Effect of Storage Conditions on the Electrochemical Behavior of Ni-Rich Cathode Materials (NMC811) for Li-Ion Batteries
Authors: Jung Roland , Morasch Robert , Karayaylali Pinar , Phillips Katherine , Maglia Filippo , Stinner Christoph , Shao-Horn Yang , Gasteiger Hubert A. 
Published: February 2020

Secondary Li-Ion batteries are considered as alternative to the combustion engine in automotive applications paving the way to electromobility. In a recent review, Andre et al. pointed out that in order to reach a driving range of 300 miles, the specific energy of today’s Li-ion batteries needs to be increased to ~250 Wh/kgcell, which corresponds to an improvement by a factor of almost 2.5 compared to today’s batteries.[1] To meet these targets, layered lithium nickel manganese cobalt oxides (LiNixMnyCozO2, NMC) are one of the most promising class of cathode materials.[1-3] Despite their very high theoretical capacity of ~275 mAh/gNMC, not all of the lithium can easily be extracted due to structural instabilities occurring at high states-of-charge (SOC).[4-6] Therefore, the accessible capacity is much lower than the theoretical one and is ~160 mAh/gNMC for LiNi1/3Mn1/3Co1/3O2 (NMC111) [6-8] when cycled up to 4.3 V vs. Li/Li+, which is insufficient to meet the required specific energy targets. With Ni-rich NMCs (Ni-content >> Mn- and Co-content), significantly higher specific capacities are accessible. In particular, 25% higher reversible capacities of up to ~200 mAh/gNMC were reported for LiNi0.8Mn0.1Co0.1O2 (NMC811) for the same upper cut-off voltage.[6, 7, 9] However, one major drawback of Ni-rich NMCs is their increased surface reactivity. Gauthier et al. rationalized this by an increasing nucleophilicity of the surface oxygen in layered LiMO2 (M = transition metals) from early to late transition metals, because the oxygen p-band is shifted to higher energies.[10] Therefore, the high Ni-content in NMC811 renders the surface oxygen chemically more reactive.[10-12] Upon storage at ambient air this surface reactivity can lead to the formation of carbonates and/or hydroxides via a reaction of CO2 and/or H2O with the NMC surface causing irreversible damage to the NMC material.

In the present study, we analyze the impact of ambient air contact on NMC811 in comparison to NMC111. It will be shown that in contrast to NMC111, NMC811 is very sensitive when stored under ambient conditions and that the formation of surface impurities detrimentally affects the performance of both, NMC811/Li and NMC811/graphite cells. We will critically investigate the chemical nature of the formed surface species and present a simple method to quantify them. Finally, we will demonstrate that a proper handling of NMC811 is much more challenging compared to NMC111 and if the latter is to be replaced by NMC811, it is of primary importance to control the amounts of surface impurities on Ni-rich NMC cathode materials to allow for low impedance and good cycling stability.

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