JP2016074972A - Stabilized lithium powder, and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stabilized lithium powder that suppresses the damage to the anode during the lithium doping, and can improve the initial charge-discharge efficiency.SOLUTION: Provided is a stabilized lithium particle, characterized, setting the average circularity of the particle as C, it is C≤0.90.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a stabilized lithium powder and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.

  The capacity of the lithium ion secondary battery mainly depends on the active material of the electrode. In general, graphite is used as the negative electrode active material, but it is necessary to use a higher capacity negative electrode active material in order to meet the above requirements. Therefore, metallic silicon (Si) having a much larger theoretical capacity (4210 mAh / g) than the theoretical capacity of graphite (372 mAh / g) has attracted attention.

On the other hand, the use of silicon oxide (SiO _x ), which has better cycle characteristics than metal silicon, is also being studied. However, silicon oxide has a larger irreversible capacity than metal silicon. Since the amount of lithium that contributes to charging and discharging is uniquely determined by the amount of lithium in the positive electrode, an increase in irreversible capacity in the negative electrode leads to a decrease in capacity of the entire battery.

  In order to reduce this irreversible capacity, there has been proposed a technique (lithium pre-doping) in which metallic lithium is brought into contact with the negative electrode in advance before starting charge / discharge and lithium is doped into the negative electrode (for example, see Patent Documents 1 and 2). ). Patent Document 1 discloses a method of doping lithium into the negative electrode by forming a film containing lithium on the negative electrode. Patent Document 2 discloses a method of doping lithium into the negative electrode by incorporating lithium particles in the negative electrode active material layer.

  Lithium used for such doping work is required to have higher safety due to its high reactivity, and the surface of lithium particles is covered with a stable coating in the atmosphere to improve safety and improve handling. Stabilized lithium powder has been proposed (see Patent Document 3).

  Usually, a negative electrode used in a lithium ion secondary battery has a step of forming a layer containing a negative electrode active material on a current collector, and then bringing it into close contact with a press, and this press exposes the lithium metal of the stabilized lithium powder. The dope into the negative electrode proceeds. Therefore, the characteristics required for the stabilized lithium powder are required not only to improve the stability of lithium but also to dope characteristics for producing excellent battery characteristics.

Japanese Patent No. 5196118 JP 2010-160986 A Japanese Patent No. 2699026

  However, when spherical stabilized lithium powder as described in the above-mentioned patent document is used, a large pressing pressure is required to break the coating layer and expose the lithium particles. Defects occur, and the initial charge / discharge efficiency when the battery is made deteriorates.

  On the other hand, when the press pressure is reduced in order to suppress damage to the negative electrode, the stabilized lithium powder cannot be crushed, so that the dope does not proceed efficiently and the reduction of the irreversible capacity is not achieved.

  This invention is made | formed in view of the subject which the said prior art has, The damage to a negative electrode is suppressed, and providing a stabilized secondary lithium powder excellent in dope efficiency, and a lithium secondary battery using the same Objective.

In order to achieve the above object, the stabilized lithium powder according to the present invention is characterized in that C ≦ 0.90, where C is the average circularity of the particles. Here, the circularity C is defined as C = 4πS / L ² where S is the particle area and L is the perimeter.

  By adopting such a configuration, stress on the stabilized lithium powder tends to concentrate on a part during the above-mentioned pressing, and lithium particles can be crushed even with a small pressing pressure. It can be sufficiently reduced, and the initial charge / discharge efficiency of the lithium ion secondary battery is greatly improved.

  The stabilized lithium powder according to the present invention preferably further satisfies FD ≦ 53.0 um when the average ferret diameter of the particles is FD. Here, the ferret diameter is defined by the length of the long side of the rectangle circumscribing the observation image when the particles are observed with a microscope or the like.

  According to this, by reducing the ferret diameter, the press pressure required for crushing the particles can be further reduced, and the irreversible capacity can be reduced.

The stabilized lithium powder according to the present invention preferably further contains a transition metal in an amount of 1.0 × 10 ⁻³ mass% to 1.0 × 10 ⁻¹ mass%.

  According to this, when the transition metal is present at a predetermined ratio, the stabilization coating is hard and brittle, and the particles are more easily crushed. As a result, the irreversible capacity can be further reduced.

According to the present invention, it is possible to provide a stabilized lithium powder excellent in doping efficiency, and by using a negative electrode doped with the stabilized lithium powder of the present invention, a lithium ion secondary whose initial charge / discharge efficiency is greatly improved. A battery can be obtained.

It is an optical microscope photograph of the stabilized lithium powder which concerns on this embodiment. It is an optical microscope photograph of the conventional stabilized lithium powder. It is a schematic cross section of the lithium ion secondary battery of this embodiment.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Stabilized lithium powder>
The stabilized lithium powder of the present embodiment has a particle form having an average circularity C of 0.90 or less. In the stabilized lithium powder, metallic lithium is coated with a single or a plurality of stable lithium compounds.

Stable lithium compounds include carbonates, hydroxides, oxides, sulfides, and the like, and specifically include LiOH, Li ₂ CO ₃ , Li ₂ O, Li ₂ S, and the like. These compounds can be identified by X-ray diffraction or Raman spectroscopy. Among them, in order to increase the safety, the lithium compound is preferably Li 2 _O as a main component.

The average circularity C of the stabilized lithium powder is more preferably 0.80 or less. The smaller the circularity, the easier it is to break the particles, the damage to the negative electrode associated with the press is further suppressed, and an excellent dope effect is obtained.

The average ferret diameter of the stabilized lithium powder is preferably 53.0 μm or less, and more preferably 25.0 μm or less, from the viewpoint of uniform dispersibility on the electrode surface after application.

From the viewpoint of the brittleness of the stabilized coating, the stabilized lithium powder contains a transition metal in an amount of 1.0 × 10 ⁻³ mass% or more and 1.0 × 10 ⁻¹ mass% or less based on the stabilized lithium powder. It is preferable. More preferably, it is contained in an amount of 1.0 × 10 ⁻³ mass% or more and 10.0 × 10 ⁻³ mass% or less. According to such a configuration, a more excellent dope effect can be obtained. This is considered to be due to the fact that the stabilizing coating becomes brittle due to the presence of foreign substances.

The transition metal is preferably contained in the stabilizing coating. According to such a configuration, it is considered that the stabilization coating becomes brittle more efficiently. The transition metal may be quantified by ICP (emission spectroscopy analysis).

Further, the transition metal is preferably a metal that is easily oxidized. Examples thereof include Mg, Al, Ti, Zr, Mn, Zn, Cr, Fe, Ni, Sn, and Cu. Among these, Fe is particularly preferable.

  In the stabilized lithium powder, it is preferable that metallic lithium is tightened to 80 parts by mass or more from the viewpoint of doping efficiency. It becomes easy for metal lithium and a negative electrode active material to contact at the time of a press, and the more excellent dope effect is acquired.

(Method for producing stabilized lithium powder)
In the stabilized lithium powder of this embodiment, a lithium ingot was added to hydrocarbon oil, this was heated to a melting point or higher of lithium, and this molten lithium-hydrocarbon oil mixture was stirred for a sufficient time to form a dispersion. After that, it is gradually cooled in a state where stirring is continued, and when the dispersion is sufficiently cooled, carbon dioxide (CO ₂ ) is brought into contact with it to form a stabilized film on the surface, and this is dried. Is done. In addition, what is necessary is just to add a carbon dioxide at the time of introduction | transduction, when adding a transition metal.

  A heat-resistant container is used, and the container is stirred at an angle of 5 to 15 degrees. The number of rotations is preferably 1000 rpm or more, and more preferably stirring is performed at 3000 rpm to 10,000 rpm.

  From the viewpoint of uniform dispersibility after melting, the hydrocarbon oil is preferably 1 to 30 parts by mass, more preferably 2 to 15 parts by mass when the lithium ingot is 1 part by mass.

  The temperature required to produce the stabilized lithium powder of the present invention is preferably equal to or higher than the temperature at which lithium metal melts. Specifically, it is 190 degreeC-250 degreeC, Preferably it is 195 degreeC-240 degreeC, More preferably, it is 200 degreeC-230 degreeC. If the temperature is too low, lithium is solidified, making it difficult to produce lithium powder. If the temperature is too high, vaporization may occur depending on the type of hydrocarbon oil, making it difficult to handle in production.

  The temperature after cooling the dispersion is preferably 100 ° C. or lower, and more preferably 50 ° C. or lower. Moreover, it is preferable that the said dispersion liquid is gradually cooled over 1 hour or more.

  When the lithium ingot is 1 part by mass, the carbon dioxide is preferably added in an amount of 0.1 to 10 parts by mass, more preferably 1 to 3 parts by mass. Carbon dioxide is preferably introduced below the surface of this mixture, and the vigorous stirring conditions necessary to produce the dispersion include carbon dioxide introduced on the molten lithium-hydrocarbon oil mixture, dispersed metallic lithium and Should be sufficient to bring in contact.

<Negative electrode>
A lithium ion secondary battery with improved initial charge / discharge efficiency can be obtained by doping the above-described stabilized lithium powder into the negative electrode during the production of the negative electrode and then completing the lithium ion secondary battery. The negative electrode 20 can be produced by forming a negative electrode active material layer 24 on a negative electrode current collector 22 as described later.

(Current collector for negative electrode)
The negative electrode current collector 22 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as copper, nickel, an alloy thereof, or stainless steel can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a negative electrode binder, and an amount of a negative electrode conductive additive as required.

(Negative electrode active material)
The negative electrode active material preferably has a large irreversible capacity, and examples thereof include metal silicon (Si) and silicon oxide (SiO _x ).

(Binder for negative electrode)
The negative electrode binder bonds the negative electrode active materials and the current collector 22 together with the negative electrode active materials. The binder is not particularly limited as long as the above-described bonding is possible, and examples thereof include fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition to the above, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, or the like may be used as the binder. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant. As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ , or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

  The content of the binder in the negative electrode active material layer 24 is not particularly limited, but is preferably 0.5 to 5 parts by mass with respect to the mass of the negative electrode active material.

(Conductive aid for negative electrode)
The conductive aid for the negative electrode is not particularly limited as long as it improves the conductivity of the negative electrode active material layer 24, and a known conductive aid can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.
(Method for producing negative electrode)
The dispersion liquid in which the above stabilized lithium powder is dispersed in a solvent is applied on the negative electrode active material layer formed on the negative electrode current collector, and dried and then pressed to dope lithium into the negative electrode active material. Advances to complete the lithium-doped negative electrode.

  As the solvent for the dispersion, those having a high vapor pressure are preferable, and examples thereof include normal heptane, normal hexane, and methyl ethyl ketone.

  The pressing method is not particularly limited, and a known method such as a hand press or a roller press can be used.

<Lithium ion secondary battery>
FIG. 3 shows a schematic cross-sectional view of the lithium ion secondary battery of the present embodiment.

  The lithium ion secondary battery 100 can be produced by producing the negative electrode 20 produced by lithium doping as described above, the positive electrode 10, and the separator 18 impregnated with the electrolyte as shown in FIG. Here, the positive electrode 10 can be produced by forming the positive electrode active material layer 14 on the positive electrode current collector 12, and the negative electrode 20 is formed by forming the negative electrode active material layer 24 on the negative electrode current collector 22. It can produce by doing. In the drawings, reference numerals 62 and 60 denote a positive electrode and a negative electrode, respectively.

<Positive electrode>
(Current collector for positive electrode)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a necessary amount of positive electrode conductive additive.

(Positive electrode active material)
Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and ^{dedoping of} lithium ions and counter anions of the lithium ions (for example, PF ⁶⁻ ). The electrode is not particularly limited as long as it can be reversibly advanced, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the general formula: LiNi _x Co _y Mn _z MaO ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Oxide, lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr, or VO) shown), and composite metal oxides of lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) , etc.

(Binder for positive electrode)
There is no limitation in particular as a binder for positive electrodes, The thing similar to the binder for negative electrodes described above can be used.

(Conductive aid for positive electrode)
There is no limitation in particular as a conductive support agent for positive electrodes, The thing similar to the conductive support agent for negative electrodes described above can be used.

<Electrolyte>
The electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited, and for example, in the present embodiment, an electrolytic solution containing a lithium salt can be used.

  It is preferable to use an organic solvent that can be operated at a high voltage as the electrolytic solution, for example, an aprotic high dielectric constant solvent such as ethylene carbonate or propylene carbonate, or an acetate ester such as dimethyl carbonate or ethyl methyl carbonate. Or aprotic low-viscosity solvents such as propionates. Furthermore, it is desirable to use these aprotic high dielectric constant solvents and aprotic low viscosity solvents in combination at an appropriate mixing ratio.

Further, an ionic liquid using imidazolium, ammonium, and pyridinium type cations may be used as the organic solvent. The counter anion is not particularly limited, and examples thereof include BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ^{− and the} like, and they may be used by mixing with the above-mentioned organic solvent.

It does not specifically limit as said lithium salt, The lithium salt used as an electrolyte of a lithium ion secondary battery can be used. For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiFSI, and LiBOB, organic acid anion salts such as LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, and the like can be used.

  Furthermore, the concentration of the lithium salt is preferably 0.5 to 2.0 M from the viewpoint of electrical conductivity. The conductivity of the electrolyte at 25 ° C. is preferably 0.01 S / m or more, and is adjusted by the type of lithium salt or its concentration.

  When the electrolyte is a solid electrolyte or gel electrolyte, poly (vinylidene fluoride) or the like can be contained as a polymer material.

Furthermore, you may add various additives in the electrolyte solution of this embodiment as needed. Examples of additives include vinylene carbonate and methyl vinylene carbonate for the purpose of improving cycle life, biphenyl and alkyl biphenyl for the purpose of preventing overcharge, various carbonate compounds for the purpose of deoxidation and dehydration, Carboxylic anhydride, various nitrogen-containing and sulfur-containing compounds can be mentioned.

The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment.

EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Example 1]
(Production of stabilized lithium powder)
A stainless steel resin container, 100 g of Kanto Chemical's lithium ingot and Witco's Carnation hydrocarbon oil were added, and the inside of the container was replaced with dry argon. The reactor was then heated to 200 ° C. to melt the lithium ingot. In a molten state, the mixture was stirred for 10 minutes at a rotational speed of 8000 rpm with the container tilted at 5 °, and then cooled to room temperature over 1 hour while maintaining stirring. After cooling, 5 g of carbon dioxide was supplied to the surface for 5 minutes with continued stirring and filled. The stirring was stopped when all of the carbon dioxide was added, and the resulting powder was washed with hexane to obtain stabilized lithium powder. A photograph of the stabilized lithium powder of Example 1 taken with an optical microscope is shown in FIG.
(Preparation of negative electrode)
83 parts by mass of silicon oxide (SiO), 2 parts by mass of acetylene black, 15 parts by mass of polyamideimide, and 100 parts by mass of N-methylpyrrolidone were mixed to prepare a slurry for forming a negative electrode active material layer. The slurry was applied to one surface of a copper foil having a thickness of 14 μm as a current collector so that the amount of the negative electrode active material applied was 2.0 mg / cm ² and dried at 100 ° C. to thereby form the negative electrode active material layer. Formed. Then, the negative electrode whose thickness of a negative electrode active material layer is 22 micrometers was obtained by pressure-molding with a roller press, and heat-processing in a vacuum at 350 degreeC for 3 hours.

A dispersion obtained by dispersing 100 parts by mass of the above stabilized lithium powder in 100 parts by mass of methyl ethyl ketone on the negative electrode obtained by the above method is applied so that the coating amount of the stabilized lithium powder becomes 0.5 mg / cm ^2. And dried at 100 ° C. Thereafter, a pressure of 10 kgf / cm ² was applied by a hand press to dope the negative electrode with lithium, and a lithium-doped negative electrode was obtained. At this time, there was no damage to the negative electrode.

(Production of evaluation lithium-ion secondary battery)
The negative electrode produced above and a counter electrode made by bonding a lithium metal foil to a copper foil as a positive electrode are placed in an aluminum laminate pack with a separator made of a polyethylene microporous film interposed therebetween, and the aluminum laminate pack is electrolyzed. After injecting a 1M LiPF ₆ solution (solvent: ethylene carbonate (EC) / diethyl carbonate (DEC) = 3/7 (volume ratio)) as a liquid, vacuum sealing was performed to produce a lithium ion secondary battery for evaluation. .

<Measurement of average ferret diameter and average circularity>
About the stabilized lithium powder produced above, particles were observed using an optical microscope. The obtained observation image was subjected to image analysis using image analysis software (software name: ImageJ) to determine the ferret diameter and circularity of the particles. The image analysis was performed on at least 500 particles and the average ferret diameter and average circularity of the synthesized stabilized lithium powder were determined. The ferret diameter is defined by the length of the long side of the rectangle circumscribing the observed image, and the circularity C is defined by C = 4πS / L ² where S is the particle area and L is the circumference.

<Measurement of initial charge / discharge efficiency>
About the lithium ion secondary battery for evaluation produced above, using a secondary battery charge / discharge test apparatus (manufactured by Hokuto Denko Co., Ltd.), the voltage range is 0.005V to 2.5V in a constant temperature bath at a temperature of 25 ° C. Charging / discharging was performed at a current value of 0.05 C when 1 C = 1600 mAh / g. Thereby, the initial charge capacity and the initial discharge capacity were obtained, and the initial charge / discharge efficiency was determined from this. The initial charge / discharge efficiency (%) is the ratio of the initial discharge capacity to the initial charge capacity (100 × initial discharge capacity / initial charge capacity). The higher this value, the lower the irreversible capacity, which means that an excellent doping effect is obtained. Table 1 shows the results together with the average ferret diameter and average circularity of the stabilized lithium powder.

[Examples 2 to 14]
The stabilized lithium powders of Examples 2 to 14 were obtained in the same manner as in Example 1 except that the production conditions of the stabilized lithium powder were changed to those shown in Table 1 below. Moreover, the lithium ion secondary battery for evaluation of Examples 2-14 was produced like Example 1 using the obtained stabilized lithium powder.

  Table 1 shows the results of various tests described in Example 1 performed on the evaluation lithium ion secondary batteries of Examples 2-14. As in Example 1, there was no damage to the negative electrode in all samples including Examples 2 to 14, and high initial charge / discharge efficiency was exhibited, and the average circularity and the average ferret diameter were further controlled. Stabilized lithium powder showing excellent dope performance was obtained.

[Examples 15 to 17]
Stabilization of Examples 15 to 17 was carried out in the same manner as in Example 1 except that commercially available Fe powder was added to the concentration shown in Table 2 simultaneously with the supply of carbon dioxide. Lithium powder was obtained. Moreover, the lithium ion secondary battery for evaluation of Examples 15-17 was produced like Example 1 using the obtained stabilized lithium powder.

  Table 2 shows the results of various tests described in Example 1 performed on the lithium ion secondary batteries for evaluation of Examples 15 to 17. In Examples 15 to 17, the initial charge and discharge efficiency superior to that of Example 1 was shown, and stabilized lithium powder exhibiting more excellent dope performance was obtained by controlling the amount of Fe to a suitable value. .

[Comparative Example 1]
A lithium ion secondary battery for evaluation of Comparative Example 1 was prepared in the same manner as in Example 1 using commercially available stabilized lithium powder (trade name: SLMP) manufactured by FMC. Moreover, the photograph of the stabilized lithium powder of the comparative example 1 image | photographed with the optical microscope is shown in FIG. From FIG. 2, it can be confirmed that the stabilized lithium powder is a true sphere.

[Comparative Example 2]
The stabilized lithium powder of Comparative Example 2 was obtained in the same manner as in Example 1 except that the production conditions for the stabilized lithium powder were changed to those shown in Table 1 below. Moreover, the lithium ion secondary battery for evaluation of the comparative example 2 was produced like Example 1 using the obtained stabilized lithium powder.

According to the stabilized lithium powder of the present invention, the doping efficiency can be improved. Moreover, the lithium ion secondary battery with which the initial stage charge / discharge efficiency was improved can be provided by using the electrode obtained by the said manufacturing method.
Moreover, the stabilized lithium powder of the present invention is not limited to lithium ion secondary battery applications, and can also be applied to electrochemical devices such as lithium ion capacitors and EDLCs (electric double layer capacitors).

Claims (4)

  A stabilized lithium powder, wherein C ≦ 0.90 when the average circularity of the particles is C.   2. The stabilized lithium powder according to claim 1, wherein FD ≦ 53.0 μm when an average ferret diameter of the particles is FD. 3. The stabilized lithium powder according to claim 1, wherein the particles contain a transition metal in an amount of 1.0 × 10 ⁻³ mass% or more and 1.0 × 10 ⁻¹ mass% or less. A lithium ion secondary battery comprising: a negative electrode obtained by doping the negative electrode using the stabilized lithium powder according to claim 1; a positive electrode; and an electrolyte.

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