Surface Contaminants on Ni-Rich Cathode Active Materials: Influence of the Storage Conditions
Authors: Pritzl Daniel , Teufl Tobias , Beyer Hans , Hartmann Louis , Sicklinger Johannes , Sommer Heino , Gasteiger Hubert A. 
Published: February 2020

Ni-rich cathode materials like NCM811 (Li1+x(Ni0.8Co0.1Mn0.1)1-xO2) are promising candidates for reaching the energy density targets of Li-Ion batteries for automotive applications [1]. However, these materials show a rapid performance decrease when stored under ambient conditions [2]. The performance decrease is often related to the formation of surface impurities containing M-CO3 and M-OH groups on the surface of the cathode active material [2, 3, 4]. In this work, we investigate the influence of different storage parameters on the formation of surface contaminants on Ni-rich cathode materials using TGA-MS and XPS.
First, we store the cathode material (NCM622) under the following conditions: (i) pristine, meaning the material as-received and stored in an argon-filled glovebox; (ii) wet, which is stored 7 days under air at a relative humidity of ~85%; and, (iii) wet without O2 at a relative humidity of ~85%. The last storage condition is realized by working under argon atmosphere.
Afterwards, TGA-MS analysis with a novel analysis protocol is performed in order to deconvolute the release of physisorbed water from the evolution of water and CO2 produced by the thermal decomposition of mixed transition metal hydroxides and carbonates, predominantly NiCO3 • x Ni(OH)2 • y H2O on the surface of the cathode material. The protocol contains a temperature hold phase at 25 °C, then a heat ramp (10K/min) to 120 °C with a hold at this temperature for 30 minutes, followed by a second temperature ramp to 450 °C and a final hold at this temperature (see panel 1, Figure 1).
In Figure 1 (panel 2), the weight loss of the NCM samples is shown. When the samples are stored for 7 days at an RH of 85%, the surface contaminants, especially Ni hydroxide species, decompose when going to 450°C, accompanied by the release of H2O (m/z = 18 from coupled MS shown in panel 3, Figure 1). The weight loss equals to 1.2 wt.%. When the pristine as-received sample is analyzed, only a very small amount of surface contaminants (< 0.1 wt.%) is present.
Surprisingly, when the material is stored under an argon atmosphere which is saturated with H2O, no Ni hydroxide species do form on the surface of the cathode material, as there is nearly no weight loss when going from 120 to 450 °C. This is confirmed by the MS analysis where no water is detected.
In our contribution we will compare the extent of surface contaminants formation as a function of Ni-content (NCM111, NCM622, and NCM811). Furthermore, we investigate the effect of the presence and absence of CO2 on the formation of surface contaminants. We will support our measurements by XPS and FTIR measurements in order to detect eventually formed LiOH and Li2CO3 on the surface of the cathode active materials.
Figure 1: TGA-MS analysis of NCM622 samples stored (i) in a glovebox under argon atmosphere, (ii) stored under air at RH~85%, and, (iii) stored in a water-saturated argon atmosphere. Panel 1 shows the TGA profile, panel 2 shows the mass loss of the different samples, and in panel 3 the mass traces of H2O (m/z = 18) are plotted.

References

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[4] Z. Chen, J. Chen, J. Wang, T. Huang, G. Fu, S. Sun, R. Lai, K. Zhou, K. Li, and J. Zhao, J. Power Sources , 363, 168, 2017

Acknowledgement

This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research.

Figure 1