Article Cite This: Chem. Mater. 2018, 30, 2174−2182
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Understanding the Formation of the Truncated Morphology of High-Voltage Spinel LiNi0.5Mn1.5O4 via Direct Atomic-Level Structural Observations Bin Chen,†,‡,§ Liubin Ben,†,‡,§ Yuyang Chen,†,‡ Hailong Yu,†,‡ Hua Zhang,†,‡ Wenwu Zhao,†,‡,∥ and Xuejie Huang*,†,‡ †
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China ‡ University of Chinese Academy of Sciences, Beijing, 100190, China ∥ Sunwoda Electronic Co., Ltd., Shenzhen, Guangdong, 518018, China S Supporting Information *
ABSTRACT: High-voltage spinel LiNi0.5Mn1.5O4 cathode materials typically exhibit a perfect octahedral morphology; i.e., only the {111} planes are observed. However, a truncated octahedral morphology is sometimes observed with the appearance of both the {100} planes and the {111} planes. The underlying mechanism of this morphological transformation is unclear. CS corrected scanning transmission electron microscopy (STEM) techniques were used to study LiNi0.5Mn1.5O4 samples lifted by a focused ion beam (FIB) to determine the atomic-level crystal and electronic structures of the octahedral and truncated octahedral morphologies. STEM images directly show that the appearance of the {100} planes in the truncated octahedral particles of LiNi0.5Mn1.5O4 is closely associated with the atomic-level migration of Ni and Mn ions in the surface region. The STEM electron energy loss spectroscopy (EELS) confirms the presence of oxygen-deficient and Ni-rich areas, particularly in the region close to the newly formed {100} planes. The formation of the {100} planes is sensitive to residual SO42− ions on the surface originating from the sulfates used to prepare LiNi0.5Mn1.5O4. The presence of a small amount of SO42− inhibits the formation of {100} planes. Firstprinciples computer simulations reveal that the adsorption of SO42− on the LiNi0.5Mn1.5O4 surface results in a reduction in the energy required for the formation of the {111} planes. Furthermore, the two O atoms of SO42− can form bonds, improving the stability of the low-coordinated Ni/Mn ions on the {111} planes.
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INTRODUCTION Of the various high-voltage cathode materials, spinel LiNi0.5 Mn1.5O4 (LNMO) is one of the most promising candidates due to its high energy and power densities, low cost, and environmental friendliness.1 The high working potential (∼4.7 V vs Li/Li+) of the Ni2+/Ni3+ and Ni3+/Ni4+ redox couples results in the highest energy density of 650 Wh/kg.2 Furthermore, the threedimensional channels in the spinel lattice enhance the lithium diffusion rates during the intercalation/deintercalation process.3 However, high-voltage spinel LNMO suffers from a limited cycle life at elevated temperatures, mainly due to surface structural distortions and side reactions between the active cathode material and the electrolyte.4 Moreover, the limited cycle life is related to the LNMO intrinsic properties, including the cation ordering degree, particle size, and surface crystal planes in contact with the electrolyte.5 Many attempts have been made to solve these problems. For example, electrolyte additives were used to preform a stable protective layer to stabilize the interface;6−9 dopants were employed to effectively adjust the degree of cation ordering, and the surface was modified to form an intermediate layer to protect the core material from attack by electrolytes.10,11 In addition, the synthesis techniques were optimized to obtain LNMO with a suitable particle size and morphology to enhance its electrochemical properties.5 © 2018 American Chemical Society
Recent studies suggested that the crystallographic surface planes of LNMO particles that are in contact with the electrolyte might have a significant effect on the electrochemical properties of the cathode materials.12−16 As shown by the schematic illustration of the spinel LNMO particles in Figure 1a, LNMO has three predominate surface planes, i.e., the {111} and the {100} planes at the faces and corners, respectively.17 Some groups suggested that the {111} planes have the lowest surface energy, contributing to the formation of a stable SEI, and thus, particles with mostly {111} planes should exhibit superior capacity retention.18 Park et al. reported that the different morphologies of LNMO are due to variations in its crystal symmetry.19 Manthiram and co-workers showed that particles with only the {111} planes have an excellent cycle life and enhanced capacity retention, which was attributed to the fact that the {111} planes have a dense arrangement of ions, the lowest surface energy, and the lowest Mn dissolution.20,21 Note that conflicting results were also reported by other groups who showed that the {100} planes are more favorable for Li+ transport.22 These researchers Received: February 22, 2018 Revised: March 5, 2018 Published: March 7, 2018 2174
DOI: 10.1021/acs.chemmater.8b00769 Chem. Mater. 2018, 30, 2174−2182
Article
Chemistry of Materials
facilely prepared method for large-scale production.28,29 Using this method, spherical particles with a high electrode density can be obtained. This method is commonly used to produce commercially layered and spinel cathode materials.30−36 Since the work of Fan et al., sulfates have been a common starting material in the preparation of LNMO via the coprecipitation method due to their low cost and superior dispersion effects.37 Many groups, including Jouanneau and Dahn,29 Manthiram and co-workers,10,33,38 Whittingham and co-workers,34 and others,40 have reported the use of sulfates in the preparation of M-(OH)2 (M = Mn, Co, and Ni) cathode precursors. In this work, micron-sized (∼3 μm), octahedral LiNi0.5 Mn1.5O4 was prepared by the coprecipitation method. It is shown for the first time that the perfect octahedral morphology of LNMO is formed by the stabilization of the {111} planes by a small amount of residual SO42−, which originates from the sulfates used in the preparation. The morphology of LNMO changes from perfectly octahedral, i.e., only the {111} planes, to truncated octahedral, i.e., the {111} and {100} planes, when the residual SO42− is removed via rinsing with deionized water. Detailed ICP, FTIR, and XRD analyses together confirm that SO42− polyanions can regulate the plane growth of LNMO. Further scanning transmission electron microscopy (STEM) investigations of samples lifted by a focused ion beam (FIB) reveal that the underlying surface atomic arrangement and its corresponding electronic structure are the key factors affecting the morphology of LNMO. The formation energies of the {111} and the {100} surface planes in the presence of SO42− were also investigated.
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RESULTS AND DISCUSSION General Structure and Morphology. The LiNi0.5Mn1.5O4 sample prepared by the coprecipitation method (O-LNMO) exhibits a typical cubic spinel structure in the Fd3̅m space group. In this structure, the lithium, manganese/nickel, and oxygen atoms are located at the 8a (tetrahedral), 16d (octahedral), and 32e Wyckoff sites, respectively, and the chemical formula is [Li+]8a[Ni2+Mn4+]16d[□]16c[O4]32e (□ indicates the empty site).39 The deionized water rinsed and thermal treated LNMO particles are denoted T-LNMO. SEM images of O-LNMO and T-LNMO, Figure 1b−g, show that both samples have similar primary particles with an average size of ∼2−5 μm. Detailed analysis of enlarged SEM images, Figure 1c,d, reveals that particles of O-LNMO exhibit a perfect spinel morphology, with only the {111} crystal planes. In contrast, particles of T-LNMO exhibit a truncated octahedral morphology with the {100} and the {111} planes, Figure 1f,g.22,23,40 In addition, a spiral growth ring is observed on the {111} surfaces but not on the newly formed {100} surfaces of T-LNMO, Figure 1g. This spiral morphology is commonly observed due to growth of crystals.41 Both O-LNMO and T-LNMO exhibit similar XRD patterns with a lattice parameter a of ∼8.1729 (2) and ∼8.1749 (2) Å, respectively (Figure S1). Morphology Transformation during Synthesis. To explain the factors that cause the transformation of morphology from octahedral (O-LNMO) to truncated octahedral (T-LNMO) during synthesis, a certain amount of O-LNMO was dispersed in deionized water under stirring for 5 h at room temperature. The dispersion was dried (without filtration) by heating in a water bath to obtain the rinsed powder, which was then thermally treated at 900 °C for 5 h before SEM investigation. The SEM images show that the morphology of the particles without filtration is the same as that of O-LNMO (Figure S2).
Figure 1. (a) Schematic illustration of LNMO with the {111} and the {100} surface planes. SEM images of (b−d) O-LNMO particles showing the octahedral morphology and (e−g) T-LNMO particles showing the truncated octahedral morphology under different magnifications.
concluded that materials with a large proportion of stable {100} planes would provide long-term cycling stability. Chen et al.23 and Yang and co-workers24 proposed that Mn dissolution from spinel compounds occurred mostly at the {111} and the {110} planes. Because of the impact of the morphology of LNMO on its electrochemical performance, many attempts have been made to modify its surface planes. Manthiram and co-workers20,21 reported that both octahedral, i.e., only the {111} planes, and truncated particles, i.e., the {111} and the {110} planes, were obtained via a method using sulfate and acetate. Liu et al.22 obtained octahedral and truncated octahedral particles by varying the heat treatment temperature. They reported that LNMO particles with the {100} planes were obtained by thermal treatment at 1000 °C. To control the proportions of the {111}, the {100}, and the {110} planes, surfactants (PEG, PS, etc.) were also employed.25,26 In addition, the substitution of elements in LNMO also changes the morphology. Deng et al.27 used NH4H2PO4 to substitute some of the Ni/Mn with P. As the P content increased, the octahedral LNMO particles gradually changed to truncated octahedral morphology. Despite these studies, the exact mechanism of the formation of the surface planes of LNMO particles remains unclear and must be determined. Of the various methods used to synthesize cathode materials, the coprecipitation method is a powerful, inexpensive, and 2175
DOI: 10.1021/acs.chemmater.8b00769 Chem. Mater. 2018, 30, 2174−2182
Article
Chemistry of Materials
Figure 2. (a) ICP analysis results for the filtrate. (b) FTIR spectra of the white recrystallized product obtained by evaporating the filtrate and the standard commercial Li2SO4·H2O. XRD patterns of (c) the recrystallized product obtained by evaporating the filtrate (Reference Li2SO4·H2O ICSD #18173) and (d) the commercial Li2SO4·H2O powder.
Figure 3. (a) Schematic illustration of LNMO with the {111} and the {100} surface planes. SEM images of T-LNMO mixed with Li2SO4·H2O (b−d) before and (e−g) after thermal treatment under different magnifications.
900 °C for 5 h. The SEM images in Figure 3 show that the truncated octahedral particles in T-LNMO with the {100} planes (Figure 3b−d) were fully transformed back into the octahedral particles without the evidence of the {100} planes (Figure 3e−g). Atomic-Level Crystal Structure of Octahedral LNMO. To understand the fundamental origins of the morphology transformation between the octahedral and truncated octahedral particles, the atomic-level crystal structures were investigated by STEM, which directly reveals information about the arrangement of atoms. In the STEM analysis, the O-LNMO and the T-LNMO samples were lifted by FIB to produce a thin film (thickness
