Article Cite This: J. Phys. Chem. C 2018, 122, 20099−20107
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Site-Selective Analysis of Nickel-Substituted Li-Rich Layered Material: Migration and Role of Transition Metal at Charging and Discharging
J. Phys. Chem. C 2018.122:20099-20107. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/06/18. For personal use only.
Hideyuki Komatsu,†,∥ Taketoshi Minato,† Toshiyuki Matsunaga,† Keiji Shimoda,† Tomoya Kawaguchi,† Katsutoshi Fukuda,† Koji Nakanishi,† Hajime Tanida,† Shunsuke Kobayashi,‡ Tsukasa Hirayama,‡ Yuichi Ikuhara,‡,§ Hajime Arai,† Yoshio Ukyo,† Yoshiharu Uchimoto,*,∥ Eiichiro Matsubara,⊥ and Zempachi Ogumi† †
Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan § Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan ∥ Graduate School of Human and Environment Studies and ⊥Department of Materials Science and Engineering, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan ‡
S Supporting Information *
ABSTRACT: Li-rich type manganese oxides are one of the most promising cathodes for lithium-ion batteries in recent years; thanks to their high energy density. In these cathodes, partial substitution of manganese by other transition metals such as nickel and cobalt has been proposed and shown to be effective in improving the performance; however, the role of such metals in the battery performance has not been clarified. We examined Ni-substituted Li2MnO3 as a model of Li2MeO3 solid-solution cathodes to understand the effect of the substituted Ni on the electrode performances by using a combination of resonant X-ray diffraction spectroscopy (RXDS) and operando X-ray absorption spectroscopy. The capacity and cyclability were improved by substituting Ni into the Li2MnO3 phase, which suggests its important roles in the cathodes. The change in the oxidation state and transbilayer migration of the transition metals as a function of the operating potential during the first charge−discharge processes were revealed by the site-selective analysis of RXDS. We discuss the influence of the irreversible and reversible migration of Ni and Mn ions on the electrode performance. scanning transmission electron microscopy (STEM) study.9 However, it is still unclear when the transition metals migrate to which crystallographic sites. Actually, the electrochemical delithiation and lithiation processes in the first cycling process can greatly alter the whole battery behavior. For example, Ito et al. reported that a gradual charge process can reduce the irreversible capacity loss in the first charge and discharge cycle.10−12 Elucidating the phenomena in the first cycle is therefore of great importance to realize the solid-solution cathodes. The changes in the crystal structure during the first cycle have been studied by the Rietveld analysis of synchrotron radiation or neutron radiation diffraction.13,14 Consequently, the disappearance of the superlattice peak is believed to be related to the rearrangement in the transition metal layer
1. INTRODUCTION Lithium-ion batteries (LIBs) have been widely used for electric devices and they are now applied as power sources for electric vehicles with high energy density for extended driving ranges. One of the effective ways for further improving the performance of the LIBs is to develop electrode active materials. Solid solutions of Li-rich manganese oxide (Li2MnO3) and other layered transition metal (TM) oxide (LiMeO2, Me = Co, Ni, Al etc.) are the most featured active material for the cathode of LIBs.1,2 A high capacity of over 250 mAh g−1 and comparatively stable cyclability have been reported for these solid-solution cathodes.3 It was currently proposed that not only the involvement of introduced transition metal but also oxygen redox reaction4−8 are its reaction mechanism. It has also been reported that transition metal migration occurs in these materials during the first charging process when the lithium in the transition metal layer is extracted, by a © 2018 American Chemical Society
Received: June 10, 2018 Revised: August 6, 2018 Published: August 17, 2018 20099
DOI: 10.1021/acs.jpcc.8b05539 J. Phys. Chem. C 2018, 122, 20099−20107
Article
The Journal of Physical Chemistry C [Li1/3Mn2/3]15 and/or the formation of a stacking fault along with c-axis.16 Such rearrangement in the transition metal layer presumably affects the evolution of the high capacity, temperature dependence of the performance,15 and voltage fading phenomenon.17 A previous X-ray absorption spectroscopy (XAS) study reported that manganese remains tetravalent and the redox reactions proceed by the valence changes of the other transition metals (e.g., Ni, Co) and oxygen.18 The combination studies of scattering and spectroscopy are complementary each other; however, it is desirable to use single analysis with both site selectivity (like scattering) and element selectivity (like spectroscopy) to understand the reaction mechanism in detail. Therefore, we attempted to semi-quantitatively comprehend the transition metal migration in Ni-substituted Li2MnO3 as a simple model of Li2MeO3 solid-solution cathodes by performing resonant X-ray diffraction spectroscopy (RXDS) analysis19 in which element selectivity and site selectivity are respectively obtained by resonant scattering. For this purpose, operando XAS were first employed to understand the average valence changes of the constituent transition metal during the whole first cycle. Then, the RXDS analyses were further performed in the selected important conditions.
ethylene carbonate and ethyl methyl carbonate with a volumetric ratio of 3:7 (Kishida Chemical) was used as the electrolyte in this study. The electrochemical cell was assembled in an Ar-filled glove box (
