Sodium Ion Conductivity in Superionic IL-Impregnated Metal-Organic Frameworks: Enhancing Stability Through Structural Disorder

Abstract

Metal-organic frameworks (Mofs) are intriguing host materials in composite electrolytes due to their ability for tailoring host-guest interactions by chemical tuning of the Mof backbone. Here, we introduce particularly high sodium ion conductivity into the zeolitic imidazolate framework ZIF-8 by impregnation with the sodium-salt-containing ionic liquid (iL) (na 0.1 eMiM 0.9)tfSi. We demonstrate an ionic conductivity exceeding 2 × 10 −4 S • cm −1 at room temperature, with an activation energy as low as 0.26 eV, i.e., the highest reported performance for room temperature na +-related ion conduction in Mof-based composite electrolytes to date. partial amorphization of the Zif-backbone by ball-milling results in significant enhancement of the composite stability towards exposure to ambient conditions, up to 20 days. While the introduction of network disorder decelerates IL exudation and interactions with ambient contaminants, the ion conductivity is only marginally affected, decreasing with decreasing crystallinity but still maintaining superionic behavior. This highlights the general importance of 3D networks of interconnected pores for efficient ion conduction in MOF/IL blends, whereas pore symmetry is a less stringent condition. Crystalline metal-organic frameworks (MOFs) consist of metal nodes as coordination centers and organic linkers which self-assemble to form a three-dimensional network. Chemical tailoring of both the inorganic node and the organic linker enables property design for a wide range of applications such as gas storage, gas separation, catalysis and ion conduction 1,2. An alternative route to tune the properties of a given MOF is post-synthetic modification, for example, by applying pressure, temperature or other exogenous stimuli 3. Depending on stimulus intensity, such post-treatment can lead to structural collapse and solid-state amorphization of the framework 4-7. The formation of amorphous MOFs through solid-solid transitions (or, similarly, through quenching of MOF-liquids) is of particular interest due to the distinct variations in chemical, mechanical and physical properties which can be obtained as a result of structural disorder 8. Amorphization of MOFs can be achieved via different techniques, including pressure-induced structural collapse, ball-milling, melt-quenching, hot-pressing, and re-melting 8-10. Of these, ball-milling, or mechanosynthesis, which can also be used to synthesize crystalline MOFs, is the most universally applicable route. The low minimum shear moduli of MOFs have previously been shown to be responsible for facile collapse of systems such as UiO-66 ([Zr 6 O 4 (OH) 4 (1,4-BDC) 6 ], BDC = benzenedicarboxylate) 11. Using calcein as a model drug incorporated into crystalline UiO-66, it was demonstrated that amorphization via ball-milling leads to delayed release of the guest molecule: the timescale of release was increased from ~2 days in the crystalline structure to one month in the amorphous composite as a result of structural collapse 12. Here, we investigate how st...

Comparison of the ionic conductivity properties of microporous and mesoporous MOFs infiltrated with a Na-ion containing IL mixture

“…12 This system has been previously studied with microcrystalline samples of ZIF-8 in our previous work. 3 NaIL was incorporated in the pores of both microporous and mesoporous samples of ZIF-8 as described in the experimental section and depicted in Fig.1, with the resulting composites being referred to as NaIL@ZIF-8micro and NaIL@ZIF-8meso.…”

“…Pellet mass g 1.07x10 -2 1.15x10 -2 Micropore + mesopore volume cm 3 6.85x10 -3 1.01x10 -2 Micropore volume cm 3 6.67x10 -3 4.49x10 -3 Mesopore volume cm 3 1.82x10 -4 5.59x10 -3 RT ionic conductivity S cm -1 1.6x10 -5 8.4x10 -6 RT pellet resistance Ω 849.7 1289.4 RT ionic conductance per total pore volume S cm -3 1.72x10 -1 7.70x10 -2 RT ionic conductance per micropore volume S cm -3 1.77x10 -1 1.73x10 -1…”

“…As a result, IL@MOF systems have been studied as pseudo-solid-state electrolytes of interest for energy storage applications. [2][3][4][5][6][7] Unlike traditional organic electrolytes, ILs are non-flammable, non-volatile, have higher thermal and chemical stabilities and have even been shown to suppress dendrite formation with metal electrodes. 8 In neat ionic liquids, leakage remains a potential issue, hence the production of a pseudo-solid-state electrolyte which retains the high ionic conductivity of the IL is an attractive solution.…”

Lithium-, Sodium-, and Potassium-ion Conduction in Polymeric and Discrete Coordination Systems

“…These conductivity values were slightly lower than that of the Li + analogs (σ = 3.8 © 10 ¹7 S cm ¹1 for EHU1(Sc,Li) and 4.2 © 10 ¹4 S cm ¹1 for EHU1(Sc,Li)¢LiBF 4 ) at approximately room temperature. However, the activation energy of the Na + electrolyte (E a = 0.64 eV), which is higher than that of the Li + electrolyte (E a = 0.25 eV), led to a higher Na + conduction than Li + conduction at temperatures over 70°C, reaching a superionic 119 They selected ZIF-8 (Zn(MeIM) 2 ) with a pore size of ³3.4 ¡ as the host framework and a mixture of NaTFSI and EMIM-TFSI as the Na-doped IL. They confirmed that the composite electrolyte exhibited a high conductivity of ³2 © 10 ¹4 S cm ¹1 at room temperature, with a low activation energy of 0.26 eV.…”

Synthesis of Metal Organic Frameworks by Ball-Milling

“…In addition, long-term ball milling may also cause the MOF structure to collapse, lose its periodic structure, and become amorphous [84,85], which is difficult to achieve in a solution phase reaction. The ball milling method can also make it more convenient to combine MOFs and other materials to form binary or even multiple composite materials [18,[86][87][88][89][90][91][92][93], such as GO/MOFs composites [94][95][96][97][98][99] Cu-BTC with truncated octahedral shape, and showed significantly reduction of reaction time and effectively improvement of yield [78].…”