Online Electrochemical Mass Spectrometry for Rechargeable Batteries
Authors: Bolli Christoph , J. Berg Erik , Novák Petr 
Published: February 2020

Operando analytical techniques for lithium ion batteries (LIBs) have proven to be valuable tools to gain insight into the processes ongoing during battery cycling. Among these techniques, operando (or online) electrochemical mass spectrometry (OEMS) is a very versatile tool for tracking reactivity of cell components during its operation.
Mass spectrometry made a first appearance as analytical technique for LIBs about two decades ago in the course of studies about the mechanisms of electrolyte reduction on graphitic surfaces (1). However, the then-used experimental cells required large amounts of electrolyte and had rather moderate detection limits. In recent years the cell design and overall experimental setup underwent significant overhaul, thus allowing the identification and quantification of various gases down to ppm level. This, in turn, allowed the more extensive application of OEMS to investigate a broad variety of LIB-related processes and it has been used by us and other groups to study: cell-component reactivity and electrolyte decomposition (2), SEI formation on graphite/silicon composite anodes (3), gas release from cathode active materials due to irreversible anionic oxygen redox (4), surface reconstruction (5) and surface impurities (6).
In this contribution we present the current OEMS setup at PSI, which can be used for measurements down to the ppm level, and highlights from our recent research. One example is the application of the electrolyte additive TMSPa as chemical probe: it allows to monitor the formation of inorganic fluorides such as HF and LiF during cell operation, which in turn made it possible to gain insight into detrimental side-reactions taking place during cell operation (7). Another example is the application of OEMS to study the mechanisms in beyond Li-ion technologies, such as sodium batteries. The study covers the surface reactivity of the graphite anode which acts as a ternary intercalation compounds in the investigated system.(8) The gas release here showed significant differences compared to the Li system stemming from the (electro)chemical reactivity of the conduction salts as well as the sodium anode. The obtained data allowed to draw conclusions for the further optimization of the Na-ion batteries as well as for future OEMS studies in the field of sodium ion batteries.
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(4) E. Castel, E. J. Berg, M. El Kazzi, P. Novák, C. Villevieille, Chem. Mater. 26, 5051-5057 (2014); K. Luo, M. R. Roberts, R. Hao, N. Guerrini, D. M. Pickup, Y. S. Liu, K. Edstrom, J. Guo, A. V. Chadwick, L. C. Duda, P. G. Bruce, Nat. Chem. 8, 684-691 (2015).
(5) D. Streich, C. Erk, A. Guéguen, P. Müller, F. Chesneau, E. J. Berg, J. Phys. Chem. C 121, 13481-13486 (2017); R. Jung, M. Metzger, F. Maglia, C. Stinner, H. A. Gasteiger, J. Electrochem. Soc. 164 A1361-A1377 (2017).
(6) S. E. Renfrew, B. D. McCloskey, J. Amer. Chem. Soc. 139, 17853-17860 (2017).
(7) C. Bolli, A. Guéguen, M. Mendez, E. J. Berg, Chem. Mater. submitted.
(8) G. Mustafa, C. Bolli, E. J. Berg, P. Novák, K. Pollok, F. Langenhorst, M. v. Roeder, O. Lenchuk, D. Mollenhauer, P. Adelhelm, Adv. Energy Mater. 8, 1702724 (2017).