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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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