Differential Electrochemical Mass Spectrometry Study of the Interface of xLi₂MnO₃·(1–x)LiMO₂ (M = Ni, Co, and Mn) Material as a Positive Electrode in Li-Ion Batteries

Abstract

Lithium-rich mixed metal layered oxides constitute a large class of promising high-potential positive electrode materials in which higher specific charges are accessed only by activation of the Li2MnO3 domains. During the activation, oxygen is extracted from the oxide and evolves at the electrode–electrolyte interface. Differential electrochemical mass spectrometry was employed to follow volatile species developed during cycling. Although typical Li-ion aprotic carbonate electrolytes already suffer from oxidative decomposition at high potentials, the presence of O2 is here confirmed to enhance its reactivity. During the first cycle, O2 and CO2 evolve and their respective amounts vary as a function of the cycling conditions. However, for ethylene carbonate-based electrolytes, the amount of O2 and CO2 is found to be independent of the electrolyte composition. Moreover, X-ray photoelectron spectroscopy revealed that carbon-based components of the solid layers are dissolved between 3.0 and 4.0 V versus Li+/Li where no gas is evolving.

Online Electrochemical Mass Spectrometry of High Energy Lithium Nickel Cobalt Manganese Oxide/Graphite Half- and Full-Cells with Ethylene Carbonate and Fluoroethylene Carbonate Based Electrolytes

“…Overall gas evolution.-Consistent with previous reports, 6,40 Figures 1a and 1b show a substantial evolution of both CO 2 (44 and 28 m/z) and O 2 (32 m/z) during the two first charge/discharge cycles of a HE-NCM/Li half-cell. An early CO 2 formation rate maximum occurs at ca.…”

“…[40][41][42] The major part of the O 2 originating from Li 2 MnO 3 domain activation is, however, released along with a second CO 2 maximum at the end of the first charge when the potential reaches values >4.5 V. The latter CO 2 evolution is also observed in the following cycles in which, in contrast, the early CO 2 formation process and the O 2 release cease to occur. CO 2 formation is slightly enhanced in FEC:DEC compared to EC:DEC, particularly when the HE-NCM electrode is cycled below 3 V. Finally, the PF 6 − decomposition product POF 3 (85 m/z) evolves in a similar fashion as CO 2 : while a slow and a rapid formation process are observed during the first charge at ∼4.5 V and at >4.5 V, respectively, only the latter process persists throughout the following cycles. Again, the extent of POF 3 formation is appreciably higher in FEC:DEC compared to EC:DEC.…”

“…In the case of FEC:DEC electrolyte being less polar than EC:DEC, the differences in POF 3 formation would suggest dominance of destabilization via equilibrium 4 over the decrease in decomposition via reactions 1 and 2 due to lower Li 2 CO 3 solubility. Since according to this line of argument both higher and lower solvent polarity can explain enhanced PF 6 − decomposition it remains an open question which effect actually is prevailing.…”

“…[5][6][7] In this context, the study of gas evolution differences arising from electrolyte additives/co-solvents such as fluoroethylene carbonate (FEC) and ethylene carbonate (EC) is of particular interest to understand the chemistry influencing electrochemical interface stability.…”

“…The impact of FEC and EC on overall gas evolution, Li 2 MnO 3 domain activation, cathodic and anodic decomposition reactions, SEI formation, and PF 6 − stability is discussed. Furthermore, cell configuration effects and the role of SuperC65 conductive carbon additive in PF 6 − decomposition are addressed. Finally, a scheme of the major parasitic reactions and their interdependence in HE-NCM batteries is proposed.…”