A lithium-sulfur battery electrolyte regulation method for promoting stable conversion of odd lithium polysulfides

Abstract

The application belongs to the technical field of new energy storage and electrochemical energy storage, and specifically discloses a lithium-sulfur battery electrolyte regulation method for promoting stable conversion of odd lithium polysulfides. The method is to introduce a high electron-donating ability polar aprotic additive (one or more of TMU, DMSO, DMF, and DMA) into a conventional ether-based electrolyte to configure a regulation electrolyte, and assemble a lithium-sulfur battery by using a sulfur positive electrode, a lithium metal negative electrode, a separator, and the regulation electrolyte. The regulation electrolyte can regulate the solvation structure and electron distribution of lithium polysulfides, promote the formation and stable existence of odd lithium polysulfides, reduce the Gibbs free energy and reaction activation energy in the conversion process of lithium polysulfides, thereby improving the sulfur utilization rate and charge-discharge performance of the lithium-sulfur battery.

Description

Technical Field

[0001] This invention belongs to the field of new energy storage and electrochemical energy storage technology, specifically relating to a method for regulating the electrolyte of a lithium-sulfur battery, and more particularly to a method for regulating the solvation structure of lithium polysulfides, promoting the stable conversion of odd-numbered lithium polysulfides, and improving the electrochemical performance of lithium-sulfur batteries by using high electron-donating polar aprotic additives. Background Technology

[0002] Lithium-sulfur batteries are characterized by their high theoretical specific capacity (1675 mAh·g). -1 ) and high theoretical energy density (2600 Wh·kg) -1 Lithium-sulfur batteries are considered an important development direction for the next generation of high-energy-density energy storage systems. However, lithium-sulfur batteries still face many key problems in practical applications, the most critical of which include:

[0003] (1) Sulfur and its discharge products have poor electrical conductivity;

[0004] (2) Lithium polysulfides (Li2S) n The dissolution and migration of (4≤n≤8) in the electrolyte leads to a severe "shuttle effect";

[0005] (3) The conversion kinetics of polysulfides to Li2S2 / Li2S are slow;

[0006] (4) An insulating passivation layer is easily formed during the liquid-solid conversion process, which limits the utilization rate of sulfur;

[0007] (5) Traditional ether electrolytes have insufficient ability to stabilize odd-numbered lithium polysulfide intermediates.

[0008] Currently, widely used DOL / DME ether-based electrolytes have low dielectric constants and limited solvation capabilities for lithium polysulfides, making it difficult to stabilize Li₂S₃, Li₂S₅, and S₃· - The presence of odd-numbered lithium polysulfide intermediates leads to a high energy barrier and slow conversion kinetics during the reaction. Furthermore, the continuous diffusion of polysulfides in the electrolyte causes loss of active material and side reactions at the lithium anode, resulting in capacity decay and reduced cycle life. Therefore, improving the charge-discharge performance of lithium-sulfur batteries by using catalytic materials to lower the energy barrier of polysulfide conversion and accelerate Li₂S… n The stepwise solid-liquid conversion can effectively suppress the shuttle effect of soluble polysulfides, reduce the irreversible loss of active sulfur and the side reaction of lithium metal anode, and improve the deposition and oxidation reversibility of Li2, avoiding the continuous accumulation of insulating dead sulfur. This can slow down the capacity decay rate during cycling to a certain extent and significantly extend the battery cycle life.

[0009] Therefore, developing an electrolyte regulation method that can stabilize odd-numbered lithium polysulfides, reduce reaction energy barriers, and promote rapid polysulfide conversion is of great significance for improving the charge-discharge performance of lithium-sulfur batteries. Summary of the Invention

[0010] The purpose of this invention is to provide a method for regulating the electrolyte of lithium-sulfur batteries to promote the stable conversion of odd-numbered lithium polysulfides. By introducing polar aprotic additives with high electron-donating ability into traditional ether-based electrolytes, the solvation structure and electron distribution of lithium polysulfides are regulated, promoting the formation and stable existence of odd-numbered lithium polysulfides, reducing the Gibbs free energy and reaction activation energy during the conversion of lithium polysulfides, thereby improving the sulfur utilization rate and charge-discharge performance of lithium-sulfur batteries.

[0011] To achieve the above objectives, the specific technical solution adopted by the present invention is as follows:

[0012] In a first aspect, the present invention provides a method for regulating a lithium-sulfur battery electrolyte to promote the stable conversion of odd-numbered lithium polysulfides, comprising the following steps:

[0013] S1. Preparation of the regulating electrolyte: A polar aprotic additive with high electron-donating ability is added to an ether solvent to obtain a mixed solvent. Lithium salt is added to the mixed solvent to form a regulating electrolyte.

[0014] The polar aprotic additive is selected from one or more of tetramethylurea (TMU), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA); the ether solvent is selected from one or more of 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), and diethyl ether (DEE); and the lithium salt is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium hexafluorophosphate (LiPF6).

[0015] S2. Constructing a catalytic cathode system: Using Co-doped Ni2P as the cathode catalytic material for lithium-sulfur batteries, a sulfur cathode was prepared;

[0016] S3. Battery assembly: A lithium-sulfur battery is assembled using a sulfur positive electrode, a lithium metal negative electrode, a separator, and the aforementioned controlled electrolyte.

[0017] Preferably, in step S1, after the lithium salt is added to the mixed solvent, LiNO3 is also added to obtain the regulated electrolyte. LiNO3 can suppress the lithium dendrite and polysulfide shuttle effect at the lithium anode, while optimizing the lithium-ion solvation structure, passivating the aluminum current collector, and stabilizing the cathode interface, thereby comprehensively improving the battery's cycle stability and electrochemical performance.

[0018] Preferably, in step S1, the volume ratio of the polar aprotic additive to the ether solvent is 1:1.

[0019] Preferably, in step S1, the polar aprotic additive is TMU and the ether solvent is DOL.

[0020] Preferably, in step S2, the Co-doped Ni2P is prepared by a hydrothermal method combined with a phosphating process. Co atoms partially replace the Ni sites in Ni2P to form a Co-Ni2P catalytic structure. The partial replacement of Ni sites in Ni2P by Co to construct Co-Ni2P significantly changes the electrode-electrolyte interface behavior, thereby directly regulating the performance of various electrolyte functions. It is a chain reaction dominated by interface properties, mainly optimizing electrolyte wetting, ion transport, interface stability, and side reaction suppression, while also changing electrolyte compatibility.

[0021] Furthermore, the specific preparation steps of the Co-doped Ni2P are as follows: Ni(NO3)2·6H2O and Co(NO3)2·6H2O are added to deionized water, NaH2PO2 is added as a phosphorus source, and a precursor is obtained after hydrothermal reaction. Then, Co-Ni2P catalytic material is obtained by high-temperature phosphating treatment.

[0022] Preferably, in step S2, the sulfur cathode is obtained by mixing sulfur powder, conductive carbon black and Co-Ni2P in a mass ratio of 7:2:1, then adding PVDF binder to form a slurry, coating it onto the surface of an aluminum foil current collector, and drying it.

[0023] Preferably, in step S3, the lithium-sulfur battery is a CR2032 type lithium-sulfur button battery, and the assembly steps are as follows: place the sulfur-based positive electrode sheet in the CR2032 positive electrode shell, add the regulating electrolyte to wet it, cover it with the Celgard separator and add electrolyte again to wet it, place the metallic lithium negative electrode sheet, and then put in the stainless steel gasket and spring sheet in sequence before covering it with the negative electrode shell and closing the shell.

[0024] Secondly, the present invention provides a controlled electrolyte for lithium-sulfur batteries, comprising a lithium salt, an ether solvent, and a polar aprotic additive; wherein the lithium salt is selected from one or more of LiTFSI, LiFSI, and LiPF6; the ether solvent is selected from one or more of DOL, DME, and DEE; and the polar aprotic additive is selected from one or more of TMU, DMSO, DMF, and DMA.

[0025] Preferably, the regulating electrolyte further includes LiNO3.

[0026] Preferably, the volume ratio of the polar aprotic additive to the ether solvent is 1:1.

[0027] Preferably, the polar aprotic additive is TMU, and the ether solvent is DOL.

[0028] The present invention has the following beneficial effects:

[0029] This invention introduces a polar aprotic additive with high electron-donating capability into a traditional ether-based electrolyte to regulate the solvation structure and electron distribution of lithium polysulfides, promote the formation and stable existence of odd-numbered lithium polysulfides, reduce the Gibbs free energy and reaction activation energy during the lithium polysulfide conversion process, improve the charge and discharge performance of lithium-sulfur batteries, and overcome the problem that currently widely used ether-based electrolytes are difficult to stabilize odd-numbered lithium polysulfide intermediates, resulting in high energy barriers and slow conversion kinetics during the reaction process. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the TMU-regulated lithium polysulfide reaction pathway in this invention (DFT calculation results).

[0031] Figure 2 The results are from the electrochemical performance tests of the assembled battery. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0033] Example 1

[0034] 1. Preparation of basic electrolyte and regulating electrolyte

[0035] LiTFSI was added to a DOL / DME mixed solvent, wherein the volume ratio of DOL to DME was 1:1 and the concentration of LiTFSI in the mixed solvent was 1 mol / L; then 2 wt.% LiNO3 was added to obtain the basic electrolyte, which is the DME system.

[0036] LiTFSI was added to a DOL / DEE mixed solvent, wherein the volume ratio of DOL to DEE was 1:1 and the concentration of LiTFSI in the mixed solvent was 1 mol / L; then 2 wt.% LiNO3 was added to obtain the basic electrolyte, which is the DEE system.

[0037] LiTFSI was added to a DOL / TMU mixed solvent, wherein the volume ratio of DOL to TMU (tetramethylurea) was 1:1, and the concentration of LiTFSI in the mixed solvent was 1 mol / L; then 2 wt.% LiNO3 was added to obtain the controlled electrolyte, which is the TMU system.

[0038] 2. Preparation of Co-Ni2P catalytic materials

[0039] Ni(NO3)2·6H2O and Co(NO3)2·6H2O were added to deionized water, and NaH2PO2 was added as a phosphorus source. After hydrothermal reaction, the mixture was phosphating under an argon atmosphere to obtain Co-Ni2P catalyst material. The specific method is as follows:

[0040] (1) Material proportions

[0041] Taking a total metal ion content of 10 mmol as an example, a Co:Ni molar ratio of 1:4 is recommended (the doping ratio can be changed according to experimental requirements):

[0042] Ni(NO3)2·6H2O: 8 mmol;

[0043] Co(NO3)2·6H2O: 2 mmol;

[0044] Phosphorus source (NaH2PO2): 4 to 6 times the total amount of metal (take 5 times, 50 mmol, to ensure sufficient phosphating);

[0045] Solvent: 40-60 mL of deionized water (solid-liquid ratio suitable for 100 mL hydrothermal reactor).

[0046] (2) Operating steps

[0047] ① Dissolving metal salts: Weigh Ni(NO3)2·6H2O and Co(NO3)2·6H2O in sequence and add them to a beaker containing 40 mL of deionized water. Stir magnetically for 10-15 min at room temperature until the solids are completely dissolved to obtain a uniform light pink mixed metal salt solution.

[0048] ② Add phosphorus source and mix well: Add the weighed NaH2PO2 to the above solution in batches, continue to stir magnetically for 15 minutes, and then sonicate for 5 minutes to eliminate local uneven concentration and ensure that the phosphorus source and metal ions are fully mixed.

[0049] ③ Hydrothermal reaction: Transfer the mixed solution to a 100 mL polytetrafluoroethylene (PTFE) hydrothermal reactor liner and seal the reactor. Set the oven temperature to 160–180 °C and maintain the reaction temperature for 10–14 h. Function: Metal ions undergo coordination and hydrolysis reactions with hypophosphite to generate a uniform Co-Ni composite precursor, achieving atomic-level dispersion of Co atoms in the precursor and laying the foundation for subsequent site substitution.

[0050] ④ Cooling, separation and washing: After the hydrothermal treatment is completed, allow the reactor to cool naturally to room temperature. Pour out the suspension and separate the solid product by centrifugation (8000 r / min, 5 min) or filtration. Wash the product 3-4 times each with deionized water and anhydrous ethanol to remove soluble impurities such as residual nitrate, sodium ions, and unreacted phosphorus sources.

[0051] ⑤ Precursor drying and phosphating: The washed solid product was transferred to a petri dish and placed in a forced-air drying oven. It was dried at 60 ℃ under vacuum or normal pressure for 12 h to completely remove the solvent, obtaining a dried Co-Ni-P composite precursor powder. After grinding, it was ready for use. Then, it was subjected to high-temperature (550 ℃) phosphating under an argon atmosphere to obtain a Co-Ni2P catalytic material in which Co atoms partially substituted Ni sites.

[0052] 3. Preparation of sulfur cathode

[0053] Sulfur powder, conductive carbon black and Co-Ni2P are mixed in a mass ratio of 7:2:1, and PVDF binder is added to form a slurry. The slurry is then uniformly coated onto the surface of an aluminum foil current collector and dried to obtain a sulfur cathode.

[0054] 4. Battery assembly

[0055] A CR2032 lithium-sulfur button battery was assembled using lithium metal as the negative electrode, a Celgard membrane as the separator, a sulfur positive electrode, a lithium metal negative electrode, a separator, and the aforementioned controlled electrolyte. The assembly steps are as follows:

[0056] (1) Place the CR2032 positive electrode shell flat, place the prepared sulfur-based positive electrode sheet in the center at the bottom of the shell, with the active material coated side facing up;

[0057] (2) Use a pipette to take an appropriate amount of electrolyte and drop it evenly onto the surface of the positive electrode to fully wet the electrode;

[0058] (3) Cut a Celgard diaphragm of matching size and flatten it on the positive electrode to ensure that the electrode is completely covered without any offset or wrinkles. Then add a small amount of electrolyte to wet the diaphragm.

[0059] (4) Place the cut lithium metal anode sheet in the center above the separator, with the bright lithium side facing the separator;

[0060] (5) Place the stainless steel washer and spring sheet in sequence;

[0061] (6) Cover the CR2032 negative electrode shell to complete the initial casing of the battery and obtain the TMU system battery;

[0062] (7) Replace the TMU system electrolyte with the DME system and DEE system base electrolyte respectively, and assemble the battery according to the above steps to obtain DME system and DEE system batteries, which are used as control groups in subsequent performance tests.

[0063] 5. Performance Testing

[0064] (1) DFT calculation

[0065] This study employs density functional theory (DFT) calculations, using the Perdew–Burke–Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) to describe the electron exchange correlation energy. Grimme D3 dispersion correction is introduced to accurately correct the van der Waals interactions between the catalyst and polysulfides. Projected fused wave (PAW) pseudopotentials are used to handle the interaction between the ionic core and valence electrons. The plane-wave basis set cutoff energy is set to 400 eV, the system energy convergence threshold is 10 eV, and the atomic force convergence threshold is 0.02 eV·Å. Spin polarization is enabled throughout the calculations to accommodate the electronic structure characteristics of the transition metal system. Structural optimization and single-point energy calculations use 3×3×1 and 5×5×1 Brillouin zone k-point grids, respectively, with a vacuum layer thickness of 15 Å to eliminate interlayer interference from adjacent periodic models.

[0066] Based on the above calculation methods and parameter settings, the conversion process of lithium polysulfides in different electrolyte systems was analyzed thermodynamically and kinetically, as follows:

[0067] ① Activation energy of reaction (kinetic energy barrier (Ea))

[0068] Apparent activation energy is defined as the difference between the electronic energy of the transition state and the reactants:

[0069] E a = E TS - E Reactant

[0070] In the formula: E TS E is the transition state electron energy. Reactant The stable structure electron energy of the initial reactants.

[0071] ② Gibbs free energy correction and ΔG calculation

[0072] At 298.15 K and standard atmospheric pressure, the Gibbs free energy is obtained by performing zero-point energy (ZPE), entropy (S), and enthalpy (H) thermal corrections on the static electron energy.

[0073] G = E ele + ZPE + H corr -TS

[0074] Zero-point energy (ZPE), vibrational entropy, and enthalpy correction are obtained from frequency analysis;

[0075] Frequency calculations are performed only on the optimized steady-state and transient structures, and not on low-frequency vibrations (<100cm). -1 The quasi-resonant approximation is corrected using this method.

[0076] Gibbs free energy change of a single elementary reaction:

[0077] ΔG = G Product / Intermediat e - G Reactant

[0078] The DFT calculation results show that the data is... Figure 1 The TMU system exhibits a lower activation energy (orange line in the left figure) and a lower ΔGmax (black line in the right figure), indicating that TMU can effectively promote the conversion kinetics of lithium polysulfides and facilitate the stable conversion of odd-numbered lithium polysulfides.

[0079] (2) Electrochemical performance testing

[0080] The assembled batteries of the three systems were subjected to constant current charge-discharge tests at a rate of 0.2 C. The results are as follows: Figure 2 As shown, the initial discharge capacity of the lithium-sulfur battery using TMU-controlled electrolyte reaches 480 mAh·g. -1 Higher than the 340mAh·g of the traditional DME system -1 360 mAh·g of the DEE system -1 This demonstrates that the electrolyte regulation method of the present invention can effectively improve the charge and discharge performance of lithium-sulfur batteries.

[0081] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. Any changes made by those skilled in the art after reading the specification of the present invention, as long as they are within the scope of the claims of the present invention, will be protected by patent law.

Claims

1. A method for regulating lithium-sulfur battery electrolyte to promote stable conversion of odd-numbered lithium polysulfides, characterized in that, Includes the following steps: S1. Preparation of the regulating electrolyte: A polar aprotic additive is added to an ether solvent to obtain a mixed solvent, and a lithium salt is added to the mixed solvent to form a regulating electrolyte; the ether solvent is selected from one or more of DOL, DME, and DEE, the polar aprotic additive is selected from one or more of TMU, DMSO, DMF, and DMA, and the lithium salt is selected from one or more of LiTFSI, LiFSI, and LiPF6; S2. Constructing a catalytic cathode system: Using Co-doped Ni2P as the cathode catalytic material for lithium-sulfur batteries, a sulfur cathode was prepared; S3. Battery assembly: A lithium-sulfur battery is assembled using a sulfur positive electrode, a lithium metal negative electrode, a separator, and the aforementioned controlled electrolyte.

2. The control method according to claim 1, characterized in that, In step S1, lithium salt is added to the mixed solvent, and then LiNO3 is added to obtain the regulated electrolyte.

3. The control method according to claim 1, characterized in that, In step S1, the volume ratio of ether solvent to polar aprotic additive in the mixed solvent is 1:

1.

4. The control method according to any one of claims 1-3, characterized in that, In step S1, the polar aprotic additive is TMU, and the ether solvent is DOL.

5. The control method according to claim 1, characterized in that, In step S2, the Co-doped Ni2P is prepared by a hydrothermal method combined with a phosphating process. The specific preparation steps are as follows: Ni(NO3)2·6H2O and Co(NO3)2·6H2O are added to deionized water, NaH2PO2 is added as a phosphorus source, and a precursor is obtained after hydrothermal reaction. Then, Co-Ni2P catalytic material is obtained by high-temperature phosphating treatment.

6. The control method according to claim 1, characterized in that, In step S2, the sulfur cathode is obtained by mixing sulfur powder, conductive carbon black and Co-doped Ni2P, adding PVDF binder to form a slurry, coating it onto the surface of an aluminum foil current collector, and then drying it.

7. The control method according to claim 1, characterized in that, In step S3, the separator is a Celgard separator, and the lithium-sulfur battery is a CR2032 lithium-sulfur button battery.

8. A controlled electrolyte for lithium-sulfur batteries, characterized in that, The controlled electrolyte includes lithium salts, ether solvents, and polar aprotic additives; The lithium salt is selected from one or more of LiTFSI, LiFSI, and LiPF6; the ether solvent is selected from one or more of DOL, DME, and DEE. The polar aprotic additive is selected from one or more of TMU, DMSO, DMF, and DMA.

9. The electrolyte for regulating electrolyte according to claim 8, characterized in that, The electrolyte also includes LiNO3.

10. The electrolyte for regulating electrolyte according to claim 8, characterized in that, The polar aprotic additive is TMU, and the ether solvent is DOL, with a volume ratio of 1:1.

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