The Promise and Challenges of Lithium-Sulfur Batteries in the Quest for Sustainable Energy Storage

Lithium-ion batteries (LIBs) have been at the forefront of energy storage solutions due to their high energy density, long cycle life, and minimal environmental impact compared to alternatives technologies such as nickel-cadmium and nickel-metal hydride batteries. Since their commercial introduction in 1991, LIBs have found innovative applications in electronic devices, as well as in the power sources for electric vehicles and railways. Their advantage of versatility and high performance have contributed significantly to the development of sustainable electric power infrastructure.

The increasing demand for batteries, driven primarily by the rapid adoption of electric vehicles (EVs), has led to rising prices for raw materials used in LIBs. Of particular concern is the impact of political instability on the production areas of key raw materials, which has triggered economic sanctions and trade restrictions. As a result, prices for battery-related minerals, including lithium, nickel, and cobalt, have surged, creating uncertainty in the clean energy sector. For instance, Russia, a major producer of high-purity nickel and cobalt, has faced economic sanctions due to geopolitical conflicts. This has further exacerbated the scarcity and cost volatility of these critical materials. These rising prices have the potential to hinder or delay the global transition toward clean energy strategies, putting the realization of environmental and climate goals at risk.

Indeed, the recent political turmoil and economic sanctions affecting major producers of these raw materials have led to soaring prices, creating an urgent need to explore alternative energy storage solutions. In a new important paper published in Journal of Power Sources by Dr. Natsuki Nakamura, Professor Toshiyuki Momma, Senior Professor Tetsuya Osaka from Waseda University in collaboration with Dr. Seongki Ahn from Hankyang National University provided expert opinion on the recent developments in lithium-sulfur (Li-S) batteries as a promising alternative to LIBs. They reviewed the principles behind Li-S batteries, discussed the challenges they face, and highlights the potential they hold for revolutionizing energy storage.

The authors explored the potential of Li-S batteries, which utilize sulfur, an abundant and cost-effective cathode active material. According to the authors Li-S batteries offer several key advantages: Firstly, high weight energy density: Li-S batteries have the potential to provide a high energy density, making them suitable for applications like battery electric vehicles (BEVs) and lightweight electric aircraft. Secondly, low cost: Sulfur is an abundant non-metal, making it a cost-effective alternative to rare metals. Thirdly, the potential for high in/output: Li-S batteries can meet the in/output performance requirements of BEVs and other high-energy-demand applications.

Understanding the mechanism of Li-S batteries is crucial to addressing their challenges and harnessing their potential. Li-S batteries rely on redox-active organic sulfur-based materials with disulfide bonds as cathode materials. These materials offer promising features such as weight reduction and high energy density. The authors discussed in detail the three main types of organic sulfur-based materials studied for energy storage: namely organodisulfide compounds: These compounds exhibit good redox reactions and relatively fast electrode reaction rates, but their electrochemical reaction can be slow, requiring high operating temperatures. Carbon sulfide compounds: Carbon sulfides offer a high theoretical capacity but face challenges related to ion and electron conductivities, as well as volume expansion during charging and discharging. Dissolution of polysulfides: A critical drawback in Li-S batteries is the dissolution of intermediate polysulfides into the electrolyte, leading to reduced active material utilization and rapid capacity deterioration. This phenomenon, known as the shuttle effect, must be mitigated for practical Li-S battery applications.

The authors provided their expert opinion on several challenges and methods to address to realize the full potential of Li-S batteries. The challenge of low ion and electron conductivities: Sulfur is inherently insulating, necessitating the use of conductive materials to facilitate electron transport. Various carbon-based composites have been explored for this purpose. Another challenge is the large volume expansion of sulfur because sulfur undergoes significant volume changes during charging and discharging, leading to capacity loss and safety concerns. According to the authors, innovative structural design and materials are being investigated to mitigate this issue. With regard to the dissolution of polysulfides and to combat the shuttle effect, various methods have been developed, including compositing with carbon materials, polymer coating, and gel/polymer electrolytes. Another important limitation is the high self-discharge rate: Li-S batteries suffer from a high self-discharge rate, leading to rapid capacity deterioration. Strategies such as surface modification and electrolyte additives are being explored to improve self-discharge performance. About the concern of hazard of metallic lithium due to the fact that metallic lithium poses safety challenges due to its reactivity with water and tendency to form dendritic structures during cycling. Protective coatings and pretreatment methods are under investigation to address these issues.

Material design plays a crucial role in improving the performance and reliability of Li-S batteries. Key considerations include the choice of carbon materials, pore structure, sulfur content, and strategies to control polysulfide dissolution. Mesoporous carbon frameworks have shown promise in providing high sulfur utilization and stability. Additionally, nitrogen doping of carbon materials has been effective in enhancing electron conductivity and suppressing polysulfide dissolution. Various synthesis methods, such as melt diffusion, ball-milling, and gas-phase injection, have been explored to create carbon-sulfur composites with optimal properties.

In conclusion, Professor Tetsuya Osaka and colleagues provided a thorough comprehensive report on the pursuit of sustainable energy solutions and how it led to the exploration of lithium-sulfur batteries as a viable alternative to conventional lithium-ion batteries. The authors acknowledged while Li-S batteries offer numerous advantages, they also face significant challenges related to low conductivity, volume expansion, polysulfide dissolution, self-discharge, and safety concerns. In a statement to Advances in Engineering, corresponding and lead author Professor Tetsuya Osaka said “The development of lithium-sulfur batteries has the potential to reshape the energy storage landscape by providing cost-effective and environmentally friendly solutions”. Researchers are actively addressing these challenges through innovative materials design, structural improvements, and electrolyte innovations.