In Situ Gas Analysis of Li-Ion NixCoyMnz Cathode Interface Reactivity

Abstract

Li-ion cathode materials based on xLi2MnO3·(1–x)(NiyCozMn1-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