Cathode and Anode Materials for Na-Ion Battery - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 410

alloy to form Na 3 Sb and Sn phase, while the Sn phase is inactive at this time and
acts as the conducting buffer to maintain structural integrity. When discharge
continues to \0.4 V, alloying reaction of Na with Sn occurs and Na 3 Sb is inactive
and serves as a buffer matrix. So, such a self-supporting network can maintain the
integrity and conductivity of the whole electrode material to provide good cycling
stability of the electrode.
Yang et al. prepared an Sb/C composite by mechanical milling, which was
composed of ~10 nm nanocrystallite Sb embedded in the carbon matrix [ 67 ]. The
naocrystalline Sb/C electrode delivered an initial capacity as high as 610 mAh g -1
based on the pure Sb, very close to the theoretical capacity (Na 3 Sb, 660 mAh g -1 ).
The Sb/C electrode in a 5 % FEC-containing electrolyte maintained an almost
constant capacity of 575 mAh g -1 over 100 cycles [ 67 ]. The excellent electro-
chemical performances enable the Sb/C nanocomposite to be used as candidate
anode for high capacity and high safety Na-ion batteries.

4.3 Metal Oxide Materials

In general, the Na storage capacities in materials with an intercalation reaction
mechanism are limited by the number of host sites in their structures. One way to
circumvent this intrinsic limitation and to achieve higher capacities would be the
use of materials with a reversible conversion reaction mechanism. The Na storage
capacity then depends on the change of the redox valence states of the material
while not the material structure. Such materials are mainly oxides, [ 69 - 71 ] fluo-
rides, [ 72 ] and sulfides [ 73 ]. Among them, oxides have the most appropriate Na
storage potentials. Tirado et al. first reported the electrochemical characteristics of
NiCo 2 O 4 spinel for Na storage [ 70 ]. It showed that the NiCo 2 O 4 electrode
delivered a reversible capacity of ca. 200 mAh g -1 with an average potential of
1.5 V. Co 3 O 4 was also investigated as Na storage anode [ 71 ]. This material
delivered 444 mAh g -1 , a reversible capacity of but with poor cycling

performance.
Johnson et al. synthesized amorphous titanium dioxide nanotube (TiO 2 NT) to
investigate its utilization for Na-ion batteries [ 74 ]. It was found that only when the
size of the nanotubes increased to [80 nm (wall thickness [15 nm), the amor-
phous TiO 2 could show relatively low specific capacity initially, which then self-
improved as cycling proceeded. The electrode delivered a reversible capacity of
75 mAh g -1 in the first cycle and the capacity increased to 150 mAh g -1 after 15
cycles (Fig. 27 a). This behavior can be explained that Na ions cannot adsorb on
TiO 2 surface and screen the charge of electrons injected into the TiO 2 matrix in the
relatively small-diameter tube, so that insufficient Na ions can not establish critical
ion concentration that supports cycling. Thus, it is important to tailor the nano-
structure of materials to enhance their Na insertion performance. Palacin et al.
reported the use of Na 2 Ti 3 O 7 as anode for Na-ion batteries [ 75 , 76 ]. Na 2 Ti 3 O 7