Optimal Composition of Li Argyrodite with Harmonious Conductivity and Chemical/Electrochemical Stability: Fine-Tuned Via Tandem Particle Swarm Optimization.

A tandem (two-step) particle swarm optimization (PSO) algorithm is implemented in the argyrodite-based multidimensional composition space for the discovery of an optimal argyrodite composition, i.e., with the highest ionic conductivity (7.78 mS cm -1 ). To enhance the industrial adaptability, an elaborate pellet preparation procedure is not used. The optimal composition (Li 5.5 PS 4.5 Cl 0.89 Br 0.61 ) is fine-tuned to enhance its practical viability by incorporating oxygen in a stepwise manner. The final composition (Li 5.5 PS 4.23 O 0.27 Cl 0.89 Br 0.61 ), which exhibits an ionic conductivity (σ ion ) of 6.70 mS cm -1 and an activation barrier of 0.27 eV, is further characterized by analyzing both its moisture and electrochemical stability. Relative to the other compositions, the exposure of Li 5.5 PS 4.23 O 0.27 Cl 0.89 Br 0.61 to a humid atmosphere results in the least amount of H 2 S released and a negligible change in structure. The improvement in the interfacial stability between the Li(Ni 0.9 Co 0.05 Mn 0.05 )O 2 cathode and Li 5.5 PS 4.23 O 0.27 Cl 0.89 Br 0.61 also results in greater specific capacity during fast charge/discharge. The structural and chemical features of Li 5.5 PS 4.5 Cl 0.89 Br 0.61 and Li 5.5 PS 4.23 O 0.27 Cl 0.89 Br 0.61 argyrodites are characterized using synchrotron X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. This work presents a novel argyrodite composition with favorably balanced properties while providing broad insights into material discovery methodologies with applications for battery development.