Computational studies of niobium-based electrodes for lithium-ion batteries

<p dir="ltr">Lithium-ion batteries (LIBs) face limitations in energy density, cost, and long-term stability, particularly under high-demand conditions. Solid-state batteries offer a promising alternative, with improved safety, higher energy densities, and enhanced cycling stability. However, conventional anodes such as graphite and Li₄Ti₅O₁₂ (LTO) are limited by low capacities and narrow voltage windows, motivating the search for high-capacity alternatives. This thesis investigates two classes of niobium-based materials: garnet-type solid-state electrolytes (SSEs) and Wadsley-Roth (WR) shear-phase oxides as anode materials for next-generation solid-state batteries.</p><p dir="ltr">The first part focuses on the garnet-phase Li₅La₃Nb₂O₁₂(LLNO), studied using density functional theory (DFT), classical molecular dynamics (MD), ab initio molecular dynamics (AMID), and structural analysis. Zn doping modifies lattice stability and Li-site occupancy, slightly lowering the Li⁺ diffusion activation energy from 0.36 to 0.32 eV. However, Li⁺ interstitial trapping limits long-range conductivity. Both undoped and Zn-doped LLNO maintain cubic symmetry and support knock-on Li⁺ transport across a wide temperature range, confirming LLNO's potential as a stable SSE.</p><p dir="ltr">WR oxides (PNb₉O₂₅ (PNO), VNb₉O₂₅ (VNO), and TiNb₂O₇ (TNO)) exhibit high Li⁺ capacity with minimal volume change. Their redox behaviour is linked to edge- and corner-sharing Nb/TiO₆ units, and second-order Jahn-Teller distortions support structural flexibility. NEB and AIMD simulations reveal low migration barriers ( 0.4 eV), which lie within the range of 0.3-0.6 eV typical for layered oxides. TNO shows the highest Li⁺ mobility due to its open framework. Diffusion coefficients confirm favourable kinetics, though mobility in PNO and VNO depends on site occupation.</p><p dir="ltr">Sodiation studies show that while TNO maintains structural integrity during Na⁺ insertion, it undergoes significant volume expansion (36%) and becomes unstable beyond moderate Na:TM ratios. PNO and VNO collapse at low Na⁺ concentrations due to the large ionic radius of Na⁺. Bader charge analysis indicates that Nb is the main redox-active species, and Na⁺ insertion induces octahedral tilting and distortion recovery. These results highlight the challenges of adapting WR oxides for sodium-ion anodes.</p><p dir="ltr"><br></p><p dir="ltr">Li⁺ diffusion in WR oxides occurs primarily through low-barrier window sites, with pocket-to-pocket transitions requiring higher activation energies (up to 0.9 eV). TNO phases (C1/2 and I1/2) demonstrate fast one-dimensional Li⁺ conduction. AIMD simulations at 1200 K show high diffusivities for VNO (2.0 × 10^-4 m² s^-¹) and TNO I1/2 (1.0 × 10^-4 m² s^-¹), supporting high-rate performance. Vanadium migration in VNO at elevated temperatures may hinder delithiation, while PNO remains stable due to its redox-inactive P⁵⁺ species.</p><p dir="ltr"><br></p><p dir="ltr">This work provides fundamental insights into niobium-based SSEs and intercalation hosts, contributing to the development of high-performance solid-state lithium- and sodium-ion batteries.</p>