Application of local high-concentration weak-solvent electrolyte in lithium-sulfur battery

Abstract

The application relates to the technical field of lithium-sulfur batteries, and discloses application of a local high-concentration weak-solvent electrolyte in a lithium-sulfur battery. The local high-concentration weak-solvent electrolyte is composed of a lithium salt, tetrahydro-pyran and toluene, wherein the volume ratio of the toluene is 20-50%. The solvent in the electrolyte is an ether solvent which is a weak solvent, high electrochemical and thermal stability, weak polarity inhibits the dissolution of polysulfides, the concentration of the solvent is regulated, the proportion of free solvent is reduced, the solvent is difficult to coordinate with polysulfides, the solubility of the lithium-sulfur battery is reduced, the shuttling of polysulfides between the positive and negative electrodes is effectively inhibited, and the contact and reaction of polysulfides and the metal lithium negative electrode are hindered. Meanwhile, the aromatic solvent toluene is introduced to increase the electrochemical reaction kinetics of the sulfur positive electrode through the characteristics of slightly dissolving sulfur, and the cycle life of the battery is prolonged.

Description

Technical Field

[0001] This invention relates to the field of lithium-sulfur battery technology, and more specifically to the application of a locally high-concentration weak solvent electrolyte in lithium-sulfur batteries. Background Technology

[0002] Lithium-sulfur batteries are considered a key candidate for next-generation high-energy-density energy storage systems due to their high theoretical energy density, abundant raw material reserves, and environmental friendliness. However, their commercialization still faces a series of key technological challenges, with the core issues stemming from the complex reaction behavior of sulfur species during charging and discharging and the high solubility of polysulfides in electrolytes.

[0003] Specifically, in ether-based electrolytes, the sulfur cathode undergoes a solid-liquid-solid reaction pathway to produce soluble long-chain polysulfides (Li2S). X (4≤X≤8). These intermediate products readily dissolve and shuttle to the lithium anode, partially reacting with metallic lithium, leading to irreversible loss of active material, reduced coulombic efficiency, and decreased cycle life—a phenomenon known as the "shuttle effect." Simultaneously, the poor intrinsic electronic conductivity of sulfur and its discharge end products (Li2S2 / Li2S) restricts reaction kinetics and reduces sulfur utilization. Furthermore, the high volume change of up to 79% of sulfur and discharge products during charging and discharging easily disrupts the structural stability of the sulfur cathode, causing separation of the sulfur active material from the cathode framework. Moreover, on the lithium anode side, polysulfides react with the lithium anode, corroding and damaging its surface solid electrolyte interphase (SEI) film and inducing lithium dendrite growth, posing safety hazards.

[0004] To fundamentally suppress polysulfide dissolution, recent research has focused on developing quasi-solid-state sulfur reactive electrolyte systems. The core strategy involves using ultra-high concentrations of lithium salts or hydrofluoroethers as diluents to reduce the number of free solvent molecules in the electrolyte to an extremely low level, thereby reducing the solubility of polysulfides and effectively suppressing the shuttle effect. This approach also promises to achieve extremely low electrolyte usage (E / S ratio). However, although polysulfides partially dissolve, the solubility of elemental sulfur is extremely low. Furthermore, quasi-solid-state reactions primarily rely on the conversion of solid sulfur into liquid polysulfides, and the poor conversion kinetics of polysulfides lead to insufficient electrochemical kinetics of the sulfur cathode in traditional quasi-solid-state reaction systems. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a novel electrolyte system and lithium-sulfur battery that can simultaneously and synergistically suppress the polysulfide shuttle effect, stabilize the lithium metal anode, and enhance the sulfur cathode reaction kinetics in lithium-sulfur batteries, thereby significantly improving the cycle stability and coulombic efficiency of lithium-sulfur batteries.

[0006] To achieve the above objectives, in a first aspect, the present invention provides an application of a locally high-concentration weak solvent electrolyte in lithium-sulfur batteries, characterized in that the locally high-concentration weak solvent electrolyte is composed of lithium salt, tetrahydropyran and toluene, wherein the volume percentage of toluene is 20-50%.

[0007] As a further preferred embodiment of the present invention, the lithium salt is at least one selected from lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0008] As a further preferred embodiment of the present invention, the concentration of the lithium salt in the electrolyte is 2 mol / L-4 mol / L. More preferably, the concentration of the lithium salt in the electrolyte is 4 mol / L.

[0009] As a further preferred embodiment of the present invention, the volume percentage of toluene is 40%.

[0010] According to a second aspect of the present invention, the present invention also provides a method for assembling a lithium-sulfur battery, comprising the following steps: (1) Under magnetic stirring, lithium salt is added to tetrahydropyran and stirred until the lithium salt is completely dissolved to obtain solution A; (2) Add toluene to solution A and continue stirring to obtain a uniform and transparent solution B; (3) Using solution B as electrolyte, lithium-sulfur batteries were assembled. After assembly, the batteries were left to stand for several hours to obtain lithium-sulfur batteries.

[0011] As a further preferred technical solution of the present invention, the standing time is 6-24 hours, the temperature is 25-35℃, and more preferably 30℃.

[0012] According to a third aspect of the present invention, the present invention also provides a lithium-sulfur battery, which employs a locally high-concentration weak solvent electrolyte composed of lithium salt, tetrahydropyran and toluene, wherein the volume percentage of toluene in the electrolyte is 20-50%.

[0013] This invention introduces tetrahydropyran, a substance with weak solvation capabilities, into the electrolyte system to adjust the electrolyte concentration and limit the dissolution of polysulfides. Simultaneously, toluene is introduced to enhance the sulfur cathode reaction kinetics. Toluene's moderate solubility for elemental sulfur creates a highly active reaction liquid environment in situ on the sulfur particle surface. This directly overcomes the bottleneck of solid-solid reaction kinetics caused by the extremely low solubility of sulfur in quasi-solid sulfur reactive electrolyte systems. This invention achieves efficient and rapid activation of the sulfur cathode at room temperature and conventional rate, yielding excellent initial capacity and rate performance.

[0014] Compared with the prior art, the present invention can achieve the following beneficial effects: The electrolyte in this invention is composed of a lithium salt and a solvent; the lithium salt is a sulfonamide lithium salt containing highly electron-withdrawing groups and having low Lewis basicity; the solvent is tetrahydropyran with weak solvation ability and toluene, an aromatic solvent. The materials for this electrolyte are readily available and simple to prepare.

[0015] The electrolyte provided by this invention uses tetrahydropyran, a weak ether solvent, as the solvent. Its high electrochemical and thermal stability, along with its weak polarity, inhibits the dissolution of polysulfides. By controlling the solvent concentration and reducing the proportion of free solvent, coordination between the solvent and polysulfides becomes difficult, reducing the solubility of lithium-sulfur batteries. This effectively inhibits the shuttle movement of polysulfides between the positive and negative electrodes, hindering the contact and reaction between polysulfides and the lithium metal negative electrode. Simultaneously, the aromatic solvent toluene, with its slightly sulfur-soluble properties, increases the electrochemical reaction kinetics of the sulfur positive electrode, extending the battery's cycle life. Attached Figure Description

[0016] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0017] Figure 1 The specific capacity performance of samples 1-6 is shown in the first cycle and after 100 cycles.

[0018] Figure 2 The specific capacity performance of samples 1 to 6 at different expansion rates.

[0019] Figure 3 The performance of galvanostatic intermittent titration (GITT) was tested for samples 5 and 6.

[0020] Figure 4 Electrolyte microstructures for samples 1, 5, and 6, as simulated by molecular dynamics.

[0021] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0022] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0023] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0024] Example The assembly method of the lithium-sulfur battery using a locally high-concentration weak solvent electrolyte provided in this embodiment is as follows: 1. Preparation of electrolyte According to the formula in Table 1, in a glove box, lithium bis(trifluoromethanesulfonyl)imide is added to solvent one and stirred thoroughly. Then solvent two is added and stirred thoroughly to obtain the electrolyte.

[0025] 2. Preparation of sulfur cathode Sulfur / CMK-3 (mass ratio 7:3) was ground and mixed to obtain a sulfur cathode composite material. The sulfur cathode composite material, binder (PVDF), and conductive agent (Super-P) were weighed at a mass ratio of 8:1:1. PVDF powder was dissolved in NMP solvent to form a homogeneous solution, and then Super-P and the composite cathode material were added sequentially. The viscosity of the slurry was controlled by adjusting the amount of NMP. After sealing, the mixture was continuously stirred for 10 hours to ensure thorough mixing and a stable, homogeneous cathode slurry. This slurry was then coated onto carbon-coated aluminum foil using a scraper and dried in a 60℃ oven for 12 hours until the NMP had completely evaporated.

[0026] 3. Assemble lithium-sulfur coin cells In an argon-filled glove box, a lithium-sulfur battery was assembled by adding the above-mentioned electrolyte, using a polypropylene membrane (PP) as the separator, a sulfur / carbon composite material as the positive electrode, and lithium metal as the negative electrode, and allowing it to stand at 30°C for 24 hours.

[0027] The assembled lithium-sulfur battery samples were subjected to charge-discharge cycle tests on a LAND-CT3002A battery testing system. The test conditions were constant current 0.2C charge-discharge, potential range of 1~3V, and 100 cycles. The results are shown below. Figure 1 As shown in the figure. The rate performance of the lithium-sulfur battery was evaluated at 0.1C, 0.2C, and 0.5C.

[0028] Table 1 shows a series of lithium-sulfur battery samples assembled with different electrolyte formulations in the examples.

[0029] Table 1

[0030] Note: The lithium salt concentration is an independent variable for the solvent system. The introduction of solvent two, or its proportion, does not change the lithium salt concentration in solvent one of the table.

[0031] The content of solvent 2 refers to its relative volume percentage in the homogeneous solution obtained after mixing with solvent 1 and lithium salt.

[0032] The battery samples in Table 1 were tested, and the results are as follows: The specific capacity performance of samples 1-6 in the first cycle and after 100 cycles is as follows: Figure 1As shown in the figure, increasing the lithium salt concentration can significantly improve the capacity retention of lithium-sulfur batteries, while the introduction of toluene effectively releases the capacity potential of the sulfur cathode.

[0033] The specific capacity properties of samples 1-6 at different expansion rates are as follows: Figure 2 The rate performance test results show that the addition of toluene can significantly improve the kinetic characteristics of the electrochemical reaction in lithium-sulfur batteries and increase the capacity release of the sulfur cathode.

[0034] The performance of samples 5 and 6 by galvanostatic intermittent titration (GITT) is as follows: Figure 3 As shown in the figure, this test further confirms that toluene can improve the sulfur cathode reaction kinetics and reduce voltage polarization in lithium-sulfur battery systems, thereby improving the overall performance of lithium-sulfur batteries.

[0035] The electrolyte microstructures of samples 1, 5, and 6 as shown in the molecular dynamics simulations are as follows: Figure 4 As shown in the figure, simulations reveal that solvent-separated ion pairs (SSIPs) are the predominant form in the 1 mol / L THP electrolyte, while in the 4 mol / L high-concentration THP electrolyte, ions mainly exist as contact ion pairs (CIPs) and aggregates (AGGs). Introducing 40% toluene into the system further increases the proportions of CIPs and AGGs, achieving a reconstruction of the electrolyte's microstructure.

[0036] The electrochemical test results of the above samples are summarized in Table 2.

[0037] Table 2

[0038] The test results show that Sample 6 has an initial discharge specific capacity of 1242 mAh / g at a discharge rate of 0.1C. Sample 6 exhibits good rate performance and capacity retention at different discharge rates of 0.1C, 0.2C, and 0.5C, demonstrating the best overall performance. In this invention, toluene not only acts as a diluent but also has a moderate solubility for elemental sulfur (i.e., "slightly soluble"). This characteristic has a positive effect on improving the cathode reaction kinetics of lithium-sulfur batteries, resulting in a significant improvement in the cycle stability and coulombic efficiency of lithium-sulfur batteries. However, compared to Sample 6, Samples 9 and 10 introduce compounds similar to toluene, ethylbenzene and p-xylene, respectively. Because they do not have the same performance effect as toluene in enhancing the sulfur cathode reaction kinetics, the rate performance and capacity retention of Samples 9 and 10 are not improved.

[0039] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is defined only by the appended claims.

Claims

1. The application of a locally high-concentration weak solvent electrolyte in lithium-sulfur batteries, characterized in that, The locally high-concentration weak solvent electrolyte is composed of lithium salt, tetrahydropyran and toluene, wherein the volume percentage of toluene is 20-50%.

2. The application of the locally high-concentration weak solvent electrolyte according to claim 1 in lithium-sulfur batteries, characterized in that, The lithium salt is lithium bis(trifluoromethanesulfonylimide).

3. The application of the locally high-concentration weak solvent electrolyte according to claim 1 in lithium-sulfur batteries, characterized in that, The concentration of the lithium salt in the electrolyte is 2 mol / L-4 mol / L.

4. The application of the locally high-concentration weak solvent electrolyte according to claim 1 in lithium-sulfur batteries, characterized in that, The concentration of the lithium salt in the electrolyte is 4 mol / L.

5. The application of the locally high-concentration weak solvent electrolyte according to claim 1 in lithium-sulfur batteries, characterized in that, The volume percentage of toluene is 40%.

6. The assembly method of the lithium-sulfur battery according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Under magnetic stirring, lithium salt is added to tetrahydropyran and stirred until the lithium salt is completely dissolved to obtain solution A; (2) Add toluene to solution A and continue stirring to obtain a uniform and transparent solution B; (3) Using solution B as electrolyte, lithium-sulfur batteries were assembled and, after standing, lithium-sulfur batteries were obtained.

7. The assembly method according to claim 6, characterized in that, The settling time is 6-24 hours, and the temperature is 25-35℃.

8. A lithium-sulfur battery assembled by the assembly method of claim 6 or 7.

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