Integration of intermittent renewable energy sources demands the development of sustainable electrical energy storage systems. The abundance and low cost of sodium (Na) make Na-ion batteries promising for smart grids and grid-scale applications. In search of electrodes for Na-ion batteries, layered Na-based oxides with the general composition of Na_xTMO₂ (TM: transition metal) have attracted significant attention for their high compositional diversity that provides tunable electrochemical performance for electrodes in Na-ion batteries.

Compared to lithium (Li)-based layered oxides of the well-known LiCoO₂ and Ni-rich LiNi_yCo_zMn(Al)_1-y-zO₂, a striking difference is that for Na-ion oxides in addition to O-type, also the P-type stacking can occur, where P-type refers to prismatic Na-ion coordination. These stackings show distinctly different electrode performance, where the most studied layered stacking configurations are P2 and O3 types, referring to the ABBA and ABCABC oxygen stacking, respectively. P2-type oxides usually provide higher Na-ion conductivity and better structural integrity against the O3 analogues, which is responsible for the high-power density and good cycling stability. In search for electrodes with good chemical/dynamic stability and high Na storage performance, various layered oxides have been synthesized and investigated. However, effective guidelines towards the design and preparation of optimal electrode materials are lacking.

Here, we introduce the “cationic potential” that captures the key interactions of layered materials, and makes it possible to predict the stacking structure. Aiming at a simple descriptor for layered oxides, we express the extent of the cation electron density and its polarizability, normalized to the ionic potential anion(O), by defining the following equation:

Ф_cation=Ф_TMФ_Na/Ф_O 

The distinct P2 and O3-type regions indicate that the cationic potential is an accurate descriptor of the inter-slab interaction, and thereby the structural competition between P2- and O3-type structures. A larger cationic potential, implies stronger TM electron cloud extend and interlayer electrostatic repulsion resulting in the P2-type structure, with more covalent TM-O bonds and an increased NaO₂ slabs. Opposing this, a larger average Na ionic potential, achieved by increasing Na content, increases the shielding of the electrostatic repulsion between the TMO₂ slabs, favouring the O3-type structure. This is demonstrated through the rational design and preparation of layered electrode materials with improved performance. As the stacking structure determines the functional properties, this methodology offers a solution towards the design of alkali metal layered materials.