3D-Printed Hierarchical Porous Frameworks for ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

3D-Printed Hierarchical Porous Frameworks for Sodium Storage Junwei Ding, Kai Shen, Zhiguo Du, Bin Li, and Shubin Yang* Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, 100191 Beijing, China S Supporting Information *

ABSTRACT: Exploring 3D printing in the field of sodium-ion batteries is a great challenge since conventionally inks cause unavoidably compact filaments or frameworks, which significantly hamper the infiltration of electrolyte and diffusion of big-size sodium ions (1.02 Å), resulting in low reversible capacities. Here, new hierarchical porous frameworks are 3D printed for sodium storage by employing well-designed GO-contained inks. The resultant frameworks possess continuous filaments, hierarchical multihole gridding. Such distinct properties render these frameworks able to facilitate the fast transportation of both sodium ion and electron. As a result, 3D-printed hierarchical porous frameworks reveal the high specific capacity as well as rate performance and periodic steadiness for up to 900 cycles for sodium storage. KEYWORDS: 3D printing, frameworks, sodium-ion batteries, cathode, anode inks inevitably lead to compact filaments and/or frameworks, which severely inhibit the infiltration of electrolyte and fast diffusion of alkali metal ions, leading to low reversible capacities and poor rate capabilities for energy storage. In particular, in the case of sodium storage, this issue becomes more severe owing to the bigger size of Na+ (0.102 nm) than that of Li+ (0.076 nm). In addition, when the GO concentrations were reduced to less than 5 mg mL−1, GO aqueous dispersions display liquid-like behavior, which are unsuitable as inks for 3D printing.20 Thus, it remains a big challenge to explore appropriate inks and to realize 3D-printed frameworks for sodium storage. Herein, we demonstrate a facile approach to 3D printing hierarchical porous frameworks for sodium storage by employing a well-designed GO-contained ink and controllable freezedrying approach. The as-prepared frameworks possess continuous filaments, hierarchical multihole gridding. Such distinct properties render these frameworks able to facilitate fast transportation of both sodium ion and electron. As a consequence, 3D-printed hierarchical porous frameworks reveal the high specific capacity as well as rate performance and periodic steadiness for up to 900 cycles for sodium storage. To the best of our knowledge, this is the first time that 3D-printing sodium-ion batteries have been realized.

1. INTRODUCTION 3D printing has attracted a wealth of attentiveness for fabrication of advanced functional materials, novel frameworks, and complex systems, with broad applications for energy storage,1−3 biological tissues and scaffolds,4 and microfluidic printheads.5 The emergence of dual-ion batteries provides a new development space for 3D printing in the field of energy storage.6−9 Amid explored additive manufacturing technologies,10−13 extrusion-based 3D printing is the most universal one owing to its straightforward printing principle and inexpensive manufacture process.14 In addition, extrusion-based 3D printing has a wide range of inks with suitable rheological properties to be chosen. Except for traditional polymer-based inks,15−18 some new inks such as graphene oxide (GO) aqueous dispersions have been also explored to construct various filaments and/or frameworks including well-defined nanowires,19 aerogel lattices,20 and networks17 owing to their gel-like behavior.21−23 Moreover, 3D-printed GO-based frameworks could be reduced chemically or thermally to graphene-based frameworks with high electrical conductivities, exhibiting wide applications for energy storage such as microsupercapacitors and microbatteries.14,24 A pioneering work of 3D-printed graphene-based microsupercapacitors was reported by Zhu and co-workers,24 where the level of GO ink was 40 mg mL−1. The 3D-printed microsupercapacitors show a gravimetric capacitance of 4.76 F g−1 at 0.4 A g−1 based on a quasi-solid-state symmetric supercapacitor.24 Subsequently, 3D-printed graphene-contained lithium-ion microbatteries were achieved on the basis of inks consisting of highly condensed GO (80 mg mL−1) and different active materials (lithium iron phosphate and lithium titanium oxide).14 Nevertheless, highly condensed GO or GO-contained © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis of GO and Na3V2(PO4)3. GO was prepared by an improved Hummers course. The resultant GO dispersion underwent Received: August 26, 2017 Accepted: November 10, 2017 Published: November 10, 2017 A

DOI: 10.1021/acsami.7b12892 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of 3D-printing hierarchical porous frameworks. NVP-GO inks with tunable NVP levels were first fabricated and then employed to print well-designed frameworks on substrates via a nozzle settled at a 3D-printing workstation. (b−d) SEM images of 3D-printed cubiclattice frameworks with different magnifications, showing continuous filaments and hierarchical porous structure. dialysis (room temperature, 14 days) and rotary evaporation (60 °C) to remove suitable water. The concentration of the final GO dispersion adopted is 5.24 mg mL−1. In a typical synthesis of Na3V2(PO4)3 (NVP), NH4VO3 (2 mmol) and oxalic acid (8.4 mmol) were added in 80 mL of water at 80 °C. After stirring for 1 h, the above GO dispersion was added into the mixture under sonication for 0.5 h. Then Na2CO3 (1.5 mmol) and NH4H2PO4 (3 mmol) were added under agitation for 36 h. The resultant product was precalcined at 350 °C for 3 h under an N2 atmosphere, followed by the powder produced being calcined at 700 °C for 10 h in H2−Ar mixture with 5% H2 (v/v). 2.2. Preparation of NVP-GO Inks with Different Levels of NVP. For the sake of obtaining desired hierarchical porous frameworks, the GO with 20 mg mL−1 was applied for all of the inks of NVP9-GO, NVP7/3-GO, and NVP1-GO. To be specific, for the inks of NVP9-GO, NVP7/3-GO, and NVP1-GO, the mass ratios of NVP to GO were 9:1, 7:3, and 1:1, respectively, and the corresponding total solid material concentrations were 200, 67, and 40 mg mL−1, respectively. Meanwhile, for the inks of NVP9-GO, NVP7/3-GO, and NVP1-GO, according to thermogravimetric analysis (TGA) (Figure S3), the corresponding resultant hierarchical porous frameworks could be denoted as NVP6.8-rGO, NVP4.2-rGO, and NVP2.8-rGO, where x represents the content of NVP in the frameworks. In a typical synthesis for the ink of NVP9-GO, 0.4716 g of NVP was added into 10 mL of the above GO dispersion (5.24 mg mL−1), and then the mixed solution was stirred and sonicated for 20 min. The above mixed solution was then controllably evaporated at 60 °C until the GO level was reached at 20 mg mL−1. 2.3. Ink Rheology. The rheological measurement parameters of NVP-GO inks were adopted according to a reported method.14 2.4. 3D Printing of Hierarchical Porous Frameworks. The process of 3D printing was achieved on the basis of a previous report.14 Note that the instrument equipment (FISNAR F4200N and DSP501N) and printing principles we applied are exactly the same as those previously reported.14 Notably, when the ink concentration is higher than 20 mg mL−1, the 3D-printed filaments or frameworks are

relatively compact without porous structure. Meanwhile, when the ink concentration is 5−20 mg mL−1, the 3D-printed filaments or frameworks are prone to collapse, due to ink containing too much water. In addition, when the GO concentrations are reduced to less than 5 mg mL−1, GO aqueous dispersions display liquid-like behavior, which are unsuitable as inks for 3D printing. To study the electrochemical performance of 3D-printed NVP-rGO frameworks, we printed 2-layer frameworks (8 mm × 8 mm, electrode width = 200 μ m, spacing = 800 μ m) on stainless steel sheets followed by freeze drying and annealing at 200 °C for 2 h in an inert atmosphere. The areal loading of 3D-printed NVP-rGO frameworks is ∼18 mg cm−2. 2.5. Characterization Methods. The morphology and structure of the obtained products were studied via a field-emission scanning electron microscope (JSM 7500, JEOL, Japan), transmission electron microscopy (TEM), X-ray diffraction (XRD, Rigaku D/max2500PC, Cu Kα), and a Vario EL cube (Elementar, Germany). The specific surface area was executed via a Quantachrome QDS-MP-30 analyzer (USA) at 77 K. The carbon content was determined by TGA using a NETZSCH TG 209 F1 Libra. 2.6. Electrochemical Measurements. Electrochemical tests were conducted in 2032 coin cells. For half-cell battery fabrication, NVPrGO frameworks on stainless steel sheet were used as the cathode or anode. For full-cell battery fabrication, the NVP6.8-rGO frameworks were used as the cathode and anode, simultaneously. The electrolyte was 1 M NaClO4 in ethylene carbon−propylene carbonate (1:1 w/w) with 5 wt % fluoroethylene carbonate and a glass fiber film as the separator. The charge−discharge tests were operated on a Land CT2001A system. The voltage ranges of the cathode and anode halfcells as well as full cell were 2.3−3.9, 1.3−2.0, and 1.0−3.0 V, respectively. Cyclic voltammetry was measured using a CHI 660D workstation. Electrochemical impedance spectroscopic measurements were executed on Autolab equipment (PGSTAT302N). B


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Figure 2. Schematics and SEM images of four types of 3D-printed hierarchical porous frameworks by using tunable inks. (a) Schematic representation and (b, c) SEM images of staggered grids printed by NVP9-GO ink, showing numerous micropores dispersed uniformly in the whole filaments. (d) Schematic representation and (e, f) SEM images of square coils printed by NVP7/3-GO ink. (g) Schematic representation and (h, i) SEM images of mosquito coils printed by NVP1-GO ink. (j) Schematic representation and (k, l) SEM images of circular arrays printed by NVP1-GO ink. SEM images of f, i, and l show the uneven distribution of the bigger sparse pores dispersed in the whole filaments.

3. RESULTS AND DISCUSSION Figure 1a shows the 3D-printing processes of hierarchical porous frameworks. Typically, homogeneous GO-contained inks with suitable rheological properties were first prepared by immobilizing the concentration of GO at 20 mg mL−1 and adjusting the concentrations of NVP from 180 to 47 to 20 mg mL−1, where NVP has been demonstrated as a promising electrode material having a capacity of 117.6 mAh g−1 for sodium storage (Figure S1 and S2). By using the above GOcontaining inks, a nozzle with a diameter of 200 μm settled at a 3D-printing workstation was employed to layer-by-layer print program-arranged frameworks on stainless steel sheets. Then the as-prepared frameworks were controllably freeze dried and thermally treated to reduce GO, generating hierarchical porous frameworks. According to TGA, the resultant hierarchical porous frameworks could be denoted as NVPx-rGO, where x represents the content of NVP in the frameworks (Figure S3). It should be noted that the average size of GO is ∼20 μm (Figure S4). The as-prepared hierarchical porous frameworks were initially investigated via FE-SEM. As shown in Figure 1b, in the case of our 3D cubic-lattice frameworks, the diameter (d) of the filaments is ∼200 μm and the interval (L) between two filaments is ∼800 μm, showing an interval-to-diameter proportion (L/d) of ∼4 (illustrated in Figure S5). It is noted that the filaments of the frameworks maintain the same

diameters before and after thermal annealing. The magnified SEM images (Figure 1c and Figure S6) further reveal that two adjacent filaments fuse together due to water evaporation under our freezing condition, retaining the structural integrity of the frameworks. Remarkably enough, the filaments are hierarchically porous and cross-linked by numerous flexible nanosheets (Figure 1c). A higher magnified SEM image (Figure 1d) discloses that the NVP particles are homogeneously dispersed in the frameworks. To confirm the widespread applicability of our GOcontaining inks in the construction of various frameworks with well-designed and complex structures, we printed four types of frameworks such as staggered grids, square coils, mosquito coils, and circular arrays via tunable NVP-GO inks, as shown in Figure 2. Obviously, all of the frameworks exhibit hierarchical porous structures in which the pore sizes are strongly dependent on the levels of NVP in the NVP-GO inks. In the case of NVP9-GO ink, there are numerous micropores with sizes of 1−20 μm dispersed uniformly in the whole filaments, constructed by many ultrathin nanosheets (Figure 2a−c). In comparison, with decreasing levels of NVP in inks, the bigger sparse pores with sizes of 5−30 μm are generated in the filaments (Figure 2d−l). This should be ascribed to the lower ratios between NVP and water that cause inhomogeneous dispersion of ice in the filaments during our freeze-drying process. The formation of a hierarchical porous structure of 3DC


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Figure 3. Rheological properties of NVP-GO inks. (a) Viscosity at different shearing speeds for the NVP-GO inks with different levels of NVP, showing the addition of NVP has less impact on the resultant inks. Storage and loss moduli for (b) NVP9-GO, (c) NVP7/3-GO, and (d) NVP1-GO inks, showing obviously different elastic and viscous response properties.

Figure 4. Electrochemical performance of 3D-printed NVP-rGO frameworks. (a) Selected 10th charge and discharge profiles at 2C, (b) cycle performance at 2C, and (c) rate capability of 3D-printed NVP-rGO frameworks. (d) Long cycling performance of NVP6.8-rGO at 1C.

printed frameworks should result from the ice-template during our freeze-drying process. Specifically, the original 3D-printed NVP-GO frameworks show a hydrogel-like state owing to the interactions between the GO functional groups and the water molecules. After a rapid freeze process, water in the frameworks was turned into ice. Though a freeze-drying process under a high-vacuum condition, solid ice was directly removed in water vapor, generating 3D-printed frameworks with different porous structure, dependent on the water residue in the frameworks.23,25

To evaluate the influence of ink nature on the microstructure of the 3D-printed frameworks, we tested the rheological properties of NVP-GO inks. Figure 3 reveals the rheological property of the NVP-GO inks. The viscosity curves of three inks with different levels of NVP are nearly overlapped (Figure 3a), demonstrating that addition of NVP can not change the shear-thinning behaviors of GO-based inks.22 Surprisingly, for the NVP9-GO ink, the yield stress value is ∼10° Pa, which is much lower than those of other NVP-GO inks (∼101−103 Pa, Figure 3b−d) and that reported for condensed GO ink (103 D


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Figure 5. (a) Nyquist plots and (b) corresponding linear fitting of Z′ versus ω−1/2 in the low-frequency range of 3D-printed NVP-rGO frameworks after 90 cycles.

Pa).14 Moreover, the variation trends of the storage and loss moduli properties vs shear stress are opposite to the reported highly condensed GO-based inks.14 Thus, it can be speculated that the low flow stress and high platform modulus are indispensable for 3D-printed frameworks with uniform porous structure. Notably, all of the NVP-GO inks have stable viscosity over a long time for up to 1 month. With prolonging the storing time over 1 month, the NVP-GO inks will gradually lose water and the viscosity of inks will become larger, which is against formation of 3D-printed hierarchical porous frameworks. Fortunately, the NVP-GO inks with high viscosity can be facilely achieved by adding the right amount of water and be reused for 3D printing. The electrochemical performance of 3D-printed NVP-rGO frameworks as cathode was primordially evaluated via cyclic voltammetry (CV). For NVP6.8-rGO framework, there is an oxidation (Na extraction) peak at 3.5 V and a reduction (Na insertion) peak at 3.3 V (Figure S7).26 To further evaluate the electrochemical performance of 3D-printed NVP-rGO frameworks for sodium storage, galvanostatic discharge−charge measurements were conducted at rates from 0.2 to 20C (1C = 2.1 mAh cm−2). Strikingly, a high capacity of 1.26 mAh cm−2 is obtained at 0.2C in the case of NVP6.8-rGO framework. This value is higher than those of NVP4.2-rGO and NVP2.8-rGO frameworks (∼0.67 mAh cm−2). Moreover, on increasing the rate from 0.2 to 5 C, the capacity of NVP6.8-rGO is 0.89 mAh cm−2. Even at the highest rate of 20C, NVP6.8-rGO still exhibits a high capacity of 0.65 mAh cm−2, superior to the most reported NVP/C composites.27−30 Such good rate capability of NVP6.8-rGO should be ascribed to the distinct hierarchical porous gridding that can efficiently shorten the transportation of Na+ especially at high current rates. As the current rate is back to 0.2 C after 80 cycles, the capacity of NVP6.8-rGO can be recovered to 1.26 mAh cm−2 again (Figure 4a−c). More importantly, NVP6.8-rGO delivers a very high capacity retention of 90.1% after 900 cycles at 1C (Figure 4d), demonstrating clearly the stable structure of our 3D-printing frameworks during long cycle processes. Note that there are no corresponding redox peak and charge−discharge voltage plateau in the CV and charge−discharge curves of NVP2.8rGO framework due to the high content of rGO relative to NVP (Figure S7 and Figure 4a). Furthermore, the traditional graphite in lithium-ion batteries typically has ∼4 mA h cm−2.31 Meanwhile, most of the materials used as electrodes have observably lower capacity toward Na+ than Li+;32 the areal capacity of 1.26 mA h cm−2 of NVP6.8-rGO framework is

comparable. Table S1 gives the electrochemical performance comparison of our 3D-printed electrodes with reported sodium-ion batteries. To identify the kinetics of 3D-printed hierarchical porous frameworks for sodium storage, EIS measurements were conducted. As shown in Figure 5a, the Nyquist plots exhibit a semicircle and a line in the altofrequency and low-frequency, respectively, which match with an improved Randles equivalent circuit (Figure S8).33,34 The semicircle belongs to the Na+ movement pass to the interfaces of the NVP-rGO frameworks and electrolyte. As summarized in Table S2, the charge transfer resistance (Rct) of NVP6.8-rGO is 157 Ω, which is much lower than those of NVP4.2-rGO (417 Ω) and NVP2.8-rGO (802 Ω), indicating the high electrochemical property. The lines with different gradients that manifest the 3D-printed frameworks have various solid-state Na+ proliferation behaviors (Figure 5b).35 The diffusion coefficient of sodium ions (DNa+) is computed adopting eq 1 D Na + = 0.5(RT /An2F 2σwC)2

(1)

where R, T, A, n, F, σw, and C are conventional parameters.36 Clearly, The DNa+ of the NVP6.8-rGO framework is calculated to be 6.12 × 10−13 cm2 s−1, higher than those of other NVPrGO frameworks (1.17−2.32 × 10−14 cm2 s−1) and reported carbon-coated NVP (1.96−4.59 × 10−13 cm2 s−1),37−39 demonstrating the fastest Na+ ions transport in our NVP6.8rGO framework owing to its ideal porous structure. Except for utilizing for cathode material, NVP can be also uased as an anode material for sodium storage since it has electrochemical activity at a low voltage of ∼1.6 V.26 Thus, we further identify the electrochemical performance of our NVPrGO frameworks as anode materials for sodium storage. NVP6.8-rGO exhibits 0.32 mAh cm−2 at 2C (Figure S9). This value is higher than those of other NVP-rGO frameworks (0.08−0.23 mAh cm−2). Meanwhile, the electrochemical performance of 3D-printed NVP6.8-rGO frameworks without freeze-drying process is also studied (Figure S10). It can be seen that the hierarchical porous frameworks contribute to the improvement of electrochemical performance.Taking into account the big voltage difference as cathode and anode, a symmetric full cell only using NVP6.8-rGO frameworks was constructed (Figure S11), which delivers a high capacity of 0.39 mAh cm−2 at 1C, comparable to previously reported graphiteNVP full cell.26 This sheds light on the direct and rapid construction of full cells via the 3D-printing approach. E


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4. CONCLUSIONS In conclusion, for the first time, hierarchical porous frameworks were 3D printed for sodium storage by employing tunable inks composed of condensed GO and NVP. In the resultant frameworks, the continuous filaments with hierarchical porous structure faciliate the rapid transfer of sodium ions, and the homogeneous dispersion of graphene gives rise to high conductivity for the whole electrode, facilitating fast transportation of electrons. As a consequence, the 3D-printed NVPrGO frameworks exhibit high reversible capacities and stable cycle performance using as both cathode and anode. It is conceivable that a variety of multifunctional hierarchical porous frameworks can be 3D printed for broad applications via tunable GO-based inks.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12892. XRD, TGA, BET, SEM, and TEM of NVP; TGA, SEM, and schematic illustration of 3D-printed frameworks, electrochemical performance of 3D-printed frameworks, and comparison of electrochemical performance between our 3D-printed electrodes and similar sodium-ion batteries (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shubin Yang: 0000-0001-9973-9785 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Science Foundation of China (Nos. 51572007 and 51622203), “Recruitment Program of Global Experts”.

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REFERENCES

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