Lithium and Sodium Intercalation with Multielectron Redox in Vacancy Ordered and Vacancy Disordered Cation-Deficient Anti-NASICON Niobium(V) Phosphates

Understanding structure-property relationships is of fundamental importance for the discovery and engineering of functional materials. In this work, two niobium(V) phosphate materials are studied for the first time as electroactive intercalation hosts after probing their crystal chemistry and defect structures with a combined high-resolution and wide-line NMR crystallography approach to resolve outstanding structural questions. The relatively rare niobyl group (Nb═O) gives an exceptionally distinct 93Nb NMR signature under the right experimental conditions, even in the presence of disorder, which should lead to its discovery and analysis in other phases. Nb5P7O30 and Nb2-xP3-yO12 provide an interesting model case study for comparative analysis because they are nearly isocompositional and both crystallize in the anti-NASICON structure, but they adopt different vacancy (dis)order patterns that lead to distinct space-group symmetries. As intercalation hosts, they both exhibit multielectron Nb5+/Nb3+ redox with lithium, with peak-to-peak separations on the order of 10 mV, and full one-electron Nb5+/Nb4+ redox with sodium. This latter observation is notable because the various niobium(V) oxide polymorphs, widely studied as battery electrode materials, are essentially inactive to sodium. Operando synchrotron diffraction with fine temporal resolution shows that Li5zNb5P7O30 undergoes a series of six minimal-strain displacive phase transitions during lithium insertion, while the lithiation of Li1.92zNb2-xP3-yO12 is purely bulk solid solution. The final volume change of Li5zNb5P7O30 to z = 1 is 1.3% and to z = 1.45 is 2.4%, while the expansion for Li1.92zNb2-xP3-yO12 to z = 1 is 3.5% and to z = 1.41 is 4.3%. Interstitial intercalation sites and percolation pathways are identified with bond valence site energy searching. Comparisons of the nonstoichiometric niobium(V) phases in this work are made to stoichiometric phases in the anti-NASICON system, as well as the effects of order and disorder in the lithium metal phosphate olivines.