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
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.
(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.
(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.