According to researchers the coating hinders undesirable side-reactions between the electrode and electrolyte, providing a good diffusion of lithium ions along the interfacial layer and a more uniform deposition of lithium when charging, and augments storage in the lithium anode via alloying reactions between lithium and indium.
Modern lithium ion batteries usually have graphite anodes that store lithium when the batteries are charged, an alternative is using batteries with metallic anodes, such as lithium metal.
While these batteries are promising significantly higher storage capacity, there are significant hurdles to their adoption such as the uneven deposition of the metal during the charging process, which leads to formation of dendrites. After longer uses of the battery, these dendrites can grow so extensive that they short-circuit the battery. There are also undesirable side-reactions between the reactive metal electrodes and the electrolyte, reducing the lifetime of the batteries.
The formation of a stable, passivating layer that prevents further contact is seen as the ideal solution, but isn't possible because of the constant expansion and contraction of the electrode upon charging and discharging which destroys the layer and exposes the metal to the electrolyte for more reactions.
Other approaches include artificial films or physical barriers.
Researchers working with Ravishankar Sundararaman at Rensselaer Polytechnic Institute and Lynden A. Archer at Cornell University have now come up with this new approach. By using straightforward electroless ion-exchange chemistry, they have produced indium coatings on lithium by immersion in a special indium salt solution. Some of the indium is deposited on the surface of the lithium electrode as metal and the lithium ion concentration in the electrolyte simultaneously increases.
The indium layer is uniform and self-healing when the electrode is in use, if small amounts of the indium salt are added to the electrolyte. It remains intact during charge/discharge cycles, its chemical composition remains unchanged, and side-reactions are prevented. Dendrites are also eliminated, leaving the surface smooth and compact.
By using computer modeling, researchers were able to show that lithium ions are very loosely bound to the indium coating. They form an alloy with the indium, which allows them to move very rapidly over the surface before they cross it and are deposited on the underlying lithium electrode.
In complete cells with commercial cathodes, these new indium-lithium hybrid electrodes were stable over more than 250 cycles, retaining about 90 % of their capacity.
