Inhomogeneous Degradation Phenomena of NMC Cathode in 18650-Type Lithium-Ion Battery: A Soft X-Ray Absorption Spectroscopic Investigation

Introduction Establishment of lifetime prediction methodology for Li-ion batteries (LIBs) is strongly required in order to prolong the lifetime of LIB and to use LIBs in safe. While “square root (SQRT) law” is proposed to predict the lifetime of LIBs [1] based on the formation of an irreversible solid electrolyte interphase (SEI) at anode surface and the electrochemically deactivation of cathode active materials, the SQRT law cannot describe the degradation behavior precisely in while period of LIB operation, especially near the end stage of degradation. In addition, detailed mechanism of degradation also remains unknown. Previously, we reported the detailed structure of SEIs in heavily deteriorated anodes by the Hard X-ray photoemission spectroscopy (HAXPES) and discuss the degradation mechanism focused on inhomogeneous SEI growth [2]. In this study, we also investigated the detailed structure of surface in heavily deteriorated cathodes by the soft X-ray absorption spectroscopy (XAS) and then, we discuss the inhomogeneous degradation phenomena of cathode. Experimental A commercial 18650-type lithium ion battery cell, which consists of a graphite anode, Li(Ni 1- x - y Mn x Co y )O 2 cathode, and LiPF 6 in a mixture solvent of EC, PC, EMC and DMC as electrolyte, was used in this study. The Cells were charged and discharged with 1C rate (0, 100, 200 and 500cycles) at 25ºC. After the cycle tests, the battery cells were disassembled in an argon glove box, in which the oxygen and water contents were maintained below 1 ppm. The electrodes were washed with highly purified DMC to remove the residual electrolyte components and then, the electrodes were vacuum-dried to evaporate the solvents. All samples were transferred to the analysis chamber by using a transfer vessel. The XAS measurements were carried out at SR Center of Ritsumeikan University on a grating beamline BL-11. We simultaneously obtained the XAS spectra in three different modes, a partial electron yield (PEY) mode, a total electron yield (TEY) mode and a fluorescence yield (FY), whose detection depth correspond to ~ 2 nm, ~ 20 nm and ~ 200 nm, respectively. Results and discussion Figure 1(a) shows the change of discharge capacity retention as a function of SQRT of the number of cycle. Although discharge capacity retention was proportional to SQRT up to 300cycles, sudden reduction was observed around 300cycles. We show the discharge curve of the reassembled coin-type cells operated in a voltage region of 4.2-2.6 V for MNC cathode at a constant current of 0.1 C at 25 °C in Fig. 1(b). In these measurements, we applied the cathode at the inside and outside of the sheet and we observed drastic fade of the cathode capacity at the outside of sheet after 500 cycled. Figure 1 (c), (d) and (e) show Mn L 2,3 –edge, Co L 2,3 - edge and Ni L 2,3 –edge XAS spectra of cathode active materials in discharge state (3.6 V) taken by PEY mode, respectively. We can easily find that ratio of lower valence state in each XAS spectra (Mn: ~640 eV, Co: ~779 eV, Ni: ~854 eV) is higher at the outside of cathode sheet after 500 cycled, especially in Mn L 2,3 –edge XAS spectra. On the other hands, we observed the no difference in XAS spectra at the outside and inside of cathode sheet taken by FY mode (shown in Fig. 1(f)). These indicate that the surface of the cathode active materials at the outside of sheet strongly degenerated and the Mn ions were selectively influenced. We also show the Ni L 2,3 –edge XAS spectra in charged state (4.2 V) taken by PEY and FY mode in Fig. 1(g) and Fig. 1(h), respectively. In the NMC cathode, it is well known that only the Ni ions change in valence form Ni 2+ to Ni 4+ during the charging process to 4.2V. However, we observed the no change in valence state of Ni ions at the surface and the outside of cathode sheet, indicating the existence of deactivated regions. In addition, the change in valence state of Ni ions at the outside of cathode sheet is smaller than that of the inside at bulky regions. From these results, the sudden reduction of discharge capacity retention was thought to be caused by the deactivation of active materials at the outside of cathode sheet, especially at the surface, in addition to the irreversible loss of Li due to rapid SEI growth at the inside of anode sheet [2]. Acknowledgement The synchrotron experiments were carried out on beamline BL-11 at SR center of Ritsumeikan University and we thank K. Yamanaka for his help in XAS measurements. Figure 1