State-of-the-art implantable Ni-Cd rechargeable batteries used in ventricular assist systems (VAS) offer barely adequate energy density, and cycle life. Advances in rechargeable battery technology would greatly improve the day-to-day comfort of patients while simultaneously reducing the need for surgery to replace worn-out cells. While rechargeable lithium batteries excel with respect to energy density, the best currently available Li systems only afford 50% of the cycle-life of Ni/Cd batteries. In Phase I it was demonstrated in two different test vehicles that novel stable solvents afford greater cycle life and that when used in conjunction with a cathode design optimized for high cycle life, unprecedented cathode turnover numbers were achieved. Phase II research will center on (1) the completion of a custom solvent synthesis, the conditions of which were determined in Phase I, (2) the building and long-term cycling at physiological temperature of higher capacity cells utilizing the same high performance cathode materials of Phase I, and (3) with electrolyte cycling the very promising high voltage cathode, LixNiO2.