Secondary battery and electric device

Abstract

A secondary battery and an electric device. The secondary battery satisfies the relation of formula (I). Me. g / Ah indicates the liquid retention coefficient of an electrolyte in the secondary battery; A indicates the ratio of the mass of a fluorine-containing additive to the mass of a cyclic carbonate additive in the electrolyte; σ mS / cm indicates the electrical conductivity of the electrolyte; α indicates the porosity of a positive electrode sheet; η mPa·s indicates the viscosity of the electrolyte at 25°C; and PD g / cm3 indicates the compaction density of a positive-electrode active material layer. The liquid retention coefficient of a battery, a positive electrode sheet, and additives in an electrolyte are controlled, so as to satisfy specific requirements, thereby significantly improving the quick-charge performance and low-temperature discharge performance of secondary batteries.

Description

Secondary battery and electrical equipment

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese invention patent application CN202311121699.2 filed on August 30, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, in particular to a secondary battery and an electrical device. Background Art

[0004] Secondary batteries, such as lithium-ion batteries, offer advantages such as higher energy density, excellent cycle performance, and no memory effect. They are now widely used in portable electronics and new energy vehicles. With the continued development of new energy vehicles, consumers are increasingly demanding faster charging. Developing fast-charging technologies that shorten charging times can significantly improve user experience and are crucial to developing the new energy vehicle market.

[0005] There are many factors that affect the development of fast charging technology. Ions escape from the positive electrode into the electrolyte, pass through the diaphragm and then enter the negative electrode for embedding. The rapid migration of a large number of ions requires the electrolyte to have high kinetic properties to ensure less resistance during the mass transfer process.

[0006] Therefore, it is necessary to develop an electrolyte with high kinetic performance that matches the positive and negative electrode materials, that is, one with higher ionic conductivity, better wettability and lower viscosity.

[0007] Summary of the Invention

[0008] In order to overcome the defects of secondary batteries that the fast charging cycle performance and low-temperature discharge performance still need to be improved, this application optimizes and improves the electrolyte of the secondary battery. By controlling the battery's liquid retention coefficient, positive electrode plate and electrolyte additives to meet specific requirements, the fast charging performance and low-temperature discharge performance of the secondary battery can be greatly improved.

[0009] To achieve the above objectives, in a first aspect of the present application, a secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the electrolyte comprises an organic solvent, a lithium salt, and an additive, wherein the additive comprises a fluorine-containing additive and a cyclic carbonate additive:

[0010] The secondary battery satisfies the following relationship:

[0011] Wherein, Me.g / Ah represents the electrolyte retention coefficient of the secondary battery, that is, the ratio of the total mass of the electrolyte to the nominal capacity of the secondary battery;

[0012] A represents the mass ratio of the fluorine-containing additive to the cyclic carbonate additive in the electrolyte;

[0013] σmS / cm represents the conductivity of the electrolyte;

[0014] α represents the porosity of the positive electrode sheet;

[0015] ηmPa·s represents the viscosity of the electrolyte at 25°C;

[0016] PD g / cm 3 It represents the compaction density of the positive electrode active material layer.

[0017] In some embodiments, the electrolyte has a retention coefficient Me.g / Ah of 2 to 5 g / Ah.

[0018] In some embodiments, the electrolyte has a conductivity σmS / cm of 7 to 13 mS / cm.

[0019] In some embodiments, the electrolyte has a viscosity η mPa·s at 25° C. of 3 to 5 mPa·s.

[0020] In some embodiments, in the electrolyte, the mass ratio A of the fluorine-containing additive to the cyclic carbonate additive is 0.1-20.

[0021] In some embodiments, the compaction density of the positive electrode active material layer is PD g / cm 3 3~4.2g / cm 3 .

[0022] In some embodiments, the porosity α of the positive electrode sheet is 0.1 to 0.5.

[0023] In some embodiments, the lithium salt in the electrolyte includes at least one of lithium hexafluorophosphate and lithium bis(oxalatoborate).

[0024] In some embodiments, the organic solvent in the electrolyte includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, diphenyl carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, ethyl fluoroacetate, fluoroether, γ-butyrolactone, acetonitrile, and sulfolane.

[0025] In some embodiments, the fluorine-containing additive in the electrolyte includes at least one of lithium fluorosulfonate, lithium difluorophosphate, and lithium difluorooxalatoborate, and the mass fraction of the fluorine-containing additive is 0.2 to 2.0% based on the mass of the electrolyte.

[0026] In some embodiments, the cyclic carbonate additive in the electrolyte includes at least one of vinylene carbonate and vinyl ethylene carbonate, and the mass fraction of the cyclic carbonate additive is 0.05 to 3% based on the mass of the electrolyte.

[0027] In some embodiments, the electrolyte further includes a silicon-containing additive, wherein the silicon-containing additive includes at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(trimethylsilyl)borate, trimethylfluorosilane, and heptamethyldisilazane. The mass fraction of the silicon-containing additive is 0.1-2% based on the mass of the electrolyte.

[0028] In some embodiments, the electrolyte additives include a fluorine-containing additive, a silicon-containing additive, and a cyclic carbonate additive, and the secondary battery satisfies the following relationship:

[0029] Wherein, A' represents the ratio of the total mass of the fluorine-containing additive and the silicon-containing additive in the electrolyte to the mass of the cyclic carbonate additive.

[0030] In some embodiments, the electrolyte further includes a sulfur-containing additive, wherein the sulfur-containing additive includes at least one of 1,3-propane sultone, vinyl sulfate, butylene sulfite, and methylene methanedisulfonate. The mass fraction of the sulfur-containing additive is 0.05-3% based on the mass of the electrolyte.

[0031] In some embodiments, in the positive electrode active material layer, the positive electrode active material includes a molecular formula of Li x Ni a Co b M c Z 1-a-b-c O2 compound, 0.95≤x≤1.2, 0<a<1, 0<b<1, 0<c<1, a+b+c≤1; in the molecular formula, M includes one or two of Mn and Al; Z includes at least one of Fe, Zr, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti.

[0032] In a second aspect of the present application, an electrical device is provided, wherein the electrical device includes the secondary battery described above.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] This application can greatly improve the fast charging performance and low-temperature discharge performance of secondary batteries by comprehensively designing the battery's liquid retention coefficient, the intrinsic parameters of the positive electrode sheet, and the intrinsic parameters of the electrolyte, and rationalizing the correlation between the above parameters. DETAILED DESCRIPTION

[0035] To better illustrate the purpose, technical solutions and advantages of the present application, the present application will be further described below in conjunction with specific examples, but the examples do not limit the present application in any form. Unless otherwise specified, the reagents, methods and equipment used in the present application are conventional reagents, methods and equipment in the art. Unless otherwise specified, the reagents and materials used in the present application are commercially available.

[0036] An embodiment of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the electrolyte comprises an organic solvent, a lithium salt, and an additive, wherein the additive comprises a fluorine-containing additive and a cyclic carbonate additive:

[0037] The secondary battery satisfies the following relationship:

[0038] Wherein, Me.g / Ah represents the electrolyte retention coefficient of the secondary battery;

[0039] A represents the mass ratio of the fluorine-containing additive to the cyclic carbonate additive in the electrolyte;

[0040] σmS / cm represents the conductivity of the electrolyte;

[0041] α represents the porosity of the positive electrode sheet;

[0042] ηmPa·s represents the viscosity of the electrolyte at 25°C;

[0043] PD g / cm 3 It represents the compaction density of the positive electrode active material layer.

[0044] In secondary batteries, the electrolyte, as the medium of secondary conduction, has an important influence on the secondary transmission performance during the charge and discharge process, which in turn affects the charge and discharge performance of the secondary battery. During the charge and discharge process of the secondary battery, on the one hand, it is necessary to increase the secondary migration speed; on the other hand, it is also necessary to improve the stability of the electrode / electrolyte interface to prevent the serious decomposition of the electrolyte during the cycle, which causes an increase in the interfacial impedance and affects the fast charge cycle lithium precipitation window. By adding film-forming additives to the electrolyte, an effective SEI film can be formed to reduce the occurrence of interfacial side reactions during the cycle, but film-forming additives often affect the low-temperature discharge performance of the battery due to the large initial impedance.

[0045] Through extensive research, the present application has found that comprehensive design of the battery's liquid retention coefficient, the intrinsic parameters of the positive electrode sheet, and the intrinsic parameters of the electrolyte, and rationalization of the correlation between the above parameters, can greatly improve the fast charging cycle performance and low-temperature discharge performance of secondary batteries.

[0046] Additives, as crucial components of secondary battery electrolytes, can optimize the composition of the SEI film and play a crucial role in its performance. Fluorine-containing additives preferentially form on the negative electrode, forming an SEI film with high ionic conductivity. Cyclic carbonate additives can polymerize and form a highly stable SEI film on the negative electrode surface. Properly controlling the composition of additives in the electrolyte can simultaneously meet the requirements for both kinetic performance and long-cycle performance of secondary batteries.

[0047] In some embodiments of the present application, the secondary battery satisfies the following relationship:

[0048] In some embodiments of the present application, the mass ratio of the fluorine-containing additive to the cyclic carbonate additive is A = m (fluorine-containing additive): m (cyclic carbonate additive) = 0.1 to 20. For example, the mass ratio A of the fluorine-containing additive to the cyclic carbonate additive can be 0.1, 1, 3, 5, 8, 10, 12, 15, 17, 20, or a range consisting of any two of the above values.

[0049] In some embodiments of the present application, the mass ratio of the fluorine-containing additive to the cyclic carbonate additive is A=m(fluorine-containing additive):m(cyclic carbonate additive)=0.8-5.

[0050] In some embodiments of the present application, the fluorine-containing additive includes at least one of lithium fluorosulfonate, lithium difluorophosphate, and lithium difluorooxalatoborate. The mass fraction of the fluorine-containing additive is 0.2% to 2.0% based on the mass of the electrolyte. For example, the mass fraction of the fluorine-containing additive can be 0.4%, 1.0%, 1.5%, 2%, or a range consisting of any two of the foregoing values.

[0051] In some embodiments of the present application, based on the mass of the electrolyte, the mass fraction of the fluorine-containing additive is 0.4-1.5%.

[0052] In some embodiments of the present application, the cyclic carbonate additive includes at least one of vinylene carbonate (VC) and vinyl ethylene carbonate; the cyclic carbonate further needs to contain vinylene carbonate (VC), and the secondary battery added with VC has better fast charging cycle performance.

[0053] In some embodiments of the present application, the mass fraction of the cyclic carbonate additive is 0.05-3% based on the mass of the electrolyte. For example, the mass fraction of the cyclic carbonate additive can be 0.1%, 0.3%, 2.8%, 3%, or a range consisting of any two of the above values.

[0054] In some embodiments of the present application, based on the mass of the electrolyte, the mass fraction of the cyclic carbonate additive is 0.1 to 2.8%.

[0055] In some embodiments of the present application, the electrolyte further includes a silicon-containing additive, which can capture HF in the electrolyte, improve the stability of the electrolyte, and inhibit HF from damaging the interface film. The silicon-containing additive includes at least one of tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) phosphite (TMSPi), tris(trimethylsilyl) borate, trimethylfluorosilane, and heptamethyldisilazane. The silicon-containing additive further needs to include tris(trimethylsilyl) phosphate (TMSP), and the secondary battery with the addition of TMSP has better fast charge cycle performance.

[0056] In some embodiments of the present application, based on the mass of the electrolyte, the mass fraction of the silicon-containing additive is preferably in the range of 0.1% to 2%. For example, the mass fraction of the silicon-containing additive can be 0.3%, 0.3%, 0.5%, 1.0%, 1.5%, 2%, or a range consisting of any two of the above values.

[0057] In some embodiments of the present application, based on the mass of the electrolyte, the mass fraction of the silicon-containing additive is 0.3-1%.

[0058] In some embodiments of the present application, the additives in the electrolyte of the secondary battery include a fluorine-containing additive, a silicon-containing additive, and a cyclic carbonate additive, wherein the ratio of the sum of the mass of the fluorine-containing additive and the silicon-containing additive to the mass of the cyclic carbonate additive is A'=0.17-40; and the secondary battery satisfies the following relationship: By adding the above three electrolyte additives and controlling their composition ratio within a suitable range, a SEI film with suitable thickness, good stability and low impedance can be obtained, thereby making the secondary battery have better fast charge cycle performance and low-temperature discharge performance.

[0059] In some embodiments of the present application, the secondary battery satisfies the following relationship:

[0060] In some embodiments of the present application, the electrolyte of the secondary battery further includes a sulfur-containing additive, and the sulfur-containing additive includes at least one of 1,3-propane sultone, vinyl sulfate, butylene sulfite, and methylene methanedisulfonate.

[0061] In some embodiments of the present application, the mass fraction of the sulfur-containing additive is 0.05-3% based on the mass of the electrolyte. For example, the mass fraction of the sulfur-containing additive can be 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, or a range consisting of any two of the above values.

[0062] In some embodiments of the present application, the conductivity σmS / cm of the electrolyte is in the range of 7 to 13mS / cm. For example, the conductivity σmS / cm of the electrolyte can be 7mS / cm, 7.5mS / cm, 8.05mS / cm, 8.30mS / cm, 8.80mS / cm, 9.5mS / cm, 11.5mS / cm, 13mS / cm, etc. or a range consisting of any two of the above values. The higher the conductivity, the faster the secondary transmission rate in the battery, which is beneficial to the improvement of fast charging performance. However, it should be noted that the conductivity of the electrolyte is mainly affected by the concentration of lithium salts and the composition of the solvent. Therefore, the conductivity of the electrolyte is within the above range, which is more conducive to the fast charging cycle performance of the secondary battery and a longer cycle life.

[0063] In some embodiments of the present application, the electrolyte has a retention coefficient Me.g / Ah in the range of 2 to 5 g / Ah. For example, the electrolyte retention coefficient Me.g / Ah can be 2.0 g / Ah, 2.3 g / Ah, 2.6 g / Ah, 3 g / Ah, 3.5 g / Ah, 4.0 g / Ah, 4.5 g / Ah, 5.0 g / Ah, or a range consisting of any two of the above values. By limiting the electrolyte retention coefficient to the above reasonable range, the cycle performance of the battery can be further improved.

[0064] It should be noted that, in this application, the liquid retention coefficient is the ratio of the total mass of the electrolyte to the nominal capacity of the secondary battery, where the nominal capacity is tested according to the following method: the secondary battery is placed at 25°C, charged to 4.35V at a constant current of 1C, and then charged at a constant voltage to a cutoff current of 0.05C, left for 10 minutes, and then discharged to 2.8V at a constant current of 1C, and the discharge capacity of the secondary battery at this time is recorded.

[0065] In some embodiments of the present application, the viscosity ηmPa·s of the electrolyte at 25°C is in the range of 3 to 5 mPa·s. For example, the viscosity ηmPa·s of the electrolyte at 25°C can be 3 mPa·s, 3.5 mPa·s, 4 mPa·s, 4.5 mPa·s, 5 mPa·s, or a range consisting of any two of the foregoing values. The viscosity of the electrolyte within the above range can further ensure that lithium plating does not occur in the secondary battery under high-rate charging conditions.

[0066] Furthermore, the intrinsic parameters of a secondary battery's positive electrode sheet also affect battery performance. For example, the greater the compaction density of the positive active material layer on the positive electrode sheet, the lower the sheet porosity per unit volume, and the less total electrolyte absorbed by the sheet. By controlling the compaction density of the positive electrode sheet within an appropriate range, the battery's energy density can be increased while also maintaining good kinetic performance.

[0067] In some embodiments of the present application, the compaction density of the positive electrode active material layer is PD g / cm 3 3~4.2g / cm 3 For example, the compacted density PD g / cm 3 Can be 3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 , 4g / cm 3 , 4.2g / cm 3 Or the range consisting of any two of the above values.

[0068] In some embodiments of the present application, the porosity α of the positive electrode sheet is in the range of 0.1 to 0.5. For example, the porosity α of the positive electrode sheet can be 0.1, 0.2, 0.35, 0.4, 0.5, or a range consisting of any two of the above values.

[0069] In some embodiments of the present application, the organic solvent in the electrolyte includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, diphenyl carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, ethyl fluoroacetate, fluoroether, γ-butyrolactone, acetonitrile, and cyclopentane sulfone.

[0070] In some embodiments of the present application, the lithium salt in the electrolyte includes at least one of lithium hexafluorophosphate and lithium bis(oxalatoborate).

[0071] In some embodiments of the present application, in the positive electrode active material layer, the positive electrode active material includes a molecular formula of Li x Ni a Co b M c Z 1-a-b-c O2 compound, 0.95≤x≤1.2, 0<a<1, 0<b<1, 0<c<1, a+b+c≤1; in the molecular formula, M includes one or two of Mn and Al; Z includes at least one of Fe, Zr, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti.

[0072] The present application also protects an electrical device including the secondary battery.

[0073] The following are specific examples of the present application, and the technical solutions of the present application are further described in conjunction with the examples, but the present application is not limited to these examples. The reagents, methods and equipment used in this application, unless otherwise specified, are conventional reagents, methods and equipment in the art.

[0074] Example 1

[0075] This embodiment provides a secondary battery, and a preparation method thereof includes the following steps:

[0076] Preparation of positive electrode sheet

[0077] The positive electrode active material LiNi 0.6 Mn 0.2 Co 0.2 O2 (denoted as NCM622), conductive agent acetylene black, and binder PVDF are mixed in a mass ratio of 94:3:3, and 1-methyl-2-pyrrolidone (NMP) is added and evenly dispersed to form a uniform black slurry. The slurry is coated on the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet with a positive electrode active material layer is obtained. The compaction density of the positive electrode active material layer is 3.6 g / cm 3 , the porosity α of the positive electrode sheet is 0.35.

[0078] Preparation of negative electrode sheet

[0079] The negative electrode active material graphite, the conductive agent acetylene black and the binder SBR were mixed uniformly in a mass ratio of 94:3:3 and evenly dispersed in deionized water to prepare a uniform black slurry. The negative electrode slurry was coated on the negative electrode current collector copper foil, dried and cold pressed to obtain a negative electrode sheet provided with a negative electrode active material layer, wherein the compaction density of the negative electrode active material layer is 1.55 g / cm 3 .

[0080] Preparation of electrolyte

[0081] At room temperature (25±5°C), in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 30:50:20 and heated to 400°C. The molecular sieve is used to remove water to obtain a mixed solvent, and LiPF6 is added gradually to the obtained mixed solvent. The mixture is stirred continuously and cooled with dry ice to ensure that the temperature of the electrolyte does not rise by more than 2°C. LiPF6 can be continued to be added to control the mass fraction of the lithium salt LiPF6 in the electrolyte to be 12.5%, and finally a colorless transparent liquid is obtained. Then, lithium fluorosulfonate, 0.3% tris(trimethylsilyl) phosphate and 0.3% vinylene carbonate equivalent to 0.4% of the total mass of the electrolyte are added, and the mixture is stirred evenly to obtain the electrolyte used in this embodiment, wherein the conductivity σ of the electrolyte is 8.26 mS / cm and the viscosity η at 25°C is 3.9 mPa·s.

[0082] Secondary battery assembly

[0083] The prepared positive electrode sheet, separator (commercially available polypropylene), and negative electrode sheet are stacked in order with the separator placed between the positive and negative electrode sheets. After winding, hot pressing and shaping, and tab welding, a bare cell is obtained. The bare cell is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10°C for 24 hours. The configured electrolyte is injected into the dried battery, allowed to stand, formed, and divided to complete the preparation of the lithium-ion battery. The battery has a liquid retention coefficient of 2.6g / Ah.

[0084] Examples 2 to 4

[0085] The preparation method is as in Example 1, with the differences shown in Table 1.

[0086] Example 5

[0087] This embodiment provides a secondary battery. The preparation method refers to that of Example 1, except that the electrolyte, specifically EC:EMC:DEC, is uniformly mixed in a mass ratio of 30:55:15 (i.e., the ratio between different solvents is changed). The conductivity σ of the obtained electrolyte is 8.5 mS / cm, and the viscosity η is 3.8 mPa·s.

[0088] Example 6

[0089] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is EC:EMC:DEC mixed evenly in a mass ratio of 30:60:10. The conductivity σ of the obtained electrolyte is 8.7 mS / cm, and the viscosity η is 3.7 mPa·s.

[0090] Example 7

[0091] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is EC:EMC mixed evenly at a mass ratio of 30:70. The conductivity σ of the obtained electrolyte is 8.8 mS / cm, and the viscosity η is 3.6 mPa·s.

[0092] Example 8 to Example 13

[0093] The preparation method is as in Example 1, with the differences shown in Table 1.

[0094] Example 14

[0095] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that 1% tris(trimethylsilyl)phosphate is added to the electrolyte. The conductivity σ of the electrolyte is 8.4 mS / cm, and the viscosity η is 3.92 mPa·s.

[0096] Example 15

[0097] This embodiment provides a secondary battery. The preparation method is similar to that of embodiment 1, except that 0.5% vinylene carbonate is added to the electrolyte. The conductivity σ of the electrolyte is 8.3 mS / cm, and the viscosity η is 3.85 mPa·s.

[0098] Example 16

[0099] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is prepared by adding 0.4% lithium difluorophosphate, 0.3% tris(trimethylsilyl)phosphate, and 0.3% vinylene carbonate. The conductivity of the electrolyte is σ of 8.18 mS / cm, and the viscosity η is 3.95 mPa·s.

[0100] Example 17

[0101] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is prepared by adding 1% lithium difluorophosphate, 0.3% tris(trimethylsilyl)phosphate, and 0.3% vinylene carbonate. The conductivity of the electrolyte is σ of 8.05 mS / cm, and the viscosity η is 4.02 mPa·s.

[0102] Example 18

[0103] The preparation method is similar to that of Example 1, except that 0.4% lithium difluorophosphate, 0.5% tris(trimethylsilyl)phosphate, and 0.3% vinylene carbonate are added to the electrolyte, and the conductivity σ of the electrolyte is 8.3 mS / cm, and the viscosity η is 3.93 mPa·s.

[0104] Example 19

[0105] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is prepared by adding 0.4% lithium difluorophosphate, 0.3% tris(trimethylsilyl)phosphate, and 0.5% vinylene carbonate. The conductivity of the electrolyte is σ of 8.28 mS / cm, and the viscosity η is 3.94 mPa·s.

[0106] Example 20

[0107] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that 0.4% lithium fluorosulfonate and 0.3% vinylene carbonate are added to the electrolyte. The conductivity σ of the electrolyte is 8.35 mS / cm, and the viscosity η is 3.87 mPa·s.

[0108] Example 21

[0109] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that 1% lithium fluorosulfonate and 0.3% vinylene carbonate are added to the electrolyte. The conductivity σ of the electrolyte is 8.49 mS / cm, and the viscosity η is 3.91 mPa·s.

[0110] Example 22

[0111] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is changed, specifically, the ratio of each component in the additive and solvent is changed (the mass ratio of EC:EMC:DMC is 35:20:45). Specific parameters are shown in Table 1.

[0112] Example 23

[0113] This embodiment provides a secondary battery. The preparation method is similar to that of Example 1, except that the electrolyte is changed, specifically, the ratio of each component in the additive and solvent is changed (the mass ratio of EC:EMC:DMC is 20:20:60). Specific parameters are shown in Table 1.

[0114] In this application, the examples and comparative examples of the preparation method are not listed. All of them are based on Example 1, and the types of raw materials or preparation processes are changed to obtain secondary batteries with the parameters shown in Table 1.

[0115] Table 1 Battery parameters of Examples and Comparative Examples

[0116] The performance of the secondary batteries obtained in the above examples and comparative examples was tested. The specific test items, test methods and results are as follows:

[0117] (1) Cycling Performance Test: The secondary batteries obtained in the Examples and Comparative Examples were charged at a 3C rate (i.e., fast charge) and discharged at a 1C rate at 25°C within the voltage range of 2.8 to 4.35V. The discharge capacity at the first cycle and the discharge capacity after 500 cycles were recorded. The capacity retention after 500 cycles = discharge capacity after 500 cycles / discharge capacity after the first cycle * 100%. The recorded data are shown in Table 2.

[0118] (2) Low-temperature discharge performance test: The secondary batteries obtained in the examples and comparative examples were placed at 25°C, charged to 4.35V at a constant current of 1C, then charged at a constant voltage to a cutoff current of 0.05C, left for 10 minutes, then discharged to 2.8V at a constant current of 1C, and the discharge capacity of the battery at 25°C was recorded. The battery was left for 10 minutes. The battery was then charged to 4.35V at a constant current of 1C, then charged at a constant voltage to a cutoff current of 0.05C, and then placed in a -20°C low-temperature box for 2 hours, discharged to 2.8V at a constant current of 1C, and the discharge capacity of the battery at -20°C was recorded. -20°C discharge capacity retention rate = -20°C discharge capacity / 25°C discharge capacity * 100%. The recorded data are shown in Table 2.

[0119] Table 2 Performance test results of secondary batteries prepared in Examples and Comparative Examples

[0120] From the above results we can see that:

[0121] From the above embodiments and comparative examples, it can be seen that the reversible capacity of the positive and negative electrode sheets of different systems, the sheet compaction density, the particle size and distribution of the active material particles will affect the electrical performance of the battery, thereby affecting the actual energy density of the battery.

[0122] Comparative Example 1, Examples 11 to 19 and Comparative Example 1 found that when When the value is lower than 0.15, the secondary battery exhibits poor fast charge cycle performance and low-temperature discharge performance. This is mainly due to the low A. At this time, the vinylene carbonate content in the electrolyte is too high, the initial film is too thick, and the impedance increases, which affects the interfacial dynamics at room temperature and low temperature, resulting in poor fast charge cycle performance and low-temperature discharge performance during the cycle.

[0123] Comparative Example 2 and Comparative Example 4 are compared with Example 1. When the value of exceeds 10, it shows good low-temperature discharge performance, but the fast charge cycle capacity at this time decreases significantly, mainly because the A value is too high. At this time, the amount of vinylene carbonate in the electrolyte is small, and it is impossible to ensure the formation of a stable SEI film.

[0124] A comparison between Example 1 and Examples 2 to 4 shows that a too high compaction density of the positive electrode will deteriorate the fast charge cycle performance. This is mainly because the too high compaction density leads to a decrease in the porosity of the electrode, making it difficult for the electrode to absorb liquid, increasing the difficulty of secondary deintercalation, and deteriorating the fast charge cycle performance and low-temperature discharge performance of the secondary battery.

[0125] Comparison between Example 1 and Examples 8 to 10 reveals that, within the range of the formula, reasonably increasing the liquid retention coefficient is beneficial to improving the charge and cycle performance of the secondary battery.

[0126] By comparing Example 1 with Examples 5 to 7, it is found that by reasonably adjusting the solvent ratio, increasing the conductivity of the electrolyte, and reducing the viscosity of the electrolyte, it is beneficial to improve the room temperature fast charging cycle performance.

[0127] Example 1 and Comparative Example 3 found that when the content of the additives lithium fluorosulfonate and tris(trimethylsilyl)phosphate was low, the secondary battery did not have a good fast charge cycle capacity retention rate and low-temperature discharge capacity retention rate, indicating that the additives lithium fluorosulfonate and tris(trimethylsilyl)phosphate have a great correlation in improving the fast charge cycle and low-temperature cycle performance.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the electrolyte includes an organic solvent, a lithium salt and an additive, and the additive includes a fluorine-containing additive and a cyclic carbonate additive; The secondary battery satisfies the following relationship: Wherein, Me.g / Ah represents the electrolyte retention coefficient of the secondary battery; A represents the mass ratio of the fluorine-containing additive to the cyclic carbonate additive in the electrolyte; σmS / cm represents the conductivity of the electrolyte; α represents the porosity of the positive electrode sheet; ηmPa·s represents the viscosity of the electrolyte at 25°C; PD g / cm 3 It represents the compaction density of the positive electrode active material layer.

2. The secondary battery according to claim 1, wherein The electrolyte satisfies at least one of the following characteristics: (1) The electrolyte has a liquid retention coefficient Me.g / Ah of 2 to 5 g / Ah; (2) The conductivity σmS / cm of the electrolyte is 7 to 13 mS / cm; (3) The viscosity of the electrolyte at 25° C. is 3 to 5 mPa·s; (4) The mass ratio A of the fluorine-containing additive to the cyclic carbonate additive in the electrolyte is 0.1 to 20.

3. The secondary battery according to claim 1, wherein Satisfy at least one of the following conditions: (1) The compaction density of the positive electrode active material layer is PD g / cm 3 3~4.2g / cm 3 ; (2) The porosity α of the positive electrode sheet is 0.1 to 0.

5.

4. The secondary battery according to claim 1, wherein The electrolyte satisfies at least one of the following characteristics: (1) The lithium salt includes at least one of lithium hexafluorophosphate and lithium bis(oxalatoborate); (2) The fluorine-containing additive includes at least one of lithium fluorosulfonate, lithium difluorophosphate, and lithium difluorooxalate borate; (3) The cyclic carbonate additive includes at least one of vinylene carbonate and vinyl ethylene carbonate. A sort of.

5. The secondary battery according to any one of claims 1 to 4, wherein: Based on the mass of the electrolyte, the amount of the additive added satisfies at least one of the following conditions: (1) The mass fraction of the fluorine-containing additive is 0.2 to 2.0%; (2) The mass fraction of the cyclic carbonate additive is 0.05-3%.

6. The secondary battery according to any one of claims 1 to 4, wherein: The additive further includes a silicon-containing additive, and the secondary battery satisfies the following relationship: In the formula, A' represents the ratio of the total mass of the fluorine-containing additive and the silicon-containing additive in the electrolyte to the mass of the cyclic carbonate additive.

7. The secondary battery according to claim 6, wherein The silicon-containing additive includes at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(trimethylsilyl)borate, trimethylfluorosilane, and heptamethyldisilazane; And / or, based on the mass of the electrolyte, the mass fraction of the silicon-containing additive is 0.1-2%.

8. The secondary battery according to any one of claims 1 to 4, wherein: The electrolyte also includes a sulfur-containing additive with a mass fraction of 0.05 to 3%; the sulfur-containing additive includes at least one of 1,3-propane sultone, vinyl sulfate, butylene sulfite, and methylene methane disulfonate.

9. The secondary battery according to any one of claims 1 to 4, wherein: In the positive electrode active material layer, the positive electrode active material includes a molecular formula of Li x Ni a Co b M c Z 1-a-b-c O2 compound, 0.95≤x≤1.2, 0<a<1, 0<b<1, 0<c<1, a+b+c≤1; in the molecular formula, M includes one or two of Mn and Al; Z includes at least one of Fe, Zr, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti.

10. The secondary battery according to any one of claims 1 to 4, wherein: The secondary battery satisfies the following relationship:

11. The secondary battery according to claim 6, wherein A'=0.17-40; and / or, the secondary battery satisfies the following relationship:

12. The secondary battery according to any one of claims 1 to 4, wherein: The organic solvent in the electrolyte includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, diphenyl carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, ethyl fluoroacetate, fluoroether, γ-butyrolactone, acetonitrile, and cyclopentane sulfone.

13. An electrical device comprising the secondary battery according to any one of claims 1 to 12.

Images