Electrochemical impedance study of graphite/electrolyte interface formed in LiBOB/PC electrolyte

Electrochemical impedance measurements were performed on graphitic anodes during their galvanostatic cycling in LiBOB/PC electrolyte, and the variation of each impedance component was studied as the function of the lithiation state. It was found that the charge-transfer component was closely related to the formation of the solid electrolyte interface (SEI), characterized by a sharp drop in the corresponding resistances between OCV and 0.60 V, while the semi-circle in the medium frequency range might arise from contact impedance components within the electrode, because it varies reversibly during the lithiation/delithiation cycle. The irreversible reduction process at 1.70 V, which is characteristic of LiBOB salt, was confirmed to originate from the reductive decomposition of oxalate ester impurities, and this process is believed to be unrelated to the protective SEI formed by LiBOB electrolytes on graphitic anode. When compared with the corresponding LiPF6-based electrolytes, it was concluded that the alkyl carbonates are not appropriate solvents for LiBOB salt, both in terms of the power and low temperature performances in lithium ion cells.