JP2009032429A - Lithium reaction electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium reaction electrode and its manufacturing method capable of obtaining a large surface area, and efficiently obtaining high conductivity without adding an conduction auxiliary material such as carbon black or a binder. <P>SOLUTION: The lithium reaction product is comprised of a metallic porous body and an oxide film formed on the surface of the metallic porous body and electrochemically reacting with a lithium ion, the metallic porous body has a specific surface area to an apparent area of ≥10, and is made of valve metal which is not alloyed with Li, especially made of Ta, and preferably has a porosity of 30-70%, an average particle size of 10 nm to 1 μm, the oxide film has a thickness of ≤100 nm. Preferably, the metallic porous body is manufactured by a vapor deposition process, and the oxide film is formed by anodic oxidation process. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium reaction electrode used for a lithium ion secondary battery and a method for producing the same.

  In secondary batteries, particularly lithium ion secondary batteries, an oxide such as lithium cobaltate or lithium nickelate is used as the positive electrode material, and graphite (theoretical capacity: 372 mAh / g, 837 mAh / cc) is used as the negative electrode material. It is used. In some cases, an oxide such as lithium titanate is also used as a negative electrode material. All of these are layered compounds capable of electrochemically inserting and extracting lithium ions.

  These materials are all powders, and are used as electrode materials by a method of forming a sheet by rolling, mixing with an appropriate conductive auxiliary agent and binder, or applying to a current collector. These powders are usually used in a size of several μm to tens of μm. The reason is that the insertion / release reaction of lithium ions is an interfacial reaction, and a large particle such as several tens of μm is surface-controlled, and a sufficient response speed cannot be obtained. Therefore, it is conceivable to use a finer powder in order to improve the reaction rate, that is, the charge / discharge rate.

  For example, Japanese Patent Laid-Open No. 2002-8647 describes that a nanoporous material using fine particles of 100 nm or less is used as a negative electrode material. Japanese Patent Laid-Open No. 11-7955 describes that a composite oxide obtained by a coprecipitation method and having an average primary particle size of 0.01 to 1 μm and having a uniform composition is used as a negative electrode material. Yes.

  However, when an oxide powder is used as the negative electrode material, the finer the powder, the more conductive auxiliary agents and binders are required to ensure conductivity, so the capacity per volume as the negative electrode material is increased. It will decline. Moreover, when apply | coating the paste slurry obtained by mixing and kneading | mixing, the thickness of a negative electrode material will be several tens of micrometers or more. In the electrode having such a thickness, the diffusion of the electrolyte is rate-determining, and it cannot be expected that the charge / discharge rate is greatly improved.

On the other hand, it has also been proposed to use an oxide thin film as an electrode. For example, JP-A-8-241707 discloses that a thin film made of a complex oxide of V, Nb, W, Mo and the like and Li (lithium) is used as an electrode. Specifically, thin film electrodes made of these complex oxides are formed by a method such as RF sputtering. Japanese Patent Application Laid-Open No. 2004-349237 describes a method of forming a negative electrode composed of a thin film of SiO _x by sputtering or ion plating.

Oxide thin films such as these have a high lithium occlusion property, and can be directly formed on a current collector such as a metal foil, which is advantageous in terms of reaction rate. There is a problem that it is difficult to secure a sufficient amount of active material. In response to such a problem, when the film thickness is increased in order to increase the amount of the active material, there is a problem that the reactivity decreases.
JP 2002-8647 A Japanese Patent Laid-Open No. 11-7955 JP-A-8-241707 JP 2004-349237 A

  The present invention provides a lithium reaction electrode capable of obtaining a large surface area and efficiently obtaining high conductivity without adding a conductive additive such as carbon black or a binder, and a method for producing the same. For the purpose.

  The lithium reaction electrode according to the present invention comprises a metal porous body and an oxide film formed on the surface of the metal porous body and electrochemically reacting with lithium ions.

  The specific surface area of the metal porous body is preferably 10 or more with respect to the apparent area.

  The metal porous body is preferably made of a valve metal that is not alloyed with lithium, and the oxide film is preferably an oxide film of the valve metal, and in particular, tantalum is preferably used as the valve metal.

  It is preferable that the porosity of the metal porous body is 30 to 70%.

  Moreover, it is preferable that the average particle diameter of the metal which comprises the said metal porous body is 10 nm or more and 1 micrometer or less.

  Furthermore, the thickness of the oxide film is preferably 100 nm or less.

  When producing the lithium reaction electrode according to the present invention, it is preferable to produce the metal porous body by a vapor phase method and to form the oxide film by an anodic oxidation method.

  The lithium reaction electrode of the present invention is porous and has a high specific surface area, and has extremely high conductivity. Further, it has a large capacity and excellent cycle characteristics, and is suitable as an electrode of an energy device in which lithium is involved.

  The lithium reaction electrode of the present invention comprises a metal porous body and an oxide film that forms the surface of the metal porous body and electrochemically reacts with lithium ions.

In the reaction at the lithium reaction electrode of the present invention, it is considered that the oxide films M _x O and Li undergo a reversible oxidation-reduction reaction and develop a reversible electric capacity. Therefore, a lithium reaction electrode can be formed without using a conductive additive or a binder.

  Furthermore, the lithium reaction electrode of the present invention has high conductivity because the material itself has higher conductivity and no contact resistance than the conductive auxiliary agent such as carbon black by conducting through the metal core. Can do.

  As described above, the oxide film on the surface of the porous body acts as an electrode of a lithium ion battery by electrochemically reacting with lithium ions.

  In the lithium reaction electrode of the present invention, the specific surface area of the metal porous body is desirably 10 or more with respect to the apparent area. The oxide film on the surface of the metal porous body is an active material, and the larger the specific surface area of the metal porous body, the larger the capacity density of the electrode per volume and per mass. Further, by increasing the surface area, the same high reactivity as when fine powder is used can be obtained.

  In the lithium reaction electrode of the present invention, the porosity of the metal porous body is desirably 30 to 70%. When the porosity is less than 30%, the surface area is small, the volume density per volume and mass is decreased, and the electrolyte does not penetrate well, resulting in poor reactivity. From the viewpoint of reactivity, it is preferable that the porosity is high. However, if the porosity exceeds 70%, the surface area becomes smaller and the capacity density per volume decreases. When the porosity exceeds 70%, the contact area between the particles forming the metal porous body becomes small, and the strength of the metal porous body also decreases.

  In the lithium reaction electrode of the present invention, it is desirable that the porous metal body be formed of a valve metal that does not alloy with Li. In the case of the same specific surface area, the electrode capacity density can be increased as the thickness of the oxide film of the metal porous body is increased. Although the thickness of the oxide film can be increased by thermal oxidation or the like, it is difficult to control the uniformity and thickness of the oxide film.

  Therefore, it is preferable that the metal porous body is formed of a valve metal that uniformly forms an oxide film of the metal on the surface by anodic oxidation. With a metal porous body made of such a valve metal, an oxide film having a uniform thickness corresponding to the anodic oxidation voltage can be easily obtained with good reproducibility by anodic oxidation. Examples of the valve metal that does not react with Li include Ti, Nb, Ta, Hf, and Zr. In particular, Ta (tantalum) is preferable from the viewpoint of stability of the oxide film and corrosion resistance.

  The oxide film formed on the surface of the metal porous body acts as an active material that electrochemically reacts with lithium ions. A valve metal such as tantalum serving as a core has high corrosion resistance and does not react with lithium, and thus acts as a good current collector.

  In addition, since transition metals such as Fe, Ni, and Cr do not react with Li and these oxides react reversibly with Li, these porous bodies can also be used as electrodes.

  In the lithium reaction electrode of the present invention, the thickness of the oxide film is desirably 100 nm or less. In the lithium reaction electrode of the present invention, the oxide film on the surface of the metal porous body becomes an active material. Therefore, the larger the surface area and the thicker the oxide film, the greater the capacity density per volume and per mass. However, when the thickness of the oxide film exceeds 100 nm, the reactivity decreases and the cycle life of charge / discharge decreases. This is due to the volume change accompanying charging and discharging.

  In the lithium reaction electrode of the present invention, it is desirable that the average particle diameter of the particles forming the metal porous body is 10 nm or more and 1 μm or less. Such an average particle diameter is obtained by a cutting method in a cross-sectional structure obtained by a scanning micrograph. When the average particle diameter is less than 10 nm, the continuity of the metal porous body is not sufficient when an oxide film is formed on the surface, and the function as a conductive current collector is lowered.

  On the other hand, when the average particle diameter exceeds 1 μm, the surface area is reduced as compared with the same amount of the metal porous body, and the capacity density per mass and per volume is reduced. In this case, in order to increase the amount of the oxide film as the active material, the film thickness must be increased, and the reactivity and cycle life are reduced as described above.

  In order to produce the lithium reaction electrode of the present invention, it is desirable to first produce a metal porous body by a vapor phase method. Examples of the vapor phase method include various methods conventionally used for film formation, such as vapor deposition, CVD, and sputtering. By using such a gas phase method, it is possible to form a lithium reaction electrode made of a metal porous body having an average particle diameter of 1 μm or less and excellent in reactivity.

  In order to form a metal porous body, a metal porous body having a large surface area can be obtained by interposing a substance that does not react with the electrode material and removing it after film formation when performing the gas phase method. it can. For example, when Ta or Nb is used as an electrode material, a metal such as Cu, Ag, or Mg, or an oxide that is thermodynamically more stable than an oxide of Ta or Nb, such as MgO or CaO, may be used. it can. When Ti, Hf, or Zr is used as the electrode material, an oxide that is thermodynamically more stable than the oxides of Mg, Ti, Hf, or Zr, such as MgO or CaO, can be used.

  Further, as described above, in the present invention, it is desirable to form the oxide film by an anodic oxidation method. In the case of the same surface area, the capacity density can be increased by increasing the thickness of the oxide film. The treatment for thickening the oxide film may be heat treatment in an oxidizing atmosphere, but oxygen diffuses inside at a high temperature. In the anodic oxidation method, applicable metals are limited, but a high-quality oxide film with a uniform film thickness can be formed with good reproducibility. The anodic oxidation voltage and the oxide film thickness are substantially proportional to each other, and the oxide film thickness and capacity can be controlled by the anodic oxidation voltage. Anodization can be performed under general conditions using an aqueous phosphoric acid solution or the like.

  Even if the oxidation treatment as described above is not performed, only the natural oxide film may be used as the oxide film. The natural oxide film is usually a very thin film of about several to several tens of nanometers although it depends on the type of metal as the core. However, since the lithium reaction electrode of the present invention has a structure with a very large surface area, a lithium reaction electrode having a large capacity density per volume and mass can be obtained even if the oxide film is thin.

  In the lithium reaction electrode of the present invention, in the electrode made of a metal porous body, as the thickness of the metal porous body is increased, the surface area is increased and the capacity density is increased. Although the film thickness of a metal porous body can be selected arbitrarily, as a standard of the film thickness of a metal porous body, it is preferable that it is 50 micrometers or less. This is because by having a thin electrode shape, the diffusion of the electrolyte solution is accelerated and a higher reactivity than the conventional slurry coating method can be obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples.

(Examples 1-8)
First, a porous metal body made of tantalum was formed on a substrate made of tantalum foil having a thickness of 50 μm by sputtering as follows.

  A substrate made of tantalum foil is attached to a direct current sputtering apparatus (manufactured by ULVAC, Inc., SH-450), and a target made of high-purity tantalum and high-purity copper is used for simultaneous sputtering in an argon atmosphere of 1.3 Pa. A tantalum-copper composite layer having a thickness of 0.4 μm, 1 μm, and 10 μm was formed. At this time, by changing the input current ratio to the tantalum target and the copper target, the composition of tantalum and copper is 35% Ta, 50% Ta or 65% Ta in terms of volume fraction. Respectively controlled.

Then, the particle diameter of the tantalum phase was changed to 0.1 to 0.7 μm by performing a heat treatment at 650 to 1000 ° C. for 2 hours in a high vacuum of 10 ⁻³ Pa. Thereafter, by immersing in 1N nitric acid, copper was removed, and a porous metal body made of tantalum having a thickness of 0.4 μm, 1 μm and 10 μm was obtained on a substrate made of tantalum foil.

  In this example, tantalum was used as the metal porous body, and copper was used as the heterogeneous component. However, copper intervened in the tantalum as the heterophasic component was tantalum in the porous body by nitric acid etching in any of the examples. On the other hand, it was confirmed by chemical analysis that it was removed to 1% by mass or less. Therefore, the volume fraction of copper at the time of film formation corresponds to the porosity of the metal porous body made of tantalum.

  Next, the obtained metal porous body was used as an integrated body with the substrate, and a tantalum plate was used as a counter electrode in a 0.6 vol% phosphoric acid aqueous solution. At 35 ° C., the applied voltage was 5 V and 10 V, respectively. By performing the treatment, an oxide film was formed on the surface of the metal porous body made of tantalum.

  In addition, the thickness of the oxide film obtained by TEM observation of the cross section in the state where only the natural oxide film is not subjected to anodization and in the state where the anodization is performed at an applied voltage of 5 V is about 3 nm, respectively. And about 9 nm. Therefore, the formation constant of tantalum under the above-described anodic oxidation conditions is 1.8 nm / V, and when the applied voltage is other than 5 V, the oxide film obtained by the anodic oxidation treatment has a film thickness of 1.8 × (anodic oxidation). The oxide film thickness was calculated for each example.

  Further, drying at 200 ° C. × 6 h was performed in vacuum to obtain a lithium reaction electrode.

The obtained lithium reaction electrode was cut to a size of 10 cm ² , a Cu lead was attached by spot welding, the counter electrode was made a 0.3 mm thick lithium foil (Honjo Metal Co., Ltd.), and a separator (Nippon Advanced Paper) An aluminum laminate cell was manufactured using TF4050). The electrolytic solution is an electrolytic solution (LBG-, manufactured by Kishida Chemical Co., Ltd.) containing LiPF ₆ at a concentration of 1 mol / L in a solvent having a volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) of 1: 2. 00018) was used to assemble and seal the cell in a dry box.

Evaluation of the produced aluminum laminate cell was performed by a charge / discharge evaluation apparatus (manufactured by Keiki Center Co., Ltd.) with a charging current of 0.3 mA / cm ² and a cut-off voltage of Li / Li + voltage of 0.01 to 3V. In the range, charge / discharge measurement was performed. Calculate the initial charge capacity and initial discharge capacity per area, the initial coulomb efficiency, and the initial discharge capacity per volume of the metal porous body from the obtained discharge capacity, electrode area, and metal porous body thickness. did.

  Furthermore, as an evaluation of cycle stability, the discharge capacity after 50 cycles of charge / discharge was measured, and the capacity represented by (discharge capacity after 50 cycles of charge / discharge per area) / (initial discharge capacity per area) × 100. Retention was calculated. The results are shown in Table 1.

  Examples 1 to 8 of the present invention all show charge / discharge behavior accompanying the absorption and release of lithium ions by the oxide film, and show good electrode characteristics. The capacity per volume is as large as several hundred mAh / cc. In particular, in Example 2, the capacity is 910 mAh / cc, which exceeds the theoretical capacity of graphite (837 mAh / cc) in an electrode state. The initial coulomb efficiency is about 70 to 85%, and some irreversible capacity is recognized, but the coulomb efficiency after the second cycle is 98% or more, which is good.

  In addition, looking at the capacity retention ratio, which is the ratio of the discharge capacity after 50 cycles of charge / discharge with respect to the initial discharge capacity, all of them are retained at 90% or more, and the cycle characteristics are also good.

  In Example 1, the active material is only a natural oxide film, but due to the structure having a large surface area, reversible charge / discharge characteristics of 510 mAh / cc are shown when viewed per volume of the metal porous body.

  In Examples 2 and 3, the thickness of the porous body and the anodic oxidation voltage are the same, and only the heat treatment temperature for changing the particle diameter is different. However, Example 2 with a larger surface area has a larger capacity. I understand that.

  In Examples 3, 7, and 8, the heat treatment temperature for changing the particle diameter is the same and only the film thickness of the metal porous body is different. However, as the film thickness of the metal porous body increases, the surface area increases. Thus, it can be seen that the capacity per area is increased.

  In Examples 3 and 6, the film thickness and particle diameter of the metal porous body are the same, and only the anodization voltage for changing the film thickness of the oxide film is different. Then, compared with Example 3, the capacity | capacitance per area and volume is about 2 times, and it turns out that the capacity | capacitance is obtained according to the film thickness of an oxide film.

  As described above, an electrode having a larger capacity per area and volume is obtained as the surface area is larger, the particle diameter is smaller, the metal porous body is thicker, the oxide film is thicker, and the anodizing voltage is higher. You can see that

(Comparative Example 1)
A lithium reaction electrode was prepared and evaluated in the same manner as in the example under the conditions shown in Table 1 except that a tantalum smooth film was prepared by sputtering using a tantalum target and 10 μm.

(Comparative Examples 2 to 5)
Under the conditions shown in Table 1, lithium reaction electrodes were prepared and evaluated in the same manner as in the examples.

  Comparative Example 1 is 100% tantalum containing no heterogeneous phase and is not made porous, so that the specific surface area is small. Therefore, the capacity is remarkably reduced. Further, Comparative Example 2 has almost the same capacity as Comparative Example 1 because the film formation Ta density is as high as 80%. Since these had a small capacity, the cycle characteristics were not measured. Moreover, although the comparative example 2 contains a heterogeneous phase, since it is small in quantity, it does not become porous even by pickling and is considered to have only a specific surface area comparable to that of the comparative example 1.

  In Comparative Example 3 with a low film formation Ta density, only the film formation Ta density was changed as compared with Example 2, but the initial capacity was smaller than that of Example 2, and the cycle deterioration was significant. This is presumably because the deposition Ta density is low, so there are few tantalum phase connection points and a stable core structure cannot be maintained.

  In Comparative Example 4 where the heat treatment temperature is low, the actual capacity is small although the particle diameter is small and a high specific surface area is expected. This is because the particles are too fine to maintain a stable core structure, and the formation of an oxide film by chemical conversion treatment increases the volume, crushes small voids, or oxidizes the metal core completely. This is thought to be due to the continuous. Further, the cycle deterioration was remarkable, and the discharge capacity was hardly exhibited after 50 cycles.

  Comparative Example 5 with a thick oxide film shows a large initial charge capacity, but the discharge amount is only 50% of the charge amount even in the first discharge. Furthermore, the charge / discharge capacity decreases with the cycle, and the capacity retention after 50 cycles is only 25%. This is thought to be because the change in volume associated with the insertion and extraction of lithium is increased due to the large thickness of the oxide film, and the oxide film is peeled off.

Claims (9)

  A lithium reaction electrode comprising a metal porous body and an oxide film formed on the surface of the metal porous body and electrochemically reacting with lithium ions.   The lithium reaction electrode according to claim 1, wherein a specific surface area of the metal porous body is 10 or more with respect to an apparent area.   3. The lithium reaction electrode according to claim 1, wherein the metal porous body is made of a valve metal that is not alloyed with lithium, and the oxide film is an oxide film of the valve metal.   The lithium reaction electrode according to claim 3, wherein the valve metal is tantalum.   The lithium reaction electrode according to any one of claims 1 to 4, wherein the porosity of the metal porous body is 30 to 70%.   The lithium reaction electrode according to any one of claims 1 to 5, wherein an average particle diameter of a metal constituting the metal porous body is 10 nm or more and 1 µm or less.   The lithium reaction electrode according to claim 1, wherein the oxide film has a thickness of 100 nm or less.   The method for producing a lithium reaction electrode according to claim 1, wherein the metal porous body is produced by a vapor phase method.   The method for producing a lithium reaction electrode according to claim 1, wherein the oxide film is formed by an anodic oxidation method.