Na-ion batteries are being extensively investigated as one of the possible alternatives to very popular Li-ion batteries. Compared with Li-ion batteries, Na-ion batteries have the advantage in cost, because of the much higher natural abundance and availability of Na than Li, which makes Na-ion batteries potentially a candidate for gird level energy storage. However, currently the energy density of Na-ion batteries is in general lower than those of Li-ion batteries, limited by multiple components in the cell, majorly by the cathode. Currently, most high capacity cathodes for Na-ion batteries are layered oxides with a general formula Na_xMO₂, where M is a transition metal or the combination of transition metals. Most of the layered oxide cathodes, however, can only stably cycle at <50% of their theoretical capacities, which greatly limits the energy density of the full cell.[1, 2] What is the limit of energy density of Na-ion batteries in current configuration? How to push the capacity of layered oxide cathodes to its theoretical limit? These are the questions to be addressed towards the goals of using Na-ion batteries to replace Li-ion batteries. In this work, we explored the fundamental reasons that limit the reversible capacity of layered oxide cathodes and demonstrated a new strategy to push their capacity to the limit, with using a group of rationally designed compounds as examples. A series of oxides, including Na_0.66(Li_0.2Mn_0.2)O₂ [3] and Na_0.66Li_0.18Fe_0.12Mn_0.7O₂, etc. were designed to verify the proposed strategies and these compounds were successfully synthesized in desired P2 type layered structures. Many of these compounds demonstrated high cycling capacities ranging from 190 to 230 mAh/g, which are very close to the theoretical limit. In-depth structure characterizations with using in situ and ex situ synchrotron X-ray diffraction, neutron diffraction and solid state NMR were performed, in order to reveal the functioning mechanism of these compounds and provide more guidance to the materials design to further improve the capacity, voltage and cycling stability of the cathode materials.

[1] B.L. Ellis, L.F. Nazar, Sodium and sodium-ion energy storage batteries, Current Opinion in Solid State & Materials Science, 16 (2012) 168-177.

[2] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X.H. Ma, G. Ceder, Voltage, stability and diffusion barrier differences between sodium-ion and lithium-ion intercalation materials, Energy & Environmental Science, 4 (2011) 3680-3688.

[3] L.F. Yang, X. Li, X.T. Ma, S. Xiong, P. Liu, Y.Z. Tang, S. Cheng, Y.Y. Hu, M.L. Liu, H.L. Chen, Design of high-performance cathode materials with single-phase pathway for sodium ion batteries: A study on P2-Na-x(LiyMn1-y)O-2 compounds, J. Power Sources, 381 (2018) 171-180.