Sodium Intercalation Behavior in Some New Transition Metal Phosphates

Despite their apparent similarities, the crystal chemistry and electrochemical intercalation behavior of sodium vs. lithium are considerably distinct. There is strong interest in developing new sodium-ion battery intercalation electrodes as electrochemical energy storage media. Designing these materials requires developing an understanding of structure–property relationships and, in that course, it is constructive to identify structural motifs where sodium is comparable to lithium and where it contrasts.

Here, we report the synthesis, electrochemistry, and structural evolution of several transition metal phosphates that have not previously been investigated as battery active materials, including materials that do not exhibit sodium capacity in analogous metal oxide phases. The host structure defect chemistry and the sodiated and lithiated materials are examined with diffraction and ⁶Li, ⁷Li, ²³Na, and ³¹P solid-state NMR spectroscopy. Compared to oxide analogues, the phosphates exhibit larger paramagnetic shifts, suggestive of a higher degree of charge localization on the transition metal centers. The intercalation voltage of these metal phosphates is higher than their oxide analogues, which can be attributed to an inductive effect. As these are unexplored electrode materials, the charge storage properties are reported in some depth to understand the impact of voltage window and current density on capacity and reversibility. In each of these respects, comparison is made between sodium and lithium guest cations.