Hierarchical Co₃O₄ Nanofiber–Carbon Sheet Skeleton with Superior Na/Li‐Philic Property Enabling Highly Stable Alkali Metal Batteries

Uncontrollable dendritic behavior and infinite volume expansion in alkali metal anode results in the severe safety hazards and short lifespan for high‐energy batteries. Constructing a stable host with superior Na/Li‐philic properties is a prerequisite for commercialization. Here, it is demonstrated that the small Gibbs free energy change in the reaction between metal oxide (Co 3 O 4 , SnO 2 , and CuO) and alkali metal is key for metal infusion. The as‐prepared hierarchical Co 3 O 4 nanofiber–carbon sheet (CS) skeleton shows improved wettability toward molten Li/Na. The 3D carbon sheet serves as a primary framework, offering adequate lithium nucleation sites and sufficient electrolyte/electrode contact for fast charge transfer. The secondary framework of Co/Li 2 O nanofibers provides physical confinement of deposited Li and further redistributes the Li + flux on each carbon fiber, which is verified by COMSOL Multiphysics simulations. Due to the uniform deposition behavior and near‐zero volume change, modified symmetrical Li/Li cells can operate under an ultrahigh current density of 20 mA cm −2 for more than 120 cycles. When paired with LiFePO 4 cathodes, the Li/Co–CS cell shows low polarization and 88.4% capacity retention after 200 cycles under 2 C. Convincing improvement can also be observed in Na/Co–CS symmetrical cells applying NaClO 4 ‐based electrolyte. These results illustrate a significant improvement in developing safe and stable alkali metal batteries.