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Honeycomb-Spherical Co3O4-TiO2 Hybrid Materials for Enhanced Lithium Storage
Accepted Manuscript Title: Honeycomb-Spherical Co3 O4 -TiO2 Hybrid Materials for Enhanced Lithium Storage Author: Youpeng Li Kaina Shang Wenjie Zhou Linyin Tan Xuexue Pan Mingna Liao Jianfei Lei Lingzhi Zhao PII: DOI: Reference:
S0013-4686(16)32515-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.11.153 EA 28443
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
30-7-2016 3-11-2016 27-11-2016
Please cite this article as: Youpeng Li, Kaina Shang, Wenjie Zhou, Linyin Tan, Xuexue Pan, Mingna Liao, Jianfei Lei, Lingzhi Zhao, Honeycomb-Spherical Co3O4-TiO2 Hybrid Materials for Enhanced Lithium Storage, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.11.153 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Honeycomb-Spherical Co3O4-TiO2 Hybrid Materials for Enhanced Lithium Storage
Youpeng Lia,d, Kaina Shanga,d, Wenjie Zhouc, Linyin Tanc, Xuexue Pana, Mingna Liaoa,d, Jianfei Leib* [email protected], Lingzhi Zhaoa,d* [email protected]
a
Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and
Devices, Institute of Opto-Electronic Materials and Technology, South China Normal University, Guangzhou 510631, China b
School of Physics and Engineering, Henan University of Science and Technology,
Luoyang 471023, China c
Guangzhou Zhixin High School, Guangzhou 510080, China
d
Guangdong Provincial Engineering Technology Research Center for Low Carbon
and Advanced Energy Materials, Guangzhou 510631, China *Corresponding authors
GRAPHICAL ABSTRACT
The honeycomb-spherical Co3O4-TiO2 composite has been successfully synthesised though a hydrothermal method with deionized water and ethanol as solvent. The Co3O4-TiO2 composite displays a enhanced electrochemical performance compared with pure Co3O4.
Highlights
We introduces an effective, inexpensive and large-scale production route to the fabrication of a novel honeycomb-spherical Co3O4-TiO2 nanocomposite anode material for use in rechargeable lithium ion batteries.
The nanocomposite electrode shows stable reversible capacity during the long cycles and improved rate performance when being applied in lithium ion half and full cells, respectively.
The unusual morphology of honeycomb-spherical Co3O4 and the introduction of secondary phase of TiO2 nanoparticles are suggested to be responsible for the enhanced electrochemical performance.
Abstract This work introduces an effective, inexpensive and large-scale production route to the fabrication of a novel honeycomb-spherical Co3O4-TiO2 nanocomposite anode material for use in rechargeable lithium ion batteries. The microstructure and phase composition of the Co3O4-TiO2 product were characterized systematically. The Co3O4-TiO2 nanocomposite electrode yielded fairly high reversible capacity of ~1000 mAh/g at 200 mA/g after 100 cycles. Moreover, the electrode exhibited extraordinary rate ability and could regain its original specific capaities as reversing to the low current densities. Meanwhile, when being applied as anode material in full cells, it still achieved remarkably improved electrochemical performance compared with traditional graphite material. Noticeably, in-situ XRD measurement verified the
conversion mechanism for the Co3O4-TiO2 electrode. The likely contributing factors to these outstanding electrochemical properties of the Co3O4-TiO2 nanocomposite could be related to the distinctive morphology including interlaced Co3O4 nanoflakes (NFs) and the TiO2 nanoparticles (NPs), which contains superior structural stability and allows more channels for the Li+ insertion/extraction reaction. This research shows that the as-prepared Co3O4-TiO2 nanocomposite could be suitable for use as a favorable cycle performance anode material for lithium ion batteries.
Keywords: Lithium-ion battery; Anode materials; Co3O4; Honeycomb-spherical; TiO2.
1. Introduction Rechargeable lithium-ion batteries (LIBs), as a promising energy storage system, are widely applied in portable electronics, electric vehicles and stationary energy storage for their high energy density, long cycling life, high efficieny and no memony effect [1-3]. Graphite-based material has been used comprehensively as the mainstream anode materials in commercial LIBs due to its low cost and encouraging capacity retention, but the limited specific capacity (372 mAh/g) could not satisfy the ever expanding requirements of individuals [4-6]. Therefore, tremendous efforts have been dedicated to heighten the electrochemical performance of anode materials [7-10]. Co3O4 has become one of the most popular candidates for anode materials to replace the commercial graphite anode owning to its high theoretical specific capacity (890 mAh/g), high tap density and excellent electrochemical property [11, 12]. However, its capacity retention and rate capability still need to be further enhanced because of its low initial coulombic efficiency, poor electrical conductivity and fast capacity fading during the Li+ insertion/extraction process [13, 14]. At present, an effective strategy to strengthen the electrochemical performance of Co3O4 anode is to construct nanomaterials with favorable morphology (nanowires, nanorods, nanosheets) [15, 16], but the poor electrical conductivity and the large volume effect have still not been solved fundamentally [17]. Thereby, it is necessary to modify Co3O4 with secondary phase, which may facilitate the electron transfer rate and accommodate volume effect during the Li+ insertion/extration reaction [18, 19]. Compared with Co3O4, TiO2 not only possesses unique merits including abundance and environmentally friendly, but
also owns attractive electronic properties and robust structural stability (less than 4% in volume change) [20]. These features make the combined Co3O4-TiO2 nanocomposite electrode a befitting anode material for long-life energy storage. So far, various Co3O4-TiO2 composites have been intensively investigated. For instance, Co3O4-TiO2 nanobelt arrays have been fabricated as an anode material. After 100 cycles, it maintained a capacity of 300 mAh/g at a current density of 100 mA/g [21]. Xia et al. Prepared the Co3O4-TiO2 core-branch hollow nanowire arrays, but it just exhibited a retention capacity of 130 mAh/g at 2 A/g [22]. Similarly, Co3O4-TiO2 hierarchical heterostructures have also been synthesized by a hydrothermal method, and delivered a reversible capacity of 663 mAh/g [23]. Meanwhile, nano-coaxial Co3O4-TiO2 arrays exhibit great high capacity of ~850 mAh/g after 50 cycles [24]. Hence, we could conclude that the peculiar morphology takes the dominant role of electrochemical performance of Co3O4-TiO2 nanocomposite and the specific capacity still needs to be further improved. Herein,
we prepared a new type of honeycomb-spherical Co3O4-TiO2
nanocomposite via a simple and low-cost approach by hydrothermal treatment and subsequent calcining process. In our previous work, we have synthesized a switch-like Co3O4-TiO2 hybrid electrode, which delivered a high capacity of 668 mAh/g after 120 cycles [25]. Here, we continue to prepare the novel honeycomb-spherical
Co3O4-TiO2
nanocomposite
to
further
improve
the
electrochemical performance. We believe that the introduction of TiO2 NPs inside the porous Co3O4 NFs could not only prevent the nanoflakes from structural damage and
provide other active sites, but also strengthen the phase stability of the whole system for the ignorable volume effect of TiO2 NPs, which will be beneficial to the long cycle performance and high rate ability. 2. Experimental Details 2.1 Materials synthesis All the reagents in this work were purchased from the Aladdin without any further purification. In a typical process, 2.5 g of Co(NO3)2·6H2O and 2.5 g of urea were dissolved in 60 ml solution containing deionized water and ethanol (4:1 in volume) with the continuous magnetic stirring. Then, 0.08 g anatase TiO2 NPs (~10 nm in diameter) and 0.3 g hexadecyltrimethylammonium chloride (CTAC) were added into the mentioned solution. After stirring for 1 h, the solution was transferred to a teflon-lined stainless autoclave and placed in an oven at 160
for 12 h. When the
reaction system was cooled to room temperature naturally, the obtained precipitate was filtered, washed for several times, and collected before being dried at 60 h. The dried powder was calcined at 400
for 10
for 3 h in air to gain the
honeycomb-spherical Co3O4-TiO2 nanocomposition. As a comparison, the pure Co3O4 was prepared by the similar steps as Co3O4-TiO2 without adding TiO2 NPs and CTAC. 2.2 Electrodes and cells fabrication The coin-type half cells and full cells were assembled in an argon-filled glove box (Mikrouna, Sukei1220/750) to evaluate the electrochemical properties of the Co3O4-TiO2 composite, the pristine Co3O4 and graphite anode materials. The anode slurry was concocted by the as-prepared product (Co3O4-TiO2, Co3O4 or graphite
anode), conductive carbon black (Super P) and Poly (vinylidene fluoride) (PVDF) in a weight ratio of 7:2:1 in the N-methyl-2-pyrrolidone (NMP) solvent. Then it was coated on a cleaned copper foil before being cut into suitable discs served as electrodes. 1M LiPF6 in a solvent mixture of ethylene carbonate and diethylene carbonate (1:1vol.%) were used as the electrolyte. Lithium metal for half cells and LiFePO4 for full cells were used as the counter electrode, respectively. 2.3 Characterization The structure and the surface properties of the obtained Co3O4-TiO2, TiO2 and Co3O4 were characterized by X-ray diffraction (XRD, BRUKER D8 ADVANCE, Cu K radiation (1.5406 nm)) and scanning electron microscopy (SEM, ZEISS Ultra 55, 5kV, Pt-spraying treatment), transmission electron microscopy (TEM, JEM-2100HR, 200kV), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD), respectively. The specific surface area and pore distribution of the samples were determined
by
the
N2
adsorption/desorption
experiment
with
the
Brunauer-Emmett-Teller method (BET Micromeritics ASAP 2020). The charge/discharge tests of the assembled cells were performed on the NEWARE BTS computer-controled galvanostat method by using different current densities at room temperature. The specific capacity of Co3O4-TiO2 electrode was calculated based on the mass of both Co3O4 and TiO2. Similarly, specific capacity of Co3O4 and graphite electrode were calculated based on the mass of Co3O4 and graphite, respectively. Cyclic voltammetry (CV) experiments were carried out with an electrochemical test analyzer (CHI 660D). Each electrochemical test was performed
with several cells. Sufficiently the operation influence of cell assembly was taken into account, the most reasonable data was selected out as the final result. 3. Results and discussion 3.1 Formation mechanism Fig.1 (a) shows the possible formation mechanism of honeycomb-spherical Co3O4-TiO2 nanocomposite. For a typical process, a host of Co2+ attach on the surface of the dispersed TiO2 NPs with continuous stirring. When a appropriate external environmental condition is achieved, large amounts of Co2+ begin to hydrolysis and then attach to the surface of TiO2 NPs to form TiO2@Co2(OH)2CO3 unite, while other Co2+ in the solution tend to hydrolyze to form honeycomb-spherical structure (5Co2+ + 5H2NCONH2 + 10H2O → 2CoCO3·3Co(OH)2·H2O + CO2↑ + 10NH4+). In this course, the formed TiO2 @Co2(OH)2CO3 unites insert to the inside of Co2(OH)2CO3 honeycomb-sphere, in which caused the widely pore distribution owning to the embedded TiO2 @Co2(OH)2CO3 unites. The rest TiO2 NPs adhere to the outside of Co2(OH)2CO3 nanosheets. All the Co2(OH)2CO3 turn into Co3O4 after a calcining process (2CoCO3·3Co(OH)2·H2O → 2Co3O4 + CO2↑ + H2O). 3.2 Physical Properties The phase composition of pure Co3O4, TiO2 NPs and Co3O4-TiO2 were investigated by X-ray diffraction (XRD) analysis (Fig.1 (b)). The XRD patterns revealed that every diffraction peaks could be indexed to anatase TiO2 (JCPDS No.21-1272) and spinel phase Co3O4 (space group: Fd-3m, lattice constant a = 8.08 Å, JCPDS No. 42-1467). From the Fig.1 (b) we can find that the well-defined peaks
of TiO2 NPs located at 25.4°, 37.9° and 48.2° could be indexed to (101), (004) and (200) plane and peaks of Co3O4 NFs located at 31.1°, 36.9°, 43.2°, 59.4° and 65.2° correspond to (220), (311), (222), (511) and (440) plane. No apparent impurity peaks were detected. The sharp and strong characteristic peaks testify the excellent crystallinity and high crystalline purity, which confirmed that the nanocomposite elements are as prepared Co3O4-TiO2 after the reasonable synthetic method. It can be clearly found from the curve of pure Co3O4 that the main peak of (311) is quite strong, corresponding to the expected crystal face exposure [26, 27]. From the curve of Co3O4-TiO2, it can be seen that some of the diffraction peaks of Co3O4 and TiO2 become weak or unconspicuous, which is attributed to the formation process of Co3O4-TiO2 composite after solution reaction under superhigh-pressure and temperature condition. As shown in Fig.1 (c), it should be noticed that the EDS measurement also detected the existence of O, Ti and Co with an atom ratio of 30:1:18.5, corresponding to the datas of XRD. To investigate the morphologies of Co3O4-TiO2 composite and pure Co3O4, scanning electron microscope (SEM) was performed at different magnifications. It could be seen from Fig.2 (a) that the Co3O4 NFs are staggered with each other and the honeycomb-spherical structure has been formed successfully. Fig.2 (b), 2 (c) and 2 (d) display the SEM images of Co3O4-TiO2 nanocomposite at different magnifications. We could discover that the unique honeycomb-spherical structure of Co3O4 has not been changed after the adding of TiO2 NPs. However, Some TiO2 NPs can be hardly found in the Co3O4 NFs in Fig.2 (c) and (d), it is mainly due to the small size (~10 nm
in diameter) of TiO2 NPs. The inner contraction of the Co3O4 NFs could provide many pores which would not only enlarge the effective electrode/electrolyte contact area and shorten the migrating distance of lithium ions, but also tolerate the volume variation during the charge/discharge process [28-29]. Transmission electron microscopy (TEM) was used to further explore the specific structure. We can clearly find that the TiO2 NPs are uniformly dispersed in the Co3O4 NFs in Fig.2 (e), confirming that the TiO2 NPs have inserted into the honeycomb-spherical Co3O4 during the process of reaction. The introduction of TiO2 NPs embedded in unique honeycomb-sphere will enhance the structural stability of the system and avoid their self-aggregation during the charge/discharge reaction[30], which expected that more outstanding synergy and advanced electrochemical performance could be realized compared with the pure Co3O4. In Fig.2 (f), some lattice fringes of composite were clearly detected by HRTEM, indicating that the TiO2 and Co3O4 have well crystalline structure. The displayed lattice spacing is measured to be ~0.35 nm corresponding to the (101) plane of TiO2, ~0.25 and ~0.29 nm corresponding to the (311) and (220) planes of Co3O4. The selected-area electron diffraction (SAED) pattern was also performed to confirm phase of Co3O4-TiO2 composite shown in Fig.2 (g). The diffraction rings of (101) for TiO2 and (311), (220), (440) for Co3O4 are distinctly described, confirming the wonderful crystalline. X-ray photoelectron spectroscopy (XPS) was also employed to estimate the surface electronic state of hybrid electrode. In Fig.3 (a), the XPS spectra of Ti double peaks shows the binding energy of 459.1 eV and 466.1 eV, respectively, which is well
coincidence with the value of Ti4+ in TiO2 [31]. Fig.3 (b) exhibits two peaks located at 780.1 eV and 795.2 eV, corresponding to Co 2p3/2 and 2p3/1, respectively, which further confirms the existence of Co3O4 [32]. Brunauer-Emmett-Teller (BET) gas-sorption measurements were also used to investigate
the
specific
surface
area
and
the
pores
dispersion
of
the
honeycomb-spherical Co3O4-TiO2 nanocomposite and the pristine Co3O4. As shown the N2 adsorption-desorption curve in Fig.4 (a), the hybrid materials enjoy a larger specific surface area of 29.07 m2/g compared with the pure Co3O4 with 14.16 m2/g. It should be owing to the introduction of TiO2 NPs decreasing the size of pores. Extensive specific surface area increases the reaction sites of the nanocomposite, contributing to the excellent electrochemical property. The Barrett-Joyner-Helanda (BJH) pore dispersion is also demonstrated in Fig.4 (b). Obviously, a smaller pore size of composite is obtained compared with the pure Co3O4, corresponding to the analysis of SEM and TEM. 3.3 Electrochemical Performance To give a more specific explanation of the property, cyclic voltammetry (CV) curves of Co3O4-TiO2 and pure Co3O4 are displayed at a scan rate of 0.2 mV/s in Fig.5 (a). In the 1st cycle of Co3O4-TiO2 electrode, a quite small peak appears at 1.6 V, which is corresponding to the reduce potential of TiO2 phase (TiO2 + xLi+ + xe- → LixTiO2, 0 ≤ x ≤ 1) [33]. A limited specific capacity of TiO2 may give a reasonable interpretation of the small peak [34]. During the following negative scanning, a sharp reduced peak appears at 0.71 V, which is coincidence with the curve of pure Co3O4
electrode, should originate from the reduction of Co3O4 to Co, accompanying the emergence of Li2O [35]. There is a great gap between the 1st and the following reduce process of both electrodes . This should be due to the irreversible loss of lithium from the deposition of electrolyte and formation of solid electrolyte interphase (SEI) [36, 37]. In the Li+ extraction reaction, the opposite peak is owning to the decomposition of Li2O, the oxidation of Co to Co3O4 (Li2O + Co → Co3O4 + Li, 0 ≤ x ≤ 1) and the extraction of Li+ from TiO2 (LixTiO2 → TiO2 + xLi+ + xe-). Generally, the value of anodic peaks are 2.1 V [23] and 2.2 V [25] for the Co3O4 and TiO2, respectively. Nevertheless, only one oxydic peak could be clearly detected at 2.1 V, which may be caused by the embezzle of the peak of Co3O4 for its alpine specific capacity [24]. After the 1st cycle, the following curves of Co3O4-TiO2 electrode are more coincident than that of pure Co3O4 electrode, indicating the better cycle ability of the unique structure. Fig.5 (b) shows the charge/discharge curves of the composite at current density of 200 mA/g. From the 1st cycle we could see that no apparent discharge platform at 1.6 V is detected owning to the negligible specific capacity contribution of TiO2. Nevertheless, it is obvious that the charge/discharge plateaus of Co3O4 are clearly shown at ~2.1 V and ~1.1 V respectively, which is similar to those reported by previous reports [23, 24]. A quite potential gap between the 1st and the 2nd curves could be easily discovered, which is corresponding to the CV profiles above. The 1st charge/discharge process of composite capacity is 1136/1458 mAh/g, exhibiting a coulombic efficiency of 77.9% caused by a more deposition of electrolyte and
formation of SEI film duo to the enhanced specific area. The 10th charge and discharge curves are nearly coincidence with those in the 2nd curves, confirming that the reversible capacity possesses wonderful steadiness after the 1st cycle, corresponding to the conclusion of CV curves. The excellent cycle performance is mainly attributed to the unique honeycomb-spherical nanostructure. After 80 cycles, a superior reversible capacity with 1009 mAh/g is obtained, delivering that the nanostructure of electrode is quite stable and not be damaged after long cycles. To further confirm the mechanism of electrochemical reactions, in-situ XRD patterns of Co3O4-TiO2 composite were recorded at different states. Fig.6 (a) shows the first discharge-charge curves at 400 mA/g. The main peaks of Co3O4-TiO2 composite were collected at specific state denoted as A-E Fig.6 (b). The black curve in Fig.6 (b) shows the peaks of composite electrode at the open circuit voltage state (A). The (101) plane of TiO2 and (220), (311) planes of Co3O4 are quiet strong and correspond to the explanations of XRD. During the following discharge process, the (101) plane of TiO2 disappears firstly, indicating that the reduce process of TiO2 takes place firstly, corresponding to the analysis of CV curves. Also, the peaks of Co3O4 become weaker for the reduce reaction of Co3O4. The peaks of Co3O4-TiO2 composite vanish in the full-discharge state (D). In the subsequent full-charge state (E), the signals of Co3O4-TiO2 composite reappear, further confirming the conversion mechanism of Co3O4-TiO2 electrode. Also, we can discover that the peaks of full-charge state (E) are still very strong and close to the open circuit voltage state
(A), indicating that the composite nearly has no change during the charge-discharge process, which may contribute to the excellent electrochemical performance. Fig.7 (a) shows the cycle performance of pure Co3O4 and Co3O4-TiO2 at a current density of 200 mA/g. It is obvious that the Co3O4-TiO2 composite delivers a markedly improved cycling property in terms of much higher initial capacity and reversible capacity after 100 cycles compared with pristine Co3O4. Apparently, the Co3O4-TiO2 composite exhibits the 1st charge/discharge capacities of 1136/1458 mAh/g at 200 mA/g, which is higher than that of Co3O4. What is worth mentioning is that both materials have suffered quite capacity damping in the second discharge process, the fundamental reason is Li-consuming reaction containing the vast decomposition of electrolyte and formation of SEI film owing to the large specific surface area. After 100 cycles, Co3O4-TiO2 composite just shows a little capacity fading and a higher specific capacity of ~1000 mAh/g (better than Co3O4) is obtained, demonstrating that the integrity of hybrid electrode is still maintained after long cycles. It may be caused by several reasons. For one thing, the reversible formation/dissolution of a gel-like film coming from electrolyte decomposition could contribute sectional capacity [38, 39]. For another, it is attributed to the increased reactive sites of Co3O4-TiO2 composite which have been activated in the subsequent cycles, giving support to lithium-ion storage [40, 41]. More importantly, the unique honeycomb-spherical Co3O4-TiO2 architecture takes an irreplaceable position in the cycles, and the two integrated phases possess wonderful synergistic effect. On the one hand, the introduction of TiO2 NPs embedded in the staggered Co3O4 NSs could make the
nanostructure more compact and stable. On the other hand, the honeycomb-spherical Co3O4 owns homogeneously dispersion of porous, which could offer enough room to place the TiO2 NPs and accommodate volume change. Therefore, the excellent cycle performance was realized, which is more superior to that of pristine Co3O4. Rate capability of the two materials was also evaluated at various current density in Fig.7 (b). It is notable that the Co3O4-TiO2 composite has a better reversible capacity than pure Co3O4 at any rate. At the beginning, the Co3O4-TiO2 electrode reveals a reversible capacity of ~1300 mAh/g at 100 mA/g. When the current density increased to 300 and 500 mA/g, there is only a rarely decline for the Co3O4-TiO2 composite. Moreover, it still maintains a higher capacity of ~700 mAh/g even if the ,current density becomes 1000 mA/g compared with pristine Co3O4 electrode with ~450 mAh/g. When the current density comes back to 100 mA/g, a great capacity of ~1300 mAh/g was obtained which correspond to the first 10 cycles of rate ability. Moreover, it still gets a incredible capacity of ~1300 mAh/g again even after a higher current density of 2 A/g and 5 A/g, which is also preferable than the pure Co3O4, indicating the excellent structural stability of hybrid electrode. Compared with pure Co3O4, the Co3O4-TiO2 electrode displays a larger specific surface area and wider pores distribution owing to the introduction of TiO2 NPs, which makes the honeycomb-spherical Co3O4 nanostructure a more dense structure and the inner pores become smaller. It could not only accelerate the transmission rate of ions and electron, but also strengthen the structural stability of the composite, contributing to a higher reversible capacity and excellent rate ability.
In order to display the actual application prospect of unique Co3O4-TiO2 composite, coin-type full cells were assembled to compare the electrochemical performance of Co3O4-TiO2 composite and traditional graphite with LiFePO4 as counter electrode in Fig. 8. As shown in Fig. 8 (a), the composite delivers higher specific capacity of 658 mAh/g at 200 mA/g after 50 cycles and the specific capacity becomes stable, while the traditional graphite just shows a specific capacity of ~200 mAh/g, confirming that the honeycomb-spherical Co3O4-TiO2 composite owns superior structural stability. Fig. 8 (b) exhibits the rate ability of Co3O4-TiO2 composite and traditional graphite. What we could discover is that the specific capacity of Co3O4-TiO2 composite is higher than traditional graphite at any rate. The composite displays high specific capacity of ~320 mAh/g at 800 mA/g and the specific capacity could come back to ~800 mAh/g when the current density becomes 200 mA/g. These promising properties of the full cells reveal that the honeycomb-spherical Co3O4-TiO2 composite is a prospective anode material for lithium ion batteries. The morphology and selected-area electron diffraction (SAED) of Co3O4-TiO2 after 100 cycles at 200 mA/g was investigated to further support the excellent cycling property of the composite shown in Fig. 9. In Fig. 9 (a), the SEM image presents that the staggered Co3O4 NFs could still be sustained, indicating that the composite delivers highly structural stability during multiple charge-discharge process. Meanwhile, selected-area electron diffraction (SAED) pattern was performed shown in Fig. 9 (b). The diffraction rings of (101) for TiO2 and (311), (220), (440) for Co3O4
could still be precisely exhibited, confirming the wonderful crystalline of the honeycomb-spherical hybrid material. Electrochemical impedance spectroscopy (EIS) was also selected to further illustrate the excellent electrochemical performance. Fig. 10 presents the Nyquist plots of both materials. Such a pattern of the EIS can be expressed by an equivalent circuit in the inset of Fig 10. The high frequency area reflects the electrolyte resistance (Rs). The semicircle of the impedance pattern is assigned to the charge-transfer impedance on electrode in the high and middle frequency region (Rct and C). The inclined line of low-frequency represents the lithium-diffusion process within electrode (Zw and Cint) [42, 43]. Accroding to the fitting results, the resistances Rs and Rct of composite are 29.18 Ω and 229.2 Ω, which is lower than the value of Co3O4 (81.42 Ω and 275.5 Ω). Noticeably, it can be seen that the composite displays a smaller semicircle than that of the pure Co3O4 in high and middle-frequency ranges, which indicates the smaller charge transfer resistance of the composite, corresponding to its good electronic conductivity. The main reason is that the introduction of TiO2 NPs process interfacial storage phenomena, which could accelerate transmission rate of Li ions and enhance the storage performance [44, 45] so that a lower impedance of composite was realized. 4. Conclusions In summary, we employed a facile and efficient route to synthesis an unprecedented honeycomb-spherical Co3O4-TiO2 nanocomposite. The nanocomposite electrode shows stable reversible capacity during the long cycles and improved rate performance when being applied in lithium ion half and full cells, respectively. The
in-situ XRD pattern reveals the mechanism of the electrochemical reactions. The key factor of the superior electrochemical performance is related to the unique honeycomb-spherical Co3O4-TiO2 including the staggered Co3O4 NFs and the TiO2 NPs, which achieves excellent structure stability and provides more channels for the Li+ insertion/extraction process. It is therefore concluded that the Co3O4-TiO2 nanocomposite electrode is a promising candidate for the applications in long cycle and high rate lithium ion batteries.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11204090), the Project of DEGP (No. 2013KJCX0050), the Scientific and Technological Plan of Guangdong Province, Longhua District of Shenzhen City, Guangzhou City and its Yuexiu District, China (Nos. 2013B040402009, 2014B040404067,
2014A040401005,
2015A040404043,
2015A090905003,
2016A040403109,
2016A040403106,
2016A050502054,
2016A050503019,
20150529A0900008, 201508030033, 2013-CY-007).
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Captions Fig. 1 (a) Schematic illustration of the possible formation mechanism of Co3O4-TiO2 composite; (b) XRD pattern of Co3O4, TiO2 and Co3O4-TiO2; (c) overall EDS of Co3O4-TiO2 . Fig. 2 (a) SEM of pure Co3O4 with scale bar 1 μm; (b, c, d) Co3O4-TiO2 composite with scale bar 10 μm, 2 μm and 2 μm; (e) TEM pattern of Co3O4-TiO2 composite; (f) HRTEM of Co3O4-TiO2 composite ; (g) SADE pattern of Co3O4-TiO2 composite. Fig. 3 (a) XPS pattern for Ti 2p; (b) Co 2p3/2 and 2p3/1 of Co3O4-TiO2 composite. Fig. 4 (a) The N2 adsorption/desorption isotherms; (b) the pore size distribution of pure Co3O4 and Co3O4-TiO2 composite. Fig. 5 (a) CV curve of pure Co3O4 and Co3O4-TiO2 composite at a scan rate of 0.2 mV/s; (b) charge/discharge curve of Co3O4-TiO2 composite at 200 mA/g. Fig. 6 (a) The discharge/charge profile of the Co3O4-TiO2 electrode at 400 mA/g. The letters A to E denote the different lithiated states for corresponding in-situ XRD pattern. (b) in-situ XRD spectra collected at various states A to E. Fig. 7 (a) Cycle performance of pristine Co3O4 and Co3O4-TiO2 composite at 200 mA/g; (b) Rate performance of pristine Co3O4 and Co3O4-TiO2. Fig. 8 (a) Cycle performance of Co3O4-TiO2 electrode and graphite in full cells; (b) Rate ability of Co3O4-TiO2 electrode and graphite in full cells.
Fig. 9 (a) The morphology of Co3O4-TiO2 electrode after 100 cycles; (b) The selected-area electron diffraction (SAED) pattern of Co3O4-TiO2 electrode after 100 cycles Fig. 10 Electrochemical impedance spectra of pristine Co3O4 and Co3O4-TiO2. Insert shows the equivalent circuit.
S0013-4686(16)32515-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.11.153 EA 28443
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
30-7-2016 3-11-2016 27-11-2016
Please cite this article as: Youpeng Li, Kaina Shang, Wenjie Zhou, Linyin Tan, Xuexue Pan, Mingna Liao, Jianfei Lei, Lingzhi Zhao, Honeycomb-Spherical Co3O4-TiO2 Hybrid Materials for Enhanced Lithium Storage, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.11.153 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Honeycomb-Spherical Co3O4-TiO2 Hybrid Materials for Enhanced Lithium Storage
Youpeng Lia,d, Kaina Shanga,d, Wenjie Zhouc, Linyin Tanc, Xuexue Pana, Mingna Liaoa,d, Jianfei Leib* [email protected], Lingzhi Zhaoa,d* [email protected]
a
Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and
Devices, Institute of Opto-Electronic Materials and Technology, South China Normal University, Guangzhou 510631, China b
School of Physics and Engineering, Henan University of Science and Technology,
Luoyang 471023, China c
Guangzhou Zhixin High School, Guangzhou 510080, China
d
Guangdong Provincial Engineering Technology Research Center for Low Carbon
and Advanced Energy Materials, Guangzhou 510631, China *Corresponding authors
GRAPHICAL ABSTRACT
The honeycomb-spherical Co3O4-TiO2 composite has been successfully synthesised though a hydrothermal method with deionized water and ethanol as solvent. The Co3O4-TiO2 composite displays a enhanced electrochemical performance compared with pure Co3O4.
Highlights
We introduces an effective, inexpensive and large-scale production route to the fabrication of a novel honeycomb-spherical Co3O4-TiO2 nanocomposite anode material for use in rechargeable lithium ion batteries.
The nanocomposite electrode shows stable reversible capacity during the long cycles and improved rate performance when being applied in lithium ion half and full cells, respectively.
The unusual morphology of honeycomb-spherical Co3O4 and the introduction of secondary phase of TiO2 nanoparticles are suggested to be responsible for the enhanced electrochemical performance.
Abstract This work introduces an effective, inexpensive and large-scale production route to the fabrication of a novel honeycomb-spherical Co3O4-TiO2 nanocomposite anode material for use in rechargeable lithium ion batteries. The microstructure and phase composition of the Co3O4-TiO2 product were characterized systematically. The Co3O4-TiO2 nanocomposite electrode yielded fairly high reversible capacity of ~1000 mAh/g at 200 mA/g after 100 cycles. Moreover, the electrode exhibited extraordinary rate ability and could regain its original specific capaities as reversing to the low current densities. Meanwhile, when being applied as anode material in full cells, it still achieved remarkably improved electrochemical performance compared with traditional graphite material. Noticeably, in-situ XRD measurement verified the
conversion mechanism for the Co3O4-TiO2 electrode. The likely contributing factors to these outstanding electrochemical properties of the Co3O4-TiO2 nanocomposite could be related to the distinctive morphology including interlaced Co3O4 nanoflakes (NFs) and the TiO2 nanoparticles (NPs), which contains superior structural stability and allows more channels for the Li+ insertion/extraction reaction. This research shows that the as-prepared Co3O4-TiO2 nanocomposite could be suitable for use as a favorable cycle performance anode material for lithium ion batteries.
Keywords: Lithium-ion battery; Anode materials; Co3O4; Honeycomb-spherical; TiO2.
1. Introduction Rechargeable lithium-ion batteries (LIBs), as a promising energy storage system, are widely applied in portable electronics, electric vehicles and stationary energy storage for their high energy density, long cycling life, high efficieny and no memony effect [1-3]. Graphite-based material has been used comprehensively as the mainstream anode materials in commercial LIBs due to its low cost and encouraging capacity retention, but the limited specific capacity (372 mAh/g) could not satisfy the ever expanding requirements of individuals [4-6]. Therefore, tremendous efforts have been dedicated to heighten the electrochemical performance of anode materials [7-10]. Co3O4 has become one of the most popular candidates for anode materials to replace the commercial graphite anode owning to its high theoretical specific capacity (890 mAh/g), high tap density and excellent electrochemical property [11, 12]. However, its capacity retention and rate capability still need to be further enhanced because of its low initial coulombic efficiency, poor electrical conductivity and fast capacity fading during the Li+ insertion/extraction process [13, 14]. At present, an effective strategy to strengthen the electrochemical performance of Co3O4 anode is to construct nanomaterials with favorable morphology (nanowires, nanorods, nanosheets) [15, 16], but the poor electrical conductivity and the large volume effect have still not been solved fundamentally [17]. Thereby, it is necessary to modify Co3O4 with secondary phase, which may facilitate the electron transfer rate and accommodate volume effect during the Li+ insertion/extration reaction [18, 19]. Compared with Co3O4, TiO2 not only possesses unique merits including abundance and environmentally friendly, but
also owns attractive electronic properties and robust structural stability (less than 4% in volume change) [20]. These features make the combined Co3O4-TiO2 nanocomposite electrode a befitting anode material for long-life energy storage. So far, various Co3O4-TiO2 composites have been intensively investigated. For instance, Co3O4-TiO2 nanobelt arrays have been fabricated as an anode material. After 100 cycles, it maintained a capacity of 300 mAh/g at a current density of 100 mA/g [21]. Xia et al. Prepared the Co3O4-TiO2 core-branch hollow nanowire arrays, but it just exhibited a retention capacity of 130 mAh/g at 2 A/g [22]. Similarly, Co3O4-TiO2 hierarchical heterostructures have also been synthesized by a hydrothermal method, and delivered a reversible capacity of 663 mAh/g [23]. Meanwhile, nano-coaxial Co3O4-TiO2 arrays exhibit great high capacity of ~850 mAh/g after 50 cycles [24]. Hence, we could conclude that the peculiar morphology takes the dominant role of electrochemical performance of Co3O4-TiO2 nanocomposite and the specific capacity still needs to be further improved. Herein,
we prepared a new type of honeycomb-spherical Co3O4-TiO2
nanocomposite via a simple and low-cost approach by hydrothermal treatment and subsequent calcining process. In our previous work, we have synthesized a switch-like Co3O4-TiO2 hybrid electrode, which delivered a high capacity of 668 mAh/g after 120 cycles [25]. Here, we continue to prepare the novel honeycomb-spherical
Co3O4-TiO2
nanocomposite
to
further
improve
the
electrochemical performance. We believe that the introduction of TiO2 NPs inside the porous Co3O4 NFs could not only prevent the nanoflakes from structural damage and
provide other active sites, but also strengthen the phase stability of the whole system for the ignorable volume effect of TiO2 NPs, which will be beneficial to the long cycle performance and high rate ability. 2. Experimental Details 2.1 Materials synthesis All the reagents in this work were purchased from the Aladdin without any further purification. In a typical process, 2.5 g of Co(NO3)2·6H2O and 2.5 g of urea were dissolved in 60 ml solution containing deionized water and ethanol (4:1 in volume) with the continuous magnetic stirring. Then, 0.08 g anatase TiO2 NPs (~10 nm in diameter) and 0.3 g hexadecyltrimethylammonium chloride (CTAC) were added into the mentioned solution. After stirring for 1 h, the solution was transferred to a teflon-lined stainless autoclave and placed in an oven at 160
for 12 h. When the
reaction system was cooled to room temperature naturally, the obtained precipitate was filtered, washed for several times, and collected before being dried at 60 h. The dried powder was calcined at 400
for 10
for 3 h in air to gain the
honeycomb-spherical Co3O4-TiO2 nanocomposition. As a comparison, the pure Co3O4 was prepared by the similar steps as Co3O4-TiO2 without adding TiO2 NPs and CTAC. 2.2 Electrodes and cells fabrication The coin-type half cells and full cells were assembled in an argon-filled glove box (Mikrouna, Sukei1220/750) to evaluate the electrochemical properties of the Co3O4-TiO2 composite, the pristine Co3O4 and graphite anode materials. The anode slurry was concocted by the as-prepared product (Co3O4-TiO2, Co3O4 or graphite
anode), conductive carbon black (Super P) and Poly (vinylidene fluoride) (PVDF) in a weight ratio of 7:2:1 in the N-methyl-2-pyrrolidone (NMP) solvent. Then it was coated on a cleaned copper foil before being cut into suitable discs served as electrodes. 1M LiPF6 in a solvent mixture of ethylene carbonate and diethylene carbonate (1:1vol.%) were used as the electrolyte. Lithium metal for half cells and LiFePO4 for full cells were used as the counter electrode, respectively. 2.3 Characterization The structure and the surface properties of the obtained Co3O4-TiO2, TiO2 and Co3O4 were characterized by X-ray diffraction (XRD, BRUKER D8 ADVANCE, Cu K radiation (1.5406 nm)) and scanning electron microscopy (SEM, ZEISS Ultra 55, 5kV, Pt-spraying treatment), transmission electron microscopy (TEM, JEM-2100HR, 200kV), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD), respectively. The specific surface area and pore distribution of the samples were determined
by
the
N2
adsorption/desorption
experiment
with
the
Brunauer-Emmett-Teller method (BET Micromeritics ASAP 2020). The charge/discharge tests of the assembled cells were performed on the NEWARE BTS computer-controled galvanostat method by using different current densities at room temperature. The specific capacity of Co3O4-TiO2 electrode was calculated based on the mass of both Co3O4 and TiO2. Similarly, specific capacity of Co3O4 and graphite electrode were calculated based on the mass of Co3O4 and graphite, respectively. Cyclic voltammetry (CV) experiments were carried out with an electrochemical test analyzer (CHI 660D). Each electrochemical test was performed
with several cells. Sufficiently the operation influence of cell assembly was taken into account, the most reasonable data was selected out as the final result. 3. Results and discussion 3.1 Formation mechanism Fig.1 (a) shows the possible formation mechanism of honeycomb-spherical Co3O4-TiO2 nanocomposite. For a typical process, a host of Co2+ attach on the surface of the dispersed TiO2 NPs with continuous stirring. When a appropriate external environmental condition is achieved, large amounts of Co2+ begin to hydrolysis and then attach to the surface of TiO2 NPs to form TiO2@Co2(OH)2CO3 unite, while other Co2+ in the solution tend to hydrolyze to form honeycomb-spherical structure (5Co2+ + 5H2NCONH2 + 10H2O → 2CoCO3·3Co(OH)2·H2O + CO2↑ + 10NH4+). In this course, the formed TiO2 @Co2(OH)2CO3 unites insert to the inside of Co2(OH)2CO3 honeycomb-sphere, in which caused the widely pore distribution owning to the embedded TiO2 @Co2(OH)2CO3 unites. The rest TiO2 NPs adhere to the outside of Co2(OH)2CO3 nanosheets. All the Co2(OH)2CO3 turn into Co3O4 after a calcining process (2CoCO3·3Co(OH)2·H2O → 2Co3O4 + CO2↑ + H2O). 3.2 Physical Properties The phase composition of pure Co3O4, TiO2 NPs and Co3O4-TiO2 were investigated by X-ray diffraction (XRD) analysis (Fig.1 (b)). The XRD patterns revealed that every diffraction peaks could be indexed to anatase TiO2 (JCPDS No.21-1272) and spinel phase Co3O4 (space group: Fd-3m, lattice constant a = 8.08 Å, JCPDS No. 42-1467). From the Fig.1 (b) we can find that the well-defined peaks
of TiO2 NPs located at 25.4°, 37.9° and 48.2° could be indexed to (101), (004) and (200) plane and peaks of Co3O4 NFs located at 31.1°, 36.9°, 43.2°, 59.4° and 65.2° correspond to (220), (311), (222), (511) and (440) plane. No apparent impurity peaks were detected. The sharp and strong characteristic peaks testify the excellent crystallinity and high crystalline purity, which confirmed that the nanocomposite elements are as prepared Co3O4-TiO2 after the reasonable synthetic method. It can be clearly found from the curve of pure Co3O4 that the main peak of (311) is quite strong, corresponding to the expected crystal face exposure [26, 27]. From the curve of Co3O4-TiO2, it can be seen that some of the diffraction peaks of Co3O4 and TiO2 become weak or unconspicuous, which is attributed to the formation process of Co3O4-TiO2 composite after solution reaction under superhigh-pressure and temperature condition. As shown in Fig.1 (c), it should be noticed that the EDS measurement also detected the existence of O, Ti and Co with an atom ratio of 30:1:18.5, corresponding to the datas of XRD. To investigate the morphologies of Co3O4-TiO2 composite and pure Co3O4, scanning electron microscope (SEM) was performed at different magnifications. It could be seen from Fig.2 (a) that the Co3O4 NFs are staggered with each other and the honeycomb-spherical structure has been formed successfully. Fig.2 (b), 2 (c) and 2 (d) display the SEM images of Co3O4-TiO2 nanocomposite at different magnifications. We could discover that the unique honeycomb-spherical structure of Co3O4 has not been changed after the adding of TiO2 NPs. However, Some TiO2 NPs can be hardly found in the Co3O4 NFs in Fig.2 (c) and (d), it is mainly due to the small size (~10 nm
in diameter) of TiO2 NPs. The inner contraction of the Co3O4 NFs could provide many pores which would not only enlarge the effective electrode/electrolyte contact area and shorten the migrating distance of lithium ions, but also tolerate the volume variation during the charge/discharge process [28-29]. Transmission electron microscopy (TEM) was used to further explore the specific structure. We can clearly find that the TiO2 NPs are uniformly dispersed in the Co3O4 NFs in Fig.2 (e), confirming that the TiO2 NPs have inserted into the honeycomb-spherical Co3O4 during the process of reaction. The introduction of TiO2 NPs embedded in unique honeycomb-sphere will enhance the structural stability of the system and avoid their self-aggregation during the charge/discharge reaction[30], which expected that more outstanding synergy and advanced electrochemical performance could be realized compared with the pure Co3O4. In Fig.2 (f), some lattice fringes of composite were clearly detected by HRTEM, indicating that the TiO2 and Co3O4 have well crystalline structure. The displayed lattice spacing is measured to be ~0.35 nm corresponding to the (101) plane of TiO2, ~0.25 and ~0.29 nm corresponding to the (311) and (220) planes of Co3O4. The selected-area electron diffraction (SAED) pattern was also performed to confirm phase of Co3O4-TiO2 composite shown in Fig.2 (g). The diffraction rings of (101) for TiO2 and (311), (220), (440) for Co3O4 are distinctly described, confirming the wonderful crystalline. X-ray photoelectron spectroscopy (XPS) was also employed to estimate the surface electronic state of hybrid electrode. In Fig.3 (a), the XPS spectra of Ti double peaks shows the binding energy of 459.1 eV and 466.1 eV, respectively, which is well
coincidence with the value of Ti4+ in TiO2 [31]. Fig.3 (b) exhibits two peaks located at 780.1 eV and 795.2 eV, corresponding to Co 2p3/2 and 2p3/1, respectively, which further confirms the existence of Co3O4 [32]. Brunauer-Emmett-Teller (BET) gas-sorption measurements were also used to investigate
the
specific
surface
area
and
the
pores
dispersion
of
the
honeycomb-spherical Co3O4-TiO2 nanocomposite and the pristine Co3O4. As shown the N2 adsorption-desorption curve in Fig.4 (a), the hybrid materials enjoy a larger specific surface area of 29.07 m2/g compared with the pure Co3O4 with 14.16 m2/g. It should be owing to the introduction of TiO2 NPs decreasing the size of pores. Extensive specific surface area increases the reaction sites of the nanocomposite, contributing to the excellent electrochemical property. The Barrett-Joyner-Helanda (BJH) pore dispersion is also demonstrated in Fig.4 (b). Obviously, a smaller pore size of composite is obtained compared with the pure Co3O4, corresponding to the analysis of SEM and TEM. 3.3 Electrochemical Performance To give a more specific explanation of the property, cyclic voltammetry (CV) curves of Co3O4-TiO2 and pure Co3O4 are displayed at a scan rate of 0.2 mV/s in Fig.5 (a). In the 1st cycle of Co3O4-TiO2 electrode, a quite small peak appears at 1.6 V, which is corresponding to the reduce potential of TiO2 phase (TiO2 + xLi+ + xe- → LixTiO2, 0 ≤ x ≤ 1) [33]. A limited specific capacity of TiO2 may give a reasonable interpretation of the small peak [34]. During the following negative scanning, a sharp reduced peak appears at 0.71 V, which is coincidence with the curve of pure Co3O4
electrode, should originate from the reduction of Co3O4 to Co, accompanying the emergence of Li2O [35]. There is a great gap between the 1st and the following reduce process of both electrodes . This should be due to the irreversible loss of lithium from the deposition of electrolyte and formation of solid electrolyte interphase (SEI) [36, 37]. In the Li+ extraction reaction, the opposite peak is owning to the decomposition of Li2O, the oxidation of Co to Co3O4 (Li2O + Co → Co3O4 + Li, 0 ≤ x ≤ 1) and the extraction of Li+ from TiO2 (LixTiO2 → TiO2 + xLi+ + xe-). Generally, the value of anodic peaks are 2.1 V [23] and 2.2 V [25] for the Co3O4 and TiO2, respectively. Nevertheless, only one oxydic peak could be clearly detected at 2.1 V, which may be caused by the embezzle of the peak of Co3O4 for its alpine specific capacity [24]. After the 1st cycle, the following curves of Co3O4-TiO2 electrode are more coincident than that of pure Co3O4 electrode, indicating the better cycle ability of the unique structure. Fig.5 (b) shows the charge/discharge curves of the composite at current density of 200 mA/g. From the 1st cycle we could see that no apparent discharge platform at 1.6 V is detected owning to the negligible specific capacity contribution of TiO2. Nevertheless, it is obvious that the charge/discharge plateaus of Co3O4 are clearly shown at ~2.1 V and ~1.1 V respectively, which is similar to those reported by previous reports [23, 24]. A quite potential gap between the 1st and the 2nd curves could be easily discovered, which is corresponding to the CV profiles above. The 1st charge/discharge process of composite capacity is 1136/1458 mAh/g, exhibiting a coulombic efficiency of 77.9% caused by a more deposition of electrolyte and
formation of SEI film duo to the enhanced specific area. The 10th charge and discharge curves are nearly coincidence with those in the 2nd curves, confirming that the reversible capacity possesses wonderful steadiness after the 1st cycle, corresponding to the conclusion of CV curves. The excellent cycle performance is mainly attributed to the unique honeycomb-spherical nanostructure. After 80 cycles, a superior reversible capacity with 1009 mAh/g is obtained, delivering that the nanostructure of electrode is quite stable and not be damaged after long cycles. To further confirm the mechanism of electrochemical reactions, in-situ XRD patterns of Co3O4-TiO2 composite were recorded at different states. Fig.6 (a) shows the first discharge-charge curves at 400 mA/g. The main peaks of Co3O4-TiO2 composite were collected at specific state denoted as A-E Fig.6 (b). The black curve in Fig.6 (b) shows the peaks of composite electrode at the open circuit voltage state (A). The (101) plane of TiO2 and (220), (311) planes of Co3O4 are quiet strong and correspond to the explanations of XRD. During the following discharge process, the (101) plane of TiO2 disappears firstly, indicating that the reduce process of TiO2 takes place firstly, corresponding to the analysis of CV curves. Also, the peaks of Co3O4 become weaker for the reduce reaction of Co3O4. The peaks of Co3O4-TiO2 composite vanish in the full-discharge state (D). In the subsequent full-charge state (E), the signals of Co3O4-TiO2 composite reappear, further confirming the conversion mechanism of Co3O4-TiO2 electrode. Also, we can discover that the peaks of full-charge state (E) are still very strong and close to the open circuit voltage state
(A), indicating that the composite nearly has no change during the charge-discharge process, which may contribute to the excellent electrochemical performance. Fig.7 (a) shows the cycle performance of pure Co3O4 and Co3O4-TiO2 at a current density of 200 mA/g. It is obvious that the Co3O4-TiO2 composite delivers a markedly improved cycling property in terms of much higher initial capacity and reversible capacity after 100 cycles compared with pristine Co3O4. Apparently, the Co3O4-TiO2 composite exhibits the 1st charge/discharge capacities of 1136/1458 mAh/g at 200 mA/g, which is higher than that of Co3O4. What is worth mentioning is that both materials have suffered quite capacity damping in the second discharge process, the fundamental reason is Li-consuming reaction containing the vast decomposition of electrolyte and formation of SEI film owing to the large specific surface area. After 100 cycles, Co3O4-TiO2 composite just shows a little capacity fading and a higher specific capacity of ~1000 mAh/g (better than Co3O4) is obtained, demonstrating that the integrity of hybrid electrode is still maintained after long cycles. It may be caused by several reasons. For one thing, the reversible formation/dissolution of a gel-like film coming from electrolyte decomposition could contribute sectional capacity [38, 39]. For another, it is attributed to the increased reactive sites of Co3O4-TiO2 composite which have been activated in the subsequent cycles, giving support to lithium-ion storage [40, 41]. More importantly, the unique honeycomb-spherical Co3O4-TiO2 architecture takes an irreplaceable position in the cycles, and the two integrated phases possess wonderful synergistic effect. On the one hand, the introduction of TiO2 NPs embedded in the staggered Co3O4 NSs could make the
nanostructure more compact and stable. On the other hand, the honeycomb-spherical Co3O4 owns homogeneously dispersion of porous, which could offer enough room to place the TiO2 NPs and accommodate volume change. Therefore, the excellent cycle performance was realized, which is more superior to that of pristine Co3O4. Rate capability of the two materials was also evaluated at various current density in Fig.7 (b). It is notable that the Co3O4-TiO2 composite has a better reversible capacity than pure Co3O4 at any rate. At the beginning, the Co3O4-TiO2 electrode reveals a reversible capacity of ~1300 mAh/g at 100 mA/g. When the current density increased to 300 and 500 mA/g, there is only a rarely decline for the Co3O4-TiO2 composite. Moreover, it still maintains a higher capacity of ~700 mAh/g even if the ,current density becomes 1000 mA/g compared with pristine Co3O4 electrode with ~450 mAh/g. When the current density comes back to 100 mA/g, a great capacity of ~1300 mAh/g was obtained which correspond to the first 10 cycles of rate ability. Moreover, it still gets a incredible capacity of ~1300 mAh/g again even after a higher current density of 2 A/g and 5 A/g, which is also preferable than the pure Co3O4, indicating the excellent structural stability of hybrid electrode. Compared with pure Co3O4, the Co3O4-TiO2 electrode displays a larger specific surface area and wider pores distribution owing to the introduction of TiO2 NPs, which makes the honeycomb-spherical Co3O4 nanostructure a more dense structure and the inner pores become smaller. It could not only accelerate the transmission rate of ions and electron, but also strengthen the structural stability of the composite, contributing to a higher reversible capacity and excellent rate ability.
In order to display the actual application prospect of unique Co3O4-TiO2 composite, coin-type full cells were assembled to compare the electrochemical performance of Co3O4-TiO2 composite and traditional graphite with LiFePO4 as counter electrode in Fig. 8. As shown in Fig. 8 (a), the composite delivers higher specific capacity of 658 mAh/g at 200 mA/g after 50 cycles and the specific capacity becomes stable, while the traditional graphite just shows a specific capacity of ~200 mAh/g, confirming that the honeycomb-spherical Co3O4-TiO2 composite owns superior structural stability. Fig. 8 (b) exhibits the rate ability of Co3O4-TiO2 composite and traditional graphite. What we could discover is that the specific capacity of Co3O4-TiO2 composite is higher than traditional graphite at any rate. The composite displays high specific capacity of ~320 mAh/g at 800 mA/g and the specific capacity could come back to ~800 mAh/g when the current density becomes 200 mA/g. These promising properties of the full cells reveal that the honeycomb-spherical Co3O4-TiO2 composite is a prospective anode material for lithium ion batteries. The morphology and selected-area electron diffraction (SAED) of Co3O4-TiO2 after 100 cycles at 200 mA/g was investigated to further support the excellent cycling property of the composite shown in Fig. 9. In Fig. 9 (a), the SEM image presents that the staggered Co3O4 NFs could still be sustained, indicating that the composite delivers highly structural stability during multiple charge-discharge process. Meanwhile, selected-area electron diffraction (SAED) pattern was performed shown in Fig. 9 (b). The diffraction rings of (101) for TiO2 and (311), (220), (440) for Co3O4
could still be precisely exhibited, confirming the wonderful crystalline of the honeycomb-spherical hybrid material. Electrochemical impedance spectroscopy (EIS) was also selected to further illustrate the excellent electrochemical performance. Fig. 10 presents the Nyquist plots of both materials. Such a pattern of the EIS can be expressed by an equivalent circuit in the inset of Fig 10. The high frequency area reflects the electrolyte resistance (Rs). The semicircle of the impedance pattern is assigned to the charge-transfer impedance on electrode in the high and middle frequency region (Rct and C). The inclined line of low-frequency represents the lithium-diffusion process within electrode (Zw and Cint) [42, 43]. Accroding to the fitting results, the resistances Rs and Rct of composite are 29.18 Ω and 229.2 Ω, which is lower than the value of Co3O4 (81.42 Ω and 275.5 Ω). Noticeably, it can be seen that the composite displays a smaller semicircle than that of the pure Co3O4 in high and middle-frequency ranges, which indicates the smaller charge transfer resistance of the composite, corresponding to its good electronic conductivity. The main reason is that the introduction of TiO2 NPs process interfacial storage phenomena, which could accelerate transmission rate of Li ions and enhance the storage performance [44, 45] so that a lower impedance of composite was realized. 4. Conclusions In summary, we employed a facile and efficient route to synthesis an unprecedented honeycomb-spherical Co3O4-TiO2 nanocomposite. The nanocomposite electrode shows stable reversible capacity during the long cycles and improved rate performance when being applied in lithium ion half and full cells, respectively. The
in-situ XRD pattern reveals the mechanism of the electrochemical reactions. The key factor of the superior electrochemical performance is related to the unique honeycomb-spherical Co3O4-TiO2 including the staggered Co3O4 NFs and the TiO2 NPs, which achieves excellent structure stability and provides more channels for the Li+ insertion/extraction process. It is therefore concluded that the Co3O4-TiO2 nanocomposite electrode is a promising candidate for the applications in long cycle and high rate lithium ion batteries.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11204090), the Project of DEGP (No. 2013KJCX0050), the Scientific and Technological Plan of Guangdong Province, Longhua District of Shenzhen City, Guangzhou City and its Yuexiu District, China (Nos. 2013B040402009, 2014B040404067,
2014A040401005,
2015A040404043,
2015A090905003,
2016A040403109,
2016A040403106,
2016A050502054,
2016A050503019,
20150529A0900008, 201508030033, 2013-CY-007).
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Captions Fig. 1 (a) Schematic illustration of the possible formation mechanism of Co3O4-TiO2 composite; (b) XRD pattern of Co3O4, TiO2 and Co3O4-TiO2; (c) overall EDS of Co3O4-TiO2 . Fig. 2 (a) SEM of pure Co3O4 with scale bar 1 μm; (b, c, d) Co3O4-TiO2 composite with scale bar 10 μm, 2 μm and 2 μm; (e) TEM pattern of Co3O4-TiO2 composite; (f) HRTEM of Co3O4-TiO2 composite ; (g) SADE pattern of Co3O4-TiO2 composite. Fig. 3 (a) XPS pattern for Ti 2p; (b) Co 2p3/2 and 2p3/1 of Co3O4-TiO2 composite. Fig. 4 (a) The N2 adsorption/desorption isotherms; (b) the pore size distribution of pure Co3O4 and Co3O4-TiO2 composite. Fig. 5 (a) CV curve of pure Co3O4 and Co3O4-TiO2 composite at a scan rate of 0.2 mV/s; (b) charge/discharge curve of Co3O4-TiO2 composite at 200 mA/g. Fig. 6 (a) The discharge/charge profile of the Co3O4-TiO2 electrode at 400 mA/g. The letters A to E denote the different lithiated states for corresponding in-situ XRD pattern. (b) in-situ XRD spectra collected at various states A to E. Fig. 7 (a) Cycle performance of pristine Co3O4 and Co3O4-TiO2 composite at 200 mA/g; (b) Rate performance of pristine Co3O4 and Co3O4-TiO2. Fig. 8 (a) Cycle performance of Co3O4-TiO2 electrode and graphite in full cells; (b) Rate ability of Co3O4-TiO2 electrode and graphite in full cells.
Fig. 9 (a) The morphology of Co3O4-TiO2 electrode after 100 cycles; (b) The selected-area electron diffraction (SAED) pattern of Co3O4-TiO2 electrode after 100 cycles Fig. 10 Electrochemical impedance spectra of pristine Co3O4 and Co3O4-TiO2. Insert shows the equivalent circuit.