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-MoSe) 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 MoSe, 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-MoSe 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-MoSe modified Na anode (Na@Gra-GC-MoSe) 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. Moreover, a Na@Gra-GC-MoSe|| NaV(PO) full cell delivers a high energy density with an excellent cycling performance.