JP7524892B2 - Electrode and method for manufacturing the same - Google Patents

Description

The present invention relates to an electrode for use in an electricity storage device and a method for manufacturing the electrode.

There are various types of power storage devices, including secondary batteries, electric double layer capacitors, redox capacitors, and hybrid capacitors. These power storage devices are being considered for a wide range of applications, including as power sources for information devices such as mobile phones and laptop computers, as motor-driven power sources for low-emission vehicles such as electric vehicles and hybrid vehicles, and as energy regeneration systems. To be used in these ranges of applications, power storage devices must meet the demand for even higher performance and smaller size. In other words, there is a demand for power storage devices with even higher energy density and cycle life.

Roughly speaking, an electricity storage device is composed of an electrolyte sandwiched between a pair of electrodes. The electrodes have an active material layer for storing energy. The electrode active material particles in the active material layer exhibit capacitance through a Faraday reaction involving the transfer of electrons with ions in the electrolyte, or through a non-Faraday reaction such as polarization that does not involve the transfer of electrons. However, electrode active material particles generally have low conductivity. Therefore, it has been considered to combine the electrode active material particles with conductive carbon and use the composite as a constituent of the active material layer.

Conductive carbon improves the conductivity of the electrode. That is, conductive carbon contributes to reducing the direct current internal resistance (DCIR) and equivalent series resistance (ESR) of the power storage device. However, conductive carbon does not contribute to the energy density of the power storage device. Therefore, it is better to minimize the amount of conductive carbon in the composite. In other words, it is better to have as many electrode active material particles per unit volume as possible while still providing good conductivity. Therefore, carbon nanotubes, which provide high conductivity even in small amounts, are attracting attention. A composite of carbon nanotubes and electrode active material particles can increase the electrode density. Furthermore, this composite can achieve low DCIR and ESR even when the electrode density is increased.

However, it has been reported that side reactions occur between the electrode active material particles and the electrolyte, resulting in a decrease in cycle life. Based on this report, it is desirable to coat a part or all of the surface of the electrode active material particles with conductive carbon in order to improve the cycle life. For example, by mixing a mother particle of a lithium composite oxide such as _LiCoO2 with a child particle of a carbon material such as acetylene black acting as a conductive agent while applying compression and shearing action, a part or all of the surface of the mother particle of the composite oxide is coated with a child particle of the carbon material (see, for example, Patent Document 1).

Japanese Patent Application Publication No. 11-283623

The idea of covering part or all of the surface of the electrode active material particles with acetylene black can improve cycle life compared to a composite of carbon nanotubes and electrode active material particles. Moreover, the electrode density is also good, although it is not as good as that of a composite of carbon nanotubes and electrode active material particles.

However, in terms of resistance, a composite in which electrode active material particles are coated with acetylene black has properties that are inferior to those of a composite of carbon nanotubes and electrode active material particles, and one has to say that it is inferior in terms of its original purpose of imparting conductivity to the electrode active material particles. Thus, there is a demand for an electrode that is highly balanced in terms of cycle life, electrode density, and resistance, but no such proposal has been made yet.

The object of the present invention is to provide an electrode having good cycle life, high electrode density and low resistance, and a method for manufacturing said electrode.

As a result of intensive research by the inventors, when the electrode active material particles were covered with a mixture of oxidized conductive carbon and another conductive carbon (hereinafter, the mixture is also referred to as a conductive carbon mixture), the cycle life and electrode density were good. However, the DCIR and ESR were significantly inferior to those when the electrode active material particles were covered with acetylene black. Therefore, in order to suppress the DCIR and ESR as much as possible, fibrous carbon such as carbon nanotubes was added to the active material composite formed by covering the electrode active material particles with the conductive carbon mixture. By doing so, it was found that the DCIR and ESR achieved by the composite of carbon nanotubes and electrode active material particles was not only close to those achieved by the composite of carbon nanotubes and electrode active material particles, but was superior to those achieved by the composite of carbon nanotubes and electrode active material particles.

The electrode of the present invention has been made based on this knowledge, and in order to solve the above-mentioned problems, it is an electrode having an active material layer, characterized in that the active material layer contains electrode active material particles, a conductive carbon mixture consisting of oxidized conductive carbon and a conductive carbon other than the oxidized conductive carbon, and fibrous carbon.

Although this mechanism is speculated and is not limited to this mechanism, the goodness of the DCIR and ESR of the present invention is speculated as follows. First, the active material complexes are connected by fibrous carbon, which is an excellent electron path, just like the composite of electrode active material particles and fibrous carbon. In the case of the composite of electrode active material particles and fibrous carbon, there was a problem with the local transfer of electrons between the fibrous carbon and the electrode active material particles. On the other hand, the electrode active material particles related to the present invention are densely coated with a conductive carbon mixture consisting of oxidized conductive carbon and another conductive carbon, forming an active material complex. Therefore, it is easy for the conductive carbon mixture to transfer electrons to the electrode active material particles. Therefore, this conductive carbon mixture first receives electrons from the fibrous carbon, and then this conductive carbon mixture transfers electrons to the electrode active material particles, thereby achieving good DCIR and ESR.

Therefore, the electrode active material particles and the conductive carbon mixture may form an active material complex in which at least a portion of the surface of the electrode active material particles is covered with the conductive carbon mixture, and the fibrous carbon may be arranged between the active material complexes to form a network structure.

The fibrous carbon may be carbon nanotubes. The oxidized conductive carbon may contain hydrophilic portions in an amount of 10% by mass or more of the entire oxidized conductive carbon.

The electrode is a negative electrode, and the electrode active material particles may be Si-based compound particles. Unlike other electrode active material particles, Si-based compound particles have a problem that the large volume change accompanying the insertion and desorption of lithium ions causes electrode destruction, pulverization of Si-based compound particles, thickening of the SEI, or a combination of these factors, which deteriorates resistance and reduces the capacity retention rate due to the number of cycles. However, the electrode can solve this problem and provide an electrode with good cycle life, high electrode density, and low resistance, as well as a manufacturing method for this electrode.

The Si-based compound particles may be particles of a compound represented by SiOx (0≦x<2).

Furthermore, the electrode manufacturing method according to the present invention has been made based on this knowledge, and in order to solve the above-mentioned problems, is characterized by comprising a slurry preparation step of preparing a slurry containing electrode active material particles, a conductive carbon mixture consisting of oxidized conductive carbon and a conductive carbon other than the oxidized conductive carbon, and fibrous carbon, and an active material layer formation step of applying the slurry to a current collector.

With this manufacturing method, the oxidized conductive carbon and another conductive carbon form a conductive carbon mixture, the electrode active material particles and the conductive carbon mixture form an active material complex in which at least a portion of the surface of the electrode active material particles is covered with the conductive carbon mixture, and the active material complex and the fibrous carbon form a network structure in which the active material complexes are connected by the fibrous carbon.

The slurry preparation process may include a first mixing process of mixing the conductive carbon mixture with the electrode active material particles, and a second mixing process of mixing the active material composite obtained by the first process with the fibrous carbon.

According to the present invention, an electrode with a good balance between electrode density and low resistance is obtained.

FIG. 2 is a schematic diagram showing a first structure taken by each material in an active material layer. FIG. 4 is a schematic diagram showing a second structure that each material takes in the active material layer. 4 is various timing charts showing a method for producing a slurry. 1 is a SEM photograph of the electrode of Example 1-1. 1 is a SEM photograph of the electrode of Example 2-1. 1 is a graph showing the capacity retention rate as a function of the number of cycles for Examples 1-4. 1 is a graph showing the capacity retention rate as a function of the number of cycles in Example 3-1.

Below, the embodiments of the electrode and manufacturing method according to the present invention are described in detail. Note that the present invention is not limited to the embodiments described below.

(electrode)
The electrode according to the present embodiment is used in an electricity storage device. An electricity storage device is a passive element that charges and discharges electric energy, and is roughly classified as including a pair of electrodes and an electrolyte interposed between the electrodes. Examples of electricity storage devices that use the electrode according to the present embodiment include secondary batteries, electric double layer capacitors, redox capacitors, and hybrid capacitors, and the electrode according to the present embodiment is applied to one or both of the positive electrode or anode and the negative electrode or cathode of the pair of electrodes.

The electrode comprises a current collector and an active material layer. The current collector is an electrical conductor and also serves as a support substrate for the active material layer. The active material layer is formed on one or both sides of the current collector. This active material layer is an energy storage layer.

The current collector may be made of a conductive material such as platinum, gold, nickel, aluminum, titanium, steel, or carbon. The current collector may be in any shape, such as a film, foil, plate, mesh, expanded metal, or cylinder.

The active material layer includes electrode active material particles, oxidized conductive carbon (hereinafter referred to as oxidized carbon), conductive carbon other than the oxidized carbon, and fibrous carbon. The electrode active material particles exhibit capacitance through a Faraday reaction involving the transfer of electrons with ions in the electrolyte, or through a non-Faraday reaction such as polarization that does not involve the transfer of electrons. The oxidized carbon, the other conductive carbon, and the fibrous carbon are conductive assistants for the active material layer.

(Electrode active material particles)
Examples of electrode active material particles used in the positive electrode of a secondary battery include layered rock salt type LiMO ₂ , layered Li ₂ MnO _{3 --LiMO2} solid solution, and spinel type LiM ₂ O ₄ (wherein M represents Mn, Fe, Co, Ni, or a combination thereof). Specific examples of these include _LiCoO2 , _LiNiO2 , _LiNi4 / _5Co1 / _{5O2 , LiNi1/3Co1 /} 3Mn1/ _3O2 , LiNi1 _{/ 2Mn1} / _{2O2 ,} LiFeO2 _, LiMnO2 _{, Li2MnO3 - LiCoO2 ,} Li2MnO3 _{- LiNiO2} , Li2MnO3 _{- LiNi1} /3Co1/ _3Mn1 / _3O2 , Li2MnO3- _LiNi1 / _{2Mn1 / 2O2 , Li2MnO3 - LiNi1} / _{2 Examples} include Mn1 _{/ 2O2} - _LiNi1 / _{3Co1 /} 3Mn1/ _3O2 , _LiMn2O4 , and LiMn3 _{/2Ni1 / 2O4} . In addition, sulfur and sulfides such as _Li2S , _TiS2 , _MoS2 , FeS2, _{VS2 ,} Cr1 _{/ 2V1/ 2S2 ,} selenides such _{as NbSe3} , _VSe2 , _NbSe3 , oxides such as _Cr2O5 , _{Cr3O8 , VO2 , V3O8 , V2O5 , V6O13 , LiNi0.8Co0.15Al0.05O2 , LiVOPO4 , LiV3O5 , LiV3O8 , MoV2O8 , Li2FeSiO4 , Li2MnSiO4} , _LiFePO4 , LiFe1 _{/2Mn1 / 2PO4} , _{LiMnPO4 , Li3V2 ( PO4} ) ₃ and other composite oxides.

Examples of active materials used in _the negative electrode of a secondary battery include oxides such as _Fe2O3 , _MnO , _MnO2 , _Mn2O3 , _Mn3O4 , CoO, _Co3O4 , _NiO , _Ni2O3 , TiO, _TiO2 , SnO, _SnO2 , SiO, _SiO2 , _RuO2 , _WO , _WO2 , and ZnO, metals such as _{Sn ,} Si, Al, and _{Zn ,} composite oxides such as _LiVO2 , _Li3VO4 , and _Li4Ti5O12 , _and nitrides such as _{Li2.6Co0.4N , Ge3N4 , Zn3N2 ,} and _Cu3N .

Examples of electrode active material particles in the polarizable electrodes of electric double layer capacitors include carbon materials with large specific surface areas, such as activated carbon, graphene, carbon nanofibers, carbon nanotubes, phenolic resin carbide, polyvinylidene chloride carbide, and microcrystalline carbon. In hybrid capacitors, the active materials used for the positive electrodes exemplified for secondary batteries can be used for the positive electrodes, in which case the negative electrodes are composed of polarizable electrodes using activated carbon or the like. Also, the negative electrode active materials exemplified for secondary batteries can be used for the negative electrodes, in which case the positive electrodes are composed of polarizable electrodes using activated carbon or the like.

Examples of the electrode active material particles in the positive electrode of the redox capacitor include metal oxides such as RuO ₂ , MnO ₂ and NiO, and the electrode active material particles in the negative electrode are composed of an active material such as RuO ₂ and a polarizable material such as activated carbon.

There are no limitations on the shape or particle size of the electrode active material particles. However, the average particle size of the electrode active material particles is preferably greater than 2 μm and less than 25 μm. Electrode active material particles with this relatively large average particle size themselves improve the electrode density. The average particle size of the electrode active material particles refers to the 50% diameter (median diameter) in the measurement of particle size distribution using a light scattering particle sizer.

In addition, it is preferable that the active material layer contains, as electrode active material particles, microparticles having an average particle size of 0.01 to 2 μm and coarse particles having an average particle size of more than 2 μm and not more than 25 μm, which can function as an active material of the same pole as the microparticles. By filling the microparticles between the coarse particles, the electrode density is further increased, and the energy density of the electricity storage device is further improved. The mixing ratio of the coarse particles to the microparticles is preferably in the range of 80:20 to 95:5 by mass, and more preferably in the range of 90:10 to 95:5.

(Oxidation treated carbon)
The oxidation-treated carbon is made from porous carbon powder, Ketjen black, furnace black having voids, carbon nanofibers, carbon nanotubes, and other carbon materials having voids, and has hydrophilic portions on the particle surface. The content of the hydrophilic portions is preferably 10% by mass or more of the entire oxidation-treated carbon. It is particularly preferable that the content of the hydrophilic portions is 12% by mass or more and 30% by mass or less of the entire carbon.

The hydrophilic parts are the parts brought about by the oxidation treatment where hydroxyl groups, carboxyl groups or ether bonds have been introduced to the carbon, where the conjugated double bonds of the carbon have been oxidized to generate carbon single bonds, and where carbon-carbon bonds have been partially broken. 0.1 g of carbon is added to 20 mL of an aqueous ammonia solution at pH 11, and ultrasonic waves are irradiated for 1 minute. The resulting solution is left for 5 hours to precipitate the solid phase. The parts that do not precipitate and remain dispersed in the aqueous ammonia solution at pH 11 can be said to be hydrophilic parts.

After leaving it for 5 hours to allow the solid phase to settle, the supernatant liquid is removed and the remaining portion is dried and the weight of the dried solid is measured. The weight of the dried solid is subtracted from the initial weight of the carbon (0.1 g) to determine the weight of the hydrophilic portion dispersed in the ammonia aqueous solution at pH 11. The weight ratio of the weight of the hydrophilic portion to the initial weight of the carbon (0.1 g) is the content of the hydrophilic portion in the carbon.

Because the oxidized carbon has a hydrophilic portion at this ratio, it spreads easily in a paste-like form, extends easily along the surface of the electrode active material particles, penetrates easily into the pores of the electrode active material particles, and is easily compacted. Therefore, the oxidized carbon can contact 80% or more, preferably 90% or more, and particularly preferably 95% or more of the surface of the electrode active material particles. In addition, the paste-like form means a state in which no grain boundaries are observed in the primary carbon particle diameter in an SEM photograph taken at a magnification of 25,000 times, and non-particulate amorphous carbon is connected. In addition, the coverage can be calculated from an SEM photograph at a magnification of 25,000 times.

The voids preferably have a specific surface area of 300 ^m2 /g or more as measured by the BET method, and the presence of such voids makes it easier for the conductive carbon to be given the property of turning into a paste by oxidation treatment. Among these, spherical particles such as Ketjen Black and furnace black having voids are preferred as raw materials. Even if solid carbon is used as a raw material and oxidation treatment is performed, it is difficult to obtain oxidation-treated carbon that turns into a paste.

(Another conductive carbon)
The other conductive carbon is distinguished from the oxidation-treated carbon, and the content of the hydrophilic portion is less than 10% by mass of the whole other conductive carbon, and is less likely to change into a paste-like form compared to the oxidation-treated carbon. If it is less than 10% by mass, it may be oxidized or unoxidized. As the other conductive carbon, carbon black such as ketjen black, acetylene black, furnace black, channel black, fullerene, graphene, amorphous carbon, natural graphite, artificial graphite, graphitized ketjen black, mesoporous carbon, etc., which are used for electrodes of conventional electricity storage devices, are used.

The other conductive carbon preferably has a spherical particle shape, and examples of the other conductive carbon include carbon black such as ketjen black, acetylene black, furnace black, and channel black, fullerene, mesoporous carbon, and artificial graphite. The other conductive carbon is less likely to change to a paste-like state and maintains a spherical shape, so it can fill spaces that cannot be filled with the paste-like oxidized carbon and densely fill the spaces between the electrode active material particles with a conductive material. It is also preferable to use a conductive carbon with a higher conductivity than the oxidized carbon, and the use of acetylene black is particularly preferable.

(fibrous carbon)
Examples of the fibrous carbon include carbon nanotubes, carbon nanofibers (hereinafter, CNF), vapor-grown carbon fibers, etc. The carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) having one graphene sheet, or multi-walled carbon nanotubes (MWCNTs) having two or more graphene sheets rolled coaxially to form multiple tube walls, or may be a mixture of these.

The outer diameter of this fibrous carbon is preferably in the range of 1 to 150 nm, more preferably 1 to 70 nm, and even more preferably 1 to 40 nm. The length of the fibrous carbon is preferably in the range of 1 to 500 μm, more preferably 5 to 400 μm, and even more preferably 5 to 200 μm. If it is smaller than these ranges, it becomes difficult to increase the electrode density.

In addition, since the fewer the number of layers of the graphene sheets of the carbon nanotube, the higher the capacity density of the carbon nanotube itself, carbon nanotubes with 50 layers or less, preferably 10 layers or less, are preferred in terms of capacity density. Note that this fibrous carbon may also be subjected to an opening treatment or activation treatment in which holes are made at the tip or wall of the fibrous carbon.

Carbon nanotubes may be single-walled or multi-walled, but in the present invention, single-walled carbon nanotubes are more preferred. When single-walled carbon nanotubes and multi-walled carbon nanotubes are used in the same weight, the single-walled carbon nanotubes contain more carbon nanotubes than the multi-walled carbon nanotubes. Therefore, more networks can be constructed between the active material composites, and the effect of reducing DCIR and ESR is enhanced. It is possible to increase the number of multi-walled carbon nanotubes by increasing the weight of the multi-walled carbon nanotubes, and obtain the same effect of reducing DCIR and ESR as that of single-walled carbon nanotubes, but the multi-walled carbon nanotubes tend to clump together, and the electrode density is reduced compared to that of single-walled carbon nanotubes. In addition, when the content of the conductive assistant contained in the active material layer is constant, if the content of the multi-walled carbon nanotubes is large, the content of the conductive carbon mixture covering the active material is relatively reduced, and the effect of improving the cycle characteristics is reduced. Therefore, in order to construct a network between active materials and reduce DCIR and ESR without impairing the cycle characteristics of conventional active materials, single-walled carbon nanotubes are preferred.

The amount of fibrous carbon is preferably 0.01% or more and 1.0% or less of the entire active material layer. At about 0.01%, the effect of reducing DCIR and ESR appears, while at more than 1.0%, the proportion of active material decreases and a decrease in capacity is observed.

(Structure of active material layer)
1 and 2 are schematic diagrams showing the structure that electrode active material particles, oxidized carbon, another conductive carbon, and fibrous carbon adopt in an active material layer. A part or the entire surface of the electrode active material particle 1 is covered with a mixture of oxidized carbon and another conductive carbon. The mixture of oxidized carbon and another conductive carbon is called a conductive carbon mixture 2. A double-shell structure particle consisting of the conductive carbon mixture 2 and the electrode active material particle 1, with the electrode active material particle 1 as the inner shell and the conductive carbon mixture as the outer shell, is called an active material complex 3.

In the active material composite 3, the oxidized carbon spreads in a paste-like form and adheres to the surface of the electrode active material particles 1. The oxidized carbon spread in a paste-like form covers the surface of the electrode active material particles 1, fills the gaps between the electrode active material particles 1, and is also extruded into the interior of the pores present on the surface of the electrode active material particles 1 to densely fill them. The pores include the gaps between primary particles found in the secondary particles. Therefore, the amount of electrode active material particles 1 per unit volume in the electrode increases, and the electrode density increases. However, the electrode of the present invention may contain oxidized carbon that has not changed into a paste-like form.

In the active material composite 3, the oxidized carbon covers not only the surface of the electrode active material particle 1, but also the surface of the other conductive carbon, and adheres to the electrode active material particle 1 by involving the other conductive carbon. In other words, the oxidized carbon makes it easier for the other conductive carbon to adhere to the surface of the electrode active material particle 1. Furthermore, the aggregation of the other conductive carbon is suppressed by being covered by the oxidized carbon. This other conductive carbon fills the gaps that could not be filled by the oxidized carbon that spreads in a paste-like form, improving the filling rate in the gaps.

Preferably, the oxidized carbon is present in gaps with a width of 50 nm or less, inside holes with a width of 50 nm or less, or both. Therefore, the surface coverage of the conductive carbon mixture 2 with respect to the electrode active material particles 1 is improved, the conductivity of the entire active material layer is improved, and the electrode density is improved. Note that the width of the gaps formed between the electrode active material particles 1 means the shortest distance between adjacent electrode active material particles 1, and the width of the holes present on the surface of the electrode active material particles 1 means the shortest distance between the opposing points of the openings of the holes.

In this active material composite 3, the mass ratio of the electrode active material particles 1 to the conductive carbon mixture 2 is preferably in the range of 90:10 to 99.5:0.5, and more preferably in the range of 95:5 to 99:1. If the proportion of the conductive carbon mixture 2 is less than the above range, the conductivity of the active material layer is insufficient, and the coverage of the electrode active material particles 1 by the paste-like oxidation-treated carbon decreases, tending to deteriorate the cycle characteristics. If the proportion of the oxidation-treated carbon is greater than the above range, the electrode density decreases, and the energy density of the electricity storage device tends to decrease. In addition, in the conductive carbon mixture 2, the ratio of the oxidation-treated carbon to another conductive carbon is preferably in the range of 3:1 to 1:3, and more preferably in the range of 2.5:1.5 to 1.5:2.5, in terms of mass ratio.

As shown in Figures 1 and 2, the fibrous carbon 4 connects the active material composites 3. In other words, the active material composites 3 and the fibrous carbon 4 form a network structure. A structure made of the active material composites 3 and the fibrous carbon 4 is called a network structure 5. The network structure 5 can be of the following two types:

First, as shown in FIG. 1, most of the conductive carbon mixture 2 covers the electrode active material particles 1, and not much of it adheres to the fibrous carbon 4, and the fibrous carbon 4 is in contact with the conductive carbon mixture 2 of the active material complex 3. Also, as shown in FIG. 2, a part of the conductive carbon mixture 2 covers the electrode active material particles 1, but another part of the conductive carbon mixture 2 also adheres to the surface of the fibrous carbon 4. The conductive carbon mixture 2 adhered to the fibrous carbon 4 is in direct contact with the electrode active material particles 1, or the conductive carbon mixture 2 adhered to the fibrous carbon 4 is in contact with the conductive carbon mixture 2 that covers the electrode active material particles 1.

It is presumed that both of these two types of network structures 5 contribute to the low resistance of the electrode. First, the active material complexes 3 are connected by fibrous carbon 4, which is an excellent electron path. The conductive carbon mixture 2 is responsible for the transfer of electrons carried by the fibrous carbon 4. Since both the fibrous carbon 4 and the conductive carbon mixture 2 are mainly made of carbon, they are easily compatible at the contact surface and have good compatibility for electron transfer. Since the conductive carbon mixture 2 is closely attached to the electrode active material particles 1, it easily transfers electrons to the electrode active material particles 1 compared to the transfer of electrons between the fibrous carbon 4 and the electrode active material particles 1. The fibrous carbon 4 is preferably 0.01% by mass or more of the entire active material layer. From about 0.01% by mass, the effect of reducing DCIR and ESR appears.

In this way, although it is a conductive assistant, the oxidized carbon and another conductive carbon are closely attached to the electrode active material particles 1, and the fibrous carbon 4 is located relative to the electrode active material particles 1 via the conductive carbon mixture 2, and is also located so as to connect the active material complexes 3. Therefore, the fibrous carbon 4 mainly functions as a highway that carries electrons close to the electrode active material particles 1, and the oxidized carbon and another conductive carbon mediates the transfer of electrons between the electrode active material particles 1 and the fibrous carbon 4, and functions as a local transfer agent that transfers electrons directly to the electrode active material particles 1. This realizes a low resistance that cannot be achieved with the fibrous carbon 4 alone, and cannot be achieved with the electrode active material particles 1 coated with the oxidized carbon and another conductive carbon alone.

In the network structure 5 shown in FIG. 1, the conductive carbon mixture 2 adheres less to the fibrous carbon 4. Therefore, the fibrous carbon 4 is less aggregated, and the volume of the fibrous carbon 4 between the electrode active material particles 1 can be made small. This further improves the electrode density. On the other hand, in the network structure 5 shown in FIG. 2, the fibrous carbon 4 is aggregated compared to the network structure 5 in FIG. 1, and the electrode density is lower. However, since the conductive carbon mixture 2 attached to the surface of the fibrous carbon 4 and the conductive carbon mixture 2 attached to the surface of the electrode active material particle 1 come into contact with each other, the transfer of electrons is further improved, and the resistance of the electrode is further reduced.

Here, Si-based compound particles are suitable as the electrode active material particles 1 constituting this active material composite 3. The Si-based compound particles are compounds expressed as SiOx (0≦x<2), such as Si or SiO, and may be doped with a different element such as Ti or P, and may further have a surface coated with carbon.

In particular, SiO particles are suitable as the electrode active material particles 1. SiO particles have a theoretical specific capacity per weight of approximately 2000 mAhg ^-1 and an operating potential of approximately 0.5 V (vs. Li/Li ⁺ ). That is, compared with graphite, the specific capacity is far greater, and the operating potential is low like that of graphite, but it is not as extremely low as the operating potential of graphite, which is approximately 0.05 V (vs. Li/Li ⁺ ). Therefore, SiO particles are easy to obtain and have a low environmental load, and are attracting attention as electrode active material particles 1 on the negative electrode side of lithium ion secondary batteries and electrode active material particles 1 on the negative electrode side of hybrid capacitors combined with a positive electrode that exerts an electric double layer action.

However, the Si-based compound particles used as the electrode active material particles 1 on the negative electrode side expand and contract due to the insertion and removal of lithium ions, and the Si particles contained in the Si-based compound particles have a volume change of about 300%. Therefore, the electrode is easily broken, and the electrode active material particles 1 are easily cracked and pulverized. In addition, a solid electrolyte interphase (SEI) is deposited on the surface of the Si-based compound particles used as the electrode active material particles 1 on the negative electrode side. The SEI is a complex consisting of inorganic lithium compounds and organic compounds formed by the reductive decomposition of the electrolyte, and suppresses the decomposition of the electrolyte beyond a certain level. When this SEI is destroyed by the expansion and contraction of the Si-based compound particles and a path of the electrolyte to the surface of the Si-based compound particles is generated, the SEI is further generated, and the SEI around the Si-based compound particles becomes too thick.

Therefore, when Si-based compound particles are used as the electrode active material particles 1 on the negative electrode side, there is a problem that the DCIR and ESR become high and the capacity decreases with each cycle due to electrode destruction, pulverization of the Si-based compound particles, thickening of the SEI, or a combination of these factors. In addition, the Si-based compound particles used as the electrode active material particles 1 on the negative electrode side cause the decomposition of the electrolyte due to the low operating voltage, and the capacity decreases with each cycle due to side reactions such as precipitation of lithium metal due to rapid charging and discharging.

On the other hand, the conductive carbon mixture 2 adheres closely to the electrode active material particles 1, so it is not easily destroyed by the expansion and contraction of the Si-based compound particles. Therefore, the expansion and contraction of the Si-based compound particles makes it difficult for a path of electrolyte to be generated that reaches the surface of the Si-based compound particles. Although this mechanism is speculative and is not limited to this mechanism alone, by using Si-based compound particles as the electrode active material particles 1 and including the conductive carbon mixture 2 and fibrous carbon 4, an electrode in which the Si-based compound particles are the electrode active material particles 1 has good electrode density and resistance, and the capacity retention rate is less likely to decrease even after a number of cycles.

(Electrode manufacturing method)
The network structure 5 as described above is produced through a process including, first, a step of preparing a conductive carbon mixture, a second step of preparing a slurry for the active material layer, and a third step of applying the slurry onto a current collector and rolling it.

(Process for preparing conductive carbon mixture)
Oxidized carbon is produced by oxidizing a carbon raw material. Any known oxidation method can be used without particular limitation. For example, oxidized carbon can be obtained by treating the carbon raw material in an acid or hydrogen peroxide solution. Examples of acids that can be used include nitric acid, a nitric acid-sulfuric acid mixture, and an aqueous hypochlorous acid solution. Oxidized carbon can also be obtained by heating the carbon raw material in an oxygen-containing atmosphere, water vapor, or carbon dioxide. Oxidized carbon can also be obtained by subjecting the carbon raw material to plasma treatment, ultraviolet light irradiation, corona discharge treatment, or glow discharge treatment in an oxygen-containing atmosphere. As the intensity of the oxidation treatment is increased, the proportion of hydrophilic portions increases.

The oxidation-treated carbon containing 10% by mass or more of a hydrophilic portion as a whole is
(a) treating a porous carbon raw material with an acid;
(b) mixing the acid-treated product with a transition metal compound;
(c) grinding the resulting mixture to cause a mechanochemical reaction;
(d) heating the product of the mechanochemical reaction in a non-oxidizing atmosphere; and
(e) removing the transition metal compound and/or its reaction products from the product after heating.

In step (a), a carbon raw material having voids, preferably Ketjen black, is immersed in acid and left to stand. Ultrasonic waves may be applied during this immersion. The acid may be an acid normally used in the oxidation treatment of carbon, such as nitric acid, a mixture of nitric acid and sulfuric acid, or an aqueous solution of hypochlorous acid. The immersion time depends on the concentration of the acid and the amount of carbon raw material to be treated, but is generally in the range of 5 minutes to 5 hours. The carbon after the acid treatment is thoroughly washed with water and dried, and then mixed with a transition metal compound in step (b).

In step (b), the transition metal compound added to the carbon raw material may be an inorganic metal salt such as a transition metal halide, nitrate, sulfate, or carbonate, an organic metal salt such as a formate, acetate, oxalate, methoxide, ethoxide, or isopropoxide, or a mixture thereof. These compounds may be used alone or in combination of two or more. Compounds containing different transition metals may be mixed and used in a predetermined amount. In addition, as long as they do not adversely affect the reaction, compounds other than the transition metal compound, such as an alkali metal compound, may be added together. Since the oxidized carbon is mixed with electrode active material particles in the manufacture of electrodes for storage devices, it is preferable to add a compound of an element constituting the active material to the carbon raw material, since this can prevent the active material from being contaminated with elements that may become impurities.

In step (c), the mixture obtained in step (b) is pulverized to cause a mechanochemical reaction. Examples of pulverizers for this reaction include a mortar mill, a stone mill, a ball mill, a bead mill, a rod mill, a roller mill, an agitator mill, a planetary mill, a vibration mill, a hybridizer, a mechanochemical composite device, and a jet mill. The pulverization time depends on the pulverizer used and the amount of carbon to be treated, and is not strictly limited, but is generally in the range of 5 minutes to 3 hours. Step (d) is performed in a non-oxidizing atmosphere such as a nitrogen atmosphere or an argon atmosphere. The heating temperature and heating time are appropriately selected depending on the transition metal compound used. In the subsequent step (e), the transition metal compound and/or the reaction product of the transition metal compound are removed from the product after heating by means of dissolving it in an acid, etc., and then the product is thoroughly washed and dried to obtain an oxidized carbon containing hydrophilic parts of 10% by mass or more of the total.

In this manufacturing method, in step (c), the transition metal compound acts to promote the oxidation of the carbon raw material through a mechanochemical reaction, and the oxidation of the carbon raw material proceeds rapidly. This oxidation produces oxidized carbon containing hydrophilic portions that account for 10% or more by mass of the total.

The oxidized carbon produced in this manner is dry-mixed with another conductive carbon to obtain a conductive carbon mixture. For dry mixing, a grinding mill, a stone grinder, a ball mill, a bead mill, a rod mill, a roller mill, an agitator mill, a planetary mill, a vibration mill, a hybridizer, a mechanochemical composite device, and a jet mill can be used.

During this dry mixing process, the oxidized carbon adheres to the surface of another conductive carbon, and the oxidized carbon undergoes partial gelatinization, resulting in a conductive carbon mixture in which at least a portion of the oxidized carbon that has been converted into a paste is adhered to the surface of another conductive carbon.

(Step of Preparing Slurry for Active Material Layer)
The slurry of the active material layer contains the electrode active material particles, the conductive carbon mixture, and the fibrous carbon, which are the material sources of the network structure, and further contains a binder, a solvent, and a diluent. The diluent is added last. There is no particular limitation on the mixing order or mixing method of the other mixing components. However, the preferred procedure differs depending on whether the active material layer mainly contains a network structure in which the conductive carbon mixture 2 is less attached to the fibrous carbon 4 with the electrode density shown in FIG. 1 being emphasized, or the active material layer mainly contains a network structure in which the conductive carbon mixture 2 is more attached to the fibrous carbon 4 than in the case shown in FIG. 1 with the low resistance shown in FIG. 2 being emphasized.

Figure 3 is a timing chart showing various slurry production methods. In (a) to (c) of Figure 3, the network structure shown in Figure 1 is the main target product, and in (d) to (f) of Figure 3, the network structure shown in Figure 2 is the main target product.

In the manufacturing methods shown in (a) to (c) of Figure 3, broadly speaking, the conductive carbon mixture and electrode active material particles are mixed to produce an active material complex, and then fibrous carbon is added to this active material complex and further mixed. Priority is given to adhering the conductive carbon mixture to the electrode active material particles, and there is a sufficient amount of conductive carbon mixture covering the electrode active material particles, while there is little conductive carbon mixture adhering to the fibrous carbon, making it less likely to aggregate.

In the manufacturing methods shown in (d) to (f) of Figure 3, the conductive carbon mixture and fibrous carbon are mixed with the electrode active material particles at the same time, or the fibrous carbon is mixed with the electrode active material particles first. The conductive carbon mixture has equal contact opportunities with the electrode active material particles and the fibrous carbon, and the conductive carbon mixture adheres to the fibrous carbon and tends to aggregate, but the conductive carbon mixture adheres to both the electrode active material particles and the fibrous carbon.

(First slurry production method)
In the manufacturing method of FIG. 3(a), dry mixing of the conductive carbon mixture and electrode active material particles and wet mixing of fibrous carbon, binder and solvent are carried out separately, and then the two mixtures are mixed together in wet mixing.

In dry mixing of the conductive carbon mixture and the electrode active material particles, the oxidized carbon adheres to and covers the surfaces of the electrode active material particles, suppressing aggregation of the electrode active material particles. In addition, the pressure applied to the oxidized carbon during the mixing process causes at least a portion of the oxidized carbon to spread in a paste-like form, partially covering the surfaces of the electrode active material particles, thereby producing an active material composite.

When the average particle size of the electrode active material particles is greater than 2 μm and less than 25 μm, the pressing force promotes the gelatinization of the oxidized carbon during the process of mixing with the oxidized carbon. In addition, when the electrode active material particles are composed of fine particles and coarse particles, the oxidized carbon adheres to and covers the surfaces of not only the coarse particles but also the fine particles, suppressing the aggregation of the electrode active material particles and making the mixed state of the electrode active material particles and the oxidized carbon uniform.

In wet mixing of fibrous carbon, binder, and solvent, it is desirable to use a dispersion liquid in which fibrous carbon is dispersed in advance. As the binder to be mixed with fibrous carbon, known binders such as polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, polyvinyl fluoride, and carboxymethyl cellulose are used. The content of the binder is preferably 1 to 30 mass% based on the total amount of the electrode material. If it is 1 mass% or less, the strength of the active material layer is insufficient, and if it is 30 mass% or more, inconveniences such as a decrease in the discharge capacity of the electrode and excessive internal resistance occur. As the solvent, any solvent that does not adversely affect other components in the electrode material, such as N-methylpyrrolidone, can be used without any particular limitation. As long as each component in the mixture is mixed uniformly, there is no particular limitation on the amount of the solvent.

The wet mixing time varies depending on the total amount of the mixture and the mixing device used, but is generally between 1 and 60 minutes. There are no particular limitations on the method of kneading with the binder and solvent, and the mixture may be mixed by hand using a mortar, or may be mixed using a known wet mixing device such as a stirrer or homogenizer. As long as the mixture is mixed uniformly, there is no problem even if the mixing time is short. However, it is preferable to finely disperse the fibrous carbon by wet mixing, and in this respect it is better to use a known wet mixing device. It is preferable to finely disperse the fibrous carbon to a length of 20 nm to 200 nm in order to improve the electrode density.

The mixture obtained by wet mixing is then mixed with the active material composite obtained by dry mixing, and further wet mixed. After wet mixing, the mixture is diluted by further adding the solvent used when wet mixing the fibrous carbon and the binder, and the viscosity of the slurry is adjusted to an easy-to-apply viscosity.

(Second slurry production method)
In the manufacturing method shown in Fig. 3(b), a conductive carbon mixture and electrode active material particles are dry mixed to produce an active material composite, and then a binder and a solvent are added to move to wet mixing. After the wet mixing is completed, a dispersion of fibrous carbon is added to the mixture obtained by the wet mixing. The mixture is diluted by further adding the solvent added with the binder, and the viscosity of the slurry is adjusted to be easy to apply.

(Third slurry production method)
In the manufacturing method shown in Fig. 3(c), the binder, the conductive carbon mixture, and the solvent are wet-mixed, and the electrode active material particles are added to the mixed liquid containing the conductive carbon mixture to continue the wet mixing, thereby producing an active material composite. After the wet mixing is completed, a dispersion of fibrous carbon is added to the mixed liquid obtained by the wet mixing. The mixed liquid is diluted by further adding the solvent added together with the binder and conductive carbon mixture, and the viscosity of the slurry is adjusted to be easy to apply.

(Fourth slurry production method)
In the manufacturing method shown in Fig. 3(d), the conductive carbon mixture, the dispersion of fibrous carbon, the binder, and the solvent are wet-mixed, and then the electrode active material particles are added to continue the wet mixing. The mixture obtained by the wet mixing is diluted by further adding the solvent used in the wet mixing of the fibrous carbon, the binder, and the conductive carbon mixture, and the viscosity of the slurry is adjusted to be easy to apply.

Because the conductive carbon mixture and fibrous carbon come into contact first, the conductive carbon mixture adheres to the surface of the fibrous carbon. The conductive carbon mixture that is not adhered to the fibrous carbon adheres to the surface of the electrode active material particles during the wet mixing stage when the electrode active material particles are added. Furthermore, while still adhering to the fibrous carbon, the conductive carbon mixture on the surface of the fibrous carbon further adheres to the electrode active material particles or to the conductive carbon mixture adhered to the electrode active material particles. Due to the pressure exerted on the oxidized carbon during the mixing process, the oxidized carbon adhered to the conductive carbon mixture also spreads out in a paste-like form, integrally connecting the electrode active material particles and the fibrous carbon.

(Fifth slurry production method)
3(e), the electrode active material particles, the conductive carbon mixture, the dispersion of fibrous carbon, the binder, and the solvent are wet-mixed, and the mixture obtained by the wet mixing is diluted by further adding the solvent used in the wet mixing, and the viscosity of the slurry is adjusted to be easy to apply.

The conductive carbon mixture comes into contact with both the fibrous carbon and the electrode active material particles at the same time, so it adheres to both the surfaces of the fibrous carbon and the electrode active material particles. Due to the pressure exerted on the oxidized carbon during the mixing process, the oxidized carbon attached to the conductive carbon mixture also spreads out in a paste-like form, integrally connecting the electrode active material particles and the fibrous carbon.

(Sixth slurry production method)
3(f), the conductive carbon mixture, the binder, and the solvent are wet-mixed, and the fibrous carbon dispersion and the electrode active material particles are wet-mixed separately, and the two mixtures are added and further wet-mixed.Then, the solvent used in the wet-mixing of the conductive carbon mixture, the binder, and the solvent is further added to dilute the mixture and adjust the viscosity of the slurry to be easy to apply.

Because the conductive carbon mixture is added to the already mixed fibrous carbon and electrode active material particles, it comes into contact with both the fibrous carbon and the electrode active material particles at the same time, and adheres to both the surfaces of the fibrous carbon and the electrode active material particles. Due to the pressure exerted on the oxidized carbon during the mixing process, the oxidized carbon attached to the conductive carbon mixture also spreads out in a paste-like form, integrally connecting the electrode active material particles and the fibrous carbon.

(Step of applying slurry onto a current collector and rolling it)
After the active material layer slurry is applied to the current collector and dried, pressure is applied to the active material layer by rolling to obtain an electrode. When pressure is applied to the active material layer, the oxidized carbon, at least a part of which has changed to a paste-like state, spreads further and densifies while covering the surfaces of the active material particles, and the active material particles approach each other, and the oxidized carbon, which has changed to a paste-like state, is pushed out and densely filled not only into the gaps formed between adjacent active material particles while covering the surfaces of the active material particles, but also into the insides of the holes present on the surfaces of the active material particles. Therefore, the amount of active material per unit volume in the electrode increases, and the electrode density increases. In addition, the densely filled paste-like oxidized carbon has sufficient conductivity to function as a conductive agent.

In addition, as pressure is applied to the active material layer, as the oxidized carbon on the electrode active material particles spreads into a paste-like shape due to rolling, some of it reaches the fibrous carbon present between the active material composites, and some of it continues to adhere to both the electrode active material particles and the fibrous carbon. Therefore, even after the rolling process is completed, some of the oxidized carbon remains stretched and spread between the electrode active material particles and the fibrous carbon. This increases the role of the fibrous carbon as an electron mediator between the electrode active material particles and the electrode active material particles, promoting low resistance. When oxidized carbon is attached to both the electrode active material particles and the fibrous carbon, rolling increases the likelihood that the oxidized carbon will spread between the electrode active material particles and the fibrous carbon, further promoting low resistance.

In addition, the coarse particles of the electrode active material particles press the oxidized carbon favorably in the rolling process, and have the effect of quickly converting the oxidized carbon into a paste-like substance and densifying it, thereby increasing the electrode density and improving the energy density of the electricity storage device. In addition, while the fine particles of the electrode active material particles press the oxidized carbon, at least a portion of which has turned into a paste, in the rolling process, they are pushed out together with the oxidized carbon that has spread into a paste-like substance and filled into the gaps formed between adjacent coarse particles, further increasing the electrode density and further improving the energy density of the electricity storage device.

The active material layer can be dried by removing the solvent under reduced pressure and heating as necessary. The pressure applied to the active material layer by rolling is generally in the range of 50,000 to 1,000,000 N/cm2, preferably 100,000 to 500,000 N/cm2. There are no particular limitations on the temperature of the rolling process, and the process may be performed at room temperature or under heated conditions.

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to the following examples.

(Example 1-1)
10 g of Ketjen Black (product name EC300J, manufactured by Ketjen Black International, BET specific surface area 800 m2/g) was added to 300 ml of 60% nitric acid, and the resulting solution was irradiated with ultrasonic waves for 10 minutes, and then filtered to recover the Ketjen Black. The recovered Ketjen Black was washed with water three times and dried to obtain acid-treated Ketjen Black.

3 g of this acid-treated Ketjen Black, 21.98 g of Fe(CH ₃ COO), 0.77 g of Li(CH ₃ COO), 1.10 g of C ₆ H ₈ O 7.H ₂ O, 1.32 _g of CH ₃ COOH, 1.31 g of H ₃ PO ₄ , and 120 ml of distilled water were mixed, and the resulting mixture was stirred with a stirrer for 1 hour, and then evaporated to dryness at 100 ° C in air to collect the mixture. The resulting mixture was then introduced into a vibration ball mill and pulverized for 10 minutes at 20 Hz. The pulverized powder was heated in nitrogen at 700 ° C for 3 minutes to obtain a composite in which LiFePO ₄ was supported on the oxidized Ketjen Black.

1 g of the composite was added to 100 ml of a 30% aqueous hydrochloric acid solution, and the resulting solution was irradiated with ultrasonic waves for 15 minutes to dissolve the _LiFePO4 in the composite, and the remaining solid was filtered, washed with water, and dried. A part of the dried solid was heated to 900°C in air by TG analysis to measure the weight loss. The above-mentioned process of dissolving _LiFePO4 in an aqueous hydrochloric acid solution, filtering, washing with water, and drying was repeated until it was confirmed that the weight loss was 100%, that is, no _LiFePO4 remained, to obtain an oxidation-treated carbon from which _LiFePO4 had been removed.

Next, 0.1 g of the obtained oxidized carbon was added to 20 ml of an aqueous ammonia solution of pH 11, and ultrasonic irradiation was performed for 1 minute. The obtained liquid was left to stand for 5 hours to allow the solid phase to precipitate. After the solid phase precipitated, the supernatant liquid was removed and the remaining part was dried, and the weight of the dried solid was measured. The weight ratio of the weight of the dried solid to the weight of the initial oxidized carbon (0.1 g) was taken as the content of the "hydrophilic portion" in the oxidized carbon. As a result, the weight ratio of the hydrophilic portion in the oxidized carbon was 15 mass%.

Next, the obtained oxidized carbon was mixed with acetylene black, which is another conductive carbon. That is, the obtained oxidized carbon and acetylene black (primary particle size: 40 nm) were introduced into a ball mill in a mass ratio of 1:1 and dry-mixed to obtain a conductive carbon mixture.

Using this conductive carbon mixture, the electrode of Example 1-1 was produced by the first slurry production method shown in Figure 3(a). In other words, the network structure of Figure 1 was the target product.

That is, 1.94 parts by mass of the obtained conductive carbon mixture and 96 parts by mass of commercially available LiNi _0.5 Mn _0.3 Co _0.2 O ₂ particles (average particle size 5 μm) as electrode active material particles were added and dry mixed to produce an active material composite. Meanwhile, 0.06 parts by mass of single-walled carbon nanotube dispersion liquid (OCSiAl, product name: TUBALL BATT) as fibrous carbon and 2 parts by weight of polyvinylidene fluoride as binder were added to an appropriate amount of N-methylpyrrolidone solvent and wet mixed. Then, the active material composite obtained by dry mixing was added to the mixture obtained by wet mixing, and wet mixing was further continued.

The mixture was then diluted with N-methylpyrrolidone to form a slurry. This slurry was then applied to an aluminum foil, dried, and rolled to obtain an electrode.

Figure 4 (a) is an SEM photograph of the electrode of Example 1-1 at a magnification of 10k, and (b) is an SEM photograph that has been processed to distinguish the active material composite and fibrous carbon shown in the SEM photograph of (a). In the photograph, the dashed line indicates the border of the active material composite, and the solid line indicates the axis of the fibrous carbon. As shown in Figure 4, it can be seen that the electrode active material particles are covered with the conductive carbon mixture, and an active material composite has been formed. It can also be seen that carbon nanotubes extend to connect the active material composites, forming a network structure.

(Example 2-1)
Using the conductive carbon mixture of Example 1-1, the electrode of Example 2-1 was produced by the fourth slurry production method shown in Fig. 3(d). That is, the network structure of Fig. 2 was used as the target product.

Specifically, 1.94 parts by mass of the obtained conductive carbon mixture, 0.06 parts by mass of a single-walled carbon nanotube dispersion (OCSiAl, product name: TUBALL BATT) as fibrous carbon, and 2 parts by weight of polyvinylidene fluoride as a binder were added to an appropriate amount of N- _{methylpyrrolidone} solvent and wet-mixed. Then, 96 parts by mass of commercially available _{LiNi0.5Mn0.3Co0.2O2 particles} (average particle size ₅ μm) were added as electrode active material particles, and the wet mixing was continued.

The mixture was then diluted with N-methylpyrrolidone to form a slurry. This slurry was then applied to an aluminum foil, dried, and rolled to obtain an electrode.

Figure 5 (a) is an SEM photograph of the electrode of Example 2-1 at a magnification of 10k, and (b) is an SEM photograph that has been processed to distinguish the active material composite and fibrous carbon shown in the SEM photograph of (a). In the photograph, the dashed line indicates the border of the active material composite, and the solid line indicates the axis of the fibrous carbon. As shown in Figure 5, it can be seen that the electrode active material particles are covered with the conductive carbon mixture, and an active material composite has been formed. It can also be seen that carbon nanotubes extend to connect the active material composites, forming a network structure.

(Example 1-2)
The electrode of Example _{1-2 was} produced by using the conductive carbon mixture of Example 1-1, changing the electrode active material particles to _{LiNi0.3Mn0.3Co0.3O2} particles (average particle size ₁₀ μm), and using the first slurry production method shown in Fig. 3(a). That is, the same network structure as in Example 1 of Fig. 1 was used as the target product.

Specifically _, 1.94 parts by mass of the obtained conductive carbon mixture and 96 parts by mass of commercially available _{LiNi0.3Mn0.3Co0.3O2 particles} (average particle size ₁₀ μm) as electrode active material particles were added and dry mixed to produce an active material composite. Meanwhile, 0.06 parts by mass of single-walled carbon nanotube dispersion liquid (OCSiAl, product name: TUBALL BATT) as fibrous carbon and 2 parts by weight of polyvinylidene fluoride as binder were added to an appropriate amount of N-methylpyrrolidone solvent and wet mixed. Then, the active material composite obtained by dry mixing was added to the mixture obtained by wet mixing, and wet mixing was further continued.

The mixture was then diluted with N-methylpyrrolidone to form a slurry. This slurry was then applied to an aluminum foil, dried, and rolled to obtain an electrode.

(Examples 1 to 3)
The electrode of Example 1-3 was produced by the second slurry production method shown in Fig. 3(b) using the conductive carbon mixture of Example 1-1. That is, the same network structure of Fig. 1 as in Example 1-1 was used as the target product.

Specifically, 1.94 parts by mass of the obtained conductive carbon mixture and 96 parts by mass of commercially available _{LiNi0.3Mn0.3Co0.3O2} particles (average particle size ₁₀ μm) as electrode active material particles were added and dry mixed. Next, the result of the dry mixing and ₂ parts by weight of polyvinylidene fluoride as a binder were added to an appropriate amount of N- _{methylpyrrolidone} solvent and wet mixed. 0.06 parts by mass of single-walled carbon nanotube dispersion (OCSiAl, product name: TUBALL BATT) as fibrous carbon was added to the result of the wet mixing and diluted with N-methylpyrrolidone to form a slurry. This slurry was applied on an aluminum foil, dried, and then rolled to obtain an electrode.

(Examples 1 to 4)
The electrode of Example 1-4 was produced by the third slurry production method shown in Fig. 3(c) using the conductive carbon mixture of Example 1-1. That is, the same network structure of Fig. 1 as in Example 1-1 was used as the target product.

Specifically, 1.94 parts by mass of the obtained conductive carbon mixture and 2 parts by weight of polyvinylidene fluoride as a binder were added to an appropriate amount of N-methylpyrrolidone solvent and wet-mixed. Next, 96 parts by mass of commercially available LiNi _0.3 Mn _0.3 Co _0.3 O ₂ particles (average particle size 10 μm) were added as electrode active material particles to the result of the dry mixing, and wet mixing was continued, thereby producing an active material complex. Then, 0.06 parts by mass of single-walled carbon nanotube dispersion (OCSiAl, product name: TUBALL BATT) was added as fibrous carbon to the result of the wet mixing, and the mixture was diluted with N-methylpyrrolidone to form a slurry. This slurry was applied to an aluminum foil, dried, and then rolled to obtain an electrode.

(Example 2-2)
The electrode of Example 2-2 _was produced by using the conductive carbon mixture of Example 1-1, changing the electrode active material particles to _{LiNi0.3Mn0.3Co0.3O2} particles (average particle size _{10 μm} ) and using the fourth slurry production method shown in Fig. 3(d). That is, the network structure of Fig. 2 was used as the target product.

Specifically, 1.94 parts by mass of the obtained conductive carbon mixture, 0.06 parts by mass of a single-walled carbon nanotube dispersion (OCSiAl, product name: TUBALL BATT) as fibrous carbon, and 2 parts by weight of polyvinylidene fluoride as a binder were added to an appropriate amount of N- _{methylpyrrolidone} solvent and wet-mixed. Then, 96 parts by mass of commercially available _{LiNi0.3Mn0.3Co0.3O2} particles (average particle size ₁₀ μm) were added as electrode active material particles, and the wet mixing was continued.

The mixture was then diluted with N-methylpyrrolidone to form a slurry. This slurry was then applied to an aluminum foil, dried, and rolled to obtain an electrode.

(Example 2-3)
The electrode of Example 2-3 was produced by the fifth slurry production method shown in Fig. 3(e) using the conductive carbon mixture of Example 1-1. That is, the same network structure of Fig. 2 as in Example 2-1 was used as the target product.

Specifically _, 96 parts by mass of commercially available _{LiNi0.3Mn0.3Co0.3O2} particles (average particle size _{10 μm} ) as electrode active material particles, 1.94 parts by mass of the obtained conductive carbon mixture, 0.06 parts by mass of single-walled carbon nanotube dispersion (OCSiAl, product name: TUBALL BATT) as fibrous carbon, and 2 parts by weight of polyvinylidene fluoride as binder were added to an appropriate amount of N-methylpyrrolidone solvent and wet mixed. Then, the mixture was diluted with N-methylpyrrolidone to form a slurry. This slurry was applied to an aluminum foil, dried, and then rolled to obtain an electrode.

(Example 2-4)
The electrode of Example 2-4 was produced by the sixth slurry production method shown in Fig. 3(f) using the conductive carbon mixture of Example 1-1. That is, the network structure of Fig. 2, which is the same as that of Example 2-1, was used as the target product.

Specifically, 96 parts _by mass of commercially available _{LiNi0.3Mn0.3Co0.3O2 particles} (average particle size ₁₀ μm) as electrode active material particles and 0.06 parts by mass of single-walled carbon nanotube dispersion liquid (OCSiAl, product name: TUBALL BATT) as fibrous carbon were added and dry mixed. Separately, 1.94 parts by mass of the obtained conductive carbon mixture and 2 parts by weight of polyvinylidene fluoride as a binder were added to an appropriate amount of N-methylpyrrolidone solvent and wet mixed. Then, both mixtures were added and further wet mixed, and diluted with N-methylpyrrolidone to form a slurry. This slurry was applied on an aluminum foil, dried, and then rolled to obtain an electrode.

(Evaluation of various characteristics 1)
The positive electrode density of the electrodes of Examples 1-2 to 1-4 and Examples 2-2 to 2-4 was measured. The positive electrode density was measured by pressing the electrode three times at 1.5 t/ ^cm3 , cutting it into ¹ cm2, and measuring the weight and thickness. The weight and thickness of the aluminum foil as the current collector were subtracted from the positive electrode density to calculate the density.

In addition, the DCIR of the electrodes of Examples 1-2 to 1-4 and Examples 2-2 to 2-4 was measured. A laminate cell of a lithium ion secondary battery was prepared as follows to measure the DCIR. That is, a counter electrode in which graphite was attached to copper foil was prepared, and a separator made of polyethylene terephthalate (PET) was interposed between the electrodes. As the electrolyte, a 1:1 solution of 1M LiPF ₆ in ethylene carbonate/diethyl carbonate was used. Then, constant current charging was performed at 25°C and a charge rate of 1.0C, and then constant current discharging was performed at 25°C and a discharge rate of 1C, and a discharge curve was measured. The DCIR was measured from this discharge curve.

In addition, the ESR of the electrodes of Examples 1-2 to 1-4 and Examples 2-2 to 2-4 was measured. A laminate cell of a lithium ion secondary battery was prepared as follows to measure the ESR. That is, a counter electrode in which graphite was attached to copper foil was prepared, and a separator made of polyethylene terephthalate (PET) was interposed between the electrodes. As the electrolyte, a 1:1 solution of 1M LiPF ₆ in ethylene carbonate/diethyl carbonate was used. Then, constant current charging was performed at a charge rate of 0.5C until the depth of charge (SOC) reached 50%, and then the value at 1 kHz was measured by AC impedance measurement.

For comparison, the following electrodes of Comparative Examples 1 to 3 were prepared, and the electrode density, DCIR, and ESR were measured under the same conditions as in each Example. In the electrode of Comparative Example 1, 2 parts by mass of acetylene black was used instead of the conductive carbon mixture. In addition, in the electrode of Comparative Example 1, fibrous carbon such as carbon nanotubes was not added. In the electrode of Comparative Example 1, the other composition, composition ratio, and preparation method are the same as in Example 1-1. In the electrode of Comparative Example 2, the amount of the conductive carbon mixture added was 2 parts by mass, and fibrous carbon such as carbon nanotubes was not added. In the electrode of Comparative Example 2, the other composition, composition ratio, and preparation method are the same as in Example 1-1. In addition, in the electrode of Comparative Example 3, the conductive carbon mixture was not added, and only 0.06 parts by mass of carbon nanotubes was used as the conductive assistant. In the electrode of Comparative Example 3, the other composition, composition ratio, and preparation method are the same as in Example 1-1. In the electrode of Comparative Example 4, 1.94 parts by mass of acetylene black was used instead of the conductive carbon mixture. The other compositions, composition ratios, and preparation methods are the same as those in Example 1-1.

The characteristics of the electrodes of Examples 1-2 to 1-4, 2-2 to 2-4, and Comparative Examples 1 to 4 are shown in Table 1 below.
(Table 1)

As shown in Table 1, the electrodes of Examples 1-2 to 1-4 exceeded those of Comparative Examples 1, 3, and 4 in terms of electrode density, and were equal to or greater than Comparative Example 2. Furthermore, the electrodes of Examples 1-2 to 1-4 showed lower resistance in terms of DCIR and ESR than Comparative Example 2 as well as Comparative Example 3. The electrodes of Examples 1-2 to 1-4 have active material layers formed from the network structure shown in Figure 1 according to the first to third slurry production methods shown in Figures 3(a) to (c). In Comparative Example 2, the active material layer is formed from an active material complex of a conductive carbon mixture and electrode active material particles, and the active material complex does not have a network structure.

The electrodes of Examples 2-2 to 2-4 had good electrode density, lower than that of Comparative Example 2 and Comparative Example 3, but higher than that of Comparative Example 1. Furthermore, in terms of DCIR and ESR, the electrodes of Examples 2-2 to 2-4 showed lower resistance than Comparative Example 2 as well as Comparative Example 3, and furthermore lower resistance than Examples 1-2 to 1-4. The electrodes of Examples 2-2 to 2-4 were formed by forming active material layers with the network structure shown in FIG. 2 according to the fourth to sixth slurry production methods shown in (d) to (f) of FIG. 3.

From the above, it has been confirmed that by using a manufacturing method in which a slurry containing electrode active material particles, oxidized carbon, a conductive carbon other than the conductive carbon, and fibrous carbon is prepared and the slurry is applied to a current collector, the oxidized carbon and the conductive carbon other than the conductive carbon form a conductive carbon mixture, the electrode active material particles and the conductive carbon mixture form an active material complex in which at least a portion of the surface of the electrode active material particles is covered with the conductive carbon mixture, and the active material complex and the fibrous carbon form a network structure in which the active material complexes are connected by the fibrous carbon.

It was also confirmed that electrodes having an active material layer formed from this network structure have good electrode density and resistance, with the resistance in particular being better than that in cases where carbon nanotubes are contained as a conductive additive.

In addition, in the slurry preparation process, a manufacturing method in which electrode active material particles are added and mixed into a conductive carbon mixture of oxidized carbon and another conductive carbon, and then fibrous carbon is added and mixed into this active material complex, has been confirmed to result in a network structure in which a large amount of conductive carbon mixture adheres to the electrode active material particles, while the amount of conductive carbon mixture adhering to the fibrous carbon is small.

It was also confirmed that a network structure in which a large amount of conductive carbon mixture adheres to the electrode active material particles while the amount of conductive carbon mixture adhered to the fibrous carbon is small results in an electrode with a particularly high electrode density.

In addition, it was confirmed that in the slurry preparation process, a network structure can be obtained in which the conductive carbon mixture adheres to both the electrode active material particles and the fibrous carbon by using a manufacturing method in which the conductive carbon mixture and the fibrous carbon are added simultaneously to the electrode active material particles, or by first mixing the fibrous carbon and the electrode active material particles and then adding the conductive carbon mixture.

It was also confirmed that a network structure in which a conductive carbon mixture adheres to both the electrode active material particles and the fibrous carbon results in an electrode with particularly low resistance.

(Cycle Characteristics 1)
The capacity retention rate for each cycle of Examples 1-4 and Comparative Examples 1 to 4 was measured. A laminate cell of a lithium ion secondary battery was prepared as follows to measure the capacity retention rate. That is, a counter electrode in which graphite was attached to copper foil was prepared, and a separator made of polyethylene terephthalate (PET) was interposed between the electrodes. As the electrolyte, a 1:1 solution of 1M LiPF ₆ in ethylene carbonate/diethyl carbonate was used. Then, the lithium ion secondary battery was charged at a constant current of 1C to 4.2V, and then charged at a constant voltage current until the current reached 0.02CA. Thereafter, a constant current discharge was performed at 1C to 3.0V, and the discharge capacity was calculated from the obtained discharge curve. Discharge cycles were performed 200 times, and the percentage of the initial discharge capacity was calculated as the capacity retention rate.

The results are shown in Figure 6. Figure 6 is a graph with the number of cycles on the horizontal axis and the capacity retention rate on the vertical axis. As shown in Figure 6, Examples 1-4 maintained a capacity retention rate of 95% or more for at least 200 charge/discharge cycles. On the other hand, in Comparative Examples 2 to 4, the capacity retention rate fell to 90% or less at the 200th charge/discharge cycle, and in Comparative Example 1, the capacity retention rate deteriorated rapidly after 110 charge/discharge cycles, and the capacity retention rate was 80% at the 200th charge/discharge cycle.

From the above, it was confirmed that an electrode in which at least a portion of the surface of the electrode active material particles is covered with this conductive carbon mixture not only maintains good cycle characteristics, but also improves cycle life compared to an electrode in which a portion or all of the surface of the electrode active material particles is covered with acetylene black.

(Si-based compound particles)
Using the conductive carbon mixture of Example 1-1, an electrode of Example 3-1 suitable for the negative electrode of a lithium ion secondary battery or a hybrid capacitor was produced. The electrode active material particles of this electrode were SiO particles. This electrode was produced by the fourth slurry production method shown in FIG.

Specifically, 2.5 parts by mass of the obtained conductive carbon mixture, a dispersion containing 2.5 parts by mass of a multi-walled carbon nanotube dispersion (JEIO, product name: JENO TUBE8) as fibrous carbon, and 15 parts by weight of polyimide as a binder were added to an appropriate amount of N-methylpyrrolidone solvent and wet-mixed. Then, 80 parts by mass of SiO particles (manufactured by Osaka Titanium) with an average particle size of 5 μm were added as electrode active material particles, and wet mixing was continued. This mixture was diluted with N-methylpyrrolidone to form a slurry. This slurry was applied to copper foil and dried, and after drying, it was rolled. After rolling, it was exposed to an inert atmosphere at 350 °C for 1 hour to obtain an electrode.

(Evaluation of various characteristics 2)
The negative electrode density of the electrode of Example 3-1 was measured. The negative electrode density was calculated by the same preparation and the same method as the positive electrode density in Evaluation of Various Characteristics 1. In addition, the DCIR and ESR of Example 3-1 were measured. The measurement conditions and measurement method of DCIR and ESR are as follows.

A coin cell of a lithium ion secondary battery was prepared for measuring the DCIR. That is, a lithium ion metal foil was prepared as a counter electrode, and a separator made of polyethylene terephthalate (PET) was interposed between the electrodes. In addition, 1 molar _LiPF6 was added as a solute to a solvent in which ethylene carbonate and diethyl carbonate were mixed in a weight ratio of 1:1 to prepare an electrolyte. Then, constant current charging was performed to SOC50% at 25°C and a charge rate of 0.2C, and then discharged for 10 seconds at 25°C, and the voltage drop was measured. The discharge current value was plotted on the horizontal axis and the voltage drop on the vertical axis, and the DCIR was calculated from the slope.

In addition, the ESR was confirmed by charging a coin cell at a constant current at a charge rate of 0.2C until the SOC reached 50%, then dismantling the coin cell that had been charged at a constant current. The electrodes removed from the dismantled cell were stacked with a separator in between to create a symmetrical cell, and the resistance value of the resulting symmetrical cell was measured at 1 kHz using AC impedance measurement.

For comparison, the following electrodes of Comparative Example 5 and Comparative Example 6 were prepared and incorporated into a lithium ion secondary battery, and the electrode density, DCIR, and ESR were measured under the same conditions as in Example 3-1. In the electrode of Comparative Example 5, 5 parts by mass of acetylene black was used instead of the conductive carbon mixture. In the electrode of Comparative Example 6, 5 parts by mass of the conductive carbon mixture was used. In the electrodes of Comparative Example 5 and Comparative Example 6, no fibrous carbon such as carbon nanotubes was added. In Comparative Examples 5 and 6, the composition, composition ratio, and preparation method of the other electrodes are the same as in Example 3-1, and the configuration, composition, and composition ratio of the lithium ion secondary battery are the same as in Example 3-1.

The characteristics of the electrodes of Example 3-1 and Comparative Example 5 are shown in Table 2 below.
(Table 2)

As shown in Table 2, the electrode of Example 3-1 exceeded that of Comparative Example 5 in terms of electrode density, and showed lower resistance in terms of DCIR and ESR than Comparative Example 5. From the above, it was confirmed that when SiO particles are used as the electrode active material particles on the negative electrode side, the deterioration of electrode density, DCIR, and ESR caused by the large volume change accompanying the insertion and extraction of lithium ions is also resolved.

(Cycle Characteristics 2)
Coin cells of lithium ion secondary batteries using the electrodes of Example 3-1, Comparative Example 5, and Comparative Example 6 as the negative electrode were prepared, and the capacity retention rate per cycle was measured. The configuration, composition, and composition ratio of the lithium ion secondary battery were the same as those in Evaluation 2 of Various Characteristics. The lithium ion secondary battery was charged at a constant current of 0.3 C to 0.01 V, and then charged at a constant current until the current reached 0.015 CA. Thereafter, constant current discharge was performed at 0.3 C to 1.5 V, and the discharge capacity was calculated from the resulting discharge curve. 50 discharge cycles were performed, and the percentage of the initial discharge capacity was calculated as the capacity retention rate.

The results are shown in Figure 7. Figure 7 is a graph with the number of cycles on the horizontal axis and the capacity retention rate on the vertical axis. As shown in Figure 7, Example 3-1 maintains a capacity retention rate of 95% or more for at least 50 charge/discharge cycles. On the other hand, the capacity retention rate of Comparative Example 5 and Comparative Example 6 deteriorates sharply, and at the time of 50 charge/discharge cycles, the capacity retention rate of Comparative Example 5 was 74.5%, and the capacity retention rate of Comparative Example 6 was 89.5%.

From the above, it was confirmed that when SiO particles are used as the electrode active material particles on the negative electrode side, the decrease in capacity retention rate caused by the large volume change accompanying the insertion and extraction of lithium ions is also resolved.

Claims (7)

An electrode having an active material layer,
the active material layer includes electrode active material particles, a conductive carbon mixture including an oxidized conductive carbon and a conductive carbon other than the oxidized conductive carbon, and fibrous carbon ;
the conductive carbon mixture is formed by covering the other conductive carbon with the oxidized conductive carbon,
the electrode active material particles and the conductive carbon mixture form an active material complex in which at least a portion of the surface of the electrode active material particles is covered with the conductive carbon mixture;
The outer diameter of the fibrous carbon is 1 nm or more and less than 70 nm,
the fibrous carbon is disposed between the active material composites to form a network structure;
An electrode comprising: The fibrous carbon is a carbon nanotube;
2. The electrode of claim 1 , The oxidized conductive carbon contains a hydrophilic portion in 10 mass % or more of the entire oxidized conductive carbon;
3. The electrode according to claim 1 or 2 , This is the negative electrode.
The electrode active material particles are Si-based compound particles;
4. The electrode according to claim 1 , wherein The Si-based compound particles are particles of a compound represented by SiOx (0≦x<2);
5. The electrode according to claim 4 , a slurry preparation step of preparing a slurry containing electrode active material particles, a conductive carbon composite including an oxidized conductive carbon and a conductive carbon other than the oxidized conductive carbon, and fibrous carbon;
an active material layer forming step of applying the slurry to a current collector;
Including,
In the slurry preparation step,
mixing the conductive carbon mixture obtained by covering the other conductive carbon with the oxidized conductive carbon, and mixing the fibrous carbon having an outer diameter of 1 nm or more and less than 70 nm;
preparing the slurry containing a network structure in which at least a portion of the surface of the electrode active material particles is covered with the conductive carbon mixture to form active material complexes and the fibrous carbon is disposed between the active material complexes;
A method for manufacturing an electrode, comprising the steps of: The slurry preparation step includes:
a first mixing step of mixing the conductive carbon mixture and the electrode active material particles;
a second mixing step of mixing the active material composite obtained in the first step with the fibrous carbon;
containing,
The method for producing an electrode according to claim 6 ,

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