Improved Lewis Acidity Using Ionic Liquid-Grafted Bimetallic Zr/Ti Uio-66 Fillers for Composite Solid Electrolytes
Authors: Ho Jeong-Won , Yoo Pil Jin 
Published: February 2024

In recent years, solid-state lithium batteries have gained significant attention due to their high energy density and stability. However, challenges such as low ionic conductivity, non-homogeneous Li ion flux, and low interfacial compatibility of solid electrolytes still remain. Composite solid electrolytes (CSEs) have emerged as a potential solution to these issues, by combining an inorganic filler with a polymer matrix. To develop high-performance CSEs, extensive research has been conducted on inorganic active fillers with an abundance of Lewis acidic sites. The acidic sites of the filler can induce Lewis acid-base interactions, which can decrease the crystallinity of fillers by weakening the attraction between polymer segments and increasing chain mobility. Additionally, they increase the degree of dissociation of lithium salt, resulting in an increase in ionic conductivity. In terms of the concentration gradient of Li-ions, the free anion of lithium salt can be trapped by an acidic filler, thereby reducing the mobility and concentration of the anion. Finally, the acidic filler prevents the oxidation of ether oxygen groups in PEO, thus, improves its electrochemical stability. Furthermore, for increasing the ionic conductivity, inorganic filler should be an active filler capable of serving as Li-ion sources to the interfacial region between polymeric matrix and inorganic fillers, thereby facilitating Li-ion transport. Consequently, Li-ions can hop along the surface of active fillers. Therefore, in order to achieve high-performance CSEs with a polymer matrix, inorganic active fillers with an abundance of acidic sites have emerged as one of the most effective strategies for designing high performance fillers with good compatibility with the matrix. To address this technological need, in this work, we present a novel CSE based on a polyethylene oxide (PEO) matrix with a bimetallic (Zr/Ti) UiO-66-ionic liquid grafted filler (BUIL) that simultaneously modified the metal and ligand sites of UiO-66 MOF particles. The bimetallic structure of BUIL was prepared by replacing the metal sites of UiO-66 with Ti metal ions using the metal exchange method, resulting in the formation of more structural defects and an increase in the number of Lewis acid sites of filler. In addition, the pre-synthesized ionic liquids containing a -OH functional group (IL-OH), N-hydroxyhexyl-N’-methyl imidazolium bis(trifluoromethylsulfonyl)imide, were grafted when synthesizing UiO-66 with a ligand containing a -Br functional group. The BUIL filler functions as a high Lewis acid filler owing to its bimetallic structure and grafted ionic liquid. Also, to utilize BUIL as an active filler, lithium bis(trifluoromethanesulfonyl)imide/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (IL, LiTFSI/EMIM-TFSI) was loaded into BUIL (IL@BUIL). In order to confirm the effect of Lewis acid filler interaction on PEO polymer matrix and lithium salt, various spectroscopic characterization methods were employed. As a result, CSEs containing IL@BUIL filler and PEO matrix exhibited markedly improved ionic conductivity, lithium-ion transference number, electrochemical stability, and long-cycle stability of Li metal electrode. After optimization, the ionic conductivity of CSEs at 30°C was enhanced to 0.458 mS cm-1, the lithium-ion transference number was raised to 0.668, and the electrochemical stability window was widened to 4.5 V. In addition, during a continuous plating/stripping test, the lithium metal symmetric cell of CSEs stably cycled for 500 h at a current density of 0.2 mA cm-2. Finally, LiFePO4/IL@BUIL/Li cell shows a capacity of 148 mA h g-1 at 1C.The present strategy of simultaneously modifying the metal and the ligand sites of MOF to improve the properties of CSEs can be used as a general platform for the design of a MOF filler in CSEs system, as well as a guideline for the next generation CSEs.