Electrolyte and lithium ion battery

Abstract

The invention provides an electrolyte and a lithium ion battery. The electrolyte comprises a compound shown in the formula 1, in the formula 1, R1 and R2 independently comprise at least one of fluoro or non-fluoro alkyl with the carbon atom number of 1-5, fluoro or non-fluoro alkenyl with the carbon atom number of 2-5 and fluoro or non-fluoro alkynyl with the carbon atom number of 2-5; and X1 and X2 are respectively and independently selected from alkyl with the carbon atom number of 1-5. According to the electrolyte, by introducing the compound as shown in the formula 1, a positive and negative electrode interface system with high stability and excellent lithium ion conduction performance can be constructed, so that the thermal shock safety and the low-temperature dynamic performance of the battery are synchronously improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to an electrolyte and a lithium-ion battery. Background Technology

[0002] With the rapid development of electric vehicles, large-scale energy storage, and high-end consumer electronics, unprecedentedly stringent requirements have been placed on the comprehensive performance of lithium-ion batteries. Among these, the safety and output capacity of batteries in a wide temperature range (especially in extreme high and low temperature environments) have become key bottlenecks restricting their further promotion and application. As the "blood" of the battery, the composition of the electrolyte directly determines the properties of the electrode / electrolyte interface, which is the core site affecting lithium-ion transport kinetics, side reactions, and battery safety.

[0003] Existing electrolytes are prone to decomposition and gas generation at high temperatures, especially under thermal shock conditions. The rapid increase in internal battery temperature triggers a violent exothermic side reaction between the electrolyte and the cathode material, potentially leading to massive gas generation, a sudden increase in internal pressure, and thermal runaway. Conversely, at low temperatures, existing electrolytes exhibit increased viscosity and decreased ionic conductivity, resulting in a significant increase in the impedance of the SEI film at the anode. This hinders lithium-ion desolvation and transmembrane transport, causing a sharp decline in usable battery capacity and power characteristics. Therefore, developing an electrolyte system that combines excellent thermal shock safety with superior low-temperature kinetic performance has become a critical technological bottleneck that the industry urgently needs to overcome. Summary of the Invention

[0004] In view of the above problems, the present invention provides an electrolyte that, by introducing the compound shown in Formula 1, can construct a highly stable interface system with excellent lithium-ion conductivity, thereby simultaneously improving the thermal shock safety and low-temperature kinetic performance of the battery.

[0005] The present invention also provides a lithium-ion battery, which, because it includes the above-mentioned electrolyte, has excellent thermal shock safety under high temperature conditions and excellent discharge performance under low temperature conditions.

[0006] In a first aspect, the present invention provides an electrolyte comprising: a compound of Formula 1:

[0007] Formula 1,

[0008] R1 and R2 each independently include at least one of the following: alkyl group with 1-5 carbon atoms (fluorinated or unfluorinated), alkenyl group with 2-5 carbon atoms (fluorinated or unfluorinated), and alkynyl group with 2-5 carbon atoms (fluorinated or unfluorinated); X1 and X2 each independently are selected from alkyl group with 1-5 carbon atoms.

[0009] The electrolyte as described above, wherein the compound represented by Formula 1 includes at least one of the following compounds:

[0010] Formula 1-1 Formula 1-2 Formula 1-3 Formula 1-4

[0011] Formulas 1-5 Formulas 1-6 Equations 1-7.

[0012] The electrolyte as described above, wherein, based on the total mass of the electrolyte, the mass content of the compound represented by Formula 1 is 0.01wt%-5wt%.

[0013] The electrolyte as described above further includes nitrile additives, wherein the mass content of the nitrile additives is 0.5wt%-8wt% based on the total mass of the electrolyte.

[0014] Preferably, the nitrile additive includes at least one selected from 1,3,6-hexanetrionitrile, glyceryltrionitrile, adiponitrile, succinic anhydride, glutaronitrile, adiponitrile, heptanonitrile, 1,4-dicyano-2-butene, ethylene glycol bis(propionitrile) ether, tri(3-cyanopropyl) phosphate, tri(3-cyanoethyl) phosphate, 1,3,5-pentanetrionitrile, 1,2,3,4-tetra(2-cyanoethoxy)butane, and xylitol penta(2-cyanoethyl) ether.

[0015] The electrolyte as described above further includes vinyl sulfate, and preferably, based on the total mass of the electrolyte, the mass content of vinyl sulfate is 0.01wt%-4wt%.

[0016] The electrolyte as described above further includes fluoroethylene carbonate; preferably, based on the total mass of the electrolyte, the mass content of the fluoroethylene carbonate is 5 wt%-25 wt%.

[0017] And / or, the electrolyte further includes 2,2-difluoroethyl acetate, preferably, based on the total mass of the electrolyte, the mass content of 2,2-difluoroethyl acetate is 5wt%-60wt%.

[0018] The electrolyte as described above further includes a boron-containing lithium salt; preferably, the boron-containing lithium salt includes at least one of lithium bis(oxalato)borate, lithium tetrafluoroborate, and lithium difluorooxalato)borate.

[0019] Preferably, the content of the boron-containing lithium salt is 0.01 wt%-3 wt% based on the total mass of the electrolyte.

[0020] In a second aspect, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described in the first aspect.

[0021] In the lithium-ion battery described above, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; preferably, the positive active material layer includes aluminum-doped lithium cobalt oxide, and the mass content of aluminum element is 7000ppm-12000ppm based on the total mass of the aluminum-doped lithium cobalt oxide.

[0022] In the lithium-ion battery described above, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector;

[0023] Preferably, the negative electrode active material layer comprises silicon-based material and carbon-based material, wherein the particle size Dv50 of the silicon-based material is 1μm-15μm.

[0024] The implementation of this invention has at least the following advantages:

[0025] This invention designs at least a portion of the electrolyte composition, specifically by introducing the compound shown in Formula 1 into the electrolyte, to construct a highly stable interfacial system with excellent lithium-ion conductivity, thereby synergistically improving the low-temperature kinetic performance and thermal shock safety of the battery. More specifically, the amine group (-N<) and methoxy group (-OCH3) in the molecular structure of the compound shown in Formula 1, through their electron-donating ability, promote the lithium-ion conductivity of the electrolyte. + The desolvation process improves the reaction kinetics of the negative electrode interface film (SEI), and this compound constructs a nitrogen- and oxygen-rich solid SEI film on the electrode surface. Nitrogen optimizes the lithium-ion conductivity of the SEI film, especially improving ion migration efficiency at low temperatures. The multi-oxygen structure of the compound further forms a fast lithium-ion transport channel, synergistically enhancing the ionic conductivity of the negative electrode interface. Simultaneously, the amino functional group (-NH-) in the molecular structure of the compound shown in Formula 1 can effectively capture free acids (such as HF, PF5, etc.) in the electrolyte, thereby significantly enhancing the thermal stability of the positive electrode interface film (CEI) and suppressing side reactions at high temperatures. These synergistic effects ultimately ensure the thermal shock safety of the battery while improving its lithium-ion transport kinetics at low temperatures. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0027] Currently, there is considerable research on electrolyte additives that aim to improve specific battery performance (such as high-temperature stability or low-temperature discharge capability). For example, phosphorus-, boron-, or sulfur-containing compounds are commonly used to capture harmful acids in the electrolyte to improve high-temperature safety; or low-impedance film-forming additives are used to improve low-temperature performance. However, these single-function additives often struggle to synergistically address multiple challenges: for instance, components that improve high-temperature safety may increase interfacial impedance, impairing low-temperature performance; while additives that improve low-temperature kinetics may be structurally unstable at high temperatures, exacerbating side reactions, making it difficult to simultaneously optimize the overall battery performance, especially the balance between high-temperature safety and low-temperature performance.

[0028] In view of the above, the purpose of this invention is to provide an electrolyte that can simultaneously and significantly improve the low-temperature kinetic performance and thermal shock safety of a battery by introducing multifunctional additive molecules.

[0029] In detail, the present invention provides an electrolyte comprising the compound shown in Formula 1:

[0030] Formula 1,

[0031] R1 and R2 each independently include at least one of the following: alkyl group with 1-5 carbon atoms (fluorinated or unfluorinated), alkenyl group with 2-5 carbon atoms (fluorinated or unfluorinated), and alkynyl group with 2-5 carbon atoms (fluorinated or unfluorinated); X1 and X2 each independently are selected from alkyl group with 1-5 carbon atoms.

[0032] This invention introduces the compound shown in Formula 1 into the electrolyte to construct a highly stable interface system with excellent lithium-ion conductivity, thereby simultaneously improving the low-temperature kinetics and thermal shock safety of the battery. Specifically, the amino group (-N<) and methoxy group (-OCH3) in the molecular structure of the compound shown in Formula 1, through their electron-donating ability, promote the desolvation process of Li⁺, improving the reaction kinetics of the SEI film. Furthermore, this compound constructs a nitrogen- and oxygen-rich solid SEI film on the electrode surface, where nitrogen optimizes the lithium-ion conductivity of the SEI film, particularly enhancing ion migration efficiency at low temperatures. The multi-oxygen structure of this compound further forms a rapid lithium-ion transport channel, synergistically enhancing the ionic conductivity of the negative electrode interface. Simultaneously, the amino functional group (-NH-) in the molecular structure of the compound shown in Formula 1 effectively captures free acids (such as HF, PF5, etc.) in the electrolyte, significantly enhancing the thermal stability of the CEI film and suppressing side reactions at high temperatures. These synergistic effects ultimately ensure the battery's thermal shock safety while improving its lithium-ion transport kinetics at low temperatures.

[0033] For example, R1 and R2 each independently include at least one of fluoromethane, fluoroethane, fluoropropane, fluorobutane, fluoropentane, unfluorinated methane, unfluorinated ethane, unfluorinated propane, unfluorinated butane, unfluorinated pentane, fluoroethylene, fluoropropylene, fluorobutene, fluoropentene, unfluorinated ethylene, unfluorinated propylene, unfluorinated butene, unfluorinated pentene, fluoroacetylene, fluoropropyne, fluorobutyne, fluoropentyne, unfluorinated acetylene, unfluorinated propyne, unfluorinated butyne, and unfluorinated pentyne; X1 and X2 each independently are selected from at least one of methane, ethane, propane, butane, and pentane.

[0034] In some embodiments, the compound represented by Formula 1 includes at least one of the following compounds:

[0035] Formula 1-1 Formula 1-2 Formula 1-3 Formula 1-4

[0036] Formulas 1-5 Formulas 1-6 Equations 1-7.

[0037] Among them, the above compounds have good solubility and low viscosity, which can further ensure the ion migration rate of the electrolyte, thereby further improving the battery dynamic performance.

[0038] The above compounds can be purchased directly or prepared conventionally. For example, the CAS number of the compound shown in Formula 1-1 is 111-95-5, the CAS number of the compound shown in Formula 1-2 is 13985-50-7, and the CAS number of the compound shown in Formula 1-4 is 960365-26-8.

[0039] In some embodiments, the mass content of the compound represented by Formula 1 is 0.01wt%-5wt% based on the total mass of the electrolyte.

[0040] The compounds shown in Formula 1 with the above content can construct an SEI film rich in nitrogen and oxygen with optimized thickness and composition on the negative electrode surface. The appropriate amount of nitrogen and oxygen can ensure the efficient establishment of lithium-ion transport channels, thereby reducing the interfacial impedance of the SEI film at low temperatures and meeting the basic requirements for improving the low-temperature kinetics of the electrolyte. At the same time, the compounds shown in Formula 1 with the above content are sufficient to ensure that acidic substances such as HF generated by electrolyte degradation are continuously and fully neutralized throughout the entire battery life cycle, inhibiting their erosion of the CEI film and positive electrode material from the source, thereby ensuring the robustness of the CEI film under thermal shock conditions and the control of side reactions.

[0041] For example, based on the total mass of the electrolyte, the mass content of the compound shown in Formula 1 is any value or a range of any two of the following: 0.01 wt%, 0.05 wt%, 0.07 wt%, 1 wt%, 1.5 wt%, 1.7 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%, 4.5 wt%, 5.0 wt%.

[0042] In some embodiments, the electrolyte also includes nitrile additives, the mass content of which is 0.5wt%-8wt% based on the total mass of the electrolyte.

[0043] Nitrile additives can form a more stable and dense CEI film on the cathode surface and suppress high-voltage oxidation, but they may increase interfacial impedance at low temperatures. Furthermore, introducing a certain amount of nitrile additives into an electrolyte containing the compound shown in Formula 1 not only synergistically strengthens the CEI film with the compound of Formula 1, but also enhances the Li-Chemical Interaction (CIE) film due to the electron-donating ability of the methoxy and amino groups of the compound of Formula 1. + Desolvation can improve the low-temperature kinetics of the electrolyte. Therefore, the above embodiments further design a certain amount of nitrile additives in the electrolyte containing the compound shown in Formula 1. This not only ensures the full utilization of the functions of the nitrile additives themselves, but also makes them functionally complementary to the compound shown in Formula 1. While achieving a leap in thermal shock safety, it successfully avoids the common dilemma of mutual constraints between low-temperature performance and high-temperature safety in traditional schemes that only add nitrile additives, and achieves simultaneous optimization of comprehensive performance over a wide temperature range.

[0044] For example, based on the total mass of the electrolyte, the mass content of the nitrile additive is any value or a range of any two of the following: 0.5 wt%, 1 wt%, 1.5 wt%, 1.7 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%, 4.5 wt%, 5.0 wt%, 6 wt%, 7 wt%, 8 wt%.

[0045] In some embodiments, the nitrile additives include at least one selected from 1,3,6-hexanetrionitrile, glyceryltrionitrile, adiponitrile, succinic anhydride, glutaronitrile, adiponitrile, heptanonitrile, 1,4-dicyano-2-butene, ethylene glycol bis(propionitrile) ether, tri(3-cyanopropyl) phosphate, tri(3-cyanoethyl) phosphate, 1,3,5-pentanetrionitrile, 1,2,3,4-tetra(2-cyanoethoxy)butane, and xylitol penta(2-cyanoethyl) ether.

[0046] The above nitrile additives contain one or more highly polar cyano groups (-CN) in their molecules, which can preferentially adsorb and oxidatively polymerize on the positive electrode surface to form a dense CEI film with good ionic conductivity. This effectively inhibits the continuous decomposition of the electrolyte and the dissolution of transition metal ions under thermal shock, thereby further improving the thermal shock safety of the battery.

[0047] In some embodiments, the electrolyte also includes vinyl sulfate.

[0048] Ethylene sulfate can preferentially reduce and decompose at higher potentials, forming a sulfur-rich solid SEI film on the negative electrode surface in advance. This SEI film exhibits excellent thermal stability and mechanical strength, effectively suppressing side reactions at the negative electrode at high temperatures and raising the battery's thermal safety boundary to a higher level. Simultaneously, this sulfur-rich SEI film also possesses high interfacial ionic conductivity, which can synergistically with the compound of Formula 1 to further reduce the lithium-ion transport barrier at the interface. This allows the electrolyte system to significantly enhance high-temperature safety while still maintaining excellent low-temperature discharge kinetics, ultimately achieving a comprehensive and balanced improvement in the electrolyte's performance over a wide temperature range.

[0049] In some embodiments, the mass content of vinyl sulfate is 0.01wt%-4wt% based on the total mass of the electrolyte.

[0050] This content range ensures a full reduction reaction, a higher sulfur content in the SEI film, and further guarantees the overall viscosity of the electrolyte, thereby further improving the low-temperature kinetics of the battery.

[0051] For example, based on the total mass of the electrolyte, the mass content of vinyl sulfate is any value or a range of any two of the following: 0.01 wt%, 0.05 wt%, 0.07 wt%, 1 wt%, 1.5 wt%, 1.7 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%.

[0052] In some embodiments, the electrolyte also includes fluoroethylene carbonate.

[0053] Fluoroethylene carbonate (FEC) can undergo a pre-reaction with the compound of Formula 1 to generate a highly active intermediate. This intermediate preferentially decomposes on the negative electrode surface during the initial stage of the battery's first charge-discharge cycle, thus rapidly constructing a LiF-rich and dense SEI film in the early stages of cycling. This significantly shortens the SEI film formation time and effectively reduces the overall and inner layer thickness of the SEI film, minimizing lithium loss and capacity loss caused by slow or uneven film formation. This optimized SEI film structure combines excellent ion conductivity and thermal stability, synergistically enhancing the battery's low-temperature kinetics in conjunction with the compound of Formula 1.

[0054] In some embodiments, the mass content of fluoroethylene carbonate is 5wt%-25wt% based on the total mass of the electrolyte.

[0055] This content range ensures sufficient pre-reaction and significant film formation, which can further guarantee the overall viscosity of the electrolyte, thereby further improving the low-temperature kinetics of the battery.

[0056] For example, based on the total mass of the electrolyte, the mass content of fluoroethylene carbonate is any value or a range of any two of the following: 5 wt%, 7 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%.

[0057] In some embodiments, the electrolyte also includes 2,2-difluoroethyl acetate.

[0058] The -CF2H group in 2,2-difluoroethyl acetate (DFEA) has a high reduction potential, which allows it to preferentially decompose and generate LiF in the early stages of film formation. This enables the rapid construction of a dense SEI film with high LiF content and inorganic components on the negative electrode surface, improving the thermal stability and mechanical integrity of the interfacial film. Simultaneously, DFEA itself possesses low viscosity and low melting point. The aforementioned DFEA content effectively reduces the overall viscosity of the electrolyte system and the ion migration barrier, thereby further enhancing the battery's discharge kinetics performance at low temperatures.

[0059] In some embodiments, the mass content of 2,2-difluoroethyl acetate is 5wt%-60wt% based on the total mass of the electrolyte.

[0060] This content range not only ensures a higher LiF content in the SEI film and a denser film, but also further guarantees the overall viscosity of the electrolyte, thereby further improving the low-temperature kinetics of the battery.

[0061] For example, based on the total mass of the electrolyte, the mass content of 2,2-difluoroethyl acetate is any value or a range of any two of the following: 5 wt%, 7 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%.

[0062] In some embodiments, the electrolyte also includes boron-containing lithium salts.

[0063] The decomposition of boron-containing lithium salts generates anions that can form stable coordination complexes with electron-donating groups in the compound of Formula 1. This process not only effectively inhibits the excessive decomposition and consumption of the compound of Formula 1 in the early stage of formation, ensuring its long-term function, but also guides the enrichment of boron-containing anions at the negative electrode interface, enhancing the mechanical strength and structural compactness of the SEI film, and can further reduce the impedance of the SEI film and improve the stability of the battery at high temperatures.

[0064] In some embodiments, the boron-containing lithium salt includes at least one of lithium bis(oxalato)borate, lithium tetrafluoroborate, and lithium difluorooxalato)borate.

[0065] The above boron-containing lithium salts decompose to produce anions (such as BOB). - DFOB - It can ensure the formation of stable coordination complexes with electron-donating groups in the compound of formula 1, and also guide the enrichment of boron-containing anions at the negative electrode interface to participate in the construction of an inorganic composite layer rich in BFO bonds. This layer significantly enhances the mechanical strength and structural compactness of the SEI film, and can further improve the stability of the battery at high temperatures.

[0066] In some implementations, the boron-containing lithium salt content is 0.01 wt%-3 wt% based on the total mass of the electrolyte.

[0067] The boron-containing lithium salts at the above concentrations ensure that the boron-containing anions generated during decomposition are sufficient to form stable complexes with the compound of Formula 1, thereby effectively inhibiting the excessive consumption of the latter.

[0068] For example, based on the total mass of the electrolyte, the content of boron-containing lithium salt is any value or a range of any two of the following: 0.01 wt%, 0.05 wt%, 0.07 wt%, 1 wt%, 1.5 wt%, 1.7 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%.

[0069] In one specific embodiment, the electrolyte further includes other lithium salts, including one or more of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), and lithium perchlorate (LiClO4).

[0070] The lithium salts described above have good electrical conductivity and chemical stability, and do not produce excessive byproducts that affect battery performance in high-temperature and low-temperature environments.

[0071] In one specific embodiment, other lithium salts include lithium hexafluorophosphate (LiPF6), with the concentration of LiPF6 ranging from 0.5 mol / L to 2 mol / L based on the electrolyte. When the content of LiPF6 is within the above range, the electrolyte typically provides sufficient lithium-ion conductivity to support efficient charge transfer and battery performance. Furthermore, an appropriate lithium salt content helps form a stable solid electrolyte interface (SEI) layer on the electrode surface. This protective film prevents direct contact between the electrolyte and the electrode material, thereby improving the battery's cycle life and safety. Exemplarily, the concentration of LiPF6 based on the electrolyte is any value or a range of any combination of 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 2 mol / L, etc.

[0072] For example, the electrolyte also includes a solvent, which includes one or both of linear carbonates and linear carboxylic acid esters.

[0073] The solvents described above can serve as solvent carriers for electrolytes, helping to separate cations and anions in the electrolyte and transfer charge between electrodes. They can also improve the overall conductivity of the electrolyte, which is beneficial to the battery's charge and discharge efficiency and energy density.

[0074] Based on the total mass of the electrolyte, the solvent content is 43wt%-85wt%. When the solvent mass percentage is within the above range, the conductivity of the electrolyte can be further improved, thereby improving the cycle performance of the battery to a greater extent.

[0075] For example, based on the total mass of the electrolyte, the solvent content is any value or a range of any two of the following: 43 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%.

[0076] In a second aspect, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described in the first aspect.

[0077] Since the aforementioned lithium-ion battery includes the electrolyte described in the first aspect, it possesses excellent thermal shock safety and low-temperature kinetic performance.

[0078] In some embodiments, the present invention does not strictly limit the choice of separator material, and can be one or more of the separator materials commonly used in lithium-ion batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene bilayer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene trilayer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0079] For example, the thickness of the diaphragm is any value or a range of any two of the following: 5µm, 15µm, 16µm, 17µm, 18µm, 19µm, 20µm, 21µm, 22µm, 23µm, 24µm, 25µm.

[0080] In some embodiments, the positive electrode includes a positive current collector and a layer of positive active material disposed on at least one side of the surface of the positive current collector.

[0081] In some embodiments, the positive electrode active material layer comprises aluminum-doped lithium cobalt oxide, wherein the mass content of aluminum element is 7000ppm-12000ppm based on the total mass of the aluminum-doped lithium cobalt oxide.

[0082] Among them, Al 3+ (Ionic radius approximately 0.535 Å) Partially substituted Co 3+ (Ionic radius approximately 0.545 Å, high spin) or Co 4 + (Smaller) Due to its different ionic radius and bond energy compared to cobalt, the Al-O bond is stronger, acting like a "pillar" to stabilize the framework structure of the cobalt oxide layer (CoO2), thus improving the stability of the cathode structure. This directly reduces microcracks caused by lattice distortion and stress accumulation, and lowers the activity of highly reactive Co. 4+ Exposure to the electrolyte reduces the chance of cobalt ion dissolution at the source; at the same time, the high bond energy of the Al-O bond enhances the overall binding energy of the lattice and increases the thermal decomposition initiation temperature of the material. Under high temperature conditions, aluminum-doped lithium cobalt oxide is more difficult to release oxygen and undergo exothermic reactions, thus improving the thermal shock safety of the battery.

[0083] Furthermore, the aluminum content mentioned above will not significantly hinder lithium-ion transport, thus ensuring the battery's low-temperature electrical performance.

[0084] For example, based on the total mass of aluminum-doped lithium cobalt oxide, the mass content of aluminum element is any value or a range of any combination of 7000ppm, 8000ppm, 9000ppm, 10000ppm, 11000ppm, 12000ppm, etc.

[0085] Preferably, the particle size Dv50 of the aluminum-doped lithium cobalt oxide is 5μm-25μm. Exemplarily, it is a range consisting of any two values ​​from 5μm, 7μm, 9μm, 11μm, 13μm, 16μm, 20μm, 25μm, etc. Aluminum-doped lithium cobalt oxide with these particle sizes can further improve the low-temperature discharge performance and thermal shock safety of the battery.

[0086] In one specific embodiment, the positive electrode active material layer also includes a positive electrode conductive agent and a positive electrode binder.

[0087] In one specific embodiment, the positive electrode current collector material can be at least one of aluminum foil and nickel foil.

[0088] In one specific embodiment, the positive electrode conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, and metal powder; the positive electrode binder includes at least one of styrene-butadiene latex, polytetrafluoroethylene, and polyethylene oxide.

[0089] In one specific embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector.

[0090] Silicon-based materials have high specific capacity, which is key to improving the overall energy density of batteries. However, they undergo huge volume expansion during lithiation, leading to particle pulverization and electrical contact failure. Carbon-based materials have excellent structural stability and moderate specific capacity. Therefore, in order to balance energy density and structural stability, in some embodiments, the negative electrode active material layer includes silicon-based materials and carbon-based materials, with the particle size Dv50 of the silicon-based materials being 1μm-15μm.

[0091] Silicon-based materials with particle sizes above these dimensions help shorten the diffusion path of lithium ions, enhance the surface bonding force between particles and conductive agents and binders, and further alleviate the volume expansion effect of silicon-based materials, thereby further improving the low-temperature electrical performance and thermal shock safety of batteries.

[0092] In one specific embodiment, the silicon-based material is selected from one or more of elemental silicon, silicon-carbon materials, and silicon-oxygen materials, preferably CVD silicon-carbon materials, which are silicon-carbon composite materials obtained by setting elemental silicon in the pores of porous carbon. This type of silicon-carbon composite material, by setting elemental silicon in the pores, is beneficial to the uniformity of lithium-ion diffusion, can further reduce the polarization of the negative electrode, and reduce the impact on the structure of the positive electrode material.

[0093] In one specific embodiment, the carbon-based material includes at least one of graphite, amorphous carbon, and graphene.

[0094] For example, the particle size Dv50 of the silicon-based material is any value or a range of any two of the following: 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm.

[0095] Here, Dv50 represents the particle size value corresponding to 50% (by volume) of the cumulative amount in the particle size distribution curve, which is generally obtained by testing with a laser diffraction particle size distribution instrument.

[0096] In one specific embodiment, the negative electrode active material layer also includes a negative electrode conductive agent and a negative electrode binder.

[0097] In one specific embodiment, the negative electrode conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, and metal powder; the negative electrode binder can be selected from at least one of sodium carboxymethyl cellulose, styrene-butadiene latex, polytetrafluoroethylene, and polyethylene oxide.

[0098] In one specific embodiment, the material of the negative electrode current collector can be at least one of copper foil, nickel foam, and copper foam.

[0099] In the fabrication of lithium-ion batteries, the positive electrode, separator, and negative electrode are sequentially wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried, electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the fabrication of the lithium-ion battery.

[0100] The lithium-ion battery provided by the present invention will be further described in detail below through specific embodiments.

[0101] The types and contents of solvents and additives in the electrolyte can be obtained using conventional testing methods in this field, such as gas chromatography-mass spectrometry.

[0102] The types and contents of lithium salts in the above electrolytes can be obtained using conventional testing methods in this field, such as ion chromatography.

[0103] The aluminum content of aluminum-doped lithium cobalt oxide can be obtained using conventional testing methods in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). Specifically, this may include the following steps:

[0104] After disassembling the lithium-ion battery, the positive electrode sheet is removed, soaked and rinsed with dimethyl carbonate, and then dried. The positive electrode active material layer can then be peeled off from the current collector. The positive electrode active material is collected, and then completely dissolved in aqua regia or concentrated nitric acid under heating conditions. The aluminum content is then obtained by inductively coupled plasma mass spectrometry (ICP-MS).

[0105] Unless otherwise specified, the reagents, materials and instruments used in the following examples are all conventional reagents, materials and instruments in the art, and can be obtained commercially. The reagents involved can also be synthesized by conventional methods in the art.

[0106] The test methods for various performance aspects of the lithium-ion batteries in the following examples and comparative examples are as follows:

[0107] (1) Low temperature discharge

[0108] Under 25℃ conditions, the battery is discharged to 3.0V at a given current of 0.2C; left to rest for 5 minutes; then charged to 4.5V at a charging current of 0.2C. When the cell voltage reaches 4.5V, it is switched to 4.5V constant voltage charging with a cutoff current of 0.02C. The battery is then discharged to 3.0V at a given current of 0.2C. The discharge capacity of the last step is taken as the room temperature capacity. Alternatively, the battery is first charged to 4.5V at a charging current of 0.2C. When the cell voltage reaches 4.5V, it is switched to 4.5V constant voltage charging with a cutoff current of 0.02C. Then the battery is placed in a -20℃ temperature chamber and left to stand for 1 hour. It is then discharged to 3.4V at a given current of 0.2C. The low temperature discharge capacity ratio = (low temperature capacity / room temperature capacity) × 100%.

[0109] (2) Thermal shock

[0110] Under 25℃ conditions, the battery was discharged to 3.0V at a given current of 0.2C; left to rest for 5 minutes; then charged to 4.45V at a charging current of 0.2C. When the cell voltage reached 4.45V, it was switched to constant voltage charging at 4.45V until the charging current was less than or equal to the given cutoff current of 0.05C. The battery state before the test was recorded by taking a picture. After the aforementioned battery was left to rest for 1 hour, the cell was placed in an oven. The oven temperature was increased to 132±2℃ at a rate of 5±2℃ / min and maintained for 30 minutes before stopping. The passing standard was that the battery did not catch fire or explode. A total of 10 batteries were tested, and the result was recorded as "number of passes / 10PASS". For example, 8 / 10PASS means that 8 out of 10 batteries passed the test.

[0111] Example 1

[0112] An electrolyte is provided, the preparation method of which includes the following steps:

[0113] In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), using 1 mol / L lithium hexafluorophosphate as the lithium salt, and a mixture of ethylene carbonate (EC), propylene carbonate (PC), propyl propionate (PP), and 2,2-difluoroethyl acetate (DFEA) as the solvent (molar ratio 1:1:3:1), the compound shown in Formula I (contents are shown in Table 1) was added. Then, 0.5 wt% ethylene sulfate based on the total mass of the electrolyte was quickly added, followed by 5 wt% fluoroethylene carbonate, 2 wt% 1,3,6-hexanetrionitrile and succinate (mass ratio 1:1), and 2 wt% lithium difluorooxalate borate. After stirring until homogeneous, the electrolyte was obtained after passing tests for moisture and free acid.

[0114] The assembly of lithium-ion batteries using the above electrolyte includes the following steps: 1) Preparation of positive electrode sheet

[0115] Aluminum-doped lithium cobalt oxide (particle size Dv50 of 7 μm, aluminum doping amount shown in Table 2), polyvinylidene fluoride (PVDF), super P (SP), and carbon nanotubes (CNT) were mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added, and the mixture was stirred under vacuum until it formed a uniform and fluid positive electrode slurry. The positive electrode slurry was uniformly coated onto both surfaces of an aluminum foil. The coated aluminum foil was dried, and then rolled and slit to obtain the desired positive electrode sheet.

[0116] 2) Preparation of negative electrode sheet

[0117] Artificial graphite, CVD silicon carbide material (particle size Dv50 of 10 μm), sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber, conductive carbon black (SP), and single-walled carbon nanotubes (SWCNTs) were mixed in a mass ratio of 89.5:5:2.5:1.5:1:0.5, and deionized water was added. The mixture was stirred in a vacuum mixer to obtain a negative electrode active slurry. The negative electrode active slurry was uniformly coated on both surfaces of a copper foil. The coated copper foil was dried at room temperature and then transferred to an oven at 80°C for 10 hours. After cold pressing and slitting, the negative electrode sheet was obtained.

[0118] 3) Preparation of lithium-ion batteries

[0119] The positive electrode sheet from step 1), the negative electrode sheet from step 2), and the separator are stacked in the order of positive electrode sheet, PP separator and negative electrode sheet, and then wound to obtain a battery cell. The battery cell is placed in an outer packaging aluminum foil, and the electrolyte from step 3) is injected into the outer packaging. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained.

[0120] Examples 2-31 and Comparative Example 1

[0121] Examples 2-31 and Comparative Example 1 provide a lithium-ion battery, the preparation method of which is the same as that of the examples, the only difference being the parameter changes in Tables 1 and 2.

[0122] Table 1

[0123]

[0124] Table 2

[0125]

[0126] The following conclusions can be drawn from Table 2: By introducing the compound shown in Formula 1 into the electrolyte, the examples can construct a highly stable interface system with excellent lithium-ion conductivity. Specifically, compared with Comparative Example 1, Examples 1-31 can simultaneously improve the low-temperature discharge capacity ratio and thermal shock pass rate of the battery.

[0127] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte comprises the compound shown in Formula 1: Formula 1, R1 and R2 each independently include at least one of the following: alkyl group with 1-5 carbon atoms (fluorinated or unfluorinated), alkenyl group with 2-5 carbon atoms (fluorinated or unfluorinated), and alkynyl group with 2-5 carbon atoms (fluorinated or unfluorinated); X1 and X2 each independently are selected from alkyl group with 1-5 carbon atoms.

2. The electrolyte according to claim 1, characterized in that, The compound represented by Formula 1 includes at least one of the following compounds: Formula 1-1 Formula 1-2 Formula 1-3 Formula 1-4 Formulas 1-5 Formulas 1-6 Equations 1-7.

3. The electrolyte according to claim 1 or 2, characterized in that, Based on the total mass of the electrolyte, the mass content of the compound represented by Formula 1 is 0.01wt%-5wt%.

4. The electrolyte according to any one of claims 1-3, characterized in that, The electrolyte also includes nitrile additives, and the mass content of the nitrile additives is 0.5wt%-8wt% based on the total mass of the electrolyte. Preferably, the nitrile additive includes at least one selected from 1,3,6-hexanetrionitrile, glyceryltrionitrile, adiponitrile, succinic anhydride, glutaronitrile, adiponitrile, heptanonitrile, 1,4-dicyano-2-butene, ethylene glycol bis(propionitrile) ether, tri(3-cyanopropyl) phosphate, tri(3-cyanoethyl) phosphate, 1,3,5-pentanetrionitrile, and xylitol penta(2-cyanoethyl) ether.

5. The electrolyte according to any one of claims 1-4, characterized in that, The electrolyte also includes vinyl sulfate, and preferably, based on the total mass of the electrolyte, the mass content of vinyl sulfate is 0.01wt%-4wt%.

6. The electrolyte according to any one of claims 1-5, characterized in that, The electrolyte further includes fluoroethylene carbonate; preferably, based on the total mass of the electrolyte, the mass content of the fluoroethylene carbonate is 5wt%-25wt%. And / or, the electrolyte further includes 2,2-difluoroethyl acetate; preferably, the mass content of 2,2-difluoroethyl acetate is 5wt%-60wt% based on the total mass of the electrolyte.

7. The electrolyte according to any one of claims 1-6, characterized in that, The electrolyte further includes a boron-containing lithium salt; preferably, the boron-containing lithium salt includes at least one of lithium bis(oxalato)borate, lithium tetrafluoroborate, and lithium difluorooxalato)borate. Preferably, the content of the boron-containing lithium salt is 0.01 wt%-3 wt% based on the total mass of the electrolyte.

8. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The lithium-ion battery further includes the electrolyte according to any one of claims 1-7.

9. The lithium-ion battery according to claim 8, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector; preferably, the positive active material layer includes aluminum-doped lithium cobalt oxide, and the mass content of aluminum element is 7000ppm-12000ppm based on the total mass of the aluminum-doped lithium cobalt oxide.

10. The lithium-ion battery according to claim 8, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector; Preferably, the negative electrode active material layer comprises silicon-based material and carbon-based material, wherein the particle size Dv50 of the silicon-based material is 1μm-15μm.

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