Batteries. We buy them at the store, use them up, and throw them away without much thought. In reality, however, batteries are remarkably complex electrochemical devices that are continually evolving. The latest example of this comes from the Lawrence Berkeley National Laboratory, where researchers have invented an advanced lithium/sulfur (Li/S) cell that offers a unique combination of energy storage, power, recharge speed, and survivability.
Lithium/sulfur rechargeable batteries offer a remarkably large capacity for energy storage, mainly because two electrons are produced each time a molecule is processed through the battery’s chemistry.
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.
That’s the good news. 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.
Another problem is that lithium and sulfur generally don’t react immediately to form Li2S, but rather get there through a series of intermediate species, such as Li2S8, Li2S6, etc. Sulfur itself and Li2S are essentially insoluble in the typical electrolyte used in Li/S cells, but these intermediate “polysulfides” often are soluble, which causes an ongoing and severe loss of sulfur at the cathode. Other problems appear, such as a roughening of the lithium anode surface with large charge or discharge currents. All of these problems result in a basic Li/S cell being a very bad battery.
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.
The Lawrence Berkeley team addressed these problems by developing a nanocomposite cathode that addresses the three main problems presented by Li/S cells. The new cathode material is a sulfur-graphene oxide nanocomposite held together using an elastic polymer binder.
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