Electrochemical properties of a new nanocrystalline NaMn2O4 cathode for rechargeable sodium ion batteries

•Nanocrystalline NaMn2O4 exhibiting a new crystalline form has been synthesized by high energy mechanical milling.•Mechanical milling for 20h directly results in nanocrystalline NaMn2O4.•Thermally treated oxide shows ∼95mAh/g capacity in the 2–4.5V window.•Capacities from ∼75 to 95mAh/g obtained with varying voltage windows.•Oxide exhibits 0.3%/cycle fade in capacity when cycled in the 2–4V window.