(76a) Sodium Octa-Vacancy Patterns in O3-NaMnO2 during Cycling

Sodium octa-vacancy patterns in O3-NaMnO2 during cycling

Masoud Aryanpour and Young-Gyoon Ryu

Advanced Materials Lab, SAIT-America

255 Main Street, Cambridge, MA-02142

Transition metal oxides have been under extensive experimental research to work as electrode materials in Na-ion batteries [1, 2]. Na-based electrochemical cells are among viable candidates to reduce the price of advanced batteries through offering similar, if not better, features provided by Li-ion chemistries.

Mere replacement of Li with Na in the known Li-TM-O compounds does necessarily lead to optimized materials for Na chemistry. Both theoretical and experimental results point to crucial differences between the two classes stemmed from the nature of their alkaline ion component. Despite the larger ionic radius of Na⁺, the ionic diffusion of Na in some layered oxide structures is even better than that of Li⁺.

As these cathode materials are charged, Na atoms are de-intercalated from the host structure, leaving behind their empty crystallographic sites. Vacant sites in the sodium layers create a driving force for the other ions to adjust their initial position, which promotes various phase changes. It is thus important to understand the effects of vacancy formation in the Na layers of electrode materials.

An important class of oxide materials is based on the layered O3-NaMnO2 [3,4], whether purely made of Mn or mixed with other transition metals such as Fe, V, Cr, Co, and Ni. It has been shown recently that the intra-layer patterns of Na atoms in this class of materials strongly depend on the chemistry of the involved transition metal [5].

In this computational work, we study the in-layer patterns formed by Na-vacancies in Na_xMnO2 at x=0.5. This Na concentration corresponds to the half charging state, where superstructures might form as a result of provided empty sites. Figure 1 shows one of the Na patterns that may form in this material. Small spheres represent vacant sites, while the large ones stand for an occupied Na site.

[1] BL Ellis and LF Nazar. “Sodium and sodium-ion energy storage batteries”, Current Opinion in Solid State and Materials Science, 16(4):168-177, 2012.

[2] C. Didier, M. Guignard, C. Denage, O. Szajwaj, S. Ito, I. Saadoune, J. Darriet, and C. Delmas, “Electrochemical Na-Deintercalation from NaVO2”, Electrochemical Solid-State Letters, 14(5), A75-A78, 2011

[3] M.M. Doeff, M.Y. Peng, Y. Ma, and L.C. De Jonghe, “Orthorhombic NaxMnO2 as Cathode Material for Secondary Sodium and Lithium Polymer Batteries”, Journal of Electrochemical Society, 141 (11), L145-L147

[4] X. Lin, et al, “Direct Visualization of the Jahn-Teller effect coupled to Na ordering in Na5/8MnO2, Nature Materials, 2014,  DOI: 10.1038/NMAT3964

[5]  M. Aryanpour, L. Miara, Y-G. Ryu, “Staging and In-Plane Superstructures Formed in Layered NaMO₂ {M = Sc, Ti, V, Cr, Mn} during Na De-Intercalation”, Journal of Electrochemical Society, 162 (4), A511-A519, 2015

Figure 1 One possible configuration in the sodium layer of O3-Na_xMnO2 (x=0.5) when used as the cathode of Na-ion batteries. The computational unit cell contains 16 Na-sites per layer, which is only half-occupied.

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