Short communicationMagnesiothermically reduced diatomaceous earth as a porous silicon anode material for lithium ion batteries
Highlights
► Porous silicon is prepared using diatomaceous earth as silicon source. ► The porous structure of the diatomaceous earth is retained. ► The carbon-coated sample shows preliminary good electrochemical performance.
Introduction
Lithium ion batteries are the most popular power sources in portable electric and electronic devices for their advantages such as high power and energy densities and environmental friendliness. Much effort is now being paid to replace the commercially used graphite with materials with higher specific lithium storage capacity and higher safety [1]. Silicon is one of the most attractive anode materials for its large specific capacity (theoretically 4200 mAh g−1) [2] and its lithium insertion potential properly above that of lithium deposition. However, the severe pulverization of the Si particles due to volume change during cycling destroys the electric conducting network and results in rapid capacity fading.
To improve the cyclability of Si-based anodes materials, several methods have been exploited, such as reducing the Si particle size to nanometer scales [3], dispersing the particles in an inactive or active matrix [4], and reforming the morphology of silicon-based anodes [5], [6], [7]. Among these strategies, constructing Si-based materials with porous structure has been proved effective [6], [8]. The void space in the porous Si can partially accommodate the huge volume change during the charge and discharge process and facilitate the transportation of lithium ions [9].
Recently, Yu et al. [10] and Jia et al. [11] synthesized 3D porous silicon using a magnesiothermic method by the equation 2 Mg + SiO2→2MgO + Si, with mesoporous silica (SBA-15) as the Si source and template. However, as the SBA-15 template is expensive, it cannot be cost-effective to use such material as the source of silicon. On the other hand, diatomaceous earth is a biogenic siliceous sedimentary rock; its main component is silica. It is very cheap and commercially available. Bao et al. [12] reported that porous silicon can be obtained by a magnesiothermic reduction of diatomaceous earth at low temperature (650 °C). The natural porous structure of diatomaceous earth works as another kind of template for porous silicon. However, their work was only focused on the conversion of three-dimensional silica into microporous silicon replicas. They did not evaluate the electrochemical performances of the reduced product.
In this work, porous silicon was synthesized by magnesiothermically reducing the diatomaceous earth as the silicon source and template. The reduction product was then coated with a layer of carbon to enhance its electric conductivity. Electrochemical evaluation indicates that the diatomaceous earth could be a promising precursor of porous silicon anode material.
Section snippets
Experimental
The diatomaceous earth was reduced by a magnesiothermic reduction. In a typical process, 0.6 g diatomaceous earth (CP) and 0.6 g Mg powder (AR, 100–200 mesh) was mixed and ground in an agate mortar for 20 min. Then the mixture was annealed in a tube furnace at 650 °C for 6 h under Ar/H2 (92:8 v/v) atmosphere at a ramping rate of 5 °C min−1. The obtained powder was immersed in 200 ml, 0.5 M HCl solution for 12 h, rinsed with distilled water and ethanol several times to remove the MgO. After
Results and discussion
The morphology of the diatomaceous earth before and after magnesiothermic reduction is compared in Fig. 1. The commercial diatomaceous earth is sunflower-like with particle size of about 25 μm and uniform pore size of about 200 nm (Fig. 1a). The shape of the sunflower was damaged after magnesiothermic reduction and HCl immersion. The size of the obtained porous Si particles decreases to less than 10 μm though the pore size remains about 200 nm. This indicates that magnesiothermic reduction does
Conclusion
Porous silicon replica has been prepared by magnesiothermically reducing diatomaceous earth. After carbon coating by the CVD method, this material shows excellent good lithium storage performance and capacity retention. Although further work is necessary to improve the capacity retention of the sample, these results demonstrate the feasibility of using the magnesiothermically reduced diatomaceous earth as a promising porous silicon anode material.
Acknowledgments
This work was financially supported by the National 973 Program (2009CB220100) and the National Science Foundation of China (NSFC, 50472072 and 20974120).
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