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Defect-Tolerant Diffusion Channels for Mg2+ Ions in Ribbon-Type Borates: Structural Insights into Potential Battery Cathodes MgVBO4 and MgxFe2–xB2O5
dataset
posted on 2015-07-14, 00:00 authored by Shou-Hang Bo, Clare P. Grey, Peter G. KhalifahThe reversible room temperature intercalation
of Mg2+ 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 (MgxFe2–xB2O5 and MgVBO4)
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
MgVBO4 nor MgxFe2–xB2O5 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 MgxFe2–xB2O5 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 MgxFe2–xB2O5 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.