Microstructure Evolution and Conversion Mechanism of Mn3

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Microstructure Evolution and Conversion Mechanism of MnO Under Electrochemical Cyclings 3

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Qingmei Su, Shixin Wang, Gaohui Du, Bingshe Xu, Shufang Ma, and Lin Shang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09412 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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The Journal of Physical Chemistry

Microstructure Evolution and Conversion Mechanism of Mn3O4 under Electrochemical Cyclings Qingmei Su,*, †, ‡ Shixin Wang,† Gaohui Du,*, † Bingshe Xu,†, § Shufang Ma,† Lin Shang† †

Institute of Atomic and Molecular Science, Shaanxi University of Science and Technology,

Xi’an 710021, China. ‡

Zhejiang Provincial Key Laboratory of Solid State Optoelectronic Devices, Zhejiang

Normal University, Jinhua 321004, China. §

Research Centre of Advanced Materials Science and Technology, Taiyuan University of

Technology, Taiyuan 030024, China.

ACS Paragon Plus Environment

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ABSTRACT: Probing the microstructure evolution, phase change and fundamental conversion mechanism of anodes for lithium ion batteries (LIBs) during lithiation-delithiation cycles are important to gain insights into understanding how the electrode works and thus how it can be improved.

The

electrochemical

reaction

and

phase

evolution

of

Mn3O4

during

lithiation-delithiation cycles remain unknown. To observe the real-time electrochemical behaviors of Mn3O4 during lithiation-delithiation cycles, a nano-sized LIB was constructed inside transmission electron microscope (TEM) using an individual Mn3O4/graphene as the anode. Upon the first lithiation, Mn3O4 nanoparticles are lithiated into the crystallized Mn nanograins embedded within Li2O matrix. While, Mn and Li2O cannot be recovered to original Mn3O4 phase but MnO after the first full delithiation, which results in an irreversible phase transformation. Such incomplete conversion reaction accounts for the huge capacity fading during the first cycle of Mn3O4-based LIBs. Excellent cyclability between Mn and MnO is also established during the subsequent lithiation-delithiation cycles, which is beneficial to the capacity retention in real battery. It provides an in-depth understanding of the phase evolution and conversion mechanism of Mn3O4 during lithiation-delithiation, holds the promise of improving the capacity for the development of durable, high-capacity, and high-rate anodes for LIBs.


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1. INTRODUCTION Li-ion batteries (LIBs) are currently being developed to power an increasingly diverse range of applications, from cars to microchips.1, 2 So, LIBs with superior energy density, power density, and good cyclability are required.3, 4 Inspired by these, transition metal oxides, such as Co3O4,5 Fe3O4,6 and TiO2,7 have extensively investigated as anode materials for LIBs through a reversible conversion reaction reported by Tarascon et al.8 Manganese oxide represents one of the most promising anodes to replace the conventional carbonaceous anodes for the next-generation LIBs, as it exhibits a low potential, abundance, low cost, and environmental benignity, 9 particularly, its high theoretical Li storage capacity (936 mAh/g for Mn3O410), which is larger than that of the carbonaceous anodes (372 mAh/g for LiC6) currently used in commercial LIBs. However, a large volume expansion of the metal oxide electrode materials occurred during electrochemical processes, resulting in pulverization of the electrode and the subsequent loss of electrical contacts between the active material and current collector, remains a serious issue that critically limits the capacity retention and cyclability of rechargeable LIBs. This has been recognized as one of the major causes of rapid capacity fading in electrode,11-13 and thus hinders the application of metal oxide anodes in LIBs. To solve the above problem, various Mn3O4 anode are prepared to improve their electrochemical performances,14-17 the capacity retention and cyclability of Mn3O4 electrode are major concerns for high-capacity anode, and the conversion mechanism of Mn3O4 remains unclear. The typical techniques to evaluate the electrochemical performances of Mn3O4 anode are the coin cell assembly, cyclic voltammetry, and galvanostatic charge/discharge measurements. However, the ex situ methods cannot satisfy the dynamic nature of the Li-insertion and Li-extraction processes of the real battery, besides they lack the direct experimental evidence and fundamental understanding of the conversion mechanism and phase evolution during charge/discharge cyclings.18 A deep understanding of the structure evolution of electrode during charge/discharge cycling is critically important. In situ TEM study has the advantage of providing direct insights into the microstructure evolution of electrode during electrochemical process, and has the ability to observe the microstructure evolution of the electrode materials for real time. Previous studies have reported that it is feasible to build a nano-LIB inside TEM, allowing for real time and atomic scale observation of Li-insertion and Li-extraction processes, which is the key to unfold the



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operation mechanism of anodes for LIBs, thus providing the critical science for designing LIBs with better electrochemical performances.11, 19-27 Recently, the number of reports on the electrochemical performance of transition metal oxide as an anode material has increased; many reports have clarified the complicated conversion reaction of Mn3O4. Some researchers suggest that the reaction mechanism between Li and Mn3O4 is a reversible conversion reaction to form lithium oxide and metal nanoparticles, as described: 14-16 Mn3O4 + 8Li+ + 8e- ↔ 3Mn + 4Li2O While some reports illustrate the reaction mechanisms of Mn3O4 are as follows: 28-31 In the first discharge: Mn3O4 +Li+ +e- → LiMn3O4 LiMn3O4 +Li++e-→ Li2O + 3MnO MnO + 2Li+ +2e- → Li2O + Mn In subsequent cycling, charge storage was proposed to involve only MnO and metallic Mn: Li2O+Mn ↔ 4MnO + 2Li+ +2eHowever, the detailed conversion mechanism of Mn3O4 was not understood, and preventing the application of Mn3O4 as a battery electrode. In this development, from both practical and fundamental standpoints, revealing the lithiation/delithiation process using in situ technique is critically important. Here, we created a Mn3O4-based all-solid nano-LIB inside TEM, the microstructure evolution of Mn3O4 nanoparticle was monitored by simultaneous determination with HRTEM, ED, and EELS. An in-depth understanding of the electrochemical process of Mn3O4 in LIBs has been achieved. Direct visualization of the lithiation process can provide important insights into how LIBs work and guide the development of advanced LIBs for powering future electrical vehicles and electrical devices.

2. EXPERIMENTAL SECTION 2.1 Preparation of Mn3O4/graphene Graphite oxide was prepared by a modified Hummer’s method.32 To prepare the sample (Mn3O4/graphene),

graphite

oxide

(20

mg),

Mn(CH3COO)2·4H2O

(0.25

g),

and

poly(ethyleneglycol) (28 mg) were dissolved into 30 mL of deionized water. After sonication for 1 h, 20 mL of NaOH aqueous solution (0.2 M) was added slowly under magnetic stirring. Finally, the solution was transferred into an autoclave and reacted at 180 ºC for 12 h. The resultant


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product was separated by centrifugation, washed with distilled water and ethanol for several times, and dried overnight at 60 ºC under vacuum. 2.2 In situ electrochemical experiments The in situ nanoscale electrochemical experiments were conducted using the Nanofactory TEM-STM holder inside TEM (JEOL JEM-2100F), with the assistance of ED pattern, HRTEM imaging and EELS. To prepare an all solid-state nanosized LIB device, the Mn3O4/graphene was attached to Au wire (0.25 mm diameter) as working electrode, metal Li was scratched on a sharp W tip and served as counter electrode. A layer of Li2O, grown on the surface of metal Li generated in the holder transferring process, used as the solid electrolyte for lithium ion transport. All materials preparation and in situ cell assembly were performed inside an Ar-filled glovebox. The holder was quickly transferred into a TEM column with minimal exposure to ambient atmosphere. Because Li2O was very sensitive to the electron beam and therefore imaging conditions were optimized to minimize beam damage. During the experiments the electron beam was blanked except for short time beam exposure for imaging to minimize the electron beam irradiation effect during the reaction. The EELS measurements were performed in the image mode using a Gatan Enfina parellel electron energy loss spectrometer attached to TEM. The oxidation state of manganese was judged by the intensity ratio of the Mn L2,3 white lines (IL3/IL2). IL3/IL2 was calculated by dividing the intensities of L3 and L2 edges (IL3 and IL2), which was integrated by IL3 and IL2 subtracted from background. The oxidation state of manganese in Mn3O4/graphene electrode in coin cell after discharged to 0.05 V and recharged to 3.0 V was also investigated by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific EscaLab 250Xi) using Al Kα radiation. Mn 3s spectra were calibrated with the C 1s photoemission peak for sp2-hybridized carbons centered at 284.5 eV.

3. RESULTS AND DISCUSSION To investigate the electrochemical behavior of Mn3O4 during lithiation-delithiation cyclings, the nano-LIB device inside TEM was constructed, as schematically illustrated in Figure 1a. In brief, the electrochemical nano-LIB consisted of three parts: a Mn3O4/graphene working electrode, a Li metal counter electrode attached to the tungsten wire, and a Li2O layer naturally formed on the surface of Li metal during the holder transferring process used as a solid electrolyte to allow Li+ transport. Graphene sheets play an important role in the in situ experiment: ensuring the better ion



and electron diffusion between Mn3O4 nanoparticls and providing improved time range for in situ observation. The Li2O/Li was moved toward and brought into contact with an individual Mn3O4/graphene driven by a piezo manipulator inside TEM. Figure 1b displays the corresponding TEM image of the nano-LIB inside TEM. The electrochemical reaction process was conducted through applying a potential between the external leads of the nano-LIB. To initiate the lithiation process, a bias of -1 V was applied to drive Li+ transport through the solid-state Li2O layer, while the bias was reversed to positive (+3 V) to facilitate delithiation. A typical HRTEM image of an individual Mn3O4 nanoparticle anchored on graphene is shown in Figure 1c. It can be clearly seen that the HRTEM image indicates a good crystallization of Mn3O4; the fringe spacing is measured to be 0.30 nm. And the corresponding fast Fourier transform (FFT) pattern demonstrated in Figure 1c inset can be well indexed as the (211), (123), ( 112) planes of the tetragonal crystal structure of Mn3O4 (JCPDS no. 24-0734). Meanwhile, the selected area electron diffraction (SAED) pattern of a single Mn3O4 nanoparticle is given in Figure 1d, corresponding to the planes of tetragonal crystal structure of Mn3O4 (JCPDS no. 24-0734) along the [ 1 20] zone axis. The electron diffraction (ED) pattern of the pristine Mn3O4/graphene electrode is shown in Figure 1e. The dominant diffraction patterns can be indexed as the planes of tetragonal crystal structure of Mn3O4, which is also well agreement with the JCPDS no. 24-0734. The morphological evolution and phase transformation of Mn3O4 induced by Li+ insertion are exhibited in Figure 2 captured from Movie S1 in Supporting Information. Figure 2a is a TEM image of two pristine Mn3O4 nanoparticles with a nearly spherical shape anchored on graphene nanosheet before contacting with Li2O solid electrolyte. The initial marked Mn3O4 nanoparticles with the diameter of 96 nm (indicated with red circle); the corresponding volume of this Mn3O4 is approximately 4.63*105 nm3 according to the spherical shape. As the front surface of Li2O layer touched this Mn3O4/graphene, a potential of -1 V was applied to initiate the lithiation process. After 17.5 s (Figure 2b), the Mn3O4 nanoparticle began to expand, and lithiation occurred on all surfaces; a gray-contrasted shell enclosing a dark Mn3O4 core indicated the lithium diffusion on the Mn3O4 nanoparticle surface. As the lithiation proceeded, Li flowed from the surface to the center of the Mn3O4 nanoparticle and numerous ultrafine nanograins were formed (Figure 2c). It suggests that the lithium diffusion (lithiation) within the Mn3O4 nanoparticles anchored on


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graphene is isotropic. At 44.0 s, the microstructural evolution of Mn3O4 nanoparticle was complete, namely the Mn3O4 nanoparticle was fully lithiated (Figure 2d). The Mn3O4 nanoparticle expanded its diameter from 96 nm to 106 nm after full lithiation, corresponding to an estimated volume expansion of ~34.6%. Movie S2 in Supporting Information give the lithiation process of another Mn3O4 nanoparticle without graphene. It shows that graphene sheets providing improved time range for in situ observation and ensuring the better ion and electron diffusion between Mn3O4 nanoparticls. The detailed microstructural and phase evolution of Mn3O4 during the first lithiation were further captured by HRTEM and ED pattern as shown in Figure 3. Figure 3a is a TEM image of Mn3O4/graphene electrode after the first full lithiation process. It can be seen that the fully lithiated electrode is featured by a thin layer with a thickness of ~3 nm formed on the surface of Mn3O4 nanoparticle (Figure 3a and b). The thin layer was identified to be Li2O from the ED pattern of the fully electrode demonstrated in Figure 3d. Figure 3b shows that the fully lithiated product consists of numerous ultrafine nanograins (black nanodots) and Li2O matrix. The corresponding HRTEM image is displayed in Figure 3c, suggesting that the lattice spacings of nanograin and matrix are measured to be 2.1 and 2.6 Å, which are in agreement with the (221) plane of Mn and (111) plane of Li2O. The ED pattern of the fully lithiated electrode is given in Figure 3d. The diffraction rings corresponds to crystalline Li2O (JCPDS, PDF no. 77-2144) and Mn (JCPDS, PDF no. 89-4857), indicating that the Mn3O4 nanoparticle has converted into the mixture of Mn and Li2O after full lithiation. So the electrochemical reaction in the first lithiation process can be expressed as: Mn3O4 + 8Li+ + 8e-→3Mn + 4Li2O. The Mn3O4 nanoparticles anchored on graphene showed excellent cyclability during discharge-charge cycles in our previous paper.33 Figure 4 presents the microstructure evolution and phase transformation of Mn3O4 nanoparticle during the first two lithiation-delithiation cycles. An original Mn3O4 nanoparticle with the diameter of ~127 nm was selected in the viewing area to study the reaction mechanism, as shown in Figure 4a. The corresponding ED pattern is displayed in Figure 4a1, all the diffraction rings are well agreed with the tetragonal crystal structure of Mn3O4 (JCPDS no. 24-0734), revealing all the nanoparticles anchored on graphene were tetragonal Mn3O4 phase. Then, a potential of -1 V was applied to initiate the first lithiation process. After the first full lithiation, the diameter increased to 141 nm, corresponding to an estimated



volume expansion of ~36.8% (Figure 4b). Fig. 4b1 presents the corresponding ED pattern of the completely lithiated Mn3O4/graphene electrode, suggesting the conversion of Mn3O4 into Mn and Li2O after the first full lithiation. Then the potential was reversed to +3 V to facilitate the delithiation. The lithiated Mn3O4 nanoparticle shrunk instantly in a uniform manner and reduced to a size of ~130 nm after complete delithiation (the complete delithiation is the state that the volume and phase no longer change) (Figure 4c), which is much larger than that of the original Mn3O4 nanoparticle, suggesting the lithium ions have not been completely extracted from the lithiated Mn3O4 nanoparticle. The fully delithiated phase was identified as MnO instead of Mn3O4, as confirmed by the corresponding ED pattern demonstrated in Figure 4c1. The irreversible conversion between Mn and Mn3O4 accounts for the capacity fading in the first cycle. After the first cycle, the repeated lithiation-delithiation cyclings were conducted by periodically reversing the applied potential. As shown in Figure 4d, the second lithiation process was conducted with the potential of -1 V again, the Mn3O4 nanoparticle expands its diameter to 142 nm and exhibits an obvious volume expansion. The ED pattern displayed in Figure 4d1 reveals that also Mn and Li2O is the main lithiated product after the second lithiation process. The lithiated Mn3O4 nanoparticle shrunk instantly again in the second delithiation process (Figure 4e). The ED patterns were also used to identify the desodiated products after the second delithiation (Figure 4e1). As expected, diffraction rings of MnO phase can be detected. It is noted that after the first two cycles, the Mn3O4 nanoparticle maintain the original nearly spherical shape, which is beneficial for the capacity retention in real battery. However, in the delithiation process, the Mn and Li2O can’t electrochemically convert to the original phase Mn3O4 but MnO, which results in a capacity fading in the first cycle because the theoretical Li storage capacity of Mn3O4 (936 mAh/g) 10 is larger than that of MnO (755.6 mAh/g) 34. After the first lithiation, the ED patterns reveal the reversible phase transformation between Mn and MnO, suggesting a good reversibility of Mn and MnO after the first lithiation-delithiation cycle. It is apparent that the volume and phase changes are reversible in the second and the subsequent cyclings. The CV curves of Mn3O4/G show that the oxidation and reduction peaks are fixed suggesting the reversible cycling of the electrode. The cycling stability of Mn3O4/G electrode was also studied. After 50 cycles at 200 mA g-1, Mn3O4/G electrode still maintains a charge capacity of 437 mAh g-1, the Mn3O4/G electrode shows an improved cycling stability.33 It agrees well with our in situ TEM results, suggesting a good reversibility after the first


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cycle. The white-line intensity ratio (IL3/IL2) has been considered to correlate energy-loss near-edge structures features with 3d occupancy of Mn to establish its oxidation states.35 The reversible phase transformation of Mn3O4 during the electrochemical reaction was further investigated by EELS results demonstrated in Figure 5. The IL3/IL2 of Mn at initial state was 2.8, corresponding to the mean valence state of 2.67+ in Mn3O4 (Figure 5a). 36 While, the IL3/IL2 of Mn after the first full lithiation process was measured to be 4.3, the change in the relative intensity of the L2,3 edge indicates that Mn has undergone a transition from 2+, 3+ to zero oxide state,36 as shown in Figure 5b. Figure 5c shows the Mn L2,3 edges after the first full delithiation process, and the IL3/IL2 decreased to 3.8 that reveals the valance state of 2+. It suggests that the lithiated product Mn converted to MnO after the first delithiation process.36 In the second lithiation-delithiation cycle, the similar change of IL3/IL2 was also observed. The repeated changes of IL3/IL2 suggest Mn undergoes a reversible transition between Mn0 and Mn2+ after the first lithiation. Combining with the ED patterns given in Figure 4, it indicates the lithiation and delithiation process of Mn3O4 is irreversible in the first cycle. While in the following cyclings, a phase transformation between Mn and MnO is reversible. So the electrochemical process of Mn3O4 can be expressed as follows: Mn3O4 + 8Li+ + 8e- → 3Mn + 4Li2O (the first lithiation); Mn + Li2O ↔ MnO + 2Li+ + 2e- (the first delithiation and the following cycles) The high-resolution TEM image and X-ray photoelectron spectra (XPS) of the Mn3O4/graphene electrode after discharged to 0.05 V and recharged to 3.0 V in coin cell after 50 cycles were ex-situ recorded. The results are shown in Fig. S1 in Supporting Information. The exsitu TEM images and XPS results also suggest the discharged product is Mn, and then recharged to MnO not Mn3O4, which agree well with the in situ TEM results. According to the in situ TEM study, the electrochemical reaction processes of Mn3O4 towards Li are illustrated in Figure 6. Figure 6a displays the first lithiation process of Mn3O4 nanoparticle anchored on graphene. Firstly, lithium ions diffusion around the surface of Mn3O4 nanoparticle under the drive of the negative potential due to the good conductivity of graphene; then lithium ions diffuse inward and electrochemically react with Mn3O4. So a visible core-shell structure is formed, including the unlithiated Mn3O4 core and the Mn/Li2O nanograins shell. The lithiation-induced expansion in Mn3O4 nanoparticle anchored on graphene was nearly isotropic. Eventually, after the first full



lithiation the whole Mn3O4 converts into Mn and Li2O (numerous Mn metal nanograins embedded in Li2O martix), but the lithiated nanoparticle maintains the original shape, which is beneficial for the cycliability of the electrode in real battery. After the first delithiation process, MnO nanograins are formed instead of the original Mn3O4. Although the irreversible reaction occurs during the first lithiation-delithiation process, the subsequent electrochemical cycles are stable and reversible between Mn/Li2O and MnO phases, as illustrated in Figure 6.

4. CONCLUSIONS In conclusion, the conversion-type electrode material of Mn3O4 has been studied by constructing a Mn3O4-based nano-LIB inside TEM to probe its microstructure and phase evolutions in real time during electrochemical lithiation-delithiation cycles. The real-time TEM images, ED patterns and EELS results confirm the large irreversibility in the first cycle, including the irreversible volume expansion and phase change. During the first lithiation, Mn3O4 nanoparticles are lithiated into the crystallized Mn nanograins embedded within Li2O matrix. Then the lithiated products Mn and Li2O cannot be recovered to original Mn3O4 phase, but MnO is identified as delithiated product. The electrochemical conversion from Mn to MnO rather than the original Mn3O4 phase accounts for the severe capacity loss during the first cycle. Impressively, a stable and reversible phase transformation was established between Mn and MnO during the subsequent lithiation-delithiation cyclings, suggesting a good cyclibility and capacity retention of Mn3O4 electrode in real battery. These findings, for the first time, provide direct evidences for deeply understanding the detailed lithiation-delithiation conversion mechanism of Mn3O4 electrode material, and is beneficial for the ultimate optimization of the practical LIBs’ anodes.

ASSOCIATED CONTENT Supporting Information Two movies show the lithiation process of individual Mn3O4 nanoparticle are provided. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant nos. 11504330 and 11574273), the Natural Science Foundation of Zhejiang Province, China (Grant nos. LQ15B01001 and LY16B030003). Thank Jian Xie’s group for preparing the samples.

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Figures

Fig. 1. (a) Schematic illustration of the in situ experimental setup inside TEM. (b) The corresponding TEM image of the nano-LIB inside TEM. (c) The HRTEM image of Mn3O4 nanoparticle anchored on the graphene, and the inset is the corresponding FFT pattern. (d) SAED pattern of a Mn3O4 nanoparticle anchored on graphene. (e) ED pattern of Mn3O4/graphene electrode.



Fig. 2. Time-resolved TEM images from video showing the direct electrochemical lithiation process of Mn3O4 nanoparticles anchored on graphene. (a) The pristine Mn3O4 particle with a diameter of about 96 nm. (b - d) Time sequence of the microstructure evolution of Mn3O4 in the first lithiation process. The scale bars in (b - d) are 20 nm.

Fig. 3. (a) TEM image, (b) a magnified TEM image, (c) HRTEM image of the fully lithiated Mn3O4. (d) ED pattern of the fully lithiated Mn3O4/graphene indicating the phase transformation.


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Fig. 4. Microstructure and phase evolutions of Mn3O4/graphene electrode during the first two cycles. (a) The selected Mn3O4 anchored on graphene with a size of 127 nm. TEM image and ED pattern of Mn3O4/graphene electrode after the first full lithiation (b and b1), and the first full delithiation (c and c1). (d and e) TEM images of Mn3O4/graphene electrode after the second cycle.



(d1 and e1) ED patterns of the electrode in (d and e).

Fig. 5. EELS spectra of Mn L2,3 edges during the first two lithiation-delithiation cycles. (a) Initial stage; after the first lithiation (b) and delithiation (c); after the second lithiation (d) and delithiation (e).


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Fig. 6. Schematic drawing of the conversion mechanism of Mn3O4 material during the delithiation-lithiation cycles (graphene is not shown here).



TOC graphic


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Microstructure Evolution and Conversion Mechanism of Mn3

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