Manufacturing of scalable, solid-state, lithium battery prototypes based on ionic liquid separators

Lithium-ion batteries are the main technology for electrochemical energy storage owing to their high energy density, good efficiency and possibility of use in numerous application fields, from portable electronics to electric mobility and stationary storage systems for energy from renewable sources. However, this technology still presents critical issues related to safety and reliability, associated with the nature of conventional organic electrolytes, which are volatile and flammable. The activity carried out during the PhD programme falls within the field of the development of innovative electrolyte technologies for electrochemical energy storage devices. In particular, the research was focused on the development, synthesis and characterization of electrolyte formulations based on molten salts at room temperature and/or sub-room temperature, known as ionic liquids. These are proposed as non-flammable and non-volatile solvents for electrolytes intended for use in lithium batteries, with the aim of increasing their level of safety and reliability. The main objective of this PhD activity was the realization of electrochemical storage systems able to: i) operate under harsh operating conditions, in terms of operating temperature and voltage, which are still unsuitable for current commercial lithium-ion batteries; ii) exhibit greater environmental sustainability; iii) offer promising future prospects toward scalable industrial processes. In this thesis work, therefore, the possibility of using ionic liquids as the main components of electrolytes was explored, due to their peculiar properties, such as non-flammability, low vapour pressure, high thermal, chemical and electrochemical stability, high ionic conductivity, and the possibility of tuning their properties through an appropriate choice of the cation–anion pair. A central aspect of the research work also concerned the development of an eco-sustainable synthesis protocol for the preparation of ionic liquids, able to: a) use deionized water as the only process solvent; b) be easily scalable; c) be easy to perform; d) obtain materials with a high degree of purity, which is a fundamental requirement for their use in electrochemical devices; e) reduce the final production cost. Moreover, the ionic liquids obtained can be confined in biodegradable polymer matrices, allowing the preparation of solid or quasi-solid electrolytes that can be used as functional separators in solid-state electrochemical devices. The activities planned within this PhD are organized into four Work Packages: WP1: design, synthesis, purification and characterization of ionic liquids intended for the formulation of advanced electrolytes; WP2–WP3: validation/qualification of the developed electrolyte formulations in batteries operating under harsh thermal conditions, namely at low temperatures (-20 °C, WP2) and high temperatures (> 100 °C, WP3); WP4: development of solid-state devices through the integration of ionic liquids into suitable polymer matrices. Main results A) Development of a sustainable synthesis strategy for high-purity ionic liquids A synthesis and purification procedure with reduced environmental impact was developed, based on the use of deionized water as the only process solvent. The method proved to be economical, flexible and scalable. The comparison with commercial ionic liquids confirmed the high quality of the obtained products, characterized by purity higher than 99.9% (lithium, halides and moisture < 2 ppm) and electrochemical stability up to approximately 4.7–4.8 V. The techniques used for quality control were: X-ray fluorescence; UV-Vis spectrophotometry; FT-IR spectroscopic analysis; Karl Fischer titration. B) Evaluation of the thermal properties of the developed ionic liquids Thermal analyses highlighted a stability ranging, depending on the chemical structure, between 100 and 250 °C, confirming the suitability of the developed ionic liquids for applications under thermally severe conditions. The effect of different parameters was evaluated: i) thermal cycle, in order to promote the crystallization of the investigated materials; ii) presence of lithium salt (LiTFSI); iii) presence of moisture traces. The following analytical techniques were used: differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). C) Development of solid and quasi-solid electrolytes through confinement in polymer matrices After appropriate selection, the ionic liquids were incorporated into PAN- and PAN/PCL-based polymer membranes, prepared by electrospinning. This methodology made it possible to obtain highly porous fibrous structures able to retain electrolyte amounts up to 85% of the total membrane volume. The polymer electrolyte membranes thus obtained showed promising ionic transport properties, reaching conductivity values up to 10⁻³ S cm⁻¹ at -20 °C, an aspect particularly relevant for use in batteries operating at low temperatures. D) Ionic transport properties The ionic conductivity of the electrolyte formulations was studied as a function of temperature, also in the presence of lithium salt (LiTFSI). It was observed that the addition of LiTFSI decreases the melting temperature of the ionic liquid, increasing its operating range in batteries, while at the same time reducing, although only moderately, the ionic conductivity in the molten state, due to the greater extent of the interactions of Li⁺ ions, compared with the cations of the ionic liquid, with the anions. Overall, the electrolytes based on the FSI anion, particularly those containing EMIFSI, exhibit the best transport properties and appear promising for low-temperature applications. E) Anodic electrochemical stability The developed electrolyte formulations were analysed by cyclic voltammetry performed on Li/electrolyte/C cells to evaluate their oxidation stability. The results show stability up to approximately 4.7 V vs Li⁺/Li, confirming the high oxidative stability of the electrolytes. F) Study of compatibility with cathode materials The performance of the developed electrolyte formulations was studied with different types of cathodes consisting of both commercial active materials (NMC111, NMC811, NMC622, LFP) and materials prepared in ENEA laboratories (LRMN), by means of cyclic voltammetry, impedance spectroscopy and charge–discharge cycling performed on Li/cathode cells. Parameters such as capacity at different current rates and after prolonged tests, cycle life, coulombic efficiency, electrode/electrolyte interfacial stability and versatility of the electrolytes based on the investigated ionic liquids were evaluated. The objective is to identify the most promising formulations. G) Realization of solid-state cells Solid-state polymer electrolytes were realized by integrating the electrolyte formulations developed within the PhD activity into polymer membranes prepared by electrospinning. H) Cell design The tests were performed on Li/cathode cells assembled in both coin-cell and pouch-cell configurations. This phase represents an important step toward configurations closer to real applications, allowing the evaluation of aspects such as processability, mechanical stability, transport properties and up-scaling of the technology toward industrial systems.