Rechargeable magnesium batteries have long been considered a promising alternative to lithium-ion systems due to magnesium's high theoretical volumetric capacity (3833 mAh/cm³ compared to lithium's 2046 mAh/cm³), abundance in the Earth's crust, and improved safety characteristics. However, despite research efforts dating back to the 1960s, fundamental electrochemical challenges have prevented their commercialization. The primary obstacles include passivation layer formation on the magnesium anode and the lack of compatible electrolyte solutions, both of which severely limit cycle life and power capability.

The first major challenge is the formation of a passivation layer on the magnesium anode surface. When magnesium metal comes into contact with most conventional electrolyte solutions, it reacts spontaneously to form a surface film composed of magnesium oxides, hydroxides, and other decomposition products. This layer is ionically insulating, preventing the reversible deposition and dissolution of magnesium ions during cycling. Unlike lithium, where the solid electrolyte interphase (SEI) can be lithium-ion conductive, the native passivation layer on magnesium blocks Mg²⁺ transport entirely. This phenomenon leads to high overpotentials during charging and ultimately renders the battery inoperable. The passivation problem persists across a wide range of solvent systems, including those based on carbonates, ethers, and other organic solvents commonly used in lithium-ion batteries.

Electrolyte compatibility represents the second critical challenge. Magnesium's divalent nature (Mg²⁺) imposes stringent requirements on electrolyte design. The strong electrostatic interaction between the doubly charged Mg²⁺ ion and solvent molecules or anions leads to high desolvation energies at the electrode-electrolyte interface. This results in poor kinetics for magnesium deposition and stripping, manifesting as low Coulombic efficiency and high polarization. Additionally, most conventional salt anions (such as PF₆⁻ or BF₄⁻) are either unstable against magnesium metal or lead to passivation. Early electrolyte systems based on Grignard reagents (RMgX) showed some reversibility but suffered from narrow electrochemical stability windows (typically below 2.5 V vs Mg/Mg²⁺), poor oxidative stability, and high corrosivity toward battery components.

The search for non-passivating electrolytes has been a central focus of magnesium battery research. Three main categories of electrolytes have shown varying degrees of success: organohaloaluminates, boron-based systems, and chloride-containing complexes. Organohaloaluminates, such as those derived from AlCl₃ and R₂Mg compounds, demonstrated reversible magnesium deposition with Coulombic efficiencies exceeding 95% in some cases. However, these electrolytes are highly sensitive to moisture and oxygen, making practical implementation difficult. Boron-based electrolytes, particularly those using fluorinated arylborates, offered improved stability but still faced limitations in voltage window and compatibility with high-voltage cathodes. Chloride-containing systems, including MgCl₂-based electrolytes, have shown promise but require careful balancing of chloride concentration to avoid corrosion of current collectors while maintaining sufficient ionic conductivity.

Cathode materials present another significant challenge. The high charge density of Mg²⁺ ions leads to slow solid-state diffusion in most host materials, resulting in poor rate capability. Many intercalation compounds that work well with lithium, such as transition metal oxides, exhibit negligible capacity with magnesium due to strong electrostatic interactions that trap the Mg²⁺ ions. Chevrel phase materials (Mo₆S₈) have shown some capability for reversible magnesium intercalation, but their low operating voltage (around 1.1 V vs Mg/Mg²⁺) limits energy density. Sulfur cathodes, while theoretically attractive due to their high capacity, face rapid capacity fading caused by polysulfide dissolution and poor redox kinetics with magnesium.

The combined effect of these challenges has resulted in magnesium batteries with substantially lower performance metrics compared to lithium-ion systems. Typical magnesium battery prototypes demonstrate cycle lives limited to a few hundred cycles at most, compared to thousands for commercial lithium-ion cells. Energy densities remain below 150 Wh/kg in most reported systems, significantly lower than the 250-300 Wh/kg of contemporary lithium-ion batteries. Power capability is particularly constrained, with most magnesium batteries showing poor performance at discharge rates above C/2 due to the kinetic limitations at both electrodes.

Materials compatibility issues extend beyond the active components. The high reactivity of many magnesium electrolytes with conventional battery materials necessitates specialized cell components. Aluminum current collectors, standard in lithium-ion batteries, corrode in many magnesium electrolyte systems. Stainless steel offers better stability but increases cost and weight. Binder materials and conductive additives must also be carefully selected to avoid degradation or unwanted side reactions.

Recent research has focused on several strategies to overcome these challenges. Surface modification of the magnesium anode aims to prevent passivation while allowing Mg²⁺ transport. Artificial SEI layers using materials such as magnesium boride or magnesium fluoride have shown some success in laboratory settings. Electrolyte engineering continues to be a major area of investigation, with efforts to develop new salt anions and solvent systems that combine wide electrochemical stability windows with high ionic conductivity. On the cathode side, research has expanded beyond intercalation compounds to conversion-type materials and organic electrodes that may better accommodate Mg²⁺ ions.

Despite these efforts, the fundamental electrochemical challenges of rechargeable magnesium batteries remain unresolved after more than five decades of research. The passivation behavior of magnesium and the difficulty in finding electrolyte solutions that combine stability, conductivity, and wide voltage windows continue to hinder progress. While magnesium batteries may eventually find niche applications where safety and cost outweigh energy density requirements, their widespread commercial adoption appears unlikely without breakthroughs in materials chemistry that address these core limitations. The history of magnesium battery development serves as a reminder that theoretical advantages in raw materials do not necessarily translate to practical energy storage solutions without overcoming fundamental electrochemical barriers.