Nanochitin-Based Separators for Enhanced Zn Batteries

• The paper demonstrates the fabrication of nanochitin-based separators with tunable amine levels (780–1740 μmol/g) that achieve low overpotentials (±20 mV) and extended cycling (>2000 h).
• Controlled deacetylation and carboxymethylation produce a dense nanofibrillar architecture that enhances electrolyte wettability and regulates ion transport (e.g., up to 3.8 mS·cm⁻¹ conductivity).
• The study establishes structure–property–performance relationships showing that optimized functional group densities and membrane thickness (25–35 μm) suppress dendrite formation compared to conventional separators.

Nanochitin-based separators are a class of electrochemical membrane materials fabricated from nanofibrillated chitin—poly-(1→4)-β-N-acetyl-D-glucosamine—extracted from waste crustacean shells and chemically functionalized to enhance compatibility with metallic anodes, notably zinc (Zn), in aqueous battery systems. By tuning the density and type of surface functional groups such as amines (–NH₂) and carboxylates (–COO⁻), these separators can regulate water coordination, ion transport, and electrodeposition dynamics. Their nanofibrillar architecture provides a mechanically robust, thin, and highly wettable scaffold capable of suppressing dendrite formation and extending Zn anode stability far beyond the capabilities of conventional glass fiber or cellulose-based separators (Kathemi et al., 22 Dec 2025).

1. Nanochitin Extraction, Functionalization, and Separator Fabrication

Nanochitin is derived from chitin flakes sourced from shrimp shells using partial deacetylation with 33 wt % NaOH at 90 °C, modulated by sodium borohydride (NaBH₄) to suppress depolymerization. Degree of amination is controlled by reaction duration: 4 h (“NH₂-Low,” 780 μmol NH₂/g) or 24 h (“NH₂-High,” 1740 μmol NH₂/g). Carboxymethylation, following a cellulose protocol, introduces ampholytic character, yielding NH₂/COOH-Low (660 μmol NH₂/g, 120 μmol COO⁻/g) and NH₂/COOH-High (1560 μmol NH₂/g, 180 μmol COO⁻/g). Fibrillation is achieved via Ultra-Turrax homogenization (10,000 rpm, 5 min) and high-pressure microfluidization (1,500 bar), followed by centrifugation and membrane casting via vacuum filtration on PVDF, resulting in films ≈ 30 μm thick. Membranes are cold-pressed for 2 days, yielding > 90 % fiber recovery by mass (Kathemi et al., 22 Dec 2025).

Nanoscale fibril morphology (AFM: 3.3–5.8 nm height) generates a dense, interwoven structure without visible micron-scale pores (SEM), contrasting with the 1–10 μm irregular macropores of glass fiber and the low porosity of cellulose. Wettability is superior: nanochitin membranes absorb < 25 μL electrolyte (1 M Zn(OTf)₂) for complete infiltration, outperforming both GF and CLL, which require ≥ 40 μL.

2. Water Coordination, Ion Transport, and Functional Group Density

Quasi-elastic neutron scattering (QENS) probes water dynamics and coordination in nanochitin membranes under pristine, hydrated, salt-plasticized, and full-electrolyte states. Low-functionalized samples (NH₂-Low, NH₂/COOH-Low) exhibit broader QENS spectral widths in electrolyte, correlating with increased local molecular dynamics and greater water and Zn²⁺ mobility. High-functionalized samples (NH₂-High, NH₂/COOH-High) show narrower QENS spectra, characteristic of restricted solvent and ion dynamics due to stronger ion pairing and trapping. NH₂-Low demonstrates “bound” water with reduced translational freedom, a feature that stabilizes Zn plating/stripping (Kathemi et al., 22 Dec 2025).

Through-plane EIS measurements reveal temperature-dependent ionic conductivities (25 °C, 1 M Zn(OTf)₂): NH₂-Low, 3.8 mS·cm⁻¹; NH₂-High, 2.6 mS·cm⁻¹; NH₂/COOH-Low, 2.2 mS·cm⁻¹; NH₂/COOH-High, 0.6 mS·cm⁻¹. GF and CLL provide ≈ 68 mS·cm⁻¹, but the transport is dominated by bulk electrolyte rather than controlled single-ion flux. Zn²⁺ transference numbers ($t_+$, Bruce–Vincent) for aminated chitin are t₊ ≈ 0.63; NH₂/COOH-Low, 0.57; GF, 0.74; CLL, 0.54. Decreased ionic conductivity at higher carboxylate density is attributed to increased fixed-charge sites immobilizing mobile anions and hindering bulk transport.

The ionic conductivity ($\sigma$) is described by:

$\sigma = \sum_i n_i z_i^2 e^2 D_i / (k_B T)$

where $n_i$, $z_i$, $D_i$ represent the number density, valence, and diffusion coefficient of ionic species $i$.

3. Zn Electrodeposition: Uniformity, Overpotential, and Dendrite Suppression

Chronoamperometry at −150 mV (vs Zn²⁺/Zn) demonstrates separator-dependent Zn²⁺ flux and deposition morphology. GF/CLL and NH₂/COOH-High show continuous current rise (indicative of 2D, coarse deposition). NH₂-Low/High undergo rapid 2D→3D transition and establish a diffusion-limited regime, characteristic of uniform nucleation. NH₂/COOH-Low displays intermediate behavior (Kathemi et al., 22 Dec 2025).

During symmetric Zn|Zn cycling (1 mA·cm⁻², 1 mAh∙cm⁻²), separators exhibit:

Separator	Overpotential (mV)	Stability (h)	Coulombic Efficiency (%)
GF	±100	30–140	96
CLL	±100	50–140	95
NH₂-Low	±20	>2,000	>99
NH₂/COOH-Low	±20	>2,000	97
NH₂-High	±60	1,200	98
NH₂/COOH-High	±80	700	97

Failure for GF/CLL occurs within 30–50 h at high current. NH₂-Low and NH₂/COOH-Low exhibit excellent polarization stability (> 900 h at 1 mA·cm⁻²) and low overpotential (±20 mV).

Zn surface examinations after 20 cycles (SEM) show that GF/CLL and highly functionalized chitin yield dendritic, rough morphologies, while NH₂-Low features an ultrasmooth, planar deposit. XRD reveals (002)/(100,101) peak ratios are enhanced in low-functionalized samples, indicating preferred basal-plane growth—an established dendrite suppression mechanism.

The governing transport and charge equations encompass Nernst–Planck and Poisson formulations:

$$J_i = -D_i \nabla c_i - \frac{D_i z_i e}{k_B T} c_i \nabla \Phi$$

$$\nabla \cdot (\epsilon \nabla \Phi) = -\sum_i z_i e c_i - \rho_\text{fixed}$$

where $\rho_\text{fixed}$ reflects separator-immobilized –NH₃⁺/–COO⁻ groups, flattening Zn²⁺ concentration profiles and mitigating local current hotspots.

4. Electrochemical Cell Performance

Nanochitin-based membranes, particularly NH₂-Low and NH₂/COOH-Low, demonstrate enhanced long-term electrochemical metrics in both symmetric and full-cell configurations (Kathemi et al., 22 Dec 2025).

High-capacity cycling (Zn|Zn, 5 mAh·cm⁻²):

• At C/5 (1 mA·cm⁻²): NH₂-Low > 700 h, NH₂/COOH-Low > 500 h.
• At 1C (5 mA·cm⁻²): Both separators > 350 h (175 cycles), GF/CLL < 20 cycles.

Zn–Cu Coulombic efficiency (1 mA·cm⁻², 1 mAh·cm⁻², 200 cycles):

• NH₂-Low: > 99 %
• NH₂/COOH-Low: 97 %
• GF/CLL: 95–96 %

In full cells (Zn‖NaV₃O₈·1.5H₂O):

• Rate capability (0.1–5 A·g⁻¹): NH₂-Low matches or slightly exceeds GF/CLL.
• Long-term cycling at 2 A·g⁻¹: NH₂-Low shows 99.9 % CE and 38.7 % capacity retention after 1000 cycles; NH₂/COOH-Low achieves 99.5 % CE, 61.5 % retention; GF/CLL fails after ~75 cycles.

5. Structure–Property–Performance Relationships

Separator effectiveness correlates with both surface chemical functionality and nanostructure. Optimal performance is associated with:

• Amine density ≈ 0.75–0.80 mmol NH₂/g (degree of deacetylation ≈ 15 %)
• Carboxylate content ≤ 0.12 mmol COO⁻/g, with higher levels (> 0.18 mmol/g) detrimental to ionic mobility
• Film thickness targeting 25–35 μm to minimize resistance without compromising integrity
• Fixed surface charge (–NH₃⁺) in mildly acidic electrolytes (pH ~ 5), providing Zn²⁺ flux uniformity and water confinement

Superior separator performance arises from balancing strong surface charge (single-ion conduction, anion immobilization) with sufficient ion mobility for sustained, stable cycling.

6. Design Guidelines, Processing Parameters, and Outlook

Key design and processing parameters include (Kathemi et al., 22 Dec 2025):

• Deacetylation: 33 wt % NaOH, 90 °C, 4 h for “Low” NH₂ content
• Carboxymethylation: monochloroacetic acid and NaOH in isopropanol/methanol, 80 °C, 1 h
• Nanofibrillation: Ultra-Turrax (10,000 rpm, 5 min) plus microfluidization (1,500 bar)
• Membrane formation: vacuum filtration, cold-press drying (2 days)

Design optimization involves fine-tuning of functional group densities and careful control of nanofibrillar assembly. Incorporation of hierarchical porosity or charge gradients may further improve uniformity of Zn plating and longevity. Extension of this methodology to other multivalent aqueous battery chemistries (e.g., Mg, Al) is plausible by tailoring both the surface charge density and pore structure.

A plausible implication is that further advances in nanochitin processing and surface engineering could allow for scalable production of separators tailored for a range of aqueous electrochemical storage and conversion systems.

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Reference:

"Tuning Separator Chemistry: Improving Zn Anode Compatibility via Functionalized Chitin Nanofibers" (Kathemi et al., 22 Dec 2025)