Page 333 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for lithium battery applications 301

extensive research for many years, their commercialization has been hampered by
severe safety issues that are due to the formation of irregular lithium dendrites during
the recharge process. Conventional liquid electrolytes facilitate the formation of
dendrites, while polymers and composites are expected to be protagonist in future
batteries as either electrolytes or electrodes, despite of the numerous challenges posed
by the all-solid-state systems. Power performances are limited in solid polymer
+
electrolytes because of the small fraction of charge carried by Li ions.
Outstanding results have been achieved by preparing a polyphosphazene-based gel
polymer electrolyte that combines easy production at low cost, thermal stability, and
good compatibility with the liquid electrolyte, resulting in good mechanical strength
of the membrane and good electrochemical stability toward repeated reduction and
oxidation. The full cell with an inorganic polyphosphazene-based gel electrolyte in
combination with a lithium-metal anode was operated for >1300 cycles [100].An
innovative and effective approach has been recently proposed in which a three-
dimensional lithium-ion-conducting ceramic network based on the garnet-type
Li 6.4 La 3 Zr 2 Al 0.2 O 12 (LLZO) lithium-ion conductor provided interconnected Li +
transfer pathways in a PEO-based composite [101]. This fiber-reinforced polymer
composite showed advantages, among the other (i) bendable structure with improved
mechanical properties, (ii) interconnected pores to enhance the ionic conductivity, and
(iii) high thermal stability. Since the 3-D structure is based on an inorganic material,
safety issues due to the melting of the polymer at high temperature (as observed in
conventional polymer batteries) that may cause direct contact between the anode
and the cathode are properly addressed. Moreover, the authors have shown that such
a membrane is effective in preventing dendrite formation in a Li/electrolyte/Li cell
during repeated Li stripping and plating over hundreds of hours. Nanocomposites with
enhanced ion transport and intrinsic safety would be an ideal solution, and polymer-
particle systems and the ubiquity of polymers can create opportunities for post Li-ion
batteries in general.
However, as far as concerns especially Li-air batteries, a significant chemical
stability of polymers is fundamental given the presence of highly reactive species.
Li-air battery technology is based on the reduction of oxygen at the cathode during dis-
charge of the cell that leads to the formation of lithium superoxide (LiO 2 ) at first and
then to lithium peroxide. The latter is reversibly decomposed during charging to evolve
oxygen. It is obvious that in such a reactive environment, polymers can undergo
unwanted side reactions that can alter their properties or even degrade irreversibly.
Recently, the chemical stability of different polymers has been investigated to ascertain
their applicability in Li-O 2 cells [102]. Starting from the most used polymers in battery
research such as PAN, PMMA, PTFE, PVC, PVDF, and PVDF-HFP and thanks to
chemical reactivity tests carried out in the presence of Li 2 O 2 , that study has highlighted
different polymer reactivity and reaction mechanisms. The order of polymer reactivity
has been determined to be PAN>>PVC PVDF>PVDF-HFP>>PVP; while
PMMA, PTFE, Nafion, and PEO appeared to be chemically stable. Although this kind
of investigation is partial because there is no information concerning reactivity toward
superoxide species (not only nucleophile like Li 2 O 2 but also a source of radicals), it
constitutes a basis for an initial selection of stable polymers and a screening method