Effect of cycling on the lithium/electrolyte interface in organic electrolytes

The successful operation of ambient temperature secondary lithium cells is primarily dependent on the lithium/electrolyte interface properties. In this study, an attempt has been made to study the effect of cell cycling on the lithium/electrolyte interface by nondestructive methods such as a.c. impedance spectroscopy and microcalorimetry. Experimental Li-TiS₂ cells were constructed and activated with different electrolytes. The cells delivered 30 to 300 cycles at 100% depth-of-discharge depending on the electrolyte. The reactivity of both uncycled and cycled lithium towards various electrolytes was studied by measuring the heat evolved from the cells under open-circuit condition at 25 °C by microcalorimetry. Cycled cells at the end of charge/discharge showed considerably higher heat output compared with the uncycled cells. After thirty days of storage, the heat output of the cycled cells is similar to that of the uncycled cells. A.c. impedance analysis results indicate that the cell internal resistance increases with cycling, and this is attributed to the degradation of the electrolyte with cycling. The value R_f was found to decrease with cycling. The observed decrease in R_f is probably due to the increase in the surface area of the lithium anode due to cycling. The peak frequency was found to be in the range, 400 to 1000 Hz for both uncycled and cycled cells suggesting that the passivating film composition does not change significantly with cycling.