Large-Scale Nanofiber Manufacturing| Hydrophilic-hydrophobic structure ofANF/MFC separator for high-stable dendritefree aqueous zinc-ion batteries

Tianjin University of Science and Technology Cheng Bowen & Yang Leixin: Hydrophilic-Hydrophobic Structured ANF/MFC Separator for Highly Stable Dendrite-Free Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are considered one of the most promising next-generation battery energy storage systems. Their high theoretical capacity (820 mAh g-1) and low redox potential (-0.762 V vs SHE) have attracted significant research attention. Additionally, AZIBs' aqueous electrolytes offer advantages like non-toxicity and high safety. However, AZIBs still face application challenges including uncontrolled zinc dendrites, side reactions, hydrogen evolution reaction (HER), and passivation. Deterioration of Zn anodes leads to reduced Coulombic efficiency and short circuits, severely hindering AZIB development.

Separators are crucial components of AZIBs, significantly affecting Zn2+ flux and deposition/stripping behavior, yet they haven't received widespread attention. Glass fiber (GF) separators are currently most commonly used in AZIBs. However, GF separators present problems like large and uneven pore structures, excessive thickness, and poor mechanical strength, ultimately causing uneven Zn2+ flux, non-uniform Zn2+ deposition, and dendrite formation. Therefore, it's necessary to develop functional separators with uniform pores and appropriate zincophilic properties.

Recently, Prof. Cheng Bowen's team at Tianjin University of Science and Technology published their latest research "Hydrophilic–hydrophobic structure of ANF/MFC separator for high-stable dendrite-free aqueous zinc-ion batteries" in the Journal of Materials Science & Technology. Tianjin University of Science and Technology is the first affiliation, with corresponding authors Prof. Cheng Bowen and Associate Prof. Yang Leixin.Through simple vacuum filtration and freeze-drying processes, the researchers designed a composite separator with hydrophilic-hydrophobic structure using hydrophobic aramid nanofibers (ANF) and hydrophilic microfibrillated cellulose (MFC). With synergistic effects from hydrophilic hydroxyl groups (MFC) and zincophilic carbonyl groups (ANF), the optimized battery features fast, low-energy-barrier hydrated Zn2+ desolvation, uniform Zn2+ deposition, and dendrite-free Zn anodes.

Fig.1: Preparation process, microstructure, and mechanical property characterization of ANF/MFC separators.

Separator structure significantly affects Zn2+ flux redistribution, thereby influencing Zn2+ deposition during cycling. SEM images of GF separators [Fig.1(e)] show large (2-7 μm) and uneven pore size distribution, potentially causing non-uniform Zn2+ flux and deposition. Based on MFC and ANF entanglement effects, ANF/MFC separators [Fig.1(g)] exhibit smoother surface morphology than GF separators.

Fig.2: Physical property characterization and theoretical calculations of ANF/MFC separators.

FT-IR results indicate coordination between Zn2+ (or some hydrated Zn2+) and -C=O on ANF backbones. XPS also verifies strong interaction between ANF and Zn2+. DFT calculations, desolvation energy barrier calculations, and activation energy experiments further confirm that ANF/MFC separators effectively optimize Zn2+ desolvation compared to GF separators.

Fig.3: Zn//Cu half-cell and symmetric cell performance characterization.

Zn//Cu half-cells with ANF/MFC separators show excellent cycling stability, achieving 99.6% average Coulombic efficiency over 800 cycles at 1 mA cm-2 and 1 mAh cm-2. Zn//Zn symmetric cells with ANF/MFC separators demonstrate outstanding cycling performance: stable cycling over 3800 hours at 1 mA cm-2/1 mAh cm-2 and over 450 hours at 5 mA cm-2/5 mAh cm-2.

Fig.4: Zn anode deposition morphology characterization.

ANF/MFC separators' multiple optimization effects promote uniform, flat Zn anode deposition. SEM and AFM characterization of cycled Zn anodes shows smoother surface morphology with ANF/MFC separators compared to GF separators. In situ optical microscopy further confirms this - while GF separators show numerous black dendrites with increasing deposition time, ANF/MFC separators maintain very smooth surfaces.

Fig.5: Schematic of ANF/MFC separator-optimized deposition.

Fig.6: Full cell performance characterization.

Assembled Zn//V2O5 full cells exhibit good cycling performance.