As the global demand for green, safe, and sustainable energy storage intensifies, aqueous magnesium–air batteries have emerged as highly promising candidates, leveraging the intrinsic merits of metallic magnesium: low cost, large volumetric capacity (3,832 mAh cm-3), a highly negative electrode potential (−2.37 V vs. SHE), and excellent operational safety. However, the practical deployment of Mg–air systems has long been hindered by severe self-discharge, poor anode utilization efficiency, and low cell voltage. Now, a comprehensive review led by Dr. Yulong Wu, Dr. Darya Snihirova, and Dr. Xiaohui Zeng at Helmholtz-Zentrum Hereon, in collaboration with Hebei University of Technology and Kiel University, provides a systematic roadmap for designing next-generation electrolyte additives that can finally unlock the full potential of aqueous Mg batteries.
Why Electrolyte Additives Matter
Traditional approaches to improve Mg–air battery performance have focused on either anode alloying or empirical electrolyte formulation. While anode microalloying has yielded notable advances—such as the "stainless magnesium" Mg-0.15Ca alloy—designing highly efficient electrolyte additives that are compatible with established clean anodes remains a formidable challenge. Individual additives typically face an inherent trade-off: complexing agents and pH buffers boost discharge potential (DP) by keeping the anode surface clean, but at the cost of accelerated hydrogen evolution and oxygen reduction reactions that degrade utilization efficiency (UE). Conversely, corrosion inhibitors and surfactants improve UE by forming protective films, yet these same films increase interfacial resistance and suppress cell voltage. This review systematically dissects these competing mechanisms and proposes a clear design concept for binary additive mixtures that enables rational, synergistic combinations rather than trial-and-error.
Innovative Design Strategies
The review categorizes electrolyte additives into four functional classes—corrosion inhibitors, Mg2+/Fe2+/3+ complexing agents, pH buffers, and surfactants—and elucidates their distinct roles in regulating the Mg–electrolyte interface. Critically, it highlights an often-overlooked contributor to anode self-discharge: the oxygen reduction reaction (ORR). While ORR has traditionally been considered negligible in NaCl electrolytes due to the blocking effect of thick Mg(OH)2 discharge films, the review demonstrates that additives promoting film dissolution can inadvertently expose the anode to dissolved oxygen, triggering substantial ORR-driven efficiency losses and metallic Mg chunk detachment.
To overcome these limitations, the authors propose two strategic frameworks for binary additive design:
(I) Mechanism-Complementary Mixtures: Combining a Mg2+ complexing agent (e.g., 5-sulfosalicylate or 3,4-dihydroxybenzoic acid) with a pH buffer or film-forming additive (e.g., CAPSO or S3S). The complexing agent maintains a clean anode surface for high DP, while the co-additive suppresses excessive HER and ORR, mitigates chunk formation, and stabilizes the interfacial pH. This approach has demonstrated simultaneous enhancement of DP, UE, specific energy, and power density in Mg-0.2Ca anodes.
(II) Oxygen-Scavenging Mixtures: Incorporating a reducing agent such as L-ascorbic acid (Vitamin C) alongside Mg2+ complexing agents. The oxygen scavenger actively removes dissolved O₂ from the electrolyte, suppressing ORR-induced self-discharge while the complexing agent preserves high cell voltage. Deaeration control experiments confirm that cleaner anode surfaces intensify ORR—validating the necessity of oxygen scavengers in additive formulations.
Outstanding Performance
The binary additive strategy has delivered remarkable results. A mixture of 0.1 M 5-sulfosalicylate + 0.1 M CAPSO achieved a full-cell voltage of 1.84 V at 1 mA cm-2 and a specific energy of 2.58 kWh kg-1 at 10 mA cm-2, with UE exceeding 65%. Even more impressively, machine learning-guided discovery identified 2,3-dihydroxynaphthalene as a powerful additive that boosted the specific energy of Mg-0.2Ca anode to 3.37 kWh kg-1 and specific capacity to 1,927 mAh g-1—approaching the theoretical maximum of 2,205 mAh g-1. Comparative analysis reveals that glutamate stands out among individual additives, simultaneously delivering high specific energy (~2.1 kWh kg-1), low toxicity (LD50 > 15,000 mg kg-1), and minimal cost—making it one of the most promising bio-compatible candidates for sustainable Mg–air systems.
AI-Driven Future Outlook
The review charts a transformative path forward by integrating high-throughput robotic experimentation with machine learning (ML) workflows. An active learning-based adaptive experimental design—employing SOAP molecular encoding and KPCovR multi-objective optimization—successfully identified high-performance additives after only three iterative loops, dramatically accelerating discovery beyond conventional Edisonian approaches. The authors envision fully automated, autonomous additive discovery platforms where robotic testing generates large-scale datasets, ML models predict optimal formulations, and quantum chemical simulations guide initial screening conditions. This convergence of artificial intelligence, robotics, and electrochemistry promises to unlock the nearly infinite compositional space of binary and multicomponent electrolyte mixtures.
Beyond conventional applications, the review highlights the rapidly growing frontier of fully biodegradable implantable Mg batteries. These aqueous primary cells leverage body fluids as electrolytes and naturally resorb after service, eliminating the need for surgical removal. However, premature degradation of non-critical battery components and excessive hydrogen evolution pose unique challenges for bio-related applications—underscoring the urgent need for precisely engineered electrolyte formulations that balance performance with biocompatibility.
This work establishes a comprehensive knowledge foundation for aqueous Mg–air battery electrolyte design, providing structured datasets to support future ML screening while offering clear, mechanistically grounded strategies for developing high-efficiency hybrid additives. By systematically coupling interfacial reaction understanding with rational mixture design, this review paves the way for next-generation aqueous Mg batteries combining high energy density, long service life, and environmental sustainability.
Stay tuned for more groundbreaking research from this collaborative team at Helmholtz-Zentrum Hereon, Hebei University of Technology, and Kiel University!
