Aqueous zinc (Zn) batteries are attractive for grid-scale energy storage because of their inherent safety and low cost. However, challenges remain, particularly in scaling up to large-format cells. The Zn anode suffers from several issues that limit its performance and lifespan: Zn dendrite growth, parasitic hydrogen evolution reaction (HER) at the anode-electrolyte interface, and electrolyte consumption. HER, driven by the thermodynamic instability of Zn in acidic electrolytes, reduces Coulombic efficiency (CE) and cycling stability. Electrolyte depletion can lead to battery failure, while hydrogen accumulation causes swelling and potential explosion. Dendrite growth, due to the "tip effect," leads to the formation of electronically isolated Zn (dead Zn), further exacerbating HER and potentially causing short circuits. These problems hinder the large-scale application of aqueous Zn batteries. Many approaches have been explored to address these issues, including electrolyte additives, highly concentrated electrolytes, non-aqueous electrolytes, hydrogel electrolytes, 3D Zn anodes, and anode/electrolyte interface modifications. However, most studies have focused on small-scale coin cells, which differ significantly from the large-format cells needed for grid-scale energy storage. Traditional sealed cell designs, adapted from Li-ion batteries, are unsuitable for aqueous Zn batteries because they don't address the issues of hydrogen accumulation and swelling inherent to the use of aqueous electrolytes and Zn anodes. An open system design offers a potential solution by allowing for hydrogen gas release and electrolyte replenishment, leveraging the inherent safety and abundance of the aqueous electrolyte. However, such a design must also prevent electrolyte leakage and evaporation. Hydrogels offer a promising approach to address this by bonding water molecules and enhancing the stability of the electrolyte, but most prior research with hydrogels has been restricted to small cell capacities and sizes. This paper introduces a refillable configuration for large-format aqueous Zn batteries based on an open system for gas release and electrolyte replenishment. A water-bonding gel electrolyte is used to mitigate electrolyte consumption, prevent swelling, and reduce dendrite growth. The electrolyte is composed of crosslinked kappa (k)-carrageenan and chitosan which provides several key advantages in terms of stability, flexibility and overall electrochemical performance.

The literature extensively documents the challenges associated with aqueous zinc-ion batteries, particularly concerning the zinc anode. Studies have explored various strategies to mitigate issues like dendrite formation, hydrogen evolution, and electrolyte decomposition. These strategies include using electrolyte additives to modify the solvation shell of zinc ions [5, 8, 22, 45, 46], employing highly concentrated electrolytes [12, 13] to suppress water activity, and exploring non-aqueous electrolytes [14] to eliminate the HER altogether. Three-dimensional (3D) anode structures [19] have been investigated to increase the surface area and reduce current density at the anode, which promotes uniform deposition and reduces dendrite formation. Modification of the anode/electrolyte interface [16, 17, 20] through the use of protective coatings or interphase layers also shows promise in improving the stability of the anode and preventing unwanted side reactions. Hydrogel electrolytes [15, 18, 33-35, 50, 51] have received significant attention due to their ability to suppress dendrite growth and provide improved ionic conductivity. However, most hydrogel research has been limited to coin-cell studies, and scaling these designs to larger formats while maintaining the desired properties has been challenging. Studies on large-format cells, such as pouch cells, are still limited, particularly those that address the issues of gas accumulation and electrolyte depletion inherent to large-scale applications. [23, 24, 26, 27, 30, 31]. The development of refillable cell architectures [18, 27-29] aims to address electrolyte consumption, but effective strategies to prevent evaporation and leakage while maintaining sufficient ionic conductivity remain an area of active research. This prior work sets the stage for the present study, which addresses the challenges of creating high-capacity, long-life aqueous zinc batteries suitable for large-scale energy storage applications.

This research involved the synthesis and characterization of a novel water-bonding gel electrolyte, followed by its implementation in both coin cells and pouch cells for electrochemical testing. The gel electrolyte, termed CarraChi, was prepared by crosslinking kappa (k)-carrageenan and chitosan using a simple mixing-casting method. The resulting gel was then soaked in 2 M ZnSO4 solution. Various characterization techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, and mechanical testing, were employed to evaluate the morphology, chemical composition, and mechanical properties of the CarraChi gel. The electrochemical performance of the CarraChi gel electrolyte was assessed using symmetric Zn|CarraChi|Zn coin cells and Zn||ZnxV2O5·nH2O coin and pouch cells. Electrochemical measurements included galvanostatic charge-discharge cycling, electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). The Zn||ZnxV2O5·nH2O cathode material was synthesized using a solvothermal method. Symmetric cells were used to evaluate the Zn anode's rate performance and cycling stability. Full cells were used to determine the overall cell performance and capacity retention. A finite element simulation was performed to analyze the concentration and electric field distribution within the cells. The pouch cell design incorporates an open configuration that allows for the release of hydrogen gas and replenishment of the electrolyte during cycling. This design directly addresses the issues of electrolyte depletion and gas accumulation that limit the performance of sealed cells. Specific steps for the CarraChi gel preparation involved dissolving chitosan in deionized water, preparing a k-carrageenan and chitosan solution, casting the solution into a mold, and drying. The resulting CarraChi gel was then immersed in 2M ZnSO4. The ZnxV2O5·nH2O cathode was synthesized using V2O5, Zn(Ac)2, acetone, and HNO3 via solvothermal treatment. Coin cells were assembled using Zn foils, the CarraChi gel or a glass fiber (GF) separator, and the ZnxV2O5·nH2O cathode material coated on carbon cloth. Pouch cells were constructed using larger Zn foils, and the CarraChi gel or GF separator. Electrochemical measurements were conducted using a Neware battery test system and an IVIUM electrochemical workstation. The ionic conductivity of the CarraChi gel was calculated using EIS data. Finite element simulations were performed in COMSOL Multiphysics 6.1 to model the ion and potential distributions in the cells.

The study demonstrated significant improvements in the performance and lifespan of aqueous zinc batteries by employing a novel crosslinked hydrogel electrolyte and a unique open-cell design. Key findings include: 1. **Water-Bonding Gel Electrolyte:** The CarraChi gel electrolyte, composed of crosslinked k-carrageenan and chitosan, exhibited superior properties compared to traditional GF separators. It demonstrated high ionic conductivity (5.3 mS cm⁻¹ at 25 °C), high mechanical strength (45% strain-to-failure and 14.2 MPa tensile strength), and excellent water retention capabilities. FTIR and XPS analysis confirmed the crosslinking and the presence of functional groups (-OH, -NH2, SO4²⁻) capable of bonding water molecules. The lower desolvation energy (27.9 kJ mol⁻¹) compared to GF (51.2 kJ mol⁻¹) suggests easier Zn²⁺ ion transport in CarraChi. Raman spectroscopy revealed a shift in hydrogen bonding in the CarraChi-ZnSO4 system, limiting HER. 2. **Uniform Zn Deposition:** Chronoamperometry and SEM analysis showed that the CarraChi gel electrolyte facilitated uniform Zn deposition, effectively suppressing the formation of dendrites. Finite element simulations supported these findings by illustrating a more homogeneous Zn²⁺ concentration and electric field distribution near the Zn-CarraChi interface compared to the Zn-GF interface. 3. **Improved Cycling Stability:** Symmetric Zn|CarraChi|Zn cells exhibited significantly enhanced cycling stability compared to Zn|GF|Zn cells, demonstrating lifetimes exceeding 2500 h at 1 mA cm⁻² and 180 h at 10 mA cm⁻². The high Coulombic efficiency (CE > 99.8% after 300 cycles at 5 mA cm⁻²) further confirms the improved reversibility of Zn plating/stripping in the presence of the CarraChi gel electrolyte. 4. **Ah-Scale Pouch Cell Performance:** The refillable pouch cells demonstrated a remarkable lifespan of >4000 h at 10 mA cm⁻² with an areal capacity of 35 mAh cm⁻², achieving a cumulative capacity of 1286 Ah. Replenishing with pure water or 2 M ZnSO4 maintained ionic conductivity and cell function. EIS confirmed the absence of short circuits. This performance significantly surpasses existing state-of-the-art results for Zn||Zn cells. 5. **High-Capacity Full Cells:** Zn||ZnxV2O5·nH2O coin cells with CarraChi showed high specific capacities (349.6 mAh g⁻¹ at 0.2 A g⁻¹ and 200 mAh g⁻¹ at 4 A g⁻¹) and excellent rate capability compared to cells with GF. Pouch cells exhibited an initial capacity of 0.9 Ah and retained 84% capacity after 200 cycles at 200 mA g⁻¹, outperforming previously reported aqueous Zn metal pouch cells with V-based cathodes. EIS demonstrated faster charge transfer kinetics in cells with the CarraChi electrolyte. These results underscore the effectiveness of the CarraChi gel electrolyte and the open-cell design in overcoming the limitations of conventional aqueous zinc batteries, paving the way for the development of practical large-scale energy storage systems.

The findings demonstrate that the combination of an innovative water-bonding gel electrolyte and a novel open, refillable cell design effectively addresses the major challenges hindering the large-scale application of aqueous zinc-ion batteries. The CarraChi gel electrolyte successfully inhibits dendrite growth, suppresses the HER, and prevents electrolyte depletion. The open design allows for the controlled release of hydrogen gas and replenishment of the electrolyte, preventing pressure buildup and extending the cell's lifespan. The superior performance of the Ah-scale pouch cells confirms the practical applicability of this technology for grid-scale energy storage. The results are significant because they demonstrate a viable path towards producing safe, high-capacity, and long-lasting aqueous zinc batteries, which are crucial for the transition to sustainable and reliable energy storage solutions. The successful implementation of this technology contributes significantly to the field of aqueous zinc batteries by providing a robust solution to the challenges associated with large-format cells. The use of biomass-derived materials in the electrolyte also adds to its sustainability. The observed performance improvements pave the way for further research into improving the energy density, cost-effectiveness, and scalability of aqueous zinc batteries for wide-spread grid-scale applications.

This research presents a significant advancement in aqueous zinc battery technology. The novel water-bonding CarraChi gel electrolyte, combined with an open, refillable pouch cell design, addresses key limitations hindering the practical application of these batteries for large-scale energy storage. The results demonstrate exceptional improvements in cycling stability, capacity retention, and lifespan, culminating in the successful fabrication of Ah-scale cells. Future research could focus on further optimizing the electrolyte composition to enhance energy density and exploring different cathode materials to improve overall cell performance and expand applications.

While the study demonstrates significant progress in addressing the challenges of large-format aqueous zinc batteries, several limitations warrant consideration. The long-term stability of the CarraChi gel electrolyte under various operating conditions requires further investigation. The impact of temperature fluctuations and environmental factors on cell performance needs to be more extensively studied. Although the refillable system addresses electrolyte depletion, the scalability of the replenishment mechanism and the practicality of its implementation in large-scale applications remain to be fully explored. Further research is also needed to optimize the cost and manufacturing processes for large-scale production.