Page 398 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanofibrous composites for sodium-ion batteries 355

materials for energy applications. Their performances as electrodes for energy storage
were already analyzed in various reviews [11,14,74] and in life-cycle assessment
studies [75], emphasizing their possible application. Even though only small percent
of the studies on NIB electrodes are on fibrous nanostructures, they show promising
results and a room for more innovations and improvement.
Following the evolution in Li-ion battery technology, carbon nanofibers are
among the most widely investigated fibrous structures for NIB. Table 12.1 summa-
rizes the materials and production methods used in the reviewed studies together
with their electrochemical performances. Even though PAN is the preferred precur-
sor for electrospun nanofibers, there is a tendency to use natural resources to
reduce the environmental impact and costs of the electrodes. Bio-based fibers
and combinations of several techniques can be used for the production of
nanofibrous carbon anodes for NIB. Doped carbon nanofibers proved to be good
approach to enhance electrochemical performance of the battery. Active materials
embedded into CNF matrix form composite material that can increase the capacity
and at the same time improve the cycling stability of the electrode. Here, the
carbon matrix acts as buffer for the volume expansion of the active material, main-
tains the structure of the material, and hence extends the lifetime and stability of
the electrodes. Transition metal oxides, dichalcogenides or multimetal oxides, sul-
fates, and phosphate nanofibrous structures exhibit better interconnection between
their nanocrystallites, which results in improved capacity and rate capability of the
anode and the cathode. Deposition of carbon layer with controlled thickness over
this 1-D structures or increasing their porous can significantly improve the cycling
stability of the electrode.
From aspect of production methodologies, various techniques can be used for the
production of nanofibrous electrodes. Among them, electrospinning is the most
widely used (Table 12.1). Its advantage is the possibility to produce freestanding
electrodes that can be used without additional current collector. Even though they
showed good performance under different conditions, the conventional slurry-coated
electrodes are still more preferred especially when active materials are the main
constituent of the electrode. The production of pliable electrodes with high content
of active material is still a challenge.
It should be noted that the investigation on fibrous composite structures as NIB
electrodes is at its beginning. Further research is needed and certainly will be done
in the upcoming years. There is a big room for new materials and fiber formation
methods to be developed and tested. Productive nanofiber spinning systems such
as centrifugal spinning and solution blowing might be preferred in the future. In gen-
eral, Na-ion technology is still novel and requires further innovations in both materials
and structures and production methods.

References

[1] Ellis BL, Nazar LF. Sodium and sodium-ion energy storage batteries. Curr Opin Solid
State Mater Sci 2012;16:168–77.
[2] SakaiM,AmanoM.Energystoragedevicesandsystems,HitachiChemicalTechnicalReport
No. 55, 2013, 9–11, http://www.hitachi-chem.co.jp/english/report/055/55_sou02.pdf.