Carbon-Coated Na3V2(PO4)3 Embedded in Porous Carbon Matrix

Mar 28, 2014 - Rational construction of Na 3 V 2 (PO 4 ) 3 nanoparticles encapsulated in 3D honeycomb carbon network as a cathode for sodium-ion batte...
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Carbon-Coated Na3V2(PO4)3 Embedded in Porous Carbon Matrix: An Ultrafast Na-Storage Cathode with the Potential of Outperforming Li Cathodes Changbao Zhu,‡ Kepeng Song,§ Peter A. van Aken,§ Joachim Maier,‡ and Yan Yu*,†,‡ †

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, P. R. China ‡ Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany § Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, Stuttgart 70569, Germany S Supporting Information *

ABSTRACT: Sodium ion batteries are one of the realistic promising alternatives to the lithium analogues. However, neither theoretical energy/power density nor the practical values reach the values of Li cathodes. Poorer performance is expected owing to larger size, larger mass, and lower cell voltage. Nonetheless, sodium ion batteries are considered to be practically relevant in view of the abundance of the element Na. The arguments in favor of Li and to the disadvantage of Na would be completely obsolete if the specific performance data of the latter would match the first. Here we present a cathode consisting of carbon-coated nanosized Na3V2(PO4)3 embedded in a porous carbon matrix, which not only matches but even outshines lithium cathodes under high rate conditions. It can be (dis)charged in 6 s with a current density as high as 22 A/g (200 C), still delivering a specific capacity of 44 mAh/g, while up to 20 C, the polarization is completely negligible. KEYWORDS: Na3V2(PO4)3, porous carbon matrix, ultrafast sodium storage, double carbon-embedding, sodium ion batteries

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Theoretical and experimental work has shown that the migration barriers can be lower for Na+ than for Li+ in various relevant structures.13,16−18 In aqueous metal-ion batteries, fast transport of Na+ or K+ has already been exploited.19−21 Especially, NASICON-type22,23 Na3V2(PO4)3 has recently been considered as a potential cathode material for SIBs24−26 because of its highly covalent 3D framework crystalline structure, high theoretical energy density (∼400 Wh/kg), and good thermal stability.27 The high sodium mobility of these compounds mentioned above is certainly an important asset; in addition, the presence of the redox variable vanadium atom confers on the phosphate a perceptible electronic conductivity and hence a substantial chemical diffusion coefficient of Na. Yet, the electronic conductivity is not high enough (cf. typical NASICON-type materials)17,28,29 to provide sufficient electronic connectivity between the particles. In principle, there are two ways of increasing the transport kinetics: the first is to vary the composition of the electroactive material (doping) in order to make it sufficiently mixed conducting; the second, which appears most promising in this case, is to reduce the size of the electroactive particles30,31 and to provide an effective mixed-

s far as sodium ion batteries (SIB) are concerned, economic and availability issues are so favorable1,2 that one puts up with the expectedly distinctly diminished performance. Hence, considerable attention has been paid to find suitable electrode materials for SIBs. As far as cathode materials are concerned, oxides and phosphates such as NaxCoO2,3 Na0.44MnO2,4,5 Na1−xNi0.5Mn0.5O2,6 NaFePO4,7,8 Na2FePO4F,9 Na3V2(PO4)2F3,10 Na0.6Fe0.5Mn0.5O2,11 and NaNi1/3Mn1/3Co1/3O212 have been extensively investigated. The greater ionic radius in conjunction with the greater molecular mass is responsible for the inferior electrochemical characteristics when compared to LIBs: lower theoretical capacity, lower cell voltage, frequently less rapid transport, more severe structural impact, and volume variation, and even if the absolute performance was comparable, a diminished specific performance (i.e., by mass or volume). However, there are also arguments referring to the kinetics that speak in favor of sodium storage. (i) In selected materials, (e.g., NASICON), the conductivity for Na+ is higher than that for Li+ and hence also the chemical diffusion coefficient of the respective component.13 (ii) The transfer reaction from liquid electrolyte to solid electrode may be easier for Na+ on account of the less extensive solvation of the bigger cation.14 On these grounds, open framework structures or 3D structures with large insertion channels for Na+ appear to be promising cathode candidates.15 © 2014 American Chemical Society

Received: February 11, 2014 Revised: March 20, 2014 Published: March 28, 2014 2175

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Figure 1. Schematic comparison of growth process. (a) Conventional solid-state process for microsized carbon-coated Na3V2(PO4)3. (b) Facile softchemistry-based double carbon-embedding approach for (C@NVP)@pC.

Figure 2. Structure, porosity, and morphology characterization. (a) XRD patterns of as-prepared Na3V2(PO4)3, Na3V2(PO4)3 annealed at 650 °C in Ar/H2 and Na3V2(PO4)3 annealed at 650 °C in Ar/H2 followed by annealed at 800 °C for in Ar. (b) Nitrogen adsorption/desorption isotherms of (C@NVP)@pC nanocomposite. Inset: the pore-size distribution plot calculated by the BJH method in the adsorption branch isotherm. (c, d) SEM images of (C@NVP)@pC annealed at 650 °C in Ar/H2 and (C@NVP)@pC annealed at 650 °C in Ar/H2 followed by annealed at 800 °C for in Ar.

conducting network.29,32 In the latter case, electrons are transported to the particles by a well-conducting current collecting network (typically carbon) and ions by providing electrolyte channels down to the nanoscale. Although the rate performance and cycling stability have been largely improved recently,32 the achieved electrochemical performance of

Na3V2(PO4)3, especially at high rate performance, is by no means comparable with high performance lithium cathodes. In this work, we propose and realize a powerful nanostructure design for Na3V2(PO4)3 that combines the advantages of carbon-coating nanosized particles with the presence of a porous carbon matrix. To achieve such a “double carbon-embedding”, we complement a facile soft-chemistry 2176

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NVP)@pC is not more than ∼17 wt %, as measured by elemental analysis (carbon sulfur determinator, ELTRA, CS800). Morphology and size of (C@NVP)@pC after annealing at 650 and 800 °C were further investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) technologies. SEM images (Figure 2c,d) show microsized clusters with agglomerated nanosized grains. The grain sizes are around 20−40 nm for (C@NVP)@pC after annealing at 650 °C, and they slightly increase to 40−100 nm after annealing at 800 °C (also see in Figure 3). Although a few

based preparation method with a post heat-treatment procedure. The resulting nanostructure consisting of carboncoated nanosized Na3V2(PO4)3 particles embedded in a highly effectively mixed conducting porous carbon matrix is denoted as (C@NVP)@pC. Its charge and discharge properties are truly impressive: up to 20 C, polarization effects are negligible (capacity of around 100 mAh/g close to the theoretical value); at a current as high as 22 A/g (200 C) the electrode can still deliver a specific capacity of 44 mAh/g. This ultrafast rate performance is comparable to that of a supercapacitor, but connected with a much higher energy density. Apart from very few exceptions, this outstanding high rate behavior even exceeds that of almost all promising lithium battery cathode materials. Results and Discussion. Figure 1 illustrates the facile fabrication process leading to this powerful (C@NVP)@pC architecture in comparison with the conventional solid-state method. In the solid-state approach, vanadate (NH4VO3) or vanadium oxide (V2O3) was employed, and during annealing, microsized Na3V2(PO4)3 (NVP) particles were obtained. Postcarbon-coating of these microsized particles usually results in inhomogeneous and incomplete coating.33 While in our “double carbon-embedding” process, we arrive at a homogeneous embedding of fully carbon-coated NVP particles in a porous carbon matrix (a description of the detailed procedure can be found in the Experimental Section of the Supporting Information). Here, VO(C5H7O2)2 was chosen as vanadium precursor, which will decompose at around 280 °C (as shown in DSC, Supporting Information, Figure S1), while in the softchemistry process, the precursor dissolved in the polyol was heated at 320 °C, leading to an amorphous NVP/C core−shell structure.34 Furthermore, tetraethylene glycol (TEG) is not only used as solvent but also applied as reducing agent and capping agent.35 As a result, the TEG molecules will cover the surface of an amorphous NVP/C core−shell structure, which on post-heat-treating transforms to the favorable nanostructure described. Additionally, the process has the advantage of preventing particle growth during annealing. Figure 2a displays the X-ray diffraction (XRD) patterns of the as-prepared Na3V2(PO4)3 samples and the ones postannealed at different temperatures and atmospheres. At the end of the polyol process, the as-prepared sample is amorphous; further annealing at 650 °C in Ar/H2 transforms it to a phasepure, crystalline Na3V2(PO4)3. In order to increase the effective conductivity of the carbon, the sample was once again heated but now at 800 °C in Ar. Na3V2(PO4)3 has the NASICON structure with R3c̅ (rhombohedral, no. 167) space group and the cell parameters a = 8.72168(9) Å and c = 21.80768(0) Å (RBragg = 8.21), as obtained by Rietveld refinement (Supporting Information, Figure S2). The pore structure and surface area of (C@NVP)@pC were investigated using a nitrogen isothermaladsorption technique (Figure 2b), the result being a type IV isotherm with hysteresis loop. This indicates a nanoporous structure with high surface area (175 m2/g) even after high temperature annealing (800 °C). The BJH pore-size distribution plot (inset of Figure 2b) shows that the carbon matrix is characterized by mesopores of 3.6 nm as mean diameter. The existence of carbon is also confirmed by Raman spectra showing peaks at 1339 cm−1 (D-band) and 1593 cm−1 (Gband) (Supporting Information, Figure S3). The ID/IG ratio is 0.939, which shows larger amount of sp2-type carbon than the sp3-type carbon, which results in high electronic conductivity and excellent rate performance. The carbon content of (C@

Figure 3. Morphology characterization by TEM technology. TEM (a, c, e) and HRTEM (b, d, f) images of (C@NVP)@pC annealed at 650 °C in Ar/H2 (a, b) and (C@NVP)@pC annealed at 650 °C in Ar/H2 followed by annealed at 800 °C for in Ar (c, d) and (C@NVP)@pC after 1000 charge−discharge cycles (e, f). Note that the surface carbon layers are marked by the red lines in HRTEM (b, d, f).

particles with sizes of hundred nanometers are also observed after annealing at 800 °C, the vast majorities of the grains are smaller and of a mean crystallite size of ∼60 nm (estimated from the width of the strongest peak of (113) by Scherrer’s equation). Furthermore, the NVP nanoparticles are welldispersed in the porous carbon matrix and exhibit 2−5 nm thin carbon layers as surface coating (see in Figure 3). The HRTEM images of (C@NVP)@pC after annealing at 650 and 800 °C (Figure 3b,d) reveal clear lattice fringes with d-spacings of around 0.43 and 0.44 nm, which correspond to the (110) 2177

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Figure 4. Electrochemical characterization of (C@NVP)@pC. (a) Galvanostatic charging−discharging profiles of (C@NVP)@pC at different current rates. (b) Excellent high rate performance and capacity retention ability of (C@NVP)@pC. (c) Long cycling stability of (C@NVP)@pC at various high current rates and Coulombic efficiency for 1000 cycles at 100 C. (d) Comparison of rate performance of (C@NVP)@pC to the recently results in the literature for NVP. (The solution-based, pyro-synthesis, graphene-supported, sol−gel, solid-state methods correspond to refs 32, 30, 29, 31, and 24, respectively. Our comparison is based on the reported capacities with different or unreported electrode thicknesses and loading masses, which can have a significant effect on the battery performance.)

than 1−20 Wh/kg.36 The cycling stability is outstanding as well (Figure 4c). After 1000 cycles, the capacities are still 83, 73, and 51 mAh/g at 10, 50, and 100 C, respectively, and Coulombic efficiency remains at 98% even after 1000 cycles at 100 C (Figure 4c). (Note that by optimization of the electrolyte, the Coulombic efficiency can be even improved.25) It is important to mention that even after 1000 charge−discharge cycles at 100 C rate, the Na3V2(PO4)3 still maintains its original morphology and structure: as can be seen in XRD (Supporting Information, Figure S6) and HRTEM (Figure 3e,f), the lattice fringes ((113) planes) are clearly identified, and only the carbon layer became slightly thicker on the outside of the particle (probably due to SEI formation, Figure 3f). This morphological invariance explains the excellent cycling stability. To the best of our knowledge, this high rate behavior is unprecedented for a nonaqueous room temperature sodium ion battery, let alone for any Na3V2(PO4)3-based electrode in the literature24,29−32 (Figure 4d). Furthermore, the ultrahigh rate performance of (C@NVP)@ pC is at least comparable to if not better than the common high-power cathode materials for lithium ion batteries (LiCoO2, LiFePO4, and LiMn2O4). In Supporting Information (Figure S7), high rate capacity and capacity retention (ratio of capacities at high C rate compared to the practical capacity at low C rates) of our (C@NVP)@pC are compared with LiCoO2,37 LiFePO4,35,38,39 and LiMn2O4.40 Note that for this comparison we picked the very best reports in the literature. The vast majority of the results for Li cathodes are not even comparable at high rates. Those rare reports that describe better performance at high rate use distinctly higher carbon contents (i.e., 45 wt % conductive carbon or higher instead of

and (104) lattice planes of NASICON-type NVP, indicating a high degree of crystallinity. In order to check the performance of this electrode, (C@ NVP)@pC/electrolyte/Na cells were constructed and tested within the electrochemical window of 2.3−3.9 V. The significance of temperatures and atmospheres of the postheat-treatment for performance is discussed in the Supporting Information. The charge and discharge profiles of (C@NVP)@ pC obtained at different current densities are shown in Figure 4a (note that 1 C means the full capacity can be charged or discharged in 1 h and 1 C = 110 mA/g in this work) and indicate that (C@NVP)@pC can be ultrarapidly charged and discharged within a few seconds, delivering both high energy and high power density. In spite of perceptible polarization issues at such high C rate (200 C), the discharge plateau around 3 V is clearly distinguished (Figure 4a). The specific discharge capacities are 104, 103, 102, 96, 91, 74, and 44 mAh/ g at current rates of 1, 10, 20, 30, 50, 100, and 200 C, respectively (Figure 4b). These capacity values are referred to the mass of NVP; if referred to the total mass (NVP plus carbon), the value is to be multiplied by 0.83. If the power density is enhanced by almost 2 orders of magnitude from 430 to 39 000 W/kg, the energy density retains about 66% of its value (from 342 to 227 Wh/kg), corresponding to a truly outstanding rate performance. Even at a power density as large as 79 kW/kg, the energy density is still 131 Wh/kg. The capacity retention is quite impressive as well: after high rate charge−discharge (200 C), the capacity can still recover 101 mAh/g at 1 C (Figure 4b). Let us compare this with a supercapacitor. There, typical power densities of around 1−20 kW/kg are connected with typical energy densities of not more 2178

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Though the position in the periodic table is of disadvantage for Na in various aspects (such as molecular weight, cell voltage), the greater size obviously offers also benefits in specific cases: (i) The Na+ ion size is optimal for fast ion transport in the NASICON structure leading to a fast diffusion therein. (ii) The larger ionic radius of Na+ compared with Li+ favors charge transfer kinetics owing to less pronounced solvation in the liquid electrolyte. These properties may explain why the abovedescribed NVP electrode not only matches the specific performance at high rates but apart from very few exceptions outperforms high power Li cathodes in spite of the intrinsic disadvantages such as the greater molecular weight. Conclusions. In summary, we propose and realize a promising nanostructure design of Na3V2(PO4)3 through a double carbon-embedding process by using a facile softchemistry based method with post-heat-treatment. The highrate charge and discharge properties are outstanding: i.e., it can be charged or discharged in 6 s with high current density of 22 A/g and delivers the specific capacity of 44 mAh/g. This ultrafast performance of (C@NVP)@pC is comparable to that of supercapacitor, but with much higher energy density (10 times higher). These excellent performance features together with long cycling stability outshine that of commonly used lithium battery cathode material including LiCoO2 and LiFePO4. In view of the much lower price for the sodium compound, we anticipate our (C@NVP)@pC to be highly relevant for energy storage. Considering the facile double carbon-embedding process, this method (as evidenced for LiFePO4) could also be extended to prepare other cathode or anode materials for lithium or sodium ion batteries.

20 wt % in this work). A more systematic comparison is enabled by considering LiFePO4, which shows a similar voltage plateau. Applying our combination polyol synthesis and postheat-treatment, we arrived at a carbon-coated LiFePO4, whose high rate performance is very close to the one of the best results in the literature,35,38 indicating the very efficient network architecture achieved by our process. (The preparation of such homemade carbon-coated LiFePO4 can be found in the Supporting Information. Note that the particle size of such homemade LiFePO4 is ∼100 nm (see Supporting Information, Figure S8), and the carbon content in the LiFePO4/C composite is around 6 wt %.) Nonetheless, this electrode is outperformed by (C@NVP)@pC as displayed in Figure 5 (also

Figure 5. Rate performance of (C@NVP)@pC in comparison with homemade carbon-coated LiFePO4. The same battery assembly procedure was used, showing the potential of outperforming Li cathodes.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details; Figures S1−S10. This material is available free of charge via the Internet at http://pubs.acs.org.

see in Supporting Information, Figure S9). As here the same battery assembly approach is used, these results shed light on the excellent kinetic parameters of the sodium compound itself. The determination of the effective diffusion coefficient is relevant here, which can be calculated by applying Randles− Sevcik analysis on CV measurements at different scan rates (in Supporting Information, Figure S10).41 Using the geometric electrode area an apparent diffusion coefficient of 1.0 × 10−10 cm2/s is obtained for (C@NVP)@pC. This is a very high room temperature value that explains the absence of substantial diffusional polarization even at 20 C (requiring D values higher than 10−13 cm2/s, estimated according to τ = L2/D, with τ ≤ 180 s (20 C) and L = 60 nm (mean particle size)). Evidently the charge transfer kinetics is fast as well. Hence, the ultrafast charge−discharge performance and long cycling stability of (C@NVP)@pC (besides the crystallographic and redox features) can be ascribed to both the advantages of the ionic properties and the advantages of the favorable morphology. Let us first list the latter ones: (i) Nanosized particles effectively decrease the ambipolar Na diffusion time in the crystal. (ii) The homogeneous and complete thin carbon-coating layer on the surface of NVP ensures fast electron transport between and within the NVP particles. (iii) The highly conductive porous carbon matrix with large surface area and mesopores filled by electrolyte facilitates fast charge transfer across the electrolyte/electrode interface by forming a well-connected 3D mixed-conducting network. (iv) The porous carbon matrix can be considered as elastic buffer to mitigate strain effects caused by volume change during Na insertion and extraction, leading to stable cycling performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Sofja Kovalevskaja award of the Alexander von Humboldt Foundation, the MaxPlanck Society, the National Natural Science Foundation of China (No. 21171015 and No. 21373195), the “1000 plan” program of the Chinese Government, the New Century Excellent Talents in University (NCET), and the Fundamental Research Funds for the Central Universities (WK2060140014, WK2060140016). K.S. acknowledges financial support from the PhD student exchange program between Max-Planck Society and Chinese Academy of Sciences and the Natural Sciences Foundation of China (Grant No. 51221264). The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement no. 312483 (ESTEEM2).

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