Na-ion batteries are important to develop lower cost batteries that deliver safe, reliable storage with long calendar life. Among various studied cathodes, layered P2-Na_xTMO₂ (TM -- 3d transition metal ions) compounds, exhibiting comparable performance as compared with Li-ion cathodes, are attractive candidates.^1-3However, they suffer from capacity loss and poor reversibility upon cycling, which results from two determinative factors structurally: oxygen layer shifts and Na-ion in-plane ordering transitions during Na insertion/extraction. The oxygen layer shifts usually happen at low Na contents and the ordering transitions form at particular Na stoichiometry. A throughout understanding of the Na/Vac ordering is necessary.

Herein we present the first atomic scale visualization of the Na-Na in-plane and out-of-plane distribution variations due to the competitions of the in-plane Na_e/f-Na_e/f interactions and the out-of-plane Na_f-TM-Na_f electrostatic forces. An alternative Na concentration distribution along the c axis has been identified in the P2-Na_2/3[Ni_1/3Mn_2/3]O₂ where (Ni are all 2+ and Mn are all 4+), while not for P2-Na_2/3[Fe_1/3Mn_2/3]O₂(both Fe and Mn are mixture of 3+ and 4+). This study visualizes for the first time the electrostatic-interaction-induced Na ordering in both ab plane and c directions. Our work demonstrates the strong interaction between alkali ordering and electronic, magnetic structure, and underlines the important role that structure plays in determining electronic behavior and electrochemical performance, opening up the new design approach for energy storage materials.

1. J.   Electrochem. Soc. 2015, 162, A2589.

2. Functional Materials Letters 2013, 06, 1330001.

3. Nat. Mater. 2012, 11, 512.