Page 370 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanofibrous composites for sodium-ion batteries 337

nanofibers containing inorganic particles can be converted into PCNFs after leaching
process that removes the inorganic part of the carbonized fibers [22].
Besides these spinning processes, 1-D structures can be obtained through other
techniques among which thermal synthesis is widely used when preparing electrodes
for energy storage devices. Thermal chemical vapor deposition (CVD) has been used
for synthesis of nanotubes (NT) or nanofibers [23] and may be combined with other
techniques in order to obtain different structures. Another approach is using solution
containing precursor salts that after heating under controlled conditions may give a
variety of nano-/microstructures such as particles, flakes, flowers, fibers, and tubes
[24,25]. In this hydro- or solvothermal synthesis, the final structure depends on
parameters, such as the temperature, time, and salt concentration. Continuous produc-
tion of fibrous structures by these methods is difficult and thus not industrially
feasible.

12.2 Nanofibrous constituents for NIB

12.2.1 Nanofibrous anodes

Anodes, or negative electrodes, are the constituent part of rechargeable batteries that
have largest impact on the capacity of the cell. It further influences the energy density
of the full cell since its redox potential affects the overall potential window of the bat-
tery. In order to achieve battery system with high energy density, an anode with low
electrochemical potential and high capacity is required. Knowing the materials that fit
these requirements, there is another issue to be solved in the case of Na-ion system.
Namely, due to the larger atomic diameter and mass, the anode also has to be able to
host Na ion without structural degradation over extended working time. This specific
issue, together with the volume expansion and low electric conductivity of the active
materials that provide high capacity, can be solved by developing composite struc-
tures that merge all these concerns. In this regard, here, we review the most attractive
active anode materials and their performances as 1-D fibrous structures.
Similarly to the anode researches for Li-ion batteries, many researchers focused
on developing CNF structures suitable for NIB. In this case, the fiber diameter, the
carbonization temperature, and the porosity of polyacrylonitrile (PAN)-based CNF
structures are important factors that affect the performances of the anode. The studies
showed that the turbostratic structure, formed at carbonization temperatures below
1300°C, provides higher capacities [26]. Even though these CNFs can be used as a
freestanding electrode without addition of current collector, they exhibit better rate
performances when casted on a current collector as a result of the improved electron
transfer when high currents are applied [27]. On the other hand, CNFs with increased
porosity (named PCNF) increase the Na-ion uptake and thus significantly enhance the
capacity of the cell. They were obtained from PAN/poloxamer mixtures by
electrospinning method, in which poloxamers act as a pore former [28].
Other carbon precursors, such as cellulose nanofibers, lignin, ramie fibers, or corn-
cobs, have also been investigated [29-31]. Carbon nanofibers with diameter below