Layered Na_xMnO_2+z in sodium ion batteries-influence of morphology on cycle performance

Due to its potential cost advantage, sodium ion batteries could become a commercial alternative to lithium ion batteries. One promising cathode material for this type of battery is layered sodium manganese oxide. In this investigation we report on the influence of morphology on cycle performance for the layered Na_xMnO_2+z. Hollow spheres of Na _xMnO_2+z with a diameter of ∼5 μm were compared to flake-like Na_xMnO_2+z. It was found that the electrochemical behavior of both materials as measured by cyclic voltammetry is comparable. However, the cycle stability of the spheres is significantly higher, with 94 mA h g^-1 discharge capacity after 100 cycles, as opposed to 73 mA h g^-1 for the flakes (50 mA g^-1). The better stability can potentially be attributed to better accommodation of volume changes of the material due to its spherical morphology, better contact with the added conductive carbon, and higher electrode/electrolyte interface owing to better wetting of the active material with the electrolyte.