Page 271 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposite materials in energy storage: Properties and applications 243

being oxidized by the charged surface of the cathode during the operation. Moreover,
LiPF 6 should be moisture-free because in the presence of moisture it from HF which is
highly corrosive. They show very high ionic conductivity and fairly stable in the work-
ing potential range. However, these electrolytes are susceptible to catch fire under
high temperature and under short circuit due to the volatile and flammable nature
of the organic solvent. There are also chances of leakage and the solvents are
toxic [40].
As mentioned earlier, the liquid electrolyte systems can be classified as (1) non-
aqueous electrolyte and (2) aqueous electrolyte. In the current commercial Li batte-
ries, the state-of-the-art electrolytes are the nonaqueous electrolyte systems where
lithium salt, 1 M LiPF 6 , is dissolved in an organic cyclic carbonate solvent or their
mixture. There have been continuous efforts to improve the stability of the carbonate
solvent in the past decades by exploring new solvent or new salt or modifying the car-
bonate salt with additives to improve the performance. One way to improve the per-
formance of the electrolyte is the synergic interaction of two solvents so that they
augment each other’s property. In this regard, a mixture of high dielectric solvents
(HDS—solvate ions and favor salt dissociation, but in common, SOTA electrolyte
is a well-known example of the synergistic effect influencing highly viscous) and a
low viscosity (LVS—facilitate ion transport, low permittivity) solvents used the ionic
conductivity of the electrolyte. One very important feature of these electrolytes has
been the formation of solid-electrolyte interphase (SEI). SEI is basically a protective
layer that is formed within initial few cycles of the charging and discharging operation
[41]. The SEI is of almost thickness 20 nm and composed of various organic and inor-
ganic salts of Li (LiF, LiOH, LiO 2 , and Li 2 CO) 3 . It protects the electrolytes from the
redox chemical reactions by electrically isolating the electrolyte from the electrodes
where the taking place at the electrode [42]. However, slow continued SEI growth
leads to the unwanted consumption of anode and electrolyte, capacity fade, and
increased cell resistance. All the faradic processes are expected to take place at the
+
electrodes. The function of the electrolyte is merely the conduction of Li ions.
A good SEI is the key toward long cycle life and safe operation of the SEI [43].
Several methods have been suggested in the literature for improving the stability
and working potential of nonaqueous electrolytes. This includes the use of the fluorine
containing organic solvents such as 2,2,2-trifluoroethyl acetate (TEFA) [44]. Apart
from flouride, boron- and phosphorus-containing solvents have also been explored
in the literature [45]. The organic solvents used in the electrolyte suffer from the sus-
ceptibility of catching fire due to its high flammability and toxic vapor release due to
reaction with the electrode. Thermal instability and extremely toxic LiPF 6 . One way to
avoid this is the use of high ionic conducting water-based solvents such as LiNO 3 in
the electrolyte. Furthermore, they are environment-friendly, show excellent safety
features, and have low capital cost investment as well [46,47]. It is built up by using
graphite coated with gel polymer membrane and LISICON as the negative electrode
and LiFePO 4 in aqueous solution as the positive electrode. Its average discharge volt-
age is up to 3.1 V, and energy density based on the two electrode materials is
1
258 Wh kg . It will be a promising energy storage system with good safety and effi-
cient cooling effects [46].Li 2 SO 4 is also used as electrolyte. Here, a coated Li metal is