High specific energy and high loading type primary lithium-sulfur battery electrolyte and preparation method thereof

Abstract

The application discloses a high-specific-energy high-loading type primary lithium-sulfur battery electrolyte and a preparation method thereof. The electrolyte comprises a lithium salt, a solvent and an additive. The lithium salt is lithium bisfluorosulfonylimide, and the additive is lithium difluoro(oxalato)borate. The preparation method comprises the following steps: in a glove box filled with inert gas, the lithium salt is added into the solvent, the additive lithium difluoro(oxalato)borate is added after uniform stirring, and the lithium-sulfur battery electrolyte is obtained after uniform mixing. The electrolyte improves the lithium ion diffusion rate and the discharge performance of the electrolyte through the synergistic effect of the lithium salt lithium bisfluorosulfonylimide and the additive lithium difluoro(oxalato)borate, so that the voltage platform of the battery discharge is higher, and the rate performance of the battery is improved.

Description

Technical Field

[0001] This invention relates to an electrolyte, and more particularly to a high-energy-density, high-capacity primary lithium-sulfur battery electrolyte and its preparation method. Background Technology

[0002] Lithium-sulfur batteries that use sulfur as the cathode material have high theoretical specific capacity and battery specific energy, reaching 1675 mAh / g and 2600 Wh / kg respectively, and have good market prospects.

[0003] Lithium-sulfur batteries have two main applications. The first is low-capacity cyclic lithium-sulfur batteries, but currently the number of cycles is very low, and they still do not have the potential to replace lithium-ion batteries. The second is high-capacity primary lithium-sulfur batteries, with the cathode fabrication referencing primary batteries such as lithium-manganese batteries, focusing on leveraging the energy storage capabilities of lithium-sulfur batteries' ultra-high energy density.

[0004] Research on lithium-sulfur primary batteries has made some progress, and the preparation process for high-capacity cathode materials is now complete. The key lies in finding a more suitable electrolyte system to maximize the performance of primary batteries. Commercial electrolytes for lithium-sulfur batteries mainly consist of lithium bis(trifluoromethanesulfonyl)imide and two solvents: 1,3-dioxane and ethylene glycol dimethyl ether. Some electrolytes add a certain mass fraction of lithium nitrate as an additive to improve battery discharge performance. Commercial electrolytes are primarily designed for low-capacity cycling lithium-sulfur batteries. When applied to high-capacity primary lithium-sulfur batteries, the overall discharge performance deteriorates significantly, mainly in three aspects: First, the specific capacity decreases significantly, dropping to about 1 / 8 of its original level; second, the rate performance deteriorates, from being able to discharge at 0.1C to performing poorly even at 0.02C; and third, the voltage plateau decreases, from around 2.1V to around 1.9V.

[0005] Therefore, there is an urgent need to invent an electrolyte system that is more suitable for high-capacity primary lithium-sulfur batteries, so that high-capacity primary lithium-sulfur batteries can have the same or even better capacity performance, rate performance and discharge voltage platform as low-capacity lithium-sulfur batteries. Summary of the Invention

[0006] Purpose of the invention: The first purpose of this invention is to provide a high-energy-density, high-capacity primary lithium-sulfur battery electrolyte that improves voltage platform and rate performance; the second purpose of this invention is to provide a method for preparing the aforementioned high-energy-density, high-capacity primary lithium-sulfur battery electrolyte.

[0007] Technical solution: The high-energy-density, high-capacity primary lithium-sulfur battery electrolyte of the present invention includes lithium salt, solvent and additive, wherein the lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI) and the additive is lithium difluorooxalate borate (LiDFOB).

[0008] Due to its smaller anionic radius, LiFSI migrates faster in solution, resulting in electrolytes with higher conductivity and better battery discharge performance compared to other organic salts. Preferably, the concentration of the lithium bis(fluorosulfonyl)imide is 0.5–5 mol / L. If the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte is below 0.5 mol / L, the electrolyte conductivity will be too low; if the concentration is above 5 mol / L, the electrolyte viscosity will be too high. Both excessively high and low concentrations of lithium bis(fluorosulfonyl)imide in the electrolyte will affect its normal performance.

[0009] Preferably, the amount of the additive lithium difluorooxalate borate is 0.01–0.1 mol / L. If the amount added is too low, the additive will not have a significant effect; if the amount added is too high, the electrolyte will become turbid.

[0010] Preferably, the solvent is a mixture of acetonitrile and fluoroether solvents.

[0011] Preferably, the molar ratio of the lithium salt to acetonitrile is 1:0.5 to 5.

[0012] Nitrile solvents possess characteristics such as high oxidation stability, wide electrochemical window, low viscosity, and high boiling point. Their dielectric constant is also significantly higher than that of ether solvents, making them a promising electrolyte component that has garnered widespread attention. However, nitrile solvents also have drawbacks such as high toxicity, low boiling point, and susceptibility to reduction at low potentials; therefore, they are generally not used as standalone electrolyte solvent components. Fluorinated ether solvents, while having drawbacks such as poor capacity retention due to dendrite formation during cycling and unsuitability for most high-voltage cathode materials due to their low oxidation stability, also benefit from the ability to simultaneously form films on both the positive and negative electrode surfaces through reductive decomposition. This reduces the dissolution of active materials from the positive electrode and protects the lithium anode, ultimately promoting the development of lithium-ion batteries. + The spread of.

[0013] Preferably, the volume ratio of acetonitrile to fluoroether is 1:0.5 to 5. Acetonitrile is the main solvent, and the fluoroether solvent is a co-solvent to adjust the overall viscosity of the electrolyte. Preferably, the fluoroether solvent is one or more of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, ethyl 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, and 1,1,2,2-tetrafluoroethyl ethyl ether. More preferably, the fluoroether solvent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0014] The preparation method of the high-energy-density, high-capacity primary lithium-sulfur battery electrolyte of the present invention includes the following steps: in a glove box filled with inert gas, lithium salt is added to a solvent, stirred evenly, and then lithium difluorooxalate borate additive is added and mixed evenly to obtain the lithium-sulfur battery electrolyte.

[0015] Electrolytes are sensitive to water and oxygen in the environment. Electrolytes with high water content will affect the discharge performance of the battery. At the same time, high water and oxygen content makes it easier for salts and solvents to produce a series of side reactions during the preparation of the electrolyte, resulting in the failure of the final electrolyte preparation. Preferably, the water and oxygen content in the glove box is less than 0.1 ppm.

[0016] Mechanism of Invention: Lithium salts commonly used in lithium-sulfur electrolytes can be broadly classified into inorganic lithium salts and organic lithium salts based on their anions. Inorganic salts exist in a dissolved form when mixed with the solvent. Since temperature significantly affects solubility, this can lead to salting-out at low temperatures, thus impacting the battery's low-temperature performance. Organic salts, on the other hand, exist in a coordinated form with the solvent, less affected by temperature, resulting in more stable performance at both high and low temperatures. Among organic anionic salts, LiFSI, due to its smaller anionic radius, migrates faster in solution, thus producing electrolytes with higher conductivity and better battery discharge performance compared to other organic salts.

[0017] Currently, due to the presence of Li in lithium salt additives + Lithium salts promote the coordination of anions, making them commonly used additives in lithium-sulfur batteries. Lithium nitrate is a commonly used lithium salt additive. In the LiFSI and acetonitrile-fluorinated ether system, LiDFOB has the highest solubility. Other additives, such as lithium nitrate, can cause electrolyte turbidity (exceeding 0.02 mol / L) if added in excess, making them unsuitable. LiDFOB forms a passivation film on the lithium anode surface through a redox reaction, isolating the anode from polysulfides and thus suppressing the shuttle effect in lithium-sulfur batteries. The Li in LiDFOB... +Its unstable five-fold coordination structure complements the anion of LiFSI, resulting in deeper coordination and tighter binding between lithium ions and the anionic salt. The stronger the Coulombic interaction between them, the higher the final discharge efficiency. Simultaneously, two anionic FSIs... - With DFOB - Collaborated with Sx 2- The conversion reduces battery polarization and forms a lower impedance SEI film on the lithium anode surface, ultimately improving the battery's discharge performance.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The electrolyte improves the lithium-ion diffusion rate and discharge performance of the electrolyte through the synergistic effect of lithium salt lithium bisfluorosulfonylimide and additive lithium difluorooxalate borate, resulting in a higher voltage platform for battery discharge and improving the rate performance of the battery. Attached Figure Description

[0019] Figure 1 This is a comparison chart of the specific capacities of the electrolytes prepared in Examples 1-4 of the present invention;

[0020] Figure 2 This is a comparison chart of the specific capacities of the electrolytes prepared in Example 1 and Example 5 of the present invention;

[0021] Figure 3 This is a comparison chart of the specific capacities of the electrolytes prepared in Examples 1 and 6-7 of the present invention;

[0022] Figure 4 This is a comparison chart of the specific capacities of the electrolytes prepared in Examples 1 and 8-9 of the present invention;

[0023] Figure 5 This is a comparison chart of the specific capacity of the electrolytes prepared in Example 1 and Examples 10-11 of the present invention;

[0024] Figure 6 This is a comparison chart of the specific capacity of the electrolytes prepared in Example 1 and Comparative Examples 1-3 of the present invention;

[0025] Figure 7 This is a comparison chart of the specific capacity (a) and specific energy (b) of the electrolyte prepared in Example 1 of the present invention at different expansion rates;

[0026] Figure 8 The diagram shows the AC impedance (a) and Weber impedance slope (b) of the electrolytes prepared in Example 1 and Comparative Examples 1-3 of this invention. Detailed Implementation

[0027] The technical solution of the present invention will be further described below with reference to the embodiments.

[0028] Example 1

[0029] The high-energy-density, high-capacity primary lithium-sulfur battery electrolyte of the present invention uses LiFSI as the lithium salt, acetonitrile and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as solvents, and LiDFOB as an additive. The molar ratio of LiFSI to acetonitrile is 1:2.5, the volume ratio of acetonitrile to fluoroether is 1:2, the amount of LiDFOB added is 0.05 mol / L, and the concentration of LiFSI in the electrolyte is 1.9 mol / L.

[0030] The preparation method of the above electrolyte is as follows: In a glove box filled with argon (water ≤ 0.1 ppm, oxygen ≤ 0.1 ppm), LiFSI and acetonitrile are placed in a sealed glass bottle in the above proportion and magnetically stirred at 600 rpm for 24 hours. Then, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and LiDFOB in the above proportion are added, and magnetically stirred at 600 rpm for 24 hours to obtain the final lithium-sulfur electrolyte.

[0031] Example 2

[0032] Based on Example 1, the amount of LiDFOB added was changed to 0.02 mol / L, while the other conditions remained unchanged.

[0033] Example 3

[0034] Based on Example 1, the amount of LiDFOB added was changed to 0.1 mol / L, while the other conditions remained unchanged.

[0035] Example 4

[0036] Based on Example 1, the amount of LiDFOB added was changed to 0.01 mol / L, while the other conditions remained unchanged.

[0037] Example 5

[0038] Based on Example 1, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether was replaced with 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, while the other conditions remained unchanged.

[0039] Example 6

[0040] Based on Example 1, the concentration of LiFSI in the electrolyte was changed to 0.5 mol / L, while the other conditions remained unchanged.

[0041] Example 7

[0042] Based on Example 1, the concentration of LiFSI in the electrolyte was changed to 5 mol / L, while the other conditions remained unchanged.

[0043] Example 8

[0044] Based on Example 1, the molar ratio of LiFSI to acetonitrile was changed to 1:0.5, while the other conditions remained unchanged.

[0045] Example 9

[0046] Based on Example 1, the molar ratio of LiFSI to acetonitrile was changed to 1:5, while the other conditions remained unchanged.

[0047] Example 10

[0048] Based on Example 1, the volume ratio of acetonitrile to fluoroether was changed to 1:0.5, while the other conditions remained unchanged.

[0049] Example 11

[0050] Based on Example 1, the volume ratio of acetonitrile to fluoroether was changed to 1:5, while the other conditions remained unchanged.

[0051] Comparative Example 1

[0052] Based on Example 1, the additive LiDFOB was replaced with LiNO3 of the same concentration, while all other conditions remained unchanged.

[0053] Comparative Example 2

[0054] Based on Example 1, lithium salt LiFSI was replaced with lithium bis(trifluoromethanesulfonylimide) (LiTFSI) of the same concentration.

[0055] Comparative Example 3

[0056] The commercial electrolyte is composed of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,3-dioxane (DOL), and dimethyl ethylene glycol (DME), wherein the volume ratio of 1,3-dioxane to dimethyl ethylene glycol is 1:1, and the concentration of LiTFSI in the electrolyte is 1 mol / L.

[0057] Fabrication of high-capacity lithium-sulfur battery cathodes

[0058] Preparation of raw materials (sulfur-carbon mixture): Weigh 50g of Ketjen black and 150g of sulfur, pour them into a pulverizer, seal it, and turn on the power. Rotate the switch and pulverize for about 20 seconds. When the pulverizer wall starts to heat up, stop pulverizing and wait for the temperature to drop before pulverizing again. Repeat this process three times. After standing for a period of time, remove the pulverizer and bottle it. Slightly open the bottle opening and place it in the glove box transition chamber. Slowly evacuate argon gas three times, then remove it from the glove box, fill it with argon gas, and tighten the screws. Place it in a forced-air drying oven and heat-melt it at 155℃ for more than 8 hours. Remove it and bottle it. The resulting powder is the raw material powder for the positive electrode of lithium-sulfur batteries.

[0059] Preparation of rolled electrode sheets: If the total mass of the mixture in one mixing step is 30g, weigh 26.7g of sulfur-carbon mixture, 1.5g of Ketjen Black, and 6g of NH4HCO3, add 80ml of alcohol, and stir with a planetary mixer for 3 hours (600r). After stirring, add 3g of PTFE suspension (60wt%), and stir again at 600r for 1 minute to complete the mixing. Pour the mixed material onto a steel plate and roll it approximately 30 times with a roller press, taking about 20 minutes. Then, place it on a roller press machine with a 2mm slit width and roll it 10 times. The upper roller begins the screen pressing operation, and the slit width is adjusted to 2-1.8-1.6-1.4-1.2-1.0-0.8-0.7-0.6-0.5-0.4-0.3mm. After forming, it is placed in a forced-air drying oven at 80℃ overnight (8-12h), and then placed in a vacuum drying oven at 80℃ for 8-12h. The sheet making is then complete.

[0060] Battery constant rate discharge test

[0061] Using the electrolytes prepared in Examples 1-11 and Comparative Examples 1-3, the above batteries were subjected to discharge tests in a constant temperature chamber at 25°C. The test rate was 0.02C for all tests. Example 1 was tested from high to low rates: 0.3C, 0.2C, 0.1C, 0.02C, and 0.05C. The cutoff voltage was 1.5V. The test results are shown in Tables 1 and 2. Figures 1-8 .

[0062] Table 1. Performance test results of the electrolytes prepared in Examples 1-11 and Comparative Examples 1-3 at 0.02C rate and 25°C.

[0063]

[0064]

[0065] As shown in Table 1, under the same temperature conditions, compared with Examples 1 to 11, Example 1 has the best ratio. That is, the molar ratio of LiFSI to acetonitrile in the design of this patent is 1:2.5, the volume ratio of acetonitrile to fluoroether is 1:2, the amount of LiDFOB added is 0.05 mol / L, and the concentration of LiFSI in the electrolyte is 1.9 mol / L. Under these conditions, the specific capacity, specific energy and discharge voltage plateau of the battery are all optimal.

[0066] Depend on Figure 1It can be seen that in Examples 1-4, as the concentration of the additive LiDFOB increases, the specific capacity, specific energy, and voltage plateau of the battery first increase and then decrease. When the amount of additive added is below 0.01 mol / L, the performance of the additive is not significant, and when the amount added is above 0.1 mol / L, the battery performance decreases. When the additive provided in Example 1 is 0.05 mol / L, the specific capacity, specific energy, and voltage plateau of the battery reach the optimal state, and the overall performance is higher than the other three.

[0067] Depend on Figure 2 As can be seen, Example 5 changed the solvent type based on Example 1. The use of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in Example 1 was more effective than the use of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether in Example 6.

[0068] Depend on Figure 3 As can be seen, in Examples 1, 6, and 7, the specific capacity, specific energy, and voltage plateau of the batteries first increased and then decreased with increasing LiFSI concentration. When the LiFSI concentration was below 0.5 mol / L, the electrolyte conductivity was too low; when the LiFSI concentration was above 5 mol / L, the electrolyte viscosity was too high, and the lithium-ion diffusion coefficient decreased, which also affected the discharge performance. The LiFSI concentration of 1.9 mol / L in the electrolyte provided in Example 1 was significantly better than that in Examples 6 and 7.

[0069] Depend on Figure 4 As can be seen, Example 1, using a lithium salt (LiFSI) to acetonitrile molar ratio of 1:2.5, exhibits a higher specific capacity and a higher discharge voltage plateau compared to Examples 8 and 9. When the LiFSI to acetonitrile molar ratio is too low, below 1:0.5, the electrolyte viscosity is too high, the lithium-ion diffusion coefficient decreases, and the battery discharge performance deteriorates. When the LiFSI to acetonitrile molar ratio is too high, above 1:0.5, the overall electrolyte concentration is too low, the conductivity is too low, and the battery discharge performance deteriorates.

[0070] Depend on Figure 5 As can be seen, in Examples 1, 10, and 11, the discharge plateau and specific capacity of the batteries differed with changes in the volume ratio of acetonitrile to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. If the volume ratio was too high, the electrolyte concentration was too low, resulting in low conductivity and decreased battery performance. If the volume ratio was too low, the electrolyte concentration was too high, leading to a low lithium-ion diffusion coefficient and decreased battery discharge performance. In Example 1, the optimal volume ratio of acetonitrile to fluoroether was 1:2.

[0071] Depend on Figure 6As can be seen, compared with the lithium nitrate added in Comparative Example 1, the specific capacity of LiDFOB added in Example 1 increased by 46.0%, the specific energy increased by 49.9%, and the discharge voltage plateau was slightly improved; Comparative Example 2 replaced lithium (LiFSI) with lithium bis(trifluoromethanesulfonylimide) of the same concentration, but the effect was not good.

[0072] The specific capacity and specific energy of the batteries prepared with the electrolytes of Examples 1 to 11 are significantly improved compared with those of the batteries prepared with the commercial electrolyte of Comparative Example 3. The specific capacity of Example 1 is increased by 8.36 times and the specific energy is increased by 8.05 times, which is far superior to that of the commercial electrolyte.

[0073] Table 2. AC impedance parameters of the electrolytes in Example 1 and Comparative Examples 1-3

[0074]

[0075] Depend on Figure 7 As shown in Table 2, with the increase of discharge rate, the specific capacity, specific energy and voltage plateau of the battery discharge all decrease. However, even at a high rate of 0.3C, the discharge capacity of the battery is still more than 60% of that at the conventional rate, which is far better than that of commercial electrolytes.

[0076] The AC impedance parameters of Examples 1 and Comparative Examples 1-3 were tested. The test method used a CHI760e electrochemical workstation manufactured by Shanghai Chenhua Instrument Co., Ltd. The frequency range was 0.01Hz-100kHz, and the voltage amplitude was 5mV. The AC impedance of different electrolytes was tested. The test results are shown in Table 3 and... Figure 8 .

[0077] Table 3. AC impedance parameters of the electrolytes in Example 1 and Comparative Examples 1-3

[0078]

[0079] Compared with Comparative Examples 1 to 3, Example 1 has lower ohmic impedance and Weber impedance slope, higher lithium-ion diffusion coefficient, and better discharge performance.

[0080] Compared to Example 1, Comparative Example 1 showed a lithium-ion diffusion coefficient that was only 1 / 4 of that of Example 1, and its discharge performance was significantly worse than that of Example 1, indicating that lithium difluorooxalate borate is irreplaceable as an additive in paint.

[0081] Compared to Example 1, Comparative Example 2 showed that the diffusion coefficient of lithium ions was about 1 / 3 of that of Example 1, and its discharge level was also slightly worse, indicating that LiFSI has a greater advantage as an organic anionic salt than LiTFSI.

[0082] Compared to Comparative Example 3 and Example 1, the lithium-ion diffusion coefficient is less than 1 / 8 of the latter, and its discharge performance is the worst among all cases.

Claims

1. A high-specific-energy, high-capacity primary lithium-sulfur battery electrolyte, characterized in that, The mixture comprises a lithium salt, a solvent, and an additive, wherein the lithium salt is lithium difluorosulfonylimide, and the additive is lithium difluorooxalate borate; the amount of the additive lithium difluorooxalate borate is 0.01~0.1 mol / L; the concentration of the lithium salt lithium difluorosulfonylimide is 1.9~5 mol / L; the solvent is a mixture of acetonitrile and fluorinated ether solvents; the molar ratio of the lithium salt to acetonitrile is 1:0.5~5; and the volume ratio of acetonitrile to fluorinated ether is 1:0.5~5.

2. The high-energy-density, high-capacity primary lithium-sulfur battery electrolyte according to claim 1, characterized in that, The fluorinated ether solvent is one or more of the following: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, ethyl 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, and 1,1,2,2-tetrafluoroethyl ethyl ether.

3. The high-energy-density, high-capacity primary lithium-sulfur battery electrolyte according to claim 1, characterized in that, The fluorinated ether solvent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

4. A method for preparing a high-specific-energy, high-capacity primary lithium-sulfur battery electrolyte according to any one of claims 1 to 3, characterized in that, Includes the following steps: In a glove box filled with inert gas, lithium salt is added to a solvent and stirred until homogeneous. Then, lithium difluorooxalate borate is added as an additive and mixed until homogeneous to obtain the lithium-sulfur battery electrolyte.

5. The method for preparing the high-energy-density, high-capacity primary lithium-sulfur battery electrolyte according to claim 4, characterized in that, The water and oxygen content in the glove box was less than 0.1 ppm.

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