Confined annealing-induced transformation of tin oxide into sulfide for sodium storage applications

As conversion-type anode materials for sodium ion batteries, metal sulfides have a lower theoretical capacity but generally deliver a much higher reversible capacity in practice than their oxide counterparts. The origin of this phenomenon has not been fundamentally understood yet. From a reaction enthalpy analysis, we reveal that the sulfidation reaction with sulfur is possible for a wide variety of metal oxides. We proceeded to convert the less anode-active tin oxide/carbon (SnO₂/C) composites into the highly anode-active tin sulfide/carbon (SnS₂/C) materials by confined annealing with sulfur powder. For anode applications in sodium ion batteries, SnS₂/C delivers a reversible capacity of 770 mA h g^-1, which is more than double the value of SnO₂/C (360 mA h g^-1). The observed superiority of SnS₂/C anodes is correlated with the narrower band gap, higher conversion potential that allows for a larger polarization, and more energetically favorable conversion process on the surface of SnS₂. In particular, we demonstrate a modelling strategy for surface conversion reactions, which could potentially be considered for the kinetics study of various conversion-type anode materials.