Electrochemical behavior of carbon black (CB) additives in 1 M LiPF6 EC/DEC (3:7 vol.) electrolyte in the operation potential range of positive electrodes in Li-ion cells was investigated. Various carbon blacks display noticeable electrochemical activity towards PF 6 − intercalation and electrolyte oxidation at potentials >4.0 V vs. Li/Li+. In situ X-ray diffraction, Raman spectroscopy, and ex situ scanning electron microscopy and Fourier Transform Infrared (FTIR) spectroscopy showed that repetitive intercalation/deintercalation of PF 6 − anions leads to structural alteration and degradation of carbon black nanoparticles. The mechanism of these interfacial processes on CB electrodes and their implications for electrochemical performance of high-voltage Li-ion electrodes during long-term cycling is evaluated.