UC Berkeley

There is an increasing need for sustainable energy storage solutions as fossil fuels are replaced by renewable energy sources. Multivalent batteries, and specifically Mg batteries, are one energy storage technology that researchers continue to develop with hopes to surpass the performance of Li-ion batteries. However, the limited energy density and transport properties of Mg cathodes remain as critical challenges preventing the realization of high-performance multivalent batteries. While spinel MgxTi2S4 and layered MgxTiS2 represented significant advancements in Mg cathodes, with improved gravimetric capacities compared to the original prototype Mg cathode, Chevrel MgxMo6S8, their performance is limited by their low voltages and poor solid-state ionic mobility. The absence of high-performance Mg cathodes motivates the need to develop new materials discovery approaches and expand material design strategies to improve multivalent ion solid-state mobility and further advance multivalent batteries.

A computational screening approach to identify high-performance multivalent intercalation cathodes among materials that do not contain the working ion of interest has been developed, which greatly expands the search space that can be considered for materials discovery. This approach has been applied to magnesium cathodes as a proof of concept and four resulting candidate materials are discussed in further detail: NASICON V2(PO4)3, birnessite NaMn4O8, tavorite MnPO4F, and spinel MnO2. This methodology includes the automated evaluation of solid-state mobility in high-throughput which enables filtering candidate cathode materials by their intrinsic transport properties such as tavorite MnPO4F and spinel MnO2, where high migration barriers were predicted. In examining the ion migration environment and associated Mg2+ migration energy in these four materials, local energy maxima are found to correspond with pathway positions of lower coordination where Mg2+ passes through a plane of anion atoms. While previous works have established the influence of local coordination on multivalent ion mobility, these results suggest that considering both the type of local bonding environment as well as the available free volume for the mobile ion along its migration pathway can be significant for improving solid-state mobility.

Two novel Mg cathode candidates were identified from the preliminary results in developing this novel computational screening approach: -VOPO4 and zircon EuCrO4. These materials were experimentally pursued to characterize their electrochemical properties and evaluate their viability as Mg cathodes. While Mg intercalation was experimentally verified in -VOPO4 and three zircon materials (YVO4, EuVO4, and EuCrO4), the measured gravimetric capacities were notably lower than expected. The properties of these materials from this initial experimental work are not yet sufficient to serve as high-performance Mg cathodes.

The computational investigation of ABO4 zircon materials (A = Y, Eu and B = V, Cr) as Mg intercalation cathodes identified remarkably good Mg-ion transport properties (migration barriers <250 meV) across multiple chemistries in this structural family. These properties were attributed to their unique structural motif of overlapping polyhedra along the diffusion pathway in zircons which appears instrumental for promoting good Mg-ion mobility. This motif results in a favorable “6-5-4” change in coordination that avoids unfavorable sites with lower coordination along the diffusion pathway. Diffusion pathways composed of polyhedra with overlapping volumes offers a promising new structural design metric for future Mg cathode development.

The introduced novel cathode computational screening approach is a significant advancement in expanding the diversity of materials considered for materials discovery and accounting for intrinsic solid-state mobility properties. Additional investigations into -VOPO4 and ABO4 zircons as Mg intercalation cathodes illustrate the value of this approach in identifying novel cathode materials. However further study and optimization will be required to overcome the limited experimental electrochemical performance in these materials. This highlights limitations in the introduced screening methodology which only considers phase stability, energy density properties, and transport properties intrinsic to the host crystal structure at the Angstrom scale. Despite its limitations, further study of materials evaluated by this screening approach yielded valuable insights into strategies for improving multivalent ion transport. Building upon these findings, such as the role of site volume and overlapping polyhedra along the diffusion pathway, may inform further advancements in promoting multivalent ion transport and the discovery of high-performance Mg cathodes.