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.
