Ionic conductivity, viscosity, and self-diffusion coefficients of novel imidazole salts for lithium-ion battery electrolytes

Abstract

Lithium-ion battery performance and longevity depend critically on the conducting salt utilized in the electrolyte. With new avenues for multifunctional integration and optimization of functional properties, conducting salts beyond lithium hexafluorophosphate (LiPF$_6$) need to be studied. Herein we elucidate on viscosity, ionicity, anion self-diffusion and ionic conductivity through variation of the length of the perfluoroalkyl side chain present in the anions of the used lithium imidazole salts. Specifically, we study LiPF$_6$ in comparison with lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium 4,5-dicyano-2-(pentafluoroethyl)imidazolide (LiPDI), and lithium 4,5-dicyano-2-(n-heptafluoropropyl)imidazolide (LiHDI). We find that the ion mobility of LiPF$_6$ depends the least on viscosity and its ionicity is the highest among the electrolytes investigated here. LiTDI shows the strongest correlation between ion mobility and viscosity and the lowest ionicity. LiPDI and LiHDI range between these two regarding their ionicity and the correlation of mobility with viscosity. The previously rarely studied anion self-diffusion coefficients exhibit a strong correlation with viscosity as it was to be expected. Differences between the LiTDI, LiPDI and LiHDI salts are minute