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 20 h directly results in nanocrystalline NaMn2O4.
• Thermally treated oxide shows ∼95 mAh/g capacity in the 2–4.5 V window.
• Capacities from ∼75 to 95 mAh/g obtained with varying voltage windows.
• Oxide exhibits 0.3%/cycle fade in capacity when cycled in the 2–4 V window.

Nanocrystalline NaMn2O4 with a crystallite size of ∼8–10 nm exhibiting a new close packed hexagonal crystalline form, different from the known stable orthorhombic (Pbam or Pmnm symmetry) or monoclinic structures common to the Na–Mn–O system, has been synthesized by a high energy mechano-chemical milling process (HEMM) using Na2O2 and Mn2O3 as starting materials. The newly synthesized structure of NaMn2O4 has been studied as a cathode for sodium ion rechargeable batteries. The HEMM derived NaMn2O4 shows a 1st cycle discharge capacity ∼75 mAh/g, ∼86 mAh/g and ∼95 mAh/g when cycled at a rate of ∼40 mA/g in the potential window ∼2.0–4.0 V, ∼2–4.2 V and ∼2–4.5 V, respectively. The nanostructured NaMn2O4 shows a fade in capacity of 0.3% per cycle and a moderate rate capability when cycled in the potential window 2–4 V. However, electrolyte decomposition occurring during charging of the electrode above ∼3.8 V needs to be resolved in order utilize the full capacity of NaMn2O4 as well as improve the stability of the electrode.