Hiroaki Urushibata

Simulation of Cathode Dissolution and Shorting for Molten Carbonate Fuel Cells

Owing to the dissolution of the nickel oxide cathodes of molten carbonate fuel cells (MCFCs), the deposited nickel particles result in electronic shorting between the anode and the cathode, which is a major problem for the long-term operation of MCFCs. The correlation between the cathode design and the amount of nickel in the matrix was studied in order to predict the cell life by nickel shorting. It was found that the nickel deposition rate depends not on the total cathode surface area but on the electrolyte fill level in the cathode void volume. It can be assumed, therefore, that the shorting process is controlled by the dissolved nickel ion transfer in the matrix. Simulating the cell life by a theoretical analysis agreed well with experimental results.

NiO Dissolution in Molten Carbonate Fuel Cells: Effect on Performance and Life

The short-circuit phenomenon caused by dissolution of the NiO cathode in the molten carbonate fuel cell was experimentally investigated. Monitoring CO{sub 2} concentration in the anode exhaust gas can be an effective way to detect cell short circuit. The effects of matrix thickness and cathode CO{sub 2} partial pressure on shorting were elucidated. The time-to-initial-short-circuit (shorting time) is approximately proportional to the second power of the matrix thickness and the reciprocal of the cathode CO{sub 2} partial pressure. This can be explained by the relationship between conductance of the short circuit and the Ni content of the matrix. A simple model to correlate the conductance with the shorting time was developed. It is concluded that part of the deposited Ni exists as lithiated NiO.

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.