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°C immediately after assembling a cell having a configuration of Li/gel-SPE/Li, the cell impedance at 100 Hz was kept below 100 Ω for the cells with a gel-SPE containing LiBF4 over 230 days at 20°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°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 Ω. 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.