Towards Improved Energy Efficiency of Aprotic Li-O2 Batteries

Aprotic Li-O₂ batteries have attracted intensive focus worldwide owing to its high energy density (up to 2-3 kWh kg^-1), which is theoretically beyond that provided by any other rechargeable devices. Non-aqueous Li-O₂ chemistry is based on reversible formation-decomposition of electrically insulating and corrosive Li₂O₂ at the cathode interface. Presumably, a chemically stable and conductive cathode interface is prerequisite for sustainable cell operation. Until very recently, carbon was the ubiquitous choice of cathode host in a Li-O₂ cell. However, it is now accepted that the corrosion processes triggered by Li₂O₂ and the in-situ generated reactive oxygen intermediates render the Li-O₂ cell employing carbon based cathodes highly energy-inefficient (~70%). And a major part of this inefficiency arises out of very high charge overpotential (1-1.5 V) and incomplete charge. In this context, non-carbonaceous cathode hosts possessing stable conductive interface for reduced polarization in O₂ evolution, and soluble oxidation catalysts capable of Li₂O₂ oxidation without a direct electrical contact with the cathode are gaining immense interest. Here, we introduce novel high surface area and conductive inorganic nanostructures as cathode host in Li-O₂ cells that improves charge overpotential to a great extent as a direct consequence of suppressed cathode corrosion. We also propose a novel redox mediator with highly favorable features for soluble oxidation catalysis. Characterization techniques ranging from electron microscopy, surface spectroscopy to operando electrochemical mass spectrometry have been applied to investigate the viability of the proposed systems. Through this presentation we not only discuss novel materials and catalysts capable of electrocatalytic oxygen reduction and evolution with improved voltage characteristics, but also present a new understanding of the critical parameters for the positive electrodes in aprotic Li-O₂ batteries.