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The stability of uncoated copper (Cu) foils and graphite-coated copper (Cu-C) foils in lithium-ion battery electrolytes were extensively studied in this dissertation. At first, the electrochemical behavior and stability of the Cu foils and Cu-C foils were studied. Cyclic voltammetry was used to study the redox behavior of the foils in the electrolyte solutions. The reduction of electrolyte and its effect on the oxidation of copper was also studied. Bulk electrolysis was used to quantitatively study the dissolution of the foils in dry electrolytes and in electrolytes doped with impurities of H2O or HF. It was found that the graphite coating greatly influenced the redox behavior of the copper substrate and provided some protection to the copper from oxidation. Impurities increased the oxidation tendency of both Cu foils and Cu-C foils and may influence the integrity of the Cu-C foil electrode. During these studies, the open-circuit voltage (OCV) of Cu foil and Cu-C foil electrodes in Li-ion battery electrolytes was found to be a variable value over time. A detailed study showed that the OCV first rapidly decreased until reaching a minimum, and then gradually increased until reaching a meta-steady or steady state. These results were compared with OCV studies of Al foil, Pt wire, glassy carbon and Cu disk and wire electrodes. The OCV variation appeared to correlate to a surface change on the electrode after being immersed into the electrolyte solutions. The influence of aging of the reference electrode, the surface condition and edge effect of the copper foil, and solution impurities on the stability of the OCV was also studied. Atomic absorption spectroscopy (AAS) was used to quantitatively evaluate the stability of Cu and Cu-C foils in lithium-ion battery electrolytes at open-circuit. Results showed that the stability of Cu and Cu-C foils was different in “fresh” electrolytes whereas it was similar in “aged” electrolytes. For Cu foils, in the “fresh” electrolyte, the copper foils showed a small amount of dissolution (up to ~ 50 ppm) during their storage in electrolyte for up to 20 weeks. On the contrary, only trace amounts (less than 1 ppm) of dissolution of Cu-C foils were found in the same “fresh” electrolyte. However, in “aged” electrolyte, large amounts of copper dissolution (up to several hundred ppm) were found in both Cu foils and Cu-C foils. The study showed that the condition of the electrolytes was critical to the stability of the copper foils. Finally, FTIR reflectance and IR microscopy surface analyses of these foils after storage in electrolyte solution were performed to further study the causative factor of the copper dissolution in “aged” electrolytes. It was found that a carboxylic copper salt (ROCO2Cu) and other possible copper salts existed in the composition of the surface films of both foils. |
