Comparative Studies of Li Versus Na System: Phase Diagram and New Polyanion Cathodes
Authors: Lu Jiechen , Chung Sai-Cheong , Barpanda Prabeer , Oyama Gosuke , Nishimura Shinichi , Okubo Masashi , Yamada Atsuo 
Published: February 2020

“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 LixFePO4, the sodium analogue NaxFePO4 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 Na2/3FePO4 one notices that the expansion from FePO4 to Na2/3FePO4 is still quite large (12.8%), nearly 2 times that for Li
x
FePO4 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.

M. Okoshi, Y. Yamada, A. Yamada, H. Nakai, J. Electrochem. Soc.
160 A2160 (2013)

S. P. Ong, V. L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma, G. Ceder, Energy Environ. Sci.
4 3680 (2011)

J. Lu, S. C. Chung, S. Nishimura, A. Yamada, Chem. Mater.
25 4557 (2013)

P. Barpanda, T. Ye, S. Nishimura, S. C. Chung, Y. Yamada, M. Ohkubo, H. Zhou, A. Yamada, Electrochem. Comm.
24 116 (2012)

T. Ye, P. Barpanda, S. Nishimura, N. Furuta, S. C. Chung, A. Yamada, Chem. Mater.
25 3623 (2013)

A. Yamada, P. Barpanda, S. Nishimura, G. Oyama, Patent JP-2013-187914 (2013)