An electrolyte and a lithium-ion battery

Abstract

This invention provides an electrolyte and a lithium-ion battery. The electrolyte comprises a fluorosulfonate lactone compound as shown in Formula 1. When applied to lithium manganese iron phosphate batteries, the electrolyte effectively inhibits the dissolution of manganese from the lithium manganese iron phosphate material, significantly improving the high-temperature storage performance and high-temperature cycling performance of the electrolyte.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, and relates to an electrolyte, and more particularly to an electrolyte and a lithium-ion battery. Background Technology

[0002] Currently, commercially available power storage batteries primarily use ternary lithium-ion batteries and lithium iron phosphate (LFP) cathode materials. Compared to these two, lithium manganese iron phosphate (LFP) offers advantages in energy density and low-temperature performance. Compared to ternary cathodes, its olivine structure provides greater stability and safety during charge and discharge. Furthermore, considering the improved energy density of LFP, its cost per watt-hour is slightly lower than that of LFP and significantly lower than that of ternary batteries. However, LFP also has some drawbacks, such as poor cycling performance at room and high temperatures, significant irreversible capacity loss after storage at both, and a noticeable voltage drop. These disadvantages greatly impact battery manufacturers' shipments and the usage by end customers.

[0003] The main reasons for the capacity decay of lithium manganese iron phosphate batteries are: ① During deep discharge or high-power charge and discharge, Mn in the lithium manganese iron phosphate material is reduced to trivalent. The formation of trivalent manganese in the delithiation state will severely distort the metal-oxygen octahedron. This distortion will change the lattice parameters, destroy the solid solution structure, and affect the life of the cathode; ② Mn 2+ It will dissolve into the electrolyte, leading to irreversible capacity loss; ③Mn 2+ The manganese is reduced and deposited on the negative electrode, damaging the SEI film and causing capacity decay. Furthermore, research shows that at room temperature, the direct capacity loss caused by manganese dissolution accounts for only a small portion, while at high temperatures, manganese dissolution causes capacity losses of up to 30% or more. Therefore, reducing manganese dissolution in lithium manganese iron phosphate materials is key to improving the lifespan of lithium manganese iron phosphate batteries.

[0004] CN108808089A discloses a lithium iron phosphate battery and its electrolyte. Three types of additives—fluorinated ethers, 2-methylmaleic anhydride, and sulfate compounds—are added to the electrolyte to improve the battery's high-temperature performance and cycle performance. However, it cannot suppress manganese ion dissolution; therefore, the improvement in high-temperature cycle life and storage life is limited.

[0005] CN106571485A discloses a lithium iron phosphate power battery, in which the electrolyte solvent is a composite of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, ethyl acetate and propylene carbonate, which improves the low-temperature performance of the lithium iron phosphate power battery. However, improving only the composition of the electrolyte solvent does not inhibit the dissolution of manganese, the positive electrode material of the lithium iron phosphate battery, and therefore it is not suitable for high-temperature cycling.

[0006] Therefore, developing an electrolyte and lithium-ion battery that can improve the high-temperature storage and high-temperature cycling performance of the electrolyte is an important research direction in this field. Summary of the Invention

[0007] In view of the shortcomings of the existing technology, the purpose of this invention is to provide an electrolyte and lithium-ion battery that improve the high-temperature storage and high-temperature cycling performance of the electrolyte.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] One object of the present invention is to provide an electrolyte comprising a fluorosulfonate lactone compound as shown in Formula 1.

[0010]

[0011] In this invention, the fluorosulfonate lactone compound shown in Formula 1 is used as an additive in the positive electrode film formation of lithium manganese iron phosphate batteries, which reduces the oxidation of the electrolyte on the surface of the positive electrode material and inhibits the dissolution of manganese. Furthermore, the fluorocarbon chain structure in the fluorosulfonate lactone compound shown in Formula 1 can improve the wettability of the electrolyte to a certain extent and reduce the internal resistance of the battery.

[0012] In this invention, the compound shown in Formula 1 can preferentially form a sulfur-containing SEI film (such as lithium alkyl sulfonate) on the negative electrode surface, which can poison many catalysts. The presence of sulfur in the SEI film can effectively reduce the activity of the graphite electrode in the reaction with the electrolyte, thereby inhibiting the occurrence of side reactions such as solvent molecule decomposition.

[0013] As a preferred embodiment of the present invention, the mass fraction of the fluorosulfonate lactone compound in the electrolyte is 0.1% to 1.5%, wherein the mass fraction can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or 1.5%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 0.5% to 1.0%.

[0014] As a preferred embodiment of the present invention, the electrolyte further includes additives.

[0015] Preferably, the additives include vinylene carbonate and lithium bis(oxalato)borate.

[0016] In this invention, lithium bis(oxalato)borate in the additive can synergistically interact with the fluorosulfonate lactone shown in Formula 1 to form a dense and uniform protective film on the surface of lithium manganese iron phosphate, effectively inhibiting the dissolution of manganese. On the other hand, lithium bis(oxalato)borate can complex manganese ions in the electrolyte, avoiding the problem of manganese ions being reduced on the negative electrode surface and damaging the negative electrode SEI film, thus hindering the lithium ion insertion channel.

[0017] As a preferred embodiment of the present invention, the mass fraction of vinylene carbonate in the electrolyte is 0.1% to 3.0%, wherein the mass fraction can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, or 3.0%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 1.0% to 2.0%.

[0018] Preferably, in the electrolyte, the mass fraction of lithium bis(oxalato)borate is 0.1% to 3.0%, wherein the mass fraction can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, or 3.0%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 0.1% to 0.5%.

[0019] As a preferred embodiment of the present invention, the electrolyte further includes lithium salt.

[0020] Preferably, the lithium salt includes any one or a combination of at least two of LiPF6, LiClO4, LiBF4, LiPO2F2, LiODFB, LiTFSI, or LiFSI, wherein typical but non-limiting examples of the combination include: a combination of LiPF6 and LiClO4, a combination of LiClO4 and LiBF4, a combination of LiPO2F2 and LiODFB, a combination of LiODFB and LiTFSI, or a combination of LiTFSI and LiFSI, etc.

[0021] Preferably, in the electrolyte, the mass fraction of the lithium salt is 8-12%, wherein the mass fraction can be 8%, 9%, 10%, 11% or 12%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0022] As a preferred embodiment of the present invention, the electrolyte further includes an organic solvent.

[0023] Preferably, the organic solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene sulfite, ethyl acetate, diethyl sulfite, or 1,3-propanesulfonate lactone.

[0024] Preferably, the organic solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

[0025] As a preferred embodiment of the present invention, the mass fraction of ethylene carbonate in the electrolyte is 30-40%, wherein the mass fraction can be 30%, 32%, 34%, 36%, 38%, or 40%, etc., but is not limited to the listed values, and other unlisted values ​​are also applicable.

[0026] Preferably, in the electrolyte, the mass fraction of methyl ethyl carbonate is 30-40%, wherein the mass fraction can be 30%, 32%, 34%, 36%, 38%, or 40%, etc., but is not limited to the listed values, and other unlisted values ​​are also applicable.

[0027] Preferably, in the electrolyte, the mass fraction of diethyl carbonate is 30-40%, wherein the mass fraction can be 30%, 32%, 34%, 36%, 38%, or 40%, etc., but is not limited to the listed values, and other unlisted values ​​are also applicable.

[0028] A second objective of the present invention is to provide a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described in one objective.

[0029] As a preferred embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive electrode material located on the positive current collector.

[0030] Preferably, the positive electrode material includes a positive electrode active material, which includes lithium manganese iron phosphate.

[0031] As a preferred embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode material located on the negative electrode current collector.

[0032] Preferably, the negative electrode material includes a negative electrode active material, which includes graphite.

[0033] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] The electrolyte prepared by this invention is applied to lithium manganese iron phosphate batteries, effectively inhibiting the dissolution of manganese in lithium manganese iron phosphate materials, and significantly improving the high-temperature storage performance and high-temperature cycling performance of the electrolyte. Specifically, after 30 days of storage at 60°C, the battery capacity retention rate can reach 85%, the capacity recovery rate can reach more than 91%, and the thickness growth rate is as low as 3.4%. Under 1C / 1C cycling at 45°C, the capacity retention rate can reach as high as 88% after 1000 cycles. Detailed Implementation

[0036] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0037] Example 1

[0038] This embodiment provides an electrolyte comprising a fluorosulfonate lactone compound as shown in Formula 1.

[0039]

[0040] This embodiment also includes additives, organic solvents, and lithium salts;

[0041] The additives, based on a mass fraction of 100% for the electrolyte, consist of 1.5% vinylene carbonate (VC) and 0.5% lithium bis(oxalato)borate (LiBOB), and the lithium salt is 10% lithium hexafluorophosphate.

[0042] The remainder consists of organic solvents, namely ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 3:4:3.

[0043] This embodiment also provides a method for preparing a lithium-ion battery, the method comprising:

[0044] Preparation of the positive electrode sheet: LiFePO4:SP:CNT:PVDF = 95.0:2.0:0.5:2.5. The positive electrode gel is prepared with a solid content of 1.327%. The first step involves adding LiFePO4, SP, and NMP, rotating at 25±1 r / min, dispersing at 500±50 r / min, and stirring for 10 min, then rotating at 25±1 r / min, dispersing at 1000±50 r / min, and stirring at 45℃ for 90 min. The second step involves adding the conductive agent CNT slurry, rotating at 25±1 r / min, dispersing at 1000±50 r / min, under a vacuum of 0.080 kPa, and stirring at 45℃ for 60 min. The third step involves adding the positive electrode gel, rotating at 25±1 r / min, dispersing at 1000±50 r / min, and stirring at 45℃ for 60 min. The process involves stirring at 2500±50 r / min, with a vacuum of 0.080 kPa, at 45℃ for 90 min; the fourth step is viscosity adjustment, adding NMP to adjust the slurry viscosity; the fifth step is slow stirring at 15±1 r / min, dispersing at 500±50 r / min, with a vacuum of 0.080 kPa, stirring for 0.5 h and then cooling to ensure the positive electrode discharge viscosity is 20000±5000 mPa·s and the fineness is ≤15 μm. Deposits on the mixing tank wall and stirring rod are scraped off promptly at each step. The positive electrode sheet is then obtained by sieving, coating, cold pressing, and slitting.

[0045] Preparation of negative electrode sheet: graphite:SP:CMC:SBR = 95.5:1.5:1.2:1.8. Negative electrode slurry preparation: The slurry solid content is 8%. Step 1: Add graphite and SP, dry mix at 20±1 r / min, disperse at 1000±50 r / min, and stir for 1 h. Step 2: Add 50% of the negative electrode slurry, rotate at 20±1 r / min, disperse at 1000±50 r / min, and stir for 1.5 h. Step 3: Add another 50% of the negative electrode slurry, rotate at 25±1 r / min, disperse at 2000±50 r / min, vacuum degree 0.085 kPa, and stir for 1 h. Step 4: Adjust viscosity by adding deionized water. Step 5: Add aqueous dispersant SBR, rotate at 25±1 r / min, disperse at 800±50 r / min, vacuum degree 0.085 kPa, and stir for 1 h. Ensure the negative electrode discharge viscosity is 4000±1500 mPa·s and the fineness is ≤20μm. Scrape away any deposited material on the mixing tank wall and stirring rod at each step. The negative electrode sheet is then obtained through sieving, coating, cold pressing, and slitting.

[0046] The above-mentioned positive electrode, negative electrode and electrolyte are assembled to form a lithium-ion battery.

[0047] Examples 2-8 and Comparative Examples 1-3 differed from Example 1 in that the electrolyte content was modified. The specific parameters are shown in Table 1.

[0048] Table 1

[0049]

[0050] The lithium-ion batteries prepared in Examples 1-8 and Comparative Examples 1-3 were tested for high-temperature storage performance and cycle performance. The test results are shown in Table 2.

[0051] The method for improving high-temperature storage performance is as follows:

[0052] 1) Let it stand for 10 minutes at 25±2℃;

[0053] 2) Charge the battery at a constant current and constant voltage of 0.5C to 4.2V, then discharge it at a constant current of 0.5C to 2.5V, repeating this cycle 5 times;

[0054] 3) Charge the battery at a constant current and constant voltage of 0.5C to 4.2V;

[0055] 4) Record the voltage, internal resistance, and thickness of the battery cell before storage;

[0056] 5) The battery should be stored at 60±2℃ for 30 days;

[0057] 6) After storage, record the voltage, internal resistance, and thickness of the battery cell after storage;

[0058] 7) Discharge the battery at a constant current of 0.5C to 2.5V at 25±2℃;

[0059] 8) Record the discharge capacity of the cell in step 7). The charge retention capability can be expressed as the percentage of the discharge capacity in step 7) to the average discharge capacity over the last 3 weeks after step 2).

[0060] 9) Cycle the battery for 5 standard cycles at 25±2℃; if it will not be stored afterward, it must be completely discharged.

[0061] 10) Record the discharge capacity of the cell in step 9). The capacity recovery capability can be expressed as the percentage of the discharge capacity in the first week of step 9) to the average discharge capacity in the last 5 weeks of step 2).

[0062] The cycle performance test method is as follows: the battery is charged at 1C constant current and constant voltage to 4.2V, discharged at 1C constant current to 2.5V, and cycled for 5 cycles.

[0063] Table 2

[0064]

[0065]

[0066] The table above shows that in Examples 2-3, both decreasing and excessively increasing the amount of the compound shown in Formula 1 resulted in a decline in the battery's high-temperature storage and cycle performance. Therefore, in this application, the optimal high-temperature performance of the battery is achieved when the compound content is between 0.5% and 1%. In Example 4, increasing the amount of the compound shown in Formula 1 did not improve the battery's electrochemical performance compared to Example 1; instead, it increased the cost. In Example 5, the absence of lithium bis(oxalato)borate and the excessively low amount of lithium bis(oxalato)borate in Example 6 both resulted in a decrease in the battery's high-temperature performance. In Examples 7-8, the absence of vinylene carbonate and the excessively low amount of vinylene carbonate both resulted in a decrease in the battery's high-temperature performance.

[0067] In Comparative Example 1, without the addition of the compound shown in Formula 1, the high-temperature performance of the battery decreased significantly. Comparative Example 2, by increasing the content of vinylene carbonate, showed a slight but not significant increase in high-temperature performance. Comparative Example 3, without the addition of lithium bis(oxalato)borate, further reduced the electrochemical performance of the battery. This indicates that lithium bis(oxalato)borate and vinylene carbonate, in the presence of the compound shown in Formula 1, further improved the performance of the electrolyte, exhibiting a synergistic effect.

[0068] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the electrolyte comprises a fluorosulfonic acid lactone compound as shown in formula 1, In the electrolyte, the mass fraction of the fluorosulfonic acid lactone compound is 0.5-1.0%; The electrolyte further comprises an additive, and the additive comprises vinylene carbonate and lithium bis(oxalato)borate; In the electrolyte, the mass fraction of the lithium bis(oxalato)borate is 0.1-0.5%; The positive electrode sheet comprises a positive electrode current collector and a positive electrode material on the positive electrode current collector; the positive electrode material comprises a positive electrode active material, and the positive electrode active material comprises lithium manganese iron phosphate.

2. The lithium-ion battery of claim 1, wherein, In the electrolyte, the mass fraction of the vinylene carbonate is 0.1-3.0%.

3. The lithium-ion battery of claim 2, wherein, In the electrolyte, the mass fraction of the vinylene carbonate is 1.0-2.0%.

4. The lithium-ion battery of claim 1, wherein, The electrolyte further comprises a lithium salt.

5. The lithium-ion battery of claim 4, wherein, The lithium salt comprises any one of LiPF6, LiClO4, LiBF4, LiPO2F2, LiODFB, LiTFSI or LiFSI or a combination of at least two thereof.

6. The lithium-ion battery of claim 4, wherein, In the electrolyte, the mass fraction of the lithium salt is 8-12%.

7. The lithium-ion battery of claim 1, wherein, The electrolyte further comprises an organic solvent.

8. The lithium-ion battery of claim 7, wherein, The organic solvent comprises at least two of vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene sulfite, ethyl acetate, diethyl sulfite or 1,3-propane sulfonic acid lactone.

9. The lithium-ion battery of claim 7, wherein, The organic solvent comprises at least two of vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate.

10. The lithium-ion battery of claim 9, wherein, In the electrolyte, the mass fraction of the vinylene carbonate is 30-40%.

11. The lithium-ion battery of claim 9, wherein, In the electrolyte, the mass fraction of the methyl ethyl carbonate is 30-40%.

12. The lithium-ion battery of claim 9, wherein, In the electrolyte, the mass fraction of the diethyl carbonate is 30-40%.

13. The lithium-ion battery of claim 1, wherein, The negative electrode sheet comprises a negative electrode current collector and a negative electrode material on the negative electrode current collector.

14. The lithium-ion battery of claim 13, wherein, The negative electrode material comprises a negative electrode active material, and the negative electrode active material comprises graphite.

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