Insight into the capacity decay mechanism of cycled LiNi_0.5Co_0.2Mn_0.3O₂ cathodes via in situ x-ray diffraction

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 conducted in situ high-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, the c-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.