Mo occupancy along crystallographic shear planes in the Wadsley–Roth compound MoxNb12W1-xO33 enables multi-electron redox behavior

Abstract

Transition metal oxide Wadsley-Roth (W-R) crystallographic shear compounds are promising alternatives to graphite for high-rate Li-ion battery applications, as fast charging can drive unsafe lithium metal plating on graphite anodes when Li+ ions deposit as metallic lithium rather than intercalating into the graphite lattice. Despite this promise, fundamental materials chemistry questions remain regarding how to tune W-R structure and composition to achieve desirable electrochemical properties such as lower working potential, enhanced capacity, and improved cycle stability. Our work is motivated by two central questions: (1) how transition-metal substitution and site occupancy modifies the electrochemically active density of states (DOS) that governs multi-electron redox and the working potential; and (2) how variations in the propensity for second-order Jahn–Teller (SOJT) distortions of transition-metal octahedra along crystallographic shear planes may influence structural stability during repeated cycling. To answer these questions, we systematically investigated a series of nearly phase pure MoxNb12W1-xO33 and defect-rich D-MoxNb12W1-xO33 samples, as evidenced by experimental and computational Raman spectroscopy, as well as X-ray diffraction and Rietveld refinement analyses. Galvanostatic cycling and differential capacity measurements revealed that Mo substitution for W alters the electrochemically active DOS and activates multi-electron redox. Mo substitution introduces new electrochemically active states at more positive potentials than the W-based compounds. Electronic structure calculations show that the states enabling multi-electron redox are highly sensitive to both the identity of the transition-metal dopant (W vs. Mo) and its crystallographic site; accordingly, we considered doping at the tetrahedral, block-center, and shear-plane sites, finding that multi-electron (Mo6+ → Mo4+) redox arises specifically from Mo occupying the edge-sharing octahedral sites along the shear planes. The defective samples generally exhibited higher capacities, likely due to the presence of Wadsley defects (e.g., intergrowth of W4Nb26O77 in a matrix of Nb12WO33) that further lower Li-ion binding energetics and alter Li-ion transport paths. Mo-rich samples exhibit greater capacity loss with additional cycling, possibly due to the inability of severely distorted Mo octahedra from “rocking” back and forth during lithiation/de-lithiation cycles. These findings are significant because they inform W-R material design strategies aimed at systematically increasing capacity and working potential via optimizing transition metal site occupancy in the structure.