Structural, electronic, and thermodynamic properties of Li3<i>X</i> (<i>X</i> = N, P, As) compounds for solid-state lithium-ion batteries

Abstract

This study investigates the structural, electronic, mechanical, and thermodynamic properties of Li3X compounds (where X represents nitrogen, phosphorus, and arsenic) using first-principles calculations based on density functional theory. Calculations were performed using the quantum espresso package within the generalized gradient approximation framework with DFT-D3 dispersion correction to account for van der Waals interactions. Results show systematic trends in the hexagonal crystal structure, with lattice parameters increasing and bulk modulus values decreasing significantly from Li3N to Li3P to Li3As. Elastic constant analysis reveals decreasing values across all constants, with C13 showing the most dramatic reduction, indicating reduced mechanical coupling between axes. The B/G ratios (1.51, 1.17, and 1.17 for Li3N, Li3P, and Li3As, respectively) classify all three compounds as brittle materials according to Pugh's criterion. Electronic structure analysis demonstrates a transition from predominantly ionic bonding in Li3N to more covalent character in Li3P and Li3As, manifested in decreasing bandgaps (1.1 eV to 0.7 eV to 0.65 eV) and increased orbital hybridization. Thermodynamic properties studied through the quasi-harmonic approximation show differences in vibrational energy, entropy (with Li3As exhibiting the highest values), and isochoric heat capacity (approaching the Dulong–Petit limit at high temperatures). This study provides crucial insights into the relationship between atomic structure and physical properties of these compounds, thereby highlighting their potential in energy storage and thermoelectric applications.