Article pubs.acs.org/cm
Nanoscale Phase Separation, Cation Ordering, and Surface Chemistry in Pristine Li1.2Ni0.2Mn0.6O2 for Li-Ion Batteries Meng Gu,† Arda Genc,‡ Ilias Belharouak,§ Dapeng Wang,§ Khalil Amine,§ Suntharampillai Thevuthasan,† Donald R. Baer,† Ji-Guang Zhang,∇ Nigel D. Browning,⊥ Jun Liu,⊥,∇ and Chongmin Wang*,† †
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States ‡ FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, Oregon 97124, United States § Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ∇ Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States ⊥ Fundamental & Computational Sciences Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States W Web-Enhanced Feature * S Supporting Information *
ABSTRACT: Li-rich layered material Li1.2Ni0.2Mn0.6O2 possesses high voltage and high specific capacity, which makes it an attractive candidate for the transportation industry and sustainable energy storage systems. The rechargeable capacity of the Li-ion battery is linked largely to the structural stability of the cathode materials during the charge−discharge cycles. However, the structure and cation distribution in pristine Li1.2Ni0.2Mn0.6O2 have not yet been fully characterized. Using a combination of aberrationcorrected scanning transmission electron microscopy, X-ray energydispersive spectroscopy (XEDS), electron energy loss spectroscopy (EELS), and complementary multislice image simulation, we have probed the crystal structure, cation/anion distribution, and electronic structure of the Li1.2Ni0.2Mn0.6O2 nanoparticle. The electronic structure and valence state of transition-metal ions show significant variations, which have been identified to be attributed to the oxygen deficiency near certain particle surfaces. Characterization of the nanoscale phase separation and cation ordering in the pristine material are critical for understanding the capacity and voltage fading of this material for battery application. KEYWORDS: Li-rich layered composite, Li1.2Ni0.2Mn0.6O2, phase separation, cation ordering, oxygen vacancies, Li-ion batteries
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INTRODUCTION The ever-growing energy demand of the modern information technology and mobile industry relies on lithium-ion batteries for power storage, because of their relatively high energy density and design flexibility. The layered LiCoO2 or spinel LiM2O4 cathodes used in current Li-ion batteries suffer from lower-than-desired capacity (∼140 mAh/g or less) and structural instability during cycling, which limits their lifetime. The Li-rich layered materials (often written as xLi2MnO3·(1 − x)LiMO2 (0 < x < 1, M = Ni, Mn, Co) in early literature) are of particular interest, because they can achieve a discharge capacity over 250 mAh/g below 3 V when they are charged above 4.6 V vs Li. One specific example, 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2 (written hereafter as Li1.2Ni0.2Mn0.6O2, or LNMO, for the sake of simplicity, throughout the text) is an important representative example of this type of electrode material;1−5 this has the potential to reduce the cost of Li-ion batteries, because of the use of Mn to replace the more-expensive Co.6 In © XXXX American Chemical Society
addition, safer operation at elevated temperatures might be achieved due to the improved stability of this cathode material.1,6,7 All of these excellent properties exhibited by LNMO make it a very promising candidate as an advanced cathode material in future lithium batteries.2,4,8−12 Cation ordering and uniformity in the cathode materials are the primary factors influencing the capacity and cyclability of lithium batteries.4,8,10,11,13,14 Currently, the atomic structure and cation ordering of LNMO are a matter of controversy between researchers who claim this material to be a solid solution or composite. Using X-ray diffraction (XRD), Lu et al. suggested that LNMO is a solid solution with an R3̅m crystal structure.15 Jarvis et al.2 analyzed the material using electron diffraction and STEM imaging, and they claimed that the Received: March 22, 2013 Revised: May 12, 2013
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dx.doi.org/10.1021/cm4009392 | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
material is composed of a solid-solution phase with Li2MO3 C2/m structure. Based on electron diffraction (ED)/STEM imaging and neutron diffraction, Bareno et al.14 and LuBeaulieu et al.7 found that Li1.2Ni0.2Mn0.6O2 cannot be assigned to one single phase. To further improve the operation and capacity of the layered LNMO cathode, clarification on the atomic structure and possible cation ordering must be made. Voltage fade and poor rate performance are the main factors inhibiting the application of layered cathodes in heavy-duty electric vehicles. In order to identify the origin of the voltage fade and poor rate performance, structure changes after charge/ discharge cycling must be elucidated. LiNi0.5Mn0.5O2 cathodes have been found to be very stable during cycling,1,5,16,17 while the pure or lightly doped phases (LiMnO2 or Li2MnO3) exhibited a transformation to spinel phase during cycling, which could decay the voltage and degrade the cycleability.5,18−20 During delithiation of the layered cathode, migration of the transition-metal (TM) cations into the Li layers occurs, which can stabilize the structure. During the high-voltage charging process, Li2MO3 phase can release Li+ cations and O2− anions, which contributes to the initial high capacity of this cathode.4,21 However, the impacts of strain and lattice distortion associated with this process are still not fully understood. Understanding the changes that these cathodes undergo during battery cycling first requires an understanding of the nature of the pristine material. In this paper, we report on the detailed structure, cation distribution, and electronic structure of pristine Li1.2Ni0.2Mn0.6O2 material. This information sets the foundation for further probing dynamic structural evolution of this category of material during the charge/discharge processes.
Figure 1. (a) HAADF Z-contrast image of a single nanoparticle and XEDS maps ((b) Mn map, (c) O map, (d) Ni map, and (e) overlaid Ni and Mn map). (f) Mn/(Mn + Ni) atomic percentage quantification map and (g) atomic percentage of Mn and Ni along the red line in panel (f). (The scale bar in panel (a) applies to all of the images.)
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RESULTS AND DISCUSSION Nanoscale Phase Separation and Cation Distribution. The contrast in a high-angle annular dark-field (HAADF) Zcontrast image shows an approximately Z2 dependence (where Z denotes the average atomic number of a certain atomic column).22 The background contrast between the atomic columns in the Z-contrast image is influenced by the thickness of the sample. However, the chemical concentration usually dominates the contrast for most thin specimens of nanometer scale.23 Therefore, it is chemical sensitive and intuitively interpretable. The Z-contrast image of the single nanoparticle in Figure 1a qualitatively identifies the contrast difference between the surface and the inner part of the particle. X-ray energy-dispersive spectroscopy (XEDS) maps of Mn, O, Ni, and overlaid Ni/Mn maps indicate that the regions with higher contrast along the surface in Figure 1a are rich in Ni. The normalized Mn/(Ni + Mn) atomic percentage map in Figure 1f identifies that the Mn(Ni) is enriched (deficient) in the core and deficient (enriched) in some regions near the surface. Therefore, an approximate core−shell shape graded concentration is formed, as quantitatively shown in Figure 1f. As illustrated by Figure 1g, the Ni concentration increases gradually from
