Ni-rich cathode materials like NCM811 (Li_1+x(Ni_0.8Co_0.1Mn_0.1)_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

[1] D. Andre, S-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny., J. Mater. Chem. A, 3, 6709, 2015.

[2] R. Jung, R. Morasch, P. Karayavlali, K. Phillips, F. Maglia, C. Stinner, Y. Shao-Horn, Hubert A. Gasteiger, J. Electrochem. Soc. 165 (2), A132 - A141, 2018.

[3] H. Liu, Y. Yang, J. Zhang., J. Power Sources. 162, 644, 2006.

[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.