WO2022244885A1

WO2022244885A1 - Coated active material for energy storage device, energy storage device, method for manufacturing coated active material for energy storage device, and coating material

Info

Publication number: WO2022244885A1
Application number: PCT/JP2022/021028
Authority: WO
Inventors: 学平澤; 雅規北川; 明博織田; 裕司小川
Original assignee: 昭和電工マテリアルズ株式会社
Priority date: 2021-05-21
Filing date: 2022-05-20
Publication date: 2022-11-24
Also published as: WO2022244272A1; JPWO2022244885A1; TW202304042A

Abstract

The present invention provides a coated active material for an energy storage device, the coated active material including: particles containing a substance that can absorb and release alkali metal ions; and a coating material containing cyclized polyacrylonitrile. The coated active material satisfies one or both of (1) and (2): (1) the yellowness, as defined by JIS K 7373:2006, of an electrolyte in which the coated active material has been immersed is 100 or less; and (2) the haze, as defined by JIS K 7136:2000, of an electrolyte in which the coated active material has been immersed is 2.5% or less.

Description

Coated active material for energy storage device, energy storage device, method for producing coated active material for energy storage device, and coating material

The present disclosure relates to an active material for an energy storage device, an energy storage device, a method for producing a coated active material for an energy storage device, and a coating material.

Energy storage devices are widely used in which charge and discharge are performed by moving alkali metal ions such as lithium ions between the positive and negative electrodes. The positive and negative electrodes of such energy storage devices generally contain particles of a material (active material) capable of absorbing and releasing alkali metal ions.

Since the volume of the active material changes as it absorbs and releases alkali metal ions, repeated charging and discharging causes cracks in the active material and causes deterioration of the electrode. Therefore, coating the surface of the active material with a flexible material such as a polymer compound has been studied.

Furthermore, some active materials have no or low electronic conductivity. Therefore, coating the surface of the active material with a polymer compound having electron conductivity has been studied.
For example, Japanese Patent Publication No. 2019-535116 and Japanese Patent No. 6635283 propose to use cyclized polyacrylonitrile (cyclized polyacrylonitrile) for electrodes of energy storage devices.

As a method for causing a cyclization reaction of polyacrylonitrile, JP 2019-535116 describes heating polyacrylonitrile in an inert atmosphere, and Japanese Patent No. 6635283 describes polyacrylonitrile and butyl acrylate. is described heating a copolymer of .
According to studies by the present inventors, it was found that there is room for improvement in the initial characteristics of energy storage devices using cyclized acrylonitrile produced by the methods described in these documents as a coating material for the active material.
In view of the above circumstances, an object of the present disclosure is to provide a coated active material for an energy storage device, an energy storage device, a method for producing the coated active material for an energy storage device, and a coating material that are excellent in initial characteristics.

Means for solving the above problems include the following embodiments.
<1> A coated active material for an energy storage device, comprising particles containing a substance capable of absorbing and releasing alkali metal ions, and a coating material containing cyclized polyacrylonitrile, and satisfying the following (1).
(1) The yellowness of the electrolytic solution in which the coated active material is immersed is 100 or less as defined in JIS K 7373:2006.
<2> A coated active material for an energy storage device, comprising particles containing a substance capable of absorbing and releasing alkali metal ions, and a coating material containing cyclized polyacrylonitrile, and satisfying the following (2).
(2) The haze defined by JIS K 7136:2000 of the electrolytic solution in which the coated active material is immersed is 2.5% or less.
<3> The coated active material for an energy storage device according to <1> or <2>, wherein the substance capable of absorbing and releasing alkali metal ions contains a silicon atom.
<4> The coated active material for an energy storage device according to any one of <1> to <3>, which has a volume average particle size (D50) of 1 μm to 50 μm.
<5> The coated active material for an energy storage device according to any one of <1> to <4>, which has a BET specific surface area of 0.5 m ² /g to 100 m ² /g.
<6> The coated active material for an energy storage device according to any one of <1> to <5>, wherein the coating material accounts for 0.1% by mass to 50% by mass of the entire coated active material. .
<7> The coated active material for an energy storage device according to any one of <1> to <6>, further comprising a coating made of a carbon material.
<8> An energy storage device comprising the coated active material for an energy storage device according to any one of <1> to <7>.
<9> The energy storage device according to <8>, comprising an electrolytic solution, wherein the electrolytic solution contains an ionic liquid as a solvent.
<10> The energy storage device according to <8>, wherein the electrolytic solution has an electrolyte salt concentration of 3 mol/L or more.
<11> A composition containing particles containing a substance capable of absorbing and releasing alkali metal ions and polyacrylonitrile is heat-treated in an atmosphere of 150° C. or more and less than 278° C. and an oxygen concentration of 5% to 30% by volume. and heat-treating the composition in an atmosphere of 278° C. to 600° C. and an oxygen concentration of 4 ppm to 100 ppm, in this order.
<12> The method for producing a coated active material for an energy storage device according to <11>, wherein each heat treatment is performed for 3 to 15 hours.
<13> A coating material for coating active material particles for an energy storage device, which contains cyclized polyacrylonitrile, and the electrolytic solution in which the coating material is immersed has a yellowness specified in JIS K 7373:2006. A covering material that is 30% or less.
<14> A coating material for coating active material particles for an energy storage device, which contains cyclized polyacrylonitrile, and the electrolytic solution in which the coating material is immersed has a haze of 0 as defined in JIS K 7136:2000. .3% or less of the coating.

According to the present invention, there are provided a coated active material for an energy storage device, an energy storage device, a method for producing the coated active material for an energy storage device, and a coating material that are excellent in initial characteristics.

1 is a conceptual diagram showing an example of the configuration of a coated active material; FIG. 1 is a conceptual diagram showing an example of the configuration of a coated active material; FIG. 1 is a conceptual diagram showing an example of the configuration of a coated active material; FIG. 1 is a conceptual diagram showing an example of the configuration of a coated active material; FIG. 1 is a conceptual diagram showing an example of the configuration of a coated active material; FIG.

A detailed description will be given below of the embodiment for implementing the present disclosure. However, the present disclosure is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, which do not limit the present disclosure.

In the present disclosure, the term "process" includes a process that is independent of other processes, and even if the purpose of the process is achieved even if it cannot be clearly distinguished from other processes. .
In the present disclosure, the numerical range indicated using "-" includes the numerical values before and after "-" as the minimum and maximum values, respectively.
In the numerical ranges described step by step in the present disclosure, the upper limit or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described step by step. . Moreover, in the numerical ranges described in the present disclosure, the upper or lower limits of the numerical ranges may be replaced with the values shown in the examples.
In the present disclosure, each component may contain multiple types of applicable substances. When there are multiple types of substances corresponding to each component in the composition, the content rate or content of each component is the total content rate or content of the multiple types of substances present in the composition unless otherwise specified. means quantity.
Particles corresponding to each component in the present disclosure may include a plurality of types. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means a value for a mixture of the multiple types of particles present in the composition, unless otherwise specified.
In the present disclosure, the term "layer" includes not only the case where the layer is formed in the entire region when observing the region where the layer exists, but also the case where it is formed only in part of the region. included.

<Coated active material for energy storage device>
The coated active material for an energy storage device of the present disclosure includes particles containing a substance capable of absorbing and releasing alkali metal ions (hereinafter also referred to as active material particles) and a coating material containing cyclized polyacrylonitrile, and is described below. A coated active material for an energy storage device (hereinafter also referred to as a coated active material) that satisfies either or both of (1) and (2).
(1) The yellowness of the electrolytic solution in which the coated active material is immersed is 100 or less as defined in JIS K 7373:2006.
(2) The haze defined by JIS K 7136:2000 of the electrolytic solution in which the coated active material is immersed is 2.5% or less.

As a result of studies by the present inventors, it was found that an energy storage device manufactured using a coated active material whose coating material contains cyclized polyacrylonitrile and satisfies the conditions (1) or (2) exhibits excellent initial characteristics. I understood it. The reason is considered as follows, for example.
When the coated active material whose coating material contains cyclized polyacrylonitrile is immersed in the electrolytic solution, components contained in the coating material (for example, low-molecular-weight compounds generated in the cyclization process of polyacrylonitrile) are eluted, and the eluted components are dissolved. Decomposition of the electrolyte salt due to the reaction, etc. may occur. These phenomena are considered to be factors affecting the color or transparency of the electrolyte.
It is considered that the coated active material satisfying the conditions (1) or (2) has a low degree of yellowness or haze of the electrolytic solution because the amount of components eluted into the electrolytic solution is small. That is, it is thought that the coated active material satisfying the condition (1) or (2) is excellent in durability against the electrolytic solution, and thus excellent initial characteristics are realized.

(Method for measuring yellowness)
In the present disclosure, the yellowness of the electrolytic solution in which the coated active material is immersed is a value specified in JIS K 7373: 2006, and the larger the value of the yellowness, the more the hue of the measurement target is colorless or white. Indicates that it is far away (yellowish).
In the present disclosure, the yellowness is the tristimulus value (X, From the results of measuring Y and Z), it is obtained by the following formula. As a tristimulus value direct-reading colorimeter, for example, Model CC-i manufactured by Suga Test Instruments Co., Ltd. can be used.
Yellowness index (YI) = 100 (1.2985X-1.1335Z)/Y

As the electrolytic solution in which the coated active material is immersed, 1M LiPF ₆ was added to a solvent in which EC (ethylene carbonate), EMC (ethyl methyl carbonate) and DEC (diethyl carbonate) were mixed at a ratio of 1:1:1 (volume ratio). used in a dissolved form.

The electrolytic solution (25 ml) is placed in a PFA bottle with a diameter of 27.6 mm, and the coated active material is immersed in the electrolytic solution. The amount of the coated active material is adjusted so that the mass of the coating material contained in the coated active material is 10 mg.
The coated active material immersed in the electrolyte is allowed to stand in a constant temperature bath at 50° C. for 24 hours. After that, the yellowness of the electrolytic solution is calculated by the method described above.
Instead of the coated active material, only the coating material may be immersed in the electrolytic solution to measure the yellowness.

From the viewpoint of initial characteristics, the lower the yellowness of the electrolytic solution in which the coated active material is immersed, the better. Therefore, the preferred lower limit of the yellowness index is 0.

(Method for measuring haze)
In the present disclosure, the haze (cloudiness value) of the electrolytic solution in which the coated active material is immersed is a value defined in JIS K 7136:2000, and the higher the haze value, the lower the transparency of the object to be measured.
In the present disclosure, haze is defined as the transmittance (total light transmittance: Tt) of light rays including all parallel components and diffuse components among the light rays that pass through the measurement object (electrolyte solution), and the total light transmittance excluding parallel components. It is obtained by the following formula from the diffused light transmittance (diffused transmittance: Td). As a measuring device, a haze meter (Model: HZ-V3 manufactured by Suga Test Instruments Co., Ltd.) can be used.
Haze (%) = (Td/Tt) x 100

The electrolytic solution (25 ml) is placed in a PFA bottle with a diameter of 27.6 mm, and the coated active material is immersed in the electrolytic solution. The amount of electrode is adjusted so that the mass of the coating material contained in the coated active material is 10 mg.
The coated active material immersed in the electrolyte is allowed to stand in a constant temperature bath at 50° C. for 24 hours. After that, the haze of the electrolytic solution is calculated by the method described above.
The haze may be measured by immersing only the coating material in the electrolytic solution instead of the coated active material.

From the viewpoint of initial characteristics, the lower the haze of the electrolyte in which the coated active material is immersed, the better. Therefore, the preferred lower limit of haze is 0%.

(Covering material)
The coating material comprises cyclized polyacrylonitrile. In the present disclosure, cyclized polyacrylonitrile means a material obtained by causing a cyclization reaction of nitrile groups contained in polyacrylonitrile.

Cyclized polyacrylonitrile can be obtained not only by ring closure of -C≡N (nitrile) group to become -C=N- group in the cyclization treatment step of raw material polyacrylonitrile, but also by dehydrogenation reaction, for example, It is preferable that the -CH ₂ -CH- group constituting the chain is changed to -CH=C- group or the like to form a double bond. Both the cyclization reaction and the dehydrogenation reaction tend to form a conjugated double bond and improve the electron conductivity.

Characterization of the above reaction can be done with infrared spectroscopy. Infrared spectroscopy may be transmission or reflection.
In infrared spectroscopy, the ^-C≡N (nitrile) group as a peak at 2240 cm ^-1 to 2243 cm ^-1 and the ring-closed -C=N- group as a peak at 1577 cm ^-1 to 1604 cm -1 by dehydrogenation A peak at 2939 cm ⁻¹ for —CH ₂ — before forming a double bond, and a peak at 806 cm ⁻¹ for a —CH═C— group after forming a double bond by dehydrogenation can be confirmed.
The ring-closed —C═N— group shifts in the range of 1577 cm ⁻¹ to 1604 cm ⁻¹ depending on the structure of the 6-membered ring formed. Specifically, the larger the number of double bonds contained in the six-membered ring structure, the shorter the bond distance of -C=N-, which tends to shift to the lower wavenumber side.

The above peaks are assigned to The influence of thermal stabilization stage on the molecular structure of polyacrylonitrile fibers prior to the carbonization stage (Fibers and Polymers 2012, Vol.13, No.3, 295-302), Structural transformation of polyacrylonitrile fibers during stabilization and low temperature carbonization (Polymer Degradation and Stability, Volume 128, June 2016, 39-45).

The cyclized polyacrylonitrile can be said to have properties intermediate between those of carbon and polymers, and the greater the degree of ring closure of the nitrile groups, the more similar the properties of the cyclized polyacrylonitrile to those of carbon.
The ratio of the nitrile group and the ring-closed -C=N- group in the cyclized polyacrylonitrile is the absorbance at the peak corresponding to the -C≡N (nitrile) group and the absorbance at the peak corresponding to the ring-closed -C=N- group. (nitrile group/ring-closed -C=N- group), and it can be said that the larger the value of this ratio, the closer the properties of the cyclized polyacrylonitrile to those of polymers. Hereinafter, this absorbance ratio is also referred to as absorbance ratio A.
The absorbance ratio A is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.03 or more.
When the absorbance ratio A is 0.01 or more, the obtained coating material has appropriate flexibility and easily follows the expansion and contraction of the active material particles.
The absorbance ratio A is preferably 6 or less, more preferably 3 or less, and even more preferably 1 or less.
When the absorbance ratio A is 6 or less, the structure of the coating material obtained by sufficiently ring-closing the nitrile groups becomes strong.

Cyclized polyacrylonitrile can be said to be a polymer imparted with electronic conductivity by a cyclization reaction.
The degree of electronic conductivity of the cyclized polyacrylonitrile is determined by the absorbance at the peak corresponding to the -CH=C- group after dehydrogenation to double bonds and at the peak corresponding to the ring-closed -C=N- group. It can be expressed by the ratio of the absorbance (-CH=C-group after double bond formation/-C=N-group after ring closure), and the larger the value of this ratio, the better the electron conduction of the cyclized polyacrylonitrile. It can be said that the sex is great. Hereinafter, this absorbance ratio is also referred to as absorbance ratio B.
The absorbance ratio B is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.03 or more.
When the absorbance ratio B is 0.01 or more, the obtained coating material exhibits sufficient electronic conductivity.
Although the upper limit of the absorbance ratio B is not particularly limited, it may be 1 or less, for example.

When performing infrared spectroscopic analysis, the cyclized polyacrylonitrile itself has various bonding species, so each peak tends to overlap with other peaks, so it is preferable to draw a baseline for calculation. There is no particular limitation on how to draw the baseline, but a method of drawing by connecting the tails of both ends of the target peak can be exemplified.

The absorbance ratio in infrared spectroscopy is a mixture of cyclized polyacrylonitrile and an active material (provided that the active material is a catalyst for the cyclization and decomposition reactions of polyacrylonitrile) even if the measurement target is only cyclized polyacrylonitrile. A similar tendency is confirmed even in the state of the electrode combined with the current collector. Therefore, the absorbance ratio may be calculated in the state where the object to be measured is a mixture of cyclized polyacrylonitrile and an active material or an electrode in combination with a current collector.

The cyclized polyacrylonitrile preferably contains an acridone structure. The acridone structure is the ring structure shown below (the wavy line indicates the binding site) generated during the cyclization reaction of polyacrylonitrile.

As can be seen from the fact that the acridone structure contains oxygen atoms, the cyclized polyacrylonitrile containing the acridone structure can be obtained by performing a heat treatment that causes a cyclization reaction of the polyacrylonitrile in an oxygen-containing environment.

The fact that the cyclized polyacrylonitrile has an acridone structure can be confirmed by pyrolysis GC/MS analysis (Pyrolysis Gas Chromatography Mass Spectrometry) and known techniques for X-ray photoelectron spectrum analysis. The presence of the acridone structure can be confirmed by a fragment of mass 177 in pyrolysis GC/MS analysis and by a peak around 532 eV in X-ray photoelectron spectroscopy.

The molecular weight of the polyacrylonitrile to be the precursor of the cyclized polyacrylonitrile is not particularly limited, but the weight average molecular weight is preferably 5,000 to 3,000,000, more preferably 10,000 to 1,000,000, and 20,000 to 60,000. 10,000 is more preferable. When the weight average molecular weight of polyacrylonitrile is 5,000 or more, a good coating material can be obtained, and when the weight average molecular weight of polyacrylonitrile is 3,000,000 or less, the viscosity is low and mixing with the active material particles becomes easy. . Furthermore, when the weight-average molecular weight of the polyacrylonitrile is 500 or more, the mass reduction due to the cyclization treatment is suppressed, and the residual amount of the cyclized polyacrylonitrile tends to increase.

Although there is no particular limitation on the molecular weight distribution of the polyacrylonitrile that is the precursor of the cyclized polyacrylonitrile, it is preferably 1 to 3, more preferably 1 to 2, and further preferably 1 to 1.5. preferable.
The closer the molecular weight distribution of polyacrylonitrile is to 1, the smaller the variation in molecular weight of polyacrylonitrile. If the variation in molecular weight of polyacrylonitrile is small, the reduction in mass due to cyclization treatment is suppressed, and the residual amount of cyclized polyacrylonitrile tends to increase.

In the present disclosure, the molecular weight distribution of polyacrylonitrile is the value (Mw/Mn) obtained by dividing the weight average molecular weight (Mw) of polyacrylonitrile by the number average molecular weight (Mn).
The weight average molecular weight (Mw) and number average molecular weight (Mn) of polyacrylonitrile in this disclosure are measured by Gel Permeation Chromatography.

Polyacrylonitrile may be a homopolymer of acrylonitrile or a copolymer of acrylonitrile and a polymerizable component other than acrylonitrile (hereinafter also referred to as an acrylonitrile copolymer).
A coated active material containing cyclized polyacrylonitrile, which is a cyclized acrylonitrile copolymer, as a coating material tends to exhibit excellent properties. The reason is considered as follows, for example.

The ring closure reaction between adjacent nitrile groups contained in polyacrylonitrile is an exothermic reaction. Since this exothermic reaction progresses abruptly at around 293°C, the temperature in the reaction system rises sharply at around 293°C. As a result, the formation and volatilization of low-molecular-weight by-products due to scission of the molecular chains of polyacrylonitrile rapidly proceed. On the other hand, when a copolymerization component is introduced into polyacrylonitrile, the amount of nitrile groups is relatively reduced and the ring closure reaction occurs at a lower temperature (for example, around 278° C.), suppressing rapid heat generation. As a result, the formation and volatilization of low-molecular-weight by-products are suppressed, and the residual amount of cyclized polyacrylonitrile increases.

Furthermore, the ring closure reaction of nitrile groups includes those that occur intramolecularly and those that occur intermolecularly. When a copolymer component is introduced into polyacrylonitrile, the intramolecular ring-closure reaction of nitrile groups is relatively reduced, and the intermolecular ring-closure reaction of nitrile groups is relatively increased. As a result, formation of a three-dimensional crosslinked structure of the cyclized polyacrylonitrile is promoted.

Alternatively, when a copolymerization component having an ionic functional group is introduced into polyacrylonitrile, in addition to the radical polymerization reaction between adjacent nitrile groups, an ionic polymerization reaction also occurs between the ionic functional group and the nitrile group. resulting in binding of the molecular chains. Ionic polymerization reactions proceed at lower temperatures than radical polymerization reactions. As a result, the formation and volatilization of low-molecular-weight by-products are suppressed, and the residual amount of cyclized polyacrylonitrile increases.

For the above reasons, it is believed that the resulting coated active material has improved strength and exhibits excellent properties.

The polymerization components other than acrylonitrile that constitute the acrylonitrile copolymer are not particularly limited. For example, it may be selected from polymer components having at least one functional group selected from the group consisting of a sulfo group, a carboxy group, an amino group and an alkyl ester group.

A sulfo group in the present disclosure is a monovalent group represented by —SO ₃ H. The sulfo group may form a salt with an alkali metal such as sodium.

A carboxy group in the present disclosure is a monovalent group represented by —COOH.

In the present disclosure, an amino group is a monovalent group represented by —NR ¹ R ² , where R ¹ and R ² are each independently a hydrogen atom or a monovalent organic group.

In the present disclosure, an alkyl ester group is a monovalent group represented by -COOR, and R is an alkyl group. The number of carbon atoms in the alkyl group is preferably 1-15, more preferably 1-5, even more preferably 1-3.

Polymerization components containing a sulfo group include allylsulfonic acid, methallylsulfonic acid, vinylbenzenesulfonic acid, and alkali metal salts (sodium salts, etc.) thereof.

Polymerization components containing a carboxy group include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, and alkali metal salts (sodium salts, etc.) thereof.

Polymerization components containing amino groups include acrylamide, methacrylamide, dimethylaminopropyl acrylamide, dimethylaminopropyl methacrylamide, and the like.

Examples of polymerizable components containing an alkyl ester group include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, and n-methacrylate. butyl, n-hexyl methacrylate, cyclohexyl methacrylate, lauryl methacrylate and the like.

Polymerization components other than the above include vinyl acetate, styrene, vinylidene chloride, and vinyl chloride.

From the viewpoint of the properties of electrodes and energy storage devices, the acrylonitrile copolymer preferably contains an ionic group such as a sulfo group, a carboxyl group and an amino group, and may contain an anionic group such as a sulfo group and a carboxyl group. More preferably, it contains a sulfo group or a carboxy group.

An acrylonitrile copolymer containing an ionic group can be obtained by using a polymerization component containing an ionic group as a polymerization component other than acrylonitrile.

From the viewpoint of the properties of the coated active material and the energy storage device, the ratio of the polymerized components other than acrylonitrile in the acrylonitrile copolymer to the total polymerized components is preferably 0.1% by mass or more, and 0.2% by mass or more. and more preferably 0.5% by mass or more.

From the viewpoint of fully exhibiting the properties of the cyclized polyacrylonitrile, the ratio of the polymer components other than acrylonitrile in the acrylonitrile copolymer to the total polymer components is preferably 20% by mass or less, and is 15% by mass or less. is more preferable, and 10% by mass or less is even more preferable.

When the polyacrylonitrile is an acrylonitrile copolymer, the proportion of acrylonitrile in the total polymerization components is preferably 80% by mass or more, more preferably 85% by mass or more, and more preferably 90% by mass or more. preferable.

The electrodes of the present disclosure may include coating materials other than cyclized polyacrylonitrile as the coating material.
Coating materials other than cyclized polyacrylonitrile include polyacrylic acid, polyvinyl acetate, polystyrene, polyvinylidene chloride, polyvinyl chloride, and polymethacrylic acid.
The proportion of the cyclized polyacrylonitrile in the entire coating material is preferably 70% by mass to 100% by mass, more preferably 80% by mass to 100% by mass, and 90% by mass to 100% by mass. is more preferred.

The proportion of the coating material in the entire coated active material (total of active material particles and coating material) is preferably 0.1% by mass to 50% by mass, more preferably 1% by mass to 20% by mass. , and more preferably 5 to 10% by mass.
When the ratio of the coating material is 0.1% by mass or more of the entire coated active material, a sufficient effect of improving the initial characteristics can be obtained. When the ratio of the coating material is 50% by mass or more of the entire coated active material, the capacity of the energy storage device is sufficiently secured.
The ratio of the coating material to the entire coated active material can be calculated from the change in mass before and after the heat treatment when the coated active material is heat-treated at a temperature at which the coating material thermally decomposes.

If necessary, the coating material may contain a conductive aid. Examples of conductive aids include carbon materials such as carbon black, carbon nanotubes, carbon nanofibers, fullerenes and carbon nanohorns, conductive oxides, and conductive nitrides.

When the coating material contains a conductive aid, the content is not particularly limited, and may be 1% by mass to 20% by mass of the entire coating material.

(Active material particles)
The active material particles contained in the coated active material of the present disclosure are not particularly limited as long as they contain a material (active material) that can occlude and release alkali metal ions.
Alkali metal ions include lithium ions, potassium ions, sodium ions, and the like. Among these, lithium ion is preferred.
The active material contained in the active material particles may be of one type or a combination of two or more types.

Examples of positive electrode active materials include lithium transition metal compounds such as lithium transition metal oxides and lithium transition metal phosphates.
Lithium transition metal oxides include compounds containing one or more of transition metals such as Mn, Ni, Co, etc., and some of the transition metals contained in these compounds, one or more of them or a lithium transition metal oxide substituted with a metal element (typical element) such as Mg or Al.
Examples of active materials for the negative electrode include carbon materials and active materials containing silicon atoms.
Carbon materials include graphite, hard carbon, soft carbon, and the like.
Examples of active materials containing silicon atoms include Si (metallic silicon) and silicon oxides represented by SiOx (0.8≦x≦1.5).
The silicon oxide may have a structure in which nano-silicon is dispersed in a silicon oxide matrix by a disproportionation reaction.
The active material containing silicon atoms may be doped with boron, phosphorus, or the like to make it a semiconductor.

The active material particles may be in a state in which silicon is present on the surfaces of the active material particles made of a carbon material.
As a method for making silicon exist on the surface of the active material particles made of a carbon material, a vapor deposition method, a plasma CVD (Chemical Vapor Deposition) method, and the like can be mentioned. The plasma CVD method may be performed by decomposing raw materials such as silane and chlorosilane.

Active materials containing silicon atoms have a large theoretical capacity and are expected to contribute to increasing the capacity of energy storage devices. In addition, active materials containing silicon atoms themselves are not electronically conductive.
The cyclized polyacrylonitrile used as a coating material for the coated active material of the present disclosure has both sufficient flexibility and electronic conductivity to accommodate changes in volume of the active material. Therefore, it can be particularly suitably used as a coating material for active material particles containing silicon atoms.

The active material particles may be further coated with a material other than cyclized polyacrylonitrile. In this case, even if the coating containing the cyclized polyacrylonitrile is located between the active material particles and the coating made of the material other than the cyclized polyacrylonitrile, it is located above the coating made of the material other than the cyclized polyacrylonitrile. Alternatively, a coating containing a cyclized polyacrylonitrile and a coating made of a material other than the cyclized polyacrylonitrile may be mixed.
The active material particles may have, for example, a coating (carbon coating) made of a carbon material. By coating the surface of the active material particles with a carbon material, for example, electronic conductivity can be imparted to active material particles that do not have electrical conductivity. For example, electron conductivity can be imparted to the silicon-containing active material particles by coating the silicon-containing active material particles with a carbon material.
When the active material particles containing silicon are coated with a carbon material, the material of the carbon material is not particularly limited, and may be graphite or amorphous carbon.
The carbon material may be obtained by carbonizing an organic compound. Examples of organic compounds include tar, pitch, and organic polymer compounds. Examples of organic polymer compounds include polyacrylonitrile, polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, starch, and cellulose.

The shape of the coated active material (or active material particles) is not particularly limited. For example, it may be spherical particles, wire-like particles, scale-like particles, massive particles, composite particles composed of a plurality of particles, or the like.

The volume average particle diameter (D50) of the coated active material (or active material particles) is preferably 1 μm to 50 μm, more preferably 3 μm to 30 μm (except for wire-shaped particles). When the volume-average particle size of the coated active material (or active material particles) is 1 μm or more, it becomes easier to prepare a slurry for forming an electrode. When the volume-average particle size of the coated active material (or active material particles) is 50 μm or less, the electrode can be easily formed into a thin film, and the input/output characteristics of the energy storage device can be easily improved.

The volume average particle size of the coated active material (or active material particles) is measured by a laser scattering diffraction method. Specifically, the volume-average particle diameter is defined as the particle diameter when the accumulation from the small diameter side is 50% in the volume-based particle diameter distribution obtained by the laser scattering diffraction method.

When the coated active material (or active material particles) are secondary particles, the volume average particle size is the volume average particle size of the secondary particles.
In the present disclosure, the term "secondary particle" means a particle that is the smallest unit of normal behavior formed by agglomeration of a plurality of primary particles, and the term "primary particle" means that it can exist alone. It means the smallest unit particle that can be made.

When the coated active material (or active material particles) are secondary particles, the particle size of the primary particles that make up the secondary particles is not particularly limited. For example, the average primary particle size is preferably 10 nm to 50 μm. More preferably, it is 30 nm to 10 μm. When the average primary particle size of the coated active material is 10 nm or more, the influence of the natural oxide film formed on the surface can be suppressed. When the average primary particle size of the coated active material is 50 μm or less, deterioration due to charging and discharging is suppressed.

In the present disclosure, the primary particle diameter of the coated active material (or active material particles) means the major diameter of the primary particles observed with a scanning electron microscope. Specifically, when the primary particles are spherical, it means the maximum diameter, and when the primary particles are tabular, it means the maximum diameter or maximum diagonal length in the projected image of the particles observed from the thickness direction. "Average primary particle diameter" is the arithmetic mean value of the measured values of the major diameters of 300 or more primary particles observed with a scanning electron microscope.

When the coated active material (or active material particles) is wire-shaped, there is no particular limitation on its length. For example, it is preferably 10 nm to 10 μm. When the length of the wire-shaped coated active material (or active material particles) is 10 nm or more, the handleability is improved, and when the length is 10 μm or less, the stress during expansion of the coated active material tends to be easily dispersed.
There is no particular limitation on the diameter of the wire-like particles. For example, it is preferably 1 nm to 5 μm. By setting the diameter of the wire-shaped particles to 1 nm or more, the self-supporting strength of the wire-shaped particles is improved, and by setting the diameter to 5 μm or less, the stress in the radial direction when the coated active material expands is suppressed, and the length direction is reduced. can relieve stress. The wire-shaped coated active material (or active material particles) may contain a catalyst component for forming the active material into a wire-like shape. A specific example of the wire-shaped coated active material (or active material particles) is a coated active material (or active material particles) containing metallic silicon.

The method for adjusting the particle size of the coated active material (or active material particles) is not particularly limited. Examples thereof include a method of selecting raw materials, a method of adjusting pulverization conditions, a vapor deposition method, a plasma method, and a method of surface treatment with silane or the like.

The BET specific surface area of the coated active material (or active material particles) is preferably 0.5 m ² /g to 100 m ² /g, more preferably 1 m ² /g to 30 m ² /g. When the BET specific surface area of the coated active material (or active material particles) is 0.5 m ² /g or more, sufficient discharge capacity can be easily obtained. When the BET specific surface area of the coated active material (or active material particles) is 100 m ² /g or less, the handleability during electrode production is excellent.
The BET specific surface area (or active material particles) of the coated active material can be calculated from the nitrogen adsorption isotherm at -196°C.

The coated active material of the present disclosure may be used in combination with another active material. For example, when the active material particles in the coated active material of the present disclosure contain silicon atoms, the coated active material of the present disclosure and active material particles made of a carbon material may be used together.

An example of the configuration of the coated active material of the present disclosure will be described based on the drawings.
The coated active material 10 shown in FIG. 1 is in a state in which a coating 2 containing cyclized polyacrylonitrile is formed on the surface of the active material particle 1 (a natural oxide film formed on the surface of the active material particle is omitted). .
The coated active material 11 shown in FIG. 2 is a modified example of the coated active material 10 shown in FIG. is formed.
A coated active material 12 shown in FIG. 3 is a modification of the coated active material 10 shown in FIG. 1, and is in a state in which a coating 3 made of a carbon material or the like is formed on a coating 2 containing cyclized polyacrylonitrile.
It is a modification of the coated active material 13 shown in FIG. 4 and the coated active material 10 shown in FIG.
The coated active material 14 shown in FIG. 5 is a modified example of the coated active material 10 shown in FIG. 1, and the active material particles 1 are in the state of secondary particles composed of a plurality of primary particles.

<Energy storage device electrodes>
An energy storage device electrode (hereinafter also referred to as an electrode) of the present disclosure includes the above-described coated active material of the present disclosure and a binder.

The type of binder is not particularly limited, and can be selected from polyacrylonitrile, carboxymethylcellulose (CMC), CMC/styrene-butadiene rubber (SBR), polyvinylidene fluoride, and the like.
When polyacrylonitrile is used as the binder, the polyacrylonitrile may be in a cyclized state.

The electrode may contain only the coated active material of the present disclosure as an active material, or may contain the coated active material of the present disclosure and an active material other than the coated active material of the present disclosure as active materials.
The type of active material other than the coated active material of the present disclosure is not particularly limited, and may be selected from the active material particles described above.

From the viewpoint of increasing the capacity of the energy storage device, the content of the active material particles contained in the electrode is preferably 50% by mass or more of the entire electrode (excluding the current collector), and is 55% by mass or more. is more preferable, and 60% by mass or more is even more preferable.

From the viewpoint of the strength maintenance effect of the electrode by the binder, the content rate of the active material contained in the electrode (the content rate including the active material other than the coated active material of the present disclosure) is the total electrode (excluding the current collector) is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less.

(Conductivity aid)
If necessary, the electrodes may contain a conductive aid. Examples of conductive aids include carbon materials such as carbon black, carbon nanotubes, carbon nanofibers, fullerenes and carbon nanohorns, conductive oxides, and conductive nitrides.

When the electrode contains a conductive aid, the content is not particularly limited, and may be 1% by mass to 20% by mass of the entire electrode (excluding the current collector).

(current collector)
The electrode may be in a state in which a layer containing an active material, a binder, and optionally a conductive aid is formed on a current collector.
The type of current collector is not particularly limited, and metals or alloys such as aluminum, copper, nickel, titanium, and stainless steel can be used. The current collector may be carbon-coated, surface-roughened, or the like.

<Energy storage device>
The energy storage device of the present disclosure comprises the electrodes of the present disclosure as described above.
The type of energy storage device is not particularly limited. Examples thereof include devices such as lithium-ion batteries, sodium-ion batteries, and potassium-ion batteries, which utilize movement of alkali metal ions between electrodes for charging and discharging.

The energy storage device of the present disclosure is composed of a positive electrode, a negative electrode, an electrolytic solution, and the like. The energy storage device electrode described above may be a positive electrode or a negative electrode, but is preferably a negative electrode.

As the electrolyte used in energy storage devices, organic solvents, ionic liquids, etc., in which electrolytes are dissolved can be used. Examples of the ionic liquid include ionic liquids that are liquid at a temperature of less than 170° C., solvated ionic liquids, and the like.

Specific examples of electrolyte salts include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiClF ₄ , LiAsF ₆ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN(FSO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ , LiN( Lithium salts that generate poorly solvated anions such as C ₂ F ₅ SO ₂ ) ₂ , LiC(CF ₃ SO ₂ ) ₃ , LiCl, and LiI are included.
Only one electrolyte salt may be used, or two or more electrolyte salts may be used.

The electrolyte salt concentration in the electrolytic solution is, for example, preferably 0.3 mol or more, more preferably 0.5 mol or more, and even more preferably 0.8 mol or more per 1 L of the electrolytic solution.
The electrolyte salt concentration in the electrolytic solution is, for example, preferably 5 mol or less, more preferably 3 mol or less, and even more preferably 1.5 mol or less per liter of the electrolytic solution.

Specific examples of organic solvents include carbonates (propylene carbonate, ethylene carbonate, diethyl carbonate, etc.), lactones (γ-butyrolactone, etc.), chain ethers (1,2-dimethoxyethane, dimethyl ether, diethyl ether, etc.), Cyclic ethers (tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane, diglyme, triglyme, tetraglyme, etc.), sulfolanes (sulfolane, etc.), sulfoxides (dimethylsulfoxide, etc.), nitriles (acetonitrile, propionitrile, etc.) , benzonitrile, etc.), amides (N,N-dimethylformamide, N,N-dimethylacetamide, etc.), polyoxyalkylene glycols (diethylene glycol, etc.), and other aprotic solvents.
Only one kind of organic solvent may be used, or two or more kinds thereof may be used.

The cation moiety constituting the ionic liquid may be either an organic cation or an inorganic cation, but is preferably an organic cation.
Specific examples of organic cations constituting the ionic liquid include imidazolium cations, pyridinium cations, pyrrolidinium cations, phosphonium cations, ammonium cations, and sulfonium cations.

The anion part constituting the ionic liquid may be either an organic anion or an inorganic anion.
Specific examples of organic anions constituting the ionic liquid include alkyl sulfate anions such as methyl sulfate anion (CH ₃ SO ₄ ⁻ ) and ethyl sulfate anion (C ₂ H ₅ SO ₄ ⁻ ); tosylate anion (CH ₃ C ₆ H ₄ SO ₃ ⁻ ); alkanesulfonate anions such as methanesulfonate anion (CH ₃ SO ₃ ⁻ ), ethanesulfonate anion (C ₂ H ₅ SO ₃ ⁻ ), butanesulfonate anion (C ₄ H ₉ SO ₃ ⁻ ); romethanesulfonate anion (CF ₃ SO ₃ ⁻ ), pentafluoroethanesulfonate anion (C ₂ F ₅ SO ₃ ⁻ ), heptafluoropropanesulfonate anion (C ₃ H ₇ SO ₃ ⁻ ), nonafluorobutanesulfonate anion (C ₄ H ₉ _SO ₃ ⁻ ) ^and _other _{perfluoroalkanesulfonate} _anions _; N ⁻ ), nonafluoro-N-[(trifluoromethane)sulfonyl]butanesulfonylimide anion ((CF ₃ SO ₂ )(C ₄ F ₉ SO ₂ )N ⁻ ), N,N-hexafluoro-1,3-di perfluoroalkanesulfonylimide anions such as sulfonylimide anions (SO ₂ CF ₂ CF ₂ CF ₂ SO ₂ N ⁻ ); acetate anions (CH ₃ COO ⁻ ); hydrogen sulfate anions (HSO ₄ ⁻ );
Specific inorganic anions constituting the ionic liquid include bis(fluorosulfonyl)imide anion (N(SO ₂ F) ₂ ⁻ ); bis(trifluorosulfonyl)imide anion (N(SO ₂ CF ₃ ) ₂ ⁻ ). hexafluorophosphate anion (PF ₆ ⁻ ); tetrafluoroborate anion (BF ₄ ⁻ ); halide anions such as chloride ion (Cl ⁻ ), bromide ion (Br ⁻ ), iodide ion (I ⁻ ); tetrachloro aluminate anion (AlCl ₄ ⁻ ); thiocyanate anion (SCN ⁻ ); and the like.

Examples of ionic liquids include those composed of a combination of any of the above cation moieties and any of the above anion moieties.

Specific examples of the ionic liquid whose cation moiety is an imidazolium cation include 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-methyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium chloride, 1-butyl-3 -methylimidazolium chloride, 1-ethyl-3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium methanesulfonate, 1,2,3-trimethylimidazolium methylsulfate, methylimidazolium chloride, methylimidazolium Hydrogen sulfate, 1-ethyl-3-methylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hydrogen sulfate, 1-ethyl-3-methyl imidazolium tetrachloroaluminate, 1-butyl-3-methylimidazolium tetrachloroaluminate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, 1-ethyl-3-methyl imidazolium ethylsulfate, 1-butyl-3-methylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium thiocyanate, 1-ethyl-2,3-dimethylimidazolium Ethyl sulfate etc. are mentioned.

Specific examples of ionic liquids in which the cation portion is a pyrrolidinium cation include 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) ) imide and the like.

Solvated ionic liquids include glyme-lithium salt complexes and the like.
Specific examples of the lithium salt in the glyme-lithium salt complex include lithium bis(fluorosulfonyl)imide (LiN(SO ₂ F) ₂ , sometimes abbreviated as “LiFSI” in the present disclosure), lithium bis(trifluoro romethanesulfonyl)imide (LiN(SO ₂ CF ₃ ) ₂ , sometimes abbreviated as “LiTFSI” in the present disclosure), and the like.
Specific examples of glyme in the glyme-lithium salt complex include triethylene glycol dimethyl ether (CH ₃ (OCH ₂ CH ₂ ) ₃ OCH ₃ , triglyme), tetraethylene glycol dimethyl ether (CH ₃ (OCH ₂ CH ₂ ) ₄ OCH ₃ , tetraglyme) and the like.
A glyme-lithium salt complex can be prepared, for example, by mixing a lithium salt and glyme such that the lithium salt:glyme (molar ratio) is preferably from 10:90 to 90:10.

The electrolytic solution may contain additives. Specific examples of additives include fluoroethylene carbonate, propanesultone, vinylene carbonate, methanesulfonic acid, cyclohexylbenzene, tert-amylbenzene, adiponitrile, and succinonitrile.
The amount of the additive in the electrolytic solution is, for example, preferably 0.1% by mass to 30% by mass, preferably 0.5% by mass to 10% by mass, based on the total amount of the electrolytic solution.

The energy storage device may further comprise commonly used members such as separators, gaskets, sealing plates, and cases in addition to the electrodes and electrolyte.

The separator used in the energy storage device is not particularly limited, and examples include polyolefin-based porous membranes such as porous polypropylene nonwoven fabrics and porous polyethylene nonwoven fabrics.

The shape of the energy storage device can be any shape, such as cylindrical, square, and button.

There are no particular restrictions on the uses of the energy storage device. For example, as a distributed or portable battery, it can be used as a power source or auxiliary power source for electronic devices, electrical devices, automobiles, power storage, and the like.

<Method for producing coated active material for energy storage device>
A method for producing a coated active material for an energy storage device of the present disclosure comprises heating a composition containing polyacrylonitrile and particles containing a substance capable of occluding and releasing alkali metal ions at a temperature of 150° C. or more and less than 278° C. and an oxygen concentration of 5. A coated active material for an energy storage device, comprising, in this order, a step of heat-treating in an atmosphere of vol% to 30% by volume, and a step of heat-treating the composition in an atmosphere of 278°C to 600°C and an oxygen concentration of 4ppm to 100ppm. is a manufacturing method.

According to the above method, it is possible to efficiently produce a coated active material that has a coating containing cyclized polyacrylonitrile and that satisfies the above (1) or (2).
Hereinafter, the step of heat-treating the composition in an atmosphere of 150° C. or more and less than 278° C. is also referred to as “pretreatment”, and the step of heat-treating the composition in an atmosphere of 278° C. to 600° C. is also referred to as “cyclization treatment”.

In thermogravimetric-differential thermal analysis, polyacrylonitrile exhibits a large exothermic peak at 278°C. Therefore, by heat-treating polyacrylonitrile at 278 ° C. or higher, the cyclization reaction of polyacrylonitrile proceeds and the dehydrogenation reaction proceeds to form double bonds, and the electron conductivity of the obtained cyclized polyacrylonitrile increases. improves.
The temperature at which the cyclization treatment is performed is 278° C. or higher, preferably 280° C. or higher, more preferably 290° C. or higher, and even more preferably 300° C. or higher.

The temperature at which the cyclization treatment is performed is 600°C or lower, preferably 500°C or lower, more preferably 450°C or lower, and even more preferably 400°C or lower. By setting the temperature at which the cyclization treatment is carried out to 600° C. or lower, carbonization of polyacrylonitrile is suppressed, and flexibility of the obtained cyclized polyacrylonitrile is maintained.

From the viewpoint of obtaining a coated active material that satisfies (1) or (2), the oxygen concentration during cyclization is 4 ppm or more, preferably 7.5 ppm or more, and 10 ppm or more. is more preferable, and 15 ppm or more is even more preferable.

From the viewpoint of suppressing the decomposition of polyacrylonitrile, the oxygen concentration when performing the cyclization treatment is preferably 100 ppm or less, more preferably 80 ppm or less, further preferably 60 ppm or less, and 40 ppm or less. is even more preferred.
Components other than oxygen in the atmosphere in which the cyclization treatment is performed are not particularly limited, and may be nitrogen, an inert gas such as argon, or a mixture thereof.

The time for performing the cyclization treatment is not particularly limited, and can be selected, for example, from 3 hours to 15 hours.
The time for performing the cyclization treatment in the present disclosure means the time during which the temperature of the composition is between 278°C and 600°C.

The method of the present disclosure includes heat treating the composition at a temperature of 150° C. or more and less than 278° C. prior to the cyclization treatment.
From the results of thermogravimetric-differential thermal analysis, polyacrylonitrile shows a large exothermic peak at 278°C. By performing the pretreatment at a temperature lower than 278°C, the yield of the cyclization reaction of polyacrylonitrile can be increased. This is because the polyacrylonitrile undergoes a rapid reaction and the main chain of polyacrylonitrile is cleaved, thereby suppressing the production of low-molecular-weight components, as compared with the case where the cyclization treatment is performed without pretreatment.

The time for performing the pretreatment is not particularly limited, and can be selected, for example, from 3 hours to 15 hours.
The time during which the pretreatment is performed in the present disclosure means the time during which the temperature of the composition is 150°C or higher and lower than 278°C.

The atmosphere during the pretreatment is not particularly limited as long as it contains 5% to 30% by volume of oxygen. For example, pretreatment may be performed in air.

When the pretreatment and the cyclization treatment are performed in this order, the pretreatment and the cyclization treatment may or may not be performed consecutively. For example, a step of cooling the composition may be performed between the pretreatment and the cyclization treatment.

If necessary, the composition may contain a conductive aid, solvent, etc. Solvents include those capable of dissolving polyacrylonitrile, such as N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and dimethylsulfoxide.

The composition may be a mixture of active material particles and polyacrylonitrile, or may be obtained by polymerizing a monomer in a state in which the active material particles and a monomer that is a raw material of polyacrylonitrile are mixed.
Examples of monomers that are raw materials for polyacrylonitrile (homopolymer or copolymer) include acrylonitrile and polymerized components of the acrylonitrile copolymer described above.

The composition may or may not be particulate when subjected to cyclization.
If the composition is not particulate at the time of cyclization, a particulate coated active material can be obtained by subjecting the composition after cyclization to pulverization, pulverization, or the like. The method of pulverization or pulverization is not particularly limited, and known means such as a mortar, ball mill, bead mill, jet mill, vibrating mill, and mixer can be employed. After pulverization or pulverization, classification treatment may be performed as necessary.

The composition may be pressurized. By pressurizing the composition, a cyclization reaction occurs in a state where the polyacrylonitrile molecules are oriented, and the molecules stack to increase the crystallinity. When polyacrylonitrile is highly crystallized, the strength of the obtained cyclized polyacrylonitrile tends to improve, and the electron conductivity also tends to improve.
The method of applying pressure to the composition is not particularly limited. A method of sandwiching between members and applying surface pressure (for example, 0.1 MPa to 10 MPa) can be used. The pressure treatment may be performed before the cyclization treatment, during the cyclization treatment, or after the cyclization treatment.

<Covering material>
The coating material of the present disclosure is a coating material for coating active material particles for an energy storage device, comprising cyclized polyacrylonitrile and satisfying either or both of the following (1) or (2): It is wood.
(1) The yellowness of the electrolytic solution in which the coating material is immersed is 100 or less as defined in JIS K 7373:2006.
(2) The haze defined by JIS K 7136:2000 of the electrolytic solution in which the coating material is immersed is 2.5% or less.

An energy storage device produced using a coated active material containing a coating material that contains cyclized polyacrylonitrile and satisfies the conditions of (1) or (2) exhibits excellent initial characteristics.

The yellowness and haze of the coating material can be measured in the same manner as the yellowness and haze of the coated active material described above.
That is, the yellowness and haze of the coating material can be measured by immersing in the electrolyte solution a coating material having the same mass as that of the coating material contained in the coated active material to be immersed in the electrolyte solution.

The details and preferred aspects of the coating material are the same as the details and preferred aspects of the coating material contained in the coated active material described above.

The present disclosure will be described in more detail below based on examples, but the present disclosure is not limited to the following examples.

<Example 1>
As polyacrylonitrile (PAN), an acrylonitrile homopolymer (manufactured by Aldrich, Mw 150,000, molecular weight distribution 2.21, atactic type) was added to N-methyl-2-pyrrolidone (NMP) and mixed at room temperature to dissolve PAN. , a PAN/NMP solution (PAN content: 10% by mass) was prepared.
As the active material particles, Si particles (manufactured by Beijing Dadi, average secondary particle diameter: about 5 μm) were used.

(Preparation of coated active material)
The Si particles and the PAN/NMP solution were mixed so that the mass ratio of the Si particles and the PAN (Si:PAN) was 70:30, and a slurry A was obtained. Slurry A was placed in a glass petri dish and dried at 80° C. for 2 hours.
The dried slurry was placed in an alumina crucible and subjected to heat treatment (pretreatment) in the air at 220° C. for 7 hours. Then, heat treatment (cyclization treatment) was performed at 350° C. for 5 hours in a nitrogen atmosphere with an oxygen concentration of 20 ppm.
The cyclized material was pulverized in an agate mortar and sieved through a 390-mesh sieve to obtain a coated active material in which Si particles were coated with cyclized polyacrylonitrile. The volume average particle diameter (D50) of the coated active material was 18.1 μm, and the BET specific surface area was 5.1 m ² /g.

(Yellowness of electrolyte)
The prepared coated active material (mass of coating material: 10 mg) was immersed in the electrolytic solution, and the yellowness of the electrolytic solution was measured by the method described above. Table 1 shows the results.

(Electrolyte haze)
The prepared coated active material (mass of binder: 10 mg) was immersed in the electrolytic solution, and the haze of the electrolytic solution was measured by the method described above. Table 1 shows the results.

(Production and evaluation of battery)
The coating active material, acetylene black (Li400 manufactured by Denka) as a conductive aid, and polyvinylidene fluoride as a binder were mixed at a mass ratio of 85:5:10 (coating active material: conductive aid: binder). mixed with. Specifically, mixing was performed 5 times for 2 minutes at 1000 rpm (rotation/minute) using a rotation/revolution mixer. In order to adjust the viscosity, N-methyl-2-pyrrolidone was appropriately added and slurried.

The resulting slurry was applied to a copper foil (20 μm thick) as a current collector and dried to obtain an electrode. The thickness of the electrode was 3.1 mAh/cm ² in terms of capacity (calculated with the capacity of Si being 3600 mAh/g). The electrodes were pressed with a roll press to an electrode density of 1.6 g/cm ³ .

LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as a positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder at a ratio of 94:3:3 (active material: conductive agent: binding agent). Material) was mixed at a mass ratio, and the composition was applied to a current collector to prepare a positive electrode. The thickness of the composition layer was 2.9 mAh/cm ² in terms of capacity (calculated based on the capacity of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ being 152 mAh/g). The positive electrode was pressed with a roll press to an electrode density of 2.8 g/cm ³ .
Using the electrode prepared above as the negative electrode and the positive electrode, a laminate type battery was prepared. A porous polypropylene film was used as the separator, and 1M LiPF ₆ dissolved in a mixed solvent containing EC, EMC and DEC in a ratio of 1:1:1 (volume ratio) was used as the electrolyte.

After performing 3 cycles of charging and discharging at a rate of 0.1 C (current for full charge and full discharge over 10 hours) for the produced battery, 0.5 C (full charge and full discharge for 2 hours) 100 cycles of charging and discharging were performed at a rate of 100 cycles. The discharge capacity after one cycle at 0.5 C (capacity conversion on the positive electrode side) and the capacity retention rate after 100 cycles (the value obtained by dividing the discharge capacity after 100 cycles by the discharge capacity after one cycle x 100) are shown. 1.

<Example 2>
The coated active material was prepared in the same manner as in Example 1 except that the pretreatment conditions were 220° C. for 10 hours in air, and the cyclization treatment conditions were 300° C. for 8 hours in a nitrogen atmosphere with an oxygen concentration of 20 ppm. was produced and evaluated. Table 1 shows the results.

<Example 3>
A PAN/NMP solution (PAN content: 10% by mass) was prepared in the same manner as in Example 1 except that an acrylonitrile homopolymer (Mw 5158, molecular weight distribution 1.92) was used as PAN. A coated active material was prepared and evaluated in the same manner as in Example 1, except that this PAN/NMP solution was used. Table 1 shows the results.

<Example 4>
A PAN/NMP solution (PAN-containing rate: 10% by mass) was prepared. A coated active material was prepared and evaluated in the same manner as in Example 1, except that this PAN/NMP solution was used. Table 1 shows the results.

<Example 5>
PAN/ An NMP solution (PAN content: 10% by mass) was prepared. A coated active material was prepared and evaluated in the same manner as in Example 1, except that this PAN/NMP solution was used. Table 1 shows the results.

<Example 6>
A copolymer (Mw 272353, molecular weight distribution 2.18) of acrylonitrile (93.9% by mass), methyl acrylate (5.8% by mass) and sodium methallylsulfonate (0.3% by mass) was used as PAN. A PAN/NMP solution (PAN content: 10% by mass) was prepared in the same manner as in Example 1 except that. A coated active material was prepared and evaluated in the same manner as in Example 1, except that this PAN/NMP solution was used. Table 1 shows the results.

<Example 7>
A PAN/NMP solution (PAN content: 10% by mass) was prepared in the same manner as in Example 1 except that an acrylonitrile homopolymer (Mw 728949, molecular weight distribution 1.704) was used as PAN. A coated active material was prepared and evaluated in the same manner as in Example 1, except that this PAN/NMP solution was used. Table 1 shows the results.

<Comparative Example 1>
A coated active material was prepared and evaluated in the same manner as in Example 1, except that the pretreatment conditions were 220° C. for 7 hours in an argon atmosphere. Table 1 shows the results.

<Comparative Example 2>
A coated active material was prepared and evaluated in the same manner as in Example 1 except that no pretreatment was performed and the cyclization treatment conditions were 300° C. for 10 hours in a nitrogen atmosphere with an oxygen concentration of 1 ppm. Table 1 shows the results.

<Comparative Example 3>
A coated active material was prepared and evaluated in the same manner as in Example 1, except that the pretreatment was not performed and the conditions for the cyclization treatment were 300° C. and 4 hours in vacuum. Table 1 shows the results.

<Comparative Example 4>
A coated active material was prepared and evaluated in the same manner as in Example 1, except that pretreatment and cyclization treatment were not performed (only drying at 80° C. was performed). Table 1 shows the results.

<Comparative Example 5>
A coated active material was prepared and evaluated in the same manner as in Example 1 except that no pretreatment was performed and the cyclization treatment was performed in a nitrogen atmosphere with an oxygen concentration of 1 ppm at 300° C. for 8 hours. Table 1 shows the results.

<Comparative Example 6>
A coated active material was produced and evaluated in the same manner as in Example 1, except that no pretreatment was performed and the conditions for cyclization treatment were 300° C. and 8 hours in vacuum. Table 1 shows the results.

"OK" in Table 1 means that the yellowness or haze satisfies the condition (1) or (2).
As shown in Table 1, the examples using the coated active material containing cyclized polyacrylonitrile and satisfying the condition (1) or (2) had a high capacity retention rate after 100 cycles, and the cycle characteristics were evaluated. is good.

The disclosure of International Application No. PCT/JP2021/019461 is hereby incorporated by reference in its entirety.
All publications, patent applications and technical standards mentioned herein are to the same extent as if each individual publication, patent application and technical standard were specifically and individually noted to be incorporated by reference. incorporated herein by reference.

Claims

A coated active material for an energy storage device, comprising particles containing a substance capable of absorbing and releasing alkali metal ions, and a coating material containing cyclized polyacrylonitrile, and satisfying the following (1).
(1) The yellowness of the electrolytic solution in which the coated active material is immersed is 100 or less as defined in JIS K 7373:2006.
A coated active material for an energy storage device, comprising particles containing a substance capable of absorbing and releasing alkali metal ions, and a coating material containing cyclized polyacrylonitrile, and satisfying the following (2).
(2) The haze defined by JIS K 7136:2000 of the electrolytic solution in which the coated active material is immersed is 2.5% or less.
The coated active material for an energy storage device according to claim 1 or 2, wherein the substance capable of absorbing and releasing alkali metal ions contains silicon atoms.
The coated active material for energy storage devices according to any one of claims 1 to 3, which has a volume average particle diameter (D50) of 1 μm to 50 μm.
The coated active material for an energy storage device according to any one of claims 1 to 4, which has a BET specific surface area of 0.5 m 2 /g to 100 m 2 /g.
The coated active material for an energy storage device according to any one of claims 1 to 5, wherein the coating material accounts for 0.1% by mass to 50% by mass of the entire coated active material.
The coated active material for an energy storage device according to any one of claims 1 to 6, further comprising a coating made of a carbon material.
An energy storage device comprising the coated active material for an energy storage device according to any one of claims 1 to 7.
The energy storage device according to claim 8, comprising an electrolytic solution, the electrolytic solution containing an ionic liquid as a solvent.
The energy storage device according to claim 8, wherein the concentration of electrolyte salt in the electrolytic solution is 3 mol/L or more.
a step of heat-treating a composition containing particles containing a substance capable of absorbing and releasing alkali metal ions and polyacrylonitrile in an atmosphere of 150° C. or more and less than 278° C. and an oxygen concentration of 5% by volume to 30% by volume; and heat-treating the composition in an atmosphere of 278° C. to 600° C. and an oxygen concentration of 4 ppm to 100 ppm, in this order.
The method for producing a coated active material for an energy storage device according to claim 11, wherein each of the heat treatments is performed for 3 to 15 hours.
A coating material for coating active material particles for an energy storage device, comprising cyclized polyacrylonitrile, wherein the electrolytic solution in which the coating material is immersed has a yellowness of 30% or less as defined in JIS K 7373:2006. , covering material.
A coating material for coating active material particles for an energy storage device, which contains cyclized polyacrylonitrile, and the electrolyte in which the coating material is immersed has a haze of 0.3% as defined in JIS K 7136:2000. A coating that is: