A carbon nanotube-modified electrode for a highly active and reversible Sn⁴⁺/Sn anode

Tin (Sn) is an attractive anode for high energy density batteries due to its four-electron redox process (Sn⁴⁺ → Sn²⁺ → Sn) without dendrite formation. However, the sluggish kinetics and poor reversibility of the Sn⁴⁺/Sn²⁺ process hinder its practical implementation. Herein, we propose a surface-engineering strategy to accelerate the Sn⁴⁺/Sn²⁺ redox kinetics and enable highly reversible Sn⁴⁺/Sn reactions. Specifically, carbon nanotubes (CNTs) enriched with edge defects and oxygen-containing groups are grown in situ on carbon felt (CF) via chemical vapor deposition (CVD), forming a high surface area electrode (denoted as CC-T). These CNTs provide abundant active sites for Sn⁴⁺ adsorption and facilitate charge transport, thereby enhancing electron transfer kinetics and redox reversibility. Consequently, the charge-transfer resistance (R _ct) of CC-T decreased by more than 55-fold compared with pristine CF (0.27 vs. 14.89 Ω). When assembled in a Sn/Br flow battery, the battery delivered an energy efficiency (EE) of 80% at 40 mA cm^-2, outperforming that of pristine CF (63%), and maintaining stable cycling for over 650 hours. Even with 4 M electrolyte, the battery achieved a discharge capacity of 373 Ah L^-1 and an areal capacity of 614 mAh cm^-2. This work provides a promising approach for developing high-capacity, dendrite-free metal anodes for next-generation flow batteries.