Geothermal Silica as a Sustainable Source for Lithium-Ion Battery Anodes: Advances, Challenges, and Future Prospects

Abstract

Geothermal silica has emerged as a promising and underutilised precursor for silicon-based lithium-ion battery anodes. Geothermal silica can be recovered from brines, scales, and solid residues generated during geothermal energy production, creating an opportunity to valorise existing waste streams while mitigating silica-scaling problems. This review examines the formation, availability, and material characteristics of geothermal silica, with particular emphasis on its high silica content, commonly reported in the range of ~50–98 wt% in solid geothermal residues, as well as its generally amorphous nature and porous structure. It then evaluates the main processing steps required to convert geothermal silica into battery-relevant silicon, including extraction, purification, and silica-to-silicon reduction, with particular focus on magnesiothermic reduction. Among the available routes, methods that provide improved impurity control while preserving porous or amorphous precursor structures appear most relevant for achieving favourable electrochemical performance. Recent comparative findings indicate that geothermal silica can, in some cases, be competitive with biomass-derived silica sources in terms of purity, composition, and morphology, although these advantages are not universal and depend on source-specific chemistry, impurity profile, and processing conditions. Reported electrochemical studies further show that geothermal-silica-derived silicon and silica-based composites can deliver electrochemically relevant capacities, in some cases exceeding the theoretical capacity of graphite (~372 mAh g⁻¹), although performance varies significantly across studies. In addition, specific surface areas of ~50–150 m² g⁻¹ reported for some geothermal silica materials may support further silicon processing and influence electrochemical behaviour. Overall, geothermal silica represents a technically relevant and sustainability-oriented pathway toward silicon-based anode materials; however, further work is needed on source consistency, impurity management, structural control, long-term cycling stability, and scalable production.

1. Introduction

Lithium-ion (Li-ion) batteries have attracted significant global attention because of their high energy density, excellent efficiency, and long service life [1]. However, despite their widespread use, current Li-ion batteries still face limitations in meeting the increasing demand for higher energy density, improved safety, longer driving range, and faster charging. A major reason is the limited capacity of conventional intercalation-type anodes. Graphite, which accounts for more than 98% of commercial Li-ion battery anodes, has a theoretical capacity of only 372 mAh g⁻¹ and therefore limits further improvements in high-energy applications such as electric vehicles [2].

Among alloying-type anodes, silicon (Si) has emerged as one of the most promising candidates for next-generation Li-ion batteries because it can alloy with up to 4.4 Li per Si, corresponding to a theoretical capacity of 3579–4200 mAh g⁻¹ [3], which is approximately eleven times higher than that of graphite [4]. However, its practical use remains challenging because of severe volume expansion during lithiation and delithiation, as well as large initial irreversible capacity loss [2]. To address these problems, various strategies have been proposed, including the use of binders [5], modification and compounding of active materials [6,7], and electrolyte additives [8].

In this context, geothermal silica has emerged as an attractive precursor for silicon production. Geothermal solid waste is generated during geothermal energy extraction, where dissolved minerals in hot water precipitate as temperature and pressure decrease [9,10]. This waste stream can contain a high silica content, with some studies reporting values as high as 88.45 wt% [11], highlighting its potential as an alternative silica source. However, this value is not universal, and the silica content varies significantly depending on the geothermal source and operating conditions, typically ranging from 50% to 98% [12,13,14].

The recovery of silica from geothermal fluids offers a dual benefit: it mitigates scaling problems in geothermal operations while simultaneously generating a potentially valuable material for advanced applications. From a sustainability perspective, geothermal silica is especially interesting because it valorises a waste stream from renewable electricity production and may reduce the environmental burden associated with mining or synthesizing conventional silicon feedstocks. In this way, geothermal silica can support more resilient and sustainable battery material supply chains. At present, however, geothermal silica is still used mainly in low-value applications such as bricks, sand, cristobalite, and fillers [15].

Among the silica-derived routes explored for silicon-based Li-ion battery anodes, geothermal silica has received comparatively limited attention despite its potential as a sustainable and technically promising precursor. This review therefore adopts a deliberately focused scope: rather than providing a comprehensive overview of all silicon-anode developments, it specifically examines geothermal silica as an emerging and still underutilised feedstock for silicon-based Li-ion battery anodes, while other silica sources and silicon-anode strategies are discussed only where necessary to provide context and comparison. In doing so, the review goes beyond descriptive summarization by critically evaluating how source characteristics, impurity profiles, purification requirements, conversion pathways, and electrochemical performance collectively determine both the opportunities and the current limitations of geothermal-silica-derived anode materials.

2. Background

2.1. Geothermal Silica Formation and Availability

In geothermal systems, silica originates from the dissolution of silicate minerals in hot water and is transported in geothermal fluids primarily as dissolved silicic acid. As the fluid rises toward the surface and undergoes cooling and pressure reduction, silica solubility decreases, leading to supersaturation and subsequent precipitation. This precipitation leads to silica scale formation in wells, pipelines, and surface equipment, making it a persistent operational issue in geothermal plants. The deposited material is typically amorphous rather than crystalline quartz [9,10,15,16,17].

Geothermal brines contain appreciable amounts of dissolved silica, and the concentration in such fluids generally ranges from 100 to 300 ppm, depending on reservoir geology and temperature conditions [17]. Across the world, geothermal facilities already generate notable quantities of silica (Table 1) either as a waste stream or as a recoverable byproduct, indicating that these systems can provide silica at scales relevant for further materials processing. Representative examples include the Dieng Geothermal Power Plant in Indonesia, which produces about 160–165 t month⁻¹ of silica-rich sludge [18,19], the Ohaaki geothermal plant in New Zealand with an annual production of about 10,500 t [20], and Icelandic geothermal facilities, where silica precipitation has been estimated to exceed 40,000 t year⁻¹ [21].

Additional examples further illustrate this scale of availability. In the Menengai geothermal field in Kenya, the annual silica potential has been estimated at about 2500 t [22], while geothermal plants in southern California were reported to send about 25,480 t year⁻¹ of silica-containing material to landfill, highlighting a substantial underutilised side stream [23].

A study of western Anatolian geothermal fields estimated a total silica reserve of 246,417 t across seven fields, indicating substantial resource potential [24]. In addition, the Cerro Prieto geothermal plant in Baja California, Mexico, generates over 70,000 t year⁻¹ of waste containing mainly amorphous nano-silica, further highlighting the large-scale availability of geothermal silica-rich by-products [25].

At the Mammoth Pacific LP geothermal power plant in California, silica recovery from geothermal fluids has been reported at approximately 7200 t year⁻¹, with product purity exceeding 99% [26].

A representative Icelandic example is provided by the HS Energy facility in Svartsengi, where about 7 million m³ of geothermal fluid are processed annually. At this site, cooling of silica-rich fluid leads to precipitation of more than 3000 t year⁻¹ of silica. The recovered material was reported to be amorphous and highly porous, with particle sizes of 20–90 nm, a specific surface area of about 50 m² g⁻¹, a pore volume of 0.24 cm³ g⁻¹, and an average pore width of about 23 nm [27]. Overall, these data show that geothermal silica is not a marginal laboratory source, but an already available and recoverable silica stream with clear potential as a precursor for large-scale upgrading and conversion.

Table 1. Representative reported geothermal silica quantities and recovery streams worldwide.

Location	Facility/Region	Reported Silica Quantity	Silica Content	Reference
Indonesia	Dieng Geothermal Power Plant	160–165 t/month (~1920–1980 t/year)	>80 wt% silica	[18,19]
New Zealand	Ohaaki Geothermal Plant	~10,500 t/year	High-quality silica	[20]
Iceland	Various geothermal facilities	>40,000 t/year	Amorphous silica	[21]
Iceland	Svartsengi (HS Energy)	~3000 t/year (from ~7 million m3 fluid)	>99% pure, 20–90 nm particles, 50 m2 g−1 SSA *	[27]
Kenya	Menengai Geothermal Field	~2500 t/year	Silica from brine precipitation	[22]
USA	Southern California geothermal plants	~25,480 t/year (sent to landfill)	~50% amorphous silica (~212 mg/kg brine)	[23]
USA	Mammoth Pacific LP, California	~7200 t/year	>99% pure, 10–100 nm colloids, 50–150 m2 g−1 SSA	[26]
Mexico	Cerro Prieto	>70,000 t/year (waste)	Amorphous nano-silica (10–50 nm), Na/K chlorides	[25]
Turkey	Western Anatolian fields (7 sites)	~246,417 t (reserve)	–	[24]

* Specific surface area = SSA.

Table 1. Representative reported geothermal silica quantities and recovery streams worldwide.

Location	Facility/Region	Reported Silica Quantity	Silica Content	Reference
Indonesia	Dieng Geothermal Power Plant	160–165 t/month (~1920–1980 t/year)	>80 wt% silica	[18,19]
New Zealand	Ohaaki Geothermal Plant	~10,500 t/year	High-quality silica	[20]
Iceland	Various geothermal facilities	>40,000 t/year	Amorphous silica	[21]
Iceland	Svartsengi (HS Energy)	~3000 t/year (from ~7 million m3 fluid)	>99% pure, 20–90 nm particles, 50 m2 g−1 SSA *	[27]
Kenya	Menengai Geothermal Field	~2500 t/year	Silica from brine precipitation	[22]
USA	Southern California geothermal plants	~25,480 t/year (sent to landfill)	~50% amorphous silica (~212 mg/kg brine)	[23]
USA	Mammoth Pacific LP, California	~7200 t/year	>99% pure, 10–100 nm colloids, 50–150 m2 g−1 SSA	[26]
Mexico	Cerro Prieto	>70,000 t/year (waste)	Amorphous nano-silica (10–50 nm), Na/K chlorides	[25]
Turkey	Western Anatolian fields (7 sites)	~246,417 t (reserve)	–	[24]

* Specific surface area = SSA.

2.2. Silicon as an Anode Material

Silicon is one of the most promising anode materials for next-generation Li-ion batteries because of its very high theoretical specific capacity (3579–4200 mAh g⁻¹), far exceeding that of commercial graphite (~372 mAh g⁻¹) [28,29]. Its lithiation potential (~0.4 V vs. Li/Li⁺) is also slightly higher than that of graphite, which helps suppress lithium plating and may improve cell safety [8]. In addition, silicon is abundant and widely available, supporting its relevance as a scalable anode material [30].

However, the practical use of silicon remains limited by major electrochemical and mechanical challenges.

During lithiation, silicon undergoes very large volume expansion, typically exceeding 300% (with some sources reporting ~400% [31]), which causes cracking, pulverization, loss of electrical contact, unstable solid electrolyte interphase (SEI) reformation, and rapid capacity fading [2,32,33,34,35,36]. Unlike the relatively stable SEI formed on graphite, the SEI on silicon repeatedly ruptures and reforms during cycling, continuously consuming electrolyte and active lithium; even at an anode coulombic efficiency of 99.9%, the available lithium inventory can fall below 80% after about 250 cycles [32,35]. Lithium trapping further contributes to irreversible capacity loss, including substantial first-cycle loss and continued lithium loss in later cycles [37,38]. Silicon also suffers from low intrinsic electrical conductivity, which hinders efficient charge transfer and reduces the utilization of its theoretical capacity [39,40]. For this reason, any silica-derived silicon pathway (including geothermal silica) must be assessed not only in terms of silicon yield but also in terms of how the resulting structure can help mitigate these limitations.

To mitigate these limitations, several improvement strategies have been developed, including silicon–carbon composites, nanostructuring, porous or hollow architectures, protective surface coatings, and advanced binder or electrolyte systems [8,41,42,43,44,45]. These approaches aim to buffer volume expansion, improve electrical conductivity, stabilize the SEI, and enhance cycling performance [8,41,42,43,44,45].

2.3. Why Geothermal Silica?

Geothermal silica is of particular interest not only because it is available as a recoverable geothermal side stream, but also because some reported sources exhibit structural and compositional characteristics that are potentially favourable for silicon production.

At Svartsengi, for example, recovered silica was found to be amorphous and porous, with nanoscale particle sizes and a relatively high specific surface area [27]. Moreover, geothermal silica has been directly converted into mesoporous silica structures such as SBA-15, with a reported surface area of 711.93 m² g⁻¹ and a pore volume of 0.85 cm³ g⁻¹, demonstrating that such feedstocks can be processed into high-surface-area silica architectures suitable for advanced materials applications [46]. These features are relevant because porous and amorphous precursor structures can be advantageous for subsequent silicon production and may help mitigate volume-expansion effects during battery cycling.

Recent comparative results from the CASBAT project support this perspective. In this work, GeoSilica (a geothermal-silica-derived sample from geothermal brine in the CASBAT project) and leached barley husk both showed high SiO₂ purity with minimal elemental contaminants, indicating that, in some cases, geothermal silica can be comparable to biomass-derived alternatives in terms of purity. The same report also showed that geothermal-brine-derived SiO₂ exhibited high purity and a mesoporous structure with an average pore size of about 5.5 nm and a porosity of about 39%, while the silicon reduced from GeoSilica exhibited a macroporous structure with an average pore diameter of 90.8 nm and a porosity of 78.2% [47]. These findings indicate that geothermal silica can provide favourable compositional and structural characteristics, although this remains strongly dependent on source-specific chemistry and processing conditions. Compared with biomass-derived sources such as barley husk, horsetail, and diatoms, geothermal silica therefore offers a potentially attractive combination of high purity, relatively low contaminant levels, favourable porous structure, and reduced dependence on variable biological feedstocks.

Although geothermal silica can exhibit favourable purity and morphology, these properties are not universal and depend strongly on reservoir chemistry, precipitation conditions, and downstream treatment. Accordingly, its performance as a precursor material can vary significantly between sources.

3. Processing of Geothermal Silica

For Li-ion battery applications, the processing of geothermal silica should not be viewed merely as a silica-recovery step. The more important objective is to recover silica in a form that can be upgraded into a battery-relevant precursor with sufficient purity, favourable morphology, and acceptable processing cost. Accordingly, the value of each processing route depends not only on silica removal efficiency, but also on its ability to limit impurity incorporation, preserve or create porous and amorphous structures, and support subsequent conversion to silicon. From this perspective, geothermal silica processing is best understood as an integrated pathway linking extraction, purification, and silica-to-silicon conversion, rather than as a set of isolated treatment steps.

3.1. Silica Extraction and Processing Methods

A range of physical, chemical, and hybrid routes has been reported for recovering silica from geothermal brines and sludge. However, these methods are not equally suitable for battery-oriented applications. Some are mainly useful for mitigating scaling and recovering silica in bulk form, whereas others are more attractive because they provide higher purity, better structural control, or more suitable precursors for subsequent conversion into silicon.

3.1.1. Seed Addition

Seed-assisted precipitation is one of the most direct strategies for recovering dissolved silica from geothermal fluids. In an ex-situ seeding approach, Setiawan et al. added fine silica gel particles to geothermal brine from the Dieng power plant and controlled pH, temperature, and agitation so that dissolved silica precipitated progressively onto the seed surface [48]. This method is attractive because it is relatively simple and can reduce dissolved silica while simultaneously generating recoverable solid particles.

An industrially oriented variant was proposed by Featherstone, where the brine pH was adjusted to induce the in situ formation of heavy-metal seed particles, onto which supersaturated silica preferentially precipitated [49]. This route is useful from an operational perspective because it combines silica removal with brine conditioning and reinjection. However, for battery applications, seed-based approaches should primarily be regarded as recovery methods rather than structure-engineering approaches. They can recover silica efficiently, but they offer limited control over pore architecture, particle morphology, and impurity incorporation compared with more advanced routes.

3.1.2. Membrane Concentration and Precipitation

Membrane-assisted concentration is more relevant when the objective is to recover silica in a purer and more controllable form. At Mammoth Lakes, Lawrence Livermore National Laboratory developed a process in which reverse osmosis concentrates the silica-rich fraction, followed by pH-induced precipitation and crossflow ultrafiltration to recover a silica product with purity above 98% and a specific surface area of 50–150 m² g⁻¹ [50]. Similarly, Bourcier et al. proposed a three-step route involving reverse osmosis, a seasoning stage to promote polymerization, and final separation of the resulting colloidal silica [51].

Compared with simple precipitation routes, membrane-based methods are more promising for battery-oriented processing because they can decouple concentration from precipitation and thereby improve control over product quality. Their main advantage is that they can yield relatively high-purity silica while avoiding some of the uncontrolled coprecipitation associated with direct chemical treatment of raw brine. As a result, they are better aligned with the need for cleaner silica feedstocks for subsequent purification and reduction.

3.1.3. High-pH Precipitation (Lime or Caustic)

High-pH precipitation using CaO, Ca(OH)₂, or NaOH has been widely explored as a silica-removal strategy. Under alkaline conditions, dissolved silica polymerizes and precipitates, often as calcium-silicate-hydrate phases, and silica removal above 80% has been reported when the pH is raised to around 10.5 and the Ca/Si ratio is sufficiently high [52,53].

This route is effective for bulk silica removal and therefore has clear relevance for geothermal scale management. However, its usefulness for battery-grade precursor production is significantly more limited. The formation of calcium silicate phases and related mixed products can complicate downstream purification and reduce the direct suitability of the recovered material for high-purity silicon production. Thus, this method is more attractive for operational silica removal than for producing a structurally refined silica precursor.

3.1.4. Electrocoagulation and Electrolysis

Electrocoagulation-based approaches remove silica by first converting dissolved silica into colloidal particles and then destabilizing these particles to induce aggregation and settling. Potapov et al. showed that this can be achieved either by adding multivalent cations chemically or by generating coagulant species electrochemically using sacrificial electrodes [54]. Sano and Yamaguchi further demonstrated that water electrolysis can promote silica precipitation in the presence of calcium and magnesium ions, forming a stable silica-containing compound without severe clogging problems [55].

These routes are promising for silica removal from geothermal fluids and may offer operational advantages, especially where continuous treatment is required. Nevertheless, from the viewpoint of battery materials, they remain primarily recovery-oriented methods. Because the precipitated products often contain additional metal species or mixed silicate phases, they do not inherently provide the high-purity or morphology-controlled silica that is most desirable for advanced silicon-anode processing.

3.1.5. Mesoporous Silica Synthesis and Sol-Gel Routes

Among the reported extraction and processing routes, mesoporous-silica and sol-gel approaches are particularly important for battery applications, because they move beyond silica recovery and begin to control precursor structure. Watanabe et al. directly synthesized mesoporous silica from geothermal water using a Cetyltrimethylammonium bromide (CTAB)-templated route, producing high-purity material with pore sizes of about 2.8 nm and specific surface areas exceeding 800 m² g⁻¹ [56]. Widiyandari et al. also recovered amorphous silica xerogel from geothermal sludge through an ultrasonic-assisted alkali-extraction/acid-precipitation method, increasing the specific surface area from about 162 to 303 m² g⁻¹ while preserving a mesoporous amorphous structure [57].

These methods are especially important because they generate silica with structural features directly relevant to downstream silicon production. High surface area, amorphous character, and mesoporosity can facilitate more effective reduction and may help produce silicon architectures better able to accommodate lithiation-induced volume change. In this sense, mesoporous and sol-gel routes are not only extraction methods but also effective precursor-design strategies.

Taken together, the extraction routes discussed above differ substantially in their relevance for battery-oriented applications. Seed addition, high-pH precipitation, and electrocoagulation are mainly effective for silica removal and recovery, but they offer limited control over the final silica structure and may introduce mixed phases or additional impurities. By contrast, membrane-based concentration and mesoporous or sol-gel routes are more promising when the objective is to obtain cleaner and more structurally suitable silica for conversion into silicon. In particular, routes that yield amorphous, porous, or mesostructured silica appear significantly more attractive for geothermal-silica-derived battery materials because they better support the requirements of subsequent reduction and electrode design.

3.2. Separating and Purifying Geothermal Silica

After recovery, purification becomes a decisive step because geothermal silica commonly contains calcium, magnesium, iron, aluminium, manganese, and related metal silicates [17]. For battery applications, this issue is particularly critical: residual impurities lower chemical purity, interfere with reduction, promote by-product formation, and deteriorate electrochemical behaviour by contributing to interfacial instability and capacity loss. Geothermal silica should therefore be judged not only by how much silicon dioxide (SiO₂) can be recovered, but also by how effectively impurities can be removed without destroying the favourable porous or amorphous features of the precursor.

Acid leaching is the most widely used purification route. In this process, the recovered silica is treated with acids such as hydrochloric acid (HCl) or sulphuric acid (H₂SO₄) (and in some cases nitric acid (HNO₃) or organic acids) to dissolve impurity phases while leaving the SiO₂ framework largely intact [11]. This is particularly important for geothermal silica because many extraction methods recover silica together with metal-containing species rather than as a fully purified product. Multi-stage leaching can further reduce residual contamination and thereby improve suitability for reduction.

However, purification cannot be treated as a uniform downstream step because geothermal brines differ markedly in fluid chemistry and trace-element inventory from one resource to another. Depending on reservoir conditions, water–rock interaction, and fluid evolution, the parent brine may be relatively enriched in Fe, B, As, Li, or other dissolved species, creating a source-specific impurity fingerprint [58,59]. As a result, different geothermal silica feedstocks require different treatment sequences before they can be regarded as suitable precursors for silicon production. In particular, B-rich brines may require selective polishing because boron in geothermal fluids is difficult to remove by simple bulk treatment alone [60]. By contrast, As-rich brines often require more targeted removal approaches, such as iron-based coagulation or electrocoagulation, rather than generic silica-cleaning procedures [61]. Fe-bearing impurities are generally more amenable to acid-leaching or precipitation-based removal at earlier stages, whereas boron becomes considerably more difficult to eliminate once silicon has already been produced [62]. This distinction is important because impurities carried into downstream silicon processing may persist as electrically active impurities in the silicon lattice and thereby compromise material quality [63].

In more aggressive purification schemes, fluoride-based treatment is used as a secondary leaching step to remove persistent Mg-containing phases and residual silica-derived impurities. Silviana et al. reported that double-stage leaching with HCl followed by a hydrofluoric acid (HF)–acetic acid mixture increased silicon purity from 58.9% to 98% [64]. Although HF treatment is effective, its safety and environmental drawbacks make it undesirable at scale, so purification remains both a technical and process-design challenge.

Control of pH during extraction influences purity indirectly by suppressing silica polymerization and limiting co-precipitation of metal silicates [17]. This is mainly a scaling-control strategy, but cleaner extraction conditions reduce the purification burden later.

Taken together, these observations show that purification in the geothermal-silica-to-silicon pathway is not merely a generic post-treatment step, but a source-dependent enabling stage. Its success depends not only on lowering total impurity content, but also on matching the treatment strategy to the chemical fingerprint of the original brine, so that trace elements do not persist into downstream silicon processing and compromise precursor quality, conversion efficiency, or final anode performance.

3.3. Silica Conversion

Converting silica to silicon can be achieved through carbothermic, magnesiothermic, aluminothermic, calciothermic, or sodiothermic reduction, with each route differing in operating temperature, reaction control, by-product formation, attainable purity, and structural retention. The choice of method determines energy demand, environmental impact, and whether the resulting silicon morphology is suitable for high-performance, porous Li-ion battery anodes. Table 2 provides a comparative overview of these methods, which are discussed below.

Table 2. Comparative Summary of Silica-to-Silicon Conversion Methods.

Feature	Carbothermal	Magnesiothermic	Aluminothermic	Calciothermic	Sodiothermic
Operating Temp.	>2000 °C	500–950 °C	650–800 °C	720–820 °C	~300 °C
Reductant	Carbon	Magnesium	Aluminium	Calcium	Sodium
Product Type	Bulk Si + SiC	Porous Si, high SSA	Crystalline Si, morphology retained	Limited crystalline Si	Amorphous Si
Yield/Purity	Moderate, needs post-purification	High porosity, tunable	Up to ~90% yield	Low yield	Low yield
Advantages	Industrially established	Lower energy, porous, scalable	High yield, preserves structure	Strong thermodynamic drive	Low temperature, direct amorphous Si
Limitations	Very energy intensive; SiC by-products	Exothermic runaway; MgO/Mg2Si residues	Stable Al2O3 hard to remove; low SSA	Poor kinetics; stable Ca–Si–O phases	Hazardous handling; not scalable

Carbothermal reduction: Carbothermal reduction is a bottom-up method for producing silicon [65]. It involves the reduction of silica using carbon to obtain silicon, typically performed in electric arc furnaces at very high temperatures exceeding 2000 °C [11,66]. The fundamental reaction is:

SiO₂(l) + 2C(s) → Si(l) + 2CO(g), ΔH = 695 kJ mol⁻¹

However, the method is highly energy-intensive, requiring continuous operation above 2000 °C, which increases both cost and environmental burden [11,66]. It also generates by-products such as silicon carbide (SiC) and requires subsequent purification to reach the high purity levels needed for advanced applications.

Magnesiothermic reduction: Magnesiothermic reduction is a metallothermic process in which silica is reduced by magnesium (Mg) to produce porous silicon. The reaction occurs at relatively low temperatures (500–950 °C), far below the >2000 °C required for carbothermic reduction, making it significantly more energy-efficient. The main reaction is:

SiO₂ + 2Mg → 2MgO + Si.

The MgO by-product is subsequently removed by acid leaching. This method produces porous silicon with high surface area and pore volume, which is particularly attractive for Li-ion battery anodes, catalysis, drug delivery, and hydrogen storage [11,65,66].

The advantages of this method include lower energy consumption, tunable porosity, morphology retention, scalability, and the ability to utilize diverse silica sources, including natural and waste materials [11,66]. However, several significant challenges remain. The reaction is highly exothermic, and uncontrolled heat release can cause thermal runaway, sintering, and pore collapse. Excess Mg leads to the formation of magnesium silicide (Mg₂Si) and other silicates, reducing yield and purity. Residual by-products such as MgO and Mg₂SiO₄ require multi-stage acid leaching, often involving hazardous acids such as HF, which increases cost and environmental risks [11,65].

Limitations include difficulties in scaling due to heat accumulation, sensitivity to Mg/SiO₂ ratio, particle size, and heating rate, and the risk of hazardous by-products (e.g., SiH₄ during leaching). Overall, this method is a promising pathway for producing porous silicon, but its industrial adoption still requires improved thermal management, optimization of reaction parameters, and safer purification strategies [11,65,66].

Aluminothermic reduction: The aluminothermic reduction method utilizes aluminium as a reducing agent to convert silica into elemental silicon at moderate temperatures, typically near the melting point of aluminium (~650–800 °C). Compared with the energy-intensive carbothermic process (>2000 °C), this method is more efficient and capable of producing high yields of silicon, reported up to ~90% under optimized conditions [65]. A major advantage of aluminothermic reduction is its strong ability for morphological retention, where the structural features of the precursor silica are preserved during the reaction. This results in silicon with crystalline domains and layered, “cabbage-like” morphologies, preventing the collapse and fusion of nanoparticles that commonly occur in magnesiothermic reduction [65,66]. Despite these benefits, significant limitations remain. The reduction inevitably generates stable by-products such as mullite and alumina (Al₂O₃), which are difficult to remove by acid leaching and may encapsulate the silicon, thereby limiting porosity [11]. Consequently, the specific surface area (≤37 m² g⁻¹) and pore volume (≤0.51 cm³ g⁻¹) are substantially lower than those obtained from magnesiothermic routes, reducing electrochemical and adsorption performance [66]. Moreover, silicon produced by this route shows relatively low drug-loading capacity and rapid release, restricting biomedical potential [65].

Calciothermic reduction: The calciothermic reduction method uses calcium as a reductant to convert silica into silicon [11]. Thermodynamically, it offers a highly favourable driving force (ΔG = −168.5 kJ mol⁻¹), surpassing both magnesiothermic (ΔG = −135.9 kJ/mol) and aluminothermic (ΔG = −157.8 kJ/mol) reductions [65]. Experimental studies in which Stöber silica nanoparticles were mixed with calcium powder and sodium chloride (NaCl) at 820 °C, followed by HCl washing, however, failed to yield crystalline silicon, largely due to calcium’s large atomic radius and high diffusion barriers [65]. Earlier work also reported granular Ca reacting with rice husk silica at ~720 °C, though with limited evidence of effective silicon production [66]. Moreover, the formation of CaO and Ca–Si–O phases during reaction introduces highly stable by-products that are difficult to remove by acid leaching, further reducing yield and purity [67]. Thus, while its advantages include strong thermodynamic feasibility and the potential for reduced reaction temperature compared to carbothermic smelting, its limitations lie in kinetic restrictions, incomplete conversion, and impurity retention. This renders calciothermic reduction far less viable than magnesiothermic or aluminothermic routes for producing porous, high-purity silicon [65,66,67].

Sodiothermic reduction: Sodiothermic reduction employs sodium metal as the reductant for converting silica into silicon. Due to sodium’s low melting point (98 °C) and moderate vapour pressure, the reaction can proceed under mild conditions compared with other metallothermic routes [66]. Wang et al. demonstrated this by reducing zeolite with small pieces of sodium, after evacuation for 2 h and heating at 300 °C for 20 h, followed by HCl washing, which yielded amorphous silicon [68]. The main advantage of this method is its relatively low operating temperature and direct formation of amorphous Si. However, it requires long reaction times and careful control of conditions, and its major drawback lies in sodium’s very high reactivity, which makes handling difficult and significantly limits scale-up [66].

Overall, the available evidence suggests that the most promising pathway is not a single isolated processing step, but a coupled strategy in which controlled silica recovery and effective purification are followed by a reduction route that preserves the structural advantages of geothermal-derived silica. Within this framework, magnesiothermic reduction presently appears to be the most relevant and practical route for geothermal-silica-derived battery materials because it combines lower processing temperature with the possibility of retaining porous and morphology-derived features that are advantageous for silicon anodes. By contrast, carbothermal reduction is too energy intensive for this purpose, while aluminothermic, calciothermic, and sodiothermic routes currently appear less suitable because of purification difficulties, limited porosity, or scale-up constraints.

4. Electrochemical Performance

The electrochemical performance of geothermal-silica-derived anode materials suggests promising potential for Li-ion battery applications, but the current evidence base remains limited and heterogeneous. Importantly, two distinct material pathways should be distinguished. In one route, geothermal silica is first reduced to silicon, and the resulting Si stores lithium primarily through an alloying mechanism. In the other, geothermal silica is used directly in SiO₂-based composite electrodes, where lithium storage involves a different reaction pathway, typically associated with conversion-type processes followed by subsequent alloying contributions from the reduced silicon domains formed during cycling. These two approaches should therefore not be interpreted as electrochemically equivalent.

Direct comparison among reported electrochemical data remains inherently difficult, because the studies differ in cell configuration, current density, electrode formulation, voltage window, cycling protocol, and whether performance is reported as gravimetric or areal capacity. As a result, the available results are more suitable for demonstrating feasibility and identifying general trends than for establishing definitive performance rankings. Table 3 summarizes the reported electrochemical performance of geothermal-derived materials.

Table 3. Comparative summary of reported electrochemical performance of geothermal-derived anode materials.

Precursor Source	Reduction/Process	Active Material	Initial Discharge Capacity	Cycle Stability/Retention	Ref.
Geothermal sludge (Dieng)	Magnesiothermic	Si	761.16 mAh g−1	Higher than partially purified Si (605.56 mAh g−1) and graphite (314.45 mAh g−1) within the same study	[15]
Geothermal waste silica	Magnesiothermic	Porous Si	~358.5 mAh g−1	Feasibility demonstrated; no strong long-term stability reported	[64]
GeoSilica (CASBAT geothermal silica sample)	Magnesiothermic	Si	1706 mAh g−1	Capacity decreased from ~1500 to ~500 mAh g−1 over 100 cycles; ~800 mAh g−1 at 2C with partial recovery to ~1000 mAh g−1 at 0.1C	[47]
Geothermal silica waste	Direct composite fabrication with carbon nanofiber architecture	SiO2/carbon composite	~4 mAh cm−2	Stable over 100 cycles	[69]
Geothermal silica crust waste	Magnesiothermic reduction + ZIF-67-derived carbonization	Si/Co-NC composite	1073 mAh g−1	93% retention after 100 cycles and 88% after 150 cycles at 200 mA g−1	[70]
Geothermal silica waste	Magnesiothermic reduction + partial oxidation + electrospinning	Si/SiOx/CNF	448 mAh g−1	66% retention after 200 cycles in half-cell testing; full cell delivered 129 mAh g−1 with 91% retention after 50 cycles	[71]

In addition to electrochemical performance, the relationship between precursor particle size, pore structure, and resulting silicon morphology is particularly important for geothermal-silica-derived anodes. Nanoscale silica particles and mesoporous structures can facilitate structure-preserving reduction during magnesiothermic processing, leading to porous silicon architectures that are better able to accommodate lithiation-induced volume expansion. Smaller silicon domains and higher porosity generally improve mechanical stability and lithium-ion transport, although excessively high surface area may increase SEI formation and first-cycle irreversible capacity loss. Accordingly, the evolution from geothermal silica particle size to final silicon structure is a key factor in determining electrochemical behaviour. Table 4 summarises the currently available information linking silica precursor characteristics, resulting silicon structures, and electrochemical performance.

Table 4. Relationship between geothermal silica precursor characteristics, resulting silicon or composite structure, and electrochemical performance.

Precursor Source	Silica Particle/Pore Characteristics	Resulting Si/Composite Structure	Electrochemical Relevance	Ref.
Svartsengi geothermal silica	20–90 nm particles; SSA ~50 m2 g−1; pore volume ~0.24 cm3 g−1; average pore width ~23 nm; amorphous/porous	Potential precursor for porous Si (not directly tested electrochemically)	Nanoscale porous silica may support structure-preserving reduction, although electrochemical performance has not yet been directly demonstrated for this specific source.	[27]
GeoSilica (CASBAT)	Mesoporous SiO2; SSA ~162 m2 g−1; pore volume ~0.27 cm3 g−1; pore size ~5.5 nm; porosity ~39%	Macroporous Si; SSA ~44 m2 g−1; pore volume ~1.54 cm3 g−1; pore diameter ~90.8 nm; porosity ~78.2%	High initial discharge capacity (~1706 mAh g−1; charge capacity ~1391 mAh g−1) but significant capacity fading over 100 cycles	[47]
Geothermal silica crust waste	Not explicitly reported	Si/Co–NC composite	Improved cycling stability, with 93% retention after 100 cycles and 88% after 150 cycles at 200 mA g−1, attributed to the buffered structure and conductive network	[70]
Geothermal silica waste	Not explicitly reported	Si/SiOx/CNF composite	Enhanced structural stability; 66% retention after 200 cycles in half-cell testing, and 129 mAh g−1 with 91% retention over 50 cycles in an LFP full cell	[71]
Geothermal sludge (Dieng)	Not explicitly reported	Si (magnesiothermic reduction)	Moderate capacity (~761 mAh g−1), demonstrating feasibility, although precursor-to-silicon structural evolution was not clearly reported	[15]

For geothermal-derived silicon systems, early studies indicate that silica recovered from geothermal sources can be converted into electrochemically active Si with capacities that, in some cases, exceed commercial graphite under the reported test conditions. For example, Widiyandari et al. reported that silicon produced from geothermal sludge delivered a specific discharge capacity of 761.16 mAh g⁻¹ in a full-cell configuration, compared with 605.56 mAh g⁻¹ for partially purified silicon and 314.45 mAh g⁻¹ for commercial graphite [15]. This study is useful in showing the influence of purification quality within the same experimental framework; however, its results should not be directly compared with half-cell data reported elsewhere.

Porous silicon structures derived from geothermal silica have also been investigated as a way to mitigate mechanical degradation during cycling. Silviana et al. synthesized high-purity (~98%) porous silicon via magnesiothermic reduction and reported an initial capacity of approximately 358.5 mAh g⁻¹ for a pure silicon anode [64]. Although this value is modest relative to more optimized silicon systems, the study still supports the feasibility of converting geothermal silica into battery-relevant porous silicon. At the same time, it illustrates that successful conversion alone does not guarantee high electrochemical performance.

More advanced silicon-based material designs have shown substantially stronger electrochemical performance. In the CASBAT project, silicon reduced from the GeoSilica sample exhibited an initial Coulombic efficiency of 81.5%, with discharge and charge capacities of 1706 and 1391 mAh g⁻¹, respectively [47]. During 100-cycle testing, the capacity gradually decreased from about 1500 mAh g⁻¹ to nearly 500 mAh g⁻¹. Rate capability tests further showed capacities of around 1500 mAh g⁻¹ at low rates and nearly 800 mAh g⁻¹ at 2C, followed by partial recovery to around 1000 mAh g⁻¹ when the rate returned to 0.1C. These findings indicate that geothermal-derived silicon can deliver high initial capacity and promising rate behaviour when combined with more advanced material engineering, although long-term structural stability remains a major limitation.

In addition to silicon-based systems, direct utilization of geothermal silica has also been explored. Hamid et al. reported a SiO₂/carbon nanofiber anode derived from geothermal silica waste that delivered an areal capacity of ~4 mAh cm⁻² with stable cycling over 100 cycles [69]. While this result is encouraging, it should be interpreted separately from gravimetric capacity data reported for silicon anodes, because it reflects a different active material system, a different storage mechanism, and a different performance metric. Rather than demonstrating superiority over geothermal-derived silicon, this study shows that geothermal silica can also be relevant in silica-based composite architectures without complete reduction to Si.

Recent studies provide more explicit evidence on long-term cycling stability than was available in the earlier geothermal-silica literature. Prakoso et al. developed a geothermal-silica-derived Si/Co–NC (cobalt–nitrogen-doped carbon nanofiber) composite in which a conductive carbon nanofiber matrix and metallic cobalt improved structural robustness; the electrode delivered an initial capacity of 1073 mAh g⁻¹, retained 93% capacity after 100 cycles, and still maintained 88% capacity after 150 cycles at 200 mA g⁻¹ [70]. Likewise, Estri et al. reported a binder-free Si/SiOx/CNF (Flexible silicon-silicon oxide-carbon nanofiber) architecture derived from geothermal silica waste; the optimized Si/SiOx/CNF electrode delivered a first-cycle discharge capacity of 448 mAh g⁻¹ and retained 66% of its capacity after 200 cycles in half-cell testing, while the corresponding full cell with LiFePO4 maintained 91% stability over 50 cycles [71]. These results indicate that long-term cycling performance can be improved when geothermal-derived silicon is incorporated into mechanically buffered composite architectures that stabilize the interface and accommodate volume change. Nevertheless, the evidence base is still limited to a small number of studies and to cycling windows of up to 150–200 cycles, so the practical service life of geothermal-silica-derived anodes under application-relevant conditions remains insufficiently established.

From a practical standpoint, the electrochemical relevance of geothermal silica cannot be judged only by whether it can be converted into silicon, but also by whether the resulting performance justifies the non-trivial processing chain required to obtain a battery-relevant precursor. Compared with simpler silica sources, geothermal feedstocks may require source-specific extraction, impurity-sensitive purification, and carefully controlled reduction before acceptable silicon quality is achieved. The currently available literature suggests that these steps can produce electrochemically active materials, but it does not yet show consistently that the added processing effort yields performance clearly superior to that obtainable from less process-intensive biomass-derived or synthetic silica routes. At present, the main justification for geothermal silica therefore appears to lie in its precursor-level advantages, including recovery from an existing industrial side stream, potentially favourable amorphous or porous morphology, and sustainability relevance, rather than in a clearly demonstrated electrochemical advantage at the electrode level.

Taken together, the available evidence suggests that geothermal silica should presently be regarded as a competitive but not yet demonstrably superior precursor for silicon-based Li-ion battery anodes. Its main potential advantages appear to arise at the precursor and supply-chain level rather than at the electrode-performance level. In several reported cases, geothermal silica is recovered as an already amorphous, porous, and relatively high-purity SiO₂ feedstock, with nanoscale features that may favour structure-preserving conversion into porous silicon [23,27,47]. In addition, it can be obtained as a non-food industrial side stream from existing geothermal operations, which strengthens its sustainability and resource-valorization relevance [21,22,23,25,26,27]. However, these advantages are not universal. The composition of geothermal silica depends strongly on brine chemistry and precipitation conditions, and impurities such as Fe, B, As, Ba, and other trace elements can increase purification demands, introduce source-to-source variability, and compromise downstream silicon quality if not effectively removed [58,59,60,61,62,63]. More importantly, the currently available electrochemical evidence remains too limited and heterogeneous to demonstrate clear superiority over silicon or silica derived from established biomass-based or synthetic sources. Although some geothermal-derived systems have shown promising capacities and, in more advanced composite designs, encouraging cycling retention [15,47,69,70,71], the reported data still span different active-material classes, testing configurations, and performance metrics, which prevents a definitive ranking against non-geothermal routes. Its practical competitiveness will therefore depend on whether these precursor-level advantages can be translated reproducibly into impurity-controlled, structurally stable, and electrochemically durable anode materials.

5. Techno-Economic Feasibility and Scale-Up Challenges

The economic feasibility of geothermal-silica-derived silicon anodes has not yet been established through a dedicated techno-economic analysis of the full pathway from silica recovery to battery-grade anode production. At present, this issue can only be discussed indirectly by combining evidence from geothermal silica recovery studies and silica-to-porous-silicon conversion analyses. For silica recovery, estimates at the Menengai geothermal field suggested that, assuming a 60% silica recovery rate and a selling price of US$2200 t⁻¹, approximately 2500 t y⁻¹ of silica could generate about US$5.5 million in annual revenue [22]. Likewise, a Lawrence Livermore assessment for the Mammoth Lakes geothermal site estimated that recovering the full geothermal fluid stream could produce approximately 7200 t y⁻¹ of silica, corresponding to a potential value of about US$11 million per year [72]. For a smaller 1.1 MGD demonstration plant, the same study reported an estimated capital cost of about US$2.3 million and a preliminary rate of return of 14%, with a payout period of around seven years [72]. Beyond silica recovery, the cost of converting silica into porous silicon is also critical. Yan et al. performed a techno-economic assessment of magnesiothermic reduction (MgTR) for battery-grade porous silicon production and reported a total cost of production of about US$85 kg⁻¹ for the baseline process, decreasing to about US$53 kg⁻¹ for an ultra-low-temperature route and to about US$42 kg⁻¹ for a two-step process [73]. Although this analysis was not specific to geothermal silica, it suggests that the conversion step itself could become economically attractive at scale. However, these studies do not yet establish the economic viability of geothermal-silica-derived silicon anodes as an integrated industrial pathway, because site-specific brine chemistry, impurity burden, silica recovery yield, purification requirements, and product specifications are all likely to influence the final economics. Accordingly, pilot-scale demonstrations and a dedicated cradle-to-gate techno-economic assessment are still needed before firm conclusions can be drawn regarding large-scale commercial competitiveness.

Although the availability of geothermal silica is promising, the feasibility of large-scale production of silicon from geothermal feedstocks has not been verified. Magnesiothermic reduction is intrinsically exothermic; without a heat moderator, batch sizes above about 1 g undergo self-heating that collapses porosity, and even with a sodium chloride moderator (typically added at ~5:1 NaCl:SiO₂), reported batch sizes have so far only reached ~30 g [74]. Early reviews emphasize that heat accumulation and mixing remain critical limitations and that simply increasing the NaCl ratio cannot keep the reaction below its melting point [75]. More recent studies propose automated mixing, heat-sink additives, and rotary-kiln furnaces to improve heat dissipation and mass transfer, but these demonstrations are limited to small reactors (≤5 L) and laboratory-scale outputs [66]. Techno-economic modelling using spiral ribbon mixers and indirect-fired rotary kilns suggests that batch sizes of 300–1500 kg and throughputs of ~90 kg h⁻¹ could be achieved cost-competitively with graphite anodes [73]. Overall, while modelling indicates that industrial-scale production is theoretically possible, no industrial process has yet been proven, and future work must develop predictive heat- and mass-transfer models, optimise mixing and reduction conditions, and validate techno-economic assumptions under industrial conditions.

6. Conclusions and Outlook

This review highlights geothermal silica as a promising and still underutilised precursor for silicon-based Li-ion battery anodes, while also showing that the current evidence base remains limited and uneven. Existing studies indicate that geothermal silica can offer attractive characteristics such as relatively high silica purity, amorphous or porous structure, and clear sustainability relevance through the valorisation of geothermal side streams. However, these advantages are not universal and depend strongly on source-specific chemistry, precipitation behaviour, impurity profile, and downstream processing conditions. For this reason, geothermal silica should not be regarded as a single uniform feedstock, but rather as a variable precursor class whose suitability must be evaluated on a case-by-case basis.

The reviewed studies indicate that geothermal silica can support two different battery-material pathways: conversion into silicon-based anodes and direct use in SiO₂-based composite architectures. Both routes appear promising, but the available electrochemical evidence remains heterogeneous, and direct comparison across studies is difficult because active material, storage mechanism, cell configuration, testing protocol, and performance metric are not consistent across the literature. Recent studies have begun to provide more explicit insights into cycling stability, particularly through advanced composite designs; however, the currently available data remain limited and are insufficient to comprehensively establish the practical long-term service life of geothermal-silica-derived anodes under application-relevant conditions.

Importantly, the main unresolved geothermal-specific issues are not the generic silicon limitations alone, but the extent to which source-dependent impurity profiles, brine-chemistry variation, and purification history influence the final silicon structure and its electrochemical durability. Future work should therefore move beyond demonstrating feasibility and instead establish clear precursor-to-performance relationships under application-relevant cycling conditions.

From a processing perspective, the available evidence suggests that the most relevant route is not a single isolated step, but an integrated pathway linking controlled silica recovery, source-specific purification, and structure-preserving reduction. In particular, the chemical fingerprint of the original geothermal brine, including impurities such as Fe, B, As, and Li, is likely to strongly influence both purification complexity and the risk of residual trace elements affecting downstream silicon quality. Within this framework, magnesiothermic reduction currently appears to be one of the most promising routes for geothermal-silica-derived battery materials because it can operate at lower temperature while retaining useful porous features. Nevertheless, its practical attractiveness remains constrained by reaction control, by-product removal, safety concerns, and continued reliance in some cases on hazardous HF-based purification.

Overall, geothermal silica can currently be considered a competitive—though not yet demonstrably superior—precursor for silicon-based Li-ion battery anodes. Its most credible advantages currently lie at the precursor and supply-chain level, including recovery from an existing industrial side stream, potentially favourable amorphous or porous morphology, and sustainability relevance, rather than in a clearly demonstrated electrode-level performance advantage. At the same time, neither a dedicated techno-economic assessment of the full geothermal-silica-to-anode pathway nor a pilot-scale industrial validation has yet been established, and the available electrochemical data remain too limited and heterogeneous to define practical long-term service life or clear superiority over non-geothermal routes.

Nevertheless, geothermal silica remains a technically relevant and sustainability-oriented pathway, and its future progress will depend on improving consistency across geothermal silica sources, strengthening impurity control and purification strategies, and developing safer and more scalable reduction routes. In particular, future work should prioritize establishing clear precursor-to-performance relationships, reproducible impurity-controlled processing, direct reporting of precursor-to-silicon size evolution, and validation under application-relevant cycling and scale conditions.