Comparison between Washing and Ambient Storage of Ni-Rich Active Materials By TGA-MS and XPS

To achieve the energy density needed for automotive applications, Ni-rich cathode active materials (CAMs), like NCM851005 (Li_1+d(Ni_0.85Co_0.10Mn_0.05)_1-dO₂), are promising candidates.^[1] However, these materials are susceptible to the formation of surface impurities upon ambient air exposure during storage.^[2–4] These surface groups were subject of various studies, but detailed information on their formation rate and on their decomposition temperatures are lacking. In this study, we investigated the changes in surface chemistry during ambient storage and during subsequent heating for the Ni-rich material NCM851005 in comparison to that of NCM111. We thus identify the surface groups after storage and also provide evidence for a Li⁺/H⁺-exchange, a phenomenon which had been observed during the washing of these materials with water.^[5]

In order to gain knowledge about the formation rate of surface impurities on NCM111 and NCM851005, the pristine powders were subjected to an accelerated storage test. The samples were stored at 25°C in ambient air with a constant relative humidity of ~85% (further on referred to as wet-storage) for increasing time intervals starting from 5 min up to 1 week. After each storage interval, a TGA-MS measurement (thermogravimetric analysis coupled with mass spectrometry) was performed under Ar (at 40 mL/min). 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 (10 K/min), and a final hold at this temperature (see red line in Fig. 1a). The plateau at 120 °C is designed to deconvolute the physisorbed from the chemisorbed water. A washed material (at a water to CAM mass ratio of 5:1) was also analyzed by the same TGA-MS procedure.

In addition to the TGA measurements, a special XPS sample holder was used to heat the samples inside the main XPS chamber to evaluate the NCM surface at different temperatures, monitoring the decomposition and desorption of surface contaminants.

It was found that the amount of surface contaminants indeed increased with storage time. Predictably, the extent of surface contaminants increased more rapidly for the Ni-rich NCM851005 material than compared to a Ni-poor NCM111 sample. Furthermore, oxygen release at already 280 °C was seen for NCM851005 kept under wet-storage for 7 days (Fig. 1c, blue line). Oxygen release for the as-received material is only observed at higher temperatures (T>650°C). This early onset of oxygen evolution is reminiscent of that of either partially delithiated NCMs or of washed NCMs (see also green line in Fig. 1c).^[5,6] We will discuss the likely mechanism for this behavior, supporting our arguments with XPS measurements, where we have found a defective oxygen-deficient phase after heating the materials above the O₂ release temperature when the materials was washed, but not in case of the as-received material. This is further evidence for a Li⁺/H⁺-exchange during storage of Ni-rich materials under ambient conditions.

We will discuss future mitigation strategies such as surface coatings or gas treatment to minimize the effects of ambient storage.

Figure 1: TGA-MS analysis of as-received (black lines), wet-stored (in blue) and washed (in green) NCM851005 under Ar. The upper panel shows the TGA profile and relative mass change of the different samples. The other panels show the MS traces of (b) H₂O (m/z = 18), (c) O₂ (m/z=32) and (d) of CO₂ (m/z=44).

Figure 1