One step closer to using the best materials for Li-ion batteries known to date
In current batteries, the capacity of positive electrodes reaches at best 200mAh/g, which remains inadequate for many users who need more autonomy for their mobile devices. Major efforts are therefore made at the engineering level among battery manufacturers to improve the performance of batteries but also at the fundamental research level to develop new materials whose chemical composition, enriched in lithium, would increase the capacity of positive electrodes, thus of the whole battery. "Li-rich" materials with capacities that are up to 300 mAh/g lets us indeed envision a 20% increase of current batteries’ capacities. Unfortunately, attempts made by BASF and 3M to manufacture these batteries have been proven difficult due to the limited lifespan of “Li-rich” materials upon the charge/discharge cycles of the battery.
General structure of the charged material (TEM imaging).
RS2E researchers are working since 2012 on the subject and have published three articles tackling the understanding of the mechanisms and outstanding performance of these materials. In 2013, they proposed in Nature Materials a first explanation of the electrochemical mechanism at the origin of their capacities, which is based on an anionic redox activity in addition to the cationic redox activity typically observed. In 2014, also in Nature Materials, they connected the voltage fade in the charge/discharge cycles to a structural instability of the materials resulting from a partially irreversible cationic migration. Finally, in early 2015, they used electronic imaging (electron paramagnetic resonance) to publish in Nature Communications the first ever experimental evidence of the anionic activity proposed in 2013. If these fundamental advances have clearly identified the anionic framework of "Li-rich" materials as an essential vector of the capacity increase, structural instability arising from the redox-active anion (O2 gas venting) must be countered in order to master the technological prowess that these materials suggest.
Visualization of oxygen dimers by TEM : small dots joined by black lines, the darker dots in the center are iridium atoms.
Today, the same group of international researchers (from Belgium, Slovenia, Russia and France) led by Jean-Marie Tarascon (Professor at Collège de France and Director of RS2E) published in Science the first experimental observation of a reversible anionic activity in a “Li-rich”material based on Iridium. The starting point of this study was to use a model of the same structural family as the "Li-rich" materials, but with much more stable chemical bonds between the cation and anion networks in order to limit cationic migration. Thanks to a special technique of transmission electron microscopy (TEM), the structural response of the material to the charge/discharge cycles could be monitored at the atomic scale. The researchers were able to prove that the anionic redox activity is accompanied by a reorganization of the oxygen sublattice in peroxidized species (O2n-), in perfect agreement with theoretical predictions published in 2013 in Nature Materials. The value of the charge of these species could be evaluated precisely (O23-). Their stability towards dioxygen degassing (irreversible mechanism interfering with a good cycle life) could also be connected to the nature of the cation-anion chemical bonds in the material and indirectly to the potential at which these species are formed (< 4.3 Volt). Finally, the capacity decay occurring upon long cycling could be correlated with the formation and accumulation of stacking defaults in the layered structure of the material. A problem to solve in priority in order to commercialize these batteries!
Superposition of the pristine material's structure (black) and its charged structure (from neutron powder diffraction study).
The question of the anionic activity is clearly settled in this article and ends a long debate in the literature on the mechanism that underlies it and the factors that promote (or limit) the life span of “Li-rich” materials. In the early 2000s, Jean-Marie Tarascon’s team had already predicted the possibility of such anionic reactions but this proposal had not been unanimously accepted. Beyond the irrefutable evidence provided here, using transmission electron microscopy, the study highlights the possibility of tuning “infinitely” the “Li-rich” materials’ properties using chemical substitutions. Incidentally, Jean-Marie Tarascon declares: "Most of these compounds’ issues are explained; now we have to propose solutions to address them. Work has already begun including researches on encapsulated nanoparticles to limit their voltage decay."
Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries
Éric McCalla, Artem M. Abakumov, Matthieu Saubanère, Dominique Foix, Erik J. Berg, Gwenaëlle Rousse, Marie-Liesse Doublet, Danielle Gonbeau, Petr Novák, Gustaaf Van Tendeloo, Robert Dominko, Jean-Marie Tarascon
Science, 18/12/2015, DOI : 10.1126/science.aac8260