Degradation Mechanism Analysis of 18650-Type Lithium-Ion Battery: A Hard X-Ray Photoelectron Spectroscopic Investigation
Establishment of lifetime prediction methodology for Li-ion batteries (LIBs) is strongly required in order to prolong the lifetime of LIBs and to use LIBs in safe. While “square root (SQRT) law” has been proposed to predict the lifetime of LIBs based on the growth mechanism of solid electrolyte interphases (SEIs) , the SQRT law cannot describe the degradation behavior precisely in whole period of LIB operation, especially near the end stage of degradation, because the detailed mechanism of degradation remains unknown.
We, therefore, investigated the detailed structure of SEIs in heavily deteriorated anodes by Hard X-ray photoelectron spectroscopy (HAXPES) technique  that enables us to investigate the chemical composition of the whole SEIs in non-destructive way, and then, we discuss the reason of deviation from the SQRT law.
A commercial 18650-type LIB cell, which consists of a graphite anode, Li(Ni1/3Mn1/3Co1/3)O2 cathode, and LiPF6 in a mixture solvent of EC, PC, EMC and DMC as electrolyte, was used in this study. Cells were charged and discharged with 1C rate (0, 100, 200 and 500cycles) at 25ºC. The cells were then disassembled in an argon glove box, in which the oxygen and water contents were maintained below 1 ppm. Then the electrodes were washed with highly purified DMC to remove the residual electrolyte components. After washing, the electrodes were vacuum-dried to evaporate the solvents and then transferred to the analysis chamber by using a transfer vessel.
The HAXPES measurements were carried out at the BL46XU beamline at SPring-8. The photon energy of incident X-rays was 14 keV. The X-ray take-off angle was 80º
Results and discussion
Figure 1(a) shows the change of discharge capacity retention as a function of SQRT of the number of charge/discharge cycle. Although discharge capacity retention was proportional to SQRT up to 300cycles, a sudden reduction was observed around 300 cycles.
We show the pictures of disassembled anodes after cycle tests in Fig. 1(b). We observed the inhomogeneous degradation in anode sheet especially on 500 cycled. At the outside, the surface of the anode layer was discolored to blue, and at the inside, we observed the depositions. We, then, investigated the distribution of SEI growth on the anode sheet in addition to the cycle dependence of SEI growth.
Figures 1(c) and (d) show C 1s spectra obtained by using 14 keV HAXPES. We can easily find the graphite peak (283.9 eV) was observed in all samples by using 14 keV HAXPES. In XPS spectra with Al Kα incident X-rays (hv = 1486.6eV), this graphite peak wasn’t observed in almost all samples. This indicates that the probing depth of 14 keV HAXPES is larger than thickness of SEI, thus 14 keV HAXPES detected whole SEI and a part of the graphite exists underneath SEI formed on all the anodes in the depth direction.
In order to look the difference in cycle dependence of SEI growth between the outside and inside of an anode sheet, we focused on the intensity of graphite peak. In Fig. 1(c), we observed that the intensity of graphite peak gradually decreases with increasing cycles. This indicates that the SEI gradually grew with increasing cycles at the outside. On the other hand, in Fig. 1(d), sudden decrease in the intensity of graphite peak was observed on 500 cycled, though the intensity of graphite peak hardly changes on 100 and 200 cycled. This result suggests that SEI dramatically grew between 200 and 500 cycles at the inside. This behavior coincides with the sudden reduction of discharge capacity retention.
From these results, one reason for the sudden reduction of discharge capacity retention was considered to be caused by the irreversible loss of Li due to rapid SEI growth at the inside of anode sheet.
At the meeting, we will also discuss about the variation of the chemical composition of SEI and the factor of rapid SEI growth.
The synchrotron experiments were carried out on beamline BL46XU at SPring-8 with approvals from Japan synchrotron radiation institute (JASRI) (Proposal No. 2014B1903).
 H. Yoshida et al., Electrochemistry 71, 1018 (2003).
 M. Matsumoto et al,. 226thECS meeting A5-422.
Fig.1 (a) Cycle dependence of discharge capacity retention. (b) Pictures of disassembled anodes after cycle tests. (c) and (d) C 1s spectra obtained by using 14 keV HAXPES at the outside and at the inside of an anode sheet, respectively.