Phase Transformations in TiS<sub>2</sub> during K Intercalation

The electrochemical performances of TiS<sub>2</sub> in potassium ion batteries (KIBs) are poor due to the large size of K ions, which induces irreversible structural changes and poor kinetics. To obtain detailed insights into the kinetics of phase changes, we investigated the electrochemical properties, phase transformations, and stability of potassium-intercalated TiS<sub>2</sub> (K<sub><i>x</i></sub>TiS<sub>2</sub>, 0 ≤ <i>x</i> ≤ 0.88). In situ XRD reveals staged transitions corresponding to distinct crystalline phases during K ion intercalation, which are distinct from those of Li and Na ions. Electrochemical (cyclic voltammetry and galvanostatic charge/discharge) studies show that the phase transitions among various intercalated stages slow down the kinetics of the discharge/charge in bulk TiS<sub>2</sub> hosts. By chemically prepotassiating the bulk TiS<sub>2</sub> (K<sub>0.25</sub>TiS<sub>2</sub>) to reduce the domain size of the crystal, these phase transitions are bypassed and more facile ion insertion kinetics can be obtained, which leads to improved Coulombic efficiency, rate capability, and cycling stability.