Operando Gas Analysis to Unpin the Root Reactions Triggering Degradation in NMC-Graphite Cells

The evolution of gases from batteries is a consequence, and also, a trigger of degradation, as well as an important safety issue. Here we show that the combination of operando gas analysis methods and advanced NMR measurements brings new understanding on previous discrepancies about the interpretation of the mechanism of degradation of NMC-graphite cells [1]. Shao-Horn and coworkers identified the formation of VC from EC dehydrogenation on NMC electrodes [2], whereas Berg, Gasteiger and coworkers observed the evolution of gases (CO₂, CO and O₂) as a result of oxygen loss from NMC [3,4]. Here, we demonstrate that both reactions indeed occur as two distinct pathways of NMC degradation at low and high potentials, respectively, and show that the formation of water as co-product in the second pathway leads to a complex chain of side-reactions initiated by the hydrolysis of the salt (LiPF₆) or co-solvent (DMC) [1].<br/>The formation (or re-formation after disruption) of the graphite SEI also involves the formation of gases (primarily, C₂H₄ and CO), but detailed gas analysis comparing reactions in half-cells and full cells with an inert LiFePO₄ electrode (which does not consume or produce gases) reveal that lithium plating on graphite can, unexpectedly, lead to gas consumption reactions [5]. This, in turn, also shows that gases can be reactants, as well as products, of SEI formation reactions.<br/>Finally, methodologies to explore and rank the stability of the graphite SEI against various disrupting degradation products, which currently limit the lifetime of NMC-graphite batteries, will also be discussed [6].<br/>References:<br/>[1] B.L. D. Rinkel, J. P. Vivek, N. Garcia-Araez, C.P. Grey. Energy Environ. Sci. 2022, 15, 3416-3438.<br/>[2] L. Giordano, P. Karayaylali, Y. Yu, Y. Katayama, F. Maglia, S. Lux, Y. Shao-Horn, J. Phys. Chem. Lett. 2017, 8, 16, 3881-3887.<br/>[3] D. Streich, C. Erk, A. Guéguen, P. Müller, F.F. Chesneau, E.J. Berg, J. Phys. Chem. C 2017, 121, 25, 13481-13486.<br/>[4] R. Jung, M. Metzger, F. Maglia, C. Stinner, H.A. Gasteiger, J. Phys. Chem. Lett. 2017, 8, 19, 4820- 4825.<br/>[5] J. P. Vivek, N. Garcia-Araez (submitted).<br/>[6] A. Kotronia, L. Lu, N. Garcia-Araez (in preparation).