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

Electronic Structure Reconfiguration of Self-Supported Polyoxometalate-Based Lithium-Ion Battery Anodes for Efficient Lithium Storage

Polyoxometalate (POM)-based materials are considered as promising candidates for lithium-ion batteries (LIBs) due to their stable and well-defined molecular structure and reversible multielectron redox properties. Currently, POM-based electrode materials suffer from high interfacial resistance and low uniformity. Herein, we reported a self-supported POM-based anode material for LIBs by electrodepositing H3PMo12O40 (PMo12) and aniline on carbon cloth (CC) for the first time. The as-prepared polyaniline (PANi)-PMo12/CC composite exhibited an excellent reversible capacity of 1092 mA h g–1 for 200 cycles at 1 A g–1. Such an outstanding performance was attributed to the rapid electron transfer and Li+ diffusion stemming from the exposure of more active sites by the self-supported structure, the strong electrostatic interaction, and electronic structure reconfiguration between the active PMo12 cluster and conductive PANi polymer. This work provides insight into the electronic structure engineering of highly efficient LIB anode materials