Lithium Sulfur (Li-S) Batteries – Insights about technology

Lithium Sulfur battery, as the name suggests, is a type of battery consisting of a Lithium metal anode and Sulfur cathode. It uses a liquid organic electrolyte, which is similar to the traditional Lithium-ion batteries. This article by Rahul Bollini discusses the basics of a Li-S battery.

Working principle of Li-S batteries

Li-S works with a shuttling effect principle, where the Lithium metal anode oxidizes (loses electrons) and forms Lithium-ions, which travel through the electrolyte towards the Sulfur cathode. They form intermediate lithium polysulfides that shuttle between anode and cathode hence, the name shuttling effect.

Why Sulfur cathode?

Sulfur has high energy density, low cost, is abundantly available and is considered to have a lower environmental footprint. For better performance, Sulfur composite such as Sulfur-carbon material is used.

Why Lithium metal anode?

Lithium metal has very high theoretical specific capacity, which is 3860 mAh/g. In comparison,  graphite used in traditional Lithium-ion batteries has a theoretical specific capacity of 372 mAh/g.

Facts about Li-S batteries

• Li-S full cell voltage is close to 2V, but the high specific capacity of cathode and anode materials used actually brings the final gravimetric energy density (Wh/Kg) above traditional Lithium-ion batteries.
• 2V cell voltage cell means a higher number of series configuration BMS is required for battery pack assembling.
• Today’s developments of Li-S are showing a volumetric energy density (Wh/L) below that of NMC Lithium-ion batteries. But the gravimetric energy density is much above 300Wh/Kg, clearly surpassing all types of Lithium-ion batteries mass manufactured today.

Why have Li-S batteries not taken off yet?

In simple words, Li-S batteries have not yet achieved the acceptable cycle life to be used for commercial purposes. It is due to the following challenges:

• Sulfur material has poor conductivity, which can slow down the kinetics of electrochemistry.
• Sulfur cathode exhibits high irreversible loss, poor cycling stability, low coulombic efficiency and high internal resistance.
• Sulfur cathode’s volume expands during charge and discharge cycles, and this exerts pressure on the electrode causing damage to the cell architecture.
• Lithium metal anode is prone to dendrite formation, where the dendrites from anode penetrate through the separator and eventually reach the cathode and short circuit the fires. This is one of the major concerns when dealing with Lithium metal.
• There is a search for a better electrolyte that slows the pace of irreversible capacity fade during cycling.

Developments and future of Li-S batteries

There are a few companies extensively working on Li-S batteries. Like most researchers working on Li-S, one of the companies I know is able to reach between 100 and 200 charge-discharge cycles with 80% retention capacity. This company hopes to solve the faster capacity degradation issue going forward. However, some companies have given up after years of hard work and huge fundraising. One of the notable companies that worked on Li-S and failed to deliver was a UK-based company, Oxis Energy.

Eventually, the cycle life will be improved in the coming times, and a few companies will set up manufacturing plants at a pilot scale. It is very difficult to say when because no company is close to commercialization as of today. Since Li-S has a higher gravimetric energy density (Wh/Kg) than traditional Lithium-ion batteries, it would be an ideal fit in drone applications and open up the gates to short-range electric planes. When ready, it would also be used to power many other applications where weight is a luxury.

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This article was originally published in EVReporter Feb 2023 Magazine that can be accessed here.