Inheritance of Crystallographic Orientation during Lithiation

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Inheritance of Crystallographic Orientation during Lithiation/Delithiation Processes of Single-Crystal #-Fe2O3 Nanocubes in Lithium-Ion Batteries Xiaowei Ma, Manyu Zhang, Chongyun Liang, Yuesheng Li, Jingjing Wu, and Renchao Che ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07547 • Publication Date (Web): 13 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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ACS Applied Materials & Interfaces

Inheritance of Crystallographic Orientation during Lithiation/Delithiation Processes of Single-Crystal α-Fe2O3 Nanocubes in Lithium-Ion Batteries Xiaowei Ma,a,§ Manyu Zhang, b,§ Chongyun Liang,c Yuesheng Li, b Jingjing Wu,a Renchao Chea * a

Laboratory of Advanced Materials, Department of Materials Science, Collaborative Innovation

Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, People’s Republic

of

China,

Tel.:

(+86)-21-51630213,

Fax:

(+86)-21-51630210,

E-mail:

[email protected] b

Department of Materials Science, Fudan University, Shanghai 200438, People’s Republic of

China c

Department of Chemistry, Fudan University, Shanghai 200438, People’s Republic of China

§ X. Ma and M. Zhang contributed equally to this work.

Abstract: Iron oxides are very promising anode materials based on conversion reactions for lithium-ions batteries (LIBs). During conversion processes, the crystal structure and composition of electrode material are drastically changed. Surprisingly, in our study, inheritance of crystallographic orientation was found during lithiation/delithiation processes of single-crystal α-

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Fe2O3 nanocubes by ex situ transmission electron microscopy (TEM). Single-crystal α-Fe2O3 was first transformed into numerous Fe nanograins embedded in Li2O matrix and then the conversion between Fe and FeO nanograins became the main reversible electrochemical reaction for energy storage. Interestingly, these Fe/FeO nanograins had almost the same crystallographic orientation indicating that the lithiated/delithiated products can inherit the crystallographic orientation of single-crystal α-Fe2O3. This finding is important for understanding the detailed electrochemical conversion processes of iron oxides and this feature may also exist during lithiation/delithiation processes of other transition metal oxides.

Keywords: alfa-iron oxide, inheritance of crystallographic orientation, conversion mechanism, high angle annular dark field imaging, lithium-ion battery INTRODUCTION

A series of transition metal oxides (MO) have attracted increased attention as potential anode materials for lithium-ion batteries (LIBs), such as CoO,1, 2 Co3O4,3, 4 Fe3O4,5, 6 Fe2O3,7, 8 NiO,9, 10 MnO11, 12 and Mn3O4.13, 14 A great number of MO with various morphologies have been reported and used as anode materials for LIBs exhibiting high performance.15,

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The energy storage

mechanisms of these MO are mainly based on conversion reactions, which are quite different from intercalation/deintercalation mechanism and alloying/dealloying mechanism.15 It involves the reduction of metal oxides and oxidation of metal nanoparticles, along with the formation and decomposition of Li2O. Usually, the first lithiation process of transition metal oxides can cause drastic destruction of crystal structure due to the formation of metal nanoparticles embedded in the Li2O matrix.17, 18 During delithiation process, the reformation of MO is accompanied by the decomposition of Li2O. Significantly, the structure of the reformed MO is drastically changed compared with the initial MO. In some cases, the oxidation state of metal elements even can’t


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return to the initial oxidation state, such as the delithiated product is FeO rather than Fe2O3 when Fe2O3 is used as electrode material in LIBs.17 However, the detailed conversion processes are still unclear. Recently, in situ transmission electron microscopy (TEM) has been developed to provide direct insights into the dynamic conversion of the lithiation/delithiation processes of electrode materials with the ability to realize the real-time observation of the microstructure evolution.17-19 Various electrode materials have been investigated by in situ TEM, such as Fe2O317, Co9S818, SnO2,19 and Si20, 21. These in situ TEM studies have provided useful information on structural evolution and valence change of electrode materials, both of which are important for understanding the electrochemical behaviour and mechanism of electrode material in LIBs. However, in the case of in situ TEM, the situation, in which the electrode material is lithiated (or delithiated), is not identical with the situation in batteries. For example, the environment surrounding the electrode material and the potential and current applied on the electrode material are not exactly the same as those in batteries. Therefore, ex situ TEM is still necessary for study on the lithiation/delithiation behaviours of electrode materials. In this article, we study the lithiated/delithiated products of single-crystal α-Fe2O3 nanocubes by ex situ TEM including selected area electron diffraction (SAED), high-angle annular dark field (HAADF), scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). It was found that the lithiated/delithiated products of single-crystal αFe2O3 exhibited obvious preferred crystallographic orientation and this feature could persist for a remarkable period (even 100 cycles). Although the lithiation/delithiation processes drastically destroyed the crystal structure and changed the composition of electrode material, the feature of


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preferred crystallographic orientation indicates that the lithiated/delithiated products can inherit the crystallographic orientation of single-crystal α-Fe2O3.

Scheme 1. Schematic illustration of the lithiation and delithiation processes of single-crystal αFe2O3 in LIBs. RESULTS AND DISCUSSION


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Figure 1. XRD spectra of α-Fe2O3 synthesized in DEG&H2O (red line). The standard data of αFe2O3 (JCPDS No. 33-0664) is shown for comparison. α-Fe2O3 nanocubes were synthesized in mixed solvents (diethylene glycol (DEG) and H2O) by a hydrothermal reaction at 200 °C for 10 h. All the well-defined diffraction peaks can be indexed to α-Fe2O3 (JCPDS No. 33-0664). Interestingly, if no H2O had been added, Fe3O4 nanoparticles would be obtained because of the weak reducibility of DEG (Figure S1). It is supposed that H2O can significantly influence the kinetics of the hydrolysis of FeCl3 and accelerate the formation of α-Fe2O3 as well as completely avoid the formation of Fe3O4.

Figure 2. (a) TEM image, (b) SEM image and (c) HRTEM image of single-crystal α-Fe2O3 nanocubes; (d) SAED pattern of a single α-Fe2O3 nanocube. As-prepared α-Fe2O3 nanocubes were characterized by TEM and scanning electron microscope (SEM). Figure 2a and 2b shows the typical TEM and SEM images. The average length of these α-Fe2O3 nanocubes was ~50 nm. These α-Fe2O3 nanocubes were further characterized by highresolution TEM (HRTEM) and selected area electron diffraction (SAED). Figure 2c shows two sets of clear lattice fringes with the same interplanar distance of 0.37 nm at an angle of 86º (close


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to the lattice spacing of (012) planes at 0.3684 nm in rhombohedrally centered hexagonal αFe2O3). There are a set of clear diffraction spots in the SAED pattern of a single α-Fe2O3 nanocube (Figure 2d). It can be concluded that as-prepared α-Fe2O3 nanocubes were singlecrystal. This facile method was also successfully used to prepare α-Fe2O3/GO/CNTs composites (Figure S2). GO and CNTs can greatly enhance the conductivities of electrode materials and guarantee their high electrochemical performance.22-24

Figure 3. Electrochemical performance of α-Fe2O3/GO/CNTs composite. (a) Rate performance at the current density from 0.1 A g-1 to 5 A g-1. (b) Cycling performance at 0.2 A g-1. (c) Galvanostatic charge/discharge curves at 0.2 A g-1. (d) Cyclic voltammetry curves at a scan rate of 0.2 mV s-1.


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As-synthesized α-Fe2O3/GO/CNTs composite exhibited high performance as an electrode material for LIBs (Figure 3). Its initial discharge capacity was ~1187 mA h g-1 at a current density of 0.1 A g-1 (approximately 0.1 C). The average discharge capacities were 906, 875, 814, 770, 677 and 467 mA h g-1 at 0.1, 0.2, 0.5, 1, 2 and 5 A g-1, respectively. There was no obvious capacity fading after 100 cycles at 0.2 A g-1 and corresponding coulombic efficiency was approaching 100%. Galvanostatic charge/discharge curves and cyclic voltammetry (CV) curves are shown in Figure 3c and 3d. The charge or discharge curves almost coincided except the first two cycles, because drastic changes on the structure and composition of electrode material occurred during the first few cycles. As previously reported,17 the electrochemical reaction during the first discharge process was 6 6 → 2 3 . However, after the first discharge process the reversible electrochemical reaction was ↔ 2 2 . It is supposed that there were two main reasons for the differences of the charge/discharge curves: (a) the change of electrochemical reactions; (b) changes on the structure of electrode material (more details will be discussed in subsequent parts).


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Figure 4. TEM image (a) and HAADF image (b) of delithiated product after charged to 3.0 V at 0.2 A g-1. TEM image (c) and HAADF image (d) of lithiated product after discharged to 0.05 V at 0.2 A g-1. The lithiated/delithiated products were further characterized by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and SAED. The contrast of HAADF images is strongly dependent on the average atomic number of the encountered-element probed by the incident electron beam.25, 26 Fe/FeO nanograins can be clearly seen (bright spots) in the HAADF images of lithiated/delithiated nanocubes (Figure 4b and 4d). Single-crystal αFe2O3 nanocubes were transformed into polycrystalline nanocubes after the first lithiation process. These lithiated polycrystalline nanocubes were consisted of numerous Fe nanograins and Li2O. The average size of these Fe nanograins was only few nanometres. After delithiation process, Fe nanograins converted to FeO nanograins rather than Fe2O3 (confirmed by the EELS and SAED data). Both Fe and FeO nanograins were embedded in Li2O matrix. Besides volume changes, the morphologies of these lithiated/delithiated products were still cubic without fracture or cracking. Maybe it was because Li2O served as both a binder and a buffer which could keep the lithiated/delithiated nanoparticles from fracture and cracking. The lithiation/delithiation processes caused drastic and irreversible changes to the crystal structure and composition of single-crystal α-Fe2O3 nanocubes. After single-crystal α-Fe2O3 nanocubes were tested as an electrode material for LIBs, some interesting phenomena were found: inheritance of crystallographic orientation existed during lithiation/delithiation processes. A set of clear diffraction spots in the SAED pattern of a single delithiated nanocube are shown in Figure 5a indicating a high degree of preferred crystallographic orientation. These diffraction spots can be indexed to (220) and (200) planes of


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face-centered cubic FeO (JCPDS no. 89-2468). As shown in Figure 5b, clear lattice fringes of the delithiated nanocube were not continuous in long range because the average size of formed FeO nanograins was only few nanometres and they were surrounded by Li2O. However, these lattice fringes were almost parallel to each other. All these results confirmed the presence of the highdegree preferred crystallographic orientation of delithiated products. The similar results were observed in lithiated products.

Figure 5. SAED pattern (a) and HRTEM image (b) of delithiated product after charged to 3.0 V at 0.2 A g-1. SAED pattern (c) and HRTEM image (d) of lithiated product after discharged to 0.05 V at 0.2 A g-1.


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Although the detailed conversion process between FeO and Fe is unclear, it can be concluded that the lithiated/delithiated products can inherit the crystallographic orientation of single-crystal α-Fe2O3 and exhibit obvious preferred crystallographic orientation. As shown in Figure S3, the feature of preferred crystallographic orientation could persist for a remarkable period (35 cycles) and became weak after dozens of lithiation/delithiation cycles (100 cycles). Samples in different oxidated states were further characterized by ex situ electron energy loss spectroscopy (EELS). EELS has been demonstrated to be a useful method to investigate the valance states of transition metal elements because the structures of their L2, 3 edges are sensitive to their valence states.27-29 The L2, 3 absorption edges arise from transition of an electron from 2p state to unoccupied 3d level (L2, from 2p1/2 to 3d3/2; L3, from 2p3/2 to 3d3/23d5/2, respectively). Both the absolute energy positions and the white-line intensity ratio (L3/L2) vary with the valence state and coordinated environment of transition metal.30-32 Numerous EELS experiments have shown that a change in valence state can introduce a drastic change in L3/L2 intensity ratio leading to the possibility of identifying the oxidation state of transition metal element.28


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Figure 6. EELS spectra of Fe-L2, 3 edges of electrode material in different states: initial state (a), delithiated state (b) and lithiated state (c). (d) EELS spectra of O-K edges of electrode material in different states. The current density was 0.2 A g-1 while the potential for lithiation and delithiation was respectively 0.05 V and 3.0 V. The EELS spectrum of Fe in single-crystal α-Fe2O3 nanocubes (initial state) is shown in Figure 6a, and the L3/L2 intensity ratio is 5.0 corresponding to the oxidation state of 3+.29 During the first lithiation process, single-crystal Fe2O3 was transformed into Fe nanograins and Li2O. Then Fe nanograins converted to FeO nanograins and the reversible electrochemical reaction could be expressed as ↔ 2 2 during the subsequent cycles.17 After the lithiation process, the L3/L2 intensity ratio decreased to 3.5 because of the reduction from FeO to Fe (Figure 6c). Compared with the reported value of L3/L2 intensity ratio in lithiated state, the


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average oxidation state of Fe was higher than 0 because of the incomplete reduction of FeO. 17 It was because the potential for lithiation was 0.05 V in our experiments (-1 V in the reported article) and there must be a certain amount of residual FeO which may be the crystal nuclei for the growth of FeO nanograins during delithiation process. The residual FeO may be important for the inheritance of crystallographic orientation. Figure 6b shows the EELS spectrum of Fe in delithiated state, and the L3/L2 intensity ratio increased to 4.3 corresponding to the oxidation state of 2+. These results confirmed that the delithiated product was FeO rather than Fe2O3 and there was residual FeO in lithiated state. Figure 6d shows the O K edges of samples in different states, which are composed of a prepeak and a broad edge. A strong prepeak can be seen in the EELS spectrum of Fe2O3 (initial state, black line). This prepeak usually decreases with the reduction in iron, which is an evidence of a weaker average hybridization for lower Fe valence state.33 In delithiated state, the prepeak was weaker than that of Fe2O3 (initial state) because the delithiated product is FeO rather than Fe2O3. The weak prepeak in O K edges of lithiated product possibly caused by the incomplete reduction of FeO. Based on our experimental results, a reasonable explanation for the inheritance of crystallographic orientation is proposed and illustrated in Scheme 1: (1) during lithiation process, Fe atoms were formed from the reduction of Fe2+ ions and departed from FeO nanograins leaving vacancies and negative charges while Li+ ions were inserted to fill the vacancies and neutralize negative charges; the sizes of FeO nanograins were decreased as lithiation process was carried out; however, some inner cores of FeO nanograins may be left because of incomplete reduction; (2) during delithiation process, Li+ ions were extracted and O2- ions were left while Fe atoms were oxidized to Fe2+ ions; these Fe2+ ions migrated to the residual FeO nanograins (might


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be considered as crystal nuclei) and formed FeO with O2- ions remained onside their surfaces. Consequently, the formed FeO nanograins inherited the crystallographic orientation of residual FeO nanograins and exhibited preferred crystallographic orientation. CONCLUSION

Single-crystal α-Fe2O3 nanocubes and their composite with GO and CNTs were successfully prepared by a one-step hydrothermal reaction. α-Fe2O3/GO/CNTs composite was used as an electrode material for LIBs and exhibited high performance. The lithiated and delithiated products of single-crystal α-Fe2O3 nanocubes were further characterized by ex situ TEM analysis. Inheritance of crystallographic orientation was found during lithiation and delithiation processes of single-crystal α-Fe2O3 nanocubes. Although the single-crystal α-Fe2O3 was transformed into numerous FeO (Fe) nanograins embedded in Li2O matrix, these FeO (Fe) nanograins had almost the same crystallographic orientation. This feature indicated that the crystallographic orientation of single-crystal α-Fe2O3 can be inherited during lithiation/delithiation processes. There was a certain amount of residual FeO after lithiation process, which was supposed to be important for the inheritance of crystallographic orientation. This finding will be valuable for understanding the detailed conversion processes of transition metal oxides in LIBs. EXPERIMENTAL SECTION

Preparation of α-Fe2O3/GO/CNTs Composite. Graphene oxide (30 mg), MWCNTs (10 mg) and FeCl3•6H2O (675 mg, 2.5 mmol) were added to 40 mL DEG. After sonication for few hours, 10 mL NaAc/DEG (0.2 g mL-1) solution was added into the dispersion. The mixture was stirred at room temperature for 1 h. Then the mixture was sealed into a 50 mL Teflon-lined stainlesssteel autoclave for hydrothermal reaction at 200°C for 10 h. The final product was collected by centrifugation, washed with distilled water and ethanol, and dried at 80°C overnight.


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Single-crystal α-Fe2O3 nanocubes were synthesized under the same conditions with no GO or MWCNTs added. Characterization. Powder X-ray diffraction (XRD) measurements were carried out using a Bruker D8 X-ray diffractometer with Ni-filtered Cu-K a radiation (40 kV, 40 mA). Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-angle annular dark field (HAADF) were performed on a JEOL JEM-2100F transmission electron microscope. Field-emission scanning electron microscopy (SEM) images were acquired on a S-4800 fieldemission scanning electron microscope operated at 1.0 kV. Thermogravimetric analysis (TGA) data were recorded at a heating rate of 10°C min-1 in air by a simultaneous thermogravimetry/differential thermal analyzers (DTG-60H). Electrochemical Characterion. Active material powder, acetylene black and PTFE, with a weight ratio of 80:10:10, were mixed into N-methyl-2-pyrrolidinone (NMP). Then the obtained slurry was cast onto a copper foil and dried at 80°C under vacuum overnight. Coin-type test cells were assembled by using lithium metal as the counter/reference electrode, 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as the electrolyte, and Celgard 2400 membrane as the separator. The cells were tested on a LAND multichannel battery system with voltage window of 0.05-3.0 V. Cyclic voltammetry was conducted on a CHI 660D electrochemical workstation. The lithiated/delithiated products for TEM were prepared in argonfilled glovebox: After electrochemical tests, cells were disassembled in an argon-filled glove box and the obtained electrode materials were washed with tetrahydrofuran (THF). Then the electrode materials were dispersed in THF by ultrasonication and dropped onto copper grids. It should be noted that the specific capacity was calculated on the basis of the total mass of the αFe2O3/GO/CNTs composite.


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Acknowledgement. This work was supported by the Ministry of Science and Technology of China (973 Project No. 2013CB932901), and the National Natural Foundation of China (Nos. 11274066, 51172047, 51102050, U1330118). This project was sponsored by Shanghai Pujiang Program and “Shu Guang” project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation (09SG01). Supporting Information. XRD, TGA, SEM and TEM data of α-Fe2O3/GO/CNTs composite, SAED pattern of a single α-Fe2O3 nanocube grown on GO, SAED patterns of samples after different lithiation/delithiation cycles. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES

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Observation of Microstructure and Phase Evolution in a SnO2 Nanowire during Lithium Intercalation. Nano Lett. 2011, 11, 1874-1880. 20. Wang, C.-M.; Li, X.; Wang, Z.; Xu, W.; Liu, J.; Gao, F.; Kovarik, L.; Zhang, J.-G.; Howe, J.; Burton, D. J.; Liu, Z.; Xiao, X.; Thevuthasan, S.; Baer, D. R. In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries. Nano Lett. 2012, 12, 1624-1632. 21. Gu, M.; Li, Y.; Li, X.; Hu, S.; Zhang, X.; Xu, W.; Thevuthasan, S.; Baer, D. R.; Zhang, J.-G.; Liu, J.; Wang, C. In Situ TEM Study of Lithiation Behavior of Silicon Nanoparticles Attached to and Embedded in a Carbon Matrix. ACS Nano 2012, 6, 8439-8447. 22. Zhang, K; Han, P; Gu, L; Zhang, L; Liu, Z; Kong, Q; Zhang, C; Dong, S; Zhang, Z; Yao, J; Xu, H; Cui, G; Chen, L. Synthesis of Nitrogen-Doped MnO/Graphene Nanosheets Hybrid Material for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4 (2), 658–664. 23. Liu, D; Liu, H; Wu, X; Hou B, Wan,F; Bao, S; Yan, Q; Xie, H; Wang, R. Constructing the optimal conductive network in MnO-based nanohybrids as high-rate and long-life anode materials for lithium-ion batteries. J. Mater. Chem. A 2015, DOI: 10.1039/C5TA03556B 24. Zhu, X ; Zhu, Y; Murali, S; Stoller, M; Ruoff, R . Nanostructured Reduced Graphene Oxide/Fe2O3 Composite As a High-Performance Anode Material for Lithium Ion Batteries ACS Nano 2011, 5 (4), 3333–3338 25. Besenbacher, F.; Brorson, M.; Clausen, B. S.; Helveg, S.; Hinnemann, B.; Kibsgaard, J.; Lauritsen, J. V.; Moses, P. G.; Nørskov, J. K.; Topsøe, H. Recent STM, DFT and HAADF-


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33. Chen, S.-Y.; Gloter, A.; Zobelli, A.; Wang, L.; Chen, C.-H.; Colliex, C. Electron Energy Loss Spectroscopy and ab Initio Investigation of Iron Oxide Nanomaterials Grown by a Hydrothermal Process. Phys. Rev. B 2009, 79, 104103.


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Inheritance of Crystallographic Orientation during Lithiation

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