Wednesday, January 15, 2014

New lithium/sulfur battery doubles energy density of lithium-ion

The lithium–sulfur battery (Li–S battery) is a rechargeable battery, notable for its high energy density. 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. Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and reduced cost from the use of sulfur
A basic Li/S cell consists of a lithium anode, a carbon-sulfur cathode, and an electrolyte that permits lithium ions to pass. The overall cell reaction during discharge converts lithium metal in the anode into Li2S at the surface of the cathode. The flow of two lithium ions from the anode to the cathode is then balanced by the flow of two electrons between the battery contacts, delivering double the current of a Li-ion battery at a voltage between about 1.7 and 2.5 volts, depending on the state of charge of the cell. Lithium polysulfides are formed at intermediate charge levels, which affect the cell voltage as indicated above.
The bad news involves a host of materials problems associated with the basic Li/S chemistry and some side reactions. When the sulfur in the cathode absorbs lithium ions from the electrolyte, the Li2S has nearly double the volume of the original sulfur. This is a very large source of mechanical stress on the cathode, which causes mechanical deterioration, reduces the electrical contact between the carbon and the sulfur (the path whereby electrons flow to allow the reaction to occur), and prevents the flow of lithium ions to the sulfur surface.
The Li/S battery chemistry, however, offers the potential for such wonderful battery performance that, since its discovery in the 1960s, a lot of work has been aimed at solving these problems. Engineers and scientists have tried putting the sulfur inside nanochannels as well as using lithium-silicon-carbon alloy anodes, sulfur polymer cathodes, and a host of other imaginative attempts at solving the interlocked Li/S battery performance limitations. While a good deal of progress has been made, development of a practical Li/S cell has eluded researchers for half a century.
Advanced technology.
OXIS Lithium Sulfur cells are the next generation of battery technology, surpassing Lithium-ion which is reaching the limit of its potential.
Battery systems using metallic Lithium are known to offer the highest specific energy.
Sulfur represents a natural cathode partner for metallic Li and, in contrast with conventional lithium-ion cells, the chemicals processes include dissolution from the anode surface during discharge and reverse lithium plating to the anode while charging. As a consequence, Lithium-Sulfur allows for a theoretical specific energy in excess of 2700Wh/kg, which is nearly 5 times higher than that of Li-ion.
OXIS’s next generation lithium technology platform offers the highest energy density among lithium chemistry:
-350 Wh/kg demonstrated at effective material level in test cells in 2013
-200 Wh/kg achieved at cell level in 2013
-400 Wh/kg achievable in the next three years
A comparison of reactions between an OXIS Energy Lithium Sulfur cell and a Lithium Ion cell when they are punctured by a nail.
Lithium Sulfur cell: Li-S, 2.1V, 4Ah developed by OXIS Energy.
Lithium Ion cell: LiCoO2, 3.7V, 4Ah from a competitor
A stack of OXIS Lithium Sulfur cells subjected to puncture by a bullet.
Test was carried out at an indoor firing range, bullet was 5.56mm NATO rifle round fired from a range of 10m.
Temperature rise of cells was 2.3C in the centre of the stack. A further test with the cells connected in parallel was performed, but we don't have the video for that. The results of that test were the same.