Fluorinated polymers and their use in electrode protection layers, solid state electrolytes

Abstract

The application discloses a fluorinated polymer and application thereof in electrode protection layer and solid-state electrolyte, and belongs to the technical field of lithium metal secondary batteries. The fluorinated polymer is obtained by the reaction of acrylate functionalized polyethylene glycol, HFBA and an initiator. When the fluorinated polymer is used as a lithium metal negative electrode protection layer, a LiFePO4 / Li full battery protected by the polymer shows more excellent cycle performance than an unprotected full battery. The battery cycle performance is stable, the capacity retention rate is high, and the capacity attenuation speed is slow. When the fluorinated polymer is applied to a solid-state electrolyte, the polymer solid-state electrolyte can form a polymer network to wrap ion-conducting plastic crystals through in-situ reaction, effectively inhibits the growth of Li dendrites, and ensures the stability of the polymer electrolyte and the lithium negative electrode. Moreover, the solid-state electrolyte can be adapted to high-nickel ternary positive electrode materials, providing a train of thought for realizing stable, safe and high-energy-density batteries.

Description

Technical Field

[0001] This invention relates to a fluorinated polymer and its application in electrode protective layers and solid electrolytes, belonging to the field of lithium metal secondary battery technology. Background Technology

[0002] Lithium metal batteries (LMBs) using organic electrolytes suffer from problems such as uncontrolled lithium dendrite growth, unstable SEI (solid electrolyte interphase) layers, formation of "dead" lithium, and negative electrode volume expansion during cycling, leading to battery failure and safety issues. Solid polymer electrolytes (SPEs), composed of a polymer matrix and lithium salts, offer higher safety compared to liquid electrolytes. Furthermore, SPEs exhibit high flexibility, resulting in better interfacial contact and compatibility with the lithium negative electrode compared to inorganic solid electrolytes. However, solid polymer electrolytes based on polyethylene oxide (PEO) operate below their crystallization temperature at room temperature, leading to a high migration barrier for lithium ions in the polymer crystal region. This results in low ionic conductivity, limiting battery performance.

[0003] To improve the performance of polymer solid electrolytes (LMBs), researchers have explored various physical and chemical methods, such as inorganic material blending, polymer blending, crosslinking, and copolymerization. A study published in Adv. Sci. by Sui Gang et al., entitled "Self-Enhancing Gel Polymer Electrolyte by In Situ Construction for Enabling Safe Lithium Metal Battery," demonstrates that in-situ polymerization using nanofibers as a framework and 1,3-dioxolane (DOL) as a monomer can construct a homogeneous gel polymer electrolyte with high ionic conductivity. Furthermore, introducing polydopamine onto the nanofiber membrane to form hydrogen bonds with PDOL (poly-DOL) and anions in lithium salts significantly improves the mechanical strength, ionic conductivity, and transport number of the gel electrolyte, thus enabling safe LMBs.

[0004] On the other hand, introducing a protective layer at the electrolyte-anode interface is an important means to induce uniform lithium-ion deposition and suppress lithium dendrite growth. The inherent viscoelastic properties of polymers give them a significant advantage in lithium metal anode protection. They exhibit excellent wettability to lithium metal and can be directly coated onto the lithium foil surface, which helps enhance interfacial contact. For example, in their study titled "Arobust all-organic protective layer towards ultrahigh-rate and large-capacity Li metal anodes" published in Nature Nanotechnology, Wu Dingcai, Liu Shaohong, and others used modified nanospheres (xPCMS-g-PEGMA) to enhance the single-ion conductive lithiation of Nafion films, preparing a novel all-organic electrode protective layer that enables efficient, dendrite-free lithium metal anodes.

[0005] The present invention aims to provide an electrode protective layer and a solid electrolyte based on fluorinated polymers. Summary of the Invention

[0006] In order to solve the problems existing in the prior art, the present invention provides a fluorinated polymer and its application in electrode protective layers and solid electrolytes, providing a solution for achieving stable, safe, and high-energy-density batteries.

[0007] The present invention achieves the above objectives by adopting the following technical solutions:

[0008] On one hand, the present invention provides a fluorinated polymer obtained by reacting acrylate-functionalized polyethylene glycol, HFBA and an initiator; HFBA, specifically named 2,2,3,4,4,4-butyl hexafluoroacrylate, CAS: 54052-90-3.

[0009] The molar ratio of acrylate-functionalized polyethylene glycol to HFBA is (1-3):1.

[0010] The molar ratio of the initiator addition to the sum of the C=C double bonds in acrylate-functionalized polyethylene glycol and HFBA is (0.5-2):100.

[0011] Optionally, the initiator is any one or more of AIBN, AMBN, ABVN, CABN, CABN, and AIBME.

[0012] On the other hand, the present invention provides the application of the fluorinated polymer in an electrode protective layer, comprising the following steps:

[0013] Acrylate-functionalized polyethylene glycol, HFBA, and an initiator are dissolved in a first solvent to prepare a reactant solution. The solution is reacted at 60-80°C under an inert gas atmosphere for 4-24 hours. The reaction product is then diluted 1-10 times with the first solvent, coated onto the electrode surface, and then vacuum dried to obtain the final product. These dilutions are mass or volume ratios.

[0014] Preferably, the sum of the mass fractions of acrylate-functionalized polyethylene glycol, HFBA, and initiator in the reactant solution is 5-20 wt%.

[0015] Optionally, the first solvent is ultra-dry THF (tetrahydrofuran) or ultra-dry DMF (N,N-dimethylformamide).

[0016] In another aspect, the present invention provides the application of the fluorinated polymer in solid electrolytes, comprising the following steps:

[0017] S1. Dissolve the lithium salt in a second solvent to prepare a lithium salt solution;

[0018] S2. Mix acrylate-functionalized polyethylene glycol and HFBA, then add an initiator and mix thoroughly to prepare a monomer mixture solution;

[0019] S3. Mix the lithium salt solution and the monomer mixed solution, and stir thoroughly at room temperature to obtain the precursor solution;

[0020] S4. The precursor solution is reacted directly, or the precursor solution is injected into the battery separator and then reacted. The reaction temperature is 40-90℃ and the reaction time is 4-24h to obtain the product.

[0021] Preferably, in step S3, the mass ratio of the lithium salt solution to the monomer mixed solution is (1-4):2.

[0022] Preferably, the second solvent is succinic anhydride.

[0023] Preferably, the concentration of lithium salt in the lithium salt solution is 0.5-5M, and the lithium salt is composed of a first lithium salt and a second lithium salt in a mass ratio of (1-9):1. The first lithium salt is any one of LiTFSI, LiFSI, LiPF6, and LiClO4, and the second lithium salt is any one of LiBOB, LiDFOB, and LiBF4.

[0024] Specifically, the preparation method of acrylate-functionalized polyethylene glycol (denoted as A-PEG) is as follows:

[0025] Polyethylene glycol and ethyl isocyanate acrylate (CAS No.:13641-96-8) are dissolved in ultra-dry THF at a molar ratio of 2:3. A catalyst is added, and the mixture is stirred and reacted under argon protection at 40-80℃ for 2-24 hours to obtain the final product.

[0026] The molar ratio of the catalyst addition to the sum of the amounts of polyethylene glycol and ethyl 2-isocyanate is (1-10):100.

[0027] The catalyst is any one or more of dibutyltin dilaurate (DBTDL), dibutyltin diacetate (DBTDA), and dioctyltin dilaurate (DOT).

[0028] Polyethylene glycol (PEG) With ethyl 2-isocyanate (ICEA) During the reaction, the hydroxyl groups at one or both ends of the polyethylene glycol molecule react with the isocyanate groups in the ethyl 2-isocyanate molecule to obtain the product A-PEG. (One end of the structural formula contains a C=C double bond) and (Both ends of the structural formula contain C=C double bonds) There are two types of monomers in A-PEG. The ratio of these two monomers in A-PEG is determined by the ratio of the reactants polyethylene glycol and ethyl 2-isocyanate. When all of the ethyl 2-isocyanate participates in the reaction, the molar amount of C=C double bonds in each unit molar amount of A-PEG is constrained by the amount of ethyl 2-isocyanate added.

[0029] This application describes the preparation of A-PEG using the aforementioned method, followed by reaction with HFBA and an initiator to prepare a fluorinated polymer. The molecular structure of HFBA is as follows: One end of the molecular structure is a C=C double bond.

[0030] The beneficial effects of the present invention include, but are not limited to:

[0031] The fluorinated polymer provided by this invention has strong antioxidant properties, can stabilize the negative electrode / electrolyte interface, and the polymer can react with Li. + Coordination, assisting Li + The transmission.

[0032] When fluorinated polymers are used as a protective layer for lithium metal anodes, polymer-protected LiFePO4 / Li full cells exhibit superior cycle performance compared to unprotected full cells. The cells demonstrate stable cycle performance, high capacity retention, and slow capacity decay. Furthermore, the fluorinated polymer structure contains self-healing urethane groups, which can automatically fill and repair cracks, maintaining the integrity of the protective layer and extending the battery's lifespan.

[0033] When fluorinated polymers are used in solid-state electrolytes, they can form a polymer network encapsulating ion-conducting crystalline polymer solid-state electrolyte through in-situ reactions, effectively suppressing the growth of Li dendrites and ensuring the stability of the polymer electrolyte and lithium anode. When applied to batteries, the solid-state electrolyte exhibits excellent ultra-long cycle performance and a high critical current density; after cycling, the electrode surface becomes smooth. Furthermore, this solid-state electrolyte is compatible with high-nickel ternary cathode materials, providing a solution for achieving stable, safe, and high-energy-density batteries.

[0034] The dual lithium salt system used in this invention further broadens the electrochemical window and stabilizes the cathode / electrolyte interface; the succinic acid used has high room temperature ionic conductivity, a wide electrochemical stability window, low flammability, and good compatibility with high-voltage cathodes. Attached Figure Description

[0035] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0036] Figure 1 The cycling performance of LFP / PFPA@Li and LFP / Li full cells at 25°C and 1C was evaluated.

[0037] Figure 2 For the self-healing process of the protective layer (1);

[0038] Figure 3 For the self-healing process of the protective layer (2);

[0039] Figure 4 Photographs of the precursor solution and the D-PFPS electrolyte obtained from the reaction of the precursor solution;

[0040] Figure 5 FT-IR spectra of monomers and D-PFPS electrolytes;

[0041] Figure 6 (a) shows the surface morphology of the PP membrane, (b) shows the cross-sectional morphology of the PP membrane, (c) shows the surface morphology of the D-PFPS@PP electrolyte, and (d) shows the cross-sectional morphology of the D-PFPS@PP electrolyte.

[0042] Figure 7 Linear potential scan curves for D-PFPS@PP, S-PFPS@PP, and D-PPS@PP;

[0043] Figure 8 Electrochemical impedance spectroscopy of a Li / D-PFPS / Li battery under open-circuit conditions at 60℃;

[0044] Figure 9 The inset shows the current-time curves of a Li / D-PFPS / Li symmetric cell, with the EIS spectra before and after polarization.

[0045] Figure 10 The figures show the long-cycle curves of Li / D-PFPS / Li, Li / S-PFPS / Li and Li / D-PPS / Li symmetric cells at 60℃. The inset shows the voltage distribution of the symmetric cells at 10–20 h, 560–570 h and 1200–1210 h.

[0046] Figure 11 In the figures (a), (b), and (c), D-PPS, S-PFPS, and D-PFPS are respectively at 0.2 mA / cm². -2 SEM image of the lithium anode surface after 20 hours of cycling;

[0047] Figure 12 (a) shows the electrochemical performance of the LFP / D-PFPS / Li full cell at 25 °C and a current density of 0.5 C; (b) shows the capacity-voltage curves of the LFP / D-PFPS / Li full cell at different cycle numbers.

[0048] Figure 13 In the middle (a) and (b), the electrochemical performance of the control group LFP / D-PFPS-1 / Li and LFP / D-PFPS-2 / Li full cells at 25 °C and a current density of 0.5 C are respectively.

[0049] Figure 14 (a) shows the electrochemical performance of LFP / D-PFPS / Li, LFP / S-PFPS / Li and LFP / D-PPS / Li full cells at 60 °C and a current density of 0.5 C, and (b) shows the capacity-voltage curves of LFP / D-PFPS / Li full cells at different cycle numbers.

[0050] Figure 15 (a) Electrochemical performance of the LFP / D-PFPS / Li full cell at 60℃ and a current density of 2C; (b) Capacity-voltage curves of the LFP / D-PFPS / Li full cell at different cycle numbers; (c) Electrochemical performance of the LFP / D-PFPS / Li full cell with a positive electrode loading of 7 mg / cm³. -2 The electrochemical performance of the LFP / D-PFPS / Li full cell at 60℃ and a current density of 0.2C;

[0051] Figure 16 (a) shows the rate performance comparison of LFP / D-PFPS / Li and LFP / S-PFPS / Li full cells at 60℃, (b) shows the capacity-voltage curves of LFP / D-PFPS / Li full cells at different cycle numbers, and (c) shows the capacity-voltage curves of LFP / S-PFPS / Li full cells at different cycle numbers.

[0052] Figure 17 (a) is a SEM image of the lithium anode of LFP / D-PFPS / Li after cycling at a current density of 0.5C for 20 cycles, and (b) is a SEM image of the lithium anode of LFP / S-PFPS / Li.

[0053] Figure 18 (a) shows the C1s spectrum on the lithium anode surface of LFP / D-PFPS / Li after 20 cycles, and (b) shows the F1s spectrum.

[0054] Figure 19 (a) is a SEM image of the cathode surface of LFP / S-PFPS / Li after 20 cycles, and (b) is a SEM image of the cathode surface of LFP / D-PFPS / Li after 20 cycles.

[0055] Figure 20 The electrochemical performance of the NCM811 / D-PFPS / Li full cell at 60 °C and a current density of 0.5 C is presented. Detailed Implementation

[0056] The present invention will be further described in detail below. However, it should be noted that the following specific embodiments are merely exemplary examples of the invention, and the scope of protection of the invention is not limited thereto. The scope of protection of the invention is defined only by the claims. It will be apparent to those skilled in the art that various other modifications and substitutions can be made to the embodiments of the invention within the scope of protection defined by the claims, and the same technical effects can still be achieved, thus achieving the ultimate technical objective of the invention.

[0057] Unless otherwise specified, all raw materials in this manual were obtained through commercial channels.

[0058] I. Application of Fluorinated Polymers in Electrode Protective Layers

[0059] 1.1 Application Method:

[0060] Example 1-1:

[0061] Acrylate-functionalized polyethylene glycol (A-PEG) and HFBA were mixed in a 3:2 molar ratio with 0.5 mol% (the molar percentage of the sum of the C=C double bonds in A-PEG and HFBA) of initiator AIBN in ultra-dry THF solvent to prepare a reaction solution. The total mass fraction of acrylate-functionalized polyethylene glycol, HFBA, and initiator in the reaction solution was 10 wt%. The reaction was carried out at 65°C under argon protection for 24 h. The reaction product was diluted 4 times with ultra-dry THF, uniformly coated on the surface of a lithium electrode, and then vacuum dried in a glove box transition chamber for 2 h to obtain the final product.

[0062] The preparation method of A-PEG is as follows: polyethylene glycol and ethyl isocyanate-2-acrylate (CAS No.: 13641-96-8) are dissolved in ultra-dry THF at a molar ratio of 2:3. A catalyst is added, and the mixture is stirred and reacted under argon protection at 40-80℃ for 2-24 hours to obtain the product. The ratio of C=C double bonds in 1 mol of A-PEG to that in 1 mol of HFBA obtained by this preparation method is approximately 1.5:1.

[0063] The molar ratio of the catalyst addition to the sum of the amounts of polyethylene glycol and ethyl isocyanate acrylate is 5:100.

[0064] The catalyst is any one or more of dibutyltin dilaurate (DBTDL), dibutyltin diacetate (DBTDA), and dioctyltin dilaurate (DOT).

[0065] It should be noted that the preparation method of acrylate-functionalized polyethylene glycol has been publicly described in the research published by the inventors of this application in Energy Storage Materials, entitled "In-Situ Formation of Quasi-Solid Polymer Electrolyte for Wide-Temperature Applicable Li-Metal Batteries".

[0066] Examples 1-2:

[0067] The difference between this embodiment and Embodiment 1-1 is that the molar ratio of acrylate-functionalized polyethylene glycol to HFBA is 2:1.

[0068] Examples 1-3:

[0069] The difference between this embodiment and Embodiment 1-1 is that the initiator used is ABVN, and the amount of initiator added is 1 mol of the sum of the C=C double bonds contained in both A-PEG and HFBA.

[0070] 1.2 Electrochemical Cycling Performance of Full Cells

[0071] Assembly of LFP / Li full cells: Using lithium foil and lithium foil protected by fluorinated polymer (PFPA@Li) prepared according to the method of Example 1-1 as counter electrodes, LFP positive electrode as working electrode, Celgard 2400 as separator, and LiPF6 / EC / DEC electrolyte added, CR2032 batteries were assembled in a glove box to make LFP / Li and LFP / PFPA@Li full cells.

[0072] Figure 1 This chart compares the long-term cycling performance of LFP / Li and LFP / PFPA@Li full cells at a current density of 1C (0.5C for the first 6 weeks). It shows that the LFP / Li battery exhibits a sharp capacity decline after 120 cycles, with a capacity retention of only 37% after 325 cycles. The poor cycling performance of the LFP / Li battery can be attributed to the accumulation of "dead" lithium and the continuous consumption of the electrolyte. In contrast, the LFP / PFPA@Li battery can cycle stably for over 325 cycles with a capacity retention of 84% and an average capacity decay of 0.049% per week. This superior full-cell cycling performance is attributed to the good stability of the fluorinated polymer and its excellent electrode protection.

[0073] 1.3 Self-healing properties of polymers

[0074] Intrinsic self-healing polymers achieve self-repair by introducing reversible interaction forces into the material to endow the cross-linked network with dynamics. When the material is damaged, the reversible forces within the material can be activated spontaneously or under external stimulation and reformed at the damaged site, thereby rebuilding the network structure at the damaged location and ultimately repairing the material's structure and properties.

[0075] This invention uses acrylate-functionalized polyethylene glycol (A-PEG) as a monomer raw material. A-PEG is obtained by reacting polyethylene glycol (PEG 600) with ethyl 2-isocyanate acrylate (ICEA). The -OH (hydroxyl group) in PEG and the -NCO (isocyanate group) in ICEA react to form a urethane group. The urethane group has hydrogen bonding interactions and is a non-covalent bond. This non-covalent bond can construct a reversible and dynamic self-healing network, achieving self-healing at lower energies, while also improving the ionic conductivity and flexibility of the polymer electrolyte.

[0076] To verify the self-healing effect of urethane groups, the inventors of this application prepared the following protective layer and tested its repair effect.

[0077] Preparation of protective layer (1): A-PEG monomer was polymerized under AIBN initiation to obtain poly(acrylate-functionalized polyethylene glycol) (poly(A-PEG), denoted as PAP).

[0078] Preparation of protective layer (2): A-PEG and HFBA are polymerized under the initiation of AIBN to obtain the fluorinated polymer described in this invention.

[0079] Next, the self-healing process of the dynamic network of protective layer (1) and protective layer (2) at room temperature was observed using optical microscopy:

[0080] Cut open the middle part of protective layer (1) and protective layer (2) with a knife, and observe them under an optical microscope for more than 15 minutes. Figure 2 Chinese ad and Figure 3 As shown in Figure ac, due to the presence of non-covalent bonds, the dynamic network of protective layer (1) and protective layer (2) completed the self-healing process within 15 minutes at room temperature, proving that the urethane group has excellent self-healing effect.

[0081] II. Application of Fluorinated Polymers in Solid Electrolytes

[0082] 2.1 Application Method:

[0083] Example 2-1:

[0084] S1. Succinate and lithium salt are melted and mixed to obtain a lithium salt solution with a concentration of 2M (i.e., 2mol / L); the lithium salt is composed of LiTFSI and LiBOB in a mass ratio of 8:2.

[0085] S2. Mix acrylate-functionalized polyethylene glycol and HFBA in a 1:1 molar ratio, add 0.5 mol% (the molar percentage of the sum of C=C double bonds in A-PEG and HFBA) of AIBN initiator, and stir thoroughly to obtain a monomer mixed solution;

[0086] S3. The lithium salt solution and the monomer mixture solution were mixed at a mass ratio of 1:1 and stirred thoroughly at room temperature to prepare the precursor solution; all the above processes were carried out in a glove box filled with argon gas, and the contents of O2 and H2O were both less than 0.1 ppm;

[0087] S4. React the precursor solution in a container to prepare a fluorinated polymer, denoted as D-PFPS electrolyte.

[0088] Example 2-2:

[0089] The precursor solution obtained in step S3 of Example 1 was injected onto the membrane, and in-situ thermal polymerization was initiated at 70°C for 6 hours to prepare a PP membrane loaded with fluorinated polymer solid electrolyte, denoted as D-PFPS@PP.

[0090] In this battery, the PP separator serves as the framework for the in-situ polymerization of the D-PFPS precursor solution, providing mechanical support for the polymer electrolyte and preventing short circuits. In this embodiment, a commercially available PP separator, Celgard 2400, is used. Other models such as Celgard 2320 and Celgard 2500 can also be used.

[0091] Examples 2-3:

[0092] The difference between this embodiment and Embodiment 2-2 is that the mass ratio of lithium salt solution to monomer mixed solution is 2:1.

[0093] Examples 2-4:

[0094] The difference between this embodiment and Embodiment 2-2 is that the lithium salt is composed of LiClO4 and LiDFOB in a mass ratio of 5:1.

[0095] To further investigate the effects of fluorine functionalization and salt regulation on polymer electrolytes and their influence on battery electrochemical performance, the following control group was also included in this application.

[0096] Comparative Example 1: This is a single-salt fluorinated polymer electrolyte, prepared using a single lithium salt, LiTFSI. The remaining steps are the same as in Example 2, and the product is designated as the S-PFPS@PP control group.

[0097] Comparative Example 2: This is a dual lithium salt non-fluorinated polymer electrolyte. BA was used instead of HFBA in its preparation. The remaining steps were the same as in Example 2. The product was designated as the D-PPS@PP control group.

[0098] Comparative Example 3: The difference between this comparative example and Example 2-2 is that trifluoroethyl methacrylate was used instead of HFBA, and the remaining steps were the same as in Example 2-2. The product was denoted as D-PFPS-1@PP control group.

[0099] Comparative Example 4: The difference between this comparative example and Example 2-2 is that 2-(perfluorooctyl)ethyl methacrylate was used instead of HFBA, and the remaining steps were the same as in Example 2-2. The product was denoted as D-PFPS-2@PP control group.

[0100] 2.2 Physicochemical properties of D-PFPS electrolyte:

[0101] Figure 4 The solidification process of D-PFPS electrolyte is shown. It can be seen that after in-situ polymerization of the precursor solution, the originally flowable liquid precursor is transformed into a non-flowable solid electrolyte.

[0102] FT-IR spectroscopic detection was performed on D-PFPS, A-PEG, and HFBA, and the results are as follows: Figure 5 As shown in the image.

[0103] FT-IR spectroscopy results indicate that after random copolymerization of A-PEG and HFBA monomers, A-PEG and HFBA exhibit a high spectral density at 1637 cm⁻¹. -1 The disappearance of the characteristic peak of the C=C stretching vibration at the corresponding point proves that the monomer polymerization was successful.

[0104] 2.3 Macroscopic and microscopic morphology of solid electrolyte@diaphragm

[0105] Obtain SEM images of the PP membrane and D-PFPS@PP as follows: Figure 6 As shown.

[0106] observe Figure 6 As shown in (a) and (b), the microstructure of the PP membrane indicates that the PP membrane has a porous structure and a thickness of 22 μm.

[0107] Figure 6 (c) shows that the surface morphology indicates that D-PFPS is uniformly covered on the PP membrane. Figure 6 (d) shows that the original pores of the PP membrane were filled with D-PFPS, the electrolyte thickness was 23 μm, and no pore structure was visible on the surface or in the cross section of the PP membrane.

[0108] 2.4 Electrochemical performance of solid electrolyte@diaphragm

[0109] The electrochemical window of the solid electrolyte@diaphragm was measured using a linear potential scanning method. The results are shown in [Figure number missing]. Figure 7 .

[0110] like Figure 7 As shown, D-PPS@PP exhibits weak antioxidant capacity and a low electrochemical stability window at 4.0V. S-PFPS@PP and D-PFPS@PP show significantly enhanced antioxidant properties at 4.5V and 4.9V, respectively, with D-PFPS@PP exhibiting the best antioxidant performance.

[0111] 2.5 Electrochemical performance of symmetric cells

[0112] Assembly of Li / Li symmetric cells: Lithium metal discs with a diameter of 11 mm and a thickness of 250 μm were used as electrodes; Celgard 2400 was used as a separator. The precursor solutions were injected into both sides of the separator using the methods in Examples 2-2 and Comparative Examples 1 and 2, respectively. CR2032 coin cells were assembled in a glove box to produce Li / D-PFPS / Li, Li / S-PFPS / Li and Li / D-PPS / Li symmetric cells.

[0113] 2.5.1 Interfacial stability of symmetric cells

[0114] To evaluate the chemical stability of the Li metal anode and D-PFPS over time, the impedance of the Li / D-PFPS / Li symmetric cell under open-circuit conditions at 60 °C was measured using EIS. The results are shown in [Figure number missing]. Figure 8 .

[0115] like Figure 8 As shown, the semicircle in the first half of the curve represents the impedance of the D-PFPS itself and the total impedance of the interface between the D-PFPS and the Li anode (originating from the SEI layer). At 60℃, the impedance of the Li / D-PFPS / Li symmetric cell remained stable at 96Ω for up to 40 days, indicating that the Li anode and the D-PFPS solid electrolyte have good interfacial stability.

[0116] 2.5.2 Li-symmetric cells + Number of migrations

[0117] The current-time curves and EIS spectra before and after polarization of the Li / D-PFPS / Li symmetric cell are shown in [reference needed]. Figure 9 .

[0118] The calculated D-PFPS The value is 0.47, indicating a low anion mobility, which is beneficial for improving battery performance in the future.

[0119] 2.5.3 Long-cycle stability test of symmetric cells

[0120] Long-cycle stability tests were conducted on Li / D-PFPS / Li, Li / S-PFPS / Li, and Li / D-PPS / Li symmetric cells to evaluate the stability of the electrolyte during deposition and dissolution.

[0121] like Figure 10 As shown, the inset is a magnified view of a portion of the voltage-time curve. The results indicate that at 60℃ and 0.2 mA cm⁻¹... -2 At the specified current density, solid-state batteries using D-PPS as the electrolyte short-circuited within a very short time (less than 20 hours); while symmetric batteries using fluorinated electrolytes (D-PFPS and S-PFPS) could cycle stably for over 500 hours, indicating that adding fluorinated monomers can significantly stabilize the lithium anode / electrolyte interface. Furthermore, during an ultra-long cycle life of 1400 hours, the D-PFPS electrolyte exhibited a more stable overpotential than the S-PFPS electrolyte.

[0122] 2.5.4 Surface morphology of lithium anode after cycling

[0123] The surface morphology of lithium anodes after cycling in different solid electrolytes for 20 hours was observed using scanning electron microscopy.

[0124] like Figure 11As shown, the lithium metal electrode surfaces after disassembly from Li / D-PFPS / Li and Li / S-PFPS / Li batteries are smooth and dendrite-free. In contrast, the lithium metal surface after disassembly from Li / D-PPS / Li batteries is rough and exhibits an uneven lithium deposition morphology. This further demonstrates that the introduction of fluorinated monomers can significantly stabilize the lithium anode / electrolyte interface.

[0125] Electrochemical performance of 2.6LFP / Li full cell

[0126] Assembly of LFP / Li full cells: Using lithium foil as the counter electrode, LFP electrode as the positive electrode, and Celgard 2400 as the separator, precursor solutions were injected into both sides of the separator using the methods described in Examples 2-2 and Comparative Examples 1, 2, 3, and 4, respectively. CR2032 batteries were assembled in a glove box to prepare LFP / D-PFPS / Li and control groups LFP / S-PFPS / Li, LFP / D-PPS / Li, LFP / D-PFPS-1 / Li, and LFP / D-PFPS-2 / Li full cells. The electrochemical performance of the electrolyte in the full cells under different current densities and temperatures was studied.

[0127] The positive electrode is prepared as follows: Active material LFP, conductive agent SP, and binder PVdF are dissolved in NMP at a mass ratio of 8:1:1 and stirred for 6 hours to mix thoroughly. The mixed slurry is then uniformly coated onto aluminum foil using a coating machine. The electrode is then vacuum-dried at 120℃ for 14 hours. The dried electrode is then cut into round pieces with a diameter of 13 mm.

[0128] 2.6.1 Cycling performance of LFP / Li full cells at room temperature

[0129] The cycling performance of LFP / D-PFPS / Li batteries at room temperature is as follows: Figure 12 As shown in (a), the LFP / D-PFPS / Li battery operates at 25°C and a current density of 0.5C (1C = 150 mAg). -1 Cycling results (with a cycle current density of 0.2C for the first 5 weeks) showed that the battery's initial discharge capacity was 132.7 mAh g. -1 After 5 weeks of activation, the capacity was 120.9 mAh g. -1 After 200 cycles, the capacity showed almost no decay. The LFP / D-PFPS / Li battery exhibited a gradual increase in capacity during the first 10 cycles, which is related to battery activation. Due to the significantly lower ionic conductivity at room temperature compared to at high temperatures, D-PFPS requires several more weeks of activation cycles at room temperature than at high temperatures, and its capacity begins to stabilize after 10 cycles.

[0130] Figure 12(b) shows the voltage-capacity curves of the LFP / D-PFPS / Li battery during 30–200 cycles. The results indicate that the voltage polarization of the LFP / D-PFPS / Li battery is very stable during cycling, and the curves almost overlap.

[0131] Figure 13 The chart shows a comparison of the long-cycle performance of the control group LFP / D-PFPS-1 / Li and LFP / D-PFPS-2 / Li full cells. It can be seen that the LFP / D-PFPS-1 / Li cell, which uses trifluoroethyl methacrylate instead of HFBA, retains approximately 81% of its capacity at 50 cycles, but its capacity declines sharply after 50 cycles, reaching only 39% after 100 cycles. The LFP / D-PFPS-2 / Li cell, which uses 2-(perfluorooctyl)ethyl methacrylate instead of HFBA, has a lower initial capacity and continues to decline after 50 cycles, with a capacity retention of approximately 65% ​​at 50 cycles.

[0132] 2.6.2 Cycling performance of LFP / Li full cells under high temperature conditions

[0133] like Figure 14 As shown in (a), at 60°C and a current density of 0.5C (1C = 170 mAg), -1 Cycling results (at 0.2C for the first 5 weeks) showed that the initial discharge capacity of the LFP / D-PPS / Li full cell was 156.5 mAh g⁻¹. -1 After 5 weeks of activation, the discharge capacity was 106.4 mAh g. -1 After 130 cycles, the capacity showed a significant decrease. The initial discharge capacity of the LFP / S-PFPS / Li battery was 150.3 mAh g. -1 After 5 weeks of activation, the discharge capacity was 137.1 mAh g. -1 It exhibits superior cycle performance compared to LFP / D-PPS / Li batteries, with capacity only showing rapid decay after 500 cycles.

[0134] In comparison, the initial discharge capacity of the LFP / D-PFPS / Li battery is 157.7 mAh g. -1 The discharge capacity after 5 weeks of activation was 142.6 mAh g. -1 After 1000 cycles, the capacity retention rate was 80%, and after 1100 cycles, the capacity retention rate was 76%, demonstrating significantly better long-cycle stability than LFP / S-PFPS / Li and LFP / D-PPS / Li batteries. Furthermore... Figure 14 As shown in (b), the voltage polarization was also relatively stable after 1000 cycles, with no significant increase, indicating that the introduction of fluorinated monomers and the regulation effect of the dual lithium salt coupling system can significantly enhance the cycle stability of the battery.

[0135] The results above demonstrate that the LFP / D-PFPS / Li full cell with dual lithium salt coupling exhibits stable electrochemical cycling performance under both room temperature and high temperature conditions.

[0136] 2.6.3 Electrochemical performance of LFP / Li full cells at high rates

[0137] To characterize the electrochemical performance of the full cell at high rates, the cycling stability of the LFP / D-PFPS / Li cell at 2C and 60℃ was tested.

[0138] like Figure 15 As shown in (a), the initial discharge capacity of the LFP / D-PFPS / Li full cell is 139.9 mAh g. -1 After 5 weeks of activation, the discharge capacity was 128.6 mAh g. -1 The discharge capacity after 500 cycles is 100.5 mAh g. -1 The capacity retention rate was 79%.

[0139] Figure 15 (b) indicates that, with increasing cycle number, the charge / discharge curves related to the LFP / D-PFPS / Li full cell did not show a significant increase in voltage polarization, further increasing the areal capacity of the positive electrode. This was achieved using an areal loading of 7 mg cm⁻¹. -2 The LFP positive electrode is matched with a D-PFPS electrolyte, and the positive electrode surface capacity is 1 mAh cm⁻¹. -2 The cycle stability of the solid-state battery was tested. Cycle performance was as follows: Figure 15 As shown in (c), the results indicate that the LFP / D-PFPS / Li battery can maintain stable cycling for 30 weeks.

[0140] 2.6.4 Rate performance of LFP / Li full cells

[0141] Figure 16 The data shows that the specific capacities of the LFP / D-PFPS / Li full cell at cycle rates of 0.2, 0.5, 1, 1.5, and 2C are 135.0, 128.3, 124.0, 119.0, and 114.2 mAh g, respectively. -1 When the current density is reset to 0.5C, the battery specific capacity recovers to 127.1 mAh g. -1 The cells continued to cycle stably for over 50 cycles. In contrast, the LFP / S-PFPS / Li full cell exhibited a relatively lower discharge capacity and slightly higher voltage polarization than the LFP / D-PFPS / Li cell. These results indicate that the LFP / D-PFPS / Li full cell demonstrates superior rate performance compared to the LFP / S-PFPS / Li cell.

[0142] 2.6.5 Surface morphology of lithium metal anode after LFP / Li cycling

[0143] Figure 17 (a) shows the surface morphology of the lithium metal anode disassembled from the LFP / D-PFPS / Li battery after 20 cycles. The macroscopic morphology in the inset shows that the lithium metal surface exhibits a metallic luster, while the SEM microscopic morphology reveals a dense and smooth surface. Figure 17 (b) shows that the surface microstructure of lithium metal disassembled from LFP / S-PFPS / Li batteries is rough and porous, and the local surface is covered with uneven lithium dendrites.

[0144] 2.6.6 Chemical Composition of SEI

[0145] Solid electrolytes can effectively enhance interfacial stability and facilitate the formation of a stable SEI layer. This application uses XPS to investigate the chemical composition of the SEI in D-PFPS.

[0146] Figure 18 (b) shows that the D-PFPS-derived SEI contains mechanically stable LiF, which is due to the introduction of fluorinated monomers, which promotes the formation of stable SEI.

[0147] 2.6.7 Surface morphology of LFP cathode after LFP / Li full cell cycling

[0148] Figure 19 (a)-(b) are surface morphology images of LFP cathodes disassembled from LFP / D-PFPS / Li and LFP / S-PFPS / Li batteries after 20 cycles.

[0149] It can be observed that the positive electrode surface of D-PFPS is smooth and covered by polymer electrolyte, while the electrode surface of the disassembled S-PFPS single salt system has some cracks.

[0150] The above results indicate that the D-PFPS electrolyte not only stabilizes the lithium metal anode interface, but also helps stabilize the electrolyte / cathode interface.

[0151] 2.7 Electrochemical performance of NCM811 cathode (nickel-cobalt-manganese ternary cathode) / Li full cell

[0152] Assembly of NCM811 / Li full cells: Using lithium foil as the counter electrode, LFP electrode as the positive electrode, and Celgard 2400 as the separator, precursor solutions were injected into both sides of the separator using the methods in Examples 2-2 and Comparative Example 1, respectively. CR2032 batteries were assembled in a glove box to prepare NCM811 / D-PFPS / Li and NCM811 / S-PFPS / Li full cells, and the electrochemical performance of the electrolyte-matched high-voltage positive electrode was studied.

[0153] The positive electrode sheet is prepared as follows: Active material NCM811, conductive agent SP, and binder PVdF are dissolved in NMP at a mass ratio of 8:1:1 and stirred for 6 hours to mix evenly. The mixed slurry is then evenly coated onto aluminum foil using a coating machine. The electrode sheet is then vacuum dried at 120℃ for 14 hours. The dried electrode sheet is then cut into round pieces with a diameter of 13 mm.

[0154] like Figure 20 As shown, at 60℃ and a current density of 0.5C (1C = 180 mAg), -1 Under the condition of a cycling current density of 0.2C for the first 5 weeks, the initial discharge capacity of the NCM811 / D-PFPS / Li battery is 206.2 mAh g. -1 After 5 weeks of activation, the discharge capacity was 162.1 mAh g. -1 After 100 cycles, the capacity retention was 75%. In contrast, the discharge capacity of the NCM811 / S-PFPS / Li full cell was significantly lower than that of the LFP / D-PFPS / Li cell. This is due to the lower voltage stability window of S-PFPS and phenomena such as electrolyte oxidation and even decomposition. The results indicate that, compared with S-PFPS, D-PFPS exhibits more stable electrochemical performance when matched with the high-voltage cathode NCM811 due to its wider electrochemical stability window.

[0155] The above specific embodiments should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, any alternative improvements or modifications made to the embodiments of the present invention shall fall within the scope of protection of the present invention.

[0156] Any aspects of this invention not described in detail are well-known to those skilled in the art.

Claims

1. The application of a fluorinated polymer in a solid electrolyte, characterized in that, Includes the following steps: S1. Dissolve the lithium salt in a second solvent to prepare a lithium salt solution; the concentration of the lithium salt in the lithium salt solution is 0.5-5M, and the lithium salt is composed of a first lithium salt and a second lithium salt in a mass ratio of (1-9):1, wherein the first lithium salt is any one of LiTFSI, LiFSI, LiPF6, and LiClO4, and the second lithium salt is any one of LiBOB, LiDFOB, and LiBF4; S2. Mix acrylate-functionalized polyethylene glycol and HFBA, then add an initiator and mix evenly to prepare a monomer mixed solution; the molar ratio of acrylate-functionalized polyethylene glycol to HFBA is (1-3):1; the molar ratio of the amount of initiator added to the sum of the C=C double bonds contained in acrylate-functionalized polyethylene glycol and HFBA is (0.5-2):

100. S3. Mix the lithium salt solution and the monomer mixed solution, and stir thoroughly at room temperature to obtain the precursor solution; S4. The precursor solution is reacted directly, or the precursor solution is injected into the battery separator and then reacted. The reaction temperature is 40-90℃ and the reaction time is 4-24 h to obtain the product. The preparation method of acrylate-functionalized polyethylene glycol is as follows: Polyethylene glycol and ethyl 2-isocyanate acrylate are dissolved in ultra-dry THF at a molar ratio of 2:

3. A catalyst is added, and the mixture is stirred and reacted under argon protection at 40-80℃ for 2-24 hours to obtain the final product. The molar ratio of the catalyst addition to the sum of the amounts of polyethylene glycol and ethyl 2-isocyanate is (1-10):

100. The catalyst is any one or more of dibutyltin dilaurate (DBTDL), dibutyltin diacetate (DBTDA), and dioctyltin dilaurate (DOT).

2. The application according to claim 1, characterized in that, In step S3, the mass ratio of the lithium salt solution to the monomer mixed solution is (1-4):

2.

3. The application according to claim 1, characterized in that, The second solvent is succinic anionyl nitrile.

4. The application according to claim 1, characterized in that, The initiator is any one or more of AIBN, AMBN, ABVN, CABN, and AIBME.

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