Preparation method of zwitterionic modified PVDF polymer electrolyte

Abstract

The present invention provides a method for preparing an amphoteric ion-modified PVDF polymer electrolyte, belonging to the technical field of lithium metal batteries. Specifically, PVDF particles, a lithium salt, pyridinium propane sulfonate and a polar organic solvent are mixed and stirred to obtain a uniform viscous solution, which is coated on the surface of a substrate and dried under vacuum to obtain an amphoteric ion-modified PVDF polymer electrolyte. Among them, the concentrations of PVDF particles, lithium salt and pyridinium propane sulfonate in the uniform viscous solution are 80-120 g / L, 10-120 g / L and 5-60 g / L respectively. The present invention uses pyridinium propane sulfonate as an additive for the PVDF electrolyte. While promoting ion transport, it takes into account the stability of PVDF and the lithium negative electrode, inhibits the side reactions of dehydrogenation and defluorination, and improves the stability of the lithium negative electrode and the performance and cycle stability of the polymer electrolyte-based lithium metal battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium metal batteries, and particularly relates to a preparation method of an amphoteric ion-modified PVDF polymer electrolyte. Background Art

[0002] Due to the high reactivity between the lithium anode and the electrolyte, the lithium metal battery (LMB) with a liquid electrolyte system is prone to thermal failure problems such as short circuit and combustion. This phenomenon is more prominent and serious in application fields with high requirements for the energy density / capacity released by LMB, strict test conditions, and high quality of battery active materials. Compared with the liquid battery system, the solid (SSE) / quasi-solid electrolyte system contains fewer or trace amounts of organic plasticizers / solvent molecules. Therefore, it has a higher battery safety factor than the liquid electrolyte system, and at the same time, it also has higher mechanical strength to resist dendrite growth. Therefore, in order to solve the high-risk safety problems existing in the LMB with a liquid electrolyte system, the LMB with a solid / quasi-solid electrolyte system has received key attention and research in recent years. Among them, the polymer (quasi)-solid electrolyte LMB has great development and application advantages due to its relatively simple preparation method, high flexibility, and lower sensitivity to air and water oxygen than inorganic solid electrolytes. However, the ionic conductivity of the polymer electrolyte (PESSE) is low and the electrochemical stability window is narrow, which limits its further development and application. Therefore, improving the ionic conductivity and electrochemical stability of PESSE is an important strategy for improving PESSE-based LMB.

[0003] Since PESSE mainly relies on the regional chain movement inside it to transport lithium ions Li + , the level of its ionic conductivity mainly depends on the structure of polar groups with adsorption properties in the polymer and the contents of lithium salts and plasticizers. Therefore, adjusting the functional group structure of the polymer to improve adsorption, or increasing the content of lithium salts / plasticizers, can improve the ionic conductivity to a certain extent. However, the regulation of the polymer structure and the content of transmission media such as lithium salts taken to improve the ionic conductivity are often accompanied by changes in the intermolecular interactions / side reactions inside the polymer and the side reaction process on the anode surface, and the chemical reaction process on the polymer and anode surface will also affect its stability level. Therefore, when regulating the ionic conductivity of the polymer, it is often necessary to take into account the regulation and control of its stability.

[0004] Polyvinylidene fluoride (PVDF), as a polymer, contains a large number of -H and -F sites. During the process of conducting Li + as an electrolyte, Li +It is extremely easy to come into contact with -H and -F, which causes the dehydrogenation and defluorination reactions of PVDF, producing HF and LiF as by-products and reducing the stability of PVDF. At the same time, HF will corrode the lithium anode and reduce the stability of the lithium anode. Therefore, in order to increase the ionic conductivity of PVDF while avoiding the reduction of its structural stability, it is necessary to consider the contact between Li + during the transmission process with -H and -F sites, and try to find a material that can transmit Li + while avoiding the contact between Li + and -H, -F, so as to achieve the effect of taking into account both the ionic conductivity and stability of PVDF. Summary of the Invention

[0005] In view of the above problems existing in the existing PVDF, the present invention provides a preparation method of an amphoteric ion modified PVDF polymer electrolyte. Using pyridinium propane sulfonate (PPS) as an additive for the PVDF electrolyte, while promoting ion transport, it takes into account the stability of PVDF and the lithium anode, inhibits the dehydrogenation and defluorination side reactions, and improves the stability of the lithium anode and the performance and cycle stability of the PESSE-based LMB battery.

[0006] In order to achieve the above object, the technical method adopted by the present invention is as follows:

[0007] A preparation method of an amphoteric ion modified PVDF polymer electrolyte, the specific process is as follows:

[0008] Mix PVDF particles, lithium salt, PPS and a polar organic solvent, stir to obtain a uniform viscous solution, coat the uniform viscous solution on the surface of a substrate, and after vacuum drying, obtain an amphoteric ion modified PVDF polymer electrolyte;

[0009] Among them, the concentrations of PVDF particles, lithium salt and PPS in the uniform viscous solution are 80 - 120 g / L, 10 - 120 g / L and 5 - 60 g / L respectively.

[0010] Further, the lithium salt is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiFSI) or lithium hexafluorophosphate (LiPF6).

[0011] Further, the polar organic solvent is N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF).

[0012] Further, the temperature of the stirring is room temperature, and the duration is 12 - 36 h.

[0013] Further, the thickness of the uniform viscous solution coated on the surface of the substrate is 500 - 1000 μm.

[0014] Furthermore, the temperature of the vacuum drying is 60 - 80 °C, and the duration is 24 - 48 h.

[0015] The present invention also proposes an application of the zwitterionic modified PVDF polymer electrolyte in a quasi-solid-state lithium metal battery.

[0016] Furthermore, the quasi-solid-state lithium metal battery is a lithium symmetric button battery, a lithium - ternary (NCM) full battery, a lithium - lithium iron phosphate (LFP) full battery, etc.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. The present invention proposes a preparation method of a zwitterionic modified PVDF polymer electrolyte. Using PPS as a PVDF electrolyte additive, while promoting ion transport, it takes into account the stability of PVDF and the lithium negative electrode, inhibits the dehydrogenation and defluorination side reactions, and improves the stability of the lithium negative electrode and the performance and cycle stability of the PESSE - based LMB battery.

[0019] 2. Specifically, PPS contains N + and SO3 - sites, which can be used as zwitterionic adsorption sites to adsorb TFSI - and Li + respectively, becoming the transport sites of LiTFSI, improving the Li + transport efficiency. At the same time, it avoids the contact and reaction between LiTFSI and the -H / F sites of PVDF, thereby inhibiting the dehydrogenation and defluorination of PVDF, enhancing the stability of PVDF, reducing the generation of HF side reaction products, and further reducing the corrosion of the lithium negative electrode, comprehensively improving the stability of the lithium negative electrode, PVDF and the battery.

[0020] 3. When the zwitterionic modified PVDF polymer electrolyte obtained in the present invention is applied to a lithium symmetric button battery, a cycle stability of up to 1000 h can be achieved at 0.2 mA cm -2 and 0.1 mAh cm -2 . When applied to a lithium - ternary button battery, a cycle stability of 145 cycles can be achieved at a NCM811 loading of 1.5 mg cm -2 and a cycling current of 0.2 C. When applied to a lithium - lithium iron phosphate button battery, stable cycling of 500 cycles can be achieved at 1 C and a loading of 1.5 mg cm -2 . Description of the Drawings

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0022] Figure 1 SEM (scanning electron microscope) micro-morphology structures of the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1; among them, (a) and (b) are the morphology structures of Comparative Example 1, and (c) and (d) are the morphology structures of Example 1;

[0023] Figure 2 XRD (X-ray diffraction) patterns of the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1;

[0024] Figure 3 Electrochemical impedance spectra of the lithium-lithium symmetric batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1; among them, (a) is for Comparative Example 1 and (b) is for Example 1;

[0025] Figure 4 Linear Scanning Voltammetry (LSV) curves of the electrochemical stability of the lithium-stainless steel batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1;

[0026] Figure 5 For the lithium-lithium symmetric batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1 at 0.1 - 1 mA cm -2 under the rate cycling curves;

[0027] Figure 6 For the lithium-lithium symmetric batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1 at 0.2 mA cm -2 、0.1 mAh cm -2 under the long cycling curves;

[0028] Figure 7 For the lithium - ternary full batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1 at 0.2 C, 1.5 mg cm -2 under the long cycling curves at the load.

[0029] Figure 8 For the lithium - lithium iron phosphate full batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1 at 1 C, 1.5 mg cm -2 under the long cycling curves at the load. Detailed implementation mode

[0030] To further understand the present invention, the preferred implementation modes of the present invention will be described below in conjunction with embodiments. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the claims of the invention. There are no special restrictions on the sources of all raw materials of the present invention, and they can be purchased on the market or prepared by conventional methods well-known to those skilled in the art.

[0031] Example 1

[0032] This example presents a preparation method of a zwitterionic modified PVDF polymer electrolyte. The specific process is as follows:

[0033] Mix powdered PVDF particles, LiTFSI, PPS and NMP solvent, stir and react at room temperature for 24 h to form a uniform viscous solution. Then, use a 750-μm thick scraper to scrape the uniform viscous solution on a glass plate, and dry it in vacuum at 60 °C for 24 h to obtain a zwitterionic modified PVDF polymer electrolyte membrane, which is stored in an Ar gas atmosphere glove box for standby. Among them, the concentrations of PVDF particles, LiTFSI and PPS in the uniform viscous solution are 100 g / L, 70 g / L and 30 g / L respectively.

[0034] Example 2

[0035] This example presents a preparation method of a zwitterionic modified PVDF polymer electrolyte. The specific process is as follows:

[0036] Mix powdered PVDF particles, LiTFSI, PPS and NMP solvent, stir and react at room temperature for 24 h to form a uniform viscous solution. Then, use a 1000-μm thick scraper to scrape the uniform viscous solution on a glass plate, and dry it in vacuum at 60 °C for 24 h to obtain a zwitterionic modified PVDF polymer electrolyte membrane, which is stored in an Ar gas atmosphere glove box for standby. Among them, the concentrations of PVDF particles, LiTFSI and PPS in the uniform viscous solution are 100 g / L, 70 g / L and 5 g / L respectively.

[0037] Example 3

[0038] This example presents a preparation method of a zwitterionic modified PVDF polymer electrolyte. The specific process is as follows:

[0039] Mix the powdered PVDF particles, LiTFSI, PPS with NMP solvent, stir and react at room temperature for 24 h to form a homogeneous viscous solution. Then, use a 500-μm thick scraper to scrape the homogeneous viscous solution on a glass plate, and dry it in vacuum at 60 °C for 24 h to obtain an amphoteric ion-modified PVDF polymer electrolyte membrane, which is stored in an Ar-gas atmosphere glove box for later use. Among them, the concentrations of PVDF particles, LiTFSI and PPS in the homogeneous viscous solution are 100 g / L, 70 g / L and 60 g / L respectively.

[0040] Example 4

[0041] This example presents a preparation method of an amphoteric ion-modified PVDF polymer electrolyte. The specific process is as follows:

[0042] Mix the powdered PVDF particles, LiFSI, PPS with NMP solvent, stir and react at room temperature for 24 h to form a homogeneous viscous solution. Then, use a 500-μm thick scraper to scrape the homogeneous viscous solution on a glass plate, and dry it in vacuum at 60 °C for 24 h to obtain an amphoteric ion-modified PVDF polymer electrolyte membrane, which is stored in an Ar-gas atmosphere glove box for later use. Among them, the concentrations of PVDF particles, LiFSI and PPS in the homogeneous viscous solution are 120 g / L, 120 g / L and 60 g / L respectively.

[0043] Example 5

[0044] This example presents a preparation method of an amphoteric ion-modified PVDF polymer electrolyte. The specific process is as follows:

[0045] Mix the powdered PVDF particles, LiPF6, PPS with NMP solvent, stir and react at room temperature for 24 h to form a homogeneous viscous solution. Then, use a 500-μm thick scraper to scrape the homogeneous viscous solution on a glass plate, and dry it in vacuum at 60 °C for 24 h to obtain an amphoteric ion-modified PVDF polymer electrolyte membrane, which is stored in an Ar-gas atmosphere glove box for later use. Among them, the concentrations of PVDF particles, LiPF6 and PPS in the homogeneous viscous solution are 80 g / L, 30 g / L and 60 g / L respectively.

[0046] Comparative Example 1

[0047] This comparative example presents a preparation method of an unmodified PVDF polymer electrolyte. The specific process is as follows:

[0048] Mix the powdered PVDF particles, LiTFSI, and NMP solvent, stir and react at room temperature for 24 h to form a homogeneous viscous solution. Then, use a 750-μm thick scraper to scrape the homogeneous viscous solution on a glass plate, and dry it in a vacuum at 60 °C for 24 h to obtain an unmodified PVDF polymer electrolyte membrane, which is stored in a glove box filled with Ar gas for later use. Among them, the concentrations of PVDF particles and LiTFSI in the homogeneous viscous solution are 100 g / L and 70 g / L, respectively.

[0049] Next, a comparative analysis is carried out on the characterization and performance of the zwitterionic modified PVDF polymer electrolyte membrane obtained in Example 1 and the unmodified PVDF polymer electrolyte membrane obtained in Comparative Example 1.

[0050] Figure 1 Figure 7 shows the SEM micro-morphology structure of the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1. Among them, Figure 1 (a) and Figure 1 (b) are the morphology structures of Comparative Example 1, Figure 1 (c) and Figure 1 (d) are the morphology structures of Example 1. It can be seen that the morphologies of both have the characteristics of being regular and flat, indicating that the PPS modification has little effect on the morphology of the PVDF polymer membrane.

[0051] Figure 2 Figure 19 shows the XRD patterns of the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1, and they are compared with the XRD pattern of pure PVDF powder. The sharp peak and broad peak around 20° in the figure are attributed to the

[110] and

[020] crystal planes of PVDF, respectively, and the broad peak around 39° is attributed to the γ-phase PVDF. It can be seen that adding PPS will, without affecting the original PVDF structure, increase the overall crystallinity of the PVDF polymer membrane to a certain extent, indicating that the introduction of amphiphilic dipole PPS molecules will enhance the dipole interaction inside the PVDF polymer.

[0052] Figure 3 Figure 23 shows the electrochemical impedance spectra of the lithium-lithium symmetric batteries assembled using the PVDF polymer electrolyte membranes obtained in Example 1 and Comparative Example 1. Among them, Figure 3 (a) is that of Comparative Example 1, Figure 3 (b) is that of Example 1; specifically, through the electrochemical test results of the lithium-lithium symmetric battery, the impedance spectrum for calculating the ionic conductivity of the PVDF polymer electrolyte membrane is calculated using the formula σ = d / (R×S) (where σ is the ionic conductivity; d is the thickness of the PVDF polymer electrolyte membrane, here the thickness of Comparative Example 1 is 40 μm, and the thickness of Example 1 is 88 μm; R is the impedance value, specifically the value at the intersection of the dotted curve and the abscissa in the figure; S is the area of the PVDF polymer electrolyte membrane, here it is 1.12 cm 2 ). According toFigure 3 The ionic conductivities of Comparative Example 1 and Example 1 are 0.21 mS cm -1 and 0.49 mS cm -1 respectively, indicating that PPS promotes the transport of Li + through amphoteric adsorption sites, that is, it improves the ionic conductivity.

[0053] Figure 4 Figure 11 is the linear sweep LSV curve of the electrochemical stability of the lithium-stainless steel battery assembled with the PVDF polymer electrolyte membranes obtained from Example 1 and Comparative Example 1. Specifically, according to the voltage critical value at which the current starts to increase steeply with the change of voltage in the electrochemical test results, the voltage resistance of the polymer membrane is obtained, that is, the electrochemical stability. From Figure 4 it can be obtained that the electrochemical stability windows of Comparative Example 1 and Example 1 are 4.2 V and 4.8 V respectively, indicating that PPS avoids the contact between Li + and the -H and -F sites of PVDF through amphoteric adsorption sites, thereby inhibiting the occurrence of dehydrogenation and defluorination side reactions and improving the stability of the polymer structure and the lithium negative electrode interface.

[0054] Figure 5 Figure 19 is the rate cycling curve of the lithium-lithium symmetric battery assembled with the PVDF polymer electrolyte membranes obtained from Example 1 and Comparative Example 1 at 0.1 - 1 mAcm -2 . It can be seen that the tolerated current of Example 1 is significantly increased, and the overpotential value is small. There is no overpotential jitter and attenuation phenomenon when the current is high in Comparative Example 1, indicating that PPS can improve the test tolerated current density and rate performance of the lithium-lithium symmetric battery by improving the ionic conductivity and stability.

[0055] Figure 6 Figure 25 is the long cycling curve of the lithium-lithium symmetric battery assembled with the PVDF polymer electrolyte membranes obtained from Example 1 and Comparative Example 1 at 0.2 mA cm -2 and 0.1 mAh cm -2 . It can be seen that Example 1 can cycle stably for 1000 h, while obvious short-circuit phenomenon occurs in Comparative Example 1 at about 138 h. This is due to the poor stability and ionic conductivity of the polymer electrolyte, resulting in too large interfacial polarization, too thick dead lithium layer and serious growth of dendrites, and finally piercing the electrolyte, indicating that PPS can improve the stability of the battery, thereby improving the cycling performance of the battery.

[0056] Figure 7 Figure 33 is the charge and discharge curve of the lithium - ternary full battery assembled with the PVDF polymer electrolyte membranes obtained from Example 1 and Comparative Example 1 at 0.2 C and 1.5 mg cm -2Long cycle curves under load show that Example 1 can stably cycle 145 times, while in Comparative Example 1, the capacity decay rate per cycle is relatively high since the start of cycling. After dozens of cycles, the remaining capacity is less than half of that in the first cycle, further demonstrating that PPS modification can significantly improve the cycling performance of the battery by increasing ionic conductivity and polymer stability.

[0057] Figure 8 Long cycle curves of the lithium - lithium iron phosphate full battery assembled with the PVDF polymer electrolyte membranes obtained from Example 1 and Comparative Example 1 under a load of 1C and 1.5 mg cm -2 Long cycle curves under load show that Example 1 can stably cycle 500 times, while in Comparative Example 1, the capacity decay rate per cycle is relatively high since the start of cycling. The battery capacity drops sharply after less than 50 cycles, further demonstrating that PPS modification can improve the cycling performance of lithium metal batteries.

[0058] In the present invention, specific examples are used to illustrate the principles and implementation manners of the present invention. The descriptions of the above embodiments are only for helping to understand the method and its core idea of the present invention, including the best mode, and also enabling any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that for those of ordinary skill in the art in the technical field of the present invention, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. The protection scope of the present invention is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements that are not different from the literal description of the claims, or if they include equivalent structural elements that have no substantial difference from the literal description of the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A method for preparing a zwitterion-modified PVDF polymer electrolyte, characterized in that: The specific process is: PVDF particles, lithium bis(trifluoromethylsulfonyl)imide, PPS and a polar organic solvent are mixed and stirred to obtain a uniform viscous solution, which is coated on the surface of a substrate and dried in a vacuum to obtain a zwitterion-modified PVDF polymer electrolyte; The concentrations of PVDF particles, lithium bis(trifluoromethylsulfonyl)imide and PPS in the uniform viscous solution are 80-120 g / L, 10-120 g / L and 5-60 g / L, respectively.

2. The method for preparing the zwitterion-modified PVDF polymer electrolyte according to claim 1, characterized in that: The polar organic solvent is N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide or tetrahydrofuran.

3. The method for preparing the zwitterion-modified PVDF polymer electrolyte according to claim 1, characterized in that: The thickness of the uniform viscous solution on the surface of the substrate is 500-1000 μm.

4. The method for preparing the zwitterion-modified PVDF polymer electrolyte according to claim 1, characterized in that: The temperature of the vacuum drying is 60-80° C. and the duration is 24-48 hours.

5. Use of the zwitterion-modified PVDF polymer electrolyte obtained according to the preparation method according to any one of claims 1 to 4 in quasi-solid-state lithium metal batteries.

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