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In-situ TEM on the coalescence of birnessite manganese dioxides nanosheets during lithiation process
Materials Research Bulletin 79 (2016) 36–40
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
In-situ TEM on the coalescence of birnessite manganese dioxides nanosheets during lithiation process Ke Caoa , Min Kuangc , Yuxin Zhangc, Jiabin Liua,* , Hongtao Wangb,* , Liang Menga a b c
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Institute of Applied Mechanics, Zhejiang University, Hangzhou 310027, China College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 November 2015 Received in revised form 23 February 2016 Accepted 28 February 2016 Available online 2 March 2016
Nanostructure is believed to produce great benefits for anode materials in lithium ion batteries (LIBs) by enhancing lithium ion transfer, accommodating large volume change and increasing surface area. Whether the nanostructure (especially the porous nanostructure) could be well held during charging/ discharging process is one of the most commonly concerned issues in LIBs research. The dynamic evolution of birnessite manganese dioxides nanosheets during lithiation process is investigated by in-situ transmission electron microscopy (TEM) for the first time. The TiO2@MnO2 core-shell nanowires are used as the anode and Li metal as the counter electrode inside the TEM. Interestingly, the lithiation process is confirmed as MnO2 and Li converting to Li2O and Mn. The original porous structure of the nanosheets is hard to preserve during lithiation process due to lithiation-induced contact flattening. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Electron diffraction C. Transmission electron microscopy (TEM) D. Crystal structure
1. Introduction Manganese dioxide has received substantial attention as anode materials in lithium ion batteries (LIBs) due to its high theoretical specific capacities, environmental benignity and natural abundance [1,2]. However, there are still two major obstacles for its practical application. The first and most important one is the large volumetric expansion during the charging/discharging processes, resulting in severe particle agglomeration and unsatisfactory cycling performance; the other one is the virtually ubiquitous large coulombic inefficiency observed in the first cycle. Nanostructure is believed to produce great benefits for the anode materials. The nanostructured materials could greatly increase the surface area and also accommodate large change in volume. This strategy is especially suitable for nanostructured MnO2, such as nanorods/ nanowires, nanotubes and nanosheets with or without porous structures [3–5]. The vacant space provided by the porous nanostructures can accommodate the structural strain and facilitate fast Li+ insertion/extraction kinetics, leading to improved rate and cycling performance. Pure nanostructured MnO2 still endures the disadvantages of low conductivity and particle agglomeration. Therefore, carbon was introduced into MnO2 nanomaterials to form homogenous
* Corresponding authors. E-mail addresses: [email protected] (J. Liu), [email protected] (H. Wang). http://dx.doi.org/10.1016/j.materresbull.2016.02.045 0025-5408/ ã 2016 Elsevier Ltd. All rights reserved.
C@MnO2 composites. An excellent example is the coaxial C@MnO2 nanotube array electrodes with good cyclic stability and capacity [6]. Various coaxial X@MnO2 nanorods/nanowires were also explored to enhance the conductivity of MnO2 and stabilize the nanostructure [1–3,7,8]. In our previous studies, TiO2@MnO2 core-shell nanowires were successfully prepared for high performance supercapacitors [9]. The TiO2@MnO2 core-shell nanowires showed a high capacitance of 120 F g1 (0.1 A g1) and excellent cycling stability (93% capacitance retention after 3000 cycles). The structure of the MnO2 nanosheets should keep unchanged during the charge/ discharge processes as suggested by the excellent cycling stability. However, considering the notable difference between electrons in supercapacitors and Li+ ions in LIBs, it is still an open question that whether the MnO2 nanosheets can retain the morphology during Li+ insertion/extraction processes, which is one of the most commonly concerned issues in LIBs research as well. In-situ transmission electron microscopy (TEM) is a powerful technique to reveal the dynamic microstructure evolution of electrode materials. Thanks to J.Y. Huang’s group’s pioneer works of the in-situ lithiation and delithiation in TEM, plenty of fresh results about the dislocation associated lithiation, layer-by-layer lithiation and even cracking were obtained continuously [10–12]. In this study, we adopted in-situ TEM to reveal the real time change of the MnO2 porous nanostructure during Li+ insertion processes. It is found that the origin MnO2 porous nanostructure could not be retained during Li+ insertion processes, which may be
K. Cao et al. / Materials Research Bulletin 79 (2016) 36–40
the main reason of the large coulombic inefficiency observed in the first cycle. 2. Experimental section 2.1. Synthesis of TiO2@MnO2 core-shell nanowires Commercial anatase TiO2 powders (0.4 g) were put into aqueous solution of 10 M NaOH (35 mL) in a Teflon-lined stainless steel autoclave (50 mL). The autoclave was kept at 180 C for 48 h. After hydrothermal treatment, the resulting precipitates were acidwashed (0.1 M HCl solution) to realize the full-ion exchange from Na+ to H+. The final white products were then dried in vacuum at 80 C for 8 h, and then calcined at 400 C for 3 h in air to obtain the anatase TiO2 nanowires. Then 25 mg TiO2 were dispersed in KMnO4 solution (40 mL, 0.01 M) by ultrasonication for 10 min. The mixed solution was then transferred into the Teflon-lined stainless steel autoclave. The autoclave was directly put in an electric oven at 140 C for 24 h and then cooled to room temperature. The black resultants were collected by centrifugation and dried at 60 C for 12 h in a vacuum oven.
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2.2. In-situ TEM setup A nanosized half-cell LIB was constructed inside TEM for in-situ investigation and is schematically shown in Fig. 1a. The in-situ TEM experiments were carried out in a JEM-2100 operating at 200 kV with the help of a Nanofactory TEM-STM holder. The TiO2@MnO2 core-shell nanowires were attached onto the tip of a gold wire, serving as the anode. Some Li metal was sticked to a tip of tungsten wire, serving as the cathode. The unavoidable surface oxidation on the Li metal served as the solid electrolyte. A negative voltage (2 V) was applied on the TiO2@MnO2 core-shell nanowires with respect to the Li counter-electrode to initiate the lithiation process. The lithiation process started instantaneously, once the contact was established between TiO2@MnO2 core-shell nanowires and the Li electrode. 3. Result and discussion Fig. 1b–f shows the microstructure of the TiO2@MnO2 core-shell nanowires. The TiO2@MnO2 nanowires are composed of quadrangular prism TiO2 nanowires and MnO2 nanosheets. The MnO2
Fig. 1. TEM images of (a) schematical illustation of the in-situ experiment, (b) a single nanowire. Inset is a typical SAED pattern of the TiO2 single crystal and MnO2 nanosheets; (c) element mapping of a nanowire, (d) the cross-sectional TEM image, (e) high resolution TEM image of the interface of MnO2 nanosheets and TiO2, (f) schematical illustration of the nanostructure of the TiO2@MnO2 core-shell nanowires.
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nanosheets have a thickness of 5–20 nm and a width of 50–100 nm. Typical selected area electron diffraction (SAED) pattern (inset in Fig. 1b) clearly indicates that TiO2 nanowire is of single crystalline tetragonal phase and MnO2 nanosheets have the monoclinic potassium birnessite phase. There is not orientation relationship between those MnO2 nanosheets and the TiO2 core. The chemical composition of a single TiO2@MnO2 nanowire is given in Fig. 1c, which clearly confirms the presence of TiO2 and MnO2. The MnO2 nanosheets tightly surround the TiO2 nanowire and are parallel with each other to form a porous nanostructure with abundant open space (Fig. 1d and e), schematically illustrated in Fig. 1f. High resolution TEM image also reveals that there is no special
orientation relationship between the MnO2 nanosheets and TiO2 core. The realtime microstructure evolution of the TiO2@MnO2 coreshell nanowires during lithiation process is recorded by video and key snapshots are extracted from the video (Fig. 2). During the lithiation process, the MnO2 nanosheets began to expand and eventually merged with each other. The thickness of the MnO2 shell increased from 47 nm to about 63 nm. The volume expansion of the MnO2 nanosheet layer is estimated to be about 38%, which is higher than previous report (26.5%) [13,14]. This value may be over estimated since the layer is porous. It is not so easy to measure the volume expansion for single sheet due to the lack of exact
Fig. 2. (a)–(c) Sequential snapshots of the lithiation process of a TiO2@MnO2 core-shell nanowire; Comparison of the high magnification TEM images of the TiO2@MnO2 coreshell structure (d) before lithiation and (e) after lithiation; The corresponding SAED patterns of the TiO2@MnO2 core-shell structure (f) before lithiation and (g) after lithiation.
K. Cao et al. / Materials Research Bulletin 79 (2016) 36–40
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Fig. 3. High resolution TEM images of the TiO2@MnO2 nanowires: (a) before lithiation and (b) after lithiation, (c) illustration of the nanosheet structure changes during the lithiation process of TiO2@MnO2 nanowires.
expansion result at different directions for the sheet in 3D space. The volume expansion of TiO2 core is neglectable, which agrees well with former studies of the high stability of TiO2 during lithium insertion/extraction processes [15–18]. High magnification TEM images of the TiO2@MnO2 core-shell structure before and after lithiation show significant coalescence of the nanosheet structure, forming a homogeneous layer of lithiated product (Fig. 2d and e). The phase change can be identified by the SAED patterns of the TiO2@MnO2 core-shell nanowires before and after lithiation in Fig. 2f and g, respectively. Fig. 2f shows two diffraction rings of (3 3 0) and (5 2 1), indicating the crystalline MnO2 phase. After lithiation, no diffraction rings from MnO2 can be observed (Fig. 2g). Instead, the diffraction rings can be indexed to the (3 0 0) crystal plane of Mn and the (111) and (2 2 0) crystal planes of Li2O. Comparison the change of SAED patterns before and after lithiation sugguests that MnO2 has reacted wh Li to form Mn and Li2O [3]: MnO2 + Li ! Mn + Li2O
(1)
Previous in-situ studies have also confirmed the similar convernsion mechanism of some transition metal oxides, fluorides, sulfides, such as RuO2 [19], FeF3 [20], CoS2 [21]. Lots of 0D nanomaterials (Si particles [22,23], Sn nanoparticles [16,24]), 1D nanomaterials (Si nanowires [10,25], carbon coated Sn nanowires [26], Al nanowires [27]) have been investigated before. Many interesting morphology changes, such as the volume changes, phase boundaries, cracks, mislocations, etc., have been observed for these individual nanostructure materials. Different from the individual nanomaterials as mentioned above, the attention is focused on the collective lithitation behavior of MnO2 nanosheets in this work. When each MnO2 nanosheet was lithiated and expanded in volume, the original porous structure changed once the nanosheet touched each other. Deeper structural information can be obtained by comparing the high resolution TEM observation before and after lithiation of the same nanowire. Fig. 3a clearly shows the (2 0 0), (3 0 1) and (3 2 1) crystal planes of MnO2 before lithiation. After lithiation, some dark clusters with a size of 2–5 nm appear, recognized as the fresh-formed Mn metal (Fig. 3b). The lattice fringe is from the
(111) crystal planes of Li2O, consistent with the SEAD patterns in Fig. 2g. Though the as-synthesized nanowires are composed of separately MnO2 nanosheets, they begin to expand and coalescence with each other once in contact with Li ions. The porous structure of the specially designed shell cannot retain its original morphology and are gradually transformed into a homogeneous solid layer of Li2O matrix with embedded Mn nanoclusters, as schematically illustrated in Fig. 3c. The uncovered microscopic process should be universe for nanostructured MnOx anodes. Exsitu experiments found that aggregation was significant for MnOx/ ACNTs and the mesoporous channels were totally blocked after 100 cycles [28]. We expect the same mechanism plays a similar key role. The aggregation makes the extraction of Li from Li2O difficult, which results in the fading of lithium ion storage capacity upon extended cycling. Moreover, the blocking of the mesoporous channels increases the mass transfer resistance of the electrolyte, which results in the decrease of rate capability. These are the main reasons for the degradation of the electrochemical performance during cycling. 4. Conclusion A typical nanostructured porous MnO2 anode is taken as a model system to study the microstructural evolution during lithiation. The in-situ TEM reveals morphology change during the conversion type of reaction in real time. The separate MnO2 nanosheets begin to expand and coalescence with each other once in contact with Li ions. The porous structure of the specially designed shell is gradually transformed into a homogeneous solid layer of Li2O matrix with embedded Mn nanoclusters. The microscopic process is universe for a wide range of metal oxide anodes. Acknowledgement The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant nos. 51104194 and 11322219).
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. materresbull.2016.02.045. References [1] C.X. Guo, M. Wang, T. Chen, X.W. Lou, C.M. Li, A hierarchically nanostructured composite of MnO2/conjugated polymer/graphene for high-performance lithium ion batteries, Adv. Energy Mater. 1 (2011) 736–741. [2] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, Threedimensional Co3O4@MnO2 hierarchical nanoneedle arrays: morphology control and electrochemical energy storage, Adv. Funct. Mater. 24 (2014) 3815–3826. [3] Q. Zhu, H. Hu, G. Li, C. Zhu, Y. Yu, TiO2 nanotube arrays grafted with MnO2 nanosheets as high-performance anode for lithium ion battery, Electrochim. Acta 156 (2015) 252–260. [4] H. Wang, Q. Zhao, X. Wang, Y. Zhang, J. Gao, Y. Fu, X. Yang, H. Shu, Preparation and performance of b-MnO2 nanorod @ nanoflake (Ni, Co Mn) oxides with hierarchical mesoporous structure, RSC Adv. 4 (2014) 42910–42916. [5] F. Tu, T. Wu, S. Liu, G. Jin, C. Pan, Facile fabrication of MnO2 nanorod/graphene hybrid as cathode materials for lithium batteries, Electrochim. Acta 106 (2013) 406–410. [6] A.L.M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, Coaxial MnO2/carbon nanotube array electrodes for high-performance lithium batteries, Nano Lett. 9 (2009) 1002–1006. [7] H. Xia, C. Hong, X. Shi, B. Li, G. Yuan, Q. Yao, J. Xie, Hierarchical heterostructures of Ag nanoparticles decorated MnO2 nanowires as promising electrodes for supercapacitors, J. Mater. Chem. A 3 (2015) 1216–1221. [8] H. Xia, D. Zhu, Z. Luo, Y. Yu, X. Shi, G. Yuan, J. Xie, Hierarchically structured Co3O4@Pt@MnO2 nanowire arrays for high-performance supercapacitors, Sci. Rep. 3 (2013). [9] Y.X. Zhang, M. Kuang, X.D. Hao, Y. Liu, M. Huang, X.L. Guo, J. Yan, G.Q. Han, J. Li, Rational design of hierarchically porous birnessite-type manganese dioxides nanosheets on different one-dimensional titania-based nanowires for high performance supercapacitors, J. Power Sour. 270 (2014) 675–683. [10] X.H. Liu, F.F. Fan, H. Yang, S.L. Zhang, J.Y. Huang, T. Zhu, Self-limiting lithiation in silicon nanowires, ACS Nano 7 (2013) 1495–1503. [11] A. Kushima, X.H. Liu, G. Zhu, Z.L. Wang, J.Y. Huang, J. Li, Leapfrog cracking and nanoamorphization of ZnO nanowires during In situ electrochemical lithiation, Nano Lett. 11 (2011) 4535–4541. [12] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, A. Subramanian, H. Fan, L. Qi, A. Kushima, J. Li, In situ observation of the electrochemical lithiation of a single SnO2 nanowire electrode, Science 330 (2010) 1515–1520. [13] F. Jiao, P.G. Bruce, Mesoporous crystalline b-MnO2—a reversible positive electrode for rechargeable lithium batteries, Adv. Mater. 19 (2007) 657–660.
[14] D. Wang, L.-M. Liu, S.-J. Zhao, B.-H. Li, H. Liu, X.-F. Lang, b-MnO2 as a cathode material for lithium ion batteries from first principles calculations, Phys. Chem. Chem. Phys. 15 (2013) 9075–9083. [15] H. Xiao, Y. Xia, Y. Gan, H. Huang, C. Liang, X. Tao, L. Xu, W. Zhang, Facile fabrication of red phosphorus/TiO2 composites for lithium ion batteries, RSC Adv. 4 (2014) 60914–60919. [16] X. Zhang, H. Chen, Y. Xie, J. Guo, Ultralong life lithium-ion battery anode with superior high-rate capability and excellent cyclic stability from mesoporous Fe2O3@TiO2 core-shell nanorods, J. Mater. Chem. A 2 (2014) 3912–3918. [17] L. Yu, Z. Wang, L. Zhang, H.B. Wu, X.W. Lou, TiO2 nanotube arrays grafted with Fe2O3 hollow nanorods as integrated electrodes for lithium-ion batteries, J. Mater. Chem. A 1 (2013) 122–127. [18] L. He, C. Wang, X. Yao, R. Ma, H. Wang, P. Chen, K. Zhang, Synthesis of carbon nanotube/mesoporous TiO2 coaxial nanocables with enhanced lithium ion battery performance, Carbon 75 (2014) 345–352. [19] M. Mao, A. Nie, J. Liu, H. Wang, S.X. Mao, Q. Wang, K. Li, X.-X. Zhang, Atomic resolution observation of conversion-type anode RuO2 during the first electrochemical lithiation, Nanotechnology 26 (2015) 125404. [20] L. Li, F. Meng, S. Jin, High-capacity lithium-ion battery conversion cathodes based on iron fluoride nanowires and insights into the conversion mechanism, Nano Lett. 12 (2012) 6030–6037. [21] Q. Su, J. Xie, J. Zhang, Y. Zhong, G. Du, B. Xu, In situ transmission electron microscopy observation of electrochemical behavior of CoS2 in lithium-ion battery, ACS Appl. Mater. Interfaces 6 (2014) 3016–3022. [22] M.T. McDowell, S.W. Lee, J.T. Harris, B.A. Korgel, C. Wang, W.D. Nix, Y. Cui, In situ TEM of two-phase lithiation of amorphous silicon nanospheres, Nano Lett. 13 (2013) 758–764. [23] X.H. Liu, L. Zhong, S. Huang, S.X. Mao, T. Zhu, J.Y. Huang, Size-dependent fracture of silicon nanoparticles during lithiation, Acs Nano 6 (2012) 1522– 1531. [24] J.W. Wang, X.H. Liu, S.X. Mao, J.Y. Huang, Microstructural evolution of tin nanoparticles during In situ sodium insertion and extraction, Nano Lett. 12 (2012) 5897–5902. [25] C.-M. Wang, X. Li, Z. Wang, W. Xu, J. Liu, F. Gao, L. Kovarik, J.-G. Zhang, J. Howe, D.J. Burton, Z. Liu, X. Xiao, S. Thevuthasan, D.R. Baer, In situ TEM investigation of congruent phase transition and structural evolution of nanostructured silicon/carbon anode for lithium ion batteries, Nano Lett. 12 (2012) 1624–1632. [26] Q. Li, P. Wang, Q. Feng, M. Mao, J. Liu, S.X. Mao, H. Wang, In situ TEM on the reversibility of nanosized Sn anodes during the electrochemical reaction, Chem. Mater. 26 (2014) 4102–4108. [27] Y. Liu, N.S. Hudak, D.L. Huber, S.J. Limmer, J.P. Sullivan, J.Y. Huang, In situ transmission electron microscopy observation of pulverization of aluminum nanowires and evolution of the thin surface Al2O3 layers during lithiationdelithiation cycles, Nano Lett. 11 (2011) 4188–4194. [28] F. Lou, H. Zhou, T.D. Tran, M.E. Melandsø Buan, F. Vullum-Bruer, M. Rønning, J.C. Walmsley, D. Chen, Coaxial carbon/metal oxide/aligned carbon nanotube arrays as high-performance anodes for lithium ion batteries, ChemSusChem 7 (2014) 1335–1346.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
In-situ TEM on the coalescence of birnessite manganese dioxides nanosheets during lithiation process Ke Caoa , Min Kuangc , Yuxin Zhangc, Jiabin Liua,* , Hongtao Wangb,* , Liang Menga a b c
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Institute of Applied Mechanics, Zhejiang University, Hangzhou 310027, China College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 November 2015 Received in revised form 23 February 2016 Accepted 28 February 2016 Available online 2 March 2016
Nanostructure is believed to produce great benefits for anode materials in lithium ion batteries (LIBs) by enhancing lithium ion transfer, accommodating large volume change and increasing surface area. Whether the nanostructure (especially the porous nanostructure) could be well held during charging/ discharging process is one of the most commonly concerned issues in LIBs research. The dynamic evolution of birnessite manganese dioxides nanosheets during lithiation process is investigated by in-situ transmission electron microscopy (TEM) for the first time. The TiO2@MnO2 core-shell nanowires are used as the anode and Li metal as the counter electrode inside the TEM. Interestingly, the lithiation process is confirmed as MnO2 and Li converting to Li2O and Mn. The original porous structure of the nanosheets is hard to preserve during lithiation process due to lithiation-induced contact flattening. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Electron diffraction C. Transmission electron microscopy (TEM) D. Crystal structure
1. Introduction Manganese dioxide has received substantial attention as anode materials in lithium ion batteries (LIBs) due to its high theoretical specific capacities, environmental benignity and natural abundance [1,2]. However, there are still two major obstacles for its practical application. The first and most important one is the large volumetric expansion during the charging/discharging processes, resulting in severe particle agglomeration and unsatisfactory cycling performance; the other one is the virtually ubiquitous large coulombic inefficiency observed in the first cycle. Nanostructure is believed to produce great benefits for the anode materials. The nanostructured materials could greatly increase the surface area and also accommodate large change in volume. This strategy is especially suitable for nanostructured MnO2, such as nanorods/ nanowires, nanotubes and nanosheets with or without porous structures [3–5]. The vacant space provided by the porous nanostructures can accommodate the structural strain and facilitate fast Li+ insertion/extraction kinetics, leading to improved rate and cycling performance. Pure nanostructured MnO2 still endures the disadvantages of low conductivity and particle agglomeration. Therefore, carbon was introduced into MnO2 nanomaterials to form homogenous
* Corresponding authors. E-mail addresses: [email protected] (J. Liu), [email protected] (H. Wang). http://dx.doi.org/10.1016/j.materresbull.2016.02.045 0025-5408/ ã 2016 Elsevier Ltd. All rights reserved.
C@MnO2 composites. An excellent example is the coaxial C@MnO2 nanotube array electrodes with good cyclic stability and capacity [6]. Various coaxial X@MnO2 nanorods/nanowires were also explored to enhance the conductivity of MnO2 and stabilize the nanostructure [1–3,7,8]. In our previous studies, TiO2@MnO2 core-shell nanowires were successfully prepared for high performance supercapacitors [9]. The TiO2@MnO2 core-shell nanowires showed a high capacitance of 120 F g1 (0.1 A g1) and excellent cycling stability (93% capacitance retention after 3000 cycles). The structure of the MnO2 nanosheets should keep unchanged during the charge/ discharge processes as suggested by the excellent cycling stability. However, considering the notable difference between electrons in supercapacitors and Li+ ions in LIBs, it is still an open question that whether the MnO2 nanosheets can retain the morphology during Li+ insertion/extraction processes, which is one of the most commonly concerned issues in LIBs research as well. In-situ transmission electron microscopy (TEM) is a powerful technique to reveal the dynamic microstructure evolution of electrode materials. Thanks to J.Y. Huang’s group’s pioneer works of the in-situ lithiation and delithiation in TEM, plenty of fresh results about the dislocation associated lithiation, layer-by-layer lithiation and even cracking were obtained continuously [10–12]. In this study, we adopted in-situ TEM to reveal the real time change of the MnO2 porous nanostructure during Li+ insertion processes. It is found that the origin MnO2 porous nanostructure could not be retained during Li+ insertion processes, which may be
K. Cao et al. / Materials Research Bulletin 79 (2016) 36–40
the main reason of the large coulombic inefficiency observed in the first cycle. 2. Experimental section 2.1. Synthesis of TiO2@MnO2 core-shell nanowires Commercial anatase TiO2 powders (0.4 g) were put into aqueous solution of 10 M NaOH (35 mL) in a Teflon-lined stainless steel autoclave (50 mL). The autoclave was kept at 180 C for 48 h. After hydrothermal treatment, the resulting precipitates were acidwashed (0.1 M HCl solution) to realize the full-ion exchange from Na+ to H+. The final white products were then dried in vacuum at 80 C for 8 h, and then calcined at 400 C for 3 h in air to obtain the anatase TiO2 nanowires. Then 25 mg TiO2 were dispersed in KMnO4 solution (40 mL, 0.01 M) by ultrasonication for 10 min. The mixed solution was then transferred into the Teflon-lined stainless steel autoclave. The autoclave was directly put in an electric oven at 140 C for 24 h and then cooled to room temperature. The black resultants were collected by centrifugation and dried at 60 C for 12 h in a vacuum oven.
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2.2. In-situ TEM setup A nanosized half-cell LIB was constructed inside TEM for in-situ investigation and is schematically shown in Fig. 1a. The in-situ TEM experiments were carried out in a JEM-2100 operating at 200 kV with the help of a Nanofactory TEM-STM holder. The TiO2@MnO2 core-shell nanowires were attached onto the tip of a gold wire, serving as the anode. Some Li metal was sticked to a tip of tungsten wire, serving as the cathode. The unavoidable surface oxidation on the Li metal served as the solid electrolyte. A negative voltage (2 V) was applied on the TiO2@MnO2 core-shell nanowires with respect to the Li counter-electrode to initiate the lithiation process. The lithiation process started instantaneously, once the contact was established between TiO2@MnO2 core-shell nanowires and the Li electrode. 3. Result and discussion Fig. 1b–f shows the microstructure of the TiO2@MnO2 core-shell nanowires. The TiO2@MnO2 nanowires are composed of quadrangular prism TiO2 nanowires and MnO2 nanosheets. The MnO2
Fig. 1. TEM images of (a) schematical illustation of the in-situ experiment, (b) a single nanowire. Inset is a typical SAED pattern of the TiO2 single crystal and MnO2 nanosheets; (c) element mapping of a nanowire, (d) the cross-sectional TEM image, (e) high resolution TEM image of the interface of MnO2 nanosheets and TiO2, (f) schematical illustration of the nanostructure of the TiO2@MnO2 core-shell nanowires.
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nanosheets have a thickness of 5–20 nm and a width of 50–100 nm. Typical selected area electron diffraction (SAED) pattern (inset in Fig. 1b) clearly indicates that TiO2 nanowire is of single crystalline tetragonal phase and MnO2 nanosheets have the monoclinic potassium birnessite phase. There is not orientation relationship between those MnO2 nanosheets and the TiO2 core. The chemical composition of a single TiO2@MnO2 nanowire is given in Fig. 1c, which clearly confirms the presence of TiO2 and MnO2. The MnO2 nanosheets tightly surround the TiO2 nanowire and are parallel with each other to form a porous nanostructure with abundant open space (Fig. 1d and e), schematically illustrated in Fig. 1f. High resolution TEM image also reveals that there is no special
orientation relationship between the MnO2 nanosheets and TiO2 core. The realtime microstructure evolution of the TiO2@MnO2 coreshell nanowires during lithiation process is recorded by video and key snapshots are extracted from the video (Fig. 2). During the lithiation process, the MnO2 nanosheets began to expand and eventually merged with each other. The thickness of the MnO2 shell increased from 47 nm to about 63 nm. The volume expansion of the MnO2 nanosheet layer is estimated to be about 38%, which is higher than previous report (26.5%) [13,14]. This value may be over estimated since the layer is porous. It is not so easy to measure the volume expansion for single sheet due to the lack of exact
Fig. 2. (a)–(c) Sequential snapshots of the lithiation process of a TiO2@MnO2 core-shell nanowire; Comparison of the high magnification TEM images of the TiO2@MnO2 coreshell structure (d) before lithiation and (e) after lithiation; The corresponding SAED patterns of the TiO2@MnO2 core-shell structure (f) before lithiation and (g) after lithiation.
K. Cao et al. / Materials Research Bulletin 79 (2016) 36–40
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Fig. 3. High resolution TEM images of the TiO2@MnO2 nanowires: (a) before lithiation and (b) after lithiation, (c) illustration of the nanosheet structure changes during the lithiation process of TiO2@MnO2 nanowires.
expansion result at different directions for the sheet in 3D space. The volume expansion of TiO2 core is neglectable, which agrees well with former studies of the high stability of TiO2 during lithium insertion/extraction processes [15–18]. High magnification TEM images of the TiO2@MnO2 core-shell structure before and after lithiation show significant coalescence of the nanosheet structure, forming a homogeneous layer of lithiated product (Fig. 2d and e). The phase change can be identified by the SAED patterns of the TiO2@MnO2 core-shell nanowires before and after lithiation in Fig. 2f and g, respectively. Fig. 2f shows two diffraction rings of (3 3 0) and (5 2 1), indicating the crystalline MnO2 phase. After lithiation, no diffraction rings from MnO2 can be observed (Fig. 2g). Instead, the diffraction rings can be indexed to the (3 0 0) crystal plane of Mn and the (111) and (2 2 0) crystal planes of Li2O. Comparison the change of SAED patterns before and after lithiation sugguests that MnO2 has reacted wh Li to form Mn and Li2O [3]: MnO2 + Li ! Mn + Li2O
(1)
Previous in-situ studies have also confirmed the similar convernsion mechanism of some transition metal oxides, fluorides, sulfides, such as RuO2 [19], FeF3 [20], CoS2 [21]. Lots of 0D nanomaterials (Si particles [22,23], Sn nanoparticles [16,24]), 1D nanomaterials (Si nanowires [10,25], carbon coated Sn nanowires [26], Al nanowires [27]) have been investigated before. Many interesting morphology changes, such as the volume changes, phase boundaries, cracks, mislocations, etc., have been observed for these individual nanostructure materials. Different from the individual nanomaterials as mentioned above, the attention is focused on the collective lithitation behavior of MnO2 nanosheets in this work. When each MnO2 nanosheet was lithiated and expanded in volume, the original porous structure changed once the nanosheet touched each other. Deeper structural information can be obtained by comparing the high resolution TEM observation before and after lithiation of the same nanowire. Fig. 3a clearly shows the (2 0 0), (3 0 1) and (3 2 1) crystal planes of MnO2 before lithiation. After lithiation, some dark clusters with a size of 2–5 nm appear, recognized as the fresh-formed Mn metal (Fig. 3b). The lattice fringe is from the
(111) crystal planes of Li2O, consistent with the SEAD patterns in Fig. 2g. Though the as-synthesized nanowires are composed of separately MnO2 nanosheets, they begin to expand and coalescence with each other once in contact with Li ions. The porous structure of the specially designed shell cannot retain its original morphology and are gradually transformed into a homogeneous solid layer of Li2O matrix with embedded Mn nanoclusters, as schematically illustrated in Fig. 3c. The uncovered microscopic process should be universe for nanostructured MnOx anodes. Exsitu experiments found that aggregation was significant for MnOx/ ACNTs and the mesoporous channels were totally blocked after 100 cycles [28]. We expect the same mechanism plays a similar key role. The aggregation makes the extraction of Li from Li2O difficult, which results in the fading of lithium ion storage capacity upon extended cycling. Moreover, the blocking of the mesoporous channels increases the mass transfer resistance of the electrolyte, which results in the decrease of rate capability. These are the main reasons for the degradation of the electrochemical performance during cycling. 4. Conclusion A typical nanostructured porous MnO2 anode is taken as a model system to study the microstructural evolution during lithiation. The in-situ TEM reveals morphology change during the conversion type of reaction in real time. The separate MnO2 nanosheets begin to expand and coalescence with each other once in contact with Li ions. The porous structure of the specially designed shell is gradually transformed into a homogeneous solid layer of Li2O matrix with embedded Mn nanoclusters. The microscopic process is universe for a wide range of metal oxide anodes. Acknowledgement The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant nos. 51104194 and 11322219).
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