Production of gas-releasing electrolyte-replenishing Ah-scale zinc metal pouch cells with aqueous gel electrolyte

The study addresses key barriers to scaling aqueous Zn metal batteries for grid-scale energy storage: hydrogen evolution reaction (HER) at the Zn/electrolyte interface, Zn dendrite growth leading to dead Zn and short circuits, electrolyte consumption, and gas accumulation causing pouch swelling or explosion in sealed formats. Recognizing that Zn metal and aqueous electrolytes are air-stable, the authors challenge the conventional sealed-cell paradigm (adapted from Li-ion batteries) and propose an open, refillable pouch cell that can vent H2 and replenish consumed electrolyte components. They hypothesize that combining this open architecture with a water-bonding hydrogel electrolyte made from crosslinked k-carrageenan and chitosan will suppress HER and dendrite formation, reduce evaporation/leakage, and extend life in large-format, multilayer cells.

Prior approaches to stabilize Zn anodes include electrolyte additives, highly concentrated and nonaqueous electrolytes, hydrogel electrolytes, 3D structured Zn anodes, and interfacial modifications. However, most validations were in small coin cells and sealed configurations derived from Li-ion systems, which neglect gas management in aqueous Zn systems and face H2 accumulation and swelling. Hydrogel electrolytes have shown promise to mitigate leakage/evaporation and stabilize Zn, yet demonstrations at practical cell sizes and capacities are limited. This work builds on those insights by introducing an open, refillable pouch design with a biomass-derived carrageenan/chitosan hydrogel to address gas release and electrolyte replenishment at scale.

- Gel electrolyte synthesis: Crosslinked CarraChi gel prepared by dispersing 1 g k-carrageenan in 100 mL saturated chitosan solution (~0.18 g L−1; chitosan pre-dissolved 1 g in 1000 mL DI water, stirred 1 week and filtered), stirring 3 days, casting 50 mL into a PTFE mold (9 cm diameter), air drying, then immersing in 2 M ZnSO4 for 12 h to obtain CarraChi-ZnSO4. - Materials characterization: SEM (morphology and thickness, ~18 μm), XPS (N1s, S2p to verify components), FTIR (functional groups and crosslinking), Raman (water H-bonding structure), zeta potential measurements (CarraChi and CarraChi-ZnSO4), mechanical testing (tensile strength/strain), and anti-corrosion tests by OCV exposure. - Electrolyte properties: Ionic conductivity measured using stainless steel blocking electrodes and EIS, yielding 5.3 mS cm−1 at 25 °C; Arrhenius analysis of Rct from EIS (25–50 °C) to estimate desolvation activation energy. - Electrochemical testing: Symmetric Zn|separator|Zn coin cells and pouch cells assembled with CarraChi gel or GF separator. Chronoamperometry at −150 mV to probe Zn deposition modes; HER onset via LSV (vs SHE). Finite-element simulations (COMSOL 6.1) to model ion concentration and electric field distributions under 10 mA cm−2. - Open, refillable pouch cell design: 8 × 8 cm2 Zn foils; CarraChi gel as electrolyte/separator; a vent/refill port to release H2 and inject water or 2 M ZnSO4 when overpotential rises. Tests at 25 °C under 370 kPa pressure. Cycling at 10 mA cm−2 with 35 mAh cm−2 areal capacity (65% DOD), with periodic electrolyte replenishment as needed. - Full cells: ZnxV2O5·nH2O (ZVO) cathode synthesized hydrothermally (V2O5 and Zn(Ac)2 in water, acetone, HNO3, autoclave at 180 °C for 24 h). Coin cells: ZVO on carbon cloth (70:20:10 ZVO:Super P:PVDF), 1 mg cm−2 loading; pouch full cells: 8 × 8 cm2 carbon cloth with ZVO (15 mg cm−2). Lamination used to assemble multilayer Zn|CarraChi|ZVO pouch cells. Electrochemical performance measured on Neware systems; EIS/LSV on specified instruments.

- CarraChi gel structure and properties: Dense 18 μm membrane with strong crosslinking between chitosan and k-carrageenan (XPS, FTIR). Mechanical strength improved upon Zn2+ incorporation; tensile strength 14.2 MPa and strain-to-failure 45% (vs GF 0.3 MPa, 6%; carrageenan-only 4.2 MPa, 32%). - Electrolyte behavior: Ionic conductivity 5.3 mS cm−1 at 25 °C. Zeta potential shifts from −9.4 mV (CarraChi gel) to −56.8 mV (CarraChi-ZnSO4). Lower charge-transfer resistance across 25–50 °C and reduced desolvation activation energy: Ea = 27.9 kJ mol−1 for Zn|CarraChi|Zn vs 51.2 kJ mol−1 with GF. Raman indicates strengthened gel–water bonding and more weakly H-bonded water, consistent with HER suppression. - Zn deposition and HER suppression: Lower HER onset overpotential observed with CarraChi (−1.37 V vs SHE) compared to GF (−1.28 V vs SHE). Chronoamperometry shows suppressed 2D surface diffusion and promotion of uniform 3D deposition with CarraChi; simulations reveal homogenized Zn2+ concentration and electric field near Zn–CarraChi interface. SEM confirms smooth, uniform deposits at 10–30 mAh cm−2 with CarraChi; GF exhibits pore-filling and shorts at 20 mAh cm−2. - Symmetric coin cells: Rate capability up to 40 mA cm−2 without shorting (GF shorts). Long life >2500 h at 1 mA cm−2, 1 mAh cm−2; at 10 mA cm−2 with 120 mAh cm−2 (65% DOD), life >180 h. Coulombic efficiency in Zn|CarraChi|Cu reaches 99.8% after 300 cycles at 5 mA cm−2 and remains stable for 600 cycles. - Symmetric refillable pouch cells (8 × 8 cm2): With open vent/refill and CarraChi gel, sustained cycling for >4000 h at 10 mA cm−2 with 35 mAh cm−2 (65% DOD). Cumulative capacity 1286 Ah; average CE >99.5%. Periodic refilling with water or 2 M ZnSO4 restores overpotential. No short circuit per EIS; GF-based pouch shorts at ≤2 mA cm−2. - Full-cell performance (Zn|CarraChi|ZVO): High rate capacities: 349.6 mAh g−1 at 0.2 A g−1; 200 mAh g−1 at 4 A g−1; 115.6 mAh g−1 at 8 A g−1. Capacity recovers to 275.8 mAh g−1 at 1 A g−1 after rate test. Cycling at 0.2 A g−1 shows 88.2% retention over 100 cycles with near-100% CE; lower Rct than GF initially and after cycling. - Multilayer pouch full cell: Initial capacity 0.9 Ah; 84% capacity retention after 200 cycles at 200 mA g−1 (25 °C, 370 kPa), outperforming GF/liquid electrolyte counterparts and prior V-based aqueous Zn pouch cells.

The open, refillable pouch architecture directly addresses gas accumulation by enabling H2 venting and mitigates electrolyte depletion through on-demand refilling, while the CarraChi hydrogel’s polar functional groups bind water and modulate the solvation structure to suppress HER and corrosion. The gel’s mechanical robustness supports practical lamination and winding in large-format cells and establishes intimate electrode–electrolyte interfaces that homogenize ion flux, reducing 2D surface diffusion and dendrite growth. These combined effects yield markedly reduced interfacial desolvation barriers, smoother Zn deposition, high Coulombic efficiency, and unprecedented longevity and cumulative capacity in Ah-scale, multilayer pouch formats. The strong rate capability and cycling stability in Zn|ZVO configurations demonstrate fast Zn2+ redox kinetics and stable interfaces, reinforcing the strategy’s relevance for grid-scale aqueous Zn storage.

An open, gas-releasing and electrolyte-replenishing Zn battery architecture using a crosslinked k-carrageenan/chitosan (CarraChi) hydrogel electrolyte enables practical large-format aqueous Zn metal cells. The CarraChi gel bonds water, suppresses HER and evaporation, homogenizes Zn2+ flux, and prevents dendrite growth. Symmetric pouch cells achieve >4000 h life at 10 mA cm−2 and 35 mAh cm−2 (65% DOD), delivering 1286 Ah cumulative capacity with >99.5% CE. Multilayer Zn||ZVO pouch cells deliver 0.9 Ah initially and retain 84% capacity after 200 cycles at 200 mA g−1. Future work could explore broader cathode chemistries, long-term stability under varying environmental conditions, optimized refill protocols, and manufacturing scale-up and durability in field environments.

The authors note that open-system configurations can face electrolyte leakage and evaporation risks; these are mitigated here by the water-bonding CarraChi gel but remain an architectural consideration. Performance assessments are primarily at 25 °C and 370 kPa in specific Zn||ZVO chemistries; broader validation across temperatures, pressures, cathode types, and extended cycle counts in full cells beyond 200 cycles would further establish generalizability. Potential long-term effects of repeated refilling on electrolyte composition and cell balance are not detailed.