Insight into the capacity decay mechanism of cycled LiNi0.5Co0.2Mn0.3O2cathodes via in situ x-ray diffraction

Yalan Huang; He Zhu; Hekang Zhu; Jian Zhang; Yang Ren; Qi Liu

doi:10.1088/1361-6528/abf2ff

Insight into the capacity decay mechanism of cycled LiNi_0.5Co_0.2Mn_0.3O₂cathodes via in situ x-ray diffraction

Nanotechnology. 2021 Apr 27;32(29). doi: 10.1088/1361-6528/abf2ff.

Authors

Yalan Huang^{1

2}, He Zhu¹, Hekang Zhu^{1

2}, Jian Zhang¹, Yang Ren³, Qi Liu^{1

2}

Affiliations

¹ Department of Physics, City University of Hong Kong, Hong Kong 999077, People's Republic of China.
² Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, People's Republic of China.
³ X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States of America.

PMID: 33780915
DOI: 10.1088/1361-6528/abf2ff

Abstract

Layered LiNi_xCo_yMn_1-x-yO₂(NCM) is expected to dominate the future cathode technology of the automotive industry, due to its high energy density and low cost. Despite its excellent prospects, however, the severe capacity decay of NCM cathodes has prevented this promising material from achieving further success. The mechanism underlying this phenomenon is controversial and has been generally understood as arising from the complex structural changes that take place upon Li-(de)intercalation. However, deeper insight has not been available due to unclear structural kinetics, in particular, in cycled NCM cathodes. For this study, we conductedin situhigh-energy synchrotron x-ray diffraction (XRD) measurements on a typical LiNi_0.5Co_0.2Mn_0.3O₂(NCM523) cathode that had been operated for 90 cycles, then compared the results with those collected from a fresh NCM532 electrode. It was revealed that the H1-H2 phase transition that only occurs at the first cycle is irreversible. Remarkably, thec-contraction triggered by the H2-H3 transition, which is expected to be the major cause of intergranular cracks in electrodes, became even more profound after cycling. Combining the above results with electrochemical testing and microscopic imaging, we discuss the interplay between structural dynamics and performance degradation in NCM532 in detail. This study provides key evidence for a mechanically induced capacity decay mechanism, which is expected to be extended to NCM materials with various compositions.

Keywords: NCM523 cathode; capacity decay mechanism; in situ XRD characterization; lithium-ion battery.