Synergistic effects of binary lithium salts on ion transport and dielectric relaxation in poly(methyl methacrylate) grafted natural rubber solid polymer electrolytes

Abstract

A series of solid polymer electrolyte films were prepared using a casting solution with a polymer matrix comprising 49% poly(methyl methacrylate)-grafted natural rubber (MG49). These films incorporated binary lithium salts: lithium tetrafluoroborate (LiBF₄) combined with either lithium trifluoromethanesulfonate (LiTf) or lithium iodide (LiI). These films' dielectric properties and ion association behavior were examined using potentiostatic electrochemical impedance spectroscopy (EIS) and Fourier transform infrared (FTIR) deconvolution. The key findings demonstrated that the increase in both the dielectric constant (ɛ_r) and dielectric loss (ɛ_i) was significantly correlated with enhanced ionic conductivity, reaching a value of 1.89 × 10⁻⁶ S cm⁻¹, which was attributed to enhanced ionic and segmental mobility. A peak observed in the M_i versus frequency plot confirmed the ionic conductor behavior. The (30:70) ratio of LiBF₄ to LiI exhibited the highest performance, with superior ionic conductivity, dielectric behavior, tangent loss, number of charge carriers, mobility, and diffusion coefficient, surpassing the performance of the single salt or LiBF₄ to LiTf. This indicates that the combination of LiBF₄ and LiI is particularly effective for applications requiring improved dielectric properties.