Secondary battery and electric device

Abstract

This application discloses a secondary battery and an electrical device, belonging to the field of secondary battery technology. The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. 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 a layered transition metal oxide. The electrolyte includes a cyclic sulfate ester compound. This application achieves this by comprehensively controlling the cell volume aÅ of the positive electrode material when the secondary battery is charged to 90% SOC. 3 The reaction of the positive electrode with ferrocene produces Fe. 3+ The amount (based on the Fe content in the solution after the reaction) 3+ The concentration of the positive electrode (b mmol / L) and the tortuosity c of the positive electrode sheet are used to characterize the positive electrode. By keeping a×c / b within a suitable range, the interfacial activity of the positive electrode material can be reduced, the stability of the positive electrode material can be improved, the internal gas production of the battery can be reduced, and the safety of the battery can be improved. It can also shorten the lithium ion transport path and reduce the DC internal resistance of the battery.

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] In the current secondary battery industry, the innovation and development of cathode materials are key to improving battery performance. Cathode materials directly affect the battery's energy density, safety, cycle life, and low-temperature performance, thus becoming a hot topic in battery technology research. Nickel-cobalt ternary materials (referred to as ternary materials) are widely used in high-end electric vehicles due to their advantages such as voltage stability, high capacity, and high energy density. However, ternary materials have high interfacial oxidation activity, making it easy for lattice oxygen to be released from the battery cathode surface. Meanwhile, Ni... 4+ The strong catalytic properties of the electrolyte make it easy for the electrolyte to undergo oxidative decomposition on the positive electrode surface, generating a large amount of gas and interface deposits, which in turn increases the battery impedance and reduces battery safety.

[0003] In view of this, it is indeed necessary to provide a technical solution to the above problems. Summary of the Invention

[0004] Based on the deficiencies of the existing technology, the purpose of this application is to provide a secondary battery and an electrical device.

[0005] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0006] In a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, 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 a layered transition metal oxide.

[0007] The electrolyte comprises a cyclic sulfate ester compound, which includes a compound represented by structural formula I below; in structural formula I, R1, R2, R3 and R4 each independently include at least one of the following groups represented by structural formula II below, a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, a haloalkoxy group of 1 to 3 carbon atoms, an alkenyl group, an aromatic group, an ester group, a cyano group, and a sulfonic acid group;

[0008] In structural formula II, R5 and R6 each independently include at least one of the following groups as shown in structural formula III: a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, a haloalkoxy group of 1 to 3 carbon atoms, an alkenyl group of 1 to 3 carbon atoms, an aromatic group, an ester group, a cyano group, and a sulfonic acid group.

[0009] In structural formula III, R7 and R8 each independently include at least one of the following: hydrogen atom, alkyl group of 1 to 6 carbon atoms, halogen atom, haloalkyl group of 1 to 3 carbon atoms, alkoxy group of 1 to 3 carbon atoms, haloalkoxy group of 1 to 3 carbon atoms, alkenyl group of 1 to 3 carbon atoms, aromatic group, ester group, cyano group, and sulfonic acid group.

[0010] Formula I, Formula II, Formula III;

[0011] When the secondary battery is charged to 90% SOC, the cell volume of the positive electrode material is aÅ. 3 ;

[0012] After the positive electrode reacts with a solution containing ferrocene, Fe2+ appears in the solution after the reaction. 3+ The concentration of ferrocene is b mmol / L, and the solution containing ferrocene is a mixed solution of ferrocene and the electrolyte, wherein the concentration of ferrocene in the solution containing ferrocene is 0.03 mol / L;

[0013] The tortuosity of the positive electrode is c;

[0014] The secondary battery satisfies: 13.8≤a×c / b≤73.7.

[0015] In a second aspect of this application, an electrical device is provided, including the secondary battery provided in the first aspect of this application.

[0016] Compared with the prior art, the beneficial effects of this application are as follows:

[0017] This application achieves comprehensive control over the cell volume aÅ of the cathode material when the secondary battery is charged to 90% SOC. 3 The reaction of the positive electrode with ferrocene produces Fe. 3+ The amount (based on the Fe content in the solution after the reaction) 3+ The concentration of the positive electrode (b mmol / L) and the tortuosity c of the positive electrode sheet are used to characterize the positive electrode. By keeping a×c / b within a suitable range, the interfacial activity of the positive electrode material can be reduced, the stability of the positive electrode material can be improved, the internal gas production of the battery can be reduced, and the safety of the battery can be improved. It can also shorten the lithium ion transport path and reduce the DC internal resistance of the battery. Detailed Implementation

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] In a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, 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 a layered transition metal oxide.

[0025] The electrolyte comprises a cyclic sulfate ester compound, which includes a compound represented by structural formula I below; in structural formula I, R1, R2, R3 and R4 each independently include at least one of the following groups represented by structural formula II below, a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, a haloalkoxy group of 1 to 3 carbon atoms, an alkenyl group, an aromatic group, an ester group, a cyano group, and a sulfonic acid group;

[0026] In structural formula II, R5 and R6 each independently include at least one of the following groups as shown in structural formula III: a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, a haloalkoxy group of 1 to 3 carbon atoms, an alkenyl group of 1 to 3 carbon atoms, an aromatic group, an ester group, a cyano group, and a sulfonic acid group.

[0027] In structural formula III, R7 and R8 each independently include at least one of the following: hydrogen atom, alkyl group of 1 to 6 carbon atoms, halogen atom, haloalkyl group of 1 to 3 carbon atoms, alkoxy group of 1 to 3 carbon atoms, haloalkoxy group of 1 to 3 carbon atoms, alkenyl group of 1 to 3 carbon atoms, aromatic group, ester group, cyano group, and sulfonic acid group.

[0028] Formula I, Formula II, Formula III;

[0029] When the secondary battery is charged to 90% SOC, the cell volume of the positive electrode material is aÅ. 3 ;

[0030] After the positive electrode reacts with a solution containing ferrocene, Fe2+ is present in the solution after the reaction. 3+ The concentration of ferrocene is b mmol / L, and the solution containing ferrocene is a mixed solution of ferrocene and the electrolyte, wherein the concentration of ferrocene in the solution containing ferrocene is 0.03 mol / L;

[0031] The tortuosity of the positive electrode is c;

[0032] The secondary battery satisfies: 13.8≤a×c / b≤73.7.

[0033] For example, in this application, a×c / b can be 13.8, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 73.7 or a range consisting of any two sets of values.

[0034] The inventors discovered that adding cyclic sulfate compounds to the electrolyte can promote CEI film formation on the positive electrode, reduce gas generation from side reactions between the positive electrode and the electrolyte, and thus improve battery safety. The positive electrode reacts with ferrocene to produce Fe. 3+ The amount (based on the Fe content in the solution after the reaction) 3+ The concentration b (characterized by the concentration of cyclic sulfate compounds) can indirectly reflect the amount of cyclic sulfate compounds added. The smaller the value of b, the better the stability of the positive electrode CEI film, indicating a higher amount of cyclic sulfate compounds added. However, excessive addition of cyclic sulfate compounds will generate viscous organic sulfides, increasing the viscosity of the electrolyte and hindering the Li... + Diffusion in the liquid phase leads to an increase in battery impedance; by controlling the cell volume aÅ of the cathode material when the secondary battery is charged to 90% SOC. 3 The tortuosity 'a' can shorten the diffusion path of lithium ions in the positive electrode material layer, promoting rapid lithium ion extraction; however, if 'a' is too small, the specific surface area of ​​the positive electrode material increases, leading to increased side reactions with the electrolyte, increased gas production, and decreased battery safety. Therefore, it is necessary to control the tortuosity 'c' of the positive electrode sheet to shorten the transport path of lithium ions in the negative electrode material layer, reduce battery impedance, and avoid the negative electrode material layer's tortuosity 'a'. 3 Gas production problems caused by being too small.

[0035] Therefore, this application achieves this by comprehensively controlling the cell volume a of the positive electrode material and the reaction of the positive electrode sheet with ferrocene to generate Fe when the secondary battery is charged to 90% SOC. 3+ The amount (based on the Fe content in the solution after the reaction) 3+ The concentration b) and the tortuosity c of the positive electrode sheet are used to characterize the positive electrode material. By keeping a×c / b within a suitable range, the interfacial activity of the positive electrode material can be reduced, the stability of the positive electrode material can be improved, the internal gas production of the battery can be reduced, the safety of the battery can be improved, and the thermal runaway temperature of the battery can be increased. It can also shorten the lithium ion transport path and reduce the DC internal resistance (discharge DCR) of the battery.

[0036] In some implementations, 28.5 ≤ a × c / b ≤ 38. When a × c / b is within this range, the battery's safety and DC internal resistance are improved.

[0037] In some implementations, 92.3 Å 3 ≤aÅ 3 ≤99.1Å 3 .

[0038] For example, aÅ 3 It could be 92.3 Å 3 92.5Å 3 93Å 3 93.5Å 3 94Å 3 94.2Å 3 94.5Å 3 95Å 3 95.5Å 3 96Å 3 96.5Å 3 97Å 3 97.5Å 3 98Å 3 98.5Å 3 99Å 3 99.1Å 3 Or a range consisting of any two sets of values.

[0039] The inventors discovered through research that this application will reduce the cell volume aÅ of the positive electrode material when the secondary battery is charged to 90% SOC. 3 By adjusting the parameters to a suitable range, the volume and specific surface area of ​​the cathode material during charging can be effectively balanced, resulting in a shorter lithium-ion diffusion path and reduced side reactions between the cathode material and the electrolyte. This, in turn, reduces the battery's DC internal resistance and improves battery safety.

[0040] Further preferred, 94.2Å 3 ≤aÅ 3 ≤96.5Å 3 .

[0041] This application does not impose any particular limitations on the methods for controlling the cell volume a of the cathode material when the secondary battery is charged to 90% SOC. This application controls aÅ 3 The methods include, but are not limited to, the following: adjusting the mass percentage of doped elements in the cathode material layer and the thickness of the coating layer on the surface of lithium nickel cobalt manganese oxide particles to achieve aÅ 3 Things have changed.

[0042] In some implementations, 2.1 mmol / L ≤ b mmol / L ≤ 7.2 mmol / L.

[0043] For example, bmmol / L can be 2.1mmol / L, 2.5mmol / L, 3mmol / L, 3.2mmol / L, 3.5mmol / L, 4mmol / L, 4.5mmol / L, 4.6mmol / L, 5mmol / L, 5.5mmol / L, 6mmol / L, 6.2mmol / L, 6.5mmol / L, 6.8mmol / L, 7mmol / L, 7.2mmol / L, or a range of any two of these values.

[0044] Layered transition metal oxides, as cathode materials, possess high oxidation potentials. Cyclic sulfate compounds can participate in the oxidation reaction at the cathode, promoting the formation of a stable CEI film. The CEI film, acting as a physical barrier, effectively blocks the contact between ferrocene and the high-potential active sites of the cathode, effectively preventing Fe... 2+ The oxidation of ferrocene is prevented by the CEI film. Furthermore, the CEI film possesses electronic insulation and ionic conductivity properties. Ferrocene oxidation is a single-electron transfer reaction, requiring electrons to transfer from the orbitals of ferrocene molecules to the conduction band of the positive electrode. The CEI film does not participate in electron exchange and therefore does not act as an intermediary for electron transfer, eliminating the possibility of ferrocene oxidation through the film. The CEI film itself has a certain dielectric constant, creating a built-in potential difference at the "positive electrode-CEI film-electrolyte" interface, thereby altering the local potential environment. Although the bulk potential of the positive electrode is relatively high, the CEI film significantly reduces the local potential near the electrolyte, significantly reducing the oxidation reaction of ferrocene. The more cyclic sulfate compounds added, the thicker the CEI film formed on the positive electrode surface, and the better the effect of isolating the oxidation reaction of ferrocene, reflected in b(Fe... 3+ The smaller the value.

[0045] The inventors discovered through research that this application reacts the positive electrode with ferrocene to generate Fe. 3+ By adjusting the amount of electrolyte to an appropriate range, the stability of the positive electrode CEI film can be improved, the interfacial impedance can be reduced, the transport of lithium ions can be promoted, the DC internal resistance of the battery can be reduced, and the side reactions between the electrolyte and the positive electrode interface can be reduced, gas production can be reduced, and the safety of the battery can be improved.

[0046] Further preferred values ​​are 3.2 mmol / L ≤ b mmol / L ≤ 4.6 mmol / L.

[0047] Understandably, this application relates to the reaction of the positive electrode with ferrocene to produce Fe. 3+ The amount (based on the Fe content in the solution after the reaction) 3+ There are no particular limitations on the methods for controlling the concentration (characterized by bmmol / L) of Fe. This application controls the reaction between the cathode and ferrocene to generate Fe. 3+The methods for controlling the amount include, but are not limited to, the following: adjusting the mass percentage of cyclic sulfate compounds in the electrolyte and the thickness of the coating layer on the surface of lithium nickel cobalt manganese oxide particles to control bmmol / L.

[0048] In some implementations, 1.0 ≤ c ≤ 1.8.

[0049] For example, c can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 or a range consisting of any two sets of values.

[0050] The inventors discovered through research that adjusting the tortuosity c of the positive electrode to a suitable range can not only reduce the local enrichment area of ​​electrolyte on the positive electrode and suppress the side reactions caused by electrolyte decomposition, but also shorten the diffusion path of lithium ions inside the positive electrode, thereby improving battery safety and reducing DCR.

[0051] For further optimization, 1.2 ≤ c ≤ 1.5.

[0052] This application does not impose any particular restrictions on the means of controlling the tortuosity c of the positive electrode sheet. The means of controlling the tortuosity c of the positive electrode sheet in this application include, but are not limited to, the following: by controlling the solid content (72%-77%) of the positive electrode slurry and the linear pressure (180~280KN / m) of the rolling during the preparation of the positive electrode sheet, the tortuosity c of the positive electrode sheet is changed.

[0053] 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 n = 2 to 4.

[0054] For example, n can be 2, 3, 4, or a range of any two sets of values.

[0055] The inventors discovered through research that the more sulfate groups there are in the compound with the structure shown in Formula I, the higher the viscosity of the electrolyte will be, which will reduce the ion diffusion coefficient of the electrolyte and increase the DC internal resistance of the battery.

[0056] In some embodiments, R2 and R4 are both hydrogen atoms, and R1 and R3 each independently include at least one of the following: a group represented by structural formula II, a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, an alkenyl group, and an aromatic group.

[0057] In some embodiments, R1 and R3 are the same substituents and are selected from any one of the groups shown in structural formula II, hydrogen atoms, alkyl groups of 1 to 6 carbon atoms, halogen atoms, haloalkyl groups of 1 to 3 carbon atoms, alkoxy groups of 1 to 3 carbon atoms, alkenyl groups, and aromatic groups.

[0058] In some embodiments, R1 and R3 are selected from any one of the following groups: -CH3, -C2H5, -C3H7, -CH=CH2, -CH2-CH=CH2, -CH=CH-CH3, -O-CH3, -O-C2H5, -O-C3H7, -F, -Cl, -Br, -I, -CF3, -CCl3, -CBr3, -C2F5, -C2Cl5, -C2Br5, -C3F7, -C3Cl7, -C3Br7, -C6H5, -C4H3S, -C4H4N, and the group shown in structural formula III.

[0059] The aforementioned R1 and R3 have no carbon chain or a short carbon chain, and a small molecular weight, which has little impact on the viscosity of the electrolyte. They can promote the transport of lithium ions, reduce the DC internal resistance of the battery, promote the formation of a stable CEI film on the positive electrode, reduce the side reaction gas production between the positive electrode and the electrolyte, and improve the safety of the battery.

[0060] In some embodiments, the cyclic sulfate compound includes at least one of the compounds shown in the following structural formulas:

[0061] , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , .

[0062] In some embodiments, the compaction density of the positive electrode is 3.2 g / cm³. 3 ~3.8g / cm 3 .

[0063] For example, the compaction density of the positive electrode sheet is 3.2 g / cm³. 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 3.6g / cm 3 3.7g / cm 3 3.8g / cm 3 Or a range consisting of any two sets of values.

[0064] The inventors discovered through research that adjusting the compaction density of the positive electrode sheet to a suitable range can ensure the wetting effect of the electrolyte on the positive electrode sheet, promote lithium ion transport, reduce side reactions between the positive electrode sheet and the electrolyte, reduce gas production, thereby reducing the DC internal resistance of the battery and improving battery safety.

[0065] In some embodiments, the layered transition metal oxide is selected from lithium nickel cobalt manganese oxide, and the ratio between the molar content of Ni element in the cathode material and the total molar content of transition metal elements in the cathode material is 0.5 to 0.9.

[0066] For example, the ratio between the molar content of Ni element in the cathode material and the total molar content of transition metal elements in the cathode material can be 0.5, 0.6, 0.7, 0.8, 0.9 or any two of these values.

[0067] The inventors discovered through research that by adjusting the ratio between the molar content of Ni element in the cathode material and the total molar content of transition metal elements in the cathode material to a suitable range, the thermal stability of the cathode material can be improved, the side reactions between the cathode material and the electrolyte can be reduced, gas production can be reduced, and battery safety can be improved.

[0068] In some embodiments, the cathode material layer includes a doping element, which includes at least one of Al, Ti, Mo, Mg, W, Nb, Zr, Ca, S, and N, and the mass percentage of the doping element in the cathode material layer is 0.03% to 3%.

[0069] For example, the mass percentage of doped elements in the cathode material layer can be 0.03%, 0.1%, 0.5%, 1.0%, 1.5%, 2%, 2.5%, 3%, or any two of these values.

[0070] The inventors discovered through research that by introducing appropriate doping elements into lithium nickel cobalt manganese oxide, this application can improve the conductivity of the cathode material, make the lithium-ion channel smoother, reduce the DCR of the battery, stabilize the crystal structure of the cathode material, reduce the volume change of the cathode material particles during fast charging, reduce the side reactions between the cathode material and the electrolyte, thereby improving battery safety.

[0071] In some embodiments, the surface of the lithium nickel cobalt manganese oxide particles has a coating layer with a thickness of 2 nm to 120 nm.

[0072] For example, the thickness of the coating layer can be 2nm, 5nm, 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 120nm, or any two of these values.

[0073] The inventors discovered through research that by setting a coating layer on the surface of lithium nickel cobalt manganese oxide particles and adjusting the thickness of the coating layer to a suitable range, the side reactions between layered transition metal oxides and electrolytes can be reduced while ensuring the lithium-ion transport path, thus reducing gas production and improving the battery's safety performance while ensuring the battery's DCR.

[0074] In some embodiments, the electrolyte contains 0.001% to 3% by mass of cyclic sulfate compounds.

[0075] For example, the mass percentage of cyclic sulfate compounds in the electrolyte is 0.001%, 0.01%, 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.

[0076] Through research, the inventors discovered that by controlling the mass percentage of cyclic sulfate compounds in the electrolyte to a suitable range, this application can promote the formation of the positive electrode CEI film, reduce side reactions between the positive electrode and the electrolyte, and improve the safety performance of the battery. It can also avoid a sudden increase in the viscosity of the electrolyte due to excessive addition of cyclic sulfate compounds, thereby ensuring the lithium ion diffusion rate and reducing the battery's DCR.

[0077] In some embodiments, the preparation method of the layered transition metal oxide includes, but is not limited to, co-precipitation.

[0078] Specifically, the preparation method of the layered transition metal oxide can be a co-precipitation method, which includes, but is not limited to, the following steps:

[0079] Nickel, cobalt, manganese and dopant sources are dissolved in deionized water and mixed evenly to prepare a metal source solution. The metal source solution, precipitant and complexing agent are mixed and subjected to co-precipitation reaction. After drying, a precursor is obtained. The lithium source and the obtained precursor are mixed and sintered in an oxygen-containing atmosphere to obtain a layered transition metal oxide.

[0080] In the coprecipitation method, the nickel source may include at least one of nickel chloride, nickel sulfate, nickel nitrate, nickel acetate, nickel carbonate, and nickel hydroxide; the cobalt source may include at least one of cobalt chloride, cobalt sulfate, cobalt nitrate, cobalt acetate, cobalt carbonate, and cobalt hydroxide; the manganese source may include at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, manganese carbonate, and manganese hydroxide; the dopant source may include at least one of chloride, sulfate, nitrate, acetate, carbonate, and hydroxide species of the dopant element; the lithium source may include at least one of lithium carbonate, lithium hydroxide, lithium oxide, and lithium acetate; the precipitant may include at least one of sodium hydroxide and potassium hydroxide; and the complexing agent may be ammonia.

[0081] In the coprecipitation method, the conditions for the coprecipitation reaction are: reaction temperature 60~80℃, reaction time 5~8h; the conditions for the sintering treatment are: sintering temperature 700~900℃, sintering time 8~12h.

[0082] In this application, the coating layer is formed by methods including but not limited to dry coating.

[0083] The coating layer is formed by dry coating, and the preparation method of the cathode material includes the following steps:

[0084] A layered transition metal oxide and a coating agent are mixed and calcined in air, then cooled, pulverized, and sieved to obtain a positive electrode material. The mixing method includes stirring or ball milling. For example, the mixing method is stirring, with a stirring speed of 1000~2000 rpm and a time of 30~60 min; or the mixing method is ball milling, with a ball-to-material ratio of (5~10):1 and a time of 1~2 h. The coating agent includes at least one of alumina, titanium oxide, and silicon oxide. The mass ratio of the coating agent to the layered transition metal oxide is (0.05~5):100. The calcination temperature is 300~600℃ and the time is 2~4 h.

[0085] In the above-mentioned method for preparing cathode materials, the thickness of the coating layer can be controlled by adjusting process parameters such as the mass ratio of the coating agent to the layered transition metal oxide, the calcination temperature, and the calcination time.

[0086] In this application, the thickness of the coating layer can be determined by the following method:

[0087] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V. The empty battery was then removed, and the positive electrode was disassembled. The positive electrode was soaked in dimethyl carbonate (DMC) solution for 4 hours, dried, and the powder on the dried electrode surface was scraped off with a scraper. The powder was then initially crushed in a mortar and pestle and passed through a 200-mesh sieve to obtain a fine powder sample. A small amount of powder (approximately 1 mg) was taken and added to anhydrous ethanol or acetone, and ultrasonically dispersed for 10-15 minutes to ensure monodispersity of the particles. The upper dispersion was then pipetted onto a copper mesh (2) with a carbon support film. The sample is dried naturally at room temperature on a 00-mesh or 300-mesh plate; then thinned using an ion thinner; the copper mesh is fixed on the sample stage and bombarded from both sides of the sample with a high-energy argon ion beam (3~5keV) at an angle, gradually thinning until electrons can penetrate; the sample is then transferred to an HRTEM to begin testing, with an accelerating voltage of 200~300kV. First, the copper mesh is quickly scanned in bright-field TEM mode to find monodisperse cathode material particles; then, the sample is switched to HRTEM mode, focusing on the interface between the coating layer and the substrate, ensuring that the lattice fringes are clearly visible in the image; the image is opened with ImageJ or Gatan Digital Micrograph software, and the thickness of the coating layer is measured on the HRTEM image along the direction perpendicular to the coating layer interface using the software's "straight line tool". Five different regions are measured, and the average value is taken.

[0088] In some embodiments, the positive electrode material layer further includes a positive electrode conductive agent, a positive electrode dispersant, and a positive electrode binder.

[0089] Specifically, in the positive electrode material layer, the mass ratio of the positive electrode material, the positive electrode conductive agent, the positive electrode binder, and the positive electrode dispersant is (94.5~98.5): (0.5~1.5): (1.3~2.2): (0.05~2).

[0090] 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.

[0091] The positive electrode dispersant includes at least one of the following: hydroxymethyl cellulose, acrylic acid, acrylate, polyether ester, phosphate ester, small molecule alkanolamine, polyurethane, modified styrene / maleic anhydride, nitrile rubber (HNBR), and polyvinylpyrrolidone (PVP).

[0092] 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.

[0093] 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.

[0094] 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 conductive agent, positive electrode binder, positive electrode dispersant and solvent to obtain a positive electrode slurry; coating the positive electrode slurry onto two opposite surfaces of a positive electrode current collector aluminum foil, and then drying, rolling and slitting to obtain a positive electrode sheet.

[0095] In the preparation process of the positive electrode sheet, the compaction density of the positive electrode sheet can be controlled by controlling the solid content (71%~77%) of the positive electrode slurry and the linear pressure of the roller (170KN / m~280KN / m).

[0096] In some embodiments, the electrolyte includes a solvent, which includes at least one of carbonate solvents and carboxylic acid ester solvents.

[0097] The carbonate solvents include at least one of ethylene carbonate, fluoroethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate.

[0098] The carboxylic acid ester solvent includes at least one of ethyl acetate, methyl isobutyrate, ethyl trimethylacetate, 2,2-difluoroethyl acetate, and 2,2,2-trifluoroethyl acetate.

[0099] In some embodiments, the electrolyte further includes additives, which include at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonyl lactone (1,3-PS), propylene sulfite (PS), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiDFOP), lithium difluorooxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), methane disulfonate (MMDS), tri(trimethylsilane) phosphate (TMSP), tri(trimethylsilane)borate (TMSB), and tri(trimethylsilane) phosphite (TMSPI), wherein the mass percentage of the additives in the electrolyte is 0.05% to 10%.

[0100] For example, the mass percentage of the additive in the electrolyte can be 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values.

[0101] In some embodiments, the electrolyte further includes a lithium salt, wherein the mass percentage of the lithium salt in the electrolyte is 11% to 15%.

[0102] For example, the mass percentage of lithium salt in the electrolyte can be 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, or any two of these values.

[0103] 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.

[0104] In some embodiments, the negative electrode sheet includes a negative electrode material layer with a porosity of 20% to 60%.

[0105] 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.

[0106] The inventors discovered through research that by adjusting the porosity of the negative electrode material layer to a suitable range, the wetting effect of the electrolyte on the negative electrode sheet can be guaranteed, promoting lithium-ion transport and reducing the battery's DCR; it can also avoid the increase of side reactions due to excessive wetting of the negative electrode sheet by the electrolyte, reduce gas production, and thus improve the safety of the battery.

[0107] In this application, the porosity of the negative electrode material layer can be determined by the following method:

[0108] 1) Pretreatment: Discharge the battery at 0.33C to the lower limit voltage of 2.5V, take the empty battery, disassemble the negative electrode, soak the negative electrode in dimethyl carbonate (DMC) solution for 4 hours, and then air dry;

[0109] 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)

[0110] In some embodiments, the negative electrode material layer contains manganese, and the mass content of manganese in the negative electrode material layer is 10ppm to 100ppm.

[0111] For example, the mass content of manganese in the negative electrode material layer can be 10ppm, 20ppm, 30ppm, 40ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm or any two of these values.

[0112] During charging and discharging, especially under high voltage or high temperature, oxygen will be lost from the surface of the positive electrode, forming a manganese-rich, easily soluble oxide layer, from which Mn will dissolve. 2+ It will migrate to the negative electrode. This application improves the structural stability of the positive electrode material, reduces side reactions between the positive electrode material and the electrolyte, and enhances battery safety by controlling the mass content of manganese in the negative electrode material layer to a suitable range, while also ensuring smooth lithium-ion transport channels and reducing the battery's DC internal resistance.

[0113] In this application, the mass content of manganese in the negative electrode material layer can be determined by the following method:

[0114] ①Pretreatment: The battery is discharged to 2.5V at 0.33C. The empty lithium-ion battery is disassembled to obtain the electrode sheet. The negative electrode sheet is soaked in DMC (dimethyl carbonate) at room temperature for 60 minutes, taken out, and dried at room temperature with humidity ≤15%. The powder on the surface of the dried electrode sheet is scraped off with a scraper, pre-crushed with a mortar and pestle, and then passed through a 200-mesh sieve to obtain a fine powder sample.

[0115] ② Accurately weigh a certain amount of powder, disperse it in 20ml of water, add 10ml of nitric acid, mix well, and heat until the powder dissolves. Then, dilute the material with water to 100mL to obtain the test solution. Perform ICP testing 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. Dilute the 1000mg / L standard solution with deionized water to different concentrations (generally 0, 1mg / 100mL, 2mg / 100mL, 3mg / 100mL), and select the element detection wavelength. Set the experimental conditions: According to the characteristics of the sample and the element to be detected, set the appropriate ICP instrument working conditions: gas flow rate 0.5L / min, power 1150W, and select the element detection wavelength, which depends on the element being tested (Mn detection wavelength is 257.61nm). The mass content of Mn in the sample can be read by the self-analysis function of the ICP testing software.

[0116] In some embodiments, the negative electrode material layer includes a negative electrode material, which includes at least one of graphite and silicon-based materials, wherein the silicon-based material includes silicon-carbon composite material, and the particle size Dv50 of the negative electrode material is 8 μm to 20 μm.

[0117] For example, the particle size Dv50 of the negative electrode material can be 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm or any two sets of values ​​therein.

[0118] This application adjusts the particle size Dv50 of the negative electrode material to a suitable range, thereby making the lithium-ion transport path moderate and reducing the side reactions between the negative electrode material and the electrolyte, which helps to reduce the battery's DCR and improve battery safety.

[0119] The particle size Dv50 of the negative electrode material can be determined by the following method:

[0120] The negative electrode sheet was disassembled from the secondary battery, then soaked in dimethyl carbonate (DMC), and dried in a fume hood for 12 hours. The powder on the dried electrode sheet was scraped off with a scraper, preliminarily crushed in a mortar, and then passed through a 200-mesh sieve to obtain a fine powder sample. 30 mg of the powder was added to 10 mL of 0.1% NP-40 (sample dispersant) aqueous solution, and the mixed sample was ultrasonicated for 1 min. The particle size of the negative electrode material was calculated by measuring the intensity of the scattered light when the laser passes through the dispersed particles using a Mastersizer 3000 laser particle size analyzer. The volumetric cumulative particle size distribution (Dv) was measured using laser diffraction. 50 This indicates the particle size corresponding to a cumulative particle size distribution percentage of 50%.

[0121] In some embodiments, the negative electrode material comprises a silicon-based material, wherein the mass percentage of silicon in the negative electrode material layer is 1% to 10%.

[0122] For example, the mass percentage of silicon in the negative electrode material layer can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values.

[0123] This application reduces the shedding of the negative electrode material caused by the volume expansion efficiency of silicon during charging and discharging by adjusting the mass percentage of silicon in the negative electrode material layer to a suitable range, improves the stability of the negative electrode SEI film, reduces side reactions between the electrolyte and the negative electrode material, lowers the DC internal resistance of the battery, and improves the safety of the battery.

[0124] 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.

[0125] Optionally, the negative electrode material layer may further include a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersant.

[0126] The mass ratio of the negative electrode material, negative electrode conductive agent, negative electrode binder and negative electrode dispersant is (94.8~97.2):(0.5~1.5):(1~2.5):(0.5~1).

[0127] 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.

[0128] 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.

[0129] The negative electrode dispersant includes at least one of the following: hydroxymethyl cellulose, acrylic acid, acrylate, polyether ester, phosphate ester, small molecule alkanolamine, polyurethane, modified styrene / maleic anhydride, nitrile rubber (HNBR), and polyvinylpyrrolidone (PVP).

[0130] Furthermore, the negative electrode sheet includes a negative electrode current collector, and at least one side surface of the negative electrode current collector is provided with the negative electrode material layer.

[0131] 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.

[0132] 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 negative electrode material, negative electrode conductive agent, negative electrode binder and solvent to obtain negative electrode slurry; coating the negative electrode slurry on at least one side surface of the negative electrode current collector, drying and then rolling and cutting to obtain the negative electrode sheet.

[0133] In some embodiments, the diaphragm includes a substrate layer comprising at least one of polyethylene, polypropylene, polyamide, and aramid.

[0134] 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.

[0135] In a second aspect, this application provides an electrical device including the aforementioned secondary battery.

[0136] In this application, the electrical device may include energy storage devices, electric ships, aircraft, laptops, power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace equipment, etc.

[0137] 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.

[0138] 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.

[0139] 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: .

[0140] Example 1

[0141] The embodiment of the lithium-ion secondary battery described in this application includes a method for preparing the lithium-ion secondary battery comprising the following steps:

[0142] S1. Preparation of cathode materials:

[0143] S11, according to LiNi 0.68 Co 0.095 Mn 0.225 The molar ratio of nickel, cobalt, and manganese in O2 was determined by dissolving nickel salt (NiSO4·6H2O), cobalt salt (CoSO4·7H2O), and manganese salt (MnSO4·H2O) in deionized water and mixing thoroughly to prepare a metal salt solution. The metal salt solution was then mixed with a precipitant (NaOH) and a complexing agent (NH3·H2O). The pH of the reaction system was adjusted to 11, and the reaction was carried out at 60°C for 6 hours. After filtration and drying, the precursor (NiSO4·6H2O) was obtained. 0.68 Co 0.095 Mn 0.225 (OH)3 powder;

[0144] S12, Weigh the precursor, zirconium oxide, and Li2CO3, and make the precursor (Ni 0.68 Co 0.095 Mn 0.225 The theoretical molar ratio of (OH)3 to lithium carbonate (Li2CO3) is 1:0.5, and the mass percentage of zirconium in the cathode material layer is 2.34%. The precursor, zirconium oxide, and Li2CO3 are added to a planetary ball mill, anhydrous ethanol is used as the dispersant (liquid-solid ratio 1:1), agate balls are used as the medium, and the mixture is ball-milled at 300 r / min for 4 h. The slurry obtained from the ball mill is dried and then sieved to obtain a mixed powder.

[0145] The mixed powder was heated to 450℃ at a rate of 3℃ / min and held for 2 hours to complete the dehydration and decomposition of the precursor and the initial reaction of the lithium salt. Then, the temperature was increased to 800℃ at a rate of 3℃ / min and held for 8 hours. After cooling to room temperature, it was crushed and sieved to obtain lithium nickel cobalt manganese oxide containing the doped element Zr.

[0146] S13. Add lithium nickel cobalt manganese oxide containing doped element Zr and alumina coating agent to a ball mill for ball milling. The mass of the coating agent is w% of the mass of lithium nickel cobalt manganese oxide containing doped element Zr, and w% is 0.25%. The ball milling conditions are: ball-to-material ratio 8:1, ball milling time 2h. Place the mixed powder obtained by ball milling into a muffle furnace and calcine at 400℃ in air atmosphere for 2h. After cooling, crush and sieve to obtain the cathode material.

[0147] S2, Preparation of the positive electrode:

[0148] The positive electrode material, conductive carbon black SP, binder polyvinylidene fluoride (PVDF), and polyvinylpyrrolidone (PVP) were mixed in a mass ratio of 96:1:2:1. N-methylpyrrolidone (NMP) was added, and the mixture was stirred under vacuum until the system was homogeneous, yielding a positive electrode slurry with a solid content of 74.5%. The positive electrode slurry was coated onto the two opposite surfaces of a positive electrode current collector aluminum foil, and then dried, rolled, and slit to obtain a positive electrode sheet. The linear pressure of the rolling was 260 kN / m, and the compaction density of the positive electrode sheet was 3.55 g / cm³. 3 .

[0149] S3. Preparation of the negative electrode:

[0150] Natural graphite, conductive carbon black SP, carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) were mixed in a mass ratio of 96:1:0.2:2.8. Deionized water was added, and the mixture was ground under vacuum at a speed of 400 r / min for 5 min, followed by grinding at a speed of 1500 r / min for 20 min. The grinding media consisted of zirconium beads with a particle size of 1 mm. The mixture was then ultrasonically treated for 15 min to obtain a negative electrode slurry. This negative electrode slurry was coated onto two opposing surfaces of a copper foil along its thickness direction. The foil was first dried at 100 °C for 2 min, and then heated to 120 °C at a rate of 3 °C / min. After rolling and cutting, the negative electrode sheet was obtained.

[0151] S4. Preparation of electrolyte:

[0152] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 7:3 to obtain an organic solvent. Then, a cyclic sulfate compound and 1,3-propanesulfonate lactone (1,3-PS) (the mass ratio of the cyclic sulfate compound and 1,3-propanesulfonate lactone was 3:1) 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 12%, and the cyclic sulfate compound was compound 1. The mass percentages of the cyclic sulfate compound and 1,3-propanesulfonate lactone in the electrolyte are shown in Table 2.

[0153] S5. Preparation of the diaphragm:

[0154] A commercially available separator is selected, with PE as the base membrane and a thickness of 9μm. A 2μm thick PVDF adhesive layer is set on one side of the base membrane, and another layer is successively set with a 3μm thick alumina coating and a 2μm thick PVDF adhesive layer. That is, the separator specification is 2+9+3+2 (adhesive layer + base membrane + coating + adhesive layer, μm).

[0155] S6. Stack the obtained positive electrode, separator, and negative electrode in sequence, so that the separator is between the positive electrode and the negative electrode to play a role in isolation, and then wind them to obtain a bare cell; place the bare cell in an outer packaging shell, dry it, inject electrolyte, and then go through vacuum sealing, standing, formation, shaping and other processes to obtain a lithium-ion secondary battery.

[0156] Examples 2-17

[0157] The difference between Examples 2-17 and Example 1 in the preparation method of lithium-ion secondary batteries is as follows:

[0158] Examples 7-15 changed the type of dopant element M, see Table 2 for details; in step S12 of Examples 2-17, the amount of oxide of dopant element M was changed, so that the mass percentage of dopant element M in the cathode material layer was changed, see Table 2 for details.

[0159] In Examples 2-3 and Examples 5-17, at least one of the following was changed in step S13: the type of coating agent, w%, calcination temperature, and calcination time. See Table 2 for details.

[0160] Examples 2-17 changed at least one of the solid content of the positive electrode slurry and the linear pressure of the roller in step S2, as detailed in Table 2;

[0161] In Examples 2-9, the selection of cyclic sulfate compounds was changed in step S4, as detailed in Table 2; in Examples 2-17, the mass percentage of cyclic sulfate compounds and 1,3-propanesulfonate lactone in the electrolyte was changed by altering the amount of cyclic sulfate compounds and 1,3-propanesulfonate lactone added in step S4, as detailed in Table 2.

[0162] Comparative Examples 1-4

[0163] The differences between Comparative Examples 1-4 and Example 1 in the preparation method of lithium-ion secondary batteries are as follows:

[0164] In Comparative Examples 1-3, the amount of oxide of dopant element M was changed in step S12, so that the mass percentage of dopant element M in the cathode material layer was changed; in Comparative Example 4, no oxide of dopant element M was added in step S12, see Table 2 for details.

[0165] Comparative Examples 1-4 also changed at least one of w% in step S13, namely the calcination temperature and the calcination time, as detailed in Table 2;

[0166] Comparative Examples 1-4 changed at least one of the solid content of the positive electrode slurry and the linear pressure of the roller pressing in step S2, as detailed in Table 2;

[0167] In Comparative Examples 1-2 and 4, the mass percentage of cyclic sulfate compounds and 1,3-propanesulfonate lactone in the electrolyte was changed by altering the amount of cyclic sulfate compounds and 1,3-propanesulfonate lactone added in step S4, as detailed in Table 2; in Comparative Example 3, no cyclic sulfate compounds and 1,3-propanesulfonate lactone were added in step S4.

[0168] The above Examples 1-17 and Comparative Examples 1-4, by adjusting the type of dopant element M and / or the amount of oxide of dopant element M added in step S12, change the mass percentage content of dopant element M in the cathode material layer; by adjusting at least one of w%, calcination temperature, and calcination time in step S13, change the thickness of the coating layer; and by changing at least one of the mass percentage content of dopant element M in the cathode material layer and the thickness of the coating layer, aÅ 3 The following changes occur: by adjusting at least one of the solid content of the positive electrode slurry and the linear pressure of the roller in step S2, the compaction density and tortuosity of the positive electrode sheet are changed; by adjusting the type and amount of cyclic sulfate compounds in step S4, the mass percentage of cyclic sulfate compounds in the electrolyte is changed; and changes in at least one of the mass percentage of cyclic sulfate compounds in the electrolyte and the coating thickness on the surface of lithium nickel cobalt manganese oxide particles will cause a change in bmmol / L.

[0169] This application uses the secondary batteries obtained in the above embodiments and comparative examples as test objects to conduct performance tests. The test results are shown in Tables 2 and 3, and the test methods are as follows:

[0170] 1. The unit cell volume of the cathode material is a Å. 3 The testing method is as follows:

[0171] The secondary battery was brought to its final capacity at 0.33C for one cycle, and the final capacity was taken as the actual capacity of the battery. It was then charged at 0.33C to 90% SOC, disassembled, the positive electrode was removed, the surface of the electrode was cleaned with dimethyl carbonate solution (DMC), and then dried.

[0172] The dried positive electrode sheet was subjected to XRD testing according to the following steps:

[0173] (1) Sample processing and loading: Fix the positive electrode sample on the glass sample holder, then insert the sample holder into the sample slot and close the sample chamber door;

[0174] (2) Software operation settings: Open the measurement and control system software and enter the main control interface. The diffractometer uses Cu-Kα line as the diffraction source. Set the working voltage to 40kV, the working current to 40mA, the scanning speed of the diffractometer to 10° / min, the scanning range to 10°~90°, set the start and end angles, and the scanning speed to 0.05℃ / min.

[0175] (3) After the instrument indicates that the test is complete and you see that the X-ray in the sample chamber has been turned off, you can open the sample chamber, take out the sample holder, and retrieve the sample;

[0176] (4) The image was processed by smoothing, filtering and background removal. The basic structure of the synthesized material was constructed using VESTA. Then, the XRD spectrum was refined using GENERALSTRUCTUREANALYSISSYSTEM software (GSAS). The atomic parameters, peak parameters and instrument parameters in the structure were adjusted by the least squares method to minimize the full spectrum weighted residual variance factor Rwp and obtain accurate structural parameters (a, b, c, α, β, γ).

[0177] (5) Calculate the unit cell volume based on the crystal form. The calculation formula is as follows: Layered NCM (hexagonal crystal system) V=3 1 / 2 ×a 2 ×c / 2; the result is aÅ. 3 See Table 3.

[0178] 2. The amount of ferric iron formed in the reaction between the positive electrode and ferrocene is determined by the amount of Fe in the solution after the reaction between the positive electrode and the ferrocene solution. 3+ The concentration b is standardized, and the test method is as follows:

[0179] Fe 3+ It reacts with potassium thiocyanate (KSCN) under acidic conditions to form a blood-red complex Fe(SCN)n. 3- (n=1~6), the complex exhibits maximum absorbance at a wavelength of 480 nm; within a certain concentration range, Fe 3+ Concentration and absorbance conform to the Lambert-Beer law. The absorbance can be measured using a spectrophotometer, and the Fe concentration in the sample can be calculated using a standard curve. 3+ The content;

[0180] (1) Prepare reagents: Prepare ferrocene-electrolyte mixed solution (the concentration of ferrocene in the ferrocene-electrolyte mixed solution is 0.03mol / L) and 20-50g blank solution (the mass ratio of each component in the electrolyte / blank solution is: EC:EMC:LiPF6:PS:BiDTD=25.22%:58.84%:13.75%:0.5%:1.2%). Prepare (NH4)Fe(SO4)2 standard working solution (100μg / mL), potassium thiocyanate (KSCN) solution (10mg / mL), and nitric acid (HNO3) solution (1mol / L).

[0181] (2) Standard curve preparation: Take 6 50mL volumetric flasks, numbered 0~5, and add the reagents listed in Table 1 below to each flask;

[0182] Table 1

[0183]

[0184] Then, 5.00 mL of 10% KSCN solution was added to each volumetric flask, and the volume was brought to the mark with deionized water. The flasks were shaken well and allowed to stand for 10 minutes. Using the blank solution (number 0) as a reference, the absorbance of each standard solution was measured at a wavelength of 480 nm. Fe... 3+ A standard curve was plotted with concentration (μg / mL) on the x-axis and absorbance on the y-axis. Linear regression was then performed to obtain the regression equation (A=kC+D) (where A is absorbance, C is concentration, k is slope, and D is intercept).

[0185] (3) Reaction of ferrocene with the positive electrode: Charge the secondary battery to the cutoff voltage of 4.4V at a rate of 0.33C, transfer the battery to the glove box for disassembly, and wait for the electrode to dry; cut the dried positive electrode into 4 small round pieces with a diameter of 14mm using a punch cutter; put the 4 positive electrode round pieces into a centrifuge tube containing 15-20mL of ferrocene-electrolyte mixed solution and blank solution, let stand for 1h, pour out the solution, and wait for testing;

[0186] (4) Sample preparation and testing: Take 0.5 mL of the ferrocene-electrolyte mixture after the reaction and add it to a 50 mL volumetric flask. Dilute to the mark with deionized water, shake well, and then take 1 mL of the diluent to a 50 mL volumetric flask. Add 5.00 mL of 1 mol / L HNO3 solution and 5.00 mL of 10% KSCN solution to the sample volumetric flask, dilute to the mark with deionized water, shake well, and let stand for 10 min. Using the blank solution as a reference, measure the absorbance at a wavelength of 480 nm. Based on the absorbance A measured in the sample, substitute it into the standard curve regression equation to calculate the Fe content in the sample solution. 3+ The concentration C (μg / mL) of Fe in the sample solution was obtained after unit conversion. 3+ The concentration is bmmol / L.

[0187] 3. The tortuosity of the positive electrode is c, and its test method is as follows:

[0188] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V. The battery was then disassembled in its empty state to obtain the positive electrode. The actual thickness (d, in cm) and area (S, in cm²) of the positive electrode material layer were measured. 2 The specific surface area (SSA) and porosity ε can be obtained by testing the positive electrode using the gas adsorption method (BET). The BET principle is based on the adsorption characteristics of gas molecules on the electrode surface. The specific surface area is calculated by measuring the adsorption amount, and then the porosity ε is derived by combining the electrode density.

[0189] The positive electrode sheets were assembled into a positive electrode-positive electrode symmetrical monocell, and a 0.2M tetraethylammonium-hexafluorophosphate DMC solution was used as the electrolyte; the conductivity of the electrolyte was measured using a conductivity meter to obtain σ.

[0190] EIS testing was performed at 0% SOC using an electrochemical workstation on the symmetric cells, with a frequency range of 10. -1 ~10 5 With an AC disturbance voltage of 5~10mV, the EIS data is fitted using the built-in software of the electrochemical workstation or professional impedance analysis software such as ZView. The ionic resistance Rion is obtained by fitting the EIS Nyquist curve using the transmission line "TLM" model. The Rion is then calculated using the following formula: c=(Rion×S×ε×σ) / d, where c is the tortuosity, Rion is the ionic resistance, S is the electrode area, ε is the electrode porosity, σ is the conductivity of the electrolyte, and d is the electrode thickness.

[0191] 4. The test method for the mass percentage content of dopant element M in the cathode material layer is as follows:

[0192] The secondary battery was discharged to 2.5V at 0.33C. The lithium-ion battery in its empty state was disassembled to obtain the positive electrode. The positive electrode was soaked in DMC (dimethyl carbonate) at room temperature for 60 minutes, then removed and dried at room temperature with humidity ≤15%. The powder on the surface of the dried electrode was scraped off with a scraper, preliminarily crushed with a mortar and pestle, and then passed through a 200-mesh sieve to obtain a fine powder sample.

[0193] Accurately weigh a certain amount of powder, disperse it in 20ml of water, add 10ml of nitric acid, mix thoroughly, and then heat to dissolve the powder. Dilute the mixture to 100mL with water to obtain the test solution. Perform ICP testing 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. Dilute the 1000mg / L standard solution with deionized water to different concentrations (generally 0, 1mg / 100mL, 2mg / 100mL, 3mg / 100mL), select the elemental detection wavelength, and set the experimental conditions according to the characteristics of the sample and requirements. For the elements to be detected, set appropriate ICP instrument operating conditions: gas flow rate 0.5 L / min, power 1150 W, and select the element test wavelength, which depends on the element being tested (e.g., Mg wavelength 279.08 nm, Al wavelength 396.15 nm, Ti wavelength 323.452 nm, Zr wavelength 336.12 nm, Nb wavelength 309.4 nm, W wavelength 239.7 nm, Y wavelength 371 nm, Mo wavelength 202.03 nm, Ca wavelength 317.933 nm, S wavelength 182.0 nm, N wavelength 149.8 nm). The content of element M in the sample can be read by the self-analysis function of the ICP testing software.

[0194] 5. The test method for the compaction density of the positive electrode is as follows:

[0195] Discharge the secondary battery at 0.33C to the lower limit voltage of 2.5V, remove the empty battery, disassemble the positive electrode, soak the positive electrode in dimethyl carbonate (DMC) solution for 4 hours; then air dry.

[0196] The pretreated positive electrode sheet is punched into circular sheets of a fixed area using a punching machine. The area is denoted as S0. Three circular sheets are taken as parallel samples, and the mass of the three circular sheets is weighed using an electronic balance. The average value is recorded as M1. The thickness of the positive electrode material layer in the three circular sheets (i.e., the total thickness excluding the current collector) is measured using a micrometer. The average value is recorded as H. Finally, an appropriate amount of deionized water is dropped onto each of the three circular sheets. The coating on the circular sheets is gently wiped off with lint-free paper to expose the copper foil. The sheets are left to stand at room temperature (or dried) for 10 minutes. After the copper foil is dry, the mass of the three copper foil sheets is weighed. The average value is recorded as M0. The compaction density A of the positive electrode sheet is calculated according to the following formula: A = (M1 - M0) / (H × S0).

[0197] 6. Test method for the mass percentage of cyclic sulfate compounds in electrolyte:

[0198] 1) Electrolyte Collection: Using the secondary batteries obtained in the above examples and comparative examples as test objects, the secondary batteries to be tested were discharged using a battery charging and discharging device. Discharge conditions: current 0.33C, cutoff lower limit voltage 2.5V. After recording the battery number / barcode, the batteries were disassembled and the electrolyte was collected in a glove box (H2O≤0.1ppm, O2≤0.1ppm). There are three methods for collecting the electrolyte: After opening the battery cover, ① if there is free electrolyte, collect the electrolyte into a 5mL sample tube with a pipette and seal it with sealing tape to prevent electrolyte from spreading. Leakage; ② If there is no free electrolyte, a hydraulic press (Beijing Heng'ao Technology Co., Ltd. FY-30 hydraulic press) can be used to continuously pressurize until free electrolyte appears. Collect the electrolyte 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;

[0199] 2) The collected electrolyte samples were injected into an Agilent Intuvo 9000 gas chromatograph-mass spectrometer using a microsyringe for testing, and the GC-MS chromatograms of the electrolyte samples were obtained. Commercially available corresponding cyclic sulfate compounds were dissolved in EMC solvents to prepare solutions of different concentrations, and these solutions were injected into the Agilent Intuvo 9000 gas chromatograph-mass spectrometer to obtain standard GC-MS chromatograms. The GC-MS chromatograms of the electrolyte samples were compared with the standard GC-MS chromatograms to confirm whether the electrolyte samples contained the corresponding cyclic sulfate compounds. Then, based on the determined structures, the mass percentage of the corresponding cyclic sulfate compounds was obtained using gas chromatography.

[0200] 7. The specific test method for the discharge DCR (DC internal resistance) of a secondary battery is as follows:

[0201] 1) Place the battery in a 25°C incubator until thermal equilibrium is reached;

[0202] 2) Charge the battery at a constant current rate of 0.33C to the upper limit voltage of 4.4V, charge it at a constant voltage rate until the current is less than 0.05C, and then discharge it at 0.33C to the lower limit voltage of 2.5V. Repeat this process more than twice to obtain the discharge capacity of the battery and record the battery capacity C1.

[0203] 3) Let stand for 5 minutes;

[0204] 4) Adjust the battery charge to 50% SOC at a discharge rate of 0.33C;

[0205] 5) Discharge at a 1C rate for 18s, record the battery voltage U2, current I before discharge stops and the battery voltage U1 after the battery voltage stabilizes. Calculate the DC internal resistance R1 according to the formula R1=(U2-U1) / I. This R1 is the DCR of 1C discharge at 25℃.

[0206] 8. The test method for the thermal runaway temperature of secondary batteries is as follows:

[0207] The secondary batteries were charged at a constant current of 0.33C to 4.4V at 25℃, then switched to constant voltage charging until the current was less than 0.05C, and allowed to stand for 30 minutes. Next, heating wires were evenly wrapped around the surface of each secondary battery, and they were placed in an adiabatic accelerated calorimeter (ARC) for adiabatic thermal stability test until thermal runaway occurred. The test ended when the heating rate of the secondary battery reached 0.02℃ / min, which is the self-heating temperature, denoted by T1, in℃. When the heating rate of the secondary battery reached 1℃ / min, the temperature of the lithium-ion battery was the thermal runaway temperature, denoted by T2, in℃.

[0208] Table 2

[0209]

[0210] Table 3

[0211]

[0212] As can be seen from Tables 2 and 3, the embodiments of this application comprehensively control the cell volume 'a' of the cathode material when the secondary battery is charged to 90% SOC, and the reaction between the cathode sheet and ferrocene to produce Fe. 3+ The amount (based on the Fe content in the solution after the reaction) 3+ The concentration b of the positive electrode and the tortuosity c of the positive electrode sheet were used to characterize the positive electrode material. By ensuring that a×c / b is within the range of 13.8~73.7, the interfacial activity of the positive electrode material can be reduced, the stability of the positive electrode material can be improved, internal gas generation in the battery can be reduced, and the battery safety can be improved. It can also shorten the lithium-ion transport path and reduce the DC internal resistance of the battery. The measured discharge DCR of the battery is ≤35.3mΩ, and the thermal runaway temperature is ≥155.2℃. When a×c / b is within the range of 28.5~38, and a is 94.2~96.5, b is 3.2~4.6, and c is 1.2~1.5, the overall improvement in battery safety and discharge DCR is even better, with the measured discharge DCR ≤32.9mΩ and the thermal runaway temperature ≥176.3℃.

[0213] Compared with the embodiments, although a, b, and c in Comparative Example 1 are within the scope of this application, a×c / b is too small, which significantly reduces the safety of the secondary battery.

[0214] Compared with the embodiments, although a, b, and c in Comparative Example 2 are within the scope of this application, a×c / b is too large, which significantly increases the DC internal resistance of the secondary battery.

[0215] Compared with the embodiments, Comparative Example 3 did not add cyclic sulfate compounds to the electrolyte, a×c / b was too small, and both a and c were too small, while b was too large, which significantly reduced the safety of the secondary battery. The measured thermal runaway temperature was even lower than that of Comparative Example 1.

[0216] Compared with the embodiments, in Comparative Example 4, a×c / b is too large and b is too small, and both a and c are too large, which significantly increases the DC internal resistance of the secondary battery, and the measured discharge DCR is even greater than that of Comparative Example 2.

[0217] 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, The device includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive electrode material layer, which in turn includes a positive electrode material comprising a layered transition metal oxide. The layered transition metal oxide includes a dopant element, which is at least one selected from Al, Ti, Mo, Mg, W, Nb, Zr, Ca, S, and N. The mass percentage of the dopant element in the positive electrode material layer is 0.03% to 3%. The electrolyte comprises a cyclic sulfate compound, wherein the mass percentage of the cyclic sulfate compound in the electrolyte is 0.001% to 3%, and the cyclic sulfate compound comprises a compound represented by the following structural formula I; in structural formula I, R1, R2, R3 and R4 each independently comprise at least one of the following structural formula II groups, hydrogen atoms, alkyl groups of 1 to 6 carbon atoms, halogen atoms, haloalkyl groups of 1 to 3 carbon atoms, alkoxy groups of 1 to 3 carbon atoms, haloalkoxy groups of 1 to 3 carbon atoms, alkenyl groups, aromatic groups, ester groups, cyano groups, and sulfonic acid groups; In structural formula II, R5 and R6 each independently include at least one of the following groups as shown in structural formula III: a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, a haloalkoxy group of 1 to 3 carbon atoms, an alkenyl group of 1 to 3 carbon atoms, an aromatic group, an ester group, a cyano group, and a sulfonic acid group. In structural formula III, R7 and R8 each independently include at least one of the following: hydrogen atom, alkyl group of 1 to 6 carbon atoms, halogen atom, haloalkyl group of 1 to 3 carbon atoms, alkoxy group of 1 to 3 carbon atoms, haloalkoxy group of 1 to 3 carbon atoms, alkenyl group of 1 to 3 carbon atoms, aromatic group, ester group, cyano group, and sulfonic acid group. Formula I, Formula II, Formula III; When the secondary battery is charged to 90% SOC, the cell volume of the positive electrode material is aÅ. 3 ; After the positive electrode reacts with a solution containing ferrocene, Fe2+ appears in the solution after the reaction. 3+ The concentration of ferrocene is b mmol / L, and the solution containing ferrocene is a mixed solution of ferrocene and the electrolyte, wherein the concentration of ferrocene in the solution containing ferrocene is 0.03 mol / L; The tortuosity of the positive electrode is c; The secondary battery satisfies: 13.8≤a×c / b≤73.

7.

2. The secondary battery as described in claim 1, characterized in that, 28.5≤a×c / b≤38.

3. The secondary battery as described in claim 1, characterized in that, 92.3Å 3 ≤aÅ 3 ≤99.1Å 3 ； And / or, 2.1 mmol / L ≤ b mmol / L ≤ 7.2 mmol / L; And / or, 1.0≤c≤1.

8.

4. The secondary battery as described in claim 1, characterized in that, In the compound with the structure shown in Formula I, the number of sulfate groups is n, where n is a positive integer and is between 2 and 4.

5. 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 the following: the group shown in structural formula II, a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a halogen atom, a haloalkyl group of 1 to 3 carbon atoms, an alkoxy group of 1 to 3 carbon atoms, an alkenyl group of 1 to 3 carbon atoms, and an aromatic group.

6. The secondary battery as described in claim 5, characterized in that, R1 and R3 are the same substituents, and are selected from any one of the groups shown in structural formula II, hydrogen atoms, alkyl groups of 1 to 6 carbon atoms, halogen atoms, haloalkyl groups of 1 to 3 carbon atoms, alkoxy groups of 1 to 3 carbon atoms, alkenyl groups of 1 to 3 carbon atoms, and aromatic groups.

7. The secondary battery as described in claim 6, characterized in that, R1 and R3 are selected from any one of the following groups: -CH3, -C2H5, -C3H7, -CH=CH2, -CH2-CH=CH2, -CH=CH-CH3, -O-CH3, -O-C2H5, -O-C3H7, -F, -Cl, -Br, -I, -CF3, -CCl3, -CBr3, -C2F5, -C2Cl5, -C2Br5, -C3F7, -C3Cl7, -C3Br7, -C6H5, -C4H3S, -C4H4N, and the group shown in structural formula III.

8. The secondary battery as described in claim 1, characterized in that, The compaction density of the positive electrode is 3.2 g / cm³. 3 ~3.8g / cm 3 .

9. The secondary battery as described in claim 1, characterized in that, The layered transition metal oxide is selected from lithium nickel cobalt manganese oxide, and the ratio between the molar content of Ni element in the cathode material and the total molar content of transition metal elements in the cathode material is 0.5~0.

9.

10. The secondary battery as described in claim 9, characterized in that, The surface of the lithium nickel cobalt manganese oxide particles has a coating layer with a thickness of 2 nm to 120 nm.

11. The secondary battery as described in claim 1, characterized in that, The electrolyte includes a solvent, which includes at least one of carbonate solvents and carboxylic acid ester solvents.

12. The secondary battery as described in claim 11, characterized in that, The carbonate solvents include at least one of ethylene carbonate, fluoroethylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate, and / or the carboxylic acid ester solvents include at least one of ethyl acetate, methyl isobutyrate, ethyl trimethylacetate, 2,2-difluoroethyl acetate, and 2,2,2-trifluoroethyl acetate.

13. The secondary battery as described in claim 1, characterized in that, The electrolyte includes additives, which include at least one of the following: vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, propylene sulfite, vinyl ethylene carbonate, vinyl sulfate, lithium bis(oxalato)borate, lithium difluorophosphate, lithium difluorooxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, methanedisulfonate, tri(trimethylsilane) phosphate, tri(trimethylsilane) borate, and tri(trimethylsilane) phosphite. The mass percentage of the additives in the electrolyte is 0.05% to 10%.

14. The secondary battery as described in claim 1, characterized in that, The electrolyte includes lithium salt, and the mass percentage of lithium salt in the electrolyte is 11% to 15%.

15. The secondary battery as described in claim 1, characterized in that, The negative electrode sheet includes a negative electrode material layer, the porosity of which is 20% to 60%.

16. The secondary battery as described in claim 15, characterized in that, The negative electrode material layer contains manganese, and the mass content of manganese in the negative electrode material layer is 10ppm to 100ppm.

17. The secondary battery as described in claim 15, characterized in that, The negative electrode material layer includes a negative electrode material, which includes at least one of graphite and silicon-based materials, wherein the silicon-based material includes silicon-carbon composite material, and the particle size Dv50 of the negative electrode material is 8μm~20μm.

18. The secondary battery as described in claim 17, characterized in that, The negative electrode material includes silicon-based materials, and the mass percentage of silicon in the negative electrode material layer is 1% to 10%.

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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