Phase evolution during a thorough Li ion's insertion of electrode materials governs their battery performance during charge and discharge. Here we investigated the lithiation pathway of titanium disulfide using in situ TEM combined with synchrotron-based pair distribution function measurement and first-principles calculations. A 2D intercalation reaction proceeds along with a transition from van der Waals interaction between Ti–S slabs to the covalent bonding of S–Li–S, with no symmetry broken. Further lithiation triggers unconventionally multiple step conversion reactions as proved: LiTiS2→TiS→Ti2S→Ti. The conversion reaction pathway is also verified in fully discharged sample in coin-cell. The expanded conversion chemistry is supposed to increase the capacity of TiS2 electrode and downgrade the cyclability, whereas the existence of intermediate phases shows the promise of improving the reversibility with a successful control of the state of charge.