The majority constituent of many sands is quartz (SiO 2 ) and sand is easily collected since it is predominantly found on the surface of the earth's crust. The sand used in this analysis was collected from the loamy surface of the shores of Cedar Creek Reservoir in the Claypan region of Texas. The soil of this region is classified as an Alfisol, specifically a Paleustalf, comprising >90% quartz with minor amounts of feldspars and chert27,28. The sand grains utilized herein have a grain size of ~0.10 mm, as in Fig. 1b. Further mechanical milling in an alumina mortar easily reduces the grain size to the micrometer and nanometer scale within minutes. Organic species are removed via calcining in air at 900°C and the sand is then sequentially washed with HCl, HF and NaOH for varying amounts of time. Unwanted silicate species are removed via the HF etch, as crystalline quartz etches much slower than other silicate species such as feldspars29. After purification, the sand assumes a bright white appearance in stark contrast to the brown hue of the unpurified sand, as in Fig. 1c. The peaks associated with unpurified sand in the XRD analysis in Fig. 2a confirm that the sample comprises mostly quartz with very minor peaks corresponding to impurities. After purifying the sand, the peaks associated with quartz greatly increase in intensity relative to the impurity peaks, confirming that most of the impurities have been successfully etched away.

Figure 2 (a) XRD plot displaying characteristic peaks of quartz in both pre-reduction samples and Si peaks in the post-reduction nano-Si. (b) EDS analysis with inset displaying weight percent of elements in nano-Si after HCl and HF etching. Low magnification (c) and higher magnification (d) SEM images of quartz powder after purification and milling. Low magnification (e) and higher magnification (f) SEM images of nano-Si after reduction and etching. Scale bars for (c),(d), (e) and (f) are 5μm, 2μm, 2μm and 500 nm, respectively. Full size image

After purification, quartz powder and NaCl is ground together in a 1:10 SiO 2 :NaCl weight ratio and ultrasonicated and vigorously stirred for 2 hours. After drying, the SiO 2 :NaCl Powder is ground together with Mg powder in a 1:0.9 SiO 2 :Mg weight ratio. The resultant powder is loaded into Swagelok-type reactors and sealed in an argon-filled (0.09 ppm O 2 ) glovebox. The reactors are immediately loaded into a 1" diameter quartz tube furnace purged with argon. The furnace is slowly heated at 5°Cmin−1 to 700°C and held for 6 hours to ensure complete reduction of all SiO 2 . After reduction the resulting brown powder is washed with DI water to remove NaCl and then etched with 1 M HCl for 6 hours to remove Mg, Mg 2 Si and MgO. The MgCl 2 that is produced via HCl etching of MgO can be easily recycled back to Mg via electrolysis, which is the predominant industrial synthesis route for Mg production30. The powder is washed several times with DI H 2 O and EtOH to remove the etchant and dried overnight under vacuum. A visual comparison, without magnification, of unpurified beach sand, purified quartz and nano-Si stored in glass vials can be seen in Fig. 1d and the entire synthesis process can be visualized in Fig. 1e.

SEM imaging in Fig. 2 reveals the broad size distribution and highly irregular morphology of the milled quartz powder before and after reduction. For the milled quartz powder, the particle size ranges from several microns to 50 nm, as in Fig. 2c and 2d. The quartz powder and nano-Si reduction product are both highly irregular in shape as expected. After reduction, the nano-Si is absent of particles with dimensions in excess several microns and has a much smaller size distribution than the quartz powder, as in Fig. 2e and 2f. We can attribute this to the breakdown of relatively larger particles during reduction and ultrasonication, which is due to the reduced mechanical integrity of the porous 3D nano-Si networks in comparison 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 Mg 2 Si 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.

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 N 2 sorption isotherms and inset showing pore diameter distribution. Full size image

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 Na 2 SiF 6 , which is produced via a reaction between residual NaCl and H 2 SiF 6 produced during HF etching of SiO 2 . 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, silicon has relatively low electrical conductivity31. Thus, nano-Si powders 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 H 2 /Ar mixture. After heating to 950°C, acetylene was introduced into the tube to produce a conformal C-coating. The weight ratio of Si to C was determined to be 81:19 after coating. Brunauer-Emmett-Teller (BET) surface area measurements were performed for nano-Si before C-coating yielding a specific surface area of 323 m2g−1, 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 surface 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 characterized using the half-cell configuration with Li-metal as the counter-electrode. Electrodes comprised nano-Si@C, acetylene black (AB) and PAA in a 7:1:2 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 electrodes demonstrate a reversible capacity of 1024 mAhg−1 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.

Figure 4 (a) Cycling data of nano-Si@C anodes with selected C-rates (C = 4Ag−1). (b) CV plot of the first 13 cycles using a scan rate of 0.02 mVs−1. (c)Charge-discharge curves for selected cycles. (d) EIS curves for selected cycles showing both experimental and fitted-model data. (e)Equivalent circuit of nano-Si@C electrodes used to produce fitted-model data. Extracted resistance values from the EIS curves for (f) charge transfer resistance and (g) SEI + INT resistance. Full size image

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 mechanical properties to that of CMC but higher concentration of carboxylic 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 interconnected network of Si to expand. Despite the fact that some of the 3D nano-Si networks have diameters of several hundreds of nanometers, the SiNPs that comprise these networks are only 8–10 nm in diameter.