Radiation Chemistry: Present Status and Future Trends (Studies in Physical and Theoretical Chemistry)
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In my opinion, this is an opportunity that physicists have not sufficiently explored. Most encouraging of all, after thirty years in existence this chemistry still offers vast and original prospects for the synthesis of eco-compatible materials.
This is the LiCoO 2 lamellar phase, used in the first commercial lithium-ion accumulators, with a layered structure as pointed out earlier. Although these developments strongly contributed to the development of Li-ion technology, they nevertheless raised many fundamental questions, first of all regarding the possibility of removing all the Li from the initial LiCoO 2 phase.
Chemical oxidation using strong oxidizing agents particularly Br 2 was unsuccessful, as it proved to be incomplete. At this stage, it was important to know which redox pair we had activated within LiCoO 2 through the total removal of Li. Had we driven the Co 3 to its higher degree of oxidation, as is customary, or had we oxidized the anionic network, a rather unusual situation in chemistry?
This was therefore quite a provocative scenario, in which copper Cu became more electronegative than oxygen Fig. The band structure of layered oxides. The displacement of the bands leads the p band of the oxygen to overlap with and spill into the d band of the metal, causing holes to form on the oxygen band. Moreover, it was on the basis of this similarity that, in an article written in in memory of Jean Rouxel, I suggested that the layered Co phases could be superconductive. From a fundamental point of view, the challenge was therefore twofold.
For these materials to be commercially viable, we needed to understand not only the origin of this enhanced capacity, which exceeded theoretical capacity, but also the drop in potential. With only one redox Ru centre left, since tin Sn is electrochemically inactive, these phases are model materials for analytical studies seeking to understand their reactional mechanisms. The identification by EPR not only of the presence of the peroxo group but also of its concentration allowed us to discover the reactional mechanism of Li insertion-extrusion in these Li-rich lamellar compounds, which amounts to a game of band structure overlapping.
This compound also includes a redox element Nb in the fourth period, which was previously overlooked, as it was too heavy. We can thus see that this activity of the anionic network broadens the range of efficient battery materials. And so it is that often, in the world of research, new concepts benefit from the emulation created by internationalization, and open up new horizons when they reach maturity.
To get further insights into these issues, we turned to high-resolution electronic microscopy, a technique to visualize atoms of a size and distance of the order of the angstrom, i. Up to this point, I have been talking about the layers and the place of the atoms as they were deduced by X-ray diffraction.
Now, thanks to high-resolution microscopy, they can be visualized directly, with both heavy atoms appearing as two white spots due to their significant electronic charge, whereas the lithium is not visible given its low atomic number.
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The question then was a matter of knowing what happened to this arrangement when lithium was inserted into and then extruded from the material during electrochemical cycling. Unexpectedly, during the charge we observed a mass migration of the cations, which nevertheless returned roughly to their initial position upon the following discharge, thus confirming the reversibility of the system but providing no clues as to the drop in potential. For tin compounds Li 2 Ru 0. This contrasts with the Li-rich metal compounds in the third period Li 1. What are these sites? Looking at the crystallographic structure, we can see that they are tetrahedral sites.
What happens, therefore, is that when the atoms migrate from the layer to the interlayer space during cycling, some of them stay trapped in the tetrahedral sites, which explains the drop in potential during cycling. Now that we have found the origin of the problem, we are in the process of developing customized materials to bypass it. I think it would be wiser for our institutions to promote a complete synergy between science and technology, so as to answer rapidly the problems faced by society, rather than setting fundamental research in opposition to applied research, as is unfortunately often the case.
I will first point out that a compound, in itself, is useless. It is only the material, which should be seen as the assemblage of a chemical composition, of a means of development and of a function that can be useful. In the case of sustainable development, this implies that: i its composition must contain only abundant and non-toxic elements; ii its production must involve only low-energy-consumption processes; and iii its performance for the targeted application must be appealing for example, with respect to potential in the case of electrode materials.
There are two options. They can either use combinatorial experimental chemistry, a tedious approach with random results, or practise deductive chemistry. The second path is the one I chose and am describing to you now, which consists in: i proceeding by analogy; ii taking advantage of the strong structure-property coupling; and iii drawing on the understanding of reaction mechanisms to make an informed synthesis.
To illustrate this approach, I will take the electrode material currently most prized, LiFePO 4 , the aim being to increase its potential, which is only 3. Based on the established electrochemical property-structure relations, we know that the potential is especially high given that the Fe-O bond is ionic. Based on the periodic table, this therefore means the phosphate entity needs to be replaced with the sulphate entity, with the concomitant addition of fluorine, which is more electronegative than oxygen.
While the compound LiFeSO 4 F met our criteria, it still needed to be synthesized, using low-energy means. Even though the aqueous medium is ideal, sulphates are soluble therein, meaning that we needed to find an alternative. While the isolation of this new compound is certainly a progress, the most exciting was determining its formation mechanism, in other words, the key to the reaction.
Once we had grasped this mechanism, we were able to generalize it, and pretty soon over 20 new Li and even Na and K fluorosulphates were obtained. Understanding reactional mechanisms is therefore crucial in chemical synthesis. In fact I will point out that if a reaction identical to the one described above is carried out without ionic liquid, the result is a compound with the same formula, LiFeSO 4 F, but with a very different structure: a polymorph with a triplite structure.
Solid-state chemistry is therefore a science at the crossroads between the expected and the unexpected. While the expected is of course intellectually pleasing, the unexpected is just as interesting, as it always opens up new perspectives. An extension of this work 17 , based on a controlled ionothermal synthesis and solid-state reactivity, allowed for a whole new class of electrode materials to be developed including, in addition to fluorosulphates: oxysulphates, hydroxosulphates and lithium-bearing sulphates, made of transition metals, which were unknown three or four years ago Fig.
Comparison of lamellar oxides and polyanionic compounds in terms of electrochemical performance.
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Oxide-based accumulators, because of their high energy density and therefore their high autonomy, are mainly used for portable electronics. By contrast, accumulators made with polyanionic compounds, because of their abundance and low cost, target larger volume applications electric vehicles and others. Solid-state chemists generally turn to the chemistry of life to try and achieve this.
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The prime example relates to well-known unicellular algae, diatoms, which are able to concentrate the silicon contained in sea-water in order to create highly textured silica shells. To broaden the spectrum of biomineralizable materials, we turned to the use of other simpler microorganisms, such as bacteria, and even yeasts, which are unicellular fungi.
Let us look at two examples. Its role here was mainly owed to the fact that its enzyme, urease, can hydrolyse urea to produce the medium basicity needed to precipitate LiFePO 4. This was confirmed by high-resolution microscopy, showing that the bacterium is surrounded by a biofilm inside of which small fine needles can be observed, with its diffraction pattern indicating the presence of LiFePO 4. It took a long time for this approach, though elegant, to make it beyond laboratory curiosity, due to issues with reproducing and upscaling it. The precipitated nanometric particles Fig.
Thanks to this original alveolar structure induced by the bacterium, these textured hematite samples display interesting electrochemical properties in terms of potential behaviour when they are used as electrode materials. The biologically assisted synthesis of textured electrodes. These two aspects highlight the fact that solid-state chemistry is a highly adaptive science, which can meet societal demands in the framework of sustainable development.
In this context I can cite the recent work of a Korean group which successfully prepared lamellar oxide particles with a concentration gradient, by combining soft chemistry and high temperatures. This is another booming aspect of solid-state chemistry. The best evidence thereof is probably the emergence of the electric vehicle. What was long thought of as an elusive idea will play a major role in the car industry in coming years. I am convinced that technological challenges bring about new scientific problems, and that they can be resolved through fundamental research.
The materials and systems we are currently designing must be more sophisticated, miniaturized, recyclable, environmentally friendly, energy saving, highly reliable, and cheap. Therefore, only with an interdisciplinary chemistry will we be able to advance in this quest for ideal materials for systems of varied complexities. The scope of solid-state chemistry is currently extending to new domains such as biology, with the development of materials produced using bio-inspired synthesis processes, and the chemistry of organic-inorganic hybrid materials.
I will therefore use this opportunity to delve further into the issue of energy. Of course, the chemical bond, the common denominator of the society of atoms constituting crystal, will be the guiding theme. I will conclude by returning to the question of time.
As I pointed out in my introduction, we need to double our energy production. Our hopes are riding on materials and we must be optimistic about our capacities to design better ones. Yet, unlike past generations, we have neither thousands of years nor centuries, but only thirty to forty years to double our energy production, with the additional constraint of sustainable development. What are the odds? What can we hope for? Fortunately, we have a periodic table full of elements.
This is certainly a great advantage, as we can design and sculpt new materials as we please, with properties exacerbated by eco-compatible approaches.
But it can also rapidly turn into a nightmare: given the large number of possible combinations, it is difficult to find the winning composition. US President Barack Obama has made this aim one of his five scientific priorities for the next decade. X-ray diffraction has allowed us to understand the arrangement of atoms, and microscopy has allowed us to see them.
Why not dream and hope that we will one day be able to see these famous electrons? This may seem like an adventurous gamble. This is an ambitious dream, the realization of which would trigger a scientific revolution comparable to the observation of the atom through microscopy. It would radically change our material design and elaboration strategies. This is an important message that I wish to repeat again and again for our future generations, who all too often blindly limit themselves to theoretical calculations, to the point of forgetting about experiments.
Theory and experiments certainly make a wonderful pair that satisfies our intellect and allows for new compounds to be prepared.
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But it is worth nothing if it is not used to make useful materials. That is why systemic approaches based on cooperation and multidisciplinarity are necessary. No proverb illustrates this point better than this citation of Kenneth G. All with the clear objective of better managing the energy resources of our planet and preserving it for future generations. I would also like to thank all the national research institutions, particularly the Ministry of Research and Higher Education and the CNRS, for their support in setting up the energy storage network RS2E.
Many thanks as well to all the brilliant and talented researchers with whom I have collaborated throughout the world. Without them, many of the advances described in this inaugural lecture could not have been exploited. Walter, P. Martinetto, G. Tsoucaris, R. Lefebvre, G.
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