Enhancing the structural durability of Ni-rich layered materials by post-process: washing and heat-treatment

Abstract

Post-process (i.e., heat-treatment after washing) is known as an effective way to improve the cycle life of Ni-rich materials. However, most of the studies on the washing and post-heating treatment mainly focus on the relationship between surface properties and electrochemical performances even though the process includes heat treatment. Therefore, to understand the effects of the washing and heat treatment (i.e., post-process) on the Ni-rich layered materials, particularly on a bulk structural point of view, untreated and treated LiNi_0.88Co_0.11Al_0.01O₂ (NCA) cathodes were investigated. The details of the crystal and electronic structures of the materials, based on synchrotron-based X-ray analyses, indicate that the treated NCA has a favorable environment for Li ions to diffuse (i.e., a larger inter-slab thickness and lower cations in the Li layer), and it goes through a relatively smooth phase transition with a wider redox range of Ni ion than the untreated NCA material during the first cycle. Furthermore, the treated NCA with a stable activation process has a lower increment in the cation disorder and a better local structural retentivity after 300 cycles, and consequently shows a superior cycle life. In addition, the XAS result underpins that the capacity loss comes from the diminution in the reduction range of Ni ions.