Material Engineering for High Energy Li-Rich Cathode Materials

Abstract

Li-rich material (Li2MnO3-LiMO2, M=Ni, Mn and Co) that has 150 % higher energy density than commercialized LiCoO¬2 is the unique next generation cathode material to solve the problem of cathode material’s low energy density. In this regard, surface modification and new material design have been considered to be effective methods to enhance the electrochemical performances of the Li-rich cathode material, since they can effectively improve active material’s electrochemical performance without the decrease of energy density of active material. Although many previous reports have been tried to stabilize Li-rich material, they could not be a comprehensive solution due to the experimental limitations, resulting low initial coulombic efficiency and voltage decline upon cycling. To overcome this barrier, new material engineering concepts with novel structure characterization method for Li-rich materials are proposed in this dissertation. 1) The new surface modification consists of (i) construction of continuous electron pathways on the bare particle’s surface by wrapping its surface with a few layers of rGO to enhance its surface electronic conductivity and suppress metal dissolution during cycles and (ii) formation of the chemically activated layer via the chemical treatment to stabilize the Li2MnO3 phase on the surface. Since the surface treatment produces stable and nanoscale uniform surface layer on pristine, the surface modified material showed superior performances compared to previous sol-gel coating methods such as AlF3 coating and spinel-layered heterostructure, which were reported in high impact factor journals. 2) The first investigation for the relationship between capacity fade and discharge voltage decline of this material during electrochemical cycling by using STEM which is a direct atomic-resolution observation method. Recently the voltage decline property has been considered as the most crucial problem of Li-rich materials. And the specific zone directions using the kikuchi map to help the analysis of epitaxial hetero-phase such as layered-spinel heterostructure are suggested in this investigation. The structural studies using direct observation method may open new understanding for cathode material’s deterioration mechanism and enable various novel material designs for high performance material. 3) Because of lack of efficient activation method for large particle, these materials should use smaller particles with high surface area, and this critical drawback leads to poor long-term energy retention with low volumetric energy density. In this regard, we propose a new material engineering concept consisted of (i) material design of 10 μm-sized secondary particles composed of sub-micron scaled flake shaped primary particle to decreased surface area without sacrifice of rate capability with (ii) a novel activation method, resulting in overcoming previous limits of large Li-rich material.