Washing of Ni-Rich Cathode Active Materials for Lithium-Ion-Batteries: Mechanistic Understanding
Authors: Hartmann Louis , Oswald Stefan , Graf Maximilian , Pritzl Daniel , Gasteiger Hubert A. 
Published: May 2020

Ni-rich cathode active materials (CAMs) for lithium-ion batteries, like NCM851005 (Li1+δ(Ni0.85Co0.10Mn0.05)1-δO2), are promising candidates for achieving the energy density target needed for battery electric vehicles [1]. However, these materials are susceptible to the formation of surface impurities and, for this reason, an additional washing step is needed to remove these surface contaminants [2,3]. It was shown that this washing step results in a H+/Li+ exchange in the near-surface region of the CAM and a metal oxohydroxide-like structure is formed [4]. In the subsequent drying/heating step, thermogravimetric analysis coupled with mass spectrometry (TGA-MS) showed the initial release of water, indicating the formation of a surface spinel-like shell; upon further heating to 250 °C, oxygen release detected by TGA-MS suggests the formation of a rocksalt-like surface layer [5].
In this work, we investigate the mechanism behind the formation of surface groups on Ni-rich CAMs during washing, using TGA-MS, on-line mass spectrometry (O-MS), and near-edge X-ray fluorescence spectroscopy (NEXAFS). In addition, we synthesized and characterized the model materials, LiNiO2 (LNO), HNiO2, and LixH1-xNiO2 to deconvolute the different effects playing a role during the drying/heating step after washing.
At first, we washed the pristine material with an excess of water (CAM:H2O mass-ratio of 1:5), to examine the effect of washing. The model material HNiO2 (Nickel oxohydroxide) was also synthesized from a simple water-based synthesis. Afterwards, TGA-MS analysis was performed in order to deconvolute the release of physisorbed water from the evolution of water and O2 produced by the thermal decomposition of the surface protonated species of the washed material. The TGA-MS protocol contains a temperature hold phase at 25 °C, then a heat ramp (10 K/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 upper panel, Figure 1). In Figure 1 (upper panel), the weight loss of the measured samples is shown. After the initial release of physisorbed water from the washed LNO at ≈ 180 °C (m/z = 18 trace, see middle panel). At higher temperatures (>250 °C), a release of oxygen (m/z=32 trace, see bottom panel) is initiating, and the overall weight loss amounts to ≈ 0.75 wt.% once a temperature of 450 °C is reached. In contrast, when the pristine unwashed LNO sample is heated to 450 °C, the overall weight loss is only minor (<0.1 wt%) and no O2 is evolved (black lines).
Surprisingly, the model HNiO2 material does not show the thermal separation of water and oxygen evolution, and both gases are starting to evolve simultaneously at ≈ 180 °C (see red lines). By chemical partial lithiation of HNiO2 to produce LixH1-xNiO2, the oxygen evolution is shifted to higher temperatures, so that water and oxygen are not being evolved anymore at the same temperature, as was the case for washed LNO. We will discuss the likely mechanism for this behavior, supporting our arguments with a surface and bulk by means of NEXAFS on pristine and washed-and-heated materials. Furthermore, the reactivity of the differently washed/heated CAMs and the model compounds with the electrolyte will be examined by O-MS.

Acknowledgement

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

References:

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Figure 1: TGA-MS analysis of pristine (in black) and washed LNO (in blue), HNiO2 (in red), and LixH1-xNiO2 (in green). The upper panel shows the TGA profile and relative mass change of the different samples. The other panels show the MS traces of H2O (m/z = 18, middle panel) and those of O2 (m/z=32, lower panel).

Figure 1