Optimization of internal standard selection for precise trace impurity quantification in Li2S via ICP-OES

Abstract

Lithium sulfide (Li₂S) is a critical precursor for sulfide-based solid electrolytes in next-generation all-solid-state batteries; accordingly, rigorous control and accurate quantification of trace-level impurities are essential. Accurate determination by inductively coupled plasma optical emission spectrometry (ICP-OES) is, however, severely complicated by matrix-induced signal suppression and enhancement in high-concentration Li₂S solutions. This study systematically evaluated the selection of internal standards (IS) to compensate for matrix effects and improve analytical precision in a 1% (w/v) Li₂S matrix. Five IS candidates, namely Sc, Y, Ru, Bi, and In, were screened on the basis of total excitation energy levels and assessed for matrix-correction efficiency across 16 target analytes. Method performance was validated through recovery, method detection limits (MDLs), and limits of quantification (LOQs). Without IS correction, matrix-induced signal depression reduced analyte recoveries to as low as 38.7%; implementation of an optimized IS scheme restored quantitative accuracy and markedly narrowed inter-replicate variability. Sc (361.383 nm) and Y (324.225 nm) demonstrated the broadest corrective efficacy, whereas Ru (240.272 nm) caused systematic overcorrection, up to 164% recovery for low-excitation-energy elements, due to its high total excitation energy (12.52 eV). IS-corrected MDLs were substantially lower than those obtained by EC. These findings establish total excitation energy matching as the governing criterion for IS selection in complex lithium-based matrices.