To elucidate what properties control and practically limit ion transport in highly concentrated electrolytes (HCEs), the viscosity, ionic conductivity, ionicity, and transport numbers were studied for nine model electrolytes and connected to the rate capability in Li-ion battery (LIB) cells. The electrolytes employed the LiTFSI salt in three molar ratio concentrations; 1 : 2, 1 : 4, and 1 : 16 (LiTFSI:X) vs. solvents (X) with different permittivities; tert-butyl methyl ether (MTBE), tetrahydrofuran (THF) and propylene carbonate (PC). While the low polarity MTBE creates liquid electrolytes, ion-pairing limits the ionic conductivity despite extremely low viscosities. For the less concentrated 1 : 16 LiTFSI:MTBE and 1 : 16 LiTFSI:THF electrolytes the ionic diffusivities decrease with increased temperature, a sign of aggregation, but still their ionic conductivities and LIB performance increase. In general, the low ionic conductivity and high viscosity both limit the use of HCEs in LIBs, and no compensating mechanism seems to be present.