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Fig. 5 a TEM image showing the rodlike Na
0.44
MnO
2
particles. b Capacity retention upon
cycling of a Na
0.44
MnO
2
/C composite electrode at a C/10 rate [
5
]
resulted from its large diameter. Hence, a solution should be downsizing the
particle of materials to shorten the diffusion path of Na ions.
Recently, Cao et al. reported the electrochemical properties of Na
0.44
MnO
2
nanowires prepared by a polymer-pyrolysis method [
4
]. The Na
0.44
MnO
2
sample
treated at 750 C was highly crystallized and showed quite uniform nanowire
morphology (~50 nm in diameter) (Fig.
6
a). The material delivered a reversible
capacity of 128 mAh g
-1
at 1/10 C and exhibited excellent cyclability (77 %
capacity retention over 1,000 cycles at 0.5 C), showing great enhancement in the
electrochemical performance than previous works [
5
]. The improvement was
considered to be attributed to the high crystallinity and uniform nanowire struc-
ture, which can provide a mechanically stable structure as well as a short diffusion
path for Na-ion insertion/extraction. An Na-ion full cell was tried by using
Na
0.44
MnO
2
as cathode and pyrolyzed carbon as anode. The full cell showed steep
charge and discharge curves with an average discharge voltage of 2.7 V and 73 %
capacity retention after 100 cycles (Fig.
7
).
3.1.3 Na
x
V
y
O
z
Vanadium oxides have variable valences and open formwork structure, which are
more suitably used as ion insertion hosts. In the late 1980s, the electrochemical
Na-ion interaction behaviors of three types of vanadium oxides were investigated:
b-Na
x
V
2
O
5
with channel structure, Na
1+x
V
3
O
8
and a-V
2
O
5
with layered structure
[
13
]. The reversible Na accommodation levels were ~1.7 mole of Na per mole of
a-V
2
O
5
, ~1.2 mole of Na per mole of b-Na
0.33
V
2
O
5
and ~1.6 mole of Na per mole
of Na
1+x
V
3
O
8
in the potential window of 1.0-3.6 V. Later, Bach et al. studied the
electrochemical Na intercalation behaviors of b-Na
0.33
V
2
O
5
prepared by two
procedures [
14
,
15
]. It showed that b-Na
0.33
V
2
O
5
prepared by the sol-gel process
can deliver reversibly ~0.7 mole of Na per mole of Na
0.33
V
2
O
5
(corresponding to
100 mAh g
-1
) in the potential window of 2.0-3.4 V, while that prepared by the
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