Enhancing Lithium Ion Conduction in LLZO-Based Solid Electrolytes through Anion Doping for Advanced Energy Storage: Insights from Molecular Dynamics Simulations

Abstract

Solid-state electrolytes (SSEs) have emerged as promising alternatives to traditional liquid electrolytes due to their enhanced safety, higher stability and energy density in energy storage applications. Among SSEs, cubic Li₇La₃Zr₂O₁₂ (LLZO) is considered particularly promising, offering high lithium ion conductivity, high chemical stability to metal anode and a wide electrochemical stability window. Nevertheless, the cubic phase converts to a less conductive tetragonal phase during cooling in pure LLZO. Doping is one of most effective methods to stabilize the cubic LLZO at lower temperatures and improve the ion conductivity. While there is extensive research on cation site substitutions, studies on anion doping are very limited. We have investigated the effects of fluorine doping on the phase stability and ion conductivity of LLZO, exploring fluorine concentrations ranging from 1 to 10% across a wide temperature range of 300–1400 K using molecular dynamics (MD) simulations based on polarizable shell model potentials. Our results indicate that 3% fluorine doping achieves the highest diffusion coefficient (3.69 × 10^–7 cm² s^–1) at room temperature, while the lowest activation energy (∼0.22 eV) also occurs at around 3% doping, which is in good agreement with experimental observations. Doping at 1% was found to be insufficient to stabilize the cubic phase, while high fluorine concentrations (>4%) inhibited ion migration pathways due to stronger electrostatic interactions between point defects V_Li^′ and F_O^•. Defect formation energies were also calculated to study defect formation and interactions and their effect on lithium ion conduction. Lithium ion diffusion pathways and mechanisms are also explored by using trajectories from MD simulations. This study provides insights into the optimization of fluorine-doped LLZO, suggesting that moderate doping levels (around 3%) offer a balance between phase stability and ionic conductivity.