Introduction Using ab-initio Density Functional Theory (DFT) simulations of both the crystalline and amorphous phases of β- Li3PS4, the cause of high conductivity of Li+ ions in the amorphous phase is explored. β- Li3PS4 and other lithium thiosulfates are promising electrolytes for all solid-state batteries, especially in amorphous phases, like 75%Li2S-25%P2S5. However, the high conductivity in the amorphous phases is not fully understood. This research aims to understand and compare diffusion mechanisms in both the amorphous and crystalline phases of β- Li3PS4. We will analyze the effect of disorder on the diffusion, both short range and long range. Methodology The crystalline and amorphous simulation cells are optimized and initialized with DFT using the Quantum ESPRESSO package. First, a crystalline phase of Li3PS4 is generated, by obtaining its minimum energy structure. In the next stage, the amorphous phase of Li3PS4 is created using simulation cells with 128 atoms using the 'melt-quench' approach. Here, the structure is melted at 1,500 K, employing the Nose-Hoover Thermostat, and then cooled to 700 K, over multiple steps at a cooling rate of 40-60 K/ps. Diffusion behavior in the crystalline and amorphous 2×2×2 supercells, are compared over three cell volumes, corresponding to lattice vectors of 23.41, 24.17 and 25.29 Bohr. Discususion mechanisms and bonding in both materials systems are then analyzed. Results and Discussion Comparison of local structure and diffusion pathways in the amorphous and crystalline phases, in particular the effect of covalent bonding and correlated Li+ motion, provide insight into the anomalous discussusion behavior in amophous lithium thiophosphates. The diffusion is analyzed by calculating the diffusion coeffcients, the activation energy barriers, jump statistics, and Li+ density plots. Maximally Localized Wannier Functions (MLWFs) are used to provide insight into the ionic/covalent interactions between the Li+ and sulfur atoms. The results presented will include the following: Li+ Mean Square Displacement (MSD) and diffusion coefficients, analysis of bonds using MLWFs, and a comparison of diffusion mechanisms, especially the effect of defects, between the crystalline and amorphous phases. These results and analyses illustrate how amorphous Li3PS4 has high Li+ diffusion, despite its disordered structure. The MLWFs centers provide insight into how the electronic structure effects correlated motion.

Understanding Li+ diffusion behavior in amorphous and crystalline Li3PS4 solid electrolytes, via ab-initio simulations

253rd National Meeting (2017)

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