Secondary battery and electric device

Abstract

This application discloses a secondary battery and an electrical device, belonging to the field of battery technology. The secondary battery provided by this application, by forming 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 on the surface of the first positive electrode active material layer, and by defining that the molar ratio P1 of iron and phosphorus in the first positive electrode active material layer is less than the molar ratio P2 of iron and phosphorus in the second positive electrode active material layer, and that the first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include titanium and / or vanadium; thereby effectively solving the problems of poor cycle performance under high temperature conditions and significant lithium plating during cycling caused by a high ethyl propionate mass percentage ≥45% in the electrolyte; the secondary battery provided by this application has high capacity, excellent high-temperature cycle performance, and low risk of lithium plating during cycling.

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 paired with high-kinetic-performance electrolytes (ethyl propionate (EP) content ≥45 wt.%) are prone to lithium plating and rapid capacity decay during 45°C / 3C cycling. Disassembly of LFP cycled batteries reveals that lithium plating interfaces typically appear after 1500–1800 cycles. Current solutions primarily address these issues by reducing the negative electrode compaction density, using soft or hard carbon coating to improve the negative electrode's kinetic performance, and optimizing the electrolyte formulation. However, these methods do not completely solve the problem because as cycling progresses, electrolyte decomposition intensifies, leading to more by-reaction products, increased battery polarization, and a gradual increase in overpotential across the positive electrode. Summary of the Invention

[0003] The purpose of this application is to solve the problem in the prior art regarding the content of ethyl propionate (EP) added to the electrolyte.

[0004] To address the technical problem of excessively rapid capacity decay in secondary batteries when improving the kinetics of secondary batteries with ≥45wt.% lithium content, this paper provides a secondary battery and power supply device that exhibits excellent cycle capacity retention and minimal lithium plating under high temperature conditions (45°C).

[0005] To achieve the above objectives, a first aspect 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.

[0006] The first positive electrode active material layer includes a first lithium iron phosphate compound, and the second positive electrode active material layer includes a second lithium iron phosphate compound;

[0007] The first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include the first element;

[0008] The first element includes titanium and / or vanadium;

[0009] The molar ratio of iron to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of iron to phosphorus in the second positive electrode active material layer, P2.

[0010] 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.

[0011] The electrolyte contains ethyl propionate, and the mass percentage of ethyl propionate is ≥45% based on the total mass of the electrolyte.

[0012] As an implementation of this application, 0.005≤P2-P1≤0.025.

[0013] As an embodiment of this application, 900ppm≤W2-W1≤2000ppm.

[0014] As an embodiment of this application, the first element includes titanium and vanadium; the mass percentage of titanium Ti1 in the first lithium iron phosphate compound is less than the mass percentage of titanium Ti2 in the second lithium iron phosphate compound;

[0015] The mass percentage of vanadium in the first lithium iron phosphate compound, V1, is less than the mass percentage of vanadium in the second lithium iron phosphate compound, V2.

[0016] As an embodiment of this application, the 400ppm≤Ti2-Ti1≤1000ppm.

[0017] As an embodiment of this application, the 400ppm≤V2-V1≤1200ppm.

[0018] As an embodiment of this application, the molar ratio P1 of iron and phosphorus in the first positive electrode active material layer is 0.950 to 0.975.

[0019] As an embodiment of this application, the molar ratio P2 of iron and phosphorus in the second positive electrode active material layer is 0.955 to 1.000.

[0020] As an embodiment of this application, the mass percentage of titanium Ti1 in the first lithium iron phosphate compound is 200 to 5000 ppm.

[0021] As an embodiment of this application, the mass percentage of titanium (Ti2) in the second lithium iron phosphate compound is 600–6000 ppm.

[0022] As an embodiment of this application, the mass percentage V1 of vanadium in the first lithium iron phosphate compound is 200 to 3000 ppm.

[0023] As an embodiment of this application, the mass percentage V2 of vanadium in the second lithium iron phosphate compound is 600 to 4000 ppm.

[0024] As an embodiment of this application, the thickness of the first positive electrode active material layer on one side is 10 to 60 μm.

[0025] As an embodiment of this application, the thickness of the second positive electrode active material layer is 10–60 μm.

[0026] As an embodiment of this application, the difference between the compaction density PD2 of the second positive electrode active material layer and the compaction density PD1 of the first positive electrode active material layer is 0.1 g / cm³. 3 ≤PD2-PD1≤0.15g / cm 3 .

[0027] As an embodiment of this application, the compaction density PD1 of the first positive electrode active material layer is 1.50–3.00 g / cm³. 3 .

[0028] As an embodiment of this application, the compaction density PD2 of the second positive electrode active material layer is 1.60–3.15 g / cm³. 3 .

[0029] In a second aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[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. Furthermore, the molar ratio of iron to phosphorus in the first positive electrode active material layer, P1, is defined as < the molar ratio of iron to phosphorus in the second positive electrode active material layer, P2. Additionally, the first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include titanium and / or vanadium. This effectively solves the problem of poor cycle performance under high temperatures caused by a high ethyl propionate mass percentage ≥45% in the electrolyte. The secondary battery provided in this application has both high capacity and good cycle capacity retention, i.e., excellent cycle 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 involved. Unless otherwise specified, the numerical ranges mentioned above are considered continuous 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 an integer, it includes every integer between the minimum and maximum values ​​of the range. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included. 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 being disposed between the positive current collector and the second positive active material layer.

[0037] The first positive electrode active material layer includes a first lithium iron phosphate compound, and the second positive electrode active material layer includes a second lithium iron phosphate compound;

[0038] The first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include the first element;

[0039] The first element includes titanium and / or vanadium;

[0040] The molar ratio of iron to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of iron to phosphorus in the second positive electrode active material layer, P2.

[0041] 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.

[0042] The electrolyte contains ethyl propionate, and the mass percentage of ethyl propionate is ≥45% based on the total mass of the electrolyte.

[0043] The secondary battery provided in this application has a first positive electrode active material layer disposed 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. Furthermore, the molar ratio of iron to phosphorus in the first positive electrode active material layer, P1, is defined as < the molar ratio of iron to phosphorus in the second positive electrode active material layer, P2. Additionally, the first lithium iron phosphate compound and the second lithium iron phosphate compound each independently include titanium and / or vanadium. This effectively solves the problems of poor cycle performance under high temperatures and significant lithium plating during cycling caused by a high ethyl propionate mass percentage ≥45% in the electrolyte. The secondary battery provided in this application has both high capacity and good high-temperature (45°C) cycle capacity retention, i.e., excellent high-temperature cycle performance and low risk of lithium plating during cycling.

[0044] Specifically, firstly, the design of the first positive electrode active material layer results in slightly lower reactivity and higher safety, reducing structural damage and interfacial side reactions during high-temperature cycling, thus minimizing the increase in CEI interface and phase transfer impedance caused by these reactions. Secondly, the design of the second positive electrode active material layer effectively improves the wettability, kinetics, and bulk and surface structural stability of the positive electrode, reducing side reactions between the positive electrode surface and the electrolyte, the increase in SEI interface and phase transfer impedance, and also reducing the increase in diffusion impedance between the positive and negative electrodes, thereby significantly reducing the risk of lithium plating on the negative electrode during high-temperature cycling. Furthermore, the design of the second positive electrode active material layer also suppresses the dissolution of iron from lithium iron phosphate during long-term high-current cycling. Thirdly, when the first and second lithium iron phosphate compounds each independently include titanium and / or vanadium, it can improve the interlayer spacing of lithium iron phosphate, enhance its bulk and surface structural stability and conductivity, improve its kinetics, and increase the capacity of the secondary battery. In summary, the secondary battery provided in this application, based on its excellent kinetics, exhibits minimal lithium plating during high-temperature fast-charging cycles and high cycle capacity retention, thus possessing both high capacity and excellent high-temperature cycling performance.

[0045] It should be noted that the method for testing and calculating the molar ratio of iron 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 iron and phosphorus in different coating powders using ICP, and then converting it into a molar ratio.

[0046] 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.

[0047] It should be noted that the method for testing the mass percentage of ethyl propionate in the electrolyte includes the following steps: taking a certain amount of electrolyte sample and measuring the characteristic peak area of ​​ethyl propionate (EP) by liquid chromatography, and then converting it into the mass fraction of ethyl propionate (EP) (mass fraction of ethyl propionate = mass of EP / mass of solvent + mass of non-lithium salt additives).

[0048] In one embodiment, the ethyl propionate comprises 45-70% by mass of the total mass of the electrolyte.

[0049] For example, the mass percentage of ethyl propionate, based on the total mass of the electrolyte, can be any point value or any two-point range between 45% and 70%, such as one or any two of 45%, 50%, 55%, 60%, 65%, and 70%.

[0050] In one embodiment, 0.005 ≤ P2 - P1 ≤ 0.025.

[0051] For example, P2-P1 can be any point value or any two-point range value between 0.005 and 0.025, such as one or any two of the following: 0.005, 0.008, 0.01, 0.012, 0.014, 0.016, 0.018, 0.02, 0.022, 0.024, and 0.025.

[0052] This study found that when the difference between the molar ratio P2 of iron and phosphorus in the second positive electrode active material layer and the molar ratio P1 of iron and phosphorus in the first positive electrode active material layer is further selected between 0.005 and 0.025, the bulk phase and surface structure stability of the positive electrode sheet can be better improved, side reactions and battery polarization during high-temperature cycling can be reduced, the risk of lithium plating in the negative electrode sheet can be reduced, and the dissolution of iron during cycling can be suppressed; thus effectively improving the cycling performance of the secondary battery at high temperatures.

[0053] In one embodiment, 900ppm ≤ W2 - W1 ≤ 2000ppm.

[0054] For example, W2-W1 can be any point value or any two-point range value between 900ppm≤W2-W1≤2000ppm, such as one or any two of 900ppm, 1000ppm, 1200ppm, 1400ppm, 1600ppm, 1800ppm, 2000ppm.

[0055] 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 900 and 2000 ppm, it can better improve the bulk phase and surface structure stability of the positive electrode, enhance the conductivity of the system, and thus improve the high-temperature cycle performance and capacity of the secondary battery.

[0056] In one embodiment, the first element comprises titanium and vanadium;

[0057] 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.

[0058] The mass percentage of vanadium in the first lithium iron phosphate compound, V1, is less than the mass percentage of vanadium in the second lithium iron phosphate compound, V2.

[0059] 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 vanadium, and the mass percentages of titanium and vanadium in the first and second lithium iron phosphate compounds satisfy the relationship of this application, it is possible to improve the bulk phase and surface structure stability and conductivity of lithium iron phosphate while increasing the interlayer spacing, thereby improving the kinetics of lithium iron phosphate and the lithium deposition phenomenon of the positive electrode; thus effectively improving the capacity, high-temperature cycle performance and fast charging performance of the secondary battery.

[0060] In one embodiment, the 400ppm≤Ti2-Ti1≤1000ppm.

[0061] 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.

[0062] In one embodiment, the 400ppm ≤ V2 - V1 ≤ 1200ppm.

[0063] For example, V2-V1 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.

[0064] 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 vanadium, and the difference in the mass percentage of titanium and vanadium 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.

[0065] In one embodiment, the molar ratio P1 of iron and phosphorus in the first positive electrode active material layer is 0.950 to 0.975.

[0066] For example, the molar ratio P1 of iron and phosphorus in the first positive electrode active material layer can be any point value or any two points between 0.950 and 0.975, such as one or any two of 0.950, 0.952, 0.954, 0.956, 0.958, 0.960, 0.962, 0.964, 0.966, 0.968, 0.970, 0.972, and 0.975.

[0067] In one embodiment, the molar ratio P2 of iron and phosphorus in the second positive electrode active material layer is 0.955 to 1.000.

[0068] For example, the molar ratio P2 of iron and phosphorus in the second positive electrode active material layer can be any point value or any two points range between 0.955 and 1.000, such as one or any two of 0.955, 0.960, 0.965, 0.970, 0.975, 0.980, 0.985, 0.990, 0.995, and 1.000.

[0069] This application research found that when the molar ratio of iron to phosphorus in the first positive electrode active material layer P1 and the molar ratio of iron to phosphorus in the second positive electrode active material layer P2 are within the above range, the difference between P2 and P1 is more conducive to the bulk phase and surface structure stability and impedance growth of the overall inner active coating and the surface active coating lithium iron phosphate.

[0070] In one embodiment, the mass percentage of titanium Ti1 in the first lithium iron phosphate compound is 200–5000 ppm.

[0071] 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.

[0072] In one embodiment, the mass percentage of titanium (Ti2) in the second lithium iron phosphate compound is 600–6000 ppm.

[0073] 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.

[0074] In one embodiment, the mass percentage V1 of vanadium in the first lithium iron phosphate compound is 200 to 3000 ppm.

[0075] For example, the mass percentage V1 of vanadium in the first lithium iron phosphate compound can be any point value or any two-point range value between 200 and 3000 ppm, such as one or any two of 200 ppm, 400 ppm, 600 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, and 3000 ppm.

[0076] In one embodiment, the mass percentage V2 of vanadium in the second lithium iron phosphate compound is any point value or any two-point range value between 600 and 4000 ppm, for example, it can be one or any two of the following: 600 ppm, 800 ppm, 1000 ppm, 1200 ppm, 1400 ppm, 1600 ppm, 2000 ppm, 3000 ppm, 3600 ppm, 4000 ppm.

[0077] This study found that when the mass percentages of titanium and vanadium in the first and second lithium iron phosphate compounds are within the above-mentioned ranges, the difference between W2 and W1 is more conducive to the bulk and surface structure stability and impedance growth of the overall inner active coating and the surface active coating lithium iron phosphate, resulting in better cycle capacity retention and slightly better negative electrode lithium plating interface.

[0078] In one embodiment, the thickness of one side of the first positive electrode active material layer is 10–60 μm.

[0079] It should be noted that the test method for the single-sided thickness of the first positive electrode active material layer is: characterization by SEM test.

[0080] 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.

[0081] In one embodiment, the thickness of the second positive electrode active material layer is 10–60 μm.

[0082] It should be noted that the thickness of the second positive electrode active material layer is tested by SEM.

[0083] 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.

[0084] 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-mentioned ranges, the battery has better 45°C cycle performance and less lithium plating.

[0085] In one embodiment, the difference between the compaction density PD2 of the second positive electrode active material layer and the compaction density PD1 of the first positive electrode active material layer is 0.1 g / cm³. 3 ≤PD2-PD1≤0.15g / cm 3 .

[0086] It should be noted that the test methods for the compaction density PD2 of the second positive electrode active material layer and the compaction density PD1 of the first positive electrode active material layer are as follows: the thickness of each coating is calibrated by SEM+CP, and the specific values ​​of PD1 and PD2 are calculated by converting the powder weight corresponding to the PD1 and PD2 coatings.

[0087] For example, the difference between the compaction density PD2 of the second positive electrode active material layer and the compaction density PD1 of the first positive electrode active material layer can be 0.1 to 0.15 g / cm³. 3 The value at any point between or between any two points, for example, 0.1 g / cm³.3 0.11 g / cm 3 0.12g / cm 3 0.13g / cm 3 0.14 g / cm 3 0.15g / cm 3 One or any two of the range values.

[0088] This application research found that when the difference between the compaction density PD2 of the second positive electrode active material layer and the compaction density PD1 of the first positive electrode active material layer is within the above range, the difference between PD2 and PD1 is more conducive to the bulk phase and surface structure stability and impedance growth of the overall inner active coating and the surface active coating lithium iron phosphate, resulting in better cycle capacity retention and slightly better negative electrode lithium plating interface.

[0089] In one embodiment, the compaction density PD1 of the first positive electrode active material layer is 1.50–3.00 g / cm³. 3 .

[0090] For example, the compaction density PD1 of the first positive electrode active material layer can be 1.50 to 3.00 g / cm³. 3 The value at any point between or between any two points, for example, 1.5 g / cm³. 3 1.6g / cm 3 1.8g / cm 3 2.0g / cm 3 2.2g / cm 3 2.4g / cm 3 2.6g / cm 3 2.8g / cm 3 3.0g / cm 3 One or any two of the range values.

[0091] In one embodiment, the compaction density PD2 of the second positive electrode active material layer is 1.60–3.15 g / cm³. 3 .

[0092] For example, the compaction density PD2 of the second positive electrode active material layer can be 1.60–3.15 g / cm³. 3 The value at any point between or between any two points, for example, 1.6 g / cm³. 3 1.8g / cm 3 2.0g / cm 3 2.2g / cm 3 2.4g / cm 3 2.6g / cm 3 2.8g / cm 3 3.0g / cm3 3.15g / cm 3 One or any two of the range values.

[0093] This study found that when the compaction density PD1 of the first positive electrode active material layer and the compaction density PD2 of the second positive electrode active material layer are further selected within the above range, the difference between PD2 and PD1 is more conducive to the bulk phase and surface structure stability and impedance growth of the overall inner active coating and the surface active coating lithium iron phosphate, resulting in better cycle capacity retention and slightly better negative electrode lithium plating interface.

[0094] 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.

[0095] For example, the conductive agent includes conductive graphite; the positive electrode binder includes polyvinylidene fluoride.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] In one embodiment, the electrolyte further includes an organic solvent and a lithium salt. This application does not limit the use of organic solvents and lithium salts in the electrolyte; any known organic solvent and lithium salt can be used.

[0100] 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.

[0101] In one embodiment, the separator of the secondary battery is disposed between the positive and negative electrodes.

[0102] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0103] 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.

[0104] Example 1

[0105] This application provides a secondary battery, the preparation method of which includes the following steps:

[0106] (1) Preparation of positive electrode sheet

[0107] S1. The first positive electrode active material layer slurry is prepared by mixing the first lithium iron phosphate compound (the molar ratio of iron to phosphorus P1 is 0.96, the mass percentage of titanium Ti1 is 1000ppm, and the mass percentage of vanadium V1 is 500ppm), conductive carbon black, and polyvinylidene fluoride in a mass ratio of 95.0:3.5:1.5.

[0108] S2. The second positive electrode active material layer slurry is prepared by mixing the second lithium iron phosphate compound (the molar ratio of iron to phosphorus P2 is 0.98, the mass percentage of titanium Ti2 is 1800ppm, and the mass percentage of vanadium V2 is 1000ppm), conductive carbon black, and polyvinylidene fluoride in a mass ratio of 95.0:3.5:1.5.

[0109] 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.

[0110] (2) Preparation of negative electrode sheet

[0111] 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.

[0112] (3) Preparation of electrolyte

[0113] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), ethyl propionate (EP), vinylene carbonate (VC), 1,3-propanesulfonate 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); wherein the molar concentration of LiPF6 in the electrolyte was 0.5 M, and the molar concentration of LiFSI was 0.7 M.

[0114] (4) Preparation of secondary batteries

[0115] 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.

[0116] Examples 2-5

[0117] This application provides a secondary battery, which differs from the one in embodiment 1 in that the molar ratio of iron to phosphorus is changed by adjusting the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0118] Examples 6-9

[0119] 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.

[0120] Examples 10-13

[0121] This application provides a secondary battery, which differs from the one in embodiment 1 in that the mass percentage of vanadium is changed by adjusting the addition of vanadium to the first element in the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0122] Examples 14-15

[0123] This application provides a secondary battery, which differs from Embodiment 1 in that the parameters in Tables 1-2 are achieved by adjusting the type and amount of the first element in the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0124] Examples 16-17

[0125] 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.

[0126] Examples 18-20

[0127] This application provides a secondary battery, which differs from Embodiment 1 in that the compaction density of the first positive electrode active material layer and the second positive electrode active material layer is changed by adjusting the coating weight.

[0128] Example 21

[0129] This application provides a secondary battery, which differs from the one in Example 1 in that the mass percentage of ethyl propionate is changed by adjusting the addition of ethyl propionate in the electrolyte.

[0130] Comparative Example 1

[0131] This application provides a secondary battery in comparison, which differs from Example 1 in that the molar ratio of iron and phosphorus is changed by adjusting the first lithium iron phosphate compound and the second lithium iron phosphate compound.

[0132] Comparative Examples 2-3

[0133] 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.

[0134] Comparative Example 4

[0135] This application provides a secondary battery in comparison, which differs from Example 1 in that the mass percentage of ethyl propionate is changed by adjusting the addition of ethyl propionate in the electrolyte.

[0136] In the examples and comparative examples, the molar ratio of iron to phosphorus in the first positive electrode active material layer (P1), the molar ratio of iron to phosphorus in the second positive electrode active material layer (P2), and the difference between them (P2-P1); the mass percentage of titanium in the first lithium iron phosphate compound (Ti1 / ppm), the mass percentage of titanium in the second lithium iron phosphate compound (Ti2 / ppm), and the difference between them (Ti2-Ti1); the mass percentage of vanadium in the first lithium iron phosphate compound (V1 / ppm), the mass percentage of vanadium in the second lithium iron phosphate compound (V2 / ppm), and the difference between them (V2-V1); the mass percentage of the first element in the second lithium iron phosphate compound (W2 / ppm) and the difference between the mass percentage of the first element in the first lithium iron phosphate compound (W1 / ppm) (W2-W1); the single-sided thickness of the first positive electrode active material layer (d1 / μm); the thickness of the second positive electrode active material layer (d2 / μm); and the compaction density of the first positive electrode active material layer (PD1 / g / cm²). 3 The compaction density PD2 / g / cm of the second positive electrode active material layer 3 The difference between the two, PD2-PD1, and the mass percentage Y / % of ethyl propionate in the electrolyte are shown in Tables 1-2.

[0137] Table 1. Parameters of Secondary Batteries

[0138]

[0139]

[0140] Table 2 Secondary Battery Parameter Table

[0141]

[0142]

[0143] The cycle performance, capacity, and lithium plating of the secondary batteries prepared in the examples and comparative examples are shown in Table 3; the test methods include the following steps:

[0144] 1) Capacity: 25±5℃, every 100cls, collect the following capacity breakdown: 1C discharge to 2.0V, then 1C charge to 3.6V, cutoff current 0.05C, 1C discharge to 2.0V; 1C=4.4A);

[0145] 2) Cycling performance: 45℃±5℃, 1cls process (3C charging to 3.6V, cutoff current 0.05C, 3C discharging to 2.0V, 1h interval between charge and discharge; 1C=4.4A); record the 25℃ / 1C capacity retention rate after 1800 cycles;

[0146] 3) Lithium plating status: Charge to 3.6V at 1C full charge, cut-off current 0.05C, then disassemble in a dry room. After disassembly, observe the interface of the negative electrode under an electron microscope. According to the lithium plating area ratio (represented by S), the degree of lithium plating is divided into the following levels.

[0147] No lithium deposition: S≤1%;

[0148] Slight lithium plating: 1% < S ≤ 10%;

[0149] Moderate lithium precipitation: 10%<S≤20%;

[0150] Severe lithium plating: 20% < S ≤ 40%;

[0151] Heavy lithium plating: S > 40%.

[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 capacity, excellent cycle performance and low risk of lithium plating; specifically, the capacity of the obtained secondary battery is above 3820mAh, the cycle retention rate is above 86.3%, and the lithium plating situation after cycling and disassembly observation is found to be no lithium plating or slight lithium plating.

[0156] The results of Examples 1-21 and Comparative Example 1 of this application show that when the molar ratio P1 of iron and phosphorus in the first positive electrode active material layer is greater than that of P2 in the second positive electrode active material layer, the capacity of the obtained secondary battery shows a significant decreasing trend, and the cycle performance also decreases, while the lithium plating phenomenon on the negative electrode is more obvious. The results of Examples 1-21 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 that W2 of the first element in the second lithium iron phosphate compound, the capacity of the obtained secondary battery shows a significant decreasing trend, and the cycle performance also decreases, while the lithium plating phenomenon on the negative electrode is more obvious. The results of Examples 1-21 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 capacity and cycle performance of the obtained secondary battery decrease significantly, and severe lithium plating occurs. The results of Examples 1-21 and Comparative Example 4 of this application show that when the mass percentage of ethyl propionate in the electrolyte is less than 45%, the capacity and cycle performance of the obtained secondary battery decrease 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 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 vanadium; The molar ratio of iron to phosphorus in the first positive electrode active material layer, P1, is less than the molar ratio of iron 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 electrolyte contains ethyl propionate, and the mass percentage of ethyl propionate is ≥45% based on the total mass of the electrolyte. The molar ratio P1 of iron and phosphorus in the first positive electrode active material layer is 0.950~0.975; The molar ratio of iron to phosphorus in the second positive electrode active material layer, P2, is 0.955~1.

000.

2. The secondary battery according to claim 1, characterized in that, 0.005≤P2-P1≤0.

025.

3. The secondary battery according to claim 1, characterized in that, 900ppm≤W2-W1≤2000ppm.

4. The secondary battery according to claim 1, characterized in that, The first element includes titanium and vanadium; 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 vanadium in the first lithium iron phosphate compound, V1, is less than the mass percentage of vanadium in the second lithium iron phosphate compound, V2.

5. The secondary battery according to claim 4, characterized in that, 400ppm≤Ti2-Ti1≤1000ppm; And / or, 400ppm≤V2-V1≤1200ppm.

6. The secondary battery according to claim 4, characterized in that, Satisfy at least one of the following: (1) The mass percentage of titanium Ti1 in the first lithium iron phosphate compound is 200~5000 ppm; (2) The mass percentage of titanium (Ti2) in the second lithium iron phosphate compound is 600~6000 ppm; (3) The mass percentage V1 of vanadium in the first lithium iron phosphate compound is 200~3000 ppm; (4) The mass percentage of vanadium V2 in the second lithium iron phosphate compound is 600~4000 ppm.

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

8. The secondary battery according to claim 1, characterized in that, The difference between the compaction density PD2 of the second positive electrode active material layer and the compaction density PD1 of the first positive electrode active material layer is 0.1 g / cm³. 3 ≤PD2-PD1≤0.15g / cm 3 .

9. The secondary battery according to claim 8, characterized in that, The compaction density PD1 of the first positive electrode active material layer is 1.50~3.00 g / cm³. 3 ; And / or, the compaction density PD2 of the second positive electrode active material layer is 1.60~3.15 g / cm³. 3 .

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

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