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

## TL;DR

A new electrode modified with carbon nanotubes improves the performance of tin-based batteries, making them more efficient and stable.

## Contribution

A surface-engineering strategy using carbon nanotubes enhances Sn4+/Sn redox kinetics and reversibility for flow batteries.

## Key findings

- CNTs on carbon felt reduce charge-transfer resistance by over 55-fold compared to pristine carbon felt.
- The Sn/Br flow battery achieved 80% energy efficiency at 40 mA cm−2 and stable cycling for over 650 hours.
- The battery delivered a discharge capacity of 373 Ah L−1 and an areal capacity of 614 mAh cm−2 with 4 M electrolyte.

## Abstract

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 (Rct) 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.

In situ-grown CNTs decorated on carbon felts overcome reversibility issues of Sn4+/Sn couple, enabling a Sn/Br flow battery with 80% energy efficiency at 40 mA cm−2, 650 h stability, 373 Ah L−1 discharge capacity and 614 mAh cm−2 areal capacity.