Interplay between Composition, Physicochemical Properties and Conductivity Mechanisms of Ionic Liquid-Based Electrolytes

Abstract

Electrochemical energy conversion and storage (EECS) devices are a family of systems that are playing a major role in today’s efforts worldwide to decarbonize the energy infrastructure, with the ultimate goal to mitigate the greenhouse effect and reduce global warming [1]. Though EECS devices include very diverse systems such as secondary batteries, fuel cells, redox flow batteries and supercapacitors, among many others, all EECS devices share the same basic components i.e., two electrodes (where redox process take place) sandwiching a separator that allows for the selective transport of ions. In particular, the separator plays a crucial role to set some of the most relevant features of every EECS device, including the maximum power density that it can express and the durability/cyclability.Ionic liquids (ILs) have demonstrated a huge potential to enable the realization of EECS devices exhibiting an improved performance and durability. This feat is possible by leveraging on the unique features of ILs, with a particular reference to their highly customizable chemistry [2]. Suitable synthetic approaches allow to obtain systems specifically tailored to a certain application. In some instances, the best electrolytes are achieved by: (i) using the IL to soak a suitable matrix (e.g., a perfluorosulfonic acid derivative); (ii) dissolving a suitable dissociable compound (e.g., LiTFSI or δ-MgCl2) into the IL; and/or (iii) dispersing a suitable inorganic filler into the IL. The resulting IL-based electrolytes exhibit the desired features in terms of: (i) chemistry of the mobile species (e.g., the proton, lithium complexes, magnesium species); (ii) facile and selective transport of such mobile species; (iii) good compatibility with the electrodes, allowing an efficient plating/stripping of the desired metal species and minimizing undesired parasitic reactions; (iv) broad electrochemical stability window (ESW); and (v) well-controlled hydrophilicity/hydrophobicity features.The present contribution overviews our activities in the synthesis, characterization and implementation into various EECS device prototypes of IL-based electrolytes. The latter include: (i) electrolytes for proton-exchange membrane fuel cells (PEMFCs) obtained by swelling a perfluorosulfonic acid derivative with proton-conducing ionic liquids (PCILs) [3]; (ii) hybrid electrolytes for lithium batteries obtained by dispersing suitable inorganic fillers into ILs [4]; (iii) catenated ILs for multivalent batteries obtained by dissolving magnesium/aluminum/titanium halides into suitable ILs [5]; and (iv) highly hydrophobic ILs for special applications [6]. Particular efforts are dedicated to elucidate the complex interplay between: (i) the synthetic parameters; (ii) the physicochemical properties; (iii) the electric response; (iv) the long-range charge transport mechanism; and (v) the performance and durability of the IL-based electrolyte upon implementation into an EECS device prototype. Acknowledgments This work received funding from the U.S. Army Research Office under the grant W911NF-21-1-0347, from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 829145 (FETOPEN-VIDICAT) and from the University of Padova under the grant BIRD2121_01.