Investigating the Multifunctional Role of Tris(trimethylsilyl)phosphite as an Electrolyte Additive via Operando Gas Chromatography/Mass Spectrometry and X-ray Photoelectron Spectroscopy

The multifunctional electrolyte additive tris(trimethylsilyl)phosphite (TMSP) is investigated with a combination of operando gas chromatography/mass spectrometry and X-ray photoelectron spectroscopy techniques, supported by cycling experiments and electrochemical impedance spectroscopy (EIS) measurements. Indications for hydrofluoric acid (HF) scavenging by TMSP could be found in the gas phase as well as on the electrode surfaces; however, it is observed that the use of TMSP leads to the production of HF, which it eventually scavenges. The investigation of the interphase formation shows that the decomposition products of TMSP are integrated into the interphases of both electrodes. This is accompanied by the formation of trimethylsilane as a decomposition product in the gas phase. TMSP also promotes the two-electron reduction of ethylene carbonate (EC), which is deduced both from an increased amount of ethene in the gas phase and from Li2CO3 on the electrode surface. The electrochemical investigations show that cells with TMSP have a lower interphase resistance after continued cycling. However, only the cells with 1 wt% of TMSP in the electrolyte outperform the TMSP-free reference cells. It is concluded that adding more than 1 wt% of TMSP increases the parasitic reactions of the additive to an extent that it partially counteracts its beneficial effect.