Page 322 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for lithium battery applications 291

group) and high dielectric constant (ε¼8.4) that facilitate better dissolution of lithium
salts. But PVDF is not stable toward lithium metal. It has been widely studied as a
separator and also as a microporous polymer electrolyte prepared by phase inversion
technique. Blending with PEO can help to tailor the pore size, porosity, and pore
connectivity of PVDF/PEO blends [64]. The shortcomings of PVDF can be overcome
by using poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), where the
HFP functions as a plasticizer. PVDF-HFP shows higher ionic conductivity and cation
transport number than pure PVDF because of the reduced crystallinity of PVDF-HFP.
Metal oxide (TiO 2 , MgO, and ZnO)/or mesoporous zeolite (MCM-41 and SBA-15)
fillers, ionic liquids, ethylene carbonate, propylene carbonate plasticizers, and LiTFSI
or LiClO 4 salt can be added to PVDF-HFP using simple direct evaporation method,
and the room-temperature ionic conductivity reached 2.1 10 3 Scm 1 [52,65].
Other polymers have been studied as electrolyte matrices, such as poly(ethylene
imine), poly(methoxy ethoxy ethyl methacrylate), poly(vinyl acetate), hyperbranched
poly[bis(hexaethylene glycol)benzoate] with terminal acetyl groups, poly
(oligoethyleneoxy phosphazane), polycarbonates, polysiloxanes, poly(acrylonitrile-
co-bis[2-(2-methoxy ethoxy) ethyl]itaconate), and carboxymethyl cellulose. Among
them, polycarbonate is promising due to the presence of carbonate groups in the
polymer backbone provide a polar environment for dissociation of salts and solvation
of ions [66].
Other emerging systems are anionic polyelectrolytes or polymeric single-ion
conductors, for example, a polymer backbone having the repeating units of anionic
moiety along with a lithium counter ion free to move, which delivers the required ionic
conductivity. Due to their single-ion nature, these systems are expected to deliver
lithium-ion transport number close to unity [67]. Various block polymer electrolyte
systems [68] have been proposed recently to replace conventional homopolymers
due to the potential tunability of molecular architecture and functionality.

10.2.3 Lithium salts
Salts are integral part of any electrolyte; the ionic conductivity is determined by
the mobility of dissociated ions that are generated from the dissociation of salt.
The properties of PCEs depend on the type of salt and its concentration in the polymer
matrix. The basic parameters that dictate the characteristics of PCEs are the ionic
conductivity, the thermal and chemical stability, the solubility of salt in the polymer
matrix, the stability of anion against oxidative decomposition at the cathode, the
ability of anion to form a stable solid-electrolyte interface (SEI) layer, and the cost
and toxicity.
Various kinds of salts with different anions have been studied; some examples are
listed below:

(1) Lithium tetrafluoroborate, LiBF 4
(2) Lithium hexafluorophosphate, LiPF 6
(3) Lithium hexafluoroarsenate, LiAsF 6
(4) Lithium perchlorate, LiCIO 4