carbon composite as high-performance anodic material for lithium-ion batteries

Journal of Alloys and Compounds 819 (2020) 153375

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A bio-inspired nanofibrous Co3O4/TiO2/carbon composite as highperformance anodic material for lithium-ion batteries Fan Wang, Hang Yuan, Jianguo Huang* Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang, 310027, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 October 2019 Received in revised form 5 December 2019 Accepted 11 December 2019 Available online xxx

A bio-inspired nanofibrous Co3O4/TiO2/carbon composite was fabricated using natural cellulose substrate (laboratory filter paper) as both the carbon source and the structural scaffold. Employing a surface sol egel process to form a thin TiO2 gel layer coating on each cellulose nanofiber of the filter paper, the resulted composite sheet was successively carbonized in inert atmosphere to give a nanofibrous TiO2/ carbon composite; afterwards, nano-sized Co3O4 particles (10e30 nm) were uniformly deposited onto the yielded composite fibers via a simple hydrothermal method, resulting in the Co3O4/TiO2/carbon hybrid. The ternary composite possessed a unique hierarchically three-dimensional porous structure which inherited precisely from the initial filter paper. This special structure with the internal conductive carbon fiber and the external nano-sized Co3O4 particles offered the nanocomposite with an excellent electrochemical performance as anodic material for lithium-ion batteries. The optimal sample with a Co3O4 mass content of 50.6% delivered an initial discharge capacity of 1239 mAh g
1 at a current density of 100 mA g
1, which was far exceeded the theoretical capacity of Co3O4. The extra capacity is ascribed to the additional lithium storage sites provided by the large specific surface area of the composite, and the formation of the solid electrolyte interface (SEI) layer in the initial discharge process causing a large consumption of Liþ which results in some irreversible capacity. And after 200 discharge/charge cycles, a high reversible capacity of 764 mAh g
1 was maintained, which is higher than most of the Co3O4/carbon hybrid materials reported. This work provided a simple and efficient strategy for designing and preparing nanoscale metal-oxide/carbon composite material with considerable potential in energy storage and conversion. © 2019 Elsevier B.V. All rights reserved.

Keywords: Biomimetic synthesis Co3O4 Nanocomposites Anodes Lithium-ion batteries

1. Introduction Owing to their high energy density, lack of memory effect, low self-discharge, environmental-friendliness and long cycling lifespan, rechargeable lithium-ion batteries (LIBs) have gradually replaced traditional batteries (lead-acid batteries, nickel-cadmium batteries, etc.) and become the most widely used energy storage devices of our daily life [1e8]. However, the commonly used graphite anodic materials cannot meet the requirements of largescaled energy storage applications such as smart grid and electric vehicles for their low theoretical capacity (372 mAh g
1) [9e15]. Thus, various efforts have been made to develop novel anodic materials with higher capacities and power densities for the new generation LIBs [16e18].

* Corresponding author. E-mail address: [email protected] (J. Huang). https://doi.org/10.1016/j.jallcom.2019.153375 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Transition metal-oxides, such as TiO2, MnO2, MoO2, Fe3O4, CoO, Co3O4, NiO, etc., are regarded as good substitutions of graphite as anodic materials for their low cost, natural abundance and environmental-friendliness [2,4,5,19e21]. For example, some TiO2 based nanoarchitectures or composites have been developed and showed improved electrochemical performances [8e10]. Among them, Co3O4 is considered as one of the most promising candidates for it stores 8 lithium atoms per unit Co3O4 upon cycling [6,12,22], thus its theoretical capacity (890 mAh g
1) is almost three times of graphite [2,12,14]. However, Co3O4 species often suffers from low electrical conductivity, large volume change and severe particle aggregation during the discharge/charge processes, which lead to rapid capacity fading, electrode pulverization and poor stability [2,6,14,21,23]. Two main strategies have been adopted so far to overcome these limitations. One strategy is to construct various Co3O4 nanostructures, such as nanoflowers [24], nanowires [25,26], nanorods [27], nanotubes [28], and nanocages [29], to increase the surface area and thus

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enhance the lithium storage capabilities. However, the poor conductivities of the electrodes still cannot be improved, resulting in inferior rate performance and poor cycling stability. For example, Co3O4 mesoporous microdisks were synthesized and gained a reversible specific capacity of 765 mAh g
1 at 100 mA g
1; however, the high capacity was held for only 30 discharge/charge cycles [30]. Another crystalline-core/amorphous-shell Co3O4 particle with hierarchical nanostructure delivered an initial discharge capacity as high as 1349 mAh g
1 at 0.2C, and the capacity maintained at 888.4 mAh g
1 after 80 discharge/charge cycles. While it still suffered the low rate performance for the capacity faded to 217 mAh g
1 when the current density rose up to 2C [31]. Another prospective way is to introduce a buffer matrix such as carbon [15,32] or conducting polymer [33] to form the corresponding composites with the Co3O4 species to promote the conductivity of the electrodes as well as alleviate the stress caused by the repeated Liþ insertion/extraction processes [2,15,16]. When nano-sized Co3O4 particles were anchored on graphene-coated carbon fiber cloth, the composite possessed good rate performance and improved cycling stability, the specific capacity remained at 400 mAh g
1 at the current density of 100 mA g
1 after 300 cycles [34]. It was also reported that a CNT@Co3O4 sponge possessed excellent reversible capacity and maintained at 1050 mAh g
1 at a current density of 100 mA g
1 after 100 cycles [35]. Therefore, combining Co3O4 with specific conductive buffer matrix significantly avoided the capacity fading during the discharge/ charge cycling process. However, carbonaceous matrix such as graphene and carbon nanotube generally possess strong surfaces activity and tend to aggregate together during the electrode preparation process. For example, graphene sheets are easily overlapped again and CNTs are always difficult to be dispersed uniformly during the commonly used slurry preparation process to prepare the electrodes. These drawbacks hindered their practical applications. Biomass-based carbon materials serve as good carbonaceous buffer matrix, possessing the advantages of widely available of raw materials, simple synthesis procedures, and the environmentally friendly property. In addition, the exquisite structures of natural biomass would be accurately transmitted into the carbonized products, and all these advantages make biomass-based carbon materials the ideal substrates to combine with transition metaloxides to form hybrid materials. In our previous works, a series of metal-oxides related nanocomposites were fabricated by decorating metal oxide precursors on natural cellulose substance, the obtained metal-oxide/carbon hybrids showed enhanced lithium storage properties when used as electrode materials for LIBs, where the unique structural features of these composites played a vital role [20,36e41]. Herein, by employing natural cellulose substance (ordinary laboratory filter paper) as both the structural scaffold and the carbon source, a three-dimensional nanofibrous Co3O4/TiO2/carbon nanocomposite was fabricated. The composite possessed a unique hierarchically porous structure which inherited precisely from the initial cellulose substance. On the nanometer scale, the TiO2 layer (ca. 5 nm, composed of nanoparticles) was coated on the conductive carbon nanofiber, and Co3O4 nanoparticles (ca. 10e30 nm) were embedded in the voids in-between TiO2 nanoparticles and in the pores of the inner carbon fibers, as well as uniformly dispersed on the surface of the TiO2 layer. The thin TiO2 layer promotes the uniformly dispersion and high mass loading of Co3O4 [20]. Owing to the synergetic effect of the carbon, TiO2, and Co3O4 components as well as the interconnected nanofibrous structure, the Co3O4/TiO2/ carbon composite showed superior electrochemical performances such as high reversible capacity, enhanced cycling stability, and excellent rate capability when applied as anodic material for LIBs.

2. Experimental 2.1. Chemicals Cobalt (II) acetate tetrahydrate (Co(Ac)2,4H2O) and titanium (IV) n-butoxide [Ti(OnBu)4] were purchased from J＆K Chemical Co., Ltd.. The laboratory quantitative filter paper made from cotton was brought from Hangzhou Xinhua Paper Industry Co., Ltd. (China). Ammonium hydroxide (26%~28%), ethanol and toluene were got from Sinopharm Chemical Reagent Co., Ltd.. All the chemicals were guaranteed reagents and used without further purification. The water used was purified by a Milli-Q Advantage A10 system (Millipore, Bedford, MA, USA) with a resistivity higher than 18.2 MU cm. 2.2. Syntheses of the nanofibrous Co3O4/TiO2/carbon composites In a typical procedure (Scheme 1), a surface solegel process was firstly taken to form an ultrathin TiO2 gel layer on each cellulose nanofiber of the commercial laboratory filter paper (Scheme 1a) according to our previous reports [42e45]. Typically, a piece of commercial filter paper was placed in a suction filtering unit, and was washed by ethanol, followed by drying with air flow. Ten milliliters of titanium n-butoxide solution (100 mM in tolueneeethanol, v: v ¼ 1 : 1) was then passed through the filter paper slowly within 2 min. During this process, nucleophilic substitution reaction took place between the hydroxyl groups exist on the cellulose nanofibers and the titanium n-butoxide. Then, ethanol was immediately filtered to remove the unreacted metal alkoxide, and 20 mL of water was suction filtrated to promote hydrolysis and condensation. Finally, the filter paper was dried in air. By repeating this filtration/deposition cycle for ten times, thin TiO2 gel layers covered the surface of the cellulose fibers. The as-obtained TiO2-gel/cellulose composite (Scheme 1b) was then carbonized at 600  C for 3h in argon atmosphere with a heating rate of 1  C min
1 to obtain the TiO2/carbon nanocomposite (Scheme 1c). The thickness of the TiO2 layer was ca. 5 nm (one filtration/deposition cycle results in 0.5 nm thick of TiO2 layer according to our previous report [43]). A hydrothermal method was then applied to deposit Co3O4 nanoparticles on each TiO2/carbon nanocomposite fiber. Typically, 30.0 mg grinded TiO2/ carbon composite was immersed into 30.0 mL cobalt (II) acetate tetrahydrate aqueous solution. Then the mixture was ultrasonicated for 1 h and stirred vigorously for 6 h to make sure that each nanofiber was fully exposed to cobalt acetate solution. After that, 4.0 mL ammonia was slowly added into the mixture. Thereafter, the mixture was transferred into a 70 mL Teflon-lined stainless-steel autoclave for the hydrothermal reaction carried out at 150  C for 4 h. The precipitates were collected and washed with pure water for several times. After drying at 80  C in air, the solids were heated in a muffle at 300  C for 2 h to yield the nanofibrous Co3O4/TiO2/carbon composite. To regulate the Co3O4 content in the Co3O4/TiO2/carbon composites, the Co2þ precursor solutions with varied concentrations were used for depositing Co3O4 nanoparticles, which gave the corresponding samples with Co3O4 contents of 21.9 wt%, 37.4 wt% and 50.6 wt%, respectively, as determined by the TGA measurement (the results are shown below). The final sample was denoted as Co3O4/TiO2/carboneA1, A2 and A3 corresponding to the Co3O4 contents of 21.9 wt%, 37.4 wt% and 50.6 wt%, respectively (Scheme 1d). Depositing Co3O4 nanoparticles before or after the carbonization process of the filter paper substrate influences the structures and electrochemical performances of the Co3O4/TiO2/carbon composites. As control materials (The detailed synthesis procedure was listed in the supporting information), Co3O4 nanoparticles were firstly deposited on the as-prepared TiO2-gel/cellulose nanofibers

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Scheme 1. Schematic illustration of the preparation process of the Co3O4/TiO2/carbon composite using natural cellulose substance (filter paper) as both the structural scaffold and the carbon source. (a) a piece of filter paper and an individual cellulose nanofiber of it; (b) the cellulose nanofiber is deposited with TiO2-gel layer; (c) the yielded TiO2/carbon nanocomposite after the carbonization of the TiO2-gel/cellulose composite; (d) the samples of the Co3O4/TiO2/carboneA series which achieved by depositing Co(OH)2 nanoparticles on the TiO2/carbon composite and then heated in air to transform Co(OH)2 to Co3O4; (e) Co(OH)2 nanoparticles was first deposited onto the as-prepared TiO2-gel/cellulose nanofiber and yielded the Co(OH)2/TiO2/cellulose composite; (f) the Co(OH)2/TiO2/cellulose composite was carbonized in Ar followed by heated in air to obtain the samples of the Co3O4/TiO2/ carboneB series.

(Scheme 1b) using a similar hydrothermal method as that for the samples of Co3O4/TiO2/carboneA series displayed, but with higher concentrations of cobalt acetate solutions (5.0, 20.0 and 60.0 mM in this case). The obtained Co(OH)2/TiO2/cellulose composite (Scheme 1e) was then carbonized in argon atmosphere followed by calcinating in air to transform Co(OH)2 to Co3O4. The prepared control materials were named as Co3O4/TiO2/carboneB1, B2 and B3 corresponding to the Co3O4 contents of 6.9 wt%, 10.7 wt% and 15.7 wt% (Fig. S1a, determined by TGA), respectively (Scheme 1f).

a VG Escalab Mark II instrument equipped with a MgKa radiation source (hn ¼ 1253.6 eV). N2 adsorption/desorption isotherms of the samples (degassed at 423 K for 12 h to remove impurities) were measured on a Micromeritics ASAP 2020 analyzer at 77 K. The specific surface areas were calculated using the BrunauereEmmetteTeller (BET) model and the pore volume was determined by the BarretteJoynereHalenda (BJH) method.

2.4. Electrochemical measurements 2.3. Characterizations For electron microscopy observations, the specimens were prepared by mixing a small amount of the corresponding sample with 1.0 mL ethanol, and ultrasonicated for few minutes to obtain a uniform suspension. Then the suspension was dropped onto an aluminum foil for field emission scanning electron microscope (FESEM) observations or a carbon-coated copper grid for transmission electron microscope (TEM) and high-resolution transmission electron microscope (HR-TEM) observations. FE-SEM was performed on a Hitachi SU-8010 instrument with an acceleration voltage of 15.0 kV, TEM was carried out on Hitachi HT-7700 with an acceleration voltage of 100 kV. HR-TEM micrographs, selected area electron diffraction (SAED) and the EDS mapping experiments were acquired on a JEM-2100F instrument working at an acceleration voltage of 100 kV. The X-ray diffraction (XRD) patterns were performed on a Philips X’ Pert Pro diffractometer with a CuKa (l ¼ 0.15405 nm) radiation source to acquire the crystalline phases of the Co3O4 and TiO2 components. Raman spectra were collected from Jobin Yvon LabRam HR UV Raman spectrometer with an excitation wavelength of 514 nm. Thermogravimetric analysis (TGA) curves were acquired from a Netzsch STA409 PC/PG instrument at a heating rate of 10  C min
1 from room temperature to 800  C in air to determine the compositions of each sample. The Xray photoelectron spectroscopy (XPS) analyses were carried out on

A slurry coating procedure was used to prepare the working electrodes. The slurry was prepared by mixing 80 wt% active material, 10 wt% acetylene black and 10 wt% PVDF (polyvinylidene fluoride) dissolved in NMP (N-methylpyrrolidinone) with several drops of ethanol. And the mixture was ultrasonicated for 1 h followed by continuous stirring overnight. After evaporating the extra ethanol, the homogeneous honey like slurry was coated uniformly on nickel foams acting as current collector. The prepared working electrodes were dried under vacuum at 80  C overnight to remove the solvent, followed by pressed under 10 atm. The mass loading density of the active material on each electrode was calculated to be ca. 1.8 mg/cm2. Celgard 2300 microporous polypropylene film and metallic lithium foil were used as the separator and the counter electrode, respectively. The CR2025 type coin cells were assembled in an argon-filled glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The electrolyte was prepared by dissolving 1.0 M LiPF6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume). The cyclic voltammetry (CV) measurement (3.0e0.01 V vs. Li/Liþ, 0.1 mV s
1) and electrochemical impendence spectroscopy (EIS) measurement (100 kHze0.01 Hz) were performed on a CHI760D electrochemical work station (CH Instruments, Shanghai, China). The galvanostatic discharge/charge tests were carried out on a Neware Battery Test System (Neware Technology Co., Ltd., Shenzhen, China) at room temperature in a voltage range of 3.0e0.01 V vs. Li/Liþ at different current densities.

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3. Results and discussion 3.1. Characterization of the nanofibrous Co3O4/TiO2/carbon composites As described in the experimental section and Scheme 1, the nanofibrous Co3O4/TiO2/carbon composite was fabricated using natural cellulose substance (filter paper) as both the carbon source and the structural scaffold. After a facile surface solegel process to coat TiO2 thin gel film on each cellulose fiber and followed by carbonized in argon, the yielded TiO2/carbon hybrid was then performed with a hydrothermal process in different concentrations of Co2þ precursor solutions to deposit Co3O4 nanoparticles on each TiO2/carbon nanofiber with varied thicknesses and coverage degrees to give the final Co3O4/TiO2/carbon composite. In order to investigate the influence of depositing Co3O4 nanoparticles prior or after the carbonization of the TiO2-gel/cellulose composite on the loading density of the Co3O4 species, cobalt precursor was first deposited on uncarbonized TiO2-gel/cellulose with higher concentrations of cobalt acetate solutions by the same hydrothermal method, and then together carbonized to form the control materials. Fig. 1a displays the TG curves of the Co3O4/TiO2/carbon composites. All the curves showed two evident weight loss. The first one occurred below 100  C, causing a weight loss of about 5.0 wt%, which is due to the release of absorbed gases and moisture on the surface of the samples [22,46]. The sharp weight loss at about 322e510  C is mainly ascribed to the pyrolysis of the carbon

component of the samples. Base on the TG curve of the TiO2/carbon sample (Fig. S2), the mass ratio of TiO2 vs. carbon was around 1:17. Accordingly, the weight ratio of Co3O4 in the samples of Co3O4/ TiO2/carboneA series were calculated to be 21.9 wt%, 37.4 wt% and 50.6 wt%, respectively. Therefore, it is clear that the deposition mass of Co3O4 on the TiO2/carbon nanofiber was evidently increased with the increment of the concentration of the Co2þ precursor solutions. Such phenomenon was also observed in the TG curves of the samples of B series (Fig. S1a), but the weight ratio of Co3O4 was counted to be 6.9 wt%, 10.7 wt% and 15.7 wt%, respectively; which are much less than those of the A series, while the concentrations of the Co2þ precursor solutions during the synthesis procedure were much higher than the A series. Depositing Co3O4 nanoparticles before or after the carbonization of the TiO2-gel/cellulose composite played a significant role of the Co3O4 loading level. The samples of the Co3O4/TiO2/carboneA series were synthesized by loading Co3O4 through hydrothermal reaction on the TiO2/carbon fibers, while the B series was performed hydrothermal process first on TiO2-gel/cellulose fibers and then carbonized. As confirmed by the nitrogen adsorption-desorption results of TiO2/carbon and TiO2-gel/cellulose composites (Table S1), the specific surface area of TiO2/carbon composite (321.5 m2 g
1) is much higher than the original TiO2-gel/cellulose composite (5.98 m2 g
1), this is owe to the leakage of CO, CO2, small molecular alkanes, etc. during the pyrolysis of cellulose, which induced numerous of pores and thus provided more reactive sites exposed to Co2þ. Thus, Co3O4 nanoparticles were not only deposited on the surface of the TiO2 layer but also embedded in the pores of the carbon nanofiber, leading to

Fig. 1. (a) TGA curves, (b) XRD patterns and (c) Raman spectra of samples Co3O4/TiO2/carboneA1, 2 and 3. (d) N2 adsorption and desorption isotherm and pore size distribution curve of sample Co3O4/TiO2/carboneA3.

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Fig. 2. Electron micrographs of the sample Co3O4/TiO2/carboneA3. (a) FE-SEM image of the sample, the inset shows the nanofiber assemblies at a higher magnification. (b) TEM image of an individual composite nanofiber, the inset shows the enlarged view of the boxed area. (c) HR-TEM image of the composite nanofiber surface, showing the lattice spacing of Co3O4 nanoparticles. (d) The corresponding SAED pattern of the composite nanofiber surface. (e,f,g) The EDX elemental mapping of C, Ti, and Co elements of the composite nanofiber, respectively.

much more high loading of Co3O4 of the samples of A series. Fig. 1b shows the XRD patterns of the samples of Co3O4/TiO2/ carboneA series, the diffraction peaks located at 2q ¼ 18.9 , 31.22 , 36.77, 44.71, 55.62 , 59.25 , and 65.13 are characteristic peaks for face-centered cubic phase Co3O4, corresponding to the (111), (220), (311), (400), (422), (511), and (440) crystalline planes of which (JCPDS# 43e1003), respectively [17,23,46,47]. These strong and sharp peaks indicate the fine crystalline nature of the Co3O4 phase. The peak located at 2q ¼ 25.3 is attributed to the (101) reflection plane of anatase phase TiO2 (JCPDS# 21e1272 [1,48]. And the broad peak located at about 26 is related to amorphous carbon contained in the composite [23,46]. All the samples displayed similar diffraction peaks, indicating the co-existence of Co3O4 and TiO2 in all the samples. The characteristic peaks for the samples of the Co3O4/TiO2/carboneB series (Fig. S1b) are similar to those of the A series. However, the diffraction peaks of Co3O4 therein are wider, suggesting that depositing Co3O4 before the carbonization process and higher calcination temperature lead to smaller particle sizes and higher crystallinity of the Co3O4 nanoparticles contained in the B-series samples, as confirmed by the FWHM values measured from the Co3O4 (311) peaks of the XRD patterns of the various Co3O4/TiO2/carbon composites and the corresponding Lhkl values calculated (Table S2). Fig. 1c shows the Raman spectra of samples Co3O4/TiO2/carboneA1, 2 and 3. Two noticeable peaks shown in all the spectra at around 1350 cm
1 (D band) and 1590 cm
1 (G band) are the typical peaks for disordered and ideal graphite carbon structure of the carbon fiber, respectively [7,23]. Peaks appeared at 515 and 671 cm
1 are ascribed to the F12g and A12g modes of well-crystallized Co3O4 phase, respectively [12,49,50]. And the Raman band at 379 cm
1 (B1g) is the stretching and bending mode of anatase phase TiO2 [51,52]. In addition, with the increase of the Co3O4 contents,

the intensity of D and G bands of carbon become weaker, which is also seen in the Raman spectra for the samples of the Co3O4/TiO2/ carboneB series (Fig. S1c). This result agrees well with the XRD measurement, indicating that the mass loading of the Co3O4 increased with the increment of the concentration of the Co2þ precursor solutions, and also reveals the successful deposition and the fine crystalline structure of the Co3O4 nanoparticles contained in the composite. In the subsequent electrochemical tests, sample Co3O4/TiO2/ carboneA3 displayed the best electrochemical performance, so it was chosen as the optimal material and its pore structure and BET specific surface area were investigated by conducting the nitrogen adsorption and desorption measurements. Fig. 1d shows the resulted isotherm, which displays a type IV isotherm with a type H4 hysteresis loop according to the IUPAC classification. The adsorption at low relative pressure is ascribed to the micropores, resulting from the pyrolysis of the cellulose fibers during the carbonization process [46,53]. The hysteresis loop located at 0.4e1.0 relative pressure reveals the existence of slit-shape mesopores which are originated from the voids in-between the fiber aggregates and inbetween the Co3O4 nanoparticles [20,53]. The corresponding BJH pore size distribution curve (Fig. 1d, the inset) centered two peaks at 3.9 and 31.8 nm. The intense peak at ca. 3.9 nm is attributed to the tensile strength effect (TSE). As a result of this effect, the pore size distribution of adsorption using BJH method will center a peak at ca. 4 nm, the formation of this peak is only related to the nature of the adsorbent (in this case, N2) [47,53], which means mesopores of 3.9 nm actually do not really exist in the sample. Aside from the TSE peak, the wide peak at range of 5e100 nm confirmed the hierarchical structure of the composite, which significantly enhanced the specific area as well as provided efficient transfer pathways for lithium ions. The relatively high specific surface area (197.6 m2 g
1)

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and total pore volume (0.29 cm3 g
1) of sample Co3O4/TiO2/carboneA3 also bring more reactive sites and a large electrolyte/ electrode contact area as being used as anodes for LIBs, which are expected to benefit the lithium storage properties. As for the control material, sample Co3O4/TiO2/carboneB3 (Fig. S1d) shows similar isotherm and hysteresis loop to that of sample Co3O4/TiO2/carboneA3, but the hysteresis loop was not closed at the range of low pressure because of the existence of irregular nanopores [54,55]. The specific surface area and the total pore volume of Co3O4/TiO2/carboneB3 are 167.5 m2 g
1 and 0.15 cm3 g
1, respectively. The relatively lower surface area is mainly resulted from the lower loading mass of Co3O4 particles (confirmed by the TG analysis discussed above). The smaller pore volume and narrower pore size are due to the smaller particle size of Co3O4 compared to that of sample Co3O4/TiO2/carboneA3 (revealed by the XRD results above). This structural defect hindered its electrical activity when used as anodic material for LIBs. The morphologies and microstructures of sample Co3O4/TiO2/ carboneA3 are displayed in Fig. 2. As seen from the FE-SEM overview of the sample (Fig. 2a), the composite consists of numerous nanofibers randomly interconnected together, forming a hierarchically structure which accurately inherited from the original filter paper. From the higher magnification image in the inset, the diameter of a single Co3O4/TiO2/carbon nanofiber was measured to be about 50e100 nm, and the nanofibers were observed uniformly coated with Co3O4 nanoparticles. The uniform coating of Co3O4 nanoparticles on the fiber surfaces as well as the nanofibrous structure of the composite were proved by the SEM image of

sample Co3O4/TiO2/carboneA3 with low magnification (Fig. S3). The TEM images reveal that the sizes of Co3O4 nanoparticles are around 10e30 nm (Fig. 2b), the enlarged view (Fig. 2b, inset) of the composite nanofiber demonstrated that there are no clear boundary between the Co3O4 nanoparticles and the TiO2 layer, confirming that partial Co3O4 nanoparticles are embedded in the voids inbetween the TiO2 nanoparticles. The morphologies of samples Co3O4/TiO2/carboneA1 and 2 (Fig. S4) present similar fibrous structures to those of sample Co3O4/TiO2/carboneA3, but with low Co3O4 loading densities, and the Co3O4 layer was not uniform. The SEM and TEM images of samples of the Co3O4/TiO2/carboneB series (Fig. S5) display similar nanofibrous structure to the A series, but with thinner Co3O4 layer, demonstrating lower Co3O4 loading density. This result is in good consistent with the TGA results discussed above. Fig. 2c shows the HR-TEM image of the Co3O4/TiO2/carboneA3 composite fiber surface. Lattice spacing of 4.67 and 2.43 Å are ascribed to the (111) and (311) planes of face-centered cubic Co3O4 (JCPDS# 43e1003), respectively [22]. No characteristic lattice stripe of TiO2 was observed for TiO2 thin layer was buried under the Co3O4 nanoparticles. The corresponding selected area electron diffraction (SAED) pattern (Fig. 2d) confirms the polycrystalline nature of the Co3O4 species. The diffraction rings from inside to outside are indexed to the (111), (220), (311), (400), (511), and (440) planes of the crystalline Co3O4 phase. This result is in good agreement with the XRD results shown above. Fig. 2eeg displayed the EDX mapping for the elements of carbon, titanium and cobalt of a composite nanofiber of the sample Co3O4/TiO2/carboneA3, respectively. It is

Fig. 3. XPS spectra of sample Co3O4/TiO2/carboneA3. (a) Overall survey spectra. (b,c,d) High-resolution XPS spectra of Co 2p, Ti 2p, and O 1s regions of the sample, respectively.

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Fig. 4. Electrochemical performances of the hierarchical nanofibrous Co3O4/TiO2/carbon composites. (a1ec1) CV curves of the samples Co3O4/TiO2/carboneA1, 2 and 3 at a scan rate of 0.1 mV s
1, respectively. (a2ec2) The galvanostatic discharge/charge profiles of Co3O4/TiO2/carboneA1, 2 and 3 over the potential range of 3.0e0.01V vs. Li/Liþ at a constant current density of 100 mA g
1, respectively.

clearly seen that carbon signal most distributed in the core region of the composite fiber, and titanium and cobalt distributed uniformly on the outer layer, for the distribution width are much broader than carbon. Evidently, the Co element distributed most densely which also suggested the large mass loading of the Co3O4 nanoparticles. X-ray photoelectron spectroscopy measurements were taken to determine the chemical states of the species of the sample Co3O4/ TiO2/carboneA3. Fig. 3a shows the overall survey spectra of the composite, and the peaks for Co 2p, O 1s, Ti 2p, and C 1s are clearly observed. The high-resolution XPS spectrum of Co 2p region of the sample provided the oxidation state of Co and proved the existence of Co (II) and Co (III) in Co3O4 (Fig. 3b). The peaks at the binding energies of 779.4 and 780.1 eV are belonged to Co (III) 2p3/2 and Co (II) 2p3/2, respectively. And the peaks at around 794.3 and 795.6 eV are indexed to Co (III) 2p1/2 and Co (II) 2p1/2, respectively [49,56,57]. The other two peaks located at 789.1 and 804.1 eV are the satellites of Co (II) [2,50]. Fig. 3c shows the high-resolution XPS spectrum of Ti 2p region of the sample, there are two characteristic peaks of Ti 2p3/2 and Ti 2p1/2 located at 459.0 and 464.1 eV, respectively, indicating a normal Ti4þ state of the TiO2 species [1,48,57]. The high-resolution XPS spectrum of O 1s region (Fig. 3d) peaked at 529.0 eV is assigned to the gaseous O2 adsorbed in the composite, the other two peaks presented at 530.7 and 532.1 eV are attributed to the lattice oxygen combined with Co (II) and Co (III), respectively [7,11,56]. The XPS results further confirmed the co-existence of Co3O4 and TiO2 components in the composites. XPS patterns for Co3O4/TiO2/carboneB3 (Fig. S6) show similar spectra features as those of sample Co3O4/TiO2/carboneA3 but with slightly different peak intensities, demonstrating the same composition as the samples of the A series. 3.2. Electrochemical study When the Co3O4/TiO2/carbon composites are employed as

anodic materials for LIBs, the bio-inspired hierarchically structures, the large loading densities of nanoscale Co3O4 particles as well as the existing of the internal porous conductive carbon nanofiber of the composite, would accelerate the electrolyte diffusion and facilitate the electron transfer during the discharge/charge processes, thus would significantly enhance the electrochemical performances. Fig. 4 shows the electrochemical performance of the nanofibrous Co3O4/TiO2/carbon composites as being used as anodic materials for LIBs. Fig. 4a1ec1 display the CV curves of the first four cycles of the Co3O4/TiO2/carboneA1, 2 and 3 at the voltage range of 3.0e0.01 V vs. Li/Liþ with a scan rate of 0.1 mV s
1, respectively. For all the samples, in the first cathodic scan, a sharp peak at 0.6e0.7 V is associated to the reduction of Co3O4 to metallic Co accompanying the formation of amorphous Li2O and the solid electrolyte interface (SEI) layer on the surface of the active materials [2,4,7]. The corresponding electrochemical reaction processes are described as follows [2,17,56]:

Co3 O4 þ 8Li þ þ 8e
44Li2 O þ 3Co

(1)

8Li 4 8Liþ þ 8e


(2)

For samples Co3O4/TiO2/carboneA1 and 2 (Fig. 4a1 and a2), a small peak at 1.6 V was also observed in the initial cathodic cycle which is ascribed to the reduction of TiO2. As for the first anodic scan, all the three samples showed two peaks at 2.1 and 1.5 V, which are assigned to the conversion reaction from metallic Co to CoO. Owing to metallic Co embedded in amorphous Li2O matrix, it is difficult to reverse to be oxidized to Co3O4 during the delithiation process [2,58]. And that is also the reason why theses peaks are wider and weaker than the reduction peak of the cathodic curves. In the second cycle, the sharp reduction peak shifted to ~0.9 V with obvious decreasing intensities. It is attributed to the reduction of CoO as well as some irreversible structure transformation during

8

Fan Wang et al. / Journal of Alloys and Compounds 819 (2020) 153375

the first cycle [46,48]. The successive cycles show very little changes compared with the second one, indicating the excellent lithiation/delithiation reversibility and the stable cycling performance of the composites. Compared with the A series of samples, the peaks in the CV curves of samples of the B series (Fig. S7a1ec1) are much weaker and wider, this phenomenon was mainly resulted from the lower Co3O4 loading densities and the relatively smaller pore volumes of these samples. Typical galvanostatic discharge/charge voltage profiles of the samples of the Co3O4/TiO2/carboneA series cycled at a current density of 100 mA g
1 with a voltage range of 3.0e0.01 V (vs. Li/Liþ) are presented in Fig. a2ec2. The first discharge curve for all the samples showed an evident voltage plateau at around 1.0 V, which is a typical characteristic for Co3O4 [22]. The plateau is mainly caused by the reduction of Co3O4 to Co and the formation of SEI film and Li2O, which leads to significant capacity loss for all the three samples, such phenomenon was reported on other Co3O4 based anodes [46,59]. As a result, the initial discharge/charge capacities are 949.5/522.0, 1089.8/670.0 and 1239.2/838.0 mAh g
1 for Co3O4/ TiO2/carboneA1, 2 and 3, respectively; with the initial Coulombic efficiencies of 55.0%, 61.5% and 67.6%. In the subsequent cycles, the voltage plateau disappeared for sample Co3O4/TiO2/carboneA1 (Fig. 4a2), and become steeper and shifted to higher potential for samples Co3O4/TiO2/carboneA2 and 3 (Fig. 4b2 and c2). This is assigned to the enhanced reversibility of the electrode reaction that gradually built after the initial Liþ uptake process [46,60]. Moreover, the discharge profiles of Co3O4/TiO2/carboneA2 and 3 in the subsequent cycles show two evident inflection points along with voltage drop at around 1.0 and 1.4 V, which is due to the reduction reaction of CoO with Liþ during the discharge process. In addition, except the initial discharge curve, the other voltage profiles overlapping well together, indicating excellent stability of the composites for high reversible lithium storage devices. All the galvanostatic discharge/charge voltage profiles of the samples Co3O4/TiO2/carboneA1, 2 and 3 are in good agreement with the corresponding CV results. In the voltage profiles of samples Co3O4/ TiO2/carboneB1, 2 and 3 (Fig. S7a2ec2), it was clearly observed that there were shorter voltage plateaus at the relatively lower lithiation potential of Co3O4 (~0.9V), this is probably ascribed to the polarization of the Co3O4 phase contained therein [61]. The galvanostatic cycling performance of samples Co3O4/TiO2/ carboneA1, 2 and 3 and the Coulombic efficiency of sample Co3O4/

TiO2/carboneA3 cycled at a current density of 100 mA g
1 are shown in Fig. 5a. The reversible specific capacity of Co3O4/TiO2/ carboneA1, 2 and 3 maintained at 373, 487 and 764 mAh g
1 after 200 discharge/charge cycles, respectively. Among these materials, Co3O4/TiO2/carboneA3 displayed the best performance (mainly due to the highest Co3O4 loading density), which is even much higher than the theoretical capacity value of the Co3O4/TiO2/carbon nanocomposites (590 mAh g
1). The theoretical capacity of this sample was calculated by multiplying the mass percentage of each component in the material by its own theoretical specific capacity, and finally add the results of each component together [49,62]. The extra reversible capacity is ascribed to the additional lithium storage site provided by the large specific surface area and high pore volume of the sample; the low-voltage capacity partly offered from the reversible formation/dissolution of the polymeric surface layer; as well as the Co metal generated from Co3O4 during discharge process served as catalysts to promote the reversible conversion of some SEI components [46,61,63]. The Coulombic efficiency for sample Co3O4/TiO2/carboneA3 rapidly rises from 67.6% at the first cycle to 98.6% in the fifth cycle, and remained above 98% at the subsequent cycles. After a slight capacity fading at the 30e50th cycles, the specific capacity continuously increased with cycles and reached to 764 mAh g
1 at the 200th cycle. This phenomenon of capacity increase upon cycling was also observed in other Co3O4 based electrode materials [46,62,64], which was attributed to the reversible growth of pseudocapacitive polymeric film and the gradual activation of some irreversible Li2O generated in the initial few cycles. Besides, after dozens of discharge/charge circles, the crystallized Co3O4 component gradually turns into amorphous phase, which offered more and more active sites available for Liþ insertion and thus improved the electrochemical performance [62,65]. As the cycling performance of the TiO2/carbon nanofiber without Co3O4 depositing displayed (Fig. S8), the specific capacity rapidly dropped from 952 mAh g
1 to 389 mAh g
1 at the initial two discharge/charge cycles, and the initial Coulombic efficiency value was only 40.9%. This result indicates that the enhanced electrochemical performances of the Co3O4/TiO2/carbon composites were ascribed to the synergetic effect of the carbon, TiO2, and Co3O4 components. Fig. 5b shows the rate performance of the samples of the Co3O4/ TiO2/carboneA series. The current density increased stepwise from 0.1 to 1.0 A g
1, and 10 cycles was measured at each current rate.

Fig. 5. (a) The discharge/charge cycling performance of the samples Co3O4/TiO2/carboneA1, 2 and 3 cycled at the current density of 100 mA g
1. (b) The rate capabilities of samples Co3O4/TiO2/carboneA1, 2 and 3 at various current densities.

Fan Wang et al. / Journal of Alloys and Compounds 819 (2020) 153375

9

Fig. 6. The Nyquist plots and the fitted curves of sample Co3O4/TiO2/carboneA3 using (a) fresh cell and (b) after charging to 3.0 V at the 20th cycles. The insets are the equivalent circuit used to model the impedance spectra, and the fitted Rs, Rf, and Rct values.

Fig. 7. (a) FE-SEM and (b) TEM images of sample Co3O4/TiO2/carboneA3 after the cycling test.

The corresponding reversible capacities of Co3O4/TiO2/carboneA3 reached 782, 704, 557 and 348 mAh g
1 at the current density of 0.1, 0.2, 0.5 and 1.0 A g
1, respectively. When the current density was turned back to 0.1 A g
1, a reversible capacity of 817 mAh g
1 was recovered, illustrating a high reversibility of the Co3O4/TiO2/ carboneA3 nanocomposite [3,12]. However, sample Co3O4/TiO2/ carboneA1 delivered capacities of 535, 392, 266 and 180 mAh g
1 at the current density of 0.1, 0.2, 0.5 and 1.0 A g
1, respectively; the corresponding capacities for sample Co3O4/TiO2/carboneA2 are 602, 534, 401 and 250 mAh g
1. It is clearly that the higher loading

densities of Co3O4 nanoparticles in the composite leads to improved specific capacity with better rate performance. The cycling and rate performance of Co3O4/TiO2/carboneB1, 2 and 3 were also tested (Figs. S9a and b). Different from the A series of samples which deposited Co3O4 after carbonization procedure, the discharge capacities of these samples declined as the cycling proceeded. Take sample Co3O4/TiO2/carboneB3 as an example, the capacity decreased sharply in the first 10 cycles, and then constantly decreased in the next 50 cycles and finally reached 428 mAh g
1 after 200 cycles. This low reversible capacity is due to the low loading density of Co3O4 in the composite. The electrochemical impedance spectroscopy (EIS) of sample Co3O4/TiO2/carboneA3 using fresh cell and that after 20th charge/ discharge cycles are presented in Fig. 6. Each profile consists of a Xaxis intercept which is related to the electrolyte resistance (Rs), two semicircles corresponded to the surface film resistance (Rf) and the charge transfer resistance (Rct), respectively; and a sloping line ascribed to the Warburg resistance (Wo), which is related to the charge transfer process and the mass transfer process of lithium ions. The plots were fitted by the equivalent circuit, and the fitting values of Rs, Rf, and Rct are listed in the insets. It is clearly seen that the Rs and Rf values of the cells after 20 discharge/charge cycles are lower than those of the fresh ones, which means that as the discharge/charge cycle proceeds, the electrolyte and the electrode

Table 1 The comparison of the electrochemical properties of the current sample compared with other reported nanostructured Co3O4/carbon and Co3O4 based matters applied as anodic materials for LIBs. Materials

Current density (mA g
1)

Cycle number

Reversible capacity (mAh g
1)

References

Partially reduced Co3O4/graphene Starfish-like Co3O4@N-doped carbon Co3O4@TiO2@C yolk-shell spheres Graphene oxides/Co3O4 Co3O4/CC@Gr Graphene/Co3O4 rose-spheres Co3O4/carbon Co3O4/NeC N-doped CNT/Co3O4 Co3O4@N-doped carbon nanocubes NiOeCo3O4 nanoplate Nano Co3O4 hollow Co3O4 microspheres Co3O4@TiO2 core-shell nanofibers Co3O4/TiO2/carbon nanofiber

21.12 500 200 200 100 180 89 1000 500 100 100 89 100 178 100

30 300 100 200 300 200 40 200 33 50 70 40 520 100 200

801.3 795 400 714.1 391 655.9 500 573 662 598 633 423 550 632 764

[66] [23] [1] [67] [34] [2] [68] [69] [70] [71] [72] [73] [74] [75] This work

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Fan Wang et al. / Journal of Alloys and Compounds 819 (2020) 153375

materials are more fully contacted. On the contrary, the Rct value of after 20 discharge/charge cycles is larger than that of the fresh cell, it is due to the formation of the SEI film on the composite surface and thus hindered the charge transfer. Furthermore, the sloped line for battery after 20 cycles is much steeper, which indicates the lower ion diffusion resistance than the fresh cell [21,47]. This result also gives evidence that the conductivity and reaction kinetics of the Co3O4/TiO2/carboneA3 electrode improved with charge/ discharge cycles. After the cycling tests, the changes in morphology and structure of the Co3O4/TiO2/carboneA3 composite were observed by SEM and TEM, the results are displayed in Fig. 7. It is seen that the threedimensional fibrous structure of the sample was finely maintained after 200 discharge/charge cycles according to the SEM observation (Fig. 7a). The corresponding TEM image shows an individual composite nanofiber isolated from the nanofiber assemblies (Fig. 7b), it is seen that the Co3O4 nanoparticles became smaller after cycling, which accelerated the electrolyte diffusion and effectively shortened the electron transfer pathway. It is highly convinced that this superior structure stability contributes to the good cycling stability and capacity reversibility of the anode. As compared with other Co3O4 based anode materials reported in the literatures (Table 1), it is clear to see that the present Co3O4/TiO2/carboeA3 nanocomposite presented high reversible specific capacity of 764 mAh g¡1 after 200 discharge/charge cycles, and delivered a remarkable initial Coulombic efficiency of 67.6%, which are both better than most of the hybrid composites reported. It is believed that the porous carbon nanofiber in the inner layer of the composites is the key factor to improve the conductivity of the nanocomposites, thus promote the fast transfer of electrons generated during electrochemical reactions. At the same time, the three-dimensional porous carbon structure also serves as a buffering matrix to prevent the large volume change and severe aggregation of the active species during cycling. Moreover, the existence of the TiO2 middle layer reduced the particle size and was essential for the uniformly disperse and the high loading content of Co3O4 [20,76,77]. Accordingly, the large amount of Co3O4 nanoparticles offered high capacity of the anodes. In addition, the unique three-dimensional hierarchical structures and the relatively high specific surface area of the nanocomposite effectively facilitated the electrolyte diffusion, benefited the sufficiently contact of the electrolyte and the electrode materials, as well as provided more active sites for Liþ insertion. These synergistic effects allowed the enhanced electrochemical performance of the Co3O4/TiO2/carbon composites. 4. Conclusion A bio-inspired nanofibrous Co3O4/TiO2/carbon composite was synthesized employing natural cellulose substance as both the carbon source and the structural scaffold. The unique threedimensional hierarchically structure and the relatively high specific surface area of the nanocomposite offered sufficient ion transport pathways for the electrode. The high loading density and the uniform dispersion of the nano-sized Co3O4 particles provided high capacity of the composite. The inner carbon nanofiber not only improved the electron transfer efficiency, but also served as a buffer layer to relief the strain induced by volume change during cycling. Given these merits, the composite showed enhanced lithium storage performance with high specific capacity, longer cycle life and good rate capability when applied as anodic material for lithium-ion batteries. This bio-inspired synthetic method provided a new pathway for designing functional materials that combining exquisite structures of biomass with the specific features of the guest species. As a consequence, this biomass derived nanomaterials would hold considerable potential for rechargeable LIBs applications.

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