Optical microscopic observation of graphite composite negative electrodes in all-solid-state lithium batteries

Composite graphite negative electrodes were prepared by mixing graphite particles and 75Li2S·25P2S5 (mol%) glass particles with weight ratios of x:100 − x (x = 50, 60 and 70). The cell with the x = 50 electrode showed the highest reversible capacity of more than 250 mAh g−1. Optical microscopy was conducted for each composite electrode after electrochemical lithiation to investigate reaction distributions by color changes of graphite particles. In the x = 50 electrodes, almost all the graphite particles changed to gold color, suggesting that the graphite particles were fully lithiated. In situ optical microscopy was also carried out for the composite graphite electrode to monitor color changes of graphite particles directly during charge-discharge tests.