The exploration of magnesium batteries represents a compelling chapter in the quest for energy storage solutions beyond lithium-ion. The journey began in the early 20th century when researchers first examined magnesium as a potential anode material due to its high theoretical volumetric capacity of 3833 mAh/cm³, nearly double that of lithium. Early experiments in the 1920s and 1930s focused on primary magnesium batteries, leveraging the metal's natural abundance and low cost. These systems, however, were plagued by rapid passivation—a phenomenon where a surface layer of magnesium oxide or hydroxide forms, severely limiting electrochemical activity. This fundamental challenge stunted progress for decades, relegating magnesium batteries to niche applications like seawater-activated reserve power sources.

A resurgence of interest emerged in the 1990s as the limitations of lithium-ion technology became apparent, particularly for grid-scale storage and electric vehicles. The pivotal breakthrough came in 2000 when a team demonstrated reversible magnesium deposition and stripping in non-aqueous electrolytes, overturning the long-held belief that magnesium electrochemistry was inherently irreversible. This achievement was enabled by the development of organohaloaluminate electrolytes, which suppressed passivation and allowed for repeated cycling. The discovery marked the birth of modern rechargeable magnesium battery research and spurred investigations into compatible cathode materials.

The early 2000s saw intensive focus on electrolyte design, as conventional lithium-ion electrolytes proved incompatible with magnesium. Researchers identified two critical requirements: electrochemical stability to prevent side reactions and the ability to facilitate magnesium ion transport. Grignard reagent-based electrolytes showed promise but suffered from poor oxidative stability. By 2010, improvements in magnesium borohydride and magnesium aluminum chloride complex electrolytes extended the voltage window to 3.0V, enabling exploration of higher-energy cathode materials. During this period, computational studies provided deeper insights into magnesium ion diffusion kinetics, revealing that strong Coulombic interactions with host lattices posed unique challenges compared to lithium.

Cathode development followed a parallel trajectory. Initial work focused on Chevrel-phase Mo₆S₈, which demonstrated reversible magnesium intercalation at moderate voltages (1.1-1.3V) with reasonable capacity (120 mAh/g). While this validated the concept of magnesium intercalation, the low voltage and energy density necessitated alternatives. Transition metal oxides, successful in lithium systems, proved problematic due to sluggish magnesium diffusion. By 2015, researchers turned to sulfur-based cathodes, capitalizing on conversion reactions to achieve higher capacities. Magnesium-sulfur systems theoretically offer 1722 Wh/kg, but practical implementations grappled with polysulfide shuttling and poor kinetics.

The past decade has witnessed three major advancements. First, the discovery of chloride-containing electrolytes like MgCl₂/AlCl₃ in ether solvents improved conductivity to 5-8 mS/cm while suppressing corrosion. Second, the development of high-voltage cathodes such as magnesium chromium oxide (2.8V) and organic quinones (2.4V) narrowed the energy density gap with lithium. Third, advanced characterization techniques including X-ray absorption spectroscopy and cryo-electron microscopy have elucidated interfacial phenomena at magnesium electrodes, guiding surface engineering strategies.

Persistent challenges remain at both fundamental and applied levels. The divalent nature of Mg²⁺ ions causes strong electrostatic interactions with host materials, resulting in slow solid-state diffusion—a stark contrast to single-charged Li⁺. This manifests in lower rate capability and power density compared to lithium counterparts. Electrolyte compatibility presents another hurdle; most high-performance magnesium electrolytes are corrosive to standard battery components, requiring specialized current collectors and cell designs. Additionally, the lack of a reliable reference electrode for magnesium systems complicates accurate voltage measurements and thermodynamic assessments.

Recent efforts have branched into hybrid approaches. Magnesium-sodium dual-ion systems exploit the fast kinetics of sodium while retaining magnesium's capacity advantage. Another direction involves solid-state magnesium batteries, where ceramic or polymer electrolytes could mitigate dendrite formation—a safety concern less prevalent than in lithium-metal systems but still present. Computational materials science has accelerated progress, with machine learning models predicting stable magnesium-ion conductors and high-capacity cathode materials.

The evolution of magnesium battery research reflects broader trends in energy storage: from empirical discovery to mechanistic understanding, from material-centric approaches to system-level engineering. While commercial viability remains uncertain, the accumulated knowledge has enriched electrochemistry as a whole, particularly in understanding multivalent ion transport. Current prototypes achieve energy densities of 250-300 Wh/kg, comparable to early lithium-ion batteries, suggesting room for growth as interfacial engineering and electrolyte formulations advance. The trajectory underscores that overcoming the limitations of incumbent technologies requires not just incremental improvements but rethinking fundamental assumptions about charge storage mechanisms.

Ongoing work focuses on four key areas: developing non-nucleophilic electrolytes with wider voltage windows, designing cathode architectures that mitigate diffusion limitations, engineering artificial interphases to prevent passivation, and establishing standardized testing protocols to enable meaningful performance comparisons across laboratories. These efforts are increasingly interdisciplinary, combining synthetic chemistry, materials science, and advanced manufacturing techniques. The historical progression demonstrates that magnesium batteries are not merely lithium-ion alternatives but represent a distinct electrochemical paradigm whose full potential remains to be unlocked.