Advanced Anode Materials for Sodium-Ion Batteries: Confining Polyoxometalates in Flexible Metal–Organic Frameworks by the “Breathing Effect”

Polyoxometalates (POMs) have shown great potential in sodium-ion batteries (SIBs) due to their reversible multielectron redox property and high ionic conductivity. Currently, POM-based SIBs suffer from the irreversible trapping and sluggish transmission kinetics of Na+. Herein, a series of POMs/metal-organic frameworks (MOFs)/graphene oxide (GO) (MOFs = MIL-101, MIL-53, and MIL-88B; POM = [PMo12O40]3-, denoted as PMo12) composites are developed as SIB anode materials for the first time. Unlike MIL-101 with large pore structures, the pores in flexible MIL-53 and MIL-88B swell spontaneously upon the accommodation of PMo12. Particularly, the PMo12/MIL-88B/GO composites deliver an excellent specific capacity of 214.2 mAh g-1 for 600 cycles at 2.0 A g-1, with a high initial Coulombic efficiency (ICE) of 51.0%. The so-called "breathing effect" of flexible MOFs leads to the relatively tight confinement space for PMo12, which greatly modulates its electronic structure, affects the adsorption energy of Na+, and eventually reduces the trapping of sodium ions. Additionally, the straight and multidimensional channels in MIL-88B significantly accelerate ion diffusion, inducing favored energetic kinetics and thus generating high-rate performance.