Evaluating bulk Nb2O2F3 for Li-battery electrode applications

Abstract

This investigation has the primary objective of elucidating the lithium intercalation process in the crystal structure of a new niobium oxyfluoride compound Nb₂O₂F₃. The framework of the density functional theory was applied in a generalized gradient approximation together with the hybrid functional method. It is revealed that lithium atoms intercalate in this material in a maximum concentration of one Li atom per formula unit forming LiNb₂O₂F₃. Moreover, octahedral positions in between the layers of Nb–O–F appear as the Li preferred occupancy resulting in a structural volume expansion of only 5%. Electronic structure evolution with the insertion of lithium displays a transformation from semi-conductor to metal when half of the lithium atoms are added. This transformation occurs due to a symmetry break induced by the transition from the +8 to +7 oxidation state of half of the Nb₂ dimers. Then, after full lithiation the symmetry is recovered and the material becomes a semiconductor again with a band gap amounting to 1 eV. The evaluated average deintercalation potential reaches 1.29 V vs. Li/Li⁺ with activation energy for lithium ion migration of 0.79 eV. The computed low potential of the redox reaction Nb₂⁸⁺ to Nb₂⁷⁺ includes niobium oxyfluoride in the map of possible materials for the anode application of Li-ion batteries.