- The paper demonstrates that low-charge chitin nanofiber separators promote robust, dendrite-free Zn SEI formation with >99% coulombic efficiency.
- Detailed analyses reveal that tailoring amine and carboxylate group densities precisely controls ionic conductivity and water confinement.
- Full-cell tests confirm extended cycling lifetimes (over 2000 h) due to improved interfacial compatibility and minimized side reactions.
Tuning Separator Chemistry for Zn Anode Compatibility via Functionalized Chitin Nanofibers
Introduction
This study presents a comprehensive investigation into separator architecture for aqueous Zn batteries (AZBs) using chitin nanofibers with tunable surface chemistry. While the interface chemistry of Zn anodes has been a persistent bottleneck, leading to dendritic growth, hydrogen evolution, and poor cycling stability, conventional separator materials such as glass fiber and cellulose lack the ability to modulate ion and water dynamics at the molecular level. By exploiting the surface chemistry of anisotropic bio-derived colloidal chitin nanofibers—precisely tuning the density of amine and carboxylate groups—the authors decouple the mechanical and chemical factors controlling separator performance, linking nanofiber functionalization directly to electrochemical stability and Zn electrode compatibility.
Separator Synthesis and Physicochemical Characterization
Chitin nanofibers are extracted from shrimp shell waste via controlled deacetylation (introducing primary amine groups) and subsequent carboxymethylation (introducing carboxylate functionalities). Four well-defined compositions are studied: NH2-Low, NH2-High, NH2/COOH-Low, and NH2/COOH-High, with defined degrees of deacetylation and carboxylation. Protonation in a mildly acidic electrolyte environment ensures the separator backbone carries predominantly positive charge due to NH3+ groups.
AFM/SEM analyses confirm consistent fibrillar morphologies with nanoscale pore exclusion superior to commercial glass fiber and cellulose analogs. The functional group content dictates colloidal stability and fiber dimensions; higher charge densities induce more pronounced nanofibrillation. Zeta potential measurements unambiguously establish ampholytic behavior in COOH-bearing separators. Mechanical characterization demonstrates that the 30 µm chitin nanofiber membranes outperform commercial benchmarks in terms of flexibility, mechanical robustness, and wetting characteristics, requiring significantly reduced electrolyte uptake for full permeation.
Effects of Functional Group Chemistry on Ionic Conductivity and Transport
The tailored chitin nanofiber separators exhibit systematically varying ionic conductivities, directly correlated with functional group type and density. NH2-Low (amine only, low charge) achieves 3.8 mS/cm at 25°C, with a decrease observed upon increasing functional group densities or COOH content (NH2/COOH-High at 0.6 mS/cm). Notably, temperature dependence studies reveal a suppression of conductivity in highly charged membranes, consistent with ion pairing and counterion binding effects. Transference numbers (tZn2+) extracted via the Bruce-Vincent approach remain in the 0.57–0.70 regime for chitin-based separators, exceeding reference values for crosslinked chitin or cellulose analogs.
Quasi-elastic neutron scattering (QENS) provides direct insight into the local water structure and ion dynamics. NH2-Low and NH2/COOH-Low separators confine water more tightly, exhibiting narrower QENS spectra, indicating hindered solvent dynamics and reduced free-water content. This “bound water” environment is interpreted as suppressive of parasitic reactions (e.g., HER) while maintaining sufficient ionic mobility for Zn2+ transport. In contrast, highly charged membranes trap ions, diminish transport, and plasticize under hydration, ultimately inhibiting battery kinetics.
Interfacial Zn Anode Behavior and Surface Analysis
Zn|Zn symmetric cell studies highlight unprecedented cycling lifetimes for NH2-Low and NH2/COOH-Low separators (â©ľ2000 h at 0.5 mA/cm2, â©ľ850 h at 1 mA/cm2), far exceeding commercial separator references (â©˝140 h). Increasing separator functional group density systematically reduces cycle life. SEM of post-cycled electrodes shows smooth, dendrite-free Zn deposits with NH2-Low/NH2/COOH-Low, while other formulations propagate corrosion, local short-circuiting, and dendrite formation. XRD analyses verify preferred growth along the Zn (002) basal plane in the presence of low-charge chitin nanofibers, attributable to uniform Zn2+ ion flux and minimized nucleation overpotential.
Surface-sensitive XPS studies further reveal that NH2-Low produces a distinct, robust solid electrolyte interphase (SEI) with fewer ZnO byproducts, dominated by Zn(OH)2 and evidence of intertwined chitin/electrolyte decomposition products, indicative of a modified SEI chemistry that prevents excessive water contact with Zn and suppresses HER.
Coulombic efficiency (CE) testing in Zn|Cu cells shows that NH2-Low maintains >99% CE over 200 cycles, outperforming all other tested separators.
Full-Cell Testing and Practical Implications
Integration of NH2-Low and NH2/COOH-Low separators into practical Zn||NaV3O8·1.5H2O full cells demonstrates compatibility with multivalent intercalation chemistries. These separators support stable charge/discharge cycling at high rates (2 A/g), with lifetimes exceeding 1000 cycles and CE ⩾99.9%. A capacity retention disparity (38.7% for NH2-Low vs. 61.5% for NH2/COOH-Low) is noted, but the suppression of side reactions and dendrites is reproducibly achieved across all full-cell protocols.
Theoretical and Practical Implications
This work articulates the critical role of separator surface chemistry, particularly the balance between functional group type and charge density, in dictating ion transport phenomena, water structuring, and Zn interfacial dynamics in AZBs. The mechanistic interplay observed—where lower density, positively charged chitin nanofibers enhance Zn SEI formation, avoid unwanted side reactions, and support extended cyclability—establishes a blueprint for separator rational design beyond the conventional passivity paradigm.
The utilization of waste biomass for scalable, tunable separator fabrication further advances the field’s movement toward sustainable and high-performance AZBs. The findings open new research trajectories in the engineering of separator/electrolyte and separator/anode interfaces, with implications for other multi-valent and aqueous battery systems.
Conclusion
The work provides direct evidence that ampholytic surface functionalization and control of charge density in chitin nanofiber separators enables previously unattainable Zn anode stability and cycle life. NH2-Low and NH2/COOH-Low compositions deliver optimal combinations of ionic conductivity, water confinement, and interfacial compatibility, resulting in state-of-the-art cell performance metrics in both half- and full-cell configurations. These results frame separator active chemistry as a core parameter in the future development of dendrite-free, high-efficiency aqueous Zn ion batteries, with the chitin nanofiber platform constituting a robust, sustainable foundation for industrial translation.
Reference:
"Tuning Separator Chemistry: Improving Zn Anode Compatibility via Functionalized Chitin Nanofibers" (2512.19449)
