Defect-Tolerant Diffusion Channels for Mg<sup>2+</sup> Ions in Ribbon-Type Borates: Structural Insights into Potential Battery Cathodes MgVBO<sub>4</sub> and Mg<sub><i>x</i></sub>Fe<sub>2–<i>x</i></sub>B<sub>2</sub>O<sub>5</sub>

The reversible room temperature intercalation of Mg<sup>2+</sup> ions is difficult to achieve but may offer substantial advantages in the design of next-generation batteries if this electrochemical process can be successfully realized. Two types of quadruple ribbon-type transition metal borates (Mg<sub><i>x</i></sub>Fe<sub>2–<i>x</i></sub>B<sub>2</sub>O<sub>5</sub> and MgVBO<sub>4</sub>) with high theoretical capacities (186 and 360 mAh/g) have been synthesized and structurally characterized through the combined Rietveld refinement of synchrotron and time-of-flight neutron diffraction data. Neither MgVBO<sub>4</sub> nor Mg<sub><i>x</i></sub>Fe<sub>2–<i>x</i></sub>B<sub>2</sub>O<sub>5</sub> can be chemically oxidized at room temperature, though Mg can be dynamically removed from the latter phase at elevated temperatures (approximately 200–500 °C). It is found that Mg diffusion in the Mg<sub><i>x</i></sub>Fe<sub>2–<i>x</i></sub>B<sub>2</sub>O<sub>5</sub> structure is more facile for the inner two octahedral sites than for the two outer octahedral sites in the ribbons, a result supported by both the refined site occupancies after Mg removal and bond valence sum difference map calculations of diffusion paths in the pristine material. Mg diffusion in this pyroborate Mg<sub><i>x</i></sub>Fe<sub>2–<i>x</i></sub>B<sub>2</sub>O<sub>5</sub> framework is also found to be tolerant to the presence of Mg/Fe disorder since Mg ions can diffuse through interstitial channels which bypass Fe-containing sites.