Design Strategies for High Voltage Spinel Cathode Materials to Enable Reversible Mg-Ion Intercalation

Reversible insertion of Mg 2+ into metal oxide frameworks stands as a critical factor for facilitating the operation of a Mg-ion battery boasting high energy density, imperative for advancing energy storage technologies. While functional Mg-ion batteries have been achieved in frameworks featuring soft anions like S 2– and Se 2– , they fall short of meeting the energy density benchmarks to rival current rechargeable lithium-ion batteries due to their low insertion potentials. This underscores the need to pinpoint an oxide-based cathode capable of operating at elevated potentials. A leading theory posits that the limited availability of oxide Mg-ion cathodes stems from the sluggish diffusion kinetics of Mg 2+ in oxides, attributed to robust electrostatic interactions between Mg2+ ions and oxide anions within the lattice. Consequently, it is hypothesized that rectifying these kinetic shortcomings could be achieved by tailoring an oxide framework to foster less stable Mg 2+ –O 2– coordination. Drawing upon theoretical computations and preliminary experimental findings, oxide spinels emerge as promising candidates for cathodes, possessing storage capacity, insertion potential, and cation mobility that align with the requisites for a secondary Mg-ion battery. However, spinels featuring a single redox metal, such as MgCr 2 O 4 or MgMn 2 O 4 , failed to exhibit adequately reversible Mg-ion intercalation at high redox potentials when paired with nonaqueous Mg-electrolytes. Consequently, a concerted effort in materials development was launched to conceive, synthesize, and assess a novel class of solid-solution oxide spinels designed to fulfill the requisite properties for a sustainable Mg-ion cathode. These materials were engineered by combining electrochemically active metals boasting stable redox potentials and charged states vis-a-vis the electrolyte, such as Mn 3+ , alongside a structural stabilization component, typically Cr 3+ . Additionally, common spinel structural defects known to compromise performance, such as antisite inversion, were managed to explore the relationship between structures and electrochemical overpotentials, thereby regulating overall hysteresis. Comprehensive structural characterization, encompassing both short- and long-range analyses in both ex-situ and in-situ settings, confirmed the nature of solid-solution and established a correlation between structural modifications and redox activity vis-a-vis electrochemical performance. Encouragingly, consistent and reproducible outcomes were noted, manifesting facile bulk Mg2+-ion activity sans phase transformations, thus bolstering energy storage capacity through reversible Mg 2+ intercalation, facilitated by an understanding of the variables governing the electrochemical performance of spinel oxide. Leveraging these insights, it becomes conceivable to engineer an oxide cathode material boasting many of the desired attributes of a Li-ion intercalation cathode, marking a significant milestone in the pursuit of high energy density Mg-ion batteries. This study delineates strategies for designing and developing novel spinel oxide intercalation materials tailored for high-energy Mg-ion battery cathodes, leveraging a blend of theoretical and experimental methodologies. We discuss the key factors dictating the kinetics of Mg2+ diffusion in spinel oxides, elucidating how material complexity aligns with the electrochemical Mg2+ activity in oxides, bolstered by supplementary characterization. The insights gleaned from fundamental inquiries into cation diffusion in oxide cathodes are poised to benefit chemists and materials scientists engaged in the advancement of rechargeable batteries. Related publication: B. Kwon et al., Accounts of Chemical Research, 2024, 57, 1, 1–9 Figure 1