In this work, we will compare LiNi1/3Mn1/3Co1/3O2 (NMC111), LiNi0.6Mn0.2Co0.2O2 (NMC622), and LiNi0.8Mn0.1Co0.1O2 (NMC811) with respect to their cycling stability in NMC-graphite cells at different upper cut-off voltages. It will be shown that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. At higher potentials, significant capacity fading was observed, which was traced back to an increase in the overpotential of the NMC electrode. We show that the increase in the overpotential occurs when the NMC materials are cycled up to a high-voltage feature in the dq/dV plot, which is described in literature as the H2 -> H3 phase transition.[6, 8] Based on gas analysis using On-line Electrochemical Mass Spectrometry (OEMS), we prove that all three materials release oxygen from the particle surface and that the onsets of oxygen evolution for the different NMCs correlate well with the H2 -> H3 phase transitions. Interestingly, the oxygen evolution coincides with the onset of CO2 and CO evolution, suggesting a correlation between the oxygen release and the formation of CO and CO2. For comparison, we show that no oxygen is released from spinel LiNi0.43Mn1.57O4 (LNMO) up to 5 V vs. Li/Li+ and, consequently, that no CO2 or CO is observed. Against the common understanding that the electrochemical oxidation of the carbonate electrolyte is catalyzed by the transition metals in metal oxide surfaces, leading to CO2 and CO evolution at potentials above 4.7 V vs. Li/Li+, our observations suggest that the observed CO2 and CO is mainly due to the chemical reaction of released reactive oxygen with the electrolyte.
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The authors gratefully acknowledge BMW for the financial support of this work. Umicore is greatly acknowledged for supplying the cathode materials. R.J. thanks TUM-IAS for their support in the frame of the Rudolf-Diesel Fellowship of Dr. Peter Lamp. M.M. gratefully acknowledges funding through the 2016 ECS Herbert H. Uhlig Summer Fellowship.
Fig. 1. (a) Cell voltage vs. time of a NMC111-Graphite cell containing 400 µL of 1.5 M LiPF6 in ethylene carbonate (EC). (b) Evolution of CO2 (dark blue), H2 (green), C2H4 (orange), CO (blue), and O2 (black, 10-fold magnified) during cycling. Solid lines indicate the gases stemming from the cathode active material (CAM; left y-axis) and dashed lines from the graphite electrode (right y-axis).