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

Abstract

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.

Recent developments in Nb‐based oxides with crystallographic shear structures as anode materials for high‐rate lithium‐ion energy storage

“…Subsequently, the Li + gradually fills the two window sites (W v and W h ). 105 In situ XRD analysis of PNb 9 O 25 during lithiation (Figure 8D) reveals three solid-solution regions (3.0-1.62, 1.54-1.11 V, and 1.08-1.0 V) and two two-phase regions (plateau at around 1.62 and 1.11 V) during the entire lithiation process. 106 Moreover, Preefer et al 107 found that an insulator-to-metal transition can occur during the lithiation process of PNb 9 O 25 , with the material remaining insulating at low levels of lithiation, owing to localization of the electron density, and showing metallic behavior after full lithiation.…”

“…(C) Crystal structure of PNb 9 O 25 and different lithium sites in PNb 9 O 25 . Reproduced with permission from Saber et al 105 Copyright: 2021 American Chemical Society. (D) Operando XRD characterization and the evolution of lattice parameters for PNb 9 O 25 during the first cycle.…”

“…According to the DFT calculations, the Li + ions occupy the P s and P e pyramidal sites at first, and then the P c sites, with some residual occupancy of the P e sites, as the Li + concentration increases. Subsequently, the Li + gradually fills the two window sites (W v and W h ) 105 . In situ XRD analysis of PNb 9 O 25 during lithiation (Figure 8D) reveals three solid‐solution regions (3.0–1.62, 1.54–1.11 V, and 1.08–1.0 V) and two two‐phase regions (plateau at around 1.62 and 1.11 V) during the entire lithiation process 106 .…”

Crystallographic design of intercalation materials

“…Other transition-metal oxides, such as PNb 9 O 25 , are also accompanied by a tetragonal distortion (from Jahn-Teller effect) of its lattices along the c−axis. These structural distortions coupled with the electronic structure of PNb 9 O 25 affects the ordered filling of multiple Li ions [13]. Overall, these abrupt crystallographic changes can induce significant misfit strains between neighboring lattices and collectively generate volume changes.…”

“…For example, intercalating ions in LiCoO 2 (a commonly used cathode) preferentially diffuse parallel to the (001) crystallographic planes. This preferential diffusion constrains ion migration to specific crystallographic channels and thus limits rapid charge/discharge properties [13,14].…”