Lithium and oxygen vacancies and their role in Li2O2 charge transport in Li–O2 batteries

Abstract

It is now well accepted that charge transport through Li₂O₂ is a serious limitation in Li–O₂ batteries. We report formation energies for the different charge states of Li, O and O₂ vacancies in Li₂O₂ that could have important implications for charge transport through Li₂O₂. Charge transition levels are given as a function of the location of the Fermi level in Li₂O₂ relative to the valence band maximum (VBM). We argue that in Li–O₂ discharge/charge electrochemistry, the Fermi level is pinned by LiO₂ interface states at ∼0.35 eV above the VBM and this causes the vacancies to be in positively charged states (weakly violating local charge neutrality). We show by non-equilibrium Greens function calculations of charge transport through a Au|Li₂O₂ + V_Li^q |Au metal–insulator–metal structure (with V_Li^q a Li vacancy of charge state q), that the +1 and 0 charge state induces considerable scattering for tunneling holes. This tunneling is the previously proposed dominant mechanism of charge transport in Li–O₂ batteries at practical current densities at room temperature, although we also proposed a contribution from hole polaron migration at very low currents and higher temperatures. We suggest that scattering of the tunneling holes by the positively charged vacancies (and possibly hole polarons) is the origin of the resistive loss observed in Li–O₂ discharges in bulk electrolysis cells (where other forms of resistance are negligible). Thus, we argue that charged vacancies hinder charge transport through Li₂O₂ in Li–O₂ electrochemical discharges.