WO2024053110A1

WO2024053110A1 - Electrode material, electrode for energy storage devices, and energy storage device

Info

Publication number: WO2024053110A1
Application number: PCT/JP2022/033953
Authority: WO
Inventors: 学平澤
Original assignee: 株式会社レゾナック
Priority date: 2022-09-09
Filing date: 2022-09-09
Publication date: 2024-03-14

Abstract

An electrode material containing composite particles that include: particles which include a substance capable of occluding and releasing alkali metal ions; and a coating portion which covers at least a portion of the surfaces of the particles and includes a polymer material and a conduction aid.

Description

Electrode materials, electrodes for energy storage devices and energy storage devices

The present disclosure relates to electrode materials, electrodes for energy storage devices, and energy storage devices.

Energy storage devices in which charging and discharging are performed by moving alkali metal ions such as lithium ions between a positive electrode and a negative electrode are widely used. The positive and negative electrodes of such energy storage devices generally include particles of a material (active material) capable of intercalating and deintercalating alkali metal ions.

Since the volume of the active material changes as it absorbs and releases alkali metal ions, repeated charging and discharging will cause cracks in the active material and cause deterioration of the electrode. Therefore, coating the surface of the active material particles with a flexible material such as a polymer compound is considered to be an effective measure for suppressing electrode deterioration.

As for active material particles whose surfaces are coated with a polymer material, for example, Japanese Patent Publication No. 2019-535116 describes coating active material particles with cyclized polyacrylonitrile.

As a result of studies conducted by the present inventors, it was found that there is room for improvement in cycle characteristics of energy storage devices manufactured using electrodes containing active material particles coated with a polymer material.
In view of the above circumstances, an object of the present disclosure is to provide an electrode material, an electrode for an energy storage device, and an energy storage device that have excellent cycle characteristics.

Means for solving the above problems include the following embodiments.
<1> An electrode comprising composite particles including particles containing a substance capable of occluding and releasing alkali metal ions, and a coating portion containing a polymeric material and a conductive agent that coats at least a portion of the surface of the particles. material.
<2> The electrode material according to <1>, wherein the coating portion has a coverage rate of 50% or more of the surface of the particles.
<3> The electrode material according to <1>, wherein the content of the conductive aid in the coating portion is 30% by mass or more.
<4> The electrode material according to <1>, wherein the polymer material includes at least one selected from polyamideimide and polyacrylonitrile.
<5> The electrode material according to <1>, wherein the particles contain silicon.
<6> The electrode material according to <1>, wherein the conductive support agent includes particles having an aspect ratio of 10 or more.
<7> An electrode for an energy storage device, comprising the electrode material according to any one of <1> to <6>.
<8> An energy storage device comprising the electrode for an energy storage device according to <7>.

According to the present invention, an electrode material, an electrode for an energy storage device, and an energy storage device with excellent cycle characteristics are provided.

1 is a SEM image of composite particles produced in Example 1. 1 is a cross-sectional SEM image of composite particles produced in Example 1. 3 is a SEM image of composite particles produced in Example 2.

Hereinafter, modes for carrying out the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, the constituent elements (including elemental steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, and they do not limit the present disclosure.

In this disclosure, the term "step" includes not only a step that is independent from other steps, but also a step that cannot be clearly distinguished from other steps, as long as the purpose of the step is achieved. .
In the present disclosure, numerical ranges indicated using "~" include the numerical values written before and after "~" as minimum and maximum values, respectively.
In the numerical ranges described step by step in this disclosure, the upper limit or lower limit described in one numerical range may be replaced with the upper limit or lower limit of another numerical range described step by step. . Furthermore, in the numerical ranges described in this disclosure, the upper limit or lower limit of the numerical range may be replaced with the values shown in the Examples.
In the present disclosure, each component may contain multiple types of corresponding substances. If 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.
In the present disclosure, each component may include a plurality of types of particles. When a plurality of types of particles corresponding to each component are present in the composition, the particle diameter of each component means a value for a mixture of the plurality of types of particles present in the composition, unless otherwise specified.
In this disclosure, the term "layer" includes not only the case where the layer is formed in the entire area when observing the area where the layer exists, but also the case where the layer is formed only in a part of the area. included.

<Electrode material>
The electrode material of the present disclosure includes particles containing a substance capable of occluding and releasing alkali metal ions (hereinafter also referred to as active material particles), a polymer material and a conductive additive that coat at least a portion of the surface of the particles. An electrode material comprising: a coating portion comprising a composite particle comprising a coating portion comprising a coating portion comprising a composite particle comprising a coating portion comprising a coating portion comprising a composite particle comprising a coating portion comprising a coating portion comprising a coating portion comprising a coating portion comprising a composite particle comprising a coating portion comprising a coating portion;

As a result of studies conducted by the present inventors, an energy storage device fabricated using composite particles, in which at least a portion of the surface of active material particles is coated with a coating portion containing a polymeric material and a conductive agent, as an electrode material. , it was found that it exhibited excellent cycle characteristics. The reason for this is thought to be, for example, as follows.

The active material particles contained in the composite particles usually undergo deterioration due to repeated expansion and contraction due to occlusion and release of alkali metal ions.
When at least a portion of the surface of the active material particles is coated with a material containing a polymeric material, the electrolytic solution resistance and mechanical strength of the active material particles are improved, and deterioration due to expansion and contraction of the active material particles is suppressed. On the other hand, the conduction paths of alkali metal ions and electrons between the active material particles are blocked by the coating, causing deterioration in cycle characteristics. It is thought that by including the conductive additive in addition to the polymeric material in the coating, a sufficient conduction path for alkali metal ions and electrons between the active material particles is ensured, resulting in good cycle characteristics.
Furthermore, in an electrode formed using active material particles coated with a coating containing a polymer material and a conductive additive, when comparing the vicinity of the surface of the active material particle with the other part, There is more conductive aid near the surface. For this reason, compared to conventional electrodes made by dispersing active material particles and conductive aids in a binder, a sufficient conduction path for alkali metal ions and electrons between active material particles is ensured, resulting in better performance. It is thought that excellent cycle characteristics can be obtained.

The entire surface of the active material particles may be covered with the coating portion, or a portion of the surface may be covered with the coating portion.
From the viewpoint of improving cycle characteristics, the coverage of the surface of the active material particles by the coating portion (hereinafter also referred to as coverage of composite particles) is preferably 50% or more, more preferably 60% or more, More preferably, it is 70% or more. The coverage of the composite particles may be 100% or less, 90% or less, or 80% or less.

In the present disclosure, the coverage of composite particles is a value measured by an image analysis method. Specifically, an image of the composite particles is acquired, and the coverage is calculated using the following formula.
Coverage rate (%) = (total area of region where coating portion exists/area of entire composite particle) x 100
If the values of coverage calculated for individual composite particles are different, the average value of the values calculated for 100 particles is taken as the coverage of the surface of the composite particle.
The coverage may be calculated using known image processing software such as ImageJ.

The content of the conductive aid contained in the coating is not particularly limited.
From the perspective of ensuring sufficient conduction paths for alkali metal ions and electrons between active material particles, the content of the conductive aid is preferably 30% by mass or more of the entire coating, and should be 40% by mass or more. is more preferable, and even more preferably 45% by mass or more.
From the viewpoint of suppressing deterioration due to expansion and contraction of active material particles, the content of the conductive additive is preferably 80% by mass or less of the entire coating, more preferably 70% by mass or less, and 65% by mass or less. % or less is more preferable.

The content of the polymer material contained in the covering portion is not particularly limited.
From the viewpoint of suppressing deterioration due to expansion and contraction of active material particles, the content of the polymer material is preferably 20% by mass or more of the entire coating, more preferably 30% by mass or more, and 35% by mass or more. % or more is more preferable.
From the viewpoint of ensuring sufficient conduction paths for alkali metal ions and electrons on the surface of the active material particles, the content of the polymer material is preferably 70% by mass or less of the entire coating, and is 60% by mass or less. It is more preferably 55% by mass or less.

The covering portion may contain only the polymeric material and the conductive aid, or may contain components other than the polymeric material and the conductive aid.
Components other than the polymer material and the conductive aid include a silane coupling agent and the like. By including the silane coupling agent, for example, the binding force of the coating portion to the active material particles can be increased.
When the covering part contains components other than the polymeric material and the conductive aid, the total proportion of the polymeric material and the conductive aid to the entire covering part is preferably 80% by mass or more, and should be 85% by mass or more. is more preferable, and even more preferably 90% by mass or more.

The proportion of the coating portion in the entire composite particle is preferably 0.1% by mass to 50% by mass, more preferably 1% by mass to 20% by mass, and 5% by mass to 10% by mass. is even more preferable.
When the proportion of the coated portion is 0.1% by mass or more of the entire composite particle, a sufficient effect of improving cycle characteristics can be obtained. When the ratio of the coating portion is 50% by mass or more of the entire composite particle, a sufficient capacity of the energy storage device is ensured.
The ratio of the coated part to the whole composite particle is determined by, for example, the change in mass before and after the treatment when the composite particle is heat-treated at a temperature at which the coated part thermally decomposes, or the mass before and after the treatment when the coated part is dissolved in a solvent. It can be calculated from the change.

The composite particles may contain only one type of active material particles or two or more types of active material particles.
The coating portion of the composite particle may contain only one kind of polymer material and conductive aid, or two or more kinds thereof.

The volume average particle diameter (D50) of the composite particles measured by a laser scattering diffraction method is preferably 1 μm to 50 μm, more preferably 3 μm to 30 μm.
When the volume average particle diameter of the composite particles is 1 μm or more, it becomes easy to prepare a slurry for forming an electrode. When the volume average particle diameter of the composite 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 diameter of the composite particles is measured by laser scattering diffraction method. Specifically, in the volume-based particle size distribution obtained by the laser scattering diffraction method, the particle size when the accumulation from the small diameter side is 50% is defined as the volume average particle size.

(Method for producing composite particles)
The method for producing composite particles is not particularly limited. For example, by mixing a predetermined amount of active material particles, a polymer material, and a conductive additive with a solvent, drying the resulting mixture, subjecting it to heat treatment as necessary, and crushing it, the surface of the active material particles can be improved. Composite particles can be obtained that are at least partially coated with a coating containing a polymeric material and a conductive additive.
The composite particles may be in a state in which individual active material particles are each covered with a coating portion, or may be in a state in which an aggregate of a plurality of active material particles is covered with a coating portion.

(active material particles)
The active material particles included in the composite particles are not particularly limited as long as they contain a substance (active material) that can occlude and release alkali metal ions.
Examples of alkali metal ions include lithium ions, potassium ions, sodium ions, and the like. Among these, lithium ions are preferred.
The active materials contained in the active material particles may be one type or a combination of two or more types.

Examples of the active material of the positive electrode include lithium transition metal compounds such as lithium transition metal oxides and lithium transition metal phosphates.
Examples of lithium transition metal oxides include compounds containing one or more transition metals such as Mn, Ni, and Co, and a portion of the transition metals contained in these compounds in combination with one or more other transition metals. Examples include lithium transition metal oxides substituted with transition metals or metal elements (typical elements) such as Mg and Al.
Examples of the active material of the negative electrode include carbon materials, active materials containing silicon atoms, and the like.
Examples of carbon materials include graphite, hard carbon, and soft carbon.
Examples of active materials containing silicon atoms include Si (metallic silicon), silicon oxide represented by SiOx (0.8≦x≦1.5), and the like.
The silicon oxide may have a structure in which nanosilicon 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 become a semiconductor.

The active material particles may be made of a carbon material and silicon may be present on the surface of the active material particles.
Examples of methods for causing silicon to be present on the surface of active material particles made of a carbon material include vapor deposition, plasma CVD (Chemical Vapor Deposition), and the like. The plasma CVD method may be performed by decomposing raw materials such as silane and chlorosilane.

Although active materials containing silicon atoms have a large theoretical capacity and are expected to contribute to increasing the capacity of energy storage devices, they undergo large volume changes during charging and discharging and are susceptible to deterioration. Furthermore, active materials containing silicon atoms do not themselves have electronic conductivity.
In the present disclosure, electron conductivity can be imparted to the active material particles by coating the surfaces of the active material particles with a coating portion containing a conductive additive.
Therefore, it is particularly suitable when the active material particles contain silicon atoms.

The shape of the active material particles is not particularly limited. For example, they may be spherical particles, scaly particles, lumpy particles, secondary particles composed of a plurality of primary particles, or the like.

The particle size of the active material particles is not particularly limited.
For example, the volume average particle diameter (D50) of the active material particles is preferably 1 μm to 50 μm, more preferably 3 μm to 30 μm. When the volume average particle diameter of the active material particles is 1 μm or more, it becomes easy to prepare a slurry for forming an electrode. When the volume average particle diameter of the 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 diameter of the active material particles is measured by laser scattering diffraction method. Specifically, in the volume-based particle size distribution obtained by the laser scattering diffraction method, the particle size when the accumulation from the small diameter side is 50% is defined as the volume average particle size.

When the active material particles are secondary particles, the volume average particle diameter is the volume average particle diameter of the secondary particles.
In the present disclosure, a "secondary particle" refers to a particle that is formed by agglomeration of a plurality of primary particles and is the smallest unit in normal behavior, and a "primary particle" refers to a particle that cannot exist alone. It means the smallest possible particle.

When the active material particles are secondary particles, the particle diameter of the primary particles constituting the secondary particles is not particularly limited. For example, the average primary particle diameter is preferably 10 nm to 50 μm. More preferably, the thickness is 30 nm to 10 μm. When the average primary particle diameter of the active material particles is 10 nm or more, the influence of a natural oxide film formed on the surface can be suppressed. When the average primary particle diameter of the active material particles is 50 μm or less, deterioration due to charging and discharging is suppressed.

In the present disclosure, the primary particle diameter of active material particles means the major diameter of the primary particles observed with a scanning electron microscope. Specifically, when the primary particle is spherical, it means its maximum diameter, and when the primary particle is plate-like, it means the maximum diameter or maximum diagonal length in a projected image of the particle observed from the thickness direction. The "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.

The method for adjusting the particle size of the active material particles is not particularly limited. Examples include a method of selecting raw materials, a method of adjusting grinding conditions, a method of vapor deposition, a plasma method, a method of surface treatment with silane, etc.

The BET specific surface area of the 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 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 active material particles is 100 m ² /g or less, handling properties during electrode production are excellent.
The BET specific surface area of the active material particles can be calculated from the nitrogen adsorption isotherm at -196°C.

The electrode material of the present disclosure may include active material particles that are in the state of composite particles and active material particles that are not in the state of composite particles.
For example, the electrode material consists of active material particles that contain silicon atoms and are covered with a coating that includes a polymeric material and a conductive agent, and a carbon material that is not covered with a coating that includes a polymeric material and a conductive agent. The active material particles may also be included.

(polymer material)
The polymer material contained in the covering portion is not particularly limited.
Specific examples of polymeric materials include polyamideimide (PAI), polyacrylonitrile, cyclized polyacrylonitrile, polyamide, polyimide, polyacrylic acid, polymethacrylic acid, polystyrene, polyvinyl chloride, polyvinylidene chloride, polyacrylate, and polymethacrylate. , polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, etc.; rubbery polymers such as SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber); polyvinylidene fluoride (PVdF), polytetra Examples include fluorine-based polymers such as fluoroethylene and fluorinated polyvinylidene fluoride; polymers having ion conductivity;

From the viewpoint of electrolyte resistance and mechanical strength, polyamideimide, polyacrylonitrile, and cyclized polyacrylonitrile are preferable as the polymer material.

Cyclized polyacrylonitrile is obtained by cyclizing polyacrylonitrile as a raw material. Specifically, cyclized polyacrylonitrile has a structure in which at least some of the nitrile groups (-C≡N) are ring-closed to become -C=N- groups. By cyclizing polyacrylonitrile as a raw material, it is possible to impart electronic conductivity to polyacrylonitrile.
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 group, the closer the cyclized polyacrylonitrile can be said to have properties to those of carbon.

Cyclized polyacrylonitrile is produced by not only ring-closing the -C≡N (nitrile) group to become a -C=N- group in the cyclization process of raw material polyacrylonitrile, but also by a dehydrogenation reaction, for example, the main It is preferable that the -CH ₂ -CH- group constituting the chain is changed to a -CH═C- group or the like to form a double bond. When both the cyclization reaction and the dehydrogenation reaction occur, a conjugated double bond is formed, which tends to improve electronic conductivity.

The presence of a -C=N- group formed by ring closure of a nitrile group in the cyclized polyacrylonitrile can be confirmed by, for example, infrared spectroscopy. Infrared spectroscopy may be a transmission method or a reflection method.
In infrared spectroscopy, the -C≡N (nitrile) group shows a peak at 2240 cm ^-1 to 2243 cm ^-1 , and the ring-closed -C=N- group shows a peak at 1577 cm ^-1 to 1604 cm ^-1 . -CH ₂ - before becoming a double bond can be confirmed as a peak at 2939 cm ^-1 , and -CH=C- group after becoming a double bond by dehydrogenation can be confirmed as a peak at 806 cm ^-1 .
The closed -C=N- group shifts in the range of 1577 cm ^-1 to 1604 cm ^-1 depending on the structure of the six-membered ring formed. Specifically, as the number of double bonds included in the six-membered ring structure increases, the bond distance of -C=N- becomes shorter and tends to shift to the lower wave number side.

The attribution of the above peak is 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 st abilization and low temperature carbonization (Polymer Degradation and Stability, Volume 128, June 2016, 39-45).

Polyacrylonitrile may be a homopolymer of acrylonitrile or a copolymer of acrylonitrile and a polymerization component other than acrylonitrile. Polymerization components other than acrylonitrile include acrylic acid, vinyl acetate, styrene, vinylidene chloride, vinyl chloride, methacrylic acid, and the like.

When polyacrylonitrile is a copolymer of acrylonitrile and a polymerization component other than acrylonitrile, the proportion of acrylonitrile in the total polymerization component is preferably 70% by mass or more, more preferably 80% by mass or more, and 90% by mass or more. More preferably, it is at least % by mass.

(Conductivity aid)
The conductive aid contained in the covering portion is not particularly limited.
Specific examples of the conductive aid include carbon materials such as carbon black, carbon fiber, carbon nanotubes, fullerenes, and carbon nanohorns, oxides exhibiting conductivity, and nitrides exhibiting conductivity.

From the viewpoint of improving cycle characteristics, the coating part preferably contains particles with an aspect ratio of 10 or more as a conductive agent, more preferably contains particles with an aspect ratio of 20 or more, and preferably contains particles with an aspect ratio of 30 or more. It is even more preferable to include.
If the covering part contains particles with an aspect ratio of 10 or more as a conductive agent, even if the active material particles repeatedly expand and contract, the conduction path of alkali metal ions and electrons between the active material particles formed by the conductive agent will be maintained. is less likely to be lost, and good cycle characteristics can be obtained.
The aspect ratio of the particles may be 500 or less, 250 or less, or 100 or less.

The shape of particles having an aspect ratio of 10 or more is not particularly limited, and may be fibrous, tubular, rod-like, columnar, or the like.
From the viewpoint of improving cycle characteristics, the length of the long axis of particles having an aspect ratio of 10 or more is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.

When the coating portion contains particles with an aspect ratio of 10 or more as a conductive aid, the proportion of the particles with an aspect ratio of 10 or more in the entire conductive aid is not particularly limited. For example, it may be within the range of 1% by mass to 50% by mass.

From the viewpoint of improving cycle characteristics, it is preferable that the coating portion contains particles having an aspect ratio of 10 or more and particles having an aspect ratio of less than 10 as a conductive aid.
When the coating portion includes particles having an aspect ratio of less than 10, the adhesion of the conductive additive to the surface of the active material particles is improved.
Examples of particles having an aspect ratio of less than 10 include carbon black.

<Electrodes for energy storage devices>
The electrode for an energy storage device (hereinafter also referred to as electrode) of the present disclosure includes the electrode material of the present disclosure described above.
If necessary, the electrode may further contain a binder, a conductive aid, and the like.

When the electrode contains a binder, the type of binder is not particularly limited, and may be selected from the polymer materials used in the electrode material of the present disclosure described above.
When the electrode contains a conductive aid, the type of the conductive aid is not particularly limited, and may be selected from the conductive aids used in the electrode material of the present disclosure described above.

When the electrode includes a binder and a conductive aid (excluding the conductive aid contained in the coating portion of the composite particle), the proportion of the conductive aid in the total of the binder and the conductive aid is not particularly limited. , for example, from the range of 1% by mass to 20% by mass.
The above proportion of the conductive aid may be smaller than the proportion of the conductive aid in the coating of the composite particles. That is, the electrode of the present disclosure may be in a state where more conductive aid exists near the surface of the active material particles when comparing the surface vicinity of the active material particles with the other portions.
Since the electrode material of the present disclosure includes active material particles coated with a conductive aid, the electrode may not further include a conductive aid.

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

From the viewpoint of maintaining the strength of the electrode, the content of the active material contained in the electrode is preferably 95% by mass or less of the entire electrode (excluding the current collector), and more preferably 90% by mass or less. The content is preferably 80% by mass or less, and more preferably 80% by mass or less.

The electrode may be in a state in which a layer containing an electrode material is formed on the current collector.
The type of current collector is not particularly limited, and examples include metals or alloys such as aluminum, copper, nickel, titanium, and stainless steel. 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 electrode of the present disclosure described above.
The type of energy storage device is not particularly limited. Examples include devices that utilize movement of alkali metal ions between electrodes for charging and discharging, such as lithium ion batteries, sodium ion batteries, and potassium ion batteries.

The energy storage device of the present disclosure is comprised of a positive electrode, a negative electrode, an electrolyte, and the like. The above-mentioned electrode for an energy storage device may be a positive electrode or a negative electrode, but is preferably a negative electrode.

As the electrolytic solution used in the energy storage device, an organic solvent in which an electrolyte is dissolved, an ionic liquid, etc. 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.

Specifically, the 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( Examples include lithium salts that generate anions that are difficult to solvate, such as C ₂ F ₅ SO ₂ ) ₂ , LiC(CF ₃ SO ₂ ) ₃ , LiCl, and LiI.
The electrolyte salt may be used alone or in combination of two or more.

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 still more preferably 0.8 mol or more per liter of 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 still more preferably 1.5 mol or less per liter of electrolytic solution.

Specifically, the 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.), and polyoxyalkylene glycols (diethylene glycol, etc.).
The organic solvent may be used alone or in combination of two or more.

The electrolyte may contain additives. Specific examples of the additive include fluoroethylene carbonate, propane sultone, 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, and preferably 0.5% by mass to 10% by mass, based on the total electrolytic solution.

In addition to the electrodes and electrolyte, the energy storage device may further include commonly used members such as a separator, gasket, sealing plate, and case.

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

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

The use of the energy storage device is not particularly limited. For example, it can be used as a power source or auxiliary power source for electronic devices, electric devices, automobiles, power storage devices, etc.

Hereinafter, the present disclosure will be described in more detail based on Examples, but the present disclosure is not limited to the Examples below.

<Example 1>
A dispersion liquid in which carbon black (CB) was dispersed in NMP and a dispersion liquid in which VGCF was dispersed in NMP were kneaded to obtain a conductive additive slurry. A conductive additive slurry was added to an SiO slurry obtained by kneading SiO particles and an NMP solution of polyamideimide (PAI), and the mixture was kneaded. The resulting mixture was poured into an alumina container and dried at 120°C under air to remove the solvent. Thereafter, heat treatment was performed at 350° C. for 5 hours in a nitrogen atmosphere. The obtained heat-treated product was crushed and passed through a sieve to obtain the desired composite particles. The mass ratio of each component contained in the composite particles is shown in the following formula.
SiO:PAI:CB:VGCF=90:5:4:1

A SEM image of the produced composite particles is shown in FIG. As shown in FIG. 1, it was observed that carbon black and fibrous VGCF were attached to the surface of the SiO particles. The coverage of the composite particles calculated by image analysis was 50% or more.
FIG. 2 shows a cross-sectional SEM image of the composite particles produced. As shown by the arrow in FIG. 2, it was observed that fibrous VGCF bridged between the SiO particles.

(Preparation of evaluation battery)
The obtained composite particles, graphite particles, and a 1% aqueous solution of carboxymethylcellulose (CMC) were uniformly mixed, and an aqueous dispersion of styrene butadiene rubber (SBR) was added dropwise to obtain a slurry for a negative electrode. The mass ratio (SiO:graphite) of composite particles and graphite particles contained in the slurry was 12:88. The obtained slurry was applied to the surface of the current collector using a coater and dried to obtain a negative electrode with the desired coating amount (80 g/m ² ). The density was set at 1.6 g/cm ³ using a press. The mass ratio of the active materials (composite particles and graphite particles), SBR, and CMC contained in the negative electrode is shown in the following formula.
Active material: SBR:CMC=97:1.5:1.5

A slurry for a positive electrode containing a positive electrode active material (NMC811), carbon black (CB), and polyvinylidene fluoride (PVDF) was obtained using NMP. The obtained slurry was coated on the surface of the current collector using a coater and dried to obtain a positive electrode with the desired coating weight (195 g/m ² ). The density was set at 2.3 g/cm ³ using a press. The mass ratio of each component contained in the positive electrode is shown in the following formula.
NMC811:CB:PVDF=94:3:3

A laminate type battery was obtained using the produced negative and positive electrodes and a separator (polypropylene porous membrane).
The capacity ratio (N/P) between the negative electrode (N) and the positive electrode (P) was designed to be 1.05.
As the electrolyte, 1M LiPF ₆ was dissolved in a mixed solvent containing EC, EMC, and DEC in a ratio of 1:1:1 (volume ratio), and further containing VC (1% by mass) and FEC (2% by mass). I used the one I made.

Details of the materials used to make the composite particles and battery are as follows.
Carbon black...Acetylene black with an average primary particle size of 48 nm (DENKA BLACK Li-400, manufactured by DENKA CORPORATION)
VGCF...Carbon fiber with an average fiber diameter of 150 nm, an average fiber length of 15.0 μm, and a carbon content of 99.99% by mass (Showa Denko K.K., VGCF-H)
SiO particles...SiO particles with a volume average particle diameter of 8.0 μm Graphite particles...Graphite particles with a volume average particle diameter of 10.0 μm

<Example 2>
A dispersion liquid in which carbon black (CB) was dispersed in NMP was kneaded to obtain a conductive additive slurry.
A conductive additive slurry was added to an SiO slurry obtained by kneading SiO particles and an NMP solution of polyamideimide (PAI), and the mixture was kneaded. The resulting mixture was poured into an alumina container and dried at 120°C under air to remove the solvent. Thereafter, heat treatment was performed at 350° C. for 5 hours in a nitrogen atmosphere. The obtained heat-treated product was crushed and passed through a sieve to obtain the desired composite particles. The mass ratio of each component contained in the composite particles is shown in the following formula.
SiO:PAI:CB=90:5:5

A SEM image of the prepared composite particles is shown in Figure 3. As shown in FIG. 3, carbon black was observed to be attached to the surface of the SiO particles. The coverage of the composite particles calculated by image analysis was 50% or more.

A battery for evaluation was produced in the same manner as in Example 1 using the obtained composite particles.

<Comparative example 1>
A battery for evaluation was produced in the same manner as in Example 1, except that the SiO particles used as the raw material for the composite particles in Example 1 were used instead of the composite particles.

<Comparative example 2>
An SiO slurry obtained by kneading SiO particles and an NMP solution of polyamideimide (PAI) was poured into an alumina container, and the solvent was removed by drying at 120° C. under air. Thereafter, heat treatment was performed at 350° C. for 5 hours in a nitrogen atmosphere. The obtained heat-treated product was crushed and passed through a sieve to obtain the desired composite particles. The mass ratio of each component contained in the composite particles is shown in the following formula.
SiO:PAI=95:5 (mass ratio)

<Comparative example 3>
An SiO slurry obtained by kneading SiO particles and an NMP solution of polyamideimide (PAI) was poured into an alumina container, and the solvent was removed by drying at 120° C. under air. Thereafter, heat treatment was performed at 350° C. for 5 hours in a nitrogen atmosphere. The obtained heat-treated product was crushed and passed through a sieve to obtain the desired composite particles. The mass ratio of each component contained in the composite particles is shown in the following formula.
SiO:PAI=90:10 (mass ratio)

<Evaluation of cycle characteristics>
Using the produced evaluation battery, a cycle characteristic evaluation test was conducted at 25°C.
First, the battery was charged under the conditions of upper limit voltage: 4.2V and current: 1/10C-CC (Constant Current). Thereafter, the battery was discharged under the conditions of lower limit voltage: 2.7V, current: 1/10C-CCCV, and cutoff current: 1/100C. After repeating this process for 3 cycles, the discharge capacity of the battery (after 3 cycles) was measured.
Next, the battery was charged under the conditions of upper limit voltage: 4.2V and current: 1C-CC. Thereafter, the battery was discharged under the conditions of lower limit voltage: 2.75V, current: 1C-CCCV, and cutoff current: 1/50C. After repeating this process for 300 cycles, the discharge capacity of the battery (after 300 cycles) was measured.
The discharge capacity retention rate (%) of the cell was calculated using the following formula. The results are shown in Table 1.
Discharge capacity maintenance rate (%) = Discharge capacity (after 300 cycles) / Discharge capacity (after 3 cycles) x 100

As shown in Table 1, the batteries of Examples 1 and 2 in which the active material particles are coated with a polymeric material and a conductive additive, the batteries of Comparative Example 1 in which the active material particles are not coated, Compared to the batteries of Comparative Examples 2 and 3 in which the coating only contained a polymeric material, the discharge capacity retention rate after 300 cycles was large and the cycle characteristics were excellent.

Claims

An electrode material comprising composite particles including particles containing a substance capable of occluding and releasing alkali metal ions, and a coating portion containing a polymeric material and a conductive agent that coats at least a portion of the surface of the particles.
The electrode material according to claim 1, wherein the coverage of the surface of the particle by the coating portion is 50% or more.
The electrode material according to claim 1, wherein the content of the conductive additive in the coating portion is 30% by mass or more.
The electrode material according to claim 1, wherein the polymer material includes at least one selected from polyamideimide and polyacrylonitrile.
The electrode material according to claim 1, wherein the particles contain silicon.
The electrode material according to claim 1, wherein the conductive additive contains particles with an aspect ratio of 10 or more.
An electrode for an energy storage device, comprising the electrode material according to any one of claims 1 to 6.
An energy storage device comprising the electrode for an energy storage device according to claim 7.