Page 371 - Polymer-based Nanocomposites for Energy and Environmental Applications

338 Polymer-based Nanocomposites for Energy and Environmental Applications

100 nm can be obtained from cellulose extracted from a pulp [29]. Such fibrous net-
works exhibit capacity of 176 mAh/g after 600 cycles at current density of 0.2 A/g.
Electrospun CNFs from lignin/PAN precursor also improve cycling stability and
maintain the structural stability of the electrode as a result of the more disordered
turbostratic microstructure imparted by the presence of lignin [30]. CNF anode
obtained from lignin/PAN nanofibers in which the PAN and lignin were present in
1:1 weight ratio and carbonized at 1300°C provides capacity of 247 mAh/g after
200 cycles at 0.1 A/g. Fibrous carbon obtained from biomass, such as ramie fibers
or corncobs, exhibits fair capacities in a Na-ion cell [31].
Despite the fact that turbostratic carbon structures can reversibly host Na ions, their
rate capability is limited. The low-rate performances of pure CNF can be improved by
nitrogen (N) doping. N-doped CNF obtained from bacterial cellulose nanofibers,
using pyrrole as N-precursor, significantly increases the capacity of the anode [32].
The increased capacity and rate capability of the N-doped anode was attributed to
the facilitated surface redox reactions in the presence of N- and O-containing func-
tional groups on the surface of the CNF, as shown in Fig. 12.3C. Imidization of
polyamic acid (PA) electrospun nanofibers can also serve as precursors for CNF with
high content of nitrogen [33]. Such carbon nanofibers provide extremely high rate
capability retaining capacity of 154 mAh/g at current density of 15 A/g or 210 mAh/g
after 7000 cycles at current density of 5 A/g. N-doped CNFs with hollow, porous, or
core-shell structures using pyrrole or aniline as N-precursors have also been investi-
gated [34,35]. The synergetic effect of hollowness, increased porosity, and the
presence of nitrogen increase the capacity and rate capability of the electrode.
CVD-grown CNF on a metallic substrate is another approach to obtain 1-D carbon
structures. Helical CNF has been grown on a texturized Ni nanowires using acetylene
as carbon precursor [36]. These CNFs arranged in a foam-like structure deliver a
capacity of 225 mAh/g at current density of 0.1 A/g and show excellent cycling
stability, retaining 98% of its capacity after 200 cycles.
Although CNFs are convenient electrode material that can be used as freestanding
electrode, due to their high conductivity, it opens a way to eliminate the usage of addi-
tional current collectors. The data summarized in Table 12.1 indicate maximum
capacity of 270 mAh/g for pure and freestanding CNF at 0.1 A/g. N-doped CNFs
do not enhance this value significantly, but they improve the rate capability of the
anode, which is an important factor for batteries for large applications. However, these
capacity values are not high enough and thus limit the performance of the NIB. There-
fore, the addition of active materials within the 1-D structures is necessary. Among
them, tin oxide (SnO 2 ), transition metal oxides and sulfides, and different antimony
(Sb)-based materials have been investigated as fibrous 1-D structured anodes for NIB.
Even though phosphorus (P) has the highest potential as NIB anode, due to its
theoretical capacity and low redox potential, it lacks electric conductivity, which leads
to low reversible capacities and poor cycling stability. Therefore, phosphorus’carbon
composites are among the most investigated P-based electrodes [10]. P-coated,
N-doped CNFs have been produced [65]. Solvothermal synthesis was selected for
the preparation of N-doped CNF, which were later thermally treated in the presence
of red phosphorus. Such P/N/CNF showed reversible capacity of about 850 mAh/g