Developing stable, low impedance interface between metallic lithium anode and polyacrylonitrile-based polymer gel electrolyte by preliminary voltage cycling

The metallic lithium anode surface facing to a gel-like polyacrylonitrile polymer electrolyte (gel-SPE) was tried to be stabilized by preliminary voltage cycling. Through voltage cycling from +0.5 to -0.5 V at a scan rate of 10 mV/s at 20 degrees C immediately after assembling a cell having a configuration of Li/gel-SPE/Li, the cell impedance at 100 Hz was kept below 100 Omega for the cells with a gel-SPE containing LiBF4 over 230 days at 20 degrees C. The cells with a gel-SPE containing LiPF6 required preliminary voltage cycling at a voltage higher than 0.5 V and at an elevated temperature of 60 degrees C to obtain and keep a lower impedance during storage. The impedance at 10 kHz which represented the bulk resistance of gel-SPE was almost the same for both cells and constant during storage, around 15 Omega. This was confirmed in the cell of Li/gel-SPE/DMcT + polyaniline composite cathode (DMcT = 2,5-dimercapto-1,3,4-thiadiazole). The metallic lithium anode surface before and after the voltage cycling was subjected to X-ray photoelectron spectroscopy (XPS) analysis: Li2CO3 disappeared and LiF-LiOH remained in the surface layer after the voltage cycling for the LiBF4 gel-SPE cell while for the LiPF6 gel-SPE cell, a dense and thin LiF surface layer was broken into a thicker LiF-LiOH layer. The layer consisting of mainly LiF and LiOH which was formed by preliminary voltage cycling was considered to stabilize the interface. (C) 1999 Elsevier Science S.A. All rights reserved.