Understanding charge storage in hydrated layered solids MOPO4 (M = V, Nb) with tunable interlayer chemistry

Hydrated layered solids are interesting charge storage hosts with potentially high electrochemical activity and interlayer tunability. Although it is often possible to tune their interlayer distance by a pillaring strategy, the poor electrochemical stability of such artificial structures remains a major issue in device operation. Here we investigate the charge storage properties of MOPO4 (M = V, Nb) hydrates with a nanosheet morphology to understand the influence of the interlayer environment on cycling stability, as well as ion selectivity. While in hydrated VOPO4 interlayer H2O molecules act as compressible springs to enable fast Li-/Na-ion transport kinetics and reasonable structural reversibility, NbOPO4 layers with bridging PO4 groups serving as permanent linkers exhibit highly stable cyclability for Li-ions, owing to a zero-volumechange ion transport process. However, the latter suffers from a much larger migration energy barrier for Na-ions. Our findings not only highlight a structurally intriguing material system, but also provide insights into reviving materials with an originally unstable interlayer chemical environment, and shed light on the design principles for creating electrochemically stable charge storage hosts.