Electrochemical reactivity of ball-milled MoO3−y as anode materials for lithium-ion batteries
Introduction
As the environmental challenge of global warming resulting from fossil-fuel consumption is growing more severe, the importance of electrochemical energy conversion/storage devices cannot be overemphasized. Among them, rechargeable lithium-ion batteries (LIB) have been widely used in various portable electronic devices. Even further, thanks to their superior performance to the other competitors such as Ni-MH batteries, their development for hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), or all electric vehicles is a current focus.
For successful application, however, electrode materials that can meet the following requirements should be developed: high-energy density, longer cycle life, high power, and safety with low cost. Regarding high capacity, conversion-type transition metal oxides (MO with M = Fe, Co, Ni, Cu, Ru, etc., MO + Li+ + e− ↔ Li2O + M) may have a capacity as high as 700 mAh g−1 (e.g. CoO: 715 mAh g−1) which is much higher than that of already-commercialized graphite (372 mAh g−1) [1], [2], [3], [4], [5], [6]. Furthermore, many transition metal oxides show outstanding cycling performance [1], [6], [7], [8], [9]. And for the purpose of more improved cycle retention, various synthetic strategies to develop nanostructure have been employed [7], [8], [9]. On the aspect of high power, the nanostructure can also significantly enhance the rate capability by reason of shortened diffusion length [7], [9], [10]. Hence, the nanostructured transition metal oxides can be regarded as one of the most promising candidate materials.
MoOx which can be also categorized as one of the conversion-type transition metal oxides has additional advantageous features of low cost and environmental benignity as well as even higher capacity. To date, various preparation methods of nanostructured MoOx and their corresponding electrochemical performance have been reported. Among them, one can quote the fissile MoO2 by hydrothermal reaction [11], [12], the porous spherical MoO2 by rheological phase reaction [12], the α-MoO3 micro-rods by vapor-transportation method [13], the carbon/MoO2 composite using tri-block copolymer as a structure directing agent and carbon source [14], MoO3 nanobelts by hydrothermal reaction [15] and utilizing poly(ethylene glycol) [16], MoO2 by reduction of MoO3 with ethanol vapor [17], MoO3 nanoparticles by hot-wire chemical vapor deposition (HWCVD) [18], [19], tremella-like MoO2 by hydrothermal reduction [20], etc. However, most research about MoOx as an anode in the potential range down to 0 V (vs. Li/Li+) has focused on MoO2 rather than MoO3 even though MoO3 can theoretically accommodate more Li (six Li) than MoO2 (four Li) (MoO3: 1117 mAh g−1, MoO2: 838 mAh g−1). Thus, the electrochemical reactivity of MoO3 utilizing a conversion reaction has not yet been fully investigated. Moreover, for the case of MoO2, their reaction is known to be restricted to the addition-type one (MOy + xLi+ + xe− ↔ LixMOy, no breakage of M–O bond) at room temperature without using nanostructured electrodes [11], [17], [21], [22].
Here we report the electrochemical performance of ball-milled MoO3 powders. High-energy ball-milling is one of the easiest ways to construct nanostructures [23], [24], [25]. The composition, morphology, and structure of ball-milled MoO3 have been characterized and their effects on the electrochemical reactivity and extent of conversion reaction are discussed.
Section snippets
Preparation
Milling was performed using SPEX 8000 Mixer/Mill. The MoO3 powders (Alfa Aesar, 3 g) as well as steel balls (two balls of 1.3 cm in diameter and six balls of 0.6 cm in diameter) were put into a hardened steel vial (72 cm3). The weight ratio between the steel ball and the MoO3 powders was 9:1. Three different ball-milled samples were prepared by ball-milling bulk MoO3 powders for 4, 6, and 8 h.
Characterization of materials
A Phillips CM-30 TEM operating at 200 kV with a 50 μm objective aperture for improved contrast was employed
Materials characterization
Fig. 1 shows the TEM images of bulk and ball-milled MoO3 powders. Different from bulk MoO3 powders (Fig. 1a), the ball-milled MoO3−y powders (Fig. 1b–d) exhibit roughened surface morphology as well as slightly reduced overall particle size (approx. 1 μm). The enlarged views (the inset in Fig. 1b and c) reveal that the overall particles of ball-milled samples consist of agglomerated nanometer-sized primary particles. Intense collisions between particles and balls during the high-energy
Conclusion
Nanostructured MoO3−y powders have been prepared by ball-milling, and their electrochemical reactivity in the 0–3 V (vs. Li/Li+) range has been studied. Ball-milling of MoO3 powders results in partially reduced MoO3−y powders that consist of nano-crystallite aggregates. Bulk and ball-milled MoO3−y can uptake more than six Li/Mo by an addition reaction followed by a conversion reaction, wherein the reactivity was found to strongly depend on the oxygen deficiency. The conversion reaction for
Acknowledgements
This work was funded by the U.S. Department of Energy under subcontract number DE-AC36-99-GO10337 through the Office of Energy Efficiency and Renewable Energy Office of the Vehicle Technologies Program. Dr. Yoon S. Jung acknowledges the Korea Research Foundation Grant funded by the Korean Government [KRF-2008-357-D00066]. Sangkyoo Lee acknowledges the Korea South-East Power Generation Co.
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