Side reactions of the charge/discharge in Li-ion batteries (LIBs) generate a solid-electrolyte interface (SEI) onto an electrode surface, resulting in the degradation of the lifetime of a cell. The suppression of SEI formations has attracted much attention for achieving longer cyclable LIBs. Our research group has previously reported that few SEI were observed at triple-phase interfaces (TPIs) consisting of BaTiO3, LiCoO2, and electrolyte interfaces in LIBs with excellent cyclability and ultrahigh-speed chargeability. An investigation on the suppression mechanisms of SEI formations at TPIs should yield important information on understanding the undesirable side reactions. Therefore, we have explored the suppression mechanisms of SEI formations by preparing epitaxial thin films and evaluating the surface of the samples after the electrochemical treatment. The results of X-ray photoelectron spectroscopy and scanning electron microscopy with energy-dispersive X-ray analysis measurements suggested that the decomposition of LiPF6 was suppressed at TPIs, implying that the generation of PF5 via the decomposition of LiPF6 contributed to SEI formation.