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Understanding Surface Structural Stabilization of the High-Temperature and High-Voltage Cycling Performance of Al3+ Modified LiMn2O4 Cathode Material Bin Chen, Liubin Ben, Hailong Yu, Yuyang Chen, and Xuejie Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14535 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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ACS Applied Materials & Interfaces

Understanding Surface Structural Stabilization of the High-Temperature and High-Voltage Cycling Performance of Al3+ Modified LiMn2O4 Cathode Material Bin Chen†‡§, Liubin Ben†‡§, Hailong Yu†‡, Yuyang Chen†‡, and Xuejie Huang†‡*

† Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China ‡ School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, 100190, China

*E-mail: [email protected] (X. J. Huang)

§ These authors contributed equally to this work

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ABSTRACT Stabilization of atomic-level surface structure of LiMn2O4 with Al3+ ions is shown to be significant in improving of cycling performance, particularly at high-temperature (55 oC) and high-voltage (5.1 V). Detailed analysis by XPS, SIMS, STEM-EDS, etc. reveals that Al3+ ions diffuse into the spinel to form a layered Li(Alx,Mny)O2 structure in the outmost surface where Al3+ concentration is the highest. Other Al3+ ions diffuse into the 8a sites of spinel to form a (Mn3-xAlx)O4 structure and the 16d sites of spinel to form Li(Mn2-xAlx)O4. These complicated surface structures, in particular the layered Li(Alx,Mny)O2, are present at the surface throughout cycling and effectively stabilize the surface structure by preventing dissolution of Mn ions and mitigating cathode-electrolyte reactions. With the Al3+ ions surface modification, stable cycle performance (~78% capacity retention after 150 cycles) and high coulombic efficiency (~99%) are achieved at 55 oC. More surprisingly, the surface stabilized LiMn2O4 can be cycled up to 5. 1 V without significant degradation, in contrast to the fast capacity degradation found in the un-modified case. Our findings demonstrate the critical role of ions coated on the surface in modifying the structural evolution of the surface of spinel electrode particles, and thus will stimulate future efforts to optimize the surface properties of battery electrodes.

KEYWORDS：：spinel LiMn2O4, STEM, Al2O3 modification, XPS, layered Li(Alx,Mny)O2


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INTRODUCTION Spinel cathode materials such as LiMn2O4 and LiNi0.5Mn1.5O4 have been considered to be the most attractive cathode materials for rechargeable lithium-ion batteries due to their low cost, thermal safety and excellent rate capability, compared with commercial LiCoO2.1-3 However, the capacity of these cathode materials fades severely upon cycling or during storage, especially at elevated temperatures (>55 oC) or high voltage (>4.3 V). Proposed mechanisms can be broadly classified as follows: 1) dissolution of Mn2+ into the electrolyte,4 2) instability of the spinel structure, e.g. Jahn-Teller distortion5 and 3) surface-electrolyte reactions.6 Recent studies indicate that the capacity degradation is directly associated with dissolution of Mn ions from the cathode/electrolyte interface and subsequent deposition of Mn2+ on the anode, increasing the cell impedance.7 Although mechanism 1 is considered as the most important factor in capacity fade, it is closely associated with mechanisms 2 and 3. To solve these problems, ion substitution8-10 and coating,11-14 have been tried in attempts to improve cycling performance. It has been shown that the interface, especially a few-nanometer region on the active material’s surface, plays a key role in the whole electrochemical process.15-17 The interface between the cathode and electrolyte is the most active zone and the region within a nanometer of the electrode surface shows significant changes during electrochemical cycling. A typical case is the layer-structured materials, which show evolution from layer to spinel, then spinel to rock-salt structural transformation, starting from the surface region and gradually extending to the whole bulk.18-20 This structural evolution is accompanied by migration of transition metal ions and loss of a small amount of oxygen in the cathode, which can be amplified by exposing a greater expanse of active surface or raising the charge-discharge rate or temperature, resulting in faster degradation of electrochemical cycling performance. Spinel LiMn2O4, the parent of many spinel-related cathode ACS Paragon Plus Environment


materials,21 shows relatively stable structure during 3-4.3 V cycling compared with the layered materials; however, recent STEM studies reveal significant structural reconstruction from spinel to Mn3O4-like on the ~5 nm surface region, which is the origin of Mn dissolution and consequently degradation of cycling performance.22 Furthermore, if the cycling voltage is raised above 4.3 V even for one cycle, the surface structure is found to distort significantly with the formation of layered Li2MnO3, giving rise to further oxygen loss, resulting in even faster capacity degradation.23-24 Since electrochemical cycling of cathode depends on the stability of surface region, surface modification is the most effective way to improve the structural stability and electrochemical performance. Surface modification with materials containing some inactive ions on the surface of LiMn2O4 particles or surface of the whole cathode electrode has been investigated.25-26 In general, the surface modification layer should be compatible with the host structure and be more stable than the host. For instance, oxide coated spinel LiMn2O4 has been studied by many groups expecting that a protective layer and a HF scavenger should form at the interface.27 In particular, Al2O3, an amphoteric oxide, is believed to effectively protect the surface from the attack of HF.28-29 Al2O3 has been coated on spinel cathodes and many layered cathodes.30-32 The common underlying mechanism is that Al3+ can combine with electrolyte molecules to form an Al-O-F like substance to inhibit the appearance of low valence Mn ions on the surface of LiMn2O4. However, our recent study of Ti4+-modified LiNi0.5Mn1.5O4 suggests that the surface structure of coated cathode still plays a dominant role in cycling performance, since electrochemical cycling performance during prolonged cycling is not improved if modification oxides are loosely bonded to the bulk structure.14 Herein, we report the application of scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectroscopy (SIMS) etc. to investigate ACS Paragon Plus Environment

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the Al2O3 modification on LiMn2O4. We observed that the electrochemical cycling performance of Al3+ modified LiMn2O4 is significantly improved during cycling (3-4.3 V) at 55 oC. More interestingly, with Al3+ surface modification, LiMn2O4 could be cycled up to 5.1 V without significant degradation of cycling performance. With the help of STEM, evolution of the crystalline surface structure of Al3+ modified LiMn2O4 can be directly observed, and we compare it with that of the surface electronic structure during electrochemical cycling.

EXPERIMENTAL SECTION Materials Synthesis. LiMn2O4 (LMO) was prepared by the conventional solid-state reaction. A mixture of a certain amount of Li2CO3 (Alfa Aesar) and electrolytic manganese dioxide (Xiangtan Electrochemical Scientific Ltd) was used as the starting material. The mixture was heated for 12 h at 950 oC in an ambient atmosphere, followed by slow cooling to room temperature. Al2O3 modified LiMn2O4 (ASD-LMO) was prepared by solution method.33-35 Certain amounts of Al(NO3)3·9H2O were dissolved in distilled water after the pH was adjusted to the desired value by ammonium hydroxide, then 2g LiMn2O4 powder was dispersed into the above solution and stirred for an hour at room temperature. The amount of coating material was controlled at 2 wt%. The suspension containing the LiMn2O4 powder was constantly stirred at 80 ℃ to evaporate water and then the resulting mass was heated at 600 oC for 3 h. For comparison, primary LiMn2O4 was also heated at 600 oC for 3 h, and this sample will henceforth be called simply LMO.

XPS Characterization. All spectra were calibrated with the C 1s photoemission peak at 284.8 eV to account for the charging effect. The peak-fitting and quantitative evaluation were performed with the Casa XPS software. The background was corrected using the Shirley method. The change in manganese valence state was interpreted from the XPS data. ACS Paragon Plus Environment


Powder XRD Diffraction. X-ray diffraction patterns of the LMO and ASD-LMO were collected on a Philips X’pert Diffractometer with Cu Kα radiation between 10 and 90° at an increment of 0.02°. Based on the X-ray diffraction data, the structural parameters of the LMO and ASD-LMO were refined using the Rietveld refinement program TOPAS.36 TEM and STEM Characterization. To investigate the morphologies and structures of the as-synthesized and cycled samples, transmission electron microscopy (TEM) and high-angle annular dark-field or low-angle bright-field STEM (HAADF/ABF STEM) measurements were carried out by JEM ARM200F (JEOL, Tokyo, Japan),equipped with two CEOS (Heidelberg, Germany) probe aberration correctors. SEM Characterization. The morphology was characterized by a scanning electron microscope (HITACHI SU-4800). The elemental composition and mapping were conducted and performed by energy dispersive X-ray (EDX) analyses on an SEM HITACHI SU-4800 instrument operated at 15 kV. SIMS Characterization. Depth profiles were obtained by using a SIMS WORKSTATION (Hiden Analytical). The ion source device was run at an operating pressure of 3x10e-5 Torr. Depth profiling was done using a 5000 eV Ar+ sputter beam giving a 260 nA target current over a 900 µm× 900 µm area. Data acquisition and post-processing analyses were performed using the Quadrupole mass spectrometer analysis. Positive and negative ion profiles were recorded using the exact same sputtering and analysis conditions.


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Electrochemical Measurements. The working electrodes were prepared by mixing LMO or ASD-LMO with conductive acetylene black and Poly(vinylidene fluoride) (PVDF) binder in a weight ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidone(NMP).The electrolyte was a 1 M solution of LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed at the volume ratio of 1 : 1. A lithium metal disc was used as the anode. Cells were assembled in an argon-filled glovebox. Charge and discharge tests of the cells were carried out at room temperature and in a 55 oC oven (Binder, Germany). The experiment was performed on the charge and discharge of constant current within the operating voltage range of 4.3-3.0 V at 0.25 C rate (Arbin Instruments). For high-voltage cycling, cells were initially cycled between 4.3-3.0 V for six cycles to stabilize then were cycled between 5.1-3.0 V for one cycle (7th cycle). After the 7th cycle, the voltage range was changed to 4.3-3.0 V. For evaluating the cathode materials, the cells were disassembled in an argon-filled glovebox, and then the cathode was rinsed with DMC solution to remove electrolyte salt. Measurements were carried out using a 2032-type coin cell.

Metal Dissolution. The amount of dissolved metal ion was investigated by soaking the fully charged electrode in the electrolyte (10 mL) at 55 °C for 7 days, followed by determining the amount of metal ions in the electrolyte with ICP. The electrolyte was filtered with filter paper (pore size 0.2 µm) before the ICP measurement.



RESULTS AND DISCUSSION

Figure 1. SEM images of (a) LMO and (b) ASD-LMO samples. (c,d) corresponding magnified images.

SEM images of LMO and ASD-LMO, Figure 1a and 1b, show the average size of the primary particles to be ~ 2-4 µm. The LMO particles exhibit smooth surfaces, Figure 1c, but a thin film is clearly visible on the surface of the ASD-LMO particles, Figure 1d, in the magnified image. SEM-EDS results (Figure S1) confirm the surface film to be rich in Al. The thin film on the surface can also be identified according to the TEM data (Figure S2). The average chemical composition is Li : Mn : O (atomic ratio) = 1.05 : 1.98 : 4 for LMO and Li : Al : Mn: O (atomic ratio) = 1.05 : 0.07 : 1.88 : 4 for ASD-LMO, as determined by inductively coupled plasma/atomic emission spectrometry (ICP/AES). XRD patterns of the LMO and ASD-LMO powders, Figure 2, exhibit a well-defined phase-pure spinel phase no extra diffraction peaks. The crystal structure of LMO is a cubic spinel structure with the space group of Fd3m. In this structure, lithium, manganese and oxygen reside on the 8a (tetrahedral sites), 16d (octahedral sites), and 32e Wyckoff sites respectively, with a chemical formula of [Li+]8a[Mn3+Mn4+]16d[□]16c[O4]32e. Rietveld refinement shows lattice parameter a of LMO and ASD-LMO to be ~8.2195(2) Å and ~8.2208(3) Å, respectively, similar to previous ACS Paragon Plus Environment

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findings.37 Detailed refinement parameters are shown in Table S1. It has been reported that Al3+ ions prefers replacement of Mn3+ in the structure.8, 38 However, incorporation of Al3+ ions at the Mn3+ sites does not result in better results, suggesting that Al3+ ions may diffuse only a few nanometers into the surface spinel structure during high-temperature post-calcination of the material.14

Figure 2. Rietveld refinement of XRD data for (a) LMO and (b) ASD-LMO samples.

To better understand the surface components after Al3+ ions modification, surface elementary analysis by SIMS and XPS was performed and compared to that of LMO. SIMS results, Figure 3a, show that the Al3+ ions concentration in the surface of the ASD-LMO particles is relatively high but decreases with increased duration of argon ion etching.39 XPS results reveal that the atomic concentration of Al element is ~1.5% and Mn element is ~0.5% in the surface region of primary ASD-LMO powders, Figure 3b, also suggesting the presence of Al-rich surface. Fitted Mn 2p3/2 spectra, Figure 3c and d, further indicate ~33.5% Mn4+, ~66.5% Mn3+ on the surface of LMO, compared to ~49.1% Mn4+, ~50.9% Mn3+ on ASD-LMO. Details of the fitting can be found in the literature.40-41 The decreased amount of surface Mn3+ ions suggests that a small amount of Al3+ ions may diffuse into the spinel to substitute for Mn3+ in the surface layer of particles, which does not result in a change of the general crystal structure, in agreement with our XRD results.



Figure 3. (a) SIMS positive ion depth profiles of ASD-LMO sample. (b) XPS atomic ratios of LMO and ASD-LMO samples. Fitted Mn 2p3/2 spectra for (c) LMO and (d) ASD-LMO samples.

The presence of high Al3+ surface concentration clearly affects electrochemical activity, possibly by stabilizing the surface structure and inhabitation of side reactions with the electrolyte. Stable cycling performance (~78% capacity retention after 150 cycles) and high coulombic efficiency (~99%) are achieved for ASD-LMO even at a high temperature (55 oC), compared with ~58% and ~98% for the bare LMO, Figure S3. More surprisingly, high-voltage cycling of ASD-LMO causes almost no degradation of cycling performance, i.e. 99% capacity retention in the 50th cycle after charging to 5.1 V, Figure S4, compared to only ~75% for LMO. The physical isolation of the cathode by Al2O3 as reflected by the XPS data is in good accord with previous work. 34-35 However, structural evolution of Al3+ modified LMO surface, which is closely related to Mn dissolution and reactions between the electrode and electrolyte, has not been reported previously. Hence this article will mainly focus on the surface structure evolution. Employing the aberration-corrected STEM technique equipped with annular bright field (ABF) and high-angle annular dark field (HAADF) detectors, the change of crystallographic structure after ACS Paragon Plus Environment

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charge/discharge could be identified directly at atomic scale, as shown in Figure 4. A typical atomic-level crystal structure of un-modified LMO viewed along the typical [110]spinel direction under STEM is shown in Figure 4a. Separated columns of Li, Mn and O atoms can be clearly observed along this direction. As indicated by the arrows in Figure 4b, two different Mn columns are designated as Mnα and Mnβ. According to the spinel structure, Mnα has twice the Mn density of Mnβ. Line profile, Figure 4c shows typical intensity of contrast associated with Mn, O and Li columns. The STEM-HAADF image is consistent with the crystal structure previously reported.42 In general, the spinel crystal structure of LMO is homogeneous from the bulk to surface before Al3+ modification.

Figure 4. STEM-HAADF image and simulated crystal structure of LMO viewed along the [110]spinel crystallographic direction. (a) Demonstration of arrangement of Mn ions in LMO resembling diamond structure. Inset is an enlarged region. (b) Schematic illustration of the standard spinel structure viewed from the [110]spinel direction. Dashed circles represent empty 16c sites, green spheres represent Li, blue for Mn, and red for O. Li occupies the 8a sites and Mn occupies the 16d sites. (c) Line profile corresponding to the red line in the enlarged region in (a). Two Mn columns are designated as Mnα, Mnβ, since they have different stacking densities.

Figure 5 shows typical STEM-HAADF images obtained from two particles of the ASD-LMO sample viewed along the most common crystallographic direction, i.e. the [110]spinel direction, of the spinel structure in the surface region. The contrast of HAADF image in region I as shown in Figure 5a and b, and its corresponding enlarged region, Figure 5c1, can be generally ascribed to the spinel ACS Paragon Plus Environment


lattice structure, as also shown in simulated image Figure 5c2. For the near-surface region, however, a significant presence of differently arranged atoms appears. In the region marked by II (Figure 5a and 5b) and its magnified image Figure 5d1, it is evident that columns associated with the 16c octahedral-sites of the spinel structure are occupied by heavy cations. The new metal columns must be formed by Mn or Al ions, because in the HAADF mode the electron-beam-scattering capacity of light elements is weak.43 Because the observed surface region is mostly along this direction, it is not easy to identify the exact structural origin. This type of arrangement of atoms (spinel lattice with 16c sites occupied) was previously suggested to be associated with a layered structure of R3m space group along the [221]layered direction, Figure 5d2.27, 44 Others reported the surface arrangement to be a rock-salt or a disorder rock-salt structure, e.g. MnO (Fm3m) showing a similar arrangement of atoms viewed along the [101]rock-salt direction.22, 45 However, it is not possible to distinguish between these two structures based on the STEM-image here. In addition, the formation of the new surface arrangement of atoms (relative to spinel structure) can be induced by two factors, one is composition change,46 and the other is dynamic effects such as damage by electrons in the beam or corrosion by electrolyte molecules.47 However, the energy of the electron beam during the experiment is not high, unlikely to cause enough damage to form the new structure, as reported in previous work.48 In addition to the new surface structure in region II, some regions show a typical defect-spinel Mn3O4-like structure (additional contrast in the 8a sites) similar to that in cycled LiMn2O4 or LiNi0.5Mn1.5O422, 45 and in primary LiMn2O4 coated by LiNi0.5Mn0.5O2,44 as shown in region III of Figure 5b and the corresponding magnified image of Figure 5e1. This is attributed to the emergence of both Li-rich (Li2Mn2O4) and Mn-rich (Mn3O4) spinel phases upon the loss of oxygen from the surface. ACS Paragon Plus Environment

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Figure 5. (a, b) STEM-HAADF images showing two typical regions in ASD-LMO particles viewed along the [110]spinel direction of spinel. (c1) Enlarged image from region I in (a, b), corresponding to the perfect spinel structure in LMO, described above. (d1) Enlarged image for region II in (a, b) showing atomic arrangement along the [221]layered direction or the [101]rock-salt direction. (e1) Enlarged image from region III in (b), showing contrast at the Li sites (8a), which is comparable to that of the Mnβ columns. Schematic of the (c2) spinel structure viewed from the [110]spinel direction, (d2) the layered-like structure viewed from the [221]layered direction or the rock-salt structure viewed from the [101]rock-salt direction and (e2) the tetragonal Mn3O4-like structure viewed from the [100]defect spinel direction.

Though STEM can directly observe the atomic-level arrangement of atoms, precise structure information can be obtained only after study of a significant amount of particles viewed along various crystallographic directions. The STEM image in Figure 6 shows an arrangement of atoms along the [211]spinel direction of the spinel structure, which is less commonly observed. In this image the sub-surface regions show the arrangements of atoms to be a perfect spinel structure, Figure 6a. However, near the surface region, the intensity associated atoms (marked by red arrows) gradually decreases, as shown in the region II in Figure 6b, suggesting a gradual transformation from spinel to ACS Paragon Plus Environment


layered structure. This rules out the surface rock-salt structure as suggested by others because the layered structure in region II of Figure 6b cannot be observed from any crystallographic direction of rock-salt, e.g, MnO with Fm3m structure. Considered together, STEM results in Figure 5 (region II) and Figure 6 (region II), strongly indicate that the new surface structure in Figure 5 is likely associated with layered R 3 m structure viewed along the [2 2 1]layered direction but along the [210]layered direction here in Figure 6.

Figure 6. (a) STEM-HAADF image of primary ASD-LMO particles in the [211]spinel direction. (b) Magnified image corresponding to the red square in a. (c) Simulated spinel crystal structure viewed along the [211]spinel crystallographic direction.

Atomic-level element information was investigated by STEM-EDS and the results are shown in Figure 7. Analysis across three regions, i.e. Mn3O4-like (similar to region III of Figure 5), layered (similar to region II of Figures 5 and 6) and spinel LiMn2O4 (similar to Region I of Figures 5 and 6), reveals the presence of Al element in all the regions, which deceases from the surface to the interior. These suggest that the diffusion of Al3+ ions into spinel may result in three possible structural modifications: partial replacement of Mn ions in the 8a sites of spinel to form Mn3O4-like structure with a possible formula of (Mn3–xAlx)O4, formation of a layered structure with a possible formula of Li(Alx,Mny)O2, and partial replacement of Mn ions in the 16d sites of spinel with a possible formula of Li(Mn2-xAlx)O4, Figure 7. Due to these atomic-level regions, the exact amount of Al in each formula is difficult to determine. The presence of Al-rich and Mn-deficient surface observed by ACS Paragon Plus Environment

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STEM-EDS is in agreement with XPS and SIMS results, Figure 3.

Figure 7. (a) STEM-HAADF image of ASD-LMO. I, II and III indicating the arrangements of atoms associated with the spinel LiMn2O4, layered-like Li(Alx,Mny)O2 and Mn3O4-like structures, respectively. (b) STEM-EDS profile of ASD-LMO showing concentration profile of Mn and Al. Red arrow in (a) shows the scanning direction.

The ASD-LMO cathode material was also investigated by STEM, after charging to 4.3 V and 5.1 V to obtain the surface structural evolution route during electrochemical process. Both the layered-like Li(Alx,Mny)O2 and the Mn3O4-like (Mn3–xAlx)O4 structures were observed on the surface, similar to Figures 5 and 6. However, the most interesting observation is that some particles show the arrangement of atoms associated with the layered-like Li(Alx,Mny)O2 structure viewed along the [100]layered direction next to the surface spinel viewed along the typical [110]spinel direction, Figure 8. The arrangement of atoms along the crystallographic direction shown in Figure 8 is the most convenient one for distinguishing between spinel LMO and layered Li(Alx,Mny)O2; however they were not commonly observed under STEM, probably due to the crystallography of the spinel and the surface oxides. Nevertheless, our STEM investigations of the surface structure of ASD-LMO suggests that the surface is stabilized by Al3+ modification, which is present on the surface of most particles even after electrochemical cycling at high-temperature and high-voltage. ACS Paragon Plus Environment


Figure 8. (a) STEM-HAADF image and (b) STEM-ABF image of ASD-LMO after cycling between 3 V and 4.3 V (charged state). (c) Simulated layered crystal structure viewed along the [100]layered crystallographic direction.

The above observed surface atomic-level structures, in particularly the surface spinel and the surface layered structures are as summarized in Figure 9. Figure 9a shows the surface layered structure viewed along the [221]layered crystallographic direction next to the surface spinel structure viewed along the [110]spinel crystallographic direction, similar to that in Figure 5. This is the most frequently observed STEM image of ASD-LMO. Figure 9b and 9c show the surface layered structure viewed along the [210]layered and [100]layered directions next to the spinel structure viewed along the [211]spinel and [110]spinel directions, similar to that in Figure 6 and Figure 8, respectively. The latter two surface structures are less frequently observed in ASD-LMO.

Figure 9. Schematic of surface layered and spinel structures observed. (a) Layered viewed along the [221]layered crystallographic direction next to the spinel observed along the [110]spinel crystallographic direction (e.g. surface structures observed in Figure 5a and 5b). (b) Layered viewed along the [210]layered crystallographic direction next to the


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spinel viewed along the [211]spinel crystallographic direction (e.g. surface structures observed in Figure 6). (c) Layered viewed along the [100]layered crystallographic direction next to the spinel viewed along the [110]spinel crystallographic direction (e.g. structures observed in Figure 8). Dashed lines show the borders of the layered and the spinel structures.

The cycled LMO and ASD-LMO cells were also surveyed by XPS to confirm the reduction of electrolyte-electrode reactions by Al3+ modification. The results show that the electrolyte decomposition deposits on the surface of particles – e.g. organic-like components and inorganic LiF/Li2CO3 layer – are also reduced by Al3+ ions modification, according to the analysis of C 1s and F 1s spectra in Figure S5.49-50 This is due to the protection of surface structure by diffusion of Al3+ ions into the spinel structure, as well as HF reduction attributable to Al3+, as reported previously. The peak around ~531 eV in ASD-LMO is broader than that in LMO, Figure S6, suggesting the presence of bonds between O and Al, likely Li(Alx,Mny)O2 and Al2O3.51 This can also be verified in the asymmetrical Al 2p peak, Figure S5, which originates from Al2O3 and Li(Alx,Mny)O2.12, 28-29, 52 These results suggest that the superior electrochemical cycling performance of Al3+ ions coated LiMn2O4 should not be attributed simply to mitigation of electrolyte-electrode reactions, but that it also involves a complex stabilized surface structure. Upon charging ASD-LMO to high-temperature or high-voltage, the layered Li(Alx,Mny)O2 structure remains intact on the surface of LMO, in contrast to un-modified LMO, in which a significant portion of Mn3+ ions migrate during cycling to form Mn3O4 and Li2MnO3. This surface layered Li(Alx,Mny)O2 has not been directly observed before but is believed to be present on the surface and remain stable during cycling.25,

53-55

Occupation of Al3+ ions in the 8a and 16d sites of spinel LMO also reduces the possibility of disproportionation of Mn3+,56-57 as well as blocking migration of Mn2+ ions from the structure to dissolve into the electrolyte.14, 45 The metal dissolution experiments, Table S2, shows clearly that the amount of Mn ions dissolved into the electrolyte after Al3+ surface modification is reduced. All ACS Paragon Plus Environment


of this results in stabilization of the spinel structure, particularly in the relatively weak surface region. Without modification of its surface structure, spinel LMO undergoes severe distortion by continuous migration of Mn ions (due to oxygen loss during charge) and interactions with the electrolyte.22 Escaped Mn ions dissolve into the electrolyte, rapidly degrading cycling performance.58, 59 A schematic of the surface modification by Al3+ ions is shown in Figure 10.

Figure 10. Schematic of the migration of Al3+ ions into the structure of Al2O3 coated spinel LMO with associated

formation of layered Li(Alx,Mny) O2 and defect-spinel (Mn3-xAlx)O4 structures in the surface. LMO is represented in green; layered Li(Alx,Mny)O2 and defect-spinel (Mn3-xAlx)O4 are indicated in red and yellow, respectively. The regions are indicated by colors; their sizes are not drawn to scale. Note that the particles observed in this work are faceted rather than spherical as they are shown in the schematic.

CONCLUSIONS In summary, made nanoscale modifications to the surface of spinel LMO with Al3+ and analyzed their effects on stabilization of the surface structure during electrochemical cycling with the help of various characterization techniques. It was found that structural evolution on the particle surface was significantly altered by Al-doping, compared to that of un-modified LiMn2O4. Specifically, a ACS Paragon Plus Environment

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phase transition from spinel to layered-like and Mn3O4-like structures was directly identified on the surface of LiMn2O4 particles after Al3+ modification via STEM. The presence of Al3+ within a few nanometers of the surface was confirmed by XPS, SIMS and STEM-EDS, suggesting that the layered-like structure is associated with layered Li(Alx,Mny)O2 and Mn3O4-like to be (Mn3-xAlx)O4 by diffusion of Al3+ ions into the spinel structure. With Al3+modification, the surface structure is stabilized and shows no significant changes after cycling at 55 oC or 5.1 V, resulting in stable cycling performance (~78% capacity retention after 150 cycles), high coulombic efficiency (~99%) at 55 oC, and ~99% capacity retention after 50 cycles at 5.1 V (room temperature). This contrasts sharply with the continuous migration of Mn2+ ions and distortion of surface structure during cycling of un-modified LMO, which exhibits fast capacity degradation. We believe that our findings advance the knowledge of the structure and chemistry of surface modification in the LMO electrode, motivating further theoretical and experimental research on the fundamental mechanisms.

ASSOCIATED CONTENT Supporting Information

TEM image of ASD-LMO, structure refinement of LMO and ASD-LMO, high-temperature and high-voltage cycling performance of LMO and ASD-LMO, XPS analysis of cycled LMO and ASD-LMO. 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) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010000).

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Table of Contents Al3+ modied spinel


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Temperature and High-Voltage Cycling ... - ACS Publications

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