JP2007012421A - Negative electrode for lithium ion battery and lithium ion battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion battery surely having a sufficient space absorbing stress caused by the expansion/contraction of a negative active material, and to provide the lithium ion battery using the negative electrode. <P>SOLUTION: The negative electrode has a current collector made of metal not alloyed with lithium and having a plurality of columnar projections 12 each having a height of 1 μm or higher, and the negative active material 13 formed on only the top of the plurality of columnar projections. Even if the lithium active material 1S repeats the volume expansion/contraction caused by the absorbing/releasing of lithium, although only surfaces of the columnar projections are affected by the expansion/contraction, since the spaces having no active material are present in the current collector, an adverse effect on the current collector by the expansion/contraction of the active material can be prevented. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion battery, and more particularly to the structure of a negative electrode.

  Currently, there is a demand for high-capacity lithium secondary batteries. In order to realize this, new development of negative electrode material (negative electrode active material) is particularly important. As a new negative electrode, a technique of applying Si or Sn alloyed with lithium or an alloy thereof to a negative electrode on a current collector is disclosed (see Patent Document 1). However, these materials have been difficult to put into practical use because of the large volume expansion / contraction associated with the insertion and extraction of lithium ions.

In order to solve this problem, attention is paid to a negative electrode in which a thin film of the above material is formed on a current collector (see Patent Document 2). In this technique, the negative electrode current collector is provided with irregularities having a thickness equal to or less than the thickness of the negative electrode active material to form columnar active material particles. This creates a space between the columnar particles, relieves stress due to expansion, and improves cycle characteristics.
Japanese Patent Laid-Open No. 10-255768 JP 2002-83594 A

  However, in the negative electrode described in Patent Document 2, since the negative electrode active material is formed on the surface of the current collector having irregularities using a CVD method, a sputtering method, or the like, the negative electrode active material also has an influence on the surface shape of the current collector. Receiving and uneven. At this time, a negative electrode active material is also formed in the concave portion of the current collector. Thus, since the negative electrode active material layer is formed over the entire surface of the current collector, if the negative electrode active material repeatedly expands and contracts as the lithium ions are absorbed and released, the surface of the current collector The whole is affected. As a result, the stress generated by the expansion / contraction of the negative electrode active material could not be relieved, and the current collector was wrinkled or cut, resulting in a deterioration in cycle characteristics.

  SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and has a sufficient space to absorb stress accompanying expansion / contraction of the negative electrode active material, and has a low cycle characteristic deterioration and a lithium ion battery using the same. An object is to provide a battery.

  In order to solve the above-described conventional problems, the negative electrode for a lithium ion battery according to the present invention is made of a metal that is not alloyed with lithium, and has a plurality of columnar protrusions having a height of 1 μm or more formed on the surface. And a negative electrode active material formed only on the upper surfaces of the plurality of columnar protrusions.

  With this configuration, even when the negative electrode active material repeatedly expands / contracts as Li is occluded / released, the surface of the columnar convex portion is affected, but the current collector does not have an active material. Therefore, the entire current collector is not affected by the expansion and contraction of the active material.

  According to the negative electrode having the structure of the present invention, the blank portion for relaxing the stress accompanying expansion / contraction of the negative electrode active material accompanying charging / discharging of the battery is provided. Thus, it is possible to provide a lithium secondary battery that can reduce deterioration of charge / discharge cycle characteristics of the battery.

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
FIG. 1 is a schematic cross-sectional view of a negative electrode according to the present invention. The negative electrode 10 has a plurality of columnar protrusions 12 provided on the current collector base 11 and having a height of 1 μm or more on the surface, and the negative electrode active material 13 is formed only on the upper surface of the columnar protrusions 12. Has been.

  As the current collector base material 11 and the columnar convex portions 12, metals such as copper, nickel, platinum, and gold, which are not alloyed with lithium, and alloys thereof can be used. The current collector base material 11 and the columnar convex portions 12 are preferably made of the same material.

  The columnar protrusions 12 are formed substantially perpendicular to the current collector base material 11. In the case where it is not substantially vertical, when the negative electrode active material 13 formed on the upper surface of the columnar protrusion 12 expands, the space for preventing the adjacent negative electrode active materials 13 from colliding with each other is substantially vertical. More are needed. As a result, the amount of the active material per unit area of the negative electrode decreases, which is not practical.

  The height of the columnar convex portion 12 is required to be 1 μm or more. Although it depends on the thickness of the negative electrode active material 13 formed on the upper portion, if the thickness is 1 μm or less, the effect of relaxing the stress cannot be sufficiently obtained. The upper limit of the height of the columnar convex part 12 is not particularly limited. However, when the cylinder or the prism is high, as a result, the ratio of the current collector to the negative electrode increases, and the capacity as the negative electrode decreases. Therefore, the upper limit of the height is determined in consideration of a practical capacity.

  In order to further enhance the effect of the present invention, the ratio of the thickness (t) of the negative electrode active material 13 formed on the upper surface of the columnar protrusion 12 to the distance (X) between the adjacent columnar protrusions 12 ( X / t) is preferably 2 or more. The distance between the adjacent columnar convex portions 12 refers to the shortest distance connecting the side surface of the columnar convex portion 12 and the side surface of the adjacent columnar convex portion 12. When the negative electrode active material 13 occludes lithium ions and expands, the expansion at the junction with the upper surface of the columnar convex portion 12 is smaller than the other portions, so that it expands in a fan shape. When the value of X / t is smaller than 2, when the negative electrode active material 13 formed on the columnar convex portion 12 expands, adjacent negative electrode active materials 13 collide with each other. As a result, stress is applied to the current collector, and in the worst case, the current collector is broken. The upper limit of X / t may be determined in consideration of the practical capacity and the ratio of the active material in the negative electrode.

  The columnar convex portion 12 may be a columnar shape or a prismatic shape, and a triangular column shape, a quadrangular column shape, a pentagonal column shape, and a hexagonal column shape are desirable in order to efficiently secure a space.

  Here, the shape of the portion (blank portion) where the columnar convex portions 12 of the current collector base material 11 are not formed is preferably flat. This is because it is easier to obtain the effect of the present invention when a shape such as a triangular groove portion where stress tends to concentrate is not formed in the blank portion.

As the negative electrode active material 13, a material generally used as a negative electrode of a lithium ion battery can be used. In particular, when a material having a large expansion when occlusion of lithium ions is used, the effect of the present application is great. Examples of the material having large expansion include a material containing at least one selected from the group consisting of Si, Sn, SiO _x (0 <X ≦ 2), and SnO _x (0 <X ≦ 2). Examples thereof include Si simple substance, Sn simple substance, alloys such as Ni ₃ Sn ₄ and Mg ₂ Sn, solid solutions, and compounds such as SiB ₄ and SiB ₆ .

  The negative electrode 10 of the present invention can be produced, for example, as follows. This will be described with reference to FIG. A resist 21 made of 2.3.5 trimethylphenol is applied on the current collector base 11 made of rolled copper foil having a smooth surface (FIG. 2a). Thereafter, the resist is patterned by an exposure method (FIG. 2b). A columnar convex portion 12 made of copper is formed thereon by vapor deposition or the like (FIG. 2c). A negative electrode active material layer 22 containing Si is formed thereon (FIG. 2d). The negative electrode active material layer 22 is formed by making a paste by adding a binder and a solvent to the powder of the active material, and forming the paste by a doctor blade method or the like. Alternatively, it may be formed by a dry thin film process such as sputtering, vacuum deposition, laser ablation, ion plating, or CVD (Chemical Vapor Deposition). Thereafter, a lift-off step of immersing in a resist stripping solution is performed (FIG. 2e).

  The negative electrode according to the present invention obtained as described above, and a positive electrode in which a positive electrode active material layer containing lithium cobaltate, which is a positive electrode active material that absorbs and releases lithium ions, is formed on a positive electrode current collector such as aluminum. Is sandwiched between separators made of porous polypropylene or the like, placed in a bag made of aluminum laminate or the like, and a lithium ion conductive electrolyte or polymer electrolyte is added to produce a lithium ion battery.

  In the lithium ion battery thus obtained, stress due to negative electrode expansion is relieved, wrinkles and cutting of the electrode plate are suppressed, and reliability is obtained because of less cycle deterioration.

  Hereinafter, the present invention will be described in more detail by way of specific examples. In addition, this invention is not limited to the Example shown below.

Example 1
The negative electrode 10 shown in FIG. 1 was produced by the following method. An electrolytic copper foil having a size of 20 mm × 20 mm and a thickness of 35 μm was used as the current collector base material 11, and a resist 21 was formed on the surface. RY-3315 manufactured by Hitachi Chemical Co., Ltd. was used as the resist. Next, ultraviolet light is irradiated through a quartz mask patterned by arranging 10 μm squares on one side (light quantity: 50 mJ / cm ² ), and immersed in an aqueous sodium carbonate solution (0.8 wt%, 25 ° C.) for 10 seconds to pattern the resist. went. The exposure machine used was EXM-1201 manufactured by Oak Manufacturing Co., Ltd., and the UV-350SD type manufactured by Oak Manufacturing Co., Ltd. was used as the light meter.

Next, copper was vapor-deposited from the top. The film thickness was 10 μm, and the film was formed by resistance heating vapor deposition. The degree of vacuum was 10 ⁻⁶ Torr, and 300 mg of copper ingot was placed thereon using a plate-like tungsten heater, and a current of 50 A was applied to the heater to melt and evaporate the copper (apparatus: SVC). -700T Sanyu Electronics Co., Ltd.).

Further, a SiO _0.3 film as a negative electrode active material layer 22 was formed thereon by an electron beam (EB) vapor deposition method with a thickness of 1 μm. Deposition conditions were EB power: 500 mA, degree of vacuum: 4 × 10 ⁻³ Pa, oxygen flow rate: 10 SCCM, film formation time: 1 hour (film formation apparatus manufactured by Shinko Seiki Co., Ltd.).

Next, it was immersed in an aqueous sodium hydroxide solution (2.0 wt%, 50 ° C.) for 10 seconds, and lift-off was performed to remove the resist, the copper film thereon, and the SiO _0.3 film thereon. Thereby, the shape of the columnar convex part 12 became a regular cube whose one side was 10 μm. The distance between the adjacent columnar convex portions 12 was 2.5 μm. The blank part was almost flat.

  Thereafter, in order to completely remove sodium hydroxide, it was taken out by being immersed in pure water for 10 minutes and dried in the atmosphere.

The negative electrode active material 13 made of SiO _0.3 formed in this manner was formed on the upper surface of the columnar convex portion 12 and was not formed in the blank portion.

(Comparative Example 1)
Next, as Comparative Example 1, a negative electrode 30 having no columnar protrusion as shown in FIG. In the electrode manufacturing method, an electrolytic copper foil having a thickness of 20 mm × 20 mm and a thickness of 35 μm is used as the current collector base material 31, and an SiO _0.3 film as the negative electrode active material layer 32 is formed thereon by an EB deposition method with a thickness of 1 μm. Filmed. Deposition conditions were EB power: 500 mA, degree of vacuum: 4 × 10 ⁻³ Pa, oxygen flow rate: 10 SCCM, film formation time: 1 hour (the film formation apparatus was manufactured by Shinko Seiki Co., Ltd.).

(Comparative Example 2)
Further, as Comparative Example 2, a negative electrode 40 having no columnar protrusions and only the active material was patterned as shown in FIG. In the electrode production method, an electrolytic copper foil having a size of 20 mm × 20 mm and a thickness of 35 μm was used as the current collector base material 41, and a resist was formed on the surface in the same manner as in Example 1 above. RY-3315 manufactured by Hitachi Chemical Co., Ltd. was used as the resist. Next, ultraviolet light is irradiated through a quartz mask patterned by arranging 10 μm squares on one side (light quantity: 50 mJ / cm ² ), and immersed in an aqueous sodium carbonate solution (0.8 wt%, 25 ° C.) for 10 seconds to pattern the resist. went. Next, a SiO _0.3 film as a negative electrode active material layer 42 was formed thereon with a thickness of 1 μm by EB vapor deposition. Deposition conditions were EB power: 500 mA, degree of vacuum: 4 × 10 ⁻³ Pa, oxygen flow rate: 10 SCCM, film formation time: 1 hour (the film formation apparatus was manufactured by Shinko Seiki Co., Ltd.).

Next, it was immersed in an aqueous sodium hydroxide solution (2.0 wt%, 50 ° C.) for 10 seconds, and lift-off was performed to remove the resist and the SiO _0.3 film thereon. Thereafter, in order to completely remove sodium hydroxide, it was taken out by being immersed in pure water for 10 minutes and dried in the atmosphere.

(Comparative Example 3)
Further, as Comparative Example 3, a negative electrode 50 having a SiO _0.3 film formed on a current collector having columnar protrusions as shown in FIG. In the electrode preparation method, an electrolytic copper foil having a size of 20 mm × 20 mm and a thickness of 35 μm was used as the current collector substrate 51, and a resist was formed on the surface. RY-3315 manufactured by Hitachi Chemical Co., Ltd. was used as the resist. Next, ultraviolet light is irradiated through a quartz mask patterned by arranging 10 μm squares on one side (light quantity: 50 mJ / cm ² ), and immersed in an aqueous sodium carbonate solution (0.8 wt%, 25 ° C.) for 10 seconds to pattern the resist. went.

  Next, copper was vapor-deposited from the top. The film thickness was 10 μm, and the film was formed by resistance heating vapor deposition. Next, it was immersed in an aqueous sodium hydroxide solution (2.0 wt%, 50 ° C.) for 10 seconds, and lift-off was performed to remove the resist and the copper film thereon. Thereafter, in order to completely remove sodium hydroxide, it was taken out by being immersed in pure water for 10 minutes and dried in the atmosphere. Thereby, the columnar convex part 53 was formed. The shape of the columnar convex portion 53 was a cube having a side of 10 μm. Further, the distance between adjacent columnar convex portions 12 was 2.5 μm. The blank part was almost flat.

Further, a SiO _0.3 film as a negative electrode active material layer 52 was formed thereon with a thickness of 1 μm by EB vapor deposition. Deposition conditions were EB power: 500 mA, degree of vacuum: 4 × 10 ⁻³ Pa, oxygen flow rate: 10 SCCM, film formation time: 1 hour (the film formation apparatus was manufactured by Shinko Seiki Co., Ltd.).

(Production of battery)
Next, a positive electrode to be combined with the negative electrodes prepared in Example 1 and Comparative Examples 1 to 3 was prepared as follows. First, a 20 mm × 20 mm platinum foil having a thickness of 50 μm is used as the substrate 12, and then LiCoO ₂ is used as the first active material 13, and a sputtering method (200 W power, Ar / O ₂ = 3/1 is 20 SCCM at a thickness of 2 μm. , 20 mTorr), and further heat-treated in a tube furnace at 800 ° C. for 2 hours in the atmosphere to obtain a positive electrode.

As an electrolytic solution, 1 mol / l LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and diethylene carbonate (mixing volume ratio = 1: 2) was used.

  As the separator, a polypropylene separator (thickness 20 μm) manufactured by Celgard was used.

The positive electrode and the negative electrodes of Examples and Comparative Examples 1 to 3 are combined so that the active materials oppose each other, a separator is disposed between the positive electrode and the negative electrode, and laminated, and then an aluminum laminate bag The electrolyte solution was injected at 1 cm ³ , the electrolyte solution inlet of the bag was sealed by heat sealing, the bag was sandwiched between 2 mm-thick glass plates, and fixed with clips to prepare a model cell.

(Evaluation)
The cycle characteristics of the obtained model cell were obtained as follows. Charging is performed at a current of 0.1 mA up to 4.2 V, and subsequent discharging is performed at a current of 0.1 mA up to 3.0 V. This charge / discharge cycle was performed 200 times, and the cycle characteristic was determined by multiplying the value obtained by dividing the discharge capacity of the first cycle by the discharge capacity of the 200th cycle by 100. The discharge capacity at the first cycle of the produced model cell was approximately 0.5 mAh. Furthermore, after 200 cycles of the charge and discharge, the aluminum laminate bag was opened, the negative electrode was taken out, and the presence or absence of wrinkles on the negative electrode current collector was examined. The results are shown in Table 1.

  As is clear from Table 1, Example 1 had higher cycle characteristics than Comparative Examples 1 to 3, and no negative electrode current collector was found. This is considered to be because in Example 1, the blank portion of the current collector relaxed the stress due to the volume expansion of the active material. In Comparative Example 2, the active material was directly formed on the current collector base material although it had a blank portion. In Comparative Example 3, it was considered that stress relaxation was insufficient because there was no blank portion. Further, in Comparative Example 1 having no blank portion, a large number of wrinkles were generated on the current collector, and cutting of the current collector was observed.

(Example 2)
Next, a result of performing the same evaluation as in Example 1 with the same structure as that of the negative electrode in Example 1 above, with the height of the columnar convex portion being changed will be shown. The height of the columnar protrusions was adjusted by adjusting the copper deposition time in Example 1. Others were the same as in Example 1. The results are shown in Table 2.

  Accordingly, it was found that if the height of the columnar convex portion is 1 μm or more, the cycle characteristics are effective. This is presumably because the stress applied to the substrate due to the expansion of the active material deposited on the columnar protrusions is reduced by increasing the columnar protrusions.

(Example 3)
Next, with the same structure as that of the negative electrode of Example 1, the distance (X) between adjacent columnar protrusions was changed, and the same evaluation as in Example 1 was performed to determine the thickness (t) of the negative electrode active material and The relationship with X was determined. The distance between adjacent columnar protrusions was changed by changing the resist patterning in Example 1. Others were the same as in Example 1. The results are shown in Table 3.

  As is apparent from Table 3, characteristics were improved when X / t was 2 or more.

  Moreover, the same result was obtained when the columnar convex portions were cylindrical, triangular, quadrangular, pentagonal, or hexagonal.

  Note that the shape of the columnar protrusions in one current collector need not all be the same. For example, even if a cylinder and a square column are mixed, the height of the columnar protrusions is 1 μm or more, and the columnar protrusions If the negative electrode active material is formed only on the upper surface, the effect of the present invention can be obtained regardless of the shape of the columnar protrusions.

  In the negative electrode according to the present invention, the current collector is provided with a plurality of columnar convex portions, the negative electrode active material is formed only on the upper surface of the columnar convex portions, and the current collectors other than the plurality of columnar convex portions have active By providing a blank area where no material is formed, stress concentration due to volume change of the active material can be alleviated, and the occurrence of wrinkling and cutting of the current collector can be prevented to suppress deterioration of cycle characteristics. Is possible. For this reason, by using the electrode of the present invention, it is possible to produce a high-capacity lithium secondary battery excellent in reliability such as cycle characteristics.

Schematic cross-sectional view of the negative electrode in Embodiment 1 of the present invention Schematic cross-sectional view showing an example of a method for manufacturing a negative electrode in Embodiment 1 of the present invention Schematic sectional view of a negative electrode in Comparative Example 1 of the present invention Schematic sectional view of the negative electrode in Comparative Example 2 of the present invention Schematic sectional view of the negative electrode in Comparative Example 3 of the present invention

Explanation of symbols

10, 30, 40, 50 Negative electrode 11, 31, 41, 51 Current collector base material 12, 53 Columnar projection 13 Negative electrode active material 21 Resist 22, 32, 42, 52 Negative electrode active material layer

Claims (5)

A current collector made of a metal that is not alloyed with lithium and having a plurality of columnar protrusions having a height of 1 μm or more formed on the surface;
A negative electrode active material formed only on the upper surfaces of the plurality of columnar convex portions;
A negative electrode for a lithium ion battery. The ratio (X / t) between the distance (X) between the adjacent columnar convex portions and the thickness (t) of the negative electrode active material is 2 or more;
The negative electrode for a lithium ion battery according to claim 1. The columnar convex portion is one selected from the group consisting of a cylinder, a triangular column, a quadrangular column, a pentagonal column, and a hexagonal column;
The negative electrode for a lithium ion battery according to claim 1 or 2, wherein: The negative electrode active material includes at least one selected from the group consisting of Si, Sn, SiO _x (0 <X ≦ 2) and SnO _x (0 <X ≦ 2);
The negative electrode for a lithium ion battery according to any one of claims 1 to 3. A negative electrode for a lithium secondary battery according to any one of claims 1 to 4,
A positive electrode having a positive electrode active material that absorbs and releases lithium ions;
A lithium ion conductive electrolyte or polymer electrolyte;
Lithium ion battery with

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