Hydrolysis-resistant functional additive, preparation method thereof and application thereof in solid-state electrolyte

Abstract

The application discloses an anti-hydrolysis functional additive, a preparation method thereof and application of the additive in a solid-state electrolyte. The preparation method comprises the following steps: adding fluorobenzenacetic acid and metal hydroxide into a mixed solution of ethanol and water, and heating, stirring and drying to obtain the anti-hydrolysis functional additive. The anti-hydrolysis functional additive prepared by the application has the characteristics of stability in air, and the additive can be oxidized and reduced into a film in a lithium metal battery, so that the interface compatibility of the positive / negative electrode and the solid-state electrolyte in the all-solid-state lithium metal battery can be greatly improved. When the functional additive is used in a polymer-based solid-state electrolyte, the assembled all-solid-state battery has double-interface stability and exhibits high rate capacity and good cycle performance.

Description

Technical Field

[0001] This invention belongs to the field of solid-state lithium battery technology, specifically relating to a hydrolysis-resistant functional additive, its preparation method, and its application. Background Technology

[0002] Polymer-based solid electrolytes have become a research hotspot and focus in all-solid-state lithium metal batteries due to their advantages such as mechanical softness, ease of film formation, good contact with electrodes, and ease of large-scale fabrication. However, polymer-based solid electrolytes, especially polyoxyethylene solid electrolytes, generally suffer from problems such as narrow electrochemical windows, instability to lithium, and inability to effectively suppress lithium dendrites. The resulting incompatibility at the electrode / electrolyte interface often leads to poor electrochemical performance in assembled polymer-based all-solid-state lithium metal batteries.

[0003] To address the interfacial incompatibility issue in polymer-based all-solid-state batteries, common strategies include electrolyte membrane structure design and composition control. Composition control, particularly the use of film-forming additives, is a simple, convenient, and low-cost modification measure. Small amounts of film-forming additives (typically 1 wt% to 5 wt%) can significantly improve the composition and structure of the electrode / electrolyte interface film, thereby stabilizing the electrode / electrolyte interface. For example, patent application CN111969247A discloses an in-situ protective solid electrolyte for lithium metal anodes and its preparation method. This method uses protective lithium salts (lithium dioxolane borate and LiNO3) as additives, ensuring continuous formation of an SEI film on the surface of the lithium metal anode, thus effectively suppressing lithium dendrite growth. Patent application CN113675477A discloses an asymmetric layered polymer-based composite solid electrolyte suitable for 4.5V all-solid-state batteries and its preparation method. This method employs an asymmetric electrolyte structure design and utilizes targeted interface stabilizing additives—lithium difluorodioxazophosphate (LiBODFP) and lithium nitrate as additives on the positive and negative electrode sides, respectively—to ensure that the prepared electrolyte membrane can be matched with a 4.5V high-voltage positive electrode and enable the battery to exhibit good electrochemical performance.

[0004] Self-sacrificing film-forming additives should undergo oxidation / reduction reactions within the battery's operating voltage range to participate in the formation of the interfacial film and alter its composition or structure. These additives are generally compounds containing boron, phosphorus, nitrogen, fluorine, or a combination of these elements. Fluorine-containing additives, in particular, are highly effective in stabilizing the positive and negative electrode interfaces because they can generate a highly electronically insulating LiF-rich layer in situ, suppressing further interfacial side reactions. However, most of these fluorine-containing additives, such as lithium dioxolane-borate, lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium difluorodioxolane-phosphate, undergo hydrolysis upon contact with water. Therefore, the preparation process of polymer electrolyte membranes using these additives requires strict environmental control, which undoubtedly increases production costs significantly. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing a novel hydrolysis-resistant functional additive and its ability to improve the performance of polymer-based all-solid-state lithium batteries.

[0006] The novel hydrolysis-resistant functional additive provided by this invention is prepared by a simple aqueous liquid-phase method and can be used as a film-forming additive on both the positive and negative electrodes. This novel film-forming additive is rich in fluorine sources and also provides metal ions that can be alloyed at the negative electrode, thereby greatly improving the lithium stability of the prepared polymer-based composite solid electrolyte and effectively inhibiting the formation and growth of lithium dendrites.

[0007] The objective of this invention is achieved by at least one of the following technical solutions.

[0008] This invention provides a method for preparing a hydrolysis-resistant functional additive for solid electrolytes, comprising the following steps:

[0009] Fluorophenylacetic acid and metal hydroxide were added to a mixed solvent of ethanol and water, and the mixture was heated, stirred, and dried to obtain the hydrolysis-resistant functional additive.

[0010] Further, the fluorophenylacetic acid includes one or more selected from 2,4,5-trifluorophenylacetic acid and 2,3,4,5,6-pentafluorophenylacetic acid. Further, the metal hydroxide includes one or more selected from lithium hydroxide, magnesium hydroxide, and aluminum hydroxide.

[0011] Furthermore, the ratio of the fluorophenylacetic acid to the metal hydroxide is determined based on the molar ratio of -COOH:-OH being 1 to 1.05:1.

[0012] Furthermore, the volume ratio of the ethanol and water mixed solvent used is 1 to 4:1.

[0013] Furthermore, the heating and evaporation temperatures during the preparation process are 50–80°C.

[0014] This invention provides a hydrolysis-resistant functional additive prepared by the above-described method.

[0015] The application of the novel hydrolysis-resistant functional additive provided by this invention in solid electrolytes.

[0016] Furthermore, the solid electrolyte is a polymer-based all-solid-state lithium metal battery.

[0017] Furthermore, the amount of hydrolysis-resistant functional additive added to the polymer-based all-solid-state lithium metal battery is 1-4 wt%.

[0018] The novel functional additive provided by this invention is stable in air and does not undergo hydrolysis. At the same time, the additive is effective in improving the compatibility of the positive / negative electrode and electrolyte interface in polymer-based all-solid-state batteries.

[0019] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0020] (1) The preparation method of the hydrolysis-resistant functional additive provided by the present invention is simple and convenient, and the raw materials and solvents used are environmentally friendly, inexpensive and can be mass-produced.

[0021] (2) The hydrolysis-resistant functional additive provided by the present invention is stable in air and does not undergo hydrolysis reaction. At the same time, the additive can provide abundant fluorine source and magnesium / aluminum source to improve the composition and structure of the interface film. The magnesium / aluminum source is alloyed on the lithium metal surface, which makes the ionic conductivity of the interface film higher and more conducive to the uniform deposition of lithium.

[0022] (3) The hydrolysis-resistant functional additive provided by the present invention is an ionic additive. It is completely dissociated in the electrolyte membrane, and the dispersion is more uniform, making the formed interface film more dense and uniform.

[0023] (4) The hydrolysis-resistant functional additive can be oxidized and reduced to form a film in lithium metal batteries, thus greatly improving the interfacial compatibility between the positive / negative electrode and the solid electrolyte in all-solid-state lithium metal batteries. When used in polymer-based solid electrolytes, it enables the assembled all-solid-state batteries to have dual interfacial stability and exhibit high rate capacity and excellent cycle performance. Attached Figure Description

[0024] Figure 1 XRD patterns of the reactant (PFPAA) and functional additive (MgPFPAA);

[0025] Figure 2 Infrared spectra of the reactant (PFPAA) and functional additive (MgPFPAA);

[0026] Figure 3 The cycling stability diagrams of the symmetrical batteries assembled in Example 1 with and without the polymer-based solid electrolyte, using functional additives and additive-free polymers, are shown at different current densities.

[0027] Figure 4 The Li-symmetric battery assembled using the polymer-based solid electrolyte as a functional additive in Example 1 at 0.2 mA cm⁻¹ -2 Cyclic stability plot at time;

[0028] Figure 5 The Li-symmetric cell assembled for Comparative Example 1 was tested at 0.2 mA cm⁻¹. -2 Cyclic stability plot at time;

[0029] Figure 6 The rate performance graph shows the lithium iron phosphate all-solid-state battery assembled with polymer-based solid electrolytes as functional additives and without additives in Example 1.

[0030] Figure 7 The graph shows the long-cycle performance of lithium iron phosphate all-solid-state batteries assembled using polymer-based solid electrolytes as functional additives and without additives, as described in Example 1. Detailed Implementation

[0031] The following examples further illustrate specific implementations of the present invention, but the implementation and protection of the present invention are not limited thereto. It should be noted that any processes not specifically described below are those that can be implemented or understood by those skilled in the art by referring to existing technology. Reagents or instruments whose manufacturers are not specified are considered to be conventional products that can be purchased commercially.

[0032] Example 1

[0033] A method for preparing a novel hydrolysis-resistant functional additive includes the following steps:

[0034] 0.0651 g of magnesium hydroxide was weighed and dispersed in 10 ml of a mixed solvent of ethanol and water (volume ratio 4:1). Then, 0.5 g of 2,3,4,5,6-pentafluorophenylacetic acid was added. The mixed solution was stirred vigorously at 80 °C for 2 h to obtain a completely clear and transparent solution. The solution was then evaporated to dryness at 80 °C to obtain a novel functional additive, magnesium 2,3,4,5,6-pentafluorophenylacetate (MgPFPAA).

[0035] Using Example 1 as a functional additive, a polymer-based composite solid electrolyte membrane was prepared by the following method:

[0036] Weigh out 0.8g of polyethylene oxide (PEO), 0.2g of polyvinylidene fluoride (PVDF), and 0.4439g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 0.15g of Li6.5 La3Zr 1.5 Ta 0.5 O 12 The solution was added to a round-bottom flask containing 15.0 g of N,N-dimethylformamide (DMF) and stirred at 50 °C for 6 h to obtain a brown slurry. Then, 0.03 g of MgPFPAA functional additive was added. The thoroughly dispersed slurry was poured into a mold and dried at 60 °C and atmospheric pressure for 3 h to evaporate most of the solvent. Then, it was transferred to an 80 °C vacuum drying oven and vacuum dried for 24 h to obtain a polymer-based composite solid electrolyte (denoted as MgPFPAA-CSE). It was then cut into 19 mm small round pieces for later use.

[0037] For comparison, a composite solid electrolyte membrane, denoted as CSE, was prepared without the addition of MgPFPAA and with all other operating steps being identical.

[0038] Comparative Example 1

[0039] Using PPFAA as an additive, a polymer-based composite solid electrolyte membrane was prepared by the following method:

[0040] Weigh out 0.8g of polyethylene oxide (PEO), 0.2g of polyvinylidene fluoride (PVDF), and 0.4439g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 0.15g of Li 6.5 La3Zr 1.5 Ta 0.5 O 12 The mixture was added to a round-bottom flask containing 15.0 g of N,N-dimethylformamide (DMF) and stirred at 50 °C for 6 h to obtain a brownish-red slurry. Then, 0.03 g of PFPAA functional additive was added. The thoroughly dispersed slurry was poured into a mold and dried at 60 °C and atmospheric pressure for 3 h to evaporate most of the solvent. Then, it was transferred to an 80 °C vacuum drying oven and vacuum dried for 24 h to obtain a polymer-based composite solid electrolyte (denoted as PFPAA-CSE). It was then cut into 19 mm small round pieces for later use.

[0041] Figure 1 The XRD patterns of Example 1 and the raw material PFPAA are shown in the figure. It can be seen from the figure that after the reaction, PFPAA is completely converted into MgPFPAA.

[0042] Figure 2 The infrared spectrum of Example 1 and the raw material PFPAA is shown in the figure. It can be seen from the figure that after the reaction, the stretching vibration of the carboxyl group in the raw material completely disappeared, proving that PFPAA was completely converted into MgPFPAA.

[0043] Figure 3The graphs show the cycle stability of lithium-symmetric batteries (Li / composite solid electrolyte / Li) assembled with and without the polymer-based solid electrolyte as functional additives in Example 1 at different current densities. It can be seen that MgPFPAA significantly improves the lithium stability of the composite solid electrolyte membrane. After using the additive, the critical current density of the composite electrolyte membrane increased from the original 0.5 mA / cm². 2 Increased to 1.0 mA / cm 2 .

[0044] Figure 4 The lithium-ion symmetric battery (Li / composite solid electrolyte / Li) assembled using polymer-based solid electrolytes as functional additives and without additives, as described in Example 1, operates at a current density of 0.2 mA / cm². 2 The long-cycle performance graph below shows that the electrolyte membrane containing the MgPFPAA functional additive has very good lithium stability.

[0045] Figure 5 The lithium-ion symmetric battery (Li / composite solid electrolyte / Li) assembled with the polymer-based solid electrolyte as Comparative Example 1 operates at a current density of 0.2 mA / cm². 2 The long-cycle performance graph below shows that the polymer electrolyte membrane containing PPFAA additive has very poor lithium stability, thus proving that PPFAA is unstable to lithium.

[0046] Figure 6 The graph shows the rate performance of the lithium iron phosphate all-solid-state battery (LFP / composite solid electrolyte / Li) assembled with and without the polymer-based solid electrolyte in Example 1 at 60°C. It can be seen that when MgPFPAA is used as a functional additive, it can significantly improve the rate performance of the assembled all-solid-state battery.

[0047] Figure 7 The graph shows the long-cycle performance of the lithium iron phosphate all-solid-state battery (LFP / composite solid-state electrolyte / Li) assembled with and without the polymer-based solid electrolyte in Example 1 at 60°C and 2C. It can be seen that the battery assembled with the composite electrolyte membrane without the additive cannot work normally for a long time because it cannot effectively suppress the growth of lithium dendrites, which leads to soft short circuits during charging. In contrast, the battery assembled with the composite solid-state electrolyte membrane with the MgPFPAA additive can work stably at a high current density of 2C.

[0048] The above embodiments are merely preferred embodiments of the present invention and are only used to explain the present invention, not to limit the present invention. Any changes, substitutions, modifications, etc., made by those skilled in the art without departing from the spirit and essence of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a hydrolysis-resistant functional additive for solid electrolytes, characterized in that, Includes the following steps: Fluorophenylacetic acid and metal hydroxide were added to a mixed solvent of ethanol and water, and the mixture was heated, stirred, and dried to obtain the hydrolysis-resistant functional additive. The fluorophenylacetic acid includes one or more of 2,4,5-trifluorophenylacetic acid and 2,3,4,5,6-pentafluorophenylacetic acid; The metal hydroxides mentioned include one or more of lithium hydroxide, magnesium hydroxide, and aluminum hydroxide.

2. The preparation method according to claim 1, characterized in that, The ratio of fluorophenylacetic acid and metal hydroxide used is determined based on the molar ratio of -COOH:-OH being 1~1.05:

1.

3. The preparation method according to claim 1, characterized in that, The volume ratio of the mixed solvent of ethanol and water used is 1 to 4:

1.

4. The preparation method according to claim 1, characterized in that, The heating and drying temperature during the preparation process is 50~80℃.

5. A hydrolysis-resistant functional additive prepared by the preparation method according to any one of claims 1-4.

6. The application of the hydrolysis-resistant functional additive as described in claim 5 in solid electrolytes.

7. The application according to claim 6, characterized in that, The solid electrolyte is a polymer-based all-solid-state lithium metal battery.

8. The application according to claim 7, characterized in that, The amount of hydrolysis-resistant functional additives added to polymer-based all-solid-state lithium metal batteries is 1-4 wt%.

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