JP6375842B2 - Stabilized lithium powder and lithium ion secondary battery using the same - Google Patents

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 (SiOx), 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 charging and discharging, 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, an electrode used for a lithium ion secondary battery has a step of forming a layer containing a negative electrode active material on a current collector and then closely contacting it with a press. In the pre-doping step, the lithium metal of the stabilized lithium powder is reduced by the press. Pre-doping progresses by exposing. Therefore, the characteristics required for the stabilized lithium powder are not only for improving the stability of lithium but also for doping characteristics for producing excellent battery characteristics.

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

  However, when the stabilized lithium powder as described in the above-mentioned patent document is used, the stabilized film of the stabilized lithium powder remains on the electrode after pressing to form a resistive film, and the high rate characteristic when the battery is formed is There was a problem of deterioration.

  This invention is made | formed in view of the subject which the said prior art has, and aims at providing the stabilized lithium powder which can improve a high rate characteristic.

  In order to achieve the above object, the stabilized lithium powder according to the present invention is characterized in that it has a stabilizing film made of an inorganic solid electrolyte on the surface of metallic lithium particles. By forming the stabilization film with an inorganic solid electrolyte having high ion conductivity, even if the stabilization film remains on the electrode after pressing, the movement of lithium ions is not inhibited, and the high-rate characteristics are greatly improved.

  The inorganic solid electrolyte is at least one oxide selected from the group consisting of Ti, Al, La, Zr, Ge, and Si, sulfide, nitride, or phosphate compound. It is preferable to contain.

Furthermore, from the viewpoint of ion conductivity, the inorganic solid electrolyte includes lithium-aluminum-titanium-phosphate (LATP), lithium lanthanum zirconate (LLZ), lithium phosphate oxynitride (LiPON), Li ₂ O— Si ₂ O, Li ₂ S—Ge ₂ S, and Li ₂ S—P ₂ S ₅ are more preferable.

  The inorganic solid electrolyte is preferably 1 part by mass or more and 10 parts by mass or less with respect to the entire stabilized lithium powder. According to this, further improvement of the high rate characteristic is expected.

  The stabilized lithium powder preferably has C ≦ 0.90 where C is the average circularity of the particles. With such a shape, stress concentrates locally on the particles, and the stabilization film made of a hard solid electrolyte can be crushed stably and easily, and the productivity of the pre-doping process can be improved.

  According to the present invention, a stabilized lithium powder capable of improving the high rate characteristics can be obtained by improving the ionic conductivity of the stabilized lithium powder remaining on the electrode as a remaining film after the pre-doping step.

It is a schematic cross section of the stabilized lithium powder of this embodiment. 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 is one in which the surface of the metallic lithium particles 1 is coated with a stabilizing film 2 made of a solid electrolyte as shown in FIG. The stabilizing membrane may be a particle coated with a single or a plurality of inorganic solid electrolytes.

The inorganic solid electrolyte is preferably a solid electrolyte of an oxide, sulfide, nitride, or phosphate compound of at least one element selected from the group consisting of Ti, Al, La, Zr, Ge, and Si. For example, β-alumina, ZrO _x , LaO _x , TiO _x , GeO _x , LiSICON, NASICON, LATP, LLZ, LiPON, Li ₂ O—Si ₂ O, Li ₂ S—Ge ₂ S, Li ₂ S—P ₂ S ₅ may be mentioned as examples of suitable solid electrolytes. These are preferably used by firing in a reducing atmosphere as necessary to cause oxygen deficiency or ion implantation to improve ion conductivity.

  In the stabilized lithium powder, the proportion of the solid electrolyte is preferably 1 part by mass or more and 10 parts by mass or less, and more preferably 1 part by mass or more and 5 parts by mass or less. According to this, it becomes possible to further improve the high rate characteristics while maintaining the protection performance of the stabilized lithium powder.

The average circularity of the stabilized lithium powder is preferably 0.90 or less. The smaller the degree of circularity, the easier the solid electrolyte membrane is crushed and the pre-doping proceeds more effectively.

(Method for producing metallic lithium particles)
Next, a method for producing metallic lithium particles will be described. First, a lithium ingot is charged into a hydrocarbon oil, heated to a temperature higher than the melting point of lithium, this molten lithium-hydrocarbon oil mixture is stirred for a sufficient time to form a dispersion, and then cooled to form a metallic lithium. Make particles.

  Use a heat-resistant container with a baffle plate, and stir the container by tilting it 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 necessary for producing 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-220 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.

(Method for producing stabilized lithium powder)
Thereafter, a solid electrolyte is added to the metal lithium particles, and dry pulverization is performed with a ball mill, thereby obtaining a stabilized lithium powder having the surface of the metal lithium particles coated with the solid electrolyte.

Examples of the ball mill include an attritor device, a planetary ball mill, a vibration mill, a conical mill, and a tube mill. In dry pulverization with a ball mill, basically, particles such as steel balls collide with each other in a pulverizer, and the particles are crushed, alloyed, or amorphized. Particles are aggregated and adhered by impact action to form composite particles.

In addition, it is preferable to perform the process by a ball mill in inert gas atmosphere, such as argon and helium, so that a metal lithium particle may not react.

The solid electrolyte for forming the stabilizing film is preferably mixed as a powder, and the particle diameter of the solid electrolyte is 0.01 when the average particle diameter of the metallic lithium particles prepared above is 1. It is preferable that it is -0.2. The average particle diameters of the solid electrolyte and the metal lithium particles can be measured by a known method such as a laser diffraction particle size distribution analyzer.

<Negative electrode>
Thus, the stabilized lithium powder described above has high ionic conductivity of the stabilizing film remaining on the negative electrode after pre-doping, and can improve the high-rate characteristics when it is made into a 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 after drying, this is pressed, so that the negative electrode active material is doped with lithium. Proceed 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. 2 shows a schematic cross-sectional view of the lithium ion secondary battery of the present embodiment.

  A lithium ion secondary battery 100 can be produced by producing the lithium-doped negative electrode 20 produced as described above, the positive electrode 10, and the separator 18 impregnated with an electrolyte as shown in FIG. It can be produced by forming the negative electrode active material layer 24 on the negative electrode current collector 22. Here, the positive electrode 10 can be produced by forming the positive electrode active material layer 14 on the positive electrode current collector 12. 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 (for example, PF ₆ ⁻ ) of the lithium ions. 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 metallic lithium particles)
A stainless steel resin flask reactor having a baffle plate attached to the inner wall was added 100 g of lithium ingot manufactured by Kanto Chemical Co. and Carnation hydrocarbon oil manufactured by Witco, and the inside of the container was replaced with dry argon. The reactor was then heated to 200 ° C. to melt the lithium ingot. The mixture was stirred for 10 minutes at a rotational speed of 8000 rpm with the container tilted 5 degrees in the molten state, and then cooled to room temperature over 1 hour with stirring maintained, and the resulting powder was washed with hexane. As a result, metallic lithium particles were obtained.

(Production of stabilized lithium powder)
5 parts by mass of LATP was added to 95 parts by mass of the obtained metal lithium particles, and the mixture was rotated at a rotational speed of 300 rpm / min for 2 minutes in a ball mill under an argon atmosphere to obtain a stabilized lithium powder.
(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 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. Then, it pressurized by the force of 30 kN with the hand press, and lithium was doped to the negative electrode, and the lithium dope negative electrode was obtained.

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

<Measurement of average circularity>
About the stabilized lithium powder produced above, particles were observed using an optical microscope. The obtained observed image was binarized, and the circularity of the particles was determined by image analysis. The above image analysis was performed on at least 500 particles, and the average circularity of the synthesized stabilized lithium powder was determined. The circularity C is defined as C = 4πS / L ² where S is the particle area and L is the perimeter.

<Measurement of initial efficiency and high rate capacity maintenance ratio>
About the lithium ion secondary battery for evaluation produced above, a secondary battery charge / discharge test apparatus (manufactured by Hokuto Denko Co., Ltd.) was used, the voltage range was 0.005 V to 2.5 V, and 1C = 1600 mAh / g. Charging / discharging was performed at a current value of 0.05 C to obtain initial efficiency. The initial 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.

  Similarly, charge and discharge are performed at current values of 0.2 C and 2 C, discharge capacities at the respective rates are obtained, and a high rate capacity retention ratio (2 C discharge capacity / 0.2 C discharge capacity × 100 is obtained from these values as a high rate characteristic. ) When the ion conductivity of the stabilized film of the stabilized lithium powder remaining as the remaining film on the negative electrode is high, the high ionization rate is not hindered and the high retention rate is exhibited. The results are shown in Table 1 together with the average circularity and the initial efficiency.

[Examples 2 to 12]
The stabilized lithium powders of Examples 2 to 12 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 2-12 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 lithium ion secondary batteries for evaluation of Examples 2 to 12. In Examples 2-6, regardless of the type of solid electrolyte, excellent high rate characteristics are shown as in Example 1, indicating that the migration of lithium ions is not inhibited by the remaining film remaining on the electrode after pre-doping. ing. Even when the solid electrolyte addition amount was changed in Examples 7 to 12, a high high-rate capacity retention ratio and an initial efficiency were exhibited, and the high-rate characteristics were improved while maintaining the protective performance of the stabilized lithium powder.

[Comparative Example 1]
A lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 1 using commercially available stabilized lithium powder (trade name: SLMP) manufactured by FMC.

  Table 1 shows the results of various tests described in Example 1 performed on the evaluation lithium ion secondary battery of Comparative Example 1. The stabilization film of the stabilized lithium powder of Comparative Example 1 is formed of Li 2 O, and high resistance Li 2 O remains as a residual film on the electrode after pre-doping, so that the high rate characteristics are deteriorated.

By using the stabilized lithium powder of the present invention for pre-doping, it is possible to provide a lithium ion secondary battery with greatly improved high rate characteristics.

DESCRIPTION OF SYMBOLS 1 ... Metal lithium, 2 ... Stabilization film, 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer 30 ... laminate, 50 ... case, 52 ... metal foil, 54 ... polymer film, 60, 62 ... lead, 100 ... lithium ion secondary battery.

Claims (3)

The surface of the metal lithium particles is provided with a stabilizing film made of an inorganic solid electrolyte, and the inorganic solid electrolyte is composed of lithium-aluminum-titanium-phosphate, lithium lanthanum zirconate, lithium phosphate oxynitride, Li ₂ O—Si. 1. A stabilized lithium powder comprising any one of ₂ O, Li ₂ S—Ge ₂ S, and Li ₂ S—P ₂ S ₅ . 3. The stabilized lithium powder according to claim 2 , wherein the inorganic solid electrolyte is 1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the entire stabilized lithium powder. 3. The stabilized lithium powder according to claim 1, wherein C ≦ 0.9 when the average circularity of the stabilized lithium powder is C. 4.

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