Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries

Non-aqueous Li-air or Li-O-2 cells show considerable promise as a very high energy density battery couple. Such cells, however, show sudden death at capacities far below their theoretical capacity and this, among other problems, limits their practicality. In this paper, we show that this sudden death arises from limited charge transport through the growing Li2O2 film to the Li2O2-electrolyte interface, and this limitation defines a critical film thickness, above which it is not possible to support electrochemistry at the Li2O2-electrolyte interface. We report both electrochemical experiments using a reversible internal redox couple and a first principles metal-insulator-metal charge transport model to probe the electrical conductivity through Li2O2 films produced during Li-O-2 discharge. Both experiment and theory show a "sudden death" in charge transport when film thickness is similar to 5 to 10 nm. The theoretical model shows that this occurs when the tunneling current through the film can no longer support the electrochemical current. Thus, engineering charge transport through Li2O2 is a serious challenge if Li-O-2 batteries are ever to reach their potential. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663385]