Secondary battery and electric device

Abstract

The application discloses a secondary battery and a power utilization device, and belongs to the technical field of secondary batteries. The secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, the positive electrode sheet comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode material, the positive electrode material comprises lithium iron phosphate, the electrolyte comprises a cyclic sulfate compound, the cyclic sulfate compound contains an electron-removing group, and the negative electrode sheet comprises a negative electrode material layer. When the secondary battery is charged to a full charge state, the negative electrode material layer contains iron elements and LiC6. According to the application, the mass percentage content a% of the electron-removing group in the electrolyte, the mass percentage content b% of LiC6 in the negative electrode material layer and the mass content cppm of the iron elements in the negative electrode material layer are regulated, so that b / (10a 1 / 2 +c) within a suitable range, active lithium is effectively compensated, the side reaction of the electrolyte with the positive and negative electrodes can be reduced, and the cycle life and safety of the battery are considered.

Description

Technical Field

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

[0002] As a type of high-efficiency and environmentally friendly battery, rechargeable batteries have been widely used in electric vehicles, portable electronic devices, aerospace, and national defense due to their high energy density and long cycle life. During the first charge of a rechargeable battery, the electrolyte reacts with the negative electrode material to form a solid electrolyte interphase (SEI) film, consuming a large number of lithium ions and converting lithium into inactive lithium compounds such as lithium carbonate, lithium fluoride, and alkyl lithium. This results in irreversible lithium loss and low initial battery efficiency.

[0003] Lithium replenishment technology involves adding lithium sources to a lithium-ion secondary battery before it operates to replenish lithium ions, offsetting irreversible lithium loss and improving the battery's capacity and energy density. However, adding too much lithium replenishment agent can easily trigger electrolyte decomposition and gas generation during battery charging and discharging, reducing the battery's safety performance and cycle life.

[0004] In view of this, it is indeed necessary to provide a rechargeable battery that can balance safety and cycle life. Summary of the Invention

[0005] Based on the deficiencies of the existing technology, the purpose of this application is to provide a secondary battery and an electrical device.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0007] The first aspect of this application provides a secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a positive electrode material layer, the positive electrode material layer includes a positive electrode material, and the positive electrode material includes lithium iron phosphate;

[0008] The electrolyte comprises a cyclic sulfate compound, which contains an electron-donating group. The electron-donating group includes at least one of a hydrogen atom, an alkyl group with 1-6 carbon atoms, an alkoxy group with 1-3 carbon atoms, an alkenyl group with 1-3 carbon atoms, a phenyl group, a thiophene group, and a pyrrole group. The mass percentage of the electron-donating group in the electrolyte is a%.

[0009] The cyclic sulfate compounds include compounds represented by structural formula I; in structural formula I, R1, R2, R3 and R4 each independently include at least one of hydrogen atom, alkyl group of 1 to 6 carbon atoms, alkoxy group of 1 to 3 carbon atoms, alkenyl group of 1 to 3 carbon atoms, phenyl group, thiophene group, and pyrrole group.

[0010] Formula I;

[0011] The negative electrode sheet includes a negative electrode material layer, which includes a negative electrode material. When the secondary battery is fully charged, the negative electrode material layer contains iron and LiC6. The mass percentage of LiC6 in the negative electrode material layer is b%, and the mass percentage of iron in the negative electrode material layer is cppm.

[0012] The secondary battery satisfies: 0.6 ≤ b / (10a) 1 / 2 +c)≤17.8.

[0013] The second aspect of this application provides an electrical device, which includes the secondary battery provided in the first aspect of this application.

[0014] Compared with the prior art, the beneficial effects of this application are as follows:

[0015] This application adjusts the mass percentage of electron-donating groups (a%) in the electrolyte, the mass percentage of LiC6 in the negative electrode material layer (b%), and the mass percentage of iron in the negative electrode material layer (cppm) to achieve b / (10a) 1 / 2 +c) Within a suitable range, the active lithium is effectively compensated, which can reduce the side reactions between the electrolyte and the positive and negative electrodes, thereby balancing the cycle life and safety of the battery. Detailed Implementation

[0016] To better illustrate the purpose, technical solution, and advantages of this application, the following description, in conjunction with specific embodiments and comparative examples, aims to provide a detailed understanding of the content of this application, rather than to limit its scope. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this application.

[0017] 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.

[0018] 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.

[0019] In the description of this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0020] Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0021] In this application, the terms "secondary battery", "lithium secondary battery", and "lithium-ion secondary battery" all have the same meaning and refer to lithium-ion secondary batteries, which typically include electrode components (such as positive electrode plates, negative electrode plates, and separators), a container (such as a housing) that houses the electrode components, and an electrolyte.

[0022] In a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode material, and the positive electrode material comprises lithium iron phosphate;

[0023] The electrolyte comprises a cyclic sulfate compound, which contains an electron-donating group. The electron-donating group includes at least one of a hydrogen atom, an alkyl group with 1-6 carbon atoms, an alkoxy group with 1-3 carbon atoms, an alkenyl group with 1-3 carbon atoms, a phenyl group, a thiophene group, and a pyrrole group. The mass percentage of the electron-donating group in the electrolyte is a%.

[0024] The cyclic sulfate compounds include compounds represented by structural formula I; in structural formula I, R1, R2, R3 and R4 each independently include at least one of hydrogen atom, alkyl group of 1 to 6 carbon atoms, alkoxy group of 1 to 3 carbon atoms, alkenyl group of 1 to 3 carbon atoms, phenyl group, thiophene group, and pyrrole group.

[0025] Formula I;

[0026] The negative electrode sheet includes a negative electrode material layer, which includes a negative electrode material. When the secondary battery is fully charged, the negative electrode material layer contains iron and LiC6. The mass percentage of LiC6 in the negative electrode material layer is b%, and the mass percentage of iron in the negative electrode material layer is cppm.

[0027] The secondary battery satisfies: 0.6 ≤ b / (10a) 1 / 2 +c)≤17.8.

[0028] For example, b / (10a) 1 / 2 +c) can be 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.1, 8.2, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 17.8, or a range consisting of any two sets of values.

[0029] During the first charge, the battery consumes active lithium to form an SEI film on the negative electrode. Once this active lithium is consumed, it cannot contribute to the battery's capacity, resulting in low initial efficiency. The inventors discovered that adding a lithium replenishing agent to the positive electrode material can compensate for the active lithium loss (m) caused by the SEI film formation, thus improving the battery's cycle life. However, this can easily lead to an excessively high LiC6 mass percentage (b%) in the negative electrode material layer. Excessive lithium replenishing agent can also cause electrolyte decomposition and gas generation during charge and discharge, resulting in poor battery safety. Therefore, the LiC6 mass percentage (b%) in the negative electrode material layer must not be too high.

[0030] The inventors further discovered that adding cyclic sulfate compounds to the electrolyte can promote the formation of an effective SEI film on the negative electrode, reduce side reactions between the negative electrode interface and the electrolyte, reduce gas production, and improve battery safety. However, the low oxidation potential of the lithium iron phosphate positive electrode prevents the cyclic sulfate compound additives from oxidizing at the positive electrode to promote CEI film formation and from repairing the exposed new interface caused by particle breakage. This leads to increased side reactions at the electrolyte-positive electrode interface, intensified iron dissolution, and decreased battery safety. By introducing electron-donating groups into the structure of the cyclic sulfate compounds and controlling the mass percentage (a%) of electron-donating groups in the electrolyte, the oxidation potential of the lithium iron phosphate positive electrode can be reduced, thereby promoting the formation of a stable CEI film at the positive electrode, improving the structural stability of the positive electrode material, reducing side reactions between the positive electrode interface and the electrolyte, further reducing gas production, and improving battery safety. However, an excessively high mass percentage (a%) of electron-donating groups in the electrolyte can lead to an excessively low reduction potential for cyclic sulfate compounds, preventing the formation of a stable SEI film at the negative electrode and increasing side reactions between the negative electrode interface and the electrolyte. By controlling the mass content (cppm) of iron in the negative electrode material layer, the amount of iron dissolution can be reduced, thereby decreasing the iron-catalyzed electrolyte decomposition and improving the stability of the SEI film, thus enhancing battery safety.

[0031] Therefore, it is necessary to comprehensively control the mass percentage of electron-donating groups in the electrolyte (a%), the mass percentage of LiC6 in the negative electrode material layer (b%), and the mass percentage of iron in the negative electrode material layer (cppm), so that b / (10a) 1 / 2 +c) Within a suitable range, the active lithium is effectively compensated, which can reduce the side reactions between the electrolyte and the positive and negative electrodes, thereby balancing the cycle life and safety of the battery.

[0032] In this application, 10a 1 / 2 This can reflect the weight of parameter a in terms of its influence on battery cycle life and safety, ensuring that it is reasonably matched with c in terms of magnitude and intensity. This avoids overemphasizing the dominant role of the iron content in the negative electrode material layer in regulating battery cycle life and safety, while weakening the role of the electron-donating group content of cyclic sulfate compounds in the electrolyte in regulating battery cycle life and safety.

[0033] Denominator term 10a 1 / 2 The parameter represented by +c can regulate the overall performance of the battery, while the molecular phase b can regulate the cycle life of the battery. The entire formula combination is b / (10a). 1 / 2 +c) better reflects the regulatory effect of each parameter on battery performance.

[0034] In this application, the mass percentage a% of electron-donating groups in the electrolyte refers to the mass percentage of electron-donating groups contained in the cyclic sulfate ester compound in the electrolyte.

[0035] In some embodiments, the secondary battery satisfies: 1.3 ≤ b / (10a) 1 / 2 +c)≤8.1.

[0036] In some implementations, 0.000008% ≤ a% ≤ 1.22%.

[0037] For example, a% can be 0.000008%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.82%, 0.9%, 1.0%, 1.1%, 1.2%, 1.22%, or a range of any two sets of values.

[0038] This application achieves a balance between reduction and oxidation potentials by controlling the mass percentage (a%) of electron-donating groups in the electrolyte to meet the aforementioned range. This promotes the formation of an effective SEI film at the negative electrode and an effective CEI film at the positive electrode, thereby reducing side reactions between the positive and negative electrodes and the electrolyte, avoiding or reducing the recombination and regeneration of the SEI and CEI films, reducing gas production, and improving battery safety and cycle performance. This application can achieve this by introducing cyclic sulfate compounds into the electrolyte. The means by which this application controls the mass percentage (a%) of electron-donating groups in the electrolyte include, but are not limited to, the following: controlling the mass percentage (a%) of electron-donating groups in the electrolyte by changing the selection of different electron-donating groups in the cyclic sulfate compounds and the amount of cyclic sulfate compounds added.

[0039] For further optimization, 0.00008%≤a%≤0.82%.

[0040] In some implementations, 60% ≤ b% ≤ 100%.

[0041] For example, b% can be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95%, 98%, 100%, or a range of any two sets of values.

[0042] LiC6 is a product of lithium intercalation in graphite anodes, and its presence indirectly proves the use of lithium replenishing agents in the cathode. Lithium replenishing agents provide additional lithium ions to the cathode, resulting in more lithium ions intercalating into the anode during charging to form LiC6. Currently, in rechargeable batteries, without the addition of lithium replenishing agents, the mass percentage of LiC6 in the anode material layer is typically low. The presence of lithium replenishing agents can be inferred by detecting abnormal LiC6 content in the anode. This application introduces a lithium replenishing agent into the cathode material layer, causing an irreversible reaction during the first charge of the battery, releasing lithium ions to compensate for the active lithium ions consumed during processes such as SEI film formation, thereby improving the battery's cycle performance. The methods used in this application to control the mass percentage b% of LiC6 in the anode material layer include, but are not limited to, the following: adjusting the amount of lithium replenishing agent added to the cathode and the porosity of the anode material layer to control the mass percentage b% of LiC6 in the anode material layer.

[0043] For further optimization, 70% ≤ b% ≤ 95%.

[0044] In some implementations, 5ppm ≤ cppm ≤ 100ppm.

[0045] For example, cppm can be 5ppm, 10ppm, 15ppm, 20ppm, 25ppm, 30ppm, 35ppm, 40ppm, 45ppm, 50ppm, 55ppm, 60ppm, 65ppm, 70ppm, 75ppm, 80ppm, 85ppm, 90ppm, 95ppm, 100ppm or any two of these values.

[0046] During charging and discharging, iron ions from the positive electrode migrate to the negative electrode, catalyzing electrolyte decomposition, consuming active lithium ions, and increasing interfacial impedance. This application controls the iron content (cppm) in the negative electrode material layer to meet the aforementioned range. An appropriate amount of Fe helps form a denser and more stable SEI film, reducing electrolyte self-decomposition, decreasing irreversible consumption of active lithium ions, improving battery cycle performance, and preventing excessive iron ions from catalyzing electrolyte decomposition, damaging SEI film stability, increasing interfacial impedance, raising battery internal resistance, increasing gas and heat generation, and improving battery safety. The methods used in this application to control the iron content (cppm) in the negative electrode material layer include, but are not limited to, the following: changing the iron content (cppm) in the negative electrode material layer by adjusting the porosity of the negative electrode material layer and the thickness of the coating layer on the surface of the positive electrode active material.

[0047] Further preferred values ​​are 10ppm≤cppm≤50ppm.

[0048] In some embodiments, the number of sulfate groups in the compound with the structure shown in Formula I is n, where n is a positive integer and 2 ≤ n ≤ 4.

[0049] In some embodiments, R2 and R4 are both hydrogen atoms, and R1 and R3 each independently include at least one of hydrogen atoms, an alkyl group with 1 to 6 carbon atoms, an alkoxy group with 1 to 3 carbon atoms, an alkenyl group with 1 to 3 carbon atoms, a phenyl group, a thiophene group, and a pyrrole group.

[0050] In some embodiments, R1 and R3 are the same substituents and are selected from any one of hydrogen atoms, alkyl groups of 1 to 6 carbon atoms, alkoxy groups of 1 to 3 carbon atoms, alkenyl groups of 1 to 3 carbon atoms, phenyl groups, thiophene groups, and pyrrole groups.

[0051] In some embodiments, R1 and R3 are selected from any one of the groups represented by structural formula III, namely -CH3, -C2H5, -C3H7, -CH=CH2, -CH2-CH=CH2, -CH=CH-CH3, -O-CH3, -O-C2H5, -O-C3H7, -C6H5, -C4H3S, and -C4H4N.

[0052] In some embodiments, the cyclic sulfate compound includes at least one of the following compounds:

[0053] , , , , , , , , , , , , .

[0054] In some embodiments, the electrolyte contains 0.001% to 3% by mass of cyclic sulfate compounds.

[0055] For example, the mass percentage of cyclic sulfate compounds in the electrolyte can be 0.001%, 0.005%, 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, or a range of any two sets of values.

[0056] In some embodiments, the porosity of the negative electrode material layer is 20% to 60%.

[0057] For example, the porosity of the negative electrode material layer can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or any two of these values.

[0058] The inventors discovered through research that by controlling the porosity of the negative electrode material layer to a suitable range, it is possible to ensure the wetting effect of the electrolyte on the negative electrode sheet, reduce or avoid the formation of "dead zones" where some active materials cannot participate in the reaction, slow down the degradation of the electrode material, and improve the cycle life of the battery. It is also possible to avoid the sudden increase of side reactions between the negative electrode material layer and the electrolyte due to excessive porosity, reduce gas production, and improve the safety of the battery.

[0059] In some embodiments, the degree of graphitization of the negative electrode material is 90-98%.

[0060] For example, the degree of graphitization of the negative electrode material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or any two of these values.

[0061] The inventors discovered that by controlling the graphitization degree of the negative electrode material to a suitable range, a large number of surface active sites can be ensured, which can improve the stability of the formed SEI film, avoid repeated rupture and regeneration of the SEI during charge-discharge cycles, reduce the consumption of active lithium, and thus improve the cycle life of the battery. The more regular arrangement of carbon atoms in the negative electrode material can reduce the resistance of the electron conduction channel, reduce the resistance, reduce Joule heating during the charge-discharge process, reduce battery temperature rise, reduce side reactions such as electrolyte decomposition, and improve battery safety.

[0062] In this application, the degree of graphitization of the negative electrode material can be determined by the following method:

[0063] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V. The negative electrode of the secondary battery in the empty state was taken and soaked in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, the negative electrode was taken out and dried in a vacuum environment. The powder on the surface of the negative electrode was scraped off with a ceramic knife.

[0064] The XRD characteristic peak spectrum of the obtained powder was tested by XRD diffraction. The graphite interplanar spacing d(002) of the powder was determined according to the spectrum. Then, the degree of graphitization of the negative electrode material was calculated according to [0.3440-d(002)] / (0.3440-0.3354).

[0065] In some embodiments, the negative electrode material includes at least one of graphite and silicon-carbon composite materials.

[0066] In some embodiments, the negative electrode material is graphite, and the electrolyte satisfies: 0.004% ≤ a% ≤ 0.86%.

[0067] The inventors discovered that graphite has a layered structure, and during the first charge, solvent components in the electrolyte (such as EC and DMC) are prone to co-intercalation, triggering a continuous reduction and decomposition reaction, leading to the continuous thickening of the SEI film and irreversible lithium-ion loss. Cyclic sulfate compounds have low reduction potential and high film-forming efficiency, and can preferentially reduce on the graphite surface to form a dense, uniform, and low-ionic-resistance inorganic-organic composite SEI film, effectively blocking solvent co-intercalation and suppressing side reactions. This film tends to stabilize after formation; therefore, it is necessary to add an appropriate amount of cyclic sulfate compounds to ensure that the mass percentage (a%) of electron-donating groups in the electrolyte is within a suitable range, thus ensuring the integrity of the SEI film and improving the battery's cycle performance and safety.

[0068] In some embodiments, the negative electrode material is a silicon-carbon composite material, and the electrolyte satisfies: 0.00008% ≤ a% ≤ 0.35%.

[0069] The inventors discovered that silicon expands by over 300% during lithium intercalation, causing the SEI film to repeatedly rupture and regenerate, forming a "dynamically consumed" interface. At this point, cyclic sulfate compounds, due to their high film rigidity and poor elasticity, cannot adapt to drastic deformation and are prone to failure at cracks. Silicon-carbon composites rely more on flexible film-forming agents, such as FEC (fluoroethylene carbonate) and VC (ethylene carbonate). Therefore, it is necessary to reduce the amount of cyclic sulfate compounds added, keeping it at 0.00008% ≤ a% ≤ 0.35%, to improve the battery's cycle performance and safety.

[0070] In some embodiments, the negative electrode material may further include at least one of hard carbon, soft carbon, mesophase carbon microspheres, elemental silicon, silicon suboxide, and lithium titanate.

[0071] Optionally, the negative electrode material layer may further include a negative electrode conductive agent and a negative electrode binder.

[0072] The mass ratio of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder is (94~96):(1~3):(2~4).

[0073] The negative electrode conductive agent may include conductive agents conventionally used in the art. For example, the negative electrode conductive agent includes at least one of multi-walled carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, conductive graphite, graphene, and conductive carbon fiber.

[0074] The negative electrode binder may include binders conventionally used in the art. For example, the negative electrode binder includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, styrene-butadiene rubber, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyimide, and polyamide-imide.

[0075] Furthermore, the negative electrode sheet includes a negative electrode current collector, and the negative electrode active material layer is disposed on at least one side surface of the negative electrode current collector.

[0076] The negative electrode current collector may include negative electrode current collectors conventionally used in the art. For example, the negative electrode current collector includes at least one of copper foil, chromium foil, nickel foil, and titanium foil.

[0077] The negative electrode sheet can be prepared according to conventional methods in the art. For example, the preparation method of the negative electrode sheet includes the following steps: mixing a negative electrode active material, a negative electrode conductive agent, a negative electrode binder and a solvent to obtain a negative electrode slurry; coating the negative electrode slurry onto at least one side surface of a negative electrode current collector, drying it, rolling it and cutting it to obtain a negative electrode sheet.

[0078] In the preparation process of the negative electrode sheet, the porosity of the negative electrode material layer can be adjusted by controlling the solid content of the negative electrode slurry and / or the rolling pressure.

[0079] In some embodiments, the compaction density of the positive electrode is 2 g / cm³. 3 ~3g / cm 3 .

[0080] For example, the compaction density of the positive electrode sheet can be 2 g / cm³. 3 2.1g / cm 3 2.2g / cm 3 2.3g / cm 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 2.8g / cm 3 2.9g / cm 3 3.0g / cm 3 Or a range consisting of any two sets of values.

[0081] The inventors discovered through research that controlling the compaction density of the positive electrode sheet to 2 g / cm³ is effective. 3 ~3g / cm 3 On the one hand, it can give the positive electrode material layer of the positive electrode a certain porosity, ensuring the wetting effect of the electrolyte on the positive electrode material layer, improving ion transport efficiency, and avoiding the phenomenon of particle breakage, pulverization or shedding of the positive electrode material during charge and discharge cycles due to excessive compaction density of the positive electrode, thereby improving the cycle life of the secondary battery; on the other hand, it can avoid the increase of side reactions between the positive electrode and the electrolyte due to excessively low compaction density of the positive electrode, reduce gas production, and improve battery safety.

[0082] In this application, the compaction density of the positive electrode sheet can be determined by the following method:

[0083] Discharge the secondary battery at 0.33C to the lower limit voltage of 2.5V, remove the empty battery, disassemble the electrode, soak the electrode in dimethyl carbonate (DMC) solution for 4 hours; air dry; use a punching machine to punch the pretreated positive electrode into circular pieces of fixed area, denoted as S0, take three circular pieces as parallel samples, and weigh the mass of the three circular pieces using an electronic balance, take the average value and record it as M1; use a micrometer to measure the thickness of the active material layer in the three circular pieces (i.e., the total thickness excluding the current collector), take the average value and record it as H, finally add an appropriate amount of deionized water to each of the three circular pieces, gently wipe the coating on the circular pieces with lint-free paper to expose the copper foil, let stand at room temperature (or dry) for 10 minutes, after the copper foil is dry, weigh the mass of the three copper foil pieces, take the average value and record it as M0, calculate the compaction density A of the positive electrode according to the following formula: A=(M1-M0) / (H×S0).

[0084] In some embodiments, the particle size Dv50 of the cathode material is 0.5 μm to 15 μm.

[0085] For example, the particle size Dv50 of the positive electrode material can be 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm or any two sets of values ​​therein.

[0086] The inventors discovered through research that by adjusting the particle size Dv50 of the cathode material to a suitable range, the migration path of lithium ions can be moderate, thereby ensuring the cycle life of the battery; it can also avoid the increase of side reactions between the cathode material and the electrolyte due to excessive specific surface area, reduce gas production, and thus improve the safety of the secondary battery.

[0087] In this application, the particle size Dv50 of the cathode material can be determined by the following method:

[0088] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V. In the empty state, the positive electrode sheet was disassembled and soaked in dimethyl carbonate (DMC) at room temperature (25℃) for 2 hours. After drying, the powder on the surface of the dried electrode sheet was scraped off with a scraper and used as a sample. The sample was fixed with conductive adhesive and polished with CP argon ion to deposit a conductive film. It was placed on the sample stage and observed under a scanning electron microscope (SEM). Three areas were selected and photographed at a magnification of 10Kx. After photographing, the particle size distribution was measured using a laser particle size distribution measuring instrument (Mastersizer 3000) according to the particle size distribution laser diffraction method (specific steps refer to GB / T19077-2016). The particle size corresponding to the cumulative particle size distribution percentage reaching 50% is the particle size Dv50 of the positive electrode material.

[0089] In some embodiments, the cathode material includes a dopant element, which includes at least one of Mg, Cr, Na, Al, Mn, Zr, Nb, Co, Ni, Ti, and V, and the mass percentage of the dopant element in the cathode material layer is 0.3% to 3%.

[0090] For example, the mass percentage of doped elements in the cathode material layer is 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, or any two of these values.

[0091] The inventors discovered that introducing doping elements into the cathode material can improve the electronic conductivity of lithium iron phosphate, expand the diffusion channels of lithium ions, reduce internal resistance, reduce polarization, and make the charge and discharge reaction more uniform and efficient, thereby improving the cycle performance of the battery. In addition, it can also stabilize the crystal structure of lithium iron phosphate particles, suppress harmful phase transitions and crystal distortions during charge and discharge, reduce the risk of thermal runaway, and improve the safety of the battery.

[0092] In this application, the mass percentage of doped elements in the cathode material layer can be determined by the following method:

[0093] The secondary battery was discharged to 2.5V at 0.33C. The empty battery was then disassembled to obtain the positive electrode. The positive electrode was immersed in DMC (dimethyl carbonate) at room temperature for 60 minutes, removed, and air-dried at room temperature with humidity ≤15%. The powder on the dried electrode surface was scraped off with a scraper, preliminarily crushed in a mortar and pestle, and then passed through a 200-mesh sieve to obtain a fine powder sample. A certain amount of powder was accurately weighed and dispersed in 20ml of water, then 10ml of nitric acid was added. After mixing thoroughly, the mixture was heated until the powder dissolved. The solution was then diluted to 100mL with water to obtain the test solution. ICP testing was performed on the test solution. Before testing, a standard solution must be prepared. The linear correlation coefficient of the standard concentration must be above 0.999 to be used as a normal standard. The 1000mg / L standard solution was diluted with deionized water to a concentration that was not... Different concentrations (typically 0, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL) are used, and the element detection wavelength is selected. Experimental conditions are set as follows: Based on the characteristics of the sample and the element to be detected, appropriate ICP instrument operating conditions are set: gas flow rate 0.5 L / min, power 1150 W, and the element detection wavelength is selected, depending on the element being tested (Mg wavelength 285.2 nm, Cr wavelength 267.72 nm, Al wavelength 308.21 nm, Mn wavelength 257.61 nm, Zr wavelength 336.12 nm, Nb wavelength 309.4 nm, Co wavelength 240.7 nm, Ni wavelength 232 nm, Ti wavelength 337.28 nm, V wavelength 292.4 nm). The content of the corresponding dopant element in the sample can be read by the self-analysis function of the ICP testing software.

[0094] In some embodiments, at least a portion of the lithium iron phosphate particles have a coating layer on their surface, the coating layer having a thickness of 10 nm to 100 nm.

[0095] For example, the thickness of the coating layer can be 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 85nm, 90nm, 95nm, 100nm, or any two of these values.

[0096] The inventors discovered that coating the surface of lithium iron phosphate particles with a coating layer and controlling the thickness of the coating layer to 1~100nm can improve the conductivity of lithium iron phosphate, avoid excessive direct contact between the electrolyte and lithium iron phosphate, reduce side reactions between lithium iron phosphate and electrolyte, reduce gas production, and improve the cycle life and safety of secondary batteries.

[0097] In some embodiments, the positive electrode material layer further includes a positive electrode lithium supplement, a positive electrode conductive agent, a positive electrode dispersant, and a positive electrode binder.

[0098] Specifically, in the positive electrode material layer, the sum of the mass percentages of the positive electrode material and the positive electrode lithium supplement is 94.5% to 98.5%, the mass percentage of the positive electrode conductive agent is 0.5% to 1.5%, the mass percentage of the positive electrode dispersant is 1.3% to 2.2%, and the mass percentage of the positive electrode binder is 0.05% to 2%.

[0099] Specifically, the mass ratio of the positive electrode lithium supplement to the positive electrode material is (1~4):100. The positive electrode lithium supplement includes at least one of lithium oxalate, Li5FeO4 (LFO), Li2NiO2 (LNO), and lithium trifluoromethyl sulfinate (LiSO2CF3).

[0100] The positive electrode conductive agent may include conductive agents conventionally used in the art. For example, the positive electrode conductive agent includes at least one of multi-walled carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, conductive graphite, graphene, and conductive carbon fiber.

[0101] The positive electrode dispersant includes at least one of the following: acrylic acid, acrylate, polyether ester, phosphate ester, small molecule alkanolamine, polyurethane, modified styrene / maleic anhydride, nitrile rubber (HNBR), and polyvinylpyrrolidone (PVP).

[0102] The positive electrode binder may include positive electrode binders conventionally used in the art. For example, the positive electrode binder includes at least one selected from polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, styrene-butadiene rubber, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyimide, and polyamide-imide.

[0103] The positive current collector may include positive current collectors conventionally used in the art. For example, the positive current collector includes at least one of aluminum foil and composite foil. The composite foil includes a middle high-density layer and metal layers disposed on both sides of the polymer layer. The polymer layer includes polymer materials, including at least one of polyamide (PA), polyterephthalate, polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PPE), polyvinyl chloride (PVC), aramid, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene terephthalate (PET), poly(p-phenylene terephthalamide) (PPTA), polypropylene (PPE), polyoxymethylene (POM), epoxy resin, phenolic resin, polytetrafluoroethylene (PTEE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate (PC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, starch, protein, their derivatives, their crosslinks, and their copolymers. The metal layers may include at least one of aluminum, copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys.

[0104] The positive electrode sheet can be prepared according to conventional methods in the art. For example, the preparation method of the positive electrode sheet includes the following steps: mixing positive electrode material, positive electrode lithium supplement, positive electrode conductive agent, positive electrode binder, positive electrode dispersant and solvent to obtain a positive electrode slurry; coating the positive electrode slurry on at least one side surface of a positive electrode current collector, drying and then rolling and cutting to obtain a positive electrode sheet.

[0105] In one embodiment, the cathode material includes lithium iron phosphate and doping elements, and the preparation method of the cathode material includes, but is not limited to, the following steps:

[0106] The precursor iron phosphate, lithium source, dopant salt and water are mixed and ground, then sintered at a temperature of 700~900℃ for 8~15h, and then crushed to obtain powder.

[0107] The obtained powder and coating are mixed, mechanically dispersed, and then calcined at a temperature of 700-900℃ for 8-15 hours. After pulverization and sieving, the positive electrode material is obtained.

[0108] In the preparation process of the cathode material, the molar ratio of Li element in the lithium source to Fe element in the precursor iron phosphate is (1.03~1.08):1, and the molar ratio of dopant element in the dopant salt to Fe element in the precursor iron phosphate is (0.005~0.05):1.

[0109] The lithium source may include at least one of the following: an inorganic salt containing Li, a hydroxide containing Li, and an oxide containing Li; the dopant salt may include at least one of the following: an inorganic salt containing a dopant element, a hydroxide containing a dopant element, and an oxide containing a dopant element; the coating material used to form the coating layer may be a carbon material, such as natural graphite or artificial graphite, and the amount of coating material added is 5% to 10% of the mass of the obtained powder;

[0110] Mechanical dispersion can be achieved through at least one of stirring, grinding, or ball milling. For example, dispersion can be achieved by stirring at a speed of 1000-2000 rpm for 30-60 minutes. Dispersion can be achieved by ball milling at a ball-to-material ratio of (5-10):1 for 1-2 hours.

[0111] It is understood that during the preparation of the cathode material, the thickness of the coating layer can be controlled by adjusting process parameters such as the amount of coating material added, the temperature of the calcination treatment, and the time of the calcination treatment.

[0112] In some embodiments, the viscosity of the electrolyte at 25°C is 2 cP to 5 cP.

[0113] For example, the viscosity of the electrolyte at 25°C can be 2cP, 2.2cP, 2.5cP, 2.8cP, 3cP, 3.2cP, 3.5cP, 3.8cP, 4cP, 4.2cP, 4.5cP, 4.8cP, 5cP, or a range consisting of any two sets of values.

[0114] The inventors discovered through research that adjusting the viscosity of the electrolyte to a suitable range can ensure good contact between the electrolyte and the electrode interface, promote the uniform formation of the SEI film, reduce side reactions between the electrolyte and the positive and negative electrode interfaces, and improve the cycle life of the battery. At the same time, it can also moderately reduce the dissolution effect of the electrolyte on the positive electrode material, reduce the dissolution of iron ions, and prevent excessive dissolved iron ions from migrating to the negative electrode and catalyzing lithium deposition, thereby ensuring the safety of the battery.

[0115] In this application, the viscosity of the electrolyte at 25°C can be determined by the following method:

[0116] The secondary battery was discharged to 2.5V at 0.33C. The battery was then disassembled and the electrolyte collected in a glove box (H2O ≤ 0.1ppm, O2 ≤ 0.1ppm). There are three methods for collecting the electrolyte: 1) After removing the battery cover: ① If there is free electrolyte, collect it into a 5mL sample tube using a pipette and seal it with adhesive tape to prevent leakage. ② If there is no free electrolyte, use a hydraulic press to continuously pressurize until free electrolyte appears, collect it into a sample tube, and seal it. ③ Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, place the battery in an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic vibrator and vibrate for 12 hours to allow the electrolyte and dichloromethane in the electrodes to mix thoroughly. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with adhesive tape.

[0117] The electrolyte was removed and tested using a Cambridge viscometer. The Cambridge viscometer is designed primarily based on electromagnetic oscillation viscosity detection technology, employing a magnetically levitated probe for viscosity measurement. Specifically, the electrolyte was placed in a beaker, and the sample temperature was controlled at 25°C for testing. The reading was taken after the displayed value stabilized.

[0118] In some embodiments, the electrolyte further includes additives, which include at least one selected from the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), methane disulfonate (MMDS), tris(trimethylsilane)borate (TMSB), propylene sulfite (PS), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiDFOP), lithium difluorooxalate borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalate borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), tris(trimethylsilane) phosphate (TMSP), tris(trimethylsilane) phosphite (TMSPI), trimethyl phosphate (TMP), and triphenyl phosphate (TPP). The mass percentage of the additives in the electrolyte is 0.05% to 10%.

[0119] Controlling the content of additives in the electrolyte within an appropriate range can promote the formation of a stable SEI film on the negative electrode, reduce side reactions between the negative electrode and the electrolyte, reduce the consumption of active lithium, and improve the cycle life of the battery; it can also prevent the electrolyte from easily decomposing and producing gas due to excessive additive content, thereby ensuring that the battery safety is not reduced.

[0120] In some embodiments, the additive is vinylene carbonate, and the negative electrode sheet satisfies: 88% ≤ b ≤ 97%. The higher the amount of lithium supplement added to the positive electrode, the easier it is to cause electrolyte decomposition and gas generation. Therefore, it is necessary to introduce vinylene carbonate (VC) into the electrolyte to promote the formation of an SEI film on the negative electrode, reduce side reactions and gas generation between the negative electrode and the electrolyte, and improve battery safety.

[0121] In some embodiments, the electrolyte further includes a lithium salt and a solvent, wherein the concentration of the lithium salt in the electrolyte is 5% to 20%.

[0122] Specifically, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium perchlorate, lithium dioxarate borate, lithium difluorooxarate borate, lithium trifluoromethanesulfonate, lithium difluorodioxarate phosphate, lithium tetrafluorooxarate phosphate, lithium difluoromethanesulfonylimide, and lithium ditrifluoromethanesulfonylimide.

[0123] The organic solvent includes at least one of carboxylic acid ester solvents, phosphate ester solvents, ether solvents, nitrile solvents, and sulfone solvents. The carbonates may include at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate; the carboxylic acid esters include at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; and the ether solvents include at least one of ethylene glycol dimethyl ether and tetrahydrofuran.

[0124] In some embodiments, the secondary battery further includes a separator located between the negative electrode and the positive electrode, the separator having an air permeability of 100s / 100mL to 400s / 100mL.

[0125] For example, the air permeability of the diaphragm can be 100s / 100mL, 120s / 100mL, 150s / 100mL, 200s / 100mL, 220s / 100mL, 250s / 100mL, 280s / 100mL, 300s / 100mL, 320s / 100mL, 350s / 100mL, 380s / 100mL, 400s / 100mL, or a range of any two sets of values ​​therein.

[0126] The inventors discovered through research that selecting a separator with suitable air permeability can ensure the electrolyte's wetting effect on the separator, improve the lithium-ion transport rate, avoid the local current density increase caused by lithium-ion transport difficulties, and thus avoid the continuous rupture-regeneration cycle of the SEI membrane, reduce the consumption of active lithium, and improve the cycle life of the battery. It can also reduce the side reactions at the interface between the electrolyte and the positive and negative electrodes, reduce heat and gas generation, and improve the safety of the secondary battery.

[0127] In some embodiments, the separator includes a substrate layer comprising at least one of polyethylene, polypropylene, polyamide, and aramid.

[0128] Optionally, the diaphragm may further include an adhesive layer and / or a ceramic layer. The adhesive layer may be made of at least one of polyvinylidene fluoride, polymethyl methacrylate, aramid, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, or polyaniline; the ceramic layer may be made of at least one of boehmite, alumina, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, aluminum nitride, and magnesium nitride.

[0129] In a second aspect, this application provides an electrical device including the aforementioned secondary battery.

[0130] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments.

[0131] In the following examples and comparative examples, the structural formula of compound 1 is as follows: The structural formula of compound 2 is: The structural formula of compound 3 is The structural formula of compound 4 is The structural formula of compound 5 is The structural formula of compound 6 is The structural formula of compound 7 is .

[0132] Unless otherwise specified, all components and raw materials used in the embodiments and comparative examples of this application are commercially available, and the same type of components and raw materials are used in each parallel experiment.

[0133] Example 1

[0134] An embodiment of the secondary battery described in this application includes a method for preparing the secondary battery comprising the following steps:

[0135] S1. Preparation of the positive electrode:

[0136] S11. The precursor iron phosphate, lithium carbonate (Fe and Li molar ratio of 1:1.03) and magnesium sulfate (Mg and Fe molar ratio of 0.01:1) are mixed and added to water to form a slurry with a solid mass percentage of 20%. The slurry is ball-milled for 10 hours. After drying, the slurry is placed in a sintering furnace and nitrogen gas containing 5% hydrogen is introduced. After the air in the furnace is exhausted, it is heated to 850°C at a heating rate of 5°C / min and held for sintering for 12 hours. After cooling, it is crushed by airflow to obtain lithium iron phosphate doped with Mg.

[0137] S12. Add Mg-doped lithium iron phosphate and coating carbon source (the mass ratio of carbon source to Mg-doped lithium iron phosphate is 7.5:100, the carbon source is a mixture of glucose and polyethylene glycol (PEG), the mass ratio of glucose and polyethylene glycol (PEG) is 2:1) to a high-speed mixer and mix at 1500 rpm for 45 min to obtain a mixed powder. Place the mixed powder in a muffle furnace and calcine at 820℃ for 10 h in air atmosphere. After cooling, pulverize with a pulverizer to obtain lithium iron phosphate cathode material.

[0138] S13. Lithium iron phosphate cathode material, lithium oxalate, binder PVDF, conductive agent SP, and dispersant CMC are mixed evenly according to the mass ratio of (lithium iron phosphate cathode material + lithium oxalate):PVDF:SP:CMC=96.2:2:1.8:0.2 and the mass ratio of lithium oxalate:lithium iron phosphate cathode material=2.8:100, and then dispersed in solvent NMP to obtain cathode slurry; the cathode slurry is coated on two opposite surfaces of aluminum foil, dried, then rolled and cut to obtain cathode sheet;

[0139] S2, Preparation of the negative electrode:

[0140] Natural graphite, conductive agent SP, binder SBR, and dispersant CMC were mixed evenly in a mass ratio of natural graphite:SP:SBR:CMC = 96.8:2:1:0.2 and then dispersed in deionized water to obtain a negative electrode slurry with a solid content of 46.3%. The negative electrode slurry was coated on two opposite surfaces of a copper foil and dried to obtain a double-sided coated negative electrode sheet. After that, it was rolled (rolling pressure of 120MPa) and cut to obtain the negative electrode sheet.

[0141] S3. Preparation of electrolyte:

[0142] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, the cyclic sulfate compounds shown in Table 1 were dissolved in the mixed organic solvent. Next, fully dried lithium salt LiPF6 was added to prepare an electrolyte. The mass percentage of lithium salt LiPF6 in the electrolyte was 10%, and the mass percentage of the cyclic sulfate compounds was shown in Table 1.

[0143] S4. Preparation of the diaphragm:

[0144] The diaphragm is commercially available. Its base membrane is a PE membrane with a thickness of 9μm. One side surface of the base membrane is sequentially provided with an alumina coating with a thickness of 3μm and a PVDF coating with a thickness of 2μm. The other side surface of the base membrane is provided with a PVDF coating with a thickness of 2μm.

[0145] S5, Assembly, Formation:

[0146] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0147] Examples 2-19 and Comparative Examples 1-2

[0148] Examples 2-19 and Comparative Examples 1-2 each provide a secondary battery. The only difference from Example 1 is that the preparation process parameters and characteristic parameters of the secondary battery are different, as shown in Tables 1-2.

[0149] The thickness of the coating layer in the above embodiments and comparative examples is controlled by adjusting at least one of the mass ratio of carbon source to Mg-doped lithium iron phosphate and the calcination temperature in step S1; the porosity of the negative electrode material layer is controlled by adjusting the solid content of the negative electrode slurry and / or the rolling pressure in step S2; and changes in the mass ratio of positive electrode lithium supplement to positive electrode material and / or the porosity of the negative electrode material layer will change the mass percentage b% of LiC6 in the negative electrode material layer; changes in the porosity of the negative electrode material layer and / or the thickness of the coating layer will change the mass content cppm of iron in the negative electrode material layer; the selection of cyclic sulfate compounds (which change the electron-donating groups they contain) and / or changes in the mass percentage of cyclic sulfate compounds in the electrolyte will change the mass percentage a% of electron-donating groups in the electrolyte.

[0150] This application uses the secondary batteries obtained in various embodiments and comparative examples as test objects, and conducts relevant performance tests. The test results are shown in Tables 1 and 2, and the test methods are as follows:

[0151] 1. The test method for the mass percentage (a%) of electron-donating groups in the electrolyte is as follows:

[0152] 1) Electrolyte Collection: The secondary battery under test is discharged using a battery charge / discharge device. Discharge conditions: current 0.33C, cutoff voltage 2.5V. After recording the battery number / barcode, the battery is disassembled and the electrolyte collected in a glove box (where the molar content of H2O ≤ 0.1ppm and the molar content of O2 ≤ 0.1ppm). There are three methods for collecting the electrolyte: After removing the battery cover, ① if there is free electrolyte, collect it into a 5mL sample tube using a pipette and seal it with sealing tape to prevent leakage. ② if there is no free electrolyte, a hydraulic press can be used to continuously pressurize until free electrolyte appears, collect it into a sample tube, and seal it. ③ Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, put the battery into an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic oscillator and oscillate for 12 hours to allow the electrolyte in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with sealing glue.

[0153] 2) The collected electrolyte samples were injected into an Agilent Intuvo 9000 gas chromatograph-mass spectrometer using a microsyringe for analysis, and GC-MS chromatograms were obtained. Cyclic sulfate ester compound standards were dissolved in EMC solvent to prepare solutions of different concentrations, and these solutions were injected into the Agilent Intuvo 9000 gas chromatograph-mass spectrometer to obtain GC-MS standard chromatograms. The GC-MS chromatograms of the electrolyte samples were compared with the GC-MS standard chromatograms to confirm whether the electrolyte samples contained the corresponding cyclic sulfate ester compounds, and the relative molar mass M2 of the cyclic sulfate ester compounds was obtained.

[0154] 3) Based on the determined structure of the cyclic sulfate ester compound, calculate the relative molar mass M1 of the corresponding electron-donating group.

[0155] 4) Based on the measured components, the mass percentage (denoted as w) of cyclic sulfate ester compounds was obtained using GC gas chromatography.

[0156] 5) Calculate the mass percentage of electron-donating groups in the electrolyte, a%, according to the following formula: a%=(M1 / M2)×w.

[0157] 2. The mass percentage (b%) of LiC6 in the negative electrode material layer is determined by the following method:

[0158] 1) Discharge the secondary battery at 0.33C to 2.5V, then charge it at a constant current of 0.33C to 3.65V, and then charge it at a constant voltage to the cutoff current of 0.05C;

[0159] Disassemble the fully charged battery and remove the negative electrode sheet; soak the removed negative electrode sheet in dimethyl carbonate (DMC) solution for 4 hours, take it out and dry it to obtain the processed negative electrode sheet;

[0160] 2) Place the treated negative electrode and anhydrous ethanol bag into an aluminum-plastic bag, seal it, evacuate it, and test the volume V1 of the aluminum-plastic bag before the reaction; apply pressure to the ethanol bag, puncture it, and the negative electrode and anhydrous ethanol react to produce gas. After 12 hours, test the volume V2 of the aluminum-plastic bag after the reaction; where the volume of gas generated = V2 - V1.

[0161] The mass of LiC6 is calculated using the following formula:

[0162] The mass of LiC6 is calculated as (V2 - V1) × 0.0899 × 79 / 1000, where 0.0899 is the hydrogen density and 79 is the molar mass of LiC6.

[0163] 3) Take a single negative electrode sheet, weigh its mass and record it as m1. Then scrape off the negative electrode material layer and measure the mass of the copper foil as m2. The mass of the negative electrode material layer in the negative electrode sheet is obtained as m1-m2. Finally, the mass percentage of LiC6 in the negative electrode material layer is calculated using the following formula.

[0164] b% = mass of LiC6 / (m1-m2) × 100%.

[0165] 3. The test method for the mass content (cppm) of iron in the negative electrode material layer is as follows:

[0166] 1) Pretreatment: Discharge the battery at 0.33C to the lower limit voltage of 2.5V, disassemble the empty secondary battery, obtain the negative electrode sheet from the secondary battery, soak the negative electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out, dry it at room temperature with humidity ≤15%, and scrape the negative electrode material powder on the surface of the negative electrode sheet with a ceramic knife.

[0167] 2) Accurately weigh a certain amount of negative electrode material powder, disperse it in 20ml of water, add 10ml of nitric acid, mix well, and then heat it. After the negative electrode material powder dissolves, dilute the material with water to 100mL to obtain the test solution. Perform ICP testing on the test solution. Before the test, a standard solution must be prepared. The linear correlation coefficient of the standard concentration must be above 0.999 to be used as a normal standard.

[0168] The 1000 mg / L standard solution was diluted with deionized water to different concentrations (generally 0, 1 mg / 100 mL, 2 mg / 100 mL, and 3 mg / 100 mL). The detection wavelength of Fe element was selected (259.94 nm). The appropriate ICP instrument operating conditions were set: gas flow rate 0.5 L / min, power 1150 W. The mass content of iron element in the sample can be read by the self-analysis function of the ICP test software.

[0169] 4. The test method for the porosity of the negative electrode material layer is as follows:

[0170] 1) Pretreatment: Discharge the secondary battery at 0.33C to the lower limit voltage of 2.5V, remove the empty battery, disassemble the negative electrode, soak the negative electrode in dimethyl carbonate (DMC) solution for 4 hours, and then air dry.

[0171] 2) Subsequently, the negative electrode sheet is cut into circular sheets with a radius r of 6 mm using an electrode sheet punching machine. Simultaneously, the thickness of the negative electrode sheet and the current collector is measured using a thickness gauge, denoted as h1 and h2 respectively. The mass is recorded as m1 using a balance with an accuracy of 0.00001 g. The negative electrode sheet is then immersed in a sealed container with a certain volume of hexadecane for 1 hour (the volume of hexadecane in the sealed solution is not critical, but the required amount must ensure complete immersion of the electrode sheet). After 1 hour, the negative electrode sheet is removed with tweezers and placed on filter paper to absorb dry until a constant weight is reached (generally, 1 hour is sufficient). The mass is then recorded as m2. The porosity is calculated using the following formula: Porosity of the negative electrode material layer = X / V × 100%, where X = (m2 - m1) / ρ, and ρ is the density of hexadecane, 0.7734 g / cm³. 3 V=πr 2 ×(h1-h2)

[0172] 5. The test method for the thickness of the coating layer is as follows:

[0173] The secondary battery was discharged to 2.5V at 0.33C. The positive electrode of the secondary battery in the empty state was taken and soaked in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, the positive electrode was taken out and dried in a vacuum environment. The powder on the surface of the positive electrode was scraped off with a ceramic knife.

[0174] The scraped powder was evenly dispersed in ethanol, and the sample was then completely dried in a vacuum drying oven. The sample was placed on the test stage and transferred to the chamber. Using TEM, the thickness of the coating layer was measured at a suitable high magnification for different coating layer thicknesses. Five regions were selected for measurement, and the average value was taken.

[0175] 6. The test method for the cycle performance of secondary batteries is as follows:

[0176] At 25°C, the secondary battery was subjected to a cycle test according to the following procedure:

[0177] 1) Charge at a constant current rate of 1C to 3.65V, and then charge at a constant voltage until the current drops to 0.05C;

[0178] 2) Let stand for 20 minutes;

[0179] 3) Discharge to 2.5V at a 1C rate;

[0180] 4) Let stand for 20 minutes;

[0181] Perform cyclic testing according to steps 1)-4) until the capacity of the secondary battery is less than 80% of the initial capacity, and record the number of cycles.

[0182] 7. The test method for the safety performance of secondary batteries is as follows:

[0183] The secondary batteries were charged at a constant current of 0.33C to 3.65V at 25℃, then switched to constant voltage charging until the current was less than 0.05C, and allowed to stand for 30 minutes. Heating wires were then evenly wrapped around the surface of each secondary battery, which were then placed in an adiabatic accelerated calorimeter (ARC) for adiabatic thermal stability test until thermal runaway occurred. The temperature at which the heating rate reached 0.02℃ / min was the self-heating temperature, denoted by T1 in℃; the temperature at which the heating rate reached 1℃ / min was the thermal runaway temperature, denoted by T2 in℃.

[0184] Table 1

[0185]

[0186] Table 2

[0187]

[0188] As can be seen from Tables 1 and 2, in each embodiment of this application, by adjusting the mass percentage of electron-donating groups in the electrolyte (a%), the mass percentage of LiC6 in the negative electrode material layer (b%), and the mass percentage of iron in the negative electrode material layer (cppm), b / (10a) 1 / 2 +c) Within a suitable range, effective compensation of active lithium can reduce side reactions between the electrolyte and the positive and negative electrodes, thus balancing the cycle life and safety of the battery, resulting in a cycle life ≥1503 cycles and a thermal runaway temperature ≥251℃ for the secondary battery. When 1.3≤b / (10a) 1 / 2 When +c)≤8.1, the improvement in cycle life and safety performance of secondary batteries is better.

[0189] Compared with the embodiments, in Comparative Example 1, b / (10a) 1 / 2If the +c) value is too high, the thermal runaway temperature of the secondary battery will drop sharply, which means that the safety performance of the secondary battery will deteriorate significantly.

[0190] Compared with the embodiments, in Comparative Example 2, b / (10a) 1 / 2 If the +c) is too small, the cycle performance of the secondary battery will deteriorate significantly.

[0191] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode material, and the positive electrode material includes lithium iron phosphate; The electrolyte comprises a cyclic sulfate compound, which contains an electron-donating group. The electron-donating group includes at least one of a hydrogen atom, an alkyl group with 1-6 carbon atoms, an alkoxy group with 1-3 carbon atoms, an alkenyl group with 1-3 carbon atoms, a phenyl group, a thiophene group, and a pyrrole group. The mass percentage of the electron-donating group in the electrolyte is a%. The cyclic sulfate compounds include compounds represented by structural formula I; in structural formula I, R1, R2, R3 and R4 each independently include at least one of hydrogen atom, alkyl group of 1 to 6 carbon atoms, alkoxy group of 1 to 3 carbon atoms, alkenyl group of 1 to 3 carbon atoms, phenyl group, thiophene group, and pyrrole group. Equation I; The negative electrode sheet includes a negative electrode material layer, which includes a negative electrode material. When the secondary battery is fully charged, the negative electrode material layer contains iron and LiC6. The mass percentage of LiC6 in the negative electrode material layer is b%, and the mass percentage of iron in the negative electrode material layer is cppm. The secondary battery satisfies: 1.2 ≤ b / (10a) 1 / 2 +c)≤9.5, 0.00008%≤a%≤0.82%, 70%≤b%≤95%, 10ppm≤cppm≤50ppm.

2. The secondary battery as described in claim 1, characterized in that, R2 and R4 are both hydrogen atoms, and R1 and R3 each independently include at least one of hydrogen atom, alkyl group with 1 to 6 carbon atoms, alkoxy group with 1 to 3 carbon atoms, alkenyl group with 1 to 3 carbon atoms, phenyl group, thiophene group, and pyrrole group.

3. The secondary battery as described in claim 2, characterized in that, R1 and R3 are the same substituents, and are selected from any one of hydrogen atom, alkyl group with 1 to 6 carbon atoms, alkoxy group with 1 to 3 carbon atoms, alkenyl group with 1 to 3 carbon atoms, phenyl group, thiophene group, and pyrrole group.

4. The secondary battery as described in claim 3, characterized in that, R1 and R3 are selected from any one of -CH3, -C2H5, -C3H7, -CH=CH2, -CH2-CH=CH2, -CH=CH-CH3, -O-CH3, -O-C2H5, -O-C3H7, -C6H5, -C4H3S, and -C4H4N.

5. The secondary battery as described in claim 1, characterized in that, The electrolyte contains 0.001% to 3% by mass of cyclic sulfate compounds.

6. The secondary battery as described in claim 1, characterized in that, The porosity of the negative electrode material layer is 20%~60%.

7. The secondary battery as described in claim 1, characterized in that, The graphitization degree of the negative electrode material is 90-98%.

8. The secondary battery as described in claim 1, characterized in that, The negative electrode material includes at least one of graphite and silicon-carbon composite materials.

9. The secondary battery as described in claim 8, characterized in that, The negative electrode material is graphite, and the electrolyte satisfies the following conditions: 0.004% ≤ a% ≤ 0.86%.

10. The secondary battery as described in claim 8, characterized in that, The negative electrode material is a silicon-carbon composite material, and the electrolyte satisfies the following conditions: 0.00008%≤a%≤0.35%.

11. The secondary battery as described in claim 1, characterized in that, The compaction density of the positive electrode is 2 g / cm³. 3 ~3g / cm 3 .

12. The secondary battery as described in claim 1, characterized in that, The particle size Dv50 of the cathode material is 0.5μm~15μm.

13. The secondary battery as described in claim 1, characterized in that, The cathode material includes doping elements, which include at least one of Mg, Cr, Na, Al, Mn, Zr, Nb, Co, Ni, Ti, and V. The mass percentage of the doping elements in the cathode material layer is 0.3% to 3%.

14. The secondary battery as described in claim 1, characterized in that, At least some of the lithium iron phosphate particles have a coating layer on their surface, and the thickness of the coating layer is 10 nm to 100 nm.

15. The secondary battery as described in claim 1, characterized in that, At 25°C, the viscosity of the electrolyte is 2 cP to 5 cP.

16. The secondary battery as described in claim 1, characterized in that, The electrolyte further includes additives, which include at least one of the following: fluoroethylene carbonate, vinylene carbonate, methane disulfonate, tris(trimethylsilane)borate, propylene sulfite, vinyl ethylene carbonate, vinyl sulfate, lithium dioxalate borate, lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalate borate), lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, trimethyl phosphate, and triphenyl phosphate. The mass percentage of the additives in the electrolyte is 0.05% to 10%. And / or, the electrolyte further includes lithium salt and solvent, wherein the mass percentage of lithium salt in the electrolyte is 5% to 20%.

17. The secondary battery as described in claim 16, characterized in that, The additives include vinylene carbonate, and the negative electrode sheet satisfies the following conditions: 88% ≤ b% ≤ 97%.

18. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes a separator, which is located between the negative electrode and the positive electrode, and the air permeability of the separator is 100s / 100mL to 400s / 100mL.

19. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 18.

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