Page 330 - Polymer-based Nanocomposites for Energy and Environmental Applications

298 Polymer-based Nanocomposites for Energy and Environmental Applications

electric connections and mechanical integrity. Si anodes using this novel polymeric
binder showed a reversible capacity of 2806 mAh g 1 at 420 mAh g 1 after
100 cycles (85.2% retention of the initial capacity).
Conducting carbon nanomaterials play key roles in structuring conducting
networks in electrodes. The carbon nanomaterials such as carbon nanotubes,
graphene, and porous carbon have been applied as either conducting additives
or scaffolds for active materials. For example, porous carbon-loaded sulfur
overcame the low conductivity and dissolvability of sulfur in electrolyte, and
the hierarchical pore structure accommodates the large volume expansion during
cycling leading to high rate and cycling stability of the lithium’sulfur batteries
[88]. Similarly, confined Si in porous carbon matrix highly accommodates the
volume change without hindering the ion transport, which led to a high reversible
capacity of 1050 mAh g 1 at a high current density of 10 A g 1 [89]. Modifying
the surface of carbon nanomaterials is also important, as it is found that pure
graphene without defects is hard to stabilize Li ions and thus affects the accessi-
bility of the ions, while the defects on graphene surface can act as electrochemi-
cally active lithium storage sites [90].
Heteroatom-doped carbon nanocomposites can further improve the performance of
electrodes. It is reported that the reversible discharge capacity of the nitrogen-doped
graphene is nearly double compared with the original graphene; the increased surface
area and defects on the graphene induced by nitrogen dopant contribute to the
improved performance [91].
Among graphene and carbon nanotube, porous nanocarbon is unique in terms of
the hierarchical porous structures. Polyphosphazene chemistry enables a wealth of
functional organic-inorganic covalent cross-linked polyphosphazene materials
with combined chemical stability, mechanical robustness, biocompatibility, and
ionic conduction properties [92]. Our research on multiheteroatom-doped micropo-
rous carbon nanospheres prepared by pyrolysis of organic-inorganic covalent
cross-linked polyphosphazene materials has demonstrated a novel electrode mate-
rial in supercapacitor and LIBs. The first related work in LIBs was released in 2009
by Gao et al. [93], who applied a thin ( 10 nm) microporous carbon coating on Si
nanoparticles (50–100 nm) in order to deal with the usual volume expansion issues
of Si during Li charge/discharge cycles. The obtained C@Si test in Li-ion half cells
achieved a discharge capacity of 1200 mAh g 1 at 0.5°C over 40 cycles. Xue et al.
achieved a performance of 700 mAh g 1 at 100 mA g 1 after 40 cycles in a C@Si-
MWCNT system [94].
Multiheteroatom-doped carbon nanospheres were successfully prepared by
carbonization of polyphosphazene nanospheres (Fig. 10.9) [95]. The porosity and
heteroatom (N, P, S, and O) content were dependent of the carbonization profile.
The treatment at 850°C produced specific surface area of the carbon spheres up to
2 1
840 m g due to the presence of abundant micropores and also generated high
content of pyridinic N and P. The as-prepared carbon spheres were tested as anodes
in LIBs and delivered a capacity of 130 mAh g 1 at a current rate of 1C and cycling
stability of >1000 cycles [95]. In addition, a remarkable coulomb efficiency higher