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Two-pot synthesis of one-dimensional hierarchically porous Co3O4 nanorods as anode for lithium-ion battery
Journal of Alloys and Compounds 735 (2018) 2446e2452
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Two-pot synthesis of one-dimensional hierarchically porous Co3O4 nanorods as anode for lithium-ion battery Xiao Li a, b, Xiaodong Tian a, Tao Yang a, b, Yan Song a, *, Yiming Liu c, **, Quangui Guo a, Zhanjun Liu a a b c
CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China University of Chinese Academy of Sciences, Beijing, 100049, China Shan Xi Academy of Analytical Science, Taiyuan, 030001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 September 2017 Received in revised form 29 November 2017 Accepted 1 December 2017 Available online 5 December 2017
Poor cyclic stability and low rate capability are two severe stumbling blocks for Co3O4-based anodes due to the insufficient structural stability of the electrode, difficult access of electrolyte to the electrode interior as well as dense structure. In this respect, one-dimensional hierarchically porous Co3O4 nanorods (HP-Co3O4 NR), which are constructed by interconnected nanoparticles, are successfully synthesized via a facile hydrothermal method following calcination. In the architecture, (i) nanoparticle-decoration is favorable to shorten the ions and charge diffusion distance, which improve the rate performance; (ii) the hierarchically porous structure can sufficiently buffer the volume change during the extended cycling, which is conducive to stable electrode structure and good cyclic stability; (iii) 1D porous nanorods ensure more exposed active sites and the contact area of the electrolyte/electrode for lithium storage, which endows the high reversible capacity. As a result, the HP-Co3O4 NR material exhibits high capacity (628 mAh g1 at 1 A g1 after 350 cycles), exceptional rate capability (247 mAh g1 at 6 A g1) and long cycle life (0.068% capacity decay per cycle after 600 cycles at 5 A g1), simultaneously. Our results indicate that the one-dimensional hierarchically porous nanoparticle-decorated nanorods structure is beneficial for improving the cycling and rate performance. © 2017 Elsevier B.V. All rights reserved.
Keywords: Cobalt oxide Hierarchically porous One-dimensional Lithium-ion battery
1. Introduction Lithium ion batteries (LIBs) are vital for developing high performance technologies, from consumer electronics, vehicles, to smart grid scale energy storage [1,2]. Unfortunately, the state-ofthe-art graphite-based LIBs are plagued by low capacity, short lifespan to meet the specific requirements [3,4]. As a result, welldesigned transition metal oxide, Co3O4, has aroused intensive research interest as an appealing candidate electrode material for LIBs because of its higher theory specific capacity (890 mAh g1 vs. 372 mAh g1 of conventional graphite) [5e11]. However, like other transition metal oxides, inferior electrical conductivity and large volumetric variation during cycling processes always result in electrode pulverization, which is responsible for the poor rate performance and capacity fading, thus hamper the practical
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Song), [email protected] (Y. Liu). https://doi.org/10.1016/j.jallcom.2017.12.001 0925-8388/© 2017 Elsevier B.V. All rights reserved.
applications of Co3O4 as LIBs anode [12,13]. In this regard, considerable attentions have been paid to tackling these issues. Typical strategy relies on an appropriate structure to obey the following virtues: (i) sufficient space for volume change, (ii) effective channel for fast Liþ ion transportation, (iii) high conductivity for electron migration. Based on these considerations, the combination with conductive carbon materials [14e16] and unique Co3O4 nanostructure construction [17,18] were usually adopted. Although the introduction of carbon materials can enhance the conductivity of the electrode and alleviate the volume variation during the charge/discharge processes, the specific capacity of Co3O4 would be reduced due to the inactivity of the carbon as well as the decreasing of mass ratio of Co3O4 in the electrode [19,20]. Furthermore, the incomplete protection of Co3O4 by carbon can also be correlated to the unsatisfied performance. As comparison, Co3O4 with unique architecture may be more attractive. Various nanostructures including one-dimensional (1 D, e.g., nanorods, nanotube, nanowires and nanofibers) [21e24], two-dimensional (2 D, nanosheets) [17,19], and three-dimensional
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(3 D, nanocubes) [25e28], have been explored to verify the above assumption. Among them, 1D nanostructures draw great attention due to their short diffusion distance for ions/electrons. And they can also alleviate the strain and accommodate volume expansion upon the redox reactions, leading to the improved cyclic stability and rate capacity. For example, Co3O4 nanowires prepared by Li et al. exhibit 836 mAh g1 at the current of 200 mA g1 after three cycles when adopted as anode for LIBs [24]. Porous Co3O4 nanotube has been prepared by using carbon nanotubes [29] or electrospun polyacrylonitrile (PAN) nanofiber [21] as the template, and the as-obtained samples displayed the reversible capacity of 1200 mAh g1 at a current density of 50 mA g1 after 20 cycles and 856.4 mAh g1 at the rate of 0.25 C after 60 cycles, respectively. The literatures shown above demonstrate that the porous nanostructure displays better electrochemical performance than that of the solid one, indicating that hierarchically porous 1D nanostructure is proposed as an effective structure because of more electrochemical reaction sites and sufficient ion/electron diffusion routes as well as effective ability to buffer the volumetric change [22,30]. However, templates were used during the fabrication processes in preceding studies which enhanced the intricacy of the produce. Besides, the specific capacities were recorded at relatively low current density [11,12,22,31e35] and the rate capability as well as cyclic performance was limited [30,36e38], which cannot sufficiently meet the practical requirements. Therefore, using low cost produce method to design porous 1D architecture of Co3O4 with high specific capacity and long cycle life still remains challenge. Herein, a simple template-free self-assembly strategy was adopted to synthesize hierarchically porous 1D Co3O4 nanorods (HP-Co3O4 NR) by hydrothermal method. The as-prepared HP-Co3O4 NR are composed of uniform nanorods with the length of 30e40 mm stacked by discontinuous nanoparticles with the diameter of 20e50 nm, which can alleviate the resistance of the lithium diffusion and electrons transport and enhance the contact area between electrode and electrolyte, thus leading to high reversible capacity and good rate performance. The hierarchically porous structure is formed mainly by the release of CO2 and H2O and the decomposition of the precursor, which can buffer the huge volume change during the repeated conversion reactions, resulting in long cycling life. Merited by the large specific surface area and abundant pores, the as-prepared HP-Co3O4 NR delivered excellent rate capability as well as high specific capacity (628 and 412 mAh g1 at the current rate of 1 and 5 A g1 after 350 and 600 cycles, respectively). In addition, the influences of reaction time and NH4F reactant concentration on the morphology, especially on the pore size distribution, were also investigated. 2. Experimental section 2.1. Materials synthesis All the regents were purchased and directly used without further purification. The typical synthetic procedure as shown in Scheme 1, 5 mM of hydrated cobalt nitrate (Co(NO3)2$6H2O), 5 mM of urea (CO(NH2)2), 2 mM of ammonium fluoride (NH4F) were blended into 75 ml deionized water. After vigorous stirring for half hour, the resulting solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and maintained at 120 C for 6 h. After cooling to room temperature naturally, the obtained precipitates were washed with anhydrous alcohol and distilled water several times, and then dried at 60 C for 6 h. Finally, the precursor was annealed in Ar at 600 C for 2 h. For comparison, a series of reference samples were prepared by varying NH4F reactant concentrations, the obtained sample remarked as 1 mM-Co3O4, 1.5 mM-Co3O4, 2 mM-Co3O4 and 3 mM-Co3O4. In addition, time-dependent morphological evolution of the HP-Co3O4
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Scheme 1. The synthetize of HP-Co3O4 NR.
NR samples was investigated by collecting samples at different reaction stages (4 h and 8 h), the samples named as 4 h-Co3O4, 6 h-Co3O4 and 8 h-Co3O4. The samples conducted under the condition of 2 mM NH4F and 6 h were donated as HP-Co3O4 NR. The obtained precursors and products were directly used for material characterizations and battery performance evaluation.
2.2. Materials characterization The as-obtained samples were characterized by powder X-ray diffraction (XRD, D8 Advance), field emission scanning electron microscopy (FESEM, JSM-7001F) and transmission electron microscope (TEM, JEM-2010), physical adsorption of N2 at 77 K using an automatic adsorption system (BET, ASAP 2020, Micromeritics), Xray photoelectron spectroscopy (XPS, PHI-5700ESCA) and thermal gravimetric analysis (TGA) system (SDT Q600, TA Instrument).
2.3. Electrochemical measurement All electrochemical measurements were executed by CR 2016 coin-type cells, which were conducted in an Ar-filled glove box (<1 ppm of oxygen and water). The cells were composed of working electrodes, lithium sheet as the counter electrode, porous polypropylene film as a separator. The working electrodes were consisted of acetylene black, polyvinylidene fluoride (PVDF) binder, and active material in a weight ratio of 1:1:8 mixed with N-methylpyrrolidone, then the mixture was coated on copper foil current collectors with diameters of ca. 10 mm. Unless otherwise noted, the specific capacity is calculated based on the total mass of as-obtained materials. The active mass loading of cell is about 1.3e1.6 mg cm2. The electrolyte used was 1 M LiPF6 in a 1:1 (V/ V) mixture of ethylene carbonate and dimethyl carbonate with 5 V% fluoroethylene carbonate. The cyclic voltammograms (CV) was conducted at 0.1 mV s1, which tested on CHI 660C (Shanghai Chenhua Co. Ltd., China) within the voltage range of 3.0e0.01 V (vs. Li/Liþ). The cycle life and rate capability tests were conducted on CT2001A battery program controlling test system (LAND, Wuhan, China) in voltage window of 0.01e3.0 V (vs. Li/Liþ).
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Fig. 1. XRD pattern of the HP-Co3O4 NR.
3. Result and discussion HP-Co3O4 NR was prepared by hydrothermal method and following calcination, as Scheme 1 shown. The crystal structure of the HP-Co3O4 NR is elaborated by XRD. Fig. 1 shows that diffraction patterns are consistent with the standard of cubic spinel phase of Co3O4 (JCPDS no. 43-1003). No other impurity peaks can be found, suggesting that the obtained Co3O4 is of high purity. In addition, XPS is carried out to further verify the element composition of the HP-Co3O4 NR. As seen in Fig. 2a, it is clearly noticed that the Co 2p spectra contains two peaks at the binding energy (BE) of 794.8 eV and 779.5 eV assigned to Co 2p1/2 and Co 2p3/2, respectively, with a spin-orbit splitting of 15.3 eV. The decomposed Co 2p spectra refer to the sharing of Co2þ and Co3þ [12,14]. The OII peak of O1s spectra located at 529.5 eV results from the Co-O bonds; the OI peak at 531 eV can be assigned to the oxygen-deficient regions in Co3O4 while the peak at higher BE of 533 eV might due to the presence of surface absorbed CO2, O2 and/or H2O (Fig. 2b) [20]. The thermal transformation behavior of Co3O4 precursor is investigated by TGA and the result is plotted in Fig. 3. The obvious mass loss appeared from 200 to 600 C can be ascribed to the decomposition of the Co3O4 precursor to Co3O4. When the temperature reached up to 600 C, the mass of the sample remains unchanged. Therefore, the calcination temperature is designed at 600 C. The total mass loss of 26.8% is close to the theoretical weight loss of 25.6% [39]. Afterwards, SEM and TEM are conducted to characterize the morphology of the as-prepared HP-Co3O4 NR. As shown in Fig. 4a, after thermal treatment, the precursor was transform into aloe-like structure, which was uniformly assembled by the nanorods. We can
Fig. 3. TGA curve of Co3O4 precursor at the rate of 10 C min1 under Ar.
deduce that the nanorods are in diameter of 100e140 nm, which is slightly larger than the precursor might ascribe to the production of the pores after calcination, as shown in Fig.S1a. Notably, the nanorods are not solid (red circle in Fig. 4b). It can be clearly seen in the TEM images. As shown in Fig. 4c and d, the nanorods are porous and composed of interconnected nanoparticles with the diameter of 20e50 nm, which might improve the rate performance of the Co3O4. Compared with Co3O4 precursor (Fig. S1b), the HP-Co3O4 NR presents large amount of pore, which might due to the gas release during the thermal decomposition of Co3O4 precursor. The HRTEM image displays the lattice fringes of 0.329 nm corresponding to the (111) lattice plane of spinel Co3O4. It is reported that the (111) crystal planes endow the Co3O4 with highly active surface to form the kinks and steps due to the large quantity of low coordinated atoms, which can effectively enhance the rate of ion diffusion from surface to the interior [26]. The SAED pattern of the sample (Fig. 4e) shows well-defined diffraction rings, indicating that the HP-Co3O4 NR is composed of polycrystalline nanoparticles. The obtained HPCo3O4 NR decorated with nanoparticles can shorten transport pathway of the lithium ions, and the nanopores can sufficiently buffer the gigantic volume changes during the lithiation/delithiation processes, leading to high reversible capacity and excellent cyclic stability. In order to further get insight into the formation mechanism of HP-Co3O4 NR, the effect of reactant time on the structure of Co3O4 is investigated. When the reaction time is 4 h, nanorods begin to aggregate together (Fig. S1a). Aloe-like structure is formed when the time increases to 6 h. As the reaction time increases further to 8 h, more and more nanorods aggregated and formed nanosheets (Fig. S1c), which might decreases the proportion of active sites
Fig. 2. XPS spectra of Co 2p and O 1s of HP-Co3O4 NR.
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Fig. 4. (a), (b) SEM, (c), (d) TEM, and (e) HRTEM images, SAED pattern (f) of HP-Co3O4 NR.
exposed, leading to the reduced electrochemical performance (Fig. S3b). The influence of the contents of NH4F is also studied. When 1 mM NH4F is added, most nanorods are casually stacked (Fig. S2a). Upon introduction of 1.5 mM NH4F, some aloe-like structures appeared (Fig. S2b). While for the concentration of the NH4F of 3 mM, the nanorods become close-grained and sheet-like structures are found (Fig. S2d). It is clear that the NH4F playing an important for the structure-directing of the sample. The N2 adsorption-desorption isotherms of different samples are carried out to determine the specific surface area and pore size distribution of the as-obtained samples. As shown in Fig. 5a, the isotherms of the HP-Co3O4 NR displays typical type IV curves with hysteresis loop at a relative pressure of 0.4e1.0, suggesting the hierarchical porous structure [38,40]. The pores of the HP-Co3O4 NR are mainly centered at 30 nm, according to the pore size distribution in Fig. 5b. The meso/macro pore feature can make more electrolyte ions accessible to the electrode and leads to improved electrochemical property of the HP-Co3O4 NR. Compared with other samples shown in Table. S1, it displays the highest specific surface area (42 m2 g1) and largest pore volume (0144 cm3 g1), which endows HP-Co3O4 NR the best energy storage ability and the strongest ability to alleviate the volumetric change during the conversion reactions. The electrochemical behaviors of the HP-Co3O4 NR as anode materials for LIBs are investigated by a half-cell test. As shown in Fig. 6a, the CV curves of the first four cycles of the as-obtained Co3O4 electrode at scan rate of 0.1 mV s1 with the cutoff voltage window over the voltage range of 3.0e0.01 V at 25 C. For the first cycle, a strong peak located at 0.84 V could be ascribed to the
formation of solid electrolyte interphase (SEI), and the weak peak at 1.25 V might due to the reduction of Co3þ/Co2þ to Co and the formation of Li2O. Meanwhile the corresponding anodic peak at 2.12 V could be assigned to the decomposition of the Li2O and the oxidation of Co [41,42]. In the subsequent cycles, the cathodic peaks obviously shift to higher potential, indicating the existence of the polarization while the anodic peak keeps unchanged [16,40,42]. The charge-discharge profiles are used to further confirm the electrochemical processes of HP-Co3O4 NR (Fig. 6b). The typical voltage plateau appears at 1.0 V in the initial discharge process and gradually shifts to 1.25 V in the following cycles, which is accordance with the CV curves in Fig. 6a. The initial charge and discharge capacity are measured to be 879 and 1402 mAh g1, respectively. The discrepancy of the charge/discharge capacities is mainly caused by the irreversible consumption of the electrolyte and the formation of SEI film [43]. Fig. 6c shows the cyclic and coulombic performance of the HPCo3O4 NR at the rate of 1 A g1. The initial coulombic efficiency is found to be 62.7%, indicating the formation of the stable SEI film [33]. In the following cycles, the average coulombic efficiency can be reached up to as high as 97%. During first 120 cycles, the capacity of the material decreases gradually to a minimum discharge capacity of 499 mAh g1, which is a normal phenomenon for metal oxide-based electrode materials. Exhilaratingly, the capacities slightly increased to 736 mAh g1 during the following 156 cycles, which might be ascribed to the activation of electrode and the reversible formation of polymeric gel-like film [19,44]. After 350 cycles, 71.4% of capacity retention was obtained, which shows best cycling performance than other various structures we have
Fig. 5. The N2 adsorption-desorption isotherms (a) and pore size distribution (b) curve of HP-Co3O4 NR.
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Fig. 6. Electrochemical performances of HP-Co3O4 NR as anode material: (a) CV curves, (b) charge-discharge curves, (c) cycling performance and coulombic efficiency at the current of 1 A g1, (d) rate capacity.
Fig. 7. The SEM images of HP-Co3O4 NR after 350 cycles.
prepared under different reaction time and the content of NH4F as shown in Fig. S3. Moreover, the structure of the sample can even maintain after 350 cycles as shown in Fig. 7. More importantly, the HP-Co3O4 NR presents remarkable rate performance at the rate densities from 0.2 to 6 A g1 (Fig. 6d). It is observed that the average reversible capacities maintained at 803, 668, 523, 440, 383 and 247 mAh g1 at the rate of 0.2, 0.5 1, 2, 4 and 6 A g1, respectively. Furthermore, when the current decreased to 0.2 A g1, the capacity resumes to 825 mAh g1. The gradually recover of the discharge capacity might result from partial lithium ions leaving the electrodes during charging with larger rate [19,45]. The HP-Co3O4 NR electrodes also exhibit excellent cyclic stability even at high current density of 5 A g1 (Fig. 8). It can be noted that the HP-Co3O4 NR anode displays high first reversible capacity of 695 mAh g1. After 600 cycles, the charge capacity remains at 412 mAh g1 (the capacity fading rate is about 0.068% per cycle), which is higher than the theoretical capacity of the commercial graphite. As shown in Fig. S4, the obtained HP-Co3O4 NR also displays better cyclic performance than previous researches even under higher current density after longer cycling numbers. As expected, due to the unique structural merits, the HP-Co3O4 NR effectively improved the electrochemical performance of the Co3O4. According to the SEM, TEM and BET results, the HP-Co3O4 NR
is composed of interconnect nanoparticles, which can shorten the lithium ion transportation pathway and provide the accessible route for the diffusion of the electrolyte, leading to excellent rate performance. In addition, abundance hierarchically porous among the particles provides sufficient space to buffer the volume variation upon extended cycling process, resulting in long cycle life.
Fig. 8. Cyclic performance of HP-Co3O4 NR at the current of 5 A g1.
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Moreover, the porous 1D nanorods with large surface area provide numerous active sites, which benefit to enlarge the storage of lithium leading to high reversible capacity. In a word, the unique structure endows the as-prepared Co3O4 excellent electrochemical properties.
[13]
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4. Conclusion [15]
We have developed a facile route for synthesis the unique structures composed of Co3O4 nanorods by hydrothermal method using NH4F as the structure-directing agent. The obtained HP-Co3O4 NR is composed of nanorods, which constructed by interconnected nanoparticles. Benefitting from the unique structure, the HP-Co3O4 NR exhibited perfect rate performance and long cyclic stability with the capacity fading of 0.717 mAh g1 per cycle at the rate of 1 A g1 after 350 cycles. Even at the current of 5 A g1, the reversible capacity maintained at 412 mAh g1 after 600 cycles, which is higher than the commercial graphite. The low cost and excellent electrochemical of the material is promising for the next generation of battery.
[16]
[17]
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[19]
[20]
Acknowledgements [21]
The work is supported by the Natural Science Foundation of China (No. U1610119, U1610252), the Key Research and Development Program of Shanxi Province (No. 201603D112007), and Youth Innovation Promotion Association, Chinese Academy of Sciences (118800QCH1).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2017.12.001.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Two-pot synthesis of one-dimensional hierarchically porous Co3O4 nanorods as anode for lithium-ion battery Xiao Li a, b, Xiaodong Tian a, Tao Yang a, b, Yan Song a, *, Yiming Liu c, **, Quangui Guo a, Zhanjun Liu a a b c
CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China University of Chinese Academy of Sciences, Beijing, 100049, China Shan Xi Academy of Analytical Science, Taiyuan, 030001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 September 2017 Received in revised form 29 November 2017 Accepted 1 December 2017 Available online 5 December 2017
Poor cyclic stability and low rate capability are two severe stumbling blocks for Co3O4-based anodes due to the insufficient structural stability of the electrode, difficult access of electrolyte to the electrode interior as well as dense structure. In this respect, one-dimensional hierarchically porous Co3O4 nanorods (HP-Co3O4 NR), which are constructed by interconnected nanoparticles, are successfully synthesized via a facile hydrothermal method following calcination. In the architecture, (i) nanoparticle-decoration is favorable to shorten the ions and charge diffusion distance, which improve the rate performance; (ii) the hierarchically porous structure can sufficiently buffer the volume change during the extended cycling, which is conducive to stable electrode structure and good cyclic stability; (iii) 1D porous nanorods ensure more exposed active sites and the contact area of the electrolyte/electrode for lithium storage, which endows the high reversible capacity. As a result, the HP-Co3O4 NR material exhibits high capacity (628 mAh g1 at 1 A g1 after 350 cycles), exceptional rate capability (247 mAh g1 at 6 A g1) and long cycle life (0.068% capacity decay per cycle after 600 cycles at 5 A g1), simultaneously. Our results indicate that the one-dimensional hierarchically porous nanoparticle-decorated nanorods structure is beneficial for improving the cycling and rate performance. © 2017 Elsevier B.V. All rights reserved.
Keywords: Cobalt oxide Hierarchically porous One-dimensional Lithium-ion battery
1. Introduction Lithium ion batteries (LIBs) are vital for developing high performance technologies, from consumer electronics, vehicles, to smart grid scale energy storage [1,2]. Unfortunately, the state-ofthe-art graphite-based LIBs are plagued by low capacity, short lifespan to meet the specific requirements [3,4]. As a result, welldesigned transition metal oxide, Co3O4, has aroused intensive research interest as an appealing candidate electrode material for LIBs because of its higher theory specific capacity (890 mAh g1 vs. 372 mAh g1 of conventional graphite) [5e11]. However, like other transition metal oxides, inferior electrical conductivity and large volumetric variation during cycling processes always result in electrode pulverization, which is responsible for the poor rate performance and capacity fading, thus hamper the practical
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Song), [email protected] (Y. Liu). https://doi.org/10.1016/j.jallcom.2017.12.001 0925-8388/© 2017 Elsevier B.V. All rights reserved.
applications of Co3O4 as LIBs anode [12,13]. In this regard, considerable attentions have been paid to tackling these issues. Typical strategy relies on an appropriate structure to obey the following virtues: (i) sufficient space for volume change, (ii) effective channel for fast Liþ ion transportation, (iii) high conductivity for electron migration. Based on these considerations, the combination with conductive carbon materials [14e16] and unique Co3O4 nanostructure construction [17,18] were usually adopted. Although the introduction of carbon materials can enhance the conductivity of the electrode and alleviate the volume variation during the charge/discharge processes, the specific capacity of Co3O4 would be reduced due to the inactivity of the carbon as well as the decreasing of mass ratio of Co3O4 in the electrode [19,20]. Furthermore, the incomplete protection of Co3O4 by carbon can also be correlated to the unsatisfied performance. As comparison, Co3O4 with unique architecture may be more attractive. Various nanostructures including one-dimensional (1 D, e.g., nanorods, nanotube, nanowires and nanofibers) [21e24], two-dimensional (2 D, nanosheets) [17,19], and three-dimensional
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(3 D, nanocubes) [25e28], have been explored to verify the above assumption. Among them, 1D nanostructures draw great attention due to their short diffusion distance for ions/electrons. And they can also alleviate the strain and accommodate volume expansion upon the redox reactions, leading to the improved cyclic stability and rate capacity. For example, Co3O4 nanowires prepared by Li et al. exhibit 836 mAh g1 at the current of 200 mA g1 after three cycles when adopted as anode for LIBs [24]. Porous Co3O4 nanotube has been prepared by using carbon nanotubes [29] or electrospun polyacrylonitrile (PAN) nanofiber [21] as the template, and the as-obtained samples displayed the reversible capacity of 1200 mAh g1 at a current density of 50 mA g1 after 20 cycles and 856.4 mAh g1 at the rate of 0.25 C after 60 cycles, respectively. The literatures shown above demonstrate that the porous nanostructure displays better electrochemical performance than that of the solid one, indicating that hierarchically porous 1D nanostructure is proposed as an effective structure because of more electrochemical reaction sites and sufficient ion/electron diffusion routes as well as effective ability to buffer the volumetric change [22,30]. However, templates were used during the fabrication processes in preceding studies which enhanced the intricacy of the produce. Besides, the specific capacities were recorded at relatively low current density [11,12,22,31e35] and the rate capability as well as cyclic performance was limited [30,36e38], which cannot sufficiently meet the practical requirements. Therefore, using low cost produce method to design porous 1D architecture of Co3O4 with high specific capacity and long cycle life still remains challenge. Herein, a simple template-free self-assembly strategy was adopted to synthesize hierarchically porous 1D Co3O4 nanorods (HP-Co3O4 NR) by hydrothermal method. The as-prepared HP-Co3O4 NR are composed of uniform nanorods with the length of 30e40 mm stacked by discontinuous nanoparticles with the diameter of 20e50 nm, which can alleviate the resistance of the lithium diffusion and electrons transport and enhance the contact area between electrode and electrolyte, thus leading to high reversible capacity and good rate performance. The hierarchically porous structure is formed mainly by the release of CO2 and H2O and the decomposition of the precursor, which can buffer the huge volume change during the repeated conversion reactions, resulting in long cycling life. Merited by the large specific surface area and abundant pores, the as-prepared HP-Co3O4 NR delivered excellent rate capability as well as high specific capacity (628 and 412 mAh g1 at the current rate of 1 and 5 A g1 after 350 and 600 cycles, respectively). In addition, the influences of reaction time and NH4F reactant concentration on the morphology, especially on the pore size distribution, were also investigated. 2. Experimental section 2.1. Materials synthesis All the regents were purchased and directly used without further purification. The typical synthetic procedure as shown in Scheme 1, 5 mM of hydrated cobalt nitrate (Co(NO3)2$6H2O), 5 mM of urea (CO(NH2)2), 2 mM of ammonium fluoride (NH4F) were blended into 75 ml deionized water. After vigorous stirring for half hour, the resulting solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and maintained at 120 C for 6 h. After cooling to room temperature naturally, the obtained precipitates were washed with anhydrous alcohol and distilled water several times, and then dried at 60 C for 6 h. Finally, the precursor was annealed in Ar at 600 C for 2 h. For comparison, a series of reference samples were prepared by varying NH4F reactant concentrations, the obtained sample remarked as 1 mM-Co3O4, 1.5 mM-Co3O4, 2 mM-Co3O4 and 3 mM-Co3O4. In addition, time-dependent morphological evolution of the HP-Co3O4
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Scheme 1. The synthetize of HP-Co3O4 NR.
NR samples was investigated by collecting samples at different reaction stages (4 h and 8 h), the samples named as 4 h-Co3O4, 6 h-Co3O4 and 8 h-Co3O4. The samples conducted under the condition of 2 mM NH4F and 6 h were donated as HP-Co3O4 NR. The obtained precursors and products were directly used for material characterizations and battery performance evaluation.
2.2. Materials characterization The as-obtained samples were characterized by powder X-ray diffraction (XRD, D8 Advance), field emission scanning electron microscopy (FESEM, JSM-7001F) and transmission electron microscope (TEM, JEM-2010), physical adsorption of N2 at 77 K using an automatic adsorption system (BET, ASAP 2020, Micromeritics), Xray photoelectron spectroscopy (XPS, PHI-5700ESCA) and thermal gravimetric analysis (TGA) system (SDT Q600, TA Instrument).
2.3. Electrochemical measurement All electrochemical measurements were executed by CR 2016 coin-type cells, which were conducted in an Ar-filled glove box (<1 ppm of oxygen and water). The cells were composed of working electrodes, lithium sheet as the counter electrode, porous polypropylene film as a separator. The working electrodes were consisted of acetylene black, polyvinylidene fluoride (PVDF) binder, and active material in a weight ratio of 1:1:8 mixed with N-methylpyrrolidone, then the mixture was coated on copper foil current collectors with diameters of ca. 10 mm. Unless otherwise noted, the specific capacity is calculated based on the total mass of as-obtained materials. The active mass loading of cell is about 1.3e1.6 mg cm2. The electrolyte used was 1 M LiPF6 in a 1:1 (V/ V) mixture of ethylene carbonate and dimethyl carbonate with 5 V% fluoroethylene carbonate. The cyclic voltammograms (CV) was conducted at 0.1 mV s1, which tested on CHI 660C (Shanghai Chenhua Co. Ltd., China) within the voltage range of 3.0e0.01 V (vs. Li/Liþ). The cycle life and rate capability tests were conducted on CT2001A battery program controlling test system (LAND, Wuhan, China) in voltage window of 0.01e3.0 V (vs. Li/Liþ).
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Fig. 1. XRD pattern of the HP-Co3O4 NR.
3. Result and discussion HP-Co3O4 NR was prepared by hydrothermal method and following calcination, as Scheme 1 shown. The crystal structure of the HP-Co3O4 NR is elaborated by XRD. Fig. 1 shows that diffraction patterns are consistent with the standard of cubic spinel phase of Co3O4 (JCPDS no. 43-1003). No other impurity peaks can be found, suggesting that the obtained Co3O4 is of high purity. In addition, XPS is carried out to further verify the element composition of the HP-Co3O4 NR. As seen in Fig. 2a, it is clearly noticed that the Co 2p spectra contains two peaks at the binding energy (BE) of 794.8 eV and 779.5 eV assigned to Co 2p1/2 and Co 2p3/2, respectively, with a spin-orbit splitting of 15.3 eV. The decomposed Co 2p spectra refer to the sharing of Co2þ and Co3þ [12,14]. The OII peak of O1s spectra located at 529.5 eV results from the Co-O bonds; the OI peak at 531 eV can be assigned to the oxygen-deficient regions in Co3O4 while the peak at higher BE of 533 eV might due to the presence of surface absorbed CO2, O2 and/or H2O (Fig. 2b) [20]. The thermal transformation behavior of Co3O4 precursor is investigated by TGA and the result is plotted in Fig. 3. The obvious mass loss appeared from 200 to 600 C can be ascribed to the decomposition of the Co3O4 precursor to Co3O4. When the temperature reached up to 600 C, the mass of the sample remains unchanged. Therefore, the calcination temperature is designed at 600 C. The total mass loss of 26.8% is close to the theoretical weight loss of 25.6% [39]. Afterwards, SEM and TEM are conducted to characterize the morphology of the as-prepared HP-Co3O4 NR. As shown in Fig. 4a, after thermal treatment, the precursor was transform into aloe-like structure, which was uniformly assembled by the nanorods. We can
Fig. 3. TGA curve of Co3O4 precursor at the rate of 10 C min1 under Ar.
deduce that the nanorods are in diameter of 100e140 nm, which is slightly larger than the precursor might ascribe to the production of the pores after calcination, as shown in Fig.S1a. Notably, the nanorods are not solid (red circle in Fig. 4b). It can be clearly seen in the TEM images. As shown in Fig. 4c and d, the nanorods are porous and composed of interconnected nanoparticles with the diameter of 20e50 nm, which might improve the rate performance of the Co3O4. Compared with Co3O4 precursor (Fig. S1b), the HP-Co3O4 NR presents large amount of pore, which might due to the gas release during the thermal decomposition of Co3O4 precursor. The HRTEM image displays the lattice fringes of 0.329 nm corresponding to the (111) lattice plane of spinel Co3O4. It is reported that the (111) crystal planes endow the Co3O4 with highly active surface to form the kinks and steps due to the large quantity of low coordinated atoms, which can effectively enhance the rate of ion diffusion from surface to the interior [26]. The SAED pattern of the sample (Fig. 4e) shows well-defined diffraction rings, indicating that the HP-Co3O4 NR is composed of polycrystalline nanoparticles. The obtained HPCo3O4 NR decorated with nanoparticles can shorten transport pathway of the lithium ions, and the nanopores can sufficiently buffer the gigantic volume changes during the lithiation/delithiation processes, leading to high reversible capacity and excellent cyclic stability. In order to further get insight into the formation mechanism of HP-Co3O4 NR, the effect of reactant time on the structure of Co3O4 is investigated. When the reaction time is 4 h, nanorods begin to aggregate together (Fig. S1a). Aloe-like structure is formed when the time increases to 6 h. As the reaction time increases further to 8 h, more and more nanorods aggregated and formed nanosheets (Fig. S1c), which might decreases the proportion of active sites
Fig. 2. XPS spectra of Co 2p and O 1s of HP-Co3O4 NR.
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Fig. 4. (a), (b) SEM, (c), (d) TEM, and (e) HRTEM images, SAED pattern (f) of HP-Co3O4 NR.
exposed, leading to the reduced electrochemical performance (Fig. S3b). The influence of the contents of NH4F is also studied. When 1 mM NH4F is added, most nanorods are casually stacked (Fig. S2a). Upon introduction of 1.5 mM NH4F, some aloe-like structures appeared (Fig. S2b). While for the concentration of the NH4F of 3 mM, the nanorods become close-grained and sheet-like structures are found (Fig. S2d). It is clear that the NH4F playing an important for the structure-directing of the sample. The N2 adsorption-desorption isotherms of different samples are carried out to determine the specific surface area and pore size distribution of the as-obtained samples. As shown in Fig. 5a, the isotherms of the HP-Co3O4 NR displays typical type IV curves with hysteresis loop at a relative pressure of 0.4e1.0, suggesting the hierarchical porous structure [38,40]. The pores of the HP-Co3O4 NR are mainly centered at 30 nm, according to the pore size distribution in Fig. 5b. The meso/macro pore feature can make more electrolyte ions accessible to the electrode and leads to improved electrochemical property of the HP-Co3O4 NR. Compared with other samples shown in Table. S1, it displays the highest specific surface area (42 m2 g1) and largest pore volume (0144 cm3 g1), which endows HP-Co3O4 NR the best energy storage ability and the strongest ability to alleviate the volumetric change during the conversion reactions. The electrochemical behaviors of the HP-Co3O4 NR as anode materials for LIBs are investigated by a half-cell test. As shown in Fig. 6a, the CV curves of the first four cycles of the as-obtained Co3O4 electrode at scan rate of 0.1 mV s1 with the cutoff voltage window over the voltage range of 3.0e0.01 V at 25 C. For the first cycle, a strong peak located at 0.84 V could be ascribed to the
formation of solid electrolyte interphase (SEI), and the weak peak at 1.25 V might due to the reduction of Co3þ/Co2þ to Co and the formation of Li2O. Meanwhile the corresponding anodic peak at 2.12 V could be assigned to the decomposition of the Li2O and the oxidation of Co [41,42]. In the subsequent cycles, the cathodic peaks obviously shift to higher potential, indicating the existence of the polarization while the anodic peak keeps unchanged [16,40,42]. The charge-discharge profiles are used to further confirm the electrochemical processes of HP-Co3O4 NR (Fig. 6b). The typical voltage plateau appears at 1.0 V in the initial discharge process and gradually shifts to 1.25 V in the following cycles, which is accordance with the CV curves in Fig. 6a. The initial charge and discharge capacity are measured to be 879 and 1402 mAh g1, respectively. The discrepancy of the charge/discharge capacities is mainly caused by the irreversible consumption of the electrolyte and the formation of SEI film [43]. Fig. 6c shows the cyclic and coulombic performance of the HPCo3O4 NR at the rate of 1 A g1. The initial coulombic efficiency is found to be 62.7%, indicating the formation of the stable SEI film [33]. In the following cycles, the average coulombic efficiency can be reached up to as high as 97%. During first 120 cycles, the capacity of the material decreases gradually to a minimum discharge capacity of 499 mAh g1, which is a normal phenomenon for metal oxide-based electrode materials. Exhilaratingly, the capacities slightly increased to 736 mAh g1 during the following 156 cycles, which might be ascribed to the activation of electrode and the reversible formation of polymeric gel-like film [19,44]. After 350 cycles, 71.4% of capacity retention was obtained, which shows best cycling performance than other various structures we have
Fig. 5. The N2 adsorption-desorption isotherms (a) and pore size distribution (b) curve of HP-Co3O4 NR.
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Fig. 6. Electrochemical performances of HP-Co3O4 NR as anode material: (a) CV curves, (b) charge-discharge curves, (c) cycling performance and coulombic efficiency at the current of 1 A g1, (d) rate capacity.
Fig. 7. The SEM images of HP-Co3O4 NR after 350 cycles.
prepared under different reaction time and the content of NH4F as shown in Fig. S3. Moreover, the structure of the sample can even maintain after 350 cycles as shown in Fig. 7. More importantly, the HP-Co3O4 NR presents remarkable rate performance at the rate densities from 0.2 to 6 A g1 (Fig. 6d). It is observed that the average reversible capacities maintained at 803, 668, 523, 440, 383 and 247 mAh g1 at the rate of 0.2, 0.5 1, 2, 4 and 6 A g1, respectively. Furthermore, when the current decreased to 0.2 A g1, the capacity resumes to 825 mAh g1. The gradually recover of the discharge capacity might result from partial lithium ions leaving the electrodes during charging with larger rate [19,45]. The HP-Co3O4 NR electrodes also exhibit excellent cyclic stability even at high current density of 5 A g1 (Fig. 8). It can be noted that the HP-Co3O4 NR anode displays high first reversible capacity of 695 mAh g1. After 600 cycles, the charge capacity remains at 412 mAh g1 (the capacity fading rate is about 0.068% per cycle), which is higher than the theoretical capacity of the commercial graphite. As shown in Fig. S4, the obtained HP-Co3O4 NR also displays better cyclic performance than previous researches even under higher current density after longer cycling numbers. As expected, due to the unique structural merits, the HP-Co3O4 NR effectively improved the electrochemical performance of the Co3O4. According to the SEM, TEM and BET results, the HP-Co3O4 NR
is composed of interconnect nanoparticles, which can shorten the lithium ion transportation pathway and provide the accessible route for the diffusion of the electrolyte, leading to excellent rate performance. In addition, abundance hierarchically porous among the particles provides sufficient space to buffer the volume variation upon extended cycling process, resulting in long cycle life.
Fig. 8. Cyclic performance of HP-Co3O4 NR at the current of 5 A g1.
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Moreover, the porous 1D nanorods with large surface area provide numerous active sites, which benefit to enlarge the storage of lithium leading to high reversible capacity. In a word, the unique structure endows the as-prepared Co3O4 excellent electrochemical properties.
[13]
[14]
4. Conclusion [15]
We have developed a facile route for synthesis the unique structures composed of Co3O4 nanorods by hydrothermal method using NH4F as the structure-directing agent. The obtained HP-Co3O4 NR is composed of nanorods, which constructed by interconnected nanoparticles. Benefitting from the unique structure, the HP-Co3O4 NR exhibited perfect rate performance and long cyclic stability with the capacity fading of 0.717 mAh g1 per cycle at the rate of 1 A g1 after 350 cycles. Even at the current of 5 A g1, the reversible capacity maintained at 412 mAh g1 after 600 cycles, which is higher than the commercial graphite. The low cost and excellent electrochemical of the material is promising for the next generation of battery.
[16]
[17]
[18]
[19]
[20]
Acknowledgements [21]
The work is supported by the Natural Science Foundation of China (No. U1610119, U1610252), the Key Research and Development Program of Shanxi Province (No. 201603D112007), and Youth Innovation Promotion Association, Chinese Academy of Sciences (118800QCH1).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2017.12.001.
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