Electroactive Materials for Rechargeable Batteries with Super-Concentrated Electrolyte

Lithium-ion batteries are the most widespread energy storage devices, offering excellent performance and versatility. However, they rely on flammable and toxic organic electrolytes, raising safety concerns such as fire and explosion risks. To address this, water-based electrolytes are being explored for their non-flammability, eco-friendliness, and affordability. Yet, their narrow electrochemical stability window limits cell potential and energy density. Promising strategies to overcome this include superconcentrated salt formulations and hybrid aqueous/non-aqueous electrolytes using cosolvents. Additionally, lithium scarcity and uneven global distribution highlight the need for alternative technologies like sodium-ion batteries. This PhD thesis investigates these approaches to develop new aqueous electrolytes for lithium and sodium-ion batteries. It also addresses the limited availability of suitable negative electrode materials for aqueous sodium systems by designing a new phase. The thesis is structured into five chapters: Chapter 1 introduces lithium-ion batteries and key concepts; Chapter 2 outlines the experimental techniques used; Chapters 3–5 present the core experimental work. Chapter 3 focuses on hybrid electrolytes for lithium-ion batteries using dimethyl sulfoxide (DMSO) as cosolvent and lithium bis(fluorosulfonyl)imide (LiFSI) as salt. DMSO was chosen for its miscibility with water, low toxicity, limited flammability, and ability to form eutectic mixtures with water, remaining liquid at very low temperatures. Three formulations combining water, DMSO, and varying LiFSI concentrations were prepared and characterized in terms of physicochemical, transport, and electrochemical properties, with emphasis on solvation and intermolecular interactions. These electrolytes showed excellent thermal and electrochemical stability and were successfully tested in full cells with LiTi₂(PO₄)₃ and LiMn₂O₄ electrodes at room temperature and –10 °C. Chapter 4 explores aqueous electrolytes for sodium-ion batteries, combining water with short-chain polyethylene glycol (PEG200) and saturating with NaTFSI or NaFSI salts. PEG200 is miscible with water, non-toxic, non-flammable, and acts as a “molecular crowding agent” by modifying water’s hydrogen-bond network. Six formulations were prepared by varying water-to-PEG ratios and salt types. Their properties were thoroughly evaluated, confirming their suitability for practical applications. The most promising electrolyte was selected for further testing with NaTi₂(PO₄)₃ and Na₃V₂(PO₄)₂F₃ electrodes, including rate performance and long-term cycling. Chapter 5 presents the development of a new negative electrode material for aqueous sodium-ion batteries, aiming to fully exploit the electrochemical window of PEG/water electrolytes. The material, NaFeNb(PO₄)₂F₃, was derived from a modified NASICON compound previously studied but discarded due to low working potential. Fluorine ions were introduced to raise the potential. After identifying a viable synthesis route, the compound was structurally confirmed via XRD and its composition validated by EDX. A preliminary electrochemical test in PEG/water electrolyte confirmed its suitability for the intended application.