Page 575 - Handbook of Battery Materials

17.2 Components of the Liquid Electrolyte 549

with joints and vents under an inert gas, which for studies on lithium generally
is purified argon. Often the preparation of the electrolyte requires both slow
addition of the salt and cooling, because dissolution of lithium salts may be highly
exothermic, entailing the decomposition of noncooled components.
Finally, recrystallization and dehydration of lithium salts deserves a remark.
Generally, lithium salts and some ILs are highly hygroscopic and often strongly
hydrated. For example, the equilibrium temperature for the dehydration pro-
cess of LiClO 4 is 136 C. Even those salts which are not thermally stable are
◦
often dried below the equilibrium temperature for dehydration of the lithium
ion, and temperatures which are too low are compensated by reduced pressure
and long drying periods at elevated temperatures. Possible side reactions such
as the hydrolysis of the anion and the formation of LiOH are not taken into
account. A common route to bypass these problems is to use dried organic
solvents with low boiling points for recrystallization and to desolvate the ob-
tained solvated salt in a vacuum with continuous weight monitoring until weight
constancy.
For ILs the situation is even worse. Synthesized ILs often contain solvents,
chloride, and water. Chloride is an impurity that results from a metathesis reaction.
As the properties of ILs are changed by these impurities, it is necessary to use
synthesis routes that avoid them.
To give an example, during a discussion in a joint project on dye solar cells
[174–177], we insisted that work with ILs has to be done under an inert at-
mosphere at low and controlled water levels. Our cooperation partners felt that
filling of thin-layer cells within a few minutes could be carried out under the
atmosphere. So we repeated an experiment to determine diffusion coefficients of
triiodide in thin-layer cells filled under the atmosphere. The result was overwhelm-
ing: diffusion coefficients increased by about 100% when compared with results
from measurements done under inert atmosphere at low and controlled water
levels.
Seddon et al. [124] stressed this important aspect: without sufficient and recorded
purity no useful data can be obtained.

17.2.5
Hydrolysis of Salts
Hydrolysis affects the anions of lithium salts and of ILs. Anions such as tetrachloroa-
luminate hydrolyze completely, and therefore it is a very hard task to determine
molar conductivities of this salt in SO 2 -based solutions at low concentrations
[178]. But even so-called ‘hydrophobic’ fluorinated ions such as tetrafluoroborate
hydrolyze. Lithium borates show at high water concentration an initial hydrolysis
that can be treated as a first-order reaction and can be studied by recording and
evaluating conductivity–time functions G(t) [179], see Figure 17.2.
At small concentrations of water, borates such as LiBOB react only very slowly
to reach equilibrium. This observation is in accordance with Yang et al. [180].
The authors state: ‘Although LiBOB electrolytes containing trace amounts (about