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 includes a compound having a sulfuryl group and an aza-aromatic group 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 compound containing sulfuryl and aza-aromatic groups in the component (A) is as shown in formula (I): # imgabs0 #, the groups X and Y represent same or different aromatic groups, one or both of the groups X and Y are selected from groups containing aza-aromatic groups, the aza-aromatic groups have one or more nitrogen atoms on an aromatic ring, and one or more of the groups X and Y are selected from groups containing one or more nitrogen atoms on an aromatic ring. The at least one nitrogen atom is directly bonded to the S atom in formula (I). The non-aqueous electrolyte provided by the invention can significantly improve the low-temperature discharge performance of the battery.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte, a diaphragm-free lithium secondary battery and a lithium secondary battery, and in particular to a low-temperature non-aqueous electrolyte for lithium ions and a diaphragm-free lithium secondary battery and a lithium secondary battery containing the non-aqueous electrolyte, belonging to the field of lithium ion batteries. Background Art

[0002] At present, various types of lithium secondary batteries have been widely used in rechargeable battery applications, especially in the fields of electric vehicles, which has become the focus of competition among countries. Most of these batteries involve ternary lithium or lithium iron phosphate as the positive electrode active material, but in actual applications, their cycle characteristics and low temperature performance have also become the focus of research and improvement.

[0003] In particular, in recent years, lithium iron phosphate batteries have gradually been favored by the market as power batteries. Although their single-cell energy density is lower than that of ternary nickel-cobalt-manganese positive electrode cells, due to the progress of cell design technology and group packaging technology, the energy density of lithium iron phosphate battery packs is already close to that of ternary products. The lower cost and safety advantages of lithium iron phosphate have rapidly increased its application share.

[0004] Since the electronic and ionic conductivity of lithium iron phosphate materials is lower than that of ternary materials, the discharge capacity of the battery cells made from them as positive electrode materials will be greatly affected under low temperatures. At -20°C, their capacity is even less than 50% of the capacity at room temperature. This has limited the large-scale commercial promotion and application of lithium iron phosphate batteries to a certain extent.

[0005] Some documents disclose a low-temperature electrolyte for lithium iron phosphate batteries. The electrolyte uses a composite lithium salt and adds a low-viscosity solvent and a fluorocarbonate additive. Although the addition of the composite lithium salt can improve the low-temperature performance of the electrolyte, it will reduce the room-temperature conductivity of the electrolyte and affect the room-temperature performance of the battery.

[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 the field has conducted certain research on electrolytes for improving the low-temperature performance of lithium iron phosphate batteries, 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, there are still deficiencies in the application of various positive electrode active materials in secondary batteries, one of which is poor low-temperature properties. For example, although lithium iron phosphate batteries have advantages such as low cost and safety, the poor electronic conductivity and low lithium ion diffusion of lithium iron phosphate materials themselves lead to poor low-temperature performance and difficulty in working in an environment of -20°C, limiting their application. This is largely due to the significant increase in the interfacial impedance of the battery at low temperatures, resulting in a significant decrease in the discharge capacity at low temperatures.

[0010] At present, the most commonly used method is to improve the low-temperature performance of the electrolyte, such as adding additives to the electrolyte or using a variety of organic solvents to dissolve and compound.

[0011] However, for the method of adding additives, the common negative electrode film-forming additives VC (vinyl carbonate) and PS (1,3-propane sultone) have the problem of large film-forming impedance, and the positive electrode film-forming additive DTD (vinyl sulfate) has the problem of stability and electrolyte discoloration. For the method of co-dissolving and compounding multiple organic solvents, since some solvents in the electrolyte are easy to crystallize at low temperatures, it makes ion migration difficult and the conductivity is low. If the electrolyte solvent components are not well matched, it is easy to reduce the diffusion of lithium ions and the speed of charge transfer, affecting the low-temperature performance of the battery.

[0012] It can be seen that adding additives or using compound organic solvents is difficult to effectively improve the low-temperature performance of the battery, and may also lead to other problems.

[0013] In order to solve the above problems, the present invention provides a non-aqueous electrolyte, which contains a compound containing a sulfone group and a nitrogen aromatic group, which can form a composite SEI film with high electronic conductivity and ionic conductivity through chemical reaction, thereby significantly improving the low-temperature discharge performance of the battery.

[0014] Furthermore, the present invention also provides a lithium secondary battery, which comprises the above-mentioned non-aqueous electrolyte.

[0015] Solutions for solving problems

[0016] The present invention first provides a non-aqueous electrolyte, wherein the non-aqueous electrolyte comprises: a compound having a sulfone group and a nitrogen heteroaromatic group as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C);

[0017] Wherein, the structure of the compound containing sulfone group and nitrogen heteroaromatic group in component (A) is shown in formula (I):

[0018]

[0019] Wherein, the groups X and Y represent the same or different aromatic groups, and one or both of them are selected from the group containing nitrogen heteroaromatic groups, wherein the nitrogen heteroaromatic groups have one or more nitrogen atoms on the aromatic ring, and the at least one nitrogen atom is directly bonded to the S atom in formula (I).

[0020] According to the non-aqueous electrolyte of the present invention, the content of the compound containing a sulfone group and an azaaromatic group as component (A) is 0.1 mass % to 15 mass % of the total mass of the non-aqueous electrolyte.

[0021] According to the non-aqueous electrolyte of the present invention, wherein, in the formula (I), the groups X and Y are the same or different, both are groups containing nitrogen heteroaromatic groups, and there is at least one nitrogen atom on the aromatic ring in each of them that is directly bonded to the S atom in the formula (I).

[0022] According to the non-aqueous electrolyte of the present invention, the nitrogen heteroaromatic group has 1 or 2 nitrogen atoms on the aromatic ring, and the aromatic ring is an aromatic ring with 5 atoms to 10 atoms.

[0023] According to the non-aqueous electrolyte of the present invention, at least one of the group X or the group Y is selected from a group having the following structure:

[0024]

[0025] Wherein, in the formula (a), R1 is selected from a group of alkyl, alkoxy or halogen atoms substituted or unsubstituted by a substituent, and x represents an integer of 0 to 2;

[0026] In the formula (b), the arc represents a 5- or 6-membered aromatic ring that shares two carbon atoms with the pyrrole ring, and the aromatic ring may optionally have a substituent.

[0027] * indicates the part connected to the S;

[0028] Preferably, the substituent in R1 is selected from at least one of an alkoxy group and a halogen-containing group.

[0029] According to the non-aqueous electrolyte of the present invention, the group X and the group Y are both selected from the group having a structure as shown in formula (a) or formula (b); or, one of the group X and the group Y is selected from the group having a structure as shown in formula (a) or formula (b), and the other is selected from a phenyl group, and optionally, the phenyl group has a substituent;

[0030] Preferably, the substituent of the phenyl group is selected from at least one of an alkoxy group and a halogen-containing group.

[0031] According to the non-aqueous electrolyte of the present invention, 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.

[0032] According to the non-aqueous electrolyte of the present invention, the concentration of the lithium salt in the non-aqueous electrolyte is 0.2 mol / L to 10 mol / L.

[0033] 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 according to the present invention.

[0034] 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 according to the present invention.

[0035] According to the lithium secondary battery of the present invention, the positive electrode material of the lithium secondary battery is lithium iron phosphate.

[0036] Effects of the Invention

[0037] By implementing the above technical solution, the present invention can achieve the following technical effects:

[0038] The non-aqueous electrolyte provided by the present invention introduces a compound containing a sulfone group and a nitrogen heteroaromatic group. On the one hand, the compound containing a sulfone group and a nitrogen heteroaromatic group can generate a high ionic conductivity layer containing alkyl lithium sulfonate or alkyl sulfonate compound through an oxidation reaction of the sulfone group. On the other hand, the nitrogen heteroaromatic group, such as a pyrrole group, can form an oligomer organic layer containing polypyrrole, and the organic layer has high electronic conductivity. The high ionic conductivity layer and the organic layer with high electronic conductivity jointly form a composite SEI film, and the SEI film has low impedance, thereby effectively improving the low-temperature discharge performance of the battery. DETAILED DESCRIPTION

[0039] 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:

[0040] 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.

[0041] In the present specification, a numerical range expressed using "above" or "below" means a numerical range including the number.

[0042] In this specification, the word "may" means both performing a certain process and not performing a certain process.

[0043] 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.

[0044] In this specification, the "normal temperature" or "room temperature" used means an indoor ambient temperature of "23±2°C".

[0045] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0046] 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%.

[0047] 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.

[0048] 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.

[0049] The present invention mainly provides a non-aqueous electrolyte, which contains a sulfone-containing pyrrole compound. The low-temperature discharge performance of a battery can be effectively improved by adding the compound.

[0050] The present invention is mainly obtained through the following insights:

[0051] As mentioned above, in view of the poor low-temperature performance of lithium iron phosphate batteries, the methods of co-dissolving and compounding of multiple organic solvents and adding additives used in the past have many shortcomings. Through the research of the present invention, it has been found that when a compound containing a sulfone group and a nitrogen heteroaromatic group is used in a non-aqueous electrolyte, on the one hand, a high ionic conductivity layer containing alkyl sulfonate lithium or alkyl sulfonate compound can be generated by oxidation reaction of the sulfone group, and on the other hand, the nitrogen heteroaromatic group can form an oligomer organic layer containing nitrogen heteroaromatic rings, which has a high electronic conductivity. The high ionic conductivity layer and the organic layer with high electronic conductivity jointly form a composite SEI film, which has a low impedance, thereby effectively improving the low-temperature discharge performance of the battery.

[0052] <First aspect>

[0053] A first aspect of the present invention provides a non-aqueous electrolyte solution comprising a compound containing a sulfone group and an azaaromatic group as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C).

[0054] The non-aqueous electrolyte of the present invention can be applied to common lithium secondary batteries in the art, especially lithium secondary batteries whose positive electrode material is lithium iron phosphate, and can effectively improve the low-temperature performance of the battery.

[0055] 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.

[0056] Component (A)

[0057] Component (A) of the present invention is a compound containing a sulfone group and an azaaromatic group, which can be dissolved in the non-aqueous solvent as component (B) described below within the range of electrolyte preparation and electrolyte operating temperature.

[0058] Furthermore, the structural formula of the compound containing a sulfone group and an azo aromatic group in component (A) is shown in formula (I):

[0059]

[0060] Wherein, the groups X and Y represent the same or different aromatic groups, and one or both of them are selected from the group containing nitrogen heteroaromatic groups, wherein the nitrogen heteroaromatic groups have one or more nitrogen atoms on the aromatic ring, and the at least one nitrogen atom is directly bonded to the S atom in formula (I).

[0061] In principle, there is no particular limitation on the aromatic group described in the present invention. The aromatic group can be selected from aromatic groups having 4 to 20 carbon atoms, and these aromatic groups can optionally have substituents such as alkyl groups, alkoxy groups, halogen-containing groups, etc. In some specific embodiments, the aromatic group can be a substituted or unsubstituted phenyl group.

[0062] For the group containing nitrogen heteroaromatic groups of the present invention, the nitrogen heteroaromatic groups, in some specific embodiments of the present invention, can be aromatic rings with 5 to 10 atoms on the aromatic ring, and the aromatic ring can have 1 or 2 nitrogen atoms. In some preferred embodiments of the present invention, the ring atoms of the aromatic ring in the nitrogen heteroaromatic groups are composed of carbon atoms and nitrogen atoms.

[0063] Further, in some specific embodiments of the present invention, in the structure (I), the group X can be the group containing a nitrogen heteroaromatic group, and the group Y is a substituted or unsubstituted phenyl group; in some other specific embodiments of the present invention, the group X and the group Y are both groups containing a nitrogen heteroaromatic group, and in both of them, there is at least one nitrogen atom on the aromatic ring directly bonded to the S atom in formula (I).

[0064] In some more specific embodiments, at least one of the group X or the group Y is selected from a group having a structure as shown in the following formula (a) or formula (b):

[0065]

[0066] Wherein, * represents the site connected to the S.

[0067] In formula (a), R1 is selected from a group consisting of substituted or unsubstituted alkyl, alkoxy or halogen atoms, and x represents an integer of 0 to 2. Furthermore, from the perspective of ultimately forming an aromatic polymer layer, R1 is not located at an ortho position to the nitrogen atom.

[0068] In principle, there is no particular limitation on the above-mentioned alkyl group, and preferably, it can be an alkyl group with 1 to 10 carbon atoms, or 2 to 6 carbon atoms; for the alkoxy group, it can be an alkoxy group with 1 to 10 carbon atoms, or 2 to 6 carbon atoms; for the halogen, it is preferably a F atom; there is no particular limitation on the substituents that can be used for the alkyl or alkoxy group, and it can be selected from at least one of an alkoxy group and a halogen-containing group, and is preferably a fluorine-containing group to further provide polarity.

[0069] In the formula (b), the arc represents a 5- or 6-membered aromatic ring that shares two carbon atoms with the pyrrole ring, and the aromatic ring optionally has a substituent, preferably, the substituent may be at least one of an alkoxy group or a halogen-containing group, especially a F-containing group. In some preferred embodiments, the aromatic ring represented by the arc is a phenyl group or a 5- or 6-membered heteroaromatic ring containing a nitrogen atom on the ring.

[0070] Furthermore, in some specific implementations of the present invention, in the formula (I), the groups X and Y are the same or different and are both groups represented by the formula (a); in other specific implementations, in the formula (I), the groups X and Y are different and are respectively a group represented by the formula (a) and a group represented by the formula (b).

[0071] In a further preferred embodiment of the present invention, the compound containing a sulfone group and an azaaromatic group in component (A) is selected from at least one of the following formulae (I-1), (I-2) and (I-3).

[0072]

[0073] The present invention introduces a compound containing a sulfone group and a nitrogen heteroaromatic group into a non-aqueous electrolyte, and can generate a high ionic conductivity layer containing alkyl lithium sulfonate or alkyl sulfonate compound through an oxidation reaction of the sulfone group. At the same time, the nitrogen heteroaromatic group, such as a pyrrole group, can form an organic layer containing an oligomer with conductive properties, and the organic layer has high electronic conductivity. The high ionic conductivity layer and the organic layer with high electronic conductivity jointly form a composite SEI film, and the SEI film has low impedance, thereby effectively improving the low-temperature discharge performance of the battery.

[0074] Component (B)

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Component (C)

[0080] The present invention does not specifically limit the type of lithium salt of component (C), 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.

[0081] 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).

[0082] Composition of non-aqueous electrolyte

[0083] In the present invention, for the content of the compound containing sulfone group and nitrogen heteroaromatic group of the component (A), in some specific embodiments of the present invention, from the perspective of controlling the film thickness and internal resistance, the content of the compound containing sulfone group and nitrogen heteroaromatic group of the component (A) is 0.1 mass% to 15 mass%, preferably 0.2 mass% to 5 mass%, for example, it can be 0.5 mass%, 1 mass%, 1.5 mass%, 2 mass%, 2.5 mass%, 5 mass%, 10 mass%, 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, it will cause the SEI film to 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 to have a sufficient impact on the impedance of the SEI film to improve the temperature reduction performance.

[0084] 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.

[0085] <Second Aspect>

[0086] A second aspect of the present invention provides a lithium secondary battery comprising the nonaqueous electrolyte according to the first aspect.

[0087] Such a secondary battery includes at least a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0088] 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.

[0089] 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.

[0090] 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 manganese iron phosphate and lithium-rich manganese-based materials, etc. More preferably, it can be lithium iron phosphate, and the improvement of the low-temperature performance of the secondary battery whose positive electrode active material is lithium iron phosphate is more obvious.

[0091] 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.

[0092] In principle, there is no particular limitation on the battery of the present invention, and a diaphragm may or may not be used. That is, the lithium secondary battery of the present invention may be a lithium secondary battery with a diaphragm or a lithium secondary battery without a diaphragm. When a diaphragm is used, it may be a diaphragm commonly used in the art, and preferably, a diaphragm 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 diaphragm, the nonaqueous electrolyte of the present invention may be used in combination with a solid electrolyte, wherein the solid electrolyte may act as a diaphragm.

[0093] Example

[0094] 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.

[0095] Example 1

[0096] (1) Preparation method of electrolyte:

[0097] 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;

[0098] (2) Preparation method of lithium ion secondary battery:

[0099] (i) Preparation of ternary material positive electrode:

[0100] 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.

[0101] (ii) Negative electrode preparation:

[0102] 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.

[0103] (iii) Battery Assembly:

[0104] 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 2Ah.

[0105] Embodiments 2 to 7

[0106] 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 7.

[0107] Wherein, the preparation method of lithium iron phosphate positive electrode is:

[0108] 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.

[0109] Comparative Examples 1 to 2 and Reference Examples

[0110] The preparation method in Example 1 was used to prepare the lithium ion batteries of Comparative Examples 1 to 2 and the reference example according to the raw material ratios in Table 1.

[0111] Performance Testing

[0112] 1. DCR test:

[0113] The ternary positive electrode material cell is charged at 0.33C constant current to 4.2V (3.6V for lithium iron phosphate positive electrode material cell) at 25℃, and then charged at constant voltage to a current of 0.05C. After being left for 1 hour, it is discharged at 0.5C for 60 minutes (50% SOC), and then left for another 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%.

[0114] 2. Low temperature discharge:

[0115] At 25°C, the packaged lithium-ion battery is charged to 4.2V (3.6V for lithium iron phosphate positive electrode material battery cell) 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%.

[0116] Table 1 Raw material ratios of the embodiments, comparative examples and reference examples

[0117]

[0118] Table 2 Test results of embodiments, comparative examples and reference examples

[0119]

[0120] It can be seen from the test data in Table 2 that the DCR and low-temperature discharge of the non-aqueous electrolyte containing the sulfone-containing pyrrole compound are improved. Specifically, by comparing Examples 1 to 4 with Comparative Example 1 and by comparing Examples 5 to 7 with Comparative Example 2, it can be seen that the introduction of the sulfone-containing pyrrole compound can effectively reduce the DCR of the non-aqueous electrolyte and improve the discharge capacity retention rate at low temperatures.

[0121] In addition, it can be seen from the reference example that although component (A) is used, its amount has exceeded 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.

[0122] 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.

[0123] 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 compound having a sulfone group and an azoaromatic group as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C); Wherein, the structure of the compound containing sulfone group and nitrogen heteroaromatic group in component (A) is shown in formula (I): Wherein, the groups X and Y represent the same or different aromatic groups, and one or both of them are selected from the group containing nitrogen heteroaromatic groups, wherein the nitrogen heteroaromatic groups have one or more nitrogen atoms on the aromatic ring, and the at least one nitrogen atom is directly bonded to the S atom in formula (I).

2. The non-aqueous electrolyte according to claim 1, characterized in that The content of the compound containing a sulfone group and an azaaromatic group as 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 In the formula (I), the groups X and Y are the same or different, both of which are groups containing nitrogen heteroaromatic groups, and in both of them, there is at least one nitrogen atom on the aromatic ring directly bonded to the S atom in the formula (I).

4. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that The azaaromatic group has 1 or 2 nitrogen atoms on the aromatic ring, and the aromatic ring has 5 to 10 atoms.

5. The non-aqueous electrolyte according to any one of claims 1 to 4, characterized in that At least one of the group X or the group Y is selected from a group having a structure as shown in the following formula (a) or formula (b): Wherein, in the formula (a), R1 is selected from a group of alkyl, alkoxy or halogen atoms substituted or unsubstituted by a substituent, and x represents an integer of 0 to 2; In the formula (b), the arc represents a 5- or 6-membered aromatic ring that shares two carbon atoms with the pyrrole ring, and the aromatic ring may optionally have a substituent. * indicates the part connected to the S; Preferably, the substituent in R1 is selected from at least one of an alkoxy group and a halogen-containing group.

6. The non-aqueous electrolyte according to any one of claims 1 to 5, characterized in that The group X and the group Y are both selected from the group having a structure as shown in formula (a) or formula (b); or, one of the group X and the group Y is selected from the group having a structure as shown in formula (a) or formula (b), and the other is selected from a phenyl group, and the phenyl group optionally has a substituent; Preferably, the substituent of the phenyl group is selected from at least one of an alkoxy group and a halogen-containing group.

7. The non-aqueous electrolyte according to any one of claims 1 to 6, 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.

8. The non-aqueous electrolyte according to any one of claims 1 to 7, characterized in that The concentration of the lithium salt in the non-aqueous electrolyte is 0.2 mol / L to 10 mol / L.

9. 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 8.

10. 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 8.

11. The lithium secondary battery according to claim 9 or 10, characterized in that: The positive electrode material of the lithium secondary battery is lithium iron phosphate.

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