Secondary battery, electric device

Abstract

The application discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries. The secondary battery provided by the application is characterized in that a first positive electrode active material layer is arranged on at least one surface of a positive electrode current collector, a second positive electrode active material layer is arranged on the surface of the first positive electrode active material layer, the molar ratio P1 of lithium and phosphorus in the first positive electrode active material layer is less than the molar ratio P2 of lithium and phosphorus in the second positive electrode active material layer, the first lithium iron phosphate compound and the second lithium iron phosphate each independently comprise titanium and / or boron, and the first positive electrode active material layer comprises aluminum titanium phosphate; thus, the gas swelling and interface deterioration of the battery during storage can be effectively improved, and the high-temperature storage capacity retention rate of the battery can be improved on the basis of ensuring the energy density of the battery.

Description

Technical Field

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

[0002] Lithium iron phosphate (LFP) batteries, with electrolytes having a viscosity ≤3.0 mPa·s at 25°C, are prone to issues such as gas formation and swelling, interface deterioration, and rapid capacity decay during high-temperature storage at 60°C and 100% SOC. Analysis and disassembly of LFP storage batteries reveal that gas formation, swelling, and purple spots at the interface are common after 300–400 days of storage. Current solutions primarily focus on optimizing the content of LFP modifying elements to improve the structural stability of the positive electrode, reducing the compaction density of the negative electrode to enhance its kinetic performance, and optimizing the electrolyte formulation. However, these measures impact battery cost, energy density, and validation timelines. With increasing storage days, the amount of by-reaction products between the positive and negative electrode surfaces and the electrolyte gradually increases, leading to a rapid increase in the interface and mass transfer resistance. Ultimately, this results in a gradual increase in overpotential on both sides of the positive and negative electrode surfaces, further increasing gas production and significantly degrading the battery's storage performance. Summary of the Invention

[0003] The purpose of this application is to solve the technical problem in the prior art that when the viscosity of the electrolyte at 25°C is ≤3.0mPa.s, the secondary battery will experience significant capacity decay and battery gas swelling during high-temperature storage at 60°C / 100% SOC. The application provides a secondary battery and power device with excellent storage performance and high energy density under high-temperature conditions (60°C).

[0004] One embodiment of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a first positive active material layer and a second positive active material layer disposed on at least one surface of the positive current collector. The first positive active material layer is disposed between the positive current collector and the second positive active material layer.

[0005] The first positive electrode active material layer includes a first lithium iron phosphate compound and titanium aluminum phosphate, and the second positive electrode active material layer includes a second lithium iron phosphate compound.

[0006] The first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include the first element;

[0007] The first element includes titanium and / or boron;

[0008] The molar ratio of lithium to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of lithium to phosphorus in the second positive electrode active material layer, P2.

[0009] The mass percentage of the first element in the first lithium iron phosphate compound, W1, is less than the mass percentage of the first element in the second lithium iron phosphate compound, W2.

[0010] The viscosity of the electrolyte at 25°C is ≤3.0 mPa·s.

[0011] As an embodiment of this application, in the first positive electrode active material layer, the mass percentage of titanium aluminum phosphate is 0.2% to 1.0%.

[0012] As an embodiment of this application, 0.01≤P2-P1≤0.06.

[0013] As an embodiment of this application, 500ppm≤W2-W1≤1500ppm.

[0014] As an embodiment of this application, the first element includes titanium and boron;

[0015] The mass percentage of titanium in the first lithium iron phosphate compound, Ti1, is less than the mass percentage of titanium in the second lithium iron phosphate compound, Ti2.

[0016] The mass percentage of boron in the first lithium iron phosphate compound, B1, is less than the mass percentage of boron in the second lithium iron phosphate compound, B2.

[0017] As an embodiment of this application, the 400ppm≤Ti2-Ti1≤1000ppm.

[0018] As an embodiment of this application, 100ppm≤B2-B1≤500ppm.

[0019] As an embodiment of this application, the molar ratio P1 of lithium and phosphorus in the first positive electrode active material layer is 1.00 to 1.05.

[0020] As an embodiment of this application, the molar ratio P2 of lithium and phosphorus in the second positive electrode active material layer is 1.01 to 1.11.

[0021] As an embodiment of this application, the mass percentage of titanium Ti1 in the first lithium iron phosphate compound is 200 to 5000 ppm.

[0022] As an embodiment of this application, the mass percentage of titanium (Ti2) in the second lithium iron phosphate compound is 600–6000 ppm.

[0023] As an embodiment of this application, the mass percentage B1 of boron in the first lithium iron phosphate compound is 100 to 1000 ppm.

[0024] As an embodiment of this application, the mass percentage of boron B2 in the second lithium iron phosphate compound is 200 to 1500 ppm.

[0025] As an embodiment of this application, the thickness of the first positive electrode active material layer on one side is 10 to 60 μm.

[0026] As an embodiment of this application, the thickness of the second positive electrode active material layer is 10–60 μm.

[0027] As an embodiment of this application, the thickness expansion rate of the first positive electrode active material layer is 3.4% to 4.3%.

[0028] As an embodiment of this application, the thickness expansion rate of the second positive electrode active material layer is 3.4% to 5.4%.

[0029] Another embodiment of this application provides an electrical device including the secondary battery described in this application.

[0030] The beneficial effects of this application are as follows:

[0031] The secondary battery provided in this application has a first positive electrode active material layer on at least one surface of the positive electrode current collector, and a second positive electrode active material layer disposed on the surface of the first positive electrode active material layer and away from the positive electrode current collector. The molar ratio of lithium to phosphorus in the first positive electrode active material layer is defined as P1 < P2, and the first and second lithium iron phosphate compounds each independently include titanium and / or boron. Furthermore, the first positive electrode active material layer includes titanium aluminum phosphate. This effectively solves the electrode polarization problem accumulated during high-temperature storage at 60°C caused by an electrolyte viscosity ≤3.0 mPa·s at 25°C. It effectively improves gas production, swelling, and interface deterioration during storage, thereby improving the high-temperature storage capacity retention rate and DCR growth rate of the battery while ensuring its energy density, thus exhibiting excellent high-temperature storage performance. Detailed Implementation

[0032] To better illustrate the purpose, technical solution, and advantages of this application, the following will provide further explanation of this application in conjunction with specific embodiments.

[0033] Unless otherwise specified, the reagents, methods and equipment used in this application are all conventional reagents, methods and equipment in the field.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] In one embodiment of this application, a secondary battery is provided, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a first positive active material layer and a second positive active material layer disposed on at least one surface of the positive current collector. The first positive active material layer is disposed between the positive current collector and the second positive active material layer.

[0038] The first positive electrode active material layer includes a first lithium iron phosphate compound and titanium aluminum phosphate, and the second positive electrode active material layer includes a second lithium iron phosphate compound.

[0039] The first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include the first element;

[0040] The first element includes titanium and / or boron;

[0041] The molar ratio of lithium to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of lithium to phosphorus in the second positive electrode active material layer, P2.

[0042] The mass percentage of the first element in the first lithium iron phosphate compound, W1, is less than the mass percentage of the first element in the second lithium iron phosphate compound, W2.

[0043] The viscosity of the electrolyte at 25°C is ≤3.0 mPa·s.

[0044] The secondary battery provided in this application comprises a first positive electrode active material layer on at least one surface of the positive electrode current collector, and a second positive electrode active material layer disposed on the surface of the first positive electrode active material layer and away from the positive electrode current collector. The molar ratio of lithium to phosphorus in the first positive electrode active material layer is defined as P1 < P2, and the first and second lithium iron phosphate compounds each independently include titanium and / or boron. Furthermore, the first positive electrode active material layer includes titanium aluminum phosphate. This effectively solves the problem of electrode polarization accumulated during high-temperature storage at 60°C when the electrolyte viscosity at 25°C is ≤3.0 mPa·s. It effectively improves gas production, swelling, and interface deterioration during storage, thereby enhancing the high-temperature storage capacity retention rate and DCR growth rate of the battery while maintaining its energy density, thus exhibiting excellent high-temperature storage performance.

[0045] Specifically, in the first aspect, the molar ratio of lithium to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of lithium to phosphorus in the second positive electrode active material layer, P2, and the first positive electrode active material layer includes titanium aluminum phosphate. This allows for the effective formation of a protected fast-ion conductor coating in the first positive electrode active material layer, while also effectively improving the kinetics of the positive electrode sheet and reducing the damage to the positive electrode active material layer structure caused by overpotential during storage and the increase in SEI interface and phase transfer impedance due to interfacial side reactions. Furthermore, the second positive electrode active material layer effectively improves the wettability, kinetics, and structural stability of the positive electrode sheet, and also reduces side reactions between the positive electrode sheet surface and the electrolyte, the increase in SEI interface and phase transfer impedance, and the increase in diffusion impedance with the negative electrode, significantly mitigating the risk of purple spots on the negative electrode interface. Simultaneously, the second positive electrode active material layer can suppress the dissolution of Fe elements and the generation of electrolyte gas during long-term high-temperature storage of lithium iron phosphate, greatly improving the battery's capacity retention rate and DCR growth rate at 60°C. Secondly, the introduction of titanium and / or boron as the first element, satisfying a specific mass percentage relationship, can effectively improve the interlayer spacing of lithium iron phosphate, as well as enhance the structural stability and conductivity of lithium iron phosphate, increase energy density, and also improve the high-temperature storage performance of the battery.

[0046] It should be noted that the method for testing and calculating the molar ratio of lithium and phosphorus in the first positive electrode active material layer and the second positive electrode active material includes the following steps: testing the content of lithium and phosphorus in different coating powders using ICP, and then converting it into a molar ratio.

[0047] It should be noted that the test method for the mass percentage of the first element in the first lithium iron phosphate compound and the second lithium iron phosphate compound includes the following steps: the molar content of the first element in different coating powders is tested by ICP, and then converted into the mass and mass fraction of the first element. The mass percentage of the first element in the lithium iron phosphate compound = the mass of the first element in the lithium iron phosphate compound / the mass of the lithium iron phosphate compound.

[0048] It should be noted that if the first element of the first lithium iron phosphate compound in the first positive electrode active material layer contains titanium, the aluminum content in the powder of the first positive electrode active material layer is first determined by ICP test, and then the titanium content is converted according to aluminum titanium phosphate. Subsequently, the total molar content of titanium in the powder of the first positive electrode active material layer obtained by ICP test is subtracted from the molar content of titanium in the converted aluminum titanium phosphate to obtain the molar content of titanium in the first lithium iron phosphate compound, and then its mass percentage in the first lithium iron phosphate compound is calculated.

[0049] It should be noted that the method for testing the viscosity of the electrolyte at 25°C includes the following steps: taking a certain amount of electrolyte sample and passing it through a viscometer to test the viscosity at room temperature.

[0050] In one embodiment, the viscosity of the electrolyte at 25°C is 1.2–2.8 mPa·s.

[0051] For example, the viscosity of the electrolyte at 25°C can be any point value or any two-point range between 1.2 and 2.8 mPa·s, such as one or any two of 1.2 mPa·s, 1.5 mPa·s, 1.8 mPa·s, 2.0 mPa·s, 2.2 mPa·s, 2.4 mPa·s, 2.6 mPa·s, and 2.8 mPa·s.

[0052] In one embodiment, the mass percentage of titanium aluminum phosphate in the first positive electrode active material layer is 0.2% to 1.0%.

[0053] It should be noted that the test method for the mass percentage of aluminum titanium phosphate in the first positive electrode active material layer is as follows: the aluminum content in the first coating powder is tested by ICP, converted into the molar ratio of aluminum titanium phosphate, and then converted into the mass percentage of aluminum titanium phosphate.

[0054] For example, in the first positive electrode active material layer, the mass percentage of titanium aluminum phosphate can be any point value or any two points between 0.2% and 1.0%, such as one or any two of 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%.

[0055] This study found that the introduction of titanium aluminum phosphate can effectively form fast ion channels in the first positive electrode active material layer, thereby effectively reducing the structural damage of the first positive electrode active material layer caused by overpotential and the increase in CEI interface and phase transfer impedance caused by interfacial side reactions; especially when the mass percentage of titanium aluminum phosphate is further selected to be 0.2-1.0%, the resulting battery has better high-temperature storage performance and higher energy density.

[0056] In one embodiment, 0.01 ≤ P2 - P1 ≤ 0.06.

[0057] For example, P2-P1 can be any point value or any two-point range value between 0.01 and 0.06, such as one or any two of 0.01, 0.02, 0.03, 0.04, 0.05, and 0.06.

[0058] This study found that when the difference between the molar ratio P2 of lithium and phosphorus in the second positive electrode active material layer and the molar ratio P1 of lithium and phosphorus in the first positive electrode active material layer is further selected between 0.01 and 0.06, the structural stability of the positive electrode sheet can be better improved, the side reactions and battery polarization during high-temperature storage can be reduced, the risk of purple spot on the negative electrode interface can be improved, and the dissolution of iron and the generation of electrolyte gas during storage can be suppressed; thus effectively improving the storage performance of the secondary battery at high temperatures.

[0059] In one embodiment, 500ppm ≤ W2 - W1 ≤ 1500ppm.

[0060] For example, W2-W1 can be any point value or any two-point range value between 500 and 1500 ppm, such as one or any two of the following: 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, and 1500 ppm.

[0061] This study found that when the difference between the mass percentage W2 of the first element in the second lithium iron phosphate compound and the mass percentage W1 of the first element in the first lithium iron phosphate compound is between 500 and 1500 ppm, the structural stability of the positive electrode sheet can be better improved, thus enhancing the high-temperature storage performance of the secondary battery.

[0062] In one embodiment, the first element comprises titanium and boron;

[0063] The mass percentage of titanium in the first lithium iron phosphate compound, Ti1, is less than the mass percentage of titanium in the second lithium iron phosphate compound, Ti2.

[0064] The mass percentage of boron in the first lithium iron phosphate compound, B1, is less than the mass percentage of boron in the second lithium iron phosphate compound, B2.

[0065] This application research found that when the first element of both the first lithium iron phosphate compound and the second lithium iron phosphate compound simultaneously includes titanium and boron, and the mass percentages of titanium and boron in the first lithium iron phosphate compound and the second lithium iron phosphate compound satisfy the relationship of this application, it is possible to improve the structural stability and conductivity of lithium iron phosphate while increasing the interlayer spacing of lithium iron phosphate, improve the kinetics of lithium iron phosphate and the lithium deposition phenomenon of the positive electrode sheet; thereby effectively improving the high-temperature storage performance of secondary batteries.

[0066] In one embodiment, the 400ppm≤Ti2-Ti1≤1000ppm.

[0067] For example, Ti2-Ti1 can be any point value or any two-point range value between 400 and 1000 ppm, such as one or any two of 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm.

[0068] In one embodiment, 100ppm≤B2-B1≤500ppm.

[0069] For example, B2-B1 can be any point value or any two-point range value between 100 and 500 ppm, such as one or any two of 100 ppm, 200 ppm, 300 ppm, 400 ppm, and 500 ppm.

[0070] This application research found that when the first element of both the first lithium iron phosphate compound and the second lithium iron phosphate compound simultaneously includes titanium and boron, and the difference in the mass percentage of titanium and boron in the first lithium iron phosphate compound and the second lithium iron phosphate compound is within the above-mentioned range, the overall performance of the obtained secondary battery is better.

[0071] In one embodiment, the molar ratio P1 of lithium and phosphorus in the first positive electrode active material layer is 1.00 to 1.05.

[0072] For example, the molar ratio P1 of lithium and phosphorus in the first positive electrode active material layer can be any point value or any two points between 1.00 and 1.05, such as one or any two of 1.00, 1.01, 1.02, 1.03, 1.04, 1.05.

[0073] In one embodiment, the molar ratio P2 of lithium and phosphorus in the second positive electrode active material layer is 1.01 to 1.11.

[0074] For example, the molar ratio P2 of lithium and phosphorus in the second positive electrode active material layer can be any point value or any two points between 1.01 and 1.11, such as one or any two of 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11.

[0075] This application research found that when the molar ratio of lithium to phosphorus in the first positive electrode active material layer, P1, and the molar ratio of lithium to phosphorus in the second positive electrode active material layer, P2, are within the above range, the difference between P2 and P1 is more beneficial to the difference in specific capacity and N / P ratio between the overall inner active coating and the surface active coating of lithium iron phosphate, as well as the stability of the surface structure and the increase in impedance. In particular, it can improve the side reactions and gas generation between the surface lithium iron phosphate coating and the electrolyte.

[0076] In one embodiment, the mass percentage of titanium Ti1 in the first lithium iron phosphate compound is 200–5000 ppm.

[0077] For example, the mass percentage of titanium Ti1 in the first lithium iron phosphate compound can be any point value or any two-point range between 200 and 5000 ppm, such as one or any two of the following: 200 ppm, 400 ppm, 600 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, and 5000 ppm.

[0078] In one embodiment, the mass percentage of titanium (Ti2) in the second lithium iron phosphate compound is 600–6000 ppm.

[0079] For example, the mass percentage of titanium Ti2 in the second lithium iron phosphate compound can be any point value or any two-point range between 600 and 6000 ppm, such as one or any two of the following: 600 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, and 6000 ppm.

[0080] In one embodiment, the mass percentage B1 of boron in the first lithium iron phosphate compound is 100 to 1000 ppm.

[0081] For example, the mass percentage B1 of boron in the first lithium iron phosphate compound can be any point value or any two-point range value between 100 and 1000 ppm, such as one or any two of 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm.

[0082] In one embodiment, the mass percentage of boron B2 in the second lithium iron phosphate compound is 200–1500 ppm.

[0083] For example, the mass percentage B2 of boron in the second lithium iron phosphate compound can be any point value or any two-point range value between 200 and 1500 ppm, such as one or any two of 200 ppm, 400 ppm, 600 ppm, 800 ppm, 1000 ppm, 1200 ppm, 1400 ppm, 1500 ppm.

[0084] This study found that when the mass percentages of titanium and boron in the first and second lithium iron phosphate compounds are within the above-mentioned ranges, the difference between W2 and W1 is more beneficial to the kinetic differences between the overall inner active coating and the surface active coating lithium iron phosphate, as well as the surface structure stability and impedance growth. In particular, it can improve the side reactions and gas generation between the surface lithium iron phosphate coating and the electrolyte.

[0085] In one embodiment, the thickness of one side of the first positive electrode active material layer is 10–60 μm.

[0086] It should be noted that the single-sided thickness of the first positive electrode active material layer is measured by CP+SEM (Cross Section Polisher-Scanning Electron Microscope).

[0087] For example, the thickness of one side of the first positive electrode active material layer can be any point value or any two-point range value between 10 and 60 μm, such as one or any two of the following: 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm.

[0088] In one embodiment, the thickness of the second positive electrode active material layer is 10–60 μm.

[0089] It should be noted that the thickness of the second positive electrode active material layer is tested by dimensional calibration using CP+SEM.

[0090] For example, the thickness of the second positive electrode active material layer can be any point value or any two-point range value between 10 and 60 μm, such as one or any two of the following: 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm.

[0091] This study found that when the single-sided thickness of the first positive electrode active material layer and the thickness of the second positive electrode active material layer are further selected within the above ranges, the storage performance of the double-coated lithium iron phosphate battery at 60°C is better. If the coating is too thin, the uniformity is poor and the equipment capacity is difficult to meet. If it is too thick, lithium plating interface is more likely to appear when the negative electrode is fully charged.

[0092] In one embodiment, the thickness expansion rate of the first positive electrode active material layer is 3.4% to 4.3%.

[0093] In one embodiment, the thickness expansion rate of the second positive electrode active material layer is 3.4% to 5.4%.

[0094] It should be noted that the thickness expansion rate of the active material layer refers to the rate of change of the thickness of the active material layer under different storage days.

[0095] The specific calculation formula is as follows:

[0096] The expansion rate of the active material layer after N days of storage = (the thickness of the active material layer after N days of storage - the thickness of the active material layer before storage) / the thickness of the active material layer before storage × 100%;

[0097] Wherein: the active material layer thickness after N days of storage refers to the active material layer thickness obtained by CP+SEM calibration after N days of storage; the active material layer thickness before storage refers to the active material layer thickness obtained by CP+SEM calibration before storage.

[0098] For example, the battery thickness expansion rate caused by the first positive electrode active material layer is any point value or any two points within a range of 3.4% to 4.3%, such as one or any two of 3.4%, 3.6%, 3.8%, 4%, 4.2%, and 4.3%, and the battery thickness expansion rate caused by the second positive electrode active material layer is any point value or any two points within a range of 3.4% to 5.4%, such as one or any two of 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, 5.2%, and 5.4%.

[0099] This application research found that when the thickness expansion rate of the first positive electrode active material layer and the second positive electrode active material layer is within the above range, the energy density and storage performance of the battery are slightly better.

[0100] In one embodiment, the first and second positive electrode active material layers further include a conductive agent and a positive electrode binder. This application does not limit the conductive agent and positive electrode binder; any known conductive agent and positive electrode binder may be used.

[0101] For example, the conductive agent includes conductive graphite; the positive electrode binder includes polyvinylidene fluoride.

[0102] In one embodiment, the positive electrode current collector can be made of a material with good conductivity and mechanical strength, serving both as a conductor and a current collector. In another embodiment, the positive electrode current collector can be a metal foil or a composite current collector (a composite current collector can be formed by depositing metal material on a polymer substrate). As an example, the positive electrode current collector is made of aluminum foil or carbon-coated aluminum foil.

[0103] In one embodiment, 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 negative active material. This application does not limit the negative active material; any known negative active material can be used. As an example, the negative active material can be at least one of artificial graphite, natural graphite, silicon-carbon composite material, elemental silicon, silicon suboxide, and hard carbon.

[0104] In one embodiment, the negative electrode current collector can be made of a material with good conductivity and mechanical strength, serving both as a conductor and a current collector. In another embodiment, the negative electrode current collector can be a metal foil or a composite current collector (a composite current collector can be formed by depositing a metal material on a polymer substrate). As an example, copper foil is used as the negative electrode current collector.

[0105] In one embodiment, the electrolyte further includes an organic solvent and a lithium salt. This application does not limit the organic solvent and lithium salt in the electrolyte; any known organic solvent and lithium salt can be used.

[0106] For example, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate; the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0107] In one embodiment, the separator of the secondary battery is disposed between the positive and negative electrodes.

[0108] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0109] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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.

[0110] Example 1

[0111] This application provides a secondary battery, the preparation method of which includes the following steps:

[0112] (1) Preparation of positive electrode sheet

[0113] S1. The first positive electrode active material layer slurry is prepared by mixing the first lithium iron phosphate compound (the molar ratio of lithium to phosphorus P1 is 1.02, the mass percentage of titanium Ti1 is 1000 ppm, and the mass percentage of boron B1 is 300 ppm), conductive carbon black, polyvinylidene fluoride, and titanium aluminum phosphate in a mass ratio of 95.0:3.0:1.5:0.5.

[0114] S2. The second positive electrode active material layer slurry is prepared by mixing the second lithium iron phosphate compound (the molar ratio of lithium to phosphorus P2 is 1.045, the mass percentage of titanium Ti2 is 1600 ppm, and the mass percentage of boron B2 is 500 ppm), conductive carbon black, and polyvinylidene fluoride in a mass ratio of 95.0:3.5:1.5.

[0115] S3. The first positive electrode active material layer slurry is coated on both sides of the positive electrode current collector aluminum foil to obtain a first positive electrode active material layer with a single-sided thickness of 30μm. Then, the second positive electrode active material layer slurry is coated on the surface of the first positive electrode active material layer to obtain a second positive electrode active material layer with a single-sided thickness of 30μm. Then, the material is dried, rolled, slit and cut to obtain the positive electrode sheet.

[0116] (2) Preparation of negative electrode sheet

[0117] Artificial graphite, conductive carbon black (SP), binder (sodium carboxymethyl cellulose), and styrene-butadiene rubber are mixed evenly in a mass ratio of 97:0.5:1.2:1.3. Then, water is added to the mixture in batches and the mixture is stirred and mixed thoroughly to prepare a negative electrode slurry. The homogeneous negative electrode slurry is then evenly coated on both sides of a 6μm copper foil. After drying, rolling, slitting, and cutting, a negative electrode sheet is obtained.

[0118] (3) Preparation of electrolyte

[0119] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), ethyl propionate (EP), vinylene carbonate (VC), 1,3-propanesulfonyl lactone (PS), ethylene sulfate (DTD), and tris(trimethylsilane) phosphate (TMSP) were mixed in a mass ratio of 20:30:45:3:1:0.5:0.5, followed by the addition of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI). The molar concentration of LiPF6 in the electrolyte was 0.5 M, and the molar concentration of LiFSI was 0.7 M. The viscosity of the electrolyte at 25 °C was 2.5 mPa·s.

[0120] (4) Preparation of secondary batteries

[0121] The prepared positive electrode, negative electrode, separator and other battery components are assembled and then processed through shaping, baking, packaging, liquid injection, formation and capacity testing to obtain a secondary battery.

[0122] Examples 2-4

[0123] This application provides a secondary battery, which differs from the one in Example 1 in that the molar ratio of lithium and phosphorus is changed by adjusting the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0124] Examples 5-8

[0125] This application provides a secondary battery, which differs from the one in embodiment 1 in that the mass percentage of titanium is changed by adjusting the addition of titanium to the first element in the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0126] Examples 9-11

[0127] This application provides a secondary battery, which differs from the one in embodiment 1 in that the mass percentage of boron is changed by adjusting the addition of boron to the first element in the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0128] Examples 12-13

[0129] This application provides a secondary battery, which differs from Embodiment 1 in that the parameters in Tables 1-2 are achieved by adjusting the first element type of the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0130] Examples 14-17

[0131] This application provides a secondary battery, which differs from Embodiment 1 in that the parameters in Tables 1-2 are achieved by adjusting the amount of titanium aluminum phosphate added to the first positive electrode active material layer.

[0132] Examples 18-19

[0133] This application provides a secondary battery, which differs from Embodiment 1 in that the parameters in Tables 1-2 are achieved by adjusting the coating thickness of the first positive electrode active material layer and the second positive electrode active material layer.

[0134] Example 20

[0135] This application provides a secondary battery, which differs from Embodiment 1 in that the parameters in Tables 1-2 are achieved by adjusting the electrolyte.

[0136] Comparative Example 1

[0137] This application provides a secondary battery in comparison, which differs from Example 1 in that the molar ratio of lithium and phosphorus is changed by adjusting the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0138] Comparative Examples 2-3

[0139] This application provides a secondary battery in comparison. The difference between the secondary battery and Example 1 is that the parameters in Tables 1-2 are achieved by adjusting the addition of the first element in the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0140] Comparative Example 4

[0141] This application provides a secondary battery in comparison. The difference between the secondary battery and Example 1 is that the parameters in Tables 1-2 are achieved by adjusting the amount of titanium aluminum phosphate added in the first positive electrode active material layer.

[0142] Comparative Example 5

[0143] This application provides a secondary battery as a comparative example. The difference between the secondary battery and Example 1 is that the parameters in Tables 1-2 are achieved by adjusting the electrolyte.

[0144] In the examples and comparative examples, the molar ratio of lithium to phosphorus in the first positive electrode active material layer is P1, the molar ratio of lithium to phosphorus in the second positive electrode active material layer is P2, and the difference between them is P2-P1; the mass percentage of titanium in the first lithium iron phosphate compound is Ti1 / ppm, the mass percentage of titanium in the second lithium iron phosphate compound is Ti2 / ppm, and the difference between them is Ti2-Ti1; the mass percentage of boron in the first lithium iron phosphate compound is B1 / ppm, the mass percentage of boron in the second lithium iron phosphate compound is B2 / ppm, and the difference between them is B2-B1. The mass percentage of the first element in the second lithium iron phosphate compound W2 / ppm and the mass percentage of the first element in the first lithium iron phosphate compound W1 / ppm, the difference W2-W1, the thickness expansion rate Y1 / %, the thickness expansion rate Y2 / %, the mass percentage of aluminum titanium phosphate in the first positive electrode active material layer Z / %, the viscosity of the electrolyte at 25℃ M / mPa.s, the single-sided thickness d1 / μm of the first positive electrode active material layer, and the thickness d2 / μm of the second positive electrode active material layer are shown in Tables 1-2.

[0145] Table 1. Parameters of Secondary Batteries

[0146] P1 P2 P2-P1 Ti1 Ti2 Ti2-Ti1 B1 B2 B2-B1 W2-W1 Z Example 1 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.5 Example 2 1 1.01 0.01 1000 1600 600 300 500 200 800 0.5 Example 3 1.05 1.11 0.06 1000 1600 600 300 500 200 800 0.5 Example 4 0.98 1.21 0.23 1000 1600 600 300 500 200 800 0.5 Example 5 1.02 1.045 0.025 200 600 400 300 500 200 600 0.5 Example 6 1.02 1.045 0.025 3000 4300 1300 300 500 200 1500 0.5 Example 7 1.02 1.045 0.025 5000 6000 1000 300 500 200 1200 0.5 Example 8 1.02 1.045 0.025 5500 6500 1000 300 500 200 1200 0.5 Example 9 1.02 1.045 0.025 1000 1600 600 100 200 100 700 0.5 Example 10 1.02 1.045 0.025 1000 1600 600 1000 1500 500 1100 0.5 Example 11 1.02 1.045 0.025 1000 1600 600 80 180 100 700 0.5 Example 12 1.02 1.045 0.025 0 0 0 1300 2100 800 800 0.5 Example 13 1.02 1.045 0.025 1300 2100 800 0 0 0 800 0.5 Example 14 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.2 Example 15 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.1 Example 16 1.02 1.045 0.025 1000 1600 600 300 500 200 800 1 Example 17 1.02 1.045 0.025 1000 1600 600 300 500 200 800 4 Example 18 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.5 Example 19 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.5 Example 20 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.5 Comparative Example 1 1.045 1.02 -0.025 1000 1600 600 300 500 200 800 0.5 Comparative Example 2 1.02 1.045 0.025 1600 1000 -600 300 500 200 -400 0.5 Comparative Example 3 1.02 1.045 0.025 0 0 0 0 0 0 0 0.5 Comparative Example 4 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0 Comparative Example 5 1.02 1.045 0.025 1000 1600 600 300 500 200 800 0.5

[0147] Table 2 Secondary Battery Parameter Table

[0148] d1 d2 Y1 Y2 M Example 1 30 30 3.5 3.8 2.5 Example 2 30 30 3.6 4.3 2.5 Example 3 30 30 3.8 4.4 2.5 Example 4 30 30 4.3 4.6 2.5 Example 5 30 30 3.8 4 2.5 Example 6 30 30 3.6 4.4 2.5 Example 7 30 30 3.7 4.4 2.5 Example 8 30 30 3.8 4.6 2.5 Example 9 30 30 3.6 4.1 2.5 Example 10 30 30 3.6 3.9 2.5 Example 11 30 30 3.6 4.9 2.5 Example 12 30 30 3.8 5.3 2.5 Example 13 30 30 3.9 5.1 2.5 Example 14 30 30 3.7 3.7 2.5 Example 15 30 30 3.8 3.8 2.5 Example 16 30 30 3.9 3.9 2.5 Example 17 30 30 3.8 3.6 2.5 Example 18 10 60 3.4 3.4 2.5 Example 19 60 10 3.8 4.1 2.5 Example 20 30 30 3.4 4.1 1.5 Comparative Example 1 30 30 4.5 5.8 2.5 Comparative Example 2 30 30 4.4 5.8 2.5 Comparative Example 3 30 30 4.6 8.3 2.5 Comparative Example 4 30 30 4.5 6.1 2.5 Comparative Example 5 30 30 4.8 8.5 3.5

[0149] The energy density and storage performance of the secondary batteries prepared in the examples and comparative examples are shown in Table 3; the testing methods include the following steps:

[0150] 1) Energy density: 25℃±5℃, 1C discharge to 2.0V, then 1C charge to 3.6V, cutoff current 0.05C, capacity tested by 1C discharge to 2.0V, 1C=4.4A; discharge plateau 3.2V, energy density=capacity*discharge plateau / battery length / battery width / battery thickness;

[0151] 2) Storage performance: 100% SOC storage at 60℃, 25℃±5℃, 1C discharge to 2.0V, then 1C charge to 3.6V, cutoff current 0.05C, capacity tested by 1C discharge to 2.0V, 1C=4.4A; battery capacity retention rate and thickness expansion rate recorded every 30 days;

[0152] Table 3 Performance Data of Secondary Batteries

[0153]

[0154]

[0155] As can be seen from Table 3, the technical solution provided in this application has high energy density and excellent storage performance; specifically, the energy density of the obtained secondary battery is above 212.6Wh / L, the capacity retention rates after 300 days, 360 days, and 400 days are above 83.4%, above 80.1%, and above 77.8%, respectively, and the thickness expansion rates after 300 days, 360 days, and 400 days are below 6.8%, below 7.5%, and below 9.2%, respectively.

[0156] The results of Examples 1-20 and Comparative Example 1 of this application show that when the molar ratio P1 of lithium to phosphorus in the first positive electrode active material layer is greater than the molar ratio P2 of lithium to phosphorus in the second positive electrode active material layer, the energy density of the obtained secondary battery shows a decreasing trend, and the capacity retention rate during storage also decreases while the thickness expansion rate increases. The results of Examples 1-20 and Comparative Example 2 of this application show that when the mass percentage W1 of the first element in the first lithium iron phosphate compound is greater than the mass percentage W2 of the first element in the second lithium iron phosphate compound, the energy density of the obtained secondary battery shows a decreasing trend. The capacity retention rate also decreases during storage, while the thickness expansion rate increases. The results of Examples 1-20 and Comparative Example 3 of this application show that when the first element is not added to the first lithium iron phosphate and second lithium iron phosphate compounds, the storage performance of the obtained secondary battery decreases significantly. The results of Examples 1-20 and Comparative Example 4 of this application show that when aluminum titanium phosphate is not added to the first positive electrode active material layer, the overall performance of the obtained secondary battery shows a certain downward trend. The results of Examples 1-20 and Comparative Example 4 of this application show that when the viscosity of the electrolyte at 25°C is >3.0 mPa·s, the storage performance of the obtained secondary battery decreases significantly.

[0157] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not 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 first positive active material layer and a second positive active material layer disposed on at least one surface of the positive current collector, wherein the first positive active material layer is disposed between the positive current collector and the second positive active material layer; The first positive electrode active material layer includes a first lithium iron phosphate compound and titanium aluminum phosphate, and the second positive electrode active material layer includes a second lithium iron phosphate compound. The first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include the first element; The first element includes titanium and / or boron; The molar ratio of lithium to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of lithium to phosphorus in the second positive electrode active material layer, P2. The mass percentage of the first element in the first lithium iron phosphate compound, W1, is less than the mass percentage of the first element in the second lithium iron phosphate compound, W2. The viscosity of the electrolyte at 25°C is ≤3.0 mPa·s.

2. The secondary battery according to claim 1, characterized in that, In the first positive electrode active material layer, the mass percentage of titanium aluminum phosphate is 0.2% to 1.0%.

3. The secondary battery according to claim 1, characterized in that, 0.01≤P2-P1≤0.

06.

4. The secondary battery according to claim 1, characterized in that, 500ppm≤W2-W1≤1500ppm.

5. The secondary battery according to claim 1, characterized in that, The first element includes titanium and boron; The mass percentage of titanium in the first lithium iron phosphate compound, Ti1, is less than the mass percentage of titanium in the second lithium iron phosphate compound, Ti2. The mass percentage of boron in the first lithium iron phosphate compound, B1, is less than the mass percentage of boron in the second lithium iron phosphate compound, B2.

6. The secondary battery according to claim 5, characterized in that, 400ppm≤Ti2-Ti1≤1000ppm; And / or, 100ppm≤B2-B1≤500ppm.

7. The secondary battery according to claim 5, characterized in that, Satisfy at least one of the following: (1) The molar ratio of lithium to phosphorus in the first positive electrode active material layer, P1, is 1.00 to 1.05; (2) The molar ratio of lithium to phosphorus in the second positive electrode active material layer, P2, is 1.01 to 1.11; (3) The mass percentage of titanium Ti1 in the first lithium iron phosphate compound is 200-5000 ppm; (4) The mass percentage of titanium (Ti2) in the second lithium iron phosphate compound is 600–6000 ppm; (5) The mass percentage of boron B1 in the first lithium iron phosphate compound is 100 to 1000 ppm; (6) The mass percentage of boron in the second lithium iron phosphate compound, B2, is 200 to 1500 ppm.

8. The secondary battery according to claim 1, characterized in that, The thickness of the first positive electrode active material layer on one side is 10–60 μm; And / or, the thickness of the second positive electrode active material layer is 10–60 μm.

9. The secondary battery according to claim 1, characterized in that, The thickness expansion rate of the first positive electrode active material layer is 3.4% to 4.3%; And / or, the thickness expansion rate of the second positive electrode active material layer is 3.4% to 5.4%.

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

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