There is considerable interest in developing solid electrolytes for rechargeable lithium batteries as they have the potential to increase both energy d. due to incorporation of a lithium metal anode and safety of batteries due to the fact that they are nonflammable. Block copolymers with a mech. hard nonconducting block and a soft ion-conducting block provide an avenue for obtaining highly conducting rigid solids. The authors add surface-modified TiO2 nanoparticles to a mixt. of polystyrene-block-poly(ethylene oxide) and bis(trifluoromethane)sulfonimide lithium salt. The presence of BF4- moieties on the surface of the particles was essential for obtaining macroscopically homogeneous electrolytes; macrophase sepn. was obsd. with the same nanoparticles with surfaces covered with oleic acid. The stability of these composite electrolytes against lithium metal electrodes was tested in sym. lithium-composite electrolyte-lithium cells. The surprising result was that electrolytes with 24% nanoparticles exhibited optimum stability; the amt. of charge passed before dendrite formation obsd. in the optimized composite electrolyte was a factor of 4.7 larger than that of the neat block copolymer electrolyte. Both tensile and shear moduli of the electrolytes were non-monotonic functions of particle concn. with peaks in the vicinity of 17 to 20%.