WO2022138861A1 - 二次電池用負極材料および二次電池 - Google Patents
二次電池用負極材料および二次電池 Download PDFInfo
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- WO2022138861A1 WO2022138861A1 PCT/JP2021/048000 JP2021048000W WO2022138861A1 WO 2022138861 A1 WO2022138861 A1 WO 2022138861A1 JP 2021048000 W JP2021048000 W JP 2021048000W WO 2022138861 A1 WO2022138861 A1 WO 2022138861A1
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- negative electrode
- carbon
- secondary battery
- carbon nanotubes
- particle
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- H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
- H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
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- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present disclosure relates to a secondary battery, and particularly to an improvement of a negative electrode used in the secondary battery.
- Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have high output and high energy density. Therefore, non-aqueous electrolyte secondary batteries are used as power sources for small consumer applications, power storage devices, and electric vehicles.
- negative electrode active materials for non-aqueous electrolyte secondary batteries.
- a negative electrode active material having a high energy density it has been conventionally proposed to use a silicon compound (for example, silicon oxide) alloyed with lithium or silicon particles (see, for example, Patent Document 1).
- Patent Document 2 describes a conductive composite containing carbon nanotubes having an outer diameter of 1 nm to 6 nm, fine particles of 1000 ⁇ m or less, and a dispersant, and the carbon nanotubes dispersed by the dispersant cover the surface of the fine particles. It has been disclosed.
- the fine particles are, for example, graphite particles having an average particle diameter of 20 ⁇ m, and it is proposed to use a conductive composite as a negative electrode of a lithium ion secondary battery.
- Patent Document 3 describes a composite of an electrode active material and carbon nanotubes, which comprises an electrode active material and a fibrous carbon nanotube aggregate having an average diameter of 10 nm to 20 ⁇ m composed of single-walled carbon nanotubes, as a lithium ion secondary. It is proposed to be used for the negative electrode of batteries.
- Patent Document 2 describes that the ratio of carbon nanotubes in the conductive composite is 0.1 to 20% by mass, and Patent Document 3 describes the mass ratio of the active material to carbon nanotubes. It is described as 1 to 500, and a large amount of carbon nanotubes are contained in the negative electrode.
- Patent Document 2 a composite in which the surface of the active particle is coated with carbon nanotubes is formed
- Patent Document 3 a composite in which the active material particles are embedded in the carbon nanotube aggregate is formed.
- a special manufacturing process is required, or when a step of applying a slurry in which various constituent materials are added to a dispersion medium and mixed, which is generally performed, is adopted.
- the slurry may become highly viscous, making uniform coating difficult. Further, since it is necessary to contain a large amount of relatively expensive carbon nanotubes in the negative electrode, the manufacturing cost is high.
- one aspect of the present disclosure includes a negative electrode active material and a carbon nanotube, the negative electrode active material contains a material containing graphite and a silicon element, and the average fiber diameter of the carbon nanotube is 0.5 to.
- the present invention relates to a negative electrode material for a secondary battery, which is 6 nm and has an average fiber length of 1.2 to 8 ⁇ m.
- Another aspect of the present disclosure relates to a secondary battery having a negative electrode including the negative electrode material for a secondary battery, a positive electrode, a separator arranged between the positive electrode and the negative electrode, and an electrolytic solution.
- the negative electrode material for a secondary battery (hereinafter, may be referred to as “negative electrode material”) according to the embodiment of the present disclosure includes a negative electrode active material and carbon nanotubes.
- Negative electrode active materials include graphite and materials containing elemental 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 ⁇ m.
- Carbon nanotubes are carbon fibers having a nano-sized and small fiber diameter and an extremely large aspect ratio (ratio of fiber length to outer diameter of the fiber).
- the contact between the active materials and the contact between the active material and the current collector are not point contact but linear contact.
- Carbon fibers with excellent conductivity intervene between the active material particles to form a linear contact portion with the particles.
- the carbon nanotubes having excellent conductivity form a linear conductive path between the active materials and between the active material and the current collector, and form a linear contact portion between the current collector and the current collector. Improves current collection.
- the average fiber length of carbon nanotubes is 1.2 ⁇ m or more. In this case, even when the volume of the negative electrode active material changes significantly due to charging and discharging, the linear contact of the carbon nanotube with the fiber is maintained following 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 ⁇ m or more, poor current collection is remarkably suppressed.
- the average fiber length may be 1.5 ⁇ m or more, preferably 2 ⁇ m or more.
- the average fiber length of the carbon nanotubes becomes longer, the carbon nanotubes tend to aggregate with each other, and poor dispersion and an increase in the viscosity of the slurry due to the aggregation of the carbon nanotubes at the time of producing the slurry tend to occur. Further, as the average fiber length becomes longer, the content of carbon nanotubes in the negative electrode material increases when the required number of carbon nanotubes is secured, and it becomes difficult to realize a high capacity.
- the average fiber length of carbon nanotubes is to achieve high capacity and to facilitate the production of a slurry in which carbon nanotubes are dispersed together with the negative electrode active material in the production of the negative electrode material, and to suppress an increase in the viscosity of the slurry.
- the average fiber length may be 6 ⁇ m or less, preferably 4 ⁇ m or less.
- the average fiber length of the carbon nanotubes is 4 ⁇ m or less, the aggregation of the carbon nanotubes is suppressed, and it is easy to obtain a slurry in a well-dispersed state.
- the average fiber length of the carbon nanotubes is 1.2 ⁇ m to 8 ⁇ m, may be 1.2 ⁇ m to 6 ⁇ m, 1.2 ⁇ m to 4 ⁇ m, or 1.5 ⁇ m to 4 ⁇ m, and is preferably in the range of 2 ⁇ m to 4 ⁇ m.
- the average fiber diameter of the carbon nanotubes may be 0.5 nm or more, preferably 1 nm or more, in that it can be manufactured without difficulty.
- the larger the average fiber diameter of the carbon nanotubes the smaller the number of carbon nanotubes contained in the negative electrode material when the content of the carbon nanotubes is the same, and it becomes difficult to suppress the poor collection of electricity.
- the content of carbon nanotubes should be increased, but the greater the content of carbon nanotubes in the slurry, the easier it is for the carbon nanotubes to aggregate in the slurry and uniformly disperse them. It becomes difficult, and the viscosity of the slurry tends to increase.
- the average fiber diameter of the carbon nanotubes is set to 6 nm or less in order to suppress an increase in the viscosity of the slurry.
- the average fiber diameter of the carbon nanotubes is preferably 4 nm or less, more preferably 3 nm or less.
- the average fiber diameter of the carbon nanotubes is 0.5 nm to 6 nm, may be 1 nm to 6 nm, and is preferably in the range of 1 nm to 4 nm or 1 nm to 3 nm.
- the average fiber length of the carbon nanotube is obtained by image analysis using a scanning electron microscope (SEM).
- the average fiber length is, for example, obtained by arbitrarily selecting a plurality of (for example, 100 to 1000) carbon nanotubes, measuring the fiber length, and averaging them.
- the fiber length refers to the length when it is linear.
- the fiber diameter refers to the length in the direction perpendicular to the fiber length direction, and means the outer diameter of the carbon nanotube. It is known that carbon nanotubes may form a bundled state called a bundled state instead of a single fiber, and the average length in this bundled state is the average length of each carbon nanotube. It may be the fiber length.
- the average fiber diameter of carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM).
- TEM transmission electron microscope
- the diameter can be determined by analyzing the RBM (Radial Breathing Mode) in which the Raman shift appears in the range of 150 to 300 cm -1 in the Raman spectroscopic spectrum. Can be estimated.
- the carbon nanotube may be a single layer (Single Wall), a double layer (Double Wall), or a multilayer (Multi Wall).
- Single-walled carbon nanotubes are preferable because they can obtain a large effect with a small amount.
- Carbon nanotubes having an average fiber diameter of 5 nm or less contain a large amount of single-walled carbon nanotubes.
- the single-walled carbon nanotubes may be 50% by mass or more of the total carbon nanotubes.
- the content of carbon nanotubes with respect to the entire negative electrode active material may be 0.005% by mass to 0.1% by mass, or 0.005% by mass to 0.07% by mass. .. According to the present embodiment, even with such a small content of carbon nanotubes, poor current collection can be suppressed by setting the average fiber length and the average fiber diameter of the carbon nanotubes within the above ranges. By setting the content of carbon nanotubes to 0.005% by mass or more, a high effect of suppressing poor current collection can be obtained.
- the content of the carbon nanotubes By setting the content of the carbon nanotubes to 0.1% by mass or less, it is easy to prepare a slurry in which the carbon nanotubes are dispersed together with the negative electrode active material, and the increase in the viscosity of the slurry is suppressed. Further, the increase in manufacturing cost due to the adoption of carbon nanotubes is suppressed, and the present negative electrode material can be obtained by minimizing the increase in manufacturing cost.
- the content of carbon nanotubes in the entire 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.
- the content of carbon nanotubes with respect to the entire 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.
- the content of carbon nanotubes in the entire negative electrode active material is obtained from a sample obtained by extracting only the negative electrode active material layer (negative electrode mixture layer) from the secondary battery in the discharged state. Specifically, first, the discharged secondary battery is disassembled and the negative electrode is taken out. Next, the negative electrode is washed with an organic solvent, further vacuum dried, and then only the negative electrode active material layer is peeled off to obtain a sample. Carbon nanotubes can be separated by dispersing the crushed sample in a dispersion medium such as water or alcohol and centrifuging. Further, by performing a thermal analysis such as TG-DTA on the sample, the ratio of the binder component and the conductive material component other than the negative electrode active material can be calculated.
- a dispersion medium such as water or alcohol
- the negative electrode active material metallic lithium, a lithium alloy, or the like may be used, but a material capable of electrochemically absorbing and releasing lithium ions is preferably used.
- the negative electrode active material includes graphite and a material containing a silicon element (hereinafter, may be referred to as a “Si-containing material”).
- graphite examples include natural graphite, artificial graphite, and graphitized mesophase carbon particles.
- known graphite used as a negative electrode active material may be used.
- a carbonaceous material other than graphite may be contained in the negative electrode active material.
- carbonaceous materials other than graphite include easy graphitized carbon (soft carbon) and non-graphitized carbon (hard carbon).
- soft carbon easy graphitized carbon
- hard carbon hard carbon
- the carbonaceous material one kind may be used alone, or two or more kinds may be used in combination.
- graphite is preferable because it has excellent charge / discharge stability and has a small irreversible capacity.
- graphite means a material having a developed graphite-type crystal structure, and generally refers to a carbon material having an average plane spacing d 002 of the (002) plane measured by an X-ray diffraction method of 0.340 nm or less.
- Si-containing material examples include Si alone, a silicon alloy, a silicon compound (silicon oxide, etc.), and a composite material in which a silicon phase is dispersed in a lithium ion conduction phase (matrix).
- silicon oxide examples include SiO X particles.
- X is, for example, 0.5 ⁇ X ⁇ 2, and may be 0.5 ⁇ X ⁇ 1.6 or 0.8 ⁇ X ⁇ 1.6.
- the lithium ion conductive phase at least one selected from the group consisting of a SiO 2 phase, a silicate phase and a carbon phase can be used.
- the first particle containing silicon oxide represented by the formula of SiO X (0.5 ⁇ X ⁇ 1.6), the lithium silicate phase and the silicon phase dispersed in the lithium silicate phase At least one particle selected from the group consisting of a second particle containing a carbon phase and a third particle containing a carbon phase and a silicon phase dispersed in the carbon phase may be used.
- the silicon-containing particles as the negative electrode active material, it is possible to increase the capacity of the battery.
- particles containing silicon have a large volume change due to occlusion and release of lithium ions, and poor current collection is likely to occur due to repeated charging and discharging.
- the electrical connection between the silicon-containing particles and another negative electrode active material or current collector is cut off, and the silicon-containing particles are isolated. Since the isolated particles cannot contribute to the capacity, the capacity retention rate decreases as a result of poor current collection.
- the negative electrode material contains carbon nanotubes having the above average fiber diameter and average fiber length, poor current collection can be effectively suppressed and the capacity retention rate can be maintained high.
- the Si-containing material may contain a plurality of types of particles selected from the group consisting of the first particle, the second particle, and the third particle.
- the Si-containing material may be composed of two types of particles selected from them, or may contain all three types of particles.
- the Si-containing material may contain a first particle and a second particle, a first particle and a third particle, or a second particle and a third particle. It may contain particles.
- 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.
- the average particle size of graphite contained in the negative electrode active material is preferably 10 to 30 ⁇ m.
- the average particle size of the Si-containing material contained in the negative electrode active material (the average particle size of each of the first particle, the second particle, and the third particle) is preferably 1 to 20 ⁇ m, preferably 5 to 15 ⁇ m. Is more preferable. In this case, the effect of suppressing poor current collection by including the carbon nanotubes having the above-mentioned fiber diameter and fiber length in the negative electrode material is remarkable.
- the average particle size is obtained by observing the cross section of the negative electrode active material layer using SEM or TEM.
- the particle type is identified by micro-Raman spectroscopy, the grain boundary of each particle is obtained from the cross-sectional photograph, and the diameter of a circle equal to the area defined by the grain boundary is defined as the particle diameter. At least 100 particles are arbitrarily selected and the average value of the particle size is obtained and used as the average particle size.
- a median diameter (D50) at which the cumulative volume becomes 50% in the volume-based particle size distribution can be used. The median diameter can be determined using, for example, a laser diffraction / scattering type particle size distribution measuring device.
- the negative electrode active material may contain other active materials other than graphite and Si-containing materials.
- other active materials include carbonaceous materials other than graphite such as graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon).
- the content of the Si-containing material may be 5 to 50% by mass with respect to the total of the graphite and Si-containing material. In this case, the capacity can be increased as compared with the case where the negative electrode active material is only graphite.
- the content of the Si-containing material with respect to the total of the graphite and Si-containing materials is preferably 6 to 20% by mass.
- the graphite content in the negative electrode active material may be in the range of 50 to 99% by mass.
- the particles of the Si-containing material contain graphite on the surface and / or inside, the graphite is not included in the graphite content.
- the graphite content is the graphite content not contained in the Si-containing material.
- the first particle is a mixture of SiO 2 and Si fine particles 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 particles may include silicon oxide particles and a carbon layer arranged around the silicon oxide particles.
- the second particle contains a lithium silicate phase and silicon particles (silicon phase) dispersed in the lithium silicate phase.
- the lithium silicate phase may contain a lithium silicate represented by the formula Li 2Z SiO (2 + Z) (0 ⁇ Z ⁇ 2), or may be composed of the lithium silicate. Z preferably satisfies the relationship of 0 ⁇ Z ⁇ 1.
- the lithium silicate phase may be composed of lithium silicate in which 50% by mass or more (for example, 60% by mass or more) satisfies 0 ⁇ Z ⁇ 0.5.
- the second particle may contain at least one element Me dispersed in the lithium silicate phase.
- 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 and the like.
- the element Me may be dispersed as a Me oxide in the lithium silicate phase.
- the Me oxide may contain 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. Then, the element Me may be dispersed in zirconium oxide.
- the amount of elemental Me contained in the second particle is calculated assuming that the elemental Me forms a stoichiometric oxide regardless of the state of the elemental Me or the type of compound of the elemental Me.
- the amount (estimated Me oxide amount) can be used as an index.
- the estimated amount of Me oxide may be in the range of 0.001% by mass to 1.0% by mass with respect to the total of the lithium silicate phase and the silicon particles. By setting the estimated Me oxide amount to 0.001% by mass or more, the effect of reducing the reaction area and improving the hardness of the lithium silicate phase is enhanced. On the other hand, by setting the estimated Me oxide amount to 1.0% by mass or less, the decrease in the initial capacity can be suppressed.
- the lithium silicate phase may contain metal compounds such as metal oxides, metal carbides, metal nitrides and metal borides. Suitable metal compounds are metal oxides and metal carbides. Among them, at least one selected from the group consisting of zirconium oxide (ZrO 2 ), aluminum oxide (Al 2 O 3 ), zirconium carbide (ZrC), tungsten carbide (WC), and silicon carbide (SiC) can be used. preferable.
- the amount of the compound of the metal element other than the element Me is in the range of 0.005% by mass to 15% by mass (for example, in the range of 0.01% by mass to 10% by mass) with respect to the total of the lithium silicate phase and the silicon particles.
- the amount of the compound of the metal element it may be in the range of 0.01% by mass to 1% by mass).
- the amount calculated on the assumption that the metal element forms a stoichiometric oxide may be obtained as in the case of the content of the element Me.
- the average particle size of the second particles may be in the range of 1 ⁇ m to 25 ⁇ m (for example, in the range of 4 ⁇ m to 15 ⁇ m). In such a range, it is easy to relieve the stress due to the volume change of the second particle due to charge / discharge, and it is easy to obtain good cycle characteristics. Further, the surface area of the second particle becomes appropriate, and the volume decrease due to a side reaction with the non-aqueous electrolyte is suppressed.
- the crystallite size of the silicon particles dispersed in the lithium silicate phase is, for example, 10 nm or more.
- Silicon particles have a particulate phase of elemental silicon (Si).
- Si elemental silicon
- the crystallite size of the silicon particle is calculated by Scherrer's equation from the half width of the diffraction peak attributed to the Si (111) plane of the X-ray diffraction (XRD) pattern of the silicon particle.
- the average particle size of the silicon particles in the second particles may be preferably 500 nm or less (more preferably 200 nm or less, still more preferably 50 nm or less) before the initial charge. After the initial charge, the average particle size of the silicon particles is preferably 400 nm or less (more preferably 100 nm or less). By refining the silicon particles, the volume change during charging and discharging becomes small, and the structural stability of the second particles is further improved.
- the content of silicon particles (elemental Si) in the second particles is in the range of 20% by mass to 95% by mass (for example, in the range of 35% by mass to 75% by mass) from the viewpoint of increasing the capacity and improving the cycle characteristics. It is preferable to be in. According to this range, the diffusivity of lithium ions is also good, and it becomes easy to obtain excellent load characteristics. Further, the surface of the silicon particles exposed without being covered with the lithium silicate phase is reduced, and the side reaction between the non-aqueous electrolyte and the silicon particles is suppressed.
- the second particle may contain a conductive material that covers at least a part of its surface. Since the lithium silicate phase has poor electron conductivity, the conductivity of the second particle also tends to be low. By covering the surface with a conductive material, the conductivity can be dramatically improved. It is preferable that the conductive layer is thin enough not to affect the average particle size of the second particles.
- the thickness of the conductive layer may be in the range of 1 nm to 200 nm (for example, in the range of 5 nm to 100 nm) from the viewpoint of ensuring conductivity and diffusing lithium ions. An example of the material of the conductive layer and an example of the forming method will be described later.
- the third particle contains a carbon phase and silicon particles (silicon phase) dispersed in the carbon phase.
- the carbon phase of the third particle may be composed of amorphous carbon (ie, amorphous carbon).
- the amorphous carbon may be hard carbon, soft carbon, or other carbon.
- Amorphous carbon (amorphous carbon) generally refers to a carbon material having an average plane spacing d 002 of planes (002) measured by an X-ray diffraction method of more than 0.34 nm.
- the carbon phase of the third particle has conductivity. Therefore, even if a void is formed around the third particle, the contact point between the third particle and its surrounding is likely to be maintained. As a result, the capacity decrease due to repeated charge / discharge cycles is likely to be suppressed.
- the content of the silicon particles 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. In such a range, a sufficiently high capacity of the negative electrode can be achieved, and the cycle characteristics can be easily improved.
- the average particle size of the silicon particles in the third particle may be, for example, 1 nm or more. Further, the average particle size of the silicon particles may be 1000 nm or less, 500 nm or less, 200 nm or less, 100 nm or less (further, 50 nm or less). The finer the silicon particles, the smaller the volume change of the third particle during charging and discharging, and the better the structural stability of the third particle.
- composition of the second and third particles and the content of the components can be analyzed by the method described in International Publication No. 2018/1796969.
- the content of each element contained in the Si-containing material may be measured by, for example, inductively coupled plasma emission spectroscopy (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. do. Subsequently, a calibration curve is prepared using a commercially available standard solution of each element, and the content of each element is calculated.
- ICP-AES inductively coupled plasma emission spectroscopy
- the second particle and the third particle each have a so-called sea-island structure.
- the silicon particles (islands) in the second and third particles are dispersed in a matrix (sea) of silicate phase and carbon phase, respectively, and are covered with a lithium ion conduction phase (silicate phase and carbon phase). ..
- the contact between the silicon particles and the electrolyte is restricted, so that side reactions are suppressed.
- the stress generated by the expansion and contraction of the silicon particles is relaxed by the matrix of the lithium ion conductive phase.
- the average particle size of graphite (graphite particles) contained in the negative electrode as an active material may be 13 ⁇ m or more and 25 ⁇ m or less.
- the average particle size of graphite is preferably larger than the average particle size 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 it is easy to obtain a negative electrode having a higher capacity. Further, the particles of the Si-containing material present in the voids contribute to the maintenance of electronic contact between the graphite particles. On the other hand, even if the particles of the Si-containing material existing in the voids expand or contract, the entire negative electrode is unlikely to expand or contract, so that deterioration due to the charge / discharge cycle is unlikely to occur.
- the first particle, SiO X can be produced, for example, by a vapor deposition method.
- the SiO X particles may be coated with carbon by the following method. First, the SiO X particles are crushed and classified to adjust the particle size. Next, the surface of the obtained particles is coated with carbon by a CVD method under an argon atmosphere. Then, by crushing and classifying this, a first particle represented as SiO X is prepared.
- the method of coating the SiO X particles with carbon is not limited to the above method, and various well-known methods can be adopted. Further, the process of coating the SiO X particles with carbon may be omitted.
- the second particle may be produced by a method other than the production method described below.
- the second particle may be produced by the method described in WO 2018/17969.
- the second particle is generally synthesized through two processes, a pre-step for obtaining lithium silicate and a post-step for obtaining second particle from lithium silicate and raw material silicon.
- the element Me When the element Me is added, the element Me may be added to the raw material of lithium silicate in the previous step, but it is preferable to add the element Me in the subsequent step so as not to affect the synthesis of lithium silicate.
- the second method for producing particles includes a step (i) of mixing silicon dioxide and a lithium compound and firing the obtained mixture to obtain lithium silicate, and lithium silicate and raw material silicon (i). It is preferable to include a step (ii) of obtaining a second particle containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase by further compounding with the element Me), if necessary. ..
- Step (i) Formula: Li 2Z
- the Z value of the lithium silicate represented by SiO 2 + Z may be controlled by the atomic ratio of silicon to lithium in the mixture of silicon dioxide and the lithium compound: Li / Si. In order to synthesize a high-quality lithium silicate with less elution of alkaline components, it is preferable to make Li / Si smaller than 1.
- Lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used as the lithium compound. These may be used individually by 1 type and may be used in combination of 2 or more type.
- the mixture containing silicon dioxide and the lithium compound is preferably heated in air at 400 ° C. to 1200 ° C., preferably 800 ° C. to 1100 ° C. to react the silicon dioxide with the lithium compound.
- the lithium silicate and the raw material silicon are combined.
- the mixture may be pulverized while applying a shearing force to the mixture of lithium silicate and the raw material silicon (which may further contain the element Me).
- the raw material silicon coarse silicon particles having an average particle size of several ⁇ m to several tens of ⁇ m may be used. It is preferable to control the finally obtained silicon particles so that the crystallite size calculated by Scherrer's equation from the half width of the diffraction peak attributed to the Si (111) plane of the XRD pattern is 10 nm or more. ..
- oxides, oxalates, nitrates, sulfates, halides, carbonates and the like of the element Me may be used.
- Me oxide is preferable because it is stable and has good ionic conductivity. More specifically, CeO 2 , Sc 2 O 3 , Y 2 O 3 , Er 2 O 3 , Tm 2 O 3 , Yb 2 O 3 , Lu 2 O 3 and the like can be mentioned.
- compounds containing elements other than the element Me and oxygen such as yttria-stabilized zirconia, may be used. These may be used individually by 1 type and may be used in combination of 2 or more type.
- lithium silicate, raw material silicon, and (if necessary, a compound of element Me) may be mixed at a predetermined mass ratio, and the mixture may be stirred while being atomized using a pulverizer such as a ball mill.
- a pulverizer such as a ball mill.
- the compounding process is not limited to this.
- silicon nanoparticles, lithium silicate nanoparticles, and (if necessary, a compound of element Me) may be synthesized and mixed without using a pulverizer.
- the finely divided mixture is heated at 450 ° C. to 1000 ° C. in an inert atmosphere (for example, an atmosphere of argon, nitrogen, etc.) and calcined.
- a sintered body of the mixture may be produced by firing while applying pressure to the mixture by hot pressing or the like.
- Lithium silicate is stable at 450 ° C to 1000 ° C and hardly reacts with silicon, so that the capacity decrease is slight even if it occurs.
- the silicate softens and flows to fill the gaps between the silicon particles. As a result, it is possible to obtain a dense block-shaped sintered body having the silicate phase as the sea portion and the silicon particles as the island portion.
- the sintered body may then be pulverized until it becomes granular to form second particles.
- the second particles having the average particle size in the above-mentioned range can be obtained.
- a step (iii) may be performed in which at least a part of the surface of the second particle is covered with a conductive material to form a conductive layer.
- the conductive material is preferably electrochemically stable, and preferably a carbon material.
- a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material may be used.
- a method of mixing coal pitch, petroleum pitch, phenol resin, or the like with the second particles and then heating them may be used. Further, carbon black may be attached to the surface of the second particle.
- a step of washing the second particles with an acid may be performed.
- the second particle may be washed with an acidic aqueous solution.
- an acidic aqueous solution By washing with an acid, a trace amount of a component such as Li 2 SiO 3 that may be generated when the raw material silicon and the lithium silicate are combined can be dissolved and removed.
- an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid or carbonic acid, or an aqueous solution of an organic acid such as citric acid or acetic acid can be used.
- the third particle may be produced by a method other than the production method described below.
- the raw material silicon and the carbon source are mixed, and the mixture of the raw material silicon and the carbon source is pulverized and composited by using a pulverizing device such as a ball mill.
- An organic solvent may be added to the mixture and wet pulverized.
- the raw material silicon is finely pulverized to generate silicon particles. Silicon particles are dispersed in a matrix of carbon sources.
- Examples of the carbon source include water-soluble resins such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyacrylic acid salt, polyacrylamide, polyvinyl alcohol, polyethylene oxide and polyvinylpyrrolidone, sugars such as cellulose and sucrose, petroleum pitch and coal pitch. , Tar and the like can be used, but are not particularly limited.
- CMC carboxymethyl cellulose
- hydroxyethyl cellulose polyacrylic acid salt
- polyacrylamide polyvinyl alcohol
- polyethylene oxide and polyvinylpyrrolidone sugars such as cellulose and sucrose
- sugars such as cellulose and sucrose
- petroleum pitch and coal pitch petroleum pitch and coal pitch.
- Tar and the like can be used, but are not particularly limited.
- organic solvent alcohol, ether, fatty acid, alkane, cycloalkane, silicate ester, metal alkoxide and the like can be used.
- the composite of silicon particles and a carbon source is heated to 700 ° C to 1200 ° C in an inert gas atmosphere (for example, an atmosphere of argon, nitrogen, etc.).
- an inert gas atmosphere for example, an atmosphere of argon, nitrogen, etc.
- This heating carbonizes the carbon source to produce amorphous carbon.
- a third particle in which silicon particles are dispersed in a carbon phase containing amorphous carbon is obtained.
- the raw material silicon and the carbon material are mixed, and the mixture of the raw material silicon and the carbon material is crushed and compounded while being made into fine particles by using a crushing device such as a ball mill.
- An organic solvent may be added to the mixture and wet pulverized.
- the raw material silicon is finely pulverized to generate silicon particles. Silicon particles are dispersed in a matrix of carbon materials.
- a third particle in which silicon particles are dispersed in the carbon phase of amorphous carbon can be obtained.
- the third particle may then be heated to 700 ° C. to 1200 ° C. in an inert gas atmosphere.
- amorphous carbon is preferable, and easily graphitized carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon black and the like can be used.
- Examples of carbon black include acetylene black and ketjen black. Even when graphite is used as the carbon material, the crystal structure of graphite is almost lost when a composite of silicon particles and the carbon material is obtained by using a pulverizer, and a carbon phase of amorphous carbon is formed.
- the secondary battery according to the embodiment of the present disclosure includes a negative electrode including the negative electrode material for the secondary battery, a positive electrode, a separator arranged between the positive electrode and the negative electrode, and an electrolytic solution.
- the negative electrode typically includes a negative electrode current collector and a negative electrode active material layer (negative electrode mixture layer) arranged on the surface of the negative electrode current collector.
- the negative electrode active material layer contains the negative electrode material for the secondary battery. That is, the negative electrode active material layer contains a negative electrode active material and carbon nanotubes, and if necessary, contains other components other than the negative electrode active material and carbon nanotubes. Examples of other components include binders, conductive agents, thickeners and the like. As those other components, components used in known secondary batteries may be used.
- the negative electrode active material includes graphite and Si-containing materials.
- the negative electrode current collector a non-perforated conductive substrate (metal foil, etc.) and a porous conductive substrate (mesh body, net body, punching sheet, etc.) are used.
- the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy.
- the thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 ⁇ m and may be 5 to 30 ⁇ m.
- the negative electrode active material layer can be formed by applying a negative electrode slurry in which a negative electrode mixture is dispersed in a dispersion medium to the surface of a negative electrode current collector to form a coating film, and then drying the coating film.
- the dried coating film may be rolled if necessary.
- the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent thereof.
- the ratio of the components in the negative electrode mixture can be adjusted by changing the mixing ratio of the materials of the negative electrode mixture.
- binder examples include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives and salts.
- conductive agents include conductive carbon materials, carbon fluorides, organic conductive materials and the like.
- thickeners include carboxymethyl cellulose (CMC), polyvinyl alcohol and the like. For these components, one kind of material may be used alone, or two or more kinds of materials may be used in combination.
- Carbon nanotubes are contained in the negative electrode active material layer as a conductive agent.
- the carbon nanotubes those having the above-mentioned average fiber length and average fiber diameter are used. Since carbon nanotubes have an extremely large aspect ratio (ratio of length to diameter), they can exhibit high conductivity even in a small amount.
- By using carbon nanotubes as the conductive material it is possible to increase the proportion of the negative electrode active material in the negative electrode active material layer while maintaining high conductivity of the negative electrode active material layer. Therefore, the capacity of the secondary battery can be increased.
- a conductive carbon material other than carbon nanotubes may be mixed with carbon nanotubes and used.
- the conductive carbon material other than carbon nanotubes include amorphous carbon and at least one selected from the group consisting of carbon fibers.
- Amorphous carbon includes hard carbon and soft carbon.
- soft carbon include carbon blacks such as acetylene black and ketjen black. A plurality of kinds of these materials may be combined and used as a conductive agent.
- the negative electrode active material layer may or may not contain a conductive material other than carbon nanotubes.
- the negative electrode active material layer may contain carbon black as a conductive material other than carbon nanotubes.
- the mass of the conductive material other than the carbon nanotubes contained in the negative electrode active material layer is 100 times or less (for example, 0 to 80 times, 0 to 50 times) the mass of the carbon nanotubes contained in the negative electrode active material layer. , Or in the range of 0 to 10 times).
- carbon nanotubes examples include carbon nanofibers. Since various carbon nanotubes are commercially available, commercially available carbon nanotubes may be used. Alternatively, the carbon nanotubes may be synthesized by a known synthesis method.
- the positive electrode typically includes 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 applying a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium to the surface of a positive electrode current collector and drying it. The dried coating film may be rolled if necessary.
- the positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components.
- 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 (for example, a rock salt type crystal structure) containing lithium and a transition metal.
- Examples of the lithium metal composite oxide include Li a CoO 2 , Li a NiO 2 , Li a MnO 2 , Li a Co b Ni 1-b O 2 , Li a Co b M 1-b O c , and Li a Ni. Examples thereof include 1-b M b O c , Li a Mn 2 O 4 , Li a Mn 2-b M b O 4 , Li GPO 4 , and Li 2 GPO 4 F.
- 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 contains at least a transition element (eg, includes at least one selected from the group consisting of Mn, Fe, Co, Ni).
- the a value indicating the molar ratio of lithium increases or decreases depending on charging and discharging.
- the lithium metal composite oxide is, for example, Li a Ni x M 1-x O 2 (where 0 ⁇ a ⁇ 1.2, 0.8 ⁇ x ⁇ 1 and M is Co.
- Lithium-nickel composite oxidation represented by (includes at least one selected from the group consisting of Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr and B). It may be a thing.
- M preferably contains at least one selected from the group consisting of Co, Mn, Al and Fe. From the viewpoint of the stability of the crystal structure, Al may be contained as M.
- Specific examples of such a composite oxide include lithium-nickel-cobalt-aluminum composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O 2 and the like).
- the ratio of Ni to the metal elements other than Li contained in the lithium metal composite oxide is 80 atomic% or more.
- the ratio of Ni to the metal element other than Li may be 85 atomic% or more, or 90 atomic% or more.
- the ratio of Ni to metal elements other than Li is preferably 95 atomic% or less, for example. When limiting the range, these upper and lower limits may be combined arbitrarily.
- Co, Mn and / or Al which can be contained as the metal element M other than Li and Ni, contributes to the stabilization of the crystal structure of the composite oxide having a high Ni content.
- the Co content is low.
- the lithium-nickel composite oxide having a low Co content or no Co content may contain Mn and Al.
- the binder and the conductive agent the same ones as those exemplified for the negative electrode can be used.
- the conductive agent graphite such as natural graphite or artificial graphite may be used.
- the shape and thickness of the positive electrode current collector can be selected 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, and titanium.
- the content of the elements constituting the lithium metal composite oxide is determined by an inductively coupled plasma emission spectroscopic analyzer (Inductive coupled plasma spectroscopy: ICP-AES), an electron probe microanalyzer (Electron Probe Micro Analyzer), or an energy dispersive X-ray. It can be measured by a type X-ray analyzer (Energy dispersive X-ray spectroscopy: EDX) or the like.
- the electrolytic solution contains a solvent and a solute dissolved in the solvent.
- the solute is an electrolyte salt that ionically dissociates in the electrolytic solution.
- the solute may include, for example, a lithium salt.
- the components of the electrolytic solution other than the solvent and solute are additives.
- the electrolyte may contain various additives.
- a known material can be used as the solvent.
- a cyclic carbonate ester, a chain carbonate ester, a cyclic carboxylic acid ester, a chain carboxylic acid ester, or the like is used.
- the cyclic carbonic acid ester include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC) and the like.
- the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and the like.
- Examples of the cyclic carboxylic acid ester include ⁇ -butyrolactone (GBL) and ⁇ -valerolactone (GVL).
- Examples of the chain carboxylic acid ester include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP) and ethyl propionate (EP).
- the non-aqueous solvent one type may be used alone, or two or more types may be used in combination.
- non-aqueous solvent examples include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.
- lithium salt examples include a lithium salt of a chlorine-containing acid (LiClO 4 , LiAlCl 4 , LiB 10 Cl 10 , etc.) and a lithium salt of a fluorine-containing acid (LiPF 6 , LiPF 2 O 2 , LiBF 4 , LiSbF 6 , LiAsF 6 ).
- LiN (FSO 2 ) 2 LiN (CF 3 SO 2 ) 2 , LiN (CF 3 SO 2 ) (C 4 F 9 SO) 2 ), LiN (C 2 F 5 SO 2 ) 2 , etc.
- lithium halide LiCl, LiBr, LiI, etc.
- One type of lithium salt may be used alone, or two or more types may be used in combination.
- the concentration of the lithium salt in the electrolytic solution may be 1 mol / liter or more and 2 mol / liter or less, or 1 mol / liter or more and 1.5 mol / liter or less.
- the lithium salt concentration is not limited to the above.
- the electrolytic solution may contain other known additives.
- the additive include 1,3-propanesarton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene and the like.
- a separator is interposed between the positive electrode and the negative electrode.
- the separator has high ion permeability and has moderate mechanical strength and insulation.
- a microporous thin film, a woven fabric, a non-woven fabric, or the like can be used.
- polyolefins such as polypropylene and polyethylene are preferable.
- aramid fiber or the like may be used to increase the mechanical strength.
- the structure of the non-aqueous electrolyte secondary battery there is a structure in which an electrode group in which a positive electrode and a negative electrode are wound via a separator is housed in an exterior body together with a non-aqueous electrolyte.
- the present invention is not limited to this, and other forms of electrodes may be applied.
- a laminated electrode group in which a positive electrode and a negative electrode are laminated via a separator may be used.
- the form of the non-aqueous electrolyte secondary battery is not limited, and may be, for example, a cylindrical type, a square type, a coin type, a button type, a laminated type, or the like.
- FIG. 1 is a schematic perspective view in which a part of a square non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure is cut out.
- the secondary battery 1 shown in FIG. 1 includes a bottomed square battery case 11, an electrode group 10 housed in 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 that is interposed between them and prevents direct contact.
- the electrode group 10 is formed by winding a negative electrode, a positive electrode, and a separator around a flat plate-shaped winding core and pulling out the winding core.
- 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 arranged 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 that also serves as the positive electrode terminal.
- a resin frame 18 is arranged on the upper part of the electrode group 10.
- the frame body 18 separates the electrode group 10 and the sealing plate 12, and also separates the negative electrode lead 15 and the battery case 11.
- the opening of the battery case 11 is sealed with a sealing plate 12.
- a liquid injection hole 17a is formed in the sealing plate 12. The electrolyte is injected into the battery case 11 from the injection hole 17a. After that, the liquid injection hole 17a is
- Negative electrode active material sodium polyacrylate (PAA-Na), sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), carbon nanotube (CNT), and water.
- PAA-Na sodium polyacrylate
- CMC-Na sodium carboxymethyl cellulose
- SBR styrene-butadiene rubber
- CNT carbon nanotube
- water was mixed at a predetermined mass ratio to prepare a negative electrode slurry.
- carbon nanotubes those having an average fiber length and an average fiber diameter shown in Tables 1 to 6 were used.
- a mixture of graphite and a Si-containing material was used as the negative electrode active material.
- a coating film was formed by applying a negative electrode slurry to the surface of a copper foil (negative electrode current collector). The coating film was dried and then 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 the mass% shown in Table 1 when the negative electrode active material (total of graphite and Si-containing material) was 100% by mass.
- the first particle, the second particle, and the third particle, which are Si-containing materials, were produced by the following methods.
- the second particle was prepared by the following method. First, silicon dioxide and lithium carbonate are mixed so that the atomic ratio: Si / Li is 1.05, and the mixture is calcined in air at 950 ° C. for 10 hours, thereby being represented by the formula: Li 2 Si 2 O 5 . Obtained a lithium silicate. The obtained lithium silicate was pulverized so as to have an average particle size of 10 ⁇ m.
- the obtained lithium silicate, raw material silicon (3N, average particle size 10 ⁇ m), and yttrium oxide (Y2O3) were mixed at a mass ratio of 50:50: 0.0005.
- the mixture was milled at 200 rpm for 50 hours.
- the powdery mixture was taken out in the inert atmosphere and fired at 800 ° C. for 4 hours in a state where pressure was applied by a hot press machine in the inert atmosphere to obtain a sintered body (mother particles) of the mixture. Obtained.
- the sintered body was crushed, passed through a mesh of 40 ⁇ m, mixed with coal pitch (MCP250 manufactured by JFE Chemical Co., Ltd.), and the mixture was fired at 800 ° C. in an inert atmosphere to obtain the crushed particles.
- the surface was coated with conductive carbon to form a conductive layer.
- the coating amount of the conductive layer was 5% by mass of the total mass of the crushed particles.
- a sieve was used to obtain second particles having a conductive layer and having an average particle size of 5 ⁇ m.
- the third particle was prepared by the following method. Coal pitch (manufactured by JFE Chemical Co., Ltd., MCP250) as a carbon source and raw material silicon (3N, average particle size 10 ⁇ m) were mixed at a mass ratio of 50:50. Fill the mixture in a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), put 24 SUS balls (diameter 20 mm) in the pot, close the lid, and in an inert atmosphere. The mixture was pulverized at 200 rpm for 50 hours to obtain a composite of silicon particles and a carbon source.
- Coal pitch manufactured by JFE Chemical Co., Ltd., MCP250
- raw material silicon 3N, average particle size 10 ⁇ m
- the composite of silicon particles and a carbon source was fired in an inert gas atmosphere to carbonize the carbon source, and a silicon-containing material in which the silicon particles were dispersed in a carbon phase containing amorphous carbon was obtained. Then, a jet mill was used to obtain a third particle having an average particle size of 10 ⁇ m.
- Tables 1 to 6 show the mass-based content of the first particles, the second particles, the third particles, and the carbon nanotubes with respect to the negative electrode active material (total of graphite and Si-containing material) used in each battery. , Average fiber length and average fiber diameter of carbon nanotubes.
- positive electrode LiNi 0.88 Co 0.09 Al 0.03 O 2 was used as the positive electrode active material.
- a positive electrode active material, acetylene black, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) were mixed at a predetermined mass ratio to prepare a positive electrode slurry.
- NMP N-methyl-2-pyrrolidone
- the positive electrode slurry was applied to the surface of the aluminum foil, which is the positive electrode current collector, the coating film was dried, and then rolled to form positive electrode mixture layers on both sides of the aluminum foil.
- Electrolyte Solution An electrolytic solution was prepared by adding LiPF 6 as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethylmethyl carbonate (EMC) in a volume ratio of 3: 7. The concentration of LiPF 6 in the non-aqueous electrolyte solution was 1.3 mol / liter.
- EC ethylene carbonate
- EMC ethylmethyl carbonate
- a plurality of secondary batteries A1 to A36 and B1 to B3 having different diameters and their contents were prepared and evaluated.
- A1 to A36 are examples, and B1 to B3 are comparative examples.
- the first particle and the second particle were mixed and used as the Si-containing material.
- the ratio of the first particle and the second particle to the whole of the negative electrode active material was 3% by mass, respectively.
- the first particles were used as the Si-containing material, and the ratio of the first particles to the entire negative electrode active material was set to 10% by mass.
- the batteries A19 to A22 shown in Table 3 the first particles were used as the Si-containing material, and the ratio of the first particles to the entire negative electrode active material was changed in the range of 7.5% by mass to 20% by mass.
- the first particle, the second particle, and the third particle were mixed and used as the Si-containing material.
- the ratios of the first particle, the second particle, and the third particle to the whole of the negative electrode active material were 2.1% by mass, 3% by mass, and 3.2% by mass, respectively.
- the third particles were used as the Si-containing material, and the ratio of the first particles to the entire negative electrode active material was set to 15% by mass.
- the batteries A33 to A35 shown in Table 6 the first particle and the second particle were mixed and used as the Si-containing material.
- the ratios of the first particle and the second particle to the whole negative electrode active material were 3% by mass and 5.5% by mass, respectively.
- Tables 1 to 6 show the evaluation results of the deterioration rates of the batteries A1 to A36 and B1 to B3.
- Table 1 Table 2, Table 4 and Table 5, the composition of the negative electrode active material (ratio of the first particle, the second particle and the third particle to the total of the graphite and the Si-containing material) was the same.
- the deterioration rate ratio based on the case where the average fiber length of the added CNT is 3.23 ⁇ m is also shown.
- 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 ⁇ m, the deterioration rate per charge / discharge cycle is determined. Can be reduced.
- FIG. 2 is a graph showing the change in the deterioration rate ratio with respect to the change in the average fiber length of carbon nanotubes based on the batteries A1 to A7, A9, A11, A15 to A18, A23, A26 to A32, A36 and B3.
- the deterioration rate ratio sharply decreases near the average fiber length of 1 ⁇ m, and when the average fiber length is 2 ⁇ m or more, the deterioration rate ratio is reduced to about 60% of the battery B3 having the average fiber length of carbon nanotubes of 1 ⁇ m. is doing.
- the average fiber length exceeds 2 ⁇ m, the amount of reduction in the deterioration rate ratio is small.
- the average fiber length of the carbon nanotubes is 8 ⁇ m or less, preferably 4 ⁇ m or less, in terms of suppressing an increase in the viscosity of the slurry and facilitating the production of the negative electrode mixture layer.
- the average fiber length of the carbon nanotubes is 4 ⁇ m or less, the aggregation of the carbon nanotubes is suppressed, and it is easy to obtain a slurry in a well-dispersed state.
- FIG. 3 is a graph showing the change in the deterioration rate with respect to the change in the average fiber diameter of the carbon nanotubes based on the batteries A5, A11, A14, B1 and B2.
- the deterioration rate is lower than that of the battery B3, but the reduction amount is small.
- the deterioration rate is remarkably reduced in the range where the average fiber diameter of the carbon nanotubes is 6 nm or less.
- the average fiber diameter of the carbon nanotubes is 4 nm or less, a remarkably low deterioration rate can be obtained, and an increase in the viscosity of the slurry can be suppressed.
- the secondary battery according to the present disclosure it is possible to provide a secondary battery having a high capacity and excellent cycle characteristics.
- the secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like.
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Abstract
Description
本発明の新規な特徴を添付の請求の範囲に記述するが、本発明は、構成および内容の両方に関し、本発明の他の目的および特徴と併せ、図面を照合した以下の詳細な説明によりさらによく理解されるであろう。
本開示の実施形態に係る二次電池用負極材料(以下において、「負極材料」と称する場合がある)は、負極活物質と、カーボンナノチューブを含む。負極活物質は、黒鉛と、ケイ素元素を含む材料を含む。カーボンナノチューブの平均繊維径が0.5~6nmであり、平均繊維長が1.2~8μmである。これにより、充放電に伴う体積変化が大きなケイ素元素を含む材料を負極活物質に用いる場合においても、体積変化に伴う負極活物質(黒鉛およびケイ素元素を含む材料)の集電不良が抑制され、容量維持率を高く維持できる。
カーボンナノチューブは、繊維径がナノサイズで小さく、そのアスペクト比(繊維の外径に対する繊維長の比)が極めて大きな炭素繊維である。アスペクト比の大きな炭素繊維は、活物質間の接触、および、活物質と集電体との接触が点接触ではなく、線状の接触になる。導電性に優れた炭素繊維が活物質粒子の間に介在し、粒子と線状の接触部を形成する。これにより、導電性に優れたカーボンナノチューブが活物質間および活物質と集電体との間に線状の導電経路を形成し、かつ集電体と線状の接触部を形成することで、集電性が向上する。
負極活物質としては、金属リチウム、リチウム合金などを用いてもよいが、電気化学的にリチウムイオンを吸蔵および放出可能な材料が好適に用いられる。このような材料として、負極活物質は、黒鉛と、ケイ素元素を含む材料(以下において、「Si含有材料」と称する場合がある)を含む。
負極活物質層を形成する前の平均粒子径には、体積基準の粒度分布において累積体積が50%になるメジアン径(D50)を用いることができる。メジアン径は、例えばレーザ回折/散乱式粒度分布測定装置を用いて求めることができる。
第1の粒子は、SiO2とSi微粒子とが微視的レベルで混合した混合物である。第1の粒子は、例えば、SiOX(0.5≦X<1.6)の式で表される酸化ケイ素を含む。第1の粒子は、酸化ケイ素の粒子と、酸化ケイ素の粒子の周囲に配置された炭素層とを含んでもよい。
第2の粒子は、リチウムシリケート相とリチウムシリケート相中に分散したシリコン粒子(シリコン相)とを含む。リチウムシリケート相は、Li2ZSiO(2+Z)(0<Z<2)の式で表されるリチウムシリケートを含んでもよく、当該リチウムシリケートで構成されてもよい。Zは、0<Z<1の関係を満たすことが好ましい。リチウムシリケート相は、その50質量%以上(例えば60質量%以上)が、0<Z≦0.5を満たすリチウムシリケートで構成されていてもよい。
第3の粒子は、炭素相と当該炭素相中に分散したシリコン粒子(シリコン相)とを含む。第3の粒子の炭素相は、無定形炭素(すなわちアモルファス炭素)で構成されてもよい。無定形炭素は、ハードカーボンでもよいし、ソフトカーボンでもよいし、それ以外でもよい。無定形炭素(アモルファス炭素)とは、一般には、X線回折法により測定される(002)面の平均面間隔d002が0.34nmを超える炭素材料を言う。第3の粒子の炭素相は、導電性を有する。そのため、第3の粒子の周囲に空隙が形成されても、第3の粒子とその周囲との接点が維持されやすい。その結果、充放電サイクルを繰り返すことによる容量低下が抑制されやすい。
第1の粒子であるSiOXは、例えば、蒸着法により製造され得る。以下の方法により、SiOX粒子を炭素で被覆してもよい。まず、SiOX粒子を粉砕・分級して粒度を調整する。次に、得られた粒子の表面を、アルゴン雰囲気下でのCVD法によって、炭素で被覆する。そして、これを解砕・分級することによって、SiOXとして表される第1の粒子を調製する。なお、SiOX粒子を炭素で被覆する方法については、上記方法に限られず、種々の周知の方法を採用することができる。また、SiOX粒子を炭素で被覆する処理については省略してもよい。
次に、第2の粒子の製造方法の一例について詳述する。第2の粒子は、以下で説明する製造方法以外の方法で製造してもよい。第2の粒子は、国際公開第2018/179969号に記載の方法で製造してもよい。
式:Li2ZSiO2+Zで表されるリチウムシリケートのZの値は、二酸化ケイ素とリチウム化合物との混合物におけるケイ素のリチウムに対する原子比:Li/Siによって制御すればよい。アルカリ成分の溶出の少ない良質なリチウムシリケートを合成するには、Li/Siを1より小さくすることが好ましい。
次に、リチウムシリケートと原料シリコンとの複合化が行われる。例えば、リチウムシリケートと原料シリコンとの混合物(さらに元素Meを含んでもよい)にせん断力を付与しながら混合物を粉砕すればよい。原料シリコンには、平均粒径が数μm~数十μm程度のシリコンの粗粒子を用いればよい。最終的に得られるシリコン粒子は、XRDパターンのSi(111)面に帰属される回析ピークの半値幅からシェラーの式により算出される結晶子サイズが10nm以上になるように制御することが好ましい。
第3の粒子を製造する方法の例として、第1および第2の方法を以下に説明する。第3の粒子は、以下で説明する製造方法以外の方法で製造してもよい。
次に、本開示の実施形態に係る二次電池について詳述する。本開示の実施形態に係る二次電池は、上記二次電池用負極材料を含む負極と、正極と、正極と負極の間に配置されたセパレータと、電解液と、を有する。
負極は、典型的には、負極集電体と、負極集電体の表面に配置された負極活物質層(負極合剤層)とを含む。負極活物質層は、上記二次電池用負極材料を含む。すなわち、負極活物質層は、負極活物質と、カーボンナノチューブを含み、必要に応じて、負極活物質およびカーボンナノチューブ以外の他の成分を含む。他の成分の例には、結着剤、導電剤、増粘剤などが含まれる。それらの他の成分には、公知の二次電池に用いられている成分を用いてもよい。負極活物質は、上述の通り、黒鉛およびSi含有材料を含む。
正極は、典型的には、正極集電体と、正極集電体の表面に形成された正極活物質層(正極合剤層)とを含む。正極合剤層は、正極合剤を分散媒に分散させた正極スラリーを、正極集電体の表面に塗布し、乾燥させることにより形成できる。乾燥後の塗膜を、必要により圧延してもよい。正極合剤は、必須成分として正極活物質を含み、任意成分として、結着剤、導電剤などを含むことができる。
電解液は、溶媒と、溶媒に溶解した溶質とを含む。溶質は、電解液中でイオン解離する電解質塩である。溶質は、例えば、リチウム塩を含み得る。溶媒および溶質以外の電解液の成分は添加剤である。電解液には、様々な添加剤が含まれ得る。
正極と負極との間には、セパレータが介在している。セパレータは、イオン透過度が高く、適度な機械的強度および絶縁性を備えている。セパレータとしては、微多孔薄膜、織布、不織布などを用いることができる。セパレータの材質としては、ポリプロピレン、ポリエチレンなどのポリオレフィンが好ましい。また、機械的強度を上げるためにアラミド繊維などを用いてもよい。
図1に示す二次電池1は、有底角形の電池ケース11と、電池ケース11内に収容された電極群10および非水電解質(図示せず)とを含む。電極群10は、長尺帯状の負極と、長尺帯状の正極と、これらの間に介在し、かつ直接接触を防ぐセパレータとを含む。電極群10は、負極、正極、およびセパレータを、平板状の巻芯を中心にして捲回し、巻芯を抜き取ることによって形成される。
負極活物質と、ポリアクリル酸ナトリウム(PAA-Na)と、カルボキシメチルセルロースナトリウム(CMC-Na)と、スチレン-ブタジエンゴム(SBR)と、カーボンナノチューブ(CNT)と、水とを所定の質量比で混合し、負極スラリーを調製した。カーボンナノチューブには、平均繊維長および平均繊維径が表1~表6に示すものを用いた。負極活物質には、黒鉛とSi含有材料との混合物を用いた。
正極活物質として、LiNi0.88Co0.09Al0.03O2を用いた。正極活物質と、アセチレンブラックと、ポリフッ化ビニリデンと、N-メチル-2-ピロリドン(NMP)とを、所定の質量比で混合し、正極スラリーを調製した。次に、正極集電体であるアルミニウム箔の表面に正極スラリーを塗布し、塗膜を乾燥させた後、圧延して、アルミニウム箔の両面に正極合剤層を形成した。
エチレンカーボネート(EC)およびエチルメチルカーボネート(EMC)を3:7の体積比で含む混合溶媒に、リチウム塩としてLiPF6を加え、電解液を調製した。非水電解液におけるLiPF6の濃度は1.3mol/リットルとした。
各電極にリードタブをそれぞれ取り付けた。次に、リードが最外周部に位置するように、セパレータを介して正極と負極とを渦巻き状に巻回した。このようにして電極群を作製した。次に、アルミニウム箔をバリア層とするラミネートフィルム製の外装体内に電極群を挿入し、真空乾燥した。次に、外装体内に非水電解液を注入し、外装体の開口部を封止した。このようにして、二次電池を得た。
表2に示す電池A15~A18およびB3では、Si含有材料として第1の粒子を用い、負極活物質の全体に対する第1の粒子の割合を10質量%とした。
表3に示す電池A19~A22では、Si含有材料として第1の粒子を用い、負極活物質の全体に対する第1の粒子の割合を7.5質量%~20質量%の範囲で変化させた。
表4に示す電池A23~A28およびA36では、Si含有材料として第1の粒子、第2の粒子、および第3の粒子を混合して用いた。負極活物質の全体に対する第1の粒子、第2の粒子、および第3の粒子の割合は、それぞれ、2.1質量%、3質量%、および3.2質量%とした。
表5に示す電池A29~A32では、Si含有材料として第3の粒子を用い、負極活物質の全体に対する第1の粒子の割合を15質量%とした。
表6に示す電池A33~A35では、Si含有材料として第1の粒子および第2の粒子を混合して用いた。負極活物質の全体に対する第1の粒子および第2の粒子の割合は、それぞれ、3質量%および5.5質量%とした。
(容量劣化率)
完成後の各電池について、25℃の環境に置き、0.5Itの電流で電圧が4.2Vになるまで定電流充電を行い、その後、4.2Vの定電圧で電流が0.02Itになるまで定電圧充電した。その後、1.0Itの電流で電圧が2.5Vになるまで定電流放電を行った。その後、20分間放置した。この操作(充放電サイクル)を100回繰り返した。充放電は25℃の環境で行った。
(劣化率(%))=100×(C50-C100)/(C0×50)
Claims (11)
- 負極活物質と、カーボンナノチューブを含み、
前記負極活物質は、黒鉛と、ケイ素元素を含む材料を含み、
前記カーボンナノチューブの平均繊維径が0.5~6nmであり、平均繊維長が1.2~8μmである、二次電池用負極材料。 - 前記黒鉛の平均粒子径が10~30μmであり、前記ケイ素元素を含む材料の平均粒子径が1~20μmである、請求項1に記載の二次電池用負極材料。
- 前記カーボンナノチューブの平均繊維径が1~4nmである、請求項1または2に記載の二次電池用負極材料。
- 前記カーボンナノチューブの平均繊維長が2~4μmである、請求項1~3のいずれか1項に記載の二次電池用負極材料。
- 前記負極活物質の全体に対する前記カーボンナノチューブの含有率は、0.005質量%~0.1質量%である、請求項1~4のいずれか1項に記載の二次電池用負極材料。
- 前記負極活物質の全体に対する前記カーボンナノチューブの含有率は、0.005質量%~0.07質量%である、請求項5に記載の二次電池用負極材料。
- 前記ケイ素元素を含む材料は、SiOX(0.5≦X<1.6)の式で表される酸化ケイ素を含む第1の粒子、リチウムシリケート相と前記リチウムシリケート相中に分散したシリコン相とを含む第2の粒子、および、炭素相と前記炭素相中に分散したシリコン相とを含む第3の粒子からなる群より選択される少なくとも一種の粒子を含む、請求項1~6のいずれか1項に記載の二次電池用負極材料。
- 前記黒鉛と前記ケイ素元素を含む材料との合計に対する前記ケイ素元素を含む材料の含有率は、5~50質量%である、請求項1~7のいずれか1項に記載の二次電池用負極材料。
- 前記黒鉛と前記ケイ素元素を含む材料との合計に対する前記ケイ素元素を含む材料の含有率は、6~20質量%である、請求項8に記載の二次電池用負極材料。
- 前記ケイ素元素を含む材料の平均粒子径が5~15μmである、請求項1~9のいずれか1項に記載の二次電池用負極材料。
- 請求項1~10のいずれか1項に記載の二次電池用負極材料を含む負極と、正極と、前記正極と前記負極の間に配置されたセパレータと、電解液と、を有する二次電池。
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JP2010212228A (ja) | 2009-02-13 | 2010-09-24 | Hitachi Maxell Ltd | 非水二次電池 |
JP2011076948A (ja) | 2009-09-30 | 2011-04-14 | Toray Ind Inc | 導電性複合体およびリチウムイオン電池用負極。 |
JP2014146519A (ja) * | 2013-01-29 | 2014-08-14 | Showa Denko Kk | 複合電極材 |
JP2015185229A (ja) * | 2014-03-20 | 2015-10-22 | 三菱マテリアル株式会社 | リチウムイオン二次電池の電極 |
JP2016139548A (ja) * | 2015-01-28 | 2016-08-04 | 日立化成株式会社 | リチウムイオン電池 |
JP2017084759A (ja) | 2015-10-30 | 2017-05-18 | 大阪瓦斯株式会社 | 電極活物質−カーボンナノチューブコンポジット及びその製造方法 |
WO2018179969A1 (ja) | 2017-03-29 | 2018-10-04 | パナソニックIpマネジメント株式会社 | 非水電解質二次電池用負極材料および非水電解質二次電池 |
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2021
- 2021-12-23 EP EP21911000.4A patent/EP4270542A1/en active Pending
- 2021-12-23 WO PCT/JP2021/048000 patent/WO2022138861A1/ja active Application Filing
- 2021-12-23 CN CN202180086840.2A patent/CN116648799A/zh active Pending
- 2021-12-23 US US18/269,095 patent/US20240047686A1/en active Pending
- 2021-12-23 JP JP2022571645A patent/JPWO2022138861A1/ja active Pending
Patent Citations (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2010212228A (ja) | 2009-02-13 | 2010-09-24 | Hitachi Maxell Ltd | 非水二次電池 |
JP2011076948A (ja) | 2009-09-30 | 2011-04-14 | Toray Ind Inc | 導電性複合体およびリチウムイオン電池用負極。 |
JP2014146519A (ja) * | 2013-01-29 | 2014-08-14 | Showa Denko Kk | 複合電極材 |
JP2015185229A (ja) * | 2014-03-20 | 2015-10-22 | 三菱マテリアル株式会社 | リチウムイオン二次電池の電極 |
JP2016139548A (ja) * | 2015-01-28 | 2016-08-04 | 日立化成株式会社 | リチウムイオン電池 |
JP2017084759A (ja) | 2015-10-30 | 2017-05-18 | 大阪瓦斯株式会社 | 電極活物質−カーボンナノチューブコンポジット及びその製造方法 |
WO2018179969A1 (ja) | 2017-03-29 | 2018-10-04 | パナソニックIpマネジメント株式会社 | 非水電解質二次電池用負極材料および非水電解質二次電池 |
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CN116648799A (zh) | 2023-08-25 |
US20240047686A1 (en) | 2024-02-08 |
EP4270542A1 (en) | 2023-11-01 |
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