Unravelling Twin Topotactic/Nontopotactic Reactive TiSe₂ Cathodes for Aqueous Batteries

Titanium selenide (TiSe₂ ), a model transition metal chalcogenide material, typically relies on topotactic ion intercalation/deintercalation to achieve stable ion storage with minimal disruption of the transport pathways but has restricted capacity (<130 mAh g^-1 ). Developing novel energy storage mechanisms beyond conventional intercalation to break capacity limits in TiSe₂ cathodes is essential yet challenging. Herein, the ion storage properties of TiSe₂ are revisited and an unusual thermodynamically stable twin topotactic/nontopotactic Cu²⁺ accommodation mechanism for aqueous batteries is unraveled. In situ synchrotron X-ray diffraction and ex situ microscopy jointly demonstrated that topotactic intercalation sustained the ion transport framework, nontopotactic conversion involved localized multielectron reactions, and these two parallel reactions are miraculously intertwined in nanoscale space. Comprehensive experimental and theoretical results suggested that the twin-reaction mechanism significantly improved the electron transfer ability, and the reserved intercalated TiSe₂ structure anchored the reduced titanium monomers with high affinity and promoted efficient charge transfer to synergistically enhance the capacity and reversibility. Consequently, TiSe₂ nanoflake cathodes delivered a never-before-achieved capacity of 275.9 mAh g^-1 at 0.1 A g^-1 , 93.5% capacity retention over 1000 cycles, and endow hybrid batteries (TiSe₂ -Cu||Zn) with a stable energy supply of 181.34 Wh kg^-1 at 2339.81 W kg^-1 , offering a promising model for aqueous ion storage.