Influence of Pristine NMC811 Secondary and Primary Particle Surfaces on the Materials Reactivity

Abstract

Vehicle electrification currently relies on lithium-ion batteries using nickel, manganese and cobalt (NMC)-based lamellar oxides as positive electrode materials. In order to achieve higher energy densities, the market is now moving towards NMC with high nickel content. Nevertheless, these materials have been widely demonstrated to suffer from serious off-gassing problems that reduce cycle life and pose safety concerns. Not only gases resulting from the formation of SEI at the negative electrode, but the production of O2, CO and CO2 have also been noted by different research groups.1–3 Despite the large number of studies analyzing the material during and after cycling, we noted a lack of in-depth characterization of the surface of Nickel-rich NMCs in their initial state. The need for such information is increasingly necessary with the recent development of new technologies for surface modification such as coating to limit the instability of high Nickel content NMCs.4,5 The surface reactivity of Ni-rich layered transition metal oxides is instrumental to the performance of batteries based on these positive electrode materials. Most often, strong surface modifications are detailed with respect to a supposed ideal initial state. The study the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material in its pristine state, hence before any contact with electrolyte or cycling, was performed thanks to advanced microscopy and spectroscopy techniques in order to fully characterize its surface down to the nanometer scale. Scanning transmission-electron microscopy-electron energy-loss spectroscopy (STEM-EELS), solid state nuclear-magnetic-resonance (SS-NMR) and X-rays photoelectron spectroscopy (XPS) are combined and correlated in an innovative manner.Specifically, using a high magnetic field NMR spectrometer, it was possible to evaluate the total amount of diamagnetic Li in the samples. Thanks to XPS it was then possible to identify the lithiated species present on the surface and to use such information to better deconvolve the NMR spectra. Li2O, Li2SO4 and LiOH were identified as the main contaminants on the surface of NMC in its initial state. On the other hand, the quantity of Li2CO3 initially estimated by XPS could be substantially re-evaluated using NMR (54% by XPS vs. only 1% by NMR). These two techniques, providing only partial and often imprecise information when used separately, have proven to be fundamental instruments in the analysis of surface species when used in a complementary manner.The results demonstrate that in usual storage conditions after synthesis, the extreme surface is already chemically different from nominal values. In particular, nickel is found in a reduced state compared to the bulk value and a Mn enrichment is determined in the first few nanometers of primary particles. Further exposition to humid air allows for quantifying the formed lithiated species per gram of active material, identifying their repartition and proposing a reaction path in relation with the instability of the surface. Modifying synthesis conditions impacts noticeably the previously identified surface particularities and, consequently, the materials reactivity.References :(1) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries. Journal of The Electrochemical Society 2017, 164 (7), A1361–A1377. https://doi.org/10.1149/2.0021707jes.(2) Renfrew, S. E.; McCloskey, B. D. Residual Lithium Carbonate Predominantly Accounts for First Cycle CO2 and CO Outgassing of Li-Stoichiometric and Li-Rich Layered Transition-Metal Oxides. J. Am. Chem. Soc. 2017, 139 (49), 17853–17860. https://doi.org/10.1021/jacs.7b08461.(3) Renfrew, S. E.; McCloskey, B. D. The Role of Electrolyte in the First-Cycle Transformations of LiNi 0.6 Mn 0.2 Co 0.2 O 2. J. Electrochem. Soc. 2019, 166 (13), A2762–A2768. https://doi.org/10.1149/2.1561912jes.(4) Zhang, H.; Liu, H.; Piper, L. F. J.; Whittingham, M. S.; Zhou, G. Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation. Chem. Rev. 2022, acs.chemrev.1c00327. https://doi.org/10.1021/acs.chemrev.1c00327.(5) Manthiram, A.; Song, B.; Li, W. A Perspective on Nickel-Rich Layered Oxide Cathodes for Lithium-Ion Batteries. Energy Storage Materials 2017, 6, 125–139. https://doi.org/10.1016/j.ensm.2016.10.007. Figure 1