Rechargeable Na/Na_xCoO₂ and Na₁₅Pb₄/Na x CoO₂ Polymer Electrolyte Cells

Cells using polyethylene oxide as a sodium ion conducting electrolyte, P2 phase Na_xCoO₂ as the positive electrode and either sodium or sodium/lead alloy as the negative electrode were assembled, discharged, and cycled. Na_xCoO₂ intercalates sodium over a range of x = 0.3-0.9, giving theoretical energy densities of 1600 Wh/liter (for sodium) or 1470 Wh/liter (for sodium/lead alloy). Cells could be discharged at rates up to 2.5 mA/cm² corresponding to 25% depth of discharge and typically were discharged and charged at 0.5 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 obtained in this laboratory. Experimental evidence suggests that the cathode is the limiting factor in determining cycle life and not the Na/PEO interface as previously thought. Estimates of practical energy and power densities based on the cell performance and the following configuration are presented: 30–45 w/o electroactive material in the positive electrode, a twofold excess of sodium, 10 μm separators, and 5 μm current collectors composed of metal coated plastic. On the basis of these calculations, practical power densities of 335 W/liter for continuous 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 exceeds that seen for some lithium/polymer systems under consideration for electric vehicle applications, but with a lower anticipated cost