Surface Chemistry and Mesopore Dual Regulation by Sulfur-Promised High Volumetric Capacity of Ti3 C2 Tx Films for Sodium-Ion Storage.

Electrochemical sodium-ion storage has come out as a promising technology for energy storage, where the development of electrode material that affords high volumetric capacity and long-term cycling stability remains highly desired yet a challenge. Herein, Ti3 C2 Tx (MXene)-based films are prepared by using sulfur (S) as the mediator to modulate the surface chemistry and microstructure, generating S-doped mesoporous Ti3 C2 Tx films with high flexibility. The mesoporous architecture offers desirable surface accessibility without significantly sacrificing the high density of Ti3 C2 Tx film. Meanwhile, the surface sulfur doping of Ti3 C2 Tx favors the diffusion of sodium ions. These merits are of critical importance to realize high volumetric capacity of the electrode material. As a consequence, as the freestanding electrode material for electrochemical sodium-ion storage, the S-doped mesoporous Ti3 C2 Tx film exhibits a high volumetric capacity of 625.6 mAh cm-3 at 0.1 A g-1 , which outperforms that of many reported electrodes. Moreover, outstanding rate capability and excellent long-term cycling stability extending 5000 cycles are achieved. The work opens the door for innovative design and rational fabrication of MXene-based films with ultrahigh volumetric capacity for sodium-ion storage.

Room-temperature assembled 3D macro-porous Ti3C2Tx/RGO hybrid hydrogel and the application as the self-standing electrode for sodium-ion storage.

Assembling two-dimensional (2D) MXene nanosheets into monolithic three-dimensional (3D) structures is an efficient pathway to transfer the nanoscale properties to practical applications. Nevertheless, the majority of the preparation schemes described in the literature are carried out at relatively high temperatures, which inevitably leads to the notorious high-temperature oxidation issue of MXenes. Preparing MXene-based hydrogels at lower temperatures or even room temperature is of great research importance. In this study, we report a novel and efficient room-temperature gelation method for fabricating 3D macro-porous Ti3C2Tx MXene/reduced graphene oxide (RGO) hybrid hydrogels, using anhydrous sodium sulfide (Na2S) as the primary reducing agent and l-cysteine as the auxiliary crosslinker. This room-temperature preparation technique successfully prevents the oxidation issue of MXenes and generates porous aerogels with excellent structural robustness after freeze-drying. As the self-standing anode for sodium-ion storage, the optimized 3D Ti3C2Tx MXene/RGO electrode possesses a specific capacity of 152 mAh/g at 0.1 A/g and good cycling stability with no significant capacity degradation after 500 cycles, which is significantly higher than that of the vacuum-filtered MXene film. This work demonstrates a straightforward room-temperature gelation method for constructing 3D MXene-based hydrogels to avoid the oxidation of MXenes, and casts new insight on the mechanism of the graphene oxide (GO)-assisted gelation.