Development of PTC Functional Cathode for Lithium Rechargeable Batteries

A new concept cathode is proposed to improve the safety of lithium rechargeable batteries. The cathode contains the Positive Temperature Coefficient (PTC) compound that can drastically increase resistivity at more than a specified temperature. A carbon black/polyethylene composite was selected for the PTC compound, because its resistivity increases at the melting point of polyethylene, since it expands and cuts off the conductive network of the carbon black. The PTC cathode containing the PTC compound was fabricated and its temperature dependence of resistivity was evaluated. The resistivity of PTC cathodes increases by several tens of times at 130-140 degreesC, which is the melting point of polyethylene. In lithium rechargeable batteries using this cathode, a level nearly reaching the rated capacity is obtained at 1 C discharge current. The discharge properties of cells using PTC cathode depend on the porosity, the heat treatment, and the PTC compound ratio of the PTC cathodes. Moreover, the cell voltage using the PTC cathode dropped at 138 degreesC under 3 C discharge condition, the same point at which the resistivity of the PTC cathode drastically increases. These results suggest that the PTC cathodes will improve the safety of lithium rechargeable batteries.