JP2006269361A - Negative electrode for lithium ion secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery enhancing cycle characteristics by preventing separation or coming off of a negative active material from copper foil which is a current collector even when charge discharge is repeated, and increasing energy density than in use of a carbon base active material. <P>SOLUTION: A Cu<SB>6</SB>Sn<SB>5</SB>phase (η phase) active material is directly formed on a negative current collector made of copper foil by Cu-Sn alloy plating using sulfosuccinic acid solution. The negative active material has composition in which the content of Sn is 35-60 mass%, and has almost a single phase of Cu<SB>6</SB>Sn<SB>5</SB>phase (η phase). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a novel negative electrode for a lithium ion secondary battery and a method for producing the same, and in particular, the active material is excellent in cycle characteristics without peeling and dropping from a copper foil as a current collector even after repeated charge and discharge. The present invention relates to a negative electrode for a lithium ion secondary battery and a method for producing the same.

  Lithium ion secondary batteries are now widely used for mobile devices. These negative electrodes include a carbon-based material on a negative electrode current collector made of copper foil or copper alloy foil (hereinafter referred to as “copper alloy foil” when referred to as “copper foil” in the present specification). Is formed as an active material.

This negative electrode material for lithium ion secondary batteries is generally provided by applying a carbon-based material dissolved in a binder and a solvent on a rolled copper foil or an electrolytic copper foil, drying, and applying a hot roll press. The In a carbon-based material, LiC ₆ which is a compound of carbon and lithium acts as an active material, and can absorb and desorb 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 Sn-based active material having a higher discharge capacity (approximately 1000 mAh / g for Li _4.4 Sn), a Si-based active material ( Li _4.4 Si (about 4000 mAh / g) has been actively studied.

In the Sn-based material, when Sn 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, There is a report that cycle characteristics are improved in order to relieve stress due to expansion and contraction of an active material during charge and discharge and suppress separation (see Non-Patent Document 1).

In addition, in a negative electrode for a lithium ion secondary battery using Cu ₆ Sn ₅ as an active material, a copper foil base material is subjected to Sn plating and subjected to heat treatment, or Cu—Sn alloy plating with a cyan plating solution. Cu ₆ Sn ₅ is formed (see Patent Document 1).
Sanyo Electric Technical Report, Vol.34, No.1, pp.87-93 (2002) JP 2004-087232 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 greatly improve the discharge capacity in the future. For this reason, as described above, Sn-based and Si-based materials have been developed. However, these materials have a drawback that volume expansion is extremely large when lithium ions are occluded. Specifically, the volume expansion of carbon material is about 1.5 times, whereas the volume expansion of Sn system is about 3.5 times and that of Si system is about 4 times. Due to this large volume change, the active material is peeled off from the copper foil, which is a current collector, and drops due to the charge / discharge cycle, resulting in a problem that the battery characteristics deteriorate sharply. This is the biggest obstacle to practical use. It was.

Therefore, in the case of Sn-based materials, an attempt is made to use it not as a pure Sn thin film but as a thin film of Sn alloy or Sn compound (hereinafter referred to as “alloy” in this specification includes “compound”). It has been. For example, Cu ₆ Sn ₅ which is a Sn compound has been studied for use as a negative electrode active material. The method of forming Cu ₆ Sn ₅ includes a method of performing a heat treatment after Sn plating is applied to a copper foil base as described in Patent Document 1, and a method of directly plating a Cu—Sn alloy with a cyan plating solution. Are known. However, although Cu ₆ Sn ₅ is formed as an active material layer by these methods, the collapse of the active material due to expansion / contraction due to charge / discharge is reduced, but it is still not sufficient.

  Furthermore, since the former method performs heat treatment after plating, the manufacturing process is complicated and expensive, and a part of the copper foil base material is alloyed by diffusion during heat treatment, resulting in a reduction in the thickness of the base material. As a result, the mechanical strength is reduced (the plastic deformation of the base material due to the expansion / contraction of the active material is increased). On the other hand, in the latter method, since a cyan plating solution is used, there is a problem in terms of safety and environment, and there is a problem that a manufacturing facility such as waste liquid treatment becomes large and causes a cost increase.

  Therefore, the negative electrode active material does not peel off from the copper foil, which is a current collector even after repeated charge and discharge, and is a negative electrode for a lithium ion secondary battery that is further excellent in cycle characteristics, and at low cost and environmentally friendly. A manufacturing method excellent in safety has been demanded.

  Accordingly, the object of the present invention is to eliminate such problems and to have excellent cycle characteristics without causing the active material to peel off and fall off from the copper foil as a current collector even if charging and discharging are repeated while having a high discharge capacity. Another object is to provide a negative electrode for a lithium ion secondary battery.

  Another object of the present invention is to provide a method for producing a negative electrode for a lithium ion secondary battery that is simple and safe in production process and does not cause environmental problems.

As a result of intensive studies to achieve the above object, the inventors of the present invention formed a Cu ₆ Sn ₅ phase (η phase) layer on a copper foil substrate by Cu—Sn alloy plating with sulfosuccinic acid. The present invention was completed based on this knowledge, which was obtained as a result of exhibiting excellent cycle characteristics as a lithium ion secondary battery compared with the case where Cu ₆ Sn ₅ was formed by diffusion by heat treatment after applying Sn plating to the material. I let you.

That is, the negative electrode for a lithium ion secondary battery according to the present invention is a negative electrode for a lithium ion secondary battery in which a Cu—Sn alloy-based active material is formed on a negative electrode current collector made of copper foil. Is composed of a substantially single-phase Cu ₆ Sn ₅ phase (η phase) with a composition of 35 mass% or more and 60 mass% or less. The term “substantially single phase” as used herein means that other than the phase in the scan range (for example, 20 ° ≦ 2θ ≦ 100 °) that is normally performed in the Xθ (X-ray diffraction) 2θ / θ scan measurement using CuKα rays. A state in which the cumulative intensity of diffraction peaks attributed to a phase is less than 10% of the cumulative intensity of diffraction peaks attributed to the phase. However, the diffraction peak due to the base material of the base is not included in a phase other than the phase.

  The surface of the active material is preferably formed in a concavo-convex structure, and the surface roughness Ra of the active material is preferably 0.5 μm or more and less than 2 μm.

  Moreover, the manufacturing method of the negative electrode for lithium ion secondary batteries of this invention is the manufacturing method of the negative electrode for lithium ion secondary batteries which formed the Cu-Sn alloy type active material by electroplating on the negative electrode collector which consists of copper foils. The active material is directly formed (precipitated) by Cu-Sn alloy plating with a sulfosuccinic acid solution.

The Cu ₆ Sn ₅ phase (η phase) is preferably formed by brush plating.

Further, the Cu ₆ Sn ₅ phase (η phase) film is uniformly formed on the surface of the Cu ₆ Sn ₅ phase formed in an uneven shape by the burn plating, and further by Cu—Sn alloy plating with a sulfosuccinic acid solution. It is preferable to form by.

  According to the present invention, the energy density is higher than when a carbon-based active material is used, and the negative electrode active material is not peeled off from the copper foil as a current collector even after repeated charge and discharge, and cycle characteristics are eliminated. The negative electrode for lithium ion secondary batteries excellent in can be provided.

  In addition, according to the present invention, it is possible to provide a method for producing a negative electrode for a lithium ion secondary battery that has a simple and safe production process and does not cause environmental problems.

In the present invention, when a Cu ₆ Sn ₅ phase coating is directly formed by Cu-Sn alloy plating with sulfosuccinic acid, compared to the case where the copper foil base material is Sn plated and Cu ₆ Sn ₅ is formed by diffusion by heat treatment, This is based on a new finding that it shows excellent cycle characteristics as a lithium ion secondary battery. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

In FIG. 1, the schematic diagram of the negative electrode for lithium ion secondary batteries which concerns on this embodiment is shown. This negative electrode 10 for a lithium ion secondary battery has a negative electrode active composed of a Cu ₆ Sn ₅ phase (η phase) having a concavo-convex structure by burnt plating on a copper foil 1 (for example, thickness 18 μm) serving as a negative electrode current collector. The material 2 is provided, and the active material smooth plating layer 3 made of Cu ₆ Sn ₅ phase (η phase) is formed on the material 2 along the uneven shape of the negative electrode active material 2 with a uniform thickness.
Hereinafter, each configuration will be described in detail.

(Formation of Cu ₆ Sn ₅ phase (η phase) film with sulfosuccinic acid)
The reason for the excellent cycle characteristics as a lithium ion secondary battery when a Cu ₆ Sn _five- phase film is directly formed by Cu—Sn alloy plating with sulfosuccinic acid is not completely clarified. In Cu—Sn alloy plating with an acid, the composition of Sn content is 35 mass% to 60 mass%, more preferably 38 mass% or more and 60 mass% or less, and still more preferably Sn content is 40 mass% or more and 60 mass% or less. A phase Cu ₆ Sn ₅ phase (η phase) is generated (precipitated). Such a phenomenon in which a substantially single-phase η phase is generated in such a wide composition region is difficult to estimate from the phase equilibrium diagram of the Cu—Sn system, and is largely deviated from the stoichiometric composition by non-equilibrium reaction. (Sn-deficient η phase structure). It is considered that this deviation from the stoichiometric composition works favorably when the crystal structure changes (volume change) accompanying the reaction with lithium (occlusion / release of lithium ions) at the battery electrode.

On the other hand, when the Cu ₆ Sn ₅ phase is formed by diffusion (heat treatment), the composition is almost stoichiometric. Further, when a Cu—Sn alloy is plated using a cyan bath, for example, a plating film having a Sn content of about 40 mass% is a mixed phase of Cu ₆ Sn ₅ (η phase) and Cu ₄₁ Sn ₁₁ (δ phase). It has been reported that When such a mixed phase Cu—Sn-based material is used as the negative electrode active material of a lithium ion secondary battery, it is presumed that the δ phase reduces the battery capacity (discharge efficiency).

Cu-Sn (40 mass%) alloy plating (speculum alloy plating) with sulfosuccinic acid is a surface technology 55 (2004) as a candidate for an alternative nickel plating process that exhibits a silvery white color to prevent nickel allergy (biological allergic reaction). p. 484 (it is the most important item to have a silver white color because it aims at an alternative process of nickel plating). According to this “surface technology”, a Cu—Sn alloy film having a speculum composition is obtained by electroplating using a sulfosuccinic acid bath, and an alloy phase and a η phase (Cu ₆ Sn ₅ ) existing only in a high temperature region of a phase equilibrium diagram. It is reported that it precipitates as a mixed phase. On the other hand, in the present invention, a substantially single-phase η phase is obtained. The reason for this is not clear at this time, but the properties of the plating film are greatly influenced by the stability of ions in the plating solution, so various additives, acidity, temperature, current density, etc. involved in ion stability This is thought to be due to the complex action of these factors. In addition, as described in this “Surface Technology”, the waste solution treatment of the plating solution with sulfosuccinic acid is safer and easier to implement than the cyan solution.

(Formation of Cu ₆ Sn ₅ phase (η phase) burnt plating film)
With reference to FIG. 2, the operation in the case where a burnt plating film of Cu ₆ Sn ₅ phase (η phase) is formed as the negative electrode active material will be described.
As shown in FIG. 2A, by forming a Cu ₆ Sn ₅ phase (η phase) plating film as the negative electrode active material 2 on the surface of the copper foil 1 by rough plating, As shown in b), even if the convex part 2a expands (the volume increases) during charging, the presence of the concave part 2b acts as a buffer to relieve the expansion in the in-plane direction of the plating film (suppress the increase in internal stress). ), Deformation of the Cu ₆ Sn _five- phase plating layer as a whole can be prevented. For this purpose, as shown in FIG. 2A, the recess 2b is preferably as deep as possible so as to reach not only the surface layer portion of the Cu ₆ Sn _five- phase plating layer but also the underlying current collector copper foil 1.

Specifically, when the surface roughness Ra is less than 0.5 μm, the surface of the substantially uniform Cu ₆ Sn _five- phase plating layer has irregularities only, and the stress relaxation effect due to expansion and contraction is insufficient. On the other hand, when the surface roughness Ra is 2.0 μm or more, dendrite-like crystals are likely to be formed (deposited) on the convex portions, and the dendrites are likely to fall off, so that improvement in charge / discharge characteristics cannot be expected, There is a risk that an electrical short circuit may occur due to the dropped substance (dendritic crystals). For these reasons, the surface roughness of the Cu ₆ Sn ₅ phase burnt plating is preferably Ra 0.5 μm or more and less than 2.0 μm. The surface roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994), and can be measured with a surface roughness meter, a scanning probe microscope (SPM), or the like.

(Formation of Cu ₆ Sn ₅ phase (η phase) smooth plating layer)
It is preferable to apply plating having a uniform thickness (hereinafter referred to as “smooth plating”) on the surface of the Cu ₆ Sn ₅ phase formed in an uneven shape by burnt plating for the following reason. In other words, for cases where the convex part falls off even in the absence of dendrite formation (for example, it is likely to occur at a high current density), the smooth plating film is formed along the rough shape of the rough plating to prevent the dropout. Because it can.

By using a negative electrode for a lithium ion secondary battery in which a Cu ₆ Sn _five- phase plating film is formed with sulfosuccinic acid as described above, instead of the conventional carbon-based active material, the energy density is higher and the size is smaller than the conventional one. Can be supplied.

A copper foil having a thickness of 0.018 mm was prepared, and Cu—Sn alloy plating (bake plating) was performed with the plating solution shown in Table 1 so as to have a thickness of 5 μm in terms of Sn plating thickness (Sample 1-1). After plating, Sn was 41 mass% in composition analysis by EDX (energy dispersive X-ray apparatus) on the surface of the plating film. Next, in order to produce Cu—Sn alloy plating films having different Sn contents, a 0.018 mm thick copper foil was separately prepared, and the ratio of the Cu component and the Sn component in Table 1 was changed. Cu—Sn alloy plating was performed so as to obtain a thickness of 5 μm in terms of plating thickness. After plating, when the composition of the plated film surface was analyzed by EDX, 50 mass% Sn (sample 1-2), 60 mass% Sn (sample 1-3), 34 mass% Sn (sample 1-4), 65 mass% Sn ( Sample 1-5). Moreover, when the surface roughness of each sample was evaluated with a stylus roughness meter, Ra = 0.9 μm for each sample. Further, for each sample, the structural analysis of the plating film was performed by 2θ / θ scan measurement using an X-ray diffraction (XRD) apparatus using CuKα rays.
FIG. 3 shows the XRD measurement results of Sample 1-1 to Sample 1-4.

As can be seen from the results in FIG. 3, in Sample 1-1 to Sample 1-3, no significant diffraction peak due to a phase other than the Cu ₆ Sn ₅ phase (η phase) was observed, and almost single-phase Cu _6. It can be confirmed that it is Sn ₅ phase (η phase). On the other hand, in Sample 1-4, diffraction peaks caused by unidentified phases other than the Cu ₆ Sn ₅ phase (η phase) were observed. In other words, Sample 1-4 is considered to be a mixed phase of Cu ₆ Sn ₅ phase (η phase) and another phase. Moreover, although not shown in FIG. 3, in Sample 1-5, in addition to the Cu ₆ Sn ₅ phase (η phase), a diffraction peak considered to be caused by an excessive Sn component was observed.

The electrode material thus obtained was punched into a 2 cm ² circle and a test cell using lithium metal as a counter electrode was manufactured, and the charge / discharge characteristics were evaluated. A polypropylene thin film was used as the separator, and a mixed solution (1: 1) of ethylene carbonate and diethyl carbonate in which 1 mol / L 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 2 shows the evaluation results of the discharge capacity retention rate after 80 cycles with respect to the discharge capacity of the initial cycle (first cycle). Table 2 also shows the Sn content of the Cu—Sn alloy plating film.

  As can be seen from the results in Table 2, Sample 1-1 to Sample 1-3 according to the present example have a higher discharge capacity retention rate after 80 cycles than Sample 1-4 and Sample 1-5, which are comparative examples. It can be seen that excellent cycle characteristics are exhibited.

Next, as a comparative example for Cu-Sn alloy plating with sulfosuccinic acid, a copper foil having a thickness of 0.018 mm was separately prepared, and Sn plating having a thickness of 5 μm was performed with a plating solution shown in Table 3 (Sample 1-6). ). After plating, heat treatment was performed at 200 ° C. for 18 hours in a vacuum in order to generate a Cu ₆ Sn ₅ phase. Formation of Cu ₆ Sn ₅ phase was confirmed by X-ray diffraction measurement. Moreover, Sn content rate was 61 mass% in the surface composition analysis by EDX.

  Thus, the charging / discharging test was done for the obtained electrode material by the method similar to the sample 1-1 to the sample 1-5. In this test, the discharge capacity retention rate after 80 cycles with respect to the discharge capacity of the initial cycle was 39%.

  Moreover, as shown in FIG. 4, when the cell after the charge / discharge test was disassembled and the surface of the electrode (negative electrode) was observed, peeling and dropping occurred in the specimen 1-6 (see FIG. 4 ( b)), such a phenomenon was not observed in the test piece of Sample 1-1, and the adhesion of the active material was maintained (FIG. 4A).

From the above results, Cu-Sn alloy plating film lithium ion secondary utilizing batteries for a negative electrode material according sulfosuccinate (e.g., sample 1-1), generating a Cu ₆ Sn ₅ phase by heating the diffusion reaction after Sn plating It can be seen that the discharge capacity retention rate in the charge / discharge test is higher than that of the sample 1-6, and excellent cycle characteristics are exhibited.

  A copper foil having a thickness of 0.018 mm was prepared, and Cu—Sn alloy plating was performed with the plating solution shown in Table 4 so as to have a thickness of 5 μm in terms of Sn plating thickness. At this time, a Cu—Sn alloy plating film having a different surface roughness was produced on the copper foil by changing the current density in burnt plating (sample 2-1 to sample 2-5). Thereafter, smooth plating of a Cu—Sn alloy was performed on a part of the samples (Sample 2-4 and Sample 2-5) with a plating solution shown in Table 5 so that the Sn plating thickness was 0.5 μm. It was. When each surface was evaluated for surface roughness (Ra) with a stylus type roughness meter, Ra = 0.2 to 2.0 μm.

FIG. 5 shows a surface SEM photograph of Sample 2-3, and FIG. 6 shows a surface SEM photograph of Sample 2-1. It can be seen that Sample 2-3 (FIG. 5) is a plating film having a deep concavo-convex structure on the surface as compared with Sample 2-1 (FIG. 6). In addition, after plating, the structure of the plating film was analyzed by X-ray diffraction measurement, and all samples (Sample 2-1 to Sample 2-5) consisted of almost single-phase Cu ₆ Sn ₅ phase (η phase). I confirmed. Furthermore, when the composition analysis of the plating film surface was performed by EDX, Sn content rate was about 41 mass% in any sample.

The electrode material thus obtained was punched into a 2 cm ² circle and a test cell using lithium metal as a counter electrode was manufactured, and the charge / discharge characteristics were evaluated. A polypropylene thin film was used as the separator, and a mixed solution (1: 1) of ethylene carbonate and diethyl carbonate in which 1 mol / L 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 6 shows the evaluation results of the discharge capacity retention rate after 80 cycles with respect to the discharge capacity of the initial cycle (first cycle). Table 6 also shows the current density in burnt plating, the presence / absence of smooth plating and the current density, and the surface roughness (Ra) of the Cu—Sn alloy plating film (negative electrode active material).

  As can be seen from the results in Table 6, Samples 2-2 to 2-4 according to this example have a higher discharge capacity maintenance rate after 80 cycles than Samples 2-1 and 2-5, which are comparative examples. It can be seen that excellent cycle characteristics are exhibited. Further, from the results of Table 6, it was found that the surface roughness Ra of the negative electrode active material is preferably 0.5 μm or more and less than 2.0 μm.

  Further, when the cell after the charge / discharge test was disassembled and the surface of the electrode (negative electrode) was observed, the specimens of Sample 2-1 and Sample 2-5 had the negative electrode active material falling off, whereas Sample 2-2 Thus, such a phenomenon was not observed in the specimen 2-4, and the soundness of the active material (adhesion with the negative electrode current collector) was maintained.

In addition, the plating solution and current density of Cu ₆ Sn ₅ phase precipitation by the sulfosuccinic acid bath described in this example are an example thereof, and the Cu ₆ Sn ₅ phase (η phase) is directly plated with the sulfosuccinic acid solution. Other plating solution compositions and current densities can be used if present.

Moreover, the plating solution composition, additive, current density, and the like of the burnt-plated Cu ₆ Sn _five- phase plating described in this example are just one example, and any conditions can be used as long as the uneven shape (roughened plating shape) is obtained. Other conditions may be used.

  Further, in this embodiment, Cu—Sn alloy plating is directly performed on the copper foil, but a base treatment for improving the adhesion of the plating, for example, base Cu plating may be performed.

It is a schematic diagram which shows one Embodiment of the negative electrode for lithium ion secondary batteries which concerns on this invention. Is a schematic diagram for explaining the operation in the case of forming the burnt plating film of Cu ₆ Sn ₅ phase (eta phase) as an anode active material, (a) shows the discharge time, (b) it is shows the state during charging is there. It is a XRD measurement result of the plating film (sample 1-1 to sample 1-4) which performed Cu-Sn alloy plating (bake plating). It is a SEM photograph of the negative electrode surface of Sample 1-1 and Sample 1-6 after the charge / discharge test. It is the surface SEM photograph of the sample 2-3 (Ra = 0.9 micrometer) before a charging / discharging test. It is the surface SEM photograph of the sample 2-1 (Ra = 0.2 micrometer) before a charging / discharging test.

Claims (6)

In a negative electrode for a lithium ion secondary battery in which a Cu-Sn alloy-based active material is formed on a negative electrode current collector made of copper foil, the active material has an Sn content of 35 mass% or more and 60 mass% or less and a substantially single-phase Cu. _A negative electrode for a lithium ion secondary battery, characterized by comprising ₆ Sn ₅ phase (η phase).   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the surface of the active material is formed in an uneven structure.   3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the surface roughness Ra of the active material is 0.5 μm or more and less than 2 μm.   In a method for manufacturing a negative electrode for a lithium ion secondary battery in which a Cu—Sn alloy active material is formed by electroplating on a negative electrode current collector made of copper foil, the active material is directly applied by Cu—Sn alloy plating with a sulfosuccinic acid solution. A method for producing a negative electrode for a lithium ion secondary battery, comprising: forming a negative electrode for a lithium ion secondary battery. The method for producing a negative electrode for a lithium ion secondary battery according to claim 4, wherein the Cu ₆ Sn ₅ phase (η phase) is formed by brush plating. The Cu ₆ Sn ₅ phase (η phase) film is formed with a uniform thickness on the surface of the Cu ₆ Sn ₅ phase formed in the uneven shape by the burn plating, and further by Cu-Sn alloy plating with a sulfosuccinic acid solution. The method for producing a negative electrode for a lithium ion secondary battery according to claim 5.

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