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 p olymer electrolyte (gel-SPE) was tried to be stabilized by preliminary volt age 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 config uration of Li/gel-SPE/Li, the cell impedance at 100 Hz was kept below 100 O mega for the cells with a gel-SPE containing LiBF4 over 230 days at 20 degr ees C. The cells with a gel-SPE containing LiPF6 required preliminary volta ge 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 impe dance 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 catho de (DMcT = 2,5-dimercapto-1,3,4-thiadiazole). The metallic lithium anode su rface before and after the voltage cycling was subjected to X-ray photoelec tron spectroscopy (XPS) analysis: Li2CO3 disappeared and LiF-LiOH remained in the surface layer after the voltage cycling for the LiBF4 gel-SPE cell w hile for the LiPF6 gel-SPE cell, a dense and thin LiF surface layer was bro ken into a thicker LiF-LiOH layer. The layer consisting of mainly LiF and L iOH which was formed by preliminary voltage cycling was considered to stabi lize the interface. (C) 1999 Elsevier Science S.A. All rights reserved.