Investigation of NaxMn0.5Fe0.5O2 Cathode Materials from Low Cost Preparation Routes

The wide use of lithium in rechargeable batteries has made it strategic and expensive and therefore different approaches have been recently investigated to overcome this issue.  Even though sodium has higher atomic weight, higher oxidation potential and bigger ionic radius than lithium, it can be a valuable alternative in less weight/volume-sensitive applications because of its wider availability and lower costs. At the present, the most promising sodium systems use a transition metal oxide and hard carbon as positive and negative electrode, respectively. In this configuration, considering the material specific capacity and the discharge profiles, the battery could be reasonably able to display an energy density of about 70-80% of lithium analogous with unexplored power density. However, several polycrystalline compounds show lower diffusivity barrier for Na⁺ compared to Li⁺ homologous structures. Among the oxide materials, different phases have been proposed like NaCrO₂, Na_xCoO₂, Na_xMnO₂, and Na_xMn_0.5Fe_0.5O₂showing capacity about 100-150 mAh/g in the potential range from 2.0 to 4.0 V vs. metallic sodium with good kinetic. In particular, the Na_0.67[Fe_0.5Mn_0.5]O₂P2 structure has received significant attention due to the presence of the relative inexpensive and environmental friendly Mn and Fe in the structure.

We report here two different low cost methods to prepare Na_0.67[Mn_0.5Fe_0.5]O₂ (NMFO); the former route is based on sol gel approach from acetate precursors in methanol while the latter one on the use of cotton as template agent. The suitable chemical precursors as well as the thermal treatment procedures were carefully selected after TGA/DSC analysis of various reaction mixtures. The resulting phases have been structural characterized by XRPD with Rietveld analysis which reveals that a small amount of hematite (< 5%) is present in both samples. The morphological analysis by SEM and TEM confirm the good crystallinity and show the characteristic platelet shape having several mm largeness and few hundred nm thickness. The electrochemical characterization was performed in 3 electrode Swagelock type cell using metallic sodium as both counter and reference electrode and a 1M solution of NaClO₄in EC:DMC 1:1 + FEC 2% as electrolyte. The charge discharge profiles show the two characteristic plateau at low (2.3/2.7 V) and high potential (3.6/4.0 V) vs. metallic sodium. The galvanostatic cycling results (see Figure 1, sol gel material) reveals a reversible specific capacity of about 120, 110, and 60 mAh/g at current rates of C/20, C/10, and C/2, respectively.

In conclusion, both low cost methods are able to produce micrometric sized NMFO materials with good electrochemical properties as cathodes in sodium ion battery.