Sodium-ion batteries have captured widespread attention for grid-scale energy storage owing to the natural abundance of sodium. The performance of such batteries is limited by available electrode materials, especially for sodium-ion layered oxides, motivating the exploration of high compositional diversity. How the composition determines the structural chemistry is decisive for the electrochemical performance but very challenging to predict, especially for complex compositions. We introduce the “cationic potential” that captures the key interactions of layered materials and makes it possible to predict the stacking structures. 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 toward the design of alkali metal layered oxides.

Rechargeable Na‐ion batteries (NIBs) are attractive large‐scale energy storage systems compared to Li‐ion batteries due to the substantial reserve and low cost of sodium resources. The recent rapid development of NIBs will no doubt accelerate the commercialization process. As one of the indispensable components in current battery systems, organic liquid electrolytes are widely used for their high ionic conductivity and good wettability, but the low thermal stability, especially the easy flammability and leakage make them at risk of safety issues. The booming solid‐state batteries with solid‐state electrolytes (SSEs) show promise as alternatives to organic liquid systems due to their improved safety and higher energy density. However, several challenges including low ionic conductivity, poor wettability, low stability/incompatibility between electrodes and electrolytes, etc., may degrade performance, hindering the development of practical applications. In this review, an overview of Na‐ion SSEs is first outlined according to the classification of solid polymer electrolytes, composite polymer electrolytes, inorganic solid electrolytes, etc. Furthermore, the current challenges and critical perspectives for the potential development of solid‐state sodium batteries are discussed in detail.

role in the high-energy-density Na-metal batteries such as Na-O 2 batteries, [9][10][11][12] Na-CO 2 batteries, [13,14] and room temperature Na-S batteries. [15][16][17][18] However, Na metal anode has been hindered owing to the obstacles of dendritic Na formation and relative large volume change during repetitive electrochemical cycling, which gives rise to poor battery cycling stability and serious safety hazards. [6][7][8]19] So far, much effort has been devoted to investigate Na dendrites under different conditions and explore the mechanism of dendritic Na deposition/dissolution. [20][21][22][23] For instance, Yui et al. utilized in situ light microscopy to observe the behavior of dendritic Na plating/stripping on Na metal or a copper electrode in propylene carbonate-based electrolytes. [22] The dendritic Na deposition/dissolution mechanism described previously can be summarized as follow. First, because of its high reactivity, metallic Na reacts spontaneously with a conventional carbonate-based electrolyte and most organic electrolyte solvents, [24] thus forming a thin and fragile solid electrolyte interphase (SEI) layer on its surface. During the plating process, due to the rough Na foil surface in microscale, Na ion flux is preferentially deposited at the protuberances (Figure 1a), which possess much higher electric field than other sites. [25] Then, Na will nucleate and grow along the protrusion, finally generating Na dendrite defined as a tree-like or branched structure of Na metal deposition. [26,27] During Na dendtrite formation, the SEI layer is fractured and the fresh Na underneath is exposed to the electrolyte, causing a new SEI layer to form. Subsequently, in the stripping process, Na which is near the base of Na dendrites will be dissolved at first due to the fact that the roots of Na dendrites have the tendency to accept electrons and dissolve early, therefore leading to break off of Na dendrites and forming so-called "dead Na." [7,22] Consequently, with the continuous Na stripping/plating and the corresponding unstable large-surface-area of SEI layer induced by the Na dendrites, Na ions from both of the working Na metal and the electrolyte are consumed, resulting in a low Coulombic efficiency and a rapid capacity decay. [26,27] Furthermore, as the Na metal does not act as a "host" for the Na ions, the accumulated dead Na and growing Na dendrites will cause a relatively large volume change, which can give rise to enormous internal stress fluctuations and inferior interface stability in the batteries. [19] In addition, Na dendrites will pierce the separator and lead to an internal short circuit and a thermal runaway with Sodium (Na) metal, which possesses a high theoretical capacity and the lowest electrochemical potential, is regarded as a promising anode material for Na-metal batteries. However, both Na dendrite growth and large volume change in cycling have severely impeded its practical applications. This study demonstrates that a 3D flexible carbon (C) felt which is already commercialized in ...