Operando XRD Observation of NMC Cathode for Reaction Distribution Analysis Under Fast Discharging
Authors: Mashino Masataka , Ikezawa Atsunori , Murakami Marina , Shimomura Takeshi , Okajima Takeyoshi , Arai Hajime 
Published: December 2024

Introduction

There is a demand for high output power of lithium-ion batteries (LIBs) as regenerative power sources for construction machinery. It is important to analyze electrode behavior during high C-rate discharge processes. The main reason why LIBs do not have sufficient capacity at high power output is thought to be that the discharge finishes when the topmost surface of the electrode reaches the cut-off potential, even if the other parts are only partially discharged.
In this study, an attempt was made to infer the reaction distribution and fast response of the entire electrode by observing the relaxation process after discharge using laboratory XRD. Operando XRD measurements were performed for a positive electrode of active material LiNi1/3Mn1/3Co1/3O2 (NMC) while discharging at 0.1C, and a correlation was observed between SOC and the peak angle derived from 101 reflection. By contrasting this with known NMC crystal lattice changes during charging and discharging, compositional changes can be captured [1-3]. Based on this, the behavior of NMC electrodes of various electrode designs during the relaxation process after fast discharge was observed, and the results are reported.

Experimental

Laminated half-cells (30 mm square) were prepared using NMC electrodes with different electrode designs (thickness and porosity) for the positive electrode, Li metal for the negative electrode, and carbonate electrolyte containing 1.0 mol dm−
3 LiPF6 as the electrolyte solution. The electrodes were made with three electrode thickness levels of approximately 20 µm, 30 µm, and 40 µm (constant porosity), and three porosity levels of approximately 30%, 40%, and 50% (constant loading). After charging these cells to 100% SOC at 0.1C, they were installed in an X-ray diffraction system (SmartLab, Rigaku) and subjected to fast discharge with the charge/discharge device. Discharge conditions were as follows: DOD 50% capacity was discharged at 5C current, followed by 1.5 hours of Relaxation. At the same time, operando XRD measurements were performed to obtain the displacement of the peak top angle originating from the (101) plane of NMC.

Result and Discussion

The obtained (101) plane peak-top angles and cell voltage displacements are shown in Figure 1. In this case, X-rays were incident from the negative electrode side. It was observed that the peak angle became smaller as the cell discharged and gradually increased during the relaxation. This suggests that the reaction was preceded on the separator-side portion of cathode, leading to the end of discharge, and that the Li imbalance in the electrode was relaxed in the subsequent open circuit state.
The same measurements were performed at different levels of NMC electrode thickness and porosity, and the peak angle displacement and relaxation time during the relaxation process were compared. The results showed that there was no clear dependence of peak angle displacement and relaxation time on thickness. On the other hand, the larger the porosity, the smaller the peak angle displacement and the larger the relaxation time. These results indicate that there is a clear correspondence between the electrode design and the reaction inhomogeneity in the electrode, and that this method can be used to evaluate the reaction distribution.
In addition, the relationship between the angular displacement of the above XRD peak and the change in cell voltage behavior during the relaxation process was also discussed by fitting with an equivalent circuit. The correlation between the two results suggests the possibility of observing the reaction distribution inside the battery from the analysis of the cell voltage behavior.

References

[1] N. Yabuuchi, et al., J. Electrochem
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Soc., 154 (4), A314-A321 (2007).
[2] H. Murayama, et al., J. Phys. Chem. C, 118, 20750-20755 (2014).
[3] K. Kitada, et al., J. Power Sources, 301, 11-17(2016).

Figure 1