In-Situ Raman Spectroscopy and Electrochemical Studies on High Energy Density xLi2MnO3-(1-x)LiNi0.66Co0.17Mn0.17O2 Composite Cathode Materials

Abstract

Development of high energy density cathode materials is the key to fabricate high power Li-ion batteries (LIBs) capable of meeting high power demands in new applications such as regenerative braking in hybrid electric vehicles, power backup, and portable power tools. Li2MnO3-based composite cathode materials are attracting much attention due to their excellent electrochemical performances [1-5]. In the present work, novel layered-layered composite cathode materials with small amount of cobalt and different amounts of Li2MnO3 e.g. xLi2MnO3-(1-x) LiNi0.66Co0.17Mn0.17O2 where x = 0.3, 0.5, and 0.7 were successfully synthesized by employing sol-gel technique and characterized using advanced techniques to investigate both the structural and electrochemical properties. Powder X-ray diffraction patterns confirmed the formation of layered-layered composite materials revealed by the presence of reflections corresponding to rhombohedral (R3m) and monoclinic (C2/m) space group symmetries. Structural analysis by Raman spectroscopy showed three distinct phonon modes. Peaks centered at 482 cm-1 and 596 cm-1 correspond to Eg and A1g modes of the rhombohedral phase and the one at 420 cm-1 represent the presence of monoclinic Li2MnO3 component. Microstructural evaluation by scanning electron microscopy and energy dispersive X-ray spectroscopy divulged the highly dense polyhedral-shaped agglomerates with respective elements. X-ray photoelectron spectroscopy analysis revealed 2+, 3+, and 4+ as the predominant oxidations of Ni, Co, and Mn respectively. Electrochemical measurements were performed using CR2032 type coin cells assembled with Li-metal foil as reference/counter electrode and spray-coated composite materials as cathode. Extended rate capability tests were also carried out at different C-rates, C/40, C/20, C/10, C/5, and 1C and compared performances of the three compositions. The results indicate that the composite cathode with x = 0.5 exhibited better electrochemical performances in terms of high discharge capacity (~250 mAh/g) , rate capability and cycleability in comparison with those with x = 0.3 and 0.7. According to the electrochemical studies, the first cycle charge profile was quite different from the subsequent cycles and irreversible capacity losses are much higher in the first cycle. The voltage in the cell increases monotonically until 4.45 V and reaches a plateau region between 4.45 and 4.6 V. To understand the local structural changes occurring in the materials and investigate the mechanisms associated with the first cycle charge-discharge profile, in situ Raman spectroscopy studies were conducted using specially designed/fabricated coin cells. Electrochemical properties of the composite cathode materials probed by charge-discharge profiles, cyclic voltammetry, cycleabilty, rate capabilty, electrochemical impedance spectroscopy and in situ Raman spectroscopic studies will be presented. Acknowledgements The financial support from NASA-URC grant #NNX10AQ17A is gratefully acknowledged. A part of the work was carried out at Jet Propulsion Laboratory under NASA’s Space Power Systems Program.