Regulating Sodium Deposition Behavior by a Triple-Gradient Framework for High-Performance Sodium Metal Batteries

An efficient method for the synthesis of a self-supporting carbon framework (denoted Gra-GC-MoSe₂) is proposed with a triple-gradient structure-in sodiophilic sites, pore volume, and electrical conductivity-which facilitates the highly efficient regulation of Na deposition. In situ and ex situ measurements, together with theoretical calculations, reveal that the gradient distribution of Se heteroatoms in MoSe₂, and its derivatives tailor the sodiophilicity, while the gradient distribution of porous nanostructures homogenizes the Na⁺ diffusion. Therefore, Na deposition occurs from the bottom to the top of the Gra-GC-MoSe₂ framework without dendrite formation. In addition, the gradient in electrical conductivity ensures the stripping process does not lead to dead Na. As a result, a Gra-GC-MoSe₂ modified Na anode (Na@Gra-GC-MoSe₂) shows impressive cycling stability with a high average Coulombic efficiency in an asymmetric cell. In symmetric cells, it also exhibits a long cycling life of 2000 h with a low polarization voltage and works stably even under a large capacity of 10 mAh cm^-2. Moreover, a Na@Gra-GC-MoSe₂|| Na₃V₂(PO₄)₃ full cell delivers a high energy density with an excellent cycling performance.