Working principle of lithium-sulfur battery and "shuttle" effect
|Specific energy||1,800,000 J/kg demonstrated|
|Energy density||1,260,000 J/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 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 solar-powered aeroplane flight in August 2008.
Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and reduced cost due to 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 range of 150–250 W·h/kg. Li–S batteries with up to 1,500 charge and discharge cycles have been demonstrated, but cycle life tests at commercial scale and with lean electrolyte are still needed. As of early 2014, none were commercially available. The key issue of Li–S 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 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 Li2S and the large amount of electrolyte needed are also issues to address.
The invention of Li–S 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 satured amines. 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. Only by the end of the 1980s a rechargeable Li–S battery was demonstrated employing ethers, in particular DOL, as solvent for the electrolyte. Thanks to the scientific improvements in the field, the potential of Li–S batteries was highlighted. Li–S batteries have experienced in the last twenty years, a renewed and growing popularity. In particular, new strategies for inhibition or mitigation of the polysulfide "shuttle" effect have been deeply investigated and was object of study by many researches. In the recent years, the number of scientific publications on the topic has grown exponentially, overcoming 700 publications in 2017.
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:
8 → Li
8 → Li
6 → Li
4 → Li
2S → Li
2 → Li
3 → Li
4 → Li
6 → Li
8 → S
These reactions are analogous to those in the sodium–sulfur battery.
The main challenges of Li–S batteries is the low conductivity of sulfur and its massive volume change upon discharging and finding a suitable cathode is the first step for commercialization of Li–S batteries. Therefore, most researchers 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 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.
Historically, the "shuttle" effect is the main cause of degradation in a Li–S battery. The lithium polysulfide Li2Sx (6<x<8) is highly soluble in the common electrolytes used for Li–S batteries. They are formed and leaked from the cathode and they diffuse to the anode, where they are reduced to short-chain polysulfide and diffuse back to the cathode where long-chain polysulfide 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 Li–S batteries, because of slow dissolution of polysulfide, which occurs also in rest state. The "shuttle" effect in Li–S 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:
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.
Conventionally, Li–S batteries employ a liquid organic electrolyte, contained in the pores of PP separator. The electrolyte plays a key role in Li–S 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 Li–S 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 Li–S batteries are conventionally employed cylic ethers (as DOL) or short-chain ethers (as DME) as well as the family of glycol ethers, including DEGDME and TEGDME. Currently, the liquid electrolyte that has been adopted as standard in the scientific community is composed by 1M LiTFSI in DOL:DME 1:1 vol. with1%w/w di LiNO3 as additive for lithium surface passivation.
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|
|Lithium metal||Polyethylene glycol coated, pitted mesoporous carbon||17 May 2009||University of Waterloo||1,110 mA⋅h/g after 20 cycles at a current 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%.|
|Lithium metal||Sulfur-coated, disordered carbon hollow carbon nanofibers||2011||Stanford University||730 mA⋅h/g after 150 cycles (at 0.5 C)||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 mA⋅h/g after 400 cycles (at 1 C)||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 mA⋅h/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 mA⋅h/g at 1,000 cycles (0.5 C)||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 (0.1 C)
800 mA·h/g at 300 cycles at 60 °C (1 C)
|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 (0.05 C discharge)
400 mA·h/g at 1,500 cycles (0.5 C charge / 1 C 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 W⋅h/kg (initial) and 250 W⋅h/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|
|Lithiated hard-carbon||Sulfur-copolymer (poly(S-co-DVB))||2019||Chungnam National University||400 mAh/g for 500 cycles at 3C||The SEI of hard-carbon prevents polysulphides deposition at anode and enables high-rate performance. |
|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|
|Lithium||Sulfur||2016||LEITAT||500 W⋅h/kg||ALISE H2020 project developing a Li–S battery for cars with new components and optimized regarding anode, cathode, electrolyte and separator|
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 W⋅h/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 1.2 kWh using 10 Ah OXIS Long Life cells, weighs 60% less than lead acid batteries with a significant increase in range. OXIS Energy has built a 3U, 3,000 W⋅h Rack-Mounted Battery that weighs only 25 kg and is fully scalable. OXIS anticipates that its Lithium-Sulfur batteries will cost about $200/kWh in mass production. OXIS has also been involved in the European Consortium for Lithium-Sulphur Power for Space Environments (ECLIPSE) H2020 project. This project is developing high-capacity Li–S batteries for satellites and launchers.
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