| specific energy | 500–600 W h/kg achievable [2] |
|---|---|
| energy density | 350 W h/l |
| Charge/discharge efficiency | C/5 nominal; up to 2C |
| 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–sulfur battery (Li–S battery) is a rechargeable galvanic cell with a very high energy density.[3] By virtue of the low atomic weight of lithium and moderate weight of sulfur, Li–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.[4] Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and the low cost of sulfur. There is much interest in using them for electric vehicles.
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The chemical processes in the Li–S cell include lithium dissolution from the anode surface (and incorporation into polysulfides) during discharge, and reverse lithium plating to the nominal anode while charging.[5] This contrasts with conventional lithium-ion cells, where the lithium ions are intercalated in the anode and cathodes, and consequently Li-S allows for a much higher lithium storage density. Polysulfides are reduced on the anode surface in sequence while the cell is discharging:
Across a porous diffusion separator, sulfur polymers form at the nominal cathode as the cell charges:
These reactions are analogous to those in the sodium–sulfur battery.
For experimental purposes most batteries are constructed with a carbon and sulfur cathode and a lithium anode.[6] Sulfur as a raw material has the advantage for mass production that it is very cheap, but it lacks electroconductivity. Sulfur alone being at 5*10−30 S cm−1 at 25°C.[7] The carbon coating on the sulfur then provides the electroconductivity missing from pure sulfur. The solution to this problem is carbon nanofibers. The carbon materials provide an effective electron conduction path and structural integrity. The downside of carbon nanofibres is the high cost.[8]
Each sulfur atom can host two lithium ions. Typically, in lithium-ion batteries, for every host atom, only 0.5–0.7 lithium ions can be accommodated.[9]
One of the primary shortfalls of most Li–S cells is intermediary reactions with the electrolytes. While S and Li2S are relatively insoluble in most electrolytes, many of the intermediary polysulfides are not. The dissolving of LiSn into electrolytes causes irreversible loss of active sulfur material.[10] The majority of research on Lithium-sulfur batteries in 2010 is to improve the choice of electrolytes to minimize this side reaction.
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 control cell operation and prevent rapid discharge.[11]
Research conducted at the University of Waterloo has produced Li–S cells with 84% of the theoretical maximum energy density for Li–S that suffer minimal degradation during charge cycling, and thus potentially offering four times the gravimetric energy density of lithium-ion. The team accomplished this through use of a mesoporous carbon cathode, full of deep pits. Sulfur and carbon were milled and heated together, causing the low surface tension sulfur to seep into the pits, with just enough room to expand. The composite was then heated to bake off residual sulfur from the surface. To further trap the polysulfides in the cathode, the surface was functionalized and coated with polyethylene glycol to repel the hydrophobic polysulfides and keep them trapped in the pits. In a "worst case scenario" test using a glyme solvent known for its affinity for dissolving polysulfides, a traditional LiS cathode lost 96% of its sulfur over 30 cycles, while the new cathode lost only 25%.[12]