Electrochemical impedance spectroscopy (EIS) is a powerful diagnostic tool for understanding the behavior of aqueous batteries, such as zinc-ion and lead-acid systems. Unlike non-aqueous batteries, aqueous electrolytes introduce unique challenges due to water decomposition, pH sensitivity, and corrosion processes. These factors significantly influence impedance signatures, requiring careful interpretation to diagnose performance limitations and degradation mechanisms.
Aqueous batteries rely on water-based electrolytes, which offer advantages like low cost, high ionic conductivity, and environmental friendliness. However, water’s electrochemical stability window is narrow, typically around 1.23 V under standard conditions. Beyond this range, water decomposes into hydrogen or oxygen, leading to gas evolution, electrolyte consumption, and pressure buildup. EIS helps characterize these processes by identifying reaction kinetics and interfacial phenomena. The impedance spectrum of an aqueous battery often includes contributions from charge transfer, diffusion, and side reactions like hydrolysis.
In zinc-ion batteries, the impedance response is heavily influenced by zinc deposition and dissolution. During cycling, zinc tends to form dendrites or undergo shape changes, increasing interfacial resistance. EIS can detect these changes through the evolution of the high-frequency semicircle in Nyquist plots, which corresponds to charge transfer resistance. Additionally, zinc corrosion in aqueous electrolytes generates hydrogen, further complicating the impedance response. The low-frequency region often reflects diffusion limitations caused by passivation layers, such as zinc oxide or hydroxide. These layers grow over time, increasing impedance and reducing battery efficiency.
Lead-acid batteries exhibit distinct EIS features due to their chemistry. The positive electrode (lead dioxide) and negative electrode (spongy lead) undergo dissolution-precipitation reactions during cycling. The impedance spectrum typically shows two semicircles: one at high frequencies representing the porous electrode structure and another at mid-frequencies corresponding to charge transfer. The low-frequency region reflects diffusion processes within the electrolyte. A key challenge in lead-acid systems is sulfation, where lead sulfate crystals form and reduce active material availability. EIS can detect sulfation through an increase in charge transfer resistance and a decrease in capacitive behavior.
Corrosion is a major concern in aqueous batteries, particularly for current collectors and electrode materials. In zinc-ion systems, the corrosion of zinc metal produces hydrogen and increases ohmic losses. EIS identifies corrosion through a combination of resistive and capacitive elements in the impedance spectrum. The formation of passivation layers, such as zinc oxide, further complicates the response by adding another time constant to the spectrum. In lead-acid batteries, grid corrosion at the positive electrode contributes to long-term degradation. This process appears as an increase in low-frequency impedance due to the accumulation of resistive corrosion products.
The pH of the electrolyte plays a critical role in aqueous battery performance. For zinc-ion batteries, alkaline electrolytes reduce hydrogen evolution but accelerate zinc corrosion and passivation. Acidic electrolytes, on the other hand, promote hydrogen evolution but may improve zinc deposition uniformity. EIS helps monitor pH-related changes by tracking shifts in charge transfer resistance and double-layer capacitance. In lead-acid batteries, sulfuric acid concentration affects ion mobility and reaction kinetics. Variations in pH alter the impedance spectrum, particularly in the mid-frequency range where charge transfer processes dominate.
Passivation layers are another key factor in aqueous battery impedance. These layers form due to side reactions or incomplete dissolution during cycling. In zinc-ion batteries, zinc hydroxide or oxide layers increase interfacial resistance and reduce ion transport. EIS reveals these layers through an additional semicircle or a distorted low-frequency tail. In lead-acid batteries, lead sulfate passivation inhibits charge transfer and reduces capacity. The impedance response shows increased resistance and depressed capacitive arcs as sulfation progresses.
Interpreting EIS data for aqueous batteries requires careful consideration of equivalent circuit models. A typical model includes resistors for ohmic losses, constant phase elements for imperfect capacitors, and Warburg elements for diffusion. For zinc-ion batteries, the circuit may include a parallel combination of charge transfer resistance and double-layer capacitance, followed by a Warburg element. Lead-acid batteries often require more complex models to account for porous electrodes and sulfation effects. Fitting experimental data to these models helps quantify degradation mechanisms and optimize battery design.
Temperature also affects EIS measurements in aqueous batteries. Higher temperatures accelerate reaction kinetics but may exacerbate corrosion or water decomposition. Lower temperatures increase electrolyte viscosity, raising ohmic resistance and slowing charge transfer. EIS spectra collected at different temperatures reveal activation energies for various processes, aiding in thermal management strategies.
Practical challenges in EIS analysis include electrode polarization and non-stationary behavior. Aqueous batteries often exhibit drifting potentials during measurements, especially under high currents or prolonged testing. Proper instrumentation and experimental design are essential to minimize artifacts and ensure reproducible results. Additionally, the choice of frequency range impacts the resolution of different processes. High frequencies capture electrolyte resistance and interfacial phenomena, while low frequencies probe diffusion and bulk effects.
Despite these challenges, EIS remains indispensable for aqueous battery research. It provides insights into charge transfer kinetics, mass transport limitations, and degradation pathways. By correlating impedance signatures with performance metrics, researchers can identify failure modes and develop mitigation strategies. For example, additives that suppress corrosion or modify passivation layers can be evaluated using EIS before long-term cycling tests.
In summary, EIS is a versatile tool for diagnosing aqueous battery behavior, particularly in zinc-ion and lead-acid systems. Water decomposition, pH effects, corrosion, and passivation layers create unique impedance signatures that differ from non-aqueous batteries. Understanding these features enables better battery design, improved durability, and enhanced performance. Future work may focus on advanced modeling techniques or in-situ EIS measurements to further unravel the complexities of aqueous battery electrochemistry.