The authors regret a mistake in the sentence “… shows a gravimetric energy density of 200 mAhr-g−1 with an average discharge voltage of ∼3.5 V”. The correct values are 180 mAhr-g−1 with an average discharge voltage of ∼3.7 V for Cr-doped Mn-oxide. The correct text should read “… shows a gravimetric energy density of 180 mAhr-g−1 with an average discharge voltage of ∼3.7 V”. The authors would like to apologise for any inconvenience caused.