C structure with high rate capability and cycling stability for sodium-ion batteries

Chemical Engineering Journal 353 (2018) 264–272

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Three-dimensional hierarchical porous Na3V2(PO4)3/C structure with high rate capability and cycling stability for sodium-ion batteries

T

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Rui Linga, Shu Caia, , Dongli Xiea, Xin Lia, Mingjing Wanga, Yishu Lina, Song Jianga, Kaier Shena, ⁎⁎ Kunzhou Xionga, Xiaohong Suna,b, a b

Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, People’s Republic of China Department of Chemistry & Biochemistry University of California, Santa Barbara, CA 93106, USA

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

hierarchical porous Na V (PO ) / • C3Dstructure is constituted by hydro3 2

4 3

thermal method.

carbon layers capped on na• In-situ nosheets assemble into 3D conductive skeleton.

hierarchical porous structure en• The sures the high ionic and electron conductivities.

delicate structure exhibits ex• The cellent rate performance and cycling performance.

A R T I C LE I N FO

A B S T R A C T

Keywords: Na3V2(PO4)3/C Hierarchical porous structure Carbon coating Sodium-ion batteries

Na3V2(PO4)3 (NVP) with an open NASICON structure has drawn worldwide attention as a potential cathode material for sodium-ion batteries (SIBs) owing to its high theoretical capacity. However, the inherently poor electronic conductivity of NVP severely restricts its electrochemical performance, particularly for rate capability and long cycle performance. Herein, high performance NVP/C cathode (denoted as NVP/C-T) is demonstrated by designing and synthesizing three-dimensional (3D) hierarchical porous NVP architecture via a facile hydrothermal technique. In this hierarchical porous structure, ultrathin NVP nanosheets capped with in-situ carbon layers are interlinked to form hierarchical pores, convenient nanochannels and 3D conductive carbon framework. This delicate structure not only provides adequate void for the intimate contact between electrode/ electrolyte, shortens ionic diffusion distances, ensures the ultrafast electron transfer but also strengthens the structural stability of electrode material. The as-prepared NVP/C-T cathode exhibits a high reversible initial capacity (114.8 mAh g−1 at 1 C approaching the theoretical capacity), excellent rate performance (89.3 mAh g−1 at 60 C and 73.2 mAh g−1 at 80 C) and long life span (93.3 mAh g−1 after 8000 cycles at 20 C). In addition, the electrochemical properties of symmetric full cell constructed with NVP/C-T/ NVP/C-T are also studied and high initial charge capacity (101.8 mAh g−1 at 0.25 C) and high stability (70.1 mAh g−1 at 2 C after 200 cycles) are achieved. Significantly, the design of the 3D hierarchical porous nanocrystal@C strategy and scalable synthesis method may pave a way to develop high performance SIBs.

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Corresponding author. Co-corresponding author: Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, People’s Republic of China. E-mail addresses: [email protected] (S. Cai), [email protected] (X. Sun). ⁎⁎

https://doi.org/10.1016/j.cej.2018.07.118 Received 11 May 2018; Received in revised form 2 July 2018; Accepted 17 July 2018 Available online 18 July 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.

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1. Introduction

layers capped on nanosheets assemble into 3D conductive skeleton, which can hinder particle growth and limit grain size during the synthesis process as well as improve the electronic conductivity for its homogeneous distribution. The ultrathin nanosheets with hierarchical porous combined with 3D conductive carbon framework not only provides adequate void for the intimate contact between electrode/electrolyte, shortens ionic diffusion distance, ensures the ultrafast electron transfer but also strengthens the structural stability of electrode material. Therefore, the 3D hierarchical porous NVP/C-T composite as cathode for SIBs exhibits outstanding Na+ intercalation/de-intercalation capacities in both the half-cells and full cells, especially for long cycle performance and significant rate performance. The relationships between the deliberate 3D hierarchical porous structure and electrochemical performances are also studied.

Among various potential energy storage technologies, batteries system is the most promising candidate with high energy efficiency for energy storage systems (ESSs) [1]. Sodium-ion batteries on the basis of its nature abundant resources, low price and safety of sodium as compared with lithium have drawn growing research attention in recent years [2]. Nevertheless, the heavier weight and larger ionic radius of sodium ion make fewer possible materials can be used as host for sodium ions de/intercalation compared with Li ion batteries [3,4]. So far, metal oxides, phosphates, sulfates, fluorophosphates, and ferrocyanides have been investigated widely as sodium hosts [5–9]. Among the polyanion-based materials for cathodes, NVP has attracted significant interest as its open NASICON framework, moderate operating potential of about 3.4 V (vs. Na+/Na), negligible volume expansion (8%), an impressive reversible gravimetric specific energy density of 400 Wh kg−1 [10,11]. However, the intrinsic low electronic conductivity of NVP severely restricts its electrochemical behavior, particularly for rate performance and long-term stability [12,13]. Up to now, many strategies including doping with alien ions, preparing nanostructured materials, coating with conductive medium, and optimizing the morphology, have been dedicated to solve the problem of poor electronic conductivity [14–17]. Duan et al. synthesized NVP@C nanocomposite particles with the size of ca. 40 nm via hydrothermal assisted sol-gel method [18]. Benefiting from small particle dimension and homogeneous carbon layer, the resulted NVP@C showed an improved rate capability (94.9 mAh g−1 at 5 C) and cycling ability (96.1% of capacity retention over 700 cycles at 5 C). Hu et al. synthesized graphene-decorated NVP with a primary grain dimension of 50–200 nm via in-situ catalytic process [19]. Due to the graphene modification on the nano-NVP particles, NVP@G composite exhibited long life span (95.8% of capacity retention over 700 cycles at 5 C) as well as excellent high rate performance (110.7 mAh g−1 at 20 C). Notwithstanding all these efforts, it is a great challenge to obtain electrode material with excellent conductivity and super electrochemical performance over continuous discharge/charge cycles. For one thing, the 2D nanoscale active materials tend to easy severely agglomeration leading to poor cycling stability during long-term employment. For another, traditional carbon-coating technology usually leads to incomplete carbon layers in the process of high temperature calcination, resulting in large polarization, which can affect the electrochemical properties of NVP, particularly for rate capability. Three-dimensional (3D) hierarchical porous morphology combined with carbon layers, are recognized as excellent matrices in virtue of their advantages such as effective electrode-electrolyte contact area and fast electronic and Na+ ion transfer. In addition, 3D conductive framework offers convenient electron pathways for rapid electron transfer and buffer the volume variation of active materials during Na+ ions insertion/deinsertion process, which can improve the structural stability and cyclability of the electrode. For example, Zhou et al. have prepared 3D flower-like architecture Na2Ti3O7 with remarkable rate capability and long cycling stability [20]. They ascribed the superior electrochemical properties to the special architecture, which can facilitate the insertion/deinsertion reactions of sodium ions and conducive the transfer of electrons from all directions. Xu et al. have synthesized carbon-coated hierarchical mesoporous microflower structure to optimize the electrochemical properties of NaTi2(PO4)3. As expected, it exhibited a superior specific capacity of 125 mAh g−1 than carboncoated NaTi2(PO4)3 particles with 2D agglomerate morphology of 86 mAh g−1 at 1 C as anodes for SIBs [21]. However, due to the structure complexity of cathode material, the 3D hierarchical porous morphology is rarely reported in cathode materials for SIBs. Targeting the aforementioned issues, for the first time, an innovative design of 3D hierarchical porous (co-existence of micropores, mesopores and macropores) NVP with in-situ carbon layers (denoted as NVP/C-T) is prepared by a simple hydrothermal strategy. In-situ carbon

2. Experimental section 2.1. Chemicals and materials All chemicals with analytical grade were used as purchased directly without further purification. V precursor (V2O5, 99%), Na source (Na2CO3 99%), P precursor (NH4H2PO4, 99%) Sodium Dodecyl Sulfonate (SDS) and polyethylene glycol 4000 (PEG-4000) were purchased from Kermel Chemicals Co. Ltd. Ultrapure deionized water was used for preparation of all aqueous solutions. 2.2. Materials synthesis Synthesis of NVP cathode material. The NVP/C-T sample was prepared by means of a hydrothermal method. First, 4 mmol V2O5, 12 mmol NH4H2PO4 and 6 mmol Na2CO3 were mixed in 70 mL of distilled water. After vigorously stirring for 20 min, a dark yellow solution was obtained. Afterward, 5 mg of PEG-4000 and 5 mmol SDS were dissolved into the aforementioned solution and further stirred for 20 min. Finally, the consequent suspension as feedstock was decanted into a 100 mL reaction autoclave and kept at 180 °C for 24 h. The products were ultrasonically treated and then dried in water bath with slow stirring at 95 °C to evaporate water. This obtained precursor was milled into power and pre-heated at 350 °C in Ar for 4 h. Then, the preheated sample was milled again to powders and subsequently calcined at 750 °C for 6 h in Ar. For the sake of contrast, the Na3V2(PO4)3/ C composite (noted as NTP/C-P) was prepared by similar method without adding SDS. 2.3. Materials characterization The X-ray diffraction data of the as-synthesized powers were recorded from using a D8 Advance X-ray diffractometer. Raman spectra for NVP/C-P and NVP/C-T were determined by using DXR Microscope with a visible laser (λ = 532 nm) at ambient temperature. The X-ray photoelectron spectroscopy (XPS) characterizations were implemented on a PHI5000 Versaprobe. Thermogravimetric analysis (TG, NETZSCH, STA 499C) were performed at a heating rate of 10 °C/min up to 700 °C in air. The morphology characteristics and particle dimension of the samples were characterized with a field-emission scanning electron microscopy (SEM, Hitachi, S4800). The TEM and HRTEM images and EDX images of the samples were obtained using a transmission electron microscopy (TEM, FEI Tecnai G2 F20). The Brunauer-Emmett-Teller (BET) surface areas and pore size distributions of NVP/C-P and NVP/CT were measured by nitrogen adsorption isotherms at 77 K using a Nova 2200e instrument (Quantachrome) with 12 h outgas at 150 °C. 2.4. Electrode fabrication and electrochemical measurements In order to investigate the electrochemical property of the prepared samples, CR2032 coin cells were assembled in a glove box filled with Ar 265

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produced during the assembly process. Finally, NVP nanocrystals start to form via in-situ crystallization and are coated with uniform carbon layers formed by the in-situ pyrolysis of SDS and PEG4000 simultaneously during the heat-treatment process. Meanwhile, the unsaturated bonds and long alkyl chains in SDS facilitate the crystallization of the NVP nanocrystals, similar phenomenon are also reported by Li et al. and Wang et al. [22,23]. The morphology characteristics and grain dimension of NVP/C-T and NVP/C-P composite are first investigated by SEM. The NVP/C-P composite exhibits rough surface and numerous non-uniform grains with the diameter range from 30 nm to 50 nm (Fig. 2a–c), indicating highly aggregated nanoparticles. The highly aggregated nanoparticles are unfavorable for electrolyte wettability and Na+ ion insertion/extraction process [24,25]. Fig. 2d shows a panoramic view of NVP/C-T composite, in which one can see that the sample present a 3D hierarchical architecture with loose surface. By careful observation, a large number of interconnected nanosheets with plenty of openings exist in NVP/C-T composite. A locally-enlarged SEM images in Fig. 2e–f demonstrate that the nanosheets arrays are loosely interconnected with obvious openings. The uniform nanosheets have smooth surface with an approximately thickness of 10–20 nm. The detailed architecture morphology features and structure of NVP/C-T are further surveyed by TEM and HRTEM. The TEM images (Fig. 2g) further reveal that the 3D architecture is actually composed of numerous ultrathin nanosheets with pores. Clear lattice fringes with an interplanar distance of 0.622 nm in Fig. 2h are observed matching well with the (0 1 2) plane of (rhombohedral structure) NVP, which verifies single-crystal nature and good crystallinity of the skeleton [26,27]. Meanwhile, a homogeneous carbon layer with the thickness ca.5 nm is clearly observed on the sample surface, which is produced from in-situ carbonation of organics. The homogeneous carbon layers not only beneficial for electronic transport but also helpful to protect the surface of NVP from electrolyte corrosion. Additionally, based on the energy dispersive X-ray (EDX) mapping results, C, V, and P elements are distributed uniformly as seen in Fig. 2i–l, further demonstrating a well-dispersed distribution of carbon on the surface of NVP. XRD patterns of both NVP/C-P and NVP/C-T samples are displayed in Fig. 3a. All diffraction peaks of NVP/C-P and NVP/C-T are matched well with a rhombohedral R-3c space group as inset of Fig. 3a (JCPDS

atmosphere The active electrodes were prepared by mixing a slurry of 80 wt% active material, 10 wt% acetylene black as electronic conducting additive and 10 wt% polyvinylidene fluoride (PVDF) as binder. They were dissolved in N-methyl-2-pyrrolidone and stirring vigorously. The homogenized slurry was further cast on Al foil and dried at 120 °C for 6 h to evaporate N-methyl-2-pyrrolidone and cut into electrode slices (diameter, 12 mm) with each coin cell was approximately 1.5–2.0 mg cm−2, subsequently. The cells were assembled in a glove box, where metallic sodium foil and NVP were used as anode and cathode for half cells, respectively. NVP/C-T was used as both cathode and anode for full cell (the weight ratio of the cathode and the anode was about 1:1.2). Glass fiber (GF/F-Whatman) was used as the separator, and 1 M NaPF6 in ethylene carbonate (EC)/dimethyl carbonate (DEC) (1:1, v/v) with 2 vol% fluoroethylene carbonate (FEC) as an additive was used as the electrolyte for both half cell and full cell. The assembled cells were tested using a LAND CT2001A battery test system (Wuhan, China). Electrochemical tests voltage was in the range of 2.5–4.0 V and 1.0–2.2 V for half cell and full cell, respectively. Cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS) spectra were carried out in a frequency range from 100 kHz to 0.1 Hz by using a CHI660E electrochemical working station. All the electrochemical measurements were performed at ambient temperature. All the capacities of cells have been calculated on the basis of the weight of the active materials in the form of NVP/C-P and NVP/C-T.

3. Results and discussion The fabrication procedure of the as-designed NVP/C-T with 3D hierarchical porous structure is presented schematically in Fig. 1. Firstly, NVP precursor and SDS employed simultaneously as a part of carbon source and surfactant to construct the hierarchical porous structure are mixed and fully stirred. The hydrophobic ends in SDS molecules are evenly and closely anchored on the interfaces of the NVP precursors with the hydrophilic group pointing to the outer reaction media. Then, these nanoparticles with SDS on the surfaces aggregate and self-assemble into nanosheets during the hydrothermal process. These nanosheets further spontaneously aggregate and assemble together to reduce the total surface energy. Multi-layered interleaved NVP/C-T nanosheets with a large number of pores are repeatedly

Fig. 1. Schematic of the fabrication procedure of NVP/C-T. 266

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Fig. 2. SEM images of (a–c) NVP/C-P. (d–f) NVP/C-T. (g) TEM images of NVP/C-T. (h) HRTEM images of NVP/C-T and (i–n) Elemental mappings of NVP/C-T.

Fig. 3b, both spectra show two typical scattering vibrational modes at about 1334 cm−1 (D-band, disordered carbon) and 1584 cm−1 (Gband, graphene carbon), thus confirming the coexistence of amorphous carbon and graphitic carbon [33,34]. According to the calculated results, the ID/IG values for NVP/C-T and NVP/C-P are 0.92 and 0.95, respectively, demonstrating the higher amount of sp2-type carbon than sp3-type one for both samples [35,36]. It implies that the carbon capped on the surface of the two samples is partially graphitized. Undoubtedly, NVP/C-T sample exhibits lower ID/IG value than that of NVP/C-P,

No. 53-0018) without any impurity phases, implying that the addition of SDS has no influence on the crystal patterns of NVP [28,29,30]. The lower peak intensity indicates the lower crystallinity of NVP/C-T sample, which attributed to its high carbon content and porous structure, similar to the phenomenon reported in Wang’s work [23]. Furthermore, because of the amorphous phase or low carbon content that out of the detection range of carbon, no significant diffraction peaks of carbon are discovered in NVP/C-P and NVP/C-T [31,32]. Raman spectra are performed to confirm the existence of carbon. As revealed in

Fig. 3. (a) XRD patterns of NVP/C-T and NVP/C-P samples. (b) Raman spectra of NVP/C-T and NVP/C-P. (c–d) X-ray photoelectron spectroscopy (XPS) measurements of NVP/C-T. (e) N2 adsorption/desorption isotherms and (f) Pore size distribution curves of NVP/C-T and NVP/C-P. 267

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sample remains the average discharge/charge coulombic efficiency (CE) at around 100% during cycling, indicating stable electrochemical property. While, NVP/C-P cathode suffers from a decay to 92.8 mAh g−1 after 500 cycles together with delivering unstable CE. The corresponding voltage profiles of NVP/C-T and NVP/C-P at different rates from 1 to 80 C are observed in Fig. S3. Obviously, as current rate increases, the voltage gap of the NVP/C-T electrode expands gradually. However, it still displays an obvious voltage platform at each current density. The distinct voltage platform and small plateau voltage gap at different current density testify low polarization of NVP/C-T electrode. Although at a high rate of 80 C, a flat voltage platform of the discharge-charge curves also can be clearly seen, demonstrating excellent dynamic electrochemical stability. In contrast with NVP/C-T, voltage profiles of NVP/C-P show a greater polarization and no apparent discharge-charge platforms can be clearly observed only above 20 C, further confirming the superiority of NVP/C-T. The rate performances of NVP/C-T and NVP/C-P are displayed in Fig. 4c. Evidently, the NVP/C-T electrode delivers much higher specific capacity than NVP/C-P at each rate. At the rates of 1, 2, 5, 10, 20, 40 and 60, NVP/C-T electrode exhibit discharge capacities of 114.6, 113.8, 112.1, 111.0, 105.8, 99.6 and 89.3 mAh g−1, respectively. Even though the current density is as high as 80 C, NVP/C-T electrode still maintains a high specific capacity of 73.2 mAh g−1. When the rate returns to 1 C after 80 cycles, a reversible capacity of 113.6 mAh g−1 can be obtained, manifesting the excellent reversibility of Na+ insertion/desertion in NVP/C-T electrode. By contrast, smaller specific surface area of the composite will undoubtedly restrict the penetration of electrolyte into the electrode material, leading to a longer Na+ ion transfer path and lower rate performance. Therefore, NVP/C-P electrode exhibits an inferior initial reversible capacity of 100.2 mAh g−1 at 1 C, followed by a rapid decrease as the rate increased from 2 to 80 C. And it only displays a negligible reversible capacity of about 0.5 mAh g−1 at 80 C. Fig. 4d shows the comparison of rate capability of NVP/C-T and other previously reported NVP-based composites. It is indicate that, the reversible capacities of the NVP/C-T electrode at both low current density and high current density are superior to those of most previously reported results [18,42,47–53]. Such great electrochemical performances suggest that the 3D hierarchical porous NVP/C-T nanocomposite has great potential in application as sodium energy storage. The cycle performances of the NVP/C-P and NVP/C-T electrodes at the rate of 10 C are also tested as illustrated in Fig. S4. The NVP/C-T electrode expresses an initial discharge capacity of 109.1 mAh g−1, and after 1000 cycles, 106.5 mAh g−1 is maintained, corresponding to 97.7% of initial capacity. For comparison, the initial discharge capacity of the NVP/C-P is 87.1 mAh g−1 and only 75.4% of the initial capacity is maintained after 1000 cycles. Additionally, ultralong cycling performance is investigated at a higher rate of 20 C for 8000 cycles for the NVP/C-T cathode (Fig. 4e). It exhibits an initial reversible capacity of 104.6 mAh g−1 at 20 C, and still retains 93.3 mAh g−1 after 8000 cycles with a capacity retention of 89.1%, while the slight capacity loss in the first few cycles is probably caused by the decomposition of electrolyte [38,39]. The average CE keeps at around 99.8% throughout cycling, demonstrating the outstanding electrochemical reversibility of NVP/CT electrode. In addition, the ultralong cycling ability of NVP/C-T is better than most of those previously reported researches on NVP-based systems (see details in Fig. 4f and Table S1) [17,18,32,42,53–58]. The outstanding long-cycling performances of NVP/C-T are ascribed to the stable hierarchical porous structure and 3D highly-conductive carbon framework, which not only provide adequate void for the intimate contact between electrode/electrolyte, shorten ionic diffusion distances, ensure the ultrafast electron transfer but also strengthen the structural stability of electrode material. In order to inquiry the different reaction kinetics of NTP/C-T and NTP/C-P electrodes, AC impedance measurements are conducted and the typical EIS responses of the samples are shown in Fig. 4(g–h) and Fig. S5. The depressed semicircle in high-frequency range and the

suggesting a higher degree of graphitization of the carbon due to the addition of SDS, which will has an affirmative effect on enhancing the electronic conductivity and facilitates fast electron transport [37]. For further examine the effects of carbon in the composition, thermo gravimetric analysis (TGA) are conducted in air atmospheres and the data of NVP/C-T and NVP/C-P are displayed in Fig. S1. The mild mass loss from 50 °C to 120 °C is attributed to the evaporation of adsorbed water. The carbon contents of NVP/C-P and NVP/C-T are calculated to be 7.15 and 9.13% as estimated from the sharp mass loss from 120 °C to 500 °C. Additionally, slight mass addition after about 550 °C is ascribed to the oxidation of V3+ to V4+ and V5+ [38,39]. For further survey the chemical components and oxidation state of vanadium in NVP/C-T, X-ray photoelectron spectroscopy (XPS) measurement is adopted as displayed in Fig. 3c. The results manifest that only C1s, O1s, Na1s, P2s, P2p, V2p3/2 and V2p1/2 peaks are observed in the XPS spectrum of NVP-T. The V2p XPS spectra (Fig. 3d) exhibit two bands at ca. 517.0 eV and 524.1 eV, corresponding to the V2p3/2 and V2p1/2 transitions, respectively. While, there is no higher binding energy peak near 519–520 eV, meaning that no V4+ or V5+ is present [11]. These results are in agreement with the unit cell parameters in previous literature reports [38,40,41]. It is further testified that the addition of SDS during the synthetic process will not influence the ultimate structure or element valence of NVP. N2 adsorption-desorption isotherm measurements are carried out to study the specific surface area and porous structure of NVP/C-T and NVP/C-P composites. As shown in Fig. 3e, the obvious hysteresis loop implies the mesoporous structure of NVP/C-T, whereas the sharp N2 uptake at a high pressure (P/P0 = 0.8–0.99) suggests the presence of macropores with a larger size in NVP/C-T [29]. The NVP/C-T composite possesses a specific surface area of 119.7 m2 g−1, which evidently exceeds that of NVP/C-P (73.2 m2 g−1). In addition, the total pore volume for NVP/C-T is 0.39 cm3 g−1, exceeding the sample of NVP/C-P (0.21 cm3 g−1) along with the values of great majority of NVP-based composites previously reported [37,42–46]. The specific pore-size distribution plots (Fig. 3f) clearly reveal that the pores of NVP/C-T can be divided into micropores (< 2 nm), mesopores (2–50 nm) and macropores (> 50 nm). Combined with the results of SEM (Fig. 2d–f) and TEM (Fig. 2g), the mesopores and macropores can be the correlated with the loosely multilayered ultrathin nanosheets of NVP/C-T. It is speculated that the micropores are ascribed to the porous carbon layer decomposed from in-situ carbonization of organics, which benefits Na+ pass through the carbon layer and react with NVP readily, similar with the phenomenon reported in Jinag’s work [2]. The above discussions manifest that a hierarchical porous architecture NVP/C-T constructed by interconnected ultrathin nanosheets with hierarchical porous and 3D conductive carbon framework has been obtained. The electrochemical behaviors of NVP/C-T and NVP/C-P are characterized in sodium half-cells. Fig. 4a exhibits initial discharge-charge profiles of NVP/C-T and NVP/C-P at 1 C in a voltage of 2.5–4.0 V. Two flat plateaus of NVP/C-T and NVP/C-P are observed at about 3.4 V, corresponding to the redox reaction of V4+/V3+. Through contrasting analysis, NVP/C-T electrode exhibits evidently longer charge–discharge plateaus and lower potential gap (35 vs. 91 mV), implying enhanced redox kinetics and decreased electrochemical polarization [39,41]. Cyclic voltammetry (CV) measurements are executed at a scan rate of 0.1 mV s−1 and the first to third typical CV curves of NVP/C-T and NVP/C-P are shown in Fig. S2(a) and Fig. S2(b), respectively. It can be seen that both NVP/C-T and NVP/C-P demonstrate similar redox peaks, suggesting analogous electrochemical behaviors. The lower voltage difference between oxidation peak and the reduction peak and the higher peak currents of NVP/C-T electrode also indicate lower polarization and faster kinetics of NVP/C-T. As expected, NVP/C-T electrode presents the initial capacity of 114.8 mAh g−1, much higher than that of NVP/C-P (105.6 mAh g−1). After 500 cycles at 1 C (Fig. 4b), NVP/C-T sample still exhibits a high discharge specific capacity of 113.1 mAh g−1. Besides the negligible capacity fading, the NVP/C-T 268

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Fig. 4. (a) The initial discharge-charge curves of NVP/C-T and NVP/C-P at 1 C. (b) Cycling performances of NVP/C-T and NVP/C-P at 1 C. (c) The rate performances of NVP/C-T and NVP/C-P. (d) Comparison of the rate capability of NVP/C-T with that of other NVP-based materials reported recently. (e) Ultralong cycling stability of the NVP/C-T for 8000 cycles at 20 C. (f) The comparison of the cycling stability of NVP/C-T in this work and previous NVP-based work in literatures. (g) Recorded impedance spectra of NTP/C-T and NTP/C-P. (h) The relationship between Z and ω−1/2 in low-frequency region.

Fig. 5. (a) Ex-situ XRD patterns and TEM images of NTP/C-T electrode before and after 8000 cycles. (b) Schematic illustration on the structure of 3D hierarchical porous NVP/C-T.

ions are calculated using the following equation [52]:

sloping line in the low-frequency range of both spectra curves are attributable to charge-transfer process and sodium ions diffusion process, respectively [47,53]. The simulation results in Table S2 indicate that the Rct of NVP/C-T (74.8 Ω) is much lower than that of NVP/C-P electrode (117.9 Ω), suggesting faster charge transfer kinetics and higher conductivity of the NVP/C-T electrode [17]. The latter straight line in the low-frequency range is associated with the Warburg impedance (Zw), which is related with sodium ions diffusion between the electrode and the electrolyte [51]. The diffusion coefficients (DNa+) of sodium

DNa + = R2T 2 2A2 n4F 4C 4d 2 Where R represents the gas constant, T represents the absolute temperature, A represents the surface area of the electrode, n represents the number of electrons per molecule during oxidization, F represents Faraday's constant and C represents the concentration of sodium ions. σ represents the Warburg factor, which is related to Z′ and ω−1/2: Fig. 4g exhibits the slope of Z′ against the reciprocal root square of frequency 269

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Fig. 6. (a) Schematic illustration of NVP/C-T//NVP/C-T full cell. (b) Galvanostatic charging-discharging profiles of NVP/C-T//NVP/C-T full cell from 0.25 C to 10 C. (c) Rate performance of NVP/C-T//NVP/C-T full cell and (d) Cyclic properties of NVP/C-T//NVP/C-T full cell at 2 C.

(ω−1/2) [48]. Notably, the calculated sodium ions diffusion coefficient value (DNa+) of NVP/C-T (1.11 × 10−11) is much larger than that of NVP/C-P (2.86 × 10−12 cm2 s−1). The AC impedance results imply that the 3D hierarchical porous structure facilitates charge transfer and sodium ions diffusion, which can account for the preferable sodium storage performance of NVP/C-T as discussed above. In order to prove the stability of the 3D hierarchical porous architecture, ex-situ XRD patterns of NTP/C-T electrode and NTP/C-P electrode after 8000 cycles are shown in Fig. 5(a) and Fig. S6. It is observed that apart from the diffraction peaks of Al, all of the remaining diffraction peaks are accurately indexed to the NASICON-type NVP structure, manifesting the excellent structural integrity after the lengthy charge/discharge processes. The TEM and SEM images as displayed in Fig. 5(a) and Fig. S7 illustrate that the 3D architecture constructed by ultrathin nanosheets with hierarchical pores maintained well even after 8000 cycles at 20 C and no aggregation or pulverization is observed, further confirming the structural stability of NVP/C-T. To further vividly explain the effect of 3D hierarchical porous structure on electrochemical performance of NVP/C-T, Fig. 5(b) presents the schematic illustration on the structure. In addition to its inherent stable and open NASICON framework, the as-prepared NVP/C-T also displays the following advantages: First, the unique ultrathin nanosheets array offers effective electrode-electrolyte contact area and prominently reduces the sodium ions diffusion pathways, which are particularly significant for the barrier-free diffusion of sodium ions in the electrode. Second, the 3D hierarchical porous structure rich in micropores, mesopores and macropores offers sufficient void for electrolyte penetration and large surface area, which efficiently guarantees the consecutive replenishment of the electrolyte and buffers large internal strain during repeated discharge/charge process to maintain the structural integrity. Last but most important, the homogeneous carbon layers build a 3D in-situ conductive framework, which promotes the electrical conductivity, limits the volume expansion and maintains the structural stability of electrode materials.

Further, the excellent sodium storage property of the NVP/C-T electrode in half cell encourages us to explore its performance in full cell (Fig. 6). Taking the fact that NVP can be reduced and oxidized with obviously different redox potentials, sodium ion full cell with the asprepared hierarchical porous NVP/C-T sample as both positive and negative electrodes (NVP/C-T//NVP/C-T) is constructed as described in Fig. 6a. The relevant electrochemical reactions are:

Anode: Na3V2 (PO4 )3 + 2Na+ + 2e− ⇋ Na5V2 (PO4 )3

Cathode: Na3V2 (PO4 )3 ⇋ NaV2 (PO4 )3 + 2Na+ + 2e− Overall reaction:2Na3V2 (PO4 )3 ⇋ Na5V2 (PO4 )3 + NaV2 (PO4 )3 Nine pieces of commercial green LEDs can be easily lighted by two series-connected NVP/C-T//NVP/C-T full-cell devices and still work well after 15 min in the normal state as shown in Fig. S8. The electrochemical performances of the symmetric NVP/C-T//NVP/C-T full sodium cell are displayed in Fig. 6b–d. The initial charge and discharge profiles are conducted from 0.1 C to 10 C with electrochemical window of 1.0–2.2 V. As shown in Fig. 6b, the distinct average voltage is around 1.7 V, which is constant with previously report of Wang et al. [31]. On the basic of the total mass of the cathode electrode, the NVP/C-T// NVP/C-T full cell delivers a reversible capacity of 101.8 and 52.8 mAh g−1 at the discharge current density of 0.25 C and 10 C, respectively (1 C means 110 mA g−1). Notably, this NVP/C-T//NVP/C-T full cell also presents high rate capability as displayed in Fig. 6c. The full cell displays discharge capacities of 102.3, 98.5, 91.5, 82.6 68.5 and 52.0 mAh g−1 at current densities of 0.25 0.5, 1, 2, 5 and 10 C, respectively. When recycles through 10 C to 0.25 C, it still delivers a reversible capacity of 98.9 mAh g−1, implying the high reversible capability of the NVP/C-T electrode in the symmetric NVP/C-T//NVP/C-T full cell system. As seen from Fig. 6d, the symmetric NVP/C-T//NVP/CT full cell displays a discharge capacity of 81.2 mAh g−1 at the 2nd cycle. The initial capacity loss is primarily owning to the formation of an SEI film on the surface of electrode during the first cycle. The 270

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symmetric NVP/C-T//NVP/C-T full cell still maintains a capacity of 70.1 mAh g−1 throughout 200 cycles, (about 87% retention) with a superior average CE of almost 98.6%, demonstrating the outstanding cycle stability of the symmetric NVP/C-T//NVP/C-T full cell.

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4. Conclusion

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In summary, 3D hierarchical porous structured NVP/C-T nanocomposite is designed and fabricated using SDS as surfactant and carbon source through hydrothermal process. The unique 3D hierarchical porous structure, formed by interconnected ultrathin nanosheets with hierarchical porous and 3D highly-conductive carbon framework provides high electrical conductivity, short ionic diffusion paths, large specific surface area and robust structural stability for NVP active cathode material. Benefiting from these features, the 3D NVP/CT nanocomposite exhibits high initial discharge capacity, excellent reversibility and stability (93.3 mAh g−1 at 20 C after 8000 cycles) and outstanding high rate-performance (73.2 mAh g−1 at 80 C). Moreover, a symmetric NVP/C-T//NVP/C-T full cell is constructed with an output voltage plateau of 1.7 V and delivers a high capacity retention of 70.1 mAh g−1 (87% retention of 2nd cycle) even throughout 200 cycles at 2C, which is comparable to most of reported results for NVP-based cathode materials.

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Acknowledgements

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The authors acknowledge the financial support by the National Natural Science Foundation of China (Project nos. 51372166 and 51572186) and Natural Science Foundation of Tianjin (Grant No. 15JCYBJC47500).

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Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cej.2018.07.118.

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