Layered Arrangement of Polyoxometalate on a Metal–Organic Framework as a High-Capacity Anode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are a potential alternative to lithium-ion batteries (LIBs), owing to their low cost and sustainability. However, developing a promising anode for sodium-ion batteries remains challenging due to the large size of Na⁺ ions and the significant volume expansion during Na⁺ insertion reactions. Polyoxometalates (POMs) can host cations on the surface and between POM clusters rather than intercalating into the crystal structure, offering their potential as an anode material for SIBs. Herein, we report a vanadium-based POM, i.e., Na₆PV₃W₉O₄₀ (PVW), stabilized on a cobalt-based metal–organic framework (CoATP), as an effective anode material for SIBs. Electrostatic interactions between CoATP and PVW are enabled by developing cationic groups (−NH₃⁺) on the surface of CoATP. The resulting PVW@CoATP exhibits a continuous layer-by-layer interconnected architecture with intimate PVW/CoATP contact. The layered arrangement provides insertion sites and ensures complete exposure of PVW clusters for redox reactions. XPS analysis indicates that apart from the simultaneous reduction of V⁵⁺ to V⁴⁺ and of W⁶⁺ to W⁴⁺, the cobalt of CoATP reduces from Co³⁺ to Co²⁺ during discharge, which demonstrates that the nanospheres of CoATP not only provide a 3D surface for the layered arrangement of PVW clusters but also synergistically enhance the performance due to the involvement of Co in the redox process. Consequently, the PVW@CoATP composite exhibits promising performance as an anode material for sodium-ion batteries, including a high reversible capacity of 413 mAh g^–1 and long-term cycling with 84% retention after 1000 cycles. This work paves a new pathway for the MOF-supported layered growth of POM, which shows promising structural prospects in energy storage applications.