- Email: [email protected]
Facile synthesis of Bi2Fe4O9@C composite as new high-performance anode materials for lithium-ion batteries
Inorganic Chemistry Communications 113 (2020) 107816
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Facile synthesis of Bi2Fe4O9@C composite as new high-performance anode materials for lithium-ion batteries Jianyin Zhanga, , Xiaoxiao Shia,b, Haohao Liua, Xingwei Shib, ⁎
a b
T
⁎
School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Bi2Fe4O9 Carbon layer coating Solid-state synthesis Lithium-ion battery anode
Faced with the increasing demand for energy density of lithium-ion batteries in the market, exploring new electrode materials is an effective way to pursue higher energy density of lithium-ion batteries. Herein, Bi2Fe4O9@C is prepared by solid state reaction in combination with mechanical ball milling. The morphology, microstructure and properties of the negative electrode Bi2Fe4O9@C of lithium ion battery were studied. As expected, the electrochemical performance of Bi2Fe4O9@C is enhanced by composite with graphite. Electrochemical tests show that the reversible charging capacity of Bi2Fe4O9@C is 743 mAh g−1 after 80 cycles at a current density of 100 mA g−1, while the pure Bi2Fe4O9 compound retains only 142 mAh g−1. Especially, the reversible delithiation capacity of Bi2Fe4O9@C composite is up to 551 mAh g−1 at 2 A g−1. This study demonstrates the potential application of Bi2Fe4O9@C composite in anode materials for lithium-ion batteries.
1. Introduction Nowadays, due to resource shortages and increasingly serious environmental problems, the scientific development and utilization of new energy has attracted more and more attention. Today, lithium-ion batteries (LIBs) have a large capacity, long cycle life and environmental friendliness, and have been widely used in electronic portable devices
⁎
[1,2]. However, large electronic devices such as vehicles urgently require the use of more portable batteries, and thus it is required to further increase the energy density of LIBs. More improvements in electrode materials are still very important to promote their practical application. At present, graphite-based carbon materials are widely used as anode materials for LIBs and exhibit excellent cycle stability [3]. However, due to the mechanism of lithium intercalation in the
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (X. Shi).
https://doi.org/10.1016/j.inoche.2020.107816 Received 30 October 2019; Received in revised form 19 January 2020; Accepted 24 January 2020 Available online 26 January 2020 1387-7003/ © 2020 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 113 (2020) 107816
J. Zhang, et al.
graphite layer, its theoretical capacity is only 372 mAh g−1. In order to solve these problems, many researchers have been working on the development of new anode materials with high energy density, such as metal oxides [4] and metal alloys [5,6]. Since Bi2O3 has a high theoretical capacity of 690 mAh g−1, involving conversion reactions and alloying reactions, it has attracted great interest [7]. In addition, bismuth-containing ternary metal oxides are also used as anode materials for LIBs [8910111213]. Song et al. [14]. prepared the Bi2Mn4O10@C composite by sol-gel method. Compared with Bi2Mn4O10 powder, the electrochemical performance was significantly improved. Rodriguez et al. [15] obtained microcrystalline Bi4Ge3O12 via sol-gel process and its electrochemical properties were first studied as anode material for LIBs. The discharge capacity is ~586 mAh g−1 even after 500 cycles test at 200 mA g−1. Li et al. [16] prepared one-dimensional Bi5Nb3O15 nanowires by electrospinning and annealing. Electrochemical characterization shows that the initial charge capacity can reach 365 mAh g−1 at 100 mA g−1. In 2014, Bi2Fe4O9-CeO2 was used as anode for second lithium batteries application [17]. Since then, little research has been done on the Li-storage properties of Bi2Fe4O9. In this communication, Bi2Fe4O9@C composite was prepared by conventional solid-state method at 800 °C in air for 12 h followed by mechanical ballmilling with graphite and studied for LIBs.
Fig. 3a shows the cyclic voltammetry (CV) profile of the Bi2Fe4O9 for the first three cycles. In the first reduction process, two strong peaks at 1.07 V and 0.75 V were obvious observed. Both are irreversible chemical reaction, which disappeared in the following cycles owing to the transformation of Bi2Fe4O9 to Bi and Fe as well as the decomposition of electrolyte. Then, a smooth peak at 0.3 V is attributed to the alloying reaction of Bi and Li, which is consistent with previous reports [20]. In the first oxidation process, one peak at 0.96 V and a broad peak between 1.0 V and 2.5 V correspond to the formation of element Bi and two oxidation state compounds of Bi and Fe, respectively [21]. During the subsequent cycles, both peak current and peak area decrease to some extent, indicating that the active ingredients were pulverized and loss of electrical contact during the lithiation and delithiation. For Bi2Fe4O9@C electrode, the CV curves are shown in Fig. 3b. From the second cycle, almost overlapping curves showed that the reversible of Bi2Fe4O9 react with Li+ has been greatly improved by carbon. The pair of redox peaks between 0.02 V and 0.20 V, which correspond to the intercalation and de-intercalation of lithium-ion in graphite. Peak M and M’ were ascribed to the mutual transformation of Fe and Fe2O3, while other peaks related to reversible transition of Bi and Bi2O3 [7 21]. Fig. 3c and d show the typical charge and discharge curves of the Bi2Fe4O9 and Bi2Fe4O9@C at 100 mA g−1. The discharge and charge capacities of the Bi2Fe4O9@C electrode were 1244 mAh g−1 and 906 mAh g−1, respectively, and the corresponding coulombic efficiency was 73%. However, the discharge and charge capacities of the Bi2Fe4O9 electrode were 984 mAh g−1 and 679 mAh g−1, respectively, and the corresponding coulombic efficiency was 69%. It demonstrates that the carbon layer maintains electrode integrity and increases surface electron conductivity, thereby increasing electrode capacity and coulombic efficiency. In order to study the practicality of the electrode, a half-cell was assembled using lithium metal as a counter electrode to test cycle life and rate performance. It can be seen from Fig. 3e that the cycle stability of the Bi2Fe4O9@C composite is significantly better than that of the Bi2Fe4O9 electrode. Bi2Fe4O9@C composite provides a reversible charge capacity of 743 mAh g−1 at a current density of 100 mA g−1 after 80 cycles, while a pure Bi2Fe4O9 compound retains only 142 mAh g−1. As shown in Table S1, we compared the Bi2Fe4O9@C composite prepared in this paper with the bismuth-based oxide anode material. It can be found that the cycle performance of Bi2Fe4O9@C is superior to many other Bi-based materials. Fig. 3f shows the rate performances of the Bi2Fe4O9@C composite and Bi2Fe4O9 compound. When the current density is 0.1 A g−1, 0.2 A g−1, 0.5 A g−1, 1 A g−1 and 2 A g−1, the corresponding specific discharge capacity of the Bi2Fe4O9@C composite is 791 mAh g−1, 738 mAh g−1, 667 mAh g−1, 618 mAh g−1 and 532 mAh g−1, respectively. On the contrary, the specific discharge capacity of the Bi2Fe4O9 electrode is 402 mAh g−1, 243 mAh g−1, 164 mAh g−1, 114 mAh g−1 and 32 mAh g−1 under the same conditions. When the current density is reset to 0.1 A g−1, the delithiation specific capacity of Bi2Fe4O9@C and Bi2Fe4O9 electrode is 824 mAh g−1 and 200 mAh g−1, respectively. The electrochemical properties of Bi2Fe4O9 have been significantly improved by the combination with carbon, which is mainly due to the fact that the carbon layer-coating not only improves its electrical conductivity, but also maintains the electrode integrity [22]. These results indicate that Bi2Fe4O9@C composite can be used as new high-performance anode materials for LIBs. In order to clarify the cycling stability of the Bi2Fe4O9@C composite electrode, ex-situ XPS combined with SEM were used to characterize its structural and morphological changes. Fig. 4a shows the XPS of the Fe 2p at the initial stage and after 80 discharge-charge cycles for the Bi2Fe4O9@C composite electrode. As seen in the figure, after repeated lithium insertion and extraction, the two peaks at ≈710.45 eV and ≈723.90 eV perfectly matched with Fe3+ [23], corresponding to Fe 2p3/2 and 2p1/2, respectively, which are consistent with the original Fe3+ state. These data indicate that the electrode is highly reversible. Fig. 4b and c show the Bi2Fe4O9@C electrode before and after cycling,
2. Experimental First, a stoichiometric amount of Bi2O3 and Fe2O3 (a molar ratio of Bi to Fe of 1:2) was sufficiently ground in an agate mortar for 30 min, and the resulting mixture was heated in air at 800 °C for 12 h to obtain a pure phase of Bi2Fe4O9. Next, the Bi2Fe4O9@C composite was prepared by planetary ball milling (QM-3SP04, Nanjing NanDa Instrument Plant, China) at 480 rpm for 10 h. The mass ratio of Bi2Fe4O9 to graphite prepared was 8:2. The total mass ratio of the ball to the material was set to 40:1. 3. Result and discussion Fig. 1a presents the XRD patterns of Bi2Fe4O9 and Bi2Fe4O9@C. All diffraction peaks can be well indexed into triclinic structured Bi2Fe4O9 (PDF#25-0090), indicating that the synthesized product is a pure phase without any impurities. Compared with pristine Bi2Fe4O9, the diffraction peak of the Bi2Fe4O9@C appears to be significantly broadened, indicating grain refinement after ball milling. No characteristic peaks of graphite appear on the X-ray diffraction pattern, indicating that the graphite after ball milling is amorphous [18]. Fig. 1b presents the particle size distribution of the Bi2Fe4O9 and Bi2Fe4O9@C. It is obvious that the mechanical ball milling process moves the particle size distribution to a larger size. This shows that ball milling can lead to agglomeration of particles. Fig. 1c shows the TG analysis of the Bi2Fe4O9@C composite. At 800 °C, the weight percentage of Bi2Fe4O9 in the composite is 78.3%, which is also roughly consistent with the feed ratio before ball milling. Fig. 1d illustrates the Raman spectra of pristine graphite and ball-milled sample. The pristine graphite appears a strong graphite band at 1580 cm−1 and a weak band at 1360 cm−1 that suggests existence of structural defects [18]. For the milled sample, the increase in intensity ratio of the 1360 cm−1 band and 1580 cm−1 band is observed, which corresponds with an increase of lattice defects in the graphitic structure [19]. The morphology of Bi2Fe4O9 prepared by the solid-state method was observed by SEM, as shown in Fig. 2a and b. It can be seen that the sample displays irregular sintered block shape. When Bi2Fe4O9 was milled with graphite (Fig. 2c and d), it showed irregular agglomeration. These results are also consistent with the above particle size distribution. The TEM (Fig. 2e) image shows that the Bi2Fe4O9 particles are coated with carbon layer. The HRTEM image is shown in Fig. 2f. The interplanar spacing is 0.308 nm, which corresponds to the (2 1 1) plane of the triclinic Bi2Fe4O9. 2
Inorganic Chemistry Communications 113 (2020) 107816
J. Zhang, et al.
Fig. 1. (a) XRD patterns, (b) particle size distribution of the synthesized Bi2Fe4O9 and ball milling with graphite, (c) TG analysis of the Bi2Fe4O9@C composite under air atmosphere (d) Raman spectra for pristine graphite and Bi2Fe4O9@C composite.
Fig. 2. SEM images of (a, b) Bi2Fe4O9, (c, d) Bi2Fe4O9@C, (e) TEM and (f) HRTEM image of Bi2Fe4O9@C.
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Fig. 3. Typical charge-discharge curves at 100 mA g−1 and CVs for the initial three cycles at scan rate of 0.1 mV s−1 of (a, c) Bi2Fe4O9 and (b, d) Bi2Fe4O9@C (e) cycling performances at 100 mA g−1 (f) rate performances.
Fig. 4. (a) XPS of the Fe 2p at the initial stage and after cycles for the Bi2Fe4O9@C electrode, SEM images of (b) pristine Bi2Fe4O9@C electrode and (c) Bi2Fe4O9@C electrode after 80 cycles at 100 mA g−1.
respectively. Both surfaces show a dense structure without significant differences, indicating that carbon layer-coating can effectively alleviate the stress during cycling.
Acknowledgments Financial supports from the Doctoral fund of Shanxi Normal University (0505/02070305) and 1331 project of Shanxi province (0109/02020001).
4. Conclusions
Appendix A. Supplementary material
In this paper, Bi2Fe4O9@C composite was synthesized via solid-state reaction accompanied by mechanical ball-milling method and investigated as new anode materials for LIBs. The results show that the electrochemical performances of Bi2Fe4O9@C composite were obviously enhanced by composite with carbon. The discharge capacity is 743 mAh g−1 after 80 cycles test at 100 mA g−1, which was more than the pure Bi2Fe4O9 electrode (142 mAh g−1). In addition, the reversible delithiation capacity of Bi2Fe4O9@C composite is up to 551 mAh g−1 at 2 A g−1.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2020.107816. References [1] R. Marom, S.F. Amalraj, N. Leifer, D. Jacob, D. Aurbach, J. Mater. Chem. 21 (27) (2011) 9938–9954. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (9) (2011) 3243–3262. [3] M. Yoshio, H. Wang, K. Fukuda, Angew. Chem. Int. Edit. 42 (35) (2003) 4203–4206. [4] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Chem. Rev. 113 (7) (2013) 5364–5457. [5] W.J. Zhang, J. Power Sources 196 (1) (2011) 13–24. [6] J. Zhang, Z. Wang, Y. Hong, S. Li, X. Jin, G.Z. Chen, Electro. Commun. 38 (2014) 36–39. [7] W. Fang, N. Zhang, L. Fan, K. Sun, J. Power Sources 333 (2016) 30–36. [8] D.P. Dubal, D.R. Patil, S.S. Patil, S.S. Patil, N.R. Munirathnam, P. Gomez-Romero, ChemSusChem 10 (21) (2017) 4163–4169.
Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work.
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J. Zhang, et al. [9] X. Zhai, J. Gao, R. Xue, X. Xu, L. Wang, Q. Tian, Y. Liu, J. Colloid Interf. Sci. 518 (2018) 242–251. [10] A. Ruud, J. Sottmann, P. Vajeeston, H. Fjellvag, Phys. Chem. Chem. Phys. 20 (47) (2018) 29798–29803. [11] Y. Zhang, G. Zhao, P. Ge, T. Wu, L. Li, P. Cai, C. Liu, G. Zou, H. Hou, X. Ji, Inorg. Chem. 58 (2019) 6410–6421. [12] D.R. Patil, S.D. Jadhav, A. Mungale, A.S. Kalekar, D.P. Dubal, Mater. Lett. 252 (2019) 235–238. [13] J. Xu, W. Wei, X. Zhang, L. Liang, M. Xu, Chinese Chem.Lett. 30 (6) (2019) 1341–1345. [14] Z. Song, H. Zhang, K. Feng, H. Wang, X. Li, H. Zhang, Dalton Trans. 47 (2018) 7739–7746. [15] J.R. Rodriguez, C. Belman-Rodriguez, S.A. Aguila, Y. Zhang, H. Liu, V.G. Pol, Chem. Commun. 54 (81) (2018) 11483–11486. [16] Y. Li, R. Zheng, H. Yu, X. Cheng, T. Liu, N. Peng, J. Zhang, M. Shui, J. Shu,
Electrochim. Acta 299 (2019) 894–901. [17] M. Liu, C. Lin, Y. Gu, T. Yang, Z. Gong, G. Yin, X. Gao, X. Zhou, W. Wen, J. Phys. Chem. C 118 (2014) 14711–14722. [18] H. Wakayama, J. Mizuno, Y. Fukushima, K. Nagano, T. Fukunaga, U. Mizutani, Carbon 37 (6) (1999) 947–952. [19] M. Ratynski, B. Hamankiewicz, M. Krajewski, M. Boczar, D. Ziolkowska, A. Czerwinski, Carbon 145 (2019) 82–89. [20] Y. Li, M.A. Trujillo, E. Fu, B. Patterson, L. Fei, Y. Xu, S. Deng, S. Smirnov, H. Luo, J. Mater. Chem. A 1 (39) (2013) 12123–12127. [21] H. Xia, F. Yan, M.O. Lai, L. Lu, Funct. Mater. Lett. 2 (04) (2009) 163–167. [22] Z. Chen, M. Zhou, Y. Cao, X. Ai, H. Yang, J. Liu, Adv. Energy Mater. 2 (1) (2012) 95–102. [23] Y. Cao, A. Zhang, H. Zhang, G. Ding, L. Zhang, Inorg. Chem. Commun. 111 (2020) 107633.
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Facile synthesis of Bi2Fe4O9@C composite as new high-performance anode materials for lithium-ion batteries Jianyin Zhanga, , Xiaoxiao Shia,b, Haohao Liua, Xingwei Shib, ⁎
a b
T
⁎
School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Bi2Fe4O9 Carbon layer coating Solid-state synthesis Lithium-ion battery anode
Faced with the increasing demand for energy density of lithium-ion batteries in the market, exploring new electrode materials is an effective way to pursue higher energy density of lithium-ion batteries. Herein, Bi2Fe4O9@C is prepared by solid state reaction in combination with mechanical ball milling. The morphology, microstructure and properties of the negative electrode Bi2Fe4O9@C of lithium ion battery were studied. As expected, the electrochemical performance of Bi2Fe4O9@C is enhanced by composite with graphite. Electrochemical tests show that the reversible charging capacity of Bi2Fe4O9@C is 743 mAh g−1 after 80 cycles at a current density of 100 mA g−1, while the pure Bi2Fe4O9 compound retains only 142 mAh g−1. Especially, the reversible delithiation capacity of Bi2Fe4O9@C composite is up to 551 mAh g−1 at 2 A g−1. This study demonstrates the potential application of Bi2Fe4O9@C composite in anode materials for lithium-ion batteries.
1. Introduction Nowadays, due to resource shortages and increasingly serious environmental problems, the scientific development and utilization of new energy has attracted more and more attention. Today, lithium-ion batteries (LIBs) have a large capacity, long cycle life and environmental friendliness, and have been widely used in electronic portable devices
⁎
[1,2]. However, large electronic devices such as vehicles urgently require the use of more portable batteries, and thus it is required to further increase the energy density of LIBs. More improvements in electrode materials are still very important to promote their practical application. At present, graphite-based carbon materials are widely used as anode materials for LIBs and exhibit excellent cycle stability [3]. However, due to the mechanism of lithium intercalation in the
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (X. Shi).
https://doi.org/10.1016/j.inoche.2020.107816 Received 30 October 2019; Received in revised form 19 January 2020; Accepted 24 January 2020 Available online 26 January 2020 1387-7003/ © 2020 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 113 (2020) 107816
J. Zhang, et al.
graphite layer, its theoretical capacity is only 372 mAh g−1. In order to solve these problems, many researchers have been working on the development of new anode materials with high energy density, such as metal oxides [4] and metal alloys [5,6]. Since Bi2O3 has a high theoretical capacity of 690 mAh g−1, involving conversion reactions and alloying reactions, it has attracted great interest [7]. In addition, bismuth-containing ternary metal oxides are also used as anode materials for LIBs [8910111213]. Song et al. [14]. prepared the Bi2Mn4O10@C composite by sol-gel method. Compared with Bi2Mn4O10 powder, the electrochemical performance was significantly improved. Rodriguez et al. [15] obtained microcrystalline Bi4Ge3O12 via sol-gel process and its electrochemical properties were first studied as anode material for LIBs. The discharge capacity is ~586 mAh g−1 even after 500 cycles test at 200 mA g−1. Li et al. [16] prepared one-dimensional Bi5Nb3O15 nanowires by electrospinning and annealing. Electrochemical characterization shows that the initial charge capacity can reach 365 mAh g−1 at 100 mA g−1. In 2014, Bi2Fe4O9-CeO2 was used as anode for second lithium batteries application [17]. Since then, little research has been done on the Li-storage properties of Bi2Fe4O9. In this communication, Bi2Fe4O9@C composite was prepared by conventional solid-state method at 800 °C in air for 12 h followed by mechanical ballmilling with graphite and studied for LIBs.
Fig. 3a shows the cyclic voltammetry (CV) profile of the Bi2Fe4O9 for the first three cycles. In the first reduction process, two strong peaks at 1.07 V and 0.75 V were obvious observed. Both are irreversible chemical reaction, which disappeared in the following cycles owing to the transformation of Bi2Fe4O9 to Bi and Fe as well as the decomposition of electrolyte. Then, a smooth peak at 0.3 V is attributed to the alloying reaction of Bi and Li, which is consistent with previous reports [20]. In the first oxidation process, one peak at 0.96 V and a broad peak between 1.0 V and 2.5 V correspond to the formation of element Bi and two oxidation state compounds of Bi and Fe, respectively [21]. During the subsequent cycles, both peak current and peak area decrease to some extent, indicating that the active ingredients were pulverized and loss of electrical contact during the lithiation and delithiation. For Bi2Fe4O9@C electrode, the CV curves are shown in Fig. 3b. From the second cycle, almost overlapping curves showed that the reversible of Bi2Fe4O9 react with Li+ has been greatly improved by carbon. The pair of redox peaks between 0.02 V and 0.20 V, which correspond to the intercalation and de-intercalation of lithium-ion in graphite. Peak M and M’ were ascribed to the mutual transformation of Fe and Fe2O3, while other peaks related to reversible transition of Bi and Bi2O3 [7 21]. Fig. 3c and d show the typical charge and discharge curves of the Bi2Fe4O9 and Bi2Fe4O9@C at 100 mA g−1. The discharge and charge capacities of the Bi2Fe4O9@C electrode were 1244 mAh g−1 and 906 mAh g−1, respectively, and the corresponding coulombic efficiency was 73%. However, the discharge and charge capacities of the Bi2Fe4O9 electrode were 984 mAh g−1 and 679 mAh g−1, respectively, and the corresponding coulombic efficiency was 69%. It demonstrates that the carbon layer maintains electrode integrity and increases surface electron conductivity, thereby increasing electrode capacity and coulombic efficiency. In order to study the practicality of the electrode, a half-cell was assembled using lithium metal as a counter electrode to test cycle life and rate performance. It can be seen from Fig. 3e that the cycle stability of the Bi2Fe4O9@C composite is significantly better than that of the Bi2Fe4O9 electrode. Bi2Fe4O9@C composite provides a reversible charge capacity of 743 mAh g−1 at a current density of 100 mA g−1 after 80 cycles, while a pure Bi2Fe4O9 compound retains only 142 mAh g−1. As shown in Table S1, we compared the Bi2Fe4O9@C composite prepared in this paper with the bismuth-based oxide anode material. It can be found that the cycle performance of Bi2Fe4O9@C is superior to many other Bi-based materials. Fig. 3f shows the rate performances of the Bi2Fe4O9@C composite and Bi2Fe4O9 compound. When the current density is 0.1 A g−1, 0.2 A g−1, 0.5 A g−1, 1 A g−1 and 2 A g−1, the corresponding specific discharge capacity of the Bi2Fe4O9@C composite is 791 mAh g−1, 738 mAh g−1, 667 mAh g−1, 618 mAh g−1 and 532 mAh g−1, respectively. On the contrary, the specific discharge capacity of the Bi2Fe4O9 electrode is 402 mAh g−1, 243 mAh g−1, 164 mAh g−1, 114 mAh g−1 and 32 mAh g−1 under the same conditions. When the current density is reset to 0.1 A g−1, the delithiation specific capacity of Bi2Fe4O9@C and Bi2Fe4O9 electrode is 824 mAh g−1 and 200 mAh g−1, respectively. The electrochemical properties of Bi2Fe4O9 have been significantly improved by the combination with carbon, which is mainly due to the fact that the carbon layer-coating not only improves its electrical conductivity, but also maintains the electrode integrity [22]. These results indicate that Bi2Fe4O9@C composite can be used as new high-performance anode materials for LIBs. In order to clarify the cycling stability of the Bi2Fe4O9@C composite electrode, ex-situ XPS combined with SEM were used to characterize its structural and morphological changes. Fig. 4a shows the XPS of the Fe 2p at the initial stage and after 80 discharge-charge cycles for the Bi2Fe4O9@C composite electrode. As seen in the figure, after repeated lithium insertion and extraction, the two peaks at ≈710.45 eV and ≈723.90 eV perfectly matched with Fe3+ [23], corresponding to Fe 2p3/2 and 2p1/2, respectively, which are consistent with the original Fe3+ state. These data indicate that the electrode is highly reversible. Fig. 4b and c show the Bi2Fe4O9@C electrode before and after cycling,
2. Experimental First, a stoichiometric amount of Bi2O3 and Fe2O3 (a molar ratio of Bi to Fe of 1:2) was sufficiently ground in an agate mortar for 30 min, and the resulting mixture was heated in air at 800 °C for 12 h to obtain a pure phase of Bi2Fe4O9. Next, the Bi2Fe4O9@C composite was prepared by planetary ball milling (QM-3SP04, Nanjing NanDa Instrument Plant, China) at 480 rpm for 10 h. The mass ratio of Bi2Fe4O9 to graphite prepared was 8:2. The total mass ratio of the ball to the material was set to 40:1. 3. Result and discussion Fig. 1a presents the XRD patterns of Bi2Fe4O9 and Bi2Fe4O9@C. All diffraction peaks can be well indexed into triclinic structured Bi2Fe4O9 (PDF#25-0090), indicating that the synthesized product is a pure phase without any impurities. Compared with pristine Bi2Fe4O9, the diffraction peak of the Bi2Fe4O9@C appears to be significantly broadened, indicating grain refinement after ball milling. No characteristic peaks of graphite appear on the X-ray diffraction pattern, indicating that the graphite after ball milling is amorphous [18]. Fig. 1b presents the particle size distribution of the Bi2Fe4O9 and Bi2Fe4O9@C. It is obvious that the mechanical ball milling process moves the particle size distribution to a larger size. This shows that ball milling can lead to agglomeration of particles. Fig. 1c shows the TG analysis of the Bi2Fe4O9@C composite. At 800 °C, the weight percentage of Bi2Fe4O9 in the composite is 78.3%, which is also roughly consistent with the feed ratio before ball milling. Fig. 1d illustrates the Raman spectra of pristine graphite and ball-milled sample. The pristine graphite appears a strong graphite band at 1580 cm−1 and a weak band at 1360 cm−1 that suggests existence of structural defects [18]. For the milled sample, the increase in intensity ratio of the 1360 cm−1 band and 1580 cm−1 band is observed, which corresponds with an increase of lattice defects in the graphitic structure [19]. The morphology of Bi2Fe4O9 prepared by the solid-state method was observed by SEM, as shown in Fig. 2a and b. It can be seen that the sample displays irregular sintered block shape. When Bi2Fe4O9 was milled with graphite (Fig. 2c and d), it showed irregular agglomeration. These results are also consistent with the above particle size distribution. The TEM (Fig. 2e) image shows that the Bi2Fe4O9 particles are coated with carbon layer. The HRTEM image is shown in Fig. 2f. The interplanar spacing is 0.308 nm, which corresponds to the (2 1 1) plane of the triclinic Bi2Fe4O9. 2
Inorganic Chemistry Communications 113 (2020) 107816
J. Zhang, et al.
Fig. 1. (a) XRD patterns, (b) particle size distribution of the synthesized Bi2Fe4O9 and ball milling with graphite, (c) TG analysis of the Bi2Fe4O9@C composite under air atmosphere (d) Raman spectra for pristine graphite and Bi2Fe4O9@C composite.
Fig. 2. SEM images of (a, b) Bi2Fe4O9, (c, d) Bi2Fe4O9@C, (e) TEM and (f) HRTEM image of Bi2Fe4O9@C.
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Fig. 3. Typical charge-discharge curves at 100 mA g−1 and CVs for the initial three cycles at scan rate of 0.1 mV s−1 of (a, c) Bi2Fe4O9 and (b, d) Bi2Fe4O9@C (e) cycling performances at 100 mA g−1 (f) rate performances.
Fig. 4. (a) XPS of the Fe 2p at the initial stage and after cycles for the Bi2Fe4O9@C electrode, SEM images of (b) pristine Bi2Fe4O9@C electrode and (c) Bi2Fe4O9@C electrode after 80 cycles at 100 mA g−1.
respectively. Both surfaces show a dense structure without significant differences, indicating that carbon layer-coating can effectively alleviate the stress during cycling.
Acknowledgments Financial supports from the Doctoral fund of Shanxi Normal University (0505/02070305) and 1331 project of Shanxi province (0109/02020001).
4. Conclusions
Appendix A. Supplementary material
In this paper, Bi2Fe4O9@C composite was synthesized via solid-state reaction accompanied by mechanical ball-milling method and investigated as new anode materials for LIBs. The results show that the electrochemical performances of Bi2Fe4O9@C composite were obviously enhanced by composite with carbon. The discharge capacity is 743 mAh g−1 after 80 cycles test at 100 mA g−1, which was more than the pure Bi2Fe4O9 electrode (142 mAh g−1). In addition, the reversible delithiation capacity of Bi2Fe4O9@C composite is up to 551 mAh g−1 at 2 A g−1.
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Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work.
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