[battery complete conquest](17) 'Half-price battery' Lithium Sulfur, a big hit as soon as it appears

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When discussing next-generation secondary batteries, one staple that invariably appears alongside solid-state batteries is the lithium-sulfur (Li-S) battery. The Ministry of Trade, Industry and Energy’s next-generation secondary battery research and development (R&D) budget, passed by the National Assembly on December 21, includes support for lithium-sulfur battery development alongside solid-state batteries. Kim Dong-myung, the newly appointed CEO of LG Energy Solution, emphasized in his inauguration speech on December 1, "We must lead innovation in future technologies and business models from a long-term perspective," adding, "We will continue to pursue the development of various future technologies such as lithium-sulfur and solid-state batteries."

Lithium-sulfur batteries are secondary batteries that use sulfur as the cathode material and lithium metal as the anode. Sulfur is a byproduct of petroleum refining, making it inexpensive and readily available. Additionally, sulfur is lightweight, which can significantly reduce the battery's weight. For these reasons, various research institutions have listed lithium-sulfur batteries as candidates for next-generation secondary batteries. Lithium-sulfur batteries are expected to have different application areas compared to lithium-ion batteries, with aircraft being a representative field. However, there are many hurdles to overcome before lithium-sulfur batteries can be commercialized. Some predict that it will take longer to commercialize than solid-state batteries.

Affordable yet High-Performance, a Staple of Next-Generation Batteries

The origin of lithium-sulfur batteries dates back to 1962 when Herbert and Ulam filed a patent for a primary battery using lithium as the anode and sulfur as the cathode. Since then, technological progress has been made, especially after the development of secondary batteries using ethers as electrolytes in 1989, leading to active research.

Like other secondary batteries, lithium-sulfur batteries consist of a cathode, anode, electrolyte, and separator. In lithium-sulfur batteries, sulfur is used as the cathode material. Lithium-ion batteries operate on the principle of intercalation, where lithium ions repeatedly insert and extract from the layered structure of the anode. In contrast, lithium-sulfur batteries are based on a chemical reduction reaction chain triggered when lithium ions meet sulfur at the cathode.

Sulfur (S8) fundamentally consists of eight sulfur atoms connected in a ring structure. When discharging begins, lithium metal at the anode releases electrons and transforms into lithium ions. These lithium ions move to the cathode and react with sulfur through a reduction reaction to form Li2S8. The ring of eight sulfur atoms breaks, and lithium ions bind at both ends of the chain.

This chain then changes into Li2S6, composed of six sulfur atoms and two lithium ions. In this way, the chain gradually shortens, producing Li2S4 → Li2S2 → Li2S. The substances formed through this continuous reduction reaction are called lithium polysulfides. During charging, the reverse oxidation reactions occur sequentially: Li2S → Li2S2 → Li2S4 → Li2S6 → Li2S8.

The average voltage of lithium-sulfur batteries is about 2.1 volts (V), lower than lithium-ion batteries (3.7V), but their low density allows for higher capacity per weight. The theoretical capacity of lithium-sulfur batteries reaches 1675 milliampere-hours per gram (mAh/g), about seven times that of lithium-ion batteries (275 mAh/g for NMC811). The theoretical gravimetric energy density is 2600 watt-hours per kilogram (Wh/kg), nearly five times that of lithium-ion batteries. The actual energy density expected upon commercialization is 400?450 Wh/kg.

Another advantage of sulfur is that it is environmentally friendly and very inexpensive. Sulfur is abundantly produced as a byproduct during petroleum refining. Unlike metals such as cobalt and nickel used in current lithium-ion battery cathodes, sulfur production does not cause environmental issues.

Images of large piles of orange sulfur waste at storage sites are easily found on the internet, illustrating how common the material is. According to the Chinese raw material price information site Sensus.com, as of December 25, the spot price of sulfur was only 1,096 yuan per ton (about 199,252 KRW). This amounts to less than 200 KRW per kilogram.

Fraunhofer ISI forecasts that lithium-sulfur batteries could be used in large drones starting in 2035 and in other electric aircraft by 2040. The institute expects that lithium-sulfur batteries will eventually reach a price below 50 euros (about 71,000 KRW) per kilowatt-hour (kWh). Considering Bloomberg's estimate that the average price of lithium-ion batteries in 2023 was 139 dollars (about 178,000 KRW) per kWh, this is less than half the price.

What is the Shuttle Effect... The Obstacle of Lithium-Sulfur Batteries

Lithium-sulfur batteries attract attention for their cost competitiveness, high energy density, and eco-friendliness, but many challenges remain for commercialization. One major issue is their low lifespan and capacity degradation. Lithium polysulfides generated through chemical reactions migrate to the anode (shuttle effect), dissolving in the organic electrolyte. This process gradually reduces the amount of sulfur material in the cathode and shortens battery life.

Additionally, sulfur is a non-conductive material with very low electrical conductivity, requiring separate manufacturing processes to improve performance. The lithium metal used in the anode also causes dendrite formation.

To enhance the electrochemical performance of lithium-sulfur batteries, active research is underway to create composites of sulfur with conductive carbon materials such as carbon, carbon nanotubes, and graphene. Various additives are also introduced into the electrolyte to prevent lithium polysulfide dissolution during discharge.

Recently, polymer solid electrolytes have been applied to lithium-sulfur batteries to suppress the shuttle effect and dendrite formation. This is why there are expectations that lithium-sulfur batteries will be commercialized in the form of solid-state batteries.

Since the 2020s, various companies and academia have published numerous research results. However, it is understood that mass production and commercialization levels have not yet been reached. Considering that it takes an additional 3?4 years from supplying samples to customers to actual commercialization, there is still a long way to go.

When Will Mass Production Begin?

One of the pioneering companies in lithium-sulfur batteries, the UK-based Oxis Energy, announced in April 2021 that after about four years of R&D, it would supply prototypes of solid lithium-sulfur batteries to customers and partners by fall of that year. In 2022, it also set a goal to supply semi-solid batteries with a high energy density of 450 Wh/kg.

However, these plans were not realized. Oxis Energy was sold to Johnson Matthey just three months later, in July 2021. Johnson Matthey stated that the acquisition aimed to expand its green hydrogen business. There was no mention of how lithium-sulfur battery technology would be utilized. Although Oxis Energy claimed to have filed nine patents related to semi-solid and solid-state lithium-sulfur batteries, these did not come to fruition.

Recently, the US startup Lyten has been emerging as a leader in the lithium-sulfur battery field. The company holds stabilization technology for lithium-sulfur batteries using 3D graphene. In September 2023, it secured a total investment of 200 million dollars (about 257.7 billion KRW) from global automotive companies including Stellantis, as well as FedEx and Honeywell. Including this, the company has raised a total of 410 million dollars (528.2 billion KRW) to date.

Various types of lithium-sulfur batteries developed by the US venture company LighTen.

Lyten announced in June 2023 that it had established its first lithium-sulfur battery pilot line in San Jose. The company aims to produce commercial cells by the end of 2023 and supply products to customers in early 2024 to generate revenue. The pilot line plans to produce prototypes for Stellantis and other automakers. Lyten also announced plans to start construction of large-scale 3D graphene and lithium-sulfur battery manufacturing facilities in the US in 2024, along with ambitious plans to build a gigafactory in Luxembourg.

In October, the US venture company Zeta Energy unveiled a lithium-sulfur battery prototype with a capacity of 20 ampere-hours (Ah) and an energy density of 300 Wh/kg. The company stated that its pilot line, scheduled to operate in 2024, will produce products exceeding energy densities of 450 Wh/kg and 800 Wh/L. To prevent the shuttle effect of lithium polysulfides, Zeta Energy explained that it applied lithiated vertical carbon nanotubes to the cathode and patented sulfide carbon technology to the anode.

Lithium-sulfur prototype unveiled by American venture company Zeta Energy in October 2023

In Japan, both government and private sectors are jointly developing lithium-sulfur batteries. GS Yuasa announced in November 2021 that it successfully developed a lithium-sulfur battery with an energy density of 400 Wh/kg. GS Yuasa is conducting research tasks for the "Next-Generation Electric Propulsion System" under the "Next-Generation Aircraft System R&D for Commercialization" project promoted by the New Energy and Industrial Technology Development Organization (NEDO). Lithium-sulfur battery development is part of this research task.

A prototype lithium-sulfur battery unveiled by Japan's GS Yuasa in November 2021.

GS Yuasa, together with researchers from Kansai University, succeeded in developing lithium-sulfur batteries after three years of R&D. The company explained that it applied porous carbon particles supporting sulfur and an electrolyte that suppresses sulfur dissolution.

In South Korea, LG Energy Solution is the most active in lithium-sulfur battery development. Other battery companies such as Samsung SDI and SK On are also understood to be conducting internal R&D. In August 2020, LG Energy Solution successfully conducted an unmanned aerial vehicle experiment equipped with a lithium-sulfur battery prototype in collaboration with the Korea Aerospace Research Institute. The aircraft flew for a total of 13 hours at the highest altitude of the stratosphere.

In January 2023, LG Energy Solution’s Next-Generation Battery Research Center, Professor Jinwoo Lee’s team at KAIST, and Professor Jungwoo Han’s team at POSTECH succeeded in developing a lithium-sulfur battery with improved performance by introducing iron (Fe) atom-based functional materials into the cathode. LG Energy Solution aims to commercialize lithium-sulfur batteries by 2027, a two-year delay from the original 2025 target. It is expected to be first applied in the aviation sector and gradually expanded to other uses.

References

Korea Institute of Science and Technology Information, KISTI Future Promising Technologies Top 10 - Lithium-Sulfur, 2014.12
PR Newswire, OXIS Energy Set To Make Solid-State Lithium-Sulfur Cell Technology A Reality, 2021.4.20
Johnson Matthey, 'Johnson Matthey acquires assets to accelerate green hydrogen scale-up', 2021.7.28
GS Yuasa Press release, GS Yuasa Achieves R&D Milestone in NEDO Advanced Aircraft System Commercialization Project ? Successful demonstration of 400Wh/kg-class lithium-sulfur battery, 2021.11.15
Stellantis Press release, Stellantis Invests in Lyten’s Breakthrough Lithium-Sulfur EV Battery Technology, 2023.5.25
Businesswire, Lyten Raises $200M in Series B Equity Round, 2023.9.12
Bloomberg, Lyten Plans US, Europe Gigafactories to Spread ‘Cleaner’ Battery, 2023.10.31
PR Newswire, Zeta Energy Demonstrates Industry-Leading Progress in Lithium-Sulfur Batteries, 2023.10.23
Fraunhofer ISI, Alternative Battery Technologies Roadmap 2030+, 2023.9
Korea Institute of S&T Planning and Evaluation (KISTEP), National Strategic Technology Sovereignty Brief: Next-Generation Secondary Batteries, 2023.11.21
Wikipedia, Lithium-sulfur battery