A Study of the Transport Properties of Ethylene Carbonate-Free Li Electrolytes

Logan, E. R.; Tonita, Erin M.; Gering, K. L.; Ma, Lin; Bauer, Michael K. G.; Li, Jing; Beaulieu, L. Y.; Dahn, J. R.

March 2018 - Recently it has been found that Li-ion cells with organic carbonate-based electrolytes that do not contain ethylene carbonate (EC) can perform exceptionally well at high voltage. This work explores the transport properties of low-EC and EC-free Li electrolytes with lithium hexafluorophosphate (LiPF6) as the conducting salt. Conductivity and viscosity were measured for electrolytes with solvent compositions EC:linear carbonate in a weight ratio of x:(100-x), where linear carbonate = {ethyl methyl carbonate (EMC), dimethyl carbonate (DMC)}, and x = {0, 10, 20, 30}. While EC-free electrolytes have lower viscosities, the maximum conductivities of these electrolytes are lower as well. Walden analysis was employed to understand the loss in conductivity as EC is removed from the electrolyte. Electrolyte properties calculated from a theoretical model, the Advanced Electrolyte Model (AEM) show excellent agreement with most of the experimental data. Differential thermal analysis (DTA) was used to investigate the phase diagram of the ternary EC:DMC:LiPF6 system. The addition of both EC and LiPF6 in the ranges studied lowered the liquidus transition temperature of the solution. Li[Ni0.4Mn0.4Co0.2]O2/graphite and single crystal Li[Ni0.5Mn0.3Co0.2]O2/graphite cells containing an EMC-based electrolyte showed good capacity retention and comparably low impedance growth when 5% w/w of fluoroethylene carbonate (FEC) was added.

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