Supervisors: Prof. Enn Lust
Ph.D. Alar Jänes
Opponent: Prof. Thierry Brousse, Université de Nantes, Nantes, Prantsusmaa/France
Supercapacitors (SC) are high-power energy storage devices, where energy is stored through reversible physical adsorption of electrolyte ions on the interface between large surface area polarized electrode and electrolyte. Various porous materials, electrolyte compositions and separator materials are being studied and electrochemically characterized to improve the performance of SCs. Finding a replacement for commercially applied harmful acetonitrile and viscous propylene carbonate based electrolytes is an important step for wider commercialization of SCs. In addition to changing solvent, possible electrolyte salt candidates have to be studied to optimize solvent/salt combination.
The aim of this thesis was to study novel electrolytes consisting of alkali metal cation and different anion based salts (LiPF6, LiClO4, LiCF3SO3, LiB(C2O4)2, NaPF6, NaClO4, CsCB11H12) in an environmentally friendly mixed solvent system of ethylene carbonate and dimethyl carbonate. The widest region of ideal polarizability (3.4 V) was established in case of NaPF6. Energy density values, calculated at identical conditions, varied only by 20% between the test systems with electrochemically stable electrolytes (35-42 Wh kg-1), but power densities depended remarkably on the electrolyte, with the highest value of 95 kW kg-1 calculated for NaPF6 based electrolyte.