A composite polymer electrolyte based on polyethylene oxide (PEO) with a hyperbranched polymer poly[bis(triethylene glycol)benzoate] capped with an acetyl group (HBP) and a ceramic filler, BaTiO3, was examined as the electrolyte in rechargeable lithium polymer batteries. The conductivity of the composite polymer electrolyte PEO-10 wt.% HBP with Li(CF3SO2)(2)N-10 wt.% LiPF6 as a lithium salt and 10 wt.% BaTiO3 was found to be 1.6 x 10(-4) S/cm at 25 degreesC and 1.5 x 10(-3) S/cm at 60 degreesC in a O/Li ratio of 10. The lithium rechargeable batteries consisted of this highly conductive composite polymer electrolyte and the 4 V class cathode, LiNi0.8CO0.2O2, showed excellent charge-discharge cycling performance. The initial cathode discharge capacity of 154 mA h/g declined only 0.1%/cycle during the first 30 cycles at 60 degreesC. (C) 2003 Elsevier Science B.V All rights reserved.