Manganese oxide materials are desirable cathode materials due to their low cost and environmentally benign character. Specifically, 2-D layered birnessite (R_yMnO₂·xH₂O, R = monovalent or divalent metal ions) materials are conceptually appealing because of their excellent ion-exchange properties. Edge-sharing MnO₆ octahedra constitute the MnO₂ layers where the interlayer spacing of 7 Å is occupied by metal ions and water molecules. Herein, we designed a study of a Mg cation containing birnessite with different crystallite sizes synthesized through a room temperature precipitation and ion exchange reaction, and characterized to determine structure and magnesium content. Electrochemical behavior was tested in lithium ion, sodium ion and magnesium ion based electrolytes, where the influence of crystallite size on electrochemical activity of Mg-birnessite was evaluated. Results and implications for future materials design will be discussed.