Magnesium metal anodes present a compelling alternative to lithium in rechargeable batteries due to magnesium’s high theoretical volumetric capacity (3833 mAh/cm³ compared to lithium’s 2046 mAh/cm³), natural abundance, and improved safety characteristics. However, the practical implementation of magnesium anodes faces several electrochemical and materials science challenges that must be addressed to enable viable magnesium-based battery systems.

A primary obstacle to efficient magnesium anode operation is the formation of passivation layers on the metal surface. When exposed to conventional electrolytes, magnesium spontaneously reacts to form surface films composed of magnesium oxides, hydroxides, and carbonates. These films are typically ionically insulating, preventing reversible magnesium deposition and stripping. Unlike lithium, where the solid electrolyte interphase (SEI) can be engineered to enable ion transport, magnesium surface layers often exhibit irreversible passivation behavior. The passivation phenomenon leads to increased interfacial resistance and significant overpotentials during cycling, sometimes exceeding 1 V in poorly compatible systems. X-ray photoelectron spectroscopy studies have identified that these layers can reach thicknesses of several nanometers after just brief air or electrolyte exposure.

Dendrite formation represents another critical challenge for magnesium anodes, though the growth morphology differs substantially from lithium systems. Magnesium dendrites tend to form as irregular, mossy deposits rather than the needle-like structures observed in lithium. This difference stems from magnesium’s higher melting point (650°C vs 180°C for lithium) and different surface diffusion characteristics. While magnesium dendrites are less prone to piercing separators, they still contribute to capacity fade through electrical disconnection from the current collector and increased surface area for side reactions. Electron microscopy has shown that magnesium deposition occurs preferentially at surface defects, with nucleation barriers approximately 3-5 times higher than lithium under comparable conditions.

The Coulombic efficiency of magnesium anodes remains lower than required for commercial applications, typically ranging from 85-98% in most reported systems. Efficiency losses arise from multiple mechanisms including chemical corrosion, incomplete stripping of deposited magnesium, and the formation of electrochemically inactive magnesium compounds. The efficiency shows strong dependence on current density, with values dropping sharply below 0.5 mA/cm² in many systems due to enhanced side reaction kinetics relative to deposition rates. At higher current densities above 5 mA/cm², efficiency improvements are offset by increased overpotentials and uneven deposition morphology.

Surface modification of magnesium anodes has emerged as a promising strategy to address these challenges. Atomic layer deposition of thin aluminum or titanium oxide coatings (2-10 nm thickness) has demonstrated improved interfacial stability, reducing passivation layer growth by up to 70% in some studies. These coatings must balance thickness to provide protection while maintaining sufficient ionic conductivity. Alternative approaches include creating artificial SEI layers through pretreatment in reactive gases or solutions, with sulfur-containing treatments showing particular promise for enhancing magnesium ion transport.

Another surface engineering approach involves nanostructuring the magnesium anode to create more uniform deposition sites. Porous magnesium structures fabricated by vapor deposition or electrochemical methods exhibit reduced nucleation overpotentials by approximately 200-300 mV compared to planar foils. The increased surface area must be carefully optimized to avoid excessive electrolyte decomposition. Three-dimensional current collectors with magnesium coatings have demonstrated cycling stability improvements of up to 50% in prototype cells.

Current collector compatibility presents unique challenges for magnesium systems. Traditional copper foils used in lithium batteries form blocking interfaces with magnesium due to alloying reactions at operating potentials. Stainless steel exhibits better stability but still shows gradual performance degradation. Titanium and molybdenum current collectors demonstrate superior interfacial stability, with titanium particularly notable for maintaining adhesion over extended cycling. However, these materials increase cost and may require surface treatments to ensure proper magnesium wetting during deposition.

The mechanical properties of magnesium introduce additional considerations for anode design. Magnesium’s higher modulus (45 GPa vs 4.9 GPa for lithium) makes it less tolerant of volume changes during cycling, potentially leading to delamination from current collectors. This has prompted development of magnesium composites incorporating flexible carbon matrices or porous metal scaffolds. Such composite anodes have shown capacity retention improvements of 20-40% over pure magnesium foils in long-term cycling tests.

Interfacial engineering between the magnesium anode and separator also requires attention. The high reactivity of magnesium necessitates chemically stable separator materials that can withstand localized pH changes and potential mechanical penetration from deposits. Ceramic-coated separators and reinforced polymer membranes have shown promise in preliminary testing, with some configurations demonstrating stable operation for over 500 cycles at practical current densities.

Recent advances in operando characterization techniques have provided new insights into magnesium anode behavior. X-ray diffraction and Raman spectroscopy during cycling have revealed the dynamic nature of the magnesium-electrolyte interface, including the formation of transient intermediate species during deposition. These studies confirm that the first few cycles critically determine long-term anode performance through interface conditioning effects.

While significant challenges remain, continued progress in understanding and controlling magnesium anode interfaces suggests potential for practical implementation. The combination of surface modification, nanostructuring, and compatible current collector design appears essential for achieving the cycle life and efficiency required for commercial applications. Future developments must address the interrelated nature of these challenges while maintaining the inherent safety and cost advantages of magnesium-based systems.