Li-ion batteries will dominate the high-end rechargeable energy storage market for a foreseeable future. Current research and development of commercially relevant electrode/electrolyte systems focus strongly on extending the lifetime of the cells. Progress is however impeded by the lack of fundamental understanding of the underlying lifetime-limiting processes. Online Electrochemical Mass Spectrometry (OEMS) provides unique means for mechanistic studies of such adverse processes by enabling an operando monitoring of the onset, rate and extent of side-reactions with unparalleled sensitivity and selectivity.

Decomposition reactions involving (1) the electrode active material, (2) conductive carbon and (3) binder, as well as (4) the electrolyte solvent, (5) salt, along with (6) eventual additives, will be discussed. The Ni-rich layered mixed transition metal oxides (LiNi_xCo_yMn_zO₂) are currently the best performing Li-ion cathode materials on the market. Surface phase transformations during the initial cycles however lead to composition- and potential-dependent evolution of O₂, which is shown to be strongly related to the Ni-content of the oxides.¹ Decomposition of electrode-processing-related residues and of electrolyte solvents leads to evolution of CO₂, particularly in the first cycles, and initiates further chemical reactions with electrolyte salt (e.g. observed as HF and POF₃).² Electrolyte additives (e.g. vinylene carbonate, silyl-phosphates, etc.) can however suppress these side-reactions, e.g. by scavenging intermediate side-products and OEMS provides further verification of their reaction mechanism. In addition, electrolyte additives may also be exploited as chemical probes enabling operando monitoring of the degradation of other cell components, such as for example the electrode binder.³

References

1. D. Streich, C. Erk, A. Guéguen, P. Müller, F. Chesneau, E. J. Berg, Submitted.
2. A. Guéguen, D. Streich, M. He, M. Mendez, F. F. Chesneau, P. Novák, E. J. Berg, J. Electrochem. Soc. 163 A1095-A1100 (2016).
3. A. Guéguen, C. Bolli, M. Mendez, F. F. Chesneau, E. J. Berg, Submitted.