Page 392 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanofibrous composites for sodium-ion batteries 351

the active material and to improve the rate capability of the electrode, solvo- or hydro-
thermal synthesis has been suggested as methods to produce 1-D structured electrodes.
During these types of thermal synthesis, the carbon content can be controlled with the
amount of carbon precursor, which may be added during or after the synthesis [24,25].
Solvothermal synthesis enables the development of microspheres into microflower
and nanofibers depending on the synthesis time [25]. The comparison of these struc-
tures emphasizes nanofibers as more suitable structure for achieving higher capacity
and more stable cycling performances, showing capacity fade of 4% after 1000 cycles.
Fig. 12.8 shows SEM images of the evolution mechanism of these structures and their
cycling performances when used as cathodes in half-cell NIB. NVP nanofibrous struc-
ture covered with tiny carbon layer, obtained by this method, exhibits enhanced elec-
trochemical performances, compared with electrospun nanofibers (110 vs. 40 mAh/g
at 10C for 3-D nanofibers and electrospun nanofibers, respectively [25,59]). Further-
more, these 3-D nanofiber structures exhibit good rate and cycling performance when
tested in a full-cell NIB, against titanium phosphate anode.
NVP nanofiber structures obtained by hydrothermal synthesis and covered with
carbon exhibited excellent cycling performances even after 10,000 cycles at 20C
[24]. Here, the limited and well-controlled carbon content enables enhanced connec-
tion between both materials and also allows full utilization of the NVP active material.
Sulfates with high electronegativity and large operating voltage window are poten-
tial active cathode materials for NIB. Iron sulfate (NFS-Na 2 Fe 2 (SO 4 ) 3 ) embedded into
porous CNF structure was produced via two steps, electrospinning-electrospraying
method [61]. PCNFs were produced by electrospinning of PAN/SiO 2 mixture and then
sprayed with Na 2 SO 4 /FeSO 4 7H 2 O aqueous solution and calcinated. The flexible
NFS/PCNFs were used as freestanding cathodes in a half-cell NIB without addition
of any binder or current collectors. The enhanced capacity and cycling stability of
this sulfate cathode was attributed to the porosity and conductivity of the structure,
which enables fast ion and electron movement during charge/discharge processes.
The optimization of the carbon content in this study was also a factor determining
the overall capacity of the electrode.
Electrospun polyvinyl acetate (PVAc) aqueous solution containing Mn(NO 3 ) 2 and
NaCH 3 COO can give Na 0.44 MnO 2 nanofibers or nanorods depending on the annealing
temperature [62]. Nanofibers with controlled nanograins were obtained after calcina-
tion at 600°C while nanorods with nucleate nanograins at 800°C. The electrochemical
tests of both structures indicated low performance of the nanorods. These low capac-
ities were attributed to the aggregation phenomena during Na-ion intercalation. On the
other hand, the stable and continuous 1-D structure of the nanofibers prevents
self-agglomeration and thus exhibited higher capacity and rate capability, even though
their cycling stability was inferior to that of the nanorods, as shown in Fig. 12.9.
Multimetal oxides, such as Na 2/3 (Fe 1/2 Mn 1/2 )O 2 (NFMO) or Li 1+ x (Mn 1/3 Ni 1/3
Fe 1/3 )O 2 (LMNFO) with layered structure, are another promising active materials
for NIB cathodes. Their 1-D nanofibrous versions were obtained by electrospinning
of PVP solutions containing their corresponding salts and subsequent annealing
[63,72]. The NFMO nanofibers with hierarchical 1-D structure showed enhanced
capacity, compared with NFMO nanoparticles, as a result of the improved