Role of Wadsley Defects and Cation Disorder to Enhance MoNb12O33 Diffusion

TL;DR

This study investigates how Wadsley defects and cation disorder in MoNb12O33 enhance lithium diffusion, revealing that defect-rich samples exhibit higher capacity, faster diffusion, and are better suited for high-rate applications.

Contribution

It provides mechanistic insights into how Wadsley defects and transition metal disorder improve ionic transport in MoNb12O33, combining experimental and machine learning approaches.

Findings

Defect-rich MoNb12O33 shows 4.66% higher capacity.

Faster diffusion rates (~3X) in defect-rich samples.

Li populates fast diffusion paths at lower lithiation levels.

Abstract

Wadsley-Roth (WR) niobates have emerged as high-rate anode materials that can combine rapid ionic diffusion with good electronic conductivity. WR compounds have been defect-enhanced by limited annealing, however, such materials often contain multiple types of defects. In particular, both Wadsley defects (variable block size) and transition metal disorder have the potential to modify transport rates, however the corresponding effects are not well understood mechanistically. Here, MoNb12O33 (MNO) was calcined at two different temperatures to compare a defect-rich condition (MNO-800) with a proximal order-rich condition (MNO-900) as assessed through XRD, XANES, EXAFS, and STEM characterizations. Galvanostatically cycled lithium half cells of MNO-800 exhibited additional capacity (307 mAh/g at 0.1C, 4.66% higher) and improved high-rate capacity of 200 mAhg-1 at 10C. ICI-based overpotential analysis identified solid state diffusion as the dominant rate limiting process where MNO-800 correspondingly exhibited ~3X faster capacity-weighted diffusivity. A machine-learning interatomic potential was trained to density functional theory and then applied with molecular dynamics (MLIP-MD) to examine the possible roles of Wadsley defects and transition metal disorder. For both defect-types, Li was found to populate and activate fast diffusion paths from window sites at lower extents of lithiation as compared to the order-rich model.