Achieving high capacity retention for SnS₂ anodes via the solvent-driven reversible conversion-alloying reactions

Despite their large theoretical capacity (typically > 1000 mAh g⁻¹), anode materials featuring Na storage via a combined mechanism of conversion and alloying reactions are practically limited in Na-ion batteries owing to their poor initial Coulombic efficiency (typically ∼50%). Using SnS₂ as an example, we present a model that elucidates the physics underpinning its inferior Coulombic efficiency by incorporating an understanding of the thermodynamics and kinetics of conversion-alloying reactions. The developed model show that conversion-alloying reactions and their reversibility can be engineered by modulating the solvation tendency of electrolyte solvents, resulting in an enhanced initial Coulombic efficiency of > 70% (corresponding to 817 mAh g⁻¹) even without expensive pretreatment and the use of nanoscale SnS₂ particle anodes. Thus, this study that correlates the solvent properties and first-cycle reversibility offers a solution for selecting appropriate electrolytes for designing high-energy-density anodes based on various sodium storage mechanisms.