Electrolyte and secondary battery

Abstract

Provided is an electrolyte, comprising a lithium salt, an organic solvent, and an additive, wherein the additive comprises a first additive, vinylene carbonate, and lithium tetrafluoroborate; the first additive is selected from a compound represented by formula 1, wherein R1, R2, and R3 are each independently selected from C1-C10 alkyl or C3-C10 cycloalkyl, and R4 is selected from C2-C5 alkenyl or C2-C5 alkynyl.

Description

Electrolyte and secondary battery

[0001] This application claims priority to Chinese Patent Application No. 202411683228.5, filed on November 22, 2024, entitled "An Electrolyte and a Secondary Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of secondary battery technology, and in particular to an electrolyte and a secondary battery. Background Technology

[0003] The rapid development and widespread application of various portable electronic devices, new energy electric vehicles, and energy storage systems in recent years have created an increasingly urgent demand for rechargeable batteries with high energy density, long cycle life, and safe operation. Increasing the charging voltage or the capacity of the active material are the main methods to improve the energy density of rechargeable batteries. However, in practical use, increasing the charging voltage or the capacity of the active material can lead to a series of problems, such as accelerating the decomposition and consumption of the electrolyte, exacerbating interfacial side reactions in the battery cell, resulting in gas generation and increased impedance, thereby affecting the cycle performance and storage performance of the rechargeable battery.

[0004] In secondary batteries, the electrolyte is a crucial factor affecting battery performance. Existing secondary batteries typically incorporate various additives into the electrolyte to form an organic passivation film on the surface of the active materials. This passivation film prevents further decomposition of the electrolyte, improving the battery's cycle life, high-temperature storage performance, and other characteristics. Therefore, developing an electrolyte for secondary batteries with excellent cycle performance and high-temperature storage capabilities is essential. Summary of the Invention

[0005] The purpose of this application is to provide an electrolyte and a secondary battery to improve the cycle performance and storage performance of the secondary battery. The specific technical solution is as follows:

[0006] The first aspect of this application provides an electrolyte comprising a lithium salt, an organic solvent, and an additive; the additive includes a first additive, vinylene carbonate, and lithium tetrafluoroborate.

[0007] The first additive is selected from the compound shown in Formula 1:

[0008] R1, R2, and R3 are each independently selected from C1-C10 alkyl or C3-C10 cycloalkyl, R4 is selected from C2-C5 alkenyl or C2-C5 alkynyl, and the dashed line represents a conjugated π bond.

[0009] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0010] The beneficial effects of this application are:

[0011] This application provides an electrolyte comprising a lithium salt, an organic solvent, and additives; the additives include a first additive, vinylene carbonate, and lithium tetrafluoroborate; the first additive is selected from compounds shown in Formula 1. Through the synergistic effect of the first additive, vinylene carbonate, and lithium tetrafluoroborate, the cycle performance and high-temperature storage performance of the secondary battery can be improved.

[0012] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0013] The technical solutions of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0014] This application provides an electrolyte comprising a lithium salt, an organic solvent, and additives; the additives include a first additive, vinylene carbonate, and lithium tetrafluoroborate; the first additive is selected from compounds shown in Formula 1:

[0015] R1, R2, and R3 are each independently selected from C1-C10 alkyl or C3-C10 cycloalkyl, R4 is selected from C2-C5 alkenyl or C2-C5 alkynyl, and the dashed line represents a conjugated π bond.

[0016] Adding vinylene carbonate to the electrolyte can form a solid electrolyte interface (SEI) film containing unsaturated bonds in organic polymers on the surface of the negative electrode, thereby improving the stability of the SEI film. However, during high-temperature storage or long-term cycling, the organic polymer components containing unsaturated bonds decompose, making the SEI film porous, increasing the impedance of the secondary battery, and degrading its performance. Boron atoms in lithium tetrafluoroborate can complex with the unsaturated bonds in the organic polymers, reducing polymer polymerization, and simultaneously forming a more stable oligomer SEI film with vinylene carbonate molecules, improving the high-temperature resistance of the secondary battery. However, the SEI film formed by vinylene carbonate and lithium tetrafluoroborate is not dense enough, resulting in a relatively high impedance in the secondary battery.

[0017] This application combines the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate. The compound of Formula 1 contains positively charged ammonium ions, which can stabilize the anionic portion of lithium tetrafluoroborate, enabling the compound of Formula 1 to form a more complex and stable oligomeric SEI film with vinylene carbonate and lithium tetrafluoroborate at the negative electrode, further improving the high-temperature resistance of the SEI. At the same time, the sulfonic acid group and phosphonofluorine group containing unsaturated bonds in the compound of Formula 1 are prone to decomposition to produce alkylated lithium sulfate, introducing sulfur and inorganic component LiF into the SEI film, further modifying the SEI film to form a low-impedance and dense SEI film, increasing ionic conductivity and improving the long-cycle performance of the secondary battery. In addition, the difluorophosphono group and boron group can improve the stability of the positive electrode interface structure of the secondary battery, reduce the extraction of transition metal ions under high voltage and high temperature conditions, thereby reducing the damage of transition metal ions to the negative electrode interface, reducing the occurrence of side reactions on the surface of the positive and negative electrode plates, effectively reducing the consumption of electrolyte during high-temperature storage and long-cycle, and improving the high-temperature storage performance and long-cycle performance of the secondary battery. This application combines the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate. Through the synergistic effect of the three, the high-temperature storage performance and long-cycle performance of the secondary battery can be improved.

[0018] In some embodiments of this application, the electrolyte satisfies at least one of the following conditions:

[0019] (1) Based on the total mass of the electrolyte, the mass percentage of the first additive is A, 0.1% ≤ A ≤ 4%, preferably 0.1% ≤ A ≤ 2%; (2) Based on the total mass of the electrolyte, the mass percentage of vinylene carbonate is B, 2% ≤ B ≤ 4%; (3) Based on the total mass of the electrolyte, the mass percentage of lithium tetrafluoroborate is C, 0.1% ≤ C ≤ 2%.

[0020] In some embodiments of this application, based on the total mass of the electrolyte, the mass percentage A of the first additive can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or a range of any two of these values; based on the total mass of the electrolyte, the mass percentage B of vinylene carbonate can be 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, or a range of any two of these values; based on the total mass of the electrolyte, the mass percentage C of lithium tetrafluoroborate can be 0.1%, 0.3%, 0.5%, 0.7%, 1%, 1.3%, 1.5%, 1.7%, 2%, or a range of any two of these values. By controlling the mass percentage content of the first additive, vinylene carbonate, and lithium tetrafluoroborate within the scope of this application, a low-resistance and dense SEI film can be formed on the surface of the negative electrode through the synergistic effect of the compound of Formula 1, vinylene carbonate, and lithium tetrafluoroborate. At the same time, it is beneficial to improve the structural stability of the positive electrode interface film, and further improve the high-temperature storage performance and long-cycle performance of the secondary battery.

[0021] In some embodiments of this application, the first additive is selected from at least one of the following compounds:

[0022] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0023] In some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer including a negative electrode active material; the specific surface area of ​​the negative electrode active material is S m 2 / g, the areal density of the negative electrode material layer is W g / m 2 During the formation and volume determination process, based on the total mass of the electrolyte, the mass percentage consumed by the first additive is a, the mass percentage consumed by vinylene carbonate is b, and the mass percentage consumed by lithium tetrafluoroborate is c. Preferably, For example, It can be 0.01, 0.02, 0.029, 0.03, 0.04, 0.045, 0.05, 0.06, 0.07, or a range of any two of these values.

[0024] Through the synergistic effect of the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate, a low-resistance and dense SEI film can be formed on the surface of the negative electrode, effectively reducing the electrolyte consumption during the formation and capacitance stage. Since the electrode reaction is mainly concentrated on the surface of the positive and negative electrodes, the larger the specific surface area of ​​the active material, the larger the contact area between the electrode and the electrolyte under the same apparent volume and sufficient electrolyte wetting conditions, the easier the electrode reaction is to proceed, the smaller the polarization, and the better the performance of the secondary battery. However, an excessively large specific surface area S of the negative electrode active material directly affects the area and thickness of the SEI film, thus affecting the film formation quality and the performance of the secondary battery. At the same time, the areal density W of the negative electrode material layer is the mass of coating per unit area on one side of the negative electrode material layer. The smaller the areal density of the negative electrode material layer, the easier it is for the electrolyte to wet the negative electrode, and the better the kinetic properties of the negative electrode surface. However, an excessively small W will directly affect the energy density of the secondary battery, and the coating process of the slurry will also be more difficult to control. Furthermore, the specific surface area of ​​the negative electrode active material and the areal density of the negative electrode material layer also affect the amount of additives consumed in the electrolyte. Therefore, the specific surface area of ​​the negative electrode active material and the areal density of the negative electrode material layer directly affect the quality of the SEI film and the performance of the secondary battery.

[0025] The formation and capacity determination process is mainly used to activate the active materials within the secondary battery and determine its capacity. During this process, a portion of the electrolyte is consumed to form an SEI film on the surface of the negative electrode, thereby adjusting the internal reaction state of the secondary battery and improving its performance and stability. The ratio of the sum of the mass percentages of additives used (A+B+C) based on the total mass of the electrolyte to the sum of the mass percentages of additives consumed during the formation and capacity determination process (a+b+c) is calculated. This method can intuitively express the degree of additive consumption during the formation and volume determination process. During experimental research, the inventors discovered that by limiting the types and amounts of additives in the electrolyte, and by limiting the amount of additives consumed during the formation and volume determination process, a relationship can be achieved between the specific surface area S of the negative electrode active material and the areal density W of the negative electrode material layer. When the electrolyte is in use, it can form a stable interfacial film on the surface of the positive and negative electrode plates, thus enhancing the interfacial stability of the positive and negative electrode plates. The addition of the compound with the structure of Formula 1 can reduce the amount of additives consumed during the formation and volume fixation process. The sulfonic acid group and phosphonofluorine group containing unsaturated bonds give the compound of Formula 1 excellent film-forming properties. It can work synergistically with vinylene carbonate and lithium tetrafluoroborate to improve the stability of the SEI film. At the same time, it can inhibit the continuous decomposition of the electrolyte during cycling and storage, effectively reducing the increase in impedance caused by the aggravation of interfacial side reactions due to electrolyte consumption, thereby affecting the cycling and storage performance.

[0026] In this application, the formation conditions are as follows: the secondary battery is placed in an environment of 25℃~45℃ and charged with a constant current to 50%~60% charge state, with the constant current being 0.05C~0.15C. The capacity determination conditions are as follows: the secondary battery is charged with a constant current to the cutoff voltage and left to stand for 5~10 minutes; it is then discharged with a constant current to the cutoff voltage; and the battery capacity is determined based on the capacity output by the 1C constant current discharge.

[0027] In some embodiments of this application, 1m 2 / g≤S≤2m 2 / g, 60g / m 2 ≤W≤110g / m 2 .

[0028] In some embodiments of this application, 1.2m 2 / g≤S≤1.5m 2 / g.

[0029] In some embodiments of this application, 80g / m 2 ≤W≤100g / m 2 .

[0030] For example, the specific surface area of ​​the negative electrode active material can be 1.0 m². 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.5m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g or any two of these values ​​can be used to define a range, and the areal density of the negative electrode material layer can be 60g / m². 2 70g / m 2 80g / m 2 90g / m 2 100g / m 2 110g / m 2 Or it can be a range consisting of any two of these values. Controlling the specific surface area of ​​the negative electrode active material and the areal density of the negative electrode material layer within the range of this application is beneficial to forming an SEI film of moderate thickness and superior quality, further suppressing the continuous decomposition of the electrolyte, and improving the high-temperature storage performance and long-cycle performance of the secondary battery.

[0031] In some embodiments of this application, the secondary battery of the second aspect of this application includes lithium-ion secondary batteries and sodium-ion secondary batteries.

[0032] In this application, the electrolyte includes a non-aqueous organic solvent. As an important carrier for ion transport, the non-aqueous organic solvent, after dissolving the electrolyte, can exhibit high electronic conductivity, 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, decanolactone, 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, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous organic solvents in the electrolyte, as long as the purpose of this application is achieved. For example, based on the total mass of the electrolyte, the mass percentage of non-aqueous organic solvents can be 70% to 85%.

[0033] In this application, the electrolyte includes an electrolyte. After dissolving in the electrolyte, the electrolyte releases lithium ions or sodium ions. These lithium ions or sodium ions form a solvation structure with the solvent, which facilitates rapid ion migration. This application does not impose any particular limitation on the electrolyte, as long as it achieves the purpose of this application. For example, the electrolyte may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, LiFSI, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluoroborate, sodium hexafluorophosphate, and sodium bis(fluorosulfonyl)imide. This application does not impose any particular limitation on the content of the electrolyte in the electrolyte, as long as it achieves the purpose of this application. For example, based on the total mass of the electrolyte, the mass percentage of the electrolyte is 8% to 18%. For example, based on the total mass of the electrolyte, the mass percentage of the electrolyte can be 8%, 10%, 12%, 14%, 16%, 18%, or a range of any two of these values. Controlling the mass percentage of the electrolyte within the range specified in this application allows for sufficient dissolution of the electrolyte in non-aqueous organic solvents, while simultaneously providing the electrolyte with both high ionic conductivity and low manufacturing cost.

[0034] The secondary battery of this application also 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 can be disposed on one surface or two surfaces in the thickness direction of the positive current collector. 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 can include a metal foil or a composite current collector. For example, the metal foil is 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 have any particular limitation on the thickness of the positive current collector and the positive electrode material layer, as long as it can achieve the purpose of this application. 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 200 μm. The positive electrode material layer of this application may also contain a conductive agent and a binder. This application does not have any particular limitations on the conductive agent and binder, as long as they can achieve the purpose of this application. 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.

[0035] In some embodiments of this application, the positive electrode active material may be any positive electrode active material known in the art for use in secondary batteries. For example, positive electrode active materials include, but are not limited to, lithium iron phosphate materials, nickel-cobalt-manganese ternary materials, lithium cobalt oxide materials, lithium manganese iron phosphate materials, lithium manganese oxide materials, lithium nickel manganese oxide materials, lithium-rich manganese-based materials, sodium iron phosphate materials, and three-dimensional tunneling Na+ materials. 0.44 At least one of MnO2.

[0036] The secondary 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.).

[0037] 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 16 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, sodium 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), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium 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.

[0038] 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.

[0039] The secondary 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.

[0040] The secondary 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.

[0041] The preparation process of the secondary battery described in 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 secondary 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 secondary 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 secondary battery from rising and overcharging / discharging.

[0042] The secondary 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.

[0043] Example

[0044] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0045] Test methods and equipment:

[0046] Specific surface area testing of negative electrode active materials

[0047] The specific surface area of ​​the negative electrode active material was tested using a specific surface area analyzer (model: Beijing Analytical Instrument Factory ST-08 BET analyzer) via nitrogen adsorption. The specific testing was conducted in accordance with the national standard GB / T 19587-2017 "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0048] Anode material layer density test

[0049] The areal density of the negative electrode material layer is tested using a weighing method, with the following steps: First, the negative electrode material layer of the secondary battery is separated from the negative electrode sheet. Then, the mass of the negative electrode material layer is weighed using a balance. Next, the area of ​​the negative electrode sheet is measured using vernier calipers. Finally, the areal density of the negative electrode material layer is obtained by dividing the mass of the negative electrode material layer by the area of ​​the negative electrode sheet.

[0050] Additive consumption test during formation and volume adjustment

[0051] The secondary batteries of each embodiment and comparative example were disassembled after being prepared and brought to a constant volume. The electrolyte was collected, and the disassembled positive electrode, negative electrode, and separator were centrifuged. The liquid obtained after centrifugation was mixed evenly with the electrolyte to obtain a liquid sample. The mass percentage of the first additive, vinylene carbonate, and lithium tetrafluoroborate in the prepared and brought to a constant volume electrolyte was determined by gas chromatography-mass spectrometry (GC-MS) / ion chromatography and recorded as x, y, and z.

[0052] During the formation and volume adjustment process, the consumption of the first additive is a = Ax, the consumption of vinylene carbonate is b = By, and the consumption of lithium tetrafluoroborate is c = Cz.

[0053] Tests on capacity retention and electrolyte consumption during ambient temperature cycling

[0054] The secondary battery was placed in a constant temperature environment of 25℃ and charged at a constant current of 1C until the voltage reached 3.65V. It was then charged at a constant voltage of 3.65V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1C until the voltage reached 2.5V. The capacity of the first discharge was recorded as C1. This charging and discharging process was repeated 1200 times, and the capacity of the 1200th discharge was recorded as C. 1200 .

[0055] Room temperature cycling capacity retention (%) = (C 1200 / C1)×100%.

[0056] After the secondary battery underwent the above-mentioned room temperature cycling performance test, its electrolyte consumption was tested. After opening the secondary battery, 3g of dichloromethane was injected into the electrolyte inlet. The secondary battery was then placed in an ultrasonic machine and sonicated for 60 minutes to ensure uniform mixing of the dichloromethane and electrolyte. After standing at room temperature for 10 minutes, the battery was disassembled in a glove box with a moisture content below 0.1ppm and an oxygen content below 0.1ppm. The electrolyte was removed, and the mass percentage of dichloromethane in the electrolyte was determined using gas chromatography-mass spectrometry (GC-MS) / ion chromatography. The mass of the remaining electrolyte was calculated using the injected dichloromethane volume and the mass percentage of dichloromethane in the electrolyte. The electrolyte injection volume refers to the amount of electrolyte injected during the secondary battery manufacturing process.

[0057] High-temperature storage capacity retention rate and high-temperature storage electrolyte consumption test

[0058] The secondary battery was placed in a constant temperature environment of 25℃ and charged at a constant current of 1C until the voltage reached 3.65V. Then, it was charged at a constant voltage at 3.65V until the cutoff current reached 0.05C. Next, it was discharged at a constant current of 1C until it reached 2.5V. The capacity of this first discharge was recorded as the capacity before storage, denoted as C0. The battery was then charged again at a constant current of 1C until the voltage reached 3.65V, and then charged at a constant voltage at 3.65V until the cutoff current reached 0.05C. The secondary battery was then stored in a 60℃ explosion-proof oven for 60 days. Finally, it was discharged at a constant current of 1C until it reached 2.5V. This capacity after storage was recorded as C. m .

[0059] High-temperature storage capacity retention rate (%) = (C m / C0)×100%.

[0060] The electrolyte consumption of the secondary battery after the high-temperature storage performance test was tested using the same method as the electrolyte consumption test of the secondary battery after the room-temperature cycling performance test. The electrolyte injection volume refers to the amount of electrolyte injected during the secondary battery manufacturing process.

[0061] Example 1

[0062] <Preparation of Electrolyte>

[0063] Under an inert atmosphere with moisture <0.1 ppm and oxygen <0.1 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 30:70 to obtain the base solvent. LiPF6 as the electrolyte, compound of formula 1-1, vinylene carbonate, and lithium tetrafluoroborate as additives were added to the base solvent, and the mixture was thoroughly mixed to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentages of compound of formula 1-1 were 0.05%, vinylene carbonate 3%, lithium tetrafluoroborate 1%, and LiPF6 12.5%, with the remainder being the base solvent.

[0064] <Preparation of the positive electrode>

[0065] Lithium iron phosphate (LiFePO4, purchased from Shenzhen Defang Nanotechnology Co., Ltd.), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) binder were mixed at 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 68 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 single-sided positive electrode sheet with a coating thickness of 198 μm was obtained. The above steps were repeated on the other surface of the aluminum foil current collector to obtain a double-sided positive 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 positive electrode sheet with dimensions of 70 mm × 54 mm.

[0066] <Preparation of Negative Electrode Sheets>

[0067] A negative electrode active material, graphite (purchased from Jiangxi Zichen Technology Co., Ltd.), conductive agent, conductive carbon black (Super P), thickener, sodium carboxymethyl cellulose (CMC), and binder, styrene-butadiene rubber (SBR), were mixed in a mass ratio of 95:2:1:2. Deionized water was added, and the mixture was stirred evenly under vacuum to obtain a negative electrode slurry with a solid content of 49 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 negative electrode sheet with a single-sided coating of negative electrode material layer with a coating thickness of 131 μm was obtained. The above steps were repeated on the other surface of the copper foil current collector to obtain a negative electrode sheet with a double-sided coating of negative electrode material layer. After drying under vacuum at 80°C, the negative electrode sheet with a specification of 74 mm × 58 mm was obtained through processes such as cold pressing, edge trimming, cutting, slitting, sheet making, and welding tabs with adhesive.

[0068] The specific surface area of ​​the negative electrode active material is 1.4 m².2 / g, the areal density of the negative electrode material layer is 90g / m² 2 .

[0069] <Preparation of the diaphragm>

[0070] The diaphragm was a polyethylene diaphragm purchased from Shenzhen Xingyuan Material Technology Co., Ltd.

[0071] <Preparation of Secondary Batteries>

[0072] 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, followed by vacuum sealing, settling, formation, shaping, and volume adjustment processes to produce the secondary battery.

[0073] The electrolyte injection volume of the secondary battery is 6g. The specific steps of the formation process are as follows: the vacuum-sealed secondary battery is charged at a constant current of 0.05C for 30 minutes, then the battery is left to rest for 5 minutes, and then the battery is charged at a constant current of 0.1C for 360 minutes.

[0074] The specific steps of the capacity determination process are as follows: Charge the secondary battery at a constant current and constant voltage of 0.2C to 3.65V, let it rest for 5 minutes, and then discharge it at a constant current of 0.2C to 2.5V. Next, charge the battery at a constant current and constant voltage of 0.5C to 3.65V, let it rest for 5 minutes, and then discharge it at a constant current of 0.5C to 2.5V. Then, charge the battery at a constant current and constant voltage of 1C to 3.65V, let it rest for 5 minutes, and then discharge it at a constant current of 1C to 2.5V. The battery capacity determination is completed based on the capacity achieved during the 1C constant current discharge.

[0075] Examples 2 to 15

[0076] Except for adjusting the amounts of the first additive, vinylene carbonate, and lithium tetrafluoroborate (A, B, and C) according to Table 1, everything else is the same as in Example 1. When the values ​​of A, B, and C in the electrolyte change, the mass percentage of the base solvent changes accordingly, while the mass percentage of the electrolyte remains unchanged.

[0077] Examples 16 to 18

[0078] Except for adjusting the type of the first additive according to Table 1, everything else is the same as in Example 1.

[0079] Examples 19 to 25

[0080] Except for adjusting the specific surface area S of the negative electrode active material according to Table 1, the rest is the same as in Example 4.

[0081] Examples 26 to 31

[0082] Except for adjusting the areal density W of the negative electrode material layer according to Table 1, the rest is the same as in Example 4.

[0083] Example 32

[0084] Except for adjusting the amounts of the first additive, vinylene carbonate, and lithium tetrafluoroborate, as well as the specific surface area S and areal density W of the negative electrode active material according to Table 1, everything else is the same as in Example 4. When the values ​​of A, B, and C in the electrolyte change, the mass percentage of the base solvent changes accordingly, while the mass percentage of the electrolyte remains constant.

[0085] Comparative Examples 1 to 3

[0086] Except that the first additive, vinylene carbonate, or lithium tetrafluoroborate was not added to the electrolyte according to Table 1, everything else was the same as in Example 1. When the values ​​of A, B, and C in the electrolyte changed, the mass percentage of the base solvent changed accordingly, while the mass percentage of the electrolyte remained unchanged.

[0087] As can be seen from Examples 1 to 15 and Comparative Examples 1 to 3, this application combines the first additive, vinylene carbonate, and lithium tetrafluoroborate, and controls the mass percentage of the three components within the range specified in this application. This reduces electrolyte consumption during cycling and high-temperature storage, thereby improving the room-temperature cycling capacity retention and high-temperature storage capacity retention of the secondary battery. As can be seen from Examples 16 to 18, using the first additive within the scope of this application, the secondary battery exhibits higher room-temperature cycling capacity retention and high-temperature storage capacity retention, while also having lower electrolyte consumption.

[0088] The specific surface area S of the negative electrode active material and the areal density W of the negative electrode material layer affect the electrolyte consumption and the performance of the secondary battery. Examples 20 to 24 show that when the specific surface area S of the negative electrode active material is controlled within the range of this application, the secondary battery exhibits lower electrolyte consumption during cycling and high-temperature storage, and higher capacity retention rates during both room-temperature cycling and high-temperature storage. Examples 19 and 25 show that when the specific surface area S of the negative electrode active material exceeds the range of this application, the secondary battery exhibits higher electrolyte consumption during cycling and high-temperature storage, and lower capacity retention rates during both room-temperature cycling and high-temperature storage. Examples 26 to 31 show that when the areal density W of the negative electrode material layer is controlled within the range of this application, the secondary battery exhibits lower electrolyte consumption during cycling and high-temperature storage, and higher capacity retention rates during both room-temperature cycling and high-temperature storage.

[0089] As can be seen from Examples 1 to 32, The value is controlled within the scope of this application. The secondary battery has low electrolyte consumption during cycling and high-temperature storage, and high capacity retention rate during room temperature cycling and high-temperature storage.

[0090] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrolyte comprising a lithium salt, an organic solvent, and an additive; said additive comprising a first additive, vinylene carbonate, and lithium tetrafluoroborate; The first additive is selected from the compound shown in Formula 1: in, R1, R2, and R3 are each independently selected from C1-C10 alkyl or C3-C10 cycloalkyl, R4 is selected from C2-C5 alkenyl or C2-C5 alkynyl, and the dashed line represents a conjugated π bond.

2. The electrolyte according to claim 1, wherein, It satisfies at least one of the following conditions: (1) Based on the total mass of the electrolyte, the mass percentage of the first additive is A, 0.1% ≤ A ≤ 4%; (2) Based on the total mass of the electrolyte, the mass percentage of the vinylene carbonate is B, 2% ≤ B ≤ 4%; (3) Based on the total mass of the electrolyte, the mass percentage of lithium tetrafluoroborate is C, 0.1% ≤ C ≤ 2%.

3. The electrolyte according to claim 1 or 2, wherein, Based on the total mass of the electrolyte, the mass percentage of the first additive is A, where 0.1% ≤ A ≤ 2%.

4. The electrolyte according to any one of claims 1 to 3, wherein, The first additive is selected from at least one of the following compounds:

5. A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 4.

6. The secondary battery according to claim 5, wherein, The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer includes a negative electrode active material; the specific surface area of ​​the negative electrode active material is Sm. 2 / g, the areal density of the negative electrode material layer is W g / m 2 ; During the formation and volume adjustment process, based on the total mass of the electrolyte, the mass percentage consumed by the first additive is a, the mass percentage consumed by the vinylene carbonate is b, and the mass percentage consumed by the lithium tetrafluoroborate is c.

7. The secondary battery according to claim 6, wherein, 1m 2 / g≤S≤2m 2 / g,60g / m 2 ≤W≤110g / m 2 。 8. The secondary battery according to claim 6 or 7, wherein, 9. The secondary battery according to any one of claims 6 to 8, wherein, 1.2m 2 / g≤S≤1.5m 2 / g。 10. The secondary battery according to any one of claims 6 to 9, wherein, 80g / m 2 ≤W≤100g / m 2 .

Images