Electrode tab, method of manufacturing the same, secondary battery including the same, and electric device

Abstract

The present application relates to an electrode tab, a preparation method thereof, a secondary battery comprising the same, and an electric device, the electrode tab comprising a current collector, an electrode film layer, and a coating layer; wherein the electrode film layer is arranged between the current collector and the coating layer, and the electrode film layer comprises an electrode active material; and the coating layer comprises a negative temperature coefficient material. By arranging the coating layer comprising the negative temperature coefficient material in the electrode tab, the negative temperature coefficient material can generate heat rapidly in the working process of the battery at low temperature due to the increase of its internal resistance, so as to rapidly increase the temperature of the battery to the comfortable working temperature range, thereby effectively improving the charge and discharge performance of the battery at low temperature.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to an electrode sheet, its preparation method, and a secondary battery and electrical device containing the electrode sheet. Background Technology

[0002] Secondary batteries, represented by lithium-ion batteries, have advantages such as high energy density and high operating voltage, and are widely used in portable electronic devices (mobile phones, computers, cameras), power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace and many other fields.

[0003] The operating environment for lithium-ion batteries is relatively complex. Among the many environmental factors, low temperature conditions are common and have a significant impact on battery performance. At low temperatures, internal polarization of lithium-ion batteries increases, hindering charge and discharge performance and potentially leading to severe lithium plating, posing a significant safety hazard. This, to some extent, limits the use of lithium-ion batteries in low-temperature environments such as winter. Summary of the Invention

[0004] The present invention was made in view of the above-mentioned technical problems, and its purpose is to provide an electrode sheet that can be rapidly heated under low temperature conditions, a method for preparing the electrode sheet, and a secondary battery, battery pack and power device containing the electrode sheet.

[0005] To this end, a first aspect of the present invention provides an electrode sheet comprising a current collector, an electrode film, and a coating; the electrode film is disposed between the current collector and the coating, the electrode film comprising an electrode active material; and the coating comprising a negative temperature coefficient material.

[0006] By incorporating a coating containing a negative temperature coefficient material into the electrode plates, the internal resistance of the negative temperature coefficient material increases under low-temperature conditions, enabling it to rapidly heat up during battery operation and quickly raise the cell temperature to the comfortable operating temperature range, thereby effectively improving the charge and discharge performance of the secondary battery under low-temperature conditions.

[0007] In some embodiments, the negative temperature coefficient material includes one or more combinations selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene polymer (PVDF-HFP) gel, polyacrylonitrile (PAN) gel, polyethylene oxide (PEO) gel, and polyphosphoric acid (LPC) gel.

[0008] In some embodiments, the coating has a porous structure, and the pore size gradient in the coating increases in the thickness direction along the electrode film layer to the direction away from the electrode film layer.

[0009] According to the technical solution of the present invention, the coating has a porous structure with varying pore size, so that after subsequent processing such as cold pressing of the electrode sheet, closed pores are not formed on the coating surface, and the number of interconnected pores can be increased to ensure that it has good ion transport channels, thereby not reducing its ionic conductivity at room temperature.

[0010] In some embodiments, the pore size in the coating is 0.1 to 1 μm.

[0011] According to the technical solution of the present invention, the heating performance of the coating under low temperature conditions can be further improved by using small pores with a pore size distribution of 0.1 to 1 μm.

[0012] In some embodiments, the porosity of the coating is 50% to 80%.

[0013] According to the technical solution of the present invention, when the porosity of the coating is 50% to 80%, it is beneficial to balance the internal resistance of the coating and the lithium-ion transport performance, so that even under ultra-low temperature conditions, the coating can meet the heating requirements without negatively affecting ion transport.

[0014] In some embodiments, the thickness of the coating is 1–5 μm.

[0015] According to the technical solution of the present invention, when the thickness of the coating is 1 to 5 μm, it can meet the heat generation requirements of the secondary battery when used at -50℃ to 0℃.

[0016] In some embodiments, the electrode sheet is a positive electrode sheet or a negative electrode sheet.

[0017] A second aspect of the present invention provides a method for preparing an electrode sheet, comprising: coating a slurry containing an electrode active material onto a current collector, drying it, and forming an electrode film layer on the current collector to obtain an initial electrode sheet; preparing a solution containing a negative temperature coefficient material raw material, coating the solution onto the surface of the initial electrode sheet, and drying it to form a coating containing a negative temperature coefficient material on the electrode film layer to obtain the electrode sheet.

[0018] In some embodiments, the initial electrode is dried before the step of coating the solution onto the surface of the initial electrode.

[0019] According to the technical solution of the present invention, pre-drying the initial electrode sheet can prevent bubbles from forming due to the initial electrode sheet not being dried, thereby damaging the structure of the coating.

[0020] In some embodiments, the solution is coated onto the surface of the initial electrode at a temperature of 50°C to 90°C.

[0021] According to the technical solution of the present invention, coating is performed when the initial electrode is at a temperature of 50°C to 90°C, which helps to ensure the smoothness and integrity of the coating.

[0022] In some embodiments, the negative temperature coefficient material is polyvinylidene fluoride-hexafluoropropylene polymer gel, and the method for preparing the electrode sheet includes:

[0023] A slurry containing electrode active material is coated onto a current collector, and after drying, an electrode film layer is formed on the current collector, thus preparing the initial electrode sheet; a polyvinylidene fluoride-hexafluoropropylene polymer solution is prepared, and the polyvinylidene fluoride-hexafluoropropylene polymer solution is coated onto the surface of the initial electrode sheet, and after drying, a coating containing polyvinylidene fluoride-hexafluoropropylene polymer gel is formed on the electrode film layer, thus preparing the electrode sheet.

[0024] In some embodiments, the polyvinylidene fluoride-hexafluoropropylene polymer solution contains 5% to 10% polyvinylidene fluoride-hexafluoropropylene polymer by mass.

[0025] In some embodiments, the method for preparing the electrode sheet includes:

[0026] S1. A slurry containing electrode active material is coated onto a current collector, and after drying, an electrode film is formed on the current collector, thus preparing the initial electrode sheet.

[0027] S2. Prepare a solution containing a material with a negative temperature coefficient, wherein the solution includes a first solvent and a second solvent, the boiling point of the first solvent is t1, the boiling point of the second solvent is t2, and t1 > t2.

[0028] S3. The solution of the negative temperature coefficient material raw material is coated onto the surface of the initial electrode, and a coating containing the negative temperature coefficient material is formed through multi-stage drying process;

[0029] Wherein, the number of stages of the multi-stage drying is n, where n is a positive integer greater than or equal to 2, the temperature of the first stage of drying is <t2, the temperature of the nth stage of drying is >(t1×0.7), and the temperature of the subsequent stage of drying is higher than the temperature of the previous stage of drying.

[0030] According to the technical solution of the present invention, by using reagents with high and low boiling points in conjunction with a multi-stage drying process, the prepared coating can have a gradient increase in pore size along the thickness direction from the initial electrode to the direction away from the initial electrode. In the initial stage of the multi-stage drying, due to the low temperature, the low-boiling-point solvent floats to the surface first. Then, as the drying temperature increases, the low-boiling-point solvent and the high-boiling-point solvent vaporize and overflow in sequence, forming a porous structure with a gradient change in pore size.

[0031] In some embodiments, the number of stages n in the multi-stage drying process is a positive integer selected from 2 to 5.

[0032] In some embodiments, the volume ratio of the first solvent to the second solvent is 1–3:7–9.

[0033] In some embodiments, the volume fraction of the first solvent in the solution is 10% to 30%.

[0034] In some embodiments, the volume fraction of the second solvent in the solution is 70% to 90%.

[0035] In some embodiments, the first solvent includes one or more combinations selected from the group consisting of N-methylpyrrolidone (NMP), ethylene carbonate (EC), propylene carbonate (PC), etc.

[0036] In some embodiments, the second solvent includes one or more combinations selected from the group consisting of acetone, tetrahydrofuran (THF), etc.

[0037] In some implementations, the difference between t1 and t2 is 20–60°C.

[0038] In some implementations, 100℃≤t1≤150℃; 50℃≤t2≤80℃.

[0039] In some embodiments, the multi-stage drying has three stages, with the temperature of the first stage drying being T1, which is 30°C to 80°C; the temperature of the second stage drying being T2, which is 50°C to 100°C; and the temperature of the third stage drying being T3, which is 100°C to 150°C; and T1 < T2 < T3.

[0040] A third aspect of the present invention provides a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode and / or the negative electrode are the electrode plates described in the first aspect of the present invention.

[0041] In some embodiments, the secondary battery is one of a lithium-ion battery, a sodium-ion battery, or a potassium-ion battery.

[0042] A fourth aspect of the present invention provides a battery module comprising the secondary battery described in the third aspect of the present invention.

[0043] A fifth aspect of the present invention provides a battery pack comprising a secondary battery as described in the third aspect of the present invention or a battery module as described in the fourth aspect of the present invention.

[0044] A sixth aspect of the present invention provides an electrical device comprising at least one of the secondary battery described in the third aspect of the present invention, the battery module described in the fourth aspect of the present invention, or the battery pack provided in the fifth aspect of the present invention.

[0045] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0046] The electrode sheet provided by the present invention has a coating containing a negative temperature coefficient material. When the secondary battery containing the electrode sheet is used under low temperature conditions, the internal resistance of the coating increases due to the low temperature conditions, thereby enabling it to heat up rapidly during battery operation and quickly raise the temperature of the cell to the comfortable operating temperature range. This effectively avoids problems such as battery polarization and lithium plating on the electrode sheet, and improves the charge and discharge performance of lithium-ion batteries under low temperature conditions. Attached Figure Description

[0047] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0048] Figure 1 This is a schematic diagram of a secondary battery according to an embodiment of the present invention;

[0049] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of the present invention is shown.

[0050] Figure 3 This is a schematic diagram of a battery module according to one embodiment of the present invention;

[0051] Figure 4 This is a schematic diagram of a battery pack according to an embodiment of the present invention;

[0052] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present invention is shown.

[0053] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to an embodiment of the present invention.

[0054] Explanation of reference numerals in the attached figures:

[0055] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Lithium-ion battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0056] Exemplary embodiments of this disclosure will now be described in more detail. It should be understood that this disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art.

[0057] The "scope" disclosed in this invention is defined by a lower limit and an upper limit. A given scope is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific scope. This method of defining a scope includes endpoints and allows for arbitrary combinations; that is, any lower limit can be combined with any upper limit to form a scope.

[0058] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0059] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0060] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially.

[0061] In existing technologies, secondary batteries, such as lithium-ion batteries, are widely used in many fields. However, under low-temperature conditions, the viscosity of the liquid electrolyte in lithium-ion batteries decreases rapidly, the ionic conductivity decreases, and the electrode interface impedance and lithium-ion migration impedance increase. This mismatch with the rapid electron migration rate in the external circuit causes severe polarization inside the lithium-ion battery, thus hindering its charge and discharge performance and potentially leading to severe lithium plating, posing a significant safety hazard. This also limits the use of lithium-ion batteries in low-temperature environments such as winter.

[0062] To address the aforementioned problems, the present invention provides an electrode sheet comprising a current collector, an electrode film, and a coating; the electrode film is disposed between the current collector and the coating, and the electrode film contains an electrode active material; the coating contains a negative temperature coefficient material.

[0063] In this article, the term "negative temperature coefficient material" refers to a material whose resistivity decreases as temperature increases.

[0064] According to the technical solution of the present invention, the coating of the electrode sheet contains a negative temperature coefficient material, which has essentially the same internal resistance as the lithium-ion battery at room temperature and does not affect the charge-discharge performance of the lithium-ion battery; however, at low temperatures, the internal resistance of the negative temperature coefficient material increases. This is because the heat generated by the battery cell itself is related to its internal resistance by the following formula: Q = I 2 R t Therefore, when lithium-ion batteries are in low-temperature conditions, the conductivity of negative temperature coefficient materials decreases and the resistance increases. The increased resistance increases the heat generated by the battery cell itself and quickly raises it to its comfortable operating temperature range, thereby improving the charge and discharge performance of lithium-ion batteries under low-temperature conditions.

[0065] According to the technical solution of the present invention, the coating mainly functions as a heating layer without affecting the normal function of the lithium-ion battery. In some embodiments, the coating is composed of a material with a negative temperature coefficient. In other embodiments, the coating may also contain other additives or other materials. In still other embodiments, the coating includes other materials, but does not include inorganic fillers (e.g., Al2O3, TiO2, molecular sieves, etc.).

[0066] In some embodiments, the negative temperature coefficient material includes one or more combinations selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene polymer (PVDF-HFP) gel, polyacrylonitrile (PAN) gel, polyethylene oxide (PEO) gel, and polyphosphoric acid (LPC) gel.

[0067] In some embodiments, the negative temperature coefficient material is a polyvinylidene fluoride-hexafluoropropylene polymer (PVDF-HFP) gel.

[0068] In this document, the terms "polyvinylidene fluoride-hexafluoropropylene polymer gel" and "PVDF-HFP gel" refer to copolymers formed from polyvinylidene fluoride and hexafluoropropylene in a gel state. In the polyvinylidene fluoride-hexafluoropropylene polymer gel, the mass ratio of polyvinylidene fluoride to hexafluoropropylene can be 1:1.

[0069] According to the technical solution of the present invention, the conductivity of the polyvinylidene fluoride-hexafluoropropylene polymer gel at room temperature is essentially on the same order of magnitude as that of the liquid organic electrolyte in current lithium-ion batteries (10). -3 The conductivity of polyvinylidene fluoride-hexafluoropropylene polymer gel is low (S / cm), and it is a negative temperature coefficient material with low conductivity at low temperatures. When lithium-ion batteries are under low temperature conditions, the conductivity of polyvinylidene fluoride-hexafluoropropylene polymer gel decreases and the resistance increases. The increased resistance leads to increased heat generation of the cell itself, which quickly rises to its comfortable operating temperature range.

[0070] In some embodiments, the coating has a porous structure, and the pore size gradient in the coating increases in the thickness direction along the electrode film layer to the direction away from the electrode film layer.

[0071] According to the technical solution of the present invention, the coating has a porous structure. The pore size gradient in the coating increases with distance from the electrode film layer, thereby preventing the formation of closed pores on the coating surface after subsequent processing such as cold pressing of the electrode sheet, and increasing the number of interconnected pores to ensure good lithium-ion transport channels, thus maintaining its ionic conductivity at room temperature.

[0072] In some embodiments, the pore size in the coating is 0.1 to 1 μm.

[0073] According to the technical solution of the present invention, the thermal performance of the coating under low-temperature conditions can be further improved by using small pores with a pore size distribution of 0.1 to 1 μm. Specifically, when a lithium-ion battery is under low-temperature conditions, the viscosity of the electrolyte increases sharply. Since small pores with a pore size distribution of 0.1 to 1 μm are not conducive to electrolyte transport, the internal resistance of the coating will increase sharply, thereby rapidly increasing its heat generation under low-temperature conditions and allowing the cell temperature to rise to the comfortable operating temperature range in a short time.

[0074] In some embodiments, the porosity of the coating is 50% to 80%; for example, about 50%, 55%, 60%, 65%, 70%, 75%, 80%, etc.

[0075] In this document, the term "porosity" refers to the ratio of the volume occupied by pores in a coating to the total volume of the coating, expressed in percentage (%). In the technical solution of this invention, the porosity is detected and calculated using the mercury intrusion porosimetry method.

[0076] According to the technical solution of the present invention, when the porosity of the coating is 50% to 80%, it is beneficial to balance the internal resistance of the coating and the lithium-ion transport performance. If the porosity of the coating is higher than 80%, the porosity is too large, the impedance is reduced, the heat generation is insufficient, resulting in insufficient temperature rise and failing to meet the application requirements under ultra-low temperature conditions (e.g., below -50°C); if the porosity of the coating is lower than 50%, the porosity is too small, the impedance increases significantly under low temperature conditions, which may lead to severe obstruction of lithium-ion transport and easy lithium plating.

[0077] In some embodiments, the thickness of the coating is 1 to 5 μm; for example, it can be selected from about 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, etc.

[0078] According to the technical solution of the present invention, the thickness of the coating is related to its heat generation capacity under low-temperature conditions. If the coating thickness is greater than 5 μm, the lithium-ion transport path is too long, making lithium plating more likely; if the coating thickness is less than 1 μm, the increase in impedance under low-temperature conditions is not significant, resulting in low heat generation and poor temperature rise. When a coating thickness of 5 μm is used, the heat generation requirements of the battery at -50°C can be met with almost no impact on lithium-ion transport. Within the range of 1 to 5 μm, a suitable coating thickness can be selected according to the temperature conditions of the target application scenario of the lithium-ion battery, thereby meeting the heat generation requirements of the battery at -50°C to 0°C.

[0079] In some embodiments, the electrode sheet is a positive electrode sheet or a negative electrode sheet.

[0080] In some embodiments, the battery electrode is a positive electrode, which includes a positive current collector, a positive electrode film, and the coating; the positive electrode film contains a positive active material.

[0081] In some embodiments, the electrode sheet is a negative electrode sheet, which includes a negative current collector, a negative electrode film layer, and the coating layer; the negative electrode film layer contains a negative electrode active material.

[0082] In some embodiments, a method for preparing the electrode sheet of the present invention is provided, comprising: coating a slurry containing an electrode active material onto a current collector, drying it, and forming an electrode film layer on the current collector to obtain an initial electrode sheet; preparing a solution containing a negative temperature coefficient material raw material, coating the solution onto the surface of the initial electrode sheet, and drying it to form a coating containing a negative temperature coefficient material on the electrode film layer to obtain the electrode sheet.

[0083] In some embodiments, the initial electrode is dried before the step of coating the solution onto the surface of the initial electrode.

[0084] According to the technical solution of the present invention, pre-drying the initial electrode sheet can prevent bubbles from forming due to the initial electrode sheet not being dried, thereby damaging the structure of the coating.

[0085] In some embodiments, the solution is coated onto the surface of the initial electrode at a temperature of 50°C to 90°C; for example, the temperature of the initial electrode can be selected from about 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, etc.

[0086] According to the technical solution of the present invention, since coating is not easy to perform under conditions of low initial electrode temperature, it is preferable to perform coating under conditions of high initial electrode temperature. In a specific embodiment, coating can be performed after the initial electrode is dried but before it cools down, thereby ensuring the smoothness and integrity of the coating.

[0087] In some embodiments, the negative temperature coefficient material is polyvinylidene fluoride-hexafluoropropylene polymer gel, and the method for preparing the electrode sheet includes:

[0088] A slurry containing electrode active material is coated onto a current collector, and after drying, an electrode film layer is formed on the current collector, thus preparing the initial electrode sheet; a polyvinylidene fluoride-hexafluoropropylene polymer solution is prepared, and the polyvinylidene fluoride-hexafluoropropylene polymer solution is coated onto the surface of the initial electrode sheet, and after drying, a coating containing polyvinylidene fluoride-hexafluoropropylene polymer gel is formed on the electrode film layer, thus preparing the electrode sheet.

[0089] In some embodiments, a polyvinylidene fluoride-hexafluoropropylene polymer is added to a solvent and stirred until homogeneous to prepare the polyvinylidene fluoride-hexafluoropropylene polymer solution; the polyvinylidene fluoride-hexafluoropropylene polymer solution can form a gel after coating and drying. The solvent includes one or more combinations selected from the group consisting of: N-methylpyrrolidone (NMP), ethylene carbonate (EC), propylene carbonate (PC), acetone, tetrahydrofuran (THF), etc.

[0090] In some embodiments, the polyvinylidene fluoride-hexafluoropropylene polymer solution contains 5% to 10% by mass of the polyvinylidene fluoride-hexafluoropropylene polymer; for example, about 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0091] According to the technical solution of the present invention, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer in the solution has a certain impact on processing performance and internal resistance at low temperatures. If the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer in the solution is higher than 30%, a gel cannot be formed and it is difficult to coat, thus it does not have the properties of a negative temperature coefficient material, resulting in a deterioration in charging capability at room temperature and low temperature; if it is lower than 30% but higher than 10%, although a gel can be formed, it is not easy to coat, the processing performance is poor, and it is easy to cause excessive impedance and increase the risk of lithium plating; if it is lower than 5%, it is not easy to shape, the processing performance is poor, and it is easy to fail to obtain sufficient heating effect due to too low internal resistance.

[0092] In some embodiments, the method for preparing the electrode sheet includes:

[0093] S1. A slurry containing electrode active material is coated onto a current collector, and after drying, an electrode film is formed on the current collector, thus preparing the initial electrode sheet.

[0094] S2. Prepare a solution containing a material with a negative temperature coefficient, wherein the solution includes a first solvent and a second solvent, the boiling point of the first solvent is t1, the boiling point of the second solvent is t2, and t1 > t2.

[0095] S3. The solution of the negative temperature coefficient material raw material is coated onto the surface of the initial electrode, and a coating containing the negative temperature coefficient material is formed through multi-stage drying process;

[0096] Wherein, the number of stages of the multi-stage drying is n, where n is a positive integer greater than or equal to 2, the temperature of the first stage of drying is <t2, the temperature of the nth stage of drying is >(t1×0.7), and the temperature of the subsequent stage of drying is higher than the temperature of the previous stage of drying.

[0097] In this document, the term "multi-stage drying" refers to performing drying several times sequentially. When the number of stages in the multi-stage drying is n, n drying times are performed sequentially. In some embodiments, the number of stages in the multi-stage drying is 2, then the first stage of drying and the second stage of drying are performed sequentially; in some embodiments, the number of stages in the multi-stage drying is 3, then the first stage of drying, the second stage of drying, and the third stage of drying are performed sequentially; in some embodiments, the number of stages in the multi-stage drying is 4, then the first stage of drying, the second stage of drying, the third stage of drying, and the fourth stage of drying are performed sequentially. In the technical solution of this invention, in the multi-stage drying, the temperature of the later stage of drying is higher than the temperature of the earlier stage of drying. Specifically, the number of stages in the multi-stage drying is n, where n is a positive integer greater than or equal to 2, and the temperature of the (m-1)th drying stage is T. (m-1) The temperature of the m-th drying cycle is T. m m takes the value of each positive integer greater than 1 and less than or equal to n, satisfying T (m-1) <T m In some embodiments, the drying is carried out in a bellows.

[0098] According to the technical solution of the present invention, by using reagents with high and low boiling points in conjunction with a multi-stage drying process, the prepared coating can have a gradient increase in pore size along the thickness direction from the initial electrode to the direction away from the initial electrode. In the initial stage of the multi-stage drying, due to the low temperature, the low-boiling-point solvent floats first. Then, as the drying temperature increases, the low-boiling-point solvent and the high-boiling-point solvent vaporize and overflow in sequence, forming a porous structure with a gradient change in pore size. Specifically, for example, when n is a positive integer > 2, the first stage of drying initially achieves the floating of the low-boiling-point solvent, thereby making the coating surface have more low-boiling-point solvent; then, through the i-th stage of drying (1 < i < n), most of the low-boiling-point solvent vaporizes and overflows, forming most of the micropores; finally, through the n-th stage of drying, the high-boiling-point solvent vaporizes and overflows, forming a small number of pores with larger pore sizes.

[0099] In some embodiments, the number of stages n in the multi-stage drying is a positive integer selected from 2 to 5; for example, the number of stages in the multi-stage drying is 2, 3, 4, 5, etc.

[0100] In some embodiments, the volume ratio of the first solvent to the second solvent is 1–3:7–9.

[0101] According to the technical solution of the present invention, when the volume ratio of the first solvent to the second solvent is 1-3:7-9, it is beneficial to achieve better heat generation and lithium-ion transport performance. If the volume ratio of the second solvent is lower than the above range, its floating is not obvious during the first stage of drying, and a large number of micropores cannot be formed, resulting in a coating with mostly macropores, which leads to low impedance and low heat generation, and the heating effect is not obvious when used at low temperatures. If the volume ratio of the second solvent is higher than the above range, the first stage of drying may cause some of the second solvent to escape, resulting in a high proportion of micropores in the prepared coating, which leads to high impedance when used at low temperatures, affecting lithium-ion transport and posing a certain risk of lithium plating.

[0102] In some embodiments, the volume fraction of the first solvent in the solution is 10% to 30%; for example, about 10%, 15%, 20%, 25%, 30%, etc.

[0103] In some embodiments, the volume fraction of the second solvent in the solution is 70% to 90%; for example, about 70%, 75%, 80%, 85%, 90%, etc.

[0104] In some embodiments, the first solvent includes one or more combinations selected from the group consisting of N-methylpyrrolidone (NMP), ethylene carbonate (EC), propylene carbonate (PC), etc.

[0105] In some embodiments, the second solvent includes one or more combinations selected from the group consisting of acetone, tetrahydrofuran (THF), etc.

[0106] In some implementations, the difference between t1 and t2 is 20–60°C.

[0107] In some implementations, 100℃≤t1≤150℃; 50℃≤t2≤80℃.

[0108] In some embodiments, the multi-stage drying has three stages. The temperature of the first stage drying is T1, which is 30℃ to 80℃, for example, 30℃, 50℃, 60℃, 70℃, 80℃, etc.; the temperature of the second stage drying is T2, which is 50℃ to 100℃, for example, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃, etc.; the temperature of the third stage drying is T3, which is 100℃ to 150℃, for example, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, etc.; and T1 < T2 < T3.

[0109] In some embodiments, a secondary battery is provided, comprising a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode and / or the negative electrode are the electrode plates described in this invention.

[0110] In some embodiments, a secondary battery is provided, comprising a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode is the electrode electrode described in this invention, and the positive electrode includes a positive current collector, a positive electrode film layer, and the coating layer; the positive electrode film layer contains a positive active material.

[0111] In some embodiments, a secondary battery is provided, comprising a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the negative electrode is the electrode electrode described in this invention, and the negative electrode includes a negative current collector, a negative electrode film layer, and the coating layer; the negative electrode film layer contains a negative electrode active material.

[0112] In some embodiments, a secondary battery is provided, which includes a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode and the negative electrode are the electrode plates described in this invention.

[0113] The positive electrode sheet includes a positive current collector, a positive electrode film layer, and the coating layer; the positive electrode film layer contains a positive electrode active material;

[0114] The negative electrode sheet includes a negative current collector, a negative electrode film layer, and the coating layer; the negative electrode film layer contains a negative electrode active material.

[0115] In some embodiments, the secondary battery is a lithium-ion battery, a sodium-ion battery, or a potassium-ion battery.

[0116] [Positive electrode plate]

[0117] In a secondary battery, when the negative electrode does not contain the coating described in this invention, the positive electrode is the electrode sheet described in this invention, which includes a positive current collector, a positive electrode film, and the coating; the positive electrode film contains a positive electrode active material.

[0118] When the negative electrode is the electrode sheet described in this invention, the positive electrode may not contain the coating described in this invention. In some embodiments, the negative electrode is the electrode sheet described in this invention, and the positive electrode includes a positive current collector and a positive electrode film layer; the positive electrode film layer contains a positive electrode active material.

[0119] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0120] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present invention is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0121] In some embodiments, when the secondary battery is a sodium-ion battery, the positive electrode active material can be a positive electrode active material known in the art for sodium-ion batteries. As an example, the positive electrode active material can be a single material or a combination of two or more. The positive electrode active material can be selected from sodium-iron composite oxide (NaFeO2), sodium-cobalt composite oxide (NaCoO2), sodium-chromium composite oxide (NaCrO2), sodium-manganese composite oxide (NaMnO2), sodium-nickel composite oxide (NaNiO2), and sodium-nickel-titanium composite oxide (NaNiO2). 1 / 2 Ti 1 / 2 O2), sodium nickel manganese composite oxide (NaNi) 1 / 2 Mn 1 / 2 O2), sodium iron manganese composite oxide (Na) 2 / 3 Fe 1 / 3 Mn 2 / 3 O2), sodium nickel cobalt manganese composite oxide (NaNi) 1 / 3 Co 1 / 3 Mn 1 / 3 O2), sodium iron phosphate (NaFePO4), sodium manganese phosphate (NaMn) P O4), sodium cobalt phosphate (NaCoPO4), Prussian blue materials, polyanionic materials (phosphates, fluorophosphates, pyrophosphates, sulfates), etc., but the present invention is not limited to these materials. The present invention can also use other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries.

[0122] In some embodiments, the positive electrode film may optionally include a binder. For example, the binder may include one or more combinations selected from the group consisting of: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0123] In some embodiments, the positive electrode film may optionally include a conductive agent. For example, the conductive agent may include one or more combinations selected from the group consisting of: Super P, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0124] [Negative electrode plate]

[0125] In a secondary battery, when the positive electrode does not contain the coating described in this invention, the negative electrode is the electrode electrode described in this invention, which includes a negative current collector, a negative electrode film layer, and the coating layer; the negative electrode film layer contains a negative electrode active material.

[0126] When the positive electrode is the electrode sheet described in this invention, the negative electrode may not contain the coating described in this invention. In some embodiments, the positive electrode is the electrode sheet described in this invention, and the negative electrode includes a negative current collector and a negative electrode film layer; the negative electrode film layer contains a negative electrode active material.

[0127] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0128] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. For example, the negative electrode active material includes one or more combinations selected from the group consisting of: natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, TiO2-Li4Ti5O 12 Li-Al alloys are used. However, this invention is not limited to these materials; other conventional materials that can be used as negative electrode active materials for lithium-ion batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0129] In some embodiments, the negative electrode film layer may optionally include a binder. For example, the binder may include one or more combinations selected from the group consisting of: styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0130] In some embodiments, the negative electrode film may optionally include a conductive agent. For example, the conductive agent may include one or more combinations selected from the group consisting of: Super P, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0131] In some embodiments, the negative electrode film may optionally include other additives. For example, other additives may be thickeners (such as sodium carboxymethyl cellulose (CMC-Na)).

[0132] Electrolyte

[0133] The electrolyte acts as a conductor of ions between the positive and negative electrodes. The electrolyte comprises an electrolyte salt and a solvent.

[0134] In some embodiments, the electrolyte salt may include at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0135] In some embodiments, the solvent may include at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0136] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0137] [Isolation membrane]

[0138] This invention does not impose any particular limitation on the type of separator membrane; any well-known porous structure separator membrane with good chemical and mechanical stability can be selected. For example, the material of the separator membrane may include one or more combinations selected from the group consisting of: glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0139] [Preparation of Secondary Batteries]

[0140] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0141] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0142] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0143] This invention does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0144] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0145] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0146] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0147] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0148] In some embodiments, the aforementioned secondary batteries can be assembled into a battery pack, and the number of secondary batteries contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0149] In some embodiments, the battery module 4 described above can be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0150] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0151] In addition, the present invention also provides an electrical device comprising a secondary battery provided by the present invention. In some embodiments, the electrical device comprises at least one of a battery module or a battery pack provided by the present invention. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0152] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0153] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0154] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0155] Example 1

[0156] 1) Preparation of positive electrode sheet

[0157] LiNi will be used as the positive electrode active material0.6 Co 0.2 Mn 0.2 O2 (NCM), superconducting carbon black SP as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are dispersed in N-methylpyrrolidone (NMP) as a solvent at a mass ratio of 96:2:2 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and after drying, cold pressing, slitting, and cutting, the initial positive electrode sheet is obtained.

[0158] A polyvinylidene fluoride-hexafluoropropylene (PVDF-HFA) polymer solution was prepared by uniformly mixing PVDF-HFA polymer, NMP, and acetone, wherein the mass fraction of PVDF-HFA polymer was 8% (w / w), and the volume ratio of NMP to acetone was 3:7. The PVDF-HFA polymer solution was stirred thoroughly at 50°C for 5 hours to prepare a PVDF-HFA polymer gel. The initial positive electrode sheet was dried at 80°C. After drying, the temperature of the initial positive electrode sheet was controlled at 50°C, and a gel coating was applied to a thickness of 5 μm. Then, staged drying was performed to form a coating on the surface of the initial positive electrode sheet, thus preparing the positive electrode sheet.

[0159] The specific steps of the graded drying process are as follows: First-stage drying: The coated initial positive electrode sheet is placed in a 30°C air box for drying; Second-stage drying: The air box temperature is adjusted to 60°C for drying; Third-stage drying: The air box temperature is adjusted to 150°C for drying.

[0160] 2) Preparation of negative electrode sheet

[0161] The negative electrode active material graphite, superconducting carbon black SP as a conductive agent, SBR as a binder, and CMC-Na as a thickener are dispersed in deionized water as a solvent at a mass ratio of 96:1:1:2 and mixed evenly to obtain a negative electrode slurry. The negative electrode slurry is then uniformly coated onto a negative electrode current collector copper foil. After drying, cold pressing, slitting, and cutting, a negative electrode sheet is obtained.

[0162] 3) Separating membrane

[0163] Polyethylene film was selected as the separator.

[0164] 4) Preparation of electrolyte

[0165] Ethyl carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) were mixed uniformly in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0166] 5) Battery manufacturing

[0167] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, lithium-ion battery I-1 is obtained.

[0168] Example 2

[0169] Except for the following parameters, the lithium-ion battery I-2 was prepared by operating in the same manner as in Example 1.

[0170] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 5% (w / w), and the volume ratio of NMP to acetone is 1:9.

[0171] Example 3

[0172] Except for the following parameters, the lithium-ion battery I-3 was prepared by operating in the same manner as in Example 1.

[0173] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 5% (w / w), the volume ratio of NMP to acetone is 1:9, and the coating thickness is 1 μm.

[0174] Example 4

[0175] Except for the following parameters, the lithium-ion battery I-4 was prepared by operating in the same manner as in Example 1.

[0176] The polyvinylidene fluoride-hexafluoropropylene polymer solution contains 5% (w / w) polyvinylidene fluoride-hexafluoropropylene polymer; the coating thickness is 1 μm. In the graded drying step, the secondary drying temperature is 70 °C.

[0177] Example 5

[0178] Except for the following parameters, the lithium-ion battery I-5 was prepared by operating in the same manner as in Example 1.

[0179] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 5% (w / w); the coating thickness is 1 μm. The specific steps of the graded drying are as follows: First-stage drying: the coated initial positive electrode sheet is placed in a 50°C air box for drying; Second-stage drying: the temperature of the air box is adjusted to 150°C for drying.

[0180] Example 6

[0181] Except for the following parameters, the lithium-ion battery I-6 was prepared by operating in the same manner as in Example 1.

[0182] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 5% (w / w).

[0183] Example 7

[0184] Except for the following parameters, the lithium-ion battery I-8 was prepared by operating in the same manner as in Example 1.

[0185] A polyvinylidene fluoride-hexafluoropropylene polymer solution was prepared by uniformly mixing polyvinylidene fluoride-hexafluoropropylene polymer and NMP, wherein the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer was 5% (w / w). The step of fractional drying was replaced by drying in a blower at 150°C.

[0186] Example 8

[0187] Except for the following parameters, the lithium-ion battery I-9 was prepared by operating in the same manner as in Example 1.

[0188] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 5% (w / w); the coating thickness is 10 μm.

[0189] Example 9

[0190] Except for the following parameters, the lithium-ion battery I-10 was prepared by operating in the same manner as in Example 1.

[0191] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 5% (w / w); the coating thickness is 0.5 μm.

[0192] Comparative Example 1

[0193] Except for the following parameters, the lithium-ion battery I-11 was prepared by operating in the same manner as in Example 1.

[0194] In the polyvinylidene fluoride-hexafluoropropylene polymer solution, the mass fraction of polyvinylidene fluoride-hexafluoropropylene polymer is 30% (w / w).

[0195] Comparative Example 2

[0196] Except for the following parameters, the lithium-ion battery I-12 was prepared by operating in the same manner as in Example 1.

[0197] The initial positive electrode is used directly as the positive electrode.

[0198] Experimental Example

[0199] The lithium-ion batteries prepared in Examples 1-9 and Comparative Examples 1-2 were tested.

[0200] Test (1):

[0201] The temperature rise rate of the lithium-ion battery was tested under low-temperature conditions. The test method is as follows: Under an ambient temperature of -20℃, the lithium-ion battery was charged at 0.3C from 20% SOC to 50% SOC, and the temperature rise rate of the battery during this process was measured.

[0202] Test (2):

[0203] The DC resistance (DCR) of the lithium-ion battery was tested under low-temperature conditions. The test method is as follows: Under an ambient temperature of -20℃, the cell was adjusted to 50% SOC and then left to stand for 2 hours. The terminal voltage U1 was measured. Then, it was charged with a 0.5C current I for 30 seconds, and the terminal voltage U2 was measured. The DC resistance R was recorded as (U2-U1) / I.

[0204] Test (3):

[0205] The test examined lithium plating in lithium-ion batteries after cyclic charge-discharge cycles at low temperatures. The test method was as follows: At an ambient temperature of -20°C, the lithium-ion batteries were charged and discharged at a current of 0.2C between 10% and 80% SOC. After 10 charge-discharge cycles (10cls), the degree of lithium plating on the electrodes was measured, and the extent of lithium plating was graded according to the following criteria:

[0206] No lithium plating: golden yellow at the interface;

[0207] Slight lithium plating: Lithium plating appears at the edge of the electrode, and the area of ​​lithium plating accounts for ≤1 / 16 of the electrode area;

[0208] Moderate lithium plating: Lithium plating occurs in the middle position, and the area of ​​lithium plating occupies ≤1 / 2 of the electrode area (1 / 16 < the area of ​​lithium plating).

[0209] Severe lithium plating: Lithium plating occurs on the entire surface of the electrode, and 1 / 2 < the area of ​​lithium plating is less than the area of ​​the electrode.

[0210] The results of the above tests are shown in Table 1.

[0211] Table 1

[0212]

[0213] Based on the above test results, it is evident that by employing the technical solution of this invention, the cell temperature can be rapidly increased when using lithium-ion batteries under low-temperature conditions, thereby effectively improving charging performance and shortening charging time. Specifically: since the rate of temperature rise is related to the heat generated by polarization, and the heat generated by polarization is directly proportional to its DCR (Discharge Rate Reduction), at the same charging rate, a faster rate of temperature rise results in a higher temperature achievable per unit time. A higher temperature leads to a more significant improvement in the charging window, allowing for a higher charging rate and thus a shorter total charging time. Therefore, by detecting the rate of temperature rise, it is possible to determine whether the overall charging performance and charging time of the battery have been improved under low-temperature conditions.

[0214] Furthermore, combined with the results of the lithium plating window determination (-20℃ 0.2C 10-80% SOC, 10cls), it can be seen that when the coating is thicker or the porosity in the coating is smaller, the DCR value under low-temperature conditions will be too high, and the risk of lithium plating will increase. This is because when the coating is too thick and the porosity is too small, Li... + Transport obstruction increases both the DCR value and the risk of lithium plating. In this case, despite a good rate of temperature rise, battery performance may be negatively affected by the safety risks associated with lithium plating.

[0215] In Comparative Example 1, the concentration of polyvinylidene fluoride-hexafluoropropylene polymer in the solution was too high, which prevented the formation of a gel-like coating on the surface of the initial electrode sheet. The polyvinylidene fluoride-hexafluoropropylene polymer in the coating was in a non-gel state and did not have the properties of a negative temperature coefficient material. Furthermore, it caused severe lithium plating at the electrode interface, affecting the normal use of the battery.

[0216] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An electrode tab, characterized by, The electrode tab comprises a current collector, an electrode film layer and a coating layer; the electrode film layer is arranged between the current collector and the coating layer, and the electrode film layer comprises an electrode active material; the coating layer comprises a negative temperature coefficient material; the coating layer has a porous structure, and the pore size gradient in the coating layer increases from the electrode film layer to a direction away from the electrode film layer in the thickness direction of the coating layer.

2. The electrode patch of claim 1, wherein, The negative temperature coefficient material comprises one or more than two combinations of the following group: polyvinylidene fluoride-hexafluoropropylene polymer gel, polyacrylonitrile gel, polyethylene oxide gel, polyphosphoric acid gel.

3. The electrode patch of claim 1, wherein, The pore size in the coating layer is 0.1-1 μm.

4. The electrode patch of claim 3, wherein, The porosity of the coating layer is 50%-80%.

5. The electrode patch of claim 1, wherein, The thickness of the coating layer is 1-5 μm.

6. The electrode patch of claim 1, wherein, The electrode tab is a positive electrode tab or a negative electrode tab.

7. A method of making an electrode, characterized by, The method comprises the following steps: A slurry comprising an electrode active material is coated on a current collector, and after drying, an electrode film layer is formed on the current collector, thereby obtaining an initial tab; a solution comprising raw materials of a negative temperature coefficient material is prepared, the solution comprising a first solvent and a second solvent, the boiling point of the first solvent being t1, and the boiling point of the second solvent being t2, t1>t2; the solution is coated on the surface of the initial tab, and a coating layer comprising a negative temperature coefficient material is formed on the electrode film layer through multi-stage drying, thereby obtaining the electrode tab; wherein the coating layer has a porous structure, and the pore size gradient in the coating layer increases from the electrode film layer to a direction away from the electrode film layer in the thickness direction of the coating layer.

8. The production method according to claim 7, wherein The step of coating the solution on the surface of the initial tab is further preceded by a drying process on the initial tab.

9. The production method according to claim 7, wherein The solution is coated on the surface of the initial tab under the condition that the temperature of the initial tab is 50-90℃.

10. The production method according to claim 7, wherein The number of stages of the multi-stage drying is n, n is a positive integer greater than or equal to 2, the temperature of the first stage of drying is <t2, the temperature of the nth stage of drying is >(t1×0.7), and the temperature of the later stage of drying is higher than that of the previous stage of drying.

11. The production method according to claim 10, wherein The volume ratio of the first solvent to the second solvent is 1-3:7-9.

12. The production method according to claim 10, wherein The first solvent comprises one or more than two combinations of the following group: N-methylpyrrolidone, ethylene carbonate, propylene carbonate.

13. The production method according to claim 10, wherein The second solvent comprises one or more than two combinations of the following group: acetone, tetrahydrofuran.

14. The production method according to claim 10, wherein The difference between t1 and t2 is 20-60℃.

15. A secondary battery comprising a positive electrode tab, a negative electrode tab, an electrolyte and a separator; wherein the positive electrode tab and / or the negative electrode tab is the electrode tab of any one of claims 1-6 or is prepared by the preparation method of any one of claims 7-14.

16. A battery module comprising the secondary battery of claim 15.

17. A battery pack comprising the secondary battery of claim 15 or the battery module of claim 16.

18. An electrical device comprising at least one of the secondary battery of claim 15, the battery module of claim 16 or the battery pack of claim 17.

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