A Comparative Study of Structural Changes during Long-Term Cycling of NCM-811 at Ambient and Elevated Temperatures
Authors: Friedrich Franziska , Strehle Benjamin , Gasteiger Hubert A. 
Published: December 2020

Ni-rich layered oxides, like NCM-811 (Li1+δ[Ni0.8Co0.1Mn0.1]1-δO2), are promising cathode active materials for lithium-ion-batteries used in applications such as portable devices and battery electric vehicles. An increased Ni-content typically allows for a higher reversible lithium usage at a given cell potential, thereby improving the specific capacity. However, pronounced capacity fading, especially at high voltages and elevated temperatures, still leads to a limited cycle life.1 The underlying degradation mechanisms and whether they are due to detrimental reactions in the bulk or at the surface, are still controversially discussed in the literature.2,3

We will show a comparison of two studies investigating the capacity fading of NCM-811/graphite full-cells over several hundreds of cycles at ambient and elevated temperatures. In order to focus on the NCM-811 material, we excluded Li loss at the anode by pre-lithiating the graphite anode. The first study was conducted in-situ with two cells cycled at ambient temperature (≈22°C) over 1000 cycles,4 while in the follow-up study a number of pouch-cells were analyzed after cycling at 45°C for up to 700 cycles. As shown in Figure 1a, all cells were cycled at a C-rate of C/2 in a potential window between 3.0 and 4.5 V vs. a Li reference electrode at both 22°C (black line in Fig. 1a) and at 45°C (blue line in Fig. 1a), with two additional C/10 cycles every 50 cycles in case of the 45°C study (blue dots in Fig. 1a). One cell was disassembled every 150 cycles to conduct ex-situ experiments, such as X-ray powder diffraction (XPD) and impedance spectroscopy. A quantitative correlation between the NCM-811 lattice parameters and the Li amount, xLi, allows for monitoring the different contributions to the capacity loss by XPD.4 In the 22°C study, it was shown that the continuous growth of a resistive surface around the NCM-811 primary particles results in (i) an irreversible capacity loss due to the material lost for its formation, and (ii) in a significant impedance growth. The former is compared for both studies according to our XPD analysis in Figure 1b, showing a clear difference in the progression and in the absolute values, suggesting a more detrimental process to kick-in after ≈550 cycles at 45°C. In contrast to the time consuming XPD analysis, the material loss was also quantified by slow C/50 cycling, a rate which is slow enough to not lead to overpotential induced capacity losses due to the gradual resistance build-up over cycling. Hence, this approach could be verified to be a far more practical method to determine the material loss during cycle-life tests. In addition, the bulk material stability in terms of the Li-Ni mixing was analyzed by Rietveld refinement of the XPD data. In contrast to the study conducted at ambient temperature, where the extent of Ni disorder was observed to remain at an essentially constant value over 1000 cycles, it was found to clearly increase when cycling the cells at 45°C over 700 cycles (see Figure 1c).

In conclusion, we can state that at both temperatures a resistive surface layer is formed, suggesting that future research should focus on stabilizing the surface of Ni-rich materials. In addition to this surface process, the bulk stability of NCM-811 is altered at elevated temperatures, which is manifesting itself in an increasing Ni disorder.

Figure 1:

Comparison of data obtained from NCM-811/graphite pouch-cells cycled at ≈22°C (black symbols) and at 45°C (blue symbols) between 3.0 and 4.5 V vs. Li+/Li (controlled vs. Li reference electrode) at the indicated C-rates. (a) Discharge capacity of two exemplary cells, (b) relative material loss of the NCM-811 calculated from XPD analysis, (c) Ni disorder determined via XPD of harvested electrodes after cycling to the indicated cycle number at 45°C (error from calculating the average of two independent measurements).

Acknowledgement:

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

References:

1. H.-J. Noh, S. Youn, C. S. Yoon, and Y.-K. Sun, J. Power Sources, 233, 121 (2013).

2. H. Liu, M. Wolf, K. Karki, Y.-S. Yu, E. A. Stach, J. Cabana, K.W. Chapman, and P. J. Chupas, Nano Lett., 17, 3452 (2017)

3. W. Li, X. Liu, H. Celio, P. Smith, A. Dolocan, M. Chi, and A. Manthiram, Adv. Energy Mater., 8, 1703154 (2018).

4. F. Friedrich, B. Strehle, A. T. S. Freiberg, K. Kleiner, S. J. Day, C. Erk, M. Piana, and H. A. Gasteiger, J. Electrochem. Soc., 166, A3760 (2019).

Figure 1