LiTDI as electrolyte salt for Li-ion batteries: transport properties in EC/DMC

Abstract

The 4,5-dicyano-2-trifluoromethyl-imidazolide (TDI−) anion is a potential lithium salt anion for lithium-ion batteries. In this paper the transport properties of LiTDI solutions in EC/DMC (50/50wt%) are compared to competing salts such as LiTFSI (lithium bis(trifluoromethylsulfonyl) imide), LiFAP (lithium tris(pentafluoroethane)-trifluorophosphate) and the most commonly used LiPF6 (lithium hexafluorophosphate). LiTDI in EC/DMC exhibits the lowest viscosity among the investigated electrolytes, but also the lowest conductivity: 6.84mScm−1 at 25°C (1molL−1). The self-diffusion coefficients (D(7Li) and D(19F)) determined by observing 7Li and 19F nuclei with the pulsed-gradient spin-echo (PGSE) NMR technique allowed access, through application of the Stokes-Einstein and Nernst-Einstein equations, to the lithium transference number (t+), the effective ionic radius and the ionic dissociation coefficient (αD) of LiTDI in EC/DMC. The ion-pair dissociation and t+ were also determined using conductivity and viscosity measurements and by following the Bruce-Vincent method respectively and were in good agreement with those obtained by PGSE-NMR. LiTDI displays in EC/DMC a cationic transference number t+=0.58, which is higher than that of LiPF6 in the same solvent mixture, meaning that the mobility of the PF6− anion is higher than the larger TDI− anion. Still, results from both methods indicate that, LiTDI, as many other lithium salts is only partially dissociated in EC/DMC. Only 31% of the ion pairs are dissociated at 25°C whereas for LiPF6, LiTFSI and LiFAP this number reaches at least 64% in 1molL−1 solutions.