Simulation results are presented for a lithium negative electrode, solid polymer separator, manganese dioxide composite positive electrode cell. In particular, we assess the effect of conductivity and transference number on cell performance, establishing criteria for situations when a trade-off between these two properties is expected. We show that polymer-electrolyte systems with a unity transference number have improved performance over systems with t+0 = 0.2, even when the conductivity is decreased by an order of magnitude. The improvements are primarily at higher rates of discharge, where the latter cell would be depleted of electrolyte due to large concentration gradients that develop. Cells with a unity transference number have larger energy densities and can attain higher peak-power densities.