Page 389 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanofibrous composites for sodium-ion batteries 349

Ti-based anodes exhibit capacities below 250 mAh/g at low current density of
0.05 A/g [54,67-70]. However, their advantage is in the extremely high cycling
stability of almost 99% after 1000 cycles [54]. Nitrogen-doped mesoporous TiO 2
nanofibers can significantly improve the rate capability and at the same time maintain
the cycling stability of the anode as a result of the reduced Na-ion diffusion and
electron transport resistance [55].
Other transition metal oxides that form conversion reactions with Na ion during
cycling have higher capacities than TiO 2 . Their high capacity combined with conduc-
tive material, such as carbon in a 1-D structured composite, also delivers high cycling
stability. Electrospun nanofibers containing Cu, Co, and MndFe precursors form
CuO [56],Co 3 O 4 [57], or MnFe 2 O 4 nanodots [58] embedded into CNF, after a
controlled thermal treatment. The capacities of these metal oxide CNFs vary from
189 to 401 mAh/g and 413 mAh/g for the Cu, Co, and MndFe oxides, respectively.
Among them, the binary metal oxide has been proved as most stable at high current
densities, retaining about 90% of its capacity after 4200 cycles [58]. This freestanding
anode paired with Na 3 V 2 (PO 4 ) 2 F 3 /C cathode in a full NIB cell exhibits capacity of
406 mAh/g at 0.5 A/g in a voltage window of 2.3 V, which corresponds to energy
density of 77.8 Wh/kg, shown in Fig. 12.7.

12.2.2 Nanofibrous cathodes

The role of the cathode in NIB is to release Na ion during charge and to accept it during
discharge while simultaneously providing and accepting an electron from the external
part of the circuit [71]. In order to do so, it needs to operate at higher potential than the
anode and in the same time to provide high specific capacity. The higher the operating
potential and the capacity of the cathode, the higher the energy and power densities of
the battery. In this regard, different cathode materials and structures have been inves-
tigated till now. Among them, materials such as transition metal oxides with layered
or tunnel structure, phosphates, sulfates, hexacyanometals, and conductive organic
compounds have been synthetized in a variety of structures [71]. From them, vana-
dium phosphates, iron sulfates, and different multitransition metal oxides have been
made as 1-D nanofibrous structures and are summarized in this part.
In situ synthetized Na 3 V 2 (PO 4 ) 3 (NVP) crystals embedded into carbon nanofibers
can be obtained by electrospinning of polyethylene oxide (PEO) solution containing
NaH 2 PO 4 ,NH 4 VO 3 , and citric acid [59]. Such nanofibers are good Na-ion conductors
that exhibit capacity of approximately 75 mAh/g at 2C and reversible Na-ion insertion/
deinsertion. NVP/CNF composite cathodes with lower carbon content would improve
the capacity of the electrode due to the enhanced utilization of the active material.
This was be obtained by changing the carbon precursor. PVP-based NVP/CNF
cathodes, containing two times less carbon, showed capacity of 110 mAh/g at the
same C-rate [60].
Regardless of the ability to produce nanofibers with controlled diameters for a rela-
tively short time through this production method, the disadvantage of these structures
is the high carbon content within the electrode. This decreases the overall capacity of
the electrode and its rate capability. In order to achieve the theoretical capacities of