Ionic liquid-loaded mof-based composite solid electrolyte, method for preparing the same, and use thereof

Abstract

The present application relates to the field of solid electrolyte, aiming at the problem of poor cycle performance of existing MOF and polymer composite solid electrolyte battery, and provides a MOF-based composite solid electrolyte loaded with ionic liquid, a preparation method and application thereof, the MOF-based composite solid electrolyte loaded with ionic liquid is characterized in that it is IL@MOF-TPU, wherein IL is 1-ethyl-3-methyl imidazoline bis(trifluoromethylsulfonyl) imide, and MOF is Ni-MOF-74.The present application uses the strategy of adsorbing ionic liquid by Ni-MOF with open metal sites, the MOF with open metal sites can generate binding force with lithium ionic liquid, so that the ionic liquid is adsorbed in the pore volume of MOF, and then TPU is added to prepare a high-performance Ni-MOF-based composite solid electrolyte loaded with ionic liquid.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of solid-state electrolytes, in particular to MOF-based composite solid-state electrolytes loaded with ionic liquids and preparation methods and applications thereof. BACKGROUND

[0002] Lithium-air battery is a kind of battery using lithium as negative electrode and oxygen in the air as positive electrode reactant. Lithium-air battery has higher energy density than lithium ion battery because its cathode (mainly porous carbon) is very light and oxygen is obtained from the environment without being stored in the battery. The energy storage of lithium-air battery is 10 times that of current lithium ion battery and has lighter mass. Most of the current lithium-air battery research is based on traditional organic system electrolyte, including ether, carbonate and amine, etc. Organic system electrolyte has certain advantages in ionic conductivity and lithium ion transference number, but organic electrolyte has stability and safety problems. At the same time, lithium anode will also form lithium dendrites during charging, which will pierce the separator and cause battery short circuit, leading to the risk of battery combustion or even explosion. Therefore, in view of the stability and safety problems of organic electrolyte in lithium-air battery and the protection of lithium anode, it is urgent to replace the currently used organic electrolyte with reliable and safe electrolyte.

[0003] Metal-organic framework (MOFs) can provide ordered ion transport channels in theory due to its adjustable porous structure and periodic crystal structure, thereby improving lithium ion transport performance, preventing sudden short circuit of battery and reducing capacity loss during charge and discharge cycles. MOFs have become a new type of solid-state electrolyte material in lithium ion battery and attracted the keen attention of many researchers. Open metal sites (OMSs) of MOFs can act as active sites for chemical reactions, and these active sites can be weakened by chemical fixation to reduce undesirable ion shuttling while continuing to support diffusion of small lithium ions. Generally, open metal sites in MOFs can complex with anions in its pores, thereby releasing flowing lithium ions and providing high lithium ion conduction ability.

[0004] The patent with publication number CN108878970A provides a preparation strategy of a composite solid electrolyte by absorbing lithium-containing ionic liquid into a metal organic framework and then mixing with polyoxyethylene. However, the patent uses polyethylene oxide as the polymer, which will react with lithium peroxide, a product in lithium-air batteries, and thus cause the failure of polyethylene oxide. The patent with publication number CN111180790A provides a polymer electrolyte, a preparation method thereof and a solid-state lithium-air battery. The solid electrolyte is obtained by absorbing lithium-containing ionic liquid into a metal organic framework and then mixing with a polyether type polyurethane elastomer. The soft segment and hard segment of the polyether type polyurethane elastomer are used to improve the performance of the polymer electrolyte. However, the metal organic framework, as a porous material, cannot generate binding force with the absorbed lithium-containing ionic liquid, resulting in leakage of the absorbed lithium ion liquid solution and failure to fully exert the performance improvement of the lithium ion liquid on the battery. Accordingly, there is a need for an ideal solution. SUMMARY

[0005] To overcome the problem of poor cycle performance of the existing MOF and polymer composite solid electrolyte battery, the present application provides a MOF-based composite solid electrolyte loaded with ionic liquid and a preparation method and application thereof. The strategy of using Ni-MOF with open metal sites to adsorb ionic liquid is used. The MOF with open metal sites can generate binding force with lithium ionic liquid, so that the ionic liquid is adsorbed in the pore volume of the MOF, and then TPU is added to prepare a high-performance Ni-MOF-based composite solid electrolyte loaded with ionic liquid.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] The MOF-based composite solid electrolyte loaded with ionic liquid is IL@MOF-TPU, wherein IL is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, and MOF is Ni-MOF-74.

[0008] As a preferred embodiment, the amount of IL@MOF accounts for 20-70% of the mass of TPU.

[0009] The present application also provides a preparation method of the MOF-based composite solid electrolyte loaded with ionic liquid, comprising the following steps:

[0010] a. Synthesis of IL@MOF: Dissolve 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide in anhydrous ethanol, uniformly disperse, and add high-temperature activated Ni-MOF-74 in batches, stir for 22-26h, centrifuge the reaction solution, wash with ethanol, and dry to obtain IL@MOF;

[0011] b.Preparation of IL@MOF-TPU composite solid electrolyte: IL@MOF is dissolved in chloroform and dispersed uniformly, and TPU is added in batches to form a film forming solution under ultrasonic stirring. After the solvent is volatilized, the formed film is taken out and vacuum dried at 70-90 DEG C to obtain IL@MOF-TPU composite solid electrolyte.

[0012] The Ni-MOF-74 with a three-dimensional porous pillar supported layer skeleton structure is induced to form an open metal stable point at high temperature, then soaked in an ionic liquid (IL), and fully wetted to adsorb sufficient ionic liquid. Centrifugation obtains an IL@MOF material loaded with ionic liquid. The IL@MOF is added as an additive to a polyether type polyurethane elastomer (TPU) to prepare a composite solid electrolyte material.

[0013] Preferably, the mass ratio of 1-ethyl-3-methyl imidazoline bis(trifluoromethylsulfonyl) imide to Ni-MOF-74 in step a is 1:(1-3).

[0014] Preferably, the preparation method of the Ni-MOF-74 in step a is as follows: 1,3,5-benzene tricarboxylic acid and 4,4'-bipyridine are used as ligands, and Ni(NO3)2·6H2O (molar ratio of the three is 1:1:1-1:1:3) is dissolved in DMF to perform a solvothermal reaction. After the reaction is completed, the reaction solution is centrifuged, and the obtained solid is sequentially washed and replaced with DMF and anhydrous methanol. After drying, the Ni-MOF-74 is obtained by vacuum activation at 60-120 DEG C. The Ni-MOF-74 with a three-dimensional porous pillar supported layer skeleton structure is synthesized by using nickel as a metal ion, using 1,3,5-benzene tricarboxylic acid (H3BTC) and 4,4'-bipyridine (4,4'-bipy) as ligands, and using a solvothermal method. Then, the Ni-MOF-74 is induced to form an open metal stable point at high temperature.

[0015] Preferably, the mass concentration of the total amount of IL@MOF and TPU in chloroform in step b is 3-8wt%.

[0016] The application also provides a use of the MOF-based composite solid electrolyte loaded with ionic liquid in a lithium air battery. The use of the MOF-based composite solid electrolyte loaded with ionic liquid in a lithium air battery can significantly improve the cycle stability of the lithium air battery.

[0017] Therefore, the application has the following advantages: (1) the open metal sites in MOFs can complex with the anions in the ionic liquid in the pores of the MOFs, thereby releasing mobile lithium ions and providing high lithium ion conductivity; (2) the MOFs enable the ionic liquid to exist in the pores of the MOFs through interaction with the ionic liquid, and the active sites of the ionic liquid can be well exposed, so that the performance of the ionic liquid is fully utilized while the problem of large viscosity and easy leakage of the ionic liquid is avoided; (3) the porous nature of the MOFs is utilized to fully adsorb 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide through the infiltration method, thereby further improving the lithium ion conductivity of the solid-state electrolyte; (4) the IL@MOF is added to the TPU, and the TPU is used as a matrix, so that the composite solid-state electrolyte has good flexibility and excellent mechanical properties, and the function of the IL@MOF in improving the lithium ion conductivity of the composite solid-state electrolyte is fully utilized. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is the XRD test diagram of Ni-MOF-74, IL@Ni-MOF-74, TPU and IL@Ni-MOF-74 / TPU in Example 1;

[0019] Figure 2 is the scanning electron microscope diagram of Ni-MOF-74, IL@Ni-MOF-74 in Example 1;

[0020] Figure 3 is the N2 adsorption isotherm measured at 77K of Ni-MOF-74, IL@Ni-MOF-74 in Example 1;

[0021] Figure 4 is the discharge capacity diagram of the lithium-air battery assembled by using two different electrolytes of TPU and IL@Ni-MOF-74 / TPU in Example 1. DETAILED DESCRIPTION

[0022] The technical solutions of the application will be further described below through specific examples.

[0023] In the application, unless otherwise specified, the raw materials and equipment used can be purchased from the market or are commonly used in the art, and the methods in the examples are conventional methods in the art, unless otherwise specified.

[0024] Example 1

[0025] A preparation method of a MOF-based composite solid-state electrolyte loaded with ionic liquid, comprising the following steps:

[0026] a. Synthesis of Ni-MOF-74: According to the prior art, 1,3,5-benzenetricarboxylic acid (H3BTC) and 4,4'-bipyridine (4,4'-bipy) were used as ligands, and Ni(NO3)2·6H2O was dissolved in N,N-dimethylformamide (DMF) at a molar ratio of 1:1:2 to carry out a solvothermal reaction. After the reaction was completed, the reaction kettle was placed at room temperature to cool naturally, and the product was collected and centrifuged once. The obtained solid was immersed in DMF for washing for 24 h, and the solvent was replaced every 12 h. Then the solid was immersed in anhydrous methanol for solvent replacement for 48 h, and the fresh solvent was replaced every 12 h. The obtained solid was dried and activated under vacuum at 100°C. The final yellowish powder obtained was Ni-MOF-74.

[0027] b. Synthesis of IL@MOF (full name: IL@Ni-MOF-74 below): 1.0 g of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (IL) was dissolved in anhydrous ethanol and magnetically stirred to uniformly disperse. Then 1.0 g of Ni-MOF-74 was added sequentially and stirred for 24 h to allow IL to fully contact and react with Ni-MOF-74. Finally, the product was centrifuged and washed with ethanol several times to remove excess IL, and then dried in an oven to obtain the product IL@Ni-MOF-74.

[0028] c. Preparation of IL@MOF-TPU (full name: IL@Ni-MOF-74 / TPU below) composite solid-state electrolyte: IL@Ni-MOF-74 accounts for 70 wt% of the mass of TPU in the film material. When preparing the film solution, chloroform (CHCl3) is used as the solvent, and the concentration of IL@Ni-MOF-74 and TPU in the CHCl3 solution is 20 wt%. The weighed IL@Ni-MOF-74 is dissolved in 5.0 mL of CHCl3 solution, and a uniformly dispersed solution is obtained under multiple ultrasonic stirring. Then TPU is added in batches under ultrasonic stirring to form the final film-forming solution. Finally, the film is formed on a watch glass, and after the solvent is volatilized, the formed film is taken out of the watch glass and dried in a vacuum oven at 80°C to obtain the IL@Ni-MOF-74 / TPU composite solid-state electrolyte.

[0029] The prepared IL@Ni-MOF-74 / TPU composite solid-state electrolyte is applied to a battery and performance tests are carried out.

[0030] The battery was assembled inside a glove box. Treated nickel foam was used as the positive electrode, and an IL@Ni-MOF-74 / TPU composite solid electrolyte membrane was used as the electrolyte. 0.1 mL of 1M LiTFSI in TEGDME liquid was added, followed by a lithium sheet as the negative electrode. The battery was then assembled. It was activated in an oven (60℃, 12h, Ar atmosphere), then removed and placed in an oxygen chamber for cyclic testing using the Wuhan Landian testing system.

[0031] Figure 1 These are XRD patterns of Ni-MOF-74, IL@Ni-MOF-74, TPU, and IL@Ni-MOF-74 / TPU from Example 1. The peaks of Ni-MOF-74 in the figures agree well with the results of Ni-MOF-74 synthesized in the literature, indicating that Ni-MOF-74 was successfully synthesized in this work. The peaks of IL@Ni-MOF-74 are consistent with those of Ni-MOF-74, proving that the structure of Ni-MOF-74 remains intact after IL loading. However, due to the loading of IL and TSIL@ in the channels, the electron density dimensions of Ni-MOF-74 have changed, resulting in a weaker peak intensity compared to IL@Ni-MOF-74.

[0032] Figure 2 These are scanning electron microscope images of Ni-MOF-74 and IL@Ni-MOF-74 from Example 1. The Ni-MOF-74 particles are 200-300 nm in size and are nearly spherical. Due to the sufficient loading of IL, the morphology of IL@Ni-MOF-74 has changed, and cross-linking has formed between the particles.

[0033] Figure 3 These are the N2 adsorption isotherms measured at 77 K for Ni-MOF-74 and IL@Ni-MOF-74 in Example 1. The BET specific surface area (S) of the synthesized material is also considered. BET The result was calculated using the N2 adsorption isotherm. Figure 3 Ni-MOF-74's S BET 1030m 2 g -1 IL@Ni-MOF-74 is 1155m 2 g -1 By comparing the S of different materials BET The S of IL@Ni-MOF-74 can be found BET Compared to SO3H-MIL-101-Cr, the specific surface area is increased. For IL@Ni-MOF-74, due to the adsorption of a certain amount of IL into the MOF macropores, forming some micropores, the specific surface area of ​​IL@Ni-MOF-74 is actually increased, and some mesopores are formed.

[0034] Figure 4 is the discharge capacity chart of lithium-air battery assembled by TPU and IL@Ni-MOF-74 / TPU of Example 1. As can be seen from the figure, the discharge capacity of IL@Ni-MOF-74 / TPU is much higher than that of TPU, which further indicates that IL@Ni-MOF-74 has better lithium ion conductivity. When the current density is 500 mA / g, the specific capacity of the first discharge is 18662 mAh / g and 7437 mAh / g, respectively. This is mainly due to the open metal sites in MOFs can complex with anions in the ionic liquid in its pores, thereby releasing flowing lithium ions and providing high lithium ion conduction capacity; and by taking advantage of the porosity of MOFs, 1-ethyl-3-methyl imidazoline bis(trifluoromethylsulfonyl) imide can be fully adsorbed by the infiltration method, which can further improve the lithium ion conductivity of the solid-state electrolyte.

[0035] Example 2

[0036] The difference from Example 1 is that IL@Ni-MOF-74 accounts for 20wt% of the mass of TPU in step c to prepare the film material.

[0037] Example 3

[0038] The difference from Example 1 is that IL@Ni-MOF-74 accounts for 40wt% of the mass of TPU in step c to prepare the film material.

[0039] Example 4

[0040] The difference from Example 1 is that IL@Ni-MOF-74 accounts for 50wt% of the mass of TPU in step c to prepare the film material.

[0041] Comparative Example 1

[0042] The difference from Example 1 is that IL@Ni-MOF-74 accounts for 10wt% of the mass of TPU in step c to prepare the film material.

[0043] Comparative Example 2

[0044] The difference from Example 1 is that IL@Ni-MOF-74 accounts for 80wt% of the mass of TPU in step c to prepare the film material.

[0045] Comparative Example 3

[0046] The difference from Example 1 is that MOF is replaced by NH2-MIL-125.

[0047] Comparative Example 4

[0048] The difference from Example 1 is that TPU is replaced by PEO.

[0049] Performance Test

[0050] The composite solid-state electrolytes prepared in the above examples and comparative examples were tested for ion conductivity and lithium ion transference number, and the composite solid-state electrolytes were assembled into lithium-air batteries to test the first discharge specific capacity and cycle stability.

[0051] The results are shown in the following table:

[0052]

[0053] As can be seen from the table, the strategy of using Ni-MOF with open metal sites to adsorb ionic liquid is used in the present application, and TPU is further added to prepare a high-performance ionic liquid-loaded Ni-MOF-based composite solid-state electrolyte. The composite solid-state electrolyte of Example 1 has excellent ion conductivity (3.4 x 10 -2 S / cm), high lithium ion transference number (0.96), and wide electrochemical window (decomposition voltage 5.31 V), and is a very promising electrolyte material. In addition, the lithium-air battery assembled using the ionic liquid-loaded Ni-MOF-based composite solid-state electrolyte exhibits excellent cycle stability, with a capacity retention rate of 97.5% after 150 charge-discharge cycles. Therefore, the strategy of using Ni-MOF with open metal sites to adsorb ionic liquid is an effective method for obtaining MOF-based solid-state electrolyte materials with excellent lithium ion conduction performance.

[0054] Compared with Example 1, in Comparative Example 1, the mass of IL@Ni-MOF-74 in TPU is lower than the preferred range, and the lithium ion conductivity and lithium ion transference number of the composite solid-state electrolyte are lower than those of Example 1, and the first discharge specific capacity and cycle capacity retention rate of the lithium-oxygen battery finally assembled are also not as good as those of Example 1, which is mainly due to the high content of IL@Ni-MOF-74 in the composite solid-state electrolyte of Example 1, which can fully utilize the improvement of IL@Ni-MOF-74 on lithium ion conductivity and battery performance. In Comparative Example 2, the mass of IL@Ni-MOF-74 in TPU is higher than the preferred range, but since IL@Ni-MOF-74 is doped into the TPU matrix by blending, when the doping amount of IL@Ni-MOF-74 reaches a certain amount, excessive doping will cause the compactness of the composite solid-state electrolyte to deteriorate, so the lithium ion conductivity and lithium-oxygen battery performance of the composite solid-state electrolyte in Comparative Example 2 actually decrease.

[0055] Compared with Example 1, the performance of the composite solid electrolyte and the performance of the lithium-oxygen battery of Comparative Example 1 are both decreased, which is because, as both are nanoporous materials, after adsorbing a certain amount of IL, NH2-MIL-125 cannot generate binding force with IL, resulting in partial leakage of IL during preparation of the composite solid electrolyte, and ultimately failing to fully play the role of IL in improving the performance of the composite solid electrolyte. The Ni-MOF-74 used in the present patent contains open metal sites that can complex with the anion in the ionic liquid in its pores, thereby releasing mobile lithium ions and providing high lithium ion conductivity.

[0056] Compared with Example 1, Comparative Example 4 uses polyethylene oxide (PEO) in patent CN108878970A to replace the TPU of the present patent. Since polyethylene oxide will react with the product lithium peroxide in the lithium-oxygen battery, thereby causing polyethylene oxide to fail, which is not conducive to maintaining excellent cycle performance of the lithium-oxygen battery, so the capacity retention rate of the battery after cycling of Comparative Example 4 is lower than that of Example 1.

[0057] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application, and any simple modification, equivalent change and modification of the above embodiments based on the technical essence of the present application are still within the scope of the technical solution of the present application.

Claims

1. An MOF-based composite solid-state electrolyte loaded with an ionic liquid, characterized in that, IL is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, MOF is Ni-MOF-74, the mass of IL@MOF is 20-70% of the mass of TPU, and TPU is a polyether type polyurethane elastomer.

2. A method of preparing a MOF-based composite solid-state electrolyte loaded with an ionic liquid according to claim 1, characterized in that, The method comprises the following steps: a. Synthesis of IL@MOF: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is dissolved in anhydrous ethanol, uniformly dispersed, and high-temperature activated Ni-MOF-74 is added in batches, stirred for 22-26 hours, the reaction solution is centrifuged, washed with ethanol, and dried to obtain IL@MOF; b. Preparation of IL@MOF-TPU composite solid-state electrolyte: IL@MOF is dissolved in chloroform, uniformly dispersed, and TPU is added in batches to form a film-forming solution under ultrasonic stirring, the formed film is taken out after the solvent is volatilized, and vacuum drying is performed at 70-90 DEG C to obtain the IL@MOF-TPU composite solid-state electrolyte.

3. The production method according to claim 2, characterized by, The mass ratio of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to Ni-MOF-74 in step a is 1:1.

2.

4. The production method according to claim 2, characterized by, The mass ratio of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to Ni-MOF-74 in step a is 1:1.

2.

5. The production method according to claim 2, characterized by, The preparation method of the Ni-MOF-74 in step a is as follows: 1,3,5-benzenetricarboxylic acid and 4,4'-dipyridine are used as ligands, and Ni(NO3)2·6H2O is dissolved in DMF to perform a solvothermal reaction, the reaction solution is centrifuged after the reaction is completed, the obtained solid is sequentially washed and replaced with DMF and anhydrous methanol, dried, and then vacuum activated at 60-120 DEG C to obtain the Ni-MOF-74.

6. The preparation method according to claim 2, characterized in that, The mass concentration of the total amount of IL@MOF and TPU in chloroform in step b is 3-8 wt%.

7. Application of the IL-loaded MOF-based composite solid-state electrolyte according to claim 1 in a lithium-air battery.

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