Lithium sulfide (Li?S) is an inorganic compound that appears as a white to yellowish crystalline powder. It has a molecular weight of 45.95 g/mol and a density of 1.66 g/cm3. Lithium sulfide is primarily used as a raw material for the synthesis of solid electrolytes and as a cathode material in lithium-sulfur batteries. It crystallizes in the antifluorite structure and is known to be deliquescent, meaning it can absorb moisture from the air. In chemical manufacturing, it acts as a precursor for lithium hydride, lithium borohydride, lithium amide, lithium thioacetate, lithium tetrafluoroborate, and others. In the battery industry, our battery grade lithium sulfide is used as a precursor to synthesize sulfide-based solid electrolytes like Li?PS?. Lithium sulfide also serves as a cathode material in lithium-sulfur batteries, offering a theoretical capacity of up to 1166 mAh/g, nearly four times that of lithium cobalt oxide. This makes it an attractive candidate for next-generation battery technologies. Additionally, lithium sulfide is valuable in the semiconductor industry for the production of thin-film transistors (TFTs) and other electronic devices, particularly lithium sulfide-phosphorus oxide (Li?S-P?O?) TFTs.

Lithium Sulfide Batteries: Addressing the Kinetic Barriers and High First Charge Overpotential

For the beyond LIB era, extensive exploration has been done to find safer, more reliable, and high capacity next generation energy storage technologies such as supercapacitors3,4 and alternative ion batteries. In the face of these challenges, research interest has grown in lithium sulfide (Li2S)-based cathodes instead. Utilizing the lithiated form of sulfur offers several benefits, including allowing for the use of nonmetallic lithium anodes, stabilizing the volume expansion of the cathode, as well as a broader range of processing methods or operation environments enabled by its substantially higher melting point. These properties can address several challenges associated with both electrodes while retaining the superior energy capacity offered by the lithium sulfur (Li–S) battery system. Nonetheless, a key challenge of Lithium sulfide cathodes is the high first charge overpotential, which must be addressed in practical batteries. The various strategies that have been adopted to tackle the high first charge overpotential will be examined, along with their effects on the cell performance as a whole. Finally, we will propose some future directions of Li2S cathode development for improved Li–S practical performance.

By using Li2S as the cathode material, it effectively creates a system where lithium already exists in the cathode, which allows for the use of lithium metal-free anodes such as graphite, silicon, or tin, to eliminate problematic dendritic growth and the formation of insulating Lithium sulfide SEIs at the anode. In this case, utilizing the accumulation of LIB anode advancements over the decades would be ideal for the practical adoption of LSB. Meanwhile, it is worth noting that the safety issues that plagued early metal-anode LIBs can also be resolved in the metal-free anode system. However, one should note that, despite the differences in physical properties between sulfur and Lithium sulfide, LSBs based on the Li2S cathode still suffer from some similar issues to varying extents. Although the electrical conductivity of Li2S is an order of magnitude higher than that of elemental sulfur, it is still very much an insulator that requires a conductive matrix or additives to function as a cathode. In addition to these common challenges, Li2S cathodes also face an activation issue, where studies have observed a high overpotential during the first charging process.

Amorphous Lithium Sulfide Deposition Accelerates Sulfur Redox Kinetics

Crucially, the final crystallization of Lithium sulfide creates fundamental kinetic barriers—its long-range ordered structure exhibits e?/Li+ isolation for poor conductivity, while dense crystalline interfaces further hinder Li+ diffusion pathways, resulting in sluggish reaction kinetics. Moreover, the slow deposition of Li2S causes LiPSs to accumulate and diffuse within the electrolyte, which not only results in the irreversible loss of active sulfur species but also promotes the formation of ″dead sulfur″ depositing on the Li anodes. Therefore, improving Lithium sulfide conversion kinetics is critical for improving the performance of Li─S batteries. By competitively weakening the binding energy between Li+ and S2?, the crystallization tendency of Lithium sulfide can be suppressed. Unlike complex catalyst design, the use of high-polarity solvents in the electrolyte. This amorphous strategy not only enables uniform Li2S deposition at the cathode/electrolyte interface but also reduces the molecular energy barrier for sulfur species. Consequently, both the nucleation and growth rates of Lithium sulfide are improved, allowing the battery to stably perform 1000 cycles at 2C with an average capacity decay of only 0.027% per cycle.[2]

In summary, this work proposes a steric hindrance-mediated strategy that achieves amorphous Lithium sulfide deposition through molecular-level structural and chemical composition modulation, significantly enhancing the sulfur conversion kinetics of Li─S batteries and enabling long-term cycling stability. By employing B15C5 as an electrolyte additive, the spatial confinement effect induced by its coordination interactions with Li+ effectively disrupts the crystallization pathway of Li2S. The amorphous Lithium sulfide phase not only forms a uniform and substrate-adaptive cathode interface to facilitate rapid Li+ diffusion but also optimizes the redox activity of sulfur species by regulating molecular energy levels. This dual structural-electronic modulation suppresses premature cathode passivation and accelerates Lithium sulfide deposition kinetics, resulting in a Li─S pouch cell with a sulfur loading of 4 mg cm?2 that delivers an initial discharge capacity of 1087 mA h g?1 while progressively activating unreacted sulfur species. Our work addresses the intrinsic limitations of sluggish conversion kinetics in sulfur cathode systems and offers a novel blueprint for designing high-energy lithium–sulfur batteries.

Lithium Sulfide: Magnesothermal Synthesis and Battery Applications

The ever-increasing demand for high energy-density rechargeable batteries that can power electric vehicles and smart electronic devices is driving a revolution in the lithium battery technology. The realization of practical Li–S batteries will reshape the battery market of billions of dollars. However, the significant volumetric expansion (80%) of S cathodes in the lithiation step and possible fire hazard for Li anodes are two big challenges. Lithium sulfide (Li2S), as the fully lithiated state of sulfur, can be an alternative cathode material to alleviate the above two issues. Hence, the above application prospects made Li2S a star material in the battery community in the past decade. However, all necessary Lithium sulfide requires artificial synthesis, because Li2S is not available in nature, highly reactive with many chemicals, and extremely sensitive to moisture. Consequently, the commercially provided battery-grade Li2S (c-Li2S) is too expensive (>$5000/kg) to enable practical applications.[3]

According to the chemical thermodynamics, this magnesothermal reduction approach is a spontaneous reaction even under ambient temperature and pressure. Through orthogonal experiments, the optimal reaction conditions are to use the molar ratio of 4:1 between Mg and Li2SO4 and heat at 550 °C for 10 min. Under other conditions, small amounts of S, MgS, and Li2O impurities are present. Further mechanistic study suggests that these impurities come from two consecutive parasitic reactions. Thus, the superior performance of s-Li2S is understandable because Li2S2 is the intermediate state, which in the shell formed outside Lithium sulfide can accelerate the charge transfer process. As a result, the s-Li2S product exhibits excellent electrode performance, particularly the lower activation barrier and rate capability. In summary, this work not only reports a new method of synthesizing Lithium sulfide but also provides a new way of improving the electrochemical performance of Li2S.

References

[1]Ting LKJ, Gao Y, Wang H, Wang T, Sun J, Wang J. Lithium Sulfide Batteries: Addressing the Kinetic Barriers and High First Charge Overpotential. ACS Omega. 2022 Oct 31;7(45):40682-40700. doi: 10.1021/acsomega.2c05477. PMID: 36406542; PMCID: PMC9670706.

[2]Wang Z, Ke J, Zhu H, Xue F, Jiang J, Huang W, Dong M, Zhu X, Zeng J, Song R, Sliz R, Ji Q, Liu Q, Fu Y, Lan S. Steric Hindrance-Induced Amorphous Lithium Sulfide Deposition Accelerates Sulfur Redox Kinetics in Lithium-Sulfur Batteries. Adv Mater. 2025 May 13:e2504715. doi: 10.1002/adma.202504715. Epub ahead of print. PMID: 40357867.

[3]Zhang X, Yang H, Sun Y, Yang Y. Lithium Sulfide: Magnesothermal Synthesis and Battery Applications. ACS Appl Mater Interfaces. 2022 Sep 14;14(36):41003-41012. doi: 10.1021/acsami.2c11196. Epub 2022 Sep 5. PMID: 36063036.