Electrolyte for lithium ion battery and lithium ion battery

Abstract

The invention provides an electrolyte for a lithium ion battery and the lithium ion battery. The electrolyte for the lithium ion battery comprises a lithium salt and an organic solvent, and further comprises a first additive # imgabs 0 # and a second additive # imgabs 1 #, wherein R1, R2 and R3 are independently selected from hydrogen, halogen, substituted or unsubstituted C1-C5 alkyl groups, substituted or unsubstituted C1-C5 alkyl ether groups and substituted or unsubstituted C1-C5 alkyl ester groups; r4 is selected from halogen, an unsubstituted aryl group of C6 to C10, or an aryl group of C6 to C10 substituted by a substituent group; the content of the first additive and the content of the second additive are respectively and independently 0.5%-5.0% of the total weight of the electrolyte for the lithium ion battery; the organic solvent comprises a carbonic ester organic solvent and a carboxylic ester organic solvent. According to the invention, the electrolyte with excellent performance is provided through the structure and combination relationship of the first additive and the second additive in combination with the variety of the organic solvent.

Description

Technical Field

[0001] The present invention relates to the field of lithium - ion batteries, and more particularly, to an electrolyte for a lithium - ion battery and a lithium - ion battery. Background Art

[0002] Low - temperature and fast - charging electrolytes are important research directions in lithium - ion battery technology. With the development of fields such as portable electronic devices, electric vehicles, and renewable energy, the requirements for battery performance are getting higher and higher. Among them, low - temperature performance, fast - charging ability, and cycle stability are particularly important.

[0003] In a traditional lithium - ion battery under low - temperature conditions, the ionic conductivity of the electrolyte decreases, resulting in a slowdown in the rate of the battery's electrochemical reaction. In severe cases, it may even cause the battery to fail to work properly. During the fast - charging process of a traditional battery, due to polarization, the problem of lithium dendrite growth usually occurs. These phenomena not only reduce the working efficiency of the battery but also may lead to safety hazards. Currently, using carboxylic esters with low viscosity, high dielectric constant, and low freezing point can, to a certain extent, solve the problem of lithium - ion transport under low - temperature and fast - charging conditions. However, during the application of the battery, carboxylic esters do not participate in the formation of a film on the negative electrode, and at the same time, they will react with physicochemical graphite (i.e., the charged state of the graphite, which is the negative - electrode active material), resulting in difficulty in meeting the requirements for fast - charging and low - temperature cycle performance. Therefore, it is particularly important to develop an electrolyte that can inhibit the reaction between carboxylic esters and physicochemical graphite.

[0004] Based on this, how to provide special electrolyte additives and electrolyte formulations to effectively inhibit the reaction between the ester - type solvent in the electrolyte for a lithium - ion battery and the negative - electrode material, thereby significantly improving the cycle stability and low - temperature performance of the corresponding lithium - ion battery, is one of the important technical problems to be solved in this field. Summary of the Invention

[0005] The main object of the present invention is to provide an electrolyte for a lithium - ion battery and a lithium - ion battery to solve the problem of poor cycle performance and low - temperature resistance of a lithium - ion battery caused by the reaction between the organic solvent in the electrolyte for a lithium - ion battery in the prior art and the negative - electrode active material in the lithium - ion battery.

[0006] To achieve the above object, in a first aspect of the present invention, there is provided an electrolyte for a lithium - ion battery, including a lithium salt and an organic solvent. The electrolyte for the lithium - ion battery further includes a first additive and a second additive; the first additive has the structure shown in Formula I:

[0007]

[0008]

[0009] In Formula I, R1, R2 and R3 are each independently selected from hydrogen, halogen, substituted or unsubstituted C1-C5 alkyl, substituted or unsubstituted C1-C5 alkyl ether group, and substituted or unsubstituted C1-C5 alkyl ester group; the second additive has the structure shown in Formula II:

[0010]

[0011] In Formula II, R4 is selected from halogen, unsubstituted C6-C10 aryl, or C6-C10 aryl substituted by substituents, and the substituents are selected from one or more of halogen, C1-C3 alkyl and C1-C3 alkyl ether group; based on the total weight of the electrolyte for a lithium-ion battery being 100%, the contents of the first additive and the second additive are each independently 0.5% to 5.0%; the organic solvent includes carbonate organic solvents and carboxylate organic solvents.

[0012] The present invention provides an electrolyte with excellent performance by carefully designing the structure and combination relationship of the first additive and the second additive, and at the same time coordinating with the type of organic solvent. When the obtained electrolyte is applied to a lithium-ion battery, the reaction between the organic solvent and the negative electrode active material in the battery electrode is inhibited, and the corresponding lithium-ion battery thus has particularly excellent cycle stability and low-temperature resistance. More importantly, the present invention provides the first additive and the second additive with the above structures and synchronously controls their dosages, so that the decomposition products of the two additives on the surface of the negative electrode can interact with each other to form a composite SEI film with particularly stable structure and good lithium-ion transport characteristics, thereby effectively inhibiting the side reactions during the application of the lithium-ion battery and improving its cycle stability and low-temperature resistance.

[0013] Furthermore, in order to better enable the first additive and the second additive to play a synergistic role and form a more stable SEI film during the battery application process, thereby further improving its cycle stability, it is preferred that the weight ratio of the first additive to the second additive is (0.5-5):1, and in Formula I, R1, R2 and R3 are each independently selected from hydrogen, halogen, unsubstituted C1-C3 alkyl, unsubstituted C1-C3 alkyl ether group, or unsubstituted C1-C3 alkyl ester group; and / or, in Formula II, R4 is selected from halogen, phenyl, para-substituted phenyl or ortho-substituted phenyl, and the substituents are selected from one or more of halogen and C1-C3 alkyl ether group.

[0014] In order to further optimize the types of additives in the electrolyte to obtain an electrolyte for lithium-ion batteries with more excellent performance, thereby improving the stability and low-temperature resistance of the corresponding lithium-ion batteries, regarding the structure of the first additive, that is, in Formula I, preferably R1, R2, and R3 each independently selected from hydrogen, fluorine, methyl, ethyl, methyl ether group, ethyl ether group, methyl ester group or ethyl ester group; and regarding the structure of the second additive, that is, in Formula II, preferably R4 is selected from fluorine, para-substituted phenyl or ortho-substituted phenyl, and the substituents are selected from one or more of fluorine, methyl ether group and ethyl ether group.

[0015] In several preferred embodiments, in Formula I, R1 and R3 each independently selected from fluorine, methyl ether group or ethyl ether group, R2 is selected from hydrogen, methyl, ethyl, methyl ester group or ethyl ester group; and / or, in Formula II, R4 is selected from fluorine, phenyl para-substituted by fluorine, phenyl para-substituted by methyl ether group or phenyl ortho-substituted by methyl ether group. The specific selection of these two types of additive structures is more effective in improving the performance of the battery in a low-temperature environment when the obtained electrolyte is used in the battery system.

[0016] In several more preferred embodiments, in order to make the synergistic effect between the two additives more prominent, so as to more effectively improve the cycle stability of the obtained electrolyte for the corresponding lithium-ion battery, preferably the weight ratio of the first additive to the second additive is (1-1.5):1, and the first additive is selected from one or more of the following structures:

[0017] and / or, the second additive is selected from one or more of the following structures:

[0018]

[0019] In order to more significantly adjust the solvation structure of the electrolyte and form a more stable lithium-ion solvation environment, further preferably the weight ratio of the carbonate organic solvent to the carboxylate organic solvent is 1:(1-9), and: the carbonate organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; and / or, the carboxylate organic solvent is selected from one or more of methyl acetate, ethyl acetate, ethyl propionate and propyl propionate.

[0020] Furthermore, the weight ratio of the carbonate organic solvent to the carboxylate organic solvent is 1:(1-4), and the carbonate organic solvent is selected from at least two of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the carboxylate organic solvent is selected from at least two of methyl acetate, ethyl acetate, and ethyl propionate. The above specific types of carbonate organic solvents and carboxylate organic solvents, as well as the dosage relationship between the two, can further optimize the lithium ion conduction mechanism and improve the overall lithium ion conductivity of the electrolyte.

[0021] Furthermore, in the electrolyte for a lithium ion battery, the molar concentration of the lithium salt is 2 mol / L to 4 mol / L, and the lithium salt is selected from at least two of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro(oxalato)phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide, so as to better adapt to the above two additives and the special organic solvent system, thereby more significantly improving the various properties of the electrolyte and enabling the corresponding lithium ion battery to exhibit more excellent fast charging ability, cycle stability, and low temperature resistance. Preferably, the lithium salt is a mixed salt formed by lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, and the concentration ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is (0.5-1.5):1, so as to promote the more effective synergistic effect of the first and second additives, and at the same time more effectively adjust the balance between the lithium salt and the solvent in the electrolyte, reduce the possibility of lithium deposition, and thus improve the cycle stability of the lithium ion battery.

[0022] Furthermore, in order to adjust and optimize the solvation structure of the obtained electrolyte, increase the content of contact ion pairs and aggregates therein, enable more anions to participate in the film-forming reaction, so as to further improve the electrode interface, increase the LiF content, and improve the lithium ion conductivity of the SEI, thereby more effectively improving the cycle performance and low temperature performance of the battery where the electrolyte is located. In the electrolyte for a lithium ion battery, the molar concentration of the lithium salt is a mol / L, the weight of the carbonate organic solvent is b, and the weight of the carboxylate organic solvent is c, and preferably a, b, and c satisfy: a*(b / c)=0.4-2.5.

[0023] The second aspect of the present invention provides a lithium ion battery, including an electrolyte, and the electrolyte is the electrolyte for a lithium ion battery as described above. The electrolyte for a lithium ion battery provided by the present invention can effectively prevent the organic solvents in the electrolyte, especially the carboxylate solvents, from reacting with the graphite of the negative electrode active material. Therefore, when this electrolyte is applied to a lithium ion battery, the service life and low temperature resistance of the obtained lithium ion battery are significantly improved.

[0024] Applying the technical solution of the present invention, through the carefully designed structure and combination relationship of the first additive and the second additive, and in conjunction with the type of organic solvent, an electrolyte with excellent performance is provided. When the obtained electrolyte is applied to a lithium-ion battery, the reaction between the organic solvent and the negative electrode active material in the battery electrode is inhibited, and the corresponding lithium-ion battery thus has particularly excellent cycle stability and low-temperature resistance. Detailed Embodiments

[0025] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0026] As described in the background art, there will be a reaction between the organic solvent in the electrolyte for lithium-ion batteries in the prior art and the negative electrode active material in the lithium-ion battery, resulting in problems of poor cycle performance and low-temperature resistance of the lithium-ion battery. To solve the above technical problems, a first aspect of the present invention provides an electrolyte for a lithium-ion battery, including a lithium salt and an organic solvent, and the electrolyte for the lithium-ion battery further includes a first additive and a second additive; the first additive has the structure shown in Formula I:

[0027]

[0028] In Formula I, R1, R2 and R3 are each independently selected from hydrogen, halogen, substituted or unsubstituted C1-C5 alkyl, substituted or unsubstituted C1-C5 alkyl ether group, and substituted or unsubstituted C1-C5 alkyl ester group; the second additive has the structure shown in Formula II:

[0029]

[0030] In Formula II, R4 is selected from halogen, unsubstituted C6-C10 aryl, or C6-C10 aryl substituted by a substituent, and the substituent is selected from one or more of halogen, C1-C3 alkyl and C1-C3 alkyl ether group; based on the total weight of the electrolyte for the lithium-ion battery being 100%, the contents of the first additive and the second additive are each independently 0.5%-5.0%; the organic solvent includes carbonate organic solvents and carboxylate organic solvents.

[0031] The present invention provides an electrolyte with excellent performance through the carefully designed structure and combination relationship of the first additive and the second additive, and in conjunction with the type of organic solvent. When the obtained electrolyte is applied to a lithium-ion battery, the reaction between the organic solvent and the negative electrode active material in the battery electrode is inhibited, and the corresponding lithium-ion battery thus has particularly excellent cycle stability and low-temperature resistance.

[0032] In the above electrolyte, the first additive has a sulfonic acid group and a benzothiazole-like structure. The sulfonic acid group can be preferentially decomposed on the surface of the negative electrode and react with lithium ions to form lithium alkyl sulfonate (RSO3Li), which is a beneficial SEI film component. Its main function is to protect the negative electrode material, prevent further decomposition of the electrolyte, and at the same time allow lithium ions to pass through. The presence of the benzothiazole-like ring structure provides additional stability to the SEI film. Its ring structure forms a network through π-π stacking, increasing the rigidity and chemical stability of the formed SEI film, and helping to form a denser and more uniform protective layer. The second additive contains an isocyanate group (-N=C=O) and a sulfonyl group (-SO2-) in its structure. The isocyanate group can cooperate with the sulfonyl group in the first additive to further enhance the formation of lithium alkyl sulfonate (RSO3Li), optimize the composition of the SEI film, and improve its conductivity. The isocyanate group can react with moisture or acidic substances that may be present in the electrolyte, reducing the adverse effects of these substances on the instability of the SEI film, accelerating battery self-discharge, and deteriorating its cycle performance. The sulfonyl group itself has good electron transport ability. When it cooperates with the sulfonic acid group in the first additive, it can further enhance the formation of lithium alkyl sulfonate (RSO3Li) in the SEI film, improving the ionic conductivity and stability of the SEI film.

[0033] More importantly, the present invention provides the first additive and the second additive with the above structures and synchronously controls their dosages, so that the decomposition products of the two additives on the surface of the negative electrode can interact with each other to form a composite SEI film with particularly stable structure and good lithium ion transport characteristics, thereby effectively suppressing side reactions during the application of lithium ion batteries and improving their cycle stability and low temperature resistance.

[0034] In particular, in the structure of the first additive provided by the present invention, it is preferably free of amino groups and / or nitro groups. The nitro group has strong oxidizing ability and is easily reduced to harmful amino groups on the negative electrode during subsequent actual application. The nitrogen-hydrogen bond in the amino group is unstable. On the one hand, it reacts and is consumed during storage or cycling, causing capacity loss; on the other hand, it is easy to generate lithium amide during application, and lithium amide dissolved in the electrolyte is likely to induce strong decomposition of carbonate organic solvents, reducing the stability of the electrolyte, which is particularly unsuitable for the electrolyte system provided by the present invention. At the same time, amino groups and nitro groups are prone to redox reactions in an electrochemical environment, especially under high voltage conditions, resulting in electrolyte decomposition, gas generation, and reducing the stability and safety of the battery.

[0035] Further, in order to better enable the first additive and the second additive to play a synergistic role and form a more stable SEI film during the battery application process, thereby further improving its cycle stability, the weight ratio of the first additive to the second additive is preferably (0.5 to 5):1. Moreover, in Formula I, R1, R2, and R3 are each independently selected from hydrogen, halogen, unsubstituted C1-C3 alkyl, unsubstituted C1-C3 alkyl ether group, or unsubstituted C1-C3 alkyl ester group; and / or, in Formula II, R4 is selected from halogen, phenyl, para-substituted phenyl, or ortho-substituted phenyl, and the substituents are selected from one or more of halogen and C1-C3 alkyl ether group.

[0036] On this basis, in order to further optimize the types of additives in the electrolyte to obtain an electrolyte for lithium-ion batteries with more excellent performance, and then improve the stability and low-temperature resistance of the corresponding lithium-ion battery, for the structure of the first additive, that is, in Formula I, it is preferably that R1, R2, and R3 are each independently selected from hydrogen, fluorine, methyl, ethyl, methyl ether group, ethyl ether group, methyl ester group, or ethyl ester group; and for the structure of the second additive, that is, in Formula II, it is preferably that R4 is selected from fluorine, para-substituted phenyl, or ortho-substituted phenyl, and the substituents are selected from one or more of fluorine, methyl ether group, and ethyl ether group.

[0037] In several preferred embodiments, in Formula I, R1 and R3 are each independently selected from fluorine, methyl ether group, or ethyl ether group, and R2 is selected from hydrogen, methyl, ethyl, methyl ester group, or ethyl ester group; and / or, in Formula II, R4 is selected from fluorine, para-fluorine-substituted phenyl, para-methyl ether group-substituted phenyl, or ortho-methyl ether group-substituted phenyl. The specific selection of these two types of additive structures is more effective in improving the performance of the battery in a low-temperature environment when the obtained electrolyte is used in the battery system.

[0038] In several more preferred embodiments, in order to make the synergistic effect between the two additives more prominent, thereby more effectively improving the cycle stability of the obtained electrolyte for the corresponding lithium-ion battery, the weight ratio of the first additive to the second additive is preferably (1 to 1.5):1, and the first additive is selected from one or more of the following structures:

[0039] and / or, the second additive is selected from one or more of the following structures:

[0040]

[0041] Through a large number of experiments, the inventor further preferably selects the first additive from one or more of the following structures: In the above structure, compound 1-2 contains fluorine in its structure, and when it decomposes, it can generate LiF (lithium fluoride) as a component of the SEI film. And LiF, as a compound with high stability and high ionic conductivity, can assist in forming a more uniform and stable protective layer, thereby more effectively reducing side reactions during the charge and discharge process of the battery and improving its cycle stability and safety. At the same time, the presence of fluorine atoms helps to lower the freezing point of the electrolyte where it is located, improve the fluidity of the electrolyte at low temperatures, and thus improve the performance of the corresponding lithium-ion battery in a low-temperature environment. And both compound 1-4 and compound 1-5 contain methoxy groups in their structures, and the ether bonds (C-O-C) on them can + form a strong solvation effect, thereby improving the transport efficiency of lithium ions in the electrolyte, reducing their binding with solvent molecules, and thus accelerating the charging speed of the battery at low temperatures and improving the fast charging performance.

[0042] Furthermore, for the above-mentioned structure preferably selected as the first additive, it is preferably and the mixture formed, and and have a weight ratio of (10:1) to (1:10), more preferably 1:(1 to 1.1); at the same time, the second additive is preferably In the above specific structural coordination relationship between the first additive and the second additive, as the first additive, compound 1-4 contains a methoxy group in its structure, which helps to form an SEI film with Li-O bonds. And when compound 1-5 is introduced in the above dosage relationship with 1-4, because compound 1-5 contains two methoxy groups in its structure, it can be coordinated to introduce a more complex ether bridge structure into the SEI film, further enhancing the stability and conductivity of the film. On this basis, the preferably selected second additive, that is, compound 2-3, also contains a methoxy group in its structure, so it can further optimize the composition of the SEI film, at the same time inhibit the excessive growth of the film, keep it thin and uniform, and ultimately significantly improve the cycle performance and safety of the corresponding lithium-ion battery.

[0043] Since in the electrolyte system provided by the present invention, the organic solvents include carbonate esters and carboxylate esters, among which the carbonate ester organic solvents have a relatively high dielectric constant and can effectively dissolve lithium salts to form an electrolyte with high ionic conductivity. The carboxylate ester organic solvents usually have a relatively low viscosity and good fluidity, which helps to improve the permeability of the electrolyte inside the battery and the transmission speed of lithium ions, especially at low temperatures. In order to more significantly adjust the solvation structure of the electrolyte and form a more stable lithium ion solvation environment, it is preferred that the weight ratio of the carbonate ester organic solvent to the carboxylate ester organic solvent is 1:(1-9), and: the carbonate ester organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and / or, the carboxylate ester organic solvent is selected from one or more of methyl acetate, ethyl acetate, ethyl propionate, and propyl propionate.

[0044] In several preferred embodiments, the weight ratio of the carbonate ester organic solvent to the carboxylate ester organic solvent is 1:(1-4), and the carbonate ester organic solvent is selected from at least two of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the carboxylate ester organic solvent is selected from at least two of methyl acetate, ethyl acetate, and ethyl propionate. The above specific types of carbonate ester organic solvents and carboxylate ester organic solvents, as well as the dosage relationship between the two, can further optimize the lithium ion conduction mechanism and improve the overall lithium ion conductivity of the electrolyte.

[0045] In several more preferred embodiments, the carbonate ester organic solvent is a mixed carbonate ester organic solvent composed of ethylene carbonate and ethyl methyl carbonate, and the weight ratio of ethylene carbonate to ethyl methyl carbonate is (1-2):1; the carboxylate ester organic solvent is a mixed carboxylate ester organic solvent composed of ethyl acetate and ethyl propionate, and the weight ratio of ethyl acetate to ethyl propionate is (0.5-1):1; or, the carbonate ester organic solvent is a mixed carbonate ester organic solvent composed of ethylene carbonate and dimethyl carbonate, and the weight ratio of ethylene carbonate to dimethyl carbonate is (1.5-2):1; the carboxylate ester organic solvent is a mixed carboxylate ester organic solvent composed of ethyl acetate and methyl acetate, and the weight ratio of ethyl acetate to methyl acetate is (1.5-2):1. The inventors have selected the corresponding solvent system based on the special structures of the first additive and the second additive through a large number of experiments. In the above two types of organic solvent systems, the components in the electrolyte have better compatibility, so that a more stable and thinner SEI film can be formed during the application of the lithium ion battery, further reducing the internal impedance and improving the cycle performance of the battery.

[0046] Regarding the concentration and type of lithium salts in the electrolyte for lithium-ion batteries, the molar concentration of the lithium salt is preferably 2 mol / L to 4 mol / L, and the lithium salt is selected from at least two of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro(oxalato)phosphate, lithium bis(fluorosulfonyl)imide salt, and lithium bis(trifluoromethylsulfonyl)imide, so as to better adapt to the above two additives and the special organic solvent system, thereby more significantly improving the various properties of the electrolyte and enabling the corresponding lithium-ion battery to exhibit more excellent fast charging ability, cycle stability, and low-temperature resistance.

[0047] In several preferred embodiments, the lithium salt is a mixed salt formed by lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide salt, and the concentration ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide salt is (0.5 to 1.5):1, more preferably (0.5 to 1):1. Among them, lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide salt can generate different anions when decomposed, namely PF6 - and FSI - . These anions react with the sulfonic acid group and benzothiazole structure in the first additive and the isocyanate and sulfonyl group structure in the second additive, and can form a more complex and stable SEI film. In particular, the FSI - anion generated by the decomposition of LiFSI can act synergistically with the sulfonic acid group, and can increase the content of lithium alkyl sulfonate (RSO3Li) and polybenzene ring in the SEI film. Mixing the two in the above preferred and more preferred dosage ratios can promote the more effective synergistic effect of the first and second additives, and at the same time more effectively adjust the balance of lithium salt and solvent in the electrolyte, reduce the possibility of lithium deposition, and thus improve the cycle stability of the lithium-ion battery.

[0048] Furthermore, in order to adjust and optimize the solvation structure of the obtained electrolyte, increase the content of contact ion pairs and aggregates therein, enable more anions to participate in the film-forming reaction, so as to further improve the electrode interface, increase the LiF content, and enhance the lithium ion conductivity of the SEI, thereby more effectively improving the cycle performance and low-temperature performance of the battery where the electrolyte is located. In the electrolyte for lithium-ion batteries, the molar concentration of the lithium salt is a mol / L, the weight of the carbonate organic solvent is b, and the weight of the carboxylic acid ester organic solvent is c, and preferably a, b, and c satisfy: a*(b / c) = 0.4 to 2.5.

[0049] On this basis, through a large number of experiments, the inventor further preferably selects a, b, and c to satisfy: a*(b / c) = 1.2 - 1.5. Because at low temperatures, the viscosity of the electrolyte increases and the lithium ion transport rate decreases. By further optimizing the value of a*(b / c) as above, the formation of polymers in the electrolyte can be more effectively increased, thus helping to maintain the fluidity of the electrolyte at low temperatures. The more optimized solvation structure formed under these conditions can further reduce the resistance of the membrane, help the battery maintain a high conductivity in a low-temperature environment, and ultimately improve the low-temperature performance of the battery.

[0050] The second aspect of the present invention provides a lithium-ion battery, including an electrolyte, which is the electrolyte for the lithium-ion battery described above. Because the electrolyte for the lithium-ion battery provided by the present invention contains a first additive and a second additive with special structures, which can effectively prevent the organic solvents in the electrolyte, especially carboxylic ester solvents, from reacting with the negative electrode active material graphite. Therefore, when this electrolyte is applied to a lithium-ion battery, the side reactions are effectively reduced during the application of the obtained lithium-ion battery, and the service life and low-temperature resistance are significantly improved.

[0051] In practical applications, the lithium-ion battery also includes a positive electrode plate and a negative electrode plate. The positive electrode plate includes a positive electrode active material, and the negative electrode plate includes a negative electrode active material. In several preferred embodiments, the positive electrode active material can be selected from one of LiNi 1-x-y-z Co x Mn y Al z O2, lithium nickel manganate, lithium cobaltate, lithium-rich manganese-based solid solution, lithium manganate, or lithium iron phosphate, where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, and 0 ≤ x + y + z ≤ 1; the negative electrode active material can be selected from one of artificial graphite, coated natural graphite, silicon-carbon negative electrode, silicon negative electrode, or lithium titanate.

[0052] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0053] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention. The specific structural formulas and CAS numbers of the first additive and the second additive adopted by the present invention are shown in Table 1.

[0054] Table 1

[0055]

[0056]

[0057] Example 1

[0058] An electrolyte for a lithium-ion battery:

[0059] Ethylene carbonate and ethyl acetate were formulated into a mixed organic solvent (ethylene carbonate: ethyl acetate = 30:70, weight ratio).

[0060] After that, lithium hexafluorophosphate (LiPF6) was added thereto at a molar concentration of 1 mol / L; lithium bis(fluorosulfonyl)imide salt (LiFSI) was added thereto at a molar concentration of 1 mol / L.

[0061] After the lithium salt was completely dissolved, 1% of Compound 1-1 and 1% of Compound 2-1 were added thereto based on the total weight of the obtained electrolyte being 100%.

[0062] Among them, the total molar concentration of the lithium salt was 2 mol / L, which was denoted as a; the weights of ethylene carbonate and ethyl acetate were denoted as b and c respectively; then a*(b / c) = 2*(3 / 7) = 0.86 (rounded to two decimal places).

[0063] The component contents of the above-mentioned electrolyte for a lithium-ion battery are shown in Table 2.

[0064] Examples 2 to 15, Comparative Examples 1 to 5

[0065] The differences between Examples 2 to 15, Comparative Examples 1 to 5 and Example 1 are only in the component contents of the electrolyte for a lithium-ion battery, as shown in Table 2 for details.

[0066] Table 2

[0067]

[0068]

[0069]

[0070] Preparation and performance test of lithium-ion battery samples:

[0071] Preparation of the positive electrode sheet: Lithium iron phosphate LFP, conductive agent Super P, binder PVDF and carbon nanotubes (CNT) were mixed evenly in a mass ratio of 97.2:1.8:1:1 to prepare a positive electrode slurry for a lithium-ion battery, which was coated on aluminum foil used as a current collector, and the coating amount was 340 g / m 2 , dried at 85 °C and then cold-pressed; then it was cut into strips and slices, and then dried in a vacuum at 85 °C for 4 h to make a positive electrode sheet for a lithium-ion battery.

[0072] Preparation of the negative electrode sheet: Artificial graphite, conductive agent Super P, thickening agent CMC, and binder SBR (styrene-butadiene rubber latex) are made into a slurry according to the mass ratio of 94.5:1.5:1.5:2.5, mixed evenly, coated on both sides of the copper foil with the prepared slurry, dried and rolled to obtain the negative electrode sheet, and then baked in a vacuum at 85°C for 4 hours to make the negative electrode sheet of the lithium-ion battery.

[0073] Preparation of lithium-ion battery samples: The positive electrode sheet, negative electrode sheet, and separator prepared according to the above process are made into a lithium-ion battery with a thickness of 0.5 mm, a width of 5 mm, and a length of 8 mm through the stacking process, with a capacity of 2.4 Ah, vacuum baked at 85°C for 48 hours, and the electrolyte obtained from the above examples and comparative examples is injected to complete the production of battery samples.

[0074] Electrical performance test: (1) Initial direct current resistance (DCR) test: After the obtained battery samples are divided into capacities, they are respectively charged to a state of charge of 50% SOC, left stationary for 30 min, and the sampling voltage V0 at the start of discharge is recorded; then discharged at a current of 3C for 10 s, and the sampling voltage V1 at the end of discharge is recorded, and the initial direct current discharge resistance DCR of each battery sample is calculated as DCR = (V1 - V0) / I. (2) 4C fast charge cycle performance test: Under the test conditions of 25°C, each battery sample is subjected to a charge-discharge cycle performance test with a 4C charge and a 1C discharge, and the charge-discharge voltage range is set to 2.0 - 3.65V. When the capacity retention rate drops to 80%, the number of cycles at this time is recorded. (3) Low-temperature test: At room temperature, each battery sample is charged at a constant current and constant voltage of 0.5C to 3.65V, left stationary at -20°C for 10 h, the charge-discharge voltage range is set to 2.0 - 3.65V, cycled 200 times at 0.5C, and finally discharged at 0.5C to 2.0V. After heating to room temperature and leaving it standing for 10 h, it is disassembled to observe the lithium deposition situation of the negative electrode sheet.

[0075] The above test results are shown in Table 3.

[0076] Table 3

[0077]

[0078] From the above description, it can be seen that the above embodiments of the present invention achieve the preparation of an electrolyte for a lithium-ion battery with excellent performance, which contains special first and second additives. When this electrolyte is applied to a lithium-ion battery, the corresponding lithium-ion battery has higher cycle stability and also exhibits particularly excellent low-temperature resistance.

[0079] Specifically, by comparing Examples 2, 7, 9, and 10 with the other examples, it can be seen that for the first additive, when it is selected from When one or more of them are present, the resulting electrolyte shows a more significant improvement in the cycle stability of the battery. Among these four, especially in Example 7, this example shows that when the first additive is and the mixture formed, and and the weight ratio of is preferably 1:(1 - 1.1), and the second additive is when the resulting electrolyte, when applied to a lithium-ion battery, the resulting lithium-ion battery has particularly excellent cycle stability.

[0080] By comparing Example 6 with Example 11 and comparing Example 1 with Example 8, it can be seen that when the weight ratio of the first additive to the second additive is further optimized from (0.5 - 5.0):1 to (1 - 1.5):1, the cooperative effect between the two is more significant, and the resulting electrolyte is also more effective in improving the cycle stability of the battery. And by comparing Example 1 with Example 11 and comparing Example 3 with Example 12, it can be seen that for the organic solvents used in the electrolyte, when the preferred carbonate organic solvents are a mixed carbonate organic solvent composed of ethylene carbonate and ethyl methyl carbonate, and the weight ratio of ethylene carbonate to ethyl methyl carbonate is (1 - 2):1; the carboxylic ester organic solvents are a mixed carboxylic ester organic solvent composed of ethyl acetate and ethyl propionate, and the weight ratio of ethyl acetate to ethyl propionate is (0.5 - 1):1, or the carbonate organic solvents are a mixed carbonate organic solvent composed of ethylene carbonate and dimethyl carbonate, and the weight ratio of ethylene carbonate to dimethyl carbonate is (1.5 - 2):1; the carboxylic ester organic solvents are a mixed carboxylic ester organic solvent composed of ethyl acetate and methyl acetate, and the weight ratio of ethyl acetate to methyl acetate is (1.5 - 2):1, these two specific organic solvent formulations can provide a more stable solvent system, and thus a more stable lithium-ion battery electrolyte can be obtained.

[0081] And by comparing Examples 14 and 15 with the other examples, it can be seen that for the molar concentration a mol / L of the lithium salt, the weight b of the carbonate organic solvent, and the weight c of the carboxylic ester organic solvent in the lithium-ion battery electrolyte, when the three satisfy a*(b / c)=0.4 - 2.5, the solvation structure of the whole electrolyte is further optimized, and finally shows better low-temperature resistance characteristics.

[0082] It should be noted that the terms "first", "second", etc. in the specification and claims of this application are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances, so that the embodiments of the present application described here can be implemented in an order other than those described here, for example.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An electrolyte for a lithium ion battery, comprising a lithium salt and an organic solvent, characterized in that: The lithium ion battery electrolyte also includes a first additive and a second additive; The first additive has a structure shown in Formula I: In the formula I, R1, R2 and R3 are each independently selected from hydrogen, halogen, substituted or unsubstituted C1-C5 alkyl, substituted or unsubstituted C1-C5 alkyl ether, and substituted or unsubstituted C1-C5 alkyl ester; The second additive has a structure shown in Formula II: In the formula II, R4 is selected from halogen, unsubstituted C6-C10 aryl, or C6-C10 aryl substituted with a substituent, and the substituent is selected from one or more of halogen, C1-C3 alkyl, and C1-C3 alkyl ether; Based on the total weight of the lithium ion battery electrolyte being 100%, the content of the first additive and the second additive is independently 0.5% to 5.0%; The organic solvent includes a carbonate organic solvent and a carboxylate organic solvent.

2. The electrolyte for lithium ion battery according to claim 1, characterized in that The weight ratio of the first additive to the second additive is (0.5-5):1, and, In the formula I, the R1, R2 and R3 are each independently selected from hydrogen, halogen, unsubstituted C1-C3 alkyl, unsubstituted C1-C3 alkyl ether, or unsubstituted C1-C3 alkyl ester; And / or, in the formula II, the R4 is selected from halogen, phenyl, para-substituted phenyl or ortho-substituted phenyl, and the substituent is selected from one or more of halogen and C1-C3 alkyl ether groups.

3. The electrolyte for lithium ion battery according to claim 1 or 2, characterized in that: In the formula I, the R1, R2 and R3 are each independently selected from hydrogen, fluorine, methyl, ethyl, methyl ether, ethyl ether, methyl ester or ethyl ester; And / or, in the formula II, the R4 is selected from fluorine, para-substituted phenyl or ortho-substituted phenyl, and the substituent is selected from one or more of fluorine, methyl ether and ethyl ether.

4. The electrolyte for lithium ion batteries according to any one of claims 1 to 3, characterized in that In the formula I, the R1 and R3 are each independently selected from fluorine, methyl ether or ethyl ether, and the R2 is selected from hydrogen, methyl, ethyl, methyl ester or ethyl ester; And / or, in the formula II, the R4 is selected from fluorine, a phenyl group substituted at the para position by fluorine, a phenyl group substituted at the para position by a methyl ether group, or a phenyl group substituted at the ortho position by a methyl ether group.

5. The electrolyte for lithium ion batteries according to any one of claims 1 to 4, characterized in that The weight ratio of the first additive to the second additive is (1-1.5):1, and, The first additive is selected from one or more of the following structures: And / or, the second additive is selected from one or more of the following structures:

6. The electrolyte for lithium ion battery according to any one of claims 1 to 5, characterized in that The weight ratio of the carbonate organic solvent to the carboxylate organic solvent is 1:(1-9), and: The carbonate organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; and / or, The carboxylate organic solvent is selected from one or more of methyl acetate, ethyl acetate, ethyl propionate and propyl propionate.

7. The electrolyte for lithium ion battery according to claim 6, characterized in that The weight ratio of the carbonate organic solvent to the carboxylate organic solvent is 1:(1-4), and the carbonate organic solvent is selected from at least two of the ethylene carbonate, the ethyl methyl carbonate and the dimethyl carbonate, and the carboxylate organic solvent is selected from at least two of the methyl acetate, the ethyl acetate and the ethyl propionate.

8. The electrolyte for lithium ion batteries according to any one of claims 1 to 7, characterized in that In the lithium ion battery electrolyte, the molar concentration of the lithium salt is 2 mol / L to 4 mol / L, and the lithium salt is selected from at least two of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorooxalatophosphate, lithium bis(fluorosulfonyl imide) salt and lithium bis(trifluoromethylsulfonyl imide); Preferably, the lithium salt is a mixed salt formed by the lithium hexafluorophosphate and the bisfluorosulfonyl imide lithium salt, and the concentration ratio of the lithium hexafluorophosphate to the bisfluorosulfonyl imide lithium salt is (0.5-1.5):

1.

9. The electrolyte for lithium ion batteries according to any one of claims 1 to 8, characterized in that In the lithium-ion battery electrolyte, the molar concentration of the lithium salt is a mol / L, the weight of the carbonate organic solvent is b, the weight of the carboxylate organic solvent is c, and a, b and c satisfy: a*(b / c)=0.4-2.

5.

10. A lithium ion battery comprising an electrolyte, characterized in that: The electrolyte is the electrolyte for a lithium ion battery according to any one of claims 1 to 9.

Images