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 comprises a nitro-containing sulfate compound as a component (A), a non-aqueous solvent as a component (B) and a lithium salt as a component (C), wherein the structural formula of the component (A) containing the nitryl sulfate compound is as shown in the formula (1): # imgabs0 #, and P1 and P2 are mutually independent or mutually bonded; p1 represents an aromatic group having at least one aromatic nitro group, P2 represents a monovalent organic group, and P2 is the same as or different from P1; or P2 represents a linking site to P1, and the component (A) is dissolved in the component (B). The non-aqueous electrolyte can form a composite SEI film, can reduce impedance, and significantly improves the fast charge performance (above 6C) and low-temperature discharge performance of the battery.

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 secondary battery and a lithium secondary battery, and specifically, to a fast-charging non-aqueous electrolyte for a lithium battery and a diaphragm-free lithium secondary battery and a lithium secondary battery containing the electrolyte. Background Art

[0002] Electric vehicles have gradually become popular in recent years. However, their lower charging rate compared to traditional fuel vehicles is still one of the key factors affecting user experience. In order to achieve 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 places higher requirements on the materials and interfaces of lithium-ion batteries.

[0003] The fast charging performance of lithium-ion batteries can be substantially improved by optimizing the electrode electrolyte materials and cell design. Many research works have shown that the electrolyte is one of the main bottlenecks restricting the fast charging performance of batteries.

[0004] Some documents disclose a fast-charging electrolyte in which the selected additive is lithium nitrate, which is beneficial to the formation of an interface film containing Li3N on the surface of the negative electrode, thereby improving the fast-charging and cycle performance. However, the inorganic salt has poor compatibility with commonly used carbonate solvents, and the preparation cost is also relatively high.

[0005] Some documents disclose an organic azide compound with excellent negative electrode film-forming properties, which can significantly improve the fast charge and rate discharge performance of the battery. However, the azide group is a highly reactive group, which is 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 high-conductivity SEI film rich in lithium nitride.

[0006] 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 ions are conducted by the electrolyte itself. From the perspective of 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.

[0007] 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

[0008] Problem that the invention aims to solve

[0009] As mentioned above, the electrolyte has an important influence on the fast charging performance of lithium-ion batteries. The requirements for the electrolyte at different charging and discharging rates are also different. In order to meet the charging rate of about 6C, 8C or even 10C, it is urgently necessary to form a low-impedance SEI film and an electrolyte with high ionic conductivity in the battery.

[0010] There are documents in the prior art that disclose using lithium nitrate or organic azide compounds as electrolyte additives to form lithium nitride on the surface of the negative electrode to improve the fast charge and rate discharge performance of the battery.

[0011] However, the present invention believes that, for lithium nitrate, as an inorganic additive, it has poor compatibility with organic electrolytes and limited solubility in conventional non-aqueous solvents; for organic azide compounds, the azide group is a highly reactive group, and as mentioned above, it has chemical instability problems. In further practice, it is also found that after a period of use, the inhibition of cyclic decay is actually not obvious. Moreover, the above problems are more obvious at high charging rates.

[0012] In order to solve the above problems, the present invention provides a non-aqueous electrolyte, which contains a nitro-containing sulfate ester compound. The compound has good antioxidant properties and can form a composite SEI film containing lithium nitride and sulfur-containing organic components on the surface of the negative electrode. The composite SEI film has high lithium ion conductivity, can reduce impedance, and can significantly improve the battery's fast charging performance (charging rate of 6C or above) and low-temperature discharge performance.

[0013] Furthermore, the present invention also provides a lithium secondary battery, which comprises 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, wherein the non-aqueous electrolyte comprises: a nitro-containing sulfate compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C);

[0017] The structural formula of the nitro-containing sulfate compound of component (A) is shown in formula (1):

[0018]

[0019] Wherein, P1 and P2 are independent of each other or bonded to each other;

[0020] P1 represents an aromatic group having at least one aromatic nitro group,

[0021] P2 represents a monovalent organic group, and P2 is the same as or different from P1; or, P2 represents a linking position to P1,

[0022] The component (A) is dissolved in the component (B).

[0023] According to the above non-aqueous electrolyte, the content of the component (A) is 0.1 mass % to 15 mass % of the total mass of the non-aqueous electrolyte.

[0024] According to the non-aqueous electrolyte described above, wherein the structural formula of the nitro-containing sulfate compound of component (A) is as shown in Formula (1a) or Formula (1b):

[0025]

[0026] in,

[0027] The aromatic group of P1 is selected from aromatic groups having 4 to 6 aromatic ring atoms, and the ring atoms of the aromatic ring have no more than two heteroatoms;

[0028] Furthermore, in formula (1a), P2 represents a substituted or unsubstituted aromatic group or an alkyl group.

[0029] According to the above non-aqueous electrolyte, wherein the P1 represents a phenyl group having at least one nitro group, and the phenyl group optionally further has other substituents different from the nitro group.

[0030] According to the non-aqueous electrolyte described above, the other substituent is selected from one or more of a halogen atom-containing group, an alkoxy group or an alkyl group.

[0031] According to the non-aqueous electrolyte described above, wherein the nitro-containing sulfate compound of component (A) has the following structure (1b-1):

[0032]

[0033] Wherein, R1 represents a substituent group of a benzene ring, and at least one R1 is -NO2;

[0034] n represents a positive integer, and 4≥n≥1.

[0035] According to the non-aqueous electrolyte described above, wherein the nitro-containing sulfate compound of component (A) is selected from one or more combinations of compounds represented by the following formulas (1b-1a), (1b-1b), (1b-1c), and (1b-1d):

[0036]

[0037]

[0038] According to the non-aqueous electrolyte described above, wherein the component (B) non-aqueous solvent is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0039] According to the non-aqueous electrolyte described above, wherein the lithium salt 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 - .

[0040] According to the above non-aqueous electrolyte, the concentration of the lithium salt in the non-aqueous electrolyte is 0.2 mol / L to 10 mol / L.

[0041] Furthermore, the present invention also provides a diaphragm-free lithium secondary battery, wherein the diaphragm-free lithium secondary battery comprises a positive electrode, a negative electrode and the non-aqueous electrolyte described above.

[0042] In addition, the present invention also provides a lithium secondary battery, wherein the lithium secondary battery comprises a positive electrode, a negative electrode, a separator and the non-aqueous electrolyte described above.

[0043] According to the lithium secondary battery described above, wherein the lithium secondary battery satisfies one or two of the following conditions:

[0044] i. The lithium secondary battery has a discharge capacity retention rate of more than 70% after 200 cycles at a constant rate of 6C at 25°C;

[0045] ii. After the lithium secondary battery is stored at -20°C for 4 hours, it is discharged at a rate of 0.33C, and the discharge capacity retention rate is more than 75%.

[0046] Effects of the Invention

[0047] By implementing the above technical solution, the present invention can achieve the following technical effects:

[0048] The nitro-containing sulfate compounds in the non-aqueous electrolyte of the present invention can undergo a reduction reaction with lithium ions at the negative electrode to generate a composite SEI film of an inorganic lithium nitride layer with high lithium ion conductivity and an organic layer of an oligomer of sulfate containing aromatic groups. The composite SEI film has low impedance characteristics, thereby significantly improving the fast charging performance and low-temperature discharge performance of the battery.

[0049] In addition, the organic components in the composite SEI film can make the film have excellent extensibility, and it is not easy to be damaged during the expansion and contraction of the film caused by repeated charging and discharging, thereby improving the cycle performance of the battery cell, which is particularly suitable for lithium secondary batteries whose negative electrode materials contain silicon. DETAILED DESCRIPTION

[0050] The following is a detailed description of the present invention. The following description of the technical features is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0051] 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.

[0052] In the present specification, a numerical range expressed using "above" or "below" means a numerical range including the number.

[0053] In this specification, the word "may" means both performing a certain process and not performing a certain process.

[0054] In this specification, the use of "optional" or "optional" indicates that certain substances, components, execution steps, application conditions and other factors are used or not used.

[0055] In this specification, the "normal temperature" or "room temperature" used means an indoor ambient temperature of "23±2°C".

[0056] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0057] 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 1%, preferably 0.8%, and more preferably 0.7%.

[0058] 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.

[0059] In this manual, the term "fast charging" refers to charging rates above 2C, above 4C, above 5C, and especially 6C to 10C.

[0060] 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.

[0061] The present invention mainly provides a non-aqueous electrolyte, which contains a nitro-containing sulfate ester compound. By adding the compound, the fast charging performance, low-temperature discharge performance and cycle performance of the battery can be significantly improved.

[0062] The present invention is mainly obtained through the following insights:

[0063] In view of the continuous development of the current demand for fast charging, it is gradually difficult to meet the usage needs by relying solely on lithium salts to form SEI film, especially when the charging rate reaches above 6C, or even 10C, 16C, etc.

[0064] The present invention believes that for the formation of SEI film, if a stability component is further introduced and the component has low impedance, it may be helpful to solve the above problems. After further research by the inventors, it was found that when the non-aqueous electrolyte contains the nitro-containing sulfate compound described in the present invention, the compound can react with lithium ions at the negative electrode to generate a composite SEI film of an inorganic layer of lithium nitride with high lithium ion conductivity and an organic layer of oligomers containing phenyl sulfate. The composite SEI film not only has better stability due to the introduction of the organic layer, but also has low impedance characteristics, which can significantly improve the fast charging performance and low-temperature discharge performance of the battery. In addition, the organic component in the composite SEI film can make the film have excellent extensibility, and it is not easy to be destroyed during the expansion and contraction of the film caused by repeated charging and discharging, thereby improving the cycle performance of the battery cell.

[0065] <First aspect>

[0066] A first aspect of the present invention provides a non-aqueous electrolyte solution comprising a nitro group-containing sulfate ester compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C).

[0067] The non-aqueous electrolyte of the present invention can be applied to conventional lithium secondary batteries in the art, and in particular, the electrolyte of the present invention can provide a lithium secondary battery with good stability, cyclability, and low-temperature discharge characteristics under conditions of higher charging rates.

[0068] 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.

[0069] Component (A)

[0070] The component (A) described in the present invention is a nitro-containing sulfate compound, which can be dissolved in the non-aqueous solvent (B) described below within the range of the electrolyte configuration and the electrolyte operating temperature.

[0071] Furthermore, component (A) has a structure shown in the following structural formula (1):

[0072]

[0073] in:

[0074] P1 represents an aromatic group having at least one aromatic nitro group. In some specific embodiments, the aromatic group may be an aromatic group having 4 to 6 aromatic ring atoms. In addition, one or two of the atoms on the aromatic ring may be heteroatoms, such as oxygen atoms, sulfur atoms or nitrogen atoms. In a further preferred embodiment of the present invention, the aromatic group may be a phenyl group.

[0075] The aromatic nitro group refers to a nitro group directly bonded to an aromatic ring. In the present invention, the number of such nitro groups in the P1 structure may be one or more, and preferably one or two.

[0076] Furthermore, for P1, the aromatic group may have other substituent groups in addition to the nitro group. In some specific embodiments, such substituent groups may be one or more of a halogen-containing group, an alkoxy-containing group or an alkyl group, preferably a halogen atom, an alkoxy group having 1 to 5 carbon atoms, or a halogenated or non-halogenated alkyl group having 1 to 5 carbon atoms.

[0077] Regarding P2, as a monovalent organic group, there is no particular limitation in principle, and P2 is the same as or different from P1; or, P2 represents a linking position to P1.

[0078] In some specific embodiments, P2 is different from P1, and the monovalent organic group may be a substituted or unsubstituted aromatic group or alkyl group, such as an aromatic group with 4 to 6 carbon atoms with or without heteroatoms, an alkyl group with 1 to 10 carbon atoms, etc., and these aromatic groups or alkyl groups may further have substituents such as halogens.

[0079] Further, in some preferred embodiments, the nitro-containing sulfate compound of component (A) may have the following structural formulas (1a) and (1b):

[0080]

[0081] in,

[0082] The aromatic group of P1 is selected from aromatic groups having 4 to 6 aromatic ring atoms, and the ring atoms of the aromatic ring have no more than two heteroatoms; and, in formula (1a), P2 represents a substituted or unsubstituted aromatic group or an alkyl group.

[0083] Further, in some more preferred embodiments of the present invention, the nitro-containing sulfate compound of component (A) may have the following structural formula (1b-1):

[0084]

[0085] Wherein, R1 represents a substituent group of the benzene ring, and each time R1 appears, it represents a substituent group of the benzene ring independently of each other, and at least one R1 is -NO2, n represents a positive integer, and 4≥n≥1.

[0086] In some specific embodiments, the benzene ring has one or two nitro groups. For other R1, it can be selected from one or more of a halogen-containing group, an alkoxy-containing group or an alkyl group, preferably a halogen atom, an alkoxy group having 1 to 5 carbon atoms, or a halogenated or non-halogenated alkyl group having 1 to 5 carbon atoms.

[0087] The above-mentioned alkyl group may preferably be a methyl group, an ethyl group, or an n-propyl group, and may be optionally substituted by a halogen atom. The above-mentioned halogen atom may preferably be a F atom.

[0088] Further, in some more specific embodiments of the present invention, the nitro-containing sulfate compound of component (A) can be selected from one or more combinations of the following formulas (1b-1a), (1b-1b), (1b-1c), (1b-1d), etc.:

[0089]

[0090]

[0091] The present invention introduces the nitro-containing sulfate compound into the electrolyte to form a composite SEI film with high lithium ion conductivity at the negative electrode, which contains lithium nitride and oligomers containing phenyl and sulfate. The composite SEI film has low impedance characteristics and can significantly improve the fast charging performance and low-temperature discharge performance of the battery.

[0092] Component (B)

[0093] 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.

[0094] In some specific embodiments, the non-aqueous solvent can be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0095] Among them, the cyclic carbonate solvent can be selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), 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), di(2,2,2-trifluoroethyl) carbonate (DFDEC), etc.; the ester solvent can be selected from methyl acetate, ethyl acetate, methyl propionate, and methyl pivalate, 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), dioxolane (DOL), etc.; the ketone solvent can be selected from polymethyl vinyl ketone, etc. These nonaqueous solvents may be used alone or in admixture of two or more.

[0096] In some preferred embodiments, the non-aqueous solvent can be selected from at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), methyl trifluoroethyl carbonate (FEMC), di(2,2,2-trifluoroethyl) carbonate (DFDEC), ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), dioxolane (DOL), etc.

[0097] Component (C)

[0098] The present invention does not specifically limit the type of lithium salt of component (D), and it can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt can be 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 - wait.

[0099] In some preferred embodiments, the lithium salt may be selected from a combination of one or more of lithium hexafluorophosphate (LiPF6) or lithium bis(fluorosulfonyl)imide (LiFSI).

[0100] Composition of non-aqueous electrolyte

[0101] In the present invention, for the content of the nitro-containing sulfate compound of the component (A), in some specific embodiments of the present invention, from the perspective of controlling the film thickness and internal resistance and improving the fast charging performance, the content of the nitro-containing sulfate compound of the component (A) can be 0.1% to 25% by mass of the total mass of the non-aqueous electrolyte, preferably 0.2% to 20% by mass, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 5%, 10%, 15%, 18%, 22%, etc. When the content of the 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 the component (A) is too low, it cannot be guaranteed that the electrolyte has sufficient ionic conductivity, thereby affecting the performance of the battery cell.

[0102] In principle, there is no special restriction on the lithium salt of the component (C). In some specific embodiments of the present invention, the concentration of the lithium salt in the non-aqueous electrolyte can be 0.2mol / L to 10mol / L, preferably 0.7mol / L to 2mol / L, for example, 0.5mol / L, 1mol / L, 1.5mol / L, 2mol / L, 2.5mol / L, 3mol / L, 3.5mol / L, 4mol / L, 5mol / L, 6mol / L, 7mol / L, 8mol / L, 9mol / L, etc. When the concentration of the lithium salt is too high, there is a concern that the viscosity and cost of the electrolyte will increase; when the concentration of the lithium salt is too low, there is a concern that the electrolyte cannot have sufficient ionic conductivity, thereby affecting the performance of the battery cell.

[0103] <Second Aspect>

[0104] A second aspect of the present invention provides a lithium secondary battery comprising the nonaqueous electrolyte according to the first aspect.

[0105] Such a secondary battery includes at least a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0106] The 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. In some preferred embodiments, various lithium-containing oxides in the art can be used. In addition to lithium, other main group, sub-group or rare earth metal elements can be added to these oxides.

[0107] Furthermore, from the perspective of wide applicability, the positive electrode active material of the present invention may be a metal-doped lithium ion positive electrode active material, and more specifically, may be a positive electrode active material containing Mn, Co, Al and Ni elements.

[0108] In some preferred embodiments, the active material of the positive electrode can be selected from at least one of lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium iron manganese phosphate and lithium-rich manganese-based materials, etc. More preferred positive electrode active materials can be ternary NCM, ternary NCA, lithium iron manganese phosphate, for example, high nickel content ternary positive electrode (6 series, 8 series, NCA) materials.

[0109] 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.

[0110] In principle, there is no particular restriction on the separator of the battery, and the separator may be used or not. That is, the lithium secondary battery of the present invention may be a lithium secondary battery with a separator or a lithium secondary battery without a separator. When a separator is used, it may be a separator commonly used in the art, and preferably, a separator having a high moisture retention capacity for the electrolyte solution and a low resistance to the transfer of electrolyte ions may be used. In the case of a lithium secondary battery without a separator, the nonaqueous electrolyte of the present invention may be used in combination with a solid electrolyte, wherein the solid electrolyte may act as a separator.

[0111] Example

[0112] 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.

[0113] Example 1

[0114] (1) Preparation method of electrolyte:

[0115] A non-aqueous electrolyte was prepared in an argon-protected glove box, wherein the moisture content in the box was less than 1 ppm. The solvent EC and EMC were mixed in a volume ratio of 1:2.5, and then lithium salt (1 mol / L) and the compound represented by formula (1-1) were added thereto. After stirring evenly, the desired electrolyte was obtained. The specific component formula is shown in Table 1;

[0116] (2) Preparation method of lithium ion secondary battery:

[0117] (i) Preparation of ternary material positive electrode:

[0118] 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 and fully stirred, and the solid content is controlled to 68% to prepare a positive electrode mixture slurry. Thereafter, the formed positive electrode slurry is coated on aluminum foil, and after drying, roll-pressed and die-cut to form a ternary material positive electrode sheet.

[0119] (ii) Negative electrode preparation:

[0120] Negative electrode preparation: Graphite, conductive agent ultrafine carbon powder (SP), thickener CMC, and adhesive 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. Thereafter, the formed negative electrode slurry was coated on a copper foil, and the negative electrode sheet was formed by rolling and die-cutting after drying.

[0121] (iii) Battery Assembly:

[0122] Take the positive electrode sheet, the negative electrode sheet and the separator, 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 cell, and finally inject the non-aqueous electrolyte prepared above into the cell to prepare a lithium battery with a capacity of 5Ah.

[0123] Embodiments 2 to 6

[0124] The preparation method in Example 1 was adopted and the raw material ratios in Table 1 were used to prepare the lithium ion batteries of Examples 2 to 6.

[0125] Wherein, the preparation method of lithium iron phosphate positive electrode is:

[0126] The positive electrode active material LiFePO4, conductive agent ultrafine carbon powder (SP), single-walled carbon nanotubes (SWNT) and binder polyvinylidene fluoride (PVDF) were added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent in a mass ratio of 94.5:2.5:2:1 and stirred thoroughly. The solid content was controlled to 65% to prepare a positive electrode mixture slurry. Thereafter, the formed positive electrode slurry was coated on aluminum foil, and after drying, the positive electrode sheet of lithium iron phosphate was formed by rolling and die-cutting.

[0127] Comparative Examples 1 to 2 and Reference Example 1

[0128] The preparation method in Example 1 was adopted and the raw material ratios in Table 1 were used to prepare the lithium ion batteries of Comparative Examples 1 to 2 and Reference Example 1.

[0129] Performance Testing

[0130] 1. Rate cycle:

[0131] At 25°C, the packaged lithium-ion battery was subjected to a 6C constant rate cycle test after the formation and capacity division steps. The battery was charged to 4.25V at 6C constant current and constant voltage. After standing for half an hour, the battery was discharged at 6C constant current to obtain the initial capacity of the first week of the battery cycle and recorded as C1. After that, the charging and discharging cycle was repeated in the CCCV / CC manner of the first week. The capacity after the 200th week of discharge was recorded as C200. The capacity retention rate (%) after 200 weeks of high-rate room temperature cycling = C200 / C1×100%.

[0132] 2. DCR test:

[0133] The cell before or after the rate cycle is charged at 25°C at a constant current of 0.5C to 4.25V, and charged at a constant voltage to a current of 0.05C. After standing for 1 hour, it is discharged at 0.5C for 60 minutes (50% SOC), and then stood for 1 hour; then it is discharged at 0.1C for 10 seconds, and the voltage V1 at the end is recorded, and then it is discharged at 1C for 1 second, and the voltage V2 at the end is recorded. The DCR of the cell before or after the cycle is (V1-V2) / (I1C-I0.1C), in mOhm. 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%.

[0134] 3. Low temperature discharge:

[0135] At 25°C, the packaged lithium-ion battery is charged to 4.25V at 1C constant current and constant voltage after formation and capacity division steps. After being left for half an hour, it is discharged at 1C constant current to obtain the initial capacity of the battery and recorded as C1. The battery is then placed in a low-temperature box at -20°C and left for 4 hours before being discharged at 0.33C. Its capacity is recorded as C2. The capacity retention rate of low-temperature discharge (%) = C2 / C1×100%.

[0136] Table 1 Formulations of Examples 1 to 6, Comparative Examples 1 to 2 and Reference Example 1

[0137]

[0138] Table 2 Test results of Examples 1 to 6, Comparative Examples 1 to 2 and Reference Example 1

[0139]

[0140] It can be seen from Table 2 that the fast charging performance and low-temperature discharge of the non-aqueous electrolyte containing the nitro-containing sulfate ester compound of the present invention are significantly improved, and the DCR growth after fast charging slows down.

[0141] In addition, it can be seen from Reference Example 1 that although component (A) is used, its amount exceeds the amount used to assist in stabilizing the SEI film, resulting in a relative decrease in lithium ion concentration and a decrease in battery performance.

[0142] 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.

[0143] 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-containing sulfate compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C); The structural formula of the nitro-containing sulfate compound of component (A) is shown in formula (1): Wherein, P1 and P2 are independent of each other or bonded to each other; P1 represents an aromatic group having at least one aromatic nitro group, P2 represents a monovalent organic group, and P2 is the same as or different from P1; or, P2 represents a linking position to P1, The component (A) is dissolved in the component (B).

2. The non-aqueous electrolyte according to claim 1, characterized in that The content of the component (A) is 0.1% to 25% by 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 structural formula of the nitro-containing sulfate compound of component (A) is shown in formula (1a) or formula (1b): in, The aromatic group represented by P1 is selected from aromatic groups having 4 to 6 aromatic ring atoms, and the ring atoms of the aromatic ring have no more than two heteroatoms; Furthermore, in formula (1a), P2 represents a substituted or unsubstituted aromatic group or an alkyl group.

4. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that The P1 represents a phenyl group having at least one nitro group, and the phenyl group may optionally further have other substituents other than the nitro group. 5 . The non-aqueous electrolyte according to claim 4 , wherein the other substituent is selected from one or more of a halogen atom-containing group, an alkoxy group-containing group or an alkyl group.

6. The non-aqueous electrolyte according to any one of claims 1 to 5, characterized in that The nitro-containing sulfate compound of component (A) has the structure of the following formula (1b-1): Wherein, R1 represents a substituent group of a benzene ring, and at least one R1 is -NO2; n represents a positive integer, and 4≥n≥1.

7. The non-aqueous electrolyte according to any one of claims 1 to 6, characterized in that The nitro-containing sulfate compound of component (A) is selected from one or more combinations of compounds represented by the following formulas (1b-1a), (1b-1b), (1b-1c), and (1b-1d):

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

9. The non-aqueous electrolyte according to any one of claims 1 to 8, characterized in that The lithium salt 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 - .

10. The non-aqueous electrolyte according to any one of claims 1 to 9, characterized in that: The concentration of the lithium salt in the non-aqueous electrolyte is 0.2 mol / L to 10 mol / L.

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 lithium secondary battery meets one or both of the following conditions: i. The lithium secondary battery has a discharge capacity retention rate of more than 70% after 200 cycles at a constant rate of 6C at 25°C; ii. After the lithium secondary battery is stored at -20°C for 4 hours, it is discharged at a rate of 0.33C, and the discharge capacity retention rate is more than 75%.

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