Spatially Resolved Post-Mortem-Analysis on Commercial Lithium-Iron-Phosphate Batteries

Three high-power commercial 8Ah Lithium Iron Phosphate (LFP) cylindrical cells with a LFP cathode and a graphite anode aged by a cycle depth of 50% at a current rate of 2C are investigated in a post-mortem-analysis (PMA). The results from the comparison between un-aged reference cells and three aged cells show a strong spatial depending aging, as shown in figure 1, which is in accordance to some reports in literature [1-6]. The examined cells have lost at least 20% of their initial capacity.

In this work the results of the life cycle tests of the full cells are discussed. This includes the capacity fade, the evolution of the internal resistance and the analysis of the differential quasi open circuit voltage curve measured at C/4 at different points of aging. The results give a first idea of the aging mechanisms and expectations with respect to the PMA.

The analytical analysis of the disassembled cells is spatial resolved. The facing sides of anode and cathode are investigated according to their position on the electrode. With respect to the electrode length, the following three positions are compared: outer part, middle part and inner part of the cylindrical cell. On each of these positions the electrode height is divided in three to five sections and compared as well.

The analysis is subdivided into several steps: At first the specific mass and thickness is surveyed. Followed by, scanning for optical anomalies by visual analysis and imaging with a confocal laser microscope. The electrochemical performance, considering available lithium, capacity and internal resistance, is tested in a half-cell arrangement of LFP vs. lithium and graphite vs. lithium.

The porosity is measured with Hg-porosimetry. The lithium distribution, stoichiometry of the cathode and trace elements were determined using an Inductively Coupled Plasma-OES (ICP-OES). The deposition layer on the anode was examined using Fourier-Transform-Infrared (FTIR) spectroscopy in attenuated total reflection (ATR) setup.

Raman spectroscopy and X-ray diffraction (XRD) measurements are performed on both electrodes in order to determine changes in crystal structures and sizes due to the high charge and discharge currents. With Raman changes in the coating of the LFP and as well the graphite of the anode are detected.

The aging of the cells is expressed in different color patterns of golden and black sections, where the black parts exhibit a thick deposition layer. The main aging can be recognized towards the electrode’s center of height and results in an inhomogeneous lithium distribution. 

References

[1]           M. Klett et al., Journal of Power Sources, vol. 257, pp. 126–137, 2014.

[2]           M. Broussely et al., Journal of Power Sources, vol. 146, no. 1–2, pp. 90–96, 2005.

[3]           G. Sarre et al., Journal of Power Sources, vol. 127, no. 1–2, pp. 65–71, 2004.

[4]           P. Arora et al., Journal of The Electrochemical Society, vol. 145, no. 10, pp. 3647–3667, 1998.

[5]           M. Ecker et al., Journal of Power Sources, vol. 248, pp. 839–851, 2014.

[6]           M. Dubarry et al., Journal of Power Sources, vol. 258, pp. 408–419, 2014.