First-Principles Study of the Thermodynamic Stability of Sodium Oxides As a Function of Temperature, Pressure and Particle Size; And Its Implications for Na−O2 Batteries

Recently, Na−O₂ batteries have shown their potential as a high energy density electrical storage. Unlike Li−O₂ batteries that have only one discharge product, Li₂O₂, the Na−O₂ battery system has three potential discharge products: Na₂O, Na₂O₂ and NaO₂. Recent experiments observed either Na₂O₂ or NaO₂ as a discharge product, and the performance of Na−O₂ batteries is undoubtedly influenced by which particular discharge product is formed.[1-4] We investigated the thermodynamic stability of each phase as a function of temperature, O₂ partial pressure, and particle size using first-principles calculations. Our results reveal that in bulk Na₂O₂ is the stable phase at the standard state (300 K, 1 atm) in agreement with experiments. Bulk NaO₂ can only be formed at P_O2 > 8.5 atm and/or T < 230 K (FIG. 1), both of which are unlikely to be accessed during the operation of typical Na−O₂ batteries. To understand the relative stability of these two competing compounds at the nanoscale, we calculated their surface energies and found that the lowest surface energies of Na₂O₂ are in the range of 30−45 meV/Ų, while that of NaO₂ is only 12 meV/Ų, rendering the possibility of stabilization of NaO₂ over Na₂O₂ at the nanoscale. To confirm this idea, we constructed the phase diagram of Na₂O₂ and NaO₂ as a function of particle size in FIG. 2. Indeed, the low surface energies of NaO₂ stabilizes NaO₂ nanoparticles, for example, up to 3 nm at P_O2 = 0.1 atm. Moreover, we found that the nucleation energy barrier for NaO₂ nanoparticles is much smaller than that of Na₂O₂, particularly when there is a small discharge overpotential and/or enough O₂ supply. We expect our findings to direct efforts towards understanding and controlling the formation of desired Na−O compounds in battery operation, and furthermore invigorate interest on the potential of Na−O₂ batteries.[5]

References

[1]       P. Hartmann, C. L. Bender, M. Vracar, A. K. Durr, A. Garsuch, J. Janek, and P. Adelhelm, Nat Mater 12, 228 (2013).

[2]       Q. Sun, Y. Yang, and Z.-W. Fu, Electrochemistry Communications 16, 22 (2012).

[3]       S. K. Das, S. Xu, and L. A. Archer, Electrochemistry Communications 27, 59 (2013).

[4]       J. Kim, H.-D. Lim, H. Gwon, and K. Kang, Physical Chemistry Chemical Physics 15, 3623 (2013).

[5]       S. Kang, Y. Mo, S. P. Ong, and G. Ceder, Nano Letters 14, 1016 (2014).