A modified composite solid electrolyte film and preparation method thereof

Abstract

The present invention provides a modified composite solid electrolyte film having an electrolyte coating provided on its surface; the composite solid electrolyte film comprises a polymer microporous membrane having a loofah-like pore structure, wherein the loofah-like pores of the polymer microporous membrane are filled with a solid electrolyte. By modifying the composite solid electrolyte film through surface coating, the present invention solves the contact issues caused by high filling of the solid electrolyte in the polymer network. While maintaining high-temperature stability and shrinkage, it successfully reduces battery impedance and significantly improves ionic conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a composite modified composite solid electrolyte film and a preparation method thereof. Background Art

[0002] Traditional lithium batteries use organic solvents with low flash points, which are prone to leakage and even combustion and explosion when impacted. This has greatly limited the development of lithium batteries. To address the safety issues of traditional lithium batteries, the industry has gradually increased its attention to solid-state electrolytes, which can significantly suppress thermal runaway in lithium batteries and have relatively excellent safety performance. Compared with liquid batteries, the interface between the electrode and the electrolyte in solid-state batteries changes from a solid / liquid interface to a solid / solid interface. The solid / solid interface has a higher contact resistance, resulting in slow ion transfer rates and affecting the battery's electrochemical performance.

[0003] In order to improve the problems of low ionic conductivity and narrow electrochemical window in solid electrolytes, some technologies use polymers and inorganic electrolytes to prepare organic-inorganic composite electrolyte materials, so as to improve the interface stability and physical contact of solid particle paper through polymers. However, it is found that after the polymer is compounded with the inorganic solid electrolyte, there are often problems such as large pores in the polymer matrix, poor strength, low filling rate of inorganic solid electrolyte particles, and large system shrinkage, and the electrochemical performance of the material is still not effectively improved. At the same time, at an ideal particle filling rate, the contact ability between the organic-inorganic composite electrolyte material and the electrode is greatly reduced, and the gaps or gaps at the contact position will have an adverse effect on the long-life performance of the battery. Therefore, while maintaining the high ion conductivity of the organic-inorganic composite electrolyte material, it is necessary to further consider how to improve its contact with the electrode. Summary of the Invention

[0004] In response to the deficiencies in the prior art, the present invention provides a modified composite solid electrolyte film, which adopts a polymer network with a continuous loofah-like pore structure to uniformly distribute the solid electrolyte in the polymer network, thereby improving the ionic conductivity and high-temperature shrinkage rate of the composite solid electrolyte. At the same time, the composite solid electrolyte film is further modified by surface coating, which solves the contact problem caused by high filling of the solid electrolyte in the polymer network, successfully reduces the battery impedance and significantly improves the ionic conductivity.

[0005] Specifically, the modified composite solid electrolyte film provided by the present invention has an electrolyte coating on the surface of the composite solid electrolyte film; the composite solid electrolyte film includes a polymer microporous membrane with a loofah-like pore structure, and the loofah-like pores of the polymer microporous membrane are filled with a solid electrolyte.

[0006] As an optional optimization scheme, the porosity of the composite solid electrolyte film is less than 15%, preferably less than 10%; the content of solid electrolyte is greater than 30%, preferably greater than 50%; the thickness is 2 to 50 μm, preferably 5 to 15 μm; the major diameter of the loofah-shaped pores is 0.1 to 5 μm, preferably 0.1 to 2.5 μm.

[0007] As an optional optimization solution, the polymer microporous membrane can be made of common polymer materials such as polyethylene, polypropylene, polyvinyl chloride, etc., and the solid electrolyte is an oxide solid electrolyte LiTi2(PO4)3, Li 0.5 La 0.5 TiO3, or sulfide solid electrolyte Li 10 GeP2S 12 、Li 10 SnP2S 12 .

[0008] As an optional optimization scheme, the composite solid electrolyte film is made of a fillable polymer microporous membrane and a solid electrolyte slurry; the porosity of the fillable polymer microporous membrane is 30% to 50%, preferably 35% to 45%; the solid content of the solid electrolyte slurry is 20% to 80%, preferably 30% to 50%.

[0009] As an optional optimization solution, the solid electrolyte slurry further comprises additives and solvents, wherein the additives include one or more of a dispersant, a silane coupling agent, a thickener, and a binder, with a content of 0 to 30%, preferably 0.5 to 5%.

[0010] As an optional optimization scheme, the electrolyte coating includes an outer solid electrolyte and an outer binder; the outer solid electrolyte is an oxide solid electrolyte LiTi2(PO4)3, Li 0.5 La 0.5 TiO3, or sulfide solid electrolyte Li 10 GeP2S 12 、Li 10 SnP2S 12 ; The outer layer adhesive is PVDF.

[0011] As an optional optimization solution, the thickness of the electrolyte coating is 2 to 15 μm.

[0012] As an optional optimization scheme, the mass ratio of the outer layer solid electrolyte to the outer layer binder is generally (1-20):1, preferably (1-10):1, and most preferably (9±1):1.

[0013] As an optional optimization solution, the average particle size of the outer layer solid electrolyte is generally 50-500 nm, preferably 50-300 nm, and most preferably 200±50 nm.

[0014] As an optional optimization solution, the solid content of the electrolyte coating slurry is generally 30 wt.%-80 wt.%, preferably 40 wt.%-60 wt.%, and most preferably (50±5) wt.%.

[0015] The present invention also provides a method for preparing the modified composite solid electrolyte film. Under a protective atmosphere, the electrolyte coating slurry is coated on the surface of the composite solid electrolyte film, and the electrolyte coating slurry is dried to form an electrolyte coating.

[0016] As an optional optimization solution, the preparation of the electrolyte coating slurry includes the steps of adding an outer layer solid electrolyte and an outer layer binder into NMP, an oily solution or a suspension and mixing them uniformly.

[0017] As an optional optimization solution, the drying temperature of the electrolyte coating slurry is above 60° C. and the drying time is above 4 h.

[0018] The present invention also provides a battery comprising the modified composite solid electrolyte film.

[0019] The present invention first provides a composite electrolyte material in which a solid electrolyte is fully filled in a continuous loofah-shaped pore structure. Through the high filling and shrinkage control of the solid electrolyte, the contact of the solid electrolyte in the polymer microporous membrane is promoted, the strength of the overall material is effectively improved, and the ionic conductivity and high-temperature performance are preliminarily improved. Furthermore, through the surface modification of the electrolyte coating, the contact problem of the composite electrolyte material is optimized and improved, the battery impedance is greatly reduced, and the ion conductivity capacity is fully utilized. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The technical solutions of the embodiments of the present invention are further described in detail below through the accompanying drawings and examples.

[0021] Figure 1 The microscopic morphology of a polyethylene microporous membrane according to an embodiment.

[0022] Figure 2 The microscopic morphology of a composite solid electrolyte film according to an embodiment. DETAILED DESCRIPTION

[0023] To facilitate understanding of the present invention, the present invention will be described in more detail below, with preferred embodiments of the present invention provided. However, it should be understood that these embodiments are merely for the purpose of further explanation and are not to be construed as limiting the present invention in any form, i.e., they are not intended to limit the scope of protection of the present invention.

[0024] The present invention first provides a composite solid electrolyte film comprising a polymer microporous membrane having a loofah-like pore structure, wherein the loofah-like pores of the polymer microporous membrane are filled with a solid electrolyte. Specifically, the loofah-like pore structure comprises continuous, open primary pores with substantially uniform orientation within the polymer film, and smaller secondary micropores on the pore walls between the primary pores, enabling partial lateral connection between adjacent primary pores.

[0025] The composite solid electrolyte film has a porosity of less than 15%, preferably less than 10%; a solid electrolyte content of greater than 30%, preferably greater than 50%; a thickness of 2 to 50 μm, preferably 5 to 15 μm; and a major diameter of the sponge-like channels of 0.1 to 5 μm, preferably 0.1 to 2.5 μm. A suitable major diameter of the sponge-like channels facilitates sufficient entry of the solid electrolyte and sufficient volatilization of the solvent during the preparation process, ensuring sufficient penetration of the solid electrolyte and thus good electrical performance. A suitable total film thickness maintains an orderly distribution of the sponge-like channels.

[0026] The polymer microporous membrane can be made of common polymer materials such as polyethylene, polypropylene, polyvinyl chloride, etc., and the solid electrolyte can be an oxide solid electrolyte such as LiTi2(PO4)3, Li 0.5 La 0.5 TiO3, etc., or sulfide solid electrolytes such as Li 10 GeP2S 12 、Li 10 SnP2S 12 And other commonly used solid electrolyte materials.

[0027] The polymer microporous membrane used for solid electrolyte filling has a porosity before filling controlled at 30% to 50%, preferably 35% to 45%. The solid electrolyte used for filling is generally in the form of a slurry with a solid content of 20% to 80%, preferably 30% to 50%. The optimized porosity of the polymer microporous membrane before filling is beneficial to the full filling of the solid electrolyte slurry on the one hand, and on the other hand, it can prevent the film from having too low a strength in the later stage due to excessive porosity, and from shrinking too much due to large residual pores after filling. The optimized solid content of the solid electrolyte slurry also ensures filling while avoiding excessive solvent, which causes large residual pores after volatilization and shrinkage too much. The present invention combines the comprehensive deployment of multi-stage means for the control of shrinkage, including the above-mentioned polymer microporous membrane porosity before filling and the solid content of the solid electrolyte slurry. At the same time, a rolling step is added during the preparation process, which further helps maintain the appropriate final porosity of the product through rolling, thereby achieving the purpose of controlling the high-temperature shrinkage of the final product.

[0028] The solid electrolyte slurry also contains additives and solvents. The types of additives include many common conventional options, such as dispersants, silane coupling agents, thickeners, binders, etc., with a content of 0 to 30%, preferably 0.5 to 5%, which can be mixed and compounded according to specific needs. The balance in the solid electrolyte slurry is solvent, which includes water and non-aqueous organic solvents. The selected solvent can coexist stably with the solid electrolyte.

[0029] According to tests, the shrinkage rate of the composite solid electrolyte film of the present invention at 110° C. is 0.45% to 0.75%, and the tensile strength of the polymer microporous membrane is above 950 MPa.

[0030] The preparation method of the above-mentioned composite solid electrolyte film includes: blending a polymer raw material with paraffin wax with a concentration of 10-50% at 150±10°C, forming a film by casting, extracting the paraffin wax with an organic solvent, and after removing the organic solvent, biaxially stretching the film at 120±10°C with a stretching ratio of (2±0.5):1, heat setting to form a polymer microporous membrane with a target thickness, porosity, and loofah-shaped pore structure, and then filling the polymer microporous membrane with a solid electrolyte slurry, first drying it at 80±10°C for 0.5-4h, and then drying it at 110±5°C for 0.5-4h, and rolling it to shrink it by 20%-60% to obtain a composite solid electrolyte film product.

[0031] For experimental verification, we prepared a series of implementation examples 1-9 by the following method for performance testing:

[0032] (1) Polyethylene resin was mixed with 30-45% paraffin wax at a high temperature of 150°C, extruded through an extruder, cast into a film, soaked in a 20°C petroleum ether solvent tank for 45 minutes, and biaxially stretched to prepare a polyethylene microporous membrane with a stretch ratio of 2:1.

[0033] (2) Preparation of solid electrolyte slurry: Add LiTi2(PO4)3 powder with a solid content of 30-50% to 1 kg of water, stir at 3000 rpm for 1 hour, add 1-5% by mass of sodium carboxymethyl cellulose, and stir to obtain a uniformly mixed solid electrolyte slurry.

[0034] (3) Immerse the microporous membrane in the solid electrolyte slurry and vacuum soak for 2 hours.

[0035] (4) Bake at 80°C for 2 h and then at 110°C for 2 h to obtain a porous composite solid electrolyte base membrane.

[0036] (5) The porous composite solid electrolyte base film is rolled to a volume shrinkage rate of 30% to 50% to obtain a composite solid electrolyte film.

[0037] (6) The relevant properties of the microporous membrane and the composite solid electrolyte film were tested, and the results are shown in Table 1 and Figure 1 shown.

[0038] Table 1 Performance test results of polymer microporous membrane and composite solid electrolyte film

[0039]

[0040] Figure 1 The microscopic morphology of the polyethylene microporous membrane of the present invention is shown, showing that it has interconnected loofah-shaped primary pore structures, and secondary micropores are distributed on the walls of the primary pore structure, confirming the pore structure characteristics of the product obtained by the present invention. Figure 2 The microscopic morphology of the composite solid electrolyte film obtained by the present invention shows that the solid electrolyte (LiTi2(PO4)3 powder) can be continuously filled in the loofah-shaped pores within the microporous membrane, forming a composite material with polyethylene with controllable porosity and shrinkage, and can also form continuous lithium ion transmission channels. The easy filling of the primary pores and the easy accessibility of the secondary pores enable the matrix to provide greater system strength while also ensuring a balanced filling and shrinkage.

[0041] Electrical performance testing was conducted using a button-type half-cell. The prepared composite solid electrolyte film was assembled into a stainless steel-stainless steel symmetrical button-type all-solid-state battery, using lithium iron phosphate (LFP:PVDF:SP=8:1:1) as the positive electrode and lithium metal as the negative electrode. Cycle life testing at 70°C involved charging and discharging at a rate of 0.01C at 70°C. The total charge and discharge time before a short circuit occurred was used to determine the cycle life at 70°C.

[0042] Combine Figure 1 The performance test results of the examples in Table 1 demonstrate that the present invention fabricates a polyethylene microporous membrane with a specific secondary pore structure through biaxial stretching and incorporates a high proportion of solid electrolyte into the membrane, enabling sufficient contact between the solid electrolyte particles, preliminarily improving ion conductivity while effectively controlling shrinkage. Furthermore, the ionic conductivity and shrinkage of the polyethylene microporous membrane can be improved by optimizing the film-forming agent ratio, adjusting the electrolyte solids content, controlling the binder addition, and controlling roller shrinkage.

[0043] Although the composite solid electrolyte film prepared as described above has good control over its shrinkage and significantly improved its cycle and high-temperature performance, the improvement in ionic conductivity is still limited. The high filling rate and controlled shrinkage rate of the solid electrolyte obviously do not fully utilize the ion transport advantages brought about by the sufficient contact between its particles. The ionic conductivity should have greater room for improvement. We found that the reason for the limited ionic conductivity may be due to the low contact ability of the composite solid electrolyte film. Polyethylene, as a polymer, has good composite contact properties. However, due to the high filling of the solid electrolyte and the introduction of the two-level pore structure of polyethylene after stretching, the contact performance of the polyethylene microporous membrane after filling with the solid electrolyte is greatly reduced, especially the increase in solid-solid point contact with other ion-conducting structures such as electrode materials, and the increase in voids or gaps, resulting in increased battery impedance and decreased cycle performance.

[0044] We tried to improve the contact performance of the composite solid electrolyte film by adding an electrolyte coating on the surface while balancing the ion transmission capacity.

[0045] The electrolyte coating is provided on at least one side of the composite solid electrolyte film in a coating manner. The electrolyte coating comprises an outer layer solid electrolyte and an outer layer binder. The material types of the outer layer solid electrolyte and the outer layer binder can be the same as or different from the solid electrolyte and binder used in the composite solid electrolyte film. For the sake of clarity, the solid electrolyte and binder used in the electrolyte coating are distinguished by adding "outer layer" before them. The outer layer solid electrolyte can also be an oxide solid electrolyte such as LiTi2(PO4)3, Li 0.5 La 0.5 TiO3, etc., or sulfide solid electrolytes such as Li 10 GeP2S 12 、Li 10 SnP2S 12 etc.; the outer layer binder can be PVDF etc.

[0046] We further prepared a series of modified composite solid electrolyte films based on the above-mentioned Example 8 with the best comprehensive performance by coating a layer of electrolyte coating on its surface for surface modification, and conducted performance testing and verification.

[0047] Specific modification methods include:

[0048] (1) Preparation of electrolyte coating slurry: Under argon atmosphere, PVDF powder and nano solid electrolyte Li 10 GeP2S 12 Add to NMP and mix at 5000 rpm.

[0049] (2) Preparation of modified composite solid electrolyte film: In an argon atmosphere, the electrolyte coating slurry is applied to the substrate of the composite solid electrolyte film using a scraper, and the coated sample is dried at 80°C for 5 hours. The coating thickness can be adjusted as needed.

[0050] The battery assembly and electrical performance test methods are the same as above, and our test results are listed in Table 2.

[0051] Table 2 Performance improvement of composite solid electrolyte film by electrolyte coating

[0052]

[0053] As can be seen from Table 2, compared with Example 8, the addition of the electrolyte coating in Examples 10-16 improves the contact performance of the composite solid electrolyte film to varying degrees. Since the coating layer promotes contact with the electrode material, the battery impedance is greatly reduced, the ionic conductivity is further improved, and the cycle capacity retention rate is also significantly improved.

[0054] In further optimization of the electrolyte coating, we found that appropriate control of the slurry solid content, outer layer solid electrolyte content, and particle size can improve the overall performance.

[0055] Change Li 10 GeP2S 12 The ratio of Li to PVDF has a great influence on the ionic conductivity and impedance of the composite solid electrolyte. For example, in Examples 10-12, when the content of the binder PVDF is too low, the Li 10 GeP2S 12 The particles will agglomerate, and the surface of the coating will fall off in powder form when twisted lightly, which will affect the contact effect between the electrolyte and the electrode. In addition, the electrolyte structure will be unstable during the cycle, causing the overall impedance of the battery to increase and the capacity retention rate to decrease. When the content of the binder PVDF is too high, the Li 10 GeP2S 12 If the content is too low, the electrolyte will have fewer ion transport channels, resulting in a decrease in ionic conductivity. Therefore, the mass ratio of the outer layer solid electrolyte to the outer layer binder is generally (1-20):1, preferably (1-10):1, and most preferably (9±1):1.

[0056] The particle size of the outer layer solid electrolyte can also affect the overall performance of the modified composite solid electrolyte film. 10 GeP2S 12As the particle size of the solid electrolyte increases, more voids or gaps will form inside the coating, the battery impedance will increase and the capacity retention rate will decrease, the constructed ion transmission channel will be reduced, and the battery performance will be affected. However, when the particle size of the solid electrolyte is too small, Li 10 GeP2S 12 The particles will agglomerate, resulting in poor contact between the coating and the composite electrolyte and high impedance. Therefore, the average particle size of the outer layer solid electrolyte is generally 50-500nm, preferably 50-300nm, and most preferably 200±50nm.

[0057] The solid content of the coating slurry will also affect the performance of the modified composite solid electrolyte film. According to Examples 13, 15, and 16, when the solid content of the coating slurry is higher, the slurry viscosity increases, and its contact with the composite solid electrolyte is poor, resulting in a larger battery impedance and a lower battery capacity retention rate. When the solid content of the coating slurry is lower, more holes are formed when the slurry is dried, and there are fewer ion transmission channels, which will still cause a larger battery impedance and a lower battery capacity retention rate. Therefore, the solid content of the coating slurry is generally 30wt.%-80wt.%, preferably 40wt.%-60wt.%, and most preferably (50±5)wt.%.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A modified composite solid electrolyte film, comprising an electrolyte coating provided on the surface of the composite solid electrolyte film; the composite solid electrolyte film comprises a polymer microporous membrane having a sponge gourd-like pore structure, wherein the sponge gourd-like pores of the polymer microporous membrane are filled with a solid electrolyte; Specifically, the sponge gourd-like pore structure comprises continuous open primary pores with substantially uniform orientation in the polymer microporous membrane, and smaller secondary micropores on the pore walls between the primary pores, which enable local lateral communication between adjacent primary pores. The major diameter of the loofah-shaped channel is 0.1-5 μm.

2. The modified composite solid electrolyte film according to claim 1, characterized in that The composite solid electrolyte film has a porosity of less than 15%, a solid electrolyte content of more than 30%, a thickness of 2 to 50 μm, and a major diameter of the loofah-shaped pores of 0.1 to 2.5 μm.

3. The modified composite solid electrolyte film according to claim 1, characterized in that The composite solid electrolyte film has a porosity of less than 10%, a solid electrolyte content of more than 50%, and a thickness of 5 to 15 μm.

4. The modified composite solid electrolyte film according to claim 1, characterized in that The major diameter of the loofah-shaped channels is 0.1~2.5μm.

5. The modified composite solid electrolyte film according to claim 1, characterized in that The polymer microporous membrane is made of polyethylene, polypropylene, or polyvinyl chloride, and the solid electrolyte is an oxide solid electrolyte or a sulfide solid electrolyte.

6. The modified composite solid electrolyte film according to claim 5, characterized in that: The oxide solid electrolyte is LiTi2(PO4)3, Li 0.5 La 0.5 One of TiO3, the sulfide solid electrolyte is Li 10 GeP2S 12 、Li 10 SnP2S 12 One of them.

7. The modified composite solid electrolyte film according to claim 1, characterized in that The electrolyte coating comprises an outer layer of solid electrolyte and an outer layer of binder.

8. The modified composite solid electrolyte membrane according to claim 7, characterized in that: The outer layer solid electrolyte is an oxide solid electrolyte or a sulfide solid electrolyte.

9. The modified composite solid electrolyte membrane according to claim 8, characterized in that The oxide solid electrolyte is LiTi2(PO4)3, Li 0.5 La 0.5 One of TiO3, the sulfide solid electrolyte is Li 10 GeP2S 12 、Li 10 SnP2S 12 One of them.

10. The modified composite solid electrolyte membrane according to claim 7, characterized in that: The outer layer binder is PVDF.

11. The modified composite solid electrolyte film according to claim 1, characterized in that: The thickness of the electrolyte coating is 2~15 μm.

12. The modified composite solid electrolyte film according to claim 7, characterized in that: The mass ratio of the outer layer solid electrolyte to the outer layer binder is (1-20):

1.

13. The modified composite solid electrolyte film according to claim 7, characterized in that: The mass ratio of the outer layer solid electrolyte to the outer layer binder is (1-10):

1.

14. The modified composite solid electrolyte membrane according to claim 7, characterized in that: The mass ratio of the outer layer solid electrolyte to the outer layer binder is (9±1):

1.

15. The modified composite solid electrolyte membrane according to claim 7, characterized in that: The average particle size of the outer layer solid electrolyte is 50-500 nm.

16. The modified composite solid electrolyte membrane according to claim 7, characterized in that: The average particle size of the outer layer solid electrolyte is 50-300 nm.

17. The modified composite solid electrolyte membrane according to claim 7, characterized in that: The average particle size of the outer layer solid electrolyte is 200±50nm.

18. The modified composite solid electrolyte membrane according to claim 1, characterized in that: The solid content of the electrolyte coating slurry is 30 wt.%-80 wt.%.

19. The modified composite solid electrolyte membrane according to claim 1, characterized in that: The solid content of the electrolyte coating slurry is 40 wt.%-60 wt.%.

20. The modified composite solid electrolyte membrane according to claim 1, characterized in that The solid content of the electrolyte coating slurry is 45 wt.%-55 wt.%.

21. The method for preparing the modified composite solid electrolyte film according to any one of claims 1 to 20, wherein the electrolyte coating slurry is applied to the surface of the composite solid electrolyte film under a protective atmosphere, and the electrolyte coating slurry is dried to form the electrolyte coating.

22. The method for preparing a modified composite solid electrolyte film according to claim 21, wherein: The preparation of the electrolyte coating slurry includes: uniformly mixing the outer layer solid electrolyte and the outer layer binder to form a slurry; The drying temperature of the electrolyte coating slurry is above 60° C. and the drying time is above 4 hours.

23. A battery comprising the modified composite solid electrolyte film according to any one of claims 1 to 20.

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