Li+ Transport in Ca-Substituted β-Li3PS4: Role of PS43- Vibrations and Rotations from Machine-Learning Molecular...

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Sulfide solid electrolytes offer high Li+ conductivity for all-solid-state batteries.
• Calcium (Ca2+) substitution in β-Li3PS4 enhances Li+ conductivity, but mechanisms are unclear.

Purpose of the Study:

• To elucidate the microscopic mechanisms behind Ca2+-enhanced Li+ diffusion in Li3PS4.
• To develop a reliable computational model for studying Ca-substituted sulfide electrolytes.

Main Methods:

• Developed a system-specific machine learning potential (MLP) for Ca-substituted Li3-2xCaxPS4.
• Performed large-scale, long-time machine learning molecular dynamics (MLMD) simulations.
• Trained MLP on ab initio molecular dynamics (AIMD) data across various Ca2+ concentrations and temperatures.

Main Results:

• MLP accurately reproduced AIMD data, confirming its reliability for Ca-substituted Li3PS4.
• Ca2+ substitution induced anisotropic lattice changes and specific coordination environments.
• Li+ diffusion coefficient showed nonmonotonic dependence on Ca2+ concentration, peaking at x = 0.05.
• PS43- dynamics, including vibrational amplitudes and rotational motion, were crucial for Li+ transport.

Conclusions:

• Optimized Ca2+ substitution (around x=0.05) maximizes Li+ diffusivity and accessible volume.
• Excessive Ca2+ can hinder Li+ conduction by fragmenting the network.
• Tuning aliovalent substitution and PS43- dynamics offers a pathway to design advanced sulfide solid electrolytes for fast Li+ conduction.