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ORIGINAL PAPER

Study on the NiO-like phase in Ni-rich cathode materials for lithium ion batteries Zexin Jiang 1,2 & Haitao Gu 3 & Jian Zhang 1 & Chunhui Cao 1 & Xiaohua Xie 1 & Ke Wang 3 & Baojia Xia 1 Received: 14 May 2020 / Revised: 20 May 2020 / Accepted: 31 May 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract Ni-rich cathode materials have become the main research focus in the field of Li-ion batteries due to the urgent need for cathode materials with higher weight and volume energy density for application of Li-ion batteries in electric vehicles. However, the conditions for the preparation of these materials are harsh, they are unstable during storage in air, and problems like capacity fading and structure deterioration are hard to overcome. All these drawbacks are related to Li/Ni disorder and NiO-like phases, which cover the surface of Ni-rich cathode materials. In this article, the influence of NiO-like phases on the electrochemical properties of Ni-rich cathode materials during preparation, cycling, and storage in air is studied, and a formation mechanism for NiO-like phases is proposed. The results confirm that the amount of NiO-like phases, especially on the surface, must be reduced for the preparation of Ni-rich cathode materials with high performance. Keywords Ni-rich . Cathode materials . NiO-like phase . Electrochemical performance

Introduction Lithium ion batteries (LIBs) have been widely used as power sources in portable devices since their commercialization by Sony in 1991. In recent years, applications of LIBs in electric automobiles have attracted much attention but require proper cathode materials to achieve higher energy densities and lower costs. Presenting these characteristics, Ni-rich cathode materials Ni1-x-yCoxMyO2 (M for Al or Mn, 1-x-y ≥ 0.6) have become one of the best choices for power batteries [1–4]. LiNi0.8Co0.15Al0.05O2 (NCA) has been successfully applied to manufacturers such as Tesla, and numerous studies on Nirich cathode materials are ongoing [5–7]. This layered material has a hexagonal structure and is isostructural with α-NaFeO2. Oxygen ions are arranged in a

* Chunhui Cao [email protected] 1

Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 201800, China

2

University of Chinese Academy of Sciences, Beijing 100049, China

3

State Key Laboratory of Space Power-Sources Technology, Shanghai Institute of Space Power-Sources, Shanghai 200245, China

hexagonal close packing, while lithium ions and transition metal ions alternately occupy octahedral interstitial sites, causing a layer sequence of O–Li–O–Ni–O along the c axis. Since Li+ and Ni2+ have similar ionic radii of 0.76 and 0.69 Å, respectively, Li+/Ni2+ disorder is facilitated, which means a small amount of Ni2+ occupies the Li+ positions in the lithium layers [8]. Ni-rich cathode materials can be considered as LiNiO2 doped with Co and Mn or Al. Therefore, the following theoretical analyses are exemplarily performed based on LiNiO2 instea