Lithium Diffusion in Niobium Tungsten Oxide Shear Structures

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 4fold and 5-fold coordinated lithium sites. We demonstrate differences regarding diffusion pathways between different cavity types; within the ReO 3 -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 m 2 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.

Fast lithium-ion conductivity in the ‘empty-perovskite’ n = 2 Ruddlesden–Popper-type oxysulphide Y2Ti2S2O5

“…Similar four-coordinate distorted square-planar-like Li + insertion geometry in 'window' positions has been reported from DFT calculations for selected sites within the Wadsley-Roth-type Nb-W-O bronzes 29,53 and TiNb 2 O 7 , 30 as well as in cubic ReO 3 under dilute conditions. 22 It is notable that a squareor rectangular-planar-like arrangement is an unusual geometry for Li + .…”

“…38 Neutron diffraction experiments indicate that inserted Li + ions occupy approximately square-planar 'window' sites in the 2D plane. 37 This is a similar insertion environment to those identied for Li + in the high-rate Wadsley-Roth-type phases from DFT calculations, 29,30 suggesting that similar mechanisms are responsible for activating high Li + mobility in both types of materials.…”

“…20,21,28 These blockedges and corners contain edge-sharing octahedra units that form W-R cavities assigned the designations type II-VI by Cava et al 21 The edge-sharing units bestow structural rigidity, so that the W-R framework does not relax signicantly during intercalation. 14,17,21,29 This structural property is key to their performance in battery applications, and means the low activation barriers for Li diffusion are retained across different states of charge, leading to exceptional charging and discharging rates. 14,17,19 Computational and experimental studies have revealed that Li diffusion is very fast down the centre of the columns in W-R phases, where type I, II or III cavities are linked through corners of their constituent octahedra.…”

“…However, the diffusion is slower into 'pocket'-like sites at block edges formed by the cornersharing motifs of type II and III cavities. 19,29 Furthermore, Liion hopping is inhibited between blocks in W-R phases, leading to effectively 1D diffusion down the columns. 29,30 The implications are thus; whilst shear planes provide structural rigidity that is key to the high Li-ion diffusion across different states of charge, they also restrict Li-ion mobility to 1D channels and introduce sites which Li-ions are slower to transfer in and out of.…”

“…19,29 Furthermore, Liion hopping is inhibited between blocks in W-R phases, leading to effectively 1D diffusion down the columns. 29,30 The implications are thus; whilst shear planes provide structural rigidity that is key to the high Li-ion diffusion across different states of charge, they also restrict Li-ion mobility to 1D channels and introduce sites which Li-ions are slower to transfer in and out of. An ideal W-R-type structure for fast Li-ion diffusion would have 2D or 3D Li-ion connectivity and would contain diffusion pathways through structural units that are connected by corner-sharing octahedra only, whilst retaining framework rigidity.…”