Page 272 - Polymer-based Nanocomposites for Energy and Environmental Applications

244 Polymer-based Nanocomposites for Energy and Environmental Applications

used as anode for an aqueous rechargeable lithium battery (ARLB) combining
LiMn 2 O 4 as cathode and 0.5 mol L 1 Li 2 SO 4 aqueous solution as electrolyte. Due
+
to the “crossover” effect of Li ions in the coating, this ARLB delivers an output volt-
age of about 4.0 V, a big breakthrough of the theoretical stable window of water,
1.229 V. Its cycling is very excellent with coulomb efficiency of 100% except in
1
the first cycle. Its energy density can be 446 Wh kg , about 80% higher than that
for traditional lithium-ion battery. Its power efficiency can be above 95% [48]. Fur-
thermore, its cost is low and safety is much reliable [48]. Aqueous electrolytes can
operate only in narrow voltage range (1.23 V), which is much less than the potential
required for Li redox chemistry, and at higher potentials, it evolutes H 2 . Suo et al.
reported a water-in-salt electrolyte obtained by dissolving bis(trifluoromethane sulfo-
nyl)imide (LiTFSI) at 20 M high concentration in water. The electrolyte could be
operated at 3 V due to the formation of electrode-electrolyte interphase. A full
Li-ion battery was constructed with LiMn 2 O 4 and Mo 6 S 8 as cathode. The open-circuit
voltage obtained was 2.3 V able to sustain 1000 cycled with 100% coulombic effi-
ciency [49]. Another very interesting type of liquid electrolyte is ionic liquid based.
Interested readers are referred to the excellent reviews [50,51].
Polymer electrolyte is largely seen as a promising alternate to the liquid electrolyte.
The liquid electrolyte based on organic solvents is highly flammable and generates
toxic vapor upon reaction and is prone to leakage. One of the major issues with
liquid-electrolyte-based lithium-ion batteries is the safety issues with the formation
of dendrites. Dendrites are the irregular microfibers of Li metal that sprout from
the Li electrode during the fast charging and discharging process and can travel
through the liquid electrolyte to the other electrode. An electric current passing
through these dendrites can short-circuit the battery, causing it to rapidly overheat
and in some instances catch fire [52,53]. An excellent review by Zhang et al. has been
recently published [54].Of course, polymer electrolyte does not suffer from any of
these drawbacks because of the absence of any liquid in the electrolyte. Moreover,
it adds several advantages such as resistance to the volume change of electrodes dur-
ing charging and discharging process; high thermal, mechanical, and chemical stabil-
ity; ease of processing; and flexibility. It suppresses or completely eliminates the
formation of dendrites [55]. Polymer electrolyte showing ionic conductivity was first
reported by Fenton et al. [56]. In this work, it was demonstrated that polyethylene
oxide (PEO) showed ionic conductivity when doped with alkali metal salts to form
a complex. Since then, lots of work have been done in this area and are summarized
in the review [57].
Among the many polymers, high-molecular-weight PEO and its derivate are con-
sidered as the best material for polymer electrolytes because of the presence of an opti-
+
mally located and oriented ether oxygen to coordinate with the cation (Li ) to promote
solvation of the salt and low glass transition temperature, their high solvation power,
and excellent transport mechanism [58,59]. It is well understood that the polymer
electrolyte should show ionic conductivity and very low electronic conductivity
[27]. It is well established that crystalline phase favors electronic conductivity,
whereas amorphous phase favors the ionic conductivity. The mechanism of ionic con-
ductivity has been closely studied by various theories [60,61]. It is now generally