Depth profiling of graphite electrode in lithium ion battery using glow discharge optical emission spectroscopy with small quantities of hydrogen or oxygen addition to argon

摘要

Depth profiling and quantification using glow discharge optical emission spectroscopy (GD-OES) were applied to a graphite electrode in a lithium ion battery. To improve the measurement time and reliability beyond conventional argon discharge plasma, reactive sputtering with the respective addition of oxygen (0.50% v/v O_2 in Ar) and hydrogen (1.00% v/v H_2 in Ar) was investigated. Samples contained dispersed 0-5 wt% LiF or 0-0.5 wt% Li_3PO_4 in graphite electrodes. Adding oxygen to argon plasma increased the sputtering rate and the sensitivity in quantitative analysis of lithium drastically. That unexpected depth profile was obtained for graphite electrode samples possibly because chemical etching by oxygen was inhomogeneous. In contrast, adding hydrogen to argon plasma exhibited benefits both for depth profiling and for quantifying Li for graphite electrode samples with a shorter measurement time and higher sensitivity than that of conventional pure argon discharge. Molecular spectra showed strong C-H and C-C bands, suggesting that formation of volatile material fragments of CH and CC increased with hydrogen addition during measurements. Surface analysis results with SEM and XPS showed that redeposition of sputtered materials and Ar~+ ion implantation that occurred in pure argon plasma were also suppressed.