This year’s Nobel Prize in Chemistry has been awarded to the fathers of the lithium-ion battery. Together, they found a way to channel lithium’s explosive nature into something useful.
Lithium’s reactivity is a double-edged sword as far as making batteries is concerned. The ease with which the metal gives up its electrons is, on the one hand, a godsend, as that process (and its reverse) is exactly what belies any conventional battery. The downside is that metallic lithium isn’t very picky about where its electrons go. Compounds such as water and oxygen will do just fine, and they take them eagerly – explosively eagerly, in fact.
While the potential of lithium for making batteries was recognized very early, it wasn’t until the 1960s that researchers dared dream about taming the element’s hotheadedness. This is exactly what this year’s winners of the Nobel Prize in Chemistry did. Many researchers contributed to the creation of the lithium-ion battery as we know it today, but according to the Nobel Committee, John Goodenough, Stanley Whittingham and Akira Yoshino provided the crucial pieces of the puzzle.
Superior alternative
Easily giving up electrons is just one of the characteristics that make lithium great for batteries. It’s also the lightest metal available, which certainly helps for increasing energy density. Lithium atoms or ions are also quite small, meaning they travel easily and can be stored in the nooks and crannies of ‘porous’ materials. This characteristic turned out to be perhaps the most essential of all.
Primitive lithium-ion batteries used metallic lithium as one of the electrodes. When discharging, this is called the anode and this is where lithium atoms give up their electrons, which go on to travel through the circuit and provide power. The lithium ions that are produced simultaneously travel through a liquid (not water!) to the other electrode (cathode), where they’re reunited with an electron and ‘stored’.
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Working in the labs of Exxon in the 1970s, Whittingham decided to try titanium disulfide (TiS2) to accommodate the lithium in the cathode: solid TiS2 has a layered structure with lots of room for guests. The material worked quite well, even showing complete reversibility of the processes involved. This boded well for creating rechargeable batteries, but alas, after a while, spikes would start to form from the surface of the metallic lithium. These dendrites could grow all the way to the TiS2 electrode, short-circuiting the battery and potentially setting it on fire.
The solution to this seemed obvious: replace the metallic lithium with another material capable of taking in lithium reversibly. As the driving force for the battery to produce current ultimately depends on the relative energy states of lithium at both electrodes, it had to be a different material than TiS2, though.
Although Goodenough didn’t discover this material, his work helped a lot in finding it. He actually found a better alternative to TiS2 cathodes. He knew that cobalt dioxide (CoO2) has a similar structure as TiS2 but he had reason to believe it would be even more hospitable to lithium. He was right, and thanks to the characteristics of CoO2, the search for a better anode could be significantly broadened.
Petroleum coke
In the end, however, the anode material that took the crown had already been extensively researched well before Goodenough’s breakthrough: graphite. It had been known for a long time that graphite can accommodate metal ions. Like the cathode materials, the carbon derivative has a layered molecular structure.
Initially, the research seemed to show that graphite is too unstable to use as an anode because the action of lithium and solvent molecules constantly moving in and out destroyed the layered structure. After screening many types of graphite-like materials in the late 1980s, however, Yoshino found one that was stable and performed well. This particular grade of petroleum coke, a byproduct of oil refining, proved to combine graphitic parts that house the lithium and non-ordered domains that keep the material stable.
Thus, all the necessary ingredients came together. The first commercial lithium battery was released in 1991, using Yoshino’s petroleum coke as the anode and Goodenough’s CoO2 as the cathode. Its descendants, however, use graphite after all because an electrolyte has been found that doesn’t destroy it.