Lithium–sulfur battery

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).

Lithium–sulfur batteries could displace lithium-ion cells because of their higher energy density and lower cost. The use of metallic lithium instead of intercalating lithium ions allows for much higher energy density, as less substances are needed to hold "lithium" and lithium is directly oxidized. Li–S batteries have a high theoretical specific energy (≈2600 Wh/kg for the Li/S redox chemistry), but practical cell-level specific energies in pouch-cell formats are typically ~300–450 Wh/kg today; values above ~400 Wh/kg generally require high sulfur loading, lean-electrolyte operation, and limited excess lithium.

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.

Several issues that have slowed acceptance. One is 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 Li₂S conversion and the large amount of electrolyte needed are also issues.

Progress has been made toward high-stability sulfurized-carbon cathodes. Sulfurized-carbon cathodes (e.g., sulfurized polyacrylonitrile, also known as SPAN) may offer some advantages. Their polysulfide shuttle free feature facilitates proper operation under lean electrolyte conditions (< 3 g·(A·h)⁻¹).

Although Li–S chemistry is attractive for its high theoretical energy density, practical pouch cells require minimizing inactive mass and operating under conditions that resemble commercial batteries. Some practical targets include: (i) high areal sulfur loading (typically ≥5 mg_s cm⁻²) to avoid overestimating capacity in thin electrodes; (ii) lean electrolyte operation, often expressed as electrolyte-to-sulfur ratio E/S ≤5 μL mg/s (or electrolyte-to-capacity ratio E/C ≲5 μL/mAh), because electrolyte can account for a large fraction of pouch-cell mass; and (iii) a controlled negative-to-positive capacity ratio (N/P), since excess lithium metal improves coin-cell cycling but strongly lowers cell-level energy density. These constraints are interdependent: increasing sulfur loading or lowering E/S improves projected energy density, but can also increase polarization and lower reversible capacity if ion/electron transport and interfacial stability are not maintained. Pouch-cell specific energies are often near 400–450 Wh/kg are reached only when these metrics are satisfied simultaneously.