N-Boronated polybenzimidazole for composite electrolyte design of highly ion conducting pseudo solid-state ion gel electrolytes with a high Li-transference number

Abstract

We prepared highly ion conducting polymer electrolytes with a high lithium transference number (t_Li⁺) using organoborane-modified polybenzimidazole (B-PBI) and an ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide (BMImTFSI). The structural characteristics of these electrolytes were examined by nuclear magnetic resonance and Fourier-transform infrared spectroscopy. Thermo-gravimetric analysis and dynamic viscosity measurements were used to investigate the thermal and rheological properties of the electrolytes. Unlike other solid polymer electrolyte systems, the addition of lithium salts was not required in the case of the electrolytes prepared in this study as the molecular designing already incorporates lithium. The amount of BMImTFSI (w/w%) affected the ion transport behaviour of the composite polymer electrolytes. The ionic conductivity of the electrolytes increased with an increase in the IL content. The electrolyte with B-PBI/BMImTFSI (w/w%) (25/75) showed the highest conductivity of 8.8 × 10⁻³ S cm⁻¹ at 51 °C. It also showed the highest t_Li⁺ value of 0.63 at room temperature with a wider electrochemical potential window of 5.45 V. The composite electrolytes were employed in anodic half-cells (Si/electrolyte/Li), and they showed a high reversible capacity up to 1300 mA h g⁻¹ which was retained even at a high charging rate. Electrochemical impedance spectroscopy and dynamic electrochemical impedance spectroscopy were performed before and after the charge–discharge measurements to confirm the formation of a stable solid electrolyte interface with B-PBI/BMImTFSI (w/w%).