Lithium ion battery based on secondary liquid injection process and preparation method

Abstract

The invention provides a lithium ion battery based on a secondary liquid injection process and a preparation method. Specifically, the lithium ion battery comprises a positive pole piece, a negative pole piece, an isolating membrane and electrolyte, the electrolyte comprises a primary liquid injection electrolyte and a secondary liquid injection electrolyte, and secondary liquid injection is adopted in the preparation process of the lithium ion battery; wherein the primary liquid injection electrolyte comprises carboxylic ester, carbonic ester and fluoroethylene carbonate; the secondary liquid injection electrolyte comprises carboxylic ester, carbonic ester, fluoroethylene carbonate and a first additive as shown in a formula (I); in the formula (I), R1 and R2 are the same or different and are independently selected from H and C1-C4 straight chain or branched chain alkyl. The performance of the high-temperature performance additive is improved by designing the primary liquid injection electrolyte and the secondary liquid injection electrolyte and adopting a secondary liquid injection mode.

Description

Technical Field

[0001] This disclosure relates to the field of battery technology, and in particular to a lithium-ion battery based on a secondary liquid injection process and its preparation method. Background Technology

[0002] Currently, lithium-ion batteries are rapidly developing in the fields of electric vehicles and large-scale energy storage, capturing a significant market share. To achieve faster charging speeds for electric vehicles and alleviate range anxiety, lithium-ion batteries are required to have higher fast-charging capabilities. Common lithium-ion batteries use cyclic carbonates (such as ethylene carbonate EC and propylene carbonate PC) and linear carbonates (such as dimethyl carbonate DMC, ethyl methyl carbonate EMC, and diethyl carbonate DEC) as solvents; the lithium salt is primarily lithium hexafluorophosphate. Due to the high viscosity of the solvent, the fast-charging performance of this combination of electrolytes is often poor. Furthermore, electrolytes currently focused on fast charging often have shortcomings in high-temperature performance, making it difficult to simultaneously achieve optimal high-temperature performance and rate capability. Summary of the Invention

[0003] In view of this, the purpose of this disclosure is to propose a lithium-ion battery based on a secondary liquid injection process and its preparation method.

[0004] To achieve the above objectives, the first aspect of this disclosure provides a lithium-ion battery based on a secondary electrolyte injection process, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the electrolyte includes a primary electrolyte injection and a secondary electrolyte injection, and the lithium-ion battery is manufactured using a secondary electrolyte injection process; The primary electrolyte solution includes carboxylic acid esters, carbonates, and fluoroethylene carbonates. The secondary electrolyte comprises carboxylic acid esters, carbonates, fluoroethylene carbonates, and a first additive shown in formula (I): Equation (I); In formula (Ⅰ), R1 and R2 may be the same or different, and are each independently selected from: H, C1-C4 straight-chain or branched alkyl groups; wherein, The mass fraction of the fluoroethylene carbonate in the primary electrolyte is less than the mass fraction of the fluoroethylene carbonate in the secondary electrolyte.

[0005] In some embodiments, based on the total mass of the secondary electrolyte, the mass fraction of the first additive is 0.1% to 0.7%, optionally 0.2% to 0.5%.

[0006] In some embodiments, the structural formula of the first additive is selected from at least one of the following structural formulas; Formula (II), Formula (III) Formula (IV), Formula (V).

[0007] In some embodiments, the fluoroethylene carbonate has a mass fraction of 0.2% to 3% in the primary electrolyte solution; and / or The fluoroethylene carbonate has a mass fraction of 15% to 40% in the secondary electrolyte; and / or The carboxylic acid ester has a mass fraction of 4% to 60% in the primary or secondary electrolyte, and optionally 8% to 40%.

[0008] In some embodiments, the ratio 'a' of the mass fraction of carboxylic acid esters in the primary electrolyte and the secondary electrolyte has the following relationship: 0.8 ≤ a ≤ 1.15; and / or The primary electrolyte injection accounts for 70% to 95% of the total mass fraction of the electrolyte injected into the lithium-ion battery.

[0009] In some embodiments, the primary electrolyte further includes a second additive; the second additive includes vinylene carbonate, ethylene ethylene carbonate, and 1,3-dimethylbenzene carbonate. Propanesulfonate lactone, 1,3 Any one or a combination of at least two of the following: propylene sulfonate lactone, vinyl sulfate, lithium difluorophosphate, lithium dioxaborate, lithium tetrafluoroborate, and lithium difluorooxaborate. The second additive has a mass fraction of 0.2% to 3.5% in the primary electrolyte solution.

[0010] In some embodiments, the carboxylic acid ester includes any one or a combination of at least two of ethyl acetate, methyl acetate, methyl formate, butyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl butyrate, butyl butyrate, isopropyl acetate, and isoamyl acetate; and / or The carbonate includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0011] In some embodiments, the primary electrolyte and the secondary electrolyte further comprise lithium salts, wherein the lithium salts include any one or a combination of at least two of LiFSI, LiTFSI, LiPF6, LiBF4, LiClO4, or LiAsF6. The lithium salt in the primary electrolyte or the secondary electrolyte has a mass fraction of 10% to 20%, optionally 12% to 18%.

[0012] In some embodiments, the positive electrode sheet includes a positive active material; the negative electrode sheet includes a negative active material; The positive electrode active material includes Li x Ni y Mn z M m O 0.5x+1.5y+1.5z+1.5m Wherein, 0.9≤x≤1.1, 0.8≤y≤1, 0≤z≤0.2, 0≤m≤0.2; where, M is selected from one or more of Fe, Ti, Co, Cr, V, Cu, Zn, Zr, Al and Nb; optionally, 0.9≤y≤1; The negative electrode active material comprises a mixture of silicon carbide and carbon-based active material; wherein the mass fraction of silicon carbide in the negative electrode active material is 5% to 35%, and the carbon-based active material comprises any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, and natural graphite.

[0013] Based on the same inventive concept, the second aspect of this disclosure also provides a method for preparing a lithium-ion battery, comprising: Prepare the positive electrode, negative electrode, and separator; The positive electrode, the negative electrode, and the separator are assembled into a bare battery cell. The primary electrolyte is injected into the bare cell, sealed, and then subjected to formation and aging to obtain a lithium-ion cell. The secondary electrolyte is injected into the lithium-ion cell, and the cell is sealed and left to stand to obtain the lithium-ion battery.

[0014] As can be seen from the above, this disclosure provides a lithium-ion battery and its preparation method based on a secondary electrolyte injection process. The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte includes a primary electrolyte injection and a secondary electrolyte injection, and the lithium-ion battery preparation process employs secondary electrolyte injection. The primary electrolyte injection includes carboxylic acid esters, carbonates, and fluoroethylene carbonates. The secondary electrolyte injection includes carboxylic acid esters, carbonates, fluoroethylene carbonates, and a first additive shown in formula (I). Formula (I); In Formula (I), R1 and R2 may be the same or different, and are each independently selected from: H, C1-C4 straight-chain or branched alkyl groups. In such lithium-ion batteries using a carboxylic acid ester fast-charging electrolyte system, the performance of high-temperature performance additives is improved by designing primary and secondary electrolytes and employing a secondary electrolyte injection method. Furthermore, the use of the first additive described in Formula (I) in the secondary electrolyte further ensures the long-term high-temperature performance of the lithium-ion battery, achieving both improved fast-charging performance and good high-temperature performance. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments.

[0016] It should be noted that, unless otherwise defined, the technical or scientific terms used in the embodiments of this disclosure should have the ordinary meaning as understood by one of ordinary skill in the art to which this disclosure pertains.

[0017] In this disclosure, the term "range" is used to define a range in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~130 and 70~120 are listed for a specific parameter, it is expected that ranges of 60~120 and 70~130 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this disclosure, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0018] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0019] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0020] Unless otherwise specified, the terms "comprising" and "including" as used in this disclosure can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0021] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B".

[0022] Terminology Explanation A solid electrolyte interface (SEI) is a passivation layer formed on the surface of a lithium-ion battery during charging and discharging, resulting from the reaction between the electrode material and the electrolyte at the solid-liquid interface. This passivation layer is an interface layer, exhibiting characteristics of a solid electrolyte; it is an electronic insulator, yet it is a Li-ion electrolyte. + Li is an excellent conductor. + It can freely embed and extract through this passivation layer.

[0023] Direct Current Resistance (DCR) is measured at a specified temperature when a battery is discharged at 1C to 50% SOC (State of Charge, reflecting the battery's remaining capacity), then the current is increased to 4C and held for 30 seconds. The difference between the updated stable voltage and the original plateau voltage is calculated, and the ratio of this difference to the 4C current value is the battery's DC resistance. The DCR test result performed after the battery's first full charge is the battery's initial DCR.

[0024] Fast-charging electrolytes utilize carboxylic acid esters to reduce the amount of high-melting-point, high-viscosity cyclic carbonates in the electrolyte solvent, effectively solving the lithium plating problem under high-rate charging conditions and improving charge / discharge capacity at low temperatures. However, carboxylic acid ester solvents can degrade high-temperature, especially high-temperature, storage performance. This is because carboxylic acid esters have high α-H activity and poor stability on the negative electrode side. Furthermore, at high temperatures, byproducts from lithium salt decomposition (such as PF5 produced from the decomposition of LiPF6) can gain electrons from the carbonyl group of the carboxylic acid ester, enhancing α-H activity and accelerating side reactions of the carboxylic acid ester solvent on the electrode surface. Fluoroethylene carbonate (FEC) is a commonly used negative electrode film-forming additive. It can decompose to form a LiF-rich SEI, which helps improve the stability of the electrolyte solvent on the negative electrode surface. However, FEC has a high film-forming potential (~1.7 V vs LiF). + / Li), fluoroethylene carbonate forms a film first, which affects the film-forming effect of other additives and limits the choice of additive combination.

[0025] In view of this, the present disclosure provides a lithium-ion battery and its preparation method based on a secondary electrolyte injection process. Specifically, the lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; the electrolyte includes a primary electrolyte injection and a secondary electrolyte injection, and the lithium-ion battery preparation process employs secondary electrolyte injection; wherein, the primary electrolyte injection includes a carboxylic acid ester, a carbonate, and a fluoroethylene carbonate; the secondary electrolyte injection includes a carboxylic acid ester, a carbonate, a fluoroethylene carbonate, and a first additive shown in formula (I): Formula (I); In Formula (I), R1 and R2 may be the same or different, and are each independently selected from: H, C1-C4 straight-chain or branched alkyl groups. In such lithium-ion batteries using a carboxylic acid ester fast-charging electrolyte system, the performance of high-temperature performance additives is improved by designing primary and secondary electrolytes and employing a secondary electrolyte injection method. Furthermore, the use of the first additive described in Formula (I) in the secondary electrolyte further ensures the long-term high-temperature performance of the lithium-ion battery, achieving both improved fast-charging performance and good high-temperature performance.

[0026] To make the technical solutions of this disclosure clearer and easier to understand, the electrolyte, lithium-ion battery and its preparation method provided in this disclosure will be described in detail below with reference to specific embodiments.

[0027] electrolyte In a first aspect, embodiments of this disclosure provide an electrolyte. Specifically, the electrolyte includes a primary injection electrolyte and a secondary injection electrolyte. Further, the primary injection electrolyte includes a carboxylic acid ester, a carbonate, and a fluoroethylene carbonate; the secondary injection electrolyte includes a carboxylic acid ester, a carbonate, a fluoroethylene carbonate, and a first additive shown in formula (I): Formula (I); in Formula (I), R1 and R2 may be the same or different, and are each independently selected from: H, C1-C4 straight-chain or branched alkyl groups. The mass fraction of fluoroethylene carbonate in the primary electrolyte is less than the mass fraction of the fluoroethylene carbonate in the secondary electrolyte.

[0028] For example, R1 and R2 can each be independently methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, or tert-butyl. For instance, R1 is methyl and R2 is ethyl; or R1 is methyl and R2 is n-propyl; or R1 is methyl and R2 is n-butyl; or R1 is ethyl and R2 is ethyl; or R1 is ethyl and R2 is isopropyl; or R1 is ethyl and R2 is sec-butyl.

[0029] It should be noted that primary electrolyte and secondary electrolyte are used for secondary electrolyte injection during the lithium-ion battery manufacturing process.

[0030] This technical solution utilizes a primary and secondary injection electrolyte to limit the film-forming ratio of fluoroethylene carbonate during the formation stage, thereby adjusting the film-forming effect of other additives and constructing a more balanced SEI. Simultaneously, the first additive shown in formula (Ⅰ) is added to the secondary injection electrolyte. The NH functional group in formula (Ⅰ) can be converted from a proton to a positively charged N group. + -H2 functional group, N +-H2 has a strong electrostatic attraction to negatively charged lithium salt anions (such as hexafluorophosphate ions). Furthermore, due to the large molecular size of formula (I), it has significant steric hindrance, making it more difficult for lithium salt anions to enter the lithium-ion solvation structure in the electrolyte. This inhibits the decomposition reaction of lithium salts (e.g., the formation of lithium fluoride and PF5 from hexafluorophosphate ions and lithium ions), thereby suppressing the catalytic process of carboxylic acid ester decomposition by byproducts (e.g., PF5), thus improving the battery's high-temperature performance. In addition, since a relatively dense SEI has already formed after the initial electrolyte injection, the first additive shown in formula (I) in the secondary electrolyte can largely avoid participating in the initial SEI formation process, minimizing the deterioration of the system's impedance and fast-charging performance. Moreover, the secondary electrolyte can repair the SEI during the later stages of cycling when it is damaged due to the continuous expansion / contraction of the electrodes, ensuring the battery's long-term cycle stability and high-temperature storage performance.

[0031] In some embodiments, based on the total mass of the secondary electrolyte, the mass fraction of the first additive is 0.1% to 0.7%, for example, 0.1%, 0.2%, 0.3%, 0.35%, 0.5%, 0.6%, 0.7%, etc.

[0032] Optionally, the mass fraction of the first additive is 0.2% to 0.5%. Using such a mass fraction results in better inhibition of carboxylic acid esters and is more conducive to improving the high-temperature performance of the battery.

[0033] In some embodiments, the structural formula of the first additive is selected from at least one of the following structural formulas; Formula (II), Formula (III) Formula (IV), Formula (V).

[0034] In some embodiments, the mass fraction of fluoroethylene carbonate in the primary electrolyte is 0.2% to 3%, such as 0.2%, 0.4%, 1%, 1.5%, 2%, 3%, etc.; and the mass fraction of fluoroethylene carbonate in the secondary electrolyte is 15% to 40%, such as 15%, 20%, 25%, 28%, 30%, 32%, 40%, etc.

[0035] Here, 0.2% to 3% fluoroethylene carbonate is used in the primary electrolyte and 15% to 40% fluoroethylene carbonate is used in the secondary electrolyte, which can form a LiF-rich SEI, which helps to improve the stability of the electrolyte solvent on the negative electrode surface. Furthermore, because its amount in the primary electrolyte is relatively small, it can effectively reduce the restrictions on other additives.

[0036] In some embodiments, the carboxylic acid ester has a mass fraction of 4% to 60% in the primary or secondary electrolyte. Exemplarily, the mass fraction of the carboxylic acid ester can be 4%, 8%, 15%, 25%, 35%, 40%, 46%, 50%, 55%, or 60%. Here, highly kinetic carboxylic acid esters typically have smaller molecules and a significant swelling effect on the SEI on the electrode surface. Using such a mass fraction of carboxylic acid ester to form the SEI can avoid swelling of the SEI by the carboxylic acid ester and further decomposition of the solvent.

[0037] Optionally, the mass fraction of the carboxylic acid ester is 8% to 40%. It should be noted that electrolytes with carboxylic acid esters in this mass fraction range have better performance.

[0038] It should be noted that the mass fraction of carboxylic acid ester in the primary and secondary electrolytes can be the same, for example, the mass fraction of carboxylic acid ester in the primary and secondary electrolytes can be 15%, 25%, or 35%, respectively; or they can be different, for example, the mass fraction of carboxylic acid ester in the primary electrolyte can be 30%, and the mass fraction in the secondary electrolyte can be 32%. This disclosure does not limit this.

[0039] In some embodiments, the ratio 'a' of the mass fraction of carboxylic acid esters in the primary electrolyte and the secondary electrolyte has the following relationship: 0.8 ≤ a ≤ 1.15.

[0040] By sampling within this ratio range, the mass fraction of carboxylic acid esters in the secondary electrolyte is relatively close to that in the primary electrolyte. Reducing the difference in the mass fraction of carboxylic acid esters between the primary and secondary electrolytes helps to prevent the carboxylic acid esters in the secondary electrolyte from swelling or even dissolving into a partial SEI after being added. This would lead to severe damage to the SEI during storage, resulting in a large increase in battery impedance and a high DCR growth rate, thus deteriorating the battery's long-term cycle and storage life.

[0041] In some embodiments, the carboxylic acid ester includes any one or a combination of at least two of ethyl acetate, methyl acetate, methyl formate, butyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl butyrate, butyl butyrate, isopropyl acetate, and isoamyl acetate.

[0042] Optionally, the carboxylic acid ester includes any one or a combination of at least two of ethyl acetate, methyl acetate, methyl propionate, and ethyl propionate, such as methyl acetate and methyl propionate, or a combination of ethyl acetate and methyl acetate, or a combination of ethyl acetate, methyl acetate, and methyl propionate.

[0043] In some embodiments, when the carboxylic acid ester comprises a combination of any one or at least two of ethyl acetate, methyl acetate, methyl propionate, and ethyl propionate, the mass fraction of the carboxylic acid ester is 8% to 40%.

[0044] It should be understood that the types of carboxylic acid esters can be the same in the primary and secondary electrolytes. For example, both the primary and secondary electrolytes may contain methyl acetate. Of course, the types of carboxylic acid esters can also be different in the primary and secondary electrolytes. For example, the primary electrolyte may contain ethyl acetate and methyl acetate, while the secondary electrolyte may contain only methyl acetate.

[0045] It should be noted that, for similar reasons as the aforementioned ratio range, the types of carboxylic acid esters in the primary and secondary electrolytes are quite similar. Reducing the difference in the types of carboxylic acid esters between the primary and secondary electrolytes helps to avoid the swelling or even dissolution of carboxylic acid esters in the secondary electrolyte into a partial SEI after its addition. This would lead to a significant increase in SEI damage during storage, resulting in a large DCR growth rate in the battery impedance and deteriorating the battery's long-term cycle and storage life.

[0046] In some embodiments, the carbonate includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0047] In some embodiments, the mass fraction P of the primary electrolyte injected into the lithium-ion battery is 70% to 95% of the total electrolyte, for example, 70%, 75%, 80%, 83%, 88%, 90%, 95%, etc.

[0048] In some embodiments, the primary electrolyte further includes a second additive; the second additive includes vinylene carbonate (VC), ethylene ethylene carbonate, 1,3 Propanesulfonate lactone (PS), 1,3 Any one or a combination of at least two of the following: propylene sulfonate lactone (PST), vinyl sulfate (DTD), lithium difluorophosphate, lithium dioxalate borate, lithium tetrafluoroborate, and lithium difluorooxalate phosphate.

[0049] For example, the second additive may be vinylene carbonate, or it may be 1,3-vinylene carbonate, ... The combination of propane sulfonate lactone and lithium difluorophosphate is not limited in this disclosure.

[0050] In some embodiments, the mass fraction of the second additive in the primary electrolyte is 0.2% to 3.5%, for example, 0.2%, 0.8%, 1.5%, 2.0%, 2.3%, 3%, 3.5%, etc.

[0051] In some embodiments, the primary electrolyte and the secondary electrolyte further comprise lithium salts, wherein the lithium salts include any one or a combination of at least two of LiFSI, LiTFSI, LiPF6, LiBF4, LiClO4, or LiAsF6. For example, the lithium salt may be LiPF6 or LiFSI.

[0052] Furthermore, the mass fraction of lithium salt in the primary or secondary electrolyte is 10% to 20%. For example, the mass fraction of lithium salt can be 10%, 12%, 13%, 15%, 16%, 17%, or 20%.

[0053] Optionally, the mass fraction of lithium salt in the primary or secondary electrolyte is 12% to 18%, for example, 12%, 14%, 15.5%, 16%, 17%, or 18%.

[0054] Lithium-ion batteries A second aspect of this disclosure provides a lithium-ion battery, including the electrolyte provided above. Here, the lithium-ion battery can be a primary lithium-ion battery or a secondary lithium-ion battery; this disclosure does not limit the specific type.

[0055] In some embodiments, a lithium-ion battery includes a positive electrode, a negative electrode, a separator located between the positive and negative electrodes, and an electrolyte. During battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0056] Positive electrode sheet The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0057] For example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0058] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (e.g., polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0059] In some embodiments, the positive electrode active material includes Li x Ni y Mn z M m O 0.5x+1.5y+1.5z+1.5m Wherein, 0.9≤x≤1.1, 0.8≤y≤1, 0≤z≤0.2, 0≤m≤0.2; and M is selected from one or more of Fe, Ti, Co, Cr, V, Cu, Zn, Zr, Al, and Nb. Optionally, 0.9≤y≤1.

[0060] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0061] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0062] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0063] Negative electrode sheet The negative electrode includes a negative current collector and a film layer optionally disposed on at least one surface of the negative current collector.

[0064] For example, the film layer may include a negative electrode active material (e.g., artificial graphite), a conductive agent, a thickener, and a binder.

[0065] In some embodiments, the negative electrode active material includes any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbide, or lithium titanate.

[0066] Optionally, the negative electrode active material is a mixture of silicon carbide and graphite; wherein, based on the total mass of the negative electrode active material, the mass fraction of the silicon carbide is 5% to 35%, for example 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%.

[0067] For example, carbon-based active materials include any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, and natural graphite.

[0068] In some embodiments, the negative electrode sheet can be prepared by dispersing the negative electrode active material, conductive agent, thickener, binder and any other components in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0069] Separating membrane In some embodiments, the secondary battery further includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0070] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0071] Preparation method The preparation method of lithium-ion batteries is illustrated by the following exemplary embodiments: The positive electrode, negative electrode, and separator are assembled into a bare cell. The bare cell is injected with a single-fill electrolyte, sealed, and then subjected to formation and aging to obtain a lithium-ion cell. The secondary electrolyte is injected into the lithium-ion cell, and the cell is sealed and left to stand to obtain the lithium-ion battery.

[0072] Example The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0073] The specific preparation method of the secondary lithium-ion battery of Embodiment 1 of this disclosure is as follows: (1) Preparation of lithium nickel cobalt manganese oxygen cathode: The positive electrode active material (LiNi) 0.9 Co 0.05 Mn 0.05 O2), polyvinylidene fluoride as a binder, and Super P as a conductive agent are mixed in a mass ratio of 98:1:1. N-methylpyrrolidone (NMP) is added, and the mixture is stirred under vacuum until the system is homogeneous and transparent to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated onto an aluminum foil. After the aluminum foil is dried at room temperature, it is transferred to an oven for drying. Then, it is cold-pressed and slit to obtain the positive electrode (electrode sheet).

[0074] (2) Preparation of graphite-silicon composite anode: Artificial graphite and silicon-carbon composite were mixed at a mass ratio of 9:1. The mixture served as the negative electrode active material, Super P as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) as the thickener, and styrene-butadiene rubber (SBR) as the binder. The mixture was then mixed at a mass ratio of 96:1:1:2, and deionized water was added. The mixture was stirred in a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto a copper foil current collector. The copper foil was dried at room temperature and then transferred to an oven for drying. After cold pressing and slitting, the negative electrode (electrode sheet) was obtained.

[0075] (3) Preparation of electrolyte: In an argon-atmosphere glove box with a water content of <10 ppm, battery-grade ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 1:1:1:7 to form a base solvent. The lithium salt in each of the primary and secondary electrolytes was 12.5% ​​lithium hexafluorophosphate based on the mass of the primary and secondary electrolytes, respectively. Other components were added quantitatively according to the electrolyte composition described in the table, and mixed thoroughly to obtain the primary / secondary electrolyte (i.e., Example 1 in Table 1). In Tables 1-3, the content of each component in the electrolyte is a mass percentage calculated based on the total mass of the primary or secondary electrolyte. The electrolytes in other examples and comparative examples were prepared in the same way as in this example, except that the component ratios are specified in the tables. It should be noted that if the total content of all components in the electrolyte is less than 100%, it is supplemented by the base solvent.

[0076] (4) Preparation of the separating membrane: Polypropylene film is used as the separator.

[0077] (5) Preparation of secondary batteries: A 12 μm thick polypropylene (PP) film was used as the separator. The prepared positive electrode, separator, and negative electrode were stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. Then, an aluminum-plastic film was wrapped around the separator, and the cells were dried in a vacuum oven at 120 °C. The total electrolyte injection coefficient was 3.0 g / Ah. After injecting 2.7 g / Ah of electrolyte into the dry cell, it was sealed and subjected to electrolyte formation and aging. The formation process involved charging the battery at 0.05C to 3.05V under 0.1 MPa pressure and 45 °C, followed by charging at 0.1C to 3.75V; then, the battery was placed in a 45 °C oven for 48 hours to complete the aging process. After formation and aging, the cell opening is injected with 0.3 g / Ah of secondary electrolyte for final sealing and settling, and finally a soft-pack battery with a capacity of 1 Ah (i.e., the lithium-ion battery of Example 1) is prepared. The electrolytes in other examples and comparative examples are prepared in the same way as in this example, except that the component ratios are specified in the table.

[0078] Test methods The secondary batteries prepared in the above embodiments and comparative examples can be tested using the following methods: (1) DC resistance (DCR) test of secondary battery at 25℃ At 25 °C, when the battery is discharged to 50% SOC (state of charge, reflecting the battery's remaining capacity) at a 1 C current, the current is increased to 4 C and maintained for 30 s. The difference between the updated stable voltage and the original platform voltage is measured, and the ratio of this difference to the 4 C current value is the battery's DC resistance. The DCR test result performed after the battery's first full charge is the battery's initial DCR.

[0079] (2) Capacity retention rate of secondary batteries under specific cycling conditions At a specific test temperature, the battery is charged at a constant current rate to 4.25 V, then charged at a constant voltage of 4.25 V until the current is less than 0.05 C. After resting for 10 minutes, it is discharged at a constant current rate of 1 C to 2.5 V. The discharge capacity of the battery at this time is tested, which is the discharge capacity of the first cycle. The battery is cycled multiple times under the above conditions, and the capacity retention rate of the battery after 400 cycles is calculated.

[0080] Calculate the capacity retention relative to the cycle using the following formula: Capacity retention rate (%) = (Discharge capacity after 400 cycles / Discharge capacity after the first cycle) × 100%.

[0081] It should be noted that the number of cycles here can be adjusted, for example, 800 cycles, and this disclosure does not limit it.

[0082] (3) Capacity recovery test of secondary battery after 60 days of full charge storage at 55℃ At 25 °C, the battery was charged at a constant current of 1 / 3C to 4.25 V, then charged at a constant voltage of 4.25 V until the current was less than 0.05 C. After resting for 10 minutes, it was discharged at a constant current of 1 / 3C to 2.5 V. The discharge capacity of the battery at this point was measured and recorded as the discharge capacity C0 for 0 days of cycling. The battery was then charged at a constant current of 1 / 3C to 4.25 V, then charged at a constant voltage of 4.25 V until the current was less than 0.05 C, and placed at 55 °C. The fully charged lithium-ion battery was stored at 55 °C for 60 days. The battery was then discharged at a constant current of 1 / 3C to 2.5 V at 25 °C, then charged at a constant current of 1 / 3C to 4.25 V, and then charged at a constant voltage until the current was 0.05 C. The battery was then discharged at a constant current of 1 / 3C to 2.5 V, and the discharge capacity was recorded as C1. The capacity recovery rate was (C1 / C0) × 100%.

[0083] (4) DCR growth rate after 90 days of storage at 60℃ After formation, the battery was charged to the cutoff voltage at room temperature using a 1C constant current and constant voltage method. It was then discharged to 50% SOC using a 1C current, followed by a 30s discharge at 4C to test the DCR, which was recorded as DCR0. Next, it was discharged to the cutoff voltage at 1C, then charged to the cutoff voltage using a 1C constant current and constant voltage method. It was then stored at 60°C for 90 days. After the battery cooled to room temperature, the DCR was tested under the same conditions, and this DCR was recorded as DCR0. 90 The DCR storage growth rate is (DCR 90 / DCR0-1)×100%.

[0084] (5) Analysis of lithium plating in secondary batteries after 50 cycles at 6C at 25℃ At 25℃, the battery was charged at a constant current rate of 6C to 4.25V, then charged at a constant voltage of 4.25V until the current was less than 0.05C. After resting for 10 minutes, it was discharged at a constant current rate of 1C to 2.5V. The discharge capacity of the battery at this point was measured, which is the discharge capacity of the first cycle. The battery was cycled 50 times under the above conditions. Finally, the battery was charged at a constant current rate of 1C to 4.25V, then charged at a constant voltage of 4.25V until the current was less than 0.05C. After resting for 10 minutes, it was disassembled to observe the lithium deposition on the negative electrode surface.

[0085] It should be noted that for lithium nickel cobalt manganese oxide / silicon-graphite batteries, the charge / discharge cutoff voltage is 2.5~4.25V.

[0086] Table 1. Performance test results of lithium-ion batteries in Examples 1-36 and Comparative Examples 1-4

[0087] Note: The mass fraction of carboxylic acid esters in the primary and secondary electrolytes in Table 1 is the same.

[0088] Comparing Examples 1 to 36 with Comparative Example 1, it can be seen that the high-rate cycle performance of the lithium-ion battery with two injections is better than that of the lithium-ion battery with a single injection.

[0089] Comparing Examples 1-6 with Example 35, it can be seen that the mass fraction of fluoroethylene carbonate in the primary electrolyte affects the performance of the lithium-ion battery. When the mass fraction of fluoroethylene carbonate in the primary electrolyte is greater than 0.1%, for example, 0.2% to 3%, the overall performance of the lithium-ion battery is better.

[0090] Comparing Examples 1, 7-13, 34 and 36, it can be seen that when the mass fraction of fluoroethylene carbonate in the secondary electrolyte is 15%-40%, the overall performance of the lithium-ion battery is better.

[0091] Comparing Examples 1, 12, and 13, it can be seen that the electrolyte system provided by the embodiments of this disclosure, which includes a primary electrolyte and a secondary electrolyte, offers more choices for the type and mass fraction of the second additive in the primary electrolyte, thereby improving the high-temperature storage performance of the battery.

[0092] Comparing Examples 1 and 14-16, it can be seen that the structural formulas (II), (III), (IV), and (V) of the first additive have similar effects on improving the cycle performance and high-temperature storage performance of lithium-ion batteries, and can all effectively improve the cycle performance and high-temperature storage performance of lithium-ion batteries.

[0093] As can be seen from Examples 17 to 21, in the total electrolyte injected into the lithium-ion battery, the lithium-ion battery exhibits better cycle performance and high-temperature storage performance when the mass fraction of the electrolyte injected in the first injection is in the range of 70% to 95%.

[0094] Comparative studies of Examples 1, 22-25, 33, 3, and 4 show that adding a first additive to the secondary electrolyte can improve the high-temperature storage performance of lithium-ion batteries. However, increasing the amount of the first additive may reduce the high-rate cycle performance of the lithium-ion batteries. The lithium-ion batteries exhibit the best overall performance when the mass fraction of the first additive in the secondary electrolyte is 0.1% to 0.7%.

[0095] Comparing Examples 1, 26-28, 32, and Comparative Example 2, it can be seen that the high-rate cycle performance of lithium-ion batteries is very poor when the primary and secondary electrolytes do not contain carboxylic acid esters. Increasing the content of carboxylic acid esters in the primary and secondary electrolytes gradually improves the high-rate cycle performance of lithium-ion batteries, but reduces their high-temperature storage performance. When the mass fraction of carboxylic acid ester in the primary or secondary electrolyte is 4% to 60%, the overall performance of the lithium-ion battery is better. Preferably, the mass fraction of carboxylic acid ester in the primary or secondary electrolyte is 8% to 40%.

[0096] Comparing Examples 1 and Examples 29-31, it can be seen that adding methyl acetate, ethyl acetate, methyl propionate, and ethyl propionate to the primary and secondary electrolytes has a similar effect on improving the performance of lithium-ion batteries.

[0097] Table 2. Performance test results of lithium-ion batteries in Examples 13, 37-46

[0098] Comparing Examples 13 and 37-46, it can be seen that when the ratio 'a' of the mass fractions of carboxylic acid esters in the primary electrolyte and the secondary electrolyte is in the range of 0.8 ≤ a ≤ 1.15, the DCR growth rate of the lithium-ion battery after high-temperature storage is better.

[0099] Table 3. Performance test results of lithium-ion batteries in Examples 26, 47-48 and Comparative Examples 5 and 6

[0100] As can be seen from Table 3, adding carboxylic acid esters to the primary and secondary electrolytes can significantly improve the problems of purple spots and lithium plating in lithium-ion batteries after high-rate cycling.

[0101] It should be noted that the compound corresponding to structural formula (II) is only representative of structural formula (I). Other compounds that satisfy structural formula (I) have similar effects in the battery cell as the compound corresponding to structural formula (II), and will not be described in detail here.

[0102] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of this disclosure (including the claims) is limited to these examples; within the framework of this disclosure, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of the embodiments of this disclosure as described above, which are not provided in detail for the sake of brevity.

[0103] This disclosure is intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure.

Claims

1. A lithium-ion battery based on a secondary electrolyte injection process, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte; the electrolyte includes a primary electrolyte and a secondary electrolyte, and the lithium-ion battery is manufactured using secondary electrolyte injection. The primary electrolyte solution includes carboxylic acid esters, carbonates, and fluoroethylene carbonates. The secondary electrolyte comprises carboxylic acid esters, carbonates, fluoroethylene carbonates, and a first additive shown in formula (I): Equation (I); In formula (Ⅰ), R1 and R2 may be the same or different, and are each independently selected from: H, C1-C4 straight-chain or branched alkyl groups; wherein, The mass fraction of the fluoroethylene carbonate in the primary electrolyte is less than the mass fraction of the fluoroethylene carbonate in the secondary electrolyte.

2. The lithium-ion battery according to claim 1, characterized in that, Based on the total mass of the secondary electrolyte, the mass fraction of the first additive is 0.1% to 0.7%, optionally 0.2% to 0.5%.

3. The lithium-ion battery according to claim 1, characterized in that, The structural formula of the first additive is selected from at least one of the following structural formulas; Formula (II), Formula (III) Formula (IV), Formula (V).

4. The lithium-ion battery according to claim 1, characterized in that, The fluoroethylene carbonate has a mass fraction of 0.2% to 3% in the primary electrolyte solution; and / or The fluoroethylene carbonate has a mass fraction of 15% to 40% in the secondary electrolyte; and / or The carboxylic acid ester has a mass fraction of 4% to 60% in the primary or secondary electrolyte, and optionally 8% to 40%.

5. The lithium-ion battery according to claim 1, characterized in that, The ratio 'a' of the mass fraction of carboxylic acid esters in the primary electrolyte and the secondary electrolyte has the following relationship: 0.8 ≤ a ≤ 1.15; and / or The primary electrolyte injection accounts for 70% to 95% of the total mass fraction of the electrolyte injected into the lithium-ion battery.

6. The lithium-ion battery according to claim 1, characterized in that, The primary electrolyte also includes a second additive; the second additive includes vinylene carbonate, ethylene ethylene carbonate, and 1,3-ethylhexylene carbonate. Propanesulfonate lactone, 1,3 Any one or a combination of at least two of the following: propylene sulfonate lactone, vinyl sulfate, lithium difluorophosphate, lithium dioxaborate, lithium tetrafluoroborate, and lithium difluorooxaborate. The second additive has a mass fraction of 0.2% to 3.5% in the primary electrolyte solution.

7. The lithium-ion battery according to claim 1, characterized in that, The carboxylic acid ester includes any one or a combination of at least two of ethyl acetate, methyl acetate, methyl formate, butyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl butyrate, butyl butyrate, isopropyl acetate, and isoamyl acetate; and / or The carbonate includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

8. The lithium-ion battery according to claim 1, characterized in that, The primary electrolyte and the secondary electrolyte further comprise lithium salts, which include any one or a combination of at least two of LiFSI, LiTFSI, LiPF6, LiBF4, LiClO4, or LiAsF6. The lithium salt in the primary electrolyte or the secondary electrolyte has a mass fraction of 10% to 20%, optionally 12% to 18%.

9. The lithium-ion battery according to claim 1, characterized in that, The positive electrode sheet includes a positive active material; the negative electrode sheet includes a negative active material; The positive electrode active material includes Li x Ni y Mn z M m O 0.5x+1.5y+1.5z+1.5m Wherein, 0.9≤x≤1.1, 0.8≤y≤1, 0≤z≤0.2, 0≤m≤0.2; where, M is selected from one or more of Fe, Ti, Co, Cr, V, Cu, Zn, Zr, Al and Nb; optionally, 0.9≤y≤1; The negative electrode active material comprises a mixture of silicon carbide and carbon-based active material; wherein the mass fraction of silicon carbide in the negative electrode active material is 5% to 35%, and the carbon-based active material comprises any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, and natural graphite.

10. The method for preparing a lithium-ion battery according to any one of claims 1 to 9, characterized in that, include: Prepare the positive electrode, negative electrode, and separator; The positive electrode, the negative electrode, and the separator are assembled into a bare battery cell. The primary electrolyte is injected into the bare cell, sealed, and then subjected to formation and aging to obtain a lithium-ion cell. The secondary electrolyte is injected into the lithium-ion cell, and the cell is sealed and left to stand to obtain the lithium-ion battery.

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