Defect engineering strategies to overcome strong electrostatic barriers of Mg²⁺ in rechargeable magnesium battery cathodes

Rechargeable magnesium batteries (RMBs) are widely regarded as a compelling “post-lithium” energy storage solution. Yet the realization of practical RMB systems has been severely constrained by the intrinsically slow solid-state diffusion of divalent Mg²⁺ within cathode host lattices, a phenomenon driven by strong electrostatic interactions with the anionic framework. A research team from Shanghai Jiao Tong University, led by Prof. Xiaowei Yang, has systematically shown that judicious defect engineering—via the controlled creation of anion and cation vacancies coupled with targeted substitutional doping—effectively mitigates these electrostatic barriers, enhances Mg²⁺ transport, and improves electronic conductivity. This defect-tailoring strategy thereby provides a robust pathway toward cathode materials capable of delivering the rate performance and cycling stability required for large-scale grid and electric-vehicle applications.

To enable improved Mg²⁺ mobility, the researchers first dissect the intrinsic migration mechanisms in a range of representative cathode structures. In Chevrel-phase Mo₆S₈, Mg²⁺ ions hop between two types of interstitial sites via ring-based pathways, encountering relatively low barriers within inner sites but facing higher barriers for transitions to outer sites. In dense-packed oxide frameworks—such as spinel Mn₂O₄ and layered NiO₂—Mg²⁺ transport follows a sequential path through tetrahedral and octahedral sites, since direct hops between like-coordination environments are energetically unfavorable. Olivine FePO₄ and certain vanadium oxides exhibit one-dimensional channels that limit diffusion, and Mg²⁺ must pass through intermediate sites of varying coordination, further raising activation energies.

Beyond crystal topology, five key factors govern Mg²⁺ diffusion coefficients: operating temperature, anion framework, geometric configuration, diffusion channel, and type of cation. Elevated temperatures can increase hopping frequency but also promote undesired side reactions and pose practical challenges for system stability. Softer anion lattices—such as sulfides and selenides—interact less strongly with Mg²⁺ and widen diffusion bottlenecks, yet they often yield lower voltages. The preference of Mg²⁺ for sixfold coordination means migration through fourfold sites in layered and olivine hosts involves high-energy penalties. Diffusion channel dimensionality—from three-dimensional interconnected networks in spinels to two-dimensional layers and one-dimensional tunnels—also dictates how readily Mg²⁺ can traverse the lattice. Finally, high-valence transition metals within the host framework exert stronger electrostatic repulsion on Mg²⁺, further impeding ion movement.

Armed with these mechanistic insights, the researchers explore defect engineering as a strategy to reduce electrostatic hindrance and improve ionic and electronic transport. They categorize defects into two primary classes: vacancies (anionic and cationic) and dopants (substitutional and interlayer). Anionic vacancies, for example, can be introduced by reducing the oxygen content in oxides. In TiO_2-x nanosheets, oxygen vacancies not only lower the bandgap, thus enhancing electronic conductivity, but also create additional Mg²⁺ insertion sites and reduce migration barriers. Similarly, oxygen-deficient V₂O₅ electrodes exhibit faster ion diffusion and lower charge-transfer resistance compared to their stoichiometric counterparts.

Cationic vacancies address phase-stability challenges, particularly in spinel oxides prone to rocksalt-phase formation upon Mg²⁺ insertion. By creating vacancies in both tetrahedral and octahedral sites, as demonstrated in ZnMnO₃, the formation of blocking phases is suppressed, preserving three-dimensional diffusion pathways and enabling more stable cycling. In anatase TiO₂, the introduction of titanium vacancies via chemical etching, combined with fluorine substitution, lowers Mg²⁺ intercalation energies at vacancy sites and dramatically increases reversible capacity compared to pristine TiO₂.

Substitutional doping further refines lattice properties. Aliovalent cation dopants, such as chromium in Li₄Ti₅O₁₂, introduce structural disorder that lowers migration barriers and reduces charge-transfer resistance, enabling faster Mg²⁺ insertion. Molybdenum doping in VS₄ expands interlayer spacing and induces sulfur vacancies, which together increase Mg²⁺ binding energies and facilitate ion diffusion. Anion substitution, such as replacing oxygen with fluorine in α-MoO₃, generates metal vacancies that unlock inert planes and improve diffusion along specific crystallographic directions. Complementary proton intercalation serves as a pillar to stabilize the structure, leading to improved conductivity and reversible capacity.

Interlayer doping strategies focus on layered materials, where pre-insertion of molecules, water, or small cations expands the interlayer spacing and shields Mg²⁺ from strong host interactions. Incorporating polymer chains, such as poly(ethylene oxide) in MoS₂, effectively widens diffusion channels and markedly improves capacity. Organic intercalants like phenylamine in VOPO₄ facilitate the insertion of MgCl⁺ complexes, significantly reducing migration barriers and enhancing cycling stability. Incorporating lattice water into VOPO₄·H₂O creates a hydrated environment that lubricates Mg²⁺ motion and lowers energy barriers by shielding the ion’s charge, thus boosting diffusivity without severely compromising structural integrity. A proton-assisted approach to cleave Mg-Cl bonds in MgCl⁺ electrolytes further enables genuine Mg²⁺ insertion into oxide lattices by lowering bond cleavage energies. Pre-intercalated Mg²⁺ and water molecules in a bilayer V₂O₅ framework have been shown to synergistically stabilize the host structure and maintain high capacity over extended cycling.

While defect engineering offers substantial performance gains, the researchers caution that uncontrolled defect concentrations can undermine structural integrity and energy density. Excessive vacancies may lead to anti-site disorder, while over-doping with inert species can dilute redox-active centers. Therefore, precise control over defect type, concentration, and spatial distribution is essential. They emphasize the importance of integrating theoretical modeling—such as high-throughput DFT and machine-learning-assisted screening—with advanced in situ characterization techniques (e.g., synchrotron X-ray diffraction, neutron scattering, and aberration-corrected transmission electron microscopy) to elucidate defect evolution and correlate microstructural changes with electrochemical behavior.

From a synthesis standpoint, scalable soft-chemical methods—including sol-gel, coprecipitation, ion-exchange, and hydro(solvo)thermal routes—are recommended for producing defect-engineered materials with fine-tuned vacancy profiles and dopant distributions. Physical methods like mechanical ball milling can also introduce vacancies, though with less precision.

Finally, the researchers assert that bulk defect strategies must be complemented by cathode-electrolyte interface engineering to fully unlock RMB potential. Tailored electrolytes that facilitate Mg²⁺ desolvation and stable interphases can minimize interfacial barriers, ensuring that the benefits of defect-tuned cathodes translate into high-rate performance and long-term cycling stability.

In summary, this comprehensive review charts a clear roadmap for tackling the intrinsic electrostatic challenges of Mg²⁺ diffusion in cathode materials through defect engineering. By combining mechanistic insights with a broad survey of vacancy- and doping-based approaches—and by highlighting future directions in modeling, characterization, synthesis, and interface design—the researchers provide a foundational framework that will guide the development of next-generation RMBs. As these strategies mature and integrate with advances in electrolyte and interphase technologies, RMBs stand poised to become a viable, sustainable energy storage solution for grid-scale and transportation applications.