Synthesis and characterization of nano-Si from RH

Preserving the nanostructure of the SiO 2 species in the RHs during the whole recovery process, especially at steps involving elevated temperature, is key to producing high quality nano-Si. In the step of converting RHs to nano-SiO 2 , simply burning RHs in air will produce bulk SiO 2 and it is necessary to first perform HCl leaching7 and then only heat at moderate temperatures to avoid the fusion of the SiO 2 product (Fig. 2a–c) while removing the organic matter. Thermogravimetric analysis (TGA) of both raw and leached RHs (Fig. 2d) shows a typical three-stage mass loss in simulated air: (i) mass loss below 100°C, which corresponds to water loss; (ii) mass loss around 300°C, which corresponds to cellulose/hemicellulose/lignin degradation; and (iii) mass loss between 350–550°C, corresponding to the burning of carbonous residues32. 23% of the mass remains as the SiO 2 product. Differential thermal analysis (DTA, Fig. 2e) confirms these three stages during heating, with an additional endothermic peak for the raw RH sample at 635°C (highlighted in the black square in Fig. 2e). This peak is due to the melting of silica catalyzed by metal impurities (such as K+, Na+ and Ca2+) in raw RHs7 and and formation of bulk SiO 2 , as shown in Fig. 2a. After optimization, heating leached RHs at 700°C in air produces amorphous SiO 2 nanoparticles with diameters distributed around 80 nm; these nanoparticles are loosely interconnected and are macroscopically preserved in the shape of the RHs (Fig. 2f and Supplementary Fig. S2). In the next step, this type of nano-SiO 2 is further reduced to obtain nano-Si.

Figure 2 Characterization of nano-SiO 2 derived from RHs. a–c, SEM images of SiO 2 obtained by heating raw or leached RHs at different temperature. d,e, TGA (d) and DTA (e) profiles of raw and leached RHs measured under flow of simulated air (20% O 2 in Ar) with a ramp rate of 5°C min−1. f, Statistical analysis of particle sizes of nano-SiO 2 shown in (c) and nano-Si (Si-RH-5) reduced from it. Full size image

Bulk Si is usually produced for industrial applications by the carbothermic reduction of silica, which requires temperatures greater than 2000°C33, which are well above the melting point of silicon (1410°C). When using magnesium as a reducing agent, the reaction temperature could decrease to 650°C, allowing the possibility of nanostructure preservation during silicon formation34. The reactions occur as below:

or

If Mg is in excess,

After reaction, MgO, Mg 2 Si and unreacted Mg is removed by hydrochloric acid, yielding silicon product. In these reactions, magnesium (b.p. = 650°C) functions as a reducing agent. Previously, the Mg and SiO 2 reactants were placed at separate locations and Mg vapor diffuses to react with SiO 2 34. Upon testing, this configuration limited the yield and scalability, because the Si product near the Mg source was further converted to Mg 2 Si (reaction (3)), while the SiO 2 far from Mg source was not reduced. Instead, we conduct the reaction with Mg and SiO 2 powder premixed together to increase the scalability and yield. Both reactions (1) and (2) takes place in this configuration, because Mg powder is in direct contact with SiO 2 and the Mg has high enough vapor pressure at elevated temperature (e.g. 1 Pa at 428°C). In a closed reactor (Supplementary Fig. S3), a slight excess of Mg (Mg : SiO 2 molar ratio = 2.5 : 1) was added, so that there would neither be a lack of Mg owing to Mg vapor escaping nor a large excess of Mg to convert Si to Mg 2 Si. We found that the temperature ramp rate significantly affects the morphology of the product. Macroporous Si was obtained with a 40°C min−1 rate (sample Si-RH-40, Fig. 3a and Supplementary Fig. S4), while partially interconnected Si nanoparticles were obtained with a 5°C min−1 rate (sample Si-RH-5, Fig. 3b–d and Supplementary Fig. S5). XRD patterns of products before (Supplementary Fig. S6) and after HCl treatment (Fig. 3e) showed that the 40°C min−1 sample had sharper peaks from Si and MgO, confirming larger grain sizes. Nitrogen adsorption (Brunauer-Emmett-Teller, BET) measurements (Fig. 3f and Supplementary Table S1) indicate that the specific surface area of the starting nano-SiO 2 was 289 m2 g−1; this value decreased dramatically to 54 m2 g−1 for sample Si-RH-40 and only slightly to 245 m2 g−1 for sample Si-RH-5. BJH (Barrett-Joyner-Halenda) analysis further revealed that the Si-RH-5 sample has significant pores less than 10 nm in size, while the Si-RH-40 sample has negligible porosity with size less than 20 nm. The different characteristics of Si produced at different temperature ramp rates can be understood by local heat accumulation. Due to the large negative enthalpies associated with magnesiothermic reduction, a large amount of heat is released in local areas. With a fast ramp rate, little time is allowed for heat radiation or conduction to occur, so local temperature increases accelerate nearby reactions, which further increases the local temperature. These high temperatures cause the fusion of Si products and the disappearance of small pores. At the same time, Si coats and fuses around the MgO that forms and after dissolving MgO with HCl, macroporous Si remains (Fig. 3a). In sample Si-RH-40, some MgO might be completely coated with Si so that HCl cannot access it during etching, which is why relatively intense MgO peaks appear in the XRD pattern of Si-RH-40 (Fig. 3e). Another result of this violent inhomogeneous reaction was the formation of Mg 2 Si in Si-RH-40 (Supplementary Fig. S6).

Figure 3 Conversion of nano-SiO 2 to nano-Si and characterization. a,b, SEM images of nano-Si obtained by magnesiothermic reduction of nano-SiO 2 with ramp rates of 40°C min−1 (a) and 5°C min−1 (b). c,d, TEM images of Si-RH-5 with different magnifications. The high-resolution TEM image (d) shows one spherical SiNP and the (111) lattice spacing of crystalline Si is clearly visible (the red line indicates the boundary of the SiNP). e, XRD patterns of Si obtained by magnesiothermic reduction of nano-SiO 2 with ramp rates of 40, 5 and 0.5°C min−1 from 400°C to 650°C (after HCl treatment). f, N 2 adsorption-desorption isotherms of nano-SiO 2 and the Si samples reduced from it. Inset, pore size distribution. g,h, TGA (g) and DTA (h) profiles of the reaction between nano-SiO 2 and Mg with different ramp rates under inert atmosphere. Full size image

TGA and DTA were employed to further investigate this reaction. The mass of the mixture did not change during the reaction with a slow ramp rate (Fig. 3g). With a higher ramp rate (20°C min−1), however, the powders were ejected out of the TGA crucible at 433°C, indicating a violent reaction. DTA directly shows that the reaction occurs around 400–450°C and that the higher ramp rate results in more severe heat accumulation (Fig. 3h), which ultimately leads to the violent reaction and ejection of reactants. Therefore, in order to obtain nano-Si, a slower heating rate of about 5°C min−1 is necessary. Further decreasing the heating rate from 5 to 0.5°C min−1 does not decrease the particle size or influence the crystallinity. The obtained Si nanoparticles (Si-RH-5) have sizes around 22 nm (Fig. 2f, Fig. 3c–d). Theoretically, spherical 22 nm single crystalline Si particles have a specific surface area of 117 m2 g−1, which is only half of the measured value (Supplementary Table S1). Hence, the Si nanoparticles obtained here are porous, which is consistent with the pore diameter analysis (Fig. 3f, inset). This naturally-inherited porous nanoparticle morphology meets the requirement of a stable Si anode to accommodate the volume change induced by lithium insertion/extraction (discussed below).

The electrical response of a pellet of nano-Si was tested and it exhibited ohmic I-V characteristics (Supplementary Fig. S7). The metallic nature of the nano-Si suggests the existence of impurities. The relatively low conductivity of 3.0 × 10−3 S m−1 could be due to the native oxide on the nanoparticles, which decreases the interparticle conductivity. To quantitatively examine the purity of the Si product, secondary ion mass spectroscopy (SIMS), which has subparts-per-million sensitivity, was used. The nano-Si pellet was subjected to a primary O− ion beam and secondary ions were ejected. Six metals common to biological systems (Na, Mg, Al, K, Ca and Fe), together with Si, were detected simultaneously by the mass spectrometer. The Si purity is determined to be higher than 99.6% based on the trace metal concentrations (Supplementary Fig. S8). The impurity with the highest concentration (less than 0.3%) is Mg. Most of the Mg is probably introduced during the magnesiothermic reduction. The element with the second highest impurity concentration was Fe (less than 0.07%). Na, Ca, K and Al were all under 0.01%. Although the nano-Si is directly recovered from a natural substance, its purity is more than enough for application in Li-ion battery anodes. The impurities may even improve the electrical conductivity of the Si anode through doping. For other applications, such as solar cells, the impurity level must be much lower, which could possibly be realized with a purer Mg reagent and additional acid leaching of the RH and the mixture after the magnesiothermic reduction6. It should be noted that previous attempts to recover silicon from RHs focused on the purity, because the purpose was to use them in solar cells35. Here, we focus primarily on preserving the nanostructure of the recovered silicon, targeting application in Li-ion batteries.

Electrochemical behavior of nano-Si from RH

Unlike bulk Si, the recovered nano-Si has high functionality due to the small size and porous nature. It exhibits excellent electrochemical behavior without any further coating or modification (Fig. 4). Upon deep galvanostatic cycling between 0.01 and 1 V, the discharge (delithiation) capacity reaches 2790 mA h g−1 for the first cycle at C/50 and stabilizes around 1750 mA h g−1 for later cycles at C/2 (Fig. 4a); these values are seven and five times the theoretical capacity of graphite (372 mA h g−1). The cycling stability is excellent as well. No capacity decay was observed over the 150 cycles after the initial lower-current cycles and the capacity retention values after 200 and 300 cycles are 95% and 86%, respectively. The voltage profiles for the different cycles are shown in Supplementary Fig. S9. The similarity of the profiles over 300 cycles indicates highly reversible Li insertion and extraction.

Figure 4 Li-ion battery anode using nano-Si recovered from RHs. a, Delithiation capacity and Coulombic efficiency (CE) of the first 300 galvanostatic cycles. The rate was C/50 for the first cycle, then C/10 for 5 cycles and C/2 for the later cycles. b, Cyclic voltammogram (CV) versus Li/Li+ at a 0.02 mV s−1 scan rate. Selected cycles are shown for clarity. c, Galvanostatic cycling of nano-Si recovered from RHs and commercial metallurgical-grade Si particles. Both electrodes were cycled at C/50 for the first cycle, C/20 for the second cycle and C/5 for the later cycles. All the nano-Si electrodes in a–c were tested between 1 V and 0.01 V versus Li/Li+ with PVDF binder. 1C = 4.2 A g−1 Si. Full size image