Page 383 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanofibrous composites for sodium-ion batteries 345

of Sn during cycling and provide efficient pathway for electron transport, as can be
seen in Fig. 12.4B. The combination of Sn and SnO 2 contributed to increased capacity
of 537 mAh/g at 0.1 A/g. Sn nanodots encapsulated into N-doped PCNFs enhance
both the rate capability and the cycling stability of the cell [39]. Such nanofibers
obtained by simple electrospinning technique of a mixture of polymers containing
Sn-salt deliver capacity of up to 543 mAh/g at 2 A/g, which falls to 483 mAh/g after
1300 cycles. These performances are the results of the defectuous C-plane formed and
the presence of Sn-atoms and three different types of NdC bonds.
Antimony, as another element that theoretically offers high capacities and energy
densities, together with its alloys is another active material for NIB anodes. The
behavior of the 1-D structures of Sb-based materials has also been the topic of research
in the last years. Electrospun PAN solutions containing SbCl 3 and converted into
Sb/CNF after a thermal treatment were used both as freestanding anodes without
any current collector [40] and as anode material coated on Cu-foil [41] and showed
good cycling stability up to 300 cycles. However, the capacities of the freestanding
and the slurry anodes differ. The former one exhibited capacity of 405 mAh/g at
0.1 A/g, while the later one 528 mAh/g. This may be attributed to the different Sb
loading and the difference in the stabilization treatment of the fibers before carbon-
ization. It should be noted that in the later study, the capacity values were calculated
on the base of the mass of the active material, which determined by TGA that was
stated as 54%. Antimony’tin alloy particles embedded into CNF exhibit similar capac-
ities [42]. In another approach, oxidative polymerization of polypyrrole (PPy) was
used as a production method for nanofibrous networks [43]. After activation in the
presence of KOH, these fibrous networks form interconnected carbon nanonetworks
(ICNN) on which Sb can be loaded via sonication. These Sb/C fibrous networks,
shown in Fig. 12.5, have enough voids to buffer the volume expansion of Sb and
provide good and traced electron path that leads to cycling stability of 96.7% after
100 cycles and high rate capability (325 mAh/g at 3.2 A/g).
Transition metal dichalcogenides are also in focus of interest for researchers due to
their unique 2-D structure that allows high cycling stability. Electrospun nanofibers
again take part of the preparation of molybdenum disulfide (MoS 2 ), iron sulfide
(FeS), or tungsten sulfide (WS x )-doped CNF as anodes for NIB. Ammonium
tetrathiomolybdate (NH 4 ) 2 MoS 4 [44,45,66], iron (III) acetylacetonate Fe(acac) 3
[47], and ammonium tetrathiotungstate (NH 4 ) 2 WS 4 [48] mixed with either PAN or
poly(styrene-acrylonitrile) (SAN) have been used as precursors. Gravimetric capaci-
ties up to 935 mAh/g MoS2 (based on the total weight of the active material) at 5 A/g
with capacity retention of 86% after 500 cycles have been reported for MoS 2 /CNF
with controlled MoS 2 structure and morphology within the CNF [45]. The specified
embodiment of MoS 2 nanoplates within the mesoporous carbon sheets obtained by
controlled thermal treatment allows high structural stability of the anode during inser-
tion and deinsertion of ions during cycling. The mechanism on which Na and Li ions
intercalate within different MoS2 structures can be seen in Fig. 12.6. In case where
carbon content is taken into account, the capacity of the flexible MoS 2 /C anodes is
about 380 mAh/g at current density of 0.1 A/g with 75% capacity retention after
600 cycles [44]. Titania (TiO 2 ) coating on electrospun MoS 2 nanofibers by atomic