Particle Shapes and Surface Structures of Olivine NaFePO4 for Sodium-Ion Batteries

The expansion of batteries into electric vehicle and grid storage applications has spurred the development of alternative cathode materials such as olivine phosphates. Building on previous studies of the lithium-ion cathode material LiFePO₄, we present atomistic simulations of olivine-structured NaFePO₄. We discuss differences in its morphology compared to its Li analogue, in the context of new simulations on the bulk properties.[1] The calculated equilibrium morphology is mostly isometric in appearance, with (010), (201) and (011) faces dominant. Exposure of a significant fraction of (010) surface is significant because it is normal to the most facile pathway for ion conduction. The equilibrium morphology of an NaFePO₄ nanoparticle is proportionally thinner along the [010] axis than that of an LiFePO₄ nanoparticle, which should also be beneficial for high ion intercalation-deintercalation rates because of the 1D nature of the conduction pathway.[2] We also simulated the effect of stabilising the (010) surface upon the particle morphology (shown below), concluding that a significant reduction in the surface energy is required to obtain the experimental “platelet” structure.

Figure: Variation in the equilibrium morphology of NaFePO₄ with the (010) surface energy, all other surface energies being held constant.

[1] R. Tripathi, S. M. Wood, M. S. Islam and L. F. Nazar, Energ. Environ. Sci., 6, 2257-2264 (2013).

[2] A. Whiteside, C. A. J. Fisher, S. C. Parker and M. S. Islam, Phys. Chem. Chem. Phys. (2014) (submitted).