Cracking the Genetic Code of Nickel on the Performance of the High Energy Layered Cathode Materials
Authors: Lee Wontae , Lee Eunkang , Kim Taewhan , Lee Kyung Lok , Jeong Mihee , Yoon Won-Sub 
Published: February 2020

Recently, the LiMO2 layered cathode material has been playing a pivotal role in the field of lithium ion battery. There have been many attempts to combine various types of atoms in layered system to discover enhanced electrochemical performance cathode materials, and three-component system (Nickel, Cobalt, and Manganese) has gained the most interests among them. Specifically, a study of increasing the Ni content in the atomic combination becomes popular trend in order to obtain advanced cathode materials with higher capacity. However, there has not been a clear elucidation on the fundamental role of nickel element in the three-component layered system yet. Thus, the Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2) layered materials were designed to unravel the role of Ni atom as a major contributor to the crystallographic and electrochemical nature of Ni-rich layered materials. The results, on the basis of synchrotron-based characterization techniques, present a decreasing trend of Ni2+ content in Li layer with increasing total Ni contents in the Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2) layered materials unlike the previously reported trend, and it disabuses the misapprehension of cation mixing in Ni-rich cathode materials. Moreover, it is discovered that a c-axis parameter of layered system is not intimately related with the real lithium ion pathway. The lithium inter-slab space increases with growing level of nickel concentration even though the c-axis parameter decreases. To investigate the structural behavior of Co fixed cathode materials during charge/discharge (de-intercalation/intercalation) process, a synchrotron-based in situ XRD technique was used. The overall structure behavior is similar to each other; however, the degree of phase transition is different depending on the nickel content. The NCM721(x=0.2) cathode material exhibited a smooth phase transition having similar lattice parameter between H1 and H2 phase. For NCM523(x=0), however, the new emerged H2 phase started to be formed exhibiting large disparities of lattice parameter between H1 and H2 phase relatively to the higher nickel content materials. To find out the whole relative integrated peak intensity of H1 and H2 phase in each material during the de-intercalation process, the fitting of (113) reflection was performed. Based on this result, the oxidation states of nickel in each phase are analogized by simplifying some values which are matched with the trend of the experimental result, and then XRD patterns are simulated to evaluate the suitability of the hypothesis. As the nickel content increases, the gap of oxidation state between H1 and H2 phase is narrowed down. According to this trend, The NCM721 has recorded the highest lithium diffusion coefficient and the lowest over-potential on the whole during the de-intercalation process, and then NCM622, NCM523 in this sequence. The NCM523 cathode material has higher Ni2+ content in the Li layer and narrower lithium inter-slab space compared to higher nickel content cathode material such as NCM622 which is even more Ni2+ content in the Li layer and narrower lithium inter-slab space than NCM721. Following this trend, the XRD patterns during the de-intercalation process also showed a distinctive trend. In relation to this, a higher nickel content material shows better electrochemical performance in terms of capacity and cycling behavior compared with lower ones.