Understanding the Formation of the Truncated Morphology of High

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00769 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Chemistry of Materials

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. § These authors contributed equally to this work * E-mail: [email protected] (X. J. Huang)

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Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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. First-principles 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 LiNi0.5Mn1.5O4 (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 an highest energy-density of 650 Wh/kg.2 Furthermore, the three-dimensional channels in the spinel lattice enhance the lithium diffusion rates during the intercalation/de-intercalation 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 pre-form 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

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 et al. 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 concluded that materials with a large proportion of stable {100} planes would provide long-term cycling stability. Chen et al.23 and Yang et al.24 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 et al.20, 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


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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 co-precipitation method is a powerful, inexpensive and facile preparation 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 commercial 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 co-precipitation method due to their low cost and superior dispersion effects.37 Many groups, including Dahn et al.29 , Manthiram et al.10, 33, 38, Whittingham et al.34 and others40 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 LiNi0.5Mn1.5O4 was prepared by the



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co-precipitation 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 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.

RESULTS AND DISCUSSION General Structure and Morphology The LiNi0.5Mn1.5O4 sample prepared by the co-precipitation method (O-LNMO) exhibits a typical cubic spinel structure in the Fd-3m 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-1g, show that both samples have similar primary particles with an


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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) Å and ~8.1749 (2) Å, respectively (Figure S1).

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.

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 thermal 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). However, T-LNMO with the truncated octahedral morphology was prepared by the same method but with the filtration procedure as shown before. These results suggest that formation of the truncated particles in T-LNMO may be caused by some components on the surface which dissolve in the deionized water and are removed during filtration.

The components on the surface of O-LNMO were identified by dispersing the O-LNMO particles in deionized water. Combined ICP analysis (Figure 2a) of the pellucid solution of this suspension and FTIR (Figure 2b) and XRD (Figure 2c and 2d) analysis of the recrystallized products from the pellucid solution indicate that the elements dissolved in the deionized water were mainly associated with Li2SO4.42 The set-up for extraction of the recrystallized products is shown in Figure S3. These results suggest that a small amount of residual SO42- originating from the transition metal sulfates used in the co-precipitation


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method might be present on the surface of O-LNMO and can be removed by rinsing with deionized water.43

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.

Furthermore, the effects of SO42- on the morphology of the crystal planes was confirmed by adding a small amount of Li2SO4·H2O into T-LNMO, followed by heating the mixture at 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-3d) were fully transformed back into the octahedral particles without the evidence of the {100} planes (Figure 3e-3g).



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.

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 < 100 nm) with an area of ~5 µm2 (Figure 4a and Figure 6a). The atomic-level structure of O-LNMO was initially investigated, as shown in Figure 4b-f. The O-LNMO thin film obtained using the FIB exhibits a sharp edge near the {111} surfaces, in agreement with the octahedral morphology. The atomic-level arrangement of the atoms near the two surfaces is shown in detail, as indicated by the red and blue boxes in Figure 4b. In general, the arrangement of the atoms is homogeneous from the {111} surfaces to the interior, Figure 4c, e. The magnified images in Figure 4d, f show the typical diamond contrast pattern of the standard spinel structure viewed along the [110] direction. The assignment of the


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contrasting regions is shown in Figure 4 and elsewhere.44-46 The STEM observations indicate that for O-LNMO particles with the perfect octahedral morphology, the arrangement of the atoms near the {111} surface region is similar to the standard spinel structure, and only the 16d sites are occupied by transition metal (TM) ions.

Figure 4. (a) Schematic showing the preparation of O-LNMO sample for STEM-EELS analysis by FIB. (b) Low-magnification STEM-HAADF image of the FIB-lifted O-LNMO sample. (c,e) correspond to the red and blue boxes near the {111} surface in panel 4b. (d,f) are the magnified images of the red and blue regions in panels 4c and 4e, respectively. The structural models in panels 4d and 4f show the diamond contrast pattern viewed along the [110] direction of the spinel. A schematic illustration of the standard spinel structure viewed along the [110] direction is also shown in the figure (bottom left). The black circles represent the empty 16c sites. The yellow/blue, red and green spheres represent Ni/Mn, O and Li, respectively. In the spinel structure, Li occupies the 8a sites, and Ni/Mn occupies the 16d sites. The two transition metal columns are denoted TMα and TMβ because they have different stacking densities. The purple box in panel 4b indicates the region selected for further STEM-EELS analysis.

The STEM-EELS mapping of O-LNMO, Figure 5, allows the distribution of the TM ions to be obtained and provides information about the valence states of the TM ions from the peak



fitting based on either the L-edge energy shift (chemical shift) or L3/L2 white-line intensity ratio.47, 48 Figure 5a shows the region corresponding to the purple box in Figure 4b above. The black box in Figure 5a indicates the region from which the EELS signals were collected. Based on the fitted results for the Mn L3/L2 white-line intensity ratio, a two-dimensional distribution of the relative Mn valence over a 161 nm × 161 nm pixelated area is shown in Figure 5b. The ratio value (0-2.5) is indicated by the color bar in the Figure 5c. Based on the relatively homogeneous distribution of the color (value), the valence state of Mn is generally uniform from the {111} surface to the bulk, indicating that the associated oxygen content is also uniform from the surface to the bulk, as confirmed by the detailed local analysis presented below. The concentration ratio of Ni and Mn, represented as INi/IMn (INi is the integrated peak area of the Ni-L3,2 and IMn is the integrated peak area of the Mn-L3,2), was added as the Z axis to this image to determine the distributions of Ni and Mn. The Z axis value is similar at each pixel, indicating that the Ni/Mn concentration ratio is uniform from the {111} surface to the bulk. Two typical regions, indicated by the black and red dots in Figure 5a, were selected for the further EELS analysis. The extracted EELS profiles in Figure 5d show that the O K-, Mn L3,2-, and Ni L3,2-edges are located at ~530, ~640, and ~850 eV, respectively. The magnified O K-edge, Figure 5e, consists of two main peaks. The pre-edge peak (labeled α) is associated with the transition of electrons from the O 1s core state to the unoccupied 2p states that are hybridized with the 3d states of the transition metals.49 The peak (labeled β) originates from the transition of electrons from the 1s state to the hybridized O 2p and metal


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4s and 4p states.50 In general, the peaks α and β are similar based on the peak position and intensity, suggesting that the electronic structures of oxygen in the bulk (black dot) and at the surface (red dot) are similar. Furthermore, the corresponding Mn L3,2-edge, Figure 5f, are also generally similar for the bulk and the {111} surface. Note that close analysis of O K-edge and Mn L3,2-edge, supporting information Figure S4, reveals that there is a tiny shift (~0.5 V) to the lower energy for the surface region compared with that for the {111} surface, which is possibly due to the slight surface oxygen vacancy induced during preparation of the samples at high temperatures. However, such a slight surface oxygen vacancy does not induce significant shift of the position of O K-edge. Note that the tiny shift (~0.5 eV) is also observed for Ni L3.2-edge for both O-LNMO (Figure S4c) and T-LNMO (Figure S5c), which is not fully understood (Ni2+ is unlikely reduced to Ni+) but may be within the errors of the measurement.



Figure 5. (a) STEM-HAADF image showing the region (indicated by the black box) from which the EELS signals were collected. This region is represented by a 161 nm×161 nm pixelated graph. (b) Two-dimensional map showing the Mn L3/L2 white-line intensity ratio. (c) Three-dimensional map showing the Ni/Mn intensity ratio (INi/IMn) as the Z axis. The X and Y axes are the same as those in panel 5b. (d) EELS spectra for the regions near the bulk (indicated by the black dot in panel 5a) and the {111} surface (indicated by the red dot in panel 5a). Magnified (e) O K-edge and (f) Mn L3,2-edge spectra.

Atomic-Level Crystal Structure of Truncated Octahedral LNMO The STEM-HAADF images of T-LNMO in Figure 6 show a thin film with a truncated edge consisting of the {100} and the {111} surfaces, in contrast to the structure observed for O-LNMO, Figure 4. Two regions with distinct contrast patterns are observed in the images, as indicated by the white dashed line in Figure 6c, e. A detailed analysis of these regions, see the magnified images in Figure 6d, f, clearly reveals that the regions close to the {100} and {111} surfaces exhibit a contrast pattern typical of that of rock-salt structures. In the region near the bulk, the contrast pattern is similar to that of the standard spinel observed for O-LNMO in this and previous studies.44-46 In summary, the structure of the regions near the {100} and the {111} surfaces transforms from the original spinel structure (Fd-3m) to the rock-salt structure (Fm-3m).


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Figure 6. (a) Schematic showing the preparation of T-LNMO sample for STEM-EELS analysis by FIB. (b) Low-magnification STEM-HAADF image of the FIB-lifted T-LNMO sample. (c,e) correspond to the blue box near the {100} surface and the green box near the {111} surface in panel 6b. (d,f) are the magnified images of the blue and green regions in (c,e), and show the structural models of the diamond contrast patterns viewed along the [110] direction of the spinel and the [101] direction of the rock-salt, respectively. The purple box in panel 6b indicates the region selected for further STEM-EELS analysis.

The transformation of the atomic arrangement from the standard spinel structure to the rock-salt structure in the {100} and {111} surface regions of T-LNMO should involve modifications to the coordination and bonding to oxygen, which would be reflected in the electronic structure.51, 52 Therefore, STEM-EELS was employed to determine the surface electronic structure, and the results are shown in Figure 7. The signals were collected from the region shown in Figure 7a, which corresponds to the purple box in Figure 6b. Based on the fitted results for the measured Mn L3/L2 white-line intensity ratio, a two-dimensional distribution of the relative Mn valence over a ~146 nm×156 nm pixelated area is shown in Figure 7b. Noticeable changes are detected in the regions near the {100} and {111} surfaces;



more dark red points, i.e., larger values, are observed in the surface regions, suggesting the presence of oxygen deficiencies and low-valence Mn in these areas. Similarly, the relative distributions of Ni and Mn, represented as INi/IMn in Figure 7c, further indicate that the Ni concentration is relatively higher in the regions near the {100} and the {111} surfaces, as shown by the significant percentage of large ratio values in the surface regions.

Detailed analyses were performed for the three regions shown in Figure 7a, i.e., the {100} surface region, the {111} surface region and the bulk region indicated by the blue, green and black dots, respectively. The extracted EELS profiles for these regions are shown in Figure 7d and the detail information is shown in Figure S5. The magnified O K-edge profile in Figure 7e shows that the intensity of peak α and the distance between α and β increase from the surface regions (blue and green dots in 7a) to the bulk region (black dot in 7a), indicating that the oxygen concentration is low in the regions near the {100} and the {111} surfaces.53 The change in the oxygen O K-edge profile is consistent with the results for the Mn L-edges, Figure 7f, which are shifted to a higher energy in the bulk region, indicating that Mn is reduced in the surface region.53 In summary, oxygen deficiencies occur in the regions near the {100} and the {111} surfaces and are charge-compensated by the reduction of Mn4+ and accumulation of a higher proportion of Ni2+ ions.


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Figure 7. (a) STEM-HAADF image showing the region (indicated by the red box) from which the EELS signals were collected. This region is represented by a 146 nm×156 nm pixelated graph. (b) Two-dimensional map showing the Mn L3/L2 white-line intensity ratio. (c) Three-dimensional map showing the Ni/Mn intensity ratio (INi/IMn) as the Z axis. The X and Y axes are the same as those in panel 7b. (d) EELS spectra for the regions near the bulk (indicated by the black dot in panel 7a), the {100} surface (indicated by the blue dot in panel 7a) and the {111} surface (indicated by the green dot in panel 7a). Magnified (e) O K-edge and (f) Mn L3,2-edge spectra.

First-principles Calculations First-principles density functional theory (DFT) calculations were performed to explain the effect of SO42- and oxygen on the crystal growth of LNMO. The slab structures of the perfectly octahedral O-LNMO with SO42- adsorbed on the {100} and the {111} surfaces and the calculated surface energies are shown in Figure 8 and compared to that of the truncated octahedral T-LNMO without surface SO42-. For the T-LNMO structures, the {100} and the



{111} surface energies are calculated to be ~1.03 and ~1.37 J/m2, respectively. The {111} surface is energetically unfavorable compared to the {100} surface, explaining the observation of the truncated morphology in this and other studies. However, for the SO42--adsorbed structures, the {100} and the {111} surface energies are reduced to ~0.7 and ~0.63 J/m2, respectively. In this case, the SO42--adsorbed {111} surface is more favorable than the {100} surface, explaining the observation of the perfect octahedral morphology.

Figure 8. Structural models and surface energies of the SO42--desorbed and the SO42--adsorbed {100} and {111} surfaces. (a,b) are the structural models for the SO42--desorbed {100} and {111} surfaces. (c,d) are the structural models for the SO42--adsorbed {100} and {111} surfaces, respectively. (e) Calculated surface energies for the SO42--desorbed and the SO42--adsorbed {100} and {111} surfaces.

These experimental and first-principles simulation results clearly suggest that the spinel surface energy is strongly influenced by the surface coverage of SO42-. Oxygen is likely absent from the surfaces during synthesis; therefore, the {100} and the {111} surfaces contain exposed Li, Ni and Mn atoms that are under-coordinated, e.g., five-coordinate and three-coordinate transition metal ions on the {100} and the {111} surfaces, respectively. The low-coordination sites result in an increase in the energy of the surface relative to that


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of the bulk, which leads to the migration of the TM ions from the surface to the bulk and the subsequent formation of the rock-salt structure. When SO42- is adsorbed on the surfaces, the coordination of the surface transition metal ions is higher, and consequently, the surface energies are reduced. Moreover, the two O atoms of SO42- can form bonds, resulting in a stronger stabilizing effect on the low-coordinated Ni/Mn ions at the {111} surface. Therefore, the surface energy of the SO42--adsorbed {111} surface is lower than that of the {100} surface, explaining the observed formation of a higher proportion of the corresponding the {111} surface morphology than the {100} surface morphology.

CONCLUSIONS High-voltage spinel LNMO cathode materials are usually prepared by the solid-state reaction of Li2CO3 with Ni0.25Mn0.75(OH)2. The latter material is prepared by a co-precipitation method using NiSO4, MnSO4, NaOH and NH4OH as the starting materials. However, a small amount of residual SO42- is present on the surface of the LNMO particles and controls the particles’ morphology, as shown by ICP, FTIR and XRD analyses. In the presence of SO42-, the morphology of the LNMO particles is a perfect octahedron, as observed by SEM. However, the SO42- ions can be removed by rinsing with deionized water. Then, after thermal treatment at 900 °C for 5 h, the morphology of the LNMO particles is a truncated octahedron. Further experimental investigations show that the truncated octahedral morphology can be transformed back into the perfect octahedral morphology by mixing the truncated octahedral particles with Li2SO4 and heat treating them. A detailed



STEM analysis of the FIB-lifted samples revealed that the fundamental origin of the morphological transformation of the LNMO particles is the distortion of the atomic-level crystal structures and associated changes of oxidation state of transition metals and oxygen stoichiometry. STEM-HAADF images reveal that the perfectly octahedral O-LNMO has an atomic arrangement that resembles the standard spinel structure from the {111} surface to the bulk. The STEM-EELS results confirm that the oxygen concentration does not vary significantly from the surface to the bulk region. In contrast, the T-LNMO particles with the truncated octahedral morphology have an atomic arrangement similar to that of rock-salt phase in the regions near the {100} and the {111} surfaces, as observed in the STEM-HAADF image. The STEM-EELS images further reveal that the rock-salt regions near the {100} and the {111} surfaces have a relatively higher concentration of Ni than Mn. The experimental results are also supported by first-principle DFT calculations, which indicate that SO42- adsorption on the {111} and the {100} surfaces of LNMO could reverse their relative surface energies. The presence or absence of SO42- on the surface, which results in a lower {111} or lower {100} surface energy, respectively, controls the ultimate morphology of the LNMO particles. These results provide a fundamental understanding of the morphology of spinel cathode materials and valuable guidelines for designing and synthesizing spinel and other cathode materials.

EXPERIMENTAL SCETION Material Synthesis. LiMn0.5Ni1.5O4 (O-LNMO) was prepared by a conventional solid-state


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reaction. A mixture of stoichiometric amounts of Li2CO3 (Alfa Aesar) and Ni0.25Mn0.75(OH)2 was used as the starting material. Ni0.25Mn0.75(OH)2 was prepared by the co-precipitation method as described in the literature. The mixture was heated for 12 h at 900 °C in an ambient atmosphere and then slowly cooled to room temperature. The truncated LiMn0.5Ni1.5O4 (T-LNMO) was prepared by dispersing a certain amount of the as-prepared O-LNMO in deionized water under stirring for 5 h at room temperature. The suspension of the O-LNMO powder was filtered to remove the solvent. The obtained powder was further heated at 900 °C for 5 h to obtain particles with the truncated octahedral morphology. To illustrate the effect of the residual SO42- on the growth of the spinel crystal planes, a certain amount of the Li2SO4·H2O crystals was dissolved in 10 mL distilled water, and T-LNMO with the truncated octahedral morphology was subsequently added to the mixture. The suspension was dried at 80 °C and then calcined at 900 °C for 5 h.

XRD. XRD patterns were obtained using a Bruker D8 ADVANCE diffractometer with a Cu Kα radiation source (λ1 = 1.54056 Å, λ2 = 1.54439 Å). The diffractometer was equipped with a LYNXEYE detector and operated at 40 kV and 40 mA. Based on the X-ray diffraction data, the structural parameters of the materials were refined using the FullProf Rietveld refinement program.

STEM/EELS. To investigate the morphologies and structures of the as-synthesized samples, high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM)



measurements were performed using a JEM ARM200F instrument (JEOL, Tokyo, Japan) equipped with two CEOS (CEOS, Heidelberg, Germany) probe aberration correctors. The incident semiangle is 25 mrad and the acceptance semiangle is 12−25 mrad. The background was subtracted from the EELS spectra shown in this work in the power law mode using the fit region of 50 eV before the peaks. The peak detection and peak area measurements for the background-corrected spectra were performed automatically by the auto peaks script. In this method, the peaks were detected by searching for downward zero-crossings in the first derivative with upward slopes exceeding a given threshold. The area of each peak was determined by the integration of the peak intensity with respect to the energy.

Focused Ion Beam (FIB) were applied to obtain samples for STEM-EELS with precise surface planes, e.g. the {100} and {111} planes. A Pt bar was initially deposited on target area of the O-LNMO and T-LNMO samples by electron beam, which protected the samples from further ion beam damage. Then thin film samples (~700 nm) of O-LNMO and T-LNMO were lifted for further ion beam thinning. The samples were transferred to the copper mesh and were further milled down to ~ 100 nm in thickness, as shown in Figure S6.

First-principles Calculations. Spin-polarized calculations were performed using the projector augmented-wave approach54 implemented in the Vienna Ab Simulation Package (VASP).55,

56

The generalized gradient approximation (GGA) of Perdew, Burke, and


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Ernzerhof (PBE) was used to describe the exchange-correlation potential.57 A Hubbard-type correction U was employed due to the strongly correlated nature of the transition metal 3d electrons.58, 59 Based on previous literature reports, the effective U values of the Ni and Mn ions were set to 6.0 and 5.0 eV, respectively.60 The plane-wave cutoff was set to 520 eV. The reciprocal space k-point mesh interval was ca. 0.03 Å-1. The geometries were optimized using a conjugate gradient minimization method with a force tolerance of less than 0.01 eV/Å per atom.

The low-index {100} and {111} facets of spinel LNMO were extracted from the bulk spinel structure and fully relaxed. The slab technique was implemented by periodically repeating a slab layer-vacuum layer model an infinite number of times in the direction perpendicular to the surface. The vacuum thickness required to ensure no interactions between the slab layers was 10 Å. The surface energies were calculated by dividing the difference between the energy of a specified amount of bulk LNMO and the energy of a slab containing the same amount of LNMO by the surface area. To construct the surface models with adsorbed sulfate, both the {111} and {100} surfaces were covered by two SO42-. All possible positions for SO42- were investigated, and the most energetically favorable positions were selected. On the {111} supercell surface slab, each SO42- bonds to one transition metal ion, whereas on the {100} supercell surface slab, each SO42- bonds to two transition metal ions.

ASSOCIATED CONTENT



Supporting Information XRD Rietveld refinement results of O-LNMO and T-LNMO, morphology of particles without the filtration process, extraction of the components dissolved in the deionized water, detailed analysis of the EEL spectra of O-K, Mn-L and Ni-L in O-LNMO, detailed Analysis of the EEL spectra of O-K, Mn-L and Ni-L in T-LNMO, SEM images of a typical FIB lifted sample. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors *Email: [email protected] (X. J. Huang)

Notes The authors declare no competing finical interest.

ACKNOWLEDGEMENTS This work was supported by the National Key R&D Program of China (Grant No. 2016YFB0100300), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010000) and the project of Science of Technology Planning of Guangdong Province, China (Grand No. 2015B010118001, 2014B010125003).


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REFERENCES 1.

Zhong, Q. M.; Bonakdarpour, A.; Zhang, M. J.; Gao, Y.; Dahn, J. R., Synthesis and

Electrochemistry of LiNixMn2-xO4. J. Electrochem. Soc. 1997, 144, (1), 205-213. 2.

Goodenough, J. B.; Kim, Y., Challenges for Rechargeable Li Batteries. Chem. Mater.

2010, 22, (3), 587-603. 3.

Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B., Lithium Insertion

into Manganse Spinels. Mater. Res. Bull. 1983, 18, (4), 461-472. 4.

Kim, J. H.; Pieczonka, N. P. W.; Li, Z. C.; Wu, Y.; Harris, S.; Powell, B. R.,

Understanding the Capacity Fading Mechanism in LiNi0.5Mn1.5O4/Graphite Li-Ion Batteries. Electrochim. Acta 2013, 90, 556-562. 5.

Manthiram, A.; Chemelewski, K.; Lee, E.-S., A Perspective on the High-Voltage

LiMn1.5Ni0.5O4 Spinel Cathode for Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, (4), 1339-1350. 6.

Li, Y.; Wan, S.; Veith, G. M.; Unocic, R. R.; Paranthaman, M. P.; Dai, S.; Sun, X.-G., A

Novel Electrolyte Salt Additive for Lithium-Ion Batteries with Voltages Greater than 4.7 V. Adv. Energy Mater. 2017, 7, (4). 7.

Sun, X.-G.; Wan, S.; Guang, H. Y.; Fang, Y.; Reeves, K. S.; Chi, M.; Dai, S., New

Promising Lithium Malonatoborate Salts for High Voltage Lithium Ion Batteries. J. Mater. Chem. A 2017, 5, (3), 1233-1241. 8.

Choi, N. S.; Han, J. G.; Ha, S. Y.; Park, I.; Back, C. K., Recent Advances in the

Electrolytes for Interfacial Stability of High-Voltage Cathodes in Lithium-Ion Batteries. Rsc Advances 2015, 5, (4), 2732-2748.



9.

Lee, H.; Choi, S.; Choi, S.; Kim, H. J.; Choi, Y.; Yoon, S.; Cho, J. J., SEI

Layer-Forming Additives for LiNi0.5Mn1.5O4/Graphite 5 V Li-ion Batteries. Electrochem. Commun. 2007, 9, (4), 801-806. 10. Shin, D. W.; Bridges, C. A.; Huq, A.; Paranthaman, M. P.; Manthiram, A., Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn1.5Ni0.5-xMxO4 (M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries. Chem. Mater. 2012, 24, (19), 3720-3731. 11. Wang, H.; Ben, L. B.; Yu, H. L.; Chen, Y. Y.; Yang, X. N.; Huang, X. J., Understanding the Effects of Surface Reconstruction on the Electrochemical Cycling Performance of the Spinel LiNi0.5Mn1.5O4 Cathode Material at Elevated Temperatures. J. Mater. Chem. A 2017, 5, (2), 822-834. 12. Zhao, Y.; Peng, L.; Liu, B.; Yu, G., Single-Crystalline LiFePO4 Nanosheets for High-Rate Li-Ion Batteries. Nano Lett. 2014, 14, (5), 2849-2853. 13. Fu, F.; Xu, G.-L.; Wang, Q.; Deng, Y.-P.; Li, X.; Li, J.-T.; Huang, L.; Sun, S.-G., Synthesis of Single Crystalline Hexagonal Nanobricks of LiNi1/3Co1/3Mn1/3O2 with High Percentage of Exposed {010} Active Facets as High Rate Performance Cathode Material for Lithium-Ion Battery. J. Mater. Chem. A 2013, 1, (12), 3860-3864. 14. Kuppan, S.; Shukla, A. K.; Membreno, D.; Nordlund, D.; Chen, G., Revealing Anisotropic Spinel Formation on Pristine Li- and Mn-Rich Layered Oxide Surface and Its Impact on Cathode Performance. Adv. Energy Mater. 2017, 7, (11). 15. Pan, X.-L.; Xu, C.-Y.; Hong, D.; Fang, H.-T.; Zhen, L., Hydrothermal Synthesis of


Page 26 of 34

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Well-Dispersed LiMnPO4 Plates for Lithium Ion Batteries Cathode. Electrochim. Acta 2013, 87, 303-308. 16. Sun, W.; Cao, F.; Liu, Y.; Zhao, X.; Liu, X.; Yuan, J., Nanoporous LiMn2O4 Nanosheets with Exposed {111} Facets as Cathodes for Highly Reversible Lithium-Ion Batteries. J. Mater. Chem. 2012, 22, (39), 20952-20957. 17. Huang, M. R.; Lin, C. W.; Lu, H. Y., Crystallographic Facetting in Solid-State Reacted LiMn2O4 Spinel Powder. Appl. Surf. Sci. 2001, 177, (1-2), 103-113. 18. Liu, R. R.; Deng, X.; Liu, X. R.; Yan, H. J.; Cao, A. M.; Wang, D., Facet Dependent SEI Formation on the LiNi0.5Mn1.5O4 Cathode Identified by In Situ Single Particle Atomic Force Microscopy. Chem. Commun. 2014, 50, (99), 15756-15759. 19. Park, S. H.; Oh, S. W.; Yoon, C. S.; Myung, S. T.; Sun, Y. K., LiNi0.5Mn1.5O4 Showing Reversible Phase Transition on 3 V Region. Electrochem. Solid-State Lett. 2005, 8, (3), A163-A167. 20. Chemelewski, K. R.; Lee, E.-S.; Li, W.; Manthiram, A., Factors Influencing the Electrochemical Properties of High-Voltage Spinel Cathodes: Relative Impact of Morphology and Cation Ordering. Chem. Mater. 2013, 25, (14), 2890-2897. 21. Chemelewski, K. R.; Shin, D. W.; Li, W.; Manthiram, A., Octahedral and Truncated High-Voltage Spinel Cathodes: the Role of Morphology and Surface Planes in Electrochemical Properties. J. Mater. Chem. A 2013, 1, (10), 3347-3354. 22. Liu, H. D.; Wang, J.; Zhang, X. F.; Zhou, D.; Qi, X.; Qiu, B.; Fang, J. H.; Kloepsch, R.; Schumacher, G.; Liu, Z. P.; Li, J., Morphological Evolution of High-Voltage Spinel



LiNi0.5Mn1.5O4 Cathode Materials for Lithium-Ion Batteries: The Critical Effects of Surface Orientations and Particle Size. ACS Appl. Mater. Interfaces 2016, 8, (7), 4661-4675. 23. Chen, Z.; Zhao, R.; Du, P.; Hu, H.; Wang, T.; Zhu, L.; Chen, H., Polyhedral LiNi0.5Mn1.5O4 with Excellent Electrochemical Properties for Lithium-Ion Batteries. J. Mater. Chem. A 2014, 2, (32), 12835-12848. 24. Chen, Z.; Qiu, S.; Cao, Y.; Ai, X.; Xie, K.; Hong, X.; Yang, H., Surface-Oriented and Nanoflake-Stacked LiNi0.5Mn1.5O4 Spinel for High-rate and Long-Cycle-Life Lithium Ion Batteries. J. Mater. Chem. 2012, 22, (34), 17768-17772. 25. Arrebola, J. C.; Caballero, A.; Cruz, M.; Hernan, L.; Morales, J.; Castellon, E. R., Crystallinity Control of a Nanostructured LiNi0.5Mn1.5O4 Spinet via Polymer-Assisted Synthesis: A method for Improving its Rate Capability and Performance in 5 V Lithium Batteries. Adv. Funct. Mater. 2006, 16, (14), 1904-1912. 26. Zhan, X.; Li, Z.; Tang, J.; Xiao, Q.; Lei, G.; Zhou, X., Cyclic Improvement of LiNi0.5Mn1.5O4 Polyhedral Spinels for 5.0 V Lithium Ion Batteries. Funct. Mater. Lett. 2010, 3, (3), 185-188. 27. Deng, Y.-F.; Zhao, S.-X.; Xu, Y.-H.; Gao, K.; Nan, C.-W., Impact of P-Doped in Spinel LiNi0.5Mn1.5O4 on Degree of Disorder, Grain Morphology, and Electrochemical Performance. Chem. Mater. 2015, 27, (22), 7734-7742. 28. Cheralathan, K. K.; Kang, N. Y.; Park, H. S.; Lee, Y. J.; Choi, W. C.; Ko, Y. S.; Park, Y.-K., Preparation of Spherical LiNi0.80Co0.15Mn0.05O2 Lithium-Ion Cathode Material by Continuous Co-Precipitation. J. Power Sources 2010, 195, (5), 1486-1494.


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29. Jouanneau, S.; Dahn, J. R., Preparation, Structure, and Thermal Stability of New NixCo1-2xMnx(OH)2 (0

Understanding the Formation of the Truncated Morphology of High

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