Sodium metal anodes have attracted significant attention due to their high specific capacity (1166 mA h g −1), low redox potential (−2.71 V vs the standard hydrogen electrode), and abundant natural resources. Nevertheless, unstable solid electrolyte interphases (SEI) and uncontrolled dendrite growth critically hinder their commercialization. Notably, SEIs play a critical role in regulating Na deposition and improving the cycling stability of rechargeable Na metal batteries. Recently, SEI research on Na metal anodes has been intensively conducted worldwide; thus, a comprehensive review is necessary. Herein, initially, the fundamentals of SEI and the related issues induced by its intrinsic instability are discussed. Thereafter, advanced characterization techniques that unveil the morphological evolution and interfacial chemistry of Na metal anodes are presented. Subsequently, efficient strategies, including liquid electrolyte engineering, artificial SEI, and solid-state electrolyte technology, to stabilize SEI films are outlined. Finally, key aspects and prospects in the development of SEI for Na metal anodes are highlighted. It is believed that this review will serve to further advance the understanding and development of SEIs for Na metal anodes.

Growth of dendrites, the low plating/stripping efficiency of Zn anodes, and the high freezing point of aqueous electrolytes hinder the practical application of aqueous Zn‐ion batteries. Here, a zwitterionic osmolyte‐based molecular crowding electrolyte is presented, by adding betaine (Bet, a by‐product from beet plant) to the aqueous electrolyte, to solve the abovementioned problems. Substantive verification tests, density functional theory calculations, and ab initio molecular dynamics simulations consistently reveal that side reactions and growth of Zn dendrites are restrained because Bet can break Zn2+ solvation and regulate oriented 2D Zn2+ deposition. The Bet/ZnSO4 electrolyte enables superior reversibility in a Zn–Cu half‐cell to achieve a high Coulombic efficiency >99.9% for 900 cycles (≈1800 h), and dendrite‐free Zn plating/stripping in Zn–Zn cells for 4235 h at 0.5 mA cm−2 and 0.5 mAh cm−2. Furthermore, a high concentration of Bet lowers the freezing point of the electrolyte to −92 °C via the molecular‐crowding effect, which ensures the stable operation of the aqueous batteries at −30 °C. This innovative concept of such a molecular crowding electrolyte will inject new vitality into the development of multifunctional aqueous electrolytes.

Owing to several advantages of metallic sodium (Na), such as a relatively high theoretical capacity, low redox potential, wide availability, and low cost, Na metal batteries are being extensively studied, which are expected to play a major role in the fields of electric vehicles and grid-scale energy storage. Although considerable efforts have been devoted to utilizing MXene-based materials for suppressing Na dendrites, achieving a stable cycling of Na metal anodes remains extremely challenging due to, for example, the low Coulombic efficiency (CE) caused by the severe side reactions. Herein, a g-C3N4 layer was attached in situ on the Ti3C2 MXene surface, inducing a surface state reconstruction and thus forming a stable hetero-interphase with excellent sodiophilicity between the MXene and g-C3N4 to inhibit side reactions and guide uniform Na ion flux. The 3D construction can not only lower the local current density to facilitate uniform Na plating/stripping but also mitigate volume change to stabilize the electrolyte/electrode interphase. Thus, the 3D Ti3C2 MXene@g-C3N4 nanocomposite enables much enhanced average CEs (99.9% at 1 mA h cm–2, 0.5 mA cm–2) in asymmetric half cells, long-term stability (up to 700 h) for symmetric cells, and stable cycling (up to 800 cycles at 2 C), together with outstanding rate capability (up to 20 C), of full cells. The present study demonstrates an approach in developing practically high performance for Na metal anodes.

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