Positive electrode sheets, secondary batteries, and electrical devices

Abstract

A positive electrode sheet (10), a secondary battery (100), and an electric device (1000). The positive electrode sheet (10) comprises a positive electrode current collector (11), and a first positive electrode coating (12) and a second positive electrode coating (13) laminated in this order on the positive electrode current collector (11). The first positive electrode coating (12) comprises a first positive electrode active material and an inorganic filler. The number of particle accumulation layers in the first positive electrode coating (12) is calculated by the following formula: n=T×(M+1) / (M×D 1 v 50+D 2 v 50), n is in the range of 4 to 7, T is the thickness of the first positive electrode coating (12) in μm, M is the mass ratio of the first positive electrode active material to the inorganic filler contained in the first positive electrode coating (12), and D 1 v 50 is the median particle diameter of the first positive electrode active material, and D 2 v 50 is the median particle size of the inorganic filler, and D 1 v 50 is 0.6 μm to 1.2 μm, and D 2 v 50 is 0.6 μm to 1.5 μm.

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority and benefits of Chinese Patent Application No. 202211047596.1, filed on 29 August 2022. The entire contents of the above-mentioned referenced application are incorporated herein by reference.

[0002] This disclosure relates to the field of battery technology, and more particularly to cathodes, secondary batteries, and electrical devices. [Background technology]

[0003] Rechargeable batteries are widely used in consumer electronics (such as mobile phones and tablet computers) and electric vehicles. However, if a rechargeable battery is pierced with a nail, crushed, subjected to impact, or otherwise mishandled, a short circuit can occur within the battery, releasing a large amount of Joule heat and potentially leading to thermal runaway. Of the common short-circuit modes in batteries, the one occurring at contact between the positive electrode current collector (e.g., aluminum foil) and the negative electrode active material layer releases the most heat and is the most dangerous short-circuit mode in batteries. Therefore, it is necessary to provide effective safety measures against this short-circuit mode. [Overview of the project] [Problems that the invention aims to solve]

[0004] Embodiments of the present disclosure therefore provide a positive electrode, a secondary battery, and an electrical device that effectively solve the problem of thermal runaway caused by the positive electrode current collector of a battery coming into contact with the negative electrode, and that control the increase in the internal resistance of the battery to a small range so as not to affect the electrochemical performance of the battery during normal operation. [Means for solving the problem]

[0005] According to the first aspect, the disclosure further states that Positive electrode current collector and A first positive electrode coating laminated on at least one surface of a positive electrode current collector and containing a first positive electrode active material and an inorganic filler having no electrochemical activity, A second positive electrode coating laminated on a surface of the first positive electrode coating facing away from the positive electrode current collector and containing a second positive electrode active material and The number n of particle filling layers in the first positive electrode coating satisfies the following relational expression n = T×(M + 1) / (M×D 1 v 50 + D 2 v 50), where n is in the range of 4 to 7, where T represents the thickness of the first positive electrode coating measured in μm, M represents the mass ratio of the first positive electrode active material to the inorganic filler, and D 1 v 50 represents the median particle diameter of the first positive electrode active material measured in μm, and D 2 v 50 represents the median particle diameter of the inorganic filler measured in μm, where D 1 v 50 is in the range of 0.6 μm to 1.2 μm, and D 2 v 50 is in the range of 0.6 μm to 1.5 μm, providing a positive electrode.

[0006] In the positive electrode according to the first aspect of the present disclosure, the relationship between the thickness of the first positive electrode coating, the mass ratio of the first positive electrode active material and the inorganic filler, and the number n of particle filling layers in the first positive electrode active material is established, and n is controlled to be within the range of 4 to 7, the median particle diameter of the particles of the first positive electrode active material is controlled to be within the range of 0.6 μm to 1.2 μm, and the median particle diameter of the inorganic filler is controlled to be within the range of 0.6 μm to 1.5 μm. This ensures that the number of filling layers and particle density in the first positive electrode coating are appropriate, effectively protecting the positive electrode current collector in abnormal situations such as when the battery is pierced with a nail or crushed, thereby preventing the positive electrode current collector from contacting the negative electrode and causing thermal runaway, and preventing the internal resistance value of the battery during normal operation from becoming too large and affecting the electrochemical performance.

[0007] In an embodiment of the present disclosure, D 1 v 50 is in the range of 0.6 μm to 0.9 μm.

[0008] In an embodiment of the present disclosure, T is in the range of 2 μm to 10 μm.

[0009] In an embodiment of the present disclosure, T is in the range of 4 μm to 7 μm.

[0010] In an embodiment of the present disclosure, M is in the range of 1 to 5.

[0011] In an embodiment of the present disclosure, the thermal decomposition temperature of the inorganic filler is 300 °C or higher.

[0012] In an embodiment of the present disclosure, the inorganic filler contains one or more of boehmite, alumina, zirconia, silica, or magnesium hydroxide.

[0013] In embodiments of the present disclosure, the first positive electrode active material comprises one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadyl phosphate, or lithium vanadium fluorophosphate, and the second positive electrode active material comprises one or more of lithium cobaltate, lithium manganeseate, lithium nickelate, lithium vanadate, lithium titanate, lithium nickel manganese oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganeseate, lithium nickel cobalt aluminum oxide, or a lithium-rich manganese-based material.

[0014] In the embodiments of this disclosure, the median particle size of the second positive electrode active material is in the range of 6 μm to 18 μm.

[0015] In embodiments of the present disclosure, the first cathode coating further comprises a first binder and a first conductive agent, and the second cathode coating further comprises a second binder and a second conductive agent.

[0016] In embodiments of the present disclosure, the weight percentage of the first binder in the first cathode coating is greater than the weight percentage of the second binder in the second cathode coating.

[0017] In embodiments of the present disclosure, the weight percentage of the first binder in the first cathode coating is in the range of 5% to 10%, and the weight percentage of the first conductive agent in the first cathode coating is in the range of 1% to 3%.

[0018] In embodiments of the present disclosure, the first binder and the second binder each separately comprise one or more of the following: polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-acrylic acid copolymer, polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyacrylate, polyvinyl alcohol (PVA), polyimide (PI), polyurethane, epoxy resin, styrene-butadiene rubber (SBR), fluororubber, or sodium carboxymethylcellulose (CMC-Na).

[0019] In embodiments of the present disclosure, the first conductive agent and the second conductive agent each comprise one or more of conductive carbon black, graphite, carbon fibers, carbon nanotubes, or graphene.

[0020] In embodiments of the present disclosure, the first cathode coating comprises, in weight percent, the following components: 50% to 85% of the first cathode active material, 10% to 40% of the inorganic filler, 5% to 10% of the first binder, and 1% to 3% of the first conductive agent.

[0021] In embodiments of the present disclosure, the second cathode coating comprises, in weight percent, the following components: 90% to 99% of the second cathode active material, 0.5% to 1.5% of the second binder, and 0.5% to 1.5% of the second conductive agent.

[0022] According to a second aspect, the Disclosure provides a secondary battery including a positive electrode according to a first aspect of the Disclosure.

[0023] A secondary battery according to a second aspect of this disclosure has low internal resistance and excellent safety performance.

[0024] According to a third aspect, the Disclosure provides an electrical device including a secondary battery according to a second aspect of the Disclosure. [Brief explanation of the drawing]

[0025] [Figure 1A] This is a schematic diagram of the negative electrode structure according to the embodiments of this disclosure. [Figure 1B] This is a schematic diagram of another negative electrode according to an embodiment of the present disclosure. [Figure 2] This is a schematic diagram of a secondary battery according to an embodiment of the present disclosure. [Figure 3] This is a schematic diagram of an electrical device according to an embodiment of the present disclosure. [Modes for carrying out the invention]

[0026] Embodiments of the present disclosure will be described below with reference to the accompanying drawings of the embodiments of the present disclosure.

[0027] Embodiments of this disclosure, with reference to Figures 1A and 1B, provide a positive electrode 10. The positive electrode 10 includes a positive electrode current collector 11 and a first positive electrode coating 12 and a second positive electrode coating 13 sequentially laminated on at least one surface of the positive electrode current collector 11. The first positive electrode coating 12 includes a first positive electrode active material and an inorganic filler that does not have electrochemical activity. The second positive electrode coating 13 includes a second positive electrode active material. The number n of particle-packed layers in the first positive electrode coating 12 is given by the following relation: n = T × (M + 1) / (M × D) 1 v 50+D 2 v It is specified that satisfies 50), and n is in the range of 4 to 7. Here, T represents the thickness of the first positive electrode coating 12 measured in μm units, M represents the mass ratio of the first positive electrode active material to the inorganic filler, and D 1 v 50 represents the particle size of the first positive electrode active material when the cumulative volume fraction of the first positive electrode active material reaches 50%, and D 2 v 50 represents the particle size of the inorganic filler when the cumulative volume fraction of the inorganic filler reaches 50% (i.e., D 1 v 50 represents the median particle size of the first positive electrode active material, D 2 v50 represents the median particle size of the inorganic filler), all measured in μm units, where D 1 v 50 is in the range of 0.6 μm to 1.2 μm, D 2 v 50 corresponds to a range of 0.6 μm to 1.5 μm.

[0028] In the positive electrode, two positive electrode coatings are simultaneously positioned on one side of the positive electrode current collector 11, and the first positive electrode coating 12, which is closer to the positive electrode current collector 11, is controlled to simultaneously contain the first positive electrode active material and an inorganic filler. The first positive electrode coating 12 can provide a safety protection function, and due to the high thermal stability and non-conductivity of the inorganic filler in the first positive electrode coating, the possibility of the positive electrode current collector 11 coming into contact with the negative electrode active material layer in abnormal situations such as when the battery is crushed, subjected to impact, or pierced with a nail is reduced, thereby reducing the possibility of a short circuit in the battery. In the initial stages of a short circuit inside the battery, the inorganic filler with good thermal conductivity can also transfer the Joule heat generated by the short circuit as quickly as possible, suppressing heat concentration and the continuous rise in battery core temperature, thereby improving the safety of the battery. In addition, the presence of the inorganic filler also reduces the distribution sites of the active material in the first positive electrode coating, reducing the risk of oxidative decomposition of the electrolyte, and thereby suppressing heat generation in the decomposition reaction. The first positive electrode active material participates in the battery's charging and discharging process during normal operation, contributing to its capacity and thereby reducing the energy density loss and increase in the internal resistance of the battery core that can occur due to the use of non-electrochemically active safety coatings (e.g., filler layers).

[0029] It has been found that the number of layers and bulk density of the particles of the first positive electrode active material and inorganic filler in the first positive electrode coating 12 have a significant impact on the safety and internal resistance of the battery, but the industry has not paid much attention to this point. In this disclosure, the thickness T of the first positive electrode coating 12, the mass ratio M of the particles of the first positive electrode active material and inorganic filler, and the median particle size (i.e., D) of the two types of particles are discussed. vA relationship with 50) has been established, and a quantitative relation is defined for the number n of particle packing layers in the first positive electrode coating 12, where n is controlled to be within the range of 4 to 7, the median particle size of the particles of the first positive electrode active material is controlled to be within the range of 0.6 μm to 1.2 μm, and the median particle size of the inorganic filler is controlled to be within the range of 0.6 μm to 1.5 μm. This ensures that the number and density of the packing layers in the first positive electrode coating are appropriate, effectively protecting the positive electrode current collector 11 without excessively sacrificing the energy density of the battery, and preventing the internal resistance of the battery from increasing excessively and affecting the battery's cycle performance and other electrochemical performance.

[0030] Specifically, if n is less than 4, the number of particle packing layers in the first positive electrode coating 12 is too small, and the particle packing is too loose. As a result, in abnormal situations such as the battery being crushed, subjected to impact, or pierced with a nail, the first positive electrode coating 12 cannot adequately protect the positive electrode current collector 11, and there is a high possibility that the positive electrode current collector 11 will come into direct contact with the negative electrode active material layer, making it impossible to guarantee the safety of the battery. If n is greater than 7, the number of particle packing layers in the first positive electrode coating 12 is too large, and the particle packing becomes too dense. As a result, the internal resistance of the battery core becomes too high, which makes it difficult for the electrolyte to penetrate the first positive electrode coating 12, affecting the battery's cycle performance, etc. In addition, in order to ensure that the packing density of the two types of particles after mixing does not become too dense, the median particle sizes of the particles of the first positive electrode active material and the particles of the inorganic filler should not differ significantly.

[0031] D of the particles of the first positive electrode active material 1 v 50 may be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, etc. D 1 v In some embodiments, 50 is in the range of 0.6 μm to 0.9 μm. D of the inorganic filler 2 v50 may be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, etc.

[0032] In this disclosure, D 1 v The specific value of 50 may be obtained from the particle size distribution graph of the first positive electrode active material obtained by laser diffraction. For the test method, please refer to GB / T19077-2016 / ISO13320:2009 "Particle size analysis - Laser diffraction". The test equipment is generally a laser particle size analyzer (for example, a Malvern 3000 laser particle size analyzer). Similarly, D 2 v 50 may be obtained from a particle size distribution graph of the inorganic filler obtained by laser diffraction. The particle size D of the material when the cumulative volume fraction of the material particles reaches 50%. v 50 may also be called the “median particle size” of the material. In addition, T may be obtained by directly or indirectly measuring the thickness of the first cathode coating 12. M may be obtained by analyzing the composition of the first cathode coating 12.

[0033] It should be noted that the laminated structure, including the first positive electrode coating 12 and the second positive electrode coating 13, may be provided on one surface of the positive electrode current collector 11 (as shown in Figure 1A) or on both surfaces of the positive electrode current collector 11 (as shown in Figure 1B). If the laminated structure is provided on each of the two surfaces of the positive electrode current collector 11, it is sufficient that only the first positive electrode coating of the laminated structure on one surface satisfies the condition that parameter n is in the range of 4 to 7. Of course, both first positive electrode coatings of the laminated structure on the two surfaces of the positive electrode current collector may satisfy the condition that parameter n is in the range of 4 to 7. The positive electrode current collector 11 may include aluminum foil, aluminum alloy foil, aluminum-plated organic film, carbon-coated aluminum foil, carbon-coated aluminum alloy foil, carbon-coated aluminum-plated organic film, etc. In some embodiments, the positive electrode current collector 11 is aluminum foil or aluminum alloy foil.

[0034] In embodiments of this disclosure, the mass ratio M of the first positive electrode active material to the inorganic filler may be in the range of 1 to 5. When M is within this range, the safety protection effect of the first positive electrode coating 12 and the energy density of the battery can be ensured.

[0035] To ensure that the first cathode coating 12 has excellent safety performance, in embodiments of this disclosure, the thickness T of the first cathode coating 12 may be controlled to be in the range of 2 μm to 10 μm. Thickness T, as herein, refers to the thickness of a single layer of the first cathode coating. In some embodiments, the thickness T of the first cathode coating 12 may be in the range of 4 μm to 7 μm. In this case, the thickness of the first cathode coating 12 is appropriate to provide an effective safety protection effect and further improve the capacitance performance of the cathode.

[0036] In embodiments of this disclosure, the thermal decomposition temperature of the inorganic filler is 300°C or higher. The inorganic filler has excellent thermal stability and does not fail to function before thermal runaway of the battery occurs, ensuring that the first positive electrode coating 12 effectively provides safety functions. In some embodiments, the inorganic filler may include one or more of boehmite, alumina, zirconia, silica, magnesium hydroxide, etc. The thermal decomposition onset temperature of boehmite is approximately 350°C. The thermal decomposition onset temperature of magnesium hydroxide is approximately 340°C.

[0037] In some embodiments, the inorganic filler is boehmite. Boehmite has excellent thermal conductivity, low hardness, and can quickly transfer Joule heat generated by short circuits in the battery core, thereby suppressing heat concentration. In addition, since the Mohs hardness of boehmite is less than 5, its low hardness virtually eliminates damage to the coating equipment when applying the suspension to produce the first positive electrode coating 12, ensures the flatness of the first positive electrode coating 12, facilitates the adhesion of the second positive electrode coating 13 to the first positive electrode coating 12, and ultimately ensures a high product yield during mass production.

[0038] In this disclosure, the first positive electrode coating 12 may further include a first binder and a first conductive agent, and the second positive electrode coating 13 may further include a second binder and a second conductive agent. The presence of a binder and a conductive agent in the two positive electrode coatings ensures excellent adhesion and conductivity of the coatings. The first binder, the second binder, the first conductive agent, and the second conductive agent may be conventional options in the battery field. The first and second binders are separately selected from one or more of the following: polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-acrylic acid copolymer, polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyacrylates (e.g., polymethyl methacrylate, polymethyl acrylate, polyethyl acrylate, etc.), polyvinyl alcohol (PVA), polyimide (PI), polyurethane, epoxy resin, styrene-butadiene rubber (SBR), fluororubber, sodium carboxymethylcellulose (CMC-Na), etc. The first and second conductive agents are separately selected from one or more of the following, but are not limited to these: conductive carbon black (e.g., acetylene black, Ketjen black, Super P, 350G carbon black, etc.), graphite, carbon fiber, carbon nanotubes, graphene, etc.

[0039] In some embodiments, the weight percentage of the first binder in the first positive electrode coating 12 is greater than the weight percentage of the second binder in the second positive electrode coating 13. The higher binder content in the first positive electrode coating 12 than in the second positive electrode coating 13 ensures stronger adhesion between the first positive electrode coating 12 and the positive electrode current collector 11. Therefore, even if the second positive electrode coating 13 peels off in an abnormal situation such as the battery being crushed, impacted, or pierced with a nail, the first positive electrode coating 12 will not peel off from the surface of the positive electrode current collector 11, thereby preventing the positive electrode current collector 11 from coming into contact with the negative electrode active material layer, increasing resistance to short circuits, reducing the risk of thermal runaway of the battery core, and further improving the safety performance of the battery.

[0040] In addition, given that the weight percentage of the binder in the first positive electrode coating 12 is greater than the weight percentage of the binder in the second positive electrode coating 13, if the weight percentage of the first positive electrode active material in the first positive electrode coating 12 is the same as the weight percentage of the second positive electrode active material in the second positive electrode coating 13, then the weight percentage of the first conductive agent in the first positive electrode coating 12 is smaller than the weight percentage of the second conductive agent in the second positive electrode coating 13. In this way, the first positive electrode coating 12, having slightly lower electrical conductivity, can better cover and passivate the positive electrode current collector 11 when the battery is handled roughly, and reduce contact between the positive electrode current collector 11 and the negative electrode of the battery.

[0041] In embodiments of this disclosure, the weight percentage of the first binder in the first positive electrode coating 12 is 5% to 10%, for example, 5%, 6%, 7%, 8%, 9%, 9.5%, etc. The weight percentage of the first binder not only ensures a good bonding effect, but also allows for an appropriate content of the first positive electrode active material in the first positive electrode coating 12, thereby further improving the energy density of the battery.

[0042] In embodiments of this disclosure, the weight percentage of the first conductive agent in the first positive electrode coating 12 is 1% to 3%. This ensures proper electrical conductivity of the first positive electrode coating 12, resulting in excellent low-temperature performance, rate performance, and cycle performance of the battery, and can also increase the short-circuit internal resistance to further improve battery safety.

[0043] In some embodiments of the present disclosure, the first cathode coating 12 may contain, in weight percent, the following components: 50% to 85% of a first cathode active material, 10% to 40% of an inorganic filler, 5% to 10% of a first binder, and 1% to 3% of a first conductive agent.

[0044] The second positive electrode coating 13 in this disclosure corresponds to the positive electrode active material layer of a conventional battery. In conventional batteries, no safety coating is provided between the positive electrode active material layer and the positive electrode current collector 11. Therefore, the second positive electrode coating 13 is a conventional component in the battery field and primarily performs a function that contributes to energy.

[0045] In some embodiments of the present disclosure, the second cathode coating 13 may contain, in weight percent, the following components: 90% to 99% of the second cathode active material, 0.5% to 1.5% of the second binder, and 0.5% to 1.5% of the second conductive agent.

[0046] In this disclosure, the first positive electrode active material and the second positive electrode active material may be the same material or different materials, and may be separately selected from one or more of the following: lithium iron phosphate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadyl phosphate, lithium vanadium fluorophosphate, lithium cobaltate, lithium manganeseate, lithium nickelate, lithium vanadate, lithium titanate, lithium nickel manganese oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganeseate, lithium nickel cobalt aluminum oxide, and lithium-rich manganese-based materials. These positive electrode active materials may or may not be doped, or may be modified by doping, and may or may not have a cladding layer on their surface. Generally, phosphate-based active materials generally have a coating layer.

[0047] To ensure superior battery safety, in embodiments of this disclosure, the structural stability of the first positive electrode active material is higher than that of the second positive electrode active material. In this case, the first and second positive electrode active materials are different materials. In some embodiments, the first positive electrode active material is a phosphate-based active material, and the second positive electrode active material is a layered transition metal oxide. Specifically, the first positive electrode active material may include one or more of the following: lithium iron phosphate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadyl phosphate, lithium vanadium fluorophosphate, etc. The second positive electrode active material may include one or more of the following: lithium cobaltate, lithium manganeseate, lithium nickelate, lithium vanadate, lithium titanate, lithium nickel manganese oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganeseate, lithium nickel cobalt aluminum oxide, lithium lithium lithium-based material. In some embodiments, the second positive electrode active material is lithium cobalt oxide having a high operating voltage and a large specific capacitance.

[0048] In embodiments of this disclosure, the median particle size (D) of the second cathode active material is specified. 3 vThe median particle size (which may be defined as 50) is in the range of 6 μm to 18 μm. In this case, the median particle size of the second positive electrode active material is large, which controls the specific surface area of ​​the second positive electrode active material within an appropriate range, thereby achieving high structural stability and facilitating lithium ion conduction. In some embodiments, the median particle size of the second positive electrode active material is in the range of 14 μm to 16 μm.

[0049] The positive electrode 10 may be manufactured by the following method: A first positive electrode suspension containing a first positive electrode active material, an inorganic filler, a first binder, and a first conductive agent is applied to at least one surface of the positive electrode current collector 11 and dried to form a first positive electrode coating 12; then a second positive electrode suspension containing a second positive electrode active material, a second binder, and a second conductive agent is applied and dried to form a second positive electrode coating 13; and then the positive electrode is obtained by pressing.

[0050] One or both sides of the positive electrode current collector may be coated. In other words, the laminated structure including the first positive electrode coating and the second positive electrode coating may be formed on one surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector. If both sides of the positive electrode current collector are coated, only the first positive electrode coating on one surface needs to satisfy the requirement that parameter n is within the above range.

[0051] Referring to Figure 2, embodiments of the present disclosure further provide a secondary battery 100. The secondary battery 100 includes the positive electrode 10 described above. Since the secondary battery 100 includes the positive electrode 10 described above, the secondary battery 100 has excellent safety performance and high energy density.

[0052] The secondary battery may be a liquid battery using a liquid electrolyte, or a semi-solid or all-solid battery using a semi-solid or solid electrolyte. Specifically, the secondary battery may be a lithium secondary battery, sodium secondary battery, potassium secondary battery, magnesium secondary battery, aluminum secondary battery, zinc secondary battery, etc.

[0053] In some embodiments, the secondary battery may include the positive electrode, negative electrode, and semi-solid or solid electrolyte between the positive and negative electrodes as described above. In addition, when a semi-solid or solid electrolyte is used, the positive and negative electrodes may further include the semi-solid or solid electrolyte material.

[0054] In some other embodiments, the secondary battery may include the positive electrode, negative electrode, and separator described above, as well as an electrolyte between the positive and negative electrodes. The secondary battery may be assembled by the following method, namely by sequentially stacking the positive electrode, separator, and negative electrode to create a battery core, placing the battery core in a battery housing, injecting the electrolyte, and then sealing the battery housing to obtain the secondary battery.

[0055] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. The negative electrode current collector may, but is not limited to, copper foil, copper alloy foil, carbon-coated copper foil, copper-plated thin film, etc. The negative electrode active material in the negative electrode active material layer may, but is not limited to, one or more of carbon materials, silicon-based materials, tin-based materials, or lithium titanate. Carbon materials may include one or more of soft carbon, hard carbon, carbon fiber, graphitized carbon microbeads, artificial graphite, or natural graphite. Silicon-based materials may include one or more of elemental silicon, silicon alloys, silicon oxides, silicon-carbon composite materials, etc. Tin-based materials may include one or more of elemental tin, tin oxide, tin-based alloys, tin-carbon compounds, etc.

[0056] A separator is generally a porous substrate through which active ions pass. The separator may have a single-layer structure or a multi-layer structure. The separator may include, but is not limited to, polymer separators such as single-layer PP film, single-layer polyethylene (PE) film, double-layer PP / PE film, double-layer PP / PP film, or triple-layer PP / PE / PP film, or nonwoven fabrics. The electrolyte generally contains an electrolyte salt containing active ions (e.g., lithium salt), a solvent (e.g., organic solvent), and optional additives. The organic solvent may include, but is not limited to, one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethylmethyl carbonate, propyl propionate, ethyl propionate, and ethyl acetate.

[0057] Referring to Figure 3, embodiments of the present disclosure further provide an electrical device 1000, which includes a secondary battery 100 according to embodiments of the present disclosure. The electrical device 1000 may be a vehicle such as an automobile (e.g., an electric vehicle), a boat, or a 3C product (e.g., a mobile phone, a tablet computer, or a smart band).

[0058] The technical solutions of this disclosure are further described below through several embodiments. [Examples]

[0059] The positive electrode was fabricated using the following steps.

[0060] (1) The first positive electrode active material (specifically, D with a median particle size of 0.8 μm, as shown in Table 1) 1 v Lithium iron phosphate having 50), inorganic filler (specifically, D with a median particle size of 0.8 μm) 2 vBoehmite (having thermal decomposition onset temperatures of 50°C and 350°C), a binder (specifically PVDF), and a conductive agent (specifically conductive carbon black) were mixed in the solvent N-methylpyrrolidone (NMP) in a mass ratio of 57:30:10:3 (where parameter M is 1.9), and homogeneously stirred to obtain the first cathode suspension.

[0061] The second positive electrode active material (specifically, D with a median particle size of 16 μm) 3 v A lithium cobalt oxide (containing 50), a binder (specifically PVDF), and a conductive agent (specifically conductive carbon black) were mixed in solvent NMP in a mass ratio of 98:1:1 and uniformly stirred to obtain a second positive electrode suspension.

[0062] (2) The first cathode suspension is applied to one surface of the aluminum foil and dried to form a first cathode coating with a thickness T of 5 μm. Next, the second cathode suspension is applied on the first cathode coating and dried to form a second cathode coating with a thickness of 47 μm. Then, the first cathode coating and the second cathode coating are similarly formed on the other surface of the aluminum foil in sequence to obtain a double-sided electrode. Finally, the double-sided electrode is rolled to obtain a cathode for later use.

[0063] A lithium secondary battery was fabricated using the following steps.

[0064] 1) Preparation of the negative electrode: The negative electrode active material (specifically artificial graphite), conductive carbon black, SBR, and carboxymethylcellulose (CMC) were dispersed in deionized water in a mass ratio of 97:1:1:1 and uniformly stirred to obtain a negative electrode suspension. The negative electrode suspension was coated onto both surfaces of a copper foil, dried, and rolled to obtain the negative electrode.

[0065] 2) Battery assembly: The positive electrode, separator, and negative electrode of the embodiment were sequentially stacked and wound to form a battery core. The battery core was packaged in an aluminum-plastic film housing. Then, electrolyte was injected into the battery housing. After formation, ventilation, secondary packaging, and capacity grading, a lithium-ion battery was obtained. The lithium-ion battery has a capacity of 4500mAh and a 2C charging system with a charge cutoff voltage of 4.45V.

[0066] Other examples Based on the parameters in Table 1 below, the positive electrodes and secondary batteries of the other embodiments were fabricated using the method for fabricating the positive electrode and secondary battery described in Example 1. The calculation results for the parameter n specified above are also summarized in Table 1.

[0067] Comparative Example 1 Comparative Example 1 differs from Example 1 in that the first positive electrode coating does not contain an inorganic filler that does not have electrochemical activity.

[0068] Comparative Example 2 Comparative Example 2 differs from Example 1 in that the positive electrode does not contain the first positive electrode coating.

[0069] [Table 1]

[0070] To highlight the beneficial effects of the technical solutions disclosed herein, the following performance tests were also performed on lithium-ion batteries. The results are summarized in Table 1 above.

[0071] (1) Internal resistance test Each lithium-ion battery was placed in a room temperature environment (23°C to 25°C) and charged to 3.95V with a constant current of 0.2C, and then charged to a cutoff current of 0.01C with a constant voltage of 3.95V. The internal resistance of the battery core was then tested using a 1kHz alternating-current (AC) internal resistance meter. Ten batteries were tested for each example and comparative example, and the average of the test results was taken. Using the internal resistance of Comparative Example 2 (approximately 25mΩ) as a baseline, the increase in internal resistance of the batteries in the example and other comparative examples was calculated. A battery was considered to have passed the internal resistance test if its increase in internal resistance was 30% or less.

[0072] (2) Nail penetration test Each lithium-ion battery was placed in a room temperature environment (23°C to 25°C) and charged to 4.45V with a constant current of 0.2C, and then charged to a cutoff current of 0.025C with a constant voltage of 4.45V. The lithium-ion batteries were then mounted in a needle-piercing test apparatus at an ambient temperature of 23°C to 25°C and a humidity of 40% to 60%. The center of the lithium-ion battery was pierced with a 3mm diameter steel needle at a constant speed of 50mm / second. The steel needle was left inside the battery for 10 minutes, then withdrawn, and it was observed whether the battery ignited or exploded. Ten batteries were tested for each example and comparative example. The number of batteries that did not ignite or explode was recorded, and the nail-piercing test pass rate was obtained. A nail-piercing test pass rate of 100% was considered to have passed the nail-piercing test.

[0073] As shown in Table 1, when the positive electrode of the embodiment of the present disclosure is constructed such that the specified particle packing parameter n of the first positive electrode coating is in the range of 4 to 7, and the average particle diameters of the first and second positive electrode active materials are in the range of 0.6 μm to 1.2 μm and 0.6 μm to 1.5 μm, respectively, a lithium-ion battery made using that positive electrode passed the nail penetration test and demonstrated the excellent safety performance of the lithium-ion battery. In addition, the rate of increase in internal resistance of the lithium-ion battery of the embodiment of the present disclosure was low, at 30% or less, compared to the battery of Comparative Example 2 which did not include the safety coating, which was advantageous in ensuring the excellent cycle performance of the battery.

[0074] Comparing Example 1 with Comparative Example 2, it can be seen that the positive electrode current collector is protected by the first positive electrode coating, and when the battery is penetrated by a steel needle, the positive electrode current collector does not come into direct contact with the negative electrode active material, and as a result, the risk of thermal runaway of the lithium-ion battery is significantly reduced. Comparing Example 1 with Comparative Example 1, it can be seen that the safety of the battery can be further improved when an inorganic filler with high thermal stability is added to the first positive electrode coating.

[0075] Comparing Examples 1 and 2 with Comparative Examples 3 to 6, when the median particle size of the particles of the first positive electrode active material and the inorganic filler particles remains unchanged, if the thickness T of the first positive electrode coating is less than 4 (as in Comparative Examples 5 to 6), the pass rate of the battery in the nail-piercing test decreases, indicating that the first positive electrode coating could not effectively provide safety protection. If n exceeds 7 (as in Comparative Examples 3 to 4), the increase in the internal resistance of the battery is too large, exceeding 30% compared to Comparative Example 2, which does not include the first positive electrode coating. If the number of particle-packed layers n is greater than 4 and less than 7 (as in Examples 1 to 2), safety can be ensured while reliably preventing an excessive increase in the internal resistance of the battery.

[0076] Comparing Examples 1 and 3 to 4 with Comparative Examples 7 to 9, the median particle size D of the first positive electrode active material (i.e., lithium iron phosphate)1 v If the thickness T of the first positive electrode coating remains unchanged, the median particle size D of the inorganic filler that does not have electrochemical activity is... 2 v It was found that a value of 50 did not significantly affect the safety of the battery, but it had a large impact on the battery's internal resistance. In Comparative Examples 7 and 8, the median particle size of the inorganic filler was too small, resulting in the number of particle packing layers n being greater than 7, and the increase in the battery's internal resistance exceeding 30%. In Comparative Example 9, the median particle size of the inorganic filler was too large, and consequently, the increase in the battery's internal resistance was also too large. This is mainly because the inorganic filler is an insulator of electrons and ions, and the excessively large particle size of the inorganic filler may have caused localized blockage of electron and ion pathways within the first positive electrode coating, leading to a significant increase in the battery's internal resistance. When the median particle size of the inorganic filler was in the range of 0.8 μm to 1.5 μm, an appropriate number of packing layers could be achieved, and the increase in the battery's internal resistance could be controlled within a small range.

[0077] Comparing Examples 1, 5 to 6, and Comparative Examples 10 to 13, the thickness T of the first cathode coating and the median particle size D of the inorganic filler are different. 2 v When 50 remained unchanged, it was found that the median particle size of the first positive electrode active material (i.e., lithium iron phosphate) had a significant impact on the battery's safety performance and internal resistance. When the median particle size of lithium iron phosphate was too small (e.g., as in Comparative Examples 10 to 11), the number n of particle packing layers in the first positive electrode coating was too large, resulting in a denser packing of small particles and an excessively high rate of increase in the battery's internal resistance. When the median particle size of lithium iron phosphate was too large (e.g., as in Comparative Examples 12 to 13), the rate of increase in the battery's internal resistance was very small, but the nail-piercing test pass rate decreased significantly. This may be due to the fact that the excessively large particle size led to an excessively small value of n, preventing the first positive electrode coating from providing effective protection.

[0078] The embodiments described above are merely illustrative of some exemplary embodiments of the Disclosure, and while the descriptions are specific and particular, they should not be construed as limiting the scope of the Disclosure. Those skilled in the art should be noted that several modifications and improvements can be made without departing from the concepts of the Disclosure, and all such modifications and improvements are deemed to fall within the scope of the Disclosure. Accordingly, the scope of the Disclosure is defined by the appended claims.

Claims

1. Positive electrode current collector (11), A first positive electrode coating (12) is laminated on at least one surface of the positive electrode current collector (11) and comprises a first positive electrode active material and an inorganic filler that does not have electrochemical activity, A second positive electrode coating (13) is laminated on the surface of the first positive electrode coating (12) that faces away from the positive electrode current collector (11) and comprises a second positive electrode active material. Equipped with, The number n of particle packing layers in the first positive electrode coating (12) is given by the following relation: n=T×(M+1) / (M×D １ ｖ 50+D ２ ｖ It is specified that the condition 50) is met, and n is in the range of 4 to 7. The inorganic filler comprises boehmite, T represents the thickness of the first positive electrode coating (12) measured in μm, M represents the mass ratio of the first positive electrode active material to the inorganic filler, D １ ｖ 50 represents the median particle diameter of the first positive electrode active material measured in μm, D ２ ｖ 50 represents the median particle diameter of the inorganic filler measured in μm, D １ ｖ 50 is in the range of 0.6 μm to 1.2 μm, D ２ ｖ A positive electrode (‎10) in which 50 is in the range of 0.6 μm to 1.5 μm.

2. D １ ｖ The positive electrode (10) according to claim 1, wherein 50 is in the range of 0.6 μm to 0.9 μm.

3. The positive electrode (10) according to claim 1 or 2, wherein T is in the range of 2 μm to 10 μm.

4. The positive electrode (10) according to claim 3, wherein T is in the range of 4 μm to 7 μm.

5. The positive electrode (10) according to claim 1 or 2, wherein M is in the range of 1 to 5.

6. The positive electrode (10) according to claim 1 or 2, wherein the thermal decomposition temperature of the inorganic filler is 300°C or higher.

7. The positive electrode (10) according to claim 1 or 2, wherein the first positive electrode active material comprises one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadyl phosphate, or lithium vanadium fluorophosphate, and the second positive electrode active material comprises one or more of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium vanadate, lithium titanate, lithium nickel manganese oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or a lithium-rich manganese-based material.

8. The positive electrode (10) according to claim 1 or 2, wherein the median particle diameter of the second positive electrode active material is in the range of 6 μm to 18 μm.

9. The positive electrode (10) according to claim 1 or 2, wherein the first positive electrode coating (12) further comprises a first binder and a first conductive agent, and the second positive electrode coating (13) further comprises a second binder and a second conductive agent.

10. The positive electrode (10) according to claim 9, wherein the weight percentage of the first binder in the first positive electrode coating (12) is greater than the weight percentage of the second binder in the second positive electrode coating (13).

11. The positive electrode (10) according to claim 9, wherein the weight percentage of the first binder in the first positive electrode coating (12) is in the range of 5% to 10%, and the weight percentage of the first conductive agent in the first positive electrode coating (12) is in the range of 1% to 3%.

12. The positive electrode (10) according to claim 9, wherein the first binder and the second binder each separately comprise one or more of the following: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-acrylic acid copolymer, polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyacrylate, polyvinyl alcohol (PVA), polyimide (PI), polyurethane, epoxy resin, styrene-butadiene rubber (SBR), fluororubber, or sodium carboxymethylcellulose (CMC-Na).

13. A secondary battery (100) comprising a positive electrode (10) according to claim 1 or 2.

14. An electrical device (1000) comprising a secondary battery (100) as described in claim 13.

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