Recent advances in semimetallic pnictogen (As, Sb, Bi) based anodes for sodium-ion batteries: Structural design, charge storage mechanisms, key challenges and perspectives

In the recent times sodium ion batteries (SIBs) have come to the forefront as an economic and resourceful alternative to lithium-ion batteries (LIBs) for powering portable electronic devices and large-scale grid storage. As the specific capacity, energy density and long cycle life of batteries depend upon the performance of anode materials: their quest is the ultimate need of the hour. Among the anode materials, the semimetallic pnictogens (As, Sb, Bi) and their compounds offer high gravimetric/volumetric capacities, but suffer from undesired volume expansion and inferior electrical conductivity. Herein, this paper reviews the recent progress in semimetallic pnictogens as alloying anodes and their compounds mainly as conversion-alloying anodes. Various debatable sodiation mechanisms (intercalation or alloying) have been presented with emphasis on in situ/ex situ advanced characterization methods well supported by theoretical modeling and calculations. The reviewed electrochemical reaction mechanisms, coherent structural designs and engineering provide a vital understanding of the electrochemical processes of Na+ ion storage. The existing challenges and perspectives are also presented, and several research directions are proposed from the aspects of special morphological design, employing conductive substrates, electrolyte additives and reducing particle size for technical and commercial success of SIBs.