Enhancing the Charge Transportation Ability of Yolk-Shell Structure for High-Rate Sodium and Potassium Storage.

The microstructure of large-capacity anodes is of great importance in determining the performance of sodium- and potassium-ion batteries. Yolk-shell nanostructures promise excellent structural stability but suffer from insufficient charge transfer rate during cycles. Herein, we tackle this challenge by constructing a single-walled carbon nanotube (SWNT) internally bridged yolk-shell structure, inside which SWNTs cover the surface of the yolk and connect the yolk and shell, for better electron/ion transportation. Combining the merits of both yolk-shell structure and conductive SWNT channels, the as-prepared Fe1-xS/SWNT@C composite manifests high reversible capacity and ultralong cycling stability up to 8700 cycles. Moreover, it displays the best rate capability (317 mA h g-1 at 20 A g-1 for Na+ and 236 mA h g-1 at 10 A g-1 for K+) among the reported yolk-shell structures and iron-sulfide-based anodes thus far. The kinetic analysis and density functional theory calculations further reveal that the Fe1-xS/SWNT heterointerface can effectively enhance the reversibility of K+ storage and decrease the K+ diffusion energy barrier, leading to excellent pseudocapacitive behavior and fast ion transportation for outstanding rate capability.