Researchers explain the high capacity of Li-rich NMC compounds
Fourteen researchers from India, USA and France gathered around Pr. Jean-Marie Tarascon lifted the veil on a chemical mechanism never explained before.
Some of the Li-ion batteries powering our smartphones are currently using Li-based compounds named NMC – since they also contain Nickel, Manganese and Cobalt – whose capacities are around 180 mAh/g. In the last five years, American and Canadian researchers showed that similar Li-rich materials could even display capacities as high as 250 mAh/g. This would translate by a 15 to 20% improvement of our cellphones autonomy. Rapidly, BASF and 3M started to commercialize such materials. Yet, they met many difficulties such as a drop of the batteries' average potential across charge-discharge cycles.
To tackle this issue and unveil the chemical mechanism causing such high capacities, many studies have been led. Until now, they have been the source of more controversies about the various proposed mechanisms than answers.
Following these issues, a team of researchers published results that settles the question of the involved mechanism and overcomes some limitations of those compounds (such as the voltage drop upon cycling). Their work has been published in Nature Materials and a patent has been filed.
To obtain such results, the team made astute chemical manipulations. They worked on compounds with a simpler chemical composition but close enough to NMC (i.e. with similar layered structure) for the conclusion of the study to be univocal and generalizable.
The team started with a Li2MnO3 compound and partially replaced manganese (Mn) with ruthenium (Ru), thus allowing the compound to become electrically conductive. The system still remaining very complex to study, researchers completely removed the manganese out of the equation replacing it with tin (Sn). Tin does not participates in the electrochemical reactions. In the end, scientists obtained oxides of general formula Li2Ru1-ySnyO3. They appear to produce capacities as high as 230 mAh/g.
Then, the compounds have been studied by experimental techniques (or "characterization" techniques: X-ray diffraction, transmission electron microscopy see below, electron paramagnetic resonance, photoemission spectroscopy, Mössbauer spectroscopy) and theoretical modeling, thus benefiting from the wide analytical expertise of the RS2E network. Those techniques have highlighted a reversible redox reaction associated with O- anions. This anionic activity, well-known in the case of sulfur-based compounds (Jean Rouxel’s work), has never been witnessed before in insertion oxides. This reaction clearly explains the high capacities displayed by Li-rich NMC compounds.
In addition to this discovery, researchers announced that the studied compound also features an improved structural stability.
Aware of the rarity and cost of ruthenium the research team will now work on its replacement. Researchers are optimistic since there are many different elements belonging to the Li2MO3 family. Scientists are also working on solutions to avoid the first cycle of charge-discharge, currently required for the electrodes to “fall” into a stable structure (see below).
Finally, this study opens the door to promising opportunities in the conception of better electrodes for Li-ion, or even Na-ion, batteries. RS2E and Alistore-ERI research networks should be able to quickly use those opportunities in the context of their researches.
Reversible anionic redox chemistry in high-capacity layered-oxide electrodes. M. Sathiya, G. Rousse, K. Ramesha, C.P. Laisa, H. Vezin, M. T. Sougrati, M. L. Doublet, D. Foix, D. Gonbeau, W. Walker, A. S. Prakash, M. Ben Hassine, L. Dupont et J.-M. Tarascon
Nature Materials, Advance Online Publication, 14 juillet 2013, DOI : 10.1038/NMAT3699.