Elucidating the Implications of Morphology on Fundamental Characteristics of Nickel-Rich NCMs: Cracking, Gassing, Rate Capability, and Thermal Stability of Poly- and Single-Crystalline NCM622

Abstract

Nickel-rich NCM (LiMO2, with M = Ni, Co, and Mn) cathode active materials for lithium-ion batteries are being increasingly commercialized due to their high specific capacity. Since the particle cracking of conventional polycrystalline NCMs is reported to be a major failure mechanism, the demand for single-crystalline materials is rising, as they are believed to provide superior cycle life. To gain comprehensive insights into the implications of NCM particle morphology on the electrochemical performance, the fundamental properties of these two material classes will be examined in this study. Krypton physisorption experiments and capacitance measurements reveal considerable differences in the change of the NCM surface area upon compression, delithiation, and charge/discharge cycling, depending on the material’s morphology. Here, a polycrystalline NCM622 exhibits changes of its specific surface area of up to 650 % when cycled to a high state of charge, while the one of a single-crystalline NCM622 remains essentially unaffected. Consequently, the difference in morphology and, therefore, in exposed NCM surface area leads to differences in the extent of gassing at high degrees of delithiation (determined via on-line electrochemical mass spectrometry), in the rate capability (evaluated in half-cell discharge rate tests), and in the thermal stability (assessed by thermogravimetric analysis).