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

Abstract

The present application provides a kind of non-aqueous electrolyte, diaphragm-free lithium secondary battery and lithium secondary battery, the non-aqueous electrolyte includes: as component (A) the nitro-containing sulfuric ester compound, as component (B) non-aqueous solvent and as component (C) lithium salt;Wherein, the structural formula of the component (A) nitro-containing sulfuric ester compound is as shown in formula (1):Wherein, P1 and P2 are independent or mutually linked;P1 indicates aromatic group with at least one aromatic nitro group, P2 indicates monovalent organic group, and the P2 is same with or different from P1;Or, P2 indicates the connecting site linked with P1, the component (A) is dissolved in the component (B).The non-aqueous electrolyte can form composite SEI film, can reduce impedance, and significantly improve the fast charging performance (more than 6C) and low temperature discharge performance of battery.

Description

TECHNICAL FIELD

[0001] The present application 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, in particular to a fast-charging type non-aqueous electrolyte for lithium batteries and a diaphragm-free lithium secondary battery and a lithium secondary battery comprising the electrolyte. BACKGROUND

[0002] Electric vehicles have gradually begun to popularize in recent years, however, the lower charging rate compared with traditional fuel vehicles is still one of the key factors affecting the user experience. In order to achieve the energy supplement speed comparable to fuel vehicles, it is required that the battery completes the charging process within 10 minutes or even shorter time, which requires a charging rate of about 6C, 8C or even 10C. Such a high charging rate puts higher requirements on the materials and interface of lithium ion batteries.

[0003] The fast-charging performance of lithium ion batteries can be essentially improved by optimizing the electrode electrolyte material and cell design of the ion battery, and many research works show that the electrolyte is one of the main bottlenecks restricting the fast-charging performance of the battery.

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

[0005] Some documents disclose an organic azide compound, which has excellent negative electrode film-forming properties and can significantly improve the fast-charging and rate discharge performance of the battery. However, the azide group is a group with high reactivity, which is easy to oxidize and decompose under the condition of transition metal existing in the positive electrode of the lithium battery, so as to generate gas, thereby failing to effectively form a high-conductivity SEI film rich in lithium nitride.

[0006] In addition, for lithium ion batteries, a diaphragm can be used or not used in the structure thereof. The diaphragm material is not conductive, and its physical and chemical properties have a great influence on the performance of the battery. For the case of using a diaphragm, the main role of the diaphragm is to separate the positive and negative electrodes of the battery, prevent the two electrodes from contacting and short-circuiting, and also has the function of allowing electrolyte ions to pass. For 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 ion conduction is also performed by the electrolyte itself. From the perspective of the energy density of the lithium secondary battery, the diaphragm-free secondary battery is advantageous. In particular, the use of solid-state electrolytes (such as quasi-solid or all-solid-state electrolytes) makes it possible to have a diaphragm-free battery.

[0007] It is evident that although some research has been conducted in this field on electrolytes for improving battery fast-charging performance, the research is not yet sufficient and there is still room for further improvement. Summary of the Invention

[0008] Problem to be solved by the invention

[0009] As mentioned above, the electrolyte has a significant impact on the fast-charging performance of lithium-ion batteries. The requirements for the electrolyte also vary at different charge and discharge rates. In order to meet charging rates of approximately 6C, 8C, or even 10C, there is an urgent need for an electrolyte that can form a low-impedance SEI film and has high ionic conductivity in the battery.

[0010] Existing technologies include literature that discloses the use of lithium nitrate or organic azide compounds as electrolyte additives to form lithium nitride on the negative electrode surface, thereby improving the fast charging and rate discharge performance of the battery.

[0011] However, this invention argues that lithium nitrate, as an inorganic additive, has poor compatibility with organic electrolytes and limited solubility in conventional non-aqueous solvents. Furthermore, organic azide compounds, with their highly reactive azide groups, exhibit chemical instability as previously mentioned, and further practical applications have shown that their ability to suppress cycle degradation becomes insignificant after a period of use. Moreover, these problems are more pronounced at high charge rates.

[0012] To address the aforementioned problems, this invention provides a non-aqueous electrolyte containing a nitro-containing sulfate ester compound. This compound exhibits excellent antioxidant properties and can form a composite SEI film containing both lithium nitride and sulfur-containing organic components on the negative electrode surface. This composite SEI film possesses high lithium-ion conductivity, reduces impedance, and significantly improves the battery's fast-charging performance (charging rate above 6C) and low-temperature discharge performance.

[0013] Furthermore, the present invention also provides a lithium secondary battery, wherein the lithium secondary battery comprises the above-mentioned non-aqueous electrolyte.

[0014] Solution to the problem

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

[0016] The present invention first provides a non-aqueous electrolyte, wherein the non-aqueous electrolyte comprises: a nitro-containing sulfate ester compound as component (A), a non-aqueous solvent as component (B), and a lithium salt as component (C);

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

[0018]

[0019] Among them, P1 and P2 are either independent of each other or interconnected;

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

[0021] P2 represents a monovalent organic group, and P2 may be the same as or different from P1; or, P2 represents a linking site to P1.

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

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

[0024] According to the non-aqueous electrolyte described above, the component (A) contains a nitro-containing sulfate ester compound with the structural formula shown in formula (1a) or formula (1b):

[0025]

[0026] in,

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

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

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

[0030] According to the non-aqueous electrolyte described above, the other substituents are selected from one or more groups containing halogen atoms, alkoxy groups, or alkyl groups.

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

[0032]

[0033] Wherein, R1 represents a substituent group on the 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 formulas (1b-1a), (1b-1b), (1b-1c), and (1b-1d):

[0036]

[0037]

[0038] According to the above-described non-aqueous electrolyte, 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.

[0039] According to the non-aqueous electrolyte described above, 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-described 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 membrane-free lithium secondary battery, wherein the membrane-free lithium secondary battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte as described above.

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

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

[0044] i. The lithium secondary battery retains more than 70% of its discharge capacity after being cycled 200 times at a constant rate of 6C at 25°C.

[0045] ii. After being stored at -20°C for 4 hours, the lithium secondary battery retains more than 75% of its discharge capacity when discharged at a rate of 0.33C.

[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 with a lithium nitride inorganic layer having high lithium ion conductivity and an oligomeric organic layer of sulfate containing aromatic groups. This 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 this composite SEI film can give the film excellent elongation, making it less susceptible to damage during repeated charge and discharge cycles that cause the film to expand and contract, thereby improving the cycle performance of the cell. It is especially suitable for lithium secondary batteries with silicon as the negative electrode material. Detailed Implementation

[0050] The present invention will now be described in detail. The descriptions of the technical features described below are 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 range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0052] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0053] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0054] In this specification, the terms "optional" or "optional" are used to indicate the use or omission of certain substances, components, procedures, application conditions, etc.

[0055] In this instruction manual, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃".

[0056] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" used refers to weight or mass percentage content.

[0057] In this specification, the terms "substantially" and "essentially" are used to indicate that the standard deviation from the theoretical model, theoretical data, or target data is within a range of 1%, preferably 0.8%, and more preferably 0.7%.

[0058] In this specification, the terms “comprising” and / or “including” are used to indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

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

[0060] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

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

[0062] This invention is mainly derived from the following insights:

[0063] Given the ever-growing demand for fast charging, relying solely on lithium salts to form an SEI film is becoming increasingly insufficient to meet usage requirements, especially when the charging rate reaches 6C or higher, or even 10C or 16C or higher.

[0064] This invention suggests that introducing a stabilizing component with low impedance could help solve the aforementioned problems in forming the SEI film. Further research by the inventors revealed that when the non-aqueous electrolyte contains the nitro-containing sulfate compound described in this invention, the compound can undergo a reduction reaction with lithium ions at the negative electrode to generate a composite SEI film consisting of a lithium nitride inorganic layer with high lithium-ion conductivity and an oligomeric organic layer containing phenyl sulfate. This composite SEI film, due to the introduction of the organic layer, not only exhibits better stability but also low impedance characteristics, thereby significantly improving the battery's fast-charging and low-temperature discharge performance. Furthermore, the organic components in this composite SEI film give it excellent extensibility, making it less susceptible to damage during repeated charge-discharge expansion and contraction, thus improving the cell's cycle performance.

[0065] <First Aspect>

[0066] A first aspect of the present invention provides a non-aqueous electrolyte comprising a nitro-containing sulfate 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. In particular, the electrolyte of the present invention can provide lithium secondary batteries with good stability, cycle performance and low-temperature discharge characteristics under higher charging rate conditions.

[0068] Furthermore, without limitation, various optional functional additives may be used in the non-aqueous electrolyte, provided that they do not impede the realization of the technical effects of the present invention.

[0069] Component (A)

[0070] The component (A) described in this invention is a nitro-containing sulfate compound that is soluble in the non-aqueous solvent (B) described below within the electrolyte preparation and electrolyte operating temperature range.

[0071] Furthermore, component (A) has the 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 can be an aromatic group with 4 to 6 aromatic ring atoms. In addition, one or two atoms on these aromatic rings can be heteroatoms, such as oxygen atoms, sulfur atoms, or nitrogen atoms. In a further preferred embodiment of the invention, the aromatic group can be a phenyl group.

[0075] For aromatic nitro groups, it refers to nitro groups that are directly bonded to aromatic rings. In this invention, there may be more than one such nitro group in the P1 structure, preferably one or two.

[0076] Furthermore, for P1, the aromatic group may have other substituents besides a nitro group. In some specific embodiments, such substituents 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 with 1 to 5 carbon atoms, or a halogenated or non-halogenated alkyl group with 1 to 5 carbon atoms.

[0077] As for P2, as a monovalent organic group, there is no particular limitation in principle, and P2 may be the same as or different from P1; or, P2 may represent a linking site to P1.

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

[0079] Furthermore, 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 with 4 to 6 aromatic ring atoms, and the aromatic ring has no more than two heteroatoms; and in formula (1a), P2 represents a substituted or unsubstituted aromatic group or alkyl group.

[0083] Furthermore, in some more preferred embodiments of the present invention, the nitro-containing sulfate ester 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 independently represents a substituent group of the benzene ring, 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 the other R1, it may be selected from one or more groups containing halogen atoms, alkoxy groups, or alkyl groups, preferably halogen atoms, alkoxy groups with 1 to 5 carbon atoms, or halogenated or non-halogenated alkyl groups with 1 to 5 carbon atoms.

[0087] The alkyl group described above is preferably methyl, ethyl, or n-propyl, and optionally substituted with a halogen atom. The halogen atom described above is preferably an F atom.

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

[0089]

[0090]

[0091] This invention introduces the nitro-containing sulfate ester compound into the electrolyte to form a composite SEI film with high lithium-ion conductivity at the negative electrode, which simultaneously contains lithium nitride and oligomers containing phenyl groups and sulfate esters. This 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 non-aqueous solvent for 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 may be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0095] The cyclic carbonate solvents can be selected from ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), fluoroethylene carbonate (FEC), etc.; the linear carbonate solvents can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC), bis(2,2,2-trifluoroethyl) carbonate (DFDEC), etc.; the ester solvents can be selected from methyl acetate, ethyl acetate, methyl propionate, methyl pentanoate, etc.; the ether solvents 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), dioxane (DOL), etc.; the ketone solvents can be selected from polymethyl vinyl ketone, etc. These non-aqueous solvents can be used alone or in mixtures of two or more.

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

[0097] Component (C)

[0098] The present invention does not particularly limit the type of lithium salt used in component (D), and it can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt may 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 one or more combinations of lithium hexafluorophosphate (LiPF6) or lithium bisfluorosulfonylimide (LiFSI).

[0100] Composition of the nonaqueous electrolyte

[0101] In this invention, regarding the content of the nitro-containing sulfate ester compound in component (A), in some specific embodiments of the invention, from the perspective of controlling membrane thickness and internal resistance and improving fast-charging performance, the content of the nitro-containing sulfate ester compound in 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% by mass, 1% by mass, 1.5% by mass, 2% by mass, 2.5% by mass, 5% by mass, 10% by mass, 15% by mass, 18% by mass, 22% by mass, etc. When the content of component (A) is too high, on the one hand, it will increase the cost of the electrolyte, and on the other hand, it will cause the SEI membrane to thicken significantly, which is not conducive to maintaining a low internal resistance; when the content of component (A) is too low, it cannot be guaranteed that the electrolyte has sufficient ionic conductivity, thus affecting the cell performance.

[0102] There are no particular limitations in principle regarding the lithium salt of component (C). In some specific embodiments of the present invention, the concentration of the lithium salt in the non-aqueous electrolyte can be 0.2 mol / L to 10 mol / L, preferably 0.7 mol / L to 2 mol / L, for example, 0.5 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, there are concerns that it will lead to an increase in the viscosity and cost of the electrolyte; when the concentration of the lithium salt is too low, there are concerns that the electrolyte may not have sufficient ionic conductivity, thus 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 non-aqueous 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 both the current collector and the positive electrode active material. There are no particular limitations on the positive electrode active material in principle; in some preferred embodiments, various lithium-containing oxides in the art can be used, in which other main group, subgroup, or rare earth metal elements may be added besides lithium.

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

[0108] In some preferred embodiments, the active material of the cathode 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. More preferably, the active material of the cathode can be ternary NCM, ternary NCA, or lithium iron manganese phosphate, for example, high-nickel-content ternary cathode (6-series, 8-series, NCA) materials.

[0109] There are no particular restrictions on the negative electrode of the battery in principle. It can be any negative electrode commonly used in the field, including a current collector and a negative electrode active material. Typically, the negative electrode active material can be a carbon material, a silicon material, or a mixture thereof.

[0110] There are no particular restrictions on the use of a separator in the battery; it may or may not be used. That is, the lithium secondary battery of the present invention can be a lithium secondary battery with a separator or a lithium secondary battery without a separator. When using a separator, it can be a separator commonly used in the art; preferably, a separator with high moisture retention capacity for the electrolyte solution and low resistance to electrolyte ion transfer can be used. In the case of a lithium secondary battery without a separator, the non-aqueous electrolyte of the present invention can be used in combination with a solid electrolyte, wherein the solid electrolyte can act as a separator.

[0111] Example

[0112] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0113] Example 1

[0114] (1) Electrolyte preparation method:

[0115] A non-aqueous electrolyte was prepared in an argon-protected glove box with a moisture content of less than 1 ppm. The solvents EC and EMC were mixed in a volume ratio of 1:2.5. Then, lithium salt (1 mol / L) and the compound shown in formula (1-1) were added and stirred until homogeneous to obtain the desired electrolyte. The specific composition formula is shown in Table 1.

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

[0117] (i) Preparation of ternary material cathode:

[0118] The positive electrode active material Li(Ni) was mixed in a mass ratio of 96:2:1:1. 0.8 Co 0.1 Mn 0.1 O2, conductive ultrafine carbon powder (SP), single-walled carbon nanotubes (SWNT), and binder polyvinylidene fluoride (PVDF) were added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent and stirred thoroughly until the solid content was controlled to 68%, thus obtaining a positive electrode mixture slurry. Subsequently, the formed positive electrode slurry was coated onto aluminum foil, and after drying, it was roll-cut to form a ternary material positive electrode sheet.

[0119] (ii) Anode preparation:

[0120] Negative electrode preparation: Graphite, conductive ultrafine carbon powder (SP), thickener CMC, and binder SBR are added to an appropriate amount of deionized water at a mass ratio of 95:2:1:2 and stirred thoroughly. The solid content is controlled to 55% to obtain a negative electrode mixture slurry. Subsequently, the formed negative electrode slurry is coated onto copper foil, and after drying, it is roll-cut to form a negative electrode sheet.

[0121] (iii) Battery assembly:

[0122] Take the positive electrode, negative electrode and separator, stack them in the order of negative electrode, separator and positive electrode, then weld the tabs and encapsulate them with aluminum-plastic film to obtain a soft-pack dry cell. Finally, inject the non-aqueous electrolyte prepared above into the cell to prepare a lithium battery with a capacity of 5Ah.

[0123] Examples 2 to 6

[0124] Using the preparation method in Example 1 and the raw material ratios in Table 1, lithium-ion batteries of Examples 2 to 6 were prepared.

[0125] The preparation method of lithium iron phosphate cathode is as follows:

[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 at a mass ratio of 94.5:2.5:2:1 and stirred thoroughly. The solid content was controlled to 65% to obtain a positive electrode mixture slurry. Subsequently, the formed positive electrode slurry was coated onto aluminum foil, and after drying, it was rolled and die-cut to form a lithium iron phosphate positive electrode sheet.

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

[0128] Using the preparation method in Example 1 and the raw material ratios in Table 1, lithium-ion batteries of Comparative Examples 1-2 and Reference Example 1 were prepared.

[0129] Performance test

[0130] 1. Multiplier Cycle:

[0131] At 25°C, the encapsulated lithium-ion battery was subjected to a 6C constant rate cycle test after formation and capacity testing. It was charged at 6C constant current and constant voltage to 4.25V, and after resting for half an hour, it was discharged at 6C constant current. The initial capacity of the battery in the first cycle was obtained and recorded as C1. Thereafter, the charging and discharging cycle was repeated in the manner of CCCV / CC of the first cycle. The capacity after the 200th cycle was recorded as C200. Then, the capacity retention rate (%) after 200 cycles of high rate at room temperature = C200 / C1×100%.

[0132] 2. DCR Test:

[0133] Before or after rate cycling, the battery cell is charged at 25°C with a constant current of 0.5C to 4.25V, then charged with a constant voltage to a current of 0.05C. After resting for 1 hour, it is discharged at 0.5C for 60 minutes (50% SOC), and then rested for another hour. Next, it is discharged at 0.1C for 10 seconds, and the voltage V1 at the end is recorded. Then, it is discharged at 1C for 1 second, and the voltage V2 at the end is recorded. The DCR before or after cycling is then calculated as (V1-V2) / (I1C-I0.1C), in mAhm. The growth rate is calculated based on the DCR values ​​before and after cycling: DCR growth rate = (Cycled battery DCR - Cycled battery DCR) / Cycled battery DCR × 100%.

[0134] 3. Low-temperature discharge:

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

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

[0137]

[0138] Table 2 shows the test results of Examples 1-6, Comparative Examples 1-2, and Reference Example 1.

[0139]

[0140] As can be seen from Table 2, the fast-charging performance and low-temperature discharge performance 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 is slower.

[0141] Furthermore, as can be seen from Reference Example 1, although component (A) was used, its amount exceeded that of the auxiliary stabilizing 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 has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0143] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A non-aqueous electrolyte, characterized in that, The non-aqueous electrolyte comprises: 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 component A, a nitro-containing sulfate ester compound, is shown in formula (1a) or (1b) below: (1a) (1b) in, P1 represents an aromatic group having at least one aromatic nitro group and having 4 to 6 aromatic ring atoms, wherein the aromatic ring of the aromatic group has no more than two heteroatoms. Furthermore, in formula (1a), P2 represents a substituted or unsubstituted aromatic group or alkyl group. Component A is dissolved in component B. Furthermore, the content of component A is 0.1% to 25% of the total mass of the non-aqueous electrolyte.

2. The non-aqueous electrolyte according to claim 1, characterized in that, P1 represents a benzene ring having at least one nitro group, and the benzene ring may or may not have other substituents different from the nitro group.

3. The non-aqueous electrolyte according to claim 2, wherein the other substituents are selected from one or more groups containing halogen atoms, alkoxy groups, or alkyl groups.

4. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that, Component A, a nitro-containing sulfate compound, has the structure of formula (1b-1): （1b-1） Wherein, R1 represents a substituent group on the benzene ring, and at least one R1 is -NO2; n represents a positive integer, and 4 ≥ n ≥ 1.

5. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that, The nitro-containing sulfate compound of component A is selected from one or more combinations of compounds represented by formulas (1b-1a), (1b-1b), (1b-1c), and (1b-1d): (1b-1a); (1b-1b); (1b-1c); (1b-1d).

6. The non-aqueous electrolyte according to any one of claims 1 to 3, 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, and ketone solvents.

7. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that, The non-aqueous solvent of component B is selected from ester solvents.

8. The non-aqueous electrolyte according to any one of claims 1 to 3, 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 - .

9. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that, The concentration of the lithium salt in the non-aqueous electrolyte is 0.2 mol / L to 10 mol / L.

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

11. The membraneless lithium secondary battery according to claim 10, characterized in that, The lithium secondary battery meets one or two of the following conditions: i. The lithium secondary battery retains more than 70% of its discharge capacity after being cycled 200 times at a constant rate of 6C at 25°C. ii. After being stored at -20°C for 4 hours, the lithium secondary battery retains more than 75% of its discharge capacity when discharged at a rate of 0.33C.

12. A lithium secondary battery, characterized in that, The lithium secondary battery comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte according to any one of claims 1 to 9.

13. The lithium secondary battery according to claim 12, characterized in that, The lithium secondary battery meets one or two of the following conditions: i. The lithium secondary battery retains more than 70% of its discharge capacity after being cycled 200 times at a constant rate of 6C at 25°C. ii. After being stored at -20°C for 4 hours, the lithium secondary battery retains more than 75% of its discharge capacity when discharged at a rate of 0.33C.

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