A Sulfonated Cellulose Separator for High-Areal-Capacity and Long-Lifespan Zn–Br2 Batteries

Abstract

Aqueous zinc-bromine (Zn-Br2) batteries are limited by Br3- shuttle, zinc dendrite growth and rapid self-discharge, leading to poor efficiency and short lifespan. Here, we design a sulfonated cellulose-nanofiber (SC-CNF) composite separator that integrates high ionic conductivity, mechanical strength, and electrostatic ion regulation. Negatively charged -SO3- groups create local electrostatic fields that repel Br3- anions and provide Zn2+ hopping sites, enabling fast, uniform ion transport and stable zinc plating/stripping. Consequently, Zn-Br2 batteries with the SC-CNF separator deliver 950 stable cycles, a Coulombic efficiency of 98.8% and excellent self-discharge resistance (98% after 12 h). Flexible pouch cells achieve 0.366 Ah initial capacity with 71.6% retention after 100 cycles, maintaining operation under folding and cutting. This work demonstrates a multifunctional, scalable separator design that effectively suppresses dendrite formation and bromine crossover, advancing safe, durable, and high-efficiency aqueous Zn-Br2 batteries for practical energy storage.