Magnesium batteries represent a promising alternative to conventional lithium-ion systems due to magnesium's natural abundance, high theoretical volumetric capacity, and potential for improved safety. The fundamental science governing these batteries revolves around multivalent ion transport, which presents unique challenges and opportunities compared to monovalent systems like lithium. Understanding the differences in solvation-desolvation energetics, charge transfer kinetics, and ion mobility is critical for advancing magnesium battery technology.

In monovalent systems, such as lithium-ion batteries, the transport of Li+ ions involves relatively straightforward processes due to the single positive charge carried by each ion. The solvation shell around a Li+ ion typically consists of four to six solvent molecules, depending on the electrolyte composition. The energy required for desolvation—the stripping of solvent molecules before ion insertion into the electrode—is moderate, allowing for efficient charge transfer at the electrode-electrolyte interface. The kinetics of Li+ intercalation or plating are fast enough to support high power densities, making lithium-ion batteries suitable for applications requiring rapid charging and discharging.

In contrast, magnesium ions (Mg2+) carry a double positive charge, which fundamentally alters their behavior in electrolytes and at interfaces. The higher charge density of Mg2+ results in stronger electrostatic interactions with solvent molecules, leading to more tightly bound solvation shells. A Mg2+ ion typically coordinates with six or more solvent molecules in common electrolytes, forming a larger and more stable solvation structure than Li+. This strong solvation has significant implications for desolvation energetics. The energy barrier for removing solvent molecules from Mg2+ is substantially higher than for Li+, making the desolvation process more difficult. This increased energy requirement can slow down the charge transfer kinetics at the electrode surface, limiting the rate capability of magnesium batteries.

The multivalent nature of Mg2+ also affects ion mobility within the electrolyte. While the Stokes radius of a solvated Mg2+ ion is larger than that of Li+, the higher charge can lead to stronger interactions with anions and other species in the electrolyte. These interactions can result in increased viscosity and reduced ionic conductivity compared to monovalent systems. For example, conventional magnesium electrolytes based on ethereal solvents like tetrahydrofuran (THF) exhibit ionic conductivities in the range of 1-5 mS/cm, which is lower than typical lithium-ion electrolytes (10-20 mS/cm). The lower conductivity further contributes to kinetic limitations in magnesium batteries.

Charge transfer kinetics at the electrode-electrolyte interface are another critical factor differentiating magnesium from lithium systems. In lithium batteries, the charge transfer process is relatively facile, with low activation energies for ion insertion or deposition. For magnesium, the divalent charge means that two electrons are involved in the reduction of Mg2+ to Mg0 during plating, or the oxidation of Mg0 to Mg2+ during stripping. This two-electron process introduces additional complexity, as it requires the simultaneous transfer of both electrons, often leading to higher overpotentials. The overpotential for magnesium deposition can exceed 0.5 V in many electrolytes, whereas lithium deposition typically occurs at overpotentials below 0.1 V. This disparity highlights the kinetic challenges inherent to multivalent systems.

The solid-state diffusion of Mg2+ within electrode materials is also markedly different from Li+. Many intercalation hosts that work well for Li+, such as graphite or layered transition metal oxides, exhibit poor performance with Mg2+ due to the stronger electrostatic interactions between the divalent ion and the host lattice. These interactions can lead to sluggish diffusion kinetics and structural distortions in the electrode material. For example, the diffusion coefficient of Mg2+ in Chevrel-phase Mo6S8—one of the few known Mg-intercalation materials—is approximately 10^-12 cm^2/s, several orders of magnitude lower than Li+ diffusion in conventional cathode materials (10^-9 to 10^-8 cm^2/s). This slow solid-state diffusion further limits the power density of magnesium batteries.

Electrolyte design plays a pivotal role in mitigating some of these challenges. Unlike lithium systems, where simple salt-solvent combinations often suffice, magnesium electrolytes frequently require more complex formulations to enhance ion transport and reduce desolvation penalties. Chloride-based electrolytes, such as MgCl2/AlCl3 in THF, have shown promise by facilitating the formation of active Mg species that lower the overpotential for deposition. However, these electrolytes often suffer from corrosion issues and limited electrochemical stability. Recent advances in non-nucleophilic electrolytes, such as those based on boron clusters or magnesium carboranes, have improved stability while maintaining reasonable ionic conductivity. These electrolytes can achieve coulombic efficiencies above 99% for magnesium plating and stripping, a critical metric for reversible cycling.

The interfacial chemistry between the electrolyte and electrode is another area where magnesium systems diverge from lithium. In lithium batteries, a passivating solid-electrolyte interphase (SEI) forms on the anode surface, which is ionically conductive but electronically insulating. This SEI prevents further electrolyte decomposition while allowing Li+ transport. In magnesium batteries, the formation of a similar SEI is complicated by the reactivity of Mg2+ and the tendency for reduction products to block ion transport. Many magnesium electrolytes are inherently reactive with conventional anode materials, leading to uncontrolled passivation that impedes Mg2+ transport. Developing stable interfaces that permit reversible magnesium plating and stripping remains a key research focus.

Despite these challenges, magnesium batteries offer intrinsic advantages that motivate continued research. The theoretical volumetric capacity of magnesium (3833 mAh/cm^3) is nearly twice that of lithium (2061 mAh/cm^3), owing to magnesium's divalent charge and high density. Additionally, magnesium metal is less prone to dendritic growth during plating compared to lithium, which could improve safety by reducing short-circuit risks. These attributes make magnesium batteries particularly appealing for applications where energy density and safety are paramount, such as grid storage or electric vehicles.

Future progress in magnesium battery technology will depend on advances in materials design, particularly in identifying new cathode materials with faster Mg2+ diffusion and electrolytes that minimize desolvation penalties. Computational modeling and high-throughput screening are valuable tools for discovering materials that can accommodate multivalent ions without excessive kinetic limitations. Pairing these materials with optimized electrolytes and stable interfaces will be essential for realizing practical magnesium batteries with performance competitive to lithium-ion systems.

In summary, the science of multivalent ion transport in magnesium batteries involves complex interactions between solvation energetics, charge transfer kinetics, and solid-state diffusion. These processes differ fundamentally from monovalent systems due to the higher charge density of Mg2+, leading to stronger solvation, slower kinetics, and more challenging interfacial chemistry. Addressing these challenges through innovative materials and electrolyte design is crucial for unlocking the potential of magnesium as a next-generation energy storage technology.