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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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