Composite polymer solid electrolyte as well as preparation method and application thereof

Abstract

The invention relates to the field of all-solid-state lithium metal batteries, and discloses a composite polymer solid electrolyte as well as a preparation method and application thereof. The solid electrolyte comprises polyoxyethylene, a lithium salt and a dithiol ligand, the lithium salt is lithium bis (trifluoromethane sulfonimide); the general formula of the dithiol molecule is HS-(CH2) n-SH, and n is equal to 2, 4, 6, 8 or 10. The composite polymer solid electrolyte provided by the invention has good ionic conductivity, and can induce generation of a LiF-rich solid electrolyte interfacial film. 1, 2-ethanedithiol, LiTFSI and PEO are adopted to form a composite polymer solid electrolyte, lithium metal is used as a negative electrode, lithium iron phosphate is used as a positive electrode, the capacity retention ratio of the obtained all-solid-state lithium metal total battery is larger than or equal to 99% after 580 cycles under the 1C multiplying power, and the cycling stability and the multiplying power performance of the all-solid-state lithium metal total battery are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of all-solid-state lithium metal batteries, specifically to a composite polymer solid electrolyte, its preparation method, and its application. Background Technology

[0002] All-solid-state lithium metal batteries (ASSLMBs) have gained unprecedented development prospects due to their intrinsic safety and potential to surpass the energy density limits of traditional liquid batteries. However, interfacial instabilities between the lithium metal anode and the solid electrolyte (such as uncontrolled lithium dendrite growth and interfacial side reactions) hinder the practical application of ASSLMBs. To address these issues, significant efforts have been made to rationally design and construct stable solid electrolyte interphase (SEI) films to significantly optimize lithium deposition / stripping behavior, thereby improving battery performance.

[0003] Tuning lithium salt decomposition in polymer-based solid electrolytes (SEEs) has become a key approach for constructing inorganic-rich SEIs (such as LiF). This principle has driven advancements in lithium salt chemistry, including the precise selection of salts and the tuning of electronic structures to optimize anion decomposition kinetics. To date, numerous lithium salts, such as lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium (benzenesulfonyl)(trifluoromethanesulfonyl)imide (LiBTFSI), have been developed to generate inorganic-rich SEI layers in polymer-based ASSLMBs. However, strategies for tuning the electronic structure of anions remain limited to the design of strongly polar groups in the filler. While ligand modification via non-covalent interactions (e.g., hydrogen bonding) offers a promising alternative for controlling anion coordination chemistry and electronic configuration, systematic ligand screening methods are currently lacking. Summary of the Invention

[0004] This invention aims to provide a design method for composite polymer solid electrolytes based on molecular docking. When adding dithiol molecules with different carbon chain lengths as fillers to polyvinyl oxide (PEO) polymer solid electrolytes, molecular docking calculations are used to screen dithiol ligands that can regulate the coordination chemistry and decomposition kinetics of lithium salt anions. This successfully optimizes the ionic conductivity and interfacial stability of the composite polymer solid electrolyte, thereby significantly improving the cycle stability and rate performance of all-solid-state lithium metal batteries.

[0005] To achieve the above objectives, the present invention provides a composite polymer solid electrolyte comprising polyethylene oxide (PEO), a lithium salt, and a dithiol ligand; wherein the average molecular weight of the polyethylene oxide is 100,000 to 5,000,000; the lithium salt is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); and the general formula of the dithiol molecule is HS-(CH2). n-SH,n = 2, 4, 6, 8 or 10.

[0006] As a further preferred embodiment of the present invention, the mass fraction of the dithiol ligand is 0.1 to 10 wt.%; and / or, the molar ratio of EO:Li in the polyethylene oxide (PEO) and lithium salt is (10 to 20):1.

[0007] As a further preferred embodiment of the present invention, the thiol molecule is 1,2-ethylenedithiol (n=2) with a mass fraction of 5wt%; and / or, the EO:Li molar ratio in the polyethylene oxide (PEO) and lithium salt is 15:1.

[0008] According to another aspect of the present invention, the present invention also provides a method for preparing a composite polymer solid electrolyte, comprising the following steps:

[0009] (1) Heating removes residual water from polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);

[0010] (2) Add lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), polyethylene oxide and dithiol ligand to anhydrous acetonitrile, stir and sonicate to mix thoroughly;

[0011] (3) Pour the fully mixed solution from step (2) into a mold and dry it to obtain a composite polymer solid electrolyte.

[0012] As a further preferred technical solution of the present invention, the drying process in step (3) includes: first, letting the solvent evaporate completely at room temperature, and then placing it on a heating table at 40-45°C for 12-24 hours.

[0013] According to another aspect of the present invention, the present invention also provides an application of a composite polymer solid electrolyte as a solid electrolyte in a lithium battery.

[0014] According to another aspect of the present invention, the present invention also provides a method for screening dithiol ligands in composite polymer solid electrolytes, using the VASP simulation software package for density functional theory (DFT) calculations, comparing a thiol molecule with a different carbon chain length with a TFSI. - The binding energy between them and the mercapto hydrogen atom and TFSI - The bond length of the hydrogen bonds between oxygen atoms is used as a descriptor to screen for thiols with the optimal bond length. The absolute value of the binding energy of the selected dithiol ligands is >0.6 eV, and the larger the absolute value, the better the performance. When the thiol molecule is 1,2-ethanedithiol (n=2), its binding energy with TFSI... - The hydrogen bond length is With a binding energy of -0.79 eV, it possesses the most stable configuration.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0016] 1) The composite polymer solid electrolyte of this invention exhibits excellent ionic conductivity and can induce the formation of a LiF-rich solid electrolyte interface film, thereby improving the electrochemical performance of the battery. Using 1,2-ethylenedithiol, LiTFSI, and PEO to form the composite polymer electrolyte, with lithium metal as the negative electrode and lithium iron phosphate as the positive electrode, the resulting all-solid-state lithium metal full battery retains ≥99% of its capacity after 580 cycles at 1C.

[0017] 2) The method for screening dithiol ligands in composite polymer solid electrolytes provided by this invention uses molecular docking calculations to screen dithiol ligands that can regulate the coordination chemistry and decomposition kinetics of lithium salt anions, thereby successfully optimizing key parameters such as ionic conductivity and interface stability of composite polymer solid electrolytes, and thus significantly improving the cycle stability and rate performance of all-solid-state lithium metal batteries. Attached Figure Description

[0018] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0019] Figure 1 The molecular formulas of thiols with different carbon chain lengths and different numbers of thiol groups involved in the embodiments of the present invention are shown below.

[0020] Figure 2 Dithiol molecules with different carbon chain lengths and multiple TFSI - A molecular docking model diagram;

[0021] Figure 3 This is a molecular docking model diagram and electron transfer number for ethanethiol;

[0022] Figure 4 Cycle performance of lithium symmetric batteries modified with dithiol molecules in PEO-LiTFSI;

[0023] Figure 5 A comparison chart of lithium metal battery performance at a 1.0C rate;

[0024] Figure 6 A comparison chart of lithium metal battery performance at different rate of increase;

[0025] Figure 7 This is a characterization diagram of SEI components.

[0026] 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

[0027] 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.

[0028] 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.

[0029] Examples 1-45

[0030] The preparation method of the composite PEO-based solid electrolyte provided in this series of embodiments is as follows:

[0031] (1) Polyethylene oxide (PEO, average molecular weight 600,000) and lithium salt (LiTFSI) were heated at 60°C and 120°C for 48 hours respectively to remove residual water.

[0032] (2) According to the formulation in Table 1, PEO, LiTFSI and dithiol ligand are dissolved in 15 mL of anhydrous acetonitrile, and the mixed solution is uniformly dispersed under the combined action of magnetic stirring and ultrasonic treatment.

[0033] (3) Pour the thoroughly mixed solution from step (2) into a polytetrafluoroethylene (PTFE) mold and let it stand at room temperature for 8 hours. Once the solvent has completely evaporated, place the mixture on a heating platform at 45°C for 24 hours. Finally, cut the resulting composite polymer solid electrolyte into the required size for subsequent battery assembly and electrochemical performance testing.

[0034] This series of embodiments also provides a method for assembling a lithium metal battery using the above-mentioned composite PEO-based solid electrolyte, including:

[0035] (1) A mixture of lithium iron phosphate (LiFePO4), polyvinylidene fluoride (PVDF), conductive carbon black (Super P), and PEO+LiTFSI (with the EO:Li molar ratio consistent with that in the corresponding PEO-based solid electrolyte of the battery) was mixed in N-methylpyrrolidone (NMP) at a mass ratio of 7:1:1:1 to form a positive electrode slurry. The slurry was homogenized by magnetic stirring and ultrasonic treatment. The polyvinylidene fluoride had a molecular weight of 700,000 and was purchased from Zhengzhou Jinghong New Energy Technology Co., Ltd. The conductive carbon black was also purchased from Zhengzhou Jinghong New Energy Technology Co., Ltd. The mass of N-methylpyrrolidone was 25 times the mass of the mixed powder.

[0036] (2) The slurry is coated onto an aluminum current collector (carbon-coated aluminum foil) and placed in a forced-air drying oven to remove most of the solvent, followed by drying in a vacuum oven at 60°C for 24 hours. After removal, it is pressed into a sheet at 12 MPa using a tablet press to obtain a sheet material. The sheet material is then cut into 12 mm diameter pieces using a cutting machine to obtain a positive electrode sheet.

[0037] (3) The battery was assembled in the order of positive electrode shell, positive electrode sheet, PEO-based solid electrolyte membrane, negative electrode sheet, nickel foam and negative electrode shell; the positive electrode shell and negative electrode shell are both model 2032 and were purchased from Henan Pengxiang Yunda Company.

[0038] The performance parameters of the composite PEO-based solid electrolyte and lithium metal battery in Examples 1-45 are shown in Table 1.

[0039] Comparative Examples 1-12

[0040] This series of comparative examples provides a method for assembling a basic PEO-based polymer solid electrolyte and a lithium metal battery without the addition of thiol molecules, including:

[0041] (1) Pretreatment of PEO and LiTFSI: Polyethylene oxide (PEO) and lithium salt (LiTFSI) were heated at 60°C and 120°C for 48 hours, respectively, to remove residual water;

[0042] (2) Preparation of PEO-based solid electrolyte: According to the formula in Table 1, PEO, LiTFSI and thiol molecules are dissolved in 15 mL of anhydrous acetonitrile. The thoroughly mixed solution is poured into a polytetrafluoroethylene (PTFE) mold and left to stand at room temperature for 8 hours. Once the solvent has completely evaporated, the mixture is placed on a heating platform at 45°C for 24 hours. The solid electrolyte membrane after molding is then cut into the required size.

[0043] (3) Battery assembly: The lithium metal battery was assembled according to the method of Example 1 and the electrochemical performance was tested. Except for the difference in the positive electrode slurry and electrolyte membrane, the other preparation parameters and assembly parameters were the same as those in Example 1.

[0044] The performance parameters of the basic PEO-based solid electrolyte and lithium metal battery in this comparative example are shown in Table 1.

[0045] Table 1. Preparation and performance parameters of Examples 1-45 and Comparative Examples 1-12

[0046]

[0047]

[0048]

[0049] Based on the above embodiments and comparative examples, for the addition of dithiol molecules with different carbon chain lengths as fillers to polyvinyl oxide (PEO) polymer solid electrolytes, molecular docking calculations were used to screen for the optimal dithiol ligands to regulate the coordination chemistry and decomposition kinetics of lithium salt anions, thereby optimizing the SEI composition and improving battery cycle stability and rate performance. Density functional theory (DFT) calculations were performed using the VASP simulation software package for dithiol molecules (HS-(CH2)) with different carbon chain lengths. n -SH, n = 2, 4, 6, 8 or 10) and TFSI - The binding energy and interactions between thiols were analyzed to screen for thiols of optimal length. The generalized gradient approximation in the form of Perdew-Burke-Ernzerhof (PBE) and the projected enhanced wave pseudopotential were used to describe the exchange correlation function and electron-ion interactions; spin polarization was included to account for magnetic effects. Furthermore, the Grimme scheme was used in conjunction with damped van der Waals corrections to improve the accuracy of non-bonding interactions. A kinetic energy cutoff of 450 eV was used for the plane-wave basis set. The convergence criterion for the total energy was set to 1.0 × 10⁶ eV. The Brillouin zone integral was performed using Monkhorst-Pack k-point sampling: 2 × 2 × 1 for relaxation and self-consistent calculations.

[0050] Performance Evaluation

[0051] 1. Thiol molecules and TFSI - Molecular docking calculations:

[0052] Molecular docking is strategically used for screening TFSI. - The ligands were designed to modulate their coordination chemistry and electronic configuration to accelerate their decomposition. Dithiols were chosen as the model system because of their bis-SH groups ( Figure 1 ) Structurally, it can simultaneously work with TFSI - The two electrophilic S=O groups in the thiol are coordinated. Table 1 summarizes the results of molecular docking simulations of thiols with different carbon chain lengths and different numbers of thiol groups. Here, an example is given with an EO:Li molar ratio of 15:1 and a thiol molecular weight fraction of 5% in a PEO-based composite electrolyte. The significantly higher absolute binding energy of 1,2-ethanedithiol (C2, 0.79 eV) compared to ethanethiol (ET, 0.38 eV) confirms this advantage. Figure 2 and Figure 3 ). Figure 2 Figure 'a' illustrates the molecular docking binding energy (ΔG) as a function of the carbon chain length of the dithiol. Figure 2The corresponding molecular docking simulation results are shown in Figure bd. Negative ΔG values ​​indicate spontaneous hydrogen bonding between the -SH group in the dithiol and the S=O moiety in TFSI, with larger |ΔG| values ​​correlated with enhanced thermodynamic stability of the dithiol-TFSI- complex. As the carbon chain length increases from 2 to 10, |ΔG| monotonically decreases from 0.79 eV (C2) to 0.57 eV (1,10-decanedithiol, C10), demonstrating an inverse relationship between alkyl chain elongation and molecular docking efficiency. This chain length-dependent trend highlights the spatial complementarity of molecular docking. Specifically, the C2-TFSI- complex ( Figure 2 b) shows a near-ideal three-dimensional arrangement, with two –SH groups forming ultrashort SH···O=S hydrogen bonds. In contrast, longer alkyl chains lead to increased steric hindrance, disrupting the optimal alignment between the -SH groups and the S=O moiety, thus lengthening the SH···O=S bond. In the cases of 1,4-butanedithiol (C4) and 1,6-hexanedithiol (C6), although both -SH groups can form hydrogen bonds with both S=O moieties, the SH···O=S bond lengths increase significantly. and ( Figure 2 In the case of 1,8-octanedithiol (C8) and C10, the distance between the two -SH groups is too large to simultaneously coordinate the S=O group within a single TFSI-, resulting in a single elongated SH···O=S bond within the complex. and ).

[0053] Charge density difference calculations demonstrate that different molecular docking configurations significantly affect intermolecular charge transfer between dithiols and TFSI. Figure 2 In the diagram (bc), the yellow area represents the charge accumulation region, while the blue area represents the charge depletion region. It's worth noting that the yellow area is related to dithiols, while the blue area primarily corresponds to TFSI. - This indicates that electrons are transferred from dithiol to TFSI. - This transfer produces electron-rich TFSI. - This promotes its decomposition. Furthermore, in C2-TFSI... - More yellow and blue areas were observed in the system. Quantitative analysis showed that each TFSI - 0.49e can be obtained from C2, C4 and ET respectively. - 0.40e - and 0.37e - These results indicate that a greater amount of charge is transferred from C2 to TFSI compared to C4 and ET. - Therefore, C2 is expected to accelerate TFSI. -Decompose and construct the optimal ligands for LiF-rich SEIs.

[0054] 2. Performance testing of Li||Li symmetric cells:

[0055] Taking a PEO-based composite electrolyte with an EO:Li molar ratio of 15:1 and a thiol molecular weight fraction of 5% as an example, this is illustrated by using a 0.1 mA cm⁻¹... -2 and 0.1mAh cm -2 The results of the molecular docking prediction were evaluated by constant current cycling tests. Figure 4 The cycling stability of Li||Li symmetric batteries equipped with dithiol-modified PEO-LiTFSI was summarized. Compared with PEO-LiTFSI, incorporating dithiol molecules with different carbon chain lengths into the electrolyte membrane generally improved the cycle life of Li||Li symmetric batteries and reduced the overpotential, demonstrating the effectiveness of dithiols. However, as the carbon chain length of the dithiol molecule increased from 2 to 10, the cycle life of the battery continuously decreased from 2000 hours to 1040 hours, accompanied by an increase in overpotential. This chain length-dependent trend is exactly the same as that obtained from molecular docking, validating molecular docking as a predictive tool for SEI engineering. The predicted champion electrolyte, PEO-LiTFSI-C2, exhibited the best performance, maintaining a stable overpotential of 59 mV after interfacial stabilization and a cumulative cycle time of 2000 hours. This indicates that PEO-LiTFSI-C2 promotes more uniform lithium deposition and reduces the occurrence of interfacial side reactions.

[0056] 3. LFP||Li full cell performance test:

[0057] LFP||Li full cells were assembled and tested at 50°C within a potential range of 2.5 to 3.8 V. The results are summarized in Table 1. Figure 5 As shown, the initial discharge specific capacity of LFP||PEO-LiTFSI-C2||Li at 1.0C reaches 152.0 mAh g. -1 After 580 cycles, the specific capacity stabilized at 150.8 mAh g⁻¹, corresponding to a capacity retention rate as high as 99.2%. Figure 6 For rate performance at different rates from 0.2 to 2.0C, the LFP||PEO-LiTFSI-C2||Li battery exhibits a high discharge specific capacity within this range. At 2.0C, the coulombic efficiency of both LFP||PEO-LiTFSI-C4||Li and LFP||PEO-LiTFSI||Li drops below 95%, indicating that their SEI cannot ensure rapid Li transport at high speeds. +Conversely, the LFP||PEO-LiTFSI-C2||Li battery continued to operate efficiently at high rates. When the current density returned to 0.2C, it maintained a mAh / g... -1 The discharge specific capacity reflects excellent rate performance.

[0058] 3. X-ray photoelectron spectroscopy and cryogenic transmission electron microscopy tests:

[0059] The effects of thiol molecules with different carbon chain lengths on TFSI were determined using X-ray photoelectron spectroscopy and cryogenic transmission electron microscopy. - The degree of catalytic decomposition demonstrates the rationality of its molecular docking. The composition of SEI is investigated as follows: Figure 7 As shown in Figure 1, in PEO-LiTFSI-C2, 73.5% of the fluoride is LiF; in PEO-LiTFSI-C4 and PEO-LiTFSI, the percentage of LiF decreases to 46.5% and 12.8%, respectively. Given TFSI... - It is the sole source of fluorine, and the varying LiF content originates from TFSI. - Different decomposition behaviors of C2 and TFSI. - Molecular docking between them facilitated the transfer of electrons from C2 to TFSI. - This accelerates the decomposition of TFSI- in SEI to form LiF.

[0060] In addition, such as Figure 7 As shown in the middle df, the SEI of Li||PEO-LiTFSI-C2||Cu exhibits a dense and uniform morphology; a similar dense morphology is observed in the SEI of Li||PEO-LiTFSI-C4||Cu; in stark contrast, the SEI of Li||PEO-LiTFSI-C4||Cu exhibits an irregular and fragmented deposition structure. High-resolution TEM images show that ( Figure 7 In PEO-LiTFSI-C2, LiF accounts for approximately 12% of the entire region; while in PEO-LiTFSI-C4, the content decreases to 4%. In contrast, the SEI of PEO-LiTFSI consists of a large number of amorphous regions and a small amount of Li2O / LiOH nanocrystals, which are generally considered unfavorable for Li. + Transmission performance.

[0061] Based on all the results, it can be concluded that molecular docking with C2 successfully designed TFSI. - The coordination chemistry of this process accelerates its decomposition and makes LiF-rich SEI possible.

[0062] 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. A composite polymer solid electrolyte, characterized in that, It includes polyethylene oxide, a lithium salt, and a dithiol ligand; the lithium salt is lithium bis(trifluoromethanesulfonylimide); the general formula of the dithiol molecule is: HS-(CH2). n -SH,n = 2, 4, 6, 8 or 10.

2. The composite polymer solid electrolyte according to claim 1, characterized in that, The mass fraction of the dithiol ligand is 0.1 to 10 wt.%; and / or, the EO:Li molar ratio in the polyethylene oxide and lithium salt is (10 to 20):

1.

3. The composite polymer solid electrolyte according to claim 1, characterized in that, The thiol molecule is 1,2-ethylenedithiol, with a mass fraction of 5 wt%, and the EO:Li molar ratio in the polyethylene oxide and lithium salt is 15:

1.

4. The composite polymer solid electrolyte according to claim 1, characterized in that, The average molecular weight of the polyethylene oxide is 100,000 to 5,000,000.

5. A method for preparing the composite polymer solid electrolyte according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Heating removes residual water from polyethylene oxide and lithium bis(trifluoromethanesulfonylimide); (2) Add lithium bis(trifluoromethanesulfonylimide), polyethylene oxide and dithiol ligand to anhydrous acetonitrile, stir and sonicate to mix thoroughly; (3) Pour the fully mixed solution from step (2) into a mold and dry it to obtain a composite polymer solid electrolyte.

6. The method for preparing the composite polymer solid electrolyte according to claim 5, characterized in that, The drying process in step (3) includes: first, letting the solvent evaporate completely at room temperature, and then placing it on a heating table at 40-45°C for 12-24 hours.

7. The application of the composite polymer solid electrolyte according to any one of claims 1-4 as a solid electrolyte in a lithium battery.

8. A method for screening dithiol ligands in the composite polymer solid electrolyte of claim 1, characterized in that, Density functional theory calculations were performed using the VASP simulation software package, with a thiol molecule of different carbon chain length and a TFSI. - The binding energy between the two ligands is used as a descriptor to screen for thiol molecules of the optimal length. The absolute value of the binding energy of the screened dithiol ligands is >0.6 eV, and the larger the absolute value, the better the performance.

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