Scholarly article on topic 'CoFe 2 O 4 /carbon nanotube aerogels as high performance anodes for lithium ion batteries'

CoFe 2 O 4 /carbon nanotube aerogels as high performance anodes for lithium ion batteries Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Xin Sun, Xiaoyi Zhu, Xianfeng Yang, Jin Sun, Yanzhi Xia, et al.

Abstract High-performance lithium ion batteries (LIBs) require electrode material to have an ideal electrode construction which provides fast ion transport, short solid-state ion diffusion, large surface area, and high electric conductivity. Herein, highly porous three-dimensional (3D) aerogels composed of cobalt ferrite (CoFe2O4, CFO) nanoparticles (NPs) and carbon nanotubes (CNTs) are prepared using sustainable alginate as the precursor. The key feature of this work is that by using the characteristic egg-box structure of the alginate, metal cations such as Co2+ and Fe3+ can be easily chelated via an ion-exchange process, thus binary CFO are expected to be prepared. In the hybrid aerogels, CFO NPs interconnected by the CNTs are embedded in carbon aerogel matrix, forming the 3D network which can provide high surface area, buffer the volume expansion and offer efficient ion and electron transport pathways for achieving high performance LIBs. The as-prepared hybrid aerogels with the optimum CNT content (20 wt%) delivers excellent electrochemical properties, i.e., reversible capacity of 1033 mAh g−1 at 0.1 A g−1 and a high specific capacity of 874 mAh g−1 after 160 cycles at 1 A g−1. This work provides a facile and low cost route to fabricate high performance anodes for LIBs.

Academic research paper on topic "CoFe 2 O 4 /carbon nanotube aerogels as high performance anodes for lithium ion batteries"

Accepted Manuscript

CoFe2O4/carbon nanotube aerogels as high performance anodes for lithium ion batteries

Xin Sun, Xiaoyi Zhu, Xianfeng Yang, Jin Sun, Yanzhi Xia, Dongjiang Yang

PII: S2468-0257(16)30126-1

DOI: 10.1016/j.gee.2017.01.008

Reference: GEE 53

To appear in: Green Energy and Environment

Received Date: 22 December 2016

Revised Date: 21 January 2017

Accepted Date: 28 January 2017

Please cite this article as: X. Sun, X. Zhu, X. Yang, J. Sun, Y. Xia, D. Yang, CoFe2O4/carbon nanotube aerogels as high performance anodes for lithium ion batteries, Green Energy & Environment (2017), doi: 10.1016/j.gee.2017.01.008.

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CoFe2O4/carbon nanotube aerogels as high performance anodes for

lithium ion batteries

Xin Sun,a Xiaoyi Zhu,a' * Xianfeng Yang,b Jin Sun,a Yanzhi Xia,a Dongjiang Yanga' c'*

a School of Environmental Science and Engineering, Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong Province, College of Automation and Electrical Engineering,Qingdao University, No. 308, Ningxia Road, Qingdao 266071, China

b Analytical and Testing Centre, South China University of Technology, Guangzhou, 510640, China

c Queensland Micro- and Nanotechnology Centre (QMNC), Griffith University,

Nathan, Brisbane, QLD 4111, Australia

Corresponding author.School of Environmental Science and Engineering, Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong Province, Qingdao University, No. 308, Ningxia Road, Qingdao 266071, China

E-mail address: xyzhu@qdu.edu.cn (X. Zhu), d.yang@qdu.edu.cn (D.Yang).

ABSTRACT

High-performance lithium-ion batteries (LIBs) require electrode material to have an ideal electrode construction which provides fast ion transport, short solid-state ion diffusion, large surface area, and high electric conductivity. Herein, highly porous three-dimensional (3D) aerogels composed of cobalt ferrite (CoFe2O4, CFO) nanoparticles (NPs) and carbon nanotubes (CNTs) are prepared using sustainable alginate as the precursor. The key feature of this work is that by using the

characteristic egg-box structure of the alginate, metal cations such as Co and Fe can be easily chelated via an ion-exchange process, thus binary CFO are expected to be prepared. In the hybrid aerogels, CFO NPs interconnected by the CNTs are embedded in carbon aerogel matrix, forming the 3D network which can provide high surface area, buffer the volume expansion and offer efficient ion and electron transport pathways for achieving high performance LIBs. The as-prepared hybrid aerogels with the optimum CNT content (20 wt%) delivers excellent electrochemical properties, i.e., reversible capacity of 1033 mA h g-1 at 0.1 A g-1 and a high specific capacity of 874 mA h g-1 after 160 cycles at 1 A g-1. This work provides a facile and low cost route to fabricate high performance anodes for LIBs.

Keywords

Alginate, Aerogels, Cobalt ferrite, Anode, Lithium-ion battery.

1. Introduction

Nowadays, lithium ion batteries (LIBs) are greatly demanded to satisfy the requirements of renewable energy storage and electric vehicle applications due to their high energy density, long cycle life, environmental friendliness and safety.[1-3] The electrochemical property of LIBs is mainly determined by both electrode materials. Therefore, developing anode materials with high ^specific capacity, excellent rate capability and cycle performance is of vital significance. Transition metal oxides (TMOs) are promising anode materials for LIBs because of their large lithium storage capacities.[4-9] Cobalt ferrite (CoFe2O4, CFO), as one of the representative binary TMOs, has stimulated extensive efforts due to its high theoretical specific capacity (916 mA h g-1), low cost, safety and non-toxicity.[10-12] Nevertheless, CFO suffers from the problems of poor electric conductivity, huge volume expansion and severe pulverization during repetitious charge/discharge process, which lead to poor electrochemical performance.[13] Conductive materials such as carbon nanotubes (CNTs), graphene, and carbon coating are adopted to form conductive composites to enhance the conductivity of CFO anodes. Zhang and his co-workers reported a layer-stacked CFO mesoporous platelets via co-precipitation of cobalt ferrous hydroxide followed by calcination method.[14] Zhao and his co-workers reported a CFO/graphene nanosheet sandwich via heating the solution followed by the hydrothermal method.[15] Zhao and his co-workers reported a p-CNTs/CFO composite via supercritical carbon dioxide expanded ethanol-assisted deposition followed by calcination method.[16] The composite exhibited remarkable electrochemical

performance due to the desirable composition and mesoporous architectures. However, the multistep fabricating process of CFO are time-consuming, complicated and high-cost, which greatly qualified its practical application.'17' Hence, developing a simple, effective and eco-friendly synthesis method to prepare CFO with unique hierarchical structure for high electrochemical performance is crucially desired.'18, 19'

In this work, we explored a facile,substantial and scalable pathway to prepare a porous three-dimensional (3D) hybrid aerogels consisted of CFO nanoparticles (NPs) and multi-walled carbon nanotubes using alginate as the carbon precursor.'20-22' The key feature for our synthesis involves the use of the "egg-box" structure of alginate which can chelate metal cations (Co2+, Fe3+) by an ion-exchange process.'23-26' After a subsequent thermal treatment, CFO NPs can be easily synthesized. In this synthesis procedure, the self-aggregated phenomenon usually occurred in nanomaterials can also be avoided due to the special "egg-box" structure of alginate.'25-31' Additionally, the porous 3D structure presents a high specific surface area and large amounts of mesopores, which provides more active sites for efficient electron and Li ions transportation.'32-35' Meanwhile, the formed 3D network can accommodate the dramatic volume change during the charge/discharge process and ultimately improve the cycling stability.'8, 14, 16, 36, 37' CNTs introduced into the hybrid aerogels can effectively enhance the electric conductivity and interconnect CFO NPs like wires, and good rate capability is achieved.'38, 39' So as expected, the as-prepared CFO/CNT aerogels exhibit a high reversible capacity (1033 mA h g-1 at 0.1 A g -1), superior

stability (874 mAh g 1 at 1 Ag 1 for 160 cycles), and excellent rate capacity (516 raA h g 1 at 5 Ag_1).

2. Experimental

2.1 Material and methods

FeCl3-6H20 (99 wt%) and CoCl2-6H20 (99 wt%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium Alginate (SA) was purchased from Aladdin. Ultra-pure water with resistivity 18 MQ cm-1 was used. Carbon nanotubes were purchased from Shandong Dazhan Nano Materials Company.

In a typical procedure, an alginate solution (1.5 wt%) with different amounts of CNTs was dispersed in ultra-pure water under vigorous stirring to form the aqueous SA/CNTs solution. The solution was dropped into a 0.1 mol L"1 aqueous solution containing Co" and Fe cations (with a Fe/Co molar ratio of 1:1) using a syringe needle at room temperature, and stirred for 40 minutes to form Fe and Co cross-linked hydrogels (Fe,Co-SA/CNT hydrogels). The obtained hydrogels were separated, and further washed clearly with ultra-pure water for several times. Then, the as-prepared hydrogels were frozen at -50 °C for 12 h, and dehydrated via a freeze-drying process to prepare (Fe,Co)-SA/CNT aerogels. The aerogels were placed in a tube furnace and heated to 580 °C for 2 h in nitrogen atmosphere at a heating rate of 2 °C min-1, and followed by oxidizing to 280 °C in air for 2 h to form the product CFO/CNT aerogels. The samples with different CNT content (10%, 15%, 20%, 25%, mass ratio) were

denoted as CFO/CNT-X (X=10, 15, 20, 25). Pure CFO aerogels were prepared without adding CNTs while keeping other parameters constant.

2.2 Structural characterization

The structural analysis, chemical composition, and morphology of the samples were strictly studied. The phase structures were characterized by XRD (DX2700, China) operating with Cu Ka radiation (L = 1.5418 Â) at a scan rate (20) of 2° min-1 ranging from 5 to 90 ° with an accelerating voltage of 40 kV and an applied current of 30 mA. Thermogravimetric analysis (TGA) measurement was carried out on an HTG-1 instrument (Beijing Hengjiu Scientific Instruments). Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw 1000 NR) with an excitation of 514.5 nm laser light. The specific surface area was calculated by the BET method from the data in a relative pressure (P/P0) range between 0.05 and 0.20. Pore size distribution plots were derived from the adsorption branch of the isotherms based on the Barret-Joyner-Halenda (BJH) model.The chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) using an ESCALab250 electron spectrometer (Thermo Scientific Corporation) with monochromatic 150 W Al Ka radiation. Themorphology and structure of the samples were investigated by Field emission scanning electron microscopy (FESEM; JSM-7001F, JEOL, Tokyo, Japan). TEM, HRTEM, and HAADF-STEM images were taken on a JE0LJEM-2100F electron microscope equipped with a Gatan CCD camera, an HAADF detector and an Energy-dispersive X-ray (EDX) spectrometer operating at 200 kV.

2.3 Electrochemical measurements

The composite powder, acetylene black and polyvinylidene fluoride (PVDF) were mixed at a weight ratio of 60:20:20 and calendered on a copper foil to fabricate

cells were assembled in a glove box filled with highly pure argon gas (H2O < 0.1 ppm and 02<0.1 ppm). The two electrode CR2016-type coin type cells were galvanostatically discharged and charged in the voltage range of 0.01-3.0 V vs. Li/Li by using a LAND CT2001A system. Cyclic voltammetry (CV) tests were carried out between 0.01 and 3.0 V at a sweep rate of 0.1 mV s-1 using a CHI 760E electrochemical workstation. The specific capacity was calculated based on the total weight of the active materials including CFO and CNTs during the cycling process. Electrochemical impedance spectroscopy (EIS) spectra were measured over a frequency range of 0.01 Hz-100 kHz.

3. Results and Discussion

3.1 CFO/CNT aerogels characterization

the working electrode. The loading amount of active material is ~0.60 mg cm . These

Fe,Co-SA/CNTs hydrogels Fe,Co-SA/CNTs aerogels

CFO/CNTs aerogels

Scheme 1. Schematic synthesis of CFO/CNT aerogels.

The synthetic procedure of the CFO/CNT aerogels is schematically illustrated in Scheme 1. First, sodium alginate (SA) and CNTs were mixed together to from the SA/CNT hybrid collosol. The Fe,Co-alginate/CNT (Fe,Co-SA/CNT) hydrogels are

formed by adding the SA/CNT solution into the aqueous solution containing Fe and

Co cations with an injector (Scheme 1a).The as-prepared hydrogels are dehydrated via a freeze-drying process to generate the Fe,Co-SA/CNT aerogels (Scheme 1b). Finally, the obtained aerogels are carbonized and subsequently oxidized to form CFO/CNT-X aerogels (X=10, 15, 20, 25, denotes as the mass content of CNTs). In the hybrid aerogels, CFO NPs together with CNTs are uniformly embedded in the carbon aerogels matrix, through which the Li ions insert and extract (Scheme 1c).

(111) (220) mi\ S" ° ~ S ^ 1 £"" « N t ^ ^ ^ ._.¡a

CFO/CNT-25

CFO/CNT-20

CFO/CNT-15

CFO/CMT-10

pure CFO , ■ I. ■ I ■ JCPDS PDF#97-016-005Ï '.U LU. ■

— pure CFO G

-CFO/CNT-20 /

1 ■ 1 ■ 1 ■ 1 ■ ■ ■ 1 ■ 1 ■ 1

800 1000 1200 1400 1600 1800 2000 2200 28 (degree) Raman shift (cm1)

Figure 1. (a) XRD patterns of pure CFO and CFO/CNT-X aerogels. (b) Raman spectra of pure CFO and CFO/CNT-20.

The X-ray diffraction (XRD) patterns of Fe,Co-SA aerogels (Fe,Co-SA) which are prepared through a ion-exchange process without adding CNTs are shown in FigureS1|. The broad diffraction peak at 20=21.0° is ascribed to the typical "egg-box" structure in a-L-guluronate (G) junction zones where Fe3+, Co2+ and H+

ions exchange with Na+ions.'40] Figurela shows XRD patterns of pure CFO and the as-prepared CFO/CNT-X aerogels. For CFO/CNT-X aerogels, the diffraction peaks at 18.3, 30.2, 35.5, 43.2, 57.1 and 62.7° correspond to the lattice planes (111), (220), (311), (400), (511) and (440), respectively, of cubic spinel structure (JCPDS No. 97-016-0059). Compared with pure CFO, the XRD patterns of CFO/CNT-X aerogels reveal the characteristic peaks of CFO as well as a diffraction peak located at 26.5° which can be attributed to (002) plane of CNTs. The sharp diffraction peaks reveal that the CFO/CNT aerogels are well-crystallized.

Raman spectra of pure CFO and CFO/CNT-20 are shown in Figure1b. The D band (1350 cm-1) and the G band (1592 cm-1), which are corresponding to the disordered and graphite bands, respectively, were observed in the spectra.'15, 41] The Id/Ig ratios were 0.84 for pure CFO and 0.69 for CFO/CNT-20, indicating higher degree of graphitization in the latter due to the addition of CNTs.

"¡3 D i-O

I 10 s 6 4 2

100 200 300 400 500 600 700 800

— CFO/CNT-20

■O > TJ OJ w\

10 Pore Width (rim) Adsorption Desorption

Temperature (°c)

0.0 0.2 0.4 0.6 0.8 Relative Pressure (P/Po)

Figure 2. (a) TG curves of pure CFO and CFO/CNT-X aerogels. (b) Nitrogen adsorption-desorption isotherms of CFO/CNT-20. (The inset shows the pore diameter distribution.)

Thermogravimetry analysis (TGA) was carried out in air from room temperature to 800 °C at a rate of 10 °C min-1 (Figure 2a). The weight loss below 200°C is ascribed to the desorption of physically adsorbed moisture on the sample surface. For pure CFO, the weight loss started at around 350 °C and ended at 450 °C with 35.0 wt% weight decrease owing to the decomposition of amorphous carbon.'42' In contrast, for CFO/CNT-X aerogels, there are two weight loss at around 350 and 450 °C, which are attributed to the decomposition of amorphous carbon and CNTs, respectively.'42' Thus, in CFO/CNT-10, CFO/CNT-15, CFO/CNT-20, and CFO/CNT-25, the contents of carbon can be roughly calculated to be around 38, 40, 44 and 52%, while the CFO content corresponding to the residual mass after 600 °C is about 62, 60, 56 and 48%.

The Brunauer-Emmett-Teller (BET) surface areas, pore volume and average pore size of pure CFO and CFO/CNT-X aerogels are shown in Table S1f. For the CFO/CNT-X aerogels, their pore size distributions range from 11.97 to 10.19 nm,

corresponding to the specific surface area from 81.69 to 360.03 m2 g-1. The specific

surface area of pure CFO without adding CNTs is measured to be 22.52 m g- , much lower than those of CFO/CNT aerogels. The increased surface area and porosity are ascribed to the interconnected voids from tangled CNTs and small mesopores which are formed after the release of H2O and CO2 during the thermal decomposition of the alginate precursors. The N2 adsorption/desorption isotherms and the corresponding pore size distribution curve of CFO/CNT-20 are shown in Figure 2b. The sample exhibit representative type IV curve, indicating the mesoporous structure with a pore size peak at ~11.44 nm. It is believed that the increase BET surface area is more

favourable for Li ions diffusion in electrodes due to the increase contact area between electrode and electrolyte, and thus provides more active sites for lithium ion storage. The highly porous structure for carbon aerogels is also beneficial to accommodate volume expansion during discharge-charge process.

Figure 3. High-resolution XPS spectra of the CFO/CNT-20 aerogels. (a) Co2p, (b) Fe2p, (c) C1s, and (d) O1s.

X-ray photoelectron spectroscopy (XPS) analysis of CFO/CNT-20 aerogels was carried out to investigate the chemical composition. The wide XPS spectrum of CFO/CNT-20 indicates the presence of C, O, Fe, and Co elements in the hybrid aerogels (Figure S2|). Figure 3a shows the High-resolution XPS spectra of Co 2p. The peaks located at 781.7 and 797.5 eV are ascribed to Co 2p3/2 and Co 2p1/2 with

two shakeup satellites (identified as "sat") at their high binding energy side, separately.'14' The spectra of Fe 2p are divided into two spin-orbit doublets, which correspond to Fe 2p3/2 and Fe 2p1/2, respectively (Figure 3b). Two satellite peaks at the high binding energy sides of Fe 2p3/2 and Fe 2p1/2 are exist.'43'''44' Figure 3c shows the spectra of C1s. It is clear that the peaks centered at 284.5 eV, 285.3 eV, 286.3 eV and 288.2 eV are assigned to graphitic carbon of the CNTs,'13' C-C, C-O and C=O bonds, respectively.'16, 42' The O1s spectra in Figure 3d show that the peak at 529.9 eV is typical of metal-oxide residues. The peak at 531.5 eV is attributed to defects, contaminants, C-OH, C-O-C coordinations or under-coordinated lattice oxygen on the surface of the spinel. The peak at 533.1 eV is associated with diversity of physic-and chemisorbed water at the surface layer.'16'

Figure 4. Microstructure characterization of CFO/CNT-20. (a, b) FESEM images, (c, d) TEM images, (e) HRTEM image, (f) IFFT and FFT (Inset) image, (g) HAADF-STEM and EDS elemental mapping images.

Figure 4a and 4b show the Field-emission scanning electron microscopy (FESEM) images of the as-prepared CFO/CNT-20 aerogels with different magnifications. Clearly, Figure4a displays CFO NPs connected by the randomly entangled CNTs, and both embedded in carbon aerogels matrix. CFO NPs are

observed to have an average diameter of 80-100 nm, and CNTs exhibit typical hollow structure with an outer diameter of approximately 20-40 nm (Figure 4b).

To further examine the architecture of the CFO/CNT aerogels, the CFO/CNT-20was investigated by Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM). Apparently, tangled CNTs and CFO NPs with octahedral structure were observed from the TEM image (Figure 4c). Figure 4d shows the magnified TEM image of an individual CFO NP, where the primary crystal size of CFO was observed around 100 nm. Figure 4e displays the HRTEM image of the selected area in the Figure 4d. It can be seen that the anchored CFO NP is well-textured with high crystallinity. The Fast Fourier Transform (FFT) and the inverse FFT (IFFT) images of the selected area in Figure 4e were presented in Figure 4f. The d-spacing value of 0.25 and 0.48 nm are attributed to (311) and (111) planes of the CFO phase, respectively. The element distribution in CFO/CNT-20 was determined by EDS mapping analysis. It is found that the elements of Co, Fe, O and C distribute uniformly on CFO/CNT aerogels. Co, Fe, and O present an approximate atomic ratio of 1: 2: 4, in good agreement with the CFO structure.

3.2 Electrochemical performance

-0.0005

1.0 1.5 2.0 Voltage (V vs. Li/Li*)

0.5 1.0 1.5

Voltage (V vs. Li/Li")

200 400 600 800 1000 1200 1400 1600 Specific Capacity (mAh g'\

40 60 80 100 120 Cycle Number

10 20 30 40 50 60 70 80 Cycle Number

—l— CFO/CNT-25 CFO/CNT-2H —«—CFO/CNT-15 —CFO/CNT-1H —»—purcCFO

50 100 150 200 250 300 350 400

Z' (D)

Figure 5. (a) CV curves of the CFO/CNT-20 electrode at a scan rate of 0.1 mV s-1. (b) Representative CV curves of the CFO/CNT-20 electrode obtained at a voltage range of 0.01 to 3.0 V (vs Li+/Li) with different potential. (c) Galvanostatic charge-discharge curves of CFO/CNT-20 electrode at the current density of 0.1 A g-1. (d) Cycle performance for the CFO/CNT-X and pure CFO electrodes at the current

density of 1 A g-1. (e) Rate capabilities of CFO/CNT-X and pure CFO electrodes cycled at different current densities from 0.1 to 5 A g-1. (f) EIS spectra of CFO/CNT-X and pure CFO electrodes.

The electrochemical performance of CFO/CNT-20 electrode was evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling techniques. CVs of CFO/CNTs-20 electrode from the 1st to 3rd cycle at 0.1 mV s-1 in the voltage window 0.01-3.0 V are shown in Figure 5a. During the first cathodic scan, the peak centered at about 0.05 V should be ascribed to the intercalation of Li ions into the carbon layers of CNTs.'16' The peak located at 0.64 V is attributed to the reduction of

Fe and Co to their metallic states, accompanied by the formation of Li2O and the formation of solid electrolyte interface (SEI) film.'44' In the subsequent scans, the cathodic peaks are shifted to 0.9 V and 1.34 V. The reduction process can be express as followings: CoFe2O4+8Li+ +8e"^2Fe+Co+4Li2O.'45, 46] During the anode scan, two oxidation peaks at approximately 1.6 and 2.0 V can be attributed to the reversible oxidations of Fe0 and Co0 to Fe3+ and Co2+, respectively.'47' The difference between the first and subsequent cycles is rated to the activation process, which causes a degree of irreversible capacity loss in the initial discharge-charge process.'41, 48' The peaks of CV reflect the redox reactions occurred during the electrochemical process. However, it can not reflect the capacitive properties of the battery. Hence, we calculated the capacitance contribution by using the relation i = avb, where i is the peak current, v is potential sweep rate, and a, b are adjustable values.'49' When b-value approaches to 1, the system is mainly controlled by capacitance; while b-value is

closed to 0.5, the Li+ insertion process dominates.'50' We read the current density i at different reduction peaks at different sweep rates from the CV curves (Figure 4b) and calculated the value of lni and corresponding lnv. The slope of the lines is just the b value (Figure S3f). As the average b value is 0.79, we confirmed that the capacitance contribution cannot be neglected. The large contribution from capacitance can be assigned to the multi-wall CNTs sand the huge specific surface areas of the aerogels.'51'

The galvanostatic charge-discharge curves of CFO/CNT-20 electrode measured at a current density of 0.1 A g-1 are illustrated in Figure 5c. The first discharge curve displays a plateau region at 0.64 V due to the formation of SEI film. The potential plateau at 0.9 V is associated with the lithiation of CFO. The galvanostatic charge-discharge profiles are in good agreement with the cyclic voltammogram characteristic. The pure CFO electrode delivered a discharge capacity of 1187.1 mA h g-1 and a charge capacity of 911.6 mA h g-1, respectively, corresponding to the Coulombic efficiency of 76.8% (Figure S4f). However, the CFO/CNT-20 electrode delivered a discharge capacity of 1525 mA h g-1 and a charge capacity of 1094 mA h g-1, respectively, corresponding to the Coulombic efficiency of 72.0%. The initial capacity loss is mainly ascribed to electrolyte decomposition and the formation of SEI layer. The overlapped curves of 2nd and 3rd indicate good stability.

Furthermore, the cycle performance of pure CFO and CFO/CNT-X electrodes at the current density of 1 A g-1 are shown in Figure 5d. The CFO/CNT-20 electrode exhibits the best cycle performance with a reversible capacity of 874 mA h g-1 after

160 discharge/charge cycles. CFO/CNT-10, CFO/CNT-15 and CFO/CNT-25 electrodes, with relative lower specific capacities, displayed a similar cycle stability to CFO/CNT-20. Pure CFO showed the lowest capacity and inferior cycle stability. The results indicate that introducing CNTs into the composite structure, capacity and cycle stability can be significantly improved. However, CFO/CNT-20 electrode exhibited the best performance indicated there would be an optimum CNT content. The CFO/CNT-20 electrode also exhibits good cycling stability at a low current density of 0.1 A g-1 (Figure S5f).

Figure 5e displays the rate performance of the pure CFO and CFO/CNT-X electrodes. Remarkably, the CFO/CNT-20 has the best capacity at different current densities. With the increase of current density from 0.1 to 0.2, 0.5, 1, 2, and 5 A g-1, the CFO/CNT-20 electrode displays reversible capacities of 1525, 969, 843, 768, 658, and 516 mA h g-1, respectively. When the current density returns to 0.1 A g-1, the capacity is regained and even increases to 1033 mA h g-1. This means that the structure of CFO/CNT-20 can be well maintained even after undergoing the high-rate charge/discharge cycles. The enhanced rate capability is attributed to the good electric conductivity of CNTs and carbon aerogels, which provide the conductive path for electron transportation. We also compared the rate performance of our work with previously reported CFO/C composites (Figure S6f). Apparently, the CFO/CNT-20 electrode presents superior rate capability at high current density.

The electrochemical impedance spectra (EIS) of CFO/CNT-X are shown in Figure 5f. CFO/CNT-X electrodes exhibit a high-frequency semicircle, a faintish

medium-frequency semicircle, and a low-frequency inclined line, which are typically assigned to the existence of the SEI films, the charge-transfer and double layer, and the Li ions diffusion process within electrodes, respectively. It also showed a smaller semicircle within the high frequency, indicating lower impedance than that of pure CFO. Additionally, among the CFO/CNT-X electrodes, impedance reduced with adding more CNTs. CFO/CNT-20 electrode possesses a faster charge transfer reaction and lower charge-transfer resistance during lithium insertion and extraction, thus lower polarization and good rate performance are predicted. The EIS analysis results are in good agreement with the previous galvanostatic charge-discharge profiles and rate performance result.

From the morphological and structural characteristics of the CFO/CNT aerogels, it is clear that the excellent electrochemical performance is attributed to the synergistic effect of CFO NPs, CNTs and carbon aerogels. CFO NPs and CNTs are embedded in carbon aerogels matrix to form a 3D network structure which favors the electrochemical performance. The characteristic structure of alginate can effectively avoid the aggregation of CFO NPs. CNTs well dispersed in the presence of alginate, significantly enhance the electronic and ionic conductivity, and thus improve the rate capability. Carbon aerogels provide the porous 3D structure which can minimize the volume expansion of CFO NPs during cycles, and long cycle stability can be obtained.

4. Conclusions

To summarize, we adopted a facile synthetic route to prepare a novel 3D CFO/CNT aerogels using sustainable alginate as precursor. The composite consisted of CFO NPs, CNTs and carbon aerogels. CFO NPs interconnected by CNTs are embedded in carbon aerogels matrix. When used as anodes for LIBs, the as-prepared product with 20 wt% CNTs exhibited a remarkable electrochemical performance with a high reversible specific capacity (1033 mA h g-1 at 0.1 A g-1), excellent rate capability (516 mA h g-1at 5 A g-1), and good cycle stability (a capacity of 874 mA h g-1 after 160 cycles at 1 A g-1). Given that the raw materials adopted in our research can be fabricated from plentiful biomass, it highlights a high efficiency, environmentally friendly and inexpensive strategy for the scale-up production of TMO anode materials for high performance LIBs.

Acknowledgement

This work is financially supported by the National Natural Science Foundation of China (No. 51473081 and 51503109), Research award fund for outstanding young scientists in Shandong province (Grant no. BS2014CL006) and Qingdao Applied Basic Research Project (16-5-1-85-jch).

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