KR102109379B1 - Electrolytic copper foil and Current collector for lithium secondary battery and Secondary battery comprising the electrolytic copper foil - Google Patents

Abstract

The electrolytic copper foil according to an embodiment of the present invention is an electrolytic copper foil applied as a current collector for a lithium secondary battery, and the electrolytic copper foil has a fracture characteristic coefficient (f) corresponding to a range of 5.0 to 18.0 when annealed at 190 ° C. for 1 hour. Wherein, the fracture characteristic coefficient (f) is defined as f = {(CB) / A} × 1000, and A is electrolytic at the time when the fracture of the electrolytic copper foil is initiated in the tensile test for the electrolytic copper foil. Represents the stress of the copper foil, B represents the elongation at the time when the fracture of the electrolytic copper foil starts, C represents the elongation at the time when the fracture of the electrolytic copper foil ends, and the fracture of the electrolytic copper foil starts The point in time corresponds to a point in time when the increased stress starts to decrease as the elongation of the electrolytic copper foil increases in the tensile test, and the fracture of the electrolytic copper foil ends. Is the time corresponds to the time when the electrolytic copper foil is destroyed separated into two or more pieces.

Description

Electrolytic copper foil, current collector for lithium secondary battery and lithium secondary battery comprising the electrolytic copper foil {Electrolytic copper foil and Current collector for lithium secondary battery and Secondary battery comprising the electrolytic copper foil}

The present invention relates to an electrolytic copper foil, a current collector for a lithium secondary battery including the electrolytic copper foil, and a lithium secondary battery, and more specifically, the breaking characteristic coefficient value is limited to a certain range, thereby improving the characteristics of the secondary battery. It relates to an electrolytic copper foil and a current collector for a lithium secondary battery and a lithium secondary battery comprising the electrolytic copper foil.

Lithium secondary batteries have many advantages such as relatively high energy density, high operating voltage, and excellent storage and life characteristics compared to other secondary batteries. It is widely used in portable electronic devices.

In general, a lithium secondary battery has a structure including a positive electrode and a negative electrode disposed with an electrolyte therebetween, the positive electrode has a structure in which a positive electrode active material is attached to a positive electrode current collector, and the negative electrode has a negative electrode active material attached to a negative electrode current collector. Has a structure.

In the lithium secondary battery, an electrolytic copper foil is mainly used as a material for the negative electrode current collector, so that the performance of the secondary battery can be maintained even if severe conditions are repeatedly formed inside the secondary battery according to charging and discharging of the secondary battery. It should have excellent physical properties.

The properties that the electrolytic copper foil should have, for example, cracks are not likely to occur even when severe conditions are repeated due to charging and discharging, and the rate of decrease of the discharge capacity retention rate as charging and discharging is not going to be too fast. There may be no fear of performance degradation and / or safety accidents of the secondary battery due to overheating.

On the other hand, the excellent physical properties of such an electrolytic copper foil can be secured according to the control of various factors, but there are many difficulties in finding out what factors can be obtained by adjusting which factors and desired ranges.

The present invention was devised in consideration of the above-mentioned problems, and it is an object of the present invention to find an important factor for expressing excellent performance of a lithium secondary battery and to control the electrolytic copper foil to have properties for expressing excellent performance of a lithium secondary battery. do.

However, the technical problem to be achieved by the present invention is not limited to the above-described problem, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the description of the invention described below.

The present inventors have repeatedly conducted studies and experiments to solve the above-described technical problems, and as a result, by applying an electrolytic copper foil with a properly adjusted fracture characteristic coefficient to the current collector for a lithium secondary battery, the characteristics of the lithium secondary battery despite repeated charge and discharge. It has been found that this can be kept excellent.

As described above, the electrolytic copper foil according to an embodiment of the present invention capable of maintaining the characteristics of a lithium secondary battery excellently is an electrolytic copper foil applied as a current collector for a lithium secondary battery, and the electrolytic copper foil is annealed at 190 ° C. for 1 hour. It has a fracture characteristic coefficient (f) corresponding to a range of 5.0 to 18.0.

Here, the fracture characteristic coefficient (f) is defined as f = {(CB) / A} × 1000, and A is an electrolytic copper foil at the time when the fracture of the electrolytic copper foil is started in the tensile test for the electrolytic copper foil. Represents the stress to have, the B represents the elongation at the time when the breakage of the electrolytic copper foil is started, C represents the elongation at the time when the breakage of the electrolytic copper foil ends, and the time at which the breakage of the electrolytic copper foil starts Silver corresponds to a point in time when the increased stress starts to decrease as the elongation of the electrolytic copper foil increases in the tensile test, and the point in time when the fracture of the electrolytic copper foil ends is broken into two or more pieces. Corresponds to the point in time.

The electrolytic copper foil has a protective layer formed on the surface, and the protective layer may be formed of at least one selected from chromate, benzotriazole (BTA), and silane coupling agents.

The electrolytic copper foil may have a thickness of less than 20 μm.

The electrolytic copper foil may have a surface roughness (Rz) of more than 0 and 2 μm or less.

Meanwhile, the electrolytic copper foil may be manufactured by the electrolytic copper foil manufacturing method according to an embodiment of the present invention. When the electrolytic copper foil manufacturing method according to an embodiment of the present invention is annealed at 190 ° C. for 1 hour, A method for preparing an electrolytic copper foil having a fracture property factor (f) corresponding to a range of 5.0 to 18.0, wherein (a) a copper sulfate aqueous solution having a copper concentration of 70 g / L to 80 g / L and a sulfuric acid concentration of 80 g / L to 110 g / L Preparing a; And (b) depositing copper (Cu) on the drum surface of the icemaker at a current density of 40 A / m ² to 70 A / m ² at a temperature of 40 ° C. to 45 ° C. using the aqueous copper sulfate solution as an electrolyte.

The electrolytic copper foil manufacturing method may include (c) forming a protective layer made of at least one selected from chromate, benzotriazole (BTA) and silane coupling agent on the surface of the electrodeposited copper layer.

Meanwhile, the above technical problem can be achieved not only by the electrode current collector for a lithium secondary battery made of the above-mentioned electrolytic copper foil, but also by a lithium secondary battery including the electrode current collector for a lithium secondary battery.

According to the present invention, even if the harsh conditions generated inside the secondary battery are repeated by charging and discharging of the lithium secondary battery, it is possible to minimize the phenomenon that the capacity retention rate of the lithium secondary battery is reduced and / or prevent the occurrence of ignition due to internal overheating. In this way, it is possible to improve the performance of the secondary battery and / or secure safety in the use of the secondary battery.

The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention together with the detailed description of the invention described below, and thus the present invention is described in such drawings. It should not be interpreted as being limited to.
1 is a cross-sectional view showing an electrolytic copper foil according to an embodiment of the present invention.
2 is a view showing a process of breaking the electrolytic copper foil in a tensile test for the electrolytic copper foil.
3 is a stress-elongation graph for explaining the fracture characteristic coefficient.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms or words used in the present specification and claims should not be interpreted as being limited to ordinary or dictionary meanings, and the inventor appropriately explains the concept of terms in order to explain his or her invention in the best way. Based on the principle that it can be defined, it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention. Therefore, the embodiments shown in the embodiments and the drawings shown in the present specification are only some of the most preferred embodiments of the present invention and do not represent all of the technical spirit of the present invention. It should be understood that there may be equivalents and variations.

The configuration of the electrolytic copper foil 10 according to an embodiment of the present invention will be described with reference to FIG. 1.

1 is a cross-sectional view showing an electrolytic copper foil according to an embodiment of the present invention.

Referring to FIG. 1, the electrolytic copper foil 10 according to an embodiment of the present invention includes a copper layer 11 and a protective layer 12 selectively formed on the surface of the copper layer 11.

The electrolytic copper foil 10 is preferably used as a negative electrode current collector of a lithium secondary battery. That is, in the lithium secondary battery, it is preferable that the electrolytic copper foil 10 is used as the negative electrode current collector combined with the negative electrode active material. On the other hand, it is common that a foil made of aluminum (Al) is used as a positive electrode current collector combined with a positive electrode active material.

Accordingly, in the present invention, a case in which the current collector for a secondary battery according to an embodiment of the present invention in which the electrolytic copper foil 10 is used corresponds to a negative electrode current collector will be described as an example.

When used as a current collector of a lithium secondary battery, the electrolytic copper foil 10 is subjected to a force due to volume expansion of the electrode due to repeated charging and discharging. In this case, even if a tensile force exceeding the breaking strength of the electrolytic copper foil 10 is applied. Breaking does not happen immediately.

That is, referring to FIG. 2, the electrolytic copper foil 10 reaches a complete fracture through a necking step, a void generation step, a growth and bonding step of the voids, and a shear generation step on the surface.

This type of fracture is referred to as ductile fracture. Referring to FIG. 3, in the stress-elongation graph showing the ductile fracture, the electrolytic copper foil 10 increases in elongation after the stress of the electrolytic copper foil reaches the breaking strength. As a result, the stress tends to decrease gradually and eventually leads to complete failure.

The breaking characteristic coefficient is the value obtained by multiplying the change in elongation from the point at which the electrolytic copper foil (10) reaches a complete fracture after the point when the stress reaches the breaking strength by the stress value at the beginning of the fracture, that is, the value obtained by dividing the breaking strength by 1000. It is called (f).

Referring to FIG. 3, the fracture characteristic coefficient f can be expressed by the following equation:

f = {(C-B) / A} × 1000

Here, A, B, and C are the stresses of the electrolytic copper foil at the time when the fracture of the electrolytic copper foil is started in the tensile test for the electrolytic copper foil, the elongation at the time when the fracture of the electrolytic copper foil is started, and the electrolytic copper foil, respectively. It represents the elongation at the time when the breakage of the end.

The point in time at which fracture of the electrolytic copper foil is started refers to a point in time corresponding to a point P1 in which the stress increased together as the elongation increases in the stress-elongation graph of FIG. 3 begins to decrease.

In addition, the point in time at which the breakage of the electrolytic copper foil ends is a point in time corresponding to the point P2 at which the decreased stress becomes 0 as the elongation increases in the stress-elongation graph of FIG. 3, and is delivered at this point The copper foil is destroyed and separated into two or more pieces.

When the breaking characteristic coefficient f has an excessively small value, the electrolytic copper foil 10 is torn in a short time when torn by external force, and accordingly, the electrolytic copper foil 10 is used as a current collector for a lithium secondary battery. In this case, it is difficult to maintain the performance of the secondary battery under severe conditions in the secondary battery due to repeated charging and discharging.

Conversely, when the fracture characteristic coefficient (f) has an excessively large value, excessive heat may be generated because the unstable condition lasts for a long time in a situation in which the electrolytic copper foil 10 is torn by an external force, thereby deteriorating the performance of the battery There may be a problem and difficult to secure safety in the use of the secondary battery.

Therefore, the fracture characteristic coefficient (f) should maintain an appropriate range, preferably 5.0 to 18.0.

The fracture characteristic coefficient (f) was measured in the state in which the electrolytic copper foil 10 was annealed at 190 ° C. for 1 hour, and this annealing was performed in the process of manufacturing a lithium secondary battery by applying the electrolytic copper foil 10 as a current collector. It is to ensure that the high temperature conditions to be applied are applied.

The fracture characteristic coefficient (f) is a change in the concentration of copper and sulfuric acid contained in the plating solution (electrolyte) for plating during the manufacturing process of the electrolytic copper foil 10, various additives (inorganic additives, levelers, which can be selectively added to the plating solution) It can be controlled by changing the concentration of Brightner, etc., the current density during plating, and the temperature of the plating solution.

On the other hand, the electrolytic copper foil 10 manufactured by electroplating is a Shiny surface 11a having a relatively low surface roughness (Rz), and a so-called acid as the opposite side thereof. It is made of a mat surface 11b, which is formed with a relatively high surface roughness Rz by the structure.

The surface roughness (Rz) of the shiny surface (11a) is determined according to the degree to which the cylindrical drum (cathode) where copper is deposited is polished, while the surface roughness (Rz) of the mat surface (11b) is prepared for electrolytic copper foil (10). It can be controlled by changing the composition of the materials constituting the plating solution and the current density during the electrolytic reaction in the process.

On the other hand, when the surface roughness (Rz) of the mat surface 11b of the electrolytic copper foil 10 is excessively high, it is difficult to make the contact between the active material and the electrolytic copper foil 10 uniform, and the discharge capacity retention rate of the lithium secondary battery decreases. Can be. Therefore, it is necessary to adjust the surface roughness (Rz) to an appropriate level, and it is desirable to suppress the surface roughness (Rz) of the mat surface 11b of the electrolytic copper foil 10 to approximately 2 μm or less.

The protective layer 12 is selectively formed on the surface of the copper layer 11 for anti-corrosion treatment of the electrolytic copper foil 10, and is made of chromate, benzotriazole (BTA) and silane coupling agent. It may be made of at least one selected from. The protective layer 12 may also serve to impart anti-corrosion properties to the electrolytic copper foil 10, as well as heat-resistance properties and / or properties to increase bonding strength with an active material.

1. Preparation of electrolytic copper foil

Electrolytic copper foil according to an embodiment of the present invention by using a copper sulfate aqueous solution as a plating solution (electrolyte) to supply a plating solution between the cathode located on the opposite side of the cylindrical cathode rotating at a constant speed and the anode located on the opposite side, the electrodeposition of copper on the rotating cylindrical electrode, It was manufactured by reduced precipitation, and was manufactured to a thickness of less than approximately 20 μm (the thinner the electrodeposited copper foil, the more the current collector with an active material can enter the secondary battery, which is advantageous for high capacity, while the thicker the thickness is applied to the secondary battery. Since it may be difficult to increase the capacity at the time, it is preferable that the thickness of the electrolytic copper foil does not exceed 20 μm). The respective conditions applied in the examples and comparative examples were as described below:

Electrolytic copper foil generally produced by electroplating has two different aspects. That is, the electrolytic copper foil has a surface (shiny side, S surface) that is in contact with the cathode drum and a surface (Matt side, M surface) located in a direction in which crystal grains grow by precipitation. The surface roughness (Rz) of the S and M surfaces of the electrolytic copper foil was formed in a range of approximately 2 μm or less.

(Example)

As shown in Table 1, a plating solution having a copper concentration of 70 to 80 g / L and a sulfuric acid concentration of 80 to 110 g / L was prepared, and various additives (inorganic metal, leveler, brightener) were added to temperature in the range of approximately 40 to 45 ° C, Electrolytic copper foil was produced under the current density condition of approximately 40 to 70 ASD (however, it is not necessarily limited to this range, and can be appropriately adjusted within a range that can achieve the object of the present invention).

Here, Fe, W, Zn, Mo, etc. were applied as inorganic additives, and gelatin, collagen, and PEG (polyethylene glycol) were used as levelers, and bisners were bis (3-sulfoproply) disulfide (SPS), MPS ( mercapto-propane sulphonic acid), DPS (3-N, N-dimethylaminodithiocarbamoyl-1-propanesulphonic acid) was used.

On the other hand, for the protection (rust prevention, etc.) for the electrolytic copper foil manufactured according to the process conditions as described above, the surface was subjected to chromate treatment.

(Comparative example)

As shown in Table 1, various additives (the same additives as in the examples) were added to the plating solution having a copper concentration of 70 to 80 g / L and a sulfuric acid concentration of 95 to 105 g / L, and a temperature of approximately 45 ° C. and a current of approximately 50 to 70 ASD An electrolytic copper foil was prepared under density conditions.

2. Measurement of fracture characteristic coefficient

The fracture characteristic coefficient (f) for the electrolytic copper foil according to the embodiment of the present invention and the electrolytic copper foil according to the comparative example was measured according to the process conditions as described above.

The fracture characteristic coefficient (f) of the electrolytic copper foil was measured through a standardized measurement method UTM, and the UTM conditions applied at this time were 50 mm mark distance, 12.7 mm width, and 2 mm / min measurement speed.

The measurement of the breaking characteristic coefficient (f) for the electrolytic copper foil was performed at 190 ° C so that the electrolytic copper foil according to the example / comparative example prepared according to the above-described conditions had the same conditions as the conditions in the completed lithium secondary battery. After annealing for an hour, it was carried out. That is, a lithium secondary battery may undergo a process in which a high temperature state is maintained for a certain period of time during its manufacture, which is to anneal the electrolytic copper foil to create similar conditions.

The results of measuring the fracture characteristic coefficient (f) of the electrolytic copper foil subjected to the annealing process are shown in Table 2 below.

Referring to the results in Table 2, in the case of the electrolytic copper foils of Examples 1 to 6, the values of the fracture characteristic coefficients in the range of 5.0 to 18.0 are shown, whereas the electrolytic copper foils of Comparative Examples 1 to 3 represent the fracture characteristic coefficient values of less than 5.0, The electrolytic copper foil of Comparative Example 4 exhibited a fracture property factor value exceeding 18.0.

3 . Production of lithium secondary battery

- Preparation of positive and negative plates

(Composition of positive electrode material mixture)

Cathode material (LiCoO2): 85wt%

Conductive material (acetylene black): 8wt%

Binder (polyvinylidene fluoride): 7wt%

(Composition of negative electrode material mixture)

Cathode material (graphite or carbon material): 95 ~ 98wt%

Binder (polyvinylidene fluoride): 2 ~ 5wt%

After adding N-methylpyrrolidone to the material to make a slurry, apply it to the surfaces of the positive electrode current collector made of aluminum foil and the negative electrode current collector made of electrolytic copper foil, and then evaporate the solvent, followed by rolling and slitting to a certain size. Thus, a positive electrode plate and a negative electrode plate were prepared.

- Lithium secondary battery assembly

A positive electrode plate, a separator (a hydrophilic porous polyethylene film), and three negative electrode plates were sequentially stacked and wound, and placed in a container to inject / seal the electrolyte solution to complete a cylindrical battery. For the specification of the battery, a general cylindrical 18650 type was used.

Here, as the electrolyte solution, 1M LiPF ₆ was dissolved in a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 1.

4. Charge / discharge test

Among the lithium secondary batteries manufactured according to the above-described conditions, a lithium secondary battery (secondary battery according to an embodiment of the present invention) produced using the negative electrode current collector to which the electrolytic copper foils according to Examples 1 to 6 shown in Table 1 was applied is used. The lithium secondary batteries according to Examples 1 to 6 were used, respectively.

Likewise, lithium secondary batteries produced using the negative electrode current collectors to which the electrolytic copper foils according to Comparative Examples 1 to 4 shown in Table 1 were applied were used as lithium secondary batteries according to Comparative Examples 1 to 4, respectively.

The lithium secondary batteries thus prepared were subjected to 500 repetitive charge / discharge tests. At this time, the charging was performed under the same conditions as CCCV (constant current constant voltage) mode, the charging voltage was 4.3 V, and the charging current was 0.2 C (current that can be fully charged by charging for 5 hours), and the discharge was CC (constant current). Mode, discharge voltage 3.0V, discharge current 0.5C (current that can be fully discharged in 2 hours) was performed under the same conditions.

As described above, after repetitive charging and discharging, a measurement was performed on the capacity retention rate of the lithium secondary battery, the degree of heat generation, and whether cracks were generated in the electrolytic copper foil forming the current collector, and are shown in Table 3.

Referring to Table 3, the lithium secondary batteries according to the embodiment of the present invention (Examples 1 to 6) having a fracture characteristic coefficient (f) of 5.0 to 18.0 in the range of the electrolytic copper foil even after 500 charges and discharges are completed No crack was generated in the current collector, and the capacity retention rate was also maintained to be approximately 84% to 93% higher than the initial capacity (in the case of 500 charge / discharge cycles, the capacity retention rate is usually approximately 80% to 90%, If there is a problem inside the secondary battery, it will fall sharply and show a big difference from these normal values).

In addition, the lithium secondary batteries of Examples 1 to 6 have a normal temperature range of approximately 25 ° C. to 28 ° C., even in the case of heat generation measurement conducted in a manner in which charging and discharging tests are performed in a constant temperature chamber (normal range if within 30 ° C.) It can be seen as, and even if the temperature rises temporarily, it does not exceed 40 ° C unless there is a special abnormality).

On the other hand, lithium secondary batteries (Comparative Examples 1 to 3) according to the comparative example in which the breaking characteristic coefficient (f) of the electrolytic copper foil was less than 5.0 caused cracks in the current collector due to repeated charging and discharging, and the capacity retention rate was also the initial capacity. Compared to the level of approximately 24% to 42%, the function as a secondary battery was significantly lost. Cracks generated in the current collector cause the electrode active material attached to the current collector to fall off, and such crack generation has a direct effect on the capacity retention rate decrease.

On the other hand, the lithium secondary battery according to the comparative example (Comparative Example 4), in which the breaking characteristic coefficient (f) of the electrolytic copper foil exceeds 18.0, does not crack even after repeated charging and discharging, and the capacity retention rate is also maintained relatively high, but for heating measurement. By showing the temperature of approximately 41 ° C, the temperature exceeded 30 ° C, which is usually seen in the normal range. Such an abnormal heat generation phenomenon may degrade the performance of the lithium secondary battery and may also increase the risk of safety accidents.

When the fracture characteristic coefficient (f) of the electrolytic copper foil 10 is maintained within a predetermined range (5.0 to 18.0) from the results of the experiment, the performance of the secondary battery can be maintained even under severe conditions due to repeated charging and discharging. It can be clearly seen that stability in use can also be ensured.

Although the present invention has been described above by way of limited examples and drawings, the present invention is not limited by this, and will be described below by the person skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the equal scope of the claims.

10: electrolytic copper foil 11: copper layer
11a: Mat surface 11b: Shiny surface
12: protective layer

Claims (10)

delete delete delete delete An electrolyte is prepared by adding 4 to 5 ppm of inorganic metal, 2 ppm or less of leveler, and 2 ppm or less of brightener as an additive to a copper sulfate aqueous solution having a copper concentration of 70 g / L to 80 g / L and a sulfuric acid concentration of 80 g / L to 110 g / L, and this electrolyte An electrolytic copper foil applied to a current collector for a lithium secondary battery manufactured by electrodepositing copper (Cu) on the drum surface of a foilmaker with an electrolysis temperature of 40 ° C to 45 ° C and a current density of 40ASD to 70ASD,
The inorganic metal includes at least one selected from the group consisting of Fe, W, Zn and Mo,
The leveler includes at least one selected from the group consisting of gelatin, collagen and PEG (polyethylene glycol),
The brightener comprises at least one selected from the group consisting of bis (3-sulfopropyl) disulfide (SPS), mercapto-propane sulphonic acid (MPS) and 3-N, N-dimethylaminodithiocarbamoyl-1-propanesulphonic acid (DPS),
When the electrolytic copper foil is annealed at 190 ° C for 1 hour, it has a fracture characteristic coefficient (f) corresponding to a range of 5.0 to 18.0,
f = {(CB) / A} × 1000,
The A represents the stress (kgf / mm ² ) of the electrolytic copper foil at the time when fracture of the electrolytic copper foil is started in the tensile test for the electrolytic copper foil,
B represents the elongation at the time when the rupture of the electrolytic copper foil is started,
C represents the elongation at the end of the fracture of the electrolytic copper foil,
The point in time at which fracture of the electrolytic copper foil is started corresponds to a point in time when the stress that was increased as the elongation of the electrolytic copper foil in the tensile test starts to decrease,
The point in time at which the breakage of the electrolytic copper foil is finished corresponds to a point in time when the electrolytic copper foil is destroyed and separated into two or more pieces. The method of claim 5,
The electrolytic copper foil,
It has a protective layer formed on the surface,
The protective layer is an electrolytic copper foil, characterized in that it comprises at least one selected from chromate (chromate), BTA (benzotriazole) and silane coupling agent. The method of claim 5,
The electrolytic copper foil,
Electrolytic copper foil, characterized in that it has a thickness of less than 20㎛. The method of claim 5,
The electrolytic copper foil,
Electrolytic copper foil characterized by having a surface roughness (Rz) of more than 0 and 2 µm or less. delete delete

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