Freezing the Metastable Sn-C Interface via Millisecond Flash Joule Heating for Ultrafast Sodium Storage Kinetics

<jats:p>Flash Joule Heating (FJH) represents a disruptive strategy for accessing nonequilibrium materials, yet atomic-scale interfacial evolution under subsecond thermal shocks remains poorly understood. Herein, millisecond FJH is employed to freeze critical metastable interfaces, unraveling the kinetic origin of stable sodium-ion storage in Sn-based anodes. Crossvalidation by 119Sn Mössbauer and X-ray photoelectron spectroscopies reveals a unique Sn–C bridging state (isomer shift ~1.2 mm s-1) that effectively suppresses volume expansion upon cycling. Notably, tuning pulse duration uncovers nonlinear kinetic evolution: metastable Sn–C structural peak at 200 ms (11.1%) and decreases sharply beyond this window due to thermodynamic relaxation or disorder reconstruction. This optimized flash-frozen interface establishes a synergistic mechanism: robust Sn–C interactions maintain structural integrity, while the Sn2+/Sn4+ dual-buffering environment enables ultrafast lattice migration, enhancing cycling stability. The engineered electrode exhibits a self-regulating behavior, achieving a highly stable electrochemical equilibrium and superior capacity retention over 700 cycles. This work provides a spectroscopic blueprint for metastable interface engineering and a universal paradigm for fabricating metal composite electrodes via FJH.</jats:p>