Application of FN-CPDs as electrolyte additive in lithium metal battery

Abstract

The invention provides an application of FN-CPDs as an electrolyte additive in a lithium metal battery, and belongs to the technical field of electrolyte. The FN-CPDs comprise four elements of C, N, O and F and take a carbon core as a center, the existence forms of the N element comprise pyridine nitrogen, pyrrole nitrogen and graphite nitrogen, the F element stably exists in a carbon skeleton in a C-F bond form, and the O element is derived from an oxygen-containing functional group on the surface. The introduction of FN-CPDs in the invention can reduce Li < + > desolvation energy and interface transmission impedance, and improve the rate capability of the battery. And meanwhile, the firm SEI can effectively inhibit dendritic crystals and side reaction gas production, so that the thermal stability and the safety performance of the battery are improved.

Description

Technical Field

[0001] This invention belongs to the field of electrolyte technology, specifically relating to the application of FN-CPDs as an electrolyte additive in lithium metal batteries. Background Technology

[0002] Lithium metal batteries have an extremely high theoretical specific capacity (3860 mAh·g). -1 With its low redox potential (-3.040 V vs. standard hydrogen electrode), lithium metal is considered one of the core candidates for next-generation high-energy-density energy storage devices, showing broad application prospects in electric vehicles, portable electronic devices, and large-scale energy storage. The electrolyte, as the "blood" of lithium metal batteries, plays a crucial role in ion transport, interface stability, and electrode protection; its performance directly determines the battery's cycle life, rate performance, safety performance, and other core indicators.

[0003] Currently, the commonly used electrolyte systems for lithium metal batteries mainly consist of carbonate solvents (such as ethylene carbonate EC, dimethyl carbonate DMC, diethyl carbonate DEC, etc.) combined with lithium salts (such as LiPF6, LiTFSI, etc.). However, this type of electrolyte has significant drawbacks: lithium metal anodes are prone to uneven deposition during cycling, forming lithium dendrites. The growth of lithium dendrites not only consumes electrolyte and lithium metal but may also puncture the separator, causing short circuits and leading to safety hazards such as battery fires and explosions. At the same time, the side reactions between the electrolyte and the lithium metal anode are intense, generating an unstable solid electrolyte interphase (SEI) film. The SEI film is prone to rupture and regeneration, further aggravating electrolyte consumption and lithium metal corrosion, and severely shortening the battery cycle life.

[0004] To address the aforementioned issues, various electrolyte additives have been developed in the prior art to improve the performance of lithium metal batteries. These mainly include fluorinated additives (such as fluoroethylene carbonate, FEC), phosphazene additives, lithium salt additives (such as LiNO3), and organosilicon additives. Among these, FEC is currently the most widely used additive. It can preferentially reduce and decompose to form a stable fluorine-rich SEI film on the lithium metal surface, thereby inhibiting lithium dendrite growth and electrolyte decomposition.

[0005] The closest existing technology to this invention is "Fluoroethylene carbonate additives can enable uniform lithium deposition in lithium metal batteries (X.-Q. Zhang, X.-B. Cheng, X. Chen, C. Yan, Q. Zhang, Adv. Funct. Mater. 2017, 27, 1605989.)". This technology improves the stability of the lithium metal anode interface and enhances battery cycle performance by adding 5%-15% FEC to the electrolyte.

[0006] However, the existing technology and other existing additive systems still have the following shortcomings: (1) The SEI film formed by existing additives has insufficient mechanical strength and is prone to cracking during the volume expansion and contraction process of lithium metal deposition / dissolution, and cannot provide long-term stable protection for the negative electrode; (2) Some additives (such as FEC) have limited improvement on battery rate performance and still have the problem of uncontrolled lithium dendrite growth under high current density; (3) Some additives (such as phosphazenes) have high synthesis costs and high toxicity, which are not conducive to large-scale application; (4) Most additives can only improve the negative electrode interface. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing lithium metal battery electrolyte additives, such as limited functionality, poor SEI film stability, limited lithium dendrite suppression effect, and inability to comprehensively improve the battery interface. This invention provides an application of FN-CPDs (fluorine-nitrogen co-doped carbonized polymer dots) as an electrolyte in lithium metal batteries. The invention aims to achieve the following objectives: (1) forming a high-strength, high-stability SEI film on the surface of the lithium metal anode, thereby suppressing lithium dendrite growth and electrolyte decomposition over a long period; (2) improving the ionic conductivity of the electrolyte and enhancing the rate performance of the battery; (3) simultaneously suppressing the dissolution of the cathode material and the oxidative decomposition of the electrolyte, comprehensively improving the cycle life, safety performance, and overall electrochemical performance of the lithium metal battery; and (4) the FN-CPDs material has a simple synthesis process, low cost, and is environmentally friendly, facilitating large-scale application.

[0008] This invention provides an application of FN-CPDs as an electrolyte additive in lithium metal batteries. The FN-CPDs include four elements: C, N, O, and F, with a carbon core at its center. The N element exists in the forms of pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen. The F element exists stably in the carbon framework in the form of CF bonds, and the O element originates from oxygen-containing functional groups on the surface.

[0009] Preferably, the particle size of the FN-CPDs is 2~5 nm.

[0010] Preferably, the preparation method of the FN-CPDs includes: Step 1: Add norfloxacin to deionized water and stir magnetically until completely dissolved to obtain a white suspension; Step 2: Transfer the white suspension obtained in Step 1 to a hydrothermal reactor lined with PTFE and seal it. Place it in an oven for hydrothermal reaction to obtain the reaction solution. Step 3: Separate and purify the reaction solution from Step 2, and dry it to obtain FN-CPDs.

[0011] Preferably, the stirring speed in step one is 500-800 r / min.

[0012] Preferably, the reaction temperature in step two is 200°C and the reaction time is 6 hours.

[0013] Preferably, the separation and purification in step three specifically involves: filtering out large particles using a 0.22-micron microporous membrane, and then dialyzing the solution through a dialysis bag for 24 hours, replacing the deionized water every 12 hours during this period.

[0014] Preferably, the drying time in step three is 24 hours.

[0015] Preferably, the application includes: Step 1: In an argon glove box, weigh the FN-CPDs powder and add it to the conventional carbonate electrolyte for lithium metal batteries at a concentration of 0.05 mg / mL to 0.5 mg / mL. Step 2: In a glove box, place the mixture on a magnetic stirrer and stir, then ultrasonically disperse to obtain a lithium metal battery electrolyte containing FN-CPDs.

[0016] Preferably, the stirring speed in step two is 500-800 r / min, and the stirring time is 1-2 h.

[0017] Preferably, the ultrasonic dispersion in step two has a power of 200-300 W, a frequency of 40-50 kHz, and a duration of 30-40 min.

[0018] Inventive Principles This invention designs a carbonized polymer dot containing both lithiophilic (N, O functional groups) and film-forming (F element) functional units, achieving dual regulation of the electrode-electrolyte interface at the molecular / nanoscale, while simultaneously optimizing the ion transport performance of the electrolyte, thereby comprehensively improving the performance of lithium metal batteries. Its working principle is as follows: (1) Interface regulation principle: FN-CPDs can preferentially adsorb onto the surface of lithium metal anode in the electrolyte. During battery charging and discharging, FN-CPDs undergo selective reduction and decomposition, forming an SEI film in synergy with electrolyte components. The SEI film contains stable components such as LiF, Li3N, and Li2O. Among them, LiF has high surface energy for lithium anodes, which can effectively suppress dendrite formation; Li3N with high ionic conductivity can significantly reduce charge transfer impedance at the interface; Li2O has good mechanical buffering and ion regulation effects in SEI, which helps to reduce stress accumulation caused by lithium insertion and extraction processes and reduce the probability of microcracks.

[0019] (2) Electrolyte optimization principle: FN-CPDs have good conductivity and dispersibility. Adding them to the electrolyte can improve the ionic conductivity of the electrolyte, reduce ion transport resistance, and improve the rate performance of the battery. At the same time, FN-CPDs can interact with free solvent molecules in the electrolyte, reduce the reduction and decomposition of solvent molecules, and reduce electrolyte consumption. In addition, FN-CPDs also have a certain modification effect on the surface of the positive electrode, which can inhibit the dissolution of positive electrode materials (such as ternary materials and lithium iron phosphate) and the oxidative decomposition of the electrolyte, and stabilize the positive electrode-electrolyte interface.

[0020] Beneficial effects of the present invention Compared with existing technologies (such as FEC additive electrolytes), the present invention, using FN-CPDs as electrolyte additives, has the following significant advantages: 1. Excellent full-cell performance: In Li / / LiFePO4 full cells, it can significantly improve capacity retention and cycle count. For example, after 200 cycles, the capacity retention increases to 90.8%.

[0021] 2. Excellent lithium dendrite suppression effect: According to the scanning electron microscope (SEM), the lithium metal anode surface of the battery of the present invention remains flat and dense after cycling, with no obvious lithium dendrite growth; while the lithium metal anode surface of the existing technology battery has a large number of needle-like lithium dendrites, and the surface roughness is significantly increased.

[0022] 3. Enhanced Interface Stability: The SEI film in the battery of this invention is a composite SEI; while the SEI film in existing batteries has a single regulation function, and obvious cracking and regeneration phenomena occur after cycling. Due to the variety of elements in the precursor norfloxacin, the synthesized FN-CPDs have abundant surface functional groups, which participate in the formation of the composite SEI during battery cycling. Among them, LiF has high surface energy for the lithium anode, which can effectively suppress dendrite formation; Li3N with high ionic conductivity can significantly reduce the charge transfer impedance at the interface; Li2O has good mechanical buffering and ion regulation effects in the SEI, which helps to mitigate the stress accumulation caused by the lithium insertion and extraction process and reduce the probability of microcracks.

[0023] 4. Excellent kinetic performance and safety: The introduction of FN-CPDs can reduce Li + Desolvation energy and interfacial transport impedance improve the rate performance of the battery. At the same time, the robust SEI effectively suppresses dendrite formation and side reaction gas generation, improving the thermal stability and safety performance of the battery.

[0024] 5. Good compatibility and economy: FN-CPDs exhibit good solubility and dispersibility in conventional electrolytes, are stable, and require only small addition amounts (<1 mg / mL). Their preparation methods (such as hydrothermal methods) are simple, the raw materials are widely available, they are environmentally friendly, and they are easy to scale up for production, demonstrating significant commercial application potential.

[0025] 6. Improved environmental friendliness: FN-CPDs materials have no obvious toxicity, no harmful gas emissions are emitted during the preparation process, and dialysis waste liquid can be discharged in compliance with standards after simple treatment; while some existing additives (such as phosphazenes) are toxic and easily cause environmental pollution. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the preparation process of FN-CPDs of the present invention.

[0027] Figure 2 This is a transmission electron microscope (TEM) image and particle size distribution statistical diagram of the FN-CPDs of this invention.

[0028] Figure 3 The X-ray photoelectron spectroscopy (XPS) total spectrum and N 1s and F 1s high-resolution spectra of the FN-CPDs of this invention are shown.

[0029] Figure 4 The ionic conductivity and ion transference number of the batteries containing FN-CPDs electrolyte and the blank electrolyte of this invention are shown.

[0030] Figure 5 The Li||Li symmetric cell of this invention, containing FN-CPDs electrolyte and blank electrolyte, was tested at a current density of 5 mA cm⁻¹. -2 , with a capacity of 1 mAh cm -2 Comparison of long-cycle performance under certain conditions.

[0031] Figure 6 The images show the surface and cross-section of the Li||Li symmetric battery containing FN-CPDs electrolyte and the blank electrolyte after 30 cycles.

[0032] Figure 7 XPS values ​​of the Li anode after 30 cycles of the Li||Li symmetric battery containing FN-CPDs electrolyte and blank electrolyte of this invention.

[0033] Figure 8 The cycling performance curves of the Li||LiFePO4 battery containing FN-CPDs electrolyte and the blank electrolyte of this invention are shown at 0.5 C.

[0034] Figure 9 These are SEM images of the positive and negative electrodes of the batteries containing FN-CPDs electrolyte and the blank group electrolyte Li||LiFePO4 after 30 cycles. Detailed Implementation

[0035] This invention provides an application of FN-CPDs as an electrolyte additive in lithium metal batteries. The FN-CPDs include four elements: C, N, O, and F. They are carbonized polymer dots with abundant functional groups on the surface, centered on a carbon core. The N element exists in the forms of pyridinic nitrogen (Pyrrolic-N), pyrrolic nitrogen (Pyrrolic-N), and graphitic nitrogen (Graphitic-N). The F element exists stably in the carbon skeleton in the form of CF bonds. The O element comes from the oxygen-containing functional groups on the surface, which are (-COOH) or hydroxyl groups (-OH).

[0036] According to the present invention, the FN-CPDs have a particle size of 2~5 nm, good dispersibility, and excellent solubility in common electrolyte solvents such as carbonates and ethers (solubility ≥20 mg / mL).

[0037] According to the present invention, the FN-CPDs of the present invention also possess a tunable surface electronic structure of nitrogen, enhancing the interaction with Li. + Adsorption of Li induces + The film possesses the ability to deposit uniformly; the interfacial modification capabilities of fluorine (enhancing the ionic conductivity and chemical stability of the SEI film); and abundant oxygen-containing functional groups on its surface enhance compatibility with the electrolyte and improve dispersion stability. This composite functionality enables it to simultaneously stabilize the negative and positive electrode interfaces, reducing side reactions.

[0038] According to the present invention, the preparation method of FN-CPDs is as follows: Figure 1 As shown, preferably including: Step 1: Add norfloxacin to deionized water and stir magnetically until completely dissolved. The preferred stirring speed is 500-800 r / min to obtain a white suspension. The ratio of norfloxacin mass (g) to deionized water volume (mL) is 0.4:25. Step 2: Transfer the white suspension obtained in Step 1 to a hydrothermal reactor lined with PTFE and seal it. Place it in an oven for hydrothermal reaction. The preferred reaction temperature is 200℃ and the preferred reaction time is 6 hours to obtain the reaction solution. Step 3: The reaction solution from Step 2 is separated and purified. Specifically, the preferred method is to filter the solution through a 0.22-micron microporous membrane to remove large particles, then dialyze the solution through a dialysis bag (molecular weight cutoff 1000 kDa) for 24 hours, replacing the deionized water every 12 hours during this period. After drying, FN-CPDs are obtained. The preferred drying time is 24 hours.

[0039] According to the present invention, the application includes: Step 1: In an argon glove box (moisture content ≤10 ppm, oxygen content ≤10 ppm), weigh the FN-CPDs powder and add it to the conventional carbonate electrolyte of lithium metal batteries at a concentration of 0.05 mg / mL to 0.5 mg / mL. Step 2: In a glove box, place the mixture on a magnetic stirrer and stir. The stirring speed is preferably 500-800 r / min and the stirring time is preferably 1-2 h. Then, ultrasonically disperse the mixture. The power is preferably 200-300 W, the frequency is preferably 40-50 kHz, and the time is preferably 30-40 min.

[0040] A lithium metal battery electrolyte containing FN-CPDs was obtained.

[0041] The present invention will be further described in detail below with reference to specific embodiments. All raw materials involved in the embodiments are commercially available.

[0042] Experimental materials: FN-CPDs (prepared in this invention, pale yellow powder); 1M LiPF6 in EC:DMC=1:1 Vol% with 1% LiNO3 (battery grade, purity ≥99.9%); lithium metal sheet (thickness 100 μm, purity ≥99.9%); LiFePO4 cathode material (purity ≥99.5%); acetylene black (conductive agent, purity ≥99.9%); polyvinylidene fluoride (PVDF, binder); N-methylpyrrolidone (NMP, analytical grade); Celgard 2400 separator (polypropylene, thickness 25 μm).

[0043] Instruments and equipment: Magnetic stirrer (HJ-6); Electronic balance (BS210S); Ultrasonic cleaner (KQ-500E); Low-temperature vacuum drying oven (DZF-6020); Tableting machine (DC-15); Battery packaging machine (MSK-11); Battery testing system (Neware) (CT-4008); Battery testing system (Blue Electric) (CT2001A); Electrochemical workstation (CHI-660E); Scanning electron microscope (SU8000); Transmission electron microscope (JEOL-2100F); X-ray photoelectron spectroscopy (ESCALAB250); Argon glove box.

[0044] Example 1: Preparation of FN-CPDs 1. Raw material preparation: Accurately weigh norfloxacin (0.4 g) and set aside; 2. Mixing and dissolving: Add norfloxacin to 25 mL of deionized water and stir magnetically (500 r / min) until completely dissolved to obtain a white suspension.

[0045] 3. Hydrothermal reaction: Transfer the white suspension to a 50 mL stainless steel hydrothermal reactor lined with polytetrafluoroethylene, seal it, place it in an oven, heat it to 200℃, and keep it at that temperature for 6 h.

[0046] 4. Separation and purification: After the reaction is completed, the solution is allowed to cool naturally to room temperature. The reaction solution is then removed and filtered through a 0.22-micron microporous membrane to remove large particles. Subsequently, the solution is dialyzed through a dialysis bag (molecular weight cutoff 1000 kDa) for 24 hours, with deionized water replaced every 12 hours to remove small molecule impurities.

[0047] 5. Drying the finished product: The dialyzed solution was placed in a freeze dryer and freeze-dried for 24 hours to obtain a light yellow powdered FN-CPDs product with a yield of over 50%.

[0048] Figure 2 Transmission electron microscopy (TEM) image of FN-CPDs prepared in Example 1 of this invention ( Figure 2 a) and particle size distribution statistics ( Figure 2 b). Figure 2 In the TEM image, the scale bar is 20 nanometers, showing that FN-CPDs are approximately spherical, monodisperse nanoparticles with a size distribution between 2 and 5 nanometers. The lattice fringes are clearly visible, proving that the preparation process of FN-CPDs is stable and the product has good uniformity.

[0049] Figure 3 The total X-ray photoelectron spectroscopy (XPS) spectrum of the FN-CPDs prepared in Example 1 of this invention is shown below. Figure 3 a) and N 1s ( Figure 3 b), F 1s ( Figure 3 c) High-resolution spectrum. Figure 3 In the XPS total spectrum, it was confirmed that FN-CPDs contain four elements: C, N, F and O. The N 1s high-resolution spectrum can be decomposed into characteristic peaks of pyrrole nitrogen (397.2 eV), pyridine nitrogen (398.0 eV) and graphitic nitrogen (398.9 eV). The F 1s high-resolution spectrum showed the presence of CF bonds, proving that the fluorine and nitrogen elements were successfully doped and exist in the target chemical form.

[0050] Example 2: Preparation and performance testing of electrolyte and battery with FN-CPDs added at a concentration of 0.2 mg / mL. Electrolyte preparation In an argon glove box (moisture ≤ 10 ppm, oxygen ≤ 10 ppm), 0.2 mg of FN-CPDs was weighed and added to 1 mL of basic electrolyte (1M LiPF6 in EC:DMC = 1:1 Vol% with 1% LiNO3 (battery grade, purity ≥ 99.9%)). The mixture was magnetically stirred (800 r / min) for 2 h, followed by ultrasonic dispersion (200 W, 40 kHz) for 30 min to obtain an electrolyte of 0.2 mg / mL FN-CPDs (denoted as E1).

[0051] Battery manufacturing Positive electrode preparation: LiFePO4, acetylene black and PVDF were mixed at a mass ratio of 8:1:1, and 400 μL of NMP was added. The mixture was magnetically stirred for 6 h to form a uniform slurry. The slurry was coated on aluminum foil and dried in an oven at 60 °C for 12 h. Then, it was pressed into a positive electrode sheet with a diameter of 14 mm using a tablet press.

[0052] Battery assembly: In an argon glove box, a Li||LiFePO4 button cell (denoted as B1) was assembled in a CR2032 button cell case using a lithium metal sheet (16 mm in diameter and 100 μm in thickness) as the negative electrode, Celgard 2400 as the separator, and E1 as the electrolyte (80 μL of electrolyte).

[0053] Comparative Example 1: Preparation and Performance Testing of Blank Electrolyte Batteries Without FN-CPDs Electrolyte preparation: In an argon glove box, take a certain volume of 1M LiPF6 in EC:DMC=1:1 Vol% with 1% LiNO3 (battery grade, purity ≥99.9%); Performance testing: The test items are the same as in Example 1.

[0054] Performance testing (1) Ionic conductivity test: The ionic conductivity of E1 in a stainless steel symmetric cell was tested using an electrochemical workstation, and the average value was taken after three repetitions. (2) Cyclic performance test: The battery test system was used to conduct a cyclic test at room temperature (25℃) and 0.5 C rate (charging voltage range 2.5~4.2 V), and the charge-discharge specific capacity and capacity retention rate were recorded for each cycle; (3) Rate performance test: At room temperature, charge and discharge tests were performed at rates of 0.1 C, 0.2 C, 0.5 C, 0.8 C, 1.0 C and 0.1 C respectively. Each rate was cycled 5 times and the discharge specific capacity was recorded. (4) Interface characterization: After the battery was cycled 30 times, it was disassembled in the glove box, and two electrodes were taken out. They were cleaned with DMC three times, vacuum dried, and then subjected to SEM and XPS tests to analyze the surface morphology of the negative electrode and the composition of the SEI film.

[0055] Test results: The ionic conductivity of E1 at 25 °C is 6.49 mS·cm. -1 The B1 battery retains 90.8% capacity after 200 cycles at 0.5 C rate, with a coulombic efficiency consistently above 99.2%; its discharge specific capacity at 0.1 C rate is 158 mAh·g. -1 At a 1.0 C rate, it is 123 mAh·g -1 SEM showed that a smooth, dense protective layer was formed on the negative electrode surface, which was free of lithium dendrites and effectively improved the porous structure of the LFP side of the positive electrode. XPS confirmed that the SEI film is a stable component of Li2O-LiF-Li3N.

[0056] Test results: The ionic conductivity of E0 at 25℃ is 1.3 mS·cm -1 The B0 battery retained 74.6% of its capacity after 200 cycles at 0.5 C; its coulombic efficiency remained stable at 98.3%; and its 0.1 C discharge specific capacity was 152 mAh·g. -1 At a 1.0 C rate, it is 118 mAh·g -1 SEM showed a large number of disordered lithium dendrites on the surface of the negative electrode, and XPS showed that harmful components such as Li2CO3 were generated on the lithium negative electrode after cycling.

[0057] Figure 4 The ionic conductivity and ion transference number of the battery containing FN-CPDs electrolyte in Example 2 of the present invention and the electrolyte in the blank group of Comparative Example 1 are shown. Figure 4 a is the impedance diagram of a symmetrical stainless steel cell at room temperature. Figure 4 b is the ionic conductivity diagram. Figure 4 c is the It curve of the Li / / Li symmetric cell assembled with the blank electrolyte group. Figure 4 d represents the impedance diagrams before and after It test of the Li / / Li symmetric cell assembled with the blank electrolyte. Figure 4 e represents the It curve of a Li / / Li symmetric cell assembled with FN-CPDs electrolyte. Figure 4 f represents the impedance diagrams before and after testing of a Li / / Li symmetric cell assembled with FN-CPDs electrolyte. It can be seen that the introduction of FN-CPDs improves the ionic conductivity compared to the blank electrolyte. Furthermore, the lithium-ion transference number (0.895) at a concentration of 0.2 mg / mL is higher than that of the blank electrolyte (0.656), indicating that the introduction of FN-CPDs can significantly improve the Li / / Li symmetric cell. + The transmission rate.

[0058] Figure 5In Example 2 of this invention, a Li||Li symmetric cell containing FN-CPDs electrolyte and the electrolyte of Comparative Example 1 (blank group) was tested at a current density of 5 mA cm⁻¹. -2 , with a capacity of 1 mAh cm -2 Comparison of long-cycle performance under certain conditions. Figure 5 a is a Li / / Li symmetric cell at a current density of 5 mA cm⁻¹ -2 , with a capacity of 1 mAh cm -2 Cycle graph under conditions Figure 5 b is a magnified view at a specific number of cycles. The comparison of long-cycle performance of Li / / Li symmetric batteries shows that at a current density of 5 mA cm⁻¹… -2 , with a capacity of 1mAh cm -2 Under these conditions, the electrolyte of this invention can reduce overpotential and significantly improve battery cycle stability.

[0059] Figure 6 The images show SEM images of the surface and cross-section of a Li||Li symmetric battery (Example 2 of the present invention) containing FN-CPDs electrolyte and the blank electrolyte (Comparative Example 1) after 30 cycles. Figure 6 a is the planar scan image after the blank group is cycled. Figure 6 b is the cyclic plane scan map with added FN-CPDs. Figure 6 c is the cross-sectional scan image after the blank group cycle. Figure 6 d represents the cyclic cross-sectional scan image with added FN-CPDs. Figure 6 In the study, SEM images of the lithium anodes after 30 cycles of the blank electrolyte and the Li / / Li symmetric battery with FN-CPDs additive showed that the electrolyte with FN-CPDs additive was smooth and dense with no obvious lithium dendrites; while the blank electrolyte showed a large number of lithium dendrites on the anode surface, proving that the electrolyte of the present invention can effectively inhibit the growth of lithium dendrites. At the same time, cross-sectional SEM verified the formation of a smooth protective layer.

[0060] Figure 7 The Li||Li symmetric cell of Example 2 of this invention, containing FN-CPDs electrolyte and the electrolyte of Comparative Example 1 (blank group), underwent XPS at the Li anode after 30 cycles. Figure 7 'a' represents the F 1s high-resolution XPS of the blank group. Figure 7 b represents a high-resolution XPS with F1s and FN-CPDs added. Figure 7 c represents the C 1s high-resolution XPS from the blank group. Figure 7 d represents a C1s high-resolution XPS with added FN-CPDs. Figure 7 e represents the S 2p high-resolution XPS of the blank group. Figure 7 f represents an S 2p high-resolution XPS with added FN-CPDs. Figure 7g represents the O 1s high-resolution XPS of the blank group. Figure 7 h represents an O 1s high-resolution XPS with added FN-CPDs. Figure 7 i represents N in the blank group, 1s high-resolution XPS. Figure 7 j is an N 1s high-resolution XPS with added FN-CPDs. Figure 7 k represents the Li 1s high-resolution XPS of the blank group. Figure 7 l is a Li 1s high-resolution XPS with added FN-CPDs. Figure 7 In this study, the composition of the protective layer after cycling was analyzed in both the blank electrolyte and the Li / / Li symmetric battery with FN-CPDs additive. XPS analysis showed that after cycling, the addition of FN-CPDs resulted in the formation of a Li₂O-LiF-Li₃N composite SEI layer at the lithium anode, reducing harmful components such as Li₂CO₃. Specifically, LiF possesses high surface energy at the lithium anode, effectively suppressing dendrite formation; Li₃N, with its high ionic conductivity, significantly reduces charge transfer impedance at the interface; and Li₂O provides good mechanical buffering and ion regulation within the SEI, helping to mitigate stress accumulation caused by lithium insertion / extraction processes and reducing the probability of microcracks.

[0061] Figure 8 The cycling performance curves of the Li||LiFePO4 battery at 0.5 C for Example 2 of this invention, containing FN-CPDs electrolyte and the blank electrolyte of Comparative Example 1, are shown. Figure 8 a is a graph showing the discharge specific capacity versus cycle number for the two electrolyte groups. Figure 8 b is the capacity-voltage curve of the blank group. Figure 8 c is the capacitance-voltage curve with FN-CPDs added. Figure 8 d represents the scaling factor diagram for the two electrolyte groups. Figure 8 Electrochemical tests were conducted on the blank electrolyte and the Li / / LiFePO4 full cell with FN-CPDs additive. The introduction of FN-CPDs at 0.5C improved the discharge specific capacity and coulombic efficiency of the full cell, with a capacity retention of 90.9% after 200 cycles. The voltage-capacity curves showed that the voltage difference between the charge-discharge plateaus of the Li / / LiFePO4 battery with added FN-CPDs remained almost the same after 200 cycles as in the first cycle, exhibiting good kinetic stability. In contrast, the voltage difference between the charge-discharge plateaus of the battery without added FN-CPDs increased significantly.

[0062] Figure 9 SEM images of the positive and negative electrodes of a Li||LiFePO4 battery containing FN-CPDs electrolyte and a blank electrolyte (Comparative Example 1) after 30 cycles. Figure 9 a and 9c are LEP scan images after the blank group cycle. Figure 9b and 9d are scan images of LEP slices after cycling with FN-CPDs added. Figure 9 e is the Li wafer scan image after the blank group cycle. Figure 9 f is a scan image of a Li film with FN-CPDs added. Figure 9 In the study, SEM characterization was performed on the two electrode sheets after cycling the blank electrolyte and the Li / / LiFePO4 full cell with FN-CPDs additive. Without FN-CPDs, a large amount of "dead" Li and a porous structure were formed on both electrodes. When FN-CPDs were used as an additive, a flatter and denser electrode surface was obtained.

[0063] In summary: (1) FN-CPDs, as an electrolyte additive, can significantly improve the cycle stability, rate performance, and ion transport rate of lithium metal batteries, with better results than existing commercial electrolyte additives that only add LiNO3; (2) The optimal addition amount of FN-CPDs is 0.2 mg / mL, at which point the overall battery performance is optimal. If the addition amount is too low (0.05 mg / mL), the interface regulation effect will be insufficient, and if the addition amount is too high (0.5 mg / mL), the electrolyte viscosity will increase, affecting the high-rate performance; (3) The FN-CPDs additive of this invention forms a stable composite SEI film Li2O-LiF-Li3N and optimizes Li + The transport and deposition processes have enabled a comprehensive improvement in the performance of lithium metal batteries, showing promising application prospects.

Claims

1. The application of FN-CPDs as an electrolyte additive in lithium metal batteries, characterized in that, The FN-CPDs include four elements: C, N, O, and F. The carbon core is the center of the carbon core. The N element exists in the form of pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. The F element exists stably in the carbon framework in the form of CF bonds. The O element comes from the oxygen-containing functional groups on the surface.

2. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 1, characterized in that, The particle size of the FN-CPDs is 2~5 nm.

3. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 1, characterized in that, The method for preparing the FN-CPDs includes: Step 1: Add norfloxacin to deionized water and stir magnetically until completely dissolved to obtain a white suspension; Step 2: Transfer the white suspension obtained in Step 1 to a hydrothermal reactor lined with PTFE and seal it. Place it in an oven for hydrothermal reaction to obtain the reaction solution. Step 3: Separate and purify the reaction solution from Step 2, and dry it to obtain FN-CPDs.

4. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 3, characterized in that, The stirring speed described in step one is 500-800 r / min.

5. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 3, characterized in that, The reaction temperature in step two is 200℃, and the reaction time is 6 hours.

6. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 3, characterized in that, The separation and purification described in step three are as follows: large particles are removed by filtration through a 0.22-micron microporous membrane, and then the solution is dialyzed through a dialysis bag for 24 hours, with the deionized water being replaced every 12 hours during this period.

7. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 3, characterized in that, The drying time described in step three is 24 hours.

8. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 1, characterized in that, The applications include: Step 1: In an argon glove box, weigh the FN-CPDs powder and add it to the conventional carbonate electrolyte for lithium metal batteries at a concentration of 0.05 mg / mL to 0.5 mg / mL. Step 2: In a glove box, place the mixture on a magnetic stirrer and stir, then ultrasonically disperse to obtain a lithium metal battery electrolyte containing FN-CPDs.

9. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 8, characterized in that, The stirring speed in step two is 500-800 r / min, and the stirring time is 1-2 h.

10. The application of FN-CPDs as an electrolyte additive in lithium metal batteries according to claim 8, characterized in that, The ultrasonic dispersion described in step two has a power of 200-300 W, a frequency of 40-50 kHz, and a duration of 30-40 min.

Images