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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, Ti 3 C 2 T x (MXene)-based films are prepared by using sulfur (S) as the mediator to modulate the surface chemistry and microstructure, generating S-doped mesoporous Ti 3 C 2 T x films with high flexibility. The mesoporous architecture offers desirable surface accessibility without significantly sacrificing the high density of Ti 3 C 2 T x film. Meanwhile, the surface sulfur doping of Ti 3 C 2 T x 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 Ti 3 C 2 T x 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., (© 2021 Wiley-VCH GmbH.)