Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries

Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries

Available online at www.sciencedirect.com CERAMICS INTERNATIONAL Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint Enhanced ...

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CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries Mengmeng Lao, Jie Shun, Lianyi Shao, Xiaoting Lin, Kaiqiang Wu, Miao Shuinn, Peng Li, Nengbing Long, Yuanlong Ren Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province, People's Republic of China Received 27 March 2014; received in revised form 3 April 2014; accepted 4 April 2014

Abstract CuCr2O4/Ag is prepared by a preliminary sol-gel formation of CuCr2O4 and a subsequent chemical deposition of Ag on the surface by electroless plating. The surface morphology, structure and electrochemical behaviors of CuCr2O4/Ag and CuCr2O4 are observed and compared by various analytical techniques. It is obvious that the electrochemical property of CuCr2O4 is enhanced by Ag coating layer. The as-prepared CuCr2O4/Ag composite shows an initial discharge specific capacity of 495.6 mA h g  1, and its coulombic efficiency evolves from 27.7% to 97.8% in the initial 40 cycles. In contrast, the initial discharge specific capacity and coulombic efficiency of CuCr2O4 are 372.8 mAh g  1 and 21.7%, respectively. During the following 40 cycles, the cycling coulombic efficiency for CuCr2O4 is merely 94.3%. Besides, the reversible charge capacity of CuCr2O4/Ag is 133.9 mA h g  1 after 40 cycles. For comparison, the reversible lithium storage capacity of bare CuCr2O4 is only 100.5 mA h g  1. It suggests that the improved reversible capacity and cycling performance should be ascribed to the Ag coating layer, which improves the electronic conductivity of CuCr2O4 and may suppress the volume change of CuCr2O4 particles during repeated cycles. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sol-gel processes; D. Batteries; Anode material; CuCr2O4; Ag coating

1. Introduction Lithium-ion batteries (LIBs) have been recognized as important energy storage devices and are widely used in both portable electronics and electric vehicles owing to their superior electrochemical performances including high power, high energy density, long cycling life and no memory effect [1]. However, the specific power and energy densities are still too poor to satisfy the industry needs, such as electric vehicles. Therefore, discovering novel electrode materials with better performance is the key to develop advanced lithium-ion batteries. Transition metal oxides with high specific capacity (600– 800 mA h g  1) are promising anode materials to replace the conventional graphite. Recently, some delafossite oxides, such n

Corresponding author. Tel.: þ86 574 87600787; fax: þ 86 574 87609987. Corresponding author. E-mail addresses: [email protected], [email protected] (J. Shu), [email protected] (M. Shui). nn

as CuCrO2 [1], CuFeO2 [2], CuFe2O4 [3,4] and CuCr2S4 [5], have shown reversible electrochemical conversion reactions with Li, but no reports about the possibility of employing CuCr2O4 as anode material for lithium-ion batteries could be found. Therefore, the feasibility of Cr-based compounds as anode materials for lithium-ion batteries remains interesting. However, these novel anode materials, such as Cr2O3, may possess poor electrochemical performance because of their poor electronic conductivity and huge volume changes of active materials during lithium insertion/extraction process. It has been reported that the coating of Ag on the electrode materials, such as LiFePO4 [6], LiCoO2 [7], LiMn2O4 [8] and Li4Ti5O12 [9,10], can increase the electronic conductivity without sacrificing power and energy densities, which means improving the rate performance and cycling stability. There fore, we try to improve the electrochemical performances of Cr-based compounds by Ag coating. In this study, CuCr2O4 is prepared via a sol-gel method using citric acid as a chelating agent. After that, it is coated by

http://dx.doi.org/10.1016/j.ceramint.2014.04.025 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: M. Lao, et al., Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.025

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Ag using electroless plating. A preliminary study is performed to identify the Li-storage characteristics for CuCr2O4/Ag as anode material for lithium-ion batteries compared to pure CuCr2O4. 15

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2. Experimental In the sample preparation, all the chemicals were analytical grade reagents (AR). The typical procedure was as follow, stoichiometric amounts of Cu(NO3)2  3H2O (AR, 99%) and Cr (NO3)3  9H2O (AR, 99%) were dissolved in citric acid solution under continuous magnetic stirring, and the molar ratio of citric acid to the total metal ions was 2:1. The pH of the mixed solution was adjusted to 7 by adding NH3  H2O (AR, 25%), and then was heated to obtain viscous gels. The viscous gels were dried at 120 1C for 24 h in a vacuum oven after stirring for 2 h, resulting in the formation of foamy dark powders. Finally, the above precursors were calcined at 900 1C for 24 h in argon (Ar) to obtain the final black products. The CuCr2O4/Ag composite was prepared as the following procedure, the prepared CuCr2O4 powder was added into a dilute glucose solution under vigorous stirring to obtain a suspension. Simultaneously, dilute NH3  H2O solution was titrated into an AgNO3 solution (0.01 mol L  1) to obtain Ag(NH3)2OH solution. Then the above suspension and solution were blended together under continuous magnetic stirring at 60 1C. After that, CuCr2O4/Ag composite was obtained by separation with a centrifuge and repeated rinsing by distilled water, and then dried at 80 1C under vacuum. The amount of glucose was about 10 wt% excessive to ensure the complete reduction of Ag þ to Ag. The weight ratio of CuCr2O4:Ag was 9:1. Differential scanning calorimetric analysis (DSC) and thermogravimetric analysis (TG) of the as-prepared precursor for CuCr2O4 were carried out on a NETZSCH STA 449C TG-DSC apparatus with a heating rate of 5 1C min  1 in flowing Ar. The crystalline structure of the samples was measured by X-ray Diffraction (XRD) using a Bruker D8 Focus diffractometer with nickel-filtered Cu Kα radiation (λ=1.5418 Å), operating at 40 kV and 40 mA. The morphology of the samples was observed by a L30 S-FEG field emission scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) detector (FEI, USA), conducting at 10 kV. The particle structure after

Fig. 3. SEM images of CuCr2O4 (a, b) and CuCr2O4/Ag (c, d). Please cite this article as: M. Lao, et al., Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.025

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(224) faces of the hexagonal structure(PDF reference code 34– 0424), appear in both patterns at 35.31, 37.91, 31.31, 29.81, 56.41, 61.71 and 65.01, respectively. In comparison, the diffraction peaks of Ag appear at 38.21 and 44.41 in the XRD pattern of CuCr2O4/ Ag, corresponding to the (111) and (200) faces (PDF reference code 87–0720), which indicates that Ag is successfully introduced in the composite. SEM images of CuCr2O4 before and after Ag coating are displayed in Fig. 3a–d. It is clear that CuCr2O4 particles are aggregated to some extent after calcination as shown

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cycles was performed with a JEOL JEM 2010 transmission electron microscopy (TEM). The electrodes for electrochemical performance testing were prepared by mixing active material (CuCr2O4/Ag or CuCr2O4), carbon black and polyvinylidene fluoride in a weight ratio of 8:1:1. After that, the mixture was dissolved in N-methylpyrolline to get slurry, and then coated onto a copper foil with the doctor blade technology. The as-prepared film was dried in vacuum oven at 120 1C for 12 h, and then cut into discs with a diameter of 15 mm. Simulated batteries for electrochemical testing were assembled in an Ar-filled Etelux glove box. In the simulated batteries, metal lithium foil was acted as counter electrode, 1 mol L  1 LiPF6 in a 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate was used as electrolyte and Whatman grass fiber was treated as separator. The moisture content and oxygen level in the glove box were controlled below 5 ppm. Charge/discharge cycles were performed at a constant current density of 50 mA g  1 in the voltage range from 0.0 to 3.0 V using multichannel Land Battery Test System. The electrochemical impedance spectra (EIS) were performed on a CHI 660D electrochemical workstation. All the electrochemical tests were carried out at room temperature.

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Fig. 1 shows the TG-DSC curves of CuCr2O4 precursor from sol-gel process. It is obvious that a weight loss of 10.0% can be observed when the temperature was increased to 200 1C, corresponding to the appearance of an exothermal peak at 170.1 1C. This weight loss is probably contributed to the loss of crystalline water in the precursor. Between 200 and 350 1C, another exothermal peak can be detected at 260.7 1C, corresponding to a weight loss of 57.9%. It suggests that the precursor decomposed and transformed into intermediate phases. After that, a weak exothermal peak can be observed at 417.3 1C, corresponding to the weight loss of 0.6%. The final product was formed at about 900 1C after a weight loss of 1.5% between 800 and 900 1C. The XRD patterns of CuCr2O4/Ag and CuCr2O4 powders are presented in Fig. 2. It shows that the typical peaks of CuCr2O4, corresponding to the (211), (202), (112), (200), (303), (400) and

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Fig. 5. The 1st (a), 10th (b) and 20th (c) charge/discharge curves of CuCr2O4/ Ag and bare CuCr2O4.

Please cite this article as: M. Lao, et al., Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.025

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in Fig. 3a and b, and their surfaces are glossy and smooth. After Ag electroless plating, Fig. 3c and d indicates that some nanosized particles are evenly distributed on the surface of CuCr2O4, which can be ensured as Ag according to the results of XRD analysis, and revealing that Ag is really coated on the surface of CuCr2O4. EDS result shows that the weight percent of Ag in the composite is about 6.28% as shown in Fig. 4.

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Fig. 5 provides the change/discharge curves of CuCr2O4/Ag and CuCr2O4 in the initial 20 cycles at 50 mA g  1 between 0.0 and 3.0 V. The voltage profiles of CuCr2O4/Ag are similar to bare CuCr2O4. It is clear that the lithiation/delithiation voltages are both at around 0.83 V. During the first discharge, the working voltage of CuCr2O4/Ag falls rapidly from the open circuit voltage (2.91 V) to 0.91 V, followed by a long plateau at about 0.85 V, and then drops slowly from 0.7 to 0.0 V. In comparison, the working voltage of CuCr2O4 decreases quickly from 2.89 V (open circuit voltage) to 0.83 V, and the following behavior is similar to CuCr2O4/Ag. However, the plateau of CuCr2O4/Ag is longer than that of CuCr2O4 during the initial discharge. These lithiation behaviors of both CuCr2O4/Ag and CuCr2O4 correspond to the reduction peak at 0.83 V as the differential capacity curves shown in Fig. 6a. It suggests that the lithium storage mechanism of CuCr2O4 is similar to that of CuFe2O4 or CuCr2S4 [3–5]. Upon the reverse charge, CuCr2O4/Ag displays an inclined slop between 0.0 and 0.25 V and a short plateau at 0.31 V, corresponding to two oxidation peaks at 0.15 and 0.33 V in Fig. 6b. In comparison, the reverse charge plot of CuCr2O4 is nearly vertical and no plateau or slop can be found on the curve. Besides, there is no oxidation peak at 0.33 V can be found for CuCr2O4, but a small oxidation peak at 0.17 V in Fig. 6b.

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Fig. 8. Cycling performance and corresponding coulombic efficiency of CuCr2O4/Ag and CuCr2O4.

Please cite this article as: M. Lao, et al., Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.025

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Fig. 9. TEM images of CuCr2O4/Ag and CuCr2O4 particles after cycling.

Fig. 5 also delivers that the discharge capacity of CuCr2O4/Ag in the first cycle is 495.6 mA h g  1, which is much higher than that of CuCr2O4 (372.8 mA h g  1). It is obvious that the lithiation/ delithiation plateaus also change with each cycle, corresponding to the differential capacity curves in Fig. 6. Upon repeated cycles, the reduction and oxidation peaks gradually decrease and become weak. Furthermore, Fig. 7 provides the EIS patterns before cycles, revealing that Ag coating obviously decreases the electrochemical resistances of CuCr2O4. Based on the simulated data from equivalent circuit, it can be found that the electrolyte resistance (Re) of bare CuCr2O4 is 6.1 Ω, which is a little larger than that of Ag-coated CuCr2O4 (5.4 Ω). Moreover, the difference in charge transfer resistance (Rct) between bare CuCr2O4 and CuCr2O4/Ag is huge. CuCr2O4/Ag shows the Rct of 56.2 Ω. In contrast, bare CuCr2O4 reveals a larger Rct of 78.6 Ω. All the results discussed above show that Ag coating does not obviously change the lithiation/delithiation characteristics of CuCr2O4, but actually improves the reversible capacity and cycling performance of CuCr2O4 anode. The cycling performance and corresponding efficiency property of CuCr2O4/Ag and CuCr2O4 are displayed in Fig. 8. It shows that the initial charge capacity of CuCr2O4/Ag is much higher than that of CuCr2O4 (137.2 mA h g  1 vs. 80.9 mA h g  1), and the tendency of reversible charge capacity is also similar to the initial charge capacity, resulting in that the initial coulombic efficiency of CuCr2O4/Ag (27.7%) is 6% higher than that of CuCr2O4 (21.7%). The capacity loss for both CuCr2O4/Ag and CuCr2O4 may come from the pulverization of active particles as the TEM images shown in Fig. 9. It can also be found that the pulverization of CuCr2O4 particles is more serious that that of CuCr2O4/Ag. It indicates that Ag coating layer may suppress the volume change of CuCr2O4 during repeated cycles. All the plots of both CuCr2O4/ Ag and CuCr2O4 keep almost plane after the initial cycle, but the specific charge capacity of CuCr2O4/Ag is visibly higher than that of CuCr2O4. After 40 cycles, the reversible charge capacity of CuCr2O4/Ag is 133.9 mA h g  1. In contrast, the reversible lithium storage capacity of bare CuCr2O4 is 100.5 mA h g  1. The much improved reversible capacity and coulombic efficiency are contributed to the existence of conductive Ag particles which improve the electronic conductivity and make CuCr2O4 react with Li completely in the following lithiation/delithiation processes. According

to the cycling performance as shown in Fig. 8, the coulombic efficiency of CuCr2O4 increases from 21.7% to 94.3% in the initial 40 cycles while CuCr2O4/Ag presents higher initial coulombic efficiency of 27.7%, and then this parameter improves to 97.8% after 40 cycles. So it can be concluded that Ag coating may facilely improve the cycling performance and efficiency property of CuCr2O4. 4. Conclusions In this paper, CuCr2O4/Ag is prepared by a preliminary sol-gel formation of CuCr2O4 and a subsequent chemical deposition of Ag on the surface. Electrochemical results show that CuCr2O4/Ag can deliver an initial discharge specific capacity of 495.6 mA h g  1 and remains a reversible charge capacity of 133.9 mA h g  1 after 40 cycles, which are much higher than those of bare CuCr2O4 (372.8 and 100.5 mA h g  1). Besides, the cycling coulombic efficiency of CuCr2O4 is also improved from 94.3% to 97.8% after Ag coating. The improved cycling stability of CuCr2O4 after Ag coating is ascribed to the improvement of electronic conductivity and the suppression of volume expansion during lithiation process. Therefore, Ag coating is a facile method to improve the reversible capacity and cycling performance of CuCr2O4 anode. Acknowledgments This work is sponsored by National Natural Science Foundation of China (No. 51104092) and National 863 Program (2013AA05 0901). The work is also supported by K.C. Wong Magna Fund in Ningbo University. References [1] X.D. Zhu, J. Tian, S.R. Le, J.R. Chen, K.N. Sun, Enhanced electrochemical performances of CuCrO2-CNTs nanocomposites anodes by insitu hydrothermal synthesis for lithium ion batteries, Mater. Lett. 107 (2013) 147–149. [2] L. Lu, J.Z. Wang, X.B. Zhu, X.W. Gao, H.K. Liu, High capacity and high rate capability of nanostructured CuFeO2 anode materials for lithium-ion batteries, J. Power Sources 196 (2011) 7025–7029.

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[3] Z. Xing, Z. Ju, J. Yang, H. Xu, Y. Qian, One-step solid state reaction to selectively fabricate cubic and tetragonal CuFe2O4 anode material for high power lithium ion batteries, Electrochim. Acta 102 (2013) 51–57. [4] Y. Ding, Y. Yang, H. Shao, Synthesis and characterization of nanostructured CuFe2O4 anode material for lithium ion battery, Solid State Ion. 217 (2012) 27–33. [5] V. Bodenez, L. Dupont, L. Laffont, A.R. Armstrong, K.M. Shaju, P.G. Bruce, J.M. Tarascon, The reaction of lithium with CuCr2S4-lithium intercalation and copper displacement/extrusion, J. Mater. Chem. 17 (2007) 3238–3247. [6] C.H. Mi, Y.X. Cao, X.G. Zhang, X.B. Zhao, H.L. Li, Synthesis and characterization of LiFePO4/(Ag þC) composite cathodes with nanocarbon webs, Powder Technol. 181 (2008) 301–306.

[7] S. Huang, Z. Wen, X. Yang, Z. Gu, X. Xu, Improvement of the high-rate discharge properties of LiCoO2 with the Ag additives, J. Power Sources 148 (2005) 72–77. [8] W.J. Zhou, B.L. He, H.L. Li, Synthesis, structure and electrochemistry of Ag-modified LiMn2O4 cathode materials for lithium-ion batteries, Mater. Res. Bull. 43 (2008) 2285–2294. [9] Z. Liu, N. Zhang, Z. Wang, K. Sun, Highly dispersed Ag nanoparticles ( o10 nm) deposited on nanocrystalline Li4Ti5O12 demonstrating highrate charge/discharge capability for lithium-ion battery, J. Power Sources 205 (2012) 479–482. [10] S. Huang, Z. Wen, J. Zhang, Z. Gu, X. Xu, Li4Ti5O12/Ag composite as electrode materials for lithium-ion battery, Solid State Ion. 177 (2006) 851–855.

Please cite this article as: M. Lao, et al., Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.025