Ultrathin lithium-doped copper foil composite tape, its preparation method and application

Abstract

An ultrathin lithium-doped copper foil composite strip, its preparation method, and its applications are disclosed. This ultrathin lithium-doped copper foil composite strip exhibits high interfacial bonding strength between metallic lithium and the copper foil substrate. The dopant elements in the copper foil undergo rearrangement and segregation on the copper foil substrate surface, self-regulating the nucleation sites for metallic lithium deposition, achieving uniform nucleation and deposition of metallic lithium, and effectively suppressing lithium dendrite growth. Furthermore, the addition of dopant elements to the copper foil not only refines the grain size and increases the toughness of the copper substrate, thus enabling the preparation of thinner copper current collectors, but also reduces the copper content in the current collector, thereby lowering the current collector cost.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal processing, and in particular relates to an ultrathin lithium-doped copper foil composite strip, its preparation method and application. Background Technology

[0002] In the field of electrochemical energy storage, lithium metal anodes are favored due to their high theoretical specific capacity (3860 mAh / g), low reduction potential (-3.04 V, relative to the standard potential of hydrogen), and low density (0.59 g / cm³). 3 Its characteristics have attracted much attention and popularity.

[0003] When lithium metal is used as the negative electrode, it must be loaded onto a metal current collector to form a lithium metal electrode. To ensure the usability and safety of the lithium metal electrode, a good interfacial bond is required between the lithium metal and the metal current collector substrate. This bond requires high interfacial bond strength, good conductivity, and a smooth, defect-free interface (e.g., bubbles, wrinkles, waves, breaks, impurity particles, etc.). A good interfacial bond between the lithium metal and the current collector substrate helps maintain a consistent current density on the electrode surface, avoids significant local dendrite growth in the lithium metal, and thus eliminates short circuits caused by dendrites.

[0004] To achieve a good interfacial bond between lithium metal and the metal current collector substrate, Chinese patent application CN109742323A incorporates an intermediate transition layer between the lithium metal and the metal current collector. While this partially improves the interfacial bond, the transition layer does not reduce the amount of copper used and may increase the thickness and overall mass of the lithium metal electrode, which is detrimental to improving the volumetric energy density and gravimetric energy density of the battery. Furthermore, the alloy metal elements in the transition layer may exhibit alloy segregation due to competitive bonding between the copper and lithium foils, significantly impacting the bonding strength between lithium and copper. This affects the electronic conductivity of the electrode surface, increases current non-uniformity, and subsequently induces rapid dendrite growth in specific regions, potentially leading to safety hazards.

[0005] Currently, the copper foil used in lithium-copper composite strips has a purity higher than 99.9%, and most of it is electrolytic copper foil. To reduce the weight of the copper foil and increase battery energy density, lithium battery companies are using increasingly thinner copper foils. However, thinner copper foils increase the difficulty of processing, which is detrimental to reducing battery costs. Furthermore, when producing high-purity copper foil using electrolysis, the copper grains are large, the grain boundaries are obvious, and the copper deposition surface (rough surface) has a high roughness. While the rough surface can improve the strength of the lithium-copper bond, lithium easily forms dendrites at the top of the copper grains, which is the main reason for the poor cycle performance of lithium-free batteries using only copper foil.

[0006] In summary, there is still a need for a lithium-copper composite strip with high interfacial bonding strength and adjustable lithium deposition sites to suppress lithium dendrite growth. Summary of the Invention

[0007] In view of this, the present invention provides an ultrathin lithium-doped copper foil composite strip. First, by using doped copper foil (copper foil doped with one or more other elements, including forming copper alloys) as the current collector, the dopant elements on the surface form positions similar to "anchor points." After the lithium metal and the dopant elements are bonded through metallic bonds, the interfacial bonding strength between the lithium metal and the current collector can be significantly improved. Second, the introduction of dopant elements can refine the grain size and improve the surface uniformity of the foil, greatly reducing the current density "hot spots" on the foil surface and effectively suppressing dendrite growth. In addition, after grain refinement, the toughness of the doped copper foil is improved, which is beneficial for preparing copper foil current collectors with thinner thicknesses. Finally, the addition of some dopant elements can also reduce the cost of the current collector.

[0008] The ultrathin lithium-doped copper foil composite strip of this invention can self-adjust lithium deposition sites, mainly because during charging, the dopant element alloys with the metallic lithium deposited on the surface of the negative electrode current collector, resulting in atomic mixing of metallic lithium and dopant elements on the current collector surface. Simultaneously, due to molecular thermal motion, lithium atoms diffuse into the interior of the doped copper foil. When the battery discharges, metallic lithium is first oxidized and removed from the current collector surface, while the surface dopant element, due to metallic bonds, undergoes redistribution and gradually segregates on the surface, providing nucleation sites for the next lithium deposition. Within a limited number of cycles, the surface segregation of the dopant element helps suppress the growth of lithium dendrites on the current collector surface; however, beyond this number of cycles, the surface segregation of the dopant element leads to severe dendrite formation on the current collector surface. This critical number of cycles varies depending on the type and mass fraction of the dopant element.

[0009] It is worth noting that the type and mass fraction of dopants in the copper foil are crucial for achieving self-regulating lithium deposition sites and maintaining current collector function. Unwilling to adhere to any particular theory, the doping elements in the copper foil of this invention can be divided into two categories. The first category consists of elements that can simultaneously form alloys with metallic copper and metallic lithium, including elements such as indium, tin, aluminum, zinc, and boron. The addition of these elements can maintain the stability of the surface of the doped copper foil. Even after the doping element dealloys with metallic lithium, the doping element can still undergo an alloying reaction with metallic copper, thereby stabilizing the element distribution state on the surface of the doped copper foil and maintaining the stability of the current collector function. The other category consists of elements that can eutectic with metallic copper and form alloys with metallic lithium, including elements such as silver, cadmium, lead, and carbon. After these elements are added, they will precipitate in the doped copper foil in the form of independent phases. The precipitated elements can undergo an alloying reaction with metallic lithium, which can reduce the nucleation energy and guide lithium deposition. Moreover, when the doping element undergoes a dealloying reaction with metallic lithium, the doping element itself undergoes rearrangement and segregation, while the position of copper atoms remains unchanged. This will lead to the formation of a large number of microstructures (including step-like, villous, and other microstructures) on the surface of the doped copper foil. In a limited number of cycles, the surface segregation of the doping element is beneficial to suppressing the growth of lithium dendrites on the surface of the current collector. Too much dopant makes the current collector prone to corrosion and breakage, affecting its conductivity. Too little dopant creates fewer alloying sites that become dendrite nucleation centers, thus promoting dendrite growth. However, when the alloy mass fraction is between 10% and 75%, it does not affect the conductivity of the current collector and can form more nucleation sites, achieving uniform lithium deposition and thus inhibiting dendrite growth.

[0010] Specifically, one aspect of the present invention provides an ultrathin lithium-doped copper foil composite strip, the ultrathin lithium-doped copper foil composite strip comprising:

[0011] Doped copper foil is a copper foil substrate containing doped elements. The doped elements are distributed in the copper foil substrate in a gradient distribution pattern with decreasing concentration from the surface to the interior or in a homogeneous distribution pattern. In the surface layer of the copper foil substrate, the doped elements are distributed in a manner with decreasing mass fraction from 75% to 10%, or in a homogeneous distribution pattern with a certain mass fraction in the range of 10% to 75%.

[0012] An ultrathin lithium metal layer with a thickness of 10 nanometers to 100 micrometers (preferably 1 to 50 micrometers, more preferably 1 to 20 micrometers, or 1 to 10 micrometers) is composited on the surface layer; and

[0013] Optionally, a protective layer is attached to the surface of the ultrathin lithium metal layer opposite to the copper foil substrate.

[0014] In some embodiments of the present invention, the doping element includes at least one of the following: elements capable of simultaneously forming alloys with metallic copper and metallic lithium (including indium, tin, aluminum, zinc, boron, etc.) and elements capable of eutectic with metallic copper and forming alloys with metallic lithium (including silver, cadmium, lead, carbon, etc.).

[0015] In some embodiments of the present invention, the thickness of the surface layer is 10 nanometers to 3 micrometers, preferably 100 nanometers to 1 micrometer.

[0016] In some embodiments of the present invention, the thickness of the copper foil substrate is 3 micrometers to 50 micrometers.

[0017] In some embodiments of the present invention, the mass fraction of the dopant element in the bulk of the copper foil substrate (the portion of the copper foil substrate excluding the surface layer) is 5% to 60%.

[0018] In some embodiments of the present invention, the protective layer comprises one of an organic polymer protective layer, an inorganic protective layer, and an organic-inorganic composite protective layer. The organic polymer protective layer includes halogenated, block, grafted, or copolymerized polymers of polyethylene oxide, dimethyl methacrylate, polyphenylene sulfide, polyacrylonitrile, polystyrene, N-methylpyrrolidone, polyvinylidene fluoride, and the above polymers. The inorganic protective layer includes oxide solid electrolytes, sulfide solid electrolytes, metal oxides, etc. The organic-inorganic composite protective layer is a product of combining the above-mentioned organic polymer protective layer and inorganic protective layer, wherein the ratio of the organic polymer protective layer to the inorganic protective layer is (15-85):(15-85).

[0019] Another aspect of the present invention provides a method for preparing an ultrathin lithium-doped copper foil composite strip as described above, comprising the following steps:

[0020] Step 1: Prepare doped copper foil with gradient or homogeneous distribution of doped elements;

[0021] Step 2: Clean the surface of the copper-doped foil;

[0022] Step 3: Apply an ultrathin lithium metal layer to the surface;

[0023] Step 4: Optionally, a protective layer is formed on the surface of the ultrathin lithium metal layer opposite to the copper foil substrate.

[0024] In some embodiments of the present invention, step 1 includes two methods:

[0025] Method 1: Expose the copper foil substrate to an environment in which doping elements coexist in gaseous, liquid, or solid phases, or any two or three phases. Perform heat treatment at a high temperature of 600℃ to 3000℃ to achieve gradient or homogeneous distribution of doping elements in the copper foil substrate through solid-phase diffusion and / or melting and mixing.

[0026] Method 2: Prepare doped copper foil by electrodeposition, wherein the concentration of dopant elements in the electrolyte during electrodeposition is controlled to achieve gradient or homogeneous distribution of dopant elements in the copper foil substrate.

[0027] In Method 1, the high-temperature heat treatment time can be from 2 hours to 96 hours, and the doped copper foil can be further processed, such as by rolling and thinning.

[0028] In some embodiments of the present invention, the surface cleaning of the copper-doped foil includes at least one of pickling, corona treatment, and plasma treatment.

[0029] In some embodiments of the present invention, the ultrathin lithium metal layer is applied by at least one of vapor deposition, magnetron sputtering, electroplating, thermal diffusion, and melt coating.

[0030] Another aspect of the present invention provides the application of the ultrathin lithium-doped copper foil composite strip as described above in lithium batteries, wherein the lithium batteries may include lithium-ion batteries, quasi-solid-state batteries, all-solid-state batteries, lithium-sulfur batteries, lithium-oxygen batteries, etc., wherein the ultrathin lithium-doped copper foil composite strip can be used as a negative electrode.

[0031] The advantages of the ultrathin lithium-doped copper foil composite strip of this invention are mainly reflected in the following aspects:

[0032] 1. Using doped copper foil as the current collector, the doped elements form positions similar to "anchors". After lithium metal bonds with the doped elements (e.g., through metallic bonds), the interfacial bonding strength between lithium metal and the current collector can be significantly improved.

[0033] 2. The introduction of doping elements can refine the grains, greatly reduce the "hot spots" of current density on the foil surface, and effectively suppress dendrite growth.

[0034] 3. The rearrangement and segregation of dopants on the surface of the copper foil substrate self-regulate the nucleation sites for lithium metal deposition, enabling uniform nucleation and deposition of lithium metal and effectively suppressing dendrite growth.

[0035] 4. After grain refinement, the toughness of copper foil is improved, which is beneficial for preparing copper foil current collectors with smaller thickness.

[0036] 5. The addition of doping elements reduces the amount of copper used, which can reduce the cost of current collectors. Attached Figure Description

[0037] Figure 1 Comparison of cycle performance of batteries prepared using ultrathin lithium-copper composite strips from Examples 4 and 5.

[0038] Figure 2The graph shows the battery cycle test results for Examples 4 and 5. Detailed Implementation

[0039] The following detailed description of the ultrathin lithium-doped copper foil composite tape of the present invention, its preparation method, and its application will be provided in conjunction with specific embodiments.

[0040] This invention utilizes doped copper foil to prepare ultrathin lithium metal-doped copper foil composite strips, which is beneficial for fabricating thin and lightweight electrodes, providing a possibility for preparing batteries with higher volumetric and gravimetric energy densities. The distribution of dopants in the copper substrate within the doped copper foil can take two forms: Form 1: The dopants are gradient-distributed within the copper substrate, with more dopants on the surface of the copper foil and fewer or no dopants inside. The higher dopant content on the surface of the copper foil substrate improves the bonding strength between the substrate and lithium metal, with the dopants forming "anchor points" that firmly fix the lithium metal to the surface. The fewer or no dopants inside the copper foil maintain the strength of the substrate and ensure rapid electron conduction within it. Form 2: The dopant element is uniformly distributed in the copper matrix, forming "anchor-like" locations that improve the interfacial bonding strength between metallic lithium and the doped copper foil substrate. Furthermore, during cycling, the elements in the doped copper foil undergo alloying and rearrangement, causing the surface of the doped copper foil substrate to be etched, forming fine microstructures such as surface vacancies and steps, and three-dimensional porous structures. The appearance of these structures can reduce the current density at the electrode surface, inhibiting lithium dendrite growth during lithium deposition. In both forms, the dopant element acts as a nucleation site for lithium deposition, which helps reduce dendrite formation.

[0041] Example 1

[0042] Borax was spread in a crucible, and a 10-micrometer-thick copper foil was laid flat on top of the borax. The copper foil was then completely covered with borax. The crucible was placed in a muffle furnace, and the temperature was raised to 800°C at a rate of 15°C / min and held for 4 hours. After cooling, a boron-doped copper substrate (copper-boron substrate) with a boron atom content decreasing from the outside to the inside was obtained.

[0043] Corona treatment is applied to the surface of the copper-boron substrate to remove impurities and particles.

[0044] A 5-micrometer-thick layer of metallic lithium was deposited on the surface of a boron alloy substrate using vapor deposition to obtain the ultrathin lithium-copper-boron composite strip of the present invention.

[0045] Example 2

[0046] Zinc and copper in a mass ratio of 3:7 were added to a crucible. The mixture was heated to 1500℃ for melting and mixing, and then cooled to obtain a copper-zinc alloy substrate. The copper-zinc alloy substrate was then rolled to obtain a copper-zinc alloy substrate with a thickness of 10 micrometers.

[0047] A 5-micrometer-thick layer of metallic lithium was deposited on the surface of a copper-zinc alloy substrate using vapor deposition to obtain the ultrathin lithium-copper-zinc alloy composite strip of the present invention.

[0048] Example 3

[0049] In a 1L electrolytic cell, the metal ion concentration was kept constant at 30g / L. The concentrations of CuCN and ZnSO4 salts were adjusted to obtain a copper-zinc alloy substrate with zinc atoms uniformly distributed within the copper substrate. The mass ratio of zinc to copper in the alloy was 3:7. During electroplating, a zinc plate was used as the anode, a titanium roller as the cathode, and the current density was 10A / m². 2 Electroplating was performed at room temperature for 4 hours. After electroplating, the copper alloy coating was peeled off from the titanium roller to obtain a copper-zinc alloy substrate with a thickness of 10 micrometers.

[0050] A 5-micrometer-thick layer of metallic lithium was deposited on the surface of a copper-zinc alloy substrate using vapor deposition to obtain the ultrathin lithium-copper-zinc alloy composite strip of the present invention.

[0051] Example 4

[0052] Indium and copper in a mass ratio of 1:6.2 were added to a crucible. The mixture was heated to 2000℃ for melting and mixing, and then cooled to obtain indium-doped copper foil. The indium-doped copper foil was then rolled to prepare a copper-indium alloy substrate with a thickness of 10 micrometers.

[0053] The ultrathin lithium-copper-indium alloy composite strip of the present invention is obtained by melt-coating a 35-micrometer-thick layer of metallic lithium onto the surface of a copper-indium alloy substrate.

[0054] Example 5

[0055] The composite lithium anode prepared in Example 2 of CN109742323 A is composed of a 10-micrometer thick copper foil, a 2-micrometer indium transition layer and a 35-micrometer thick lithium foil.

[0056] The ultrathin lithium-doped copper foil composite strips and composite lithium anodes obtained in the above embodiments were subjected to peel strength tests. The peel strength test method followed GB 2792-1998. First, the samples obtained in the above embodiments were cut into strips of 25mm*125mm. Then, pressure-sensitive double-sided adhesive was applied to the surface of the test sample, with the other side adhered to a stainless steel plate. The test sample and the stainless steel plate were fixed on the fixture of the peel strength testing equipment, and a 180-degree peel test was performed at a relative speed of 300mm / min. The peel strength test results of the above embodiments are listed in Table 1.

[0057] Table 1. Peel strength test results for each embodiment.

[0058] Example Peel strength N / 25mm Example 1 6.9 Example 2 7.2 Example 3 7.5 Example 4 8.3 Example 5 2.5

[0059] As shown in Table 1, the doped copper foil obtained through doping can produce an ultrathin lithium-doped copper foil composite tape with high peel strength. Comparison of Examples 4 and 5 also reveals that the ultrathin lithium-doped copper foil composite tape prepared by this invention not only eliminates the need for a transition layer structure, resulting in a lightweight and thin electrode structure, but also exhibits very high interfacial bonding strength between metallic lithium and the doped copper foil substrate. This is entirely due to the bonding effect of the metallic lithium and the dopant elements in the doped copper foil substrate at the "anchor points".

[0060] CR2032 coin cells were prepared using the ultrathin lithium-doped copper foil composite strip and composite lithium anode obtained in Examples 4 and 5. The steps are as follows: First, the ultrathin lithium-copper composite strip and composite lithium anode obtained in Examples 4 and 5 were punched into circular pieces with a diameter of 15 mm. These were used as working electrodes and assembled into coin cells with commercially available lithium metal sheets, which served as counter electrodes. The electrolyte composition was: 1 mol / L LiPF6 solute and EC and EMC solvents (volume ratio 1:1). The separator was a polypropylene membrane. Electrochemical impedance spectroscopy was first performed on the assembled coin cells at a test temperature of 25°C. The test results are as follows: Figure 1 As shown. The battery was subjected to charge-discharge cycle testing after the initial test. The program was set as follows: 5 hours of rest, 2 hours of constant current charging, 2 hours of constant current discharging, and a cycle current of 1.5 mA / cm². The battery cycle test results after 100 hours are shown below. Figure 2 As shown.

[0061] from Figure 1 It can be seen that after assembling coin cells using the ultrathin lithium-doped copper foil composite strip and composite lithium anode as anodes in Examples 4 and 5, the cells exhibit significant differences in the mid-frequency region of the electrochemical impedance spectroscopy. The interface impedance of Example 4 is approximately 9.5 Ω, while that of Example 5 is approximately 15 Ω. This indicates that the lithium metal anode prepared using the ultrathin lithium-doped copper foil composite strip of Example 4 has a smaller interface impedance. That is, under the same lithium-ion transport environment, the electron transport inside the electrode made of the ultrathin lithium-doped copper foil composite strip is faster. This is mainly due to the good interfacial bonding between lithium metal and the doped copper foil substrate.

[0062] from Figure 2It can be seen that when the anodes prepared by the ultrathin lithium-doped copper foil composite strip and the composite lithium anode in Examples 4 and 5 are used for cycle performance testing, the initial polarization voltage of the electrode prepared by the ultrathin lithium-doped copper foil composite strip in Example 4 is lower than that of the electrode prepared by the composite lithium anode in Example 5. As the cycle progresses, the polarization voltage of the electrode prepared by the ultrathin lithium-doped copper foil composite strip in Example 4 does not change much, and its polarization voltage is still less than 0.05V after 100 hours of cycling. However, the polarization voltage of the electrode prepared by the composite lithium anode in Example 5 increases continuously, and its polarization voltage reaches close to 0.1V after 100 hours of cycling.

[0063] As can be seen from the above, the negative electrode prepared by the ultrathin lithium-doped copper foil composite strip of the present invention has strong interfacial bonding and low interfacial impedance between metallic lithium and the doped copper foil substrate, resulting in a stable electrode structure and good battery cycle performance after battery preparation.

[0064] It is understood that, although the ultrathin lithium-doped copper foil composite tape of the present invention has been described in detail in conjunction with specific doping elements and preparation methods of doped copper foil substrates in the embodiments of the present invention, the above description is merely for the purpose of meeting legal requirements, and the present invention is not limited to the given embodiments. Those skilled in the art can replicate the ultrathin lithium-doped copper foil composite tape through appropriate operations based on the disclosure and teachings of the specification.

[0065] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make appropriate changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. An ultrathin lithium-doped copper foil composite strip, characterized in that, The composite tape includes: Doped copper foil is a copper foil substrate containing dopant elements, wherein the dopant elements are distributed in the copper foil substrate in a gradient distribution with decreasing concentration from the surface inwards, wherein the mass fraction of the dopant elements in the copper foil substrate is 5% to 60%, and wherein in the surface layer of the copper foil substrate, the dopant elements are distributed in a distribution with a mass fraction decreasing from 75% to 10%; and An ultrathin lithium metal layer with a thickness of 10 nanometers to 100 micrometers is composited on the surface layer. The doping element includes at least one of the following: an element capable of forming alloys with both metallic copper and metallic lithium, and an element capable of eutectic with metallic copper and forming alloys with metallic lithium.

2. The ultrathin lithium-doped copper foil composite strip according to claim 1, characterized in that, The composite strip also includes a protective layer attached to the surface of the ultrathin lithium metal layer opposite to the copper foil substrate.

3. The ultrathin lithium-doped copper foil composite strip according to claim 1, characterized in that, The elements capable of forming alloys with both copper and lithium include indium, tin, aluminum, zinc, and boron; the elements capable of eutectic with copper and forming alloys with lithium include silver, cadmium, lead, and carbon.

4. The ultrathin lithium-doped copper foil composite strip according to claim 1, characterized in that, The composite tape satisfies at least one of the following conditions: (1) The thickness of the surface layer is 10 nanometers to 3 micrometers; (2) The thickness of the copper foil substrate is 3 micrometers to 50 micrometers.

5. The ultrathin lithium-doped copper foil composite strip according to claim 2, characterized in that, The protective layer includes at least one of an organic polymer protective layer, an inorganic protective layer, and an organic-inorganic composite protective layer.

6. A method for preparing an ultrathin lithium-doped copper foil composite strip as described in any one of claims 1-5, characterized in that, The method includes the following steps: Step 1: Prepare a doped copper foil with a gradient distribution of doped elements; Step 2: Clean the surface of the copper-doped foil; Step 3: Apply an ultrathin lithium metal layer to the surface.

7. The method according to claim 6, characterized in that, Step 1 includes two methods: Method 1: Expose the copper foil substrate to an environment in which doping elements coexist in gaseous, liquid, or solid phases, or any two or three phases. Perform heat treatment at a high temperature of 600℃ to 3000℃ to achieve a gradient distribution of doping elements in the copper foil substrate through solid-phase diffusion and / or melting and mixing. Method 2: Prepare doped copper foil by electrodeposition, wherein the concentration of dopant elements in the electrolyte during electrodeposition is controlled to achieve a gradient distribution of dopant elements in the copper foil substrate.

8. The method according to claim 6, characterized in that, The cleaning of the surface of the doped copper foil includes at least one of pickling, corona treatment, and plasma treatment.

9. The method according to claim 6, characterized in that, The application of ultrathin lithium metal layers includes at least one of vapor deposition, magnetron sputtering, electroplating, thermal diffusion, and melt coating.

10. The method according to claim 6, characterized in that, The method further includes: Step 4: Form a protective layer on the surface opposite to the copper foil substrate of the ultrathin lithium metal layer.

11. The application of the ultrathin lithium-doped copper foil composite strip as described in any one of claims 1 to 5 in lithium batteries.

12. The application according to claim 11, characterized in that, The lithium batteries include lithium-ion batteries, quasi-solid-state batteries, all-solid-state batteries, lithium-sulfur batteries, and lithium-oxygen batteries.

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