Recent advances in Li₂S@C nanocomposites for lithium–sulfur batteries

Abstract

Lithium–sulfur batteries (LSBs) are considered as promising next-generation energy-storage systems because of their high theoretical energy density, low cost, material abundance, and environmental compatibility. Over the past decade, intensive research has substantially mitigated key sulfur-cathode limitations, including poor electronic/ionic transport, large volume changes, and the polysulfide shuttle, enabling near-commercial performance in selected studies. These advances have been achieved predominantly in elemental sulfur-based LSBs (S-LSBs), but practical deployment remains largely constrained by reliance on lithium-metal anodes. Lithium sulfide (Li₂S)-based LSBs (Li₂S-LSBs) offer an attractive alternative because they can eliminate lithium-metal anodes while retaining the same overall sulfur redox chemistry. However, Li₂S-LSBs face distinct challenges, most notably the moisture sensitivity of Li₂S and the high first-charge activation overpotential, which often reduces accessible capacity and compromises cycling stability. The central barrier is the preparation of well-defined Li₂S@C nanocomposites with Li₂S uniformly embedded within nanoscale porous carbon hosts, a performance-dictating architecture that is readily achieved for S@C via melt infiltration but is difficult for Li₂S because of its high melting point and limited processability. This review summarizes the current state of Li₂S@C synthesis, critically comparing major physical and chemical routes (e.g., ball milling, carbothermal methods, lithiation of S@C, sulfuration strategies, solution infiltration, and precursor infiltration–decomposition), and evaluates their advantages, limitations, and scalability. Emerging developments in Li₂S@C nanocomposites for all-solid-state Li₂S batteries are also discussed, with emphasis on design strategies for addressing sluggish solid-state reaction kinetics. Finally, we outline complementary directions needed to advance Li₂S-LSBs toward practical implementation, including Li₂S-compatible binders and additives that couple shuttle suppression with kinetic promotion, lean-electrolyte cell designs, lithium-free full-cell configurations, and opportunities enabled by integrating Li₂S@C nanocomposites with solid-state electrolytes.

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1 Introduction

Lithium-ion batteries (LIBs) are widely used in portable electronics, electric vehicles, and grid-level storage.^1–3 Continued improvements in LIBs based on lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminum oxide (NCA)^4–7 have enhanced performance and safety, but further increases in practical cell-level energy density are increasingly constrained by intrinsic material limits and by the cost and supply risks of transition-metal resources.^8,9 These considerations motivate the exploration of alternative chemistries that can deliver substantially higher energy density with improved sustainability.

Elemental sulfur-based lithium–sulfur batteries (S-LSBs) are leading candidates because sulfur offers an exceptionally high theoretical specific capacity (1672 mA h g⁻¹) and is abundant, low-cost, and environmentally benign.^10–13 When paired with a lithium metal anode, S-LSBs provide a theoretical energy density of 2600 W h kg⁻¹ and practical values of 400–600 W h kg⁻¹.^11,14,15 Although the concept of S-LSBs appeared in early patents in the 1960s,^16–18 which predates that of the Li-intercalation-based rechargeable LIBs first reported in the 1970–1980s,^19,20 the development of S-LSBs lagged far behind the LIB technology commercialized by Sony in 1991. This gap largely stems from several intrinsic challenges of sulfur cathodes. First, both elemental sulfur (S₈) and the fully discharged product (Li₂S) are electronic and ionic insulators, leading to sluggish solid-state redox kinetics. Second, the S₈ ↔ Li₂S conversion involves a large volume change (up to ∼78%), which can damage cathode integrity. Third, in commonly used ether-based liquid electrolytes, discharge proceeds through soluble lithium polysulfides (Li₂S_x, x = 4–8). These species can detach from the conductive framework, remain electrochemically inactive in the electrolyte, and migrate to the lithium anode, where they are reduced to insoluble Li₂S deposits. The resulting loss of active material and parasitic reactions, collectively termed the “polysulfide shuttle”, cause rapid capacity decay and poor coulombic efficiency.

A major advance was reported by Nazar and co-workers in 2009, who mitigated these limitations by thermally infiltrating sulfur into a conductive mesoporous carbon host to form a sulfur-embedded carbon (S@C) nanocomposite.²¹ Nanoscale confinement shortens electron/ion transport pathways, buffers volume changes, and delays polysulfide escape from the cathode. However, because polysulfides interact weakly with nonpolar carbon, shuttling typically persists. Accordingly, extensive efforts have been directed toward strengthening sulfur-species confinement and adsorption using heteroatom-doped carbons,^22–24 polar/metallic trapping compounds,^25–27 and functional binders,^28–30 enabling impressive cycle life (some with ≥80% capacity retention over 1000 cycles). In parallel, covalent immobilization of sulfur in polymeric matrices (e.g., sulfurized polyacrylonitrile, PAN)^31,32 has proven highly effective for suppressing polysulfide dissolution, while the most definitive strategy is to replace liquid electrolytes with solid electrolytes.^33–35 Despite these advances, large scale commercialization has not been realized in part because conventional S-LSBs rely on lithium metal anodes, whose high reactivity and dendrite-related safety risks complicate manufacturing and long-term operation.^36–39

These challenges have stimulated growing interest in Li₂S-based LSBs (Li₂S-LSBs), which use Li₂S, the fully discharged product of sulfur cathodes, as the cathode active material, enabling lithium-metal-free cell configurations.^40–43 This approach is compatible with existing LIB manufacturing infrastructure and can be paired with high-capacity anode hosts such as silicon and tin. Moreover, Li₂S undergoes volume shrinkage during delithiation, which can generate internal free volume that partially accommodates subsequent expansion upon lithiation, offering a potentially favorable mechanical pathway for improved cycling stability. Nevertheless, Li₂S-LSBs introduce new challenges, including the scalable and cost-effective preparation of Li₂S-embedded carbon (Li₂S@C) nanocomposites and the high activation overpotential during the first charge, both of which impede practical implementation. While the origin of the first-charge activation overpotential and related mitigation strategies have been reviewed extensively elsewhere,⁴⁴ this article focuses on recent advances in nanostructure engineering of Li₂S@C nanocomposites, a particularly effective approach for alleviating the activation barrier and addressing other challenges in Li₂S-LSBs. Through these perspectives, we distill design principles and remaining bottlenecks governing Li₂S cathode performance and outline research directions toward practically relevant Li₂S-LSBs.

2 Lithium sulfide-based vs. sulfur-based Li–S batteries

In an ether-based liquid electrolyte, the ideal S-LSBs and Li₂S-LSBs should undergo identical redox cycles and share the same steady-state voltage profile.^44,47–49 The only difference is the starting point: S-LSBs begin with discharge from elemental sulfur, whereas Li₂S-LSBs begin with charge from Li₂S (Fig. 1a). The discharge proceeds through multiple steps and is commonly represented by two plateaus: a shorter high-voltage plateau associated with the conversion of S₈ to soluble polysulfides (one-quarter of the theoretical capacity) and a longer low-voltage plateau corresponding to the further reduction of polysulfides (e.g., Li₂S₄) to insoluble Li₂S₂/Li₂S (three-quarters of the theoretical capacity).

Fig. 1 Discharge and charge reactions and galvanostatic discharge curves of (a) ideal S-LSBs and Li₂S-LSBs,⁴⁵ where DoD is the degree of discharge. Reprinted with permission.⁴⁵ Copyright © 2013, Springer Nature Limited. All rights reserved. (b) Typical real S-LSBs and Li₂S-LSBs, where specific capacity is based on sulfur (modified from the original graph, showing that, in Li₂S-LSBs, the first-charge specific capacity often exceeds the theoretical value due to parasitic reactions). Reprinted with permission.⁴⁶ Copyright © 2023 Elsevier B.V. All rights reserved.

In practice, however, S-LSBs and Li₂S-LSBs show markedly different first-cycle behaviors (Fig. 1b).⁴⁶ Li₂S-LSBs using commercial Li₂S typically require significantly higher charging potentials and exhibit a pronounced initial voltage spike.⁴⁴ This first-charge overpotential is generally attributed to (i) the high thermodynamic stability of crystalline Li₂S (antifluorite cubic structure), which makes delithiation energetically unfavorable and necessitates the nucleation of sulfur/polysulfide phases;^50–52 (ii) sluggish interfacial charge transfer and Li⁺ transport through the oxidizing Li₂S surface region, which further increases the potential at practical rates;⁵³ and (iii) passivating surface contaminants on Li₂S particles (e.g., LiOH/Li₂O and sometimes Li₂SO₃/Li₂SO₄) that increase interfacial resistance.^46,54 By contrast, during the first discharge of an S-LSB, the initial reduction of solid sulfur rapidly generates soluble polysulfides, enabling a favorable solid–liquid pathway and sometimes delivering a near-theoretical first plateau capacity.²⁵ The subsequent low-voltage conversion to Li₂S₂/Li₂S is kinetically limited, so the first discharge typically ends with a mixture of poorly crystalline/amorphous Li₂S₂/Li₂S. These freshly formed discharge products also lack the thick impurity layers found on commercial Li₂S. Therefore, S-LSBs generally delithiate more easily, and the following charge does not exhibit the large activation spike characteristic of Li₂S-LSBs.

The first-cycle spike of Li₂S-LSBs can reach ∼4 V vs. Li⁺/Li, and the first-charge capacity often significantly exceeds the theoretical value, indicating severe parasitic reactions and/or cathode structural degradation (Fig. 1b). Accordingly, numerous strategies have been developed to mitigate the activation barrier,⁴⁴ including heteroatom/cation–anion doping (e.g., Fe, Co, Se, and Te) to introduce defects, weaken Li–S bonding, and improve transport;^43,55 incorporation of polar electrocatalysts (e.g., metal sulfides, phosphides, and carbides) to accelerate Li₂S oxidation;^56–58 electrolyte additives that remove LiOH/Li₂O and promote interfacial reactions;^59,60 and redox mediators that facilitate charge transfer through solution pathways.^61–64 Reducing Li₂S crystallinity and particle size can further lower the activation energy and shorten Li⁺ diffusion lengths, improving kinetics and sulfur utilization.^65,66

As first demonstrated by Nazar and co-workers for S-LSBs,²¹ confining sulfur within a mesoporous conductive carbon host to form an S@C nanocomposite is crucial for achieving high specific capacity and cycle stability by improving electronic/ionic transport, buffering cathode volume changes, and suppressing polysulfide shuttling.²¹ Elemental sulfur can be readily infused into porous carbon by simple melt infiltration (typically at ∼155 °C) above its melting point (∼115 °C). In contrast, Li₂S has a much higher melting point (∼938 °C), rendering melt infiltration impractical for preparing Li₂S@C nanocomposites. Moreover, Li₂S is highly sensitive to moisture, which further complicates synthesis and handling. These challenges have been major obstacles to the development of high-performance Li₂S-LSBs. In the following sections, representative strategies for preparing Li₂S@C nanocomposites are introduced and discussed.

3 Strategies for preparing Li₂S@C nanocomposites

Cathodes made from commercial Li₂S often suffer from high initial charge overpotentials, low capacity, limited rate capability, and poor cycling stability. These issues stem primarily from the large particle size of Li₂S (10–20 µm),^67,68 which prolongs Li⁺ diffusion distances, reduces the interfacial reaction area, and aggravates transport and kinetic limitations. In contrast, nanostructured Li₂S-based cathodes, especially Li₂S@C nanocomposites, have shown significantly improved electrochemical performance. Here, a Li₂S@C nanocomposite refers to an architecture in which Li₂S is confined within, or uniformly distributed throughout, a conductive carbon framework to create intimate and continuous Li₂S-carbon interfaces. In contrast, Li₂S/C denotes a simple physical mixture or surface-deposited composite with limited confinement and weaker interfacial integration. The performance improvements of Li₂S@C nanocomposites arise from nanoscale Li₂S dispersion in a conductive matrix, which increases the active surface area, shortens ion-transport lengths, and provides efficient electronic pathways for charge transfer.⁶⁹ Equally important, uniform distribution and strong connectivity between Li₂S and carbon help lower the first-charge activation barrier, accelerate redox kinetics, and increase sulfur utilization.

To realize such structures, a range of synthesis strategies have been developed, including solid-state routes (e.g., high-energy ball milling, carbothermal reduction, and thermal decomposition) and liquid-phase approaches (e.g., precipitation, infiltration, and in situ conversion). The following subsections summarize representative preparation strategies, structural designs, and the resulting electrochemical benefits of Li₂S@C nanocomposites, showing their key role in enabling high-performance and practically relevant Li₂S-LSBs.

3.1 Ball milling

Ball milling is a mechanical technique that utilizes repeated collisions between powder particles and milling media to reduce particle size and induce defects.^70,71 For Li₂S cathodes, it has been widely used to downsize commercial Li₂S and promote intimate mixing with conductive carbon, thereby improving interfacial contact and enhancing electrochemical reactivity.⁷² High-energy ball milling can typically reduce Li₂S to the submicron range (∼0.2–2 µm), shortening Li⁺ diffusion lengths and facilitating electron transport.

Cai et al. prepared a nanostructured Li₂S/C composite by ball milling commercial Li₂S with carbon black at 1060 rpm for 2 hours, resulting in particles ranging from 200 to 500 nm as shown in Fig. 2a.⁷³ This composite showed a reduced activation potential of 2.6 V (vs. Li⁺/Li) compared to commercial Li₂S; however, high cutoff voltages up to 4.0 V were still required to complete the initial charge. In another study, Chen et al. (Fig. 2b) combined high-energy ball milling of Li₂S and carbon black with pyrrole-assisted carbonization to form ∼400 nm Li₂S/C particles encapsulated by a N-doped carbon shell.⁷⁴ This core–shell structure delivered a high initial capacity of 1029 mA h g⁻¹ and retained 652 mA h g⁻¹ after 100 cycles, indicating improved cycling stability.⁷⁴ Similarly, Li et al. synthesized nanosized Li₂S by ball milling a LiH + S₈ precursor mixture (Fig. 2c).⁷⁵ The Li₂S particles were subsequently mixed with mesoporous carbon matrices by milling with polyacrylonitrile (PAN), followed by high-temperature carbonization (∼1000 °C), forming conductive and mechanically robust nanocomposites. A practical concern is that the reaction between LiH and S₈ can generate hydrogen gas (H₂) during milling, posing a safety risk for scale-up. Ball milling also enables incorporation of functional additives. For example, Cupid et al. introduced polar SnS₂ into Li₂S/C via a scalable milling process to chemically trap lithium polysulfides, suppress shuttling, and stabilize interfacial reactions.⁷⁶

Fig. 2 (a) Schematic of the high-energy ball milling process for preparing Li₂S-C nanocomposites. Reprinted with permission.⁷³ Copyright © 2012, American Chemical Society. All rights reserved. (b) Schematic of the approach for synthesizing the Li₂S/C composite particles encapsulated by a nitrogen-doped carbon shell. Reprinted with permission.⁷⁴ Copyright © 2014, Royal Society of Chemistry. All rights reserved. (c) Schematic illustration of the fabrications of Li₂S and the Li₂S/C hybrid. Reprinted with permission.⁷⁵ Copyright © 2017, Royal Society of Chemistry. All rights reserved.

Despite its simplicity, ball milling has notable limitations: it is energy-intensive and time-consuming, achieving uniform Li₂S dispersions below ∼100 nm remains difficult,⁷⁷ and the process cannot effectively infiltrate Li₂S into the internal pore network of mesoporous carbon hosts.

3.2 Carbothermal reduction

Carbothermal reduction is a high-temperature process in which carbon serves as a reducing agent to convert metal oxides, sulfates, or other precursors into lower-valence products such as metals, sulfides, or carbides under inert or reducing atmospheres (e.g., Ar, N₂, or H₂).⁷⁸ In addition to traditional carbon sources such as graphite and carbon black, carbon-rich polymers such as glucose, polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and polyacrylonitrile (PAN) are often used as carbon precursors.^79,80 Upon thermal decomposition, these polymers generate carbon in situ, enabling intimate precursor–carbon contact and thereby enhancing reduction efficiency and compositional control.

In 2013, Yang et al. pioneered a cost-effective carbothermal route to synthesize Li₂S@C composites from Li₂SO₄ via the reaction described in eqn (1):⁸¹

Li₂SO₄ (s) + xC (s) → Li₂S (s) + xCO_y (g) ((x, y) = (1, 1) or (4,2)) (1)

As shown in Fig. 3a, the carbon framework is formed by pyrolyzing a resorcinol–formaldehyde (RF) gel, which is synthesized via condensation polymerization.⁸¹ Due to its high surface area, porosity, and conductivity, the abundant oxygen groups in the RF gel can coordinate with Li⁺ in Li₂SO₄, promoting uniform distribution of the salt in the carbon matrix. TEM analysis shows that the cross-linked RF-derived carbon forms spherical particles with sizes ranging from ∼500 nm to 2 µm. STEM-EDX mapping in Fig. 3b confirms that sulfur is homogeneously distributed throughout the carbon spheres, indicating successful incorporation rather than surface deposition. The Li₂S@C composite exhibits higher reversible capacity and significantly improved suppression of the polysulfide shuttle compared to the physical mixture. At 0.5C, it retains 280 mA h g⁻¹ after 40 cycles (from 330 mA h g⁻¹), demonstrating improved cycling stability, though further optimization is still needed.

Fig. 3 (a) In situ synthesis scheme for a Li₂S@C composite. (b) HAADF-STEM image of Li₂S@C particles (1), EDX spectrum showing the presence of the carbon K edge and sulfur K edge (2), and EDX elemental mapping of carbon (3) and sulfur (4). Reprinted with permission.⁸¹ Copyright © 2013, Royal Society of Chemistry. All rights reserved. (c) Ellingham diagram for different carbothermal reduction reactions. The numbers at the end of the lines are ΔG_R values at 820 °C. (d) SEM images from a Li₂SO₄·H₂O sample before and after heat treatment at 820 °C under an argon atmosphere. Reprinted with permission.⁸² Copyright © 2015, Royal Society of Chemistry. All rights reserved. (e) Schematic of the fabrication process of Li₂S@C strips and the structural evolution of PVA during PVA-assisted carbothermal reduction of Li₂SO₄. Reprinted with permission.⁷⁹ Copyright © 2018, Royal Society of Chemistry. All rights reserved.

Temperature is a key parameter in carbothermal reduction because it strongly influences the crystallinity and particle size of Li₂S. The Ellingham diagram (Fig. 3c) indicates that reduction of Li₂SO₄ by carbon becomes thermodynamically favorable above ∼300 °C.⁸² SEM images (Fig. 3d) reveal a morphological evolution from monoclinic Li₂SO₄·H₂O to well-defined octahedral particles, indicating the successful formation of Li₂S crystals. Nonetheless, many studies conduct the reaction at around ∼800 °C, which accelerates grain growth and yields larger, highly crystalline particles.⁸³ In contrast, Ye et al. showed that using PVA as a carbon source and conducting the reaction below the melting point of Li₂SO₄ (635 °C) helps preserve morphology and yields smaller Li₂S particles (10–20 nm), likely due to gradual oxygen removal (Fig. 3e).⁷⁹ The unsaturated C=C and C=O bonds in the partially carbonized polymer enable efficient reduction at considerably low temperatures. Notably, Li₂S@C prepared under these milder conditions exhibited a reduced activation potential (2.63 V) and a higher initial discharge capacity (805 mA h g⁻¹) compared to the material produced at 900 °C (3.2 V and 760 mA h g⁻¹).

Carbothermal reduction has several drawbacks. Evolution of CO and CO₂ gases during reduction can generate excessive porosity in the carbon matrix, decreasing cathode volumetric capacity.^78,82 The process may also release hazardous sulfur-containing gases (e.g., SO₂, SO₃, and H₂S), raising safety and environmental concerns.⁸⁴ In addition, morphology control of Li₂S remains challenging, particularly for high-temperature syntheses, where particle coarsening is difficult to suppress.

3.3 Carbothermal reduction-derived methods

PVP can serve as both a carbon source and structural template in the carbothermal reduction of Li₂SO₄ to Li₂S, forming Li₂S@C composites. However, conventional solid-state mixing yields inhomogeneous dispersion and poorly controlled morphologies of Li₂S particles. To address this, electrospinning is used to fabricate uniform Li₂SO₄/PVP nanofibers as structured precursors (Fig. 4a).⁸⁵ The nanofiber geometry promotes nanoscale dispersion of Li₂SO₄ and creates an interconnected architecture. Subsequent annealing under a reducing atmosphere converts Li₂SO₄ into finely distributed Li₂S within conductive carbon nanofibers, which can be collected as free-standing, binder-free electrodes. A major limitation is the low mass loading of typical electrospun mats, which restricts areal capacity unless multiple layers are stacked. In addition, scaling-up remains challenging due to the limited throughput of traditional electrospinning setups. The solvent system must also be carefully chosen to balance PVP solubility, Li₂SO₄ dispersion, and electrospinnability.⁸⁶ Moreover, the final fiber morphology is highly sensitive to processing parameters and ambient conditions, necessitating tight process control.^87,88

Fig. 4 (a) Schematic illustration of the production of freestanding flexible Li₂S@NCNF paper electrodes via Ar-protected carbothermal reduction of Li₂SO₄@PVP fabrics made by electrospinning under ambient conditions. Reprinted with permission.⁸⁵ Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. All rights reserved. (b) Illustrative process of the magnesothermal synthesis of Li₂S and purification. Reprinted with permission.⁸⁹ Copyright © 2022, American Chemical Society. All rights reserved.

Also inspired by the traditional carbothermal reduction method, the magnesothermal synthesis of Li₂S replaces carbon with magnesium (Mg) as the reducing agent (Fig. 4b).⁸⁹ While both approaches reduce lithium-sulfur-containing precursors such as Li₂SO₄ to Li₂S, Mg offers a much stronger reduction driving force, allowing the reaction to proceed at lower temperatures and with more favorable thermodynamics. Unlike carbothermal methods, which often generate gaseous byproducts such as CO and CO₂ and leave highly porous carbon residues, magnesothermal synthesis produces solid MgO as the main byproduct, avoiding greenhouse-gas emissions. After the reaction, high-purity Li₂S is typically obtained through thorough removal of MgO, and an ethanol-based dissolution/recrystallization step is often employed to improve purity and narrow the particle-size distribution, which helps ensure reproducible electrochemical performance. Despite these advantages, magnesothermal synthesis has practical limitations. Mg powder is pyrophoric and must be handled under inert conditions. The reaction is highly exothermic, complicating heat management and scale-up. After the reaction, MgO byproducts must be fully removed, and an additional ethanol-based recrystallization step is often needed to purify Li₂S. The method also offers little control over morphology and uniformity.

3.4 Lithiation of S@C nanocomposites

Li₂S@C nanocomposites can be directly synthesized by lithiating S@C nanocomposites using lithium metal or reducing lithium reagents (Table 1). For example, Yang et al. lithiated a S@CMK-3 nanocomposite prepared by thermal infiltration using n-butyllithium at 65 °C for 2 h and then at 105 °C for 18 h (Fig. 5a).⁹¹ In the XRD pattern of the lithiated product, the diffraction peaks of sulfur disappeared, indicating complete conversion. Notably, no Li₂S reflections were observed either, which was attributed to Li₂S being confined within the sub-5 nm CMK-3 pores, thereby suppressing crystallite growth and long-range ordering. To verify Li₂S formation, the same lithiation was performed on an S/carbon composite based on macroporous carbon (200–300 nm pores). In this case, clear Li₂S diffraction peaks were observed, supporting Li₂S formation in S/CMK-3 as well. The composite delivers an initial discharge capacity of 573 mA h g⁻¹, with the capacity stabilizing after approximately five cycles. Additionally, a small potential difference of only ∼200 mV between the first charge and subsequent charges further indicates the significantly improved reaction kinetics of Li₂S upon incorporation into the mesoporous carbon nanocomposite. Hwa et al. reported the synthesis of Li₂S/GO@C nanospheres via lithiation of sulfur using LiET₃BH.⁹³ Briefly, an S/single-layer graphene oxide (S/SLGO) nanocomposite was prepared by combining a sulfur solution in toluene with an SLGO dispersion in THF, followed by slow addition into a LiEt₃BH solution in THF to yield Li₂S/GO nanospheres after solvent removal. The Li₂S/GO nanospheres were subsequently treated under hydrogen to generate a carbon shell, producing Li₂S/GO@C nanospheres (Fig. 5b). The carbon-coated Li₂S/GO@C and Li₂S/GO@C-NR electrodes deliver high initial capacities of 964 and 896 mA h g⁻¹ (based on Li₂S), respectively, significantly exceeding those of uncoated electrodes. The carbon shell enhances sulfur utilization by suppressing polysulfide dissolution and improving electronic conductivity.

Table 1 Lithiation agents for elemental sulfur and corresponding reactions

Lithiation agent	Formula	Conditions	Reaction	Ref.
Lithium metal	Li	50–70 °C, inert gas, dry ether solvent	S + 2Li → Li2S	90
n-Butyllithium	C4H9Li	Room temp., dry THF or hexane, inert atmosphere	S + 2C4H9Li → Li2S + C4H9 − C4H9	91
Lithium hydride	LiH	High-energy ball milling	S + 2LiH → Li2S + H2	75
Lithium naphthalenide	LiC10H8	In THF, under Ar	S + 2LiC10H8 →Li2S + 2C10H8	92
Lithium triethylborohydride	LiBEt3H	Anhydrous ether solvents, ambient conditions	S + 2LiBEt3H → Li2S + H2 + 2BEt3	93 and 94

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Fig. 5 (a) X-ray diffraction characterization of Li₂S/mesoporous carbon nanocomposite particles. Reprinted with permission.⁹¹ Copyright © 2010, American Chemical Society. All rights reserved. (b) Schematic illustration of the synthesis of Li₂S/GO@C nanospheres. Reprinted with permission.⁹³ Copyright © 2015, American Chemical Society. All rights reserved.

The chemical lithiation route enables low-temperature synthesis compared to carbothermal approaches and can achieve uniform dispersion of Li₂S within a conductive carbon matrix, especially when nanostructured carbon hosts are used.⁹⁵ Importantly, this strategy is frequently combined with chemical vapor deposition (CVD) to further engineer the interfacial structure. After sulfur infiltration, or even after partial lithiation, a conformal carbon layer can be deposited via CVD to reinforce electronic percolation, seal residual surface defects, and stabilize the newly formed Li₂S phase.^93,96 However, the practical adoption of lithiation is constrained by the safety hazards and high cost of typical lithiation reagents, which pose significant barriers to scale-up and commercialization.

3.5 Sulfuration of lithium compounds

Sulfuration has emerged as a versatile and potentially scalable route to synthesize Li₂S from a wide range of lithium-containing precursors. In this approach, reactive sulfur-containing gases, such as H₂S, CS₂, and sulfur vapor, convert lithium precursors in the solid, molten, or vapor state into Li₂S under controlled thermal and atmospheric conditions.

Air-stable lithium salts such as LiOH, Li₂CO₃, and LiNO₃ are attractive starting materials because they are easy to handle and stable under ambient conditions.⁹⁷ In addition, several lithium salts with relatively low melting points, such as lithium nitrite (LiNO₂, 222 °C) and lithium acetate (CH₃COOLi, 286 °C),⁹⁷ can be melt-infiltrated into carbon hosts to form Li-salt@C composites, which are subsequently sulfurized to yield Li₂S@C. Representative conversion reactions are summarized in Table 2.

Table 2 Chemical sulfuration agents for lithium-rich precursors

Sulfuration agent	Reaction	Ref.
H2S	2ROLi (sol) + H2S (g) → 2Li2S (s) + H2 (g)	97
	2LiX (s) + H2S (g) → 2Li2S (s) + 2HX (g)	98
CS2	4Li (l) + CS2 (g) → 2Li2S (s) + C (s)	99
	2LiOH (s) + CS2 (g) → Li2S (s) + CO2 (g) + H2S (g)	100
	4LiH (s) + CS2 (g) → 2Li2S (s) + C (s) + 2H2 (g)	101
S (gas)	2Li + S (g) → Li2S (s)	102
	6LiOH (s) + 3S (g) → 2Li2S (s) + Li2SO3 (g) + 3H2O (g)	84

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The required reaction temperature depends strongly on the lithium precursor: strong Brønsted base salts (e.g., LiOH, LiH, and LiNH₂) can react at ∼100 °C, while Li₂CO₃ and CH₃COOLi require substantially higher temperatures of ∼400–725 °C. Lower conversion temperatures are generally preferred as they help preserve the original morphology, minimize Li₂S particle coarsening, and reduce energy consumption. Pre-processing steps such as ball milling or recrystallization can further reduce and homogenize precursor particle size prior to sulfuration. For example, Dressel et al. used high-energy ball milling to decrease the particle size of LiOH and then mixed it with carbon black (Fig. 6a).⁹⁸ The LiOH/C mixture was dispersed with a binder to form a slurry, cast onto an aluminum current collector, and subsequently sulfurized under continuous H₂S gas flow at 100 or 150 °C. The resulting Li₂S/C electrodes delivered discharge capacities of up to 770 mA h g⁻¹ and retained >410 mA h g⁻¹ after 100 cycles at 0.2C.

Fig. 6 (a) Schematic illustration of routes for preparing Li₂S/C composite and electrodes. Reprinted with permission.⁹⁸ Copyright © 2016 Elsevier B.V. All rights reserved. (b) Schematic illustration of Li₂S@graphene capsules by burning lithium in CS₂. (c) TEM images of Li₂S@graphene capsules, with the inset showing bulk nanocapsules, revealing single-crystal Li₂S (50–100 nm) encapsulated by about 10 to 20 graphite layers. (d) EDXS spectra and (e) EELS spectra of Li₂S@graphene capsules shown in 6c. Reprinted with permission.⁹⁹ Copyright © 2017, Springer Nature Limited. All rights reserved. (f) Schematic illustration of the synthesis procedure for Li₂S@PC-CNT. Reprinted with permission.¹⁰¹ Copyright © 2018, Royal Society of Chemistry. All rights reserved.

Tan et al. synthesized 50–80 nm Li₂S nanocrystals encapsulated by few-layer graphene (Li₂S@graphene) via combustion of lithium foil in CS₂ vapor (Fig. 6b).⁹⁹ TEM images reveal rhombic Li₂S@graphene nanoparticles (50–80 nm) with highly crystalline Li₂S cores tightly encapsulated by 10–20 layers of graphene, forming a capsule-like core–shell nanostructure (Fig. 6c). The EDXS results confirm such a high active mass percentage, where the Li₂S/C ratio is determined to be 88 : 12 by weight (Fig. 6d). The EELS results further confirm the presence of the Li element in the Li₂S@graphene composite (Fig. 6e). At a high Li₂S loading of 10 mg cm⁻², the electrode delivers a high reversible capacity of 1160 mA h g⁻¹. Liang et al. used CS₂ to sulfurize LiH to Li₂S at temperatures below 250 °C (Fig. 6f).¹⁰¹ As a result, the electrode achieved 820 mA h g⁻¹ at 0.1 A g⁻¹ after 10 cycles and showed excellent cycling stability, retaining 502 mA h g⁻¹ at 0.5 A g⁻¹ after 300 cycles (relative to an initial capacity of ∼650 mA h g⁻¹ at 0.5 A g⁻¹).

The most direct route to Li₂S is the reaction between lithium and sulfur. Although rarely used in practice due to difficult handling, sulfur vapor can react with Li metal in a sealed vacuum vessel at ∼300 °C to form Li₂S.¹⁰³

Overall, sulfuration offers several advantages, including the use of air-stable lithium precursors, tunable reaction conditions, and control over Li₂S particle size and crystallinity. It also facilitates integration with conductive carbon frameworks and is, in principle, scalable. However, the use of toxic and flammable gases such as H₂S poses significant safety and environmental risks, necessitating specialized reactors and stringent gas-handling infrastructure. For some precursors, high processing temperatures are required, which can promote particle agglomeration and coarsening, compromising electrochemical performance. In addition, incomplete conversion and residual precursors/byproducts often necessitate post-treatment and purification.

3.6 Solution infiltration of Li₂S

Li₂S can be dissolved in a suitable solvent and infiltrated into porous carbon hosts to form Li₂S@C nanocomposites after solvent evaporation. Graphene has been frequently used as the carbon host. In 2014, Wu et al. reported a simple solution-based route to graphene–Li₂S composites.¹⁰⁴ Briefly, commercial Li₂S powder was dissolved in anhydrous ethanol, followed by the addition of graphene. Solvent evaporation under continuous mixing deposited Li₂S onto the graphene sheets, yielding a well-integrated Li₂S composite (Fig. 7). The Li₂S–graphene composites exhibited excellent electrochemical performance in both 5 M and 7 M electrolytes. With increasing graphene content and reduced Li₂S particle size, capacity utilization improved significantly, delivering a maximum specific capacity exceeding 1100 mA h g⁻¹ (S) at C/20, comparable to or higher than those of previously reported S–CNT and S–graphene cathodes. Wang et al. drop-cast a Li₂S–ethanol solution onto reduced graphene oxide (rGO) paper to uniformly infiltrate Li₂S into rGO's porous framework (Fig. 8a).¹⁰⁵ HAADF-STEM and elemental mapping shown in Fig. 8b confirm the uniform distribution of Li₂S nanoparticles on graphene sheets, with C, S, and O elements homogeneously dispersed throughout the nano-Li₂S/rGO structure. The flexible rGO paper not only prevented particle agglomeration but also helped accommodate volume changes during cycling, thereby enhancing structural stability and electrochemical performance.

Fig. 7 Schematic of the Li₂S and Li₂S–graphene composite synthesis process. Reprinted with permission.¹⁰⁴ Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. All rights reserved.

Fig. 8 (a) Schematic illustration of the material preparation processes of nano-Li₂S/rGO paper and structure changes during cycling of nano-Li₂S/rGO paper. (b) HAADF-STEM image of nano-Li₂S/rGO paper (1) and corresponding EDS elemental mapping of (2) C, (3) S, and (4) O Reprinted with permission.¹⁰⁵ Copyright © 2015, American Chemical Society. All rights reserved.

He et al. incorporated carbon nanotubes (CNTs) into a graphene oxide (GO) suspension to construct a three-dimensional conductive network.¹⁰⁶ As shown in Fig. 9a, the one-dimensional CNTs served as nanoscale pillars that separated and supported the two-dimensional graphene sheets, yielding a robust 3D architecture with enhanced conductivity and mechanical integrity. Li₂S was then incorporated by infiltrating a Li₂S–ethanol solution followed by vacuum-assisted evaporation, enabling uniform Li₂S recrystallization within the interlayer voids. TEM images reveal uniformly dispersed ultrafine Li₂S nanoparticles (∼8 nm) anchored on the three-dimensional CNT/graphene conductive network, with a lattice spacing of 0.33 nm corresponding to the Li₂S (111) plane, as shown in Fig. 9b–g.

Fig. 9 (a) Synthetic procedure for the 3DCG–Li₂S composite. Reprinted with permission.¹⁰⁶ Copyright © 2016, American Chemical Society. All rights reserved. SEM images of (b) 3DG, (c) 3DCG, (d) 3DG–Li₂S, and (e) 3DCG–Li₂S composite. (f) Low-magnification TEM images of 3DCG–Li₂S. (g) TEM image of Li₂S nanoparticles on 3DCG and a high-resolution TEM image of Li₂S nanocrystals in the inset. Reprinted with permission.¹⁰⁶ Copyright © 2016, American Chemical Society. All rights reserved.

Wu et al. applied four infiltration/vacuum-drying cycles to load Li₂S into rGO, improving its distribution and promoting the formation of smaller Li₂S nanoparticles, and then deposited a protective carbon layer by CVD to form a core–shell Li₂S@C nanocomposite (Fig. 10a).¹⁰⁷ The conductive graphene framework enhanced rate capability, while the vapor-deposited carbon shell together with an in situ formed passivation layer provided effective protection during cycling. As a result, the composite cathode retained approximately 97% of its initial capacity (∼1040 mA h g⁻¹ (S) at 0.5C) after 700 cycles, demonstrating exceptional long-term stability.

Fig. 10 (a) Cycling stability of the graphene–nano Li₂S@C electrode at 0.5C and its morphological characterization. Reprinted with permission.¹⁰⁷ Copyright © 2016, American Chemical Society. All rights reserved. (b) Cycling stability of free-standing Li₂S electrodes at 0.5C and a schematic illustration of their preparation using infiltration of active materials into porous carbonized biomass sheets. Reprinted with permission.¹⁰⁸ Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. All rights reserved.

They further extended this strategy to fabricate freestanding Li₂S electrodes (Fig. 10b).¹⁰⁸ Porous cellulose sheets were used as the scaffold and carbonized at 500 °C. Carbonization mitigated Li₂S hydrolysis during processing and generated additional porosity, increasing surface area and facilitating Li₂S loading and ion transport. The resulting cells exhibited minimal capacity decay at 0.5C.

A key limitation of this strategy is the very low solubility of Li₂S in common solvents. For example, the solubility of Li₂S in the most widely used solvent, ethanol, is around 25 mg mL⁻¹ (∼0.5 M).^{104–106,108–114} This limited solubility constrains the amount of Li₂S that can be infiltrated into the nanopores of porous carbon hosts, so a substantial fraction tends to precipitate outside the pore network. As a result, Li₂S@C composites produced by this route often employ two-dimensional hosts, such as graphene-based frameworks, to facilitate uniform deposition and electrical contact.^104–108

3.7 Precursor solution infiltration–decomposition method

To overcome the limited solubility of Li₂S in conventional solvents, which restricts direct solution infiltration, we recently developed a precursor solution infiltration–decomposition method. As shown in Fig. 11a, Li₂S is first converted to lithium trithiocarbonate (Li₂CS₃) via reaction with CS₂ in ethanol at room temperature. The resulting Li₂CS₃ is highly soluble and largely amorphous, which greatly facilitates its infiltration into the mesoporous carbon host, Super P (SP).⁶⁵ Subsequent thermal decomposition regenerates a Li₂S@SP nanocomposite with Li₂S confined within the mesoporous carbon framework. Thermal decomposition of Li₂CS₃ at 400 °C produced Li₂S@SP-400 nanocomposites with ∼11 nm Li₂S uniformly confined in Super P. The cathodes delivered a high discharge capacity of 821 mA h g⁻¹ (Li₂S) (∼1190 mA h g⁻¹ (S)) with improved rate and cycling performance compared to commercial Li₂S and melt-infiltrated S@SP, demonstrating the effectiveness and scalability of this nanoconfinement strategy. This simple, low-cost strategy is broadly applicable to diverse porous carbons, enabling uniformly dispersed Li₂S@C nanocomposites with intimate interfacial contact, and offering a practical pathway toward Li₂S-LSBs.

Fig. 11 (a) Preparation of Li₂S@SP nanocomposites through a precursor solution infiltration–decomposition method: (i) mixing SP with a Li₂CS₃ solution in anhydrous ethanol at room temperature, (ii) vacuum drying at room temperature, and (iii) thermal decomposition at 200–450 °C. Reprinted with permission.⁶⁵ Copyright © 2025, American Chemical Society. All rights reserved. (b) Schematic illustration of the synthesis of Li₂S@C nanocomposites with high in-pore Li₂S loading via a multi-cycle Li₂CS₃ infiltration–decomposition strategy. This strategy achieves high in-pore Li₂S loading and uniform distribution, leading to superior performance in lithium–sulfur batteries. Reprinted with permission.¹¹⁵ Copyright © 2026, American Chemical Society. All rights reserved.

Nonetheless, thermal decomposition of Li₂CS₃ releases a substantial amount of CS₂ gas (∼62 wt% of the precursor), which generates internal voids and limits in-pore Li₂S formation. As a result, the in-pore filling is only ∼38%, with a significant fraction of Li₂S deposited outside the pores.

To increase the in-pore loading, a multi-cycle infiltration strategy was developed (Fig. 11b).¹¹⁵ In this approach, Li₂CS₃ infiltration and decomposition are repeated for multiple cycles, allowing the newly created void space to be refilled in subsequent steps. This sequential “infiltrate–decompose–refill” process increased the pore-filling factor to 91% after five cycles. SEM and elemental mapping of the first-charged Li₂S@SP-5 cathode confirm a homogeneous sulfur distribution within the carbon matrix, indicating uniform Li₂S confinement and effective nanoscale dispersion throughout the SP framework. The resulting Li₂S@SP-5 nanocomposite exhibited reduced activation overpotential, improved charge-transfer kinetics, and enhanced cycling stability, demonstrating the multi-cycle method as an effective and scalable route to high-loading, high-performance Li₂S cathodes. Li₂S@SP-5 retained 376 mA h g⁻¹ after 500 cycles, compared with an initial capacity of 598 mA h g⁻¹ at 1.0C, representing a significant improvement over the cell prepared with a single infiltration step.

As discussed above, considerable efforts have been devoted to developing Li₂S@C nanocomposites through various synthesis strategies, including ball milling, carbothermal reduction, lithiation of sulfur/carbon composites, solution infiltration, and other advanced approaches. These strategies differ significantly in terms of Li₂S loading, particle size control, structural design, synthesis complexity, and electrochemical performance. To provide a clearer overview and facilitate comparison, Table 3 summarizes the key synthesis strategies and corresponding electrochemical performances discussed in this review.

Table 3 Representative examples of Li₂S@C nanocomposites prepared by different synthesis strategies

Strategy	Cathode	Li2S content (wt%)	Loading (mg cm^−2)	Activation voltage (V)	Initial capacity (mA h g^−1)	Capacity after cycling (mA h g^−1)	Ref.
Ball milling	Li2S-C	67.5	0.54	4.0	1144@0.02C	411@0.1C@50 cycles	73
Ball milling	Li2S/CB@NC	72	∼1	4.0	1029@0.2C	652@0.2C@100 cycles	74
Ball milling	Li2S/C	74	3–3.5	4.0	971@0.1C	570@0.1C@200 cycles	75
Ball milling	Li2S/C/SnS2	75	∼1	∼3.5	712@0.1C	391@0.1C@100 cycles	76
Ball milling	Li2S-C-PVP	60	∼1.5	∼4.2	∼500@0.1C	∼460@0.1C@50 cycles	116
Carbothermal reduction	Li2S-C	72	2	3.8	600@0.2C	400@0.2C@200 cycles	78
Carbothermal reduction	Li2S@C-CNT	N/A	1.86	3.2	805@0.1C	595@0.2C@150 cycles	79
			3.7 (high loading)
Carbothermal reduction	Li2S@C	62	0.54	3.0	330@0.5C	280@0.5C@40 cycles	81
Carbothermal reduction	Li2S/KB	68–78	3.5–4.0	3.4	938@0.1C	∼650@140cycles	82
Carbothermal reduction	Li2S	60	1.0	4.0	643@0.05C	459@0.05C@100 cycles	84
Carbothermal reduction-derived methods	Li2S@NCNF	50.6	3.0	3.5	720@0.2C	597.6@0.2C@50 cycles	85
Carbothermal reduction-derived methods	Li2S@Li2S2	60	∼1	4.0	∼750@0.05C	>400@0.5C@200 cycles	89
Carbothermal reduction-derived methods	TG-Li2S	53	1.3	∼3.4	1119@0.1C	791@0.1C@100 cycles	117
Carbothermal reduction-derived methods	Li2S@C-LPS-AB	38	1.75	2.4	680@2.0 mA cm^−2	632@2.0 mA cm^−2@700 cycles	118
Lithiation of S@C nanocomposites	PPy/Li2S/KB	N/A	N/A	4.0	∼850@0.2C	∼700@0.2C@80 cycles	90
Lithiation of S@C nanocomposites	Li2S/CMK-3	N/A	N/A	2.8	573@C/8	∼310@C/8@20 cycles	91
Lithiation of S@C nanocomposites	Li2S/KB/CNT	∼83.8	N/A	4.0	∼977@0.1C	414@0.1C@100 cycles	92
Lithiation of S@C nanocomposites	Li2S/GO@C	60	0.7–0.9	4.0	964@0.2C	∼700@0.2C@50 cycles	93
Lithiation of S@C nanocomposites	Li2S@C	60	1.0–1.5	4.0	972@0.2C	737@0.2C@100 cycles	94
Lithiation of S@C nanocomposites	Nano-Li2S/GA	69	3.66	3.6	838.5@0.1C	462.8@0.1C@100 cycles	95
Lithiation of S@C nanocomposites	Li2S (ALD)	67	N/A	3.0	∼860@55 mA g^−1	∼800@55 mA g^−1@36 cycles	96
Sulfuration of lithium compounds	Li2S-C	71.6	∼1.4	3.5	∼650–770@117 mA g^−1	540@117 mA g^−1@200 cycles	97
Sulfuration of lithium compounds	Li2S/C	∼56	∼2.68	4.0	∼770@0.2C	>410@0.2C@100 cycles	98
Sulfuration of lithium compounds	Li2S@graphene	∼88	10	3.5	1160@0.05C	>600@0.1C@200 cycles	99
Sulfuration of lithium compounds	Li2S@PC-CNT	68.2	1.34	3.8	1017@0.1 A g^−1	502@0.5 A g^−1@300 cycles	101
Sulfuration of lithium compounds	Li2S@C	92	N/A	4.0	1163@0.05C	954@0.1C@100 cycles	102
Sulfuration of lithium compounds	Li2S-rGO	∼66	∼0.96	3.5	982@0.1C	315@0.1C@100 cycles	119
Sulfuration of lithium compounds	Li2S-KB	71	3–3.2	3.4	∼868@0.05C	∼566@0.1C@100 cycles	120
Sulfuration of lithium compounds	HNG-Li2S	60	∼1.2	3.8	1067@0.05C	596@0.2C@500 cycles	121
Sulfuration of lithium compounds	Carbon-coated Li2S	51.3	∼1	∼3.2	∼700@0.1C	∼800@0.1C@15 cycles	122
Solution infiltration of Li2S	Graphene-Li2S	82–94	∼1	4.0	∼759@0.05C	∼698@0.1C@100 cycles	104
Solution infiltration of Li2S	Li2S/rGO	50–60	0.8–1.5	3.6	1119@0.1C	692@0.5C@145 cycles	105
Solution infiltration of Li2S	CNT/graphene-Li2S	81.4	∼4.0	3.6	1052.1@0.2C	958.3@0.2C@300 cycles	106
Solution infiltration of Li2S	Graphene-Li2S@C	55	1.3	2.8	742@0.2C	∼719@0.5C@700 cycle	107
Solution infiltration of Li2S	Li2S/C	∼50	∼1.3	2.8	∼870@0.5C	∼772@0.5C@200 cycles	108
Solution infiltration of Li2S	C-Li2S	∼73	∼1.4	3.8	947@0.2C	828@0.2C@100 cycles	109
Solution infiltration of Li2S	Li2S/N-doped graphene	50–55	∼2	4.0	801@0.3C	635@0.3C@100 cycles	110
Solution infiltration of Li2S	Li2S/graphene	∼50	1	3.6	894.7@	784.7@0.2C@300 cycles	111
Solution infiltration of Li2S	Li2S@Ni-P-S@G cage	60.6	5.2	4.0	980@0.1C	543@4C@100 cycles	112
Solution infiltration of Li2S	C-Li2S@C	∼60	N/A	2.8	∼850@0.2C	∼850@0.2C@300 cycles	113
Solution infiltration of Li2S	Li2S@C-Co-N	∼52	2	3.6	1137.1@0.2C	929.6@0.2C@300 cycles	114
Solution infiltration of Li2S	C-Li2S	90	1.35–1.62	3.8	826.1@0.1C	∼677@1C@500 cycles	123
Solution infiltration of Li2S	Nano-Li2S@C	72.3	2.8	2.8	1083.5@0.2C	766.4@0.2C@200 cycles	124
Solution infiltration of Li2S	3D-rGO-Li2S@C	75	2.5–3.5	3.8	856@0.1C	563.2@0.1C@100 cycles	125
Solution infiltration of Li2S	CF-CB-Li2S@C	N/A	7	3.8	943.7@0.1C	567.5@1C@200cycles	126
Precursor solution infiltration-decomposition method	Li2S@SP-400	60	1.0–1.2	4.0	821@0.1C	411@0.1C@100 cycles	65
Precursor solution infiltration-decomposition method	Li2S@SP-5	70	1.0–1.2	4.0	807@0.1C	402.5@0.1C@200 cycles	115

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4 Li₂S@C cathodes for all-solid-state batteries

Li₂S cathodes are particularly attractive for all-solid-state Li–S batteries (ASSLSBs) because Li₂S is already in the fully lithiated state and can therefore be paired with lithium-free anodes, enabling safer and more practical full-cell configurations. In addition, replacing flammable liquid electrolytes with solid electrolytes can effectively suppress polysulfide dissolution and improve battery safety.^35,127–129 Therefore, integrating Li₂S cathodes with solid-state electrolytes represents a promising route toward high-energy-density and safer sulfur-based batteries.

However, Li₂S activation, which is a major challenge even in conventional liquid-electrolyte systems, has become even more difficult in solid-state batteries. In liquid-electrolyte cells, electrolyte penetration and soluble polysulfide intermediates can partially assist Li₂S oxidation by providing continuous ion transport and additional reaction pathways. These electrolyte-mediated processes can, to some extent, compensate for the sluggish reaction kinetics of Li₂S.⁵³ In contrast, all-solid-state systems rely primarily on direct solid–solid interfacial reactions, where the absence of liquid-phase transport and soluble redox intermediates significantly increases the kinetic barrier for Li₂S activation. As a result, the already challenging oxidation process of Li₂S in liquid systems becomes even more demanding under solid-state conditions.

Eom et al. developed a Li₂S–VGCF nanocomposite using a solution-assisted synthesis route followed by thermal treatment, in which nanosized Li₂S particles were uniformly dispersed within a vapor-grown carbon fiber network (Fig. 12a).¹³⁰ The cathode composite, prepared by mixing Li₂S–VGCF with a Li₂S–P₂S₅ solid electrolyte, was used to assemble ASSLSBs with Li₂S–P₂S₅ as the solid electrolyte and a Li–In alloy as the anode. Compared with cells using oxide-based cathodes, these cells showed reduced interfacial resistance and avoided irreversible capacity loss. The multidimensional conductive framework improved electron transport and maintained stable conductive pathways during cycling, leading to capacities approaching 600 mA h g⁻¹ with stable cycling up to 20 cycles. Wang et al. used an N-doped carbon-coated Li₂S nanocomposite, Li₂S@NC, to prepare a Li₂S cathode for ASSLSBs using Li₇P₃S₁₁ as the solid electrolyte and Li–In as the anode. In Li₂S@NC, a thin nitrogen-doped carbon shell enhanced electrical conductivity and facilitated Li-ion transport (Fig. 12b). The resulting batteries achieved high Li₂S utilization (∼91%) even at practical loadings exceeding 8 mg cm⁻² and a capacity retention of 80% after 100 cycles.¹³¹ Jiang et al. synthesized ultrasmall Li₂S nanoparticles (∼15 nm) anchored on CNTs using a liquid-phase method by mixing Li₂S and CNTs in ethanol.¹³² The nanoscale Li₂S and one-dimensional CNT conductive framework effectively improved reaction kinetics and reduced transport resistance. ASSLSBs with a Li/75%Li₂S-24%P₂S₅-1%P₂O₅/Li₁₀GeP₂S₁₂/Li₂S-53%CNT architecture achieved a high initial capacity of 1160 mA h g⁻¹ at 0.1C and retained a capacity of 651.4 mA h g⁻¹ at 1C after 300 cycles at 60 °C.

Fig. 12 (a) Schematic of the nanocomposite evolution versus temperature. Reprinted with permission.¹³⁰ Copyright© 2017 Elsevier B.V. All rights reserved. (b) SEM image of the as obtained Li₂S@NC composite. Reprinted with permission.¹³¹ Copyright© 2020 Elsevier B.V. All rights reserved. (c) Schematic illustration of the preparation process for the Li₂S-CNT cathode. (d) Cycling stability of the Li₂S-53%CNT cathode at 1.0C and 60 °C. Reprinted with permission.¹³² Copyright © 2022, American Chemical Society. All rights reserved.

Catalytic strategies have been explored to overcome the sluggish solid–solid conversion kinetics of Li₂S cathodes in ASSLSBs. By introducing catalytic species into Li₂S cathodes, the activation barrier of Li₂S can be reduced and the conversion between Li₂S and sulfur can be accelerated, thereby improving active-material utilization and long-term cycling stability. Liu et al.¹³³ developed an ASSLSB using a solid polymer electrolyte through in situ polymerization of DOL and a Li₂S@Co-C@MHF cathode, which was prepared by first thermally infiltrating elemental sulfur into Co nanoparticle-decorated carbon nanocages encapsulated within MXene hollow fibers (Co-C@MHF), followed by converting sulfur into Li₂S using lithium naphthalide. The metallic Co in the cathode served as catalytic sites to facilitate Li₂S activation and sulfur conversion while MXenes improved charge transport and confined reaction intermediates. The catalytic effect significantly reduced the Li₂S activation barrier to ∼2.32 V and enabled stable cycling over 500 cycles with high-capacity retention. Similarly, Hao et al.¹³⁴ prepared nanosized Li₂S embedded within an amorphous LiFeS₂ matrix (Li₂S@LiFeS₂) through an in situ solid-state reaction between Li₂S and FeCl₃ using ball milling and mixed it with Li₆PS₅Cl and carbon black to prepare a cathode composite. The amorphous LiFeS₂ simultaneously functioned as a catalytic phase and a mixed ionic/electronic conductor, while the solid electrolyte Li₆PS₅Cl improved the lithium-ion conductivity within the cathode. The battery using Li₆PS₅Cl as the solid electrolyte showed substantially improved transport properties and nearly 99% capacity retention after 300 cycles. However, the discharge voltages are between 1.6 and 0.8 V, significantly lower than that of the liquid electrolyte LSBs (between 2.4 and 1.7 V), reducing cathode energy density.

Redox mediators have also been used to improve the kinetics of ASSLSBs. Yu et al.¹³⁵ introduced In₂S₃ as a mediator into Li₂S cathodes, forming Li₂S–Li_xIn₂S₃ composites during cycling. Li₂S–Li_xIn₂S₃ was prepared by ball milling Li₂S with In₂S₃, then with vapor-grown carbon fiber (VGCF), and finally with a solid electrolyte Li₇P₃S₁₁. ASSLSBs with this cathode were assembled with a Li₇P₃S₁₁ solid electrolyte and a Li/In anode. The battery performance indicated that Li_xIn₂S₃ simultaneously functioned as a redox mediator and a charge-carrier mediator for Li⁺ and electrons, improving transport kinetics and actively regulating electrode volume variation.

Compared with catalyst-assisted systems that mainly lower reaction barriers at interfaces, mediator-assisted strategies actively participate in electrochemical processes and create alternative reaction pathways, simultaneously improving reaction kinetics and transport characteristics. However, ensuring long-term mediator stability while minimizing inactive mass remains an important consideration for practical applications.

It should be noted that most Li₂S cathodes reported for ASSLSBs are based on Li₂S/C composites, that is, physical mixtures of Li₂S and conductive carbon, rather than true Li₂S@C nanocomposites. This is presumably due to the synthetic challenges associated with preparing Li₂S@C nanocomposites. Although catalyst- and mediator-assisted activation approaches can effectively improve Li₂S oxidation, they may not simultaneously address the intrinsic limitations of Li₂S, including poor electronic conductivity and severe electrochemical polarization. Therefore, integrating catalyst- or mediator-assisted activation strategies with Li₂S@C nanocomposites may provide additional advantages for all-solid-state systems.

5 Conclusions and future developments

LSBs are widely considered as promising next-generation energy-storage systems because of their high theoretical energy density, low cost, material abundance, and environmental compatibility. However, intrinsic limitations of sulfur cathodes, including extremely low electronic/ionic conductivity, large volume change during cycling, and the polysulfide shuttle, have long constrained commercially viable performance. Over the past decade, substantial progress has been made to address these challenges, and near-commercial metrics have been demonstrated in certain reports. Most of these advances, however, have been achieved in S-LSBs, whose practical deployment remains largely hindered by the use of lithium-metal anodes.

In this context, Li₂S-LSBs have drawn increasing attention because they can, in principle, eliminate lithium-metal anodes. Li₂S cathodes may also help mitigate cathode volume change and, to some extent, reduce polysulfide dissolution, offering potential benefits for cycle life. Despite sharing the same overall sulfur redox chemistry, Li₂S-LSBs face distinct challenges, notably the moisture sensitivity of Li₂S (complicating electrode fabrication and handling) and the well-known first-charge activation overpotential, which often leads to lower accessible capacity and poorer cycling stability. As a result, Li₂S-LSBs still lag well behind S-LSBs in overall maturity. Meanwhile, recent developments in all-solid-state batteries have also attracted growing interest toward Li₂S cathodes because solid electrolytes can potentially suppress polysulfide dissolution and improve cell safety.

A primary origin of the high overpotential and suboptimal performance is the difficulty of preparing well-defined Li₂S@C nanocomposites in which Li₂S is uniformly embedded within nanoscale porous carbon hosts. Unlike S@C nanocomposites, which can be conveniently produced via melt infiltration, Li₂S has a high melting point and is challenging to thermally infiltrate into porous frameworks. As reviewed here, a range of physical and chemical routes has been explored for Li₂S@C synthesis, including ball milling, carbothermal reduction and derived routes, lithiation of preformed S@C nanocomposites, sulfuration of lithium-containing precursors, solution-based infiltration of Li₂S, and the precursor solution infiltration–decomposition method.

Among these routes, ball milling is simple but energy intensive, typically cannot produce Li₂S particles below ∼100 nm and is ineffective for deeply embedding Li₂S into mesoporous hosts. Solution infiltration is straightforward and can be effective for 2D hosts (e.g., graphene), but the low solubility of Li₂S limits infiltration into nanoporous carbons. Carbothermal approaches using inexpensive sulfur-containing compounds can form Li₂S@C in situ and offer scalability potential, although high processing temperature/energy consumption and limited architectural control remain concerns. Sulfuration of lithium compounds using H₂S, CS₂, or sulfur is another promising scalable strategy, but constructing suitable precursor@C architectures and achieving complete conversion require further efforts. Direct lithiation of preformed S@C nanocomposites can yield well-defined nanostructures but commonly relies on hazardous and costly lithiating reagents (e.g., butyllithium), compromising practicality. Notably, the precursor solution infiltration–decomposition method using in situ generated Li₂CS₃ from Li₂S and CS₂ appears low-cost, operationally simple, and potentially scalable. It can produce Li₂S particles with sizes governed by the carbon host pore structure (e.g., ∼30 nm with Super P) and with uniform distribution. Li₂S-LSBs prepared via this route have delivered performance comparable to, or better than, S-LSBs using melt infiltrated S@C, demonstrating the promise of this method for advancing Li₂S-LSBs toward practical implementation.

Beyond conventional liquid-electrolyte systems, recent studies have extended Li₂S cathodes into all-solid-state battery configurations. Unlike liquid-electrolyte Li₂S-LSBs, where suppressing polysulfide dissolution remains a major focus, the development of Li₂S cathodes for ASSLSBs faces additional challenges associated with sluggish solid-state reaction kinetics. Existing studies have mainly focused on confining nanosized Li₂S particles on CNTs and carbon fibers and introducing redox catalysts or mediators to improve ionic/electronic transport, reaction pathways, and conversion kinetics. Nonetheless, the application of Li₂S@C nanocomposites in ASSLSBs remains relatively limited. Rational design of Li₂S@C architectures is expected to provide more effective strategies for improving sulfur utilization and electrochemical performance in all-solid-state systems.

Besides Li₂S@C synthesis, additional bottlenecks must be addressed before Li₂S-LSBs can be commercialized. Cycle stability remains generally inferior to that of S-LSBs, with the polysulfide shuttle still being a dominant degradation pathway. While numerous shuttle-mitigation strategies have been developed for S-LSBs, their implementation in Li₂S cathodes is constrained by Li₂S's moisture sensitivity and chemical reactivity. For example, aqueous-processable binders¹³⁶ are incompatible with Li₂S electrode fabrication, and some functional binders contain groups that readily react with Li₂S.²⁵ Even PVDF, the most widely used stable binder, has been reported to react with Li₂S during slurry preparation, consuming active material and degrading performance.⁴⁶ Consequently, only a limited set of binders has been successfully adopted in Li₂S-LSBs,⁴⁵ and many catalytic or polysulfide-trapping additives effective in S-LSBs cannot be directly incorporated due to undesirable side reactions.

Future progress in Li₂S-LSBs will likely rely on: (i) optimizing existing Li₂S@C synthesis routes and developing more practical, scalable methods; (ii) developing Li₂S-compatible functional binders and additives that simultaneously immobilize polysulfides and accelerate redox kinetics; and (iii) advancing cell-level engineering, including reduced electrolyte-to-sulfur ratios and full-cell configurations with lithium-free anodes. In parallel, advances in solid electrolytes and Li₂S@carbon nanocomposite design may further accelerate the development of all-solid-state Li₂S-LSBs. Future studies should focus on improving solid-state Li₂S reaction kinetics, stabilizing cathode/electrolyte interfaces, and constructing effective ionic/electronic transport networks. Given the currently limited number of reported Li₂S@carbon nanocomposite systems in all-solid-state batteries, further development of nanoscale Li₂S confinement strategies and optimized carbon architectures represents a promising direction for future research. These developments could ultimately provide a pathway toward high stability, improved safety, and high energy density by fundamentally suppressing polysulfide shuttle and enabling efficient solid-state sulfur redox reactions.

Author contributions

Zhe Huang: conceptualization, data curation, formal analysis, investigation, validation, visualization, writing – original draft, writing – review and editing. Yixuan Zhao: data curation, formal analysis. Yonglin Wang: data curation, formal analysis. Yuning Li: conceptualization, formal analysis, funding acquisition, investigation, visualization, project administration, resources, supervision, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

No new data were generated or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgements

The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support through a Discovery Grant (RGPIN-2022-03835) and an Alliance Grant (ALLRP 581429–23).

Notes and references

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