Method for plasma-assisted in-situ construction of PVDF-HFP-based composite solid electrolyte NaF-rich interface layer and application thereof

Abstract

The invention discloses a method for plasma-assisted in-situ construction of a PVDF-HFP-based composite solid electrolyte NaF-rich interface layer and application thereof, and relates to the technical field of solid-state batteries. The surface of the PVDF-HFP composite solid electrolyte is modified by adopting a dielectric barrier discharge plasma technology to realize optimization of interface components, so that the interface compatibility of the composite solid electrolyte and an electrode material is improved, and the ion transmission performance of the PVDF-HFP composite solid electrolyte is enhanced; and the interface impedance between the composite solid electrolyte and the electrode is reduced. The ratio of NaF on the surface of the PVDF-HFP-based composite solid electrolyte obtained after plasma treatment modification is increased from 21.49% to 67.48%, compared with a contrast 1 in an embodiment 3, the ionic conductivity is increased by nearly two times, the interface impedance of a solid-state sodium symmetric battery is reduced by nearly 40%, and excellent rate and cycle performance are shown when the PVDF-HFP-based composite solid electrolyte is applied to a sodium metal solid-state battery. The method is easy to operate, short in period and low in cost, and commercial application of the PVDF-HFP-based composite solid electrolyte is promoted.

Description

Technical Field

[0001] This invention relates to the field of solid-state battery materials, and more particularly to a method for creating a NaF-rich interface layer in a PVDF-HFP-based composite solid-state electrolyte and its application. Background Technology

[0002] With the rapid development of the new energy industry, energy storage devices with high safety and high energy density have become a research hotspot. Solid-state sodium metal batteries are considered one of the most promising next-generation energy storage technologies due to the abundance of sodium resources, low cost, and excellent safety. Solid electrolytes, as the core component of solid-state sodium metal batteries, directly determine the battery's energy density, cycle stability, and safety performance. Among them, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) is often used as the matrix material for composite solid electrolytes due to its good electrochemical stability, thermal stability, machinability, and electrode compatibility.

[0003] In existing technologies, the preparation of PVDF-HFP-based composite solid electrolytes is mostly achieved through processes such as solution casting and drying molding of raw materials such as blended polymers, inorganic electrolyte powders, and sodium salts. To improve its ion conductivity, inorganic electrolyte powders such as NASICON (sodium superionic conductor) are added to construct ion conduction channels, and PEG (polyethylene glycol) is introduced to improve the flexibility of the polymer matrix and the ion migration environment. However, the solid electrolyte interface layer generated during battery cycling of PVDF-HFP-based composite solid electrolytes prepared by this process is usually dominated by organic components. This solid electrolyte interface layer typically has poor mechanical properties, resulting in poor compatibility with the electrode interface and insufficient interface stability. This exacerbates interfacial side reactions and forms a large interfacial impedance, hindering ion transport and reducing battery cycle performance. Therefore, developing a method for preparing a PVDF-HFP-based composite solid electrolyte and its stable interface layer is of great significance for improving the performance of sodium-based solid-state batteries and promoting their industrial application. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies, such as poor interfacial compatibility between PVDF-HFP-based composite solid electrolytes and electrodes, high interfacial impedance, difficulty in suppressing sodium dendrite growth, and limited battery performance. This invention provides a method and application for plasma-assisted in-situ construction of a NaF-rich interfacial layer for PVDF-HFP-based composite solid electrolytes. Through dielectric barrier discharge plasma technology with specific parameters, a NaF-rich interfacial layer is formed in-situ at the solid electrolyte interface, optimizing the interfacial composition, reducing interfacial impedance, improving interfacial stability and resistance to sodium dendrite growth, and ultimately significantly improving the cycle life and electrochemical performance of sodium-based solid-state batteries.

[0005] The technical solution of this invention is: A PVDF-HFP-based composite solid electrolyte containing a NaF-rich interface layer comprises a matrix and a NaF-rich interface layer formed on both the front and back surfaces of the matrix. The matrix comprises a PVDF-HFP polymer matrix, a PEG plasticizer, and NASICON (Na3Zr2Si2PO4). 12 The composite electrolyte matrix is ​​treated with dielectric barrier discharge plasma with specific parameters. Through the collision of high-energy particles generated by the plasma and the reaction with highly active components, the CF covalent bonds in the PVDF-HFP matrix are dissociated and NaPF6 is decomposed in situ, thereby achieving the in-situ formation of a high-performance solid electrolyte interface layer rich in NaF. The process parameters of the dielectric barrier discharge plasma include: discharge voltage of 50-100V, discharge current of 1A, and nitrogen as the plasma source.

[0006] To achieve the above-mentioned method for plasma-assisted in-situ construction of a NaF-rich interface layer for a PVDF-HFP-based composite solid electrolyte, the specific steps include: (1) Preparation of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) based composite solid electrolyte: PVDF-HFP, inorganic filler and additive powder are weighed according to a preset ratio. The inorganic filler, PVDF-HFP and additive are dissolved in a solvent and stirred for a first predetermined time to obtain a first mixed solution. The first mixed solution is then ultrasonically mixed for a second preset time to obtain a composite solid electrolyte precursor slurry. The composite solid electrolyte precursor slurry is then poured onto a preset template and vacuum dried to remove the solvent and form. The PVDF-HFP based composite solid electrolyte is taken out from the preset template and rolled to obtain a PVDF-HFP based composite solid electrolyte membrane. The additive includes polyethylene glycol (PEG) and sodium hexafluorophosphate (NaPF6). (2) Plasma surface modification treatment: The surface of the PVDF-HFP-based composite solid electrolyte membrane obtained in step (1) is subjected to plasma surface treatment using dielectric barrier discharge plasma under a nitrogen atmosphere, so as to convert the PVDF-HFP and the additive components in the PVDF-HFP-based composite solid electrolyte into sodium fluoride (NaF), thereby obtaining a PVDF-HFP-based composite solid electrolyte with a NaF-rich interface layer.

[0007] Compared to existing technologies, the above-mentioned method for plasma-modified PVDF-HFP-based composite solid electrolyte utilizes plasma to perform double-sided plasma-assisted in-situ construction on the surface of the PVDF-HFP-based composite solid electrolyte to optimize the interface composition. This enhances the solid-solid interface compatibility of the PVDF-HFP-based composite solid electrolyte, reduces interface impedance, decreases battery polarization, and extends the cycle life of solid sodium metal batteries using the plasma-modified PVDF-HFP-based composite solid electrolyte. Furthermore, the above-mentioned method for plasma-modified PVDF-HFP-based composite solid electrolyte has a simple process flow, does not involve complex reaction processes, reduces energy consumption and equipment investment, and virtually eliminates the generation of waste in any process step, aligning with green industry principles and being environmentally friendly.

[0008] In some embodiments, the preset ratio is the mass percentage of the sum of the masses of the poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix, the inorganic solid electrolyte filler, and the additives. Specifically, the preset ratio of the PVDF-HFP matrix is ​​40% of the total mass of the PVDF-HFP matrix, inorganic solid electrolyte filler, and additives, and the preset ratio of the inorganic filler is also 40% of the total mass of the PVDF-HFP matrix, inorganic solid electrolyte filler, and additives. By following the above preset ratio, the crystallinity of the PVDF-HFP matrix can be effectively reduced, creating continuous and uniform ion transport channels internally, resulting in excellent mechanical properties and high ionic conductivity, and the battery exhibits superior cycle performance.

[0009] In some embodiments, the inorganic solid electrolyte filler is a sodium-ion superconductor (NASICON) type inorganic solid electrolyte Na3Zr2Si2PO synthesized by solid-state sintering. 12 Specifically, the aforementioned inorganic filler, with its excellent thermal stability, three-dimensional structure, and superior electrochemical stability, enables the obtained PVDF-HFP-based composite solid electrolyte to exhibit excellent ion transport and electrochemical performance.

[0010] In some embodiments, the additives are polyethylene glycol (PEG) and sodium hexafluorophosphate (NaPF6). Specifically, the preset proportion of polyethylene glycol (PEG) is 10% of the total mass of the PVDF-HFP matrix, inorganic solid electrolyte filler, and additives, and the preset proportion of sodium hexafluorophosphate (NaPF6) is 10% of the total mass of the PVDF-HFP matrix, inorganic solid electrolyte filler, and additives. PEG (polyethylene glycol) acts as a plasticizer, effectively improving the flexibility and ion migration environment of the composite solid electrolyte. The fluorinated sodium salt sodium hexafluorophosphate (NaPF6) provides a sodium source and fluorine as an interfacial component for the composite solid electrolyte, achieving stable interfacial layer construction and sodium ion flux.

[0011] In some embodiments, the solvent comprises a blend of acetone and N,N-dimethylformamide (DMF) mixed in a 1:1 volume ratio. Specifically, the solvent described above exhibits good compatibility with the polymer, resulting in a PVDF-HFP-based composite solid electrolyte with superior microstructure and mechanical properties.

[0012] In some embodiments, a first mixed solution is obtained by weighing the PVDF-HFP, inorganic filler, and additive powder according to a preset ratio, dissolving them in the solvent, and stirring for a first predetermined time. Specifically, the stirring time is 8 to 12 hours, and the stirring rotor speed is 300 to 500 r / min. Using the above stirring scheme allows the polymer powder to dissolve fully and initially disperses the inorganic filler, reducing bubble generation and obtaining a uniformly mixed first mixed solution.

[0013] The first mixed solution is transferred to an ultrasonic cleaner and ultrasonicated for a second predetermined time to further disperse the inorganic solid electrolyte powder, thereby obtaining a precursor slurry of the composite solid electrolyte. Specifically, the second predetermined ultrasonic time is 60 minutes. Ultrasonic oscillation for the second predetermined time can further uniformly disperse the inorganic solid electrolyte filler powder, resulting in a PVDF-HFP-based composite solid electrolyte precursor slurry with uniformly distributed components. This ensures a uniform internal component distribution of the obtained PVDF-HFP-based composite solid electrolyte film, resulting in excellent microstructure, mechanical properties, and electrochemical performance.

[0014] In some embodiments, the thickness of the PVDF-HFP-based composite solid electrolyte is 30 μm to 60 μm. The PVDF-HFP-based composite solid electrolyte with a NaF-rich solid electrolyte interface layer after plasma modification has a thickness of 30 μm to 60 μm. Specifically, the PVDF-HFP-based composite solid electrolyte with a NaF-rich inorganic solid electrolyte interface layer has a thickness of 30 μm to 60 μm, which makes it easier to realize high-energy-density energy storage devices in sodium metal solid-state batteries, thus enabling wider application.

[0015] In some embodiments, the PVDF-HFP-based composite solid electrolyte is subjected to plasma modification treatment to achieve plasma-assisted in-situ construction of a NaF-rich interface layer. Specific steps include: The obtained PVDF-HFP-based composite solid electrolyte matrix was placed in a dielectric barrier discharge plasma treatment device to perform double-sided plasma modification treatment. The plasma modification treatment adopted a preset plasma atmosphere, preset gas flow rate, preset voltage, preset current, and a third preset time. The plasma atmosphere was nitrogen, the preset plasma atmosphere pressure was atmospheric pressure, the preset gas flow rate was 0.5 L / min, the preset voltage was the voltage applied to both sides of the PVDF-HFP-based composite solid electrolyte with a voltage range of 50V to 100V, the discharge frequency was 50 to 60 Hz, the preset current was the current applied to both sides of the PVDF-HFP-based composite solid electrolyte with a current of 1 A, and the third preset time was 10 seconds to 60 seconds. Specifically, the plasma modification treatment described above can alter the surface composition of the PVDF-HFP-based composite solid electrolyte. The non-equilibrium high-energy particles generated by the dielectric barrier discharge plasma promote the dissociation of NaPF6 chemical bonds, causing it to decompose and generate F⁻. Furthermore, the dissociation of the CF covalent bonds in the surface PVDF-HFP matrix also provides more F. In addition, plasma can activate the active sites on the PVDF-HFP matrix surface, guiding F⁻ to migrate directionally with Na⁺ in the system and bind in situ, forming a NaF-rich interface layer on the electrolyte surface; thus obtaining a PVDF-HFP-based composite solid electrolyte containing a NaF-rich interface layer.

[0016] This invention also discloses the application of plasma-assisted in-situ construction of a PVDF-HFP-based composite solid electrolyte with a NaF-rich interface layer in a solid sodium metal battery system. The invention is characterized in that the composite solid electrolyte is used as a sodium ion conducting medium to assemble a solid sodium metal battery, and the positive and negative electrode materials of the battery are at least one of sodium metal, sodium vanadium phosphate, and sodium iron pyrophosphate.

[0017] The beneficial effects of this invention are: This invention relates to a plasma-assisted in-situ construction of a NaF-rich PVDF-HFP-based composite solid electrolyte. It represents the first application of plasma surface treatment technology in in-situ modification of composite solid electrolytes, aiming to leverage its energy-saving, high-efficiency, and highly customizable advantages to optimize the interface composition of the composite solid electrolyte, thereby improving its performance. The high activity and high energy of the non-equilibrium high-energy particles in nitrogen plasma enable the construction of a NaF-rich interface layer, significantly enhancing the surface mechanical and electrochemical properties of the composite solid electrolyte and ensuring its efficient operation.

[0018] 1) Innovative construction of NaF-rich interface layer: In-situ construction of NaF-rich interface layer was achieved through dielectric barrier discharge plasma-assisted technology. When the plasma treatment time reached 60 seconds, the NaF content of the interface exceeded 60% (Example 5), and showed a clear regularity with the plasma treatment time. By precisely controlling the plasma discharge parameters, the composition of the composite solid electrolyte interface layer was precisely controlled, which solved many limitations of traditional physicochemical methods, such as low efficiency, uncontrollable composition and difficulty in controlling by-products. 2) Unique role of plasma technology: The non-equilibrium high-energy particles generated by dielectric barrier discharge plasma promote the dissociation of chemical bonds in NaPF6, causing it to decompose and generate F⁻. Furthermore, the dissociation of CF covalent bonds in the surface PVDF-HFP matrix also provides more F. In addition, plasma can activate active sites on the surface of the PVDF-HFP matrix, guiding F⁻ to migrate directionally with Na⁺ in the system and bind in situ, forming a NaF-rich interface layer on the electrolyte surface; thus obtaining a PVDF-HFP-based composite solid electrolyte containing a NaF-rich interface layer. 3) Optimized interfacial compatibility: The NaF-rich interfacial layer can reduce the interfacial impedance between the electrolyte and the sodium metal electrode by nearly 40% compared to the untreated state (Example 3), and the room temperature sodium ion conductivity of the electrolyte is reduced from 1.75 × 10⁻⁻⁻⁶. 4 S / cm (Comparative Example 1) increased to 3.35×10⁻ 4 The S / cm (Example 3) effectively suppresses sodium dendrite growth, resulting in stable cycling of the sodium symmetric battery for over 1300 hours, significantly improving the overall battery cycle capacity and cycle stability. The inorganic solid electrolyte interface, primarily composed of NaF, provides excellent mechanical stability due to its high Young's modulus, effectively blocking sodium dendrite growth and penetration, and reducing the ion migration barrier, thus improving interfacial ion transport efficiency. This interfacial composition reduces side reactions between the electrolyte and electrodes, optimizes the interfacial contact state, thereby improving battery cycle stability and extending battery life, providing crucial support for the practical application of solid-state sodium metal batteries. 4) Simple and controllable preparation process: The electrolyte preparation and plasma-assisted construction process of this invention are simple and easy to implement, requiring no complex equipment, with mild reaction conditions (atmospheric pressure and low temperature), low cost, and no "three wastes" generated, which is in line with the concept of green production and easy to scale up for industrial production. 5) The NaF-rich interface layer of this invention is directly derived from the matrix of the PVDF-HFP-based composite solid electrolyte. In contrast, if the interface product is derived from the electrode sheet, it will consume the contribution of the electrode sheet to the battery capacity to a certain extent. Moreover, it is limited by the transformation of the electrode sheet components. Compared with chemical additives or physical deposition, this plasma technology for treating the electrolyte has the advantages of high efficiency while avoiding the limitations brought about by modified electrode sheets.

[0019] 6) Compared with chemical and physical methods such as in-situ reaction with chemical additives and atomic layer deposition, the method of plasma-assisted precise in-situ construction of NaF-rich interface layer on the surface of PVDF-HFP-based composite solid electrolyte of the present invention has the advantages of uniform NaF dispersion, fewer by-products and high preparation efficiency. Attached Figure Description

[0020] Figure 1 A schematic diagram illustrating the method and steps for plasma-assisted in-situ construction of a NaF-rich interface layer for a PVDF-HFP-based composite solid electrolyte, provided by this invention. Figure 2 The diagram shows the ionic conductivity of Examples 1 to 5 and Comparative Example 1 of the present invention. Figure 3 These are scanning electron microscope (SEM) images of the surfaces of Embodiments 1 to 5 and Comparative Example 1 of the present invention; Figure 4 The surface scanning electron microscope (SEM) energy dispersive spectroscopy (EDS) spectra of Na and F in Example 3 are shown. Figure 5 The results of surface X-ray photoelectron spectroscopy analysis of Na1s in Examples 1 to 5 are compared with those of Comparative Example 1. Figure 6 The graph shows the electrochemical impedance results of the symmetrical cells assembled with sodium metal electrodes in Example 3 and Comparative Example 1 at room temperature. Figure 7 For Example 3 and Comparative Example 1, the symmetrical battery assembled with sodium metal as the electrode achieved an A / cm² reading of 0.1 mA / cm² at room temperature (30°C). 2 Long-cycle voltage-time curves at current density; Figure 8 The graphs show the cycle performance of the full cells assembled at room temperature with sodium metal anode and sodium vanadium phosphate cathode in Example 3 and Comparative Example 1 at stepped rate (first 35 cycles) and 1C rate. Detailed Implementation

[0021] To make the technical means, creative features, objectives and effects of the present invention easier to understand, the present invention will be further described in detail below with reference to specific embodiments. However, the scope of protection of the present invention is not limited to the contents described. The following embodiments are only for further detailed description of the present invention and do not constitute any limitation on the present invention. This experiment aims to investigate the effects of different plasma modification times on the surface composition of PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene) based composite solid electrolytes, and further explore their impact on electrochemical performance. The additives used in this experiment included alkali metal salts and plasticizers. Sodium hexafluorophosphate (NaPF6) was chosen as the alkali metal salt, and polyethylene glycol (PEG) was chosen as the plasticizer. The main components were the PVDF-HFP polymer and NASICON (Na3Zr2Si2PO4). 12 Inorganic fillers, alkali metal salts, and plasticizers were used as additives. All examples were prepared according to the preparation scheme of this invention.

[0022] Example 1: The specific steps for plasma treatment of PVDF-HFP-based composite solid electrolyte for 10 seconds include: a) Inorganic solid electrolyte filler NASICON (Na3Zr2Si2PO) 12 Preparation of ) Inorganic solid electrolyte filler NASICON (Na3Zr2Si2PO4) was synthesized by solid-state sintering. 12 Specifically, trisodium phosphate (Na3PO4(H2O)) 12 Zirconia (ZrO2) and silicon dioxide (SiO2) were weighed in a mass ratio of 6:4:2, and ethanol was used as a dispersant. The mixture was then placed in a zirconia ball mill jar and milled in a planetary ball mill at 350 r / min for 5 hours to obtain a mixed slurry. The slurry was then placed in an 80℃ constant temperature drying oven for 24 hours to remove the ethanol, yielding a solid electrolyte powder raw material. The obtained powder was then transferred to a muffle furnace for two sintering stages to obtain the finished inorganic solid electrolyte powder. The sintering temperatures and times for the two stages were 1100℃ for 12 hours and 1225℃ for 15 hours.

[0023] b) Preparation of PVDF-HFP based composite solid electrolyte: 1) First, the raw material PVDF-HFP and the inorganic solid electrolyte filler NASICON (Na3Zr2Si2PO4) are added. 12PEG and NaPF6 were weighed and mixed in a mass ratio of 4:4:1:1 and placed in a reagent bottle. Acetone and N,N-dimethylformamide (DMF) in a volume ratio of 1:1 were used as the mixing solvent. The mixture was then magnetically stirred at 300 r / min for 8 hours to obtain the first mixed solution.

[0024] 2) Place the first mixed solution above in an ultrasonic cleaner and sonicate for 30 minutes to obtain a composite solid electrolyte precursor slurry.

[0025] 3) The precursor slurry is uniformly coated onto the template using a scraper, and the solvent is evaporated. After the surface solvent has evaporated, the obtained coating film is transferred to a 60℃ constant temperature drying oven and dried for 12 hours to finally obtain the PVDF-HFP based composite solid electrolyte.

[0026] c) Plasma treatment modification: 1) The obtained PVDF-HFP-based composite solid electrolyte was placed in the mold of the dielectric barrier discharge plasma device. Nitrogen plasma was used as the plasma source to modify the surface of the composite solid electrolyte by double-sided plasma treatment. Specifically, the plasma discharge parameters were: discharge voltage 80 V, discharge current 1 A, and the treatment time started from the time the plasma arc reached a stable state and lasted for 10 seconds.

[0027] 2) Repeat the plasma discharge parameters of the previous step, and perform reverse treatment on the composite solid electrolyte for the same time (10 seconds) to obtain the PVDF-HFP-based composite solid electrolyte after plasma treatment for 10 seconds, which is Example 1.

[0028] Example 2: The PVDF-HFP-based composite solid electrolyte was modified by plasma treatment for 20 seconds, specifically: Repeat the steps in Example 1, but adjust the plasma treatment time to 20 seconds to obtain a PVDF-HFP-based composite solid electrolyte after plasma treatment for 20 seconds, which is Example 2.

[0029] Example 3: The PVDF-HFP-based composite solid electrolyte was modified by plasma treatment for 30 seconds. Specifically: Repeat the steps in Example 1, but adjust the plasma treatment time to 30 seconds to obtain a PVDF-HFP-based composite solid electrolyte after plasma treatment for 30 seconds, which is Example 3.

[0030] Example 4: The PVDF-HFP-based composite solid electrolyte was modified by plasma treatment for 40 seconds, specifically: Repeat the steps in Example 1, but adjust the plasma treatment time to 40 seconds to obtain a PVDF-HFP-based composite solid electrolyte after plasma treatment for 40 seconds, which is Example 4.

[0031] Example 5: Plasma treatment of modified PVDF-HFP-based composite solid electrolyte for 60 seconds, specifically: Repeat the steps in Example 1, but adjust the plasma treatment time to 60 seconds to obtain a PVDF-HFP-based composite solid electrolyte after plasma treatment for 60 seconds, which is Example 5.

[0032] Comparative Example 1: Repeat steps a) and b) in Example 1 to obtain a PVDF-HFP-based composite solid electrolyte without plasma treatment, which is Comparative Example 1.

[0033] Performance testing: The ionic conductivity results for the above embodiments and comparative examples at room temperature are as follows: Ionic conductivity: such as Figure 2 The ionic conductivity of Examples 1-5 was significantly higher than that of Comparative Example 1. As the plasma treatment time increased, the ionic conductivity gradually increased until Example 3 reached the highest value. After further extending the plasma treatment time (Examples 4 and 5), the ionic conductivity decreased slightly, but was still higher than that of Comparative Example 1. This is because the inherent etching effect of plasma and the excessive accumulation of modified products hindered the efficient transport of sodium ions at the solid electrolyte interface.

[0034] like Figure 3 The surface morphology changes of Comparative Example 1 and Examples 1-5 were tested using scanning electron microscopy. It can be concluded that with the extension of plasma treatment time, the surface of the composite solid electrolyte exhibits a change towards a tightly bonded, integral structure. Example 3 shows a more uniform and smooth morphology, demonstrating the best treatment effect, and the surface components are evenly distributed. Figure 4 However, if the processing time is too long (Example 5), the surface structure of the composite solid electrolyte will suffer severe etching, causing interface failure. This is because the high energy and high activity of the nitrogen plasma cause damage to the surface structure of the composite solid electrolyte.

[0035] like Figure 5The surface composition of Comparative Example 1 and Examples 1-5 was characterized by X-ray photoelectron spectroscopy. From the peak fitting results and bar charts of the binding energy spectrum obtained by the 1s orbital (F1s) of fluorine and the 1s orbital (Na1s) of sodium, it can be concluded that the proportion of NaF component on the surface of Examples 1-5 is higher than that of Comparative Example 1. Moreover, the proportion of NaF shows a significant regular increase with the extension of plasma treatment time. This is beneficial for the precise control of the component proportion by adjusting the treatment time, further demonstrating the strong controllability advantage of the present invention.

[0036] Further performance tests were conducted using Na||Na symmetric cells and Na||Na3V2(PO4)3 cells assembled with sodium metal anode and sodium vanadium phosphate cathode, as described in Example 3 and Comparative Example 1. Figure 6 (AC impedance spectrum) It can be seen that, with the real value of AC impedance represented by the horizontal axis and the imaginary value represented by the vertical axis, combined with the test frequency and equivalent circuit fitting results, the reduction of interface impedance after constructing a NaF-rich interface layer through plasma surface treatment enables the sodium-symmetric cell to exhibit better cycle stability and a smaller polarization voltage. Specifically, the sodium-symmetric cell corresponding to Comparative Example 1 achieves a polarization voltage of 0.1 mA / cm² at room temperature. 2 At the current density, short-circuit failure occurred after only 580 hours of cycling, significantly lower than Example 3 (>1400 hours), as shown in Figure 7. Furthermore, for Na || Na3V2(PO4)3 batteries, such as Figure 8 Comparative Example 1 showed significantly lower discharge specific capacity than Example 3 (first 35 cycles) under 0.1C–2C step rate charge-discharge test conditions. Comparative Example 1 exhibited an initial discharge specific capacity of only 86.1 mAh / g at 1C current density, which decreased to only 80.7 mAh / g after 200 cycles, with a capacity retention of 93%. In contrast, the battery in Example 3 achieved an initial discharge specific capacity of 102.2 mAh / g at 1C current density, and maintained a capacity retention of 96% after 200 cycles. This is attributed to the beneficial effects of plasma-assisted construction of a stable Na-rich interface layer, which reduced interfacial impedance and enhanced interfacial sodium ion transport.

[0037] In summary, the present invention achieves in-situ construction of a NaF-rich interface layer PVDF-HFP-based composite solid electrolyte through efficient plasma treatment modification technology, which exhibits excellent performance in terms of ionic conductivity and electrochemical stability. In particular, the plasma treatment time of 30 seconds (Example 3) shows the best electrochemical performance.

[0038] Compared to chemical and physical methods such as in-situ reaction with chemical additives and atomic layer deposition, the plasma-assisted precise in-situ construction of a NaF-rich interface layer on the surface of a PVDF-HFP-based composite solid electrolyte of this invention has advantages such as uniform NaF dispersion, fewer byproducts, strong interfacial bonding, and high preparation efficiency. Specifically, it includes: (1) NaF is evenly dispersed Plasma is a quasi-neutral gas composed of high-energy electrons, ions, and neutral particles. During in-situ construction, it can provide a uniform energy field, allowing for a more uniform distribution of nucleation sites when the NaF precursor undergoes vapor deposition or reaction on the substrate surface, thus avoiding localized agglomeration caused by solution concentration gradients or surface energy differences in chemical methods.

[0039] (2) Few by-products The plasma "dry" process eliminates the need for solvents or other chemical additives, resulting in a cleaner reaction pathway. It primarily generates NaF by bombarding and activating precursors with high-energy particles, reducing issues such as salt residues and solvent side reactions inherent in liquid-phase reactions.

[0040] (3) Strong interface bonding The high-energy particles of plasma etch and activate the substrate surface, increasing surface roughness and active sites. When NaF is deposited on this activated surface, it can form chemically bonded or mechanically interlocked interfaces, rather than simple physical adhesion, thus significantly improving the bonding force.

[0041] (4) High preparation process efficiency Plasma has a fast reaction rate and can quickly complete deposition under normal pressure or low vacuum. Furthermore, the process parameters (power, gas flow rate, time) are easy to control precisely, enabling continuous production. Compared with methods such as atomic layer deposition, it greatly shortens the preparation cycle.

[0042] The NaF-rich interface layer of this invention is directly derived from the matrix of the PVDF-HFP-based composite solid electrolyte. In contrast, if the interface products are derived from the electrode sheet, they will consume the contribution of the electrode sheet to the battery capacity to a certain extent. Moreover, it is limited by the transformation of the electrode sheet components. Compared with chemical additives or physical deposition, this plasma technology for treating the electrolyte has the advantages of high efficiency while avoiding the limitations brought about by modified electrode sheets.

[0043] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing PVDF-HFP-based composite solid electrolytes, characterized in that, Specifically, the steps include the following: (1) Preparation of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) based composite solid electrolyte: PVDF-HFP, inorganic filler and additive powder are weighed according to a preset ratio. The inorganic filler, PVDF-HFP and additive are dissolved in a solvent and stirred for a first predetermined time to obtain a first mixed solution. The first mixed solution is then ultrasonically mixed for a second preset time to obtain a composite solid electrolyte precursor slurry. The composite solid electrolyte precursor slurry is then poured onto a preset template and vacuum dried to remove the solvent and form. The PVDF-HFP based composite solid electrolyte is taken out from the preset template and rolled to obtain a PVDF-HFP based composite solid electrolyte membrane. The additive includes polyethylene glycol (PEG) and sodium hexafluorophosphate (NaPF6). (2) Plasma surface modification treatment: The surface of the PVDF-HFP-based composite solid electrolyte membrane obtained in step (1) is constructed in situ with plasma assistance using dielectric barrier discharge plasma under a nitrogen atmosphere, so as to convert the PVDF-HFP and the additive on the surface of the PVDF-HFP-based composite solid electrolyte to form sodium fluoride (NaF), thereby obtaining a PVDF-HFP-based composite solid electrolyte with a NaF-rich interface layer.

2. The method according to claim 1, characterized in that: The preset ratio is the mass percentage of PVDF-HFP, the inorganic filler and additive powder, and the sum of the masses of PVDF-HFP, the inorganic filler and additive powder. Specifically, the preset ratio is PVDF-HFP (wt. 40%), inorganic filler (wt. 40%), and additives polyethylene glycol (PEG) and sodium hexafluorophosphate (NaPF6) each accounting for (wt. 10%).

3. The method according to claim 1, characterized in that: The inorganic filler is a sodium-ion superconductor (NASICON) type inorganic solid electrolyte Na3Zr2Si2PO synthesized by solid-state sintering. 12 .

4. The method according to claim 1, characterized in that: The solvents include acetone and N,N-dimethylformamide (DMF), and the additives are polyethylene glycol (PEG) and sodium hexafluorophosphate (NaPF6).

5. The method according to claim 1, characterized in that: The step of dissolving the PVDF-HFP, inorganic filler, and additive in the solvent and stirring for the first predetermined time includes: using a magnetic stirrer to dissolve the PVDF-HFP, inorganic filler, and additive in the solvent and stirring for the first predetermined time to obtain a first mixed solution; the step of transferring the first mixed solution for ultrasonic mixing for a second predetermined time includes: using an ultrasonic instrument to sonicate the first mixed solution for the second predetermined time to obtain a composite solid electrolyte precursor slurry; the first predetermined time ranges from 8 hours to 12 hours; the second predetermined time ranges from 30 to 60 minutes.

6. The method according to claim 1, characterized in that: The step of plasma modification treatment of the surface of the PVDF-HFP-based composite solid electrolyte includes: plasma modification treatment of both the front and back surfaces of the PVDF-HFP-based composite solid electrolyte, wherein the plasma modification treatment adopts a preset gas flow rate, preset voltage, preset current and a third preset time under atmospheric pressure. The preset gas flow rate is the preset gas flow rate of the nitrogen atmosphere. The preset voltage is the voltage applied to the front and back surfaces of the PVDF-HFP-based composite solid electrolyte and the voltage range is 50V to 100V. The preset current is the current of the front and back surfaces of the PVDF-HFP-based composite solid electrolyte and the current is 1A. The third preset time is 10 seconds to 60 seconds.

7. The method according to claim 1, characterized in that: The steps of casting the precursor slurry into the preset template and then vacuum drying it to form a PVDF-HFP-based composite solid electrolyte membrane include: casting the precursor slurry onto the surface of the preset template and placing it in a vacuum drying oven to dry for a fourth preset time; then adjusting the temperature in the vacuum drying oven to a preset temperature and maintaining it for a fifth preset time to obtain the PVDF-HFP-based composite solid electrolyte membrane; the fourth preset time ranges from 1 hour to 2 hours; the preset temperature is 60°C; and the fifth preset time ranges from 12 hours.

8. The method according to claim 1, characterized in that: The thickness of the PVDF-HFP-based composite solid electrolyte membrane is 30 μm to 60 μm; the plasma surface modification treatment modifies both sides of the PVDF-HFP-based composite solid electrolyte membrane to obtain the PVDF-HFP-based composite solid electrolyte with a NaF-rich interface layer and a thickness of 30 μm to 60 μm.

9. A PVDF-HFP-based composite solid electrolyte having a NaF-rich interface layer, comprising a matrix and a NaF-rich interface layer formed on the front and back surfaces of the matrix, wherein the matrix comprises a PVDF-HFP polymer matrix, a PEG plasticizer, and NASICON (Na3Zr2Si2PO4). 12 The inorganic filler and sodium salt additive of NaPF6 are used, and the NaF-rich interface layer is formed by the dissociation of the CF covalent bonds on the positive and negative surfaces of the matrix and the in-situ decomposition of NaPF6.

10. A sodium metal battery, characterized in that: The NaF-rich interface layer PVDF-HFP-based composite solid electrolyte described in claims 1 to 9 is assembled with a sodium metal anode and a sodium vanadium phosphate cathode to form a sodium metal battery.

Images