017] For particles that can be separated individually (e.g., particles before forming the negative electrode mixture), the median diameter (D50) at which the cumulative volume in the volume-based particle size distribution reaches 50% can be used as the average particle size. The median diameter can be determined, for example, using a laser diffraction / scattering particle size distribution analyzer.

[0018] The average circularity of particles that can be separated individually (for example, particles before the formation of the negative electrode mixture) can be measured by the following method. First, the particles to be measured are dispersed in a resin. Next, the cross-section of the resin is exposed, and an image is obtained by photographing the cross-section with a scanning electron microscope. Then, the area of ​​the cross-section of the particle and the perimeter of the cross-section of the particle are obtained by image analysis of the particle image (cross-section of the particle) in the image.

[0019] Increasing the circularity of the silicon-containing material (S) is desirable, for example, because the rounded shape increases bulk density, which in turn increases packing density when used as a negative electrode. However, the inventors of this invention have found that it is difficult to achieve high charge-discharge cycle characteristics when using only silicon-containing material (S) with high circularity. As a result of repeated studies to solve this problem, the inventors of this invention have newly discovered that a secondary battery with high charge-discharge cycle characteristics can be obtained by satisfying the above conditions (1) and (2). This disclosure is based on this new finding.

[0020] The reason why high cycle characteristics can be achieved by satisfying the above conditions (1) and (2) is not currently clear. However, it is possible to consider the following: Generally, silicon-containing materials have lower conductivity than carbon materials. Therefore, when silicon-containing materials (S) and carbon materials (C) are used in a mixture, it is important that there are many contact points between the two in order to form many conductive paths. In particular, when charge-discharge cycles are repeated, the silicon-containing materials (S) and carbon materials (C) expand and contract repeatedly, making it easy for the contact points between them to break. When the above conditions (1) and (2) are satisfied, it is thought that the silicon-containing materials (S) can easily enter the gaps between carbon materials (C), and the number of contact points between them increases. As a result, it is thought that conductive paths are maintained even when charge-discharge cycles are repeated, and high charge-discharge cycle characteristics are achieved.

[0021] Furthermore, the negative electrode of the secondary battery of this disclosure includes two types of silicon-containing materials with different average circularities. Compared to silicon-containing materials with low average circularities, silicon-containing materials with high average circularities are less prone to stress concentration due to expansion and contraction associated with charging and discharging, thus reducing particle cracking and preventing a high specific surface area. Therefore, using silicon-containing materials with high average circularities can suppress an increase in side reactions of the silicon-containing material. On the other hand, using silicon-containing materials with low average circularities makes it easier to maintain the conductive path even after repeated charge-discharge cycles. Therefore, by using two types of silicon-containing materials with different average circularities, the cycle characteristics can be particularly improved.

[0022] Furthermore, when conditions (1) and (2) above are met, voids are formed between the particles of the larger carbon material (C), and the silicon-containing material (S) is easily accommodated in these voids. This makes it easier to increase the packing density of the active material in the negative electrode and to obtain a high-capacity negative electrode.

[0023] The average circularity of the carbon material (C) may be 0.7 or higher, or 0.8 or higher, and is 1 or less.

[0024] The average circularity Zs1 of the first silicon-containing material may be 0.6 or less, or 0.5 or less, and may be 0.2 or more, 0.3 or more, or 0.4 or more. In one example of a negative electrode, the average circularity Zc of the carbon material (C) is 0.7 or more, and the average circularity Zs1 of the first silicon-containing material is 0.6 or less. With this configuration, the charge-discharge cycle characteristics are particularly good.

[0025] The average particle size of the carbon material (C) may be 5 μm or more, or 10 μm or more, or 50 μm or less, or 30 μm or less. The average particle size of the silicon-containing material (S) may be 1 μm or more, or 5 μm or more, or 20 μm or less, 15 μm or less, or 10 μm or less. In one example of a negative electrode, the average particle size of the carbon material (C) is in the range of 10 to 30 μm, and the average particle sizes of the first silicon-containing material and the second silicon-containing material are in the range of 1 to 15 μm, respectively. With this configuration, the charge-discharge cycle characteristics are particularly good.

[0026] The value of (average particle size of silicon-containing material (S)) / (average particle size of carbon material (C)) is less than 1 and may be 0.8 or less, 0.6 or less, or 0.5 or less. There is no particular lower limit to this value, but it may be 0.05 or more, 0.1 or more, or 0.2 or more. Setting this value to 0.5 or less results in particularly good charge-discharge cycle characteristics.

[0027] The ratio Zs1 / Zs2 between the mean circularity Zs1 and the mean circularity Zs2 may be 0.4 or greater, or 0.5 or greater, or 0.9 or less, or 0.8 or less. For example, the ratio Zs1 / Zs2 may be in the range of 0.4 to 0.9.

[0028] The ratio of average circularity Zs1 to average circularity Zc, Zs1 / Zc, may be 0.4 or greater, or 0.5 or greater, or 0.9 or less, or 0.8 or less. For example, the ratio Zs1 / Zc may be in the range of 0.4 to 0.9. Furthermore, the average circularity Zs2 may be less than the average circularity Zc, or greater than or equal to the average circularity Zc.

[0029] A preferred negative electrode example satisfies at least one of the following conditions (J1) to (J4). For example, it may satisfy (J1) and (J2), or (J1) and (J2) in addition to (J3) and / or (J4). (J1) The average circularity Zc of carbon material (C) is 0.7 or higher, and the average circularity Zs2 of silicon-2 material is 0.6 or lower. (J2) The average particle size of the carbon material (C) is in the range of 10 to 30 μm, and the average particle size of the silicon-containing material (S) is in the range of 1 to 15 μm. (J3) The ratio Zs1 / Zs2 is in the range of 0.4 to 0.9. (J4) The ratio Zs1 / Zc is in the range of 0.4 to 0.9.

[0030] (Carbon material (C)) The carbon material (C) included in the negative electrode as an active material may include at least one selected from the group consisting of graphite, soft carbon (easily graphitizable carbon), and hard carbon (difficult to graphitize carbon), or it may be at least one of these. The carbon material (C) may include at least one selected from the group consisting of graphite and hard carbon, or it may be at least one of these. The carbon material (C) may be used alone or in combination of two or more types. Graphite is preferred because it has excellent charge-discharge stability and low irreversible capacity. The proportion of graphite in the carbon material (C) may be 50% by mass or more, or 80% by mass or more.

[0031] Graphite refers to a material with a well-developed graphite-type crystal structure, and generally refers to carbon materials with an average interplanar spacing d002 of (002) planes, measured by X-ray diffraction, of 0.34 nm or less. For example, natural graphite, artificial graphite, and graphitized mesophase carbon particles are typical examples of graphite. On the other hand, hard carbon is a carbon material in which minute graphite crystals are arranged in random directions, and further graphitization hardly progresses, and the average interplanar spacing d002 of 002 planes is greater than 0.38 nm. Hard carbon is preferable because it has low resistance and high capacitance.

[0032] As carbon material (C), various types with different average particle sizes and average circularities are commercially available and may be used. Alternatively, the average particle size and / or average circularity of commercially available carbon material (C) may be adjusted.

[0033] (Silicon-containing material (S)) Examples of silicon-containing materials (S) include silicon alloys, silicon compounds, and composite materials. Silicon-containing materials (S) may also be composite materials having a so-called sea-island structure.

[0034] The silicon-containing material (S) may be a composite particle comprising an ion-conducting phase and a silicon phase (silicon particles in one respect) dispersed within the ion-conducting phase. The ion-conducting phase is a phase that conducts ions. The ion-conducting phase may be at least one selected from the group consisting of a silicate phase, a carbon phase, and a silicon oxide phase.

[0035] The first silicon-containing material and the second silicon-containing material may each be independently a first composite material comprising a silicate phase and a first silicon phase dispersed within the silicate phase, or a second composite material comprising a carbon phase and a second silicon phase dispersed within the carbon phase. Hereinafter, the composite material comprising a silicate phase and a first silicon phase dispersed within the silicate phase (first composite material) may be referred to as "silicon-containing material (Ss)", and the composite material comprising a carbon phase and a second silicon phase dispersed within the carbon phase (second composite material) may be referred to as "silicon-containing material (Sc)". The first silicon-containing material and the second silicon-containing material may both be silicon-containing material (Ss), or both may be silicon-containing material (Sc). In a preferred example, one of the first silicon-containing material and the second silicon-containing material is silicon-containing material (Ss), and the other is silicon-containing material (Sc). For example, the first silicon-containing material may be a silicon-containing material (Ss), and the second silicon-containing material may be a silicon-containing material (Sc). Alternatively, the first silicon-containing material may be a silicon-containing material (Sc), and the second silicon-containing material may be a silicon-containing material (Ss).

[0036] The ion-conducting carbon phase may be composed of amorphous carbon. Examples of amorphous carbon that make up the carbon layer include hard carbon, soft carbon, and other amorphous carbons. Amorphous carbon is a carbon material in which the average interplanar spacing d002 of (002) planes, as measured by X-ray diffraction, is greater than 0.34 nm.

[0037] The main component of the silicon oxide phase (e.g., 95-100% by mass) may be silicon dioxide. The overall composition of the composite material containing the silicon oxide phase and the silicon phase dispersed therein is SiO2. x It can be expressed as follows. SiOx has a structure in which silicon nanoparticles are dispersed in amorphous SiO2. The oxygen content ratio x to silicon is, for example, 0.5 ≤ x < 2.0, and more preferably 0.8 ≤ x ≤ 1.5.

[0038] The silicate phase may satisfy the following conditions (3) and / or (4): (3) The silicate phase includes at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long-period periodic table). (4) The silicate phase contains element L. Element L is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanides, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanides are a collective term for 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0039] Regarding condition (3) above, 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 alkali metal elements and / or group 2 elements significantly reduces the irreversible capacity of the silicate phase. A lithium-containing silicate phase (hereinafter sometimes referred to as the "lithium silicate phase") is preferred because it has a low irreversible capacity and high initial charge-discharge efficiency.

[0040] The silicon-containing material (S) may include a coating layer disposed on its surface. The silicon-containing material (Ss) (first composite material) may include composite particles comprising a silicate phase and a first silicon phase, and a coating layer covering at least a portion of the surface of the composite particles.

[0041] Examples of coating layers present on the surface of silicon-containing materials (S) (e.g., silicon-containing materials (Ss)) include conductive layers, such as conductive layers formed from conductive carbon materials. By forming a conductive layer on the surface of silicon-containing materials (S), the conductivity of the silicon-containing materials (S) can be dramatically increased. As the conductive material constituting the conductive layer, conductive materials containing carbon are preferred. Examples of conductive materials containing carbon include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, and amorphous carbon with low crystallinity. Amorphous carbon is preferred because it has a large buffering effect on the silicon phase, which changes volume during charging and discharging. Amorphous carbon may be easily graphitizable carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Examples of carbon black include acetylene black and Ketjen black.

[0042] The thickness of the conductive layer is preferably thin enough not to affect the average particle size of the silicon-containing material (S). Considering the ensuring of conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably in the range of 1 to 200 nm (for example, in the range of 5 to 100 nm). The thickness of the conductive layer can be measured by observing the cross section of the silicon-containing material (S) using SEM or TEM (transmission electron microscope).

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

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

[0045] The total proportion of the silicon-containing material (S) and the carbon material (C) in the negative electrode active material may be 60% by mass or more, 80% by mass or more, 90% by mass or more, or even 90% by mass or more, and may be 100% by mass or less.

[0046] In the negative electrode (negative electrode active material), the mass Ws of the silicon-containing material (S) may be 3% or more, 5% or more, or 10% or more of the sum of the mass Wc of the carbon material (C) and the mass Ws of the silicon-containing material (S), and may also be 40% or less, 30% or less, or 20% or less. For example, in the negative electrode, the mass Ws of the silicon-containing material (S) may be in the range of 5 to 30% of the sum of the mass Wc of the carbon material (C) and the mass Ws of the silicon-containing material (S). It is believed that this range allows for more favorable control of the expansion and contraction of the entire negative electrode, and allows for the maximum enjoyment of the high capacity benefits of the silicon-containing material (S).

[0047] At the negative electrode, the mass Ws1 of the first silicon-containing material may be 0.2 times or more, 0.5 times or more, or 1 time or more than the mass Ws2 of the second silicon-containing material, and may be 5 times or less, 2 times or less, or 1 time or less. For example, the mass Ws1 may be in the range of 0.2 to 5 times the mass Ws2. Within this range, the respective effects of the first silicon-containing material and the second silicon-containing material can be effectively obtained. The ratio of mass Ws1 to mass Ws2 corresponds to the ratio of the content of the first silicon-containing material to the content of the second silicon-containing material in the negative electrode.

[0048] The compositional analysis of silicon-containing material (S) should preferably be performed using silicon-containing material (S) from the negative electrode in the discharge state. Furthermore, from the viewpoint of eliminating the influence of electrolyte decomposition products, it is desirable to analyze a sample of silicon-containing material (S) in the battery before or at the beginning of the charge-discharge cycle.

[0049] The B, Na, K, and Al content in the silicate layer can be determined by quantitative analysis in accordance with, for example, JIS (Japanese Industrial Standard) R3105 (1995) (Analytical Method for Borosilicate Glass). The Ca content can be determined by quantitative analysis in accordance with JIS R3101 (1995) (Analytical Method for Soda-Lime Glass).

[0050] The content of each element in silicon-containing material (S) can be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, a sample of silicon-containing material (S) is completely dissolved in a heated acid solution, the carbon in the solution residue is filtered out, and then the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of each element, and the content of each element is calculated.

[0051] When analyzing the composition of the silicate phase, the silicon-containing material (S) may be extracted from the battery by, for example, the following method. Specifically, the battery is disassembled and the negative electrode is removed. The negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode mixture layer is peeled off from the negative electrode current collector and crushed in a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and immersed in weakly boiled 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be contained in the binder, etc. Next, the sample powder is washed with deionized water, filtered off, and dried at 200°C for 1 hour. In this way, the silicon-containing material (S) is extracted.

[0052] Silicon-containing materials (S) may contain silicate phases, silicon oxide phases, and silicon phases. These can be distinguished and quantified using Si-NMR. As described above, the Si content obtained by ICP-AES is the sum of the Si content constituting the first silicon phase, the Si content in the silicate phase, and the Si content in the silicon oxide phase. On the other hand, the Si content constituting the silicon phase and the Si content in the silicon oxide phase can be quantified separately using Si-NMR. Therefore, the Si content in the silicate phase can be quantified by subtracting the Si content constituting the silicon phase and the Si content in the silicon oxide phase from the Si content obtained by ICP-AES. For quantification, a mixture containing silicate and silicon phase with known Si content in predetermined proportions can be used as the standard substance.

[0053] The following describes the desirable measurement conditions for Si-NMR. <Si-NMR Measurement Conditions> Measuring device: Manufactured by Varian, solid nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupling) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Number of integrations: 560 Sample amount: 207.6 mg

[0054] Also, the quantification of each element in the silicon-containing material (S) can also be performed by SEM-EDX analysis, Auger electron spectroscopy (AES), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0055] The average particle size of the silicon phase (for example, the first silicon phase and the second silicon phase) dispersed in the ion conduction phase may be 1 nm or more, or 5 nm or more. The average particle size may be 1000 nm or less, 500 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less. The fine silicon phase is preferable in that the volume change during charge and discharge is small and the structural stability of the silicon-containing material (S) is improved. The average particle size of the silicon phase can be determined by the method for determining the average particle size of the above-mentioned particles.

[0056] The crystallite size of the silicon phase (e.g., the first silicon phase and the second silicon phase) is preferably 30 nm or less. When the crystallite size is 30 nm or less, the volume change of the silicon-containing material (S) due to the expansion and contraction of the silicon phase during charging and discharging can be reduced. The crystallite size is more preferably 30 nm or less, and even more preferably 20 nm or less. When the crystallite size is 20 nm or less, the expansion and contraction of the silicon phase are made more uniform, fine cracks in the silicon phase are reduced, and the cycle characteristics can be further improved.

[0057] The crystallite size of the silicon phase is calculated using Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the (111) plane of the silicon phase (elementary Si) in the X-ray diffraction pattern.

[0058] Examples of silicon-containing materials (S) with a sea-island structure include silicon-containing materials (Ss) and silicon-containing materials (Sc).

[0059] Silicon-containing material (Ss) comprises a silicate phase and a silicon phase (first silicon phase) dispersed within the silicate phase. Similarly, silicon-containing material (Sc) comprises a carbon phase and a silicon phase (second silicon phase) dispersed within the carbon phase. The first and second silicon phases can each be considered as silicon particles.

[0060] Silicon-containing materials (Ss) and silicon-containing materials (Sc) can each exist in the form of particles having a so-called sea-island structure. The first or second silicon phase (island) is dispersed in a matrix (sea) of silicate or carbon phase and covered by a lithium-ion conducting phase (silicate or carbon phase). In the sea-island structure, side reactions are suppressed because contact between the first or second silicon phase and the electrolyte is limited. In addition, stresses caused by the expansion and contraction of the silicon phase are relieved by the lithium-ion conducting phase matrix.

[0061] Silicon-containing materials (Ss) can contain a considerable amount of primary silicon phase, have few sites that trap lithium ions which cause irreversible capacity, and are less prone to side reactions. However, the silicate phase of silicon-containing materials (Ss) has low conductivity. Therefore, expansion and contraction due to charging and discharging can form voids around the silicon-containing material (Ss), or cracks can form in the silicon-containing material (Ss) due to stress caused by expansion and contraction. In such cases, a portion of the silicon-containing material (Ss) becomes isolated, reducing the contact area between that portion and its surroundings, which tends to decrease capacity.

[0062] In contrast, silicon-containing material (Sc) contains a carbon phase and a secondary silicon phase dispersed within the carbon phase. The carbon phase of silicon-containing material (Sc) has high conductivity. Therefore, even if voids are formed around the silicon-containing material (Sc) or cracks occur in the silicon-containing material (Ss), parts of the silicon-containing material (Sc) are less likely to become isolated, and the contact between the silicon-containing material (Sc) and its surroundings is easily maintained. Thus, by replacing part of the silicon-containing material (Ss) with silicon-containing material (Sc), the contact between the entire silicon-containing material and its surroundings is more easily maintained, and the decrease in capacity when repeated charge-discharge cycles are suppressed.

[0063] The relative masses of silicon-containing material (Ss) A, silicon-containing material (Sc) B, and carbon material (C) C in the negative electrode can be determined by cross-sectional SEM-EDX analysis. First, the silicon-containing material (Ss) particles (particle A), silicon-containing material (Sc) particles (particle B), and carbon material (C) particles (particle C) are distinguished. A magnification of 2000 to 20000x is desirable.

[0064] To perform cross-sectional SEM-EDX analysis, for example, disassemble the battery, take out the negative electrode, and obtain a cross-section of the negative electrode using a cross-section polisher (CP). Observe the cross-section of the negative electrode using a scanning electron microscope (SEM). Perform elemental mapping analysis of the cross-sectional image of the backscattered electron image of the negative electrode using energy-dispersive X-ray (EDX). Calculate the total areas A to C occupied by particles A to C respectively using image analysis software. The area ratios of the total areas A to C may be regarded as the volume ratios of particles A to C.

[0065] In SEM-EDX analysis, it is also possible to quantify the elements in particles A to C. Randomly select 10 particles A to C each with a maximum diameter of 5 μm or more from the cross-sectional image of the backscattered electron image of the negative electrode, and perform elemental mapping analysis of each using energy-dispersive X-ray (EDX). Calculate the area containing the target element using image analysis software. Average the measured values of the areas containing the predetermined elements for the 10 particles. Convert the area containing the element into the number of atoms and calculate the composition. Determine the specific gravity of particles A to C from the compositions of particles A to C respectively. Next, calculate B / A and (A + B) / (A + B + C) using the total areas A to C and the specific gravities A to C.

[0066] It is desirable to perform the above analysis and the analysis of the negative electrode described below using the negative electrode in the discharged state. Also, from the perspective of excluding the influence of the decomposition products of the electrolyte, it is desirable to analyze the sample of the negative electrode in the battery before or at the initial stage of the charge-discharge cycle.

[0067] Note that a film is formed on the surface of the silicon-containing material due to decomposition of the electrolyte or the like during the charge-discharge process. Also, the silicon-containing material may have a conductive layer on its surface. Therefore, the mapping analysis by EDX is performed on a region 1 μm or more inside from the peripheral edge of the cross-section of the silicon-containing material so that the film and the conductive layer are not included in the measurement range.

[0068] The following shows the measurement conditions for desirable cross-sectional SEM-EDX analysis. <SEM-EDX Measurement Conditions> Processing device: JEOL, SM-09010 (Cross Section Polisher) Processing conditions: Acceleration voltage 6kV Current value: 140μA Vacuum degree: 1×10 -3 ~2×10 -3 Pa Measurement device: Electron microscope, Hitachi SU-70 Acceleration voltage during analysis: 10kV Field: Free Mode Probe current mode: Medium Probe current range: High Anode Ap.:3 OBJ Ap.:2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS:20500 Lsec:50 Time constant: 3.2

[0069] The average particle size Da of the silicon-containing material (Ss) may be 2 μm or more, 3 μm or more, or 5 μm or more, and may also be 15 μm or less, 12 μm or less, or 10 μm or less. Within this range, it is considered that voids that may be created due to the expansion and contraction of the silicon-containing material (Ss) are appropriately suppressed, and cracks in the silicon-containing material (Ss) that may be created due to expansion and contraction are also easily suppressed.

[0070] The average particle size Db of the silicon-containing material (Sc) may be 3 μm or more, 6 μm or more, or 8 μm or more, and may be 18 μm or less, 15 μm or less, or 12 μm or less. When the negative electrode active material contains both silicon-containing material (Ss) and silicon-containing material (Sc), within the above range, even if cracks are generated in the silicon-containing material (Sc), it is considered that a portion of the silicon-containing material (Sc) is likely to penetrate into the voids created by the contraction and cracks after expansion of the silicon-containing material (Ss). As a result, it is considered that the effect of maintaining the electrical connection between the silicon-containing material (Ss) and its surroundings will be significantly enhanced.

[0071] The content of the first silicon phase in the silicon-containing material (Ss) may be 30% by mass or more, 40% by mass or more, or 50% by mass or more, and may be 80% by mass or less, or 70% by mass or less. Within this range, not only is a sufficiently high capacity of the negative electrode achieved, but the side effects due to the expansion and contraction of the first silicon phase are limited, thus improving the cycle characteristics. This is because the silicon-containing material (Ss) contains a sufficient amount of the first silicon phase, while the proportion of the silicate phase in the silicon-containing material (Ss) does not become too small. By maintaining the proportion of the silicate phase above a certain level, contact between the first silicon phase and the electrolyte is significantly limited, and side reactions are also significantly suppressed. Furthermore, the stress generated by the expansion and contraction of the first silicon phase is easily relieved by the silicate phase matrix.

[0072] The content of the second silicon phase in the silicon-containing material (Sc) may be 30% by mass or more, 40% by mass or more, or 50% by mass or more, and may be 80% by mass or less, or 70% by mass or less. Within this range, as with the silicon-containing material (Ss), a sufficiently high capacity of the negative electrode can be achieved, and the cycle characteristics are also easily improved. Furthermore, by maintaining a considerable proportion of the carbon phase, the carbon phase can easily penetrate into voids that are subsequently generated due to charging and discharging, for example, the electrical connection between the silicon-containing material (Ss) and its surroundings can be easily maintained.

[0073] Silicon-containing materials (Sc) typically exhibit a higher amount of side reactions than silicon-containing materials (Ss). Therefore, it is preferable that the average particle size of the second silicon phase be larger than that of the first silicon phase, for example, 1.1 to 2 times larger. By increasing the average particle size of the second silicon phase, it is possible to reduce the contact area with the electrolyte and thereby reduce the amount of side reactions.

[0074] (Method for adjusting the average particle size and average circularity of carbon material (C)) There are no particular limitations on the method for adjusting the average particle size and average circularity of the carbon material (C), and known methods may be used. For example, methods similar to the mechanofusion method or grinding methods may be used alone or in combination. For example, the average particle size may be adjusted using a jet mill or the like, and then the average circularity may be adjusted using equipment used in mechanofusion or the like.

[0075] (Method for adjusting the average particle size and average circularity of silicon-containing material (S)) There are no particular limitations on the method for adjusting the average particle size and average circularity of silicon-containing materials (S) (silicon-containing materials (Ss) and silicon-containing materials (Sc)), and known methods may be used. For example, when silicon-containing materials are ground into particles using a jet mill or the like, the average particle size and average circularity may be adjusted by changing conditions such as the grinding time. For example, it is possible to lower the average circularity by shortening the grinding time.

[0076] Next, we will describe in detail an example of a method for producing silicon-containing material (Ss). Here, we will explain the case in which a first silicon phase is dispersed in a lithium silicate phase.

[0077] Process (i) The raw materials for lithium silicate are a mixture containing Si raw materials and Li raw materials in predetermined proportions. The raw material mixture may also contain the alkali metal elements, group 2 elements, and / or element L mentioned above. The raw material mixture is dissolved, and the melt is passed through a metal roll to flake it and produce lithium silicate. The flake silicate is then crystallized by heat treatment in an atmospheric atmosphere at a temperature above the glass transition point and below the melting point. It is also possible to use the flake silicate without crystallization. Alternatively, silicate can be produced by solid-phase reaction by firing the raw material mixture at a temperature below the melting point without dissolving it.

[0078] Silicon oxide can be used as the Si raw material. For Li raw materials, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc., can be used. These may be used individually or in combination of two or more. For alkali metal elements, group 2 elements, and element L, oxides, hydroxides, carbonates, hydrides, nitrates, sulfates, etc., of each element can be used as raw materials.

[0079] Process (ii) Next, the lithium silicate is compounded with raw silicon. For example, silicon-containing material (Ss), which is a composite particle of lithium silicate and the first silicon phase (hereinafter also referred to as silicate composite particle), is produced through the following steps (a) to (c).

[0080] Process (a) The raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. For the raw silicon, coarse silicon particles with an average particle size of several micrometers to several tens of micrometers should be used.

[0081] Process (b) Next, the mixture of raw silicon and lithium silicate is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. At this time, an organic solvent may be added to the mixture for wet pulverization. The organic solvent serves to prevent the material to be pulverized from adhering to the inner wall of the pulverizing container.

[0082] Suitable organic solvents include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides.

[0083] Alternatively, the raw materials, silicon and lithium silicate, may be separately atomized into fine particles before being mixed. Furthermore, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared and mixed without using a pulverizing device. Known methods such as gas-phase methods (e.g., plasma methods) or liquid-phase methods (e.g., liquid-phase reduction methods) can be used to prepare the nanoparticles.

[0084] Process (c) Next, the mixture is heated to 600°C to 1000°C in an inert gas atmosphere (e.g., argon, nitrogen, etc.) and pressurized to sinter it. For sintering, a sintering apparatus that can pressurize under an inert atmosphere, such as a hot press, can be used. During sintering, the silicate softens and flows to fill the gaps between the first silicon phases. As a result, a dense, block-shaped sintered body can be obtained, with the silicate phase forming the "sea" and the first silicon phase forming the "islands". By crushing the obtained sintered body, silicate composite particles can be obtained.

[0085] Process (iii) Subsequently, at least a portion of the surface of the composite particles may be coated with a conductive material to form a conductive layer. Examples of methods for coating the surface of composite particles with a conductive carbon material include the CVD method using hydrocarbon gases such as acetylene and methane as raw materials, and a method in which coal pitch, petroleum pitch, phenolic resin, etc., are mixed with the composite particles and heated at 700°C to 950°C in an inert atmosphere (for example, an atmosphere of argon or nitrogen) to carbonize them. Alternatively, carbon black may be attached to the surface of the composite particles.

[0086] Process (iv) A step of washing the composite particles (including those having a conductive layer on their surface) with acid may be performed. For example, washing the composite particles with an acidic aqueous solution can dissolve and remove trace amounts of alkaline components that may be generated when the raw silicon and lithium silicate are compounded. As the acidic aqueous solution, aqueous solutions of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, or aqueous solutions of organic acids such as citric acid and acetic acid can be used.

[0087] Through the above process, a silicon-containing material (Ss) is obtained. As an example of a silicon-containing material (Ss), a schematic cross-section of silicate composite particles 20 coated with a conductive layer is shown in Figure 1.

[0088] The silicate composite particle (mother particle) 23 comprises a lithium silicate phase 21 and a silicon phase 22 dispersed within the lithium silicate phase 21. The silicate composite particle (mother particle) 23 has a sea-island structure in which fine silicon phases 22 are dispersed in a matrix of lithium silicate phase 21. The surface of the silicate composite particle (mother particle) 23 is coated with a conductive layer 26.

[0089] A silicon oxide phase (not shown) may be dispersed in the lithium silicate phase 21. The SiO2 content in the silicate composite particles (mother particles) 23, as measured by Si-NMR, is preferably 30% by mass or less, and more preferably less than 7% by mass.

[0090] The silicate composite particles (mother particles) 23 may contain other components besides those mentioned above. For example, reinforcing materials such as carbon materials, oxides such as ZrO2, and carbides may be present in less than 10% by mass relative to the mother particles 23.

[0091] Next, we will illustrate two methods for producing silicon-containing materials (Sc). (a) Method 1 The raw silicon and carbon source are mixed, and the mixture of raw silicon and carbon source is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. An organic solvent may be added to the mixture. At this time, the raw silicon is finely pulverized to form a second silicon phase. The second silicon phase is dispersed in the matrix of the carbon source.

[0092] As a carbon source, examples of water-soluble resins such as carboxymethylcellulose (CMC), hydroxyethylcellulose, polyacrylates, polyacrylamide, polyvinyl alcohol, polyethylene oxide, and polyvinylpyrrolidone, as well as sugars such as cellulose and sucrose, petroleum pitch, coal pitch, and tar may be used, but are not particularly limited.

[0093] Suitable organic solvents include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides.

[0094] Next, the composite of the second silicon phase and the carbon source is heated to 700°C to 1200°C in an inert gas atmosphere (e.g., argon, nitrogen, etc.) to carbonize the carbon source and generate amorphous carbon. This yields a silicon-containing material (Sc) in which the second silicon phase is dispersed in a carbon phase containing amorphous carbon.

[0095] (b) Second method The raw silicon and carbon material are mixed, and the mixture is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. An organic solvent may be added to the mixture. At this time, the raw silicon is finely pulverized to form a second silicon phase. The second silicon phase is dispersed in the matrix of the carbon material.

[0096] By compounding the raw silicon and carbon material as described above, a silicon-containing material (Sc) is obtained in which secondary silicon is dispersed in the carbon phase of amorphous carbon. Subsequently, the silicon-containing material (Sc) may be heated to 700°C to 1200°C in an inert gas atmosphere.

[0097] As the carbon material, amorphous carbon is preferred, and easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), and carbon black can be used. Examples of carbon black include acetylene black and Ketjen black. Even when graphite is used as the carbon material, when a composite of silicon-secondary material and the carbon material is obtained using a grinding device, the crystalline structure of the graphite is almost completely lost, and an amorphous carbon phase is formed.

[0098] (Example of a secondary battery configuration) An example of the configuration of a secondary battery according to this disclosure is described below. Aside from using the negative electrode active material described above, various other components can be selected. For example, known components may be used for components other than the negative electrode active material. The secondary battery according to this disclosure typically includes a positive electrode, a negative electrode, an electrolyte, and a separator placed between the positive and negative electrodes. These components are described below. Note that, apart from using the negative electrode active material described above, the method of manufacturing the secondary battery is not limited, and known manufacturing methods may be used.

[0099] [Negative electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying it. The dried coating may be rolled as needed.

[0100] The negative electrode mixture contains a negative electrode active material as an essential component and may contain optional components such as binders, conductive agents, and thickeners. The negative electrode active material contains a carbon material (C) and a silicon-containing material (S).

[0101] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) and porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0102] Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives. These may be used individually or in combination of two or more.

[0103] Examples of conductive materials include carbon black, conductive fibers, fluorinated carbon, and organic conductive materials. These may be used individually or in combination of two or more.

[0104] Examples of the thickening agent include carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. These may be used alone or in combination of two or more kinds.

[0105] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof.

[0106] [Positive electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which components of the positive electrode mixture are dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying it. The dried coating film may be rolled as necessary.

[0107] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, etc. as optional components.

[0108] As the positive electrode active material, a material capable of occluding and releasing lithium ions can be used. Known positive electrode active materials used in non-aqueous electrolyte secondary batteries may be used as the positive electrode active material. Examples of the positive electrode active material include lithium composite metal oxides. Examples of the lithium composite metal oxides include, for example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c 、Li a Ni 1-b M b O c 、Li a Mn2O4、Li a Mn 2-b M b O 4、 LiMePO 4、Li2MePO4F is one example. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me includes at least one transition element (for example, at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. Note that the value of a, which indicates the molar ratio of lithium, is the value immediately after the active material is prepared and will increase or decrease with charging and discharging.

[0109] For the binder and conductive agent, the same substances as those exemplified for the negative electrode can be used. For the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0110] A conductive substrate having a shape similar to that described for the negative electrode current collector can be used for the positive electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0111] [Electrolyte] The electrolyte (or electrolyte solution) contains a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5 to 2 mol / L. The electrolyte may also contain known additives.

[0112] Aqueous or non-aqueous solvent may be used as the solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, and cyclic carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). A single non-aqueous solvent may be used, or two or more may be used in combination.

[0113] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts of fluorine-containing acids (such as LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0114] The secondary battery of this embodiment may be a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte. The non-aqueous electrolyte can be obtained, for example, by dissolving a lithium salt in a non-aqueous solvent.

[0115] [Separator] It is desirable to interpose a separator between the positive and negative electrodes. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene can be used.

[0116] An example of a secondary battery includes an electrode group, an electrolyte, and an outer casing that houses them. The electrode group may be a wound electrode group in which the positive and negative electrodes are wound around each other with a separator in between. Alternatively, the electrode group may take other forms. For example, the electrode group may be a stacked electrode group in which the positive and negative electrodes are stacked with a separator in between. There are no particular limitations on the form of the secondary battery, and it may take the form of cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0117] Figure 2 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure. Note that the battery shown in Figure 2 is an example, and the secondary batteries of the present disclosure are not limited to the battery shown in Figure 2.

[0118] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 and an electrolyte (not shown) housed within the battery case 4, and a sealing plate 5 that seals the opening of the battery case 4. The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. The sealing plate 5 has an electrolyte inlet sealed with a plug 8 and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0119] One end of the negative electrode lead 3 is attached to the negative electrode current collector by welding or other means. One end of the positive electrode lead 2 is attached to the positive electrode current collector by welding or other means. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6. The other end of the positive electrode lead 2 is electrically connected to the sealing plate 5. [Examples]

[0120] The present disclosure will be described below in detail based on examples and comparative examples, but the present disclosure is not limited to the following examples. Multiple secondary batteries were manufactured using the following procedure.

[0121] <Battery A1> [Preparation of silicon-containing materials (Ss)] A mixture was obtained by mixing silicon dioxide and lithium carbonate in an atomic ratio of Si / Li of 1.05. The resulting mixture was calcined in air at 950°C for 10 hours to obtain lithium silicate represented by the formula Li2Si2O5 (z=0.5). The obtained lithium silicate was pulverized to a certain size (average particle size of 100 μm or less).

[0122] Crushed lithium silicate (Li2Si2O5) and raw silicon (3N, average particle size 10μm) were mixed in a mass ratio of 40:60. The mixture was filled into a 500mL pot (made of stainless steel) of a planetary ball mill (Fritsch, P-5). Next, 24 stainless steel balls (20mm in diameter) were placed in the pot, the lid was closed, and the mixture was ground in an inert atmosphere at 200rpm for 50 hours.

[0123] Next, the powdered mixture was removed in an inert atmosphere and fired at 800°C for 4 hours under pressure from a hot press in the inert atmosphere to obtain a sintered mixture (silicon-silicate composite).

[0124] Subsequently, the silicon-silicate composite was crushed and passed through a 40 μm mesh. The resulting silicate composite particles were then mixed with coal pitch (JFE Chemical Corporation, MCP250), and the mixture was calcined at 800°C in an inert atmosphere to coat the surface of the silicate composite particles with conductive carbon, forming a conductive layer. The amount of conductive layer coating was 5% by mass relative to the total mass of the silicate composite particles and the conductive layer. In this way, silicate composite particles P1 (silicon-containing material (Ss)) having a conductive layer were obtained. After that, the average particle size of the silicate composite particles P1 was adjusted using a sieve.

[0125] XRD analysis of silicate composite particle P1 revealed that the crystallite size of silicate composite particle P1 was 15 nm, calculated using Scherrer's equation from the diffraction peaks attributed to the Si(111) plane.

[0126] When the composition of the lithium silicate phase of silicate composite particle P1 was analyzed by the above method (ICP-AES), the Si / Li ratio was 1.0, and the content of Li2Si2O5 measured by Si-NMR was 70% by mass (the content of the first silicon phase was 30% by mass).

[0127] [Preparation of silicon-containing materials (Sc)] Coal pitch (MCP250, manufactured by JFE Chemical Corporation), a carbon source, and raw silicon (3N, average particle size 10 μm) were mixed in a 50:50 mass ratio. The mixture was filled into a 500 mL pot (made of stainless steel, volume: SUS) of a planetary ball mill (P-5, manufactured by Fritsch). 24 stainless steel balls (20 mm in diameter) were placed in the pot, the lid was closed, and the mixture was ground at 200 rpm for 50 hours in an inert atmosphere to obtain a composite of a second silicon phase and a carbon source.

[0128] Next, the composite of the second silicon phase and the carbon source was calcined in an inert gas atmosphere to carbonize the carbon source and obtain composite particles P2 (silicon-containing material (Sc)) in which the second silicon phase was dispersed in a carbon phase containing amorphous carbon. Subsequently, the average particle size of the composite particles P2 was adjusted using a jet mill.

[0129] XRD analysis of the second silicon-containing particles revealed that the crystallite size of the second silicon phase, calculated using Scherrer's equation from the diffraction peaks attributed to the Si(111) plane, was 15 nm.

[0130] [Carbon material (C)] Spherical graphite particles with an average particle size Dc of 24 μm were prepared.

[0131] [Fabrication of the negative electrode] A negative electrode active material was obtained by mixing silicate composite particles P1, composite particles P2, and graphite particles in a mass ratio of silicate composite particles P1:composite particles P2:graphite particles = 3:3:94. The obtained negative electrode active material was mixed with carboxymethylcellulose sodium (CMC-Na) and styrene-butadiene rubber (SBR) in a mass ratio of negative electrode active material:CMC-Na:SBR = 97.5:1:1.5 to obtain a mixture. After adding water to this mixture, it was stirred using a mixer (TK Hibiscus Mix, manufactured by Primix). In this way, a negative electrode slurry was prepared.

[0132] Next, 1m 2 A negative electrode slurry was applied to the surface of a copper foil to form a coating film, with each coating film containing 190g of negative electrode mixture. After drying the coating film, rolling was performed to create a negative electrode mixture layer (density: 1.5g / cm³) on both sides of the copper foil. 3 A negative electrode was obtained in which a ) was formed.

[0133] [Fabrication of the positive electrode] Lithium nickel composite oxide (LiNi 0.8 Co 0.18 Al 0.02A mixture was obtained by mixing lithium nickel composite oxide (O2), acetylene black, and polyvinylidene fluoride in a mass ratio of lithium nickel composite oxide:acetylene black:polyvinylidene fluoride = 95:2.5:2.5. N-methyl-2-pyrrolidone (NMP) was added to this mixture, and then the mixture was stirred using a mixer (TK Hibismix, manufactured by Primix Corporation) to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil to form a coating. After drying the coating, rolling was performed to form a positive electrode mixture layer (density: 3.6 g / cm³) on both sides of the aluminum foil. 3 A positive electrode was obtained in which a ) was formed.

[0134] [Preparation of electrolyte solution] The electrolyte was prepared by dissolving a lithium salt in a non-aqueous solvent. The non-aqueous solvent used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of EC:DMC:MA = 20:40:40. The concentration of LiPF6 in the electrolyte was 1.0 mol / L.

[0135] [Manufacturing of secondary batteries] Tabs were attached to each electrode. Next, an electrode group was fabricated by spirally winding the positive and negative electrodes with a separator in between. The winding was done so that the tabs were located on the outermost part. Next, the electrode group was inserted into an aluminum laminate film enclosure and vacuum-dried at 105°C for 2 hours. Then, electrolyte was injected into the enclosure and the opening of the enclosure was sealed. In this way, battery A1 was obtained.

[0136] <Batteries A2-A8 and CA1-CA4> Except for changing the average particle size and average circularity of the silicate composite particles P1 and P2 (silicon-containing material (S)) and the average particle size and average circularity of the graphite particles (carbon material (C)) and their mixing ratio, multiple negative electrodes were fabricated using the same method and conditions as those used for the negative electrode of battery A1. Then, batteries A2 to A8 and batteries CA1 to CA4 were fabricated using the same method and conditions as those used for the negative electrode of battery A1, except for using the fabricated negative electrodes.

[0137] The average particle size and average circularity of silicate composite particles P1 (silicon-containing material (Ss)) and composite particles P2 (silicon-containing material (Sc)) were modified by the method described above. Carbon material (C) with different average particle size and average circularity was purchased and used.

[0138] The average particle size and average circularity of silicate composite particles P1 (silicon-containing material (Ss)), composite particles P2 (silicon-containing material (Sc)), and graphite particles (carbon material (C)) were measured by the following method. For the measurement of the average particle size, first, the volume-based particle size distribution of each particle was measured using a laser diffraction particle size distribution analyzer (MT3300EXII manufactured by Microtrac Co., Ltd.). Then, the particle size at a cumulative volume of 50% (median diameter D50) was taken as the average particle size. The average circularity was measured by the method described above, i.e., by dispersing the particles in resin and measuring them.

[0139] [evaluation] The fabricated batteries were evaluated using the following method.

[0140] (Charge-discharge cycle test) Each manufactured battery underwent a charge-discharge cycle test under the following conditions. First, as a charging process, constant current charging was performed with a current of 0.3 It until the voltage reached 4.2 V, and then constant voltage charging was performed with a constant voltage of 4.2 V until the current reached 0.015 It. Subsequently, as a discharging process, constant current discharge was performed with a current of 0.3 It until the voltage reached 2.75 V. A rest period of 10 minutes was observed between charging and discharging. Charging and discharging were performed in an environment of 25°C.

[0141] Note that (1 / X)It represents the current, and (1 / X)It(A) = rated capacity (Ah) / X(h), where X represents the time required to charge or discharge electricity equal to the rated capacity. For example, 0.5It means that X=2, and the current value is rated capacity (Ah) / 2(h).

[0142] The batteries were repeatedly charged and discharged under the above conditions. For each battery, the discharge capacity C1 after the first cycle and the discharge capacity C300 after 300 cycles were measured. The capacity degradation rate (%) was then calculated using the following formula. Capacity deterioration rate (%)=100×(C1-C300) / C1

[0143] The evaluation results are shown in Tables 1 to 3. Note that the capacity degradation rate of the batteries in Table 1 is shown as a relative value with the capacity degradation rate of battery C1 set to 100. The capacity degradation rate of the batteries in Table 2 is shown as a relative value with the capacity degradation rate of battery C3 set to 100. The capacity degradation rate of the batteries in Table 3 is shown as a relative value with the capacity degradation rate of battery C4 set to 100. A smaller capacity degradation rate indicates a higher capacity retention rate. In this example, silicate composite particle P1 (silicon-containing material (Ss)) corresponds to the first silicon-containing material, and composite particle P2 (silicon-containing material (Sc)) corresponds to the second silicon-containing material.

[0144] [Table 1]

[0145] [Table 2]

[0146] [Table 3]

[0147] Batteries C1 to C4 are batteries of the comparative example, and batteries A1 to A8 are batteries of the example. As shown in the table, for batteries A1 to A7 that satisfy the following conditions (1) and (2), the capacity degradation rate was smaller than that of batteries C1 to C4. That is, for batteries A1 to A8, the capacity retention rate was higher than that of batteries C1 to C4. (1) The average particle size of the first silicon-containing material and the average particle size of the second silicon-containing material are each smaller than the average particle size of the carbon material (C). (2) The average circularity Zc of the carbon material (C), the average circularity Zs1 of the first silicon-containing material, and the average circularity Zs2 of the second silicon-containing material satisfy Zs1 < Zc and Zs1 < Zs2.

[0148] As shown in the table, it is preferable that the average circularity Zc of the carbon material (C) is 0.70 or more and the average circularity of the first silicon-containing material is 0.60 or less. Further, as shown in the table, the value of the ratio Zs1 / Zs2 of the average circularities is preferably 0.80 or less, more preferably 0.60 or less. The value of the ratio Zs1 / Zc of the average circularities is preferably 0.80 or less, more preferably 0.60 or less.

Industrial Applicability

[0149] The present disclosure can be used for secondary batteries and is useful, for example, as a main power source for mobile communication devices, portable electronic devices, and the like. The present invention has been described with respect to the preferred embodiments at the present time, but such disclosure should not be construed in a limiting sense. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the technical field to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to include all modifications and alterations without departing from the true spirit and scope of the present invention.

Explanation of Signs

[0150] 1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Sealing plug, 20: Silicate composite particles coated with a conductive layer, 21: Lithium silicate phase, 22: First silicon phase (silicon particles), 23: Silicate composite particles, 26: Conductive layer

Claims

1. It is a secondary battery, It includes a positive electrode and a negative electrode containing a negative electrode active material. The aforementioned negative electrode active material comprises a carbon material and a silicon-containing material. The silicon-containing material includes a first silicon-containing material and a second silicon-containing material. The average particle size of the first silicon-containing material and the average particle size of the second silicon-containing material are each smaller than the average particle size of the carbon material. A secondary battery in which the average circularity Zc of the carbon material, the average circularity Zs1 of the first silicon-containing material, and the average circularity Zs2 of the second silicon-containing material satisfy Zs1 < Zc and Zs1 < Zs2.

2. The secondary battery according to claim 1, wherein the average circularity Zc is 0.7 or greater.

3. The secondary battery according to claim 1 or 2, wherein the average circularity Zs1 is 0.6 or less.

4. The average particle size of the carbon material is in the range of 10 to 30 μm. The secondary battery according to claim 1 or 2, wherein the average particle size of the first silicon-containing material and the average particle size of the second silicon-containing material are each in the range of 1 to 15 μm.

5. The secondary battery according to claim 1 or 2, wherein the ratio Zs1 / Zs2 of the average circularity Zs1 to the average circularity Zs2 is in the range of 0.4 to 0.

9.

6. The secondary battery according to claim 1 or 2, wherein the ratio Zs1 / Zc of the average circularity Zs1 to the average circularity Zc is in the range of 0.4 to 0.

9.

7. The secondary battery according to claim 1 or 2, wherein the carbon material comprises at least one selected from the group consisting of graphite, soft carbon, and hard carbon.

8. The secondary battery according to claim 1 or 2, wherein the silicon-containing material is a composite particle comprising an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.

9. The secondary battery according to claim 8, wherein the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, and a silicon oxide phase.

10. The secondary battery according to claim 1 or 2, wherein the first silicon-containing material and the second silicon-containing material are each independently a first composite material comprising a silicate phase and a first silicon phase dispersed within the silicate phase, or a second composite material comprising a carbon phase and a second silicon phase dispersed within the carbon phase.

11. The secondary battery according to claim 10, wherein the silicate phase includes at least one selected from the group consisting of alkali metal elements and group 2 elements.

12. The secondary battery according to claim 10, wherein the silicate phase includes at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanide, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

13. The secondary battery according to claim 10, wherein the first composite material comprises composite particles comprising the silicate phase and the first silicon phase, and a coating layer covering at least a portion of the surface of the composite particles.

14. The secondary battery according to claim 1 or 2, wherein in the negative electrode, the mass of the silicon-containing material is in the range of 5% to 30% of the sum of the mass of the carbon material and the mass of the silicon-containing material.

15. The secondary battery according to claim 1 or 2, wherein in the negative electrode, the mass of the first silicon-containing material is in the range of 0.2 to 5 times the mass of the second silicon-containing material.

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