This report describes the first detailed electrochemical examination of a series of copper birnessite samples under lithium-based battery conditions, allowing a structure/function analysis of the electrochemistry and related material properties. To obtain the series of copper birnessite samples, a novel synthetic approach for the preparation of copper birnessite, CuxMnOy center dot nH(2)O is reported. The copper content (x) in CuxMnOy center dot nH(2)O, 0.28 >= x >= 0.20, was inversely proportional to crystallite size, which ranged from 12 to 19 nm. The electrochemistry under lithium-based battery conditions showed that the higher copper content (x = 0.28) and small crystallite size (similar to 12 nm) sample delivered similar to 194 mAh/g, about 20% higher capacity than the low copper content (x = 0.22) and larger crystallite size (similar to 19 nm) material. In addition, CuxMnOy center dot nH(2)O displays quasi-reversible electrochemistry in magnesium based electrolytes, indicating that copper birnessite could be a candidate for future application in magnesium-ion batteries. (C) The Author(s) 2015. Published by ECS.