Fabrication of free-standing NiCo2O4 nanoarrays via a facile modified hydrothermal synthesis method and their applications for lithium ion batteries and high-rate alkaline batteries

Materials Research Bulletin 63 (2015) 211–215

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Short communication

Fabrication of free-standing NiCo2O4 nanoarrays via a facile modified hydrothermal synthesis method and their applications for lithium ion batteries and high-rate alkaline batteries Qingyun Zheng * , Xiangyang Zhang, Youming Shen Department of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde, Hunan 415000, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 November 2014 Accepted 8 December 2014 Available online 10 December 2014

Self-supported NiCo2O4 nanoflake arrays on nickel foam are prepared by a facile hydrothermal method. The obtained NiCo2O4 nanoflakes with thicknesses of
25 nm grow vertically to the nickel foam substrate and form an interconnected porous network with pore diameters of 50–500 nm. As anode material of LIBs, the NiCo2O4 nanoflake arrays show a high initial coulombic efficiency of 76%, as well as good cycling stability with a capacity of 880 mAh g1 at 0.5 A g1, and 523 mAh g1 at 1.5 A g1 after 50 cycles. As the cathode of alkaline batteries, a high capacity of 95 mAh g1 is achieved at 2 A g1 and 94% retention is maintained after 10,000 cycles. The superior electrochemical performance is mainly due to the unique nanoflake arrays structure with large surface area and shorter diffusion length for mass and charge transport. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: Oxides Thin films Nanostructures Energy storage Electrochemical properties

1. Introduction Over the past decades, the emerging need for green energy and sophisticated electronics has motivated scientific researchers to explore and fabricate new classes of nanomaterials with enhanced properties. A significant progress in nanomaterials is a smart construction of bespoke nanostructures with various materials [1–3]. Among the explored systems, self-supported micro/nanoarrays are of both fundamental and technological interest because the functionalities derived from their unique array architectures enable them to hold a huge promise for applications in electronics, photonics and electrochemical energy storage devices. The realization of these devices is highly dependent on the ability to scrupulously design and fabricate high-quality micro/nanoarrays. Nanostructured nickel cobalt oxide (NiCo2O4) is of great importance in many applications such as supercapacitors [4,5], oxygen reduction reaction [6], Li-air batteries [7], alkaline batteries and lithium ion batteries (LIBs) [8,9]. Over the recent years, in particular, NiCo2O4 has been widely studied as anode materials for LIBs and cathode for alkaline batteries due to its high conductivity, high capacity/capacitance, and a good cycling life. The performance of batteries is mainly determined by the electrochemical activity and kinetic feature of the electrodes [10,11]. To improve the

* Corresponding author. Tel.: +86 736 78186715; fax: +86 736 78186716. E-mail address: [email protected] (Q. Zheng). http://dx.doi.org/10.1016/j.materresbull.2014.12.024 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.

power and energy density of batteries, it is crucial to enhance the kinetics of ion and electron transport in electrodes and at the electrode/electrolyte interface [12,13]. Therefore, the electrodes with proper pore structure and good electrical conductivity are highly desirable. Recently, fabrication of vertically aligned transition metal oxide arrays directly grown on conductive substrate represents the feasibility of improving the electrode kinetics and reaction activity [14]. Especially, self-supported nanoflake structures have been considered as one of the most promising structures due to their higher surface-to-volume ratio than other one-dimensional nanostructures such as nanowires and more difficult for aggregation in comparison with nanoparticles. Meanwhile, for LIBs application, despite high capacity of NiCo2O4 (theoretic capacity of
890 mAh g1), its practical use is still hindered by large initial irreversible loss and poor capacity retention resulting from the large specific volume change causing pulverization and deterioration of active materials over extended cycling [4,7,8]. Fortunately, this problem can be overcome by constructing nanoflake architectures with the ability of buffering the volume change upon cycles. In addition, for high-rate alkaline battery application, these nanoporous structures are favorable for improving high-rate characteristics because the nanoflake porous architecture could provide shortened ion diffusion path as well as lowered inner stress leading to an enhanced cycling life. Prompted by these interests, to date, lots of NiCo2O4 nanostructures, including nanoparticles [15], nanowires [9], nanorods

212

Q. Zheng et al. / Materials Research Bulletin 63 (2015) 211–215

[16], and nanospheres [17], have been synthesized by various routes and demonstrated to exhibit superior characteristics to the bulk counterparts. Previously, Jadhav et al. [18] and Mondal et al. [19] reported NiCo2O4 nanowire and nanobelt powder materials with enhanced performance (a high reversible capacity of 799 mAh g1). However, in the case of powder materials for batteries, the active materials need to be mixed with carbon and polymer binders and compressed into pellets. This process risks negating the benefits associated with the reduced particle size and introduces supplementary, undesirable interfaces. In the present work, we develop a novel modified hydrothermal method for synthesis of self-supported NiCo2O4 nanoflake arrays with hierarchical pore system. The obtained NiCo2O4 nanoflake arrays are evaluated as anode of LIBs and cathode of alkaline batteries and show good electrochemical properties with high capacity. 2. Experimental All solvents and chemicals were of reagent quality and used without further purification. The nickel nitrate, ammonium fluoride and urea were obtained from Shanghai Chemical Reagent Co. All aqueous solutions were freshly prepared with high purity water (18 MV cm resistance). In a typical synthesis of NiCo2O4 nanoflake arrays, 1 mmol Ni(NO3)2, 2 mmol Co(NO3)2, 6 mmol of NH4F, 1 mmol cetyl trimethylammonium bromide (CTAB) and 15 mmol of CO(NH2)2 were dissolved in 70 mL of water under stirring, respectively. Then, the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave. Then, a piece of clean nickel foam was immersed into the reaction solution. Hereafter, the autoclave was sealed and maintained at 120  C for 6 h, and allowed to cool down to room temperature spontaneously. After the reaction, the substrate was taken out, completely washed with deionized water, and dried in the air. Finally, the substrate was annealed at 350  C in common argon for 2 h.

The morphology and microstructure of the sample was characterized by a field emission scanning electron microscopy (FESEM, Hitachi S-4700), transmission electron microscopy (TEM, JEM 200CX 200 kV), and X-ray diffraction (XRD, Philips PC-APD with Cu Ka radiation). The loading weight for the sample was approximately 2.5 mg cm2. 2.1. Electrochemical measurements for LIBs Test cells were assembled in a glove box filled with argon using the NiCo2O4 nanoflake arrays as the working electrode, Li foil as the counter electrode, and polypropylene film as the separator. The electrolyte was a mixed solution containing ethylene carbonate and diethyl carbonate, in which dissolved 1 mol L1 of LiPF6. The galvanostatic charge/discharge tests were conducted on LAND battery program-control test system from 0.02 to 3.0 V (versus Li/ Li+) at room temperature (25  1  C). Cyclic voltammetry (CV) tests were carried out using the CHI660C electrochemical workshop at a scanning rate of 0.1 mV s1. 2.2. Electrochemical measurements for alkaline batteries test The electrochemical measurements were carried out in a threeelectrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV) measurements were performed on a CHI660c electrochemical workstation (Chenhua, Shanghai). CV measurements were carried out at different scanning rates at 25  C, NiCo2O4 nanoflake arrays grown on the nickel foam were used as the working electrodes, Hg/HgO as the reference electrode and a Pt foil as the counter-electrode. The electrolyte was 2 M KOH. The galvanostatic charge/discharge tests were conducted on a LAND battery program-control test system. The NiCo2O4 nanoflake arrays electrode, together with a nickel

Fig. 1. (a) XRD pattern and (b and c) SEM images of NiCo2O4 nanoflake arrays on nickel foam.

Q. Zheng et al. / Materials Research Bulletin 63 (2015) 211–215

213

Fig. 2. (a and b) TEM image and SAED pattern of NiCo2O4 nanoflake.

mesh counter electrode and an Hg/HgO reference electrode were tested in a three-compartment system. Specific capacity could also be calculated from the galvanostatic discharge curves using the following equation: C specific ¼

Q IðDt=3600Þ I Dt ¼ ¼ M M 3600M

(1)

1

where C (mAh g ) was the specific capacity, Q was the quantity of charge, I (mA) represented the discharge current, and M (g), Dt (s) designated the mass of active materials and total discharge time, respectively.

Fig. 1a shows the XRD pattern of NiCo2O4 nanoflake arrays grown on nickel foam. Obviously, except for three representative peaks from nickel foam, the other diffraction peaks at 18.9, 31.1, 36.6, 55.4, 59.02, 64.9 correspond to the (111), (2 2 0), (3 11), (4 2 2), (5 11), (4 4 0) crystal planes of spinel NiCo2O4 phase (JCPDS 20-0781), respectively, indicating that the crystalline NiCo2O4 film has formed after heat treatment. Fig. 1b–d shows the SEM images of NiCo2O4 nanoflake arrays. Notice that the entire surface of nickel foam is uniformly covered by interconnected two-dimensional

b 1.0 1.56 V

2.30 V

+

0.5

Potential / V (vs. Li /Li)

Current density / mA cm

-2

a

3. Results and discussion

Anodic process

0.0 Cathodic process

-0.5

1.42V

-1.0

0.78 V

3.0 2.5 2.0 1.5 1.0 0.5 0.0

-1.5 0.0

0.5

1.0

1.5

2.0

2.5

3.0

200

c

d

800

1st

1000

1200

1250 -1

800 600

0.5 A g -1 1.0 A g -1 1.5 A g -1 2.0 A g

-1

Capacity / mAh g !

-1

Capacity ! / mAh g

600

-1

0.5 A g -1 1.5 A g

1000

1000 750 500 250

400 200

400

Capacity / mAh g-1

+

Potential / V (vs. Li /Li) 1200

2nd

50th

0

0

10

20

30

Cycle number

40

50

0

0

5

10

15

20

25

30

35

Cycle number

Fig. 3. Electrochemical performance of NiCo2O4 nanoflake arrays as anode of LIBs: (a) CV at a scanning rate of 0.1 mV s1 at the second cycle. (b) Discharge/charge curves of NiCo2O4 nanoflake arrays. (c) Cycling performance of NiCo2O4 nanoflake arrays. (d) Rate capability.

214

Q. Zheng et al. / Materials Research Bulletin 63 (2015) 211–215

a

b

40

Potential / V vs. Hg/HgO)

ess roc cp i d o An

20 0

!

Current density / A cm

-2

60

-20

ess roc cp i d tho Ca

-40 -60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2 A/g 4 A/g 8 A/g 16 A/g 32 A/g

0.5 0.4 0.3 0.2 0.1 -20 0

20 40 60 80 100 120 140 160 180

Time / s

Potential / V (vs. Hg/HgO)

d

120

-1

120

Specific capacity/ mAh g

Specific capacity / mAh g

-1

c

100 80 60 40 20 0

0

5

10

15

20

25

30

35

100 80 60 40 20 0

0

2000

4000

6000

8000

10000

Cycle number

-1

Current density/A g

Fig. 4. Electrochemical properties of NiCo2O4 nanoflake arrays as cathode of alkaline batteries: (a) CV at a scanning rate 20 mV s1. (b) Discharge curves of NiCo2O4 nanoflake arrays at different current densities. (c) Specific capacity at different current densities. (d) Cycling performance of NiCo2O4 nanoflake arrays at 2 A g1.

nanoflake arrays (Fig. 1b and c). The as-prepared NiCo2O4 nanoflakes have thicknesses of
25 nm (Fig. 1d). The NiCo2O4 nanoflakes are connected with each other and form highly porous structure with pore diameters of 50–500 nm. Further insight into the microstructure of NiCo2O4 nanoflake is performed by TEM analysis (Fig. 2a and b). The individual NiCo2O4 nanoflake shows a smooth structure and the selected area electronic diffraction (SAED) pattern of the NiCo2O4 nanoflake are well indexed as NiCo2O4 phase, which is consistent with the XRD result above. As an anode for LIBs, the electrochemical properties of the NiCo2O4 nanoflake arrays were evaluated by cyclic voltammetry (CV) and galvanostatic discharge/charge measurements. It has been reported that the electrochemical reaction mechanism of Li with NiCo2O4 differs from the classical mechanisms of reversible lithium ion insertion/deinsertion into host structures or on lithium alloying reactions, but involves the formation and decomposition of Li2O accompanying the reduction and oxidation of metal Ni and Co. The involved electrochemical reactions are described as follows [18–21]: NiCo2O4 + 8Li+ + 8e ! 2Co + Ni + 4Li2O

(2)

Ni + Li2O $ NiO + 2Li+ + 2e

(3)

Co + Li2O $ CoO + 2Li+ + 2e

(4)

CoO + 1/3Li2O $ 1/3Co3O4 + 2/3Li+ + 2/3e

(5)

Fig. 3a shows the CV curve of the NiCo2O4 nanoflake arrays at a scanning rate of 0.1 mV s1. The NiCo2O4 nanoflake arrays exhibit two reduction peaks in the cathodic process and two oxidation peaks in the anodic process, respectively. The intense reduction peaks at 0.78 V and 1.42 V can be ascribed to the reduction of Ni and Co ions into metallic Ni and Co, respectively, and the formation of amorphous Li2O and partially reversible solid electrolyte interface (SEI) layer [22,23]. During the following anodic process, the peak at 1.56 V is attributed to the partial decomposition of SEI layer. The broad peak around 2.30 V corresponds to the decomposition of Li2O and oxidation of metal Ni and Co leading to the reformation of nickel oxide and cobalt oxide [18,19]. The discharge/charge curves of the NiCo2O4 nanoflake arrays at 0.5 A g1 are shown in Fig. 3b. Two discharge voltage plateaus and a sloping potential range are noticed during the first discharge. The long discharge plateau at 0.75 V corresponds to the reduction of NiCo2O4 to metallic Ni and Co nano-domains embedded in the Li2O matrix, which is consistent with the CV result. The first discharge capacity of the NiCo2O4 nanoflake arrays is 1158 mAh g1, higher than the theoretical value (
890 mAh g1), which is due to the formation of solid electrolyte interphase (SEI) in the first discharge. This phenomenon exists in all transition metal oxides and binary metal oxides, including NiO [24], Fe3O4 [25], CoO [26], Co3O4 [27], CuO [28], and ZnCo2O4 [29]. The nanoflake arrays exhibit a capacity of 880 mAh g1 at the first charge process, less than that of the first discharge process above. The irreversible capacity loss is attributed to the incomplete decomposition of both of the SEI and Li2O. More importantly, Hu et al. [30] have done a careful research about the additional capacity of metal oxides. It is found that the additional capacity may come from reversible SEI formation, and the

Q. Zheng et al. / Materials Research Bulletin 63 (2015) 211–215

formation of LiOH and its subsequent reversible conversion to Li2O and LiH. In our case, the corresponding initial coulombic efficiency is
76%. The discharge specific capacity is 880 mAh g1 at the second cycle and remains at 657 mAh g1 after 50 cycles. These values are much better than the NiCo2O4 powder materials in the literature [18,19]. Fig. 3c shows the cycling properties of the NiCo2O4 nanoflake arrays. Observe that the NiCo2O4 nanoflake arrays present good cycling stability with a capacity of 646 mAh g1 at 0.5 A g1, and 523 mAh g1 at 1.5 A g1 after 50 cycles, maintaining 72.6% and 58.7% of the theoretical values, respectively. The NiCo2O4 nanoflake arrays also exhibit good high-rate capability at higher current rates. A reversible and steady capacity of 541 mAh g1 is obtained at 2 A g1, demonstrating its good capability for a high cycling rate (Fig. 3d). In addition, the electrochemical performances of the NiCo2O4 nanoflake arrays as cathode of alkaline batteries were also investigated. Fig. 4a shows the CV curve of NiCo2O4 nanoflake arrays at a scanning rate of 20 mV s1. The redox couple is attributed to the Faradaic redox reactions related to M—O/M—O—OH (M represents Ni or Co) [31]. The specific capacity values of the NiCo2O4 nanoflake arrays are calculated from the galvanostatic discharge curves at various current densities (Fig. 4b and c). The NiCo2O4 nanoflake arrays deliver a specific capacity of 95 mAh g1 at 2 A g1 to 66 mAh g1 at 32 A g1, respectively. It should be mentioned that the Ni foam shows a small contribution to the capacity arising from the oxidation of Ni. In our work, the contribution of the nickel foam has been subtracted. Additionally, the NiCo2O4 nanoflake arrays exhibit an excellent cycling stability with 94% capacity retention (89.5 mAh g1) after 10,000 cycles at 2 A g1 (Fig. 4d). In a nutshell, these results demonstrate the high specific capacity, superior high-rate capability of NiCo2O4 nanoflake arrays. The noticeable electrochemical performance is mainly due to the unique porous nanoflake arrays structure [32,33]. First, each nanoflake directly grown on nickel foam has its own contact with the substrate and ensures that every nanoflake participates in the electrochemical reaction. Second, the nanoflake arrays configuration provides fast ion/electron transfer, sufficient contact between active materials and electrolyte, and enhanced flexibility, resulting in reduced internal resistance and improved high-power performance. 4. Conclusions In summary, we have developed a facile hydrothermal method for preparation of self-supported NiCo2O4 nanoflake arrays with hierarchical porous structure. Due to their unique composition and architecture, the NiCo2O4 nanoflake arrays display excellent electrochemical performances in LIBs and alkaline batteries, respectively. Impressively, high capacity, good cycling stability and high rate capability are demonstrated for the above devices. The enhanced performance is due to the unique porous nanoflake arrays architecture with high electrode–electrolyte contact area and fast electron/ion diffusion path. With their ease of fabrication and good performance, the self-supported NiCo2O4 nanoflake

215

arrays will be interesting in potential applications for chemical sensing and catalysis. Acknowledgements This work was supported by the construct program of the key discipline in Hunan province (Applied Chemistry). References [1] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge, I.M. Bacho, J. Tu, H.J. Fan, Nanoscale 6 (2014) 5008. [2] G.X. Fan, H.L. Li, S.P. Dai, C.X. Zhang, X.M. Guan, Z.R. Chang, B.Z. Liu, Mater. Res. Bull. 57 (2014) 274. [3] S. Kabi, A. Ghosh, Mater. Res. Bull. 48 (2013) 3405. [4] J. Zhu, Z. Xu, B.N. Lu, Nano Energy 7 (2014) 114. [5] Y.R. Zhu, X.B. Ji, Z.P. Wu, W.X. Song, H.S. Hou, Z.B. Wu, X. He, Q.Y. Chen, C.E. Banks, J. Power Sources 267 (2014) 888. [6] Y. Xiao, C.G. Hu, L.T. Qu, C.W. Hu, M.H. Cao, Chem. Eur. J. 19 (2013) 14271. [7] L.X. Zhang, S.L. Zhang, K.J. Zhang, G.J. Xu, X. He, S.M. Dong, Z.H. Liu, C.S. Huang, L. Gu, G.L. Cui, Chem. Commun. 49 (2013) 3540. [8] B. Sun, X.D. Huang, S.Q. Chen, Y.F. Zhao, J.Q. Zhang, P. Munroe, G.X. Wang, J. Mater. Chem. A 2 (2014) 12053. [9] F.L. Lu, X.C. Cao, Y.R. Wang, C. Jin, M. Shen, R.Z. Yang, RSC Adv. 4 (2014) 40373. [10] X. Xia, D. Chao, Z. Fan, C. Guan, X. Cao, H. Zhang, H.J. Fan, Nano Lett. 14 (2014) 1651. [11] X. Xia, D. Chao, X. Qi, Q. Xiong, Y. Zhang, J. Tu, H. Zhang, H.J. Fan, Nano Lett. 13 (2013) 4562. [12] Q. Zhang, E. Uchaker, S.L. Candelaria, G. Cao, Chem. Soc. Rev. 42 (2013) 3127. [13] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Adv. Mater. 22 (2010) E28. [14] X.H. Xia, J.P. Tu, Y.Q. Zhang, X.L. Wang, C.D. Gu, X.B. Zhao, H.J. Fan, ACS Nano 6 (2012) 5531. [15] L.Y. Wang, L.H. Zhuo, C. Zhang, F.Y. Zhao, ACS Appl. Mater. Interfaces 6 (2014) 10813. [16] B. Sun, J.Q. Zhang, P. Munroe, H.J. Ahn, G. Wang, Electrochem. Commun. 31 (2013) 88. [17] J.F. Li, S.L. Xiong, Y.R. Liu, Z.C. Ju, Y.T. Qian, ACS Appl. Mater. Interfaces 5 (2013) 981. [18] H.S. Jadhav, R.S. Kalubarme, C.N. Park, J. Kim, C.J. Park, Nanoscale 6 (2014) 10071. [19] A.K. Mondal, D.W. Su, S.Q. Chen, X.Q. Xie, G.X. Wang, ACS Appl. Mater. Interfaces 6 (2014) 14827. [20] Y.J. Chen, J. Zhu, B.H. Qu, B.A. Lu, Z. Xu, Nano Energy 3 (2014) 88. [21] R. Ding, L. Qi, H.Y. Wang, RSC Adv. 3 (2013) 12581. [22] Y. Li, L.L. Zou, J. Li, K. Guo, X.W. Dong, X.W. Li, X.Z. Xue, H.F. Zhang, H. Yang, Electrochim. Acta 129 (2014) 14. [23] J. Liu, C.P. Liu, Y.L. Wan, W. Liu, Z.S. Ma, S.M. Ji, J.B. Wang, Y.C. Zhou, P. Hodgson, Y.C. Li, CrystEngComm 15 (2013) 1578. [24] Y.F. Yuan, X.H. Xia, J.B. Wu, J.L. Yang, Y.B. Chen, S.Y. Guo, Electrochem. Commun. 12 (2010) 890. [25] Q.Q. Xiong, Y. Lu, X.L. Wang, C.D. Gu, Y.Q. Qiao, J.P. Tu, J. Alloys Compd. 536 (2012) 219. [26] C. Guan, X.H. Xia, N. Meng, Z.Y. Zeng, X.H. Cao, C. Soci, H. Zhang, H.J. Fan, Energy Environ. Sci. 5 (2012) 9085. [27] X.H. Xia, Q.Q. Xiong, Y.Q. Zhang, J.P. Tu, C.F. Ng, H.J. Fan, Small 10 (2014) 2419. [28] D.W. Su, X.Q. Xie, S.X. Dou, G.X. Wang, Sci. Rep. 4 (2014) . [29] C.W. Lee, S.D. Seo, D.W. Kim, S. Park, K. Jin, D.W. Kim, K.S. Hong, Nano Res. 6 (2013) 348. [30] Y.-Y. Hu, Z. Liu, K.-W. Nam, O.J. Borkiewicz, J. Cheng, X. Hua, M.T. Dunstan, X. Yu, K.M. Wiaderek, L.-S. Du, K.W. Chapman, P.J. Chupas, X.-Q. Yang, C.P. Grey, Nat. Mater. 12 (2013) 1130. [31] C.Z. Yuan, J.Y. Li, L.R. Hou, X.G. Zhang, L.F. Shen, X.W. Lou, Adv. Funct. Mater. 22 (2012) 4592. [32] X. Xia, C. Zhu, J. Luo, Z. Zeng, C. Guan, C.F. Ng, H. Zhang, H.J. Fan, Small 10 (2014) 766. [33] X.H. Xia, J.P. Tu, Y.Q. Zhang, J. Chen, X.L. Wang, C.D. Gu, C. Guan, J.S. Luo, H.J. Fan, Chem. Mater. 24 (2012) 3793.