Nature Communications 5: Article number: 3990 (2014); Published 2 June 2014; Updated 7 December 2015

In Fig. 2a of the main manuscript and in Supplementary Fig. 3, the unit label for C−2 (on the vertical axis) should read (cm4/μF2) instead of (m4/μF2).

This correction only applies to the labels in these two figures and does not affect any of the calculations or conclusions in the paper.