- Email: [email protected]
Double-carbon coated Na3V2(PO4)3 as a superior cathode material for Na-ion batteries
Applied Surface Science 487 (2019) 1159–1166
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Double-carbon coated Na3V2(PO4)3 as a superior cathode material for Naion batteries
T
⁎
Hong-bo Huanga,d, Shao-hua Luob,c,d, , Cai-ling Liua,d, Yue Yange, Yu-chun Zhaib, ⁎⁎ Long-jiao Changf, Ming-qi Lig, a
School of Metallurgy, Northeastern University, Shenyang 110819, PR China School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, PR China c School of Materials Science and Engineering, Northeastern University, Shenyang 110819, PR China d Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province, Qinhuangdao 066004, PR China e Procurement Center for Police Equipment of Ministy of Public Security, Shanghai 201100, PR China f College of New Energy, Bohai University, Jinzhou 121013, PR China g College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637009, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Dopamine Nitrogen-doped carbon-coating Na3V2(PO4)3 Cathode material Na-ion batteries
Na super ionic conductor (NASICON)-type Na3V2(PO4)3 (NVP) has been considered as a potential positive electrode material for sodium ion batteries (NIBs) owing to its high theoretical specific capacity. Nonetheless, the practical application of NVP is hindered by the intrinsically poor electronic conductivity. Herein, polydopaminederived nitrogen-doped carbon-covered Na3V2(PO4)3/C composites (NVP/C/NC) have been prepared through a self-polymerization of dopamine on the NVP/C surface and subsequent calcination at high temperature. The assynthesized NVP/C/NC composite cathode exhibits a high initial reversible capacity (109.2 mAh/g at 0.2C), superior rate performance (87.2 mAh/g at a rate up to 20C), and excellent cycling capability (91.2% of the initial capacity is kept after 500 cycles at 2C) in NIBs. Furthermore, compared with NVP/C electrode, the NVP/C/NC electrode presents low resistance and high sodium ions diffusion coefficient. The good performance can be ascribed to the nitrogen-doped carbon layer in improving the electronic conductivity, shortening diffusion length of Na+ ions and electrons, and relieving the volume changes of electrode materials. These preliminary results suggest that the as-obtained NVP/C/NC composite is a novel promising electrode material for low-cost sodium energy storage.
1. Introduction Nowadays, lithium ion batteries (LIBs), owing to their large energy density and long lifespan, have been utilized in a broad range of applications, such as portable electronic equipment, electric vehicles and smart grid [1,2]. However, the limited and unevenly distribution of lithium resources has led to the relatively high cost for LIBs in largescale applications [3,4]. Therefore, the development of new alternative energy storage system is not only desirable but also extremely essential. In recent years, due to the low price and natural abundant of sodium sources, sodium ion batteries (NIBs) have attracted great attention for future large-scale energy storage [5,6]. In addition, the similar physical and chemical properties between lithium and sodium elements further promote the development of NIBs [7]. Hitherto, many cathode compounds such as layered transition metal ⁎
oxides NaxMO2 (M = Co, Mn) [8–10], Na super ionic conductor (NASICON) Na3V2(PO4)3 [11] and olivine-type NaMPO4 (M = Mn, Fe) [12,13] have been widely investigated for NIBs. Among them, Na3V2(PO4)3 (NVP) has proved to be one of the most promising electrode materials because of its outstanding thermal stability, high theoretical energy density (400 Wh/kg), and fast sodium ions diffusion tunnel (NASICON structure) [14]. Additionally, NVP has a large theoretical capacity (117.6 mAh/g) and a flat potential plateaus at around 3.4 V [15]. Nevertheless, the intrinsic defect of NVP is the extremely low electronic conductivity, which leads to inferior electrochemical capability, especially rate performance, and hence, greatly inhibits its practical applications. To solve this issue, many studies such as doping with secondary ions, carbon coating and fabricating nanostructure have been conducted. In the case of other polyanionic materials, carbon layer coating has been considered to be an economical and effective strategy
Correspondence to: S.H. Luo, School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, PR China. Corresponding author. E-mail addresses: [email protected] (S.-h. Luo), [email protected] (M.-q. Li).
⁎⁎
https://doi.org/10.1016/j.apsusc.2019.05.224 Received 7 April 2019; Received in revised form 15 May 2019; Accepted 18 May 2019 Available online 20 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
to overcome this disadvantage. For example, Mai et al. prepared threedimensional graphene frameworks covered LVP (LVP-G-FD) composite, which exhibits large energy density (501.3 Wh/kg) and superior cycle performance with a capacity retention of 65% after 500 cycles at 500 mA g−1 [16]. Chen et al. fabricated core-shell structured Li3V2(PO4)3@C nanocomposite with approximately 86% capacity retention after 1000 cycles at 5C (1C = 197 mAh g−1), showing outstanding rate capability [17]. Wan et al. prepared carbon-coated LiMn0.5Fe0.5PO4@C core-shell composite through in-situ resorcinolformaldehyde polymerization method, exhibiting a remarkable high rate capability (155 mAh g−1 at 17 mA g−1 and 105 mAh g−1 at 1700 mA g−1) [18]. Chung et al. reported the preparation of polythiophene coated NaFePO4, which displays a first discharge specific capacity of 142 mAh g−1 at 10 mA g−1 with a capacity retention of 94% over 100 cycles and superior rate capability (70 mAh g−1 at 150 mA g−1 and 42 mAh g−1 at 300 mA g−1) [19]. Herein, we prepared a nitrogen-doped carbon-coated NVP/C positive electrode material by employing dopamine as the nitrogen source and carbon source simultaneously through a versatile and simple method. Owing to the good film-forming ability of dopamine, a homogeneous coating layer was successfully covered on the surface of NVP/C particles, which would suppress structural collapse resulting from the volume changes during ions insertion/extraction. The coated nitrogen-doped carbon layer could enhance the electrical conductivity greatly. Also, it will further improve the cycling stability, particularly the high rate performance. As expected, the NVP/C/NC composite cathode shows a larger first discharge specific capacity of 109.2 mAh/g at 0.2C with capacity retention of 95.4% over 100 cycles. Even at a current density of 20C, a high specific capacity of 87.2 mAh/g is delivered.
Fig. 1. Schematic illustration for the synthesis process of the NVP/C/NC composite.
centrifugation and washed three times with deionized water and ethanol, and then dried at 80 °C for 24 h. Finally, the nitrogen-doped carbon-wrapped NVP/C (NVP/C/NC) was obtained after sintering in a tube furnace at 350 °C for 3 h followed by calcining at 750 °C for 3 h in an Ar atmosphere. The fabrication process of the NVP/C/NC composite is illustrated in Fig. 1.
2.2. Material characterization X-ray powder diffraction (XRD) measurements were conducted with a Rigaku SMARTLAB diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5406 Å) in the scanning range of 10–60°. SEM with energy dispersive spectrometer (EDS) was performed for morphology analysis using ZEISS SUPRA55 field-emission scanning electron microscope (ZEISS, Germany) working at 15 kV. TEM was carried out on a JEOL 2100F transmission electron microscope (JEOL, Japan) working at 200 kV. Thermogravimetric analysis (TGA) of the compound was achieved by an HCT-2 device (Hengjiu Instrument Co., Ltd., China) under air from ambient temperature to 700 °C, with a ramp rate of 10 °C/min. Raman spectrum is measured by Renishaw 2000 spectrometer (Renishaw, UK) with an excitation wavelength of 532 nm. The carbon content is measured by carbon sulfur analyzer (ELTRA, CS800). The Brunauer-Emmett-Teller (BET) surface area and porous structure were estimated by SSA-4000 automatic specific surface area and pore size distribution analyzer (Builder, China). Tap density was measured by tapping 1 g NVP 1000 times in a cylindrical container.
2. Experimental 2.1. Material synthesis Citric acid and NaOH were purchased from Aladdin Industrial Corporation. NH4VO3 and NH4H2PO4 were provided by Sinopharm Chemical Reagent Co., Ltd. Tris(hydroxymethyl)aminomethane and dopamine hydrochloride were obtained from J&K Scientific Ltd. All chemicals were of analytical grade and employed without any purification. Citric acid was used not only as a carbon source but also as a chelating agent.
2.3. Electrochemical measurements The working electrode was mixed by dispersing the active material (80 wt%), acetylene black (10 wt%), and poly(vinylidene fluoride) binder (PVDF, 10 wt%) in N-methyl-2-pyrrolidone (NMP) solution. The acquired homogenous slurry was pasted on an Al foil, followed by drying at 120 °C in a vacuum oven for 12 h. Then the electrode was cut into circles with a diameter of 10 mm. CR2032 type button batteries were assembled in an Ar-filled glove-box (O2 ≤ 0.1 ppm and H2O ≤ 0.1 ppm) with sodium metal foils as the counter/reference electrodes. 1 M NaClO4 in the ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by vol.) mixed solvent, with 5 wt% fluoroethylene carbonate (FEC) as an additive was used as the electrolyte. Glass fiber filter (Whatman, GF/D) was chosen as the separator. The typical mass loading of the active material is about 1.3 mg/cm2. The galvanostatic charging and discharging testing were performed in the voltage window of 2.5–3.8 V vs. Na+/Na using a Land CT2001A battery test system at room temperature. In this work, C rates are utilized for characterizing the current rate, where 1C corresponds to a current rate of 117.6 mA g−1. Cyclic voltammetry (CV) experiments were conducted with a CHI660C electrochemical workstation at a scanning rate of 0.1 mV/s between 2.5 V and 3.8 V (vs. Na+/Na). Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range from 100 kHz to 0.1 Hz with an AC voltage of 10 mV by using a Solartron 1260 electrochemical analyzer.
2.1.1. Preparation of Na3V2(PO4)3/C (NVP/C) The pristine Na3V2(PO4)3/C sample was fabricated by the sol-gel approach according to the previous work [20]. In brief, citric acid, NH4VO3, NH4H2PO4 and NaOH with the molar ratio of 1:2:3:3 were sequentially dissolved in 75 mL of deionized water to get an orange solution. Subsequently, the solution was agitated magnetically at 80 °C for 8 h to obtain a dark blue sol. Then, the sol was dried in a drying oven at 80 °C overnight to form a gel. After grinding, the dried gel powder was calcined in a tube furnace at 350 °C for 3 h, then annealed at 800 °C for 16 h under an Ar atmosphere to achieve the NVP/C material. 2.1.2. Preparation of nitrogen-doped carbon-wrapped Na3V2(PO4)3/C (NVP/C/NC) NVP/C/NC composite was fabricated through a self-polymerization process of dopamine monomers. 1.5 g of the as-prepared NVP/C powder was dispersed in a tris(hydroxymethyl)aminomethane buffer aqueous solution (75 mL, 0.5 M) by ultrasound for 30 min. Then, HCl (1 M) was injected drop by drop into the solution under magnetic stirring until the pH value of the tris-buffer solution reached 8.7. Later, 0.15 g of dopamine hydrochloride was added into the suspension with continuous and vigorous stirring for 24 h under ambient temperature. The polydopamine-coated NVP/C/NC precursor was harvested via 1160
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 2. (a) XRD patterns, (b) TGA curves and (c) Raman spectra of NVP/C and NVP/C/NC; (d) Nitrogen adsorption-desorption isotherms of NVP/C/NC, the insert was pore size distribution curve.
3. Results and discussion
for NVP/C and NVP/C/NC are about 1.04 and 1.02, respectively, implying the partial graphitization of the deposited carbon [15,27]. There is no doubt that the ID/IG value of NVP/C/NC is lower than that of NVP/ C, suggesting the high degree of graphitization and high conductivity of carbon, which result from the additional coated nitrogen-doped carbon layer. It is worth mentioning that this could efficiently improve the electrical conductivity and accelerate the electron transport [28]. The nitrogen adsorption-desorption isotherms were utilized to investigate the porous structure of the NVP/C/NC composite. Fig. 2d exhibits the nitrogen isotherm adsorption-desorption plot and the pore size distribution of the NVP/C/NC. The isotherms of the sample are assigned to the type IV curve with an H3 hysteresis loop between the P/ P0 range of 0.4 and 1.0, which corresponds to the mesoporous structure [29]. Based on the Barrett-Joyner-Halenda (BJH) equation, the specific surface area of the NVP/C/NC was estimated to be 49.1 m2 g−1, much larger than that of pristine NVP/C (25.66 m2 g−1, Fig. S1). The enlarged surface area of NVP/C/NC provides more active sites for sodium ions insertion/extraction and facilitates the effective contact area between electrode and electrolyte, leading to improved reaction kinetics [30]. In addition, the pore size of the NVP/C/NC composite is distributed mainly below 10 nm with an average pore diameter of approximately 3 nm (inset in Fig. 2d). X-ray photoelectron spectroscopy (XPS) is carried out to analyze the surface chemical composition of the NVP/C/NC, as shown in Fig. 3. The high-resolution C 1s spectrum (Fig. 3a) displays four types of carbon: C]C at 284.6 eV, CeC at 285.4 eV, CeN at 287.8 eV, and O–C=O at 288.6 eV. [31] This demonstrates that nitrogen has been successfully doped into the carbon layer. For the high-resolution N 1s spectrum of NVP/C/NC in Fig. 3b, three peaks located at about 398.5, 399.5 and 400.4 eV correspond to pyridinic N, pyrrolic N and graphitic N, respectively. [32] Additionally, the nitrogen content for NVP/C/NC is
Fig. 2 shows the XRD patterns of the as-prepared NVP/C and NVP/ C/NC composite samples. All the diffraction peaks can be well indexed to a rhombohedral NASICON structure with a space group of R-3c (JCPDS card No. 62–0345), which is consistent with previous reports [21–23]. In addition, both the peak intensities and positions of the two samples are similar, suggesting that the carbon coating does not significantly change the crystallinity and NASICON-type structure of NVP. Simultaneity, no other peaks from the impurities is detected in the XRD patterns, manifesting that the nitrogen-doped carbon in the sample is amorphous. The precise carbon contents measured from the elemental analyzer are 2.11% and 5.88% for NVP/C and NVP/C/NC composite, respectively. The carbon content in the NVP/C and NVP/C/NC composite was also estimated from the TGA test in air as displayed in Fig. 2b. In the TGA curves, the slight weight loss at around 100 °C is attributed to the volatilization of adsorbed water molecules. The carbon contents are 2.20% for NVP/C and 5.91% for NVP/C/NC, which are in good agreement with the results of carbon sulfur determinator. After 500 °C, both samples show the same tendency of a slight weight increase, which can be ascribed to the oxidation of V ions in the NVP from 3+ to 4+ and 5+ [24]. Moreover, the packing density of NVP/C and NVP/C/NC samples measured is 0.93 and 0.87 g cm−3, respectively. The nature of carbon layer in the NVP/C and NVP/C/NC composites is further characterized by Raman spectroscopy and the result is shown in Fig. 2c. Two characteristic peaks centered at 1360 and 1582 cm−1 are seen for both of them, corresponding to the D (disordered carbon) band and G (graphite carbon) band, respectively [25]. Generally, the intensity ratio of D/G peak (ID/IG) is often defined to evaluate the structural disordering of carbon materials [26]. And the values of ID/IG 1161
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 3. High-resolution XPS of (a) C1s and (b) N1s spectra of NVP/C/NC composite.
Fig. 4. SEM images of (a) NVP/C and NVP/C/NC; (c) EDS mapping of the NVP/C/NC.
1162
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 5. TEM images of the NVP/C/NC composite.
Fig. 6. (a) The CV curves of NVP/C and NVP/C/NC electrodes at a scan rate of 0.1 mV/s. (b) Cycling performance of NVP/C and NVP/C/NC at 0.2C. (c) Rate capability from 0.2 to 20C for 10 cycles, and (d, e) corresponding charge/discharge curves of NVP/C and NVP/C/NC. (f) Cycling performance of NVP/C and NVP/C/ NC at 2C.
1163
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 7. (a) The Nyquist plots, and (b) the relationship curves between Zre and ω−1/2 in the low-frequency range of the NVP/C and NVP/C/NC samples.
spectrometry (EDS) mappings indicate that Na, V, P, C, and N elements are homogeneously distributed in NVP/C/NC composite. The detailed microstructure of the NVP/C/NC sample was characterized by transmission electron microscopy (TEM). Fig. 5 displays the TEM images of the NVP/C/NC composite. It is observed that the surface of grains is wrapped and connected by nitrogen-doped carbon derived from polydopamine, which provides a facial channel for electrons transport [34]. The appropriate carbon layer is a key factor on the enhancement of electrochemical capabilities of NVP. A coating of the carbon layer on the surface of NVP can effectively prevent the particles from further growing and aggregating so as to acquire smaller grains [35]. It also reveals that the particle size is about 100–180 nm, which is in accordance with the above SEM result. As seen in Fig. 5b, the NVP/ C/NC composite has a relatively uniform amorphous nitrogen-doped carbon layer with the thickness of < 10 nm, demonstrating the effectiveness of the carbon coating strategy. The cyclic voltammetry (CV) (Fig. 6a) was conducted in a potential range of 2.5–3.8 V at a scanning rate of 0.1 mV/s. As observed from the CV curves, there appears a couple of well-defined redox peaks on both the electrodes, which correspond to the reversible transformation of V3+/V4+ [36,37]. However, in the reduction (sodiation) process, the single peak splits into two parts, which is owing to the structural rearrangements when sodium ions transfer from Na(1) to Na(2) sites [38–40]. The voltage distance (ΔV) between the reduction and oxidation peaks of NVP/C and NVP/C/NC is 243 mV and 216 mV, respectively, suggesting the lower polarization and easier migration of Na+ in NVP/C/NC composite electrode. Meanwhile, the peak current of NVP/ C/NC is much higher than that of pristine NVP/C, proving the faster
Table 1 Rct and DNa+ values from EIS date of the as-prepared NVP/C and NVP/C/NC samples. Samples
NVP/C
NVP/C/NC
Rct (Ω) DNa+ (cm2 s−1)
719.58 3.05 × 10−13
291.93 5.41 × 10−13
about 1.69 at.% by the results of XPS. The N-doped carbon layer with multiple type of nitrogen is beneficial to enhance the electronic conductivity of composite and promote the transportation of Na ions in the interface due to defects caused by N doping [33]. The morphological characteristic of the as-obtained NVP/C and NVP/C/NC samples was studied by a field emission scanning electron microscopy (FESEM). As displayed in Fig. 4a and b, both the two samples are composed of plate-like nano-particles with a size of about 100–375 nm. Although both of them showed no significant difference in the morphology, the NVP/C/NC composite has a porous structure after the carbonization procedure of the polydopamine, which can not only shorten the sodium ion diffusion distance, but also facilitate the infiltration of electrolyte, thus providing better performance. Moreover, the pristine NVP/C occurs in an apparent agglomeration compared to NVP/C/NC composite, suggesting that the coated nitrogen-doped carbon layer can inhibit particle growth to a great extent. Meanwhile, smoke-like carbon grains were seen for NVP/C/NC composite but were hardly seen for NVP/C. Therefore, it was evident that the nitrogendoped carbon, derived from polydopamine, can inhibit the particle growth and agglomeration of NVP/C. In Fig. 4c, the energy dispersive
Fig. 8. (a) CV curves of NVP/C/NC at different scan rates from 0.1 to 1 mV/s; and (b) the corresponding linear relationship of the cathodic/anodic peak current (Ip) and the square root of the scan rate (v1/2). 1164
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
the calculated diffusion coefficients of the sodium ions (DNa+) are displayed in Table 1. It can be found that the DNa+ of NVP/C/NC (5.41 × 10−13 cm2 s−1) is larger than that of NVP/C (3.05 × 10−13 cm2 s−1), suggesting faster Na+ ions transport in the NVP/C/NC composite. Additionally, this EIS results well interpret the better electrochemical capacities of NVP/C/NC composite electrode. CV plots of the NVP/C/NC composite at different scan rates from 0.1 to 1 mV/s were acquired with fresh half cells (Fig. 8a). In all the curves, the single defined redox peaks are related to the V3+/V4+ redox couple, corresponding to a two-phase transformation between Na3V2(PO4)3 and NaV2(PO4)3 [45]. With the increase of the scan rates, the oxidation peak moves to higher potential and the reduction peak shifts to lower potential, suggesting larger polarization at higher scanning rates. Meanwhile, the intensities of oxidation and reduction peaks increase with increasing scan rates. The corresponding correlation between the peak current (Ip) and the square root of the scanning rate (v1/2) is presented in Fig. 8b. Obviously, the Ip and v1/2 coincide with a linear relationship, revealing that the electrode reaction of NVP/NC is mainly controlled by ions diffusion [46]. The apparent diffusion coefficients of Na+ are estimated according to the classic Randles-Sevcik equation [47,48]:
kinetics improved by the high conductivity and the shortened diffusion length for Na+ ions [41]. Fig. 6b shows the cycling performances of NVP/C and NVP/C/NC at a low rate of 0.2C between 2.5 V and 3.8 V vs. Na+/Na. The NVP/C/NC composite electrode delivered a discharge specific capacity of 109.2 mAh/g for the first cycle, with 95.4% capacity retention over 100 cycles. For the pristine NVP/C electrode, the first discharge capacity was 106.7 mAh/g and after 100 cycles the capacity had faded to 93.3% of its initial value. The coulombic efficiency reaches almost 100% throughout the whole cycling, manifesting the good reversibility of the NVP/C/NC composite electrode. It also should be noted that the capacity contribution from the carbon is negligible in the potential range measured. Therefore, the specific capacity was estimated based on the mass of the electroactive NVP. Fig. 6c presents the rate capability of NVP/C and NVP/C/NC composite from 0.2C to 20C. Obviously, the NVP/C/NC composite exhibits higher average reversible capacity of 109.8, 107.1, 104.3, 101.5, 97.2, 93.3, and 87.2 mAh/g at current rates of 0.2, 0.5, 1, 2, 5, 10, and 20C, respectively, much higher than that of NVP/C at the same rate. When the current rate returns to 0.2C again, the specific capacity of 107.7 mAh/g (91.6% of its theoretical capacity) could be recovered, indicating the excellent rate reversibility of the NVP/C/NC composite electrode. The superior rate performance can be attributed to the fast electronic transport network of the nitrogen-doped carbon layer. The corresponding charge/discharge profiles of NVP/C/NC and NVP/C composite electrode from 0.2C to 20C are shown in Fig. 6d and e, respectively. In Fig. 6d, all the charge/discharge plateaus can be clearly observed at low current rates. Even though the plateaus become vague, the NVP/C/NC composite electrode can still exhibit large reversible capacities when the current rate increased to 10C or 20C. In addition, the relatively small gap between the two potential plateaus at different rates implies the lower polarization of NVP/C/NC compared to that of pristine NVP/C (Table S1). The long-term cycling performance of NVP/C and NVP/C/NC composite electrodes at 2C is displayed in Fig. 6f. The NVP/C and NVP/ C/NC composite cathodes deliver the initial reversible capacity of 82.9 and 113.6 mAh/g, respectively. After 500 cycles, the capacity retention of the NVP/C/NC composite electrode keeps at 91.2%, which is higher than that of pristine NVP/C electrode (70%). Both the coulombic efficiencies of the two compounds approach 100% in the whole charge/ discharge process, demonstrating the excellent reversibility of the NVP electrode material. The good cycling capability may be due to the open NASICON structure and/or the nitrogen-doped carbon coating layer. The kinetics processes of the two electrodes were studied by electrochemical impedance spectroscopy (EIS) measurements. Fig. 7a exhibits the Nyquist plots and the corresponding equivalent circuit model of the NVP/C and NVP/C/NC electrodes. All curves are composed of a semicircle in the high frequency and a slanted line in the low frequency. The charge transfer impedance (Rct) of NVP/C/NC is 291.93 Ω, much smaller than that of NVP/C (719.58 Ω), implying that carbon coating is beneficial for the fast ions/electrons transport and results in lower polarization of NVP/C/NC. Furthermore, the sodium ion diffusion coefficients of the electrodes were calculated using the following equations [42–44]:
D=
(3)
wherein Ip is the peak current, n stands for the number of electrons involved in the half-reaction for the redox couple, A is the contact area between the electrode and electrolyte, D denotes the Na+ ion diffusion coefficient, C is the concentration of Na+ in a solid, and v is the voltage scanning rate. From the slope of the lines calculated on the basis of the Eq. (3), the diffusion coefficient Danodic (positive) = 3.47 × 10−11 cm2 s−1, and the diffusion coefficient Dcathodic (negative) = 2.47 × 10−11 cm2 s−1 were determined. The relative value is in accordance with the sodium ion coefficient achieved from EIS measurement. Furthermore, the DNa+ of NVP/C/NC composite is in good agreement with the best cycle stability and rate performance.
4. Conclusion In summary, polydopamine-derived nitrogen-doped carbon coating NVP/C/NC cathode material has been synthesized by a simple sol-gel and post-sintering process. The obtained NVP/C/NC composite exhibits high specific capacity, good cycling stability and superior rate performance, which are ascribed to the expanding contact area between active material and electrolyte and enhanced electronic conductivity of the in situ generated nitrogen-doped carbon coating. The relatively uniform and stable coated nitrogen-doped carbon layer restrains the volume change and agglomeration of the electrode during cycling. Therefore, this polydopamine-derived nitrogen-doped carbon coating approach developed here represents a promising technique to improve the electrochemical properties of NVP or to design new electrode materials.
Acknowledgments
R2T 2 2A2 n2F 4C 2σ 2
Z ′ = R s + R ct +
Ip = 2.69 × 105n3/2AD1/2 Cv1/2
(1)
σω−1/2
This work was financially supported by the National Natural Science Foundation of China (Nos. 51674068, 51774002, 51771046, 51874079, 51871046, 51804035), Natural Science Foundation of Hebei Province (No. E2018501091), the Science and Technology Project of Hebei Province (No. 15271302D), the Training Foundation for Scientific Research of Talents Project, Hebei Province (No. A2016005004), the Fundamental Research Funds for the Central Universities (No. N172302001). The authors would like to thank Chen Weiwei from Shiyanjia Lab (www.shiyanjia.com) for the XPS analysis.
(2) −1
−1
where R is the gas constant (8.314 J mol K ), T is the absolute temperature (298.15 K), A denotes the surface area of the electrode, n stands for the number per molecule during oxidization, F is the Faraday constant (96,485C mol−1), C is the concentration of sodium ions, and σ is the Warburg impedance factor which is related to Z' and ω. Fig. 7b displays the relationship between Z' and the reciprocal square root of frequency (ω−1/2) in the low frequency zone. Based on Eqs. (1) and (2), 1165
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Appendix A. Supplementary data [25]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.05.224.
[26]
References
[27]
[1] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. [2] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [3] S.-W. Kim, D.-H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries, Adv. Energy Mater. 2 (2012) 710–721. [4] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma, G. Ceder, Voltage, stability and diffusion barrier differences between sodium-ion and lithiumion intercalation materials, Energy Environ. Sci. 4 (2011) 3680–3688. [5] Y. Fang, L. Xiao, X. Ai, Y. Cao, H. Yang, Hierarchical carbon framework wrapped Na3V2(PO4)3 as a superior high-rate and extended lifespan cathode for sodium-ion batteries, Adv. Mater. 27 (2015) 5895–5900. [6] D. Buchholz, A. Moretti, R. Kloepsch, S. Nowak, V. Siozios, M. Winter, S. Passerini, Toward Na-ion batteries—synthesis and characterization of a novel high capacity Na ion intercalation material, Chem. Mater. 25 (2013) 142–148. [7] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-ion batteries, Adv. Funct. Mater. 23 (2013) 947–958. [8] W. Olszewski, M.A. Perez, C. Marini, E. Paris, X.F. Wang, T. Iwao, M. Okubo, A. Yamada, T. Mizokawa, N.L. Saini, L. Sirnonelli, Temperature dependent local structure of NaxCoO2 cathode material for rechargeable sodium-ion batteries, J. Phys. Chem. C 120 (2016) 4227–4232. [9] D. Su, C. Wang, H.J. Ahn, G. Wang, Single crystalline Na0.7MnO2 nanoplates as cathode materials for sodium-ion batteries with enhanced performance, Chem. Eur. J. 19 (2013) 10884–10889. [10] X. Ma, H. Chen, G. Ceder, Electrochemical properties of monoclinic NaMnO2, J. Electrochem. Soc. 158 (2011) A1307–A1312. [11] L. Wu, S. Shi, X. Zhang, Y. Yang, J. Liu, S. Tang, S. Zhong, Room-temperature prereduction of spinning solution for the synthesis of Na3V2(PO4)3/C nanofibers as high-performance cathode materials for Na-ion batteries, Electrochim. Acta 274 (2018) 233–241. [12] Y. Fang, Q. Liu, L. Xiao, X. Ai, H. Yang, Y. Cao, High-performance olivine NaFePO4 microsphere cathode synthesized by aqueous electrochemical displacement method for sodium ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 17977–17984. [13] R. Tripathi, S.M. Wood, M.S. Islam, L.F. Nazar, Na-ion mobility in layered Na2FePO4F and olivine Na[Fe,Mn]PO4, Energy Environ. Sci. 6 (2013) 2257–2264. [14] Z. Jian, W. Han, X. Lu, H. Yang, Y.-S. Hu, J. Zhou, Z. Zhou, J. Li, W. Chen, D. Chen, L. Chen, Superior electrochemical performance and storage mechanism of Na3V2(PO4)3 cathode for room-temperature sodium-ion batteries, Adv. Energy Mater. 3 (2013) 156–160. [15] J. Li, X. Cao, A. Pan, Y. Zhao, H. Yang, G. Cao, S. Liang, Nanoflake-assembled threedimensional Na3V2(PO4)3/C cathode for high performance sodium ion batteries, Chem. Eng. J. 335 (2018) 301–308. [16] F. Xiong, S. Tan, Q. Wei, G. Zhang, J. Sheng, Q. An, L. Mai, Three-dimensional graphene frameworks wrapped Li3V2(PO4)3 with reversible topotactic sodium-ion storage, Nano Energy 32 (2017) 347–352. [17] W. Duan, Z. Hu, K. Zhang, F. Cheng, Z. Tao, J. Chen, Li3V2(PO4)3@C core-shell nanocomposite as a superior cathode material for lithium-ion batteries, Nanoscale 5 (2013) 6485–6490. [18] Z.-X. Chi, W. Zhang, X.-S. Wang, F.-Q. Cheng, J.-T. Chen, A.-M. Cao, L.-J. Wan, Accurate surface control of core–shell structured LiMn0.5Fe0.5PO4@C for improved battery performance, J. Mater. Chem. A 2 (2014) 17359–17365. [19] G. Ali, J.-H. Lee, D. Susanto, S.-W. Choi, B.W. Cho, K.-W. Nam, K.Y. Chung, Polythiophene-wrapped olivine NaFePO4 as a cathode for Na-ion batteries, ACS Appl. Mater. Interfaces 8 (2016) 15422–15429. [20] S.H. Luo, J.Y. Li, S. Bao, Y.Y. Liu, Z.Y. Wang, Na3V2(PO4)3/C composite prepared by sol-gel method as cathode for sodium ion batteries, J. Electrochem. Soc. 165 (2018) A1460–A1465. [21] X. Rui, W. Sun, C. Wu, Y. Yu, Q. Yan, An advanced sodium-ion battery composed of carbon coated Na3V2(PO4)3 in a porous graphene network, Adv. Mater. 27 (2015) 6670–6676. [22] Z. Jian, L. Zhao, H. Pan, Y.-S. Hu, H. Li, W. Chen, L. Chen, Carbon coated Na3V2(PO4)3 as novel electrode material for sodium ion batteries, Electrochem. Commun. 14 (2012) 86–89. [23] Q. Wang, B. Zhao, S. Zhang, X. Gao, C. Deng, Superior sodium intercalation of honeycomb-structured hierarchical porous Na3V2(PO4)3/C microballs prepared by a facile one-pot synthesis, J. Mater. Chem. A 3 (2015) 7732–7740. [24] L. Chen, Y. Zhao, S. Liu, L. Zhao, Hard carbon wrapped Na3V2(PO4)3@C porous
[28] [29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
1166
composite extending cycling lifespan for sodium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 44485–44493. Z. Chu, C. Yue, Core–shell structured Na3V2(PO4)3/C nanocrystals embedded in multi-walled carbon nanotubes: a high-performance cathode for sodium-ion batteries, Solid State Ionics 287 (2016) 36–41. B. Guo, Q. Liu, E. Chen, H. Zhu, L. Fang, J.R. Gong, Controllable N-doping of graphene, Nano Lett. 10 (2010) 4975–4980. M. Li, Z. Zuo, J. Deng, Q. Yao, Z. Wang, H. Zhou, W.-B. Luo, H.-K. Liu, S.-X. Dou, A high rate capability and long lifespan symmetric sodium-ion battery system based on a bipolar material Na2LiV2(PO4)3/C, J. Mater. Chem. A 6 (2018) 9962–9970. Z. Chu, C. Yue, Graphene oxide wrapped Na3V2(PO4)3/C nanocomposite as superior cathode material for sodium-ion batteries, Ceram. Int. 42 (2016) 820–827. T.-F. Hung, W.-J. Cheng, W.-S. Chang, C.-C. Yang, C.-C. Shen, Y.-L. Kuo, Ascorbic acid-assisted synthesis of mesoporous sodium vanadium phosphate nanoparticles with highly sp2-coordinated carbon coatings as efficient cathode materials for rechargeable sodium-ion batteries, Chem. Eur. J. 22 (2016) 10620–10626. M.-X. Jing, J. Zhang, C. Han, H. Yang, S.-S. Yao, L. Zhu, L.-L. Chen, Q.-L. Xie, X. Chen, X.-Q. Shen, S.-B. Qin, A flexible Na3V2(PO4)3/C composite fiber membrane cathode for Na-ion and Na-Li hybrid-ion batteries, J. Electrochem. Soc. 165 (2018) A1761–A1769. L.-L. Zhang, D. Ma, T. Li, J. Liu, X.-K. Ding, Y.-H. Huang, X.-L. Yang, Polydopaminederived nitrogen-doped carbon-covered Na3V2(PO4)2F3 cathode material for highperformance Na-ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 36851–36859. X. Liu, Y. Wang, Z. Wang, T. Zhou, M. Yu, L. Xiu, J. Qiu, Achieving ultralong life sodium storage in amorphous cobalt-tin binary sulfide nanoboxes sheathed in Ndoped carbon, J. Mater. Chem. A 5 (2017) 10398–10405. F. Yang, Z. Zhang, K. Du, X. Zhao, W. Chen, Y. Lai, J. Li, Dopamine derived nitrogen-doped carbon sheets as anode materials for high-performance sodium ion batteries, Carbon 91 (2015) 88–95. L.-L. Zheng, Y. Xue, L. Deng, G.-R. Wu, S.-E. Hao, Z.-B. Wang, Na3V2(PO4)3 with specially designed carbon framework as high performance cathode for sodium-ion batteries, Ceram. Int. 45 (2019) 4637–4644. C. Miao, P. Bai, Q. Jiang, S. Sun, X. Wang, A novel synthesis and characterization of LiFePO4 and LiFePO4/C as a cathode material for lithium-ion battery, J. Power Sources 246 (2014) 232–238. W. Duan, Z. Zhu, H. Li, Z. Hu, K. Zhang, F. Cheng, J. Chen, Na3V2(PO4)3@C core–shell nanocomposites for rechargeable sodium-ion batteries, J. Mater. Chem. A 2 (2014) 8668–8675. W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Selfsacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for highrate sodium-ion full batteries, Nano Energy 25 (2016) 145–153. J.-Z. Guo, X.-L. Wu, F. Wan, J. Wang, X.-H. Zhang, R.-S. Wang, A superior Na3V2(PO4)3-based nanocornposite enhanced by both N-doped coating carbon and graphene as the cathode for sodium-ion batteries, Chem. Eur. J. 21 (2015) 17371–17378. K. Saravanan, C.W. Mason, A. Rudola, K.H. Wong, P. Balaya, The first report on excellent cycling stability and superior rate capability of Na3V2(PO4)3 for Sodium Ion Batteries, Adv. Energy Mater. 3 (2013) 444–450. L.-L. Zheng, Y. Xue, S.-E. Hao, Z.-B. Wang, Porous Na3V2(PO4)3 prepared by freezedrying method as high performance cathode for sodium-ion batteries, Ceram. Int. 44 (2018) 9880–9886. Y. Jiang, Z. Yang, W. Li, L. Zeng, F. Pan, M. Wang, X. Wei, G. Hu, L. Gu, Y. Yu, Nanoconfined carbon-coated Na3V2(PO4)3 particles in mesoporous carbon enabling ultralong cycle life for sodium-ion batteries, Adv. Energy Mater. 5 (2015) 1402104. Y. Chen, Y. Xu, X. Sun, B. Zhang, S. He, C. Wang, F-doping and V-defect synergetic effects on Na3V2(PO4)3/C composite: a promising cathode with high ionic conductivity for sodium ion batteries, J. Power Sources 397 (2018) 307–317. H. Chen, B. Zhang, X. Wang, P. Dong, H. Tong, J.-C. Zheng, W. Yu, J. Zhang, CNTdecorated Na3V2(PO4)3 microspheres as a high-rate and cycle-stable cathode material for sodium ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 3590–3595. L. Deng, G. Sun, K. Goh, L.-L. Zheng, F.-D. Yu, X.-L. Sui, L. Zhao, Z.-B. Wang, Facile one-step carbothermal reduction synthesis of Na3V2(PO4)2F3/C serving as cathode for sodium ion batteries, Electrochim. Acta 298 (2019) 459–467. Z. Jian, C. Yuan, W. Han, X. Lu, L. Gu, X. Xi, Y.-S. Hu, H. Li, W. Chen, D. Chen, Y. Ikuhara, L. Chen, Atomic structure and kinetics of NASICON NaxV2(PO4)3 cathode for sodium-ion batteries, Adv. Funct. Mater. 24 (2014) 4265–4272. K. Kretschmer, B. Sun, J. Zhang, X. Xie, H. Liu, G. Wang, 3D interconnected carbon fiber network-enabled ultralong life Na3V2(PO4)3@carbon paper cathode for sodium-ion batteries, Small 13 (2017) 1603318. Y.Q. Qiao, X.L. Wang, J.Y. Xiang, D. Zhang, W.L. Liu, J.P. Tu, Electrochemical performance of Li3V2(PO4)3/C cathode materials using stearic acid as a carbon source, Electrochim. Acta 56 (2011) 2269–2275. T. Jiang, W. Pan, J. Wang, X. Bie, F. Du, Y. Wei, C. Wang, G. Chen, Carbon coated Li3V2(PO4)3 cathode material prepared by a PVA assisted sol–gel method, Electrochim. Acta 55 (2010) 3864–3869.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Double-carbon coated Na3V2(PO4)3 as a superior cathode material for Naion batteries
T
⁎
Hong-bo Huanga,d, Shao-hua Luob,c,d, , Cai-ling Liua,d, Yue Yange, Yu-chun Zhaib, ⁎⁎ Long-jiao Changf, Ming-qi Lig, a
School of Metallurgy, Northeastern University, Shenyang 110819, PR China School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, PR China c School of Materials Science and Engineering, Northeastern University, Shenyang 110819, PR China d Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province, Qinhuangdao 066004, PR China e Procurement Center for Police Equipment of Ministy of Public Security, Shanghai 201100, PR China f College of New Energy, Bohai University, Jinzhou 121013, PR China g College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637009, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Dopamine Nitrogen-doped carbon-coating Na3V2(PO4)3 Cathode material Na-ion batteries
Na super ionic conductor (NASICON)-type Na3V2(PO4)3 (NVP) has been considered as a potential positive electrode material for sodium ion batteries (NIBs) owing to its high theoretical specific capacity. Nonetheless, the practical application of NVP is hindered by the intrinsically poor electronic conductivity. Herein, polydopaminederived nitrogen-doped carbon-covered Na3V2(PO4)3/C composites (NVP/C/NC) have been prepared through a self-polymerization of dopamine on the NVP/C surface and subsequent calcination at high temperature. The assynthesized NVP/C/NC composite cathode exhibits a high initial reversible capacity (109.2 mAh/g at 0.2C), superior rate performance (87.2 mAh/g at a rate up to 20C), and excellent cycling capability (91.2% of the initial capacity is kept after 500 cycles at 2C) in NIBs. Furthermore, compared with NVP/C electrode, the NVP/C/NC electrode presents low resistance and high sodium ions diffusion coefficient. The good performance can be ascribed to the nitrogen-doped carbon layer in improving the electronic conductivity, shortening diffusion length of Na+ ions and electrons, and relieving the volume changes of electrode materials. These preliminary results suggest that the as-obtained NVP/C/NC composite is a novel promising electrode material for low-cost sodium energy storage.
1. Introduction Nowadays, lithium ion batteries (LIBs), owing to their large energy density and long lifespan, have been utilized in a broad range of applications, such as portable electronic equipment, electric vehicles and smart grid [1,2]. However, the limited and unevenly distribution of lithium resources has led to the relatively high cost for LIBs in largescale applications [3,4]. Therefore, the development of new alternative energy storage system is not only desirable but also extremely essential. In recent years, due to the low price and natural abundant of sodium sources, sodium ion batteries (NIBs) have attracted great attention for future large-scale energy storage [5,6]. In addition, the similar physical and chemical properties between lithium and sodium elements further promote the development of NIBs [7]. Hitherto, many cathode compounds such as layered transition metal ⁎
oxides NaxMO2 (M = Co, Mn) [8–10], Na super ionic conductor (NASICON) Na3V2(PO4)3 [11] and olivine-type NaMPO4 (M = Mn, Fe) [12,13] have been widely investigated for NIBs. Among them, Na3V2(PO4)3 (NVP) has proved to be one of the most promising electrode materials because of its outstanding thermal stability, high theoretical energy density (400 Wh/kg), and fast sodium ions diffusion tunnel (NASICON structure) [14]. Additionally, NVP has a large theoretical capacity (117.6 mAh/g) and a flat potential plateaus at around 3.4 V [15]. Nevertheless, the intrinsic defect of NVP is the extremely low electronic conductivity, which leads to inferior electrochemical capability, especially rate performance, and hence, greatly inhibits its practical applications. To solve this issue, many studies such as doping with secondary ions, carbon coating and fabricating nanostructure have been conducted. In the case of other polyanionic materials, carbon layer coating has been considered to be an economical and effective strategy
Correspondence to: S.H. Luo, School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, PR China. Corresponding author. E-mail addresses: [email protected] (S.-h. Luo), [email protected] (M.-q. Li).
⁎⁎
https://doi.org/10.1016/j.apsusc.2019.05.224 Received 7 April 2019; Received in revised form 15 May 2019; Accepted 18 May 2019 Available online 20 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
to overcome this disadvantage. For example, Mai et al. prepared threedimensional graphene frameworks covered LVP (LVP-G-FD) composite, which exhibits large energy density (501.3 Wh/kg) and superior cycle performance with a capacity retention of 65% after 500 cycles at 500 mA g−1 [16]. Chen et al. fabricated core-shell structured Li3V2(PO4)3@C nanocomposite with approximately 86% capacity retention after 1000 cycles at 5C (1C = 197 mAh g−1), showing outstanding rate capability [17]. Wan et al. prepared carbon-coated LiMn0.5Fe0.5PO4@C core-shell composite through in-situ resorcinolformaldehyde polymerization method, exhibiting a remarkable high rate capability (155 mAh g−1 at 17 mA g−1 and 105 mAh g−1 at 1700 mA g−1) [18]. Chung et al. reported the preparation of polythiophene coated NaFePO4, which displays a first discharge specific capacity of 142 mAh g−1 at 10 mA g−1 with a capacity retention of 94% over 100 cycles and superior rate capability (70 mAh g−1 at 150 mA g−1 and 42 mAh g−1 at 300 mA g−1) [19]. Herein, we prepared a nitrogen-doped carbon-coated NVP/C positive electrode material by employing dopamine as the nitrogen source and carbon source simultaneously through a versatile and simple method. Owing to the good film-forming ability of dopamine, a homogeneous coating layer was successfully covered on the surface of NVP/C particles, which would suppress structural collapse resulting from the volume changes during ions insertion/extraction. The coated nitrogen-doped carbon layer could enhance the electrical conductivity greatly. Also, it will further improve the cycling stability, particularly the high rate performance. As expected, the NVP/C/NC composite cathode shows a larger first discharge specific capacity of 109.2 mAh/g at 0.2C with capacity retention of 95.4% over 100 cycles. Even at a current density of 20C, a high specific capacity of 87.2 mAh/g is delivered.
Fig. 1. Schematic illustration for the synthesis process of the NVP/C/NC composite.
centrifugation and washed three times with deionized water and ethanol, and then dried at 80 °C for 24 h. Finally, the nitrogen-doped carbon-wrapped NVP/C (NVP/C/NC) was obtained after sintering in a tube furnace at 350 °C for 3 h followed by calcining at 750 °C for 3 h in an Ar atmosphere. The fabrication process of the NVP/C/NC composite is illustrated in Fig. 1.
2.2. Material characterization X-ray powder diffraction (XRD) measurements were conducted with a Rigaku SMARTLAB diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5406 Å) in the scanning range of 10–60°. SEM with energy dispersive spectrometer (EDS) was performed for morphology analysis using ZEISS SUPRA55 field-emission scanning electron microscope (ZEISS, Germany) working at 15 kV. TEM was carried out on a JEOL 2100F transmission electron microscope (JEOL, Japan) working at 200 kV. Thermogravimetric analysis (TGA) of the compound was achieved by an HCT-2 device (Hengjiu Instrument Co., Ltd., China) under air from ambient temperature to 700 °C, with a ramp rate of 10 °C/min. Raman spectrum is measured by Renishaw 2000 spectrometer (Renishaw, UK) with an excitation wavelength of 532 nm. The carbon content is measured by carbon sulfur analyzer (ELTRA, CS800). The Brunauer-Emmett-Teller (BET) surface area and porous structure were estimated by SSA-4000 automatic specific surface area and pore size distribution analyzer (Builder, China). Tap density was measured by tapping 1 g NVP 1000 times in a cylindrical container.
2. Experimental 2.1. Material synthesis Citric acid and NaOH were purchased from Aladdin Industrial Corporation. NH4VO3 and NH4H2PO4 were provided by Sinopharm Chemical Reagent Co., Ltd. Tris(hydroxymethyl)aminomethane and dopamine hydrochloride were obtained from J&K Scientific Ltd. All chemicals were of analytical grade and employed without any purification. Citric acid was used not only as a carbon source but also as a chelating agent.
2.3. Electrochemical measurements The working electrode was mixed by dispersing the active material (80 wt%), acetylene black (10 wt%), and poly(vinylidene fluoride) binder (PVDF, 10 wt%) in N-methyl-2-pyrrolidone (NMP) solution. The acquired homogenous slurry was pasted on an Al foil, followed by drying at 120 °C in a vacuum oven for 12 h. Then the electrode was cut into circles with a diameter of 10 mm. CR2032 type button batteries were assembled in an Ar-filled glove-box (O2 ≤ 0.1 ppm and H2O ≤ 0.1 ppm) with sodium metal foils as the counter/reference electrodes. 1 M NaClO4 in the ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by vol.) mixed solvent, with 5 wt% fluoroethylene carbonate (FEC) as an additive was used as the electrolyte. Glass fiber filter (Whatman, GF/D) was chosen as the separator. The typical mass loading of the active material is about 1.3 mg/cm2. The galvanostatic charging and discharging testing were performed in the voltage window of 2.5–3.8 V vs. Na+/Na using a Land CT2001A battery test system at room temperature. In this work, C rates are utilized for characterizing the current rate, where 1C corresponds to a current rate of 117.6 mA g−1. Cyclic voltammetry (CV) experiments were conducted with a CHI660C electrochemical workstation at a scanning rate of 0.1 mV/s between 2.5 V and 3.8 V (vs. Na+/Na). Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range from 100 kHz to 0.1 Hz with an AC voltage of 10 mV by using a Solartron 1260 electrochemical analyzer.
2.1.1. Preparation of Na3V2(PO4)3/C (NVP/C) The pristine Na3V2(PO4)3/C sample was fabricated by the sol-gel approach according to the previous work [20]. In brief, citric acid, NH4VO3, NH4H2PO4 and NaOH with the molar ratio of 1:2:3:3 were sequentially dissolved in 75 mL of deionized water to get an orange solution. Subsequently, the solution was agitated magnetically at 80 °C for 8 h to obtain a dark blue sol. Then, the sol was dried in a drying oven at 80 °C overnight to form a gel. After grinding, the dried gel powder was calcined in a tube furnace at 350 °C for 3 h, then annealed at 800 °C for 16 h under an Ar atmosphere to achieve the NVP/C material. 2.1.2. Preparation of nitrogen-doped carbon-wrapped Na3V2(PO4)3/C (NVP/C/NC) NVP/C/NC composite was fabricated through a self-polymerization process of dopamine monomers. 1.5 g of the as-prepared NVP/C powder was dispersed in a tris(hydroxymethyl)aminomethane buffer aqueous solution (75 mL, 0.5 M) by ultrasound for 30 min. Then, HCl (1 M) was injected drop by drop into the solution under magnetic stirring until the pH value of the tris-buffer solution reached 8.7. Later, 0.15 g of dopamine hydrochloride was added into the suspension with continuous and vigorous stirring for 24 h under ambient temperature. The polydopamine-coated NVP/C/NC precursor was harvested via 1160
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 2. (a) XRD patterns, (b) TGA curves and (c) Raman spectra of NVP/C and NVP/C/NC; (d) Nitrogen adsorption-desorption isotherms of NVP/C/NC, the insert was pore size distribution curve.
3. Results and discussion
for NVP/C and NVP/C/NC are about 1.04 and 1.02, respectively, implying the partial graphitization of the deposited carbon [15,27]. There is no doubt that the ID/IG value of NVP/C/NC is lower than that of NVP/ C, suggesting the high degree of graphitization and high conductivity of carbon, which result from the additional coated nitrogen-doped carbon layer. It is worth mentioning that this could efficiently improve the electrical conductivity and accelerate the electron transport [28]. The nitrogen adsorption-desorption isotherms were utilized to investigate the porous structure of the NVP/C/NC composite. Fig. 2d exhibits the nitrogen isotherm adsorption-desorption plot and the pore size distribution of the NVP/C/NC. The isotherms of the sample are assigned to the type IV curve with an H3 hysteresis loop between the P/ P0 range of 0.4 and 1.0, which corresponds to the mesoporous structure [29]. Based on the Barrett-Joyner-Halenda (BJH) equation, the specific surface area of the NVP/C/NC was estimated to be 49.1 m2 g−1, much larger than that of pristine NVP/C (25.66 m2 g−1, Fig. S1). The enlarged surface area of NVP/C/NC provides more active sites for sodium ions insertion/extraction and facilitates the effective contact area between electrode and electrolyte, leading to improved reaction kinetics [30]. In addition, the pore size of the NVP/C/NC composite is distributed mainly below 10 nm with an average pore diameter of approximately 3 nm (inset in Fig. 2d). X-ray photoelectron spectroscopy (XPS) is carried out to analyze the surface chemical composition of the NVP/C/NC, as shown in Fig. 3. The high-resolution C 1s spectrum (Fig. 3a) displays four types of carbon: C]C at 284.6 eV, CeC at 285.4 eV, CeN at 287.8 eV, and O–C=O at 288.6 eV. [31] This demonstrates that nitrogen has been successfully doped into the carbon layer. For the high-resolution N 1s spectrum of NVP/C/NC in Fig. 3b, three peaks located at about 398.5, 399.5 and 400.4 eV correspond to pyridinic N, pyrrolic N and graphitic N, respectively. [32] Additionally, the nitrogen content for NVP/C/NC is
Fig. 2 shows the XRD patterns of the as-prepared NVP/C and NVP/ C/NC composite samples. All the diffraction peaks can be well indexed to a rhombohedral NASICON structure with a space group of R-3c (JCPDS card No. 62–0345), which is consistent with previous reports [21–23]. In addition, both the peak intensities and positions of the two samples are similar, suggesting that the carbon coating does not significantly change the crystallinity and NASICON-type structure of NVP. Simultaneity, no other peaks from the impurities is detected in the XRD patterns, manifesting that the nitrogen-doped carbon in the sample is amorphous. The precise carbon contents measured from the elemental analyzer are 2.11% and 5.88% for NVP/C and NVP/C/NC composite, respectively. The carbon content in the NVP/C and NVP/C/NC composite was also estimated from the TGA test in air as displayed in Fig. 2b. In the TGA curves, the slight weight loss at around 100 °C is attributed to the volatilization of adsorbed water molecules. The carbon contents are 2.20% for NVP/C and 5.91% for NVP/C/NC, which are in good agreement with the results of carbon sulfur determinator. After 500 °C, both samples show the same tendency of a slight weight increase, which can be ascribed to the oxidation of V ions in the NVP from 3+ to 4+ and 5+ [24]. Moreover, the packing density of NVP/C and NVP/C/NC samples measured is 0.93 and 0.87 g cm−3, respectively. The nature of carbon layer in the NVP/C and NVP/C/NC composites is further characterized by Raman spectroscopy and the result is shown in Fig. 2c. Two characteristic peaks centered at 1360 and 1582 cm−1 are seen for both of them, corresponding to the D (disordered carbon) band and G (graphite carbon) band, respectively [25]. Generally, the intensity ratio of D/G peak (ID/IG) is often defined to evaluate the structural disordering of carbon materials [26]. And the values of ID/IG 1161
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 3. High-resolution XPS of (a) C1s and (b) N1s spectra of NVP/C/NC composite.
Fig. 4. SEM images of (a) NVP/C and NVP/C/NC; (c) EDS mapping of the NVP/C/NC.
1162
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 5. TEM images of the NVP/C/NC composite.
Fig. 6. (a) The CV curves of NVP/C and NVP/C/NC electrodes at a scan rate of 0.1 mV/s. (b) Cycling performance of NVP/C and NVP/C/NC at 0.2C. (c) Rate capability from 0.2 to 20C for 10 cycles, and (d, e) corresponding charge/discharge curves of NVP/C and NVP/C/NC. (f) Cycling performance of NVP/C and NVP/C/ NC at 2C.
1163
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Fig. 7. (a) The Nyquist plots, and (b) the relationship curves between Zre and ω−1/2 in the low-frequency range of the NVP/C and NVP/C/NC samples.
spectrometry (EDS) mappings indicate that Na, V, P, C, and N elements are homogeneously distributed in NVP/C/NC composite. The detailed microstructure of the NVP/C/NC sample was characterized by transmission electron microscopy (TEM). Fig. 5 displays the TEM images of the NVP/C/NC composite. It is observed that the surface of grains is wrapped and connected by nitrogen-doped carbon derived from polydopamine, which provides a facial channel for electrons transport [34]. The appropriate carbon layer is a key factor on the enhancement of electrochemical capabilities of NVP. A coating of the carbon layer on the surface of NVP can effectively prevent the particles from further growing and aggregating so as to acquire smaller grains [35]. It also reveals that the particle size is about 100–180 nm, which is in accordance with the above SEM result. As seen in Fig. 5b, the NVP/ C/NC composite has a relatively uniform amorphous nitrogen-doped carbon layer with the thickness of < 10 nm, demonstrating the effectiveness of the carbon coating strategy. The cyclic voltammetry (CV) (Fig. 6a) was conducted in a potential range of 2.5–3.8 V at a scanning rate of 0.1 mV/s. As observed from the CV curves, there appears a couple of well-defined redox peaks on both the electrodes, which correspond to the reversible transformation of V3+/V4+ [36,37]. However, in the reduction (sodiation) process, the single peak splits into two parts, which is owing to the structural rearrangements when sodium ions transfer from Na(1) to Na(2) sites [38–40]. The voltage distance (ΔV) between the reduction and oxidation peaks of NVP/C and NVP/C/NC is 243 mV and 216 mV, respectively, suggesting the lower polarization and easier migration of Na+ in NVP/C/NC composite electrode. Meanwhile, the peak current of NVP/ C/NC is much higher than that of pristine NVP/C, proving the faster
Table 1 Rct and DNa+ values from EIS date of the as-prepared NVP/C and NVP/C/NC samples. Samples
NVP/C
NVP/C/NC
Rct (Ω) DNa+ (cm2 s−1)
719.58 3.05 × 10−13
291.93 5.41 × 10−13
about 1.69 at.% by the results of XPS. The N-doped carbon layer with multiple type of nitrogen is beneficial to enhance the electronic conductivity of composite and promote the transportation of Na ions in the interface due to defects caused by N doping [33]. The morphological characteristic of the as-obtained NVP/C and NVP/C/NC samples was studied by a field emission scanning electron microscopy (FESEM). As displayed in Fig. 4a and b, both the two samples are composed of plate-like nano-particles with a size of about 100–375 nm. Although both of them showed no significant difference in the morphology, the NVP/C/NC composite has a porous structure after the carbonization procedure of the polydopamine, which can not only shorten the sodium ion diffusion distance, but also facilitate the infiltration of electrolyte, thus providing better performance. Moreover, the pristine NVP/C occurs in an apparent agglomeration compared to NVP/C/NC composite, suggesting that the coated nitrogen-doped carbon layer can inhibit particle growth to a great extent. Meanwhile, smoke-like carbon grains were seen for NVP/C/NC composite but were hardly seen for NVP/C. Therefore, it was evident that the nitrogendoped carbon, derived from polydopamine, can inhibit the particle growth and agglomeration of NVP/C. In Fig. 4c, the energy dispersive
Fig. 8. (a) CV curves of NVP/C/NC at different scan rates from 0.1 to 1 mV/s; and (b) the corresponding linear relationship of the cathodic/anodic peak current (Ip) and the square root of the scan rate (v1/2). 1164
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
the calculated diffusion coefficients of the sodium ions (DNa+) are displayed in Table 1. It can be found that the DNa+ of NVP/C/NC (5.41 × 10−13 cm2 s−1) is larger than that of NVP/C (3.05 × 10−13 cm2 s−1), suggesting faster Na+ ions transport in the NVP/C/NC composite. Additionally, this EIS results well interpret the better electrochemical capacities of NVP/C/NC composite electrode. CV plots of the NVP/C/NC composite at different scan rates from 0.1 to 1 mV/s were acquired with fresh half cells (Fig. 8a). In all the curves, the single defined redox peaks are related to the V3+/V4+ redox couple, corresponding to a two-phase transformation between Na3V2(PO4)3 and NaV2(PO4)3 [45]. With the increase of the scan rates, the oxidation peak moves to higher potential and the reduction peak shifts to lower potential, suggesting larger polarization at higher scanning rates. Meanwhile, the intensities of oxidation and reduction peaks increase with increasing scan rates. The corresponding correlation between the peak current (Ip) and the square root of the scanning rate (v1/2) is presented in Fig. 8b. Obviously, the Ip and v1/2 coincide with a linear relationship, revealing that the electrode reaction of NVP/NC is mainly controlled by ions diffusion [46]. The apparent diffusion coefficients of Na+ are estimated according to the classic Randles-Sevcik equation [47,48]:
kinetics improved by the high conductivity and the shortened diffusion length for Na+ ions [41]. Fig. 6b shows the cycling performances of NVP/C and NVP/C/NC at a low rate of 0.2C between 2.5 V and 3.8 V vs. Na+/Na. The NVP/C/NC composite electrode delivered a discharge specific capacity of 109.2 mAh/g for the first cycle, with 95.4% capacity retention over 100 cycles. For the pristine NVP/C electrode, the first discharge capacity was 106.7 mAh/g and after 100 cycles the capacity had faded to 93.3% of its initial value. The coulombic efficiency reaches almost 100% throughout the whole cycling, manifesting the good reversibility of the NVP/C/NC composite electrode. It also should be noted that the capacity contribution from the carbon is negligible in the potential range measured. Therefore, the specific capacity was estimated based on the mass of the electroactive NVP. Fig. 6c presents the rate capability of NVP/C and NVP/C/NC composite from 0.2C to 20C. Obviously, the NVP/C/NC composite exhibits higher average reversible capacity of 109.8, 107.1, 104.3, 101.5, 97.2, 93.3, and 87.2 mAh/g at current rates of 0.2, 0.5, 1, 2, 5, 10, and 20C, respectively, much higher than that of NVP/C at the same rate. When the current rate returns to 0.2C again, the specific capacity of 107.7 mAh/g (91.6% of its theoretical capacity) could be recovered, indicating the excellent rate reversibility of the NVP/C/NC composite electrode. The superior rate performance can be attributed to the fast electronic transport network of the nitrogen-doped carbon layer. The corresponding charge/discharge profiles of NVP/C/NC and NVP/C composite electrode from 0.2C to 20C are shown in Fig. 6d and e, respectively. In Fig. 6d, all the charge/discharge plateaus can be clearly observed at low current rates. Even though the plateaus become vague, the NVP/C/NC composite electrode can still exhibit large reversible capacities when the current rate increased to 10C or 20C. In addition, the relatively small gap between the two potential plateaus at different rates implies the lower polarization of NVP/C/NC compared to that of pristine NVP/C (Table S1). The long-term cycling performance of NVP/C and NVP/C/NC composite electrodes at 2C is displayed in Fig. 6f. The NVP/C and NVP/ C/NC composite cathodes deliver the initial reversible capacity of 82.9 and 113.6 mAh/g, respectively. After 500 cycles, the capacity retention of the NVP/C/NC composite electrode keeps at 91.2%, which is higher than that of pristine NVP/C electrode (70%). Both the coulombic efficiencies of the two compounds approach 100% in the whole charge/ discharge process, demonstrating the excellent reversibility of the NVP electrode material. The good cycling capability may be due to the open NASICON structure and/or the nitrogen-doped carbon coating layer. The kinetics processes of the two electrodes were studied by electrochemical impedance spectroscopy (EIS) measurements. Fig. 7a exhibits the Nyquist plots and the corresponding equivalent circuit model of the NVP/C and NVP/C/NC electrodes. All curves are composed of a semicircle in the high frequency and a slanted line in the low frequency. The charge transfer impedance (Rct) of NVP/C/NC is 291.93 Ω, much smaller than that of NVP/C (719.58 Ω), implying that carbon coating is beneficial for the fast ions/electrons transport and results in lower polarization of NVP/C/NC. Furthermore, the sodium ion diffusion coefficients of the electrodes were calculated using the following equations [42–44]:
D=
(3)
wherein Ip is the peak current, n stands for the number of electrons involved in the half-reaction for the redox couple, A is the contact area between the electrode and electrolyte, D denotes the Na+ ion diffusion coefficient, C is the concentration of Na+ in a solid, and v is the voltage scanning rate. From the slope of the lines calculated on the basis of the Eq. (3), the diffusion coefficient Danodic (positive) = 3.47 × 10−11 cm2 s−1, and the diffusion coefficient Dcathodic (negative) = 2.47 × 10−11 cm2 s−1 were determined. The relative value is in accordance with the sodium ion coefficient achieved from EIS measurement. Furthermore, the DNa+ of NVP/C/NC composite is in good agreement with the best cycle stability and rate performance.
4. Conclusion In summary, polydopamine-derived nitrogen-doped carbon coating NVP/C/NC cathode material has been synthesized by a simple sol-gel and post-sintering process. The obtained NVP/C/NC composite exhibits high specific capacity, good cycling stability and superior rate performance, which are ascribed to the expanding contact area between active material and electrolyte and enhanced electronic conductivity of the in situ generated nitrogen-doped carbon coating. The relatively uniform and stable coated nitrogen-doped carbon layer restrains the volume change and agglomeration of the electrode during cycling. Therefore, this polydopamine-derived nitrogen-doped carbon coating approach developed here represents a promising technique to improve the electrochemical properties of NVP or to design new electrode materials.
Acknowledgments
R2T 2 2A2 n2F 4C 2σ 2
Z ′ = R s + R ct +
Ip = 2.69 × 105n3/2AD1/2 Cv1/2
(1)
σω−1/2
This work was financially supported by the National Natural Science Foundation of China (Nos. 51674068, 51774002, 51771046, 51874079, 51871046, 51804035), Natural Science Foundation of Hebei Province (No. E2018501091), the Science and Technology Project of Hebei Province (No. 15271302D), the Training Foundation for Scientific Research of Talents Project, Hebei Province (No. A2016005004), the Fundamental Research Funds for the Central Universities (No. N172302001). The authors would like to thank Chen Weiwei from Shiyanjia Lab (www.shiyanjia.com) for the XPS analysis.
(2) −1
−1
where R is the gas constant (8.314 J mol K ), T is the absolute temperature (298.15 K), A denotes the surface area of the electrode, n stands for the number per molecule during oxidization, F is the Faraday constant (96,485C mol−1), C is the concentration of sodium ions, and σ is the Warburg impedance factor which is related to Z' and ω. Fig. 7b displays the relationship between Z' and the reciprocal square root of frequency (ω−1/2) in the low frequency zone. Based on Eqs. (1) and (2), 1165
Applied Surface Science 487 (2019) 1159–1166
H.-b. Huang, et al.
Appendix A. Supplementary data [25]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.05.224.
[26]
References
[27]
[1] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. [2] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [3] S.-W. Kim, D.-H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries, Adv. Energy Mater. 2 (2012) 710–721. [4] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma, G. Ceder, Voltage, stability and diffusion barrier differences between sodium-ion and lithiumion intercalation materials, Energy Environ. Sci. 4 (2011) 3680–3688. [5] Y. Fang, L. Xiao, X. Ai, Y. Cao, H. Yang, Hierarchical carbon framework wrapped Na3V2(PO4)3 as a superior high-rate and extended lifespan cathode for sodium-ion batteries, Adv. Mater. 27 (2015) 5895–5900. [6] D. Buchholz, A. Moretti, R. Kloepsch, S. Nowak, V. Siozios, M. Winter, S. Passerini, Toward Na-ion batteries—synthesis and characterization of a novel high capacity Na ion intercalation material, Chem. Mater. 25 (2013) 142–148. [7] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-ion batteries, Adv. Funct. Mater. 23 (2013) 947–958. [8] W. Olszewski, M.A. Perez, C. Marini, E. Paris, X.F. Wang, T. Iwao, M. Okubo, A. Yamada, T. Mizokawa, N.L. Saini, L. Sirnonelli, Temperature dependent local structure of NaxCoO2 cathode material for rechargeable sodium-ion batteries, J. Phys. Chem. C 120 (2016) 4227–4232. [9] D. Su, C. Wang, H.J. Ahn, G. Wang, Single crystalline Na0.7MnO2 nanoplates as cathode materials for sodium-ion batteries with enhanced performance, Chem. Eur. J. 19 (2013) 10884–10889. [10] X. Ma, H. Chen, G. Ceder, Electrochemical properties of monoclinic NaMnO2, J. Electrochem. Soc. 158 (2011) A1307–A1312. [11] L. Wu, S. Shi, X. Zhang, Y. Yang, J. Liu, S. Tang, S. Zhong, Room-temperature prereduction of spinning solution for the synthesis of Na3V2(PO4)3/C nanofibers as high-performance cathode materials for Na-ion batteries, Electrochim. Acta 274 (2018) 233–241. [12] Y. Fang, Q. Liu, L. Xiao, X. Ai, H. Yang, Y. Cao, High-performance olivine NaFePO4 microsphere cathode synthesized by aqueous electrochemical displacement method for sodium ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 17977–17984. [13] R. Tripathi, S.M. Wood, M.S. Islam, L.F. Nazar, Na-ion mobility in layered Na2FePO4F and olivine Na[Fe,Mn]PO4, Energy Environ. Sci. 6 (2013) 2257–2264. [14] Z. Jian, W. Han, X. Lu, H. Yang, Y.-S. Hu, J. Zhou, Z. Zhou, J. Li, W. Chen, D. Chen, L. Chen, Superior electrochemical performance and storage mechanism of Na3V2(PO4)3 cathode for room-temperature sodium-ion batteries, Adv. Energy Mater. 3 (2013) 156–160. [15] J. Li, X. Cao, A. Pan, Y. Zhao, H. Yang, G. Cao, S. Liang, Nanoflake-assembled threedimensional Na3V2(PO4)3/C cathode for high performance sodium ion batteries, Chem. Eng. J. 335 (2018) 301–308. [16] F. Xiong, S. Tan, Q. Wei, G. Zhang, J. Sheng, Q. An, L. Mai, Three-dimensional graphene frameworks wrapped Li3V2(PO4)3 with reversible topotactic sodium-ion storage, Nano Energy 32 (2017) 347–352. [17] W. Duan, Z. Hu, K. Zhang, F. Cheng, Z. Tao, J. Chen, Li3V2(PO4)3@C core-shell nanocomposite as a superior cathode material for lithium-ion batteries, Nanoscale 5 (2013) 6485–6490. [18] Z.-X. Chi, W. Zhang, X.-S. Wang, F.-Q. Cheng, J.-T. Chen, A.-M. Cao, L.-J. Wan, Accurate surface control of core–shell structured LiMn0.5Fe0.5PO4@C for improved battery performance, J. Mater. Chem. A 2 (2014) 17359–17365. [19] G. Ali, J.-H. Lee, D. Susanto, S.-W. Choi, B.W. Cho, K.-W. Nam, K.Y. Chung, Polythiophene-wrapped olivine NaFePO4 as a cathode for Na-ion batteries, ACS Appl. Mater. Interfaces 8 (2016) 15422–15429. [20] S.H. Luo, J.Y. Li, S. Bao, Y.Y. Liu, Z.Y. Wang, Na3V2(PO4)3/C composite prepared by sol-gel method as cathode for sodium ion batteries, J. Electrochem. Soc. 165 (2018) A1460–A1465. [21] X. Rui, W. Sun, C. Wu, Y. Yu, Q. Yan, An advanced sodium-ion battery composed of carbon coated Na3V2(PO4)3 in a porous graphene network, Adv. Mater. 27 (2015) 6670–6676. [22] Z. Jian, L. Zhao, H. Pan, Y.-S. Hu, H. Li, W. Chen, L. Chen, Carbon coated Na3V2(PO4)3 as novel electrode material for sodium ion batteries, Electrochem. Commun. 14 (2012) 86–89. [23] Q. Wang, B. Zhao, S. Zhang, X. Gao, C. Deng, Superior sodium intercalation of honeycomb-structured hierarchical porous Na3V2(PO4)3/C microballs prepared by a facile one-pot synthesis, J. Mater. Chem. A 3 (2015) 7732–7740. [24] L. Chen, Y. Zhao, S. Liu, L. Zhao, Hard carbon wrapped Na3V2(PO4)3@C porous
[28] [29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
1166
composite extending cycling lifespan for sodium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 44485–44493. Z. Chu, C. Yue, Core–shell structured Na3V2(PO4)3/C nanocrystals embedded in multi-walled carbon nanotubes: a high-performance cathode for sodium-ion batteries, Solid State Ionics 287 (2016) 36–41. B. Guo, Q. Liu, E. Chen, H. Zhu, L. Fang, J.R. Gong, Controllable N-doping of graphene, Nano Lett. 10 (2010) 4975–4980. M. Li, Z. Zuo, J. Deng, Q. Yao, Z. Wang, H. Zhou, W.-B. Luo, H.-K. Liu, S.-X. Dou, A high rate capability and long lifespan symmetric sodium-ion battery system based on a bipolar material Na2LiV2(PO4)3/C, J. Mater. Chem. A 6 (2018) 9962–9970. Z. Chu, C. Yue, Graphene oxide wrapped Na3V2(PO4)3/C nanocomposite as superior cathode material for sodium-ion batteries, Ceram. Int. 42 (2016) 820–827. T.-F. Hung, W.-J. Cheng, W.-S. Chang, C.-C. Yang, C.-C. Shen, Y.-L. Kuo, Ascorbic acid-assisted synthesis of mesoporous sodium vanadium phosphate nanoparticles with highly sp2-coordinated carbon coatings as efficient cathode materials for rechargeable sodium-ion batteries, Chem. Eur. J. 22 (2016) 10620–10626. M.-X. Jing, J. Zhang, C. Han, H. Yang, S.-S. Yao, L. Zhu, L.-L. Chen, Q.-L. Xie, X. Chen, X.-Q. Shen, S.-B. Qin, A flexible Na3V2(PO4)3/C composite fiber membrane cathode for Na-ion and Na-Li hybrid-ion batteries, J. Electrochem. Soc. 165 (2018) A1761–A1769. L.-L. Zhang, D. Ma, T. Li, J. Liu, X.-K. Ding, Y.-H. Huang, X.-L. Yang, Polydopaminederived nitrogen-doped carbon-covered Na3V2(PO4)2F3 cathode material for highperformance Na-ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 36851–36859. X. Liu, Y. Wang, Z. Wang, T. Zhou, M. Yu, L. Xiu, J. Qiu, Achieving ultralong life sodium storage in amorphous cobalt-tin binary sulfide nanoboxes sheathed in Ndoped carbon, J. Mater. Chem. A 5 (2017) 10398–10405. F. Yang, Z. Zhang, K. Du, X. Zhao, W. Chen, Y. Lai, J. Li, Dopamine derived nitrogen-doped carbon sheets as anode materials for high-performance sodium ion batteries, Carbon 91 (2015) 88–95. L.-L. Zheng, Y. Xue, L. Deng, G.-R. Wu, S.-E. Hao, Z.-B. Wang, Na3V2(PO4)3 with specially designed carbon framework as high performance cathode for sodium-ion batteries, Ceram. Int. 45 (2019) 4637–4644. C. Miao, P. Bai, Q. Jiang, S. Sun, X. Wang, A novel synthesis and characterization of LiFePO4 and LiFePO4/C as a cathode material for lithium-ion battery, J. Power Sources 246 (2014) 232–238. W. Duan, Z. Zhu, H. Li, Z. Hu, K. Zhang, F. Cheng, J. Chen, Na3V2(PO4)3@C core–shell nanocomposites for rechargeable sodium-ion batteries, J. Mater. Chem. A 2 (2014) 8668–8675. W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Selfsacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for highrate sodium-ion full batteries, Nano Energy 25 (2016) 145–153. J.-Z. Guo, X.-L. Wu, F. Wan, J. Wang, X.-H. Zhang, R.-S. Wang, A superior Na3V2(PO4)3-based nanocornposite enhanced by both N-doped coating carbon and graphene as the cathode for sodium-ion batteries, Chem. Eur. J. 21 (2015) 17371–17378. K. Saravanan, C.W. Mason, A. Rudola, K.H. Wong, P. Balaya, The first report on excellent cycling stability and superior rate capability of Na3V2(PO4)3 for Sodium Ion Batteries, Adv. Energy Mater. 3 (2013) 444–450. L.-L. Zheng, Y. Xue, S.-E. Hao, Z.-B. Wang, Porous Na3V2(PO4)3 prepared by freezedrying method as high performance cathode for sodium-ion batteries, Ceram. Int. 44 (2018) 9880–9886. Y. Jiang, Z. Yang, W. Li, L. Zeng, F. Pan, M. Wang, X. Wei, G. Hu, L. Gu, Y. Yu, Nanoconfined carbon-coated Na3V2(PO4)3 particles in mesoporous carbon enabling ultralong cycle life for sodium-ion batteries, Adv. Energy Mater. 5 (2015) 1402104. Y. Chen, Y. Xu, X. Sun, B. Zhang, S. He, C. Wang, F-doping and V-defect synergetic effects on Na3V2(PO4)3/C composite: a promising cathode with high ionic conductivity for sodium ion batteries, J. Power Sources 397 (2018) 307–317. H. Chen, B. Zhang, X. Wang, P. Dong, H. Tong, J.-C. Zheng, W. Yu, J. Zhang, CNTdecorated Na3V2(PO4)3 microspheres as a high-rate and cycle-stable cathode material for sodium ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 3590–3595. L. Deng, G. Sun, K. Goh, L.-L. Zheng, F.-D. Yu, X.-L. Sui, L. Zhao, Z.-B. Wang, Facile one-step carbothermal reduction synthesis of Na3V2(PO4)2F3/C serving as cathode for sodium ion batteries, Electrochim. Acta 298 (2019) 459–467. Z. Jian, C. Yuan, W. Han, X. Lu, L. Gu, X. Xi, Y.-S. Hu, H. Li, W. Chen, D. Chen, Y. Ikuhara, L. Chen, Atomic structure and kinetics of NASICON NaxV2(PO4)3 cathode for sodium-ion batteries, Adv. Funct. Mater. 24 (2014) 4265–4272. K. Kretschmer, B. Sun, J. Zhang, X. Xie, H. Liu, G. Wang, 3D interconnected carbon fiber network-enabled ultralong life Na3V2(PO4)3@carbon paper cathode for sodium-ion batteries, Small 13 (2017) 1603318. Y.Q. Qiao, X.L. Wang, J.Y. Xiang, D. Zhang, W.L. Liu, J.P. Tu, Electrochemical performance of Li3V2(PO4)3/C cathode materials using stearic acid as a carbon source, Electrochim. Acta 56 (2011) 2269–2275. T. Jiang, W. Pan, J. Wang, X. Bie, F. Du, Y. Wei, C. Wang, G. Chen, Carbon coated Li3V2(PO4)3 cathode material prepared by a PVA assisted sol–gel method, Electrochim. Acta 55 (2010) 3864–3869.