Page 325 - Polymer-based Nanocomposites for Energy and Environmental Applications

294 Polymer-based Nanocomposites for Energy and Environmental Applications

framework within the electrodes [74,75]. Further thermal treatment of PAA-/PVA-
based binders is generally required in order to produce chemical cross-linking
networks of the binders. In addition of using PAA/PVA blends or PAA-co-PVA
copolymers, catechol-grafted PAA and alginate have further improved the capacity
and cycling stability of the Si anodes [74] that is due to the improved adhesion and
redox functions of the binders. In any case, the current binder technologies either
rely on the intrinsic hydrogen bonding or postthermal cross-linking, which involves
complicated reactions and multistep purification processes.
Thermally stable copolyimide (P84) (Fig. 10.5F) was used as an adhesive binder
1
for Si anode, which delivered higher initial discharge capacity (2392 mAh g ), plus
fairly improved coulomb efficiency (71.2%) compared with the Si anode using
conventional PVDF binder (2148 mAh g 1 and 61.2%, respectively) [76]. As a result,
the Si anode could deliver 647 mAh g 1 until the 300th cycle, which is still two times
1
higher than the theoretical capacity of graphite at 372 mAh g .
Electric conducting polymers such as polyacetylene, polyaniline, and polypyrrole
have been applied as both electric conductive additives and binders. The chemical
structures of conducting polymers are shown in Fig. 10.6. Various conducting-
polymer-based heterostructures, such as V 2 O 5 /PEDOT [77], MoS 2 /PANI [78],
a-Fe 2 O 3 /PPy [79], and Fe 3 O 4 /poly[3-(potassium-4-butanoate)thiophene] (PPBT)
[80], have been developed as promising electrodes for energy storage; some examples
are shown in Fig. 10.7. A thin layer of poly(3,4-ethylenedioxythiophene) polystyrene
sulfonate (PEDOT/PSS) on an electroactive LiCoO 2 (LCO) powder provides elec-
tronic/ionic pathways and delivers higher volumetric capacities [81]. The conducting
coating on Si surface alleviates the volume changes of Si anodes. However, the
conducting polymers are limited by the low stability during the charge/discharge
process and solubility in electrolyte [82].
Amphiphilic polymeric binders composed of electron- and ion-conducting poly
(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) were applied for
1
carbon-free V 2 O 5 cathodes [83], which shows a highest capacity of 190 mAh g ,
2.5-fold higher than V 2 O 5 electrode. The unique architecture of P3HT-b-PEO,
wherein P3HT and PEO blocks are covalently bonded, promotes the uniform distri-
bution of conductive binders within the V 2 O 5 structure, whereas the analogous P3HT/
PEO blend suffers from phase separation. This shows that the copolymerization of
conducting polymers improves the stability and performance of the conducting
binders.
Recently, 3-D conductive interpenetrated gel network has been studied as a novel
binder system for Si anodes, which either through the encapsulation by graphene
sheets, in situ polymerization of conductive polypyrrole gel onto commercial lithium
iron phosphate particles (Fig. 10.7A) [84], or chemically cross-linking of acrylic acid
monomer followed by in situ polymerization of aniline (Fig. 10.7B) [85]. The 3-D
conducting framework electrodes exhibited greatly improved rate and cyclic perfor-
mance because the highly conductive and hierarchically porous network promotes
both electron and ion transport. In addition, it shows the potential to mitigate the
aggregation of additives and maintain both electric and ionic connections among
the active materials with current collectors [10]. Three-dimensional porous PANI