Article pubs.acs.org/JPCC
Li-Removal Mechanism and Its Effect on Oxygen Stability Influencing the Electrochemical Performance of Li1.17Ni0.17Mn0.67O2: Experimental and First-Principles Analysis Tanmay Sarkar,†,‡,§ K. R. Prakasha,‡,§ Mridula Dixit Bharadwaj,† and A. S. Prakash*,§ †
Center for Study of Science, Technology and Policy (CSTEP), Bangalore 560094, India Academy of Scientific and Innovative Research (AcSIR), CSIR Madras Complex, Chennai 600113, India § CSIR Central Electrochemical Research Institute, Chennai Unit, Chennai 600113, India ‡
S Supporting Information *
ABSTRACT: The high capacity of Li-rich layered cathode materials is always accompanied by the removal of oxygen from the crystal structure. These oxygen vacancies alter the structural stability, which subsequently deteriorates the electrochemical performance. The electronic origin of oxygen stability with partial delithiation has not been extensively studied so far in the presence of multiple d-orbital elements. Current work presents the experimental and density functional theory based study of the Li-rich phase, Li1.17Ni0.17Mn0.67O2. This study reveals the lithium removal mechanism and its influence on the oxygen stability. Further, the study suggests how lithium removal from different lithium sites, i.e., 2b, 2c, and 4h Wyckoff positions, influence the partial intercalation potential. On higher degree of delithiation, electrochemical potential increases and oxygen binding energy decreases. Thus, the oxygen stability reduces in the compound. At this stage, the material becomes metallic with zero band gap, which facilitates oxygen loss. This affectively influences the charge transfer process and redox center of the compound, which has been captured in this study.
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morphology or with coating or doping.8,11−13 There are limited studies to understand the origin of oxygen stability in Li-rich cathodes in the presence of multiple d-orbital elements. Density Functional Theory (DFT) studies on Li2MO3 (M = Ti, Mn, Co) revealed that the band gap determines oxygen stability of a Li-rich cathode. The study shows oxygen loss is suppressed in the Ti-phase due to the large band gap, whereas it is favored in the Co-phase due to zero band gap.14 In the present study, experimental observations together with theoretical calculations have been carried out to investigate the (a) changes in local structural environment of Mn and Ni, (b) phase transformation, (c) electrochemical properties, and (d) oxygen binding energy on Li-ion intercalation/deintercalation in Li1.17Ni0.17Mn0.67O2. It is anticipated that the electrochemical potential depends on removal of Li from the distinct sites (3dmetal layer and lithium layer). Li+ removal from Li and LiM2layers influence the oxygen binding energy, which affects the structural stability. The current study proposes a mechanism on how the oxygen binding energy changes with the Li removal process and its effect on charge transfer.
INTRODUCTION Li-rich layered compounds are the most promising cathode materials for automotive applications on account of their high capacities, high operating voltage, and relatively low cost.1−4 The Li-rich layered phases are represented by the general formula of xLiMO2·(1−x)Li2MnO3 (where M = Mn, Ni, etc.). In this phase, both monoclinic Li2MnO3 (space group C2/m) and hexagonal LiMO2 (space group R3̅m) crystallize in layered α-NaFeO2-type rock salt structures with a change in the ordering of ions in the unit cell. The Li2MnO3 phase can be reformulated as Li[Li1/3Mn2/3]O2 because of the excess lithium present in the transition-metal (LiM2) layer. This cation mixing makes lithium-rich phases differ from the conventional layered transition metal oxides.5−7 In the high capacity Li-rich cathodes, Li deintercalation starts from the matrix of LiMO2, followed by the Li2MnO3 with oxygen redox participation in the voltage window of 4.4−4.8 V. This activation of the Li2MnO3 matrix does bring in high capacity but is also responsible for the huge irreversible capacity loss of 40−100 mAh/g. The release of oxygen radicals from the Li2MnO3 matrix accelerates the electrolyte decomposition at the cathode surface and makes it unsafe.8−10 Among the Li-rich cathodes, Li1.17Ni0.17Mn0.67O2 has been selected to study the oxygen stability in the present work. So far, much effort has been made to enhance its rate performance and the first-cycle efficiency by changing the material © XXXX American Chemical Society
Received: June 9, 2017 Revised: August 30, 2017
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DOI: 10.1021/acs.jpcc.7b05662 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a) Rietveld refinement of the powder X-ray diffraction pattern, (b) FESEM image, (c−f) elemental mapping images, and (g) EDAX profile of Li1.17Ni0.17Mn0.67O2 sample.
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respectively, consistent with early work on LiNi0.5Mn0.5O2.17 A 2 × 1 × 1 supercell was chosen with 48 atoms to represent Li1.17Ni0.17Mn0.67O2. For all the materials, a plane-wave energy cutoff of 500 eV and the Brillouin Zone integrations in k-mesh grids of 4 × 2 × 4 were used. All the lattice parameters and ionic positions were fully optimized until the pressure is
