Electrolyte and lithium iron phosphate battery

Abstract

The application discloses an electrolyte and a lithium iron phosphate battery. The electrolyte is suitable for the lithium iron phosphate battery, and comprises a first additive and a second additive. The first additive comprises a compound with a structure shown in formula (1). The second additive comprises a compound with a structure shown in formula (2). The electrolyte provided by the application contains the first additive and the second additive. Through the synergistic effect of the first additive and the second additive, the low-temperature conductivity of the electrolyte can be improved during the operation of the battery, the polarization voltage at the instant of discharge can be reduced, the composition of the electrode interface film can be regulated, and the diffusion rate of lithium ions can be improved, so that the improvement of the low-temperature discharge capacity retention rate of the battery is promoted.

Description

Technical Field

[0001] This invention relates to the field of electrolyte technology, and more particularly to an electrolyte and a lithium iron phosphate battery. Background Technology

[0002] Lithium-ion batteries, as a high-energy-density chemical power source, have been widely used in portable electronic devices, large-scale energy storage devices, electric vehicles, and other fields in recent years. A lithium-ion battery mainly consists of a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte, primarily composed of electrolyte and organic solvent, is the active component connecting the positive and negative electrodes and is a crucial factor affecting battery performance.

[0003] Lithium iron phosphate batteries have become a widely used power battery technology in electric vehicles and energy storage systems due to their advantages such as high safety, low cost, and long cycle life. However, these batteries generally suffer from problems such as significantly reduced discharge capacity, decreased operating voltage plateau, poor rate performance, accelerated capacity decay, increased electrolyte viscosity, and reduced conductivity in low-temperature environments, which severely limit their applicability in cold regions.

[0004] The aforementioned problems are further exacerbated, particularly in "starved electrolyte" lithium iron phosphate batteries. "Starved electrolyte" refers to a design where the amount of electrolyte in the battery is relatively small. While this design can increase energy density and reduce cost, it leads to limited ion conduction channels and decreased electrolyte distribution uniformity. At low temperatures, the increased viscosity and decreased conductivity of the electrolyte make it even more difficult for the already limited electrolyte to effectively wet the electrodes, resulting in a significant increase in internal resistance and a sharp slowdown in lithium-ion migration rate, thus severely impacting the battery's actual performance.

[0005] Furthermore, in extremely cold environments, such as the frigid regions of northern my country where winter temperatures often drop below -20°C, lithium iron phosphate batteries face even more severe challenges. After a vehicle has been idle for an extended period, the battery temperature becomes extremely low, significantly weakening the reaction kinetics of the active materials. During high-current discharge at startup, severe battery polarization occurs, causing a rapid voltage drop, especially towards the end of the discharge cycle where the protection voltage (e.g., 2.0V) is easily reached prematurely. This leads the system to determine that the battery is depleted, resulting in the vehicle failing to start or experiencing multiple start-up failures. Simultaneously, frequent high-current discharges in low-temperature environments accelerate battery aging, causing irreversible capacity loss.

[0006] Therefore, in response to the dual technical challenges of low-temperature conditions and cold environments, the development of lithium iron phosphate batteries with good low-temperature adaptability has become an urgent need for the industry. Summary of the Invention

[0007] In view of this, the main objective of the present invention is to provide an electrolyte comprising a first additive and a second additive, which can significantly improve the low-temperature conductivity and low-temperature discharge capacity retention of lithium iron phosphate batteries, thereby at least partially solving the above-mentioned technical problems.

[0008] To achieve the above objectives, a first aspect of the present invention provides an electrolyte suitable for lithium iron phosphate batteries, the electrolyte comprising a first additive and a second additive; the first additive comprising a compound having the structural formula shown in formula (1):

[0009]

[0010] In formula (1), X1 is independently selected from halogens, and X2 is independently selected from at least one of naphthyl and substituted phenyl groups, wherein the substituents in the substituted phenyl group are selected from amino and oxysulfonyl halogen groups;

[0011] The second additive comprises a compound having the structural formula shown in formula (2):

[0012] .

[0013] Optionally, the first additive comprises at least one compound having the structural formulas shown in formulas (1-1) to (1-3):

[0014] .

[0015] Optionally, based on the total mass of the electrolyte, the mass percentage content of the first additive is P1, and the mass percentage content of the second additive is P2, wherein 0.1%≤P1≤1% and 0.05%≤P2≤3%.

[0016] Optionally, 0.2% ≤ P1 ≤ 0.5%; and / or, 0.1% ≤ P2 ≤ 1.5%.

[0017] Optionally, the electrolyte further includes a third additive; the third additive is selected from at least one of fluoroethylene carbonate, ethylene sulfate, tris(trimethylsilane) phosphate, vinylene carbonate, ethylene sulfate, ethylene sulfite, and lithium difluorodioxazophosphate; and / or, based on the total mass of the electrolyte, the mass percentage content of the third additive is P3, 0.5% ≤ P3 ≤ 5%.

[0018] Optionally, the electrolyte further includes a lithium salt; the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetraphenylborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)methyl lithium, lithium bis(fluorosulfonyl)imide, lithium hexafluorosilicate, lithium bis(oxalateborate), lithium bis(trifluoromethanesulfonyl)imide, and lithium difluoroborate; and / or, based on the total mass of the electrolyte, the lithium salt has a mass percentage content of 8% to 18%.

[0019] Optionally, the electrolyte further includes a lithium salt; the lithium salt includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, wherein the mass ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is (2~5):1.

[0020] A second aspect of this application provides a lithium iron phosphate battery, comprising the electrolyte described in the first aspect.

[0021] Optionally, based on the total mass of the electrolyte, the mass percentage content of the first additive is P1, the mass percentage content of the second additive is P2, and the electrolyte filling coefficient of the lithium iron phosphate battery is I. c The lithium iron phosphate battery satisfies 0.15≤ ≤120, and / or, 2.8≤Ic≤4.0.

[0022] Alternatively, 2.9 ≤ Ic ≤ 3.2, and / or 4 ≤ ≤20.

[0023] The electrolyte provided in this application contains a first additive and a second additive. Through the synergistic effect of the first additive and the second additive, the low-temperature conductivity of the electrolyte can be improved during the operation of the lithium iron phosphate battery, the polarization voltage at the moment of discharge can be reduced, and the electrode interface film composition can be regulated to improve the diffusion rate of lithium ions, thereby promoting the improvement of the low-temperature discharge capacity retention rate of the lithium iron phosphate battery. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] To gain a more complete understanding of this application and its beneficial effects, the following description will be provided in conjunction with the accompanying drawings, wherein the same reference numerals in the following description denote the same parts.

[0026] Figure 1 This is a schematic diagram of solvation structure types such as SSIP, CIP, and AGG. Detailed Implementation

[0027] The technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. Furthermore, it should be understood that the specific implementation methods described herein are only for illustration and explanation of this application and are not intended to limit this application.

[0028] In this application, "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural.

[0029] In this application, "at least one" means one or more, and "more than one" means two or more. "One or more", "at least one of the following", or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0030] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0031] A first aspect of this application provides an electrolyte suitable for lithium iron phosphate batteries. The electrolyte includes a first additive and a second additive; the first additive includes a compound having the structural formula shown in formula (1):

[0032]

[0033] In formula (1), X1 is independently selected from halogens, and X2 is independently selected from at least one of naphthyl and substituted phenyl groups, wherein the substituents in the substituted phenyl group are selected from amino and oxysulfonyl halogen groups;

[0034] The second additive comprises a compound having the structural formula shown in formula (2):

[0035] .

[0036] This application combines the first additive and the second additive. Through their synergistic effect, the low-temperature conductivity of the electrolyte can be improved during battery operation, the polarization voltage at the moment of discharge can be reduced, and the electrode interface film composition can be regulated to improve the diffusion rate of lithium ions, thereby promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0037] Among the existing lithium iron phosphate battery systems, the core issue of poor low-temperature performance is that low temperatures disrupt the mass transfer (Li) within the entire battery system. + The transport (transport in the electrolyte and solid phase) and charge transfer (electrode interface reaction) processes are significantly slowed down. This manifests as a sharp increase in internal resistance, incomplete reaction, and exacerbated side reactions. The sharp increase in internal resistance is manifested in increased ohmic resistance, increased charge transfer resistance, and increased concentration polarization resistance, leading to a decrease in the discharge voltage plateau and poor rate performance. Incomplete reaction is specifically manifested in slow interface reactions and slow solid-phase diffusion, which leads to reduced utilization of active materials, resulting in low discharge capacity. Exacerbated side reactions are specifically manifested in increased risk of lithium plating at the negative electrode and instability of the SEI film, leading to rapid capacity decay of the battery. Moreover, based on the olivine structure of lithium iron phosphate, the migration path of lithium ions is a one-dimensional channel, resulting in a very low lithium-ion diffusion coefficient (approximately 10). -14 cm 2 The internal resistance increases significantly, resulting in poor output / input capability and a small voltage margin. It is also prone to triggering protection limits due to polarization, limiting low-temperature performance. These problems are exacerbated in cold operating environments. Therefore, optimizing the electrolyte (low viscosity, low freezing point, high conductivity) and providing a lithium-ion migration environment can significantly improve the low-temperature performance of lithium iron phosphate batteries.

[0038] In common electrolyte systems with a lithium salt concentration of 1 mol / L, lithium ions typically exist as solvent-separated ion pairs (SSIPs), primarily exhibiting Li ions as Li ions. + - Solvent-forming structures of solvent molecules. These structures are prone to initiating Li-induced degradation in batteries using graphite anodes. + It co-intercalates with solvent molecules between graphite layers, leading to damage to the negative electrode and a decrease in battery performance.

[0039] The inventors discovered that, upon introduction of the first additive, due to its strong coordination ability, this additive can competitively enter the first solvation layer of lithium ions and promote the addition of more lithium salt anions (such as PF6). - It also enters the first solvation layer, forming a layer composed of Li + -PF6 - Li + - Solvents and Li + The solvation structure formed by the first additive transforms the original high-proportion solvent-separated ion pairs (SSIP) into a solvation type dominated by contact ion pairs (CIP) and supplemented by solvent-separated ion pairs (SSIP). This change effectively weakens the dipole interaction between lithium ions and solvent molecules, not only improving the lithium ion transport rate but also significantly reducing the risk of solvent co-intercalation into the graphite anode. However, the first additive may cause excessive solvent molecules to be expelled from the first solvation layer, leading to an increase in the free solvent concentration in the system, which may inadvertently trigger side reactions at the electrode-electrolyte interface.

[0040] Therefore, to reduce potential side reactions, this application includes a second additive comprising a compound having the structural formula shown in formula (2). The compound shown in formula (2) is lithium fluorosulfonate. See, for example, [example missing]. Figure 1 In this process, the molecules or anions of the second additive can strongly coordinate with the nearly free solvent molecules in the electrolyte and combine with the already formed contact ion pair (CIP) structure (such as the coordination form containing the first additive) to assemble into smaller, more compact aggregates (AGGs). That is, the smaller aggregates (AGGs) can significantly reduce the desolvation barrier of lithium ions at the interface, reduce the resistance of ions to passing through the electrode-electrolyte interface film, and simultaneously promote rapid lithium ion migration, thus improving lithium ion transport.

[0041] In summary, the first additive, through its strong coordination ability, can enhance ion migration rate and suppress solvent co-intercalation. However, it increases the possibility of side reactions at the electrode-electrolyte interface. Therefore, this application introduces a second additive, which, through its strong coordination with the free solvent, assembles with contact ion pairs (CIP) to form compact AGG aggregates, thereby further reducing the desolvation barrier and interfacial transport resistance of lithium ions, thus comprehensively optimizing the transport kinetics of lithium ions at the bulk electrolyte and electrode interface. Therefore, through the synergistic effect of the first and second additives, a more stable, lower impedance, and more efficient lithium ion migration environment is constructed, providing a feasible electrolyte solution for low-temperature lithium iron phosphate batteries with a lean electrolyte. Furthermore, the homo (highest occupied molecular orbital) energy levels of the first and second additives are higher than those of other electrolyte components, preferentially decomposing at the positive electrode. The decomposition products entering the CEI membrane can improve membrane stability and enhance the cycle stability of the lithium iron phosphate battery.

[0042] The electrolyte in this application contains both a first additive and a second additive, which can improve the low-temperature conductivity and low-temperature discharge capacity retention of lithium iron phosphate batteries. Through the synergistic effect of the first and second additives, the low-temperature conductivity of the electrolyte can be improved during the operation of the lithium iron phosphate battery, the polarization voltage at the moment of discharge can be reduced, and the electrode interface film composition can be controlled to improve the diffusion rate of lithium ions, thereby promoting the improvement of the low-temperature discharge capacity retention of lithium iron phosphate batteries.

[0043] The first additive comprises a compound having the structure shown in formula (1), wherein X1 is independently selected from halogens and X2 is independently selected from at least one of naphthyl and substituted phenyl groups, wherein the substituents in the substituted phenyl group are selected from amino and oxysulfonyl halogen groups; the second additive comprises a compound having the structure shown in formula (2).

[0044] For example, the halogen mentioned above refers to halogen elements, such as fluorine (F) and chlorine (Cl).

[0045] For example, a substituted phenyl group is formed by substituting an H atom on a benzene ring. An oxysulfonyl halide is formed, for example, by halogenating an H atom in an oxysulfonyl group. For example, an amino-substituted phenyl group may be selected from an aminobenzene (-C6H6N), and an oxysulfonyl halide-substituted phenyl group may be selected from a phenyl fluorosulfonate group (-C6H5OSO2F).

[0046] In some examples, the first additive may be selected from compounds with the following structures:

[0047] .

[0048] Among them, formula (1-1) is 1,4-phenylenebis(fluorosulfonic acid), formula (1-2) is naphth-2-ylfluorosulfonyl fluoride, and formula (1-3) is 4-aminophenyl thiofluoride ester.

[0049] It is understood that the compounds shown in the above structural formulas can be obtained commercially or synthesized in the laboratory using existing methods. Furthermore, the compounds shown in the above structural formulas are merely illustrative and can be either cis or trans structures.

[0050] In some specific embodiments, in the electrolyte, based on the total mass of the electrolyte, the mass percentage content of the first additive is P1, and the mass percentage content of the second additive is P2, wherein 0.1% ≤ P1 ≤ 1%, and 0.05% ≤ P2 ≤ 3%. When the mass percentage content of the first additive is between 0.1% and 1%, it ensures sufficient participation of the first additive in SEI film formation (below 0.1% results in lower film strength) while avoiding excessive amounts leading to excessively high electrolyte viscosity. When the mass percentage content of the second additive is between 0.05% and 3%, it avoids both insufficient content affecting the wetting degree of the electrolyte and excessive content leading to an overly thick SEI film, resulting in significant capacity loss in the lithium iron phosphate battery. The electrolyte of this application includes both a first additive and a second additive, and the contents of the first additive and the second additive are controlled within the range of this application. The synergistic effect of the two additives can improve the low-temperature conductivity of the electrolyte and reduce the polarization voltage at the moment of discharge during the operation of the lithium iron phosphate battery. At the same time, it can regulate the composition of the electrode interface film and improve the diffusion rate of lithium ions, thereby promoting the improvement of the low-temperature discharge capacity retention rate of the lithium iron phosphate battery.

[0051] For example, based on the total mass of the electrolyte, the mass percentage content P1 of the first additive can be any value among 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1%, and the range between any two of the above values.

[0052] For example, based on the total mass of the electrolyte, the mass percentage content P2 of the second additive is any one of the following values: 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, and a range between any two of the above values.

[0053] In some examples, based on the total mass of the electrolyte, the mass percentage content of the first additive, P1, is 0.2%–0.5%, and the mass percentage content of the second additive, P2, is 0.1%–1.5%. When the mass percentage content of the first additive is 0.2%–0.5%, it facilitates lithium-ion desolvation and charge transfer, thereby improving the cycle stability and high-temperature storage performance of the lithium iron phosphate battery. When the mass percentage content of the second additive is 0.1%–1.5%, it better reduces side reactions of the electrolyte on the electrode surface and mitigates the hindering effect of the first additive on lithium-ion migration.

[0054] Therefore, based on the total mass of the electrolyte, when the mass percentage content of the first additive P1 is 0.2% to 0.5% and the mass percentage content of the second additive P2 is 0.1% to 1.5%, the first and second additives can fully exert a synergistic effect, significantly improve the low-temperature conductivity of the electrolyte, reduce the polarization voltage at the moment of discharge, and at the same time regulate the composition of the electrode interface film, improve the diffusion rate of lithium ions, thereby promoting the improvement of the low-temperature discharge capacity retention rate of lithium iron phosphate batteries.

[0055] For example, based on the total mass of the electrolyte, the mass percentage content P1 of the first additive can be any value among 0.2%, 0.3%, 0.4%, 0.5%, and any range between any two of the above values.

[0056] For example, based on the total mass of the electrolyte, the mass percentage content P2 of the second additive can be any value among 0.1%, 0.2%, 0.5%, 0.8%, 1.1%, 1.5%, and any range between any two of the above values.

[0057] In some specific embodiments, the electrolyte further includes a third additive. The third additive is selected from at least one of fluoroethylene carbonate (FEC), ethylene sulfate (DTD), tris(trimethylsilane) phosphate (TMSP), vinylene carbonate, ethylene sulfate, ethylene sulfite, and lithium difluorodioxane phosphate. By adding the third additive to the electrolyte, the uniformity of the SEI film formation in lithium iron phosphate batteries can be balanced, promoting improved low-temperature discharge capacity retention.

[0058] In some cases, the mass percentage content of the third additive is P3, based on the total mass of the electrolyte, where 0.5% ≤ P3 ≤ 5%.

[0059] For example, based on the total mass of the electrolyte, the mass percentage content P3 of the third additive is 0.5%, 1%, 1.5%, 2%, 2.1%, 2.3%, 2.5%, 2.7%, 2.9%, 3%, 3.5%, 4%, 4.5%, 5%, and any range between any two of the above values.

[0060] In some cases, the mass percentage content (P3) of the third additive is 0.5% to 1.5% based on the total mass of the electrolyte. When the mass percentage content (P3) of the third additive is 0.5% to 1.5%, the third additive can better balance the SEI film uniformity of lithium iron phosphate batteries and promote the improvement of battery low-temperature discharge capacity retention.

[0061] For example, based on the total mass of the electrolyte, the mass percentage content P3 of the third additive can be any value among 0.5%, 0.7%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, and 1.5%, as well as a range between any two of the above values.

[0062] In this application, the electrolyte includes lithium salts. After dissolving in the solvent of the electrolyte, the lithium salt releases lithium ions, which form a solvated structure with the solvent, facilitating rapid migration of the lithium ions. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetraphenylborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)methyl lithium, lithium bis(fluorosulfonyl)imide, lithium hexafluorosilicate, lithium bis(oxalato)borate, lithium bis(trifluoromethanesulfonyl)imide, and lithium difluoroborate.

[0063] In some embodiments of this application, the lithium salt may include lithium hexafluorophosphate and lithium bisfluorosulfonylimide. Simultaneous addition of lithium hexafluorophosphate and lithium bisfluorosulfonylimide to the electrolyte can further reduce battery gas production and improve battery cycle performance.

[0064] In some embodiments of this application, the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide is (2~5):1. For example, the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide is 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, or any range of two of these values. Controlling the ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide within the above range can enable lithium iron phosphate batteries to have excellent cycle performance and high-temperature storage performance, while also taking into account the battery manufacturing cost, reducing gas production, and improving the battery's cycle performance.

[0065] In some embodiments of this application, the mass percentage content of lithium salt is 8% to 18%, preferably 10% to 16%, based on the total mass of the electrolyte. For example, the mass percentage content of lithium salt can be 8%, 10%, 12%, 14%, 16%, 18%, or a range of any two of these values, based on the total mass of the electrolyte. Controlling the mass percentage content of lithium salt within the range specified in this application allows for sufficient dissolution of the lithium salt in non-aqueous organic solvents, while also providing the electrolyte with both high ionic conductivity and low manufacturing cost.

[0066] In some specific embodiments, the electrolyte includes a non-aqueous organic solvent. As the main component of the electrolyte, the non-aqueous organic solvent should have high lithium salt solubility to ensure high ionic conductivity. As an important carrier for ion transport, the non-aqueous organic solvent can exhibit high electronic conductivity after the lithium salt dissolves, improving battery cycle life, charge / discharge rate, high-temperature performance, low-temperature performance, and energy density. This application does not impose any particular limitation on the non-aqueous organic solvent, as long as it achieves the purpose of this application. For example, the non-aqueous organic solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds or cyclic carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate, or butylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, ethyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and 1,2-dimethoxyethane.

[0067] In some specific embodiments, the mass percentage of the non-aqueous organic solvent can be 62% to 91% based on the total mass of the electrolyte. The non-aqueous solvent can fully dissolve the lithium salt, maintain high ionic conductivity, and provide a medium for uniform dispersion of the first and second additives.

[0068] Based on the total mass of the electrolyte, the mass percentage of non-aqueous solvent can be any value from 62%, 67.5%, 71%, 74.5%, 78%, 81.5%, 85%, 88.5%, 90%, 91%, or any range between any two of the above values.

[0069] According to a second aspect of this application, a lithium iron phosphate battery is provided, comprising the electrolyte of the first aspect. The electrolyte filling coefficient of the lithium iron phosphate battery is I. c (Unit: g / Ah), lithium iron phosphate batteries meet the following requirement: 0.15 ≤ ≤120. For example, The value is 0.15, 1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, or a range of any two of these values. Here, P1 refers to the mass percentage content of the first additive, and P2 refers to the mass percentage content of the second additive.

[0070] In this application, the electrolyte injection coefficient I c This directly affects the sufficiency of contact between the first and second additives and the surfaces of the positive and negative electrode material layers, as well as the consumption, decomposition, and diffusion rates of the first and second additives on the surfaces of the positive and negative electrode material layers during the formation and capacity building stage. By using I... c P1 and P2 are associated and controlled within the scope of this application. They can improve the low-temperature conductivity of the electrolyte during battery operation, reduce the polarization voltage at the moment of discharge, and at the same time regulate the composition of the electrode interface film to improve the diffusion rate of lithium ions, thereby promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0071] In some embodiments of this application, the lithium iron phosphate battery satisfies 4≤ ≤20. For example, The range is 4, 5, 10, 15, 20, or any two of these values. Through optimization... This range can further promote the improvement of battery low-temperature discharge capacity retention rate.

[0072] In some embodiments of this application, 2.8 ≤ Ic ≤ 4.0. For example, Ic can be 2.8, 3.0, 3.2, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or a range of any two of these values. Preferably, 2.9 ≤ Ic ≤ 3.2.

[0073] Ic is an important parameter affecting the relative quality of the positive and negative electrode material layers and the non-aqueous electrolyte in lithium iron phosphate batteries. Controlling Ic within the scope of this application can ensure that the positive electrode, negative electrode, and separator in the lithium iron phosphate battery are fully wetted by the electrolyte, and that the first and second additives can form a stable interface film on the surface of the positive and negative electrode, which is beneficial to the full utilization of the capacity of the lithium iron phosphate battery, thereby improving the fast-charging cycle stability of the lithium iron phosphate battery under low-temperature conditions.

[0074] In some specific embodiments, the battery of this application includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. In this application, the positive electrode material layer may be disposed on one surface or on two surfaces in the thickness direction of the positive current collector. For example, the positive electrode material layer includes a positive electrode active material, such as lithium iron phosphate. This application does not have any particular limitation on the positive current collector, as long as it can achieve the purpose of this application. For example, the positive current collector may include a metal foil or a composite current collector. For example, the metal foil may be aluminum foil. The composite current collector may include a polymer material base layer and a metal material layer disposed on at least one surface of the polymer material base layer. For example, the material of the metal material layer may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy. The polymer material base layer may include at least one of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, or polyethylene. This application does not impose any particular limitation on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm. The thickness of the single-sided positive electrode material layer is 30 μm to 120 μm. The positive electrode material layer of this application may also contain a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and the binder, as long as the purpose of this application can be achieved. For example, the conductive agent may include, but is not limited to, at least one of superconducting carbon (Super P), acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The binder may include, but is not limited to, at least one of polyvinyl chloride, polyacrylamide, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0075] The battery of this application includes a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. In this application, the negative electrode material layer may be disposed on one surface or on two surfaces in the thickness direction of the negative electrode current collector. This application does not impose any particular limitation on the negative electrode current collector; any negative electrode current collector known in the art can be used, as long as it can achieve the purpose of this application. The negative electrode current collector may be a metal foil or a composite current collector. For example, as a metal foil, it may include at least one of aluminum foil, copper foil, nickel foil, and titanium foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by laminating a metal material (copper, copper alloy, nickel, nickel alloy, etc.) onto a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.).

[0076] In this application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm, and the thickness of the single-sided negative electrode material layer is 30 μm to 150 μm. Optionally, the negative electrode active material includes a thickener, which may include, but is not limited to, lithium carboxymethyl cellulose (CMC-Na). The negative electrode material layer of this application may also contain a conductive agent and a binder. There are no particular limitations on the conductive agent and binder in this application, as long as the purpose of this application can be achieved. For example, the binder may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), lithium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), lithium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). 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.

[0077] The negative electrode material layer of this application includes a negative electrode active material. This application does not particularly limit the type of negative electrode active material; any negative electrode active material known in the art can be used, as long as it achieves the purpose of this application. For example, the negative electrode active material may include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon-based materials, tin-based materials, and lithium titanate. The aforementioned silicon-based materials may include, but are not limited to, at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys; the aforementioned tin-based materials may include at least one of elemental tin, tin oxide compounds, or tin alloys.

[0078] The lithium iron phosphate battery of this application also includes a separator to separate the positive and negative electrode plates, prevent internal short circuits, allow ions to pass freely, and not affect the electrochemical charging and discharging process. This application does not impose any particular limitation on the type of separator; any porous structure separator with good chemical and mechanical stability can be selected. For example, the separator material can include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator type can include, but is not limited to, at least one of woven membrane, nonwoven membrane (nonwoven fabric), microporous membrane, composite membrane, rolled membrane, or spun membrane. The separator can be a single-layer thin film or a multi-layer composite thin film. In this application, the thickness of the separator is not particularly limited, as long as it achieves the purpose of this application; for example, the thickness can be from 10 μm to 25 μm.

[0079] The lithium iron phosphate battery of this application also includes a packaging bag for containing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the art. This application does not limit the aforementioned other components. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it achieves the purpose of this application. For example, an aluminum-plastic film packaging bag can be used.

[0080] The preparation process of the lithium iron phosphate battery of this application is well known to those skilled in the art, and this application has no particular limitations. For example, it may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and winding and folding them as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a lithium iron phosphate battery; or, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a lithium iron phosphate battery. In addition, overcurrent protection components, conductive plates, etc., may be placed in the packaging bag as needed to prevent the internal pressure of the lithium iron phosphate battery from rising and overcharging / discharging.

[0081] The battery of this application may be in the form of a single battery cell, a battery module, or a battery pack. A single battery cell can be assembled into a battery module, and a battery module may contain one or more battery cells; the specific number can be selected by those skilled in the art based on the application and capacity of the battery module. The battery modules of this application can also be assembled into a battery pack, and a battery pack may contain one or more battery modules; the specific number can be selected by those skilled in the art based on the application and capacity of the battery pack.

[0082] The electrolyte and battery provided in this application will be described in detail below through specific embodiments.

[0083] Example 1

[0084] Preparation of electrolyte

[0085] Under an inert atmosphere with moisture < 0.1 ppm and oxygen < 0.1 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) were mixed in a mass ratio of 3:4:3 to obtain the base solvent. Lithium salts (lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide), compound (1-1), and compound (2) were added to the base solvent and mixed thoroughly to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of compound (1-1) was 0.1%, the mass percentage of compound (2) was 1.0%, the mass percentage of lithium bis(fluorosulfonyl)imide (LiFSI) was 3%, the mass percentage of lithium hexafluorophosphate (LiPF6) was 9%, and the remainder was the base solvent.

[0086] In Table 1, A1 lithium salt is a combination of 9% lithium hexafluorophosphate and 3% lithium difluorosulfonyl imide; B1 solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) in a mass ratio of 3:4:3; and C1 third additive is a combination of fluoroethylene carbonate (FEC) and ethylene sulfate (DTD).

[0087] The electrolyte compositions of Examples 2 to 40 and Comparative Examples 1 to 6 are basically the same as those of Example 1, with the differences shown in Table 1; wherein, in Table 1:

[0088] In Example 28, the first additive is a compound with the structural formula (1-2), and the second additive is a compound with the structural formula (2).

[0089] In Example 29, the first additive is a compound with the structural formula (1-3), and the second additive is a compound with the structural formula (2).

[0090] In Example 30, lithium salt A2 was selected as 12% lithium hexafluorophosphate;

[0091] In Example 31, lithium salt A3 was selected as 12% lithium difluorosulfonylimide;

[0092] In Example 32, solvent B2 is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 3:7;

[0093] In Example 33, solvent B3 is a mixture of ethylene carbonate (EC) and ethyl acetate (EA) in a mass ratio of 3:7;

[0094] In Examples 34-40, a third additive was added; wherein, in Examples 34-36, the third additive C1 was a combination of fluoroethylene carbonate (FEC) and vinyl sulfate (DTD); in Example 37, the third additive was vinyl sulfate (DTD); in Example 38, the third additive was fluoroethylene carbonate (FEC); in Example 39, the third additive C2 was a combination of fluoroethylene carbonate (FEC) and tris(trimethylsilane) phosphate (TMSP); and in Example 40, the third additive C3 was a combination of vinyl sulfate (DTD) and tris(trimethylsilane) phosphate (TMSP).

[0095] In Comparative Example 1, the first additive, the second additive, and the third additive were not added;

[0096] In Comparative Example 2, a second additive was added, but neither the second nor the third additive was added.

[0097] In Comparative Examples 3-4, the mass percentage content of the first additive was adjusted, and the third additive was not added;

[0098] In Comparative Examples 5-6, the mass percentage content of the second additive was adjusted, and the third additive was not added;

[0099] Table 1. Additives and their contents in Examples 1-40 and Comparative Examples 1-6

[0100]

[0101] Test case

[0102] Batteries were prepared by combining the electrolytes of the examples and comparative examples with positive electrode plates, negative electrode plates, and separators;

[0103] 1. Preparation of positive electrode sheet

[0104] Lithium iron phosphate (LiFePO4), a positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF), a binder, were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum until a homogeneous and fluid positive electrode slurry was obtained, yielding a positive electrode slurry with a solid content of 60 wt%. The positive electrode slurry was uniformly coated onto one surface of a 16 μm thick aluminum foil current collector. After drying at 80°C, a positive electrode sheet with a single-sided coating of positive electrode material was obtained. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying under vacuum at 80°C, the sheet underwent cold pressing, edge trimming, cutting, slitting, sheet forming, and electrode tab welding and adhesive bonding processes to obtain a positive electrode sheet with dimensions of 70 mm × 54 mm. The compaction density of the positive electrode material layer was 2.4 g / cm³.3 .

[0105] 2. Preparation of negative electrode sheet

[0106] Artificial graphite (negative electrode active material), conductive carbon black (SuperP) (conductive agent), lithium carboxymethyl cellulose (CMC) (thickener), and styrene-butadiene rubber (SBR) (binder) (mass ratio 94.5:2:1.5:2) were mixed with deionized water and stirred evenly under vacuum to obtain a negative electrode slurry with a solid content of 50 wt%. The negative electrode slurry was uniformly coated onto one surface of a 9 μm thick copper foil current collector. After drying at 80°C, a single-sided negative electrode sheet with a 100 μm coating thickness was obtained. The above steps were repeated on the other surface of the copper foil to obtain a double-sided negative electrode sheet. After drying under vacuum at 80°C, the sheet underwent cold pressing, edge trimming, cutting, slitting, sheet forming, and tab welding and adhesive bonding processes to obtain a 74 mm × 58 mm negative electrode sheet. The compaction density of the negative electrode material layer was 1.6 g / cm³. 3 .

[0107] 3. Preparation of the diaphragm

[0108] The diaphragm is a polypropylene (PP) diaphragm purchased from Dongguan Kelude, with a thickness of 16μm.

[0109] 4. Preparation of Lithium Iron Phosphate Batteries

[0110] The separator, positive electrode, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to isolate them. They are then wound, with the positive tab connected to the positive electrode and the negative tab connected to the negative electrode, thus obtaining the electrode assembly. The electrode assembly is placed in an aluminum foil packaging bag, with the positive and negative tabs extended from the inside of the bag to the outside. After drying at 85°C for 36 hours to remove moisture, it is heat-sealed to obtain the cell ready for electrolyte injection. The prepared electrolyte is then injected into the dried cell. Following vacuum sealing, settling, formation, shaping, and capacity testing, a lithium iron phosphate battery is obtained.

[0111] The following battery performance tests were conducted on the lithium iron phosphate batteries using the electrolytes from the examples and comparative examples. The test results are summarized in Table 2.

[0112] 1. High-temperature storage performance test

[0113] The lithium iron phosphate battery was placed in a constant temperature environment of 25℃ and charged at a constant current of 1C to a voltage of 3.65V. It was then charged at a constant voltage at 3.65V until a cutoff current of 0.05C was reached. Next, it was discharged at a constant current of 1C to 2V. The capacity of the first discharge was recorded as the capacity before storage, denoted as C0. The battery was then charged again at a constant current of 1C to a voltage of 3.65V, and then charged at a constant voltage at 3.65V until a cutoff current of 0.05C was reached. The lithium-ion battery was then stored in a 60℃ explosion-proof oven for 30 days. Finally, it was discharged at a constant current of 1C to 2V. This capacity after storage was recorded as Cm. High-temperature storage capacity retention rate = (Cm / C0) × 100%.

[0114] 2. Low-temperature discharge performance test

[0115] The lithium iron phosphate battery was placed in a constant temperature environment at 25℃ and charged at a constant current of 1C to a voltage of 3.65V. Then, it was charged at a constant voltage at 3.65V until the cutoff current reached 0.05C, and then discharged at a constant current of 1C to 2V. This charge-discharge cycle was repeated three times, and the maximum discharge capacity of the first three cycles was recorded as Q1. The battery was then charged again at a constant current of 1C to a voltage of 3.65V, charged at a constant voltage at 3.65V until the cutoff current reached 0.05C, and then discharged at a constant current of 1C for 40 minutes. After being placed at -25℃ for 5 hours, it was discharged at a current of 0.66*Q1 to 2V, and the discharge time was recorded as T.

[0116] In the low-temperature discharge performance and high-temperature storage performance tests, three batteries were tested in parallel for each performance test and the average value was taken as the test result. The absolute deviation of the lithium iron phosphate battery test was within ±1%.

[0117] Table 2 Performance test results of Examples 1-40 and Comparative Examples 1-6

[0118]

[0119] Combining Table 1 and Table 2, we can see that:

[0120] Examples 1-15, 25-27 and Comparative Examples 3-6 show that optimizing the mass percentage content of the first and second additives can improve the low-temperature conductivity of the electrolyte and promote the improvement of the battery's low-temperature discharge capacity retention rate.

[0121] As can be seen from Examples 16-24, optimizing the range of the electrolyte injection coefficient Ic is beneficial to improving the low-temperature conductivity of the electrolyte and promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0122] Comparing Examples 4 and 30-31, it can be seen that optimizing the lithium salt ratio and mass percentage content is beneficial to improving the low-temperature conductivity of the electrolyte and promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0123] Comparing Examples 4 and 32-33, it can be seen that optimizing the solvent ratio and mass percentage content is beneficial to improving the low-temperature conductivity of the electrolyte and promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0124] Comparing Examples 4 and 34-40, it can be seen that adding a third additive and optimizing the mass percentage content of the third additive are beneficial to improving the low-temperature conductivity of the electrolyte and promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0125] By comparing Examples 1-7 and Comparative Examples 1-2, it can be seen that adding the first additive and the second additive can improve the low-temperature conductivity of the electrolyte and promote the improvement of the battery's low-temperature discharge capacity retention rate.

[0126] In summary, the electrolyte provided in this application, with the addition of a first additive and a second additive, can improve the low-temperature conductivity of the electrolyte during battery operation, reduce the polarization voltage at the moment of discharge, and simultaneously regulate the composition of the electrode interface film to improve the diffusion rate of lithium ions, thereby promoting the improvement of the battery's low-temperature discharge capacity retention rate.

[0127] The above are merely optional embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. An electrolyte, characterized by, The electrolyte is suitable for a lithium iron phosphate battery, and comprises a first additive and a second additive; the first additive comprises a compound as shown in a structural formula of formula (1): In formula (1), X1 is independently selected from halogen, and X2 is independently selected from at least one of a naphthyl group and a substituted phenyl group, wherein the substituent group of the substituted phenyl group is selected from an amino group and an oxysulfonyl halide group; the second additive comprises a compound as shown in a structural formula of formula (2): ； The mass percentage content of the first additive is P1, and the mass percentage content of the second additive is P2, based on the total mass of the electrolyte, wherein 0.1%≤P1≤1%, and 0.05%≤P2≤3%.

2. The electrolyte according to claim 1, characterized in that, The first additive comprises at least one of compounds as shown in structural formulas of formula (1-1) to formula (1-3): 。 3. The electrolyte of claim 1, wherein 0.2%≤P1≤0.5%, and / or, 0.1%≤P2≤1.5%.

4. The electrolyte of claim 1, wherein The electrolyte further comprises a third additive; The third additive is selected from at least one of fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)phosphate, vinylene carbonate, ethylene sulfate, vinyl sulfite, lithium difluorodioxalate phosphate, and / or, the mass percentage content of the third additive is P3, based on the total mass of the electrolyte, wherein 0.5%≤P3≤5%. The electrolyte further comprises a lithium salt; 5. The electrolyte of claim 1, wherein The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium tetraphenylborate, lithium methane sulfonate, lithium triflate, lithium bistrifluoromethanesulfonimide, lithium tris(trifluoromethylsulfonyl)methide, lithium bisfluorosulfonimide, lithium hexafluorosilicate, lithium bisoxalate borate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluoroborate, and / or, the mass percentage content of the lithium salt is 8%~18%, based on the total mass of the electrolyte. The electrolyte further comprises a lithium salt; The lithium salt comprises lithium hexafluorophosphate and lithium bisfluorosulfonimide, and the mass ratio of the lithium hexafluorophosphate to the lithium bisfluorosulfonimide is (2~5):

1.

6. The electrolyte of claim 1, wherein The electrolyte comprises the electrolyte according to any one of claims 1 to 6. ​ 7. A lithium iron phosphate battery, characterized in that, ​ 8. The lithium iron phosphate battery of claim 7, wherein, The mass percentage content of the first additive is P1 and the mass percentage content of the second additive is P2 based on the total mass of the electrolyte, and the injection coefficient of the lithium iron phosphate battery is I c , the lithium iron phosphate battery satisfies 0.15≤ ≤120, and / or 2.8≤Ic≤4.

0.

9. The lithium iron phosphate battery of claim 7, wherein, 2.9 < Ic < 3.2, and / or, 4 < Ic < 20. ≤ 20.

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