Electrolyte and secondary battery

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to an electrolyte and a secondary battery, the electrolyte comprises a low-boiling-point organic solvent, a high-boiling-point organic solvent, a lithium salt and an additive, and the additive comprises a first additive with a structure as shown in a formula I and a second additive with a structure as shown in a formula II; wherein R1 is independently selected from one or a combination of more of alkyl, alkoxy, alkenyl, alkenyloxy, alkynyl, alkynyloxy, halogen, aryl, hydrogen, silyl and siloxane; wherein R2 is independently selected from one or a combination of more of alkyl, alkenyl, alkynyl, hydrogen, nitrile group, ether group, halogen group and aryl. The electrolyte suitable for the extreme temperature environment is obtained by increasing the proportion of the high-boiling-point solvent and adding the first additive and the second additive into the electrolyte. Wherein the high-temperature performance of the battery is improved by increasing the proportion of the high-boiling-point solvent, the cycle performance of the whole temperature section is improved by the first additive, and the low-temperature performance of the battery is improved by the second additive.

Description

Technical Field

[0001] The present invention relates to the field of secondary batteries, and in particular to an electrolyte and a secondary battery. Background Art

[0002] Traditional lithium batteries usually have a discharge temperature range of -20°C to 60°C. However, for some outdoor applications, such as outdoor cameras, outdoor doorbells, vehicle power supplies, drones, etc., the temperature may be between -40°C and 80°C. Ordinary lithium-ion batteries cannot work under extremely high and low temperature conditions, which limits the applicability of the product and cannot bring a better user experience. At present, there is no better solution to achieve such a wide operating temperature range without the help of a temperature control system and relying solely on the battery's own capabilities.

[0003] Lithium-ion batteries have been widely used in mobile devices, electric vehicles, and energy storage systems due to their high energy density and long cycle life. However, the performance of lithium-ion batteries is greatly affected by the ambient temperature, especially under extreme temperature conditions (below 0°C or above 40°C), the battery performance will decay rapidly, and even safety accidents may occur. This limits the application of lithium-ion batteries in high-cold or high-temperature areas. Under low temperature conditions, the viscosity of the electrolyte inside the battery increases, and ion migration and electrode wettability decrease, resulting in a decrease in ratio performance; under high temperature conditions, the electrolyte is prone to chemical changes and reacts with charged electrodes, affecting the safety and stability of the battery.

[0004] Therefore, it is necessary to develop an electrolyte suitable for use in extreme temperature environments. Summary of the invention

[0005] The purpose of the present invention is to provide an electrolyte suitable for use in extreme temperature environments in view of the deficiencies of the prior art.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme: an electrolyte, comprising a high boiling point organic solvent, a low boiling point organic solvent, a lithium salt and an additive, wherein the additive comprises a first additive having a structure shown in formula I and a second additive having a structure shown in formula II;

[0007]

[0008] Wherein, R1 is independently selected from one or a combination of linear or cyclic alkyl or alkoxy, linear or cyclic alkenyl or alkenyloxy, linear or cyclic alkynyl, alkynyloxy, halogen, aryl, hydrogen, silane, siloxane;

[0009] Wherein, R2 is independently selected from one or a combination of alkyl, alkenyl, alkynyl, hydrogen, nitrile, ether, halogen, and aryl.

[0010] Preferably, the first additive accounts for W1 of the total mass of the electrolyte, the second additive accounts for W2 of the total mass of the electrolyte, and the mass of the high boiling point organic solvent is W3, wherein W1, W2 and W3 satisfy the relationship: 13≤W3 / (W1+W2)≤40.

[0011] Preferably, the first additive accounts for 1-5wt% of the total mass of the electrolyte, and the second additive accounts for 1-5wt% of the total mass of the electrolyte.

[0012] Preferably, the organic solvent accounts for 40-80 wt % of the electrolyte.

[0013] Preferably, the organic solvent includes a high boiling point organic solvent and a low boiling point organic solvent.

[0014] Preferably, the high boiling point organic solvent includes one or more of propylene carbonate, ethylene carbonate, ethylene carbonate, propylene carbonate and γ-butyrolactone, and the low boiling point organic solvent includes one or more of diethyl carbonate, ethyl methyl carbonate, ethyl propionate, propyl propionate, ethyl acetate and ethyl butyrate.

[0015] Preferably, the mass of the low boiling point organic solvent is W4, wherein W3 and W4 satisfy the relationship: 1.6≤W3 / W4≤4.

[0016] Preferably, the lithium salt accounts for 0.1-15.0% of the total mass of the electrolyte.

[0017] Preferably, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, bisfluorosulfonyl imide lithium salt and bisfluorosulfonyl imide lithium.

[0018] In addition, the present invention also provides a secondary battery, comprising a battery core wound with a negative electrode sheet, a positive electrode sheet and a separator, an electrolyte, and a shell encapsulating the battery core and the electrolyte, wherein the electrolyte is the above-mentioned electrolyte.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The present invention obtains an electrolyte suitable for use in extreme temperature environments by adding a first additive and a second additive to the electrolyte. The sulfur-oxygen bond reaction in the first additive participates in the negative electrode film formation, making the negative electrode graphite more stable, improving the cycle performance of the entire temperature range, improving the first effect, inhibiting expansion and irreversible capacity loss, and reducing internal resistance. The carbon-nitrogen bond in the second additive reacts during formation to form a protective film on the positive electrode surface, which protects the positive electrode. The second additive has a high dielectric constant, low viscosity, and good dissociation characteristics, which effectively improves the low-temperature performance of the battery.

[0021] (2) The present application obtains an electrolyte suitable for use in extreme temperature environments by controlling the ratio of low-boiling-point solvent to high-boiling-point solvent in the organic solvent while adding the first additive and the second additive to the electrolyte. By setting the ratio of high-boiling-point solvent to low-boiling-point solvent in the organic solvent, an electrolyte that remains stable at high temperatures is obtained, but the low-temperature performance is affected at this time. Therefore, the second additive is added to expand the discharge temperature to -40°C while maintaining the high-temperature performance. The addition of the first additive ensures the cycle performance in the entire temperature range. DETAILED DESCRIPTION

[0022] In order to make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] According to a first aspect of the present application, the present application provides an electrolyte, including an organic solvent, a lithium salt and an additive, wherein the additive includes a first additive having a structure shown in Formula I and a second additive having a structure shown in Formula II;

[0024]

[0025] Wherein, R1 is independently selected from one or a combination of linear or cyclic alkyl or alkoxy, linear or cyclic alkenyl or alkenyloxy, linear or cyclic alkynyl, alkynyloxy, halogen, aryl, hydrogen, silane, siloxane;

[0026] Wherein, R2 is independently selected from one or a combination of alkyl, alkenyl, alkynyl, hydrogen, nitrile, ether, halogen, and aryl.

[0027] Among them, the sulfur-oxygen bond reaction in the first additive participates in the negative electrode film formation, making the negative electrode graphite more stable, improving the cycle performance in the entire temperature range, improving the first efficiency, inhibiting expansion and irreversible capacity loss, and reducing internal resistance; the carbon-nitrogen bond in the second additive reacts during formation to form a protective film on the positive electrode surface, which protects the positive electrode, and its high dielectric constant, low viscosity and good dissociation characteristics effectively improve the low temperature performance of the battery.

[0028] In some embodiments, the first additive can be selected from one or more of vinyl sulfate, fluoroethylene carbonate, vinyl carbonate, methyl vinyl sulfate, fluoroethylene sulfate, n-propyl vinyl sulfate, disulfate vinyl, acetylene sulfate, glyoxal disulfate, glycerol sulfate, 4-acetylene sulfate; the second additive can be selected from one or more of ethanedinitrile, succinonitrile, 1,3,6-hexanetrinitrile, 1,3,5-pentanetrinitrile, p-fluorobenzonitrile, p-methylbenzonitrile, bis(cyanoethyl)sulfone, trans-ethylene dinitrile, and crotononitrile.

[0029] In some embodiments, the first additive accounts for W1 of the total mass of the electrolyte, the second additive accounts for W2 of the total mass of the electrolyte, and the mass of the high boiling point organic solvent is W3, wherein W1, W2 and W3 satisfy the relationship: 13≤W3 / (W1+W2)≤40, for example, it can be 13, 15, 20, 25, 30, 35 or 40. Controlling the numerical value of the relationship within a preset range can improve the performance of the battery in extreme temperature environments. When the value is less than the preset range, the electrolyte conductivity will decrease and the battery performance will be affected. When the value is greater than the preset range, the low temperature performance of the battery will be affected, and the cycle performance of the entire temperature range will decrease.

[0030] In some embodiments, the first additive accounts for 1-5wt% of the total mass of the electrolyte, for example, it can be 1wt%, 2wt%, 3wt%, 4wt% or 5wt%; the second additive accounts for 1-5wt% of the total mass of the electrolyte, for example, it can be 1wt%, 2wt%, 3wt%, 4wt% or 5wt%.

[0031] In some embodiments, the organic solvent accounts for 40-80 wt % of the electrolyte, for example, 40 wt %, 50 wt %, 60 wt %, 70 wt % or 80 wt %.

[0032] In some embodiments, the organic solvent includes a high boiling point organic solvent and a low boiling point organic solvent.

[0033] In some embodiments, the high boiling point organic solvent includes one or more of propylene carbonate, ethylene carbonate, ethylene carbonate, propylene carbonate and γ-butyrolactone; the low boiling point organic solvent includes one or more of diethyl carbonate, ethyl methyl carbonate, ethyl propionate, propyl propionate, ethyl acetate and ethyl butyrate.

[0034] In some embodiments, the mass of the low-boiling point organic solvent is W5, wherein W3 and W4 satisfy the relationship: 1.6≤W3 / W4≤4, for example, it can be 1.6, 1.8, 2, 2.5, 3, 3.5 or 4. By controlling the numerical value within the preset range, it is possible to improve the stability of the electrolyte at high temperatures and inhibit solvent gas production. When the numerical value is less than the preset range, the low-boiling point solvent of the electrolyte will produce gas at high temperatures, and the battery performance will be seriously reduced. When the numerical value is greater than the preset range, the viscosity of the electrolyte will be too high, the conductivity will be reduced, and the performance of the electrolyte will be seriously reduced.

[0035] In some embodiments, the lithium salt accounts for 0.1-15.0% of the total mass of the electrolyte, for example, 0.1%, 1%, 5%, 10% or 15%.

[0036] In some embodiments, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, bisfluorosulfonyl imide lithium salt, and bisfluorosulfonyl imide lithium.

[0037] According to the second aspect of the present application, the present application provides a secondary battery, including a battery cell formed by winding a negative electrode sheet, a positive electrode sheet and a separator, an electrolyte, and a shell that encapsulates the battery cell and the electrolyte, wherein the electrolyte is the above-mentioned electrolyte.

[0038] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include but is not limited to a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive active material may also be modified. The method of modifying the positive active material should be known to those skilled in the art. For example, the positive active material may be modified by coating, doping, etc. The material used for the modification may include but is not limited to a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive current collector is usually a structure or part for collecting current. The positive current collector may be various materials suitable for use as a positive current collector for lithium-ion batteries in the art. For example, the positive current collector may include but is not limited to metal foil, etc., and more specifically may include but is not limited to aluminum foil, etc.

[0039] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include but is not limited to graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate or other metals that can form alloys with lithium. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is usually a structure or part that collects current. The negative electrode current collector can be various materials in the art that are suitable for use as a negative electrode current collector for lithium-ion batteries. For example, the negative electrode current collector may include but is not limited to metal foil, and more specifically may include but is not limited to copper foil.

[0040] Example 1

[0041] Preparation of electrolyte:

[0042] S1. In a glove box filled with argon, EC, PC, DEC and EMC were mixed uniformly according to the following components and mass ratios to obtain an organic solution: ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC): ethyl methyl carbonate (EMC) = 4:2:1:1;

[0043] S2, adding lithium salt LiPF6 to the organic solution, wherein the lithium salt accounts for 13.2% of the total mass of the electrolyte, and stirring until it is completely dissolved to obtain a mixed solution;

[0044] S3. Based on the mass of the electrolyte, 1% of the first additive (ADN) and 1% of the second additive (DTD) are added to the mixed solution, and the mixture is evenly mixed to obtain an electrolyte.

[0045] Preparation of positive electrode:

[0046] 97.4% lithium nickel cobalt manganese oxide, 0.8% conductive carbon, 0.8% carbon nanotubes, and 1.0% polyvinylidene fluoride are evenly dispersed in N-methylpyrrolidone by a double planetary mixer, and evenly coated on a 9μm aluminum foil by an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, the positive electrode sheet is obtained.

[0047] Preparation of negative electrode:

[0048] 97.7% graphite, 1.2% polystyrene butadiene copolymer, and 1.1% lithium carboxymethyl cellulose are evenly dispersed in deionized water by a double planetary mixer, and evenly coated on a 5μm copper foil by an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, a negative electrode sheet is obtained.

[0049] Preparation of secondary batteries:

[0050] The positive electrode sheet and negative electrode sheet prepared in the above steps and the separator of a certain width are welded to the pole ears, glued, wound, cut, hot pressed, etc. on a winding machine to obtain a bare battery cell, and the aluminum-plastic film of suitable size is punched, and the bare battery cell is encapsulated in the punched aluminum-plastic film. The encapsulated battery cell is injected with the electrolyte prepared in the above steps and pre-sealed, and then hot-pressed after standing at high temperature and room temperature for a certain period of time, and then degassing and re-sealing are performed.

[0051] Among them, the preparation steps of Examples 2-11 and Comparative Examples 1-7 are the same as those of the Examples, except that the parameters in the electrolyte are different. In Example 11, the first additive is glyoxal disulfate, and the second additive is 1,3,5-pentanetrinitrile. The other specific parameters are shown in Table 1 below.

[0052] Table 1

[0053] Group W1 / wt% W2 / wt% W3 / wt% W3 / (W1+W2) W4 / wt% W3 / W4 Example 1 1 1 60 30 20 3.0 Example 2 2 2 60 15 20 3.0 Example 3 1 2 60 20 20 3.0 Example 4 2 3 75 15 20 3.8 Example 5 2 1 60 20 20 3.0 Example 6 1 1 60 30 20 3.0 Example 7 1 1 70 35 20 3.5 Example 8 1 1 40 20 25 1.6 Example 9 1 1 70 35 20 3.5 Example 10 1 1 78 39 20 3.9 Embodiment 11 1 1 60 30 20 3.0 Comparative Example 1 / / 60 / 20 3.0 Comparative Example 2 1 / 60 / 20 3.0 Comparative Example 3 / 1 60 / 20 3.0 Comparative Example 4 1 1 20 10 20 1.0 Comparative Example 5 0.5 0.5 60 60 20 3.0 Comparative Example 6 1 1 50 25 50 1.0 Comparative Example 7 1 1 80 40 10 8.0

[0054] The following performance tests were performed on the batteries prepared in the examples and comparative examples:

[0055] (1) High temperature performance test:

[0056] High temperature cycle retention rate: Charge at 0.5C at 60°C and discharge at 0.5C at 84°C. After 100 cycles, the cycle retention rate is measured.

[0057] High-temperature storage expansion rate: 100% SOC state is stored at 80°C for 1008h without gas generation, and the thickness of the battery after the reaction is measured. High-temperature storage expansion rate = (battery thickness after reaction - battery thickness before reaction) * 100%.

[0058] (2) Low temperature performance test:

[0059] Low temperature cycle retention rate: The battery is placed at 0°C and charged at 0.06C and discharged at 1.0C. After 100 cycles, the cycle retention rate is obtained.

[0060] Low temperature storage retention rate: After the battery is stored at -40°C for 1008h, the low temperature storage retention rate is measured.

[0061] Among them, the above performance test results are shown in Table 2 below.

[0062] Table 2

[0063]

[0064]

[0065] From the test data of Examples 1-11 and Comparative Examples 1-7 in Table 2, it can be seen that the present application obtains an electrolyte suitable for use in extreme temperature environments by adding the first additive and the second additive to the electrolyte while controlling the ratio of the low-boiling point solvent to the high-boiling point solvent in the organic solvent.

[0066] By comparing the data of Example 1 with Comparative Examples 1-3, it can be seen that when the first additive and the second additive are not added, both the high-temperature and low-temperature cycles will result in NG; when only the first additive is added without the second additive, the low-temperature cycle will result in NG; when only the second additive is added without the first additive, the high-temperature cycle will result in NG.

[0067] From the data comparison of Examples 1-2, 6-11 and Comparative Examples 4-5, it can be seen that when the value of the W3 / (W1+W2) relationship is less than the preset range, both the high-temperature cycle and the high-temperature storage will be NG; when the value of the W3 / (W1+W2) relationship is greater than the preset range, the high-temperature storage will be NG;

[0068] From the data comparison of Examples 1-2, 6-11 and Comparative Examples 6-7, it can be seen that when the content ratio of each organic solvent is less than the preset range, both the high-temperature cycle and the high-temperature storage will be NG; when the content ratio of each organic solvent is greater than the preset range, the high-temperature cycle will be NG;

[0069] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. An electrolyte, characterized in that: It includes a high boiling point organic solvent, a low boiling point organic solvent, a lithium salt and an additive, wherein the additive includes a first additive with a structure shown in formula I and a second additive with a structure shown in formula II; Wherein, R1 is independently selected from one or a combination of alkyl, alkoxy, alkenyl, alkenyloxy, alkynyl, alkynyloxy, halogen, aryl, hydrogen, silane, siloxane; Wherein, R2 is independently selected from one or a combination of alkyl, alkenyl, alkynyl, hydrogen, nitrile, ether, halogen, and aryl.

2. An electrolyte according to claim 1, characterized in that: The first additive accounts for W1 of the total mass of the electrolyte, the second additive accounts for W2 of the total mass of the electrolyte, and the mass of the high boiling point organic solvent is W3, wherein W1, W2 and W3 satisfy the relationship: 13≤W3 / (W1+W2)≤40.

3. An electrolyte according to claim 1, characterized in that: The first additive accounts for 1-5 wt % of the total mass of the electrolyte, and the second additive accounts for 1-5 wt % of the total mass of the electrolyte.

4. An electrolyte according to claim 1, characterized in that: The organic solvent accounts for 40-80 wt % of the electrolyte.

5. An electrolyte according to claim 4, characterized in that: The organic solvent includes a high boiling point organic solvent and a low boiling point organic solvent.

6. An electrolyte according to claim 5, characterized in that: The high boiling point organic solvent includes one or more of propylene carbonate, ethylene carbonate, ethylene carbonate, propylene carbonate and γ-butyrolactone; the low boiling point organic solvent includes one or more of diethyl carbonate, ethyl methyl carbonate, ethyl propionate, propyl propionate, ethyl acetate and ethyl n-butyrate.

7. An electrolyte according to claim 5, characterized in that: The mass of the low boiling point organic solvent is W4, wherein W3 and W4 satisfy the relationship: 1.6≤W3 / W4≤4.

8. An electrolyte according to claim 1, characterized in that: The lithium salt accounts for 0.1-15.0% of the total mass of the electrolyte.

9. An electrolyte according to claim 1, characterized in that: The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, bisfluorosulfonyl imide lithium salt and bisfluorosulfonyl imide lithium.

10. A secondary battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 9.

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