Accordion-like polyoxometalate hybrid architectures for capacity-dense and flexible Zn-Ion battery cathodes

Polyoxometalates (POMs) have received significant attention as functional materials owing to their tunable structures and sizes, versatile electronic properties, and multiple electrochemical redox behaviours. However, the hierarchical architecture of POMs and their unique Zn storage mechanism have yet to be explored for Zn-ion battery (ZIB) applications. Herein, we demonstrate an accordion-like hierarchical architecture of POM-based hybrids (L-rGO/PPy@POM) via a redox-relay Heck reaction, where multi-layered reduced graphene oxide (rGO)/polypyrrole (PPy) hybrid nanosheets are assembled with ultrafine POM nanoparticles through specific interaction. As confirmed through in situ and ex situ spectroscopic methods, the Mo=O bond is more implicated in Zn2+ insertion than the two Mo–O single bonds and two edge-sharing and corner-sharing Mo–Mo bonds. Owing to a fast and facile redox kinetics, the ZIB cells with l-rGO/PPy@POM delivered high specific capacity of 269.3 mAh g−1 at 100 mA g−1, high-rate capacity of 68.0 mAh g−1 at 10,000 mA g−1, and 80.4% of capacity retention over 22,000 cycles in water-in-bisalt electrolytes. Furthermore, flexible quasi-solid-state ZIB full cells with l-rGO/PPy@POM cathode achieved the maximum volumetric energy and power densities of 51.1 mWh cmcell−3 and 1159.9 mW cmcell−3, demonstrating long-term cyclic stability and functional operation of various electronic devices.