Capacity Fading Mechanism of Ni-Rich Cathode Materials Focusing on Particle Interior
Authors: Park Nam-Yung , Han Sang-Mun , Sun Yang-Kook 
Published: February 2024

With the prevalence of electric vehicles, Ni-rich layered cathodes have been extensively developed to increase the energy density of Li-ion batteries (LIBs). However, cathodes with a high Ni content suffer from inherent structural and chemical instabilities, which lead to rapid capacity fading and thermal instability.1 The degradation of Ni-rich cathode materials is largely caused by the high proportion of reactive Ni4+ combined with the mechanical instability originated from microcracks. This unstable Ni4+ can be readily reduced to Ni2+ by parasitic reactions with the electrolyte concurrently with oxygen release, forming a NiO-like rocksalt impurity phase.2 This phase increases the charge-transfer resistance and thus deteriorates the electrochemical performance of Ni-rich cathodes. As the degradation mainly occurs at the surface of cathode by deleterious electrolyte attack, the rate of degradation is significantly determined by the area exposed to the electrolyte. For cathodes with low Ni contents of ≤60%, the cathode−electrolyte interphase is only limited to the exterior surface of cathode particles in which microcracks are not severe; thus, the decrease in the cycling stability of cathodes is directly proportional only to their Ni4+ fraction. However, as the Ni content in the cathode increases above 60%, the extent of microcracks rapidly increases owing to the buildup of anisotropic strain induced by abrupt lattice volume changes during the H2−H3 phase transition.2 The resultant microcracks in Ni-rich cathode particles can create channels through which the electrolyte infiltrates into the particle interior, thereby increasing the surface area exposed to the electrolyte attack. Janek et al. reported that the specific surface area of 0.2 m2 g−1 for pristine LixNi0.8Co0.1Mn0.1O2 cathode increased to ∼1.4 m2 g−1 when charged to 4.2 V (vs Li/Li+) owing to crack formation.3 This increased surface area further accelerated the rate of capacity fading for Ni-rich cathodes (especially Ni fraction ≥90%), as not only the particle exteriors but also the interiors were exposed to deleterious electrolyte attack and were thus also passivated by NiO-like impurity phases caused by surface degradation.

In this study, we performed comparative post-mortem analyses of Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) and Li[Ni0.90Co0.05Mn0.05]O2 (NCM90) cathodes after long-term cycling to elucidate the degradation mechanism of Ni-rich cathodes, focusing on the particle interior. Furthermore, we demonstrated correlations between the degradation of cathodes and the resultant changes in electrochemical properties (e.g., ionic and electrical conductivities). This systematic approach enables us to identify the mechanism by which cathode degradation deteriorates the electrochemical performance, which we expect to lead to breakthroughs in developing Ni-rich cathodes appropriate for EV applications.

Reference
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[1] H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources, 2013, 233, 121.

[2] H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155.

[3] E. Trevisanello, R. Ruess, G. Conforto, F. H. Richter, J. Janek, Adv. Energy Mater.
2021, 11, 2003400.