Bimetallic selenide nanocages covered by carbon layer deliver high rate performance for sodium ion storage

Abstract

Sodium-ion batteries (SIBs) have been acknowledged as a particularly promising supplement to lithium-ion batteries, thanks to the abundant sodium resources, geographically-wide distribution and low cost. However, the large volume expansion hence poor structural stability of the anode in the Na+ insertion/deinsertion process severely compromises the rate performance of SIBs. Herein, we report the hierarchical, hollow structured nanocages of nickel-cobalt bimetallic selenide covered by nitrogen-doped carbon shell (NiCo3Se4@NC) as anode material for SIB. The hollow structure and the abundant holes on the cage walls greatly shorten the transfer distance for Na+ ions and buffer the volume expansion during the Na+ storage process. The bimetallic form expands the lattice spacing meanwhile increase the density of states near Fermi level. Moreover, the highly-conductive NC shell further benefits the charge transfer and structural stability upon Na+ storage, and promotes the cycle stability. As a result, surface-responsive capacitive-dominated kinetic process is revealed from the NiCo3Se4@NC anode, delivering a high reversible capacity of 416 mAh/g−1 at a current density of 0.05 A/g−1 and 314 mAh/g−1 at 2 A/g−1. This work proposes an efficient strategy to substantially increase the rate performance and cycle stability of SIBs.