Direct internal pressure measurements of the cylindrical Li-ion cells with a mixture of LiCoO2 and Li1.167Ni0.233Co0.1Mn0.467Mo0.033O2 (a solid solution between 0.4 Li2Mn0.8Ni0.1Mo0.1O3 and 0.6 LiNi0.4Co0.2Mn0.4O2) as cathode and graphite as anode have been performed during cell charging. Cell internal pressure at the end of charging is greatly reduced from 2.85 to 0.84–1.84 bar by adding a small amount of phosphorus derivatives such as triphenyl phosphine (TPP), ethyl diphenylphosphinite (EDP), and triethyl phosphite (TEP) into a carbonate-based electrolyte. The phosphorus derivatives are supposed to react with O2 generated from the decomposition of the Li2MnO3 component. The chemical states of additive molecules before and after the charging process have been characterized with a nuclear magnetic resonance (NMR) spectroscopy and gas chromatography–mass spectrometry (GC–MS). It has also been shown that those additives improve the cycle life when applied in coin full cells.