As we venture into the next generation of energy storage technologies, all-solid-state batteries have positioned themselves at the forefront due to their unparalleled safety profile and potential for high energy density. The transition from conventional liquid electrolytes to solid counterparts mitigates risks associated with leakage, flammability, and thermal runaway, which are critical concerns in large-scale applications. Among the various solid electrolyte systems, sulfide-based materials have garnered significant attention for their exceptional ionic conductivities, often rivaling or exceeding those of organic liquid electrolytes. This remarkable property stems from the unique structural frameworks and the high mobility of lithium ions within sulfide matrices. In this comprehensive analysis, I will delve into the pivotal role of lithium sulfide (Li₂S) as the foundational precursor for these high-performance sulfide solid electrolytes, which are indispensable for the realization of practical and commercially viable all-solid-state batteries.
The strategic importance of Li₂S is multifaceted, encompassing both technical performance and economic viability. In the broader context of solid-state battery development, the choice of solid electrolyte directly influences the battery’s energy density, power capability, cycle life, and safety. Sulfide solid electrolytes, such as those derived from the Li₂S-P₂S₅ system, have demonstrated room-temperature ionic conductivities exceeding 10 mS/cm in some advanced compositions. This performance is crucial for achieving the high power densities required for electric vehicles and grid storage applications. However, the synthesis of these superior electrolytes is intrinsically dependent on the availability of high-purity Li₂S. Any deviation in the purity or stoichiometry of the Li₂S precursor can lead to the formation of secondary phases, grain boundary resistances, and ultimately, a degradation in the overall performance of the solid-state battery. Therefore, a deep understanding of Li₂S properties, coupled with the development of robust and scalable synthesis methods, is not merely an academic exercise but a industrial imperative.
From an economic perspective, Li₂S constitutes a significant portion of the raw material cost for sulfide solid electrolytes. In a typical composition like Li₆PS₅Cl, Li₂S can account for over 40% of the mass, and its cost can represent more than 80% of the total raw material expense. This cost structure highlights the sensitivity of the entire solid-state battery value chain to the price and availability of high-quality Li₂S. As global efforts intensify to commercialize all-solid-state batteries, establishing a reliable and cost-effective supply chain for Li₂S becomes a critical success factor. In the following sections, I will systematically explore the fundamental characteristics of Li₂S, establish a framework for its quality assessment, evaluate various synthesis pathways from an industrial feasibility standpoint, and discuss future directions for its sustainable production.
Fundamental Physicochemical Properties of Lithium Sulfide
Lithium sulfide (Li₂S) is an inorganic compound with the chemical formula Li₂S and a molecular weight of 45.95 g/mol. In its pure form, it appears as a white crystalline solid, but commercial samples often exhibit off-white, gray, or yellowish tints due to the presence of impurities. The compound crystallizes in the cubic anti-fluorite structure (space group Fm3m), where the sulfur anions (S²⁻) form a face-centered cubic lattice, and the lithium cations (Li⁺) occupy all the tetrahedral interstitial sites. This specific arrangement is represented by the following structural formula, emphasizing the coordination:
$$ \text{Li}_2\text{S} \equiv \text{Li}^+_2 \cdot \text{S}^{2-} $$
The lattice parameter (a) for the cubic unit cell is approximately 5.71 Å, resulting in a theoretical density of 1.66 g/cm³. The melting point of Li₂S is notably high at 938 °C, which is advantageous for high-temperature processing but also necessitates energy-intensive synthesis conditions. The thermal stability of Li₂S can be described by its Gibbs free energy of formation, which is highly negative, indicating its thermodynamic stability under standard conditions:
$$ \Delta_f G^\circ(\text{Li}_2\text{S}) \approx -439 \, \text{kJ/mol} $$
However, this thermodynamic stability belies its kinetic reactivity, particularly towards protic solvents and oxidizing agents. The most significant chemical reaction from a handling perspective is its hydrolysis with water. This reaction is exothermic and proceeds in steps, initially forming lithium hydrosulfide and lithium hydroxide:
$$ \ce{Li2S + H2O -> LiHS + LiOH} \quad \Delta H < 0 $$
Further reaction with water can lead to the complete hydrolysis and release of hydrogen sulfide gas, a toxic and flammable compound:
$$ \ce{LiHS + H2O -> LiOH + H2S ^} $$
The overall hydrolysis reaction can be summarized as:
$$ \ce{Li2S + 2H2O -> 2LiOH + H2S ^} $$
This propensity for hydrolysis mandates that Li₂S must be handled, stored, and processed in strictly controlled, anhydrous, and inert atmospheres, typically with water and oxygen levels maintained below 0.1 ppm. Exposure to atmospheric oxygen leads to surface oxidation, which can be represented by the following reaction, forming lithium oxide and elemental sulfur, the latter often imparting a yellow color:
$$ \ce{2Li2S + O2 -> 2Li2O + 2S} $$
At elevated temperatures (above 300 °C), the oxidation can proceed further to form lithium sulfate:
$$ \ce{Li2S + 2O2 -> Li2SO4} $$
The electronic structure of Li₂S is that of a wide bandgap semiconductor, with a bandgap estimated to be around 4.0 eV. This large bandgap is beneficial for its application in solid-state batteries, as it contributes to a low electronic conductivity, thereby minimizing leakage currents and self-discharge within the electrolyte. The primary charge carriers in pure Li₂S are lithium ions, which migrate via vacancy or interstitial mechanisms within the crystal lattice, though its intrinsic ionic conductivity is relatively low compared to optimized solid electrolytes. The key properties are summarized in the table below.
| Property | Value / Description | Implication for Solid-State Batteries |
|---|---|---|
| Chemical Formula | Li₂S | Stoichiometric precursor for sulfide electrolytes |
| Crystal Structure | Cubic Anti-fluorite | Provides a framework for Li⁺ ion mobility |
| Melting Point | 938 °C | Enables high-temperature sintering processes |
| Density | 1.66 g/cm³ | Affects the volumetric energy density of composites |
| Band Gap | ~4.0 eV | Contributes to low electronic conductivity, desirable for electrolytes |
| Solubility in H₂O | Reacts (Hydrolysis) | Requires strict moisture control during processing |
Comprehensive Quality Assessment Framework for Li₂S
The performance of sulfide-based solid electrolytes, and consequently the solid-state batteries they enable, is exquisitely sensitive to the purity and physical characteristics of the starting Li₂S material. Even trace amounts of certain impurities can act as nucleation sites for undesired phases, create high-resistance grain boundaries, or introduce electronic conduction pathways, all of which degrade the electrolyte’s function. Based on my analysis of the failure modes in solid-state battery cells, I have identified several key parameters that form a critical quality assessment framework for Li₂S.
Color and Whiteness: Pure, anhydrous Li₂S is a brilliant white solid. The presence of color, such as yellow, gray, or brown hues, is a primary visual indicator of impurities. Yellow tints often suggest the formation of polysulfides (Li₂S_x, x > 2) or elemental sulfur, which can arise from non-stoichiometric reactions or partial oxidation. Gray or black discolorations typically point towards carbonaceous residues from organic precursors or reduction processes, or metallic impurities from reactor corrosion. These colored impurities are not merely cosmetic; they can participate in side reactions during solid electrolyte synthesis. For instance, polysulfides can lead to off-stoichiometry in the final electrolyte composition, while carbon residues can significantly increase the electronic conductivity (σ_e) of the electrolyte. The transference number for lithium ions (t_Li⁺), a key metric for solid-state battery performance, is given by:
$$ t_{\text{Li}^+} = \frac{\sigma_{\text{Li}^+}}{\sigma_{\text{Li}^+} + \sigma_e} $$
where σ_Li⁺ is the ionic conductivity. An increase in σ_e directly reduces t_Li⁺, promoting internal shunt currents and facilitating lithium dendrite growth.
Phase Purity and Crystallinity: X-ray diffraction (XRD) is the definitive technique for assessing the phase purity of Li₂S. The diffraction pattern should match the reference pattern for cubic Li₂S without extraneous peaks. Common impurity phases detected by XRD include:
- Lithium hydroxide (LiOH): from reaction with atmospheric moisture.
- Lithium oxide (Li₂O): from oxidation by air.
- Lithium carbonate (Li₂CO₃): from reaction with atmospheric CO₂.
- Lithium sulfate (Li₂SO₄): from over-oxidation during synthesis or storage.
The presence of these lithium-containing impurities is particularly detrimental because they consume the acidic P₂S₅ precursor during electrolyte synthesis via neutralization reactions, altering the delicate Li/P/S ratio required for forming the high-conductivity phase. For example, the reaction with Li₂O:
$$ \ce{Li2O + P2S5 -> 2Li3PO4 + …} $$
forms lithium phosphate (Li₃PO₄), an electrochemically inert and ionically resistive phase that segregates at grain boundaries, severely impeding Li⁺ transport and lowering the overall ionic conductivity of the solid-state battery electrolyte.
Moisture Content: The hygroscopic nature of Li₂S makes moisture content a paramount concern. The allowable moisture level is exceptionally low, typically targeted to be below 100 mg/kg (ppm). Moisture analysis is performed using Karl Fischer coulometric titration. The insidious nature of moisture is that even at low levels, it can catalyze slow degradation during storage. The hydrolysis reaction follows pseudo-first-order kinetics under constant humidity:
$$ -\frac{d[\ce{Li2S}]}{dt} = k [\ce{Li2S}] [\ce{H2O}] $$
where k is the rate constant. This means that the degradation rate is proportional to both the Li₂S and moisture concentrations. In a sealed container, the reaction is self-accelerating as the produced LiOH is also hygroscopic, further increasing the local water activity. I have observed that batches with moisture content exceeding 1000 ppm can develop significant amounts of LiOH and Li₂CO₃ within weeks, even in nominally dry environments, rendering them unsuitable for producing high-performance solid electrolytes for all-solid-state batteries.
Carbon and Solvent Residues: For synthesis routes involving carbonaceous reductants or organic solvents, the residual carbon and solvent content must be meticulously controlled. Carbon content is typically measured using a high-frequency combustion infrared absorption method (e.g., with an infrared carbon-sulfur analyzer). Solvent residues, such as ethanol or N-methyl-2-pyrrolidone (NMP), are quantified using techniques like thermogravimetric analysis coupled with mass spectrometry (TGA-MS) or gas chromatography-mass spectrometry (GC-MS). During the high-temperature sintering step of electrolyte synthesis (often between 400-550 °C), residual organic solvents can decompose and carbonize. This in-situ formed carbon creates a percolating network for electrons, raising the electronic conductivity (σ_e) of the electrolyte. As previously mentioned, this compromises the Li⁺ transference number. The effective conductivity of a composite containing an ionic conductor and an electronic conductor can be modeled using percolation theory. The threshold for a significant increase in σ_e occurs when the volume fraction of the electronic conductor (φ_c) exceeds the percolation threshold (φ_c, crit), which for a random 3D network is approximately 0.16. Even small amounts of finely dispersed carbon can approach this threshold, posing a serious risk to the solid-state battery’s coulombic efficiency and cycle life.
Elemental and Ionic Impurities: Trace metallic impurities (e.g., Fe, Ni, Cu) originating from raw materials or reactor wear can act as redox centers, catalyzing the decomposition of the solid electrolyte or the electrode materials. Their concentration must be kept in the low ppm range, typically analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES). Similarly, anions like chloride (Cl⁻) or sulfate (SO₄²⁻) from precursor salts can incorporate into the electrolyte lattice, potentially stabilizing or destabilizing certain phases and affecting the ionic conductivity.
| Quality Parameter | Target Specification | Analytical Technique | Impact on Solid-State Battery Electrolyte |
|---|---|---|---|
| Whiteness (L* value) | > 90 (Hunter Lab scale) | Colorimetry / Visual Inspection | Indicator of polysulfide and transition metal impurities |
| Phase Purity (XRD) | > 99.5% Li₂S phase | X-ray Diffraction (XRD) | Prevents formation of resistive secondary phases (e.g., Li₃PO₄) |
| Moisture Content | < 100 mg/kg | Karl Fischer Titration | Prevents hydrolysis and degradation during storage and processing |
| Total Carbon Content | < 500 mg/kg | Combustion-IR Analysis | Minimizes electronic conductivity, prevents dendrite initiation |
| Solvent Residue | < 100 mg/kg | GC-MS / TGA-MS | Avoids in-situ carbonization and pore formation during sintering |
| Metallic Impurities (Fe, Ni, Cu) | Each < 10 mg/kg | ICP-OES | Prevents catalytic decomposition of electrolyte and electrodes |
Industrial Synthesis Pathways for Lithium Sulfide
The quest for a scalable, cost-effective, and high-purity synthesis method for Li₂S is a central challenge in the solid-state battery supply chain. The chosen synthesis route dictates not only the cost structure but also the inherent safety profile, environmental footprint, and final product quality. I have categorized the prominent synthesis strategies based on the reactivity of the lithium and sulfur sources, providing a detailed examination of their mechanisms, advantages, and limitations for industrial adoption.
High-Reactivity Lithium and Sulfur Sources
These methods employ highly reactive precursors, such as metallic lithium or hydrogen sulfide, which benefit from high conversion yields and straightforward reaction pathways but are often associated with significant safety hazards and higher costs.
Direct Lithium-Sulfur Reaction: This is the most direct synthesis method, involving the exothermic combination of elemental lithium and sulfur. The reaction is highly favorable thermodynamically:
$$ \ce{2Li_{(s)} + S_{(s)} -> Li2S_{(s)}} \quad \Delta H^\circ \approx -430 \, \text{kJ/mol} $$
The large negative enthalpy change drives a self-propagating high-temperature synthesis (SHS) process, where the reaction, once initiated, generates enough heat to sustain itself, often reaching temperatures exceeding 1000 °C locally. While this ensures high conversion efficiency and yields a product with exceptional purity (>99.9%) and whiteness, the process is notoriously difficult to control at scale. The extreme exothermicity poses a severe risk of thermal runaway, and the molten lithium and sulfur can aggressively corrode reactor materials. Furthermore, the handling of pyrophoric lithium metal requires an entirely inert atmosphere, adding substantial capital and operational costs to the production line. This method is currently best suited for small-scale, high-purity production where cost is a secondary concern to quality.
Hydrogen Sulfide (H₂S) Neutralization: This route utilizes the acid-base reaction between gaseous H₂S and a lithium base, typically lithium hydroxide monohydrate (LiOH·H₂O):
$$ \ce{H2S_{(g)} + 2LiOH·H2O_{(s)} -> Li2S_{(s)} + 4H2O_{(g)}} $$
The process is typically conducted in a fluidized bed or rotary kiln reactor, where a stream of dry, diluted H₂S is passed over solid LiOH·H₂O. The primary advantage of this method is the potential for low-cost sulfur sourcing if H₂S is available as a by-product from natural gas processing or petroleum refining (e.g., via the Claus process). However, the method is fraught with challenges. H₂S is an extremely toxic and flammable gas, requiring stringent safety protocols and specialized equipment for containment and scrubbing. A critical side reaction involves the catalytic decomposition of H₂S on the surface of the forming Li₂S product, leading to the generation of polysulfides and hydrogen gas:
$$ \ce{H2S ->[\text{Li2S catalyst}] 1/x S_x^{2-} + H2 ^} $$
This side reaction not only introduces polysulfide impurities, which degrade the product’s color and purity, but also creates an explosive H₂ atmosphere. Consequently, precise control of reaction temperature, gas concentration, and residence time is essential. The geographical constraint of needing a proximate H₂S source also limits its widespread adoption for the general solid-state battery market.
Stable Lithium and Sulfur Source Routes
These methods utilize more stable and often less expensive precursors, such as lithium sulfate or sodium sulfide. They generally offer better safety profiles and lower raw material costs but may involve more complex reaction sequences or require post-synthesis purification.
Carbothermal Reduction: This is a highly promising route for large-scale production due to the low cost and wide availability of its raw materials: lithium sulfate (Li₂SO₄) and a carbon source (e.g., petroleum coke, graphite, or organic polymers). The overall reduction reaction is:
$$ \ce{Li2SO4_{(s)} + 2C_{(s)} -> Li2S_{(s)} + 2CO2_{(g)}} $$
The thermodynamics of this reaction become favorable at temperatures above approximately 635 °C. The standard Gibbs free energy change (ΔG°) can be expressed as a function of temperature (T):
$$ \Delta G^\circ(T) = \Delta H^\circ – T\Delta S^\circ $$
Where ΔH° is positive, making the reaction endothermic, and thus requiring significant energy input. The Boudouard reaction (C + CO₂ ⇌ 2CO) often plays a role in the reduction mechanism at higher temperatures. The key advantage of this method is its dramatically lower raw material cost compared to metal-based routes. However, the process faces several hurdles:
- Impurity Formation: Incomplete reduction can leave behind Li₂SO₄, while over-reduction or side reactions can produce Li₂O, Li₂CO₃, and residual carbon.
- Low Yield: The evolution of CO₂ gas means that a significant mass (over 60%) is lost, resulting in a relatively low mass yield of Li₂S from Li₂SO₄.
- Energy Intensity: The high operating temperatures (700-900 °C) contribute to high energy consumption.
Recent advances have focused on optimizing the carbon source and reactor design to achieve higher purity directly from the primary reaction, thereby avoiding costly secondary purification steps. For instance, using nanostructured carbon or polymer-derived carbons can enhance the reduction kinetics and lower the required temperature, making the process more efficient and better suited for the cost-sensitive solid-state battery industry.
Hydrazine Hydrate Reduction: This wet-chemical method employs hydrazine hydrate (N₂H₄·H₂O) as a powerful reducing agent to convert sulfur in the presence of a lithium base (LiOH) into Li₂S. The complex redox chemistry can be simplified as:
$$ \ce{2S + 4LiOH + N2H4 -> 2Li2S + N2 ^ + 4H2O} $$
The reaction can be conducted at or near room temperature, which is a significant energy-saving advantage. The process typically proceeds through the formation of soluble polysulfide intermediates (Li₂S_x), which are subsequently reduced by excess hydrazine to the final sulfide (S²⁻). The major challenge is the complete reduction of these intermediates to prevent polysulfide contamination in the final product. Furthermore, hydrazine is highly toxic, carcinogenic, and explosive, demanding extreme care in handling and sophisticated waste treatment systems. Despite these drawbacks, its potential for low-temperature operation and relatively simple infrastructure makes it an active area of process intensification for solid-state battery material production.
Liquid-Phase Metathesis (Ion Exchange): This method exploits the differences in solubility between reactants and products in a suitable solvent. A common reaction involves sodium sulfide (Na₂S) and lithium chloride (LiCl) in an anhydrous, polar aprotic solvent like ethanol or tetrahydrofuran (THF):
$$ \ce{Na2S_{(s)} + 2LiCl_{(s)} ->[Solvent] Li2S_{(s)} + 2NaCl_{(s)}} $$
The driving force is the precipitation of Li₂S from the solution, while the co-product NaCl remains soluble or is easily separated. The solubility product constant (K_sp) governs the reaction completeness. For the dissolution of Li₂S, we have:
$$ \ce{Li2S_{(s)} <=> 2Li+_{(sol)} + S^{2-}_{(sol)}} \quad K_{sp} = [\ce{Li+}]^2[\ce{S^{2-}}] $$
By using a solvent where K_sp(Li₂S) is very small and the lithium salt is highly soluble, the equilibrium is shifted strongly towards the solid Li₂S product. The primary advantages are the mild reaction conditions and the potential for continuous processing. The significant disadvantages include:
- Solvent Removal: Removing the solvent from the fine Li₂S precipitate is energy-intensive and often incomplete, leading to solvent residues.
- Product Morphology: The precipitated Li₂S often has a high surface area and small particle size, which, while reactive, can be more prone to oxidation and hydrolysis.
- Purification: The initial Na₂S raw material often contains impurities like sulfites and sulfates that can carry through to the final product, necessitating additional washing steps.
The economic viability of this route is highly dependent on the efficiency of solvent recovery and recycling within the process, which is crucial for minimizing the operational cost for solid-state battery precursor manufacturing.
| Synthesis Method | Reaction Equation | Typical Purity | Safety Concerns | Estimated Raw Material Cost per ton Li₂S | Scalability Potential |
|---|---|---|---|---|---|
| Lithium-Sulfur Reaction | 2Li + S → Li₂S | > 99.9% | Very High (Pyrophoric Li, Exothermic) | ~212,600 CNY | Low (Safety-limited) |
| H₂S Neutralization | H₂S + 2LiOH·H₂O → Li₂S + 4H₂O | High (99%+) | Very High (Toxic H₂S, H₂ by-product) | ~146,100 CNY | Moderate (Source-dependent) |
| Carbothermal Reduction | Li₂SO₄ + 2C → Li₂S + 2CO₂ | Moderate-High (99%+, may need purification) | Low (High Temp., CO₂ emission) | ~136,100 CNY | Very High |
| Hydrazine Reduction | 2S + 4LiOH + N₂H₄ → 2Li₂S + N₂ + 4H₂O | High (99.9%) | High (Toxic, Explosive N₂H₄) | ~147,500 CNY | Moderate-High |
| Liquid Metathesis | Na₂S + 2LiCl → Li₂S + 2NaCl | Moderate-High (Solvent-dependent) | Moderate (Solvent handling) | ~150,100 CNY | High (if solvent is recycled) |
Multidimensional Evaluation and Future Outlook
Synthesizing the information from the various synthesis pathways, a multidimensional evaluation is essential to guide industrial investment and research priorities for the solid-state battery ecosystem. The optimal choice is not universal but depends on the specific constraints of cost, quality, safety, and scale.
From a product quality perspective, methods starting from elemental lithium currently set the benchmark for purity and whiteness, making them indispensable for R&D and premium applications. However, for the mass market of solid-state batteries, the carbothermal reduction and refined liquid-phase metathesis routes show the most promise for achieving the necessary purity levels (≥99.5%) after process optimization, at a fraction of the cost.
The safety and environmental dimension clearly favors the carbothermal reduction and metathesis methods. While they have their own hazards (high temperatures, solvent flammability), they avoid the extreme risks associated with handling metallic lithium, hydrogen sulfide, or hydrazine. The carbon footprint of the carbothermal process is a concern due to CO₂ emissions and high energy use, but this can be mitigated by using renewable energy sources and implementing carbon capture technologies, or by utilizing sustainable carbon sources like biomass.
Economic viability is arguably the most decisive factor for the widespread adoption of all-solid-state batteries. The cost analysis reveals a stark contrast. The carbothermal reduction method holds a decisive raw material cost advantage. Its viability is further enhanced by the possibility of integrating it with upstream lithium extraction processes, either from mineral concentrates (e.g., spodumene) or from recycled lithium-ion batteries, creating a circular economy for lithium. In such an integrated plant, lithium sulfate, a common intermediate in both extraction and recycling flowsheets, can be directly channeled into Li₂S production, bypassing the more expensive lithium carbonate or hydroxide stages. This synergy could potentially lower the cost of Li₂S below 100,000 CNY per ton, which would be a game-changer for the solid-state battery industry.
Looking forward, the development of Li₂S production technology will likely focus on the following key areas:
- Process Intensification for High-Reactivity Routes: For the lithium-sulfur and H₂S routes, engineering solutions such as microreactors, staged feeding, and advanced heat management systems could tame the reaction violence and enable safer scaling. The use of lithium nitride (Li₃N) as a porosity-inducing and heat-conducting additive in the Li-S reaction is one such innovative approach being explored.
- Purity Breakthroughs in Carbothermal Reduction: The main thrust for the low-cost route will be to achieve “one-step” high purity. This involves sophisticated control over the carbon reactivity, gas atmosphere, and temperature profile to suppress all side reactions. The use of novel carbon sources, such as polyvinyl alcohol (PVA), which decomposes to a highly reactive carbon network, has recently been shown to produce Li₂S with purity exceeding 99.7% directly from the furnace.
- Green Chemistry in Liquid-Phase Synthesis: For metathesis and reduction routes, the future lies in developing greener, less toxic solvents and more efficient solvent recovery systems. Supercritical fluid processing or the use of ionic liquids could offer alternatives to conventional organic solvents.
- Modular and Distributed Production: The H₂S neutralization method finds its niche in modular, on-site production facilities located at natural gas processing plants, effectively converting a waste stream into a valuable precursor for the energy storage revolution.
- Advanced Quality Control and Sensing: As production scales, implementing real-time, in-line sensors for monitoring critical parameters like moisture, oxygen, and particle size during synthesis and packaging will be crucial for maintaining consistent quality essential for reliable solid-state battery manufacturing.
In conclusion, lithium sulfide is far more than a simple chemical precursor; it is the foundational pillar upon which the high-performance sulfide solid electrolyte segment of the all-solid-state battery industry is being built. The journey from a laboratory curiosity to a commoditized industrial material is fraught with technical and economic challenges. However, the clear roadmap that prioritizes the development of safe, scalable, and cost-effective synthesis methods, particularly those based on stable raw materials like lithium sulfate, is within sight. By conquering the synthesis of Li₂S, we remove a major bottleneck, paving the way for the accelerated commercialization of all-solid-state batteries that are safer, more powerful, and capable of unlocking new frontiers in electric mobility and grid storage. The progress in this field will be a key indicator of the overall health and trajectory of the next-generation solid-state battery industry.