# Carbon-halogen bond substitution enables high-utilization four-electron iodine redox in noncorrosive dilute electrolytes

## TL;DR

This paper introduces a new method to improve the performance of aqueous Zn-I2 batteries by using a noncorrosive additive that enhances iodine utilization and battery lifespan.

## Contribution

A novel carbon-halogen bond substitution pathway is introduced to enable efficient four-electron iodine redox in noncorrosive dilute electrolytes.

## Key findings

- High iodine utilization (55%–80%) is achieved at high current densities (5.8–46.4 mA cm⁻²).
- The battery demonstrates a long lifespan (>400 cycles) with 99.5% capacity retention at 47.5 mA cm⁻².
- The method enables sustainable operation with high iodine loading (8.6–24.0 mg cm⁻²).

## Abstract

Aqueous Zn | |I2 batteries, involving I-/I0/I+ redox, are promising yet usually facing low I2 utilization dominated by I0/I+ redox, especially under high loadings. Unlocking alternative pathway to I0/I+ redox, preferably in noncorrosive dilute electrolytes, is a crucial solution. Here, we report a pathway towards more thermodynamically favorable I0/I+ redox, via a unique carbon-halogen bond substitution. This pathway is realized with a low-concentrated (0.7 M), noncorrosive organohalide additive (2-bromoacetamide, BrAce), triggering a reversible Br-C···I(0) and C-I(+)-Br bond substitution. Compared with conventional interhalogen bonding (I-Br) pathway, this pathway synchronously lowers the barrier for I⁰/I⁺ redox and strengthens the anti-hydrolysis of I+ species, by elaborately regulating axial δ hole activity of interhalogen bond (I(δ+)-Br). Notably, this pathway enables sustainable operation of four-electron Zn | |I2 batteries with high I2 loading (8.6 ~ 24.0 mg cm-2), featuring improved performances: (1) high I2 utilizations (55% ~ 80%) at high rates (5.8 ~ 46.4 mA cm-2), (2) long lifespan (\documentclass[12pt]{minimal}
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Aqueous Zn | |I2 batteries, involving I-/I0/I+ redox, are promising yet usually facing low I2 utilization dominated by I0/I+ redox, especially under high loadings. Here, authors report a carbon-halogen bond substitution with a noncorrosive organohalide additive (2-bromoacetamide, BrAce) for a more thermodynamically favourable I0/I + , improving battery cycling under high loading and current conditions.