Lithium–sulfur battery

Lithium–sulfur battery

Specific energy	500 W·h/kg demonstrated
Energy density	350 W·h/L
Charge/discharge efficiency	C/5 nominal
Cycle durability	disputed
Nominal cell voltage	cell voltage varies nonlinearly in the range 2.5–1.7 during discharge; batteries often packaged for 3V

The lithium–sulphur battery (Li–S battery) is a type of rechargeable battery, notable for its high energy density. 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–sulphur 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.

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. 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:

S
₈ → Li
₂S
₈ → Li
₂S
₆ → Li
₂S
₄ → Li
₂S
₃

Across a porous diffusion separator, sulfur polymers form at the cathode as the cell charges:

Li
₂S → Li
₂S
₂ → Li
₂S
₃ → Li
₂S
₄ → Li
₂S
₆ → Li
₂S
₈ → S
₈

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, 5×10⁻³⁰ S 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 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
₂S
_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.

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.

Research

Research

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
	Porous TiO 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	240 mA·h/g (1000 cycles) 25 A·h/cell	Passivation layer prevents sulfur loss
Lithium sulfur batteries	Carbon nanotube/Sulfur	2014	Tsinghua University	15.1 mA·h cm −2 at a sulfur loading of 17.3 mgS cm-2	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−1 (50 cycles)	Glass coating prevents lithium polysulfides from permanently migrating to an electrode

Commercialization

So far very few companies, if any have been able to commercialize the technology on an industrial scale as of October 2015. Some companies however, such as Tucson, Arizona based Sion Power have partnered with other such as Airbus Defense and Space in order to test their lithium sulfur battery technology. Airbus Defense and Space successfully launched their prototype High Altitude Pseudo-Satellite (HAPS) aircraft which was powered by solar energy during the day and by such lithium sulfur batteries at night in real life conditions during an 11 day flight. The batteries used in the test flight utilized Sion Powers high specific energy Li-S cells which provide 350 Wh/kg

Other companies, such as England-based Oxis Energy Ltd, have also developed prototype lithium sulfur batteries which are currently operating in small scale, commercial, test applications. Oxis plans to sell its energy storage batteries in 2016.

Sony, which also commercialized the first lithium-ion battery, plans to introduce lithium–sulfur batteries to the market in 2020.