Patent Grant
10205170
The present disclosure is directed to copper foils possessing a puncture strength making the foils suitable for a wide variety of uses. These copper foils find particular utility as current collectors in rechargeable secondary batteries, particularly in lithium secondary battery with high capacity. Methods of making the copper foils, method of producing negative electrode for use in lithium secondary battery and lithium secondary battery of high capacity are also described.
Electrification is the most viable way to achieve clean and efficient transportation that is crucial to the sustainable development of the entire world. In the near future, electric vehicles (EV), including hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and pure battery electric vehicles (BEVs) will dominate the clean vehicle market. By 2020, it is expected that more than half of new vehicle sales will likely be EV models. The key and the enabling technology to this revolutionary change in transportation is the battery. EV batteries are quite different from those used in consumer electronic devices, such as laptops and cellphones. They are required to handle high power (up to a hundred kW) and have high energy capacity (up to tens of kW) within a limited space and weight and at an affordable price. The current two major battery types used in EVs today are nickel metal hydride (NiMH) and lithium secondary battery. Nearly all HEVs available in the market today use NiMH batteries because of its mature technology. Due to the potential of obtaining higher specific energy and energy density, the adoption of lithium secondary batteries is expected to grow fast in EVs, particularly in PHEVs and BEVs.
The negative electrode current collector of a lithium secondary battery is generally made using copper foil. The negative electrode is formed by coating the surface of the copper foil (current collector) with carbon powder, or other negative electrode active materials processed to a paste, drying this paste, then press flattening the negative electrode active materials by rolling or other pressure application. This composite of copper foil and compressed negative electrode active material is then, together with a separator, and an aluminum foil coated with a positive electrode active material, rolled up to form a cylindrical lithium secondary battery.
An alternative form of the lithium secondary battery is the laminated type lithium secondary battery. A laminated type lithium secondary battery boasts of advanced heat dissipation as compared to conventional cylindrical batteries. Because the laminated type lithium secondary battery has a broad surface area, the laminated type lithium secondary battery is better able to dissipate heat, and increases in the overall temperature of the battery due to charging and discharging can be kept low. Therefore, electric vehicles that adopt batteries of laminated type can simplify countermeasures against heat.
With an increase in the energy density and the capacity of the lithium secondary battery, and the need for the volumetric shrinkage rate of the negative electrode active material increasing, the required strength for the current collector is also increasing. Particularly, a metal alloy-based negative electrode active material containing silicon (“Si”), tin (“Sn”) or the like has several to several ten times the volumetric expansion higher than the conventional carbon material during the charging/discharging reaction.
For manufacturing a copper foil with stable, excellent properties for battery performance, one should impart to the copper foil during its manufacture various important properties. Included among such properties is a puncture strength that is suitable for employing the copper foil as a current collector in rechargeable secondary batteries of high capacity. With suitable puncture strength, the likelihood or probability that the copper foil will fracture during the charging/discharging cycle will be reduced. On the other hand, the negative electrode active material is closely adhered to the copper foil. Therefore, it suffers from the disadvantage that a large stress acts upon the negative electrode when the active material layer expands in volume due to the repetition of the charging/discharging cycle. When an electrode, with large expansion and contraction states is used in a rechargeable secondary battery, the charging/discharging cycle is repeated many times over the batteries' useful life repeatedly imparting stresses to the copper foil component of the electrode, whereby the negative electrode may be broken, resulting in a tremendous capacity reduction in the battery. For lithium secondary batteries with high capacity, which require a relatively high pressure to compress the negative electrode active materials onto the copper foil during production of the negative electrode, the copper foil is more likely to break during the pressing step of the manufacturing process of compressing the negative electrode active materials onto the copper foil.
After diligent research, the present inventors have found that copper foils suitable for use as current collectors in lithium secondary battery of high capacity must have a puncture strength within a range of values, but yet must not be too brittle as to fracture during the manufacturing of the consolidation of the negative electrode active material onto the copper foil by pressing. Various factors come into play when attempting to produce suitable copper foils for use in lithium secondary battery of high capacity as discussed in detail below. It is desirable to provide a rechargeable secondary battery, an electric tool, an electric vehicle, and a power storage system which can obtain an excellent battery capacity characteristic and cycle characteristics.
The present inventors have produced copper foils having an acceptable puncture strength values suitable for use as current collectors in lithium secondary battery, but not being too brittle so as to fracture during the manufacturing process of consolidating the active materials onto the copper foil by pressing.
The present inventors have discovered a process of producing a copper foil by electrodepositing copper onto a drum from a liquid copper-containing electrolyte solution under the influence of an electric current to produce a copper foil having a drum side (the side of the foil in contact with the drum during production) and an opposed side or deposit side, where the grain size difference between the deposit side and the drum side is small.
The presence of an inorganic metal and/or organic additive in the liquid copper-containing electrolyte solution reduces the grain size difference between the deposit side and the drum side of the copper foil, and the number of voids produced by electrodeposition. The copper foils produced according to this disclosure have suitable puncture strength for use as current collectors in lithium secondary battery.
The present inventors have also discovered that if the surface roughness (Rz) of the copper foil is large, it becomes difficult to coat the copper foil with the negative electrode active materials in uniform thickness. Therefore, it is desirable that the surface roughness (Rz) of the deposit side is 2.0 μm or less. If the surface roughness Rz of the deposit side is less than 0.4 μm, adhesion to a negative electrode material tends to decrease. Therefore, it is more preferable that the surface roughness Rz is 0.4 μm or more. A preferred range of surface roughness of the copper foil is 0.8 μm to 1.7 μm. A more preferred range is 1.0 μm to 1.6 μm.
As used throughout the various drawing figures, like elements in different views may be given a common numeral labeling, to assist the reader in understanding the various embodiments.
As used throughout this specification and claims, surface roughness is measured and provided as “Rz” standard, (utilizing JIS B 0601-1994, using an α-type surface roughness and contour measuring instrument manufactured by Kosaka Laboratory Ltd. (Model Type: SE1700), although there are other systems available to measure surface roughness. Not all measurement systems of surface roughness are equivalent. According to this Rz standard, the results are presented as an average of 10 points.
As shown in
The copper-containing electrolyte solution 12 can be formed by adding copper wires to a sulfuric acid solution to prepare a copper sulfate containing solution. By the use of copper wire, the copper content of the copper can be easily regulated to contain more than 99.9% pure copper. Inorganic metal can be added, such as nickel, for the purpose of controlling the presence of voids in the electrodeposited copper foil 21. If the nickel content is too high, the aforementioned voids can easily be formed and the conductivity of the copper foil is reduced. If the nickel content is too low, the grain size difference between the drum side and the deposit side of the copper foil is increased, which tends to diminish the cycle characteristics of the battery. Generally, the nickel should be present in an amount between 200-500 ppm (“parts per million”). Other additives can also be added to the copper-containing electrolyte solution to regulate the grain size difference between the drum side and the deposit side of the resulting copper foil 21. These additives include, but are not limited to gelatin, chloride ion, sodium 3-mercapto-1-propanesulfonate (MPS: HOPAX Company), polyethylene glycol (PEG-2000 (available from Sigma-Aldrich Company) and trimethyl thiourea.
As shown in the below Table 3 of Examples and Comparative Examples, the varying of these components will affect the resultant properties of the copper foils produced.
On the other hand the laminated type battery 60 schematically illustrated in
The decrease in thickness of the copper foil 21 means that any voids in the copper foil volume will have the effect of a pinhole through the copper foil. This is illustrated in
Test Methods
1. Puncture strength—A test piece with a size of 120 mm in length×30 mm in width was tailored from each of the electrodeposited copper foils, and needle penetration test is performed using a universal testing machine (TKS-20N made by IMADA), measurement was performed at room temperature (15˜35° C.), with a needle (MS2-1R7 made by RENISHAW or Mitutoyo), entry speed being 50 mm/min. The maximum load acting on the copper foil when the specimen underwent penetration of the copper foil was read. A value obtained by the load of the specimen is determined as puncture strength (N). The measurement was performed 5 times each for the deposit side and for the drum side of each sample. Puncture strength for deposit side and for the drum side were then calculated by averaging the respective side's 5 measurements. The puncture strength of electrodeposited copper foil was then determined by taking average of the puncture strength values from each side.
2. Area weight and Thickness—A test piece with a size of 100 mm in length×100 mm in width was tailored from each of the electrodeposited copper foils, and an AG-204 type microbalance manufactured by Mettler Toledo International Inc. was used to measure the test piece. For each of the test pieces, the numerical value in the reading taken was multiplied by 100 to obtain an area weight (g/m2). A thickness of the electrodeposited copper foil means Mean average thickness using Test Method 2.4.18 of IPC-TM-650.
Mean average thickness (μm)=Weight of sample in grams/(Area of sample in square meter×Density of electrodeposited copper)
The density of electrodeposited copper in present invention is 8.909 gm/cm3
3. Grain size—The cross-section of the copper foil is processed by CP (“Cross-section Polisher”), Gatan Ilion II. A specific section processed by CP is observed by EBSD (“Electron Back Scatter Diffraction”) to measure the grain size of the copper foil. As the SEM, JSM-6500F manufactured by JEOL Ltd., with EBSD detector, NordlysNano manufactured by Oxford Instruments was used. The analysis software had Axtec HKL, HKL Channel5. The measurement used herein used the HKL Channel5. Grain size at the drum and deposit side obtained from the analysis by an EBSD in a depth direction of 2 μm from the surface.
4. Charge-discharge cycle test—A laminated type lithium secondary battery was prepared as follows and subjected to a high c-rate charging and discharging test. N-methyl-2-pyrollidone (NMP) was used as a solvent for a positive electrode material (at a solid to liquid ratio of 195 wt. % (100 g of the positive electrode material: 195 g of NMP)) as shown in Table 1 below, so as to obtain a positive electrode slurry. NMP was used as a solvent for a negative electrode material (at a solid to liquid ratio of 60 wt. % (100 g of the negative electrode material: 60 g of NMP), so as to obtain a negative electrode slurry.
The foregoing negative electrode active substance, as a slurry, was coated on copper foil, and a positive electrode slurry was coated on an aluminum foil. After the solvents were evaporated, the positive electrode and negative electrode were individually pressed, and slitted into certain sizes. Afterwards, negative electrode and positive electrode are alternately stacked, with a separator (manufactured by Celgard Company) sandwiched between them, and placed in a container molded of laminate film. The container was filled with electrolyte, and sealed to form the battery. The size of the laminated type battery was 41 mm×34 mm×53 mm. The charging mode was the constant current-constant voltage (“CCCV”) mode, the charging voltage was 4.2 V, and the charging current constant was 5 C. The discharging mode was the constant current (“CC”) mode, the discharging voltage was 2.8 V, and the discharging current was 5 C. The charging-discharging test was conducted at an elevated temperature (at 55° C.). Cycle life is defined as the number of charge-discharge cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. Cycle life test will be performed on a copper foil only if it has passed fracture of negative electrode test.
5. Surface roughness (Rz)—The measurement was conducted based on the method of JIS B 0601-1994 by using α Type Surface Roughness Measuring Instrument (Kosaka Laboratory Ltd.: SE1700 series. Rz (Ten points Mean Roughness) is obtained from the total (in micron meter (μm)) of the mean value of the each distance mean line and 5 peaks from the highest one, and the mean value of each distance between the mean line and 5 valleys from the lowest one, of the roughness curve in the range of sampled reference length.
6. Number of voids—Using a Nova NanoLab 200 DB-FIB (“Dual-beam focused ion beam)-SEM (“Scanning Electron Microscope”), manufactured by FEI Company, a cross-section of the copper foil was processed by FIB. The SEM images of an area of 1 μm2, at a magnification of 50,000 times, the Nova NanoLab 200 DB-FIB, operated at 3 kV using secondary electron detection are shown to measure the number of voids. The number of voids each having a 0.01 μm or more of a circle diameter were counted for each of the Examples and Comparative Examples in the below Table 3.
7. Fracture of negative electrode—A negative electrode slurry was prepared using N-methyl-2-pyrollidone (NMP) as a solvent and the negative electrode materials listed in Table 2 below with a solid-liquid ratio of 60% (100 g of negative electrode materials; 60 g of NMP). After the components of the negative electrode material formulation are mixed, the carbon material slurry is coated on the surface of copper foil at a speed of 5 meters per minute (“mpm”) to a thickness of 200 μm and then dried through a 160° C. oven. The negative electrode (copper foil+negative electrode active material) was then pressed. The dimension of the rollers of the pressing apparatus were ϕ250 mm×250 mm, hardness of the rollers was 62˜65 HRC, and the roller material was high-carbon chrome bearing steel (SUJ2). A 1 M/min (“meter per minute”) pressing speed and 3000 psi pressure were used and then observed to see whether the interface between the copper foil and negative electrode active material was not broken (referred to in Table 4 as “OK”), or if it was broken (it was referred to in Table 4 as “NG”).
The following Table 3 illustrates the parameter condition for producing copper foils in the Examples and Comparative Examples. The following Table 4 illustrates the effect of varying parameter condition on the resulting properties of the copper foils produced in the Examples and Comparative Examples. It should be clearly understood that these are additional and more favorable conditions for achieving the electrodeposited copper foil for a lithium secondary battery according to the present invention. It should be understood within the scope of the present disclosure, the above-mentioned technical features and technical features mentioned below (such as example) can be combined freely and mutually to form new or preferred technical solutions, which are omitted for brevity.
It is apparent that various changes in parameters have an effect on the resulting properties of the electrodeposited copper foil produced and the cycle life of the electrodeposited copper foil when used as a current collector in a secondary battery during charge-discharge test of the secondary battery. The following Table 4 illustrates the change in grain size and grain size difference for the deposit side and the drum side, as well as the result on puncture strength (N) properties on the deposit side and drum side (as well as average) of a electrodeposited copper foil as changes occur in the presence and amounts of the PEG-2000, trimethyl thiourea, and nickel components of the copper foil, while maintaining all other variables constant.
It can therefore be seen that the puncture strength of the electrodeposited copper foil must be high, but not too high such that the copper foil is brittle leading to failures of the copper foil during manufacture of a negative electrode by pressing the active materials onto the copper foil. Nor can the foil have too low puncture strength such that it fails during repeated charging-discharging cycles when used in a secondary battery. Furthermore, the copper foil must be manufactured without too many voids which also lead to battery failure.
The area weight value (X axis) and puncture strength value of the electrodeposited copper foil (Y axis) at this time were subjected to polynomial regression analysis (in
The value for Y1 is calculated by the following formula 1:
Y1=−0.00007*X2+0.075*X−1.3373, wherein X=the area weight of the copper foil (g/m2), which is calculated by weight basis. Formula 1:
The value Y2 is calculated by the following formula 2:
Y2=0.00003*X2+0.0247*X−0.6454, Formula 2:
wherein X in each of formulas 1 and 2 represents the area weight of the electrodeposited copper foil in g/m2, and Y in each of formulas 1 and 2 represents the puncture strength of the electrodeposited copper foil in N.
In making the copper foil by electrodepositing a copper foil from a copper-containing electrolyte solution, the electrolyte solution comprises nickel in the amount in the range of 200-500 ppm.
It is furthermore important that the electrodeposited copper foil contains less than 5 voids in an area of 1 μm2 of the copper foil, wherein the voids are defined as each having a diameter of 0.01 μm or more.
In order to obtain an electrodeposited copper foil suitable for use as a current collector in a rechargeable secondary battery having a cycle life in excess of 900 charge/discharge cycles, the grains constituting the internal layer are preferable to be not only fine but also uniform. It is important that the grain size difference between the drum side and the deposit side of the copper foil be in the range of 0.05-0.55 μm. The uniform grains make a load applied on the foil disperse without concentration on specific grains and contribute to the strength enhancement. Note that, the grain size can be an average value of grain sizes determined from the grain diameters of grains present in a prescribed measurement visual field.
The disclosure of specific, including preferred embodiments, is intended to be exemplary of the embodiments described herein, but should not be construed as limiting the disclosure, parameters, process or method steps, copper foils, current collectors, batteries or other products incorporating the same.
It is to be further understood that one skilled in the art, upon reading the present disclosure, could envision modifications thereof, including equivalents of the parameters, steps and copper foils described herein, without the exercise of inventive effort and without departing from the spirit and scope of the appended claims.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
It should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations.
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