Mesoporous Silicon Anodes Prepared by Magnesiothermic Reduction for Lithium Ion Batteries Wei Chen, Zhongli Fan, Abirami Dhanabalan, Chunhui Chen, and Chunlei Wang * ,z Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, USA Porous Si prepared by the magnesiothermic reaction was used as anode material for lithium ion batteries. The synthesis conditions were investigated to obtain high purity of silicon phase. The structures of porous Si were characterized by TEM, Raman and N 2 adsorption-desorption. The electrochemical performance was tested by cyclic voltammetry (CV) and discharge–charge measure- ment. The porous Si exhibits larger storage capacity and improved cyclability compared to commercial Si powders. The enhanced capacity and cyclability are attributed to the loose mesoporous structure of porous Si, which can accommodate large volume change in the lithiation/delithiation process. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3611433] All rights reserved. Manuscript submitted May 5, 2011; revised manuscript received June 24, 2011. Published July 20, 2011. Rechargeable lithium-ion batteries (LIB) are the most popular power sources for their high energy density, high voltage and envi- ronmental friendliness. 1–3 Much effort has been devoted to replac- ing the currently commercialized graphite anode in LIB by the materials allowing more Li-storage capacity. 4–7 Silicon is one of the attractive anode materials for LIB compared to graphite because it can provide a theoretical capacity as high as 4200 mAh g 1 (Li 4.4 Si). 6,8 However, large volume change of silicon upon insertion and extraction of lithium can cause pulverization and breakdown of the electrical conductive network, which results in rapid capacity fading. Extensive efforts have been made to accommodate the vol- ume change of silicon, such as reducing the silicon particle size to nanoscales or preparing silicon-acitve/inactive composites. 9–13 For instance, Cui et al. reported that a electrode based on binder-free Si nanowires with 100 nm diameter improved the cycling stability, which allowed rapid transport of Li ions. 12 Liu et al. reported that car- bon-coated Si nanocomposites exhibited capacity of 1489 mAh g 1 after 20 cycles. 13 Studies showed that these methods can control the volume expansion of Si powders and extensive researches have been carried out in order to achieve larger gravimetric capacity, higher coulombic efficiency and better cylability in LIB. Circumventing the volume change of Si by controlling its morphologies has recently been reported. 14,15 Chen et al. reported that nest-like Si nanospheres were prepared by a modified solvothermal method, exhibiting a supe- rior lithium-storage capacity and long cycling properties. 14 Cho et al. prepared 3D porous Si particles by thermal annealing of SiO 2 and butyl-capped Si particles. 15 These particles with ordered structures facilitate faster transport and better intercalation kinetics of Li ions, resulting in a high specific capacity. These studies showed that the loose structure of Si can effectively buffer the volume change during the cycling and reduce the pulverization of the electrode. Recently, Sandhage et al. proposed a facile magnesiothermic reduction to prepare microporous Si structure from 3D silica micro- assemblies. 16 However, the low yield and micron scale of the final Si products made them unsuitable as the candidates of electrode materials. Here, we propose a modified strategy using mesoporous silica SBA-15 as the template and converting silica nanostructure into porous silicon replica by the magnesiothermic reduction. By optimizing the synthesis conditions, we can obtain porous Si with dozens of gram once at lab-scale. The as-prepared porous Si exhibits high capacity and stable cyclability, indicating it is a promising candi- date as the anode material of rechargeable LIB. We anticipate that it will provide an alternative approach for preparing porous silicon with high battery performance. Experimental Commercial silicon powder was purchased from Sigma-Aldrich (98.5%). Porous Si was prepared by the magnesiothermic reduction adapted from the literatures. 16,17 Briefly, mesoporous molecular sieve SBA-15 (Novel Chemistry Corp.) were mixed with Mg pow- der and sealed inside a stainless steel ampoule in a glove box under argon atmosphere. The mixture was then heated to desired tempera- tures in nitrogen environment. After being cooled down to room temperature, the resultant was dissolved in a solution of hydrochlo- ric acid (37 wt %)/ethanol with 1:10 volume ratio, then being filtered and washed with deionized water until the pH value of the filtrate was around 7. The porous Si product was dried at 80  C for 12 h. X-ray diffraction (XRD) patterns were recorded using a D-5000 diffractometer with Cu Ka radiation (k ¼ 0.154056 nm). Transmis- sion electron microscopy (TEM) was performed using a Philips CM200 FEG microscope operating at 200 kV. N 2 adsorption– desorption isotherms were collected on a Quantachrome NOVA instrument at 77 K. Raman spectroscopy measurements were carried out with an argon ion (Ar þ ) laser system (Spectra Physics, model 177G02) of k ¼ 514.5 nm at a laser power of ca.7 mW. The working electrode was prepared by casting slurry containing 40 wt % active materials (porous Si or commercial Si particles), 40 wt % conductive carbon black (Super P Li V R ) and 20 wt. % PVDF onto a nickel foam, which acts as a current collector with larger sur- face area and three-dimensional network structure and increases the utilization of active materials. The working electrode disc was dried in air at 60  C and further heated in argon environment at 230  C for 2 h. A typical electrode disc (U ¼ 15 mm) was loaded with 2.5 mg active materials. CR-2032-type coin cells were assembled in a glove box under argon atmosphere, where lithium discs were used as counter electrode and reference electrode. The electrolyte was 1 M lithiumbis(perfluoroethylsulfonyl)imide dissolved in the mixed sol- vent of ethylene carbonate: dimethyl carbonate: Diethyl carbonate in a 1:1:1 volume ratio. Cyclic voltammetry (CV) was performed on a Versatile Multichannel Potentiostat (VMP3). All electrochemical cells were galvanostatically cycled at room temperature by using an NEWARE BTS-610 battery tester. The capacity data were calcu- lated by subtracting the contribution of carbon black based on the blank experiments using 80 wt % carbon black and 20 wt % PVDF under the same conditions. Results and Discussion During the magnesiothermic reaction, Si is formed via the solid- state redox reaction between SiO 2 and Mg SiO 2 ðsÞþ 2MgðgÞ! SiðsÞþ 2MgOðsÞ [1] Excessive Mg could react with the product Si into Mg 2 Si and SiO 2 could react with MgO into Mg 2 SiO 4 MgðgÞþ SiðsÞ! Mg 2 SiðsÞ [2] SiO 2 ðsÞþ 2MgOðsÞ! Mg 2 SiO 4 ðsÞ [3] * Electrochemical Society Active Member. z E-mail: wangc@fiu.edu Journal of The Electrochemical Society, 158 (9) A1055-A1059 (2011) 0013-4651/2011/158(9)/(1)/5/$28.00 V C The Electrochemical Society A1055 Downloaded 01 Aug 2011 to 109.171.137.211. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp