Lithium–sulfur battery

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.||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−1800 mA·h/g at 300 cycles at 60 °C (1 C) 400 mA·h/g at 1,500 cycles (0.5 C charge / 1 C discharge)25 A·h/cell 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 '60, 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 '80 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: 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 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.