KR20180102177A - Cathode material for power storage device - Google Patents

Abstract

A negative electrode material for a power storage device is provided. The negative electrode material is made of powder that is a set of a plurality of particles, and the material of the particle is a Si-based alloy containing Si, Cr and X, and the Si-based alloy is composed of (1) 2) compound phase, and the compound phase (2) is at least one compound selected from the group consisting of (2-1) a Si- (Cr, X) compound phase or (2-2) a Si- (I): RCr-RX / RCr? 0.2 (I) wherein RCr represents an atomic radius of Cr, RX represents an element X Of the atomic radius). According to the present invention, a negative electrode for a power storage device having a large storage capacity and a high storage ratio of such storage capacity can be obtained.

Description

Cathode material for power storage device

The present invention relates to a material suitable for a negative electrode of a battery device accompanied with movement of lithium ions during charging and discharging of a lithium ion secondary battery, a full solid lithium ion secondary battery, a hybrid capacitor, and the like.

2. Description of the Related Art In recent years, mobile phones, portable music players, portable terminals and the like have been rapidly spreading. These portable devices have a lithium ion secondary battery. Electric vehicles and hybrid vehicles also have lithium-ion secondary batteries. Further, lithium ion secondary batteries and hybrid capacitors are used as residential static electricity accumulating devices. In a lithium ion secondary battery, the negative electrode occludes lithium ions upon charging. When the lithium ion secondary battery is used, lithium ions are released from the negative electrode. The negative electrode has a current collector and an active material fixed to the surface of the current collector.

As the active material in the negative electrode, carbon-based materials such as natural graphite, artificial graphite, and coke have been used. However, the theoretical capacity of the carbon-based material for lithium ions is only 372 mAh / g. A large amount of active material is desired. On the other hand, attention has been focused on Si as an active material in the negative electrode. Si reacts with lithium ions. Compounds are formed by this reaction. A typical compound is Li ₂₂ Si ₅ . By this reaction, a large amount of lithium ions are stored in the negative electrode. Si can increase the storage capacity of the negative electrode.

When the active material layer containing Si stores lithium ions, the active material layer expands due to the above-described formation of the compound. The expansion ratio of the active material is about 400%. When lithium ions are released from the active material layer, the active material layer shrinks. By repeating expansion and contraction, the active material falls off the current collector. Such dropout degrades the storage capacity. By repeated expansion and contraction, the conductivity between the active materials may be impaired. The lifetime of a conventional lithium ion secondary battery in which the negative electrode contains Si is not long. Furthermore, the electrical conductivity in a single body of Si is lower than that of a carbonaceous material and a metal-based material. Therefore, in the case of a negative electrode material containing Si, the efficiency in charging and discharging is insufficient.

Therefore, proposals have been made to improve defects of the anode material including Si. Japanese Unexamined Patent Application Publication No. 2016-004715 discloses a negative electrode material in which an amorphous alloy phase exists between clusters of amorphous metals composed of Si or Sn. In such an anode material, the amorphous alloy phase suppresses the expansion and contraction of Si or Sn.

Patent Document 1: JP-A-2016-004715

However, in the anode material disclosed in JP-A-2016-004715, the amorphous alloy phase may inhibit the movement of electrons or ions. The same problem also occurs in a variety of power storage devices other than lithium ion secondary batteries.

Therefore, an object of the present invention is to provide a material which has a large storage capacity and in which a reduction in storage capacity due to repetition of charging and discharging is suppressed.

According to the present invention, there is provided a negative electrode material for a power storage device comprising powder as a set of a plurality of particles,

Wherein the material of the particles is a Si-based alloy containing Si, Cr and an element X,

The Si-

(1) a Si phase mainly composed of Si and

(2) a compound phase,

The compound phase (2)

(2-1) a Si- (Cr, X) compound phase or

(2-2) a composite phase of a Si- (Cr, X) compound and an Si-X compound,

(I): < EMI ID =

(In the above formula (I), RCr represents the atomic radius of Cr, and RX represents the atomic radius of the element X).

Is satisfied. ≪ / RTI >

In the negative electrode containing such a material, the storage capacity is large. Furthermore, in such a negative electrode, a decrease in the storage capacity due to repetition of charging and discharging is suppressed. Such cathodes can be applied not only to lithium ion secondary batteries, but also to various power storage devices such as pre-solid lithium ion secondary batteries and hybrid capacitors.

1 is a conceptual diagram showing a lithium ion secondary battery using a cathode material according to an embodiment of the present invention.
2 is an enlarged cross-sectional view showing a part of a cathode of the battery of FIG.
3 is a chart showing an X-ray diffraction pattern.
4 is a chart showing another X-ray diffraction pattern.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the drawings as appropriate.

The lithium ion secondary battery 2 conceptually shown in Fig. 1 includes a water tank 4, an electrolyte 6, a separator 8, a positive electrode 10 and a negative electrode 12. The electrolytic solution (6) is stored in the water tank (4). This electrolyte solution 6 contains lithium ions. The separator 8 divides the water tank 4 into an anode chamber 14 and a cathode chamber 16. The separator 8 prevents the anode 10 and the cathode 12 from abutting against each other. The separator 8 has a plurality of holes (not shown). Lithium ions can pass through this hole. The anode 10 is immersed in the electrolyte solution 6 in the anode chamber 14. The cathode 12 is immersed in the electrolyte solution 6 in the cathode chamber 16.

In Fig. 2, a part of the cathode 12 is shown. The negative electrode 12 includes a current collector 18 and an active material layer 20. The active material layer 20 includes a plurality of particles 22. The particles 22 are adhered to other particles 22 that abut these particles 22. The particles 22 contacting the current collector 18 are fixed to the current collector 18. The active material layer 20 is porous.

The aggregate of the plurality of particles 22 before forming the active material layer 20 is powder. In the present invention, such a powder is referred to as an " anode material ".

The material of the particles 22 is a Si-based alloy. These Si-based alloys include Si, Cr and element X. Preferably, the balance of Si, Cr and the element X in this Si-based alloy is an inevitable impurity.

These Si-based alloys have (1) a Si phase and (2) a compound phase. Preferably, the Si-based alloy is composed of only the Si phase (1) and the compound phase (2).

The main component of the Si phase 1 is Si. Si reacts with lithium ions. Therefore, the negative electrode 12 including such a Si phase 1 can store a large amount of lithium ions. The Si phase 1 can increase the storage capacity of the cathode 12. [ From the viewpoint of the storage capacity, the content of Si in the Si phase 1 is preferably at least 50 at.%, More preferably at least 60 at.%, Particularly preferably at least 70 at.%.

From the viewpoint of the storage capacity, the content of Si in the alloy is preferably at least 50 at.%, More preferably at least 60 at.%, Particularly preferably at least 70 at.%. From the viewpoint that the alloy can sufficiently contain Cr and element X to be described later, the Si content is preferably 95 at.% Or less, and particularly preferably 90 at.% Or less.

The Si phase 1 may contain an element other than Si. It is preferable that the Si phase 1 contains an element having excellent conductivity. In an alloy containing an element having excellent conductivity, a large electric capacity is achieved, and excellent electrical conductivity is also achieved. Preferably, the element having excellent conductivity is contained in Si.

The compound phase (2) includes a compound in which a part of Cr of the Si-Cr compound is substituted with another element. The Si-Cr compound is an intermetallic compound of Si and Cr. The electric resistance value of the Si-Cr compound is smaller than the electric resistance value of Si. Alloys containing Si-Cr compounds are excellent in conductivity. The Si-Cr compound further alleviates the stress generated in accordance with the change in volume during charging and discharging. By the relaxation of the stress, the undifferentiation of the Si phase is suppressed, and further, the dropping of the particles 22 is suppressed. The Si-Cr compound phase suppresses the decrease of the storage capacity due to repetition of charging and discharging.

Si and Cr can cause a process reaction. In the process structure, the Si phase and the Si-Cr compound phase are fine. In the microstructure, the decomposition of the Si phase hardly occurs depending on the volume change at the time of charging and discharging.

The compound phase (2) contains a Si- (Cr, X) compound. In the Si- (Cr, X) compound, a part of Cr in the Si-Cr compound is substituted with the element X. Also in the Si- (Cr, X) compound, the microstructure according to the process reaction can be attained as in the case of the Si-Cr compound having the parent structure. In the compound phase (2), the Si- (Cr, X) compound is a compound of the formula (2-1) Si- (Cr, X) Compound < / RTI > The compound phase 2 may include both the Si- (Cr, X) compound phase 2-1 and the composite phase 2-2.

The element X is represented by the following formula (I):

. In the above formula (I), RCr represents the atomic radius of Cr, and RX represents the atomic radius of the element X.

In the cathode material satisfying the above formula (I), the difference between the atomic radius RX of the element X and the atomic radius RCr of Cr is small. This element X can be easily substituted with Cr in the Si-Cr compound.

Specific examples of the element X include one or two or more selected from the group consisting of Ag, Al, Co, Cu, Fe, Mn, Mo, Nb, Ni, Re, V, . Preferred elements X are Co, Ni and Ti. The atomic radius RX of each element X is as follows.

Ag: 1.44 Angstrom

Al: 1.43 Angstrom

Co: 1.25 Angstrom

Cu: 1.28 angstroms

Fe: 1.24 angstroms

Mn: 1.12 angstroms or 1.50 angstroms (Mn is polymorphic)

Mo: 1.36 Angstroms

Nb: 1.43 angstroms

Ni: 1.25 angstroms

Re: 1.37 Angstrom

V: 1.32 angstroms

U: 1.38 angstroms or 1.50 angstroms (U is polymorphic)

Ta: 1.43 Angstrom

Ti: 1.47 angstroms

W: 1.37 angstroms

On the other hand, the atomic radius RCr of Cr is 1.25 angstroms.

Fig. 3 shows an X-ray diffraction pattern of the Si-Cr compound and the Si-Cr-Ti compound. As is clear from Fig. 3, the angle of the XRD peak of the Si-Cr-Ti compound is smaller than the angle of the XRD peak of the Si-Cr compound. From these charts, it can be seen that Cr and substituted Ti shift the XRD peak in the low angle direction. In other words, Cr and substituted Ti enlarge the lattice spacing of the crystal plane of the compound.

In the present invention, the lattice spacing d of the crystal plane is calculated on the basis of the equation: 2 · d · sin θ = n · λ in accordance with Bragg's law. In this expression,? Represents the angle formed by the crystal plane and the X-ray,? Represents the wavelength of the X-ray, and n represents an integer.

FIG. 4 shows an X-ray diffraction pattern of a Si-Cr-Ti compound and a Si-Cr-Co-Ti compound. 4, the angle of the XRD peak of the Si-Cr-Co-Ti compound is smaller than the angle of the XRD peak of the Si-Cr-Ti compound. From these charts, it can be seen that Cr and substituted Co shift the XRD peak in the low angle direction. In other words, Cr and substituted Co increase the lattice spacing of the crystal plane of the compound.

In the alloy having the compound phase (2-1) or the composite phase (2-2) described above, since the element X exists, the lattice spacing is large. In such an alloy, lithium ions are liable to move. Such an alloy is excellent in conductivity.

From the viewpoint of the lattice spacing, the ratio of the total amount of the compound phase (2-1) and the composite phase (2-2) in the alloy is preferably 30% or more and 80% or less. This ratio is a ratio of the sum of the area of the compound phase (2-1) and the area of the composite phase (2-2) in the enlarged photograph of the section of the alloy to the area of the alloy.

From the viewpoint of the storage capacity holding performance, the total content of Cr and the element X in the Si-based alloy is preferably at least 5 at.%, More preferably at least 8 at.%, Particularly preferably at least 10 at.%. From the viewpoint that the alloy contains sufficient Si and, therefore, the initial storage capacity is large, this content is preferably 30 at.% Or less.

From the viewpoint of conductivity, the content of the element X in the Si-based alloy is preferably 0.01 at.% Or more, and particularly preferably 1.0 at.% Or more. If the element X is excessive, the element X is not substituted with Cr at all, and the element X is precipitated singly. From the standpoint of preventing precipitation into a single body, the content of the element X is preferably 20 at.% Or less, and particularly preferably 15 at.% Or less.

The crystallite size of the Si phase 1 is preferably 10 nm or less. In the case of the negative electrode material having a crystallite size of 10 nm or less, cracks, electrical independence and drop-off of the particles 22 due to the stress during charging and discharging are suppressed. From this viewpoint, the crystallite size is more preferably 7 nm or less, and particularly preferably 5 nm or less.

The crystallite size of the compound phase (2) is preferably 20 nm or less. In the negative electrode material having a crystallite size of 20 nm or less, lithium ions can easily migrate in the compound phase. From this viewpoint, the crystallite size is more preferably 17 nm or less, and particularly preferably 15 nm or less.

The crystallite size can be confirmed by X-ray diffraction. In X-ray diffraction, a CuK? Ray having a wavelength of 1.54059 angstroms is used as an X-ray source. The measurement is performed in the range where 2? Is not less than 20 degrees and not more than 80 degrees. In the obtained diffraction spectrum, the smaller the crystallite size, the larger the diffraction peak is observed. From the half width of the peak obtained by the powder X-ray diffraction analysis, the crystallite size can be obtained by using Scherrer's equation: D = (K x lambda) / (x x cos?). In this equation, D represents the crystallite size (angstrom), K represents the Scherrer constant,? Represents the wavelength of the X-ray tube,? Represents the diffraction of the diffraction line depending on the crystallite size, .

Control of the crystallite size on the Si phase and the compound phase can be achieved by adjusting the raw material component. The crystallite size can also be controlled by controlling the cooling rate at the time of solidification after the raw material powder is dissolved.

The BET specific surface area (SS) of the powder is preferably 3.0 m ² / g or more and 9.0 m ² / g or less. In a powder having a specific surface area (SS) of 3.0 m ² / g or more, the Si-based alloy can react with lithium ions over a wide area. Therefore, in the negative electrode 12 using such a powder, the storage capacity is large. Furthermore, in the powder having the specific surface area SS of 3.0 m ² / g or more, the difference in stress between the inside of the particles 22 during charging and discharging and the surface of the particles 22 is small. Therefore, in the cathode 12 using such a powder, the particleization of the particles 22 is suppressed, and the charge storage capacity is maintained. In view of this, the specific surface area (SS) is 3.5m ² / g or more is more preferable, 4.0m ² / g or more is particularly preferable. In the powder having a specific surface area (SS) of 9.0 m ² / g or less, the decomposition reaction of the electrolytic solution on the surface of the particles 22 is suppressed. Therefore, in the cathode 12 using such a powder, reduction of lithium ions is suppressed, and the shape of the solid electrolytic material (SEI) is suppressed. In this negative electrode 12, the charge storage capacity is maintained. In view of this, the specific surface area (SS) is 8.0m ² / g or less, more preferably, 7.5m ² / g or less is particularly preferred. The BET specific surface area (SS) is measured according to the standard of JIS Z 8830: 2013.

As a method for producing the powder, water atomization method, monocotyl quenching method, twin-roll quenching method, gas atomization method, disk atomization method and centrifugal atomization method are exemplified. The powder obtained by these methods may be subjected to mechanical milling or the like. Examples of the milling method include a ball mill method, a bead mill method, a planetary ball mill method, an atrito method and a vibrating ball mill method. The preferred production methods are the simple roll cooling method, the gas atomization method and the disk atomization method. Hereinafter, one example of these production methods will be described in detail. The manufacturing conditions are not limited to those described below.

In the single roll cooling method, a raw material is introduced into a quartz tube having pores at the bottom. This raw material is heated and melted in a high-frequency induction furnace in an argon gas atmosphere. The raw material flowing out from the pores falls on the surface of the dowel and is cooled to obtain a ribbon. This ribbon is put into the port with the ball. Examples of the material of the balls include zirconia, SUS304 and SUJ2. Examples of the material of the port include zirconia, SUS304 and SUJ2. The port is filled with argon gas, and this port is sealed. This ribbon is pulverized by milling to obtain a particle 22. Examples of the milling include a ball mill, a bead mill, a planetary ball mill, an atrito, and a vibrating ball mill.

In the gas atomization method, raw materials are introduced into a quartz crucible having pores at the bottom. This raw material is heated and melted in a high-frequency induction furnace in an argon gas atmosphere. In the argon gas atmosphere, argon gas is injected into the raw material flowing out of the pores. The raw material is quenched and solidified to obtain particles (22).

In the disk atomization method, a raw material is charged into a quartz crucible having pores at the bottom. This raw material is heated and melted in a high-frequency induction furnace in an argon gas atmosphere. In the argon gas atmosphere, the raw material flowing out from the pores falls on the disk rotating at high speed. The rotation speed is 60000 rpm at 40000 rpm. The raw material is quenched by the disk and solidified to obtain a powder. This powder is put into the port with the ball. Examples of the material of the balls include zirconia, SUS304 and SUJ2. Examples of the material of the port include zirconia, SUS304 and SUJ2. The port is filled with argon gas, and this port is sealed. This powder is pulverized by milling to obtain a particle 22. Examples of the milling include a ball mill, a bead mill, a planetary ball mill, an atrito, and a vibrating ball mill.

Example

Hereinafter, the effects of the present invention will be clarified by examples, but the present invention should not be construed on the basis of the description of these examples.

The effect of the anode material according to the present invention was confirmed by using a bipolar coin type cell. First, raw materials having compositions shown in Tables 1 and 2 were prepared. Powders were prepared from each raw material by gas atomization method and mechanical milling. Each powder, a conductive material (acetylene black), a binder (polyimide, polyvinylidene fluoride, etc.) and a dispersion (N-methylpyrrolidone) were mixed to obtain a slurry. This slurry was applied on a copper foil as a current collector. This slurry was dried under reduced pressure using a vacuum drier. The drying temperature was 200 占 폚 or higher when the polyimide was a binder and 160 占 폚 or higher when the polyvinylidene fluoride was the binder. The solvent was evaporated by this drying to obtain an active material layer. The active material layer and the copper foil were pressed with a roll. The active material layer and the copper foil were perforated in a shape suitable for the coin type cell to obtain a negative electrode.

As an electrolytic solution, a mixed solvent of ethylene carbonate and dimethyl carbonate was prepared. The mass ratio of both was 3: 7. Further, lithium hexafluorophosphate (LiPF ₆ ) was prepared as a supporting electrolyte. The amount of the supporting electrolyte is 1 mole per 1 liter of the electrolytic solution. This supporting electrolyte was dissolved in the electrolytic solution.

A separator and a positive electrode of a shape suitable for a coin-shaped cell were prepared. This positive electrode was punched out from a lithium foil. The separator was immersed in the electrolytic solution under reduced pressure and allowed to stand for 5 hours to sufficiently penetrate the electrolytic solution into the separator.

A negative electrode, a separator and a positive electrode were put in a water tank. An electrolytic solution was filled in the water tank, and a coin-shaped cell was obtained. On the other hand, the electrolytic solution needs to be treated in a dew-point controlled inert atmosphere. Therefore, the cells were assembled in a glove box in an inert atmosphere.

The cell was charged with the coin-shaped cell at a temperature of 25 캜 and a current density of 0.50 mA / cm ² until the potential difference between the positive electrode and the negative electrode reached 0 V. Thereafter, discharge was performed until the potential difference became 1.5 V. This charging and discharging were repeated for 50 cycles. The initial discharge capacity A and the discharge capacity B after 50 cycles of charging and discharging were measured. Furthermore, the ratio of the discharge capacity B to the discharge capacity A (maintenance rate) was calculated. The results are shown in Tables 1 and 2 below.

In the following Tables 1 and 2, Nos. 1 to 23 are the composition of the negative electrode material according to the embodiment of the present invention, and Nos. 24 to 32 are the composition of the negative electrode material according to the comparative example. The remainder of the components listed in Table 1 and Table 2 are Si and inevitable impurities.

[Table 1] Evaluation results

[Table 2] Evaluation results

The atomic radii of the additional elements other than the element X shown in Table 2 are as follows.

Mg: 1.60 angstroms

Zr: 1.62 angstroms

Y: 1.82 angstroms

La: 1.88 Angstrom

The formula (I) shown in Tables 1 and 2 is as follows. In the formula (I), RCr represents the atomic radius of Cr, and RX represents the atomic radius of the element X.

In Table 1 and Table 2, the initial discharge capacity is preferably 500 mAh / g or more, and the retention ratio is preferably 70% or more.

The anode member of each embodiment is a Si-based alloy in which the material of the particles is Si, Cr, and element X. Such an element X satisfies the above-mentioned formula (I). This Si-based alloy has a Si phase (1) and a compound phase (2), which are main components of Si. Such a compound phase 2 contains a compound phase 2-2 of a Si- (Cr, X) compound phase 2-1 or a Si- (Cr, X) compound and a Si-X compound .

For example, in the negative electrode material of No. 11, the initial discharge capacity is sufficiently large at 1314 mAh / g. The discharge capacity retention rate of this negative electrode material after 50 cycles is 88%, which has a sufficient cycle life.

The negative electrode material of each comparative example does not satisfy any of the inventive aspects of the present invention. In Table 2, undefined inventive particulars are underlined. Among the elements in the composition, those which do not satisfy claim 4 are double underlined.

For example, the negative electrode material of No. 29 does not have a Si-Cr compound phase. This negative electrode material does not have a Si- (Cr, X) compound phase. Furthermore, this negative electrode material does not have a mixed phase of a Si- (Cr, X) compound and an Si-X compound. The initial discharge capacity of this negative electrode material is 429 mAh / g, which is not sufficient. The discharge capacity retention ratio of the negative electrode material after 50 cycles is 9%, and the cycle life is short.

From the above evaluation results, the superiority of the present invention is clear.

Claims (6)

1. A negative electrode material for a power storage device comprising powder as a group of a plurality of particles,
Wherein the material of the particles is a Si-based alloy containing Si, Cr and an element X,
The Si-
(1) a Si phase mainly composed of Si and
(2) a compound phase,
The compound phase (2)
(2-1) a Si- (Cr, X) compound phase or
(2-2) a composite phase of a Si- (Cr, X) compound and an Si-X compound,
(I): < EMI ID =

(In the above formula (I), RCr represents the atomic radius of Cr, and RX represents the atomic radius of the element X).
Lt; RTI ID = 0.0 >%< / RTI > The method according to claim 1,
Wherein the total content of Cr and the element X in the Si-based alloy is not less than 5 at.% And not more than 30 at.%. 3. The method according to claim 1 or 2,
The content of the element X in the Si-based alloy is 0.01 at.% Or more and 20 at.% Or less. 4. The method according to any one of claims 1 to 3,
Wherein the element X is one or more selected from the group consisting of Ag, Al, Co, Cu, Fe, Mn, Mo, Nb, Ni, Re, V, 5. The method according to any one of claims 1 to 4,
Wherein a crystallite size of the Si phase (1) is 10 nm or less and a crystallite size of the compound phase (2) is 20 nm or less. 6. The method according to any one of claims 1 to 5,
A BET specific surface area of the powder than 3.0m ² / g 9.0m ² / g greater than the anode material.

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