A method for modifying the interface of a solid-state polymer lithium metal battery using lithium fluorozirconate

Abstract

The application discloses a modification method of a solid-state polymer lithium metal battery interface by using lithium fluorozirconate and a composite electrolyte, and belongs to the field of solid-state electrolyte materials of lithium metal batteries. 4+ The unoccupied d orbitals are coordinated with free radicals generated by electrochemical cycles, efficiently dissipating the free radicals to inhibit polymer chain degradation; meanwhile, a stable SEI film rich in LiF is induced to be generated, and a continuous "ion bridge" channel is constructed to homogenize Li+ flux. The obtained composite electrolyte has an ionic conductivity of 8.83*10 ‑4 S·cm ‑1 , and lithium ion transference number is increased to 0.54, which significantly inhibits lithium dendrite growth and improves interface chemical stability, and is suitable for high-safety solid-state lithium metal batteries.

Description

Technical Field

[0001] This invention relates to the field of polymer electrolyte technology for solid-state lithium metal batteries, specifically to a method for modifying the interface of solid-state polymer lithium metal batteries using lithium fluorozirconate, which is particularly suitable for high-safety, long-cycle lithium iron phosphate solid-state lithium metal batteries. Background Technology

[0002] Polymer solid electrolytes (PSSEs) are considered an ideal solution to address the safety concerns of traditional liquid electrolytes due to their excellent interfacial compatibility and mechanical flexibility. Among them, polymethyl methacrylate (PMMA)-based electrolytes have attracted much attention due to their high ionic conductivity and electrode affinity. However, PMMA-based PSSEs still face two major challenges when applied to high-energy-density lithium metal anodes:

[0003] First, the space charge effect induces dendrite growth. The enrichment of anions at the electrode / electrolyte interface creates a local electric field distortion, which drives lithium ions to preferentially deposit at microscopic defect sites, eventually evolving into lethal dendrites that pierce the electrolyte and cause a short circuit in the battery.

[0004] Secondly, and more seriously, is the chain reaction initiated by free radicals. During charge and discharge, the ester groups of the PMMA side chains are catalytically decomposed at the lithium / electrolyte interface, generating a large number of highly reactive free radicals (mainly alkoxy radicals). These free radicals not only directly consume active lithium but also trigger continuous oxidative degradation and chain breakage of the polymer backbone. This parasitic reaction continues throughout the battery's lifespan, leading to a sharp increase in interfacial impedance, low coulombic efficiency, and rapid decay of the full battery capacity.

[0005] Existing solutions mostly employ physical filling with inorganic fillers (such as Al2O3 and SiO2) or simple LiF coatings. However, inorganic fillers can only provide mechanical reinforcement or physical adsorption, and cannot chemically eliminate generated free radicals; simple LiF coatings lack dynamic repair capabilities and are prone to cracking and failure during long-term cycling. Therefore, there is an urgent need to design an interface layer with a specific electronic structure that can fundamentally chemically dissipate free radicals and simultaneously homogenize lithium-ion flux to overcome the long cycle life bottleneck of lithium iron phosphate solid-state batteries. Summary of the Invention

[0006] The main objective of this invention is to provide a method for modifying the interface of solid-state polymer lithium metal batteries using lithium fluorozirconate. This method introduces lithium fluorozirconate (Li₂ZrF₆, LZF) with unoccupied 4d orbitals. LZF's unique electronic structure coordinates with free radicals generated during electrochemical cycling, achieving chemical dissipation of these free radicals. Simultaneously, it constructs an "ion bridge" to homogenize lithium flux, inducing the formation of a stable LiF-rich SEI film. This significantly suppresses lithium dendrite growth and substantially improves the cycle life and safety performance of lithium iron phosphate solid-state batteries.

[0007] To address the aforementioned technical problems, the present invention provides the following technical solutions:

[0008] A method for modifying the interface of a solid polymer lithium metal battery using lithium fluorozirconate includes the following steps:

[0009] (1) Preparation of lithium fluorozirconate particles

[0010] Lithium source and fluorozirconate source were reacted in aqueous solution, and lithium fluorozirconate (Li2ZrF6) particles were obtained after separation, washing and drying.

[0011] Preferably, the lithium source is lithium carbonate (Li₂CO₃), and the fluorozirconic acid source is hexafluorozirconic acid (H₂ZrF₆); the mass ratio of lithium carbonate to hexafluorozirconic acid is 1:6 to 1:9 (specifically 2.8-3.2 mg : 21-25 mg). The reaction temperature is 25-30℃, and the reaction time is 20-28 h. Washing is performed with anhydrous ethanol 5-8 times to ensure the removal of residual acidic substances.

[0012] (2) Preparation of polymer electrolyte precursors

[0013] Methyl methacrylate (MMA), polyvinylidene fluoride (PVDF), and polyvinylidene fluoride-hexafluoropropylene copolymer (P(VDF-HFP)) are dissolved in an organic solvent (such as DMF), and lithium salt (such as LiTFSI), inorganic filler (such as organomontmorillonite), and initiator (such as BPO) are added to carry out a prepolymerization reaction to obtain a polymer electrolyte precursor (denoted as precursor C).

[0014] Preferably, the mass ratio of MMA, PVDF, and P(VDF-HFP) is 40-60:1:3-5 (optimal is 50:1:4); the mass percentage of organomontmorillonite is 5wt%-10wt%; and the mass percentage of initiator BPO is 0.5wt%-1wt%. The prepolymerization temperature is 90-100℃, and the time is 10-15 min.

[0015] (3) Construction of the fluorinated lithium zirconate interface layer

[0016] The lithium fluorozirconate particles obtained in step (1) are dispersed in the polymer electrolyte precursor C obtained in step (2) to form an interface-modified precursor (denoted as precursor D).

[0017] (4) Preparation of composite electrolyte membrane

[0018] The polymer electrolyte precursor C from step (2) is used to make a polymer electrolyte base film (denoted as PFPE), and the interface modification precursor D from step (3) is coated on the surface of the base film. After curing, a composite polymer electrolyte with a lithium fluorozirconate functional interface layer is formed (denoted as PFPE-LZF).

[0019] Preferably, the thickness of the base film is 150-250 μm, and the thickness of the functional interface layer is 5-15 μm. Further, the lithium source in step (1) is lithium carbonate (Li2CO3), and the fluorozirconic acid source is hexafluorozirconic acid (H2ZrF6); the mass ratio of lithium carbonate to hexafluorozirconic acid is 1:6 to 1:9.

[0020] The present invention also provides a solid lithium metal battery comprising a lithium iron phosphate cathode, a lithium metal anode, and a composite polymer electrolyte having a lithium fluorozirconate functional interface layer prepared by the above method.

[0021] The solid-state lithium metal battery has the following excellent properties:

[0022] The critical current density (CCD) at room temperature is not less than 0.85 mA cm⁻¹. -2 ;

[0023] In 0.1-0.2 mA cm -2 At current density, the stable cycle life of lithium symmetric batteries is no less than 400 hours;

[0024] At 1C rate, the initial discharge specific capacity is not less than 140 mAh g. -1 After 650 cycles, the capacity retention rate is no less than 70%.

[0025] Based on the above technical solution, the present invention has the following outstanding substantive features and significant progress (inventive embodiment):

[0026] A pioneering free radical chemical dissipation mechanism: Unlike the physical adsorption of existing technologies, this invention utilizes Zr in lithium fluorozirconate. 4+ The unoccupied empty 4d orbitals strongly coordinate with the lone pairs of electrons on the free radicals generated by the electrochemical cycle, transferring the single electron of the free radical to the empty d orbitals and thus deactivating it. EPR spectra confirm that this mechanism can cut off the chain reaction propagation at its source, a qualitative change that cannot be expected by those skilled in the art based on existing technology.

[0027] Synergistic effect of "ion bridge" and dynamic SEI: The LZF interface layer not only acts as an "ion bridge" to regulate Li⁺ flux distribution and weaken the space charge effect, but also continuously releases ZrF₆ under the action of an electric field. 2- This induces the formation of a stable SEI film rich in LiF and Li3N. This dynamic self-healing mechanism solves the problem of easy cracking and failure of traditional coatings.

[0028] Unexpected technical effects: Experimental data show that this invention increases the cycle life of lithium iron phosphate solid-state batteries from less than 100 cycles in the comparative example to over 650 cycles (an improvement of more than 6 times), and the critical current density from 0.5 mA cm⁻¹ -2 Increased to 0.85 mA cm -2 (70% improvement). This magnitude of performance improvement demonstrates a significant synergistic effect between "radical dissipation" and "flux equalization," rather than a simple functional additive effect, and is highly innovative. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the mechanism by which free radicals are dissipated and lithium dendrite growth is suppressed in the examples and comparative cases.

[0030] Figure 2 X-ray diffraction (XRD) pattern of lithium fluorozirconate prepared as an example;

[0031] Figure 3 Scanning electron microscope (SEM) image of lithium fluorozirconate prepared for the example;

[0032] Figure 4 SEM images of the surface and cross-section of the composite polymer electrolyte (PFPE-LZF) prepared for the example;

[0033] Figure 5 SEM image of the surface of the pure polymer electrolyte (PFPE) prepared for Comparative Example 1;

[0034] Figure 6 Thermogravimetric analysis (TGA) curves of the composite polymer electrolyte prepared for the example;

[0035] Figure 7 Stress-strain curves of the polymer electrolytes prepared in Examples and Comparative Example 1;

[0036] Figure 8 The electrochemical impedance spectroscopy (EIS) spectra of the polymer electrolytes prepared in Examples and Comparative Example 1 are shown.

[0037] Figure 9 Electron paramagnetic resonance (EPR) spectra of the polymer electrolytes prepared in Examples and Comparative Example 1 after cycling at different voltage states;

[0038] Figure 10 The following are the Fourier Transform Infrared (FTIR) spectra of the polymer electrolytes prepared in Examples 1 and Comparative Example 1 after cycling at different voltage states.

[0039] Figure 11The full and partial X-ray photoelectron spectroscopy (XPS) spectra of the polymer electrolytes prepared in Examples and Comparative Example 1 after cycling under different conditions;

[0040] Figure 12 The lithium-ion transport number test graphs of the polymer electrolytes prepared in Examples and Comparative Example 1 are shown.

[0041] Figure 13 Linear sweep voltammetry (LSV) curves (electrochemical stability window) of the polymer electrolytes prepared for Examples and Comparative Example 1.

[0042] Figure 14 Arrhenius curves showing the ionic conductivity of the polymer electrolytes prepared in Examples and Comparative Example 1 as a function of temperature;

[0043] Figure 15 Tafel curves of the solid-state lithium metal batteries assembled in Examples 1 and Comparative Example 1;

[0044] Figure 16 Cyclic volt-ampere (CV) curves of the solid-state lithium metal batteries assembled in Examples 1 and Comparative Example 1;

[0045] Figure 17 Nucleation overpotential curves of the lithium copper batteries assembled in Examples 1 and Comparative Example 1;

[0046] Figure 18 Constant current charge-discharge cycle curves of the lithium symmetric batteries assembled for Examples 1 and Comparative Example 1.

[0047] Figure 19 The critical current density (CCD) test curves of the lithium symmetric batteries assembled in Examples 1 and Comparative Example 1 are shown.

[0048] Figure 20 The graph shows the long-cycle performance of the lithium iron phosphate solid-state batteries assembled in Examples 1 and Comparative Example 1.

[0049] Figure 21 The rate performance graphs of the lithium iron phosphate solid-state batteries assembled in Examples 1 and Comparative Example 1 are shown.

[0050] Figure 22 The high-rate and high-area-load cycle performance of the lithium iron phosphate solid-state battery assembled for the example is shown in the figure.

[0051] Figure 23 SEM images of the lithium anode surface of the lithium iron phosphate solid-state batteries assembled in Examples and Comparative Example 1 after cycling;

[0052] Figure 24 The diagram shows a comparison of the interface impedance of the lithium iron phosphate solid-state batteries assembled in Examples 1 and Comparative Example 1 before and after cycling.

[0053] Figure 25 XPS spectra of F 1s, N 1s, and Li 1s in the SEI film of the lithium iron phosphate solid-state battery assembled in Examples and Comparative Example 1. Detailed Implementation

[0054] The technical content of the present invention will be described in detail below through embodiments and comparative examples, but the scope of protection of the present invention is not limited thereto.

[0055] Experimental materials and instruments

[0056] The reagents used in the experiment are shown in Table 1, and the instruments used in the experiment are shown in Table 2.

[0057] Table 1 Experimental Drugs

[0058] name Chemical formula and abbreviation purity supplier hexafluorozirconic acid [H2ZrF6] <![CDATA[45% in H2O]]> Aladdin Shanghai Century Co., Ltd. lithium carbonate <![CDATA[Li2CO3]]> 99.9% Aladdin Shanghai Century Co., Ltd. Nano-organic montmorillonite OMMT Analytical Pure Zhejiang Fenghong New Materials Co., Ltd. Acetylene black AB Battery level Taiyuan Lizhiyuan Technology Co., Ltd. Methyl methacrylate MMA 99% Fuchen Chemical Reagent Co., Ltd. Polyvinylidene fluoride-hexafluoropropylene P(VDF-HFP) Battery level Aladdin Shanghai Century Co., Ltd. polyvinylidene fluoride PVDF Battery level Taiyuan Lizhiyuan Technology Co., Ltd. Benzoyl peroxide BPO Analytical Pure Guangfu Fine Chemical Research Institute Lithium iron phosphate <![CDATA[LiFePO4]]> Battery level Taiyuan Lizhiyuan Technology Co., Ltd. N,N-Dimethylformamide DMF Analytical Pure Fuyu Fine Chemical Co., Ltd. N-Methylpyrrolidone NMP Analytical Pure Zhiyuan Chemical Reagent Co., Ltd. Lithium bis(trifluoromethanesulfonylimide) LiTFSI 99% Shanghai Chengjie Chemical Co., Ltd. Lithium tablets Li Battery level Kelude New Energy Technology Co., Ltd.

[0059] Table 2 Experimental Equipment

[0060] Instrument Name model factory Analytical balance FC-204 Shanghai Jingke Balance Magnetic stirrer CL-200 Gongyi Yuhua Instrument Co., Ltd. Vacuum drying oven ZK-82BB Shanghai Experimental Instrument Factory Co., Ltd. Button battery sealing machine MSK-110 Shenzhen Kejing Zhida Technology Co., Ltd. Electrode punching machine MSK-T10 Shenzhen Kejing Zhida Technology Co., Ltd. LAND Battery Testing System CT2001A Wuhan Jinno Electronics Co., Ltd. Electrochemical workstation CHI760E Shanghai Chenhua Instrument Co., Ltd. Scanning electron microscope FEI sirion200 FEI Company Vacuum glove box ZKX Nanjing University Instrument Factory low-speed centrifuge LC-LX-L40B Shanghai Lichen Bangxi Instrument Co., Ltd.

[0061] II. Example: Preparation of a solid-state battery using lithium iron phosphate modified with lithium fluorozirconate

[0062] Step 1: Preparation of lithium fluorozirconate (LZF) particles

[0063] Weigh 3.0 mg of lithium carbonate (Li₂CO₃) and 23 mg of hexafluorozirconic acid (H₂ZrF₆), and dissolve them in 40 mL of deionized water. The mixture was magnetically stirred at 25–30 °C for 24 h to obtain a mixture containing a white precipitate. The mixture was centrifuged, the supernatant was discarded, and the precipitate was collected. The precipitate was washed six times with anhydrous ethanol to thoroughly remove any residual acidic substances. Finally, the precipitate was placed in a vacuum drying oven and dried at 60 °C for 24 h to obtain white powdery lithium fluorozirconate (Li₂ZrF₆) particles.

[0064] Characterization results: such as Figure 2 As shown, the XRD pattern indicates that the main diffraction peaks are completely consistent with the standard card (No. 24-0689), with no impurity peaks, proving that high-purity LZF was successfully prepared. Figure 3 As shown in the figure, SEM reveals that LZF crystals have uniform grain size and regular morphology.

[0065] Step 2: Preparation of polymer electrolyte precursor and interface layer precursor

[0066] MMA, PVDF, and P (VDF-HFP) were weighed at a mass ratio of 50:1:4 and dissolved in DMF solvent. The mixture was stirred for 12 h to form a homogeneous and transparent solution A. Separately, 7 wt% of organomontmorillonite was dissolved in DMF and stirred to form solution B. Solutions A and B were mixed, and an appropriate amount of LiTFSI (lithium salt) was added. The mixture was stirred until a light yellow homogeneous precursor was formed. Subsequently, 0.8 wt% of initiator BPO was added, and the mixture was stirred at 95 °C for 12 min to carry out prepolymerization, yielding a viscous polymer electrolyte precursor, denoted as precursor C.

[0067] Take a portion of precursor C, add LZF particles prepared in step 1 (accounting for 5% of the total mass of precursor), ultrasonically disperse and stir evenly to obtain interfacial precursor D.

[0068] Step 3: Preparation of composite polymer electrolyte

[0069] Precursor C was poured into a glass mold using a solution casting method and dried in a vacuum drying oven at 80°C for 10 hours to prepare a polymer electrolyte base film with a thickness of approximately 200 μm, named PFPE (for comparative example).

[0070] A layer of interface precursor D was uniformly coated on the surface of the PFPE base film and then cured and dried again to form a functional interface layer with a thickness of about 8 μm, thus obtaining a composite polymer electrolyte, named PFPE-LZF (used in the example).

[0071] Characterization results: such as Figure 4 As shown, the PFPE-LZF surface exhibits a uniform, regular wrinkled structure, increasing the effective contact area; while the comparative PFPE ( Figure 5 The surface is relatively messy. Figure 7 Mechanical tests showed that the tensile strength (3.92 MPa) and elongation (129%) of PFPE-LZF were significantly better than those of PFPE (2.05 MPa, 88%).

[0072] Step 4: Preparation of Lithium Iron Phosphate (LFP) Cathode

[0073] Lithium iron phosphate active material, conductive agent acetylene black, and binder PVDF were added to NMP solvent at a mass ratio of 8:1:1, and ball milled to form a uniform black slurry. The slurry was coated onto carbon-coated aluminum foil, vacuum dried for 24 hours, and then cut into round sheets for later use.

[0074] Step 5: Assembly of solid-state lithium metal batteries

[0075] In an argon-protected glove box (water and oxygen content <0.1 ppm), CR2025 coin cells were assembled using lithium foil as the negative electrode, LFP prepared in step 4 as the positive electrode, and PFPE-LZF or PFPE prepared in step 3 as the electrolyte.

[0076] III. Comparative Example 1

[0077] The process is exactly the same as in the previous example, except that in step 3, the LZF-containing interface precursor D is not coated, and the battery is assembled directly using a pure PFPE base film.

[0078] IV. Performance Characterization and Creative Data Analysis

[0079] The performance of the above embodiments and comparative examples was characterized:

[0080] like Figure 1 The diagram illustrates the mechanism by which LZF dissipates free radicals and inhibits lithium dendrite growth in embodiments and comparative examples of the present invention. Introducing an LZF interface between the electrolyte and the lithium anode promotes uniform lithium ion deposition, thereby effectively inhibiting lithium dendrite formation. This is mainly attributed to the effective dissipation of free radicals generated during cycling by LZF, preventing interface deterioration. Specifically, under electrochemical cycling conditions, a small number of ester groups in the side chains of PMMA may decompose to generate free radicals (mainly alkoxy radicals). These free radicals further induce the continuous decomposition of PMMA and interface deterioration; while the Zr in this invention... 4+ Unoccupied d orbitals can capture and dissipate these free radicals.

[0081] like Figure 2 The image shows the X-ray diffraction pattern of lithium fluorozirconate prepared in the example. The main diffraction peaks are consistent with those of the standard card (PDF#24-0689), the peak intensities are similar, and no extra impurity peaks are observed, indicating that pure-phase Li2ZrF6 (LZF) was successfully prepared.

[0082] like Figure 3 The image shown is a scanning electron microscope (SEM) image of the lithium fluorozirconate (LZF) prepared in the example. It can be seen that the LZF crystals have a uniform particle size. This uniform particle size is beneficial for constructing continuous ion transport channels at the interface, thereby regulating the deposition behavior of lithium ions.

[0083] like Figure 4 The image shows scanning electron microscope (SEM) images of the surface and cross-section of the composite polymer electrolyte (PFPE-LZF) prepared in the example. The planar SEM images show that the PFPE-LZF surface is uniform and exhibits a regular wrinkled shape, increasing the specific surface area and providing more lithium-ion deposition sites. The cross-sectional images show that the interface layer is tightly bonded to the base film, with no obvious delamination.

[0084] like Figure 5The image shown is a scanning electron microscope (SEM) image of the surface of the polymer electrolyte (PFPE) prepared in comparison. Figure 4 As can be seen from the comparison, the internal morphology of the PFPE sample is relatively disordered and has poor uniformity, lacking a continuous and effective ion transport network, which is not conducive to the uniform migration and deposition of lithium ions.

[0085] like Figure 6 The figure shows the thermogravimetric analysis (TGA) curve of the composite polymer electrolyte prepared in the example. The data shows that the weight loss rate of PFPE-LZF is approximately 5.52% up to 300°C. This weight loss is mainly due to the residual liquid solvent DMF, indicating that the electrolyte has good thermal stability.

[0086] like Figure 7 The figure shows the stress-strain curves of the polymer electrolytes prepared in the examples and comparative examples. The test results show that the tensile strength of the PFPE polymer electrolyte is 2.05 MPa and the elongation is 88%. After introducing an interface layer rich in lithium fluorozirconate, the tensile strength of the PFPE-LZF polymer electrolyte is increased to 3.92 MPa and the elongation is increased to 129%, showing a significant enhancement in mechanical properties.

[0087] like Figure 8 The figure shows the electrochemical impedance spectroscopy (EIS) spectra of the polymer electrolytes prepared in the examples and comparative examples. As can be seen from the figure, the polymer electrolyte membrane of the example (PFPE-LZF) has a lower impedance value, only 6.13 Ω. Using the ionic conductivity calculation formula, the ionic conductivity of the polymer electrolyte membrane of the example is calculated to be 8.83 × 10⁻⁶. -4 S·cm⁻¹; while the ionic conductivity of the comparative (PFPE) polymer electrolyte membrane is only 4.89×10⁻¹. -4 S·cm⁻¹.

[0088] like Figure 9 The figures show the electron paramagnetic resonance (EPR) spectra of the polymer electrolytes prepared in the examples and comparative examples under different charge and discharge states. As the charge and discharge process progresses, the free radical content in PFPE gradually increases, reaching its maximum at a discharge state of 2.6 V. In contrast, in PFPE-LZF, the free radical content only slightly increases during charging and decreases significantly during discharging, reaching its lowest point at a discharge state of 2.6 V, demonstrating the efficient dissipation of free radicals by LZF.

[0089] like Figure 10The figures show the infrared spectra of the polymer electrolytes prepared in the examples and comparative examples under different charge-discharge states. In PFPE, a sharper shoulder peak appears next to the original broad C=O main peak. This shoulder peak originates from the main chain breakage and free radical-induced oxidative degradation of PMMA during battery cycling; while this phenomenon is significantly weakened in PFPE-LZF.

[0090] like Figure 11 The figures show XPS spectra of the polymer electrolytes prepared in the examples and comparative examples under different charge-discharge states. In the C 1s spectrum, after the addition of Li2ZrF6, the relative proportion of OC=O groups shows a stable trend as free radicals dissipate, corresponding to the stabilization of PMMA decomposition. However, for PFPE without Li2ZrF6, the relative proportion of OC=O groups continuously decreases as free radicals are generated from PMMA decomposition and the resulting continuous decomposition occurs, especially under full charge, where the degree of PMMA decomposition is greatest, which corresponds to the EPR results. The O 1s spectrum results also correspond to the C 1s spectrum.

[0091] like Figure 12 The figure shows the lithium-ion transference number (LTN) test results for the polymer electrolytes prepared in the examples and comparative examples. The calculated LTN of PFPE-LZF is 0.54, significantly higher than that of PFPE (0.42).

[0092] like Figure 13 The figure shows the linear sweep voltammetry (LSV) curves of the polymer electrolytes prepared in the examples and comparative examples. The reduction potential of PFPE-LZF is 0.12 V, the oxidation potential is 4.24 V, and its electrochemical stability window is 4.12 V, which is greater than that of PFPE (3.75 V). This indicates that the introduction of the LZF interfacial layer broadens the electrochemical stability window of the electrolyte.

[0093] like Figure 14 The figure shows the ionic conductivity of the polymer electrolytes prepared in the examples and comparative examples as a function of temperature. The activation energy of PFPE-LZF was calculated to be 11.55 kJ·mol⁻¹ using the Arrhenius equation. -1 It is lower than PFPE's 17.37 kJ·mol⁻¹. -1 This indicates that the LZF interface layer lowers the migration barrier of Li⁺.

[0094] like Figure 15The figure shows the Tafel curves for the solid-state lithium metal batteries assembled in the examples and comparative examples. During the redox process, the absolute values ​​of the slopes of the Li / PFPE-LZF / LFP batteries (oxidized state 0.33, reduced state 0.70) were both lower than those of the Li / PFPE / LFP batteries (oxidized state 0.5, reduced state 0.75). This indicates that PFPE-LZF exhibits superior catalytic kinetics in achieving rapid redox conversion of lithium ions and significantly reduces battery polarization.

[0095] like Figure 16 The figure shows the cyclic voltammetry (CV) curves of the solid-state lithium metal batteries assembled in the examples and comparative examples. The cyclic voltammetry curves of the solid-state lithium battery assembled with the PFPE-LZF polymer electrolyte membrane show lower polarization voltage, better redox peak separation, and better reversibility.

[0096] like Figure 17 The figure shows the nucleation overpotential curves of the lithium-copper batteries assembled in the examples and comparative examples. The nucleation overpotential of PFPE-LZF is 32 mV, and the plateau overpotential is 83 mV, which are significantly lower than the nucleation overpotential (44 mV) and plateau overpotential (107 mV) of PFPE. This indicates that after introducing the LZF interface layer, the nucleation and growth barrier of lithium ions is lower, which is beneficial to the uniform deposition of lithium ions and the stability of the SEI.

[0097] like Figure 18 The figure shows the constant current charge-discharge cycle curves of the lithium symmetric batteries assembled in the examples and comparative examples. At 0.2 mA·cm⁻¹ -2 At the specified current density, the polarization voltage of the Li / PFPE / Li battery continuously increases, leading to short-circuit failure after 90 hours. In contrast, the Li / PFPE-LZF / Li battery can cycle stably for over 400 hours, and its polarization voltage remains stable and lower than that of the Li / PFPE / Li battery. This indicates that the LZF interface layer has superior compatibility with the lithium anode.

[0098] like Figure 19 The figure shows the critical current density (CCD) test results for the lithium-ion symmetric batteries assembled in the examples and comparative examples. The critical current density of PFPE is 0.5 mA·cm⁻¹. -2 The CCD of PFPE-LZF can reach up to 0.85 mA·cm⁻¹. -2 Higher CCD resolution provides a solid guarantee for assembling full cells for high-rate and high-load cycling.

[0099] like Figure 20 The figure shows the long-cycle performance of the lithium metal batteries assembled in the examples and comparative examples. At 1 C rate and room temperature, the initial discharge specific capacity of PFPE-LZF reaches as high as 142.8 mAh·g.-1 It can cycle stably for over 650 cycles, maintaining a capacity retention of 70% and an average coulombic efficiency of over 99%. In contrast, PFPE has an initial discharge specific capacity of only 124 mAh·g at 1 C. -1 And it decays rapidly after 100 cycles.

[0100] like Figure 21 The figure shows the rate performance of the lithium metal batteries assembled in the examples and comparative examples. The PFPE-LZF in the example exhibits ultra-high discharge specific capacity of 158.5, 154.3, 145.1, 135.0, and 120.7 mAh·g⁻¹ under 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C conditions, respectively. Furthermore, it still exhibits a stable discharge specific capacity of 158 mAh·g⁻¹ at 0.2 C, and its polarization voltage is also lower, which is better than that of the comparative example.

[0101] like Figure 22 The figure shows the cycling performance of the lithium metal battery assembled in the example under high rate and high areal loading conditions. The Li / PFPE-LZF / LFP battery can stably cycle 20 times at 5 C, achieving a capacity of up to 109 mAh·g. -1 Furthermore, the polarization voltage consistency is good; when the active substance loading reaches 10 mg·cm⁻¹ -2 At this time, the assembled Li / PFPE-LZF / LFP battery has an initial capacity of up to 120 mAh·g at 0.5 C. -1 After 20 stable cycles, the capacity retention rate is still as high as 82%, and the polarization voltage is stable at 0.3V.

[0102] like Figure 23 The image shows scanning electron microscope (SEM) images of the surface and cross-section of the lithium anode after cycling in the examples and comparative examples of lithium metal batteries. After prolonged charge-discharge cycling, the Li / PFPE-LZF / LFP battery, thanks to the removal of free radicals, exhibits more uniform lithium deposition, which to some extent suppresses excessive lithium dendrite growth and further ensures stable subsequent cycling. In contrast, the Li / PFPE / LFP battery, under prolonged cycling, shows irregular lithium deposition, cracks, and uneven spherical deposition, with severe dendrite growth.

[0103] like Figure 24The figures show the electrochemical impedance spectroscopy (EIS) spectra of the lithium metal batteries assembled in the examples and comparative examples before and after cycling. The interfacial impedance of the Li / PFPE-LZF / LFP battery decreased from 140.9 Ω before cycling to 113.7 Ω. A stable and lower interfacial impedance is beneficial for better ion transport and charge transfer at the interface. However, the interfacial impedance of the Li / PFPE / LFP battery (comparative example) increased sharply after cycling, surging from an initial 224.5 Ω to 570.3 Ω. Severe deterioration occurred at the interface, and the SEI film was severely damaged, which is the key factor leading to the instantaneous failure of the battery's cycling performance. (Note: The subject error in the original text has been corrected here.)

[0104] like Figure 25 The figure shows the XPS spectra of the SEI films of the lithium metal batteries assembled in the examples and comparative examples after cycling. Combining the elemental spectra of F1s, N1s, and Li1s, it can be seen that the electrode surface with the LZF interface is more conducive to the formation of stable SEIs rich in LiF and Li3N, especially with a significant increase in the LiF content. This verifies the mechanism of LZF-induced LiF-rich SEI formation.

Claims

1. A method for modifying the interface of a solid polymer lithium metal battery using lithium fluorozirconate, characterized in that, Includes the following steps: (1) Preparation of lithium fluorozirconate particles: Lithium source and fluorozirconate source are reacted in aqueous solution, and lithium fluorozirconate (Li2ZrF6) particles are obtained after separation, washing and drying. (2) Preparation of polymer electrolyte precursor: Methyl methacrylate (MMA), polyvinylidene fluoride (PVDF), and polyvinylidene fluoride-hexafluoropropylene copolymer (P(VDF-HFP)) were dissolved in an organic solvent, and lithium salt and initiator were added to carry out a prepolymerization reaction to obtain polymer electrolyte precursor; (3) Construction of the fluorolithium zirconate interface layer: Take part or all of the polymer electrolyte precursor liquid obtained in step (2), disperse the lithium fluorolithium zirconate particles obtained in step (1) in it, stir evenly, and form an interface-modified precursor liquid. (4) Preparation of composite electrolyte membrane: First, the remaining polymer electrolyte precursor liquid from step (2) is formed into a film and cured to form a polymer electrolyte base membrane; then, the interface modification precursor liquid obtained in step (3) is coated on one side of the polymer electrolyte base membrane, and after curing, a composite polymer electrolyte with a lithium fluorozirconate functional interface layer is formed. The lithium fluorozirconate functional interface layer contains lithium fluorozirconate particles, which are configured to utilize the unoccupied d orbitals of their zirconium ions to coordinate with free radicals generated by electrolyte decomposition, thereby inhibiting free radical-induced polymer chain degradation reactions.

2. The modification method according to claim 1, characterized in that, The lithium source in step (1) is lithium carbonate (Li2CO3), and the fluorozirconic acid source is hexafluorozirconic acid (H2ZrF6); the mass ratio of lithium carbonate to hexafluorozirconic acid is 1:6 to 1:

9.

3. The modification method according to claim 1, characterized in that, The reaction temperature in step (1) is 25-30℃ and the reaction time is 20-28h; the washing is done with anhydrous ethanol and the number of washing cycles is 5-8.

4. The modification method according to claim 1, characterized in that, The mass ratio of MMA, PVDF and P(VDF-HFP) in step (2) is 40-60:1:3-5; and organic montmorillonite with a mass ratio of 5wt%-10wt% is also added.

5. The modification method according to claim 1, characterized in that, The initiator in step (2) is benzoyl peroxide (BPO), with a mass percentage of 0.5wt%-1wt%; the temperature of the prepolymerization reaction is 90-100℃ and the time is 10-15min.

6. The modification method according to claim 1, characterized in that, The thickness of the polymer electrolyte base film in step (4) is 150-250 μm, and the thickness of the lithium fluorozirconate functional interface layer is 5-15 μm.

7. A solid-state lithium metal battery, characterized in that, A solid-state lithium metal battery comprises a positive electrode, a lithium metal negative electrode, and a composite polymer electrolyte with a lithium fluorozirconate functional interface layer as described in any one of claims 1-6; wherein the lithium fluorozirconate functional interface layer of the composite polymer electrolyte is in direct contact with the lithium metal negative electrode; the solid-state lithium metal battery has a critical current density (CCD) of not less than 0.85 mA cm⁻¹ at room temperature. -2 And at 0.2 mA cm -2 The stable cycle life of the symmetrical battery under the specified current density is no less than 400 hours.

8. The solid-state lithium metal battery according to claim 7, characterized in that, The lithium iron phosphate cathode is composed of lithium iron phosphate active material, conductive agent and binder, wherein the mass ratio of lithium iron phosphate active material is 80%-90%.

9. The solid-state lithium metal battery according to claim 7, characterized in that, The solid-state lithium metal battery has an initial discharge specific capacity of no less than 140 mAh g at a 1C rate. -1 Furthermore, the capacity retention rate after 650 cycles at a 1C rate is not less than 70%.

10. The application of a solid-state lithium metal battery as described in any one of claims 7-9 in electric vehicle power battery packs, grid-scale energy storage devices, or portable electronic devices.

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