Aqueous batteries have gained attention as a promising alternative to traditional non-aqueous lithium-ion systems due to their inherent safety, lower cost, and environmental friendliness. However, their performance is often limited by parasitic reactions such as hydrogen evolution at the anode, which degrades efficiency and cycle life. To mitigate these challenges, pH modifiers play a critical role in stabilizing the electrolyte environment. Common additives like lithium hydroxide (LiOH) and phosphoric acid (H3PO4) are employed to adjust the pH, influencing reaction kinetics and interfacial stability.

The hydrogen evolution reaction (HER) is a major obstacle in aqueous batteries, particularly in systems using low-potential anodes like zinc or lithium metal. In neutral or acidic electrolytes, the thermodynamic driving force for HER increases, leading to rapid corrosion and capacity loss. By introducing alkaline pH modifiers such as LiOH, the electrolyte shifts to a higher pH, raising the overpotential required for HER and suppressing water decomposition. For instance, in zinc-based batteries, increasing pH to 12–14 with LiOH reduces HER rates significantly, extending cycle life. However, excessive alkalinity can trigger other side reactions, such as zincate formation or precipitation of metal hydroxides, which impair electrode kinetics.

Conversely, acidic modifiers like H3PO4 are used in certain aqueous systems to enhance proton activity and improve ionic conductivity. Phosphoric acid also serves a dual role by passivating electrode surfaces through phosphate layer formation, further inhibiting corrosion. Yet, highly acidic conditions accelerate HER and may dissolve active materials, necessitating careful optimization. The choice of pH modifier thus involves balancing suppression of undesirable reactions with maintaining electrochemical stability.

A critical distinction exists between pH-modified aqueous electrolytes and water-in-salt (WiS) systems. WiS electrolytes employ ultrahigh salt concentrations to expand the electrochemical stability window by restructuring the solvation sheath and reducing free water molecules. While pH modifiers adjust proton activity, WiS systems primarily rely on limiting water’s reactivity through solvation effects. Combining pH control with WiS strategies has shown synergistic effects, further widening stability windows. For example, adding LiOH to a WiS electrolyte can reinforce the suppression of HER while maintaining high ionic conductivity.

Non-aqueous electrolytes, typically using organic carbonates or ethers, operate outside aqueous constraints, enabling wider voltage windows and compatibility with high-energy electrodes. However, they suffer from flammability, toxicity, and higher costs. pH modifiers are irrelevant in these systems since proton activity is negligible. Instead, non-aqueous systems rely on SEI-forming additives to stabilize interfaces.

The stability trade-offs of pH modifiers must be carefully evaluated. Alkaline conditions, while effective against HER, may induce cathode dissolution or promote oxygen evolution at high potentials. Acidic environments risk corroding current collectors or forming resistive byproducts. Buffer systems, such as borate or phosphate salts, are sometimes introduced to stabilize pH dynamically during cycling. For instance, a borate buffer in zinc electrolytes maintains near-neutral pH, mitigating both HER and cathode degradation.

Material compatibility is another consideration. Aluminum current collectors, common in non-aqueous systems, corrode in alkaline media, necessitating alternative materials like stainless steel or carbon-coated substrates. Electrode materials must also resist dissolution or phase transitions at extreme pH levels. For example, manganese-based cathodes exhibit better stability in neutral or mildly acidic electrolytes, while nickel-rich cathodes tolerate higher alkalinity.

Recent research has explored hybrid approaches where pH modifiers are combined with other additives, such as polymers or inorganic coatings, to enhance interfacial stability. Gel electrolytes incorporating pH buffers have demonstrated improved cycle life by maintaining a stable local environment around electrodes. Additionally, advanced characterization techniques like in-situ pH mapping help elucidate dynamic changes during operation, guiding better electrolyte design.

In summary, pH modifiers are a vital tool for optimizing aqueous battery performance by controlling HER and interfacial reactions. Their effectiveness depends on the specific chemistry, electrode materials, and operational conditions. While alkaline additives like LiOH are widely used for HER suppression, acidic modifiers like H3PO4 offer alternative mechanisms for stability. The trade-offs between reaction suppression and material compatibility underscore the need for tailored electrolyte formulations. Differentiating these strategies from WiS or non-aqueous systems highlights the unique challenges and opportunities in aqueous battery development. Future advancements may focus on multifunctional additives and real-time pH regulation to further improve energy density and longevity.