C hierarchical nanofibers for ultrafast

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Porous NaTi2(PO4)3/C hierarchical nanofibers for ultrafast electrochemical energy storage Peng Wei, Yanxiang Liu, Zhihao Wang, Yangyang Huang, Yu Jin, Yi Liu, Shixiong Sun, Yuegang Qiu, Jian Peng, Yue Xu, Xueping Sun, Chun Fang, Jiantao Han, and Yunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08415 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Applied Materials & Interfaces

Porous NaTi2(PO4)3/C hierarchical nanofibers for ultrafast electrochemical energy storage Peng Wei┴, Yanxiang Liu┴, Zhihao Wang, Yangyang Huang, Yu Jin, Yi Liu, Shixiong Sun, Yuegang Qiu, Jian Peng, Yue Xu, Xueping Sun, Chun Fang, Jiantao Han*, and Yunhui Huang School of Materials Science and Engineering, HuaZhong University of Science and Technology, Wuhan, Hubei 430074, China

NaTi2(PO4)3 (NTP) with a sodium superionic conductor (NASICON) 3D framework is a promising anode material for sodium-ion batteries (SIBs), due to its suitable potential and stable structure. Although its 3D structure enables high Na-ion diffusivity, low electronic conductivity severely limits NTP’s practical application in SIBs. Herein, we report porous NTP/C nanofibers (NTP/C-NFs) obtained via an electrospinning method. The NTP/C-NFs exhibit high reversible capacity (120 mAh g−1 at 0.2 C) and long cycling stability (capacity retention of ~93% after 700 cycles at 2 C). Furthermore, sodium-ion full cells and hybrid sodium-ion capacitors (NICs) have also been successfully assembled, both of which exhibit high-rate capabilities and remarkable cycling stabilities due to the high electronic/ionic conductivity and impressive structural stability of NTP/C-NFs. The results show that the nanoscale-tailored NTP/C-NFs could deliver new insights into the design of high-performing and highly-stable anode materials for room-temperature SIBs. ABSTRACT:

KEYWORDS: Sodium-ion batteries, NaTi2(PO4)3, Nanofiber, Sodium-ion full cell, Hybrid sodium-ion capacitors

INTRODUCTION In recent years, Na super ionic conductors (NASICON) have been widely investigated as electrode materials for sodium-ion batteries (SIBs).1-8 Due to high ionic conductivity and an open stable 3D framework, the NASICON family holds high promise for use in SIBs.9 Among these compounds, NaTi2(PO4)3 (NTP) has been regarded as a potential electrode material for SIBs by virtue of NTP’s structural stability, environmental friendliness, and low cost.10-14 However, the actual application of NTP is always hampered by its low inherent electronic conductivity, which results in low specific capacity, poor cycling stability, and poor rate performance.15-16 To address these issues, considerable efforts have been made to improve NTP’s electrochemical performance through reducing particle size, coating conductive materials, and so on.17-21 Among them, electrospinning is a flexible and scalable method

to fabricate some unique nanocomposites that enhance low-conductive polyanion materials.22-25 Here, we synthesized 1D porous NaTi2(PO4)3/C nanofibers (denoted as NTP/CNFs) via an electrospinning method followed by annealing. As comparisons, obtained via the same method were bare, carbonless NaTi2(PO4)3 nanofibers (denoted as NTP-NFs), conventional bulk composite NaTi2(PO4)3/C (denoted as NTP/C) was obtained by a traditional solid-state reaction. As we know, nanostructures combine a variety of advantages: small particle size shortens ionic/electronic diffusion pathways; porous structure improves electrolyte permeation; and 1D carbon skeletons facilitate rapid electron transport. Thus, NTP/C-NFs exhibit excellent electrochemical performance with a large reversible capacity (120 mAh g−1 at a rate of 0.2 C, with flat voltage plateaus located at ~2.1 V), long cycle stability (~93% capacity retention after 700 cycles at 2 C), and high rate capability (71 mAh g−1 at 20 C). In addition, sodium-ion full cells were successfully fabricated with

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NTP/C-NFs as anode material and NiHCF (Nickel Hexacyanoferrate) as cathode material, obtaining a high initial Coulombic efficiency of ~90% and 90% capacity retention after 500 cycles at 150 mA g-1. Furthermore, hybrid sodium-ion capacitors (NICs) were assembled with commercial activated carbon as counter electrode, achieving the maximum specific energy density of 56 Wh kg-1 of this system at a specific power density of 174 W kg-1, and 91.4% capacity retention after 500 cycles at 1 A g-1. These superior NTP/C-NF performance results present potential for practical application in a variety of high-rate energy storage systems.

EXPERIMENTAL SECTION Chemicals and reagents. Anhydrous sodium acetate (CH3COONa, 98%, Aladdin), Titanium isopropoxide (TTIP, 97%, Aladdin), Phosphorus pentoxide (P2O5, 98%, Aladdin), and Polyvinyl pyrrolidone (PVP, Aladdin, Mw = 1,300,000 g mol-1). Other reagents were chemically pure and were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. Materials Synthesis. The NTP/C-NFs were prepared through electrospinning, followed by a heat treatment. After optimizing synthetic conditions, 7.5 mmol phosphorus pentoxide (P2O5, 98%, Aladdin), 5 mmol titanium isopropoxide (TTIP, 97%, Aladdin), 2.5 mmol anhydrous sodium acetate (CH3COONa, 98%, Aladdin), and 0.5 g PVP (Aladdin, Mw = 1,300,000 g mol-1) were dissolved in 10 mL ethanol by vigorous stirring at 60 °C for 12 h to obtain a homogeneous viscous solution. Then, the precursor solution was loaded into a plastic syringe with 23-gauge tip needle, which was fixed at a position 20 cm from the grounded collector. A high voltage of 20 kV was applied at the needle tip with the solution feeding rate set to 0.6 mL h-1. Then, the as-collected membrane was stabilized at 400 °C for 2 h with a temperature ramp rate of 2 °C min−1 and was annealed at 700 °C for 4 h under nitrogen (N2) atmosphere with a temperature ramp rate of 5 °C min−1 to obtain NaTi2(PO4)3/C nanofibers (denoted as NTP/CNFs). For comparison, the bulk NaTi2(PO4)3/C composite particles (denoted as NTP/C) sample

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was prepared by a traditional solid-state reaction. A mixture of powdered Na2CO3, TiO2, (NH4)H2PO4, and glucose was heat treated at 800 °C under nitrogen (N2) atmosphere for 5 h. The bare NaTi2(PO4)3 nanofibers (denoted as NTPNFs) were obtained by calcination of the as-spun membrane in air at 700 °C for 4 h with a temperature ramp rate of 5 °C min−1. The NiHCF was synthesized by a usual coprecipitation method as we reported before.38 An appropriate amount of NiCl2·6H2O, sodium citrate, NaCl, and PVP were dissolved in Deionized water and Na4Fe(CN)6 also was dissolved in Deionized water. Then the two solutions were mixed into a beaker and keep stirring. After aged for 24 h, the precipitate was separated by centrifugation and washed with DI water and ethanol for three times, then dried at 100 °C in vacuum oven for 24 h. Finally, the NiHCF products was obtained. Material Characterization. X-ray diffraction patterns were obtained by PANalytical Empyrean diffractometer equipped with a Cu Kα radiation. Morphology and particle size were measured by scanning electron microscope (SEM, FEI Nova NanoSEM 450) coupled with an energydispersive X-ray (EDX) spectrometer. Transmission electron microscopy (TEM) observations were carried out with a JEM-2100 electron microscope. The Brunauer-Emme-Teller (BET) surface area and porosity were determined by nitrogen-sorption using a Micromeritics ASAP 2020 analyzer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Multi-Lab 2000 system with a monochromatic Al Ka X-ray source (Thermo VG Scientific). Raman spectroscopy were measured using LabRAM HR800 with an operating power level of 2 mW on a confocal Raman spectrometer. ThermogravimetryDifferential Scanning Calorimetry (TG-DSC) measurement was analyzed with a Netzsch STA 449F3 analyzer from 30 to 700 °C at a rate of 10 °C min-1 in air. Electrochemical measurements. The electrochemical performances were measured on 2032 coin cells. The working electrodes were made by reeling a mixture of active material – Ketjen black, Super P, and polytetrafluoroethylene (PTFE) (70: 10: 10: 10


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wt%) into a film and press it onto copper net, sodium metal was used as counter electrode. A sheet of porous glass fiber (GF/A) served as the separator. 1 mol L-1 NaClO4 in ethylene carbonate (EC) /diethyl carbonate (DEC) solution (v/v = 1:1) with 2 wt% fluoroethylene carbonate (FEC) was used as the electrolyte. All the cells were assembled in an argon-filled glove box and tested at room temperature. Cyclic voltammetry (CV) profiles were obtained on an electrochemical workstation (Princeton, US) at a scan rate of 0.1 mV s-1 within a voltage window of 1.5 – 3.0 V. Electrochemical impedance spectroscopy (EIS) was also measured on the Princeton over a frequency range of 100 kHz – 10 MHz with a potential amplitude of 5 mV. The charge/discharge tests were performed on a battery testing system (Land Electronics and Neware Electronics, China) between 1.5 and 3 V versus Na+/Na.

RESULTS AND DISCUSSION Figure 1 illustrates the synthesis process of NTP/C-NFs. Firstly, phosphorus pentoxide (P2O5), titanium isopropoxide (TTIP), anhydrous sodium acetate (CH3COONa), and polyvinyl pyrrolidone (PVP) were dissolved into ethanol to form a precursor solution, and then a voltage of 20 kV was connected to power supply and used to begin electrospinning. The precursor fibers were carbonized at a low temperature of 400 °C and further thermally annealed at 700 °C in N2 to obtain NTP/C-NFs. During annealing, PVP carbonized and released gas, forming a lot of nanopores in the carbon matrix.26-27 Meanwhile, CH3COONa, TTIP, and P2O5 reacted and formed NaTi2(PO4)3 nanoparticles uniformly dispersed in the carbon matrix. Figure S1 indicates the scanning electron microscopy (SEM) images of the precursor nanofibers with different molar concentrations of 1.25, 2.5, and 3.75 mmol, respectively, which suggests that the product morphology greatly depends on the concentration of precursor solution and experimental parameters,22, 28 such as spinning temperature, spinning speed, and fiber fineness.

Figure 1. Schematic illustration of the formation process for NTP/C-NFs.

Difficulties usually develop for filaments when their precursor concentration is more than 3.75 mmol, resulting in exorbitant viscidity. Whereas, low molar concentration leads to excessive carbon content and decreases energy density. Therefore, an appropriate content of 2.5 mmol NTP precursor was used to prepare these samples. Meanwhile, the influences of calcination temperature and time on the morphology and crystallinity of the NTP/C-NFs are systemically investigated, as respectively shown in Figures S2 and S3. Figure S2 presents the SEM images of NTP/C-NFs annealed at 400 °C for 2 h, and then carbonized at 650 °C, 700 °C, 750 °C for 4 h in N2. The corresponding Xray diffraction (XRD) patterns are presented in Figure S3a and b; the results show that the NTP phase can form at 700 °C, and its diffraction peaks strengthen and sharpen with increased carbonization time. Based on XRD results, the most suitable calcination condition is at about 700 °C for 4 h. Figure 2a shows SEM images of the as-spun precursor fibers, with smooth surfaces and an average diameter of ~200 nm. Figure 2 (b-c) shows a structural evolution of the fibers: They keep fiber morphology after thermal treatment, but with a slightly rough surface arising from the pyrolysis of PVP and the grain growth of NTP ploycrystalline.29-24 Figure 2 (d-f) shows the transmission electron microscopy (TEM) images of NTP/C-NFs. Obviously, these nanofibers are porous nanostructures with a large number of NTP nanocrystals evenly dispersed throughout (Figure 2e). The HRTEM images and the



selected area electron diffraction (SAED) patterns in the inset in Figure 2f all indicate the high crystallization, the clear lattice fringes with a d-spacing of 0.36 nm, assigning to the (113) planes of NTP. Amorphous components peak between 15° and 35°. The well-crystallized are also observed, corresponding to carbon in NTP/C-NFs. The energy dispersive spectrometer (EDS) is measured and shown in Figure 2g. The results indicate that Na, Ti, P, O, and C elements are homogeneously distributed in these nanofibers.

Figure 2. The morphology of NTP/C-NFs (a) precursor nanofibers, (b–c) SEM images of NTP/CNFs at different magnifications. (d–f) TEM images at different magnifications. (g) SEM and EDS mapping images of NTP/C-NFs.

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patterns of the precursor nanofibers (black) and NTP/C-NFs (red), respectively. The precursor shows amorphous characteristics with a broad NTP/C-NFs were obtained by annealing, and all of their diffraction peaks are indexed in NTP (JCPDS no. 01-085-2265). Notably, except for the crystalline NTP, no other impurity phase can be detected. The average crystal size calculated by the Scherrer equation is ~15 nm. Figure S4a shows the crystal structure of NTP, which is rhombohedral with an open 3D framework of TiO6 octahedra sharing all corners with PO4 tetrahedra. The two kinds of structural sites can store sodium ions: M1 sites (6b) surrounded by six oxygen atoms in an antiprism, and M2 sites (18f) with irregular oxygen coordination that are disposed to symmetrical dispersal around ternary axes.30-32 Sodium ions occupy the M1 site firstly; however, sodium ions occupy the vacant M2 sites after a large amount of sodium ions are accommodated. Moreover, Figure S4b presents the Rietveld refinements of XRD. The experimental data well agree with the refinement results, indicating that the NTP is in the R3c space group, with a NASICON-type framework and lattice parameters a = b = 8.5839 Å and c = 22.1003 Å.

Figure 3a shows the X-ray diffraction (XRD)

Figure 3 (a) XRD patterns of the precursor nanofibers, NTP/C-NFs. (b) Nitrogen adsorption/desorption isotherms (inset: pore size distribution) of NTP/C-NFs. (c) Raman spectra of the NTP/C-NFs nanofibers. Two characteristic bands of carbon at around 1340 and 1590 cm-1 are assigned to amorphous carbon and graphitic carbon, respectively. (d) Survey XPS spectrum and (e) high-resolution XPS spectrum of C 1s


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for the resulting NTP/C-NFs. (f) TG profiles of NTP/C-NFs, NTP-NFs and NTP/C from 30 to 700 °C at the heating rate of 10 °C min-1 in air. To investigate the microstructural band value (ID/IG =1.015) further suggests carbon characteristics of NTP/C-NFs, Brunauer-Emmettamorphous features in NTP/C-NFs, which is Teller (BET) measurement was carried out and consistent with the TEM results. shown in Figure 3b. The specific surface area of Thermogravimetric analysis (TGA) was 2 -1 NTP/C-NFs is up to ~94 m g . Figure 3b shows conducted to measure the specific carbon content in NTP/C-NFs, as well as to measure the carbon the Barrett-Joyner-Halenda (BJH) pore-sizecontent in the two contrasting samples, NTP/C distribution of NTP/C-NFs, with a main peak of and NTP-NFs. As shown in Figure 3f, the carbon ~3.8 nm. The nitrogen sorption isotherms exhibit content in the three samples (NTP/C-NFs, type-IV isotherms with an obvious hysteresis NTP/C, NTP-NFs) is approximately 5.2 wt%, 2.2 loop in a P/P0 range of 0.6 – 1.0, suggesting that wt%, 0.8 wt%, respectively. Figure S6 displays the composite is a typical mesoporous material, that the NTP/C is a single phase, wellwhich will benefits electrolyte infiltration.6, 16, 3334 crystallized with particle size distribution within X-ray photoelectron spectroscopy (XPS) 0.5 – 2 µm and covered by amorphous carbon. measurements were also conducted to evaluate Figure S7 shows NTP-NF is also purely phased the chemical bonding states of NTP/C-NFs. As with a NASICON structure. The Raman spectrum shown in Figure 3d, Na, Ti, P, O, and C are (Figure S7b) shows the disappearance of only the detected in the survey spectrum, which is D and G bands, strongly suggesting that no consistent with the EDS results. Figure 3e shows obvious carbon exists. In brief, the above results the high-resolution C1s spectrum. The fitted demonstrate that NTP/C-NFs have a 1D porous peaks locate at 284.9, 286.5, and 289 eV, structure with high specific surface area and low associating with a typical of non-oxygenated ring 35 carbon content, both of which are beneficial to of C, C-O, and O-C=O bonds, respectively. high electrochemical performance. Figure 3c displays the Raman spectra of NTP/CNFs with an appearance of disordered amorphous carbon (D band) and crystalline graphic carbon (G band).16, 36 The relative intensity of D to G

Figure 4 (a) Cyclic voltammetry curves at a scan rate of 0.1 mV s−1 of NTP/C-NFs. (b) The rate performance of NTP/C, NTP/C-NFs and NTP-NFs. (c) Charge/discharge profiles at various C-rates in the voltage range of 1.5 – 3 V ACS Paragon Plus Environment


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versus Na+/Na. (d) Cycling performance and corresponding coulombic efficiency of the NTP/C-NFs at a charge/discharge rate of 2 C.

Figure 4a presents cyclic voltammetry profiles of NTP/C-NFs measured at a scan rate of 0.1 mV s−1 in a voltage range of 1.5 – 3.0 V. A pair of sharp peaks at 2.21/2.06 V are observed, which is characteristic of a Ti4+/Ti3+ redox couple.37 The CV curves of NTP/C and NTP-NFs give the twopeak separation values (∆E), ~187 and ~176 mV, respectively, as shown in Figures S6d and S7d. It’s noted that the ∆E of NTP/C-NFs is the lowest (~150 mV) among them, suggesting a fast kinetic process of electrode reaction in the Na/NTP-NFs battery system. Figure 4c shows the charge/discharge voltage profiles of NTP/C-NFs with increasing C-rate from 0.2 to 20 C. Figure 4d shows the long cycling performance of NTP/C-NFs at the current rate of 2 C, remaining ~110 mAh g−1 after 700 cycles, 93% capacity retention. Meanwhile, the Coulombic efficiency approaches nearly 100% in the long cycle process. Figure 4b shows the rate performances of NTP/C, NTP/C-NFs, and NTP-NFs at a C-rate range of 0.2 – 20 C. NTP/C-NFs present the best rate performance with stable discharge capacities of 120, 118, 111, 98, 71 mAh g−1 at the rates of 0.2, 2, 5, 10, 20 C, respectively. Significantly, when the current rate is finally reset to 1 C, its capacity resumes the initial value, ~118 mAh g−1, indicating superior reversibility. In addition, the electrochemical impedance spectra (EIS) were taken and shown in Figure S8a. The chargetransfer resistance of NTP-NFs and NTP/C-NFs is smaller than that of NTP/C, which suggests that the improved performances arise from better electronic conductivity. Meanwhile, the 1D porous nanostructure of NTP-NFs and 3D crosslinked carbon nanofibers can shorten ionic/electronic transport paths, and thereby decrease electrode/electrolyte interface impedances. Based on the above results, sodium-ion fullcells were fabricated with NiHCF as cathode and NTP/C-NFs as anode, along with NaClO4 (1 M) electrolyte in ethylene carbonate (EC), diethyl carbonate (DEC) (v/v = 1:1), and 2 wt% fluoroethylene carbonate (FEC) additive. Figure S9 shows that the NiHCF displays one pair of oxidation/redox peaks located at 3.58/3.05 V, delivering a stable capacity of ~83 mAh g-1.13

Figure 5a shows a typical charge/discharge profile of NiHCF (10% excess) and NTP/C-NFs versus Na+/Na, respectively, in Na-half cell.38-40 Figure 5b shows the charge/discharge profiles of full cell NTP/C-NFs//NiHCF operated between 0.5 V and 2.5 V at a current density of 50 mA g−1. The full cell exhibits an initial charge capacity of ~122 mAh g-1 and a discharge capacity of ~110 mAh g-1, corresponding to an initial Coulombic efficiency of ~90%. It can be seen that the following charge/discharge profiles coincide well, indicating excellent reversibility. Figure 5c shows superior cycling stability of the full cell at a current density of 150 mA g-1 (with the first 5-cycle activation under a small current density of 50 mA g-1), in which the Coulombic efficiency approaches nearly 100% and the capacity retains 93 mA h g−1 after 500 cycles (~90% of its initial capacity). As shown in Figure 5d, the NTP/C-NFs//NiHCF full cell delivers reversible capacities of 103.1, 101.6, 100.5, 98.6, and 95.6 mAh g-1 at the current densities of 50, 100, 150, 300 and 500 mA g-1, respectively, revealing a high-rate capability. Even if the current density is up to 1000 mA g-1, the capacity still approaches 84.8 mAh g-1. After the current density is reset to 100 mA g−1, the specific capacity can be restored to 98.7 mA h g−1, demonstrating that the full cell has an excellent reversibility. The above electrochemical performance of NTP/C-NFs demonstrates potential application in SIBs.

Figure 5 (a) Schematic illustration of the NTP/CNFs//NiHCF full cell. (b) Charge-discharge curves and (c) cycling stability, and (d) various charge/discharge rates of NTP/C-NFs//NiHCF full cell.


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To exploit a new application of NTP/C-NFs in fast energy-storage devices, an asymmetric hybrid NIC was fabricated with commercial activated carbon (AC) as cathode, NTP/C-NFs as anode, and organic electrolyte of NaClO4 (1 M) in ethylene carbonate (EC), diethyl carbonate (DEC) (v/v = 1: 1), and 2 wt% fluoroethylene carbonate (FEC) additive. According to the specific capacity of AC (Figure S10), the mass loading of cathode and anode were set at 2.6: 1. As shown in Figures S10b and S10c, the AC was evaluated by CV and galvanostatic charge/discharge measurement at a voltage window of 2.5 – 4 V.41-42 The schematic diagram and electrochemical performance of the AC//NTP/C-NFs hybrid NIC are shown in Fig. 6. As shown in Figure 6a, Na+ is inserted into the NTP/C-NFs anode, relating to Ti4+ reduced to Ti3+, nevertheless the AC cathode contains a double-electric-layer formation with anion (ClO4) crossing the electrolyte/electrode interface during the charging process.43-45 During the subsequent discharge process, Na+ is extracted

reversibly from NTP/C-NFs, and the anion (ClO4) is removed from the surface of active carbon. Figure 6b presents the galvanostatic charge/discharge profiles with a current density range from 0.3 to 2 A g-1. The charge/discharge curves of NIC are neither triangular nor linear in shape, exhibiting a mixed capacitor- and batterybased charge storage mechanism.46-48 Figure 6c displays the cycling performance of the hybrid NIC at a current density of 1 A g-1 with a capacity retention of 91.4% after 500 cycles, and a near 100% Coulombic efficiency. Figure 6d shows the Ragone plots of performance compared with the other hybrid NICs in previous reports.49-54 The calculated energy and power densities are based on the total active mass in both electrodes. The results show that at a specific power density of 174 W kg-1, the hybrid NIC of NTP/C-NFs delivers the maximum specific energy density of 56 Wh kg-1. Even at a higher specific power density of 1162 W kg-1, the hybrid NIC retains a decent energy density of ~30 W h kg-1.

Figure 6. (a) Schematic illustration of the NTP/C-NFs//AC hybrid supercapactior configuration. (b) Charge/discharge profiles of the hybrid supercapacitor at various current densities. (c) Cycling stability and (d) Ragone plot of the hybrid supercapacitor at various charge/discharge rates..

CONCLUSION

In summary, we successfully synthesized NTP/C-NFs through feasible electrospinning, ACS Paragon Plus Environment


followed by a carbonization process. As an anode for SIBs, the NTP/C-NFs exhibit h25igh reversible capacity of 118 mA h g−1 at 2 C. Even at 20 C, NTP/C-NFs deliver a specific discharge capacity of 71 mA h g−1. After 700 cycles at 2 C, the NTP/C-NFs achieve high capacity retention of 93%. In addition, the sodium-ion full cell, assembled by NTP/C-NFs anode and NiHCF cathode, delivers a high initial specific capacity of 121.7 mA h g−1 at 50 mA g−1, as well as high capacity retention of 90% over 500 cycles at 150 mA g−1. Furthermore, an asymmetric sodium-ion hybrid supercapacitor was also fabricated, which delivers the maximum specific energy density of 56 Wh kg-1 of the system at a specific power density of 174 W kg-1. At a current density of 1 A g-1, the specific capacitance can remain at 91.4% after 500 cycles with nearly 100% Coulombic efficiency. Overall, these remarkable characteristics are clearly due to a porous crosslinked 3D nanostructure of NTP/C-NFs, which significantly facilitates ionic/electronic transport and thus allows the high-rate and long-term cycling. The NTP/C-NFs consist of NTP nanoparticles, homogeneously dispersed in the carbon matrix, which enhances conductivity and capacity and could offer a aussichtsreich method for achieving advanced electrode materials in SIBs and other electrochemical energy storage systems.

ASSOCIATED CONTENT S Supporting Information ○ The Supporting Information is available free of charge on the ACS Publication website at DOI.

AUTHOR INFORMATION Corresponding Author Jiantao Han E-mail: [email protected] ORCID Peng Wei: 0000-0002-4432-8978 Jian Peng: 0000-0002-4624-054X Jiantao Han: 0000-0002-9509-3785 Yunhui Huang: 0000-0003-3852-7038

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT P.W. and Y.X.L. contributed equally to this work. This work was supported by the Natural Science Foundation of China (Grant No. 2016YFB0700600, 2016YFB010030X) and the National Natural Science Foundation of China (Grant No. 51732005 and 51772117). The authors also thank the Analytical and Testing Centre of HUST and the State Key Laboratory of Materials Processing and Die & Mould Technology of HUST for Raman, XRD, SEM, TEM, TGA, and other measurements.

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