RECHARGEABLE NA NAXCOO2 AND NA15PB4 NAXCOO2 POLYMER ELECTROLYTE CELLS

Cells using polyethylene oxide as a sodium ion conducting electrolyte, P2 phase NaxCoO2 as the positive electrode ans either sodium or sodiu m/lead alloy as the negative electrode were assembled, discharged, and cycled. NaxCoO2 intercalates sodium over a range of x = 0.3-0.9, givi ng theoretical energy densities of 1600 Wh/liter (for sodium) or 1470 Wh/liter (for sodium/lead alloy). Cells could be discharged at rates U P to 2.5 mA/cm2 Corresponding to 25% depth of discharge and typically were discharged and charged at 6.5 mA/cm2 (100% depth of discharge) or approximately 1-2 C rate. Over one hundred cycles to 60% utilization or more, and two hundred shallower cycles at this rate have been obtai ned in this laboratory. Experimental evidence suggests that the cathod e is the limiting factor in determining cycle life and not the Na/PEO interface as previously thought. Estimates of practical energy and pow er densities based on the cell performances and the following configur ation are presented: 30-45 w/o electroactive material in the positive electrode, a twofold excess of sodium, 10 mum separators, and 5 mum cu rrent collectors composed of metal coated plastic. On the basis of the se calculations, practical power densities of 335 W/liter for continuo us discharge at 0.5 mA/cm2 and up to 2.7 kW/liter for short periods of time should be attainable. This level of performance approaches or ex ceeds that seen for some lithium/polymer systems under consideration f or electric vehicle applications, but with a lower anticipated cost.