Structural and Electrochemical Studies on NaMnO2 for Na-Ion Batteries

Introduction
  Layered Na_xMeO₂ (Me = transition metals) compounds have been intensively studied as electrode materials for Na-ion batteries. Among the layered oxides materials, O’3-type (distorted α-NaFeO₂ type) NaMnO₂ is a promising candidate as a positive electrode material for rechargeable Na batteries due to relatively high reversible capacity of more than 180 mAh g^-1 and material abundance.^1,2 Electrode reversibility is, however, significantly deteriorated during cycles at upper cut-off voltages of 3.4, 3.8 and 4.2 V. The structural change during charge was also reported by using ex-situ X-ray diffraction. While the formation of monoclinic Na_5/8MnO₂ superstructure with Mn charge ordering is observed,³ relationship between the phase transition and cycle stability for NaMnO₂is still unclear. In this study, the detailed structural change has been examined by in-situ and ex-situ X-ray diffraction to elucidate the reaction mechanism.

Experimental
  NaMnO₂ was synthesized by solid-state reaction. Na₂CO₃ and Mn₂O₃ were mixed, pelletized and then heated at 800 °C for 10 h in air. Crystal structures and morphology of the samples were examined by X-ray diffraction measurement and scanning electron microscopy (SEM). Galvanostatic charge/discharge tests were conducted using coin-type cells. Positive electrodes consisted of the active material, acetylene black and polyvinylidenefluoride (PVdF) with a gravimetric ratio of 80:10:10. Metallic sodium was used as a counter electrode. The electrolyte used was 1.0 mol dm^-3 NaClO₄ / PC : FEC (98 : 2 by volume) and 1.0 mol dm^-3 NaPF₆/ PC.

Results and Discussion
  Figure 1 shows galvanostatic charge and discharge curves of O’3-NaMnO₂ at a voltage range of 2.0 – 3.8 V. As reported by Ma et al.², the Na cell showed ca. 180 mAh g^-1 of initial discharge capacity with step-wise charge curve and the reversible capacity drastically decayed in the voltage rage. The plateaux during initial charge irreversibly disappeared and smooth curves were observed in initial discharge and subsequent cycles. In order to understand the phase transition during initial charge, in-situ X-ray diffraction measurement was carried out. The crystal structure of O’3-Na_xMnO₂ changed at least 6 times as shown in Fig. 2. The peaks of all the phase can be indexed as O3 or O’3-type structure and no phase transition into P3-type was observed, which is different from the formation energy of P3-type Na_0.5MnO₂ lower than that of O3-type estimated by first principles calculations.⁴ The thermodynamic stability of the phases would be different form that in electrochemical (de)intercalation. By charge beyond 3.6 V, no diffraction peaks were observed and broad peaks were seen after discharge. The reversibility of Na extraction/insertion from/into O’3-NaMnO₂ is, therefore, improved by change in the upper cut-off voltage to below 3.55 V. From these results, we will further discuss charge/discharge mechanisms of O’3-NaMnO₂as electrode materials for Na-ion batteries.

References
1.  A. Mendiboure, C. Delmas and P. Hagenmuller, J. Solid State Chem., 57, 323 (1985).
2.  X. H. Ma, H. L. Chen and G. Ceder, J. Electrochem. Soc., 158, A1307 (2011).
3.  X. Li, X. H. Ma, D. Su, L. Liu, R. Chisnell, S. P. Ong, H. L. Chen, A. Toumar, J. C. Idrobo, Y. C. Lei, J. M. Bai, F. Wang, J. W. Lynn, Y. S. Lee and G. Ceder, Nat. Mater., 13, 586 (2014).
4.  S. Kim, X. H. Ma, S. P. Ong and G. Ceder, Phys. Chem. Chem. Phys., 14, 15571 (2012).