Synergy Between Surface Confinement and Heterointerfacial Regulations with Fast Electron/Ion Migration in InSe-PPy for Sodium-Ion Storage.

Layered indium selenide (InSe) is a new 2D semiconductor material with high carrier mobility, widely adjustable bandgap, and high ductility. However, its ion storage behavior and related electrochemical reaction mechanism are rarely reported. In this study, InSe nanoflakes encapsulated in conductive polypyrrole (InSe@PPy) are designed in consideration of restraining the severe volume change in the electrochemical reaction and increasing conductivity via in situ chemical oxidation polymerization. Density functional theory calculations demonstrate that the construction of heterostructure can generate an internal electric field to accelerate electron transfer via additional driving forces, offering synergistically enhanced structural stability, electrical conductivity, and Na + diffusion process. The resulting InSe@PPy composite shows outstanding electrochemical performance in the sodium ion batteries system, achieving a high reversible capacity of 336.4 mA h g -1 after 500 cycles at 1 A g -1 and a long-term cyclic stability with capacity of 274.4 mA h g -1 after 2800 cycles at 5 A g -1 . In particular, the investigation of capacity fluctuation within the first cycling reveals the alternating significance of intercalation and conversion reactions and evanescent alloying reaction. The combined reaction mechanism of insertion, conversion, and alloying of InSe@PPy is revealed by in situ X-ray diffraction, ex situ electrochemical impedance spectroscopy, and transmission electron microscopy.