The lithium–sulfur battery (Li–S battery) is a type of rechargeable battery. It is notable for its high specific energy. The low atomic weight of lithium and moderate atomic weight of sulfur means that Li–S batteries are relatively light (about the density of water). They were used on the longest and highest-altitude unmanned solar-powered aeroplane flight (at the time) by Zephyr 6 in August 2008.
Lithium–sulfur batteries may displace lithium-ion cells because of their higher energy density and reduced cost. This is due to the use of sulfur instead of cobalt, a common element in lithium-ion batteries. Li–S batteries offer specific energies on the order of 550Wh/kg, while lithium-ion batteries are in the range of 150260Wh/kg.
Li–S batteries with up to 1,500 charge and discharge cycles were demonstrated in 2017, but cycle life tests at commercial scale and with lean electrolyte have not been completed. As of early 2021, none were commercially available.
Issues that have slowed acceptance include the polysulfide "shuttle" effect that is responsible for the progressive leakage of active material from the cathode, resulting in too few recharge cycles. Also, sulfur cathodes have low conductivity, requiring extra mass for a conducting agent in order to exploit the contribution of active mass to the capacity. Volume expansion of the sulfur cathode during S to LiS conversion and the large amount of electrolyte needed are also issues.
History
LiS batteries were invented in the 1960s, when Herbert and Ulam patented a primary battery employing lithium or lithium alloys as anodic material, sulfur as cathodic material and an electrolyte composed of aliphatic saturated amines. A few years later the technology was improved by the introduction of organic solvents as PC, DMSO and DMF yielding a 2.35-2.5 V battery. By the end of the 1980s a rechargeable LiS battery was demonstrated employing ethers, in particular DOL, as the electrolyte solvent.
In 2020 Manthiram identified the critical parameters needed for achieving commercial acceptance. Specifically, Li-S batteries need to achieve a sulfur loading of >5 mg cm−2, a carbon content of <5%, electrolyte-to-sulfur ratio of <5 μL mg−1, electrolyte-to-capacity ratio of <5 μL (mA h)−1, and negative-to-positive capacity ratio of <5 in pouch-type cells.
In 2021, researchers announced the use of a sugar-based anode additive that prevented the release of polysulfide chains from the cathodes that pollute the anode. A prototype cell demonstrated 1,000 charge cycles with a capacity of 700 mAh/g.
In 2022, an interlayer was introduced that claimed to reduce polysulfide movement (protecting the anode) and facilitate lithium ion transfer to reduce charge/discharge times. Also that year, researchers employed aramid nanofibers (nanoscale Kevlar fibers), fashioned into cell membrane-like networks. This prevented dendrite formation. It addressed polysulfide shuttle by using ion selectivity, by integrating tiny channels into the network and adding an electrical charge.
Also in 2022, Researchers at Drexel University produced a prototype lithium-sulfur battery that did not degrade over 4000 charge cycles. Analysis has shown that the battery contained monoclinic gamma-phase sulfur, which has been thought to be unstable below 95 degrees Celsius, and only a few studies have shown this type of sulfur to be stable longer than 20 to 30 minutes.
Chemistry
Chemical processes in the Li–S cell include lithium dissolution from the anode surface (and incorporation into alkali metal polysulfide salts) during discharge, and reverse lithium plating to the anode while charging.
Anode
At the anodic surface, dissolution of the metallic lithium occurs, with the production of electrons and lithium ions during the discharge and electrodeposition during the charge. The half-reaction is expressed as:
Li <=> Li+ + e-
In analogy with lithium batteries, the dissolution / electrodeposition reaction causes over time problems of unstable growth of the solid-electrolyte interface (SEI), generating active sites for the nucleation and dendritic growth of lithium. Dendritic growth is responsible for the internal short circuit in lithium batteries and leads to the death of the battery itself.
Cathode
In Li-S batteries, energy is stored in the sulfur cathode (S8). During discharge, the lithium ions in the electrolyte migrate to the cathode where the sulfur is reduced to lithium sulphide (Li2S). The sulfur is reoxidized to S8 during the recharge phase. The semi-reaction is therefore expressed as:
S + 2Li+ + 2e- <=> Li2S (E ° ≈ 2.15 V vs Li / Li+ )