Makiko Kise

Alternating Current Impedance Behavior and Overcharge Tolerance of Lithium-Ion Batteries Using Positive Temperature Coefficient Cathodes

To improve the safety of lithium-ion battery, a new conceptual cathode, which contains a positive temperature coefficient (PTC) compound consisting of a carbon black/polyethylene composite as the conductive material, was fabricated. Cells that incorporated PTC cathodes not had only good discharge characteristics but also high safety performance. To investigate the safety mechanism of PTC cathodes, alternating current (ac) impedance spectra were measured and analyzed. Based on the results of fitting, the resistance of a PTC cell which corresponds to ohmic resistance increased several fold and the resistance which corresponds to charge transfer resistance increased more than one order of magnitude at 140°C, because the electrical resistance of PTC cathodes increased at high temperature. Moreover, an overcharge test was performed for laminated prismatic PTC cells under a charge rate of 1.5 C to 10 V. The cell temperature did not increase after the short circuit, because the cell voltage reached the set voltage early and the short-circuit current barely flowed due to an increase in the cell impedance. These results indicate that cells which incorporate PTC cathodes are safer than conventional cells.

Effect of the Addition of Conductive Material to Positive Temperature Coefficient Cathodes of Lithium-Ion Batteries

A conceptual positive temperature coefficient (PTC) cathode has been proposed to improve the safety of large-scale lithium-ion batteries. The PTC cathode contains the PTC compound as the conductive material, which increases its resistivity at temperatures above its melting point. In this paper, to improve the performance of cells using PTC cathodes, acetylene black (AB), which supports conductivity in the cathode as a secondary conductive material, was added. Cells using PTC cathodes containing a small amount of AB (PTC-AB cell) had better discharge characteristics and a longer cycle life than a cell using a PTC cathode without AB (PTC cell). For a basic evaluation of battery safety, an external short-circuit test and a discharge test were performed at a temperature higher than the melting point of the PTC compound. The short-circuit current of the PTC-AB cells was lower than 1 A at 140°C, which was almost the same as the current of the PTC cell. Moreover, under a discharge rate of 3C, the voltage of PTC-AB cells dropped sharply at 135°C due to a drastic increase in PTC cathode resistivity. These results indicate that the addition of AB to PTC cathodes improves cell performance while maintaining battery safety.