Structural and Electrochemical Characterization of New Hexagonal Form of NaMn2O4 Cathodes for Sodium Ion Batteries
Lithium ion batteries have thus far been the torchbearers in the field of electrochemical energy storage for mobile applications. The large scale commercialization of lithium ion batteries, however has the associated ‘cost of business’ including possible socio-economic concerns to various lithium rich nations such as Bolivia(1). There has therefore been a gradual realization in the battery community at large for the need to switch to more abundant raw-material based chemistries. Sodium and magnesium batteries are the fore-runners therein(2). In order for the smooth transition and for Na-ion to compete with the Li-ion batteries, considerable improvements in cathode and anode energy density, cyclability and rate capability are still required.
Nanocrystalline NaMn2O4 has been synthesized by a high energy mechano-chemical milling process (HEMM) using Na2O2 and Mn2O3 as starting materials for use as cathodes for sodium ion batteries. As previously reported by this group(3), the material exhibits a new close packed hexagonal crystalline form, different from the commonly known stable orthorhombic or monoclinic structures in the Na–Mn–O system, identified for the first time.
In this work, we study the stabilization of the hexagonal phase and its evolution to an orthorhombic phase upon heat treatment. Using in-depth solid-state NMR studies in conjunction with detailed electrochemical evaluation, the nature of the charge-storage behavior of this phase is evaluated herein. Figure 1 depicts the evolution of the hexagonal NaMn2O4 to a conventional orthorhombic phase upon heat-treatment. Alteration in sodium co-ordination chemistry and its effect on insertion/deinsertion and capacity will be presented and discussed.
Figure 1: Evolution of structure of hexagonal NaMn2O4 into the more known orthorhombic phase upon heat-treatment as observed using scanning electron microscopy.
1. R. Aguilar-Fernandez, Estimating the Opportunity Cost of Lithium extraction in the Salar de Uyuni, Bolivia, in, Duke University (2009).
2. B. L. Ellis and L. F. Nazar, Current Opinion in Solid State and Materials Science, 16, 168 (2012).