Electrolyte membrane for alkaline metal batteries

Abstract

The present disclosure relates to an electrolyte membrane for an alkali metal or alkali metal ion battery, the electrolyte membrane comprising a composite membrane including an ion-conducting polymer, an alkali metal salt, and a filler including carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof.

Description

[Technical Field]

[0001] <Cross-reference of related applications> This application claims the benefits of U.S. Provisional Application No. 63 / 084,068, filed on 28 September 2020.

[0002] <Description of research and development funded by the federal government> Not applicable. [Background technology]

[0003] <Description regarding the joint development agreement> The claimed invention was made by, on behalf of, and / or in connection with, either Enpower Greentech Inc. or the University of Texas (Austin), which are parties to the joint development agreement. This agreement became effective prior to the date on which the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of this agreement.

[0004] This disclosure relates, in general terms, to electrolyte membranes. More specifically, this disclosure relates to polymer-based electrolyte membranes having fillers including carbon nitride nanosheets, oxygenated carbon nitride nanosheets, or combinations thereof.

[0005] The development of highly safe, high-energy-density lithium metal batteries is in very high demand to meet the ever-growing market for intelligent electronics, electric vehicles, and grid-scale energy storage systems. Replacing current liquid organic electrolytes with safe solid electrolytes is currently the most viable and effective strategy to achieve this development. Compared with ceramic electrolytes, polymer electrolytes such as poly(ethylene oxide) (PEO) have been widely studied. PEO electrolytes exhibit low interfacial resistance, high flexibility, and simple processability. However, PEO-based polymer electrolytes have not yet been commercialized due to several disadvantages, including: 1) low ionic conductivity at room temperature, requiring extra energy to heat the electrolyte to temperatures above 50°C; 2) poor ability to suppress lithium dendrite growth, especially at high current densities; 3) low thermal stability, leading to short-circuit thermal runaway; and 4) a narrow electrochemical window, limiting their use in combination with high-voltage nickel-manganese-cobalt (NMC) based cathodes.

[0006] Therefore, the development of another polymer electrolyte system with improved performance at room temperature remains highly desirable. Polyvinylidene difluoride (PVDF)-based polymer electrolytes have attracted attention due to their high polarity in the dissociation of lithium salts at room temperature. Specifically, Li7La3Zr2O 12 Li (LLZO) + PVDF-based electrolytes containing conductive fillers have high Li at room temperature. +It exhibits conductivity, good fire resistance, and a broad electrochemical window. The increase in ionic conductivity is mainly due to the interfacial interaction between the LLZO filler and the PVDF / LiTFSI matrix, rather than from the contribution of the Li-conducting LLZO itself. Therefore, in PVDF-based composite electrolytes, a considerable amount of LLZO (about 30 wt%) is required to provide a sufficient surface-active region to adequately interact with the PVDF / LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) matrix, which comes at a significant cost to the electrolyte's energy density. Furthermore, PVDF is unstable with lithium metal and gradually decomposes during cycling upon contact with the lithium metal anode, which considerably limits its long-term cyclability. + Since conductive ceramic fillers contribute little to the total conductivity of polymer electrolytes, inert fillers with a large surface area and strong interaction with the PVDF / LiTFSI matrix are expected to be effective in increasing the ionic conductivity of the electrolyte. [Overview of the project]

[0007] In a first aspect of this disclosure, an electrolyte membrane for an alkali metal ion battery is provided herein, comprising a matrix containing an ion-conducting polymer, an alkali metal salt, and a filler containing carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof.

[0008] Throughout this specification and the claims, carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, and combinations thereof are discussed. As will be apparent to those skilled in the art, this does not mean a single sheet of material. Instead, as used in the present invention, the material is understood to include a plurality of sheets. Thus, for convenience, it is referred to as carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, and combinations thereof, even though it is composed of a plurality of sheets of these materials. Further, the preferred embodiments of the present disclosure do not include a non-ion-conductive polymer in the electrolyte membrane, and preferably, only an ion-conductive polymer is used to form the electrolyte membrane according to the present disclosure.

[0009] In one embodiment of the first aspect of the present disclosure, the carbon nitride is graphitic carbon nitride (g-C3N4).

[0010] In one embodiment of the first aspect of the present disclosure, the filler includes oxygen-containing carbon nitride (OCN) nanosheets.

[0011] In some embodiments of the first aspect of the present disclosure, the filler includes oxygen-containing carbon nitride nanosheets, and the filler is present in an amount of approximately 0.5 wt% to 10 wt%, more preferably approximately 2 wt% to 5 wt%, based on the total weight of the conductive polymer. In an exemplary embodiment of the present disclosure, the amount of oxygen-containing carbon nitride nanosheets is approximately 2 wt% based on the total weight of the conductive polymer.

[0012] In one embodiment of the first aspect of the present disclosure, the carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or combinations thereof are inert to alkali metal salts and have a large surface area exceeding 100 m 2 / g. In an example of the present disclosure, the surface area is about 171 m 2 / g. The carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or combinations thereof are sheet-like and porous.

[0013] In one embodiment of the first aspect of the present disclosure, carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof are dispersed throughout the matrix. In an exemplary embodiment of the present disclosure, carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof are uniformly dispersed throughout the matrix.

[0014] In one embodiment of the first aspect of the present disclosure, the polymer is polyvinylidene difluoride (PVDF) or polyacrylonitrile (PAN).

[0015] In one embodiment of the first aspect of the present disclosure, the alkali metal salt is selected from the group consisting of LiTFSI (lithium bis(fluorosulfonyl)imide), LiFSI (lithium bis(trifluoromethanesulfonyl)imide), LiClO4 (lithium perchlorate), NaTFSI (sodium bis(fluorosulfonyl)imide), NaFSI (sodium bis(trifluoromethanesulfonyl)imide), and NaClO4 (sodium perchlorate).

[0016] In one embodiment of the first aspect of the present disclosure, the amount of the alkali metal salt is present in the range of approximately 50% to 80% by weight based on the total weight of the conductive polymer.

[0017] In the second aspect of the present disclosure, provided herein is a method for preparing an electrolyte membrane for an alkali metal ion battery, the method comprising: mixing a matrix containing an ion-conductive polymer, an alkali metal salt, and a solvent to obtain a composition; combining the composition with a filler containing carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof to obtain a suspension; and shaping and drying the suspension to form an electrolyte membrane.

[0018] In the third aspect of the present disclosure, provided herein is an alkali metal ion battery comprising an anode, a cathode, and an electrolyte membrane according to the present disclosure disposed between the anode and the cathode.

[0019] The novel features in this disclosure are described in detail in the appended claims. A better understanding of the features and advantages of this disclosure can be obtained by referring to the following detailed description and appended drawings which describe exemplary embodiments in which the principles of this disclosure are utilized. [Brief explanation of the drawing]

[0020] [Figure 1] The inset shows a scanning electron microscope (SEM) image of the surface of a PVDF / LiTFSI / OCN film according to one embodiment described herein, illustrating the excellent flexibility of the film when wound around a glass tube. [Figure 2] The X-ray diffraction (XRD) patterns of OCN (oxygen-containing carbon nitride) nanosheet material alone, a PVDF / LiTFSI film, a PVDF / LiTFSI / OCN film containing 2 wt% OCN, and a PVDF film alone are shown. [Figure 3] Impedance spectroscopy ionic conductivity of PVDF / LiTFSI / OCN films containing different amounts of OCN is shown. [Figure 4] The current-to-time profile of a Li-symmetric cell with a PVDF / LiTFSI / OCN film containing 2 wt% OCN is shown, and the inset shows the impedance spectra of the PVDF / LiTFSI before and after the addition of the OCN filler. [Figure 5] The linear sweep voltammetry curves of a PVDF / LiTFSI film and a PVDF / LiTFSI / OCN film containing 2 wt% OCN are shown. [Figure 6] This shows ignition tests of a PVDF / LiTFSI / OCN film (top) and a PVDF / LiTFSI film (bottom) containing 2 wt% OCN. [Figure 7] The images show the temperature distribution of a PVDF / LiTFSI / OCN film (top) and a PVDF / LiTFSI film (bottom) containing 2 wt% OCN. [Figure 8]Three panels (a), (b), and (c) are shown. Panel (a) shows the XRD pattern of bulk C3N4 and nanosheet C3N4 after exposure of bulk C3N4 to air. Panel (b) shows a photograph of 20 milligrams of bulk C3N4 and a photograph of nanosheet C3N4 material. Panel (c) shows traces of N2 adsorption / desorption isotherm curves for testing the respective surface areas of bulk C3N4 and nanosheet C3N4. [Figure 9] Five panels (a) through (e) and two subpanels of panel (e) are shown. Panel (a) shows a scanning electron microscope image of bulk C3N4 material. Panel (b) shows a scanning electron microscope image of nanosheet C3N4 material. Panel (c) is a transmission electron microscope (TEM) image of nanosheet C3N4 material. Panel (d) is an energy-dispersive X-ray spectroscopy (EDX) spectrum of nanosheet C3N4 material. Panel (e) is a scanning electron microscope image of nanosheet C3N4 material, with the upper subpanel showing the carbon distribution from this image and the lower subpanel showing the nitrogen distribution. [Figure 10] Panels (a) through (f) are shown. Panel (a) shows a vial of PVDF / LiTFSI solution. Panel (b) shows a vial of suspension of PVDF / LiTFSI / CN nanosheet material. Panel (c) shows an electrolyte membrane formed from the material composition in the vial shown in panel (b). Panel (d) shows the membrane from panel (c) wound around a glass tube. Panel (e) shows a series of membranes formed from PVDF with different amounts of carbon nitride nanosheet filler. Panel (f) shows a series of traces of room-temperature conductivity of membranes having the composition shown in panel (e). [Modes for carrying out the invention]

[0021] Some aspects of this disclosure are described below with reference to illustrative examples. It should be understood that numerous specific details, relationships, and methods are provided to provide a complete understanding of this disclosure. However, a person skilled in the art will readily recognize that this disclosure can be implemented without including one or more specific details or in other ways. This disclosure is not limited by the illustrated order of actions or events, because some actions may occur in a different order and / or simultaneously with other actions or events.

[0022] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art in which this disclosure pertains. The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit this disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural form unless the context clearly indicates otherwise.

[0023] Furthermore, where the terms “contains,” “includes,” “possess,” or variations thereof are used in the detailed description and / or claims, such terms are intended to be similarly encompassed by the term “equipped with.”

[0024] The term "approximately" means that a particular value is within an acceptable margin of error as determined by those skilled in the art, and this depends on how that value is measured or determined, i.e., in part on the limits of the measuring system. For example, "approximately" can mean within one standard deviation or within one standard deviation, in accordance with the practices of the art. Alternatively, "approximately" can mean within ±20%, preferably ±10%, more preferably ±5%, and most preferably ±1% of a given value. Or, particularly with respect to biological systems or processes, the term can mean within 10 times, preferably 5 times, and more preferably 2 times the value. If a particular value is described in the specification and claims, unless otherwise stated, the term "approximately" should be assumed to mean within an acceptable margin of error for that particular value.

[0025] The term "g-C3N4" refers to graphite carbon nitride, a family of carbon nitride compounds that has a general formula similar to C3N4 and two main substructures based on heptadine and poly(triazine imide) units.

[0026] The alkali metal salt may be any suitable alkali metal salt available in the art, including (but not limited to) LiTFSI, LiFSI, LiClO4, NaTFSI, NaFSI, and NaClO4.

[0027] The polymer may be any suitable ion-conducting polymer available in the art, including (but not limited to) polyvinylidene difluoride (PVDF), polyacrylonitrile (PAN), and poly(ethylene oxide) (PEO).

[0028] The solvent for preparing the electrolyte membrane of this disclosure may be any suitable solvent available in the art, including but not limited to N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF).

[0029] In a further aspect of the present disclosure, the alkali metal ion battery can include an anode, a cathode, and the electrolyte membrane of the present disclosure disposed between the anode and the cathode.

[0030] The alkali metal ion battery may be a solid battery, and the electrolyte membrane may be a solid electrolyte.

[0031] The anode includes a metal or metal alloy electrode containing any suitable alkali metal available in the art. In an exemplary embodiment of the present disclosure, the anode is a lithium metal electrode. Alternatively, the anode may be a sodium metal or sodium alloy electrode.

[0032] The cathode may be any cathode containing an alkali metal or a nickel-rich high-voltage cathode suitable for an alkali metal ion battery available in the art. In an exemplary embodiment of the present disclosure, the cathode is LiNi 0.8 Mn 0.1 Co 0.1 O2 (NMC811).

[0033] Example

[0034] Example 1. Preparation of OCN nanosheets (2D g-C3N4 nanosheets)

[0035] Those skilled in the art should understand that the OCN nanosheet material is commercially available and can be prepared by any suitable method known in the art. For example, the method for preparing OCN nanosheets is reported in the reference She, X.; Wu, J.; Zhong, J.; Xu, H.; Yang, Y.; Vajtai, R.; Lou, J.; Liu, Y.; Du, D.; Li, H.; Ajayan, P. M. Nano Energy 2016, 27, 138.

[0036] In this example, OCN nanosheets were prepared by a modified layer-by-layer thermal oxidation slicing process, as reported in reference M. Wang, Z. Li, L. Tian, ​​Y. Xie, J. Han, T. Liu, C. Jin, Z. Wu, International Journal of Hydrogen Energy 2019, 44, 4102. In the first step, melamine was thermally condensed, 5 ml of diluted nitric acid (20%) and 3 g of melamine were thoroughly mixed in hot water, and then the mixture was cooled to room temperature for approximately 25 m 2 A precipitate of bulk C3N4 powder was produced, which was a yellowish precipitate with a surface area of ​​1 / g. The collected precipitate was sintered in a warming furnace under three sequential calcination treatments: 10°C / min at 550°C for 2h, then 5°C / min at 550°C for 1h, and finally 2°C / min at 500°C for 1h. After this treatment, OCN nanosheets were obtained, which is the thermal exfoliation treatment discussed herein. The obtained OCN nanosheets are two-dimensional (2D) g-C3N4 nanosheets, which are white, fluffy, ultralight powders with high porosity and a large surface area.

[0037] The preparation of OCN is much easier compared to conventional ceramic fillers such as LLZO and lithium titanate (LLTO) currently used in PVDF / LiTFSI electrolytes, and the precursors for OCN nanosheet materials are very inexpensive.

[0038] The surface area of ​​the OCN nanosheet created by this disclosure is 171 m². 2 The ratio is / g, while the surface area of ​​g-C3N4 bulk particles is only 25m². 2 It is / g.

[0039] Example 2. Fabrication of PVDF / LiTFSI / OCN films

[0040] In this example, the present disclosure provides exemplary embodiments of PVDF / LiTFSI / OCN films having different amounts of OCN filler and methods for preparing them. Those skilled in the art should understand that PVDF / LiTFSI / OCN films may also be prepared by any suitable method available in the art.

[0041] In this example, a PVDF / LiTFSI / OCN film was prepared by a readily available solution casting method involving vacuum drying of the solution. First, 1 g of polyvinylidene difluoride (PVDF, HSV900 provided by Arkema) and 0.8 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (Sigma-Aldrich) were dissolved in N-methyl-2-pyrrolidone (NMP) solvent (Sigma-Aldrich) to obtain a composition. Then, OCN nanosheets (20 mg of 2Dg-C3N4 nanosheets prepared by the treatment in Example 1) were added to the clear PVDF / LiTFSI solution to obtain a PVDF / LiTFSI / OCN suspension. After stirring for 5 hours, the PVDF / LiTFSI / OCN suspension was poured into a polytetrafluoroethylene dish and dried in a vacuum oven at 60°C for 48 hours to obtain a PVDF / LiTFSI / OCN film (Sample 1). Next, the PVDF / LiTFSI / OCN film was transferred from the vacuum oven to a glove box and stored for use.

[0042] The obtained sample 1 was a PVDF / LiTFSI / OCN film containing a PVDF matrix, LiTFSI, and 2Dg-C3N4 nanosheets as fillers, where the amount of g-C3N4 nanosheets was approximately 2% by weight relative to the total weight of the PVDF.

[0043] As shown in Figure 1, the obtained sample 1 is mechanically robust, bendable, and fits into a glass tube as shown in the inset. SEM images showed no aggregates of OCN nanosheets on the surface of the film, indicating that the OCN nanosheets were uniformly dispersed in the PVDF / LiTFSI matrix.

[0044] PVDF / LiTFSI / OCN membranes containing 5 wt% OCN and 10 wt% OCN were prepared using the same method as Sample 1, except that the amount of OCN nanosheets added to the PVDF / LiTFSI solution differed. For the PVDF / LiTFSI / OCN membrane containing 5 wt% OCN (Sample 2), a suspension was obtained by adding 50 mg of OCN nanosheets to a clear PVDF / LiTFSI solution. For the PVDF / LiTFSI / OCN membrane containing 10 wt% OCN (Sample 3), a suspension was obtained by adding 100 mg of OCN nanosheets to a clear PVDF / LiTFSI solution.

[0045] Finally, a PVDF / LiTFSI film without OCN material was prepared as a control sample using the same method as Sample 1, except that the OCN nanosheet was not added to the transparent PVDF / LiTFSI solution.

[0046] Example 3. Evaluation of the electrochemical properties of electrolyte membranes

[0047] 1. X-ray diffraction (XRD)

[0048] X-ray diffraction (XRD) was performed on electrolyte membrane sample 1 prepared in Example 2, a control sample membrane, a pure OCN nanosheet prepared in Example 1, and a pure PVDF membrane. The results are shown in Figure 2, which characterize the detailed structure and crystallinity of the membranes and materials.

[0049] As shown in Figure 2, in the XRD pattern of the control sample PVDF / LiTFSI film without OCN, the second trace from the bottom labeled PVDF / LiTFSI, no peak for LiTFSI salt was detected, and a very weak peak for the PVDF polymer was shown, indicating that LiTFSI was well dissolved in the PVDF polymer and the crystallinity of PVDF was reduced. The PVDF / LiTFSI / OCN film containing 2 wt% OCN of sample 1, the top trace labeled PVDF / LiTFSI / OCN, showed a further reduction in crystallinity in PVDF, indicating that OCN disrupts the order of the PVDF chains and converts the PVDF structure from a crystalline to an amorphous phase. By generating more amorphous segments, Li along the interface between the OCN sheet and the PVDF chains... + Transport is facilitated, thereby increasing the Li ion conductivity of the PVDF / OCN composite polymer electrolyte. Pure PVDF is shown in the bottom trace, and the g-C3N4 nanosheet material is shown in the third trace from the bottom.

[0050] 2. Measurement of ionic conductivity

[0051] Ionic conductivity was measured by applying a voltage bias of 10 millivolts (mV) in a frequency range of approximately 1 hertz (Hz) to approximately 1 megahertz (MHz) to electrolyte membrane samples 1 through 3 and the control sample membrane. The ionic conductivity of the electrolyte membranes measured at room temperature is shown in Figure 3 and Table 1.

[0052] [Table 1]

[0053] As shown in Table 1 and Figure 3, all PVDF / LiTFSI / OCN films (Samples 1 to 3) have higher Li ion conductivity than the PVDF / LiTFSI film with 0% OCN nanosheet filler. Specifically, the PVDF / LiTFSI film has a Li ion conductivity of 3.2 × 10⁻¹⁰ at room temperature. -5It has a low conductivity of S / cm. After the introduction of OCN nanosheets, the composite PVDF / LiTFSI / OCN has increased significantly. + Conductivity is shown. PVDF / OCN with a small amount of 2 wt% OCN has a conductivity of 1.6 × 10⁻⁶ at room temperature. -4 It has a high ionic conductivity of S / cm, which is due to the PVDF / OCN (7.5 × 10) having an OCN content of 5 wt% OCN. -5 PVDF / OCN (4.6 × 10) is 10 wt% OCN (S / cm) -5 It is higher than (S / cm).

[0054] It is found that a mere 2 wt% of OCN is sufficient to alter the composite PVDF / LiTFSI / OCN electrolyte by providing ample surfactant sites for interaction with the polymer matrix. Therefore, PVDF / LiTFSI / OCN films containing 2 wt% OCN exhibit the highest conductivity, while conventional PVDF / LiTFSI / LLZO films require 10 wt% or more of LLZO to yield any benefits. Furthermore, with just 2 wt% of OCN, Li + It has the effect of increasing conductivity. If the amount of OCN is greater, Li + The conductivity decreases because excess OCN tends to self-aggregate, which dilutes the polymer electrolyte and Li at the interface between OCN and PVDF polymer. + This is because transport is blocked, reducing the ionic conductivity of the PVDF / OCN polymer electrolyte. The Nyquist plot of OCN-based PVDF polymer electrolytes supports the effect of OCN sheet content on ionic conductivity.

[0055] OCN nanosheet materials are quite cotton-like and have a large surface area due to their graphene-like planar structure, providing ample active sites for interaction with the PVDF / LiTFSI matrix. The larger the surface area of ​​OCN, the stronger the interaction with LiTFSI. OCN is Li + It interacts strongly with LiTFSI, which promotes the dissociation of LiTFSI and allows more free Li to be released. +The oxygen atom can occupy various sites within the layered structure, resulting in a highly diversified OCN structure. The complexity of the OCN structure provides many binding sites. The binding energy of the lithium ion to oxy-C3N4 varies from 0 to several electron volts. The wide range of binding energies of the lithium ion to oxy-C3N4 can facilitate the dissociation of LiTFSI.

[0056] 3. Ion Transference Measurement

[0057] Electrochemical impedance spectroscopy (EIS) was performed on electrolyte membrane sample 1 and the control sample to evaluate their electrochemical stability.

[0058] As shown in the inset of Figure 4, the PVDF / LiTFSI / OCN membrane with an amount of 2 wt% OCN, which is the upper trace labeled "post", has a Li ion transport rate of (tLi + The value is 0.58, which is higher than the Li ion transport fraction of 0.34 for the PVDF / LiTFSI membrane without OCN, which is the lower trace labeled "previous". This clearly indicates that OCN can efficiently release more Li ions.

[0059] 2Dg-C3N4 nanosheets are Li + It interacts strongly with LiTFSI, and therefore promotes the dissociation of LiTFSI, resulting in more free Li. + This will be released. Therefore, by adding 2Dg-C3N4 nanosheets, the lithium transport rate of PVDF / LITFSI can be increased.

[0060] 4. Linear Sweep Voltammetry (LSV)

[0061] Linear sweep voltammetry (LSV) was performed on electrolyte membrane sample 1 and a control sample to evaluate their electrochemical stability. The results are shown in Figure 5 and Table 2. LSV was performed at a temperature of approximately 25°C, within a voltage range of approximately 2.5 volts (V) to approximately 6.0 V (V vs. Li / Li), and at a scan speed of approximately 1 mV / second (millivolts per second).

[0062] [Table 2]

[0063] As shown in Table 2 and Figure 5, the interaction between OCN and PVDF / LiTFSI also increases the electrochemical window from 4.85 V (control sample: PVDF / LiTFSI film shown in the leftmost trace) to 5.3 V (sample 1: PVDF / LiTFSI / OCN film with 2 wt% OCN shown in the rightmost trace) by suppressing anionic oxidation.

[0064] OCN nanosheets are 171m 2 Due to its large surface area of ​​1 / g, the nanosheet structure provides ample active sites for PVDF / LiTFSI. As verified by density functional theory (DFT) calculations, OCN nanosheets are Li + It interacts strongly with LiTFSI, thereby releasing free Li + This facilitates the dissociation of the Li into movable PVDF chain segments, thereby enabling high ionic conductivity and Li + A transport rate is introduced. Furthermore, by introducing OCN nanosheets into PVDF / LiTFSI, the order of the PVDF chains is disrupted, and the structure of the PVDF is converted from a crystalline to an amorphous phase. The generation of more amorphous segments leads to Li along the interface between the OCN sheet and the PVDF chains. + Transport is facilitated, thereby increasing the Li ion conductivity of the PVDF / OCN composite polymer electrolyte. The strong interaction between OCN and PVDF also increases the electrochemical stability of the membrane to higher voltages, making it an ideal electrolyte candidate when combined with a high-voltage cathode.

[0065] In conclusion, the addition of OCN nanosheets has the following advantages: (1) it reduces the crystallinity of the polymer and promotes segment mobility, and (2) it provides Li on the large surface area of ​​the OCN. + (3) It has several attractive advantages, including (4) facilitating the dissociation of LiTFSI by trapping (5) g-C3N4 and providing a fast pathway for lithium ion transport through the tri-s-triazine unit of C3N4. Incorporating C3N4 into the PVDF / LiTFSI film significantly increases Li ion conductivity, broadens the stable voltage range, and increases the lithium transport rate.

[0066] Example 4. Evaluation of the fire resistance of electrolyte membranes

[0067] Fire resistance evaluation

[0068] Ignition tests were conducted on electrolyte membrane sample 1 and a control sample to evaluate their fire resistance. The results are shown in Figure 6.

[0069] As shown in Figure 6, the PVDF / LiTFSI film (control sample) shown in the lower panel set easily ignites upon contact with a flame, while the PVDF / LiTFSI / OCN film (Sample 1: PVDF / LiTFSI / OCN film with 2 wt% 2Dg-C3N4 nanosheets) shown in the upper panel set does not ignite and only turns black. This demonstrates that incorporating OCN nanosheets significantly improves the fire resistance of the film compared to the original PVDF / LiTFSI film.

[0070] 2. Heat distribution evaluation

[0071] The thermal distribution of electrolyte membrane sample 1 and the control sample was evaluated. The results are shown in Figure 7.

[0072] As shown in Figure 7, the temperature at the hot spot of the PVDF / LiTFSI film (control sample) shown in the lower box labeled PVDF / LiTFSI rose to 62°C, while the measured temperature of the PVDF / LiTFSI / OCN film (Sample 1: PVDF / LiTFSI / OCN film with 2 wt% 2Dg-C3N4 nanosheets) shown in the upper box labeled PVDF / CN was only 45°C. A more uniform heat distribution may also contribute to improved fire resistance of the PVDF / OCN film. Furthermore, it can be seen that the thermally conductive OCN nanosheets also improve the uniformity of the film's temperature distribution.

[0073] OCN, with its large surface area, is known as a good flame retardant and good thermal conductor. Therefore, by adding 2Dg-C3N4 nanosheets to PVDF / LiTFSI, the fire resistance of the electrolyte membrane is greatly improved, and the excellent thermal conductivity and stability of OCN provide excellent fire resistance to the PVDF / LiTFSI / OCN membrane.

[0074] Example 5. Lithium metal ion battery with PVDF / LiTFSI / OCN film

[0075] This example provides an exemplary lithium metal-ion battery comprising a PVDF / LiTFSI / OCN film. Those skilled in the art should understand that the PVDF / LiTFSI / OCN film provided herein can be used in any suitable solid-state battery.

[0076] In this example, the battery consists of an anode, which is a lithium metal electrode, and an NMC electrode (LiNi 0.8 Mn 0.1 Co 0.1 PVDF / LiTFSI / OCN membrane sample 1 comprises a cathode which is O2, and an electrolyte membrane positioned between the anode and the cathode, wherein the electrolyte membrane contains a PVDF matrix, LiTFSI, and 2 wt% OCN nanosheets (2Dg-C3N4 nanosheets) as fillers.

[0077] By adding OCN nanosheets, PVDF / LiTFSI / OCN membranes become ideal electrolytes for alkali metal batteries; therefore, the introduction of 2Dg-C3N4 nanosheets brings batteries using PVDF-based polymer electrolytes one step closer to commercial applications. Preferably, in such batteries, the electrolyte membrane thickness is in the range of 10 to 100 microns. This applies to all membranes fabricated according to this disclosure, regardless of the polymer, alkali metal salt, carbon nitride nanosheet, oxygenated carbon nitride nanosheet, or any combination thereof used to form the membrane.

[0078] As-assembled symmetrical batteries containing PVDF / LiTFSI / OCN electrolytes have a lifespan of 300 hours and a capacitance of 0.05 mA / cm². 2 From 0.4 mA / cm 2 Its exceptional rate capability, as well as 0.1 mA / cm² 2 With long-term cycle stability of up to 2500 hours and 0.1mAh / cm² 2 This indicates the capacity. Furthermore, batteries using NMC as the cathode and PVDF / LiTFSI / OCN as the electrolyte demonstrate long-term cycle stability at high rates and 1.8 mg / cm³. 2 From 8.2 mg / cm³ 2 It exhibited extremely excellent electrochemical performance at room temperature, including a high NMC loading.

[0079] In Figure 8, panels (a) to (c) show the differences in characteristics between bulk C3N4 and C3N4 nanosheet material. Panel (a) shows the XRD patterns of bulk C3N4 and C3N4 nanosheet material produced after thermal exfoliation of bulk C3N4 in air via the sintering process described herein. Both have similar crystalline structures with two typical peaks (100) and (002), but the C3N4 nanosheet material has much lower and broader peak intensities, which indicates that thermal exfoliation, by forming thin nanosheets, destroys the periodic stacking and structural order of bulk C3N4 to a considerable extent. Viewed in panel (b), it can be seen that the C3N4 nanosheet material occupies a much larger volume at the same weight of 20 mg due to its cottony characteristics. In panel (c), the Brunauer-Emmett Teller (BET) analysis showed that nanosheet C3N4 had a specific surface area of ​​25 m², while bulk C3N4 had a specific surface area of ​​25 m². 2 Compared to / g, 171m 2 It was shown to be a much larger value, / g.

[0080] In Figure 9, panels (a) through (e) and the two subpanels related to panel (e) illustrate other differences between bulk C3N4 and nanosheet C3N4. Panel (a) shows an SEM image of bulk C3N4, while panels (b) and (c) show SEM and transmission electron microscope (TEM) images of nanosheet C3N4. Panel (d) shows energy-dispersive X-ray spectroscopy (EDX) of nanosheet C3N4. Panel (e) shows an SEM image of nanosheet C3N4, with the two subpanels showing the dispersion of C in the upper subpanel and the dispersion of N in the lower subpanel. From panel (a), it can be seen that bulk C3N4 exhibits a dense, thick aggregate morphology after formation via thermal condensation of melamine. Then, in panel (b), it can be seen that the nanosheet C3N4 material produced by thermal exfoliation of bulk C3N4 has a very different morphology and structure. It does not have aggregates, but instead is sheet-like with numerous pores within the sheet. The TEM image in panel (c) shows that the sheet layer is very thin and almost transparent to the electron beam. As shown in panel (d), the EDX spectrum shows only the elements C and N. As shown in panel (e) and its subpanels, C and N are uniformly dispersed in the nanosheet C3N4.

[0081] Figure 10 shows a series of panels (a) through (f). Panel (a) is a photograph of a vial of a clear solution of the PVDF / LiTFSI mixture. Panel (b) is a photograph of the PVDF / LiTFSI mixture after the addition of the nanosheet C3N4 material. Panel (c) is a photograph of the film recovered after the solvent was evaporated from the PVDF / LiTFSI / CN in a vacuum oven at 60°C for 24 hours. Panel (d) shows the flexibility of the film wound around a glass tube. Panel (e) shows a series of films described herein containing 0%, 2%, 5%, and 10% by weight of nanosheet C3N4 relative to the total weight of the conductive polymer. Panel (f) shows the room-temperature conductivity of each film prepared as shown in panel (e).

[0082] While exemplary embodiments of the present disclosure are described and illustrated herein, it will be apparent to those skilled in the art that these embodiments are for illustrative purposes only. It will also be apparent to those skilled in the art that numerous variations, modifications, and substitutions can be made to these embodiments without departing from the scope and spirit of the present disclosure. The scope of the present disclosure is defined by the appended claims, and methods and structures included in the claims, together with their equivalents, are intended to be encompassed by the appended claims.

Claims

1. An electrolyte membrane for an alkali metal ion battery, A matrix containing an ion-conducting polymer, Alkali metal salts and A filler comprising carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof, The electrolyte membrane is a solid electrolyte membrane having an ionic conductivity of 4.6 × 10⁻⁵ S / cm or more at 25°C.

2. The carbon nitride contained in the carbon nitride nanosheet, oxygenated carbon nitride nanosheet, or combination thereof is g-C ３ N ４ The electrolyte membrane according to claim 1, comprising:

3. The electrolyte membrane according to claim 1, wherein the filler comprises an oxygenated carbon nitride nanosheet.

4. The electrolyte membrane according to claim 1, wherein the amount of the filler is in the range of approximately 0.5% to 10% by weight relative to the total weight of the ion-conducting polymer.

5. The electrolyte membrane according to claim 4, wherein the amount of the filler is in the range of approximately 2% to 5% by weight relative to the total weight of the ion-conducting polymer.

6. The electrolyte membrane according to claim 5, wherein the amount of the filler is approximately 2% by weight relative to the total weight of the ion-conducting polymer.

7. The surface area of ​​the carbon nitride nanosheet, the oxygen-containing carbon nitride nanosheet, or a combination thereof is 100 m². ２ The electrolyte membrane according to claim 1, which is greater than / g.

8. The electrolyte membrane according to claim 7, wherein the carbon nitride nanosheet, the oxygen-containing carbon nitride nanosheet, or a combination thereof is porous.

9. The electrolyte membrane according to claim 1, wherein the carbon nitride nanosheet, the oxygen-containing carbon nitride nanosheet, or a combination thereof is uniformly dispersed in the matrix.

10. The electrolyte membrane according to claim 1, wherein the ion-conducting polymer comprises at least one of polyvinylidene difluoride (PVDF), polyacrylonitrile (PAN), or polyethylene oxide (PEO).

11. The alkali metal salt is LiTFSI, LiFSI, LiClO ４ NaTFSI, NaFSI and NaClO ４ An electrolyte membrane according to claim 1, selected from the group consisting of the following.

12. The electrolyte membrane according to claim 1, wherein the amount of the alkali metal salt is in the range of approximately 50% to 80% by weight relative to the total weight of the ion-conducting polymer.

13. A method for creating a solid electrolyte membrane for an alkali metal ion battery, A step of obtaining a composition by mixing a matrix containing an ion-conducting polymer, an alkali metal salt, and a solvent, A step of obtaining a suspension by combining the above composition with a filler containing carbon nitride nanosheets, oxygen-containing carbon nitride nanosheets, or a combination thereof, A method comprising the step of forming and drying the suspension to form a solid electrolyte membrane having an ionic conductivity of 4.6 × 10⁻⁵ S / cm or more at 25°C.

14. A-scatter, Cathode and, An alkali metal battery comprising an electrolyte membrane according to claim 1 disposed between the anode and the cathode.

15. The alkali metal battery according to claim 14, wherein the electrolyte membrane has a thickness of approximately 10 to 100 microns.

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