Polyimide-modified PVDF-based organic high-conductivity solid electrolyte and preparation method thereof

Abstract

The invention discloses a polyimide-modified PVDF (polyvinylidene fluoride)-based organic high-conductivity solid electrolyte and a preparation method thereof, and particularly, nano flower-shaped polyimide particles are uniformly filled in a polyvinylidene fluoride-based electrolyte by adopting a solution casting method. The nano flower-shaped PI has a synergistic effect with chemical adsorption (a C = O group is used as a lithium ion conduction site) through physical filling (shortening an ion conduction distance and reducing a transition potential barrier), so that the room-temperature ion conductivity is improved to 1.41 * 10 <-3 > S / cm from 1.2 * 10 <-4 > S / cm of pure PVDF (Polyvinylidene Fluoride), and the ion conduction and interface properties are remarkably optimized. Meanwhile, the nickel-cobalt-manganese ternary high-voltage lithium battery assembled based on the electrolyte can stably circulate for 600 circles at the multiplying power of 5 C, the capacity retention ratio is greater than 90%, and the specific capacity reaches 116.85 mAh / g, which is close to the level of a liquid battery.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of solid-state batteries, in particular to a polyimide modified PVDF-based organic high-conductivity solid-state electrolyte and a preparation method thereof. BACKGROUND

[0002] With the rapid development of high-energy-density energy storage devices and electric vehicle industries, the safety hazards (such as leakage and combustion) caused by the flammable and explosive characteristics of traditional liquid electrolyte lithium ion batteries have become a technical bottleneck, and solid-state batteries have become a core development direction due to their intrinsic safety and higher energy density. Although polyvinylidene fluoride (PVDF)-based polymer electrolytes have the advantages of good flexibility and easy processing, their room-temperature ionic conductivity is extremely low (1.2×10 -4 S / cm), and the inter-particle gaps (200-500 nm) and rough interfaces (roughness Ra=2.5 μm) caused by high crystallinity hinder ion transmission and result in high interfacial impedance (>50 Ω), which limits their practical application.

[0003] The introduction of fillers is a key to improving the performance of PVDF, but traditional fillers (such as inorganic oxides and spherical polymer particles) have problems such as poor dispersibility, small specific surface area, and insufficient surface active groups, which makes it difficult to effectively fill the gaps and construct ion conduction channels. As a high-performance polymer, polyimide (PI) has electrochemical stability and processability, but the particles generated by conventional preparation methods (such as solution polymerization) are mostly irregular in shape and cannot form efficient conduction structures.

[0004] To solve the above problems, the application discloses a method for preparing nanoflower PI particles by a solvothermal method, which utilizes the unique advantages of high specific surface area, multi-level pores, and surface oxygen-containing groups to solve the problems of low ionic conductivity and high interfacial impedance of pure PVDF electrolyte at room temperature, thereby improving the ionic conductivity, interface performance, and cycle stability of the corresponding solid-state battery. SUMMARY

[0005] The application aims to provide a PI nanoflower modified PVDF-based electrolyte with high ionic conductivity and low interfacial impedance, and a simple preparation process thereof, so as to meet the needs of solid-state batteries for efficient ion transmission and stable interfaces.

[0006] The application discloses a polyimide modified PVDF-based organic high-conductivity solid-state electrolyte, which is composed of polyvinylidene fluoride (PVDF), lithium trifluoromethyl sulfonamide imide (LiTFSI), and nanoflower polyimide (PI) particles. The PI content is 25-50% of the total content of PVDF and LiTFSI.

[0007] Preferably, the PI particles are prepared by a solvothermal method, have a size of 550-650 nm, and have C=O groups on the surface.

[0008] The application also provides a preparation method of a polyimide-modified PVDF-based organic high-conductivity solid-state electrolyte, characterized by comprising the following steps: S1. Preparation of nanoflower-shaped polyimide particles: biphenylamine and 3,3',4,4'-benzophenonetetracarboxylic dianhydride are added to N,N-dimethylformamide (DMF) at a molar ratio of 1:1, and magnetic stirring is performed at 200-300 rpm at room temperature for 10 h to form a viscous prepolymerization solution; the prepolymerization solution is transferred to a high-pressure reactor with a polytetrafluoroethylene liner, sealed, heated to 180℃, and kept at a constant temperature for 10 h to complete the solvothermal polymerization; the reaction product is separated by centrifugation at 8000 rpm for 10 min, washed with DMF and anhydrous ethanol each for more than 3 times in sequence, and finally dried in a vacuum oven at 80℃ for 12 h to obtain nanoflower-shaped PI particles; S2. Solution mixing: PVDF powder and LiTFSI are added to DMF solvent in a certain proportion, and the nanoflower-shaped PI particles prepared in S1 are added, and magnetic stirring is performed at room temperature to form a uniformly dispersed mixed solution; S3. Casting into a film: the mixed solution is uniformly cast into a clean glass culture dish, dried in a vacuum oven, and after the solvent is completely volatilized, a PI-modified PVDF-based organic solid-state electrolyte film with uniform thickness, i.e., a PI-PVDF electrolyte, is obtained by peeling off from the culture dish.

[0009] Preferably, the mass ratio of the PVDF powder to LiTFSI in step S2 is 3:2.

[0010] Preferably, the stirring speed in step S2 is 300-400 rpm, and the stirring time is 20-25 h.

[0011] Preferably, the drying process in step S3 is drying at 60℃ for 15-25 h.

[0012] The application has the following advantages: 1. The application is prepared by a simple solution casting method, raw materials are easy to obtain, and the process is simple, so that large-scale production can be realized without complex equipment, the size and thickness of the electrolyte film can be flexibly controlled, the solvent recovery rate is >95%, it is green and environmentally friendly, and the cost is low, which meets the needs of industrial application.

[0013] 2, The nano-flower-shaped PI particles prepared by the application as a reinforcing phase have rigidity support and flexible deformation ability through physical entanglement and chemical action (hydrogen bonding between the C=O group of PI and the -CF2- group of PVDF), and are suitable for flexible battery scenes such as wearable devices. At the same time, the ion conduction distance is effectively shortened, the transition barrier is reduced, a high-efficiency ion conduction channel is constructed, and the room temperature ionic conductivity is improved compared with pure PVDF.

[0014] 3, The composite film prepared by the application has excellent flatness, and the microstructure optimization improves the electrode / electrolyte interface contact area, and the ion conduction network constructed by the nano-flower-shaped PI particles finally realizes excellent interface impedance characteristics with an internal resistance as low as 6.23Ω. The nickel-cobalt-manganese ternary high-voltage lithium battery assembled based on the electrolyte exhibits excellent electrochemical performance: stable cycle for 600 cycles at 5 C high rate, capacity retention of 116.85 mAh / g, far exceeding the pure PVDF system. The performance not only breaks through the rate performance bottleneck of the traditional PVDF-based electrolyte, but also realizes the long cycle stability of the high-energy-density battery system, which provides key technical support for the industrialization application of solid-state batteries. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments described in the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0016] Figure 1 The SEM images of the PI-PVDF electrolyte film obtained in Example 1 of the application and the pure PVDF electrolyte film obtained in Comparative Example 1 are shown; Figure 2 The SEM cross-sectional images of the PI-PVDF electrolyte film obtained in Example 1 of the application and the pure PVDF electrolyte film obtained in Comparative Example 1 are shown; Figure 3 The impedance and ionic conductivity results of the PI-PVDF electrolyte film obtained in Example 1 of the application and the pure PVDF electrolyte film obtained in Comparative Example 1 are shown; Figure 4 The cycle stability results of the PI-PVDF electrolyte film obtained in Example 1 of the application and the pure PVDF electrolyte film obtained in Comparative Example 1 are shown. DETAILED DESCRIPTION

[0017] With reference to the accompanying drawings on the basis of the embodiments of the application, the technical solutions in the embodiments of the application will be described in detail. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments of the application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the application.

[0018] Example 1 Preparation of S1 nanoflower-shaped polyimide particles: 1.78 g of benzidine and 3.11 g of 3,3',4,4'-benzophenonetetracarboxylic dianhydride were added to 60 ml of N,N-dimethylformamide (the mass-volume concentration of total solids was 8.15%) and magnetically stirred at a speed of 300 rpm at 25°C for 10 h to form a viscous prepolymerization solution; 60 ml of the prepolymerization solution was transferred to a 100 mL high-pressure reactor with a polytetrafluoroethylene liner and polymerized at 180°C for 10 h; the reaction product was separated and filtered by centrifugation (8000 rpm, 10 min), washed with DMF and ethanol each 3 times, and dried in a vacuum oven at 80°C for 12 hours to obtain uniform-sized nanoflower-shaped PI particles; S2 solution mixing: 240 mg of PVDF powder and 160 mg of LiTFSI were added to 15 ml of DMF solvent, and then 100 mg of the above-prepared nanoflower-shaped PI particles were added, and the mixture was magnetically stirred at a speed of 300-400 rpm at room temperature for 24 hours to form a uniformly dispersed mixed solution; S3 casting into a film: the mixed solution was uniformly cast into a clean glass culture dish, vacuum dried at 60°C for 20 h, and after the solvent was completely volatilized, a PI modified PVDF-based organic solid-state electrolyte (PI-PVDF electrolyte) with uniform thickness was obtained by peeling off from the culture dish. Example 2

[0019] The content of the nanoflower-shaped PI particles in step S2 in Example 1 was adjusted to 150 mg, and the rest was consistent with Example 1, to obtain a PI-PVDF electrolyte with uniform thickness. Example 3

[0020] The content of the nanoflower-shaped PI particles in step S1 in Example 1 was adjusted to 200 mg, and the rest was consistent with Example 1, to obtain a PI-PVDF electrolyte with uniform thickness. Comparative Example 1

[0021] 240 mg of PVDF and 160 mg of LiTFSI were mixed in 15 mL of DMF solution, dissolved and stirred for 24 hours, then the mixed solution was cast into a glass Petri dish, vacuum dried at 60°C for 20 hours, and finally the solid film was peeled off to obtain a pure PVDF-based organic solid electrolyte.

[0022] The solid electrolytes obtained in Example 1 and Comparative Example 1 were subjected to morphological analysis. The results are as follows: Figure 1 、2: Figure 1 It can be seen from the figure that the surface of the pure PVDF membrane is loose and rough, and the local magnification shows that the gaps between the particles are large (>1μm); the surface of the PI-PVDF membrane is dense and uniform, and the local magnification shows that the nano-flower-like PI particles fill the gaps in the PVDF, forming a continuous ion conduction network, and the surface smoothness is significantly improved (the gaps between the particles are small and there are no large pores); Figure 2 From the cross-sectional diagram, we can see that the PI-PVDF membrane is 148 μm thick, with a uniform and dense cross-section and no obvious pores; the pure PVDF membrane is 169 μm thick, with a loose and porous cross-section and a high porosity; this indicates that PI particles can not only fill the surface gaps of PVDF, but also penetrate into the interior of the PVDF membrane, making the PVDF membrane more compact and substantial, thereby effectively reducing the transmission distance of ions.

[0023] The electrochemical performance of the solid electrolytes obtained in Example 1 and Comparative Example 1 was analyzed. Figure 3 The impedance and conductivity analysis results show that the impedance arc of PI-PVDF is smaller than that of PVDF, the interface impedance is reduced from >50 Ω to 6.23 Ω, and the ionic conductivity is reduced from 0.12×10 -3 S / cm increased to 1.41×10 -3 S / cm (↑11.7 times), which directly proves that the nano-flower-like PI particles construct efficient conduction channels through high specific surface area and oxygen-containing groups, optimize interface contact, and improve ion conduction efficiency. Figure 4 The cycling performance analysis results shown in the figure indicate that the capacity retention rate of the PI-PVDF battery is >90% and the Coulombic efficiency is nearly 100% after 600 cycles at a rate of 5 C, while the capacity of the pure PVDF battery decays rapidly, verifying that the PI modification significantly enhances the cycling stability, the dense structure inhibits side reactions, and the low impedance ensures the reversibility of ion transport.

[0024] PI addition amount (as a percentage of the total amount of PVDF + LiTFSI) Room temperature ionic conductivity (x 10 -3 S / cm)]]> Interfacial impedance (Ω) Capacity retention rate after 600 cycles at 5C rate (%) Specific capacity at 5C rate (mAh / g) Comparative Example 1 0% 0.12 >50 <30 52.3 Example 1 25% 1.41 6.23 >90 116.85 Example 2 37.5% 1.35 7.02 >86 112.5 Example 3 50% 1.28 8.36 >82 108.3 The performance corresponding to different nano-flower PI particle addition amounts in Examples 1-3 and Comparative Example 1 is compared, and the data shows that with the change of the PI addition amount, the performance of the solid-state electrolyte and the corresponding battery presents a "first better and then stable" trend, and the performance of all PI-containing examples is significantly better than that of the pure PVDF system. It is proved that the addition of PI particles can improve the ionic conductivity of the electrolyte, improve the interface performance, and enhance the cycle stability of the corresponding solid-state battery.

[0025] The present embodiment is only an exemplary description of the patent and does not limit the protection scope thereof, and the person skilled in the art can also make partial changes thereto, as long as it does not exceed the spirit and essence of the patent, it is considered as an equivalent replacement of the patent, and it is within the protection scope of the patent.

Claims

1. A polyimide-modified PVDF-based organic high-conductivity solid electrolyte, characterized in that: The invention is composed of polyvinylidene fluoride (PVDF), lithium trifluoromethanesulfonimide (LiTFSI) and nano-flower-shaped polyimide (PI) particles, wherein the amount of PI added is 25-50% of the total content of PVDF and LiTFSI.

2. The solid electrolyte according to claim 1, characterized in that The PI particles were prepared by a solvothermal method, had a size of 550-650 nm, and contained C=O groups on the surface.

3. The method for preparing a solid electrolyte according to claim 1, wherein: The following steps are involved: Preparation of S1 Nanoflower-like Polyimide Particles: Benzidine and 3,3',4,4'-benzophenonetetracarboxylic dianhydride were added to N,N-dimethylformamide (DMF) at a molar ratio of 1:1 and magnetically stirred at 200-300 rpm for 10 h at room temperature to form a viscous prepolymer solution. The prepolymer solution was transferred to a high-pressure reactor with a polytetrafluoroethylene liner, sealed, heated to 180°C, and reacted at this temperature for 10 h to complete solvent thermal polymerization. The reaction product was separated by centrifugation for 10 min at 8000 rpm, washed with DMF and anhydrous ethanol for more than three times each, and finally dried in a vacuum oven at 80°C for 12 h to obtain nanoflower-like PI particles. S2 solution mixing: PVDF powder and LiTFSI were added to DMF solvent in proportion, and then the nanoflower-like PI particles prepared in S1 were added, and magnetic stirring was performed at room temperature to form a uniformly dispersed mixed solution; S3 film casting: The mixed solution is uniformly cast into a clean glass culture dish, placed in a vacuum oven to dry, and after the solvent is completely evaporated, it is peeled off from the culture dish to obtain a PI-modified PVDF-based organic solid electrolyte with uniform thickness, namely PI-PVDF electrolyte.

4. The method for preparing polyimide particles according to claim 3, wherein: The mass ratio of PVDF powder to LiTFSI in step S2 is 3:

2.

5. The method for preparing polyimide particles according to claim 3, wherein: The stirring speed in step S2 is 300-400 rpm, and the stirring time is 20-25 hours.

6. The method for preparing polyimide particles according to claim 3, wherein: The drying process in step S3 is: drying at 60° C. for 15 to 25 hours.

Images