Compatibility of ionic liquid electrolytes towards LI-NION and NA-ION electrodes

One of the major challenges of LIBs and SIBs is related to safety and reliability, which are mostly connected to the nature of the electrolyte. Commercial organic electrolytes show several critical issues, therefore different alternatives have been considered, such as polymers, ceramics, and ionic liquids. The experimental activities of the present PhD thesis have been structured in five sections: the first one was focused on synthesis, characterization, and selection of IL electrolytes. The other sections were addressed to investigation of compatibility and behaviour of the selected IL electrolytes with LIB (2nd and 3rdsections) and SIB (4th and 5thsections) systems, respectively. During the first section, different IL families were studied: in particular, imidazolium, tetra-alkyl-ammonium, and piperidinium cations, coupled with bis(perfluroalkylsulfonyl)imide anions, were considered as innovative safety electrolyte components for being addressed to LIB and SIB systems. The synthesis purification route is reported in detail and the quality level of the synthetized ILs was validated through X-Ray fluorescence analysis, UV-Vis spectrophotometry and Karl-Fisher titration. The thermal stability of the pure ILs was analyzed by thermogravimetric analysis (TGA) under helium atmosphere, through both temperature heating scan and isothermal steps. Li+ and Na+-conducting electrolyte formulations, based on 1-ethyl-3-methyl-imidazolium (EMI), trimethyl-butyl-ammonium (N1114), and N-alkyl-N-methyl-piperidinium (PIPn1) ionic liquid (IL) families were designed and investigated. Lithium bis(trifluoromethylsulfonyl)imide, LiTFSI, and sodium bis(trifluoromethylsulfonyl)imide, NaTFSI, were selected as the salts. The ion transport properties and electrochemical stability have been investigated, analyzing the dependence from the nature of the anion and the cation aliphatic side chain length. In the second section, LIB silicon nanowire anodes were investigated in lithium metal cells using electrolyte formulations (selected in the frame of the 1st section) based on EMITFSI, EMIFSI and N1114FSI ionic liquids. The lithium insertion process in the silicon anode was analyzed by cyclic voltammetry measurements, performed at different scan rates and for prolonged CV tests, combined with impedance spectroscopy analysis. The electrochemical performances were investigated by galvanostatic charge-discharge cycling tests. X-ray photoelectron spectroscopy (XPS) measurements were carried out onto the silicon nanowire electrode surface to gain knowledge about the SEI (Solid Electrochemical Interface) layer. In the third section, LIB cobalt-free, lithium-rich, layered oxide Li1.2Ni0.2Mn0.6O2 (LRLO) cathodes were investigated in the selected IL electrolytes. The battery performances, after previous screening of the IL formulations, were evaluated by prolonged galvanostatic charge-discharge cycling in Li/LRLO cells. The effect of the lithium salt was also assessed. The electrochemical process of Li+-insertion was analyzed through cyclic voltammetry measurements at increasing scan rates and for prolonged cycles. In the fourth section, SIB hard carbon (HC) anodes, obtained from natural biowaste, have been investigated in EMIFSI and N1114FSI electrolytes. The Na+ intercalation process was analyzed by cyclic voltammetry tests, performed at different scan rates for hundreds of cycles, in combination with impedance spectroscopy measurements to decouple bulk and interfacial resistances of the cells. Also, the Na+ diffusion coefficient in the HC host was determined via the Randles-Sevcik equation. The cell performance was evaluated by room temperature galvanostatic charge/discharge cycling tests. The evolution of the SEI (solid electrochemical interface) layer grown on the HC surface, cycled in different electrolytes, has been studied by Raman spectroscopy, XPS and focused ion beam milling scanning electron microscopy (FIB-SEM) analysis. In the last section, SIB monocline sodium manganite, α-NaMnO2, cathodes were investigated in EMIFSI and N1114FSI electrolytes. The Na+ insertion process in α-NaMnO2 was analyzed through cyclic voltammetry tests, carried out at different scan rates, combined with impedance spectroscopy measurements. The cell performance of α-NaMnO2 electrodes was validated by galvanostatic charge-discharge cycling tests whereas the surface composition and the morphology of post-mortem cathodes were analyzed through XPS and FIB-SEM measurements. My PhD research activities were carried out in the frame of two Projects: i) Si-DRIVE (Silicon Alloying Anodes for High Energy Density Batteries comprising Lithium Rich Cathodes and Safe Ionic Liquid based Electrolytes for Enhanced High VoltagE Performance), H2020-NMBP-ST-IND-2018” (Topic LC-NMBP-30-2018) Grant Agreement 814464, founded by the European Union’s Horizon 2020; ii) National Program (Electric Research System) Agreement (Energy storage systems for electrical network, Activity LA1.7: Ionic liquid electrolytes for sodium batteries) between the Italian Ministry of Ecological Transition and ENEA.