Electrolyte, secondary battery and electric equipment

Abstract

The embodiment of the invention provides an electrolyte, a secondary battery and electric equipment, and the electrolyte provided by the embodiment of the invention comprises a lithium salt, an organic solvent and a first additive, the first additive comprises an anacardol polyoxyethylene ether additive, and the anacardol polyoxyethylene ether additive comprises a compound shown as a formula (I) and a compound shown as a formula (II), so that the stability of the electrolyte under the low-temperature condition and the high-temperature condition can be improved; and therefore, the impedance of the secondary battery prepared based on the electrolyte under the low-temperature condition and the high-temperature condition can be reduced, and the battery capacity of the secondary battery under the low-temperature condition and the high-temperature condition can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to an electrolyte, a secondary battery, and an electrical device using the same. Background Art

[0002] With the rapid development of pure electric vehicles and hybrid electric vehicles, the requirements for the electrochemical performance of secondary batteries are also continuously increasing. The electrolyte in a secondary battery is not only a medium for ion transport in the secondary battery, but also forms a stable solid electrolyte interface (SEI) film on the surface of the electrode plate of the secondary battery, playing a role in protecting the structure of the electrode plate, and is the basis for improving the electrochemical performance of the secondary battery.

[0003] In related technologies, the electrolyte mainly consists of an organic solvent, a lithium salt, and a carbonate additive. The viscosity of such an electrolyte will increase significantly or even solidify under low-temperature conditions, making the migration resistance of ions in the electrolyte larger, and further resulting in an increase in the interfacial impedance and charge transfer impedance between the electrode plate and the electrolyte. Under high-temperature conditions, the electrolyte will undergo chemical reactions such as decomposition and oxidation, and the products of the chemical reactions will damage the solid electrolyte interface film and the structure of the electrode plate, making the impedance of the secondary battery prepared based on such an electrolyte increase and the battery capacity deteriorate under low-temperature and high-temperature conditions, and it is difficult to meet the requirements for the electrochemical performance of secondary batteries in current pure electric vehicles and hybrid electric vehicles. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an electrolyte, a secondary battery, and an electrical device using the same, so as to solve the problem that the electrochemical performance of the secondary battery deteriorates due to the low stability of the electrolyte in related technologies under low-temperature and high-temperature conditions.

[0005] To solve the above problems, the present invention is implemented by the following technical solutions:

[0006] The present invention provides an electrolyte, which includes a lithium salt, an organic solvent, and a first additive; the first additive includes a cardanol polyoxyethylene ether additive, and the cardanol polyoxyethylene ether additive includes a compound having formula (I) and a compound having formula (II):

[0007] C n H 2n+1 —O—R Formula (I);

[0008]

[0009] Among them, R in formula (I) represents a hydrogen element or a sulfate compound; n in formula (I) represents the number of carbon atoms in the compound having formula (I), and n is an integer greater than 2;

[0010] m in formula (II) represents the addition number of —CH 2 —CH 2 —O—, and m is any integer from 3 to 200.

[0011] Furthermore, in the electrolyte, the cardanol polyoxyethylene ether additive is selected from at least one of cardanol polyoxyethylene ether, sodium cardanol polyoxyethylene ether sulfate, and ammonium cardanol polyoxyethylene ether sulfate.

[0012] Furthermore, in the electrolyte, the sulfate compound includes sodium sulfate and / or ammonium sulfate.

[0013] Furthermore, in the electrolyte, the mass ratio of the first additive is 0.1 wt% to 2 wt%.

[0014] Furthermore, in the electrolyte, the electrolyte further includes a second additive, and the second additive includes at least one of vinylene carbonate, propylene sulfite, ethylene sulfate, ethylene vinylene carbonate, fluorinated ethylene carbonate, lithium bis(oxalato)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0015] Furthermore, in the electrolyte, the mass ratio of the second additive is 2 wt% to 10 wt%.

[0016] Furthermore, in the electrolyte, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium difluoro(oxalato)borate.

[0017] Furthermore, in the electrolyte, the mass ratio of the lithium salt is 8 wt% to 16 wt%.

[0018] The present invention also provides a secondary battery, wherein the secondary battery includes a positive electrode sheet, a negative electrode sheet, and the electrolyte as described in any one of the above.

[0019] The present invention also provides an electrical device, wherein the electrical device includes the above secondary battery, and this secondary battery serves as the power supply of the electrical device.

[0020] Compared with the related art, the embodiments of the present invention have the following advantages:

[0021] The electrolyte provided by the embodiment of the present invention includes a lithium salt, an organic solvent, and a first additive; the first additive includes a cardanol polyoxyethylene ether additive, and the cardanol polyoxyethylene ether additive includes a compound having the formula (I) and a compound having the formula (II); wherein, the long-chain alkyl group (C n H 2n+1 ) in the compound having the formula (I) has certain flexibility and hydrophobicity, so that the long-chain alkyl group can provide a certain steric hindrance under low-temperature conditions, prevent excessive aggregation and crystallization between molecules in the electrolyte, and is beneficial to improving the low-temperature fluidity of the electrolyte. Moreover, the polyoxyethylene group (—CH 2 —CH 2 —O—) in the compound having the formula (II) is a hydrophilic group, which can improve the solubility and dispersibility of the cardanol polyoxyethylene ether additive in water. Under low-temperature conditions, the cardanol polyoxyethylene ether additive with good solubility and dispersibility can prevent the problem of increased viscosity of the electrolyte caused by the freezing of water in the electrolyte, which is beneficial to reducing the freezing temperature of the electrolyte and improving the stability of the electrolyte under low-temperature conditions; under high-temperature conditions, the long-chain alkyl group in the compound having the formula (I) can also increase the distance between molecules in the electrolyte, reduce the intermolecular interaction force, and reduce the probability of violent collision and chemical reaction caused by molecular thermal motion, thereby improving the stability of the electrolyte under high-temperature conditions. The ether bond (—O—) in the polyoxyethylene group of the compound having the formula (II) has high chemical stability and is not easily oxidized and hydrolyzed, which can reduce the probability of chemical reactions such as decomposition and oxidation of the electrolyte under high-temperature conditions, and is beneficial to further improving the stability of the electrolyte under high-temperature conditions. Furthermore, it is beneficial to reduce the impedance of the secondary battery prepared based on the electrolyte under low-temperature and high-temperature conditions and improve the battery capacity of the secondary battery under low-temperature and high-temperature conditions.

[0022] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present invention. Detailed Embodiments

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0024] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with specific embodiments.

[0025] In the related art, the electrolyte mainly consists of an organic solvent, a lithium salt, and a carbonate additive. The viscosity of this type of electrolyte will increase significantly or even solidify under low-temperature conditions, resulting in a greater migration resistance of ions in the electrolyte, and further leading to an increase in the interfacial impedance and charge transfer impedance between the electrode and the electrolyte. Under high-temperature conditions, however, chemical reactions such as decomposition and oxidation will occur in the electrolyte (for example, hydrolysis reaction and pyrolysis reaction of the lithium salt, etc.). The products of the chemical reaction will damage the solid electrolyte interface film and the electrode structure. For example, hydrofluoric acid (HF), the product of the hydrolysis reaction and pyrolysis reaction of the lithium salt in the electrolyte, will corrode the solid electrolyte interface film and the electrode. The solid products generated during the oxidation reaction of the electrolyte will block the pores in the solid electrolyte interface film and the electrode, affecting the migration of ions in the secondary battery, thereby resulting in an increase in the impedance and deterioration of the battery capacity of the secondary battery prepared based on this type of electrolyte under low-temperature and high-temperature conditions.

[0026] In addition, the surface tension of the electrolyte in the related art is usually relatively large, and its wettability with the separator and the electrode is poor, resulting in a relatively high interfacial resistance between the electrolyte and the separator and the electrode, further deteriorating the impedance and battery capacity of the secondary battery prepared based on this type of electrolyte, and it is difficult to meet the requirements of the current pure electric vehicles and hybrid electric vehicles for the electrochemical performance of the secondary battery.

[0027] In order to solve the above problems, the embodiments of the present invention provide an electrolyte, which includes a lithium salt, an organic solvent, and a first additive. Among them, the first additive includes a cardanol polyoxyethylene ether additive, and the cardanol polyoxyethylene ether additive includes a compound having the formula (I) and a compound having the formula (II):

[0028] C n H 2n+1 —O—R Formula (I);

[0029]

[0030] It should be noted that R in the formula (I) represents a hydrogen (H) element or a sulfate compound; specifically, when R in the formula (I) represents the H element, the compound having the formula (I) is a fatty alcohol compound C n H 2n+1-OH, such that the compound of formula (I) includes both hydrophilic hydroxyl groups (—OH) and hydrophobic long-chain alkyl groups. This structure enables the compound of formula (I) to form stable aggregates (micelles), thereby facilitating the improvement of the stability of the electrolyte. Further, when the cashew phenol polyoxyethylene ether additive further includes a compound of formula (II), the polyoxyethylene group in the compound of formula (II) can adjust the hydrophilic-lipophilic balance value of the compound of formula (I), further enhancing the stability of the electrolyte. When the stability of the electrolyte is improved, the impedance of the secondary battery prepared based on the electrolyte under low-temperature and high-temperature conditions can be reduced, and the battery capacity of the secondary battery under low-temperature and high-temperature conditions can be increased.

[0031] When R in formula (I) represents a sulfate compound, it indicates that the H element in the hydroxyl group of the fatty alcohol compound C n H 2n+1 -OH is replaced by a sulfate compound to form a group including the sulfate compound, and the group including the sulfate compound has a hydrophilic effect the same as or similar to that of the hydroxyl group. In terms of improving the stability of the electrolyte, it can achieve the same or similar effect as C n H 2n+1 -OH. To avoid repetition, it will not be elaborated here.

[0032] In the embodiments of the present invention, n in formula (I) represents the number of carbon atoms in the compound of formula (I), and n is any integer greater than 2; in some embodiments, n can be any integer from 8 to 22.

[0033] m in formula (II) represents the addition number of polyoxyethylene groups, and m is any integer from 3 to 200.

[0034] Specifically, in the cashew phenol polyoxyethylene ether additive, the compound of formula (I) includes a long-chain alkyl group. The long-chain alkyl group has a certain flexibility and hydrophobicity. Under low-temperature conditions, the long-chain alkyl group can increase the steric hindrance in the electrolyte, prevent excessive aggregation and crystallization between molecules in the electrolyte, facilitate the reduction of the viscosity of the electrolyte under low-temperature conditions, improve the low-temperature fluidity of the electrolyte, and further enhance the stability of the electrolyte under low-temperature conditions. Under high-temperature conditions, the long-chain alkyl group in the compound of formula (I) can also increase the distance between molecules in the electrolyte, reduce the intermolecular interaction force, and reduce the probability of violent collisions and chemical reactions caused by molecular thermal motion, thereby improving the stability of the electrolyte under high-temperature conditions.

[0035] The compound of formula (II) is formed by repeatedly connecting multiple polyoxyethylene groups. The oxygen (O) atom in the ether bond of the polyoxyethylene group has a lone pair of electrons, and the lone pair of electrons can interact with water (H 2O) The hydrogen atoms in the molecule form hydrogen bonds, and the formation of hydrogen bonds enhances the hydrophilicity of the polyoxyethylene group. When the polyoxyethylene group has hydrophilicity, the solubility and dispersibility of the cardanol polyoxyethylene ether additives including the compound of formula (II) in water can be improved. Under low-temperature conditions, the cardanol polyoxyethylene ether additives with good solubility and dispersibility can prevent the problem of increased electrolyte viscosity caused by the freezing of water in the electrolyte, which is beneficial to reducing the freezing temperature of the electrolyte, thereby further improving the stability of the electrolyte under low-temperature conditions. In addition, the ether bond in the polyoxyethylene group of the compound of formula (II) has high chemical stability and is not easily oxidized and hydrolyzed, which can reduce the probability of chemical reactions such as decomposition and oxidation of the electrolyte under high-temperature conditions, and is beneficial to further improving the stability of the electrolyte under high-temperature conditions.

[0036] When the stability of the electrolyte under low-temperature conditions is improved, the viscosity of the electrolyte under low-temperature conditions can be reduced, the freezing temperature of the electrolyte under low-temperature conditions can be reduced, the migration resistance of ions in the electrolyte can be reduced, and then the interfacial impedance and charge transfer impedance between the electrode sheet and the electrolyte can be reduced, which is beneficial to reducing the impedance of the secondary battery prepared based on the electrolyte and improving the battery capacity of the secondary battery; when the freezing temperature of the electrolyte is reduced, it can also avoid the damage to the secondary battery structure caused by the volume expansion generated by the freezing of the electrolyte under low-temperature conditions, which is beneficial to improving the service life of the secondary battery under low-temperature conditions.

[0037] Correspondingly, when the stability of the electrolyte under high-temperature conditions is improved, the probability of chemical reactions such as decomposition and oxidation of the electrolyte under high-temperature conditions can be reduced, and then the degree of damage to the solid electrolyte interface film and the electrode sheet structure by the products of the chemical reaction can be reduced, which is beneficial to improving the battery capacity of the secondary battery prepared based on the electrolyte; in addition, when the stability of the electrolyte under high-temperature conditions is improved, the volatilization of the electrolyte under high-temperature conditions can also be reduced, which is beneficial to improving the service life of the secondary battery under high-temperature conditions.

[0038] Furthermore, the chemical bond between the carbon atom and the hydrogen atom in the long-chain alkyl group of the compound with formula (I) has a strong non-polarity, which enables a strong interaction force to be generated between the long-chain alkyl group and the oil molecule with non-polarity based on the principle of "like dissolves like", improving the lipophilicity of the cardanol polyoxyethylene ether additive. The oxygen atom in the ether bond of the compound with formula (II) contains lone pairs of electrons, and the lone pairs of electrons can form hydrogen bonds with the hydrogen atoms in water molecules. The formation of hydrogen bonds enhances the solubility of the polyoxyethylene group in water, thereby enabling the cardanol polyoxyethylene ether additive to have good hydrophilicity. When the cardanol polyoxyethylene ether additive has both good lipophilicity and hydrophilicity, the cardanol polyoxyethylene ether additive in the electrolyte can be oriented at the two-phase interface, reducing the surface tension of the electrolyte and improving the wettability between the electrolyte and the separator and the electrode sheet, enabling the electrolyte to contact and penetrate the separator and the electrode sheet faster, which is not only beneficial to reducing the injection efficiency of the electrolyte, but also beneficial to improving the charge transfer efficiency, energy density and capacity retention rate of the secondary battery.

[0039] In the embodiments of the present invention, the two-phase interface includes the solid-liquid interface between the electrolyte and the separator, and the solid-liquid interface between the electrolyte and the electrode sheet.

[0040] It should be noted that the low-temperature condition involved in the embodiments of the present invention refers to the low-temperature condition during the operation of the secondary battery, and the temperature range corresponding to the low-temperature condition can be less than or equal to 0 °C. For example, the low-temperature condition can be -20 °C.

[0041] The high-temperature condition involved in the embodiments of the present invention refers to the high-temperature condition during the operation of the secondary battery, and the temperature range corresponding to the high-temperature condition can be 40 °C to 60 °C.

[0042] Optionally, in some embodiments, the cardanol polyoxyethylene ether additive is selected from at least one of cardanol polyoxyethylene ether (Cashew Nutshell Liquid Polymer, CNSL), sodium cardanolpolyoxyethylene ether sulfate (SNAP), and ammonium cardanolpolyoxyethylene ether sulfate.

[0043] Among them, when the cardanol polyoxyethylene ether additive is selected from cardanol polyoxyethylene ether, R in formula (I) represents hydrogen element; the structural formula of cardanol polyoxyethylene ether can be represented by formula (III) or formula (IV):

[0044]

[0045] When the cardanol polyoxyethylene ether additive is selected from sodium cardanol polyoxyethylene ether sulfate, R in formula (I) represents a sulfate compound, and the sulfate compound is specifically sodium sulfate (—SO 3 Na); the structural formula of sodium cardanol polyoxyethylene ether sulfate can be represented by formula (V):

[0046]

[0047] When the cardanol polyoxyethylene ether additive is selected from ammonium cardanol polyoxyethylene ether sulfate, R in formula (I) represents a sulfate compound, and the sulfate compound is specifically ammonium sulfate (—SO 3 NH 4 ); the structural formula of ammonium cardanol polyoxyethylene ether sulfate can be represented by formula (VI):

[0048]

[0049] Optionally, in some embodiments, when R in formula (I) represents a sulfate compound, the sulfate compound includes sodium sulfate and / or ammonium sulfate; specifically, when the sulfate compound includes sodium sulfate, the cardanol polyoxyethylene ether additive is selected from sodium cardanol polyoxyethylene ether sulfate; when the sulfate compound includes ammonium sulfate, the cardanol polyoxyethylene ether additive is selected from ammonium cardanol polyoxyethylene ether sulfate.

[0050] Optionally, in the electrolyte provided by the embodiments of the present invention, the mass ratio of the first additive is 0.1 wt% to 2 wt%; specifically, in the electrolyte, the mass ratio of the first additive can be one of 0.1 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.6 wt% and 2 wt% or the range value of any two of them. Within this range value, the effect of the cardanol polyoxyethylene ether additive in improving the stability of the electrolyte under low-temperature and high-temperature conditions and the wettability between the electrolyte and the separator and the electrode can be further exerted.

[0051] Optionally, the electrolyte provided by the embodiments of the present invention further includes a second additive, and the second additive includes at least one of vinylene carbonate (VC), propylene sulfite (PS), 1,3,2-dioxathiolane 2,2-dioxide (DTD), vinylethylene carbonate (VEC), 4-fluoro-1,3-dioxolan-2-one (FEC), lithium bis(oxalate)borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis((trifluoromethyl)sulfonyl)azanide (LiTFSI).

[0052] Among them, vinylene carbonate and / or vinylethylene carbonate have good electrochemical stability and thermal stability, can reduce the decomposition and volatilization of the electrolyte, further improve the stability of the electrolyte, and extend the service life of the secondary battery; in addition, vinylene carbonate and / or vinylethylene carbonate can also form a stable SEI film on the surface of the electrode plate of the secondary battery, reduce the side reactions on the surface of the electrode plate, and improve the charge and discharge cycle performance of the secondary battery; further, vinylene carbonate and / or vinylethylene carbonate can also improve the battery capacity and energy density of the secondary battery by increasing the conductivity of the electrolyte.

[0053] As a second additive, propylene sulfite can improve the performance of the electrolyte under low-temperature conditions and further improve the low-temperature cycle stability of the secondary battery.

[0054] As a second additive, 1,3,2-dioxathiolane 2,2-dioxide can inhibit the decline of the initial capacity of the secondary battery and improve the initial discharge capacity of the secondary battery; 1,3,2-dioxathiolane 2,2-dioxide can also improve the cycle stability and storage performance of the secondary battery by reducing the swelling of the secondary battery after being placed at high temperature.

[0055] 4-fluoro-1,3-dioxolan-2-one can form an effective SEI film on the surface of the electrode plate of the secondary battery, increase the migration rate of lithium ions in the secondary battery, and improve the charge and discharge performance of the secondary battery at high rates.

[0056] Lithium bis(oxalate)borate can avoid the dissolution of transition metal ions caused by the erosion of hydrofluoric acid and maintain the structural integrity of the electrode plate.

[0057] Lithium bis(fluorosulfonyl)imide can effectively reduce the low-temperature resistance of the SEI layer formed on the surface of the electrode sheet, improving the low-temperature performance of the secondary battery; lithium bis(fluorosulfonyl)imide can also improve the capacity retention rate and storage performance of the secondary battery by reducing the capacity loss during the storage of the secondary battery.

[0058] Lithium bis(trifluoromethanesulfonyl)imide has high conductivity and chemical stability, which can enhance the overall performance of the electrolyte.

[0059] It can be understood that the above-mentioned second additive can form a stable SEI film on the surface of the negative electrode sheet of the secondary battery. It synergizes with the first additive to improve the stability of the electrolyte under low-temperature and high-temperature conditions, and the wettability between the electrolyte and the separator and the electrode sheet. At the same time, it can also improve the protection of the electrode sheet structure during the charge and discharge cycles of the secondary battery, which is beneficial to reducing the impedance of the secondary battery, increasing the battery capacity of the secondary battery, and extending the service life of the secondary battery.

[0060] Optionally, in the electrolyte provided by the embodiment of the present invention, the mass ratio of the second additive is 2wt% - 10wt%; specifically, in the electrolyte, the mass ratio of the second additive can be one of 2wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt% and 10wt% or the range value of any two of them. Within this range value, the effect of the second additive in improving the protection of the electrode sheet structure during the charge and discharge cycles of the secondary battery and improving the stability of the electrolyte can be further exerted.

[0061] Optionally, in the electrolyte provided by the embodiment of the present invention, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF 6 )、lithium tetrafluoroborate (LiBF 4 ), and lithium difluoro(oxalato)borate (LiODFB).

[0062] Among them, lithium hexafluorophosphate has high ionic conductivity and good electrochemical stability; lithium tetrafluoroborate has good thermal stability, can maintain its chemical properties stable under low-temperature and high-temperature conditions, prevent the corrosion of the electrode sheet, and improve the cycle life and safety of the secondary battery; lithium difluoro(oxalato)borate has excellent high and low temperature performance and can maintain stable electrochemical performance within a wide temperature range; in addition, lithium difluoro(oxalato)borate as the lithium salt in the electrolyte can also provide high ionic conductivity, thereby improving the overall performance of the secondary battery.

[0063] Optionally, in the electrolyte provided by the embodiments of the present invention, the mass ratio of the lithium salt is 8 wt% to 16 wt%; specifically, in the electrolyte, the mass ratio of the lithium salt can be one of 8 wt%, 10 wt%, 11 wt%, 13 wt%, 15 wt% and 16 wt% or the range value of any two of them. Within this range value, while improving the conductivity of the electrolyte, the viscosity of the electrolyte can be made moderate, effectively taking into account both the conductivity and viscosity of the electrolyte, and further exerting the effect of the lithium salt in improving the electrolyte performance.

[0064] Optionally, in the electrolyte provided by the embodiments of the present invention, the organic solvent includes at least two of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0065] Among them, ethylene carbonate has a high film-forming potential on the negative electrode sheet and can participate in the formation of the SEI film on the surface of the graphite negative electrode sheet, reducing side reactions on the electrode sheet surface. Moreover, ethylene carbonate has a relatively high dielectric constant, which can improve the ionic conductivity of the electrolyte.

[0066] Propylene carbonate can enhance the stability of the secondary battery, protect the negative electrode sheet, reduce the polarity of the electrolyte, and reduce the possibility of secondary battery failure; in addition, propylene carbonate can also reduce the operating temperature of the secondary battery, further improving the stability of the secondary battery.

[0067] Dimethyl carbonate can mix with electrolyte components with low polarity, such as ethylene carbonate and diethyl carbonate, to form a balanced dissolution, enabling the secondary battery to reach a stable state faster, improving the cycle life and capacity retention rate of the secondary battery; dimethyl carbonate can also increase the electrochemical window of the secondary battery, reduce the film formation rate on the electrode sheet surface, reduce the interfacial resistance, and improve the battery capacity and cycle life of the secondary battery.

[0068] Diethyl carbonate can be used in combination with other solvents, such as ethylene carbonate and dimethyl carbonate, to improve the solubility of each component in the electrolyte.

[0069] Ethyl methyl carbonate mainly plays a role in providing an ion conduction channel in the electrolyte, enabling the ions in the electrolyte to be transmitted between the positive and negative electrode sheets for chemical reactions, which is beneficial to improving the efficiency and stability in the electrochemical reaction; in addition, ethyl methyl carbonate also has advantages such as higher conductivity, higher stability, and lower explosion risk.

[0070] Optionally, in the electrolyte provided by the embodiments of the present invention, the mass ratio of the organic solvent is 74wt% - 85wt%; specifically, in the electrolyte, the mass ratio of the organic solvent can be one of 74wt%, 76wt%, 78wt%, 80wt%, 83wt% and 85wt%, or the range value of any two of them. Within this range value, it is beneficial to fully dissolve the lithium salt and additives, provide a necessary migration medium for lithium ions, improve the conductivity of the electrolyte, thereby improving the safety performance of the secondary battery and extending the service life of the secondary battery.

[0071] It should be noted that in the electrolyte provided by the embodiments of the present invention, the sum of the mass ratios of the lithium salt, organic solvent, first additive and second additive is 100wt%.

[0072] The present invention also provides a method for preparing an electrolyte, which includes: under the protection of an inert gas, mixing a lithium salt and an organic solvent to obtain a colorless transparent liquid; mixing the first additive, the second additive and the colorless transparent liquid to obtain an electrolyte.

[0073] Among them, during the process of mixing the lithium salt and the organic solvent, since the addition of the lithium salt will cause the temperature of the electrolyte to rise, resulting in a certain degree of thermal decomposition of the lithium salt, so during the addition of the lithium salt, it is necessary to control the temperature of the electrolyte; specifically, when the temperature of the electrolyte rises above 2°C, stop adding the lithium salt, and when the temperature of the electrolyte rises below 2°C, continue to add the lithium salt.

[0074] Among them, the temperature control of the above electrolyte can be achieved by ice bath, dry ice circulation cooling, liquid nitrogen cooling, etc.

[0075] Optionally, in some embodiments, the above organic solvent includes multiple solvents. Before mixing the lithium salt and the organic solvent, it is necessary to pre-mix and dehydrate the multiple solvents, and then under the protection of an inert gas, mix the lithium salt and the above organic solvent to obtain the above colorless transparent liquid; then mix the first additive, the second additive with the above colorless transparent liquid to obtain an electrolyte. Among them, the organic solvent can be dehydrated by means of molecular sieve adsorption.

[0076] The present invention also proposes a secondary battery, wherein the secondary battery includes a positive electrode plate, a negative electrode plate and the above electrolyte.

[0077] Among them, the lithium salt in the electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate.

[0078] The above-mentioned positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes a lithium-ion transition metal oxide, a ternary positive electrode material, etc. The lithium-ion transition metal oxide includes at least one of lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide (LiMn 2 O 4 ), and lithium iron phosphate (LiFePO 4 ). The ternary positive electrode material may specifically be a high-nickel layered ternary positive electrode material.

[0079] Optionally, in some embodiments, the positive electrode sheet further includes a conductive agent and a binder. The conductive agent may include at least one of conductive carbon black, acetylene black (Super P), Ketjen black, carbon nanotubes, graphene, carbon fiber, and carbon microspheres. The binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

[0080] In some embodiments, the positive electrode sheet is prepared as follows: The components for preparing the positive electrode sheet, such as the positive electrode active material, the binder, and any other components, are dispersed in a solvent such as N-methylpyrrolidone (NMP) to form a positive electrode slurry; the positive electrode slurry is coated on both sides of a positive electrode current collector such as aluminum foil; after processes such as baking, rolling, and cutting, the positive electrode sheet can be obtained.

[0081] Among them, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. The negative electrode active material layer may adopt a negative electrode active material for a secondary battery. The negative electrode active material includes any one or a combination of at least two of hard carbon, soft carbon, graphite, and silicon monoxide.

[0082] In the secondary battery provided by the embodiments of the present invention, the negative electrode sheet further includes a conductive agent and a binder; optionally, the conductive agent includes one or more of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, and graphene, and the binder includes a carboxymethyl cellulose (CMC)-based binder and a resin-based binder.

[0083] Optionally, in some embodiments, the carboxymethyl cellulose-based binder includes one or more of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose; and / or the resin-based binder includes one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and polyacrylonitrile (PAN).

[0084] In some embodiments, the negative electrode sheet is prepared as follows: The components for preparing the negative electrode sheet described above, such as the negative electrode active material, binder, and conductive agent, are dispersed in a solvent such as deionized water to form a negative electrode slurry; the negative electrode slurry is coated on both sides of a negative electrode current collector such as a copper foil; after processes such as baking, rolling, and cutting, the negative electrode sheet can be obtained.

[0085] It can be understood that the secondary battery provided by the embodiments of the present invention further includes a separator.

[0086] In practical applications, the negative electrode sheet, separator, and positive electrode sheet are stacked in sequence and wound to obtain a wound core, the wound core is encapsulated to obtain a bare battery cell, and the bare battery cell is baked and then filled with electrolyte, formed, secondarily sealed, and sorted to obtain the above-mentioned secondary battery.

[0087] The present invention also proposes an electrical device, wherein the electrical device includes the above-mentioned secondary battery, and the secondary battery serves as the power supply of the electrical device.

[0088] For the above-mentioned secondary battery embodiments and electrical device embodiments, they include the above-mentioned electrolyte and can achieve the same technical effects. To avoid repetition, they will not be elaborated here. For related parts, refer to the partial description of the electrolyte embodiments.

[0089] In order to make the invention purpose, technical solution, and beneficial effects of the present invention clearer, the present invention will be further described below in conjunction with embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0090] The present invention will be described in detail below through embodiments.

[0091] Example 1

[0092] (1) Preparation of electrolyte:

[0093] At room temperature, in a glove box filled with argon (H 2 O < 1 ppm, O 2(1 ppm or less), ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed uniformly to obtain an organic solvent; lithium hexafluorophosphate was added to the organic solvent successively, with continuous stirring and cooling using dry ice. When the temperature of the electrolyte increased by no more than 2 °C, lithium hexafluorophosphate could be added continuously to obtain a colorless transparent liquid; a first additive, cardanol polyoxyethylene (4) ether, and a second additive, vinylene carbonate (VC), fluoroethylene carbonate (FEC), and vinyl ethylene carbonate (VEC) were added to the colorless transparent liquid and stirred evenly to obtain an electrolyte; wherein, in the electrolyte, the mass ratio of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, lithium hexafluorophosphate, vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, and cardanol polyoxyethylene (4) ether was 27.86:4.98:7.96:43.78:12.44:0.5:1.49:0.5:0.49.

[0094] (2) Preparation of the positive electrode sheet:

[0095] The positive electrode active material Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 , conductive agent acetylene black, and binder polyvinylidene fluoride were mixed uniformly in a mass ratio of 95:3:2, and uniformly dispersed in 1-methyl-2-pyrrolidone to prepare a positive electrode paste. The positive electrode paste was coated on both sides of an aluminum foil, baked, rolled, and cut into pieces to obtain a positive electrode sheet.

[0096] (3) Preparation of the negative electrode sheet

[0097] The negative electrode active material graphite, conductive agent acetylene black, binder styrene-butadiene rubber, and carboxymethyl cellulose were mixed uniformly in a mass ratio of 95:2:1.5:1.5, and uniformly dispersed in deionized water to prepare a negative electrode paste. The negative electrode paste was coated on both sides of a copper foil, baked, rolled, and cut into pieces to obtain a negative electrode sheet.

[0098] (4) Preparation of the secondary battery

[0099] The prepared positive electrode sheet, separator, and negative electrode sheet were stacked in sequence, with the separator placed in the middle of the positive and negative electrode sheets. After winding, hot pressing and shaping, and tab welding, a bare battery core was obtained. The bare battery core was placed in an outer packaging aluminum plastic film and baked in an oven at 85 ± 10 °C for 24 h. The electrolyte prepared in step (1) was injected into the dried battery, and after standing, forming, and grading, a secondary battery was obtained.

[0100] Examples 2 to 5

[0101] Examples 2 to 5 are different from Example 1 in that in step (1), in the electrolyte, the mass ratios of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, lithium hexafluorophosphate, vinylene carbonate, fluoroethylene carbonate, ethylene vinylene carbonate and cardanol polyoxyethylene (4) ether are 27.72:4.95:7.92:43.56:12.38:0.5:1.49:0.5:0.98, 27.59:4.93:7.88:43.35:12.32:0.49:1.48:0.49:1.47, 28.03:5:7.99:43.96:12.42:0.5:1.5:0.5:0.1, and 27.45:4.9:7.84:43.1:12.26:0.49:1.47:0.49:2, respectively.

[0102] Comparative Example 1

[0103] Comparative Example 1 is different from Example 1 in that in step (1), the first additive is not included in the electrolyte, and in the electrolyte, the mass ratios of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, lithium hexafluorophosphate, vinylene carbonate, fluoroethylene carbonate and ethylene vinylene carbonate are 28:5:8:44:12.5:0.5:1.5:0.5.

[0104] The components and parameters of the electrolytes in Examples 1 to 5 and Comparative Example 1 are shown in Table 1.

[0105] Table 1

[0106]

[0107] Testing method:

[0108] (1) Wettability test:

[0109] At 45 ± 5°C, the same weight of electrolyte is injected into the bare battery cell. Under the same injection time and the same wetting time, the unabsorbed electrolyte is weighed to prove the wetting effect. The less the amount of unabsorbed electrolyte (liquid loss, unit: g), the better the wettability of the electrolyte.

[0110] (2) Direct Current Resistance (DCR) test:

[0111] ① At room temperature (25 ± 2°C), first discharge the secondary battery at a current of 1 / 3C to 2.8V and let it stand for 30 min. Then charge it at a current of 1 / 3C to 4.35V and perform constant voltage charging at 4.35V until the current ≤ 0.05C.

[0112] ②First, perform a standard charge and discharge to obtain the actual discharge capacity C0 of the secondary battery. Then, charge the secondary battery at a constant current of 1 / 3C0 to the maximum voltage of 4.35V per cell, and then continue charging at a constant voltage of 4.35V. Terminate the charging when the current drops to 0.05C0 during the constant voltage process; after leaving it standing for 30 min, discharge the secondary battery at a constant current of 1 / 3C0 to a capacity of (1 - n%)C0, where n is any value greater than 0 and less than 100.

[0113] ③At room temperature (25 ± 2°C), after charging the secondary battery according to ①, let it stand for 30 min, and then adjust the battery charge of the secondary battery to 90%, 50%, 20%, and 10% SOC respectively according to ②, and perform 1C10s charge and discharge pulses to calculate the 10s DC internal resistance of the secondary battery.

[0114] (3) Low-temperature cycle performance test:

[0115] At room temperature (25 ± 2°C), discharge the secondary battery at a current of 1 / 3C to 2.8V, let it stand for 30 min, then charge it at a current of 1 / 3C to 4.35V, and perform constant voltage charging at 4.35V until the current ≤ 0.05C; place the secondary battery in an environment of -20 ± 2°C and leave it standing for 3 h, then discharge it at a current of 1C to 2.8V, and record the discharge capacity.

[0116] (4) High-temperature cycle performance test:

[0117] At room temperature (25 ± 2°C), discharge the secondary battery at a current of 1 / 3C to 2.8V, let it stand for 30 min, then charge it at a current of 1 / 3C to 4.35V, and perform constant voltage charging at 4.35V until the current ≤ 0.05C; place the secondary battery in an environment of 40 ± 2°C and leave it standing for 3 h, then discharge it at a current of 1C to 2.8V, and record the discharge capacity.

[0118] The wettability test was carried out on the secondary batteries prepared through Examples 1 to 5 and Comparative Example 1, and the test data are shown in Table 2.

[0119] Table 2

[0120] Example Injection time / s Injection volume / g Infiltration time / h Liquid loss volume / g Example 1 750 435 14 19.3 Example 2 750 435 14 12.4 Example 3 750 435 14 5.4 Example 4 750 435 14 20.5 Example 5 750 435 14 3.7 Comparative Example 1 750 435 14 21.1

[0121] According to the test data in Table 2, it can be seen that the average liquid loss of the secondary batteries prepared through Examples 1 to 5 is 13.7 g, which is 35% lower than the liquid loss of 21.1 g of the secondary batteries prepared through Comparative Example 1; and as the mass percentage of the first additive in the electrolyte increases, the liquid loss gradually decreases. When the mass percentage of the first additive in the electrolyte increases to 2 wt%, the liquid loss decreases to 3.7 g. Thus, it can be seen that the wetting effect of the electrolyte provided in the embodiments of the present invention on the separator and the electrode sheet is significantly improved.

[0122] The DCR tests were conducted on the secondary batteries prepared in Examples 1 to 5 and Comparative Example 1, and the test data are shown in Table 3.

[0123] Table 3

[0124] Example DCR / mΩ Example 1 0.59 Example 2 0.56 Example 3 0.54 Example 4 0.60 Example 5 0.52 Comparative Example 1 0.62

[0125] According to the test data in Table 3, the average value of the DCR of the secondary batteries prepared in Examples 1 to 5 is 0.56 mΩ, which is 9.7% lower than the DCR of 0.62 mΩ of the secondary battery prepared in Comparative Example 1; and, as the mass percentage of the first additive in the electrolyte increases, the DCR of the secondary battery gradually decreases. Thus, it can be seen that the electrolyte provided based on the embodiments of the present invention is beneficial to reducing the DC internal resistance and impedance of the secondary battery.

[0126] The low-temperature cycle performance tests were conducted on the secondary batteries prepared in Examples 1 to 5 and Comparative Example 1, and the test data are shown in Table 4.

[0127] Table 4

[0128] Example Initial capacity / Ah Discharge capacity at -20°C / Ah Capacity retention rate % Example 1 173.5 121.5 70.03 Example 2 173.7 123.7 71.21 Example 3 173.6 129.4 74.54 Example 4 173.5 120.8 69.63 Example 5 173.7 131.5 75.71 Comparative Example 1 173.8 119.8 68.93

[0129] According to the test data in Table 4, the average value of the discharge capacity of the secondary batteries prepared in Examples 1 to 5 at low temperature (-20 °C) is 124.5 Ah, which is 3.9% higher than the discharge capacity of 119.8 Ah of the secondary battery prepared in Comparative Example 1; further, the average value of the capacity retention rate of the secondary batteries prepared in Examples 1 to 5 at low temperature is 71.7%, which is 2.77% higher than the capacity retention rate of 68.93% of the secondary battery prepared in Comparative Example 1; and, as the mass percentage of the first additive in the electrolyte increases, the discharge capacity and capacity retention rate of the secondary battery at low temperature gradually increase. Thus, it can be seen that the low-temperature performance of the secondary battery prepared based on the electrolyte provided in the embodiments of the present invention is significantly improved.

[0130] The high-temperature cycle performance tests were conducted on the secondary batteries prepared in Examples 1 to 5 and Comparative Example 1, and the test data are shown in Table 5.

[0131] Table 5

[0132] Example Initial capacity / Ah Discharge capacity at 40°C / Ah Capacity retention rate / % Example 1 173.5 174.6 100.63 Example 2 173.7 175.9 101.27 Example 3 173.6 176.2 101.50 Example 4 173.5 174.4 100.52 Example 5 173.7 177.5 102.19 Comparative Example 1 173.8 174.1 100.17

[0133] According to the test data in Table 5, the average discharge capacity of the secondary batteries prepared by Examples 1 to 5 at high temperature (40 °C) is 175.5 Ah, which is 0.8% higher than the discharge capacity of 174.1 Ah of the secondary battery prepared by Comparative Example 1; further, the average capacity retention rate of the secondary batteries prepared by Examples 1 to 5 at low temperature is 101.22%, which is 1.05% higher than the capacity retention rate of 100.17% of the secondary battery prepared by Comparative Example 1; and, as the mass ratio of the first additive in the electrolyte increases, the discharge capacity and capacity retention rate of the secondary battery at high temperature gradually increase. Thus, it can be seen that the high-temperature performance of the secondary battery prepared based on the electrolyte provided in the embodiments of the present invention is significantly improved.

[0134] In summary, the electrolyte provided in the embodiments of the present invention includes a lithium salt, an organic solvent, and a first additive; the first additive includes a cardanol polyoxyethylene ether additive, and the cardanol polyoxyethylene ether additive includes a compound having the formula (I) and a compound having the formula (II); wherein, the long-chain alkyl group in the compound having the formula (I) has a certain flexibility and hydrophobicity, so that the long-chain alkyl group can provide a certain steric hindrance at low temperature, preventing excessive aggregation and crystallization between molecules in the electrolyte, which is beneficial to improving the low-temperature fluidity of the electrolyte, and the polyoxyethylene group in the compound having the formula (II) is a hydrophilic group, which can improve the solubility and dispersibility of the cardanol polyoxyethylene ether additive in water. At low temperature, the cardanol polyoxyethylene ether additive with good solubility and dispersibility can prevent the problem of increased viscosity of the electrolyte caused by the freezing of water in the electrolyte, which is beneficial to reducing the freezing temperature of the electrolyte and improving the stability of the electrolyte at low temperature; at high temperature, the long-chain alkyl group in the compound having the formula (I) can also increase the distance between molecules in the electrolyte, reduce the intermolecular interaction force, and reduce the probability of violent collision and chemical reaction caused by thermal motion of molecules, thereby improving the stability of the electrolyte at high temperature. The ether bond in the polyoxyethylene group of the compound having the formula (II) has high chemical stability and is not easily oxidized and hydrolyzed, which can reduce the probability of chemical reactions such as decomposition and oxidation of the electrolyte at high temperature, which is beneficial to further improving the stability of the electrolyte at high temperature, and further beneficial to reducing the impedance of the secondary battery prepared based on the electrolyte at low temperature and high temperature, and improving the battery capacity of the secondary battery at low temperature and high temperature.

[0135] The above has introduced in detail an electrolyte, a secondary battery and an electrical device provided by the present invention. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. An electrolyte, characterized in that: It includes a lithium salt, an organic solvent and a first additive; the first additive includes a cardanol polyoxyethylene ether additive, and the cardanol polyoxyethylene ether additive includes a compound having formula (I) and a compound having formula (II): C n H 2n+1 -OR formula (I); Wherein, R in formula (I) represents hydrogen or a sulfate compound; n in formula (I) represents the number of carbon atoms in the compound having formula (I), and n is an integer greater than 2; In formula (II), m represents the number of addition of —CH2—CH2—O—, and m is any integer from 3 to 200.

2. The electrolyte according to claim 1, characterized in that The cardanol polyoxyethylene ether additive is selected from at least one of cardanol polyoxyethylene ether, cardanol polyoxyethylene ether sodium sulfate and cardanol polyoxyethylene ether ammonium sulfate.

3. The electrolyte according to claim 1, characterized in that The sulfate compound includes sodium sulfate and / or ammonium sulfate.

4. The electrolyte according to claim 1, characterized in that In the electrolyte, the mass percentage of the first additive is 0.1 wt % to 2 wt %.

5. The electrolyte according to claim 1, characterized in that The electrolyte further includes a second additive, which includes at least one of vinylene carbonate, propylene sulfite, vinyl sulfate, vinylene carbonate, fluoroethylene carbonate, lithium bis(oxalatoborate), lithium bis(fluorosulfonyl imide), and lithium bis(trifluoromethanesulfonyl imide).

6. The electrolyte according to claim 5, characterized in that In the electrolyte, the mass percentage of the second additive is 2 wt % to 10 wt %.

7. The electrolyte according to claim 1, characterized in that The lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium difluorooxalatoborate.

8. The electrolyte according to claim 1, characterized in that In the electrolyte, the mass percentage of the lithium salt is 8wt% to 16wt%.

9. A secondary battery, characterized in that: The secondary battery comprises a positive electrode sheet, a negative electrode sheet and the electrolyte according to any one of claims 1 to 8.

10. An electrical device, characterized in that: The electrical device comprises the secondary battery according to claim 9, and the secondary battery serves as a power supply for the electrical device.

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