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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - OMICON (Organic Mixed Ion and Electron Conductors for High-Energy Batteries)

Teaser

Summary

Energy storage is undeniably amongst the greatest societal challenges. Batteries will be key enablers but require major progress. Battery materials that promise a step-change in energy density compared with current Li-ion batteries rely on fundamentally different reactions to store charge. These include replacing the graphite anode of a Li-ion battery with Si or Sn alloying. Currently used intercalation cathode materials may be replaced by the O2 cathode or the S cathode. Next higher energy stored per volume and mass of battery, they avoid scarce and expensive chemical elements in favour of amply available ones. These new storage principles are, however, much more difficult to realize in practice than the currently used charge storage materials. The new charge storage materials have in common high volume changes on cycling and poor conductivity. For the active component of a battery electrode to function it must be simultaneously in contact with ionic and electronic pathways to electrolyte and current collector. State-of-the-art conducting additives and binders in the composite electrodes cannot ensure ideal contact for such materials and fail to exploit their full potential. In this project we directly target these fundamental challenges of high-energy batteries by replacing now used conducting additives and binders with flexible organic mixed ion and electron conductors that follow volume changes to ensure at any stage intimate contact with ions and electrons. The significant advantage, next to intimate contact, is that the packing density of active material can be maximized. This boosts energy stored by total electrode mass and volume by rigorously cutting the amount of non-active materials compared with current approaches. The expected overriding scientific impact includes improved understanding of mixed conductors concerning synthesis, structure, conductivity and their behaviour in the complex battery environment. This opens up new perspectives for the realm of high-capacity battery materials that demand such a breakthrough to succeed.

Work performed

In the first period of the project we could achieve major progress with both mechanistic understanding of electrode processes and the synthesis of new mixed conducting materials. These are summarized as follows:
(I) With respect to the O2 cathode major progress has been achieved with redox mediators to address the insulating electrode material Li2O2. A fundamental study on mediators for high rate energy storage is now in press in the top journal Nature Materials. To make further progress with the O2 cathode it is crucial to master parasitic reactions (in any battery the reason for capacity fading). We could now clarify the so far unknown major source of irreversibility to be singlet oxygen. It is highly difficult to detect which is why we developed new analytical methods and could show how to suppress the parasitic reactions. The was published in the top journal Nature Energy and overturned the previous belief expressed in thousands of papers that superoxide/peroxide were the main sources of parasitic chemistry. The Na-O2 cell suffers a similar problem (published in Angew. Chem. Int. Ed.)
(II) Small molecule and polymeric mixed conductors: we synthesized entirely new organic mixed conductors and could so far demonstrate appropriate charge carrier mobility and initial characterization to address high energy electrode materials. We could demonstrate very favourable capacity, efficiency, rate capability, and cycle life for Si alloying. We are also the first to demonstrate electron conduction in a liquid organic medium. For characterization we developed new experimental methods. Further publications resulting so far from the project are an invited review article and a comment piece in the top journals Chemical Sciences and Nature Energy and a book chapter.

Final results

(I) The lithium-oxygen battery promises the highest theoretical energy storage per cell mass. Realization in practice is, however, hampered by various issues. Parasitic reactions are one of the biggest obstacle. Only better knowledge of parasitic reactions may allow them to be inhibited so that progress towards fully reversible cell operation can continue. We could identify the highly reactive singlet oxygen (an excited form of normal oxygen) as being mostly responsible for short cycle and we could show how to suppress it. These results form now the foundation for future research towards achieving highly reversible cell operation, which is required for further development of metal-O2 batteries towards a major energy storage technology.
(II) There is a well-known gap in energy and power between supercapacitors and batteries. High-energy batteries have limited power and high-power supercapacitors provide little energy. The problem is rooted in slow ion mobility in solid charge storage materials. We could now demonstrate that a liquid redox material with solid like redox density can close some of the gap between supercapacitors and batteries and open the way to high-capacity/high-rate charge storage.
(III) Further research is underway to make flexible mixed conducting electrode by designing organic mixed conductors. Results so far show that the key properties of electronic and ionic conductivity and mechanical flexibility can be achieved, which are required to make beyond-intercalation electrode with maxium packing density, and thus to achieve a step-change in energy stored per mass and volume of battery.