Atomic-level insights into the first cycle irreversible capacity loss of Ni-rich layered cathodes for Li-ion batteries

Abstract

Nickel-rich layered cathode materials and their correlation with the electronic structure are crucial to understanding the functionality of Li-ion batteries in the commercial deployment of electric vehicles. In this study, we employ synchrotron X-ray diffraction and hard X-ray absorption spectroscopy measurements, along with the corresponding first-principles calculations, to provide atomic-level insights into the first cycle irreversible capacity (IRC) loss of a LiNi_0.90Mn_0.05Co_0.05O₂ (NCM900505) cathode. Our results identify that the first cycle IRC loss is strongly related to the irreversibility of the Ni charge state, which limits the capacity and energy density. We show that the shift to higher energies of the Ni K-edge X-ray absorption spectrum during charging is a result of a decreased number of Ni²⁺/Ni³⁺ ions, while the number of Ni⁴⁺ ions increases. Our findings also indicate that the increased number of Ni⁴⁺ ions shortens the averaged Ni–O bond length after the first charging, and these changes in the atomic structure are irreversible after discharging. We expect that these new insights of Ni-rich cathodes (Ni > 80%) provide a fundamental understanding of their cycling behavior and offer strategies to understand this first cycle IRC loss and suggest structural design routes of Li-ion batteries for electric vehicles.