The development of soft, wearable, and implantable electronic devices for continuous health monitoring presents a significant challenge in power sourcing. Conventional batteries are too bulky and rigid for skin interfacing, while flexible energy harvesting systems offer low and inconsistent power output. While flexible batteries using lithium-ion or alkaline chemistries are being explored, concerns about long-term operation, toxicity, and electrolyte leakage persist. Aqueous rechargeable batteries, particularly aqueous zinc metal batteries (AZMBs), offer a safer and more cost-effective alternative due to the use of non-toxic and abundant zinc. However, AZMBs are plagued by issues such as zinc dendrite growth, parasitic side reactions (like hydrogen evolution), and short lifespan, limiting their practical applications. Various strategies have been explored to improve Zn reversibility, but these often involve toxic chemicals. Hydrogel electrolytes offer improved flexibility and potentially suppress parasitic reactions, but existing hydrogel electrolytes suffer from low ionic conductivity and poor control over Zn deposition. This research aims to overcome these limitations by developing a biocompatible hydrogel electrolyte using hyaluronic acid (HA) to enhance the performance and safety of AZMBs for bio-interfaced applications.

Existing literature highlights the limitations of conventional batteries for wearable applications, emphasizing the need for biocompatible and flexible alternatives. Several studies have explored aqueous zinc-ion batteries as a promising solution due to their inherent safety and cost-effectiveness. However, the challenges associated with zinc dendrite formation, parasitic side reactions (especially hydrogen evolution), and poor cycle life have been consistently reported. Researchers have attempted to address these challenges through various strategies, including the use of hybrid electrolytes, functional additives, and high-concentration electrolytes. While some success has been achieved, concerns regarding the biocompatibility and safety of these solutions remain. The use of hydrogel electrolytes has emerged as a potential solution to the issues of flexibility and leakage, but existing hydrogel electrolytes are often limited by low ionic conductivity and insufficient control over zinc deposition.

The researchers prepared a 2 M ZnSO₄ liquid electrolyte and a hyaluronic acid (HA) gel electrolyte with varying HA concentrations (6.0, 15.0, and 30.0 wt%). The gelation process was attributed to strong hydrogen bonding between HA and water. Physicochemical properties of the electrolytes, including ionic conductivity, mechanical strength, and FTIR spectroscopy, were characterized. Electrochemical properties were assessed using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), chronoamperometry (CA), and Tafel tests. The corrosion behavior of zinc anodes was investigated through pH measurements, XRD, SEM, and electrochemical studies. The nucleation and growth behavior of zinc was studied using CV, CA, SEM, and confocal laser microscopy. Molecular dynamics (MD) simulations and density functional theory (DFT) calculations were used to understand the interactions between Zn²⁺ ions, HA, and water molecules. Biocompatibility was evaluated using a WST-1 cell proliferation assay with primary epidermal keratinocytes (HEKn) and RAW 264.7 macrophage cells. Electrochemical performance was tested using Zn//Zn symmetric cells and Zn//LiMn₂O₄ (LMO) and Zn//I₂ full cells, including pouch cells and flexible cells. In-situ synchrotron-based FTIR was employed to monitor chemical changes at the electrode/electrolyte interface during cycling. The Zn²⁺ transference number was determined using the Bruce-Vincent method, and activation energy for desolvation was obtained from Arrhenius plots.

The HA gel electrolyte exhibited a slightly lower ionic conductivity (47.7 mS cm⁻¹) compared to the liquid electrolyte (57.0 mS cm⁻¹), but it still outperformed other reported gel electrolytes. The HA gel demonstrated excellent mechanical properties, including a moderate elongation-at-break of 220% and a compressive strength of 0.18 MPa. FTIR and Raman spectroscopy, along with MD simulations and DFT calculations, confirmed the formation of strong hydrogen bonds between HA and water molecules, leading to a reduction in water reactivity. The HA gel electrolyte effectively suppressed the hydrogen evolution reaction (HER) and improved the electrochemical stability window. The HA gel electrolyte significantly improved the anti-corrosion ability of the zinc anode, with a corrosion current of 0.297 mA cm⁻² compared to 1.632 mA cm⁻² for the liquid electrolyte. Analysis showed significantly reduced zinc corrosion rates in the HA gel electrolyte. The HA gel electrolyte regulated Zn nucleation and growth, leading to a more uniform and compact Zn deposition layer compared to the liquid electrolyte. The higher nucleation overpotential in the HA gel electrolyte (50 mV higher than liquid) suggested the formation of smaller Zn nuclei, leading to fine-grained and uniform Zn particles. The Zn//Zn symmetric cells with the HA gel electrolyte demonstrated exceptional long-term cycling stability, exceeding 5500 h at 1 mA cm⁻² with 1 mAh cm⁻² capacity. The average Coulombic efficiency was 99.71% after 2000 cycles. The Zn//LiMn₂O₄ full cells using the HA gel electrolyte exhibited superior cycle life and capacity retention compared to those with liquid electrolyte, with an 82% capacity retention after 1000 cycles at 3 C. In-situ synchrotron-based FTIR revealed that the HA gel electrolyte facilitated rapid removal of water and SO₄²⁻ from the Zn²⁺ solvation shell during Zn deposition. The HA gel electrolyte showed excellent biocompatibility with minimal cytotoxicity towards HEKn and RAW 264.7 cells. Flexible Zn//LiMn₂O₄ pouch cells with the HA gel electrolyte maintained excellent performance under various mechanical deformations.

The findings demonstrate the effectiveness of the hyaluronic acid-based hydrogel electrolyte in addressing the key challenges associated with zinc anodes in aqueous zinc-ion batteries. The superior performance is attributed to the unique properties of HA, including its ability to reduce water reactivity, regulate zinc ion flux, and shield the zinc surface from unwanted reactions. The combination of experimental characterization, theoretical calculations, and in-situ measurements provides a comprehensive understanding of the mechanism by which the HA electrolyte enhances zinc anode reversibility. The excellent biocompatibility of the HA electrolyte makes it a promising candidate for applications in wearable and implantable energy storage devices. The results of the full cell tests, particularly the pouch cell and flexible cell tests, showcase the potential for practical applications in the field of biocompatible energy storage.

This study successfully demonstrates a biocompatible HA-based hydrogel electrolyte for high-performance aqueous zinc-ion batteries. The electrolyte's unique properties effectively mitigate zinc corrosion, regulate zinc deposition, and significantly enhance battery performance. The results highlight the potential for practical applications in biocompatible energy storage, particularly in wearable and implantable devices. Future research could focus on further optimizing the electrolyte formulation, exploring other biocompatible polymers, and investigating the long-term in vivo performance of these batteries.

While the study demonstrates excellent performance in vitro, further investigation is needed to assess the long-term in vivo biocompatibility and performance of the HA gel electrolyte. The relatively lower ionic conductivity compared to liquid electrolytes might limit the rate capability at extremely high current densities. The scalability and manufacturing cost of the HA gel electrolyte also need to be considered for wider applications.