Comparative Studies of Li Versus Na System: Phase Diagram and New Polyanion Cathodes

“Abundance” is the important key word in materials development. This is particularly the case for energy storage sector, where materials themselves function as a storage host; the amount of materials directly link to the amount of energy stored in a device.  At the present stage, Na-ion batteries in not a strong competitor for Li-ion chemistry by only considering large abundance compared to the well established state-of-art Li-ion power packs. However, it has rapidly attract many material scientist; both weak Lewis acidity and large polarizability of Na-ion allow fast charge transfer reaction at electrode/electrolyte interface and in bulk diffusion.^1,2Furthermore, unlike an incremental progress of the mature Li-ion systems, the research of the immature Na-ion systems based on Na-ion intercalation is likely to discover new electrodes with ordered functional structure due to difference of ionic radius of Na and 3d transition metal, which may enable Na-ion batteries to surpass the Li-ion batteries. Abundance of Na should not be compensated for by combination with rare transition metals, thereby Fe is definitely the best to functionalize as a redox center.

In spite of the great success of olivine Li_xFePO₄, the sodium analogue Na_xFePO₄ with the same structure has turned out to be unsuitable for an electrode with completely different phase diagram³. The existence of an intermediate phase at x = 2/3 in Na case, which is very stable up to 500°C, is somehow anticipated to relax the large lattice strain between x= 0 and x = 1 phases. However, even with the existence of an intermediate phase at Na_2/3FePO₄ one notices that the expansion from FePO₄ to Na_2/3FePO₄ is still quite large (12.8%), nearly 2 times that for Li_xFePO₄ to change from x=0 to 1, which may disturb to sustain a coherent interface for smooth electrode reaction. Although switch to Na from Li is the conceptually obvious way to follow, simple extension by just replacing lithium to sodium does not always lead to similar performances, and in many cases ended up with in failure. Therefore, strategies should be directed toward specific exploration of new materials suitable for sodium system. One of our recent achievements was new material, Na₂FeP₂O₇, with high rate performance and extreme thermal stability⁴, and Sumitomo electric company chose Na₂FeP₂O₇ in parallel with NaCrO₂as cathode materials for their prototype Na-ion battery pack, but its operating voltage was limited below 3.0 V versus Na.

Maximizing the redox function of Fe has been intensively studied in the lithium system, from which we can expect to realize ~3.6 V operation in Na system⁵. However, the present status is still far from this. Operating voltage and reversible capacity of various known iron based cathode for sodium-“ion” (cathode functions as whole sodium source) battery system are summarized in Figure 1. Here, we report a log-awaited new sodium battery material to reach primary target of >3.6 V operation and capacity around 120 mAh/g (dashed circle in Figure 1) being able to lead to energy density of >450 Wh/kg, which is comparable to those of LiMn₂O₄ (430 Wh/kg) and LiFePO₄ (500 Wh/kg) in lithium system⁶. 

1. M. Okoshi, Y. Yamada, A. Yamada, H. Nakai, J. Electrochem. Soc. 160 A2160 (2013)
2. S. P. Ong, V. L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma, G. Ceder, Energy Environ. Sci. 4 3680 (2011)
3. J. Lu, S. C. Chung, S. Nishimura, A. Yamada, Chem. Mater. 25 4557 (2013)
4. P. Barpanda, T. Ye, S. Nishimura, S. C. Chung, Y. Yamada, M. Ohkubo, H. Zhou, A. Yamada, Electrochem. Comm. 24 116 (2012)
5. T. Ye, P. Barpanda, S. Nishimura, N. Furuta, S. C. Chung, A. Yamada, Chem. Mater. 25 3623 (2013)
6. A. Yamada, P. Barpanda, S. Nishimura, G. Oyama, Patent JP-2013-187914 (2013)