Triboelectric nanogenerators (TENGs) effectively convert mechanical energy to electric power or signals. In the presented work, films of sulfated cellulose nanocrystals (CNC) in combination with additives, triethanolamine (TEOA), and methylimidazole (MI) were turned into a tribo-positive layer in combination with polyethylene terephthalate (PET) (tribo-negative layer) to develop CNC-PET TENG. The additives improved the bendability of the formed films. Under optimized additive concentrations, cellulose nanocrystals (CNC) film with 4 wt % TEOA and 4.5 wt % MI shows tensile strength of 68 MPa with 4.8% elongation at break values in comparison to 47 MPa strength and 1% elongation at break values for neat CNC films. The CNC-PET film-based vertical contact-separation TENG device shows output voltages of 80 and 123 V at frequencies of 4 and 8 Hz, respectively. The TENG based on modified CNC films was stable for 4000 contact-separation cycles. Unlike TENG based on CNF and other nanocellulose composites, in this work, CNC films consist of more than 90 wt % cellulose and show good electromechanical performance, which makes them promising candidates as substrates for flexible electronics with recyclability over earlier published reports on cellulose composite with synthetic additives described in the introduction section.