The dependence on Na/P(EO)(n)NaX/NaxMnO2 (P(EO)=poly(ethylene oxide), X=CF3SO3 or (CF3SO2)(2)N)) cell cycle life and rate capability on polymer electrolyte composition is described. Transition time experiments and mathematical modeling indicate that failure due to salt precipitation occurs at it(1/2)=10.5 to 21.4 mA s(0.5)/cm(2), when high initial concentrations of NaCF3SO3 are used in operating cells. Evidence for large ionic clusters in concentrated PEO/NaCF3SO3 solutions is also seen in the Raman spectroscopic data. Salt precipitation is a direct consequence of the concentration gradients that arise during operation, due to the negative cationic transference numbers (t(t)(0)) of the binary saltipolymer electrolyte. By decreasing the initial salt concentration, t(+)(0) is increased, cell rate capability is doubled, and the cycle life is enhanced nearly threefold. Similar improvements are obtained when PEO/NaN(CF3SO2)(2) electrolytes are used.