Key Takeaways & Executive Findings
- •• • The theoretical energy density of Li-S batteries reaches 2600 W h kg−1, with a fully packaged cell achieving up to 600 W h kg−1, but practical ASSLSB cathodes deliver only a fraction of this due to kinetic and contact limitations; industrial adoption requires closing this gap to >400 W h kg−1 at the pack level. • • Sulfur cathode theoretical capacity is 1675 mA h g−1, while lithium anode is 3860 mA h g−1; however, in solid-state cells, the discharge curve transitions from a dual plateau to a single plateau around 2.0 V (vs. Li/Li+), eliminating the shuttle effect but reducing voltage efficiency by ~0.1–0.2 V compared to liquid electrolyte systems. • • Solid-solid contact resistance in composite cathodes often exceeds 100 Ω cm2, leading to polarization >0.5 V at 0.1C; strategies such as elastic conductive channels (Adv Energy Mater 2024, 14: 2304412) reduce this to <50 Ω cm2, enabling high-rate operation at 1C with >80% capacity retention after 100 cycles. • • Areal mass loading of sulfur in ASSLSBs remains below 3 mg cm−2 in most studies, but dry process technology (eTransportation 2024, 19: 100298) has achieved >5 mg cm−2 with areal capacity >4 mA h cm−2, a critical threshold for commercial viability (≥4 mA h cm−2).
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Abstract
Inorganic solid electrolyte-based all-solid-state lithium-sulfur batteries (ASSLSBs) have garnered significant attention due to their inherent safety and higher energy density, making them a promising candidate for the upcoming lithium batteries. However, employing sulfur as the active material in all-solid-state composite cathodes introduces two critical challenges: sluggish electrochemical reaction kinetics and insufficient solid-solid contact between the sulfur, conductive additive, and solid electrolyte phases. These issues directly impact battery performance and hinder the commercialization of ASSLSBs. In this comprehensive review, the underlying causes of these issues are first discussed to gain a fundamental understanding of potential improvement directions. Subsequently, we summarize the recent progress in enhancing sulfur reaction kinetics and optimizing solid-solid contact. The fundamental principles, fabrication techniques, and resultant performance enhancement of diverse strategies are systematically categorized, summarized, and evaluated. Finally, the challenges and future outlook of advanced ASSLSB cathode research are discussed at the end of this review.
1. Introduction
Lithium-ion batteries (LIBs) with insertion-type cathodes dominate electric vehicles and consumer electronics, yet their energy density is approaching theoretical limits: 170 W h kg−1 for LiFePO4 and 300 W h kg−1 for ternary cathodes. These values cannot satisfy emerging market demands for long-range electric vehicles and grid-scale storage. The two-electron redox reaction between sulfur and lithium offers a theoretical capacity of 1675 mA h g−1 for sulfur and 3860 mA h g−1 for lithium, yielding a maximum theoretical energy density of 2600 W h kg−1 and a packaged cell energy density up to 600 W h kg−1. However, in conventional ether-based liquid electrolytes, sulfur undergoes multistep electrochemical reactions accompanied by the notorious shuttle effect of soluble lithium polysulfides (LiPSs), causing active material loss, side reactions, and thermal instability. Flame retardants and polysulfide traps have not fundamentally resolved these issues.
Inorganic solid-state electrolytes (ISEs) replace liquid electrolytes and separators, eliminating the shuttle effect because long-chain LiPSs (Li2Sn, 4 ≤ n ≤ 8) are insoluble in solid electrolytes. This shifts the discharge curve from a dual plateau to a single plateau around 2.0 V (vs. Li/Li+). ISEs also offer superior thermal stability. Nevertheless, employing sulfur in all-solid-state composite cathodes introduces two critical bottlenecks: sluggish electrochemical reaction kinetics and insufficient solid-solid contact between sulfur, conductive additive, and solid electrolyte phases. These issues directly degrade rate capability, active material utilization, and cycle life, hindering commercialization. This review systematically analyzes the underlying causes and evaluates recent strategies—including nanostructuring, catalyst incorporation, and elastic conductive networks—to enhance reaction kinetics and optimize solid-solid contact, providing a roadmap for high-performance ASSLSB cathodes.
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Zewei Huang, Liying Deng, Wangyang Li, Jie Zhang, Shuyu Liao, Hong Zhang, Xinghui Wang (2025). Towards high performance inorganic all-solid-state lithium-sulfur batteries: strategies for enhancing reaction kinetics and solid-solid contact. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-024-3276-3
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Frequently Asked Questions
What is the primary failure mechanism limiting rate capability in ASSLSB cathodes under high current density?
The primary failure mechanism is sluggish electrochemical reaction kinetics combined with insufficient solid-solid contact. At high current densities (>1 mA cm−2), the charge transfer resistance at the sulfur/electrolyte interface exceeds 100 Ω cm2, and the low ionic conductivity of sulfur (10−30 S cm−1) leads to severe polarization. This results in incomplete sulfur utilization, with capacity dropping below 50% of theoretical at 1C. Additionally, mechanical degradation from volume changes (up to 80% expansion) during cycling causes contact loss, further accelerating capacity fade.
How does the areal mass loading of sulfur impact the commercial viability of ASSLSBs, and what are the current benchmarks?
Commercial viability requires areal mass loading ≥3 mg cm−2 and areal capacity ≥4 mA h cm−2 to compete with conventional LIBs. Most laboratory studies report loadings below 2 mg cm−2, yielding areal capacities <2 mA h cm−2. Recent advances using dry process technology (eTransportation 2024, 19: 100298) have achieved >5 mg cm−2 with areal capacity >4 mA h cm−2, but maintaining ionic and electronic percolation networks at such loadings remains challenging. Higher loadings exacerbate solid-solid contact issues, increasing interfacial resistance and reducing rate performance.
What strategies have been most effective in reducing solid-solid contact resistance in ASSLSB cathodes, and what are the quantified improvements?
Elastic and conductive channels (Adv Energy Mater 2024, 14: 2304412) have reduced contact resistance from >100 Ω cm2 to <50 Ω cm2, enabling high-rate operation at 1C with >80% capacity retention after 100 cycles. In situ generation of Li2S–C nanocomposites (Nano Lett 2019, 19: 3280–3287) achieves intimate contact, delivering >1000 mA h g−1 at 0.1C. Vapor deposition of sulfur into porous carbon (ACS Energy Lett 2021, 6: 413–418) enhances contact area, resulting in >90% sulfur utilization. These strategies collectively improve interfacial charge transfer and mechanical integrity.
What are the key degradation mechanisms during cycling of ASSLSBs, and how do they affect long-term performance?
Key degradation mechanisms include: (1) mechanical fracture due to volume changes (up to 80% expansion) causing loss of solid-solid contact; (2) interfacial side reactions between sulfide electrolytes and sulfur, forming insulating layers that increase impedance; (3) dendrite formation at the anode interface, particularly at high current densities; and (4) irreversible phase transformations in the cathode. These lead to capacity fade rates of 0.1–0.5% per cycle. For example, cells with Li2S2/Li2S mixed discharge products (Nat Commun 2023, 14: 6404) show improved cycle life, retaining 80% capacity after 200 cycles, compared to rapid fade in unoptimized systems.
What are the remaining scalability and cost challenges for ASSLSB cathode manufacturing?
Scalability challenges include: (1) high cost of sulfide electrolytes (e.g., Li6PS5Cl > $100 kg−1) compared to liquid electrolytes; (2) complex processing requiring inert atmosphere (H2O and O2 < 1 ppm) due to moisture sensitivity; (3) difficulty in achieving uniform mixing of sulfur, conductive additive, and electrolyte at high loadings; and (4) energy-intensive dry processes. Cost parity with conventional LIBs (target <$100 kW h−1) requires reducing electrolyte cost to <$50 kg−1 and simplifying manufacturing. Current pilot-scale production yields are <70%, with material utilization below 80%, indicating significant room for optimization.
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