Negative electrode material for secondary batteries and secondary batteries

Abstract

Provided are: a negative-electrode material for a secondary battery, the negative-electrode material for a secondary battery including a negative electrode active material and carbon nanotubes, the negative electrode active material containing graphite and elemental silicon, and the carbon nanotubes having an average fiber diameter of 0.5-6 nm and an average fiber length of 1.2-8 μm; and a secondary battery having a negative electrode that includes the negative-electrode material for a secondary battery, a positive electrode, a separator arranged between the positive electrode and the negative electrode, and an electrolyte solution.

Description

[Technical Field]

[0001] This disclosure relates to secondary batteries, and more particularly to improvements to negative electrodes used in secondary batteries. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, offer high power output and high energy density. Therefore, they are used in small consumer applications, power storage devices, and as power sources for electric vehicles.

[0003] Various materials have been proposed as negative electrode active materials for non-aqueous electrolyte secondary batteries. As negative electrode active materials with high energy density, silicon compounds that alloy with lithium (e.g., silicon oxide) or silicon particles have been conventionally proposed (see, for example, Patent Document 1).

[0004] Patent Document 2 discloses a conductive composite comprising carbon nanotubes with an outer diameter of 1 nm to 6 nm, fine particles of 1000 μm or less, and a dispersant, wherein the carbon nanotubes dispersed by the dispersant coat the surface of the fine particles. The fine particles are, for example, graphite particles with an average particle diameter of 20 μm, and it is proposed to use the conductive composite as the negative electrode of a lithium-ion secondary battery.

[0005] Patent Document 3 proposes using a composite of an electrode active material and carbon nanotubes, which contains an electrode active material and a fibrous carbon nanotube aggregate consisting of single-walled carbon nanotubes with an average diameter of 10 nm to 20 μm, as the negative electrode of a lithium-ion secondary battery. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2010-212228 [Patent Document 2] Japanese Patent Publication No. 2011-076948

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] Regarding the content of carbon nanotubes in the negative electrode, Patent Document 2 states that the ratio of carbon nanotubes in the conductive composite is 0.1 to 20% by mass, and Patent Document 3 states that the mass ratio of the active material to carbon nanotubes is 1 to 500, and a large amount of carbon nanotubes are included in the negative electrode.

[0008] Also, in Patent Document 2, a composite in which the surface of the active particles is coated with carbon nanotubes is formed, and in Patent Document 3, a composite in which the active material particles are embedded in the carbon nanotube aggregate is formed. In order to form such a composite, a special manufacturing process may be required, or when adopting a process of applying a slurry in which various constituent materials generally used are added to a dispersion medium and mixed, the slurry may have an increased viscosity, making it difficult to apply uniformly. Furthermore, since a relatively expensive carbon nanotube needs to be included in a large amount in the negative electrode, the manufacturing cost becomes high.

Means for Solving the Problems

[0009] : In view of the above, one aspect of the present disclosure relates to a negative electrode material for a secondary battery, which includes a negative electrode active material and carbon nanotubes, the negative electrode active material includes graphite and a material containing a silicon element, and the average fiber diameter of the carbon nanotubes is 0.5 to 6 nm and the average fiber length is 1.2 to 8 μm.

[0010] Another aspect of the present disclosure relates to a secondary battery having a negative electrode including the above negative electrode material for a secondary battery, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolytic solution.

Effects of the Invention

[0011] According to the present disclosure, a secondary battery having excellent characteristics can be realized at a low cost. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic perspective view showing a portion of a secondary battery according to one embodiment of the present disclosure. [Figure 2] This graph shows the change in the degradation rate with respect to the change in the average fiber length of carbon nanotubes in a secondary battery according to one embodiment of the present disclosure, as a ratio based on the degradation rate when the average fiber length is 3.23 μm. [Figure 3] This graph shows the change in degradation rate with respect to a change in the average fiber diameter of carbon nanotubes in a secondary battery according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0013] [Negative electrode material for secondary batteries] The negative electrode material for a secondary battery according to the embodiment of this disclosure (hereinafter sometimes referred to as "negative electrode material") comprises a negative electrode active material and carbon nanotubes. The negative electrode active material comprises graphite and a material containing silicon. The average fiber diameter of the carbon nanotubes is 0.5 to 6 nm, and the average fiber length is 1.2 to 8 μm. As a result, even when a material containing silicon, which undergoes a large volume change due to charging and discharging, is used as the negative electrode active material, current collection failure of the negative electrode active material (graphite and material containing silicon) due to volume change is suppressed, and a high capacity retention rate can be maintained.

[0014] (Carbon nanotubes) Carbon nanotubes are carbon fibers with extremely small fiber diameters (nano-sized) and a very large aspect ratio (ratio of fiber length to outer diameter). Carbon fibers with a large aspect ratio result in linear contact between active materials and between active materials and current collectors, rather than point contact. Highly conductive carbon fibers are interposed between active material particles, forming linear contact points with the particles. This allows the highly conductive carbon nanotubes to form linear conductive paths between active materials and between active materials and current collectors, as well as linear contact points with the current collector, thereby improving current collection performance.

[0015] The average fiber length of the carbon nanotubes is 1.2 μm or more. In this case, even when the volume of the negative electrode active material changes significantly due to charging and discharging, linear contact with the carbon nanotube fibers is maintained in accordance with the volume change, and the electrical connection with the negative electrode active material can be maintained. When the average fiber length of the carbon nanotubes is 1.2 μm or more, current collection failure is significantly suppressed. The average fiber length may be 1.5 μm or more, but 2 μm or more is preferable.

[0016] On the other hand, as the average fiber length of carbon nanotubes increases, they tend to aggregate more easily, leading to poor dispersion and increased slurry viscosity during slurry preparation. Furthermore, the longer the average fiber length, the higher the carbon nanotube content in the anode material becomes when securing the required number of carbon nanotubes, making it difficult to achieve high capacity. To achieve high capacity and to facilitate the preparation of a slurry in which carbon nanotubes are dispersed together with the anode active material during the manufacturing of the anode material, as well as to suppress the increase in slurry viscosity, the average fiber length of carbon nanotubes should be 8 μm or less. The average fiber length may be 6 μm or less, but 4 μm or less is preferred. When the average fiber length of carbon nanotubes is 4 μm or less, the aggregation of carbon nanotubes is suppressed, making it easy to obtain a slurry with good dispersion.

[0017] The average fiber length of the carbon nanotubes is 1.2 μm to 8 μm, and may also be 1.2 μm to 6 μm, 1.2 μm to 4 μm, or 1.5 μm to 4 μm, with a range of 2 μm to 4 μm being preferred.

[0018] The average fiber diameter of carbon nanotubes can be 0.5 nm or greater, and preferably 1 nm or greater, as long as it can be manufactured without difficulty. On the other hand, the larger the average fiber diameter of carbon nanotubes, the fewer carbon nanotubes are contained in the anode material when the carbon nanotube content is the same, making it difficult to suppress current collection failure. To suppress current collection failure, the carbon nanotube content should be increased, but the higher the carbon nanotube content in the slurry, the more easily the carbon nanotubes aggregate in the slurry, making it difficult to disperse them uniformly, and the viscosity of the slurry also tends to increase. Therefore, in the manufacture of the anode material, the average fiber diameter of the carbon nanotubes should be 6 nm or less in order to facilitate the production of a slurry in which carbon nanotubes are dispersed together with the anode active material, and to suppress the increase in slurry viscosity. By setting the average fiber diameter to 6 nm or less, a negative electrode material can be obtained in which current collection failure is significantly suppressed and which is easy to manufacture. The average fiber diameter of carbon nanotubes should preferably be 4 nm or less, and more preferably 3 nm or less.

[0019] The average fiber diameter of carbon nanotubes is 0.5 nm to 6 nm, and may also be 1 nm to 6 nm, with a preferred range being 1 nm to 4 nm or 1 nm to 3 nm.

[0020] Here, the average fiber length of carbon nanotubes can be determined by image analysis using a scanning electron microscope (SEM). The average fiber length can be determined, for example, by arbitrarily selecting several carbon nanotubes (e.g., 100 to 1000), measuring their fiber lengths, and averaging them. Fiber length refers to the length when the nanotube is in a straight line. Fiber diameter refers to the length perpendicular to the fiber length direction and represents the outer diameter of the carbon nanotube. It is known that carbon nanotubes may form bundles of multiple fibers, and the average length in this bundled state may be used as the fiber length of each carbon nanotube. The average fiber diameter of carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM). Furthermore, for carbon nanotubes with two or fewer layers on the tube wall, the Raman shift in the Raman spectroscopy spectrum is 150-300 cm². -1 By analyzing the RBM (Radial Breathing Mode) that appears within this range, its diameter can be estimated.

[0021] Carbon nanotubes may be single-walled, double-walled, or multi-walled. Single-walled carbon nanotubes are preferred because a large effect can be obtained with a small amount. Carbon nanotubes with an average fiber diameter of 5 nm or less contain a large amount of single-walled carbon nanotubes. Single-walled carbon nanotubes may account for 50% or more of the total mass of carbon nanotubes.

[0022] In the negative electrode material, the carbon nanotube content relative to the total negative electrode active material may be 0.005% to 0.1% by mass, or 0.005% to 0.07% by mass. According to this embodiment, even with such a low carbon nanotube content, poor current collection can be suppressed by setting the average fiber length and average fiber diameter of the carbon nanotubes within the above range. A high level of suppression of poor current collection can be obtained by setting the carbon nanotube content to 0.005% by mass or more. A carbon nanotube content of 0.1% by mass or less makes it easier to prepare a slurry in which carbon nanotubes are dispersed together with the negative electrode active material, and the increase in viscosity of the slurry is suppressed. Furthermore, the increase in manufacturing costs due to the use of carbon nanotubes is suppressed, and the negative electrode material can be obtained with minimal increase in manufacturing costs.

[0023] The carbon nanotube content relative to the total negative electrode active material may be 0.005% by mass or more, 0.01% by mass or more, or 0.02% by mass or more. Alternatively, the carbon nanotube content relative to the total negative electrode active material may be 0.1% by mass or less, 0.07% by mass or less, 0.05% by mass or less, or 0.04% by mass or less. These lower and upper limits can be combined in any way.

[0024] The carbon nanotube content relative to the total negative electrode active material can be determined from a sample obtained by extracting only the negative electrode active material layer (negative electrode mixture layer) from a discharged secondary battery. Specifically, first, the discharged secondary battery is disassembled and the negative electrode is extracted. Next, the negative electrode is washed with an organic solvent and then vacuum-dried, after which the negative electrode active material layer is peeled off to obtain a sample. The pulverized sample is dispersed in a dispersion medium such as water or alcohol, and the carbon nanotubes can be separated by centrifugation. Furthermore, by performing thermal analysis such as TG-DTA on the sample, the ratio of binder components and conductive material components other than the negative electrode active material can be calculated. By performing micro-Raman spectroscopy on the cross-section of the negative electrode active material layer, carbon species such as carbon nanotubes and acetylene black can be identified, and their proportions can be calculated from thermal analysis such as TG-DTA on the peeled sample.

[0025] (Negative electrode active material) As the negative electrode active material, metallic lithium, lithium alloys, etc., may be used, but a material capable of electrochemically intercalating and releasing lithium ions is preferably used. As such a material, the negative electrode active material includes graphite and a material containing the element silicon (hereinafter sometimes referred to as "Si-containing material").

[0026] Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Known graphite used as a negative electrode active material may also be used. Other carbonaceous materials may be included in the negative electrode active material. Examples of other carbonaceous materials include easily graphitizable carbon (soft carbon) and difficult-to-graphitize carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination. Among the carbonaceous materials, graphite is preferred because it exhibits excellent charge-discharge stability and has low irreversible capacity.

[0027] Note that graphite refers to a material in which a graphite-type crystal structure has developed, and generally refers to the average interplanar spacing d of the (002) plane measured by X-ray diffraction. 002 This refers to carbon materials with a wavelength of 0.340 nm or less.

[0028] Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which the silicon phase is dispersed within the lithium-ion conductive phase (matrix). Examples of silicon oxides include SiO2. X Particles are an example. X is, for example, 0.5 ≤ X < 2, and may also be 0.5 ≤ X < 1.6 or 0.8 ≤ X ≤ 1.6. As the lithium ion conducting phase, at least one selected from the group consisting of SiO2 phase, silicate phase, and carbon phase may be used.

[0029] Examples of Si-containing materials include SiO X At least one particle selected from the group consisting of a first particle containing silicon oxide represented by the equation (0.5 ≤ X < 1.6), a second particle containing a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase, and a third particle containing a carbon phase and a silicon phase dispersed in the carbon phase may be used. By using silicon-containing particles as a negative electrode active material, it is possible to increase the capacity of the battery. On the other hand, silicon-containing particles undergo large volume changes due to the intercalation and release of lithium ions, and are prone to current collection failure due to repeated charging and discharging. When current collection failure occurs, the electrical connection between the silicon-containing particles and other negative electrode active materials or current collectors is broken, and the silicon-containing particles become isolated. Isolated particles cannot contribute to capacity, and as a result of current collection failure, the capacity retention rate decreases. However, by including carbon nanotubes having the above-mentioned average fiber diameter and average fiber length in the negative electrode material, current collection failure can be effectively suppressed, and a high capacity retention rate can be maintained.

[0030] The Si-containing material may contain multiple types of particles selected from the group consisting of first particles, second particles, and third particles. For example, the Si-containing material may consist of two types of particles selected from these, or it may contain all three types of particles. Specifically, the Si-containing material may contain first particles and second particles, first particles and third particles, or second particles and third particles. Alternatively, the Si-containing material may contain all of the first, second, and third particles. The Si-containing material is preferably used as a negative electrode active material in combination with graphite.

[0031] The average particle size of the graphite contained in the negative electrode active material is preferably 10 to 30 μm. The average particle size of the Si-containing material contained in the negative electrode active material (the average particle size of the first, second, and third particles) is preferably 1 to 20 μm, and more preferably 5 to 15 μm. In this case, the effect of suppressing current collection failure by including carbon nanotubes having the above fiber diameter and fiber length in the negative electrode material is significant. The average particle size is determined by observing the cross-section of the negative electrode active material layer using SEM or TEM. The particle species is identified by micro-Raman spectroscopy, and the grain boundaries of each particle are determined from the cross-sectional image. The diameter of the circle equal to the area defined by the grain boundaries is taken as the particle size. At least 100 particles are arbitrarily selected, and the average value of the particle size is determined and taken as the average particle size. Before forming the negative electrode active material layer, the average particle diameter can be the median diameter (D50) at which the cumulative volume in the volume-based particle size distribution reaches 50%. The median diameter can be determined, for example, using a laser diffraction / scattering particle size distribution analyzer.

[0032] The negative electrode active material may contain active materials other than graphite and Si-containing materials. Examples of other active materials include carbonaceous materials other than graphite, such as easily graphitizable carbon (soft carbon) and difficult-to-graphitize carbon (hard carbon).

[0033] When the negative electrode active material contains graphite and a Si-containing material, the content rate of the Si-containing material with respect to the total of the graphite and the Si-containing material may be 5 to 50% by mass. In this case, a higher capacity can be achieved as compared with the case where the negative electrode active material is only graphite. The content rate of the Si-containing material with respect to the total of the graphite and the Si-containing material is preferably 6 to 20% by mass.

[0034] The content rate of graphite in the negative electrode active material may be in the range of 50 to 99% by mass. When the particles of the Si-containing material contain graphite on the surface and / or inside, the graphite is not included in the above content rate of graphite. The content rate of graphite is the content rate of graphite not contained in the Si-containing material.

[0035] (First particle) The first particle is a mixture in which SiO2 and Si fine particles are mixed at a microscopic level. The first particle contains, for example, silicon oxide represented by the formula SiO X (0.5 ≦ X < 1.6). The first particle may include silicon oxide particles and a carbon layer disposed around the silicon oxide particles.

[0036] (Second particle) The second particle contains a lithium silicate phase and silicon particles (silicon phase) dispersed in the lithium silicate phase. The lithium silicate phase contains, or may be composed of, lithium silicate represented by the formula Li 2Z SiO (2+Z) (0 < Z < 2). It is preferable that Z satisfies the relationship 0 < Z < 1. At least 50% by mass (for example, at least 60% by mass) of the lithium silicate phase may be composed of lithium silicate satisfying 0 < Z ≦ 0.5.

[0037] The second particle may contain at least one element Me dispersed in the lithium silicate phase. The at least one element Me is at least one element selected from the group consisting of rare earth elements and alkaline earth metal elements. Examples of alkaline earth metal elements include Mg, Ca, Sr, Ba, etc.

[0038] The element Me may be dispersed as Me oxide in the lithium silicate phase. The Me oxide may include at least one selected from the group consisting of yttrium oxide, cerium oxide, calcium oxide, and magnesium oxide. The lithium silicate phase may contain zirconium oxide, and the element Me may be dispersed in the zirconium oxide.

[0039] The amount of elemental Me contained in the second particle can be indicated by the amount calculated assuming that elemental Me forms a stoichiometric oxide, regardless of the state of elemental Me or the type of compound of elemental Me (estimated Me oxide amount). The estimated Me oxide amount may be in the range of 0.001 mass% to 1.0 mass% relative to the total of the lithium silicate phase and silicon particles. By setting the estimated Me oxide amount to 0.001 mass% or more, the effect of reducing the reaction area and improving the hardness of the lithium silicate phase is greatly increased. On the other hand, by setting the estimated Me oxide amount to 1.0 mass% or less, the decrease in initial volume can be suppressed.

[0040] The lithium silicate phase may contain metal compounds such as metal oxides, metal carbides, metal nitrides, and metal borides. Preferred metal compounds are metal oxides and metal carbides. In particular, it is preferable to use at least one selected from the group consisting of zirconium oxide (ZrO2), aluminum oxide (Al2O3), zirconium carbide (ZrC), tungsten carbide (WC), and silicon carbide (SiC). The amount of metal element compounds other than element Me may be in the range of 0.005 mass% to 15 mass% (for example, in the range of 0.01 mass% to 10 mass% or 0.01 mass% to 1 mass%) relative to the total of the lithium silicate phase and silicon particles. The amount of metal element compounds can be determined by assuming that the metal elements form stoichiometric oxides, similar to the content of element Me.

[0041] The average particle size of the second particle may be in the range of 1 μm to 25 μm (for example, 4 μm to 15 μm). In this range, stress due to volume changes of the second particle during charging and discharging is easily relieved, making it easier to obtain good cycle characteristics. Furthermore, the surface area of ​​the second particle becomes appropriate, and capacity reduction due to side reactions with non-aqueous electrolytes is suppressed.

[0042] The crystallite size of silicon particles dispersed within the lithium silicate phase is, for example, 10 nm or larger. The silicon particles have a particulate phase of elemental silicon (Si). When the crystallite size of the silicon particles is 10 nm or larger, the surface area of ​​the silicon particles can be kept small, making it less likely for the silicon particles to degrade, which is associated with the generation of irreversible capacitance. The crystallite size of the silicon particles is calculated using Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0043] The average particle size of the silicon particles in the second particle is preferably 500 nm or less (more preferably 200 nm or less, and even more preferably 50 nm or less) before the first charge. After the first charge, the average particle size of the silicon particles is preferably 400 nm or less (more preferably 100 nm or less). By miniaturizing the silicon particles, the volume change during charging and discharging is reduced, and the structural stability of the second particle is further improved.

[0044] The content of silicon particles (elementary Si) in the second particle is preferably in the range of 20% to 95% by mass (for example, 35% to 75% by mass) from the viewpoint of increasing capacity and improving cycle characteristics. Within this range, lithium ion diffusion is also good, making it easier to obtain excellent loading characteristics. Furthermore, the surface area of ​​silicon particles that is not covered by the lithium silicate phase is reduced, and side reactions between the non-aqueous electrolyte and silicon particles are suppressed.

[0045] The second particle may include a conductive material covering at least a portion of its surface. Since the lithium silicate phase has poor electronic conductivity, the conductivity of the second particle tends to be low as well. By covering the surface with a conductive material, the conductivity can be dramatically increased. Preferably, the conductive layer is thin enough not to substantially affect the average particle size of the second particle. For example, from the viewpoint of ensuring conductivity and lithium ion diffusion, the thickness of the conductive layer may be in the range of 1 nm to 200 nm (e.g., 5 nm to 100 nm). Examples of materials for the conductive layer and examples of methods for forming it will be described later.

[0046] (The third particle) The third particle comprises a carbon phase and silicon particles (silicon phase) dispersed within the carbon phase. The carbon phase of the third particle may consist of amorphous carbon. Amorphous carbon may be hard carbon, soft carbon, or something else. Amorphous carbon is generally defined as the average interplanar spacing d of the (002) plane as measured by X-ray diffraction. 002 This refers to carbon materials with a thickness exceeding 0.34 nm. The carbon phase of the third particle is conductive. Therefore, even if a void is formed around the third particle, the contact between the third particle and its surroundings is easily maintained. As a result, capacity degradation due to repeated charge-discharge cycles is easily suppressed.

[0047] The silicon particle content in the third particle may be 30% by mass or more and 80% by mass or less, or 40% by mass or more and 70% by mass or less. Within this range, a sufficiently high capacity of the negative electrode can be achieved, and the cycle characteristics are also easily improved.

[0048] The average particle size of the silicon particles in the third particle can be, for example, 1 nm or larger. Alternatively, the average particle size of the silicon particles may be 1000 nm or less, 500 nm or less, 200 nm or less, or 100 nm or less (and even 50 nm or less). The finer the silicon particles, the smaller the volume change of the third particle during charging and discharging, and the improved structural stability of the third particle.

[0049] The composition and component content rates of the second and third particles can be analyzed by the method described in International Publication No. 2018 / 179969.

[0050] The content rate of each element contained in the Si-containing material may be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, the Si-containing material is dissolved in a heated acid solution, carbon in the solution residue is removed by filtration, and then the obtained filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using a standard solution of each commercially available element, and the content rate of each element is calculated.

[0051] The second and third particles each have a so-called sea-island structure. The silicon particles (islands) in the second and third particles are each dispersed in a matrix (sea) of a silicate phase and a carbon phase and are covered with a lithium ion conduction phase (silicate phase and carbon phase). In the sea-island structure, since the contact between the silicon particles and the electrolyte is restricted, side reactions are suppressed. Further, the stress generated by the expansion and contraction of the silicon particles is relaxed by the matrix of the lithium ion conduction phase.

[0052] The average particle diameter of the graphite (graphite particles) contained as an active material in the negative electrode may be 13 μm or more and 25 μm or less. The average particle diameter of the graphite is preferably larger than the average particle diameter of the Si-containing material. According to this configuration, voids are formed between the relatively large graphite particles, and the particles of the Si-containing material are easily accommodated in the voids. Therefore, it is easy to increase the filling rate of the active material in the negative electrode and to obtain a higher-capacity negative electrode. Further, the particles of the Si-containing material present in the voids contribute to maintaining the electronic contact between the graphite particles. On the other hand, even if the particles of the Si-containing material present in the voids expand and contract, the negative electrode as a whole is less likely to expand and contract, and thus deterioration due to charge-discharge cycles is less likely to occur.

[0053] (Method for producing the first particles) SiO which is the first particle XFor example, it can be manufactured by vapor deposition. SiO X The particles may be coated with carbon. First, SiO X The particles are crushed and classified to adjust the particle size. Next, the surface of the resulting particles is coated with carbon by CVD under an argon atmosphere. Then, by crushing and classifying this, SiO is obtained. X Prepare the first particle represented as SiO. X The method for coating the particles with carbon is not limited to the method described above, and various well-known methods can be employed. Also, SiO X The process of coating the particles with carbon may be omitted.

[0054] (Method for producing the second particle) Next, an example of a method for producing the second particle will be described in detail. The second particle may be produced by methods other than those described below. The second particle may also be produced by the method described in International Publication No. 2018 / 179969.

[0055] The second particles are generally synthesized through two processes: a pre-process to obtain lithium silicate and a post-process to obtain the second particles from lithium silicate and raw silicon. When element Me is added, element Me may be added to the raw materials for lithium silicate in the pre-process, but it is preferable to add it in the post-process so as not to affect the synthesis of lithium silicate. More specifically, the method for producing the second particles preferably includes the steps of (i) mixing silicon dioxide and a lithium compound, and calcining the resulting mixture to obtain lithium silicate, and (ii) compounding the lithium silicate with raw silicon (and element Me as needed) to obtain second particles containing a lithium silicate phase and silicon particles dispersed within the lithium silicate phase.

[0056] (Step (i)) Formula: Li 2Z SiO 2+ZThe Z value of lithium silicate, represented by [formula], can be controlled by the atomic ratio of silicon to lithium in the mixture of silicon dioxide and lithium compound: Li / Si. To synthesize high-quality lithium silicate with minimal leaching of alkaline components, it is preferable to make Li / Si less than 1.

[0057] Lithium compounds such as lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride can be used. These may be used individually or in combination of two or more.

[0058] It is preferable to heat the mixture containing silicon dioxide and a lithium compound in air at 400°C to 1200°C, preferably 800°C to 1100°C, to react the silicon dioxide and the lithium compound.

[0059] (Step (ii)) Next, the lithium silicate is compounded with the raw silicon. For example, the mixture of lithium silicate and raw silicon (which may also contain the element Me) can be pulverized while applying shear force. For the raw silicon, coarse silicon particles with an average particle size of several μm to several tens of μm can be used. It is preferable that the silicon particles obtained in the end have a crystallite size of 10 nm or more, calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane of the XRD pattern.

[0060] For the element Me used in the preparation, any oxide, oxalate, nitrate, sulfate, halide, or carbonate of element Me may be used. Among these, Me oxides are preferred because they are stable and have good ionic conductivity. More specifically, examples include CeO2, Sc2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. Compounds containing elements other than Me and oxygen, such as yttria-stabilized zirconia, may also be used. These may be used individually or in combination of two or more.

[0061] For example, lithium silicate and raw silicon (and optionally a compound of element Me) can be mixed in a predetermined mass ratio, and the mixture can be stirred while being atomized using a grinding device such as a ball mill. However, the compounding process is not limited to this. For example, silicon nanoparticles and lithium silicate nanoparticles (and optionally a compound of element Me) can be synthesized without using a grinding device, and these can then be mixed.

[0062] Next, the particulated mixture is heated and sintered at 450°C to 1000°C in an inert atmosphere (e.g., an atmosphere such as argon or nitrogen). At this time, pressure may be applied to the mixture using a hot press or the like during sintering to produce a sintered body of the mixture. Lithium silicate is stable at 450°C to 1000°C and hardly reacts with silicon, so any decrease in capacity is minor. During sintering, the silicate softens and flows to fill the gaps between silicon particles. As a result, a dense, block-shaped sintered body can be obtained in which the silicate phase forms the "sea" and the silicon particles form the "islands".

[0063] The sintered body can then be crushed into granular material to obtain second particles. By appropriately selecting the crushing conditions, second particles with an average particle size within the range described above can be obtained.

[0064] After step (ii), step (iii) may be performed to form a conductive layer by coating at least a portion of the surface of the second particles with a conductive material. The conductive material is preferably electrochemically stable, and carbon material is preferred. As a method for coating the surface of the particulate material with carbon material, a CVD method using hydrocarbon gases such as acetylene and methane as raw materials may be used. Alternatively, a method may be used in which coal pitch, petroleum pitch, phenolic resin, etc., are mixed with the second particles and then heated. In addition, carbon black may be attached to the surface of the second particles.

[0065] A step of washing the second particles with acid may be performed. For example, the second particles may be washed with an acidic aqueous solution. Washing with acid can dissolve and remove trace amounts of components such as Li2SiO3 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.

[0066] (Method for producing the third particle) The first and second methods are described below as examples of methods for producing the third particle. The third particle may also be produced by methods other than those described below.

[0067] In the first method, the raw silicon and carbon source are first 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. Alternatively, an organic solvent may be added to the mixture for wet pulverization. At this stage, the raw silicon is finely pulverized to produce silicon particles. These silicon particles are then dispersed in the matrix of the carbon source.

[0068] 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.

[0069] Suitable organic solvents include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides.

[0070] Next, the composite of silicon particles and a carbon source is heated to 700°C to 1200°C in an inert gas atmosphere (e.g., argon or nitrogen). This heating carbonizes the carbon source, generating amorphous carbon. This yields a third particle in which silicon particles are dispersed in a carbon phase containing amorphous carbon.

[0071] In the second method, first, the raw silicon and carbon material are mixed, and the mixture of raw silicon and carbon material is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. Alternatively, an organic solvent may be added to the mixture for wet pulverization. At this stage, the raw silicon is finely pulverized to produce silicon particles. These silicon particles are then dispersed in a matrix of carbon material.

[0072] By compounding the raw silicon and carbon material as described above, a third particle is obtained in which silicon particles are dispersed in the carbon phase of amorphous carbon. Subsequently, the third particle may be heated to 700°C to 1200°C in an inert gas atmosphere.

[0073] 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 particles 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.

[0074] [Secondary battery] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery according to an embodiment of the present disclosure comprises a negative electrode containing the negative electrode material for secondary batteries, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0075] (Negative electrode) The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer (negative electrode mixture layer) disposed on the surface of the negative electrode current collector. The negative electrode active material layer includes the negative electrode material for secondary batteries described above. That is, the negative electrode active material layer includes the negative electrode active material and carbon nanotubes, and optionally includes other components other than the negative electrode active material and carbon nanotubes. Examples of other components include binders, conductive agents, and thickeners. These other components may be components used in known secondary batteries. As described above, the negative electrode active material includes graphite and Si-containing materials.

[0076] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) or 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. The thickness of the negative electrode current collector is not particularly limited, but for example, it can be 1 to 50 μm, or 5 to 30 μm.

[0077] The negative electrode active material layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector to form a coating, and then drying the coating. The dried coating may be rolled if necessary. Examples of dispersion media include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or mixed solvents thereof. The ratio of components in the negative electrode mixture can be adjusted by changing the mixing ratio of the materials in the negative electrode mixture.

[0078] Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives and salts. Examples of conductive agents include conductive carbon materials, fluorinated carbon, and organic conductive materials. Examples of thickeners include carboxymethylcellulose (CMC) and polyvinyl alcohol. These components may be used individually or in combination of two or more materials.

[0079] Carbon nanotubes are included in the negative electrode active material layer as a conductive agent. The carbon nanotubes used have the average fiber length and average fiber diameter described above. Because carbon nanotubes have an extremely large aspect ratio (ratio of length to diameter), they can exhibit high conductivity even in small quantities. By using carbon nanotubes as a conductive material, it is possible to increase the proportion of negative electrode active material in the negative electrode active material layer while maintaining high conductivity. Therefore, secondary batteries can be made to have higher capacity.

[0080] As a conductive agent, conductive carbon materials other than carbon nanotubes may be used in mixture with carbon nanotubes. Examples of conductive carbon materials other than carbon nanotubes include at least one selected from the group consisting of amorphous carbon and carbon fibers. Amorphous carbon includes hard carbon and soft carbon. Examples of soft carbon include carbon black such as acetylene black and Ketjen black. Multiple types of these materials may be combined and used as conductive agents.

[0081] The negative electrode active material layer may or may not contain conductive materials other than carbon nanotubes. In addition to carbon nanotubes, the negative electrode active material layer may also contain carbon black as a conductive material other than carbon nanotubes. However, if these are included in large quantities, the proportion of negative electrode active material in the negative electrode active material layer will decrease. Therefore, the mass of conductive materials other than carbon nanotubes included in the negative electrode active material layer may be 100 times or less the mass of carbon nanotubes included in the negative electrode active material layer (for example, in the range of 0 to 80 times, 0 to 50 times, or 0 to 10 times).

[0082] Examples of carbon nanotubes include carbon nanofibers. Since various types of carbon nanotubes are commercially available, commercially available ones may be used. Alternatively, carbon nanotubes may be synthesized using known synthesis methods.

[0083] (positive electrode) A positive electrode typically comprises a positive electrode current collector and a positive electrode active material layer (positive electrode mixture layer) formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by coating the surface of the positive electrode current collector with a positive electrode slurry, which is obtained by dispersing the positive electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary. The positive electrode mixture contains positive electrode active material as an essential component and may contain binders, conductive agents, etc., as optional components.

[0084] A lithium metal composite oxide can be used as the positive electrode active material. The lithium metal composite oxide may be a composite oxide having a layered structure (e.g., a rock salt crystal structure) containing lithium and a transition metal. Examples of lithium metal composite oxides include Li a CoO2, Li a KiO2, 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、 LiGPO 4、 Li2GPO4F 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. G 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, increases or decreases with charging and discharging.

[0085] More specifically, lithium metal composite oxides are, for example, Li a Ni x M 1-xIt may also be a lithium-nickel composite oxide represented by O2 (where 0 < a ≤ 1.2, 0.8 ≤ x ≤ 1, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B). Among these, it is preferable that M contains at least one selected from the group consisting of Co, Mn, Al, and Fe. From the perspective of the stability of the crystal structure, Al may be included as M. Specific examples of such composite oxides include lithium-nickel-cobalt-aluminum composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.).

[0086] Here, from the perspective of obtaining high capacity, it is desirable that the proportion of Ni in the metal elements other than Li contained in the lithium metal composite oxide is 80 atomic% or more. The proportion of Ni in the metal elements other than Li may be 85 atomic% or more, or may be 90 atomic% or more. The proportion of Ni in the metal elements other than Li is preferably, for example, 95 atomic% or less. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0087] In the lithium-nickel composite oxide, Co, Mn, and / or Al that can be included as the metal element M other than Li and Ni contribute to the stabilization of the crystal structure of the composite oxide with a high Ni content. However, from the perspective of reducing manufacturing costs, it is more desirable that the Co content is less. The lithium-nickel composite oxide with a low Co content or without Co may contain Mn and Al.

[0088] As the binder and conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite may be used.

[0089] The shape and thickness of the positive electrode current collector can be selected respectively from the shape and range according to the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0090] The elemental content of lithium metal composite oxides can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), electron probe microanalyzer (EPMA), or energy dispersive X-ray spectroscopy (EDX), among other methods.

[0091] (electrolyte) The electrolyte contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ionic dissociation in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and solute are additives. Various additives may be included in the electrolyte.

[0092] Any known material can be used as a solvent. Examples of solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). 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). Examples of linear carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). Non-aqueous solvents may be used individually or in combination of two or more.

[0093] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0094] 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, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, 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.

[0095] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0096] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0097] (Separator) A separator is interposed between the positive and negative electrodes. The separator has high ion permeability and possesses appropriate mechanical strength and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as the separator. Polyolefins such as polypropylene and polyethylene are preferred as the material of the separator. In addition, aramid fibers may be used to increase mechanical strength.

[0098] One example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, is housed together with the non-aqueous electrolyte in an outer casing. However, it is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The form of the non-aqueous electrolyte secondary battery is also not limited, and may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0099] Figure 1 is a schematic perspective view showing a portion of a rectangular non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. The secondary battery 1 shown in Figure 1 includes a bottomed rectangular battery case 11, an electrode group 10 housed within the battery case 11, and a non-aqueous electrolyte (not shown). The electrode group 10 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 10 is formed by winding the negative electrode, positive electrode, and separator around a flat core and then removing the core.

[0100] One end of the negative electrode lead 15 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of the positive electrode lead 14 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the negative electrode lead 15 is electrically connected to the negative electrode terminal 13 provided on the sealing plate 12. A gasket 16 is placed between the sealing plate 12 and the negative electrode terminal 13 to insulate them. The other end of the positive electrode lead 14 is connected to the sealing plate 12 and electrically connected to the battery case 11, which also serves as the positive electrode terminal. A resin frame 18 is placed on top of the electrode group 10. The frame 18 isolates the electrode group 10 from the sealing plate 12 and also isolates the negative electrode lead 15 from the battery case 11. The opening of the battery case 11 is sealed by the sealing plate 12. An injection hole 17a is formed in the sealing plate 12. The electrolyte is injected into the battery case 11 through the injection port 17a. After that, the injection port 17a is sealed by the seal 17.

[0101] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0102] (1) Fabrication of the negative electrode A negative electrode slurry was prepared by mixing a negative electrode active material, sodium polyacrylate (PAA-Na), sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), carbon nanotubes (CNTs), and water in a predetermined mass ratio. The carbon nanotubes used had average fiber length and average fiber diameter as shown in Tables 1 to 6. A mixture of graphite and a Si-containing material was used as the negative electrode active material.

[0103] Next, a coating film was formed on the surface of the copper foil (negative electrode current collector) by applying the negative electrode slurry. After drying the coating film, it was rolled. In this way, negative electrode active material layers were formed on both sides of the copper foil. The mixing ratio of carbon nanotubes in the negative electrode slurry was set to the mass percentages shown in Table 1, with the negative electrode active material (total of graphite and Si-containing material) being 100% by mass. The mixing ratio of negative electrode active material, sodium polyacrylate, sodium carboxymethylcellulose, and styrene-butadiene rubber in the negative electrode slurry was set to a mass ratio of negative electrode active material:PAA-Na:CMC-Na:SBR = 100:0.5:1:1.

[0104] The first, second, and third particles, which are Si-containing materials, were prepared by the following method.

[0105] The first particles were prepared by the following method: First, SiO was deposited by vapor deposition. X (X=1.20) was prepared. The obtained SiO X The material was crushed and classified to adjust the particle size. Next, the surface of the resulting particles was coated with carbon by CVD under an argon atmosphere. Then, the first particles were prepared by crushing and classifying this material.

[0106] The second type of particle was prepared by the following method. First, silicon dioxide and lithium carbonate were mixed in an atomic ratio of Si / Li of 1.05, and the mixture was calcined in air at 950°C for 10 hours to obtain lithium silicate represented by the formula Li2Si2O5. The obtained lithium silicate was pulverized to an average particle size of 10 μm.

[0107] Next, the obtained lithium silicate, raw silicon (3N, average particle size 10 μm), and yttrium oxide (Y2O3) were mixed in a mass ratio of 50:50:0.0005. The mixture was filled into a pot (made of stainless steel, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch, P-5), 24 stainless steel balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was ground in an inert atmosphere at 200 rpm for 50 hours. 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 an inert atmosphere to obtain a sintered body (master particle) of the mixture.

[0108] Subsequently, the sintered body was crushed, passed through a 40 μm mesh, and mixed with coal pitch (JFE Chemical Corporation, MCP250). The mixture was fired at 800°C in an inert atmosphere, and the surface of the crushed particles was coated with conductive carbon to form a conductive layer. The amount of conductive layer coating was 5% by mass of the total mass of the crushed particles. Then, using a sieve, second particles with an average particle size of 5 μm and a conductive layer were obtained.

[0109] The third type of particle was prepared by the following method: Coal pitch (JFE Chemical Corporation, MCP250) as 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 pot (made of stainless steel, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 stainless steel balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was ground in an inert atmosphere at 200 rpm for 50 hours to obtain a composite of silicon particles and carbon source.

[0110] Next, a composite of silicon particles and a carbon source was calcined in an inert gas atmosphere to carbonize the carbon source, obtaining a silicon-containing material in which silicon particles were dispersed in a carbon phase containing amorphous carbon. Subsequently, a third particle with an average particle size of 10 μm was obtained using a jet mill.

[0111] Tables 1 to 6 show the mass-based content of the first, second, and third particles, as well as the carbon nanotubes, relative to the negative electrode active material (total of graphite and Si-containing material) used in each battery, along with the average fiber length and average fiber diameter of the carbon nanotubes.

[0112] (2) Preparation of the positive electrode As the positive electrode active material, LiNi 0.88 Co 0.09 Al 0.03 O2 was used. A positive electrode slurry was prepared by mixing the positive electrode active material, acetylene black, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) in a predetermined mass ratio. Next, the positive electrode slurry was applied to the surface of an aluminum foil, which was to be used as the positive electrode current collector. After the coating was dried, the foil was rolled to form positive electrode mixture layers on both sides of the aluminum foil.

[0113] (3) Preparation of electrolyte An electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. The concentration of LiPF6 in the non-aqueous electrolyte was 1.3 mol / liter.

[0114] (4) Manufacturing of secondary batteries Lead tabs were attached to each electrode. Next, the positive and negative electrodes were wound in a spiral pattern via a separator so that the leads were located on the outermost edge. The electrode group was thus fabricated. Next, the electrode group was inserted into an outer casing made of laminate film with aluminum foil as a barrier layer and vacuum dried. Then, a non-aqueous electrolyte was injected into the outer casing and the opening of the casing was sealed. In this way, a secondary battery was obtained.

[0115] In this manner, several secondary batteries A1-A36 and B1-B3 with different content ratios of Si-containing material in the total negative electrode active material, ratios of first, second, and third particles in the Si-containing material, average fiber length, average fiber diameter, and content ratios of carbon nanotubes were fabricated and evaluated. A1-A36 are examples, and B1-B3 are comparative examples.

[0116] In batteries A1-A14, B1, and B2 shown in Table 1, a mixture of first and second particles was used as the Si-containing material. The proportion of the first and second particles to the total negative electrode active material was 3% by mass, for each. In batteries A15-A18 and B3 shown in Table 2, the first particle was used as the Si-containing material, and the proportion of the first particle to the total negative electrode active material was set to 10% by mass. In batteries A19 to A22 shown in Table 3, the first particle was used as the Si-containing material, and the ratio of the first particle to the total negative electrode active material was varied in the range of 7.5% by mass to 20% by mass. In batteries A23-A28 and A36 shown in Table 4, a mixture of first, second, and third particles was used as the Si-containing material. The proportions of the first, second, and third particles to the total negative electrode active material were 2.1% by mass, 3% by mass, and 3.2% by mass, respectively. In batteries A29 to A32 shown in Table 5, a third type of Si-containing material was used, and the proportion of the first type of particle to the total negative electrode active material was set to 15% by mass. In batteries A33 to A35 shown in Table 6, a mixture of first and second particles was used as the Si-containing material. The proportions of the first and second particles to the total negative electrode active material were 3% by mass and 5.5% by mass, respectively.

[0117] (5) Evaluation (Capacity deterioration rate) Each completed battery was placed in a 25°C environment and charged with a constant current of 0.5 It until the voltage reached 4.2V. Then, it was charged with a constant voltage of 4.2V until the current reached 0.02 It. After that, it was discharged with a constant current of 1.0 It until the voltage reached 2.5V. After that, it was left to stand for 20 minutes. This operation (charge-discharge cycle) was repeated 100 times. Charging and discharging were performed in a 25°C environment.

[0118] Let C0 be the discharge capacity during the initial discharge, and let C0 be the discharge capacity after repeating the above charge-discharge cycle 50 and 100 times, respectively. 50 and C 100 The following was used for measurement. The change in capacity retention rate per cycle was derived from the formula below and evaluated as the degradation rate (%). (Deterioration rate (%))=100×(C 50 -C 100 ) / (C0×50)

[0119] Tables 1 to 6 show the evaluation results of the degradation rate for batteries A1 to A36 and B1 to B3. In Tables 1, 2, 4, and 5, the degradation rate ratio is also shown based on the case where the average fiber length of the added CNTs is 3.23 μm, assuming the same composition of the negative electrode active material (ratio of the first, second, and third particles to the total of graphite and Si-containing material). For each composition, the degradation rate per charge-discharge cycle can be reduced when the average fiber diameter of the carbon nanotubes contained in the negative electrode mixture layer is 0.5 to 6 nm and the average fiber length is 1.2 to 8 μm.

[0120] Figure 2 is a graph showing the change in the degradation rate ratio with respect to the change in the average fiber length of carbon nanotubes, based on batteries A1-A7, A9, A11, A15-A18, A23, A26-A32, A36, and B3. As shown in Figure 2, the degradation rate ratio decreases sharply when the average fiber length is around 1 μm, and at 2 μm or more, the degradation rate ratio is reduced to about 60% of that of battery B3 with an average fiber length of 1 μm. On the other hand, when the average fiber length exceeds 2 μm, the reduction in the degradation rate ratio is small. As the average fiber length of carbon nanotubes increases, the carbon nanotubes tend to aggregate with each other, and the viscosity of the slurry increases. In terms of suppressing the increase in slurry viscosity and facilitating the production of the negative electrode mixture layer, the average fiber length of carbon nanotubes is 8 μm or less, and preferably 4 μm or less. When the average fiber length of carbon nanotubes is 4 μm or less, the aggregation of carbon nanotubes is suppressed, and it is easy to obtain a slurry with a good dispersion state.

[0121] Figure 3 is a graph showing the change in degradation rate with respect to the change in the average fiber diameter of carbon nanotubes, based on batteries A5, A11, A14, B1, and B2. As shown in Figure 3, in the range where the average fiber diameter of carbon nanotubes exceeds 6 nm, the degradation rate is reduced compared to battery B3, although the reduction is small. On the other hand, in the range where the average fiber diameter of carbon nanotubes is 6 nm or less, the degradation rate is significantly reduced. When the average fiber diameter of carbon nanotubes is 4 nm or less, a significantly low degradation rate is obtained, and the increase in slurry viscosity can be suppressed.

[0122] [Table 1]

[0123] [Table 2]

[0124] [Table 3]

[0125] [Table 4]

[0126] [Table 5]

[0127] [Table 6] [Industrial applicability]

[0128] The secondary battery described herein provides a secondary battery with high capacity and excellent cycle characteristics. The secondary battery described herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0129] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of Symbols]

[0130] 1: Non-aqueous electrolyte secondary battery, 2: Negative electrode, 10: Electrode group, 11: Battery case, 12: Sealing plate, 13: Negative electrode terminal, 14: Positive electrode lead, 15: Negative electrode lead, 16: Gasket, 17: Sealing plug, 17a: Injection hole, 18: Frame

Claims

1. It contains a negative electrode active material and carbon nanotubes. The aforementioned negative electrode active material includes graphite and a material containing silicon. The carbon nanotubes have an average fiber diameter of 0.5 to 6 nm and an average fiber length of 1.2 to 4 μm. The average particle size of the graphite is 10 to 30 μm. The average particle size of the silicon-containing material is 1 to 20 μm. A negative electrode material for a secondary battery, wherein the carbon nanotube content relative to the total negative electrode active material is 0.005% by mass to 0.01% by mass.

2. The negative electrode material for a secondary battery according to claim 1, wherein the average fiber diameter of the carbon nanotubes is 1 to 4 nm.

3. The negative electrode material for a secondary battery according to claim 1 or 2, wherein the average fiber length of the carbon nanotubes is 2 to 4 μm.

4. The material containing the silicon element is SiO Ｘ A negative electrode material for a secondary battery according to any one of claims 1 to 3, comprising at least one particle selected from the group consisting of a first particle containing silicon oxide represented by the formula (0.5 ≤ X < 1.6), a second particle containing a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase, and a third particle containing a carbon phase and a silicon phase dispersed in the carbon phase.

5. The negative electrode material for a secondary battery according to any one of claims 1 to 4, wherein the content of the silicon-containing material relative to the total of the graphite and the silicon-containing material is 5 to 50% by mass.

6. The negative electrode material for a secondary battery according to claim 5, wherein the content of the silicon-containing material relative to the total of the graphite and the silicon-containing material is 6 to 20% by mass.

7. The negative electrode material for a secondary battery according to any one of claims 1 to 6, wherein the average particle size of the silicon-containing material is 5 to 15 μm.

8. A secondary battery comprising a negative electrode containing the negative electrode material for secondary batteries described in any one of claims 1 to 7, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

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