Lithium Diffusion in Niobium Tungsten Oxide Shear Structures.

Can P Koçer,Andrew J Morris,Clare P Grey,Kent J Griffith

doi:10.1021/acs.chemmater.0c00483

Abstract

Niobium tungsten oxides with crystallographic shear structures form a promising class of high-rate Li-ion anode materials. Lithium diffusion within these materials is studied in this work using density functional theory calculations, specifically nudged elastic band calculations and ab initio molecular dynamics simulations. Lithium diffusion is found to occur through jumps between 4-fold coordinated window sites with low activation barriers (80–300 meV) and is constrained to be effectively one-dimensional by the crystallographic shear planes of the structures. We identify a number of other processes, including rattling motions with barriers on the order of the thermal energy at room temperature, and intermediate barrier hops between 4-fold and 5-fold coordinated lithium sites. We demonstrate differences regarding diffusion pathways between different cavity types; within the ReO3-like block units of the structures, cavities at the corners and edges host more isolated diffusion tunnels than those in the interior. Diffusion coefficients are found to be in the range of 10–12 to 10–11 m2 s–1 for lithium concentrations of 0.5 Li/TM. Overall, the results provide a complete picture of the diffusion mechanism in niobium tungsten oxide shear structures, and the structure–property relationships identified in this work can be generalized to the entire family of crystallographic shear phases.

Highlights

Li-ion batteries with short charge times and high power density are required to accelerate consumer adoption of electric vehicles and relieve intermittency of renewable energy resources.[1,2]While there are many factors determining the charge/discharge rate of a device,[1] and not all materials with high-rate capability are suited for each application, the ionic and electronic conduction within the active materials represent fundamental limits to the achievable rate
The results provide a complete picture of the diffusion mechanism in niobium tungsten oxide shear structures, and the structure−property relationships identified in this work can be generalized to the entire family of crystallographic shear phases
The low activation barriers lead to high room temperature diffusivities (10−12 to 10−11 m2 s−1 for stoichiometries probed in this work) and are responsible for the excellent high-rate capability of the niobium tungsten oxides in lithium ion batteries

Summary

Introduction

Li-ion batteries with short charge times and high power density are required to accelerate consumer adoption of electric vehicles and relieve intermittency of renewable energy resources.[1,2]While there are many factors determining the charge/discharge rate of a device,[1] and not all materials with high-rate capability are suited for each application, the ionic and electronic conduction within the active materials represent fundamental limits to the achievable rate. A number of niobium-based complex oxides with open framework structures show very fast lithium diffusion and are promising for applications as high-rate, high-voltage anodes. These include T-Nb2O5,4 TiNb2O7,5 and the recently discovered niobium tungsten oxides Nb16W5O55 and Nb18W16O93,6 among others. The present work focuses on niobium tungsten oxides with Wadsley−Roth crystallographic shear structures, Nb12WO33, Nb14W3O44, Nb16W5O55, and Nb18W8O69. The type VI cavity is special because, in comparison to the others, the open space within it is blocked by the tetrahedral site

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