Correlation of Structural Changes in Li Rich Cathode Materials to Their Electrochemical Performance

Abstract

Li rich oxides xLi2MnO3.(1-x)LiMO2, (M=Ni, Co, Fe, Al, Cr etc.) have attracted significant attention in recent years as the promising cathodes for Li-ion batteries because of their higher energy storage capacity, thermal stability, and lower costs. These materials are structurally integrated composites of two phases: monoclinic C2/m Li2MnO3 phase and trigonal R-3m LiMO2 phase [1]. The Li2MnO3 phase provides structural integrity to the LiMO2 during Li intercalation and also can be activated to LixMnOy at potentials higher than 4.5 V to further boost the energy storing capacity [2]. The reported performance of Li[Li1/6Fe1/6Ni1/3Mn1/2]O2 cathode is 282 mAh/g at first discharge [3], which is double the capacity of LiCoO2 cathode currently utilized in commercial Li ion batteries. However, before those cathodes could replace traditional materials two issues with Li-rich cathodes need to be resolved: gradual capacity loss (71% of capacity after 50 cycles [3]), and drop off in discharge voltage upon cycling (3.57 V of midpoint voltage at the 4th discharge decreases to 3.38 V at the 100thdischarge [4]). Both degradation phenomena are closely related to structural changes within the cathode materials during cycling. To improve the performance of Li-rich cathode detailed understanding of the degradation mechanisms in those materials is required. In this talk we will report on the experimental study of evolution of structure as the function of cycle number, charge modes and composition in low cost, Li rich, Co free Li(Li0.2Nix/2Mn0.8-xFex/2)O2, 0.2<x<0.4 layered-layered oxide cathode. Our study employs x-ray diffraction for crystallographic phase analysis, scanning electron microscopy for particle size and morphology characterization, infrared spectroscopy for molecular bonding and in-situ x-ray absorption spectroscopy for probing the oxidation state and local structure within the materials, during charge and discharge cycles.