Non-aqueous electrolyte, non-membrane lithium secondary battery, and lithium secondary battery

Abstract

Provided are a nonaqueous electrolyte, a non-membrane lithium secondary battery, and a lithium secondary battery. The nonaqueous electrolyte of the present invention comprises: a nitro group-containing compound as component (A), an isocyanate group-containing compound as component (B), a nonaqueous solvent as component (C), and a lithium salt as component (D); wherein the component (A) and the component (B) are dissolved in the component (C). The present invention significantly improves the fast charging performance and low-temperature discharge performance of the battery by using a nitro group-containing compound in combination with an isocyanate group-containing compound.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a non-aqueous electrolyte, membrane-free lithium secondary battery and lithium secondary battery. Background Technology

[0002] Electric vehicles have gradually become more widespread in recent years. However, the charging time for pure electric vehicles using lithium-ion batteries and membrane-less lithium-ion batteries is generally around 1 hour, which is about 20 times longer than the time it takes to fill up a regular gasoline car. Therefore, compared with traditional gasoline vehicles, the lower charging rate of electric vehicles remains one of the key factors affecting the user experience. To achieve an energy replenishment speed comparable to gasoline vehicles, the battery needs to complete the charging process in 10 minutes or even less, which requires a charging rate of approximately 6C, 8C, or even 10C. Such a high charging rate also places higher demands on the electrode materials, cell design, and electrolyte of lithium-ion batteries.

[0003] Optimizing the electrode materials, electrolyte, and cell design of lithium-ion batteries can fundamentally improve their fast-charging performance. Among these advancements, some studies in existing technologies have shown that the electrolyte is one of the main bottlenecks restricting the fast-charging performance of batteries.

[0004] From the perspective of facilitating the formation of the SEI film, some studies have disclosed the use of lithium nitrate in the electrolyte, which is beneficial for forming a Li3N-containing interfacial film on the negative electrode surface, improving fast charging and cycle performance. However, inorganic salts have poor compatibility with commonly used carbonate solvents, and the preparation cost is also high. Other studies have disclosed the use of organic azide compounds in non-aqueous electrolytes, which have been found to have excellent negative electrode film-forming properties. Using these organic azide compounds as electrolyte additives can significantly improve the fast charging and low-temperature discharge performance of batteries. However, due to the highly reactive azide groups contained in the electrolyte, organic azide compounds are easily oxidized and decomposed to produce gas in the presence of transition metals at the positive electrode of lithium batteries. As a result, they cannot effectively form a high-conductivity SEI film rich in lithium nitride, leading to very limited improvement in fast charging and low-temperature discharge performance.

[0005] Furthermore, in lithium-ion batteries, the structure typically includes or excludes the use of a separator. The separator material is non-conductive, and its physicochemical properties significantly influence battery performance. When a separator is used, its primary function is to separate the positive and negative electrodes, preventing short circuits caused by contact between them. It also allows electrolyte ions to pass through. When a separator is not used, the electrolyte itself typically separates the positive and negative electrodes to prevent short circuits and facilitates ion conduction. From the perspective of improving the energy density of lithium-ion rechargeable batteries, separator-less batteries are advantageous. In particular, the use of solid-state electrolytes (such as quasi-solid-state or all-solid-state electrolytes) makes separator-less batteries possible.

[0006] It is evident that although some research has been conducted in this field on electrolytes for improving battery fast-charging performance, the research is not yet sufficient and there is still room for further improvement. Summary of the Invention

[0007] The problem the invention aims to solve

[0008] As mentioned above, the electrolyte has a significant impact on the fast-charging performance of lithium-ion batteries (including those with and without separators). The requirements for the electrolyte vary depending on the charge / discharge rate. Currently, the requirements for fast charging have evolved from 2C to 4C and even higher rates like 6C, 8C, and even 10C and above in the future. Faster charge / discharge requires the SEI film formed on the electrode surface to have better ion conductivity and better suppressed reactivity between the electrolyte components and the SEI film.

[0009] As mentioned above, in the prior art, lithium nitrate or organic azide compounds are used as electrolyte additives, which can form lithium nitride (Li3N) on the surface of the electrode through electrochemical reaction. As a fast ion conductor, lithium nitride can reduce the impedance of the SEI film, thereby significantly improving the fast charging and rate discharging performance of the battery.

[0010] However, through long-term practical research, this invention has found that: 1) Lithium nitrate, as an inorganic additive, has poor compatibility with organic electrolytes and limited solubility in conventional non-aqueous solvents; 2) For organic azide compounds, the azide group is a highly reactive group, while the transition metal of the lithium-ion battery cathode contains a large number of empty d orbitals, and the ions carry multiple positive charges. The lone pair electrons of the azide group in the electrolyte can easily enter the empty orbitals of the transition metal ions, which has a significant impact on the polarization of the azide group, leading to its decomposition. Ultimately, this prevents the azide group from reacting with lithium to form a Li3N layer. This phenomenon is particularly pronounced under fast charging conditions, as higher charging rates lead to higher charging current densities, making the aforementioned oxidation and decomposition phenomena more likely to occur. Therefore, the use of lithium nitrate or organic azide compounds as electrolyte additives has very limited effect on improving the fast charging performance of the battery cell, especially considering charging rates above 4C, such as 6C or even 10C.

[0011] To address the aforementioned problems, this invention provides a non-aqueous electrolyte comprising compounds containing nitro groups and compounds containing isocyanate groups. On one hand, the nitro group-containing compounds possess a nitro group (NO2) and two double-bonded oxygen atoms (N=O and C=O), giving them a dual effect of strong electron-withdrawing and coordination of transition metal ions (e.g., transition metal ions dissolved from the positive electrode). This helps improve the oxidative stability of the electrolyte and the dissociation of lithium salts, effectively preventing the nitro group-containing compounds from undergoing oxidative decomposition under transition metal catalysis. This allows for the formation and maintenance of a high lithium-ion conductivity SEI film rich in lithium nitride at the negative electrode. On the other hand, the lone pair of electrons (N atoms) in the isocyanate group-containing compounds not only reduce the decomposition of lithium salts (e.g., LiPF6) but also allow them to react with Lewis acids (e.g., PF5) obtained from their decomposition, thus preventing these substances from ultimately generating HF and reacting with Li3N, thereby improving the overall stability of the SEI film.

[0012] Under such circumstances, even with faster charging rates, such as 4C or higher, such as 6C or even 10C, it still has excellent effects in removing decomposition products generated by lithium salts and enhancing the SEI film. Therefore, by using compounds containing nitro groups and compounds containing isocyanate groups in the electrolyte, the fast charging performance (charging rate of 6C or higher) and low-temperature discharge performance of the battery can be significantly improved.

[0013] Furthermore, the present invention also provides a lithium secondary battery, including lithium secondary batteries with and without separators, wherein the lithium secondary battery contains the above-mentioned non-aqueous electrolyte.

[0014] Solution for solving the problem

[0015] After in-depth research, it has been found that the above-mentioned technical problems can be solved by implementing the following technical solutions:

[0016] This invention first provides a non-aqueous electrolyte, the non-aqueous electrolyte comprising:

[0017] Compounds containing nitro groups as component (A),

[0018] Compounds containing isocyanate groups as component (B),

[0019] As a non-aqueous solvent for component (C), and

[0020] Lithium salt as component (D);

[0021] Component (A) and component (B) are dissolved in component (C).

[0022] According to the non-aqueous electrolyte described above, the compound containing nitro groups in component (A) has the structure shown in general formula I:

[0023]

[0024] Wherein, R1 represents a monovalent organic group, and the monovalent organic group contains at least one group selected from carboxylic acid ester group and carbonate group.

[0025] According to the non-aqueous electrolyte described above, the carboxylic acid ester group includes one or more of aliphatic carboxylic acid ester groups and aromatic carboxylic acid ester groups.

[0026] According to the non-aqueous electrolyte described above, the compound containing the nitro group is selected from one or more combinations of the following compounds:

[0027]

[0028] According to the non-aqueous electrolyte described above, the isocyanate-containing compound as component (B) has one or more isocyanate groups.

[0029] According to the non-aqueous electrolyte described above, the isocyanate-containing compound as component (B) is selected from one or more of chain aliphatic isocyanates, alicyclic isocyanates, and aromatic isocyanates.

[0030] According to the non-aqueous electrolyte described above, the content of the compound containing nitro groups in component (A) is 0.1% to 10% of the total mass of the non-aqueous electrolyte, and the content of the compound containing isocyanate groups in component (B) is 0.1% to 10% of the total mass of the non-aqueous electrolyte.

[0031] According to the non-aqueous electrolyte described above, the non-aqueous solvent of component (C) is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, carboxylic acid ester solvents, and ether solvents.

[0032] According to the above-described non-aqueous electrolyte, the component (C) non-aqueous solvent contains a combination of cyclic carbonate solvents and linear carbonate solvents.

[0033] According to the non-aqueous electrolyte described above, the lithium salt includes one or more fluorinated lithium salts.

[0034] Furthermore, the present invention provides a membrane-free lithium secondary battery, which includes a positive electrode, a negative electrode and the aforementioned non-aqueous electrolyte.

[0035] Furthermore, the present invention also provides a lithium secondary battery, comprising a positive electrode, a negative electrode, a separator, and the aforementioned non-aqueous electrolyte. According to the lithium secondary battery described above, the positive electrode active material includes nickel.

[0036] Furthermore, the present invention also provides an electric vehicle or transport vehicle that uses the aforementioned lithium secondary battery.

[0037] The effects of the invention

[0038] By implementing the above technical solution, the present invention can achieve the following technical effects:

[0039] The present invention provides a combination of nitro-containing compounds and isocyanate-containing compounds used in non-aqueous electrolytes. The nitro-containing compounds, with their strong electron-withdrawing nitro groups, can lower the highest occupied orbital (HOMO) energy level, resulting in high oxidation stability. It has been unexpectedly discovered that this strong electron-withdrawing effect effectively prevents the nitro-containing compounds from decomposing under transition metal catalysis, even at high current densities under high charging rates (2C, 4C and above, even 6C and 10C). This allows them to preferentially form a lithium-nitride-rich, highly conductive SEI film on the negative electrode compared to non-aqueous solvents (such as carbonate solvents). The isocyanate-containing compounds are Lewis bases with unshared electron pairs of nitrogen atoms, which can easily remove Lewis acids (e.g., PF5) generated during lithium salt decomposition inside the battery during charging and discharging, and form a robust film on the positive electrode surface, thereby suppressing side reactions between the positive electrode and the electrolyte. The combined use of these two compounds can effectively reduce the impedance of the SEI film formed on the electrode surface, significantly improving the battery's fast-charging performance and low-temperature discharge performance. Detailed Implementation

[0040] The present invention will now be described in detail. The description of the technical features described below is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples.

[0041] It should be noted that:

[0042] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0043] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0044] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0045] In this instruction manual, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃".

[0046] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" used refers to weight or mass percentage content.

[0047] In this specification, the terms "substantially" and "essentially" are used to indicate that the standard deviation from the theoretical model, theoretical data, or target data is within a range of 2%, preferably 1%, and more preferably 0.8%.

[0048] In this specification, the terms “comprising” and / or “including” are used to indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0049] In this manual, the term "fast charging" refers to charging rates of 2C or higher, 4C or higher, 5C or higher, such as 6C to 18C.

[0050] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0051] The term "aryl" refers to a monovalent group, either a monocyclic or fused polycyclic aromatic ring, whose ring atoms are all carbon atoms, for example, it can have 6 to 20, 6 to 12, or 6 to 10 carbon atoms. Non-limiting examples of aryl groups include (but are not limited to) phenyl, naphthyl, anthracene, and 1,2,3,4-tetrahydronaphthalene, etc. The term "C 6-12 "Aryl" refers to an aryl group having 6 to 12 carbon atoms. 6-12 Non-limiting examples of aryl groups include (but are not limited to) phenyl, naphthyl, and 1,2,3,4-tetrahydronaphthalene. When situated between or used to connect two other structural segments, it can also be a divalent group.

[0052] The term "heteroaryl" refers to a monovalent group of aromatic monocyclic or fused polycyclic nature, wherein at least one (e.g., 1, 2, 3, or 4) ring atoms are heteroatoms selected from N, O, S, and P, and the remaining ring atoms are C, for example, 5 to 12-membered rings, especially 5 to 10-membered rings. Non-limiting examples of heteroaryls include (but are not limited to)

[0053] etc. When it is located between two other structural segments or used to connect two other structural segments, it can also be a divalent group.

[0054] The term "alkyl" refers to a straight-chain or branched monovalent hydrocarbon group that does not contain unsaturation. The term "C"... 1-4 "Alkyl" refers to an alkyl group having 1 to 4 carbon atoms; the term "C" is used in this context. 1-8 "Alkyl" refers to an alkyl group having 1 to 8 carbon atoms, C 1-8 Non-limiting examples of alkyl groups include (but are not limited to) methyl (-CH3), ethyl (-CH2CH3), n-propyl (-CH2CH2CH3), isopropyl (-CH(CH3)2), n-butyl (-CH2CH2CH2CH3), sec-butyl (-CH(CH3)CH2CH3), isobutyl (-CH2CH(CH3)2), tert-butyl (-C(CH3)3), n-pentyl (-CH2CH2CH2CH2CH3), neopentyl (-CH2C(CH3)3), n-hexyl (-CH2CH2CH2CH2CH2CH3), etc. When it is located between or used to connect two other structural segments, it can also be a divalent group.

[0055] The term "halogen" refers to fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), which are located in Group VII of the periodic table.

[0056] The term "carbonyl" refers to the "-C(=O)-" group.

[0057] In this specification, the terms "optional," "optionally," or "independently" are used to indicate that the events or conditions described below may or may not occur, including both the occurrence and non-occurrence of the events or conditions. For example, the phrase "optionally" substituted with a halogen means that the alkyl group may be unsubstituted or monosubstituted, polysubstituted, or fully substituted with a halogen atom. Those skilled in the art will understand that for any group containing one or more substituents, no substitution or substitution pattern that is spatially impossible and / or cannot be synthesized is introduced.

[0058] The main objective of this invention is to provide a non-aqueous electrolyte and a lithium secondary battery (including lithium secondary batteries with and without separators) comprising the non-aqueous electrolyte. The non-aqueous electrolyte comprises: a compound containing nitro groups, a compound containing isocyanate groups, a non-aqueous solvent, and a lithium salt. By using the combination of the compound containing nitro groups and the compound containing isocyanate groups, it not only has excellent effects in removing decomposition products generated by lithium salts and enhancing the SEI film, but also significantly improves the battery's fast charging performance, low-temperature discharge performance, and cell cycle performance.

[0059] This invention is mainly derived from the following insights:

[0060] It has been observed that the amount of available lithium ions in the battery decreases due to changes in the positive electrode structure caused by repeated charging and discharging during the operation of lithium secondary batteries, while transition metal ions are easily eluted from the positive electrode into the electrolyte, resulting in a deterioration of battery capacity.

[0061] This phenomenon has a more pronounced impact on battery performance during fast charging. In particular, Lewis acids (such as PF5) produced by the thermal decomposition of lithium salts degrade the passivation film (such as the solid electrolyte interphase (SEI) film), exacerbating the elution of transition metal ions. These eluted transition metal ions can redeposit on the positive electrode to increase its resistance, or electrodeposit on the surface of the negative electrode to damage the SEI film, potentially leading to internal short circuits. The electrolyte decomposition reactions caused by this series of reactions are further promoted, increasing gas generation, the interfacial resistance of the negative electrode, and its self-discharge, resulting in low-voltage failure.

[0062] This invention provides a non-aqueous electrolyte comprising: a compound containing nitro groups, a compound containing isocyanate groups, a non-aqueous solvent, and a lithium salt. The inventors have discovered that when the compound containing nitro groups and the compound containing isocyanate groups are used together in the electrolyte, firstly, because the compound containing nitro groups has two double oxygen bonds (N=O and C=O) in its structure, it can coordinate in situ with high-valence transition metals (such as cobalt, manganese, and nickel), inhibiting the dissolution of cobalt, manganese, and nickel ions, thereby preventing the electrolyte from being oxidized and decomposed to produce gas. This improves the cycle performance of the battery during high-voltage charge and discharge processes, extends battery life, and enhances battery safety. Secondly, because the compound containing nitro groups has a strong electron-withdrawing nitro group... The presence of nitro groups gives the electrolyte a low oxidation potential, allowing it to form a dense and stable passivation film on the positive electrode surface during the first charge. This effectively suppresses the oxidative decomposition of organic solvents (such as carbonates) in the electrolyte on the positive electrode surface, effectively solving the problem of rapid capacity decay during high-voltage charge and discharge. Furthermore, the combined effect of the electron-withdrawing and metal-ion coordination effects effectively prevents the nitro group-containing compounds from undergoing oxidative decomposition under transition metal catalysis, thus forming a high lithium-ion conductivity SEI film rich in lithium nitride on the negative electrode, significantly improving the battery's fast-charging performance. In addition, compounds containing isocyanate groups are Lewis bases with unshared electron pairs of nitrogen atoms, which can easily remove Lewis acids (such as PF5) generated as electrolyte decomposition products inside the battery during charge and discharge, and form a robust film on the positive electrode surface, thereby suppressing side reactions between the positive electrode and the electrolyte, resulting in a significant improvement in battery performance.

[0063] <First Aspect>

[0064] A first aspect of the present invention provides a non-aqueous electrolyte comprising a nitro group-containing compound as component (A), an isocyanate group-containing compound as component (B), a non-aqueous solvent as component (C), and a lithium salt as component (D).

[0065] The non-aqueous electrolyte of the present invention can be applied to conventional lithium secondary batteries (including lithium secondary batteries with and without separators) in the art. In particular, the electrolyte of the present invention can provide lithium secondary batteries with good stability, cycle performance and low-temperature discharge characteristics under higher charging rate conditions.

[0066] Furthermore, without limitation, various optional functional additives may be used in the non-aqueous electrolyte, provided that they do not impede the realization of the technical effect of the present invention.

[0067] Component (A)

[0068] The general structural formula of the nitro group-containing compound of component (A) in this invention is shown in Formula I:

[0069]

[0070] Wherein, R1 represents a monovalent organic group, and more specifically, from the perspective of improving ion conduction efficiency, the monovalent organic group contains at least one group selected from carboxylic acid ester group and carbonate group.

[0071] For the carboxylic acid ester group, in some specific embodiments, the carboxylic acid ester group in the monovalent organic group may be derived from at least one of linear or branched chain aliphatic carboxylic acid ester groups, alicyclic carboxylic acid ester groups, or aromatic carboxylic acid ester groups, or a mixture thereof.

[0072] For such ester groups, in some preferred embodiments, R1 can be -X1-COO-X2 or -X1-OOC-X2. X1 and X2 can be the same or different, and under these conditions, X1 or X2 can be derived from a substituted or unsubstituted straight-chain or branched alkyl group, a group having a substituted or unsubstituted cycloalkyl group, or a group having a substituted or unsubstituted aromatic ring. For the alkyl or cycloalkyl groups described above in this invention, C1 is preferred. 1-10 Alkyl or C 3-10 Cycloalkyl groups, etc. The aromatic rings described above can be aryl or heteroaryl groups as defined above, preferably phenyl.

[0073] There are no particular restrictions on the substituents that can be used for the above substitutions without affecting the technical effect of the present invention. They can usually be halogens, especially F atoms.

[0074] In further exemplified embodiments, the R1 group may be a group derived from alkyl esters of alkyl acids, aryl esters of alkyl acids, alkyl esters of aryl acids, aryl esters of aryl acids, etc., and these alkyl and aryl groups may also have the substituents described above.

[0075] In some preferred embodiments, the nitro group-containing compound includes (but is not limited to) the following compounds:

[0076]

[0077] This invention introduces the nitro group-containing compound into the electrolyte, which can form a lithium-ion-conducting SEI film rich in lithium nitride at the negative electrode, thereby effectively improving the fast-charging performance of the battery.

[0078] In the case of carbonate groups, there are no particular restrictions in principle. In principle, R1 can include groups derived from chain carbonates or cyclic carbonates. From the perspective of stability, it is preferred that R1 can include groups derived from cyclic carbonates.

[0079] Furthermore, the inventors also discovered that, compared with compounds such as carbonates that contain only carbonyl groups, the compounds of the present invention contain not only carbonyl groups but also nitro groups. The strong electron-withdrawing nitro groups can lower the energy level of the highest occupied orbital (HOMO), thus exhibiting higher oxidation stability. Even under the catalysis of the positive electrode transition metal, they are not easily oxidized and decomposed. However, they readily react with lithium at the negative electrode to undergo a reduction reaction to generate a lithium nitride (Li3N) layer. This inorganic SEI film component with high ionic conductivity can significantly improve the fast charging performance of the battery.

[0080] Component (B)

[0081] The isocyanate group-containing compound of component (B) in this invention may be selected from one or more of chain aliphatic isocyanates, alicyclic isocyanates, and aromatic isocyanate compounds.

[0082] Furthermore, for such compounds containing isocyanate groups, their molecular structure may have one or more isocyanate groups, preferably 1 to 3 isocyanate groups.

[0083] In a further preferred embodiment, the compound containing the isocyanate group has the general structural formula shown in Formula II:

[0084] R2(N=C=O)n

[0085] (II)

[0086] Where n = 1 or 2.

[0087] R2 is selected from substituted or unsubstituted alkyl, alkoxy, cycloalkyl or their corresponding divalent groups, preferably, R2 is selected from substituted or unsubstituted straight-chain or branched alkyl or alkylene groups.

[0088] For the alkyl / alkylene groups described above in this invention, C is preferably preferred. 1-10 Alkyl / alkylene groups, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl, or their divalent groups, can be listed.

[0089] There are no particular restrictions on the substituents that can be used for the above substitution without affecting the technical effect of the present invention; preferably, they can be F atoms.

[0090] In some preferred embodiments, the isocyanate-containing compound includes (but is not limited to) the following compounds:

[0091]

[0092] This invention introduces the isocyanate-containing compound into the electrolyte. The isocyanate-containing compound has a lone pair of electrons in its structure, which not only reduces the decomposition of lithium salts (e.g., LiPF6) but also allows it to react with Lewis acids (e.g., PF5) obtained from its decomposition. This prevents these substances from ultimately generating HF and reacting with Li3N, thereby improving the overall stability of the SEI film.

[0093] Component (C)

[0094] The present invention does not particularly limit the type of non-aqueous solvent for component (C), as long as it is a non-aqueous solvent commonly used as a non-aqueous electrolyte.

[0095] In some specific embodiments, the non-aqueous solvent may be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, carboxylic acid ester solvents, and ether solvents.

[0096] In some specific embodiments, the cyclic carbonate solvent may be selected from ethylene carbonate, propylene carbonate, butyl carbonate, ethylene carbonate (EC), and fluoroethylene carbonate (FEC), etc.; the linear carbonate solvent may be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC), and bis(2,2,2-trifluoroethyl) carbonate (DFDEC), etc.; the carboxylic acid ester solvent may be selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, and methyl butyrate, etc.; and the ether solvent may be selected from dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), and dioxapentane (DOL), etc. These non-aqueous solvents can be used alone or in mixtures of two or more, preferably a combination of cyclic carbonate solvents and linear carbonate solvents. The cyclic carbonate solvents have a high dielectric constant and can form solvated lithium ion molecules well with lithium ions; the linear carbonate solvents have a low viscosity, which is beneficial for lithium ion conduction.

[0097] In some preferred embodiments, the non-aqueous solvent may be selected from one or more combinations of ethylene carbonate (EC), fluoroethylene carbonate (FEC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), methyl trifluoroethyl carbonate (FEMC), bis(2,2,2-trifluoroethyl) carbonate (DFDEC), ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), and dioxane (DOL), preferably a combination of EC and EMC, or a combination of FEC and FEMC.

[0098] Component (D)

[0099] The present invention does not particularly limit the type of lithium salt in component (D), and it can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt includes one or more fluorinated lithium salts. Specifically, the lithium salt is selected from one or more salts formed by lithium ions and the following anions: PF6 - BF4 - AsF6 - CF3SO3 - N(CF3SO2)2 - N(FSO2)2 - C(CF2SO2)3 - C2BF2O4 - .

[0100] In some preferred embodiments, the lithium salt may be selected from one or more combinations of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(fluorooxalateborate) (LiDFOB).

[0101] Other functional additives

[0102] There are no particular limitations in principle on other functional additives that can be used in the non-aqueous electrolyte of the present invention. For example, the use of some additives can promote film formation.

[0103] Examples of such additives include ethylene ethylene carbonate (VC), trimethylsilyl phosphate (TMSP), and sulfur-containing additives. The sulfur-containing additives can be selected from 1,3-propanesulfonate lactone (1,3-PS), 1,4-butanesulfonate lactone (1,4-BS), 2,4-butanesulfonate lactone (2,4-BS), 1,3-propenesulfonate lactone (PST), vinyl sulfate (DTD), methanedisulfonate methylene ester (MMDS), and vinyl sulfite (ES). These additives can be used alone or in mixtures of two or more.

[0104] Composition of non-aqueous electrolyte

[0105] In this invention, the content of the nitro-containing compound in component (A) is not particularly limited in principle, and the dosage range used in various scenarios already employed in the art can be referenced. In some specific embodiments of this invention, from the perspective of improving the stability of the nitro-containing compound in the electrolyte, the content of the nitro-containing compound in component (A) can be 0.1% to 10% by mass of the total mass of the non-aqueous electrolyte, preferably 0.2% to 5% by mass. For example, it can be 0.5% by mass, 1% by mass, 1.5% by mass, 2% by mass, 2.5% by mass, 3% by mass, 5% by mass, 8% by mass, 9% by mass, etc. When the content of component (A) is too high, on the one hand, it will increase the cost of the electrolyte, and on the other hand, it will cause the SEI film to thicken significantly, which is not conducive to maintaining a low internal resistance; when the content of component (A) is too low, it cannot be guaranteed that the electrolyte has sufficient ionic conductivity, thus affecting the cell performance.

[0106] In this invention, the content of the isocyanate-containing compound in component (B) is mainly determined in conjunction with component (A), therefore, in principle, there is no particular limitation on the amount of component (B). In some specific embodiments of this invention, from the perspective of controlling SEI film thickness and internal resistance and improving fast charging performance, the content of the isocyanate-containing compound in component (B) can be 0.1% to 10% by mass of the total mass of the non-aqueous electrolyte, preferably 0.2% to 5% by mass. For example, it can be 0.5% by mass, 1% by mass, 1.5% by mass, 2% by mass, 2.5% by mass, 3% by mass, 5% by mass, 8% by mass, 9% by mass, etc. Compared with component (A), when the content of component (B) is too high, there is a concern that it will lead to a significant increase in the SEI film, which is not conducive to the performance of fast charging of the battery; when the content of component (B) is too low, there is a concern that the electrolyte may not have sufficient ionic conductivity, thus affecting the performance of the battery cell.

[0107] There are no particular restrictions on the content of the lithium salt in component (D), as long as the purpose of this invention can be achieved. In some specific embodiments of this invention, the concentration of the lithium salt in the non-aqueous electrolyte is 0.2–10 mol / L, preferably 0.7–2 mol / L. For example, it can be 0.3 mol / L, 0.5 mol / L, 0.7 mol / L, 0.9 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, etc. When the concentration of lithium salt is too high, it will lead to excessively high viscosity and cost of the electrolyte; when the concentration of lithium salt is too low, sufficient ionic conductivity of the electrolyte cannot be guaranteed, thereby affecting the performance of the battery cell.

[0108] The present invention does not impose any particular limitation on the content of the functional additives, and the content can be selected as needed.

[0109] <Second aspect>

[0110] A second aspect of the present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte according to the first aspect.

[0111] There are no particular restrictions on the positive electrode of the battery in principle; it can be any positive electrode commonly used in the art, including a current collector and a positive electrode active material. There are also no particular restrictions on the positive electrode active material in principle; in some preferred embodiments, various lithium-containing oxides in the art can be used, and these oxides may contain other main group, subgroup, or rare earth metal elements in addition to lithium.

[0112] Furthermore, from the perspective of wide applicability, the above-mentioned positive electrode active material of the present invention can be a ternary lithium-ion positive electrode active material, and more specifically, it can be a positive electrode active material containing Mn, Co and Ni elements.

[0113] In some preferred embodiments, the positive electrode active material is lithium nickel cobalt manganese oxide, for example, Li(Ni) 0.6 Co 0.2 Mn 0.2 O2, Li(Ni) 0.7 Co 0.2 Mn 0.1 O2, Li(Ni) 0.8 Co 0.1 Mn 0.1 O2, Li(Ni) 0.9 Co 0.05 Mn 0.05 O2, etc.

[0114] There are no particular restrictions on the negative electrode of the battery in principle. It can be any negative electrode commonly used in the field, including a current collector and a negative electrode active material. Typically, the negative electrode active material can be a carbon material, a silicon material, or a mixture thereof.

[0115] There are no particular restrictions on the use of a separator in the battery; it may or may not be used. In some specific embodiments, a separator is further used in the lithium secondary battery. When using a separator, it can be a separator commonly used in the art. Preferably, a separator with high moisture retention capacity for the electrolyte solution and low resistance to electrolyte ion transfer can be used. In other specific embodiments, the lithium secondary battery is a separator-less lithium secondary battery. Preferably, the lithium secondary battery can be an all-solid-state battery, that is, it is permissible not to contain a separator, and the solid electrolyte plays the role of isolating the positive and negative electrodes and transporting lithium ions.

[0116] <Third aspect>

[0117] A third aspect of the invention provides an electric vehicle or transport vehicle that uses a lithium secondary battery according to the second aspect, for example, a passenger car, an off-road vehicle, or a sports utility vehicle.

[0118] Example

[0119] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0120] Example 1:

[0121] Preparation of electrolyte: In an argon atmosphere glove box with a moisture content of less than 1 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of EC:EMC = 1:3 to obtain a mixed solvent. Then, 1 mol / L lithium hexafluorophosphate (LiPF6), 1.0% of a nitro group-containing compound (compound 1) based on the total mass of the electrolyte, and 1.0% of an isocyanate group-containing compound (compound 3) based on the total mass of the electrolyte were added to the mixed solvent. After stirring evenly, the non-aqueous electrolyte of Example 1 was obtained.

[0122] Examples 2-4:

[0123] Examples 2-4 are also specific examples of electrolyte preparation. Except that the composition ratio of each component of the electrolyte is added as shown in Table 1, the other parameters and preparation methods are the same as in Example 1.

[0124] Comparative Examples 1-3:

[0125] Comparative Examples 1-3 were identical to Example 1 except that the electrolyte components were added in the proportions shown in Table 1.

[0126] Table 1. Composition ratio of electrolyte components in Examples 1-4 and Comparative Examples 1-3

[0127] Group non-aqueous solvents lithium salts Compounds containing nitro groups Compounds containing isocyanate groups Example 1 EC / EMC (1:3) <![CDATA[LiPF6(1mol / L)]]> Compound 1 (1%) Compound 3 (1%) Example 2 EC / EMC (1:3) <![CDATA[LiPF6(1mol / L)]]> Compound 2 (1%) Compound 4 (1%) Example 3 EC / EMC (1:3) <![CDATA[LiPF6(1mol / L)]]> Compound 2 (3%) Compound 3 (3%) Example 4 FEC / FEMC (1:3) <![CDATA[LiPF6(1mol / L)]]> Compound 2 (2%) Compound 3 (2%) Comparative Example 1 EC / EMC (1:3) <![CDATA[LiPF6(1mol / L)]]> -- -- Comparative Example 2 EC / EMC (1:3) <![CDATA[LiPF6(1mol / L)]]> Compound 1 (15%) -- Comparative Example 3 EC / EMC (1:3) <![CDATA[LiPF6(1mol / L)]]> -- Compound 4 (15%)

[0128] Preparation of lithium secondary batteries

[0129] Cathode preparation: The cathode active material Li(Ni) is prepared... 0.8 Co 0.1 Mn 0.1 O2, conductive ultrafine carbon powder (SP), single-walled carbon nanotubes (SWNT), and binder polyvinylidene fluoride (PVDF) were added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent at a mass ratio of 96:2:1:1 and stirred thoroughly until the solid content was controlled to 68%, thus obtaining a positive electrode mixture slurry. The formed positive electrode slurry was then coated onto aluminum foil and, after drying, rolled and die-cut to form a positive electrode sheet.

[0130] Negative electrode preparation: Graphite, conductive agent ultrafine carbon powder (SP), thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) are added to an appropriate amount of deionized water at a mass ratio of 95:2:1:2 and stirred thoroughly until the solid content is controlled to 55%, thus obtaining a negative electrode mixture slurry. Then, the formed negative electrode slurry is coated onto copper foil, and after drying, it is roll-cut to form a negative electrode sheet.

[0131] Battery assembly: Take the positive electrode sheet, negative electrode sheet and separator, stack them in the order of negative electrode, separator and positive electrode, then weld the tabs and encapsulate them with aluminum-plastic film to obtain a soft-pack dry cell. Finally, inject the non-aqueous electrolyte prepared in Examples 1-4 and Comparative Examples 1-3 into the cell to prepare a lithium-ion battery with a capacity of 5Ah.

[0132] Performance testing

[0133] The performance test was conducted as follows, and the test results are shown in Table 2.

[0134] (1) Rate Cycling: At 25°C, the packaged lithium-ion battery was subjected to a 6C constant rate cycle test after formation and capacity testing. It was charged at 6C constant current and constant voltage to 4.25V, rested for 0.5h, and then discharged at 6C constant current. The initial capacity of the battery in the first cycle was recorded as C1. Thereafter, the charging and discharging cycles were repeated in the manner of CCCV / CC from the first cycle. The capacity after the 200th discharge cycle was recorded as C.200 Then, the capacity retention rate (%) after 200 cycles at high rate and room temperature = C 200 / C1×100%.

[0135] (2) Low temperature discharge: At 25℃, after the lithium-ion battery has been formed and capacity tested, it is charged to 4.25V by constant current and constant voltage at 1C. After resting for 0.5h, it is discharged by constant current at 1C to obtain the initial capacity of the battery and recorded as C0. Then the cell is placed in a low temperature chamber at -20℃ and left for 4h before being discharged at a rate of 0.33C. The capacity obtained by the discharge is recorded as C1. Then the capacity retention rate (%) of low temperature discharge at -20℃ = C1 / C0 × 100%.

[0136] (3) DCR test: DC internal resistance DCR includes the battery's internal ohmic resistance, charge transfer resistance, diffusion resistance and polarization resistance, reflecting the battery's comprehensive resistance. Therefore, the value of DCR is extremely important for evaluating the fast charging performance of the battery cell.

[0137] Table 2. Battery performance test results for each embodiment and comparative example

[0138] Group Capacity retention rate (%) after 200 cycles of constant rate charge and discharge at 6C 25℃ Cell DCR (mΩ) Discharge capacity retention rate (%) at 0.33C-20℃ Example 1 90.5 19.3 83.4 Example 2 91.8 20.4 85.2 Example 3 88.6 21.8 82.7 Example 4 85.9 22.6 84.8 Comparative Example 1 78.5 24.2 74.9 Comparative Example 2 77.1 25.9 76.0 Comparative Example 3 73.3 27.8 70.6

[0139] As shown in Table 2, compared to Comparative Example 1, Examples 1-4, due to the use of the nitro-containing and isocyanate-containing compounds of the present invention in their electrolytes, exhibited not only better high-rate cycling performance but also better low-temperature performance at -20°C. Furthermore, in Comparative Examples 2 and 3, the excessively high content of the nitro-containing or isocyanate-containing compounds may have led to a relative decrease in lithium salt concentration, thus affecting internal resistance and cycle performance. Therefore, Comparative Examples 2 and 3 provide some references regarding the upper limits of the content of the nitro-containing and isocyanate-containing compounds of the present invention.

[0140] Furthermore, the addition of compounds containing nitro groups and compounds containing isocyanate groups in this invention significantly reduces the initial DCR value of the battery cell, thereby significantly improving the fast charging performance of the battery.

[0141] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0142] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A non-aqueous electrolyte, characterized in that, The non-aqueous electrolyte comprises: Compounds containing nitro groups, as component A Compounds containing isocyanate groups, as component B As a non-aqueous solvent for component C, and Lithium salt as component D; Component A and component B are dissolved in component C. Wherein, component A, containing a nitro group, has the structure shown in general formula I: （I） Wherein, R1 represents a monovalent organic group, and the monovalent organic group contains an aliphatic carboxylic acid ester group. Furthermore, the content of the nitro group-containing compound in component A is 0.1% to 10% by mass of the total mass of the non-aqueous electrolyte. The content of the isocyanate group-containing compound in component B is 0.1% to 10% of the total mass of the non-aqueous electrolyte.

2. The non-aqueous electrolyte according to claim 1, characterized in that, Component A, containing a nitro group, is selected from one or more of the following compounds in combination: 。 3. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The isocyanate-containing compound that is component B has one or more isocyanate groups.

4. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The isocyanate-containing compound that is component B is selected from one or more of chain aliphatic isocyanates, alicyclic isocyanates, and aromatic isocyanates.

5. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The non-aqueous solvent of component C is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, carboxylic acid ester solvents, and ether solvents.

6. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The non-aqueous solvent of component C contains a combination of cyclic carbonate solvents and linear carbonate solvents.

7. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The lithium salt includes one or more of fluorine-containing lithium salts.

8. A membraneless lithium secondary battery, characterized in that, The membraneless lithium secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte according to any one of claims 1 to 7.

9. The membraneless lithium secondary battery according to claim 8, characterized in that, The positive electrode active material in the positive electrode includes nickel.

10. A lithium secondary battery, characterized in that, The lithium secondary battery includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte according to any one of claims 1 to 7.

11. The lithium secondary battery according to claim 10, characterized in that, The positive electrode active material in the positive electrode includes nickel.

12. An electric vehicle that uses a membraneless lithium secondary battery according to claim 8 or a lithium secondary battery according to claim 10.

13. A means of transport that uses a membraneless lithium secondary battery according to claim 8 or a lithium secondary battery according to claim 10.

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