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during reduction and ultrasonication, which is due to the reduced mechanical integrity of the porous 3D nano-Si networks in compar- ison to the solid pre-reduction quartz particles. In lieu of the solid crystalline particles found in the quartz powder, the nano-Si powder is composed of a highly porous network of interconnected crystalline silicon nanoparticles (SiNPs). HRTEM in Fig. 3a and 3b reveals the interconnected SiNPs that comprise the 3D Si networks, and the diameter of the SiNPs is ,8–10 nm, with larger particles existing sparingly. This high porosity can be attributed to the selective etching of imbedded MgO and Mg2Si particles after reduction. Through the use a NaCl as a heat scavenger during the reduction process, we are able to synthesize a highly uniform porous structure throughout the width of the particle by avoiding localized melting of Si. This uniform 3D network is achieved via removal of oxygen (53.3% by weight) from the original quartz particles through reduction and a conservation of volume via the heat scavenger (NaCl). The XRD peaks in Fig. 2a indicate a successful reduction to silicon after Mg reduction. Energy Dispersive X-ray Spectroscopy (EDS) in Fig. 2b reveals the weight percentage of elements present in the nano-Si powder. The quantitative analysis shows Si is the predominant element present with non-negligible amounts of F, Na, Mg, Al, and O. The F and Na peaks may be due to the existence of Na2SiF6, which is produced via a Figure 3 | Low magnification (a) and high magnification (b) TEM images of nano-Si. (c) HRTEM image of nano-Si showing the conformal carbon coating and characteristic lattice spacing of Si(111). (d) HRTEM image of C-coated nano-Si showing thickness of the carbon layer. Scale bars for (a), (b), (c), and (d) are 20 nm, 10 nm, 2 nm, and 2 nm, respectively. (e) BET surface area measurements of nano-Si with type IV N2 sorption isotherms and inset showing pore diameter distribution. reaction between residual NaCl and H2SiF6 produced during HF etching of SiO2. The existence of Al may be derived from the original sand or from the alumina mortar. While the existence of metallic contaminants at these levels may present deleterious effects for some applications, for battery applications these metallic impurities may increase the conductivity of nano-Si. Despite silicon’s relatively high surface diffusion capability with respect to bulk diffusion of Li, sil- icon has relatively low electrical conductivity31. Thus, nano-Si pow- ders were conformally coated with a ,4 nm amorphous carbon coating to enhance conductivity across all surfaces, as in Fig. 3c and 3d. Briefly, nano-Si powder was loaded into a quartz boat and placed in the center of a quartz tube furnace purged with an H2/Ar mixture. After heating to 950uC, acetylene was introduced into the tube to produce a conformal C-coating. The weight ratio of Si to C was determined to be 81519 after coating. Brunauer-Emmett-Teller (BET) surface area measurements were performed for nano-Si before C-coating yielding a specific surface area of 323 m2g21, as in Fig. 3e. The inset in Fig. 3e reveals a pore diameter distribution with a peak centred at 9 nm. The pore diameter is in good agreement with the TEM images of porous nano-Si. This high surface area confirms that NaCl effectively scavenges the large amount of heat generated during Mg reduction, preventing agglomeration of nano-Si. The high sur- face area and pore volume distribution also confirm the existence of large internal porosity available for volume expansion buffering and, thus, minimal capacity fading due to SEI layer degradation and active material pulverization. Nano-Si@C derived from sand was electrochemically character- ized using the half-cell configuration with Li-metal as the counter- electrode. Electrodes comprised nano-Si@C, acetylene black (AB), and PAA in a 75152 nano-Si@C:AB:PAA weight ratio. Fig. 4a demonstrates the rate capability of the C-coated nano-Si electrodes up to the C/2 rate, with additional cycling up to 1000 cycles at the C/2 rate. Initial cycling at C/40 is necessary for proper activation of all Si and development of a stable SEI layer. This activation process is confirmed via cyclic voltammetry measurements, as in Fig. 4b. The peaks corresponding to the lithiation (0.22 V and 0.10 V) and delithiation (0.33 V and 0.50 V) grow in intensity over the first 12 cycles before stabilizing, which suggests a kinetic enhancement occurs in the electrode. After a kinetic enhancement is achieved via this low current density activation process, the electrodes are cycled at much higher rates. Even at the C/2 rate the nano-Si electro- des demonstrate a reversible capacity of 1024 mAhg21 and a Coulombic efficiency of 99.1% after 1000 cycles. We attribute the excellent cycle stability of the nano-Si@C electrodes to a combination of the conformal C-coating, PAA binder, and the porous 3D nano-Si network. The addition of a C-coating alters the makeup of the SEI layer and may also partially alleviate the lithiation-induced volume expansion effects in nano-Si32. The use of PAA as the binder also greatly enhances the cyclability of the electrodes. Magasinski et al. have recently reported on the improved cycling performance of PAA- bound electrodes relative to conventionally used binders such as poly(vinylidene fluoride) (PVDF) and carboxymethylcellulose (CMC)33. The improved stability is attributed to PAA’s similar mech- anical properties to that of CMC but higher concentration of carb- oxylic functional groups. The mechanical properties of PAA prevent the formation of large void spaces created during lithiation and delithiation of Si. The higher concentrations of carboxylic groups form strong hydrogen bonds with hydroxyl groups on C and Si, minimizing separation of binder from active material during cycling. The porous nature of the nano-Si is also partly responsible for the good cyclability due to the internal void space available for the inter- connected network of Si to expand. Despite the fact that some of the 3D nano-Si networks have diameters of several hundreds of nan- ometers, the SiNPs that comprise these networks are only 8–10 nm in diameter. www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 5623 | DOI: 10.1038/srep05623 4PDF Image | Scalable Synthesis of Nano-Silicon from Beach Sand
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