A carbon-coated aluminum foil and an electrode sheet for a secondary battery

Abstract

The present application relates to the field of battery technologies, and particularly to a carbon-coated aluminum foil and an electrode tab for a secondary battery. The carbon-coated aluminum foil includes an aluminum foil and a functional carbon layer coated on at least one surface of the aluminum foil. It is characterized in that the functional carbon layer includes graphene quantum dots and conductive carbon black, and the graphene quantum dots are dispersed in the conductive carbon black; the weight percentage A of the graphene quantum dots, the weight percentage B of the conductive carbon black, the volatile content X of the graphene quantum dots, the volatile content Y of the conductive carbon black, the sheet diameter distribution width S1 of the graphene quantum dots, and the secondary particle size distribution width S2 of the conductive carbon black in the mixed slurry for forming the functional carbon layer satisfy the following relational expression: 0.35 ≤ 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 ≤ 5.5. The technical solution of the present application can optimize the peel strength of the functional carbon layer while achieving excellent conductivity of the functional carbon layer, reduce the tab resistance and the AC internal resistance, and improve the rate performance.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly relates to a carbon-coated aluminum foil and an electrode sheet for a secondary battery. Background Art

[0002] Carbon-coated aluminum foils are widely used in the field of secondary batteries and can collect the microcurrents of active substances, thereby reducing the contact resistance between the positive / negative electrode materials and the current collector. In current secondary batteries, a conductive carbon black / graphene composite conductive paste is commonly used to coat the surface of the aluminum foil to prepare the carbon-coated aluminum foil, so as to improve the static conductivity and further improve the overall performance of the battery. However, the contact between graphene and the aluminum foil in this carbon-coated aluminum foil is poor, and there are a large number of voids on the contact surface, resulting in the inability of electrons to pass smoothly, affecting the high-rate charge and discharge performance, and the improvement of battery performance is not ideal; moreover, the large-diameter graphene sheets are mainly SP2 hybridized, and the interaction force with the active substance coating is small, resulting in a low peeling force. Summary of the Invention

[0003] In view of the above problems of the prior art, the present application provides a carbon-coated aluminum foil and an electrode sheet for a secondary battery, which can effectively reduce the resistance of the carbon-coated aluminum foil, improve the electrical performance of the battery, and improve the peeling strength of the carbon-coated aluminum foil. The specific technical solutions are as follows:

[0004] On the one hand, the present application provides a carbon-coated aluminum foil, including an aluminum foil and a functional carbon layer coated on at least one surface of the aluminum foil, where the functional carbon layer includes graphene quantum dots and conductive carbon black, and the graphene quantum dots are dispersed in the conductive carbon black;

[0005] The weight percentage A of the graphene quantum dots, the weight percentage B of the conductive carbon black, the volatile content X of the graphene quantum dots, the volatile content Y of the conductive carbon black, the particle size distribution width S1 of the graphene quantum dots, and the secondary particle size distribution width S2 of the conductive carbon black in the mixed slurry for forming the functional carbon layer satisfy the following relational expression: 0.35 ≤ 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 ≤ 5.5.

[0006] Specifically, the weight percentage A of the graphene quantum dots, the volatile content X of the graphene quantum dots, and the particle size distribution width S1 of the graphene quantum dots satisfy 0.15 ≤ 100A*(1 - X) / S1 ≤ 1.5; the weight percentage B of the conductive carbon black in the functional carbon layer, the volatile content Y of the conductive carbon black, and the secondary particle size distribution width S2 of the conductive carbon black satisfy 0.2 ≤ 100B*(1 - Y) / S2 ≤ 4.0.

[0007] Specifically, the weight percentage A of the graphene quantum dots in the mixed slurry for forming the functional carbon layer satisfies 0.5% ≤ A ≤ 1.5%.

[0008] Specifically, the volatile content X of the graphene quantum dots satisfies 2% ≤ X ≤ 7%.

[0009] Specifically, the weight percentage B of the conductive carbon black in the mixed slurry for forming the functional carbon layer is 2% ≤ B ≤ 6%.

[0010] Specifically, the volatile content Y of the conductive carbon black satisfies 1% ≤ Y ≤ 4%.

[0011] Specifically, the particle size distribution width S1 of the graphene quantum dots satisfies 1.1 ≤ S1 ≤ 2.8.

[0012] Specifically, the secondary particle size distribution width S2 of the conductive carbon black satisfies 1.5 ≤ S2 ≤ 9.6.

[0013] Specifically, the calculation formula for the particle size distribution width S1 of the graphene quantum dots is: S1 = (D3 - D1) / D2; D1 represents the particle size corresponding to when the cumulative volume percentage of the graphene quantum dots in the aqueous solvent reaches the first preset ratio, D2 represents the particle size corresponding to when the cumulative volume percentage of the graphene quantum dots in the aqueous solvent reaches the second preset ratio, D3 represents the particle size corresponding to when the cumulative volume percentage of the graphene quantum dots in the aqueous solvent reaches the third preset ratio, and the first preset ratio, the second preset ratio, and the third preset ratio increase in sequence;

[0014] The particle size of the graphene quantum dots satisfies at least one of 5 ≤ D1 ≤ 10 nm, 25 ≤ D2 ≤ 40 nm, and 55 ≤ D3 ≤ 75 nm.

[0015] Specifically, the calculation formula for the secondary particle size distribution width S2 of the conductive carbon black is: S2 = (D6 - D4) / D5; D4 represents the particle size corresponding to when the cumulative volume percentage of the conductive carbon black in the aqueous solvent reaches the fourth preset ratio, D5 represents the particle size corresponding to when the cumulative volume percentage of the conductive carbon black in the aqueous solvent reaches the fifth preset ratio, D6 represents the particle size corresponding to when the cumulative volume percentage of the conductive carbon black in the aqueous solvent reaches the sixth preset ratio, and the fourth preset ratio, the fifth preset ratio, and the sixth preset ratio increase in sequence;

[0016] The particle size of the conductive carbon black satisfies at least one of 0.2 ≤ D4 ≤ 0.4 μm, 1 ≤ D5 ≤ 5 μm, and 8 ≤ D6 ≤ 15 μm.

[0017] On the other hand, the present application provides an electrode tab for a secondary battery, and the electrode tab includes the carbon-coated aluminum foil as described above.

[0018] On the other hand, the present application provides a secondary battery, and the secondary battery includes the carbon-coated aluminum foil as described above.

[0019] On the other hand, the present application provides an electronic device, and the electronic device includes the secondary battery as described above.

[0020] Based on the above technical solution, the present application has the following beneficial effects:

[0021] In the technical solution of the present application, a functional carbon layer including graphene quantum dots and conductive carbon black is coated on the aluminum foil, and the graphene quantum dots are dispersed in the conductive carbon black. By controlling the weight percentage A of the graphene quantum dots, the weight percentage B of the conductive carbon black, the volatile content X of the graphene quantum dots, the volatile content Y of the conductive carbon black, the particle size distribution width S1 of the graphene quantum dots, and the secondary particle size distribution width S2 of the conductive carbon black to satisfy the following relational expression: 0.35 ≤ 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 ≤ 5.5, it is possible to synergistically regulate the dispersibility of the graphene quantum dots in the conductive carbon black and the ratio between the SP2 hybridization and the SP3 hybridization in the functional carbon layer to an optimal state, improve the binding force between the carbon layer and the active material layer, thereby enhancing the interlayer peeling force, increasing the contact area between the aluminum foil and the functional carbon layer in the carbon-coated aluminum foil, reducing the voids in the contact surface, further increasing the electron channel amount, and optimizing the electrical properties. In addition, a "plane-plane" two-dimensional conductive network structure is formed between the graphene quantum dots, and a "point-line" (zero-dimensional and one-dimensional) conductive structure is formed between the graphene quantum dots and the conductive carbon black, thereby constituting a three-dimensional conductive network structure in space. While improving the contact between the functional carbon layer and the aluminum foil and optimizing the peeling force of the functional carbon layer, excellent conductivity of the functional carbon layer is achieved, the pole piece resistance and the AC internal resistance are reduced, and the rate performance is improved. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0023] Figure 1 It is a SEM image of the aluminum foil provided by the embodiment of the present application;

[0024] Figure 2 It is a SEM image of the carbon-coated aluminum foil provided by the embodiment of the present application;

[0025] Figure 3 It is the 25°C / 3C discharge curves of Examples 1 to 3, Comparative Examples 1 to 4, and Blank Example 1 provided by the embodiment of the present application. Detailed Embodiments

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the scope of protection of the present application.

[0027] For the terms defined below, unless a different definition is given elsewhere in the claims or in this specification, these definitions shall apply. All numerical values, whether or not explicitly indicated, are hereby defined as being modified by the term "about". The term "about" generally refers to a numerical range that a person of ordinary skill in the art would consider equivalent to the stated value to produce substantially the same properties, functions, results, etc. A numerical range indicated by a lower value and a higher value is defined to include all the numerical values included in that numerical range and all the sub-ranges included in that numerical range.

[0028] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances so that the embodiments of the present application described here can be implemented in an order other than those illustrated or described here. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion.

[0029] The following introduces the carbon-coated aluminum foil provided by the embodiments of the present application, which includes an aluminum foil and a functional carbon layer coated on at least one surface of the aluminum foil. The functional carbon layer includes graphene quantum dots and conductive carbon black, and the graphene quantum dots are dispersed in the conductive carbon black. The weight percentage A of the graphene quantum dots, the weight percentage B of the conductive carbon black, the volatile content X of the graphene quantum dots, the volatile content Y of the conductive carbon black, the sheet diameter distribution width S1 of the graphene quantum dots, and the secondary particle size distribution width S2 of the conductive carbon black in the mixed slurry for forming the functional carbon layer satisfy the following relational formula: 0.35 ≤ 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 ≤ 5.5. In this way, it is possible to synergistically regulate the dispersibility of the graphene quantum dots in the conductive carbon black and the ratio between the sp2 hybridization and sp3 hybridization in the functional carbon layer to an optimal state, improve the binding force between the carbon layer and the active material layer, thereby enhancing the interlayer peeling force, and increasing the contact area between the aluminum foil and the functional carbon layer in the carbon-coated aluminum foil, reducing the voids in the contact surface, thereby increasing the electron channel volume, optimizing the electrical properties. In addition, a "plane-plane" two-dimensional conductive network structure is formed between the graphene quantum dots, and a "point-line" (zero-dimensional and one-dimensional) conductive structure is formed between the graphene quantum dots and the conductive carbon black, thereby constituting a three-dimensional conductive network structure in space. While improving the contact between the functional carbon layer and the aluminum foil and optimizing the peeling force of the functional carbon layer, the resistivity of the carbon-coated aluminum foil is reduced, the excellent conductivity of the functional carbon layer is realized, the resistance of the electrode sheet and the AC internal resistance are reduced, and the rate performance is improved.

[0030] Specifically, the value of 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 can be any point value between 0.35 and 5.5. The lower limit value of 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 can include the following values: 0.35, 0.37, 0.95, or 1.51, etc. The upper limit value of 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 can include the following values: 5.5, 5.3, 4.5, or 3.96, etc.

[0031] In the embodiments of the present application, the graphene quantum dots are uniformly dispersed in the conductive carbon black, and some of the graphene quantum dots are in contact with the surface of the aluminum foil. In this way, good contact between the functional carbon layer and the aluminum foil is achieved, which is beneficial to improving the peeling strength of the carbon-coated aluminum foil and increasing the effective electron transfer area, thereby improving the battery performance, especially significantly improving the high-rate charge and discharge performance.

[0032] In the embodiments of the present application, some graphene quantum dots are filled in the protrusions or depressions on the surface of the aluminum foil. For example, when the size of the protrusions or depressions on the surface of the aluminum foil is less than or equal to 100 nm, some graphene quantum dots with a diameter less than or equal to 100 nm and a narrow diameter distribution width S1 value (such as S1 ≤ 2.8) can be filled on the above-mentioned protrusions or depressions to achieve effective filling of the aluminum foil surface, thereby increasing the number of electron channels between active substances such as conductive carbon black in the functional carbon layer and the aluminum foil, and significantly improving the battery performance. At the same time, by adjusting each parameter to meet the above relationship, effective filling of the aluminum foil surface is achieved, the gap amount between the aluminum foil and the functional carbon layer is reduced, and at the same time, the coating voids caused by the overlapping structure of the graphene quantum dots and the conductive carbon black are weakened, the contact area between the two is increased, and thus the bonding strength between the two is improved, and the peel strength of the carbon-coated aluminum foil is optimized.

[0033] Please refer to Figure 1 , Figure 1 shows an SEM image of an aluminum foil. It can be seen that its surface is uneven, with depression and protrusion structures formed (marked by the dotted square in the middle of the figure), which affect its contact with the coating, as well as the conductivity and peel force. Please refer to Figure 2 , Figure 2 provides an SEM image of the carbon-coated aluminum foil prepared by the technical solution of the present application. As shown in the figure, on the surface of the carbon-coated aluminum foil, graphene quantum dots are filled in the gaps such as the depressions of the aluminum foil and form a three-dimensional conductive network structure with the conductive carbon black.

[0034] In the embodiments of the present application, the weight percentage A of graphene quantum dots, the volatile content X of graphene quantum dots, and the diameter distribution width S1 of graphene quantum dots in the mixed slurry for forming the functional carbon layer satisfy 0.15 ≤ 100A*(1 - X) / S1 ≤ 1.5; the weight percentage B of conductive carbon black, the volatile content Y of conductive carbon black, and the secondary particle size distribution width S2 of conductive carbon black in the functional carbon layer satisfy 0.2 ≤ 100B*(1 - Y) / S2 ≤ 4.0. In this way, the dispersion of graphene quantum dots in conductive carbon black is further optimized, and the ratio between SP2 hybridization and SP3 hybridization in the functional carbon layer is further optimized. Moreover, the effective carbon content, volatile content, and diameter distribution width of graphene quantum dots are balanced, thereby balancing the effective carbon content, the proportion of SP2 / SP3 hybridization, and the dispersion in the solvent of graphene quantum dots. At the same time, the effective carbon content, the proportion of SP2 / SP3 hybridization, and the dispersion in the solvent of conductive carbon black are balanced, ensuring that both graphene quantum dots and conductive carbon black have good intrinsic conductivity and solvent dispersion, and achieving better improvement in battery performance.

[0035] Specifically, the value of 100A*(1-X) / S1 can be any point value between 0.15 and 1.5. The lower limit value of 100A*(1-X) / S1 can be, for example, 0.15, 0.17, 0.3, or 0.5, etc. The upper limit value of 100A*(1-X) / S1 can be, for example, 1.5, 1.4, 1.3, or 1.2, etc. When 100A*(1-X) / S1 is too large, there is a situation where the content of graphene quantum dots is too high, resulting in difficult dispersion; or the volatile content of graphene quantum dots is too low, resulting in difficult dispersion; or the particle size distribution range is too wide, leading to poor processability and stability of the slurry. When 100A*(1-X) / S1 is too small, there is a situation where the content of graphene quantum dots is too low to achieve sufficient coating of the uneven parts on the surface of the aluminum foil; or the volatile content of graphene quantum dots is too high and the effective carbon content is low, resulting in poor conductivity; or the particle size distribution is too narrow, and the production efficiency of quantum dots is low and the cost is too high.

[0036] Specifically, the value of 100B*(1-Y) / S2 can be any point value between 0.2 and 4.0. The lower limit value of 100B*(1-Y) / S2 can be, for example, 0.2, 0.5, 0.9, or 1.3, etc. The upper limit value of 100B*(1-Y) / S2 can be, for example, 4.0, 3.5, 3.1, or 2.9, etc. When 100B*(1-Y) / S2 is too large, there is a situation where the carbon black addition amount is too high, resulting in difficult dispersion; or the volatile content of graphene quantum dots is too low, resulting in difficult dispersion; or the particle size distribution range is too wide, resulting in poor processability and stability of the slurry. When 100B*(1-Y) / S2 is too small, there is a situation where the carbon black content is too low and it is not easy to form a good conductive network structure; or the volatile content of carbon black is too high and the effective carbon content is low, resulting in poor conductivity; or the particle size distribution is too narrow and the oil absorption value of carbon black is too low, which is not conducive to the infiltration of the electrolyte.

[0037] In the embodiments of the present application, the weight percentage A of graphene quantum dots in the mixed slurry for forming the functional carbon layer satisfies 0.5% ≤ A ≤ 1.5%. By controlling the weight percentage of graphene quantum dots within the above range, the good dispersion degree of graphene quantum dots in the functional carbon layer is ensured, and at the same time, the effective coverage and filling of the gaps and protrusions on the surface of the aluminum foil are ensured.

[0038] Specifically, A can be any point value between 0.5% and 1.5%. The lower limit value of A can be, for example, 0.5%, 0.6%, 0.7%, or 0.8%, etc. The upper limit value of A can be, for example, 1.5%, 1.4%, 1.3%, or 1.2%, etc. When A is too large, it is easy to cause difficult dispersion of graphene quantum dots and easy secondary aggregation. When A is too small, it is impossible to achieve reasonable coverage of the gaps and protrusions on the surface of the aluminum foil, reducing the filling degree, and further reducing the conductivity and peel strength.

[0039] In the embodiments of the present application, the volatile content X of the graphene quantum dots satisfies 2% ≤ X ≤ 7%. By setting X within the above range, a reasonable effective carbon content and the SP2 / SP3 hybridization ratio of the graphene quantum dots can be ensured, synergistically optimizing their electrical conductivity and dispersibility. At the same time, in combination with the reasonable value of the above-mentioned A, the proportion of the "plane-plane" two-dimensional conductive network structure between the graphene quantum dots and the "point-line" conductive structure between the graphene quantum dots and the conductive carbon black in the three-dimensional conductive network structure can be optimized, thereby reducing the resistance and improving the electrical performance of the battery.

[0040] Specifically, X can be any point value between 2% and 7%. The lower limit value of X can be, for example: 2%, 2.2%, 2.5% or 2.8%, etc. The upper limit value of X can be, for example: 7%, 6.5%, 6% or 5.5%, etc. When X is too large, the effective content of the graphene quantum dots is low, the proportion of SP2 hybridization is too small, and the proportion of SP3 hybridization is too large, resulting in a decrease in intrinsic electrical conductivity and affecting the electrical performance. When X is too small, the effective carbon content of the graphene quantum dots is too high, the proportion of SP2 hybridization is high, it is difficult to disperse, and secondary aggregation is likely to occur, thereby affecting the stability of the slurry.

[0041] In the embodiments of the present application, the weight percentage B of the conductive carbon black in the mixed slurry forming the functional carbon layer is 2% ≤ B ≤ 6%. By controlling the weight percentage of the conductive carbon black within the above range, effective lap joint between the conductive carbon black and the graphene quantum dots is ensured to form a zero-dimensional and one-dimensional conductive structure, and then a three-dimensional conductive network is formed with the two-dimensional graphene quantum dots, ensuring the electron channel amount in the three-dimensional conductive network structure.

[0042] Specifically, B can be any point value between 2% and 6%. The lower limit value of B can be, for example: 2%, 2.2%, 2.8% or 3%, etc. The upper limit value of B can be, for example: 6%, 5.5%, 5% or 4.5%, etc. When B is too large, the carbon black content is too high and it is difficult to disperse. When B is too small, the carbon black content is too low and it is not easy to form a good conductive network structure.

[0043] In the embodiments of the present application, the volatile content Y of the conductive carbon black satisfies 1% ≤ Y ≤ 4%. By setting Y within the above range, a reasonable effective carbon content and the SP2 / SP3 hybridization ratio of the conductive carbon black can be ensured, synergistically optimizing its electrical conductivity and dispersibility. At the same time, in combination with the reasonable values of the above-mentioned A, B and X, the proportion of the "point-line" conductive structure between the conductive carbon black and the graphene quantum dots in the three-dimensional conductive network structure can be optimized, thereby reducing the resistance and improving the electrical performance of the battery.

[0044] Specifically, Y can be any point value between 1% and 4%. The lower limit value of Y can be, for example: 1%, 1.2%, 1.5% or 1.8%, etc. The upper limit value of Y can be, for example: 4%, 3.5%, 3% or 2.5%, etc. When Y is too large, the effective content of conductive carbon black is low, the proportion of SP2 hybridization is too small, and the proportion of SP3 hybridization is too large, resulting in a decrease in intrinsic conductivity, making it difficult to form a good conductive network structure and affecting the electrical properties. When Y is too small, the effective carbon content of conductive carbon black is too high, increasing the dispersion difficulty and prone to secondary aggregation, thereby affecting the stability of the slurry.

[0045] In the embodiments of the present application, the particle size distribution width S1 of the graphene quantum dots satisfies 1.1 ≤ S1 ≤ 2.8. S1 characterizes the particle size distribution of the graphene quantum dots and is determined based on the particle diameters of the graphene quantum dots at different cumulative volume percentages in the aqueous solvent. By setting the particle size distribution parameter of the graphene quantum dots to the above values, a reasonable grading of the particle sizes can be achieved, enabling it to meet the filling and covering requirements of various size gaps and protrusions on the aluminum foil surface, reducing the amount of gaps and increasing the contact area between the carbon layer and the aluminum foil. At the same time, the graphene quantum dots are evenly interspersed in the conductive carbon black to form a good three-dimensional conductive network structure, and improve the processability, stability of the slurry, and the production efficiency of the quantum dots.

[0046] Specifically, S1 can be any point value between 1.1 and 2.8. The lower limit value of S1 can be, for example: 1.1, 1.3, 1.5 or 1.6, etc. The upper limit value of S1 can be, for example: 2.8, 2.6, 2.3 or 2.0, etc. When S1 is too narrow, the particle size distribution is too narrow, the production efficiency of the quantum dots is low, and the cost is too high. When S1 is too wide, the dispersion difficulty of the graphene quantum dots is increased, prone to aggregation, reducing the stability and conductivity of the slurry.

[0047] In the embodiments of the present application, the secondary particle size distribution width S2 of the conductive carbon black satisfies 1.5 ≤ S2 ≤ 9.6. S2 characterizes the particle size distribution of the conductive carbon black and is determined based on the particle diameters of the conductive carbon black at different cumulative volume percentages in the aqueous solvent. By setting the secondary particle size distribution width of the conductive carbon black to the above values, a reasonable grading of the secondary particle sizes of the conductive carbon black can be achieved, enabling it to form a reasonable structural construction with the graphene quantum dots, weakening the coating voids caused by the overlapping structure of the graphene quantum dots and the conductive carbon black, so as to achieve good peel strength of the carbon-coated aluminum foil, prevent the problem of a large increase in the internal resistance of the battery cell caused by volume expansion during high-temperature storage, and prevent the possibility of voltage drop caused by low peel strength during cycling.

[0048] Specifically, S2 can be any point value between 1.5 and 9.6. The lower limit value of S2 can be, for example: 1.5, 2, 2.5, or 3, etc. The upper limit value of S2 can be, for example: 9.6, 9, 8.3, or 7.7, etc. In the case where S2 is too narrow, the particle size distribution is too narrow, and the oil absorption value of the carbon black is too low, which is not conducive to the infiltration of the electrolyte. In the case where S2 is too wide, the particle size distribution is too large, and the processing performance of the carbon black and the stability of the slurry are poor.

[0049] In the embodiments of the present application, the calculation formula for the sheet diameter distribution width S1 of the graphene quantum dots is: S1 = (D3 - D1) / D2; D1 represents the sheet diameter corresponding to the graphene quantum dots when the cumulative volume percentage in the water solvent reaches the first preset ratio, D2 represents the sheet diameter corresponding to the graphene quantum dots when the cumulative volume percentage in the water solvent reaches the second preset ratio, D3 represents the sheet diameter corresponding to the graphene quantum dots when the cumulative volume percentage in the water solvent reaches the third preset ratio, and the first preset ratio, the second preset ratio, and the third preset ratio increase in sequence; specifically, the first preset ratio is 5% - 15%, preferably 10%, the second preset ratio is 45% - 60%, preferably 50%, and the third preset ratio is 80% - 95%, preferably 90%.

[0050] Optionally, the sheet diameter of the graphene quantum dots satisfies at least one of 5 ≤ D1 ≤ 10 nm, 25 ≤ D2 ≤ 40 nm, and 55 ≤ D3 ≤ 75 nm. In this way, by setting the above-mentioned grading sizes of the graphene quantum dots, it is possible to ensure that the size of the graphene quantum dots matches the size between the surface gaps and protrusions of the aluminum foil, meeting the requirements for surface filling and covering of the aluminum foil.

[0051] In the embodiments of the present application, the calculation formula for the secondary particle size distribution width S2 of the conductive carbon black is: S2 = (D6 - D4) / D5; D4 represents the sheet diameter corresponding to the conductive carbon black when the cumulative volume percentage in the water solvent reaches the fourth preset ratio, D5 represents the sheet diameter corresponding to the conductive carbon black when the cumulative volume percentage in the water solvent reaches the fifth preset ratio, D6 represents the sheet diameter corresponding to the conductive carbon black when the cumulative volume percentage in the water solvent reaches the sixth preset ratio, and the fourth preset ratio, the fifth preset ratio, and the sixth preset ratio increase in sequence; specifically, the fourth preset ratio is 5% - 15%, preferably 10%, the fifth preset ratio is 45% - 60%, preferably 50%, and the sixth preset ratio is 80% - 95%, preferably 90%.

[0052] Optionally, the secondary particle size of the conductive carbon black satisfies at least one of 0.2 ≤ D4 ≤ 0.4 μm, 1 ≤ D5 ≤ 5 μm, and 8 ≤ D6 ≤ 15 μm. Thus, by setting the above-mentioned grading size of the conductive carbon black, good dispersion of the conductive carbon black as a one-dimensional conductive carbon material can be achieved, and it is ensured that the carbon black has a sufficient oil absorption value, which is beneficial to the infiltration of the electrolyte. In the embodiment of the present application, the functional carbon layer may further include a binder. Preferably, the binder may include but is not limited to PAA; the functional carbon layer may further include a dispersant. Preferably, the dispersant may include but is not limited to PVP.

[0053] In summary, by controlling the addition amount, volatile content, and sheet diameter distribution width value of the graphene quantum dots, a good "plane-plane" two-dimensional conductive network structure is achieved; then, by controlling the addition amount, volatile content, and secondary particle size distribution width value of the conductive carbon black, a good "point-line" zero-dimensional and one-dimensional conductive structure is achieved. By combining the zero-dimensional, one-dimensional, and two-dimensional conductive network structures, excellent conductivity of the functional carbon layer is realized. Moreover, by limiting that the addition amount, volatile content, and particle size distribution width value of the graphene quantum dots and conductive carbon black used in the functional carbon layer satisfy the foregoing relational expressions and value ranges, the peel strength of the electrode sheet is significantly improved, the sheet resistance is reduced, and the internal resistance growth of the battery cell before and after high-temperature storage and the rate discharge performance are improved. In addition, graphene quantum dots with a sheet diameter < 100 nm and a narrow sheet diameter distribution width value can fill the bumps or gaps on the surface of the aluminum foil, achieving good contact between the functional carbon layer and the aluminum foil, reducing the resistance of the carbon-coated aluminum foil, and reducing the contact resistance between the foil and the positive active material coating, improving the conductivity and peel strength of the carbon-coated aluminum foil, and using conductive carbon black with a smaller secondary particle size and a moderate secondary particle size distribution width value to weaken the coating voids caused by the overlapping structure of the graphene quantum dots and the conductive carbon black, optimizing and enhancing the peel strength of the carbon-coated aluminum foil, preventing a significant increase in the internal resistance of the battery cell caused by volume expansion during high-temperature storage, and preventing the diving problem caused by low peel strength during the cycling process. Based on the technical solution of the present application, the peel strength of the cold-pressed electrode sheet can be increased to more than 0.47 N, at least increased by more than 30%, the resistance of the cold-pressed positive electrode sheet is reduced to less than 0.62 Ω, at least reduced by more than 15%, the internal resistance of the battery cell before and after storage at 65 °C for 30 d is reduced to less than 8.7, at least reduced by 5%, and the discharge specific capacity at 25 °C and 3C reaches more than 113 mAh / g, at least increased by 2%.

[0054] On the other hand, the present application provides an electrode sheet, and the electrode sheet includes the carbon-coated aluminum foil as described above. Specifically, the electrode sheet may be a positive electrode sheet. Correspondingly, a positive active material coating is coated on at least one surface of the carbon-coated aluminum foil.

[0055] On the other hand, the present application provides a secondary battery, including the above-mentioned electrode sheet, separator, and electrolyte. The secondary battery may include but is not limited to lithium-ion secondary batteries, sodium-ion secondary batteries, etc.

[0056] On the other hand, the present application provides an electronic device, which includes the electrode tab or the secondary battery as described above. Specifically, the electronic device may include, but is not limited to, a computer, a mobile phone, a wearable electronic device, a vehicle-mounted terminal device, a VR device, and other electronic terminal devices that require secondary batteries, etc.

[0057] On the other hand, the present application provides a preparation method of carbon-coated aluminum foil, including the following steps: mixing graphene quantum dots, conductive carbon black, PAA, and PVP according to a weight ratio of (0.5~1.5):(2~6):20:0.2 to obtain a mixed slurry, adding deionized water to mix and stir these materials evenly, and then coating at least one surface of the aluminum foil to form a functional carbon layer, and drying to obtain the carbon-coated aluminum foil.

[0058] Specifically, the single-sided coating areal density of the aluminum foil is 0.2~0.4g / m 2 , and the coating thickness is 0.2~0.8μm. Preferably, the single-sided coating areal density is 0.3±0.06g / m 2 , and the coating thickness is 0.5±0.2μm.

[0059] The main function of the carbon-coated aluminum foil is to provide excellent static conductive performance, collect the microcurrent of the active material, greatly reduce the contact resistance between the positive electrode material and the current collector, and improve the adhesion between the two, preventing the diving problem caused by low peeling force during the cycle. The present application uses a one-time coating method to prepare the carbon-coated aluminum foil, which not only improves the peeling force of the carbon-coated aluminum foil, but also reduces the internal resistance of the foil and the contact resistance between the foil and the positive electrode active material coating, ensuring the conductive consistency of the electrode tab. At the same time, the process is simple, the coating layer is thin, reducing the process complexity and material cost of mass production.

[0060] On the other hand, the present application provides a preparation method of a positive electrode tab, including the following steps: mixing lithium iron phosphate, conductive carbon black, and PVDF according to a first preset weight ratio, adding NMP as a solvent, mixing and stirring these materials evenly, and then coating them on the above-mentioned carbon-coated aluminum foil respectively, and obtaining the positive electrode tab after processes such as drying, cold pressing, and die cutting.

[0061] Preferably, the first preset weight ratio can be 97:1:2.

[0062] On the other hand, the present application provides a preparation method of a negative electrode tab, including the following steps: mixing graphite, CMC, SBR, and conductive carbon black according to a second preset weight ratio, adding deionized water as a solvent to mix and stir these materials evenly, and then coating them on a copper foil, and obtaining the negative electrode tab after processes such as drying, cold pressing, and die cutting.

[0063] Preferably, the second preset weight ratio can be 96:1.3:1.7:1.

[0064] On the other hand, the present application provides a method for preparing a secondary battery, including the following steps: stacking the above-mentioned negative electrode sheet, separator, and positive electrode sheet in sequence, with the separator acting as an insulator between the positive and negative electrode sheets, and then obtaining a core by stacking or winding; placing the core in a soft package for casing, injecting electrolyte after drying, vacuum packaging, and forming and grading, namely the secondary battery.

[0065] The following introduces the examples and comparative examples of the present application in combination with the above technical solutions.

[0066] The carbon-coated aluminum foils in Examples 1-3 and Comparative Examples 1-2 were prepared by the following method: mixing graphene quantum dots, conductive carbon black, PAA, and PVP in a weight ratio of (0.5-1.5):(2-6):20:0.2, adding deionized water to mix and stir these materials evenly, and then coating them on the aluminum foil. The single-sided coating surface density is 0.3±0.06 g / m 2 , and the coating thickness is 0.5±0.2 μm, and then drying to obtain the carbon-coated aluminum foil.

[0067] The carbon-coated aluminum foil in Comparative Example 3 was prepared by the following method: mixing graphene quantum dots, PAA, and PVP in a weight ratio of 1.0:20:0.2, adding deionized water to mix and stir these materials evenly, and then coating them on the aluminum foil. The single-sided coating surface density is 0.3±0.06 g / m 2 , and the coating thickness is 0.5±0.2 μm, and then drying to obtain the carbon-coated aluminum foil.

[0068] The carbon-coated aluminum foil in Comparative Example 4 was prepared by the following method: mixing conductive carbon black, PAA, and PVP in a weight ratio of 4:20:0.2, adding deionized water to mix and stir these materials evenly, and then coating them on the aluminum foil. The single-sided coating surface density is 0.3±0.06 g / m 2 , and the coating thickness is 0.5±0.2 μm, and then drying to obtain the carbon-coated aluminum foil.

[0069] Blank Example 1: Using a bare aluminum foil without a functional elastic layer.

[0070] Furthermore, the secondary batteries in Examples 1-3, Comparative Examples 1-4, and the blank example were prepared by the aforementioned method for preparing a secondary battery. The first preset weight ratio can be 97:1:2, and the second preset weight ratio can be 96:1.3:1.7:1.

[0071] Specifically, the material parameter testing methods in the examples, comparative examples, and blank example are as follows:

[0072] 1) Volatile matter testing of graphene quantum dots and conductive carbon black:

[0073] The graphene quantum dots (with a weight of m1) that have been dried in advance at 80 °C for 5 h are placed in a muffle furnace at 900 °C under nitrogen protection and heated for 15 min. Then, they are weighed again at room temperature as m2, and the volatile content X = (m 1- m2) / m1 * 100%;

[0074] The conductive carbon black (with a weight of m3) that has been dried in advance at 80 °C for 5 h is placed in a muffle furnace at 900 °C under nitrogen protection and heated for 15 min. Then, it is weighed again at room temperature as m4, and the volatile content Y = (m4 - m3) / m3 * 100%

[0075] 2) Tests on D1, D2, D3 of graphene quantum dots and D4, D5, D6 of conductive carbon black: Using a Malvern 3000 laser particle size analyzer, measure and calculate S1 and S2.

[0076] Specifically, the battery performance test methods in the examples, comparative examples, and blank examples are as follows:

[0077] 1) Test the internal resistance of the battery cell before and after storing at 65 °C for 30 d: At 25 °C, after fully charging at 0.5C, place the secondary battery cell in a 65 °C incubator for 30 d; test the internal resistance of the battery cell before and after storage;

[0078] 2) 25 °C rate discharge test: Test the 3C discharge performance at 25 °C and calculate the discharge capacity in grams.

[0079] The performance parameters of each example, comparative example, and blank example are shown in Table 1 and Table 2.

[0080] Table 1

[0081]

[0082] Table 2

[0083]

[0084]

[0085] Based on the above data and Figure 3 it can be seen that compared with Comparative Example 1, Comparative Example 3, and Blank Example 1, the peel strength of Examples 1 - 8 has increased significantly; compared with Comparative Example 2, the peel strength, pole piece resistance, and performance after storing at 65 °C for 30 d of Example 3 have all been optimized; compared with Comparative Example 4, the pole piece resistance of Examples 1 - 8 has been significantly optimized; compared with Blank Example 1, Examples 1 - 8 have better performance at 25 °C / 3C rate.

[0086] In summary, through a large number of experiments, the present application has found that the addition amounts, volatile contents, and particle size distribution width values of graphene quantum dots and conductive carbon black have a strong correlation with the peel strength and resistivity of the functional carbon layer. In the functional carbon layer, by regulating the volatile contents of the two to balance the proportions of SP2 hybridization and SP3 hybridization, and by regulating the addition amounts, volatile contents, particle size values, and particle size distributions of graphene quantum dots and conductive carbon black to meet their respective range requirements and the correlation relationship between them, a "surface-to-surface" two-dimensional conductive network structure of graphene quantum dots and a "point-to-line" zero-dimensional and one-dimensional conductive structure of conductive carbon black are constructed, thereby forming a three-dimensional conductive network structure with excellent performance, achieving excellent conductivity of the carbon-coated aluminum foil, thereby reducing the resistance of the electrode sheet and the AC internal resistance, and improving the rate performance; and ensuring that the graphene quantum dots can fill the bumps or gaps on the surface of the aluminum foil to achieve good contact between the carbon layer and the aluminum foil, and weakening the coating voids caused by the overlapping structure of the graphene quantum dots and conductive carbon black, improving the peel strength and conductivity of the carbon-coated aluminum foil, preventing a significant increase in the internal resistance of the battery cell caused by volume expansion during high-temperature storage, and preventing the diving problem caused by low peel strength during the cycling process.

[0087] The above description has fully disclosed the specific implementation manners of the present application. It should be noted that any modification made by those skilled in the art to the specific implementation manners of the present application does not depart from the scope of the claims of the present application. Correspondingly, the scope of the claims of the present application is not limited solely to the foregoing specific implementation manners.

Claims

1. A carbon-coated aluminum foil, comprising an aluminum foil and a functional carbon layer coated on at least one surface of the aluminum foil, characterized in that, The functional carbon layer includes graphene quantum dots and conductive carbon black, and the graphene quantum dots are dispersed in the conductive carbon black; For the weight percentage A of the graphene quantum dots, the weight percentage B of the conductive carbon black, the volatile content X of the graphene quantum dots, the volatile content Y of the conductive carbon black, the particle size distribution width S1 of the graphene quantum dots, and the secondary particle size distribution width S2 of the conductive carbon black in the mixed slurry for forming the functional carbon layer, the following relational expression is satisfied: 0.35 ≤ 100A*(1 - X) / S1 + 100B*(1 - Y) / S2 ≤ 5.5; The weight percentage A of the graphene quantum dots in the mixed slurry for forming the functional carbon layer satisfies 0.5% ≤ A ≤ 1.5%; The volatile content X of the graphene quantum dots satisfies 2% ≤ X ≤ 7%; The weight percentage B of the conductive carbon black in the mixed slurry for forming the functional carbon layer is 2% ≤ B ≤ 6%; The volatile content Y of the conductive carbon black satisfies 1% ≤ Y ≤ 4%; The calculation formula for the particle size distribution width S1 of the graphene quantum dots is: S1 = (D3 - D1) / D2; D1 represents the particle size corresponding to the graphene quantum dots when the cumulative volume percentage in the aqueous solvent reaches the first preset ratio, D2 represents the particle size corresponding to the graphene quantum dots when the cumulative volume percentage in the aqueous solvent reaches the second preset ratio, D3 represents the particle size corresponding to the graphene quantum dots when the cumulative volume percentage in the aqueous solvent reaches the third preset ratio, and the first preset ratio, the second preset ratio, and the third preset ratio increase in sequence; The particle size of the graphene quantum dots satisfies at least one of 5 ≤ D1 ≤ 10 nm, 25 ≤ D2 ≤ 40 nm, and 55 ≤ D3 ≤ 75 nm; The calculation formula for the secondary particle size distribution width S2 of the conductive carbon black is: S2 = (D6 - D4) / D5; D4 represents the particle size corresponding to the conductive carbon black when the cumulative volume percentage in the aqueous solvent reaches the fourth preset ratio, D5 represents the particle size corresponding to the conductive carbon black when the cumulative volume percentage in the aqueous solvent reaches the fifth preset ratio, D6 represents the particle size corresponding to the conductive carbon black when the cumulative volume percentage in the aqueous solvent reaches the sixth preset ratio, and the fourth preset ratio, the fifth preset ratio, and the sixth preset ratio increase in sequence; The particle size of the conductive carbon black satisfies at least one of 0.2 ≤ D4 ≤ 0.4 μm, 1 ≤ D5 ≤ 5 μm, and 8 ≤ D6 ≤ 15 μm.

2. The carbon-coated aluminum foil according to claim 1, wherein For the weight percentage A of the graphene quantum dots, the volatile content X of the graphene quantum dots, and the particle size distribution width S1 of the graphene quantum dots, 0.15 ≤ 100A*(1 - X) / S1 ≤ 1.5 is satisfied; for the weight percentage B of the conductive carbon black in the functional carbon layer, the volatile content Y of the conductive carbon black, and the secondary particle size distribution width S2 of the conductive carbon black, 0.2 ≤ 100B*(1 - Y) / S2 ≤ 4.0 is satisfied.

3. The carbon-coated aluminum foil according to claim 1 or 2, characterized in that, The particle size distribution width S1 of the graphene quantum dots satisfies 1.1 ≤ S1 ≤ 2.

8.

4. The carbon-coated aluminum foil according to claim 1 or 2, characterized in that, The secondary particle size distribution width S2 of the conductive carbon black satisfies 1.5 ≤ S2 ≤ 9.

6.

5. An electrode sheet for a secondary battery, characterized in that, The electrode plate includes the carbon-coated aluminum foil as described in any one of claims 1-4.

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