Water-enabled crystallization of mesoporous SnO₂ as a binder-free electrode for enhanced sodium storage

Water-enabled crystallization of amorphous anodic SnO₂ is reported. After low-temperature water soaking for a short period of time (e.g., 60 °C for 2 h), mesoporous rutile SnO₂ with a remarkably increased surface area is achieved (nearly 2-fold that of the as-anodized SnO₂ and 3.3 times that of SnO₂ annealed at high temperature). Closer examination reveals that the water-crystallized SnO₂ possesses a hierarchical nanostructure that features 1D nanochannels (e.g., ∼30 nm in diameter) and thin channel walls (e.g., 20 nm thick) that are comprised of tiny nanoparticles (e.g., ∼4 nm big). The water-crystallized SnO₂ directly grown on the copper foil can be directly applied as a novel type of binder-free electrode for sodium-ion storage, delivering a high reversible capacity of 514 mA h g^-1 after 100 cycles at a current rate of 0.1C.