As portable energy storage systems, Lithium-ion batteries are particularly attractive due to their high energy densities. Developing as well as improving appropriate cathode materials are important as the energy density of the commercial Li-ion batteries are limited mainly by the positive electrode materials. Several cathode material classes such as spinels (LiMxMn2-xO4, M = Mn, Ni, Co etc.)[1–4], layered oxides (LiMO2, M = V, Cr, Co, Ni, Ni/Co/Mn, Al/Co/Mn etc.)[5], Li-rich layered oxides [xLi2MnO3 ∙(1-x)LiMO2, M = Mn, Co, Ni, Fe][5] and olivines (LiMPO4, M = Fe, Mn, Co, Ni etc.)[3] have been widely investigated in the recent years. Out of these materials, the layered Li-rich materials are particularly attractive for high energy Li-ion batteries as they offer high capacities (~200 mAh/g) in a voltage range 2.0-4.8 V. Up to now, layered Li-rich materials containing Co are broadly used as positive electrode materials in several commercial Li-ion batteries. Many literature reports show that these materials can exhibit a stable cycling stability and in addition, for some special compositions of the transition metals, a high rate capability[6][7]. Nevertheless, Co is expensive and not environmentally friendly. Hence, keeping the advantages of Li-rich materials, together with reducing or eliminating Co from these Li-rich layered materials can be a big step to develop potential cathode materials for Li-ion batteries which are cost effective and environmentally friendly.
The Li-rich materials containing Fe are particularly attractive as cathode materials for Li-ion batteries as they are more “green” and cost effective[8]. In 2002, Fe substituted Li2MnO3 was 1st synthesized as 4 V cathode materials. The material delivered only a very low specific capacity (~70 mAh/g) and a very severe capacity fading[8]. Later, it was found that the capacity retention of the above material can be improved by Ni or Al substitution. However, the reports on the long term cycling stability are scarce and those on high rate cycling stability (> 5C) and electrochemical mechanism are missing.
Results and Discussion
In the present work, Fe and Ni containing Li-rich materials with a general formula, xLi2MnO3. (1-x)LiNi0.4Mn0.4Fe0.2O2, were synthesized through a modified citric-acid assisted sol-gel process reported elsewhere[1]. The precursors obtained were subjected to final calcination temperatures of 800 °C, 850 °C, 900 °C, respectively. For a part of the material annealed at 900 °C, a quenching step with liquid N2 was applied. The annealing temperature was found to have an influence on the phase purity and morphology of the material. Rietveld refinement result based on the synchrotron diffraction experiments conducted at MSPD beamline, ALBA revealed that the amount of impurity phases increases with increase in annealing temperature. However, the quenching step was found to suppress the formation of impurity phase successfully, even after the high-temperature annealing. Moessbauer spectroscopy confirms the above observation and concludes the existence of Fe only in +3 oxidation state. After synthesis optimization, electrode optimization was done and the materials were investigated electrochemically using cyclic voltammetry and galvanostatic cycling. The details on the influence of annealing temperature on the morphology, cycling stability and rate capability will be discussed. In addition, a first insight into the electrochemical mechanism will be given.
Acknowledgement:
This research has benefitted from beamtime allocation at the Materials Science and Powder Diffraction Beam Line (MSPD) at ALBA (Barcelona, Spain). The kind support of the beamline scientist Dr. Francois Fauth is gratefully acknowledged.
References:
[1] A. Bhaskar, N. N. Bramnik, A. Senyshyn, H. Fuess, H. Ehrenberg, J. Electrochem. Soc. 2010, 157, A689
[2] M. M. Thackeray, Prog. Solid State Chem. 1997, 25, 1
[3] C. M. Julien, A. Mauger, Ionics 2013, 19, 951
[4] M. J. Iqbal, S. Zahoor, J. Power Sources 2007, 165, 393
[5] M. M. Thackeray, S.-H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek, S. A. Hackney, J. Mater. Chem. 2007, 17, 3112
[6] J. Wang, X. He, R. Kloepsch, S. Wang, B. Hoffmann, S. Jeong, Y. Yang, J. Li, Energy Technol. 2014, 2, 188
[7] J. Li, R. Klöpsch, M. C. Stan, S. Nowak, M. Kunze, M. Winter, S. Passerini, J. Power Sources 2011, 196, 4821
[8] M. Tabuchi, A. Nakashima, H. Shigemura, K. Ado, H. Kobayashi, H. Sakaebe, H. Kageyama, T. Nakamura, M. Kohzaki, A. Hirano, R. Kanno, J. Electrochem. Soc. 2002, 149, A509