JP2009205888A - Negative electrode for lithium secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium secondary battery having excellent cycle characteristics, and to provide its manufacturing method. <P>SOLUTION: The thin film negative electrode has an alloy thin-film active material layer of Sn alloyed with Li and a metal not alloyed with Li formed on one side or both sides of a copper or a copper alloy foil substrate. The half-value width of the maximum peak in X-ray diffraction method of the negative electrode active material using Cukα beam is 0.2-0.55° at 2θ. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a negative electrode for a lithium secondary battery, particularly a lithium secondary battery formed by forming an alloy thin film active material of Sn alloyed with Li and a metal not alloyed with Li on one surface or both surfaces of copper or a copper alloy foil. The present invention relates to a negative electrode for a secondary battery and a method for producing the same.

Lithium secondary batteries have been actively researched and developed for higher performance, and various electrode materials have been proposed. When metallic lithium is used as the negative electrode, a high-capacity battery can be obtained, but there is a problem in that lithium precipitates in a dendrite state during charging and causes an internal short circuit. In addition, negative electrode materials using simple substances such as Si and Sn, which are alloyed with lithium during charging, have been reported as high capacity negative electrode materials. When an alloy itself is pulverized, current collection characteristics deteriorate, and only low cycle characteristics can be obtained. Therefore, in order to suppress pulverization of the alloy negative electrode itself, by forming an intermetallic compound of a metal that does not alloy with lithium such as Co, Ni, Cu, and Ag and a metal that alloy with lithium such as Si and Sn. In addition, negative electrodes with reduced pulverization during charging / discharging have been studied (see, for example, Patent Documents 3 and 4), and Sn and Si, which are high capacity negative electrode active materials, are alloyed with metals that do not alloy with lithium. By making the active material into a fine crystal structure and an amorphous structure, pulverization is suppressed and cycle characteristics are improved.
JP 2002-198091 A JP 2002-373647 A Japanese Patent Laid-Open No. 10-223221 JP 2001-143761 A JP 2001-256968 A

  In Patent Document 3, “as the negative electrode material, an intermetallic compound of at least one element selected from the group of elements of Al, Ge, Pb, Si, Sn, and Zn and a metal or metalloid other than the above elements is used. A secondary battery characterized in that the intermetallic compound used in the negative electrode active material is low crystalline, and a secondary battery characterized in that the intermetallic compound used in the negative electrode active material is amorphous. The proposed secondary battery is characterized "([007])," The intermetallic compound used in the present invention is more preferably low crystalline or amorphous "(" 0009 "). It has been proposed that when the negative electrode active material has a low crystalline or amorphous crystal structure, the cycle characteristics are good. Furthermore, examples of negative electrode active materials having a highly crystalline crystal structure are also described as examples.

In Patent Document 4, “the non-aqueous electrolyte secondary battery according to claim 1, wherein the metal material contains CoSn ₂ whose crystallite size in the [211] direction is 20 nm or less” (“Bill Item 4 ”), the crystallite size of 20 nm or less as described later corresponds to the low crystalline or amorphous crystal structure in Patent Document 4, and the negative electrode active material is It is considered to have a low crystalline structure similar to that of Patent Document 4.

  On the other hand, Patent Document 5 uses a copper foil subjected to electroplating as a negative electrode material (“Claim 1”), heat-treats the copper foil subjected to electroplating (“Claim 3”), and has a fine grain of 1 μm or less. Is defined as a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery (“Claim 3”).

  However, as will be described in detail later, the crystal structure of low crystallinity or amorphous described in Patent Document 3 (hereinafter, “low crystallinity or amorphous” is referred to as “low crystallinity” in the present invention). If it has, since the crystallinity is low, the active material and the current collector foil are likely to be peeled off because cracks that relieve stress in the active material layer due to volume expansion during charge and discharge are difficult to enter, In the case of having the highly crystalline crystal structure in Patent Document 3 and the crystal structure in Patent Document 5, cracks on the surface of the active material are likely to occur due to volume expansion during charge / discharge, but cracks are caused by the presence of grain boundaries. Propagation is easy, and when the crack propagates to the current collector, the active material and the current collector foil are easily separated. As described above, all of the measures for peeling between the active material layer and the current collector foil are insufficient, and sufficient cycle characteristics are not obtained.

  The problem to be solved by the present invention is that the active material in the negative electrode for a lithium secondary battery tends to generate cracks for relaxing the stress due to volume expansion associated with charge / discharge, but it is good that crack propagation is difficult. It is providing the negative electrode for lithium secondary batteries which has cycling characteristics, and its manufacturing method.

  The present inventor has intensively studied to solve the above problems, and as a result, an active material having a crystal structure different from an active material having a low crystalline crystal structure or an active material having a high crystalline crystal structure in a thin film negative electrode. It has been found that the cycle characteristics are better by using the active material than the active material having a high crystalline structure or the active material having a low crystalline structure. That is, the invention is as follows.

  (1) In a negative electrode for a lithium secondary battery in which an active material layer made of an alloy of Sn alloyed with Li and a metal not alloyed with Li is formed on one surface or both surfaces of a copper or copper alloy foil base material. A negative electrode for a lithium secondary battery, wherein the half-value width of the maximum peak in the X-ray diffraction method of the active material layer using CuKα rays is 0.2 to 0.55 ° in 2θ.

  (2) The negative electrode for a lithium secondary battery according to (1), wherein the metal not alloyed with Li is Co.

  (3) The negative electrode for a lithium secondary battery according to (1) or (2), wherein the Sn composition of the active material is 50 to 70% by mass.

  (4) The negative electrode for a lithium secondary battery according to any one of (1) to (3), wherein the thickness of the active material layer is 2 to 15 μm per side.

  (5) It consists of an alloy of Sn alloyed with Li and a metal not alloyed with Li on one side or both sides of the copper or copper alloy foil base material, and has the maximum peak in the X-ray diffraction method using CuKα rays. An active material layer having a half width of 2θ of 0.6 ° or more is formed, and then the half width of the maximum peak in the X-ray diffraction method of the active material layer is 0.2θ to 0.5 to 0.55 ° (1)-(4) The manufacturing method of the negative electrode for lithium secondary batteries in any one of (1)-(4) characterized by performing heat processing on conditions of temperature and time which become.

  The method for producing a negative electrode for a lithium secondary battery according to (5), wherein the active material layer is formed by electroplating.

  In accordance with the present invention, an alloy-based negative electrode having an active material that forms an intermetallic compound with an alloy of Sn that is alloyed with Li and a metal that is not alloyed with Li on one surface or both surfaces of copper or a copper alloy foil, particularly an SnCo alloy As a result of suppressing the pulverization and separation of the active material accompanying charging / discharging of the negative electrode having the active material, a thin film negative electrode having a high capacity and good cycle characteristics can be obtained.

  Embodiments of the present invention will be described below.

(1) Composition of negative electrode active material In the present invention, the negative electrode active material formed on the collector copper foil or copper alloy foil is a compound of Sn alloyed with Li and a metal not alloyed with Li. Yes, and thus, pulverization caused by the volume change of the active material accompanying charge / discharge can be suppressed. Examples of metals that do not alloy with lithium include Ti, V, Cr, Mn, Fe, Ni, Cu, Ag, and alloys thereof. However, although the negative electrode active material having a low crystalline crystal structure formed on the current collector is heat-treated, it becomes the crystal structure of the present invention and the cycle characteristics are improved. Alloys such as Mn, Fe, Ni, Cu, and Ag that are not alloyed with Li are relatively difficult to form a thin film having a low-crystalline crystal structure. Co is preferably used as the metal that is not alloyed.
Although the composition depends on the production method, the Sn composition can be produced in the range of 50 to 85% by mass. In this invention, it is preferable that Sn composition is 50-70 mass%, and it is more preferable that it is 60-70 mass%.

(2) Thickness of the negative electrode active material The negative electrode active material in the present invention has a low crystalline crystal structure when formed on the current collector copper foil or copper alloy foil. In order to obtain a crystal structure, it is desirable to form a thin film. Moreover, the active material layer by a thin film can be made thinner than a powdery active material layer. The film thickness is 2 to 15 μm. The thicker the active material layer, the larger the charge / discharge capacity per unit area of the current collector, but the cycle characteristics tend to deteriorate. A more preferable range is a film thickness of 4 to 12 μm, and further 5 to 10 μm. In addition, a film thickness can be measured by observing the cross section of a thin film negative electrode by SEM.

(3) Crystal Structure of Negative Electrode Active Material The present invention is a crystal structure having a high crystallinity in Patent Document 3, even if it is an active material having a low crystal structure in Patent Document 3 or an active material having a crystal structure as in Patent Document 4. And an active material having a crystal structure that is not an active material as disclosed in Patent Document 5.
That is, in the present invention, as the crystal structure of the active material layer, the half-value width of the maximum peak of X-ray diffraction is 0.2 to 0.55 ° in 2θ. In addition, it is desirable that the crystallite calculated from the half width of the peak of X-ray diffraction exceeds 20 nm.
“Low crystallinity” in Patent Document 3 means that the half width of an X-ray diffraction peak is 0.6 ° or more, and the crystallite size calculated from the half width of the X-ray diffraction peak is as small as 20 nm or less. Means. On the other hand, “high crystallinity” described in Patent Document 4 and “crystal grain size” as described in Patent Document 6 include crystal grains that are observed by, for example, SIM having a resolution of 9000 to 27000 times, and X-ray The half width of the diffraction peak is less than 0.2 °.
The crystallite size was calculated from the half width of the X-ray diffraction peak using the Scherrer equation as follows: D = K · λ / (β · cos θ)
D: Crystallite size (Å, maximum peak crystallite size)
λ: Measurement X-ray wavelength (Å)
β: broadening of diffraction lines depending on crystallite size. In the present invention, half-width of maximum peak in the present invention. θ: maximum peak position in the present invention K: 0.9 in the case of using a half-width for β.
For example, Invention Example No. In the case of 3, the full width at half maximum of the maximum peak is 0.35 ° and the maximum peak position is 36 ° from FIG.
D (size of crystallite) nm = 0.90 × 0.15404 / (0.35 / 180 × π × cos (36/180 × π)) = 31.7 nm
It becomes.
The reason why an active material having a low crystalline crystal structure is inferior in cycle characteristics to the active material of the present invention is that, in the case of a low crystalline active material, if the stress in the active material layer due to volume expansion during charging increases, Since the crystallites are too small, cracks that occur on the surface of the active material to relieve the stress are difficult to occur and stress that is not relaxed concentrates at the interface between the current collector and the active material layer, This is thought to be because peeling tends to occur.
On the other hand, in an active material having a highly crystalline crystal structure or crystal grains, cracks are generated along the crystal grain boundaries on the surface of the active material to relieve stress in the active material layer generated by volume expansion during charging. Although it is easy, the generated cracks are likely to proceed along the grain boundary, and the active material is likely to be peeled off from the interface between the current collector and the active material layer.
In contrast, in the active material of the present invention, although clear crystal grains cannot be observed, the crystallites grow to a certain size, so that the crystallites play the same role as the crystal grain boundaries. Therefore, when stress is generated in the active material layer due to volume expansion during charging, cracks are likely to occur along the boundary of the surface crystallites. However, once a crack has occurred, the active material of the present invention has a crystallite boundary that is not as clear as the crystal grain boundary, so the propagation of cracks is slow, and the active material is more active than the active material having a highly crystalline crystal structure. It is considered that peeling is slow and cycle characteristics are improved.

(4) Copper foil and copper alloy foil The copper or copper alloy foil of the current collector used in the present invention may be either an electrolytic copper foil or a rolled copper foil, and should be appropriately selected according to the application and required characteristics. Can do. For example, the rolled copper foil is preferably used when high strength and bending resistance are required. When used as a current collector for a lithium secondary battery negative electrode, it is possible to obtain a battery with a higher capacity by reducing the thickness of the copper foil. However, if the thickness is reduced, there is a risk of breakage due to a decrease in strength. In such a case, it is preferable to use a rolled copper foil that is superior in strength to the electrolytic copper foil.
Moreover, there is no restriction | limiting in particular in the kind of copper alloy used for copper or copper alloy foil, What is necessary is just to select suitably according to a use or a required characteristic. For example, but not limited to, Cu-Ni-Si based copper alloy with addition of high purity copper (oxygen-free copper, tough pitch copper, etc.), Sn-containing copper, Ag-containing copper, Ni, Si, etc., Cr, Examples thereof include a copper alloy such as a Cu—Cr—Zr copper alloy to which Zr or the like is added.
The thickness of the copper or copper alloy foil is not particularly limited, and may be appropriately selected depending on the application and required characteristics. Generally, the thickness is 1 to 100 μm, but when used as a current collector for a negative electrode of a lithium secondary battery, a battery having a higher capacity can be obtained by thinning the copper foil. From such a viewpoint, it is preferably 2 to 50 μm, more preferably about 5 to 20 μm.

(5) Roughening treatment When forming a negative electrode active material on a current collector in a lithium secondary battery negative electrode, a roughening treatment is performed on copper or copper alloy foil in order to increase the adhesion between the current collector and the active material. In general, the negative electrode active material is formed after the treatment.
The roughening treatment layer may be a general Cu roughening treatment layer, and further, Ni plating and Sn plating are sequentially applied on individual Cu particles forming the general Cu roughening treatment layer, It has a two-layer structure in which a Ni—Sn alloy layer is formed at both plating interfaces by heat treatment, and a Sn—Ni layer that is a fine protrusion generated by removing the remaining Sn layer by electrolytic polishing or the like is formed. It may be a layer.
In the case of only the Cu roughening layer, Cu may be eluted into the plating solution depending on the type of the plating solution. When elution is not desired, Ni plating is performed after the Cu roughening treatment and before forming the active material layer. There is a case.

(6) Negative Electrode Manufacturing Method The negative electrode for a lithium secondary battery in the present invention is formed by forming a roughening treatment layer on a copper or copper alloy foil of a current collector, forming a negative electrode active material, and performing a heat treatment. can get.
Specifically, it is as follows.
a) Roughening treatment The roughening treatment method may be a general Cu roughening treatment method. Moreover, about the method of forming a Ni-Sn diffused layer on Cu roughening process layer, the process 1 which makes a some Cu particle adhere on a copper or copper alloy foil base material, and Ni plating layer on the surface of this Cu particle | grain Forming a Sn plating layer on the Ni plating layer, forming a NiSn alloy layer by heat treatment at the interface between the Ni plating layer and the Sn plating layer, and a residual Sn plating layer It can be formed by performing Step 5 for removing. The copper or copper alloy foil having such a surface structure has adhesiveness with the negative electrode active material, and can relieve stress concentration at the current collector interface when the active material expands.
b) Formation of Active Material In order to obtain a negative electrode according to the present invention, a negative electrode having an active material having a crystal structure that is neither high crystalline nor low crystalline, heat treatment is performed after the active material is formed. However, it is necessary that the active material layer before heat treatment has a low crystalline structure. Here, as for the crystallinity of the active material layer, the full width at half maximum of the maximum peak of X-ray diffraction using CuKα rays needs to be 0.6 ° or more.
Therefore, the method for forming the active material constituent element and the thin film is limited. As described above, an alloy such as Mn, Fe, Ni, Cu, and Ag that is not alloyed with Sn and Li is relatively difficult to form a thin film having an amorphous or low crystalline crystal structure, depending on the manufacturing method. On the other hand, Co can form a thin film having a low crystalline crystal structure relatively easily. The formation method is the half of the maximum peak of X-ray diffraction among the electroplating method, which is a chemical thin film formation method, the electroless plating method, and the sputtering method, vapor deposition method, thermal spraying method, and CVD method, which are physical thin film formation methods. The method is not limited as long as the value width can be 0.6 ° or more, but electroplating and sputtering that facilitate formation of a thin film having a low crystalline crystal structure are preferred, and industrially, the formation rate of the thin film is high. In the electroplating method which is high and easy to control, a method using a pyrophosphate bath is preferable.
c) Heat treatment It is important that the heat treatment is performed under a condition that the crystal structure of the active material does not change from low crystallinity to high crystallinity. If the heat treatment is insufficient, the active material remains in a low crystalline crystal structure. If the heat treatment is excessive, the active material has a highly crystalline structure in which crystal grains are observed. Furthermore, excessive heat treatment may soften the copper or copper alloy foil that is the current collector, or may cause diffusion of atoms between the active material and the current collector.
Accordingly, the heat treatment conditions are preferably 230 ° C. to 350 ° C. and 0.5 to 2 hours. More preferably, it is 250-300 degreeC. After the heat treatment, it is desirable that the crystal grains cannot be confirmed by observation at 27000 times the SIM image. (See Figure 2)

Examples of the present invention will be described below, but the present invention is not limited to these examples.
A copper alloy foil having a composition of Cu-3.0Ni-0.65Si-0.15Mg and having a size of 60 mm × 45 mm × 18 μm (rolled copper foil manufactured by Nikko Metals Co., Ltd .: product name C7025) was prepared as a current collector. After the copper alloy foil of the current collector was alkaline degreased and pickled, Cu plating was performed on the entire surface of the foil in a copper sulfate bath. Cu plating was performed in the order of base plating 2.3 A / dm ² × 118 s, roughening plating 4.6 A / dm ² × 77 s, and Kabuse plating 2.3 A / dm ² × 94 s. Next, Ni plating was performed on the entire surface of the substrate in a sulfamic acid bath.
These manufacturing conditions are shown in Table 1.

  An Sn—Co alloy thin film was formed on the copper alloy foil subjected to the roughening treatment by electroplating in a plating bath shown in Table 3 to produce a Sn—Co alloy thin film negative electrode. The Sn—Co alloy thin film thickness was 4 μm, and the Sn concentration was 65.2% by mass. Table 2 shows the conditions of each plating bath.

  Invention Example No. 1-6, Comparative Example No. 2-8, no. 10, no. No. 12 was heat-treated at each temperature and time shown in Table 3. The heat treatment was performed using a quartz tube furnace in an Ar atmosphere in which Ar gas as an inert gas was flowed at a flow rate of 2 liters / minute. After the heating time was over, it was cooled slowly.

The measuring method is as follows.
CuKα X-ray diffraction of the surface of the thin film negative electrode active material layer was performed with an X-ray diffraction apparatus for X-ray diffraction maximum peak half width thin film evaluation (apparatus: RINT2000 manufactured by Rigaku), and the half width of the maximum peak was measured. The full width at half maximum of the maximum peak of the sample without heat treatment after the plating of Comparative Example 1 was 1.83 °.
The average grain size of the Sn-Co alloy was measured at an area of 3 μm × 5 μm at 9000 to 27000 times the average grain size SIM image (apparatus: SMI3050SE manufactured by SII), and averaged by the number of crystal grains. did. When the average crystal grain size was 27000 times and it was not confirmed, it was judged as “not judged”.
About the capacity maintenance rate comparison example and the example, a battery cell for charge / discharge test was assembled using an electrolytic solution of metallic lithium, polyethylene separator, EC / DMC3: 71 1MLiPF6 on the counter electrode under an Ar atmosphere in the glove box, and the cycle. The test was conducted. The charge / discharge conditions are as follows: current density at charge 0.25 mA / cm ² , cut-off voltage 3 mV (vs Li / Li +), current density at discharge 1.00 mA / cm ² , cut-off voltage 2.0 V (vs Li / Li +) ) Up to 30 cycles. The discharge capacity at the 30th cycle relative to the discharge capacity at the 1st cycle was defined as the capacity retention rate.

Table 3 shows the results of the examples.
Invention Example No. 1-7, Comparative Example No. In Nos. 1 to 8, since the Sn—Co film was formed in the pyrophosphoric acid bath, the half-width of the maximum peak of the active material before the heat treatment was 1.83 °, which was a low-crystalline crystal structure. Furthermore, Invention Example No. As a result of heat treatment at 240 to 350 ° C. for Nos. 1 to 7, the full width at half maximum of the active material peak was in the range of 0.2 to 0.55 °, indicating good cycle characteristics.
Comparative Example No. In No. 1, since the thin film formed by plating was not subjected to heat treatment, the half-width of the maximum peak remained at 1.83 ° and remained a low crystalline crystal structure. Therefore, the cycle characteristics of the 30-cycle charge / discharge test were as low as 81%.
Comparative Example No. In 2 to 7, heat treatment temperatures of 150 ° C., 200 ° C., and 220 ° C. are not sufficient, and a low crystalline crystal structure having a maximum peak half width of 0.6 ° or more remains, so that the cycle characteristics are 85 to 89%. Although it is slightly improved as compared with Comparative Example 1, it is inferior to the product of the present invention.
Comparative Example No. No. 8 was carried out at a heat treatment temperature of 400 ° C., and therefore, an average crystal grain size of 0.35 μm was observed in the SIM observation after the heat treatment, and the half-width of the maximum peak of the active material was 0.19 ° and the crystallinity was high. You can see that The maintenance rate was as low as 67%.
Comparative Example No. In Nos. 9 and 11, since a chloride bath or a cyan bath was used to form the Sn—Co film, the maximum peak half-value widths were both 0.16, and the average crystal grain sizes of 0.23 μm and 0.21 μm were also observed by SIM observation. The maintenance rate was as low as 72% and 77%.
Further, Comparative Example No. 9 and no. Comparative Example No. 11 was heat-treated. 10 and no. For No. 12, no. 9 and No. 10 are not active materials having a low crystalline crystal structure. Therefore, even when the heat treatment temperature is 300 ° C., the half widths are 0.12 ° and 0.11 °, and the crystal grain sizes are 1.18 μm and 1 .33 μm, Comparative Example No. 9 and no. Since the crystallinity was higher than 11, the maintenance ratio decreased to 64% and 59%.

Invention Example No. 3 is a result of X-ray diffraction of the Sn—Co active material layer in FIG. Invention Example No. 3 is a SIM image of a cross section in FIG.

Claims (6)

  In a negative electrode for a lithium secondary battery in which an active material layer made of an alloy of Sn alloyed with Li and a metal not alloyed with Li is formed on one side or both sides of a copper or copper alloy foil base material, A negative electrode for a lithium secondary battery, wherein the active material layer used has a maximum peak half-value width of 2θ of 0.2 to 0.55 ° in the X-ray diffraction method.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the metal not alloyed with Li is Co.   The negative electrode for a lithium secondary battery according to claim 1 or 2, wherein the Sn composition of the active material layer is 50 to 70 mass%.   The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the thickness of the active material layer is 2 to 15 µm per side.   It consists of an alloy of Sn alloyed with Li and a metal not alloyed with Li on one or both surfaces of a copper or copper alloy foil base material, and the half-width of the maximum peak in the X-ray diffraction method using CuKα rays is An active material layer that is 0.6 ° or more at 2θ is formed, and then the half-value width of the maximum peak in the X-ray diffraction method of the active material layer is a range of 0.2 to 0.55 ° at 2θ The method for producing a negative electrode for a lithium secondary battery according to any one of claims 1 to 4, wherein the heat treatment is performed under conditions of time and time.   6. The method for producing a negative electrode for a lithium secondary battery according to claim 5, wherein the active material layer is formed by electroplating.