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

Abstract

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.

Electron Storage Performance of Hybrid Materials Comprising Polyoxometalates and Carbon Nanohorns as Cathode‐Active Materials

“…The N 1s and P 2p peaks were observed at approximately 532 and 134 eV. The Mo 3d and P 2p peak positions were consistent with those in the previously reported XPS profiles of POM [27] . This indicates that the Mo 3d and P 2p peaks result from the presence of POM molecules, suggesting that the electronic structure of the POM molecules does not change after nanohybridization with CNHox.…”

“…The Mo 3d and P 2p peak positions were consistent with those in the previously reported XPS profiles of POM. [27] This indicates that the Mo 3d and P 2p peaks result from the presence of POM molecules, suggesting that the electronic structure of the POM molecules does not change after nanohybridization with CNHox. Herein, the C 1s, O 1s, and N 1s peaks are assigned to CNHox, POM molecules, and tetrabutylammonium (TBA) molecules used as POM countercations, respectively.…”

Insight into the Structural Variation and Sodium Storage Behavior of Polyoxometalates Encapsulated within Single‐Walled Carbon Nanotubes

“…Upon application of a potential of 3.0 V (Figure S7C), Mo 4+ was fully oxidized to Mo 5+ and Mo 6+ states. The results indicate that PMo 12 acts as an “electron sponge” and exhibits highly reversible valence change during charging/discharging [40,48,49] …”

“…The results indicate that PMo 12 acts as an "electron sponge" and exhibits highly reversible valence change during charging/discharging. [40,48,49] The cycling performance of PMo 12 , SWNTs, a physical mixture of PMo 12 and SWNTs, and the as-prepared PMo 12 @SWNT were compared in Figure 3B. At a current density of 750 mA g À 1 , the initial specific capacity of PMo 12 @SWNT was around 580 mA h g À 1 .…”

Advances of Electroactive Metal–Organic Frameworks

“…[ 460 ] Moreover, a hybrid Fe MOF/H 3 PMo 12 O 40 /GO in the presence of carbon black and sodium carboxymethyl cellulose as anodes in SIBs was also reported to exhibit a specific capacity of 214.2 mAh g −1 for 600 cycles, which benefited from dynamic structures of the Fe MOF that could well confine multielectron redox1PMo 12 O 40 ] 3− in pores. [ 461 ]…”