Lithium–sulfur battery

The lithium–sulfur 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.
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, 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 LiS conversion and the large amount of electrolyte needed are also issues.
Progress has been made toward high-stability sulfurized-carbon cathodes. Sulfurized-carbon cathodes may offer some advantages. Their polysulfide shuttle free feature facilitates proper operation under lean electrolyte conditions.
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: high areal sulfur loading to avoid overestimating capacity in thin electrodes; lean electrolyte operation, often expressed as electrolyte-to-sulfur ratio E/S ≤5 μL mg/s, because electrolyte can account for a large fraction of pouch-cell mass; and a controlled negative-to-positive capacity ratio, 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.

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.
The critical parameters needed for achieving commercial acceptance have been described. 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·⁻¹, and negative-to-positive capacity ratio of <5 in pouch-type cells.
They were used on the longest and highest-altitude unmanned solar-powered aeroplane flight by Zephyr 6 in August 2008.

Chemistry

Chemical processes in the Li–S cell include lithium dissolution from the anode surface 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:
In analogy with lithium batteries, the dissolution / electrodeposition reaction causes over time problems of unstable growth of the solid-electrolyte interface, 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

One idealized concept for Li–S batteries, energy is stored in the sulfur cathode. During discharge, the lithium ions in the electrolyte migrate to the cathode where the sulfur is reduced to lithium sulphide. The sulfur is reoxidized to S₈ during the recharge phase. This idealized semi-reaction is therefore expressed as:
In reality the sulfur is reduced not to lithium sulphide but to lithium polysulphides at decreasing chain length according to:
Depending on the battery, the final product is a mixture of Li₂S₂ and Li₂S. This contrasts with conventional lithium-ion cells, where the lithium ions are intercalated in the anode and cathodes. Each sulfur atom can host two lithium ions. Typically, lithium-ion batteries accommodate only 0.5–0.7 lithium ions per host atom. Consequently, LiS allows for a much higher lithium storage density. Polysulfides are reduced on the cathode surface in sequence while the cell is discharging:
Across a porous diffusion separator, sulfur polymers form at the cathode as the cell charges:
These reactions are analogous to those in the sodium–sulfur battery.
The main challenges of LiS batteries is the low conductivity of sulfur and its considerable volume change upon discharging and finding a suitable cathode is the first step for commercialization of LiS batteries. Therefore, carbon/sulfur cathode and a lithium anode are common. Sulfur is very cheap, but has practically no electroconductivity, 5S⋅cm⁻¹ at 25°C. A carbon coating provides the missing electroconductivity. Carbon nanofibers provide an effective electron conduction path and structural integrity, at the disadvantage of higher cost.
One problem with the lithium–sulfur design is that when the sulfur in the cathode absorbs lithium, volume expansion of the Li_xS compositions occurs, and predicted volume expansion of Li₂S is nearly 80% of the volume of the original sulfur. This causes large mechanical stresses on the cathode, which is a major cause of rapid degradation. This process negatively affects the contact between the carbon and the sulfur, and prevents the flow of lithium ions to the carbon surface.
Mechanical properties of the lithiated sulfur compounds are strongly contingent on the lithium content, and with increasing lithium content, the strength of lithiated sulfur compounds improves, although this increment is not linear with lithiation.
One of the primary shortfalls of most Li–S cells is unwanted reactions with the electrolytes. While S and are relatively insoluble in most electrolytes, many intermediate polysulfides are not. Dissolving into electrolytes causes irreversible loss of active sulfur. Use of highly reactive lithium as a negative electrode causes dissociation of most of the commonly used other type electrolytes. Use of a protective layer in the anode surface has been studied to improve cell safety, i.e., using Teflon coating showed improvement in the electrolyte stability, LIPON, Li₃N also exhibited promising performance.

Polysulfide "shuttle"

Historically, the "shuttle" effect is the main cause of degradation in a LiS battery. The lithium polysulfide Li₂S_x is highly soluble in the common electrolytes used for LiS batteries. They are formed and leaked from the cathode and they diffuse to the anode, where they are reduced to short-chain polysulfides and diffuse back to the cathode where long-chain polysulfides are formed again. This process results in the continuous leakage of active material from the cathode, lithium corrosion, low coulombic efficiency and low battery life. Moreover, the "shuttle" effect is responsible for the characteristic self-discharge of LiS batteries, because of slow dissolution of polysulfide, which occurs also in rest state. The "shuttle" effect in a LiS battery can be quantified by a factor f_c, evaluated by the extension of the charge voltage plateau. The factor f_c is given by the expression:
where k_s, q_up, and I_c are respectively the kinetic constant, specific capacity contributing to the anodic plateau, the total sulfur concentration and charge current.
Its initial capacity was 800 Ah/kg. It decayed only very slowly, on average 0.04% each cycle, and retained 658 Ah/kg after 4000 cycles.

Electrolyte

Conventionally, LiS batteries employ a liquid organic electrolyte, contained in the pores of PP separator. The electrolyte plays a key role in LiS batteries, acting both on "shuttle" effect by the polysulfide dissolution and the SEI stabilization at anode surface. It has been demonstrated that the electrolytes based on organic carbonates commonly employed in Li-ion batteries are not compatible with the chemistry of LiS batteries. Long-chain polysulfides undergo nucleophilic attack on electrophilic sites of carbonates, resulting in the irreversible formation of by-products as ethanol, methanol, ethylene glycol and thiocarbonates. In LiS batteries are conventionally employed cyclic ethers or short-chain ethers as well as the family of glycol ethers, including DEGDME and TEGDME. One common electrolyte is 1M LiTFSI in DOL:DME 1:1 vol. with 1%w/w di LiNO₃ as additive for lithium surface passivation.
Solid-state and gel electrolytes are being explored to improve safety and to suppress polysulfide shuttling by immobilizing sulfur species. Sulfide solid electrolytes provide high ionic conductivity and can enable shuttle-free operation, while polymer or composite electrolytes offer better mechanical compliance and wider processing windows. However, solid-state Li–S cells introduce new challenges, including large interfacial resistance at the sulfur/solid-electrolyte boundary and mechanical loss of contact due to the ~80% volume change during S ↔ Li₂S conversion. Recent all-solid-state Li–S prototypes therefore rely on engineered composite cathodes and interface designs to maintain continuous ionic and electronic pathways during cycling.