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Fig. 9 Voltage-composition curves for a NaVO
2
(O3) c Na
0.7
VO
2
(P2) and associated
respectively b and d derivative curves. Insets indicate the capacity evolution versus number of
cycles. The cells using 5-6 mg of positive electrode composites were cycled at C/20. The small
polarization between charge and discharge for the P2 phase suggests a high rate capability. This
was confirmed by power rate measurements (e.g. signature curves) as this electrode can deliver
90 % of its capacity at 1 C rate [
17
]
about 130 mAh g
-1
. For both materials, multiple step phase transitions can be
easily observed on the charge/discharge curves and the derivative curves (Fig.
9
).
A careful observation can find that at low potential range, the voltage variation is
about 200 mV for O3-NaVO
2
, compared to 50 mV for P2-Na
0.7
VO
2
. The large
difference probably due to O3-NaVO
2
is electron insulative, while P2-Na
0.7
VO
2
is semiconductive [
18
,
19
].
3.1.4 Mixed Metal Oxide
Except for Na
x
MO
2
, mixed metal oxides have also been surveyed in consideration
of the combination of the respective properties of different elements. Carlier et al.
used Mn to partly replace Co in P2-Na
0.67
CoO
2
to form P2-Na
0.67
Co
0.67
Mn
0.33
O
2
based on the consideration of the stabilization of Mn
4+
ions in the lattice [
20
]. The
charge/discharge curves of the P2-Na
0.67
Co
0.67
Mn
0.33
O
2
electrode are shown in
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