Tris(trimethylsilyl) phosphite (TMSP) is well known as an effective electrolyte additive that significantly improves the electrochemical performance of high-voltage lithium-ion batteries. This work reveals that TMSP is oxidized more readily than an electrolyte solvent, is more difficult to be reduced, and exhibits high reactivity with HF. The significant role of TMSP in HF removal is demonstrated by full-cell experiments. We suggest a mechanism through which TMSP removes HF molecules from the electrolyte, forms a cathode–electrolyte interphase with outstanding properties on the cathode surface while not undergoing a side reaction at the anode surface. In addition, the fact that TMSP can be oxidized more readily than vinylene carbonate is what results in the synergistic effect observed when the two additives are used together.