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 LiPF 6 (lithium hexafluorophosphate). LiTDI in EC/DMC exhibits the lowest viscosity among the investigated electrolytes, but also the lowest conductivity: 6.84 mS cm −1 at 25 °C (1 mol L −1 ). The self-diffusion coefficients ( D ( 7 Li ) and D ( 19 F )) determined by observing 7 Li and 19 F 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 (α D ) 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 LiPF 6 in the same solvent mixture, meaning that the mobility of the PF 6 − 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 LiPF 6 , LiTFSI and LiFAP this number reaches at least 64% in 1 mol L −1 solutions.