Negative electrode and secondary battery

Abstract

Provided is a negative electrode or the like with which a secondary battery having excellent cycle characteristics can be manufactured.　The present invention provides: a negative electrode that includes a silicon-based negative electrode active material and a carbon nanotube aggregate, and that satisfies conditions (1) and (2); and applications thereof. (1) At least part of the silicon-based negative electrode active material is connected to a substance contained in the negative electrode via the carbon nanotube aggregate. (2) The carbon nanotube aggregate that contributes to the connection is in contact with a surface of the silicon-based negative electrode active material connected to the substance contained in the negative electrode by 0.01 or more per 1 μｍ2 of the surface.

Description

Anode and secondary battery

[0001] The present disclosure relates to a negative electrode and a secondary battery.

[0002] Carbon nanotubes have excellent mechanical and electronic properties and are expected to be used in a variety of applications. For example, Patent Document 1 describes a silicon-carbon-based anode material in the form of grape-like secondary particles, the secondary particles consisting of primary particles, carbon nanotubes, and graphite, and the surfaces of the primary particles and the graphite are connected by bridges of the carbon nanotubes. Patent Document 2 describes a negative electrode comprising a negative electrode active material layer, the negative electrode active material layer including a first active material, the first active material including a silicon-based material and a shell layer conductive binder, the shell layer conductive binder being encapsulated on the outer surface of the silicon-based material, the shell layer conductive binder including the first carbon nanotubes and a polymer, and the polymer being grafted to the first carbon nanotubes.

[0003] Chinese Patent Application Publication No. 116314732 Chinese Patent Application Publication No. 115966653

[0004] When a secondary battery is produced using a negative electrode containing an aggregate of carbon nanotubes, better cycle characteristics are required.

[0005] The present disclosure has been made in consideration of the above circumstances. An object of one embodiment of the present disclosure is to provide a negative electrode that can be used to fabricate a secondary battery having excellent cycle characteristics. An object of another embodiment of the present disclosure is to provide a secondary battery using the negative electrode.

[0006] Specific means for solving the above problems include the following aspects: [1] A negative electrode comprising a silicon-based negative electrode active material and a carbon nanotube aggregate, and satisfying the following conditions (1) and (2): (1) At least a part of the silicon-based negative electrode active material is connected to a substance contained in the negative electrode via the carbon nanotube aggregate; (2) With respect to the surface of the silicon-based negative electrode active material connected to the substance contained in the negative electrode, the carbon nanotube aggregate contributing to the connection has an area of ​​1 μm ２ [2] The anode according to [1], wherein the aggregate of carbon nanotubes includes carbon nanotubes having a maximum length of 1 μm to 1000 μm. [3] A secondary battery comprising the anode according to [1] or [2].

[0007] According to one embodiment of the present disclosure, there is provided a negative electrode capable of producing a secondary battery having excellent cycle characteristics. According to another embodiment of the present disclosure, there is provided a secondary battery using the above negative electrode.

[0008] 1 is a scanning electron microscope photograph of negative electrode 1 in Example 1. FIG. 2 is a scanning electron microscope photograph of negative electrode 1A in Comparative Example 1.

[0009] The negative electrode and secondary battery according to the present disclosure will be described in detail below. The following description of the requirements may be based on representative embodiments of the present disclosure, but the present disclosure is not limited to such embodiments and can be implemented with appropriate modifications within the scope of the present disclosure.

[0010] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples.

[0011] In the present disclosure, a combination of two or more preferred aspects is a more preferred aspect.

[0012] In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0013] In the present disclosure, "carbon nanotubes" are also referred to as "CNTs," "single-walled carbon nanotubes" are also referred to as "SWCNTs," "multi-walled carbon nanotubes" are also referred to as "MWCNTs," and "multi-walled carbon nanotube aggregates" are also referred to as "MWCNT aggregates."

[0014] [Negative electrode] The negative electrode according to the present disclosure includes a silicon-based negative electrode active material and a CNT aggregate, and satisfies the following conditions (1) and (2): (1) At least a part of the silicon-based negative electrode active material is connected to a substance contained in the negative electrode via the CNT aggregate; (2) With respect to the surface of the silicon-based negative electrode active material connected to the substance contained in the negative electrode, the CNT aggregate contributing to the connection has an area of ​​1 μm² on the surface. ２ There must be at least 0.01 contact.

[0015] The negative electrode according to the present disclosure can produce a battery having excellent cycle characteristics. The reason why such an effect is obtained is not clear, but is presumed to be as follows.

[0016] In the negative electrode, the CNT aggregate connects the silicon-based negative electrode active material to the substance contained in the negative electrode, thereby establishing electrical communication, and as a result, even if the silicon-based negative electrode active material repeatedly expands and contracts during charging and discharging, the electrical communication is maintained by the CNT aggregate, which is thought to improve the cycle characteristics of the secondary battery. In the negative electrode according to the present disclosure, in particular, the CNT aggregate contributing to the connection is located on the surface of the silicon-based negative electrode active material connected to the substance contained in the negative electrode (hereinafter also referred to as "connected silicon-based negative electrode active material") within an area of ​​1 μm ２ Since 0.01 or more CNT aggregates are in contact per unit area, there is a lot of electrical communication by the CNT aggregates in the linked silicon-based negative electrode active material, which is thought to improve the cycle characteristics of the secondary battery.

[0017] In contrast, Patent Documents 1 and 2 disclose that the CNT aggregates that contribute to the connection to the surface of the connected silicon-based negative electrode active material are within an area of ​​1 μm ２ There is no mention of contact of more than 0.01 hits.

[0018] <CNT aggregate> In the present disclosure, the CNTs contained in the CNT aggregate may be SWCNTs or MWCNTs, but it is preferable that the CNTs contain MWCNTs, which have a wall number distribution and are slightly less uniform, making it easier to improve dispersibility.

[0019] In general, SWCNTs have a lower volume resistivity than MWCNTs, and the lower the volume resistivity, the smaller the change in resistivity due to applied load tends to be. Therefore, in the CNT aggregate, SWCNTs and MWCNTs may be mixed and used as needed. The number of CNT walls can be controlled by selecting the CNT manufacturing method.

[0020] The longer the CNT, the lower the volume resistivity tends to be. The length of the CNT can be controlled by selecting the method for producing the CNT.

[0021] The CNT aggregate preferably contains CNTs having a maximum length of 1 μm to 1000 μm (i.e., specific CNTs). The specific CNTs are longer than general-purpose CNTs and tend to take on a fibrous shape.

[0022] Here, the term "fiber" is generally used to refer to a structure in which one dimension is larger than the other two. Fibers may be thread-like fibers with a circular cross section, ribbon-like fibers with a rectangular cross section, hollow, or have other shapes. From the viewpoint of low volume resistivity, the cross section of CNTs is preferably circular, and hollow is preferred.

[0023] When the maximum length is 1 μm or more, the silicon-based negative electrode active material is easily connected to the substance contained in the negative electrode via the CNT aggregate, further improving the cycle characteristics of the secondary battery.When the maximum length is 1000 μm or less, the entanglement between the CNT aggregates is not too strong, and the proportion of CNTs used to connect the silicon-based negative electrode active material to the substance contained in the negative electrode increases, making it easier to ensure electrical connection, which is advantageous.

[0024] The CNT aggregate preferably contains CNTs having a maximum length of 1 μm to 1000 μm, more preferably contains CNTs having a maximum length of 3 μm to 900 μm, further preferably contains CNTs having a maximum length of 5 μm to 800 μm, and particularly preferably contains CNTs having a maximum length of 10 μm to 700 μm.

[0025] The length of a CNT can be measured by focusing on a single CNT and observing multiple scanning electron microscope (SEM) photographs taken at adjacent viewing angles. Here, "CNT length" refers to the measured length of the CNT in the longitudinal direction, and the maximum value of the measured lengths is referred to as the "maximum length." When one or more CNTs with a maximum length in the range of 1 μm to 1000 μm are observed within the viewing angle of the SEM photograph, it can be confirmed that the observed CNTs include a specific CNT.

[0026] It is preferable that a plurality of specific CNTs are present within the viewing angle of the SEM photograph. Focusing on 100 CNTs included in the viewing angle of the SEM photograph, the maximum length of each is measured, and among the observed CNTs, it is preferable that MWCNTs (i.e., specific CNTs) having a maximum length in the range of 1 μm to 1000 μm are present in 10% or more in terms of number, from the viewpoint of further improving the cycle characteristics of the secondary battery, since the CNT aggregates facilitate bonding of the silicon-based negative electrode active material with the material contained in the negative electrode. It is more preferable that they are present in 20% or more, even more preferably 30% or more, and particularly preferably 50% or more.

[0027] The diameter of the CNTs in the CNT aggregate according to the present disclosure can be measured by observing an SEM photograph or a transmission electron microscope (TEM) photograph. Here, the diameter refers to the length in the direction perpendicular to the longitudinal direction of the CNT, and the diameter is measured at 10 different locations on one CNT, and the average value is taken as the diameter of that CNT.

[0028] The diameter of the CNT is preferably 1 nm to 100 nm, more preferably 2 nm to 80 nm, even more preferably 3 nm to 50 nm, and particularly preferably 5 nm to 30 nm.

[0029] The length / diameter ratio of the CNT, or the so-called aspect ratio, is preferably 30 or more, more preferably 100 or more, even more preferably 300 or more, and particularly preferably 500 or more. The aspect ratio can be calculated from the ratio of the maximum length to the diameter of a single CNT. From the viewpoint of measurement accuracy, it is preferable to use the average value of the measurements of 20 or more CNTs.

[0030] The length and diameter of the CNTs contained in the negative electrode can also be measured by observing multiple SEM or TEM photographs taken at adjacent viewing angles, as described above.

[0031] Furthermore, from the viewpoint of dispersibility, the specific gravity of the CNT aggregate is preferably in the range of 1.5 to 2.5, more preferably in the range of 1.7 to 2.4, and even more preferably in the range of 1.8 to 2.2. The specific gravity of the CNT aggregate can be measured by the method described in JIS Z8807:2012 "Method for measuring density and specific gravity of solids."

[0032] The purity of the CNT can be measured by thermogravimetric analysis. For example, a thermal analyzer (Shimadzu Corporation, DTG-60) is used to obtain a thermogravimetric (TG) curve and a differential thermal analysis (DTA) curve of the CNT aggregate. The largest exothermic peak in the DTA curve, which appears at a peak top near 650°C to 750°C, is considered to represent the combustion of CNT, and any other exothermic peaks are considered to represent the combustion of substances other than CNT. The purity of the CNT is determined from the weight loss rate of the TG curve. From the viewpoint of the resulting conductivity, the purity of the CNT aggregate is preferably 50% by mass or more, more preferably 65% ​​by mass or more, even more preferably 80% by mass or more, and particularly preferably 93% by mass or more.

[0033] The CNTs are preferably flexible and strong, and the electrical conductivity of the CNTs themselves is 5000 ohms. －１ ・m －１ It is preferable that the resistance is 10,000 ohms or more. －１ ・m －１ It is more preferable that the conductivity of the CNT itself is 1,000,000 ohms or more. －１ ・m －１ The following is the result.

[0034] <Method for Producing CNTs> The method for producing CNTs in the present disclosure is not particularly limited. For example, conventionally known methods such as chemical vapor deposition (CVD) and a method of reacting a gaseous reactant containing a carbon source in the presence of a catalyst can be used as the method for producing CNTs in the present disclosure.

[0035] The CNTs in the present disclosure can be produced by referring to the methods described in, for example, JP-A-2016-102047 and JP-T-2021-527611.

[0036] The CNT manufacturing method according to the present disclosure will be described below using examples, although the CNT manufacturing method according to the present disclosure is not limited to the following examples.

[0037] =Manufacturing Method X= An example of a CNT manufacturing method referenced in this disclosure is the manufacturing method described in JP 2016-102047 A. That is, the manufacturing method (hereinafter also referred to as "Manufacturing Method X") includes the steps of passing gaseous reactants containing one or more carbon sources through a reactor, reacting the one or more gaseous reactants in a reaction zone of the reactor in the presence of a catalyst to form product particles containing carbon, aggregating the product particles into aggregates, and applying force to the aggregates to continuously move the aggregates out of the reaction zone.

[0038] According to production method X, CNTs can be obtained in the form of fibrous aggregates or other aggregate forms that are easy to handle.

[0039] In production method X, the force applied to the product particles may be a mechanical force. When the agglomerates are fibrous CNTs, the mechanical force applied to the product particles may be applied by a rotating spindle around which the agglomerates are wound. The fibrous CNTs may be collected on the spindle, or may be accumulated elsewhere by rotating around the spindle one or more times and then continuously unwinding the spindle.

[0040] The spindle is preferably oriented with its axis perpendicular or parallel to the flow direction of the gaseous reactant(s), although other orientations are also possible, for example, a spindle with its axis oriented at an angle of 25° to the flow direction of the gaseous reactants may also be suitable for applying mechanical forces to the product particles.

[0041] The spindle can rotate about two axes (e.g., two perpendicular axes). In particular, the spindle can rotate about axes perpendicular and parallel to the flow direction of the gaseous reactants. Such a spindle can pull and twist the fibrous CNT aggregates to control the twist number and length.

[0042] The spindle material may be made of metal, ceramic, or resin. The spindle can have different suitable shapes depending on the properties of the material and the intended use of the CNTs. The spindle can be used as a mold for producing carbon products, for example, by a spin coating process. Preferred spindle shapes are rod-shaped or box-shaped.

[0043] Fibrous CNTs are deposited on a spindle or elsewhere, and the coating thickness and orientation can be controlled by controlling the reaction time and conditions, or by the conditions under which an electric or other field is applied to the carbon product. The coating thickness and orientation of the carbon product can be controlled, for example, by gas flow forces.

[0044] The spindle rotation speed is preferably 0.01 rpm (revolutions per minute; hereinafter the same) to 10,000 rpm, and more preferably 0.1 rpm to 100 rpm. The spinning speed (i.e., the spindle rotation speed) may be adjusted so that the material is collected at a similar rate to that at which it is produced. The spindle rotation speed may also control the thickness of the accumulated CNT fiber. In a preferred embodiment, as the spindle rotates, the CNT fiber is processed in the axial direction of the spindle. In this processing, the CNT fiber is evenly wrapped along the spindle, rather than being wrapped only at one specific point on the spindle.

[0045] The CNT fibers may be collected, for example, on the reactor wall by a substrate placed in the reactor. The substrate may be a fixed substrate or a rotating guide used to apply a strong and equal force to the CNT fibers as they are collected. Suitable substrate configurations for fiber technology include a substrate consisting of two guides positioned at right angles to each other.

[0046] In manufacturing method X, the mechanical force applied to the product particles may be a force applied by an accelerating gas flow. The accelerating gas flow may be generated by passing the product particles through a reactor having a narrow diameter or through a capillary tube downstream of the reaction zone. A vacuum may be applied to the product particles.

[0047] Other forces that can be applied to the product particles include electrostatic forces, suitably applied by a charged plate. Electrostatic forces require that the product particles be charged. The use of a charged plate allows the CNTs to grow in the form of intertwined sheets on the charged plate.

[0048] Other forces applied to the product particles may also be magnetic forces or photon pressure applied by a light source.

[0049] Instead of a gaseous reactant containing a carbon source, the CNT source may be injected in the form of a liquid containing a carbon source. When a liquid is used as the CNT source, it can be injected through a single inlet or multiple inlets, for example, in a showerhead configuration.

[0050] Preferably, the gaseous reactant or reactants are reacted at a temperature between 500° C. and 1600° C., more preferably between 1000° C. and 1500° C. or 1600° C. (especially between 1000° C. and 1500° C.). A temperature gradient is preferably maintained within the reactor, with the reaction zone being maintained at a higher temperature than the product zone of the reactor.

[0051] The gaseous reactants may be mixed with one or more gases that act as diluents. The gaseous reactants may also be mixed with gases that play a supporting but not direct role in the reaction. It is also preferred to use a diluent gas that can react with the amorphous carbon by-product, if any, to maintain active sites on the catalyst for nanotube production.

[0052] Examples of gases that can be used as diluents include argon or other inert gases, hydrogen, nitrogen, ammonia, carbon dioxide, and helium. Among these, a mixed gas of nitrogen and argon or hydrogen is particularly preferred as a gas that can be used as a diluent. The flow rate of the gas used as a diluent is preferably 2000 mL (milliliters) / min or less, and more preferably 400 mL / min to 800 mL / min.

[0053] The gas pressure of the gaseous reactants and any diluents is preferably between 0.1 bar and 50 bar, more preferably between 0.5 bar and 5 bar, and even more preferably between 1 bar and 2 bar. If there is a gas effluent from the furnace, the effluent gas can be recycled with or without cleaning.

[0054] The composition of the product particles can be controlled by monitoring the agglomerates and modifying the reaction conditions based on the information obtained. For example, the agglomerates can be monitored by online Raman spectroscopy, which provides data indicating whether the CNTs are single-walled or multi-walled. It also provides data indicating the diameter and crystallinity of the CNTs. The agglomerates can also be monitored by online conductivity measurements, gas analysis, measuring the opacity of the reaction zone, and / or measuring the winding force.

[0055] When the agglomerate is removed from the reactor, it is preferable to prevent air from entering the reactor. When the diluent gas contains hydrogen, preventing air from entering the reactor is desirable, for example, from the viewpoint of preventing an explosive mixture of hydrogen and air from being formed in the reactor.

[0056] The product particles in Production Method X contain MWCNTs. Depending on the production conditions, SWCNTs may also be contained in addition to MWCNTs.

[0057] The product particles may be produced by chemical vapor deposition, where a gaseous reactant, a carbon source, is reacted in the presence of a catalyst.

[0058] Suitable carbon-containing compounds as the carbon source include carbon monoxide, carbon dioxide, aromatic hydrocarbons (e.g., benzene, toluene, xylene, cumene, ethylbenzene, naphthalene, or mesitylene), non-aromatic hydrocarbons (e.g., methane, ethane, propane, butane, pentane, hexane, cyclohexane, ethylene, propylene, or acetylene), and oxygen-containing hydrocarbons (e.g., formaldehyde, acetaldehyde, acetone, methanol, ethanol, diethyl ether, polyethylene glycol, 1-propanol, ethyl formate, and hydrocarbons containing mixtures of two or more thereof). Preferred carbon-containing compounds are carbon monoxide, methane, ethylene, or acetylene.

[0059] Preferably, the carbon source contains oxygen. Ethanol is a particularly preferred carbon source. Oxygen can be introduced into the reactor by other methods, such as by using a diluent gas or a carbon source containing water.

[0060] The gaseous reactant, which is a carbon source, is preferably injected into the reactor at a rate of 0.01 mL / min to 10 mL / min, more preferably 0.08 mL / min to 0.25 mL / min.

[0061] The catalyst is preferably a transition metal, particularly a Group VIB transition metal such as chromium (Cr), molybdenum (Mo), tungsten (W), or a Group VIIIB transition metal. Specifically, the catalyst may be, for example, iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), or manganese (Mn), or a mixture thereof. Metals from the lanthanide and actinide series, such as yttrium (Y), may also be used as catalysts. Fe, Ni, Co, Mo, and mixtures thereof, such as a 50 / 50 mixture of Ni and Co, a mixture of Fe and Ni, or a mixture of Fe and Mo, are more preferred. Any of these transition metals, alone or in combination with any of the other transition metals listed, may serve as a catalyst for CNT growth. It is particularly preferred that the catalyst is a mixture of two or more of the metals listed.

[0062] The catalyst is preferably formed by decomposition of a precursor. The precursor is preferably a thermally, photo-, or plasma-decomposable compound of one or more of the above metals, such as a carbonyl or cyclopentadienyl organometallic compound. Ferrocene, iron pentacarbonyl, nickelocene, and cobaltocene are particularly preferred precursors. Suitably, at least 0.01% by weight of the precursor is contained in the carbon source, and preferably 0.2% to 2.5% by weight of the precursor is contained in the carbon source. In one embodiment, 0.23% to 2.3% by weight of the precursor is contained in the carbon source. The catalyst may be supported on a carrier. Preferred carriers include silica and magnesium oxide.

[0063] The carbon source is preferably reacted in the presence of a promoter. Suitable promoters include one or more of sulfur, phosphorus, molybdenum, and organic compounds of these elements. Thiophene is one of the preferred promoters. Suitably, up to 10% by weight (10% by weight or less) of the promoter is contained in the carbon source. Preferably, 0.2% to 6% by weight of the promoter is contained in the carbon source. MWCNTs are successfully formed when a high or low concentration of thiophene is used as the promoter. For example, MWCNTs are successfully formed using ethanol containing 0% by weight or 1.5% to 4.0% by weight of thiophene and 1.0% to 10.0% by weight (particularly 2.3% by weight) of ferrocene, with an injection rate of 5.0 mL / hour to 30.0 mL / hour (particularly 7.5 mL / hour), a hydrogen flow rate of 400 mL / min to 800 mL / min, and a synthesis temperature of 1100°C to 1180°C.

[0064] According to production method X, it is possible to obtain fibrous CNTs having a length of at least 500 μm, for example, at least 1 mm. The fibrous CNTs can be in the form of threads or sheets. The length of the fibrous CNTs can be controlled, for example, by the winding capacity of the spindle used to produce the fibrous CNTs.

[0065] The manufacturing method X preferably includes the steps of reacting a carbon source in a reaction zone of a reactor to produce CNTs, and aggregating the CNTs into aggregates by applying force to the CNTs. This manufacturing method makes it possible to easily produce fibrous CNTs.

[0066] Another embodiment may include generating CNTs in a reaction zone by the above method, followed by condensation to form CNTs, and continuously withdrawing the CNTs from near the reaction zone. Another embodiment may include generating CNTs in a reaction zone, continuously electrostatically withdrawing the CNTs from the reaction zone, and recovering the CNTs.

[0067] =Production Method Y= In the present disclosure, as an example of a method for producing CNTs, the production method described in JP-A No. 2021-527611 can be referred to. That is, a mixture containing a main catalyst precursor and a co-catalyst precursor is mixed with γ-Al ２ O ３ a production method (hereinafter also referred to as "production method Y") that includes a step (1) of supporting a carbon nanotube on a support to produce an active support, a step (2) of drying the active support by multi-stage drying including vacuum drying, a step (3) of subjecting the dried active support to a heat treatment to produce a supported catalyst, and a step (4) of producing CNTs in the presence of the supported catalyst.

[0068] Step (1) In step (1), a mixture containing a main catalyst precursor and a co-catalyst precursor is added to a γ-Al ２ O ３ to produce an active support.

[0069] The main catalyst precursor and the co-catalyst precursor were ２ O ３ In order to uniformly support the catalyst precursor and the co-catalyst precursor on the catalyst support, the mixture may further contain a solvent, and the main catalyst precursor and the co-catalyst precursor may be dissolved in the solvent. The solvent may be one or more selected from the group consisting of water, methanol, and ethanol, and water is preferred.

[0070] γ-Al ２ O ３ has high porosity and a spinel structure, and therefore the main catalyst and co-catalyst are γ-Al.２ O ３ The CNTs grown from the randomly arranged primary catalyst can be produced in an entangled state.

[0071] The main catalyst may be one or more selected from the group consisting of cobalt, iron, nickel, manganese and chromium, with cobalt being preferred.

[0072] The main catalyst precursor may be one or more selected from the group consisting of nitrates, sulfates, carbonates and acetates of the main catalyst, with nitrates of the main catalyst being preferred.

[0073] The main catalyst precursor is Co(NO ３ ) ２ , Co(NO ３ ) ２ ・6H ２ O, Co ２ (CO) ８ , Co ２ (CO) ６ [HC=C(C(CH ３ ) ３ )], Co(CH ３ CO ２ ) ２ , Fe(NO ３ ) ３ , Fe(NO ３ ) ２ ・nH ２ O, Fe(CH ３ CO ２ ) ２ , Ni(NO ３ ) ２ , Ni(NO ３ ) ２ ・6H ２ O, Mn(NO ３ ) ２ , Mn(NO ３ ) ２ ・6H ２ O, Mn(CH ３ CO ２ ) ２ ・n (H ２ O) and Mn(CO)5Br, and among these, Co(NO ３ ) ２ ・6H ２ O, Fe(NO ３ )２ ・nH ２ O, Ni(NO ３ ) ２ ・6H ２ O is preferred.

[0074] The promoter improves the dispersibility of the main catalyst, and may be one or more selected from the group consisting of vanadium and molybdenum.

[0075] The cocatalyst precursor is NH ４ VO ３ , NaVO ３ , V ２ O ５ , V(C ５ H ７ O ２ ) ３ , and (NH ４ ) 6Mo ７ O ２４ ・4H ２ O, and NH ４ VO ３ and (NH ４ ) 6Mo ７ O ２４ ・4H ２ O is preferred.

[0076] When the mixture contains two or more promoter precursors, i.e., when it contains both a vanadium precursor and a molybdenum precursor, the molar ratio of the sum of vanadium and molybdenum to vanadium can be 1:0.45 to 1:0.95 or 1:0.5 to 1:0.9, and preferably 1:0.5 to 1:0.9. When the above conditions are met, the CNT structure can be stably maintained and CNTs having the desired pore volume can be produced.

[0077] The mixture may contain the main catalyst precursor and the co-catalyst precursor such that the molar ratio of the main catalyst to the co-catalyst is 1:0.01 to 1:0.5, 1:0.1 to 1:0.4, or 1:0.1 to 1:0.25, and preferably 1:0.1 to 1:0.25. Satisfying the above molar ratios can improve the dispersibility of the main catalyst, allowing the production of CNTs with a desired pore volume.

[0078] The mixture may further include an organic acid which serves to inhibit precipitation of the main catalyst precursor and the co-catalyst precursor.

[0079] The organic acid may be one or more selected from the group consisting of citric acid, tartaric acid, fumaric acid, malic acid, acetic acid, butyric acid, palmitic acid and oxalic acid, with citric acid being preferred.

[0080] The mixture may contain the organic acid and the co-catalyst precursor in a molar ratio of 1:1 to 1:20, 1:2 to 1:10, or 1:3 to 1:6, preferably 1:3 to 1:6. When the above range is satisfied, it is possible to prepare a transparent catalyst metal solution during catalyst preparation, and it is possible to prepare a catalyst with reduced fine particles during impregnation.

[0081] After step (1), a step of aging may be further included.

[0082] The aging may be carried out for 1 minute to 60 minutes or 10 minutes to 50 minutes. It is preferably carried out for 10 minutes to 50 minutes. When the above conditions are satisfied, γ-Al ２ O ３ In addition, bubbles present in the support are removed to the maximum extent possible, allowing the main catalyst precursor and the co-catalyst precursor to be sufficiently supported even in the fine pores inside the support.

[0083] Step (2) Next, the active support is dried by multi-stage drying including vacuum drying.

[0084] Multi-stage drying may mean that a drying process including vacuum drying is performed two or more times. Specifically, multi-stage drying may include atmospheric drying and vacuum drying, or may include vacuum drying two or more times.

[0085] The vacuum drying may be carried out at a temperature of 80° C. to 300° C. or 120° C. to 250° C., preferably 120° C. to 250° C. If the above conditions are met, the main catalyst precursor, i.e., the coordinate bond of the main catalyst, can be easily decomposed to form the main catalyst oxide, and energy consumption can be minimized.

[0086] The vacuum drying may be performed at 1 mbar to 200 mbar or 30 mbar to 150 mbar, preferably 30 mbar to 150 mbar. When the above conditions are met, the main catalyst precursor, i.e., the main catalyst coordinated compound, is rapidly decomposed and discharged, making it easier to form the main catalyst oxide under vacuum conditions and minimizing energy consumption.

[0087] The vacuum drying can be performed for 10 minutes to 3 hours or 10 minutes to 2 hours, preferably 10 minutes to 2 hours. If the above conditions are met, the main catalyst precursor can be easily decomposed and converted into the main catalyst oxide, and energy consumption can be minimized.

[0088] On the other hand, when the multi-stage drying includes atmospheric drying and vacuum drying, atmospheric drying can be performed before the above-mentioned vacuum drying, and the atmospheric drying can remove solvent that may be present in the active support.

[0089] Drying at atmospheric pressure may be carried out at 80° C. to 160° C. or 100° C. to 140° C., and is preferably carried out at 100° C. to 140° C. If the above conditions are met, the solvent present in the active support can be sufficiently removed, and energy consumption can be minimized.

[0090] Drying under atmospheric pressure may be carried out at 900 mbar to 1,100 mbar, preferably 950 mbar to 1,050 mbar. If the above conditions are met, the solvent present in the active support can be sufficiently removed and energy consumption can be minimized.

[0091] Drying under atmospheric pressure may be carried out for 1 to 12 hours, preferably 3 to 9 hours. If the above conditions are met, the solvent present in the active support can be sufficiently removed and energy consumption can be minimized.

[0092] On the other hand, when the multi-stage drying includes two or more vacuum dryings, the multi-stage drying may include two or more vacuum dryings performed at different temperatures, more specifically, a primary vacuum drying performed at a first temperature and a secondary vacuum drying performed at a second temperature higher than the first temperature.

[0093] The primary vacuum drying can remove any solvent that may be present in the active support.

[0094] The first temperature may be 80° C. to 160° C., and preferably 100° C. to 140° C. If the above conditions are met, the solvent present in the active support can be sufficiently removed, and energy consumption can be minimized.

[0095] The primary vacuum drying can be performed for 1 to 12 hours, preferably 3 to 9 hours. If the above conditions are met, the solvent present in the active support can be sufficiently removed and energy consumption can be minimized.

[0096] The primary vacuum drying may be performed at 1 mbar to 200 mbar, 1 mbar to 150 mbar, or 80 mbar to 150 mbar, and is preferably performed at 80 mbar to 150 mbar. If the above conditions are met, the solvent present in the active support can be sufficiently removed and energy consumption can be minimized.

[0097] The secondary vacuum drying is as described above in the description of the vacuum drying.

[0098] The second temperature may be 175 to 300° C., and preferably 180 to 280° C. If the above conditions are met, the main catalyst precursor, i.e., the coordinate bond of the main catalyst, can be easily decomposed to form the main catalyst oxide, and energy consumption can be minimized.

[0099] The secondary vacuum drying may be performed at 1 mbar to 200 mbar, 1 mbar to 150 mbar, or 1 mbar to 70 mbar, and more preferably at 1 mbar to 70 mbar. When the above conditions are met, the main catalyst precursor, i.e., the main catalyst coordinated compound, is rapidly decomposed and discharged, making it easier to form the main catalyst oxide under vacuum conditions and minimizing energy consumption.

[0100] The secondary vacuum drying may be performed for 10 minutes to 3 hours or 10 minutes to 2 hours, preferably 10 minutes to 2 hours. If the above conditions are met, the main catalyst precursor can be easily decomposed and converted into the main catalyst oxide, and energy consumption can be minimized.

[0101] Step (3) Next, the dried active support is subjected to a heat treatment to produce a supported catalyst.

[0102] When heat treatment is performed, the main catalyst and the co-catalyst are converted into γ-Al ２ O ３ A supported catalyst is produced in which the catalyst is present in a coated state on the surface and in the pores of the catalyst.

[0103] The heat treatment may be carried out at 600° C. to 800° C. or 620° C. to 750° C., and is preferably carried out at 620° C. to 750° C. When the above conditions are satisfied, the main catalyst and the co-catalyst are γ-Al. ２ O ３ Therefore, the supported catalyst can be produced with the surface and pores of the catalyst uniformly coated, and energy consumption can be minimized.

[0104] The heat treatment may be carried out for 1 to 12 hours or 2 to 8 hours, and is preferably carried out for 2 to 8 hours. ２ O ３ A supported catalyst can be produced in which the catalyst is present in a uniform coating on the surface and in the pores of the catalyst.

[0105] Step (4) Next, CNTs are produced in the presence of a supported catalyst.

[0106] Specifically, CNTs can be produced by contacting a supported catalyst with a carbon-based compound, and specifically, by chemical vapor synthesis.

[0107] To explain the steps of producing CNTs in detail, first, a supported catalyst can be loaded into a horizontal fixed-bed reactor or a fluidized-bed reactor. Then, at a temperature above the thermal decomposition temperature of the gaseous carbon-based compound or below the melting point of the catalyst supported on the supported catalyst, a gaseous carbon-based compound or a mixture of the gaseous carbon-based compound, a reducing gas (e.g., hydrogen), and a carrier gas (e.g., nitrogen) is injected to grow CNTs by chemical vapor synthesis through the decomposition of the gaseous carbon-based compound.

[0108] The CNTs produced by the above-mentioned chemical vapor synthesis method have a crystal growth direction that is nearly parallel to the tube axis, and the graphite structure has high crystallinity along the tube length. As a result, CNTs with small unit diameters and high electrical conductivity and strength can be produced.

[0109] The chemical vapor synthesis method may be carried out at a temperature of 600° C. to 800° C. or 650° C. to 750° C., and is preferably carried out at a temperature of 650° C. to 750° C. If the above temperature is satisfied, CNTs can be produced while minimizing the generation of amorphous carbon.

[0110] The heat source for the reaction may be induction heating, radiant heat, laser, IR, microwave, plasma, surface plasmon heating, or the like.

[0111] Any carbonaceous compound can be used without particular limitation as long as it can supply carbon and can exist in a gaseous state at a temperature of 300° C. or higher.

[0112] The carbon-based compound may be a carbon-based compound having 6 or less carbon atoms, and may be one or more compounds selected from the group consisting of carbon monoxide, methane, ethane, ethylene, ethanol, acetylene, propane, propylene, butane, butadiene, pentane, pentene, cyclopentadiene, hexane, cyclohexane, benzene, and toluene.

[0113] After growing CNTs through the above-described reaction, a cooling step may be optionally performed to arrange the CNTs more regularly. Specifically, the cooling step may be performed by natural cooling by removing the heat source or by using a cooler.

[0114] The above manufacturing method X and manufacturing method Y are examples, and the manufacturing method of CNTs that can be contained in a CNT aggregate is not limited to the above.

[0115] <CNT Dispersion> When producing the negative electrode according to the present disclosure, it is preferable to prepare a CNT dispersion in advance in order to disperse the CNT aggregate in the negative electrode. The CNT dispersion preferably contains a CNT aggregate and a dispersion medium.

[0116] The CNT dispersion preferably contains a dispersion medium. The dispersion medium preferably contains water, more preferably contains water as a main component, and even more preferably is water. "Contains water as a main component" means that the proportion of water in the dispersion medium is greater than 50 mass%. The proportion of water in the dispersion medium is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more, and may be, for example, 100 mass%.

[0117] The water is not particularly limited, but is preferably distilled water, ion-exchanged water, pure water, or the like, which contains fewer impurities.

[0118] The dispersion medium may be a mixture of water and a hydrophilic solvent. Examples of the hydrophilic solvent include carbonate compounds such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and butylene carbonate; ether compounds such as tetrahydrofuran; ketone compounds such as acetone; lower alcohol compounds such as methanol and ethanol; and solvents such as acetonitrile. When the dispersion medium contains a hydrophilic solvent, the proportion of the hydrophilic solvent in the dispersion medium is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0119] In addition to the CNT aggregate and the dispersion medium, the CNT dispersion may further contain other components that can be used in the dispersion. Examples of other components include dispersants, antifoaming agents, antistatic agents, and conductive additives other than the conductive additives according to the present disclosure. The CNT dispersion may also contain trace amounts of impurity components, so-called inevitable impurities.

[0120] The CNT dispersion may contain a dispersant for the purpose of further improving the dispersibility and dispersion stability of the CNT aggregate. The dispersant is not particularly limited, and examples thereof include various surfactants. Further examples of the dispersant include polymer compounds such as resins. The dispersant is preferably a surfactant. The surfactant may be an ionic surfactant or a nonionic surfactant, and is not particularly limited. In the CNT dispersion, the surfactant may be used alone or in combination of two or more types.

[0121] Examples of ionic surfactants include anionic surfactants, cationic surfactants, and amphoteric surfactants. Examples of anionic surfactants include aromatic sulfonic acid surfactants such as alkylbenzene sulfonates (e.g., dodecylbenzene sulfonate) and dodecylphenyl ether sulfonates; ether sulfate surfactants; phosphate surfactants; and carboxylic acid surfactants. Examples of cationic surfactants include alkylamine salts and quaternary ammonium salts. Examples of amphoteric surfactants include alkylbetaine surfactants and amine oxide surfactants. Examples of ionic surfactants include ionic surfactants having an aromatic ring (so-called aromatic ionic surfactants), and more preferably aromatic sulfonic acid surfactants such as alkylbenzene sulfonates and dodecylphenyl ether sulfonates. Aromatic ionic surfactants tend to have excellent CNT dispersibility, dispersion stability, and high concentration.

[0122] Examples of nonionic surfactants include sugar ester surfactants such as sorbitan fatty acid esters and polyoxyethylene sorbitan fatty acid esters; fatty acid ester surfactants such as polyoxyethylene resin acid esters and polyoxyethylene fatty acid diethyl esters; ether surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, and polyoxyethylene-polypropylene glycols; and aromatic nonionic surfactants such as polyoxyalkylene octyl phenyl ether, polyoxyalkylene nonyl phenyl ether, polyoxyalkyl dibutyl phenyl ether, polyoxyalkyl styryl phenyl ether, polyoxyalkyl benzyl phenyl ether, polyoxyalkyl bisphenyl ether, polyoxyalkyl cumyl phenyl ether, and polyoxyalkylene phenyl ether. Nonionic surfactants are preferably ionic surfactants having an aromatic ring (so-called aromatic nonionic surfactants), more preferably polyoxyalkylene phenyl ether, and even more preferably polyoxyethylene phenyl ether. Aromatic nonionic surfactants tend to be excellent in CNT dispersibility, dispersion stability, and high concentration.

[0123] Other dispersants that are excellent in CNT dispersibility, dispersion stabilization, and concentration enhancement include β-naphthalenesulfonic acid formalin condensate sodium salts, such as Demol N, Demol RN, and Demol T (manufactured by Kao Corporation), polyoxyethylene stearyl ether Brij S 100 (manufactured by Sigma-Aldrich), polyvinylpyrrolidone K30 (manufactured by, for example, Fujifilm Wako Pure Chemical Industries, Ltd.), carboxymethylcellulose (CMC) (manufactured by, for example, Daicel Miraize Co., Ltd.), sodium deoxycholate (manufactured by, for example, Fujifilm Wako Pure Chemical Industries, Ltd.), and SOLSPERSE ＴＭ W100, SOLSPERSE ＴＭ W150 (manufactured by Lubrizol Japan, Inc.) and the like. CMC is particularly preferred from the viewpoint of excellent CNT dispersibility, dispersion stabilization ability, and ability to increase the concentration.

[0124] When the CNT dispersion liquid contains a dispersant, the amount of the dispersant used is not particularly limited and can be set appropriately depending on the type of dispersant, the amount of CNT, the amount of dispersion medium, etc.

[0125] [Method for Producing CNT Dispersion] The method for producing the CNT dispersion is not particularly limited. The CNT dispersion can be produced by dispersing a CNT aggregate in a dispersion medium. That is, the CNT dispersion can be produced by a method including a step of dispersing a CNT aggregate in a dispersion medium (also referred to as a "dispersion step"). The dispersion medium that can be used in the dispersion step is as described above.

[0126] The dispersion method is not particularly limited. Examples of the dispersion method include methods using dispersion devices such as a stirrer, homogenizer, colloid mill, flow jet mixer, dissolver, paint conditioner, Manton emulsifier, jet mill, and ultrasonic device. Examples of the dispersion method include methods using known pulverization means, such as ball milling (e.g., ball mill, vibration ball mill, planetary ball mill, bead mill, etc.), sand milling, colloid milling, jet milling, roller milling, vertical or horizontal agitator mill, attritor, colloid mill, three-roll mill, pearl mill, super mill, impeller, disperser, KD mill, dynatron, and pressure kneader. As a dispersion method, a method using a jet mill is preferred, and a method using a wet jet mill is more preferred. A wet jet mill is a dispersion device that pressure-feeds a mixture in a solvent as a high-speed flow from a nozzle arranged in a sealed pressure-resistant container. In a wet jet mill, CNTs are dispersed in a pressure-resistant vessel by collisions between opposing flows, collisions with the vessel wall, turbulence caused by high-speed flows, shear flows, etc. As a wet jet mill, an ultra-high-pressure homogenizer (model numbers: NAGS20, NAGS100, JAGS200, NAGS1000, etc.) manufactured by Joko Co., Ltd. can be suitably used. However, the wet jet mill is not limited to this. When using the ultra-high-pressure homogenizer as a dispersing device, the dispersion treatment pressure is preferably 10 MPa to 250 MPa.

[0127] The method for producing a CNT dispersion may include a step of drying the CNT aggregate (also referred to as a "drying step") before the dispersion step.

[0128] If moisture adheres to the CNTs, the surface tension of the water makes them more likely to adhere to each other, which raises concerns about reduced dispersibility. Therefore, by performing a drying process on the conductive additive body before the dispersion process, moisture adhered to the CNTs is removed, preventing adhesion of CNTs to each other due to moisture adhesion, and further improving the dispersibility of the CNTs in the dispersion medium. The drying method is not particularly limited. Examples of drying methods include heat drying, vacuum drying, and heat vacuum drying. Heat vacuum drying is preferred as the drying method. The drying temperature is not particularly limited, and is preferably, for example, 40°C to 100°C. The drying time is not particularly limited, and can be set appropriately depending on the drying temperature, the degree of moisture adhesion to the CNTs, etc.

[0129] An example of producing a CNT dispersion is shown below, but the production of a CNT dispersion is not limited to the following.

[0130] <Production Example 1: Example of Production of CNT Dispersion> 0.040 g of the CNT aggregate according to the present disclosure is weighed and placed in a three-neck flask. After the CNT aggregate is placed in the flask, a large excess of ion-exchanged water (for example, 20 mL) is poured into the flask and stirred at room temperature (25°C, the same applies below). At this time, a known dispersant (for example, carboxymethyl cellulose) may be added as appropriate. Next, a conductive additive is dispersed in the dispersion medium using a known dispersion device (for example, an ultrasonic irradiation device or a wet jet mill). The obtained dispersion is further stirred with a stirrer at room temperature for a long period of time (for example, 1 hour to 48 hours). In this way, a CNT dispersion is obtained.

[0131] <Silicon-Based Negative Electrode Active Material> The silicon-based negative electrode active material is preferably silicon-based negative electrode active material particles. As the silicon-based active material particles, at least one selected from the group consisting of Si, SiOx (0<x<2), a Si-C composite, and a Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof) may be used. The use of silicon-based active material particles can increase the capacity of the battery.

[0132] The average particle size of the silicon-based negative electrode active material particles is preferably 100 nm to 50 μm, more preferably 500 nm to 40 μm, and even more preferably 1 μm to 30 μm. The average particle size of the silicon-based negative electrode active material particles is measured, for example, using a laser diffraction / scattering particle size distribution measuring device (Mastersizer 3000+ Lab, manufactured by Spectris Inc.). The particle size distribution of the silicon-based negative electrode active material particles is measured, and the 50% cumulative volume particle size (D ５０ (μm)) can be used as the average particle size.

[0133] The average particle size of the silicon-based negative electrode active material particles contained in the negative electrode can be measured, for example, by observing an SEM photograph. The imaging method using an SEM is not particularly limited and can be performed by a known method. As an example, imaging is performed using an SEM device (S-4800, manufactured by Hitachi High-Technologies Corporation) under the following conditions: Acceleration voltage: 5 kV Emission current: 10 μA Measurement magnification: 5000 times. 50 or more silicon-based negative electrode active material particles are imaged, and the length of the perpendicular line sandwiched between two parallel lines in a fixed direction (Ferret diameter (μm)) is measured in the image projected with the silicon-based negative electrode active material particles. The average value of the Feret diameters can be used as the average particle size.

[0134] The negative electrode may contain a negative electrode active material other than the silicon-based negative electrode active material. Specifically, it may contain graphite-based active material particles. The graphite-based active material particles may be at least one selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbeads. Using artificial graphite as the graphite-based active material particles can improve rate characteristics.

[0135] The average particle size of the graphite-based active material particles is preferably 100 nm to 100 μm, more preferably 500 nm to 80 μm, and even more preferably 1 μm to 50 μm. The average particle size of the graphite-based active material particles is measured, for example, using a laser diffraction / scattering particle size distribution measuring device (Mastersizer 3000+ Lab, manufactured by Spectris Inc.). The particle size distribution of the graphite-based active material particles is measured, and the 50% cumulative volume particle size (D ５０ (μm)) can be used as the average particle size.

[0136] The average particle size of the graphite-based active material particles contained in the negative electrode can be measured, for example, by observing an SEM photograph. The imaging method using an SEM is not particularly limited and can be performed by a known method. As an example, an SEM device (S-4800, manufactured by Hitachi High-Technologies Corporation) is used to image under the following conditions: Acceleration voltage: 5 kV Emission current: 10 μA Measurement magnification: 5000 times 50 or more graphite-based active material particles are imaged, and in the image projected onto the graphite-based active material particles, the length of the perpendicular line sandwiched between two parallel lines in a fixed direction (Ferret diameter (μm)) is measured. The average value of the Feret diameters can be used as the average particle size.

[0137] <Other Components> The negative electrode may include a negative electrode active material layer, or may include a current collector and a negative electrode active material layer disposed on the current collector.

[0138] The current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. Examples of the current collector include copper, stainless steel, aluminum, nickel, titanium, baked carbon, and aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like. Specifically, transition metals such as copper and nickel that have good carbon adsorption properties may be used as the current collector.

[0139] The negative electrode active material layer may further contain a binder. The binder is not particularly limited, and the negative electrode active material layer may contain a binder that is commonly used in electrode materials. Examples of the binder include at least one polymer selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and polyacrylic acid, as well as polymers in which the hydrogen atoms of these polymers have been substituted with Li, Na, Ca, or the like.

[0140] <Relationship between Silicon-Based Negative Electrode Active Material and CNT Aggregate> In the negative electrode according to the present disclosure, at least a portion of the silicon-based negative electrode active material is connected to a substance contained in the negative electrode via a CNT aggregate. That is, there is a silicon-based negative electrode active material (connected silicon-based negative electrode active material) connected to a substance contained in the negative electrode via a CNT aggregate. The substance contained in the negative electrode that is connected to the silicon-based negative electrode active material is not particularly limited as long as it is other than the silicon-based negative electrode active material itself. That is, a form in which the silicon-based negative electrode active material and another silicon-based negative electrode active material are connected via a CNT aggregate may also be used.

[0141] In the negative electrode according to the present disclosure, the CNT aggregates that contribute to the connection are arranged on the surface of the connected silicon-based negative electrode active material within an area of ​​1 μm ２ From the viewpoint of improving the cycle characteristics of the secondary battery, the surface area of ​​the linked silicon-based negative electrode active material is 1 μm or more. ２ The number of CNT aggregates that are in contact and contribute to the connection per unit area of ​​the surface of the connected silicon-based negative electrode active material is preferably 0.02 or more, more preferably 0.03 or more, even more preferably 0.05 or more, and particularly preferably 0.1 or more. ２ The upper limit of the number of CNT aggregates that are in contact and contribute to the connection may be 10 or less per 1 μm2 of the surface of the connected silicon-based negative electrode active material. ２ The number of CNT aggregates that are in contact and contribute to the connection is measured by the following method.

[0142] The negative electrode is imaged using an SEM. The imaging method using an SEM is not particularly limited, and can be performed using a known method. As an example, an SEM device (S-4800, manufactured by Hitachi High-Technologies Corporation) is used to image the negative electrode under the following conditions: Acceleration voltage: 1 kV, Emission current: 10 μA, Measurement magnification: 10,000 times.

[0143] Methods for identifying the silicon-based negative electrode active material contained in the negative electrode include methods that utilize backscattered electron images and energy dispersive X-ray spectroscopy (EDX) in addition to the shape of the material.

[0144] A backscattered electron image is an image constructed based on backscattered electrons (backscattered electrons) emitted by elastic scattering of incident electrons, and the contrast of the image is determined by the difference in average atomic number. In the region where the average atomic number is 40 or less, the change in the backscattered electron emission rate is large, making it easier to obtain contrast and information about the composition. In the present disclosure, since the substances contained in the negative electrode often have an average atomic number of 40 or less, and silicon has a higher backscattered electron emission rate than carbon, silicon-based negative electrode active materials can be identified from the difference in contrast in the backscattered electron image.

[0145] EDX is a technique for performing elemental analysis by detecting characteristic X-rays generated by electron beam irradiation and analyzing them by energy. In the present disclosure, since Si is detected from silicon-based negative electrode active materials, the silicon-based negative electrode active materials can be identified from the results of elemental analysis.

[0146] CNTs contained in the anode can be identified based on the shape of the material. Materials that have a high aspect ratio, are fibrous, and have a diameter of 100 nm or less can be identified as CNTs. As mentioned above, in addition to the shape of the material, backscattered electron images and EDX can also be used.

[0147] In this way, the silicon-based negative electrode active material and CNTs are distinguished from the materials contained in the negative electrode. The presence or absence of a linked silicon-based negative electrode active material is determined from the captured SEM photograph. When one or more linked silicon-based negative electrode active materials are observed in the SEM photograph, it can be confirmed that the observed negative electrode contains a linked silicon-based negative electrode active material. It is preferable that a plurality of linked silicon-based negative electrode active materials are present in the negative electrode.

[0148] In the linked silicon-based negative electrode active material, the number of CNT aggregates that are in contact with the linked silicon-based negative electrode active material and contribute to the connection are counted. In addition, in an image projecting the linked silicon-based negative electrode active material, the area (μm ２ ) is calculated. Image analysis software (for example, ImageJ) may be used as a method for calculating the area. The number of CNT aggregates that are in contact with the linked silicon-based negative electrode active material and contribute to the connection is divided by the area of ​​the linked silicon-based negative electrode active material, thereby calculating the area per μm2 on the surface of the linked silicon-based negative electrode active material. ２ From the viewpoint of measurement accuracy, it is preferable to adopt an average value of the measured values ​​of a plurality of connected silicon-based negative electrode active materials, and it is preferable to adopt an average value of the measured values ​​of 10 or more connected silicon-based negative electrode active materials.

[0149] The surface area of ​​the linked silicon-based negative electrode active material is 1 μm ２When the number of CNT aggregates that contribute to the connection and are in contact is 0.01 or more per unit area, there is a large amount of electrical communication through the CNT aggregates in the connected silicon-based negative electrode active material, improving the cycle characteristics of the secondary battery. In the negative electrode according to the present disclosure, at least a portion of the silicon-based negative electrode active material is connected to the material contained in the negative electrode via the CNT aggregate. That is, at least a portion of the CNT aggregate is in contact with both the silicon-based negative electrode active material and the material contained in the negative electrode. The presence of multiple such structures allows electrical communication to be maintained by the CNT aggregate even when the silicon-based negative electrode active material repeatedly expands and contracts during charge and discharge, further improving the cycle characteristics of the secondary battery.

[0150] The surface area of ​​the linked silicon-based negative electrode active material is 1 μm ２ An example of a method for making the number of contacting CNT aggregates contributing to connection 0.01 or more per layer is to control the proportion of CNT aggregates in the total mass of the negative electrode active material layer. The proportion of CNT aggregates in the total mass of the negative electrode active material layer is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and even more preferably 0.05% by mass or more. The upper limit of the proportion is not particularly limited, but from the viewpoint of cost, it is preferably 2.0% by mass or less.

[0151] [Secondary Battery] The secondary battery may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode is the negative electrode according to the present disclosure.

[0152] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. It is not particularly limited as long as it is typically used as a separator in a secondary battery. The separator preferably has low resistance to ion migration of the electrolyte and excellent electrolyte humidification capacity. Specific examples of the separator include porous polymer films. The porous polymer film may be, for example, a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure comprising two or more layers of these films. The separator may also be a conventional porous nonwoven fabric, such as a nonwoven fabric made from high-melting-point glass fiber or polyethylene terephthalate fiber. The separator may also be coated with a ceramic component or a polymeric substance to ensure heat resistance or mechanical strength. The separator may be selectively formed into a single-layer or multi-layer structure.

[0153] The electrolyte is not particularly limited, and examples thereof include electrolytes such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in producing lithium secondary batteries.

[0154] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt. Examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0155] Among carbonate organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferred as non-aqueous organic solvents because they have high viscosity, high dielectric constants, and effectively dissociate lithium salts. It is more preferred to use a non-aqueous organic solvent obtained by mixing such cyclic carbonates with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, in order to obtain an electrolyte having high electrical conductivity.

[0156] The metal salt may be a lithium salt. The lithium salt is a substance that is easily dissolved in a non-aqueous electrolyte solution. The anion portion of the lithium salt may be, for example, F － , Cl － , I － , NO ３ － , N (CN) ２ － , B.F. ４ － , ClO ４ － , P.F. ６ － , (CF ３ ) ２ PF ４ － , (CF３ ) ３ PF ３ － 、(CF ３ ) ４ PF ２ － 、(CF ３ ) ５ PF － 、(CF ３ ) ６ P[[ID=2四十八]] － 、CF ３ SO ３ － 、CF ３ CF ２ SO ３ － ] － [[ID=三十]] ３ SO ２ ) ２ N － 、(FSO ２ )< ２ N － 、CF ３ CF[[ID=5十四]] ２ 、(CF ３ ) ２ CO － 、(CF ３ [[ID=6三]]SO ２ ) ２ 二十六]] － 、(SF ５ ) ３ － C － 、(CF ３ SO ２ ) ３ C － 、CF ３ 、(CF ２ ７ ) ７ SO ３ － 、CF ３ CO ２ [[ID=9六百]] － 、CH ３ CO ２ － 、SCN － 、及び、(CF<0000]] ３ CF ２ SO<0000]] ２ ) ２ N － が挙げられる。

[0157] In addition to the non-aqueous organic solvent and metal salt, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, for the purposes of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0158] The above-described secondary battery can be used to form a battery module including the secondary battery as a unit cell, and a battery pack including the battery module. The battery module and the battery pack can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0159] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these examples.

[0160] Example 1 - Production of CNT aggregate - The MWCNT aggregate 1 of Example 1 was produced by a floating catalyst method (CVD method) that directly interacts with the self-assembly of CNT bundles in the gas phase. First, ferrocene as a metal catalyst precursor containing Fe atoms and thiophene as a promoter were introduced into a continuous flow of carrier gas in a once-through reactor whose temperature was controlled to 400°C to 700°C. A mixture of nitrogen and argon was used as the carrier gas, and the flow rate of the carrier gas was 30,000 sccm (standard cubic centimeter per minute). By maintaining the temperature in the once-through reactor within the above range, the metal catalyst precursor was produced as a particulate metal catalyst. The region where the metal catalyst was produced is referred to as the first temperature zone. Next, methane, a carbon source, was released into the carrier gas flow. A metal catalyst and a carbon source were supplied to a second temperature zone downstream of the first temperature zone, which was controlled at 1400°C. The second temperature zone was maintained at a temperature sufficient to produce CNT aggregates. In the second temperature zone, an electric field was generated in a temperature-controlled flow reactor, thereby producing MWCNT aggregates. The produced aggregates were continuously discharged through an outlet of the flow reactor, which was controlled at a temperature of 100°C to 500°C, and collected as sheet-like MWCNT aggregates by continuous discharge. The collected sheet-like MWCNT aggregate was washed with deionized water for 10 seconds. In this manner, MWCNT aggregate 1 was obtained. When an SEM photograph (magnification: 5000x) of MWCNT aggregate 1 was taken, it was confirmed that multiple MWCNTs with maximum lengths ranging from 500 μm to 10,000 μm were present within the viewing angle of the SEM photograph. Furthermore, a plurality of SEM photographs (magnification: 5000x) of the MWCNT aggregate 1 were taken at adjacent viewing angles, and focusing on 100 MWCNTs included in the viewing angle of the SEM photographs, the maximum length of each was measured. Of the observed MWCNTs, the proportion of MWCNTs having a maximum length in the range of 500 μm to 10000 μm was 90%, and the maximum value of the maximum length of the 100 observed MWCNTs was 3500 μm.

[0161] -Production of CNT Dispersion 1- 1.1 g of MWCNT aggregate 1, 1.65 g of carboxymethyl cellulose (trade name: Carboxymethyl Cellulose Sodium Salt High Viscosity, manufactured by MP Biomedicals) as a dispersant, and 547.25 g of ion-exchanged water were mixed. The resulting mixture was subjected to a dispersion treatment at 10,000 rpm for 1 hour using an Ace Homogenizer (product name) manufactured by Nippon Seiki Seisakusho Co., Ltd. as a pre-dispersion before the main dispersion, thereby obtaining dispersion A. Note that, in order to prevent the MWCNT aggregate 1 from entanglement in the blades of the homogenizer, the MWCNT aggregate 1 was cut into small pieces of about 1 cm square using scissors before mixing. Next, the dispersion liquid A obtained above was subjected to a main dispersion treatment under the following conditions using an ultra-high pressure homogenizer (model number: NAGS100) manufactured by Joko Co., Ltd., which is a wet jet mill, to obtain CNT dispersion liquid 1.

[0162] -Conditions- Nozzle diameter: 0.22 mm Pressure: 85 MPa Number of times: 8 Method: Circulation method

[0163] - Preparation of Lithium-ion Secondary Battery Negative Electrode 1 - For the lithium-ion secondary battery negative electrode, a 9:1 (weight ratio) mixture of artificial graphite MAG-E and carbon-coated SiO was used as the negative electrode active material, CLPA-C07 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the binder, and the CNT dispersion 1 was used as the conductive additive. The components were added and kneaded to form a negative electrode active material: binder: conductive additive = 94.9:5.0:0.1 (mass ratio), to prepare an electrode slurry. Ion-exchanged water was used as the solvent when preparing the electrode slurry. The obtained electrode slurry was applied to a 20 μm thick Cu foil serving as a current collector using a single-sided continuous coater, dried at 120 ° C., and then roll-pressed to obtain a lithium-ion secondary battery negative electrode 1. The coating weight was 9.3 mg / cm ２ , density is 1.4 g / cm ３ It was adjusted so that

[0164] Example 2 Similar to the sheet-like MWCNT aggregate collected in Example 1, MWCNT aggregate 2 in Example 2 was produced by a floating catalyst method (CVD method) that directly interacts with the self-assembly of CNT bundles in the gas phase. First, ferrocene as a metal catalyst precursor containing Fe atoms and thiophene as a promoter were introduced into a continuous flow of carrier gas in a once-through reactor whose temperature was controlled between 400°C and 700°C. A mixture of nitrogen and argon was used as the carrier gas. The carrier gas flow rate was 30,000 sccm. By maintaining the temperature in the once-through reactor within the above range, the metal catalyst precursor was produced as a particulate metal catalyst. The region where the metal catalyst was produced is referred to as the first temperature zone. Next, methane, a carbon source, was released into the carrier gas flow. The metal catalyst and carbon source were supplied to a second temperature zone downstream of the first temperature zone, whose temperature was controlled at 1,400°C. The second temperature zone was maintained at a temperature sufficient to produce CNT aggregates. In the second temperature zone, an electric field was generated in a temperature-controlled flow reactor, thereby producing MWCNT aggregates. The produced aggregates were continuously discharged through an outlet of the flow reactor, which was temperature-controlled at 100°C to 500°C, and collected as fibrous MWCNT aggregates by continuous discharge. The collected fibrous MWCNT aggregates were washed with deionized water for 10 seconds. In this manner, MWCNT aggregate 2 was obtained. When an SEM photograph (magnification: 5000x) of MWCNT aggregate 2 was taken, it was confirmed that multiple MWCNTs with maximum lengths in the range of 500 μm to 10,000 μm were present within the viewing angle of the SEM photograph. Furthermore, a plurality of SEM photographs (magnification: 5000x) were taken of the MWCNT aggregate 2 at adjacent viewing angles, and focusing on 100 MWCNTs included in the viewing angles of the SEM photographs, the maximum length of each was measured. Of the observed MWCNTs, the proportion of MWCNTs having a maximum length in the range of 500 μm to 10000 μm was 90%, and the maximum value of the maximum length of the 100 observed MWCNTs was 4000 μm.

[0165] - Production of CNT Dispersion 2 - 0.55 g of the MWCNT aggregate 2 of Example 2, 0.825 g of carboxymethyl cellulose (trade name: Carboxymethyl Cellulose Sodium Salt High Viscosity, manufactured by MP Biomedicals) as a dispersant, and 273.625 g of ion-exchanged water were mixed. The resulting mixture was subjected to a dispersion treatment at 10,000 rpm for 3.5 hours using an Ace Homogenizer (product name) manufactured by Nippon Seiki Seisakusho Co., Ltd. as a pre-dispersion before the main dispersion, thereby obtaining a dispersion B. Note that the MWCNT aggregate 2 was cut to a length of about 1 cm using scissors before mixing so that the MWCNT aggregate 2 would not become entangled in the blades of the homogenizer. Next, the dispersion liquid B obtained above was subjected to a main dispersion treatment under the following conditions using an ultra-high pressure homogenizer (model number: NAGS100) manufactured by Joko Co., Ltd., which is a wet jet mill, to obtain CNT dispersion liquid 2.

[0166] -Conditions- Nozzle diameter: 0.22 mm Pressure: 85 MPa Number of times: 8 Method: Circulation method

[0167] —Production of Lithium Ion Secondary Battery Negative Electrode 2— A lithium ion secondary battery negative electrode 2 was obtained in the same manner as in Example 1, except that the CNT dispersion liquid 1 was changed to the CNT dispersion liquid 2.

[0168] <Comparative Example 1> A MWCNT aggregate 1A (product number: FT7000, MWCNT aggregate, manufactured by C-nano Co., Ltd.) was prepared.

[0169] -Production of CNT Dispersion 1A- 1.1 g of the MWCNT aggregate 1A of Comparative Example 1, 1.65 g of carboxymethyl cellulose (trade name: Carboxymethyl Cellulose Sodium Salt High Viscosity, manufactured by MP Biomedicals) as a dispersant, and 547.25 g of ion-exchanged water were mixed. The resulting mixture was subjected to a dispersion treatment at 10,000 rpm for 1 hour using an Ace Homogenizer (product name) manufactured by Nippon Seiki Seisakusho Co., Ltd. as a pre-dispersion before the main dispersion, thereby obtaining Dispersion 1A. Next, the dispersion 1A obtained above was subjected to a main dispersion treatment under the following conditions using an ultra-high pressure homogenizer (model number: NAGS100) manufactured by Joko Co., Ltd., which is a wet jet mill, to obtain CNT Dispersion 1A.

[0170] -Conditions- Nozzle diameter: 0.22 mm Pressure: 85 MPa Number of times: 8 Method: Circulation method

[0171] -Preparation of Lithium Ion Secondary Battery Negative Electrode 1A- A lithium ion secondary battery negative electrode 2 was obtained in the same manner as in Example 1, except that the CNT dispersion liquid 1 was changed to the CNT dispersion liquid 1A.

[0172] <Comparative Example 2> A MWCNT aggregate 2A [product number: FT9100, MWCNT aggregate, manufactured by C-nano Co., Ltd.] was prepared. A negative electrode for a lithium ion secondary battery 2A was obtained in the same manner as in Comparative Example 1A except that the MWCNT aggregate 1A was changed to the MWCNT aggregate 2A.

[0173] [Evaluation] <Presence of linked silicon-based negative electrode active material and area 1 μm2 on the surface of linked silicon-based negative electrode active material> ２The negative electrode was imaged using an SEM under the following conditions. SEM device: S-4800 (manufactured by Hitachi High-Technologies Corporation) Acceleration voltage: 1 kV Emission current: 10 μA Measurement magnification: 10,000 times Furthermore, in the same field of view as that in which the SEM was imaged, backscattered electron images were taken and EDX measurements were also performed. Based on the obtained shape information and composition (element) information, the silicon-based negative electrode active material and CNTs contained in the negative electrode were distinguished. From the captured SEM photograph, it was determined whether or not there was a silicon-based negative electrode active material (linked silicon-based negative electrode active material) that was linked to a substance contained in the negative electrode by a CNT aggregate. Furthermore, in the linked silicon-based negative electrode active material, the number of CNT aggregates that were in contact with the linked silicon-based negative electrode active material and contributed to the linkage were counted. In the image in which the linked silicon-based negative electrode active material was projected, the area (μm ２ The area of ​​the surface of the connected silicon-based negative electrode active material (1 μm2) was calculated by dividing the number of CNT aggregates that are in contact with the connected silicon-based negative electrode active material and contribute to the connection by the area of ​​the connected silicon-based negative electrode active material. ２ The number of contacting CNT aggregates contributing to the connection was calculated.

[0174] An SEM photograph of negative electrode 1 in Example 1 is shown in Figure 1. It was observed that a linked silicon-based negative electrode active material was present, and that the number of CNT aggregates that were in contact with the linked silicon-based negative electrode active material and contributed to the connection was greater than a certain level. An SEM photograph of negative electrode 1A in Comparative Example 1 is shown in Figure 2. It was observed that no linked silicon-based negative electrode active material was present.

[0175] Next, the presence of CNTs (specific CNTs) with a maximum length in the range of 1 μm to 1000 μm was determined in multiple SEM photographs taken at adjacent viewing angles of the negative electrodes obtained above. As a result, it was confirmed that the specific CNTs were present in the negative electrodes of Examples 1 and 2. On the other hand, it was confirmed that the specific CNTs were not present in the negative electrodes of Comparative Examples 1 and 2.

[0176] <Preparation of Secondary Battery> A lithium ion secondary battery was prepared as follows.

[0177] (1) Preparation of Positive Electrode for Lithium Ion Secondary Battery Li metal (manufactured by Honjo Metals Co., Ltd.) was used as the positive electrode for the lithium ion secondary battery.

[0178] (2) Preparation of Lithium-ion Secondary Battery The positive electrode for the lithium-ion secondary battery was placed on the bottom cover of a coin-type battery R2032 part (manufactured by Hosen Co., Ltd.), and a laminated film separator was placed on top of it, which was a polyethylene porous film with a 16 μm heat-resistant porous layer laminated on top. 300 μL of electrolyte was poured into this. The electrolyte was a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 16:10:74, to which 1% by volume of vinylene carbonate was added, and LiPF ６ The lithium secondary battery negative electrode was then placed on top of a laminated film separator, a top lid was placed on top via a gasket, and the battery was crimped using a crimping machine to prepare a coin-type half-cell R2032 lithium secondary battery (hereinafter sometimes referred to as a "half cell"). These operations were carried out in a glove box under an argon atmosphere. The assembled half-cell was then left to stand at room temperature for 12 hours to allow the separator and electrodes to be fully impregnated with the electrolyte.

[0179] <Cycle Characteristics> Using the above half cell, 15 cycle tests were performed under the conditions shown below, and the discharge capacity retention rate after 15 cycles was calculated from the first discharge capacity and the 15th discharge capacity using the following formula. Note that a higher discharge capacity retention rate after 15 cycles indicates better cycle characteristics because the decrease in battery capacity after repeated charging and discharging is suppressed. Discharge capacity retention rate after 15 cycles (%) = 15th discharge capacity (mAh / g) / 1st discharge capacity (mAh / g) × 100

[0180] - Cycle test conditions - Test temperature: 25°C Charging conditions: constant current / constant voltage charging, minimum charging voltage 5 mV, charging current 120 mAh / g Discharging conditions: constant current discharging, maximum discharging voltage 1.5 V, discharging current 120 mAh / g In this test, one cycle was defined as a cycle in which charging, charging pause, discharging, and discharging pause were carried out in that order.

[0181] The test results are shown in Table 1.

[0182]

[0183] As shown in Table 1, the negative electrodes of Examples 1 and 2 have a linked silicon-based negative electrode active material, and the CNT aggregates contributing to the linkage are located on the surface of the linked silicon-based negative electrode active material within an area of ​​1 μm ２ Since 0.01 or more wires were in contact per cell, it was found that the discharge capacity retention rate after 15 cycles was high and the cycle characteristics were excellent.

Claims

1. A negative electrode comprising a silicon-based negative electrode active material and a carbon nanotube aggregate, and satisfying the following conditions (1) and (2): (1) At least a portion of the silicon-based negative electrode active material is connected to a substance contained in the negative electrode via the carbon nanotube aggregate; (2) The carbon nanotube aggregate contributing to the connection has an area of ​​1 μm2 on the surface of the silicon-based negative electrode active material connected to the substance contained in the negative electrode. ２ There must be at least 0.01 contact.

2. The negative electrode according to claim 1, wherein the carbon nanotube aggregate includes carbon nanotubes having a maximum length of 1 μm to 1000 μm.

3. A secondary battery comprising the negative electrode according to claim 1 or 2.

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