Secondary batteries and electrical devices

Abstract

This application discloses a secondary battery and an electrical device, relating to the field of battery technology. The application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The modified positive and negative active materials specified in this application result in a secondary battery that exhibits excellent high-rate charge-discharge capacity retention under ultra-low temperature conditions, effectively reduces charging internal resistance, and suppresses lithium dendrite growth.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0002] Lithium-ion rechargeable batteries are widely used due to their high safety, long cycle life, and cost advantages. However, their poor low-temperature performance has been a long-standing technical bottleneck, especially for lithium iron phosphate (LiFePO4, LFP) batteries. In extreme low-temperature environments below 0°C, especially below -30°C, the intrinsic electronic conductivity and lithium-ion diffusion coefficient of LFP batteries decrease sharply. Simultaneously, the electrolyte viscosity increases and conductivity decreases, leading to a significant increase in internal resistance and severe capacity decay during charging and discharging. Furthermore, during low-temperature charging, the lithium-ion insertion kinetics in the graphite anode are extremely slow, far lower than the rate of extraction from the cathode, easily leading to the precipitation of metallic lithium on the anode surface (lithium dendrites), causing capacity loss and safety hazards. Summary of the Invention

[0003] Based on this, the purpose of this application is to overcome the shortcomings of the prior art and provide a secondary battery and power device that has excellent high-rate charge and discharge capacity retention in ultra-low temperature environments, and effectively reduces charging internal resistance and suppresses lithium dendrite growth.

[0004] To achieve the above objectives, in a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a modified positive active material. The modified positive active material includes a positive active material and a coating layer. The coating layer includes a coating material. The coating material has a core-shell structure. The core includes a first sulfide, and the shell includes a second sulfide. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative active material layer includes a modified negative active material, which includes a negative active material, a third sulfide, and a fourth sulfide. The first sulfide and the third sulfide each independently comprise tungsten-based sulfides, and the second sulfide and the fourth sulfide each independently comprise copper-based sulfides.

[0005] In some embodiments of this application, the mass ratio of the core to the shell in the core-shell structure is 1:(0.5-6).

[0006] In some embodiments of this application, the mass percentage of the coating layer is 0.3-3% based on the total mass of the modified positive electrode active material.

[0007] In some embodiments of this application, the mass ratio of the third sulfide to the fourth sulfide in the modified negative electrode active material is 1:(1-4).

[0008] In some embodiments of this application, the mass percentage of the third sulfide and the fourth sulfide is 0.2-2% based on the total mass of the modified negative electrode active material.

[0009] In some embodiments of this application, the particle size Dv50 of the first sulfide is ≤100nm.

[0010] In some embodiments of this application, the particle size Dv50 of the second sulfide is ≤50nm.

[0011] In some embodiments of this application, the particle size Dv50 of the third sulfide is ≤100nm.

[0012] In some embodiments of this application, the particle size Dv50 of the fourth sulfide is ≤50nm.

[0013] In some embodiments of this application, the tungsten-based sulfide has a mass percentage content of ≥50% based on the total mass of the first sulfide.

[0014] In some embodiments of this application, the tungsten-based sulfide has a mass percentage content of ≥50% based on the total mass of the third sulfide.

[0015] In some embodiments of this application, the first sulfide and the third sulfide also independently include at least one of molybdenum-based sulfide, vanadium-based sulfide, and titanium-based sulfide.

[0016] In some embodiments of this application, the tungsten-based sulfide is at least one of WS2 and WS3.

[0017] In some embodiments of this application, the copper-based sulfide is at least one of CuS, Cu2S, and CuS2.

[0018] In some embodiments of this application, the molybdenum-based sulfide is at least one of MoS2, MoS3, and MoS.

[0019] In some embodiments of this application, the vanadium-based sulfide is at least one of VS, Ti2S3, VS2, and VS4.

[0020] In some embodiments of this application, the titanium-based sulfide is at least one of TiS, TiS2, and TiS3.

[0021] In some embodiments of this application, the positive electrode active material layer further includes a positive electrode conductive agent and a positive electrode binder; the modified positive electrode active material has a mass percentage content of 90-99% based on the total mass of the positive electrode active material layer.

[0022] In some embodiments of this application, the negative electrode active material layer further includes a negative electrode conductive agent and a negative electrode binder; the mass percentage of the modified negative electrode active material is 90-99% based on the total mass of the negative electrode active material layer.

[0023] In a second aspect, this application provides an electrical device including the aforementioned secondary battery.

[0024] Compared to existing technologies, the beneficial effects of this application are as follows: On the one hand, the modified positive electrode active material includes a positive electrode active material and a core-shell structured coating material. Tungsten-based sulfide forms the core, enhancing electronic conductivity through interlayer electron transitions and providing a high-speed electron conduction channel. Copper-based sulfide forms the shell, reducing the lithium-ion migration barrier through interface engineering, such as optimizing the lithium-ion migration barrier and suppressing side reactions through Cu-S bond polarity regulation. On the other hand, the modified negative electrode active material includes a negative electrode active material, tungsten-based sulfide, and copper-based sulfide. The modified negative electrode active material formed by these components can increase lithium-ion insertion sites and alleviate low-temperature volume expansion. This application provides modified positive and negative electrode active materials, and the resulting secondary battery exhibits excellent high-rate charge-discharge capacity retention under ultra-low temperature conditions, effectively reducing charging internal resistance and suppressing lithium dendrite growth. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0026] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0027] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0028] An embodiment of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte; The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a modified positive active material. The modified positive active material includes a positive active material and a coating layer. The coating layer includes a coating material. The coating material has a core-shell structure. The core includes a first sulfide, and the shell includes a second sulfide. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative active material layer includes a modified negative active material, which includes a negative active material, a third sulfide, and a fourth sulfide. The first sulfide and the third sulfide each independently comprise tungsten-based sulfides, and the second sulfide and the fourth sulfide each independently comprise copper-based sulfides.

[0029] On the one hand, the modified positive electrode active material includes a positive electrode active material and a core-shell structured coating material. Tungsten-based sulfide forms the core, enhancing electronic conductivity through interlayer electron transitions and providing a high-speed electron conduction channel. Copper-based sulfide forms the shell, reducing the lithium-ion migration barrier through interface engineering, such as optimizing the lithium-ion migration barrier and suppressing side reactions through Cu-S bond polarity regulation. On the other hand, the modified negative electrode active material includes a negative electrode active material, tungsten-based sulfide, and copper-based sulfide. The modified negative electrode active material formed by these components can increase lithium-ion insertion sites and alleviate low-temperature volume expansion. This application provides modified positive and negative electrode active materials. The resulting secondary battery exhibits excellent high-rate charge-discharge capacity retention under ultra-low temperature conditions and effectively reduces charging internal resistance, suppressing lithium dendrite growth.

[0030] The preparation method of the modified positive electrode active material described in this application is not limited, and all preparation methods that can obtain the modified positive electrode active material are applicable to this application; in some embodiments, the preparation method of the modified positive electrode active material is as follows: S1. Mix the first sulfide and the second sulfide (ball milling, 100-300 rpm, 4-8 h), heat-treat at 250-350℃ for 1-3 h in an argon atmosphere to form a core-shell structure coating material. Grind the dried coating material through a 300-500 mesh sieve to obtain core-shell powder. S2. Thoroughly mix the positive electrode active material with the core-shell powder, ultrasonically disperse for 20-40 minutes, and heat-treat at 400-600℃ for 1-3 hours under an argon atmosphere to form a coating layer. Grind the above material through a 300-500 mesh sieve to obtain the final product, modified positive electrode active material—positive electrode active material with a core-shell structure coating layer on its surface.

[0031] The preparation method of the modified negative electrode active material described in this application is not limited, and all preparation methods that can prepare the modified negative electrode active material are applicable to this application; in some embodiments, the preparation method of the modified negative electrode active material is as follows: the negative electrode active material, the third sulfide and the fourth sulfide are thoroughly mixed, ultrasonically dispersed for 20-40 min, heat-treated at 400-600℃ for 1-3 h in an argon atmosphere, and the above material is ground through a 300-500 mesh sieve to prepare the modified negative electrode active material.

[0032] In some embodiments, the mass ratio of the core to the shell in the core-shell structure is 1:(0.5-6); in some embodiments, the mass ratio of the core to the shell in the core-shell structure is 1:(1-3); for example, the mass ratio of the core to the shell in the core-shell structure is one of 1:0.5, 1:1, 1:1.5, 1:2, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:6 or a range consisting of any two of the above values.

[0033] The modified positive electrode active material described in this application includes a positive electrode active material and a coating layer. The coating layer includes a core-shell structured coating material. When the mass ratio of the core to the shell in the core-shell structure is within the above-mentioned range, the coating layer improves electronic conductivity through interlayer electronic transitions, and at the same time reduces the migration barrier of lithium ions through interface engineering, thereby improving ionic conductivity and reducing interface impedance, thus improving the charge and discharge capability of the positive electrode active material.

[0034] In some embodiments, the mass percentage of the coating layer is 0.3-5% based on the total mass of the modified positive electrode active material; for example, the mass percentage of the coating layer is one of 0.3%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or a range consisting of any two of the above values; in some embodiments, the mass percentage of the coating layer is 1-3% based on the total mass of the modified positive electrode active material.

[0035] The modified positive electrode active material described in this application includes a positive electrode active material and a coating layer. When the mass percentage of the coating layer is within the aforementioned range, the coating layer can provide a high-speed electron conduction channel, promote lithium-ion diffusion, and reduce interfacial impedance. Simultaneously, the coating layer can alleviate the volume expansion of the material during charging and discharging, maintain the integrity of the crystal structure, reduce microcrack formation, reduce interfacial side reactions, and extend cycle life. When the mass percentage of the coating layer is 1-3%, the coating effect is optimal, the processability of the positive electrode active material during electrode preparation is better, and ultimately, the cycle life is longer.

[0036] In some embodiments, the mass ratio of the third sulfide to the fourth sulfide in the modified negative electrode active material is 1:(1-4); in some embodiments, the mass ratio of the third sulfide to the fourth sulfide in the modified negative electrode active material is 1:(2-3); for example, the mass ratio of the third sulfide to the fourth sulfide is one of 1:1, 1:1.5, 1:1.8, 1:2, 1:2.5, 1:2.8, 1:3, 1:4 or a range consisting of any two of the above values.

[0037] The modified negative electrode active material described in this application includes a negative electrode active material, a third sulfide, and a fourth sulfide. When the mass ratio of the third sulfide to the fourth sulfide is within the above range, it can increase lithium ion insertion sites, reduce charge transfer impedance, and alleviate low-temperature volume expansion.

[0038] In some embodiments, the mass percentage of the third sulfide and the fourth sulfide is 0.2-2% based on the total mass of the modified negative electrode active material; for example, the mass percentage of the third sulfide and the fourth sulfide is one of 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, or a range consisting of any two of the above values; in some embodiments, the mass percentage of the third sulfide and the fourth sulfide is 0.8-1.6% based on the total mass of the modified negative electrode active material.

[0039] The modified negative electrode active material described in this application includes a negative electrode active material, a third sulfide, and a fourth sulfide. When the mass percentage of the third sulfide and the fourth sulfide is within the above-mentioned range, it can enhance the structural stability of the negative electrode active material, reduce surface defects of the negative electrode active material, reduce interlayer peeling caused by volume expansion during charging and discharging, optimize the kinetic performance of lithium ion insertion / extraction, reduce interface impedance, and promote lithium ion transport.

[0040] In some embodiments, the particle size Dv50 of the first sulfide is ≤100nm, and the particle size Dv50 of the second sulfide is ≤50nm; for example, the particle size Dv50 of the first sulfide is 10-100nm, and the particle size Dv50 of the second sulfide is 10-50nm.

[0041] In some embodiments, the particle size Dv50 of the third sulfide is ≤100nm, and the particle size Dv50 of the fourth sulfide is ≤50nm; for example, the particle size Dv50 of the third sulfide is 10-100nm, and the particle size Dv50 of the fourth sulfide is 10-50nm.

[0042] In some embodiments, the particle size Dv50 of the first sulfide is one of 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, and 100nm, or a range consisting of any two of the above values; the particle size Dv50 of the second sulfide is one of 10nm, 20nm, 30nm, 40nm, and 50nm, or a range consisting of any two of the above values; the particle size Dv50 of the third sulfide is one of 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, and 100nm, or a range consisting of any two of the above values; and the particle size Dv50 of the fourth sulfide is one of 10nm, 20nm, 30nm, 40nm, and 50nm, or a range consisting of any two of the above values.

[0043] The particle size Dv50 of the first, second, third and fourth sulfides in this application is within the above-mentioned range. The combination of sulfides with particle size ranges of Dv50 ensures the uniformity of coating and improves the processability of the negative electrode active material in the preparation of the electrode sheet.

[0044] In some embodiments, the particle size Dv50 of the first sulfide is 30-60 nm, the particle size Dv50 of the second sulfide is 20-30 nm, the particle size Dv50 of the third sulfide is 30-60 nm, and the particle size Dv50 of the fourth sulfide is 20-30 nm.

[0045] This application reduces surface defects in the coating layer and achieves better coating effect by combining the particle sizes of the first, second, third and fourth sulfides, while also improving the processability of the negative electrode active material in the preparation of the electrode sheet.

[0046] In some embodiments, the tungsten-based sulfide has a mass percentage content of ≥50% based on the total mass of the first sulfide; for example, the tungsten-based sulfide has a mass percentage content of 50%-100%.

[0047] In some embodiments, the tungsten-based sulfide has a mass percentage content of ≥50% based on the total mass of the third sulfide; for example, the tungsten-based sulfide has a mass percentage content of 50%-100%.

[0048] In some embodiments, the mass percentage of the tungsten-based sulfide is one of 50%, 60%, 70%, 80%, 85%, 90%, 95%, 100% or a range consisting of any two of the above values.

[0049] When the mass percentage of the tungsten-based sulfide described in this application is within the above range, the excellent electronic conductivity of the tungsten-based sulfide can serve as an electronic conduction channel, forming a continuous conductive network and reducing the interfacial resistance of lithium ion migration.

[0050] In some embodiments, the first sulfide and the third sulfide also independently include at least one of molybdenum-based sulfide, vanadium-based sulfide, and titanium-based sulfide.

[0051] In some embodiments, the tungsten-based sulfide is at least one of WS2 and WS3.

[0052] In some embodiments, the copper-based sulfide is at least one of CuS, Cu2S, and CuS2.

[0053] In some embodiments, the molybdenum-based sulfide is at least one of MoS2, MoS3, and MoS.

[0054] In some embodiments, the vanadium-based sulfide is at least one of VS, Ti2S3, VS2, and VS4.

[0055] In some embodiments, the titanium-based sulfide is at least one of TiS, TiS2, and TiS3.

[0056] In some embodiments, the positive electrode active material layer further includes a positive electrode conductive agent and a positive electrode binder; the modified positive electrode active material has a mass percentage content of 90-99% based on the total mass of the positive electrode active material layer; for example, the modified positive electrode active material has a mass percentage content of 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or a range consisting of any two of the above values.

[0057] This application limits the mass percentage of the modified positive electrode active material to the above range to ensure the stability of the positive electrode structure, improve conductivity and ion transport efficiency, and at the same time increase the battery energy density.

[0058] In some embodiments, the negative electrode active material layer further includes a negative electrode conductive agent and a negative electrode binder; the mass percentage of the modified negative electrode active material is 90-99% based on the total mass of the negative electrode active material layer; for example, the mass percentage of the modified negative electrode active material is one of 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or a range consisting of any two of the above values.

[0059] This application limits the mass percentage of the modified negative electrode active material to the above range to ensure the stability of the negative electrode structure, improve conductivity and ion transport efficiency, and at the same time increase the battery energy density.

[0060] In some embodiments, the mass percentage of the positive conductive agent is 0.01-5%, for example, it can be one of 0.01%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5% or a range consisting of any two of the above values.

[0061] In some embodiments, the mass percentage of the positive electrode binder is 0.01-5%, for example, it can be one of 0.01%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5% or a range consisting of any two of the above values.

[0062] In some embodiments, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, polyacrylic acid, polyacrylonitrile, polyimide, polyurethane, polyvinyl butyral, polyvinylpyrrolidone (PVP), acrylic acid-acrylonitrile-acrylamide copolymer, and acrylic acid-acrylonitrile-acrylate copolymer. The positive electrode binder of this application is not limited to the above materials, but also includes other materials that can be used as battery positive electrode binders.

[0063] In some embodiments, the positive electrode conductive agent includes at least one of carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, activated carbon, and mesoporous carbon. The positive electrode conductive agent in this application is not limited to the above materials, but also includes other materials that can be used as positive electrode conductive agents in batteries.

[0064] In some embodiments, the mass percentage of the negative electrode conductive agent is 0.01-5%, for example, it can be one of 0.01%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5% or a range consisting of any two of the above values.

[0065] In some embodiments, the mass percentage of the negative electrode binder is 0.01-5%, for example, it can be one of 0.01%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5% or a range consisting of any two of the above values.

[0066] In some embodiments, the negative electrode conductive agent may include at least one of carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, activated carbon, and mesoporous carbon. The negative electrode conductive agent of this application is not limited to the above materials, but also includes other materials that can be used as battery negative electrode conductive agents.

[0067] In some embodiments, the negative electrode binder may include at least one of polyacrylic acid, polymethacrylic acid, polyacrylate, polymethacrylate, polyacrylamide, styrene-butadiene rubber, acrylic styrene-butadiene rubber, acrylic acid-acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylonitrile-butadiene rubber, nitrile rubber, acrylonitrile-styrene-butadiene copolymer, acryloyl rubber, butyl rubber, fluororubber, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, polystyrene, latex, acrylic resin, phenolic resin, epoxy resin, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl cellulose, carboxymethyl chitosan, polyester, polyamide, polyether, polyimide, polycarboxylic acid ester, polycarboxylic acid, polyurethane, alginate, fluorinated polymer, chlorinated polymer, polyvinylidene fluoride, and poly(vinylidene fluoride)-hexafluoropropylene. The negative electrode binder of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as battery negative electrode binders.

[0068] In some embodiments, the negative electrode active material may include natural graphite particles, synthetic graphite particles, hard carbon, soft carbon, mesophase carbon microspheres (MCMB), Sn, SnO2, SnO, or Li4Ti5O. 12 The negative electrode active material is selected from at least one of LTO, Si material, silicon-carbon (Si-C) composite material, silicon-nitrogen (Si-N) composite material, and silicon-oxygen (Si-O) composite material. The negative electrode active material of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as negative electrode active materials for batteries.

[0069] In some embodiments, the positive electrode active material may include lithium transition metal oxides or lithium transition metal phosphates. Positive electrode active materials include LiCoO2, LiNiO2, and LiNi... x Mn y O2, Li 1+z Ni x Mn y Co 1-x-y O2, LiNi x Co y Al z At least one of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, Li2MnO3, LiFeO2, LiFePO4, and LiMnPO4, wherein each x is independently 0.4 to 0.6; each y is independently 0.2 to 0.4; and each z is independently 0 to 0.1. The positive electrode active material of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as positive electrode active materials for batteries.

[0070] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and additives. This application does not limit the organic solvent, lithium salt, and additives in the electrolyte; any known organic solvent, lithium salt, and additives can be used. Exemplarily, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, and diethyl carbonate; the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium hexafluoroarsine phosphate; the additives are film-forming additives for the positive and / or negative electrodes, or additives that can improve certain battery performance, such as additives that improve the high or low temperature performance of the battery.

[0071] In the secondary batteries mentioned in this application, a separator is typically provided between the positive and negative electrodes to prevent short circuits. In some embodiments, the separator comprises a porous sheet-like or non-woven material with excellent liquid retention properties. Materials for resin or glass fiber separators include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone.

[0072] In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the above-described diaphragm can be used alone or in any combination.

[0073] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and electrolyte described above.

[0074] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0075] In a second aspect, this application provides an electrical device including the aforementioned secondary battery. Exemplary examples of such electrical devices may include mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0076] Example 1 This embodiment provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of positive electrode sheet Modified positive electrode active material, conductive agent (conductive carbon black Super P), and binder (PVDF) are added to 1-methyl-2-pyrrolidone (NMP) solvent in a mass ratio of 97.5:0.5:2 and stirred thoroughly to form a positive electrode slurry with a solid content of 58%. The positive electrode slurry is then coated on both sides of an aluminum foil (12μm), baked, rolled, and cut to obtain the positive electrode sheet.

[0077] The modified positive electrode active material consists of lithium iron phosphate (LFP) positive electrode active material and a coating layer. The coating layer includes a coating material with a core-shell structure, wherein the core is WS2 and the shell is CuS, with a core-to-shell mass ratio of 0.6:0.4. Based on the total mass of the modified positive electrode active material, the coating layer comprises 1% by mass. (The core-to-shell mass ratio in the core-shell structure is adjusted by controlling the mass ratio of WS2 and CuS added to the raw materials, which is 0.65:0.62. The coating layer's mass percentage is controlled by controlling the amount of WS2 / CuS core-shell powder added, which is 1.3% by mass.) Preparation method of modified positive electrode active material: S1. Mix WS2 and CuS (ball mill, 200 rpm, 6 h), heat treat at 300 °C for 2 h under argon atmosphere to form WS2 / CuS core-shell structure coating material, grind the dried coating material through a 400 mesh sieve to obtain WS2 / CuS core-shell powder. S2. The LFP and WS2 / CuS core-shell powders are thoroughly mixed and ultrasonically dispersed for 30 min. The mixture is then heat-treated at 500℃ for 1.5 h under an argon atmosphere to form a coating layer. The above material is then ground through a 400-mesh sieve to obtain the final product, modified positive electrode active material (LFP@WS2 / CuS) – a lithium iron phosphate positive electrode material with a WS2 / CuS core-shell structure coating layer on its surface. (2) Preparation of negative electrode sheet Modified negative electrode active material, conductive carbon black Super P, sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were mixed evenly in a mass ratio of 96.5:0.5:1.5:1.5, and deionized water was added to disperse the mixture evenly to prepare a slurry with a solid content of 30%. The slurry was coated on both sides of a copper foil (8μm), and after baking, rolling, and cutting, the negative electrode sheet was obtained. The modified negative electrode active material includes graphite, WS2, and CuS, with a mass ratio of WS2 to CuS of 0.5:0.5. Based on the total mass of the modified negative electrode active material, the mass percentage of the third and fourth sulfides is 1%. (The mass ratio of the third and fourth sulfides is adjusted by controlling the mass ratio of WS2 to CuS added, with the WS2 to CuS added in the raw material at a mass ratio of 0.5:0.5; the mass percentage of the third and fourth sulfides is 1%, meaning the total mass percentage of WS2 and CuS added to the negative electrode active material is 1%.) Preparation method of modified negative electrode active material: Graphite, WS2 and CuS were thoroughly mixed and ultrasonically dispersed for 30 min. The mixture was then heat-treated at 500℃ for 1.5 h in an argon atmosphere. The material was then ground through a 400-mesh sieve to obtain the modified negative electrode active material.

[0078] Modified negative electrode active material was prepared; (3) The diaphragm is made of 8μm PE membrane; (4) Preparation of electrolyte; In an argon atmosphere glove box (H2O < 10 ppm, O2 < 0.1 ppm), the organic solvent ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) were mixed evenly in a volume ratio of 1:1:1. LiPF6 and the low-temperature additive fluoroethylene carbonate (FEC) were added and dissolved in the organic solvent and stirred evenly to obtain the electrolyte. The concentration of LiPF6 in the electrolyte was 1.2 mol / L.

[0079] (5) Secondary battery assembly The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrode sheets. After winding, hot pressing and shaping, and electrode tab welding, a bare battery is obtained. The bare battery is placed in a square aluminum shell and baked in an oven at 85±10℃ for 24 hours to remove moisture. The electrolyte is injected into the dried battery. After standing, formation, and capacity testing, the preparation of the square aluminum shell lithium-ion battery is completed, and the final secondary battery is obtained.

[0080] Examples 2-5 The process for preparing the secondary battery is the same as in Example 1, except for the parameter adjustments shown in Table 1. Specifically, the mass ratio of the core to the shell in the core-shell structure is adjusted by controlling the mass ratio of WS2 and CuS added to the raw materials; the mass ratio of the core to the shell in the core-shell structure gradually decreases, that is, the mass ratio of WS2 and CuS in the raw materials gradually decreases.

[0081] Examples 6-9 The process for preparing the secondary battery is the same as in Example 1, except that the parameters are adjusted as shown in Table 1. Specifically, the mass percentage of the coating layer, based on the total mass of the modified positive electrode active material, is adjusted by controlling the mass ratio of added WS2 / CuS core-shell powder; the mass percentage of the coating layer gradually increases, meaning the proportion of WS2 / CuS core-shell powder added to the raw materials gradually increases.

[0082] Examples 10-12 The process for preparing the secondary battery is the same as in Example 1, except for the parameter adjustments shown in Table 1. Specifically, in the modified negative electrode active material, the mass ratio of the third sulfide to the fourth sulfide is adjusted by controlling the mass ratio of WS2 and CuS added to the raw materials; an increase in the mass ratio of the third sulfide to the fourth sulfide means an increase in the mass ratio of the third sulfide and the fourth sulfide added to the raw materials.

[0083] Examples 13-16 The process for preparing the secondary battery is the same as in Example 1, except for the parameter adjustments shown in Table 1. Specifically, based on the total mass of the modified negative electrode active material, the mass percentage of the third and fourth sulfides is controlled by adjusting the amounts of WS2 and CuS in the raw materials. An increase in the mass percentage of the third and fourth sulfides indicates an increase in the amounts of WS2 and CuS in the raw materials.

[0084] Examples 17-22 The process for preparing the secondary battery is the same as in Example 1, except for the parameter adjustments shown in Table 1. The particle sizes Dv50 of the first, second, third, and fourth sulfides are achieved by controlling the Dv50 particle size of the raw materials, which were purchased from Aladdin.

[0085] Examples 23-28 The process for preparing the secondary battery is the same as in Example 1, except that the parameters are adjusted as shown in Table 1.

[0086] Comparative Example 1 The process for preparing the secondary battery is the same as in Example 1, except that the modified positive electrode active material is different. The modified positive electrode active material includes a positive electrode active material and a coating layer. The coating layer includes a coating material, which does not form a core-shell structure but is simply mixed conventionally.

[0087] Preparation method of modified positive electrode active material: LFP, WS2, and CuS are thoroughly mixed, ultrasonically dispersed for 30 min, and heat-treated at 500℃ for 1.5 h under argon atmosphere to obtain the modified positive electrode active material. Comparative Example 2 The process for preparing the secondary battery is the same as in Example 1, except that the modified positive electrode active material is different. The modified positive electrode active material is composed of lithium iron phosphate (LFP) positive electrode active material and a coating layer. The coating layer includes a coating material with a core-shell structure. The core is MoS2 and the shell is CuS. The mass ratio of the core to the shell is 0.6:0.4. The coating layer accounts for 1% of the total mass of the modified positive electrode active material.

[0088] Preparation method of modified positive electrode active material: S1. Mix MoS2 and CuS (ball mill, 200 rpm, 6 h), heat-treat at 300℃ for 2 h under argon atmosphere to form MoS2 / CuS core-shell structure coating material, grind the dried coating material through a 400 mesh sieve to obtain MoS2 / CuS core-shell powder. S2. Thoroughly mix LFP with MoS2 / CuS core-shell powder, ultrasonically disperse for 30 min, and heat-treat at 500℃ for 1.5 h under argon atmosphere to form a coating layer. Grind the above material through a 400-mesh sieve to obtain the final product modified positive electrode active material (LFP@MoS2 / CuS) - a lithium iron phosphate positive electrode material with a MoS2 / CuS core-shell structure coating layer on the surface. Comparative Example 3 The process for preparing the secondary battery is the same as in Example 1, except that the positive electrode active material is lithium iron phosphate (LFP) used directly without modification. Comparative Example 4 The process for preparing the secondary battery is the same as in Example 1, except that the modified negative electrode active material is different. The modified negative electrode active material includes graphite, MoS2, and CuS, with a mass ratio of MoS2 to CuS of 0.5:0.5. Based on the total mass of the modified negative electrode active material, the mass percentage of the third and fourth sulfides is 1%. The preparation method of the modified negative electrode active material is as follows: Graphite, MoS2 and CuS were thoroughly mixed, ultrasonically dispersed for 30 min, and heat-treated at 500℃ for 1.5 h in an argon atmosphere. The above materials were then ground through a 400-mesh sieve to prepare the modified negative electrode active material. Comparative Example 5 The process of preparing the secondary battery is the same as in Example 1, except that the negative electrode active material is graphite directly without modification. The parameter testing method is as follows: ① Electrode separation experiment: Sample preparation; take a complete electrode (approximately 1 cm) 2 Clean the surface with anhydrous ethanol to remove impurities and ensure there are no contaminants; place the electrode in a desiccator to ensure the surface is completely dry.

[0089] ② Freezing treatment: Place the electrode in liquid nitrogen (-196℃) and freeze for 10 minutes to make the binder brittle. Quickly cut the electrode with tweezers or a cutting knife to separate the electrode.

[0090] ③ Verify the separation effect; observe the interfaces of each layer using SEM to confirm that there are no residues or structural damage.

[0091] (1) Test method for the mass ratio of core and shell in core-shell structure of modified positive electrode active material: After separating the positive electrode sheet, the intensity of characteristic peaks of WS2 (002) and CuS (102) is analyzed by XRD, and the mass ratio is calculated in combination with the standard sample. Mass ratio = (mass of W element / mass of Cu element) × (molecular weight of WS2 / molecular weight of CuxS).

[0092] (2) Test method for the mass ratio of the third sulfide and the fourth sulfide in the modified negative electrode active material: After separating the negative electrode sheet, the intensity of the characteristic peaks of WS2 (002) and CuS (102) is analyzed by XRD, and the mass ratio is calculated in combination with the standard sample. Mass ratio = (mass of W element / mass of Cu element) × (molecular weight of WS2 / molecular weight of CuxS).

[0093] (3) The method for testing the mass percentage of the coating layer based on the total mass of the modified positive electrode active material is as follows: Thermogravimetric analysis (TGA) is performed on the separated positive electrode sample, and the mass loss of the coating layer in the range of 300-500℃ is recorded. The mass percentage of the coating layer = (mass loss of the coating layer / total mass of the electrode) × 100%.

[0094] (4) The method for testing the mass percentage of the third sulfide and the fourth sulfide based on the total mass of the modified negative electrode active material is as follows: Thermogravimetric analysis (TGA) is performed on the separated negative electrode sample, and the mass loss of the third sulfide and the fourth sulfide in the range of 300-500℃ is recorded. The percentage of the third sulfide and the fourth sulfide is calculated as (total mass loss of the third sulfide and the fourth sulfide / total mass of the electrode) × 100%.

[0095] (5) The method for testing the mass percentage of the tungsten-based sulfide based on the total mass of the first sulfide is as follows: EDS surface scanning is performed on the separated coating area to quantitatively analyze the elemental content of W (WS2) in the first sulfide. The mass percentage of the tungsten-based sulfide = W (WS2) content / total content of the first sulfide × 100%, where the total content of the first sulfide is the sum of the contents of WS2, MoS2, VS2, etc.

[0096] The test method for the mass percentage of the tungsten-based sulfide, based on the total mass of the third sulfide, is the same as above.

[0097] (6) Test method for particle size Dv50 of the first, second, third and fourth sulfides: GB-T 19077-2016.

[0098] Battery performance test: (1) Low-temperature discharge capacity retention test: At 25±1℃, the secondary batteries obtained in the examples and comparative examples were charged to 3.65V at 0.33C and then discharged to 2.5V at 3C current. The discharge capacity was calculated as n1. At 25±1℃, the batteries were charged to 3.65V at 0.33C and then placed in an environment of -40℃±1℃ for 3 hours. After the battery body temperature was consistent with the ambient temperature, it was discharged to 2.0V at 3C current. The discharge capacity was calculated as n2. The discharge capacity retention rate was calculated as (n2 / n1)×100%.

[0099] (2) Low-temperature charging capacity retention test: At 25±1℃, the secondary batteries obtained in the examples and comparative examples were discharged to 2.5V at 0.33C and then charged to 3.65V at 0.33C. The charging capacity was recorded as n1. At 25±1℃, the battery was discharged to 2.5V at 0.33C and then placed in an environment of -30℃±1℃ for 3 hours. After the battery body temperature was consistent with the ambient temperature, it was charged to 3.65V at 0.33C. The charging capacity was recorded as n2. The charging capacity retention rate was calculated as (n2 / n1)×100%.

[0100] (3) Low temperature charging DCR test: At 25±1℃, the secondary batteries obtained in the examples and comparative examples were charged to 3.65V at 0.33C, and then discharged at 0.33C current for 90 min. After adjusting to 50% SOC, they were placed in an environment of -30℃±1℃ and left to stand for 3h. After the battery body temperature was consistent with the ambient temperature, they were charged with 0.33C constant current pulse for 10 s. The DCR was calculated as (voltage after pulse charging - voltage before pulse charging) / charging current.

[0101] (4) Room temperature cycle performance test: At 25±1℃, the secondary batteries obtained in the examples and comparative examples were subjected to charge-discharge cycle tests at a charge-discharge rate of 1C / 1C within the range of 2.5-3.65 V, and the number of cycles in which the battery capacity retention rate was 80% was recorded. n-cycle capacity retention rate = n-cycle discharge specific capacity / first-cycle discharge specific capacity × 100%.

[0102] In Table 1, for the sake of simplification, the modified positive electrode active material is defined as follows: the coating material has a core-shell structure, the core includes a first sulfide, the shell includes a second sulfide, and the mass ratio of the core to the shell in the core-shell structure is set to 0; the mass percentage of the coating layer is set to P based on the total mass of the modified positive electrode active material.

[0103] In the modified negative electrode active material: the mass ratio of the third sulfide to the fourth sulfide in the modified negative electrode active material is set as M; the total mass percentage of the third sulfide and the fourth sulfide is set as N based on the total mass of the modified negative electrode active material.

[0104] The particle size of the first sulfide is set to Dv50-1, and the particle size of the second sulfide is set to Dv50-2; The particle size of the third sulfide is set to Dv50-3, and the particle size of the fourth sulfide is set to Dv50-4; In the modified positive electrode active material, the mass percentage of the tungsten-based sulfide is set as X, based on the total mass of the first sulfide; in the modified negative electrode active material, the mass percentage of the tungsten-based sulfide is set as Y, based on the total mass of the third sulfide.

[0105] Table 1 Table 2 As shown in the table above, when the technical solution provided in this application is adopted, the secondary battery has excellent comprehensive performance; specifically, the low-temperature discharge capacity retention rate is above 60%, the low-temperature charging capacity retention rate is above 57%, the DCR is below 4.25mΩ, and the number of cycles is above 1400.

[0106] As can be seen from Examples 1-28 and Comparative Examples 1-5, when the positive and negative electrode active materials in the electrode sheet do not use the active materials specific to this application, the resulting secondary battery cannot achieve the effect of this application.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a modified positive active material. The modified positive active material includes a positive active material and a coating layer. The coating layer includes a coating material. The coating material has a core-shell structure. The core includes a first sulfide, and the shell includes a second sulfide. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative active material layer includes a modified negative active material, which includes a negative active material, a third sulfide, and a fourth sulfide. The first sulfide and the third sulfide each independently comprise tungsten-based sulfides, and the second sulfide and the fourth sulfide each independently comprise copper-based sulfides.

2. The secondary battery as described in claim 1, characterized in that, Includes at least one of the following (1)-(2): (1) The mass ratio of the core to the shell in the core-shell structure is 1:(0.5-6); (2) The mass percentage of the coating layer is 0.3-3% based on the total mass of the modified positive electrode active material.

3. The secondary battery as described in claim 1, characterized in that, Includes at least one of the following (I)-(II): (I) In the modified negative electrode active material, the mass ratio of the third sulfide to the fourth sulfide is 1:(1-4). (II) The mass percentage of the third sulfide and the fourth sulfide is 0.2-2% based on the total mass of the modified negative electrode active material.

4. The secondary battery as described in claim 1, characterized in that, Includes at least one of the following (a)-(d): (a) The particle size Dv50 of the first sulfide is ≤100 nm; (b) The particle size Dv50 of the second sulfide is ≤50 nm; (c) The particle size Dv50 of the third sulfide is ≤100 nm; (d) The particle size of the fourth sulfide is Dv50≤50nm.

5. The secondary battery as described in claim 1, characterized in that, Includes at least one of the following (A)-(B): (A) The tungsten-based sulfide has a mass percentage content of ≥50% based on the total mass of the first sulfide; (B) The tungsten-based sulfide has a mass percentage content of ≥50% based on the total mass of the third sulfide.

6. The secondary battery as described in claim 1, characterized in that, The first sulfide and the third sulfide each independently include at least one of molybdenum-based sulfides, vanadium-based sulfides, and titanium-based sulfides.

7. The secondary battery as described in claim 6, characterized in that, Includes at least one of the following (5)-(9): (5) The tungsten-based sulfide is at least one of WS2 and WS3; (6) The copper-based sulfide is at least one of CuS, Cu2S, and CuS2; (7) The molybdenum-based sulfide is at least one of MoS2, MoS3, and MoS; (8) The vanadium-based sulfide is at least one of VS, Ti2S3, VS2, and VS4; (9) The titanium-based sulfide is at least one of TiS, TiS2, and TiS3.

8. The secondary battery as described in claim 1, characterized in that, The positive electrode active material layer further includes a positive electrode conductive agent and a positive electrode binder; based on the total mass of the positive electrode active material layer, the mass percentage of the modified positive electrode active material is 90-99%.

9. The secondary battery as described in claim 1, characterized in that, The negative electrode active material layer further includes a negative electrode conductive agent and a negative electrode binder; based on the total mass of the negative electrode active material layer, the mass percentage of the modified negative electrode active material is 90-99%.

10. An electrical device, characterized in that, Includes the secondary battery as described in any one of claims 1-9.

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