The worldwide demand to develop electrical energy storage is growing as renewable energy technologies such as wind and solar energy conversion become increasingly prevalent. The large Na abundance, low cost, and suitable redox potential of rechargeable Na-ion batteries (NIBs) indicate its great promise for energy storage applications. Among many cathode candidates, layered oxides (Na_xTMO₂) offer many advantages due to their simple structures, high capacities, and ease of synthesis.¹ It has been shown that P2-Na_2/3[Ni_1/3Mn_2/3]O₂ used as cathode in Na cells reversibly exchanges all of the Na ions, leading to a capacity of 160 mAh g^-1 between 2.0 and 4.5 V.² Komaba et al. reported that the O3-NaNi_0.5Mn_0.5O₂ electrode exhibits 105 mAh g^-1 at 1C (240 mA g^-1) and 125 mAh g^-1 at C/30 (8 mA g^-1) in the voltage range of 2.2-3.8V and displays 75% of the capacity after 50 cycles.³ The same group also reported that Na_2/3[Fe_1/2Mn_1/2]O₂ delivers an exceptional initial capacity of 190 mAh g^-1 between 1.5-4.2 V.⁴However, most of these materials undergo phase transformations leading to several voltage steps in their electrochemical profiles, ultimately, shortening the cycle life and reduce rate capabilities.

Herein, we propose a new P2-type Na_0.78Ni_0.23Mn_0.69O₂ as a potential cathode for Na-ion batteries, which shows superb rate performance. The material is synthesized through solid state method, and the crystal structure has been studied by X-ray diffraction. As shown in Figure 1 (a), when working between 2.0 to 4.5 V, it exhibits 135 mAh g^-1of specific capacity at a lower rate of 0.1 C, yet it still delivers 120 mAh/g of the reversible capacity up to 5 C. In addition, the material also shows very stable cycling performance. According to Figure 1 (b), 90% of the initial capacity can be maintained up to 50 cycles. Various materials characterizations are being conducted to investigate the working mechanisms of this material. The understanding of its high power property as cathode material will shed light on the future of Na-ion technologies.

Acknowledgement

The authors acknowledge the support by the National Science Foundation under Award Number DMR-1057170. Also thanks goes to AGEP GSR fellowship, which is the supplement fund to the DMR1057170.

References

(1)        Han, M. H.; Gonzalo, E.; Singh, G.; Rojo, T. A Comprehensive Review of Sodium Layered Oxides: Powerful Cathodes for Na-Ion Batteries. Energy Environ. Sci. 2015, 8, 81–102.

(2)        Lu, Z.; Dahn, J. R. In Situ X-Ray Diffraction Study of P2-Na_2/3[Ni_1/3Mn_2/3]O₂. J. Electrochem. Soc. 2001, 148, A1225.

(3)        Komaba, S.; Yabuuchi, N.; Nakayama, T.; Ogata, A.; Ishikawa, T.; Nakai, I. Study on the Reversible Electrode Reaction of Na(1-x)Ni(0.5)Mn(0.5)O2 for a Rechargeable Sodium-Ion Battery. Inorg. Chem. 2012, 51, 6211–6220.

(4)        Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-Type Na_x[Fe_1/2Mn_1/2]O₂ Made from Earth-Abundant Elements for Rechargeable Na Batteries. Nat. Mater. 2012, 11, 512–517.