Figures to show typical FE-SEM images of as-prepared NixCo1-xMoO4·nH2O precursors with various cationic compositions. All of the samples were of nanowire morphology regardless of their compositional differences. However, the diameters and lengths of the nanowires tended to decrease with increasing Ni concentration. As shown in Figure , the NiMoO4·nH2O nanowires were agglomerated acutely and even formed clusters that were several microns in size. The agglomeration of the nanowires might be due to strong attractive forces between the small-sized nanowire particles. The crystalline structures of the as-prepared NixCo1-xMoO4·nH2O nanowires were confirmed from their XRD patterns, as seen in Figure . The XRD patterns taken from all NixCo1-xMoO4·nH2O nanowires agreed well with previously reported patterns for CoMoO4·nH2O (JCPDS no.: 26-0477).
Figure 1 Morphologies and crystal structures of NixCo1-xMoO4·nH2O nanowires. (a-e) Typical FE-SEM images of as-prepared NixCo1-xMoO4·nH2O nanowire precursors with various x values and (f) their corresponding X-ray diffraction patterns (by KS Park (more ...)
The dehydration process was investigated using the thermogravimetric [TG] technique; the results of which are shown in Figure . The typical TG curves obtained from the Nix
O nanowires in the temperature range from 30°C to 800°C showed net weight losses of 7% to 9% for each sample. This weight loss was mainly attributed to the evolution of species related to water molecules, such as reversibly bound water molecules (low temperature), water molecules forming an integral part of the crystal structure of Nix
O (medium temperature), and water molecules reversibly bound to the hydrate crystal phase (high temperature) [14
]. On the basis of the TG results, the post-annealing temperature for all the hydrate samples was 500°C, at which almost all water molecules were removed sufficiently to form Nix
Thermal behavior of NixCo1-xMoO4·nH2O nanowires. TG curves for as-prepared NixCo1-xMoO4·nH2O nanowire precursors with various values of x, heating at a rate of 10°C/min in air (by KS Park et al.).
Figures to show FE-SEM images of NixCo1-xMoO4 (0 ≤ x ≤ 1) nanowires obtained by post-annealing the NixCo1-xMoO4·nH2O nanowire precursors at 500°C for 2 h. All post-annealed samples maintained their pristine nanowire morphologies. Although a marked change in the diameter or length of the nanowires after post-annealing was not observed, they seemed to be agglomerated slightly and were forming bundles, which could lead to a decrease in their surface area. In order to confirm the change in surface area that may have occurred during post-annealing of NixCo1-xMoO4·nH2O nanowire precursors, the Brunauer-Emmett-Teller [BET] (Belsorp-mini, BEL Japan Inc., Osaka, Japan) technique was carried out at liquid nitrogen temperature. From the results presented in Table it can be confirmed that the surface areas of NixCo1-xMoO4 nanowires in all compositions were slightly decreased compared with those of the corresponding NixCo1-xMoO4·nH2O nanowires. This was ascribed to the aggregation and growth of the nanowires. Furthermore, it was found that the surface areas of the nanowires gradually increased with increasing Ni concentration because of the corresponding decrease in their diameter and length.
Figure 3 Morphologies and crystal structures of NixCo1-xMoO4 nanowires. (a-e) FE-SEM images of NixCo1-xMoO4 nanowires with various x values and (f) their corresponding X-ray diffraction patterns. Filled circle, empty circle, filled inverted triangle, and empty (more ...)
Surface areas of prepared samples
To investigate the crystal structures of the Nix
nanowires, their XRD patterns were examined carefully. The XRD patterns obtained for the CoMoO4
nanowires in Figure corresponded well with their bulk materials (JCPDS nos.: 25-1434, 21-0868, 33-0948, and 45-0142 for α-CoMoO4
, and β-NiMoO4
, respectively). However, as can be seen in Figure , both end-members had α- and β-phases corresponding to each material together. It is known that single-phase β-CoMoO4
can be formed by fast cooling and by slow cooling, respectively, to room temperature after post-annealing of their hydrates [14
]. Meanwhile, D. Vie et al. reported that the α-CoMoO4
phase became detectable as a minority phase after heat treatment of the amorphous solid precursor at a temperature above 700°C [17
]. On the basis of these reports, we believe that the coexistence of α- and β-phases in CoMoO4
nanowires can be attributed to the medium cooling rate (≈10°C/min to 30°C/min) applied after post-annealing or to a change of phase transition temperature with the unique morphologies of the samples.
Among the Nix
nanowires with a medium composition, the XRD pattern of the Ni0.25
nanowires agreed well with that of CoMoO4
. However, in contrast with that observed for pure CoMoO4
, the peaks corresponding to α-CoMoO4
became more dominant compared with those of β-CoMoO4
, indicating that relative intensities of the diffraction peaks of the α-phase increase with Ni content [17
]. In the case of the Ni0.5
nanowires, it was found that the XRD pattern corresponded closer with pure NiMoO4
than with pure CoMoO4
(Figure ). This implies that the basic crystal structure of the nanowires was transformed from CoMoO4
-related structures into NiMoO4
-related structures. The XRD pattern of the Ni0.75
nanowires also agreed with that of pure NiMoO4
. It should be noted that no secondary phases, such as NiO and CoO, were detected in any of the XRD patterns, which indicated that CoMoO4
formed solid solutions perfectly.
In general, most reports on the lithium reactivity of molybdates have been focused on their application as anode materials for lithium-ion batteries because they could deliver high reversible capacity through a conversion reaction with lithium [19
]. Moreover, NiO and CoO have also been known as anode materials, reacting with lithium by an equivalent mechanism with a molybdate [3
]. On the basis of such knowledge, we expected the following electrochemical reaction mechanism of Nix
To evaluate the electrochemical performance of the NixCo1-xMoO4 nanowire electrodes, typical voltage-specific capacity curves were recorded at a current rate of C/5 (≈196 mA/g; note that 1 C ≈ 980 mA/g on the basis of the reaction in Equation 2) in a potential window between 0.01 and 3.0 V (Figure ). In the first discharge reaction, samples containing nickel reacted with lithium at a lower potential region (≈0.4 V to 0.01 V) compared with the CoMoO4 nanowire electrode (≈0.3 V to 0.01 V). Notably, the first discharge capacities for Ni0.75Co0.25MoO4 and NiMoO4 were higher than their theoretical capacities; this might be ascribed to the formation of the solid electrolyte interface-like organic layer. Subsequent second discharge-charge reactions occurred reversibly on the basis of the reaction in Equation 2. These highly reversible capacities of NixCo1-xMoO4 nanowire electrodes could be an evidence for delivering capacity by conversion mechanism.
Charging-discharging curves of NixCo1-xMoO4 nanowire electrodes. Charging-discharging curves of the NixCo1-xMoO4 nanowire electrodes with various x values at a rate of C/5 (by KS Park et al.).
Figure shows the specific capacity of NixCo1-xMoO4 nanowire electrodes versus the cycle number at a current rate of C/5. When the values of x were 0, 0.25, and 0.5, the reversible capacities of NixCo1-xMoO4 nanowire electrodes faded rapidly even during the initial cycles, resulting in a poor reversible capacity of ≈290 mA h/g after 20 cycles. This result was attributed to the small surface areas of the nanowire electrodes. From the BET results summarized in Table it was found that their surface areas were very small compared with those of the other nanowires, which led to a small electrode/electrolyte interface, resulting in a poor reversible capacity. Meanwhile, the reversible capacity of the NixCo1-xMoO4 nanowire electrodes with x values of 0.75 and 1 (≈520 mA h/g after 20 cycles) was larger than that of the previously mentioned three nanowire electrodes because of the increased electrode/electrolyte interface that resulted from the larger surface area of the nanowires (Table ). In accordance with the BET results, the electrochemical performance of the NiMoO4 nanowire electrode was expected to be vastly superior compared with that of the Ni0.75Co0.25MoO4 nanowire electrode. However, the electrochemical performances of both electrodes were similar, as shown in Figure due to the aggregation of the NiMoO4 nanowires. When we fabricated the positive electrode, the carbon black that is commonly used as a conducting additive was mechanically mixed with the nanowires as the active material. At that time, clusters formed by aggregation of the NiMoO4 nanowires could not be mixed efficiently with carbon black, which resulted in impairing their electrochemical performance by restriction of the electronic conduction paths from each nanowire to a current collector. In contrast, the Ni0.75Co0.25MoO4 nanowires did not form such clusters during the synthesis process. For this reason, the Ni0.75Co0.25MoO4 nanowire electrode could deliver a reversible capacity as high as the NiMoO4 nanowire electrode. Furthermore, the Ni0.75Co0.25MoO4 nanowire electrode exhibited a superior coulombic efficiency of ≈95% (Figure ).
Figure 5 Electrochemical properties of NixCo1-xMoO4 nanowire electrodes. Variation of the discharge (open circles)-charge (solid circles) specific capacity and coulombic efficiency (solid triangle) versus the cycle number at a rate of C/5 for the NixCo1-xMoO4 (more ...)