Characterizing Mg2+ Conduction in Metal-Organic Frameworks

Abstract

Next generation solid-state materials for fast ion conduction have the potential to revolutionize battery technology. Metal-organic frameworks (MOF) are a promising material for achieving this goal. Given their structural diversity and complexity, design of efficient MOF-based ion conductors can be accelerated by detailed understanding and quantitative prediction of ion conductivity. To this end, the chemical behavior and transport properties of an Mg(TFSI)2/DME electrolyte system inside Mg-MOF-74 were studied by a combination of computational and experimental methods.The dominant minimum energy pathway (MEP) for Mg2+ conduction inside Mg-MOF-74 was found to be “solvent hopping” mechanism, with an energy barrier of 4.4 kcal/mol.However, due to the polycrystalline nature of solid-state materials, specifically MOF-74 in our case, grain boundary effects need to be taken into consideration, which is complicated by challenges in grain boundary characterization, as well as modeling of ensemble transport properties. To address these issues, we developed an approach for modeling ion transport at grain boundaries and predicting their contribution to conductivity. In particular, Mg2+ conduction in Mg-MOF-74 thin film was studied as a representative system. Using computational techniques and guided by experimental data, we investigated the structural details of MOF grain boundary interfaces to determine accessible Mg2+ transport pathways. Computed transport kinetics were input into a simplified model of the MOF nanocrystal, which combined ion transport in bulk structure and at grain boundaries. The model was able to predict Mg2+ conductivity in MOF-74 thin film within chemical accuracy (<1 kcal/mol difference for apparent activation energy), validating our approach. The results indicate that Mg2+ conduction in MOF-74 is inhibited due to strong Mg2+ binding at grain boundaries. Only favorable alignments of the grain boundary interface allow for fast Mg2+ transport through MOF and contribute notably to ion conductivity. The relative scarcity (~0.1% frequency) of these favorable alignments results in a reduction of conductivity by 2-3 orders of magnitude, illustrating the large impact of the grain boundary contribution. Grain boundary structure and its interaction with ions are critical for a molecular-level understanding of solid-state ion transport and quantitative prediction of conductivity.Our work provides a computation-aided synergistic platform for quantifying the role of grain boundaries in mediating ion transport, which can serve as a complementary characterization tool for the solid-state ion conductor community and providing insights for MOFs’ application in Mg battery area.