Flexible Sub-Micro Carbon Fiber@CNTs as Anodes for Potassium-ion

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Energy, Environmental, and Catalysis Applications

Flexible Sub-Micro Carbon Fiber@CNTs as Anodes for Potassium-ion Batteries Chao Shen, Kai Yuan, Te Tian, Maohui Bai, Jian-Gan Wang, Xifei Li, Keyu Xie, Qian-Gang Fu, and Bingqing Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18834 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Flexible Sub-Micro Carbon Fiber@CNTs as Anodes for Potassium-ion Batteries Chao Shen, 1, † Kai Yuan, 1, † Te Tian, 1 Maohui Bai, 3 Jian-Gan Wang, 1 Xifei Li, 2 Keyu Xie,*, 1 Qian-Gang Fu*, 1 and Bingqing Wei*1, 4

1State

Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of

Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi’an 710072, China. 2Institute

of Advanced Electrochemical Energy, Xi’an University of Technology, Xi’an 710048,

China. 3School

of Metallurgy and Environment, Central South University, Changsha 410083, P. R.

China. 4Department †Chao

of Mechanical Engineering, University of Delaware, Newark, DE19716, USA.

Shen and Kai Yuan contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: Potassium-ion batteries (KIBs) with potential cost benefits are a promising alternative to lithium-ion batteries (LIBs). However, due to the large radius of K+, current anode materials usually undergo large volumetric expansion and structural collapse during the charging-discharging process. Self-supporting carbon nanotubes encapsulated in sub-micro carbon fiber (SMCF@CNTs) are utilized as the KIB anode in this study. The SMCF@CNTs anode exhibits high specific capacity, good rate performance, and cycling stability. The SMCF@CNT electrode has specific capacities of 236 mAh g-1 at 0.1 C and 164 mAh g-1 at 5 C, and maintains over 193 mAh g-1 after 300 cycles at 1 C. Furthermore, a combined capacitive and diffusion-controlled K+ storage mechanism is proposed based on the investigation using in-situ Raman and quantitative analyses. By coupling the SMCF@CNTs anode with the K0.3MnO2 cathode, a pouch cell with good flexibility delivers a capacity of 74.0 mAh g-1 at 20 mA g-1. This work is expected to promote the application of KIBs in wearable electronics.

KEYWORDS: potassium ion batteries, anode, sub-micro carbon fibers, CNT, flexible batteries


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INTRODUCTION Rechargeable lithium-ion batteries (LIBs) are widely used in mobile devices, including laptops, cell phones, and electric vehicles for its lightweight and high-capacity.1-3 However, lithium is a scarce resource with high extraction costs and abundance of approximately 20 ppm in the Earth’s crust.4 Therefore, finding alternatives to LIBs that are inexpensive and have high crustal abundance has become necessary.5 This has inspired research on alternative ion batteries based on Earth’s abundant alkali metals, such as Na (~23,000 ppm) and K (~17,000 ppm).6 Compared with the potassium-ion battery (KIB) system, the sodium-ion battery (NIB) system exhibits remarkable improvements in the cathode, anode, and electrolyte.7 However, sodium cannot form stable intercalation compounds in graphite, which is commonly used as the anode material in commercial batteries, and previous work found that one K+ can combine with 8 C in graphite to form KC8.8–10 Furthermore, the redox potential of K+/K (−2.93 V vs. SHE) is lower than that of Na+/Na (−2.71 V vs. SHE) and 0.15 V lower than that of Li+/Li in ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte.11 Thus, KIB is highly competitive. However, a large ionic radius (K+ to Na+ and Li+: 1.38 Å to 1.02 Å and 0.76 Å) leads to structural collapses of K ion-intercalated graphite and eventually results in the rapid decay of capacity during charge-discharge.9, 11 Researchers constantly search for solutions to this problem. Non-graphite soft carbon appears to be more suitable for K+ storage and demonstrates better cycle stability than


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graphite.10–12 However, only a few studies have used soft carbon as the anode of KIBs, and its cycle stability and rate performance still require improvement. The capacitive behavior of LIBs or NIBs at the surface or interface of anode materials has elicited the great attention of researchers recently.13–17 Compared with insertion chemistry that involves diffusion and insertion/extraction of alkali metal ions, capacitive charge storage occurs primarily at the surface or interface and provides appropriate reaction kinetics that is minimally affected by ion radius. Along this line, anode materials with a large specific surface area and high electronic conductivity are favorable for advanced surface energy storage.18 Micro- and nano-carbon materials, such as carbon nanotubes (CNTs), carbon nanospheres, carbon fibers (CFs), and graphene, with high electronic conductivity and the favorable specific surface area, can store charges at the interface to improve the specific capacity and rate performance of batteries. These anode materials demonstrate the promising potential for use in LIBs and NIBs.19-22 According to previous research, surface energy storage can be enhanced through improving the surface area of electrode materials and doping heteroatoms (e.g., O, N, S, and F).23–27 And developing self-supporting electrodes can increase the energy density of batteries in the absence of a binder, conductive agent, and current collector.24,

28, 29

Furthermore, the current KIBs are generally used coin-type cell and inflexible. Recently, flexible and inexpensive energy storage devices have been urgently needed to supply energy for flexible and wearable electronics. Therefore, the self-supporting flexible electrode and its assembled flexible KIBs are pretty suitable. Inspired by these approaches, we synthesized self-supporting sub-micro carbon fibers wrapped in carbon nanotubes


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(SMCF@CNTs) via electrospinning to introduce CNTs into the precursor solution of polyacrylonitrile (PAN). The SMCF@CNT composite held excellent electrical conductivity and a relatively high specific surface area of 49.8 m2 g-1 due to the addition of CNTs and the 3D network characteristic. The self-supporting SMCF@CNT film exhibited advanced electrical conductivity, high flexibility, and was directly used as the anode for KIBs. As a result, the SMCF@CNT anode exhibited high specific capacity (236 mAh g-1 at 0.1 C), good cycling stability (98% retention after 300 cycles at 1 C), and outstanding rate performance (164 mAh g-1 at a high current density of 5 C). Moreover, we assembled a pouch cell using the SMCF@CNTs anode and the K0.3MnO2 cathode, which showed good flexibility and electrochemical stability. Our work promotes the fabrication of binder-free electrodes and application of KIBs in flexible devices.

RESULTS AND DISCUSSION SMCFs and flexible SMCF@CNTs were prepared via electrospinning and carbonizing with PAN and CNTs as precursors (Figure 1). Digital photos of SMCFs and SMCF@CNTs are shown in Figure 2a, 2b, S1a, and S1b. The SMCF@CNTs film could be wrapped around a glass rod without breaking, and the SMCFs film was broken after bending a small curvature. This is probably caused by the toughening effect of CNTs. When the force is applied, the stress on the SMCFs is concentrated on the defects and the stress on the SMCF@CNTs is transferred to the high-strength CNTs.30, 31 Therefore, the flexibility of SMCF@CNTs has been greatly improved.


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Figure 1. Illustration of the fabrication of SMCF@CNTs via electrospinning and subcarbonization. The SEM images of the fabricated fibers before carbonization are shown in Figure S1c (SMCFs) and S1d (SMCF@CNTs). The samples without CNTs exhibit a smooth, curved, randomly arranged fiber network with an average diameter of 367 ± 53 nm. The CNTcontaining samples have an average diameter of 309 ± 92 nm and show rough, curved, randomly arranged fiber networks with irregular surfaces. After carbonization, the bare SMCF sample (Figure S1e) possesses a smooth surface with a diameter of 188 ± 30 nm, and the SMCF@CNT sample (Figure S1f) has a diameter of 258 ± 114 nm with a rougher and more curved fiber than that before carbonization. Microstructures were further studied by TEM, as shown in Figure 2c (SMCFs) and 2d (SMCF@CNTs). It is easily noticed that the fibers have a uniform diameter of 300 nm. An amorphous structure was observed in the SMCFs and SMCF@CNTs shown in the HRTEM images in Figure 2e (SMCFs) and 2f (SMCF@CNTs), further confirming the amorphous nature of the carbonized PAN.


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Figure 2. Digital photos illustrating the flexibility of (a) SMCFs and (b) SMCF@CNTs. TEM images of (c) SMCFs and (d) SMCF@CNTs and HRTEM images of (e) SMCFs and (f) SMCF@CNTs. Further investigation of the crystallinity and molecular structure of SMCFs and SMCF@CNTs were performed via X-ray diffraction (XRD) and Raman spectroscopy. As shown in Figure 3a, both materials show broad peaks of 25.8° and 42.9° corresponding to crystallographic Miller planes of (002) and (100) in the disordered carbon structure, respectively. The (002) peak indicates the appearance of a graphitic layer in the fiber.32 Comparison of the SMCFs and SMCF@CNTs shows that almost no effect is exerted on the XRD peak when CNTs were added, but the peak of (002) was enhanced due to the presence of CNTs. The Raman spectra of SMCFs, CNTs, and SMCF@CNTs are shown in Figure 3b, which exhibit


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a G-band (crystalline graphite band, ~1573 cm−1) and a D-band (defect-induced band, ~1348 cm−1). The G-band is attributed to E2g vibration of sp2 carbon, and the D-band corresponds to the finite-size graphene domain with A1g oscillation. The 2D band, that is, the harmonics of the D band has vibrations that require a defect-free graphene domain. Comparison of the SMCFs and SMCF@CNTs shows that the addition of CNTs results in a more pronounced 2D band, which further confirms the presence of CNTs in [email protected] Nitrogen-doped carbon from PAN decomposition is conducive, increasing the conductivity of carbon materials, and enhances the electrochemical properties of the electrode.34 X-ray photoelectron spectroscopy (XPS) was performed to investigate the elemental composition and bonding configuration of the SMCF@CNTs (Figure S2 (a-c), Figure 3c). The major component elements of SMCF@CNTs are carbon, oxygen, and nitrogen with atomic contents of 89.84 %, 8.58 %, and 1.58 %, respectively. The high-resolution N1s spectrum in Figure 3c shows that the three independent peaks of 398.47, 400.07, and 403.27 eV can be further deconvoluted to correspond to pyridine N, pyrrole N, and graphite N, respectively.35 The atomic contents of pyridinic N, pyrrolic N and graphitic N in the material are 0.44%, 0.70%, and 0.44%, respectively.


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Figure 3. (a) XRD patterns of SMCFs and SMCF@CNTs. (b) Raman spectra of SMCFs, CNTs, and SMCF@CNTs. (c) XPS spectrum of SMCF@CNTs of the N1s region. (d) Nitrogen adsorption isotherms of the SMCFs and SMCF@CNTs. BET analysis was performed to determine the effect of CNTs on the specific surface area and pore structure of the SMCFs. The N2 adsorption/desorption isotherms are shown in Figure 3d, S3a, and S3b. The specific surface areas of the SMCFs and SMCF@CNTs are 18.0 and 49.8 m2 g-1, respectively. The BJH model suggests that the pore size distributions of SMCFs (Figure S3c) and SMCF@CNTs (Figure S3d) are mainly in the range of 2 to 100 nm. However, the pore volumes of meso- and micropores in SMCF@CNTs are larger than those in SMCFs. Thus, the addition of CNT increased the BET surface area by increasing the pore volume of mesopores and micropores, thereby resulting in high diffusion and appropriate adsorption/desorption of K+.36, 37


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The electrochemical performance of SMCF@CNTs as non-aqueous KIB anode materials was evaluated via half-coin cells. Figure 4a and 4b show the first three CV curves for the SMCF and SMCF@CNT anodes, respectively, with a potential window of 0.01 to 2.00 V (vs. K+/K) at a scan rate of 0.2 mV s-1. They show wide cathodic peaks below 1 V in the first CV scan, but the peaks disappear in the subsequent scans because of the formation of solid electrolyte interface (SEI) films. The sharper peak at 0.46 V of SMCF@CNTs can be attributed to the formation of more SEI film.24 However, it is worth noting that near 0 V, the first CV curve of the SMCFs is a broad peak, while the SMCF@CNTs is sharper. This phenomenon is caused by the increase in the capacity contribution of the reversible K+ insertion reaction by the introduction of CNTs, indicating that SMCF@CNTs as the anode material for KIBs have better reversibility than SCMFs. In both cases, the CV curves from the 2nd to the 3rd cycle almost overlapped, indicating good electrochemical repeatability. The CNT-added SMCFs exhibit a good rate performance with galvanostatic cycling from 0.1 C (1 C=279 mAh g-1) to 5 C (Figure 4c). The SMCF@CNTs shows an extremely high reversible specific capacity of 236 mAh g-1 at 0.1 C compared with 192 mAh g-1 for SMCFs. The reversible specific capacity of SMCF@CNTs remains at 210, 209, 192, and 164 mAh g-1 at current densities of 0.2, 0.5, 1, 2, and 5 C, respectively, showing excellent rate performance. However, the SMCFs only represents 192, 149, 129, 81, and 25 mAh g-1 at the respective current density. Compared with other carbon materials reported as anode of KIBs, such as graphite, mesoporous carbon, hard-soft composite carbon, S and O codoped porous hard carbon microspheres, polynanocrystalline graphite, F-


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doped graphene foam, soft carbon, hard carbon, the SMCF@CNTs composite anode exhibits the one of the best cycle stability and rate performance for KIBs to date.10, 12, 26, 38-41 (Table 1)

Figure 4. Cyclic voltammetry (CV) curves of (a) SMCFs and (b) SMCF@CNTs. (c) Rate performance of SMCFs and SMCF@CNTs from 0.1 C to 5 C. (d) Cycling performance of SMCFs and SMCF@CNTs at a current rate of 1 C The cycling performance of SMCF@CNTs was further studied at 1 C. Compared with SMCFs (Figure 4d), SMCF@CNTs has a stable and higher storage performance and good reversibility. The initial capacity of the SMCF@CNTs approaches 196 mAh g-1, and maintains more than 193 mAh g-1 after 300 cycles. The SMCFs shows an initial capacity of 168 mAh g-1 and a capacity of only 130 mAh g-1 after 300 cycles.


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Table 1. Comparison of recent literature reports on the electrochemical performances of carbon materials as anode for K-ion battery. The calculation of capacity and active material content considered binder and conductive agent. Materials

Initial capacity

Capacity retention

Rate capability

Voltage

Active material content

Ref.

SMCF@CNTs

196 mAh g-1 at 1 C

193 mAh g-1

236 mAh g-1 at 0.1 C

0.01-2 V

100%

after cycles

164 mAh g-1 at 5 C

This work

0.01-2.5 V

80%

12

0.01-2 V

80%

38

0.01-2.5 V

90%

39

0.01-3 V

70%

26

0.01-2 V

80%

40

0.01-1.5 V

90%

10

0.01-1.5 V

90%

10

0.01-1.5 V

80%

41

Mesoporous Carbon

216 mAh g-1 at 0.72 C

Hard-Soft Composite Carbon

~160 mAh g-1 at 1 C

S and O Codoped Porous Hard Carbon

198 mAhg-1 at 0.72 C

F-Doped Graphene Foam

~227 mAh g-1 at 1.8 C

Polynanocrystalline Graphite

~120 mAh g-1 at 0.36 C

Graphite

177 mAh g-1 at 0.5 C

Soft Carbon

Hard Carbon

~175 mAh g-1 at 2 C

208 mAh g-1 at 0.1 C

300

158 mAh g-1

229 mAh g-1 at 0.18 C

after cycles

115 mAh g-1 at 3.6 C

200

149 mAh g-1

~208 mAh g-1 at 0.1 C

after cycles

97 mAh g-1 at 5 C

200

~180 mAhg-1

207 mAh g-1 at 0.18 C

after cycles

142 mAh g-1 at 3.6 C

200

116 mAh g-1

228 mAh g-1 at 0.18 C

after cycles

145 mAh g-1 at 1.8 C

200

~60 mAh g-1

177 mAh g-1 at 0.07 C

after cycles

11.2 mAh g-1 at 3.6 C

300

90 mAh g-1

236 mAh g-1 at 0.1 C

after 50 cycles

72 mAh g-1 at 1 C

~140 mAh g-1

237 mAh g-1 at 0.1 C

after 50 cycles

126 mAh g-1 at 5 C

173 mAh g-1

183 mAh g-1 at 0.1 C

after cycles

109 mAh g-1 at 5 C

100


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In-situ Raman spectroscopy was performed to reveal the storage mechanism of K+, as shown in Figure 5a. During the discharge process, the decrease in the intensity of the D-band can be attributed to K+ occupying the defects in the SMCF@CNTs, such as amorphous structures and nanopores.42 The G-band shows an insignificant redshift, which can be explained by the charge transfer effect from K+ ion adsorption and the corresponding formation of the SEI layer. However, the D and 2D peaks become less pronounced at 0.3 V, proving that the introduction of K+ affected the structure of the SMCF@CNT composites, that is, the ion insertion reaction occurred.23 The increase in the intensity of the G peak during charging and the decrease during discharge agree well with the findings. Therefore, the “adsorption–insertion mechanism” for K+ storage in the SMCF@CNT electrode was confirmed by in-situ Raman analysis.

Figure 5. (a) In-situ Raman spectra of the SMCF@CNT electrode. Quantitative capacitive and kinetics analysis. CV curves of (b) SMCFs and (c) SMCF@CNTs at different scan rates.


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Capacitive contributions to the charge storage of (d) SMCFs and (e) SMCF@CNTs at a scan rate of 0.2 mV s −1. In Figure 5b and 5c, CVs with a scan rate of 0.2 mV s-1 to 0.8 mV s-1 were used to assess the contribution of surface adsorption to charge storage. The two storage modes for electrode materials are Faradaic and non-Faradaic-processes.43 Typically, the non-Faradaic process is the reversible charge storage of the electric double layer. The Faradaic process includes a pseudocapacitance process occurring at the active material surface and a diffusion control process (typically a conventional cell) in the material. The contribution of capacitance can be qualitatively analyzed based on the relationship between the peak current (i) and the scan speed (v) on the CV curve; i = avb, where a and b are constants. The b value can be determined from the slope of the log(i)–log(v) curves (Figure S4), which is typically between 0.5 and 1.0.

b=1.0 indicates that the charge storage is completely contributed by the capacitive process, and b = 0.5 indicates that it is contributed by the solid-state diffusion process. The b values of SMCFs and SMCF@CNTs were determined to be 0.94 and 0.89, respectively, by using anodic peak current in the CV curve of 0.2 mV s-1 to 0.8 mV s-1, indicating that the capacity was obtained from both capacitive and diffusional contributions. The results of the Trasatti analysis performed by Dunn et al. showed that the specific contribution from capacitance and diffusion-controlling charge at a fixed voltage could be estimated at a certain scan rate, as follows:17


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where I (V), k1ν, and k2ν1/2

(1) I(V) = k1 v + k2 v1/2 represent the total current at a particular voltage V, the current

caused by the surface capacitance, and the current generated by diffusion-controlled K+ insertion, respectively. For analytical purposes, Eq. 1 can be rearranged as (2) I(V)/v1/2 = k1 v1/2 + k2 The values of k1 (slope) and k2 (intercept) at a given voltage can be calculated by plotting the I (V)/v1/2–v1/2 curves. The series values of k1 and k2 are substituted into k1v and k2v1/2 at different voltages, and after integrating the closed CV region, the amount of stored charges from different energy storage modes can be calculated. The contribution of capacitive charges (shaded areas) is presented in Figure 5d and 5e to be compared with the total charges at a scan rate of 0.2 mV s-1. The capacitive contribution of the SMCFs (77%) to the total capacitance was slightly higher than that of the SMCF@CNTs (69%). This is also the reason why the peak at the first cycle of the CV curve nearing 0 V (Figure 4a and 4b) becomes sharper by the introduction of CNTs. However, the capacitive contribution of the SMCF@CNTs is higher than that of the SMCFs (e.g. 144 vs. 115 mAh g-1 at 0.5 C). This result indicates that the addition of the CNTs also enhanced the capacitive and diffusion-controlled charge storage.


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Figure 6. (a) Galvanostatic cycling at 20 mA g-1 of the SMCF@CNT/K0.3MnO2 pouch cell before and after bending. LEDs lit by the SMCF@CNT/K0.3MnO2 pouch cell (b)-(d) bending and (e)(g) releasing. The self-supporting SMCF@CNT film has excellent electrical and mechanical properties; thus, it can be directly used as an electrode of a flexible KIB battery. To this end, we used the SMCF@CNT film as the anode and K0.3MnO2 as the cathode to assemble the pouch battery as shown in Figure S5. K0.3MnO2 was synthesized via annealing of KMnO4.44 Before assembling


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the full pouch battery, the K0.3MnO2 electrode was assembled into a battery with K metal and charged-discharged twice. Because the initial state of K0.3MnO2 is close to the state of potassium depletion.44 The capacity of K0.3MnO2 for the first charge is only ~4 mAh g-1 (Figure S6), so the activation and prepotassiaion process is required. Finally, K0.3MnO2 was in the discharged state. As shown in Figure 6a, the pouch battery exhibits an initial capacity of 74.0 mAh g-1 at 20 mA g-1. After 15 and 30 cycles, the 1st and 2nd bent soft pack battery was recharged and discharged under the same conditions, and the discharge capacity was almost stable. And the Galvanostatic cycling of the pouch cell was operated with a releasing state after bending There was a slight increase in capacity at the 16th and 31st cycles. This is presumably because of the decrease in concentration polarization resulting from the long interval between the 15th and 16th cycles (or 30th and 31st cycles) and the bending-releasing process. The flexibility of the pouch KIB was demonstrated by simple visual cues that are used to illuminate a light-emitting diode (LED). When the pouch battery was bent (Figure 6b-d) and released (Figure 6e-g), the LED was still lit well, indicating the flexibility of the pouch cell. CONCLUSIONS We fabricated SMCF@CNTs through electrospinning. The flexible self-supporting SMCF@CNTs used as the anode of KIB exhibited high reversible specific capacity, good cycling stability, and high rate performance. The capacitance contribution to the SMCF and SMCF@CNT electrodes was more than 60%, and the addition of CNTs increased the capacitive and diffusion-controlled charge storage. The coexistence of two types of K+ storage


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mechanism, namely, capacitive and diffusion-controlled, resulted in the excellent electrochemical performance of SMCF@CNTs for KIBs. Furthermore, the pouch cell based on SMCF@CNTs and K0.3MnO2 provided considerable reversible capacity and good flexibility. This work can be employed to promote the use of low-cost, flexible KIBs.

EXPERIMENTAL SECTION Fabrication of SMCFs and SMCF@CNTs: SMCFs and SMCF@CNTs were prepared via electrospinning. In a typical procedure, PAN (Mw15000) and CNTs were dissolved in N, N-dimethylformamide (DMF) to obtain a 12 wt% PAN solution and a 10 wt% CNT dispersed solution, respectively. Subsequently, 1.5 g of the CNT dispersion was added to 25 g of the PAN solution and stirred for 6 h to obtain a black precursor, which was then transferred to a syringe with the distance of 20 cm between the needle and copper collector. During electrospinning, the precursor solution was fed at a flow rate of 1.0 mL h-1 and voltage of 20 kV. Pre-oxidation of the collected PAN@CNT fiber film was conducted by heating the film to 240 °C at a heating rate of 1 °C min-1 in the air and maintaining for 10 h. Then, the pre-oxidized fiber was heated to 1100 °C and maintained for 3 h in argon at a heating rate of 5 °C min-1 for carbonization. For comparison, the SMCF film was synthesized through the same procedure but without the addition of CNTs.

Fabrication of K0.3MnO2:


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K0.3MnO2 was synthesized via the annealing of KMnO4.42 In a typical synthesis, KMnO4 was dispersed in an alumina crucible, heated to 1050 °C at 1 °C min -1, maintained for 5 h in air, washed several times with deionized water, and dried overnight at 80 °C in the air.

Characterizations: The morphologies and structures of the SMCFs and SMCF@CNTs were investigated using scanning electron microscopy (SEM, FEI Nano SEM 450) and transmission electron microscopy (TEM, JEOL JEM-2100F). XRD patterns were collected with an X'Pert PRO MPD diffractometer equipment (Cu Kα radiation, λ = 1.54060 Å). Raman spectra were obtained with Renishaw inVia RM200 at an excitation wavelength of 532 nm. In-situ Raman spectroscopy was performed from 1200 cm-1 to 3200 cm-1 by using modified 2032-coin cells with a hole drilled on the top. The Nrelated bonding on the surface of SMCFs was characterized by X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Scientific). The pore size distribution and specific surface area of the SMCFs and SMCF@CNTs were determined with an ASAP-2020 surface area analyzer by using the methods of Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH), respectively.

Electrochemical Characterization: CR 2032-type coin cells were assembled in an argon-filled glove box for electrochemical performance evaluation. The prepared SMCFs or SMCF@CNTs film was dried overnight at 60°C in a vacuum oven, and the electrode was punched out of the film with a diameter of 12 mm as a working electrode. The average weight of the SMCFs and SMCF@CNTs electrode is about 4.5 mg. Potassium metal was used as the counter electrode, glass fiber membrane (Whatman) was adopted as the separator, and 0.8M KPF6 dissolved in a mixture of ethylene carbonate (EC) and


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diethyl carbonate (DEC) with a volume ratio of 1:1 was employed as the electrolyte. A LAND CT2001A cell tester was used to perform charge and discharge at different current densities with a voltage window of 0.01 V to 2.0 V (vs. K+/K). The K0.3MnO2 electrode was prepared by mixing K0.3MnO2, Super P and polyvinylidene difluoride (PVDF) at a weight ratio of 90:3:7 and uniformly coated on a 2.9 cm×1.2 cm SMCF@CNTs film. The mass loading of K0.3MnO2 electrode is 6.5 mg cm-2. A pouch cell was assembled with the same separator and electrolyte. A K0.3MnO2/K pouch cell was assembled in an argon-filled glove box and charged-discharged twice at 20 mA g1

from 1.5 V to 3.5 V. A full pouch cell was assembled using the SMCF@CNTs film as the anode

and the K0.3MnO2 electrode from the discharged K0.3MnO2/K pouch cell as the cathode. The dimensions of separator, anode, and battery are 3.1 cm×1.3 cm, 3 cm×1.25 cm, and 4.5 cm×2.5 cm, respectively. And the weight of the anode is 14.7 mg, so the configuration of the full battery is cathode-restricted. The charge-discharge was performed at 20 mA g-1 from 0.5 V to 3.5 V. The initial charge-discharge of the cell for in-situ Raman spectroscopy was conducted with a LAND CT2001A cell test instrument at 0.1 C from 0.01 V to 2.0 V (vs. K+/K). Cyclic voltammetry (CV) curves were measured by a Solartron Electrochemical Workstation (UK 1260+1287) in the range of 0.01 V to 2.0 V at rates of 0.2, 0.4, 0.6, and 0.8 mV s-1.

ASSOCIATED CONTENT Supporting Information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author


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Email: [email protected]; [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support provided by the National Key R&D Program of China (2018YFB0104204), National Natural Science Foundation of China (51674202, 51804259 and 51521061), Fundamental Research Funds for Central Universities (G2018KY0002, 3102018jgc004 and 3102017HQZZ007), Key R&D Program of Shaanxi (2017ZDCXL-GY-08-03), and Natural Science Foundation of the province of Shaanxi (2018JQ2037). REFERENCES (1) Radin, M. D.; Hy, S.; Sina, M.; Fang, C.; Liu, H.; Vinckeviciute, J.; Zhang, M.; Whittingham, M. S.; Meng, Y. S.; Van der Ven, A. Narrowing the Gap Between Theoretical and Practical Capacities in Li-Ion Layered Oxide Cathode Materials. Adv. Energy Mater. 2017, 7, 1602888.


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Flexible Sub-Micro Carbon Fiber@CNTs as Anodes for Potassium-ion

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