Uncovering the underlying science behind dimensionality in the potassium battery regime

Abstract

Compared to the significant advances in its lithium and sodium analogue, the practical application of potassium ion batteries (KIBs) has been hindered by the sluggish K ion diffusion. Electrochemical storage via insertion/de-insertion reactions in lamellar electrode materials critically rests upon the host materials with wide interlayer spacing and the sizes of the guest ions. To uncover the underlying science behind the dimensionality in the KIBs regime, we investigated MoS2/K half-cell using uniform MoS2 rod, sheet, and sphere structures synthesized by a facile and controllable method for the first time. Impressively, when MoS2 is in the shape of a hierarchical rod, there is more intercalation pseudocapacitance (74.8% at the scan rate of 1.5 mV s−1) and the optimized electrode resulted in high-rate capacity (320 and 183 mAh g−1 at 0.1 and 1 A g−1, respectively). In contrast, MoS2 sheet and sphere contribute little pseudocapacitance (69.1% and 61.4%, respectively) and therefore exhibit low-rate capacity (232 and 110 mAh g−1 for MoS2 sheet, 185 and 96 mAh g−1 for MoS2 sphere at 0.1 and 1 A g−1, respectively). Density functional theory (DFT) results uncover that the K atom is much more liable to occupy the octahedral (Oh) site. What’s more, non-destructively 3D reconstruction of MoS2 rod electrode strongly demonstrated the integrity of the rod structure after K+ insertion. The work paves a new way for the design of two dimensional (2D) materials and offers profound insights into the charge storage mechanism of KIBs.