Silicon, Germanium, and Renewable Energy Materials

As the second-most common element in the Earth’s crust, silicon is a virtually unlimited resource (Figure 1.3). In contrast, the share of germanium in the Earth’s crust amounts to only 6.7 ppm and the element is only the 53^rd-most common element.^[25] Naturally, silicon occurs in various silicates and as silicon dioxide (mostly quartz). Germanium is typically found in sulfidic minerals such as argyrodite (Ag8GeS6) and germanite (Cu13Fe2Ge2S16).

Figure 1.3. Composition of the Earth’s crust by element.^[26]

Elemental silicon and germanium both crystallize in the diamond structure type and are intrinsic semiconductors characterized by indirect bandgaps of 1.12 eV and 0.67 eV, respectively.^[27] Silicon is highly oxophilic and forms a stable SiO2 passivation layer on any surfaces with air contact. In contrast, germanium is not passivated in air and can be dissolved in oxidizing acids via GeO formation.

1.1 SILICON AND GERMANIUM AND THEIR RELEVANCE IN THE CONTEXT OF RENEWABLE ENERGY

These physical and chemical properties determine some of the largest applications of the two elements. Ferrosilicon, for example, is an alloy of iron and silicon, which is available with silicon contents between 15 and 90 %. Exploiting the oxophilicity of silicon, ferrosilicon is largely used in the steel production as a deoxidizing agent. The semiconducting properties of silicon and germanium are exploited in the photovoltaics and electronics industries. Other important applications of silicon include the production of aluminum alloys as well as silanes, silicones, and other silicon-containing compounds. Germanium is further used in fiber and infrared optics as well as in catalysts for the pro-duction of polyethylene.^[25,28]

Industrially, silicon is typically produced by reducing quartz with coal at > 2000 °C in an electric furnace.^[25] The obtained metallurgical grade silicon (98.5–99.7 %) is primarily used in aluminum production and the chemical industry.^[25] For the production of ferrosilicon, iron turnings are added to the reactants. Semiconductor application of elemental silicon demands significantly larger purities. The Siemens process is the most important method of silicon purification. Exploiting the reversible reaction of Si with HCl to form trichlorosilane SiHCl3, purity levels of up to 9N to 11N suitable for electronic components can be achieved. Alternatively, silicon can be purified by conversion to highly pure monosilane. This process consumes much less energy than the Siemens process and affords 6N to 9N silicon, which suffices for photovoltaics applications. Due to the increasing demand for solar grade silicon, the market share of this so-called fluidized bed reactor production is growing rapidly.^[29]

The Siemens and the fluidized bed reactor processes both produce polycrystalline silicon. However, monocrystalline silicon is necessary for microchips and for some photovoltaics technologies. Thus, polycrystalline Si is converted by growing large single crystals around a seed crystal from molten Si (Czochralski process) or with the floating zone process.^[25] The latter technique further purifies silicon, which benefits applications in the electronics industry.

In 2014, 8,200 kilotons of ferrosilicon were produced,^[30] rendering it by far the largest application of silicon. Typically, the silicon content in ferrosilicon makes up about 65 % of the total silicon production which was 7,200 megatons in 2016.^[31] In addition, 2,700 kt of metallurgical grade Si were produced in 2014, of which 228 kt were converted to highly pure polycrystalline silicon for semiconductor pur-poses.^[30]

Due to its relatively rare occurrence, germanium is quite expensive at about 1500 $ kg⁻¹. In contrast, ultrapure silicon was sold for 25 $ kg⁻¹ in 2012.^[25] Thus, germanium is only used for specialty applica-tions and the total worldwide production in 2013 was 145 t.^[28] Industrially, germanium is obtained as a side product during processing of zinc ores. Purification of germanium can be achieved in a floating zone process.^[25]

As mentioned earlier, the production of solar photovoltaic (PV) cells is currently rising steeply (Figure 1.4). In 2015, the cumulated global capacity of installed solar PV facilities amounted to 229.3 GW, which can already provide more than 1 % of the global primary energy demand.^[32,33] The most probable scenario for the following years predicts a further increase in solar PV capacity of around 20

% per year, yielding 613 GW by 2020.^[33]

Figure 1.4. Cumulative installed capacity of global solar photovoltaics.^[33-35] Projections from 2016 to 2020 represent the most probable scenario according to SolarPower Europe’s 2016 Global Market Outlook.^[33] For comparison: the total global primary consumption in 2015 corresponds to 17,510 GW.^[32]

The vast majority of solar cells uses crystalline or amorphous silicon for the p-n junctions, which are the energy conversion components. However, the maximum efficiency of solar cells based on silicon only is physically restricted by the Shockley-Queisser limit.^[36] In AM1.5 solar irradiation¹ the maximum theoretical efficiency for Si p-n junctions is 32 %.^[37] The most efficient solar cell based on crystalline Si without sunlight concentration was recently developed by Panasonic and reaches an efficiency of 25.6 %.^[38]

Significantly larger energy conversion efficiencies can be achieved using multijunction photovoltaic cells. They employ several p-n junctions fabricated from materials with different band gaps in order to make better use of the whole solar irradiation spectrum. Using concentrator optics, Fraunhofer ISE has recently achieved an energy conversion efficiency of 46.0 % using a quadruple junction cell.^[39,40] In order to obtain sets of semiconductor materials with band gaps tuned for the highest possible efficiency, III-V semiconductors such as Ga1−xInxAs and Ga1−xInxP deposited on germanium are typically used.^[25,40] These compounds are, however, much more expensive so that a widespread use of highly efficient multijunction PV cells is still far away. Therefore, many research groups investigate cheaper semiconductors, ideally with tunable bandgaps, which could eventually disrupt the solar PV market.^[41]

Electric vehicles (EVs) using LIBs for energy storage are currently the most promising technology in the attempt to shift the transportation sector away from fossil fuels. Fuel cells have not yet come close to a comparable cost structure and large-scale production of biomass fuels strongly competes with food production in terms of land-use. Thus, many countries support LIB research and have created incen-tives to increase the demand of EVs. For example, the German Federal Government’s National Electromobility Development Plan introduced in 2009 aims for one million EVs sold in Germany by 2020.^[42]

However, by December 2015 only 51,600 additional EVs had actually been sold,² endangering the im-plementation of this plan.^[43] Evidently, EVs do not yet attract many customers for a variety of economic

1.1 SILICON AND GERMANIUM AND THEIR RELEVANCE IN THE CONTEXT OF RENEWABLE ENERGY

and life-style reasons. In EVs, energy storage still represents a large portion of price, weight, and volume and their driving ranges are typically shorter than those of conventional vehicles. Meanwhile, recharging EVs requires much more time than simple refueling at a gas station. Therefore, LIBs still need to be significantly improved in order to be cheap, light, and long-lived while still offering fast recharge options and a long driving range for EVs. Many materials scientists and electrochemists today thus investigate new materials for LIB cathodes, anodes, and electrolytes.

Solid-state electrolytes represent such a heavily investigated class of materials.^[44] The flammable organic electrolytes currently used in commercial LIBs pose a significant safety hazard, which can be overcome with solid-state electrolytes. In addition, they could mitigate the stability issues associated with organic electrolytes. However, the biggest challenge here is to find materials with similarly large Li⁺ ion diffusivities. Several inorganic Si- and Ge-containing compounds have shown great potential in this context. Li14Zn(GeO4)4 was the first prominent candidate for a solid Li⁺ ion conductor.^[45] Its properties were more and more optimized by adjusting the stoichiometry in Li2+2xZn1−xGeO4 and adding various dopants.^[46] More recently, oxides were replaced by sulfides, which exhibit even better Li⁺ ion conduction properties. Li10GeP2S12 is currently one of the most promising solid electrolyte materials with Li⁺ ion conductivities even exceeding those of liquid organic electrolytes.^[47] Additional studies have shown that Ge can also be replaced by Si or Sn in this structure.^[48,49]

Both silicon and germanium also attract much attention as potential anode materials.^[50,51] Upon formation of Li15Si4, the specific theoretical capacity of Si anodes amounts to 3579 mAh g⁻¹,^[52]

representing an almost ten-fold capacity compared to 372 mAh g⁻¹ for graphite anodes (LiC6 forma-tion) which are commercially used today.^[53] Ge anodes are characterized by a specific theoretical capacity of 1385 mAh g⁻¹ with isostructural Li15Ge4 as the most lithiated phase.^[54] Its high electrical conductivity (10⁴ x greater than for Si) and Li⁺ ion diffusivity (400 x higher than for Si) render Ge another very interesting LIB anode material.^[55] In contrast to graphite anodes, however, lithiation and de-lithiation of silicon and germanium do not occur via an intercalation/deintercalation mechanism, causing a number of practical difficulties. Most prominently, Si and Ge anodes suffer from extreme volume changes of > 300 % and 230%, respectively, upon charge and discharge.^[56] If this issue can be overcome in the upcoming years, silicon and germanium may have a bright future not only in solar cells but also in LIBs.