Self-stratified aqueous biphasic Zn–I and Zn–Br batteries enabled by spontaneous phase separation and halogen extraction effects of ionic liquids
Kaiqiang Zhang, Yang Ge, Qianchuan Yu, Pengbo Zhang, Yuge Feng, Zuoxiu Tie, Jing Ma, Zhong Jin
Abstract
Self-stratified liquid electrode batteries are considered as a viable solution for large-scale energy storage applications due to their high safety and low cost. However, achieving long-term operation stability with high efficiency of selective ion migration/separation in multiple liquid phases remains a challenge. Here, we report an aqueous biphasic system based on imidazolium ionic liquids (ILs) for constructing membrane-free self-stratified aqueous biphasic Zn–I and Zn–Br batteries. The limited intersolubility and density difference between the IL-dominated catholyte and the aqueous anolyte create a self-stratified liquid-liquid phase separation structure, promising a reliable battery construction. Molecular dynamics simulations identify the internal association of IL, which forms a 3D H–bond network with the C–H···O interaction being the primary contributor, along with a thermodynamic electron transfer of ≤0.03 e. Additionally, a unique π + –bond effect is found between the positively charged imidazole ring of cation and the anion of IL. The as-constructed self-stratified aqueous biphasic Zn–halogen batteries work stably without the necessity of air isolation and exhibit long-term cycling stability (with a capacity mentation of 81.0 % for Zn–I batteries and 76.8 % for Zn–Br batteries over 1000 cycles), high energy efficiencies (93.5 % for Zn–I batteries and 86.7 % for Zn–Br batteries), inherent fire resistance and good scalability, demonstrating great potential for large-scale energy storage applications.