Positive electrode sheet and secondary battery

Abstract

The application provides a positive electrode sheet and a secondary battery. The positive electrode sheet comprises a positive electrode current collector and an active material layer arranged on at least one surface of the positive electrode current collector; the positive electrode sheet further comprises a functional layer arranged on the side of the active material layer away from the positive electrode current collector; the functional layer comprises a phase change material, the phase change material comprises a phase change material matrix and a shell coated on at least part of the surface of the phase change material matrix; the latent heat of phase change of the phase change material matrix is not less than 150 J / g, and the supercooling degree of the phase change material matrix is not more than 10 DEG C; the Young's modulus of the shell is not less than 2.0 GPa; and the positive electrode sheet satisfies b / a=1.6-30%. The positive electrode sheet of the application introduces the phase change material into the functional layer, realizes the synchronous control of heat generation and heat absorption, significantly delays the triggering time of thermal runaway, and simultaneously accurately controls the area density ratio of the functional layer and the active material layer, so as to balance the thermal safety, the cycle performance and the energy density of the secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, and particularly relates to a positive electrode and a secondary battery. Background Technology

[0002] Secondary batteries are widely used in high-performance fields such as electric vehicles and energy storage systems, especially in batteries that use high-energy-density cathode materials such as high-nickel ternary batteries and lithium cobalt oxide. When these batteries operate under high power, high temperature, or abnormal conditions, the redox reactions of the cathode materials and interfacial side reactions continuously generate a large amount of heat.

[0003] If the battery system's heat dissipation design is inadequate, heat will accumulate internally, causing the battery temperature to rise sharply. This will not only accelerate exothermic side reactions such as positive electrode structure damage and electrolyte decomposition, but in severe cases, it may also trigger thermal runaway, leading to combustion or explosion, directly threatening safety and battery life.

[0004] Therefore, effective thermal management of high-energy-density battery cathode materials has become a key technical challenge to ensure their safe and stable operation. Summary of the Invention

[0005] This invention provides a positive electrode that enables in-situ, real-time thermal management of internal heat generation, significantly improving the thermal safety and cycle life of secondary batteries while also taking into account the energy density of secondary batteries.

[0006] The present invention also provides a secondary battery that has excellent thermal safety, cycle life and energy density.

[0007] A first aspect of the present invention provides a positive electrode sheet, comprising a positive current collector and an active material layer disposed on at least one surface of the positive current collector; the positive electrode sheet further comprises a functional layer disposed on the side of the active material layer away from the positive current collector;

[0008] The functional layer includes a phase change material, which includes a phase change material matrix and a shell covering at least a portion of the surface of the phase change material matrix; the latent heat of phase change of the phase change material matrix is ​​not less than 150 J / g, and the supercooling of the phase change material matrix is ​​not greater than 10°C; the Young's modulus of the shell is not less than 2.0 GPa.

[0009] The positive electrode sheet satisfies: b / a = 1.6-30%; where b is the areal density of the functional layer and a is the areal density of the active material layer.

[0010] The positive electrode plate described above satisfies the following condition: b / a = 3.5-20%;

[0011] And / or, the thickness ratio of the functional layer to the active material layer is 1.6-30%.

[0012] In the positive electrode as described above, the areal density of the functional layer is 0.5-5 mg / cm³. 2 Preferably 1-2 mg / cm 2 ; and / or, the thickness of the functional layer is 1-25 μm, preferably 3-10 μm; and / or, the areal density of the active material layer is 7-30 mg / cm³. 2 ; and / or, the thickness of the active material layer is 19-150 μm.

[0013] The positive electrode as described above, wherein the average particle size of the phase change material is 0.5-8 μm; and / or, the phase change temperature of the phase change material is 45-65 °C;

[0014] Preferably, the phase change material matrix includes at least one of paraffin wax, fatty acid, and polyethylene glycol;

[0015] Preferably, the outer shell comprises at least one of melamine-formaldehyde resin, polymethyl methacrylate, and silica; and / or,

[0016] The active material layer includes a positive electrode active material, which includes at least one of lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium iron phosphate, and lithium manganese oxide.

[0017] The positive electrode sheet as described above, wherein the functional layer further includes a thermally conductive filler and a binder;

[0018] Preferably, by mass percentage, the functional layer comprises: 50-90% phase change material, 5-40% thermally conductive filler, and 5-10% binder;

[0019] Preferably, the thermally conductive filler includes at least one of boron nitride, alumina, and carbon nanotubes, and the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylic acid, and polyacrylonitrile.

[0020] As described above, in the thickness direction of the positive electrode current collector, the active material layer includes a first active material layer and a second active material layer stacked together, with the second active material layer located between the positive electrode current collector and the first active material layer;

[0021] Based on the total weight of the active material layers, the content of the first active material layer is 5-50%, and the content of the second active material layer is 50-95%.

[0022] The positive electrode sheet satisfies: C1>C2; where C1 is the mass percentage of the phase change material in the first active material layer, and C2 is the mass percentage of the phase change material in the second active material layer.

[0023] The positive electrode sheet as described above, wherein the thickness ratio of the first active material layer and the second active material layer is 0.0526-1; and / or, C1 / C2≥3.

[0024] The positive electrode as described above, wherein 1%≤C1≤8%; and / or, 0%≤C2≤2%.

[0025] As described above, the positive electrode sheet includes a first active material layer and a second active material layer, wherein the second active material layer is close to the positive current collector, and the first active material layer is disposed on the surface of the second active material layer away from the positive current collector;

[0026] The first active material layer includes a first phase change material, the second active material layer includes a second phase change material, and the positive electrode sheet satisfies: L1 > L2; wherein, L1 is the latent heat of phase change of the phase change material matrix in the first phase change material, and L2 is the latent heat of phase change of the phase change material matrix in the second phase change material.

[0027] A second aspect of the present invention provides a secondary battery comprising the positive electrode sheet described in the first aspect above.

[0028] The positive electrode of this invention introduces a phase change material into the functional layer. By utilizing the phase change endothermic characteristics of the phase change material, in-situ thermal suppression is achieved from the heat generation source, realizing simultaneous control of heat generation and endothermic processes. This significantly delays the thermal runaway trigger time and improves the thermal safety and cycle performance of the secondary battery. Simultaneously, by precisely controlling the areal density ratio of the functional layer and the active material layer, a dynamic balance between the thermal runaway threshold and energy output density is achieved in the secondary battery. This synergistically optimizes the thermal management performance and electrochemical performance of the secondary battery, maintaining high energy density while ensuring safety. Attached Figure Description

[0029] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0030] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet provided in Embodiment 1 of the present invention;

[0031] Figure 2 This is a schematic diagram of the structure of the positive electrode sheet provided in Embodiment 20 of the present invention.

[0032] Figure label:

[0033] f - Active material layer; f1 - First active material layer; f2 - Second active material layer; e - Functional layer; c - Positive current collector; d - Phase change material.

[0034] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] To further improve the thermal safety and cycle performance of rechargeable batteries, the inventors conducted in-depth research on existing technologies. Currently, the industry commonly uses external thermal management methods (such as liquid cooling and air cooling) to address battery thermal issues. However, the inventors noted that these traditional methods still have significant limitations: for example, liquid cooling systems based on circulating coolants (such as water-glycol mixtures), while able to remove heat generated by the battery cell, have a lag in response, making it difficult to achieve real-time control of the heat sources inside the battery cell; while air cooling systems relying on air convection have limited heat dissipation efficiency due to the low thermal conductivity of air, often failing to meet thermal management requirements, especially in high-power applications.

[0037] Therefore, the inventors turned to research on modifying the cathode material, attempting to coat the cathode material with oxides such as alumina and manganese oxide, or to dope the cathode material with elements such as Mg, Al, and Ti. However, the research found that surface coating mainly serves to delay the trigger time of thermal runaway and cannot directly absorb heat; while the element doping method still has a relatively limited ability to absorb heat from the heat source.

[0038] Based on this, the first aspect of the present invention provides a positive electrode sheet, including a positive current collector c and an active material layer f disposed on at least one surface of the positive current collector c; the positive electrode sheet further includes a functional layer e, which is disposed on the side of the active material layer f away from the positive current collector c.

[0039] The functional layer e includes a phase change material d, which includes a phase change material matrix and a shell covering at least part of the surface of the phase change material matrix; the latent heat of phase change of the phase change material matrix is ​​not less than 150 J / g, and the supercooling of the phase change material matrix is ​​not greater than 10℃; the Young's modulus of the shell is not less than 2.0 GPa.

[0040] The positive electrode sheet satisfies: b / a = 1.6-30%; where b is the areal density of the functional layer e and a is the areal density of the active material layer f.

[0041] In detail, the active material layer includes the positive electrode active material. During the operation of the secondary battery, the redox reaction and interfacial side reactions of the positive electrode active material continuously generate a large amount of heat. If this heat cannot be dissipated in time, it may lead to a chain of exothermic reactions such as structural degradation of the positive electrode material, oxygen evolution, and electrolyte decomposition, ultimately causing thermal runaway and threatening the safety and lifespan of the battery system.

[0042] For example, the supercooling of the phase change material matrix is ​​10°C, 9.5°C, 9°C, 8.5°C, 8°C, 7.5°C, 7°C or below, or a range of any two of these values. Preferably, the supercooling of the phase change material matrix is ​​not greater than 5°C.

[0043] For example, the Young's modulus of the shell is 2.0 GPa, 2.1 GPa, 2.2 GPa, 2.3 GPa, 2.4 GPa, 2.5 GPa or above, or a range consisting of any two of these values. Preferably, the Young's modulus of the shell is not less than 2.5 GPa.

[0044] For example, b / a is 1.6%, 5%, 10%, 15%, 20%, 25%, or 30%, or a range of any two of these values.

[0045] For example, the latent heat of phase change of the phase change material matrix is ​​150 J / g, 160 J / g, 170 J / g, 180 J / g, 190 J / g, 200 J / g or above, or a range of any two values ​​therein. Preferably, the latent heat of phase change of the phase change material matrix is ​​not less than 180 J / g.

[0046] The positive electrode provided by this invention achieves in-situ, real-time thermal management of internal heat generation, significantly improving the thermal safety and cycle life of the secondary battery while maintaining its energy density. The reason is:

[0047] On the one hand, by introducing phase change materials (PCMs) into the functional layer, these PCMs absorb the high-temperature heat generated inside the positive electrode active material through phase change, achieving simultaneous control of heat generation and absorption, and significantly improving the thermal response speed to local high-temperature regions. The PCM matrix, with its high latent heat of phase change and low supercooling, absorbs a large amount of latent heat during the phase change process. Its high Young's modulus ensures the mechanical strength of the outer shell, making it less prone to breakage and effectively preventing leakage of the PCM matrix. This ensures its chemical stability in the battery environment and improves the thermal safety and cycle performance of the secondary battery. On the other hand, by controlling the areal density ratio of the functional layer and the active material layer, a balance is achieved between thermal management performance and the electrochemical performance of the secondary battery, taking into account both safety and energy density. Furthermore, unlike the conventional approach of setting up thermal management systems outside the battery or at the cell level, this invention integrates the thermal management function into the positive electrode material layer, avoiding the use of complex external thermal management systems and thus minimizing the impact on the energy density of the secondary battery.

[0048] The present invention does not specifically limit the material of the positive electrode current collector, which can be at least one of aluminum foil and nickel foil.

[0049] The present invention does not impose any particular limitation on the thickness of the positive electrode current collector. For example, the thickness of the positive electrode current collector is 5-20 μm, preferably 6-15 μm.

[0050] In one specific implementation, the positive electrode sheet satisfies: b / a = 3.5-20%. Within this range, b / a further balances the heat absorption capacity of the phase change material and the content of active material in the positive electrode sheet, thereby balancing the thermal safety and energy density of the secondary battery.

[0051] For example, b / a is 3.5%, 5%, 7%, 9%, 11%, 13%, 15%, 17%, or 20%, or a range of any two of these values.

[0052] In one specific embodiment, the thickness ratio of the functional layer e to the active material layer f is 1.6-30%. Within this range, the thickness ratio balances the content of phase change material and active material in the positive electrode, thereby taking into account both the thermal safety and energy density of the secondary battery.

[0053] For example, the thickness ratio of the functional layer to the active material layer is 1.6%, 5%, 10%, 15%, 20%, 25%, or 30%, or a range of any two of these values.

[0054] In one specific embodiment, the areal density of functional layer e is 0.5-5 mg / cm³. 2 Preferably 1-2 mg / cm 2 Within this range, the areal density of the functional layer allows the phase change material to fully absorb the heat generated by the positive electrode.

[0055] For example, the areal density of the functional layer is 0.5 mg / cm³. 2 1mg / cm 2 1.5 mg / cm 2 2mg / cm 2 2.5 mg / cm 2 3mg / cm 2 3.5 mg / cm 2 4mg / cm 2 4.5 mg / cm 2 Or 5mg / cm 2 , or a range consisting of any two of the values.

[0056] In one specific embodiment, the thickness of the functional layer e is 1-25 μm, preferably 3-10 μm. A thickness within this range is more conducive to balancing the heat absorption capacity of the phase change material and the thickness of the positive electrode, thereby ensuring both the thermal safety and energy density of the secondary battery.

[0057] For example, the thickness of the functional layer is 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, 17 μm, 19 μm, 22 μm, 24 μm or 25 μm, or a range of any two of these values.

[0058] In one specific embodiment, the areal density of the active material layer f is 7-30 mg / cm³. 2 The areal density of the active material layer is within this range, further ensuring the energy density of the secondary battery.

[0059] For example, the areal density of the functional layer is 7 mg / cm³. 2 10mg / cm 2 11mg / cm 2 13mg / cm 2 15mg / cm 2 17mg / cm 2 19mg / cm 2 21mg / cm 2 23mg / cm 2 25mg / cm 2 27mg / cm 2 Or 30mg / cm 2 , or a range consisting of any two of the values.

[0060] In one specific embodiment, the thickness of the active material layer f is 19-150 μm. Within this range, the thickness of the active material layer is more conducive to balancing the thickness of the positive electrode and the functional layer, thereby ensuring both the thermal safety and energy density of the secondary battery.

[0061] For example, the thickness of the active material layer is 19 μm, 40 μm, 60 μm, 80 μm, 100 μm, 120 μm, 140 μm or 150 μm, or a range of any two of these values.

[0062] To further enhance the heat absorption capacity of phase change materials, the average particle size, phase change temperature, matrix type, and shell type of the phase change material can be controlled.

[0063] In one specific embodiment, the average particle size of the phase change material d is 0.5-8 μm. An average particle size within this range allows for more uniform dispersion of the phase change material in the slurry, enhancing its encapsulation stability and thermal conductivity, thus laying the foundation for subsequent cathode fabrication.

[0064] For example, the average particle size of the phase change material is 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm or 8 μm, or a range of any two of these values.

[0065] In one specific embodiment, the phase change material d has a phase change temperature of 45-65°C. This range of phase change temperatures better matches the thermal management requirements of secondary batteries. Maintaining the battery temperature within this range can effectively prevent battery overheating, thereby inhibiting electrolyte decomposition and SEI film damage, helping to extend the cycle life of secondary batteries, and improving safety under fast charging conditions.

[0066] For example, the phase transition temperature of the phase change material is 45°C, 48°C, 51°C, 54°C, 57°C, 60°C, 63°C or 65°C, or a range of any two of these values.

[0067] In one specific embodiment, the phase change material matrix includes at least one of paraffin wax, fatty acid, and polyethylene glycol. These materials exhibit high latent heat of phase change, excellent heat absorption, and stable physicochemical properties, effectively improving the thermal safety and cycle life of secondary batteries.

[0068] When the phase change material matrix is ​​a mixture of the aforementioned substances, the present invention does not specifically limit the proportion of each specific substance in the mixture.

[0069] The parameters that affect the latent heat of phase change and supercooling of paraffin include carbon chain length (molecular weight) and grafting rate. Therefore, the paraffin used in this invention is n-alkanes or refined paraffins with carbon chain length of C22-C28 and molecular weight of 250-400, or modified paraffins with grafting rate ≥10% through grafting modification.

[0070] The parameters that affect the latent heat of phase transition and supercooling of fatty acids include carbon chain length and functional group structure. Therefore, the fatty acids used in this invention are saturated fatty acids with carbon chain length of C10-C18, preferably lauric acid (C12, phase transition temperature about 44°C) and palmitic acid (C16, phase transition temperature about 63°C).

[0071] The parameters affecting the latent heat of phase change and supercooling of polyethylene glycol include molecular weight and crystallinity. Therefore, the polyethylene glycol used in this invention has an average molecular weight of 400-20000, preferably 4000-15000.

[0072] In one specific embodiment, the outer shell comprises at least one of melamine-formaldehyde resin, polymethyl methacrylate, and silicon dioxide. These materials possess high mechanical strength and stable chemical properties, making them resistant to breakage under mechanical stress, further ensuring the phase change material matrix remains leak-proof and guaranteeing the heat absorption effect of the heat-absorbing material. Simultaneously, these materials exhibit excellent thermal conductivity, further enhancing the thermal response speed and preventing heat accumulation, thereby improving the thermal safety and cycle life of the secondary battery.

[0073] When the aforementioned outer shells are mixtures of the aforementioned substances, the present invention does not specifically limit the proportion of each specific substance in the mixture.

[0074] The parameters affecting the Young's modulus of melamine-formaldehyde resin (shell) include crosslinking density, flexible chain length and ratio, and amount of nanofiller added. Therefore, the melamine-formaldehyde resin used in this invention is modified by internal or external toughening, preferably by adding 5-15wt% nano SiO2 or 5-20wt% polyethylene glycol copolymer modified MF resin (melamine-formaldehyde resin) to achieve a Young's modulus of 2.0 GPa or higher while maintaining appropriate toughness.

[0075] The parameters affecting the Young's modulus of polymethyl methacrylate (shell) include dielectric constant, stereoregularity, and molecular chain conformation. Therefore, the polymethyl methacrylate used in this invention is PMMA (polymethyl methacrylate) with an isotactic or syndiotactic regular structure. By adjusting the dielectric constant, electrostatic interactions are enhanced, preferably with a dielectric constant of 3.5-4.5, so that its Young's modulus reaches above 3.0 GPa while maintaining appropriate toughness.

[0076] The parameters affecting the Young's modulus of silica (shell) include porosity, pore wall thickness (ligament length), and crack orientation. Therefore, the silica used in this invention is: dense amorphous silica with a porosity of <10%, preferably with its Young's modulus increased by densification treatment to reach 7.0 GPa or higher.

[0077] The phase change material used in this invention can be a commercially available product or can be prepared using conventional methods in the art. This invention does not specifically limit the preparation method of the phase change material. In one embodiment, the preparation method of the phase change material includes (taking melamine-formaldehyde resin coated paraffin as an example):

[0078] (1) Mix melamine and formaldehyde solutions at a molar ratio of 1:(3-4) and adjust the pH to 8.5-9.0. React at a water bath temperature of 70-75℃ for 60-90 min to form an aqueous solution of melamine-formaldehyde (MF) prepolymer.

[0079] (2) Mix paraffin wax, emulsifier (SDS) and deionized water evenly at a mass ratio of 1:(0.01-0.1):(5-20), and then emulsify for 5-10 minutes at a speed of 10000-12000 rpm using a high-speed shear emulsifier to obtain paraffin wax emulsion.

[0080] (3) Add the prepared MF prepolymer aqueous solution to the paraffin emulsion and mix evenly. Adjust the pH value to 3.5-4.0 and then react at 75-80℃ for 2-3 hours to obtain the phase change material.

[0081] In one specific embodiment, the active material layer f includes a positive electrode active material, which includes at least one of lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium iron phosphate, and lithium manganese oxide. The aforementioned positive electrode active materials have the advantages of high thermal stability or low heat generation from side reactions, which helps reduce the risk of thermal runaway and thus facilitates in-situ, real-time thermal management of heat generation within the positive electrode sheet.

[0082] In one specific embodiment, the functional layer e further includes a thermally conductive filler and a binder. The addition of the thermally conductive filler further improves the thermal response speed, and the binder adheres the phase change material and the thermally conductive filler to the surface of the active material layer.

[0083] In one specific embodiment, the functional layer e comprises, by mass percentage: 50-90% phase change material, 5-40% thermally conductive filler, and 5-10% binder. The proportions of each component in the functional layer are within this range, while simultaneously ensuring excellent heat transfer and heat absorption rates.

[0084] For example, the mass percentage of phase change material in the functional layer is 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%, or a range of any two of these values.

[0085] For example, the thermally conductive filler content in the functional layer is 5%, 10%, 15%, 20%, 25%, 30%, 35%, or 40% by mass, or a range of any two of these values.

[0086] For example, the adhesive content in the functional layer by mass percentage is 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5% or 10%, or a range of any two of these values.

[0087] In one specific embodiment, the thermally conductive filler includes at least one of boron nitride, alumina, and carbon nanotubes, and the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylic acid, and polyacrylonitrile. The aforementioned thermally conductive filler is a highly thermally conductive material, capable of forming a thermally conductive network in the functional layer, increasing the rate at which heat is conducted from the heat source (such as the positive electrode active material) to the phase change material, further preventing localized overheating. The aforementioned binder has strong adhesive force, further improving the peel strength between the functional layer and the active material layer, thereby enhancing the structural stability of the positive electrode sheet.

[0088] To further optimize the thermal safety and energy density of the secondary battery, a gradient distribution of the phase change material in the active material layer was configured.

[0089] In one specific embodiment, in the thickness direction of the positive current collector, the active material layer f includes a first active material layer f1 and a second active material layer f2 stacked together, with the second active material layer f2 located between the positive current collector c and the first active material layer f1.

[0090] Based on the total weight of the active material layers, the content of the first active material layer f1 is 5-50%, and the content of the second active material layer f2 is 50-95%.

[0091] The positive electrode sheet satisfies the condition: C1 > C2; where C1 is the mass percentage of phase change material in the first active material layer, and C2 is the mass percentage of phase change material in the second active material layer. This constraint ensures a gradient distribution of the phase change material within the active material layers, further optimizing thermal management efficiency. It allows the phase change material to be concentrated in the heat-generating region of the positive electrode (e.g., away from the current collector), rapidly absorbing localized high-temperature heat. While maintaining the heat absorption effect of the phase change material, its usage is reduced, balancing the thermal safety and energy density of the secondary battery.

[0092] For example, based on the total weight of the active material layer, the content of the first active material layer is 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%, or a range of any two of these values.

[0093] For example, based on the total weight of the active material layers, the content of the second active material layer is 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%, or a range of any two of these values.

[0094] In one specific embodiment, the thickness ratio of the first active material layer f1 and the second active material layer f2 is 0.0526-1.

[0095] For example, the thickness ratio of the first active material layer and the second active material layer is 0.0526, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, or a range of any two of these values.

[0096] In one specific implementation, C1 / C2 ≥ 3. Within this range, C1 / C2 further balances the proportions of active materials and phase change materials, thereby balancing the energy density and thermal safety of the secondary battery.

[0097] For example, C1 / C2 is 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8 or more, or a range consisting of any two of these values.

[0098] In one specific implementation, 1% ≤ C1 ≤ 8%. Within this range, C1 ensures that the phase change material is concentrated on the side away from the current collector, preferentially absorbing heat from the heat-generating area.

[0099] For example, C1 is 1%, 2%, 3%, 4%, 5%, 6%, 7%, or 8%, or a range of any two of these values.

[0100] In one specific embodiment, 0% ≤ C2 ≤ 2%. Within this range, C2 reduces the interference of the phase change material on the electron transport path near the current collector, thus maintaining electrochemical performance.

[0101] For example, C2 is 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, or 2%, or a range of any two of these values.

[0102] In one specific embodiment, the active material layer f includes a first active material layer f1 and a second active material layer f2, the second active material layer f2 being close to the positive current collector c, and the first active material layer f1 being disposed on the surface of the second active material layer f2 away from the positive current collector c;

[0103] The first active material layer f1 includes a first phase change material, and the second active material layer f1 includes a second phase change material. The positive electrode sheet satisfies: L1 > L2; where L1 is the latent heat of phase change of the phase change material matrix in the first phase change material, and L2 is the latent heat of phase change of the phase change material matrix in the second phase change material. This configuration ensures that the heat absorption capacity of the first active material layer is greater than that of the second active material layer. By achieving a gradient distribution of the phase change material in the active material layers, thermal management efficiency is further improved. The phase change material is mainly concentrated in the region of high heat generation at the positive electrode (e.g., the side away from the current collector), thus enabling rapid absorption of localized high-temperature heat. This design reduces the amount of phase change material used while ensuring efficient heat absorption, thereby balancing the thermal safety and energy density of the secondary battery.

[0104] This invention does not specifically limit the preparation method of the positive electrode sheet. In one embodiment, the preparation method of the positive electrode sheet includes the following steps:

[0105] (1) Disperse the positive electrode active material, the first conductive agent and the first binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent in proportion, and stir and mix thoroughly to form a uniform positive electrode slurry; coat the positive electrode slurry uniformly on the positive electrode current collector, and obtain the basic electrode sheet after drying and rolling.

[0106] (2) The phase change material, the second conductive agent, and the second binder are dispersed in NMP solvent and ultrasonically treated to obtain a suspension. The suspension is coated on the surface of the base electrode and then dried and rolled to obtain the positive electrode.

[0107] The present invention does not specifically limit the type of the first conductive agent, which can be a conventional material in the art. For example, the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber.

[0108] The present invention does not specifically limit the type of the first adhesive, which can be a conventional material in the art. For example, the adhesive can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0109] This invention does not specifically limit the type of the second conductive agent, which can be a conventional material in the art. For example, the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber.

[0110] The present invention does not specifically limit the type of the second adhesive, which can be a conventional material in the art. For example, the adhesive can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0111] A second aspect of the present invention provides a secondary battery, including the positive electrode sheet described in the first aspect. This secondary battery combines excellent thermal safety, cycle life, and high energy density, achieving an optimized balance between performance and safety.

[0112] It is conceivable that, in addition to the aforementioned positive electrode, the secondary battery of the present invention also includes a negative electrode, an electrolyte, and a separator.

[0113] The present invention does not specifically limit the structure of the negative electrode sheet. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least a portion of the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent.

[0114] This invention does not specifically limit the material of the negative electrode current collector; it can be any conventional material in the art. For example, the negative electrode current collector can be any of copper foil, nickel foam, or copper foam.

[0115] This invention does not specifically limit the type of negative electrode active material, and it can be any negative electrode active material commonly used in batteries. For example, the negative electrode active material can be selected from at least one of graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrodes), tin-based negative electrode materials (mainly including tin and tin alloys).

[0116] This invention does not specifically limit the type of binder; it can be any binder commonly used in battery negative electrodes. For example, the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0117] This invention does not specifically limit the type of conductive agent; it can be any conductive agent commonly used in the negative electrode of batteries. For example, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0118] This invention does not specifically limit the preparation method of the negative electrode sheet. In one embodiment, the negative electrode sheet can be prepared by a method including the following steps:

[0119] The negative electrode active material, conductive agent, and binder are dispersed in deionized water and thoroughly stirred to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector, and after drying, rolling, and slitting, a negative electrode sheet is obtained.

[0120] This invention does not impose specific limits on the amount of negative electrode active material, conductive agent, and binder, and these amounts can be adjusted according to actual conditions.

[0121] This invention does not specifically limit the composition of the electrolyte, which may include one or more solvents commonly used in current battery electrolytes, as well as lithium salts commonly used in current battery electrolytes. For example, the solvent may include at least one of ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, and γ-butyrolactone; the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0122] This invention does not specifically limit the material of the separator; it can be any separator material commonly used in batteries. For example, the separator can be selected from any of the following: polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven fabric separator, and separator with ceramic coating.

[0123] This invention does not specifically limit the method of battery preparation. In one embodiment, the battery can be prepared by a method including the following steps:

[0124] The above-mentioned positive electrode sheet, separator, and negative electrode sheet are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried, electrolyte is injected into the dried battery. After the battery is placed, formed, and resealed, the secondary battery is completed.

[0125] The present invention will be further described below through specific embodiments.

[0126] Example 1

[0127] The method for preparing the positive electrode in this embodiment includes:

[0128] (1) Preparation of basic electrode: NCM811, Super P and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 95:3:2 to form a slurry, which is then coated on both sides of an aluminum foil. After drying and rolling, the basic electrode is obtained. The basic electrode includes a positive current collector and an active material layer. The areal density of the active material layer is 20 mg / cm³. 2 .

[0129] (2) Preparation of the functional layer: Paraffin microcapsules (the phase change material matrix is ​​paraffin-C22, and the shell is melamine-formaldehyde resin), thermally conductive filler carbon nanotubes, and binder PVDF are dispersed in NMP solvent at a mass percentage of 50%:40%:10% and ultrasonically treated to obtain a uniform suspension. A high-precision slit extrusion coating machine is used to coat both sides of the base electrode surface, and after drying and rolling, a positive electrode is obtained. This positive electrode includes a positive current collector, a functional layer coated on both sides of the positive current collector, and a functional layer coated on the surface of the active material layer. The areal density of the functional layer is 1.5 mg / cm³. 2 The thickness is 4.2 μm. A schematic diagram of the positive electrode structure is shown below. Figure 1 .

[0130] Examples 2-27 and Comparative Examples 1-6 are basically the same as Example 1, with differences shown in Tables 1A, 1B, 2A, 2B, 2-1A, and 2-1B.

[0131] A schematic diagram of the positive electrode structure in Example 20 is shown below. Figure 2 .

[0132] Experimental Example 1

[0133] 1. The relevant parameter tests for the examples and comparative examples are shown in Tables 1A, 1B, 2A, 2B, 2-1A, and 2-1B.

[0134] 2. Testing Methods

[0135] 1) Test methods for the areal density of the functional layer and the areal density of the active material layer

[0136] After disassembling the battery under test, the positive electrode sheet was removed and rinsed with dimethyl carbonate (DMC) to remove residual electrolyte. It was then vacuum-dried at 80°C for 2 hours. The dried electrode sheet was then stamped into a circular disc with an area of ​​S using a stamping machine. The disc was immersed in liquid nitrogen for 5 minutes, then quickly removed and sheared along the edge of the positive electrode sheet to brittlely peel off the functional layer and active material layer (after each peeling, the active layer was observed to be exposed; peeling was repeated until the functional layer and active material layer were separated). The separated functional layer and active material layer were collected, dried, and weighed separately. The mass of the functional layer was m1, and the mass of the active material layer was m2. The areal density of the functional layer was calculated using the formula m1 / S, and the areal density of the active material layer was calculated using the formula m2 / S.

[0137] 2) Test methods for the thickness of the functional layer and the active material layer

[0138] The cross-section was observed using a scanning electron microscope (SEM). Ten points were randomly selected at 500-2000x magnification to measure the thickness of the active material layer (or functional layer), and the arithmetic mean was taken.

[0139] 3) Test methods for phase transition temperature, latent heat of phase transition, and undercooling of phase change materials

[0140] Determination of latent heat of phase change and supercooling: Differential scanning calorimetry (DSC) was used for determination. 10 mg of the separated functional layer or scraped active material layer powder was placed in an aluminum crucible. Under a nitrogen atmosphere (flow rate 50 mL / min), the temperature was increased from 0℃ to 80℃ at a rate of 5℃ / min, held at this temperature for 3 min to eliminate thermal history, and then cooled back to 0℃ at the same rate. The heat flow curves during the heating and cooling processes were recorded.

[0141] Latent heat of phase change: The area of ​​the endothermic peak on the integral heating heat flux curve, expressed in J / g.

[0142] Supercooling: The difference between the peak temperature of heating (Tm) and the peak temperature of cooling (Tc), i.e., ΔT = Tm - Tc.

[0143] Determination of Young's modulus of the outer shell: Atomic force microscopy (AFM) nanoindentation method was used according to ISO 14577 standard. The sample to be tested (electrode cross-section) was fixed on the stage. Microcapsules with a smooth surface and a diameter greater than 5 μm were selected. Indentation tests were performed using a diamond indenter in load-controlled mode, with a maximum load of 500 μN, a loading / unloading rate of 50 μN / s, a holding time of 10 s, and an indentation depth controlled within 10% of the shell thickness. The unloading segment of the load-displacement curve was analyzed using the Oliver-Pharr method to calculate the Young's modulus value. At least 10 different microcapsules were tested for each sample, and the average value was taken.

[0144] Table 1A

[0145]

[0146] Table 1B

[0147]

[0148] Table 2A

[0149]

[0150] Table 2B

[0151]

[0152] Table 2-1A

[0153]

[0154] Table 2-1B

[0155]

[0156] Experimental Example 2

[0157] 1. The positive electrode sheets of the examples and comparative examples were prepared into secondary batteries and subjected to mass energy density testing, cycle performance testing, heating testing, short circuit testing, and overcharge testing. The results are shown in Tables 3A and 3B.

[0158] Methods for preparing secondary batteries:

[0159] Negative electrode preparation: By weight percentage, 95% graphite, 1% conductive carbon black, 2% polyacrylic acid (PAA) binder, and 2% styrene-butadiene rubber (SBR) binder were mixed, and deionized water was added and stirred evenly to obtain a slurry. The slurry was coated onto a 6μm thick copper foil, dried at 95℃, rolled (after rolling, the porosity of the negative electrode was 32%), and slit to obtain the negative electrode. The active material layer density of this negative electrode was 11 mg / cm³. 2 .

[0160] The positive electrode sheets from the examples and comparative examples were stacked together with the aforementioned negative electrode sheet and a 10μm thick polyethylene (PE) separator (purchased from Yunnan Enjie New Material Co., Ltd.), and packaged into a soft-pack model with a thickness of 10mm, a width of 100mm, and a height of 100mm. Moisture was removed by baking, and then 60g of electrolyte (model QCA50, purchased from Zhuhai Saiwei Electronic Materials Co., Ltd.) was injected. After formation, secondary sealing and degassing, and capacity testing, a secondary battery was obtained.

[0161] 2. Testing Methods

[0162] 1) Test method for mass energy density

[0163] The secondary battery was charged to 4.2V at 1C constant current and constant voltage at 25℃, then charged at constant voltage until the current dropped to 0.05C (cutoff). After standing for 5 minutes, it was discharged to 2.5V at 1C. This discharge energy is recorded as E (unit: Wh). The battery weight is recorded as W (unit: kg). The mass energy density is calculated as E / W, unit: Wh / kg.

[0164] 2) Cyclic performance testing methods

[0165] The battery is charged at a constant current of 1C to the upper limit voltage of 4.2V at 45°C, then charged at a constant voltage, with the cut-off current being 0.05C. It is left to stand for 10 minutes, then discharged at a constant current of 1C to the lower limit voltage of 2.5V and left to stand for 10 minutes. The charging - standing - discharging - standing steps are repeated, and the capacity retention rate comparison results for 500 cycles are recorded. At the same time, the maximum temperature on the battery surface is monitored. Measure the discharge capacity Q1 during the first cycle and the discharge capacity Q during the 500th cycle. 500 .

[0166] The capacity retention rate Q = Q 500 / Q1 * 100%.

[0167] 3) Test method for the heating test

[0168] After the battery cell is fully charged (charged at 0.33C until 4.2V and cut off at 0.05C), the battery is placed in a forced-air oven and heated from room temperature to (130 ± 2)°C at a rate of (5 ± 2)°C / min, and kept at this temperature for 30 minutes. If the battery does not explode or catch fire, it passes; otherwise, it fails. 10 batteries of each embodiment and comparative example are tested in parallel, and the passing rate is calculated.

[0169] Table 3A

[0170]

[0171] Table 3B

[0172]

[0173] As can be seen from Tables 3A and 3B, compared with the comparative examples, the secondary batteries made from the positive electrode sheets of the embodiments of the present invention have excellent thermal safety, cycle life, and energy density.

[0174] Finally, it should be noted that: After considering the specification and practicing the invention disclosed herein, those skilled in the art will readily think of other embodiments of the present invention. The present invention aims to cover any variations, uses, or adaptive changes of the present invention, which follow the general principles of the present invention and include the common knowledge or conventional technical means in the technical field not disclosed in the present invention. It is not limited to the precise structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A positive electrode plate, characterized in that, The positive electrode includes a positive current collector and an active material layer disposed on at least one surface of the positive current collector; the positive electrode further includes a functional layer disposed on the side of the active material layer away from the positive current collector. The functional layer includes a phase change material, which includes a phase change material matrix and a shell covering at least a portion of the surface of the phase change material matrix; the latent heat of phase change of the phase change material matrix is ​​not less than 150 J / g, and the supercooling of the phase change material matrix is ​​not greater than 10°C; the Young's modulus of the shell is not less than 2.0 GPa. The positive electrode sheet satisfies: b / a = 1.6-30%; where b is the areal density of the functional layer and a is the areal density of the active material layer.

2. The positive electrode sheet according to claim 1, characterized in that, The positive electrode plate satisfies: b / a = 3.5-20%; And / or, the thickness ratio of the functional layer to the active material layer is 1.6-30%.

3. The positive electrode sheet according to claim 1 or 2, characterized in that, The areal density of the functional layer is 0.5-5 mg / cm³. 2 Preferably 1-2 mg / cm 2 ; and / or, the thickness of the functional layer is 1-25 μm, preferably 3-10 μm; and / or, the areal density of the active material layer is 7-30 mg / cm³. 2 ; and / or, the thickness of the active material layer is 19-150 μm.

4. The positive electrode sheet according to any one of claims 1-3, characterized in that, The phase change material has an average particle size of 0.5-8 μm; and / or, the phase change temperature of the phase change material is 45-65℃; Preferably, the phase change material matrix includes at least one of paraffin wax, fatty acid, and polyethylene glycol; Preferably, the outer shell comprises at least one of melamine-formaldehyde resin, polymethyl methacrylate, and silica; and / or, The active material layer includes a positive electrode active material, which includes at least one of lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium iron phosphate, and lithium manganese oxide.

5. The positive electrode sheet according to any one of claims 1-4, characterized in that, The functional layer also includes thermally conductive fillers and binders; Preferably, by mass percentage, the functional layer comprises: 50-90% phase change material, 5-40% thermally conductive filler, and 5-10% binder; Preferably, the thermally conductive filler includes at least one of boron nitride, alumina, and carbon nanotubes, and the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylic acid, and polyacrylonitrile.

6. The positive electrode sheet according to any one of claims 1-5, characterized in that, In the thickness direction of the positive electrode current collector, the active material layer includes a first active material layer and a second active material layer stacked together, with the second active material layer located between the positive electrode current collector and the first active material layer; Based on the total weight of the active material layers, the content of the first active material layer is 5-50%, and the content of the second active material layer is 50-95%. The positive electrode plate satisfies: C1>C2; Wherein, C1 is the mass percentage of the phase change material in the first active material layer, and C2 is the mass percentage of the phase change material in the second active material layer.

7. The positive electrode sheet according to claim 6, characterized in that, The thickness ratio of the first active material layer to the second active material layer is 0.0526-1; and / or, C1 / C2≥3.

8. The positive electrode sheet according to claim 6 or 7, characterized in that, 1%≤C1≤8%; and / or, 0%≤C2≤2%.

9. The positive electrode sheet according to any one of claims 1-5, characterized in that, The active material layer includes a first active material layer and a second active material layer, the second active material layer being close to the positive electrode current collector, and the first active material layer being disposed on the surface of the second active material layer facing away from the positive electrode current collector; The first active material layer includes a first phase change material, the second active material layer includes a second phase change material, and the positive electrode sheet satisfies: L1 > L2; wherein, L1 is the latent heat of phase change of the phase change material matrix in the first phase change material, and L2 is the latent heat of phase change of the phase change material matrix in the second phase change material.

10. A secondary battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1-9.

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