In 2008, rechargeable lithium-sulfur (Li-S) batteries were used in a solar-powered airplane demo flight. Known to some scientists as the cousin of lithium-ion (Li-On) batteries, Li-S batteries have higher energy density, lighter weight and are cheaper in comparison to Li-On. Although not yet commercially available, Li-S cells are said to be the next state-of-the-art batteries that will soon power up our electronic devices and electric vehicles.
Li-S Pouch Cell
Image Source: OXIS Energy Website
Basic Features and Characteristics
Lithium-sulfur batteries use sulfur mainly on its cathode. Sulfur is a chemical element that usually comes from waste products of petroleum processing making Li-S a cheaper alternative for secondary cells. Because of the sulfur’s light atomic weight, it can be packed more than twice as densely as lithium ions in a given volume, and so its energy density is several times better than that of Li-On batteries. In general, Li-S batteries have higher storage capacity, cheaper to manufacture, and they are less toxic than conventional Li-On batteries.
Element – Sulfur
Image Source: PHYS.ORG Website
Chemical Properties and Processes
During discharging, a chemical process known as lithium dissolution occurs on the anode surface of Li-S cells. During charging, the reverse lithium plating that occurs on the same anode surface. The property of the sulfur atom to accommodate 2 lithium ions (only 0.5 to 0.7 for Li-On), which gives Li-S battery the higher lithium storage density compared to Li-On.
Design Challenges and Developments
One reason why lithium-sulfur batteries are not yet commercially available is safety considerations. Sulfur easily combines with lithium to form compounds that crystallize and gum up the battery’s cathode. This event occurs repeatedly during the charge and discharge cycles. This causes mechanical stress on the cathode which leads to rapid degradation and in worst cases, cracks on the battery. Another problem scientists are seeing is the occurence of unwanted chemical reactions between sulfur and the electrolytes used. When Li-S is combined with some type of electrolytes, irreversible reaction causes loss of active sulfur.
At present, research and development on improving the performance of Li-S batteries are on-going. In Germany and Canada, a study focuses on the surface-to-volume ratio of sulfur. Its objective is to enhance the efficiency of the electrochemical processes that occur in the course of charge-discharge cycles in Li-S battery. A company called Oxis is currently improving the energy density, safety features and cycle life of Li-S cells. In terms of safety, they use a ceramic lithium sulfide passivation layer and a non-flammable electrolyte in order to withstand extreme abuse situations. In addition, they claim that their Li-S cells products will have 100% available depth-of-discharge, indefinite shelf-life and are eco-friendly.
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