Non-aqueous electrolyte solution, separator-free lithium secondary battery, and lithium secondary battery

Abstract

The invention provides a non-aqueous electrolyte, a diaphragm-free lithium secondary battery and a lithium secondary battery. The non-aqueous electrolyte solution of the present invention contains a nitro group-containing compound as a component (A), an isocyanate group-containing compound as a component (B), a non-aqueous solvent as a component (C), and a lithium salt as a component (D), wherein the component (A) and the component (B) are dissolved in the component (C). According to the invention, the compound containing the nitro group and the compound containing the isocyanate group are combined for use, so that the fast charging performance and the low-temperature discharging performance of the battery are remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to a non-aqueous electrolyte, a diaphragm-free lithium secondary battery and a lithium secondary battery. Background Art

[0002] Electric vehicles have gradually become popular in recent years. However, the charging time of lithium-ion batteries and non-diaphragm lithium-ion batteries for pure electric vehicles is generally about 1 hour, and the time required to fully charge is about 20 times the time it takes to fill up an ordinary fuel vehicle. Therefore, compared with traditional fuel vehicles, the lower charging rate of electric vehicles is still one of the key factors affecting user experience. In order to achieve an energy replenishment speed comparable to that of fuel vehicles, the battery is required to complete the charging process in 10 minutes or even less, which requires a charging rate of about 6C, 8C or even 10C. Such a high charging rate also puts higher requirements on the electrode materials, cell design and electrolyte of lithium-ion batteries.

[0003] The fast charging performance of lithium-ion batteries can be substantially improved by optimizing the electrode materials, electrolytes and cell designs of lithium-ion batteries. Among them, some studies in the prior art 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 layer, some studies have disclosed the use of lithium nitrate in the electrolyte, which is conducive to the formation of an interface film containing Li3N on the surface of the negative electrode, improving the fast charging and cycle performance, but the inorganic salt has poor compatibility with commonly used carbonate solvents, and the preparation cost is also high. Some studies have also disclosed the use of organic azide compounds in non-aqueous electrolytes, which have been found to have excellent negative electrode film-forming properties. Using the organic azide compounds as electrolyte additives can significantly improve the fast charging and low-temperature discharge performance of the battery. However, due to the highly reactive azide groups contained in the organic azide compounds in the electrolyte, they are easily oxidized and decomposed to produce gas under the condition of the presence of transition metals at the positive electrode of the lithium battery, so that it is impossible to effectively form a highly conductive SEI film rich in lithium nitride, resulting in very limited improvements in fast charging performance and low-temperature discharge performance.

[0005] In addition, for lithium-ion batteries, a diaphragm may or may not be used in its structure. The diaphragm material is non-conductive, and its physical and chemical properties have a great influence on the performance of the battery. In the case of using a diaphragm, the main function of the diaphragm is to separate the positive and negative electrodes of the battery to prevent the two electrodes from contacting and short-circuiting. In addition, it also has the function of allowing electrolyte ions to pass through. In the case of not using a diaphragm, the positive and negative electrodes are usually separated by the electrolyte itself to prevent short-circuiting, and the ions are conducted by the electrolyte itself. From the perspective of improving the energy density of lithium secondary batteries, secondary batteries without diaphragms are advantageous. In particular, the use of solid electrolytes (such as quasi-solid or all-solid electrolytes) makes diaphragm-free batteries possible.

[0006] It can be seen that although some research has been conducted in this field on electrolytes for improving battery fast charging performance, the research cannot be said to be sufficient and there is still room for further improvement. Summary of the invention

[0007] Problem that the invention aims to solve

[0008] As mentioned above, the electrolyte has an important influence on the fast charging performance of lithium-ion batteries (including lithium-ion batteries with and without diaphragms). The requirements for electrolytes at different charge and discharge rates are also different. The current requirements for fast charging have further developed from 2C to 4C and even higher charging rates of 6C, 8C, and even 10C in the future. Faster charging and discharging requires that the SEI film formed on the electrode surface has better ion conductivity and better suppressed reaction activity between electrolyte components and SEI film.

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

[0010] However, after long-term practical research, the present invention found that: 1) For lithium nitrate, as an inorganic additive, it 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, and the positive transition metal of the lithium-ion battery contains a large number of empty d orbitals, and the ions carry multiple positive charges. The lone pair of electrons of the azide group in the electrolyte can easily enter the empty orbitals of the transition metal ions, which has a great effect on the polarization of the azide group, thereby causing the decomposition of the azide group, and ultimately making the azide group unable to react with lithium to form a Li3N layer. This phenomenon is particularly obvious under fast charging conditions. A higher charging rate leads to a greater charging current density, which makes the above-mentioned oxidation and decomposition phenomena more likely to occur. Therefore, the use of lithium nitrate or organic azide compounds as electrolyte additives has very limited improvement on the fast charging performance of the battery cell, especially considering that the charging rate is above 4C, such as 6C or even 10C.

[0011] In order to solve the above problems, the present invention provides a non-aqueous electrolyte, which includes a nitro group-containing compound and an isocyanate group-containing compound. On the one hand, the nitro group-containing compound has a nitro group (NO2) and two double-bonded oxygens (N=O and C=O) in its structure, which makes it have a dual effect of strong electron-withdrawing effect and coordination effect on transition metal ions (such as transition metal ions dissolved from the positive electrode), which helps to improve the oxidation stability of the electrolyte and the dissociation of lithium salts, and effectively avoids the oxidative decomposition of the nitro group-containing compound under transition metal catalysis, so that a high lithium ion conductivity SEI film rich in lithium nitride can be formed and maintained at the negative electrode; on the other hand, the N atom with lone pair electrons in the structure of the isocyanate group-containing compound not only reduces the decomposition of lithium salts (such as LiPF6), but also can react with Lewis acids (such as PF5) obtained by its decomposition, thereby avoiding these substances from eventually generating HF and reacting with Li3N, thereby improving the stability of the SEI film as a whole.

[0012] In this case, even in the face of faster charging rates, such as above 4C, such as 6C or even 10C, it has excellent effects of removing decomposition products produced by lithium salts and enhancing SEI film. Therefore, by using a nitro group-containing compound and an isocyanate group-containing compound in combination in the electrolyte, the fast charging performance (charging rate of 6C or above) and low-temperature discharge performance of the battery can be significantly improved.

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

[0014] Solutions for solving problems

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

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

[0017] As the nitro group-containing compound of component (A),

[0018] As component (B) the isocyanate group-containing compound,

[0019] as a non-aqueous solvent of component (C), and

[0020] A lithium salt as component (D);

[0021] Wherein, the components (A) and (B) are dissolved in the component (C).

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

[0023]

[0024] Wherein, the R1 represents a monovalent organic group, and the monovalent organic group contains at least one of a carboxylate group and a carbonate group.

[0025] According to the non-aqueous electrolyte described above, the carboxylate group includes one or more of an aliphatic carboxylate group and an aromatic carboxylate group.

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

[0027]

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

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

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

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

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

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

[0034] Furthermore, the present invention provides a separator-free lithium secondary battery, which comprises a positive electrode, a negative electrode and the above-mentioned non-aqueous electrolyte.

[0035] Furthermore, the present invention also provides a lithium secondary battery, which comprises a positive electrode, a negative electrode, a separator and the above-mentioned non-aqueous electrolyte. According to the above-mentioned lithium secondary battery, the positive electrode active material in the positive electrode comprises nickel element.

[0036] Furthermore, the present invention also provides an electric vehicle or transportation tool, which uses the above-mentioned lithium secondary battery.

[0037] 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 nitro group-containing compound and an isocyanate group-containing compound used in combination in a non-aqueous electrolyte, wherein the strongly electron-withdrawing nitro group in the nitro group-containing compound can reduce the energy level of the highest occupied orbital (HOMO) and have higher oxidation stability, and it has been unexpectedly found that such a strong electron-withdrawing effect can effectively prevent the nitro group-containing compound from decomposing under transition metal catalysis even in the face of high current density at a high charging rate (2C, 4C or above, even 6C, or even 10C), thereby forming a lithium nitride-rich high lithium ion conductivity SEI film at the negative electrode in preference to non-aqueous solvents (such as carbonate solvents); the isocyanate group-containing compound is a Lewis base compound having a nitrogen atom with an unshared electron pair, which can easily remove the Lewis acid (such as PF5) generated by the decomposition of lithium salts inside the battery during charging and discharging, and form a strong film on the positive electrode surface, thereby inhibiting the side reaction between the positive electrode and the electrolyte, and the combined use of the two can effectively reduce the impedance of the SEI film formed on the electrode surface, thereby significantly improving the fast charging performance of the battery, and at the same time can also effectively improve the low-temperature discharge performance of the battery. DETAILED DESCRIPTION

[0040] The following is a detailed description of the present invention. The technical features described below are described 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 numerical range expressed using "a numerical value A to a numerical value B" means a range including the endpoints numerical values ​​A and B.

[0043] In this specification, the numerical range expressed using "above" or "below" means a numerical range including the number.

[0044] In this specification, the word "may" includes both performing a certain process and not performing a certain process.

[0045] In this specification, the term "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2°C".

[0046] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0047] In the present specification, the use of “substantially” or “essentially” means that the standard deviation from a theoretical model, theoretical data or target data is within a numerical range of 2%, preferably 1%, and more preferably 0.8%.

[0048] In this specification, when the terms “include” and / or “comprise” are used, they indicate the existence of features, steps, operations, devices, components and / or their combinations.

[0049] In this manual, the term "fast charging" refers to a charging rate of 2C or above, 4C or above, or 5C or above, for example, 6C to 18C.

[0050] In this specification, the references to "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", etc., mean that the specific elements (e.g., features, structures, properties and / or characteristics) described in connection with the embodiments are included in at least one embodiment described herein, and may or may not exist in other embodiments. In addition, it should be understood that the elements may be combined in various embodiments in any suitable manner.

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

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

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

[0054] The term "alkyl" refers to a linear or branched monovalent hydrocarbon group containing no unsaturation. 1-4 "Alkyl" refers to an alkyl group having 1 to 4 carbon atoms. 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 between two other structural fragments or used to connect two other structural fragments, 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 of Elements.

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

[0057] In this specification, the use of "optional", "optionally" or "optional" indicates that the event or situation described subsequently may or may not occur, and the description includes the occurrence of the event or situation and the non-occurrence of the event or situation. For example, an alkyl group is "optionally" substituted with a halogen, which means that the alkyl group may be unsubstituted, or may be monosubstituted, polysubstituted or fully substituted with a halogen atom. It will be understood by those skilled in the art that for any group containing one or more substituents, no substitution or substitution pattern that is sterically impossible to exist and / or cannot be synthesized will be introduced.

[0058] The main purpose of the present invention is to provide a non-aqueous electrolyte and a lithium secondary battery comprising the non-aqueous electrolyte (including lithium secondary batteries with and without diaphragms). The non-aqueous electrolyte comprises: a compound containing a nitro group, a compound containing an isocyanate group, a non-aqueous solvent and a lithium salt. The combined use of the nitro group-containing compound and the isocyanate group-containing compound not only has excellent effects of removing decomposition products generated by the lithium salt and enhancing the SEI film, but also can significantly improve the battery's fast charging performance, low-temperature discharge performance and cycle performance of the battery cell.

[0059] The present invention is mainly obtained through the following insights:

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

[0061] This phenomenon has a more obvious impact on battery performance during fast charging. In particular, the Lewis acid (such as PF5) produced by the thermal decomposition of lithium salts degrades the passivation film (such as the solid electrolyte interface (SEI) film), exacerbating the elution of transition metal ions. Such eluted transition metal ions are redeposited on the positive electrode to increase the resistance of the positive electrode, or are electro-deposited on the surface of the negative electrode to destroy the SEI film, which may cause an internal short circuit. The decomposition reaction of the electrolyte caused by this series of reactions is promoted to increase gas generation, the interface resistance of the negative electrode, and the self-discharge of the negative electrode, resulting in low voltage failure.

[0062] The present invention provides a non-aqueous electrolyte, which comprises: a nitro group-containing compound, an isocyanate group-containing compound, a non-aqueous solvent and a lithium salt. The inventors have found through research that when the nitro group-containing compound and the isocyanate group-containing compound are used in combination in the electrolyte, firstly, since the nitro group-containing compound has two double bond oxygens (N=O and C=O) in its structure, it can be in-situ coordinated with high-valent transition metals (such as cobalt, manganese, nickel, etc.), inhibiting the dissolution of transition metal cobalt, manganese, and nickel ions, thereby avoiding the electrolyte from being oxidized and decomposed to produce gas, which can improve the cycle performance of the battery during high-voltage charging and discharging, extend the battery life, and improve the safety performance of the battery; furthermore, since the nitro group-containing compound has a strong electron-withdrawing nitro group in its structure, the nitro group-containing compound can be used to dissolve the nitro group. The base makes the electrolyte have a lower oxidation potential, and can form a dense and stable passivation film on the positive electrode surface during the first charging process, which can effectively inhibit the oxidative decomposition of the organic solvent (such as carbonate) of the electrolyte on the positive electrode surface, and effectively solve the problem of rapid capacity decay of the battery during high-voltage charging and discharging; further, the combined effect of the above-mentioned electron-withdrawing effect and metal ion coordination effect can also effectively avoid the oxidative decomposition of the compound containing the nitro group itself under the catalysis of the transition metal, so that a high lithium ion conductivity SEI film rich in lithium nitride can be formed at the negative electrode, significantly improving the fast charging performance of the battery. In addition, the compound containing the isocyanate group is a Lewis base compound with a nitrogen atom with an unshared electron pair, which can easily remove the Lewis acid (such as PF5) generated as an electrolyte decomposition product inside the battery during charging and discharging, and form a solid film on the positive electrode surface, thereby inhibiting the side reaction between the positive electrode and the electrolyte, so that the performance of the battery can be significantly improved.

[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 in the art (including lithium secondary batteries with and without separators). In particular, the electrolyte of the present invention can provide a lithium secondary battery with good stability, cyclability, and low-temperature discharge characteristics under higher charging rate conditions.

[0066] In addition, without limitation, various optional functional additive components may be used in the non-aqueous electrolyte as long as they do not hinder the realization of the technical effects of the present invention.

[0067] Component (A)

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

[0069]

[0070] The R1 represents a monovalent organic group. More specifically, from the perspective of improving ion conduction efficiency, the monovalent organic group contains at least one of a carboxylate group and a carbonate group.

[0071] For the carboxylate group, in some specific embodiments, the carboxylate group in the monovalent organic group can be derived from at least one of a linear or branched aliphatic carboxylate group, an alicyclic carboxylate group or an aromatic carboxylate group, or an ester group formed by a mixture of these.

[0072] For such an ester group, in some preferred embodiments, the R1 may be -X1-COO-X2 or -X1-OOC-X2. Wherein, X1 and X2 may be the same or different. Under this condition, X1 or X2 may be derived from a substituted or unsubstituted straight 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 above-mentioned alkyl or cycloalkyl group of the present invention, it is preferred that C 1-10 Alkyl or C 3-10 The aromatic ring may be an aryl group or a heteroaryl group as defined above, and preferably a phenyl group.

[0073] There is no particular limitation on the substituents that can be used in the above substitutions, provided that the technical effects of the present invention are not affected. The substituents can usually be halogens, especially F atoms.

[0074] In further specific embodiments, the R1 group may be a group derived from alkyl alkyl ester, aryl alkyl ester, alkyl aryl ester, aryl aryl ester, etc. These alkyl and aryl groups may also have the above-mentioned substituents.

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

[0076]

[0077] The present invention introduces the nitro group-containing compound into the electrolyte to form a lithium nitride-rich SEI film with high lithium ion conductivity at the negative electrode, thereby effectively improving the fast charging performance of the battery.

[0078] In the case of carbonate groups, there is no particular limitation in principle. In principle, R1 may include a group derived from a linear carbonate or a cyclic carbonate. From the perspective of stability, preferably, R1 may include a group derived from a cyclic carbonate.

[0079] In addition, the inventors also found that compared with compounds such as carbonates that only contain carbonyl groups, the structure of the compound of the present invention contains not only carbonyl groups but also nitro groups. The strongly electron-withdrawing nitro group can reduce the energy level of the highest occupied orbital (HOMO), and thus has higher oxidation stability. It is not easy to oxidize and decompose even under the catalysis of the positive electrode transition metal, but it is easy to react with lithium at the negative electrode to undergo a reduction reaction to form 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 the present invention may be selected from one or more of chain aliphatic isocyanate, alicyclic isocyanate and aromatic isocyanate compounds.

[0082] Furthermore, such a compound containing an isocyanate group may have one or more isocyanate groups in its molecular structure, and preferably, has 1 to 3 isocyanate groups in its molecular structure.

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

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

[0085] (Ⅱ)

[0086] Wherein, 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 and alkylene.

[0088] For the above-mentioned alkyl / alkylene group of the present invention, preferably C 1-10 The alkyl / alkylene group may include methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl or divalent groups thereof.

[0089] There is no particular limitation on the substituents that can be used in the above substitutions, provided that the technical effects of the present invention are not affected. Preferably, the substituents can be F atoms.

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

[0091]

[0092] The present invention introduces the isocyanate group-containing compound into the electrolyte. The isocyanate group-containing compound has a lone pair of electrons in its structure, which not only reduces the decomposition of lithium salts (such as LiPF6) but also reacts with Lewis acids (such as PF5) obtained by its decomposition, thereby preventing these substances from eventually generating HF and reacting with Li3N, thereby improving the stability of the SEI film as a whole.

[0093] Component (C)

[0094] The present invention does not particularly limit the type of the non-aqueous solvent of 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, carboxylate solvents and ether solvents.

[0096] In some specific embodiments, the cyclic carbonate solvent can be selected from ethylene carbonate, propylene carbonate, butylene carbonate, ethylene carbonate (EC) and fluoroethylene carbonate (FEC), etc.; the linear carbonate solvent can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC) and di(2,2,2-trifluoroethyl) carbonate (DFDEC), etc.; the carboxylate solvent can be selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate and methyl butyrate, etc.; the ether solvent can 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 dioxolane (DOL), etc. These non-aqueous solvents can be used alone or in the form of a mixture of two or more, preferably a combination of a cyclic carbonate solvent and a linear carbonate solvent. The cyclic carbonate solvent has a higher dielectric constant and can form solvated lithium ion molecules with lithium ions; the linear carbonate solvent has a lower viscosity, which is conducive to the conduction of lithium ions.

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

[0098] Component (D)

[0099] The present invention does not specifically limit the type of the lithium salt of 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 fluorine-containing 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 a combination of one or more of lithium hexafluorophosphate (LiPF6), lithium bistrifluoromethylsulfonyl imide (LiTFSI), lithium bisfluorosulfonyl imide (LiFSI) and lithium bisfluorooxalatoborate (LiDFOB).

[0101] Other functional added ingredients

[0102] In principle, there is no particular limitation 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] For such additives, vinyl ethylene carbonate (VC), trimethylsilyl phosphate (TMSP), sulfur-containing additives, etc. can be cited. Among them, the sulfur-containing additive can be selected from 1,3-propane sultone (1,3-PS), 1,4-butane sultone (1,4-BS), 2,4-butane sultone (2,4-BS), 1,3-propylene sultone (PST), vinyl sulfate (DTD), methylene disulfonate (MMDS), vinyl sulfite (ES), etc. These additives can be used alone or in the form of a mixture of two or more.

[0104] Composition of non-aqueous electrolyte

[0105] In the present invention, there is no particular restriction on the content of the nitro group-containing compound of the component (A), and the dosage range in various scenarios that have been used in the art can be referred to. In some specific embodiments of the present invention, from the perspective of improving the stability of the nitro group-containing compound in the electrolyte, the content of the nitro group-containing compound of the 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%, 1%, 1.5%, 2%, 2.5%, 3%, 5%, 8%, 9%, etc. When the content of component (A) is too high, on the one hand, the cost of the electrolyte will increase, and on the other hand, the SEI film will be significantly thickened, 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 to affect the performance of the battery cell.

[0106] In the present invention, the content of the compound containing an isocyanate group of the component (B) is mainly determined by coordinating with the above-mentioned component (A). Therefore, in principle, the amount of component (B) is not particularly limited. In some specific embodiments of the present invention, from the perspective of controlling the thickness and internal resistance of the SEI film and improving the fast charging performance, the content of the compound containing an isocyanate group of the 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%, 1%, 1.5%, 2%, 2.5%, 3%, 5%, 8%, 9%, etc. Relative to component (A), when the content of component (B) is too high, there is a concern that the SEI film will be significantly thickened, which is not conducive to the performance of the fast charging performance of the battery; when the content of component (B) is too low, there is a concern that the electrolyte cannot be guaranteed to have sufficient ionic conductivity to affect the performance of the battery cell.

[0107] In principle, there is no particular restriction on the content of the component (D) lithium salt, as long as the purpose of the present invention can be achieved. In some specific embodiments of the present invention, the concentration of the lithium salt in the non-aqueous electrolyte is 0.2 to 10 mol / L, preferably 0.7 to 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 the lithium salt is too high, the viscosity and cost of the electrolyte will be too high; when the concentration of the lithium salt is too low, it cannot be guaranteed that the electrolyte has sufficient ionic conductivity, thereby affecting the performance of the battery cell.

[0108] The content of the functional additive is not particularly limited in the present invention and 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 the nonaqueous electrolyte according to the first aspect.

[0111] In principle, there is no particular limitation on the positive electrode of the battery, and it can be a positive electrode commonly used in the art, and such a positive electrode includes a current collector and a positive electrode active material. In principle, there is no particular limitation on the positive electrode active material, and in some preferred embodiments, various lithium-containing oxides in the art can be used, and in addition to lithium, other main group, subgroup or rare earth metal elements can be added to these oxides.

[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, 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] In principle, there is no particular limitation on the negative electrode of the battery, and it can be any negative electrode commonly used in the art. Such a negative electrode includes 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] In principle, there is no special restriction on the separator of the battery, and the separator may be used or not. In some specific embodiments, the lithium secondary battery further uses a separator, and when the separator is used, it can be a separator commonly used in the art, preferably, a separator with high moisture retention capacity for electrolyte solution and low resistance to electrolyte ion transfer can be used; in some other specific embodiments, the lithium secondary battery is a separator-free lithium secondary battery, and preferably, the lithium secondary battery can be an all-solid-state battery, that is, it is allowed not to contain a separator inside, and the solid electrolyte plays the role of isolating the positive and negative electrodes and transmitting lithium ions.

[0116] <Third Aspect>

[0117] The third aspect of the present invention provides an electric vehicle or transportation tool, which uses the lithium secondary battery according to the <second aspect>, for such vehicles or transportation tools, such as a family 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 in conjunction with the examples, but those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. If no specific conditions are specified in the examples, they are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0120] Embodiment 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) are mixed in a volume ratio of EC:EMC=1:3 to obtain a mixed solvent, and then 1 mol / L lithium hexafluorophosphate (LiPF6) is added to the mixed solvent, 1.0% of a nitro group-containing compound (Compound 1) based on the total mass of the electrolyte is added, and 1.0% of an isocyanate group-containing compound (Compound 3) based on the total mass of the electrolyte is added. After stirring evenly, the non-aqueous electrolyte of Example 1 is obtained.

[0122] Embodiment 2~4:

[0123] Examples 2 to 4 are also specific examples of preparing the electrolyte. Except that the components of the electrolyte are added in the proportions shown in Table 1, other parameters and preparation methods are the same as those of Example 1.

[0124] Comparative Examples 1 to 3:

[0125] In Comparative Examples 1 to 3, except that the components of the electrolyte were added in the proportions shown in Table 1, other parameters and preparation methods were the same as those in Example 1.

[0126] Table 1. Composition ratio of each component of the electrolyte of Examples 1 to 4 and Comparative Examples 1 to 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] Positive electrode preparation: The positive electrode active material Li(Ni 0.8 Co 0.1 Mn 0.1 )O2, conductive agent ultrafine carbon powder (SP), single-walled carbon nanotubes (SWNT) and binder polyvinylidene fluoride (PVDF) are added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent in a mass ratio of 96:2:1:1 and stirred thoroughly. The solid content is controlled to 68% to prepare a positive electrode mixture slurry. Then, the formed positive electrode slurry is coated on aluminum foil, and the positive electrode sheet is formed by rolling and die-cutting after drying.

[0130] Negative electrode preparation: Graphite, conductive agent ultrafine carbon powder (SP), thickener sodium carboxymethyl cellulose (CMC) and binder styrene butadiene rubber (SBR) were added to an appropriate amount of deionized water in a mass ratio of 95:2:1:2 and stirred thoroughly. The solid content was controlled to 55% to prepare a negative electrode mixture slurry. Then, the formed negative electrode slurry was coated on copper foil, and the negative electrode sheet was formed by rolling and die-cutting after drying.

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

[0132] Performance Testing

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

[0134] (1) Rate Cycle: At 25°C, the packaged lithium-ion battery was subjected to a 6C constant rate cycle test after the formation and capacity separation steps. The battery was charged to 4.25V at 6C constant current and constant voltage. After 0.5h of standing, the battery was discharged at 6C constant current. The initial capacity of the first cycle of the battery was obtained and recorded as C1. After that, the charge and discharge cycle was repeated in the CCCV / CC manner of the first cycle. The capacity after the 200th cycle of discharge was recorded as C200 , 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°C, the packaged lithium-ion battery is charged to 4.25V at 1C constant current and constant voltage after the formation and capacity division steps. After 0.5h, it is discharged at 1C constant current to obtain the initial capacity of the battery and recorded as C0. Then the battery cell is placed in a low temperature box at -20°C. After 4h, it is discharged at a rate of 0.33C. The capacity obtained by discharge is recorded as C1. The capacity retention rate (%) of low temperature discharge at -20°C = C1 / C0×100%.

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

[0137] Table 2. Battery performance test results of various embodiments and comparative examples

[0138] Group 6C 25℃ constant rate 200-cycle charge and discharge capacity retention rate (%) Cell DCR(mΩ) 0.33C-20℃ discharge capacity retention rate (%) 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 can be seen from Table 2, compared with Comparative Example 1, Examples 1 to 4 use the nitro group-containing compound and the isocyanate group-containing compound of the present invention in the electrolyte, so the lithium secondary battery not only has good high-rate cycle performance, but also has good low-temperature performance at -20°C. In addition, in Comparative Examples 2 and 3, since the content of the nitro group-containing compound or the isocyanate group-containing compound is too high, this may lead to a relative decrease in the lithium salt concentration, and therefore, the internal resistance and cyclability are affected. Therefore, Comparative Examples 2 and 3 respectively provide some references for the upper limits of the content of the nitro group-containing compound and the isocyanate group-containing compound of the present invention.

[0140] In addition, the addition of the nitro group-containing compound and the isocyanate group-containing compound of the present invention also 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 is introduced with specific examples, those skilled in the art will appreciate that the present invention should not be limited thereto.

[0142] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A non-aqueous electrolyte, characterized in that: The non-aqueous electrolyte comprises: As the nitro group-containing compound of component (A), As component (B) the isocyanate group-containing compound, as a non-aqueous solvent of component (C), and A lithium salt as component (D); Wherein, the components (A) and (B) are dissolved in the component (C).

2. The non-aqueous electrolyte according to claim 1, characterized in that The nitro group-containing compound of component (A) has a structure shown in the following general formula I: Wherein, the R1 represents a monovalent organic group, and the monovalent organic group contains at least one of a carboxylate group and a carbonate group.

3. The non-aqueous electrolyte according to claim 2, characterized in that The carboxylate group includes one or more of an aliphatic carboxylate group and an aromatic carboxylate group.

4. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that The nitro group-containing compound of component (A) is selected from one or more combinations of the following compounds:

5. The non-aqueous electrolyte according to any one of claims 1 to 4, characterized in that The isocyanate group-containing compound as component (B) has one or two or more isocyanate groups.

6. The non-aqueous electrolyte according to any one of claims 1 to 5, characterized in that The isocyanate group-containing compound as component (B) is selected from one or more of chain aliphatic isocyanate, alicyclic isocyanate and aromatic isocyanate.

7. The non-aqueous electrolyte according to any one of claims 1 to 6, characterized in that 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 as component (B) is 0.1% to 10% by mass of the total mass of the non-aqueous electrolyte.

8. The non-aqueous electrolyte according to any one of claims 1 to 7, characterized in that The non-aqueous solvent of component (C) is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, carboxylate solvents and ether solvents.

9. The non-aqueous electrolyte according to any one of claims 1 to 8, characterized in that The non-aqueous solvent of component (C) comprises a combination of a cyclic carbonate solvent and a linear carbonate solvent.

10. The non-aqueous electrolyte according to any one of claims 1 to 9, characterized in that: The lithium salt includes one or more fluorine-containing lithium salts.

11. A diaphragm-free lithium secondary battery, characterized in that: The separator-free lithium secondary battery comprises a positive electrode, a negative electrode and the nonaqueous electrolyte according to any one of claims 1 to 10.

12. A lithium secondary battery, characterized in that: The lithium secondary battery comprises a positive electrode, a negative electrode, a separator and the nonaqueous electrolyte according to any one of claims 1 to 10.

13. The lithium secondary battery according to claim 11 or 12, characterized in that: The positive electrode active material in the positive electrode includes nickel element.

14. An electric vehicle or transportation tool using the lithium secondary battery according to claim 11 or 12.

Images