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reduced graphene oxide hollow spheres with enhanced sodium storage performance
Journal Pre-proofs Fabrication of porous Na3V2(PO4)3/reduced graphene oxide hollow spheres with enhanced sodium storage performance Jingyi Xu, Erlong Gu, Zhuangzhuang Zhang, Zhenhua Xu, Yifan Xu, Yichen Du, Xiaoshu Zhu, Xiaosi Zhou PII: DOI: Reference:
S0021-9797(20)30135-1 https://doi.org/10.1016/j.jcis.2020.01.121 YJCIS 25994
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
19 October 2019 29 January 2020 30 January 2020
Please cite this article as: J. Xu, E. Gu, Z. Zhang, Z. Xu, Y. Xu, Y. Du, X. Zhu, X. Zhou, Fabrication of porous Na3V2(PO4)3/reduced graphene oxide hollow spheres with enhanced sodium storage performance, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.121
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Fabrication of porous Na3V2(PO4)3/reduced graphene oxide hollow spheres with enhanced sodium storage performance Jingyi Xu a, 1, Erlong Gu a, 1, Zhuangzhuang Zhang a, Zhenhua Xu a, Yifan Xu a, Yichen Du a, Xiaoshu Zhu b, *, Xiaosi Zhou a, **
a
School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China
b
Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, China
Corresponding authors *
E-mail addresses: [email protected] (X. Zhou), [email protected] (X. Zhu).
1 These
authors equally contributed to this work.
Abstract Sodium-ion batteries (SIBs) have long been recognized as a potential substitute for lithium-ion batteries, while their practical application is greatly hindered owing to the absence of suitable cathode materials with improved rate capability and prolonged cycling life. Na3V2(PO4)3 (NVP) has drawn extensive attention among the cathode materials for SIBs because of its fast Na+-transportable framework which enables high-speed charge transfer, but the poor electric conductivity of NVP significantly restricts the Na+ diffusion. To tackle this issue, in this work, porous NVP/reduced graphene oxide hollow spheres (NVP/rGO HSs) are constructed via a spray drying strategy. Due to the unique porous hollow architecture, the synthesized compound manifests a high reversible capacity of 116 mAh g−1 at 1 C (1 C = 118 mA g−1), an outstanding high-rate capability of 107.5 mAh g−1 at 10 C and 98.5 mAh g−1 at 20 C, as well as a stable cycling performance of 109 mAh g−1 after 400 cycles at 1 C and 73.1 mAh g−1 after 1000 cycles at 10 C. Moreover, galvanostatic intermittent titration technique demonstrates that the Na+ diffusion coefficient of NVP/rGO HSs is an order of magnitude larger than the pristine NVP. The remarkable electrochemical properties of NVP/rGO HSs in full cells further enable it a potential cathode for SIBs.
Keywords: Na3V2(PO4)3; hollow structure; cathode; sodium-ion batteries; energy storage
1. Introduction Lithium-ion batteries (LIBs) have dominated the market of portable power sources for decades because of their high energy density and specific capacity [1]. In spite of these advantages, LIBs cannot meet the increasing demands because lithium resources are relatively scarce and their distribution is fairly uneven [2]. Located at the same group of alkali metals, sodium shares similar electrochemical properties with lithium, but the amount of sodium storage in the earth shell ranks fourth among all the metals, greatly reducing its costs [3]. By virtue of the merits mentioned above, sodium-ion batteries (SIBs) have long been recognized as a potential substitute for LIBs, especially in some large-scale energy storage systems [4,5]. Unfortunately, sodium ions (Na+) are more kinetically sluggish than lithium ions (Li+), so Na+ intercalation/deintercalation reactions are generally kinetically inferior, causing poor rate performance and limited cycling life [6]. Recently, vanadium based composites have been extensively explored as potential electrode material candidates for batteries [7-9]. Among these candidates, Na3V2(PO4)3 (NVP) has aroused wide attention because the Na+-superionic conductor (NASICON) structure can promote Na+ transfer and well address the kinetic drawbacks [10,11]. Though the ion diffusion coefficient is high in NVP, the electronic conductivity of it is far from satisfying as the electron transitions are impeded by the vast V-O orbital energy difference [12]. A lot of studies focused on boosting the conductivity of NVP, and carbon wrapping is a frequently adopted strategy. Yu et al. confined NVP nanoparticles within mesoporous CMK-3 matrix and the product showed a high rate capacity and cycling stability [13]. Mai and co-workers fabricated a layer-by-layer structured NVP/reduced graphene oxide and the obtained capacity was as high as 118 mAh g−1 at 0.5 C [14]. Recently, our group systematically explored the effects of diverse carbon matrix on the electrochemical performance of NVP and
incredible cycling stability (48 mAh g−1 after 20000 cycles at 50 C) was achieved in the NVP/expanded carbon system [15]. All the examples above indicate that carbon coating is an efficient strategy towards the practical use of NVP-based cathodes. Despite the fact that progress has been made in improving the electrochemical performance of NVP, its superionic conductivity potential needs further exploration [16-18]. It is generally assumed that a porous hollow structure is more favorable for the penetration of electrolyte, and can accelerate the Na+ diffusion [19,20]. Herein, we report the fabrication of NVP/reduced graphene oxide hollow spheres (NVP/rGO HSs) as an advanced cathode material for SIBs by spray drying and subsequent pyrolysis. The rGO layer connected with NVP forms a network for high-speed electron transfer, and the spherical hollow structure enables a thorough impregnating of electrolyte, thereby facilitating the penetration of Na+. Interestingly, it is found via galvanostatic intermittent titration technique (GITT) that the Na+ diffusion coefficient of NVP will increase by an order of magnitude once it is coated by rGO to form the hollow spheres. As a consequence, NVP/rGO HSs display a series of remarkable electrochemical performance, such as a large reversible capacity of 116 mAh g−1 at 1 C (1 C = 118 mA g−1), superior rate performance (107.5 mAh g−1 at 10 C), and remarkable cycling stability (109 mAh g−1 after 400 cycles at 1 C; 73.1 mAh g−1 after 1000 cycles at 10 C). Notably, this is a precursory work on hollowstructured high-performance cathode for SIBs, and may shed new insights into the design and construction of high-performance energy storage applications. 2. Experimental 2.1. Synthesis of NVP/rGO HSs In the first step, 684.8 mg V2O5 was dispersed in 37.5 mL deionized water and added with 7.5 mL H2O2 (30%) under vigorous stirring. After 10 min, 932 mg CH3COONa and 1307 mg NH4H2PO4
were introduced to the system, forming a bright yellow precursor solution. Afterwards, 15 mL graphene oxide (GO) (10 mg mL1) was ultrasonically dispersed in the precursor of NVP for 20 min to produce a homogeneous suspension, which was subsequently subjected to the spray drying process with the pyrogenic temperature being 270 ℃. Finally, the obtained faint yellow micro-balls were annealed at 900 ℃ for 8 h under a H2/Ar (5:95) flow to yield NVP/rGO HSs. For comparison, pristine NVP was synthesized in the same way except the use of GO. 2.2. Material Characterization. X-ray diffraction (XRD) patterns were determined by a Rigaku SmartLab diffractometer using a Cu Kα radiation (λ =1.5406 Å). Raman spectra were collected on a Labram HR800 instrument (λ = 514 nm). The morphology of the samples was imaged by a field-emission scanning electron microscope (FESEM, JEOL JSM-7600F) operated at 10 kV. Transmission electron microscope (TEM) images and elemental mappings were achieved with a JEOL JEM-2100F operated at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded with an ESCALab250Xi electron spectrometer with an Al Kα radiation. Thermogravimetric analysis (TGA) was conducted with a NETZSCH STA 449 F3. Nitrogen sorption isotherms were obtained by an ASAP 2020 surface analyzer. 2.3. Electrochemical Characterization To prepare the working electrodes, active material, super-P carbon black, and polyvinylidene fluoride in a weight ratio of 8:1:1 were hand milled with a suitable amount of N-methyl-2-pyrrolidone into a homogeneous slurry, which was spread on an aluminum foil and then dried in the vacuum oven at 80 °C for 12 h. The average mass loading of each electrode was controlled to be around 1 or 5 mg cm−2. The working electrodes were assembled as the cathodes of coin cells (CR2032) in an argon-
filled glove box (MBRAUN, Unilab1200). The counter electrode was sodium; the separator was glass fiber (Whatman); and the electrolyte was 1 M NaClO4 in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) (95:5 in volume). To study the sodium storage performance of the full cell, sulfur-doped carbon microtubes (SCMTs) were selected as the anode material. The S-CMTs were synthesized through a previously reported strategy [21]. To prepare the anode, the S-CMTs, carboxymethyl cellulose sodium, and super-P were hand milled in a weight ratio of 70:15:15 into a slurry. The slurry was pasted onto a copper foil and vacuum-dried at 40 °C for 12 h. The counter electrode, the separator, and the electrolyte were the same as those in the NVP/rGO HSs cathode. Prior to assembling the full batteries, the NVP/rGO HSs was firstly activated in the half cell, and the S-CMTs anode was also operated between 0.01 and 3 V to eliminate the irreversible capacity from the initial cycle. Finally, the half cells were coupled together to obtain the full cell. In order to avoid Na planting on the anode, the capacity of cathode to anode is controlled to be 1:1.05. The cells were galvanostatically charged/discharged on a Land CT2001A testing system over a voltage range of 2.0−3.9 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on a PARSTAT 4000 workstation. GITT was used to investigate the Na+ diffusion coefficients with a pulse current of 10 mA g−1 for 0.5 h between rest intervals for 2 h. 3. Results and discussion Fig. 1 demonstrates the synthetic process of NVP/rGO HSs. Briefly, the hollow spheres were fabricated via spray drying a blend of GO and NVP precursor and subsequent annealing. The preparation steps are detailed in the Experimental Section. When the dispersion droplets were spray dried in the decomposition column, the solvent water exposed to the hot air was first vaporized, and
the precursor of NVP and graphene oxide was solidified in situ into spherical particles. During the subsequent pyrolysis process, the NVP precursor and graphene oxide decomposed to lose water, ammonia and methane. The NVP nanoparticles formed in the central part had higher surface energy than the NVP nanoparticles formed on the surface, so they can easily migrate to the surface and deposit, finally producing the hollow NVP/rGO microspheres. It is worth noting that the rGO plays an important role in bridging NVP nanoparticles and maintaining the morphology of the composite even though its content is as low as 5 wt % (Fig. S1) [22,23]. When the NVP precursor was spray dried without the addition of rGO, the regular spherical morphology cannot be observed (Fig. S2). Besides, the Barrett−Joyner−Halenda (BJH) pore-size-distribution show that a large number of nanopores are formed on NVP/rGO HSs with their sizes falling in the range of 2 to 45 nm (Fig. S3), indicating that the hollow spheres are highly porous. Fig. 2a displays the XRD patterns of NVP/rGO HSs and the pure NVP. All the characteristic peaks of NVP/rGO HSs are in good agreement with those of the pure NVP [24,25], mirroring that the crystal structure of NVP remains intact when wrapped by graphene. By applying the Scherrer equation (D = Kλ/B cos θ) to the (116) crystal plane of NVP/rGO HSs, the NVP nanoparticles impregnated in the hollow carbon spheres are calculated to be approximately 42 nm in diameter. The Raman spectrum of the pure NVP shows sharp peaks at around 1000 and 440 cm1, which can be assigned to the modes of vibration and bending for PO4 tetrahedral (Fig. 2b). The Raman fingerprints below 400 cm1 may be caused by the vibration/bending of the VO6 octahedral [26]. The characteristic peaks of NVP do not appear in the Raman spectrum of NVP/rGO HSs, which can be attributed to the following two reasons: (1) the rGO coating shields the signals of NVP to some extent; (2) the particle size of NVP nanocrystals is too small to be precisely detected [27,28]. The two evident
bands located at 1346 and 1601 cm1 stand for the disordered carbon band (D band) and the graphitic carbon band (G band), respectively, and the ID/IG ratio is calculated to be 0.91, which suggests the carbon spheres are amorphous with partial graphitization [29]. Fig. 3a and b show the SEM images of NVP/rGO HSs where the sizes of the spheres are observed to be in the range of 4 to 10 m and some NVP/rGO nanoparticles are found to be present onto the outer surface of the microspheres. The intimate contact between the NVP and rGO coatings can increase the electric conductivity of the NVP nanoparticles and prevent them from aggregation. The TEM image (Fig. 3c) clearly illustrates the unique hollow architecture of NVP/rGO HSs. The HRTEM image (Fig. 3d) further reveals that the rGO is about 3.3 nm thick and in close contact with the NVP nanoparticles. The interlayer distance of 0.37 nm corresponds to the (113) plane of NVP nanocrystals (JCPDS card No. 76-1962). The elemental mappings (Fig. 3e) unveil the even distribution of all the elements presenting in NVP/rGO HSs. In addition, the surface area of NVP/rGO HSs was determined as 23.26 cm2 g1 (Fig. S3), over ten times the value of the pristine NVP (1.75 cm2 g1). This result suggests that the contacting area between the NVP/rGO HSs and the electrolyte is larger, which probably enables faster charge transfer. XPS was performed to detect the surface chemical state of the NVP/rGO HSs. The presence of C, V, O and P can be clearly observed in the XPS survey scan (Fig. S4), which is in accordance with the result of elemental mappings. Notably, it is found that the V in the NVP/rGO HSs is in a mixed valence state rather than a pure trivalent state, and the atomic content of V3+ and V4+ is calculated to be 71.0% and 29.0%, respectively, indicating that the surface V3+ is partially oxidized under air atmosphere (Fig. 4a) [30]. Meanwhile, in the high-resolution C 1s spectrum (Fig. 4b), the C=O functional group (288.4 eV) which is usually detected in rGO disappears [31]. Such phenomenon
indicates that graphene oxide is almost completely reduced to generate rGO by heat treatment at 900 °C in a H2/Ar (5:95) atmosphere for 8 h [32]. From the point of view of defect chemistry, multivalent V can increase electron transfer and have a positive impact on reaction kinetics [33]. Except for C−O functional group, the main carbon atoms are in the sp2 hybrid state, and the binding energy of C=C bond is approximately 284.7 eV, which means that the NVP/rGO HSs are surrounded by electron clouds from the conjugated 2p orbital, increasing the electronic conductivity to a higher level [34]. To investigate the sodium storage properties of NVP/rGO HSs, this composite was assembled into coin cells, sodium metal being the counter electrode. Fig. 5a demonstrates the cyclic voltammetry (CV) curves of NVP/rGO HSs and NVP. The pair of characteristic redox peaks centered at 3.4 V represents
the
transformation
reaction
between
V3+
and
V4+,
companied
by
the
intercalation/deintercalation of two sodium ions [35]. In addition, the higher peak current intensity also manifests the better Na+ diffusion kinetics in NVP/rGO HSs, which can be attributed to the rGO shell and the porous hollow structure, enabling high-speed ionic and electronic transportation [36]. The potential gap between the initial charge/discharge profiles of the bare NVP (240 mV) is three times that of NVP/rGO HSs (80 mV), showing that the NVP/rGO HSs cathode has a much smaller polarization (Fig. 5b), highly agreeing with the CV curves. It is also worth mentioning that the capacity of NVP/rGO HSs and the pristine NVP are measured to be 124 and 119 mAh g−1, and the extra capacity over the theoretical value (118 mAh g−1) may derive from the contribution of the rGO and the carbon black. The rate capability (Fig. 5c) of NVP/rGO HSs is measured to be 121, 117.1, 113.2, 107.5 and 98.5 mAh g−1 at the rate of 1, 2, 5, 10 and 20 C, respectively. Such excellent rate capability has surpassed almost all the NVP-based cathodes reported to date (Table S1). On the contrary, although
the capacity of pure NVP is roughly the same as that of NVP/rGO HSs at 1 C, it drops quickly to 79 mAh g−1 at 10 C and 59.3 mAh g−1 at 20 C, which is significantly lower than that of NVP/rGO HSs. Furthermore, despite the drastic fluctuations of currents, a high reversible capacity of 116.3 mAh g−1 is still maintained for NVP/rGO HSs after 60 cycles, recovering to 96.1% of its original capacity. We also checked the rate performance of electrodes with higher mass loading of 5 mg cm−2. When the mass loading is increased from 1 to 5 mg cm−2, the capacity of NVP/rGO HSs is 109.3, 105.7, 96.8, 88.7 and 105 mAh g−1 at the rate of 1, 2, 5, 10 and 20 C, respectively (Fig. 5c). Though the rate performance of the high loading electrode is not as satisfying as that of the low loading electrode, especially at low current densities, the high loading electrode still displays decent capacities of 96.8 and 88.7 mAh g−1 at the high rates of 10 and 20 C, far exceeding those of the pure NVP. This outcome demonstrates that the extraordinary rate capability is mainly attributed to the special structure rather than the low mass loading. For the purpose of comprehensively assessing its stability, the cycling properties of NVP/rGO HSs were tested at 1 C and 10 C. As depicted in Fig. 5d, even though the NVP/rGO HSs cathode has been cycled for 400 cycles at 1 C, it can still exhibit a large capacity of 109 mAh g−1, greatly exceeding that of NVP (96.6 mAh g−1). Based on the lower tap density (0.91 g cm−3 vs. 1.02 g cm−3) and higher average output voltage (3.33 V vs. 3.28 V) of NVP/rGO HSs compared with NVP, the volumetric energy density of NVP/rGO HSs is still slightly larger than that of NVP (330.3 Wh L−1 vs. 323.2 Wh L−1). After 1000 cycles at 10 C, the NVP/rGO HSs electrode showed a decent capacity of 73.1 mAh g−1 and an average Coulombic efficiency approaching 100% (Fig. 5e). After the long cycling test, neither morphological changes (Fig. S5) nor structural variation (Fig. S6) were detected in NVP/rGO HSs, signifying a good structural stability. The outstanding sodium storage properties
of NVP/rGO HSs mentioned above can be attributed to their unique architecture. On the one hand, the porous hollow sphere not only enlarges the interface between electrode and electrolyte, but also contributes to a complete infiltration of the electrolyte into the NVP/rGO HSs electrode [37]. On the other hand, the tight combination of NVP and the graphene shells imparts a high-speed electron conduction network and a robust skeleton to the encapsulated NVP nanocrystals, thereby increasing the rate performance and cycling stability of NVP/rGO HSs. To evaluate the electronic and ionic conductivity of NVP/rGO HSs, we measured the EIS curves over the frequency range from 100 kHz to 10 mHz. The Nyquist plot depicted in Fig. 6a consists of a depressed semicircle at the high-frequency region and a quasi-straight line at the low-frequency region. The radius of the semicircle for NVP/rGO HSs is much smaller than that of the pristine NVP, implying that the charge transfer at the electrode/electrolyte interphase can be prominently facilitated by the intimate rGO coating and the porous hollow structure [38]. Furthermore, the diffusion coefficient of Na+ (DNa+) within NVP/rGO HSs and the pristine NVP were calculated according to following equation:
DNa +
R 2T 2 2 4 4 2 2 2A n F C
where R refers to the gas constant, T is 298.15K, A represents the surface area of the electrode, n represents the number of electrons transfer in one molecule of the active material during intercalation/deintercalation, F is the Faraday constant, C refers to the concentration of Na+, and σ is the Warburg factor, which can be determined by the slope of the Zre ⁓ ω−1/2 plot [39,40]. As shown in Fig. 6b, the σ of the NVP/rGO HSs electrode is smaller (231) than that of the pure NVP electrode (311), mirroring that the DNa+ value of NVP/rGO HSs (5.10 10−9 cm2 s−1) is larger than that of the pristine NVP (2.81 10−9 cm2 s−1).
Fig. 7a and b show the potential responses for NVP/rGO HSs and the pristine NVP utilizing GITT. Fig. 7c and d illustrate the evolution of the computed Na+ diffusion coefficients (DNa+) during Na+ insertion/extraction processes by using the following equation: 2
DNa +
4 mBVM Es M B S Et
2
L /D 2
Na +
where τ is the time of the employed galvanostatic current; mB, MB, and VM represent the mass, molecular weight, and molar volume of NVP, respectively; S is the total area between the electrode and the electrolyte; ΔEs is the variation of steady-state voltage after a single step; ΔEt represents the total transient voltage variation after imposing the galvanostatic current for the time τ; and L is the sample thickness [41]. Remarkably, the DNa+ values achieved from the GITT curves are in the range of 10−9−10−6 cm2 s−1, and the values of NVP/rGO HSs are an order of magnitude larger than those of the pristine NVP in both charging and discharging processes [42]. This finding is consistent with the EIS measurements shown above. The extraordinary properties of NVP/rGO HSs inspire us to investigate the sodium storage performance of the full cell. We selected S-CMTs as the anode material and coupled it with the NVP/rGO HSs cathode to form full cells. The SEM image and XRD pattern of the synthesized SCMTs are shown in Fig. S7a and b. The S-CMTs half-cell delivers an initial charge and discharge capacity of 532.1 and 850.3 mAh g−1 (Fig. S7c), and can cycle at 200 mA g−1 for 100 cycles with negligible capacity decrease (Fig. S7d). Fig. 8a depicts the charge and discharge profiles of the NVP/rGO HSs//S-CMTs full battery. The curves of the second and third cycles are overlapped, indicating the electrochemical reactions are reversible. In the third cycle, the charge and discharge capacities of the full cell are measured to be 97.8 and 98.8 mAh g−1, normalized by the weight of the NVP/rGO HSs. The full battery also presents a decent rate capability of 100.6, 92.4, 77.8, 53.5 and
33.8 mAh g−1 at the current densities of 1, 2, 5, 10 and 20 C, revealing that the rate performance of the NVP/rGO HSs composite is also competitive in full cells (Fig. 8b). The cycling stability of the full cell is demonstrated in Fig. 8c, where the reversible capacity remains 84.2 mAh g−1 after 100 cycles at 1 C, corresponding to a high capacity retention of 85.4%. 4. Conclusion In summary, we have fabricated Na3V2(PO4)3/reduced graphene hollow spheres (i.e., NVP/rGO HSs) as a high-performance cathode material for SIBs. The unique porous hollow structure benefits the penetration of electrolyte and significantly enlarges the electrolyte/electrode interface. Moreover, the close contact between the rGO shell and NVP nanocrystals helps to boost the electronic transport. As a result, the NVP/rGO HSs electrode exhibits superior electrochemical properties, including large reversible capacity (116 mAh g−1 at 1 C), excellent rate capability (107.5 mAh g−1 at 10 C and 98.5 mAh g−1 at 20 C), and long cycling life (109 mAh g−1 after 400 cycles at 1 C and 73.1 mAh g−1 after 1000 cycles at 10 C). The excellent electrochemical performance of NVP/rGO HSs in full batteries also enables it to be a competitive cathode for SIBs. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 51577094), the Natural Science Foundation of Jiangsu Province of China (BK20180086), and the 100 Talents Program of Nanjing Normal University. References [1] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: A battery of choices, Science 334 (2011) 928−935. [2] Y. You, H.R. Yao, S. Xin, Y.X. Yin, T.T. Zuo, C.P. Yang, Y.G. Guo, Y. Cui, L.J Wan,. J.B. Goodenough, Subzero-temperature cathode for a sodium-ion battery, Adv. Mater. 28 (2016)
7243−7248. [3] S. Bao, S. Luo, Z. Wang, S. Yan, Q. Wang, Improving the electrochemical performance of layered cathode oxide for sodium-ion batteries by optimizing the titanium content, J. Colloid Interf. Sci. 544 (2019) 164−171. [4] Q.C. Wang, J.K. Meng, X.Y. Yue, Q.Q. Qiu, Y. Song, X.J. Wu, Z.W. Fu, Y.Y. Xia, Z. Shadike, J. Wu, X.Q. Yang, Y.N. Zhou, Tuning P2-structured cathode material by na-site Mg substitution for Na-ion batteries, J. Am. Chem. Soc. 141 (2019) 840−848. [5] H. Wan, X. Hu, Sulfur-doped honeycomb-like carbon with outstanding electrochemical performance as an anode material for lithium and sodium ion batteries, J. Colloid Interf. Sci. 558 (2019) 242−250. [6] W. Chen, X. Zhang, L. Mi, C. Liu, J. Zhang, S. Cui, X. Feng, Y. Cao, C. Shen, High-performance flexible freestanding anode with hierarchical 3D carbon-networks/Fe7S8/graphene for applicable sodium-ion batteries, Adv. Mater. 31 (2019) 1806664. [7] L. Li, W. Zhang, X. Wang, S. Zhang, Y. Liu, M. Li, G. Zhu, Y. Zheng, Q. Zhang, T. Zhou, W. K. Pang, W. Luo, Z. Guo, J. Yang, Hollow-carbon-templated few-layered V5S8 nanosheets enabling ultrafast potassium storage and long-term cycling, ACS Nano 13 (2019) 7939−7948. [8] Y. Jiang, X. Zhou, D. Li, X. Cheng, F. Liu, Y. Yu, Highly reversible Na storage in Na3V2(PO4)3 by optimizing nanostructure and rational surface engineering, Adv. Energy Mater. 8 (2018) 1800068. [9] W. Li, Z. Yao, C. Zhou, X. Wang, X. Xia, C. Cu, J. Tu, Boosting high-rate sodium storage performance of N-doped carbon-encapsulated Na3V2(PO4)3 nanoparticles anchoring on carbon cloth, Small 15 (2019) 1902432. [10] J. Liu, K. Tang, K. Song, P. A. van Aken, Y. Yu, J. Maier, Electrospun Na3V2(PO4)3/C nanofibers as stable cathode materials for sodium-ion batteries, Nanoscale 6 (2014) 5081−5086. [11] P.N. Didwal, R. Verma, C.W. Min, C.J. Park, Synthesis of 3-dimensional interconnected porous Na3V2(PO4)3@C composite as a high-performance dual electrode for Na-ion batteries, J. Power Sources 413 (2019) 1−10. [12] W.X. Song, H.S. Hou, X.B. Ji, Progress in the investigation and application of Na3V2(PO4)3 for electrochemical energy storage, Acta Phys. Chim. Sin. 33 (2017) 103−129. [13] 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 sodiumion batteries, Adv. Energy Mater. 5 (2015) 1402104. [14] Y. Xu, Q. Wei, C. Xu, Q. Li, Q. An, P. Zhang, J. Sheng, L. Zhou, L. Mai, Layer-by-layer Na3V2(PO4)3 embedded in reduced graphene oxide as superior rate and ultralong-life sodium-ion battery cathode, Adv. Energy Mater. 6 (2016) 1600389. [15] E. Gu, J. Xu, Y. Du, X. Ge, X. Zhu, J. Bao, X. Zhou, Understanding the influence of different carbon matrix on the electrochemical performance of Na3V2(PO4)3 cathode for sodium-ion batteries, J. Alloys Compd. 788 (2019) 240−247. [16] Q. Hu, J.Y. Liao, X.D. He, S. Wang, L.N. Xiao, X. Ding, C.H. Chen, In situ catalytic formation of graphene-like graphitic layer decoration on Na3V2-xGax(PO4)3 (0 ≤ x ≤ 0.6) for ultrafast and high energy sodium storage, J. Mater. Chem. A 7 (2019) 4660−4667. [17] J. Huang, X. Tang, Z. Li, K. Liu, Metal organic frameworks derived cobalt sulfide/reduced graphene oxide composites with fast reaction kinetic and excellent structural stability for sodium storage, J. Colloid Interf. Sci. 532 (2018) 407−415. [18] W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Self-
sacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for high-rate sodium in full batteries, Nano Energy 25 (2016) 145−153. [19] D. Ni, W. Sun, Z. Wang, Y. Bai, H. Lei, X. Lai, K. Sun, Heteroatom-doped mesoporous hollow carbon spheres for fast dodium dtorage with an ultralong cycle life, Adv. Energy Mater. 9 (2019) 1900036. [20] Y. Liu, X.-Y. Yu, Y. Fang, X. Zhu, J. Bao, X. Zhou, X.W. Lou, Confining SnS2 ultrathin nanosheets in hollow carbon nanostructures for efficient capacitive sodium storage, Joule 2 (2018) 725−735. [21] Q. Wang, X. Ge, J. Xu, Y. Du, X. Zhao, L. Si, X. Zhou, Fabrication of microporous sulfur-doped carbon microtubes for high-performance sodium-ion batteries, ACS Appl. Energy Mater. 1 (2018) 6638−6645. [22] Z. Pan, G. Zhang, X. Wang, Polymeric carbon nitride/reduced graphene oxide/Fe2O3: all-solidstate Z-scheme system for photocatalytic overall water splitting, Angew. Chem. Int. Ed. 58 (2019) 7102−7106. [23] J. Zhang, Y. Fang, L. Xiao, J. Qian, Y. Cao, X. Ai, H. Yang, Graphene-Scaffolded Na3V2(PO4)3 microsphere cathode with high rate capability and cycling stability for sodium ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 7177−7184. [24] X. Li, S. Wang, X. Tang, R. Zang, P. Li, P. Li, Z. Man, C. Li, S. Liu, Y. Wu, G. Wang, J. Colloid Interf. Sci. 539 (2019) 168−174. [25] H. Zhang, I. Hasa, D. Buchholz, B. Qin, S. Passerini, Effects of nitrogen doping on the structure and performance of carbon coated Na3V2(PO4)3 cathodes for sodium-ion batteries, Carbon 124 (2017) 334−341. [26] W. Song, X. Ji, Y. Yao, H. Zhu, Q. Chen, Q. Sun, C.E. Banks, A promising Na3V2(PO4)3 cathode for use in the construction of high energy batteries, Phys. Chem. Chem. Phys. 16 (2014) 3055−3061. [27] 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 roomtemperature sodium-ion batteries, Adv. Energy Mater. 3 (2013) 156−160. [28] E. Gu, S. Liu, Z. Zhang, Y. Fang, X. Zhou, J. Bao, An efficient sodium-ion battery consisting of reduced graphene oxide bonded Na3V2(PO4)3 in a composite carbon network, J. Alloys Compd. 767 (2018) 131−140. [29] 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. [30] H. Li, X. Yu, Y. Bai, F. Wu, C. Wu, L.Y. Liu, X.Q. Yang, Effects of Mg doping on the remarkably enhanced electrochemical performance of Na3V2(PO4)3 cathode materials for sodium ion batteries, J. Mater. Chem. A 3 (2015) 9578−9586. [31] N. Wang, Q. Liu, B. Sun, J. Gu, B. Yu, W. Zhang, D. Zhang, N-doped catalytic graphitized hard carbon for high-performance lithium/sodium-ion batteries, Sci. Rep. 8 (2018) 9934. [32] L. Luo, B. Cheng, S. Chen, Z. Ge, H. Zhuo, Three-dimensional Na3V2(PO4)3/carbon nanofiber networks prepared by electrospinning as self-standing cathodes for high performance Na-ion batteries, Mater. Lett. 232 (2018) 153−156. [33] L. Zhao, H. Zhao, Z. Du, J. Wang, X. Long, Z. Li, K. Swierczek, Delicate lattice modulation enables superior Na storage performance of Na3V2(PO4)3 as both an anode and cathode material for sodium-ion batteries: Understanding the role of calcium substitution for vanadium, J. Mater. Chem.
A 7 (2019) 9807−9814. [34] W. Shen, C. Wang, Q. Xu, H. Liu, Y. Wang, Nitrogen-doping-induced defects of a carbon coating layer facilitate Na-storage in electrode materials, Adv. Energy Mater. 5 (2015) 1400982. [35] W. Song, X. Ji, Z. Wu, Y. Zhu, Y. Yang, J. Chen, M. Jing, F. Li, C.E. Banks, First exploration of Na-ion migration pathways in the NASICON structure Na3V2(PO4)3, J. Mater. Chem. A 2 (2014) 5358−5362. [36] Y. Fang, X. Xu, Y. Du, X. Zhu, X. Zhou, J. Bao, Novel nitrogen-doped reduced graphene oxidebonded Sb nanoparticles for improvedsodium storage performance, J. Mater. Chem. A 6 (2018) 11244−11251. [37] P. Hu, X. Wang, J. Ma, Z. Zhang, J. He, X. Wang, S. Shi, G. Cui, L. Chen, NaV3(PO4)3/C nanocomposite as novel anode material for Na-ion batteries with high stability, Nano Energy 26 (2016) 382−391. [38] L. Yu, X. Zhang, Electrochemical insertion of magnesium ions into V2O5 from aprotic electrolytes with varied water content, J. Colloid Interf. Sci. 278 (2004) 160−165. [39] Q. Wang, X. Zhu, Y. Liu, Y. Fang, X. Zhou, J. Bao, Rice husk-derived hard carbons as highperformance anode materials for sodium-ion batteries, Carbon 127 (2018) 685−666. [40] Y. Zhang, P. Wang, Y. Yin, N. Liu, L. Fan, N. Zhang, K. Sun, Carbon coated amorphous bimetallic sulfide hollow nanocubes towards advanced sodium ion battery anode, Carbon 150 (2019) 378−387. [41] J. Huang, X. Tang, Z. Li, K. Liu, Metal organic frameworks derived cobalt sulfide/reduced graphene oxide composites with fast reaction kinetic and excellent structural stability for sodium storage, J. Colloid Interf. Sci. 532 (2018) 407−415. [42] J. Zeng, D. Wu, X. Wang, J. Wu, J. Li, J. Wang, J. Zhao, Insights into the Mg storage property and mechanism based on the honeycomb-like structured Na3V2(PO4)3/C/G in anhydrous electrolyte, Chem. Eng. J. 372 (2019) 37−45.
Fig. and Table Captions
Fig. 1. Schematic illustration for the fabrication of NVP/rGO HSs. Fig. 2. (a) XRD patterns and (b) Raman spectra of NVP/rGO HSs and pure NVP.
Fig. 3. (a, b) SEM images, (c) TEM image, (d) HRTEM images, and (e) elemental mappings of C, Na, V, P, and O for NVP/rGO HSs. Fig. 4. High-resolution (a) V 3d and (b) C 1s XPS spectra of NVP/rGO HSs. Fig. 5. (a) The initial CV curves, (b) the first charge and discharge profiles, (c) rate capabilities, and (d) cycling performance at 1 C of the NVP/G HSs and the pristine NVP. (e) Prolonged cycling performance of NVP/rGO HSs at 10 C. Fig. 6. (a) Nyquist plots and (b) linear fitting line of Zre versus ω−1/2 of NVP/rGO HSs and the pristine NVP. Fig. 7. (a, b) GITT curves and (c, d) the corresponding Na+ diffusion coefficients of NVP/rGO HSs and the pristine NVP. Fig. 8. (a) Charge and discharge profiles, (b) rate capability, and (c) cycling performance of the NVP/rGO HSs//S-CMTs full battery.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
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Fig. 8.
Credit Author Statement
The author declares that all the data in this work is authentic and repeatable.
Declaration of Interest Statement
The authors declare no competing financial interest.
Graphical abstract
S0021-9797(20)30135-1 https://doi.org/10.1016/j.jcis.2020.01.121 YJCIS 25994
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
19 October 2019 29 January 2020 30 January 2020
Please cite this article as: J. Xu, E. Gu, Z. Zhang, Z. Xu, Y. Xu, Y. Du, X. Zhu, X. Zhou, Fabrication of porous Na3V2(PO4)3/reduced graphene oxide hollow spheres with enhanced sodium storage performance, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.121
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Fabrication of porous Na3V2(PO4)3/reduced graphene oxide hollow spheres with enhanced sodium storage performance Jingyi Xu a, 1, Erlong Gu a, 1, Zhuangzhuang Zhang a, Zhenhua Xu a, Yifan Xu a, Yichen Du a, Xiaoshu Zhu b, *, Xiaosi Zhou a, **
a
School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China
b
Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, China
Corresponding authors *
E-mail addresses: [email protected] (X. Zhou), [email protected] (X. Zhu).
1 These
authors equally contributed to this work.
Abstract Sodium-ion batteries (SIBs) have long been recognized as a potential substitute for lithium-ion batteries, while their practical application is greatly hindered owing to the absence of suitable cathode materials with improved rate capability and prolonged cycling life. Na3V2(PO4)3 (NVP) has drawn extensive attention among the cathode materials for SIBs because of its fast Na+-transportable framework which enables high-speed charge transfer, but the poor electric conductivity of NVP significantly restricts the Na+ diffusion. To tackle this issue, in this work, porous NVP/reduced graphene oxide hollow spheres (NVP/rGO HSs) are constructed via a spray drying strategy. Due to the unique porous hollow architecture, the synthesized compound manifests a high reversible capacity of 116 mAh g−1 at 1 C (1 C = 118 mA g−1), an outstanding high-rate capability of 107.5 mAh g−1 at 10 C and 98.5 mAh g−1 at 20 C, as well as a stable cycling performance of 109 mAh g−1 after 400 cycles at 1 C and 73.1 mAh g−1 after 1000 cycles at 10 C. Moreover, galvanostatic intermittent titration technique demonstrates that the Na+ diffusion coefficient of NVP/rGO HSs is an order of magnitude larger than the pristine NVP. The remarkable electrochemical properties of NVP/rGO HSs in full cells further enable it a potential cathode for SIBs.
Keywords: Na3V2(PO4)3; hollow structure; cathode; sodium-ion batteries; energy storage
1. Introduction Lithium-ion batteries (LIBs) have dominated the market of portable power sources for decades because of their high energy density and specific capacity [1]. In spite of these advantages, LIBs cannot meet the increasing demands because lithium resources are relatively scarce and their distribution is fairly uneven [2]. Located at the same group of alkali metals, sodium shares similar electrochemical properties with lithium, but the amount of sodium storage in the earth shell ranks fourth among all the metals, greatly reducing its costs [3]. By virtue of the merits mentioned above, sodium-ion batteries (SIBs) have long been recognized as a potential substitute for LIBs, especially in some large-scale energy storage systems [4,5]. Unfortunately, sodium ions (Na+) are more kinetically sluggish than lithium ions (Li+), so Na+ intercalation/deintercalation reactions are generally kinetically inferior, causing poor rate performance and limited cycling life [6]. Recently, vanadium based composites have been extensively explored as potential electrode material candidates for batteries [7-9]. Among these candidates, Na3V2(PO4)3 (NVP) has aroused wide attention because the Na+-superionic conductor (NASICON) structure can promote Na+ transfer and well address the kinetic drawbacks [10,11]. Though the ion diffusion coefficient is high in NVP, the electronic conductivity of it is far from satisfying as the electron transitions are impeded by the vast V-O orbital energy difference [12]. A lot of studies focused on boosting the conductivity of NVP, and carbon wrapping is a frequently adopted strategy. Yu et al. confined NVP nanoparticles within mesoporous CMK-3 matrix and the product showed a high rate capacity and cycling stability [13]. Mai and co-workers fabricated a layer-by-layer structured NVP/reduced graphene oxide and the obtained capacity was as high as 118 mAh g−1 at 0.5 C [14]. Recently, our group systematically explored the effects of diverse carbon matrix on the electrochemical performance of NVP and
incredible cycling stability (48 mAh g−1 after 20000 cycles at 50 C) was achieved in the NVP/expanded carbon system [15]. All the examples above indicate that carbon coating is an efficient strategy towards the practical use of NVP-based cathodes. Despite the fact that progress has been made in improving the electrochemical performance of NVP, its superionic conductivity potential needs further exploration [16-18]. It is generally assumed that a porous hollow structure is more favorable for the penetration of electrolyte, and can accelerate the Na+ diffusion [19,20]. Herein, we report the fabrication of NVP/reduced graphene oxide hollow spheres (NVP/rGO HSs) as an advanced cathode material for SIBs by spray drying and subsequent pyrolysis. The rGO layer connected with NVP forms a network for high-speed electron transfer, and the spherical hollow structure enables a thorough impregnating of electrolyte, thereby facilitating the penetration of Na+. Interestingly, it is found via galvanostatic intermittent titration technique (GITT) that the Na+ diffusion coefficient of NVP will increase by an order of magnitude once it is coated by rGO to form the hollow spheres. As a consequence, NVP/rGO HSs display a series of remarkable electrochemical performance, such as a large reversible capacity of 116 mAh g−1 at 1 C (1 C = 118 mA g−1), superior rate performance (107.5 mAh g−1 at 10 C), and remarkable cycling stability (109 mAh g−1 after 400 cycles at 1 C; 73.1 mAh g−1 after 1000 cycles at 10 C). Notably, this is a precursory work on hollowstructured high-performance cathode for SIBs, and may shed new insights into the design and construction of high-performance energy storage applications. 2. Experimental 2.1. Synthesis of NVP/rGO HSs In the first step, 684.8 mg V2O5 was dispersed in 37.5 mL deionized water and added with 7.5 mL H2O2 (30%) under vigorous stirring. After 10 min, 932 mg CH3COONa and 1307 mg NH4H2PO4
were introduced to the system, forming a bright yellow precursor solution. Afterwards, 15 mL graphene oxide (GO) (10 mg mL1) was ultrasonically dispersed in the precursor of NVP for 20 min to produce a homogeneous suspension, which was subsequently subjected to the spray drying process with the pyrogenic temperature being 270 ℃. Finally, the obtained faint yellow micro-balls were annealed at 900 ℃ for 8 h under a H2/Ar (5:95) flow to yield NVP/rGO HSs. For comparison, pristine NVP was synthesized in the same way except the use of GO. 2.2. Material Characterization. X-ray diffraction (XRD) patterns were determined by a Rigaku SmartLab diffractometer using a Cu Kα radiation (λ =1.5406 Å). Raman spectra were collected on a Labram HR800 instrument (λ = 514 nm). The morphology of the samples was imaged by a field-emission scanning electron microscope (FESEM, JEOL JSM-7600F) operated at 10 kV. Transmission electron microscope (TEM) images and elemental mappings were achieved with a JEOL JEM-2100F operated at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded with an ESCALab250Xi electron spectrometer with an Al Kα radiation. Thermogravimetric analysis (TGA) was conducted with a NETZSCH STA 449 F3. Nitrogen sorption isotherms were obtained by an ASAP 2020 surface analyzer. 2.3. Electrochemical Characterization To prepare the working electrodes, active material, super-P carbon black, and polyvinylidene fluoride in a weight ratio of 8:1:1 were hand milled with a suitable amount of N-methyl-2-pyrrolidone into a homogeneous slurry, which was spread on an aluminum foil and then dried in the vacuum oven at 80 °C for 12 h. The average mass loading of each electrode was controlled to be around 1 or 5 mg cm−2. The working electrodes were assembled as the cathodes of coin cells (CR2032) in an argon-
filled glove box (MBRAUN, Unilab1200). The counter electrode was sodium; the separator was glass fiber (Whatman); and the electrolyte was 1 M NaClO4 in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) (95:5 in volume). To study the sodium storage performance of the full cell, sulfur-doped carbon microtubes (SCMTs) were selected as the anode material. The S-CMTs were synthesized through a previously reported strategy [21]. To prepare the anode, the S-CMTs, carboxymethyl cellulose sodium, and super-P were hand milled in a weight ratio of 70:15:15 into a slurry. The slurry was pasted onto a copper foil and vacuum-dried at 40 °C for 12 h. The counter electrode, the separator, and the electrolyte were the same as those in the NVP/rGO HSs cathode. Prior to assembling the full batteries, the NVP/rGO HSs was firstly activated in the half cell, and the S-CMTs anode was also operated between 0.01 and 3 V to eliminate the irreversible capacity from the initial cycle. Finally, the half cells were coupled together to obtain the full cell. In order to avoid Na planting on the anode, the capacity of cathode to anode is controlled to be 1:1.05. The cells were galvanostatically charged/discharged on a Land CT2001A testing system over a voltage range of 2.0−3.9 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on a PARSTAT 4000 workstation. GITT was used to investigate the Na+ diffusion coefficients with a pulse current of 10 mA g−1 for 0.5 h between rest intervals for 2 h. 3. Results and discussion Fig. 1 demonstrates the synthetic process of NVP/rGO HSs. Briefly, the hollow spheres were fabricated via spray drying a blend of GO and NVP precursor and subsequent annealing. The preparation steps are detailed in the Experimental Section. When the dispersion droplets were spray dried in the decomposition column, the solvent water exposed to the hot air was first vaporized, and
the precursor of NVP and graphene oxide was solidified in situ into spherical particles. During the subsequent pyrolysis process, the NVP precursor and graphene oxide decomposed to lose water, ammonia and methane. The NVP nanoparticles formed in the central part had higher surface energy than the NVP nanoparticles formed on the surface, so they can easily migrate to the surface and deposit, finally producing the hollow NVP/rGO microspheres. It is worth noting that the rGO plays an important role in bridging NVP nanoparticles and maintaining the morphology of the composite even though its content is as low as 5 wt % (Fig. S1) [22,23]. When the NVP precursor was spray dried without the addition of rGO, the regular spherical morphology cannot be observed (Fig. S2). Besides, the Barrett−Joyner−Halenda (BJH) pore-size-distribution show that a large number of nanopores are formed on NVP/rGO HSs with their sizes falling in the range of 2 to 45 nm (Fig. S3), indicating that the hollow spheres are highly porous. Fig. 2a displays the XRD patterns of NVP/rGO HSs and the pure NVP. All the characteristic peaks of NVP/rGO HSs are in good agreement with those of the pure NVP [24,25], mirroring that the crystal structure of NVP remains intact when wrapped by graphene. By applying the Scherrer equation (D = Kλ/B cos θ) to the (116) crystal plane of NVP/rGO HSs, the NVP nanoparticles impregnated in the hollow carbon spheres are calculated to be approximately 42 nm in diameter. The Raman spectrum of the pure NVP shows sharp peaks at around 1000 and 440 cm1, which can be assigned to the modes of vibration and bending for PO4 tetrahedral (Fig. 2b). The Raman fingerprints below 400 cm1 may be caused by the vibration/bending of the VO6 octahedral [26]. The characteristic peaks of NVP do not appear in the Raman spectrum of NVP/rGO HSs, which can be attributed to the following two reasons: (1) the rGO coating shields the signals of NVP to some extent; (2) the particle size of NVP nanocrystals is too small to be precisely detected [27,28]. The two evident
bands located at 1346 and 1601 cm1 stand for the disordered carbon band (D band) and the graphitic carbon band (G band), respectively, and the ID/IG ratio is calculated to be 0.91, which suggests the carbon spheres are amorphous with partial graphitization [29]. Fig. 3a and b show the SEM images of NVP/rGO HSs where the sizes of the spheres are observed to be in the range of 4 to 10 m and some NVP/rGO nanoparticles are found to be present onto the outer surface of the microspheres. The intimate contact between the NVP and rGO coatings can increase the electric conductivity of the NVP nanoparticles and prevent them from aggregation. The TEM image (Fig. 3c) clearly illustrates the unique hollow architecture of NVP/rGO HSs. The HRTEM image (Fig. 3d) further reveals that the rGO is about 3.3 nm thick and in close contact with the NVP nanoparticles. The interlayer distance of 0.37 nm corresponds to the (113) plane of NVP nanocrystals (JCPDS card No. 76-1962). The elemental mappings (Fig. 3e) unveil the even distribution of all the elements presenting in NVP/rGO HSs. In addition, the surface area of NVP/rGO HSs was determined as 23.26 cm2 g1 (Fig. S3), over ten times the value of the pristine NVP (1.75 cm2 g1). This result suggests that the contacting area between the NVP/rGO HSs and the electrolyte is larger, which probably enables faster charge transfer. XPS was performed to detect the surface chemical state of the NVP/rGO HSs. The presence of C, V, O and P can be clearly observed in the XPS survey scan (Fig. S4), which is in accordance with the result of elemental mappings. Notably, it is found that the V in the NVP/rGO HSs is in a mixed valence state rather than a pure trivalent state, and the atomic content of V3+ and V4+ is calculated to be 71.0% and 29.0%, respectively, indicating that the surface V3+ is partially oxidized under air atmosphere (Fig. 4a) [30]. Meanwhile, in the high-resolution C 1s spectrum (Fig. 4b), the C=O functional group (288.4 eV) which is usually detected in rGO disappears [31]. Such phenomenon
indicates that graphene oxide is almost completely reduced to generate rGO by heat treatment at 900 °C in a H2/Ar (5:95) atmosphere for 8 h [32]. From the point of view of defect chemistry, multivalent V can increase electron transfer and have a positive impact on reaction kinetics [33]. Except for C−O functional group, the main carbon atoms are in the sp2 hybrid state, and the binding energy of C=C bond is approximately 284.7 eV, which means that the NVP/rGO HSs are surrounded by electron clouds from the conjugated 2p orbital, increasing the electronic conductivity to a higher level [34]. To investigate the sodium storage properties of NVP/rGO HSs, this composite was assembled into coin cells, sodium metal being the counter electrode. Fig. 5a demonstrates the cyclic voltammetry (CV) curves of NVP/rGO HSs and NVP. The pair of characteristic redox peaks centered at 3.4 V represents
the
transformation
reaction
between
V3+
and
V4+,
companied
by
the
intercalation/deintercalation of two sodium ions [35]. In addition, the higher peak current intensity also manifests the better Na+ diffusion kinetics in NVP/rGO HSs, which can be attributed to the rGO shell and the porous hollow structure, enabling high-speed ionic and electronic transportation [36]. The potential gap between the initial charge/discharge profiles of the bare NVP (240 mV) is three times that of NVP/rGO HSs (80 mV), showing that the NVP/rGO HSs cathode has a much smaller polarization (Fig. 5b), highly agreeing with the CV curves. It is also worth mentioning that the capacity of NVP/rGO HSs and the pristine NVP are measured to be 124 and 119 mAh g−1, and the extra capacity over the theoretical value (118 mAh g−1) may derive from the contribution of the rGO and the carbon black. The rate capability (Fig. 5c) of NVP/rGO HSs is measured to be 121, 117.1, 113.2, 107.5 and 98.5 mAh g−1 at the rate of 1, 2, 5, 10 and 20 C, respectively. Such excellent rate capability has surpassed almost all the NVP-based cathodes reported to date (Table S1). On the contrary, although
the capacity of pure NVP is roughly the same as that of NVP/rGO HSs at 1 C, it drops quickly to 79 mAh g−1 at 10 C and 59.3 mAh g−1 at 20 C, which is significantly lower than that of NVP/rGO HSs. Furthermore, despite the drastic fluctuations of currents, a high reversible capacity of 116.3 mAh g−1 is still maintained for NVP/rGO HSs after 60 cycles, recovering to 96.1% of its original capacity. We also checked the rate performance of electrodes with higher mass loading of 5 mg cm−2. When the mass loading is increased from 1 to 5 mg cm−2, the capacity of NVP/rGO HSs is 109.3, 105.7, 96.8, 88.7 and 105 mAh g−1 at the rate of 1, 2, 5, 10 and 20 C, respectively (Fig. 5c). Though the rate performance of the high loading electrode is not as satisfying as that of the low loading electrode, especially at low current densities, the high loading electrode still displays decent capacities of 96.8 and 88.7 mAh g−1 at the high rates of 10 and 20 C, far exceeding those of the pure NVP. This outcome demonstrates that the extraordinary rate capability is mainly attributed to the special structure rather than the low mass loading. For the purpose of comprehensively assessing its stability, the cycling properties of NVP/rGO HSs were tested at 1 C and 10 C. As depicted in Fig. 5d, even though the NVP/rGO HSs cathode has been cycled for 400 cycles at 1 C, it can still exhibit a large capacity of 109 mAh g−1, greatly exceeding that of NVP (96.6 mAh g−1). Based on the lower tap density (0.91 g cm−3 vs. 1.02 g cm−3) and higher average output voltage (3.33 V vs. 3.28 V) of NVP/rGO HSs compared with NVP, the volumetric energy density of NVP/rGO HSs is still slightly larger than that of NVP (330.3 Wh L−1 vs. 323.2 Wh L−1). After 1000 cycles at 10 C, the NVP/rGO HSs electrode showed a decent capacity of 73.1 mAh g−1 and an average Coulombic efficiency approaching 100% (Fig. 5e). After the long cycling test, neither morphological changes (Fig. S5) nor structural variation (Fig. S6) were detected in NVP/rGO HSs, signifying a good structural stability. The outstanding sodium storage properties
of NVP/rGO HSs mentioned above can be attributed to their unique architecture. On the one hand, the porous hollow sphere not only enlarges the interface between electrode and electrolyte, but also contributes to a complete infiltration of the electrolyte into the NVP/rGO HSs electrode [37]. On the other hand, the tight combination of NVP and the graphene shells imparts a high-speed electron conduction network and a robust skeleton to the encapsulated NVP nanocrystals, thereby increasing the rate performance and cycling stability of NVP/rGO HSs. To evaluate the electronic and ionic conductivity of NVP/rGO HSs, we measured the EIS curves over the frequency range from 100 kHz to 10 mHz. The Nyquist plot depicted in Fig. 6a consists of a depressed semicircle at the high-frequency region and a quasi-straight line at the low-frequency region. The radius of the semicircle for NVP/rGO HSs is much smaller than that of the pristine NVP, implying that the charge transfer at the electrode/electrolyte interphase can be prominently facilitated by the intimate rGO coating and the porous hollow structure [38]. Furthermore, the diffusion coefficient of Na+ (DNa+) within NVP/rGO HSs and the pristine NVP were calculated according to following equation:
DNa +
R 2T 2 2 4 4 2 2 2A n F C
where R refers to the gas constant, T is 298.15K, A represents the surface area of the electrode, n represents the number of electrons transfer in one molecule of the active material during intercalation/deintercalation, F is the Faraday constant, C refers to the concentration of Na+, and σ is the Warburg factor, which can be determined by the slope of the Zre ⁓ ω−1/2 plot [39,40]. As shown in Fig. 6b, the σ of the NVP/rGO HSs electrode is smaller (231) than that of the pure NVP electrode (311), mirroring that the DNa+ value of NVP/rGO HSs (5.10 10−9 cm2 s−1) is larger than that of the pristine NVP (2.81 10−9 cm2 s−1).
Fig. 7a and b show the potential responses for NVP/rGO HSs and the pristine NVP utilizing GITT. Fig. 7c and d illustrate the evolution of the computed Na+ diffusion coefficients (DNa+) during Na+ insertion/extraction processes by using the following equation: 2
DNa +
4 mBVM Es M B S Et
2
L /D 2
Na +
where τ is the time of the employed galvanostatic current; mB, MB, and VM represent the mass, molecular weight, and molar volume of NVP, respectively; S is the total area between the electrode and the electrolyte; ΔEs is the variation of steady-state voltage after a single step; ΔEt represents the total transient voltage variation after imposing the galvanostatic current for the time τ; and L is the sample thickness [41]. Remarkably, the DNa+ values achieved from the GITT curves are in the range of 10−9−10−6 cm2 s−1, and the values of NVP/rGO HSs are an order of magnitude larger than those of the pristine NVP in both charging and discharging processes [42]. This finding is consistent with the EIS measurements shown above. The extraordinary properties of NVP/rGO HSs inspire us to investigate the sodium storage performance of the full cell. We selected S-CMTs as the anode material and coupled it with the NVP/rGO HSs cathode to form full cells. The SEM image and XRD pattern of the synthesized SCMTs are shown in Fig. S7a and b. The S-CMTs half-cell delivers an initial charge and discharge capacity of 532.1 and 850.3 mAh g−1 (Fig. S7c), and can cycle at 200 mA g−1 for 100 cycles with negligible capacity decrease (Fig. S7d). Fig. 8a depicts the charge and discharge profiles of the NVP/rGO HSs//S-CMTs full battery. The curves of the second and third cycles are overlapped, indicating the electrochemical reactions are reversible. In the third cycle, the charge and discharge capacities of the full cell are measured to be 97.8 and 98.8 mAh g−1, normalized by the weight of the NVP/rGO HSs. The full battery also presents a decent rate capability of 100.6, 92.4, 77.8, 53.5 and
33.8 mAh g−1 at the current densities of 1, 2, 5, 10 and 20 C, revealing that the rate performance of the NVP/rGO HSs composite is also competitive in full cells (Fig. 8b). The cycling stability of the full cell is demonstrated in Fig. 8c, where the reversible capacity remains 84.2 mAh g−1 after 100 cycles at 1 C, corresponding to a high capacity retention of 85.4%. 4. Conclusion In summary, we have fabricated Na3V2(PO4)3/reduced graphene hollow spheres (i.e., NVP/rGO HSs) as a high-performance cathode material for SIBs. The unique porous hollow structure benefits the penetration of electrolyte and significantly enlarges the electrolyte/electrode interface. Moreover, the close contact between the rGO shell and NVP nanocrystals helps to boost the electronic transport. As a result, the NVP/rGO HSs electrode exhibits superior electrochemical properties, including large reversible capacity (116 mAh g−1 at 1 C), excellent rate capability (107.5 mAh g−1 at 10 C and 98.5 mAh g−1 at 20 C), and long cycling life (109 mAh g−1 after 400 cycles at 1 C and 73.1 mAh g−1 after 1000 cycles at 10 C). The excellent electrochemical performance of NVP/rGO HSs in full batteries also enables it to be a competitive cathode for SIBs. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 51577094), the Natural Science Foundation of Jiangsu Province of China (BK20180086), and the 100 Talents Program of Nanjing Normal University. References [1] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: A battery of choices, Science 334 (2011) 928−935. [2] Y. You, H.R. Yao, S. Xin, Y.X. Yin, T.T. Zuo, C.P. Yang, Y.G. Guo, Y. Cui, L.J Wan,. J.B. Goodenough, Subzero-temperature cathode for a sodium-ion battery, Adv. Mater. 28 (2016)
7243−7248. [3] S. Bao, S. Luo, Z. Wang, S. Yan, Q. Wang, Improving the electrochemical performance of layered cathode oxide for sodium-ion batteries by optimizing the titanium content, J. Colloid Interf. Sci. 544 (2019) 164−171. [4] Q.C. Wang, J.K. Meng, X.Y. Yue, Q.Q. Qiu, Y. Song, X.J. Wu, Z.W. Fu, Y.Y. Xia, Z. Shadike, J. Wu, X.Q. Yang, Y.N. Zhou, Tuning P2-structured cathode material by na-site Mg substitution for Na-ion batteries, J. Am. Chem. Soc. 141 (2019) 840−848. [5] H. Wan, X. Hu, Sulfur-doped honeycomb-like carbon with outstanding electrochemical performance as an anode material for lithium and sodium ion batteries, J. Colloid Interf. Sci. 558 (2019) 242−250. [6] W. Chen, X. Zhang, L. Mi, C. Liu, J. Zhang, S. Cui, X. Feng, Y. Cao, C. Shen, High-performance flexible freestanding anode with hierarchical 3D carbon-networks/Fe7S8/graphene for applicable sodium-ion batteries, Adv. Mater. 31 (2019) 1806664. [7] L. Li, W. Zhang, X. Wang, S. Zhang, Y. Liu, M. Li, G. Zhu, Y. Zheng, Q. Zhang, T. Zhou, W. K. Pang, W. Luo, Z. Guo, J. Yang, Hollow-carbon-templated few-layered V5S8 nanosheets enabling ultrafast potassium storage and long-term cycling, ACS Nano 13 (2019) 7939−7948. [8] Y. Jiang, X. Zhou, D. Li, X. Cheng, F. Liu, Y. Yu, Highly reversible Na storage in Na3V2(PO4)3 by optimizing nanostructure and rational surface engineering, Adv. Energy Mater. 8 (2018) 1800068. [9] W. Li, Z. Yao, C. Zhou, X. Wang, X. Xia, C. Cu, J. Tu, Boosting high-rate sodium storage performance of N-doped carbon-encapsulated Na3V2(PO4)3 nanoparticles anchoring on carbon cloth, Small 15 (2019) 1902432. [10] J. Liu, K. Tang, K. Song, P. A. van Aken, Y. Yu, J. Maier, Electrospun Na3V2(PO4)3/C nanofibers as stable cathode materials for sodium-ion batteries, Nanoscale 6 (2014) 5081−5086. [11] P.N. Didwal, R. Verma, C.W. Min, C.J. Park, Synthesis of 3-dimensional interconnected porous Na3V2(PO4)3@C composite as a high-performance dual electrode for Na-ion batteries, J. Power Sources 413 (2019) 1−10. [12] W.X. Song, H.S. Hou, X.B. Ji, Progress in the investigation and application of Na3V2(PO4)3 for electrochemical energy storage, Acta Phys. Chim. Sin. 33 (2017) 103−129. [13] 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 sodiumion batteries, Adv. Energy Mater. 5 (2015) 1402104. [14] Y. Xu, Q. Wei, C. Xu, Q. Li, Q. An, P. Zhang, J. Sheng, L. Zhou, L. Mai, Layer-by-layer Na3V2(PO4)3 embedded in reduced graphene oxide as superior rate and ultralong-life sodium-ion battery cathode, Adv. Energy Mater. 6 (2016) 1600389. [15] E. Gu, J. Xu, Y. Du, X. Ge, X. Zhu, J. Bao, X. Zhou, Understanding the influence of different carbon matrix on the electrochemical performance of Na3V2(PO4)3 cathode for sodium-ion batteries, J. Alloys Compd. 788 (2019) 240−247. [16] Q. Hu, J.Y. Liao, X.D. He, S. Wang, L.N. Xiao, X. Ding, C.H. Chen, In situ catalytic formation of graphene-like graphitic layer decoration on Na3V2-xGax(PO4)3 (0 ≤ x ≤ 0.6) for ultrafast and high energy sodium storage, J. Mater. Chem. A 7 (2019) 4660−4667. [17] J. Huang, X. Tang, Z. Li, K. Liu, Metal organic frameworks derived cobalt sulfide/reduced graphene oxide composites with fast reaction kinetic and excellent structural stability for sodium storage, J. Colloid Interf. Sci. 532 (2018) 407−415. [18] W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Self-
sacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for high-rate sodium in full batteries, Nano Energy 25 (2016) 145−153. [19] D. Ni, W. Sun, Z. Wang, Y. Bai, H. Lei, X. Lai, K. Sun, Heteroatom-doped mesoporous hollow carbon spheres for fast dodium dtorage with an ultralong cycle life, Adv. Energy Mater. 9 (2019) 1900036. [20] Y. Liu, X.-Y. Yu, Y. Fang, X. Zhu, J. Bao, X. Zhou, X.W. Lou, Confining SnS2 ultrathin nanosheets in hollow carbon nanostructures for efficient capacitive sodium storage, Joule 2 (2018) 725−735. [21] Q. Wang, X. Ge, J. Xu, Y. Du, X. Zhao, L. Si, X. Zhou, Fabrication of microporous sulfur-doped carbon microtubes for high-performance sodium-ion batteries, ACS Appl. Energy Mater. 1 (2018) 6638−6645. [22] Z. Pan, G. Zhang, X. Wang, Polymeric carbon nitride/reduced graphene oxide/Fe2O3: all-solidstate Z-scheme system for photocatalytic overall water splitting, Angew. Chem. Int. Ed. 58 (2019) 7102−7106. [23] J. Zhang, Y. Fang, L. Xiao, J. Qian, Y. Cao, X. Ai, H. Yang, Graphene-Scaffolded Na3V2(PO4)3 microsphere cathode with high rate capability and cycling stability for sodium ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 7177−7184. [24] X. Li, S. Wang, X. Tang, R. Zang, P. Li, P. Li, Z. Man, C. Li, S. Liu, Y. Wu, G. Wang, J. Colloid Interf. Sci. 539 (2019) 168−174. [25] H. Zhang, I. Hasa, D. Buchholz, B. Qin, S. Passerini, Effects of nitrogen doping on the structure and performance of carbon coated Na3V2(PO4)3 cathodes for sodium-ion batteries, Carbon 124 (2017) 334−341. [26] W. Song, X. Ji, Y. Yao, H. Zhu, Q. Chen, Q. Sun, C.E. Banks, A promising Na3V2(PO4)3 cathode for use in the construction of high energy batteries, Phys. Chem. Chem. Phys. 16 (2014) 3055−3061. [27] 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 roomtemperature sodium-ion batteries, Adv. Energy Mater. 3 (2013) 156−160. [28] E. Gu, S. Liu, Z. Zhang, Y. Fang, X. Zhou, J. Bao, An efficient sodium-ion battery consisting of reduced graphene oxide bonded Na3V2(PO4)3 in a composite carbon network, J. Alloys Compd. 767 (2018) 131−140. [29] 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. [30] H. Li, X. Yu, Y. Bai, F. Wu, C. Wu, L.Y. Liu, X.Q. Yang, Effects of Mg doping on the remarkably enhanced electrochemical performance of Na3V2(PO4)3 cathode materials for sodium ion batteries, J. Mater. Chem. A 3 (2015) 9578−9586. [31] N. Wang, Q. Liu, B. Sun, J. Gu, B. Yu, W. Zhang, D. Zhang, N-doped catalytic graphitized hard carbon for high-performance lithium/sodium-ion batteries, Sci. Rep. 8 (2018) 9934. [32] L. Luo, B. Cheng, S. Chen, Z. Ge, H. Zhuo, Three-dimensional Na3V2(PO4)3/carbon nanofiber networks prepared by electrospinning as self-standing cathodes for high performance Na-ion batteries, Mater. Lett. 232 (2018) 153−156. [33] L. Zhao, H. Zhao, Z. Du, J. Wang, X. Long, Z. Li, K. Swierczek, Delicate lattice modulation enables superior Na storage performance of Na3V2(PO4)3 as both an anode and cathode material for sodium-ion batteries: Understanding the role of calcium substitution for vanadium, J. Mater. Chem.
A 7 (2019) 9807−9814. [34] W. Shen, C. Wang, Q. Xu, H. Liu, Y. Wang, Nitrogen-doping-induced defects of a carbon coating layer facilitate Na-storage in electrode materials, Adv. Energy Mater. 5 (2015) 1400982. [35] W. Song, X. Ji, Z. Wu, Y. Zhu, Y. Yang, J. Chen, M. Jing, F. Li, C.E. Banks, First exploration of Na-ion migration pathways in the NASICON structure Na3V2(PO4)3, J. Mater. Chem. A 2 (2014) 5358−5362. [36] Y. Fang, X. Xu, Y. Du, X. Zhu, X. Zhou, J. Bao, Novel nitrogen-doped reduced graphene oxidebonded Sb nanoparticles for improvedsodium storage performance, J. Mater. Chem. A 6 (2018) 11244−11251. [37] P. Hu, X. Wang, J. Ma, Z. Zhang, J. He, X. Wang, S. Shi, G. Cui, L. Chen, NaV3(PO4)3/C nanocomposite as novel anode material for Na-ion batteries with high stability, Nano Energy 26 (2016) 382−391. [38] L. Yu, X. Zhang, Electrochemical insertion of magnesium ions into V2O5 from aprotic electrolytes with varied water content, J. Colloid Interf. Sci. 278 (2004) 160−165. [39] Q. Wang, X. Zhu, Y. Liu, Y. Fang, X. Zhou, J. Bao, Rice husk-derived hard carbons as highperformance anode materials for sodium-ion batteries, Carbon 127 (2018) 685−666. [40] Y. Zhang, P. Wang, Y. Yin, N. Liu, L. Fan, N. Zhang, K. Sun, Carbon coated amorphous bimetallic sulfide hollow nanocubes towards advanced sodium ion battery anode, Carbon 150 (2019) 378−387. [41] J. Huang, X. Tang, Z. Li, K. Liu, Metal organic frameworks derived cobalt sulfide/reduced graphene oxide composites with fast reaction kinetic and excellent structural stability for sodium storage, J. Colloid Interf. Sci. 532 (2018) 407−415. [42] J. Zeng, D. Wu, X. Wang, J. Wu, J. Li, J. Wang, J. Zhao, Insights into the Mg storage property and mechanism based on the honeycomb-like structured Na3V2(PO4)3/C/G in anhydrous electrolyte, Chem. Eng. J. 372 (2019) 37−45.
Fig. and Table Captions
Fig. 1. Schematic illustration for the fabrication of NVP/rGO HSs. Fig. 2. (a) XRD patterns and (b) Raman spectra of NVP/rGO HSs and pure NVP.
Fig. 3. (a, b) SEM images, (c) TEM image, (d) HRTEM images, and (e) elemental mappings of C, Na, V, P, and O for NVP/rGO HSs. Fig. 4. High-resolution (a) V 3d and (b) C 1s XPS spectra of NVP/rGO HSs. Fig. 5. (a) The initial CV curves, (b) the first charge and discharge profiles, (c) rate capabilities, and (d) cycling performance at 1 C of the NVP/G HSs and the pristine NVP. (e) Prolonged cycling performance of NVP/rGO HSs at 10 C. Fig. 6. (a) Nyquist plots and (b) linear fitting line of Zre versus ω−1/2 of NVP/rGO HSs and the pristine NVP. Fig. 7. (a, b) GITT curves and (c, d) the corresponding Na+ diffusion coefficients of NVP/rGO HSs and the pristine NVP. Fig. 8. (a) Charge and discharge profiles, (b) rate capability, and (c) cycling performance of the NVP/rGO HSs//S-CMTs full battery.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
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Fig. 8.
Credit Author Statement
The author declares that all the data in this work is authentic and repeatable.
Declaration of Interest Statement
The authors declare no competing financial interest.
Graphical abstract