Li-ion cathode materials based on xLi₂MnO₃·(1–x)(Ni_yCo_zMn_1-y-z)O₂ (NCM), including both the stoichiometric (x = 0) as well as the Li-rich derivatives (x > 0), are currently under intensive research and development to be implemented in the next-generation batteries because of their high specific charge and operating potential [1, 2]. By tuning the transition metal composition, the cathode properties can be tailored according to application specific requirements: specific charge, rate capability and cycle life. However, compositions offering higher cell energies (e.g. Li- and Ni-rich) generally display increased reactivity towards the electrolyte that may unfavorably influence the thermal stability, impedance and capacity retention of the cells [3]. We demonstrate in situ gas analysis to be a powerful tool to both characterize and in turn mitigate performance limiting interface side-reactions of NCM cathodes and carbonate based electrolytes.

Online electrochemical mass spectrometry (OEMS) enables the in situ analysis of gases evolving inside the electrochemical cell during cycling [4]. The nature, onset, and extent of evolving volatile species provide crucial mechanistic understanding of transformation/decomposition reactions of both the electrode and electrolyte. Side-reaction products associated with the electrolyte LiPF₆ salt (e.g. POF₃, …), solvent (H₂, C₂H₄, …), and electrode (O₂, CO₂,…) are readily discernable with high sensitivity and correlated to the electrochemical response during the charge/discharge process. We demonstrate the initiation of a continuous LiPF₆mediated electrolyte decomposition cycle at high cell voltages, which drastically increases acidity of the electrolyte and dissolution of the active materials [5]. The influence of the transition metal composition of the active material [6], electrolyte solvents [7], and additives [8] (EC, FEC, VC, …) with respect to the anodic electrolyte decomposition is demonstrated.

[1] P. Rozier, J.M. Tarascon, Journal of The Electrochemical Society, 162 (2015) A2490-A2499.
[2] E.J. Berg, C. Villevieille, D. Streich, S. Trabesinger, P. Novák, Journal of The Electrochemical Society, 162 (2015) A2468-A2475.
[3] H.-J. Noh, S. Youn, C.S. Yoon, Y.-K. Sun, Journal of Power Sources, 233 (2013) 121-130.
[4] E. Castel, E.J. Berg, M. El Kazzi, P. Novák, C. Villevieille, Chemistry of Materials, 26 (2014) 5051-5057.
[5] A. Guéguen, D. Streich, M. He, M. Mendez, F. Chesneau, P. Novák, E. J. Berg, Submitted.
[6] D. Streich, J-Y. Shin, F. Chesneau, P. Novák, E. J. Berg, Submitted.
[7] D. Streich, A. Guéguen, M. Mendez, F. Chesneau, P. Novák, E. J. Berg, Submitted.
[8] A. Guéguen, C. Bolli, M. Mendez, F. Chesneau, P. Novák, E. J. Berg, Submitted