A High Power–High Energy Na3V2(PO4)2F3 Sodium Cathode

May 30, 2017 - The rather ideal bicontinuous electrode architecture, NVPF nanoparticles (∼30–50 nm) embedded in a 3D graphene network, provides pr...
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A High Power−High Energy Na3V2(PO4)2F3 Sodium Cathode: Investigation of Transport Parameters, Rational Design and Realization Changbao Zhu,‡ Chao Wu,‡ Chia-Chin Chen,‡ Peter Kopold,‡ 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, Anhui 230026, P. R. China ‡ Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart, 70569, Germany S Supporting Information *

ABSTRACT: Sodium ion batteries are realistic and promising alternatives to lithium due to the abundance of Na and the similar intercalation chemistry of Na when compared to the lithium counterpart. Developing high-power and high-energy sodium batteries is still a significant challenge. Na3V2(PO4)2F3 (NVPF) has been shown to combine excellent charge−discharge kinetics with a competitively high voltage. However, the major issue is, as for the vast majority of electrode materials, the lack of distinct knowledge of fundamental transport parameters, on which an optimized strategy for developing a high-power and high-energy sodium cathode can be based. This work aims at filling this gap. We experimentally investigate the intrinsic ionic and electronic conductivities, as well as the chemical diffusion coefficient of sodium of Na3V2(PO4)2F3 by impedance and dc polarization. On the basis of these results, we develop an optimized design. As the electronic conductivity is found to be much smaller than the ionic one, electronic wiring of the particles (by a graphene network) has higher priority than providing electrolyte contact. This is important since the contact by graphene and electrolyte wetting is partly antagonistic, not so much because of interfacial tensions rather because of the introduced heterogeneities on the nanoscale (cf. Lotus effect). We develop and apply a one-step costeffective low temperature hydrothermal method without any postheat treatment for the fabrication of a nanoparticulate (∼30−50 nm) NVPF electrode, which provides sufficient porosity and in which every nanoparticle is connected to the graphene network. In terms of rate capability, the performance of this electrode is excellent and at least belongs to the best Na storage performances reported for Na3V2(PO4)2F3 so far.

1. INTRODUCTION

At present, sodium layered oxides and polyanionic-type compounds have been extensively investigated as sodium cathodes.4−21 Particularly, NASICON-type Na3V2(PO4)3 has shown its potential as cathode material for high-power and high-energy SIBs.22,23 We have demonstrated that at high rates Na3V2(PO4)3 can even outperform well-established lithium cathodes (i.e., LiFePO4) in terms of specific capacity.24 As we aim at even higher energy densities, the high voltage

Notwithstanding the fact that lithium ion batteries (LIBs) have become popular in consumer electronics and electric vehicles, the relatively high cost and the geographic constraints associated with limited lithium resources are major limitations for large-scale application.1,2 Sodium ion batteries (SIBs) are promising alternatives due to the abundance of Na and the farreachingly analogous intercalation chemistry when compared to their lithium counterparts.3 Owing to the often astonishingly good performance, SIBs are indeed serious competitors to LIBs, especially for large-scale energy storage application. © 2017 American Chemical Society

Received: March 7, 2017 Revised: May 29, 2017 Published: May 30, 2017 5207

DOI: 10.1021/acs.chemmater.7b00927 Chem. Mater. 2017, 29, 5207−5215

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Figure 1. (a) Ion blocking cell used in this work, where Ti is reversible for electronic exchange but blocks sodium ions. (b) ac impedance spectra of Ti/NVPF/Ti at T = 130 °C with its equivalent circuit. For ac spectra, the fitting result with the given equivalent circuit is displayed by the red line. (c) dc polarization curve of Ti/NVPF/Ti at T = 130 °C. An applied current for dc polarization is 1 × 10−11 A. (d) dc polarization curve of Ti/ NVPF/Ti at T = 130 °C; the measured voltage nicely follows a square root time behavior (i.e., semi-infinite diffusion) over the entire range (linear fitting by red lines) (cf. Supporting Information).

fluorophosphate Na3V2(PO4)2F3 is a good candidate as already suggested by Ponrouch et al.25 The theoretical energy density of Na3V2(PO4)3 is ∼400 Wh/kg, while for Na3V2(PO4)2F3, due to the higher ionicity and the higher equilibrium voltage, the (de)intercalation of 2 Na+ ions corresponds to a theoretical energy density of ∼507 Wh/kg, which is comparable to commercial lithium cathodes, such as LiFePO4 (580 Wh/kg) and LiMn2O4 (480 Wh/kg).26 Na3V2(PO4)2F3 is a promising polyanionic compound. The crystal structure of Na3V2(PO4)2F3 was first established by Le Meins et al.27 in 1999 with the space group P42/mnm. However, the subtle orthorhombic distortion in the space group of Amam for Na3V2(PO4)2F3 was observed by Bianchini et al. by using a high angular resolution synchrotron radiation diffraction technique.11 Recently, the crystal structures of stoichiometric Na3V2(PO4)2F3 and the compounds with an impact on oxygen content were carefully investigated by Broux et al. as well.29 The crystal structure of Na3V2(PO4)2F3 is composed of pairs of corner shared VO4F2 octahedra, which are equatorially connected to the PO4 tetrahedra via O atoms. Na ions reside in the open tunnel sites along [110] and [110̅ ] directions,11 which offers efficient diffusion pathways.28 Such covalent 3D open framework leads to pronounced advantages for Na3V2(PO4)2F3, such as high structure stability, thermal safety, and high sodium mobility.30 There are various approaches to prepare a well-working sodium cathode based on Na3V2(PO4)2F3. The most frequently used method involves a solid state high temperature reaction step31,32 resulting in rather large particle sizes and nonuniform carbon coating and hence in poor rate performance and cycling stability. Furthermore, this process is rather energy consuming.33 Recently, a low temperature soft chemistry (solvothermal) route to prepare Na3V2(PO4)2F3 was reported, avoiding the costly high temperature step.34 However, rate capability and

cycling stability are still far from being satisfactory, when compared to Na3V2(PO4)3. A major shortcoming, as far as a strategic electrode design, here and for other electrode materials, is concerned, is the lack of knowledge of fundamental transport parameters, viz., electronic conductivity, ionic conductivity, and chemical diffusivity of Na. Note that the latter quantity determines the rate of redistribution of the element in the particle, while the other quantities are decisive for electron or ion transport through reacted particles to the particle to be (dis)charged. (Note that the transport properties are expected to change during the charge, because the diffusivity of Na in Na3V2(PO4)2F3 (NVPF) varies with different sodium contents owing to the variations of sodium/vacancy orderings.35,36) Here, for the first time, we directly and experimentally investigate these parameters by ac impedance and dc polarization using the ion blocking Ti/Na3V2(PO4)2F3/Ti cell and use this information strategically to design and realize a nanoparticulate (∼30−50 nm) Na3V2(PO4)2F3 electrode, in which every nanoparticle is connected to the graphene network, demonstrating superior energy and power densities as a sodium cathode.

2. RESULTS AND DISCUSSION According to the procedure described in the Experimental Section, well crystalline, single phase Na3V2(PO4)2F3 powder was obtained (cf. X-ray diffraction (XRD) in Figure S1) and then isostatically (∼500 MPa) pressed to a dense pellet that was used for transport measurements. In order to separate ionic and electronic conductivities, we used the cell Ti/NVPF/Ti (Figure 1a), in which Ti is reversible for electronic exchange but blocks sodium ions. Figure 1b presents a typical impedance spectrum at 130 °C, where the high frequency semicircle is followed by a Warburg response at low frequencies. According to the corresponding capacity (2.8 × 10−11 F), the semicircle 5208

DOI: 10.1021/acs.chemmater.7b00927 Chem. Mater. 2017, 29, 5207−5215

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Chemistry of Materials can be assigned to the bulk response of NVPF, while the Warburg diffusion tail is due to blocking of ionic species. The total conductivity can be obtained by fitting the ac spectra with the equivalent circuit, as shown in the inset of Figure 1b, with σtotal = 1.2 × 10−7 S cm−1. The capacitance calculated from the maximum frequency (ω = 1/RC) yield has a dielectric constant of εr ≈ 86 (which is rather high, probably due to the polarity introduced by fluorine), but still bulk-typical excluding that the arc is dominated by grain boundaries. The Warburg response at low frequencies can be attributed to the blocking effect and indicates that the ionic conductivity is much higher than the electronic conductivity in NVPF, leading to the conclusion that σionic = 1.2 × 10−7 S cm−1. To better investigate the polarization, we turned to the time domain (corresponding to times/frequencies that are not accessible by impedance spectroscopy). Figure 1c shows a typical galvanostatic polarization curve. The saturation limit at which the current is solely carried by electrons is not reached even after waiting times of ∼10 days. However, from the very well-fulfilled square root behavior, an upper limit for the electronic conductivity and the Na chemical diffusion coefficient can be extracted. (For details of the evaluation, see the Supporting Information and ref 37.) The results are σeon < 2 × 10−11 S cm−1 and DδNa < 1 × 10−9 cm2/s. Hence, we can confirm that the ionic conductivity in NVPF is at least 3−4 orders of magnitude higher than the electronic conductivity. It is important to note the influence of porosity (∼40%). If the particles are well connected and the microstructure is invariant, the absolute conductivity values should be corrected by a constant factor of the order of 0.6 (owing to a porosity of 40%). If the microstructure is such that the pores result in serious blocking/constriction effects, the absolute value might be strongly depressed. However, the observation of bulk-like effective dielectric constant indicates such effects to be less significant. This is also corroborated by the reproducibility of the results. Parameters such as activation energy or electronic transport number should be invariant anyway. The temperature dependence of the ionic conductivity obtained from impedance spectra of NVPF in the temperature range of 22−180 °C is shown in Figure 2. To make sure that we refer to equilibrium values, we investigated various heating

and cooling cycles. In this way, a reliable activation energy of ionic conduction of NVPF can be given as 0.68 eV. The dominance of the ionic behavior of NVPF is maintained down to room temperature (Figures 1b, 2, and S10). The above results now give us a clear advice how toward an optimized electrode architecture. When typical activation energies for Dδ are invoked, it is certainly reasonable to assume that at room temperature Dδ does not exceed 10−12 cm2/s meaning that achieving a stoichiometric redistribution within a few minutes (cf. rate of 10C) requires particle sizes below a micrometer. Making full use of particle downsizing requires one to quickly bring ions and electrons to the electroactive particle under regard. As the electronic conductivity is very low, electronic wiring has the highest priority. For ion access, it suffices to establish liquid electrolyte access through pores close to the particles. As the infiltration of electrolyte might be partially hindered by the local presence of the current collecting phase, probably not so much by interfacial tension effects but rather certainly due to the nanoheterogeneity introduced (“Lotus effect”),38 it is very helpful to realize that the ionic conductivity is perceptible and ion transport to the particle under (dis)charge can also occur through neighboring NVPF particles. This leads to the necessity of a porous architecture composed of nanosized particles, in which it is key that every particle is directly connected with the percolating electronic current collecting phase, even if this might be partly at the expense of ionic access, because the ionic boundary condition is less strict. (Nonetheless, electrolyte wetting is fine as well, see below.) Graphene offers an ideal conductive matrix to build a 3D conductive network (GN) replacing the traditional amorphous carbon coating, due to high electronic conductivity, high surface area, and excellent chemical stability.39 For this purpose, we propose and realize a low temperature hydrothermal method to prepare 0D-NVPF ⊂3D-GN and compare it with (i) the conventional solid state high temperature reaction and (ii) a low temperature synthetic route without an additional electronic conducting network. In the conventional solid state method (Figure 3a), due to the high temperature annealing process (600−900 °C), microsized NVPF with inhomogeneous and incomplete carbon coating is obtained. In samples synthesized by the hydrothermal method without carbon coating (Figure 3b), the overall electronic conduction is insufficient and so is the contact of the nanoparticle agglomerate with the current collector. By the procedure recommended here, a well-suited architecture is arrived at: NVPF nanoparticles are homogeneously embedded in a 3D graphene to form microsized clusters of ∼5−10 μm. Such 0DNVPF ⊂3D-GN is easily obtained from the precursor salts (i.e., V(III) acetylacetonate, NaF, NH4H2PO4) and graphene oxide (GO) by a single simple self-assembly hydrothermal step at low temperature (180 °C) without any postheat treatment (Figure 3c), which is superior to the previously reported methods. Figure 4a shows the XRD patterns of NVPF and NVPF ⊂GN prepared by the hydrothermal method without postheat treatment. All the characteristic peaks of the two samples are well indexed to the Na3V2(PO4)2F3 structure with the space group P42/mnm or Amam, which can not be distinguished due to the resolution of conventional XRD used here and the small particle sizes. No impurity phases could be detected. For NVPF ⊂GN, no extra graphite phase (2θ = 26°, corresponding to the (002) plane of graphite) is observed, demonstrating that the NVPF particles can prevent graphene aggregation into graphite.

Figure 2. Temperature-dependent ionic conductivity of NVPF obtained through ac impedance by using ion blocking cell Ti/ NVPF/Ti. Three heating and cooling cycles were performed in order to get stable conductivity values. (Inset: temperature-dependent ionic conductivity of NVPF obtained through ac impedance by using the ion blocking cell Ti/NVPF/Ti with the purpose of investigating the impact of the reported phase transition at 400 K.11 Obviously, there is no influence observed in this temperature range.) 5209

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Figure 3. Schematic comparison of various synthetic routes. (a) Conventional solid state high temperature synthesis method for microsized NVPF. (b) Low temperature solvothermal method for nanosized NVPF without carbon coating. (c) Low temperature hydrothermal approach to prepare NVPF ⊂GN with effective mixed-conducting networks.

Figure 4. (a) XRD patterns of NVPF (obtained by the hydrothermal method without GO) and NVPF ⊂GN. (b) Raman spectra of NVPF and NVPF ⊂GN. (c, d) Nitrogen adsorption/desorption isotherms of NVPF and NVPF ⊂GN; insets: the pore-size distribution plot calculated by the BJH method.

respectively.40 The broad peaks at 1342 and 1596 cm−1 for NVPF indicate the existence of a small amount of amorphous carbon. The pore structures and surface areas of NVPF and NVPF ⊂GN were investigated by using nitrogen isothermal adsorption (Figure 4c,d). All these samples exhibit type IV isothermal adsorption−desorption curves. The NVPF sample displays a Brunauer-Emmett-Teller (BET) surface area of 81 m2/g, with an average pore size of around 10 nm according to the Barrett-Joyner-Halenda (BJH) pore-size distribution curve. While for NVPF ⊂GN, its BET surface area is as high as 166

Estimated from the XRD patterns using Scherrer’s equation, the average grain sizes of NVPF and NVPF ⊂GN are obtained as ∼31 and ∼28 nm, respectively. The carbon content of NVPF and NVPF ⊂GN is around 0.6 and 15.8 wt % respectively, as measured by elemental analysis. The existence of carbon is also confirmed by Raman spectra (Figure 4b). The peaks at 1343 cm−1 (D-band) and 1599 cm−1 (G-band) in the NVPF ⊂GN root in characteristic sp2-like carbon material. In addition, the peaks localized at 2686, 2928, 3196 cm−1 can be assigned as 2D, D+G, and 2D′ modes of reduced graphene oxide, 5210

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Figure 5. (a−c) SEM images of NVPF ⊂GN at various magnifications: (a) overview at lower magnification; (b) side view and (c) top view at higher magnification. (d−f) TEM images of NVPF ⊂GN: (d, e) TEM images of NVPF ⊂GN; (f) HRTEM image of NVPF ⊂GN cathode.

Figure 6. (a) Galvanostatic charge−discharge profiles of NVPF ⊂GN cathode at different current densities. Capacities refer to the mass of the composite, and the mass of NVPF is shown by the upper and lower x axis, respectively. (b) Rate capability of NVPF and NVPF ⊂GN. (c) Long cycling stability of NVPF ⊂GN for 1000 cycles at 10C.

m2/g, and the BJH pore-size distribution plot shows that the sample is characterized by mesopores of 4.4 nm as mean radius. The morphology and microstructure of NVPF ⊂GN is investigated by scanning electron microscopy (SEM). The low magnification SEM image (Figure 5a) shows microsized clusters (∼5−10 μm) composed of graphene sheets and NVPF nanoparticles, which is important for practical application in order to increase the volumetric energy density compared to low packing density of nanoparticles. At higher

magnification, the side (Figure 5b) and top (Figure 5c) views show that the microsized clusters are formed by cube-like nanoparticles of ∼50 nm. The NVPF nanoparticles are wrapped by graphene sheets, which are themselves connected in a 3D conductive network. The morphology of the NVPF sample without graphene can be seen in Figure S2; however, due to the low electronic conductivity, the SEM image of NVPF is not sufficiently clear. For this reason, transmission electron microscopy (TEM) and high-resolution TEM 5211

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Figure 7. (a) CV curves of NVPF ⊂GN for various scan rates. (b) Corresponding relationship between the peak current Ip and the square root of the scan rate v1/2. (c) Comparison of rate performance of our NVPF ⊂GN to the typical ones obtained by different methods in the literature. (The samples of NVPF/C-high T, NVPF/rGO-high T, NVPF/CMK-3/C-high T, and NVPF-low T correspond to the references of 31, 42, 41, and 34, respectively.) (d) Comparison of energy densities of NVPF and NVP at various rates, indicating the advantages of NVPF.

0.5C to 10C, two pairs of charge and discharge plateaus can still be clearly distinguished, demonstrating the excellent kinetics for Na (de)intercalation in NVPF ⊂GN. However, for NVPF without graphene, the charge and discharge plateaus are not obvious and exhibit larger polarizations compared to NVPF ⊂GN at all current densities, indicating the significance of constructing efficient electronic networks. The rate capabilities of NVPF and NVPF ⊂GN at various current densities are shown in Figure 6b. The discharge specific capacities of NVPF are 109, 97, 88, 73, and 57 mAh/g at 0.5C, 1C, 2C, 5C, and 10C. In contrast to NVPF ⊂GN, the rate capability is largely improved. For example, at a current density of 0.5C, the discharge capacity is 114 mAh/g, which is close to the theoretical capacity. When the current densities are increased to 1C, 2C, and 5C, the capacities are still as high as 113, 110, and 105 mAh/g. Even at a high current density of 10C (charge and discharge within 6 min), the discharge capacity is still 99 mAh/ g, which presents 87% capacity retention compared to the value at 0.5C. The graphene content can be reduced in order to further increase volumetric energy density. A preliminary study shows that, if the carbon content is reduced to 6.6 wt %, the good rate performance of NVPF is still obtained (Figure S11). Obviously, such low temperature-synthesized 3D graphenedecorated NVPF is a promising candidate for a high-power and high-energy cathode material for SIBs. Furthermore, the cycling stability of NVPF ⊂GN is outstanding as well (Figure 6c). After 1000 cycles at 10C, the specific capacity is still 80 mAh/g, corresponding to a capacity retention of 75% relative to the first cycle (only 0.025% loss per cycle). The Coulombic efficiency remains at 99.3% after 1000 cycles. The cyclic voltammetry (CV) was carried out with various scan rates from 0.1 to 1 mV/s for NVPF (Figure S5a) and NVPF ⊂GN (Figure 7a). For both samples, the two pairs of

(HRTEM) were applied to investigate the detailed structures and morphologies of NVPF and NVPF ⊂GN. Figure S3 shows the TEM image of NVPF without graphene. The NVPF particles display a nanorod morphology with large size distribution ranging from 20 to 100 nm. For NVPF ⊂GN, however, as shown in Figure 5d, cube-like nanoparticles with uniform sizes of ∼30−50 nm are observed, which is consistent with the SEM results. Such cube-like NVPF nanoparticles are embedded in the 3D graphene network (Figure 5d,e) that is beneficial for the electronic transport. The HRTEM image of NVPF ⊂GN (Figure 5f) displays clear lattice fringes with a dspace of 0.53 nm, which corresponds to the (002) lattice planes of NVPF, indicating a high degree of crystallinity. In short, obviously, every particle has access to the percolating graphene network. The porosity and pore distribution is adequate for providing electrolyte access at least to the immediate neighborhood of a given particle. In order to check the sodium cathode performance of NVPF ⊂GN, half cells of NVPF ⊂GN/electrolyte/Na were constructed and tested in the electrochemical window of 2.3−4.3 V. The charge and discharge profiles of NVPF and NVPF ⊂GN at various current densities are displayed in Figures S4 and 6a. Note that 1C means that the full capacity is charged or discharged in 1 h, which corresponds to 110 mAh/g in this work. The capacity values given here are calculated by referring to the mass of NVPF. The values referring to the mass of the whole composite can be seen in Table S2. For NVPF ⊂GN at a current density of 0.5C, it presents two distinct charge and discharge plateaus located at 4.03 V/3.98 and 3.65 V/3.51 V with small polarizations. The energy density was calculated to be about ∼414 Wh/kg, which is increased by ∼16% compared to the state-of-the-art high rate Na3V2(PO4)3 (i.e., 358 Wh/kg).24 If the current density is increased from 5212

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3. CONCLUSIONS In summary, we systematically investigate the temperaturedependent transport behavior of NVPF through impedance and dc polarization by using an ion blocking cell. We confirm that the ionic conductivity in NVPF is at least 3−4 orders of magnitude higher than the electronic conductivity. The chemical sodium diffusion coefficient is also rather small. The activation energy of ionic conduction of NVPF is around 0.68 eV and limited by the electronic mobility. The low chemical diffusion coefficient, the low electronic conductivity, and the perceptible ionic conductivity give clear advice on optimized electrode architecture. As far as the realization is concerned, we proposed a facile one-step, low temperature hydrothermal route without any postheat treatment to prepare 0D-NVPF ⊂3D-GN in which every nanoparticle has good contact with the percolating graphene network where it is embedded. Such electrode exhibits high-power and high-energy properties, as well as long cycling stability. For example, in terms of power density, this electrode is superior to literature results. At high current density (6 min charge/discharge), the energy density of NVPF ⊂GN can reach values as high as 348 Wh/kg, which is even comparable to the value of the lithium cathode LiFePO4. In this work, decisive transport parameters for charge/discharge kinetics are measured and it is shown how this knowledge leads to an optimized electrode architecture. As far as application is concerned, not only does this work highlight the efficiency of the architecture of 0D-nanoparticle ⊂3D-GN, but also the one-step, low temperature hydrothermal approach is very cost-effective, making NVPF ⊂GN a promising candidate as a high-power and high-energy cathode material for SIBs.

redox peaks (O1/R1 and O2/R2) correspond to reversible extraction/insertion of two sodium ions. Especially, for NVPF ⊂GN, more defined and sharp redox peaks compared to NVPF are observed, indicating a much better kinetics of the sodium (de)intercalation process. The good linear relationships for plots of peak current (Ip) vs square root of the scan rate (v1/2) as displayed in Figures S5b and 7b demonstrate a diffusioncontrolled process of sodium (de)intercalation for both NVPF and NVPF ⊂GN. The determination of the effective diffusion coefficients can be done by Randles-Sevcik analysis based on the above CV tests. Such values are calculated for NVPF and NVPF ⊂GN for different oxidation (O1 and O2) and reduction (R1 and R2) processes in Table S1. In Figures S6−S9, one can also find detailed fitting results. The effective diffusion coefficients of NVPF ⊂GN are 1.7 × 10−10 (O1), 1.8 × 10−10 (O2), 9.8 × 10−11 (R1), and 1.0 × 10−10 (R2) cm2/s, respectively, and hence are almost 1 order of magnitude higher than NVPF without graphene. Figure 7c compares the rate capabilities of our NVPF sample with the typical literature examples obtained by different synthesis methods.31,34,41,42 It is clear that the high rate capability of our NVPF ⊂GN belongs to the best ones reported in the literature. Although the NVPF/ CMK-3/C sample shows higher capacities,41 the high temperature preparation is rather energy consuming when compared to our cost-effective low temperature synthesis. Figure 7d compares the energy densities for NVPF ⊂GN and the state-ofthe-art high NVP24 at various C rates. For each C rate, the energy density of NVPF ⊂GN is higher than that for NVP, demonstrating the great potential of such low temperaturesynthesized 3D graphene decorated NVPF. Concerning practical application, (dis)charging rates of 10C (charge and discharge in 6 min) are very fast. At such high current density (10C), the energy density of NVPF ⊂GN is 348 Wh/kg, which is even comparable to the value of the popular lithium cathode LiFePO4 (351 Wh/kg, for a discharge in 6 min, corresponding to one of the best literature results).24 Hence, the outstanding electrochemical performance of highpower and high-energy NVPF ⊂GN can be attributed to the effective design of mixed-conducting network according to the intrinsic transport behavior of NVPF investigated in this work. The rather ideal bicontinuous electrode architecture, NVPF nanoparticles (∼30−50 nm) embedded in a 3D graphene network, provides pronounced advantages: (1) Nanosized NVPF particles effectively reduce the ambipolar Na diffusion time in the crystals. (2) The 3D graphene network offers efficient electronic pathways between and within the NVPF particles. (3) The 3D graphene network with large surface area and mesopores filled by electrolyte facilitates fast charge transfer across the interface of electrolyte/electrode. (4) The 3D graphene encapsulation prevents self-aggregation of NVPF particles, ensures good electronic contact, and mitigates strain effects due to volume change during Na (de)intercalation. Furthermore, from the practical application point of view, such a one-step, low temperature hydrothermal approach will greatly reduce the cost of material production compared to the conventional two-step, high temperature method, promoting the application of NVPF ⊂GN as a high-power and high-energy sodium battery cathode. In short, the results for the so-prepared NVPF electrode are, in terms of energy and power density, as well as in terms of fabrication cost issues, promising.

4. EXPERIMENTAL SECTION Synthesis of Bulk NVPF for Conductivity Measurements. The carbon free NVPF was prepared by dissolving a stoichiometric amount of NH4VO3, NH4H2PO4, and NaF in water. Using a hot plate, the water was slowly evaporated under continuous stirring. The powder was ground with mortar and pestle. Afterward, the powder was sintered at 350 °C for 2 h in Ar (95%)/H2 (5%). The powder was ground again and sintered at 600 °C for 6 h in Ar (95%)/H2 (5%). Soft Chemistry Method Synthesis of NVPF ⊂GN. GO was prepared by the modified Hummers’ method. In a typical process, GO (100 mg), 3 mmol of NaF, and 2 mmol of NH4H2PO4 were dissolved into 15 mL of water to prepare the solution 1. V(III) acetylacetonate (V(III) (acac)3) was dissolved in 10 mL of dimethylformamide (DMF) to form the solution 2. Solution 1 and solution 2 were mixed by magnetic stirring and ultrasonic treatment to form the solution 3. Solution 3 was heated at 180 °C for 12 h in a hydrothermal autoclave with a Teflon vessel, and a monolithic gel-like product was obtained. The resulting product was repeatedly washed with ethanol and water several times, followed by drying in a vacuum oven at 80 °C overnight. If no GO was used in this synthesis route, NVPF instead of NVPF ⊂GN can be obtained. Structural and Electrochemical Characterization. XRD was performed with a Philips PW 3020 diffractometer using Cu Kα radiation. SEM was done by using a JEOL 6300F field-emission scanning electron microscope (JEOL, Tokyo, Japan). TEM and HRTEM were carried out by using a JEOL 4000FX transmission electron microscope (JEOL, Tokyo, Japan) operated at 400 kV. The carbon content is measured by a carbon sulfur determinator (ELTRA, CS800), and a Raman spectrum is measured by Labram V010 with a 532 nm diode laser. Conductivity Measurements. For conductivity measurements, the polycrystalline powder sample of NVPF was pressed isostatically (∼500 MPa) to pellets, and the relative density was around 63%. The 5213

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Chemistry of Materials symmetric ion blocking cell was made by depositing 400 nm titanium films on both side of the pellets through the evaporation technique. Impedance measurements were done in the frequency range of 106− 10−2 with a High Frequency Dielectric Analyzer (Novocontrol) at a temperature range of 22−180 °C. Typical waiting times to allow the system to equilibrate have been 1 day. Three heating and cooling cycles were performed in order to get stable conductivity values. The dc polarization experiments in galvanostatic mode were carried out with a Keithley 2634B SourceMeter. Due to the sluggish kinetics, we investigate the transport situation at 130 °C. For both ac and dc conductivity measurements, the cell was put in an airtight quartz tube with a gas inlet and outlet, which was arranged in a furnace to control the temperature. The measurements were carried out in pure Ar, and the oxygen value is less than 2 ppm monitored by a commercial oxygen measuring system (ZR5). Electrochemical Tests. NVPF or NVPF ⊂GN (70 wt %), carbon black (20 wt %, Super-P, Timcal), and poly(vinylidene fluoride) binder (10 wt %, Aldrich) in N-methylpyrrolidone were mixed together. The obtained slurry was pasted on an Al foil, followed by drying in a vacuum oven overnight at 80 °C. Electrochemical test cells (Swageloktype) were assembled in an argon-filled glovebox (O2 ≤ 0.1 ppm, H2O ≤ 1 ppm) with the electroactive material coated Al foil as working electrode, sodium metal foil as the counter/reference electrode, and 1 M solution of NaClO4 in the propylene carbonate (PC) with 5% fluoroethylene carbonate (FEC) as the electrolyte. Glass fiber (Whatman) was used as a separator. The batteries were charged and discharged galvanostatically in the electrochemical window between 2.3 and 4.3 V using an Arbin MSTAT battery tester at room temperature. Cyclic voltammetry was carried out with a Voltalab system (D21 V032, Radiometer Analytical SAS, France) on Swageloktype cells.

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Experts, the New Century Excellent Talents in University (NCET), and the Fundamental Research Funds for the Central Universities (WK3430000004), as well as the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 312483 (ESTEEM2).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00927. Powder XRD pattern of NVPF sample; SEM and TEM images of NVPF; charge−discharge profiles of NVPF; CV curves of NVPF; relationship between the peak current Ip and the square root of the scan rate v1/2; fitting results of oxidation and reduction peak currents Ip as a function of scan rates of NVPF and NVPF ⊂GN; Ac impedance spectra of Ti/NVPF/Ti; rate capability of NVPF ⊂GN; apparent diffusion coefficients of both NVPF and NVPF ⊂GN; specific capacity of sodium storage; estimation of the upper limit of electronic conductivity and diffusion coefficient from dc polarization (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yan Yu: 0000-0002-3685-7773 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Anette Fuchs for SEM investigations and Dr. Helga Hoier for XRD measurements. This work was financially supported by the Sofja Kovalevskaja award of the Alexander von Humboldt Foundation, the Max-Planck Society, the National Natural Science Foundation of China (No. 51622210, No. 21373195), the Recruitment Program of Global 5214

DOI: 10.1021/acs.chemmater.7b00927 Chem. Mater. 2017, 29, 5207−5215

Article

Chemistry of Materials

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DOI: 10.1021/acs.chemmater.7b00927 Chem. Mater. 2017, 29, 5207−5215