Aqueous Electrolytes for Next Generation Batteries

Abstract

In today’s society the demand for green energy is increasing, which in turn increases the demand for batteries to store the energy with. Facing a large expansion in battery production, the demands on batteries being safe, environmentally friendly and economical with natural resources are raised. This thesis aims to investigate the possibility of using water-based electrolytes in batteries, to mitigate safety problems usually associated with solvents for electrolytes. The thesis digs deeper into the area of highly concentrated aqueous electrolytes and investigates which properties in the salts are responsible for the expanded voltage window seen in previous research on the subject. The study comprises a range of combinations in anions and cations chosen by cost, abundance and previous results. The anions chosen were triflate (Tf), thiocyanate (SCN−) and bis(trifluoromethane)sulfonimide (TFSI) and the cations were lithium (Li+), sodium (Na+) and magnesium (Mg2+). Three main properties were in focus; the ionic conductivity, the solvation structure and the electrochemical stability window (ESW). The properties were examined experimentally with Raman spectroscopy, dielectric broadband spectroscopy and linear sweep voltammetry, and computationally with semi-empirical and density functional theory (DFT) calculations. The study found LiTf to have the largest ESW at 3.25 V. The majority of the investigated systems show potential for solidelectrolyte interphase (SEI) formation for highly concentrated aqueous electrolytes, and an expanded ESW, possibly as a result of this SEI formation.