Page 273 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposite materials in energy storage: Properties and applications 245

agreed that chain dynamics plays a critical role in the conductivity mechanism, and the
ionic conduction was limited mainly to the amorphous polymer electrolytes above
their glass transition temperature T g . It has been reported that the conductivity in
amorphous phase is two or three orders higher than that in crystalline phase
[62,63]. The prerequisite for an ideal polymer electrolyte are as follows: (1) it should
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show high ionic conductivity (>10 4 Scm ) at ambient conditions; (2) has mini-
1
mum electronic conductivity (<10 6 Scm ); (3) should be nonvolatile; (4) has high
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Li ion transference number (close to unity); (5) has thermal, chemical, electrochem-
ical, and mechanical stability; (6) has good compatibility with anode material; (7) has
low cost of fabrication; (8) has low glass temperature to allow high ionic conductivity
at lower temperatures; and (9) has low toxicity [64,65]. Lithium-ion batteries based on
polymer electrolytes are designed to be flexible. The battery can be fabricated in the
form of cylinder, coin, flat cells, etc. There are three widely used types of polymer
electrolytes: (1) solid polymer electrolyte, (2) gel polymer electrolyte, and (3) poly-
mer composites [65]. As it is mentioned that solid polymer electrolyte comprising a
polymer matrix and a lithium salt is called dry SPE. These types of electrolytes have
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very small ionic conductivity (<10 8 Scm ). Several ways have been adopted to
increase the ionic conductivity of the dry SPE. An approach to increase the ionic con-
ductivity by increasing the concentration of the Li salt by dissolving small amount of
polymer matrix in large amount of salt is called polymer-in-salt electrolytes or
rubbery electrolytes [66–68]. In these electrolytes, the ionic conductivity as high as
10 5 Scm 1 has been reported for polymer electrolytes composed of an acrylonitrile
and butyl acrylate copolymer poly(AN-co-BuA) with addition of LiN(CF 3 SO 2 ) 2
(LiTFSI) or LiI and LiTFSI salt mixture [67]. The content of Li salt was very high
(around 80%). In another attempt, the biionic solid polymer electrolyte arising from
the cations and anions present in the Li salt and cations attached to the polymer back-
bone and anions is coordinated nearby the cations. The anions are relatively free and
move faster. This leads to the concentration polarization as anions also migrate toward
the anode. This leads to gradual decay of the ionic conductivity of the SPEs. In single-
ion conductor polymers [68–70], the anions are attached to the polymer backbone so
that they are not able to move and minimize concentration polarization [68]. Method
has been employed, in which an anion receptor is introduced into electrolytes to
increase their ionic conductivity and/or cation transference number. In these systems,
although anions are trapped by an anion receptor, the interaction between the anion
and anion receptor promotes the further dissociation of lithium salts, which might lead
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to an increase in both ionic conductivity and Li transference number at the same
time [35].
Gel polymer electrolytes are one of the most promising polymer electrolytes [71].
In this type of electrolyte, in addition to polymer electrolyte and Li salt, an external
plasticizer or solvent is added. The solvent or the plasticizer can be either attached
physically or chemically. As usual, lithium salts provide conduction in polymer
matrix, polymer provides mechanical strength, and the plasticizer improves the ion
conductivity [72]. The advantage of gel polymer electrolytes is their improved ionic
conductivity: as high as 10 3 Scm 1 has been reported [73] and high mechanical
strength. The polymer electrolytes show a crucial drawback that has low ionic