Copper foil and preparation method therefor, negative electrode comprising same, and lithium-ion battery

Abstract

The present invention relates to the technical field of lithium-ion batteries, and in particular to a copper foil and a preparation method therefor, a negative electrode comprising same, and a lithium-ion battery. The copper foil comprises ultra-fine grains and nanotwins, and has an average grain size of 0.2-0.4 μm and an area-weighted average grain size of 0.3-0.8 μm, wherein the proportion of the nanotwins in the copper foil is greater than 50%. The copper foil is prepared by means of a pulse electroplating technique, which can control the average grain size and the proportion of nanotwins in the copper foil, thereby making the copper foil have high tensile strength and a high elongation rate. Moreover, a specific electrolyte is used in the preparation method, enabling the tensile strength of the copper foil to reach 600-900 MPa and the elongation rate thereof to be greater than 5%. Applying the copper foil to a lithium-ion battery can reduce the volume expansion rate of a negative electrode material after charging and discharging cycles, prevent the copper foil from cracking due to repeated volume changes during the charging and discharging processes of the negative electrode material, prolong the cycle life of the battery and improve the safety of the battery.

Description

Copper foil, preparation method thereof and negative electrode and lithium ion battery comprising same TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, and in particular to a copper foil, a preparation method thereof and a negative electrode and lithium ion battery comprising same. BACKGROUND

[0002] With the increasing number of electric vehicles and installed capacity of renewable energy, the demand for lithium ion batteries is rapidly increasing.

[0003] Electrolytic copper foil is widely used as the negative electrode current collector of lithium ion batteries due to its excellent electrical conductivity. In addition, it is also the main stress bearer during battery manufacturing and use due to its good mechanical properties.

[0004] The main trend of today's lithium ion battery development is to improve energy density and safety. The theoretical specific capacity of traditional graphite negative electrode is only 372 Ah / kg, while the new silicon-carbon negative electrode is expected to achieve an energy density of more than 3000 Wh / kg. The iteration of negative electrode materials will be the main way to improve the energy density of batteries in the future. However, the volume of silicon-carbon negative electrode material will shrink / swell significantly with the deintercalation / intercalation of lithium ions. This phenomenon leads to volume change during battery charging and discharging, thereby repeatedly stretching the copper foil as the stress carrier. The traditional copper foil current collector is prone to breakage due to excessive stretching during this process, resulting in internal short circuit of the battery and affecting the safety and service life of the battery.

[0005] SUMMARY

[0006] The present application provides a copper foil, a preparation method thereof and a negative electrode and lithium ion battery comprising same, to solve the problem that the existing copper foil as a current collector is prone to breakage due to excessive stretching during battery charging and discharging.

[0007] According to a first aspect of the present application, the present application provides a copper foil, the copper foil comprising ultrafine grains and nanotwins, the average grain size of the copper foil being 0.2-0.4 μm, the area-weighted average grain size being 0.3-0.8 μm, and the proportion of nanotwin grains measured by EBSD in the copper foil being greater than 50%.

[0008] Further, the elongation of the copper foil is greater than 5%.

[0009] Further, the tensile strength of the copper foil is 600-900 MPa.

[0010] Further, the thickness of the copper foil is 4-10 μm.

[0011] According to a second aspect of the present application, the present application further provides a preparation method of a copper foil, the preparation method comprising the following steps:

[0012] The original foil is obtained by electrolyzing the electrolyte with a pulse power source at a predetermined pulse width and duty cycle; the current density of the pulse power source is 8000-15000 A / m 2 , the pulse width is 1-50 ms, and the duty cycle is 1%-30%; the electrolyte comprises an electrolytic additive, and the electrolytic additive comprises a brightener with a content of 12-20 mg / L, an auxiliary brightener with a content of 4-10 mg / L, a leveling agent with a content of 2-8 mg / L, polyethylene glycol with a content of 2-10 mg / L, and an inhibitor with a content of 6-30 mg / L.

[0013] The original foil is washed and passivated to obtain the electrolytic copper foil.

[0014] Further, the electrolyte further comprises copper ions 60-120 g / L, sulfuric acid 90-150 g / L, and chloride ions 20-40 mg / L. Further, the brightener comprises an organic sulfide; preferably, the brightener comprises one or more of polydithiopropane sulfonate sodium, polydithioethane sulfonate sodium, and 3-mercapto-1-propane sulfonate sodium.

[0015] Further, the auxiliary brightener comprises a nitrogen-containing organic sulfide; preferably, the auxiliary brightener comprises one or more of polyisothioureine propane sulfonate inner salt, 3-benzothiazyl-2-thiol propane sulfonate sodium, N,N-dimethyl-propane sulfonate sodium, and tetrahydrothiazole thione.

[0016] Further, the leveling agent comprises a nitrogen-containing small molecule compound; preferably, the leveling agent comprises one or more of 2-mercapto pyridine 2-, amino-4-methyl benzothiazole, and tricyclic azole.

[0017] Further, the molecular weight of the polyethylene glycol is 4000-10000.

[0018] Further, the inhibitor comprises a polyether compound, and the molecular weight of the polyether compound is less than or equal to 2000; preferably, the inhibitor comprises one or more of polyethylene glycol with a molecular weight of 400-1000, polypropylene glycol with a molecular weight of 800-2000, and a copolymer of ethylene glycol and propylene glycol with a molecular weight of 400-1000.

[0019] Further, the mass ratio of the polyethylene glycol to the polyether compound is 1:2-1:4.

[0020] Further, the temperature of the electrolytic treatment is 50-60℃; and during the electrolytic treatment, the upflow rate of the electrolyte is 40-50 m 3 / h.

[0021] According to a third aspect of the present application, the present application further provides a negative electrode, comprising a negative electrode current collector, wherein the negative electrode current collector comprises a copper foil according to any one of the embodiments of the first aspect.

[0022] According to a fourth aspect of the present application, the present application further provides a lithium ion battery, comprising a negative electrode, wherein the negative electrode comprises a negative electrode according to any one of the embodiments of the third aspect.

[0023] The technical solution provided by the present application has the following beneficial effects:

[0024] The copper foil provided by the present application has a highly refined grain size, and the mechanical properties of the electrolytic copper foil can be adjusted by introducing a nano-twin structure into the micro-grain structure. The coherent grain boundary of the nano-twin can effectively hinder dislocation movement, thereby strengthening the material. When the deformation stress is high enough, the dislocation can also slip at the coherent twin boundary, thereby effectively improving the plasticity and work hardening capacity of the material, and ensuring that the electrolytic copper foil has high elongation.

[0025] The preparation method of the copper foil provided by the present application adopts pulse plating technology. Compared with the traditional direct current plating technology process, the present application realizes controllable adjustment of the crystalline microstructure. Different micro-grain sizes of electrolytic copper foils are constructed by adopting pulse plating technology. By adjusting the power parameters, including peak current, pulse width and duty cycle, the growth cycle and stress release process of the micro-copper grains can be changed, thereby adjusting the size of the grain size and the proportion of the nano-twin, so that the tensile strength of the electrolytic copper foil can be adjusted in the range of 600-900 MPa. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the present application or prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0027] Fig. 1 is an EBSD crystalline structure diagram of a cross section of an electrolytic copper foil prepared according to Example 4 of the present application;

[0028] Fig. 2 is an EBSD crystalline structure diagram of a cross section of an electrolytic copper foil prepared according to Comparative Example 1 of the present application;

[0029] Fig. 3 is a comparison diagram of the tensile strength and elongation of the electrolytic copper foils prepared according to Example 4 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0030] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be clearly and completely described below. Obviously, the described embodiments are some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0031] As analyzed in the background, the electrolytic copper foil in the prior art is prone to breakage due to excessive stretching during the charging and discharging process of the battery, thereby causing internal short circuit of the battery, affecting the safety and service life of the battery and the like.

[0032] Generally, the grain refinement of the polycrystalline metal material can effectively improve the tensile strength thereof, but the plasticity of the material will be greatly reduced. Thus, it is difficult to obtain a copper foil with both ultra-high tensile strength and high elongation by the traditional method of introducing grain boundary impurities. The nanotwinned metal has unique mechanical properties, which can have both extremely high tensile strength and good elongation, which is due to the interaction between the coherent grain boundaries and dislocation defects in the nanotwinned metal. On the one hand, the twin boundary can effectively hinder the movement of dislocations, and when the stress is high enough, the dislocations react with the twin boundary and pass through the twin boundary; on the other hand, the twin boundary is also the slip plane of the face-centered cubic (FCC) packed metal material, and the dislocations can not only move on the twin boundary, but also the coherent grain boundary can provide storage space for the dislocations generated in the deformation, thereby effectively improving the elongation of the copper foil.

[0033] However, it is not easy to prepare nanoscale twin lamellae and realize the controllable growth of nanotwinned structure. The formation of nanotwins is driven by kinetics, and increasing the cathode overpotential is beneficial to increasing the nucleation rate of electrodeposition, thereby obtaining a sample with smaller grain size. According to the description of the Butler-Volmer equation in electrochemical theory, when the mass transfer is not considered, the polarization current increases exponentially with the increase of overpotential, so increasing the current density is a traditional method to increase the cathode polarization. Further considering the mass transfer process, the Cu 2+The fast consumption of lithium will result in a significant lower concentration than that in the bulk phase, thus a large concentration polarization and overpotential. However, studies have shown that the thickness of the twin lamella does not show a strong correlation with the current density in the direct current electroplating process. This may be related to the lower energy of the coherent grain boundary than the common high-angle grain boundary. Twins often nucleate at grain boundaries or triple junctions, and reduce the overall grain boundary energy by twin-induced grain orientation transformation. The continuous current in the direct current electroplating process will generate a high cumulative stress, thus often having a high grain boundary energy. In contrast, the stress accumulation and relaxation in the pulse electroplating process is conducive to reducing the grain boundary energy and promoting the growth of twins. When the pulse current is on, a large number of nuclei form and generate tensile stress in the coating, accumulating elastic strain energy; and when the pulse current is off, the tensile stress is released and the coating produces nanotwins in a recrystallization manner. Thus, adjusting the pulse width and duty cycle is the key to controlling the proportion of nanotwins in the sample and obtaining extremely high tensile strength and high elongation. The pulse electroplating technology can also achieve a larger instantaneous current density, thus obtaining smaller grain size and higher tensile strength.

[0034] In a first exemplary embodiment of the present application, the present application provides a copper foil comprising ultra-fine grains and nanotwins, the average grain size of which is 0.2-0.4 μm, the area-weighted average grain size of which is 0.3-0.8 μm, and the proportion of nanotwin grains measured by EBSD in the copper foil is greater than 50%.

[0035] It should be noted that the grain size in the conventional double-sided optical lithium copper foil is usually greater than 1 μm, and the grain size less than 1 μm is defined as ultra-fine grain. The ultra-fine grain in the present application refers to the grain with a grain size less than 1 μm. The average grain size and the area-weighted average grain size are measured by electron backscatter diffraction (EBSD). Both are calculated by the following formula

[0036] where d is the size of a single grain, n d is the number of grains with a size of d, S d is the area of the grain with a size of d. In the above scheme, the copper foil with the above average grain size and area-weighted average grain size has a highly refined grain size, thus ensuring that the electrolytic copper foil has high tensile strength and high elongation. When the average grain size is too large, the grain boundary density contained therein is reduced, resulting in that the tensile strength cannot reach the expected level, however, too small average grain size will also introduce excessive grain boundary impurities, reduce the proportion of twin boundaries, and result in a decrease in elongation.

[0037] In the above scheme, the electrolytic copper foil of the present application has a high twin crystal ratio. By introducing a large number of nano twin crystal structures into the micro grain structure, the mechanical properties of the electrolytic copper foil can be regulated. The coherent grain boundary of the nano twin crystal can effectively hinder the movement of dislocations, thereby strengthening the material. When the deformation stress is high enough, the dislocations can also slip at the coherent twin crystal interface, thereby effectively improving the plasticity and work hardening capacity of the material. A high twin crystal ratio (greater than 50%) helps to improve the elongation of the copper foil, while a low twin crystal ratio (less than or equal to 50%) will result in a decrease in elongation.

[0038] In some embodiments of the present application, the elongation of the copper foil is greater than 5%.

[0039] In some embodiments of the present application, the tensile strength of the copper foil is 600-900 MPa.

[0040] In the above scheme, the electrolytic copper foil of the present application has high tensile strength and / or high elongation. The high tensile strength can effectively reduce the volume expansion rate of the silicon-carbon negative electrode material, while the high elongation can reduce the risk of battery cycle life reduction caused by copper foil rupture during battery use, thereby improving the cycle stability and safety of the battery.

[0041] In some specific embodiments of the present application, the thickness of the electrolytic copper foil is 4-10 μm.

[0042] In the above scheme, by limiting the thickness of the electrolytic copper foil within the above range, the battery processing yield can be improved, and the battery performance can be improved. If the thickness of the copper foil is less than 4 μm, the probability of abnormality in the coating process of the battery will increase, and the elongation will decrease. If the thickness of the copper foil is greater than 10 μm, the active material content in the battery will decrease due to the high thickness, and the capacity density of the battery will be affected.

[0043] In a second typical embodiment of the present application, the present application further provides a preparation method of a copper foil, which comprises the following steps:

[0044] The original foil is obtained by electrolyzing the electrolyte using a pulse power source under a predetermined pulse width and duty cycle. The current density of the pulse power source is 8000-15000 A / m 2 , the pulse width is 1-50 ms, and the duty cycle is 1%-30%. The electrolyte comprises an electrolytic additive, and the electrolytic additive comprises a brightener with a content of 12-20 mg / L, an auxiliary brightener with a content of 4-10 mg / L, a leveling agent with a content of 2-8 mg / L, polyethylene glycol with a content of 2-10 mg / L, and an inhibitor with a content of 6-30 mg / L.

[0045] The original foil is washed and passivated to obtain the electrolytic copper foil.

[0046] It should be noted that the pulse width of the pulse power source is defined as the time of the power source being turned on in each cycle, and the duty cycle refers to the proportion of the power source being turned on (ton) in each cycle to the total time cycle (ton+toff), i.e., ton / (ton+toff) x 100%.

[0047] In the above scheme, the copper foil is electrolytically prepared by using the pulse electroplating technology, and the pulse power source effectively makes up for the problem that the grain size is reduced when the tensile strength of the copper foil is improved by adding electrolytic additives in the past, a large number of high-angle grain boundaries are introduced, and the elongation of the copper foil is significantly reduced. The current density, pulse width and duty cycle of the pulse power source in the pulse electroplating process are further limited in the above scheme based on the electrolytic additive of a specific composition. A higher peak current density can increase the overpotential of the electrodeposition reaction and increase the driving force of the copper ion deposition on the surface of the cathode roller, so that smaller crystalline particles can be obtained. On the other hand, too fast deposition speed will also lead to uneven crystalline growth speed, so the pulse width needs to be controlled to control the size of the crystalline growth.

[0048] The above preparation method not only can obtain electrolytic copper foil with extremely high tensile strength, but also can ensure that the elongation is greater than 5%, so as to better resist the volume change caused by material expansion in battery use, and improve the cycle stability and safety of the battery.

[0049] Preferably, in some embodiments, the peak current density of the pulse power source used includes but is not limited to 8000 A / m 2 , 9000 A / m 2 , 10000 A / m 2 , 11000 A / m 2 , 12000 A / m 2 , 13000 A / m 2 , 14000 A / m 2 , 15000 A / m 2 , which is much higher than the current density (6000-7000 A / m 2 ) used in conventional electrolysis conditions.

[0050] Higher current density and smaller pulse width can significantly reduce the generation of large-size crystals. Preferably, in some embodiments, the pulse width used includes but is not limited to 1 ms, 3 ms, 5 ms, 7 ms, 10 ms, 20 ms, 30 ms, 40 ms, 50 ms. A single pulse can only generate a few nanometer-sized crystalline particles, and a large number of nanometer twin crystal stacking structures are formed in the process of repeated pulses.

[0051] A suitable duty cycle can reduce the concentration polarization on the electrode surface, and the copper ions and additives are consumed and deposited on the electrode surface during the power-on period and replenished during the power-off period. Preferably, in some embodiments, the duty cycle used includes, but is not limited to, 1%, 5%, 10%, 13%, 15%, 20%, 25%, 30%. Higher pulse peak current density or longer pulse width will consume more active substances, and thus the above duty cycle can ensure better enrichment of copper ions and additives on the surface.

[0052] The present application can be combined with the above-mentioned pulse electrolysis conditions to achieve the above effects. Alternatively, the electrolyte can be prepared by a commonly used method, such as dissolving elemental copper in sulfuric acid, adding electrolyte additives and hydrochloric acid thereto, stirring uniformly to obtain a first solution, and filtering and purifying the first solution to obtain the electrolyte.

[0053] In some embodiments, the electrolyte further comprises copper ions 60-120 g / L, sulfuric acid 90-150 g / L, and chloride ions 20-40 mg / L.

[0054] In the above scheme, by limiting the amount of each component in the electrolyte to a reasonable range, the components can better synergize with each other, and it is more conducive to preparing electrolytic copper foil with high tensile strength and high elongation.

[0055] In some embodiments of the present application, the brightener includes an organic sulfide; preferably, the brightener includes one or more of polydithiopropyl sulfide sodium (SPS), polydithioethane sulfide sodium (SES), and 3-mercapto-1-propane sulfonic acid sodium (MPS).

[0056] In some embodiments of the present application, the auxiliary brightener includes a nitrogen-containing organic sulfide; preferably, the auxiliary brightener includes one or more of polyisothiourethane propane sulfonic acid inner salt (UPS), 3-benzothiazyl-2-thiol propane sulfonic acid sodium (ZPS), N,N-dimethyl-propane sulfonic acid sodium (DPS), and tetrahydrothiazole thione (TTT).

[0057] In the above scheme, the electrolytic additive uses an organic sulfide brightener in combination with a nitrogen-containing organic sulfide auxiliary brightener, which can obtain an electrolytic copper foil with extremely high tensile strength and effectively improve the elongation of the electrolytic copper foil. The reason is that during the copper crystallization growth process, the organic sulfide tends to specifically adsorb on the (111) crystal plane with high atomic density, and the grain refinement effect is achieved by inhibiting the growth of the specific crystal plane. This specific adsorption often leads to the crystallization inside the copper foil showing obvious (111) texture, which causes the grains to easily slip along the specific crystal plane during stretching, resulting in lower tensile strength and elongation. The nitrogen-containing organic sulfide will change its adsorption characteristics due to the influence of heteroatoms, and will gather on the (200) crystal plane with low atomic density, reducing the proportion of (111) crystal plane texture. In summary, the use of organic sulfide brightener and nitrogen-containing organic sulfide auxiliary brightener can effectively reduce the texture phenomenon generated during the formation of nanotwins.

[0058] Preferably, in some embodiments, the organic sulfide brightener used includes but is not limited to 12 mg / L, 13 mg / L, 14 mg / L, 15 mg / L, 16 mg / L, 17 mg / L, 18 mg / L, 19 mg / L, 20 mg / L in terms of concentration in the electrolyte, and the nitrogen-containing organic sulfide auxiliary brightener used includes but is not limited to 4 mg / L, 5 mg / L, 6 mg / L, 7 mg / L, 8 mg / L, 9 mg / L, 10 mg / L in terms of concentration in the electrolyte.

[0059] In some specific embodiments of the present application, the leveling agent includes a nitrogen-containing small molecule compound; preferably, the leveling agent includes one or more of 2-mercaptopyridine (2-MP), 2-amino-4-methylbenzothiazole (AMBT), and tricyclozole (TCA).

[0060] In the above scheme, the electrolytic additive uses a nitrogen-containing small molecule compound to replace the traditional collagen leveling agent, so as to better meet the demand for higher diffusion speed of the additive molecules under the condition of higher frequency and peak current in pulse plating. At the same time, the nitrogen-containing small molecule compound leveling agent has a stronger adsorption capacity on the surface of the copper foil due to its higher charge density, and can be introduced into the grain boundary as a dopant, which plays a role in improving the tensile strength to a certain extent.

[0061] Preferably, in some embodiments, the nitrogen-containing small molecule compound leveling agent used includes but is not limited to 2 mg / L, 3 mg / L, 4 mg / L, 5 mg / L, 6 mg / L, 7 mg / L, 8 mg / L in terms of concentration in the electrolyte.

[0062] In some specific embodiments of the present application, the molecular weight of the polyethylene glycol is 4000-10000 (hereinafter referred to as macromolecular weight polyethylene glycol).

[0063] In some embodiments of the present application, the inhibitor comprises a polyether compound having a molecular weight of 2000 or less. The polyether compound can be a homo-polyether compound or a co-polyether compound commonly used in electrolyte inhibitors. Preferably, the inhibitor comprises one or more of polyethylene glycol having a molecular weight of 400-1000, polypropylene glycol having a molecular weight of 800-2000, and a copolymer of ethylene glycol and propylene glycol having a molecular weight of 400-1000.

[0064] In the above scheme, the electrolytic additive uses a low molecular weight polyether or a copolymer thereof as an auxiliary inhibitor. The low molecular weight polyether has a higher diffusion speed than high molecular weight polyethylene glycol, and thus can timely supplement the consumed inhibitor on the cathode surface within a short off time of high frequency pulse.

[0065] Experiments show that by using a macromolecular polyethylene glycol and a small molecular weight polyether in a specific mass ratio (such as 1:2-1:4, further examples being 1:2, 1:3 or 1:4) as a plating inhibitor, the synergistic effect of the two can further effectively increase the proportion of nanotwins in the electrolytic copper foil, thereby achieving extremely high tensile strength while maintaining high elongation.

[0066] Preferably, in some embodiments, the polyethylene glycol used includes but is not limited to 2 mg / L, 4 mg / L, 6 mg / L, 8 mg / L, 10 mg / L in terms of concentration in the electrolyte, and the polyether inhibitor used includes but is not limited to 6 mg / L, 12 mg / L, 18 mg / L, 24 mg / L, 30 mg / L in terms of concentration in the electrolyte.

[0067] The temperature and the flow rate of the electrolyte used in the electrolytic treatment refer to the conventional direct current electrolytic copper foil conditions. In some embodiments of the present application, the temperature of the electrolytic treatment is 50-60°C; and the flow rate of the electrolyte during the electrolytic treatment is 40-50 m 3 / h.

[0068] In a third typical embodiment of the present application, the present application further provides a negative electrode comprising a negative electrode current collector, the negative electrode current collector comprising a metal foil, which comprises the above-mentioned copper foil.

[0069] In some embodiments, the negative electrode further comprises an active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer can comprise carbon-based active materials such as graphite and hard carbon. In addition, the negative electrode active material layer can comprise one or more of the elements in the group consisting of Si, Ge, Sn, Zn, Ni, Co and Li in the form of simple substance or oxide. In addition, the negative electrode active material layer can further comprise a conductive agent and a binder.

[0070] In some embodiments of the present application, the preparation method of the negative electrode is as follows:

[0071] The commercially available silicon-carbon negative electrode material, conductive carbon black, SBR (styrene-butadiene rubber) and CMC (carboxymethyl cellulose) are mixed in high-purity water to prepare a slurry; preferably, the amount of each raw material of the slurry is as follows: 95 parts of commercially available silicon-carbon negative electrode material, 2 parts of conductive carbon black, 2 parts of SBR (styrene-butadiene rubber) and 1 part of CMC (carboxymethyl cellulose) by weight.

[0072] Then, the obtained slurry is coated on one side of the copper foil using a doctor blade, and the single-sided loading amount is 7±0.5 mg / cm 2 The electrode piece coated with the negative electrode material is dried by gradient heating at 115°C. Then the other side of the copper foil is coated, and after drying is completed, the dried copper foil sample is pressed using a roll press to make the electrode density 1.75±0.05 g / cm 3 Then, the coated copper foil is die-cut into a negative electrode piece with a size of 60 mm×80 mm, and the thickness of the negative electrode piece is measured using a micrometer.

[0073] In a fourth typical embodiment of the present application, the present application further provides a lithium ion battery comprising the above-mentioned negative electrode.

[0074] The beneficial effects of the present application will be described below in combination with specific examples and comparative examples.

[0075] In the present application, if a specific technology or condition is not specified in the examples, it is performed according to the technology or condition described in the literature in the field, or according to the product manual. If the manufacturer of the instrument used is not specified, it is a conventional product that can be purchased through a regular channel. The raw materials used in the present application can be easily purchased on the domestic market.

[0076] The test method of the microstructure and basic physical properties of the electrolytic copper foil prepared in all embodiments of the present application is as follows:

[0077] EBSD (Electron Backscatter Diffraction) test: The microstructure of part of the copper foil samples of each example and comparative example is observed using a C-Swift EBSD detector manufactured by Oxford Instruments, UK. In order to facilitate the observation of the grain boundary profile of the material, the cross section is polished for 20 min in advance using an ion mill.

[0078] Tensile strength and elongation test: according to test method GB / T29847-2013, using HY-0230 universal material testing machine manufactured by Shanghai Hengyi Precision Instrument Co., Ltd., the tensile strength and elongation of electrolytic copper foil are tested at a strain rate of 0.8 mm / s and room temperature (about 25℃). The tensile test sample is a strip-shaped sample with a total length of 100 mm and a width of 15 mm. The sample is measured three times and the average value is taken as the final tensile strength and elongation of the sample.

[0079] The preparation method and test method of the lithium ion battery are as follows:

[0080] According to parts by weight, commercially available silicon-carbon negative electrode material 95 parts, conductive carbon black 2 parts, SBR (styrene-butadiene rubber) 2 parts, CMC (carboxymethyl cellulose) 1 part are mixed in high-purity water to prepare a slurry.

[0081] Then, the obtained slurry is coated on one side of the copper foil using a doctor blade, and the single-sided loading amount is 7±0.5 mg / cm 2 The electrode piece coated with the negative electrode material is dried by gradient heating at 115℃. Then the other side of the copper foil is coated, and after drying is completed, the dried copper foil sample is pressed using a roll press to make the electrode density 1.75±0.05 g / cm 3 . Then the coated copper foil is die-cut into a negative electrode piece with a size of 60 mm×80 mm, and the thickness of the negative electrode piece is measured using a micrometer.

[0082] Then, the obtained negative electrode piece is assembled with a commercially available separator, electrolyte and positive electrode piece to manufacture a soft package battery. The battery is subjected to 100 charge-discharge cycles at a rate of 2C. Then the battery is disassembled to take out the negative electrode piece, and the thickness of the cycled electrode piece is measured using a micrometer, and the expansion rate is calculated according to the difference in thickness of the electrode piece before and after cycling. At the same time, whether the electrode piece is damaged and transmits light is observed with a strong light flashlight.

[0083] The present application is a double-sided copper foil D(Drum) and A(Air) surface, the D surface is the side close to the cathode roller, and the other side is the A surface.

[0084] Example 1

[0085] The present application provides an electrolytic copper foil, and the preparation method thereof comprises the following steps:

[0086] (1) Dissolve elemental copper metal in sulfuric acid to form a sulfuric acid-copper sulfate solution, then add electrolytic additives and hydrochloric acid and stir uniformly to obtain a first solution; add 1 g / m 3 of activated carbon and diatomite to the obtained first solution and stir, then remove organic impurities and suspended solids with stainless steel filter cloth, and then pass through twice to obtain a pure electrolyte for electrolysis.

[0087] The concentration of copper ions in the obtained electrolyte is 86 g / L, the concentration of sulfuric acid is 120 g / L, the concentration of chloride ions (hydrochloric acid) is 20 mg / L, the electrolyte is added with 13 mg / L of organic sulfide brightener SPS, 6 mg / L of nitrogen-containing organic sulfide auxiliary brightener UPS, 4 mg / L of small molecule leveling agent 2-MP, 6 mg / L of polyethylene glycol with a molecular weight of 4000, and 18 mg / L of polyether type inhibitor (polyethylene glycol with a molecular weight of 400).

[0088] (2) The above electrolyte is subjected to electrolytic treatment by using a titanium roller as a cathode and an iridium-tantalum coated titanium plate as an anode, and by using a pulse power source. The peak current density is 13000 A / m 2 , the pulse width is 3 ms, the duty cycle is 5%, and a copper raw foil with a thickness of 6 μm is prepared. The temperature of the electrolytic treatment is 55°C, and the flow rate of the electrolyte is 45 m 3 / h.

[0089] (3) The obtained copper raw foil is washed, passivated, and dried to obtain a finished electrolytic copper foil without cutting. Referring to FIG. 1, the obtained electrolytic copper foil has a super-fine grain size and a uniform distribution.

[0090] Next, the microstructure of the cross section of the electrolytic copper foil obtained by the above pulse plating is observed and studied by using EBSD, and the average grain size, the area-weighted average grain size, and the twin grain proportion of the sample are measured.

[0091] Next, the mechanical properties of the above obtained electrolytic copper foil are tested, and the tensile strength and the elongation of the sample are obtained.

[0092] Next, the above electrolytic copper foil is assembled into a soft-pack battery for cycle testing to check whether the negative copper foil is damaged.

[0093] Example 2

[0094] The present embodiment provides an electrolytic copper foil, and a preparation method thereof, which comprises the following steps:

[0095] (1) Dissolve metallic copper into sulfuric acid to form a sulfuric acid-copper sulfate solution, and then add electrolytic additives and hydrochloric acid to obtain a first solution; add 1 g / m 3 of activated carbon and diatomite to the obtained first solution and stir, and then remove organic impurities and suspended solids by using a stainless steel filter cloth, and then pass through twice to obtain a pure electrolyte for electrolysis.

[0096] The copper ion concentration in the obtained electrolyte is 109 g / L, the sulfuric acid concentration is 130 g / L, the chloride ion (hydrochloric acid) concentration is 22 mg / L, 16 mg / L of SES and SPS with a mass ratio of 1:1 is added to the electrolyte, 4 mg / L of auxiliary brightener TTT, 3 mg / L of small molecule leveling agent AMBT, 4 mg / L of polyethylene glycol with a molecular weight of 8000, and 12 mg / L of polyethylene glycol with a molecular weight of 1000.

[0097] (2) The above electrolyte is subjected to electrolytic treatment by using a titanium roller as a cathode and an iridium-tantalum coated titanium plate as an anode, and using a pulse power source. The peak current density is 9000 A / m 2 , the pulse width is 30 ms, the duty cycle is 25%, and a copper raw foil with a thickness of 6 μm is prepared. The temperature of the electrolytic treatment is 55°C, and the flow rate of the electrolyte is 45 m 3 / h.

[0098] (3) The obtained copper raw foil is washed, passivated, and dried to obtain a finished electrolytic copper foil without cutting.

[0099] Next, the microstructure of the cross section of the electrolytic copper foil obtained by the above pulse plating is observed and studied by using EBSD, and the average grain size, the area-weighted average grain size, and the twin grain proportion of the sample are measured.

[0100] Next, the mechanical properties of the above obtained electrolytic copper foil are tested, and the tensile strength and elongation of the sample are obtained.

[0101] Next, the above electrolytic copper foil is assembled into a soft package battery for cycle test to check whether the negative copper foil is damaged.

[0102] Example 3

[0103] The present embodiment provides an electrolytic copper foil, and a preparation method thereof, which comprises the following steps:

[0104] (1) Dissolve the metal elemental copper in sulfuric acid to form a sulfuric acid-copper sulfate solution, then add electrolytic additives and hydrochloric acid and stir uniformly to obtain a first solution; add 1 g / m 3 of activated carbon and diatomite to the obtained first solution and stir, then remove organic impurities and suspended solids by using a stainless steel filter cloth, and then filter twice to obtain a pure electrolyte for electrolysis.

[0105] The copper ion concentration in the obtained electrolyte is 63 g / L, the sulfuric acid concentration is 105 g / L, the chloride ion (hydrochloric acid) concentration is 38 mg / L, 15 mg / L of SPS and MPS with a mass ratio of 1:1 is added to the electrolyte, 8 mg / L of DPS, 2 mg / L of small molecule leveling agent TCA, 3 mg / L of polyethylene glycol with a molecular weight of 80000, and 9 mg / L of propylene glycol with a molecular weight of 800.

[0106] (3) with titanium roller as cathode, iridium tantalum coated titanium plate as anode, electrolysis treatment is carried out on the above electrolyte by pulse power. The peak current density is 11000 A / m 2 , pulse width is 10 ms, duty cycle is 20%, and the prepared copper raw foil has a thickness of 6 μm. The temperature of electrolysis treatment is 55℃, and the upflow rate of electrolyte is 45 m 3 / h.

[0107] (3) The obtained copper raw foil is washed, passivated and dried to obtain the finished electrolytic copper foil without cutting.

[0108] Then, the microstructure of the cross section of the electrolytic copper foil obtained by pulse plating is observed and studied by EBSD, and the average grain size, area weighted average grain size and twin grain proportion of the sample are measured.

[0109] Then, the mechanical properties of the electrolytic copper foil are tested, and the tensile strength and elongation of the sample are obtained.

[0110] Then, the electrolytic copper foil is assembled into a soft package battery for cycle test to check whether the negative copper foil is damaged.

[0111] Example 4

[0112] The present embodiment provides an electrolytic copper foil, and the preparation method thereof comprises the following steps:

[0113] (1) Dissolve the metal elemental copper in sulfuric acid to form a sulfuric acid-copper sulfate solution, then add electrolytic additives and hydrochloric acid and stir uniformly to obtain a first solution; add 1 g / m 3 of activated carbon and diatomite to the obtained first solution and stir, then remove organic impurities and suspended solids with stainless steel filter cloth, and then filter twice to obtain a pure electrolyte for electrolysis.

[0114] Among them, the copper ion concentration in the obtained electrolyte is 117 g / L, the sulfuric acid concentration is 110 g / L, the chloride ion (hydrochloric acid) concentration is 30 mg / L, 19 mg / L of SPS and MPS with a mass ratio of 1:1, 7 mg / L of DPS and TTT with a mass ratio of 1:1, 6 mg / L of small molecule leveling agent 2-MP, 7 mg / L of polyethylene glycol with a molecular weight of 10000, and 21 mg / L of copolymer of ethylene glycol and propylene glycol with a molecular weight of 1000 are added to the electrolyte.

[0115] (2) with titanium roller as cathode, iridium tantalum coated titanium plate as anode, electrolysis treatment is carried out on the above electrolyte by pulse power. The peak current density is 14000 A / m 2 , pulse width is 7 ms, duty cycle is 13%, and the prepared copper raw foil has a thickness of 6 μm. The temperature of electrolysis treatment is 55℃, and the upflow rate of electrolyte is 45 m3 / h.

[0116] (3) The obtained copper raw foil is washed, passivated and dried to obtain the uncut finished electrolytic copper foil.

[0117] Then, the cross section of the electrolytic copper foil obtained by pulse plating is observed and studied by EBSD, and the average grain size, area-weighted average grain size and twin grain proportion of the sample are measured.

[0118] Then, the mechanical properties of the electrolytic copper foil obtained above are tested, and the tensile strength and elongation of the sample are obtained.

[0119] The electrolytic copper foil is then assembled into a soft package battery for cycle testing to check whether the negative copper foil is damaged.

[0120] Example 5

[0121] The electrolytic copper foil of the present embodiment is prepared by the method different from that of Example 4 only in that the duty cycle of the pulse power source is 1%, and the rest is the same as that of Example 4.

[0122] Example 6

[0123] The electrolytic copper foil of the present embodiment is prepared by the method different from that of Example 4 only in that the duty cycle of the pulse power source is 30%, and the rest is the same as that of Example 4.

[0124] Example 7

[0125] The electrolytic copper foil of the present embodiment is prepared by the method different from that of Example 4 only in that the pulse width of the pulse power source is 5ms, and the rest is the same as that of Example 4.

[0126] Example 8

[0127] The electrolytic copper foil of the present embodiment is prepared by the method different from that of Example 4 only in that the pulse width of the pulse power source is 50ms, and the rest is the same as that of Example 4.

[0128] Example 9

[0129] The electrolytic copper foil of the present embodiment is prepared by the method different from that of Example 4 only in that the peak current density of the pulse power source is 8000A / m 2 , and the rest is the same as that of Example 4.

[0130] Example 10

[0131] The electrolytic copper foil of the present embodiment is prepared by the method different from that of Example 4 only in that the peak current density of the pulse power source is 15000A / m 2 , and the rest is the same as that of Example 4.

[0132] Example 11

[0133] This example provides an electrolytic copper foil, the preparation method of which is different from that of Example 4 only in that the concentration of the organic sulfide brightener in the electrolyte is 20 mg / L, the concentration of the nitrogen-containing organic sulfide auxiliary brightener is 6 mg / L, and the rest is the same as that of Example 4.

[0134] Example 12

[0135] This example provides an electrolytic copper foil, the preparation method of which is different from that of Example 4 only in that the concentration of the organic sulfide brightener in the electrolyte is 18 mg / L, the concentration of the nitrogen-containing organic sulfide auxiliary brightener is 10 mg / L, and the rest is the same as that of Example 4.

[0136] Example 13

[0137] This example provides an electrolytic copper foil, the preparation method of which is different from that of Example 4 only in that the concentration of the small molecule leveling agent in the electrolyte is 5 mg / L, and the rest is the same as that of Example 4.

[0138] Example 14

[0139] This example provides an electrolytic copper foil, the preparation method of which is different from that of Example 4 only in that the concentration of the polyethylene glycol in the electrolyte is 2 mg / L, the concentration of the polyether type inhibitor is 6 mg / L, and the rest is the same as that of Example 4.

[0140] Example 15

[0141] This example provides an electrolytic copper foil, the preparation method of which is different from that of Example 4 only in that the concentration of the polyethylene glycol in the electrolyte is 10 mg / L, the concentration of the polyether type inhibitor is 30 mg / L, and the rest is the same as that of Example 4.

[0142] Comparative Example 1

[0143] Comparative Example 1 is a comparative example of Example 4, which provides a high-tensile copper foil prepared by a conventional direct current plating method, and the specific preparation steps are as follows:

[0144] (1) Dissolve elemental copper metal in sulfuric acid to form a sulfuric acid-copper sulfate solution, and then add electrolytic additives and hydrochloric acid to obtain a first solution after stirring uniformly; add 1 g / m 3The activated carbon and diatomite are stirred, and then the organic impurities and suspended matters are removed by using a stainless steel filter cloth, and then the electrolyte is purified by twice filtration. The copper ion concentration in the obtained electrolyte is 117 g / L, the sulfuric acid concentration is 110 g / L, and the chloride ion (hydrochloric acid) concentration is 30 mg / L. The obtained electrolyte is added with 19 mg / L of SPS and MPS in a mass ratio of 1:1, 7 mg / L of DPS and TTT in a mass ratio of 1:1, 6 mg / L of a small molecule leveler 2-MP, 7 mg / L of polyethylene glycol with a molecular weight of 10000, and 21 mg / L of a copolymer of ethylene glycol and propylene glycol with a molecular weight of 1000.

[0145] (2) The electrolyte is electrolyzed by using a titanium roller as a cathode and an iridium-tantalum coated titanium plate as an anode, and a constant current power supply, and the current density is 6500 A / m 2 . The thickness of the prepared copper raw foil is 6 μm. The temperature of the electrolysis is 55℃, and the flow rate of the electrolyte is 45 m 3 / h.

[0146] (3) The obtained copper raw foil is washed, passivated and dried to obtain a finished electrolytic copper foil without cutting. As shown in FIG. 2, the obtained electrolytic copper foil has a large average grain size, and the grain size distribution shows an obvious increasing trend from the D surface to the A surface.

[0147] Then, the microstructure of the cross section of the obtained electrolytic copper foil is observed by using EBSD, and the average grain size, the area weighted average grain size and the twin grain ratio of the sample are measured.

[0148] Then, the mechanical properties of the obtained electrolytic copper foil are tested, and the tensile strength and the elongation of the sample are obtained.

[0149] Then, the electrolytic copper foil is assembled into a soft package battery for cycle test to check whether the negative copper foil is damaged.

[0150] Comparative Example 2

[0151] The present comparative example provides an electrolytic copper foil, and the difference between the preparation method thereof and that of Example 4 is that the pulse width of the pulse power supply is 60 ms, and the rest is the same as that of Example 4.

[0152] Comparative Example 3

[0153] The present comparative example provides an electrolytic copper foil, and the difference between the preparation method thereof and that of Example 4 is that the duty cycle of the pulse power supply is 40%, and the rest is the same as that of Example 4.

[0154] Comparative Example 4

[0155] This comparative example provides an electrolytic copper foil, the difference between its preparation method and that of Example 4 is only that the peak current of the pulse power supply is 7000 A / m 2 The rest is the same as Example 4.

[0156] Comparative Example 5

[0157] This comparative example provides an electrolytic copper foil, the difference between its preparation method and that of Example 4 is only that the concentration of the small molecule leveling agent in the electrolyte is 10 mg / L, and the rest is the same as Example 4.

[0158] Comparative Example 6

[0159] This comparative example provides an electrolytic copper foil, the difference between its preparation method and that of Example 4 is only that the concentration of the small molecule leveling agent in the electrolyte is 10 mg / L, and the rest is the same as Example 4.

[0160] Comparative Example 7

[0161] This comparative example provides an electrolytic copper foil, the difference between its preparation method and that of Example 4 is only that the concentration of the small molecule leveling agent in the electrolyte is 10 mg / L, and the rest is the same as Example 4.

[0162] Table 1 below lists the micro-grain size, basic physical properties and battery expansion rate test results of the electrolytic copper foil samples manufactured according to the examples and comparative examples of the present application.

[0163] Table 1

[0164] Firstly, Example 4 and Comparative Example 1 are samples prepared by pulse plating method and direct current plating method respectively in the same electrolyte. Comparing Figure 1 and Figure 2, the electrolytic copper foil prepared by direct current plating method has larger average grain size, and the grain size distribution shows a clear increasing trend from the D surface to the A surface, which shows that the pulse plating method adopted in Example 4 can effectively realize grain refinement, and the intermittent deposition of copper ions under the action of pulse electric field also helps to obtain consistent grain size distribution from the D surface to the A surface. On the other hand, the higher twin grain proportion of Comparative Example 1 is due to the uneven distribution of grain size, and the individual larger twin crystals contribute to more grain boundary length and twin area. Figure 3 is a comparison of the tensile elongation curves of the electrolytic copper foils of Example 4 and Comparative Example 1, it can be seen that although the elongation of Comparative Example 1 is greater than that of Example 4, its tensile strength is lower, which is consistent with the law of average grain size and twin grain proportion shown in EBSD. Lower tensile strength leads to larger volume change during charging and discharging, and metal fatigue caused by repeated stretching leads to rupture.

[0165] In addition, Example 4 has similar tensile strength as Comparative Example 6. However, due to the unreasonable electrolyte ratio, an excess of doped grain boundaries are introduced, which reduces the proportion of nanotwins, and thus the elongation is low, which is prone to rupture during battery charging cycles. After the current collector is broken, the swelling of the negative electrode material is not limited, and thus Comparative Example 6 also has a high volume expansion rate.

[0166] In addition, Examples 1-15 achieve a tensile strength covering the range of 600-900 MPa by adjusting the power parameters (duty cycle, pulse width, and peak current density) and / or electrolyte ratio, while maintaining an elongation greater than 5%.

[0167] In summary, the pulse plating technology of the present application can prepare electrolytic copper foil with highly refined grain size, high nanotwin proportion, and ultra-high tensile strength, which can effectively suppress the volume change of silicon-carbon negative electrode material during the charging and discharging process. At the same time, the electrolytic copper foil of the present application has a high elongation, which can effectively prevent the copper foil from breaking due to repeated volume changes during the charging and discharging process of the silicon-carbon negative electrode material. The lithium ion battery negative electrode containing the electrolytic copper foil of the present application thus has better cycle life and safety.

[0168] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A copper foil, characterized by, The copper foil comprises ultra-fine grains and nano-twins, the average grain size of the copper foil is 0.2-0.4 μm, the area-weighted average grain size is 0.3-0.8 μm, and the proportion of nano-twin grains measured by EBSD in the copper foil is greater than 50%.

2. The copper foil according to claim 1, characterized by, The elongation of the copper foil is greater than 5%.

3. The copper foil according to claim 1 or 2, characterized by, The tensile strength of the copper foil is 600-900 MPa.

4. The copper foil according to claim 1, characterized by, The thickness of the copper foil is 4-10 μm.

5. A method for producing a copper foil, characterized by, The preparation method comprises the following steps: The original foil is obtained by electrolyzing electrolyte with a pulse power source under a predetermined pulse width and duty cycle; the current density of the pulse power source is 8000-15000 A / m 2 , the pulse width is 1-50 ms, and the duty cycle is 1%-30%; the electrolyte comprises an electrolytic additive, and the electrolytic additive comprises a brightener with a content of 12-20 mg / L, an auxiliary brightener with a content of 4-10 mg / L, a leveling agent with a content of 2-8 mg / L, polyethylene glycol with a content of 2-10 mg / L, and an inhibitor with a content of 6-30 mg / L. The original foil is washed and passivated to obtain an electrolytic copper foil.

6. The production method according to claim 5, wherein The electrolyte further comprises copper ions 60-120 g / L, sulfuric acid 90-150 g / L, and chloride ions 20-40 mg / L.

7. The production method according to claim 6, characterized by, The brightener comprises an organic sulfide; preferably, the brightener comprises one or more of sodium polydithiopropyl sulfide, sodium polydithioethane sulfide, and sodium 3-mercapto-1-propane sulfide; And / or, the auxiliary brightener comprises a nitrogen-containing organic sulfide; preferably, the auxiliary brightener comprises one or more of polyisothioureine propane sulfide inner salt, 3-benzothiazyl-2-thiol propane sulfide sodium, N,N-dimethyl-propane sulfide sodium, and tetrahydrothiazole thione; And / or, the leveling agent comprises a nitrogen-containing small molecule compound; preferably, the leveling agent comprises one or more of 2-mercaptopyridine 2-, amino-4-methyl benzothiazole, and tricyclic azole; And / or, the molecular weight of the polyethylene glycol is 4000-10000; And / or, the inhibitor comprises a polyether compound, the molecular weight of the polyether compound is less than or equal to 2000; preferably, the inhibitor comprises one or more of polyethylene glycol with a molecular weight of 400-1000, polypropylene glycol with a molecular weight of 800-2000, and a copolymer of ethylene glycol and propylene glycol with a molecular weight of 400-1000; And / or the mass ratio of the polyethylene glycol to the polyether compound is 1:2-1:

4. The metal foil comprises the copper foil according to any one of claims 1-4.

8. The production method according to any one of claims 5 to 7, characterized by, The temperature of the electrolytic treatment is 50-60℃; during the electrolytic treatment, the upflowing flow rate of the electrolyte is 40-50m 3 / h.

9. A negative electrode comprising a negative electrode current collector, the negative electrode current collector comprising a metal foil, characterized in that, The negative electrode comprises the negative electrode according to claim 9.

10. A lithium-ion battery comprising a negative electrode, characterized in that ​

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