Novel Charge Storage Mechanism in Lithium-ion Batteries Revealed

Lithium batteries work by shuttling Li+ ions and electrons around between two electrode materials (the anode and cathode). The beauty of a lithium battery is that the flow of Li+ ions and electrons is separated with the Li+ ions moving inside the battery between the two electrodes while the electrons flow through the outside connectors and make your phone or computer work. A challenge for battery scientists has been to create electrode materials that can store all these Li+ ions and electrons being exchanged between the electrodes. There are no mysteries as to how Li+ ions are stored: they sit in empty spaces in the crystal structure of materials (graphite on the anode, and some metal oxides on the cathode). Electrons, on the other hand, have more options to be accommodated by a solid. Traditionally it has been assumed that they are stored on so called transition metals. These are metals in the middle of the periodic table that can exist in different charge states, thereby accommodating a different number of electrons. Voilà, by changing the charge state of a transition metal, one can store the electron. This is why batteries contain oxides with metals such as Cobalt, Nickel and Manganese. These are transition metals that are light and store electrons with the proper energy. Hence, a simple paradigm of how to make positive electrode materials existed: mix Lithium with transition metals with oxygen as the glue to keep them all together. Ceder always thought there was more to the oxygen ions than just being a glue. In a paper published in Nature in 1998[1] he already indicated that the oxygen ions play a very significant role in determining the energy with which the electron is stored on the transition metal, but they did not go as far as to assign the electron-storage role to the oxygen. In their recent paper in Nature Chemistry,[2] Ceder and co-authors Seo, Lee and three others now find that the initial hunch was right and that there is indeed more to oxygen than just being a glue. The authors show that in some cases, the oxygen ion is actually the storage vehicle for the electron. Normally, oxygen ions bind their electrons too strongly for them to be released upon battery charging. But the authors found, using advanced quantum mechanical modeling, that in certain environments an oxygen ion can release and absorb an electron during battery operation. So after 18 years the picture is complete. The fact that it is now exactly known how to exchange electrons with oxygen in electrode materials opens up the possibility for electrode materials with higher energy density and lower cost, as less transition metal may be necessary to host lots of those needed electrons!

[1] Ceder, G., Y-M. Chiang, D. R. Sadoway, M. K. Aydinol, Y-I. Jang, and B. Huang, Nature 392 (6677) 694 (1998)

[2] Seo, D.-H., Lee, J., Urban, A., Malik, R., Kang, S., Ceder, G., Nature Chemistry (2016) http://dx.doi.org/10.1038/nchem.2524