Li-Ion batteries have recently been used to power battery electric vehicles (BEVs). However, in order to penetrate the mass market, further improvements have to be achieved with respect to the driving range of BEVs.^[1] The latter highly depends on the choice of the cathode active material, with layered lithium nickel manganese cobalt oxides (LiNi_xMn_yCo_zO₂, NMC) being one of the most promising class of candidate materials.^{[2, 3]} Even though the theoretical capacity of NMC is as high as ~275 mAh/g_NMC, not all of the lithium can easily be extracted due to structural instabilities occurring at high states-of-charge (SOC).^{[4, 5]} Additionally, the sloped voltage profile requires very high voltages to achieve complete removal of lithium. In order to improve the accessible capacity at reasonable upper cut-off voltages, Ni-rich NMCs (Ni-content >> Mn- and Co-content) recently came into focus, as more lithium can be extracted from their structure within the same voltage window. Therefore, they provide larger specific capacities and specific energies, which are crucial for a longer driving range of electric vehicles.^{[2, 6, 7]}

In this work, we will compare LiNi_1/3Mn_1/3Co_1/3O₂ (NMC111), LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622), and LiNi_0.8Mn_0.1Co_0.1O₂ (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 LiNi_0.43Mn_1.57O₄ (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.

References:

1. O. Groeger, H. A. Gasteiger, J.-P. Suchsland, J. Electrochem. Soc. 2015, 162, A2605.
2. D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny, J. Mater. Chem. A 2015, 3, 6709.
3. K. G. Gallagher, S. Goebel, T. Greszler, M. Mathias, W. Oelerich, D. Eroglu, V. Srinivasan, Energy Environ. Sci. 2014, 7, 1555.
4. S.-K. Jung, H. Gwon, J. Hong, K.-Y. Park, D.-H. Seo, H. Kim, J. Hyun, W. Yang, K. Kang, Adv. Energy Mater. 2014, 4, 1300787/1.
5. H. Gabrisch, T. Yi, R. Yazami, Electrochem. Solid-State Lett. 2008, 11, A119.
6. H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources 2013, 233, 121.
7. W. Liu, P. Oh, X. Liu, M.-J. Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Angew. Chem., Int. Ed. 2015, 54, 4440.
8. W. Li, J. N. Reimers, J. R. Dahn, Solid State Ionics 1993, 67, 123.

Acknowledgements:

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).