Self-standing carbon nanofiber and SnO2 nanorod composite as high

Inc., Pennsylvania, United States). The chemical and crystalline properties of CNF@SnO2 and pristine CNFs were examined via Raman spectroscopy (inVia ...
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Self-standing carbon nanofiber and SnO2 nanorod composite as high-capacity and high-rate-capability anode for lithium-ion batteries Jyunichiro Abe, Keisuke Takahashi, Koki Kawase, Yuta Kobayashi, and Seimei Shiratori ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00586 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Nano Materials

Self-standing Carbon Nanofiber and SnO2 Nanorod Composite as High-capacity and High-rate-capability Anode for Lithium-ion Batteries Jyunichiro Abe, Keisuke Takahashi, Koki Kawase, Yuta Kobayashi, and Seimei Shiratori* Department of Integrated Design Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan KEYWORDS: Electrospinning, Battery, Anode, Self-standing, SnO2, Polyacrylonitrile, Carbon nanofiber, Hydrothermal synthesis ABSTRACT: We fabricated a composite of self-standing carbon nanofibers (CNF) and nanorod-like SnO2 (CNF@SnO2) for use in an anode for a lithium-ion battery (LIB), via electrospinning and hydrothermal synthesis methods using naturally abundant, environmentally friendly, and cost-effective materials. The composite electrode is flexible and can be directly used as an LIB anode without a metal collector. The nanorod-like structure of SnO2 accommodates the dramatic volume expansion intrinsic to SnO2 during charge–discharge cycles, increases the specific surface area, and decreases the charge-transfer resistance. CNF@SnO2 exhibits the discharge capacity of 800 mAhg−1 under 0.5 Ag–1 during the second cycle, 2.8 times higher than the capacity of the CNF-only electrode (285 mAhg−1 under the same condition). This high capacity is realized by the high reversibility of the conversion reaction of SnO2, arising from its well-organized nanostructure. Further, CNF@SnO2 shows excellent rate capability; CNF@SnO2 maintains 49% of its second discharge capacity at current densities reaching 4.0 Ag−1. This high rate capability is attributed to the high degree of CNF graphitization. Overall, CNF@SnO2 exhibits a high capacity, good rate capability, and excellent potential as a candidate LIB anode material.

INTRODUCTION Lithium-ion batteries (LIBs) have been widely used energy storage devices for portable electronic devices such as cell phones, laptop computers, or electric vehicles.1 With the rapid development of electric vehicles and stationary power storage, LIBs are required to have a higher power density and a longer cycle life. Therefore, various types of nanomaterials have been studied for energy storage2–5. Commercial LIBs use graphite as the anode, but graphite has a low theoretical specific capacity (372 mAhg−1),6 which is too low to meet the demands for electric vehicles requiring a high energy density and power density. Hence, a new anode material that has a high specific capacity, high rate capability, and long cycle life is needed. Extensive research has been carried out on metals (i.e., silicon),7–10 metal oxides,11–13 and metal sulfides14–16 that can be potential LIB anode materials. Among these new anode candidates, SnO2 has been intensively investigated in recent years because of its higher theoretical capacity (782 mAhg−1)11 than that of graphite as well as because of other practical advantages such as high abundance, low toxicity, and low cost. The lithium storage mechanism of the SnO2 anode is divided into two reactions: the conversion reaction and the subsequent alloying reaction, as shown below.

 +   + 4 →  + 2 

(1)

 +   +  →   0 ≤  ≤ 4.4

(2)

1

Based on previous research results, the first reaction is considered electrochemically irreversible with no contribution to the cell’s capacity. If this conversion reaction were highly reversible, the theoretical capacity of SnO2 would be significantly improved from 782 to 1494 mAhg−1. The alloying reaction between Sn and Li is highly reversible, but causes a large volume expansion (~259%).17 This large volume change pulverizes the active material,14 which rapidly degrades the capacity. Several strategies have been reported to solve the poor reversibility of the conversion reaction the volume expansion accompanying alloying. One strategy utilizes nanostructured SnO2, such as nanoparticles,18 nanosheets,19 nanowires,20 nanotubes,21 and core–shell particles.22 Nanostructured SnO2 can accommodate the volume change during the charge–discharge process because of its buffer space. Furthermore, some studies have suggested that the conversion reaction described in Eq. (1) becomes partially reversible for well-organized nanostructured SnO2. The other effective way to improve SnO2 anode performance is combination with carbon materials, such as graphene,20-24 carbon nanotubes,18,25,26 and carbon nanofibers (CNFs).27,28 These materials exhibit outstanding high conductivity, which can compensate for the low conductivity of SnO2. In recent years, many studies on nanostructured composite SnO2/carbon materials for LIB anodes have been reported, including hollow particles, carbon nanotubes and SnO2 hierarchical structures,25 and yolk–shell spheres. These materials have exhibited good electrochemical properties as anodes of LIBs. However, these types of electrodes are not self-supporting; they require other cell components, such as binders and current collectors, which do not contribute to capacity. The reduction of these battery

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components leads to an enhanced specific capacity. Furthermore, such electrodes require additional processes like ball-milling or pasting for practical use in LIBs. Some researchers reported on a self-standing CNF/SnO2 composite electrode.8,29–33 They fabricated a CNF membrane containing SnO2 using a one-step fabrication method, calcining nanofibers composed of a carbon precursor polymer and SnO2 (or a precursor thereof). The electrode material showed a high specific capacity and long cycle life, but performance at high current densities could be improved. The composite nanofiber could only be calcined to the temperature of 700°C because metal oxides or precursors react and catalyze fiber decomposition. Therefore, the anode showed poor rate capability because of the low carbonization degree from the low calcination temperature. Therefore, in this study, we prepared a core-shell structure nanocomposite anode (denoted as CNF@SnO2) by a two-step method in order to realize a well-graphitized CNF membrane. The fabrication process is divided into two steps, as shown in Fig. 1. First, we fabricated CNFs from an as-spun nanofiber mat of polyacrylonitrile (PAN) at a high calcination temperature (1000°C). Then, a nanorod-like SnO2 layer was deposited on the CNFs via hydrothermal synthesis followed by heat treatment. By dividing the fabrication into two stages, the PAN nanofibers were heated to 1000°C, allowing a high degree of CNF graphitization. In addition, the electrode surface morphology was changed by coating SnO2 nanorods on the CNF via hydrothermal reactions. With the nanorod-like structures on the surface, the specific surface area of the electrode increased, and the resistance decreased. This CNF@SnO2 anode showed an excellent capacity retention rate under a high current density (48% of initial capacity under 4000 mAg−1) because of its high degree graphitization from the high-temperature calcination. The anode is also flexible and self-standing, with double the capacity of the CNF-only electrode; thus, it is a potential candidate anode material for high-capacity LIBs.

Sigma-Aldrich (USA). N,N-dimethylformamide (DMF; 99.5%) 10 wt.%, tin(II) oxalate (SnC2O4), and SnO2 nanoparticles were purchased from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). Hexamethylenetetramine (HMTA, C6H12N4) was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Plastic syringes and needles (21G ½) were purchased from Termo Co. (Tokyo, Japan). LiPF6 (1 mol L−1) in an ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 v/v%) solution was purchased from Kishida Chemical Co. (Tokyo, Japan). Preparation of CNFs. The complete experimental procedure is illustrated in Fig. 1. PAN (10 wt.%) was dissolved in DMF with stirring for 24 h at room temperature. The solution was loaded into a plastic syringe and electrospun at a voltage of 10 kV, a tip-to-collector distance of 15 cm, and feed rate of 1 mL h−1 for 10 h. The as-spun fiber mats were stabilized at 280°C for 2 h in air with a heating rate of 5°C min−1, and subsequently, carbonized at 1000°C for 1 h under a flowing N2 atmosphere with a heating rate of 5°C min−1. Fabrication of CNF@SnO2. A layer of nanorod-like SnO2 was deposited onto the CNF by hydrothermal synthesis. The precursor solution was prepared by dissolving SnC2O4 and HMTA in 100 mL deionized water at concentrations of 0.012 M and 0.018 M, respectively. The formation mechanism of the SnO2 nanorods has been reported in detail in a previous study34. The CNF was immersed into the precursor solution and SnO2 crystal growth proceeded in an electric furnace at 95°C for 3 h. After completing the growth reaction, the sample was washed with deionized water and then annealed at 300°C for 1 h in air at a heating rate of 5°C min−1 to oxidize Sn or SnO to SnO2. Photographs of the fabricated electrodes are shown in Fig. 2

Fig. 2 Photograph of self-standing CNF@SnO2

Fig. 1 Schematic of the entire experimental procedure.

EXPERIMENTAL Materials. All reagents were used without further purification. PAN (average Mw ~150,000) was purchased from 2

Material Characterization. The surface morphology of carbon nanofibers and CNF@SnO2 was investigated via fieldemission scanning electron microscopy (FE-SEM, S-4700, Hitachi Ltd., Tokyo, Japan) and transmission electron microscopy (TEM, FP 5360/22, E.A. Fischione Instruments, Inc., Pennsylvania, United States). The chemical and crystalline properties of CNF@SnO2 and pristine CNFs were examined via Raman spectroscopy (inVia confocal Raman microscope, Renishaw, PLC, Gloucestershire, United Kingdom), X-ray diffraction (XRD, D8 ADVANCE, Bruker Co., Massachusetts, United States), and X-ray photoelectron spectroscopy (XPS, JEOL, Ltd., JPS-9010TR, Tokyo, Japan). The N2 adsorption/desorption isotherms were acquired at −196°C with a surface area and porosity analyzer (MIC2010M, Shimazu Co., Tokyo, Japan) to detect the specific surface area and pore size distribution. The Brunauer– Emmett–Teller (BET) method was used to calculate the

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ACS Applied Nano Materials surface area and the Barrett–Joyner–Halenda (BJH) method was used to calculate the pore size distribution. Electrochemical tests. Electrochemical measurements were performed using CR2032 coin cells, which were assembled in a dry air-filled glove box (Galaxy, Matsuura Manufactory Corporation, Tokyo, Japan). Both CNF and CNF@SnO2 were punched into small disks with diameters of 12 mm, and these fibrous mats were directly used as working electrodes in the cells with no current collector, conductive agent, or polyvinylidene fluoride (PVDF) binder. All electrodes were adjusted to 3 mg ± 1 mg. For comparison, a SnO2 nanoparticle electrode was prepared using commercial 80 wt.% SnO2 nanoparticles, 10 wt.% conductive carbon black, and 10 wt.% PVDF binder in an N-methyl-2-pyrrolidone solvent. A pure Li foil was used as the counter electrode and a microporous monolayer membrane (Celgard 2500, Celgard, Charlotte, North Carolina, USA) was used as the separator. The electrolyte was 1 mol L−1 LiPF6 in EC/DMC (1:1 v/v%). Galvanostatic charge/discharge cycles were tested by a multichannel charge–discharge device (HJ-1001SMB, Hokuto Denko Corporation, Tokyo, Japan) at various current densities with a voltage range of 0.05–3.00 V (vs. Li/Li+) at room temperature. The weight of the total electrode was used to calculate the capacity in this experiment. Cyclic voltammetry (CV) measurements were performed potentiostatically on an ECstat-301 (EC FRONTIER Co., Ltd, Kyoto, Japan) over the potential range of 0 to 3.5 V at the scan rate of 0.5 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were performed using an impedance analyzer (IM 353-01, HIOKI E.E. Co., Nagano, Japan) by employing an AC voltage of 0.1 V in the frequency range 0.01 Hz to 200 kHz. RESULTS AND DISCUSSION

To confirm the morphologies and elemental compositions of CNF and CNF@SnO2, SEM and TEM observations and energy-dispersive X-ray spectroscopy (EDX) analysis were performed. Figs. 3a, b, c and d show SEM and TEM images of CNF and CNF@SnO2. CNF has a uniform, continuous morphology and a smooth surface with a diameter of ~400 nm. After the hydrothermal reaction, the fiber diameter is slightly increased and a nanorod-like SnO2 crystal layer is uniformly and densely deposited on the CNF surface. The SnO2 crystal deposition onto CNF changes the surface structure dramatically, affecting the charge-transfer resistance (Rct) of the electrode because of the increase in the specific surface area. As shown in the TEM image (Fig. 3f), CNF@SnO2 has a core–shell structure with a CNF core and SnO2 shell. The thickness of the SnO2 layer is some tens of nanometers. As shown in Fig. 3f, C, O, and Sn peaks were confirmed by EDX analysis (the weak peak around 2.0 keV arises from the Os coating deposited on CNF@SnO2 for SEM imaging). The SnO2 content in the CNF@SnO2 is estimated as 41.3% by thermogravimetric-differential thermal analysis (TG-DTA, Fig. 3g). Since the DTA peak below 200°C is an endothermic reaction, the first mass loss below 200°C is attributed to the evaporation of moisture from CNF, and peaks at higher temperature indicate the reaction of hydroxyl groups on the surface of SnO2. The DTA peak at 490°C indicates the oxidation of Sn or SnO, which are incompletely oxidized during annealing after hydrothermal synthesis. N2 adsorption/desorption isotherm analysis was performed to measure the surface areas and porosities of the CNF@SnO2 and pristine CNFs. Their BET surface areas and BJH pore volumes are summarized in Table 1.

Fig. 3. FE-SEM images of (a) CNF and (b) CNF@SnO2. (c–e) FE-TEM image of CNF@SnO2. (f) EDX spectral graph of CNF@SnO2. (g) TG-DTA analysis of CNF@SnO2.

Table 1. BET surface areas and BJH pore volumes of CNF and CNF@SnO2

2

−1

BET surface area (m g )

3

CNF

CNF@SnO2

164.9

216.7

BJH adsorption: cumulative pore volume of pores between 17–3000  (cm3 g−1)

0.011

0.029

The higher surface area of CNF@SnO2 is attributed to the nanorod-like structure of the outermost SnO2 layer. The cumulative pore volume of CNF@SnO2 is approximately

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three times larger than that of the pristine CNFs. The increase in both the surface area and pore volume is due to the nanorod-like structure, which contributes to the decreased Rct and the improved reversibility of the conversion reaction. The XRD patterns of CNF and CNF@SnO2 are shown in Fig. 4a. The intense peak around 26° is ascribed to amorphous C, and the three diffraction peaks of CNT@SnO2 at 26.2°, 33.5°, and 51.6° are well assigned to the (110), (101), and (211) planes of the tetragonal structure of SnO2 (JCPDS Card No. 41-1445), respectively. These peaks derived from SnO2 are not observed immediately after hydrothermal synthesis. After calcination following the hydrothermal synthesis, the crystallinity of SnO2 is slightly improved and very weak and broad peaks appear,

indicating that SnO2 deposited onto CNF is mostly amorphous in structure. The degree of graphitization of the CNFs was investigated by Raman spectroscopy. The D band (1350 cm−1) is attributed to carbon defects, and the G band (1590 cm−1) is attributed to graphitic structure. In general, the intensity ratio of the D band to the G band (ID/IG) is used to evaluate the degree of graphitization of carbon materials. CNF calcined at 1000°C shows a stronger G peak than that calcined at 700°C with ID/IG of 0.89 and 1.08 in Fig. 4b, respectively. This difference in the ID/IG value indicates increased graphitization that affects the electrochemical performance of the anode under high current densities.

Fig. 4 (a) XRD patterns of CNF and CNF@SnO2. (b) Raman spectra of CNF annealed at 700°C and 1000°C.

To confirm the composition and oxidation state, CNF and CNF@SnO2 were investigated by XPS analysis. As shown in the wide-scan XPS spectra (Fig. 5a), C 1s, N 1s, and O 1s peaks appear for pristine CNF; strong Sn peaks appear only for CNF@SnO2. After the hydrothermal synthesis, the C 1s peak is dramatically weakened, and the N 1s peak disappears in the spectrum for CNF@SnO2. On the other hand, O 1s peaks are dramatically enhanced, and strong Sn peaks appear. These results indicate that the CNF is coated by a SnO2/SnO layer with a thickness of tens of nanometers, which is

consistent with the TEM image. In the high-resolution XPS spectra of O 1s and Sn 3d, characteristic peaks of SnO2 are observed. The O 1s spectrum of CNF@SnO2 is deconvoluted into two peaks of O–Sn (Sn4+). The Sn 3d spectrum of CNF@SnO2 is deconvoluted into two main peaks and one very weak peak, corresponding to Sn4+ 3d3/2, Sn4+ 3d5/2, and Sn0 3d3/2, respectively. From the peak intensity of Sn 3d, it is considered that most of the Sn deposited on the CNF is in the SnO2 state, which can act as an LIB anode material.

Fig. 5 (a) Low-resolution XPS spectrum: high-resolution XPS spectra of (b) C 1s, (c) O 1s, and (d) Sn 3d of CNF and CNF@SnO2.

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ACS Applied Nano Materials Fig. 6 shows the cyclic voltammetry (CV) curves and charge–discharge profiles of pristine CNF and CNF@SnO2 for the first five cycles. Pristine CNF shows a cathodic peak at 0.5 V only in the first cycle, which is attributed to the formation of the solid electrolyte interphase (SEI), and a cathodic peak around 2.5 V is attributed to the reaction of Li and O2. A cathodic peak at 0 V and an anodic peak at 0.3 V are attributed to the intercalation of Li+ into pristine CNF. For CNF@SnO2, a cathodic peak corresponding to the formation of SEI is observed only in the first cycle. In addition, two characteristic pairs indicating redox peaks are observed for CNF@SnO2. The first pair, located at 0.05 and 0.58 V, is attributed to the highly reversible alloying/ dealloying reaction between Li and Sn (Eq. 2). The second pair located at 1.0 and 1.25 V (red circle in Fig. 6b) is attributed to the conversion reaction between SnO2 and Sn (Eq. 1). The CV curves from the second to fifth cycles nearly overlap, indicating the high reversibility of both the alloying and conversion reactions of SnO2, as well as the good cycling performance of CNF@SnO2. From these CV curves, we can conclude that the composite electrode has

high reversibility. The charge–discharge profiles of CNF and CNF@SnO2 are shown in Fig. 6c and d, respectively. Both CNF and CNF@SnO2 show large discharge capacity losses from the first to second cycle of 712 mAhg−1 to 282 mAhg−1 and from 1210 mAhg−1 to 793 mAhg−1, respectively. These large capacity losses are caused by the inevitable formation of SEI in the first cycle. After the second cycle, the charge– discharge curves almost overlap, indicating good cyclic stability. The charge–discharge curves of CNF@SnO2 exhibit two slopes at ~0.5 V and ~1.25 V during charging. The electrochemical reactions of SnO2-based anodes at low voltages (1.25 V) are the alloying and conversion reactions, respectively. Moreover, from the CV result of CNF@SnO2, it is inferred that the conversion reaction of this CNF@SnO2 anode occurs above 1.0 V. In the charge process, the charge profiles of CNF@SnO2 after the second cycle almost overlap, indicating that the reversibility of the conversion reaction is well maintained.

Fig. 6 Cyclic voltammetry curves for the first five cycles of (a) CNF and (b) CNF@SnO2 at a scan rate of 0.5 mV s−1 in the voltage range of 0.05–3.0 V. First five charge–discharge curves of (c) CNF and (d) CNF@SnO2 at current density of 50 mAg−1.

The rate capabilities of the CNF, CNF@SnO2, and SnO2 nanoparticle electrodes were investigated through galvanostatic measurements at various current densities. As shown in Fig. 7, the reversible capacities of the CNF are 296, 187, 170, 159, and 131 mAhg−1 at current densities of 0.1, 0.25, 0.5, 1, and 2.5 Ag−1, respectively. Those of CNF@SnO2 are 822, 616, 553, 492, and 402 mAhg−1 at current densities of 0.1, 0.25, 0.5, 1, and 2.5 Ag−1, respectively. Under all current densities, CNF@SnO2 exhibits a capacity 250% higher than that of the CNF-only electrode. Furthermore, even at the current density of 2.5 Ag−1, CNF@SnO2 exceeds the theoretical capacity of graphite (372 mAhg−1). This high 5

specific capacity is derived from two main factors: the use of a high-capacity material, i.e., SnO2, and the improved reversibility of the conversion reaction of SnO2 from the wellorganized nanostructured SnO2 layer on the CNF. Further, the SnO2 nanoparticle electrode exhibits the capacity of 446 mAhg−1 under 0.1 Ag−1 in the first cycle, but the capacity deteriorates rapidly after the second cycle, dropping below that of the CNF-only electrode at the current density of 0.5 Ag−1. Moreover, the SnO2 nanoparticle electrode does not show any capacity at the current density of 1.0 Ag−1 because of its high resistance. In addition, as shown in Fig. 8, the

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characteristics are improved by increasing the carbonization temperature of CNF.

Fig. 7 Rate capabilities of CNF, CNF@SnO2, and commercial SnO2 nanoparticle electrode in the voltage range of 0.05–3.0 V.

As shown in Fig. 9, the CNF@SnO2 electrode still exhibits a specific capacity of 485 mAhg−1 after 850 cycles, indicating excellent cycling performance. Moreover, as shown in Fig. 10, since the SEI is homogeneously formed on the CNF@SnO2 surface, this electrode has good cyclability. EIS measurements were performed on fresh cells with CNF and CNF@SnO2 electrodes, with results shown in Fig. 11. The EIS curves consist of semicircles in the high- to middle-frequency range and inclined slopes in the low-frequency range; these are attributed to the combination of charge transfer resistance (Rct) and electrical double-layer capacitance (Cdl), and to the diffusion of Li+ in the electrode, respectively. The diameter of the semicircle for the CNF@SnO2 electrode is obviously smaller than that of CNF, indicating that CNF@SnO2 has a lower Rct than CNF. This is caused by the nanorod-like structure and high specific surface area of CNF@SnO2, which yield a high rate capability for the composite electrode. Table 2 presents a comparison of previously reported SnO2-based LIB anodes. The table clearly shows that the nanocomposite reported in this study has the highest rate capability. This outstanding high rate capability is due to the high degree of graphitization of CNF, and the self-standing ability of this CNF@SnO2 composite makes it a suitable LIB anode material.

Fig. 8 Rate capabilities of CNF@SnO2 calcined at 700°C (green) and 1000°C (red) in the voltage range of 0.05–3.0 V.

The rate capability of CNF@SnO2 at higher current densities is investigated (Fig. 7). Even when the current density is increased to 4.0 Ag−1, CNF@SnO2 maintains the discharge capacity of 359 mAhg−1, 49% of the second discharge capacity of 735 mAhg−1, which indicates excellent rate capability. This arises from the high degree of graphitization of the CNF calcined at 1000°C. This composite electrode also exhibits good cycle performance.

Fig. 10 Cycling performance of CNF@SnO2 as monitored every 50 cycles at a current density of 0.1 Ag−1.

Fig. 11 FE-SEM images of CNF@SnO2 composite after cycling Fig. 9 Rate capability of CNF@SnO2 at higher current densities in the voltage range of 0.05 to 3.0 V.

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ACS Applied Nano Materials *E-mail: [email protected]. Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources No funds were used to support the research contained in this manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Fig. 12 EIS curves of CNF, SnO2, and CNF@SnO2 fresh cells.

We are grateful to the members of the Battery team of Shiratori Laboratory, Yuka Abe, and Fumika Abe. REFERENCES

Table 2. Comparison of rate capability obtained in present study with that of C/SnO2 composite anodes Materials

Self-standing ability

Capacity retention

Refs.

C/SnO2

No

43% at 2.0 C

35

C/SnOx/ZnO

Yes

36% at 0.9 C

32

C/SnO2

Yes

45% at 3.6 C

33

48% at 5.4 C

This work

C/SnO2

Yes

CONCLUSIONS In this work, a free-standing nanocomposite electrode of CNF and well-organized SnO2 nanorods was fabricated by electrospinning and hydrothermal synthesis. The nanofibrous mat electrode material exhibited improved reversible capacity and excellent rate capability compared to previously reported composite anodes of SnO2 and carbon. SEM observation and XPS analysis results revealed that nanorod-like SnO2 was deposited onto CNF, and the CV curves clearly showed that the conversion reaction of this composite electrode was highly reversible. The CNF@SnO2 electrode exhibited a capacity 250% higher than that of CNF, with second-cycle discharge capacities of 800 mAhg−1 and 286 mAhg−1, respectively, and excellent rate capability, retaining 49% of the second discharge capacity at the current density of 4.0 Ag−1. CNF@SnO2 showed a low charge-transfer resistance because of the increase in surface area by SnO2 deposition, and further promoting excellent electrochemical properties. CNF@SnO2 maintained the capacity of 485 mAhg−1 after 850 cycles, indicating long cycle lifetime. CNF@SnO2 was prepared from naturally abundant, environmentally friendly, and costeffective materials and was self-standing, meaning it does not require cell components such as binders or current collectors. This composite electrode is thus a promising candidate LIB anode material.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author 7

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