JP2008010364A - Anode for lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode for a lithium ion secondary battery with excellent cycle characteristics. <P>SOLUTION: For the anode for the lithium ion secondary battery 1 having an active material layer 3 consisting of a lithium-active active material of either a tin, silicon, or antimony system formed on a collector layer 2 made of copper or a copper alloy, a reinforcing layer 4 made of a conductive and lithium-inactive matter is formed on the above active material layer 3, a second active material layer 3 is formed on the reinforcing layer 4, with that second active material layer 3 as an outermost layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery excellent in cycle characteristics.

  Lithium ion secondary batteries are widely used in a wide range of applications including mobile devices. As a negative electrode of the lithium ion secondary battery, a carbon-based active material (lithium active material) is formed on a current collecting layer (also referred to as a negative electrode current collector) made of copper or a copper alloy such as a copper foil or a copper alloy foil. In general, an active material layer made of That is, a solution obtained by dissolving a carbon-based active material with a binder and a solvent is applied to a current collecting layer such as a rolled copper foil or an electrolytic copper foil, and the solution is dried and then subjected to a hot roll press to form a lithium ion secondary. A negative electrode for a battery is used.

  Note that the carbon-based active material is an active material containing at least one of graphite, hard carbon, and soft carbon.

An example of the carbon-based active material is LiC ₆ which is a compound of carbon and lithium. The carbon-based active material can occlude and desorb (release) lithium ions. At this time, the theoretical discharge capacity (maximum capacity) per unit weight of LiC ₆ is said to be 372 mAh / g. Since the capacity of the carbon-based active material cannot be increased beyond this value, recently a tin-based active material having a larger discharge capacity (for example, Li4.4Sn has a theoretical discharge capacity of about 1000 mAh / g), Studies on practical application of silicon-based active materials (for example, Li4.4Si has a theoretical discharge capacity of about 4000 Ah / g) are being actively conducted.

  The tin (Sn) -based active material is an active material containing either tin or a tin alloy. The silicon (Si) -based active material is an active material containing either silicon or a silicon alloy, and the antimony (Sb) -based active material is an active material containing either antimony or an antimony alloy. That is.

The tin-based active material is disclosed in Non-Patent Document 1. That is, when a tin plating layer is formed on the copper foil surface by electrolytic plating and heat treatment is performed at 200 ° C. for 24 hours, the plating layer changes to a multilayer structure of Sn—Cu ₆ Sn ₅ —Cu ₃ Sn. Since the stress due to the expansion and contraction of the active material during discharge is relieved to suppress separation, cycle characteristics are improved.

  In addition, the volume expansion due to the reaction with lithium is reduced by finely pulverizing a tin-based or silicon-based active material in advance and applying the active material mixed with a conductive binder to the current collecting layer. Attempts have also been made to improve cycle characteristics. For example, Patent Document 1 discloses that particles containing tin are used as the negative electrode active material.

JP 2004-087232 A Sanyo Electric Technical Report, Vol. 34, no. 1, pp. 87-93 (2002)

  As described above, the development of carbon-based active materials has progressed to a point close to the theoretical discharge capacity, and it is difficult to significantly improve the discharge capacity in the future. For this reason, tin-based and silicon-based active materials are being developed. However, these active materials have a drawback that volume expansion is extremely large when lithium ions are occluded. Specifically, the volume expansion of the carbon-based active material is about 1.5 times, whereas the volume expansion of the tin-based active material is 3.5 times, and the volume expansion of the silicon-based active material is 4 times. It also becomes. Because of this large volume change, the active material peels off from the copper foil, which is the current collecting layer, and drops as the charge / discharge cycle occurs, resulting in a problem that the battery characteristics deteriorate sharply. It is an obstacle.

  The countermeasure of Non-Patent Document 1 in which the copper foil on which the tin plating layer is formed is heat-treated is not sufficient, and only reduces the peeling when the heat treatment is not performed.

  Further, the technique of Patent Document 1 requires a process such as mechanical alloying and gas atomization to form particles, and has a problem that the manufacturing cost is significantly increased. Further, since the active material is mixed with the binder, the filling amount of the active material that reacts with lithium is limited, and a performance defect such as a reduction in battery capacity is unavoidable.

  Then, the objective of this invention is providing the negative electrode for lithium ion secondary batteries which solved the said subject and was excellent in cycling characteristics.

  In order to achieve the above object, in the present invention, an active material layer made of a lithium-based active material that is any of tin, silicon, and antimony is formed on a current collecting layer made of copper or a copper alloy. In the negative electrode for a lithium ion secondary battery, a reinforcing layer made of a conductive and lithium inactive material is formed on the active material layer, and a second active material layer is formed on the reinforcing layer. The second active material layer is the outermost layer.

  A second reinforcing layer is formed on the second active material layer, and a third active material layer is formed on the second reinforcing material layer. Layers may be alternately formed, and the last active material layer may be the outermost layer.

  The material forming the reinforcing layer may be a single metal of nickel, cobalt, iron, silver, or copper, or an alloy of any two or more.

  The reinforcing layer may have a thickness of 0.05 μm or more and less than 0.5 μm.

  The thickness of the active material layer may be less than 5 μm.

  The present invention exhibits the following excellent effects.

  (1) Excellent cycle characteristics.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, a negative electrode 1 for a lithium ion secondary battery according to the present invention is a tin-based, silicon-based, or antimony-based lithium on a current collecting layer 2 made of copper or a copper alloy. In the negative electrode 1 for a lithium ion secondary battery in which an active material layer 3 made of an active active material is formed, a reinforcing layer 4 made of a conductive and lithium inactive material is formed on the active material layer 3, A second active material layer 3 is formed on the reinforcing layer 4, and the second active material layer 3 is the outermost layer.

  As shown in the figure, the negative electrode 1 for a lithium ion secondary battery of the present invention has a reinforcing layer 4 made of a conductive and inactive lithium material between the two active material layers 3 and 3. It can be said that the reinforcing layer 4 is provided inside the active material layer (a combination of the active material layers 3 and 3).

  If the active material layer 3 made of a tin film is formed on the copper foil as the current collecting layer 2 by plating or the like, or the active material layer 3 made of an intermetallic compound of copper and tin is formed, the charge / discharge cycle can be performed. The active material layer 3 suddenly collapses and falls off as it repeats. In order to solve this problem, the present inventor can obtain new knowledge that the charge / discharge cycle characteristics are drastically improved by providing the reinforcing layer 4 inside the active material layer after intensive studies. did it. This effect is considered to be due to the presence of the reinforcing layer 4 that does not contribute to charging / discharging, so that the active material can be fixed by the reinforcing layer 4 and collapse can be suppressed.

  When the reinforcing layer 4 is not electrically conductive, conduction with the current collecting layer 2 cannot be obtained, and therefore the material of the reinforcing layer 4 needs to be a conductive material. In addition, when the material of the reinforcing layer 4 is lithium active, the reinforcing layer 4 expands and contracts with charge / discharge, so that the effect of suppressing the collapse of the active material layer 3 cannot be obtained. In that respect, the present invention forms the reinforced layer 4 made of a material that is conductive and inactive to lithium, so that conduction with the current collecting layer 2 can be obtained, and the collapse of the active material layer 3 can be suppressed. Is obtained.

  If the reinforcing layer 4 is the outermost layer, normal charging / discharging does not occur. In that respect, in the present invention, since the second active material layer 3 is provided as the outermost layer on the reinforcing layer 4 (= the reinforcing layer 4 is provided inside the active material layer), normal charging / discharging occurs. be able to.

  As described above, the negative electrode for a lithium ion secondary battery according to the present embodiment is formed on the active material layer 3 formed on the current collecting layer 2 and the reinforcing layer 4 made of a conductive and lithium inactive material. And the second active material layer 3 is formed on the reinforcing layer 4 to provide a structure in which the reinforcing layer 4 is provided inside the active material layer. Thereby, the collapse of the active material layer 3 can be eliminated, and the cycle characteristics are superior to those using conventional tin-based or silicon-based active materials. In addition, it is possible to supply a lithium ion secondary battery that has higher energy density, excellent cycle characteristics, and can be miniaturized as compared with a conventional one using a carbon-based active material.

  In this embodiment, the material that forms the reinforcing layer 4 may be any material that is conductive and lithium-inactive. For example, nickel (Ni), cobalt (Co), iron (Fe), silver ( Ag) and copper (Cu). These may be used as a single metal, or an alloy of any two or more may be used, and the above-described effects can be achieved.

  The thickness of the reinforcing layer 4 is desirably in the range of 0.05 μm or more and less than 0.5 μm. This is because if the thickness is less than 0.05 μm, the surface region of the active material layer 3 that is not covered with the reinforcing layer 4 inevitably expands, and the reinforcing effect in that region cannot be obtained. Moreover, since the intensity | strength of reinforcement | strengthening layer 4 itself will become it low that it is less than 0.05 micrometer, collapse of an active material cannot be suppressed effectively. On the other hand, when the thickness exceeds 0.5 μm, the reinforcing layer 4 becomes substantially pinhole-free and covers the entire surface of the active material layer 3, so that lithium ions cannot pass through the reinforcing layer 4. The active material layer 3 sandwiched between the layers cannot contribute to charge / discharge. In that respect, in the present invention, the thickness of the reinforcing layer 4 is in the range of 0.05 μm or more and less than 0.5 μm, so that the surface of the active material layer 3 is appropriately covered with the reinforcing layer 4.

  It is more desirable that the thickness of the reinforcing layer 4 is in the range of 0.1 μm or more and less than 0.3 μm. This is because a margin can be taken with respect to the aforementioned range.

  The thickness of the active material layer 3 is preferably less than 5 μm. As the negative electrode 21 for the lithium ion secondary battery in FIG. 2, the reinforcing layers 24 and the active material layers 23 are alternately formed in multiple layers, whereby the thickness of the active material layer 23 can be reduced. Since the thickness of the active material is small, it becomes difficult for the active material to fall off.

  The thickness of the active material layer 3 is more preferably less than 3 μm. This is because a margin can be taken with respect to the above-described upper limit.

As a method of forming the active material layer 3, for example, when an alloy of copper and tin is used as an active material, Cu ₆ Sn or the like is formed by thermal diffusion after separately performing copper plating and tin plating on the current collecting layer 2. The active material layer 3 made of an alloy of the above may be formed, or the active material layer 3 made of an alloy of copper and tin is directly formed on the current collecting layer 2 with a plating solution capable of simultaneously plating copper and tin. May be.

  Next, another embodiment of the present invention will be described.

  As shown in FIG. 2, the negative electrode 21 for a lithium ion secondary battery according to the present invention is a tin-based, silicon-based, or antimony-based lithium on a current collecting layer 22 made of copper or a copper alloy. In the negative electrode 21 for a lithium ion secondary battery in which an active material layer 23 made of an active active material is formed, a reinforcing layer 24 made of a conductive and lithium inactive material is formed on the active material layer 23, A second active material layer 23 is formed on the reinforcing layer 24, a second reinforcing layer 24 is formed on the second active material layer 23, and the second reinforcing layer 24 is formed. A third active material layer 23 is formed thereon, and the reinforcing layers 24 and the active material layers 23 are alternately formed in multiple layers, with the last active material layer 23 being the outermost layer.

  With this configuration, each active material layer 23 can contribute to the charge / discharge reaction. Therefore, the total amount of active materials in each active material layer 23 may be set to a target active material amount in battery design. For this reason, as the number of the active material layers 23 is increased, the thickness of one active material layer 23 can be reduced, and by reducing the thickness of the active material layer 23, it is difficult to drop off. The thickness of each active material layer 23 does not need to be the same.

A copper foil having a thickness of 18 μm roughened to a surface roughness Ra = 0.12 μm was prepared as a current collecting layer. On the current collecting layer, a copper plating solution shown in Table 1 was used to make a thickness of 2.7 μm. Copper plating was performed, and then tin plating with a thickness of 5 μm was performed with a tin plating solution shown in Table 1. Next, this plated sample was heat-treated in vacuum at 200 ° C. for 20 hours to obtain a negative electrode. The negative electrode after this heat treatment is subjected to structural analysis of the plating layer with an X-ray diffraction (XRD) apparatus and compared with JCPDS data, whereby an intermetallic compound of Cu ₆ Sn ₅ as a desired active material is formed. It was confirmed. The thickness of this active material layer was 8 μm. This negative electrode has a conventional structure in which only one active material layer is formed on the current collecting layer, and this is used as the negative electrode of Comparative Example 1-1.

  On the same current collecting layer as above, copper plating with a thickness of 1.3 μm is performed with the same plating solution, then tin plating with a thickness of 2.5 μm is performed, and then the nickel plating solution in Table 1 is used. After nickel plating with a thickness of 0.1 μm and tin plating with a thickness of 2.5 μm, heat treatment was performed in the same manner as above to obtain a negative electrode. This negative electrode has the form of FIG. 1 in which an active material layer, a reinforcing layer, and an active material layer are formed in this order on a current collecting layer. This is the negative electrode of Example 1-1.

  On the same current collecting layer as above, 0.9 μm-thick copper plating, 1.6 μm-thick tin plating, 0.1 μm-thickness nickel plating, 0.9 μm-thickness with the same plating solution Thickness copper plating, 1.6 μm thickness tin plating, 0.1 μm thickness nickel plating, 0.9 μm thickness copper plating, 1.6 μm thickness tin plating in order, In the same manner as above, heat treatment was performed to obtain a negative electrode. This negative electrode has the form of FIG. 2 in which an active material layer, a reinforcing layer, an active material layer, a reinforcing layer, and an active material layer are formed in this order on a current collecting layer, and this is used as the negative electrode of Example 1-2. .

  A cross-sectional SEM photograph of the negative electrode (after plating) of Example 1-2 is shown in FIG. As shown in the figure, an active material layer made of copper and tin and a reinforcing layer made of nickel are alternately laminated on a current collecting layer made of copper.

The negative electrodes of Comparative Example 1-1, Example 1-1, and Example 1-2 obtained in this way were each punched out into a 2 cm ² circle, and a prototype cell (lithium ion secondary battery) using metallic lithium as the positive electrode for the negative electrode. Secondary battery) was manufactured, and charge / discharge tests and evaluations of each prototype cell were performed. The measuring device is HJ1001SM8A manufactured by Hokuto Denko, the separator is Celgard # 2400, and the electrolyte is Fuji Chemical Industry LIPASTER-EDMC / PF1 (1 mol / 1000 cm ³ of LiPF ₆ mixed solution of ethylene carbonate and diethyl carbonate ( 1: 1 vol.)) Was used. Charging / discharging was performed at a constant current density of 0.25 mA / cm ^{2 in} the range of 0.01 to 1 V vs Li / Li +.

  Table 2 shows the evaluation results of the discharge capacity retention ratio expressed by the discharge capacity after 5 cycles and after 20 cycles with respect to the discharge capacity (hereinafter referred to as initial discharge capacity) of the initial cycle (first cycle). As shown in Table 2, compared to Comparative Example 1-1 in which only one active material layer is provided, Example 1-1 and Example 1-2 in which a reinforcing layer is provided have a higher discharge capacity retention rate. From this result, it can be seen that the cycle characteristics are improved by providing the reinforcing layer.

  In addition, when the prototype cell after the charge / discharge test was disassembled and the negative electrode surface was observed, the negative electrode of Comparative Example 1-1 had fallen off the active material, whereas Example 1-1 and Example 1-2. In the negative electrode, the active material did not fall off and the soundness was maintained.

  In the same manner as in Example 1-1, a negative electrode having the form of FIG. 1 in which an active material layer, a reinforcing layer, and an active material layer were sequentially formed on a current collecting layer was prepared. However, a negative electrode having a reinforcing layer thickness of 0.01, 0.05, 0.1, 0.3, 0.5 μm was prepared, and Comparative Example 2-1, Example 2-1, It was set as Example 2-2, Example 2-3, and Comparative Example 2-2.

  Using each of these negative electrodes, prototype cells were prepared and subjected to the same charge / discharge test as described above. Here, the initial discharge capacity and the discharge capacity retention rate were evaluated.

  As shown in Table 3, the initial discharge capacity is about half compared to 286 mAh / g in Comparative Example 2-2 and around 530 mAh / g in the other examples. This is presumably because the thickness of the reinforcing layer in Comparative Example 2-2 is as thick as 0.5 μm, so that lithium ion permeation is inhibited and the active material in the active material layer does not contribute to charge / discharge. In Comparative Example 2-1, Example 2-1, Example 2-2, and Example 2-3, the lithium ion easily penetrates the reinforcing layer, so that the active material layer contributes well to charge and discharge, and the initial discharge capacity. Becomes higher.

  When compared with the discharge capacity maintenance rate, the thickness of the reinforcing layer is as small as 0.01 μm, but Comparative Example 2-1 has a low discharge capacity maintenance rate and poor cycle characteristics, whereas the thickness of the reinforcing layer is 0.05 μm or more. In certain Example 2-1, Example 2-2, Example 2-3, and Comparative Example 2-2, the discharge capacity retention rate is high and the cycle characteristics are good.

  From this test result, it can be seen that the thickness of the reinforcing layer is preferably in the range of 0.05 μm or more and less than 0.5 μm.

It is sectional drawing of the negative electrode for lithium ion secondary batteries which shows one Embodiment of this invention. It is sectional drawing of the negative electrode for lithium ion secondary batteries which shows other embodiment of this invention. It is an image figure of the cross-sectional SEM photograph of the negative electrode (after plating) of Example 1-2.

Explanation of symbols

1,21 Negative electrode for lithium ion secondary battery 2,22 Current collecting layer 3,23 Active material layer 4,24 Strengthening layer

Claims (5)

  A negative electrode for a lithium ion secondary battery in which an active material layer made of a lithium-active active material that is either tin-based, silicon-based, or antimony-based is formed on a current collecting layer made of copper or a copper alloy. A reinforcing layer made of a material that is conductive and lithium-inactive is formed on the material layer, a second active material layer is formed on the reinforcing layer, and the second active material layer is formed on the outermost layer. A negative electrode for a lithium ion secondary battery, characterized in that it is an outer layer.   A second reinforcing layer is formed on the second active material layer, and a third active material layer is formed on the second reinforcing material layer. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the layers are alternately formed in multiple layers, and the last active material layer is the outermost layer.   3. The lithium ion secondary battery according to claim 1, wherein the material forming the reinforcing layer is a single metal of nickel, cobalt, iron, silver, or copper, or an alloy of any two or more thereof. Negative electrode.   4. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the reinforcing layer has a thickness of 0.05 μm or more and less than 0.5 μm. 5. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the active material layer has a thickness of less than 5 μm.

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