A method for preparing a solid electrolyte for lithium metal batteries

Abstract

The present invention relates to the field of lithium batteries, and specifically to a method for preparing a solid electrolyte for lithium metal batteries, comprising the following steps: synthesizing a PI polymer; synthesizing a COF; synthesizing a solid electrolyte: immersing the COF in a 1 mol / L LiTFSI acetonitrile solution, washing, and vacuum drying to obtain a NKCOF compound; dissolving the NKCOF compound, the PI polymer, and LiTFSI powder in DMAc, filtering, spreading, and evaporating the solvent under vacuum conditions; soaking in methanol, and drying to obtain a finished product. The present invention integrates an ion conduction pathway through homologous imide bond molecules, and the polar microenvironment makes Li + / TFSI − The transport has structurally selective decoupling, which solves the long-standing problem in the field of solid electrolytes for lithium metal batteries, achieves high ionic conductivity, stable interface contact and mechanical strength at room temperature, and prepares a polymer-covalent organic framework solid electrolyte.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and in particular to a method for preparing a solid electrolyte for lithium metal batteries. Background Art

[0002] Lithium (Li) metal batteries (LMBs) are considered to be attractive next-generation energy storage devices because Li metal has an extremely high theoretical capacity (3860 mAh g −1 ) and a low redox potential (−3.04 V vs. standard hydrogen electrode). However, the commercialization of lithium metal batteries using organic electrolyte systems is severely limited by safety concerns and ongoing challenges associated with the lithium metal anode, including uncontrolled dendrite formation, dead lithium accumulation, and large volume fluctuations during cycling. To address these challenges, researchers in the field have conducted extensive research on methods such as porous scaffolds, artificial solid electrolyte interlayers, and solid-state electrolytes (SSEs). Among these approaches, SSEs have attracted particular attention due to their inherent safety and potential to suppress Li dendrites.

[0003] Covalent organic frameworks (COFs), with their highly tunable chemical structures and well-defined porosity, have emerged as competitive candidates for next-generation solid electrolytes (SSEs), providing a unique platform for designing efficient lithium-ion conductors. The characteristic stacking of COFs enables the formation of directional, open channels throughout the COF backbone, promoting directional ion transport. To date, various COF-based solid electrolytes have been developed, including anionic COFs, cationic COFs, zwitterionic COFs, PEO-intercalated COFs, and plasticizer-COF composites. These materials demonstrate efficient ion conduction by introducing mobile charge carriers and modifying the internal pore environment, thereby enhancing Li dissolution and mobility. However, most of them exhibit limited room-temperature ionic conductivity and cycling performance in all-solid-state lithium metal batteries (LMBs). Therefore, high-performance all-solid-state lithium metal batteries still require a new strategy to fabricate COF solid electrolytes that can effectively promote ion pair dissociation and ionic component transport. Summary of the Invention

[0004] Despite the promising prospects of current solid polymer electrolytes (SPEs) and solid inorganic electrolytes (SIEs), they still face inherent limitations and cannot meet the various requirements of solid-state LMBs. Compared with SIEs, solid polymer electrolytes (SPEs) such as polyethylene oxide (PEO) exhibit superior interfacial compatibility and processability. However, the ion transport mechanism relies on the coupling of lithium migration and polymer segmental motion, resulting in insufficient ionic conductivity (<10−4 S cm −1) and low lithium transference number (0.2~0.5). To alleviate these problems, conventional solid polymer electrolytes are usually operated at high temperatures to promote segmental motion and improve ionic conductivity. However, this approach can compromise the mechanical integrity of the battery, resulting in reduced structural stability and causing safety issues. Despite various strategies such as adding inorganic fillers, cross-linking polymers, introducing flame retardants, and designing synthetic block copolymers, the balance between ionic conductivity, ion transference number and physicochemical stability exhibited by the materials continues to cause trouble in the industry. This long-standing challenge highlights the urgent need for innovative methods to develop materials that can simultaneously achieve high efficiency Li + SSE with excellent transport, high ion selectivity and thermal stability.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing a solid electrolyte for a lithium metal battery comprises the following steps:

[0007] Synthesis of PI: Under inert gas, 4 mmol of 2,4,6-trimethyl-1,3-phenylenediamine and 10 mL of N,N-dimethylacetamide (DMAc) were added to a three-necked flask equipped with a mechanical stirrer. Then, 3.8 mmol of 4,4'-(hexafluoroisopropenyl)diphthalic anhydride (6FDA) and 0.2 mmol of 4,4'-(acetylene-1,2-diyl)diphthalic anhydride were added in sequence and mechanically stirred until a viscous polyamic acid was generated. Then, 4.8 mL of acetic anhydride and 4 mL of pyridine were slowly added to the polyamic acid and stirred at room temperature to obtain a mixture. The mixture was slowly poured into a methanol aqueous solution to obtain an intermediate product. The intermediate product was crushed and washed with a methanol aqueous solution, and then vacuum dried to obtain a PI polymer.

[0008] Synthesis of COF: 0.15 mmol of pyromellitic acid, 0.60 mmol of benzoic acid, and 0.10 mmol of 1,3,5-tris(4-aminophenyl) were reacted at 180-210 degrees Celsius for 5 days. The product was washed and Soxhlet extracted with tetrahydrofuran, and then vacuum dried to obtain COF.

[0009] Synthesis of solid-state electrolyte: COF is immersed in a 1 mol / L LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) acetonitrile solution, washed and vacuum dried to obtain an NKCOF compound. The NKCOF compound, PI polymer and LiTFSI powder are dissolved in DMAc to prepare a casting solution, which is then filtered, spread and the solvent evaporated under vacuum conditions to obtain a thin film. Finally, it is soaked in methanol and then dried to obtain the finished product.

[0010] As a further embodiment of the present invention: the inert gas is argon.

[0011] As a further solution of the present invention: the mechanical stirring time is 48 hours, and the room temperature stirring time is 24 hours.

[0012] As a further solution of the present invention, the volume ratio of methanol to water in the methanol aqueous solution is 1:1, and the number of washing times of the methanol aqueous solution is 5 times.

[0013] As a further solution of the present invention: the vacuum drying temperature is 80 degrees Celsius.

[0014] As a further embodiment of the present invention, the 1,3,5-tris(4-aminophenyl) compound is one of 1,3,5-tris(4-aminophenyl)benzene, 1,3,5-tris(4-aminophenyl)oxazine and 1,3,5-tris(4-aminophenyl)amine.

[0015] As a further embodiment of the present invention, the NKCOF compound is one of NKCOF-28, NKCOF-29 and NKCOF-30.

[0016] As a further solution of the present invention, the filtration was performed using a polytetrafluoroethylene watch glass (10 cm×10 cm).

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention integrates the ion conduction pathway through homologous imide bond molecules, and the polar microenvironment makes Li + / TFSI − The structure-selective decoupling of transport solves a long-standing challenge in the design and application of solid-state electrolytes (SSEs) for lithium metal batteries—simultaneously achieving high ionic conductivity, stable interfacial contact, and mechanical strength at room temperature. A polymer-covalent organic framework (COF) solid electrolyte constructed through homologous imide bonding was prepared. This molecularly integrated architecture can form continuous ion conduction pathways, a unified chemical interface, and stable electrode compatibility, providing a new design strategy for high-performance SSEs. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The present invention is a flow chart of a method for preparing a solid electrolyte for lithium metal batteries.

[0020] Figure 2 Schematic diagram of the synthesis of three representative imide-bonded COFs.

[0021] Figure 3 PXRD and nitrogen adsorption data of three representative imide-bonded COFs.

[0022] Figure 4HR-TEM images of three representative imide-bonded COFs, where a and b are images of NKCOF-28, c and d are images of NKCOF-29, and e and f are images of NKCOF-30.

[0023] Figure 5 Figure 4 shows the Fourier transform infrared spectra of three representative imide-bonded COFs. a is the image of NKCOF-28, b is the image of NKCOF-29, and c is the image of NKCOF-30.

[0024] Figure 6 The solid-state NMR images of three representative imide-bonded COFs are shown in Figure 1. (a) is the image of NKCOF-28, (b) is the image of NKCOF-29, and (c) is the image of NKCOF-30.

[0025] Figure 7 Schematic diagram of the composite morphology of PI, PI@COF0.5 and PI@COF.

[0026] Figure 8 The ionic conductivity and activation energy results of PI, PI@COF0.5 and PI@COF.

[0027] Figure 9 The lithium ion migration number and electrochemical window results of PI, PI@COF0.5 and PI@COF.

[0028] Figure 10 The critical current density results of three Li / Li symmetric batteries.

[0029] Figure 11 The graph shows the Li deposition / stripping experimental results of three Li / Li symmetric batteries.

[0030] Figure 12 This is the long cycle stability test data result diagram of Li / PI@COF / Li symmetric battery.

[0031] Figure 13 The discharge capacity results of solid-state batteries with electrolytes of different components at 0.2 C.

[0032] Figure 14 The chart shows the battery cycle stability and capacity retention data of three batteries.

[0033] Figure 15 Figure 2 shows the rate capability results of batteries using PI, PI@COF0.5, and PI@COF electrolytes. DETAILED DESCRIPTION

[0034] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0035] The preparation of high-performance polymer COF solid electrolytes with homologous imide bonds requires the rational selection of COF frameworks and imide-rich polymer matrices. In this application, three representative imide-bonded COFs (NKCOF-28, NKCOF-29, and NKCOF-30) were first selected and synthesized to systematically evaluate their applicability as COF frameworks for solid electrolyte construction. Tris(4-aminophenyl)amine (TAPA), 1,3,5-tris(4-aminophenyl)benzene (TAPB), and 2,4,6-tris(4-aminophenyl)-triazine (TAPT) were used as linkers to construct the COF frameworks. Figure 2 Models of three structurally related COFs are shown.

[0036] The crystal structure analysis of these three structurally related COF models was performed. Figure 3 The powder X-ray diffraction (PXRD) and nitrogen adsorption data of the three structurally related COF models are shown in Figure 2. Figure 3 It can be seen that all imide-linked COFs have a well-defined crystalline structure, exhibiting sharp peaks at low angles, which closely match the simulated pattern of overlapping AA stacking modes. The permanent porosity of COFs was evaluated by nitrogen adsorption-desorption experiments. All adsorption isotherms showed typical type IV characteristics, which are characteristic of mesoporous materials. The BET surface areas of NKCOF-28, NKCOF-29, and NKCOF-30 were 941 m 2 / g, 1025 m 2 / g and 1234 m 2 This result indicates that there are long-range ordered crystalline channels in these imine-bonded COFs, which is beneficial for the Li + transportation.

[0037] The three structurally related COF models were photographed using high-resolution transmission electron microscopy (HR-TEM). Figure 4 For HR-TEM images, Figure 4 It can be seen that the three COFs have good crystallinity and the pore morphology is clearly visible.

[0038] The three COFs were characterized by Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance. Figure 5 The infrared spectra of three COFs are shown in Figure 2. Figure 6Solid-state NMR results of three COFs.

[0039] The polymer-COF solid electrolyte was prepared by mixing NKCOF compound, PI polymer and LiTFSI powder in different mass ratios (0:1:0.2, 0.5:1:0.2 and 1:1:0.2), and PI, PI@COF0.5 and PI@COF were obtained respectively. Figure 7 Schematic diagram of the composite morphology of PI, PI@COF0.5 and PI@COF. + Ions also migrate between coordination sites through the segmented motion of amorphous polymer chains, similar to the conductive behavior of other polymers. After the formation of the polymer COF solid electrolyte, the number of ion adsorption sites and carriers is expected to increase significantly, and its ion conduction behavior is also expected to change.

[0040] The ionic conductivity and activation energy of PI, PI@COF0.5 and PI@COF were tested. The test results are shown in Figure 8 .from Figure 8 It can be seen that the ionic conductivity at room temperature is PI (6.0×10 −5 S cm −1 ) <PI@COF0.5(1.5×10 −4 S cm −1 ) <PI@COF(3.3×10 −4 S cm −1 ), indicating that PI@COF plays an important role in promoting the + It has advantages in conductivity and helps to form a continuous conductive network between the COF skeleton and the polymer chain through the homologous imide bond. Compared with most reported COF-based composite polymer electrolytes, PI@COF exhibits significantly enhanced ionic conductivity. In addition, the activation energies of PI, PI@COF0.5 and PI@COF are 0.35 eV, 0.22 eV and 0.14 eV, respectively, indicating that Li + The migration barrier for transport in PI@COF is low.

[0041] DC polarization and AC impedance were performed on PI, PI@COF0.5 and PI@COF. The results are shown in Figure 9 .from Figure 9 It can be seen that PI@COF exhibits smaller interfacial resistance, lower charge transfer impedance and higher tLi than PI and PI@COF0.5. + This suggests that combining polymers and COFs with homologous imide bonds results in a stable electrode / electrolyte interface and enhances Li +Directed migration. Linear sweep voltammetry (LSV) shows that PI@COF has a high electrochemical stability greater than 4.6 V. This shows that PI@COF significantly improves the + conductivity and increased Li + mobility, making it a suitable electrolyte material for solid-state lithium metal batteries.

[0042] PI, PI@COF0.5 and PI@COF were applied in Li / Li symmetric batteries, and their critical current density (CCD) was measured. The critical current density (CCD) of the Li / Li symmetric battery was determined by constant current cycling with a step increase in current density. -2 Set the step value and keep the time as one hour per step. The measurement results are shown in Figure 10 ,from Figure 10 It can be seen that the voltage of Li / PI / Li battery is 0.8 mA cm -2 The CCD of the Li / PI@COF / Li battery is 1.4 mA cm -2 , which is much higher than the test values ​​of PI and PI@COF0.5, which are 0.8 mA cm -2 and 1.0 mA cm -2 .

[0043] Then the Li / Li symmetric battery was subjected to Li deposition / stripping experiments and long cycle stability tests. Figure 11 and Figure 12 Li deposition / stripping experiments can study the stability of the interface between solid electrolyte and Li metal. Figure 11 and Figure 12 As can be seen from the figure, the Li / Li symmetric battery with PI@COF exhibits stable and reliable lithium plating / stripping behavior for more than 2000 hours at 0.1 mA cm -2 There is no obvious increase in current density and irreversible fluctuation of overpotential, indicating a significantly superior cycle life. In contrast, the PI and PI@COF0.5 derived batteries exhibit a rapid voltage increase, which lasts for about 600 h and 1000 h. Even at 0.5 mA cm -2 At a higher current density, the symmetrical cell with PI@COF also maintains a high cycling stability for up to 1000 h, which means that a continuous and uniform deposition behavior of ion flux has been created. These results indicate that the positive effect of PI@COF promotes the + The uniform transmission of Li +The results show that the transfer resistance is reduced and the dendrite growth during repeated plating and stripping is significantly suppressed, thus enhancing the stability of lithium metal batteries.

[0044] The NCM811 cathode was prepared by a slurry coating method. A slurry mixture consisting of 90 wt% NCM811, 5 wt% SuperP, and 5 wt% PVDF binder in anhydrous N-methyl-2-pyrrolidone (NMP) was cast onto aluminum foil, dried at 80 degrees Celsius for 1 hour, and then vacuumed at 120 degrees Celsius overnight. The mass loading of active material was approximately 3.0 mg cm −2 The prepared cathode, solid electrolyte, and lithium metal anode were used to assemble CR2032 coin cells in an argon-filled glove box. After assembly, charge and discharge tests were performed to measure the battery's capacity and coulombic efficiency. Figure 13 The discharge capacity diagram of solid-state batteries with different electrolyte components at 0.2 C, from Figure 13 As can be seen from the figure, the battery with PI@COF provides a high initial discharge capacity of 180.6 mAh g at 0.2 C. -1 , much higher than PI and PI@COF0.5. Figure 14 The data diagram of battery cycle stability and capacity retention rate is shown in Figure 2. Figure 14 It can be seen that after 200 cycles of stable operation, the battery capacity of PI@COF remains at 156.1 mAh g -1 , showing a retention of 86.6% and a Coulombic efficiency of 99.0%, indicating a stable interface between the electrolyte / electrode during long cycling.

[0045] Rate capability is a key indicator for achieving higher power density. Figure 15 shows the rate capability of solid-state lithium metal batteries using PI, PI@COF0.5, and PI@COF electrolytes. Figure 15 As can be seen from the graph, the current density gradually increases from 0.2 C to 0.4, 0.6, 0.8 and 1.0 C, and finally returns to 0.2 C. The discharge capacity of the battery using PI@COF is 180.2 mAh g -1 、175.3mAh g -1 、161.5mAh g -1 、149.8mAh g -1 and 135.4 mAh g -1 , which is much higher than that of solid-state batteries using PI and PI@COF0.5 as solid-state electrolytes. This result confirms that the superionic conductivity of PI@COF electrolyte enables solid-state lithium metal batteries to achieve high power.

[0046] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for preparing a solid electrolyte for a lithium metal battery, characterized in that: The following steps are involved: Synthesis of PI: Under inert gas, 4 mmol of 2,4,6-trimethyl-1,3-phenylenediamine and 10 mL of N,N-dimethylacetamide (DMAc) were added to a three-necked flask equipped with a mechanical stirrer. Then, 3.8 mmol of 4,4'-(hexafluoroisopropenyl)diphthalic anhydride (6FDA) and 0.2 mmol of 4,4'-(acetylene-1,2-diyl)diphthalic anhydride were added in sequence and mechanically stirred until a viscous polyamic acid was generated. Then, 4.8 mL of acetic anhydride and 4 mL of pyridine were slowly added to the polyamic acid and stirred at room temperature to obtain a mixture. The mixture was slowly poured into a methanol aqueous solution to obtain an intermediate product. The intermediate product was crushed and washed with a methanol aqueous solution, and then vacuum dried to obtain a PI polymer. Synthesis of COF: 0.15 mmol of pyromellitic acid, 0.60 mmol of benzoic acid, and 0.10 mmol of 1,3,5-tris(4-aminophenyl) were reacted at 180-210 degrees Celsius for 5 days. The product was washed and Soxhlet extracted with tetrahydrofuran, and then vacuum dried to obtain COF. Synthesis of solid-state electrolyte: COF is immersed in a 1 mol / L LiTFSI (lithium bistrifluoromethanesulfonyl imide) acetonitrile solution, washed and vacuum-dried to obtain an NKCOF compound. The NKCOF compound, PI polymer and LiTFSI powder are dissolved in N,N-dimethylacetamide (DMAc) to prepare a casting solution, which is then filtered, spread and the solvent evaporated under vacuum conditions to obtain a thin film. Finally, the film is immersed in methanol and then dried to obtain the finished product. The NKCOF compound is one of NKCOF-28, NKCOF-29 and NKCOF-30.

2. The method for preparing a solid electrolyte for a lithium metal battery according to claim 1, wherein: The inert gas is argon.

3. The method for preparing a solid electrolyte for a lithium metal battery according to claim 1, wherein: The mechanical stirring time was 48 h, and the room temperature stirring time was 24 h.

4. The method for preparing a solid electrolyte for a lithium metal battery according to claim 1 or 2, wherein: The volume ratio of methanol to water in the methanol aqueous solution is 1:

1.

5. The method for preparing a solid electrolyte for lithium metal batteries according to claim 1 or 3, characterized in that: The vacuum drying temperature is 80 degrees Celsius.

6. The method for preparing a solid electrolyte for a lithium metal battery according to claim 1, wherein: The 1,3,5-tris(4-aminophenyl) compound is one of 1,3,5-tris(4-aminophenyl)benzene, 1,3,5-tris(4-aminophenyl)oxazine and 1,3,5-tris(4-aminophenyl)amine.

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