Polymer solid electrolyte and preparation method thereof

Abstract

The invention relates to a polymer solid electrolyte and a preparation method thereof, the polymer solid electrolyte comprises a structural unit I and a structural unit II, the structural formula of the structural unit I is # imgabs 0 #, the structural formula of the structural unit II is # imgabs 1 #, R1, R2, R3, R4, R5, R6, R7 and R8 are respectively and independently selected from H or-SO3Li, and at least one group of R1, R2, R3, R4, R5, R6, R7 and R8 is-SO3Li. The polymer solid electrolyte provided by the invention has high ionic conductivity and good electrochemical stability, and can effectively prevent the formation of lithium dendrites in a lithium metal battery, thereby significantly improving the cycle stability of the battery. In addition, the polymer solid electrolyte can be well compatible with a lithium metal negative electrode, and the interface stability of the battery is optimized.

Description

Technical Field

[0001] The present invention relates to the technical field of solid electrolytes, and particularly relates to a polymer solid electrolyte and a preparation method thereof. Background Art

[0002] With the increasing demand for energy storage, lithium metal batteries (LMBs) have become an important development direction for next-generation battery technologies due to their high energy density. However, LMBs face many challenges in practical applications, especially the selection of electrolytes. Traditional liquid electrolytes have problems such as flammability, leakage, and low electrochemical stability, which greatly limit the safety and long-term stability of LMBs. As an alternative, solid electrolytes have received increasing attention due to their good mechanical strength, safety, and wide electrochemical stability window.

[0003] Currently, the research on solid electrolytes mainly focuses on three fields: inorganic materials, polymer materials, and composite materials. Among them, polymer solid electrolytes (PSEs) are gradually becoming a key direction to overcome many challenges of LMBs due to their excellent processing performance, outstanding flexibility, and relatively high ionic conductivity. However, traditional polymer electrolytes still have many problems to be solved, such as the ionic conductivity not fully meeting the actual application requirements, the stability needs to be further improved, and the interfacial compatibility with lithium metal electrodes is poor. Summary of the Invention

[0004] The first object of the present invention is to provide a polymer solid electrolyte with high ionic conductivity, good stability, and good interfacial compatibility with lithium metal electrodes.

[0005] The second object of the present invention is to provide a preparation method of the polymer solid electrolyte as described above.

[0006] The third object of the present invention is to provide a polymer solid electrolyte membrane comprising the polymer solid electrolyte as described above.

[0007] The fourth object of the present invention is to provide a preparation method of the polymer solid electrolyte membrane as described above.

[0008] To achieve the above objects, the technical solutions adopted by the present invention are as follows:

[0009] On the one hand, the present invention provides a polymer solid electrolyte, which comprises structural unit I and structural unit II, and the structural formula of structural unit I is

[0010] The structural formula of structural unit II is

[0011] Among them, R1, R2, R3, R4, R5, R6, R7, and R8 are each independently selected from H or -SO3Li, and at least one of the groups of R1, R2, R3, R4, R5, R6, R7, and R8 is -SO3Li.

[0012] By introducing structural unit I, the present invention significantly enhances the dissociation ability of lithium ions and helps to construct a stable ion transport channel, thereby greatly improving the ionic conductivity of the electrolyte. At the same time, the main chain rigid structure of structural unit I can remain stable during the long-term use of the battery, effectively preventing the degradation of the electrolyte, thereby ensuring the long-cycle stability of the battery. Further introducing structural unit II significantly optimizes the electrochemical performance of the material. The fluorine side group not only significantly improves the polarity of the material, promotes the dissociation and migration of lithium ions, but also endows the electrolyte with excellent thermal stability and chemical stability. The fluorine atoms play a strong electron-attracting role in the electrolyte, which can effectively improve the chemical stability of the electrolyte and prevent it from having an adverse reaction with the lithium metal electrode, thereby significantly improving the cycle stability of the lithium metal battery.

[0013] Preferably, the molar ratio of structural unit I to structural unit II in the polymer solid electrolyte is (1.5 - 3):1.

[0014] More preferably, the molar ratio of structural unit I to structural unit II in the polymer solid electrolyte is (2 - 2.5):1, such as 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1.

[0015] Preferably, at least one of R1 and R4 is -SO3Li, at least one of R5 and R7 is -SO3Li, and R2, R3, R6, and R8 are each independently selected from H or -SO3Li.

[0016] The second aspect of the present invention provides a preparation method of the polymer solid electrolyte as described above, and the preparation method includes the following steps:

[0017] (1) Dissolve 4,4'-biphenyl ether dicarboxylic acid, tetrafluoroterephthalic acid, and hydrazine sulfate in fuming sulfuric acid, heat for copolymerization reaction to obtain a polymer matrix, wherein the molar ratio of the fed 4,4'-biphenyl ether dicarboxylic acid to tetrafluoroterephthalic acid is (1.5 - 3):1, and the molar amount of hydrazine sulfate is 1.1 - 1.5 times the total molar amount of 4,4'-biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid;

[0018] (2) Immerse the polymer matrix in a lithium hydroxide solution for a neutralization reaction, wash it to neutral after the reaction, and finally dry it to obtain the polymer solid electrolyte.

[0019] Preferably, the molar ratio of the 4,4'-biphenylene ether dicarboxylic acid to the tetrafluoroterephthalic acid is (2 to 2.5):1, such as 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1.

[0020] Preferably, the molar amount of the hydrazine sulfate is 1.2 to 1.4 times the total molar amount of the 4,4'-biphenylene ether dicarboxylic acid and the tetrafluoroterephthalic acid.

[0021] Preferably, the temperature of the copolymerization reaction is 100 to 140 °C, and the time is 2 to 6 h.

[0022] More preferably, the temperature of the copolymerization reaction is 110 to 130 °C, and the time is 3 to 5 h.

[0023] Even more preferably, the temperature of the copolymerization reaction is 115 to 125 °C, and the time is 3.5 to 4.5 h.

[0024] Preferably, the mass ratio of the polymer matrix to the lithium hydroxide in the lithium hydroxide solution is 1:(0.2 to 0.5).

[0025] The third aspect of the present invention is to provide a polymer solid electrolyte membrane, which includes the polymer solid electrolyte as described above.

[0026] Preferably, the polymer solid electrolyte membrane further includes a lithium salt.

[0027] In some embodiments, the lithium salt is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), and lithium hexafluorophosphate (LiPF6).

[0028] Preferably, the mass ratio of the polymer solid electrolyte to the lithium salt in the polymer solid electrolyte membrane is (8 to 10):1.

[0029] The fourth aspect of the present invention provides a method for preparing the polymer solid electrolyte membrane as described above, which includes dissolving the polymer solid electrolyte and the lithium salt in an organic solvent, then coating to form a film, and drying to obtain the polymer solid electrolyte membrane.

[0030] Preferably, the organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, and dimethylformamide.

[0031] Preferably, the polymer solid electrolyte is first dissolved in the organic solvent to form a solution with a mass concentration of 1 to 10%, and then the lithium salt is added.

[0032] Preferably, the feeding mass ratio of the polymer solid electrolyte to the lithium salt is (8-10):1.

[0033] Preferably, the temperature for dissolution is controlled to be 70-90°C, and more preferably 75-85°C.

[0034] Preferably, the temperature for drying is 70-90°C.

[0035] More preferably, the drying time is 40-56h.

[0036] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0037] The polymer solid electrolyte of the present invention has high ionic conductivity and good electrochemical stability, can effectively prevent the formation of lithium dendrites in lithium metal batteries, and thus significantly improves the cycle stability of the battery. In addition, the polymer solid electrolyte can also be well compatible with the lithium metal negative electrode and optimize the interfacial stability of the battery.

[0038] The preparation method of the present invention is simple, easy to operate, and low in cost, and is suitable for mass production. Description of the Drawings

[0039] Figure 1 It is the reaction route diagram of the polymer solid electrolyte related to Example 1 of the present invention;

[0040] Figure 2 It is the infrared spectrum diagram of the polymer solid electrolyte of Example 1 of the present invention;

[0041] Figure 3 It is the linear sweep voltammetry (LSV) curve diagram of the polymer solid electrolyte membrane of Example 1 of the present invention;

[0042] Figure 4 It is the EIS spectrum of the polymer solid electrolyte membrane of Example 1 of the present invention at different temperatures;

[0043] Figure 5 It is the EIS spectrum of the polymer solid electrolyte membrane of Comparative Example 2 of the present invention at different temperatures;

[0044] Figure 6 It is the schematic diagram of the lithium ion activation energy of the polymer solid electrolyte membranes of Example 1 and Comparative Example 2 of the present invention;

[0045] Figure 7 It is the constant current cycle diagram of the Li||Li symmetric battery of the polymer solid electrolyte membranes of Example 1 and Comparative Example 2 of the present invention. Detailed Embodiments

[0046] The present invention will be further described below in conjunction with embodiments. However, the present invention is not limited to the following embodiments. The implementation conditions adopted in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0047] Unless otherwise specified, the reagents and instruments involved in the following text are commercially available products or can be prepared by referring to the existing technical methods.

[0048] In the following text, the dosage of fuming sulfuric acid as the reaction solvent is preferably 600-800% of the total mass of the reaction raw materials (4,4'-biphenyl ether dicarboxylic acid, tetrafluoroterephthalic acid and hydrazine sulfate), or can be referred to the existing technology without special limitation.

[0049] In the following text, the alkali solution is an aqueous lithium hydroxide solution with a mass concentration of 0.05 mol / L, or can be referred to the existing technology without special limitation.

[0050] Example 1

[0051] Mix 4,4'-biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid in a molar ratio of 7:3, then add hydrazine sulfate with a molar amount 1.3 times the total molar amount of 4,4'-biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid, add it to fuming sulfuric acid and heat to 75 °C with stirring to dissolve, then raise the temperature to 120 °C for copolymerization. After reacting for 4 hours, pour the product into deionized water for washing, and add an alkali solution to soak and neutralize the residual sulfuric acid in the polymer. Finally, wash until neutral and dry to obtain SFPOD. The reaction equation of SFPOD refers to Figure 1 , and the infrared spectrum is shown in Figure 2 .

[0052] Dissolve 0.9 g of the above-synthesized SFPOD material in N-methylpyrrolidone to prepare a solution with a mass concentration of 6% (SFPOD has a better solubility in conventional solutions such as N-methylpyrrolidone, dimethyl sulfoxide, and dimethylformamide, and can also be prepared into a higher concentration, such as a 10% concentration solution), add 0.1 g of lithium salt LiTFSI, stir at 80 °C until the lithium salt is completely dissolved, and prepare a solid electrolyte film through a coating process. Place the wet film in a vacuum oven at 80 °C for 48 hours to remove the solvent.

[0053] Comparative Example 1

[0054] Mix tetrafluoroterephthalic acid and hydrazine sulfate in a molar ratio of 1:1.3, add it to fuming sulfuric acid and heat to 75 °C with stirring to dissolve, then raise the temperature to 120 °C for copolymerization. After reacting for 4 hours, pour the product into deionized water for washing, and add an alkali solution to soak and neutralize the residual sulfuric acid in the polymer. Finally, wash until neutral and dry.

[0055] The product was added to N-methylpyrrolidone, but it could not dissolve and was also insoluble in other conventional organic solvents, making it difficult to form a film.

[0056] Comparative Example 2

[0057] 4,4'-Biphenyl ether dicarboxylic acid and hydrazine sulfate were mixed in a molar ratio of 1:1.05, added to fuming sulfuric acid, heated to 75 °C and stirred to dissolve, then heated to 120 °C for copolymerization. After reacting for 4 hours, the product was poured into deionized water for washing, and an alkaline solution was added for soaking to neutralize the residual sulfuric acid in the polymer. Finally, it was washed to neutral and dried to obtain SPOD.

[0058] 0.9 g of the above-synthesized SPOD material was dissolved in N-methylpyrrolidone to prepare a solution with a mass concentration of 6%. 0.1 g of lithium salt LiTFSI was added, and it was stirred at 80 °C until the lithium salt was completely dissolved. A solid electrolyte film was prepared by a coating process, and the wet film was placed in a vacuum oven at 80 °C for 48 hours to remove the solvent.

[0059] Comparative Example 3

[0060] 4,4'-Biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid were mixed in a molar ratio of 1:9, and then hydrazine sulfate with a molar amount 1.3 times the total molar amount of 4,4'-biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid was added. It was added to fuming sulfuric acid, heated to 75 °C and stirred to dissolve, then heated to 120 °C for copolymerization. After reacting for 4 hours, the product was poured into deionized water for washing, and an alkaline solution was added for soaking to neutralize the residual sulfuric acid in the polymer. It was washed to neutral and dried to obtain SFPOD19.

[0061] 0.9 g of the above-synthesized SFPOD19 material was dissolved in N-methylpyrrolidone to prepare a solution with a mass concentration of 6%. 0.1 g of lithium salt LiTFSI was added, and it was stirred at 80 °C until the lithium salt was completely dissolved. A solid electrolyte film was prepared by a coating process, and the wet film was placed in a vacuum oven at 80 °C for 48 hours to remove the solvent.

[0062] Comparative Example 4

[0063] 4,4'-Biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid were mixed in a molar ratio of 5:5, and then hydrazine sulfate with a molar amount 1.3 times the total molar amount of 4,4'-biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid was added. It was added to fuming sulfuric acid, heated to 75 °C and stirred to dissolve, then heated to 120 °C for copolymerization. After reacting for 4 hours, the product was poured into deionized water for washing, and an alkaline solution was added for soaking to neutralize the residual sulfuric acid in the polymer. It was washed to neutral and dried to obtain SFPOD55.

[0064] Dissolve 0.8 g of the synthesized SFPOD55 material above in N-methylpyrrolidone to prepare a solution with a mass concentration of 3% (the solubility of SFPOD55 in conventional organic solvents is not high, so a solution with a higher concentration cannot be prepared). Add 0.1 g of lithium salt LiTFSI and stir at 80 °C until the lithium salt is completely dissolved. Then, prepare a solid electrolyte film through a coating process. Place the wet film in a vacuum oven at 80 °C for 48 hours to remove the solvent. Due to the low solubility of SFPOD55, the surface uniformity of the obtained film is poor.

[0065] Performance testing:

[0066] (1) Place the dried film between a lithium sheet and a stainless-steel sheet and measure the linear sweep voltammetry (LSV) curve. As Figure 3 shown, SFPOD decomposes only at 4.65 V, indicating good electrochemical stability of this polymer.

[0067] (2) Assemble batteries by placing the dried films in the above examples and comparative examples between stainless-steel sheets respectively, and test the EIS spectra at different temperatures (test conditions: voltage amplitude 10 mV, frequency range 100 kHz to 0.01 Hz, temperature range 10 °C to 70 °C, heating step 10 °C).

[0068] As Figure 4 、 Figure 6 shown, through fitting calculation, the lithium-ion activation energy of SFPOD in Example 1 is only 0.1077 eV, and the ionic conductivity at 30 °C reaches 1.25×10 -4 S cm -1 , proving the superiority of the SFPOD structure for lithium-ion transport. As Figure 5 、 Figure 6 shown, the lithium-ion activation energy of SPOD in Comparative Example 2 is 0.1412 eV, greater than that of SFPOD; the lithium-ion conductivity at 30 °C is only 6.98×10 -5 S cm -1 , far lower than that of SFPOD. This shows that when the polymer has both structural unit I and structural unit II, it can have better ionic conductivity.

[0069] Furthermore, the lithium-ion conductivity of SFPOD19 in Comparative Example 3 at 30 °C is 8.66×10 -5 S cm-1, and the lithium-ion conductivity of SFPOD55 in Comparative Example 4 at 30 °C is 3.58×10 -5S cm-1 are not high and are much lower than those in Example 1 of the present invention. It shows that when the polymer has both structural unit I and structural unit II, if the amount of structural unit I is too small or too large, it will have an adverse effect on the ionic conductivity of the polymer. By optimizing the ratio of 4,4'-biphenyl ether dicarboxylic acid and tetrafluoroterephthalic acid, the present invention optimizes the ratio of structural unit I and structural unit II, which is conducive to achieving the balance between the segmental motion and the content of electronegative groups, thereby being beneficial to improving the conductivity of the solid electrolyte.

[0070] (3) The dried film was subjected to a constant current cycle (1 mA cm -2 ; 1 mAh cm -2 ) test of Li||Li symmetric battery as Figure 7 shown. The polarization voltage of SFPOD is 65 mV, and the polarization voltage of SPOD is 127 mV. As for the films in Comparative Example 3 and Comparative Example 4, since their lithium ion conductivities are much lower than that in Example 1, it can be predicted that their polarization voltages are higher than that in Example 1. It can be seen that the interface contact between the polymer solid electrolyte film of Example 1 of the present invention and the lithium metal electrode is better, the interfacial side reactions are fewer, and the battery can maintain stable performance during long-term use.

[0071] The above has described the present invention in detail. The purpose is to enable those skilled in this field to understand the content of the present invention and implement it. However, it should not be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A polymer solid electrolyte, characterized in that: The polymer solid electrolyte includes structural unit I and structural unit II, and the structural formula of the structural unit I is The structural formula of the structural unit II is Wherein, R1, R2, R3, R4, R5, R6, R7, and R8 are each independently selected from H or -SO3Li, and at least one of the groups R1, R2, R3, R4, R5, R6, R7, and R8 is -SO3Li.

2. The polymer solid electrolyte according to claim 1, wherein: The molar ratio of structural unit I to structural unit II in the polymer solid electrolyte is (1.5 - 3):

1.

3. The polymer solid electrolyte according to claim 2, wherein: The molar ratio of structural unit I to structural unit II in the polymer solid electrolyte is (2 - 2.5):

1.

4. The polymer solid electrolyte according to claim 1, wherein: At least one of R1 and R4 is -SO3Li, at least one of R5 and R7 is -SO3Li, and R2, R3, R6, and R8 are each independently selected from H or -SO3Li.

5. The preparation method of the polymer solid electrolyte according to any one of claims 1 to 4, characterized in that: The preparation method includes the following steps: (1) Dissolve 4,4'-biphenyl ether dicarboxylic acid, tetrafluoro terephthalic acid, and hydrazine sulfate in fuming sulfuric acid, and heat for copolymerization reaction to obtain a polymer matrix. Among them, the molar ratio of the charged 4,4'-biphenyl ether dicarboxylic acid to tetrafluoro terephthalic acid is (1.5 - 3):1, and the molar amount of hydrazine sulfate is 1.1 - 1.5 times the total molar amount of 4,4'-biphenyl ether dicarboxylic acid and tetrafluoro terephthalic acid; (2) Immerse the polymer matrix in a lithium hydroxide solution for neutralization reaction. After the reaction ends, wash it to neutral, and then dry it to obtain the polymer solid electrolyte.

6. The preparation method of the polymer solid electrolyte according to claim 5, characterized in that: The temperature of the copolymerization reaction is 100 - 140 °C, and the time is 2 - 6 h.

7. The preparation method of the polymer solid electrolyte according to claim 5, wherein: The feeding mass ratio of the polymer matrix to lithium hydroxide in the lithium hydroxide solution is 1:(0.2 - 0.5).

8. A polymer solid electrolyte membrane, characterized in that: The polymer solid electrolyte membrane includes the polymer solid electrolyte according to any one of claims 1 to 4 or the polymer solid electrolyte prepared by the preparation method according to any one of claims 5 to 7.

9. The polymer solid electrolyte membrane according to claim 8, characterized in that: The polymer solid electrolyte membrane further includes a lithium salt.

10. The method for preparing a polymer solid electrolyte membrane according to claim 8 or 9, characterized in that: The preparation method includes dissolving the polymer solid electrolyte and the lithium salt in an organic solvent, then coating to form a film, and drying to obtain the polymer solid electrolyte membrane.

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