|Specific energy||500 W·h/kg demonstrated|
|Energy density||350 W·h/L|
|Charge/discharge efficiency||C/5 nominal|
|Nominal cell voltage||cell voltage varies nonlinearly in the range 2.5–1.7 V during discharge; batteries often packaged for 3 V|
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 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 solar-powered aeroplane flight in August 2008.
Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and reduced cost from the use of sulfur. Currently the best Li–S batteries offer specific energies on the order of 500 W·h/kg, significantly better than most lithium-ion batteries which are in the 150 to 200 range. Li–S batteries with up to 1,500 charge and discharge cycles have been demonstrated. As of early 2014, none were commercially available.
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. 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, Li-S 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.
Most use a carbon/sulfur cathode and a lithium anode. Sulfur is very cheap, but has practically no electroconductivity, 5×10 S cm 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 happens, 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.
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 Li
2S are relatively insoluble in most electrolytes, many intermediate polysulfides are not. Dissolving Li
n 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.
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.
|Anode||Cathode||Date||Source||Specific Capacity after cycling||Notes|
|Polyethylene glycol coated, pitted mesoporous carbon||17 May 2009||University of Waterloo||1,110 mAh/g after 20 cycles at a current rate of 168 mA g-1||Minimal degradation during charge cycling. To retain polysulfides in the cathode, the surface was functionalized to repel (hydrophobic) polysulfides. In a test using a glyme solvent, a traditional sulfur cathode lost 96% of its sulfur over 30 cycles, while the experimental cathode lost only 25%.|
|Silicon nanowire||Sulfur-coated, disordered carbon nanotubes||2011||Stanford University||730mAh/g after 150 cycles (at 0.5C)||An electrolyte additive boosted the faraday efficiency from 85% to over 99%.|
|Silicon nanowire/carbon||Sulfur-coated, disordered carbon nanotubes made from carbohydrates||2013||CGS||1,300 mAh/g after 400 cycles (at 1C)||Microwave processing of materials and laser-printing of electrodes.|
|Silicon carbon||Sulfur||2013||Fraunhofer Institute for Material and Beam Technology IWS||? after 1,400 cycles|
|Copolymerized sulfur||2013||University of Arizona||823 mAh/g at 100 cycles||Uses "inverse vulcanization" on mostly sulfur with a small amount of 1,3-diisopropenylbenzene (DIB) additive|
2-encapsulated sulfur nanoparticles
|2013||Stanford University||721 mAh/g at 1,000 cycles (0.5C)||shell protects the sulfur-lithium intermediate from electrolyte solvent. Each cathode particle is 800 nanometers in diameter. Faraday efficiency of 98.4%.|
|Sulfur||June 2013||Oak Ridge National Laboratory||1200 mA·h/g at 300 cycles at 60 °C (0.1C)
800 mA·h/g at 300 cycles at 60 °C (1C)
|Solid lithium polysulfidophosphate electrolyte. Half the voltage of typical LIBs. Remaining issues include low electrolyte ionic conductivity and brittleness in the ceramic structure.|
|Lithium||Sulfur-graphene oxide nanocomposite with styrene-butadiene-carboxymethyl cellulose copolymer binder||2013||Lawrence Berkeley National Laboratory||700 mA·h/g at 1,500 cycles (0.05C discharge)
400 mA·h/g at 1,500 cycles (0.5C charge/ 1C discharge)
|Voltage between about 1.7 and 2.5 volts, depending on charge state. Lithium bis(trifluoromethanesulfonyl)imide) dissolved in a mixture of nmethyl-(n-butyl) pyrrolidinium bis(trifluoromethanesulfonyl)-imide (PYR14TFSI), 1,3-dioxolane (DOL), dimethoxyethane (DME) with 1 M lithium bis-(trifluoromethylsulfonyl)imide (LiTFSI), and lithium nitrate (LiNO
3). High porosity polypropylene separator. Specific energy is 500 Wh/kg (initial) and 250 Wh/kg at 1,500 cycles (C=1.0)
|Lithiated graphite||Sulfur||February 2014||Pacific Northwest National Laboratory||400 cycles||Coating prevents polysulfides from destroying the anode.|
|Lithiated graphene||Sulfur/Lithium-sulfide passivation layer||2014||OXIS Energy||240 mA·h/g (1000 cycles)
|Passivation layer prevents sulfur loss|
|Lithium sulfur batteries||Carbon nanotube/Sulfur||2014||Tsinghua University||15.1 mA·h cm at a sulfur loading of 17.3 mgS cm||A free-standing CNT–S paper electrode with a high areal sulfur-loading was fabricated, in which short MWCNTs served as the short-range electrical conductive network and super-long CNTs acted as both the long-range conductive network and intercrossed binders.|
|Glass-coated sulfur with mildly reduced graphene oxide for structural support||2015||University of California, Riverside||700 mA h g (50 cycles)||Glass coating prevents lithium polysulfides from permanently migrating to an electrode|
As of 2015 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 Li-S cells that provide 350 Wh/kg. Sion claims to be in the process of volume manufacturing with availability by end of 2017.
British firm OXIS Energy developed prototype lithium sulfur batteries that are currently operating in small scale, commercial, test applications. As of June 2015, OXIS Energy planned to sell its energy storage batteries from 2016. Together with Imperial College London and Cranfield University, OXIS Energy 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 OXIS Long Life cells, weighs 60% less than lead acid batteries with a significant increase in range.
Sony, which also commercialized the first lithium-ion battery, plans to introduce lithium–sulfur batteries to the market in 2020.
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