JP2007227239A - Anode for a lithium secondary battery and lithium secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode for a lithium secondary battery capable of restraining deterioration of a current-collecting property inside an electrode due to exfoliation of a mixture layer from a collector, with a large charging and discharging capacity, and with excellent charge/discharge cycle characteristics, as well as a lithium secondary battery. <P>SOLUTION: A mixture obtained by mixing a metal material with its volume expanding/contracting by absorbing/releasing lithium, and scaly graphite with shearing stress applied in a dry state is used as an active material. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery using the same.

  In recent years, as one of new secondary batteries with high output and high energy density, there has been a lithium secondary battery that uses a non-aqueous electrolyte and moves lithium ions between a positive electrode and a negative electrode for charging and discharging. It's being used.

  In such a lithium secondary battery, graphite having a theoretical capacity of 372 mAh / g is used as the negative electrode material, but in order to increase the capacity of the battery, it is essential to increase the capacity of the negative electrode material. As one of negative electrode materials having a theoretical capacity larger than that of graphite, a material in which a negative electrode active material is alloyed with lithium and the negative electrode active material is applied to a negative electrode current collector is used.

  However, in such an electrode for a lithium secondary battery, separation of the mixture layer from the current collector occurs due to stress generated in the mixture layer due to expansion / contraction of the volume of the active material accompanying the charge / discharge reaction. In some cases, the current collecting property in the electrode is lowered and the charge / discharge characteristics are lowered.

  In Patent Document 1, a lithium secondary battery negative electrode having a three-layer structure in which a metal capable of inserting and extracting lithium is fixed to the surface of graphite particles by mechanochemical treatment, and a carbon layer is further formed on the surface. A composite carbon material is disclosed.

However, even if such a conventional composite material is used, it is not possible to suppress a decrease in the current collecting property in the electrode due to the peeling of the mixture layer from the current collector, and good charge / discharge cycle characteristics are obtained. I could not.
JP 2004-185975 A

  An object of the present invention is to provide a lithium secondary battery that can suppress a decrease in current collecting property in an electrode due to peeling of a mixture layer from a current collector, has a large charge / discharge capacity, and is excellent in charge / discharge cycle characteristics. And providing a lithium secondary battery using the same.

  The negative electrode for a lithium secondary battery of the present invention uses a mixture obtained by mixing a metallic material whose volume expands and contracts by occluding and releasing lithium and scaly graphite while applying a shear stress in a dry process. It is characterized by being used as a substance.

  In the present invention, a mixture obtained by mixing the metal material and scaly graphite while applying a shear stress in a dry process is used as an active material. Since mixing is performed while applying a shearing stress in a dry manner, the metal material is pressed against the scaly graphite with a strong force, and the metal material and the scaly graphite adhere firmly. In particular, since scaly graphite has a layered structure, the metal material can be firmly held by the layered structure. Therefore, even if the metal material expands / shrinks during charging / discharging, the active material layer can be prevented from peeling from the current collector.

  The flaky graphite is a graphite material having a primary particle shape that is flaky and highly oriented. The thickness in the c direction of the flaky graphite is generally 3 μm or less, preferably 0.1 to 3 μm. Moreover, the average value of each width | variety of scale-like graphite a direction and b direction is 3 times or more of the thickness of c direction.

  In the present invention, the mixing ratio of the flaky graphite in the mixture of the metal material and the flaky graphite is preferably 70% or more and less than 100% by volume, more preferably in the range of 70 to 95%. More preferably, it is 70 to 90% of range.

  As the metal material in the present invention, a metal material such as Si, Sn, Ge, Al, etc., which has an energy density larger than that of the carbon material and whose volume expands / contracts as lithium is occluded / released is used. Among these metal materials, particularly, Si has a large theoretical density of 4200 mAh / g and is suitable for increasing the capacity of the battery. In addition, since Si has low electrical conductivity, the voltage can be lowered by using it together with flake graphite.

  The metal material used in the present invention may be an alloy material containing Si or Sn. Examples of such an alloy material include an intermetallic compound of Si or Sn and one or more other elements, and a eutectic alloy of Si or Sn and one or more other elements. Examples of the method for producing the alloy include an arc melting method, a liquid quenching method, a mechanical alloying method, a sputtering method, a chemical vapor deposition method, and a firing method. In particular, examples of the liquid quenching method include a single roll quenching method, a twin roll quenching method, and various atomizing methods such as a gas atomizing method, a water atomizing method, and a disk atomizing method.

  In addition, the metal material particles used in the present invention may have a surface coated with a metal or the like. Examples of the coating method include an electroless plating method, an electrolytic plating method, a chemical reduction method, a vapor deposition method, a sputtering method, and a chemical vapor deposition method.

  The average particle diameter of the metal material particles used in the present invention is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less. As the particle size of the metal material particles is smaller, good cycle characteristics tend to be obtained.

  The negative electrode for a lithium secondary battery in the present invention is formed by applying a slurry containing a mixture of the above metal material and scaly graphite and a binder on a metal foil current collector to form a mixture layer on the current collector. Can be manufactured.

  The binder in the mixture layer is preferably a thermoplastic resin.

  Examples of the metal foil current collector used in the present invention include an alloy made of a metal such as copper, nickel, iron, titanium, cobalt, or a combination thereof. In particular, a metal foil containing a copper element is preferable, and a copper foil or a copper alloy foil is more preferable.

  As a method for producing a negative electrode for a lithium secondary battery of the present invention, the above metal material and scaly graphite are mixed while applying a shear stress in a dry process to produce a mixture, and the resulting mixture and binder are combined. Examples of the method include a step of uniformly mixing to prepare a slurry and a step of applying the slurry on a current collector and drying to form a mixture layer.

  A lithium secondary battery according to the present invention includes the above-described negative electrode according to the present invention, a positive electrode including a positive electrode active material, and a nonaqueous electrolyte.

The solvent of the electrolyte used in the lithium secondary battery of the present invention is not particularly limited, but a cyclic carbonate such as ethylene carbonate, propylene carbonate, and butylene carbonate, and a chain such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. A mixed solvent with carbonate is exemplified. Further, mixed solvents of the cyclic carbonate and ether solvents such as 1,2-dimethoxyethane and 1,2-diethoxyethane are also exemplified. The electrolyte solutes include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , and mixtures thereof Illustrated. Among these, LiPF ₆ is particularly preferably used. Further, examples of the electrolyte include gel polymer electrolytes in which a polymer electrolyte such as polyethylene oxide and polyacrylonitrile is impregnated with an electrolytic solution, and inorganic solid electrolytes such as LiI and Li ₃ N. The electrolyte of the lithium secondary battery of the present invention is not limited as long as the lithium compound as a solute that exhibits ionic conductivity and the solvent that dissolves and retains the lithium compound do not decompose at the time of battery charging, discharging, or storage. Can be used.

As the positive electrode active material of the lithium secondary battery of the present _{invention, LiCoO 2, LiNiO 2, LiMn} 2 O 4, LiMnO 2, LiCo 0.5 Ni 0.5 O 2, LiNi 0.7 Co 0.2 Mn 0. Examples include lithium-containing transition metal oxides such as ₁ O ₂ and metal oxides such as MnO ₂ that do not contain lithium. In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery which can suppress the fall of the current collection property in an electrode by peeling of the mixture layer from a collector, has a large charging / discharging capacity | capacitance, and was excellent in charging / discharging cycling characteristics. Negative electrode and lithium secondary battery.

  Hereinafter, the present invention will be described in more detail on the basis of examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications without departing from the scope of the present invention. Is.

[Production of Negative Electrode 1]
Of the active material, scaly artificial graphite having an average particle diameter of 20 μm was used as the carbon material, and Si powder having an average particle diameter of 2.5 μm was used as the metal material. These materials were weighed so that the volume ratio was carbon: alloy = 9: 1 and mixed to obtain an electrode mixture. Then, it mixed with the N-methylpyrrolidone solution containing the polyvinylidene fluoride as a binder, and was set as the negative mix slurry. The weight ratio of the electrode mixture to the binder was 95: 5.

  This negative electrode mixture slurry was applied onto a copper foil as a current collector and dried. The obtained product was cut into a 20 × 20 mm square shape, rolled and attached with a current collecting lead tab to obtain a negative electrode.

[Preparation of negative electrode 2]
A negative electrode 2 was produced in the same procedure as the negative electrode 1 except that mechanofusion (manufactured by Hosokawa Micron) was used to mix the carbon material and the alloy material with the negative electrode 1 and mixed while applying shear stress.

[Preparation of Negative Electrode 3]
A negative electrode 3 was produced in the same procedure as the negative electrode 1 except that artificial spherical graphite was used as the carbon material for the negative electrode 1.

[Preparation of Negative Electrode 4]
A negative electrode 4 was produced in the same procedure as the negative electrode 2 except that artificial spherical graphite was used as the carbon material for the negative electrode 2.

[Preparation of Negative Electrode 5]
A negative electrode 5 was produced in the same procedure as the negative electrode 2 except that the volume ratio of carbon material: alloy material was 3: 7 with respect to the negative electrode 2 and was used.

[Preparation of Negative Electrode 6]
A negative electrode 6 was produced in the same procedure as the negative electrode 2 except that the volume ratio of carbon material: alloy material was 7: 3 with respect to the negative electrode 2.

(Preparation of electrolyte)
LiPF ₆ was dissolved in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 so as to be 1 mol / liter to prepare an electrolytic solution.

[Production of 3-electrode cell]
Using the negative electrodes 1 to 6 as working electrodes, a three-electrode cell was produced. The above electrolytic solution was used as the electrolytic solution, and lithium metal was used as the counter electrode and the reference electrode.

(Charge / discharge test)
A charge / discharge test was conducted using the above three-electrode cell. After charging at a constant current up to 0 V (vs. Li / Li ⁺ ) at 1 mA / cm ² , charging was performed at a constant current up to 0 V at 0.125 mA / cm ² . Discharging was performed at a constant current up to 2.0 V at 1 mA / cm ² .

  The first cycle discharge capacity / first cycle charge capacity measured in the charge / discharge test is shown in Table 1 as the initial efficiency. In addition, Table 5 shows the discharge capacity at the 5th cycle / the charge capacity at the 1st cycle as a discharge rate after 5 cycles.

  The charge capacity of the electrode of Example 1 was 700 mAh / g, and the discharge capacity was 616 mAh / g. Therefore, it can be seen that the charge / discharge capacity is significantly increased as compared with the case of using scaly graphite alone.

  As shown in Table 1, the negative electrode 2 of Example 1 in which flaky graphite and Si powder were mixed while applying shear stress was higher in initial efficiency (initial value) than the negative electrode 1 of Comparative Example 1 that was mixed without applying shear stress. (Charge / discharge efficiency) is high, and the discharge rate after 5 cycles is also high. Therefore, it can be seen that the charge / discharge cycle characteristics are also improved.

  In Comparative Example 2 and Comparative Example 3 using spheroidal graphite as the carbon material, the improvement rate of the discharge rate after 5 cycles is lower than when scaly graphite is used, even if mixed by shear stress, It can be seen that the improvement of the charge / discharge cycle characteristics is an effect that is significantly obtained when scaly graphite is used. Since the scaly graphite has a surface area larger than that of the spherical graphite, the contact area with the metal material is large, and since it has a layered structure, the metal material can be held more firmly, and the effect of the present invention can be achieved. It seems to be obtained.

  Further, as apparent from the comparison of the negative electrode 2 (Example 1), the negative electrode 5 (Example 2), and the negative electrode 6 (Example 3), the mixing ratio of the flake graphite is 70% or more and less than 100%. Thus, it can be seen that the initial efficiency and the discharge rate after 5 cycles are high, and the effects of the present invention can be obtained more effectively.

[Microscopic observation of negative electrode]
FIG. 1 is a photomicrograph showing the surface of the negative electrode 2 (Example 1), and FIG. 2 is a photomicrograph showing the surface of the negative electrode 1 (Comparative Example 1). 1 and 2, the bright particles are Si powder. As is clear from comparison between FIG. 1 and FIG. 2, in the negative electrode 2 in which Si powder and flaky graphite are mixed while applying shear stress, it is observed that Si powder is taken into the flaky graphite. Therefore, it is considered that when the Si powder is firmly taken into the flaky graphite in this way, peeling of the mixture layer from the current collector is suppressed, and good charge / discharge cycle characteristics can be obtained.

The microscope picture which shows the surface of the negative electrode 2 (Example 1). The microscope picture which shows the surface of the negative electrode 1 (comparative example 1).

Claims (4)

  Lithium secondary characterized by using, as an active material, a mixture obtained by mixing a metallic material whose volume expands and contracts by inserting and extracting lithium and scaly graphite while applying shear stress in a dry process Battery negative electrode.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the mixture ratio of the flaky graphite in the mixture is 70% or more and less than 100% by volume ratio.   The negative electrode for a lithium secondary battery according to claim 1 or 2, wherein the metal material contains Si.   A lithium secondary battery comprising the negative electrode according to claim 1, a positive electrode, and a nonaqueous electrolyte.