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Article

Atomic Insights into Ti Doping on the Stability Enhancement of Truncated Octahedron LiMn2O4 Nanoparticles

by
Wangqiong Xu
1,
Hongkai Li
1,
Yonghui Zheng
1,
Weibin Lei
1,
Zhenguo Wang
1,
Yan Cheng
1,
Ruijuan Qi
1,*,
Hui Peng
1,
Hechun Lin
1,
Fangyu Yue
1 and
Rong Huang
1,2,*
1
Key Laboratory of Polar Materials and Devices (MOE) and Department of Electronics, East China Normal University, Shanghai 200062, China
2
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, Shanxi, China
*
Authors to whom correspondence should be addressed.
Nanomaterials 2021, 11(2), 508; https://doi.org/10.3390/nano11020508
Submission received: 19 January 2021 / Revised: 10 February 2021 / Accepted: 15 February 2021 / Published: 17 February 2021
(This article belongs to the Special Issue State-of-the-Art Nanophotonics Materials and Devices in China)
Browse Figures
Review Reports Versions Notes

Abstract

:
Ti-doped truncated octahedron LiTixMn2-xO4 nanocomposites were synthesized through a facile hydrothermal treatment and calcination process. By using spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM), the effects of Ti-doping on the structure evolution and stability enhancement of LiMn2O4 are revealed. It is found that truncated octahedrons are easily formed in Ti doping LiMn2O4 material. Structural characterizations reveal that most of the Ti4+ ions are composed into the spinel to form a more stable spinel LiTixMn2−xO4 phase framework in bulk. However, a portion of Ti4+ ions occupy 8a sites around the {001} plane surface to form a new TiMn2O4-like structure. The combination of LiTixMn2−xO4 frameworks in bulk and the TiMn2O4-like structure at the surface may enhance the stability of the spinel LiMn2O4. Our findings demonstrate the critical role of Ti doping in the surface chemical and structural evolution of LiMn2O4 and may guide the design principle for viable electrode materials.

Graphical Abstract

1. Introduction

Rechargeable lithium-ion batteries (LIBs) have been regarded as promising energy storage and conversion devices for wearable mobile devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), and stationary energy storage wells [1,2,3]. Among the various lithium-ion battery cathode materials, spinel LiMn2O4 is believed to hold huge potential for fulfilling the field-use requirements because of its good thermal stability, low cost, environmental friendliness, and three-dimensional channel structure [4,5,6]. Nevertheless, the practical applications of LiMn2O4 cathodes are restricted by the capacity fading during charge–discharge cycles, especially at elevated temperatures (≥ 55 °C), which can be ascribed to the Mn dissolution and Jahn–Teller distortion [7,8].
In order to tackle these challenges, efforts have been paid to stabilize the structure of LiMn2O4. By doping with monovalent (e.g., Li+ [9]), divalent (e.g., Mg2+ [10] and Ni2+ [11]) or trivalent (e.g., Al3+ [12], Co3+ [13] and Fe3+ [14]) metal ions, the average manganese ion valence is slightly increased, and therefore the Jahn–Teller effect is suppressed and a promoted cycling performance is obtained in the 4 V region. However, when LiMn2O4 works in the 2.0–4.8 V, inactive Mn4+ ions in the 4 V regions are further reduced to Mn3+ ions, and the cycle performance of low-valent ions doped materials is not that satisfactory. For example, Lee et al. [15] found that the LiAl0.1Mn1.9O4 achieved capacity retention of 70% after 50 cycles in the 2.0–4.3 V range. When cycled between 2.0 and 5.0 V, the LiNi0.5Mn1.5O4 shows a capacity retention value of about 65% [16]. Since the bond energy of Ti-O (662 kJ mol−1) is higher than that of Mn-O (402 kJ mol−1), the Mn4+ in the lattice of LiMn2O4 could be partly replaced by Ti4+ to form a more stable spinel framework, i.e., [Mn2−xTix]O4, therefore enhancing the stability of the spinel LiMn2O4. Recently, He et al. [17] reported that 72% capacity retention was achieved with the LiTi0.5Mn1.5O4 electrode after 150 cycles performed between 2.0 and 4.8 V. By using an in situ X-ray diffraction technique, Wang et al. [18] found that Ti4+ ions can also suppress the Jahn–Teller distortion and stabilize the spinel structure during the charging/discharging process. Moreover, Ti substitution improves the structural stability of spinel cathode material as reported at large [19,20,21]. Although these findings are important and intriguing, a deep understanding on how Ti doping contributes to the stability enhancement of LiMn2O4 is still lacking.
To date, various experimental and computational results show that the structural stability of LiMn2O4 is strongly related to its surface structure [22,23,24,25]. Karim et al. [26] ascribed the improved stability of LiMn2O4 to the creation of a partial inverse spinel arrangement in the (111) surface. A further example by Ouyang et al. [27] showed that covering the LiMn2O4 (001) surface with Al2O3 changed the oxidation state of surface Mn atoms from +3 to +4, which is beneficial for the improvement in LiMn2O4 stability. Nevertheless, few studies have been undertaken to reveal the surface structure and chemical evolution of Ti-doped LiMn2O4 at atomic levels.
In this work, Ti-doped truncated octahedron LiMn2O4 samples are synthesized through a facile hydrothermal treatment and calcination process. To reveal the underlying mechanism of Ti-doping on the structure evolution and stability enhancement of LiMn2O4, morphology and phase characterization are performed by powder X-ray diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) further reveals that Ti ions are in a tetravalent oxidation state; after Ti ion doping, the percentage of Mn4+ in LiTi0.5Mn1.5O4 reduced, suggesting the successful replacement of Mn4+ by Ti4+. The surface evolution of LiTixMn2−xO4 (001) planes was investigated using the spherical aberration-corrected scanning transmission electron microscopes (Cs-STEM) technique. It is found that there is a more stable spinel LiTixMn2−xO4 formed in bulk, as well as at the {111} and {110} planes. In addition, for the first time, a TiMn2O4-like structure formed at {001} surface is observed by the Cs-STEM technique, which can reduce the surface energy of {001} planes and accelerate the growth rate of {001} planes. In addition, the TiMn2O4-like structure at {001} surface might improve the stability of LiMn2O4. According to the electron energy-loss spectroscopy (EELS) analysis, the appearance of the TiMn2O4-like phase is associated with the enrichment of Ti4+. This work provides a comprehensive understanding of the influence of Ti doping on the evolutions of morphology, surface structure, and electronic structure of LiMn2O4 cathodes, which will benefit the further optimization of the electrochemical performance.

2. Materials and Methods

2.1. Sample Preparation

The LiTixMn2−xO4 (0 ≤ x ≤ 0.5) samples were synthesized by hydrothermal treatment and a calcination process [28,29], as depicted in Figure 1. First, to get Mn3O4 nanoparticles with better reaction activity and smaller particle size, commercially purchased Mn3O4 powders (1.0 g) were dispersed into NaOH aqueous solution (30 mL, 5 mol dm−3) and magnetically stirred for 1 h. Afterward, the dispersion was transferred to a Teflon-lined stainless-steel autoclave (50 mL) and heated at 205 °C for 4 d in an oven. The final precipitated products were washed repeatedly with deionized water. The obtained Mn3O4 precursor was subsequently dried at 70 °C for 12 h in air. Then, the as-prepared Mn3O4 precursor, LiNO3, LiCl·H2O, and TiO2 (rutile) were ground in a mortar for 30 min and burned in the air at 500 °C for 3 h. The obtained Ti-doped LiMn2O4 precursors were washed repeatedly in deionized water to remove chlorion and nitrate impurities. Finally, the obtained Ti-doped LiMn2O4 precursor was calcined in air at 700 °C for 6 h. The final products were obtained after cooling to room temperature.

2.2. Sample Characterization

The crystal structures were characterized by X-ray diffraction (XRD, D8, Bruker, Germany) with Cu Kα radiation; the data were collected between 10 and 80 degrees at an increment of 0.02 degrees. The size and morphology of the samples were observed by scanning electron microscope (SEM, S-4800, Hitachi, Japan). The crystal quality and defects were characterized by Raman spectra using a micro-Raman spectrometer (Jobin Yvon LabRAM HR 800UV, Longjumeau, France) with a 532 nm laser source. EDS mapping was performed with an Oxford Inca EDS detector on the JEOL 2100F, operated in the dark field scanning transmission electron microscopy (STEM, JEOL) mode. X-ray photoelectron spectroscopy (XPS, Thermo Fischer, ESCALAB 250Xi, Walham, MA, USA) measurements were performed to investigate the valence states of the materials, using the value of 284.8 eV as the C 1s peak reference. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and electron energy-loss spectroscopy (EELS) were performed with a spherical aberration-corrected (Cs-corrected) scanning transmission electron microscopy (STEM) operated at 300 kV (JEM-ARM300F, JEOL).

3. Results and Discussions

The phase and crystal structure of the samples are examined by XRD, as shown in Figure 2a. The diffraction peaks of all samples can be indexed to the standard pattern of spinel LiMn2O4 (JCPDS card No.35-0782; space group Fd-3m (No. 227)) without any impurity phases. More importantly, the relative peak intensities reflect the dominant surface orientations of each sample. Compared to the octahedron, the peaks on the (400), (440), and (311) lattice planes (Figure S1a–c) are more obvious in truncated octahedron samples after normalizing peaks to the dominant (111) octahedral orientation. With the increase in Ti content, the diffraction peaks shift toward lower angles, suggesting the increase in lattice parameters. The detailed lattice parameters of the LiTixMn2−xO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples were calculated and are listed in Table S1. Since the atom radius of Ti4+ (0.061 Å) is larger than that of Mn4+ (0.053 Å) [30], the enlargement of the lattice constant indicates the substitution of Mn4+ by Ti4+ in the lattice, in line with previous reports [31].
Furthermore, the microstructure vibration of LiTixMn2−xO4 with different Ti doping content (Figure 2b) is investigated by Raman spectroscopy. The medium peak at about 480 cm−1 has F2g(2) symmetry, while the weak bands observed at 400 and 370 cm−1 have the Eg and F2g(3) symmetry, respectively [32,33]. The weak peak at 370 cm−1 is related to the Li-O symmetric vibration, i.e., connecting to the tetrahedral cation movements (F2g(3)) [34]. A very weak band at 285 cm−1 might be associated with the translation mode of lattice vibration [35]. A strong Raman peak at ~640 (±5) cm−1 could be assigned the symmetric Mn–O stretching vibration of [MnO6] octahedron (A1g mode). Moreover, a blue shift below x = 0.2 and a redshift above x = 0.2 are observed (Figure S1d), which further confirms the substitution of Ti atoms.
As shown in Figure 3a-f, the surface morphology and particle size of LiTixMn2-xO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were studied by SEM. The pristine LiMn2O4 (Figure 3a) is the prototype octahedral shape, which is bounded by eight {111} planes. It is reported that the truncated octahedral structure is beneficial for improving the high-rate capability and prolonging the cycle stability of LIBs, as the {111} planes can mitigate Mn dissolution while the truncated {110} and {001} planes facilitate Li+ diffusion [8]. Though several strategies have been proposed to obtain truncated octahedral structures [5,36,37,38], in this report we find that Ti doping is beneficial to synthesize a truncated octahedral shape. With the increase in Ti concentration, the growth rate is increased in the (001) plane (red rectangle), reduced in the (111) plane (blue lines), and remains the same in the (110) plane (green lines), implying that the Ti doping can reduce the surface energy of the (001) planes. In addition, the particle size is also increased with the doping of the Ti element (Figure S2), which may result from the substitution of Ti with Mn element.
To further examine the valence states of elements in the mixed-valence compounds, XPS was performed for LiMn2O4 and LiTi0.5Mn1.5O4, respectively. Figure 4a shows that the peaks of Ti 2p3/2 and Ti 2p1/2 in LiTi0.5Mn1.5O4 are located at 458.2 and 463.8 eV, respectively, with 5.6 eV spin-orbit components, indicating that the Ti ions are in the tetravalent oxidation state [31,39]. As for the Mn 2p XPS spectra, two main peaks corresponding to the spin-orbit splitting of Mn 2p3/2 and Mn 2p1/2 are observed in LiMn2O4 and LiTi0.5Mn1.5O4 (Figure 4b) [40]. Since the full width at half-maximum (FWHM) of the Mn 2p3/2 peaks are both larger than 3.5 eV, the oxidation states of Mn are expected to be between +3 and +4 valence. Furthermore, curve-fitting was conducted on the Mn 2p3/2 spectra (Figure 4c,d, see the fitting parameters in the supporting information in Tables S2 and S3) to evaluate the percentage of Mn3+ and Mn4+ ions [41,42]. The results show that the concentration of Mn4+ reduces from 47.28% to 42.13%, while the concentration of Mn3+ increases from 52.72% to 57.86% due to the substitution of Ti4+ ions (x = 0.5), as observed in Figure 4a.
To reveal the underlying mechanism of Ti-doping on the structure evolution and stability enhancement of LiMn2O4, samples with different Ti doping concentrations were systematically investigated using the Cs-STEM technique [43]. Figure 5a verifies the octahedron characteristic of LiMn2O4 composed of {111} facets, and Figure 5b is the enlarged HAADF image, taken along the [110] direction around the (111) surface. Since the contrast of the HAADF-STEM image is roughly proportional to the square of the atomic number Z [44], the Li (Z = 3) and O (Z = 8) are invisible, while the Mn (Z = 25) could be detected. The Mn diamond configuration was clearly observed (Figure 5b), in line with the previously reported [45], showing a homogeneous microstructure from the bulk to the surface.
As for the LiTi0.5Mn1.5O4, truncated octahedrons composed of {111} facets, {110} and {001} facets were observed. A uniform distribution of the Mn, O, and Ti elements is also shown in Figure S3. Similar to Figure 5b, the spinel crystal structure in LiTi0.5Mn1.5O4 is also stable from the bulk to the surface in (111) planes according to the HAADF image (Figure 5d) taken along [110] orientation. This homogenous situation also happened in the (110) plane, as depicted in Figure 5e. However, a phase transition from the bulk to the surface appears progressively in (001) planes, as indicated by the cyan line. Though the atomic configuration in the bulk region (red rectangle) is similar to Figure 5d–e, the surface region (purple rectangle) is quite different. The contrast of the atoms at Li tetrahedral sites becomes brighter and visible, which can be attributed to the substitution of heavy Ti or Mn (TM) ions [46]. This is also further confirmed by the line profiles shown in Figure 5i, in which the spacing in the surface area (d = 8.69 Å) is larger than the bulk area (d = 8.20 Å).
We inspected the crystal structure of LiMn2O4, TiMn2O4, and Mn3O4 along the [110] direction, as shown in Figure 6a–c. Though the atomic arrangement is similar, the long diagonals (n) for TiMn2O4 (n = 8.679 Å) is significantly higher than that of LiMn2O4 (n = 8.245 Å) and Mn3O4 (n = 8.15 Å). Thus, the new phase formed at (001) surface is expected to be TiMn2O4, which can help to combat the impedance growth [47] and promote the electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4. In short, the majority of the Ti atoms could replace Mn element in the bulk area and form a stable LiTixMn2−xO4 framework, which complies well with the XRD results. In addition, there is a new phase similar to TiMn2O4 formed at the spinel LiMn2O4 (001) surface.
It is known that the surface energy is gradually reduced in the sequence of {001}, {110} and {111} [48], thus the presence of the TiMn2O4-like spinel phase on the (001) surface may be related to the surface energy difference. Thus, the {001} plane is in accordance with the most unstable surfaces, favoring the Ti cations shift. Moreover, the Li-terminated LiMn2O4 {001} surfaces are also very unstable due to the increased dangling bonds and lower bonding energy with the oxygen anions [49]. Therefore, a small amount of TM cations can exchange the position with Li+, resulting in the formation of reconstruction layers in these regions. This reconstruction layers (TiMn2O4-like) are able to produce a more stable cathode/electrolyte interfacial layer due to the stronger Ti-O bond, promoting the stability of cathode materials [21].
To further unveil the change in surface chemical states around different crystal planes, the pristine LiMn2O4 and LiTi0.5Mn1.5O4 samples are characterized using EELS, and the results are shown in Figure 7a–c. Figure 7a shows the Ti-L2,3, O-K, and Mn-L2,3 energy-loss near-edge fine structure (ELNES) around the (111) facet surface for the LiMn2O4, and the (111), (110), and (001) facet surfaces for the LiTi0.5Mn1.5O4 after background subtraction and normalization. In LiTi0.5Mn1.5O4, the pre-peak intensity of the O-K edge in LiTi0.5Mn1.5O4 is slightly less than that in LiMn2O4, which is correlated with a slight decrease in Mn valence [50]. Moreover, the O-K spectrum in LiMn2O4 shows a sharp peak followed by a shoulder structure, while two peaks at 532.4 and 534.2 eV in LiTi0.5Mn1.5O4 are observed, which can be assigned to the transition to the 3d bands of tetravalent Ti. Four peaks in the Ti-L2,3 ELNES for LiTi0.5Mn1.5O4 at different facets are also shown in Figure 7a, a fingerprint of Ti4+, which is also consistent with the XPS measurement. By using the pristine LiMn2O4 as a reference to extract the k-factors (Figure S4), the Mn/O ratios (RMn/O) at different surface planes were quantified (Figure 7b), in which RMn/O(110) > RMn/O(111) > RMn/O(001) planes. In the (001) plane, RMn/O(001) is approximately 0.39, indicating that Ti4+ is enriched in the (001) plane. Furthermore, the relationship between the Mn (L3/L2) intensity ratio and the Mn valence state at different facets is investigated (Figure 7c). A higher L3/L2 value results in a decreased Mn valence state [51,52,53,54], originating from the Ti doping effect as claimed in Figure 7a.

4. Conclusions

In conclusion, Ti-doped truncated octahedron LiMn2O4 samples were synthesized by a facile hydrothermal treatment and calcination process. Cs-STEM and chemical analysis techniques were carried out to reveal the underlying mechanism of Ti doping on the structure evolution and the stability enhancement of LiMn2O4 samples with different contents of Ti doping. It is found that Ti doping is beneficial to forming truncated octahedron LiTixMn2−xO4. Among the samples, LiTi0.5Mn1.5O4 samples exhibit the most obvious truncated octahedron structure. After Ti ion doping, the percentage of Mn4+ in LiTi0.5Mn1.5O4 reduced, suggesting the successful replacement of Mn4+ with Ti4+. Based on detailed surface structural analysis of the {111}, {110}, and {001} planes of LiTi0.5Mn1.5O4 at the atomic scale, it is found that there is a more stable spinel LiTixMn2−xO4 framework formed in bulk, as well as at the (111) and (110) planes. In addition, a TiMn2O4-like structure at the {001} surface is observed and thoroughly analyzed by Cs-STEM combined with EELS techniques. The new TiMn2O4-like structure can reduce the surface energy of (100) planes and accelerate the growth rate of (100) planes, therefore enhancing the stability of the spinel LiMn2O4. According to the EELS analysis, the appearance of the TiMn2O4-like phase can be associated with the enrichment of Ti4+. Our findings demonstrate the critical role of the Ti ion doping in the surface chemical and structural evolution of LiMn2O4, which provides a facile method for high-stability cathode materials design and growth.

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/11/2/508/s1, Figure S1: Enlarged view of XRD patterns (a–c) at (311), (400) and (440), and microscopic view of raman spectra at 620 cm−1 (d), Figure S2: SEM images of the LiTixMn2−xO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) at low magnification, and their corresponding to particle size distribution, Figure S3: STEM image and EDS elemental mappings of the LiTi0.5Mn1.5O4 samples, Figure S4: Details to quantification the ratio of Mn and O, Table S1: Lattice parameters of LiTixMn2−xO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5), Table S2: Mn 2p3/2 peak parameters for Mn in LiMn2O4 sample, Table S3: Mn 2p3/2 peak parameters for Mn in LiTi0.5Mn1.5O4 sample.

Author Contributions

W.X., R.Q. and R.H. conceived and designed the research and wrote the manuscript. W.X. carried out all of the experiments. Y.Z., Y.C., and F.Y. helped with the analysis of the STEM results. H.L. (Hongkai Li ), W.L., and Z.W. were involved in the XPS experiments. H.P. and H.L. (Hechun Lin) were involved in the material syntheses. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Research and Development Program of China (2017YFA0303403), Shanghai Science and Technology Innovation Action Plan (No.19JC1416700), and the National Natural Science Foundation of China (Grant No. 61974042 and 11774092).

Data Availability Statement

The data presented in this study are openly available in [repository name e.g., FigShare] at [doi], reference number [reference number].

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the preparation process of the LiTixMn2−xO4 samples.
Figure 1. Schematic diagram of the preparation process of the LiTixMn2−xO4 samples.
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Figure 2. XRD patterns (a) and Raman spectra (b) of the LiTixMn2−xO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples.
Figure 2. XRD patterns (a) and Raman spectra (b) of the LiTixMn2−xO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples.
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Figure 3. SEM images of LiTixMn2−xO4 at different concentrations (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) (a–f).
Figure 3. SEM images of LiTixMn2−xO4 at different concentrations (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) (a–f).
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Figure 4. Ti 2p XPS spectra of LiTi0.5Mn1.5O4 (a). Mn 2p XPS spectra (b) of LiMn2O4 and LiTi0.5Mn1.5O4. Fitted spectra of LiMn2O4 (c) and LiTi0.5Mn1.5O4 (d).
Figure 4. Ti 2p XPS spectra of LiTi0.5Mn1.5O4 (a). Mn 2p XPS spectra (b) of LiMn2O4 and LiTi0.5Mn1.5O4. Fitted spectra of LiMn2O4 (c) and LiTi0.5Mn1.5O4 (d).
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Figure 5. Low-magnification (a) and high-resolution (b) high-angle annular dark-field (HAADF) images of the LiMn2O4 viewed from the [110] crystallographic direction in (111) planes. Low-magnification (c) and high-resolution HAADF images of the LiTi0.5Mn1.5O4 particles viewed from the [110] crystallographic direction in the (111) plane (d), (001) plane (e), and (110) plane (f). Magnified views of selected regions are shown in the right panels, where the contrast corresponding to the Mn columns at 16d and 8a sites are indicated by blue and orange spheres, respectively. The boundary between the bulk and the surface regions is marked by the green dashed line. Line profiles (g–j) correspond to the sky blue lines in panel (b,d–f), respectively.
Figure 5. Low-magnification (a) and high-resolution (b) high-angle annular dark-field (HAADF) images of the LiMn2O4 viewed from the [110] crystallographic direction in (111) planes. Low-magnification (c) and high-resolution HAADF images of the LiTi0.5Mn1.5O4 particles viewed from the [110] crystallographic direction in the (111) plane (d), (001) plane (e), and (110) plane (f). Magnified views of selected regions are shown in the right panels, where the contrast corresponding to the Mn columns at 16d and 8a sites are indicated by blue and orange spheres, respectively. The boundary between the bulk and the surface regions is marked by the green dashed line. Line profiles (g–j) correspond to the sky blue lines in panel (b,d–f), respectively.
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Figure 6. Crystal structure of the LiMn2O4 (a), TiMn2O4 (b), and Mn3O4 (c) viewed along the [110] direction.
Figure 6. Crystal structure of the LiMn2O4 (a), TiMn2O4 (b), and Mn3O4 (c) viewed along the [110] direction.
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Figure 7. ELNES spectra of Ti-L2,3, O-K, and Mn-L2,3 from the surface of the (111) facet of the LiMn2O4, and (111), (110), and (001) facets of the LiTi0.5Mn1.5O4 (a). Mn/O atomic ratio of LiTi0.5Mn1.5O5 (b). Pristine LiMn2O4 was used as a reference to extract the k factors. Dependence of the Mn (L3/L2) intensity ratio vs. the Mn valence state in the (111) facet of the LiMn2O4, and (111), (110), and (001) facets of the LiTi0.5Mn1.5O4 (c).
Figure 7. ELNES spectra of Ti-L2,3, O-K, and Mn-L2,3 from the surface of the (111) facet of the LiMn2O4, and (111), (110), and (001) facets of the LiTi0.5Mn1.5O4 (a). Mn/O atomic ratio of LiTi0.5Mn1.5O5 (b). Pristine LiMn2O4 was used as a reference to extract the k factors. Dependence of the Mn (L3/L2) intensity ratio vs. the Mn valence state in the (111) facet of the LiMn2O4, and (111), (110), and (001) facets of the LiTi0.5Mn1.5O4 (c).
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Xu, W.; Li, H.; Zheng, Y.; Lei, W.; Wang, Z.; Cheng, Y.; Qi, R.; Peng, H.; Lin, H.; Yue, F.; et al. Atomic Insights into Ti Doping on the Stability Enhancement of Truncated Octahedron LiMn2O4 Nanoparticles. Nanomaterials 2021, 11, 508. https://doi.org/10.3390/nano11020508

AMA Style

Xu W, Li H, Zheng Y, Lei W, Wang Z, Cheng Y, Qi R, Peng H, Lin H, Yue F, et al. Atomic Insights into Ti Doping on the Stability Enhancement of Truncated Octahedron LiMn2O4 Nanoparticles. Nanomaterials. 2021; 11(2):508. https://doi.org/10.3390/nano11020508

Chicago/Turabian Style

Xu, Wangqiong, Hongkai Li, Yonghui Zheng, Weibin Lei, Zhenguo Wang, Yan Cheng, Ruijuan Qi, Hui Peng, Hechun Lin, Fangyu Yue, and et al. 2021. "Atomic Insights into Ti Doping on the Stability Enhancement of Truncated Octahedron LiMn2O4 Nanoparticles" Nanomaterials 11, no. 2: 508. https://doi.org/10.3390/nano11020508

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