Page 369 - Polymer-based Nanocomposites for Energy and Environmental Applications

336 Polymer-based Nanocomposites for Energy and Environmental Applications

energy storage devices enable stable and ionically conductive solid-electrolyte inter-
phase (SEI) layer during cycling and hence improve the electrochemical performances
of the battery. The possibility to design and modify their structures and properties has
strong impact on the improvement of battery life and capacity.
Fibrous materials, as opposed to particle-form materials, have been reported as
constituent parts of different energy storage systems, including secondary batteries
[11]. Doped or blended nanofiber webs, nanotubes, nanorods, and/or nanowires have
been considered as electrodes and/or separators in these devices. The enhanced
characteristics of fibrous materials arise from the combination of compound charac-
teristics and unique morphologies and structures formed in the fibers. The
one-dimensional morphology, high surface area, adjustable density, and high porosity
of fibers decrease the length of Na-ion diffusion pathways and thus enhance the power
capability and kinetic properties of the battery [12]. Thus, the research and develop-
ment of 1-D structures for energy storage devices, including NIB, shows a
growing trend.

12.1.2.1 Nanofiber structures and production methods
The synthesis of 1-D structures allows the formation of a wide range of different
configurations (doped, porous, hollow, and core-shell) using a variety of organic
and inorganic materials [13]. NF can be obtained by different methods. Among them,
electrospinning, solution blowing, and centrifugal spinning are the most promising for
industrial production and enable formation of nanofibrous mats that can be used as
freestanding electrodes in energy storage devices [13,14]. These methods use polymer
solutions or melts together with specific forces to shape the polymer chains into
fibrous form. In the case of electrospinning, electric force is used for attenuation of
the polymer jets [12]. Solution-blowing method uses compressed air, while centri-
fugal spinning uses centrifugal force to overcome the surface tension of the polymer
fluid and form fine polymer jet that solidifies into fiber form on its way to the collector
[13]. The last two production methods enable high production rate, safer operation,
and lower power consumption. However, their studies for energy application are
not that widely investigated compared with electrospinning.
Polymer nanofiber mats produced via spinning methods are usually used as precur-
sor fibers for obtaining carbon nanofibers (CNF), which can be further used in many
areas, including energy storage. Such CNF mats can be doped or coated with a variety
of materials that improve the electrochemical performances of the device. Despite the
possibility for spinning neat, doped, or blended polymer solutions by using coaxial
nozzle, various polymer solutions can be simultaneously spun, which enables the for-
mation of porous, hollow, core-shell, and multilayered nanofibrous structures [15-18].
These spun polymer nanofibers can also serve as templates for organic, inorganic, or
porous carbon nanofiber (PCNF) formation. Composite nanofibers can be produced
by using spun nanofibers as template for in situ deposition of other polymer in the
presence of oxidizing agent [19]. Another alternative is removing the organic part
by annealing the polymeric nanofibers that contain inorganic salts [20,21]. Polymeric