Activation of nano-Ca 2 MnO 4 for electrochemical lithium intercalation
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Activation of nano-Ca2MnO4 for electrochemical lithium intercalation Yuri Surace1, Mário Simões1, Lassi Karvonen1, Corsin Battaglia1, Simone Pokrant1, Anke Weidenkaff,2 1 Laboratory Materials for Energy Conversion, EMPA - Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland 2 Materials Chemistry, Institute for Materials Science, University of Stuttgart, Heisenbergstr. 3, DE-70569 Stuttgart; Germany ABSTRACT Ca2MnO4 nanoparticles were prepared by the Pechini method and acid treated to extract Ca2+ ions. Structural, morphological and spectroscopic analyses by XRD, SEM/EDX, TEM/EDS and Raman revealed the formation of an amorphous outer layer at the particles surface with a preserved inner crystalline bulk. Thanks to the outer layer, which is electrochemically active, the acid-treated compounds showed capacity up to 150 Ah/kg. The crystalline bulk improved cycling stability, allowing reaching capacity retention up to 70% after 30 cycles. INTRODUCTION Since the introduction of LiCoO2-based Li-ion batteries in the global market, much effort has been made on developing metal oxides as cathode materials. Nowadays, three main metal oxides families are employed in Li-ion batteries: layered oxides LiMO2 (M=Co, Ni, Mn, etc), spinels LiM2O4 (M=Mn, Ni, etc), and olivines LiMPO4 (M=Fe, Co, Ni, Mn, etc) [1]. However, their performances are still not suitable for large scale Li-ion battery applications (i.e. electric vehicles) [2]. Consequently, exploration of new energy storage materials is a mandatory step to obtain Li-ion batteries having high energies and power densities. Ruddlesden–Popper phase Ca2MnO4 is a promising compound unexplored as battery cathode material. It contains low toxic and cheap elements as Ca and Mn, is thermally stable, has a layered structure and contains Mn which is an easily reducible/oxidable ion. On the other hand its extremely high resistivity[3] (> 20 MΩ * cm) prevents its use in Li-ion batteries. Ca2MnO4 having micron-sized particles has been already synthesized, modified for lithium intercalation, and characterized electrochemically [4]. It is well known, however, that lowering the particle size increases the electrode/electrolyte contact area improving total capacity and rate capability [5], especially in materials with low conductivity. For this reason, Ca2MnO4 was synthesized as nanoparticles and subsequently activated removing calcium ions from the surface of the samples by acid treatment similar to the treatment in Ref.[4] . EXPERIMENTAL Nano Ca2MnO4 (n-Ca2MnO4) was prepared by a soft chemistry method [6] using analytical grade citric acid (CA) (Sigma-Aldrich 99%), ethylene glycol (EG) (VWR 98%), Ca(NO3)2·4H2O (Sigma-Aldrich >99%) and Mn(NO3)2·4H2O (Sigma-Aldrich 98%) with a CA/EG ratio of 1 and a CA/metal ion ratio of 1. The precursors were dissolved in a round bottom flask containing 50 mL of high purity water. The solution was heated to allow the evaporation of water and the formation of the gel. The gel was heated at 250 °C for
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