The lithiumâ€"sulphur battery (Liâ€"S battery) is a rechargeable battery, notable for its high energy density. The low atomic weight of lithium and moderate weight of sulfur means that Liâ€"S batteries are relatively light (about the density of water). They were demonstrated on the longest and highest-altitude solar-powered airplane flight in August 2008.
Lithiumâ€"sulfur batteries may succeed lithium-ion cells because of their higher energy density and reduced cost from the use of sulfur. Currently the best Liâ€"S batteries offer energy densities on the order of 500 W·h/kg, significantly better than most lithium-ion batteries which are in the 150 to 200 range. Liâ€"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 Liâ€"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 0.5â€"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 â†' Li
2S
8 â†' Li
2S
6 â†' Li
2S
4 â†' Li
2S
3
Across a porous diffusion separator, sulfur polymers form at the cathode as the cell charges:
- Li
2S â†' Li
2S
2 â†' Li
2S
3 â†' Li
2S
4 â†' Li
2S
6 â†' Li
2S
8 â†' S
8
These reactions are analogous to those in the sodiumâ€"sulfur battery.
Most use a carbon/sulfur cathode and a lithium anode. Sulfur is very cheap, but lacks electroconductivity. Sulfur alone is 5×10âˆ'30 S cmâˆ'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 lithiumâ€"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 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 sulfur 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 Liâ€"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 negative electrode causes dissociation of most of the commonly used ether type electrolytes. Use of 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
References
External links
- "OXIS Energy". OXIS Energy. Retrieved 2013-10-30.Â
- "Sion Power". Sion Power. Retrieved 2013-04-06.Â
- "PolyPlus Lithium Sulfur". Polyplus.com. Retrieved 2013-04-06.Â
- "Winston Battery Limited". En.winston-battery.com. Retrieved 2013-04-06.Â
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