Revealing the degradation mechanism of Ni-rich cathode materials after ambient storage and related regeneration method

Abstract

The widespread application of Li-ion batteries (LIBs) in electric vehicles requires high energy density and high stability of batteries in prolonged and diverse storage and service conditions. As one of the most practical and promising cathode materials for LIBs, Ni-rich layered oxide cathodes possess high capacity together with low cost. Nevertheless, due to its high sensitivity toward exposure to ambient air, structural evolution will occur accompanied with the generation of an impurity layer on the surface of the cathode. The structural instability of such Ni-rich cathodes in the storage process inevitably deteriorates their performance in subsequent service occasions. Herein, we carefully investigated the degradation mechanism of the Ni-rich cathode (LiNi_0.8Co_0.15Al_0.05O₂, NCA) after storage in the ambient air by means of in situ and ex situ characterization. The formed impurity layer on the surface of NCA has been determined as “residual lithium” composed of lithium carbonate and lithium bicarbonate, and nickel carbonate species due to the H⁺/Li⁺ exchange reaction followed by carbonation. Moreover, lithium and nickel extracted from the lattice to the surface further aggravated the formation of surface impurities, leaving a Li deficiency layer accompanied with spinel and rock salt phases near the surface. Simultaneously, the degraded NCA possessed increased c-axis and decreased a-axis lattice parameters, a lower degree of Ni/Li anti-sites and more surface Ni²⁺ ions, which caused the deterioration of its electrochemical performances. In addition, we proposed a facile method for regenerating the degraded NCA after ambient storage. By mixing the degraded NCA with a certain amount of additional lithium source and calcining the mixture in an oxygen atmosphere at a given temperature for a certain time, the degraded NCA was restored to its original state and regained superior electrochemical performances. It is highly expected that this work may help deepen the understanding of the storage failure and regeneration for Ni-rich cathode materials, and promote the practical industrialization and application of LIBs.