Insight 08: Lithium-Sulfur Batteries: Lightweight Technology for Multiple Sectors
Summary
Lithium-sulfur technology has the potential to offer cheaper, lighter-weight batteries that also offer safety advantages. After initially finding use in niche markets such as satellites, drones and military vehicles, the technology has the potential to transform aviation in the long-term. Electric aircraft offering short-range flights or vertical take-off and landing (including personalised aviation and flying taxis in cities) are distinct possibilities by 2050. The UK, which is already home to established lithium-sulfur battery manufacturers and to leading academics in the field, has a great opportunity to be the global leader in this ground-breaking technology.
Focus of the Insight
Batteries that extend performance beyond the fundamental limits of lithium-ion (Li-ion) technology are essential for the transition away from fossil fuels. Amongst the most mature of these ‘beyond Li-ion’ technologies are lithium-sulfur (Li-S) batteries. Li-S cells replace the metal rich cathode of Li-ion cells with comparatively cheap and abundant elemental sulfur, a material that also offers the theoretical potential for a five-fold improvement in capacity for the same weight compared with materials widely used in Li-ion cells. By using sulfur, lightweight cells can be produced using more cost-effective materials, while also reducing the environmental and social concerns surrounding the production of nickel and cobalt.
Li-S batteries offer a number of advantages in comparison to current battery technology including (1) an improved gravimetric energy density, (2) a significantly reduced raw materials cost, (3) improved safety characteristics and (4) a reduced environmental burden associated with the cell materials.
Conclusion
Despite being researched since the 1970s, Li-S technology has not achieved the widespread commercialisation of other battery types due to a number of issues, which occur across the cell.
The power offered by Li-S cells has historically been insufficient to enable the benefit of the lightweight nature of the cell to be fully realised. As the sulfur used in the cathode is not electrically conductive, carbon is typically added in relatively high quantities to improve the cells’ performance. Improved strategies to reduce the amount of carbon required and design structured electrodes will improve the power density of Li-S cells. Further challenges to the power of cells arise due to the low solubility of polysulfides in the electrolyte and the rate at which the polysulfide products react during the discharge in current cell designs. To overcome these issues, materials that increase the rate of the reactions can be incorporated into the cathode to promote improved performance while extensive research is underway to improve the electrolytes used.
The cell suffers from a phenomenon known as the polysulfide shuttle effect, in which the polysulfides formed during cell operation shuttle back and forth between the electrodes, causing a loss of active sulfur from the cathode. This shuttle effect is part of a wider challenge with the Li metal anode, which can react with the electrolyte reducing the lifetime of the cell. Indeed, the challenges faced at the interface between the anode and electrolyte are perhaps the most crucial to solve in order to maximise the commercial prospects of Li-S cells. The use of a solid layer to protect the Li anode and the design of materials to ‘trap’ polysulfides and to reduce the shuttle effect are amongst the most promising ongoing research areas.
The sulfur used at the cathode also undergoes significant expansion as it reacts during discharge. This expansion can result in fracturing of the cathode and a loss of electrical connectivity in the electrode, in turn, reducing the number of charge and discharge cycles a cell can undertake. By designing expansion tolerant electrodes, the negative effects of this reaction can be reduced, improving the lifetime of the cell. This approach should also facilitate an increased amount of sulfur in the cathode and consequently a higher cell performance.
Developments in any one of these areas will result in improved cell performance; however, to maximise the benefits offered by Li-S cells improvements must be made across all the components of the battery.