Sodium containing 2 x 4 (0.46nm x 0.92nm) tunnel structured manganese dioxide (Na-2 x 4-MnO2) nanofibers are hydrothermally synthesized. The nanofibrous Na-2 x 4-MnO2 was thoroughly characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), inductively coupled plasma optical emission spectroscopy (ICP-OES), N-2 sorption, and electron microscopy (SEM and TEM) and is assigned a formula of Na0.25MnO2.0.16H(2)O. The electrochemistry of Na-2 x 4-MnO2 in non-aqueous Li, Na and Mg batteries is compared for the first time and first cycle delivered capacities were found to be 152mAh/g, 86mAh/g, and 307mAh/g, respectively. Characterization of the active material after electrochemical tests demonstrated that Na-2 x 4-MnO2 preserved both its crystallographic structure and nanofibrous morphology after cycling in Na and Li cells, however, Na-2 x 4-MnO2 lost its crystalline structure after cycling in a Mg based system. (C) 2019 The Electrochemical Society.