Sulfur: The Industry Status - part 1
This week we’ve got Roman Healy back with an industry starter pack of what’s going on with lithium sulfur, why it matters and who the players are. Roman is a PhD researcher at AK Adelhelm (Humboldt University of Berlin) studying the interaction between carbon and sulfur to prepare improved cathodes for metal-sulfur batteries.
Lithium–sulfur batteries (LSBs) are a type of rechargeable battery consisting of a cathode with sulfur, an electrolyte containing a lithium salt dissolved in ether or carbonate-based solvents, and a lithium metal anode.
Unlike conventional Li-ion batteries, which rely on intercalation of lithium into a host material, LSBs operate through a conversion reaction. In this process, elemental sulfur (S8) chemically reacts with lithium to form lithium sulfide (Li2S) during discharge. This transformation is not a direct reaction as it proceeds through several soluble lithium polysulfide (LP) intermediates, which, as we will discuss later, represent one of the major challenges in LSB development, known as the shuttle effect.
Graphite is generally not used in LSBs because it does not contain lithium. As a result, using graphite would require employing Li₂S instead of S₈ in the cathode. However, Li₂S is challenging to handle on a large scale. Both S₈ and Li₂S are insulating materials and they undergo significant volume expansion during cycling. Li₂S has a melting point of 950 °C, compared to approximately 113 °C for S₈. This high melting point makes melt infiltration, a common method for preparing Li₂S-containing electrodes, impractical at scale. Consequently, achieving a well-mixed and homogeneous cathode becomes increasingly difficult.
Furthermore, graphite is incompatible with ether-based electrolytes commonly employed in LSBs, as these solvents form strong lithium-ion complexes that can co-intercalate into the graphite layers and disrupt its structural integrity
Why sulfur?
Sulfur is arguably one of the most extensively studied elements in the periodic table due to its rich and cross-disciplinary chemistry. In the field of energy storage, LSBs have attracted significant interest since their initial patent by Herbert and Ulam in 1960. The appeal of this chemistry stems primarily from its low cost and exceptional theoretical performance.
Economically, sulfur is abundant and inexpensive, as it is a byproduct of the petroleum refining industry. From a performance standpoint, LSBs offer impressive theoretical specific capacity of 1675 mAh/g (in practice only about 800–1200 mAh/g), which is significantly higher than the practical capacity of commercial Li-ion battery cathodes like NMC (160–200 mAh/g) or LFP (160 mAh/g). In terms of energy density, LSBs could potentially reach 500–1000 Wh/kg, compared to the practical energy densities of commercial Li-ion batteries, which typically range from 150 to 300 Wh/kg depending on the chemistry and configuration.
Reading about these benefits naturally raises the question: why has this technology not yet reached commercialization? The bottlenecks hindering LSBs from reaching this stage can be categorized into fundamental scientific challenges and commercialization demands.
Fundamental challenges
Formation of lithium metal dendrites, which are needle-like structures, that can pierce the separator and cause short circuits. This problem is more pronounced in LSBs due to the use of lithium metal, which undergoes plating and stripping during cycling. This process can result in uneven lithium deposition and create hotspots that promote dendrite growth. In contrast, conventional Li-ion batteries typically use host materials such as graphite, where lithium ions intercalate and deintercalate, thereby minimizing dendrite formation. This is a problem for any batteries needing a Li-metal anode, and not just sulfur.
Both elemental sulfur and its discharge product, Li2S, are inherently insulating and electrochemically inactive. This limits electron transport through the active material, unless conductive additives are used. Furthermore, Li2S coats the sulfur particles, reducing the electrochemically active surface area. Overall, these factors directly affect charge transport and reaction kinetics.
The previously mentioned lithium polysulfide (LP) shuttle effect involves the dissolution of intermediate LPs into the electrolyte, followed by their migration to the anode where they react, resulting in a loss of active material.
Conversion of sulfur to Li2S involves an 80% volume expansion, which can lead to mechanical stress, loss of contact in the cathode, and further capacity degradation.
Commercialization demands
From an industrial perspective, commercialization requires the technology to meet several key specifications. These are essential to maximising volumetric and gravimetric energy densities, as they tackle issues like minimization of inactive components such as excess electrolyte or conductive additives, therefore increasing areal capacities while reducing weight and volume.
Sulfur loadings of at least 5 mg/cm².
Sulfur utilization rates above 80%.
Electrolyte-to-sulfur ratios below 5 µL/mg.
Sulfur fractions in the cathode of at least 70%.
To address these fundamental issues, numerous strategies have been explored. For example, the use of carbonaceous or other conductive hosts has shown promise in mitigating the poor conductivity of sulfur and enhancing its utilization. To suppress polysulfide shuttling, researchers have developed catalysts (such as TiO2 or MoS2) and employed functional separators capable of trapping polysulfides. To address the problem of volume expansion porous carbon matrices and/or binders engineered with suitable mechanical resilience have been demonstrated.
Thanks to such intensive research efforts, several companies have emerged with innovative cathode and anode designs aimed at driving Li-S battery technology toward commercialization.
In the next installment, we will discuss 5 companies in depth, providing insights into their developments and assessing their competitive potential.
Summary
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