Porous MXene monoliths with locally laminated structure for enhanced pseudo-capacitance and fast sodium-ion storage

Highlights: • With the assistance of KOH, MXene monoliths with balanced laminated and porous structures are produced by MXene gelation. • The KOH-assisted porous MXene monolith shows enhanced sodium-ion storage capacity and competitive rate performance. • KOH promotes the stability of MXene nanosheets against oxidation during the process of assembly. MXenes are regarded as typical pseudocapacitive materials that store charges via the intercalation mechanism and have arisen extensive research in sodium-ion batteries. However, it is still challenging to rationally design the function-oriented structure of MXenes to enhance their sodium-ion storage capability under the premise of ensuring superior rate performance. Herein, porous MXene monoliths with locally laminated structure are produced by the alkali-assisted self-assembly of MXene from liquid phase, wherein alkali promotes the "face-to-face" stacking of MXene nanosheets by weakening the electrostatic repulsion during the process of self-assembly. As an anode for sodium-ion storage, it shows an enhanced capacity of 188 mA h g⁻¹ with competitive rate performance compared to that without alkali (110 mA h g⁻¹), which is ascribed to the considerable intercalation pseudo-capacitance provided by the laminated MXene structures. The balance of laminated and porous structures in MXene monoliths is the key to simultaneously provide sufficient Na⁺ storage sites and multi-dimensional ion transport pathways. We also reveal that the K⁺ adsorbed on the MXene nanosheets can recede their oxidation process to promote the inherent stability of MXene flakes. This study is promising to inspire researchers to design advanced porous MXene macro-assembly and targeted structures of other 2D materials for electrochemical energy storage systems.