JP2013185228A - Electrolytic copper foil and negative electrode collector for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolytic copper foil by which adhesion between a current collector (copper foil) and an active material is maintained and the current collector (copper foil) does not break upon high expansion and contraction of the active material, and to provide an excellent lithium secondary battery by using the electrolytic copper foil as a negative electrode collector.SOLUTION: An electrolytic copper foil has a ratio of peak intensities I (220)/I (200) of at least 3 obtained by X-ray diffraction and contains tungsten in the foil. A tungsten concentration in the electrolytic copper foil is preferably 15-540 ppm, initial tensile strength is preferably at least 600 MPa, and the tensile strength after heated at 300°C for one hour is preferably at least 450 MPa.

Description

  The present invention relates to an electrolytic copper foil excellent in heat resistance, and particularly relates to an electrolytic copper foil excellent as a current collector for a lithium secondary battery electrode.

A lithium secondary battery is basically composed of a positive electrode, a negative electrode, and an electrolyte, and the negative electrode is formed by coating a surface of a copper foil used as a current collector with a negative electrode active material layer made of a carbon-based material. The
As a method for forming the negative electrode, a negative electrode active material and a binder resin (added for the purpose of binding the active material and the copper foil substrate) dissolved in a solvent are applied onto the copper foil substrate, and the binder resin is then formed. The slurry method is generally formed by pressing after drying at a temperature equal to or higher than the curing temperature.

  As the binder resin, polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and the like are widely used. However, in recent years, the theoretical capacity is higher than that of carbon-based materials, which are attracting attention as the capacity of batteries increases. Silicon, tin, germanium alloy materials, etc. have a very large volume expansion coefficient due to lithium insertion / desorption during charge / discharge, and binder resins that have been used so far have insufficient strength to peel off active materials. Therefore, a polyimide resin having a high adhesive strength with a copper substrate has been preferably used.

However, unlike binder resins that have been widely used so far, the polyimide resin has a very high curing temperature of about 300 ° C., and a negative electrode current collector (copper foil) that can withstand this heating condition is required.
In response to such demands, the present inventors improved the heat resistance of the copper foil by adding tungsten to the copper foil, and the electrolytic copper foil for a current collector coated with an active material having a polyimide resin as a binder resin. Tried development. However, tungsten is a metal that is very difficult to incorporate into the electrolytic copper foil.

Patent Documents 1 and 2 exist as documents in which tungsten is added to an electrolytic solution for forming an electrolytic copper foil.
Patent Documents 1 and 2 both relate to a copper foil for a printed circuit, and in the examples, a copper foil made of an electrolytic solution in which 20 to 100 mg / l of tungsten (W) and chloride ions are added to the electrolytic solution is Although it is disclosed that there is no pinhole, excellent adhesion to a resin substrate, and high hot elongation at 180 ° C., tungsten is incorporated into the copper foil, that is, a Cu—W alloy foil is manufactured. There is no description that it was done.

Japanese Patent No. 3238278 JP-A-9-67693

Unlike the binder resins that have been widely used so far, the polyimide resin has a very high curing temperature of about 300 ° C., and a negative electrode current collector (copper foil) that can withstand this heating condition is required.
As a condition of the copper foil that can withstand the heating process at the time of electrode formation as described above, the present inventors have targeted the production of a highly heat-resistant copper foil having a tensile strength after heating at 300 ° C. for 1 hour of 450 MPa or more. , Worked hard on development.

  As a result of various developments for the above target, a copper foil that can maintain a tensile strength of 450 MPa or more after heating at 300 ° C. for 1 hour is achieved by adding tungsten having a high melting point to obtain a copper alloy foil. As a result, the present invention was achieved.

The electrolytic copper foil of the present invention has a peak intensity ratio I (220) / I (200) obtained by X-ray diffraction.
Is an electrolytic copper foil having 3 or more and containing tungsten in the foil.

  The tungsten concentration contained in the electrolytic copper foil of the present invention is preferably 15 ppm or more, and more preferably 15 ppm or more and 540 ppm or less.

  The initial tensile strength of the electrolytic copper foil of the present invention is preferably 600 MPa or more, more preferably an electrolytic copper foil having an initial tensile strength of 600 MPa or more and a tensile strength after heating at 300 ° C. for 1 hour of 450 MPa or more. is there.

  The negative electrode current collector for a secondary battery of the present invention is a current collector using the electrolytic copper foil of the present invention.

  The present invention can provide an alloy electrolytic foil having excellent heat resistance with a tensile strength of 450 MPa or more after heating at 300 ° C. for 1 hour by incorporating tungsten into the foil.

By incorporating tungsten into the copper foil, coarsening of crystal particles during heating is suppressed and heat resistance is improved, but tungsten is very difficult to incorporate into the electrolytic copper foil.
In the present invention, a peak intensity ratio obtained by X-ray diffraction of an electrolytic copper foil obtained by adding a predetermined organic additive to an electrolytic solution and having a crystal orientation of 3 or more, I (220) / I (200) As a result, tungsten was successfully incorporated into the electrolytic copper foil, and a tungsten-copper alloy foil containing tungsten and excellent in heat resistance was completed.

The electrolytic copper foil for a current collector on which an active material having a polyimide resin as a binder resin is deposited is an electrolytic copper alloy foil having a tensile strength of 450 MPa or more after heating at 300 ° C. for 1 hour.
As described above, the current collector (copper foil) constituting the negative electrode current collector of the lithium secondary battery needs to withstand heat treatment at 300 ° C. for 1 hour usually when a polyimide binder is used. That is, an active material composition prepared by adding a solvent or the like to a mixture of an active material, a conductive material and a binder resin is applied to the surface of a current collector for a lithium secondary battery, and after passing through a drying process, Although it is set as the negative electrode of a battery, the drying process requires heat treatment at 300 ° C. for 1 hour. As a copper foil that can withstand the heating conditions of the drying step and withstand expansion and contraction due to charge / discharge cycles of the active material, an electrolytic copper foil having a tensile strength of 450 MPa or more after heating at 300 ° C. for 1 hour is required.

The inventors repeated various experiments in order to produce Cu-W alloy foils. As a result, in an electrolytic solution containing chlorine ions, even if a large amount of tungsten is added to the solution, tungsten is not taken into the formed copper foil, and naturally the copper foil formed with such an electrolytic solution is used. The tensile strength of the foil after heating at normal temperature and after heating was not improved.
However, even if the electrolytic solution contains chlorine ions, the opinion was obtained that when a thiourea compound such as ethylenethiourea was added to the solution, tungsten was taken into the foil depending on the foil-making conditions.
When analyzing the factor that tungsten is taken into the copper foil based on such a view, the peak intensity ratio I (220) / I (200) obtained by X-ray diffraction of the copper foil is 3 or more. The result that tungsten was taken in in foil was obtained.

Based on these findings, we succeeded in producing an electrolytic copper foil with excellent heat resistance by making an electrolytic copper foil under the following conditions.
That is, the peak intensity ratio obtained from the X-ray diffraction of the electrolytic copper foil by producing a copper alloy foil having a tensile strength of 450 MPa or more after heating at 300 ° C. for 1 hour with the following basic electrolytic bath composition and current density. An electrolytic copper foil in which I (220) / I (200) is 3 or more and tungsten is taken into the foil as a Cu-W alloy can be manufactured.

Basic electrolytic bath composition:
Cu = 50 to 150 g / L
H ₂ SO ₄ = 20 to 200 g / L
Cl = 15-50ppm
Sodium tungstate (as tungsten) = 10-200ppm
Thiourea compounds = 3-20ppm
Current density:
Current density = 20-60 A / dm ²

  10 to 200 ppm of tungsten is added to the electrolytic solution. The reason why the added amount of tungsten is set to 10 ppm or more is that the effect of containing tungsten does not appear below this, and the effect of improving the tensile strength or the like does not improve even if contained at 200 ppm or more. Therefore, the addition amount of tungsten is preferably 10 to 200 ppm.

  The first purpose of adding the thiourea compound to the electrolytic solution is to incorporate tungsten into the copper foil to obtain a Cu-W alloy foil. As described above, the electrolyte containing chlorine ions cannot take tungsten into the copper foil. However, in the present invention, W was successfully incorporated into the copper foil by adding a thiourea compound. The amount of thiourea compound to be added is 3 ppm to 20 ppm because if 3 ppm or less, a prescribed amount of tungsten cannot be taken into the copper foil, the tensile strength after heating at 300 ° C. for 1 hour is 450 MPa or less, and 20 ppm. If it is added in excess of 1, tungsten enters the copper foil too much, the tensile strength becomes too high, or the elongation becomes small, and undesirable properties appear. Therefore, the addition amount is preferably 3 ppm to 20 ppm.

  The addition amount of chloride ions is 15 to 50 ppm. Addition of chlorine ions of 15 ppm or less is not preferable because many pinholes are generated in the foil, and addition of chlorine ions of 50 ppm or more causes problems such as a marked increase in surface roughness. The ions are preferably in the range of 15 to 50 ppm, particularly preferably 20 to 45 ppm.

The electrolytic copper alloy foil has a current density of 20 using, as an electrolyte, a copper sulfate solution containing tungsten, a thiourea compound, and chlorine ions added in the above-described amounts, a noble metal oxide-coated titanium as an anode, and a titanium rotating drum as a cathode. It is made into a foil by electrolytic treatment under the conditions of ˜60 A / dm ² and a liquid temperature of 30 to 75 ° C. By making the foil under such conditions, the peak intensity ratio I (220) / I (200) obtained by X-ray diffraction can be made 3 or more, and a prescribed amount of W can be taken into the electrolytic copper foil. it can.

  Hereinafter, the present invention will be described in detail based on examples.

<Examples 1-36>
The following bath composition was used as the basic bath composition of the copper sulfate electrolyte.
Cu = 50 to 150 g / L
H ₂ SO ₄ = 20 to 200 g / L
Cl = 15-50ppm
Organic additives shown in Table 1 = 3 to 20 ppm

  To the above bath, sodium tungstate as an additive and thiourea, ethylenethiourea, N, N-diethylthiourea, and tetramethylthiourea shown in Table 1 were added to the concentrations shown in Table 2, and shown in Table 2. An electrolytic copper alloy foil was formed to a thickness of 12 μm at a current density.

The electrolytic copper alloy foil thus formed was subjected to rust prevention treatment under the following conditions.
The formed electrolytic copper alloy foil (untreated copper alloy foil) was immersed in a CrO ₃ ; 1 g / l aqueous solution for 5 seconds, subjected to chromate treatment, washed with water and dried.
Although the chromate treatment is performed here, it goes without saying that the silane coupling agent treatment may be performed after the benzotriazole-based treatment, the silane coupling agent treatment, or the chromate treatment.
The copper alloy foil thus prepared was subjected to the following various measurements and tests, and the results are shown in Table 2.

(1) Crystal orientation measurement (XRD)
The crystal orientation of the copper foil was measured by X-ray diffraction (XRD).
Equipment used: RAD-B (Rigaku)
Condition CuKα
Scan method θ-2θ
Tube voltage 40 kV
Tube current 20mA
Measurement range 20-100 °

(2) Measurement of W concentration in foil (ICP)
The W concentration in the copper foil was measured using an inductively coupled plasma (ICP) emission spectrometer on a sample obtained by dissolving a certain amount of copper foil with an acid and then diluting with distilled water.
Equipment used: ICPS-7000 (Shimadzu Corporation)

(3) Tensile strength measurement Tensile strength of copper foil is measured before and after heating the foil based on IPC-TM-650. Equipment used: AG-I (Shimadzu Corporation)

(4) Battery Performance Test Next, a lithium secondary battery was prepared using the electrolytic copper foil produced in the example as a current collector, and a cycle life test was performed.
Powdered Si alloy-based active material (average particle size 0.1 μm to 10 μm) is mixed in a ratio (weight ratio) of 85 and binder (polyimide) at 15 (weight ratio), and dispersed in N-methylpyrrolidone (solvent). A slurry was obtained.
Next, this slurry was applied to both sides of a 12 μm-thick electrolytic copper foil prepared in the Examples, dried and compressed by a roller press, and then sintered at 300 ° C. for 1 hour in a nitrogen atmosphere to form a negative electrode. . In this negative electrode, the negative electrode mixture after molding had the same film thickness of 20 μm on both sides.

Creation of Lithium Secondary Battery A three-electrode cell for evaluation was constructed with the following configuration in a glove box under an argon atmosphere.
Negative electrode: Si alloy negative electrode prepared above Counter electrode, reference electrode: Lithium foil Electrolytic solution: 1 mol / L LiPF ₆ / EC + DEC (3: 7 vol%)

The constructed cell was taken out from the box into the atmosphere, and charge / discharge measurement was performed in an atmosphere at 25 ° C.
Charging was carried out at CC (constant current) up to 0.02 V with respect to the Li standard unipolar potential reference, and thereafter, charging was terminated when the CV (still at constant potential) current decreased by 0.05 C. Discharge was performed at CC (constant current) at 0.1 C to 1.5 V (Li standard). The charge and discharge were repeated with the same current equivalent to 0.1 C.
As an evaluation of the charge / discharge performance, the number of cycles until the discharge capacity reaches 70% of the discharge capacity of the first cycle was determined, and it was determined that an electrode having a cycle number of 100 or more could be used practically, and was regarded as an acceptable level. . Tables 2 and 4 show the cycle numbers of the electrodes manufactured under each condition.

  Moreover, the battery was decomposed | disassembled after completion | finish of a charging / discharging test, and the presence or absence of a deformation | transformation was observed about the electrode (copper foil) used as a negative electrode collector material. The results were marked with “◯” for those without deformation such as wrinkles, and with “x” for those with deformation such as wrinkles.

(Comparative Examples 1-15)
The following bath composition was used as the basic bath composition of the copper sulfate electrolyte.
Cu = 50 to 150 g / L
H ₂ SO ₄ = 20 to 200 g / L
Cl = 15-50ppm
Organic additives shown in Table 3 = 3 to 20 ppm

  In the same manner as in the examples, sodium tungstate and thiourea, ethylenethiourea, N, N-diethylthiourea, and glue were added to the above baths at the concentrations shown in Table 4, and currents shown in Table 4 were added. An electrolytic copper foil having a density of 12 μm was formed.

  Evaluation similar to the Example was performed with respect to the produced copper foil, the crystal orientation of the foil, the W concentration in the foil, the tensile strength before and after the heat treatment, and the battery characteristics were measured, and the results are shown in Table 4.

  In Table 2, the evaluation result of the copper foil produced in Examples 1-36 is shown. In each sample, it can be confirmed that the peak intensity ratio I (220) / I (200) obtained by X-ray diffraction changes due to changes in additive concentration and current density. In the copper foil shown in Table 2, the peak intensity ratio obtained from X-ray diffraction, I (220) / I (200), is all 3 or more, and the crystal orientation in which tungsten is easily incorporated is controlled. In the foil, 15 ppm or more and 540 ppm or less of tungsten is incorporated.

It can be seen that increasing the amount of tungsten in the electrolytic bath tends to increase the amount of tungsten taken into the foil. Moreover, the tensile strength (MPa) after a heating was 450 MPa or more, and it turned out that it is excellent in heat resistance.
When the tensile strength (MPa) heated at 300 ° C. for 1 hour was observed, all foils were excellent in heat resistance of 450 MPa or more, and deformation of the foil after the cycle test could not be confirmed under all conditions. More preferably, under the condition that the tungsten uptake amount is 15 ppm or more, the tensile strength after heating at 300 ° C. for 1 hour is 450 MPa or more, which is more excellent in heat resistance, and the battery cycle number is 100 or more that can be used practically. Preferred properties were shown. Furthermore, in a foil having a tungsten uptake amount of 540 ppm or more, it is most preferable that the tungsten uptake amount in the foil is 500 ppm or less because the foil is easily cut and difficult to produce.

On the other hand, Table 4 shows the evaluation results of Comparative Examples 1-15.
The copper foils of Comparative Examples 1 to 6 are the results of samples prepared under different electrodeposition conditions (current density) using the same electrolytic solution as in Examples, but the peak intensity ratio obtained from X-ray diffraction, I (220) / I (200) was 3 or less, and it was confirmed that the crystal orientation considered to contribute to tungsten uptake was not shown. The tungsten uptake is less than 10 ppm regardless of the tungsten concentration in the electrolytic bath, and tungsten is hardly taken up. The tensile strength after heating at 300 ° C. for 1 hour is 450 MPa or less, and the heat resistance is very high. It became clear that it was low.

In addition, the copper foils of Comparative Examples 7 to 15 made by adding glue commonly used as an additive, the peak intensity ratio obtained by X-ray diffraction, I (220) / The crystal orientation in which I (200) is 3 or more is not exhibited, all samples have a tungsten uptake of 0 ppm, and the tensile strength after heating is very low, 250 MPa or less. .
In the battery using the foil of the comparative example as a current collector, all the foils were deformed after the cycle test. Moreover, the cycle number of the battery was 85 or less, which was not a preferable characteristic.

  With the W-Cu alloy foil having the above characteristics, the current collector (copper foil) and the active material are kept in close contact with the current collector (copper foil) against the large expansion and contraction of the silicon-based or tin-alloy active material. The copper foil) can provide an electrolytic copper foil that does not break, and by using the electrolytic copper foil as a negative electrode current collector, an excellent lithium secondary battery can be provided.

Claims (6)

Peak intensity ratio I (220) / I (200) obtained by X-ray diffraction
Is an electrolytic copper foil having 3 or more and containing tungsten in the foil.   The electrolytic copper foil according to claim 1, wherein the tungsten concentration in the foil is 15 ppm or more.   The electrolytic copper foil according to claim 1, wherein the tungsten concentration in the foil is 15 ppm or more and 540 ppm or less.   The electrolytic copper foil according to any one of claims 1 to 3, wherein an initial tensile strength is 600 MPa or more.   The electrolytic copper foil according to any one of claims 1 to 3, wherein the initial tensile strength is 600 MPa or more, and the tensile strength after heating at 300 ° C for 1 hour is 450 MPa or more.   The negative electrode collector for secondary batteries using the electrolytic copper foil in any one of Claims 1-5.