Aging Analysis of LiNi1/3Mn1/3Co1/3O2-Graphite Cells Via X-Ray Diffraction
Authors: Buchberger Irmgard , Seidlmayer Stefan , Hattendorff Johannes , Pokharel Aneil , Piana Michele , Kudejova Petra , Gilles Ralph , Gasteiger Hubert A. 
Published: February 2020

Although lithium ion batteries (LiBs) are being used successfully in portable electronics, automobiles, and stationary power storages, safety and degradation issues are still impeding their long term application. The main LiB aging mechanisms that have been reported include structural changes of the cathode, loss of active material, loss of cycleable lithium due to SEI growth, and impedance rise of the cell [1].
A study by Dubarry et al. and Burns et al. on LiNi1/3Mn1/3Co1/3O2 (NMC) based cells showed that during early cycles, the loss of lithium inventory is the main cause of capacity fade and that it follows a linear relationship. A rapid capacity roll-over failure, however, was attributed to deterioration of the interfacial kinetics [2,3]. Further studies on the graphite/NMC system showed that also transition metal dissolution has to be considered when cycling to high charging potential limits [4] or at elevated temperatures [5]. In both cases, the capacity loss which can be contributed to the loss of cyclable lithium was not quantified.
In this work, we focus on the performance degradation of graphite/NMC cells charged and discharged for 300 cycles at different temperatures (25 °C and 60 °C) and with different charging voltage limits (4.2 V, 4.6 V). Cycling data clearly suggest that two degradation mechanisms can be distinguished: (i) a linear capacity fade at a 4.2 V limit (at both 25 and 60 °C) and, (ii) a rapid capacity roll-over failure at the more positive 4.6 V limit. To analyze the major contribution to cell failure, different diagnostic techniques comprising structural investigations via powder X-ray diffraction (XRD) and neutron based elemental analysis via prompt gamma activation analysis (PGAA) to determine transition metal deposition on graphite are applied.
As NMC shows no severe structural changes like phase transition after cycling (ex-situ XRD), the material exhibits good crystal structure stability at the different tested operating conditions, which is consistent with Zheng et al. [4]. Slight differences observed in peak shifts are directly associated with changes in the lattice constants a and c.
Figure 1 shows an in-situ XRD analysis of the Li/NMC system. XRD pattern collection during delithiation and lithiation of NMC demonstrates a clear correlation between the lattice ratio c/a and the corresponding lithium content of the cathode. We will show that this correlation allows for the quantification of the loss of cyclable lithium via post-mortem ex-situ XRD measurements. In addition, Fig. 1 also shows that NMC exhibits an irreversible capacity loss (ICL) in the initial cycle which goes along with structural irreversibility. As this is important for the in-situ XRD calibration, this issue will be discussed in more detail.
In conclusion, loss of lithium inventory is most detrimental for NMC/graphite cells cycled at different temperatures, whereas transition metal dissolution and active material inactivation is most severe when cycling with a charge limit of 4.6 V.

Figure 1: In-situ XRD calibration: Potential profile of Li/NMC (top) and the corresponding ratio of the lattice parameters a and c (bottom) determined by in-situ XRD during OCV steps (indicated by spikes) as a linear function of lithium content.

References:

[1] J. Vetter, P. Novák, M. Wagner, C. Veit, K-C. Möller, J. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Volger, and A. Hammouche, J. Power Sources, 147, 269 (2005).
[2] M. Dubarry, C. Truchot, B. Y. Liaw, K. Gering, S. Sazhin, D. Jamison, and C. Michelbacher, J. Power Sources, 196, 10336 (2011).
[3] J. C. Burns, A. Kassam, N. N. Sinha, L. E. Downie, L. Solnickova, B. M. Way, and J. R. Dahn, J. Electrochem. Soc., 160, A1451 (2013).
[4] H. Zheng, Q. Sun, G. Liu, X. Song, and V.S. Battaglia, J. Power Sources, 207, 134 (2012).
[5] K. Amine, Z. Chen, Z. Zhang, J. Liu, W. Lu, Y. Qin, J. Lu, L. Curtis, and Y-K. Sun, J. Mater. Chem. 21, 17754 (2011).

Acknowledgement:

Funding was provided by the BMBF (Federal ministry of Education and research, Germany) of the ExZellTUM project, grant number 03X4633A.

Figure 1