Herein, we report on the 2-phenoxy-1,3,2-dioxaphospholane (PhEPi) molecule as a novel functional phospholane-based electrolyte additive. The presence of PhEPi in optimal concentration in the baseline electrolyte (EC:EMC 3:7, 1 M LiPF₆) significantly enhances the electrochemical performance of the NMC811||Si-graphite cell chemistry. To further optimize the cell's overall performance, two well-known film-forming additives, namely, vinylene carbonate (VC) and fluoroethylene carbonate (FEC), were used as co-additives with PhEPi, and the resulting cells thoroughly characterized by selected electrochemical and spectroscopic techniques. The addition of PhEPi to the VC-containing electrolytes showed the complementary advantages of high specific discharge capacity and prolonged cycle life of the considered cells and outperformed the overall performance of the FEC counterpart. To gain better understanding of the role of each additive on the individual positive and negative electrode interphases, we conducted a systematic investigation of both electrodes, which were pre-cycled against Li-metal electrode. Post mortem spectroscopic investigations of the corresponding interphases provided insights into the synergistic effect of VC and PhEPi in the NMC811||Si-graphite cell chemistry.