A bimetallic induced enhanced 3D electron transport network supported by micro constrain area of balls-in-ball structure used for high performance sodium storage

Abstract

Metal selenides' reversible capacity and rate performance are greatly constrained by their low intrinsic conductivity, severe volume expansion, and low initial coulombic efficiency, therefore selenides with high theoretical capacities cannot be employed as efficient anode for sodium ions batteries. Here, we report a simple and feasible methodology for synthesizing (M−Mn)Se/C (M = Co, Ni) heterogeneous anode materials based on Kirkendall diffusion process. A heterogeneous balls-in-ball structure is created when the nanospheres are concentrated in a microsphere. The confinement effect of the nano-scale enables each microsphere to act an effective reaction micro-region. The transport of sodium ions is constrained more by electrons when the electrode material reaches the nanoscale. In terms of composition and structure, the balls-in-ball structure with heterogeneous composition greatly promotes the electron transfer. The bimetal-regulated heterogeneous (M−Mn)Se/C electrode exhibits bonding energy changed under the synergistic effect between the two metal metals, thus forming significant quantity of built-in electric field to promote electron transport. The amount of MnSe is distributed uniformly around each MSe in every single micro-sphere. The built-in electric field establish an efficient 3D electron transport network, thus making the optimization in composition substantially boosted by a sensible structural design. (M−Mn)Se/C showed excellent sodium storage performance through the cooperative optimization of composition and structure. To our expectation, (CoMn)Se/C delivered a high reversible capacity (597 mAh·g−1 at 100 mA⋅g−1) and enduring cyclic stability (retention rate of 97.8% after 2000 cycles at 5000 mA⋅g−1). In addition, a full cell assembled by Na3V2(PO4)3 showed a high capacity of 350 mAh/g and a ultra-high energy density of 162.2Wh/kg. This work provides a simple method for constructing the anode electrode of high-performance sodium ions batteries.