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2017-08-21

MIT, Toyota team clarifies role of iodide in Li-air batteries




The schematic of Li-oxygen discharge process in presence of H2O and LiI in DME based electrolyte a) chemical route by mixing KO2 with electrolyte containing Li+ cations and b) electrochemical route by discharging Li-O2 battery

Lithium-air (or lithium-oxygen) batteries potentially could offer three times the gravimetric energy of current Li-ion batteries (3500 Wh/kg at the cell level); as such, they are looked to a potential solution for long-range EVs. However, tests of various approaches to creating such batteries have produced conflicting and confusing results, as well as controversies over how to explain them.
Now, researchers from MIT, with a colleague from Toyota Motor Europe’s R&D group, have carried out detailed tests that seem to resolve the questions surrounding one promising material for such batteries: lithium iodide (LiI). The compound was seen as a possible solution to some of the lithium-air battery’s problems, including an inability to sustain many charging-discharging cycles, but conflicting findings had raised questions about the material’s usefulness for this task. The new study, published in the RSC journal Energy & Environmental Science, explains these discrepancies. Although the results suggest that the material might not be suitable after all, the work provides guidance for efforts to overcome LiI’s drawbacks or to find alternative materials.
Li-air batteries operate on a different principle than the Li-ion batteries, which are based on the intercalation of Li ions into a metal oxide host. In a Li-air battery, lithium ions react with reduced oxygen species to form Li2O2 (lithium peroxide) deposits inside a porous cathode. The specific energy is limited by the capacity of the cathode material to accommodate large quantities of Li2O2.
One of the main problems related to Li2O2 precipitation is its insulating nature. Li2O2 precipitate passivates the electrode surface hindering further electron transfer. This obstacle can be overcome, to some extent, by designing high surface area structures of the carbon matrix, modifying the surface chemistry and the morphology of the deposited Li2O2. However, the insulating nature of Li2O2 brings further complications during the charge process. In particular, the sluggish kinetics of charge transfer result in high charging over potentials, low rate capability, low energy efficiency, numerous parasitic reactions and poor cycle life.
LiI has been studied intensively to decrease charging over- potential and improve the cycle life of Li-O2 batteries. Ambiguities exist in terms of its effect on the reversibility, cyclability, and reaction mechanism of lithium-oxygen redox chemistry. … In this study, we focus on examining the role of iodide on the formation of LiOH under two experimental conditions: i) reacting KO2 with lithium salts in DME solution and ii) the electrochemical reduction of Li+-oxygen in lithium-oxygen batteries at high potentials (~2.7 V), where Li2O2 is formed largely by disproportionation of LiO2, and at low potentials (~2.2 V), where Li2O2 is formed directly through electrochemical steps.
The team looked at the role of LiI on lithium-air battery discharge, using a different approach from most other studies. One set of studies was conducted with the components outside of the battery, which allowed the researchers to zero in on one part of the reaction, while the other study was done in the battery, to help explain the overall process.
They then used ultraviolet and visible-light spectroscopy and other techniques to study the reactions that took place. Both of these processes foster the production of different lithium compound such as LiOH (lithium hydroxide) in the presence of both LiI and water, instead of Li2O2.
LiI can enhance water’s reactivity and make it lose protons more easily, which promotes the formation of LiOH in these batteries and interferes with the charging process. These observations show that finding ways to suppress these reactions could make compounds such as LiI work better.
This study could point the way toward selecting a different compound instead of LiI to perform its intended function of suppressing unwanted chemical reactions at the electrode surface, said co-author Graham Leverick, adding that this work demonstrates the importance of “looking at the detailed mechanism carefully.”
Professor Yang Shao-Horn said that the new findings “help get to the bottom of this existing controversy on the role of LiI on discharge. We believe this clarifies and brings together all these different points of view.”
Lithium-oxygen batteries that run on oxygen and lithium ions are of great interest because they could enable electric vehicles of much greater range. However, one of the problems is that they are not very efficient yet. [In this study] it is shown how adding a simple salt, lithium iodide, can potentially be used to make these batteries run much more efficiently. They have provided new insight into how the lithium iodide acts to help break up the solid discharge product, which will help to enable the development of these advanced battery systems. There are still significant barriers remaining to be overcome before these batteries become a reality, such as getting long enough cycle life, but this is an important contribution to the field.