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LITHIUM-AIR TECHNOLOGY comes as close as possible to the theoretical limits of battery energy density; this, according to Science, the weekly magazine published by the American Association for the Advancement of Science. Over the past decade, researchers in academe and industry have been working on this Li-O2 concept. However, there have been lots of challenges bringing it from the lab into the real world.
Researchers in the Department of Chemistry, University of Cambridge, have met several of these challenges through innovative combinations of chemical and electrochemical means. Here’s what I can glean from their paper, “Cycling Li-O2 Batteries via LiOH Formation and Decomposition,” that appeared in Science, October 30, 2015. I added a bit from some Internet sleuthing. (There’s also an “EV Glossary” I did a while back, many of its entries still relevant.)
First, note that lithium-air technology is different from the lithium-ion technology that is used in everything from smart phones to battery electric vehicles. Briefly, Li-ion batteries move lithium ions through an electrolyte from negative electrode to positive electrode during discharge and back when being recharged.
Li-ion energy density depends profoundly on the specific lithium compound selected for the positive electrode. Alas, the highest energy density is accompanied by inherent safety risks, especially when a battery is damaged. Increased safety and enhanced longevity come with other electrode choices, albeit with lower energy density.
Lithium-air technology is fundamentally different. On discharge, its lithium ions combine with oxygen in the air and produce lithium oxide, Li2O2, at the positive electrode. In recharging, this oxidation is reversed, the ions returning to the negative electrode and the oxygen released to the air.
In a typical Li-O2 cell, the negative electrode is composed of lithium and the positive electrode is a porous material. This porous electrode isn’t active in the process; rather, it’s a stable conductive framework that hosts the Li2-O2 produced in discharging.
One of the challenges of Li-O2 technology concerns volume of these electrode pores and the size of Li2-O2 crystals produced in discharge. In theory, the larger the pores, the greater the energy density and the less likelihood of pore clogging during charge/discharge cycling. On the other hand, larger crystals giving greater energy density may promote this clogging.
Another challenge concerns severe side reactions occurring during cycling that degrade electrode material and electrolyte. A third one is large hysteresis; that is, there are lags in charging and discharging that result in low energy efficiencies. Last, Li-O2 cells have hitherto been very sensitive to moisture and carbon dioxide.
Cambridge researchers have addressed each of these with optimization of the porous electrode and electrolyte. Inclusion of a lithium-iodine additive and water to the electrolyte promotes a somewhat different set of reactions in the charge/discharge cycle.
The LiI additive and water transform purely electrochemical reactions on discharge and charge into a pair of reactions for each process. Intermediately, there’s a beneficial chemical reaction involving Li-OH, lithium hydroxide crystals whose size takes advantage of porous material gaps.
The researchers conclude that the combination plays “a decisive factor in the capacity and rechargeability of the resulting Li-O2 battery.” In the magazine’s “Research” section, it’s noted that energy density “is roughly 10 times higher than conventional lithium-ion batteries and would be sufficient to power cars with a range comparable to those with gasoline engines.” ds
© Dennis Simanaitis, SimanaitisSays.com, 2015