Influence of binary lithium salts on 49% poly(methyl methacrylate) grafted natural rubber based solid polymer electrolytes

Abstract

Effect of binary lithium salts (lithium tetrafluoroborate, LiBF4 with lithium trifluoromethanesulfonate, LiCF3SO3) and (lithium tetrafluoroborate, LiBF4 with Lithium iodide, LiI) as charge carriers in solid polymer electrolyte based 49% poly(methyl methacrylate) grafted natural rubber (MG49) for Li-ion battery application has been investigated. The polymer electrolytes were prepared by solution casting technique. The effect of binary lithium salts on chemical interaction, structural, thermal studies, ionic conductivity and ion transference number of MG49 films are analyzed by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and electrochemical impedance spectroscopy (EIS). Infrared analysis indicated the interaction occurred between Li ions and oxygen atoms at the carbonyl group (CO) and the ether group (COC) on methyl methacrylate (MMA) segments. XRD studies exhibited a reduction of the MMA peak intensity at 29.5° after the addition of different ratios of binary Li salts due to the plasticizing effect of the salts. The larger anion size tends to create bigger free volume in the polymer electrolyte. In addition, this confirms that the degree of crystallinity of the electrolyte films is reduced leading to enhancement of ionic conductivity. DSC results revealed the highest conductivity sample has the lowest Tg implying the ions can flow with more ease throughout the polymer chain. The ratios of LiBF4:LiI presenting the higher overall performance in terms of ionic conductivity comparing to LiBF4:LiCF3SO3 ratios in MG49. The highest room temperature conductivity was obtained at 1.89 × 10−6 S cm−1 for (30:70) LiBF4:LiI percentages ratio. Moreover, tion is observed to increase with the ionic conductivities.