Li rich oxides xLi₂MnO_3.(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 Li_xMnO_y at potentials higher than 4.5 V to further boost the energy storing capacity [2]. The reported performance of Li[Li_1/6Fe_1/6Ni_1/3Mn_1/2]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 4^th discharge decreases to 3.38 V at the 100^thdischarge [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(Li_0.2Ni_x/2Mn_0.8-xFe_x/2)O_2, 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.

References:

1. Thackeray, Michael M., et al. "Li 2 MnO 3-stabilized LiMO 2 (M= Mn, Ni, Co) electrodes for lithium-ion batteries." Journal of Materials Chemistry17.30 (2007): 3112-3125.

2. Armstrong, A. Robert, et al. "Demonstrating oxygen loss and associated structural reorganization in the lithium battery cathode Li [Ni0. 2Li0. 2Mn0. 6] O2." Journal of the American Chemical Society128.26 (2006): 8694-8698.

3. Zhao, Taolin, et al. "Organic-acid-assisted fabrication of low-cost Li-rich cathode material (Li [Li1/6Fe1/6Ni1/6Mn1/2] O2) for lithium–ion battery." ACS applied materials & interfaces6.24 (2014): 22305-22315.

4. Zheng, Jianming, et al. "Corrosion/fragmentation of layered composite cathode and related capacity/voltage fading during cycling process." Nano letters 13.8 (2013): 3824-3830.