Electrolyte for lithium secondary battery, secondary battery, and electric device

Abstract

Disclosed is a lithium secondary battery electrolyte, a secondary battery, and an electric device, the lithium secondary battery electrolyte including a sulfuric acid ester and a fluorosulfate ion, the molar ratio of the sulfuric acid ester to the fluorosulfate ion being (8-223): 1.

Description

Technical Field

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

[0002] Secondary batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, but also widely used in electric vehicles such as electric bicycles, electric motorcycles and electric cars, as well as in military equipment and aerospace and other fields.

[0003] However, existing secondary batteries experience a significant decrease in storage and cycle performance after multiple cycles, and their DC internal resistance also increases significantly. Summary of the Invention

[0004] In view of the technical problems existing in the background art, this application provides an electrolyte for lithium secondary batteries, which aims to solve the problems that the storage performance and cycle performance of secondary batteries containing the electrolyte are significantly reduced and the DC internal resistance is significantly increased after multiple cycles.

[0005] To achieve the above objective, the electrolyte for the lithium secondary battery comprises sulfate ester and fluorosulfonate ions, wherein the molar ratio of sulfate ester to fluorosulfonate ions is (8-223):1.

[0006] This application includes at least the following beneficial effects: the electrolyte for lithium secondary batteries includes sulfate esters and fluorosulfonate ions, and the electrolyte for lithium secondary batteries includes sulfate esters and fluorosulfonate ions in the above-mentioned proportions. The sulfate esters and fluorosulfonate ions jointly participate in the formation of the positive and negative electrode interface films, forming a dense composite film, which can improve the thermal stability and mechanical stability of the interface film and significantly improve the cycle life of the secondary battery; in addition, the formed interface film has strong ionic conductivity, which reduces the DC internal resistance of the secondary battery. Thus, the simultaneous use of sulfate esters and fluorosulfonate ions can reduce the DC internal resistance of the secondary battery and improve the storage performance and cycle performance of the secondary battery.

[0007] In some embodiments of this application, the molar ratio of the sulfate ester to the fluorosulfonate ion in the electrolyte for lithium secondary batteries is (10-60):1, optionally (16.7-50):1. This reduces the DC internal resistance of the secondary battery and improves its storage and cycle performance.

[0008] In some embodiments of this application, the molar concentration of sulfate ester in the electrolyte for lithium secondary batteries is 0.08 mol / L-0.2 mol / L, optionally 0.1 mol / L-0.15 mol / L. Therefore, when the molar concentration of sulfate ester in the electrolyte for lithium secondary batteries is within the above range, the DC internal resistance of the secondary battery can be reduced, and the storage performance and cycle performance of the secondary battery can be improved.

[0009] In some embodiments of this application, the molar concentration of fluorosulfonate ions in the electrolyte for lithium secondary batteries is 0.0009 mol / L-0.009 mol / L, optionally 0.003 mol / L-0.006 mol / L. Therefore, having a molar concentration of fluorosulfonate ions in the electrolyte for lithium secondary batteries within the above range can reduce the DC internal resistance of the secondary battery and improve its storage and cycle performance.

[0010] In some embodiments of this application, the sulfate ester includes at least one selected from vinyl sulfate, 4-methyl vinyl sulfate, 4-fluorovinyl sulfate, 4-n-propyl vinyl sulfate, and 4,4'-divinyl sulfate.

[0011] In some embodiments of this application, the fluorosulfonate ion is ASO3F, where A includes at least one of Li, Na, K, and H.

[0012] In some embodiments of this application, the electrolyte for lithium secondary batteries further includes a positive electrode film-forming additive. Therefore, the positive electrode film-forming additive can participate in the formation of an interfacial film on the surface of the positive electrode of the secondary battery, improving the storage performance and cycle performance of the secondary battery.

[0013] In some embodiments of this application, the positive electrode film-forming additive includes at least one of fluoroethylene carbonate and 1,3-propanesulfonate lactone. Therefore, by using at least one of the above substances, the positive electrode film-forming additive can participate in the film formation on the positive electrode surface of the secondary battery, thereby improving the storage performance and cycle performance of the secondary battery.

[0014] In some embodiments of this application, the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of sulfate ester and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.05-0.37):1, optionally (0.06-0.3):1. Therefore, when the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of sulfate ester and fluorosulfonate ions in the electrolyte for lithium secondary batteries is within the above range, it can participate in the formation of the interface film on the positive electrode surface of the secondary battery, thereby improving the storage performance and cycle performance of the secondary battery.

[0015] In some embodiments of this application, the molar concentration of the positive electrode film-forming additive in the electrolyte for lithium secondary batteries is 0.01 mol / L to 0.03 mol / L. Therefore, when the concentration of the positive electrode film-forming additive in the electrolyte for lithium secondary batteries is within the above range, it can participate in the formation of the interface film on the surface of the positive electrode of the secondary battery, thereby improving the storage performance and cycle performance of the secondary battery.

[0016] In some embodiments of this application, the electrolyte for lithium secondary batteries further includes difluorophosphate ions. This alters the solvation structure of the electrolyte, allowing more alkali metal ions to participate in film formation and improving the ionic conductivity of the interfacial film. Furthermore, the difluorophosphate ions, along with fluorosulfonate ions and sulfate esters, participate in the formation of the positive and negative electrode interfacial films, enhancing the thermal stability of the interfacial films, reducing the DC internal resistance of the secondary battery, and improving the storage and cycle performance of the secondary battery.

[0017] In some embodiments of this application, the molar ratio of the difluorophosphate ions to the sum of the molar ratios of the sulfate and fluorosulfonate ions is (0.048-1):1, optionally (0.13-0.58):1. Therefore, when the molar ratio of the difluorophosphate ions to the sum of the molar ratios of the sulfate and fluorosulfonate ions in the electrolyte for lithium secondary batteries is within the above range, the DC internal resistance of the secondary battery can be reduced, and the storage performance and cycle performance of the secondary battery can be improved.

[0018] In some embodiments of this application, the molar concentration of difluorophosphate ions is 0.01 mol / L-0.08 mol / L, optionally 0.02 mol / L-0.06 mol / L. Therefore, when the molar concentration of difluorophosphate ions in the electrolyte for lithium secondary batteries is within the above range, the DC internal resistance of the secondary battery can be reduced, and the storage performance and cycle performance of the secondary battery can be improved.

[0019] In some embodiments of this application, the difluorophosphate ion is taken from the following substance: MPO2F2, wherein M includes at least one of Li, Na, K and H.

[0020] In some embodiments of this application, the electrolyte for lithium secondary batteries further includes tetrafluoroborate ions. Tetrafluoroborate ions are compatible with the electrolyte system for lithium secondary batteries and can participate in the formation of the negative electrode interface film of the secondary battery, opening ion channels and reducing the DC internal resistance of the secondary battery.

[0021] In some embodiments of this application, the molar ratio of tetrafluoroborate ions to the sum of the molar ratios of sulfate and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.0024-0.012):1, optionally (0.0038-0.008):1. Therefore, when the molar ratio of tetrafluoroborate ions to the sum of the molar ratios of sulfate and fluorosulfonate ions in the electrolyte for lithium secondary batteries is within the above range, it can participate in the formation of the negative electrode interface film of the secondary battery, thereby reducing the DC internal resistance of the secondary battery.

[0022] In some embodiments of this application, the molar concentration of tetrafluoroborate ions in the electrolyte for lithium secondary batteries is 0.0005 mol / L-0.001 mol / L, optionally 0.0006 mol / L-0.0008 mol / L. Therefore, when the molar concentration of tetrafluoroborate ions in the electrolyte for lithium secondary batteries is within the above range, it can participate in the formation of the negative electrode interface film of the secondary battery, thereby reducing the DC internal resistance of the secondary battery.

[0023] In some embodiments of this application, the tetrafluoroborate ion is provided by at least one of the following substances: tetrafluoroborate, sodium tetrafluoroborate, potassium tetrafluoroborate, and lithium tetrafluoroborate.

[0024] In some embodiments of this application, the water content in the electrolyte for lithium secondary batteries is less than or equal to 20 ppm. Therefore, having a water content within this range in the electrolyte for lithium secondary batteries can reduce the damage of water to the interfacial film, especially at high voltages, thus mitigating the damage and improving the storage and cycle performance of the secondary battery.

[0025] In some embodiments of this application, the HF content in the electrolyte for lithium secondary batteries is less than or equal to 150 ppm. Therefore, having the HF content in the electrolyte for lithium secondary batteries within this range can reduce the damage of HF to the interfacial film, especially at high voltages, thus mitigating the damage and improving the storage and cycle performance of the secondary battery.

[0026] A second aspect of this application provides a secondary battery comprising the electrolyte for lithium secondary batteries described in the first aspect of this application. Consequently, this secondary battery exhibits low DC internal resistance and excellent storage and cycle performance.

[0027] In some embodiments of this application, the secondary battery includes a positive electrode active material, which includes Li x Ni (1-y-z) Co y M z O 2-bM includes at least one of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, Nb and Zr, 0 < x ≤ 1.2, 0.05 ≤ y ≤ 0.8, 0.01 ≤ z ≤ 0.5, and 0 ≤ b ≤ 0.2.

[0028] In some embodiments of this application, the BET specific surface area of ​​the positive electrode active material is less than or equal to 3m². 2 / g, optionally, the BET specific surface area of ​​the positive electrode active material is 1m². 2 / g-2.5m 2 / g.

[0029] A third aspect of this application provides an electrical device that includes the secondary battery described in the second aspect.

[0030] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0031] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0032] Figure 1 This is a schematic diagram of the structure of a secondary battery according to one embodiment of this application;

[0033] Figure 2 This is a schematic diagram of the structure of a battery module according to one embodiment of this application;

[0034] Figure 3 This is a schematic diagram of the structure of a battery pack according to one embodiment of this application;

[0035] Figure 4 yes Figure 3 Exploded view;

[0036] Figure 5 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source.

[0037] Explanation of reference numerals in the attached figures:

[0038] 1: Secondary battery; 2: Battery module; 3: Battery pack; 4: Upper casing; 5: Lower casing. Detailed Implementation

[0039] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0040] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0041] For the sake of brevity, this article only discloses some specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0042] In the description of the embodiments in 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, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0044] Secondary batteries are not only used in energy storage systems for hydropower, thermal power, wind power, and solar power plants, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of applications for secondary batteries, the market demand for them is also constantly increasing.

[0045] During the initial charge and discharge of a secondary battery, the electrode material (such as the negative electrode active material) reacts with the electrolyte in the lithium secondary battery at the solid-liquid interface, forming a passivation layer covering the surface of the electrode material. This passivation layer is an interfacial film that exhibits the characteristics of a solid electrolyte. It is an electronic insulator but an excellent conductor of alkali metal ions. Alkali metal ions can freely intercalate and deintercalate through this passivation layer; therefore, this passivation film is called the positive and negative electrode interfacial film.

[0046] However, existing secondary batteries have the following problems: On the one hand, with the charge and discharge cycles of the secondary battery, the interfacial film at the positive and negative electrode interfaces becomes unstable and easily damaged, and the transport resistance of alkali metal ions also increases, resulting in an increase in the DC impedance of the secondary battery and a deterioration in storage and cycle performance; on the other hand, during the charge and discharge of the secondary battery, under high temperature and high pressure, the positive electrode active material has strong oxidizing properties, which will oxidize the electrolyte used in the lithium secondary battery and generate a large amount of gas, causing the loss of the electrolyte and positive electrode active material used in the lithium secondary battery, affecting the storage and cycle performance of the secondary battery.

[0047] In this application, sulfate esters and fluorosulfonate ions jointly participate in the formation of the positive and negative electrode interface film, forming a dense composite film. This improves the thermal and mechanical stability of the interface film, significantly enhancing the cycle life of the secondary battery. Furthermore, the formed interface film exhibits strong ionic conductivity, reducing the DC internal resistance of the secondary battery. Thus, the simultaneous use of sulfate esters and fluorosulfonate ions can reduce the DC internal resistance of the secondary battery and improve its storage and cycle performance. On the other hand, the interface film possesses thermal and mechanical stability, reducing the probability of interface film rupture. This, in turn, reduces the reaction between the positive electrode active material and the electrolyte used in the lithium secondary battery, suppresses gas generation in the secondary battery, reduces the loss of the electrolyte and positive electrode active material, and improves the storage and cycle performance of the secondary battery.

[0048] The electrolyte for lithium secondary batteries disclosed in this application is applicable to secondary batteries, and the secondary batteries disclosed in this application can be used in electrical devices that use secondary batteries as a power source or in various energy storage systems that use secondary batteries as energy storage elements. Electrical devices may include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys may include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft may include airplanes, rockets, space shuttles, and spacecraft, etc.

[0049] The first aspect of this application discloses an electrolyte for lithium secondary batteries, wherein the electrolyte comprises sulfate and fluorosulfonate ions, and the molar ratio of sulfate to fluorosulfonate ions is (8-223):1.

[0050] This application includes at least the following beneficial effects: the electrolyte for lithium secondary batteries includes sulfate esters and fluorosulfonate ions, and the electrolyte for lithium secondary batteries includes sulfate esters and fluorosulfonate ions in the above-mentioned proportions. The two together participate in the formation of the positive and negative electrode interface film, forming a composite film, which improves the mechanical and thermal stability of the interface film; it can also improve the ionic conductivity of the interface film. Thus, the simultaneous use of sulfate esters and fluorosulfonate ions can reduce the DC internal resistance of the secondary battery and improve the storage performance and cycle performance of the secondary battery.

[0051] DC internal resistance (DCR) refers to the resistance encountered by current flowing through the battery cell. After the discharge process ends, the voltage of a secondary battery will rebound due to polarization. DC impedance technology uses the voltage difference between the moment before the end of the discharge and the voltage after the discharge stabilizes during the intermittent discharge process to calculate the internal resistance of the secondary battery.

[0052] In some embodiments of this application, the sulfate ester may include at least one selected from vinyl sulfate, 4-methyl vinyl sulfate, 4-fluorovinyl sulfate, 4-n-propyl vinyl sulfate, and 4,4'-divinyl sulfate. Therefore, by using the above-mentioned sulfate esters in the electrolyte for lithium secondary batteries of this application, the sulfate ester can participate in the formation of the positive and negative electrode interface film, resulting in a flexible interface film that can adapt to the expansion and contraction of the secondary battery electrodes during cycling, improving the mechanical stability of the interface film, and enhancing its ion conductivity, thereby improving the cycle performance of the secondary battery and reducing its DC internal resistance. In other embodiments of this application, the sulfate ester includes 4,4'-divinyl sulfate.

[0053] In some embodiments of this application, the fluorosulfonate ions are derived from ASO3F, wherein A includes at least one of Li, Na, K, and H. Specifically, the aforementioned substance containing fluorosulfonate ions can provide fluorosulfonate ions, and the interface formed by the fluorosulfonate ions is very dense and has good thermal stability, thereby improving the storage performance of the secondary battery while reducing its DC resistance.

[0054] In some embodiments of this application, the molar ratio of sulfate to fluorosulfonate ions can be (8.9-222):1, (10-200):1, (20-190):1, (30-180):1, (40-170):1, (50-160):1, (60-150):1, (70-140):1, (80-130):1, (90-120):1, (100-110):1, etc. In other embodiments of this application, in lithium secondary batteries... In the electrolyte, the molar ratio of sulfate to fluorosulfonate ions is (10-60):1, for example (16.7-50):1. Specifically, when the molar ratio of sulfate to fluorosulfonate ions is within the above range, it can reduce the decrease in the thermal stability of the interfacial film caused by excessive sulfate, thereby improving the storage performance of the secondary battery. It can also reduce the defects of insufficient interfacial film formation and poor ion conductivity caused by insufficient sulfate, thereby reducing the DC internal resistance of the secondary battery and improving the storage performance and cycle performance of the secondary battery.

[0055] In some embodiments of this application, the molar concentration of sulfate in the electrolyte for lithium secondary batteries is 0.08 mol / L-0.2 mol / L. For example, the molar concentration of sulfate in the electrolyte for lithium secondary batteries can be 0.04 mol / L-0.18 mol / L, 0.09 mol / L-0.17 mol / L, 0.1 mol / L-0.16 mol / L, 0.11 mol / L-0.15 mol / L, 0.12 mol / L-0.14 mol / L, etc. Therefore, when the molar concentration of sulfate in the electrolyte for lithium secondary batteries is within the above range, the decrease in the thermal stability of the interfacial film caused by excessive sulfate can be reduced, thereby improving the storage performance of the secondary battery. It can also reduce the defects of insufficient interfacial film formation and poor ion conductivity caused by insufficient sulfate, thereby reducing the DC internal resistance of the secondary battery and improving the storage performance and cycle performance of the secondary battery. In some other embodiments of this application, the molar concentration of the sulfate ester in the electrolyte for lithium secondary batteries is 0.1 mol / L to 0.15 mol / L.

[0056] In some embodiments of this application, the molar concentration of fluorosulfonate ions in the electrolyte for lithium secondary batteries is 0.0009 mol / L-0.009 mol / L. For example, the molar concentration of fluorosulfonate ions in the electrolyte for lithium secondary batteries can be 0.001 mol / L-0.009 mol / L, 0.002 mol / L-0.008 mol / L, 0.003 mol / L-0.007 mol / L, 0.004 mol / L-0.006 mol / L, etc. Specifically, having a molar concentration of fluorosulfonate ions in the electrolyte for lithium secondary batteries within the above range can reduce the problem of increased viscosity of the electrolyte for lithium secondary batteries caused by the addition of substances containing fluorosulfonate ions, thereby reducing the DC internal resistance of the secondary battery, while also improving the thermal stability of the interface film and enhancing the storage performance of the secondary battery. In some other embodiments of this application, the molar concentration of fluorosulfonate ions in the electrolyte for lithium secondary batteries is 0.003 mol / L to 0.006 mol / L.

[0057] In some embodiments of this application, the electrolyte for lithium secondary batteries further includes a positive electrode film-forming additive. Specifically, the positive electrode film-forming additive can participate in the formation of a stable positive electrode interface film, reduce the DC impedance of the secondary battery, and improve the cycle performance of the secondary battery.

[0058] In some embodiments of this application, the positive electrode film-forming additive includes at least one of fluoroethylene carbonate and 1,3-propanesulfonate lactone. Specifically, the positive electrode interface film formed by fluoroethylene carbonate and 1,3-propanesulfonate lactone exhibits excellent performance, forming a tight interface film without increasing impedance. This interface film can prevent the decomposition of the electrolyte in lithium secondary batteries, reduce the DC internal resistance of the secondary battery, and improve the storage and cycle performance of the secondary battery.

[0059] In some embodiments of this application, the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of the sulfate ester and the fluorosulfonate ion in the electrolyte for lithium secondary batteries is (0.05-0.37):1. For example, the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of the sulfate ester and the fluorosulfonate ion can be (0.05-0.36):1, (0.08-0.35):1, (0.1-0.33):1, (0.12-0.3):1, (0.15-0.29):1, (0.13-0.25):1, (0.15-0.22):1, (0.17-0.2):1, etc. Therefore, when the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of sulfate ester and fluorosulfonate ions in the electrolyte for lithium secondary batteries is within the aforementioned range, it reduces the impact of excessive positive electrode film-forming additives on the function of sulfate ester and fluorosulfonate ions, thus preventing an increase in the DC resistance of the secondary battery. It also reduces the impact of insufficient positive electrode film-forming additives, which would prevent them from participating in the formation of the positive electrode interface film. This reduces the DC internal resistance of the secondary battery and improves its storage and cycle performance. In some other embodiments of this application, the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of sulfate ester and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.06-0.3):1.

[0060] In some embodiments of this application, the molar concentration of the positive electrode film-forming additive in the electrolyte for lithium secondary batteries is 0.01 mol / L-0.03 mol / L. For example, the molar concentration of the positive electrode film-forming additive in the electrolyte for lithium secondary batteries can be 0.01 mol / L-0.025 mol / L, 0.013 mol / L-0.024 mol / L, 0.015 mol / L-0.023 mol / L, 0.018 mol / L-0.02 mol / L, etc. Therefore, the concentration of the positive electrode film-forming additive in the electrolyte for lithium secondary batteries within the above range reduces both the increase in DC resistance of the secondary battery caused by excessive amounts of the positive electrode film-forming additive and the inability to participate in the formation of the positive electrode interface film due to insufficient dosage. This reduces the DC internal resistance of the secondary battery and improves its storage and cycle performance.

[0061] In some embodiments of this application, the electrolyte for lithium secondary batteries further includes difluorophosphate ions. Difluorophosphate ions can alter the solvation structure of the electrolyte for lithium secondary batteries, allowing more alkali metal ions to participate in film formation, improving the ionic conductivity of the interfacial film, and reducing the transport resistance of alkali metal ions. Furthermore, difluorophosphate ions can also participate in the formation of the positive and negative electrode interfacial films, enhancing the thermal stability of the interfacial films, thereby improving the storage performance of the secondary battery while reducing its DC impedance.

[0062] In some embodiments of this application, the ratio of the molar amount of difluorophosphate ions to the sum of the molar amounts of sulfate ions and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.048-1):1. For example, the ratio of the molar amount of difluorophosphate ions to the sum of the molar amounts of sulfate ions and fluorosulfonate ions in the electrolyte for lithium secondary batteries can be (0.048-0.95):1, (0.05-0.9):1, (0.1-0.8):1, (0.2-0.7):1, (0.3-0.6):1, (0.4-0.5):1, etc. Specifically, when the molar ratio of difluorophosphate ions to the sum of the molar ratios of sulfate ions and fluorosulfonate ions in the electrolyte for lithium secondary batteries is within the aforementioned range, the solvation structure of the electrolyte for lithium secondary batteries can be altered, allowing more alkali metal ions to participate in film formation, improving the ionic conductivity of the interfacial film, and reducing the transport resistance of alkali metal ions. Furthermore, difluorophosphate ions can also participate in the formation of the positive and negative electrode interfacial films, enhancing the thermal stability of the interfacial films, thereby improving the storage performance of the secondary battery while reducing its DC impedance. In some other embodiments of this application, the molar ratio of difluorophosphate ions to the sum of the molar ratios of sulfate ions and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.13-0.58):1.

[0063] In some embodiments of this application, the molar concentration of difluorophosphate ions in the electrolyte for lithium secondary batteries is 0.01 mol / L-0.08 mol / L. For example, the molar concentration of difluorophosphate ions in the electrolyte for lithium secondary batteries is 0.01 mol / L-0.07 mol / L, 0.02 mol / L-0.06 mol / L, 0.03 mol / L-0.05 mol / L, 0.04 mol / L-0.05 mol / L, etc. Specifically, the concentration of difluorophosphate ions in the electrolyte for lithium secondary batteries within the above range can reduce the impact of excessive addition of difluorophosphate ions on the function of fluorosulfonate ions and sulfate esters, while ensuring sufficient concentration of difluorophosphate ions to function effectively. This can reduce the DC internal resistance of the secondary battery and improve its storage and cycle performance. In other embodiments of this application, the molar concentration of difluorophosphate ions in the electrolyte for lithium secondary batteries is 0.02 mol / L-0.06 mol / L.

[0064] In some embodiments of this application, the difluorophosphate ions are derived from MPO2F2, wherein M includes at least one of Li, Na, K, and H. Specifically, the aforementioned substance containing difluorophosphate ions can provide difluorophosphate ions, altering the solvation structure of the electrolyte for lithium secondary batteries, allowing more alkali metal ions to participate in film formation, improving the ionic conductivity of the interfacial film, and reducing the transport resistance of alkali metal ions. Furthermore, difluorophosphate ions can also participate in the formation of the positive and negative electrode interfacial films, enhancing the thermal stability of the interfacial films, thereby improving the storage performance of the secondary battery while reducing its DC resistance.

[0065] In some embodiments of this application, the electrolyte for lithium secondary batteries further includes tetrafluoroborate ions. Tetrafluoroborate ions are compatible with the electrolyte system for lithium secondary batteries and can participate in the formation of the negative electrode interface film of the secondary battery, opening ion channels and reducing the DC internal resistance of the secondary battery.

[0066] In some embodiments of this application, the substance containing the tetrafluoroborate ion may be at least one of tetrafluoroborate, sodium tetrafluoroborate, potassium tetrafluoroborate, and lithium tetrafluoroborate.

[0067] In some embodiments of this application, the ratio of the molar amount of tetrafluoroborate ions to the sum of the molar amounts of sulfate ions and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.0024-0.012):1. For example, the ratio of the molar amount of tetrafluoroborate ions to the sum of the molar amounts of sulfate ions and fluorosulfonate ions can be (0.0024-0.011):1, (0.003-0.01):1, (0.004-0.009):1, (0.005-0.008):1, (0.006-0.007):1, etc. Therefore, when the molar ratio of tetrafluoroborate ions to the sum of the molar ratios of sulfate and fluorosulfonate ions in the electrolyte for lithium secondary batteries is within the aforementioned range, it reduces the impact of excessive tetrafluoroborate ions on the function of sulfate and fluorosulfonate ions, thus preventing an increase in the DC resistance of the secondary battery. Conversely, it reduces the inability of tetrafluoroborate ions to participate in the formation of the negative electrode interface film due to insufficient tetrafluoroborate ions. This reduces the DC internal resistance of the secondary battery and improves its storage and cycle performance. In some other embodiments of this application, the molar ratio of tetrafluoroborate ions to the sum of the molar ratios of sulfate and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.0038-0.008):1.

[0068] In some embodiments of this application, the molar concentration of tetrafluoroborate ions in the electrolyte for lithium secondary batteries is 0.0005 mol / L to 0.001 mol / L. For example, the molar concentration of tetrafluoroborate ions in the electrolyte for lithium secondary batteries can be 0.0005 mol / L to 0.0009 mol / L, 0.0006 mol / L to 0.0008 mol / L, 0.0007 mol / L to 0.0008 mol / L, etc. Therefore, by maintaining the molar concentration of tetrafluoroborate ions in the electrolyte for lithium secondary batteries within the above range, the deterioration of the DC resistance of the secondary battery caused by excessive tetrafluoroborate ions is reduced. Simultaneously, the tetrafluoroborate ions can participate in the formation of the negative electrode interface film, thereby reducing the DC internal resistance of the secondary battery and improving its storage and cycle performance. In some other embodiments of this application, the molar concentration of tetrafluoroborate ions in the electrolyte for lithium secondary batteries is 0.0006 mol / L to 0.0008 mol / L.

[0069] In some embodiments of this application, the water content in the electrolyte for lithium secondary batteries is less than or equal to 20 ppm. For example, the water content in the electrolyte for lithium secondary batteries can be 1 ppm-19 ppm, 2 ppm-18 ppm, 3 ppm-17 ppm, 4 ppm-16 ppm, 5 ppm-15 ppm, 6 ppm-14 ppm, 7 ppm-13 ppm, 8 ppm-12 ppm, 9 ppm-11 ppm, etc. Therefore, having a water content in the electrolyte for lithium secondary batteries within the above range can reduce the damage of water to the interface film of the secondary battery, especially at high voltages. This reduces the DC internal resistance of the secondary battery and improves its storage and cycle performance.

[0070] In some embodiments of this application, the HF content in the electrolyte for lithium secondary batteries is less than or equal to 150 ppm. For example, the HF content in the electrolyte for lithium secondary batteries can be 10 ppm-140 ppm, 20 ppm-130 ppm, 30 ppm-120 ppm, 40 ppm-110 ppm, 50 ppm-100 ppm, 60 ppm-90 ppm, 70 ppm-80 ppm, etc. Therefore, having the HF content in the electrolyte for lithium secondary batteries within the above range can reduce the damage of HF to the secondary battery interface film, especially at high voltages. This reduces the DC internal resistance of the secondary battery and improves its storage and cycle performance.

[0071] Understandably, the water and HF content in the electrolyte for lithium secondary batteries can be controlled by using molecular sieves to remove water and HF. Specifically, molecular sieves can be added to the electrolyte for lithium secondary batteries to ensure full contact between the molecular sieves and the electrolyte, adsorbing the water and HF in the electrolyte. Then, the molecular sieves and the electrolyte can be separated by filtration, centrifugation, or other methods.

[0072] Understandably, the water and HF content in the electrolyte for lithium secondary batteries can be measured using the following methods:

[0073] Water content test: The electrolyte from a lithium-ion secondary battery is injected into an equilibrium electrolytic cell. Upon detection of H₂O by the indicator electrode, electrode oxidation occurs. - The water content is calculated by the quantitative chemical reaction between I2 and H2O, according to HG / T4067-2015 hexafluorophosphate electrolyte 5.9 Determination of water content; the quantitative reaction formula of I2 and H2O is: H2O+I2+SO2+3C5H5N=2C5H5N·HI+C5H5N·SO3; the water content is calculated by the amount of iodine consumed.

[0074] Hydrofluoric acid content test: Under dry conditions, the free acid in the electrolyte is titrated with triethylamine standard solution and calculated as HF content;

[0075] HG / T4067-2015 Lithium hexafluorophosphate electrolyte 5.10 Determination of free acid content;

[0076] HF(ppm)=(V2-V1) / 1000×C×20 / m×1000000=(V2-V1)×C / m×20000;

[0077] Where: 0.01—concentration of the standard solution, 0.02 mol / L;

[0078] V1—The volume reading of the burette before the start of titration, in ml;

[0079] V2—The volume reading of the burette, in ml, when the electrolyte sample is added and titration reaches the endpoint;

[0080] 20 — Molar molecular weight of HF, g / mol;

[0081] m — the amount of electrolyte weighed, in grams;

[0082] 20000 — the coefficient for converting to ug / g.

[0083] In some embodiments of this application, the electrolyte for lithium secondary batteries may further include electrolyte salts and solvents.

[0084] When the secondary battery is a lithium-ion battery, as an example, the electrolyte salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0085] As an example, the solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0086] In some embodiments, the electrolyte for lithium secondary batteries may also include additives. For example, additives may include additives that can improve certain performance characteristics of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high-temperature performance of the secondary battery, and additives that improve the low-temperature performance of the secondary battery.

[0087] A second aspect of this application provides a secondary battery comprising the electrolyte for lithium secondary batteries described in the first aspect of this application. Consequently, this secondary battery exhibits low DC internal resistance, excellent storage performance, and good cycle performance.

[0088] A rechargeable battery is a battery that can be recharged after being discharged to activate its active materials and continue to be used.

[0089] Typically, a secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. During the charging and discharging process, active ions (alkali metal ions) repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, serves as a barrier. The electrolyte, acting as a conductor of ions, lies between the positive and negative electrodes.

[0090] In some embodiments of this application, the secondary battery may be a lithium-ion battery.

[0091] [Positive electrode plate]

[0092] In a secondary battery, the positive electrode typically includes a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes a positive active material.

[0093] The positive electrode current collector can be a conventional metal foil or a composite current collector (a composite current collector can be formed by setting metal material on a polymer substrate). As an example, the positive electrode current collector can be aluminum foil.

[0094] The specific type of positive electrode active material is not limited. Any active material known in the art that can be used as the positive electrode of a secondary battery can be used. Those skilled in the art can select according to actual needs.

[0095] For example, when the secondary battery is a lithium-ion battery, the positive electrode active material may include, but is not limited to, at least one of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, at least one of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their modified compounds. All of these materials are commercially available.

[0096] In some embodiments of this application, the positive electrode active material includes Li x Ni (1-y-z) Co y M z O 2-b M includes at least one of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, Nb and Zr, 0 < x ≤ 1.2, 0.05 ≤ y ≤ 0.8, 0.01 ≤ z ≤ 0.5, and 0 ≤ b ≤ 0.2.

[0097] For example, x can satisfy: 0.2≤x≤1.2, 0.4≤x≤1.1, 0.5≤x≤1.0, 0.6≤x≤0.9, 0.7≤x≤0.8, etc.; y can satisfy: 0.05≤y≤0.7, 0.1≤y≤0.6, 0.2≤y≤0.5, 0.3≤y≤0.4, etc.; z can satisfy: 0.01≤z≤0.45, 0.1≤z≤0.4, 0.2≤z≤0.3, etc.; b can satisfy: 0≤b≤0.15, 0.02≤b≤0.13, 0.05≤b≤0.1, 0.07≤b≤0.09, etc.

[0098] It should be noted that in positive electrode sheets, secondary batteries, or electrical devices, lithium ions are consumed during the formation and cycling processes of secondary batteries, resulting in a measured lithium content (x) in the positive electrode active material being less than 1. Conversely, if lithium replenishment agents are used on both the positive and negative electrode sheets, the measured lithium content (x) in the positive electrode active material will be greater than 1 after the secondary battery undergoes formation and cycling processes.

[0099] In addition, due to the formation and cycling processes of secondary batteries, oxygen may be released from the lattice of the positive electrode active material, resulting in oxygen loss. Therefore, the oxygen content in the positive electrode active material may be less than 2.

[0100] In some embodiments of this application, the BET specific surface area of ​​the positive electrode active material is less than or equal to 3m². 2 / g, for example, the BET specific surface area of ​​the positive electrode active material can be 0.3m². 2 / g-3m 2 / g, 1m 2 / g-2.5m 2 / g, 1.2m 2 / g-2.3m 2 / g, 1.5m 2 / g-2m 2 Specifically, when the BET specific surface area of ​​the positive electrode active material is within the above-mentioned range, it can reduce the side reactions of the positive electrode active material under high voltage, thereby improving the storage performance and cycle performance of the secondary battery. In some other embodiments of this application, the BET specific surface area of ​​the positive electrode active material is 1m². 2 / g-2.5m 2 / g.

[0101] Understandably, the BET specific surface area of ​​the positive electrode active material can be obtained by the following method: using a US-made multi-station fully automated BET specific surface area and porosity analyzer Gemini VII2390, take about 7g of sample and put it into a 9cc long tube with a bulb, degas at 200℃ for 2h, and then put it into the main unit to test and obtain the BET (specific surface area) data of the positive electrode active material.

[0102] The positive electrode active material layer may also optionally include a binder, a conductive agent, and other optional additives.

[0103] As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, SuperP(SP), graphene, and carbon nanofibers.

[0104] As an example, the adhesive may include at least one of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0105] [Negative electrode plate]

[0106] In a secondary battery, the negative electrode typically includes a negative current collector and a negative active material layer disposed on the negative current collector, wherein the negative active material layer includes a negative active material.

[0107] The negative electrode current collector can be a conventional metal foil or a composite current collector (for example, a metal material can be disposed on a polymer substrate to form a composite current collector). As an example, the negative electrode current collector can be a copper foil.

[0108] The specific type of negative electrode active material is not limited; any active material known in the art that can be used as a negative electrode in secondary batteries can be used. Those skilled in the art can select according to actual needs. As an example, the negative electrode active material may include, but is not limited to, at least one of artificial graphite, natural graphite, hard carbon, soft carbon, silicon-based materials, and tin-based materials. The silicon-based material may include at least one of elemental silicon, silicon oxide compounds (e.g., silicon suboxide), silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may include at least one of elemental tin, tin oxide compounds, and tin alloys. These materials are all commercially available.

[0109] In some embodiments, the negative electrode active material may include a silicon-based material in order to further improve the energy density of the secondary battery.

[0110] The negative electrode active material layer may also optionally include binders, conductive agents, and other optional additives.

[0111] As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0112] As an example, the adhesive may include at least one of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0113] As an example, other optional additives may include thickeners and dispersants (such as sodium carboxymethyl cellulose CMC-Na) and PTC thermistor materials.

[0114] As for the aforementioned separator, this application does not have any particular limitations. Any known porous structure separator with electrochemical and mechanical stability can be selected according to actual needs. For example, it may include a single-layer or multi-layer film containing at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.

[0115] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 1.

[0116] In some embodiments, the secondary battery may include an outer packaging. This outer packaging is used to encapsulate the positive electrode, the negative electrode, and the electrolyte.

[0117] In some embodiments, the outer packaging may include a shell and a cover. The shell may include a base plate and side plates attached to the base plate, the base plate and side plates enclosing a receiving cavity. The shell has an opening communicating with the receiving cavity, and the cover can be placed over the opening to close the receiving cavity.

[0118] The positive electrode, negative electrode, and separator can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is encapsulated within the receiving cavity. The secondary battery can contain one or more electrode assemblies, which can be adjusted according to requirements.

[0119] In some implementations, the outer casing of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell.

[0120] The outer packaging of secondary batteries can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0121] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module.

[0122] Figure 2 This is battery module 2 as an example. (See reference...) Figure 2 In battery module 2, multiple secondary batteries 1 can be arranged sequentially along the length of battery module 2. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 1 can be fixed in place using fasteners.

[0123] The battery module 2 may also include a housing with a receiving space in which multiple secondary batteries 1 are housed. In some embodiments, the battery modules may also be assembled into a battery pack, the number of battery modules contained in the battery pack being adjustable according to the application and capacity of the battery pack.

[0124] Figure 3 and 4 This is battery pack 3 as an example. (See reference...) Figure 3 and 4 The battery pack 3 may include a battery box and multiple battery modules 2 disposed within the battery box. The battery box includes an upper body 4 and a lower body 5, with the upper body 4 covering the lower body 5 to form a closed space for accommodating the battery modules 2. The multiple battery modules 2 can be arranged in any manner within the battery box.

[0125] A third aspect of this application provides an electrical device comprising the secondary battery described in the second aspect. Specifically, the secondary battery can serve as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks), electric trains, ships and satellites, and energy storage systems.

[0126] Figure 5 This is an example of an electrical appliance. The electrical appliance includes pure electric vehicles, hybrid electric vehicles, or plug-in hybrid electric vehicles.

[0127] Another example of a power-consuming device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and may use rechargeable batteries as their power source.

[0128] To make the technical problems, technical solutions, and beneficial effects solved by the embodiments of this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0129] Example 1

[0130] Preparation of secondary batteries containing electrolyte for lithium secondary batteries

[0131] 1. Preparation of electrolyte for lithium secondary batteries

[0132] Ethyl carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a mass ratio of 30:70 to obtain an organic solvent. The fully dried electrolyte salt LiPF6, sulfate, lithium fluorosulfonate, positive electrode film-forming additive, lithium difluorophosphate and lithium tetrafluoroborate are dissolved in the above solvent and mixed evenly to obtain an electrolyte for lithium secondary batteries.

[0133] Molecular sieves are added to the electrolyte for lithium secondary batteries to ensure full contact between the molecular sieves and the electrolyte, thereby adsorbing water and HF from the electrolyte. Then, the mixture is filtered to separate the molecular sieves from the electrolyte, removing water and HF from the electrolyte.

[0134] 2. Preparation of secondary batteries

[0135] Preparation of positive electrode sheet

[0136] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed evenly in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 96.2:2.7:1.1 to obtain a positive electrode slurry. The positive electrode slurry is coated onto the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained through drying, cold pressing, slitting, and cutting processes.

[0137] Preparation of negative electrode sheet

[0138] Artificial graphite (anode active material), carbon black (Super P) (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (mass ratio 96.4:0.7:1.8:1.1) are mixed evenly in an appropriate amount of deionized water to obtain a cathode slurry. The cathode slurry is then coated onto copper foil (anode current collector), and the cathode sheet is obtained through drying, cold pressing, slitting, and cutting processes.

[0139] The separator is made of polyethylene film.

[0140] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up. The electrode assembly is placed in an outer package, and the electrolyte of the lithium secondary battery is injected into the dried secondary battery. After vacuum sealing, settling, formation, and shaping processes, the secondary battery is obtained.

[0141] The secondary batteries of Examples 2-49 and Comparative Examples 1-2 are the same as those of Example 1, except for the different parameters (see Table 1).

[0142] The composition of the gel polymer electrolyte in the secondary batteries of Examples 1-49 and Comparative Examples 1-2 of this application is shown in Table 1.

[0143] Table 1

[0144]

[0145]

[0146]

[0147]

[0148] In Table 1, " / " indicates that the corresponding substance is not added or the amount added is 0.

[0149] Secondary battery performance test:

[0150] 1. Secondary battery capacity and cycle performance

[0151] Taking Example 1 as an example, at 25°C, the secondary battery was charged at a constant current rate of 0.5C to the charging cutoff voltage of 4.2V, then charged at a constant voltage until the current was ≤0.05C, and allowed to stand for 5 minutes. Next, it was discharged at a constant current rate of 0.33C to the discharge cutoff voltage of 2.8V, and allowed to stand for 5 minutes. The discharge capacity C0 of the secondary battery at this point was recorded, which is the initial capacity of the secondary battery. The secondary battery was subjected to 200 charge-discharge cycles using this method, and the discharge capacity of the secondary battery after 200 cycles was recorded as C1, which is the capacity of the secondary battery after 200 cycles.

[0152] Cycle capacity retention of a secondary battery = C1 / C0 × 100%

[0153] The testing procedures for the capacity of the secondary batteries in Examples 2-49 and Comparative Examples 1-2 are the same as above.

[0154] 2. DC internal resistance of secondary battery

[0155] Taking Example 1 as an example, the secondary battery is charged at 25°C with a constant current of 1C to 4.15V, then charged at a constant voltage of 4.15V until the current drops to 0.05C, and then discharged at a constant current of 1C to 2.5V. After resting for 5 minutes (stabilization time), the next cycle is continued. The secondary battery voltage before the discharge stops and the secondary battery voltage after the secondary battery voltage stabilizes are recorded in each cycle. Then, the DC impedance R = ΔU / I is calculated using the following formula (where: ΔU is the voltage difference, R is the DC resistance, and I is the discharge current).

[0156] The rate of increase in the internal resistance of a secondary battery = (Internal resistance of the secondary battery after 200 cycles - Initial internal resistance of the secondary battery) / Initial internal resistance of the secondary battery × 100%

[0157] The testing process for the DC internal resistance of the secondary batteries in Examples 2-49 and Comparative Examples 1-2 is the same as above.

[0158] The performance test results of the secondary batteries in each embodiment and comparative example are shown in Table 2.

[0159] Table 2

[0160]

[0161]

[0162] As shown in Table 2, in Examples 1-49 of this application, sulfate and fluorosulfonate ions are added to the electrolyte for lithium secondary batteries, and the electrolyte includes sulfate and fluorosulfonate ions in the above-mentioned proportions. The simultaneous use of both can reduce the DC internal resistance of the secondary battery and improve its storage and cycle performance. Compared to Examples 1-49, the molar ratio of sulfate to fluorosulfonate ions in Comparative Examples 1 and 2 is outside the range of this application, resulting in significantly reduced storage and cycle performance and significantly increased DC internal resistance in the secondary batteries.

[0163] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. An electrolyte for lithium secondary batteries, characterized in that, The electrolyte for the lithium secondary battery comprises sulfate ester and fluorosulfonate ions, wherein the molar ratio of sulfate ester to fluorosulfonate ions is (10-60):1; and the molar concentration of sulfate ester in the electrolyte for the lithium secondary battery is 0.08mol / L-0.2mol / L. The electrolyte for lithium secondary batteries also includes difluorophosphate ions, and the molar concentration of difluorophosphate ions in the electrolyte for lithium secondary batteries is 0.01 mol / L-0.08 mol / L; The electrolyte for lithium secondary batteries also includes tetrafluoroborate ions, and the ratio of the molar amount of tetrafluoroborate ions to the sum of the molar amounts of sulfate ester and fluorosulfonate ions in the electrolyte for lithium secondary batteries is (0.0024-0.012):

1.

2. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, In the electrolyte for lithium secondary batteries, the molar concentration of the sulfate ester is 0.1 mol / L to 0.15 mol / L.

3. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, In the electrolyte for the lithium secondary battery, the molar concentration of the fluorosulfonate ions is 0.0009 mol / L-0.009 mol / L.

4. The electrolyte for lithium secondary batteries according to claim 3, characterized in that, In the electrolyte for the lithium secondary battery, the molar concentration of the fluorosulfonate ions is 0.003 mol / L to 0.006 mol / L.

5. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, The sulfate ester includes at least one selected from vinyl sulfate, 4-methyl vinyl sulfate, 4-fluorovinyl sulfate, 4-n-propyl vinyl sulfate, and 4,4'-divinyl sulfate.

6. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, The fluorosulfonate ion is made of the following substance: ASO3F, where A includes at least one of Li, Na, K and H.

7. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, The electrolyte for lithium secondary batteries also includes positive electrode film-forming additives.

8. The electrolyte for lithium secondary batteries according to claim 7, characterized in that, The positive electrode film-forming additive includes at least one of fluoroethylene carbonate and 1,3-propanesulfonate lactone.

9. The electrolyte for lithium secondary batteries according to claim 7 or 8, characterized in that, In the electrolyte for lithium secondary batteries, the ratio of the molar amount of the positive electrode film-forming additive to the sum of the molar amounts of the sulfate ester and the fluorosulfonate ion is (0.05-0.37):

1.

10. The electrolyte for lithium secondary batteries according to claim 9, characterized in that, In the electrolyte for lithium secondary batteries, the molar ratio of the positive electrode film-forming additive to the sum of the molar amounts of the sulfate ester and the fluorosulfonate ion is (0.06-0.3):

1.

11. The electrolyte for lithium secondary batteries according to claim 7, characterized in that, In the electrolyte for lithium secondary batteries, the molar concentration of the positive electrode film-forming additive is 0.01 mol / L-0.03 mol / L.

12. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, In the electrolyte for lithium secondary batteries, the molar ratio of the difluorophosphate ion to the sum of the molar amounts of the sulfate ion and the fluorosulfonate ion is (0.048-1):

1.

13. The electrolyte for lithium secondary batteries according to claim 12, characterized in that, In the electrolyte for lithium secondary batteries, the molar ratio of the difluorophosphate ion to the sum of the molar amounts of the sulfate ion and the fluorosulfonate ion is (0.13-0.58):

1.

14. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, In the electrolyte for lithium secondary batteries, the molar concentration of difluorophosphate ions is 0.02 mol / L to 0.06 mol / L.

15. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, The difluorophosphate ion is made of the following substance: MPO2F2, wherein M includes at least one of Li, Na, K and H.

16. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, In the electrolyte for lithium secondary batteries, the molar ratio of the tetrafluoroborate ion to the sum of the molar amounts of the sulfate ester and fluorosulfonate ions is (0.0038-0.008):

1.

17. The electrolyte for lithium secondary batteries according to claim 15 or 16, characterized in that, In the electrolyte for lithium secondary batteries, the molar concentration of tetrafluoroborate ions is 0.0005 mol / L to 0.001 mol / L.

18. The electrolyte for lithium secondary batteries according to claim 17, characterized in that, In the electrolyte for the lithium secondary battery, the molar concentration of tetrafluoroborate ions is 0.0006 mol / L to 0.0008 mol / L.

19. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, The tetrafluoroborate ion is provided by at least one of the following substances: tetrafluoroboric acid, sodium tetrafluoroborate, potassium tetrafluoroborate, and lithium tetrafluoroborate.

20. The electrolyte for lithium secondary batteries according to claim 1, characterized in that, The electrolyte for lithium secondary batteries meets at least one of the following conditions: The water content in the electrolyte for the lithium secondary battery is less than or equal to 20 ppm. The HF content in the electrolyte for the lithium secondary battery is less than or equal to 150 ppm.

21. A secondary battery, characterized in that, The secondary battery includes the electrolyte for lithium secondary batteries according to any one of claims 1-20.

22. The secondary battery according to claim 21, characterized in that, The secondary battery includes a positive electrode active material, which includes Li. x Ni (1-y-z) Co y M z O 2-b M includes at least one of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, Nb and Zr, 0 < x ≤ 1.2, 0.05 ≤ y ≤ 0.8, 0.01 ≤ z ≤ 0.5, and 0 ≤ b ≤ 0.

2.

23. The secondary battery according to claim 22, characterized in that, The BET specific surface area of ​​the positive electrode active material is less than or equal to 3m². 2 / g.

24. The secondary battery according to claim 23, characterized in that, The BET specific surface area of ​​the positive electrode active material is 1m². 2 / g-2.5m 2 / g.

25. An electrical appliance, characterized in that, The secondary battery includes any one of claims 21-24.

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