The lithium–sulfur battery (Li–S battery) is a type of rechargeable battery, 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.Amos, J. (24 August 2008) "Solar plane makes record flight" BBC News
Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and reduced cost due to the use of sulfur instead of  cobalt, which is commonly used in lithium-ion batteries.
Some Li–S batteries offer specific energies of the order of 550Wh/kg, while most 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, In 2017: "can be cycled approximately 1500 times ... In the next 2 years, we expect this to reach 2500 cycles".
In 2021: "Within the next two years we aim to double the current cycle life to achieve upwards of 500 cycles" but cycle life tests at commercial scale and with lean electrolyte are still needed.
As of early 2021, none were commercially available.
The key issue of LiS battery is the polysulfide "shuttle" effect that is responsible for the progressive leakage of active material from the cathode resulting in low life cycle of the battery.
Moreover, the extremely low electrical conductivity of a sulfur cathode requires an extra mass for a conducting agent in order to exploit the whole contribution of active mass to the capacity.
Large volume expansion of sulfur cathode from S to LiS and the large amount of electrolyte needed are also issues to address.
History
The invention of LiS batteries dates back to the 1960s, when Herbert and Ulam patented in 1962, a primary battery employing lithium or lithium alloys as anodic material, sulfur as cathodic material and an electrolyte composed of aliphatic saturated amines.US 3043896A A few years later the technology was improved by the introduction of organic solvents as PC, DMSO and DMF obtaining 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 solvent for the electrolyte.
Thanks to scientific improvements in the field, the potential of LiS batteries was highlighted.
LiS batteries have experienced in the last twenty years a renewed and growing popularity.
In particular, strategies for inhibition or mitigation of the polysulfide "shuttle" effect have been deeply investigated and studied by many researchers.
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.
, 700 related publications had appeared.
In 2021, researchers announced the use of a sugar-based anode additive that prevented the release of polysulfide chains from the cathode that pollute the anode.
A prototype cell demonstrated 1,000 charge cycles with a capacity of 700 mAh/g.
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.Tudron, F.B., Akridge, J.R., and Puglisi, V.J. (2004) "Lithium-Sulfur Rechargeable Batteries: Characteristics, State of Development, and Applicability to Powering Portable Electronics" (Tucson, AZ: Sion Power) 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+ )
Actually the sulfur reduction reaction to lithium sulphide is much more complex and involves the formation of lithium polysulphides (Li2Sx, 2 ≤ x ≤ 8) at decreasing chain length according to the order:
Li2S8->Li2S6->Li2S4->Li2S2->Li2S
The final product is actually a mixture of Li2S2 and Li2S rather than pure Li2S, due to the slow reduction kinetics at 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, most researchers use a carbon/sulfur cathode and a lithium anode.
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 LixS 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 reduces the contact between the carbon and the sulfur, and prevents the flow of lithium ions to the carbon surface.Brian Dodson, "New lithium/sulfur battery doubles energy density of lithium-ion", NewAtlas, 1 December 2013
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.
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Historically, the "shuttle" effect is the main cause of degradation in a LiS battery.
The lithium polysulfide Li2Sx (6≤x≤8) is highly soluble Extract of page 224 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  fc (0<fc<1), evaluated by the extension of the charge voltage plateau.
The factor fc is given by the expression:
fc =\frac{k_\text{s}q_\text{up}[S_\text{tot}]}{I_c}
where ks, qup, [Stot] and Ic are respectively the kinetic constant, specific capacity contributing to the anodic plateau, the total sulfur concentration and charge current.
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 (i.e. PC, EC, DEC and mixtures of them) 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 (as DOL) or short-chain ethers (as DME) as well as the family of glycol ethers, including DEGDME and TEGDME.
One common electrolyte is 1M LiTFSI in DOL:DME 1:1 vol. with1%w/w di LiNO3 as additive for lithium surface passivation.
Safety
Because of the high potential energy density and the nonlinear discharge and charging response of the cell, a microcontroller and other safety circuitry is sometimes used along with voltage regulators to manage cell operation and prevent rapid discharge.Akridge, J.R. (October 2001) "Lithium Sulfur Rechargeable Battery Safety" Battery Power Products & Technology Research
Research
Commercialization
As of 2021 few companies had been able to commercialize the technology on an industrial scale.
Companies such as Sion Power have partnered with Airbus Defence and Space to test their lithium sulfur battery technology.
Airbus Defense and Space successfully launched their prototype High Altitude Pseudo-Satellite (HAPS) aircraft powered by solar energy during the day and by lithium sulfur batteries at night in real life conditions during an 11-day flight.
The batteries used in the test flight utilized Sion Power's LiS cells that provide 350 W⋅h/kg.Kopera, J (September 2014) "Sion Power's Lithium-Sulfur Batteries Power High Altitude Pseudo-Satellite Flight" Sion Power Company Press Release Sion originally claimed to be in the process of volume manufacturing with availability by end of 2017; however more recently it can be seen that they have dropped work on their lithium sulfur battery in favor of a lithium-metal battery.
British firm OXIS Energy developed prototype lithium sulfur batteries.
Together with Imperial College London and Cranfield University, they published equivalent-circuit-network models for its cells.
With Lithium Balance of Denmark they built a prototype scooter battery system primarily for the Chinese market.
The prototype battery has a capacity of 1.2kWh using 10Ah Long Life cells, weighs 60% less than lead acid batteries with a significant increase in range.
They also built a 3U, 3,000W⋅h Rack-Mounted Battery that weighs only 25kg and is fully scalable.
They anticipate their Lithium-Sulfur batteries will cost about $200/kWh in mass production.
The firm entered bankruptcy (insolvency) status in May 2021.
Sony, which also commercialized the first lithium-ion battery, planned to introduce lithium–sulfur batteries to the market in 2020, but has provided no updates since the initial announcement in 2015.
Monash University’s Department of Mechanical and Aerospace Engineering in Melbourne, Australia  developed an ultra-high capacity Li-S battery that has been manufactured by partners at the Fraunhofer Institute for Material and Beam Technology in Germany.
It is claimed the battery can provide power to a smartphone for five days.
See also
List of battery types
References
External links
"EEMB Battery".
EEMB Battery.
Retrieved 2018-04-13.
