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 Nb_2-<i>x</i>P_3-<i>y</i>O₁₂ 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. <i>Operando</i> synchrotron diffraction with fine temporal resolution shows that Li_5<i>z</i>Nb₅P₇O₃₀ undergoes a series of six minimal-strain displacive phase transitions during lithium insertion, while the lithiation of Li_1.92<i>z</i>Nb_2-<i>x</i>P_3-<i>y</i>O₁₂ is purely bulk solid solution. The final volume change of Li_5<i>z</i>Nb₅P₇O₃₀ to <i>z</i> = 1 is 1.3% and to <i>z</i> = 1.45 is 2.4%, while the expansion for Li_1.92<i>z</i>Nb_2-<i>x</i>P_3-<i>y</i>O₁₂ to <i>z</i> = 1 is 3.5% and to <i>z</i> = 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.