Utilizing an ionic liquid for synthesizing a soft matter polymer “gel” electrolyte for high rate capability lithium-ion batteries

Abstract

A cross-linked polymer “gel” electrolyte obtained from free radical polymerization of a vinyl monomer (acrylonitrile; AN) in a room temperature ionic liquid electrolyte (N,N-methyl butyl pyrrolidinium-bis(trifluoromethanesulphonyl)imide-lithium bis(trifluoromethanesulphonyl)imide; LiTFSI-[Py_1,4-TFSI]) for application in high rate capability rechargeable lithium-ion batteries is discussed here. This is a novel alternative compared to the often employed approach of using a molecular liquid as the medium for performing the polymerization reaction. The polymer “gel” electrolytes (AN:Py_1,4-TFSI = 0.16–0.18, w/w) showed remarkable compliable mechanical strength and higher thermal stability compared to LiTFSI-[Py_1,4-TFSI]. Despite two orders increase in magnitude of viscosity of polymer “gels”, the room temperature ionic conductivity of the “gels” (1.1 × 10⁻³–1.7 × 10⁻³ Ω⁻¹ cm⁻¹) were nearly identical to that of the ionic liquid (1.8 × 10⁻³ Ω⁻¹ cm⁻¹). The present “gel” electrolytes did not exhibit any ageing effects on ionic conductivity similar to the conventional polymer gel electrolytes (e.g. high molecular weight polymer + salt + high dielectric constant molecular solvent). The disorder (ionic liquid) to a relative order (cross-linked polymer electrolyte) transformation does not at all influence the concentration of conducting species. The polymer framework is still able to provide efficient pathways for fast ion transport. Unlike the ionic liquid which is impossible to assemble without a conventional separator in a cell, the polymer “gel” electrolyte could be conveniently assembled without a separator in a Li|lithium iron phosphate (LiFePO₄) cell. Compared to the ionic liquid, the “gel” electrolyte showed exceptional cyclability and rate capability (current density: 35–760 mA g⁻¹ with LiFePO₄ electronically wired with carbon (amorphous or multiwalled nanotube [MWCNT]).