Electrolytes in modern lithium ion batteries (LIBs) mostly consist of a mixture of a linear and a cyclic organic carbonate, a conducting salt and several electrolyte additives. The latter fulfill a variety of different roles in LIBs. They are for example used to form a more stable SEI on the anode, they can act as flame retardant or redox shuttle to improve the safety of the battery, they can be used as sacrificial components which are oxidized at high cell voltages before the regular electrolyte and they can be applied to scavenge molecules which are detrimental to performance of LIBs, such as HF.(1)

Tristrimethylsilylphosphite (TMSPi) and Tristrimethylsilylphosphate (TMSPa) have gained increasing interest as electrolyte additives in recent years. Both are known to improve the performance of LIBs and it was shown that both act as HF scavengers.(2) However, also other roles such as their oxidation and formation of a protective interphase on the cathode (3) or the complexation of transition metals on the surface of the cathode (4) have been described.

Online electrochemical mass spectrometry (OEMS) was previously shown to be a valuable tool to monitor the gas evolution in LIBs. It allows to study processes in the electrolyte such as solvent decomposition processes (5) or the hydrolysis of the conducting salt,(6) as well gas release resulting from structural changes in the active material as shown for the surface reconstruction of Ni-rich NCM cathodes(7).

In this contribution the effect of TMSPa and TMSPi on the gassing of HENCM/graphite full cells as well as cells with carbon model electrodes is presented. According to our results, the formation of a protective surface layer or complexation of surface transition metal centers are secondary effects in these systems, we rather show that these TMS based compounds act mainly as scavenging agents for harmful reaction intermediates such as HF and LiF. This leads to increased electrolyte stability and improved cycling performance.

(1) A. M. Haregewoin, A. S. Wotango, B.-J. Hwang, Energ. Environ. Sci. 9, 1955-1988 (2016).

(2) J.-G. Han, S. J. Lee, J. Lee, J.-S. Kim, K. T. Lee, N.-S. Choi, ACS Appl. Mater. Interfaces 7, 8319−8329 (2015).

(3) S. Mai, M. Xu, X. Liao, J. Hu, H. Lin, L. Xing, Y. Liao, X. Li, W. Li, Electrochim. Acta 147, 565-571 (2014).

(4) C. Peebles, R. Sahore, J. A. Gilbert, J. C. Garcia, A. Tornheim, J. Bareño, H. Iddir, C. Liao, D. P. Abraham, J. Electrochem. Soc. 164, A1579-A1586 (2017).

(5) D. Streich, A. Guéguen, M. Mendez, F. Chesneau, P. Novák, E. J. Berg, J. Electrochem. Soc. 163, A964-A970 (2016).

(6) A. Guéguen, D. Streich, M. He, M. Mendez, F. F. Chesneau, P. Novák, E. J. Berg, J. Electrochem. Soc. 163 A1095-A1100 (2016).

(7) D. Streich, C. Erk, A. Guéguen, P. Müller, F. Chesneau, E. J. Berg, J. Phys. Chem. C 121, 13481-13486 (2017).