Orchestrating Sodium Deposition and SEI Passivation via Bifunctional Mo2TiC2Tx MXene for High-Efficiency Anode-Free Sodium Batteries.

The operational instability of anode-free sodium metal batteries (AFSMBs), stemming from uncontrolled sodium dendrite growth and unstable solid electrolyte interphase (SEI) formation, presents a formidable barrier to their commercialization. Herein, we address these critical bottlenecks by employing a bimetallic MXene (Mo2TiC2Tx) as a dual-functional interfacial modifier on planar copper current collectors. Unlike conventional single-metal MXenes, Mo2TiC2Tx exploits the synergistic electronic interaction between Mo and Ti to create a highly sodiophilic surface, drastically reducing the nucleation overpotential and guiding a lateral, spherical growth mode of sodium that precludes dendritic protrusions. Beyond physical nucleation control, the unique surface chemistry of Mo2TiC2Tx actively modulates the solvation/decomposition pathway of the electrolyte, fostering an ultrathin SEI that is predominantly composed of highly ion-conductive NaF and Na2O nanocrystallites. This electrochemically robust interface ensures rapid Na+ desolvation and transport, as confirmed by a six-fold increase in the Na+ diffusion coefficient compared to conventional MXenes. Benefiting from this synergistic orchestration of bulk deposition and interfacial chemistry, the Mo2TiC2Tx@Cu electrode achieves an unparalleled cycling longevity of over 1400 hours and a high average Coulombic efficiency of 99.61%. When paired with a Na3V2(PO4)3 cathode, the full cell demonstrates outstanding capacity retention over 600 cycles. This work underscores the critical role of bimetallic MXenes in engineering both the nucleation thermodynamics and SEI formation kinetics, offering a transformative strategy for developing durable and energy-dense AFSMBs.