Electrochemical performance of Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowire anodes for lithium-ion batteries
© Park et al; licensee Springer. 2012
Received: 2 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowire electrodes for lithium-ion rechargeable batteries have been synthesized via a hydrothermal method, followed by thermal post-annealing at 500°C for 2 h. The chemical composition of the nanowires was varied, and their morphological features and crystalline structures were characterized using field-emission scanning electron microscopy and X-ray powder diffraction. The reversible capacity of NiMoO4 and Ni0.75Co0.25MoO4 nanowire electrodes was larger (≈520 mA h/g after 20 cycles at a rate of 196 mA/g) than that of the other nanowires. This enhanced electrochemical performance of Ni x Co1-xMoO4 nanowires with high Ni content was ascribed to their larger surface area and efficient electron transport path facilitated by their one-dimensional nanostructure.
Among the types of anode materials available for rechargeable lithium-ion batteries, graphite has been commercialized. However, because of its drawbacks, such as capacity limitations (theoretical capacity of 372 mA h/g), initial loss of capacity, and structural deformation [1, 2], one of the current areas of interest in lithium-ion battery research is the search for new anode candidates that have large reversible capacities. In the past decade, transition metal oxides (M x O y , M = Co, Ni, Cu, Fe) that can deliver a high reversible capacity by conversion reaction mechanisms (M x O y + ne- + n Li+ ↔ x M0 + Li n O y ) have been considered as an alternative to commercial graphite anodes for lithium-ion batteries [3–10].
Metal molybdates, AMoO4-type compounds (where A is a divalent metal ion), have attracted the interest of researchers because of their electronic and magnetic properties and their many applications, such as catalysis and photoluminescence [11–14]. Recently, CoMoO4 and NiMoO4, synthesized with nanowire morphology by a simple hydrothermal method, were exploited as materials for lithium-ion batteries but were only applied as cathodes [15, 16].
In this paper, we report the fabrication of Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowire electrodes by a hydrothermal method, followed by thermal post-annealing. We also demonstrate the superior electrochemical performance of Ni x Co1-xMoO4 nanowires for lithium-ion battery anodes.
Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowires were synthesized by a simple hydrothermal method, in which high purity Ni(NO3)2·6H2O (99.999%; Sigma-Aldrich, Saint Louis, MO, USA), Co(NO3)2·6H2O (98%; Sigma-Aldrich, Saint Louis, MO, USA), and Na2MoO4·2H2O (99.5%; Sigma-Aldrich, Saint Louis, MO, USA) were used as source materials, followed by post-annealing at an elevated temperature. Initially, to prepare a clean solution (total cationic concentration of 0.1 M) with a molar fraction (x) of Ni (x = 0, 0.25, 0.5, 0.75, and 1), controlled amounts of Co- and Ni-containing reagents were dissolved in deionized water (80 mL) under constant magnetic stirring; then, the solution was added to an aqueous solution (80 mL) containing 0.1 M of Na2MoO4·2H2O. This resulting solution was transferred into a Teflon-lined stainless steel autoclave, sealed, and maintained at 180°C for 8 h. After the reaction was completed, the resulting solid products were harvested by centrifugation, washed with deionized water and acetone several times, and then dried at 60°C for 6 h in a vacuum oven. Finally, the as-prepared hydrate nanowire precursors were post-annealed at 500°C for 2 h to dehydrate them.
The morphologies and crystal structures of the prepared Ni x Co1-xMoO4·n H2O and Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowires were investigated using field-emission scanning electron microscopy [FE-SEM] (10 kV; FEI NOVA, Tokyo, Japan) and X-ray powder diffraction [XRD] (λCuKa = 1.5405 Å; Miniflex II, Rigaku, Tokyo, Japan). The thermal behavior of the as-prepared hydrate samples was analyzed by thermogravimetric analysis [TGA] (DTG-60H, Shimadzu, Kyoto, Japan). For TGA, the samples were heated from room temperature up to 800°C at a heating rate of 10°C/min in air.
For the electrochemical evaluation of the Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowires, positive electrode films were cast on a Cu foil by mixing each nanowire powder (1 to 2 mg) with Super P carbon black (MMM Carbon, Brussels, Belgium) and Kynar 2801 binder (PVdF-HFP, Arkema Inc., King of Prussia, PA, USA) in a mass ratio of 70:15:15. The assembled Swagelok-type cells composed of a positive electrode, negative electrode (lithium metal-foil), and separator film (Celgard 2400, Celgard LLC, Charlotte, NC, USA) saturated with a liquid electrolyte consisting of LiPF6 (1 M) dissolved in a solution of ethylene carbonate and dimethyl carbonate (1:1 v/v) were cycled at voltages between 0.01 and 3.0 V using an automatic battery cycler (WBCS 3000, WonaTech, Seoul, South Korea).
Results and discussion
Surface areas of prepared samples
Ni x Co1-xMoO4·n H2O
Ni x Co1-xMoO4
Surface area (m 2 /g)
To investigate the crystal structures of the Ni x Co1-xMoO4 nanowires, their XRD patterns were examined carefully. The XRD patterns obtained for the CoMoO4 and NiMoO4 nanowires in Figure 3f corresponded well with their bulk materials (JCPDS nos.: 25-1434, 21-0868, 33-0948, and 45-0142 for α-CoMoO4, β-CoMoO4, α-NiMoO4, and β-NiMoO4, respectively). However, as can be seen in Figure 3f, both end-members had α- and β-phases corresponding to each material together. It is known that single-phase β-CoMoO4 and α-NiMoO4 can be formed by fast cooling and by slow cooling, respectively, to room temperature after post-annealing of their hydrates [14, 17, 18]. 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 . On the basis of these reports, we believe that the coexistence of α- and β-phases in CoMoO4 and NiMoO4 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 Ni x Co1-xMoO4 nanowires with a medium composition, the XRD pattern of the Ni0.25Co0.75MoO4 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 . In the case of the Ni0.5Co0.5MoO4 nanowires, it was found that the XRD pattern corresponded closer with pure NiMoO4 than with pure CoMoO4 (Figure 3f). 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 Ni0.75Co0.25MoO4 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 and NiMoO4 formed solid solutions perfectly.
In summary, we have successfully synthesized Ni x Co1-xMoO4 (0 ≤ x ≤ 1) nanowire electrodes by employing a hydrothermal method to prepare hydrate nanowires, followed by post-annealing of the as-prepared hydrate nanowires. Although α- and β-phases coexisted in all of the nanowire samples, no secondary phases were detected, indicating that CoMoO4 and NiMoO4 formed perfect solid solutions. Compared with the reversible capacity (≈290 mA h/g after 20 cycles) of the Ni x Co1-xMoO4 nanowire electrodes with x values of 0, 0.25, and 0.5, the reversible capacity of the nanowire electrodes containing a higher Ni content (x = 0.75 and 1) increased more (≈520 mA h/g after 20 cycles). Despite the difference in surface area between the NiMoO4 and Ni0.75Co0.25MoO4 nanowires, the reason for the observation of their similar reversible capacities was because of the agglomeration of NiMoO4 nanowires and the resulting restriction of the electronic conduction paths from each nanowire to a current collector. We anticipate that this approach will facilitate the tailoring of other electrodes based on solid solution materials, to provide electrodes with superior electrochemical performance.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST; nos. 2010-0029617 and 2011-0005776).
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