We introduce 2-(triphenylphosphoranylidene) succinic anhydride (TPSA) as a lithium (Li) metal anode surface-stabilizing electrolyte additive for improving the cycle performance of Li metal secondary batteries. We verified that TPSA is readily decomposed on Li metal anodes before the decomposition of liquid electrolytes, and forms stable surface layers, i.e., a solid electrolyte interface (SEI). Consequently, TPSA was found to improve the cycle performance of unit cells consisting of LiCoO2/Li metal. The unit cells containing 3 wt.% of TPSA retain 85% of the initial discharge capacity after 100 cycles, whereas the unit cells that do not contain TPSA showed catastrophic failure after only 50 cycles. Each unit cell was operated at C/2 rate (0.8 mA cm−2) in the voltage range 3.0–4.2 V vs. Li/Li+. The TPSA-derived SEI suppresses continuous electrolyte decomposition, and thereby Li dendrite formation as well. The effect of TPSA was characterized by scanning electron microscopy, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy.