Hollow NiFe₂O₄nanospheres anchored on carbon nanorods as high performance anodes.

Na metal is considered as a promising anode material for Na metal batteries (NMBs) due to its high theoretical specific capacity and low electrochemical potential. However, the Na metal anode faces several challenges, including: (1) safety concerns related to dendrite formation; (2) low coulombic efficiency (CE) and poor cycling performance caused by the unstable solid electrolyte interphase (SEI); (3) an infinite volume change due to its hostless nature. In this paper, we demonstrate for the first time a facile and in situ solution-based method to synthesize an artificial protective layer of Na₃PS₄ (NaPS) on the surface of Na metal. Promisingly, the thickness and composition of the NaPS can be controlled through adjustment of the precursor concentration and reaction times. The thin amorphous NaPS layers on the surface of the Na metal will reduce parasitic side reactions with electrolyte components, increase the homogeneity of the SEI layer and improve the Na⁺ flux on the surface. Meanwhile, the ionically conductive NaPS can eliminate the strong electric field buildup at the surface, resulting in a more homogeneous Na plating/stripping process with suppressed Na dendrite growth. The design of the NaPS-coated Na anode opens up new opportunities for the realization of next-generation high-energy density Na metal batteries.

Covalent organic frameworks (COFs) have emerged as promising electrode materials for rechargeable metal-ion batteries and have gained much attention in recent years due to their high specific surface area, inherent porosity, tunable molecular structure, robust framework, and abundant active sites. Moreover, compared with inorganic materials and small organic molecules, COFs have the advantages of multi-electron transfer, short pathways, and high cycling stability. Although great progress on COF-based electrodes has been made, the corresponding electrochemical performance is still far from satisfactory for practical applications. In this review, we first summarize the fundamental background of COFs, including the species of COFs (different active covalent bonds) and typical synthesis methods of COFs. Then, the key challenges and the latest research progress of COF-based cathodes and anodes for metal-ion batteries are reviewed, including Li-ion batteries, Na-ion batteries, K-ion batteries, Zn-ion batteries, et al. Moreover, the effective strategies to enhance electrochemical performance of COF-based electrodes are presented. Finally, this review also covers the typical superiorities of COFs used in energy devices, as well as providing some perspectives and outlooks in this field. We hope this review can provide fundamental guidance for the development of COF-based electrodes for metal-ion batteries in the further research.