C Nanocomposite as a High-Performance

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F-doped NaTi2(PO4)3/C nanocomposite as a highperformance anode for sodium-ion batteries Peng Wei, Yanxiang Liu, Yarui Su, Ling Miao, Yangyang Huang, Yi Liu, Yuegang Qiu, YuYu Li, Xiaoyu Zhang, Yue Xu, Xueping Sun, Chun Fang, Qing Li, Jiantao Han, and Yunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19637 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on January 6, 2019

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F-doped NaTi2(PO4)3/C Nanocomposite as a High-Performance Anode for Sodium-Ion Batteries Peng Wei, † Yanxiang Liu, †, Yarui Su, †† Ling Miao, †† Yangyang Huang, † Yi Liu, † Yuegang Qiu, † Yuyu Li, † Xiaoyu Zhang, † Yue Xu, † Xueping Sun, † Chun Fang, † Qing Li, † Jiantao Han,*, † and Yunhui Huang† †School

of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China ††School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China

We’re presenting a sol-gel method for building novel nanostructures made of nanosized Fdoped Na1-2xTi2(PO4)3-xFx (NTP-Fx, x = 0, 0.02, 0.05, 0.10) particles embedded in 3D carbon matrices (NTP-Fx/C). This technique combines advantages of both 0D materials and 3D-carbon networks. Proper fluorine doping stabilizes the NTP structure and greatly enhances ion/electron transportation, leading to super-high rate electrochemical performance and ultra-long cycle life. The composite electrode delivers high specific capacities of 121, 115, 112.2, 110.1, 107.7, 103.1, 85.8, and 62.5 mAh g-1 at 0.2, 0.5, 1, 2, 5, 10, 20, and 30 C, respectively. It retains an unbelievable ~70 % capacity after a thousand cycles at a rate of as high as 10 C. Electroanalytical results reveal that fluorine doping significantly enhances Na+ diffusion kinetics. Meanwhile, density functional theory calculations demonstrate F-doped NTPs own outstanding electrochemical properties, which is due to the enhanced intrinsic ionic/electronic conductivity. The results show that anion doping is an efficient way to make high-performance NTP anodes for sodium-ion batteries. ABSTRACT:

KEYWORDS: sodium-ion batteries, NaTi2(PO4)3, anion doping, F-doping, sodium-ion full cell

 INTRODUCTION For embedded rocking-chair batteries, sufficient space for ion transfer are highly preferable for electrode materials, therefore, it’s key to find proper electrode materials for sodiumion transfer. Sodium-ion batteries are intensely researched because of sodium’s abundance and low cost.1-3 Research, so far, has focused on anode materials – such as hard carbon, metal alloys, oxides, and sulfides – that, due to their low capacities or unstable structures, can’t meet the requirements of the energy storage systems.4-7 For example, the radius of Na+ (1.02 Å) is greater than the radius of Li+ (0.76 Å). Sodium super ionic conductors (NASICON) are important electrode materials because of their threedimensional crystal framework, which is consisted of three [XO4] tetrahedrons (X = P, S, Si, etc.) connected to two [MO6] octahedrons (M = V, Zr, Ti, etc.), forming three-dimensional ion transport tunnels that help sodium-ions rapidly

migrate.8-10 Among NASICON, NaTi2(PO4)3 (NTP) is a aussichtsreich candidate for sodiumion batteries with the advantages of thermal stability, high theoretical capacity, low cost, environmental friendliness and so on. Nevertheless, the inherent slow charge transfer kinetics and low electron conductivity results in low-capacity release and poor rate performance. There have been attempts to enhance the electrochemical properties of NTP by reducing particle size and combining it with the conductive network, but it’s rare to improve the properties of NTP by doping.11-16 It’s well-known that doping effectively improves the intrinsic properties of electrochemical-active materials.17-19 Lippens et al. reported Na1.5Fe0.5Ti1.5(PO4)3/C composite with a steady specific capacity of 120 mAh g-1 at a current density of C/10, and the reversible capacity at 5C is 91 mAh g-1.20 In addition, anion doping is a flexible choice to improve kinetics and structural stability. Fluorine doping is widely

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used in electrodes to improve electrochemical properties.21-24 All of this encouraged us to fabricate NTP with fluorine doping. As far as we know, there is no literatures on the influence of anion doping on the electrical performance of NTP. During our research, we first synthesized F-doped Na1-2xTi2(PO4)3-xFx (denoted as NTP-Fx/C, x = 0, 0.02, 0.05, 0.10) as anode materials. The influences of fluorine substitution on NTP crystal structure, Na+ ion diffusion, and reaction kinetics were experimentally investigated and theoretically studied. By optimizing experimental conditions, NTP-F0.05/C showed the best power capability, cycling performance, and Na+ ion diffusion kinetics. Excellent electrochemical properties were demonstrated with high specific capacity, superior rate-capability, and long cycle-life time. Density functional theory (DFT) calculations show clearly fluorine substitution improves the electronic conductivity of NTP, comporting with the experimental results.

 EXPERIMENTAL SECTION Chemicals and reagents. Tetrabutyl titanate (TBOT, 99%, Aladdin), anhydrous sodium acetate (CH3COONa, Aladdin), phosphoric acid (H3PO4, 85%, Aladdin), citric acid monohydrate (C6H8O7∙H2O), sodium fluoride (NaF). All chemicals and reagents were used directly without purification and other treatments. Materials Synthesis. In a typical procedure, Na1-2xTi2(PO4)3-xFx/C (x = 0, 0.02, 0.05, 0.10) composites were made by a sol-gel method with CH3COONa, TBOT, H3PO4, NaF, and citric acid monohydrate. The molar ratio was (1-2x): 2: (3x): x: 2 (x = 0, 0.02, 0.05, 0.10). After optimizing synthetic conditions, 2 mmol citric acid monohydrate, 2 mmol TBOT, 1 mmol CH3COONa, and 3 mmol H3PO4 were respectively dissolved in 20 ml, 20 ml, 10 ml, and 10 ml ethyl alcohol by vigorous stirring for about 30 minutes. Then, the above solutions were mixed sequentially and stirred at 300 rpm for several hours. 15 ml deionized water (DI) with stoichiometric NaF was slowly added to the solution and then dried at 70 °C. The obtained dry-gel was ground and calcined at 350 °C at a heating speed of 2 °C per minute for 4 hours and further calcined at 700 °C for 8 hours in a

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nitrogen (N2) atmosphere to obtain the Na12xTi2(PO4)3-xFx/C (denoted as NTP-Fx/C). Material Characterization. We analyzed the crystalline phase of as-prepared samples by Xray powder diffraction (XRD) measurements with a Panalytical X’pert PRO MRD using CuKα radiation. Morphology and particle size of asprepared samples were studied using a JSM 6700F field-emission scanning electron microscope (FE-SEM, JEOL) coupled with an energy-dispersive X-ray spectrometer (EDX). Nitrogen adsorption and desorption isotherms were tested with a Micromeritics ASAP 2020 analyzer at the temperature of liquid nitrogen. Transmission electron microscopy (TEM) images were obtained using a JEM-2100 electron microscope. The surface chemical compositions were carried out by X-ray photoelectron spectroscopy measurements by using a KRATOS AXIS DLD spectrometer on a VG Multi-Lab 2000 system. Thermogravimetry (TG) measurements were tested in air from 30 to 700 °C at a heating speed of 10 °C min-1 on a Netzsch STA 449F3 analyzer. Raman spectrum analysis (LabRAM HR800, with a wavelength of 532 nm sweep from 200 to 2000 cm-1) distinguished the characteristic vibrational modes of the synthesized materials, mainly for the samples’ Dbands and G-bands. Electrochemical measurements. We used CR2032 coin cells to value the electrochemical properties of F-doped NTP/C. The coin cells are galvanostatic charged/discharged by the Land Electronics battery testing system and are tested in a voltage range of 1.5 - 3 V versus Na+/Na. We made working electrodes by mixing 70 wt% active material, 20 wt% Super P, and 10 wt% polytetrafluoroethylene (PTFE) binder and rolling them out onto a soft film and press it on to copper mesh. In half-cells, metallic sodium was used as counter electrode. Electrolyte is made up of 1 mol L-1 NaClO4 in ethylene carbonate (EC) /diethyl carbonate (DEC) (1:1 vol) solution with 2 wt% fluoroethylene carbonate (FEC). Whatman glass fiber (Whatman, GF/A) as the separator is used. Cyclic voltammetry curves were performed with a scan speed of 0.1 mV s-1 in the voltage range from 1.5 V to 3.0 V on an Princeton electrochemical workstation. Electrochemical impedance spectroscopy (EIS) is also obtained in


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the PARSTAT MC Princeton with a potential amplitude of 5 mV over a frequency range from 105 to 10-1 Hz.

 RESULTS AND DISCUSSION In order to study the effect of fluorine doping on the structure and electrical performance of NTP, we prepared a series of samples of NTPFx/C by the sol-gel method (see experimental section). We used critic acid as a carbon source to obtain samples with different fluorine contents by adding NaF in proportion to stoichiometric ratios. We marked the samples as NTP/C, NTPF0.02/C, NTP-F0.05/C, and NTP-F0.10/C, respectively. Figure S1 (see supporting information) shows the Rietveld refinement results that were used to determine the crystal structure of as-prepared NTP-Fx/C based on XRD

data. All samples have similar rhombohedral structures with the R3C space group; Figure 1a shows the corresponding refined structural model , which consists of PO4 tetrahedrons and TiO6 octahedrons that form a three-dimensional stable framework with open ion channels for rapid sodium ions migration. In the basic NASICON framework, there are two different sites for Na+ ions, mainly on site 6b (M1) and partially on site 18f (M2).25-26 The lattice parameters of the Rietveld refinement results are presented on the Table S1. With increase of F doping amount, the lattice parameters of a (b), c and cell volume (V) decrease obviously, especially in the direction of the c axis because the radius of the F ion is smaller than the PO4 group further confirming that F enters the NTP lattice.27-28

Figure 1. (a) Schematic crystal structure of Na1-2xTi2(PO4)3-xFx/C (The figure is created by VESTA29); (b) XRD patterns and partially-enlarged view. Figure 1b displays XRD patterns and, in the enlargement, the F-doped NTP/C. No impurity phase is detected in the XRD patterns, and all peaks match well with the rhombohedral NTP (JCPDS no. 01-085-2265), which shows the crystal structure didn’t change with F doping. The enlargement indicates the peak (113) shifts slightly and gradually to the larger angle as F content increases, suggesting the F-doped samples have fewer lattice parameters, which comports with the Rietveld refinement results.21, 24

We took Raman measurements of the NTPFx/C composites. Figure S2a shows the Raman spectra of the pristine NTP/C and NTP-F0.05/C. The pristine sample exhibits some bands located at ~1200 cm-1, consisting with the Raman

fingerprint characteristics of NTP. The bands located at ~1005 cm-1 correspond to the stretching vibration bands of the PO4. The bands below 426 cm-1 can be attributed to external modes arising from the TiO6 octahedral. Pyrolysis of citric acid forms carbon, and obvious bands at 1336 cm-1 (D-band) and 1591 cm-1 (Gband) are observable in the two composites; the two bands are typical Raman characteristics of carbon matrix materials.8, 30 Fourier-transformed infrared spectra (FT-IR) of as-prepared NTPFx/C are shown in Figure S2b, tested in the range of 500 and 2500 cm-1. All as-prepared composites show similar vibrations of chemical bonds Ti4+- O2- in the TiO6 octahedron are evidenced at 644 and 997 cm-1, respectively. The appearance of P–O bonds in PO4 tetrahedron is demonstrated at 572, 1022, and 1224 cm-1 .31 X-



ray photoelectron spectroscopy (XPS) explored chemical and electronic states. In Figure S2c, the survey spectrum demonstrates the sample is made up of Na, Ti, P, O, C, and F elements. As shown in Figure S2d, two strong fitted peaks located at 460.6 and 466.4 eV are assigned to typical Ti (IV) 2p3/2 and Ti (IV) 2p1/2, respectively.32-33 The thermogravimetry (TG) curves of NTP-Fx/C are used to detect the samples’ carbon content, which is shown in

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Figure S3. The weight losses of 3.49 %, 5.06 %, 3.64 %, and 3.63 % occur in NTP/C, NTPF0.02/C, NTP-F0.05/C, and NTP-F0.10/C from 350 to 700 °C, respectively, corresponding to the relevant carbon content in each sample. The disappearance of carbon caused each sample to lose mass. Similar mass loss proves that the carbon content in the synthesis process is controllable.34

Figure 2. The morphology of Na1-2xTi2(PO4)3-xFx/C (a) TEM and (b) HRTEM images of NTP/C; (c) TEM and (d) HRTEM images of NTP-F0.05/C; (e - f) EDS mapping and elements catalyst of NTPF0.05/C. ACS Paragon Plus Environment

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We explored the morphology and detailed structure of NTP-Fx/C composites by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure S4 exhibits field-emission SEM images of as-prepared samples. All the samples were homogeneous and formed by small-size nanoparticles aggregating, indicating fluorine doping did not change the particle size and morphology. As a result, the improvement of the electrochemical properties of the F-doped composites could be attributable to intrinsic properties rather than morphology and grain sizes. Figure 2a-d presents the TEM images of NTP/C and NTP-F0.05/C, respectively. It can be seen clearly that both of the samples were formed by the aggregation of uniform carbon-coated nanoparticles, and the average particle size range of 15 – 20 nm, which comports well with the SEM results. As shown in Figure 2b,d, the high-resolution TEM (HRTEM) images obviously show intense crystallization, a group of fringes with lattice spacing of 0.437 and 0.342 nm, corresponding to the crystal planes of NTP, (104), and (202) respectively.35-36 It is noteworthy that the lattice spacing of the (202) planes of NTP/C is ~0.348 nm, larger than that of NTP-F0.05/C, indicating the introduction of fluorine reduces plane spacing, which is consistent with XRD results. What’s more, uniform thin amorphous carbon layered on the surface of the nanoparticles in all samples, arising from the pyrolysis of citric acid, benefits electrochemical performance by improving electron transfer.37-39 As shown in Figure 2e, TEM elemental mapping images of NTP-F0.05/C further confirm Na, Ti, P, O, C, and F are uniform distribution throughout the composite, which corresponds with the XPS data. As displayed in Figure 2f, which the content of Na, Ti, P, O, C, and F in NTP-F0.05/C are 4.8, 15.48, 17.6, 37.91, 10.3, and 0.19 wt%, respectively, indicating the elements uniformly distribute, and fluorine is successfully introduced. All asprepared composites exhibit typical type IV isothermal adsorption-desorption curves, which is

shown in Figure S5. The Brunauer-EmmettTeller (BET) specific surface areas of NTP-Fx/C composites are 117.81, 120.23, 90.19, and 93.18 m2g-1, respectively. The high surface area can shorten Na+ ion diffusion path and benefit the sufficient contact of active materials and electrolytes, which is beneficial to electrochemical performance.14, 40 The electrochemical performance of the different NTP-Fx/C electrodes was evaluated in a half cell using the sodium metal as a counter electrode. Figure 3a shows voltage profiles of the four samples at 0.5 C (where 1 C corresponds to complete discharge in 1 h, 1 C = 133 mAh g-1), with the initial discharge capacities of 108.5, 113.1, 120.9, and 119.9 mAh g-1, respectively. The NTP-F0.05/C electrode shows the highest specific capacity compared to other electrodes. There are many factors that affect the specific capacity of electrodes, such as the molar mass, the electron conductivity, and so on. The specific capacity has been obviously improved by fluorine substitution, and the result testifies fluorine substitution is beneficial for Na+ ion insertion/extraction. All electrodes present a dominant charge/discharge plateau at about 2.1 V, which indicates the typical two-phase reaction of NTP and Na3Ti2(PO4)3 system. Due to low current density, the polarization of various NTPFx/C electrodes show slight difference. In order to confirm the influences of F-doping on the lifecycle of NTP electrodes, cycling properties of the synthetic NTP/C and various F-doped samples were investigated at a rate of as high as 10 C between 1.5 – 3 V versus Na+/Na, which is shown in Figure 3b. Significantly, the NTPF0.05/C presents the best steady cyclic life with a high first discharge capacity of 106.9 mAh g-1; even after 500 cycles, its specific capacity can restrain 102 mAh g-1 and only 4.5% of the initial capacity is lost. The superior cycle performance further displays that F-doping is conducive to structural stability and restrains volume change during successive charging/discharging.



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Figure 3 Electrochemical performance of Na1-2xTi2(PO4)3-xFx/C. (a) The charge and discharge profiles of Na1-2xTi2(PO4)3-xFx/C at 0.5 C in the voltage range of 1.5 – 3 V versus Na+/Na. (b) Cycling performance of Na1-2xTi2(PO4)3-xFx/C at 10 C; (c) Rate performance of Na1-2xTi2(PO4)3-xFx/C from 0.2 C to 30 C; (d) Comparison of the rate performances of NTP-F0.05/C with the rest NaTi2(PO4)3 composites reported in literatures; (e) Long cycle life performance and coulombic efficiency of NTP-F0.05/C at the rate of 10 C. To demonstrate rate capability of the NTPFx/C composites at different C-rates, the rate properties at rates from 0.2 C to 30 C of all the composites is shown in Figure 3c. It is obvious that the fluorine doping samples show better rate capability, compared to the NTP/C, and the content of fluorine doping has an important impact on electrochemical performance. Among the various F-doped composites, the NTP-F0.05/C composite shows the highest discharge capacities, followed by NTP-F0.02/C and NTPF0.10/C. It’s noteworthy that NTP-F0.10/C shows

higher discharge capacities than NTP-F0.02/C at 30 C. The values of discharge capacities of NTPF0.05/C are 121, 115, 112.2, 110.1, 107.7, 103.1, 85.8, and 62.5 mAh g-1 at 0.2, 0.5, 1, 2, 5, 10, 20, and 30 C, respectively. Impressively, when the current rate finally returns to 1C, the capacity can resume 108.5 mAh g-1, suggesting its excellent reversibility. The reason for obtaining such outstanding rate performance for the NTP-F0.05/C composite is that F-doping improves fast kinetics and structural stability. Figure 3d compares these rate performances of the synthetic F-doped NTP-


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Fx/C samples with the rest NaTi2(PO4)3 composites published in literatures. As far as we know, the rate properties of the NTP-F0.05/C electrode is better than many previously reported electrode materials, for example, the NTP/CMK3 composites,31 NTP/C nanosheets hybrids,12 NaFe0.5Ti1.5(PO4)3/C composites,20 and NTP nanoparticles/graphene composites 11 for NIBs. The superior sodium-storage behavior showed by F-doped NTP/C can be ascribed to its fast sodium-ion kinetics and improved conductivity, which will be discussed in detail later. Figure 3e

presents the ultra-long cycling performance of the NTP-F0.05/C electrode. To research the longterm cycling stability at high C-rate, the NTPF0.05/C electrode was tested, which still delivers outstanding cycling performance at the rate of 10 C. A high reversible storage capacity can be still obtained even after 1000 cycles, corresponding to a capacity decay of only 0.01% per cycle. What’s more, the coulombic effciency nearly approaches 100% during the long-term GCD cycling, further confirming its outstanding cycling ability and reversibility.

Figure 4. (a) GITT measurements of NTP/C and NTP-F0.05/C at 13.3 mA g-1; (b) CV profiles with a scan speed of 0.1 mV s−1 of NTP/C and NTP-F0.05/C electrode. (c) The Nyquist plots and equivalent circuit of Na1-2xTi2(PO4)3-xFx/C. (d) Real parts of the impedance (Z′) versus the reciprocal square root of the angular frequency (ω) in the low frequency region of Na1-2xTi2(PO4)3-xFx/C. Figure 4 further investigates Na-ion kinetics of the as-prepared electrodes using various electroanalytical techniques, such as cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectra (EIS). Figure 4a shows the GITT charge/discharge curves of NTP/C and NTP-F0.05/C at the first cycle in a voltage range of 1.5 – 3.0 V. During the GITT measurements, the cell is executed in a constant current density of 13.3 mA g-1 for an interval of 10 min followed

by an open circuit relaxation of 60 min to obtain the steady-state voltage.41-42 Both samples present a smooth plateau at about 2.1 V at the lower current density. Obviously, the curve of NTP-F0.05/C shows less over-potential than NTP/C, indicating F-doping introduces faster Na+ ion reaction kinetics. Figure 4b, Figure S6a and b show the CV curves of NTP-Fx/C electrodes with a scan speed of 0.1 mV s-1 and the voltage range is between1.5



and 3.0 V, respectively. All the samples exhibit a couple of redox peaks at ~2.1 V coming from Ti4+ to Ti3+, which agrees with the charge/discharge profiles. Compared to the NTP/C, the various F-doped composites show much smaller potential intervals among the various F-doped composites. The NTP-F0.05/C composite shows the smallest potential interval, ~0.133 V, suggesting that the redox kinetics are enhanced by the improved conductivity from Fdoping. CV, with the various scan rates of NTPF0.05/C, was measured from 0.1 to 10 mV s-1, as shown in Figure S6c. There’s an obvious peak shift and increase in peak separation between cathodic and anodic peaks with increased sweep current rates. It is known to all that the current (i) and scanning rate (v) of CV curves are shown by the formula: i = avb, b value depends on the storage mechanism of solvent ions (b = 1, the extreme pseudo capacity control process; b = 0.5, polar diffusion control process). Figure S6d shows the logarithm peak currents against logarithm scan rates with linear relation. The bvalues of cathodic and anodic peaks are 0.67 and 0.68, respectively, indicating that the Na+ insertion kinetics process in NTP-F0.05/C is a combination of capacitive and intercalation characteristics.43-45 The GITT and CV results convince us that Fdoping makes the electrochemical reaction of NTP/C more reversible and more easily achievable. As we know, electron conductivity and Na+ ion diffusion are main factors that influence electrode performance.46-47 To clarify the kinetics process of NTP-Fx/C electrodes, electrochemical impedance spectroscopy (EIS) measurements were generated. All the impedance curves include a semicircle in high frequency regions and a straight line in low frequency regions, indicating charge transfer and diffusion of sodium ions are key factors to affect the electrochemical process. To investigate the impedance spectra furtherly, the equivalent circuit model was applied to the impedance data, which is shown in the inset of Figure 4c. Rs is the electrolyte resistance, and CPE represents the capacity of the surface layer and double-layer capacitance. Rct is the charge transfer resistance. Wo is the Warburg impedance, representing

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diffusion behavior in the low frequencies. Rct of the various F-doped composites is lower than the NTP/C composite, and especially, the NTPF0.05/C electrode exhibits the lowest Rct (70.2 Ω) among these samples. The result reveals Fdoping could increase the electron conductivity of NTP/C electrodes. Moreover, according to the Warburg impedances at low frequency range of the EIS diagram, the diffusion coefficient of sodium ions (DNa+) can be calculated and the equations as follows: D=

𝑅2𝑇2 2𝐴2𝑛4𝐹4𝐶2𝜎2

Z′ = 𝑅𝐷 + 𝑅𝐿 + 𝜎𝜔 ―1/2 where R, T, F are constants, A is electrode area, n is electrons transfer number, C is Na+ ion concentration, σ is the Warburg factor at the low frequency.48-49 Figure 4d displays the connection between Z’ and ω-1/2 in the low-frequency range. The values of σ and DNa+ for all samples are shown in Table S2. Impressively, DNa+ of the various F-doped composites are higher than the un-doped sample, suggesting F-doping improves Na+ ion diffusion and kinetics. To further prove the effect of fluorine doping on NTP, sodium ions full battery were made with Nickel hexacyanoferrate as cathodes and NTPFx/C as anodes. Diethyl carbonate (DEC), ethylene carbonate (EC), 2 wt% fluoroethylene carbonate (FEC) containing 1 M NaClO4 was used as the electrolyte. In order to fully release the capacity of NTP-Fx/C, we made NiHCF excess 10%. Figure S7 shows the electrical performance of the NiHCF, made in accordance with our previous work. Figure S7a shows a pair of symmetric oxidation/redox peaks of the CV curve of the NiHCF electrode, which situated at 3.60/3.16 V, respectively. Figure S7b displays the first three charge/discharge profiles of the NiHCF, and the potential platform is consistent with the CV result. The rate performance and cycling performance of the NiHCF electrode are displayed in Figure S7c and S7d, respectively. As shown in the Figure S7c, the reversible capacity of the NiHCF is 61.2, 60.7, 58, 45.6 mAh g-1 at different current density (1, 2, 5, 10 C, respectively). The galvanostatic charge/discharge


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curves of Nickel hexacyanoferrate and NaTi2(PO4)3/C electrodes are shown in Figure 5a, using sodium metal as counter electrode. As shown in Figure 5b, the sodium ions full cell charge/discharge curves were set in the voltage range of 0.5 V - 2.2 V. It can be seen that the initial charge and discharge capacities are ~120

mAh g-1 and ~116 mAh g-1 at a current density of 66.5 mA g−1. As a result, the initial coulombic efficiency can approach 96.7%. It is observed that the NTP-F0.05/C displays the highest discharge capacity among the various F-doped composites, which is consistent with the result in Na-half cells.50-52

Figure 5 (a) Schematic diagram of the NiHCF//NTP-F0.05/C full cell. (b) The galvanostatic charge and discharge profiles of NiHCF//NTP-F0.05/C under a current density of 66.5 mA g-1, (c) Cycling performance of the NiHCF//NTP-F0.05/C at 1330 mA g-1, and (d) Rate performance of NiHCF//NTPF0.05/C full cell. An outstanding cycle performance over 1000 cycles at a current density of 1330 mA g-1 is presented in Figure 5c. The NTP-F0.05/C composite exhibits the best performance, with the highest initial capacity (96.98 mAh g-1) and over 1000 cycles, NiHCF//NTP-F0.05/C full cell still maintains ~70% capacity retention corresponding to the capacity of 67.3 mA h g−1. The rate performances are tested at the current densities of 66.5, 133, 266, 665, 1330, 2660, and 3990 mA g1, and the full cell demonstrates specific capacities of 114, 110.7, 109.7, 105.8, 93.2, 75.7, and 56.2 mAh g-1, respectively (Figure 5d), indicating the good rate capability of the full cell. The reversible capacity can return to 74.4 mAh g−1, when the rate returns to 2660 mA g−1,

revealing an excellent full-cell reversibility. Figure S8 presents the charge/discharge profiles of NTP-Fx/C at various current densities; compared to undoped samples, the F-doped composites perform better. The all electrochemical properties of NTP-Fx/C indicates F-doping improves effectively full-cell performance. The first-principles calculations are based on DFT using the Vienna ab initio simulation package (VASP).53-54 The gradient corrected Perdew-Burke-Ernzerhof functionals (PBE)55 is employed to handle the exchange correlation interaction and the projector augmented wave (PAW) method was used for the electronic



states.56 The brillouin zone is sampled with 3 × 3 × 1 Monkhorst mesh,57 and the force convergence thresholds are 0.03 eV Å-1. The calculated structure illustration is shown in Figure 6a. The density of states (DOS) and band structure of F-doped NaTi2(PO4)2.83F0.17 and undoped NTP are shown in Figure 6b-d. Compared with the un-doped NTP, the band structure of Fdoped NaTi2(PO4)2.83F0.17 alters dramatically with the Fermi level’s upward shift into the

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conduction bands, hence exhibiting metallic characteristics. As show in Figure S9, the conduction bands are mainly contributed by Ti atoms. The F atom replaces a phosphate radical, leading to lower chemical valance of adjacent Ti atoms. The Fermi level shifts up, and electrons occupy some empty conduction orbitals, leading to higher electron conductivity, which is consistent with prior experiments.

Figure 6 (a) Structural illustration of Na0.66Ti2(PO4)2.83F0.17 (The figure is created by VESTA29). (b) The density of states of NaTi2(PO4)3 and F-doped NaTi2(PO4)3, where the Fermi energy is chosen as 0 eV. (c) Band structure of NaTi2(PO4)3. (d) Band structure of F-doped NaTi2(PO4)3.

 CONCLUSION In summary, we successfully synthesized a series of NTP-Fx/C (x = 0, 0.02, 0.05, 0.10) by a typical sol-gel method, which towards sodiumion batteries (SIBs) as a anode, the electrochemical performance and morphologies and structural characteristics of NTP-Fx/C were investigated. F-doping doesn’t change the XRD phase and particle morphology of NTP, while the lattice parameters of NTP reduces as the content of fluorine doped increased. Compared to the undoped sample of NTP, it has been found that the intrinsic electric conductivity and Na+ ion

diffusion coefficient of NTP are significantly improved after F-doping, drastically improving rate capability and cycle performance. DFT calculations demonstrate F-doped NTPs own outstanding electrochemical properties, which is due to improved intrinsic ionic/electronic conductivity, and Na+ ion diffusion kinetics are significantly enhanced by F-doping, according to the results of various electroanalytical techniques. Among the F-doped composites, NTP-F0.05/C exhibits the best electrochemical performance: The specific capacity decreased from 121 mAh g-1 to 85.8 mAh g-1 as current density increased from 0.2 C to 20 C,. Even at 30 C, it can deliver a specific discharge capacity of


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62.5 mAh g−1, demonstrating superior rate capability. NTP-F0.05/C also exhibits high reversible capacity of 102.5 mAh g−1 at 10 C and retained capacity of 90 % after 1000 cycles, indicating superior cycle performance and structural stability. These results and remarkable electrochemical performances strongly confirm that anion doping efficiently optimizes NASICON type electrode materials as stable and high rate anode materials for SIBs. Furthermore, anion doping reveals new insights into Na+ ion kinetics that can be able to improve the properties of electrode materials.

 ASSOCIATED CONTENT 𝑆 Supporting Information ○ The Supporting Information is attainable free of charge on the ACS Publication website at DOI:. Supporting Information Available: Rietveld refinement patterns, Raman spectra, Fourier transform infrared spectra, XPS spectrum, TG images, SEM images and BET images.

 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (Jiantao Han *) ORCID Peng Wei: 0000-0002-4432-8978 Jiantao Han: 0000-0002-9509-3785

Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENT This manuscript is supported by the National Natural Science Foundation of China (Grants No. 51772117 and 51732005), National Key R&D Program of China (Grants No: 2016YFB0700600 and 2016YFB010030X). We also thank the State Key Laboratory of Materials Processing and Die & Mould Technology of HUST and the Analytical and Testing Centre of HUST for TEM, SEM, XRD, TG, XPS and other measurements.

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C Nanocomposite as a High-Performance

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