Investigating Structural Evolution and Reaction Kinetics of LixNi0.8Co0.15Al0.05O2 Cathode Materials during Initial Charge/Discharge Using TEM Coupled with Electron Energy Loss Spectroscopy
Authors: Jo Eunmi , Hwang Sooyeon , Chung Kyung Yoon , Stach Eric A. , Kim Seung Min , Chang Wonyoung 
Published: February 2020

Lithium-ion batteries have been widely utilized as the power sources in portable devices such as laptops and cellular phones and have attracted great interests for large-scale applications, representatively electric vehicles (EV), which require higher capacity, higher power, longer cycle life, and in particular lower cost and better safety characteristics. LiNi0.8Co0.15Al0.05O2 (NCA) is considered as one of the promising cathode candidates for EV applications due to its high discharge capacity (200 mAhg-1). However, NCA cathode materials have a disadvantage of structural instability, which results in a drastic capacity fade and impedance rise with cycling. Thus it is important to understand the degradation mechanisms of NCA cathode materials under various electrochemical conditions.
Our previous study delineated the structural evolution of NCA as a function of the extent of first charge. It was found that the crystallographic structures are modified from the layered structure (space group R-3m) to the disordered spinel structure (Fd-3m), and eventually to the rock-salt structure (Fm-3m). Changes in crystal structures are accompanied by significant changes in the electronic structures, which are reflected in oxygen K-edge electron energy-loss (EEL) spectra.1

In this work, we further investigate the evolution of surface structure that occurs within the NCA cathode materials during the initial charge/discharge under the various conditions of C-rates (C/10 to 10C) and state of charge (SOC 50% to 90%) using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). X-ray based techniques, such as x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) are very powerful in examining the average structural and chemical changes of the bulk electrode materials, but often inefficient in studies of local structural disorder or reorganization. As the structural changes initiate at the nanoscale, we take advantage of TEM, which is an ideal tool to investigate incipient structural changes with a high spatial resolution. Modifications in crystallographic and electronic structures occurring in NCA materials at the subsurface are investigated with selected-area electron diffraction (SAED) and EELS, which is used to obtain spectra of the O K edge and Ni L2,3 edge. Structural changes occurred at the edge of NCA particles are also examined at the nanoscale with high-resolution electron microscopy (HREM) and fast Fourier transformation (FFT). In particular, we take advantage of scanning transmission electron microscopy (STEM) and EELS to directly compare the extent of surface degradation that occurs in discharged NCA cathode at different C-rates. The NCA cathode materials were electrochemically charged to either SOC 50% or 90% at a same rate of C/10 and then discharged to 2.0V at different C-rates (C/10 to 10C) for evaluating the reaction kinetics during the lithiation process. We acquired EELS by scanning a sub-nanometer sized electron probe from the surface to the center of the particles. All the details will be available at the meeting.

Acknowledgement

This work was supported by the Korea Institute of Science and Technology (KIST) Institutional Program (Project Nos. 2Z04670 & 2E26292). E.A.S. acknowledges support to the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.

References

1. S Hwang, W Chang, S. M. Kim, D. Su, D. H. Kim, J. Y. Lee, K. Y. Chung, and E. A. Stach, Chem. Mater., 26, 1084−1092 (2014).