Sodium-ion battery electrolyte, and sodium-ion battery using secondary electrolyte injection process and preparation method therefor

Abstract

A sodium-ion battery electrolyte, and a sodium-ion battery using a secondary electrolyte-injection process and a preparation method therefor. The sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte, and the primary electrolyte comprises a first organic solvent, a sodium salt and a first additive; the first organic solvent is propylene carbonate, and the concentration of the sodium salt in the primary electrolyte is 2-6 wt.%; the second electrolyte comprises a second organic solvent, a sodium salt and a second additive; and the content of propylene carbonate in the second organic solvent is not less than 70 wt.%, the concentration of the sodium salt in the secondary electrolyte is 30-60 wt.%, and the second additive contains fluoroethylene carbonate. The primary electrolyte has good wettability to an electrode sheet, and the boiling point of propylene carbonate is relatively high, so that the thermal stability of the electrolyte can be effectively improved; and an electrolyte with relatively high sodium salt concentration is used as the secondary electrolyte to ensure the conductivity, and the sodium ion battery obtained by secondary electrolyte injection has relatively good cycle stability and high-temperature stability.

Description

A sodium-ion battery electrolyte, a secondary sodium-ion battery, and a method for preparing the same. Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a sodium-ion battery electrolyte, a secondary-filled sodium-ion battery, and a method for preparing the same. Background Technology

[0002] With the gradual depletion of traditional fossil fuels and increasingly serious environmental problems, the development of new renewable energy sources has become an inevitable trend. Lithium-ion batteries, due to their advantages such as high energy density, long cycle life, and environmental friendliness, have been widely used in electric vehicles, laptops, and energy storage. However, lithium resources are limited and unevenly distributed across the Earth, and will eventually be exhausted. Sodium, on the other hand, is one of the most abundant elements on Earth. Sodium-ion batteries operate on a similar principle to lithium-ion batteries and offer advantages such as low cost, good safety, and long-term large-scale storage, making them increasingly attractive to researchers.

[0003] However, due to the instability of the solid electrolyte interface (SEI) formed by hard carbon at the negative electrode in current sodium-ion batteries, and the easy dissolution of SEI components, side reactions between the negative electrode and the electrolyte increase, affecting battery performance. In particular, during cycling, the SEI is repeatedly damaged and repaired, generating a large amount of gas, resulting in decreased cycle performance, rapid capacity decay, and impact on battery safety. Existing technologies attempt to improve the cycle performance of sodium-ion batteries by adjusting additives in the primary and secondary electrolytes, but in practice, it has been found that this has limited effect on improving the cycle stability of sodium-ion batteries, and performance is poor under high-temperature conditions. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects or deficiencies existing in the above-mentioned sodium-ion batteries and to provide a sodium-ion battery electrolyte that can significantly improve the cycle performance of sodium-ion batteries when applied to them.

[0005] Another object of the present invention is to provide a secondary liquid-filled sodium-ion battery.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A sodium-ion battery electrolyte includes a primary electrolyte and a secondary electrolyte. The primary electrolyte comprises a first organic solvent, a sodium salt, and a first additive. The first organic solvent is propylene carbonate, and the concentration of the sodium salt in the primary electrolyte is 2 wt.% to 8 wt.%. The second electrolyte comprises a second organic solvent, a sodium salt, and a second additive. The propylene carbonate content in the second organic solvent is not less than 70 wt.%, the concentration of the sodium salt in the secondary electrolyte is 25 wt.% to 70 wt.%, and the second additive contains fluoroethylene carbonate.

[0008] The method of injecting the sodium-ion battery electrolyte includes: first injecting the primary electrolyte into the battery, then placing it at high temperature and forming it; and then injecting the secondary electrolyte into the battery.

[0009] This invention provides a sodium-ion battery electrolyte, comprising a primary electrolyte and a secondary electrolyte. The primary electrolyte uses only pure propylene carbonate (PC) as a solvent, which has a high boiling point and good thermal stability, improving the thermal stability of the electrolyte. However, its high viscosity is not conducive to the wetting of the electrode. By adjusting the sodium salt concentration, the viscosity of the primary electrolyte is made to achieve better wettability of the electrode, enabling the formation of a complete SEI film on the negative electrode surface. This avoids the SEI film from being damaged during long-term cycling, preventing direct contact between the negative electrode and the electrolyte and the occurrence of side reactions. The secondary electrolyte has a higher sodium salt content, ensuring the electrolyte conductivity and ensuring unimpeded sodium ion transport during battery cycling, with minimal increase in polarization. It also contains fluoroethylene carbonate (FEC), which allows the battery to continuously self-repair the SEI film during long-term cycling, thereby improving battery cycle life.

[0010] It should be noted that the concentration of the sodium salt in the primary electrolyte in this invention is 2 wt.% to 8 wt.%, for example, but not limited to 2 wt.%, 2.5 wt.%, 3 wt.%, 3.5 wt.%, 4 wt.%, 4.5 wt.%, 5 wt.%, 5.5 wt.%, 6 wt.%, 6.5 wt.%, 7 wt.%, 7.5 wt.%, or 8 wt.%, etc., can all achieve this invention.

[0011] Furthermore, the concentration of the sodium salt in the primary electrolyte is 3 wt.% to 6 wt.%.

[0012] The concentration of sodium salt in the secondary electrolyte described in this invention is 25 wt.% to 70 wt.%, for example, but not limited to 25 wt.%, 28 wt.%, 30 wt.%, 32 wt.%, 35 wt.%, 38 wt.%, 40 wt.%, 42 wt.%, 45 wt.%, 48 wt.%, 50 wt.%, 52 wt.%, 55 wt.%, 58 wt.%, 60 wt.%, 62 wt.%, 65 wt.%, 68 wt.%, or 70 wt.%, etc., all of which can achieve the present invention.

[0013] Furthermore, the concentration of the sodium salt in the secondary electrolyte is 30 wt.% to 60 wt.%.

[0014] Specifically, the total sodium ion concentration in the sodium-ion battery electrolyte is not less than 8 wt.%.

[0015] Specifically, in the sodium-ion battery electrolyte, the mass ratio of the primary electrolyte to the secondary electrolyte is (7-9):(1-3).

[0016] Furthermore, the mass ratio of the primary electrolyte to the secondary electrolyte is (7.5–8.5):(1.5–2.5).

[0017] The sodium salts described in this invention can be selected from commonly used sodium salts with reference to the technology, such as, but not limited to, sodium hexafluorophosphate, sodium tetrafluoroborate, sodium difluorophosphate, sodium bis(oxalate-borate), sodium difluorooxalate-borate, sodium difluorosulfonylimide, sodium bis(trifluoromethylsulfonyl)imide, and sodium perchlorate, or one or more of these.

[0018] In some specific embodiments, the first additive includes one or more of vinylene carbonate, fluorovinyl carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, vinyl sulfate, propylene sulfate, methanedisulfonate, tris(trimethylsilane) phosphate, or tris(trimethylsilane) borate.

[0019] In some specific embodiments, the content of the first additive in the primary electrolyte is 2 wt.% to 10 wt.%.

[0020] In some preferred embodiments, the first additive is a mixture of fluoroethylene carbonate, ethylene sulfate, and methylene disulfonate in a mass ratio of (1-3):(1-3):1.

[0021] In some preferred embodiments, the content of the fluoroethylene carbonate in the secondary electrolyte is 5 wt.% to 20 wt.%.

[0022] In some specific embodiments, the second additive further includes one or more of vinylene carbonate, 1,3-propanesulfonate lactone, 1,3-propylene sulfonate lactone, vinyl sulfate, propylene sulfate, methanedisulfonate, tris(trimethylsilane) phosphate, or tris(trimethylsilane) borate.

[0023] In some specific embodiments, the total content of the second additive in the secondary electrolyte is 10 wt.% to 25 wt.%.

[0024] In some specific embodiments, the second organic solvent further includes one or more of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl butyrate, γ-butyrolactone, difluoroethyl acetate, or ethyl 2,2,2-trifluoroacetate.

[0025] In some preferred embodiments, the second organic solvent is propylene carbonate and ethyl methyl carbonate in a mass ratio of 7:3 to 9:1.

[0026] The present invention also provides a secondary electrolyte sodium-ion battery containing the above-mentioned sodium-ion battery electrolyte.

[0027] In some specific embodiments, the electrolyte injection method for the sodium-ion battery is as follows: the positive electrode, negative electrode, and separator are stacked to obtain a bare cell. The cell is then installed in the casing. The electrolyte is injected once using a single electrolyte solution. After injection, the cell is left at room temperature for 24-36 hours, then left at 40-50°C for 12-24 hours for formation (formation process: charge at 0.2C to 3.0V at 40-60°C, let stand for 15 minutes, then charge at 0.5C to 3.35V). After formation, a second electrolyte injection is performed using a second electrolyte solution. Then, the cell is sealed, aged, shaped, and capacity tested.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] This invention provides a sodium-ion battery electrolyte. By injecting a primary electrolyte with a low sodium salt content using pure PC as a solvent, the electrolyte exhibits good wettability on the electrode. PC has a high boiling point and good thermal stability, thus improving the thermal stability of the electrolyte. The secondary electrolyte with a higher sodium salt content ensures the electrolyte conductivity, guaranteeing unimpeded sodium ion transport during battery cycling and thus ensuring the battery's cycle performance. Attached Figure Description

[0030] Figure 1 shows the cycling performance of the sodium-ion batteries prepared in Example 20 and Comparative Example 10 at 60°C.

[0031] Figure 2 shows the hot plate test results of the sodium-ion batteries prepared in Example 20 and Comparative Example 10 at 220°C. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are only used to explain the present invention and are not intended to limit the scope of the present invention. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0033] Example 1

[0034] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0035] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0036] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0037] Example 2

[0038] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 70:30.

[0039] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0040] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0041] Example 3

[0042] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 90:10.

[0043] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0044] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0045] Example 4

[0046] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0047] The primary electrolyte comprises 92 wt.% of a first organic solvent (propylene carbonate), 3 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0048] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0049] Example 5

[0050] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0051] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0052] The secondary electrolyte comprises 55 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 30 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0053] Example 6

[0054] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0055] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0056] The secondary electrolyte comprises 30 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 55 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0057] Example 7

[0058] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 70:30.

[0059] The primary electrolyte comprises 96 wt.% of a first organic solvent (propylene carbonate), 2 wt.% of a sodium salt (sodium tetrafluoroborate), and 2 wt.% of a first additive (2 wt.% of vinylene carbonate).

[0060] The secondary electrolyte comprises 20 wt.% of a second organic solvent (ethylene carbonate and dimethyl carbonate in a mass ratio of 8:2), 60 wt.% of a sodium salt (sodium bis(trifluoromethanesulfonyl)imide) and 20 wt.% of a second additive (fluoroethylene carbonate).

[0061] Example 8

[0062] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 90:10.

[0063] The primary electrolyte comprises 84 wt.% of a first organic solvent (propylene carbonate), 6 wt.% of a sodium salt (sodium perchlorate), and 10 wt.% of a first additive (5 wt.% of fluoroethylene carbonate, 3 wt.% of 1,3-propanesulfonate lactone, and 2 wt.% of tris(trimethylsilane) phosphate).

[0064] The secondary electrolyte comprises 60 wt.% of a second organic solvent (ethylene carbonate and diethyl carbonate in a mass ratio of 8:2), 30 wt.% of a sodium salt (sodium difluorooxalate borate) and 10 wt.% of a second additive (7 wt.% fluoroethylene carbonate and 3 wt.% vinylene carbonate).

[0065] Example 9

[0066] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0067] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0068] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and methyl ethyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium hexafluorophosphate), and 15 wt.% of a second additive (fluoroethylene carbonate).

[0069] Example 10

[0070] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0071] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (3 wt.% of vinylene carbonate, 1 wt.% of vinyl sulfate, and 1 wt.% of methanedisulfonate).

[0072] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0073] Example 11

[0074] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0075] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0076] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide), and 15 wt.% of a second additive (fluoroethylene carbonate).

[0077] Example 12

[0078] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0079] The primary electrolyte comprises 87 wt.% of a first organic solvent (propylene carbonate), 8 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0080] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0081] Example 13

[0082] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0083] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0084] The secondary electrolyte comprises 60 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 25 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0085] Example 14

[0086] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0087] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0088] The secondary electrolyte comprises 15 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 70 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0089] Comparative Example 1

[0090] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0091] The primary electrolyte comprises 94 wt.% of a first organic solvent (propylene carbonate), 1 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0092] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0093] Comparative Example 2

[0094] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0095] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 3:7), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0096] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0097] Comparative Example 3

[0098] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0099] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0100] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 2:8), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (fluoroethylene carbonate).

[0101] Comparative Example 4

[0102] A sodium-ion battery electrolyte comprises a primary electrolyte and a secondary electrolyte in a mass ratio of 85:15.

[0103] The primary electrolyte comprises 90 wt.% of a first organic solvent (propylene carbonate), 5 wt.% of a sodium salt (sodium hexafluorophosphate), and 5 wt.% of a first additive (2 wt.% of fluoroethylene carbonate, 2 wt.% of ethylene sulfate, and 1 wt.% of methane disulfonate).

[0104] The secondary electrolyte comprises 45 wt.% of a second organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 8:2), 40 wt.% of a sodium salt (sodium bis(fluorosulfonyl)imide) and 15 wt.% of a second additive (ethylene carbonate).

[0105] Examples 15-28 and Comparative Examples 5-8

[0106] Sodium-ion batteries were provided in Examples 15-28 and Comparative Examples 5-8, and the preparation methods are as follows:

[0107] (1) Preparation of positive electrode sheet: The positive electrode material Na4Fe(PO4)2P2O7 (NFPP), binder (PVDF), and conductive agent (SP) were dispersed in NMP organic solvent at a mass ratio of 96:2:2. The mixture was stirred until stable and homogeneous under vacuum stirring and uniformly coated onto a carbon-coated aluminum foil with a thickness of 15μm. After the aluminum foil was dried at room temperature, it was transferred to a forced-air oven at 120℃ and dried for 1 hour. Then, it was cold-pressed and die-cut to form a positive electrode sheet.

[0108] (2) Preparation of negative electrode sheet: Hard carbon, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive agent (SP) were mixed together in a mass ratio of 96:1:2:1 and dispersed in deionized water. The mixture was stirred until stable and homogeneous using a vacuum mixer to form a negative electrode slurry. The negative electrode slurry was then uniformly coated onto an aluminum foil with a thickness of 12 μm. After the aluminum foil was dried at room temperature, it was transferred to a forced-air oven at 120°C and dried for 1 hour. Then, it was cold-pressed and die-cut to form a negative electrode sheet.

[0109] (3) The positive electrode, negative electrode and separator are stacked to obtain a bare cell. The cell is installed in the casing. The electrolyte is injected once using the first electrolyte. After the electrolyte is injected, the cell is left at room temperature for 24h to 36h, and then left at 40℃-50℃ for 12-24h for formation (formation process: charge at 0.2C to 3.0V at 40℃-60℃, let stand for 15 minutes, and then charge at 0.5C to 3.35V). After formation, the electrolyte is injected a second time using the second electrolyte. Then the cell is sealed, aged, shaped and capacity tested to obtain a sodium-ion battery. The batteries made with the electrolyte in Examples 1 to 14 correspond to Examples 15 to 28, respectively. The batteries made with the electrolyte in Comparative Examples 1 to 4 correspond to Comparative Examples 5 to 8, respectively.

[0110] Comparative Example 9

[0111] A sodium-ion battery electrode sheet differs from Example 1 in that:

[0112] The electrolyte described in Example 10 of CN117543085A was used.

[0113] Comparative Example 10

[0114] A sodium-ion battery electrode sheet differs from that in Example 15 in that:

[0115] Sodium-ion batteries are produced by a single electrolyte injection process followed by sealing, aging, shaping, and capacity testing of the cells.

[0116] The electrolyte comprises: 15 wt.% sodium salt (sodium hexafluorophosphate), 80 wt.% organic solvent (propylene carbonate and ethyl methyl carbonate in a mass ratio of 3:7) and additives (2 wt.% fluoroethylene carbonate, 2 wt.% ethylene sulfate and 1 wt.% methanedisulfonate).

[0117] Performance testing

[0118] 1. Testing Method

[0119] (1) 60℃ Cyclic Test: After fully charging the sodium-ion battery cells prepared in the above examples and comparative examples, the cell thickness D0 was measured. The cells were then placed in an environment of (60±3)℃ and left to stand for 3 hours. When the cell body reached (60±3)℃, the cell was charged to 3.65V at a constant current of 1C, then charged to a cutoff current of 0.05C at a constant voltage of 3.65V, and then discharged to 1.5V at 1C. The initial capacity Q0 was recorded. This cycle was repeated until 1000 cycles were completed, at which point the capacity Q1 was recorded. The capacity retention rate (%) was calculated. The cell was then fully charged again, and the full-charge thickness D1 was measured. The thickness change rate (%) was calculated, and the results are shown in Table 1. The calculation formulas used are as follows:

[0120] Capacity retention rate (%) = Q1 / Q0 × 100%; Thickness change rate (%) = (D1-D0) / D0 × 100%.

[0121] (2) 85℃ High-Temperature Storage Experiment: The sodium-ion cells prepared in the above examples and comparative examples were charged at 25℃ to 3.65V using a 1C method, then charged at a constant voltage of 3.65V to a cutoff current of 0.02C, and then discharged at a constant current of 1C to 1.5V, which was recorded as the initial capacity Q2. The cells were then charged again to 3.65V using a 1C method, and then charged at a constant voltage of 3.65V to a cutoff current of 0.02C. The thickness D2 of the fully charged cell was measured. The fully charged battery was then placed at 85℃. After being left in the environment for 15 days, the thickness D3 of the fully charged cell was tested. It was then discharged at a constant current of 1C to 1.5V, and this was recorded as the retention capacity Q3. At 25℃, the battery was charged at 1C to 3.65V, then charged at a constant voltage of 3.65V to the cutoff current of 0.02C, and finally discharged at a constant current of 1C to 1.5V, which was recorded as the recovery capacity Q4. The cell capacity retention rate (%), capacity recovery rate (%), and thickness change rate (%) were calculated, and the results are shown in Table 1. The calculation formulas used are as follows:

[0122] Capacity retention rate (%) = Q3 / Q2 × 100%; Capacity recovery rate (%) = Q4 / Q2 × 100%; Thickness change rate (%) = (D3-D2) / D2 × 100%.

[0123] (3) 220℃ hot plate test: The sodium-ion battery cells prepared in the above examples and comparative examples were charged to 3.65V at 25℃ at 1C, and then charged to the cutoff current of 0.02C at a constant voltage of 3.65V. The fully charged battery cells were then placed on a hot plate and heated to 220℃ for testing, and the battery cell voltage was monitored.

[0124] 2. Test Results

[0125] The test results of each embodiment and comparative example are shown in Figures 1-2 and Table 1.

[0126] Table 1. Experimental data for each embodiment and comparative example.

[0127] As can be seen from Table 1, the sodium-ion battery made using the sodium-ion battery electrolyte described in this invention exhibits significantly improved cycle stability and safety performance. When the cell is stored at 85°C for 15 days, the cell thickness expansion rate does not exceed 10%, and the capacity retention rate and recovery rate are both not less than 80%. Simultaneously, when the cell is cycled at 60°C for 1000 cycles, the capacity retention rate is not less than 80%, the cell thickness expansion rate does not exceed 12%, and when the cell is heated to 220°C with a hot plate, no fire or explosion occurs, and the cell voltage is not less than 3.2V.

[0128] As can be seen from Comparative Example 5, if the sodium salt concentration in the primary electrolyte is too low, the SEI formed by formation is unstable, resulting in a significant decrease in cell performance.

[0129] As can be seen from Comparative Example 6, if the content of propylene carbonate in the primary electrolyte is low, the thermal stability of the electrolyte will decrease, resulting in a significant decrease in the performance of the battery cell, and the safety performance will also be significantly deteriorated.

[0130] As can be seen from Comparative Example 7, if the content of the organic solvent propylene carbonate in the secondary electrolyte is low, the thermal stability of the electrolyte will decrease, resulting in a significant decrease in the performance of the battery cell, and at the same time, the safety performance will also be significantly deteriorated.

[0131] As can be seen from Comparative Example 8, if the second additive in the secondary electrolyte does not contain fluoroethylene carbonate, the electrolyte will not wet sufficiently, resulting in a decrease in the performance of the battery cell.

[0132] As can be seen from Comparative Example 9, the battery cells made using the electrolyte described in the prior art have poor electrical performance and poor safety performance.

[0133] As can be seen from Comparative Example 10, the battery cells produced by a single injection have poor electrical performance and poor safety performance.

[0134] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A sodium-ion battery electrolyte comprising a primary electrolyte and a secondary electrolyte, characterized in that, The primary electrolyte comprises a first organic solvent, a sodium salt and a first additive; the first organic solvent is propylene carbonate, the concentration of the sodium salt in the primary electrolyte is 2wt.%-8wt.%; the secondary electrolyte comprises a second organic solvent, a sodium salt and a second additive; the content of propylene carbonate in the second organic solvent is not less than 70wt.%, the concentration of the sodium salt in the secondary electrolyte is 25wt.%-70wt.%, and the second additive contains fluoroethylene carbonate; The injection method of the sodium ion battery electrolyte comprises: firstly injecting the primary electrolyte into the battery, and then placing and forming the battery at high temperature; Then, the secondary electrolyte is injected into the battery. 2.The sodium-ion battery electrolyte of claim 1, wherein, The total concentration of sodium ions in the sodium ion battery electrolyte is not less than 8wt.%. 3.The sodium-ion battery electrolyte of claim 1, wherein, The mass ratio of the primary electrolyte to the secondary electrolyte is (7-9):(1-3). 4.The sodium-ion battery electrolyte of claim 1, wherein, The sodium salt is one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium difluorophosphate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide and sodium perchlorate.

5. The sodium-ion battery electrolyte of claim 1, wherein, The first additive comprises one or more of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propene sultone, ethylene sulfate, propylene sulfate, methylene methane disulfonate, tris(trimethylsilyl)phosphate or tris(trimethylsilyl)borate. 6.The sodium-ion battery electrolyte of claim 1, wherein, The content of the first additive in the primary electrolyte is 2wt.%-10wt.%. 7.The sodium-ion battery electrolyte of claim 1, wherein, The second additive further comprises one or more of vinylene carbonate, 1,3-propane sultone, 1,3-propene sultone, ethylene sulfate, propylene sulfate, methylene methane disulfonate, tris(trimethylsilyl)phosphate or tris(trimethylsilyl)borate. 8.The sodium-ion battery electrolyte of claim 1, wherein, The total content of the second additive in the secondary electrolyte is 10wt.%-25wt.%. 9.The sodium-ion battery electrolyte of claim 1, wherein, The second organic solvent further comprises one or more of vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl n-butyrate, γ-butyrolactone, difluoroethyl acetate or 2,2,2-trifluoroethyl acetate.

10. A secondary liquid-impregnated sodium-ion battery, characterized by, The sodium ion battery electrolyte comprises the sodium ion battery electrolyte according to any one of claims 1-9.

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