Non-aqueous electrolyte, diaphragm-free lithium ion battery and lithium ion battery

Abstract

The invention provides a non-aqueous electrolyte, a diaphragm-free lithium ion battery and a lithium ion battery. The non-aqueous electrolyte solution of the present invention contains a nitro group-containing compound as a component (A), a non-aqueous solvent as a component (B), and a lithium salt as a component (C), wherein the structure of the component (A) compound containing the nitro group further contains at least one carbonyl group. The non-aqueous electrolyte provided by the invention has relatively good oxidation stability, and can effectively reduce the impedance of a negative electrode SEI film and the growth rate in the cycle process in a relatively wide temperature range, so that the fast charge performance of the battery is remarkably improved, and the high-temperature cycle performance and the low-temperature discharge performance at-20 DEG C of the lithium ion battery can also be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and relates to a non-aqueous electrolyte, a diaphragm-free lithium ion battery and a lithium ion battery, and specifically to a fast-charging non-aqueous electrolyte for a lithium battery and a diaphragm-free lithium ion battery and a lithium ion battery containing the electrolyte. 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 the 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 materials and interfaces of lithium-ion batteries, and an electrolyte that can form a low-impedance SEI film and has high ionic conductivity is urgently needed.

[0003] From the perspective of facilitating the formation of SEI film layers, some studies in the prior art disclose that organic azide compounds can be used in non-aqueous electrolytes. It has been found that the organic azide compounds have excellent negative electrode film-forming properties, and the use of the organic azide compounds as electrolyte additives can significantly improve the fast charge and rate 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 in 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 limited improvement in fast charge performance. In addition, it may also reduce electrochemical stability and affect the service life of lithium-ion batteries.

[0004] 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.

[0005] 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

[0006] Problem that the invention aims to solve

[0007] 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 charging and discharging 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.

[0008] As mentioned above, in the prior art, organic azide compounds are used as electrolyte additives to form lithium azide (LiN) on the surface of the electrode through electrochemical reaction. 3 ) and eventually form lithium nitride (Li 3 N), 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. However, the azide group is a highly reactive group, and the transition metal in the positive electrode 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 influence on the polarization of the azide group, thereby causing the azide group to decompose, and ultimately making it impossible for the azide group to react with lithium to form lithium nitride Li 3 N layer, this phenomenon is particularly obvious under fast charging conditions. Higher charging rates lead to greater charging current density, making the above-mentioned oxidation and decomposition phenomena more likely to occur. Therefore, the use of organic azide compounds as electrolyte additives has very limited improvement on the fast charging performance of the battery cell, especially considering the charging rate is above 4C, such as 6C or even 10C.

[0009] In order to solve the above problems, the present invention provides a non-aqueous electrolyte, wherein the non-aqueous electrolyte includes a compound containing a nitro group, wherein a nitro group (NO 2), two double-bonded oxygens (N=O and C=O), which make it have the dual effects of strong electron-withdrawing and metal ion coordination, which helps to improve the oxidation stability of the electrolyte and the dissociation of lithium salts, and effectively avoid the oxidative decomposition of compounds containing nitro groups 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. In this case, even in the face of faster charging rates, such as above 4C, such as 6C or even 10C charging rates, it is sufficient to suppress the decomposition of the nitro-containing compound caused by high current density, and therefore, the fast charging performance and high and low temperature characteristics of the battery can be significantly improved.

[0010] Furthermore, the present invention also provides a lithium ion battery, including a lithium ion battery with a diaphragm and a lithium ion battery without a diaphragm, wherein the lithium ion battery contains the above-mentioned non-aqueous electrolyte.

[0011] Solutions for solving problems

[0012] After in-depth research, it has been found that the above technical problems can be solved by implementing the following technical solutions:

[0013] The present invention first provides a non-aqueous electrolyte, which comprises: a nitro group-containing compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C);

[0014] The compound containing a nitro group in component (A) further contains at least one carbonyl group in its structure.

[0015] According to the non-aqueous electrolyte described above, the content of the nitro group-containing compound as component (A) is 0.1 mass % to 15 mass % of the total mass of the non-aqueous electrolyte.

[0016] 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:

[0017]

[0018] Wherein, the R represents a monovalent organic group, and the monovalent organic group contains at least one of an ester group and a carbonate group.

[0019] According to the non-aqueous electrolyte described above, the ester group includes one or more of an aliphatic ester group and an aromatic ester group.

[0020] According to the non-aqueous electrolyte described above, the nitro group-containing compound is selected from one or more combinations of the following compounds:

[0021]

[0022] According to the non-aqueous electrolyte described above, the component (B) non-aqueous solvent is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, carboxylate solvents and ether solvents.

[0023] According to the non-aqueous electrolyte described above, the non-aqueous solvent is selected from at least one 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).

[0024] According to the non-aqueous electrolyte described above, the component (C) lithium salt is selected from one or more salts formed by lithium ions and the following anions: PF 6 - , BF 4 - , Cl - Br - ,I - , ClO 4 - , AsF 6 - , CH 3 CO 2 - CF 3 SO 3 - 、N(CF 3 SO 2 ) 2 - 、N(FSO 2 ) 2 - , C(CF 2 SO 2 ) 3 - , C 2 BF 2 O 4 - .

[0025] According to the non-aqueous electrolyte described above, the lithium salt is selected from lithium hexafluorophosphate (LiPF 6 ), one or more of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(fluorooxalatoborate) (LiDFOB).

[0026] Furthermore, the present invention provides a diaphragm-free lithium-ion battery, which comprises a positive electrode, a negative electrode and the above-mentioned non-aqueous electrolyte.

[0027] Furthermore, the present invention also provides a lithium-ion battery, which comprises a positive electrode, a negative electrode, a separator and the above-mentioned non-aqueous electrolyte.

[0028] Effects of the Invention

[0029] 1) The nitro group-containing compound used in the non-aqueous electrolyte provided by the present invention has high oxidative stability because the strong electron-withdrawing nitro group can reduce the energy level of the highest occupied orbital (HOMO), and it has been unexpectedly found that such 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 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), and can effectively reduce the impedance of the negative electrode SEI film and the growth rate during the cycle process in a wide temperature range, thereby significantly improving the fast charging performance of the battery, and at the same time effectively improving the low-temperature discharge performance of the battery.

[0030] 2) The nitro group-containing compound in the non-aqueous electrolyte provided by the present invention has two double-bonded oxygens (N=O, C=O) that can be in-situ coordinated with the positive electrode transition metal, significantly reducing the side reactions between the electrolyte and the positive electrode at high temperatures, thereby significantly improving the high-temperature cycle performance of the battery.

[0031] 3) The non-aqueous electrolyte provided by the present invention can be used in lithium-ion batteries with diaphragms or in lithium-ion batteries without diaphragms. DETAILED DESCRIPTION

[0032] 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.

[0033] It should be noted that:

[0034] 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.

[0035] In the present specification, a numerical range expressed using "above" or "below" means a numerical range including the number.

[0036] In this specification, the word "may" means both performing a certain process and not performing a certain process.

[0037] In this specification, the "normal temperature" or "room temperature" used means an indoor ambient temperature of "23±2°C".

[0038] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0039] 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%.

[0040] In this specification, when the terms “include” and / or “comprises” are used, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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)

[0045] When it is between two other structural fragments or used to connect two other structural fragments, it can also be a divalent group.

[0046] 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 (—CH 3 ), ethyl (-CH 2 CH 3 ), n-propyl (-CH 2 CH 2 CH 3 ), isopropyl (-CH(CH 3 ) 2 ), n-butyl (-CH 2 CH 2 CH 2 CH 3 ), sec-butyl (-CH(CH 3 )CH 2 CH 3 ), isobutyl (-CH 2 CH(CH 3 ) 2 ), tert-butyl (-C(CH 3 ) 3 ), n-pentyl (-CH 2 CH 2 CH 2 CH 2 CH 3 ), neopentyl (-CH 2 C(CH 3 ) 3 ) etc. When it is between two other structural fragments or used to connect two other structural fragments, it can also be a divalent group.

[0047] 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.

[0048] The term "cyano" refers to a "-CN" group.

[0049] The term "carbonyl" refers to a "-C(=O)-" group.

[0050] 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.

[0051] The present invention provides a non-aqueous electrolyte, which comprises: a nitro group-containing compound, a non-aqueous solvent and a lithium salt, wherein the nitro group-containing compound has a nitro group (NO 2 ), two double-bonded oxygens (N=O and C=O), the use of compounds containing nitro groups can significantly improve the fast charging performance and low-temperature discharge performance of the battery.

[0052] The present invention is mainly obtained through the following insights:

[0053] The present invention provides a non-aqueous electrolyte, which comprises: a compound containing a nitro group, a non-aqueous solvent and a lithium salt, wherein the compound containing a nitro group has a nitro group (NO 2 ) and two double-bonded oxygens (N=O and C=O), which give it the dual effects of strong electron-withdrawing and metal ion coordination, helping to improve the oxidation stability of the electrolyte and the dissociation of lithium salts, thereby significantly improving the fast charging performance and high-temperature stability of the battery.

[0054] The inventors have found that when the nitro group-containing compound is used in an electrolyte, firstly, since the nitro group-containing compound has two double-bonded oxygens (N=O and C=O) in its structure, it can coordinate with high-valent transition metals (such as cobalt, manganese, nickel, etc.) in situ, inhibiting the dissolution of transition metal cobalt, manganese, and nickel ions, thereby preventing 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 electrolyte has a lower oxidation potential, and a dense and stable passivation film can be formed on the positive electrode surface during the first charging process, which can effectively inhibit the oxidation and decomposition of the organic solvent (such as carbonates) of the electrolyte on the positive electrode surface, effectively solving the problem of rapid capacity decay of the battery during high-voltage charging and discharging. Furthermore, the combined effect of the above-mentioned electron-withdrawing effect and metal ion coordination effect can also effectively prevent the oxidative decomposition of compounds containing nitro groups under transition metal catalysis, thereby forming a highly lithium-ion conductive SEI film rich in lithium nitride at the negative electrode, significantly improving the fast charging performance of the battery. In addition, the introduction of nitro groups increases the boiling point of the electrolyte, which significantly improves the high-temperature performance of the battery.

[0055] <First aspect>

[0056] A first aspect of the present invention provides a non-aqueous electrolyte solution comprising a nitro group-containing compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C).

[0057] The non-aqueous electrolyte of the present invention can be applied to conventional lithium-ion batteries in the art (including lithium-ion batteries with and without separators). In particular, the electrolyte of the present invention can provide lithium-ion batteries with good stability and cyclability under higher charging rate conditions.

[0058] 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.

[0059] Component (A)

[0060] The general structural formula of the nitro group-containing compound of component (A) of the present invention is shown in Formula I:

[0061]

[0062] Wherein, the R represents a monovalent organic group, and R has at least one carbonyl group. More specifically, the R has at least one or more ester groups, carbonate groups or a combination thereof.

[0063] The present invention does not particularly limit the monovalent organic group as long as it has the above structure. In some specific embodiments, the ester group in the monovalent organic group can be derived from at least one of an aliphatic ester group and an aromatic ester group.

[0064] For aliphatic ester groups, in some preferred embodiments, the R may be -R1-COO-R2 or -R1-OOC-R2. Wherein, R1 and R2 may be the same or different, under which condition, R1 or R2 may be a substituted or unsubstituted alkyl group. For the above-mentioned alkyl group of the present invention, preferably, it may be methyl, ethyl, n-propyl, isopropyl, n-butyl, etc.

[0065] For aromatic ester groups, in some preferred embodiments, the R may be -R3-COO-R4, or -R3-OOC-R4. Wherein, at least one of R3 or R4 is an aryl group, under which condition, R3 or R4 may also be a substituted or unsubstituted alkyl group. For the above-mentioned aryl group of the present invention, it may be a substituted or unsubstituted carboaryl or heteroaryl group without affecting the technical effect of the present invention, and preferably, the aryl group may be a phenyl group. For the above-mentioned alkyl group of the present invention, it may be preferably a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, etc.

[0066] 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, and they can generally be halogen, cyano, etc.

[0067] In further specific embodiments, the R 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.

[0068] In some preferred embodiments, the nitro group-containing compound includes (but is not limited to) the following compounds:

[0069]

[0070] 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.

[0071] In addition, the inventors have also found that compared with compounds such as carbonates that only contain carbonyl groups, the nitro group-containing compound structure of the present invention contains not only carbonyl groups but also nitro groups. The strong electron-withdrawing nitro group can reduce the energy level of the highest occupied orbital (HOMO), and thus has higher oxidation stability. Even under the catalysis of the positive electrode transition metal, it is not easy to oxidize and decompose, but it is easy to react with lithium at the negative electrode to generate lithium nitride Li 3N layer, this inorganic SEI film component with high ionic conductivity can significantly improve the fast charging performance of the battery.

[0072] Component (B)

[0073] The present invention does not particularly limit the type of the non-aqueous solvent of component (B), as long as it is a non-aqueous solvent commonly used as a non-aqueous electrolyte.

[0074] 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.

[0075] 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 nonaqueous solvents may be used alone or in admixture of two or more.

[0076] In some preferred embodiments, the non-aqueous solvent can be selected from at least one 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).

[0077] Component (C)

[0078] The present invention does not specifically limit the type of the lithium salt of component (C), and it can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt is selected from one or more salts formed by lithium ions and the following anions: PF 6 - , BF 4 - , Cl - Br- ,I - , ClO 4 - , AsF 6 - , CH 3 CO 2 - CF 3 SO 3 - 、N(CF 3 SO 2 ) 2 - 、N(FSO 2 ) 2 - , C(CF 2 SO 2 ) 3 - , C 2 BF 2 O 4 - .

[0079] In some preferred embodiments, the lithium salt may be selected from lithium hexafluorophosphate (LiPF 6 ), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(fluorooxalatoborate) (LiDFOB).

[0080] Other functional added ingredients

[0081] 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.

[0082] 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.

[0083] Composition of non-aqueous electrolyte

[0084] In the present invention, there is no particular limitation on the content of the nitro group-containing compound of the component (A) in principle, and reference may be made to the dosage ranges used in various scenarios in the art.

[0085] 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 15% by mass of the total mass of the non-aqueous electrolyte, preferably 0.2% to 10% by mass. For example, it can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 5%, 8%, 10%, 15%, etc. When the content of component (A) is too high, there is also a concern that the viscosity and cost of the electrolyte will be significantly increased; when the content of component (A) is too low, there is a concern that a low-impedance SEI film cannot be fully formed at the negative electrode.

[0086] In principle, there is no particular restriction on the content of the component (C) 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 0.5 to 5 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.

[0087] The content of the functional additive is not particularly limited in the present invention and can be selected as needed.

[0088] <Second Aspect>

[0089] A second aspect of the present invention provides a lithium ion battery comprising a positive electrode, a negative electrode, and the nonaqueous electrolyte according to the first aspect.

[0090] In principle, there is no particular restriction on the positive electrode sheet of the battery, and it can be a positive electrode sheet commonly used in the art, and such a positive electrode sheet includes a current collector and a positive electrode active material. In principle, there is no particular restriction 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, sub-group or rare earth metal elements can be added to these oxides.

[0091] 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.

[0092] In some preferred embodiments, the positive electrode active material can be selected from at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate and lithium-rich manganese-based materials.

[0093] In principle, there is no particular limitation on the negative electrode sheet of the battery. It can be any negative electrode sheet commonly used in the art. Such a negative electrode sheet 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.

[0094] 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-ion 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-ion battery is a separator-free lithium-ion battery, and the separator-free lithium-ion battery at least includes a positive electrode plate, a negative electrode plate, a solid electrolyte and a non-aqueous electrolyte, wherein the solid electrolyte is between the positive electrode plate and the negative electrode plate, and acts as a substitute for the separator.

[0095] Example

[0096] 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.

[0097] Embodiment 1:

[0098] 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 = 1:2.5 to obtain a mixed solvent, and then 1 mol / L lithium hexafluorophosphate (LiPF 6 ), and finally, 1.0% of a nitro group-containing compound (Compound 1) based on the total mass of the electrolyte was added, and the mixture was stirred evenly to obtain the non-aqueous electrolyte of Example 1.

[0099] Embodiments 2 to 8:

[0100] Examples 2 to 8 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.

[0101] Comparative Example 1 and Reference Example 1:

[0102] In Comparative Example 1 and Reference Example 1, 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.

[0103] Table 1. Composition ratio of each component of the electrolyte of Examples 1 to 8, Comparative Example 1 and Reference Example 1

[0104] Group Non-aqueous solvents Lithium salts Compounds containing nitro groups Example 1 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 1 (1%) Example 2 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 2 (1%) Example 3 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 3 (3%) Example 4 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 4 (1%) Example 5 EC / EMC(1:2.5) LiTFSI(1mol / L) Compound 1 (3%) Example 6 FEC / FEMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 3 (3%) Example 7 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 1 (8%) Example 8 DX / DEE(1:2.5) <![CDATA[LiPF 6 (0.9mol / L)+LiTFSI(0.1mol / L)]]> Compound 1 (1%) Comparative Example 1 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> — Reference Example 1 EC / EMC(1:2.5) <![CDATA[LiPF 6 (1mol / L)]]> Compound 1 (30%)

[0105] Preparation of lithium-ion secondary batteries

[0106] Positive electrode preparation: The positive electrode active material Li(Ni 0.8 Co 0.1 Mn 0.1 ) 2 , 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 fully stirred, and 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.

[0107] Negative electrode preparation: Graphite, conductive agent ultrafine carbon powder (SP), thickener sodium carboxymethyl cellulose (CMC) and adhesive 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.

[0108] Battery assembly: Take the positive electrode sheet, the negative electrode sheet and the separator, and stack them in order of the negative electrode, the separator and the positive electrode, then weld the tabs and use aluminum-plastic film to package them to obtain a soft-package dry battery cell, and finally inject the non-aqueous electrolyte prepared in Examples 1 to 8, Comparative Example 1 and Reference Example 1 into the battery cell to prepare a lithium-ion battery with a capacity of 5Ah.

[0109] Performance Testing

[0110] The performance test was performed as follows, and the test results are shown in Table 2.

[0111] (1) High temperature cycle: At 45°C, the packaged lithium-ion battery was subjected to the formation and capacity separation steps and then cycled. The battery was charged to 4.25 V at a constant current of 1 C, then charged at a constant voltage until the current reached 0.05 C. After 0.5 h of standing, the battery was discharged at a constant current of 1 C. The initial capacity of the battery in the first cycle was obtained and recorded as C. 1 After that, the charge and discharge cycle is repeated in the CCCV / CC manner of the first week. The capacity after the 200th week of discharge is recorded as C 200 , then the capacity retention rate after 200 cycles of high temperature cycling (%) = C 200 / C 1 ×100%.

[0112] (2) Cycling rate: At 25°C, the packaged lithium-ion battery was subjected to a 6C constant rate cycling 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 C. 1 After that, the charge and discharge cycle is repeated in the CCCV / CC manner of the first week. The capacity after the 200th week of discharge is recorded as C 200 , then the capacity retention rate (%) after 200 cycles at high rate and room temperature = C 200 / C 1 ×100%.

[0113] (3) 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 separation steps. After 0.5h, it is discharged at 1C constant current to obtain the initial capacity of the battery and recorded as C. 0 Then put the battery into a -20℃ low temperature box and discharge it at a rate of 0.33C after 4 hours. The capacity obtained by discharge is recorded as C 1 , then the capacity retention rate of low temperature discharge at -20℃ (%) = C 1 / C 0 ×100%.

[0114] (4) DCR test: At 25°C, charge the cell before or after high temperature cycling at a constant current of 0.5C to 4.25V, charge at a constant voltage to a current of 0.05C, leave it for 1 hour, then discharge it at 0.5C for 1 hour (50% SOC), leave it for another 1 hour; then discharge it at 0.1C for 10 seconds, and record the voltage V at the end. 1 , then discharge at 1C for 1 second, and record the voltage V at the end 2 , then the DCR of the battery before or after the cycle = (V 1 -V 2 ) / (I 1C -I 0.1C), the unit is mΩ. The growth rate is calculated based on the DCR values ​​before and after the cycle, DCR growth rate = (battery DCR after the cycle - battery DCR before the cycle) / battery DCR before the cycle × 100%.

[0115] 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. In addition, the heat generated by the battery per unit time is also directly related to the DCR of the battery. The greater the DC internal resistance, the greater the heat generated, and the influence of temperature on fast charging performance is very obvious. Therefore, the value of DCR and its growth with cycles are extremely important for the evaluation of the fast charging performance of the battery cell.

[0116] Table 2. Battery performance test results of various embodiments, comparative examples and reference examples

[0117]

[0118] As shown in Table 2, compared with Comparative Example 1, Examples 1 to 8 use the nitro group-containing compound of the present invention in the electrolyte, so the lithium ion battery not only has good high temperature cycle performance and high rate cycle performance, but also has good low temperature performance at -20°C. This may be because the nitro group-containing compound can be adsorbed on the positive electrode surface to form an adsorption layer resistant to high voltage and high temperature, thereby improving the high voltage and high temperature characteristics of the battery; at the same time, the nitro group-containing compound can form a high lithium ion conductivity SEI film rich in lithium nitride at the negative electrode, improve the lithium ion transmission rate, and significantly improve the low temperature performance of the lithium ion battery at -20°C. In addition, in Reference Example 1, since the content of the nitro 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, the Reference Example provides some references to the upper limit of the nitro group-containing compound of the present invention.

[0119] In addition, the addition of the nitro group-containing compound of the present invention not only reduces the initial DCR value of the battery cell, but also significantly inhibits the growth of the battery cell impedance before and after high-temperature cycling, thereby significantly improving the fast charging performance of the battery.

[0120] 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.

[0121] 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: a nitro group-containing compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C); The compound containing a nitro group in component (A) further contains at least one carbonyl group in its structure.

2. The non-aqueous electrolyte according to claim 1, characterized in that The content of the nitro group-containing compound of component (A) is 0.1 mass % to 15 mass % of the total mass of the non-aqueous electrolyte.

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

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

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

6. The non-aqueous electrolyte according to any one of claims 1 to 5, characterized in that: The non-aqueous solvent of component (B) is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, carboxylate solvents and ether solvents.

7. The non-aqueous electrolyte according to claim 6, characterized in that The non-aqueous solvent is selected from at least one of ethylene carbonate, fluoroethylene carbonate, diethyl carbonate, ethyl methyl carbonate, methyl trifluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, dioxane and dioxolane.

8. The non-aqueous electrolyte according to any one of claims 1 to 7, characterized in that: The lithium salt of component (C) is selected from one or more salts formed by lithium ions and the following anions: PF6 - 、BF4 - , Cl - Br - ,I - 、ClO4 - 、AsF6 - 、CH3CO2 - CF3SO3 - 、N(CF3SO2)2 - 、N(FSO2)2 - 、C(CF2SO2)3 - 、C2BF2O4 - .

9. The non-aqueous electrolyte according to claim 8, characterized in that The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide and lithium bis(fluorooxalatoborate).

10. A diaphragm-free lithium-ion battery, characterized in that: The membrane-free lithium-ion battery comprises a positive electrode, a negative electrode and the non-aqueous electrolyte according to any one of claims 1 to 9.

11. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode, a negative electrode, a separator and the nonaqueous electrolyte according to any one of claims 1 to 9.

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