Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 19298−19308
Strain-Induced Stabilization of Charged State in Li-Rich Layered Transition-Metal Oxide for Lithium-Ion Batteries Tomoya Kawaguchi,*,† Masashi Sakaida,‡ Masatsugu Oishi,†,§ Tetsu Ichitsubo,‡,∥ Katsutoshi Fukuda,† Satoshi Toyoda,‡ and Eiichiro Matsubara†,‡ †
Office of Society-Academia Collaboration for Innovation, Kyoto University, Kyoto 611-0011, Japan Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
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ABSTRACT: Li-rich layered oxide (LLO) is a promising cathode material for lithium-ion batteries because of its large capacity in comparison with conventional layered rock-salt structure materials. In contrast to the conventional materials, it is known that LLO of 3d transition metal has a nanodomain microstructure; however, roles of each domain and effects of strain, induced by the microstructure, on electrode properties are still unclear. In this study, the influence of the strain on an electronic structure is studied to elucidate the stabilization mechanism of LLO material Li[Li0.2Ni0.2Mn0.6]O2 in the charged state by using resonant X-ray diffraction spectroscopy (RXDS), X-ray diffraction, and X-ray absorption spectroscopy (XAS) in combination with ab initio calculation. RXDS of a superlattice peak and XAS at Mn and Ni K-edges unveil that this material has a microstructure consisting of Mn-rich and Ni-rich domains, whose structures are similar to Li2MnO3 and LiNiO2, respectively. In the Ni-rich domain, trigonal distortion in the NiO6 octahedral cluster is induced by an elastic constraint due to the microstructure. Hybridization between oxygen p- and nickel d-orbitals is enhanced by the distortion as revealed both by XAS and by ab initio calculation, accounting for stabilization of the charged state by alleviating the direct hole formation on oxygen p-orbital that usually destabilizes the charged material.
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complete solid solution.3−8 In the layered rock-salt structure of LiMO2, Li and TM form respective layers by occupying the octahedral sites among oxygens of the face-centered cubic. The other end member, Li2MnO3, is similar except for the Licentered honeycomb structure of Mn in the TM layer. Because of the structural similarity of Li2MnO3 and LiMO2, most X-ray diffraction (XRD) peaks of the LLO can be indexed by R3̅m layered rock-salt structure, except for weak upsurges in the range of Q = 1.4−1.8 Å−1.9 The weak upsurges, referred to as superlattice peaks in the perspective of the rhombohedral structure, indicate the presence of the honeycomb order and delineate the LLOs from the conventional layered rock-salt structure materials. Li1.2Co0.4Mn0.4O27 and Li1.2Cr0.4Mn0.4O23 are further reported to have a microstructure of Co (or Cr)rich and Mn-rich domains as a local structure. The intensity of the superlattice peaks associated with the microstructure is known to exhibit a strong correlation to the electrode properties.6 Understanding the roles of each domain in the microstructure is, therefore, a key to reveal the stabilization mechanism in the LLOs. However, because of the structural complexity of the materials, the domain-specific analysis is challenging. To address this problem, the element-specific
INTRODUCTION Intercalation chemistry is of great importance in addressing current material issues for rechargeable batteries such as lithium-ion batteries, where further improvement of capacity is required for applications in electric vehicles, robots, and largescale batteries. The capacity of the battery is determined not only by the Li composition of the material but also by the tolerance to valence changes of constituent atoms for charge compensation during Li insertion and extraction. In a cathode oxide material consisting of Li and transition metals (TMs) such as LiCoO2, valence change of the TMs apparently compensates the change in the total charge of the material, for example, LiCo(III)O2 → Co(IV)O2 + Li+ + e. However, it was theoretically1 and experimentally2 reported that oxygen oxidation to the total charge compensation can be considerable because the energy level of oxygen 2p-orbital is close to that of TM 3d orbital. The oxygen oxidation tends to destabilize the electronic structure of the material and limits the reversible capacity, especially in the conventional layered rock-salt structure materials. Recently, the Li-rich layered oxide (LLO) family, whose composition is represented as xLi2MnO3·(1 − x)LiMO2 (M = Cr, Mn, Co, Ni), has attracted much attention as promising electrode materials having large capacity and good stability. The LLOs are reported to have microstructures consisting of domains similar to those of the end members rather than the © 2018 American Chemical Society
Received: April 4, 2018 Revised: August 7, 2018 Published: August 8, 2018 19298
DOI: 10.1021/acs.jpcc.8b03205 J. Phys. Chem. C 2018, 122, 19298−19308
Article
The Journal of Physical Chemistry C
and 2.0 V vs Li+/Li for discharge. Electrode samples for the Xray analyses were obtained by disassembling the tested cells in an Ar-filled dry box and washing the electrodes with EC/DMC (1:2 by volume) and with DMC, followed by drying under vacuum in an antechamber of the dry box. For the synchrotron XRD measurements, the samples were prepared by encapsulating the pulverized composite electrode into Lindeman glass capillaries of 0.3 mm in diameter in the Ar-filled dry box. For the laboratory XRD measurements of the superlattice peak, the washed electrodes were used. For the RXDS and XAS measurements, the composite electrode was pelletized into a 6 mm-diameter disk with boron nitride powder as a diluent. X-ray Diffraction Measurement. Synchrotron XRD was performed at beamline BL28XU, SPring-8, Japan. The XRD profile was acquired from the rotating capillary sample by a one-dimensional detector (Mythen, Dectris), with an incident X-ray of 11.0008(4) keV (wavelength: 1.12704(4) Å) monochromated by a Si(111) channel-cut monochromator. XRD profiles of the superlattice and 003 peaks were acquired from the as-synthesized sample (0 mAh g−1) and samples separately charged to 50, 100, 200, and 290 mAh g−1 in an Ar atmosphere by a laboratory XRD apparatus (SmartLab, Rigaku) equipped with a Cu Kα X-ray tube and a multilayer mirror. Resonant X-ray Diffraction Spectroscopy. Diffraction anomalous fine structure (DAFS) spectra of the superlattice peak were measured at Mn and Ni K-edges at beamline BL28XU, SPring-8, Japan. The details of the data collection and analyses of DAFS were described elsewhere.14,17,18 The XRD profiles at each X-ray energy were acquired with transmission geometry from a rotating sample by a onedimensional detector (Mythen, Dectris). The intensities of incident and transmitted X-rays were simultaneously monitored by ionization chambers with He and N2 gas flows. The superlattice XRD peak was measured from 6.233 to 7.124 and 8.039 to 8.930 keV for Mn and Ni K-edges, respectively. The areas of the XRD peaks were determined by fitting a split pseudo-Voigt function19 with a linear background. The energy spectra of the XRD intensity were further corrected by the incident X-ray intensity, Lorentz-polarization factor, and absorption factor to obtain the DAFS spectra. The occupancy of Ni at the Li 2b site, which is a Li site in the TM layer, was determined by fitting the DAFS spectrum of the superlattice peak around Ni K-edge. Li2MnO3 that has the space group of C2/m (No. 12) was used as a model structure for the fit. The DAFS spectra were calculated by evaluating | Fhkl|2, where Fhkl is an energy-dependent complex structure factor for hkl diffraction. The structure factors of 020 and 110 in C2/m were used for determining the Ni occupancy from the equations F020 = −2f Mn(4g) + 2gNi f Ni(2b) + 2f Li(2c) − 2f Li(4h) + FOX,020 and F110 = 2f Mn(4g) − 2gNi f Ni(2b) + 2f Li(2c) − 2f Li(4h) + FOX,110, where f x(y) is an atomic form factor of element x at y site, gNi is the occupancy of Ni at 2b site, and FOX,hkl is the structure factor from oxygen in hkl diffraction. f is further expanded to f = f 0 + f ′ + if″, where f 0 is an energy-independent nonresonant term and f ′ + if″ are energy-dependent resonant terms. The theoretical values of the nonresonant20 and resonant21 terms were used to calculate the DAFS spectra. The occupancy of Ni at Li 2b site, edge energy, and a scale factor of the theoretical curve were determined by fitting the calculated spectrum to the experimental data below the absorption-edge energy. The X-ray absorption fine structure (XAFS) spectrum of the superlattice peak was extracted from
structural analysis, which is able to distinguish crystallographic sites and domains, is required. In the present study, we analyzed both crystalline and electronic structures of Li[Li0.2Ni0.2Mn0.6]O2 (LNMO) as a representative model widely studied both experimentally10−12 and theoretically13 because it shows typical structural and electrochemical features of the LLOs. The structural origin for the superlattice peak is studied in detail by recently developed resonant X-ray diffraction spectroscopy (RXDS),14 which enables site- and phase-selective analyses. Further structural analyses of X-ray absorption spectroscopy (XAS) are performed, revealing that the microstructure consists of Nirich and Mn-rich domains and there is a trigonal distortion of NiO6 cluster in the Ni-rich domain. X-ray absorption nearedge structure (XANES) analyses indicate that the orbital hybridization between nickel and oxygen is enhanced in this material. Ab initio calculation is performed in order to clarify the relationship between the trigonal distortion and orbital hybridization in the experimentally determined structure models. Finally, we discuss a future possibility of manipulating material properties by means of controlling the microstructure and orbital hybridization of oxide materials.
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METHODS Synthesis of Li[Li0.2Ni0.2Mn0.6]O2. The polycrystalline Li[Li0.2Ni0.2Mn0.6]O2 was synthesized by the solid-reaction method using mixed carbonate precursors of the transition metals prepared by a coprecipitation method in accordance with the reported procedure.15 A 0.2 mol dm−3 (M) aqueous solution of NiSO4 and MnSO4 (Ni/Mn = 1:3) was dropped to a 1 M aqueous solution of NaHCO3 followed by continuous stirring for 24 h at 50 °C. The deposited precipitation was filtered, washed, and dried at 110 °C for 12 h to obtain the TM precursor. The final product was obtained by firing the mixed powders of the TM precursor and 2 wt % excess Li2CO3 at 600 °C for 6 h in air. The XRD peaks of the as-synthesized sample were indexed by a layered α-NaFeO2 (space group: R3̅m) structure and the reported superlattice peak without any impurity phase.16 The inductively coupled plasma method was performed to determine the cation ratio of the as-synthesized sample. The result was Li[Li0.19Ni0.22Mn0.59]Ox, and this is in good agreement with the target composition. No sulfur was detected (
