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Organic Mixed Ion and Electron Conductors for High-Energy Batteries

Periodic Reporting for period 4 - OMICON (Organic Mixed Ion and Electron Conductors for High-Energy Batteries)

Reporting period: 2019-10-01 to 2020-03-31

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 (Figure 1a,b) 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. Equally important is in depth understanding of the mechanisms during discharge and charge, particularly of metal-O2 cathodes, which are poorly understood both in terms of the ideal reactions and parasitic reactions.
We have achieved soft mixed conductors that allow for extended cyclability of Si anodes with minimum volume at every state of charge. For the O2 cathode we could unveil the single biggest barrier for long term operation: inherent formation of the highly reactive singlet oxygen. We developed a set of detection methods, which are now universally used by other groups. This way we identified formation mechanisms in great detail and laid the mechanistic foundations to mitigate singlet oxygen with redox mediators as mixed conductors.
We further established in-situ SAXS a powerful in situ metrology tool to quantitatively characterize morphologies and growth mechanisms in complex multi-phase systems in general, not limited to batteries or electrochemistry.
We achieved major progress with both mechanistic understanding of electrode processes, the synthesis of new mixed conducting materials, and new methods to investigate the processes. These are summarized as follows:
(I) With respect to the O2 cathode major progress has been achieved with mastering parasitic reactions. We identified singlet oxygen as the so far unknown source of irreversibility. To detect it we had to develope new methods and could show how to suppress the parasitic reactions. The results published in Nature Energy overturned the previous belief expressed in thousands of papers that superoxide/peroxide were the main sources of parasitic chemistry. In a series of papers, we clarified reaction mechanisms in depth which culminated into understanding how mixed conducting electrolytes using mediators can overcome the singlet oxygen issue.
(II) Small molecule and polymeric mixed conductors: we synthesized entirely new organic mixed conductors and demonstrated appropriate charge carrier mobility and mechanical flexibility to address high energy electrode materials with large volume expansions. We could demonstrate very favourable capacity (particularly based on total electrode mass and volume), 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.
(III) We established in-situ small angle X-ray scattering (SAXS) as a powerful tool for conversion type batteries by expanding the possibilities of in situ small/wide angle X-ray scattering (SAXS/WAXS) by developing a sophisticated data analysis strategy that makes accessible the rich quantitative information in the scattering data of complex electrochemical multi-phase systems, seamlessly covering atomic to sub-micron scales. SAXS is sensitive towards any means that generate electron density contrast between <1 to ~100 nm. The data contains hence rich structural and kinetic information, but inferring back to the complex multi-phase system is highly challenging. We demonstrated the power of the method using conversion type battery chemistries which are the core of this project. We used it in a work which overturns previous believes in O2 reduction mechanism.
(I) The lithium-oxygen battery promises the highest theoretical energy storage per cell mass. Realization in practice is, however, hampered by parasitic reactions Only better knowledge of parasitic reactions may allow them to be inhibited so that progress towards fully reversible cell operation can continue. We identified the highly reactive singlet oxygen (an excited form of oxygen) as being responsible for short cycle life and we showed how to suppress it based on detailed mechanistic understanding of its formation. These results are the foundation for to achieve highly reversible cell operation, 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 demonstrated a liquid redox material with solid like redox density to close some of the gap between supercapacitors and batteries.
(III) High capacity electrode materials go along with large volume expansions for which we elaborate flexible mixed conducting electrode by designing organic mixed conductors. These include mixed conducting polymers, small molecules, and mediators. We could show that for mixed conducting polymers the key properties of electronic and ionic conductivity and mechanical flexibility can be achieved in conjunction to allow for breathing electrodes, 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. In terms of mediators, we could estabilish the mechanistic foundation how they can both allow for forming mixed conducting electrodes to cycle O2 cathodes at high rates and at the same time how they need to work to suppress singlet oxygen as the major obstacle for reversible operation.
(IV) In terms of new methods a suite of methods to quantify singlet oxygen in non-aqueous battery chemistry as well as in-situ small angle X-ray scattering as an in situ metrology tool to quantitatively characterize morphologies and growth mechanisms in complex multi-phase systems in general, not limited to batteries or electrochemistry.
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