Breaking the Conversion Limit in an Intercalation-Type Cathode by Loosening Aqueous Cation Coordination

Intercalation-type cathodes continue to dominate aqueous multivalent ion storage, despite higher theoretical capacities being available from conversion reactions involving multiple electron transfer. Pushing intercalation-type cathodes into conversion is certainly desirable, conventionally with great sacrificing cycling-stability, kinetics, and discharge potential. To date, limited progress has been achieved in overcoming this longstanding and formidable performance trade-off. Herein, it is demonstrated that by loosening cation coordination in dilute aqueous systems, typical intercalation-type electrodes (Bi₂Se₃) can be unexpectedly extended into the conversion regime, yielding a twofold capacity increase of 417.6 mAh g^-1 with an excellent combination of reversible lifespan (20 000 cycles with a decay rate of 0.013‰), rate capability (314.6 mAh g^-1 at 30 A g^-1), and enhanced operating potential. Composited operando synchrotron X-ray diffraction technology, first-principal calculations, and ex situ X-ray absorption spectroscopy with electron microscopy analyses indicated that hydrophobic perchlorate optimizes the solvation coordination and charge transfer of cations, which overall decreases the interfacial reaction barrier and enhances the reaction potential, hence initiating unique reversible and depth intercalation‒conversion mechanisms. Well-characterized loosening cation coordination further ensures high ion mobility, extraordinary low-temperature performance, and robust operation in quasi-solid-state pouch cells, offering key insights for improving the energy density of aqueous batteries.