Development of innovative electrolytes for safer lithium-ion batteries
2014
Lithium-ion batteries are dominating both the nomad device and electric vehicle markets.
However they raise safety concerns related to their electrolyte, which consists of flammable
and volatile carbonate mixtures and toxic salts. The replacement of the latter by ionic liquids
(IL), liquid salts claimed to be thermally stable and non-flammable, could provide a safer
alternative. Yet this often claimed feature has been poorly examined by experiments. The
work of this thesis investigates IL behaviour under abuse conditions such as overheating, fire
or overcharge. Decomposition temperatures of IL based on differently substituted
imidazolium or pyrrolidinium cations and the bis(trifluoromethanesulfonyl)imide anion were
determined by thermogravimetric analysis (TGA). A critical study of gathered data (from
literature and our work) led to the determination of an optimised procedure to obtain
reproducible and comparable results. Electrolytes based on carbonates mixtures or IL and
containing lithium salt were studied by dynamic and isothermal TGA, and their
decomposition products were identified. Their combustion behaviour was also tested by
measuring heats of combustion and ignition delays. Emitted gases were analysed and
quantified. Electrochemical cycling tests were carried out with these electrolytes in
lithium-ion systems based on Li4Ti5O12 and LiNi1/3Mn1/3Co1/3O2 electrodes. The evolution of
the electrolytes and electrodes surface was also examined under overcharge.
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