We report the current dependence of the fundamental kinetic overpotentials for Li–O2 discharge and charge (Tafel plots) that define the optimal cycle efficiency in a Li-air battery. Comparison of the unusual experimental Tafel plots obtained in a bulk electrolysis cell with those obtained by first-principles theory is semiquantitative. The kinetic overpotentials for any practical current density are very small, considerably less than polarization losses due to iR drops from the cell impedance in Li–O2 batteries. If only the kinetic overpotentials were present, then a discharge–charge voltaic cycle efficiency of ∼85% should be possible at ∼10 mA/cm2 superficial current density in a battery of ∼0.1 m2 total cathode area. We therefore suggest that minimizing the cell impedance is a more important problem than minimizing the kinetic overpotentials to develop higher current Li-air batteries.