JP2007172963A - Negative electrode for lithium-ion secondary battery, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Sn based negative electrode material for a lithium-ion secondary battery which is superior in cycle characteristics, and provide its manufacturing method. <P>SOLUTION: This Sn based negative electrode material for the lithium-ion secondary battery has a negative electrode structure that at the surface of a negative electrode current collector 2 consisting of Cu foil, that a layer 3 composed of an intermetallic compound of Cu and Sn is formed, and furthermore that a layer consisting of porous Sn oxide film 4 is formed on it. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a manufacturing method thereof.

Lithium ion batteries are now widely used, especially for mobile devices. As a negative electrode of this lithium ion battery, as a negative electrode material obtained by applying a carbon-based material dissolved in a binder and a solvent on a negative electrode current collector made of Cu foil onto a Cu foil, drying, and pressing with a hot roll Provided. The carbon-based material can also occlude and desorb lithium ions by intercalation, and acts as an active material. At this time, the discharge capacity per unit weight of LiC ₆ which is a compound of carbon and lithium is 372 mAh / g. Since it is not possible to increase the capacity beyond this value, recently, an Sn-based active material having a larger discharge capacity (approximately 1000 mAh / g for Li _4,4 Sn), an Si-based active material (Li _4,4 A practical application of Si (about 4000 mAh / g) has been actively studied.

For example, Non-Patent Document 1 reports the status of investigation of Sn-based materials. Here, Sn was formed by electrolytic plating on the surface of the Cu foil, and the characteristics were evaluated as the negative electrode material as it was and heat-treated at 200 ° C. for 24 hours. When the heat-treatment was performed, the plating layer was Sn—Cu It changes to a multilayer structure of ₆ Sn ₅ —Cu ₃ Sn, and the cycle characteristics are improved because the stress due to expansion and contraction of the active material during charge and discharge is relieved to suppress peeling.

Nobuyuki Tamura and 4 others, “Electrochemical characteristics of high capacity tin negative electrode material for lithium secondary battery”, Sanyo Electric Technical Report, Vol. 34, No1, pp. 87-93 (2002) JP 2004-111202 A Japanese Patent No. 2887632 JP 2004-139768 A

  The development of batteries is progressing to a point where the carbon-based material is almost close to the theoretical capacity, and it is difficult to further improve the discharge capacity. For this reason, Sn-based and Si-based materials have been developed. However, these materials are characterized by extremely large volume expansion when occluded lithium ions. The carbon-based material has a volume expansion of 1.5 times at most, whereas the Sn-based material has a volume expansion of about 3.5 times and the Si-based material has a volume expansion of about 4 times. For this reason, the active material is pulverized from the Cu foil which is a current collector along with the charging / discharging cycle, and the problem that the characteristics are rapidly deteriorated is caused. This is the biggest obstacle to practical use. It has become. The above-described example of heat-treating the Sn-plated Cu foil is considered as one countermeasure for this, but even this is not a sufficient countermeasure, and only reduces the peeling when the Sn-plated film is used as it is. .

  In addition, Sn-based and Si-based materials are pulverized in advance, and these are mixed with a conductive binder and applied to the current collector to reduce volume expansion due to reaction with lithium and improve cycle characteristics. Attempts have also been made to try. For example, Patent Document 1 discloses an example in which Sn-containing particles are used as a negative electrode active material. However, this method requires a process such as mechanical alloying and gas atomization in order to form particles, resulting in a significant increase in manufacturing cost. Moreover, since it mixes with a binder etc. also from performance, the filling amount of the active material which reacts with lithium is restrict | limited, and the fall of a battery capacity will be forced.

  Accordingly, an object of the present invention is to provide an Sn-based negative electrode material for a lithium ion secondary battery that is free from such problems and has excellent cycle characteristics, and a method for manufacturing the same.

  The present invention has been devised to achieve the above object, and the invention of claim 1 forms a layer made of an intermetallic compound of Cu and Sn on the surface of a negative electrode current collector made of Cu foil, Furthermore, it is a negative electrode for a lithium ion secondary battery having a negative electrode structure in which a layer made of a porous Sn oxide film is formed thereon.

  According to a second aspect of the present invention, in the negative electrode for a lithium ion secondary battery according to the first aspect of the present invention, the pore diameter of the layer made of the porous Sn oxide film is several hundred nm or less.

  A third aspect of the present invention is the negative electrode for a lithium ion secondary battery according to the first or second aspect, wherein the Cu foil is preliminarily plated with Cu for reacting with Sn to form an intermetallic compound.

  A fourth aspect of the present invention is the negative electrode for a lithium ion secondary battery according to any one of the first to third aspects, wherein the Cu foil is subjected to a roughening treatment in advance by Cu plating or etching.

  The invention according to claim 5 is a diffusion made of Ni, Co, or the like for preventing the roughened surface from being changed into an intermetallic compound by thermal diffusion in advance on the roughened surface formed by performing the roughening treatment. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the barrier layer is formed with a thickness of 1 µm or less.

  The invention of claim 6 has an anodic oxidation step of anodizing the Sn plating film after the Sn plating is applied to the Cu foil, and a subsequent thermal diffusion step, on the surface of the negative electrode current collector made of the Cu foil. , Forming a negative electrode structure for a negative electrode for a lithium ion secondary battery by forming a layer made of an intermetallic workpiece of Cu and Sn and further forming a layer made of a porous Sn oxide film thereon.

  The invention of claim 7 is the method for producing a negative electrode for a lithium ion secondary battery according to claim 6, wherein the anodic oxidation step is a step of anodizing the surface side of the Sn plating film.

  The invention of claim 8 is the method for producing a negative electrode for a lithium ion secondary battery according to claim 6 or 7, wherein the thickness of the Sn plating film is 5 μm or more.

  The invention according to claim 9 is the lithium ion secondary according to any one of claims 6 to 8, wherein the thickness of the layer made of the porous Sn oxide film is 1/3 or less of the thickness of the Sn plating film. It is a manufacturing method of the negative electrode for batteries.

  According to the present invention, by using the negative electrode according to the present invention instead of the conventional carbon-based active material, it is possible to supply a lithium ion secondary battery that has a higher energy density and can be downsized.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  The main point of the present invention is that the intermetallic compound layer of Cu and Sn formed on the negative electrode current collector is covered with a porous Sn oxide.

  If a Sn film is formed on the Cu foil as the negative electrode current collector by plating or the like, or if this is thermally diffused to form an intermetallic compound of Cu and Sn, the charge / discharge cycle is repeated rapidly as described above. In addition, these films are pulverized and fall off. In order to solve this problem, the present inventors have made various studies, and as a result, a structure in which the intermetallic compound layer of Cu and Sn is covered with a porous Sn oxide is used to dramatically charge and discharge. New knowledge that cycle characteristics are improved was obtained.

At this time, the oxide film may be SnO, SnO ₂ , crystalline, or amorphous. It is known that SnO or SnO ₂ acts as a negative electrode active material in a lithium ion battery. For example, Patent Document 2 discloses an example in which LiXSnO (0 ≦ X) is used as the negative electrode active material. In Patent Document 2, LiXSnO is generated through complicated processes such as using Sn or lithium alone or a compound as a starting material, and heat-treating them under the atmosphere control. Since these are powders, in order to actually form a battery, it is necessary to mix them with a conductive agent such as graphite or a binder of resin and press-mold them, and then adhere them to the current collector.

  On the other hand, in the present invention, rather than taking a complicated process using a conventional powder, as will be described later, from the state in which the Sn plating film is formed on the Cu foil as the negative electrode current collector, The Sn oxide film is formed by a simple and simple process called a thermal diffusion process.

  Fig.1 (a)-FIG.1 (c) are figures which show the manufacturing method of the negative electrode for lithium ion secondary batteries which is suitable embodiment of this invention.

  As shown in FIG.1 (c), the negative electrode 1 for lithium ion secondary batteries which concerns on this embodiment is an intermetallic compound which consists of an intermetallic compound of Cu and Sn on the surface of the negative electrode collector 2 which consists of Cu foil. This is a negative electrode structure in which the layer 3 is formed and the Sn oxide film layer 4 made of a porous (porous) Sn oxide film is further formed thereon.

  In addition, the Sn oxide film layer 4 is characterized in that an infinite number of extremely fine holes are formed perpendicular to the film thickness direction.

  These fine holes contribute to a significant increase in reaction sites during the reaction between lithium ions and the Sn oxide film layer 4 via the electrolyte, and smooth lithium ion doping / de-doping (intercalation / deintercalation). To improve charge / discharge characteristics. In addition, it is considered that a LiXSn compound is finally generated by the reaction between the lithium ions and the Sn oxide film layer 4. In the past, the active material was pulverized due to the significant volume expansion at this time and the cycle characteristics deteriorated. However, the porous structure eased the volume expansion and the characteristics in this respect were also greatly improved.

  The size of the holes needs to be extremely fine, and the diameter needs to be several hundred nm or less, preferably 100 nm or less.

  In order to form such fine holes perpendicular to the film thickness direction, a method of anodizing the Sn oxide film layer 4 can be employed. For example, the Sn plating film can be formed by performing constant potential electrolysis in an oxalic acid solution. The size of the hole and the oxide film formation thickness are controlled by the potential and time at this time.

  The surface of the negative electrode current collector 2 made of Cu foil may be preliminarily plated with Cu for reacting with Sn to form an intermetallic compound.

  In the thermal diffusion process described later, an example in which the plated Sn and Cu foil react to form an intermetallic compound will be described. However, in this case, the Cu foil becomes thin, causing a problem in strength. It is preferable to plate Sn after forming Cu that reacts in advance on the foil by plating.

  At this time, in order to maintain the adhesion between the negative electrode current collector 2 made of the Cu plating film and the Cu foil, the surface of the negative electrode current collector 2 made of the Cu foil is previously roughened by Cu plating or etching. Is preferred. The effect is demonstrated if the surface roughness is Ra = 0.1 μm or more.

  This roughened surface is also effective for ensuring adhesion during the charge / discharge cycle of the intermetallic compound layer 3. Therefore, in order to prevent the roughened unevenness from changing to the intermetallic compound layer 3 due to diffusion, a diffusion barrier layer made of Ni, Co or the like is formed to a thickness of 1 μm or less on the roughened surface formed by the roughening treatment. It may be formed on the roughened surface.

Here, the intermetallic compound layer 3 will be described in more detail. As shown in FIG. 2 in order from the negative electrode current collector 2 side, a multilayer film composed of a Cu ₃ Sn layer 31, a Cu ₆ Sn ₅ layer 32, and a Sn layer 33 It is a layer. That is, in the present embodiment, a multilayer film layer is formed under the Sn oxide film layer 4 by thermal diffusion. The multilayer film layer formed by such thermal diffusion has better adhesion to the Cu foil than the case of a single plating film.

  Next, the manufacturing method of the negative electrode 1 for lithium ion secondary batteries is demonstrated.

First, as shown in FIG. 1A, Sn plating is performed on the surface of the negative electrode current collector 2 made of Cu foil to form a Sn plating film 21. The thickness of the Sn plating film 21 is preferably 5 μm or more. Moreover, when performing Sn plating to Cu foil, electroplating or electroless plating may be used. Usually, electroplating is performed in a sulfuric acid bath in which SnSO ₄ is dissolved.

As shown in FIG. 1B, the surface side of the Sn plating film 21 is anodized to form a SnO _x film (Sn oxide film layer) 22. At this time, it is necessary to leave a part of Sn on the Cu foil side instead of using the entire Sn plating film 21 as an oxide film.

  Subsequently, as shown in FIG. 1 (c), the remaining Sn and Cu on the Cu foil side are reacted to form these intermetallic compound layers 3 and the Sn oxide film layer. 4 is formed into a negative electrode structure, a negative electrode 1 for a lithium ion secondary battery is obtained.

By doing so, the cycle characteristics are greatly improved as compared with the case of only the intermetallic compound layer 3. The temperature of the thermal diffusion process at this time may be equal to or lower than the melting point of Sn, and the time is taken so that Cu ₆ Sn ₅ as an intermetallic compound is generated as much as possible and Cu ₃ Sn and remaining Sn are reduced as much as possible. To do.

  As described above, it is necessary to form Sn with a thickness of 5 μm on the Cu foil in order to ensure the normal charge / discharge capacity. The thickness to be polished is preferably 1/3 or less of the entire Sn plating film 21. Sn oxide reacts with Li to produce Li oxide and Sn, but Li oxide does not contribute to the charge / discharge cycle because it is not reduced again. Therefore, if the Sn oxide film layer 22 is too thick, the irreversible capacity increases, which is not preferable.

  Thus, the negative electrode 1 for a lithium ion secondary battery according to the present embodiment is a Sn oxide film layer in which the intermetallic compound layer 3 of Cu and Sn formed on the negative electrode current collector 2 made of Cu foil is porous. 4 is covered. For this reason, a lithium ion secondary battery having a high energy density, excellent cycle characteristics, and capable of being miniaturized can be supplied as compared with a conventional negative electrode made of a carbon-based active material.

  In addition, since the adhesion between the negative electrode current collector 2 made of Cu foil and the intermetallic compound layer 3 made of a multilayer film by thermal diffusion is better than that of a conventional single plating film, the negative electrode can be obtained even after repeated charge / discharge cycles. The intermetallic compound layer 3 does not fall off from the current collector 2 and has a long life.

A rolled Cu foil having a thickness of 0.018 mm was prepared, and first, roughening treatment was performed by electrodeposition of Cu. The conditions were an electrolytic solution of copper sulfate 150 g / L and sulfuric acid 150 g / L. After electrolysis at a liquid temperature of 30 ° C. and a current density of 20 A / dm ² , copper sulfate 250 g / L and sulfuric acid 100 g / L of electrolytic solution Cover plating was performed at a liquid temperature of 30 ° C. and a current density of 10 A / dm ² , and the surface roughness Ra was set to 0.12 μm.

Thereafter, Ni plating with a thickness of 0.3 μm was first applied in a normal watt bath to form a diffusion barrier layer. A 2.7 μm-thick Cu plating was performed with the plating solution subjected to the above-described covering Cu plating, and further a current density of 3 A / dm ^{2 in} a plating solution containing 50 g / L of stannous sulfate and 100 g / L of sulfuric acid and an appropriate amount of additive. Sn plating was performed under the following conditions. The film thickness at this time was set to 7 μm.

  The Sn-plated Cu foil prepared in this way is further used as a negative electrode sample for those not anodized under predetermined conditions in a 0.5 M oxalic acid solution, and those subjected to subsequent thermal diffusion treatment and those not. A test cell with metallic lithium as the counter electrode was fabricated and the charge / discharge characteristics were evaluated. That is, a sample subjected to both anodization and diffusion heating was used as an example, a sample subjected only to anodization was compared to Comparative Example 1, a sample subjected only to diffusion heating was compared to Comparative Example 2, and a sample subjected to neither anodization nor diffusion heating was compared to Comparative Example 3. did.

A polypropylene thin film was used as the separator, and a mixed solution (1: 1) of ethylene carbonate and diethyl carbonate in which 1M LiPF ₆ was dissolved was used as the electrolyte. Charging / discharging was performed at a constant current density of 0.25 mA / cm ^{2 in} the range of 0.01 to 1V.

  Table 1 shows the breakdown of the test material and the discharge capacity maintenance ratio with respect to the initial cycle after 5 and 20 charge / discharge tests.

  FIG. 3 shows an example of an SEM photograph of the sample surface of the example in which the Cu foil was subjected to Sn plating and then anodized. As shown in FIG. 3, countless fine pores of 100 nm or less are observed on the surface.

  Further, when X-ray diffraction was performed from the surface of the sample after the anodic oxidation of the example and before diffusion heating, the remaining Sn peak was confirmed, and no clear Sn oxide peak was observed. The sample was black in appearance, and it was presumed that amorphous SnO was probably formed. Moreover, when the cross section was observed with a microscope, 15 to 20% of the surface of the Sn plating film was oxidized.

  As is apparent from Table 1, it can be seen that in the examples, the capacity of 90% or more was maintained even after 20 cycles of the charge / discharge test, and the cycle characteristics were excellent. On the other hand, Comparative Example 1 in which diffusion heat treatment was not performed, Comparative Example 2 in which anodization was not performed, and Comparative Example 3 in which neither treatment was performed were inferior in cycle characteristics, and the effectiveness of the present invention was proved. It was done.

The Cu foil used in the present invention may be a rolled Cu foil or an electrolytic Cu foil. However, when the active material expands with the charge / discharge cycle, a large tensile stress is applied to the Cu foil. Recommended. In this sense, a rolled Cu foil to which a high-strength Cu alloy can be applied is preferable. For example, a normal rolled Cu foil or electrolytic Cu foil has a tensile strength of about 300 to 400 N / mm ² , but if a Cu—Ni—Si based so-called Corson Cu alloy is used, a high strength of 700 N / mm ² or more is obtained. A rolled Cu foil can be obtained. Therefore, it is preferable to use such a Cu foil as the negative electrode current collector 2.

Fig.1 (a)-FIG.1 (b) are schematic diagrams which show the manufacturing method of the negative electrode for lithium ion secondary batteries which shows suitable embodiment of this invention. It is a schematic diagram of the intermetallic compound layer shown in FIG. It is the surface SEM photograph of an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode for lithium ion secondary batteries 2 Negative electrode collector 3 Intermetallic compound layer 4 Sn oxide film layer

Claims (9)

  A negative electrode structure in which a layer made of an intermetallic compound of Cu and Sn is formed on the surface of a negative electrode current collector made of Cu foil, and a layer made of a porous Sn oxide film is further formed thereon. A negative electrode for a lithium ion secondary battery.   2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein a diameter of a hole of the layer made of the porous Sn oxide film is several hundred nm or less.   The negative electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the Cu foil is previously subjected to Cu plating for reacting with Sn to form an intermetallic compound.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the Cu foil is subjected to a roughening treatment by Cu plating or etching in advance.   On the roughened surface formed by the above roughening treatment, a diffusion barrier layer made of Ni, Co or the like for preventing the roughened surface from being changed into an intermetallic compound by thermal diffusion in advance is 1 μm or less in thickness. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4, which is formed in advance.   After Sn plating is applied to the Cu foil, an anodizing step for anodizing the Sn plating film and a subsequent thermal diffusion step are performed. A method for producing a negative electrode for a lithium ion secondary battery, comprising forming a layer made of a workpiece and further forming a layer made of a porous Sn oxide film thereon to form a negative electrode structure.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 6, wherein the anodizing step is a step of anodizing the surface side of the Sn plating film.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 6 or 7, wherein the Sn plating film has a thickness of 5 µm or more.   The method for producing a negative electrode for a lithium ion secondary battery according to any one of claims 6 to 8, wherein the thickness of the layer made of the porous Sn oxide film is 1/3 or less of the thickness of the Sn plating film.

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