LiF Formation and Cathode Swelling in the Li/CFx Battery

Abstract

The Li/CFx cell has been traditionally used for low rate applications at discharge rates of C/1000 or lower. The development of cells capable of operating at discharge rates closer to C/10 is underway for communication applications. The Li/CFx cell is known to produce a significant amount of thermal power during high rate discharge and cathode swelling that can result in mechanical deformation of the cell. Previously, we characterized the thermal power during discharge in both coin cells and large format cells and proposed that the discharge is a two phase reaction with the direct formation of LiF. In this paper we measure the mechanical cathode swelling as a function of discharge depth, rate, and temperature. Discharged cathodes are then characterized using X-ray diffraction, scanning electron microscopy, and N2 adsorption surface area measurements. These results support a mechanism where the discharge product is LiF deposited on the internal surfaces of the carbon layers left behind after electrochemical reduction with this deposition leading directly to the measured cathode swelling. The recrystallization of LiF observed by X-ray diffraction over a period of days results in a long time constant thermal power that can be measured after discharge in large format cells.