Construction of the POMOF@Polypyrrole Composite with Enhanced Ion Diffusion and Capacitive Contribution for High-Performance Lithium-Ion Batteries.

Polyoxometalate (POM) as an "electronic sponge" can store a great number of electrons; however, shortcomings of poor conductivity and solubility in electrolytes cause a significant decrease in specific capacity and poor rate capability. To address the aforementioned disadvantages, a dual strategy was proposed, including coating the conductive polypyrrole (PPy) and utilizing nitrogenous ligands (1,10-phenanthroline monohydrate = 1,10-phen) for metal-organic frameworks (MOFs) to fabricate a [Cu(1,10-phen)(H2O)2]2[Mo6O20]@PPy (Cu-POMOF@PPy) composite, effectively confining the POM in MOFs to avoid dissolution of POM in the electrolyte and improve electrochemical stability. Simultaneously, the PPy shell could improve the conductivity, contribute extra capacity, and alleviate volume variation of Cu-POMOF during cycling. Therefore, the final Cu-POMOF@PPy composite provides an excellent specific capacity of around 769 mA h g-1 at 0.1 A g-1 after 160 cycles and good rate performance, associated with great cycling stability (319 mA h g-1 at 2 A g-1 after 500 cycles). Moreover, the electrochemical reaction mechanism of Cu-POMOF@PPy was investigated by ex situ XPS measurements, indicating that storage of electrons results from the reduction/oxidation of Mo atoms (Mo6+ ↔ Mo4+) and Cu atoms (Cu2+ ↔ Cu0). As a consequence, this work not only proposes a novel method for preparing POM-based lithium-ion batteries but also expands the variety of anode materials.