Boosting energy density of MXenes: Atomically-resolved ion polar rotation within electrolyte-specific-width interlayers

Layer-structured pseudocapacitive electrode materials show excellent potential for overcoming the kinetic constraints of batteries while achieving charge storage capacities greater than those of electrical double-layer capacitors. However, real-world applications are limited by the lack of effective mechanisms to strip electrolyte ions from solvent atoms and control their interactions and charge exchange with the layered electrode materials. Here, we discover the ion polar rotation within sub-nanometer interlayer space of two-dimensional MXene sheets by directly imaging interlayer ionic structure and measuring the local charge transfer. Unlike common ion desolvation, ion polar rotation rearranges solvent molecules and creates more exposure of intercalated ions, eliminating the need for long-range ion diffusion and desolvation. This effect enhances ion-electrode charge transfer at a unique interlayer spacing for each solvent. We demonstrated the efficacy of the polar rotation mechanism by designing a hierarchically structured MXene electrode with the optimal interlayer spacing, which shows Li + storage capacitance of up to 310 F g −1 (744 F cm −3 ) in LiTFSI/PC system and good rate performance. The schematic explaining the key concept of the ion polar rotation and how we made “both ends meet” and boosted energy density of MXene is shown in Fig. 1 . Fig. 1 Conceptual novelty, key mechanisms and achievements in our work. • Through novel iDPC technique, ion desolvation and ion polar rotation are directly imaged at atomic scale. • The evolution of ion desolvation mechanism is figured out while increasing the d -spacing from 1.0 to 2.2 nm. • Outstanding capacitance of 310 F g −1 (744 F cm −3 ) is achieved by utilizing ion polar rotation mechanism.