Lithium-sulfur is already being developed on several continents and is expected to reach mass production by 2033. IDTechEx forecasts that, by 2035, the lithium-sulfur market will exceed US$1.3 billion.
Degradation Modes:
Lithium-sulfur batteries have been the subject of development efforts in the past. However, their chemistry has been limited as a result of an intrinsic degradation method: polysulfide shuttle. Polysulfides of the Li₂Sx form are produced at the cathode and transported to the electrolyte, causing leaching of the active materials. These polysulfides can also reach the anode and initiate their own redox reactions, reducing the cell's effective redox potential. Polysulfides can also form an insoluble Li₂S layer at the anode, preventing ion transport. The overall effect of polysulfide shuttle is to significantly reduce the cell's coulombic efficiency, severely impacting battery life.
The formation of metallic lithium dendrites is also a problem, although it tends to be less significant than polysulfide shuttle degradation. Lithium dendrites form at the anode and leach into the electrolyte, reacting irreversibly and reducing the cell's active material. Furthermore, during charging and discharging, the sulfur cathode undergoes significant swelling, up to 80% during discharge. This places considerable stress on the cathode's architecture and can reduce the overall cell's contact conductivity due to crack formation and nucleation.
Solutions to degradation
Polysulfide transport can be countered in several ways. The most obvious approach may be the use of a solid electrolyte, as this prevents polysulfide transport. However, this can lead to a significant reduction in conductivity at the interface between the electrolyte and the cathode, since sulfur is a poor conductor. Alternative liquid electrolytes are a more attractive option. Polysulfides are soluble in the liquid electrolytes used in lithium-ion batteries with graphite anodes. However, there are other solutions in which polysulfides are not soluble, such as cyclic ethers, short-chain ethers, and glycol ethers.
Alternatively, a separator layer or membrane can be used to prevent the transport of polysulfides. The chosen membrane must be selective, allowing the passage of lithium ions but not polysulfides. This can be achieved in three ways:
Charge repulsion: taking advantage of the fact that polysulfides are negatively charged and lithium ions are positively charged.
Shielding: physically blocking the ions from coming into direct contact with the active materials, thus preventing polysulfide leaching (for example, using carbon nanotubes). Since polysulfides are much larger than lithium ions, they can be selectively blocked.
Adsorption: trapping the polysulfides at the cathode using materials that attract them. Micropores can be used for this method.
Cathode expansion can be addressed by using alternative cathode structures, such as expansion-tolerant lattices or stronger binders. Alternative materials can enable the development of single-material structures without binders, significantly improving the current collector's rigidity. One example is sulfurized polyacrylonitrile, or SPAN.
Key points
Lithium-sulfur batteries have attracted the interest of major players, notably LG Chem, though they have remained silent since then. Now, the industry is dominated by startups poised to bring their products to market. Here are a few examples:
Lyten began as a developer of 3D graphene materials but has since focused on using its materials for lithium-sulfur batteries. The 3D graphene is used as a substrate for the sulfur cathode, enabling higher conductivity and reducing the polysulfide shuttle effect. The company aims to commercialize it for aerospace and marine applications by the end of 2025 and already has plans underway for a gigafactory.
theion is a German startup developing sulfur cathodes formed from a wafer of pure sulfur. The wafers are grown directly from molten sulfur. The technology is still in the development stage, but the company is leveraging an AI platform for battery testing to significantly reduce time to market.
Li-S Energy is an Australian company that works with lithium-sulfur. The company uses boron nitride nanotubes (BNNTs) to form the cathode structure. This improves strength and conductivity. Li-S recently completed the construction of a 2 MWh bag cell production plant.
Lithium-sulfur's higher specific energy, but lower energy density, makes it particularly suitable for aviation, defense, and maritime applications, especially in unmanned aerial vehicles (UAVs) or drones. However, this chemistry is also expected to be used in electric vehicles, particularly heavy-duty electric vehicles. IDTechEx forecasts that more than 14 GWh will be produced globally by 2035.
Author: Daniel Parr, technology analyst at IDTechEx
