Abstract
Metal Organic Frameworks (MOFs), as a novel class of crystalline porous material have gained great attention in the fields of gas storage, separation, drug delivery, catalysis, sensors and so forth. The supramolecular design of MOFs allow a number of organic ligands, with two or more functional groups, to be linked to metal ion coordination centers giving myriad variations and tunable properties in synthesized structures. Metal Organic Frameworks (MOFs) are of great interest due to several unique features and their applications have been extended to the field of Li-ion batteries. Simultaneously, ionic liquids (ILs) are becoming popular in a range of areas including biphasic reaction catalysis, electromechanical actuator membranes and diluents and separation membranes. Many studies have been conducted for the application of MOFs in energy storage devices [1]-[3], but scarcely any investigation was performed to understand the effect of ionic liquid incorporated MOFs in electrochemical storage devices especially batteries. Hence, an attempt has been made by our group with ionic liquid incorporated modified MOFs to serve as a better electrolyte system for Li-ion batteries with increased thermal and electrochemical stability, high ionic conductivity and other electrochemical aspects. Further, dimensionally and morphologically controlled MOFs will be carefully used to ease the ion transport in the electrolyte system by the regulation of ion conductive path. MOF-1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (AMImTFSI), synthesized by a facile in-situ electrochemical method [4] was doped with a lithium salt, lithium bis (triflouromethylsulfonyl) imide (LiTFSI) by a modified procedure for electrochemical measurements. Morphological features and crystal size of these matrices were evaluated by TEM (Transmission Electron Microscopy) and XRD measurements. Temperature dependence of ionic conductivity of the MOF/ionic liquid matrix was studied by AC-impedance method. Combination of AC-impedance and DC-polarization method was used to determine the lithium ion transference number of the samples. Electrochemical window of different electrolyte compositions was studied by linear sweep voltammetry. A coin cell was fabricated to study the charge discharge characteristics of the resulting cell.
Synthesis of MOF-5/ionic liquid matrix by electrochemical method - MOF was successfully synthesized by a mild in-situ electrochemical method using 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (AMImTFSI) as a templating agent employing a modified procedure of reported method [4]. Synthesis of the MOF (AMImTFSI) was achieved by a constant dissolution of Zn2+ ion from Zn anode. The dissolution was carried out by applying a constant direct current (DC) of 0.15A. A titanium electrode was employed as cathode. The electrolyte used in this procedure was a solution containing 0.07 M terephthalic acid and 0.04 M zinc nitrate hexahydrate in DMF. The electrochemical galvanostatic procedure was carried out for 3hrs. A white powder formed during the above mentioned procedure, was filtered and washed with DMF first and then, with chloroform. The sample was then dried in oven at 80 oC to get pure MOF (IL). For further electrochemical analysis, MOF (IL) was doped with lithium salt, LiTFSI. A set of samples were synthesized with varying content of MOF (IL) in ionic liquid system and ionic conductivity was measured by AC-impedance method.
Three samples with different weight % of MOF (IL) in AMImTFSI were prepared and temperature dependent ionic conductivity measurements were performed by AC impedance method. All the samples showed linear profiles in Arrhenius plots. As expected the ionic conductivity decreased with increase in wt% of MOF (IL) because of change in state from liquid to gel. Interestingly a decrease in activation energy of ion transport was observed. The ionic conductivities of the samples were calculated by the Vogel-Fulcher-Tammann (VFT) equation, resulting out to be highest for the sample with 10 wt% of MOF (IL) in AMImTFSI i.e. 1.0 x 10-2 Scm-1 at 51 oC . The highest tLi+ of 0.31 was observed for 20 wt% MOF (IL) in AMImTFSI. The systems showed a potential window of 5.22-5.55 V. Charge discharge studies were also carried out after fabricating a coin anodic half-cell composed of Si/electrolyte/Li which showed 90% coulombic efficiency in the presence of 30 mL of EC: DC=1:1 in electrolyte.
Conclusion
Modified MOF with ionic liquid framework was prepared in ionic liquid system by electrochemical method. Measurements of temperature dependence of ionic conductivity using AC impedance showed relatively high ionic conductivity. These encouraging results affirm the prepared modified MOF (IL) to be a prospective electrolyte in energy storage devices.
References
[1] C. Yuan et al., J. Power Sources, 2013, 240, 653-658.
[2] B. M.Wiers et al., J. Am. Chem. Soc., 2011, 133, 14522–14525.
[3] C. Gerbaldi et al., J. Mater. Chem. A., 2014, 2, 9948–9954.
[4] H. M. Yang et.al., RSC Adv., 2014, 4, 15720-15726.
Figure 1