Direct evidence on anomalous expansion of graphite-negative electrodes on first charge by dilatometry

Dilatometer was fabricated and applied to measure change in electrode thickness of the graphite-negative electrode for lithium-ion batteries. Anomalous expansion was observed in the initial part of the first reduction, i.e. so-called irreversible capacity region in voltages of 1.1–0.5 V versus Li. During further reduction to 0 V versus Li, the thickness of electrode increased monotonously as the lithium-intercalation reaction proceeded. The observed change in electrode thickness was larger than that of 10% expected from XRD data, due mainly to the expansion at the initial part of the first reduction. One-to-one correspondence between the electrochemical and dilatometric data was observed. The acoustic events occurring inside the Li/graphite cells were also measured. The effect of electrolyte on the anomalous expansion was also examined by dilatometry in relation to the irreversible capacity for the first reduction of graphite, and the significance of introducing the dilatometry together with acoustic emission technique to insertion materials research for advanced lithium-ion batteries was discussed.