Design Strategies of Spinel Oxide Frameworks Enabling Reversible Mg-Ion Intercalation

ConspectusReversible Mg²⁺ intercalation in metal oxide frameworks is a key enabler for an operational Mg-ion battery with high energy density needed for the next generation of energy storage technologies. While functional Mg-ion batteries have been achieved in structures with soft anions (e.g., S^2- and Se^2-), they do not meet energy density requirements to compete with the current rechargeable lithium-ion batteries due to their low insertion potentials. It emphasizes the necessity of finding an oxide-based cathode that operates at high potentials. A leading hypothesis to explain the limited availability of oxide Mg-ion cathodes is the belief that Mg²⁺ has sluggish diffusion kinetics in oxides due to strong electrostatic interactions between the Mg²⁺ ions and oxide anions in the lattice. From this assessment, it can be hypothesized that such rate limiting kinetic shortcomings can be mitigated by tailoring an oxide framework through creating less stable Mg²⁺-O^2- coordination.Based on theoretical calculations and preliminary experimental data, oxide spinels have been identified as promising cathode candidates with storage capacity, insertion potential, and cation mobility that meet the requirements for a secondary Mg-ion battery. However, spinels with a single redox metal, such as MgCr₂O₄ or MgMn₂O₄, were not found to demonstrate sufficiently reversible Mg-ion intercalation at high redox potentials when coupled with nonaqueous Mg-electrolytes. Therefore, a materials development effort was initiated to design, synthesize, and evaluate a new class of solid-solution oxide spinels that can satisfy the required properties needed to create a sustainable Mg-ion cathode. These were designed by bringing together electrochemically active metals with stable redox potentials and charged states against the electrolyte, for instance, Mn³⁺, in combination with a structural stabilization component, typically Cr³⁺. Furthermore, common spinel structural defects that degrade performance, i.e., antisite inversion, were controlled to see correlation between structures and electrochemical overpotentials, thus controlling overall hysteresis. The designed materials were characterized by both short- and long-range structure in both ex situ and in situ modes to confirm the nature of solid-solution and to correlate structural changes and redox activity to electrochemical performance. Consistent and reproducible results were observed for facile bulk Mg²⁺-ion activity without phase transformations, leading to enhanced energy storage capability based on reversible intercalation of Mg²⁺, enabled by understanding the variables that control the electrochemical performance of the spinel oxide. Based on these observations, with proper materials design, it is possible to develop an oxide cathode material that has many of the desired properties of a Li-ion intercalation cathode, representing a significant mile marker in the quest for high energy density Mg-ion batteries.This Account describes strategies for the design and development of new spinel oxide intercalation materials for high-energy Mg-ion battery cathodes through a combination of theoretical and experimental approaches. We will review the key factors that govern the kinetics of Mg²⁺ diffusion in spinel oxides and illustrate how material complexity correlates with the electrochemical Mg²⁺ activity in oxides and is supported by secondary characterization. The understanding gained from the fundamental studies of cation diffusion in oxide cathodes will be beneficial for chemists and materials scientists who are developing rechargeable batteries.