No abstract

To improve the safety of lithium-ion battery, a new conceptual cathode, which contains a positive temperature coefficient ͑PTC͒ 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 ͑ac͒ 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.

A conceptual positive temperature coefficient ͑PTC͒ 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 ͑AB͒, 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 cell͒ had better discharge characteristics and a longer cycle life than a cell using a PTC cathode without AB ͑PTC cell͒. 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.Lithium-ion batteries ͑LIBs͒ are widely used as the power source for a variety of portable electronic devices, such as cellular phones and mobile personal computers. These batteries are expected to be the main power storage devices for hybrid electric vehicles and fuel cell vehicles. [1][2][3][4][5] For LIBs to be used as such power storage devices, greater battery safety is required. To improve LIB safety, we proposed a new concept cathode, as shown in Fig. 1. 6 The cathode contains a positive temperature coefficient ͑PTC͒ compound of a carbon black/ polyethylene composite, the resistivity of which increases nonlinearly at 130-140°C, since the polyethylene expands due to the phase change at the melting point. This cuts off the conductive network of the carbon black. During the discharge test, the voltage of the PTC-12 wt % cell dropped at 138°C. This result indicates that the electrode reaction was aborted by the increase in cathode resistivity above the melting point of polyethylene. 7 However, the discharge characteristics of PTC cells were not good under high discharge rates because the contact points between the PTC compound and active material were insufficient due to the large particle size of the PTC compound. Consequently, current collection in the PTC cathode deteriorated and the cathode resistivity at room temperature became high.In this study, to improve current collection, we added acetylene black ͑AB͒ particles to the PTC cathodes as a secondary conductive material, and evaluated the resistivity of cathodes, cell performance, and battery safety of PTC and PTC-AB cells. Particles of AB are smaller than those of PTC compound, which leads to an expected increase in contact points and current collection. ExperimentalPTC and PTC-AB cathodes.-High density polyethylene resin ͑melting point 136°C͒ and...

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