The role of ionic liquids and deep eutectic solvents in addressing modern battery systems challenges

The aim of this thesis is to explore the potential of neoteric materials such as ionic liquids (ILs) and deep eutectic solvents (DESs) to address some of the critical challenges faced by modern battery technologies. The work will illustrate how these materials can play a pivotal role in developing both advanced lithium batteries and post-lithium technologies (zinc-ion batteries, ZIBs). The thesis is organized into three chapters. The first one will provide an introduction to the general landscape of lithium batteries, with particular emphasis on modern challenges. Then, the fundamentals of ionic liquids and deep eutectic solvents will be discussed, highlighting their potential to overcome these challenges. Finally, this chapter will explore side aspects of the battery field, including post-lithium technologies and battery recycling. In the second chapter, the focus will be on oxalatoborate-based ILs, containing bis(oxalato)borate (BOB) or difluoro(oxalato)borate (DFOB) anions and different cations properly designed. This family of ILs is explored for their potential to form robust and efficient cathode-electrolyte interfaces (CEIs), helping to mitigate the instability issues at the electrolyte-electrode interphase. First, a sustainable and optimized synthetic route for these ILs will be developed, aiming to minimize the use of organic solvents. After synthesis, their thermal properties will be evaluated through differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and Knudsen effusion mass spectrometry (KMES). Additionally, the results will be complemented by theoretical studies through computational studies, including molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The combination of all these methods, including spectra obtained through ATR-FTIR, will provide a comprehensive understanding of the thermal behavior of ILs, from their phase transitions at low temperatures to the mechanisms underlying their decomposition at high temperatures. The synthesized ILs will then be tested in lithium metal battery systems. To lower the amount of ILs required, and thereby reduce the potential cost of future industrial applications, the ILs will first be used as additives in LP30 (1M LiPF in EC:DMC 1:1 v/v), the most common electrolyte in LIBs. Furthermore, to minimize cost even further, the ILs will be applied as additives directly during the preparation of the cathodes, resulting in "IL-enriched electrodes." This innovative approach allows for a smaller amount of IL to be used, with the cathodes tested in combination with pure LP30 as the electrolyte. The main cathode material tested will be LNMO. Additionally, preliminary results on their use for LRLO, provided by ENEA. The third chapter will shift the focus on a study of a low-transition temperature mixture composed of Zn(TFSI)2 and ethylene glycol (EG), which will be evaluated as a potential electrolyte for zinc-ion batteries (ZIBs). Different mixtures with varying molar ratios of Zn(TFSI)2 and EG will be prepared and characterized using infrared (IR) and Raman spectroscopy to investigate zinc coordination within the solvent. The thermal behavior of the mixture too will be explored, in particular to asses if it can be described as a DES. The most promising mixtures, based on their spectroscopic profiles, will undergo electrochemical testing to evaluate their performance during continuous zinc stripping and deposition, as well as their Coulombic efficiency. The best-performing DES mixture will then be tested in a ZIB system, paired with an innovative potassium-doped vanadium oxide cathode (K0.5V2O5, KVO). Both these two chapters will begin with an introduction to the general context of the research, followed by detailed descriptions of the materials and methods used. The results of the experiments will be presented and discussed in depth, leading to conclusions of the respective works. In addition, Appendix B will present the results obtained during my six-month tenure at Eco Recycling s.r.l., a company specializing in the recycling of materials from spent batteries, as part of my PhD program. This period focused primarily on the characterization and testing of materials produced within the framework of the LIFE DRONE European Project [1], which started in September 2020 and concluded in September 2024. Specifically, the appendix will detail results from testing half-cells containing graphite or NMC111 derived from recycling processes, along with preliminary data from full-cells incorporating both recycled materials.