Aqueous batteries have gained significant attention due to their inherent safety, cost-effectiveness, and environmental friendliness compared to non-aqueous systems. Among the key components, anode materials play a critical role in determining the performance, stability, and longevity of these batteries. Zinc (Zn) and molybdenum disulfide (MoS₂) are two prominent anode materials for aqueous batteries, each offering distinct advantages and challenges. This article explores the behavior of these materials in mild and acidic electrolytes, the impact of pH on their stability, and strategies to overcome performance limitations.

Zinc is one of the most widely studied anode materials for aqueous batteries due to its high theoretical capacity (820 mAh/g), low redox potential (−0.76 V vs. SHE), and natural abundance. In mild neutral or slightly acidic electrolytes, zinc anodes exhibit reversible plating and stripping, making them suitable for rechargeable systems. However, zinc anodes face several challenges, including dendrite formation, hydrogen evolution, and corrosion. Dendrite growth during cycling can lead to internal short circuits, while hydrogen evolution reduces Coulombic efficiency and damages the electrode structure. Corrosion, accelerated in acidic environments, further degrades the anode over time.

The pH of the electrolyte significantly influences zinc anode behavior. In strongly acidic conditions, zinc corrosion becomes severe due to increased hydrogen evolution. Neutral or mildly acidic electrolytes (pH 4–6) are preferred to mitigate these issues, but even in these ranges, parasitic reactions persist. Additives such as inorganic salts (e.g., ZnSO₄) or organic inhibitors (e.g., polyethylene glycol) have been employed to suppress dendrite growth and hydrogen evolution. Additionally, electrode modifications, including the use of porous conductive scaffolds or protective coatings, enhance zinc deposition uniformity and cycling stability.

Molybdenum disulfide (MoS₂) has emerged as an alternative anode material, particularly for aqueous batteries requiring higher energy density and improved rate capability. MoS₂ operates via intercalation or conversion mechanisms, depending on the electrolyte pH and battery configuration. In mild electrolytes, MoS₂ demonstrates intercalation behavior, where ions insert between its layered structure without disrupting the host framework. This mechanism provides good reversibility and moderate capacity (∼100–200 mAh/g). In acidic electrolytes, MoS₂ undergoes a conversion reaction, yielding higher capacities (∼400–600 mAh/g) but with greater structural degradation.

The stability of MoS₂ anodes is highly pH-dependent. In neutral or alkaline conditions, the material maintains structural integrity over multiple cycles, but its capacity is limited. Acidic electrolytes enhance capacity by facilitating proton intercalation or conversion reactions, yet they also accelerate material dissolution and sulfur loss. Strategies to mitigate these issues include nanostructuring MoS₂ to shorten ion diffusion paths, doping with heteroatoms to improve electronic conductivity, and compositing with carbon materials to buffer volume changes.

Comparing Zn and MoS₂ anodes, zinc offers higher capacity and simpler processing but suffers from dendrite and corrosion issues. MoS₂ provides better structural stability in mild conditions but requires optimization to achieve competitive capacity in acidic systems. The choice between these materials depends on the specific battery application, electrolyte composition, and performance requirements.

Recent research has explored hybrid systems combining zinc and MoS₂ to leverage their complementary strengths. For example, zinc anodes paired with MoS₂ cathodes in mild electrolytes demonstrate improved cycling stability and energy density. Alternatively, MoS₂-coated zinc anodes exhibit suppressed dendrite growth and enhanced interfacial kinetics. These hybrid approaches highlight the potential for tailored anode designs to address stability and performance challenges in aqueous batteries.

The development of advanced characterization techniques has provided deeper insights into anode degradation mechanisms. In-situ microscopy and spectroscopy reveal real-time structural changes during cycling, while electrochemical impedance spectroscopy quantifies interfacial resistance. These tools enable precise optimization of electrode materials and electrolytes for improved performance.

Despite progress, several challenges remain for aqueous battery anodes. For zinc, achieving long-term cycling stability without dendrites or corrosion requires further electrolyte engineering and interfacial control. For MoS₂, enhancing capacity retention in acidic electrolytes demands innovative material designs and protective strategies. Future research may focus on novel anode compositions, such as alloyed zinc or defect-engineered MoS₂, to overcome current limitations.

In summary, anode materials for aqueous batteries must balance high capacity, stability, and compatibility with mild or acidic electrolytes. Zinc and MoS₂ represent promising candidates, each with distinct advantages and challenges. Advances in material design, electrolyte formulation, and interfacial engineering are critical to unlocking their full potential for next-generation energy storage systems. The continued exploration of these materials will drive the development of safer, more efficient, and sustainable aqueous batteries.