Influence of lithium bis(oxalato)borate salt on the structure, thermal properties and ionic conductivity of poly(ethylene oxide)

Abstract

Polymer electrolytes based on poly(ethylene oxide) (PEO) and a lithium bis(oxalato)borate salt (LiBOB) have shown their potential for applications as electrolytes for lithium-ion polymer batteries. However, there are not enough studies to clarify the direct impact of LiBOB on the properties of PEO in polymer electrolytes. Therefore, the aim of this study is to clarify the dependence of the addition of the LiBOB salt on the structure, thermal properties and ionic conductivity of PEO. Solid polymer electrolytes with different ether oxygen to lithium ions molar ratios (EO/Li) (PEX) were prepared. Fourier transform infrared spectroscopy (FTIR) analysis showed that the salt concentration has affected the structure of PEO, especially its crystallinity as one of the most important properties affecting the ionic conductivity of PEO-based polymer electrolytes. These results were confirmed by differential scanning calorimetry (DSC) analysis as the degree of crystallinity decreases with the addition of salt. Moreover, DSC analysis revealed the composition of the samples at which crystallinity of PEO completely disappeared. Scanning electron microscopy (SEM) showed the modifications in surface morphological features of PEX as a function of LiBOB concentration. The results obtained from these studies were correlated with the most important result for the application of polymer electrolyte, that of electrochemical impedance spectroscopy (EIS). It revealed the maximum ionic conductivity at room temperature as high as 1.68∙10^–4 S cm^⁻1 for sample with EO/Li molar ratio 5. The importance of this result lies in the promising increase of five orders of magnitude compared to the ionic conductivity of PEO and in reaching the practical applicable ionic conductivity. Furthermore, thermogravimetric analysis (TGA) showed that the addition of salt causes a more complex non-isothermal decomposition of the prepared samples compared to pure PEO.