The electrolyte plays an important role in governing the high-current performance of Li-ion batteries. Normally, battery electrolytes are optimized for maximum conductivity. In order to gain a more profound understanding of the role of the electrolyte, properties such as the Li salt diffusion coefficient, the Li+ transference number, and the Li salt activity all need to be measured in addition to the conductivity. The situation is further complicated by the fact that high currents change the cell temperature and also create strong concentration gradients in the electrolyte. A full set of transport properties for LiPF6 in a propylene carbonate/ ethylene carbonate/dimethyl/carbonate mixture were measured as a function of temperature and LiPF6 concentration. The Li+ transference number was found to be fairly constant with concentration. The activity and diffusion coefficient were both found to vary strongly with temperature and concentration. The temperature dependence of the transport properties is shown to be crucial for making predictions of cell performance at high currents. (c) 2005 The Electrochemical Society.