Role of Electronic Structure in Li Ordering and Chemical Strain in the Fast Charging Wadsley-Roth Phase PNb9O25

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Chemistry of Materials

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Wadsley-Roth crystallographic shear phases are a family of transition-metal oxides that show tremendous promise as electrode materials in Li-ion batteries. Despite their ability to intercalate lithium at high rates, little is known about their structural, thermodynamic, and electronic properties as a function of Li concentration. In this study, we use first-principles statistical mechanics methods to explore the lithium-site preference, lithiation strain, and electronic structure of PNb9O25, a Wadsley-Roth phase that has been shown to reversibly cycle at a rate of 60 C and that can accommodate more than one Li per Nb. We find that Li ions can occupy five symmetrically distinct interstitial sites within the PNb9O25 crystal structure, with three being pyramidal sites coordinated by five oxygen and two being window sites with square-planar oxygen coordination. The insertion of Li into PNb9O25 leads to a complex site filling sequence, with pyramidal sites preferred at low Li concentrations, followed by the filling of window sites at higher Li concentrations. Our findings are aided by neutron diffraction where pyramidal sites are found to be filled at low compositions. The order in which sites are filled is strongly influenced by the chemical strain due to Li insertion. The strain arises from the delocalization of donated electrons over the d orbitals of the structure's edge-sharing niobium, which leads to a tetragonal distortion along the c-axis, thereby making vertical window sites favorable for Li occupancy at intermediate to high Li concentrations. Given the crystallographic similarities among different shear phases, we expect that the results of this study will also shed light on the electrochemical properties of other Wadsley-Roth chemistries.

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