Capturing the differences between lithiation and sodiation of nanostructured TiS₂ electrodes

In this study TiS₂ is chosen as a model electrode material to investigate the relationship between the electrochemical and mechanical performance of layered cathodes for Na-ion batteries. Employing NaFP₆ in EC/DMC as the electrolyte allowed for the most promising electrochemical properties recorded in the literature, namely a reversible capacity of 203 mAh g⁻¹ at 0.2 C and 88 mAh g⁻¹ at 10 C with a capacity retention of 92% over 50 cycles. Despite this promising performance the capacity still decayed during long term cycling. In-situ x-ray diffraction and high-resolution transmission electron microscopy imaging revealed that TiS₂ underwent a large expansion of 17.7% along the c direction and irreversible phase transformations took place during the sodiation/de-sodiation process, which lead to severe mechanical strains and intragranular cracks. In comparison, the mechanical stability of TiS₂ in Li-ion cells was significantly higher. The experimental results are interpreted within a continuum mechanics model which revealed that the maximum effective von Mises stress that is present at the interface between the ion-intercalated TiS₂ and pristine TiS₂ is about four times higher during sodiation than lithiation indicating that the electrode is more susceptible to failure/fracture during sodiation.