Electrochemical properties of a new nanocrystalline NaMn_2O_4 cathode for rechargeable sodium ion batteries

摘要

Nanocrystalline NaMn_2O_4 with a crystallite size of ～8-10nm 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 Na_2O_2 and Mn_2O_3 as starting materials. The newly synthesized structure of NaMn_2O_4 has been studied as a cathode for sodium ion rechargeable batteries. The HEMM derived NaMn_2O_4 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 NaMn_2O_4 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 NaMn_2O_4 as well as improve the stability of the electrode.