Nanometallurgical Silicon for Energy Application

FUTURE ENERGY

Nanometallurgical Silicon for Energy Application

and microelectronic industries. The steady increase in demand for high-purity silicon can be attributed to the compound annual growth rate of both PV and microelectronics, which is well above 15%. However, the production of high-purity silicon requires high-energy payback time coupled with the need to resolve the heavy burden of pollution and CO2 emissions created in the process.

Xiaopeng Li1 and Ralf B. Wehrspohn2,*

Professor Ralf B. Wehrspohn was appointed jointly by the Martin Luther University (MLU) Halle-Wittenberg and the Fraunhofer Gesellschaft to work in Halle. As the youngest institute director in the Fraunhofer Gesellschaft, he has headed the Fraunhofer Institute for Microstructure of Materials and Systems IMWS since 2006. He holds the chair of Microstructure-Based Material Design at the MLU. His main areas of work are nanostructure materials and components, such as those used in microelectronics, sensors, photonics, and photovoltaics.

Xiaopeng Li studied in Professor Wehrspohn’s group from 2010 to 2013. He received his doctorate degree with highest honor Summa Cum Laude. Since 2014, he has been working in Shanghai Advanced Research Institute, CAS. He is currently an associate professor and focuses on developing advanced metal-air batteries, electrolyzers, fuel cells, and silicon nanomaterials.

Silicon, as the second most abundant element on earth (27.5%), has been essential for human civilization in the past and continues to be so in the present as well as for the future. Silicon production increased steadily from 1.1 million tons in 1964 to 7.2 million tons in 2016. At present, China is a major producer of silicon, accounting for more than 60% of the market. The low-grade ferrosilicon, with a purity of 15%–90%, and metallurgical silicon (MG-Si), with a purity of 99% (2N), are mainly used in aluminum, steel, and chemical industries. A few percentages of MG-Si are further purified to solar-grade 99.9999% (6N) and electronic-grade >99.9999999% (>9N) for the applications in photovoltaic (PV)

1172 Joule 3, 1172–1179, May 15, 2019 ª 2019 Elsevier Inc.

Here, we will explore the possibilities to reduce carbon emissions from the silicon industry by coupling with renewable energies and adopting energysaving wet chemical and electrochemical processes. We will also discuss the use of nanostructured low-grade silicon in water splitting and lithium-ion batteries (LIBs). We believe resourceful silicon can act as an energy carrier and has increasing value in the future energy system. Si as an Energy Carrier Industrial production of silicon starts with carbothermic reduction (CRSiO2: SiO2 (s) + 2C (s) / Si (s) + CO2 (g)) in a submerged arc furnace at a high temperature of 1900 C and requires the energy input of 15 MWh/t of silicon. Of note, the hydrogen element is not essential in this process. Major Chinese silicon producers are located close to the solar and wind energy plants, quartz mines, and carbon-rich (e.g., coal) but water-poor regions such as Inner Mongolia and Ningxia provinces. Integration of renewable electricity in the CRSiO2 is thus of great potential. However, the use of carbon reductants causes intensive CO2 emissions. Electrochemical reduction of SiO2 (ERSiO2) in molten salts (e.g., LiCl and CaCl2) is an attractive alternative approach involving the use of electrons to deoxidize silicate: SiO2 (s) + 4e / Si (s) + 2O2 .1,2 ERSiO2 requires a mild reaction temperature (500 C–900 C).

the product to 6N. The energy consumption of metallurgical process is just one-fourth of that of Siemens process.5 Therefore, the combined route (Figure 1) is attractive with nearly zero carbon emission and an estimated electrical energy input of 40 MWh/t.

Figure 1. Dependence of Estimated Production Energy Input and Current Silicon Price on Silicon Purity The price information of silicon ingot or chunk was obtained from Alibaba.com and PVinsights.com. The energy input for wafer cut was not included. The electrochemical route indicates ERSiO 2 and electrochemical refining for MG-Si and upgraded MG (UMG)-Si, respectively. The combined route represents the metallurgical treatment of the Si from ERSiO 2 or the etched Si from CRSiO2 .

Although ERSiO2 also consumes electrical energy of 13 MWh/t, the energy can be fully supplied with renewable energy. The first pilot plant of ERSiO2 was operated between 1960 and 1966 by Monnier and co-workers.1,3 The system used the SiO2-cryolite system. The deposition current density as high as 800 mA cm 2 was achieved, which is comparable to the current density used in the Hall-He´roult process for aluminum production. Solid Si with a crystal size of 1–3 mm was obtained.3 The major obstacle for commercialization was a slow deposition rate. Recently, Homma’s group developed a novel ERSiO2 reactor operating at 850 C (Figure 2) using CaCl2 as the electrolyte.4 The charging of silica powder or granules (at the top of the cell) and the recovery to a crystalline MG-Si product with nanostructure features can be operated continuously, thereby enabling high-throughput production. The Si formation rate is 1.8–3 mm/h, which is comparable to that of polycrystalline Si growth rate of 1–2 mm/h in the Siemens process. The power cost will be the major factor influencing the commercial viability of an ERSiO2 plant.

With proper design of the electrochemical reactor, the Joule heating from the electrical deposition current should be sufficient to maintain the reactor temperature. Si Purification Routes for the Photovoltaic and Solar WaterSplitting Applications Reducing impurity concentration into the ppb-ppt range in Si is essential for PV applications. The value of MG-Si increases by 10- to 20-fold after purification at the expense of intensive energy input (Figure 1). Industrial silicon refining mostly involves Siemens or Siemens-like process, which upgrades MG-Si to solar-grade poly-Si (6N) with an energy input of 100–120 MWh/t.5 Here, we suggest several attractive routes for Si refining. The first is combining ERSiO2 and metallurgical processes. The Si product from ERSiO2 using pure quartz feedstock can achieve 99.94%.2 The major impurities are Ca and Cl from molten salt. One- or twostep metallurgical process (e.g., vacuum induction melting and directional solidification) could be enough to purify

The second route is based on wet chemical etching that can process MG-Si from CRSiO2. There are two types, including acid etching and metal-assisted chemical etching (MACE).6–9 For the acid etching, the metal removal efficiency increases proportionally with the surface area of MG-Si particles. Zong and co-workers obtained nano-Si of 99.999% (5N) purity from low-grade ferro silicon (84%) using an acidic etchant consisting of HF, HNO3, and HCl.6 However, high-energy ball milling is required to grind the low-grade Si into a size range less than 150 nm. By contrast, MACE can process coarse MG-Si particles since it is a self-catalyzing porosification process.7–9 The MG-Si particle size can be tens of micrometers without compromising purification effect as the metal nanoparticles can drill deep into the silicon particles and expose new surfaces. Our group achieved MG-Si purification from 99.74% to 99.9884% via Ag-based MACE.8 Recently, we observed chemical cracking effect induced by Cu-based MACE.9 Coarse MG-Si particles can be disintegrated into fine nanoparticles. Therefore, this method may replace the ball milling process. Recycling of acidic etchant and precious metals can be achieved by (electro-)dialysis, which is technologically matured and requires low energy input. Thus, we estimate that the energy input of purification via the etching route could be less than 5 MWh/t. The subsequent metallurgical process is sufficient to increase purity to 6N for PV application (Figure 2). The third route is electrochemical refining that also takes place in molten salts (800 C–1,500 C). Controlled dissolution of MG-Si and its alloy

Joule 3, 1172–1179, May 15, 2019 1173

bon emissions. However, substantial efforts are still required to fabricate Si wafers based on these routes and investigate their electronic properties and corresponding solar cell performances.

Figure 2. Schematic Process of ERSiO2, MACE, and Directional Solidification for Producing SolarGrade Si Red liquid, white spheres, and blue spheres represent molten salt, SiO2 , and Si particles, respectively. In MACE, colorful tiny particles represent various metal impurities. Yellow spheres are electroless-deposited metal nanoparticles (e.g., Cu and Ag).

(e.g., Cu3Si) occurs at the anode, and silicon ions are then electrochemically deposited at the cathode. This process only demands an electrical energy input of 2 MWh/t, and the Si purity can reach

99.9996%.10 There are several key challenges (e.g., anode passivation and metal contamination) to be addressed. The three routes have advantages in reduction of production cost and car-

Figure 3. Schematic of the Roll-to-Roll Nano-MG-Si Battery Material Production Process

1174 Joule 3, 1172–1179, May 15, 2019

Si is also a viable candidate for solar water splitting since it absorbs light over a broad range of solar spectrum. Although Si is unstable in an aqueous electrolyte, the state-of-the-art Si photoelectrode with a protective overcoating has demonstrated over 2,000-h stability under 1-sun illumination.11 Past research has been mainly focusing on electronic-grade Si for water splitting. We believe there is a great potential of utilizing cheap UMG-Si. Our preliminary results indicated MACE can produce silicon nanowires with high purity at the UMG-Si surface.8 The etched UMG-Si wafer exhibited enhanced performance with an identical onset potential for hydrogen evolution reaction as that of the etched electronic-grade Si. In addition, the high-aspect-ratio Si nanowires can act as an excellent support to deposit thin metal oxides to construct tandem cells with coreshell structures for unassisted water splitting. Lithium-Ion Battery Application The addition of Si into graphite anode in LIBs delivers the promise of at least

30%–40% increase of capacity. With the upsurge in the demand for high-capacity batteries, Si-based LIBs are expected to foresee a dramatic increase. The price of battery Si is estimated to be $150/kg, and its purity requirement is low (R2N). Wet chemical etching that can produce nano-MG-Si for battery application is therefore of great economic potential (Figure 1). To date, nanostructured MG-Si produced by wet chemical etching has shown satisfactory performance with a specific capacity of >1,000 mAh/g and cycling stability of 1,000 cycles.12 Multiple studies have found that the metal impurities in MG-Si particles had a direct impact in the formation of interior porosity, which was vital for accommodating the large volume change.6,7 Intentional control over the concentration and distribution of metal impurities during MG-Si solidification is essential for reaching the desired nanostructures especially in large-scale production. However, little attention has been paid to this issue. Besides, the particle size of nano-MG-Si should be not significantly larger than 2 mm for achieving long-term cycling stability. To our knowledge, there is one commercial example using the etched MGSi for LIBs.12 ERSiO2 is another potential route to produce nano-MG-Si for LIBs.2 Here, we propose a productive roll-to-roll ERSiO2 process (Figure 3). The SiO2 particles are premixed or anchored at the porous carbon electrode. The carbon electrode provides an electronconducting framework for electrolytic reduction of SiO2 as well as LIB operation. Removal of O2 ions in SiO2 leads to the formation of interior porosity. Subsequent chemical etching removes contamination from molten salt and unreduced SiO2. Reducing carbon emissions from the current Si industry and expanding the Si value chain demands technological revolutions in Si production and purifi-

cation. We discussed several promising routes that can produce and purify MG-Si with a significantly reduced carbon footprint. The nano-MG-Si products also open new possibilities in LIBs and solar water-splitting applications. We expect the formation of a new regional Si economy if current Si producers can take advantage of energy transition and technology advancement.

10. Olson, J.M., and Carleton, K.L. (1981). A semipermeable anode for silicon electrorefining. J. Electrochem. Soc. 128, 2698–2699. 11. Zhou, X., Liu, R., Sun, K., Papadantonakis, K.M., Brunschwig, B.S., and Lewis, N.S. (2016). 570 mV photovoltage, stabilized n-Si/ CoOx heterojunction photoanodes fabricated using atomic layer deposition. Energy Environ. Sci. 9, 892–897. 12. Elkem, A.S. Silicon-carbon composite anode for lithium ion batteries. US patent US20180040880A1, https://patents.google. com/patent/US20180040880A1/en. 1CAS

ACKNOWLEDGMENTS This work was supported by BMBF Struktursolar and Youth Innovation Promotion Association CAS.

1. Elwell, D., and Rao, G.M. (1988). Electrolytic production of silicon. J. Appl. Electrochem 18, 15–22. 2. Yang, X., Ji, L., Zou, X., Lim, T., Zhao, J., Yu, E.T., and Bard, A.J. (2017). Toward costeffective manufacturing of silicon solar cells: electrodeposition of high-quality Si films in a CaCl2-based molten salt. Angew. Chem. Int. Ed. 56, 15078–15082. 3. Monnier, R., and Barakat, D. US Patents 3219561 (1965) and 3254010 (1966). 4. Homma, T., Matsuo, N., Yang, X., Yasuda, K., Fukunaka, Y., and Nohira, T. (2015). High purity silicon materials prepared through wet-chemical and electrochemical approaches. Electrochim. Acta 179, 512–518.

Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS), Shanghai 201210, China

2Fraunhofer

Institute for Microstructure of Materials and Systems (IMWS), Halle 06120, Germany *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.03.025

FUTURE ENERGY

Fusion Energy: Research at the Crossroads Martin Greenwald1,*

5. Safarian, J., Tranell, G., and Tangstad, M. (2012). Processes for upgrading metallurgical grade silicon to solar grade silicon. Energy Procedia 20, 88–97. 6. Zong, L., Zhu, B., Lu, Z., Tan, Y., Jin, Y., Liu, N., Hu, Y., Gu, S., Zhu, J., and Cui, Y. (2015). Nanopurification of silicon from 84% to 99.999% purity with a simple and scalable process. Proc. Natl. Acad. Sci. USA 112, 13473–13477. 7. Ge, M., Lu, Y., Ercius, P., Rong, J., Fang, X., Mecklenburg, M., and Zhou, C. (2014). Large-scale fabrication, 3D tomography, and lithium-ion battery application of porous silicon. Nano Lett. 14, 261–268. 8. Li, X., Xiao, Y., Bang, J.H., Lausch, D., Meyer, S., Miclea, P.T., Jung, J.Y., Schweizer, S.L., Lee, J.H., and Wehrspohn, R.B. (2013). Upgraded silicon nanowires by metalassisted etching of metallurgical silicon: a new route to nanostructured solar-grade silicon. Adv. Mater 25, 3187–3191. 9. Guan, B., Sun, Y., Li, X., Wang, J., Chen, S., Schweizer, S., Wang, Y., and Wehrspohn, R.B. (2016). Conversion of bulk metallurgical silicon into photocatalytic nanoparticles by copperassisted chemical etching. ACS Sustainable Chem. Eng 4, 6590–6599.

Martin Greenwald has worked in the field of magnetic fusion energy for more than 40 years. He is the Deputy Director of MIT’s Plasma Science & Fusion Center and a cofounder of Commonwealth Fusion Systems, a recent startup. Dr. Greenwald’s research has focused on the turbulent transport of energy in magnetically confined plasmas. He led a research team that was the first to exceed the Lawson’s criteria for density-confinement product—a key fusion goal. He

Joule 3, 1172–1179, May 15, 2019 ª 2019 Elsevier Inc. 1175